Online Mass Spectrometric Monitoring of Carbon Corrosion in PEMFC Electrodes Subjected to Accellerated Stress Testing

Polymer electrolyte membrane fuel cells (PEMFCs) are considered to be almost market-ready for automotive applications. The main obstacles which decelerate a broad application arise from the high costs for the production of membrane electrode assemblies (MEAs). Another aspect, which is related to operation costs of a fuel cell system, is its lifetime. The lifetime of PEMFCs especially in automotive applications, which include alternating load conditions and frequent start-stop-cycles, is still too short to make them more attractive vehicle power sources than combustion engines. Regarding the lifetime of PEMFCs the US DoE set the durability target of 5000 h by 2017 (with less than 10% loss of initial performance) for automotive applications [1]. Carbon (C) support corrosion was identified as one main contributing mechanism to the degradation of PEMFCs i.e. membrane electrode assemblies (MEAs) [2]. It is known that C support corrosion is dominantly an issue of the fuel cell cathode. It leads to (i) decrease of electrochemically active surface area and (ii) damage of porous electrode structure causing a deterioration of mass transport [2]. Both named phenomena contribute to a decrease of reaction rate and hence fuel cell performance. Beside the applied support material fuel cell operating conditions have a significant influence on C support corrosion rate e.g. high cathode potential conditions during fuel cell start/stop accelerate C support corrosion rates tremendously compared usual equilibrium operating conditions [2-3]. There is still a lack of understanding on how different operating conditions affect the C support corrosion rate. For this reason carbon corrosion rates of PEMFC cathodes were measured by means of inline mass spectrometry. The method is based on a measurement of the release i.e. generation rate of CO 2 at the cathode fuel cell exhaust. A correlation to the C corrosion rate is possible since CO 2 is known to be final product of the carbon corrosion reaction. The C corrosion rate of fuel cell cathodes was investigated for different conditions. Here, the investigations were focused on cathode potential. On one hand, corrosion rates for cathode potentials corresponding to usual operating potentials were probed and compared to each other. On the other hand, accelerated stress tests (ASTs) [4-5] involving potential cycling at unusually high cathode potentials were applied in order to simulate fuel cell start/stop conditions. It was found that the C corrosion rate during the simulated start/stop conditions is roughly 30-60 times higher compared to usual fuel cell operating conditions. Another focus was put on a separate measurement of C corrosion rate in the catalyst layer (CL) and gas diffusion layer (GDL). From the results of these investigations it can be concluded that the applied AST conditions dominantly accelerate carbon corrosion rates in the CL and hardly in the GDL. [1] US DOE, Multi-Year Research, Development and Demonstration Plan (2012) [2] P.T. Yu et. al, Carbon-Support Requirements for Highly Durable Fuel Cell Operation in F.N. Büchi et. al, Polymer Electrolyte Fuel Cell Durability, Springer (2009). [3] X.-Z. Yuan et. al, A review of polymer electrolyte membrane fuel cell durability test protocols, Journal of Power Sources. 196 (2011), 9107–9116. [4] Cell Component Accelerated Stress Test and Polarization Curve Protocols for PEM Fuel Cells, http://www.uscar.org/commands/files_download.php?files_id=348 (2013). [5] New Energy and Industrial Technology Development Organization (NEDO) - Development of PEFC Technologies for Commercial Promotion - PEFC Evaluation Project - Cell Evaluation and Analysis Protocol Guideline (2014). Figure 1