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,...

Multiblock copolymer with long head-group tethers were synthesized as anion exchange membranes with high ionic conductivity and good alkaline stability.

The combination of proton exchange membrane (PEM) and anion exchange membrane (AEM) materials to form a bipolar membrane (BPM) is of interest in hybrid electrochemical devices to mitigate the disadvantages of their monopolar counterparts. The PEM-AEM interface is a critical component in bipolar membrane fuel cell operation. In this study, mono-and di-membrane bipolar membranes were fabricated. Interfacial materials with varying conductivities were used in order to control the location of the junction within the di-membrane BPMs. Mono-membrane BPMs were constructed via conversion of a single face of a monopolar membrane (Nafion). The membranes were used in fully functional fuel cells and characterized via electrochemical impedance spectroscopy (EIS). For the di-membrane BPMs, use of a conductive interface consisting of a single ion conductive material resulted in devices with lower interfacial resistance as compared to a neutral interface. When comparing conductive interface materials, anion-conductive materials provided lower total membrane resistance than proton-conductive materials. This decrease is due to positioning the junction closer to the anode and farther from the air-cathode. These results show that the formation of the optimal junction is critically dependent on fabrication technique and location. Polymeric membrane-based fuel cells are a promising candidate to provide clean, efficient, and energy dense power sources. Of the primary types of these fuel cells, proton exchange membrane (PEM) fuel cells have capabilities of producing extremely high power densities, and the materials used as the proton exchange membrane (e.g. Nafion) are thermally and mechanically robust, allowing for long-term operation. Despite these features, there are several drawbacks relating to the operation of these devices under acidic conditions. First, the migration of protons from anode to cathode results in electro-osmotic drag of water (and methanol, where applicable) which can lead to a phenomenon known as "cathode flooding" or a blockage of transport passageways with water. Second, both the catalytic and polymeric materials required to survive the harsh acidic operating environment are expensive to manufacture. Third, both the metal catalyst and its carbon support are subject to corrosion at low pH.One alternative to the PEM fuel cell is the anion exchange membrane (AEM) fuel cell. This configuration addresses many of the shortcomings of the PEM cell simply by operating under alkaline (high pH) conditions. AEM fuel cells potentially address many of the issues inherent with PEM devices. The basic operating environment is much more conducive to the oxygen reduction reaction (ORR), which opens the possibility of using non-platinum based (cheaper) catalysts.1-3 Additionally, anion exchange membranes are typically hydrocarbon based, and as such are less expensive to manufacture than Nafion and other perfluorosulfonic acid (PFSA) membranes used in PEM devices. Another benefit is the reversed direction of ion transport through th...

The design of new anion exchange ionomers for use in electrochemical devices is a critical step in improvement of anionic fuel cells and electrolyzers. Although these materials share some targeted goals with anion exchange membranes (AEMs), there are several different property requirements. Since ionomers are used within electrode layers, they do not serve as a primary separator for fuel and air. Rather, they act as a transport facilitator in the electrode where a high degree of mass transport is required. In this study, a series of anionic ionomers is synthesized and tested in hybrid fuel cells and AEM electrolyzers. These ionomers are based on a series of materials which include block copolymer AEMs with alkyl tethers that have been modified to be used as anion conductors. The newly synthesized ionomers were tested in both fuel cell and electrolysis devices. In situ testing of these materials shows that lower molecular weight materials outperform their higher molecular weight counterparts. Additionally, the use of a block copolymer with the introduction of a hydrophobic spacer further increased device performance.

The properties of the ionomer used to construct electrodes for direct methanol anionic fuel cells are critically important to the fuel cell performance. In this study, a new polymer backbone with a higher degree of fluorination for increased hydrophobicity has been shown to improve both the alkaline anode and cathode performance. It was also shown that decreasing the molecular weight of the ionomer improves fuel cell performance. Finally, higher fuel cell performance was observed with quinuclidinium head groups on the anionic ionomer, in comparison to the more traditional trimethyl ammonium cation. The improvements in performance appear to be due to improved mass transport of reactants and products through the electrode as a result of increased free volume within the electrode and a more efficient construction of the electrical double layer at the ionomer/catalyst interface.