Electrochemical oxidation of Pt(111) beyond the place-exchange model

Leon Jacobse,Vedran Vonk,Ian T Mccrum,Christoph Seitz,Marc T.M Koper,Marcel J Rost,Andreas Stierle

doi:10.1016/j.electacta.2022.139881

Abstract

Oxide formation plays an important role in the degradation of Pt electrocatalysts. However, the exact oxide structure and reaction mechanism are not fully understood. Here, we used in situ surface X-ray diffraction experiments to resolve the oxide formation at a Pt(111) model electrode at potentials near the onset of the oxygen evolution reaction. Fast experiments are possible by using X-ray photons with a high kinetic energy in combination with a large 2D detector. By employing very low potential sweep rates we obtain a more ordered oxidized surface compared to literature data from potential step experiments. This demonstrates that the oxidation process is strongly governed by the reaction kinetics. The increased surface order enables us to disentangle two subsequent oxidation process; initially the place-exchange process, followed by the formation of a partially disordered oxide in which still 50% of the surface atoms reside on sites commensurate to the Pt(111) surface. The reduction experiments indicate that the place-exchange process is structurally reversible, whereas the disordered oxide causes the surface roughening observed during potential cycling. Despite the increased surface order, oxide superstructures are not observed. These results provide important insights in the oxidation and degradation process of Pt(111), which are valuable for the design of improved electrocatalysts and they rationalize operating procedures.

Highlights

Platinum electrodes are extensively studied because of their high reactivity for reactions involved in sustainable energy technologies, such as hydrogen oxidation and oxygen reduction in fuel cells
crystal truncation rod (CTR) are scattering rods in reciprocal space running perpendicular to the surface that originate from the broken periodicity at the interface between bulk and electrolyte
The oxidation and subsequent reduction of Pt(111) electrodes leads to a severe surface roughening. This roughening has been described in detail, much less is known about the underlying cause, i.e. the structure of the electrochemically formed oxide

Summary

Introduction

Platinum electrodes are extensively studied because of their high reactivity for reactions involved in sustainable energy technologies, such as hydrogen oxidation and oxygen reduction in fuel cells Their widespread technological application, is hindered by degradation of the expensive electrodes. 1,2 The effect of the degradation on the electrode surface structure, can be assessed in potential cycling experiments, where oxidizing and reducing potentials are applied alternatingly Studies employing such procedures using model electrodes have shown that at mildly oxidizing potentials only small amounts of Pt are lost in the electrolyte solution 3–5. 45 in the electrochemical SXRD experiments the potential is applied in a stepwise manner and the sample is at room temperature, such that kinetic barriers may drastically slow down the formation of the stripe structures. We analyze the structure of the oxidized Pt(111) surface resulting from slow potential sweeps and

Experimental

Surface oxidation via Place-Exchange

Surface oxidation dynamics

Reduction and roughening

Absence of oxide superstructure

Conclusions