Reversible anion exchange membrane fuel cells (AEMFCs) have been widely considered as both energy conversion and energy storage devices due to their low cost and high-energy storage capacity, especially when combined with renewable resources. The oxygen electrode reactions have long been one of the primary limiting factors of AEMFCs because of their sluggish kinetics and high overpotential. In our previous study, bifunctional non-platinum group metal (PGM) electrocatalysts for oxygen reduction and evolution reactions have been developed to lower the overpotential on the oxygen electrode, using hybrid structure of metal oxide supported on advance carbon nanotubes [1], transition metal derived graphene tubes [2] or structurally modified transition metal oxides. In this study, the membrane electrode assemblies (MEAs) of reversible AEMFCs using these non-PGM catalysts were designed exclusively, to be applied in real operating environment.

In addition to hardware and cell component down-selection, catalyst layer composition and distribution with different combination of anion exchange membranes and ionomers were optimized extensively by varying parameters including ionomer/catalyst ratio, catalyst ink solvent, mixing method, deposition method and annealing condition. Both catalyst-coated membrane (CCM) and gas diffusion layer (GDE) configurations were compared. Figure 1 shows the highlights of fuel cell and electrolyzer performance improvements. Fig. 1a indicates higher fuel cell internal resistance but better mass transfer in GDE configuration than that in CCM configuration. Fig. 1b demonstrates that electrolyzer performance was improved at elevated temperature through lowering the resistivity of membrane electrode. Fig. 1c provides the optimized ionomer/catalyst ratio for nickel cobalt oxide in electrolyzer mode. Fig. 1d presents the significant enhancement of anion conductivity in the electrolyzer cell with the assistance of KOH solution. This work may provide systematic design perspectives for bifunctional non-PGM catalyst-based MEA fabrication used for reversible AMFCs.

Figure 1. Fuel cell and electrolyzer performances of reversible AEMFC with under different configurations, temperatures, ionomer/catalyst ratios and electrolyte compositions.

Acknowledgement: The project is financially supported by the Department of Energy’s Fuel Cell Technology Office under the Grant DE-EE0006960.

References:

[1] Zhao, Shuai, et al. "Highly durable and active Co3O4 nanocrystals supported on carbon nanotubes as bifunctional electrocatalysts in alkaline media." Applied Catalysis B: Environmental 203 (2017): 138-145.

[2] Gupta, Shiva, et al. "Highly Active and Stable Graphene Tubes Decorated with FeCoNi Alloy Nanoparticles via a Template‐Free Graphitization for Bifunctional Oxygen Reduction and Evolution." Advanced Energy Materials 6.22 (2016).