Single-Atom Cobalt on N-Doped Reduced Graphene Oxide Pushes the Oxygen Reduction Reaction toward 4-Electron Pathway

Single-Atom Cobalt on N-Doped Reduced Graphene Oxide Pushes the Oxygen Reduction Reaction toward 4-Electron Pathway | 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 Article Single-Atom Cobalt on N-Doped Reduced Graphene Oxide Pushes the Oxygen Reduction Reaction toward 4-Electron Pathway Francesca Risplendi, Nadia Garino, Juqin Zeng, Adriano Sacco, and 7 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6904882/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 29 Sep, 2025 Read the published version in npj 2D Materials and Applications → Version 1 posted 12 You are reading this latest preprint version Abstract We present a cobalt-based single-atom catalyst (SAC) anchored on nitrogen-doped reduced graphene oxide (Co-N-rGO), synthesized via a rapid, one-pot microwave-assisted method. Comprehensive characterization by FESEM, TEM, Raman spectroscopy, and X-ray photoelectron spectroscopy confirms the successful formation of atomically dispersed Co(II) centers, coordinated to oxygen-containing functional groups within the graphene matrix, with no evidence of metallic clusters or oxide nanoparticles. The atomically dispersed Co sites act as highly active centers for the oxygen reduction reaction (ORR) in alkaline media, delivering near four-electron transfer efficiency, a positive onset potential, and outstanding durability that surpasses commercial Pt/C catalysts. First-principles density functional theory (DFT) calculations corroborate the XPS findings and reveal the electronic structure of the Co–O coordination environment, offering atomistic insight into the catalytic mechanism. The synergy between precise site isolation, optimized local coordination, and electronic modulation enables the superior electrocatalytic performance of this Co SAC. This study establishes a versatile and scalable framework for engineering high-performance ORR catalysts featuring non-noble metal single-atom active sites. Physical sciences/Materials science/Condensed matter physics/Surfaces interfaces and thin films Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis Physical sciences/Materials science/Theory and computation/Atomistic models Physical sciences/Physics/Chemical physics Physical sciences/Physics/Techniques and instrumentation/Characterization and analytical techniques Physical sciences/Physics/Techniques and instrumentation/Design synthesis and processing Oxygen reduction reaction (ORR) cobalt functionalization N-doping reduced graphene oxide catalytic activity Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction In recent years, extensive research has focused on the development of sustainable and efficient energy technologies to address the global demand for cleaner energy sources. Among these, fuel cells have attracted significant attention due to their high energy density and potential to serve as practical alternatives to conventional energy ​[ 1 , 2 ]​. However, one of the major challenges in fuel cell technology is the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode, which significantly affects the overall efficiency of the ​[ 3 – 6 ]​. To enhance ORR efficiency, electrocatalysts are required to reduce overpotentials and facilitate the four-electron pathway over the two-electron one, leading to the direct formation of water ​[ 7 – 10 ]​. Traditionally, noble metals such as platinum (Pt) and palladium (Pd) have been employed as ORR catalysts due to their known high catalytic activity ​[ 11 – 15 ]​. However, their high cost and limited availability have severely restricted their widespread applications. Consequently, alternative materials, particularly carbon-based supports decorated with metal species, have emerged as promising ORR electrocatalyst candidates ​[ 7 , 16 – 20 ]​. Carbon-based materials exhibit excellent stability, tunable electronic properties, and significantly lower costs compared to noble metals ​[ 21 – 24 ]​. These properties can be further enhanced by doping with heteroatoms or transition metals. A particularly interesting platform of such a kind is nitrogen-doped reduced graphene oxide (N-rGO), which exhibits ease of further functionalization with transition metals​[ 25 – 28 ]​. Moreover, the presence of nitrogen introduces additional active sites and strengthens electronic interactions between the carbon support and metal species, thereby enhancing both catalytic activity and durability ​[ 29 – 32 ]​. Similarly, cobalt (Co)-based materials have attracted considerable interest for ORR applications due to their ability to facilitate efficient electron transfer and thus, enhance catalytic performance ​[ 33 – 36 ]​. In particular, various studies have demonstrated that Co, in the form of metal, oxides, or carbides, can significantly boost ORR activity when incorporated into carbonaceous frameworks such as porous carbons, carbon nanotubes, or graphene-based materials ​[ 37 – 40 ]​. As a result, several studies have reported on the functionalization of N-rGO with Co in different oxidation states for electrocatalytic applications. A. Ejaz ​[ 41 ]​ investigated a cobalt hydroxide [Co(OH)₂] nanoflower-decorated N-rGO system for electrocatalytic sensing, demonstrating the synergistic effects between N doping and Co(OH)₂ in enhancing surface catalytic activity. Similarly, P. Yaengthip et al. ​[ 42 ]​ reported ORR activity in acidic media for a Co-decorated N-rGO material, showing that thermal treatment at 300°C optimized the catalytic performance by preserving N functionalities. In an alkaline medium, Zhang et al. ​[ 43 ]​ observed an ORR peak potential of -0.26 V vs. Ag/AgCl for a cobalt oxide (Co₃O₄)-decorated N-rGO catalyst, demonstrating an electron transfer number close to 4, which is ideal for ORR in the fuel cell applications. In this work, we present a combined experimental and theoretical investigation aimed at elucidating the structure–activity relationship responsible for the outstanding catalytic performance of Co-functionalized N-rGO (Co-N-rGO). The material is synthesized via a rapid and environmentally sustainable microwave (MW)-assisted hydrothermal process, during which GO undergoes simultaneous reduction, N incorporation, and Co coordination. This one-step synthesis results in the formation of a well-defined tetraphenyl porphyrin (TPP)-like coordination environment, akin to that observed in other metal-containing N-doped systems ( e.g. , Cu-functionalized rGO)​[ 24 , 44 , 45 ]​. Our findings highlight the distinct role of Co incorporation within this coordination framework in driving the superior catalytic activity observed. This insight offers a new design principle for the development of efficient single-atom catalysts based on non-noble metals. Comprehensive characterization of the synthesized Co-N-rGO —performed via field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy— confirms the successful incorporation of Co ions into the graphene lattice, with no evidence of crystalline structures such as metallic nanoclusters or oxide nanoparticles. The Co is stabilized in a + 2-oxidation state and coordinated to oxygen (O)-containing functional groups anchored to the rGO matrix. To identify the atomistic structure of the active site and gain deeper insight into its electronic properties and exceptional electrocatalytic performance, density functional theory (DFT) calculations were performed. These simulations not only support the experimental XPS findings but also elucidate the fundamental role of cobalt in the catalysis of the oxygen reduction reaction. Electrochemical and analytical measurements further confirm the high ORR activity and long-term stability of Co-N-rGO in alkaline media. The integration of advanced characterization techniques with theoretical modelling provides a thorough understanding of the underlying structure–activity relationship, offering valuable guidance for the rational design of next-generation electrocatalysts. 2. Results and Discussion 2.1. Physical and chemical characterization The Co-N-rGO was synthesized using a previously established and reproducible microwave assisted method, as reported in our earlier work (refs. [ 24 , 44 , 45 ]) and described in detail in the Supporting Information. To investigate the morphological and structural properties of the synthesized material, FESEM and TEM were first employed. As shown in Fig. 1 a, FESEM imaging reveals multilayered graphene-like flakes with extended lateral size at the micrometer scale, consistent with high-quality 2D graphene derivatives. Notably, no signs of morphological degradation or fragmentation were observed following the microwave-assisted synthesis, confirming the mildness of the process. The presence of cobalt was clearly detected by EDX spectroscopy (Fig. 1 b), which reveals distinct peaks for Co, C, and N, along with minor signals from the copper grid (Cu) and the holder (Al). TEM micrographs (Figs. 1 c) further confirm the integrity of the carbonaceous framework at the nanometer scale, showing the absence of crystalline domains that could be attributed to cobalt-based or cobalt oxide phases. Electron diffraction patterns (Fig. 1 d) exhibit the typical ring-like features of multilayered graphene with no additional diffraction spots, reinforcing the conclusion that no crystalline phases are present and confirming successful Co inclusion without nanoparticle formation. Nevertheless, Raman spectroscopy provides complementary insights into the structural properties of the catalyst. Spectra of the Co-N-rGO material and a Co-free N-rGO reference (synthesized as in [ 46 , 47 ]) are compared in Fig. 2 . In the low-frequency region (< 1000 cm⁻¹)[ 48 ], the absence of cobalt oxide fingerprint bands—typically found between 190 and 700 cm⁻¹ ( Figure S1 ) —supports the conclusion drawn from TEM imaging. The only detectable signal in this region is a weak, broad band at ~ 640 cm⁻¹, also present in the reference N-rGO, possibly originating from a high density of vibrational states in defective graphitic domains [ 49 , 50 ] rather than Co-based species. Confocal Raman setup excludes any contribution from the glass substrate ( Figure S2 ). The main spectral features of Co-N-rGO are typical of rGO, displaying a D band at 1349 cm⁻¹ and a G band at 1605 cm⁻¹. The D band arises from the defect-activated A 1g mode (ring breathing), indicative of disordered sp² carbon, while the G band corresponds to the doubly-degenerate E 2g mode (sp² C–C stretching), which is highly sensitive to disruptions in the planar sp² graphitic network[ 51 , 52 ] and to variations in charge carrier concentration [ 53 ]. The second-order Raman features, including the 2D (~ 2688 cm⁻¹), D + D′ (~ 2935 cm⁻¹), and 2D′ (~ 3197 cm⁻¹) bands, match those of the Co-free N-rGO reference material, suggesting that the fundamental graphitic structure is preserved. The intensity of the D and G bands (I D /I G ) ratio (0.95) of Co-N-rGO is nearly identical to that of N-rGO (0.94), indicating a comparable level of reduction and structural disorder in both [ 54 ]. However, both D and G bands exhibit significant blueshifts in the Co-containing sample (two-tailed t-test, p-value << α, with α = 0.05, and a statistical power of 0.9999 for the D band and 1.0000 for the G band, see Table S1 and Table S2 ), implying lattice alterations induced by cobalt incorporation. G-band blueshifts are generally attributed to phonon stiffening due to compressive strain or increased doping levels, suggesting that Co atoms or ions may either intercalate or chemically interact with the rGO [ 49 , 55 ]. While an increase in doping could be directly attributed to cobalt incorporation into the carbon lattice—alongside nitrogen, i.e., co-doping—the presence of compressive strain may stem from a broader and more complex set of factors, such as the intercalation of coordinating ions or surface functionalization by coordinating or electrostatically interacting species[ 25 , 56 ]. Moreover, as reported in Table S3 , a noticeable narrowing of the G band’s full width at half maximum (FWHM) is observed in Co-N-rGO (90 cm − 1 ) compared to the reference (114 cm − 1 ), potentially arising from an increased fraction of ordered sp² domains [ 52 ] or from high dopant concentrations suppressing phonon–electron decay via Pauli blocking [ 25 ]. Although the specific origin remains ambiguous, these spectral changes reflect significant structural and electronic modifications, highlighting the need for further investigation via XPS. XPS was employed to probe the chemical states of the elements and get an insight on Co binding configuration. XPS survey spectra (Fig. 3 a) confirm the presence of C, N, O, and Co elements. The relative atomic concentrations are consistent with those reported for other metal ion-functionalized N-rGO materials (e.g., Cu-N-rGO in [ 44 ]), highlighting the reproducibility and versatility of the synthesis strategy. High-resolution C1s spectra (Fig. 3 b) exhibit five distinct components corresponding to sp² carbon (284.5 eV), C–O/H/N (285.8 eV), C = O (288.2 eV), COOH (290.2 eV), and the π–π* shake-up satellite (291.8 eV), mirroring previously reported deconvolutions for related systems [ 24 , 44 ] . The N1s region (Fig. 3 c) of Co-N-rGO overlaps almost perfectly with that of the cobalt-free reference, suggesting no significant modification in nitrogen speciation or bonding environments. This contrasts with previous findings on Mn-doped samples, where metal coordination with nitrogen atoms was evident from spectral shifts ([ 24 ]). Therefore, cobalt interaction with N-rGO does not directly involve nitrogen. To probe the chemical state of cobalt, high-resolution Co2p spectra were acquired (Fig. 3 d). The Co2p ₃/₂ peak, accompanied by characteristic satellite features between 782 and 795 eV, is consistent with Co(II) oxidation state [ 58 ]. However, fitting using standard models for cobalt oxides failed to reproduce the experimental line shape accurately, likely because these models assume nanoparticulate or crystalline Co oxide phases—absent in our material as verified by TEM. Instead, we adopted a multiplet-splitting-based model from Briggs and Gibson [ 59 ], better suited to describe Co in molecular or coordination environments. The measured 2p ₃/₂ –2p ₁/₂ separation of 16 eV further confirms Co(II) assignment and suggests a paramagnetic coordination state. To refine our interpretation, we conducted a targeted literature review on cobalt-containing carbon complexes. The study by Ivanova [ 57 ]] emerged as particularly relevant, offering a comparative analysis of five structurally distinct Co(II) species. The best agreement with our experimental spectrum was obtained using parameters corresponding to their compound 4, Co₉II(µ₃-OH)₆(OOCCMe₃)₁₂(OCMe₂)₂, where Co²⁺ ions are coordinated by a mixture of carboxylate, carbonyl, and hydroxyl ligands. Further support for this assignment was provided by the satellite-to-main peak intensity ratio, which was 1.85—closely matching the value reported for high-spin Co(II) species in the same study. Complete fitting parameters are listed in Table S4 (Supporting Information). While these XPS results confirm the oxidation and coordination state of Co, they do not reveal whether cobalt is substitutionally doped into the graphene lattice or merely coordinated at the surface. To gain further insight into the binding nature of cobalt in the Co–N-rGO system, we subjected the Co-N-rGO sample to chemical treatment in H 2 SO 4 at room temperature. This procedure, previously employed by H. Wu et al. [ 60 ] for a nitrogen–cobalt co-doped graphene catalyst, serves to selectively remove Co metallic particles or Co ions bound to the oxygen containing functional groups present at the rGO basal plane without affecting the N-doped framework. Post-treatment XPS analysis ( Figure S3a in the Supporting Information) revealed the complete disappearance of the Co2p signal, indicating that all cobalt was removed from the surface. In contrast, the N1s signal ( Figure S3b ) remained detectable, confirming the stability of nitrogen within the graphene structure, consistent with previous findings [ 24 ]. A slight shift in the N1s peak toward higher binding energy was observed, likely due to protonation of nitrogen atoms by the acidic environment. This behaviour confirms that nitrogen is chemically embedded in the structure, while cobalt is only superficially coordinated. Further confirmation comes from valence band (VB) XPS spectra ( Figure S3c ), which compare the electronic structure of the as-prepared Co-N-rGO with that of the acid-treated sample and a reference N-rGO. The VB spectrum of the post-treatment sample closely matches that of the reference N-rGO material, while the as-prepared Co-N-rGO shows a distinct increase in spectral intensity near the Fermi level. This enhancement is thus attributed to Co 3d-derived states, which disappear after acid treatment. Taken together, these observations indicate that Co is not embedded within the graphene framework as a substitutional dopant but rather coordinated to functional groups naturally present at sample surface. 2.2. DFT-Model of Co-rGO Building on this experimental evidence, we performed DFT calculations to propose structural models for Co²⁺ coordination with the rGO surface. The loss of cobalt after acid treatment ( Figure S3c , Supporting Information) and the absence of direct Co-C or Co-N bonding signatures in XPS ( Figure S3b , Supporting Information) rule out substitutional doping in the rGO honeycomb structure. Instead, Co²⁺ is coordinated to the rGO basal plane through residual oxygen functionalities, such as epoxide and hydroxide groups, present on the rGO surface, as confirmed by XPS. Given this evidence, we considered Co-N-rGO structural models in which Co²⁺ forms coordination complexes with ligands consisting of oxygen groups from the rGO basal plane or water molecules and hydroxide ions from the surrounding solution to completing its coordination shell. We modeled several alternative octahedral complexes where the Co 2+ ion is bound to two vicinal oxygen containing groups of the rGO (Co-OrGO) besides four from OH − ions (Co-OH) or water molecules (Co-OH 2 ) in the electrolyte solution. The geometries of these configurations are shown in Figure S5 of Supporting Information. Our computed Co-O bonding configuration was consistent with earlier findings, which demonstrated that interactions between transition metals and surface oxygen groups lead to stable structures[ 44 ]. In the most stable configuration, reported in Fig. 4 a and Fig. 4 b, the coordination shell of the Co 2+ anchored to N-rGO is completed by four water molecules, as illustrated in Figure S5a and Figure S5b . Upon structural relaxation, the Co-OrGO bond lengths were found to be 1.91 Å, whereas the Co–OH₂ bond distances ranged from 2.19 Å to 2.25 Å. The resulting complex maintains a + 2-oxidation state for cobalt and closely reproduces the local coordination environment deducted from XPS analysis[ 57 ], reinforcing the consistency between experimental observations and computational modeling. To explore the impact of Co²⁺ coordination on the electronic properties of rGO, we examined the total and projected density of states (DOS and pDOS), shown in Fig. 4 c and Fig. 4 d, respectively. The DOS comparison between pristine rGO (blue curve) and Co²⁺-decorated rGO (black curve) reveals a significant increase in occupied energy states just below the Fermi level upon introduction of Co 2+ , consistent with the XPS results presented in Figure S3c of Supporting Information. The pDOS analysis in Fig. 4 d demonstrates that the increased energy states are predominantly localized on the Co 2+ ion and result from the hybridization of Co-3d and O-2p orbitals. The increased density of occupied states near the Fermi level implies greater electronic availability, which may facilitate charge transfer processes and thereby enhance the material's catalytic activity. 2.3. Electrochemical characterization Electrocatalytic activity was systematically investigated through a comprehensive set of electrochemical measurements, comparing the Co-functionalized N-rGO with its metal-free counterpart (N-rGO). Experimental details, including cell configuration and electrode preparation, are provided in the Supporting Information. All potentials are referenced to the reversible hydrogen electrode (RHE). ORR performance was first assessed by cyclic voltammetry (CV) at a scan rate of10 mV s⁻¹ in O₂-saturated 0.1 M KOH, as shown in Fig. 5 a. Co-N-rGO exhibits a distinct cathodic peak centered at ~ 0.8 V, which vanishes under N 2 -saturated conditions, unambiguously confirming its catalytic activity toward ORR. Notably, the onset of reduction for Co-N-rGO is positively shifted by ~ 200 mV compared to the N–rGO counterpart, highlighting the crucial role of Co in enhancing reaction kinetics and lowering the overpotential. These features are consistent with benchmark behavior for high-performance metal–N–C catalysts. [ 61 , 62 ] The selectivity and efficiency of the ORR were further elucidated via Rotating Ring Disk Electrode (RRDE) measurements, conducted in O₂-saturated 0.1 M KOH at a scan rate of 5 mV s⁻¹ and a constant rotation speed of 2500 rpm, following established protocols [ 63 ]. As shown in Figure S4a–b , the disk current reflects the total ORR activity, encompassing both the 2-electron and 4-electron pathways (R1 and R2)[ 64 ]: O 2 + H 2 O + 2e − → HO 2 − + OH − (R1) O 2 + 2 H 2 O + 4e − → 4 OH − (R2) Simultaneously, the ring current monitors the oxidation of peroxide intermediates, a parasitic product with well-known corrosive effects [ 65 ]. Both Co-N-rGO and the commercial Pt/C benchmark display ring currents that are two orders of magnitude lower than their corresponding disk currents, pointing to a dominant 4-electron pathway and minimal peroxide formation [ 66 ]. In contrast, the N-rGO sample exhibits a ring current only one order of magnitude lower than its disk current, indicating a substantial contribution from the 2-electron route. Quantitative analysis based on equations S1 and S2 (Supporting Information) enabled calculation of the average number of transferred electrons and the peroxide yield. As summarized in Fig. 5 b, Co-N-rGO achieves an electron transfer number close to 4 and a peroxide yield as low as 6%, closely matching the performance of Pt/C. This result underscores the critical role of cobalt in steering the reaction toward the desirable 4-electron reduction, confirming the synergistic interplay between Co and N dopants in modulating the electronic structure and catalytic behavior of the rGO framework. To gain insight into the catalytic behavior of the Co-N-rGO system, DFT calculations were performed to evaluate the Gibbs free energy ORR profile. Both the 2-electron (partial reduction to HO₂ − ) and 4-electron (complete reduction to OH − ) pathways in alkaline solution were considered and calculated according to the computational hydrogen electrode (CHE) model[ 3 , 67 , 68 ], as depicted in Fig. 6 . The reaction steps modeled along the 4e⁻ pathway were as follows: *Co(H 2 O) 4 + O 2 (g) → *Co(H 2 O) 3 O 2 + H 2 O (l) *Co(H 2 O) 3 O 2 + H 2 O (l) + e − → *Co(H 2 O) 3 OOH + OH − (l) *Co(H 2 O) 3 OOH + + e − → *Co(H 2 O) 3 O + + OH − (l) *Co(H 2 O) 3 O + H 2 O (l) + e − → *Co(H 2 O) 3 OH + OH − (l) *Co(H 2 O) 3 OH + H 2 O (l) + e − → *Co(H 2 O) 4 + OH − (l) For the 2e⁻ pathway, the modeled steps were: *Co(H 2 O) 4 + O 2 (g) → *Co(H 2 O) 3 O 2 + H 2 O (l) *Co(H 2 O) 3 O 2 + H 2 O (l) + e − → *Co(H 2 O) 3 OOH + OH − (l) *Co(H 2 O) 3 OOH + + e − + H 2 O (l) → *Co(H 2 O) 4 + HO 2 − (l) Where * indicated the rGO substrate anchoring the Co 2+ ion through two residual surface oxygen atoms (O rGO ). The computed free energy diagrams revealed that both ORR pathways are thermodynamically downhill, confirming their spontaneity. However, a marked difference emerged in the limiting potentials: the 4e⁻ pathway exhibited a significantly higher limiting potential (0.44 V) compared to just 0.05 V for the 2e⁻ route. This energetic preference clearly favors the complete 4-electron reduction of oxygen to water on the Co-N-rGO catalyst. These theoretical insights are fully consistent with experimental RRDE data, both converging toward the conclusion that Co functionalization enables highly selective and efficient 4e⁻ ORR catalysis. To further investigate the origin of the enhanced ORR performance, electrochemical impedance spectroscopy (EIS) measurements were carried out in O₂-saturated 0.1 M KOH at 0.38 V and 2500 rpm (Fig. 5 c)[ 69 ]. The Nyquist plots exhibit two distinct regions: a high-frequency (HF) semicircle, associated with charge transport within the electrode material, and a low-frequency (LF) arc, reflecting charge transfer processes at the electrode–electrolyte interface and mass transport limitations. The intercept at high frequency provides information on electrolyte and contact resistances[ 70 ]. An equivalent circuit model (inset of Fig. 5 c) was used to fit the impedance spectra and deconvolute the contributions to the overall resistance. The model includes: (i) a series resistance (Rₛ) accounting for electrolyte and wiring contributions, (ii) a parallel Rₜ//Cₜ element describing bulk transport in the material, and (iii) a parallel combination of a constant phase element with a series of charge transfer resistance (R ct ) and diffusion impedance (Z d ), representing interfacial and diffusive processes [ 71 ]. To capture the effects of porosity and surface heterogeneity, constant phase elements with exponents > 0.85 were used instead of ideal capacitors [ 72 ]. The fitted curves (Fig. 5 c) closely match the experimental data, and the extracted parameters are summarized in Table 1 . A marked reduction in both Rₜ and R ct is observed upon Co incorporation, indicating improved electronic conductivity and enhanced charge transfer kinetics. This is consistent with previous findings on Mn- and Cu-functionalized rGO systems[ 24 , 44 ] and highlights the synergistic role of Co and N-doping in optimizing the electrochemical response of graphene-based electrodes. The long-term durability of the Co-N-rGO catalyst was assessed via chronoamperometry (CA) at 0.68 V under O₂ saturation and continuous rotation (2500 rpm). As shown in Fig. 5 d, the Co-N-rGO sample retained 94% of its initial current after prolonged operation, outperforming the commercial Pt/C benchmark, which exhibited a substantial activity loss. These results confirm the excellent stability of the Co-N-rGO catalyst and underscore its suitability for practical ORR applications, with performance comparable to—or even superior to—that of other state-of-the-art Co-based carbon materials [ 37 , 73 – 75 ]. Table 1 Resistances obtained from the EIS analysis. Sample R s [Ω] R t [Ω] R ct [Ω] R d [Ω] N-rGO 47.7 78.3 35420.0 100.5 Co-N-rGO 46.8 54.2 12732.6 97.1 3. Conclusions In this study, we have provided a comprehensive characterization of the morphology, structure, and chemical environment of a Co-N-rGO electrocatalyst synthesized via microwave-assisted methods. High-resolution FESEM and TEM analyses confirmed the preservation of a multilayered, graphene-like morphology and the absence of cobalt nanoparticles or crystalline cobalt oxide phases, demonstrating the mild and effective nature of the synthesis protocol. Raman spectroscopy revealed slight yet significant modifications to the graphitic framework upon cobalt incorporation, including G-band blueshifts and narrowing, indicative of lattice strain and/or electronic perturbations likely arising from Co coordination at the surface. XPS analysis provided a detailed picture of the elemental speciation, clearly identifying cobalt in the Co(II) oxidation state. The Co2p spectral lineshape, satellite structure, and valence band features collectively suggest the presence of a high-spin, paramagnetic Co(II) species, distinct from conventional cobalt oxide environments. Importantly, chemical leaching experiments in acidic media eliminated the cobalt signal while leaving the nitrogen signature intact, confirming that cobalt is not incorporated substitutionally into the carbon lattice, but is instead coordinated to surface functional groups. DFT modeling supported these findings, proposing a stable coordination geometry where Co²⁺ is anchored by oxygen-containing functionalities on the rGO surface and solvated by water molecules. This coordination environment is consistent with the spectroscopic data and the known chemistry of cobalt in aqueous systems. The obtained experimental and computational insight challenge conventional assumptions about substitutional metal doping in carbon-based electrocatalysts, suggesting that surface coordination—rather than bulk incorporation—dominates the local environment of catalytically active metal centers. The unique coordination environment of Co in the Co-N-rGO catalyst directly translates into an enhanced ORR performance. The catalyst demonstrates a high limiting potential in alkaline media, comparable to those of its Co-free counterpart, alongside excellent current densities and durability over extended operation, comparable to those of commercial Pt/C. Crucially, both experimental data and DFT calculations coherently support a dominant four-electron reduction pathway, confirming the efficiency of the atomically dispersed Co²⁺ active sites. The absence of metallic cobalt nanoparticles and the prevalence of atomically dispersed Co²⁺ species are critical to the observed electrocatalytic activity, underscoring the importance of surface coordination in determining the electrocatalytic behavior. Our findings not only refine the understanding of Co speciation in N-doped graphene-based systems, but also provide key design principles for the development of future catalysts based on atomically dispersed transition metals. By tuning surface coordination, the catalytic properties can be modulated without compromising the integrity of the carbon-based matrix. These results establish a clear correlation between the local geometry of active sites and their electrocatalytic performance, offering valuable insight for the rational design of efficient and sustainable metal-based single atom electrocatalysts for energy conversion applications. 4. Methods Materials, methods and any associated references are available as Supporting Information. Declarations Author Contribution Author contributionsConceptualization and study design: F.R., N.G., M.C.Project administration and supervision: F.R., M.C., G.C.Methodology development: F.R., N.G., S.M., G.C., M.C.Formal analysis: F.R., A.S., S.M., C.D., M.F., M.C.Validation: A.S., S.M., C.D., M.F., M.C.Investigation and data acquisition: N.G., J.Z., A.S., S.M., C.D., M.F., A.C., M.C.Visualization and figure preparation: F.R., M.C.Funding acquisition: L.F., G.C.Writing – original draft: F.R., N.G., J.Z., A.S., S.M., C.D., M.C.Writing – review and editing: C.D., M.F., G.C., M.C.Resources: All authors.All authors reviewed and approved the final manuscript. Acknowledgements We acknowledge funding provided under Italy’s National Recovery and Resilience Plan (NRRP), Mission 4 - Component 2, “From Research to Business,” through Investment 3.1, “Fund for the Creation of an Integrated System of Research and Innovation Infrastructures” (Call No. 3264, 28/12/2021, from the Italian Ministry of Research), funded by the European Union –NextGenerationEU (Project Code: IR0000027, Concession DecreeNo. 128, 21/06/2022, CUP: B33C22000710006, Project Title: iENTRANCE). We acknowledge the “Italian Research Center on High Performance Computing, Big Data and Quantum Computing” (ICSC) funded by the European Union – NextGenerationEU and established under the NRRP, as well as high-performance computing resources and support provided by CINECA through the ISCRA initiative and HPC@POLITO. We also acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 2.2.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union – NextGenerationEU– Project Title: A Combined Experimental and Theoretical Approach for Single-Atom Catalyst Engineering Towards Tuneable Activity and Selectivity in CO2 electroreduction (RECYCLE-CO2) - Grant Assignment Decree No. 2022FM3LXT. Moreover, this study was also developed in the framework of the research activities carried out within the Project “Network 4 Energy Sustainable Transition—NEST”, Spoke 4, Project code PE0000021, funded under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.3— Call for tender No. 1561 of 11.10.2022 of Ministero dell’Universita` e della Ricerca (MUR); funded by the European Union—NextGenerationEU. 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(c) Bright Field TEM image of a rGO flake, (d) corresponding electron diffraction pattern typical of multilayered graphene.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/3c98176064a5fc2e10711d57.png"},{"id":85042582,"identity":"0b2132f9-14c6-428b-a229-cd04d059b352","added_by":"auto","created_at":"2025-06-20 09:39:05","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":133734,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Average Raman spectra of the N-rGO reference (top, red spectrum, N = 33) and of the cobalt-functionalized N-rGO nanomaterial (bottom, blue spectrum, N = 33). Shaded profiles represent the standard deviation of the mean normalized Raman intensity. The spectra are vertically offset for clarity. A low intensity band at around 640 cm\u003csup\u003e-1\u003c/sup\u003e (marked with an asterisk) appears in both the cobalt-functionalized N-rGO and the reference N-rGO samples, indicating no univocal relationship with the cobalt-functionalized nanomaterial. This band has been assigned to the density of phonon states in reduced graphene oxide and does not match any characteristic cobalt oxide band.\u0026nbsp;\u0026nbsp;\u0026nbsp;\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/ffb5b2a9e3ea147e56c45340.png"},{"id":85042583,"identity":"346ddc3b-30a5-4896-9e97-de341a6496af","added_by":"auto","created_at":"2025-06-20 09:39:05","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":227057,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XPS survey spectrum of Co-N-rGO sample with relative atomic concentration values; (b) XPS HR spectrum for C1s region for Co-N-rGO sample: a fitting procedure has been applied to raw data, which gives as a result five components due to C-C (sp\u003csup\u003e2\u003c/sup\u003e), C-O/H/N, C=O, COOH and shake-up satellite due to p-p* transition bonds; (c) XPS HR N1s region for N-rGO and Co-N-rGO samples, showing the almost perfect overlap of the two spectra;\u0026nbsp;\u0026nbsp; (d) XPS HR Co2p region with the Co2p\u003csub\u003e3/2\u003c/sub\u003e component fitted with four curves due to Co(II) oxidation state and three related satellites as reported in [57]. The distance ∆=16 eV is typical for Co(II) paramagnetic arrangement.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"image3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/8c3700ac497db71d0c82f3b6.jpeg"},{"id":85042575,"identity":"7e215228-3262-476c-a315-2d49855c629a","added_by":"auto","created_at":"2025-06-20 09:39:04","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":199348,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Top and (b) side views of the modeled structure for Co-functionalized reduced graphene oxide. \u0026nbsp;Gray, red, white, and blue spheres represent carbon, oxygen, hydrogen, and cobalt atoms, respectively. (c) Comparison of the total density of states (DOS) for rGO and Co-rGO. in the case of Co-rGO, the DOS is averaged over the spin-up and spin-down components. (d) DOS and projected DOS (pDOS) of Co-rGO, with spin-up and spin-down contributions plotted separately.\u003c/p\u003e","description":"","filename":"image4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/eb998752637c52bdc04ba5c3.jpeg"},{"id":85042586,"identity":"5dce063b-e1ff-421c-afa3-29645f724278","added_by":"auto","created_at":"2025-06-20 09:39:06","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":185053,"visible":true,"origin":"","legend":"\u003cp\u003eResults of electrochemical measurements for ORR: (a) CV in O\u003csub\u003e2\u003c/sub\u003e-saturated and N\u003csub\u003e2\u003c/sub\u003e-saturated solutions; (b) comparison of electron transfer number (left axis) and peroxide percentage (right axis) evaluated from RRDE measurements; (c) impedance spectra (the points are experimental data while the continuous lines are the curves obtained from a fitting procedure using the equivalent circuit shown as inset); (d) CA curves normalized with respect to the initial current density value.\u0026nbsp;\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/da314eb6aab64c9866ab923d.png"},{"id":85042579,"identity":"5044f302-708a-451a-af52-fef15b31c1b0","added_by":"auto","created_at":"2025-06-20 09:39:05","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":198269,"visible":true,"origin":"","legend":"\u003cp\u003eGibbs free energy profile for ORR on the Co-rGO catalyst. The top panel shows the Gibbs free energy diagram for the 2e⁻ (blue curve) and 4e⁻ (red curve) pathways of O₂ reduction, evaluated at 0 V. The bottom panels display the structural configurations of the reaction intermediates along the 4e⁻ ORR pathway. Gray, red, white, and blue spheres represent carbon, oxygen, hydrogen, and cobalt atoms, respectively.\u003c/p\u003e","description":"","filename":"image6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/9367601400dea12fd7d457bf.jpeg"},{"id":92884622,"identity":"6d102b00-92a0-4d85-9efa-8b1f74979713","added_by":"auto","created_at":"2025-10-06 16:13:39","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2648709,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/24ab1c60-09c7-4d02-870f-968b6ffa8c44.pdf"},{"id":85042537,"identity":"71cd15cb-d38d-4d15-ad83-97896f985c5e","added_by":"auto","created_at":"2025-06-20 09:39:03","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":651496,"visible":true,"origin":"","legend":"","description":"","filename":"supportinginfo16062025.docx","url":"https://assets-eu.researchsquare.com/files/rs-6904882/v1/1b32bdff880583bd0544259d.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Single-Atom Cobalt on N-Doped Reduced Graphene Oxide Pushes the Oxygen Reduction Reaction toward 4-Electron Pathway","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn recent years, extensive research has focused on the development of sustainable and efficient energy technologies to address the global demand for cleaner energy sources. Among these, fuel cells have attracted significant attention due to their high energy density and potential to serve as practical alternatives to conventional energy ​[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]​. However, one of the major challenges in fuel cell technology is the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode, which significantly affects the overall efficiency of the ​[\u003cspan additionalcitationids=\"CR4 CR5\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]​. To enhance ORR efficiency, electrocatalysts are required to reduce overpotentials and facilitate the four-electron pathway over the two-electron one, leading to the direct formation of water ​[\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]​. Traditionally, noble metals such as platinum (Pt) and palladium (Pd) have been employed as ORR catalysts due to their known high catalytic activity ​[\u003cspan additionalcitationids=\"CR12 CR13 CR14\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]​. However, their high cost and limited availability have severely restricted their widespread applications. Consequently, alternative materials, particularly carbon-based supports decorated with metal species, have emerged as promising ORR electrocatalyst candidates ​[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan additionalcitationids=\"CR17 CR18 CR19\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]​. Carbon-based materials exhibit excellent stability, tunable electronic properties, and significantly lower costs compared to noble metals ​[\u003cspan additionalcitationids=\"CR22 CR23\" citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]​. These properties can be further enhanced by doping with heteroatoms or transition metals. A particularly interesting platform of such a kind is nitrogen-doped reduced graphene oxide (N-rGO), which exhibits ease of further functionalization with transition metals​[\u003cspan additionalcitationids=\"CR26 CR27\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]​. Moreover, the presence of nitrogen introduces additional active sites and strengthens electronic interactions between the carbon support and metal species, thereby enhancing both catalytic activity and durability ​[\u003cspan additionalcitationids=\"CR30 CR31\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]​.\u003c/p\u003e \u003cp\u003eSimilarly, cobalt (Co)-based materials have attracted considerable interest for ORR applications due to their ability to facilitate efficient electron transfer and thus, enhance catalytic performance ​[\u003cspan additionalcitationids=\"CR34 CR35\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]​. In particular, various studies have demonstrated that Co, in the form of metal, oxides, or carbides, can significantly boost ORR activity when incorporated into carbonaceous frameworks such as porous carbons, carbon nanotubes, or graphene-based materials ​[\u003cspan additionalcitationids=\"CR38 CR39\" citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]​. As a result, several studies have reported on the functionalization of N-rGO with Co in different oxidation states for electrocatalytic applications. A. Ejaz ​[\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]​ investigated a cobalt hydroxide [Co(OH)₂] nanoflower-decorated N-rGO system for electrocatalytic sensing, demonstrating the synergistic effects between N doping and Co(OH)₂ in enhancing surface catalytic activity. Similarly, P. Yaengthip et al. ​[\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]​ reported ORR activity in acidic media for a Co-decorated N-rGO material, showing that thermal treatment at 300\u0026deg;C optimized the catalytic performance by preserving N functionalities. In an alkaline medium, Zhang et al. ​[\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]​ observed an ORR peak potential of -0.26 V vs. Ag/AgCl for a cobalt oxide (Co₃O₄)-decorated N-rGO catalyst, demonstrating an electron transfer number close to 4, which is ideal for ORR in the fuel cell applications.\u003c/p\u003e \u003cp\u003eIn this work, we present a combined experimental and theoretical investigation aimed at elucidating the structure\u0026ndash;activity relationship responsible for the outstanding catalytic performance of Co-functionalized N-rGO (Co-N-rGO). The material is synthesized via a rapid and environmentally sustainable microwave (MW)-assisted hydrothermal process, during which GO undergoes simultaneous reduction, N incorporation, and Co coordination. This one-step synthesis results in the formation of a well-defined tetraphenyl porphyrin (TPP)-like coordination environment, akin to that observed in other metal-containing N-doped systems (\u003cem\u003ee.g.\u003c/em\u003e, Cu-functionalized rGO)​[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]​. Our findings highlight the distinct role of Co incorporation within this coordination framework in driving the superior catalytic activity observed. This insight offers a new design principle for the development of efficient single-atom catalysts based on non-noble metals.\u003c/p\u003e \u003cp\u003eComprehensive characterization of the synthesized Co-N-rGO \u0026mdash;performed via field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy\u0026mdash; confirms the successful incorporation of Co ions into the graphene lattice, with no evidence of crystalline structures such as metallic nanoclusters or oxide nanoparticles. The Co is stabilized in a\u0026thinsp;+\u0026thinsp;2-oxidation state and coordinated to oxygen (O)-containing functional groups anchored to the rGO matrix. To identify the atomistic structure of the active site and gain deeper insight into its electronic properties and exceptional electrocatalytic performance, density functional theory (DFT) calculations were performed. These simulations not only support the experimental XPS findings but also elucidate the fundamental role of cobalt in the catalysis of the oxygen reduction reaction. Electrochemical and analytical measurements further confirm the high ORR activity and long-term stability of Co-N-rGO in alkaline media. The integration of advanced characterization techniques with theoretical modelling provides a thorough understanding of the underlying structure\u0026ndash;activity relationship, offering valuable guidance for the rational design of next-generation electrocatalysts.\u003c/p\u003e"},{"header":"2. Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1. Physical and chemical characterization\u003c/h2\u003e\n\u003cp\u003eThe Co-N-rGO was synthesized using a previously established and reproducible microwave assisted method, as reported in our earlier work (refs. [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e45\u003c/span\u003e]) and described in detail in the Supporting Information. To investigate the morphological and structural properties of the synthesized material, FESEM and TEM were first employed. As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea, FESEM imaging reveals multilayered graphene-like flakes with extended lateral size at the micrometer scale, consistent with high-quality 2D graphene derivatives. Notably, no signs of morphological degradation or fragmentation were observed following the microwave-assisted synthesis, confirming the mildness of the process. The presence of cobalt was clearly detected by EDX spectroscopy (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003eb), which reveals distinct peaks for Co, C, and N, along with minor signals from the copper grid (Cu) and the holder (Al).\u003c/p\u003e\n\u003cp\u003eTEM micrographs (Figs.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ec) further confirm the integrity of the carbonaceous framework at the nanometer scale, showing the absence of crystalline domains that could be attributed to cobalt-based or cobalt oxide phases. Electron diffraction patterns (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed) exhibit the typical ring-like features of multilayered graphene with no additional diffraction spots, reinforcing the conclusion that no crystalline phases are present and confirming successful Co inclusion without nanoparticle formation. Nevertheless, Raman spectroscopy provides complementary insights into the structural properties of the catalyst. Spectra of the Co-N-rGO material and a Co-free N-rGO reference (synthesized as in [\u003cspan class=\"CitationRef\"\u003e46\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e47\u003c/span\u003e]) are compared in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e. In the low-frequency region (\u0026lt;\u0026thinsp;1000 cm⁻\u0026sup1;)[\u003cspan class=\"CitationRef\"\u003e48\u003c/span\u003e], the absence of cobalt oxide fingerprint bands\u0026mdash;typically found between 190 and 700 cm⁻\u0026sup1; (\u003cstrong\u003eFigure \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/strong\u003e) \u0026mdash;supports the conclusion drawn from TEM imaging. The only detectable signal in this region is a weak, broad band at ~\u0026thinsp;640 cm⁻\u0026sup1;, also present in the reference N-rGO, possibly originating from a high density of vibrational states in defective graphitic domains [\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e50\u003c/span\u003e] rather than Co-based species. Confocal Raman setup excludes any contribution from the glass substrate (\u003cstrong\u003eFigure S2\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eThe main spectral features of Co-N-rGO are typical of rGO, displaying a D band at 1349 cm⁻\u0026sup1; and a G band at 1605 cm⁻\u0026sup1;. The D band arises from the defect-activated A\u003csub\u003e1g\u003c/sub\u003e mode (ring breathing), indicative of disordered sp\u0026sup2; carbon, while the G band corresponds to the doubly-degenerate E\u003csub\u003e2g\u003c/sub\u003e mode (sp\u0026sup2; C\u0026ndash;C stretching), which is highly sensitive to disruptions in the planar sp\u0026sup2; graphitic network[\u003cspan class=\"CitationRef\"\u003e51\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e] and to variations in charge carrier concentration [\u003cspan class=\"CitationRef\"\u003e53\u003c/span\u003e]. The second-order Raman features, including the 2D (~\u0026thinsp;2688 cm⁻\u0026sup1;), D\u0026thinsp;+\u0026thinsp;D\u0026prime; (~\u0026thinsp;2935 cm⁻\u0026sup1;), and 2D\u0026prime; (~\u0026thinsp;3197 cm⁻\u0026sup1;) bands, match those of the Co-free N-rGO reference material, suggesting that the fundamental graphitic structure is preserved.\u003c/p\u003e\n\u003cp\u003eThe intensity of the D and G bands (I\u003csub\u003eD\u003c/sub\u003e/I\u003csub\u003eG\u003c/sub\u003e) ratio (0.95) of Co-N-rGO is nearly identical to that of N-rGO (0.94), indicating a comparable level of reduction and structural disorder in both [\u003cspan class=\"CitationRef\"\u003e54\u003c/span\u003e]. However, both D and G bands exhibit significant blueshifts in the Co-containing sample (two-tailed t-test, p-value \u0026lt;\u0026lt; \u0026alpha;, with \u0026alpha;\u0026thinsp;=\u0026thinsp;0.05, and a statistical power of 0.9999 for the D band and 1.0000 for the G band, see \u003cstrong\u003eTable \u003cspan class=\"InternalRef\"\u003eS1\u003c/span\u003e\u003c/strong\u003e and \u003cstrong\u003eTable S2\u003c/strong\u003e), implying lattice alterations induced by cobalt incorporation. G-band blueshifts are generally attributed to phonon stiffening due to compressive strain or increased doping levels, suggesting that Co atoms or ions may either intercalate or chemically interact with the rGO [\u003cspan class=\"CitationRef\"\u003e49\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e55\u003c/span\u003e]. While an increase in doping could be directly attributed to cobalt incorporation into the carbon lattice\u0026mdash;alongside nitrogen, i.e., co-doping\u0026mdash;the presence of compressive strain may stem from a broader and more complex set of factors, such as the intercalation of coordinating ions or surface functionalization by coordinating or electrostatically interacting species[\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e56\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eMoreover, as reported in \u003cstrong\u003eTable S3\u003c/strong\u003e, a noticeable narrowing of the G band\u0026rsquo;s full width at half maximum (FWHM) is observed in Co-N-rGO (90 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) compared to the reference (114 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), potentially arising from an increased fraction of ordered sp\u0026sup2; domains [\u003cspan class=\"CitationRef\"\u003e52\u003c/span\u003e] or from high dopant concentrations suppressing phonon\u0026ndash;electron decay via Pauli blocking [\u003cspan class=\"CitationRef\"\u003e25\u003c/span\u003e]. Although the specific origin remains ambiguous, these spectral changes reflect significant structural and electronic modifications, highlighting the need for further investigation via XPS.\u003c/p\u003e\n\u003cp\u003eXPS was employed to probe the chemical states of the elements and get an insight on Co binding configuration. XPS survey spectra (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea) confirm the presence of C, N, O, and Co elements. The relative atomic concentrations are consistent with those reported for other metal ion-functionalized N-rGO materials (e.g., Cu-N-rGO in [\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e]), highlighting the reproducibility and versatility of the synthesis strategy. High-resolution C1s spectra (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003eb) exhibit five distinct components corresponding to sp\u0026sup2; carbon (284.5 eV), C\u0026ndash;O/H/N (285.8 eV), C\u0026thinsp;=\u0026thinsp;O (288.2 eV), COOH (290.2 eV), and the \u0026pi;\u0026ndash;\u0026pi;* shake-up satellite (291.8 eV), mirroring previously reported deconvolutions for related systems [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e] .\u003c/p\u003e\n\u003cp\u003eThe N1s region (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ec) of Co-N-rGO overlaps almost perfectly with that of the cobalt-free reference, suggesting no significant modification in nitrogen speciation or bonding environments. This contrasts with previous findings on Mn-doped samples, where metal coordination with nitrogen atoms was evident from spectral shifts ([\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]). Therefore, cobalt interaction with N-rGO does not directly involve nitrogen.\u003c/p\u003e\n\u003cp\u003eTo probe the chemical state of cobalt, high-resolution Co2p spectra were acquired (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ed). The Co2p\u003csub\u003e₃/₂\u003c/sub\u003e peak, accompanied by characteristic satellite features between 782 and 795 eV, is consistent with Co(II) oxidation state [\u003cspan class=\"CitationRef\"\u003e58\u003c/span\u003e]. However, fitting using standard models for cobalt oxides failed to reproduce the experimental line shape accurately, likely because these models assume nanoparticulate or crystalline Co oxide phases\u0026mdash;absent in our material as verified by TEM. Instead, we adopted a multiplet-splitting-based model from Briggs and Gibson [\u003cspan class=\"CitationRef\"\u003e59\u003c/span\u003e], better suited to describe Co in molecular or coordination environments. The measured 2p\u003csub\u003e₃/₂\u003c/sub\u003e\u0026ndash;2p\u003csub\u003e₁/₂\u003c/sub\u003e separation of 16 eV further confirms Co(II) assignment and suggests a paramagnetic coordination state.\u003c/p\u003e\n\u003cp\u003eTo refine our interpretation, we conducted a targeted literature review on cobalt-containing carbon complexes. The study by Ivanova [\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e]] emerged as particularly relevant, offering a comparative analysis of five structurally distinct Co(II) species. The best agreement with our experimental spectrum was obtained using parameters corresponding to their compound 4, Co₉II(\u0026micro;₃-OH)₆(OOCCMe₃)₁₂(OCMe₂)₂, where Co\u0026sup2;⁺ ions are coordinated by a mixture of carboxylate, carbonyl, and hydroxyl ligands. Further support for this assignment was provided by the satellite-to-main peak intensity ratio, which was 1.85\u0026mdash;closely matching the value reported for high-spin Co(II) species in the same study. Complete fitting parameters are listed in \u003cstrong\u003eTable S4\u003c/strong\u003e (Supporting Information).\u003c/p\u003e\n\u003cp\u003eWhile these XPS results confirm the oxidation and coordination state of Co, they do not reveal whether cobalt is substitutionally doped into the graphene lattice or merely coordinated at the surface. To gain further insight into the binding nature of cobalt in the Co\u0026ndash;N-rGO system, we subjected the Co-N-rGO sample to chemical treatment in H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e at room temperature. This procedure, previously employed by H. Wu et al. [\u003cspan class=\"CitationRef\"\u003e60\u003c/span\u003e] for a nitrogen\u0026ndash;cobalt co-doped graphene catalyst, serves to selectively remove Co metallic particles or Co ions bound to the oxygen containing functional groups present at the rGO basal plane without affecting the N-doped framework. Post-treatment XPS analysis (\u003cstrong\u003eFigure S3a\u003c/strong\u003e in the Supporting Information) revealed the complete disappearance of the Co2p signal, indicating that all cobalt was removed from the surface. In contrast, the N1s signal (\u003cstrong\u003eFigure S3b\u003c/strong\u003e) remained detectable, confirming the stability of nitrogen within the graphene structure, consistent with previous findings [\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e]. A slight shift in the N1s peak toward higher binding energy was observed, likely due to protonation of nitrogen atoms by the acidic environment. This behaviour confirms that nitrogen is chemically embedded in the structure, while cobalt is only superficially coordinated.\u003c/p\u003e\n\u003cp\u003eFurther confirmation comes from valence band (VB) XPS spectra (\u003cstrong\u003eFigure S3c\u003c/strong\u003e), which compare the electronic structure of the as-prepared Co-N-rGO with that of the acid-treated sample and a reference N-rGO. The VB spectrum of the post-treatment sample closely matches that of the reference N-rGO material, while the as-prepared Co-N-rGO shows a distinct increase in spectral intensity near the Fermi level. This enhancement is thus attributed to Co 3d-derived states, which disappear after acid treatment. Taken together, these observations indicate that Co is not embedded within the graphene framework as a substitutional dopant but rather coordinated to functional groups naturally present at sample surface.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2. DFT-Model of Co-rGO\u003c/h2\u003e\n\u003cp\u003eBuilding on this experimental evidence, we performed DFT calculations to propose structural models for Co\u0026sup2;⁺ coordination with the rGO surface. The loss of cobalt after acid treatment (\u003cstrong\u003eFigure S3c\u003c/strong\u003e, Supporting Information) and the absence of direct Co-C or Co-N bonding signatures in XPS (\u003cstrong\u003eFigure S3b\u003c/strong\u003e, Supporting Information) rule out substitutional doping in the rGO honeycomb structure. Instead, Co\u0026sup2;⁺ is coordinated to the rGO basal plane through residual oxygen functionalities, such as epoxide and hydroxide groups, present on the rGO surface, as confirmed by XPS.\u003c/p\u003e\n\u003cp\u003eGiven this evidence, we considered Co-N-rGO structural models in which Co\u0026sup2;⁺ forms coordination complexes with ligands consisting of oxygen groups from the rGO basal plane or water molecules and hydroxide ions from the surrounding solution to completing its coordination shell. We modeled several alternative octahedral complexes where the Co\u003csup\u003e2+\u003c/sup\u003e ion is bound to two vicinal oxygen containing groups of the rGO (Co-OrGO) besides four from OH\u003csup\u003e\u0026minus;\u003c/sup\u003e ions (Co-OH) or water molecules (Co-OH\u003csub\u003e2\u003c/sub\u003e) in the electrolyte solution. The geometries of these configurations are shown in \u003cstrong\u003eFigure S5\u003c/strong\u003e of Supporting Information. Our computed Co-O bonding configuration was consistent with earlier findings, which demonstrated that interactions between transition metals and surface oxygen groups lead to stable structures[\u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eIn the most stable configuration, reported in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003eb, the coordination shell of the Co\u003csup\u003e2+\u003c/sup\u003e anchored to N-rGO is completed by four water molecules, as illustrated in \u003cstrong\u003eFigure S5a\u003c/strong\u003e and \u003cstrong\u003eFigure S5b\u003c/strong\u003e. Upon structural relaxation, the Co-OrGO bond lengths were found to be 1.91 \u0026Aring;, whereas the Co\u0026ndash;OH₂ bond distances ranged from 2.19 \u0026Aring; to 2.25 \u0026Aring;. The resulting complex maintains a\u0026thinsp;+\u0026thinsp;2-oxidation state for cobalt and closely reproduces the local coordination environment deducted from XPS analysis[\u003cspan class=\"CitationRef\"\u003e57\u003c/span\u003e], reinforcing the consistency between experimental observations and computational modeling.\u003c/p\u003e\n\u003cp\u003eTo explore the impact of Co\u0026sup2;⁺ coordination on the electronic properties of rGO, we examined the total and projected density of states (DOS and pDOS), shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec and Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed, respectively. The DOS comparison between pristine rGO (blue curve) and Co\u0026sup2;⁺-decorated rGO (black curve) reveals a significant increase in occupied energy states just below the Fermi level upon introduction of Co\u003csup\u003e2+\u003c/sup\u003e, consistent with the XPS results presented in \u003cstrong\u003eFigure S3c\u003c/strong\u003e of Supporting Information. The pDOS analysis in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ed demonstrates that the increased energy states are predominantly localized on the Co\u003csup\u003e2+\u003c/sup\u003e ion and result from the hybridization of Co-3d and O-2p orbitals. The increased density of occupied states near the Fermi level implies greater electronic availability, which may facilitate charge transfer processes and thereby enhance the material's catalytic activity.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003e2.3. Electrochemical characterization\u003c/h2\u003e\n\u003cp\u003eElectrocatalytic activity was systematically investigated through a comprehensive set of electrochemical measurements, comparing the Co-functionalized N-rGO with its metal-free counterpart (N-rGO). Experimental details, including cell configuration and electrode preparation, are provided in the Supporting Information. All potentials are referenced to the reversible hydrogen electrode (RHE).\u003c/p\u003e\n\u003cp\u003eORR performance was first assessed by cyclic voltammetry (CV) at a scan rate of10 mV s⁻\u0026sup1; in O₂-saturated 0.1 M KOH, as shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea. Co-N-rGO exhibits a distinct cathodic peak centered at ~\u0026thinsp;0.8 V, which vanishes under N\u003csub\u003e2\u003c/sub\u003e-saturated conditions, unambiguously confirming its catalytic activity toward ORR. Notably, the onset of reduction for Co-N-rGO is positively shifted by ~\u0026thinsp;200 mV compared to the N\u0026ndash;rGO counterpart, highlighting the crucial role of Co in enhancing reaction kinetics and lowering the overpotential. These features are consistent with benchmark behavior for high-performance metal\u0026ndash;N\u0026ndash;C catalysts. [\u003cspan class=\"CitationRef\"\u003e61\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e62\u003c/span\u003e]\u003c/p\u003e\n\u003cp\u003eThe selectivity and efficiency of the ORR were further elucidated via Rotating Ring Disk Electrode (RRDE) measurements, conducted in O₂-saturated 0.1 M KOH at a scan rate of 5 mV s⁻\u0026sup1; and a constant rotation speed of 2500 rpm, following established protocols [\u003cspan class=\"CitationRef\"\u003e63\u003c/span\u003e]. As shown in \u003cstrong\u003eFigure S4a\u0026ndash;b\u003c/strong\u003e, the disk current reflects the total ORR activity, encompassing both the 2-electron and 4-electron pathways (R1 and R2)[\u003cspan class=\"CitationRef\"\u003e64\u003c/span\u003e]:\u003c/p\u003e\n\u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;2e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; HO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e + OH\u003csup\u003e\u0026minus;\u003c/sup\u003e (R1)\u003c/p\u003e\n\u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2 H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;4e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; 4 OH\u003csup\u003e\u0026minus;\u003c/sup\u003e (R2)\u003c/p\u003e\n\u003cp\u003eSimultaneously, the ring current monitors the oxidation of peroxide intermediates, a parasitic product with well-known corrosive effects [\u003cspan class=\"CitationRef\"\u003e65\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eBoth Co-N-rGO and the commercial Pt/C benchmark display ring currents that are two orders of magnitude lower than their corresponding disk currents, pointing to a dominant 4-electron pathway and minimal peroxide formation [\u003cspan class=\"CitationRef\"\u003e66\u003c/span\u003e]. In contrast, the N-rGO sample exhibits a ring current only one order of magnitude lower than its disk current, indicating a substantial contribution from the 2-electron route.\u003c/p\u003e\n\u003cp\u003eQuantitative analysis based on equations S1 and S2 (Supporting Information) enabled calculation of the average number of transferred electrons and the peroxide yield. As summarized in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb, Co-N-rGO achieves an electron transfer number close to 4 and a peroxide yield as low as 6%, closely matching the performance of Pt/C. This result underscores the critical role of cobalt in steering the reaction toward the desirable 4-electron reduction, confirming the synergistic interplay between Co and N dopants in modulating the electronic structure and catalytic behavior of the rGO framework.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo gain insight into the catalytic behavior of the Co-N-rGO system, DFT calculations were performed to evaluate the Gibbs free energy ORR profile. Both the 2-electron (partial reduction to HO₂\u003csup\u003e\u0026minus;\u003c/sup\u003e) and 4-electron (complete reduction to OH\u003csup\u003e\u0026minus;\u003c/sup\u003e) pathways in alkaline solution were considered and calculated according to the computational hydrogen electrode (CHE) model[\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e67\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e68\u003c/span\u003e], as depicted in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e. The reaction steps modeled along the 4e⁻ pathway were as follows:\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e4\u003c/sub\u003e + O\u003csub\u003e2 (g)\u003c/sub\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eOOH + OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eOOH\u0026thinsp;+\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eO\u0026thinsp;+\u0026thinsp;+\u0026thinsp;OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eO + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eOH + OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eOH + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e4\u003c/sub\u003e + OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003eFor the 2e⁻ pathway, the modeled steps were:\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e4\u003c/sub\u003e + O\u003csub\u003e2 (g)\u003c/sub\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eOOH + OH\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003e*Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e3\u003c/sub\u003eOOH\u0026thinsp;+\u0026thinsp;+\u0026thinsp;e\u003csup\u003e\u0026minus;\u003c/sup\u003e + H\u003csub\u003e2\u003c/sub\u003eO \u003csub\u003e(l)\u003c/sub\u003e \u0026rarr; *Co(H\u003csub\u003e2\u003c/sub\u003eO)\u003csub\u003e4\u003c/sub\u003e + HO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e \u003csub\u003e(l)\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003eWhere * indicated the rGO substrate anchoring the Co\u003csup\u003e2+\u003c/sup\u003e ion through two residual surface oxygen atoms (O\u003csub\u003erGO\u003c/sub\u003e). The computed free energy diagrams revealed that both ORR pathways are thermodynamically downhill, confirming their spontaneity. However, a marked difference emerged in the limiting potentials: the 4e⁻ pathway exhibited a significantly higher limiting potential (0.44 V) compared to just 0.05 V for the 2e⁻ route. This energetic preference clearly favors the complete 4-electron reduction of oxygen to water on the Co-N-rGO catalyst. These theoretical insights are fully consistent with experimental RRDE data, both converging toward the conclusion that Co functionalization enables highly selective and efficient 4e⁻ ORR catalysis.\u003c/p\u003e\n\u003cp\u003eTo further investigate the origin of the enhanced ORR performance, electrochemical impedance spectroscopy (EIS) measurements were carried out in O₂-saturated 0.1 M KOH at 0.38 V and 2500 rpm (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec)[\u003cspan class=\"CitationRef\"\u003e69\u003c/span\u003e]. The Nyquist plots exhibit two distinct regions: a high-frequency (HF) semicircle, associated with charge transport within the electrode material, and a low-frequency (LF) arc, reflecting charge transfer processes at the electrode\u0026ndash;electrolyte interface and mass transport limitations. The intercept at high frequency provides information on electrolyte and contact resistances[\u003cspan class=\"CitationRef\"\u003e70\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eAn equivalent circuit model (inset of Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec) was used to fit the impedance spectra and deconvolute the contributions to the overall resistance. The model includes: (i) a series resistance (Rₛ) accounting for electrolyte and wiring contributions, (ii) a parallel Rₜ//Cₜ element describing bulk transport in the material, and (iii) a parallel combination of a constant phase element with a series of charge transfer resistance (R\u003csub\u003ect\u003c/sub\u003e) and diffusion impedance (Z\u003csub\u003ed\u003c/sub\u003e), representing interfacial and diffusive processes [\u003cspan class=\"CitationRef\"\u003e71\u003c/span\u003e]. To capture the effects of porosity and surface heterogeneity, constant phase elements with exponents\u0026thinsp;\u0026gt;\u0026thinsp;0.85 were used instead of ideal capacitors [\u003cspan class=\"CitationRef\"\u003e72\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003eThe fitted curves (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec) closely match the experimental data, and the extracted parameters are summarized in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e. A marked reduction in both Rₜ and R\u003csub\u003ect\u003c/sub\u003e is observed upon Co incorporation, indicating improved electronic conductivity and enhanced charge transfer kinetics. This is consistent with previous findings on Mn- and Cu-functionalized rGO systems[\u003cspan class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e44\u003c/span\u003e] and highlights the synergistic role of Co and N-doping in optimizing the electrochemical response of graphene-based electrodes.\u003c/p\u003e\n\u003cp\u003eThe long-term durability of the Co-N-rGO catalyst was assessed via chronoamperometry (CA) at 0.68 V under O₂ saturation and continuous rotation (2500 rpm). As shown in Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ed, the Co-N-rGO sample retained 94% of its initial current after prolonged operation, outperforming the commercial Pt/C benchmark, which exhibited a substantial activity loss. These results confirm the excellent stability of the Co-N-rGO catalyst and underscore its suitability for practical ORR applications, with performance comparable to\u0026mdash;or even superior to\u0026mdash;that of other state-of-the-art Co-based carbon materials [\u003cspan class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e73\u003c/span\u003e\u0026ndash;\u003cspan class=\"CitationRef\"\u003e75\u003c/span\u003e].\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eResistances obtained from the EIS analysis.\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eSample\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e [Ω]\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003et\u003c/sub\u003e [Ω]\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e [Ω]\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e [Ω]\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN-rGO\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e47.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e78.3\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e35420.0\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e100.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCo-N-rGO\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e46.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e54.2\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e12732.6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"char\" char=\".\"\u003e\n\u003cp\u003e97.1\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e"},{"header":"3. Conclusions","content":"\u003cp\u003eIn this study, we have provided a comprehensive characterization of the morphology, structure, and chemical environment of a Co-N-rGO electrocatalyst synthesized via microwave-assisted methods. High-resolution FESEM and TEM analyses confirmed the preservation of a multilayered, graphene-like morphology and the absence of cobalt nanoparticles or crystalline cobalt oxide phases, demonstrating the mild and effective nature of the synthesis protocol. Raman spectroscopy revealed slight yet significant modifications to the graphitic framework upon cobalt incorporation, including G-band blueshifts and narrowing, indicative of lattice strain and/or electronic perturbations likely arising from Co coordination at the surface.\u003c/p\u003e \u003cp\u003eXPS analysis provided a detailed picture of the elemental speciation, clearly identifying cobalt in the Co(II) oxidation state. The Co2p spectral lineshape, satellite structure, and valence band features collectively suggest the presence of a high-spin, paramagnetic Co(II) species, distinct from conventional cobalt oxide environments. Importantly, chemical leaching experiments in acidic media eliminated the cobalt signal while leaving the nitrogen signature intact, confirming that cobalt is not incorporated substitutionally into the carbon lattice, but is instead coordinated to surface functional groups.\u003c/p\u003e \u003cp\u003eDFT modeling supported these findings, proposing a stable coordination geometry where Co\u0026sup2;⁺ is anchored by oxygen-containing functionalities on the rGO surface and solvated by water molecules. This coordination environment is consistent with the spectroscopic data and the known chemistry of cobalt in aqueous systems. The obtained experimental and computational insight challenge conventional assumptions about substitutional metal doping in carbon-based electrocatalysts, suggesting that surface coordination\u0026mdash;rather than bulk incorporation\u0026mdash;dominates the local environment of catalytically active metal centers.\u003c/p\u003e \u003cp\u003eThe unique coordination environment of Co in the Co-N-rGO catalyst directly translates into an enhanced ORR performance. The catalyst demonstrates a high limiting potential in alkaline media, comparable to those of its Co-free counterpart, alongside excellent current densities and durability over extended operation, comparable to those of commercial Pt/C. Crucially, both experimental data and DFT calculations coherently support a dominant four-electron reduction pathway, confirming the efficiency of the atomically dispersed Co\u0026sup2;⁺ active sites. The absence of metallic cobalt nanoparticles and the prevalence of atomically dispersed Co\u0026sup2;⁺ species are critical to the observed electrocatalytic activity, underscoring the importance of surface coordination in determining the electrocatalytic behavior.\u003c/p\u003e \u003cp\u003eOur findings not only refine the understanding of Co speciation in N-doped graphene-based systems, but also provide key design principles for the development of future catalysts based on atomically dispersed transition metals. By tuning surface coordination, the catalytic properties can be modulated without compromising the integrity of the carbon-based matrix. These results establish a clear correlation between the local geometry of active sites and their electrocatalytic performance, offering valuable insight for the rational design of efficient and sustainable metal-based single atom electrocatalysts for energy conversion applications.\u003c/p\u003e"},{"header":"4. Methods","content":"\u003cp\u003eMaterials, methods and any associated references are available as Supporting Information.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAuthor contributionsConceptualization and study design: F.R., N.G., M.C.Project administration and supervision: F.R., M.C., G.C.Methodology development: F.R., N.G., S.M., G.C., M.C.Formal analysis: F.R., A.S., S.M., C.D., M.F., M.C.Validation: A.S., S.M., C.D., M.F., M.C.Investigation and data acquisition: N.G., J.Z., A.S., S.M., C.D., M.F., A.C., M.C.Visualization and figure preparation: F.R., M.C.Funding acquisition: L.F., G.C.Writing \u0026ndash; original draft: F.R., N.G., J.Z., A.S., S.M., C.D., M.C.Writing \u0026ndash; review and editing: C.D., M.F., G.C., M.C.Resources: All authors.All authors reviewed and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eWe acknowledge funding provided under Italy\u0026rsquo;s National Recovery and Resilience Plan (NRRP), Mission 4 - Component 2, \u0026ldquo;From Research to Business,\u0026rdquo; through Investment 3.1, \u0026ldquo;Fund for the Creation of an Integrated System of Research and Innovation Infrastructures\u0026rdquo; (Call No. 3264, 28/12/2021, from the Italian Ministry of Research), funded by the European Union \u0026ndash;NextGenerationEU (Project Code: IR0000027, Concession DecreeNo. 128, 21/06/2022, CUP: B33C22000710006, Project Title: iENTRANCE). We acknowledge the \u0026ldquo;Italian Research Center on High Performance Computing, Big Data and Quantum Computing\u0026rdquo; (ICSC) funded by the European Union \u0026ndash; NextGenerationEU and established under the NRRP, as well as high-performance computing resources and support provided by CINECA through the ISCRA initiative and HPC@POLITO. We also acknowledge financial support under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.1, Call for tender No. 104 published on 2.2.2022 by the Italian Ministry of University and Research (MUR), funded by the European Union \u0026ndash; NextGenerationEU\u0026ndash; Project Title: A Combined Experimental and Theoretical Approach for Single-Atom Catalyst Engineering Towards Tuneable Activity and Selectivity in CO2 electroreduction (RECYCLE-CO2) - Grant Assignment Decree No. 2022FM3LXT. Moreover, this study was also developed in the framework of the research activities carried out within the Project \u0026ldquo;Network 4 Energy Sustainable Transition\u0026mdash;NEST\u0026rdquo;, Spoke 4, Project code PE0000021, funded under the National Recovery and Resilience Plan (NRRP), Mission 4, Component 2, Investment 1.3\u0026mdash; Call for tender No. 1561 of 11.10.2022 of Ministero dell\u0026rsquo;Universita` e della Ricerca (MUR); funded by the European Union\u0026mdash;NextGenerationEU. The Raman analysis portion of this project has received funding from the European Research Council (ERC) under the European Union\u0026rsquo;s Horizon 2020 research and innovation programme (grant agreement No 865819).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003e​​Liu, Y. \u003cem\u003eet al.\u003c/em\u003e (2020). A CO₂/H₂ fuel cell: reducing CO₂ while generating electricity. \u003cem\u003eJ. Mater. Chem. 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Synthesis of cobalt and nitrogen co\u0026ndash;doped carbon nanotubes and its ORR activity as the catalyst used in hydrogen fuel cells. \u003cem\u003eInt. J. Hydrogen Energy\u003c/em\u003e \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.ijhydene.2019.03.271\u003c/span\u003e\u003cspan address=\"10.1016/j.ijhydene.2019.03.271\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e.\u003c/span\u003e\u003c/li\u003e\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":"npj-2d-materials-and-applications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npj2dmaterials","sideBox":"Learn more about [npj 2D Materials and Applications](http://www.nature.com/npj2dmaterials/)","snPcode":"41699","submissionUrl":"https://submission.springernature.com/new-submission/41699/3","title":"npj 2D Materials and Applications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Oxygen reduction reaction (ORR), cobalt functionalization, N-doping, reduced graphene oxide, catalytic activity","lastPublishedDoi":"10.21203/rs.3.rs-6904882/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6904882/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eWe present a cobalt-based single-atom catalyst (SAC) anchored on nitrogen-doped reduced graphene oxide (Co-N-rGO), synthesized via a rapid, one-pot microwave-assisted method. Comprehensive characterization by FESEM, TEM, Raman spectroscopy, and X-ray photoelectron spectroscopy confirms the successful formation of atomically dispersed Co(II) centers, coordinated to oxygen-containing functional groups within the graphene matrix, with no evidence of metallic clusters or oxide nanoparticles. The atomically dispersed Co sites act as highly active centers for the oxygen reduction reaction (ORR) in alkaline media, delivering near four-electron transfer efficiency, a positive onset potential, and outstanding durability that surpasses commercial Pt/C catalysts. First-principles density functional theory (DFT) calculations corroborate the XPS findings and reveal the electronic structure of the Co\u0026ndash;O coordination environment, offering atomistic insight into the catalytic mechanism. The synergy between precise site isolation, optimized local coordination, and electronic modulation enables the superior electrocatalytic performance of this Co SAC. This study establishes a versatile and scalable framework for engineering high-performance ORR catalysts featuring non-noble metal single-atom active sites.\u003c/p\u003e","manuscriptTitle":"Single-Atom Cobalt on N-Doped Reduced Graphene Oxide Pushes the Oxygen Reduction Reaction toward 4-Electron Pathway","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-20 09:38:29","doi":"10.21203/rs.3.rs-6904882/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-02T01:37:46+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-29T21:42:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"118027277334485913651425171044086473828","date":"2025-06-24T14:54:49+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-23T14:47:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"269770702545126954666250372248571543604","date":"2025-06-23T07:11:23+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"221017446435816373884994312726341715858","date":"2025-06-23T01:26:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"315268388561042571279234196872333068709","date":"2025-06-20T12:22:29+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"124654175268954514810254194047097038279","date":"2025-06-20T11:36:28+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-18T09:54:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-18T07:01:54+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-17T16:56:39+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj 2D Materials and Applications","date":"2025-06-16T10:59:51+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-2d-materials-and-applications","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npj2dmaterials","sideBox":"Learn more about [npj 2D Materials and Applications](http://www.nature.com/npj2dmaterials/)","snPcode":"41699","submissionUrl":"https://submission.springernature.com/new-submission/41699/3","title":"npj 2D Materials and Applications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"5d3e9fb9-bbbd-4ea2-ad37-900bf8370405","owner":[],"postedDate":"June 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":50233579,"name":"Physical sciences/Materials science/Condensed matter physics/Surfaces interfaces and thin films"},{"id":50233580,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis"},{"id":50233581,"name":"Physical sciences/Materials science/Theory and computation/Atomistic models"},{"id":50233582,"name":"Physical sciences/Physics/Chemical physics"},{"id":50233583,"name":"Physical sciences/Physics/Techniques and instrumentation/Characterization and analytical techniques"},{"id":50233584,"name":"Physical sciences/Physics/Techniques and instrumentation/Design synthesis and processing"}],"tags":[],"updatedAt":"2025-10-06T16:11:22+00:00","versionOfRecord":{"articleIdentity":"rs-6904882","link":"https://doi.org/10.1038/s41699-025-00604-x","journal":{"identity":"npj-2d-materials-and-applications","isVorOnly":false,"title":"npj 2D Materials and Applications"},"publishedOn":"2025-09-29 15:57:30","publishedOnDateReadable":"September 29th, 2025"},"versionCreatedAt":"2025-06-20 09:38:29","video":"","vorDoi":"10.1038/s41699-025-00604-x","vorDoiUrl":"https://doi.org/10.1038/s41699-025-00604-x","workflowStages":[]},"version":"v1","identity":"rs-6904882","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6904882","identity":"rs-6904882","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}