High-Performance Direct Borohydride Fuel Cells Using Non-PGM Electrocatalysts

Abstract

The Direct Borohydride Fuel Cell (DBFC) is investigated since several years as a potential power generator technology for portable and mobile applications. It takes advantage of the high intrinsic energy density of the borohydride fuel: the Borohydride Oxidation Reaction (BOR), that occurs at the anode can theoretically provide 8 electrons and has a really low theoretical onset potential of -0.42 V vs RHE. Although Pt-group metal (PGMs) catalysts are the benchmarks for this reaction, they are severely limited by the competition of the desired BOR with the unwanted hydrogen evolution reaction (HER) at potential below 0 V vs RHE. In order to limit this competition, one should consider a catalyst with a poor HER activity. In this regard, Ni showed impressive performance at low potentials but more limited at higher potentials [1]. Such performance has been obtained by precisely controlling the state of the nickel surface as described by Oshchepkov et al. [1–4]. This catalyst has been used in a single cell configuration and promising results were obtained [1] (NiED/GDL, figure 1.A).In order to increase even more the performance of the cell, Ni-foam structures have been employed to obtain highly porous and totally metallic (nickel) anodes. This new electrode morphology allowed to reach performance competing with that of Pt electrodes will be shown (NiED/Ni Felt, figure 1.A). Another aspect limiting the performance of the DBFC is the use of inappropriate membranes such as commercial Nafion 212 (in Na+ form) in figure 1.A. The group of Ramani developed a bipolar interface allowing to maintain highly different pH at the anode and the cathode, which improves drastically the performance of the system [5]. The fully-nickel-based anodes presented earlier have been used with such interfaces and performance greatly surpassing noble-based electrodes has been obtained (figure 1.B). Fully non-noble cells using Fe-N-C catalysts (from the group of Jaouen) at the cathode have also been tested and presented interesting results, when used in conjunction with an anion-exchange membrane. Acknowledgments This work has been performed in the frame of the MobiDiC project, funded by the French National Research Agency (ANR, grant # ANR-16-CE05-0009-01) and the DGA. The authors also thank the Russian Science Foundation (RSF 18-73-00143) and the US Office of Naval Research (ONR) under grant number N00014-16-1-2833 for their financial support.