Octahedral Co2+-O-Co3+ in mixed cobalt spinel promotes active and stable acidic oxygen evolution

doi:10.21203/rs.3.rs-4530526/v1

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Octahedral Co2+-O-Co3+ in mixed cobalt spinel promotes active and stable acidic oxygen evolution

https://doi.org/10.21203/rs.3.rs-4530526/v1

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Cobalt (Co)-based oxides show promising activity as precious metal-free catalysts for the oxygen evolution reaction in proton exchange membrane water electrolysis, but the dissolution of Cohas limited the durability of Co₃O₄ at industrially relevant current densities. This work demonstrates that cation in an octahedral coordination environment accounts for the oxygen evolution activity. Using a mixed inverse-normal phase spinel Co_xGa_(3-x)O₄ as a proof-of-concept example, the designed Co²⁺-O-Co³⁺ motifs in octahedral sites trigger oxygen evolution through a kinetically favorable radical coupling pathway. Furthermore, lattice oxygen exchange, a leading factor in catalyst structural degradation for normal Co₃O₄, is suppressed, as evidenced by isotopic labeling experiments and theoretical calculations. With the optimized catalyst, Co_1.8Ga_1.2O₄, we report an overpotential of 310 mV at 10 mA/cm², stable operation at 200 mA/cm²for 200 hours in a three-electrode setup, and a proton exchange membrane electrolyzer operating at 200 mA/cm²for 450 hours.

Physical sciences/Chemistry/Electrochemistry/Electrocatalysis

Physical sciences/Materials science/Materials for energy and catalysis/Electrocatalysis

Oxygen evolution reaction (OER) at the anode of pure water electrolyzer requires active and stable catalysts^1,2. It is an ongoing challenge to explore platinum-group-metal-(PGM-)free catalysts to replace traditional iridium (Ir)- and ruthenium (Ru)-based materials^3-5. PGM-free electrocatalysts have been witnessed with exciting progress, while electrocatalysts that can achieve low overpotential and stable operation at industry-relevant current density for at least one hundred hours are still desired.

Cobalt (Co)-based oxides have recently emerged as the most promising alternative among all PGM-free acidic OER electrocatalysts, for example, Mg, Cr, La and Mn have been doped in to Co₃O₄, showing continuously improved activity and stability^6,7,but the rapid dissolution of Co species, although it may not influence the overall electrolysis performance, possibly due to the overloaded Co-based electrocatalyst at anode, can potentially lead to the poisoning of the proton exchange membrane (PEM) and ultimately result in the breakdown of the entire PEM water electrolyzer (PEMWE). We observed that Co²⁺ in tetrahedral coordination not only fail to contribute to catalytic activity but also dissolve easily in acidic conditions. Whereas, in an octahedral environment, strong cobalt-oxygen (Co-O) sigma bonds protect Co against easy dissolution. Furthermore, Co-based metal oxides are prone to triggering OER through a kinetically favorable lattice oxygen mechanism (LOM), by sacrificing lattice oxygen stability. It seems that the simultaneous realization of both high activity and stability in Co-based catalysts remains elusive. Therefore, a strategic design of active sites that harness the stability of octahedral Co-O sigma bonds while facilitating faster reaction kinetics is imperative.

Considering the need for active and stable OER catalysts, inverse spinel oxides drew our attention. Inverse spinels have the +2 metal ions occupying the octahedral sites and forming strong sigma bonds with oxygen, while the +3 metal ions occupy both tetrahedral and octahedral sites. Our design principle was to promote the transformation of Co from tetrahedral to octahedral sites by incorporating a +3 metal, aiming to improve both activity and stability. As a proof-of-concept, we designed and prepared a series of Co and Ga-based mixed normal-inverse spinel structures, such as Co_1.8Ga_1.2O₄.^8,9 The site migration of Co²⁺ from T_d Co²⁺-O to O_h Co²⁺-O is supported by Co L-edge, soft X-ray Absorption spectra, and Raman spectra. Electroanalytical studies show a volcano relationship between overpotential and Co²⁺ concentration in the octahedral sites, and the OER activity reaches its apex when Co²⁺ and Co³⁺ have a similar proportion in octahedral sites. In situ Raman further confirms that a dual-site active center, which is identified as Co-O-Co, favors OER through a low energy barrier radical coupling pathway. Lattice oxygen exchange – prominent in Co₃O₄ – is suppressed to 1/6 in Co_1.8Ga_1.2O₄, in which major oxygen comes from the electrolyte (differential electrochemical mass spectra, DEMS). We report as a result an overpotential of 310 mV at 10 mA/cm², and document the durability at 200 mA/cm²in a single cell for 200 hours. Taking the strong anti-dissolution properties of Co in active Co_1.8Ga_1.2O₄mixed spinel, we report a PEMWE operating at 200 mA/cm²for 450 hours. The site occupancy modulation strategy reported in this work can motivate and encourage the succeeding design of materials for various applications.

Transforming Co₃O₄ into the inverse spinel by Ga³⁺ substitution

To implement the desired octahedral environment, we aimed to synthesize Co-based oxides with an inverse spinel crystal structure (Scheme 1).

We reasoned that incorporating a Group III metal could act as an inductor to preferentially occupy the tetrahedral sites, leaving Co²⁺ to reside in the desired octahedral sites⁹. We synthesized CoGa hydroxide precursor with different concentrations of Ga³⁺ using a co-precipitation method and then annealed the precursor in air at 450 ^oC for 1 hour to yield Co_xGa_(3−x)O₄.

An initial oxygen evolution reaction (OER) activity evaluation of Co-spinel oxides with different Co to Ga ratios was performed. The results in Supplementary Fig. 1 indicate a volcano-shaped trend in OER activity as a function of increasing Ga percentage, with Co_1.8Ga_1.2O₄ exhibiting the highest activity. We then investigated the structural and morphological characteristics of Co_1.8Ga_1.2O₄. X-ray diffraction (XRD) pattern and refinement in Fig. 1a show that Co_1.8Ga_1.2O₄ has a well-ordered cubic spinel crystal structure, with each diffraction peak located between the corresponding peak of Co₃O₄ (#65-3103) and CoGa₂O₄ (#11–0689). The lattice parameter of Co_1.8Ga_1.2O₄ (8.21 Å) also falls between those of Co₃O₄ (8.0837 Å) and CoGa₂O₄ (8.3250 Å), confirming a structural transformation from Co₃O₄ to CoGa₂O₄ upon Ga³⁺ substitution. The transmission electron microscopy (TEM) image in Fig. 1b exhibits the lattice fringe of Co_1.8Ga_1.2O₄, with a 0.245 nm fringe corresponding to the (311) facet, which is also the dominant exposed facet.

Raman spectroscopy was used to reveal the site occupancy change after Ga³⁺ substitution. The results in Fig. 1c show that the introduction of Ga³⁺ neither forms Ga₂O₃ nor brings additional peaks to the Raman spectra of Co-based spinel. However, the peak corresponding to the octahedral site in the spinel structure at 680 cm^− 1 undergoes a red shift upon the introduction of Ga^3 + 10, which can be attributed to the site transformation of Co²⁺ with a larger ionic radius compared to Co³⁺, indicating minor lattice expansion¹¹. As we increase the Ga³⁺ dopant concentration, the tetrahedron peak (at ~ 205 cm^− 1) shifts to higher wavenumbers, suggesting that the smaller Ga³⁺ preferentially occupy the tetrahedral sites, leading to lattice shrinkage¹².

Soft X-ray absorption spectroscopy (sXAS) was conducted to ascertain the Co site occupancy transformation by comparing the Co L-edge spectra for different Co-based materials. A peak centered at 776.1 eV is present only in Co_1.8Ga_1.2O₄, a feature that indicates the occupation of Co²⁺ at the octahedron site, akin to Co(OH)₂ and CoO^13–15. The spin state of the mixed Co_1.8Ga_1.2O₄ spinel, revealed by analyzing the branching ratio L₃/(L₂ + L₃) from Fig. 1d, confirms a higher spin state, which is positively correlated with the increased number of unpaired electrons (Supplementary Fig. 2) and potentially high redox activity in Co_1.8Ga_1.2O₄^16,17. We find a higher average coordination number for Co of 5.6 in the case of Co_1.8Ga_1.2O₄ compared to 5.3 in the case of Co₃O₄ normal spinel (K-edge in Fig. 1e and 1f). Furthermore, the Co_oh-Co_oh distance in Co_1.8Ga_1.2O₄ falls between those of Co₃O₄ and CoO, consistent with Co^2 + _Oh-Co^3 + _Oh. Additionally, X-ray photoelectron spectroscopy (XPS, Supplementary Fig. 3) reveals that the peak at 780.2 eV assigned to Co³⁺ diminishes as the Ga³⁺ ratio increases, while the peak at 782.2 eV assigned to Co²⁺ intensifies. This again indicates the partial replacement of Co³⁺ ions by Ga³⁺ ions at the octahedral sites.

In sum, Raman, sXAS, and XPS data collectively indicate a mixed CoGa spinel structure, where partial Co²⁺ transfers from tetrahedral sites to octahedral sites upon Ga³⁺ substitution.

Electrochemical investigations and performance

To investigate whether electrochemical behavior provides evidence of the structural transformations noted above, we used linear sweep voltammetry to analysis the Co site transformation in Co_xGa_(3−x)O_y in comparison to Co(OH)₂ and Co₃O₄. The oxidation peaks at 1.1, 1.4, and 1.6 V (vs RHE) (Fig. 2a) are attributed to Co_Oh^2+/3+, Co_Td^2+/3+, and Co_Oh^3+/4+, respectively¹⁸. With increased Ga³⁺, the oxidation of Co_Oh^3+/4+ shifts cathodically, consistent with electron transfer from Ga³⁺ to Co_Oh³⁺, a shift that produces electron-rich and easily oxidized Co_Oh^{3+ 19}. The oxidation peak of Co_Td²⁺ also shifts cathodically and becomes closer to the Co_Oh^2+/3+ oxidation peak in Co(OH)₂, which can be possibly attributed to the more exposed Co²⁺ in octahedral sites.

The lowest OER overpotential observed at a current density of 10 mA/cm², corrected for iR drop and normalized by electrochemical surface area (ECSA data is shown in Supplementary Fig. 4), is 310 mV (Fig. 2b). This lowest overpotential was achieved at 30% of Co²⁺ occupied octahedral sites, corresponding to Co_1.8Ga_1.2O₄. The integrated formula for this mixed spinel is (Co^2 + _0.4Ga³⁺_0.6)[Co^2 + _0.6Co³⁺_0.8Ga³⁺_0.6] (note that only half of the total Ga³⁺ will occupy the tetrahedral site⁸). The best-performing Co_1.8Ga_1.2O₄ also has a lower Tafel slope (57 mV/dec compared to 106 mV/dec, as shown in Supplementary Fig. 5) and a smaller charge transfer resistance than Co₃O₄ (Supplementary Fig. 6).

The OER stability (Fig. 2c) is significantly improved in Co_1.8Ga_1.2O₄ compared to Co₃O₄. Co_1.8Ga_1.2O₄ operates at 200 mA/cm² for 200 hours, whereas Co₃O₄ normal spinel operates only at 50 mA/cm² for 20 hours before exhibiting noticeable decay in OER performance. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) results in Supplementary Fig. 7 compare the dissolution amount of Co in normal and mixed spinel under the same working conditions, showing that Co in Co_1.8Ga_1.2O₄ dissolves at 1/6 the rate of Co₃O₄. Post-mortem characterization shows that the crystal structure, morphology and valence state of the Co_1.8Ga_1.2O₄ inverse spinel are well-maintained (Supplementary Fig. 8). Compared to previous reports of PGM-free acidic OER electrocatalysts, Co_1.8Ga_1.2O₄ exhibits a union of low overpotential at 10 mA/cm² and superior stability when considering both test time and current density^6,20–31 (Fig. 2d and Supplementary Table 1).

When incorporated into the proton exchange membrane water electrolysis (PEMWE) system (Fig. 2e and Supplementary Fig. 9), Co_1.8Ga_1.2O₄ operates stably for 200 mA/cm² for 450 h, while Co₃O₄ fails to operate after 25 hours. We sought to deconstruct the contributions to voltage from kinetics, ohmic loss, and mass transfer in the PEMWE system (Fig. 2g-2h) and investigate these components before vs. after a 12-hour runtime at 200 mA/cm². We found that both the kinetics and mass transfer overpotential show a negligible increase over this timeframe for both the reference and Co_1.8Ga_1.2O₄ inverse spinel catalysts, while the ohmic overpotential is significantly increased following operation in the case of the reference catalyst.

Taking into account the fast dissolution of Co from Co₃O₄ normal spinel, we propose that the dissolved Co ions, which can be adsorbed to the proton exchange membrane by electrostatic force between the cathode and metal cations and serve as the inhibitor to proton transfer, is the driving force of the increased ohmic overpotential. The removal of the increased ohmic overpotential by acid wash (Fig. 2i) confirms that Co₃O₄ suffered from this effect. In contrast, with Co_1.8Ga_1.2O₄, PEMWE exhibits consistent performance (Fig. 2j), and thus sees no benefit from acid wash, underscoring the critical role of stability of Co_1.8Ga_1.2O₄ against dissolution in maintaining the functionality of PEMWE.

Mechanistic study of Co-based metal oxide catalysts

The volcano shape of OER activity as a function of octahedrally-coordinated Co²⁺ mentioned above (Supplementary Fig. 3) prompts us to further investigate the origin of the optimal Co²⁺:Co³⁺ ratio.

We used in situ Raman spectroscopy to study OER intermediates. After background removal, Co-O vibration peaks at 210, 488, 520, and 685 cm^− 1, and peaks corresponding to H₂SO₄ at 980 cm^− 1 (S-O signal) are present across the applied potentials. When we increase the anodic potential to 1.35 V (vs RHE), two additional peaks centered at 926 and 1067 cm^− 1, corresponding to µ-OO peroxide (Co-OO-Co) and superoxide intermediates²⁰, appear in the Co_1.8Ga_1.2O₄ inverse spinel; while these features are absent in Co₃O₄ (Fig. 3a and 3b). This suggests a dual OER active site involvement in Co_1.8Ga_1.2O₄, wherein water deprotonation occurs on adjacent Co sites, enabling O-O radical coupling without the need for lattice oxygen participation or extra *OOH involvment³². The appearance of µ-OO peroxide intermediates in the Co_1.8Ga_1.2O₄ inverse spinel compared to the Co₃O₄ normal spinel is in line with the OER polarization curves that show higher OER activity in the case of Co_1.8Ga_1.2O₄ (Fig. 2).

Previous studies on cobalt hydroxides for alkaline OER reported that lattice oxygen participates in the OER process, resulting in excellent activity but sacrificing the structural stability of the Co-based electrocatalysts³³. To investigate the possibility of lattice O exchange during the OER in the present system, we carried out differential electrochemical mass spectroscopy (DEMS) and in situ Raman using H₂¹⁸O. From DEMS (Fig. 3c), we find that when using the Co₃O₄ catalyst and H₂¹⁸O, the molecular weight distribution of the evolved O₂ is: 47% m/z 32, 32% m/z 34, and 21% m/z 36. In contrast, when using the Co_1.8Ga_1.2O₄ catalyst, 84% of the evolved O₂ has a molecular weight of m/z 36. From these results, we estimate that the rate of lattice oxygen exchange in Co_1.8Ga_1.2O₄ is four times lower than in Co₃O₄.

Theoretical study of catalytic activity and stability of Co-based metal oxide catalysts

We first conducted density functional theory (DFT) calculations to study the enhanced catalytic activity (Fig. 4a,b) and underlying mechanisms of OER on (311) surface of Co_1.8Ga_1.2O₄ inverse spinel compared to Co₃O₄ normal spinel (Supplementary Fig. 10). We adopted a four-step adsorbate evolution mechanism (AEM) through the *OH, *O, and *OOH (Supplementary Figs. 11 and 12). Additionally, we integrated the insights from in situ Raman and DEMS results (Fig. 3) and considered a five-step oxide path mechanism (OPM). The OPM mechanism involves *OH, *OH-*OH, *OH-*O, and µ-OO peroxide (*O-*O) (Supplementary Figs. 13 and 14), and we compared its energetics with that of the conventional AEM. The calculated free energy diagrams of OER at 1.23 V vs RHE for different sites following the suggested reaction pathways are depicted in Fig. 4a,b. Specifically, for Co₃O₄, we determined that the potential-determining step (PDS) for AEM is the oxidation of *O to *OOH with a ΔG_max of 0.44 eV on the Co_Td²⁺ site. For comparison, we also included the free energy diagram of Co_1.8Ga_1.2O₄ which gives a significantly lower ΔG_max for OPM, underscoring the essential role of Ga atoms in facilitating the OER of Co₃O₄. On the Co_1.8Ga_1.2O₄ surface, the most thermodynamically favored mechanism is OPM, with the PDS being the first step of *OH formation on the Co_Oh²⁺–Co_Oh³⁺ site, exhibiting a ΔG_max of 0.34 eV. Our simulation results suggest that the catalytic activity and reaction pathway are dependent on the identity of Co sites in Co-based metal oxide catalysts.

We also performed DFT calculations to assess the energetics of Co dissolution and oxygen vacancy formation, to understand why Co_1.8Ga_1.2O₄ inverse spinel is more stable than Co₃O₄ normal spinel (Fig. 2c and 2e). The stability of a catalyst is determined by the surface dissolution rate (Fig. 4c), which includes metal (Co) dissolution, *OOH formation from the lattice oxygen (O_l) through H₂O deprotonation, and oxygen vacancy formation (O₂ release).³⁴ We found the deprotonation step to be the most thermodynamically unfavorable in both pathways on Co₃O₄ and Co_1.8Ga_1.2O₄ surfaces (Supplementary Figs. 15–17). As shown in Fig. 4d, we defined the experimental degradation rate, a metric that describes how fast a catalyst breaks down (degradation rate, µV h^− 1) under experimental conditions (a current density of 200 mA cm^− 2_geo), and confirmed its correlation with the reaction free energy of least thermodynamically favored step in the M and O dissolution pathway (ΔG^′_max). Additionally, we also calculated the oxidation states on both Co₃O₄ and Co_1.8Ga_1.2O₄ surfaces to determine the bonding strength. Compared to the Co₃O₄ surface, we observed that more electrons are transferred from Co to O on the Co_1.8Ga_1.2O₂ surface which is evidenced by the lower oxidation states on Co and O according to the Bader charge analysis, indicating a stronger Co–O bond (Fig. 4e). We further computed the project crystal orbital Hamilton population (− COHP) and its integrated value (ICOHP), to better understand the metal-oxygen bonding strength and adsorbate-surface interaction. As demonstrated in Fig. 4e, we found ICOHP values Co-O and Ga-O bonds on the Co_1.8Ga_1.2O₄ surface (Co₁–O₁: −1.09 eV, Ga₁–O₁: −2.51 eV) are more negative compared to the Co-O bonds on Co3O4 surfaces (Co₁–O₁: −1.07 eV, Co₂–O₁: −1.06 eV). We, therefore, conclude that the enhanced thermodynamic stability can also be ascribed to the stronger Ga-O bond compared to the Co-O bond.

In conclusion, this work reports the combination of good activity (310 mV overpotential at 10 mA/cm²) and increased stability (450 hours of PEMWE operation at 200 mA/cm²) in a PGM-free oxide catalyst, linked to the Co geometrical configuration and electronic structure. By incorporating Ga³⁺ to facilitate the transformation of Co²⁺ from tetrahedral to octahedral sites, the Co_1.8Ga_1.2O₄ mixed normal and inverse spinel, with its unpaired electrons, strong Co-O bond, and adjacent Co-Co dual active sites, promotes a favored OER pathway at its surface. Looking ahead, we expect that the consideration of the coordination environment, as demonstrated in this work, will open up a new strategy to rationally design active and stable catalysts for electrocatalysis.

Conflicts of Interest

The authors declare no competing interests.

Author Contributions

D.Z., J.Y., and J.T. designed and performed experiments. X.-Y.L. and P.O. performed the DFT calculations. D.Z., J.Y., and P.O. discussed the concept and analyzed the data. All the authors contributed to the draft, and D.Z., J.Y., J. T. and P.O. finalized the manuscript.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (21935001, 22175012), the Young Elite Scientists Sponsorship Program by CAST (2022QNRC001), the Program for Changjiang Scholars and Innovation Research Team in the University (No. IRT1205), the Fundamental Research Funds for the Central Universities, and Beijing Advanced Innovation Center for Soft Matter Science and Engineering is acknowledged as well. P.O. acknowledges the support from the National University of Singapore Presidential Young Professorship start-up grant and white space funding.

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Chemicals. Cobalt(II)-chloride hexahydrate, gallium nitrate, sodium hydroxide, sodium carbonate were purchased from Sigma-Aldrich. Platinized titanium felt (592799) were purchased from the Fuel Cell Store. All chemicals were used without further purification. Millipore water (18.2 MΩ cm) was used in all experiments.

Synthesis of Co₃O₄ and Co_xGa_(3-x)O_y catalysts

A co-precipitation method is used to synthesize the precursors of Co₃O₄ and Co_xGa_(3-x)O_y. A solution contains Co, or CoGa cations, and the B solution contains NaOH and Na₂CO₃. Then solution A and B were mixed, and the pH was kept at 9. The mixed solution was stirred at 50^oC, open air condition for 12 hours, then the solution was centrifuged. The products were washed by water for two time, ethanol for one time, the the products were dried at 80^oC overnight, The pink products were annealed in air at 450^oC for 1h to obtain Co₃O₄or Co_xGa_(3-x)O_y.

Characterization. The surface scanning electron microscopy (SEM) images of thin-film samples were acquired using a Hitachi SU8230 scanning electron microscope operated at 5.0 kV. The physical phases and structure of the catalysts were determined by analysing the X-ray diffraction (XRD) data collected with a point step of 0.02 degree using a Bruker D8 using Cu-Kα radiation (λ = 0.15406 nm). The collected data were analyzed using Jade 6 software. X-ray photoelectron spectroscopy (XPS) studies were conducted using a Thermo Scientific K-Alpha equipped with an Al Kα X-ray source (1486.6 eV) for excitation. The background pressure is 10^-6 Torr. Survey scans were collected at 1 eV resolution. The peak energies were calibrated against the binding energy of the adventitious C 1s peak, which was set at 284.8 eV. The surface elemental ratios were calculated using the Thermo Avantage Software package. The atomic composition of samples and Co leaching concentration were determined using an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-AES).

The sXAS measurements were performed using Beamline 6.3.1.1 at the Advanced Light Source (Berkeley, CA). X-ray absorption fine structure (XAFS) of Co K-edge were acquired at the 8C nanoprobe XAFS beamline (BL8C) of Pohang Accelerator Laboratory (PAL) on a 3.0 GeV storage ring with 250 mA. The X-ray beam was monochromated by a Si(111) double crystal where the beam intensity was reduced by 30% to eliminate the higher harmonics. An energy calibration was conducted before the measurement using reference foil.

In-situ Raman measurements were carried out with a Renishaw inVia Raman spectrometer in a modified single cell and a water immersion objective (×63) with a 785 nm laser. Pt wire and saturated Hg/Hg₂SO₄ (saturated K₂SO₄ is electrolyte) were used as the counter and reference electrodes, respectively. The as-prepared Co-based catalysts were used as the working electrode. 0.5 M H₂SO₄ was used as the electrolyte.

Operando DEMS with isotope labelling. The operando DEMS system The catalyst ink was directly dropped into the Au film and then dried. The electrochemical cell is a typical three-electrode system, and its volume is around 3 ml. The small volume is suitable for the isotope experiments. The gaseous products including ³²O₂, ³⁴O₂ and ³⁶O₂ were monitored by the mass spectrometer. For isotope labelling studies, 2 ml of 0.5 M H₂SO₄ was prepared using H₂¹⁸O as the solvent. The catalysts including CoGa mixed spinel and the reference Co₃O₄ normal spinel were subjected to three LSV cycles in the potential range of 1.17-1.72 V versus RHE at a scan rate of 10 mV/s, while the mass signals of the gaseous products ³²O₂, ³⁴O₂ and ³⁶O₂ were recorded. Before the electrochemical measurements, all the electrolytes were purged with high-purity Ar to remove the dissolved oxygen.

The dissolution of cations was tested by ICP-AES. Calibration solutions were prepared by diluting the Co standard solutions for ICP with 10% nitric acid. The electrodeposited catalyst samples were dissolved into 10 ml of fresh aqua regia and microwave digestion. Then 1 ml of sample solution was taken out and diluted it to 10 ml to test the elemental ratio. In order to test the leaching Co and Ga concentration from the catalysts, we run the acidic OER stability test in 35 ml electrolyte. We took 1 ml of electrolyte out during the stability test and diluted it to 10 ml in a separate centrifuge tube to test the concentration.

Electrochemical characterization. Electrochemical data were collected using a three-electrode system connected to an electrochemical workstation (Autolab PGSTAT302N) with a built-in electrochemical impedance spectroscopy (EIS) analyser. 5 mg catalysts, 10 ul Nafion ionomer and 1 ml ethanol were mixed together. The catalysts ink was sonicated until a homogeneous ink was prepared. 200 ul ink was drop-coated on a 5 mm*5mm Ti felt and it was used as the working electrode. Ag/AgCl and a carbon rod were used as the reference and the counter electrodes, respectively. Glass electrochemical cells were cleaned with aqua regia prior to use. 35 ml of 0.5 M H₂SO₄ was used as the electrolyte and the stirring rate was kept constant at 400 r/min. Cyclic voltammetry (CV) measurements at 50 mV/s were performed for 5 cycles prior to recording linear scan voltammetry (LSV) at 5 mV/s for each sample. Each sample was analysed through at least three independent measurements. Electrochemical impedance spectroscopy data were collected in a static solution at 1.5 V (vs. RHE) with a frequency scan range from 100 kHz to 0.01 Hz. EIS spectra were analysed using the Voigt circuit model in Nova software package. Unless otherwise stated, all experiments were performed at ambient temperature (22 ± 2 °C), and electrode potentials were converted to the RHE scale using .

A commercial PEM system (purchased from Dioxide Materials) was used to carry out the water splitting reaction at 0.2 A/cm². The OER catalyst substrate was Ti fibre with a loading of 2 mg/cm², and Pt/C-20 wt% deposited onto carbon paper (loading is 1 mg/cm²) was used as the HER catalyst. The Nafion 212 cation exchange membrane (purchased from Fuel Cell Store) was used as the ion exchange membrane. During the test, the cell was maintained at 80 ℃, and the pre-heated electrolyte was fed to the anode at a flow rate of 40 ml/ min.

Electrochemical active surface area (ECSA). The ECSA of each catalyst was determined by measuring the electrochemical double-layer capacitances (C_dl) from the scan rate CV-dependence plot. The CV cycle potential window was 0.7 to 0.9 V vs. RHE, and the scan rates were 10, 20, 30, 40, and 50 mV/s. The C_dl was estimated at the average potential in the selected range against the scan rates. The slope of the linear fit was calculated as the C_dl. A specific capacitance of 40 μF/cm² was used here. The ECSA of the catalyst is calculated from the following equation,

IR correction. The correction was done according to the given equation:

where E_Corrected is the iR-corrected potential, E is the experimentally measured potential and R is the series resistance of measurement.

Density functional theory (DFT) calculations. Spin-polarized density functional theory (DFT) calculations were performed with a periodic slab model using the Vienna Ab initio Simulation Package (VASP)^35,36. The generalized gradient approximation (GGA)³⁷ was adopted with the Perdew–Burke–Ernzerhof exchange-correlation functional to describe the behaviour of electrons and nuclei. The projector-augmented wave method³⁶ was utilized to describe the electron-ion interactions, and the cut-off energy for the plane-wave basis set was 400 eV. All geometries were optimized using a quasi-Newton (variable metric) algorithm³⁸. The Monkhorst-Pack k-points of 1 × 2 × 1 were applied for all the surface optimization and electronic structure calculations for the spinel (311) surfaces with the convergence of force and energy set to 0.05 eV Å⁻¹ to 10⁻⁶ eV. In addition, the effects of the Hubbard U corrections³⁹ were considered beyond the accuracy of the DFT calculations of GGA, where U values (employed as U − J) of 3.32 were applied for Co, whereas 0 was applied for Ga and O due to their non-magnetization.

To determine the thermodynamically stable structure, spin polarization calculations were first performed to obtain the initial magnetic moment, and 31 different spin states near the initial magnetic moment were considered by varying the NUPDOWN parameter in the VASP.^20,40 This approach was used consistently to find the most stable structure in the calculations of intermediates in AEM and OPM as well as the dissolution pathways.

The potential-determining step (PDS) of AEM and OPM was determined by calculating the highest reaction free energy (ΔG_max) for the most thermodynamic unfavoured step in the OER process. Similar dissolution pathways were taken from ref. 34 and computed to determine the theoretical stability, described by the reaction free energy of the most thermodynamically unfavourable step (ΔG^′_max).

Scheme 1 is available in the Supplementary Files section.

There is NO Competing Interest.

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Octahedral Co2+-O-Co3+ in mixed cobalt spinel promotes active and stable acidic oxygen evolution

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Abstract

Figures

Introduction

Results

Transforming Co₃O₄ into the inverse spinel by Ga³⁺ substitution

Electrochemical investigations and performance

Mechanistic study of Co-based metal oxide catalysts

Theoretical study of catalytic activity and stability of Co-based metal oxide catalysts

Declarations

References

Materials and Methods

Scheme

Additional Declarations

Supplementary Files

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Version 1

Octahedral Co2+-O-Co3+ in mixed cobalt spinel promotes active and stable acidic oxygen evolution

Status:

Version 1

Abstract

Figures

Introduction

Results

Transforming Co3O4 into the inverse spinel by Ga3+ substitution

Electrochemical investigations and performance

Mechanistic study of Co-based metal oxide catalysts

Theoretical study of catalytic activity and stability of Co-based metal oxide catalysts

Declarations

References

Materials and Methods

Scheme

Additional Declarations

Supplementary Files

Status:

Version 1

Transforming Co₃O₄ into the inverse spinel by Ga³⁺ substitution