Effects of Catalyst Layer and Porous Transport Layer Properties on Performance of Unitized Regenerative Fuel Cells

Abstract

A unitized regenerative fuel cell (URFC) that uses a single cell that functions in electrolysis or fuel cell mode provides potential advantages including lower cost, mass, and volume compared to separate discrete systems. However, significant challenges for URFCs include lower performance, lower round-trip efficiency, and higher degradation compared to separate systems. The lower performance and enhanced degradation of URFCs are generally considered to consist of catalyst degradation and water management issues that arise during continuous operation and switching between electrolyzer and fuel cell modes. Underlying URFC performance and degradation are complex interconnected processes that involve the catalysts, catalyst layers, gas diffusion layer (GDL), porous transport layer (PTL), and operating conditions within both electrolyzer and fuel cell modes. To improve performance and stability, we modified the structure of the bifunctional oxygen catalyst layer (CL) and porous-transport layer (PTL) within proton exchange membrane (PEM) URFC membrane electrode assemblies (MEAs). We investigated the effects of different experimental conditions (e.g. catalyst (Pt-IrO2) ratio catalyst loading, spraying/drying conditions, porosity, and content of polytetrafluoroethylene within the CL and/or PTL) and their influence on catalyst layer thickness, porosity, wettability, mass transport, and cell resistance which were correlated to round-trip efficiency and stability over repeated switching between electrolyzer and fuel cell modes. The understanding of the effects of CL and PTL structure on PEM URFC MEAs performance and stability underlies the ability to fabricate improved URFCs and lays the foundations of experimental conditions necessary for the application of advanced oxygen electrocatalysts, as recently reported by our group.1,2,3 References Godínez-Salomón, F.; Albiter, L.; Mendoza-Cruz, R.; Rhodes, C.P. Bimetallic Two-dimensional Nanoframes: High Activity Acidic Bifunctional Oxygen Reduction and Evolution Electrocatalysts. ACS Applied Energy Materials 2020, 3, 2404-2421. http://dx.doi.org/10.1021/acsaem.9b02051 Godínez-Salomón, F.; Albiter, L.; Alia, S. M.; Pivovar, B. S.; Camacho-Forero, L. E.; Balbuena, P. B.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Rhodes, C. P., Self-Supported Hydrous Iridium–Nickel Oxide Two-Dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts. ACS Catal. 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171 Godinez-Salomon, F.; Mendoza-Cruz, R.; Arellano-Jimenez, M. J.; Jose-Yacaman, M.; Rhodes, C. P., Metallic Two-Dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability. ACS Appl. Mater. Interfaces 2017, 9, 18660-18674. DOI: 10.1021/acsami.7b00043