Advancement in Distribution and Control Strategy of Phosphoric Acid in Membrane Electrode Assembly of High-Temperature Polymer Electrolyte Membrane Fuel Cells

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have the unique advantages of fast electrode reaction kinetics, high CO tolerance, and simple water and thermal management at their operating temperature (140-200 degrees C), which are considered as one of the important research directions of PEMFCs. Membrane electrode assemblies (MEAs), as the core component of HT-PEMFCs, are usually fabricated by sandwiching phosphoric acid (PA)-doped polymer membrane (HT-PEM) between two electrodes. Technically, high PA content is required in HT-PEMs to ensure fast proton conduction, since PA acts as a proton transport carrier, while a high content of PA decreases the interaction among polymer molecules, thus enhancing the movement of the polymer molecules and leading to a decrease in the mechanical strength of the polymer membranes. In addition, PA is driven into catalyst layers owing to capillary force caused by micropore structures, crack connectivity, and accessibility. The PA content in the electrodes is also affected by the hydrophilic/hydrophobic characteristics of the catalyst layers and the surface tension of the acid when it is in close contact with the catalyst layers. Furthermore, PA plays an important role in the construction of electrochemical triple-phase boundaries to promote electrochemical reactions in the catalyst layers. Simultaneously, as a liquid or free molecule, the migration of PA may be accelerated by the current and the water produced, owing to the formation of charged phosphates or hydronium ions. This process encourages the redistribution of PA within the catalyst layers, and results in acid flooding of the catalytic layers and adsorption on the surface of the platinum catalyst, leading to increased mass transfer resistance for the gas reaction and reduced catalyst activity. Moreover, the increase in supplied absolute flow rate and the temperature elevation in the HT-PEMFC process could accelerate the evaporation of PA from the electrolyte membrane, resulting in a decrease in the stability of HT-PEMFC and corrosion of the metal end plate. Therefore, it is crucial to regulate the distribution and migration of PA in MEAs for the construction of HT-PEMFCs with high performance and stability. Hence, this paper reviews the research status of PA distribution in HT-PEM electrodes in recent years, and summarizes the corresponding regulations and optimization strategies as well as its future development trend.

Automated summary

High-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) have unique advantages, but the control of phosphoric acid content is crucial for their performance. The distribution and migration of phosphoric acid in the electrodes are influenced by various factors, requiring precise regulation in order to construct high-performance HT-PEMFCs.