Structure-Dependence of the Atomic-Scale Mechanisms of Pt Electrooxidation and Dissolution

Abstract

For current low-temperature hydrogen-oxygen fuel cells, as used e.g. in the upcoming generation of commercial fuel cell vehicles, platinum is still the catalyst of choice, especially for the cathode side. However, catalyst lifetime and performance remain a problem for large-scale commercialization. It has been known for some time that electrochemical oxidation and subsequent oxide reduction lead to irreversible structural changes as well platinum dissolution but the fundamental understanding of these complex processes is still rudimentary. A detailed atomistic picture of the steps by which these ultrathin oxide layers form existed up to now only for the close-packed Pt(111) surface [1]. Here, previous in situ studies by surface X-ray diffraction (SXRD) techniques established the precise locations of the first Pt atoms that move out of their lattice sites in a place-exchange process [2] and found good agreement with density functional theory (DFT) calculations [3]. Provided the potential is not too high, these atoms can move back to their original locations, but at higher potentials an irreversible restructuring of the surface occurs.To elucidate the role of the Pt surface structure, we performed in-depth comparative studies of Pt(111) and Pt(100) in perchloric acid solution by in situ SXRD, online inductively coupled plasma mass spectrometry (ICP-MS) and DFT [4]. Clear differences were found, which mirror the differences in the dissolution behavior of the two electrode surfaces (Figure 1a). Pt(100) not only start to dissolve at lower potential but its oxidation also immediately changes its surface structure in an irreversible manner. These observations demonstrate clearly that Pt dissolution is linked to the way Pt atoms are initially extracted from the surfaces during oxidation.For a better understanding of the extraction process on Pt(100), we performed surface diffraction studies at ESRF ID31 with very high photon energies of 70 keV (HESXRD) [5]. This technique was employed for the first time to a solid-liquid interface and proofed to be indispensable for clarifying the oxide’s structure. The oxidation-induced changes, exemplified in figure 1b for one out of twelve simultaneously measured crystal truncation rods, can be unambiguously assigned to a process in which the extracted Pt atom moves laterally away from its original site. This initiates the immediate extraction of a second atom, leading to the formation of atomic oxide stripes on the surface (see figure 1c). DFT calculations support this scenario and show that, differently from Pt(111), this mechanism produces unstable surface atoms at stripe ends that can dissolve during the oxidation as well as during the subsequent oxide reduction. Consequently, the extraction process on Pt(100) is irreversible from its very onset.These results show a high sensitivity of Pt electrocatalyst oxidation and degradation to the precise surface structure. Rational strategies for the design of catalysts with improved stability will have to take the different mechanisms into account.[1] J. Drnec, D.A. Harrington, O.M. Magnussen, Curr. Op. Electrochem., 4, 69-75 (2017)[2] M. Ruge, et al., J. Electrochem. Soc., 164, H608-H614 (2017)[3] M. J. Eslamibidgoli,, M. H. Eikerling, Electrocatalysis, 7, 345–354 (2016)[4] T. Fuchs, et al., Nat. Catal., 3, 754-761 (2020)[5] J. Gustafson, et al., Science, 343, 758–761 (2014) Figure 1. (a) correlation between Pt(hkl) dissolution and restructuring behavior in 0.1 M HClO4. (b) Example of Pt(100) crystal truncation rod changes with increasing oxidation, measured by in situ high-energy surface X-ray diffraction. (c) Pt extraction mechanisms, obtained from DFT calculations (adapted from T. Fuchs, et al., Nat. Catal., 3, 754 (2020)). Figure 1