A General Strategy for Decoupled Hydrogen Production from Water Splitting by Integrating Oxidative Biomass Valorization

Conventional water electrolyzers produce H-2 and O-2 simultaneously, such that additional gas separation steps are needed to prevent H-2/O-2 mixing. The sluggish anodic O-2 evolution reaction (OER) always results in low overall energy conversion efficiency and the product of OER, O-2, is not of significant value. In addition, the potential formation of reactive oxygen species (ROS) may lead to degradation of cell membranes and thus premature device failure. Herein we report a general concept of integrating oxidative biomass upgrading reactions with decoupled H-2 generation from water splitting. Five representative biomass substrates, ethanol, benzyl alcohol, furfural, furfuryl alcohol, and S-hydroxymethylfurfural (HMF), were selected for oxidative upgrading catalyzed by a hierarchically porous Ni3S2/Ni foam bifunctional electrocatalyst (Ni3S2/NF). All the five organics can be oxidized to value-added liquid products at much lower overpotentials than that of OER. In particular, the electrocatalytic oxidation of HMF to the value-added 2,5-furandicarboxylic acid (FDCA) was further studied in detail. Benefiting from the more favorable thermodynamics of HMF oxidation than that of OER, the cell voltage for integrated H-2 production and HMF oxidation was significantly reduced by-100 mV relative to pure water splitting to achieve 100 mA cm(-2), while the oxidation product (FDCA) at the anode was much more valuable than O-2. When utilized as electrocatalysts for both cathode and anode, Ni3S2/NF demonstrated outstanding durability and nearly unity Faradaic efficiencies for both H-2 and FDCA production. Overall, such an integration of oxidative biomass valorization and HER via earth-abundant electrocatalysts not only avoids the generation of explosive H-2/O-2 mixture and ROS, but also yields products of high value at both electrodes with lower voltage input, maximizing the energy conversion efficiency.