The efficiency of water electrolysis for hydrogen production is limited in part by the slow kinetics of the oxygen evolution reaction (OER).^1,2 Nickel-iron and cobalt-iron (oxy)hydroxides have been shown to be the most active, low-cost electrocatalysts for the OER in alkaline conditions.^3–5 Although it is evident that Fe is essential for high activity,^6,7 its role in the catalytic active site is still largely debated.^8,9 We investigate the role of Fe in NiOOH by comparing the effects of Ti, Mn, La, and Ce incorporation on the OER activity and electrochemical behavior of NiOOH in Fe-free 1 M KOH.¹⁰ We show that other metal cations in NiOOH can increase its reduction potential in a similar way to Fe, but without the same orders of magnitude increase in OER activity. This suggests that Ni is not likely the active site as its electronic properties are not correlated to activity. We also evaluate the OER activity and Tafel behavior of Fe³⁺ impurities on different noble metal substrates. We find that the activity varies by substrate suggesting that the local atomic and electronic structure of [FeO₆] units play an important role in catalysis of the OER as it can be tuned by substrate interactions. Finally, through in situ and in operando X-ray absorption spectroscopy experiments, we find that the local structure around Fe atoms in Co(Fe)OOH changes during OER while that of Co stays the same. Given that the OER activity on a per Fe basis is independent of [Fe] in Co(Fe)OOH, the potential dependent structural changes further indicate that Fe is essential to the catalytic active site for the OER on transition-metal (oxy)hydroxides.

References

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(4) Burke, M. S.; Zou, S.; Enman, L. J.; Kellon, J. E.; Gabor, C. A.; Pledger, E.; Boettcher, S. W. J. Phys. Chem. Lett. 2015, 6(18), 3737.

(5) Burke, M. S.; Enman, L. J.; Batchellor, A. S.; Zou, S.; Boettcher, S. W. Chem. Mater. 2015, 27(22), 7549.

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(9) Chen, J. Y. C.; Dang, L.; Liang, H.; Bi, W.; Gerken, J. B.; Jin, S.; Alp, E. E.; Stahl, S. S. J. Am. Chem. Soc. 2015, 137(48), 15090.

(10) Enman, L. J.; Burke, M. S.; Batchellor, A. S.; Boettcher, S. W. ACS Catal. 2016, 6 (4), 2416.