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.