(Invited) Activity Trends and Design Principles for Multi-Transition-Metal (Oxy)Hydroxide Oxygen Evolution Catalysts

Abstract

Oxygen evolution reaction (OER) catalysis limits the efficiency of H2 production through water electrolysis/photoelectrolysis, a route to large-scale energy storage. The factors governing the activity of OER catalysts are not well understood. We found Ni-Fe oxyhydroxides are the fastest known water oxidation catalysts under basic conditions when compared to others using a quantitative electrochemical-microbalance approach. In-situ electrical, photoelectron spectroscopy, x-ray diffraction, and electrochemical analyses on Ni1–x Fe x OOH films show that the reaction mechanism relies on the local electronic structure of a Ni-O(H)-Fe active site. Experiments with rigorous exclusion of Fe electrolyte impurities reveal that, contrary to common belief, pure NiOOH is a poor OER catalyst. FeOOH was found to be a poor electrical conductor, explaining its low apparent OER activity. Measurements on Co1–x Fe x OOH support the hypothesis that NiOOH and CoOOH provide a conductive host for the Fe-related active sites. In sum, these results establish a new activity trend for OER catalysts, opposite to previous ones, that informs catalyst design. (1) Trotochaud, L.; Ranney, J. K.; Williams, K. N.; Boettcher, S. W. Solution-cast metal oxide thin film electrocatalysts for oxygen evolution. J. Am. Chem. Soc. 2012, 134, 17253. (2) Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. Nickel–iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation. J. Am. Chem. Soc. 2014, 136, 6744. (3) Burke, M. S.; Kast, M.; Trotochaud, L.; Smith, A.; Boettcher, S. W. Cobalt-iron (oxy)hydroxide oxygen evolution electrocatalysts: The role of structure and composition on activity, stability and catalytic mechanism. To be submitted 2014.