Self-Supporting Nanosheet Electrode for Efficient Oxygen Evolution in a Wide pH Range: Engineering Electronic Structure of Co 3 O 4 by Fe Doping

Self-Supporting Nanosheet Electrode for Efficient Oxygen Evolution in a Wide pH Range: Engineering Electronic Structure of Co 3 O 4 by Fe Doping | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Self-Supporting Nanosheet Electrode for Efficient Oxygen Evolution in a Wide pH Range: Engineering Electronic Structure of Co 3 O 4 by Fe Doping Xiaobo Cheng, Ningning Han, Guangli He, Zhuang Xu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3896219/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 10 You are reading this latest preprint version Abstract Developing low-cost and efficient non-precious metal-based electrocatalysts for oxygen evolution reaction (OER) is of great significance for large-scale application of water electrolysis technology. Herein, we present a facile and scalable one-step pyrolysis strategy to fabricate a self-supporting nanosheet electrode involving Fe-doped Co 3 O 4 catalyst (Fe-Co 3 O 4 ) in-situ grown on carbon paper for efficient and durable OER catalysis in both alkaline and acidic electrolyte. Results show that doping Fe induces the formation of uniform a nanosheet-like morphology with larger specific surface area that facilitates the full exposure of active sites with accessible contact with electrolyte. Electrochemical test results show that the obtained Fe-Co 3 O 4 exhibits superior activity and high stability for OER catalysis in wide pH range, showing the low overpotentials of 263 and 295 mV in 1.0 M KOH and in 0.5 M H 2 SO 4 , respectively, outperforming commercial IrO 2 , and also exhibiting outstanding electrochemical stability up to 420 h in 1.0 M KOH and 15 h in 0.5 M H 2 SO 4 at 10 mA cm − 2 . X-ray photoelectron spectroscopy and DFT theoretical calculations reveal that doping Fe modifies the electronic structure of Co 3 O 4 by decreasing the valence state of Co, which upwards d band center of Co site and then promotes adsorption intensity of oxygen intermediates, leading to an enhanced OER activity. Furthermore, doping Fe also increases the cobalt vacancy formation energy in Fe-Co 3 O 4 , which inhibits the thermodynamics of Co dissolution, thus improving the structural stability during OER catalysis. This work provides a new insight into the design of high-performance of Co 3 O 4 -based non-precious electrocatalysts in both alkaline and acidic electrolyte for large-scale application of water electrolysis. Fe-doping Co3O4 electronic interaction oxygen evolution reaction wide pH range water electrolysis Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1 Introduction The production of green hydrogen by water electrolysis with renewable power is expected to be an effective way to replace traditional fossil energy and realize the transition to low-carbon clean energy [ 1 ] . Water splitting involves two electrochemical half-reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the cathode and anode [ 1 ] , respectively. However, the anode OER with sluggish reaction kinetics usually induces a high overpotential, which limits the efficiency of water splitting and increases the energy consumption for producing hydrogen. Currently, noble metal materials, including Ir and Ru or their oxides, are still regarded as the pH-universal electrocatalysts with the highest activity in both acidic and alkaline electrolytes [ 2 ] . However, their high cost and limited reserve greatly hinders the application in water electrolysis. Therefore, exploring high-performance non-precious metal electrocatalysts toward OER catalysis in wide pH electrolyte is of vital significance in large-scale application of water electrolysis [ 3 , 4 ] . In recent years, cobalt oxide-based materials, especially Co 3 O 4 [ 5 , 6 ] , have attracted much attention and become a kind of promising non-precious metal electrocatalysts for OER catalysis due to their flexibility of structure/valence state and relatively high catalytic activity. However, the intrinsic activity of pure Co 3 O 4 is still low, which limits its application in water electrolysis [ 7 ] . Therefore, various modification strategies have been proposed to improve OER activity of Co 3 O 4 , such as introducing oxygen vacancy defects, doping other metals to tune electronic structure of Co sites, and modifying the surface properties ( e.g ., pore structure, morphology) through surface engineering strategy. Sengeni et al. prepared a Ti-doped Co 3 O 4 catalyst through the co-deposition and pyrolysis strategy, showing the overpotential of 513 mV in 0.5 M H 2 SO 4 . Detailed pre- and post-OER material characterization suggested that significant surface reconstruction has taken place on the surface of Co 2 TiO 4 , which lowered the OER activity [ 8 ] . Yan et al. prepared a mesoporous Ag-doped Co 3 O 4 nanowire arrays (Ag-Co/FTO) by electrodeposition-hydrothermal method, showing the potential of 1.91V at 10 mA cm − 2 and negligible attenuation of current density within 10 h in 0.5 M H 2 SO 4 . [ 9 ] . Marco et al. prepared Li-doped Co 3 O 4 thin films on FTO glass by thermal decomposition of nitrate. The Li-doped Co 3 O 4 can reach 10 mA cm − 2 at overpotential of 510 mV and keep the overpotential within 16 h, the improvement in performance can be attributed to the high electrochemical surface area and electronic modification caused by Li doping [ 10 ] . However, the reported Co 3 O 4 -based catalysts still exhibits unsatisfied OER activity, probably due to insufficient exposure of active sites, and unfavorable morphology and pore structure for mass transport. Furthermore, the obtained catalysts also show relatively low durability during OER catalysis, especially in acidic electrolyte, which might due to the dissolution of Co site, the microstructure change induced by electric field in strong corrosive electrolyte, and the detachment originating from the weak catalyst/substrate binding induced by bubbles effect at high current density. Therefore, exploring highly efficient and durable Co 3 O 4 -based non-precious electrocatalyst in wide pH range is imperative. Herein, we present a facile pyrolysis strategy to fabricate a self-supporting nanosheet-structured Fe-doping Co 3 O 4 electrode in-situ growth on carbon paper without addition of binders for OER catalysis. The doping of Fe regulates the morphology and modifies the electronic structure, offering an efficient and durable OER catalysis in wide pH range. The porous nanosheets structure is beneficial for mass transportation and thus promote the shedding of oxygen bubbles from the electrode. X-ray photoelectron spectroscopy and DFT theoretical calculations reveal that doping of Fe changes the electronic structure of Co with decreased valence state and increases d band center of Co site, leading to an enhanced adsorption strength of oxygen intermediates with faster OER kinetics. Moreover, Fe doping also induces the increase of vacancy formation energy of Co site, resulting into an improved electrochemical durability. Therefore, the optimized Fe-Co 3 O 4 electrode exhibits a superior pH-universal OER performance with a low overpotential of 263 mV in KOH and 295 mV in 0.5 M H 2 SO 4 , and outstanding stability up to 420 h and 15 h at 10 mA cm − 2 in 1 M KOH and 0.5 M H 2 SO 4 , respectively. 2 Experimental section 2.1 Materials and reagents Cobalt(II) nitrate hexahydrate (Co(NO 3 ) 2 ·6H 2 O), iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) and sodium hydroxide (NaOH) were all purchased from Sinopharm Chemical Reagent limited corporation. Carbon paper (CP, Sigma), sulfuric acid (H 2 SO 4 , 95%-98%, Sinopharm Chemical Reagent limited corporation), ethanol (C 2 H 6 O, 99%), and commercial iridium (Ⅳ) dioxide (IrO 2 , Heraeus Group). All chemicals used were of analytical grade, and all solutions used for experiments were prepared with deionized water (DI water, resistivity ≥ 18 MΩ cm). 2.2 Synthesis of Fe-Co 3 O 4 electrocatalyst growth on CP The self-supporting electrode was synthesized by coating Fe-Co 3 O 4 electrocatalyst onto CP surface via a facile one-step pyrolysis process. Firstly, CP was first soaked with ethanol and then pretreated by acid oxidation using electrochemical cyclic voltammetry (CV) in 0.5 M H 2 SO 4 for 10 cycles between 1.5 and 2.3 V ( vs . Ag/AgCl, in KCl saturation solution) to clean CP surface and increase its hydrophilicity. Afterwards, Co(NO 3 ) 2 ·6H 2 O and Fe(NO 3 ) 3 ·9H 2 O with a metal molar ratio of 3:1 (Co/Fe) (total metal content: 0.3 mmol) were added into 300 µL of DI water and ultrasonicated for 0.5 h. The pretreated CP was heated at 250 ℃ for 5 min (Fig. 1 a). Then, the above solution was sprayed onto CP within the area of 1 cm 2 . After that, the final product was annealed in air at 250 ℃ for 4 h. For comparison, Co 3 O 4 without Fe addition was also prepared with the same procedure. 2.3 Material characterization The crystal phase of the as-prepared samples was identified by X-ray diffraction (XRD, D/max-2500 XRD, Cu K α radiation). The morphologies were examined using the scanning electron microscopy (SEM, JEOL FE-JSM-6701F). Surface element composition and chemical state were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250) using an Al Kα source, and the binding energies were referenced to the C 1 s peak (~ 284.4 eV). 2.4 Electrochemical measurements All the electrochemical measurements for OER were carried out on CHI760E electrochemical workstation (Shanghai Chenhua Instrument Corporation, China) in a three-electrode system in acidic (0.5 M H 2 SO 4 ) and alkaline (1.0 M KOH) electrolyte at room temperature. The electrocatalyst coated CP was used as the working electrode (area = 1 cm 2 ), while a graphite rod and saturated calomel electrode (SCE) were employed as counter and reference electrodes, respectively. All current densities were calibrated with the geometric surface area. Unless otherwise specified, all the labeled voltages in this work were converted to the reversible hydrogen electrode ( vs. RHE) with the following equation: E RHE =E SCE +0.241 + 0.059pH. The OER activity of samples was evaluated by measuring linear sweep voltammetry (LSV) curves at a scan rate of 1 mV s − 1 in the potential range of 1.2–1.6 V ( vs. RHE) in 0.5 M H 2 SO 4 and 1.0 M KOH electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a constant current density of 10 mA cm − 2 from 100 KHz to 0.1 Hz. Chronopotentiometric measurement was recorded under a constant current density of 10 mA cm geo −2 in 0.5 M H 2 SO 4 and 1.0 M KOH. To activate the catalysts, cyclic voltammetry (CV) was first performed on the catalysts with 20 cycles at a scan rate of 50 mV s − 1 in the potential range of 0.9–1.2 V. 2.5 DFT calculations To investigate the effect of Fe doping on electronic structure of Co 3 O 4 , all calculations reported in this work were performed by the Cambridge Sequential Total Energy Package (Castep) module in Materials Studio (MS) software. The Perdew-Burke-Ernzerh (PBE) functional [ 13 ] theory under the Generalized Gradient Approximation (GGA) [ 12 ] framework was used to optimize geometric structure, calculate total energy, and analyze electronic structure of the constructed model. The cutoff energy of plane waves was set to 400 eV and the Brillouin zone integration was sampled via the Monkhorst-Pack (MP) method with 2×2×2 k -points mesh for bulk. The convergence criteria for all geometry optimization calculation were set as following: total energy per atom<2×10 − 5 eV atom − 1 , maximum atomic force<0.05 eV Å −1 and maximum displacement<0.002 Å. Calculations were subjected to Van der Waals force correction through the semiempirical D2 method of Grimme ( DFT-D2). 3 Results and discussion 3.1 Morphology and structural properties As shown in Fig. 1 b, the crystal structure of the as-prepared samples was firstly investigated by XRD. For pure Co 3 O 4 , several main diffraction peaks appear at 2 θ of 31.38°, 36.98°, 44.96°, 59.54° and 65.18°, corresponding to (220), (311), (400), (511) and (440) crystal planes of spinel-type Co 3 O 4 (JCPDS card no. 76-1802), respectively, without other detectable crystal phases, proving the successful preparation of pure-phase Co 3 O 4 by this pyrolysis method. After addition of Fe, the crystal structure of Fe-Co 3 O 4 was basically consistent with Co 3 O 4 . Notably, the main diffraction peaks negatively shift as compared with the corresponding diffraction peaks of Co 3 O 4 , which can be explained by the slightly larger ionic radius of Fe 3+ (0.645 Å) than that of Co 3+ (0.61 Å) [ 11 ] , confirming the successful incorporation of Fe into Co 3 O 4 [ 12 ] . The morphologies of the as-synthesized Co 3 O 4 and Fe-Co 3 O 4 catalysts were further investigated by SEM (Fig. 1 c-h). As shown in Fig. 1 c-e, Co 3 O 4 shows a spherical-like morphology formed by the aggregation of small particles with uneven sizes. After adding Fe, the morphology of Fe-Co 3 O 4 undergoes a significant change and exhibits a nanosheet-like structure with the length of ~ 400–600 nm and thickness of ~ 100 nm. It implies that doping Fe induces the formation of a uniform nanosheet-like morphology with a larger specific surface area by inhibiting the aggregation of Co 3 O 4 particles, which facilitates the full exposure of active sites accessible to the electrolyte. The water droplet contact angle measurement results show that the contact angle of Fe-Co 3 O 4 electrode ( θ = 79.3 °) is lower as compared with Co 3 O 4 ( θ = 118.9 °) (Fig. 2 ), indicating the increase of hydrophilicity, which may originate from the increased oxygen vacancies after Fe-doping. The increase of hydrophilicity is believed to benefit the sufficient contact of active sites with electrolyte because oxygen vacancies have an affinity for water molecules, leading to an enhanced OER activity. [ 16 ] . Furthermore, the increase of hydrophilicity could promote the bubble removal especially at high current density, which is favorable for mass transfer and re-exposure of active sites [ 17 ] , all of which will ultimately contribute to the enhancement of OER activity. To investigate the surface chemical composition and element valence states of the as-prepared Co 3 O 4 and Fe-Co 3 O 4 samples, XPS tests were conducted. The full survey XPS spectra shows the signals of Co 2 p , Fe 2 p and O 1 s orbitals (Fig. 3 a), without other impurity element peaks, confirming the coexistence of Co, Fe and O elements. As shown in Fig. 3 b, the high resolution XPS spectra of Co 2 p show the peaks at binding energy (B.E.) of 796.15 and 780.93 eV, attributable to Co 2 p 1/2 and Co 2 p 3/2 of Co 3 O 4 , respectively, whereas the peaks at 797.45 and 781.51 eV correspond to Co 2 p 1/2 and Co 2 p 3/2 of Fe-Co 3 O 4 , respectively. It is worth noting that, compared with the Co 2 p peak of Co 3 O 4 (780.93 eV), the Co 2 p of Fe-Co 3 O 4 (781.51 eV) shifts to a higher binding energy with the increase value of 0.58 eV, suggesting the modification of electronic structure of Co 3 O 4 induced by Fe doping. The fitted Co 2 p spectra show that the presence of Co 2+ and Co 3+ species for both Co 3 O 4 and Fe-Co 3 O 4 samples. Notably, Fe-Co 3 O 4 exhibits an increased Co 2+ content due to the electronic modification effect by Fe doping, showing the increase ratio of Co 2+ /Co 3+ (Co 3 O 4 : 2.047; Fe-Co 3 O 4 : 5.683)(Table 1 ), which is attributed to the electron donating effect of Fe that enriches electron around Co sites based on the higher electronegativity of Co (1.88) than Fe (1.83) [26] . It has been reported that the lower valence state of Co has a positive effect on OER catalysis [18, 19] . For the fitted Fe 2 p spectrum, the peaks at binding energies of 715.5 and 721.35 eV are corresponded to Fe 2 p 3/2 and Fe 2 p 1/2 orbitals, respectively (Fig. 3 c). It is evident that Fe-Co 3 O 4 exhibits more oxygen vacancies (47.9%) in comparison with Co 3 O 4 (37.9%) as indicated from the fitted O 1 s spectrum (Fig. 3 d), which may be caused by the increased ratio of Co 2+ /Co 3+ , favorable for the enhancement of OER activity [ 7 , 13 ] . Table 1 High-resolution spectra peaking data of Co 2 p . Sample B.E. / eV Co 2+ /% Co 3+ /% Co 2+ /Co 3 Co 2 p 1/2 Co 2 p 3/2 Co 3 O 4 796.15 780.93 67.179 32.820 2.047 Fe-Co 3 O 4 797.45 781.51 85.037 14.963 5.683 2.2 Electrochemical performance The OER electrocatalytic performance of as-prepared Fe-Co 3 O 4 , Co 3 O 4 and commercial IrO 2 electrodes were assessed in 1.0 M KOH electrolyte in a three-electrode system (Fig. 4 ). CV curves show that Fe-Co 3 O 4 exhibits a larger capacitive current than others (Fig. 4 a), implying a larger electrochemical active area with more accessible active sites. LSV measurements show that the overpotential at current density of 10 mA cm − 2 ( η 10 ) of Fe-Co 3 O 4 is determined to be as low as 263 mV, remarkably superior than that of Co 3 O 4 ( η 10 = 315 mV) and even outperforming commercial IrO 2 ( η 10 = 329 mV) (Fig. 4 b), suggesting an improved OER activity after Fe doping. Electrochemical impedance spectroscopy (EIS) measurements show that Fe-Co 3 O 4 shows a much lower charge transfer resistance ( R ct =5.131 Ω) than Co 3 O 4 ( R ct =7.127 Ω), and comparable to commercial IrO 2 ( R ct =5.57 Ω), indicating a faster OER kinetic with a faster charge transfer capability at the electrode-electrolyte interface. Furthermore, chronopotentiometry measurement tested at 10 mA cm − 2 shows an outstanding electrochemical stability of Fe-Co 3 O 4 for up to 420 h without obvious degradation (Fig. 4 e). In addition, the OER performance of the as-prepared catalyst was also assessed in acidic electrolyte (0.5 M H 2 SO 4 ) to verify whether as-prepared catalysts exhibit superior performance in acidic electrolyte. As expected, Fe-Co 3 O 4 sample shows different CV curve compared Co 3 O 4 and commercial IrO 2 (Fig. 5 ), and the η 10 value of Fe-Co 3 O 4 is determined to be the smallest (295 mV) among all the samples (Co 3 O 4 : 359 mV, commercial IrO 2 : 351 mV). Meanwhile, the Nyquist plots show that Fe-Co 3 O 4 has the smallest charge transfer resistance, accounting for the superior OER activity. Chronopotentiometric measurement also show that Fe-Co 3 O 4 exhibits the electrochemical stability for ~ 15 h at the current density of 10 mA cm − 2 . 2.3 DFT calculations To further illustrate the effect of Fe doping on the improvement of OER performance, we also performed DFT calculations to analyze the contribution of Fe to activity and stability of Co 3 O 4 from the perspectives of vacancy formation energy, density of states and charge distribution changes. The schematic diagrams of the crystal structure were presented in Fig. 6 a, showing the spinel structure of Co 3 O 4 that consists of Co 2+ and Co 3+ normally located at the tetrahedral and octahedral sites surrounded by oxygen ions, respectively [ 14 ] . Based on the XRD analyses, doping Fe only causes partial lattice expansion without altering the basic structure of Co 3 O 4 , thus the model of Fe-Co 3 O 4 was constructed for optimization and calculation based on the spinel structure of Co 3 O 4 . First, we evaluated the vacancy formation energy of cobalt and oxygen (Fig. 6 b), and the results is depicted schematically in Fig. 6 c. The cobalt vacancy formation energy ( E VCo ) of Fe-Co 3 O 4 (4.611 eV) is higher than that of Co 3 O 4 (4.248 eV). The increased E VCo of Fe-Co 3 O 4 makes it difficult for cobalt ions to dissolve and thus improving the structural stability of catalysts, consistent with the electrochemical stability test results. In contrast, the oxygen vacancy formation energy ( E VO ) of Fe-Co 3 O 4 is determined to be 3.488 eV, which is lower than Co 3 O 4 ( E VO =3.957 eV). The reduced E VO of Fe-Co 3 O 4 implies that doping Fe makes it easier for Co 3 O 4 to form oxygen vacancies, probably leading to the increase of O 2 p band as indicated by the projected density of state (PDOS) results, which is consistent with the XPS analysis with increased oxygen vacancy [ 15 ] . It reveals that the reactivity of lattice oxygen was improved during the OER process due to the decreased E VO induced by Fe doping [ 15 ] . PDOS calculations found that the atomic density distribution of Fe-Co 3 O 4 located near the Fermi level is higher than that of Co 3 O 4 (Fig. 6 d-e), indicating that more charges are directly involved during OER catalysis, which is beneficial for the improvement of OER activity. Meanwhile, the Co 3 d band center was found to be upshifted after Fe doping, which enhances the adsorption strength of oxygen intermediates during OER catalytic process and thus resulting in a higher OER activity [ 16 , 17 ] . To further reveal the reason for the improvement of OER activity, Mulliken charge population analysis was further carried out to investigate the charge distribution of each atomic orbital by uniformly distributing charge in the overlapping region to the relevant atomic orbitals. As shown in Fig. 6 f, the charge value on Co sites was found to be decreased from + 0.83 |e| (Co 3 O 4 ) to + 0.63 |e| (Fe-Co 3 O 4 ) after Fe doping, implying the presence of high content low-valence state of Co sites (Co 2+ ) in Fe-Co 3 O 4 sample due to the electron donation from Fe sites forming the Fe-O-Co bond, which is consistent with the XPS analysis. Previous reports have shown that the presence of high content Co 2+ species is beneficial to promoting the improvement of OER catalytic activity [ 18 ] . Liu. et al reported that the Co 2+ species located in high spin state can release electrons under the applied potential, which improves the affinity of oxygen ions on catalyst surface and is beneficial to the interaction between catalyst surface and oxygen intermediates to form hydroxyl cobalt oxide (CoOOH) that is the real active site for OER catalysis. Therefore, the enrichment of electrons near Co sites reduces the valence state of Co and promotes the increase of OER activity. Additionally, this electron-supply effect induced by the formed Fe-O-Co structure effectively suppresses the over-oxidation of Co sites and results in an outstanding long-term durability during OER catalysis. 4 Conclusion In summary, we present a facile and scalable one-step pyrolysis strategy to fabricate self-supporting Fe-doped Co 3 O 4 nanosheet in-situ grown on CP surface for efficient and durable OER catalysis. The introduction of Fe induced the formation of nanosheet-like morphology, facilitating the full exposure of active sites on the surface with enlarged contact area between catalyst and electrolyte. Furthermore, the presence of Fe also decreases the valence state of Co with a higher proportion of Co 2+ /Co 3+ for Fe-Co 3 O 4 compared with Co 3 O 4 due to the electron donating effect of Fe to Co. DFT calculation show that the Co-O interaction was changed after doping Fe showing the upwards d band center of Co site, thus promoting adsorption intensity of oxygen intermediates, leading to an enhanced OER activity. Besides, the formation energy of cobalt vacancies in Fe-Co 3 O 4 was increased, which inhibits the Co dissolution and improves the structural stability of Fe- Co 3 O 4 during OER durability test. The obtained Fe-Co 3 O 4 electrode exhibits superior OER activity in a wide pH range, showing an overpotential of 263 mV at 10 mA cm − 2 in 1.0 M KOH (lower than Co 3 O 4 and commercial IrO 2 by 52 mV and 66 mV, respectively) and 295 mV at 10 mA cm − 2 in 0.5 M H 2 SO 4 (lower than that of Co 3 O 4 and commercial IrO 2 by 64 mV and 56 mV, respectively). Furthermore, the electrode also exhibits outstanding electrochemical durability up to 420 h at 10 mA cm − 2 in 1.0 M KOH and 15 h in 0.5 M H 2 SO 4 . This work offers promising potential as a high-performance non-noble-metal based anode material for application of water splitting. Declarations Author Contribution Xiaobo Cheng did major experiments and wrote the manuscript text.Ningning Han assisted in analyzing experimental results.Guangli He reviewed the manuscriptZhuang Xu gave suggestions for the experiment and reviewed the manuscript Ethical Approval : This declaration is not applicable. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3896219","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":269722333,"identity":"ff855ea0-a14a-4ce4-a6f2-09dddbd76811","order_by":0,"name":"Xiaobo Cheng","email":"","orcid":"","institution":"NATIONAL INSTITUTE OF CLEAN AND LOW CARBON ENERGY","correspondingAuthor":false,"prefix":"","firstName":"Xiaobo","middleName":"","lastName":"Cheng","suffix":""},{"id":269722334,"identity":"a1d38fc7-69e6-4c14-b3ff-93fb88b185c7","order_by":1,"name":"Ningning Han","email":"","orcid":"","institution":"Guoneng Yuedian Taishan Power generation Co.,Ltd","correspondingAuthor":false,"prefix":"","firstName":"Ningning","middleName":"","lastName":"Han","suffix":""},{"id":269722335,"identity":"4b3f9685-5ebc-429d-b8f9-ecaf2c2a6f01","order_by":2,"name":"Guangli He","email":"","orcid":"","institution":"NATIONAL INSTITUTE OF CLEAN AND LOW CARBON ENERGY","correspondingAuthor":false,"prefix":"","firstName":"Guangli","middleName":"","lastName":"He","suffix":""},{"id":269722336,"identity":"32beb26a-de22-4c7f-a1e5-860dd58ca203","order_by":3,"name":"Zhuang Xu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABIElEQVRIiWNgGAWjYDACCQST8QFjA4lamA1I1sImQZQW+dk9hp8LfjEkbjjA/Kzi5447cubsvY9f8zDYyek2MD97gEUL45wzxtIz+xiMDQ6wmd3sPfPM2LLnuJk1D0OysdkBNnMDLFqYJXIMpHl7GOQMDjCY3WZsO5y44UYamzEPw4HEbQd42CSwaGGTyDH+DdTCY3CA/VsxUEv9hvvP8Gvhkcgxk+b5AbKFx4wZqCXB4AYb82N8WiQk0sqseRskjCUP8xRL9rYdNtxwJo2NcY4B0C+H2cywaZGfkbz5Ns8fm8S+4+0bP/xsOyxvcPwY84c3FXZyZsebn2HTAgm2NqAUM5IHpXhAQcWMSz0I/EHlMn/8gU/1KBgFo2AUjDQAACYgXLHLOIi3AAAAAElFTkSuQmCC","orcid":"","institution":"NATIONAL INSTITUTE OF CLEAN AND LOW CARBON ENERGY","correspondingAuthor":true,"prefix":"","firstName":"Zhuang","middleName":"","lastName":"Xu","suffix":""}],"badges":[],"createdAt":"2024-01-25 06:29:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3896219/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3896219/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":50394450,"identity":"95d496d4-4b16-443c-8357-dd805827d97c","added_by":"auto","created_at":"2024-01-30 21:05:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":702606,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic diagram of catalyst preparation process, (b) XRD patterns of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e,\u003csub\u003e \u003c/sub\u003e(c-e) SEM images of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003esamples at different magnifications, and (f-h) SEM images of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eat different magnifications.\u003c/p\u003e","description":"","filename":"floatimage152.png","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/46cf844e239e51ba25e66916.png"},{"id":50394452,"identity":"ea2ecc20-848e-492d-b5b1-0215236526de","added_by":"auto","created_at":"2024-01-30 21:05:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":228107,"visible":true,"origin":"","legend":"\u003cp\u003eWater droplet contact angle measurement results of (a) Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and\u003csub\u003e \u003c/sub\u003e(b) Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage249.png","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/658159f32f2e3fdb49806f81.png"},{"id":50394453,"identity":"a31d15ec-c313-4859-970c-4e50113f0693","added_by":"auto","created_at":"2024-01-30 21:05:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":282308,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XPS survey spectra of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and\u003csub\u003e \u003c/sub\u003eFe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, (b) high-resolution spectra of Co 2\u003cem\u003ep\u003c/em\u003e, (c) Fe 2\u003cem\u003ep\u003c/em\u003e and (d) O 1\u003cem\u003es.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage347.png","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/45d07bf6b2f215b943f48cac.png"},{"id":50394449,"identity":"fd47260d-818b-43e5-8a66-f93d8a57ddd3","added_by":"auto","created_at":"2024-01-30 21:05:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":248069,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical OER activity of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and IrO\u003csub\u003e2\u003c/sub\u003e in 1.0 M KOH electrolyte: (a) CV curves, (b) polarization curves, (c) Nyquist plots, (d) corresponding overpotential and charge transfer resistance, and (e) chronopotentiometry test at constant current density of 10 mA cm\u003csup\u003e-2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/a992ea3f29fedf60252f2753.png"},{"id":50394451,"identity":"bf1b4a84-b17c-45bc-b00e-4dffcfc31c7a","added_by":"auto","created_at":"2024-01-30 21:05:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":237076,"visible":true,"origin":"","legend":"\u003cp\u003eElectrochemical performance of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and IrO\u003csub\u003e2\u003c/sub\u003e in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e electrolyte: (a) CV curves, (b) polarization curves, (c) Nyquist plots, (d) histogram of overpotential and charge transfer resistance, and (e) chronopotentiometry test at constant current density of 10 mA cm\u003csup\u003e-2\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/9ccde8cfa77c0c16f40c73ad.png"},{"id":50394454,"identity":"d47c7c3a-7f11-40a9-9bc1-5a6095743ab5","added_by":"auto","created_at":"2024-01-30 21:05:16","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":756557,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Crystal structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eand Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4, \u003c/sub\u003e(b) histogram of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eand Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003evacancy formation energy, (c) structural schematic diagram of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003evacancy formation, (d) PDOS of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eand Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eO 2\u003cem\u003ep\u003c/em\u003e,\u003cem\u003e \u003c/em\u003e(e) PDOS of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eand Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eCo 3\u003cem\u003ed\u003c/em\u003e, and (f) Mulliken charge distribution of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4 \u003c/sub\u003eand Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/26839915f108256c05bf7778.png"},{"id":50394684,"identity":"fe307b1f-716e-4e6b-91d0-5df3c2f0e569","added_by":"auto","created_at":"2024-01-30 21:13:17","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3129461,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3896219/v1/e4714dbc-27d9-4997-b4dc-9cec0b65a9f6.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Self-Supporting Nanosheet Electrode for Efficient Oxygen Evolution in a Wide pH Range: Engineering Electronic Structure of Co 3 O 4 by Fe Doping","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eThe production of green hydrogen by water electrolysis with renewable power is expected to be an effective way to replace traditional fossil energy and realize the transition to low-carbon clean energy\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e. Water splitting involves two electrochemical half-reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) at the cathode and anode \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e, respectively. However, the anode OER with sluggish reaction kinetics usually induces a high overpotential, which limits the efficiency of water splitting and increases the energy consumption for producing hydrogen. Currently, noble metal materials, including Ir and Ru or their oxides, are still regarded as the pH-universal electrocatalysts with the highest activity in both acidic and alkaline electrolytes \u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. However, their high cost and limited reserve greatly hinders the application in water electrolysis. Therefore, exploring high-performance non-precious metal electrocatalysts toward OER catalysis in wide pH electrolyte is of vital significance in large-scale application of water electrolysis\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eIn recent years, cobalt oxide-based materials, especially Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e, have attracted much attention and become a kind of promising non-precious metal electrocatalysts for OER catalysis due to their flexibility of structure/valence state and relatively high catalytic activity. However, the intrinsic activity of pure Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is still low, which limits its application in water electrolysis\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. Therefore, various modification strategies have been proposed to improve OER activity of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, such as introducing oxygen vacancy defects, doping other metals to tune electronic structure of Co sites, and modifying the surface properties (\u003cem\u003ee.g\u003c/em\u003e., pore structure, morphology) through surface engineering strategy. Sengeni \u003cem\u003eet al.\u003c/em\u003e prepared a Ti-doped Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e catalyst through the co-deposition and pyrolysis strategy, showing the overpotential of 513 mV in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. Detailed pre- and post-OER material characterization suggested that significant surface reconstruction has taken place on the surface of Co\u003csub\u003e2\u003c/sub\u003eTiO\u003csub\u003e4\u003c/sub\u003e, which lowered the OER activity\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. Yan \u003cem\u003eet al.\u003c/em\u003e prepared a mesoporous Ag-doped Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanowire arrays (Ag-Co/FTO) by electrodeposition-hydrothermal method, showing the potential of 1.91V at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and negligible attenuation of current density within 10 h in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. \u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. Marco \u003cem\u003eet al.\u003c/em\u003e prepared Li-doped Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e thin films on FTO glass by thermal decomposition of nitrate. The Li-doped Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e can reach 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at overpotential of 510 mV and keep the overpotential within 16 h, the improvement in performance can be attributed to the high electrochemical surface area and electronic modification caused by Li doping\u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. However, the reported Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-based catalysts still exhibits unsatisfied OER activity, probably due to insufficient exposure of active sites, and unfavorable morphology and pore structure for mass transport. Furthermore, the obtained catalysts also show relatively low durability during OER catalysis, especially in acidic electrolyte, which might due to the dissolution of Co site, the microstructure change induced by electric field in strong corrosive electrolyte, and the detachment originating from the weak catalyst/substrate binding induced by bubbles effect at high current density. Therefore, exploring highly efficient and durable Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-based non-precious electrocatalyst in wide pH range is imperative.\u003c/p\u003e\u003cp\u003eHerein, we present a facile pyrolysis strategy to fabricate a self-supporting nanosheet-structured Fe-doping Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e electrode in-situ growth on carbon paper without addition of binders for OER catalysis. The doping of Fe regulates the morphology and modifies the electronic structure, offering an efficient and durable OER catalysis in wide pH range. The porous nanosheets structure is beneficial for mass transportation and thus promote the shedding of oxygen bubbles from the electrode. X-ray photoelectron spectroscopy and DFT theoretical calculations reveal that doping of Fe changes the electronic structure of Co with decreased valence state and increases \u003cem\u003ed\u003c/em\u003e band center of Co site, leading to an enhanced adsorption strength of oxygen intermediates with faster OER kinetics. Moreover, Fe doping also induces the increase of vacancy formation energy of Co site, resulting into an improved electrochemical durability. Therefore, the optimized Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e electrode exhibits a superior pH-universal OER performance with a low overpotential of 263 mV in KOH and 295 mV in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, and outstanding stability up to 420 h and 15 h at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e in 1 M KOH and 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, respectively.\u003c/p\u003e"},{"header":"2 Experimental section","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials and reagents\u003c/h2\u003e \u003cp\u003eCobalt(II) nitrate hexahydrate (Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO), iron(III) nitrate nonahydrate (Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO) and sodium hydroxide (NaOH) were all purchased from Sinopharm Chemical Reagent limited corporation. Carbon paper (CP, Sigma), sulfuric acid (H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 95%-98%, Sinopharm Chemical Reagent limited corporation), ethanol (C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eO, 99%), and commercial iridium (Ⅳ) dioxide (IrO\u003csub\u003e2\u003c/sub\u003e, Heraeus Group). All chemicals used were of analytical grade, and all solutions used for experiments were prepared with deionized water (DI water, resistivity\u0026thinsp;\u0026ge;\u0026thinsp;18 MΩ cm).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e electrocatalyst growth on CP\u003c/h2\u003e \u003cp\u003eThe self-supporting electrode was synthesized by coating Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e electrocatalyst onto CP surface via a facile one-step pyrolysis process. Firstly, CP was first soaked with ethanol and then pretreated by acid oxidation using electrochemical cyclic voltammetry (CV) in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e for 10 cycles between 1.5 and 2.3 V (\u003cem\u003evs\u003c/em\u003e. Ag/AgCl, in KCl saturation solution) to clean CP surface and increase its hydrophilicity. Afterwards, Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e\u0026middot;6H\u003csub\u003e2\u003c/sub\u003eO and Fe(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e3\u003c/sub\u003e\u0026middot;9H\u003csub\u003e2\u003c/sub\u003eO with a metal molar ratio of 3:1 (Co/Fe) (total metal content: 0.3 mmol) were added into 300 \u0026micro;L of DI water and ultrasonicated for 0.5 h. The pretreated CP was heated at 250 ℃ for 5 min (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Then, the above solution was sprayed onto CP within the area of 1 cm\u003csup\u003e2\u003c/sup\u003e. After that, the final product was annealed in air at 250 ℃ for 4 h. For comparison, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e without Fe addition was also prepared with the same procedure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Material characterization\u003c/h2\u003e \u003cp\u003eThe crystal phase of the as-prepared samples was identified by X-ray diffraction (XRD, D/max-2500 XRD, Cu K\u003cem\u003eα\u003c/em\u003e radiation). The morphologies were examined using the scanning electron microscopy (SEM, JEOL FE-JSM-6701F). Surface element composition and chemical state were characterized by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250) using an Al Kα source, and the binding energies were referenced to the C 1\u003cem\u003es\u003c/em\u003e peak (~\u0026thinsp;284.4 eV).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Electrochemical measurements\u003c/h2\u003e \u003cp\u003eAll the electrochemical measurements for OER were carried out on CHI760E electrochemical workstation (Shanghai Chenhua Instrument Corporation, China) in a three-electrode system in acidic (0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) and alkaline (1.0 M KOH) electrolyte at room temperature. The electrocatalyst coated CP was used as the working electrode (area\u0026thinsp;=\u0026thinsp;1 cm\u003csup\u003e2\u003c/sup\u003e), while a graphite rod and saturated calomel electrode (SCE) were employed as counter and reference electrodes, respectively. All current densities were calibrated with the geometric surface area. Unless otherwise specified, all the labeled voltages in this work were converted to the reversible hydrogen electrode (\u003cem\u003evs.\u003c/em\u003e RHE) with the following equation: E\u003csub\u003eRHE\u003c/sub\u003e=E\u003csub\u003eSCE\u003c/sub\u003e+0.241\u0026thinsp;+\u0026thinsp;0.059pH. The OER activity of samples was evaluated by measuring linear sweep voltammetry (LSV) curves at a scan rate of 1 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the potential range of 1.2\u0026ndash;1.6 V (\u003cem\u003evs.\u003c/em\u003e RHE) in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and 1.0 M KOH electrolyte. Electrochemical impedance spectroscopy (EIS) measurements were carried out at a constant current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e from 100 KHz to 0.1 Hz. Chronopotentiometric measurement was recorded under a constant current density of 10 mA cm\u003csub\u003egeo\u003c/sub\u003e\u003csup\u003e\u0026minus;2\u003c/sup\u003e in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e and 1.0 M KOH. To activate the catalysts, cyclic voltammetry (CV) was first performed on the catalysts with 20 cycles at a scan rate of 50 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the potential range of 0.9\u0026ndash;1.2 V.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.5 DFT calculations\u003c/h2\u003e \u003cp\u003eTo investigate the effect of Fe doping on electronic structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, all calculations reported in this work were performed by the Cambridge Sequential Total Energy Package (Castep) module in Materials Studio (MS) software. The Perdew-Burke-Ernzerh (PBE) functional \u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e theory under the Generalized Gradient Approximation (GGA)\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e framework was used to optimize geometric structure, calculate total energy, and analyze electronic structure of the constructed model. The cutoff energy of plane waves was set to 400 eV and the Brillouin zone integration was sampled via the Monkhorst-Pack (MP) method with 2\u0026times;2\u0026times;2 \u003cem\u003ek\u003c/em\u003e-points mesh for bulk. The convergence criteria for all geometry optimization calculation were set as following: total energy per atom\u0026lt;2\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e eV atom\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, maximum atomic force\u0026lt;0.05 eV \u0026Aring;\u003csup\u003e\u0026minus;1\u003c/sup\u003e and maximum displacement\u0026lt;0.002 \u0026Aring;. Calculations were subjected to Van der Waals force correction through the semiempirical D2 method of Grimme \u003cb\u003e(\u003c/b\u003eDFT-D2).\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Results and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Morphology and structural properties\u003c/h2\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, the crystal structure of the as-prepared samples was firstly investigated by XRD. For pure Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, several main diffraction peaks appear at 2\u003cem\u003eθ\u003c/em\u003e of 31.38\u0026deg;, 36.98\u0026deg;, 44.96\u0026deg;, 59.54\u0026deg; and 65.18\u0026deg;, corresponding to (220), (311), (400), (511) and (440) crystal planes of spinel-type Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (JCPDS card no. 76-1802), respectively, without other detectable crystal phases, proving the successful preparation of pure-phase Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e by this pyrolysis method. After addition of Fe, the crystal structure of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was basically consistent with Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. Notably, the main diffraction peaks negatively shift as compared with the corresponding diffraction peaks of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, which can be explained by the slightly larger ionic radius of Fe\u003csup\u003e3+\u003c/sup\u003e (0.645 \u0026Aring;) than that of Co\u003csup\u003e3+\u003c/sup\u003e (0.61 \u0026Aring;)\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e, confirming the successful incorporation of Fe into Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe morphologies of the as-synthesized Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e catalysts were further investigated by SEM (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec-h). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec-e, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e shows a spherical-like morphology formed by the aggregation of small particles with uneven sizes. After adding Fe, the morphology of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e undergoes a significant change and exhibits a nanosheet-like structure with the length of ~\u0026thinsp;400\u0026ndash;600 nm and thickness of ~\u0026thinsp;100 nm. It implies that doping Fe induces the formation of a uniform nanosheet-like morphology with a larger specific surface area by inhibiting the aggregation of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e particles, which facilitates the full exposure of active sites accessible to the electrolyte. The water droplet contact angle measurement results show that the contact angle of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e electrode (\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;79.3 \u0026deg;) is lower as compared with Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (\u003cem\u003eθ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;118.9 \u0026deg;) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), indicating the increase of hydrophilicity, which may originate from the increased oxygen vacancies after Fe-doping. The increase of hydrophilicity is believed to benefit the sufficient contact of active sites with electrolyte because oxygen vacancies have an affinity for water molecules, leading to an enhanced OER activity. \u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e. Furthermore, the increase of hydrophilicity could promote the bubble removal especially at high current density, which is favorable for mass transfer and re-exposure of active sites \u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e, all of which will ultimately contribute to the enhancement of OER activity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo investigate the surface chemical composition and element valence states of the as-prepared Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples, XPS tests were conducted. The full survey XPS spectra shows the signals of Co 2\u003cem\u003ep\u003c/em\u003e, Fe 2\u003cem\u003ep\u003c/em\u003e and O 1\u003cem\u003es\u003c/em\u003e orbitals (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea), without other impurity element peaks, confirming the coexistence of Co, Fe and O elements. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, the high resolution XPS spectra of Co 2\u003cem\u003ep\u003c/em\u003e show the peaks at binding energy (B.E.) of 796.15 and 780.93 eV, attributable to Co 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e and Co 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e3/2\u003c/sub\u003e of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, respectively, whereas the peaks at 797.45 and 781.51 eV correspond to Co 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e and Co 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e3/2\u003c/sub\u003e of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, respectively. It is worth noting that, compared with the Co 2\u003cem\u003ep\u003c/em\u003e peak of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (780.93 eV), the Co 2\u003cem\u003ep\u003c/em\u003e of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (781.51 eV) shifts to a higher binding energy with the increase value of 0.58 eV, suggesting the modification of electronic structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e induced by Fe doping. The fitted Co 2\u003cem\u003ep\u003c/em\u003e spectra show that the presence of Co\u003csup\u003e2+\u003c/sup\u003e and Co\u003csup\u003e3+\u003c/sup\u003e species for both Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e samples. Notably, Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e exhibits an increased Co\u003csup\u003e2+\u003c/sup\u003e content due to the electronic modification effect by Fe doping, showing the increase ratio of Co\u003csup\u003e2+\u003c/sup\u003e/Co\u003csup\u003e3+\u003c/sup\u003e (Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e: 2.047; Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e: 5.683)(Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), which is attributed to the electron donating effect of Fe that enriches electron around Co sites based on the higher electronegativity of Co (1.88) than Fe (1.83) \u003csup\u003e[26]\u003c/sup\u003e. It has been reported that the lower valence state of Co has a positive effect on OER catalysis \u003csup\u003e[18, 19]\u003c/sup\u003e. For the fitted Fe 2\u003cem\u003ep\u003c/em\u003e spectrum, the peaks at binding energies of 715.5 and 721.35 eV are corresponded to Fe 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e3/2\u003c/sub\u003e and Fe 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e orbitals, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). It is evident that Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e exhibits more oxygen vacancies (47.9%) in comparison with Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (37.9%) as indicated from the fitted O 1\u003cem\u003es\u003c/em\u003e spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), which may be caused by the increased ratio of Co\u003csup\u003e2+\u003c/sup\u003e/Co\u003csup\u003e3+\u003c/sup\u003e, favorable for the enhancement of OER activity\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eHigh-resolution spectra peaking data of Co 2\u003cem\u003ep\u003c/em\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e \u003cp\u003eB.E. / eV\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCo\u003csup\u003e2+\u003c/sup\u003e/%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCo\u003csup\u003e3+\u003c/sup\u003e/%\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCo\u003csup\u003e2+\u003c/sup\u003e/Co\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCo 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e1/2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCo 2\u003cem\u003ep\u003c/em\u003e\u003csub\u003e3/2\u003c/sub\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCo\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e796.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e780.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e67.179\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e32.820\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2.047\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e797.45\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e781.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e85.037\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e14.963\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5.683\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Electrochemical performance\u003c/h2\u003e \u003cp\u003eThe OER electrocatalytic performance of as-prepared Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and commercial IrO\u003csub\u003e2\u003c/sub\u003e electrodes were assessed in 1.0 M KOH electrolyte in a three-electrode system (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). CV curves show that Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e exhibits a larger capacitive current than others (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), implying a larger electrochemical active area with more accessible active sites. LSV measurements show that the overpotential at current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (\u003cem\u003eη\u003c/em\u003e\u003csub\u003e10\u003c/sub\u003e) of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is determined to be as low as 263 mV, remarkably superior than that of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (\u003cem\u003eη\u003c/em\u003e\u003csub\u003e10\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;315 mV) and even outperforming commercial IrO\u003csub\u003e2\u003c/sub\u003e (\u003cem\u003eη\u003c/em\u003e\u003csub\u003e10\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;329 mV) (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), suggesting an improved OER activity after Fe doping. Electrochemical impedance spectroscopy (EIS) measurements show that Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e shows a much lower charge transfer resistance (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e=5.131 Ω) than Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e=7.127 Ω), and comparable to commercial IrO\u003csub\u003e2\u003c/sub\u003e (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ect\u003c/sub\u003e=5.57 Ω), indicating a faster OER kinetic with a faster charge transfer capability at the electrode-electrolyte interface. Furthermore, chronopotentiometry measurement tested at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e shows an outstanding electrochemical stability of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e for up to 420 h without obvious degradation (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn addition, the OER performance of the as-prepared catalyst was also assessed in acidic electrolyte (0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) to verify whether as-prepared catalysts exhibit superior performance in acidic electrolyte. As expected, Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e sample shows different CV curve compared Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and commercial IrO\u003csub\u003e2\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), and the \u003cem\u003eη\u003c/em\u003e\u003csub\u003e10\u003c/sub\u003e value of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is determined to be the smallest (295 mV) among all the samples (Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e: 359 mV, commercial IrO\u003csub\u003e2\u003c/sub\u003e: 351 mV). Meanwhile, the Nyquist plots show that Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e has the smallest charge transfer resistance, accounting for the superior OER activity. Chronopotentiometric measurement also show that Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e exhibits the electrochemical stability for ~\u0026thinsp;15 h at the current density of 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e2.3 DFT calculations\u003c/h2\u003e \u003cp\u003eTo further illustrate the effect of Fe doping on the improvement of OER performance, we also performed DFT calculations to analyze the contribution of Fe to activity and stability of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e from the perspectives of vacancy formation energy, density of states and charge distribution changes. The schematic diagrams of the crystal structure were presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, showing the spinel structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e that consists of Co\u003csup\u003e2+\u003c/sup\u003e and Co\u003csup\u003e3+\u003c/sup\u003e normally located at the tetrahedral and octahedral sites surrounded by oxygen ions, respectively\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. Based on the XRD analyses, doping Fe only causes partial lattice expansion without altering the basic structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, thus the model of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was constructed for optimization and calculation based on the spinel structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e. First, we evaluated the vacancy formation energy of cobalt and oxygen (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb), and the results is depicted schematically in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec. The cobalt vacancy formation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVCo\u003c/sub\u003e) of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (4.611 eV) is higher than that of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (4.248 eV). The increased \u003cem\u003eE\u003c/em\u003e\u003csub\u003eVCo\u003c/sub\u003e of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e makes it difficult for cobalt ions to dissolve and thus improving the structural stability of catalysts, consistent with the electrochemical stability test results. In contrast, the oxygen vacancy formation energy (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVO\u003c/sub\u003e) of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e is determined to be 3.488 eV, which is lower than Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eVO\u003c/sub\u003e=3.957 eV). The reduced \u003cem\u003eE\u003c/em\u003e\u003csub\u003eVO\u003c/sub\u003e of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e implies that doping Fe makes it easier for Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e to form oxygen vacancies, probably leading to the increase of O 2\u003cem\u003ep\u003c/em\u003e band as indicated by the projected density of state (PDOS) results, which is consistent with the XPS analysis with increased oxygen vacancy \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. It reveals that the reactivity of lattice oxygen was improved during the OER process due to the decreased \u003cem\u003eE\u003c/em\u003e\u003csub\u003eVO\u003c/sub\u003e induced by Fe doping\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. PDOS calculations found that the atomic density distribution of Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e located near the Fermi level is higher than that of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed-e), indicating that more charges are directly involved during OER catalysis, which is beneficial for the improvement of OER activity. Meanwhile, the Co 3\u003cem\u003ed\u003c/em\u003e band center was found to be upshifted after Fe doping, which enhances the adsorption strength of oxygen intermediates during OER catalytic process and thus resulting in a higher OER activity\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo further reveal the reason for the improvement of OER activity, Mulliken charge population analysis was further carried out to investigate the charge distribution of each atomic orbital by uniformly distributing charge in the overlapping region to the relevant atomic orbitals. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef, the charge value on Co sites was found to be decreased from +\u0026thinsp;0.83 |e| (Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) to +\u0026thinsp;0.63 |e| (Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) after Fe doping, implying the presence of high content low-valence state of Co sites (Co\u003csup\u003e2+\u003c/sup\u003e) in Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e sample due to the electron donation from Fe sites forming the Fe-O-Co bond, which is consistent with the XPS analysis. Previous reports have shown that the presence of high content Co\u003csup\u003e2+\u003c/sup\u003e species is beneficial to promoting the improvement of OER catalytic activity \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e. Liu. \u003cem\u003eet al\u003c/em\u003e reported that the Co\u003csup\u003e2+\u003c/sup\u003e species located in high spin state can release electrons under the applied potential, which improves the affinity of oxygen ions on catalyst surface and is beneficial to the interaction between catalyst surface and oxygen intermediates to form hydroxyl cobalt oxide (CoOOH) that is the real active site for OER catalysis. Therefore, the enrichment of electrons near Co sites reduces the valence state of Co and promotes the increase of OER activity. Additionally, this electron-supply effect induced by the formed Fe-O-Co structure effectively suppresses the over-oxidation of Co sites and results in an outstanding long-term durability during OER catalysis.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4 Conclusion","content":" \u003cp\u003eIn summary, we present a facile and scalable one-step pyrolysis strategy to fabricate self-supporting Fe-doped Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e nanosheet in-situ grown on CP surface for efficient and durable OER catalysis. The introduction of Fe induced the formation of nanosheet-like morphology, facilitating the full exposure of active sites on the surface with enlarged contact area between catalyst and electrolyte. Furthermore, the presence of Fe also decreases the valence state of Co with a higher proportion of Co\u003csup\u003e2+\u003c/sup\u003e/Co\u003csup\u003e3+\u003c/sup\u003e for Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e compared with Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e due to the electron donating effect of Fe to Co. DFT calculation show that the Co-O interaction was changed after doping Fe showing the upwards \u003cem\u003ed\u003c/em\u003e band center of Co site, thus promoting adsorption intensity of oxygen intermediates, leading to an enhanced OER activity. Besides, the formation energy of cobalt vacancies in Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e was increased, which inhibits the Co dissolution and improves the structural stability of Fe- Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e during OER durability test. The obtained Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e electrode exhibits superior OER activity in a wide pH range, showing an overpotential of 263 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e in 1.0 M KOH (lower than Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and commercial IrO\u003csub\u003e2\u003c/sub\u003e by 52 mV and 66 mV, respectively) and 295 mV at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (lower than that of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e and commercial IrO\u003csub\u003e2\u003c/sub\u003e by 64 mV and 56 mV, respectively). Furthermore, the electrode also exhibits outstanding electrochemical durability up to 420 h at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e in 1.0 M KOH and 15 h in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e. This work offers promising potential as a high-performance non-noble-metal based anode material for application of water splitting.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eXiaobo Cheng did major experiments and wrote the manuscript text.Ningning Han assisted in analyzing experimental results.Guangli He reviewed the manuscriptZhuang Xu gave suggestions for the experiment and reviewed the manuscript\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eEthical Approval\u003c/strong\u003e\u003cstrong\u003e: \u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis declaration is not applicable. We don\u0026rsquo;t have any humans and Animal experiments.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and materials\u003c/strong\u003e\u003cstrong\u003e:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analyzed during this study are included in this article\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLI H, GUO J, LI Z, et al. Research Progress of Hydrogen Production Technology and Related Catalysts by Electrolysis of Water [J/OL] 2023, 28(13):\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.3390/molecules28135010\u003c/span\u003e\u003cspan address=\"10.3390/molecules28135010\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJAMESH M-I, HARB M. Tuning the electronic structure of the earth-abundant electrocatalysts for oxygen evolution reaction (OER) to achieve efficient alkaline water splitting \u0026ndash; A review [J]. Journal of Energy Chemistry, 2021, 56: 299\u0026ndash;342.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYANG J, JI Y, SHAO Q, et al. A Universal Strategy to Metal Wavy Nanowires for Efficient Electrochemical Water Splitting at pH-Universal Conditions [J]. 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Estimation of Electronegativity Values of Elements in Different Valence States [J]. Phys. Chem. A, 2006, 110, 11332\u0026ndash;11337.\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":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Fe-doping Co3O4, electronic interaction, oxygen evolution reaction, wide pH range, water electrolysis","lastPublishedDoi":"10.21203/rs.3.rs-3896219/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3896219/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDeveloping low-cost and efficient non-precious metal-based electrocatalysts for oxygen evolution reaction (OER) is of great significance for large-scale application of water electrolysis technology. Herein, we present a facile and scalable one-step pyrolysis strategy to fabricate a self-supporting nanosheet electrode involving Fe-doped Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e catalyst (Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e) in-situ grown on carbon paper for efficient and durable OER catalysis in both alkaline and acidic electrolyte. Results show that doping Fe induces the formation of uniform a nanosheet-like morphology with larger specific surface area that facilitates the full exposure of active sites with accessible contact with electrolyte. Electrochemical test results show that the obtained Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e exhibits superior activity and high stability for OER catalysis in wide pH range, showing the low overpotentials of 263 and 295 mV in 1.0 M KOH and in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, respectively, outperforming commercial IrO\u003csub\u003e2\u003c/sub\u003e, and also exhibiting outstanding electrochemical stability up to 420 h in 1.0 M KOH and 15 h in 0.5 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e at 10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. X-ray photoelectron spectroscopy and DFT theoretical calculations reveal that doping Fe modifies the electronic structure of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e by decreasing the valence state of Co, which upwards \u003cem\u003ed\u003c/em\u003e band center of Co site and then promotes adsorption intensity of oxygen intermediates, leading to an enhanced OER activity. Furthermore, doping Fe also increases the cobalt vacancy formation energy in Fe-Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, which inhibits the thermodynamics of Co dissolution, thus improving the structural stability during OER catalysis. This work provides a new insight into the design of high-performance of Co\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e-based non-precious electrocatalysts in both alkaline and acidic electrolyte for large-scale application of water electrolysis.\u003c/p\u003e","manuscriptTitle":"Self-Supporting Nanosheet Electrode for Efficient Oxygen Evolution in a Wide pH Range: Engineering Electronic Structure of Co 3 O 4 by Fe Doping","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-30 21:05:11","doi":"10.21203/rs.3.rs-3896219/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-03-11T13:36:03+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-03-06T01:51:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"c1356bf5-9a81-4148-a6c1-42a8fa28ed20","date":"2024-03-06T00:04:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"efdac8a1-d873-4738-ada4-fe2941ed780c","date":"2024-01-31T15:44:56+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-01-31T05:46:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"8fd38ba0-2cac-4ec2-a564-dc83c96f06da","date":"2024-01-26T23:08:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-01-26T15:43:57+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-01-26T07:37:41+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-01-26T07:37:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Ionics","date":"2024-01-25T06:19:30+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"ionics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":" Learn more about [Ionics](https://www.springer.com/journal/11581) ","snPcode":"11581","submissionUrl":"https://mc.manuscriptcentral.com/ionics","title":"Ionics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"0aff918e-42cb-4f5e-ac2f-e6273b60d348","owner":[],"postedDate":"January 30th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2024-04-08T17:11:50+00:00","versionOfRecord":[],"versionCreatedAt":"2024-01-30 21:05:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3896219","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3896219","identity":"rs-3896219","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}