Poly(aryl piperidinium) Based Hydroxide Exchange Membranes and Ionomers
Authors: Wang Junhua , Zhao Yun , Setzler Brian P. , Wang Lan , Hu Keda , Rojas-Carbonell Santiago , Xu Bingjun , Yan Yushan 
Published: February 2020

The first commercial sales of fuel cell automobiles based on high-performance proton exchange membrane fuel cells (PEMFCs) mark an important milestone for the field, bringing to the market a zero-emission vehicle with the advantages of long driving range and short refuelling time.1 Chiefly due to the use of Pt electrocatalysts in PEMFCs, they are not yet cost competitive with gasoline engines. Thus, further cost reductions are needed to facilitate PEMFCs’ wider adoption. Current efforts have been focused on the reduction of the Pt loading in membrane electrode assemblies (MEAs), however, the acidic environment has been a significant barrier.2 A fundamentally different approach to lower the cost of fuel cells is to switch from the acidic proton exchange membranes (PEMs) to the alkaline hydroxide exchange membranes (HEMs) in HEM fuel cells (HEMFCs), which could enable the employment of platinum-group-metal (PGM)-free anode and cathode catalysts.3
,
4 A key challenge in the development of high performance HEMFCs is the lack of chemically stable, ionically conductive, and mechanically robust HEMs.

HEMs are typically composed of organic cations tethered to a polymer backbone, with the charge balanced by free OH− anions. The alkaline stability of both the organic cation and the polymer backbone limit the overall stability of the HEM.5 Recently, Marino and Kreuer reported that piperidinium exhibited a 21-fold increase in alkaline stability as compared to benzyl trimethyl ammonium, the benchmark ammonium.6 Meanwhile, a reliable and scalable synthetic route to tether piperidinium to a stable polymer backbone remains elusive. It is generally accepted that functionalized ether-bond-containing engineering plastics, e.g., poly(arylene ether) and poly(phenylene oxide), are susceptible to chemical degradation via ether-bond scission and the subsequent decomposition.7 Another key challenge in the development of HEMs is the tradeoff between ionic conductivity and mechanical strength.

In this work, we report a new family of poly(aryl piperidinium) (PAP) HEMs/HEIs in which alkaline stable piperidinium cations were tethered to an ether-bond-free, rigid and hydrophobic aryl backbone. PAP’s excellent properties originate from the combination of the stable piperidinium cation and the rigid ether-bond-free aryl backbone which enables an unprecedented combination of a high ion exchange capacity (IEC) and a low water uptake/swelling ratio. PAP HEMs have demonstrated excellent alkaline stability (e.g., no degradation in 1 M KOH for 2000 h at 100 °C), high OH− conductivity (e.g., 200 mS cm−1 at 95 °C in water), and high mechanical strength (140 MPa). PAP also has the selective solubility to make ionomer solutions and can be cast into membranes with thicknesses down to 5 µm. MEAs using PAP HEM and the corresponding ionomer showed a peak power density of 860 mW cm−2 at 95 °C with minimal optimization. Further, discharge at 90 °C revealed no voltage degradation at constant current density of 200 mA cm−2 for 60 h, in contrast to the rapid degradation of a commercial membrane/ionomer under identical conditions (within minutes).

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