Membrane Electrode Assembly Development in High-Temperature Proton Exchange Membrane Fuel Cells

Abstract

High-temperature proton exchange membrane fuel cells (HT-PEMFCs) are studied worldwide as efficient energy conversion devices. Great efforts have been made in the area of designing and developing phosphoric acid (PA)-based high-temperature proton exchange membranes (HT-PEMs). However, the current HT-PEMs suffer limited proton conductivity, acid leaching, along with chemical and mechanical degradations. This thesis focuses on the incorporation of inorganic clay-based fillers (silica nanosheets and muscovite) into HT-PEMs in order to improve proton conductivity, reduce acid leaching and alleviate membrane degradation. The first experimental work of this thesis is based on the optimization of polyethersulfone-polyvinylpyrrolidone (PES-PVP) membrane preparation procedures. It was found that the membranes prepared using high-molecular-weight polymers and indirect mixing method exhibited better mechanical properties and durability than the membrane prepared using low-molecular-weight polymer or direct mixing method. The possible reason is that the membrane prepared using high-molecular-weight polymer or the indirect-mixing method has a dense and homogenous structure and therefore has a longer lifetime. Later, silica nanosheets (SN), functionalized and exfoliated SN, and as-received muscovite were introduced into PES-PVP membranes and tested at 150°C without humidification. The surface functional groups (hydroxyl or sulfone groups) and the layered structures of clay-based fillers appear to facilitate the proton conductivity and protect the membrane from radical attack to get improved power density and durability. To better understand how the clay-based fillers work in HT-PEMFCs and alleviate the acid leaching in HT-PEMs, polybenzimidazole (PBI) membranes which have better durability and longer lifetime than PES-PVP membranes were used as HT-PEMs in this work. In addition, since muscovite has similar properties with SN in terms of the improvement of membrane properties and cell performance, the as-received muscovite which is a lot cheaper and has simpler preparation procedure than SN was chosen as the inorganic filler and added into PBI membranes to investigate the acid leaching issue. Accelerated stress test was employed to accelerate acid leaching and facilitate the evaluation of acid leaching issue in HT-PEMs. Due to the muscovite-PA and muscovite-PBI crosslinks confirmed by energy dispersive X-ray spectroscopy, thermogravimetric analysis, and X-ray photoelectron spectroscopy, the inorganic-organic composite membranes showed improved performance, alleviated acid leaching and better durability.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Holmes (Supervisor)