Compositional Optimization of Alloy FexNiy(OH)2 Nanoparticles for Alkaline Electrochemical Oxygen Evolution

Abstract

It has recently been demonstrated by Bell and co-authors [1] that intentional iron doping into a nickel hydroxide film causes significant enhancement in electrochemical performance for the oxygen evolution half reaction (OER) of alkaline electrolysis. A subsequent study by Boettcher and co-authors [2] demonstrated that unintentional iron doping of a nickel hydroxide catalyst occurs from ppm- to ppb-level iron contaminants in alkaline electrolytes; the iron incorporates into the nickel hydroxide structure, and causes at least an order of magnitude increase in current density. Freibel et al. subsequently demonstrated that iron may in fact be the active site of OER, with nickel hydroxide acting as a host [3]. The majority of the work thus far investigating how iron content in an FexNiy(OH)2 catalyst affects OER has been performed on thin film catalysts; Burke et al. specifically describe the need for catalyst architectures that reduce mass transport limitations and allow improved access to more active sites per unit mass of catalyst [4]. As a result, there is an interest in developing nanoscale catalysts that are composed of iron and nickel and result in greatly improved performance metrics for OER over their thin film counterparts. Here, we will present results for the optimization of a FexNiy(OH)2 nanoparticle catalyst. In this work, the nanoparticles are synthesized with an alloy morphology and the atomic ratio of iron to nickel is varied from 1:5 to 5:1. Once synthesized, the nanoparticles are evaluated in 1 M KOH that has been purified to remove any iron impurities. Electrochemical performance of this series of nanoparticle catalysts will be discussed as well as our efforts to characterize the nanoparticles with electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction. [1] M.W. Louie, A.T. Bell, An investigation of thin-film Ni-Fe oxide catalysts for the electrochemical evolution of oxygen, J. Am. Chem. Soc., 135 (2013) 12329-12337. [2] L. Trotochaud, S.L. Young, J.K. Ranney, S.W. Boettcher, Nickel-iron oxyhydroxide oxygen-evolution electrocatalysts: The role of intentional and incidental iron incorporation, J. Am. Chem. Soc., 136 (2014) 6744-6753. [3] D. Friebel, M.W. Louie, M. Bajdich, K.E. Sanwald, Y. Cai, A.M. Wise, M.-J. Cheng, D. Sokaras, T.-C. Weng, R. Alonso, R.C. Davis, J.R. Bargar, J.K. Norskov, A. Nilsson, A.T. Bell, Identification of highly active Fe sites in (Ni,Fe)OOH for electrocatalytic water splitting, J. Am. Chem. Soc., 137 (2015) 1305–1313. [4] M.S. Burke, L.J. Enman, A.S. Batchellor, S.H. Zou, S.W. Boettcher, Oxygen evolution reaction electrocatalysis on transition metal oxides and (oxy)hydroxides: Activity trends and design principles, Chemistry of Materials, 27 (2015) 7549-7558.