Characterization of Iron-Nickel Alloy Nanoparticles for the Oxygen Evolution Reaction As a Function of Iron-Nickel Composition

Iron-doped nickel hydroxide catalysts are now recognized as the leading catalyst composition for alkaline electrochemical water splitting, and in particular, the anodic reaction of oxygen evolution [1]. The iron-nickel bimetallic composition of these hydroxide catalysts has been demonstrated to be critical for optimal catalyst performance [2]. However, few studies thus far have investigated if the same trend in catalyst activity is observed when catalyst morphology is scaled from bulk films to the nanoscale [3], where the overall field of iron-nickel hydroxide/oxide catalysts is in its infancy in terms of developing, synthesizing, and fully characterizing these nanoparticulate bimetallic hydroxides. Further, preliminary stability studies performed on iron-nickel [4] and iron-cobalt [5] thin films suggests that an evaluation of the stability of the bimetallic catalysts as a function of composition will be critical to identifying the best-performing catalyst compositions to pursue. In particular, Burke et al. report that while iron atomic compositions of less than 54 % resulted in high catalytic activities, the activities of these iron-cobalt bimetallic catalysts decreased by up to 62 % over two hours [5]. In the iron-nickel catalyst field, little work has evaluated how catalyst stability is affected by bimetallic composition in either thin film or nanocatalyst morphologies. Thus far, most studies have primarily focused on how the iron-nickel bimetallic composition affects catalyst performance from the perspective of lowering the overpotential, thereby lowering the kinetic limitation to the oxygen evolution reaction [2, 6]. In this talk, our ongoing work to develop and characterize a suite of bimetallic iron-nickel alloy hydroxide nanoparticle catalysts for the oxygen evolution reaction will be discussed. In particular, results will be presented for three iron-nickel compositions (1:5, 1:1, and 5:1 mol:mol Fe:Ni) and trends in both overpotential and stability will be discussed. Nanoparticle characterization via high resolution transmission electron microscopy, elemental analysis, and synchrotron-based x-ray absorption spectroscopy will be presented and discussed as a function of bimetallic composition. The role of nanoparticle structure in controlling activity and stability will be discussed, and electrochemical performance data will be used to demonstrate that both activity (i.e., overpotential) and stability (i.e., degradation rate as mV/hr) are critical to evaluate as we develop these catalysts for water splitting. References [1] 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. [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] J.A. Bau, E.J. Luber, J.M. Buriak, Oxygen Evolution Catalyzed by Nickel–Iron Oxide Nanocrystals with a Nonequilibrium Phase, ACS Applied Materials & Interfaces, 7 (2015) 19755-19763. [4] M. Gong, Y. Li, H. Wang, Y. Liang, J.Z. Wu, J. Zhou, J. Wang, T. Regier, F. Wei, H. Dai, An Advanced Ni–Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation, Journal of the American Chemical Society, 135 (2013) 8452-8455. [5] M.S. Burke, M.G. Kast, L. Trotochaud, A.M. Smith, S.W. Boettcher, Cobalt-iron (oxy)hydroxide oxygen evolution electrocatalysts: The role of structure and composition on activity, stability, and mechanism, J. Am. Chem. Soc., 137 (2015) 3638-3648. [6] 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.