Understanding the Stability and Surface Restructuring of Platinum in an Electrochemical Environment

Abstract

Platinum is a commonly used catalyst in aqueous environments, particularly in fuel cells as an electrocatalyst for hydrogen oxidation [1] and oxygen reduction [2]. To maximize the activity per unit mass (and per unit cost), the electrocatalyst is typically dispersed as small Pt nanoparticles. These nanoparticles may change in size and shape during use in an electrochemical environment due to dissolution/deposition, oxide growth/reduction, and reconstruction [3]. The resulting size and shape of the nanoparticles are dictated in part by the surface energy of the various platinum surface facets. Additionally, many techniques exist to synthesize Pt nanoparticles in an aqueous electrochemical environment, where again the relative stability of the platinum facets is plays a role in controlling the size and shape of the resulting Pt nanoparticles. To understand the driving forces for surface faceting and reconstruction, we have previously used density functional theory to calculate the surface energy of Pt(111), Pt(100), and Pt(110) in an aqueous environment as a function of potential, including the adsorption of hydrogen, hydroxide, and oxygen, spanning the full electrochemical window of water [4]. While the Pt(111) surface is the most stable facet in vacuum, we found the strong binding of hydrogen to Pt(100) and hydroxide/oxygen to Pt(110) drives these surfaces to be more stable at low and high potentials, respectively [4]. We use these relative surface energies to explain the experimentally observed growth in the fraction of 110 step sites seen on cycling a Pt(111) single crystal electrode to high potentials into the oxide formation region, where the 110 sites are most stable at potentials where this oxide is reduced [5]. Further, the strong adsorption of hydrogen to Pt(100) at low potentials may explain the growth of 100 sites (and formation of tetrahexahedral nanoparticles) seen on square wave cycling of spherical Pt nanoparticles [6,7]. We have recently extended this work to include stepped surfaces (Pt(553), Pt(533), and Pt(110) 1x2), as well as to examine high coverages of hydrogen (> 1ML) adsorbed at low potentials, where sub-surface hydrogen may drive surface reconstruction and the appearance of the anomalous “third peak¨ seen in cyclic voltammograms measured on platinum.[1] H. A. Gasteiger, J. E. Panels, and S. G. Yan, "Dependence of PEM fuel cell performance on catalyst loading," Journal of Power Sources, vol. 127, pp. 162-171, 2004.[2] H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, "Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs," Applied Catalysis B: Environmental, vol. 56, pp. 9-35, 2005.[3] Y. Shao-Horn, W.C. Sheng, S. Chen, P.J. Ferreira, E.F. Holby, D. Morgan, “Instability of supported platinum nanoparticles in low-temperature fuel cells,” Top. Catal., vol. 46, pp. 285-305, 2007.[4] I. T. McCrum, M. A. Hickner, and M. J. Janik, “First-principles calculation of Pt surface energies in an electrochemical environment: thermodynamic driving forces for surface faceting and nanoparticle reconstruction,” Langmuir, vol 33, pp. 7043-7052, 2017.