Direct borohydride fuel cells: A selected review of their reaction mechanisms, electrocatalysts, and influence of operating parameters on their performance

Abstract

Direct borohydride fuel cells (DBFC) oxidize an easily-stored energy-dense borohydride fuel (sodium borohydride: NaBH 4 ), that in theory reacts ca . 400 mV below H 2 and produces 8 electrons per BH 4 - anion. However, the borohydride oxidation reaction (BOR) does not fully meet these promises in practice: the electrocatalyst nature, structure and state-of-surface, and the operating conditions (pH, BH 4 - concentration, temperature, fluxes) noticeably influence the BOR kinetics and mechanism. Nickel and platinum-based catalysts both have assets for the BOR. DBFCs can only yield decent performance if their separator combines high ion-conductivity and efficient separation of the reactants; cation-exchange membranes, anion-exchange membranes, bipolar membranes and porous separators all have their own advantages and drawbacks. Besides the anode, the choice of separator must consider the DBFC cathode reaction, where oxygen (usually from air) or hydrogen peroxide are reduced, provided adapted catalysts are used. All these aspects drive the DBFC performance and stability/durability. • The BOR kinetics/mechanism strongly depends on the catalyst and operating parameters. • Metallic nickel is highly efficient at low potential values but leads to high H 2 escape. • Platinum leads to fast kinetics of the H 2 production from BH 4 - and fast H 2 oxidation. • Depending on the membrane/separator, DBFC must employ BH 4 - tolerant cathodes. • The morphology of the electrode, diffusion medium and bipolar plate channels impact mass-transport hence DBFC performance.