(512i) Effect of Azole Structure for Membranes Doping Phosphoric Acid to Single Cell Performance at HT-Pemfcs

High temperature proton exchange membrane fuel cells (HT-PEMFCs) operate between 120-180℃ and thus the phosphoric acid (PA) is used as proton-conducting media. The hydrogen bonding among N and NH sites of benzimidazole groups plays an important role in the proton hopping with PA [1-3]. Azole groups containing the benzimidazole are also used as proton-conducting media with PA. In this study, the performance of membrane electrode assemblies (MEAs) using polymer membranes with various azole groups are measured and compared according to membrane properties.

First, triazole membranes consisted of different cross-linking ratios are compared [2]. The degree of cross-linking in the membranes was controlled by varying the triazole functionality in the side chains. The degree of cross-linking, to some extent, affected the proton conductivities. The single-cell level test results showed the less cross-linked membranes showed better performances compared to more cross-linked membranes. The less cross-linked membranes could make more dynamic chain movement for proton conduction. And it might be due to the PA movement from the membrane during MEA activation, which blocks the active sites in the catalyst layer, causing mass transport resistance to increase [4,5].

Second, three types of membranes from the polymers having triazole, benzimidazole, and imidazole groups [3] are investigated. The membranes are chemically cross-linked using an alkyne-azide click reaction to relieve the issue of declining mechanical strength after PA doping [2]. Even though the membranes had the same PA doping level in the first place, the different PA loss caused different three-phase interfaces, influencing a resistance for the charge transfer and the mass transfer (R_ct + R_mt). The imidazole membrane showed the highest peak power density of 0.20W/cm². Because it had the highest proton conductivity and the lowest R_ct + R_mt.

Therefore, it was suggested that PA-doped membranes have two requirements for high performance of MEAs; (1) high PA uptake to enhance proton conductivity and (2) low PA loss from membrane for a three-phase boundary in the electrode.

[1] Y.L. Ma, J.S. Wainright, M.H. Litt, R.F. Savinell, J. Electrohem. Soc. 151(1) (2004) A8-A16.

[2] J. Jang, D.-H. Kim, M.-K. Ahn, C.-M. Min, S.-B. Lee, J. Byun, C. Pak, J.-S. Lee, J. Membr. Sci. 595 (2020) 117508.

[3] J. Jang, D.-H. Kim, C.-M. Min, C. Pak, J.-S. Lee, J. Membr. Sci. 605 (2020) 118096.

[4] K. Kwon, T.Y. Kim, D.Y. Yoo, S. Hong, J.O. Park, J. Power Sources 188 (2009) 463-467.

[5] R. Zeis, Beilstein J. Nanotechnol. 6 (2015) 68-83.