Characterizing SOFC Stack Electrochemical Performance at Elevated Pressure

Abstract

In this report, we present our work to characterize the electrochemical performance of solid-oxide fuel cell stacks under elevated-pressure operation. The effort is part of a five-year U.S. DoE ARPA-E INTEGRATE program to combine the high efficiency of solid-oxide fuel cells with the low-cost of reciprocating engines to demonstrate an 70%-efficient hybrid electric generator. Within this hybrid system, solid-oxide fuel cells (SOFCs) convert natural gas into electricity. Unconverted fuel leaving the SOFC stacks feeds a downstream reciprocating engine that drives an electric generator. In this architecture, the stacks are hydraulically placed upstream of the engine, necessitating stack operation at elevated pressures of 2-5 barg. SOFC stack performance at elevated pressure has witnessed limited study. The team at the Colorado School of Mines is characterizing SOFC stack performance at elevated pressure through design and commissioning of the pressurized stack electrochemical test stand shown in the figure. The facility is used to explore the performance of 5-kW SOFC stack test modules (Ceres Power, Ltd., Horsham, UK) at the unusual operating conditions presented by the hybrid system, including pressurized operation at high anode tail-gas recycle.A simulant of steam-reformed natural gas with anode tail-gas recycle is synthesized using mass flow controllers. A fuel processor will be incorporated into the stand at a later stage of the research program. Reactants flow through a preheat furnace and into a pressure vessel to reach the insulated solid-oxide fuel cell stack test module. The pressure within this vessel is matched to the operating pressure of the SOFC stack, minimizing pressure differentials between the internal flow channels of the stack and the surrounding environment. Following electrochemical conversion, exhaust gases exit the stack and the pressure vessel, and are cooled through a series of heat exchangers. Back-pressure regulators near the end of the flow stream regulate stack and vessel operating pressure.The stand has been used to explore the impacts of elevated pressure operation on the electrochemical performance of Ceres SteelCell stacks. Additional parameters being explored include fuel composition, anode recycle, and internal fuel reforming. In this talk, we will present our findings on the effects of these variables on stack performance, and their impact in achieving the aggressive performance targets of the SOFC-engine hybrid electric generator.A principal concern during performance testing at elevated pressures is minimizing pressure differentials across the anode and cathode gas-flow streams. Such differentials can bring substantial mechanical stress to the metal-supported membrane-electrode assemblies. Pressure differentials in this work are below +/- 25 mbar at 5 barg operating pressure, well below manufacturer’s specifications. The stacks show steady, stable performance over all conditions tested.In addition to exploring the electrochemical-performance impacts brought by pressurization and anode recycle, experiments explore the roles of methane internal reforming and cathode air flow on stack cooling. The endothermicity of methane steam reforming can bring meaningful reductions in cooling-air flow rate, raising system efficiency. The results of these studies and the trajectory of the research will be presented. Figure 1