Integrating Electrochemical CO₂ Reduction on α-NiS with the Water or Organic Oxidations by Its Electro-Oxidized NiO(OH) Counterpart to an Artificial Photosynthetic Scheme

Efficient hydrogen production, biomass up-conversion, and CO 2 -to-fuel generation are the key challenges of the present decade. Electrocatalysis in aqueous electrolytes by choosing suitable transition-metal-based electrode materials remains the green approach for the trio of sustainable developments. Given that, finding electrode materials with multifunctional capability would be beneficial. Herein, the nanocrystalline α-NiS, synthesized solvothermally, has been chosen as an electrode material. As the first step to construct an electrolyzer, α-NiS deposited on conducting nickel foam (NF) has been used as an anode, and under the anodic potential, the α-NiS particles have lost sulfides to the electrolyte and transform to amorphous electro-derived NiO(OH) (NiO(OH) ED ), confirmed by different spectroscopic and microscopic studies. In situ transformation of α-NiS to amorphous NiO(OH) ED results in an enhancement of the electrochemical surface area and not only becomes active toward oxygen evolution reaction (OER) at a moderate overpotential of 264 mV (at 20 mA cm –2 ) but also can convert a series of biomass-derived organic compounds, namely, 2-hydroxymethylfurfural (HMF), 2-furfural (FF), ethylene glycol (EG), and glycerol (Gly), to industrially relevant feedstocks with a high (∼96%) Faradaic efficiency. During these organic oxidations, NiO(OH) ED /NF participate in the multiple-electron oxidation process (up to 8e – ) including C–C bond cleavages of EG and Gly. During the cathodic performance of the α-NiS/NF, the structural integrity has been retained and the unaltered α-NiS/NF electrode remains more effective cathode for alkaline hydrogen evolution reaction (HER) and CO 2 reduction (CO 2 R) compared to its in situ-derived NiO(OH) ED /NF. α-NiS/NF can reduce the CO 2 predominantly to CO even at a higher potential like −0.8 V ( vs RHE). The fabricated cell with α-NiS and its electro-oxidized NiO(OH) ED counterpart, α-NiS/NF(−)/(+)NiO(OH) ED /NF, is able to show an artificial photosynthetic scheme in which the NiO(OH) ED /NF anode oxidizes water to O 2 and the α-NiS cathode reduces CO 2 majorly to CO in a moderate cell potential. In this study, α-NiS has been utilized as a single electrode material to perform multiple sustainable transformations.