Bifunctional Oxygen Electrodes for PEM-Unitized Regenerative Fuel Cell Fabricated By Inkjet Printing
Authors: Padilla Luis , Liu Jiafei , Semagina Natalia , Secanell Marc 
Published: December 2024

Hydrogen energy storage has proven to be a promising technology to balance the intermittency of renewable energy sources. The high theoretical specific energy density achieved by fuel cell-based systems makes them attractive for energy production, especially for weight and space-sensitive applications.
A unitized regenerative fuel cell (URFC) is a single device that works as both an electrolyzer and a fuel cell. The cost of a URFC could be significantly reduced compared to a separate electrolyzer and a fuel cell, since material costs are nearly halved. However, working with a single unit means that the system needs to be designed for both processes, which requires compromises to optimize the overall performance. The performance of URFC systems is significantly affected by the electrode’s fabrication method, so to further develop new and more efficient, durable, and cost-effective bifunctional catalyst layers (CL) for these systems, a more fundamental understanding of the relationship between the CL structure, and performance is required.
The most common approach in the literature for fabricating bifunctional CLs for the oxygen electrode in a URFC is to mix both ORR and OER catalysts in a single ink. An alternative method, however, is to deposit layers containing different catalysts next to each other. Even though the latter approach has shown promising results, with the potential for optimization, limited work has been published to date [1-5]; moreover, no study has been done using the inkjet printing technique for the fabrication of either mixed or multilayer arrangement electrodes.
In this work, inkjet printing is evaluated as a feasible technique for unitized regenerative fuel cells (URFCs) electrode fabrication and to study the optimal ionomer and catalyst loading of mixed and multi-layer URFC electrodes. A physical mixture of Pt, IrOx, and Nafion ionomer was inkjet printed directly on a Nafion membrane to fabricate varying loading bifunctional oxygen electrodes for a URFC and characterized via scanning electrode microscopy and cyclic voltammetry. The results show coverage of the Pt active sites by IrOx when the catalysts are mixed, which means that reducing the IrOx catalyst compared to platinum, i.e., 3:1 Pt-IrOx ratio is necessary for optimum performance [6]. A catalyst loading study shows that a round-trip efficiency of 51% at 500 mA/cm2 can be obtained using 0.67 mgPtIrOx/cm2 PMG catalyst loading in the oxygen electrode, resulting in one of the highest efficiencies by the amount of catalyst reported. The results are compared with a multilayer arrangement electrode, showing that the partial coverage of Pt sites by the IrOx can be avoided by printing the catalysts in separate layers. The Pt to Ir ratio was critical to the successful operation of the multilayer arrangement when the IrOx layer was in contact with the GDL, due to the poor conductivity of the CL, which at the highest loadings provided a barrier for the electrons to reach the Pt layer near the membrane. A catalyst ratio study showed optimum performance with a 9:1 Pt to Ir ratio, obtained with the electrode with the IrOx layer on top of the Pt CL and these on top of the PEM. In this case, the total catalyst loading was 0.56 mgPtIrOx/cm2, reaching a round-trip efficiency of 51% at 500 mA/cm2 and 44% at 1000 mA/cm2, one of the highest RT efficiency for a multi-layer arrangement and by the amount of catalyst of any constant gas bifunctional electrode reported in the literature.
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