Bipolar Membrane Development for Fuel Cells and Electrolyzers

Abstract

Bipolar membranes (BPMs) are composed of a cation exchange layer (CEL), an anion exchange layer (AEL), and a nanometer scale bipolar junction (Fig 1). At reverse bias polarization the potential drop within both the AEL and CEL are almost negligible when compared to the potential drop within the junction. This large potential drop within the bipolar junction results in a high electric field, which, in combination with water dissociation catalyst, allow BPMs to facilitate water dissociation (into H+ and OH- ions) at a rate seven orders of magnitude greater than the rate in bulk water [1]. BPMs have been studied at the laboratory and pilot scale for a wide variety of applications, including: electrodialysis for water treatment [2], CO2 reduction [3], solar water splitting [4], and fuel cells [5]. Despite the commercial availability of BPMs, significant hurdles exist both in the fundamental understanding of limiting processes and their subsequent incorporation into electrochemical devices. The main technological challenges for BPMs at present include: i) low membrane stability, ii) insufficient water transport at high current densities, and iii) high membrane resistance. Here we will discuss the optimization of BPM junctions and related water management issues. An example of the improved interfacial charge transfer for BPMs fabricated using 3-D electrospun junctions is shown in Fig 2. The AC impedance spectra shown in Fig 2, highlights both the similarity in the high frequency resistance (membrane + electrolyte resistance) of the two samples, along with the significantly different low frequency (membrane + electrolyte + interfacial charge transfer) resistance. The experimental results from both 4-electrode and membrane electrode assembly (MEA) level operando testing of BPM junctions fabricated using 2-D and 3-D approaches both with and without water dissociation promoters will be shown. Reference [1] Strathmann, H., Krol, J. J., Rapp, H. J., & Eigenberger, G. (1997). Limiting current density and water dissociation in bipolar membranes. Journal of Membrane Science, 125(1), 123-142. [2] Reig, M., Valderrama, C., Gibert, O., & Cortina, J. L. (2016). Selectrodialysis and bipolar membrane electrodialysis combination for industrial process brines treatment: Monovalent-divalent ions separation and acid and base production. Desalination, 399, 88-95. [3] Li, Y. C., Zhou, D., Yan, Z., Gonçalves, R. H., Salvatore, D. A., Berlinguette, C. P., & Mallouk, T. E. (2016). Electrolysis of CO2 to syngas in bipolar membrane-based electrochemical cells. ACS Energy Letters, 1(6), 1149-1153. [4] Luo, J., Vermaas, D. A., Bi, D., Hagfeldt, A., Smith, W. A., & Grätzel, M. (2016). Bipolar Membrane‐Assisted Solar Water Splitting in Optimal pH. Advanced Energy Materials, 6(13), 1600100. [5] Peng, S., Xu, X., Lu, S., Sui, P. C., Djilali, N., & Xiang, Y. (2015). A self-humidifying acidic–alkaline bipolar membrane fuel cell. Journal of Power Sources, 299, 273-279. Figure 1