Developments in direct borohydride fuel cells and remaining challenges

Over the last twenty years, there has been a resurgent research interest in direct borohydride fuel cells (DBFCs) highlighting the fundamental aspects that need to be addressed to achieve their optimal performance. The main problem is the hydrolysis of borohydride ions, which generates hydrogen, decreases the energy efficiency and reduces the power density. The electrons released during borohydride oxidation, the cell potential difference and the power output are strongly influenced by the choice of anode and cathode, including three-dimensional and nanostructured electrodes, the electrolyte composition and the operating conditions. Extensive investigations on various anodic electrocatalysts and their effect on the oxidation and hydrolysis have been quantified as well as the cathode catalyst and its influence on the overall fuel cell performance. Computational methods such as ab-initio and physical modelling could play prominent roles in the design and fundamental characterisation of DBFCs but are currently underused and only small number of studies in well-defined materials such as Pt (111) or Au (111) exist. Cell design and configuration have also been considered but the basic requirement to engineer a selective catalyst able to suppress the hydrogen evolution and the elucidation of the mechanism of borohydride ion oxidation, remain. ► We review aspects of the borohydride fuel cell that have not been revised previously. ► Aspects of the borohydride hydrolysis, modelling, simulation and recycling are discussed. ► Future trends and recommendations to improve the technology are suggested