High Temperature PEM Fuel Cells (HTPEMFC) typically operate in the temperature range of 100-200°C while Low Temperature PEM Fuel Cells (LTPEMFC) operate below 100°C. Due to the higher operating temperature, HTPEMFCs benefit from several advantages over LTPEMFCs, including enhanced electrode kinetics, increased tolerance to impurities in the fuel feed and improved heat and water management. However, there are also many challenges preventing their large-scale implementation. Some of the issues related to the increased temperature of operation include corrosion of catalyst and support, degradation of cell components, and difficulties in selecting a viable polymer electrolyte membrane where applicable. Currently, polymer membranes filled (doped) with a low vapor pressure acid, such as concentrated phosphoric acid, are used as electrolytes in HTPEMFCs operating in the 140-200°C temperature range. The acid bound to the membrane molecules acts as the ionic conductor in the electrolyte, while the free or “unbound” acid functions as the ionic conductor in the electrodes. In order to form triple phase boundaries in the electrode, the free acid must be in contact with the catalyst particles, and as a result the oxygen has to dissolve in and diffuse through the acid film to reach the catalyst site. This can result in significant mass transport losses in the cathode due to the low solubility and diffusivity of oxygen in concentrated phosphoric acid. With the use of phosphoric acid, additional issues affecting cell performance arise due to phosphate adsorption on the Pt catalyst, migration of acid during operation and the loss of acid from the stack. The performance losses resulting from the mass transport losses and anion adsorption on Pt are two major issues affecting the development of HTPEMFC technology as they necessitate the use of higher Pt catalyst loading compared to state of the art LTPEMFCs in order to match their performance, which increases the stack cost significantly [1-4].
The effect of perfluorinated carbon compounds on improving the oxygen solubility and diffusivity in phosphoric acid is well documented [5, 6]. In this study, pottasium nonafuorobutanesulfonate (C4F9SO3K) and potassium perfluorohexanesulfonate (C6F13SO3K) will be used to modify the phosphoric acid prior to doping the electrolyte membrane in order to analyze the effect of the additive on ORR kinetics and fuel cell performance. The modified electrolytes are also expected to exhibit less anion adsorption on the Pt surface. Polarization curves, Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) will be used to evaluate the effect of varying the concentration and type of additive on cell performance, the ECSA and mass transport resistance in the cathode catalyst layer. A drawback to using additives is that the wetting angle of the modified acid on PTFE increases significantly, leading to electrode flooding issues at higher additive concentrations in the acid. So, optimization of the wet-proofing of the electrode by controlling the PTFE content in the catalyst layer will also be studied.
References
[1] Myles, L. Bonville, R. Maric, Catalysts 7 (2017) pp.16-43
[2] G. Waller, M. R. Walluk, T. A. Trabold, International Journal of Hydrogen Energy 41 (2016) pp.2944-2954
[3] S. Araya, F. Zhou, V. Liso, S. L. Sahlin, J. R. Vang, S. Thomas, X. Gao, C. Jeppesen, S. K. Kaer, International Hournalof Hydrogen Energy 41 (2016) pp. 21310-21344
[4] Kim, T. D. Myles, H. R. Kunz, D. Kwak, Y. Wang, R. Maric, Electrochimica Acta 177 (2015) pp.190-200
[5] Gang, H. A. Hjuler, C. Olsen, R. W. Berg, N. J. Bjerrum, Journal of the Electrochemical Society 140 (1993), pp.896-902
[6] Razaq, A. Razaq, E. Yeager, D. D. Desmarteau, S. Singh, Journal of the Electrochemical Society 136 (1989) pp.385-390
