Ultra-low voltage bipolar hydrogen production from biomass-derived aldehydes and water in membrane-less electrolyzers

Water electrolysis using renewable energy inputs is being actively pursued as a green route for hydrogen production. However, it is limited by the high energy consumption due to the sluggish anodic oxygen evolution reaction (OER) and safety issues associated with H₂ and O₂ mixing. Here, we replaced the OER with an electrocatalytic oxidative dehydrogenation (EOD) of aldehydes for bipolar H₂ production and achieved industrial-level current densities at cell voltages much lower than during water electrolysis. Experimental and computational studies suggest a reasonable barrier for C-H dissociation on Cu surfaces, mainly through a diol intermediate, with a potential-dependent competition with the solution-phase Cannizzaro reaction. The kinetics of the EOD reaction was further enhanced using a porous CuAg catalyst prepared from a galvanic replacement method. Through Ag incorporation and its modification on the Cu surface, the geometric current density and electrocatalyst durability were significantly improved. Finally, we engineered a bipolar H₂ production system in membrane-electrode assembly-based flow cells to facilitate mass transport, achieving maximum current densities of 248 and 390 mA cm⁻² at cell voltages of 0.4 V and 0.6 V, respectively. The faradaic efficiency of H₂ from both the cathode and anode reactions attained ∼100%. Taking advantage of the bipolar H₂ production without the issues associated with H₂/O₂ mixing, an inexpensive, easy-to-manufacture dialysis porous membrane was demonstrated to substitute the costly anion exchange membrane, achieving an energy-efficient and cost-effective process in a simple reactor for H₂ production. An estimated H₂ price of $2.51/kg from an initial technoeconomic assessment is competitive with US DoE's “Green H₂” targets.