Impact of the Catalyst Type and Dopant Composition on the Performance of High-Temperature PEM Fuel Cell

Increasing interest in hydrogen-powered fuel cells for electric energy generation led to the identification of serious limitations of state-of-the-art technologies. In particular, low-temperature fuel cell with a proton-exchange membrane (LT-PEM FC), which is widely considered as an optimal type for mobility, is reaching its technological limits. This is due to its operating temperature up to 90 °C, defined by properties of perfluorinated, sulfonated membranes used as PEM, requiring liquid water for ensuring their ionic conductivity. In the case of heavy duty applications, high-power LT-PEMFC stacks suffer from local overheating, which is currently solved by intensive cooling. Ineffective heat-exchange and necessity of water regime control lead to limitations in total energy utilization. Such problem can be solved by increasing operating temperature up to 180 °C, which is suitable for heat cogeneration. This requires application of appropriate PEM conducting even at an elevated temperature. Current state-of-the-art high-temperature PEM fuel cells (HT-PEM FCs) utilize PEMs based on polymer doped with phosphoric acid. Polymers, such as polybenzimidazole or pyridine aromatic polyether, act as a matrix and a bonding agent for the phosphoric acid dopant. Such PEMs can operate within the temperature range of 120 to 180 °C. In terms of the catalyst, HT-PEM FCs use Pt-based, nanoparticle catalysts on carbon support as in LT-PEM FCs. However, due to the mobile nature of phosphoric acid, which is responsible for pronounced catalyst degradation, poisoning as well as local catalytic layer flooding, Pt loading used in HT-PEM FCs is within an order of magnitude higher than in LT-PEM FCs. Due to its high operating temperature, HT-PEM FC technology offers numerous advantages, such as heat and electric energy cogeneration, simple water regime and, due to improved tolerance to catalyst poisons like CO, the possibility of utilization of contaminated H 2 produced from fossil fuels and biomass. However, degradation of components, in particular the catalyst and the membrane, remain the main bottlenecks for wide HT-PEM FC commercialization. With respect to the latest advances in technology, two critical aspects were identified. Firstly, the necessity of new catalysts immobilized on durable supports, with improved stability, activity and resistance against poisoning by phosphate anions. Secondly, membrane with improved fixation of phosphoric acid and greater durability. A special case of the membrane degradation is the reduction of phosphoric acid, by hydrogen or at low potentials on the anode, to phosphorous acid. This compound can poison the anode catalyst, incorporate into the membrane as well as oxidize on the cathode to phosphoric acid, completing dopant lifecycle. Nevertheless, the exact impact of phosphorous acid on HT-PEM FC performance is still not fully understood. Accordingly, the study had two major goals. A first one was to investigate the performance and degradation of Pt-based alloy catalyst on graphene-type support using thin-film modified rotating disc electrode, half-cell and single cell setups. All experiments have been performed at HT-PEM FC relevant conditions, i.e. in concentrated phosphoric acid electrolyte within temperature range of 120 to 180 °C. On the other hand, the second one goal investigated the influence of the dopant composition in terms of phosphorous acid addition to phosphoric acid electrolyte on the activity of the catalyst. Performance of experimental catalysts based on Pt-Co on various supports, including carbon black and graphene was compared to commercial Pt/C catalyst. Experimental catalysts exhibited superior activity for oxygen reduction. The overall performance of single cells with experimental catalyst on the cathode was better than the one with the commercial catalyst, with best results obtained for the graphene supported Pt-Co. The impact of phosphorous acid on oxygen reduction on commercial Pt/C catalyst was significant, due to simultaneous oxidation of acid and reduction of oxygen, as confirmed using rotating disc electrode and half-cell setups. Using the addition of phosphorous acid after single cell break-in on the cathode, the anode or both electrodes at once, it was determined that after stabilization the performance improved. Electrochemical impedance spectroscopy showed that addition of phosphorous acid decreases cell Ohmic resistance, but the comparative I-U curves and CO stripping analysis performed on cathode indicated significant decrease of available Pt surface. This reflects the ambivalent influence of phosphorous acid on HT-PEM FC. This study was supported by the Grant Agency of the Czech Republic under project No. 22-23668K.