Method for Measuring Platinum Electrochemically Active Surface Area and Understanding Surface Restructuring

Abstract

Platinum is a widely used catalyst in aqueous environments, particularly in fuel cells as an electrocatalyst for hydrogen oxidation [1] and oxygen reduction [2]. Key to developing new and improved catalysts is a method to accurately measure the surface area of various catalysts so their activity can be properly compared. Further, platinum catalysts can undergo changes in size and shape in an aqueous electrochemical environment, changing the surface area and surface faceting, impacting catalytic performance. We have developed a simple method, based on our prior DFT results [3, 4] which can be used to deconvolute experimentally measured cyclic voltammograms on platinum electrodes in non-adsorbing electrolytes (acid and alkaline) to more accurately estimate electrochemically active surface area (ECSA) than the traditional “H-upd” method, without any additional measurements. This method can also give the relative proportion of 111-terrace sites, 100 step and terrace sites, and 110 step and terrace sites. We validate this method using cyclic voltammograms measured on single crystal electrodes, then extend to Pt nanoparticles and polycrystalline Pt. We also use this method to examine the experimentally measured restructuring of a polycrystalline platinum electrode in an alkaline electrolyte upon potential cycling. To understand the driving forces for restructuring, we use DFT to calculate the surface energy of Pt(111), Pt(100), and Pt(110) in an aqueous environment as a function of potential, considering the adsorption of hydrogen, hydroxide, and oxygen, spanning a large range of potential (the full electrochemical window of water). With these results, we explain the experimentally observed growth in primarily 110 sites, as well as 100 sites, due to the lower energy of these sites near potentials where surface oxide is reduced. We can also use these DFT results to gain insight into nanoparticle growth and restructuring, where for example, the strong binding of hydrogen to Pt(100) at low potentials contributes to the growth of tetrahexahedral nanoparticles on square wave cycling of spherical Pt nanoparticles [5-7]. [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] I. T. McCrum and M. J. Janik, "pH and Alkali Cation Effects on the Pt Cyclic Voltammogram Explained Using Density Functional Theory," The Journal of Physical Chemistry C, vol. 120, pp. 457-471, 2016. [4] I. T. McCrum and M. J. Janik, "First Principles Simulations of Cyclic Voltammograms on Stepped Pt(553) and Pt(533) Electrode Surfaces," ChemElectroChem, vol. 3, pp. 1609-1617, 2016. [5] N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, and Z. L. Wang, "Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity," Science, vol. 316, pp. 732-735, 2007. [6] S. Liu, N. Tian, A.-Y. Xie, J.-H. Du, J. Xiao, L. Liu, et al., "Electrochemically Seed-Mediated Synthesis of Sub-10 nm Tetrahexahedral Pt Nanocrystals Supported on Graphene with Improved Catalytic Performance," Journal of the American Chemical Society, vol. 138, pp. 5753-5756, 2016. [7] T. Zhu, E. J. M. Hensen, R. A. van Santen, N. Tian, S.-G. Sun, P. Kaghazchi, et al., "Roughening of Pt nanoparticles induced by surface-oxide formation," Physical Chemistry Chemical Physics, vol. 15, pp. 2268-2272, 2013.