Using Theory to Understand the Origins of the High Activity of Earth-Abundant Transition Metal Oxide Electrocatalysts for the Hydrogen and Oxygen Evolution Reactions in Alkaline Media
Authors: Tryk Donald A. , Shi Guoyu , Kakinuma Katsuyoshi , Uchida Makoto , Iiyama Akihiro 
Published: December 2024

A global hydrogen economy (GHE) was envisioned by Haldane in 1923 but has only recently captured wide attention. As the movement toward a GHE accelerates, the supply vs. demand of precious metals such as platinum and iridium will inevitably force their prices skyward. In order to ensure the future of the GHE, it is crucial to intensify the R&D of earth-abundant element-based electrocatalysts. We have recently focused on electrocatalysts for the oxygen evolution and hydrogen evolution reactions (OER, HER) based on Fe, Co, Ni and Mo-containing oxides.1-3 The use of density functional theory (DFT) calculations has enabled us to understand more deeply the origins of high catalytic activity.
From OER experimental results, we have found that the activity is higher for Ni-Co oxides with lower structural order.1 This was explained by calculations indicating that the edges of NiOOH sheets are the active sites, at which the rate-determining step M-O + MOH  M-OOH-M is facilitated by the favorable geometry, in which the initial ) O—O distance is significantly shorter than that in the ideal structure (Fig. 1A). The presence of Co ("b" structure) further lowers the activation energy, to 0.15 eV. With the addition of Mo to NixCo1-xOOH, the rds M-O + M-O  M-O-O-M at the layer edges was found to become spontaneous, due to the electron-withdrawing effect of Mo.2 However, it was found that Mo slowly leached out of the structure during actual long-term operation. Ni-Fe oxide was found to be much more stable, and the corresponding DFT calculations showed that the activation energy for the M-O + M-OH  M-OOH-M step was lowered to 0.042 eV.3

In our work on the HER with Ni-Fe alloy/Ni-Fe oxide, we found experimentally that the activity was highest when the alloy was present in contact with a more crystalline form of the oxide.3 The DFT calculations showed that it is necessary to consider both the water dissociation step on the surface of the NixFe1-x(OH)2 sheets and the transfer of a proton to a metallic Ni-Fe alloy particle, on which two adsorbed hydrogens combine to produce H2 (Fig. 1B). This mechanism, which is similar to the bifunctional mechanism proposed by Markovic and coworkers for Pt/NiO,4 we have termed "reverse spillover."
Thus, through the combination of experiment and theory, we are developing an understanding of how further increases in electrocatalytic activity for the OER and HER can be achieved with earth-abundant element-based materials.
Acknowledgments
This work was partially based on results obtained from project JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and a JSPS grant, KAKENHI (23H02059).
References

G. Shi, et al., ACS Catal., 12, 14209 (2022).

G. Shi, et al., ACS Appl. Energy Mater., 6, 10742 (2023).

G. Shi, et al., ACS Omega, 8, 13068 (2023).

Z. Zeng, et al., Nature Energy, 2, 17070 (2017).

Figure 1. (A) Reaction profile for the rate-determining step M-O + M-OH  M-O-OH-M on defective NixCo1-xOOH sheets with (a) only Ni and (b) Ni and Co in the active site (taken from Ref 1). (B) Reverse spillover reaction in which a proton is transferred from a water molecule at the surface of the Ni(OH)2 sheet to the Ni7Fe cluster that has a high coverage of H, from which H2 can subsequently desorb (taken from Ref. 3).

Figure 1