(472h) Design Strategies for Efficient Mixed Metal Oxide Electrocatalysts: Correlating Measurable Oxide Properties with Electrocatalytic Performance

Non-stoichiometric mixed metal oxides belonging to the perovskite family of the general form A_n+1B_nO_3n+1 (A = rare earth/alkaline earth metal; B = transition metal; n = 1, 2, 3... ∞) have been explored as cost-effective catalysts for various thermochemical and electrochemical reactions.^1,2 In particular, they have shown promise for electrochemical transformation of molecular oxygen (i.e., oxygen reduction and evolution reactions (ORR/OER)), which remain cornerstones for sustainable energy conversion and storage technologies.^3,4 This rapid evolution in their application stems from their ability to encompass >90% of the metals in the periodic table, resulting in a large phase space to tailor catalytic performance. Such a vast phase space has deemed development of design criteria for these oxides rather challenging. Despite these challenges, experimentally measurable oxide properties such as the e_g orbital filling of the transition metal, as well as the theoretically calculated adsorption energies of oxygenated intermediates has been used to predict electrocatalytic performance.^5,6 However, varying electrocatalytic performance has been observed for different materials with similar e_g filling, suggesting its limitations in describing the overall activity. In addition, most adsorption energy-based descriptors are limited by a direct link between idealistic model systems used to determine their values, and the complex surface structure of working oxide surfaces.

In this presentation, we show that experimentally measurable oxide properties (such as surface reducibility) can be used to predict the electrocatalytic activity and stability of non-stochiometric mixed metal oxides. This is demonstrated through ORR in alkaline environments as a probe reaction.² The underlying factors that govern the catalytic performance of A_n+1B_nO_3n+1 (n = 1 and n = ∞) oxides are investigated using a combined theoretical and experimental approach. Periodic density functional theory (DFT) calculations suggest that the surface oxygen vacancy formation energy (E_VO), which describes the oxide surface reducibility, exhibits a linear scaling relationship with adsorption energetics of ORR intermediates (O*, OH* and OOH*) and thus describes the ORR activity. A linear correlation between theoretically calculated E_VO and the experimentally measured surface reduction temperature via H₂-temperature programmed reduction (H₂-TPR) is found, thus providing a link between the surface reducibility and the electrocatalytic activity of the oxides. In addition, a correlation between the oxide surface reduction temperature and oxide stability under electrochemical conditions is observed. These findings demonstrate that the experimentally measured surface reducibility can be used to predict the activity and stability of non-stochiometric mixed metal oxide electrocatalysts for targeted electrochemical reactions.

References

(1) Gu, X. K.^†; Samira, S.^†; Nikolla, E. Chem. Mater. 2018, 30, 2860-2872.

(2) Samira, S.^†; Gu, X. K.^†; Nikolla, E. ACS Catal. 2019, 9, 10575–10586.

(3) Gu, X. K.^†; Carneiro, J. S. A.^†; Samira, S.; Das, A.; Ariyasingha, N. M.; Nikolla, E. J. Am. Chem. Soc. 2018, 140, 8128–8137.

(4) Samira, S.^†; Deshpande, S.^†; Roberts, C. A.; Nacy, A. M.; Kubal, J.; Matesić, K.; Oesterling, O.; Greeley, J.; Nikolla, E. Chem. Mater. 2019, 31, 7300–7310.

(5) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Yabuuchi, N., Nakanishi, H., Goodenough, J. B.; Shao-Horn, Y. Nat. Chem. 2011, 3, 546–550.

(6) Man, I. C.; Su, H. Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. ChemCatChem 2011, 3, 1159-1165.