Electrochemical co-upgrading CO₂ and glycerol for selective formate production with 190% overall Faradaic efficiency

The overall energy efficiency (EE) is critical for commercializing promising electrochemical technologies, such as the carbon dioxide reduction reaction (CO₂RR). Despite the rapid development of advanced catalysts and reactors for CO₂RR, its commercial potential is still hindered by the sluggish oxygen evolution reaction (OER), which causes high cell voltages and low EEs. Herein, we developed a NiOOH@Ni₃S₂ catalyst on the surface of nickel foam (NF) via an electrochemical surface reconstruction strategy. We observed that the oxidation of glycerol (GLY) to formate (FA) is more thermodynamically favorable than the OER on the developed NiOOH@Ni₃S₂/NF catalysts. The Ni²⁺/Ni³⁺ redox couples within the NiOOH@Ni₃S₂ heterojunction enhance the charge transfer kinetics between the active sites and adsorbed reaction intermediates, facilitating the highly selective and active generation of FA from GLY oxidation reaction (GOR), with a remarkable Faradaic efficiency (FE) of 94% achieved at 100 mA·cm⁻². Comprehensive mechanistic studies identified that the reaction pathway towards FA generation starts from glyceraldehyde intermediates, and glycolate was considered as the key species. Moreover, benefiting from the efficient conversion of CO₂ to FA on bismuth nanosheets, the GOR//CO₂RR paired electrolysis system realizes a remarkable overall FE of ca. 190% for FA co-production at 160 mA·cm⁻² (cathodic FE: 91.25% and anodic FE: 98.70%). This proceeds at a cell voltage of ca. 2.32 V, which is ca. 0.85 V lower than that of OER-assisted CO₂RR system at the same current density. This work provides new insights for co-upgrading CO₂ and biomass to value-added chemicals.