Evaluation of Poly(phenylene oxide)-Based Anion Exchange Membranes in Fuel Cells
Authors: Carlson Annika Elisabet , Dang Hai-Son , Lindbergh Göran , Lagergren Carina , Jannasch Patric , Lindström Rakel Wreland 
Published: February 2020

Anion exchange membrane fuel cells (AEMFC) or hydroxide exchange membrane fuel cells (HEMFC) are emerging as possible alternatives to proton exchange membrane fuels cells [1]. The alkaline environment in these fuel cells make possible the use of platinum free catalysts since the oxygen reduction is more easily catalysed. In addition, the higher pH is less corrosive towards the stainless steel bipolar plates. Thereby, AEMFC has potential to be produced at a lower cost. However, further research is still needed to improve cell components and to commercialize the technology.

The polymeric material is a very important part in AEMFC, and new polymers are continuously being developed to improve the fuel cells [1-3]. The polymer electrolyte membranes have to be impermeable to gases and need to have a high hydroxide conductivity. If used as ionomers in the electrodes, high hydroxide conductivity, low swelling and a good interaction with the catalyst powder are desirable. During operation water is produced at the anode and consumed at the cathode meaning that the water will affect the polymer performance and, so electrochemical out-put. Further, the water balance across the cell is dependent on the membrane properties [4]. Thus, for better understanding of these materials, they have to be evaluated in a real fuel cell set-up.

In this study the performance of novel poly(phenylene oxide) (PPO)-based polymer electrolytes with quaternary amines is evaluated. These materials have been synthesized to avoid hydroxide degradation of the backbone by attaching the quaternary amine at the end of a longer side-chain. Distancing it from the polymer backbone results in steric hindering of hydroxide attack. The chain structures compared in this study are shown in figure 1. Their polymer backbone remains the same, but the side-chains altered to have varying length or a different quaternary amine group. These materials have previously shown very promising properties in ex-situ tests at 80 °C. Among these, high conductivity for fully humidified membranes, over 100 mS cm-1, and high stability up to 200 h in 1 M NaOH with very low loss of ion-exchange capacity were measured [3].

Figure 1 -The polymeric backbone investigated is shown in a), and the three side chains that replaces the R in the backbone are shown in b), c) and d).

Membrane electrode assemblies (MEAs) are prepared using the PPO-based membranes and in house made electrodes using Tokuyama AS-4 ionomer and Pt/C catalyst prepared on the gas diffusion layer. The ion-exchange of the membranes synthesized in bromide form was performed in carbon dioxide free environment prior to MEA preparation. The investigation focuses on electrochemical measurements of the cell, including in-cell gas crossover, cell resistance and fuel cell performance. Among other methods polarization curves, cyclic voltammograms and impedance spectra are used for cell characterization. In addition the water production and flux across the MEA is measured using humidity sensors.

References:

[1] B. Britton and S. Holdcroft, J. Electrochem. Soc., vol. 163, issue 5, pp. F353-F358, 2016.

[2] J. R. Varcoe, et al., Energy Environ. Sci., vol. 7, pp. 3135–3191, 2014.

[3] H.-S. Dang and P. Jannasch, Macromolecules, vol. 48, pp. 5742–5751, 2015.

[4] T.J. Omasta, et al. J. Power sources, xxx, pp 1-9, In press 2017

Figure 1