PEM unitised reversible/regenerative hydrogen fuel cell systems: State of the art and technical challenges

A Unitised Regenerative Fuel Cell (URFC) is a single cell that can function both as a fuel cell and electrolyser. URFCs have mass and volume savings compared to conventional hydrogen systems employing a separate electrolyser and fuel cell. Hence they have potential applications in electrical energy storage systems for a wide range of terrestrial and space applications. This paper reviews the current state of art of this promising technology and identifies the main technical challenges to overcome to make the URFC system more technologically and economically viable. The review reveals that the oxygen-side catalyst layer in a URFC is typically more challenging to design and construct, since a bifunctional catalyst layer and gas diffusion layer (GDL) must be made that functions equally well in electrolyser and fuel cell modes. The prime goal in URFC development is to obtain a roundtrip energy efficiency very close that of a system with a separate electrolyser and fuel cell. It is important to develop light weight, long-lived cells resistant to corrosion or other degradation, and retention of structural integrity and strength after repeated cycling and mode switching. There is still a lack of practical designs and operating experience for URFC stacks scaled up to multi-cell stacks with supply power in the kW range. In addition, development of URFC stacks able to deliver hydrogen at elevated pressure up to 20 bar in E-mode is essential. So that hydrogen can be stored directly as compressed gas or in metal hydride cylinder to reduce the cost of external compressor and parasitic power in standalone power supply systems. If these challenges can be overcome, URFC based on proton exchange membrane (PEM) technology has potential for reducing the cost of overall hydrogen fuel cell system and can offer clean, reliable and onsite hydrogen and power generation.