Experimental Analysis of Operating Conditions of Proton Exchange Membrane Based Unitized Regenerative Fuel Cells for Efficient and Economic Energy Conversion

Abstract

Unitized regenerative fuel cells (URFCs) are energy conversion and storage devices usually promoted in applications such as remote locations, off-grid and unmanned vehicles. However, with cost reductions and efficiency improvements they have the potential to offer a techno-economic advantage over battery storage and discrete fuel cell+electrolyzer systems opening up new application areas. A single URFC device comprising the same membrane electrode assembly (MEA) based stack can convert excess grid energy during off peak hours or from renewable sources while operating as an electrolyzer and store H2; then convert the stored chemical energy in H2 to electricity when in fuel cell mode. A regenerative unit requires overcoming scientific and engineering challenges such as fluid management, stable materials for various components, and reliable switching between the two modes of operation. URFCs also require bifunctional catalysts in order to operate under fuel cell and electrolysis conditions on the same MEA. Most commonly investigated URFCs operate in a constant gas mode where the hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) are performed on one electrode and oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) take place on the other electrode. However, URFCs can also be assembled to perform in constant polarity mode where HOR and OER take place on the anode and ORR and HER take place on the cathode. Platinum deposited on high surface area carbon (Pt/C) has been extensively engineered over the years to efficiently catalyze HER, ORR, and HOR in acid without severe degradation over relevant operation times. Thus, Pt/C can be used as a bifunctional catalyst when the reaction pairs are HER/HOR or HER/ORR in proton exchange membrane (PEM) based URFCs. However, when the reaction pairs are either ORR/OER or OER/HOR, Pt/C is not sufficient. Additionally, carbon cannot be used as support material because it readily oxidized under OER conditions and the catalyst degrades rapidly. Thus, there is an urgent need to identify efficient and economic bifunctional catalyst for OER/HOR and ORR/OER in order to realize URFCs as functional devices. In this study we have investigated bifunctional catalyst inks in PEM based membrane electrode assemblies (MEA). Initially, various atomic ratios of unsupported Pt and Ir are investigated to identify a mixture that provide the best performance compromise when operated in standalone fuel cell and electrolyzer modes. The physical mixtures are compared against standalone fuel cell and electrolyzers with similar catalyst loadings and membranes. Subsequently, the optimized ratio is implemented in MEAs in reversible mode and performance assessed in comparison to state-of-the-art fuel cells and electrolyzers. Additionally, various aspects of MEA fabrication and cell assemblies are also discussed including ionomer to catalyst ratio, membrane pretreatments, flow-field geometry, and various combinations of PTL and MPL in MEA. The study will provide pathways towards future materials improvements and system optimizations for URFCs.