Electrocatalytic Activity of Tetravalent Fe–Co Mixed Oxide for Oxygen and Hydrogen Evolution Reactions

Abstract

Electrochemical water splitting is a useful way for sustainable hydrogen production, whereas sluggish kinetics of oxygen/hydrogen evolution reactions (OER/HER) limits the efficiency; thus, active electrocatalysts to reduce overpotentials are desired. Mixing of multiple transition-metal elements is a promising way to enhance electrochemical catalysis. In the present study, we investigated the OER and HER catalytic activities of tetravalent Fe–Co mixed perovskite oxide CaFe_0.5Co_0.5O₃. CaFe_0.5Co_0.5O₃ demonstrated a higher OER activity than those of the parent compounds CaFeO₃ and CaCoO₃. In contrast, the HER activity of CaFe_0.5Co_0.5O₃ was not significantly enhanced. These observations suggest that the mixing of Fe⁴⁺ and Co⁴⁺ ions is an efficient way to activate OER.

1. Introduction

Electrochemical water splitting is a useful method for sustainable hydrogen production,¹⁾ although sluggish kinetics of oxygen/hydrogen evolution reactions (OER/HER) disturbs efficient energy conversions.^2,3) Fundamental understanding of the mechanism of OER/HER is necessary to develop efficient catalysts. Platinum-group metals and their oxides (e.g. Pt, IrO₂, RuO₂) have been widely utilized as practical catalysts for OER and HER to suppress intrinsic large overpotentials and resulting energy loss.^4–6) On the other hand, transition-metal oxides have been extensively investigated because of their earth abundance and low cost.⁷⁾ Exploration of novel catalysts for OER and HER has been eagerly conducted among perovskite oxide family (ABO₃), some of which display high catalytic activity.^8–10)

Mixing of multiple transition-metal elements is a promising way to enhance catalytic activity, which is considered as a synergistic effect between mixed elements. For example, Ba_0.5Sr_0.5Co_0.2Fe_0.8O_3−δ (BSCF) is proposed as a highly active OER catalyst⁸⁾ and further mixing of multiple transition metals enhances stability.¹⁰⁾ However, the origin of the synergistic effect has not been elucidated well because of complex chemical compositions and crystal structures including substantial amount of oxygen deficiency.¹¹⁾ Suntivich et al. also proposed electron occupancy in the e_g orbital of the transition metal ion at B site in the perovskite structure can be a simple descriptor; the catalytic activity of BSCF (e_g^∼1.2) locates at the top of the volcano-like plot as a function of e_g electron number.⁸⁾ On the other hand, it has been recently suggested that the oxygen deficiencies in the perovskite-related phases also play an important role in promoting OER.⁹⁾ A brownmillerite oxide Ca₂FeCoO₅ consisting of randomly mixed Fe and Co ions displays a high catalytic activity for OER,^12–14) implying that tetrahedral (Fe³⁺/Co³⁺)O₄ and octahedral (Fe³⁺/Co³⁺)O₆ units affect catalytic activity for OER in addition to the mixing of Fe³⁺ and Co³⁺ ions. It is expected that the mixing of intrinsically active ions such as Fe⁴⁺ and Co⁴⁺ further improves the electrochemical catalysis of perovskite oxides for OER,¹⁵⁾ but detailed study has not been conducted yet because of the difficulty in synthesis of high-valent metal oxides under ambient conditions. In the present study, we synthesized CaBO₃ (B = Fe, Co, and Fe_0.5Co_0.5) under high-pressure and high-temperature conditions at 8 GPa and 1373 K to examine the coexistence effects of randomly mixed Fe⁴⁺ and Co⁴⁺ ions on OER and HER catalysts. The OER catalytic activity was significantly improved by mixing Fe⁴⁺ and Co⁴⁺ ions in CaFe_0.5Co_0.5O₃ whereas a smaller enhancement was observed for HER. This finding proposes the mixing of high-valent Fe⁴⁺ and Co⁴⁺ ions is a useful way for activating OER rather than HER.

2. Experimental Procedure

The powder sample of CaFe_0.5Co_0.5O₃ was synthesized from its stoichiometric mixture, which was prepared by complex polymerized method in advance.¹⁶⁾ High-pressure and high-temperature treatment (8 GPa and 1373 K) with a oxidizing agent KClO₄ was utilized to stabilize Fe⁴⁺ and Co⁴⁺ valence states for CaBO₃ (B = Fe, Co, and Fe_0.5Co_0.5) without oxygen deficiency. X-ray powder diffraction patterns at room temperature were collected using a X-ray diffractometer with Cu Kα a radiation (Ultima IV, Rigaku, Japan). Specific surface area was estimated by the Brunauer-Emmett-Teller (BET) analysis of Kr gas adsorption data. Specific surface area was determined by BET analysis of Kr-gas absorption data. Oxidation states of the constituent transition metal ions were determined by iodometric titration with Hiranuma automatic titrator (COM-1700A, Hiranuma, Japan).

OER catalytic activity was evaluated by using rotating disk electrode system (RRDE-3A, BAS, Japan) in the same manner as the previous studies.^15,17,18) Electrochemical measurements were conducted in 1 M KOH solution under O₂ saturation (for OER) and N₂ saturation (for HER) at room temperature.

3. Results and Discussions

Figure 1 shows the XRD pattern and Rietveld refinement result of CaFe_0.5Co_0.5O₃. All the Bragg reflection peaks were indexed with the GdFeO₃-type orthorhombic perovskite structure with the space group of Pnma (No. 62). Based on the refinement, the lattice constants were determined to be a = 5.3106(5) Å, b = 7.4850(7) Å, c = 5.2921(5) Å. The calculated unit cell volume (210.36(3) ų) for CaFe_0.5Co_0.5O₃ was an intermediate value between CaFeO₃ (214.97 ų) and CaCoO₃ (207.65 ų), confirming that the obtained oxide is the 1:1 solid solution of CaFeO₃ and CaCoO₃.^19,20) CaFe_0.5Co_0.5O₃ probably consists of Fe⁴⁺ (t_2g³e_g¹ configuration) and Co⁴⁺ (t_2g⁴e_g¹ configuration) ions as well as the parent compounds.^19,20) Since there was no sign of additional peaks attributed to atomic ordering, Fe and Co ions are distributed randomly at the same crystallographic sites. Oxygen content for CaFe_0.5Co_0.5O₃ was determined to be about 2.98 by iodometric titration, confirming the tetravalency of the transition metals.

Fig. 1

Observed XRD pattern of CaFe_0.5Co_0.5O₃ and the Rietveld refinement result. The circles and solid lines represent observed and calculated patterns, respectively. The difference between the observed and calculated patterns is shown at the bottom. The vertical marks indicate the Bragg reflection positions.

Figure 2 shows linear sweep voltammograms in OER conditions for CaBO₃ (B = Fe, Co, and Fe_0.5Co_0.5). Current densities per surface area of oxides (in a unit of mA cm⁻²_oxide) were adopted to evaluate the intrinsic catalytic activities. The OER onset potentials (E_OER) were determined using the potentials reaching 1 mA cm⁻², and OER overpotentials (η_OER) were calculated as follows: η_OER = E_OER −1.23 V. The overpotential of CaFeO₃ (η_OER = 0.39 V) for OER was superior to that of CaCoO₃ (η_OER = 0.47 V) as well as the previous study measured in 0.1 M KOH conditions.¹⁸⁾ CaFe_0.5Co_0.5O₃ exhibited a lower overpotential (η_OER = 0.31 V) than both CaFeO₃ and CaCoO₃, clearly indicating a significant Fe⁴⁺–Co⁴⁺ mixing effect on OER activity. It is notable that the present result cannot be explained by other descriptors such as e_g electron configuration and oxygen deficiency because the e_g electron for CaFe_0.5Co_0.5O₃ is retained in the mixture of Fe⁴⁺ (t_2g³e_g¹) and Co⁴⁺ (t_2g⁴e_g¹) with identical e_g electron number.

Fig. 2

Linear sweep voltammograms in OER conditions for CaBO₃ (B = Fe, Co, and Fe_0.5Co_0.5). The disk potential was controlled between 0.3 and 0.9 V versus Hg/HgO at a scan rate of 10 mV s⁻¹ and the disk rotation rate was set at 1600 rpm.

Figure 3 shows linear sweep voltammograms in HER conditions for CaBO₃ (B = Fe, Co, and Fe_0.5Co_0.5). HER overpotentials (η_HER) were determined based on the absolute values of the onset potentials reaching at −1 mA cm⁻² (E_HER; η_HER = |E_HER|), as well as η_OER. The HER overpotential of CaCoO₃ for HER (η_HER = 0.26 V) was lower than that of CaFeO₃ (η_HER = 0.35 V) in contrast to OER overpotentials. The disagreement between OER and HER activity implies that the descriptors for each reaction are different. Since systematic trend in HER activity for perovskite oxides has not been elucidated yet, we do not focus on the detailed HER mechanism in this study. CaFe_0.5Co_0.5O₃ exhibited a lower overpotential (η_HER = 0.22 V) than CaFeO₃, but almost the same as CaCoO₃. This is apparent from the result that both the voltammograms of CaFe_0.5Co_0.5O₃ and CaCoO₃ are almost overlapping. These observations indicate that the synergistic effect of the Fe⁴⁺–Co⁴⁺ mixing is subtle on HER activity in contrast to the significant increase in the OER activity of CaFe_0.5Co_0.5O₃.

Fig. 3

Linear sweep voltammograms in HER conditions for CaBO₃ (B = Fe, Co, and Fe_0.5Co_0.5). The disk potential was controlled between −0.9 and −1.5 V versus Hg/HgO at a scan rate of 10 mV s⁻¹ and the disk rotation rate was set at 1600 rpm.

4. Conclusion

We successfully synthesized the Fe⁴⁺–Co⁴⁺ mixed oxide CaFe_0.5Co_0.5O₃ using a high-pressure synthesis method, exhibiting that the OER catalytic activity is significantly improved by mixing Fe⁴⁺ and Co⁴⁺ ions. This work first demonstrated the synergistic effect between Fe⁴⁺ and Co⁴⁺ on the OER activity. In contrast, the HER activity was slightly enhanced by Fe⁴⁺–Co⁴⁺ mixing. This finding suggests a simple and new design principle to enhance the OER catalysis rather than HER catalysis.

Acknowledgments

This work was supported by JSPS KAKENHI (grant number 18H03835 and 19H02438).

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