The anion exchange membrane (AEM) has attracted considerable attention in the application of fuel cell technology. Compared with proton exchange membrane fuel cells, AEM fuel cells (AEMFCs) have advantages in cost, catalyst stability, efficiency and system size¹. However, one challenge in the application of AEMFCs is the absorption of CO₂ from the cathode air supply, which forms carbonate and bicarbonate species and reduces the ionic conductivity of membrane.

In this study, to better understand the effect induced by CO₂ on AEM performance, the ion transport mechanism was investigated by using electrical conductivity and Nuclear Magnetic Resonance (NMR) techniques. A typical commercial anion exchange membrane Tokuyama A201 has been used for all experiments. OH^- form membranes are prepared by soaking in low concentration (10^-4 M) KOH solution. CO₃^2- and HCO₃^- form membranes are used after soaking in 1M K₂CO₃ or KHCO₃ solution. Both electrical conductivity and NMR experiments have been performed on a series of OH^- form, CO₃^2- and HCO₃^- form samples with different water contents from 20 °C to 80 °C. Figure 1 shows the electrical conductivity of HCO₃^- form A201 at different relative humility as a function of temperature, which displays an increasing trend as the relative humility and temperature increase. Also note the low values of conductivity relative to that expected from a hydroxyl ion conductor. This is in accord with previous reports that indicate a significant decrease in conductivity upon uptake of CO₂ into hydroxide-form membranes. ¹H and ¹³C pulse-field-gradient NMR are used to characterize the speciation and diffusion coefficient D_(NMR) of water, CO₃^2- and HCO₃^- respectively. Difference in ionic conductivity and diffusion coefficient of each species in the membrane between three different forms can be observed. The estimated diffusion coefficient D(σ) from ionic conductivity obtained by using Nernst-Einstein equation is sometimes different from that measured by NMR. The comparison between D_(NMR) and D(σ) provides valuable information on ionic transport mechanism ². Obtaining dynamic information on the behavior of CO₂– derived species in the membrane will allow modeling to the distribution of such species in the membrane.

Acknowledgement.

We gretefully acknowledge the support of this work by the Office of Naval Research. We also appreciate the assistance of Tokuyama Soda in providing A201 membrane samples.

Reference:

1. X. M. Ren, S. C. Price, A. C. Jackson, N. Pomerantz and F. L. Beyer, Acs Appl Mater Inter, 6, 13330 (2014).

2. S. Ochi, O. Kamishima, J. Mizusaki and J. Kawamura, Solid State Ionics, 180, 580 (2009).