Estimating the energy density of direct borohydride–hydrogen peroxide fuel cell systems for air-independent propulsion applications

•Maximum power density of a unit cell is 243 mW/cm2 at 25 °C.•The estimated energy density of a DBPFC system is 63.2 Wh/kg (87.1 Wh/L).•Effect of various parameters on the energy density of a DBPFC system is evaluated.•Performance target allowing DBPFCs to compete with other power sources are proposed.•The energy density at the proposed target is 378.5 Wh/kg (541.4 Wh/L).

The energy density of DBPFCs (direct borohydride–hydrogen peroxide fuel cells) is estimated for air-independent propulsion applications. The performance of DBPFCs containing heat-treated electrocatalysts supported on multiwalled carbon nanotubes is evaluated. The open circuit voltage and the maximum power density are 1.44 V and 243 mW/cm2, respectively, whereas the voltage efficiency is 17.9% at the maximum power density. Based on experimental results, the energy density is estimated to be 63.2 Wh/kg (87.1 Wh/L), which is much lower than those of other power sources. The degree of influence of various parameters on the energy density is ranked in the following order: fuel utilization efficiency > voltage efficiency > H2O2 (hydrogen peroxide) concentration > NaBH4 (sodium borohydride) concentration > maximum power density. Performance targets comprising of fuel utilization efficiency of 90%, voltage efficiency of 40%, H2O2 concentration of 50 wt%, NaBH4 concentration of 20 wt%, and maximum power density of 650 mW/cm2, have been determined. Under these conditions, the energy density and fuel volume are expected to be 378.5 Wh/kg (541.4 Wh/L) and 1.57 L, respectively, which would allow them to be used widely as power sources for air-independent propulsion.