Electrocatalyst for Ethanol Oxidation Reaction Based on Isatin-modified Chitosan

Electrocatalyst for Ethanol Oxidation Reaction Based on Isatin-modified Chitosan | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Electrocatalyst for Ethanol Oxidation Reaction Based on Isatin-modified Chitosan Brenda Antunes Louriçal Paixão, Nathalia Biazotto Sá, Jardel Ramos Encarnação, and 9 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4986733/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 23 Nov, 2024 Read the published version in Journal of Applied Electrochemistry → Version 1 posted 10 You are reading this latest preprint version Abstract Direct Ethanol Fuel Cells (DEFCs) have garnered significant attention in recent years due to their potential for producing clean and renewable energy. DEFCs are particularly appealing because of their low toxicity and high current density. This study aims to investigate the catalytic activity of an isatin-modified chitosan Schiff-base in ethanol electrooxidation for possible applications in DEFCs. This study utilizes density functional theory calculations and experimental results to analyze the relative stability, through Gibbs free energy, between two protonated models of an isatin-modified chitosan Schiff-base. The synthesized isatin-modified chitosan Schiff base was structurally characterized through nuclear magnetic resonance spectroscopy, Fourier transform infrared vibrational spectroscopy, ultraviolet-visible spectroscopy, and thermogravimetric analysis. Morphological aspects of the compound, such as formation and structure, were assessed by scanning electron microscopy. In electrocatalytic evaluation experiments, the carbon paste electrode with isatin-modified chitosan (5%-CPE) demonstrated efficiency in oxidizing ethanol, especially at pH 3. At this pH, protonation of hydroxyl and amino groups present in chitosan favored ethanol oxidation. Current density increased proportionally to ethanol concentration. Electrochemical impedance spectroscopy pointed to a capacitive behavior of the sensor, indicated by high values of electrical double layer capacitance. Our results offer new insights into the isatin-modified chitosan Schiff base (CIS), indicating its potential as an electrocatalyst for ethanol oxidation. This material shows promise for use in direct ethanol fuel cells (DEFCs), providing a more sustainable and efficient alternative to traditional platinum catalysts. Chitosan Isatin Fuel Cells Electrocatalysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 INTRODUCTION The energy crisis faced by humanity and its consequent damage to the environment requires that new, more sustainable technologies using degradable materials be developed. Among the alternative power sources are Direct Alcohol Fuel Cells (DAFCs), particularly Direct Ethanol Fuel Cells (DEFCs). DEFCs offer several advantages over cells that use methanol and formic acid. They are safer for human use due to their lower toxicity, provide high current density and energy conversion efficiency, have reduced ethanol fuel crossover, and can be sourced from renewable plant materials [ 1 – 4 ]. However, a major challenge in developing DEFCs is the release of poisonous CO into the environment by metallic electrocatalysts during the process. To overcome this, it is essential to improve the anodic catalysts employed in ethanol oxidation, by either bettering or replacing Pt electrocatalysts, which are widely used due to their high efficiency [ 5 – 7 ]. To meet the demand for novel electrocatalysts capable of overcoming the aforementioned challenges, polymeric materials have garnered the attention of researchers worldwide. These materials offer significant benefits across various industries, including good performance and low processing costs [ 8 , 9 ]. In particular, biopolymers stand out as more sustainable alternatives as they are biodegradable, naturally available, and renewable. In addition, they exhibit low toxicity and important bioactive properties among other characteristics, making them suitable for multiple applications [ 10 – 14 ]. Among existing biopolymers, chitosan (Chi) is widely applicable as it can be obtained by means of deacetylation of chitin, extracted mostly from exoskeletons of crustaceans and other marine animals [ 15 – 17 ] but also from fungi and insect cuticles [ 18 – 20 ]. Its linear structure composed of glucosamine and acetylglucosamine confers important physicochemical properties such as high intrinsic rigidity, large spacing between the charges, and high propensity to form intra and intermolecular hydrogen bonds [ 21 ]. The literature contains numerous accounts of uses of modified chitosan electrodes in various electrochemical applications. These include nitrite sensing [ 2 – 23 ], metal detection [ 24 – 26 ], ciprofloxacin detection in biological samples [ 27 ], glucose oxidation [ 28 ], and electrocatalytic oxidation of formaldehyde [ 29 ], methanol [ 30 ], and ethanol [ 31 ]. For example, Guibal provides a comprehensive review on the use of chitosan and its derivatives in heterogeneous catalysis, highlighting their significance for industry [ 32 ]. Vedula and Yadav discuss the challenges and opportunities associated with chitosan-based membranes, particularly in catalysis and fuel cells [ 33 ]. They conclude that these membranes can address many issues related to direct alcohol fuel cells (DAFCs) [ 4 , 33 – 37 ]. Additionally, the functional groups found in chitosan make it a versatile material for synthesizing derivatives capable of enhancing its properties [ 38 ]. By way of illustration, polymeric Schiff bases can be produced by condensing the primary amine in chitosan with a carbonyl group, aldehyde, or ketone. One way to functionalize chitosan to obtain Schiff bases is by using isatin (IS − 1H-indole-2,3-dione). Recent studies have explored the use of isatin and its derivatives in anionic polymeric membranes, with potential applications in power and fuel cells [ 39 – 41 ]. Our research group has focused on studying the catalytic activity of uranyl complexes with Salen-type Schiff bases as co-catalysts in ethanol electrooxidation, achieving promising results [ 42 , 43 ]. Given the lack in the literature of studies on the use of polymeric Schiff bases derived from chitosan and isatin in DEFC catalysts, this work aims to explore the catalytic activity of these Schiff bases in ethanol electrooxidation, with potential application in DEFCs. MATERIALS AND METHODS Reagents Chitosan (with an average molar mass and a deacetylation degree of 76%) and isatin (97%) were obtained from Sigma-Aldrich (USA). HPLC-grade acetonitrile, obtained from JT Baker, was used without prior purification. Other analytical-grade (P.A.) solvents were obtained from Synth. Synthesis of the Isatin-modified Chitosan Schiff Base (CIS) The modification of chitosan with isatin was carried out using a method adapted from Araújo and colleagues [ 44 ]. In this process, 0.5 g of chitosan was dissolved in 75 mL of a 0.15 mol L⁻¹ acetic acid solution and stirred magnetically at 55°C for 24 hours. After this time, an ethanolic solution of isatin (at a 1.0:1.5 amino/isatin ratio) was added dropwise to the mixture. The system was then stirred magnetically at 55°C for an additional 24 hours. The general synthesis route is shown in Fig. 1 . Figure 1 . General route for synthesis of Schiff base after modification (QI) of chitosan (1) with isatin (2). The reaction mixture was transferred to a dialysis membrane (Spectra/Por® 6, MWCO = 14,000 g/mol) and dialyzed in deionized water for 5 days. The modified polymer was then transferred to petri dishes and dried in an oven at 50°C until fully dry. ¹H NMR (400 MHz, D₂O/TFA, 25°C, ppm): δ = 1.9 (s, H7), 3.0 (s, H2), 3.6–3.7 (m, H3-H6), 4.3 (s, H1), 6.8 (d, H9), 6.9 (t, HR), 7.3 (t, H10), 7.4 (d, H11), 8.0 (s, H8). IR (cm⁻¹): 3600 − 3200 (O-H and N-H), 2919 (C-H asymmetric), 2872 (C-H symmetric), 1643 (C = O acetamide), 1620 (C = N), 1453 (C = C), 1153 and 897 (C-O β(1–4) glycosidic bonds), 1061 and 1025 (C-OH). UV-Vis (λ max , H₂O/HCl 1% (v/v), 25°C, nm): 205 (π→π*), 239 (π→π*), 313 (n→π*), 340 (n→π*). \(\:\stackrel{-}{GS}\) : 17%. Physical Measurements UV-Vis absorption spectra were recorded using a SHIMADZU UV-1800 spectrophotometer on a 1% (v/v) HCl/H₂O solution at a concentration of 0.1 g·L − 1 . Quartz cuvettes with 1.0 cm optical path length were employed, ensuring that the measurements remained within the transparency range of the solvent used to prepare the samples. The Fourier transform infrared (FT-IR) vibrational spectrum was obtained by attenuated total reflectance (ATR) using a PerkinElmer Frontier spectrometer. Measurements were performed over a range of 4,000 to 450 cm⁻¹, with 4 cm⁻¹ resolution and 16 scans. Hydrogen nuclear magnetic resonance (¹H-NMR) spectra were obtained using a Brucker ARX (400 MHz; 9.4 T) and a Brucker Ultrashield (300 MHz). The solvent used was D₂O with two drops of trifluoroacetic acid (TFA). Thermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were conducted using a PerkinElmer 4000 thermogravimetric analyzer. Samples were placed on a ceramic sample holder and heated from 30 to 800°C at a rate of 10°C·min − 1 . The oven atmosphere consisted of synthetic air and N₂ gases, flowing at an average rate of 20 mL·min − 1 . The samples were in film form, with a mass of 2.2 ± 0.1 mg. The morphologies of chitosan and the modified film were examined by means of scanning electron microscopy (SEM) with a Tescan instrument operated with Vega software at 20 kV. The solid samples were previously coated with Au to ensure a conductive surface. The morphologies of chitosan and the modified film were analyzed under a Tescan scanning electron microscope (SEM) operated with Vega software at 20 kV. The solid samples were previously coated with Au to ensure a conductive surface. Voltammetric analyses in solution were performed using an Autolab Type III potentiostat/galvanostat at 25°C. The electrochemical system consisted of a Pt disk electrode as working electrode (d = 4 mm), a Pt counter electrode (d = 2 mm), and an Ag/AgCl reference electrode. Dimethylformamide solutions containing 1.0 × 10⁻³ mol·L − 1 tetrabutylammonium hexafluorophosphate (TBAPF₆) served as supporting electrolyte. Ethanol concentration in the cyclic voltammetry measurements of the synthesized compounds was 1.0 × 10⁻³ mol·L − 1 . Theoretical Calculations Density functional theory (DFT) was employed for molecular modeling, using the hybrid functional B3LYP [ 45 – 47 ] and the Pople basis set 6-311 + G(2d,p) [ 48 ]. The solvent effect was modeled using the implicit solvation method IEF-PCM (integral equation formalism for polarized continuum method) with a cavity constructed based on UFF (universal force field) radii [ 49 ], using ethanol as solvent (dielectric constant ε = 24.8520) for all calculations of Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (TΔS). To obtain IR spectra and the UV-Vis spectrum profile, solvation by dimethyl sulfoxide (ε = 46.826) was employed, applying the TD-DFT (time-dependent density functional theory) approximation at the same level of theory. All calculations were conducted using Gaussian 16® software [ 50 ]. Preparation of the Carbon Paste Electrode (CPE) The carbon paste electrode (CPE) was prepared by thoroughly mixing 0.02 g of graphite powder with 25 µL of mineral oil in a mortar using a pestle until a homogeneous paste was achieved. The paste was then packed into the cavity of a glass electrode (diameter = 3.32 mm; depth = 2.2 mm). A platinum wire was inserted to ensure electrical contact between the electrode and the potentiostat, as illustrated in Fig. 2 . Figure 2 . Schematic representation of preparation of CIS-modified CPEs. The CIS-CPE was prepared similarly, but without the addition of the CIS. The CIS content in the modified CPE was fitted to 5%, 15%, 30%, and 50% (w/w). Assessment of the Electrochemically Active Area in the Graphite Electrode The electrochemically active area of the CIS-CPE was determined using cyclic voltammetry with the ferricyanide/ferrocyanide redox system. The oxidation process at + 300 mV was analyzed at various scan rates, using a K₄Fe(CN)₆ concentration of 1.0 × 10⁻³ mol·L − 1 . Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy was conducted using an Autolab 128N potentiostat/galvanostat at 25°C. The CPE sensor served as the working electrode, with a Pt disk (d = 2 mm) as counter electrode, an Ag/AgCl electrode as reference electrode, and a dimethylformamide solution containing 1.0 × 10⁻³ mol·L − 1 Tetrabutylammonium Hexafluorophosphate (TBAPF₆) as supporting electrolyte. A sine wave amplitude of 10 mV was applied. Impedance measurements were performed potentiostatically at the open circuit potential in a Faraday cage. The applied frequency ranged from 10 kHz to 0.01 Hz. The results are presented as Nyquist and Bode plots. RESULTS AND DISCUSSION Characterization of the Schiff Base (CIS) Nuclear magnetic resonance The ¹H NMR spectrum of chitosan (Fig. S1 a) shows chemical shifts at approximately 1.9 ppm for the H7 acetamide-CH₃ group in N-acetylglucosamine residues [ 51 ], 3.0 ppm for H2, 3.6 ppm for H3-H5 hydrogens, and 3.7 ppm for H6 [ 52 ]. Additionally, a signal at approximately 4.3 ppm corresponds to H1 [ 52 ]. In the ¹H NMR spectrum of the CIS (Fig. S1 b), signals indicative of isatin-modified moiety are observed at 6.76 ppm for H9, 6.92 ppm for HR [ 52 ], 7.38 ppm for H11, and 7.41 ppm for H10. The degree of isatin substitution in chitosan was approximately 17%. This was calculated by comparing the integration of the H2 signal associated with glucosamine and N-acetylated glucosamine to the integrations of signals corresponding to isatin units. Theoretical Calculations Molecular modeling was employed to create a model of a polymer structure modified with isatin, focusing on a simplified segment of a functionalized adduct. In this model, the C = N-isatin bond was positioned adjacent to a carbonyl group (acetylated) on the right-hand side and to an amine group on the left-hand side, as illustrated in Fig. 3 . The figure also highlights the polymer cut, indicated by black hydroxyl groups and their respective arrows. Figure 3 . Proposed models for structural investigation and thermodynamic analysis. A relative stability analysis based on Gibbs free energy was performed between two protonated models: (OH - Model 01) and (NH - Model 02). The model with lower relative energy was selected for further investigation into spectroscopic profiles and molecular orbitals. Orbital population analysis was conducted, considering only orbitals with oscillator strength greater than 0.03, and focusing on those with significant contributions (values greater than 0.5) at each frontier orbital. Based on the reaction thermodynamics analysis, the structure identified as more stable — considering enthalpic variation and relative negative Gibbs free energy (ΔG = -15.37 kcal/mol and ΔH = -14.21 kcal·mol − 1 , respectively) — was the one described in Model 02, corresponding to the NH-isatin tautomer, as shown in Fig. 4 . This finding is further supported by the favorable entropic term (TΔS = + 1.16 kcal·mol − 1 ). In the final geometry-optimized structure, a slight deviation of approximately 5 degrees in the dipole moment vector was observed, with charge concentration remaining higher within the acetylated region. This deviation resulted in an increase of 15.6% in dipole moment magnitude, from µOH = 18.42 Debye in Model 01 to µNH = 21.85 Debye in Model 02. Figure 4 . Electrostatic potential surface (isovalue = 0.02) and thermodynamic analysis for Model 02. For that reason, Model 02 was chosen to assign the characteristic spectroscopic signals of the described polymer type in the UV-Vis region. For clarity, the spectral regions were divided into three bands, labeled A, B, and C in order of increasing energy. This division allowed for a more detailed analysis of spectroscopic behaviors in each region. Fourier Transfer Infrared Spectroscopy (FT-IR) The FT-IR spectrum of the CIS (Fig. S2) exhibits the main bands associated to functional groups in chitosan, as well as those characteristics of Schiff base formation. A broad band between 3,600 and 3,200 cm⁻¹ corresponds to axial stretching vibrations of O-H and N-H bonds in the amine, where νs(NH) and νas(NH) signals are masked by overlapping O-H stretches in the same region [ 53 ]. The band at 1,643 cm⁻¹ indicates the C = O stretching vibration of the acetoamide group in chitosan [ 54 ]. Polysaccharide characteristics are evident in bands at 1,153 and 897 cm⁻¹, corresponding to the stretching of glycosidic C-O bonds β(1–4) in the biopolymeric structure [ 55 ]. Peaks at 1,061 and 1,025 cm⁻¹ are attributed to axial deformation of the C-OH group, characteristic of secondary alcohols containing a 5- or 6-carbon alicyclic ring [ 55 ]. The primary distinction between unmodified chitosan (CS) and the modified polymer (the CIS) is the presence of a band at 1,520 cm⁻¹, which corresponds to C = N imine vibrations indicative of Schiff base formation [ 52 , 56 ]. Additionally, a band attributed to symmetrical C = N stretching is observed at 1,591 cm⁻¹. A further band at 1,453 cm⁻¹, characteristic of aromatic C = C bonds present in isatin (IS), also supports the polymer modification. The expected band around 1,720 cm⁻¹, corresponding to C = O stretching in the isatin amide, is not observed, suggesting that the enol-imine tautomer is predominant in the solid state [ 52 , 56 ]. Moreover, a low-intensity band at 1,226 cm⁻¹ suggests C-O vibrations associated with the enol form of the CIS [ 57 ]. UV-Vis Spectra In the UV-Vis region, the CIS in 1% (v/v) H₂O/HCl solution exhibited four absorption bands (Fig. S3). Both calculated and experimental UV-Vis spectra revealed an absorption profile spanning three distinct regions: Band A (300–500 nm, corresponding to n→π* transitions), Band B (250–300 nm, corresponding to π→π* transitions), and Band C (200–250 nm, corresponding to n→σ* transitions). Each transition type, as discussed above, can be characterized as the frontier orbital with the most significant contribution (weight greater than 0.5, considering f > 0.03) to the absorption signal, in accordance with the signal intensity refining strategy discussed in recent literature [ 58 ]. In this model, the total number of filled orbitals was 178, with the highest occupied molecular orbital (HOMO: H) at this level, and the lowest unoccupied molecular orbital (LUMO: L) at level 179. Specifically, for the absorption transition at approximately 236 nm, Band C corresponds to a HOMO→LUMO + 1 transition. Band B, observed at 295 nm, is primarily due to a HOMO-6→LUMO transition. Lastly, Band A, located at 386 nm, corresponds to a HOMO→LUMO transition. Additionally, the UV-Vis profile shows the movement of electrons between molecular orbitals associated with Bands A and C, where electron transitions occur primarily within the derivatized isatin segments. In contrast, Band B corresponds to an electron transfer from the polymeric chain to the derivatized isatin portion, as illustrated in Fig. 5 . Figure 5 . Comparison between theoretical and experimental UV-Vis spectra for isatin-modified chitosan, based on Model 02, highlighting the electronic states most effectively represented in the transitions for each band. Scanning Electron Microscopy The morphologies of both the chitosan and CIS were examined using SEM on the film samples. The micrograph of unmodified chitosan (Fig. 6 A) reveals a smooth, nonporous surface. Upon examining a bordering region of the sample, it becomes evident that chitosan is organized in stacked layers with thicknesses on the order of a few nanometers (Fig. 6 B-C), which accounts for film formation of this material using the employed methodology. The CIS film also exhibited a stacked layer structure; however, it displayed a gapped surface with small agglomerates (Fig. 6 D). Figure 6 . Micrographs of chitosan (A, C, and D) and CIS (B). Thermogravimetric Analysis The thermal properties of chitosan and the CIS were analyzed by TG/DTG under synthetic air and nitrogen atmospheres, and their curves are presented in Figure. Unlike purified chitosan, which presented three thermal events in synthetic air atmosphere, the CIS presented four thermal events. The first stage (~ 10.7%) up to 115°C is due to loss of water adsorbed on the polymer surface. This value was higher than that found for this material in the literature (~ 8%, < 100°C) [ 56 ] and may be due to the film sample having more adsorbed water molecules as compared to powder samples. Furthermore, the CIS film exhibited a thermal event (~ 3.2%) between 140–220°C (DTG = 268.7°C), which may be related to the loss of the substituted isatin portion, since thermal decomposition of isatin in synthetic air can be observed in a nearby region (DTG = 278.8°C). The third degradation stage (~ 36.8%) is associated to decomposition of free monomers. In this stage, DTG was 268.7°C, while for unmodified chitosan it was 297.0°C. Lastly, the fourth degradation stage (~ 45.3%) refers to decomposition of condensed monomers [ 56 , 59 ]. Under nitrogen atmosphere, the CIS film behaved similarly to chitosan, exhibiting two thermal events. The first mass loss stage (~ 13%) below 130°C is due to water loss from the polymer surface, while the second, larger mass loss (~ 49.4%) is due to polymer decomposition. The thermal event data obtained from the TG/DTG curves are compiled in Table 1 . Table 1 TG/DTG data for chitosan and Schiff base biopolymer under synthetic air and nitrogen atmospheres. Sample Air Nitrogen Temperature range (°C) Mass loss (%) Highest DTG (°C) Residue at 800°C (%) Temperature range (°C) Mass loss (%) Highest DTG (°C) Residue at 800°C (%) Chitosan 30–157 11.1 33.2 - 30–120 13.0 31.9 215–417 42.2 297.0 - 157–565 46.8 301.3 27.8 441–732 43.2 583.8 3.5 - - - - CIS 30–115 10.7 32.0 - 30–130 13.0 32.6 - 140–220 2.3 178.8 - 160–517 49.4 303.6 32,5 220–439 36.8 268.7 - - - - - 439–719 45.3 604.5 3.2 - - - - Table 1 . TG/DTG data for chitosan and Schiff base biopolymer under synthetic air and nitrogen atmospheres. Determination of Catalyzer The effective surface area of the electrode was calculated according to the Randles–Sevcik equation [ 60 ]. I p = 2.687×10 5 · n 3⁄2 · A eff · D 1⁄2 · C · v 1⁄2 (1) where A eff is the electroactive area (cm 2 ), I p is the peak current, n the number of transferred electrons, D the diffusion coefficient for K 3 [Fe(CN) 6 ], C the concentration of redox species in solution (mol·cm − 3 ), and v the scan rate (V·s − 1 ). The anodic (Ipa) and cathodic (Ipc) peak currents were derived from the linear fit of the voltammogram in regions where no electrochemical activity of the analyte was detected [ 61 , 62 ]. Assuming the conditions for Eq. (1) were met, the slope values were obtained for the catalysts under investigation. By substituting these values into Eq. (1), and considering n = 1 and D = 6.56 × 10⁻⁶ cm²·s⁻¹, the effective area ( A eff ) values were determined (Table 2 ). Table 2 Active electrochemical areas of catalysts. AREA (cm 2 ) 5%-CPE 15%–CPE 30%–CPE 50%–CPE 0.26 0.26 0.22 0.0053 Table 2 . Active electrochemical areas of catalysts. The 5%-CPE mass ratio was chosen as the optimal electrode for use as a catalyst in the electrochemical oxidation reactions of ethanol in this study. Ethanol Electrooxidation Effect of pH The cyclic voltammograms in Figs. 7 A-B depict the voltammetric behavior of the 5%-CPE modified electrode at pH levels of 0.3, 2.0, and 4.0, recorded at a scan rate of 50 mV·s⁻¹. Figure 7 . Cyclic voltammograms obtained for several concentrations of aqueous solutions of H 2 SO 4 at a scan rate of 50 mV·s⁻¹: (A) in the absence of ethanol and (B) in the presence of 1.0 mol·L − 1 of ethanol. Temperature = 25°C. Electrode area = 0.26 cm 2 . Figure 7 A shows that voltammetric peak currents varied with pH. In a more acidic medium, a higher current density was observed, likely due to the protonation of hydroxyl and amino groups in chitosan that were not substituted by isatin, since electrochemical oxidation and reduction for isatin occur at approximately 1.0 V and − 0.6 V, respectively. This protonation effect in acidic conditions was also observed in CPE containing only chitosan [ 63 ]. Figure 7 B indicates the dependence of the EOR (ethanol oxidation reaction) peak current on ethanol concentration, as observed in the cyclic voltammograms (CVs) for the 5%-CPE catalyst in the presence of 1.0 mol L − 1 of ethanol at pH levels of 0.3, 2.0, and 4.0. An increase in current density was observed, likely due to oxidation-reduction processes of ethanol molecules. These electrochemical processes were most efficient at pH 0.3, suggesting that the protonation of functional groups enhances the electrochemical oxidation of ethanol [ 64 – 67 ]. The voltammograms for ethanol electrocatalytic oxidation at various pH values indicate that the best catalytic performance is associated to the protonation of hydroxyl and amino groups. The behavior of the catalytic system across different pH levels suggests that maximum efficiency occurs at pH 0.3, implying that higher concentrations of H + ions facilitate EOR by aiding in the removal of adsorbed intermediates. The scan rate effect on the voltammetric behavior of the modified 5%-CPE electrode in ethanol oxidation was further investigated over a range of 0.01 to 0.3 V·s − 1 in 0.5 mol·L − 1 H 2 SO 4 solution (Fig. 8 A). The anodic peak current densities ( J pa ) increased with the scan rate; however, the plot of J pa / J pc vs. scan rate ( v ) did not exhibit a strong linear relationship (Fig. 8 B). Conversely, the plot of J pa / J pc vs. the square root of the sweep rate (v 1/2 ) displayed a strong linear correlation, with the following equations: J pa /mA cm − 2 = 6.16423 − 0.0381 v 1/2 (V·s − 1 ) 1/2 com R 2 = 0.9926 and J pc /mA cm − 2 = -6.0545 + 0.2333 v 1/2 (V·s − 1 ) 1/2 with R 2 = 0.9961 (Fig. 8 C). This result is consistent with a diffusion-controlled transfer process of the electroactive species, indicating a relatively slow diffusion of H + ions onto the electrode surface. Figure 8 . (A) Cyclic voltammetry of the 5%-CPE electrode in 0.5 mol·L − 1 H 2 SO 4 solution at varying scan rates (0.01, 0.025, 0.05, 0.15, 0.2, 0.25, and 0.3 V·s − 1 ). (B) Peak current density vs. scan rate. (C) Peak current density vs. square root of scan rate. (D) Anodic and cathodic peak potential vs. logarithm of scan rate. (E) Difference between anodic and cathodic potential vs. scan rate. Temperature = 25°C. Electrode area = 0.26 cm². The slopes of the log J pa vs. log v curves were determined using the modified Randles-Sevcik equation [ 69 ]. For the 5%-CPE electrode, the slope was found to be 0.59 ± 0.04 (R² = 0.983). This value is close to the theoretical value of 0.5, indicating a process governed by linear diffusion conditions [ 70 ]. As shown in Fig. 8 D, at higher scan rates ( v > 0.2 V·s − 1 ), the peak potential ( Ep ) becomes proportional to the logarithm of the scan rate, suggesting kinetic limitations as predicted by Laviron’s theory [ 71 ]. The electron transfer coefficient (α), which reflects the symmetry of the energy barrier in the redox reaction. When the peak separation ΔEp is greater than 200/n mV, the plot Ep = f(log v ) yields two linear regions with slopes of − 2.3 RT / αnF for the cathodic peak and 2.3 RT /(1 − α) nF for the anodic peak, where R is the ideal gas constant, F is the Faraday constant, and n is the number of electrons involved in the redox process. The slopes obtained for Ep vs. log v were − 0.7406 and 0.3197 for the cathodic and anodic peaks, respectively, resulting in a calculated α value of 0.60. The difference between the anodic and cathodic peak potentials ( ΔE ) increases with scan rate (Fig. 8 E). As the scan rate increases, the redox peaks broaden, indicating higher polarization effects at faster scan rates. This is further evidenced by the growing voltage difference between the anodic and cathodic peaks at higher scan rates. Effect of EtOH concentration The electrocatalytic oxidation of ethanol carried out by 5%-CPE was also investigated via cyclic voltammetry for different ethanol concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mol L − 1 ) as shown in Fig. 9 A. These results indicate that the EOR peak increases with ethanol concentration. Figure 9 . (A) Cyclic voltammetry (anodic direction) for 5%-CPE at varying ethanol concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mol·L − 1 ) in 0.5 mol·L − 1 of H 2 SO 4 solution. (B) Maximum peak current density as a function of ethanol concentration. Scan rate = 0.025 V·s − 1 . Electrochemically active area of electrode = 0.26 cm 2 . Figure 9 B shows the linear correlation between the catalytic current EOR and the ethanol concentration ranging from 0 to 1.0 mol·L − 1 , suggesting that the peak current density is dependent on ethanol concentration. Impedance Spectroscopy To generate the Nyquist diagram, both the frequency values and the real ( Z’ ) and imaginary ( -Z” ) parts of impedance are analyzed. The frequency corresponds to the specific point in time when the impedance is measured, since different electrochemical processes and reactions may occur across various frequency ranges. The real part of impedance ( Z’ ) reflects the resistive component of the system. In contrast, the imaginary part ( -Z” ) represents the reactive nature of the system, revealing the capacitive or inductive behavior at the interface. Thus, the values of Z’ and -Z” represent the complex impedance of the system at the frequency under investigation. Figure 10 illustrates the Nyquist diagram for the CPE sensor. Figure 10 . Nyquist diagram obtained via EIS for 5%-CPE. The EIS results were fitted via the equivalent electric circuit model. The fitting was achieved using the Q1 ( R1Q2 )( R2Q3 ) circuit, which consists of three serial components, to interpret the physicochemical behavior of the sensor. The contribution of Q1 is shown on the left-hand side of the Nyquist diagram, which represents the regions of high frequencies where it is possible to assess the capacitive behavior of the interface. The initial Z’ and Z” values (-4.64 Ω and 359.51 Ω, respectively) indicate a high capacitance within the electric double layer. The negative Z’ value suggests a predominant capacitive component at the interface. Typically, the real part of impedance is associated to the system resistance, and a negative value implies that capacitive behavior is overriding. This behavior is likely linked to the electric double layer formed at the electrode-electrolyte interface. The positive value of the imaginary part ( Z” ) further confirms the predominance of the capacitive behavior, reflecting the reactive nature of system, such as the accumulation and release of electric charges at the interface. In sensor applications, this capacitive behavior offers significant advantages, including enhanced sensitivity, rapid response, improved accuracy, and greater versatility. The parallel elements Q2 and R1 , associated with the high-frequency semicircle, correspond to the constant-phase element and charge transfer resistance, respectively. Meanwhile, the parallel elements Q3 and R2 , related to the inclined line in the low-frequency region, represent the diffusion processes of electroactive species in the electrolyte as they move toward the electrode. This electrical circuit was designed specifically to characterize porous structures. Q1 and Q2 are interpreted as non-ideal capacitances, influenced by surface geometry, roughness, and porosity. The relationship between these constant-phase elements ( Q ) and impedance ( Z ) is described by the equation below. Z = 1/ Q ( j·ω ) n (2) where j is the imaginary unit, ω is the angular frequency, and n is a correction factor (0 < n < 1). As the value of n approaches 0, Q exhibits less capacitive behavior [ 72 , 73 ]. The equivalent circuit was chosen based on its ability to fit the Nyquist plot with minimal error. The estimated parameter values obtained after fitting the equivalent circuit model to the impedance spectra are presented in Table 3 . The fitting results show that n1 and n2 are both less than 1, which corroborates the roughness and heterogeneity of the sensor surface. Additionally, the increase in resistance R2 indicates that the occupation of complementary sites on the sensor makes the electrode surface more resistant to charge transfer. Table 3 Values ​​of the elements that make up the equivalent electrical circuit proposed for the CPE electrode with their respective errors. Q1 R1 Q2 R2 Q3 Y (F) (± error) 4.0 × 10 − 4 (± 1.58%) --- 4.15 × 10 − 5 (± 2.43%) --- 1.94 × 10 − 8 (± 2.59%) n (Rad) (± error) 0.673 (± 1.01%) --- 0.646 (± 1.56%) --- 1.1 (± 0.23%) R (Ω) (± error) --- 802.34 (± 3.45%) --- 822.71 (± 1.20%) --- Table 3 . Values ​​of the elements that make up the equivalent electrical circuit proposed for the CPE electrode with their respective errors. The phase angle at low frequencies provides specific information on the behavior of the electrode. Ideally, a capacitor should exhibit a phase angle close to 90°. This angle is particularly desirable for insulating electrodes because it represents the maximum phase difference between the applied voltage and the resulting current [ 74 ]. Significant deviations from this value suggest that the dielectric layer is losing its effectiveness in retaining electric charge, indicating the onset of electron flow. As shown in the Bode plot (Fig. 11 ), the phase angle starts at approximately − 58° at low frequencies and drops to values below − 20° in the mid-frequency region, indicating a shift towards resistive behavior. This shift suggests increased ionic permeability and electron flow through the CPE electrode. This finding is further supported by the higher impedance values observed in the low-frequency region of the same plot. Figure 11 . Bode plot obtained via EIS for 5%-CPE. To ensure the validity of all analyses, it is fundamental that the measured spectrum accurately represents a linear, time-invariant, and causal system. The Kramers-Kronig relations are used to verify compliance with these requirements, as they establish a connection between the real and imaginary parts of the impedance spectrum for such systems. This allows for a thorough and accurate evaluation of the system under investigation [ 75 ]. Generally, values below 10 − 6 typically indicate an excellent fit, values between 10 − 5 and 10 − 6 are considered reasonable, and values in the range of 10 − 4 to 10 − 5 are marginal. Higher values suggest a poor fit. As shown in Table 4 , the linearity, stability, and causality data obtained for the CPE sensor indicate an excellent fit, validating the analyses performed. Table 4 Kronig-Kramers analysis values ​​for the proposed CPE electrode circuit. Kronig-Kramers X 2 (Z) X 2 (Z’) X 2 (-Z”) Electrode CPE 1.91 × 10 − 7 1.30 × 10 − 7 6.16 × 10 − 8 During the Kramers-Kronig test, the experimental data points are fitted using the proposed circuit model, which satisfies the Kramers-Kronig relations. If the experimental data can be accurately represented by the circuit model Q1 ( R1Q2 )( R2Q3 ), then the data set should also satisfy the Kramers-Kronig assumptions. Table 4 . Kronig-Kramers analysis values ​​for the proposed CPE electrode circuit. CONCLUSIONS This study explored the synthesis, characterization, and application of an isatin-modified chitosan Schiff base (CIS) as an electrocatalyst for the ethanol oxidation reaction (EOR) in direct ethanol fuel cells (DEFCs). The primary motivation was to address the challenges of CO poisoning by metal electrocatalysts and to enhance the efficiency of the ethanol oxidation process. Characterization of the isatin-modified chitosan confirmed the successful formation of the Schiff base, evidenced by the presence of new bands characteristic of the C = N bond. Scanning electron microscopy (SEM) revealed that the CIS exhibited a gap-like morphology on its surface, which contrasts with the smooth surface of unmodified chitosan. Thermogravimetric analysis (TG/DTG) demonstrated that the CIS possesses greater thermal stability compared to pure chitosan, with four distinct thermal events. Electrochemical performance testing identified the CIS-modified electrode (5%-CPE) as the ideal candidate for catalyzing the electrochemical oxidation of ethanol. The catalyst efficiency was assessed at various pH levels, exhibiting optimal performance in an acidic medium (pH 3). At this pH, protonation of hydroxyl and amine groups enhances ethanol oxidation. The linear relationship between EOR peak current and ethanol concentration indicated that peak current density increases with ethanol concentration. Furthermore, electrochemical impedance spectroscopy results revealed a dominant capacitive behavior and confirmed the roughness and heterogeneity of the sensor surface. Overall, this study demonstrated that the isatin-modified chitosan Schiff base (CIS) is a promising electrocatalyst for EOR in DEFCs. The modification of chitosan with isatin not only improved the electrocatalytic properties but also enhanced the thermal stability of the material, positioning the CIS as a sustainable and efficient alternative for applications in direct ethanol fuel cells. Declarations Acknowledgements The authors would like to thank the Professional Master’s Program in Chemistry in a national network with the Federal University of Rio de Janeiro (PROFQUI-UFRJ), the Postgraduate Program in Science and Technology of Amazonian Resources of the Federal University of Amazonas (PPGCTRA-UFAM), the Multiuser Laboratory of Nuclear Magnetic Resonance (LAMRMN), the Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro (FAPERJ grants No. E-26/210.070/2022-DCTR, SEI-260003/015957/2021, SEI-260003/015018/2021-APQ1, SEI-260003/003411/2022–BBP, and SEI-260003/014874/2023 - APQ1), the Amazonas State Research Support Foundation for their support (FAPEAM), the Coordination for the Improvement of Higher Education Personnel – Brazil (CAPES) and the National Council for Scientific Development – CNPq for financial support. Funding This work was supported by grants numbers . E-26/210.070/2022-DCTR, SEI-260003/015957/2021, SEI-260003/015018/2021-APQ1, SEI-260003/003411/2022–BBP, and SEI-260003/014874/2023 - APQ1 (FAPERJ). Competing Interests The authors have no relevant financial or non-financial interests to disclose. Author Contributions All authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by Eduardo Guilherme Cividini Neiva; Nathalia Biazotto Sá, José Wimo da Cruz Jr; Rodrigo de Siqueira Melo and Jardel Ramos da Encarnação; Brenda Antunes Louriçal Paixão, Joel dos Santos Batista and Letícia Oliveira Laier. The theoretical calculus was realized by Willian Tássio Gomes Novato. The first draft of the manuscript was written by Paulo José Sousa Maia, Ismael Casagrande Bellettini, José Wimo da Cruz Jr and Eduardo Guilherme Cividini Neiva. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. 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Federal de Santa Catarina","correspondingAuthor":false,"prefix":"","firstName":"Nathalia","middleName":"Biazotto","lastName":"Sá","suffix":""},{"id":350696451,"identity":"2fbbd1e8-f507-4566-b3cd-6c7af8ef1f84","order_by":2,"name":"Jardel Ramos Encarnação","email":"","orcid":"","institution":"Universidade Federal do Amazonas","correspondingAuthor":false,"prefix":"","firstName":"Jardel","middleName":"Ramos","lastName":"Encarnação","suffix":""},{"id":350696452,"identity":"97e95822-1edb-4f71-b63b-244447b643ac","order_by":3,"name":"Joel dos Santos Batista","email":"","orcid":"","institution":"Universidade Federal do Amazonas","correspondingAuthor":false,"prefix":"","firstName":"Joel","middleName":"dos Santos","lastName":"Batista","suffix":""},{"id":350696453,"identity":"24d7ff39-8ae8-4d95-ad4a-ba37d5154a3c","order_by":4,"name":"Letícia Oliveira Laier","email":"","orcid":"","institution":"Universidade Federal do Rio de Janeiro","correspondingAuthor":false,"prefix":"","firstName":"Letícia","middleName":"Oliveira","lastName":"Laier","suffix":""},{"id":350696455,"identity":"807aaf95-75ff-4ff2-a904-a8481080fccf","order_by":5,"name":"Eduardo Guilherme Cividini Neiva","email":"","orcid":"","institution":"Universidade Regional de Blumenau","correspondingAuthor":false,"prefix":"","firstName":"Eduardo","middleName":"Guilherme Cividini","lastName":"Neiva","suffix":""},{"id":350696457,"identity":"cc3f833f-c1d0-454f-9b69-08733b99c540","order_by":6,"name":"Elson Almeida Souza","email":"","orcid":"","institution":"Universidade Federal do Amazonas","correspondingAuthor":false,"prefix":"","firstName":"Elson","middleName":"Almeida","lastName":"Souza","suffix":""},{"id":350696458,"identity":"47f2bc01-1588-43fa-9b1e-411c68b43ed7","order_by":7,"name":"Rodrigo Siqueira Melo","email":"","orcid":"","institution":"Universidade Federal do Rio de Janeiro","correspondingAuthor":false,"prefix":"","firstName":"Rodrigo","middleName":"Siqueira","lastName":"Melo","suffix":""},{"id":350696459,"identity":"6f4456a7-bb11-417d-8a7b-530a10421edf","order_by":8,"name":"Willian Tássio Gomes Novato","email":"","orcid":"","institution":"Universidade Federal do Rio de Janeiro","correspondingAuthor":false,"prefix":"","firstName":"Willian","middleName":"Tássio Gomes","lastName":"Novato","suffix":""},{"id":350696462,"identity":"5e2c23c0-7559-47f2-baec-bb903f320676","order_by":9,"name":"Ismael Casagrande Bellettini","email":"","orcid":"","institution":"Universidade Federal de Santa Catarina","correspondingAuthor":false,"prefix":"","firstName":"Ismael","middleName":"Casagrande","lastName":"Bellettini","suffix":""},{"id":350696463,"identity":"17e980fa-4d2a-4e45-a671-5515ae452d07","order_by":10,"name":"José Wimo Cruz","email":"","orcid":"","institution":"Universidade Federal de Santa Catarina","correspondingAuthor":false,"prefix":"","firstName":"José","middleName":"Wimo","lastName":"Cruz","suffix":""},{"id":350696465,"identity":"7e335d1a-220f-4bef-a9c9-a929a8a20653","order_by":11,"name":"Paulo José Sousa Maia","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5UlEQVRIiWNgGAWjYHACxgMMPMxAmvkAkJCQIUoPVAtbAkgLD5FaGEBaeAxAHMJazNnPGBz4IGMtJz/tzOdXN2oseBjYDx/dgE+LZU+OwcEZPOnGBrdzt1nnHAM6jCct7QY+LQYHcgwO8/AcTtwgnbvNOIcNqEWCxwy/lvNvwFrq58/OeWac848YLTcgtiQw3M5hfpzbRoQWyxnPCkB+MdxwO82MObdPgoeNkF/M+ZM3PvjYYy0vPzv58eecb3Vy/OyHj+F3GIhg7AGz2STAJD7lcC0MP8Ak8wdCqkfBKBgFo2BkAgCSb0hqHMUD4wAAAABJRU5ErkJggg==","orcid":"","institution":"Universidade Federal de São Carlos – Campus Sorocaba","correspondingAuthor":true,"prefix":"","firstName":"Paulo","middleName":"José Sousa","lastName":"Maia","suffix":""}],"badges":[],"createdAt":"2024-08-27 20:31:45","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4986733/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4986733/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10800-024-02228-3","type":"published","date":"2024-11-23T15:58:15+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":65385405,"identity":"a57ae9f9-5f7a-4672-80aa-b57ce4a7181b","added_by":"auto","created_at":"2024-09-26 19:38:08","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":50481,"visible":true,"origin":"","legend":"\u003cp\u003eGeneral route for synthesis of Schiff base after modification (QI) of chitosan (1) with isatin (2).\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/62185908846fe151620f8589.png"},{"id":65385210,"identity":"0e047821-4fba-40dd-9520-b9d35578b502","added_by":"auto","created_at":"2024-09-26 19:30:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":162193,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of preparation of CIS-modified CPEs.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/1882d8c56a83415d254e4660.png"},{"id":65385209,"identity":"b3019cfb-0883-45c7-92e1-945f78f4f44c","added_by":"auto","created_at":"2024-09-26 19:30:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":87130,"visible":true,"origin":"","legend":"\u003cp\u003eProposed models for structural investigation and thermodynamic analysis.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/29d4854e78475f9bf0311289.png"},{"id":65386008,"identity":"53f9bd57-1ed8-472b-b4dc-db0eff249d83","added_by":"auto","created_at":"2024-09-26 19:54:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":137850,"visible":true,"origin":"","legend":"\u003cp\u003eElectrostatic potential surface (isovalue = 0.02) and thermodynamic analysis for Model 02.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/7495062dc18e834a55d614a6.png"},{"id":65385217,"identity":"0dd5c56f-3712-49bf-8096-5f125f4b1122","added_by":"auto","created_at":"2024-09-26 19:30:08","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":187599,"visible":true,"origin":"","legend":"\u003cp\u003eComparison between theoretical and experimental UV-Vis spectra for isatin-modified chitosan, based on Model 02, highlighting the electronic states most effectively represented in the transitions for each band.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/4a694bac14d04f02983984d9.png"},{"id":65385721,"identity":"5a5f4815-dffc-488e-88e4-18f92ff001d8","added_by":"auto","created_at":"2024-09-26 19:46:08","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":132345,"visible":true,"origin":"","legend":"\u003cp\u003eMicrographs of chitosan (A, C, and D) and CIS (B).\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/12308f82eb61d59d8ce01f75.png"},{"id":65385408,"identity":"443b4660-3fc2-4b7d-a3a6-6c89e8540474","added_by":"auto","created_at":"2024-09-26 19:38:08","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":211924,"visible":true,"origin":"","legend":"\u003cp\u003eCyclic voltammograms obtained for several concentrations of aqueous solutions of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e at a scan rate of 50 mV·s⁻¹: (A) in the absence of ethanol and (B) in the presence of 1.0 mol·L\u003csup\u003e-1\u003c/sup\u003e of ethanol. Temperature = 25 °C. Electrode area = 0.26 cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage7.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/c64efea1e41f1e6d0ef34924.png"},{"id":65385722,"identity":"52853de0-48d1-4f89-b399-4e6319a7bd3e","added_by":"auto","created_at":"2024-09-26 19:46:08","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":247887,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Cyclic voltammetry of the 5%-CPE electrode in 0.5 mol·L\u003csup\u003e−1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution at varying scan rates (0.01, 0.025, 0.05, 0.15, 0.2, 0.25, and 0.3 V·s\u003csup\u003e−1\u003c/sup\u003e). (B) Peak current density vs. scan rate. (C) Peak current density vs. square root of scan rate. (D) Anodic and cathodic peak potential vs. logarithm of scan rate. (E) Difference between anodic and cathodic potential vs. scan rate. Temperature = 25 °C. Electrode area = 0.26 cm².\u003c/p\u003e","description":"","filename":"floatimage8.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/f92649a641ad3b71ccf24efc.png"},{"id":65385415,"identity":"74e38fd6-ee15-4b89-8f2c-146d92dc24ae","added_by":"auto","created_at":"2024-09-26 19:38:08","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":207021,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Cyclic voltammetry (anodic direction) for 5%-CPE at varying ethanol concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mol·L\u003csup\u003e-1\u003c/sup\u003e) in 0.5 mol·L\u003csup\u003e-1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution. (B) Maximum peak current density as a function of ethanol concentration. Scan rate = 0.025 V·s\u003csup\u003e-1\u003c/sup\u003e. Electrochemically active area of electrode = 0.26 cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/a4ce51fc9158eb20be307c63.png"},{"id":65385214,"identity":"3a5ba42a-b4ec-45c5-99c5-aae898b87190","added_by":"auto","created_at":"2024-09-26 19:30:08","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":69045,"visible":true,"origin":"","legend":"\u003cp\u003eNyquist diagram obtained via EIS for 5%-CPE.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/8228969f3bf2462b61dd2511.png"},{"id":65385219,"identity":"c435e299-834e-4168-b356-f970aff58381","added_by":"auto","created_at":"2024-09-26 19:30:08","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":141207,"visible":true,"origin":"","legend":"\u003cp\u003eBode plot obtained via EIS for 5%-CPE.\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/d8d6e76b2354b77d3c80f0ab.png"},{"id":69835117,"identity":"bf71be7a-83a2-4756-b109-5d6790e93b2e","added_by":"auto","created_at":"2024-11-25 16:12:10","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2705548,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/3c330340-707e-40e6-8af6-740b06fe8a88.pdf"},{"id":65385409,"identity":"2e0f251a-36e8-4b33-9fc9-20e5990bcfdd","added_by":"auto","created_at":"2024-09-26 19:38:08","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":410149,"visible":true,"origin":"","legend":"","description":"","filename":"MaiaPJSISATINMODIFIEDCHITOSANSupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-4986733/v1/2ff33375046727b1e0480748.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eElectrocatalyst for Ethanol Oxidation Reaction Based on Isatin-modified Chitosan\u003c/p\u003e","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eThe energy crisis faced by humanity and its consequent damage to the environment requires that new, more sustainable technologies using degradable materials be developed. Among the alternative power sources are Direct Alcohol Fuel Cells (DAFCs), particularly Direct Ethanol Fuel Cells (DEFCs). DEFCs offer several advantages over cells that use methanol and formic acid. They are safer for human use due to their lower toxicity, provide high current density and energy conversion efficiency, have reduced ethanol fuel crossover, and can be sourced from renewable plant materials [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, a major challenge in developing DEFCs is the release of poisonous CO into the environment by metallic electrocatalysts during the process. To overcome this, it is essential to improve the anodic catalysts employed in ethanol oxidation, by either bettering or replacing Pt electrocatalysts, which are widely used due to their high efficiency [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo meet the demand for novel electrocatalysts capable of overcoming the aforementioned challenges, polymeric materials have garnered the attention of researchers worldwide. These materials offer significant benefits across various industries, including good performance and low processing costs [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. In particular, biopolymers stand out as more sustainable alternatives as they are biodegradable, naturally available, and renewable. In addition, they exhibit low toxicity and important bioactive properties among other characteristics, making them suitable for multiple applications [\u003cspan additionalcitationids=\"CR11 CR12 CR13\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAmong existing biopolymers, chitosan (Chi) is widely applicable as it can be obtained by means of deacetylation of chitin, extracted mostly from exoskeletons of crustaceans and other marine animals [\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] but also from fungi and insect cuticles [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Its linear structure composed of glucosamine and acetylglucosamine confers important physicochemical properties such as high intrinsic rigidity, large spacing between the charges, and high propensity to form intra and intermolecular hydrogen bonds [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe literature contains numerous accounts of uses of modified chitosan electrodes in various electrochemical applications. These include nitrite sensing [\u003cspan additionalcitationids=\"CR3 CR4 CR5 CR6 CR7 CR8 CR9 CR10 CR11 CR12 CR13 CR14 CR15 CR16 CR17 CR18 CR19 CR20 CR21 CR22\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], metal detection [\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], ciprofloxacin detection in biological samples [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], glucose oxidation [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], and electrocatalytic oxidation of formaldehyde [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], methanol [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], and ethanol [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. For example, Guibal provides a comprehensive review on the use of chitosan and its derivatives in heterogeneous catalysis, highlighting their significance for industry [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Vedula and Yadav discuss the challenges and opportunities associated with chitosan-based membranes, particularly in catalysis and fuel cells [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. They conclude that these membranes can address many issues related to direct alcohol fuel cells (DAFCs) [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan additionalcitationids=\"CR34 CR35 CR36\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAdditionally, the functional groups found in chitosan make it a versatile material for synthesizing derivatives capable of enhancing its properties [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. By way of illustration, polymeric Schiff bases can be produced by condensing the primary amine in chitosan with a carbonyl group, aldehyde, or ketone. One way to functionalize chitosan to obtain Schiff bases is by using isatin (IS \u0026minus;\u0026thinsp;1H-indole-2,3-dione). Recent studies have explored the use of isatin and its derivatives in anionic polymeric membranes, with potential applications in power and fuel cells [\u003cspan additionalcitationids=\"CR40\" citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eOur research group has focused on studying the catalytic activity of uranyl complexes with Salen-type Schiff bases as co-catalysts in ethanol electrooxidation, achieving promising results [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. Given the lack in the literature of studies on the use of polymeric Schiff bases derived from chitosan and isatin in DEFC catalysts, this work aims to explore the catalytic activity of these Schiff bases in ethanol electrooxidation, with potential application in DEFCs.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eReagents\u003c/h2\u003e \u003cp\u003eChitosan (with an average molar mass and a deacetylation degree of 76%) and isatin (97%) were obtained from Sigma-Aldrich (USA). HPLC-grade acetonitrile, obtained from JT Baker, was used without prior purification. Other analytical-grade (P.A.) solvents were obtained from Synth.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis of the Isatin-modified Chitosan Schiff Base (CIS)\u003c/h2\u003e \u003cp\u003eThe modification of chitosan with isatin was carried out using a method adapted from Ara\u0026uacute;jo and colleagues [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. In this process, 0.5 g of chitosan was dissolved in 75 mL of a 0.15 mol L⁻\u0026sup1; acetic acid solution and stirred magnetically at 55\u0026deg;C for 24 hours. After this time, an ethanolic solution of isatin (at a 1.0:1.5 amino/isatin ratio) was added dropwise to the mixture. The system was then stirred magnetically at 55\u0026deg;C for an additional 24 hours. The general synthesis route is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. General route for synthesis of Schiff base after modification (QI) of chitosan (1) with isatin (2).\u003c/p\u003e \u003cp\u003eThe reaction mixture was transferred to a dialysis membrane (Spectra/Por\u0026reg; 6, MWCO\u0026thinsp;=\u0026thinsp;14,000 g/mol) and dialyzed in deionized water for 5 days. The modified polymer was then transferred to petri dishes and dried in an oven at 50\u0026deg;C until fully dry. \u003cb\u003e\u0026sup1;H NMR\u003c/b\u003e (400 MHz, D₂O/TFA, 25\u0026deg;C, ppm): δ\u0026thinsp;=\u0026thinsp;1.9 (s, H7), 3.0 (s, H2), 3.6\u0026ndash;3.7 (m, H3-H6), 4.3 (s, H1), 6.8 (d, H9), 6.9 (t, HR), 7.3 (t, H10), 7.4 (d, H11), 8.0 (s, H8). \u003cb\u003eIR\u003c/b\u003e (cm⁻\u0026sup1;): 3600\u0026thinsp;\u0026minus;\u0026thinsp;3200 (O-H and N-H), 2919 (C-H asymmetric), 2872 (C-H symmetric), 1643 (C\u0026thinsp;=\u0026thinsp;O acetamide), 1620 (C\u0026thinsp;=\u0026thinsp;N), 1453 (C\u0026thinsp;=\u0026thinsp;C), 1153 and 897 (C-O β(1\u0026ndash;4) glycosidic bonds), 1061 and 1025 (C-OH). \u003cb\u003eUV-Vis\u003c/b\u003e (λ\u003csub\u003emax\u003c/sub\u003e, H₂O/HCl 1% (v/v), 25\u0026deg;C, nm): 205 (π\u0026rarr;π*), 239 (π\u0026rarr;π*), 313 (n\u0026rarr;π*), 340 (n\u0026rarr;π*). \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{GS}\\)\u003c/span\u003e\u003c/span\u003e: 17%.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003ePhysical Measurements\u003c/h2\u003e \u003cp\u003eUV-Vis absorption spectra were recorded using a SHIMADZU UV-1800 spectrophotometer on a 1% (v/v) HCl/H₂O solution at a concentration of 0.1 g\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Quartz cuvettes with 1.0 cm optical path length were employed, ensuring that the measurements remained within the transparency range of the solvent used to prepare the samples.\u003c/p\u003e \u003cp\u003eThe Fourier transform infrared (FT-IR) vibrational spectrum was obtained by attenuated total reflectance (ATR) using a PerkinElmer Frontier spectrometer. Measurements were performed over a range of 4,000 to 450 cm⁻\u0026sup1;, with 4 cm⁻\u0026sup1; resolution and 16 scans.\u003c/p\u003e \u003cp\u003eHydrogen nuclear magnetic resonance (\u0026sup1;H-NMR) spectra were obtained using a Brucker ARX (400 MHz; 9.4 T) and a Brucker Ultrashield (300 MHz). The solvent used was D₂O with two drops of trifluoroacetic acid (TFA).\u003c/p\u003e \u003cp\u003eThermogravimetric (TG) and derivative thermogravimetric (DTG) analyses were conducted using a PerkinElmer 4000 thermogravimetric analyzer. Samples were placed on a ceramic sample holder and heated from 30 to 800\u0026deg;C at a rate of 10\u0026deg;C\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The oven atmosphere consisted of synthetic air and N₂ gases, flowing at an average rate of 20 mL\u0026middot;min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The samples were in film form, with a mass of 2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg.\u003c/p\u003e \u003cp\u003eThe morphologies of chitosan and the modified film were examined by means of scanning electron microscopy (SEM) with a Tescan instrument operated with Vega software at 20 kV. The solid samples were previously coated with Au to ensure a conductive surface.\u003c/p\u003e \u003cp\u003eThe morphologies of chitosan and the modified film were analyzed under a Tescan scanning electron microscope (SEM) operated with Vega software at 20 kV. The solid samples were previously coated with Au to ensure a conductive surface.\u003c/p\u003e \u003cp\u003eVoltammetric analyses in solution were performed using an Autolab Type III potentiostat/galvanostat at 25\u0026deg;C. The electrochemical system consisted of a Pt disk electrode as working electrode (d\u0026thinsp;=\u0026thinsp;4 mm), a Pt counter electrode (d\u0026thinsp;=\u0026thinsp;2 mm), and an Ag/AgCl reference electrode. Dimethylformamide solutions containing 1.0 \u0026times; 10⁻\u0026sup3; mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e tetrabutylammonium hexafluorophosphate (TBAPF₆) served as supporting electrolyte. Ethanol concentration in the cyclic voltammetry measurements of the synthesized compounds was 1.0 \u0026times; 10⁻\u0026sup3; mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eTheoretical Calculations\u003c/h2\u003e \u003cp\u003eDensity functional theory (DFT) was employed for molecular modeling, using the hybrid functional B3LYP [\u003cspan additionalcitationids=\"CR46\" citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e] and the Pople basis set 6-311\u0026thinsp;+\u0026thinsp;G(2d,p) [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. The solvent effect was modeled using the implicit solvation method IEF-PCM (integral equation formalism for polarized continuum method) with a cavity constructed based on UFF (universal force field) radii [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e], using ethanol as solvent (dielectric constant ε\u0026thinsp;=\u0026thinsp;24.8520) for all calculations of Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (TΔS). To obtain IR spectra and the UV-Vis spectrum profile, solvation by dimethyl sulfoxide (ε\u0026thinsp;=\u0026thinsp;46.826) was employed, applying the TD-DFT (time-dependent density functional theory) approximation at the same level of theory. All calculations were conducted using Gaussian 16\u0026reg; software [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003ePreparation of the Carbon Paste Electrode (CPE)\u003c/h2\u003e \u003cp\u003eThe carbon paste electrode (CPE) was prepared by thoroughly mixing 0.02 g of graphite powder with 25 \u0026micro;L of mineral oil in a mortar using a pestle until a homogeneous paste was achieved. The paste was then packed into the cavity of a glass electrode (diameter\u0026thinsp;=\u0026thinsp;3.32 mm; depth\u0026thinsp;=\u0026thinsp;2.2 mm). A platinum wire was inserted to ensure electrical contact between the electrode and the potentiostat, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Schematic representation of preparation of CIS-modified CPEs.\u003c/p\u003e \u003cp\u003eThe CIS-CPE was prepared similarly, but without the addition of the CIS. The CIS content in the modified CPE was fitted to 5%, 15%, 30%, and 50% (w/w).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eAssessment of the Electrochemically Active Area in the Graphite Electrode\u003c/h2\u003e \u003cp\u003eThe electrochemically active area of the CIS-CPE was determined using cyclic voltammetry with the ferricyanide/ferrocyanide redox system. The oxidation process at +\u0026thinsp;300 mV was analyzed at various scan rates, using a K₄Fe(CN)₆ concentration of 1.0 \u0026times; 10⁻\u0026sup3; mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eElectrochemical Impedance Spectroscopy\u003c/h2\u003e \u003cp\u003eElectrochemical impedance spectroscopy was conducted using an Autolab 128N potentiostat/galvanostat at 25\u0026deg;C. The CPE sensor served as the working electrode, with a Pt disk (d\u0026thinsp;=\u0026thinsp;2 mm) as counter electrode, an Ag/AgCl electrode as reference electrode, and a dimethylformamide solution containing 1.0 \u0026times; 10⁻\u0026sup3; mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e Tetrabutylammonium Hexafluorophosphate (TBAPF₆) as supporting electrolyte. A sine wave amplitude of 10 mV was applied. Impedance measurements were performed potentiostatically at the open circuit potential in a Faraday cage. The applied frequency ranged from 10 kHz to 0.01 Hz. The results are presented as Nyquist and Bode plots.\u003c/p\u003e \u003c/div\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eCharacterization of the Schiff Base (CIS)\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003eNuclear magnetic resonance\u003c/h2\u003e \u003cp\u003eThe \u0026sup1;H NMR spectrum of chitosan (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea) shows chemical shifts at approximately 1.9 ppm for the H7 acetamide-CH₃ group in N-acetylglucosamine residues [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e], 3.0 ppm for H2, 3.6 ppm for H3-H5 hydrogens, and 3.7 ppm for H6 [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. Additionally, a signal at approximately 4.3 ppm corresponds to H1 [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. In the \u0026sup1;H NMR spectrum of the CIS (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb), signals indicative of isatin-modified moiety are observed at 6.76 ppm for H9, 6.92 ppm for HR [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e], 7.38 ppm for H11, and 7.41 ppm for H10.\u003c/p\u003e \u003cp\u003eThe degree of isatin substitution in chitosan was approximately 17%. This was calculated by comparing the integration of the H2 signal associated with glucosamine and N-acetylated glucosamine to the integrations of signals corresponding to isatin units.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eTheoretical Calculations\u003c/h2\u003e \u003cp\u003eMolecular modeling was employed to create a model of a polymer structure modified with isatin, focusing on a simplified segment of a functionalized adduct. In this model, the C\u0026thinsp;=\u0026thinsp;N-isatin bond was positioned adjacent to a carbonyl group (acetylated) on the right-hand side and to an amine group on the left-hand side, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The figure also highlights the polymer cut, indicated by black hydroxyl groups and their respective arrows.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Proposed models for structural investigation and thermodynamic analysis.\u003c/p\u003e \u003cp\u003eA relative stability analysis based on Gibbs free energy was performed between two protonated models: (OH - Model 01) and (NH - Model 02). The model with lower relative energy was selected for further investigation into spectroscopic profiles and molecular orbitals. Orbital population analysis was conducted, considering only orbitals with oscillator strength greater than 0.03, and focusing on those with significant contributions (values greater than 0.5) at each frontier orbital.\u003c/p\u003e \u003cp\u003eBased on the reaction thermodynamics analysis, the structure identified as more stable \u0026mdash; considering enthalpic variation and relative negative Gibbs free energy (ΔG = -15.37 kcal/mol and ΔH = -14.21 kcal\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively) \u0026mdash; was the one described in Model 02, corresponding to the NH-isatin tautomer, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. This finding is further supported by the favorable entropic term (TΔS\u0026thinsp;=\u0026thinsp;+\u0026thinsp;1.16 kcal\u0026middot;mol\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the final geometry-optimized structure, a slight deviation of approximately 5 degrees in the dipole moment vector was observed, with charge concentration remaining higher within the acetylated region. This deviation resulted in an increase of 15.6% in dipole moment magnitude, from \u0026micro;OH\u0026thinsp;=\u0026thinsp;18.42 Debye in Model 01 to \u0026micro;NH\u0026thinsp;=\u0026thinsp;21.85 Debye in Model 02.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Electrostatic potential surface (isovalue\u0026thinsp;=\u0026thinsp;0.02) and thermodynamic analysis for Model 02.\u003c/p\u003e \u003cp\u003eFor that reason, Model 02 was chosen to assign the characteristic spectroscopic signals of the described polymer type in the UV-Vis region. For clarity, the spectral regions were divided into three bands, labeled A, B, and C in order of increasing energy. This division allowed for a more detailed analysis of spectroscopic behaviors in each region.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eFourier Transfer Infrared Spectroscopy (FT-IR)\u003c/h2\u003e \u003cp\u003eThe FT-IR spectrum of the CIS (Fig. S2) exhibits the main bands associated to functional groups in chitosan, as well as those characteristics of Schiff base formation. A broad band between 3,600 and 3,200 cm⁻\u0026sup1; corresponds to axial stretching vibrations of O-H and N-H bonds in the amine, where νs(NH) and νas(NH) signals are masked by overlapping O-H stretches in the same region [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. The band at 1,643 cm⁻\u0026sup1; indicates the C\u0026thinsp;=\u0026thinsp;O stretching vibration of the acetoamide group in chitosan [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. Polysaccharide characteristics are evident in bands at 1,153 and 897 cm⁻\u0026sup1;, corresponding to the stretching of glycosidic C-O bonds β(1\u0026ndash;4) in the biopolymeric structure [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. Peaks at 1,061 and 1,025 cm⁻\u0026sup1; are attributed to axial deformation of the C-OH group, characteristic of secondary alcohols containing a 5- or 6-carbon alicyclic ring [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe primary distinction between unmodified chitosan (CS) and the modified polymer (the CIS) is the presence of a band at 1,520 cm⁻\u0026sup1;, which corresponds to C\u0026thinsp;=\u0026thinsp;N imine vibrations indicative of Schiff base formation [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. Additionally, a band attributed to symmetrical C\u0026thinsp;=\u0026thinsp;N stretching is observed at 1,591 cm⁻\u0026sup1;. A further band at 1,453 cm⁻\u0026sup1;, characteristic of aromatic C\u0026thinsp;=\u0026thinsp;C bonds present in isatin (IS), also supports the polymer modification. The expected band around 1,720 cm⁻\u0026sup1;, corresponding to C\u0026thinsp;=\u0026thinsp;O stretching in the isatin amide, is not observed, suggesting that the enol-imine tautomer is predominant in the solid state [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e]. Moreover, a low-intensity band at 1,226 cm⁻\u0026sup1; suggests C-O vibrations associated with the enol form of the CIS [\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003eUV-Vis Spectra\u003c/h2\u003e \u003cp\u003eIn the UV-Vis region, the CIS in 1% (v/v) H₂O/HCl solution exhibited four absorption bands (Fig. S3). Both calculated and experimental UV-Vis spectra revealed an absorption profile spanning three distinct regions: Band A (300\u0026ndash;500 nm, corresponding to n\u0026rarr;π* transitions), Band B (250\u0026ndash;300 nm, corresponding to π\u0026rarr;π* transitions), and Band C (200\u0026ndash;250 nm, corresponding to n\u0026rarr;σ* transitions). Each transition type, as discussed above, can be characterized as the frontier orbital with the most significant contribution (weight greater than 0.5, considering f\u0026thinsp;\u0026gt;\u0026thinsp;0.03) to the absorption signal, in accordance with the signal intensity refining strategy discussed in recent literature [\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this model, the total number of filled orbitals was 178, with the highest occupied molecular orbital (HOMO: H) at this level, and the lowest unoccupied molecular orbital (LUMO: L) at level 179. Specifically, for the absorption transition at approximately 236 nm, Band C corresponds to a HOMO\u0026rarr;LUMO\u0026thinsp;+\u0026thinsp;1 transition. Band B, observed at 295 nm, is primarily due to a HOMO-6\u0026rarr;LUMO transition. Lastly, Band A, located at 386 nm, corresponds to a HOMO\u0026rarr;LUMO transition.\u003c/p\u003e \u003cp\u003eAdditionally, the UV-Vis profile shows the movement of electrons between molecular orbitals associated with Bands A and C, where electron transitions occur primarily within the derivatized isatin segments. In contrast, Band B corresponds to an electron transfer from the polymeric chain to the derivatized isatin portion, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Comparison between theoretical and experimental UV-Vis spectra for isatin-modified chitosan, based on Model 02, highlighting the electronic states most effectively represented in the transitions for each band.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eScanning Electron Microscopy\u003c/h2\u003e \u003cp\u003eThe morphologies of both the chitosan and CIS were examined using SEM on the film samples. The micrograph of unmodified chitosan (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eA) reveals a smooth, nonporous surface. Upon examining a bordering region of the sample, it becomes evident that chitosan is organized in stacked layers with thicknesses on the order of a few nanometers (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eB-C), which accounts for film formation of this material using the employed methodology. The CIS film also exhibited a stacked layer structure; however, it displayed a gapped surface with small agglomerates (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eD).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Micrographs of chitosan (A, C, and D) and CIS (B).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003eThermogravimetric Analysis\u003c/h2\u003e \u003cp\u003eThe thermal properties of chitosan and the CIS were analyzed by TG/DTG under synthetic air and nitrogen atmospheres, and their curves are presented in Figure. Unlike purified chitosan, which presented three thermal events in synthetic air atmosphere, the CIS presented four thermal events. The first stage (~\u0026thinsp;10.7%) up to 115\u0026deg;C is due to loss of water adsorbed on the polymer surface. This value was higher than that found for this material in the literature (~\u0026thinsp;8%, \u0026lt; 100\u0026deg;C) [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e] and may be due to the film sample having more adsorbed water molecules as compared to powder samples. Furthermore, the CIS film exhibited a thermal event (~\u0026thinsp;3.2%) between 140\u0026ndash;220\u0026deg;C (DTG\u0026thinsp;=\u0026thinsp;268.7\u0026deg;C), which may be related to the loss of the substituted isatin portion, since thermal decomposition of isatin in synthetic air can be observed in a nearby region (DTG\u0026thinsp;=\u0026thinsp;278.8\u0026deg;C). The third degradation stage (~\u0026thinsp;36.8%) is associated to decomposition of free monomers. In this stage, DTG was 268.7\u0026deg;C, while for unmodified chitosan it was 297.0\u0026deg;C. Lastly, the fourth degradation stage (~\u0026thinsp;45.3%) refers to decomposition of condensed monomers [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e, \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eUnder nitrogen atmosphere, the CIS film behaved similarly to chitosan, exhibiting two thermal events. The first mass loss stage (~\u0026thinsp;13%) below 130\u0026deg;C is due to water loss from the polymer surface, while the second, larger mass loss (~\u0026thinsp;49.4%) is due to polymer decomposition. The thermal event data obtained from the TG/DTG curves are compiled in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTG/DTG data for chitosan and Schiff base biopolymer under synthetic air and nitrogen atmospheres.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eAir\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c9\" namest=\"c6\"\u003e \u003cp\u003eNitrogen\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTemperature range\u003c/p\u003e \u003cp\u003e(\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMass loss\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eHighest DTG\u003c/p\u003e \u003cp\u003e(\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eResidue at 800\u0026deg;C\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eTemperature range\u003c/p\u003e \u003cp\u003e(\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMass loss\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eHighest DTG\u003c/p\u003e \u003cp\u003e(\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eResidue at 800\u0026deg;C\u003c/p\u003e \u003cp\u003e(%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eChitosan\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e30\u0026ndash;157\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e11.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e33.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e30\u0026ndash;120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e31.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e\u0026nbsp;\u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e215\u0026ndash;417\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e42.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e297.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e157\u0026ndash;565\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e46.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e301.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e27.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e441\u0026ndash;732\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e43.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e583.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCIS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e30\u0026ndash;115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e32.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e30\u0026ndash;130\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e13.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e32.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e140\u0026ndash;220\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e178.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e160\u0026ndash;517\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e49.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e303.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e32,5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e220\u0026ndash;439\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e36.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e268.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e439\u0026ndash;719\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e45.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e604.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. TG/DTG data for chitosan and Schiff base biopolymer under synthetic air and nitrogen atmospheres.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003eDetermination of Catalyzer\u003c/h2\u003e \u003cp\u003eThe effective surface area of the electrode was calculated according to the Randles\u0026ndash;Sevcik equation [\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003cem\u003eI\u003c/em\u003e \u003csub\u003e \u003cem\u003ep\u003c/em\u003e \u003c/sub\u003e = 2.687\u0026times;10\u003csup\u003e5\u003c/sup\u003e\u0026middot;\u003cem\u003en\u003c/em\u003e\u003csup\u003e3\u0026frasl;2\u003c/sup\u003e\u0026middot;\u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003eeff\u003c/em\u003e\u003c/sub\u003e\u0026middot;\u003cem\u003eD\u003c/em\u003e\u003csup\u003e1\u0026frasl;2\u003c/sup\u003e\u0026middot;\u003cem\u003eC\u003c/em\u003e\u0026middot;\u003cem\u003ev\u003c/em\u003e\u003csup\u003e1\u0026frasl;2\u003c/sup\u003e (1)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003eeff\u003c/em\u003e\u003c/sub\u003e is the electroactive area (cm\u003csup\u003e2\u003c/sup\u003e), \u003cem\u003eI\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e is the peak current, \u003cem\u003en\u003c/em\u003e the number of transferred electrons, \u003cem\u003eD\u003c/em\u003e the diffusion coefficient for K\u003csub\u003e3\u003c/sub\u003e[Fe(CN)\u003csub\u003e6\u003c/sub\u003e], \u003cem\u003eC\u003c/em\u003e the concentration of redox species in solution (mol\u0026middot;cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e), and \u003cem\u003ev\u003c/em\u003e the scan rate (V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e).\u003c/p\u003e \u003cp\u003eThe anodic (Ipa) and cathodic (Ipc) peak currents were derived from the linear fit of the voltammogram in regions where no electrochemical activity of the analyte was detected [\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e, \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e]. Assuming the conditions for Eq.\u0026nbsp;(1) were met, the slope values were obtained for the catalysts under investigation. By substituting these values into Eq.\u0026nbsp;(1), and considering \u003cem\u003en\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1 and \u003cem\u003eD\u003c/em\u003e\u0026thinsp;=\u0026thinsp;6.56 \u0026times; 10⁻⁶ cm\u0026sup2;\u0026middot;s⁻\u0026sup1;, the effective area (\u003cem\u003eA\u003c/em\u003e\u003csub\u003e\u003cem\u003eeff\u003c/em\u003e\u003c/sub\u003e) values were determined (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eActive electrochemical areas of catalysts.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAREA (cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e5%-CPE\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e15%\u0026ndash;CPE\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e30%\u0026ndash;CPE\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e50%\u0026ndash;CPE\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0053\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Active electrochemical areas of catalysts.\u003c/p\u003e \u003cp\u003eThe 5%-CPE mass ratio was chosen as the optimal electrode for use as a catalyst in the electrochemical oxidation reactions of ethanol in this study.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec19\" class=\"Section2\"\u003e \u003ch2\u003eEthanol Electrooxidation\u003c/h2\u003e \u003cdiv id=\"Sec20\" class=\"Section3\"\u003e \u003ch2\u003eEffect of pH\u003c/h2\u003e \u003cp\u003eThe cyclic voltammograms in Figs.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eA-B depict the voltammetric behavior of the 5%-CPE modified electrode at pH levels of 0.3, 2.0, and 4.0, recorded at a scan rate of 50 mV\u0026middot;s⁻\u0026sup1;.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Cyclic voltammograms obtained for several concentrations of aqueous solutions of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e at a scan rate of 50 mV\u0026middot;s⁻\u0026sup1;: (A) in the absence of ethanol and (B) in the presence of 1.0 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of ethanol. Temperature\u0026thinsp;=\u0026thinsp;25\u0026deg;C. Electrode area\u0026thinsp;=\u0026thinsp;0.26 cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eA shows that voltammetric peak currents varied with pH. In a more acidic medium, a higher current density was observed, likely due to the protonation of hydroxyl and amino groups in chitosan that were not substituted by isatin, since electrochemical oxidation and reduction for isatin occur at approximately 1.0 V and \u0026minus;\u0026thinsp;0.6 V, respectively. This protonation effect in acidic conditions was also observed in CPE containing only chitosan [\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eB indicates the dependence of the EOR (ethanol oxidation reaction) peak current on ethanol concentration, as observed in the cyclic voltammograms (CVs) for the 5%-CPE catalyst in the presence of 1.0 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of ethanol at pH levels of 0.3, 2.0, and 4.0.\u003c/p\u003e \u003cp\u003eAn increase in current density was observed, likely due to oxidation-reduction processes of ethanol molecules. These electrochemical processes were most efficient at pH 0.3, suggesting that the protonation of functional groups enhances the electrochemical oxidation of ethanol [\u003cspan additionalcitationids=\"CR65 CR66\" citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe voltammograms for ethanol electrocatalytic oxidation at various pH values indicate that the best catalytic performance is associated to the protonation of hydroxyl and amino groups. The behavior of the catalytic system across different pH levels suggests that maximum efficiency occurs at pH 0.3, implying that higher concentrations of H\u0026thinsp;+\u0026thinsp;ions facilitate EOR by aiding in the removal of adsorbed intermediates.\u003c/p\u003e \u003cp\u003eThe scan rate effect on the voltammetric behavior of the modified 5%-CPE electrode in ethanol oxidation was further investigated over a range of 0.01 to 0.3 V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in 0.5 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eA). The anodic peak current densities (\u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epa\u003c/em\u003e\u003c/sub\u003e) increased with the scan rate; however, the plot of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epa\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epc\u003c/em\u003e\u003c/sub\u003e vs. scan rate (\u003cem\u003ev\u003c/em\u003e) did not exhibit a strong linear relationship (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eB). Conversely, the plot of \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epa\u003c/em\u003e\u003c/sub\u003e/\u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epc\u003c/em\u003e\u003c/sub\u003e vs. the square root of the sweep rate (v\u003csup\u003e1/2\u003c/sup\u003e) displayed a strong linear correlation, with the following equations: \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epa\u003c/em\u003e\u003c/sub\u003e/mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e = 6.16423\u0026thinsp;\u0026minus;\u0026thinsp;0.0381 v\u003csup\u003e1/2\u003c/sup\u003e (V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003csup\u003e1/2\u003c/sup\u003e com \u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.9926 and \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epc\u003c/em\u003e\u003c/sub\u003e/mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e = -6.0545\u0026thinsp;+\u0026thinsp;0.2333 \u003cem\u003ev\u003c/em\u003e\u003csup\u003e1/2\u003c/sup\u003e (V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003csup\u003e1/2\u003c/sup\u003e with \u003cem\u003eR\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.9961 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eC). This result is consistent with a diffusion-controlled transfer process of the electroactive species, indicating a relatively slow diffusion of H\u0026thinsp;+\u0026thinsp;ions onto the electrode surface.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. (A) Cyclic voltammetry of the 5%-CPE electrode in 0.5 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution at varying scan rates (0.01, 0.025, 0.05, 0.15, 0.2, 0.25, and 0.3 V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). (B) Peak current density vs. scan rate. (C) Peak current density vs. square root of scan rate. (D) Anodic and cathodic peak potential vs. logarithm of scan rate. (E) Difference between anodic and cathodic potential vs. scan rate. Temperature\u0026thinsp;=\u0026thinsp;25\u0026deg;C. Electrode area\u0026thinsp;=\u0026thinsp;0.26 cm\u0026sup2;.\u003c/p\u003e \u003cp\u003eThe slopes of the log \u003cem\u003eJ\u003c/em\u003e\u003csub\u003e\u003cem\u003epa\u003c/em\u003e\u003c/sub\u003e vs. log \u003cem\u003ev\u003c/em\u003e curves were determined using the modified Randles-Sevcik equation [\u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e]. For the 5%-CPE electrode, the slope was found to be 0.59\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04 (R\u0026sup2; = 0.983). This value is close to the theoretical value of 0.5, indicating a process governed by linear diffusion conditions [\u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eD, at higher scan rates (\u003cem\u003ev\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.2 V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), the peak potential (\u003cem\u003eEp\u003c/em\u003e) becomes proportional to the logarithm of the scan rate, suggesting kinetic limitations as predicted by Laviron\u0026rsquo;s theory [\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e]. The electron transfer coefficient (α), which reflects the symmetry of the energy barrier in the redox reaction. When the peak separation \u003cem\u003eΔEp\u003c/em\u003e is greater than 200/n mV, the plot \u003cem\u003eEp\u003c/em\u003e\u0026thinsp;=\u0026thinsp;f(log \u003cem\u003ev\u003c/em\u003e) yields two linear regions with slopes of \u0026minus;\u0026thinsp;2.3\u003cem\u003eRT\u003c/em\u003e/\u003cem\u003eαnF\u003c/em\u003e for the cathodic peak and 2.3\u003cem\u003eRT\u003c/em\u003e/(1\u0026thinsp;\u0026minus;\u0026thinsp;α)\u003cem\u003enF\u003c/em\u003e for the anodic peak, where \u003cem\u003eR\u003c/em\u003e is the ideal gas constant, \u003cem\u003eF\u003c/em\u003e is the Faraday constant, and \u003cem\u003en\u003c/em\u003e is the number of electrons involved in the redox process. The slopes obtained for \u003cem\u003eEp\u003c/em\u003e vs. log \u003cem\u003ev\u003c/em\u003e were \u0026minus;\u0026thinsp;0.7406 and 0.3197 for the cathodic and anodic peaks, respectively, resulting in a calculated α value of 0.60.\u003c/p\u003e \u003cp\u003eThe difference between the anodic and cathodic peak potentials (\u003cem\u003eΔE\u003c/em\u003e) increases with scan rate (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eE). As the scan rate increases, the redox peaks broaden, indicating higher polarization effects at faster scan rates. This is further evidenced by the growing voltage difference between the anodic and cathodic peaks at higher scan rates.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec21\" class=\"Section2\"\u003e \u003ch2\u003eEffect of EtOH concentration\u003c/h2\u003e \u003cp\u003eThe electrocatalytic oxidation of ethanol carried out by 5%-CPE was also investigated via cyclic voltammetry for different ethanol concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eA. These results indicate that the EOR peak increases with ethanol concentration.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. (A) Cyclic voltammetry (anodic direction) for 5%-CPE at varying ethanol concentrations (0.2, 0.4, 0.6, 0.8, and 1.0 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in 0.5 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e of H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e solution. (B) Maximum peak current density as a function of ethanol concentration. Scan rate\u0026thinsp;=\u0026thinsp;0.025 V\u0026middot;s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Electrochemically active area of electrode\u0026thinsp;=\u0026thinsp;0.26 cm\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eB shows the linear correlation between the catalytic current EOR and the ethanol concentration ranging from 0 to 1.0 mol\u0026middot;L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, suggesting that the peak current density is dependent on ethanol concentration.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section2\"\u003e \u003ch2\u003eImpedance Spectroscopy\u003c/h2\u003e \u003cp\u003eTo generate the Nyquist diagram, both the frequency values and the real (\u003cem\u003eZ\u0026rsquo;\u003c/em\u003e) and imaginary (\u003cem\u003e-Z\u0026rdquo;\u003c/em\u003e) parts of impedance are analyzed. The frequency corresponds to the specific point in time when the impedance is measured, since different electrochemical processes and reactions may occur across various frequency ranges. The real part of impedance (\u003cem\u003eZ\u0026rsquo;\u003c/em\u003e) reflects the resistive component of the system. In contrast, the imaginary part (\u003cem\u003e-Z\u0026rdquo;\u003c/em\u003e) represents the reactive nature of the system, revealing the capacitive or inductive behavior at the interface. Thus, the values of \u003cem\u003eZ\u0026rsquo;\u003c/em\u003e and \u003cem\u003e-Z\u0026rdquo;\u003c/em\u003e represent the complex impedance of the system at the frequency under investigation. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e illustrates the Nyquist diagram for the CPE sensor.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Nyquist diagram obtained via EIS for 5%-CPE.\u003c/p\u003e \u003cp\u003eThe EIS results were fitted via the equivalent electric circuit model. The fitting was achieved using the \u003cem\u003eQ1\u003c/em\u003e(\u003cem\u003eR1Q2\u003c/em\u003e)(\u003cem\u003eR2Q3\u003c/em\u003e) circuit, which consists of three serial components, to interpret the physicochemical behavior of the sensor. The contribution of \u003cem\u003eQ1\u003c/em\u003e is shown on the left-hand side of the Nyquist diagram, which represents the regions of high frequencies where it is possible to assess the capacitive behavior of the interface.\u003c/p\u003e \u003cp\u003eThe initial \u003cem\u003eZ\u0026rsquo;\u003c/em\u003e and \u003cem\u003eZ\u0026rdquo;\u003c/em\u003e values (-4.64 Ω and 359.51 Ω, respectively) indicate a high capacitance within the electric double layer. The negative \u003cem\u003eZ\u0026rsquo;\u003c/em\u003e value suggests a predominant capacitive component at the interface. Typically, the real part of impedance is associated to the system resistance, and a negative value implies that capacitive behavior is overriding. This behavior is likely linked to the electric double layer formed at the electrode-electrolyte interface. The positive value of the imaginary part (\u003cem\u003eZ\u0026rdquo;\u003c/em\u003e) further confirms the predominance of the capacitive behavior, reflecting the reactive nature of system, such as the accumulation and release of electric charges at the interface. In sensor applications, this capacitive behavior offers significant advantages, including enhanced sensitivity, rapid response, improved accuracy, and greater versatility.\u003c/p\u003e \u003cp\u003eThe parallel elements \u003cem\u003eQ2\u003c/em\u003e and \u003cem\u003eR1\u003c/em\u003e, associated with the high-frequency semicircle, correspond to the constant-phase element and charge transfer resistance, respectively. Meanwhile, the parallel elements \u003cem\u003eQ3\u003c/em\u003e and \u003cem\u003eR2\u003c/em\u003e, related to the inclined line in the low-frequency region, represent the diffusion processes of electroactive species in the electrolyte as they move toward the electrode. This electrical circuit was designed specifically to characterize porous structures. \u003cem\u003eQ1\u003c/em\u003e and \u003cem\u003eQ2\u003c/em\u003e are interpreted as non-ideal capacitances, influenced by surface geometry, roughness, and porosity. The relationship between these constant-phase elements (\u003cem\u003eQ\u003c/em\u003e) and impedance (\u003cem\u003eZ\u003c/em\u003e) is described by the equation below.\u003c/p\u003e \u003cp\u003e \u003cem\u003eZ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;1/\u003cem\u003eQ\u003c/em\u003e(\u003cem\u003ej\u0026middot;ω\u003c/em\u003e)\u003cem\u003en\u003c/em\u003e (2)\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003ej\u003c/em\u003e is the imaginary unit, \u003cem\u003eω\u003c/em\u003e is the angular frequency, and \u003cem\u003en\u003c/em\u003e is a correction factor (0\u0026thinsp;\u0026lt;\u0026thinsp;\u003cem\u003en\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1).\u003c/p\u003e \u003cp\u003eAs the value of \u003cem\u003en\u003c/em\u003e approaches 0, \u003cem\u003eQ\u003c/em\u003e exhibits less capacitive behavior [\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e, \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e73\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe equivalent circuit was chosen based on its ability to fit the Nyquist plot with minimal error. The estimated parameter values obtained after fitting the equivalent circuit model to the impedance spectra are presented in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The fitting results show that \u003cem\u003en1\u003c/em\u003e and \u003cem\u003en2\u003c/em\u003e are both less than 1, which corroborates the roughness and heterogeneity of the sensor surface. Additionally, the increase in resistance \u003cem\u003eR2\u003c/em\u003e indicates that the occupation of complementary sites on the sensor makes the electrode surface more resistant to charge transfer.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eValues ​​of the elements that make up the equivalent electrical circuit proposed for the CPE electrode with their respective errors.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eQ1\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eR1\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eQ2\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cem\u003eR2\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cem\u003eQ3\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eY\u003c/b\u003e \u003cb\u003e(F)\u003c/b\u003e\u003c/p\u003e \u003cp\u003e\u003cb\u003e(\u0026plusmn;\u0026thinsp;error)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e4.0 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;1.58%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4.15 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;2.43%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.94 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;2.59%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003en\u003c/b\u003e \u003cb\u003e(Rad)\u003c/b\u003e\u003c/p\u003e \u003cp\u003e\u003cb\u003e(\u0026plusmn;\u0026thinsp;error)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.673\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;1.01%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.646\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;1.56%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;0.23%)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cb\u003eR\u003c/b\u003e \u003cb\u003e(Ω)\u003c/b\u003e\u003c/p\u003e \u003cp\u003e\u003cb\u003e(\u0026plusmn;\u0026thinsp;error)\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e802.34\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;3.45%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e822.71\u003c/p\u003e \u003cp\u003e(\u0026plusmn;\u0026thinsp;1.20%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e---\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Values ​​of the elements that make up the equivalent electrical circuit proposed for the CPE electrode with their respective errors.\u003c/p\u003e \u003cp\u003eThe phase angle at low frequencies provides specific information on the behavior of the electrode. Ideally, a capacitor should exhibit a phase angle close to 90\u0026deg;. This angle is particularly desirable for insulating electrodes because it represents the maximum phase difference between the applied voltage and the resulting current [\u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e74\u003c/span\u003e]. Significant deviations from this value suggest that the dielectric layer is losing its effectiveness in retaining electric charge, indicating the onset of electron flow.\u003c/p\u003e \u003cp\u003eAs shown in the Bode plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e), the phase angle starts at approximately \u0026minus;\u0026thinsp;58\u0026deg; at low frequencies and drops to values below \u0026minus;\u0026thinsp;20\u0026deg; in the mid-frequency region, indicating a shift towards resistive behavior. This shift suggests increased ionic permeability and electron flow through the CPE electrode. This finding is further supported by the higher impedance values observed in the low-frequency region of the same plot.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. Bode plot obtained via EIS for 5%-CPE.\u003c/p\u003e \u003cp\u003eTo ensure the validity of all analyses, it is fundamental that the measured spectrum accurately represents a linear, time-invariant, and causal system. The Kramers-Kronig relations are used to verify compliance with these requirements, as they establish a connection between the real and imaginary parts of the impedance spectrum for such systems. This allows for a thorough and accurate evaluation of the system under investigation [\u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e75\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eGenerally, values below 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e typically indicate an excellent fit, values between 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e and 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e are considered reasonable, and values in the range of 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e to 10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e are marginal. Higher values suggest a poor fit. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the linearity, stability, and causality data obtained for the CPE sensor indicate an excellent fit, validating the analyses performed.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eKronig-Kramers analysis values ​​for the proposed CPE electrode circuit.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c4\" namest=\"c1\"\u003e \u003cp\u003eKronig-Kramers\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eX\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e \u003cem\u003e(Z)\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eX\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e \u003cem\u003e(Z\u0026rsquo;)\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eX\u003c/em\u003e\u003csup\u003e\u003cem\u003e2\u003c/em\u003e\u003c/sup\u003e \u003cem\u003e(-Z\u0026rdquo;)\u003c/em\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eElectrode CPE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.91 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.30 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;7\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.16 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;8\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eDuring the Kramers-Kronig test, the experimental data points are fitted using the proposed circuit model, which satisfies the Kramers-Kronig relations. If the experimental data can be accurately represented by the circuit model \u003cem\u003eQ1\u003c/em\u003e(\u003cem\u003eR1Q2\u003c/em\u003e)(\u003cem\u003eR2Q3\u003c/em\u003e), then the data set should also satisfy the Kramers-Kronig assumptions.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Kronig-Kramers analysis values ​​for the proposed CPE electrode circuit.\u003c/p\u003e \u003c/div\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eThis study explored the synthesis, characterization, and application of an isatin-modified chitosan Schiff base (CIS) as an electrocatalyst for the ethanol oxidation reaction (EOR) in direct ethanol fuel cells (DEFCs). The primary motivation was to address the challenges of CO poisoning by metal electrocatalysts and to enhance the efficiency of the ethanol oxidation process.\u003c/p\u003e \u003cp\u003eCharacterization of the isatin-modified chitosan confirmed the successful formation of the Schiff base, evidenced by the presence of new bands characteristic of the C\u0026thinsp;=\u0026thinsp;N bond. Scanning electron microscopy (SEM) revealed that the CIS exhibited a gap-like morphology on its surface, which contrasts with the smooth surface of unmodified chitosan. Thermogravimetric analysis (TG/DTG) demonstrated that the CIS possesses greater thermal stability compared to pure chitosan, with four distinct thermal events.\u003c/p\u003e \u003cp\u003eElectrochemical performance testing identified the CIS-modified electrode (5%-CPE) as the ideal candidate for catalyzing the electrochemical oxidation of ethanol. The catalyst efficiency was assessed at various pH levels, exhibiting optimal performance in an acidic medium (pH 3). At this pH, protonation of hydroxyl and amine groups enhances ethanol oxidation.\u003c/p\u003e \u003cp\u003eThe linear relationship between EOR peak current and ethanol concentration indicated that peak current density increases with ethanol concentration. Furthermore, electrochemical impedance spectroscopy results revealed a dominant capacitive behavior and confirmed the roughness and heterogeneity of the sensor surface.\u003c/p\u003e \u003cp\u003eOverall, this study demonstrated that the isatin-modified chitosan Schiff base (CIS) is a promising electrocatalyst for EOR in DEFCs. The modification of chitosan with isatin not only improved the electrocatalytic properties but also enhanced the thermal stability of the material, positioning the CIS as a sustainable and efficient alternative for applications in direct ethanol fuel cells.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch1\u003eAcknowledgements\u003c/h1\u003e\n\u003cp\u003eThe authors would like to thank the Professional Master\u0026rsquo;s Program in Chemistry in a national network with the Federal University of Rio de Janeiro (PROFQUI-UFRJ), the Postgraduate Program in Science and Technology of Amazonian Resources of the Federal University of Amazonas (PPGCTRA-UFAM), the Multiuser Laboratory of Nuclear Magnetic Resonance (LAMRMN), the Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro (FAPERJ grants No. E-26/210.070/2022-DCTR, SEI-260003/015957/2021, SEI-260003/015018/2021-APQ1, SEI-260003/003411/2022\u0026ndash;BBP, and SEI-260003/014874/2023 - APQ1), the Amazonas State Research Support Foundation for their support (FAPEAM), the Coordination for the Improvement of Higher Education Personnel \u0026ndash; Brazil (CAPES) and the National Council for Scientific Development \u0026ndash; CNPq for financial support.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThis work was supported by\u0026nbsp;grants numbers\u003csup\u003e.\u003c/sup\u003e E-26/210.070/2022-DCTR, SEI-260003/015957/2021, SEI-260003/015018/2021-APQ1, SEI-260003/003411/2022\u0026ndash;BBP, and SEI-260003/014874/2023 - APQ1\u0026nbsp;(FAPERJ).\u003c/p\u003e\n\u003ch2\u003eCompeting Interests\u003c/h2\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003ch2\u003eAuthor Contributions\u003c/h2\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation, data collection and analysis were performed by\u0026nbsp;Eduardo Guilherme Cividini Neiva; Nathalia Biazotto S\u0026aacute;, Jos\u0026eacute; Wimo da Cruz Jr; Rodrigo de Siqueira Melo and Jardel Ramos da Encarna\u0026ccedil;\u0026atilde;o; Brenda Antunes Louri\u0026ccedil;al Paix\u0026atilde;o, Joel dos Santos Batista and Let\u0026iacute;cia Oliveira Laier. The\u0026nbsp;theoretical calculus was realized by Willian T\u0026aacute;ssio Gomes Novato.\u0026nbsp;The first draft of the manuscript was written by Paulo Jos\u0026eacute; Sousa Maia,\u0026nbsp;Ismael Casagrande Bellettini,\u0026nbsp;Jos\u0026eacute; Wimo da Cruz Jr\u0026nbsp;and\u0026nbsp;Eduardo Guilherme Cividini Neiva. All authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003ch2\u003eData Availability\u003c/h2\u003e\n\u003cp\u003eIt does not apply\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLyu, F., Cao, M., Mahsud, A., \u0026amp; Zhang, Q. Interfacial engineering of noble metals for electrocatalytic methanol and ethanol oxidation. \u003cem\u003eJ. Mater\u003c/em\u003e. Chem.\u003cem\u003e A.\u003c/em\u003e \u003cstrong\u003e8(31)\u003c/strong\u003e, 15445\u0026ndash;15457 (2020).\u003c/li\u003e\n\u003cli\u003eMendon\u0026ccedil;a, I. M., Paes, O. A., Maia, P. J., Souza, M. P., Almeida, R. A., Silva, C. C., Duvoisin Jr, S., \u0026amp; de Freitas, F. A. New heterogeneous catalyst for biodiesel production from waste tucum\u0026atilde; peels (Astrocaryum aculeatum Meyer): Parameters optimization study. Renew. Energy. \u003cstrong\u003e130\u003c/strong\u003e, 103\u0026ndash;110 (2019).\u003c/li\u003e\n\u003cli\u003eSong, S. Q., Zhou, W. J., Zhou, Z. H., Jiang, L. H., Sun, G. Q., Xin, Q., Leontidis, V., Kontou, S., \u0026amp; Tsiakaras, P. Direct ethanol PEM fuel cells: The case of platinum based anodes. \u003cem\u003eInt. J. Hydrog. Energy\u003c/em\u003e. \u003cstrong\u003e30(9)\u003c/strong\u003e, 995\u0026ndash;1001 (2005). \u003c/li\u003e\n\u003cli\u003eZakaria, Z., Kamarudin, S. K., \u0026amp; Wahid, K. A. A. Polymer electrolyte membrane modification in direct ethanol fuel cells: An update. J. Appl. Polym. Sci. \u003cstrong\u003e140(4), \u003c/strong\u003e1-20 (2023).\u003c/li\u003e\n\u003cli\u003eFeng, C., Takeuchi, T., Abdelkareem, M. A., Tsujiguchi, T., \u0026amp; Nakagawa, N. Carbon\u0026ndash;CeO\u003csub\u003e2\u003c/sub\u003e composite nanofibers as a promising support for a PtRu anode catalyst in a direct methanol fuel cell. \u003cem\u003eJ. 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Acta. \u003cstrong\u003e131\u003c/strong\u003e, 20\u0026ndash;27 (2014). \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-applied-electrochemistry","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jach","sideBox":"Learn more about [Journal of Applied Electrochemistry](http://link.springer.com/journal/10800)","snPcode":"10800","submissionUrl":"https://submission.nature.com/new-submission/10800/3","title":"Journal of Applied Electrochemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Chitosan, Isatin, Fuel Cells, Electrocatalysis","lastPublishedDoi":"10.21203/rs.3.rs-4986733/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4986733/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDirect Ethanol Fuel Cells (DEFCs) have garnered significant attention in recent years due to their potential for producing clean and renewable energy. DEFCs are particularly appealing because of their low toxicity and high current density. This study aims to investigate the catalytic activity of an isatin-modified chitosan Schiff-base in ethanol electrooxidation for possible applications in DEFCs. This study utilizes density functional theory calculations and experimental results to analyze the relative stability, through Gibbs free energy, between two protonated models of an isatin-modified chitosan Schiff-base. The synthesized isatin-modified chitosan Schiff base was structurally characterized through nuclear magnetic resonance spectroscopy, Fourier transform infrared vibrational spectroscopy, ultraviolet-visible spectroscopy, and thermogravimetric analysis. Morphological aspects of the compound, such as formation and structure, were assessed by scanning electron microscopy. In electrocatalytic evaluation experiments, the carbon paste electrode with isatin-modified chitosan (5%-CPE) demonstrated efficiency in oxidizing ethanol, especially at pH 3. At this pH, protonation of hydroxyl and amino groups present in chitosan favored ethanol oxidation. Current density increased proportionally to ethanol concentration. Electrochemical impedance spectroscopy pointed to a capacitive behavior of the sensor, indicated by high values of electrical double layer capacitance. Our results offer new insights into the isatin-modified chitosan Schiff base (CIS), indicating its potential as an electrocatalyst for ethanol oxidation. This material shows promise for use in direct ethanol fuel cells (DEFCs), providing a more sustainable and efficient alternative to traditional platinum catalysts.\u003c/p\u003e","manuscriptTitle":"Electrocatalyst for Ethanol Oxidation Reaction Based on Isatin-modified Chitosan","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-26 19:30:03","doi":"10.21203/rs.3.rs-4986733/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-10-25T23:52:45+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-21T17:12:30+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-21T09:09:39+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"133681827244692892588717056336829287674","date":"2024-10-06T18:05:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"230418740753911008364942002095054959801","date":"2024-09-30T14:50:58+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"332021212379375566402960030829953050646","date":"2024-09-29T04:12:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-09-28T12:55:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-30T00:34:54+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-28T05:05:18+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Applied Electrochemistry","date":"2024-08-27T20:30:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-applied-electrochemistry","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jach","sideBox":"Learn more about [Journal of Applied Electrochemistry](http://link.springer.com/journal/10800)","snPcode":"10800","submissionUrl":"https://submission.nature.com/new-submission/10800/3","title":"Journal of Applied Electrochemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"eed21fb3-b207-4f97-83e0-6f9d57ba743e","owner":[],"postedDate":"September 26th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-11-25T16:05:36+00:00","versionOfRecord":{"articleIdentity":"rs-4986733","link":"https://doi.org/10.1007/s10800-024-02228-3","journal":{"identity":"journal-of-applied-electrochemistry","isVorOnly":false,"title":"Journal of Applied Electrochemistry"},"publishedOn":"2024-11-23 15:58:15","publishedOnDateReadable":"November 23rd, 2024"},"versionCreatedAt":"2024-09-26 19:30:03","video":"","vorDoi":"10.1007/s10800-024-02228-3","vorDoiUrl":"https://doi.org/10.1007/s10800-024-02228-3","workflowStages":[]},"version":"v1","identity":"rs-4986733","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4986733","identity":"rs-4986733","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}