Research on Unitized Regenerative Fuel Cell Stack with Producing High Pressure H2 and O2 in WE Mode

A unitized regenerative fuel cell (URFC) is an electrochemical energy storage device that combines a fuel cell and a water electrolyzer in a single system. The URFC is an excellent clean, quite energy source and are promising energy storage systems for uninterrupted power supplies, solar powered aircraft, and satellites. In the paper, the high pressure unitized regenerative fuel cell was developed and validated with a capacity of producing hydrogen and oxygen at a pressure of up to 5MPa in the water electrolysis mode.

The bifunctional performance of URFC stack in the hydrogen and oxygen fuel cell (FC) and water electrolysis (WE) modes and the generated gases pressure influence on URFC stack were investigated. The main electrochemical element is bifunctional membrane electrode assembly (MEA) with an active area of 200cm² which consists of polymer electrolyte membrane from Dupont and the electrode layer on both sides of the membrane by catalyst coated on membrane technique (CCM). Oxygen electrode layer was prepared by Pt/IrO₂mixing catalysts. Hydrogen electrode layer was prepared by 40%Pt/C (J.M.). The diffusion layer of MEA was the porous sintered metal foil with corrosion resistance.

Primary test results illustrated that high pressure URFC stack exhibited the promising bifunctional performance. The static seal level of URFC stack was up to 10MPa and the dynamic gas pressure reached 5MPa in the water electrolysis mode. Figure1 showed that the stack output electric power was greater than 1.2kW at a current density of 1000mA/cm²in the fuel cell mode.

Figure 2 showed that the generated gases pressure had few influence on the performance of the stack in the water electrolysis mode under working conditions. The single cell voltage was 5~12mV higher at the generated gases pressure of up to 5MPa when the current density was less than 700mA/cm². However the single cell voltage was 5~10mV lower at the generated gases pressure of up to 5MPa when the current density greater than 700mA/cm². The stack showed good pressure adaptability. The round-trip energy conversion efficiency of high pressure URFC stack was greater than 45% at the current density of 500mA/cm².

In addition, the cycling test of the stack during URFC operation was performed in both fuel cell and water electrolysis modes. The results indicated that high pressure URFC stack had the promising cycling stability. Further research and validation are still underway.