Bipolar Hydrogen Production from a Hybrid Alkaline-Acidic Formaldehyde-Proton Fuel Cell

Due to a positive standard reaction Gibbs free energy (Δ_rG_m ^θ) of 237.1 kJ mol^-1, electric energy input is indispensable for hydrogen production by conventional electrochemical water splitting. This energy requirement can be reduced by replacing the anodic oxygen evolution reaction to thermodynamic favorable small-molecules oxidation reactions. In this work, anodic formaldehyde oxidation reaction (FOR) in alkaline media was paired with cathodic hydrogen evolution reaction (HER) in acidic media to establish a thermodynamically downhill system. The utilization of electrochemical neutralization energy in a hybrid alkaline-acidic electrolyte configuration enables a further decrease in Δ_rG_m ^θ. Therefore, the resulting hybrid alkaline-acidic formaldehyde-proton fuel cell (FPFC) exhibits a significantly reduced Δ_rG_m ^θ of -101.5 kJ mol^-1. A bifunctional Ru-doped Cu catalyst (Ru─Cu NTs@CM) was designed and synthesized to simultaneously promote the kinetics of acidic HER and alkaline FOR, demonstrating superior catalytic activity and durability to pristine Cu and Ru catalysts. This catalyst enabled concurrent bipolar H₂ production and electricity generation from the assembled FPFC, reaching a peak power density of 18.3 mW cm^-2 at 53.4 mA cm^-2. A combination of (quasi) in situ characterizations and theoretical calculations unveiled the important mechanistic role of Ru-doping in enhancing the Cu catalyst's activity and stability.