Beware of cyclic voltammetry! Measurement artefact in accelerated stress test of fuel cell cathode revealed by operando X-ray diffraction

Abstract

Efficient screening of proton exchange membrane fuel cell (PEMFC) cathode catalysts long-term stability at the laboratory scale requires the use of accelerated stress tests (ASTs), mostly consisting in cathode potential cycling in the device operational range. However, AST protocols must be complemented with auxiliary in situ electrochemical characterization techniques, such as cyclic voltammetry, at least at the beginning and at the end of test. Supposedly non-destructive, such techniques are necessary to interpret the AST in terms of catalyst performance stability.Here, by combining operando X-ray diffraction, on-line inductively coupled plasma mass spectrometry (ICP-MS) and ex situ characterization techniques, we provide experimental evidence that the cyclic voltammetry experiment performed at the end of AST controls the increase of the coherent domain size monitored on the aged catalyst. We propose this process originates from coalescence of crystallites during low potential excursions of the cathode far below the operational range of the device. This study reveals a ‘measurement artefact’ in typical AST protocols, where the investigated state of the aged catalyst is largely impacted by the cyclic voltammetry measurement itself, without relevance of fuel cell device practical operation.