Research progress and prospects of unitized regenerative fuel cells

With its high calorific value, stable energy output, and zero-emission byproducts, hydrogen energy is seen as a leading alternative to fossil fuels in the 21st century. By combining hydrogen energy with renewable sources such as solar and wind power, hydrogen can be produced through water electrolysis, allowing for long-term storage and long-distance transportation. Unitized Regenerative Fuel Cells (URFCs) combine the functionalities of both fuel cells (FC) for power generation and water electrolyzers (WE) for hydrogen production in a single device, providing benefits such as high specific energy, high conversion efficiency, and zero emissions. With advancements in hydrogen technology, URFCs are poised to play a crucial role in energy storage and supply, long-duration power systems, and space applications. Despite these advantages, several challenges hinder their large-scale implementation. One major obstacle is the difficulty URFC components face in simultaneously handling the bidirectional reaction and gas-water transport required in both FC and WE operations. The high usage of costly metals and the relative inefficiencies of the bifunctional modes, compared to dedicated fuel cells and electrolyzers, present significant barriers. Additionally, the severe operational conditions and frequent cycling can lead to accelerated degradation, affecting long-term performance. This paper summarizes the current state of research on URFCs both domestically and internationally, and offers insights into future research directions. It begins by discussing the working principles and classifications of URFCs, then delves into the advancements in component and material research for URFCs with a constant gas configuration. The study details how bifunctional modes influence the materials, structure, and lifespan of CLs, PTLs, and BPPs. For bifunctional oxygen catalysts to efficiently support both ORR and OER, the use of Pt and IrO is common, though this combination tends to reduce the activity of the supporting material. Metal oxides are preferred as they provide better support than commercial carbon supports, despite having lower specific surface areas and conductivity. The increased dissolution rate of Pt in a bidirectional mode significantly contributes to URFC degradation, although the specific mechanisms of degradation are not fully understood. An important research focus for improving the bidirectional performance and durability of PEM-URFCs is the development of ultra-low loading noble metal mixed catalysts on metal supports or the creation of separate OER and ORR catalysts. For the PEM, existing membrane materials have difficulty simultaneously achieving high proton conductivity, low gas permeability, and strong mechanical strength for FC and WE modes. There remains a need for better membrane materials and reinforced structural designs. For the PTL at oxygen side, typical materials include carbon-based substances with anti-corrosion coatings and chemically stable titanium-based materials. Titanium felt with an MPL is favored for the oxygen side in URFCs, but its porosity, wettability, and conductivity require further optimization. For the BPPs, surface treatments such as coating with gold, silver, or platinum, and nitridation are common methods to improve durability of BPPs. Analyzing the water-gas transmission requirements and the mode-switching demands of dual working modes is essential for enhancing overall performance. Water-gas transmission behavior under dual-mode operation necessitates coordinated regulation of materials and structures for membrane, CLs, PTLs, and BPPs to satisfy the requirements of both operational modes. Amphiphilic PTLs facilitate water-gas transmission for both WE and FC modes, providing solutions for URFC water-gas management. In addition, a promising approach is to explore materials that can dynamically adjust their hydrophilic and hydrophobic properties in response to differences between the two modes, such as changes in electron flow and temperature. Transitioning from WE mode to FC mode is critical for URFCs, requiring comprehensive consideration of gas consumption, safety, and additional factors. This paper systematically reviews the research advancements in bifunctional components of URFCs and delineates future research directions. These insights are crucial for guiding the enhancement of URFC performance.