Integrating Electrochemical CO2 Reduction on -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 CO2-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 alpha-NiS, synthesized solvothermally, has been chosen as an electrode material. As the first step to construct an electrolyzer, alpha-NiS deposited on conducting nickel foam (NF) has been used as an anode, and under the anodic potential, the alpha-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 alpha-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 (similar to 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 alpha-NiS/NF, the structural integrity has been retained and the unaltered alpha-NiS/NF electrode remains more effective cathode for alkaline hydrogen evolution reaction (HER) and CO2 reduction (CO2R) compared to its in situ derived NiO(OH)ED/NF. alpha-NiS/NF can reduce the CO2 predominantly to CO even at a higher potential like -0.8 V (vs RHE). The fabricated cell with alpha-NiS and its electro-oxidized NiO(OH)ED counterpart, alpha-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 O2 and the alpha-NiS cathode reduces CO2 majorly to CO in a moderate cell potential. In this study, alpha-NiS has been utilized as a single electrode material to perform multiple sustainable transformations.