Graphene and Graphene Related Materials for PBI-Membrane-Based High-Temperature Hydrogen Fuel Cells

Abstract

The high-temperature proton exchange membrane fuel cell (HT-PEMFC) is a promising alternative to low temperature PEMFC, due to its low demand for hydrogen purity, simple water management, and high waste heat utilization. However, HT-PEMFC based on phosphoric acid (PA)-doped polybenzimidazole (PBI) membrane still has problems such as insufficient mechanical strength under high PA doping level, insufficient proton conductivity, and phosphoric acid leaching. This thesis explores the influence of PA distribution and migration on HT-PEMFC, and the application of single layer graphene (SLG) and electrochemical exfoliated (functionalized) graphene oxide (GO) in HT-PEMFC. This work is the first to apply high-quality SLG to HT-PEMFC. The coverage of the SLG on the electrode surface and the impact of SLG on the performance and durability when loaded between membrane and electrodes at anode, cathode or both sides of the HT-PEMFC are tested. After nearly 70 hours of accelerated stress testing, the peak power density at 150 ℃ of the membrane electrode assembly with SLG on both sides, with SLG on anode, with SLG on cathode and no SLG was measured as 480 mW cm-2, 367 mW cm-2, 365 mW cm-2 and 249 mW cm-2, respectively. Moreover, through electrochemical characterization, X-ray micro-computed tomography (CT) and Raman spectroscopic mapping, this work proposes the mechanism by which monolayer graphene improves the performance and durability of high-temperature fuel cell by controlling its PA leaching and hydrogen crossover. Also, a reactor based on 3D printing was designed and manufactured to utilize natural graphite flakes as raw materials to synthesize GO with a reasonable oxygen content through one-step electrochemical exfoliation in a rapid and high-volume manner. Based on the established reactor-based one-step electrochemical exfoliation method to prepare GO, phosphonated (P)GO with a reasonable P content was prepared by using appropriate electrolytes. The as-prepared GO and PGO were doped in the PBI membranes to explore their effects on the mechanical properties of PBI, and the performance and durability of HT-PEMFC. Compared with the pure PBI membrane, 0.5wt%, 1wt% and 2wt% of GO doping increased the peak power density of HT-PEMFC at 150℃ by 13.8%, 24.4% and 29.2%, respectively. The performance study of PGO adopts different PBI membrane material and preparation process from the GO study. The doping of 1.5wt% PGO in the PBI membrane increases the peak power density by 35.4%. The thesis is presented as a collection of four published or processing papers.

Date of Award	31 Dec 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Holmes (Supervisor) & James Winterburn (Supervisor)