(Invited) Phosphoric Acid Redistribution in HT-Pemfcs: Scratching the Surface

There is a growing consensus that electrochemical energy conversion of hydrogen and high-energy-density fuels will be essential in the global transition towards a sustainable energy future. High-temperature proton exchange membrane fuel cells (HT-PEMFCs) that use doped phosphoric acid can operate above 100 °C under anhydrous conditions and provide an ideal solution for heat rejection in automotive fuel cell applications. An imperative but poorly understood research area of HT-PEMFCs is phosphoric acid redistribution in the membrane electrode assembly (MEA). While the phosphoric acid redistribution during pre-conditioning in MEAs improves fuel cell performance, further acid redistribution can be detrimental to the lifetime of the fuel cells. Here, we present our recent discovery regarding phosphoric acid redistribution in phosphoric acid doped membrane-based HT-PEMFCs. We first show the phosphoric acid concentration change in polybenzimidazole-based MEAs under various operating conditions. This result explains more than 10,000 hours of stable operation for HT-PEMFCs under certain operating conditions. Next, we investigate the phosphoric acid redistribution behavior in the MEA by calculating the energetics of phosphoric acid clusters in the membrane and electrodes, emphasizing the importance of phosphoric acid retention in the membrane. Lastly, we explain how acid redistribution can be controlled by introducing ion-pair coordinated membranes, providing future directions for obtaining more durable HT-PEMFCs for various applications.