Modeling the performance of an ideal NaBH4-H2O2 direct borohydride fuel cell

A 2D direct borohydride fuel cell (DBFC) model has been developed to explore the prospective performance of this technology, for a cell with fast selective electrocatalysts and a selective membrane. In the modeled DBFC, a Nafion membrane in the Na+ form separates flow channels with aqueous fuel (0.1-0.5 M NaBH4/4 M NaOH) and oxidizer (4 M H2O2/4 M H2SO4). Electrochemical reactions occur on catalyst-coated channel walls. The electrocatalysts are selective for complete BH4− oxidation and H2O2 reduction, the reactions have fast forward rate constants, and only Na+ and H2O cross the membrane. The model captures interfacial charge transfer reactions and complex transport in the flow channels and membrane. Results show that current density and voltage efficiency vary by >50% from inlet to outlet due to concentration boundary layer development. The BH4− concentration boundary layer limits peak power density, despite migration and fuel utilizations below 10%. Power density increases with BH4− inlet concentration and fuel flow rate, but at the expense of lower fuel utilization. Water crosses the membrane up to 14 times its production rate at the anode. Low fuel utilization and water imbalance suggest the importance of system designs with reactant recirculation and water recovery.