Unitized Regenerative Fuel Cells in Constant Gas and Constant Polarity Modes for Performance Optimization

Abstract

Unitized regenerative fuel cells (URFC) can operate as both fuel cells and electrolyzers. In fuel cell mode, a proton exchange membrane based device can convert chemical energy in hydrogen molecules to electricity. Alternatively, when switched to the electrolyzer mode, the same device can convert excess energy from the grid to chemical energy for storage. Thus, URFCs can operate as rechargeable batteries with infinite capacity as long as storage and delivery of hydrogen, oxygen and water are not limited. URFCs have traditionally been configured in constant gas (CG) mode. In CG mode, hydrogen is either oxidized via hydrogen oxidation reaction (HOR) or protons are reduced via hydrogen evolution reaction (HER) on the same electrode using carbon supported Pt as the catalyst. Similarly, at the other electrode, oxygen is either reduced via oxygen reduction reaction (ORR) or water is oxidized via oxygen evolution reaction (OER) using unsupported mixture of Ir and Pt as the catalyst. Such configuration is intuitively the simplest because overall gas and product management is simplified. However, the water condensation at the ORR electrode is an issue. However, URFCs can also be configured in constant polarity (CP) mode. In CP mode, the polarity of the electrodes remains unchanged with oxidation reaction, HOR and OER, taking place on the anode, and reduction reactions, ORR and HER, occurring at the anode. The overarching advantage of the CP mode is that oxygen evolution and oxygen reduction reactions are catalyzed by two different catalyst layers unlike in CG mode. Thus, the thermodynamic and electrochemical burdens on the catalyst layers are less strenuous. Beyond catalysis, each component of the device also has to be compatible to perform bifunctional and multifunctional tasks. For example, in CP mode, the same anode porous transport layer (PTL) needs to facilitate transport of liquid water in and let oxygen diffuse out in electrolyzer mode; and in fuel cell mode, the same layer needs to allow hydrogen delivery without significant transport hindrance to the active sites in the catalyst layer. Thus, materials and geometry of each component needs to be based on optimum performance, durability and compatibility in overall electrochemical conditions necessary in both fuel cell and electrolyzer modes. In this presentation we will discuss methodology used to select various components, geometry and conditions in both modes of URFC operations. We will discuss overall system performance, performances in stand-alone modes and discuss operational limitations. Finally, we will also discuss system efficiencies and importance of cost benefit analysis when considering complex yet highly desirable systems like URFCs. For example, as apparent from Figure 1, URFC in CP mode has been optimized to reach almost 60% efficiency at 1 A/cm2 while in CG mode round trip efficiencies are <50%. We will discuss how optimizing each component and improving device integration can lead to enhanced performance. Figure 1