Comparing Voltage Cycling–Induced Performance Losses of a Porous Carbon Supported Pt Catalyst Under H2/N2 and H2/Air at Different Relative Humidities
Authors: Harzer Carla Sophie , Hnyk Franziska Carmen , Gasteiger Hubert Andreas 
Published: December 2024

Accelerated stress tests (ASTs) mimic the dynamic operation of a PEM fuel cell stack by inducing degradation in different parts of the system. Voltage cycling–based ASTs are performed to specifically induce degradation in the cathode catalyst layer of a membrane electrode assembly (MEA). During voltage cycling, the electrochemical surface area (ECSA) of the catalyst is reduced by Pt particle growth via Ostwald ripening and by loss of the Pt into the ionomer/membrane phase.[1] However, the degradation rate during such tests can be significantly affected by different voltage cycling AST conditions, such as cell temperature, relative humidity, catalyst morphology, upper/lower potential limit, and whether the cycling is conducted under H2/N2 or H2/air (anode/cathode).[2–6]

The objective of this study is to compare the induced performance losses of a Pt on Ketjenblack catalyst (Pt/KB) by voltage cycling under H2/N2 or H2/Air at different relative humidities (RHs). Under reduced RH, Pt particles inside the nanopores of a KB carbon support are neither in contact with ionomer nor in contact with water.[7] During voltage cycling under H2/N2, no additional water is produced due to the absence of the oxygen reduction reaction (ORR). Under these conditions, it is hypothesized that the mobility of Pt may be hindered, decreasing the rate of ECSA loss. On the other hand, when cycling under H2/Air, water produced in the nanopores of the catalyst may enhance Pt movement. However, the local RH within the catalyst layer may be affected by the heat of the reaction at high current densities,[8] influencing the Pt mobility.

The decrease of the ECSA over the course of voltage cycling results in a significant decline in H2/air performance, mainly due to the increasing O2 transport resistance (RO2,local).[4] We found a strong H2/air performance dependence on the cathode roughness factor (rf), regardless of the upper potential limit[2] and the RH
[3]
during cycling. In the course of this study, the ECSA loss difference between voltage cycling conducted under H2/N2 and H2/Air at different RHs for a porous carbon-supported Pt catalyst will be discussed.

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Acknowledgement

C.S. H. gratefully acknowledges funding from the German Federal Ministry for Digital and Transport (BMDV) under the funding scheme H2Sky (funding code 03B10706). Furthermore, F. C. H. gratefully acknowledge financial support from the Fuel Cells and Hydrogen 2 Joint Undertaking (JU) under the MORELife grant agreement 101007170 that enabled us to conduct these studies. The JU receives support from the European Union’s Horizon 2020 research and innovation program and Hydrogen Europe and Hydrogen Europe Research.