Electrolytic hydrogenation at 100 mA cm−2 and 1.6 volts

Abstract

Electrochemical hydrogenation (ECH) could decarbonize hydrogenation reactions across energy, chemical, and pharmaceutical sectors by avoiding high-temperature reactors and sourcing hydrogen from water instead of fossil fuels. However, ECH has yet to be commercialized due to (i) impractically high operating voltages and (ii) the parasitic crossover of organic reagents across the membrane separating the cathode from the anode. We address both of these issues here with a continuous-flow zero-gap palladium (Pd) membrane electrolyzer (“Pd membrane electrolyzer”). This reactor is unique because it consists of two membranes: (i) a proton-exchange membrane (“PEM”); and (ii) a hydrogen-permeable Pd membrane. During operation, water in the anode chamber is oxidized to form H⁺. The H⁺ was then transported through the PEM to be reduced into H atoms or hydride (“reactive hydrogen”) at the Pd membrane. The reactive hydrogen passes through the Pd membrane to hydrogenate unsaturated species on the opposing side of the membrane. With the anode and cathode pressed against the PEM akin to high-performance water electrolyzers, the voltage required to generate reactive hydrogen is minimized. The use of a solid Pd membrane precludes the unwanted crossover of organic species and electrolytes, thereby overcoming a persistent shortcoming of ECH. We support these claims by demonstrating the hydrogenation of styrene as a model reaction. These experiments show a low cell voltage (1.6 V at 100 mA cm⁻²), high faradaic efficiency (>95%), fast reaction rates (1.66 mmol h⁻¹ cm⁻²), and high energy efficiency (47%) for over 30 h of continuous hydrogenation.