Development of a Unitized Reversible Fuel Cell Stack for High Round-Trip Efficiency Energy Storage and Conversion

Unitized reversible fuel cells are a promising clean energy storage and conversion technology to support intermittent renewable energy resources; however, their adoption is prevented by low round-trip efficiency and expensive materials. Round-trip efficiency suffers due to the contradictory roles a single porous transport layer must perform; in fuel cell mode, hydrophobicity is preferred for effective gas transport and liquid water drainage, whereas in electrolysis cell mode, hydrophilicity is suitable for transporting aqueous reactants to the catalyst layer. Furthermore, unitized reversible fuel cells necessitate a bifunctional oxygen electrode with expensive platinum-group metal catalysts due to the distinct requirements of the oxygen reduction reaction and oxygen evolution reaction. In this project, we will develop a high round-trip efficiency unitized reversible fuel cell stack by scaling up and optimizing our amphiphilic titanium porous transport layers that contain both hydrophobic domains for gas transport in fuel cell mode and hydrophilic domains for water transport in electrolysis cell mode. The optimized transport layers will be integrated with lowloading catalysts to create low-cost electrodes. In the Phase I project, we achieved >52% round-trip efficiency in a 5cm2 single-cell unitized reversible fuel cell with our amphiphilic titanium porous transport layer and dual-layer bifunctional oxygen electrode design. This transport layer was fabricated by embedding microporous Teflon particles into a porous titanium sinter to form naturally hydrophobic gas transport paths, while the naturally hydrophilic sinter skeleton provides an effective pathway for liquid water transport. In the Phase II project, the amphiphilic porous transport layers will be scaled up and optimized to 50cm2 and low-loading catalysts will be integrated to reduce cost. The ratio and design of hydrophobic vs. hydrophilic regions will be explored to maximize performance in a larger stack where mass transport and heat management become more challenging. These transport layers will be integrated with a novel method for catalyst deposition that reduces loading and simplifies electrode fabrication while maintaining performance. A round-trip efficiency of >52% in a 50cm2 3-cell unitized reversible fuel cell stack will be demonstrated at the end of the project. The usage of fossil fuels and resulting emissions would be significantly reduced if usable energy were readily available from clean sources like solar and wind. The proposed project will develop and deliver a low-cost, high-efficiency unitized reversible fuel cell. It can be used to store renewable energy as hydrogen and reversely convert the chemical energy in hydrogen back into electricity. In addition, the developed technology can be used for power plants to store off-peak electricity, thus reducing plant component stress and promoting efficient plant operations.