Hypophosphites as Eco‐Compatible Fuels for Membrane‐Free Direct Liquid Fuel Cells

Abstract

Crossover of liquid fuel remains a severe problem for conventional direct liquid fuel cells even when polymer electrolyte membranes are applied. Herein, we report for the first time a membrane-free direct liquid fuel cell powered by alkaline hypophosphite solution. The proof-of-concept fuel cell yields a peak power density of 32 mW cm under air flow at room temperature. The removal of the polymer electrolyte membrane is attributed to the high reactivity and selectivity of Pd and α-MnO towards the hypophosphite oxidation on the anode and oxygen reduction on the cathode, respectively. The discharge products are analyzed by P NMR spectroscopy and the faradaic efficiencies have been calculated after discharging at 10 mA cm for 20 hours. The non-toxicity of hypophosphite and membrane-free fuel cell structure provide huge potential for future applications.

Theory-Guided Design of Anode Catalysts for Hydrogenous Liquid Fuels

“…Hypophosphite is newly reported as a novel fuel for membrane-free direct liquid fuel cells (DLFCs) . Its high theoretical energy density (185 W h/L for 1 mol/L hypophosphite), low cost, nontoxicity, and zero carbon-emission have won advantages over conventional fuels such as methanol, − ethanol, formate, , borohydride, − hydrazine, and ammonia borane .…”

“…Hypophosphite oxidation represents an interesting system: it shows great correlation with HOR for hydride oxidation, , but displays an unusual catalyst selectivity on Pd rather than Pt for a normal hydride oxidation. ,, The chemical decomposition of hypophosphite is fast on Pd, the generated hydrides show great influence on the electrode process: first, the open circuit potential of the Pd electrode in 0.5 M NaH 2 PO 2 + 1 M KOH solution is −1.09 V versus standard hydrogen electrode (SHE), which is equal to the Pd-hydride potential in the same condition, but is deviated from the thermodynamic potential of −1.51 V versus SHE in the standard alkaline condition . Second, whether the electric field is applied, hydrogen evolution resulted by combining hydrides is vigorous on Pd surface.…”

“…Hypophosphite is newly reported as a novel fuel for membrane-free direct liquid fuel cells (DLFCs). 1 Its high theoretical energy density (185 W h/L for 1 mol/L hypophosphite), low cost, nontoxicity, and zero carbonemission have won advantages over conventional fuels such as methanol, 2−4 ethanol, 5 formate, 6,7 borohydride, 8−10 hydrazine, 11 and ammonia borane. 12 However, similar to all of these liquid fuels based on hydrogen compounds, hypophosphite oxidation also suffers from a low faradaic efficiency and a low energy efficiency in the fuel cell operation.…”

“…13 Second, whether the electric field is applied, hydrogen evolution resulted by combining hydrides is vigorous on Pd surface. In our previous works, 1,13 it is estimated that this dissipation of hydrogen from Pd electrode causes low faradaic efficiencies (<30%) in fuel cell operations. Both the deviated potential of electrode and faradaic efficiency loss correlates to hydride indicate that the Volmer reaction *H → H + + e − is of great importance in hypophosphite's anodic process.…”