Proton exchange membrane unitized regenerative fuel cells (URFCs) combine the ability to both produce power (in fuel cell mode) and generate fuel and oxidant (in electrolysis mode) from the same cell. The further development and utilization of URFCs is hindered by the high costs and low stability of bifunctional oxygen electrocatalysts. Distributing the oxygen electrocatalyst on a high surface area support can reduce the loading of the active noble metal (e.g., platinum and iridium) and influence the efficiency and stability of the catalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Carbon, which is typically used as a support material, is highly unstable under the conditions required for OER which makes carbon unfeasible for long-term use within URFCs. Therefore, alternative carbon-free catalyst supports are needed. Niobium oxide, Nb2O5, is stable in the oxidative potentials and highly corrosive acidic conditions of URFCs; however, the low electronic conductivity of Nb2O5 significantly limits its use as a catalyst support material. We investigated approaches to obtain high surface area and conductive niobium oxides, NbOx, as stable supports for bifunctional oxygen electrocatalysts. The effects of synthesis conditions, drying methods, and temperature/atmosphere treatments on the structure, morphology, surface area, and electronic conductivity were evaluated. The structure, morphology, composition, and surface area were determined by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and nitrogen physisorption analysis. Sol-gel synthesis and supercritical drying resulted in a NbOx aerogel with high surface area and porous morphology. Thermal treatment of the NbOx aerogel under H2 increased the electronic conductivity of NbOx when compared to the as-prepared NbOx. Different methods to deposit noble metals on the NbOx support were evaluated. The NbOx-supported catalysts were tested as bifunctional ORR/OER electrocatalysts to determine their activity and stability. The development supported bifunctional oxygen electrocatalysts that can provide lower noble metal loadings, higher activity and improved stability under harsh acidic conditions and highly oxidizing potentials furthers the development of URFCs with lower cost, improved performance, and enhanced durability.

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. 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. 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.

Proton exchange membrane unitized regenerative fuel cells (PEM-URFCs) combine the ability to both produce power (in fuel cell mode) and generate fuel and oxidant (in electrolysis mode) from the same cell. Compared with discrete fuel cell and electrolyzer systems, URFCs provide the potential for lower mass, volume, and cost. The composition and structure of the bifunctional oxygen catalyst layer (CL), which facilitates the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the same electrode, significantly affects performance and stability, particularly given the wide potential differences and different mass transport phenomena involved under operation in fuel cell or electrolysis mode. We previously showed bimetallic nanoframes provide bifunctional oxygen electrocatalysts with significantly higher activity compared with monometallic structures, evaluated using a rotating disk electrode configuration.1 Here, we investigated the effects of the oxygen CL composition, loading, and spray parameters on the performance of URFC membrane electrode assemblies (MEAs). Pt black and IrO2 catalysts were combined at different ratios and deposited at different mass loadings. Catalyst layers were sprayed directly onto membranes using an ultrasonic spray system. Within ultrasonically sprayed catalyst layers, higher spray rates resulted in improved fuel cell performance. Higher catalyst loadings improved both fuel cell and electrolyzer performance, but reached a region of diminishing returns at higher loadings. The ratio of the ORR catalyst (Pt) to the OER catalyst (IrO2) modulated the performance in both fuel cell and electrolyzer modes. Different catalyst ratios influence the catalyst utilization at higher loadings. The comparison of URFC performance at same Pt or IrO2 loadings indicates that in addition to the active catalyst itself, the non-catalytically active component influences MEA performance. This result is in line with our previous evaluation of synergistic effects of Pt and IrO2 within rotating disk electrode measurements1 and supports that in addition to the significant effects of catalyst material structure on performance, the electrode structure plays a critical role in fuel cell and electrolyzer performance in URFC MEAs. Repeated switching of URFC MEAs between fuel cell and electrolysis modes indicates that reasonable performance is maintained over the initial cycles within our single cell test configuration. 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 Appl. Energy Mater. 2020, 3, 2404-2421. http://dx.doi.org/10.1021/acsaem.9b02051