Bipolar Membrane Development for Reversible Fuel Cells

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 (electrolysis mode) 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]. At forward bias polarization (fuel cell mode), H+ and OH- produced on the electrodes transport through the membrane and meet at bipolar junction forming H2O. 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. This is particularly true for BPM reversible fuel cell applications where the requirements for an optimized bipolar junction are specialized for each case. The main technological challenges to enable BPM reversible fuel cell operation include: i)Optimizing bipolar junction to perform in both fuel cell mode and electrolysis mode; ii)Achieving high bipolar junction stability at high current densities; iii)Achieving high membrane durability when cycling between each mode; IV)Optimizing ion exchange layers to decrease membrane resistance Here we will discuss the optimization of BPM junctions to enable reversible fuel cells. Both 2-D and 3-D approaches with and without water dissociation promoters will be applied to fabricate BPM junctions. An example of the improved interfacial charge transfer for BPMs fabricated using 3-D electrospun junctions is shown in Fig 2. Both membranes show high enough water transport into bipolar junction at current densities up to 500 mA/cm2. BPM#2 with 3-D junction shows lower activation overpotential and 0.8 V less at 500 mA/cm2 than BPM#1 with 2-D junction. Different junction designs are preferred for fuel cell mode and electrolysis mode. Therefore, it is important to find a balance in between that could work in both modes and achieve reversible operation. BPM stability of recycling between fuel cell mode and electrolysis mode will also 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