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

Conventional water electrolyzers produce H2 and O2 simultaneously, such that additional gas separation steps are needed to prevent H2/O2 mixing. The sluggish anodic O2 evolution reaction (OER) always results in low overall energy conversion efficiency and the product of OER, O2, 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 H2 generation from water splitting. Five representative biomass substrates, ethanol, benzyl alcohol, furfural, furfuryl alcohol, and 5-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 H2 production and HMF oxidation was significantly reduced by ∼200 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 O2. When utilized as electrocatalysts for both cathode and anode, Ni3S2/NF demonstrated outstanding durability and nearly unity Faradaic efficiencies for both H2 and FDCA production. Overall, such an integration of oxidative biomass valorization and HER via earth-abundant electrocatalysts not only avoids the generation of explosive H2/O2 mixture and ROS, but also yields products of high value at both electrodes with lower voltage input, maximizing the energy conversion efficiency.