Measurement of Protonic Conductivity of PEM Water Electrolyzer Electrodes

Abstract

Polymer electrolyte membrane water electrolyzer (PEMWE) performance mostly depends on the catalyst layer (CL) composition and microstructure. The CL microstructure is influenced by catalyst ink composition and fabrication process [1]. Due to the use of an unsupported catalyst (IrOx) in PEMWE, the porosity of the CL is low, resulting in a low CL surface area and high activation loss. By increasing the porosity of the CL, it is hypothesized that more catalyst surface area will be exposed to the reactant and electrolyte. One method to increase the CL porosity is to use pore formers: sacrificial particles that are added to the ink and then removed after CL fabrication, leaving additional pores. Limited work has been done on the use of pore formers in fuel cell applications [2–6] which resulted in an increase of the porosity [3,5] and the surface area [2–4]. No work has been done using pore formers in PEMWE application. In this study, a pore former is used to study the effect of pore former addition on PEMWE microstructure and performance. In order to fabricate the catalyst coated membranes (CCMs), an anode catalyst ink was prepared by dispersing iridium oxide powder (Tanaka Kikinzoku Kogyo, ELC-0110) and pore former in a dispersion media as described in ref. [7]. The pore former to IrOx volume ratio was varied between 0 to 0.25 (Table 1). The ionomer content was kept constant at 35 wt.%. The cathode ink was prepared using the method and material described in ref. [8]. The CCMs were prepared by inkjet printing method as described in ref. [7] with catalyst loading as shown in Table 1. Two CCMs for each pore former to IrOx volume ratio were tested and the results are reported. The effect of the pore former on the electrochemical surface area (ECSA) was studied using cyclic voltammetry (CV). ECSA was estimated, after conditioning and obtaining a polarization curve, using the method developed by Tan et al. [9]. The average ECSA of CCM-0.1 is increased by 38 % compared to the CCM-0 as shown in Figure 1b and Table 1. While CCM-0.1 shows the highest ECSA, a similar improvement is not seen from CCM-0.25, indicating that the removal of pore former is having an impact beyond increasing the ECSA. To study the electrode performance without the effect of varying loading between electrodes, the normalized electrochemical performance with respect to IrOx loading are compared in Figure 1a. CCM-0.1 exhibits the highest performance and shows an average improvement of 30 mV at 2 A/mg, compared to the CCM-0. CCM-0.25 shows an improved performance in the kinetic region, however, the performance in the ohmic region is significantly worse than the other two CCMs. This is despite having a similar high frequency resistance (HFR) to the other CCMs. The pore former has a complex impact on the CL microstructure. These preliminary results indicate that the PEMWE performance can be improved by using pore formers, and that there might be an optimal pore former content. References K.-H. Kim, K.-Y. Lee, H.-J. Kim, E. Cho, S.-Y. Lee, T.-H. Lim, S. P. Yoon, I. C. Hwang and J. H. Jang, Int. J. Hydrog. Energy, 35, 2119 (2010). Y. Song, Y. Wei, H. Xu, M. Williams, Y. Liu, L. J. Bonville, H. R. Kunz and J. M. Fenton, J. Power Sources, 141, 250 (2005). T. V. Reshetenko, H.-T. Kim and H.-J. Kweon, J. Power Sources, 171, 433 (2007). Q. Huang, J. Jiang, J. Chai, T. Yuan, H. Zhang, Z. Zou, X. Zhang and H. Yang, J. Power Sources, 262, 213 (2014). A. Fischer, J. Jindra and H. Wendt, J. Appl. Electrochem., 28, 277 (1998). Y.-H. Cho, N. Jung, Y. S. Kang, D. Y. Chung, J. W. Lim, H. Choe, Y.-H. Cho and Y.-E. Sung, Int. J. Hydrog. Energy, 37, 11969 (2012). M. Mandal, A. Valls, N. Gangnus and M. Secanell, J. Electrochem. Soc., 165, F543 (2018). S. Shukla, K. Domican, K. Karan, S. Bhattacharjee and M. Secanell, Electrochimica Acta, 156, 289 (2015). X. Tan, J. Shen, N. Semagina and M. Secanell, J. Catal., 371, 57 (2019). Figure 1