Understanding Transport at the Acid-Alkaline Interface of Bipolar Membranes

Abstract

The combination of cation exchange membrane (CEM) and anion exchange membrane (AEM) electrolytes to form of a hybrid, or bipolar membrane (BPM) electrolyte, can have unique advantages for electrochemical systems including fuel cells, electrolyzers, electrodialysis, and photovoltaic solar-to-fuel devices. However, a major challenge for this approach is the development of a stable and active interfacial region (i.e., junction) that adjoins the CEM and AEM layers. Moreover, a fundamental understanding of transport at the CEM-AEM junction is lacking. Therefore, the present study focuses on the theoretical development and analysis of the nature of the BPM interface. A Poisson-Nernst-Planck (PNP) theory is formalized and applied to a representative BPM interface. The findings are reported in terms of bias (i.e., overpotential) in a galvanic device with respect to CEM and AEM material requirements. Specific attention is paid to our interests in the application of the BPM to a fuel cell device with an acidic (CEM) anode and alkaline (AEM) cathode. We demonstrate that a BPM with an acidic CEM anode and alkaline AEM cathode must promote a trap-assisted type of recombination mechanism under forward bias. Without such a mechanism, large overpotentials are needed to drive ionic recombination processes. Low temperature fuel cells have had difficulty in reaching the mass market despite promise as an efficient and scalable power source. Issues associated with costs, reliability, and ease of integration have made it difficult to disrupt established energy storage and conversion technologies. Fuel cell costs are often driven by the use of noble metal catalysts and fluorinated polymeric electrolytes.1 Numerous research groups explore catalyst materials in search of methods to remove platinum (Pt) and other precious metal electrocatalysts from low temperature fuel cell systems. To their credit, Pt content in polymer electrolyte membrane hydrogen/air fuel cells have dropped from upwards of 5 mg/cm 2 in the 1980s to approximately 0.125-0.3 mg/cm 2 at present. 1,2Other portions of the research community have turned their attention from acidic polymer electrolytes, a type form of cation exchange membrane (CEM), to alkaline anion exchange membrane (AEM) materials. While less mature, AEM materials have been steadily improving with notable improvements in metrics such as conductivity and stability. 3-6The motivation for the move to AEM materials is the recognition that oxygen reduction reaction (ORR) can be performed without Pt-based electrocatalysts. However, the hydrogen and methanol oxidation reaction at an alkaline ionomer-catalyst interface can also experience a voltage penalty associated with specific adsorption of cationic groups and/or other intermediates. [7][8][9] In addition to challenges with catalysts, a fuel cell system's balance of plant (BoP) can turn a simple device into a complex system. The BoP, which is typically comprised of radiators or heat exchangers, humidifiers, flow regulators, and power conditioning components,...

PEM/AEM Junction Design for Bipolar Membrane Fuel Cells

“…Two possible reasons have been considered for this device failure: the most likely explanation is the enormously large width (∼2 μm) of the neutral region relative to the desired junction, which is 3 orders of magnitude narrower (per the computational supplement to this work done in Ref. 6), resulting in excessive resistance over the interfacial region. Another possible reason is thermal stressing of Nafion due to repeated application of one face to a dry, heated surface.…”

“…A recent publication by Grew 6 provides a computational guideline for transport at the critical PEM/AEM junction in BPM-based devices. In this work, it is shown that for a device with acidic anode and alkaline cathode (Figure 1b), the recombination rate of the proton/hydroxyl groups at the interface is the limiting component for device performance.…”

Conversion of Visible Light into Ionic Power Using Photoacid-Dye-Sensitized Bipolar Ion-Exchange Membranes

“…Assuming E oc is dictated solely by the potential drop across the space-charge region, the value of V oc is less than the magnitude of E oc , 12 and therefore, a larger magnitude E oc also translated to larger V oc values. The decrease in the magnitude of the photocurrent for PSBM could be due to formation of a well-defined and stable space-charge region that is expected to be on the order of several nanometers thick, 32,33 beyond which cPFSA is strongly acidic. Most photogenerated carriers in the space-charge region will charge separate and be collected if the drift length is larger than the width of the space-charge region.…”

“…Reported current densities were calculated by dividing the external current passed through the potentiostat by the geometric area of the membrane exposed to the aqueous electrolyte solution (1.86 cm 2 ). To follow the J-E convention used in our previous publication 20 and prior literature reports of J-E behavior of bipolar membranes where the working electrode is on the acidic side of the bipolar membrane, and therefore P1 is the working electrode in Figure 7, 33,[46][47][48] current and voltage data that were measured with P2 as the working electrode were multiplied by À1. This correction was applied to the data reported in Figures 5, S6, and S7, and the data from mem2S0 and mem7S1 in Table S1.…”

Catalytic, Conductive Bipolar Membrane Interfaces through Layer‐by‐Layer Deposition for the Design of Membrane‐Integrated Artificial Photosynthesis Systems

“…Whereas the graphene BPM interface has been proven to enhance WD significantly and offers the potential to provide conductive properties, the spin‐coat deposition method does not lead to high‐level molecular organization in these films, and the interfaces between the materials is not uniform or well defined. It was reported that the structure of the interface, including molecular orientation and roughness, impacts WD in terms of forming efficient pathways for water and ions, distributing the active catalytic sites, and maximizing the electric‐field intensity . For these same reasons, the interface thickness is also very important and also cannot be controlled on the molecular scale by using spin coating.…”