Short O–O separation in layered oxide Na_0.67CoO₂ enables an ultrafast oxygen evolution reaction

Significance

The development of a low-cost, stable, and more active electrocatalyst for the oxygen evolution reaction (OER) is critical for the practical storage of electric power in hydrogen gas produced by the electrolysis of water. We demonstrate a stable OER of higher rate at a lower voltage with a low-cost oxide that provides, as an alternative to the conventional reaction route, a faster route that is greatly enhanced by an unusually short O–O separation.

Abstract

The layered oxide Na_0.67CoO₂ with Na⁺ occupying trigonal prismatic sites between CoO₂ layers exhibits a remarkably high room temperature oxygen evolution reaction (OER) activity in alkaline solution. The high activity is attributed to an unusually short O–O separation that favors formation of peroxide ions by O⁻–O^– interactions followed by O₂ evolution in preference to the conventional route through surface O–OH^– species. The dependence of the onset potential on the pH of the alkaline solution was found to be consistent with the loss of H⁺ ions from the surface oxygen to provide surface O⁻ that may either be attacked by solution OH⁻ or react with another O⁻; a short O–O separation favors the latter route. The role of a strong hybridization of the O–2p and low-spin Co^III/Co^IV π-bonding d states is also important; the OER on other Co^III/Co^IV oxides is compared with that on Na_0.67CoO₂ as well as that on IrO₂.

Highly active oxygen evolution reaction (OER) catalysts with a long-term stability are required to reduce the energy loss, increase the rate performance, and improve the cycling stability of different energy conversion and storage systems, particularly in electrochemical water electrolysis and rechargeable metal–air aqueous electrolyte batteries (1–4). The most active OER catalyst, IrO₂, is expensive and shows a high overpotential of 0.3 V at 10 mA⋅cm⁻²; moreover, it is unstable at the applied potential in an alkaline electrolyte, which degrades its activity and limits its application (5–7).

Low-cost transition metal (TM) oxides are promising candidates for OER catalysts (8–17). AMO₃ perovskites with controllable chemical compositions and electronic structures by substituting the cations on A and M sites have been extensively investigated as OER catalysts (18–22). Some perovskite oxides (e.g., Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ, Hg₂Ru₂O₇, and CaCu₃Fe₄O₁₂) exhibit comparable or even better OER catalytic activity than that of IrO₂ (3, 5, 23). However, the obtained OER descriptor of e_g ≈ 1 on the d-orbital manifold of Mⁿ⁺ ions in AMO₃ perovskites for their excellent OER activity is challenged by the good OER on 4d/5d TM oxide catalysts with zero antibonding e_g electrons such as IrO₂ and Hg₂Ru₂O₇ (24–27). In addition to the OER activity, the stability and the preparation condition of the catalysts are 2 other critical parameters for their large-scale application. For example, the crystalline Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ phase becomes amorphous after OER testing in an alkaline electrolyte, and the stable Hg₂Ru₂O₇ and CaCu₃Fe₄O₁₂ catalysts with high valence Ru⁵⁺ and Fe⁴⁺ ions can only be prepared at an extremely high pressure of 6 and 15 GPa, respectively (23). The instability of TM oxides originates from cations that can dissolve into the alkaline solution and from lattice oxygen loss during the OER (28). Increasing the Mⁿ⁺–O^2– bond strength of oxides helps to improve their stability. Therefore, developing an easily prepared, efficient, and durable OER catalyst based on Earth-abundant elements is still a challenge.

We have recently studied the onset potentials and OER activities of 2 cubic perovskites, CaCoO₃ and SrCoO₃, prepared under high pressure (29); both have Co^IV ions having similar intermediate spin states t⁴σ*¹, but CaCoO₃ had a significantly shorter Co^IV–O bond (1.87 Å) and showed a higher OER activity than SrCoO₃ as a result of its reduced lattice parameter. After surface deprotonation at a charging potential V_ch = V_on, where V_on was the onset potential, the surface Co^IV–O^– were attacked by solution OH⁻,

$$\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}-{\mathrm{O}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{-}=\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}\mathrm{OO}{\mathrm{H}}^{-},$$ [1]

but the competitive reaction

$$2\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}-{\mathrm{O}}^{-}+\mathrm{O}{\mathrm{H}}^{-}=\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}{\left({\mathrm{O}}_2\right)}^{2-}+\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}\mathrm{O}{\mathrm{H}}^{-}$$ [2]

followed by

$$2\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}{\left({\mathrm{O}}_2\right)}^{2-}-2{\mathrm{e}}^{-}=2\mathrm{C}{\mathrm{o}}^{\mathrm{IV}}{\mathrm{O}}^{-}+{\mathrm{O}}_2\uparrow$$ [3]

was much stronger on CaCoO₃ because of its much smaller lattice parameter, which reduced the surface O–O separation. The O⁻ + O⁻ = (O₂)^2– reaction is faster the shorter the O–O separation of the catalyst. In the conventional reaction route, the O⁻ ion is attacked by a solution OH⁻ to form OOH⁻, and the OOH⁻ + OH⁻ = (O₂)^2– + H₂O; this mechanism is independent of the O–O separation.

These findings recommended to us a search for stable oxides with a shorter surface oxygen separation that can be prepared at ambient pressure. Therefore, we prepared a metallic layered oxide Na_0.67CoO₂ with low-spin Co^III/IV ions (Co^III: π*⁶σ*⁰; Co^IV: π*⁵σ*⁰) and a much shorter O–O separation than on CaCoO₃ to study further the influence of zeta potential, surface oxygen separation, and the pH of the aqueous medium on the onset potential and OER activity. Metallic, low-spin Na_0.67CoO₂ has a strong Co^III/IV–O interaction as a result of empty σ-antibond e_g orbitals and itinerant π-bonding electrons, which reduces the charge transfer resistance during the OER; Na_0.67CoO₂ has a zero zeta potential at pH = 4 and shows a pH-dependent onset potential for the OER consistent with the potential for the CoOH⁻ – e^– + OH⁻ = CoO⁻ + H₂O reaction.

Results

The Crystal and Electronic Structure of Na_0.67CoO₂.

Synchrotron powder X-ray diffraction (XRD) and powder X-ray diffraction patterns of as-prepared Na_0.67CoO₂ (Fig. 1 and SI Appendix, Fig. S1) confirm the layered structure of Na_0.67CoO₂. Two-dimensional CoO₂ layers with edge-sharing CoO₆ octahedra in trigonal Na_0.67CoO₂ (space group: R-3c) are separated by sodium layers. The Na⁺ occupy 3 different trigonal prismatic sites, Na1 and Na2 (Na2a and Na2b) in SI Appendix, Table S1; each shares 6 edges and 2 triangular faces along the c axis with the CoO₆ octahedra vertically above and below. The Na1 ions are energetically less favorable because of the coulombic repulsions from 2 Co ions in face-sharing octahedra. The Co ions of Na_0.67CoO₂ have 4 different positions; the distorted Co–O octahedra have a similar average Co–O bond distance, but one shortest O–O separation (2.30 Å; Fig. 1F) than that of cubic CaCoO₃ (O–O: 2.64 Å) in which the Co^IV have an intermediate spin state (t⁴σ*¹). About 0.3 wt% Co₃O₄ impurity was found to exist in the sample by refining the synchrotron data. The Na_0.67CoO₂ sample had an average particle size of 15 μm (SI Appendix, Fig. S3), and the energy dispersive spectroscopy (EDS) mapping revealed a uniform distribution of Na, Co, and O elements.

Fig. 1.

Crystal structure and magnetic and transport properties of Na_0.67CoO₂. (A) Observed, calculated patterns, and their difference for the Rietveld refinement of the synchrotron XRD of Na_0.67CoO₂; (Inset) its crystal structure (pink, Na; blue, Co; red, O). (B and C) TEM images of Na_0.67CoO₂ before and after OER measurements. (D) Temperature dependence of resistivity and magnetization. (E) Electronic spin states of Co^IV ions and schematic band diagrams of Na_0.67CoO₂. (F–I) The O–O bonds shorter than (2.4 to 2.64) Å are highlighted with thick red lines.

The electronic conductivity and magnetic properties of Na_0.67CoO₂ were investigated to determine the d-electron configuration of the Co^III/IV ions (Fig. 1D). The temperature dependence of resistivity of Na_0.67CoO₂ shows a metallic behavior down to 2 K, with a large residual resistance ratio; a nonlinear magnetic susceptibility curve from 2 to 300 K is also consistent with itinerant d electrons. A spin-polarized first-principle calculation was performed, and the density of states of Co ions from Na_0.67CoO₂ is shown in Fig. 1E. The conduction band of Na_0.67CoO₂, which contains one spin component of t_2g orbitals, is not full, indicating a metallic conductivity of Na_0.67CoO₂. The higher electronic conductivity of Na_0.67CoO₂ reduces the charge transfer resistance (R_ct) and the ohmic potential drop of the OER; the R_ct of Na_0.67CoO₂ is one order of magnitude smaller than that of the electronic insulator Co₃O₄, which is above 1.6 V (SI Appendix, Fig. S4).

Soft X-ray absorption spectroscopy (sXAS), which is quite sensitive to the TM 3d states because of the strong dipole-allowed 2p–3d (L edge) excitations (30–33), was performed to explore the valence and spin states of the Co ions of Na_0.67CoO₂ (Fig. 2). With a probe depth of 10 nm, the total electron yield (TEY) mode of sXAS is quite an effective surface-sensitive probe, as shown in Fig. 2 A and B. Due to the core-hole spin–orbit coupling, the Co L edge in Fig. 2A is well separated into 2 branches, that is, L₃ (between 774 and 787 eV) and L₂ (between 788 and 799 eV). The branching ratio of the L₃ and L₂ edges, which is greatly affected by the spin–orbit coupling and especially by the 2p–3d multiplet effects, can be utilized to deduce the spin state of Co (34, 35). Quantitatively, for the statistical ratio of the integrated intensity of 2 edges, namely I_L3/(I_L3 + I_L2), the high-spin state has a higher one than the low-spin state. Respectively for the samples before and after catalysis reaction, the statistical branching ratio of the Co L inverse partial fluorescence yield (iPFY) is 0.689 ± 0.005 and 0.686 ± 0.005. Based on the atomic multiplet calculations (36, 37), the standard spectra of LS Co within the octahedral (CoO₆) structure is achieved as shown by dashed lines in Fig. 2 A and B, and that of HS is done as shown in SI Appendix, Fig. S6. While the calculated spectrum of LS Co (either Co³⁺ or Co⁴⁺) has a branching ratio of 0.698 ± 0.005, very close to the experimental Co L iPFY, that of HS has a much higher one of 0.780 ± 0.005, suggesting that Na_0.67CoO₂ has a low-spin state (Co^III: t⁶e⁰, Co^IV: t⁵e⁰). The density of states of Co ions from Na_0.67CoO₂, which is shown in Fig. 1E, also indicates low-spin Co^III/IV ions. The valence state of the Co ions can be inferred by the Co L₃ peak position. As shown in Fig. 2B, the peak A located at 777.6 eV, which is 0.2 eV from the standard Co³⁺ (777.4 eV) and 0.4 eV from the standard Co⁴⁺ (778.0 eV). This position indicates that the valence of the Co ions is about 3.3+, consistent with the expected stoichiometry as prepared.

Fig. 2.

The absorption profiles of Co^III and Co^IV in Na_0.67CoO₂ probed by sXAS and mRIXS. (A and B) The sXAS TEY of (A) Co L edge and (B) magnified L₃ edge obtained with a surface probe having a probe depth of 10 nm. (C and D) The mRIXS iPFY of (C) Co L edge and (D) magnified L₃ edge by a bulk probe with a probe depth of 100 nm.

In addition, we demonstrate the bulk information as a supplement. Resulting from the self-absorption and saturation effects (38, 39), the bulk probe of sXAS, that is, total fluorescence yield mode with a probe depth of 100 nm (40), provided a noisy lineshape and an unreliable L₂/L₃ intensity ratio as shown in SI Appendix, Fig. S5. The iPFY is theoretically an undistorted absorption profile that can be extracted from the map of resonant inelastic X-ray scattering (mRIXS). In this work, we performed the Co L mRIXS measurement on Na_0.67CoO₂, and achieved the iPFY for characterizing the bulk Co status. (For the convenience of the reader, we introduce how to extract the Co L iPFY from the mRIXS in SI Appendix, Fig. S5.) As shown in Fig. 2 C and D, the Co L iPFY spectra demonstrate a consistent lineshape with the TEY spectra, indicating that Co ions in the bulk of Na_0.67CoO₂ present the same low-spin electronic and 3.3+ valence state as those on the surface.

The OER Activity of Na_0.67CoO₂.

The OER performance of Na_0.67CoO₂ was compared with that of rutile IrO₂, spinel Co₃O₄, and layered Co(OH)₂ (Fig. 3). The current densities of all of the samples were normalized to the electrochemically active surface area to exclude geometric effects (SI Appendix, Fig. S7 and Table S2). The catalysts with different particle size have similar OER activities when the current density is normalized to the electrochemical surface area or the surface area confirmed by Brunauer–Emmett–Teller measurement (41). Na_0.67CoO₂ with an onset potential of 1.5 V vs. a reversible hydrogen electrode (RHE) had the smallest overpotential (0.29 V) at 10 mA cm⁻² and the highest current density at voltages above 1.6 V; the layered Co(OH)₂ and spinel Co₃O₄ exhibited a negligible catalytic current density compared with Na_0.67CoO₂. The smallest Tafel slope of 39 mV⋅dec⁻¹ (Fig. 3C) for Na_0.67CoO₂ also indicates its excellent OER kinetics. The layered Li_1-xCoO₂ has a structure and Co^III/IV–O bond similar to that in Na_0.67CoO₂, but the Li⁺ are in octahedral sites and no O–O separation is reduced; it has a much higher onset potential and a smaller OER current density than Na_0.67CoO₂ (6).

Fig. 3.

OER performance of Na_0.67CoO₂, IrO₂, Co(OH)₂, and Co₃O₄. (A) Linear sweep voltammograms at 1,600 revolutions per minute. in 0.1 M KOH. (B) The overpotential at 10 mA⋅cm⁻² (dashed line). (C) Tafel plots. (D) The chronoamperometric curves in an O₂-saturated 0.1 M KOH electrolyte at 1.6 V vs. RHE and the CV curves of first, 500th, and 1,000th cycles (Inset).

The Surface Charge Density of Na_0.67CoO₂.

The mean surface charge density of Na_0.67CoO₂ (Fig. 4A) was evaluated by its zeta potential in water with different pH. Na_0.67CoO₂ has a zero zeta potential at pH = 4; the strong covalence of the Co ^III/IV–O bond of Na_0.67CoO₂ makes it more acidic than the spinel Co₃O₄, which has a weaker Co^II/III–O bond and a high zero-zeta potential at pH = 7.5. IrO₂ with a strong Ir^IV–O bond has a similar zeta potential curve to that of Na_0.67CoO₂, indicating analogous acidity of the surface states and pH influence on their OER.

Fig. 4.

The pH-dependent OER behavior of Na_0.67CoO₂. (A) Zeta potential of the catalysts. (B and C) CV measurements of Na_0.67CoO₂ in O₂-saturated KOH with pH 12.5 to 14. Inset shows the enlarged CV part from 1.3 to 1.6 V. (D) The slope change of the linear CV curves at voltages above 1.7 V in B and C and SI Appendix, Fig. S8 with different pH.

The onset potential and activity of Na_0.67CoO₂ at different pH are compared with those of Co₃O₄, Co(OH)₂ and IrO₂ in Fig. 4 and SI Appendix, Fig. S8. The oxidation voltage of surface Co^III to Co^IV and the onset potential of Na_0.67CoO₂ were reduced with increasing pH (Fig. 4B) because of the easier deprotonation process at higher pH; the activities of Na_0.67CoO₂ and IrO₂ show a similar pH-dependent behavior on the RHE scale, and their OER currents increase significantly at high pH; however, both Co₃O₄ and Co(OH)₂ have no significant OER current increase. The slope of all CV curves of these catalysts at voltages above 1.6 V increases exponentially with pH (Fig. 4D), and the most active Na_0.67CoO₂ has the biggest slope of all of the catalysts at different pH. Both Na_0.67CoO₂ and IrO₂ have a much larger slope change than Co₃O₄ and Co(OH)₂, indicating different OER mechanisms in these catalysts.

The OER Stability of Na_0.67CoO₂.

Na_0.67CoO₂ shows an excellent OER stability in an O₂-saturated electrolyte with pH = 13 (Fig. 3D). More than 90% of its initial current density after 20,000 s was retained; and, after 1,000 cyclic voltammetry (CV) cycles, Na_0.67CoO₂ showed almost the same OER activity. The transmission electron microscopy (TEM) image (Fig. 1C) and XRD (SI Appendix, Fig. S1) results confirmed the same crystalline structure of bulk and surface Na_0.67CoO₂ before and after OER testing. The X-ray photoelectron spectroscopy (XPS) spectra of fresh Na_0.67CoO₂, Na_0.67CoO₂ soaked in KOH for a week, and Na_0.67CoO₂ after 1,000 CV cycles are shown in SI Appendix, Fig. S9; the Na, Co, and O peaks of Na_0.67CoO₂ after cycling are much weaker because of the covering Nafion binder on the particle surface; all of these XPS peaks retain the same positions, verifying the good stability of Na_0.67CoO₂ during OER. Na_0.67CoO₂ shows a stable peak position in sXAS, and the valence state of Co sXAS remains unchanged in the Na_0.67CoO₂ powder before and after OER (Fig. 2). The strong Co–O bonds of Na_0.67CoO₂ increase its stability in alkaline solution.

The Surface of Na_0.67CoO₂ after OER.

Because surface oxygens of Na_0.67CoO₂ participate in the OER, bulk lattice oxygen can diffuse to the surface oxygen vacancies after O₂ gas release and capture a proton from solution before the solution OH⁻ enters, and then the vacancies will be generated on these oxygens. The Co and O ions of a minimum 14-nm-thick surface of perovskite SrCoO_3-x have been reported to participate in the OER (42).

Time-of-flight secondary ion mass spectrometry (TOF-SIMS), which is an ultrahigh elemental and surface sensitive technique, was employed to study whether any chemical composition change occurs on the surface of Na_0.67CoO₂ before and after OER testing. Given the destructive nature of TOF-SIMS, all ionized fragments detected imply chemical bonding between the fragment elements prior to sputtering (43). TOF-SIMS depth profiling and high-resolution mapping were used to show the presence of CoOH and CoO₂H on the surface of the Na_0.67CoO₂ particles following OER (Fig. 5 and SI Appendix, Fig. S10). Due to the naturally high surface corrugation of Na_0.67CoO₂, both CoOH⁻ and CoO₂H⁻ secondary ion depth profiles were normalized by the corresponding Co⁻ profile in each sample to account for the topography changes between the Na_0.67CoO₂ surfaces before and after OER. As a proxy for bulk Na_0.67CoO₂, the Co⁻ signal was selected for normalization. Finally, we used the ratio between the Co⁻–normalized CoOH⁻ and CoO₂H⁻ profiles before and after OER to demonstrate their surface localization after the OER in a ∼70-nm layer; the peak position of this ratio in Fig. 5 A and B provided the localization. In comparison, the CoO⁻/Co⁻ profile appears less enhanced at the surface, which suggests the OER produces only a limited amount of CoO at the Na_0.67CoO₂ surface (Fig. 5C). However, given the natural fragmentation of Na_0.67CoO₂ upon sputtering, the CoO⁻ signal could also be used as a marker for the bulk Na_0.67CoO₂. As such, TOF-SIMS high-resolution mapping of CoO⁻ and CoOH⁻ secondary ion fragments was performed to show directly the formation of CoOH at the surface of Na_0.67CoO₂ (Fig. 5D). Indeed, deep sputtering of the Na_0.67CoO₂ particles confirms the continuous presence of CoOH at their surface, as presented in Fig. 5D in the dual color overlay as a function of depth, that is, of Cs⁺ sputtering time. The Na/Co ratio of Na_0.67CoO₂ was confirmed, by inductively coupled plasma mass spectrometry, not to change from before to after OER testing (SI Appendix, Table S3).

Fig. 5.

TOF-SIMS depth profiling and high-resolution mapping of Na_0.67CoO₂ particles before and after OER. (A–C) TOF-SIMS depth profiling of Na_0.67CoO₂ before and after cycling. The intermediates CoOH⁻, CoO₂H⁻, and CoO⁻ increase after cycling. (D) Dual color overlays of high-resolution maps of the CoO⁻ and CoOH⁻ secondary ion signals demonstrating that CoOH⁻ mainly exists on the surface of the catalyst particles.

Discussion

In air, an exposed surface cation M of an oxide, especially one with a strong octahedral site preference energy, attracts water to complete its oxygen coordination; the hydrogen atoms of the adsorbed water H₂O are dispersed over the surface oxygen to create surface M–OH^–. In alkaline solution, an exposed surface cation attracts solution OH⁻ when it is oxidized during charge. Continuing removal of electrons from the oxide during charge in KOH solution induces the reaction (Fig. 6A)

$$\mathrm{M}-\mathrm{O}{\mathrm{H}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{\mathrm{-}}=\mathrm{M}-{\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}\mathrm{.}$$ [4]

This reaction occurs at a critical potential V_c ≤ V_on, where V_on is the onset potential for the OER to occur with continuing charge; the resulting MO^– may be attacked by the solution OH⁻,

$$\mathrm{M}-{\mathrm{O}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{-}=\mathrm{M}-\mathrm{OO}{\mathrm{H}}^{-},$$ [5]

and the subsequent OER activity then depends on the relative rates of removal of the H⁺,

$$\mathrm{M}-\mathrm{OO}{\mathrm{H}}^{-}-2{\mathrm{e}}^{-}+2\mathrm{O}{\mathrm{H}}^{-}=\mathrm{MO}{\mathrm{H}}^{-}+{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\uparrow ,$$ [6]

and displacement of the O₂H⁻ by a solution OH⁻. An O₂H⁻ can be an unwanted by-product of reaction 5.

Fig. 6.

Competing OER mechanisms on Na_0.67CoO₂. OER mechanism with surface lattice oxygen activated for OER to form peroxide either (A) in a CoO₂ layer or (B) between 2 neighboring CoO₂ layers.

Alternatively, 2 neighboring surface O⁻ may react with one another (Fig. 6B),

$$2\mathrm{M}{\mathrm{O}}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{-}=\mathrm{M}{\left({\mathrm{O}}_2\right)}^{-}+\mathrm{MO}{\mathrm{H}}^{-}$$ [7]

followed by

$$\mathrm{M}{\left({\mathrm{O}}_2\right)}^{-}-{\mathrm{e}}^{-}+\mathrm{O}{\mathrm{H}}^{-}=\mathrm{MO}{\mathrm{H}}^{-}+{\mathrm{O}}_2\uparrow .$$ [8]

The rate of reaction 7 competes with that of reaction 5; it increases strongly with decreasing surface O–O separation. Therefore, the activity of the OER above the onset potential depends strongly on the surface O–O separation. The observation of an ultrafast OER in Na_0.67CoO₂ with an unusually short O–O separation demonstrates this dependence. A layer oxide Na_xCoO₂ with x = 0.52, 0.65, and 0.75 has a different crystal structure (space group: P63/mmc) with our Na_0.67CoO₂ (R-3c); all of these 3 materials show almost the same OER activity after cycling. Our Na_0.67CoO₂ shows a much higher OER activity than the reported Na_xCoO₂; the overpotential at 10 mA⋅cm⁻² of our Na₀.₆₇CoO₂ is 0.29 V, which is much smaller than their Na_xCoO₂ (0.45 to 0.47 V) (44). In Na_xCoO₂ with space group P63/mmc, for example, Na_0.65CoO₂, Na and Co ions occupy 2 and 1 different positions, respectively; however, there are 3 Na and 4 Co positions in Na_0.67CoO₂ with space group R-3c. The O–O separation in Na_0.65CoO₂ is 2.57 Å, while the O–O separation of our Na_0.67CoO₂ is much smaller (the shortest O–O is 2.30 Å); the large O–O separation difference is caused by the different crystal structure and Na ordering. Their results also well support our conclusion that short O–O separation determines the OER activity. The relationship between the catalytic performance and the O–O bond length in Na_xCoO₂ and Li_xCoO₂ with different Na⁺ and Li⁺ proportion according to the previous report is shown in SI Appendix, Fig. S2. The shorter the O–O bond length in these oxides, the smaller the overpotentials at 5 mA⋅cm⁻², which confirms the key role of the short O–O separation in the OER performance of the Na_0.67CoO₂.

Reaction 4, which sets the onset potential, depends on the zeta potential of the oxide and the pH of the solution. Reaction 4 is favored the higher the pH of the solution and the greater the acidity of the oxide. The stronger the M–O bond, the more acidic is the oxide. The existence of itinerant electrons in π-bonding orbitals of d-wave symmetry not only lowers the resistance to the OER, but also testifies to a strong O-2p hybridization in the π-bonding as well as σ-bonding orbitals of d-wave symmetry.

Summary

The excellent OER activity of Na_0.67CoO₂ is the result of a short O–O separation that increases the rate of reaction 7 relative to reaction 5 and demonstrates the superior rate capability of reaction 7. The stability of the catalyst is the result of strong Co–O σ-bonding with the Co^IV configuration on π*⁵σ*⁰.

Materials and Methods

Na_0.67CoO₂ was prepared by a typical solid-state reaction with analytical grade Na₂CO₃ and Co₃O₄ as raw materials. First of all, the mixture of Na₂CO₃ and Co₃O₄ in a stoichiometric ratio with an excess of 5 mol% Na₂CO₃ was thoroughly ground in agate bowls, pressed into pellets, and sintered at 700 °C for 2 d, 900 °C for 15 h, and 950 °C for 10 h at a heating rate of 3 °C⋅min⁻¹ with intermediate grinding. These pellets, after sintering, were crushed and powdered to obtain a fine particle size. For comparison, commercial IrO₂, Co(OH)₂, and Co₃O₄ were purchased from Alfa Aesar and used without further purification.