The Role of Local Heating and Phosphate Poisoning during the Oxygen Reduction Reaction in Solid Acid Fuel Cells

Abstract

With the ongoing effort to decarbonize and decentralize the electricity infrastructure by shifting to renewable energy sources, fuel cells and in particular intermediate temperature fuel cells receive a growing interest as a promising technology for a highly efficient, on demand transformation of chemical into electrical energy. Since the 2000s, fuel cells have cycled from high expectations to disillusion, caused by the high production cost an insufficient stability. High temperature polymer electrolyte membrane fuel cells (HT PEM FC) are a prominent and well investigated candidate for the intermediate temperature range. However, the usage of concentrated, highly corrosive phosphorous acid as membrane dopant required expensive production materials and causes deterioration of the catalyst over time. Changing from a liquid to a solid electrolyte can simultaneously improve the long term stability and reduce the construction price. A suitable solid acid is CsH2PO4. It is a non-toxic and less corrosive electrolyte with a proton conductivity of 2*10-2 S/cm at 240°C.[1] Since first demonstrated for fuel cell fabrication in 2003,[2] the platinum utilization and power densities have been improved ever since.[3,4] Solid acid fuel cells (SAFC) are already used in industry in a small scale, but the development has been hampered by the poor stability of the cathode electrode. The rate limiting reaction step is the complex oxygen reduction reaction (ORR) at the cathode side. During operation, it is the largest source of overpotential and hence waste heat generation.[5] For an active site, the current collector, electrolyte and gas phase need to be in direct contact with each other to provide all necessary reactants, thereby the interface resistance between the current collector and the catalyst is crucial for the performance.In the work we are going to present, we investigated electrodes based on a platinum thin film catalyst layer.[6,7] These electrodes enable us to investigate degradation effects on a reasonable time scale. We will show that only a few areas close to the current collectors meet the requirements for active sites and consequently generating most of the measured current, leading to a localized heating. We observed a current density depended morphology chance of CsH2PO4 during operation at these active sites, located close to the current collector. While a variety of degradation processes can result from this process, we determined a phosphate adsorption on the catalyst as the most pronounced one. Similar to HT PEM FC, phosphate species from the electrolyte can adsorb at the platinum surface and act as catalyst poison.[8–10] We will present, that these poisoning effects can be reversed but not prevented by cyclic voltammetry (CV) measurements. After reactivating the cell by CV measurements, the cell performance increased to the initial value before degrading again. The reversibility of the process was shown for five reactivations over a period of 50 h. Overall, we identified the cause and nature of the main degradation mechanism in solid acid fuel cells with platinum thin film electrodes and will present different design optimizations for a stable, high performance fuel cell which arise from our findings.