Application of Novel Materials in Fuel Cells

Abstract

Methanol crossover significantly affects the performance and efficiency of direct methanol fuel cells (DMFC). The main goal of this project was improving the performance of a DMFC by reducing the methanol crossover of the proton exchange membrane without reducing its proton conductivity. Therefore, various electrolyte configurations of proton exchange membranes with and without methanol barrier materials were fabricated and evaluated. Standard membrane electrode assemblies (MEAs) were fabricated using commercial single layer Nafion membranes. Optimum operating conditions were found by testing the DMFC under different methanol flow rates, cell temperatures, air flow rates, and methanol concentrations. Multi-layer membranes with the total thickness equal to Nafion 115 (127 μm) were fabricated from individual commercial layers of one Nafion NR211 and two Nafion NR212s by simple, fast and cost effective pressing (hot and cold) technique, leading enhancement of 19% when they were hot pressed. Prepared membranes show equal methanol flux density and 20% higher proton conductivity than Nafion 115 membrane. Methanol crossover of the membranes was reduced by using two types of barrier materials, graphene oxide (GO) and cellulose nanocrystals (CNC). GO was added to the single and multi-layer membranes (at different locations) by spraying and physical deposition techniques. Multi-layer membrane spray coated by the optimum amount of 0.3 wt% GO (wt% of total final membrane) on its outer surface presents the maximum power density of 71 ± 0.5 mW/cm2, which is 30% higher than Nafion 115 membrane. To investigate the effects of thermal treatment on the proton conductivity and barrier properties of the CNC, two types of multi-layer membranes were prepared by hot and cold pressing methods. Results indicate that proton conductivity drops sharply after thermal treatment, while barrier properties remain constant. The addition of 1.5 wt% CNC (wt% of total final membrane) to the cold pressed multi-layer membrane leads to a 38 % higher maximum power density in comparison to standard Nafion 115 membrane. GO, and CNC can be used as practical barriers for reducing the methanol crossover of the DMFC. By reducing the methanol crossover, DMFC can be operated at either higher power densities or lower fuel volumes for the same power density. This means the total size of the DMFC system can be reduced, which is an essential issue for mobile applications. Also, the long term operation of the prepared membranes indicates that they show appropriate performance, which is essential for their commercialization.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Stuart Holmes (Supervisor) & James Winterburn (Supervisor)