Review Developments in direct borohydride fuel cells and remaining challenges I. Merino-Jiménez a , C. Ponce de León a, * , A.A. Shah b , F.C. Walsh a a Electrochemical Engineering Laboratory, Energy Technology Research Group, Engineering Sciences, University of Southampton, Highfield Rd., Southampton SO17 1BJ, UK b School of Engineering, University of Warwick, Coventry CV4 7AL, UK highlights < 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 article info Article history: Received 27 April 2012 Received in revised form 13 June 2012 Accepted 27 June 2012 Available online xxx Keywords: Direct borohydride fuel cells Hydrolysis inhibition Mathematical modelling Membranes Surfactants Recycling abstract 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 borohy- dride 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 engi- neer a selective catalyst able to suppress the hydrogen evolution and the elucidation of the mechanism of borohydride ion oxidation, remain. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Fuel cells offer a promising alternative to incumbent electrical power generation technologies (primarily based on fossil fuels), for medium-scale applications such as remote or backup power, as well as small-scale applications, such as portable consumer elec- tronics. Widespread adoption of the direct hydrogen-oxygen fuel cell, the most highly-developed fuel cell, is hindered by a number of problems related to the sourcing, storage and safe handling of hydrogen (particularly for mobile applications) [1]. Fuel reformers integrated with fuel cell stacks can generate the hydrogen from hydrocarbon fuels but present additional engineering complica- tions and add further volume, weight and cost to the overall power system. An alternative solution to the aforementioned issues is the replacement of gaseous hydrogen with a liquid hydrogen carrier, often an alcohol such as ethanol or methanol. The latter leads to the direct methanol fuel cell (DMFC), which, on the other hand, suffers from high rates of reactant crossover and low power densities. Other hydrogen-containing compounds under consideration are metal hydrides (in solution), such as LiBH 4 , NaBH 4 or KBH 4 [2]. Table 1 shows the theoretical energy densities for different fuels and oxidants. Comparing these values, the maximum corresponds to the NaBH 4 /H 2 O 2 system (up to 17 kW h kg 1 ), followed by the NaBH 4 /O 2 system (up to 9.3 kW h kg 1 ). The theoretical energy density of a NaBH 4 /H 2 O 2 cell is at least five-fold that of a H 2 /O 2 fuel cell, two-fold that of an ethanol/O 2 cell and three-fold that of a methanol/O 2 cell. The direct borohydride fuel cell (DBFC) has been studied since 1960, when Pt and Ni were investigated by Elder and Hickling [3] and Indig and Snyder [4], respectively, as anode catalysts for borohydride oxidation. Fig. 1 shows a histogram of the number of papers published on the DBFC and the reactions of borohydride * Corresponding author. Tel.: þ44 0 23 8059 8931/6727; fax: þ44 0 23 8059 7051. E-mail address: capla@soton.ac.uk (C. Ponce de León). Contents lists available at SciVerse ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour 0378-7753/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jpowsour.2012.06.091 Journal of Power Sources 219 (2012) 339e357