Direct borohydride fuel cells (DBFCs) may offer high energy density due to the potential release of 8 electrons when borohydride (BH₄^-) has been fully oxidized (overall reaction: BH₄^- + 8OH^- à BO₂^- + 6H₂O + 8e^-). However, the borohydride oxidation reaction (BOR) is complex producing several reaction intermediates and solid borates¹, and may compete with the heterogeneous hydrolysis of BH₄^- to produce H₂, which can be lost. In consequence, the full 8-electron oxidation reaction may be practically inaccessible. Incomplete BOR is largely due to transport of intermediate species away from the electrocatalyst before complete oxidation can occur. The escape of incompletely oxidized species leads to BH₄^- and intermediate species concentrations (including H₂) which vary as the fuel solution progresses through the fuel cell flow field ². For this reason, the optimal catalyst layer composition and porosity are likely different at the fuel inlet vs outlet, making the catalyst layer and overall cell design a challenge.

In this study, the direct electrooxidation of sodium borohydride (NaBH₄) is investigated within a liquid flow-through DBFC having a catalyst composition which varies from inlet to outlet. Electrochemical and spray coating deposition are used to form catalyst layers composed of a gradient of both thin-film and carbon-supported catalysts. The efficacy of the fuel cell design is evaluated electrochemically to understand which catalyst layer system maximizes fuel diffusion, minimizes hydroxide and hydrogen adsorption, and facilitates the removal of borate from the catalyst layer.

The characteristics of catalyst layer structure/composition before and after testing, and electrochemical efficiency for BOR are observed using several analytical techniques including scanning electron microscopy (SEM), X-Ray energy dispersive spectroscopy (X-EDS), Braunauer-Emmett-Teller sorptometry (BET), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS).

(1) Olu, P. Y.; Bonnefont, A.; Rouhet, M.; Bozdech, S.; Job, N.; Chatenet, M.; Savinova, E. Electrochim. Acta 2015, 179, 637.

(2) Stroman, R. O.; Jackson, G. S. J. Power Sources 2014, 247, 756.