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

Abstract

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 5-hydroxymethylfurfural (HMF), were selected for oxidative upgrading catalyzed by a hierarchically porous Ni 3 S 2 /Ni foam bifunctional electrocatalyst (Ni 3 S 2 /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 ∼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 O 2 . When utilized as electrocatalysts for both cathode and anode, Ni 3 S 2 /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.