High-Temperature Self-Humidifying Proton Exchange Membrane Fuel Cell
Authors: Han Wei , Yeung King Lun 
Published: February 2020

Proton exchange membrane fuel cell (PEMFC) is considered as a clean and high-efficient power source for portable and on-site applications. Perfluorosulfonic acid (PFSA) polymer is the most common used electrolyte membrane material in PEMFC due to its high proton conductivity and excellent long-term stability under fully hydrated condition. However, its low glass transition temperature limits the operating temperature of PEMFC below 80oC. High operating temperature results in lowered mechanical and dimensional stabilities of PFSA polymer though high temperature operation has many other advantages for PEMFC including higher catalytic and anti-poisoning activities of electrical catalysts, and better controlled water and heat management. Moreover, proton transport capability of PFSA polymer is sensitive to environmental humidity. Membrane dehydration at high temperature sharply decreases its proton conductivity. The required external humidification equipment complicates the design and operation of fuel cell systems and lowers system reliability and energy utilization efficiency, as well as increases their weight and size. Therefore, to develop proton exchange membrane which can be operated under high temperature and low humidity conditions has become a very active research topic.
PEMFC has been stably operated under high temperature and low humidity conditions by adding inorganic fillers (e.g., zirconium phosphate, metal oxides, zeolites) into PFSA polymer to prepare polymer-inorganic composite membranes. However, main issues are the increase of proton transport resistance, the aggregation of inorganic filler particles and the appearance of phase separation for the above-mentioned composite membranes. Recently, we have developed and patented a new kind of high-temperature self-humidifying proton exchange membrane (i.e., confined structural composite membrane) by confining PFSA polymer into zeolite-coated porous substrate. Confined structural composite membrane harmonizes the structure and function of different membrane compositions to utilize water retention property of zeolite and avoid the formation of discontinuous phase within PFSA matrix. Confinement effect further improves the performance of structural composite membrane. New confined structural composite membrane can be stably operated up to 160oC without humidification. Self-humidifying hydrogen PEMFC and passive direct methanol fuel cell with new confined structural composite membrane output one order of magnitude higher maximum power densities than those with commercial Nafion 117 membrane. The effect of preparation parameters including zeolite type (e.g., silicate-1, NaZSM-5, HZSM-5, HY), coating thickness, coating morphology (e.g., single-layered, multi-layered, continuous, discontinuous) and PFSA precursor (e.g., solvent, incorporated material) on membrane physicochemical properties and fuel cell performance is also investigated to explore formation mechanisms of high thermal stability and self-humidifying property. On one hand, zeolite coating regulates water within confined structural composite membrane through adsorption of reaction generated water and catalytic formation of water. On the other hand, the confinement of PFSA polymer within zeolite-coated porous substrate limits the shrinkage and swelling of polymer caused by temperature and humidity changes, and induces the oriented rearrangement of polymer chains within confined space.