and component manufacturing costs, which are driven by the fact that PEMFCs require specialized component materials due to the highly acidic operating environment. To alleviate these costs, anion exchange membrane fuel cells (AEMFCs) have recently emerged as an alternative to PEMFCs. The higher pH operating environment offers several advantages such as lower material and manufacturing costs. For example, AEMFCs can use platinum group metal (PGM) free cathodes, have more facile oxygen reduction kinetics, [3,4] and may enable a wider range of fuels (e.g., methanol, hydrazine). However, early AEMFC developments suffered from very low overall performance and durability. [5,6] From a durability perspective, cells were mostly believed to be limited by chemical degradation of the anion exchange membrane (AEM) and ionomer at high pH. Because of this, there has been a significant amount of investment targeting the design and manufacturing of stable AEMs and ionomers. As a result, several AEMs have been shown to be stable both at elevated temperature (≥80 °C) and pH for hundreds or even thousands of hours during ex situ testing. [7-11] Another issue that has started to be addressed in the AEMFC literature is water management. Water is formed at the AEMFC hydrogen anode and consumed at the oxygen cathode. As water is produced at the anode, it must be removed to avoid flooding-either through the anode gas stream or, preferably, through the AEM to the cathode (via backdiffusion) where it is needed to react. [12] If too little water is supplied to the cathode by back-diffusion it can dry out; if more water arrives to the cathode than can be reacted, it is also possible for the cathode to flood. Therefore, there is a need for both the anode and cathode catalyst layers to be able to passively transport water. The ability of the ionomer in the catalyst layer to facilitate such ion transport is related to the overall hydrophilicity of the monomers used in the polymer membranes and ionomer. One example where ionomer water uptake has been limiting is the well-known ethylene-tetrafluoroethylene copolymer (ETFE) based powder ionomer. [13] Though the ion exchange capacity (IEC) of that material is modest (only 1.24 meq g −1), The primary function of the ionomers that are incorporated into fuel cell electrode catalyst layers is to provide pathways for ion transport between the catalyst active sites and the electrolyte. This is influenced by many variables, including the ion-exchange capacity, water uptake, and molecular weight. In anion exchange membrane fuel cells (AEMFCs), controlling ionomer water uptake is particularly important and tailoring this property in each electrode is an important consideration when looking to maximize cell performance. In this study, three poly(norbornene) tetrablock copolymer ionomers with a range of physical properties are synthesized and incorporated into AEMFC anode and cathode electrodes. Systematic electrode engineering with these ionomers allows the peak power density to be increased by 100% (1.6 W ...

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Alkaline fuel cells and electrolyzers are of interest because they have potential advantages over their acid counterparts. Highconductivity anion conducting membranes were analyzed and used in alkaline hydrogen/oxygen fuel cells. The membranes were composed of reinforced block copolymers of poly(norbornenes) with pendant quaternary ammonium head-groups. It was found that membranes with light cross-linking provided excellent mechanical stability and allowed very high ion exchange capacity polymers to be used without penalty of excessive water uptake and swelling. The optimum membrane and fuel cell operating conditions were able to achieve a peak power density of 3.4 W/cm 2 using hydrogen and oxygen. The performance increase was greater than expected from minimizing ohmic losses. Mechanical deformations within the membrane due to excess water uptake can disrupt full cell operation. Cells were also run for over 500 h under load with no change in the membrane resistance and minimal loss of operating voltage.

High ionic conductivity membranes can be used to minimize ohmic losses in electrochemical devices such as fuel cells, flow batteries, and electrolyzers. Very high hydroxide conductivity was achieved through the synthesis of a norbornene-based tetrablock copolymer with an ion-exchange capacity of 3.88 meq/g. The membranes were cast with a thin polymer reinforcement layer and lightly crosslinked with N,N,N ,N -tetramethyl-1,6-hexanediamine. The norbornene polymer had a hydroxide conductivity of 212 mS/cm at 80°C. Light cross-linking helped to control the water uptake and provide mechanical stability while balancing the bound (i.e. waters of hydration) vs. free water in the films. The films showed excellent chemical stability with <1.5% conductivity loss after soaking in 1 M NaOH for 1000 h at 80°C. The aged films were analyzed by FT-IR before and after aging to confirm their chemical stability. A H 2 /O 2 alkaline polymer electrolyte fuel cell was fabricated and was able to achieve a peak power density of 3.5 W/cm 2 with a maximum current density of 9.7 A/cm 2 at 0.15 V at 80°C. The exceptionally high current and power densities were achieved by balancing and optimizing water removal and transport from the hydrogen negative electrode to the oxygen positive electrode. High water transport and thinness are critical aspects of the membrane in extending the power and current density of the cells to new record values.

Cross-linked (XL) anion-exchange membranes (AEMs) synthesized by vinyl addition polymerization of norbornene were prepared for use in anion-exchange membrane electrochemical devices, including fuel cells and electrolyzers. Tetrablock copolymers composed of an all-hydrocarbon backbone with a very high ion-exchange capacity (IEC), 3.46 mequiv/g, were synthesized. Light cross-linking was found to be adequate for providing critical control over unwanted water uptake. This enabled use of very high IEC membranes. Without light cross-linking, the unwanted water uptake would cause swelling and softening of the membrane. The best performing membrane had no significant drop in ionic conductivity over 1000 h of aging in 1 M NaOH at 80 °C and a record high ionic conductivity of 198 mS/cm at 80 °C (for a chemically stable AEM). The number of bound and free water molecules per ion pair is described along with ion mobility comparisons to previous materials. The membranes are suitable for electrochemical devices and were used in AEM fuel cells.