Enhancing Energy Efficiency in Bicarbonate Electrolysis through The Development of an Au-NiO-CNT Catalyst for Glycerol Oxidation

Reducing emitted CO₂ through direct air capture is crucial to mitigate climate change. Bicarbonate electrolysis is a method that facilitates CO₂ reduction by utilizing bicarbonate generated from aqueously captured CO₂, ultimately yielding syngas or CO as a product. However, the overall energy efficiency of bicarbonate electrolysis is impeded by the high theoretical voltage associated with the oxygen evolution reaction (OER) occurring at the anode. To enhance energy efficiency and bolster the economic viability of this process, it is important to design reactions and catalysts that can simultaneously produce valuable products at the anode while reducing operating voltage.<br/>Here, we proposed a novel cell architecture that combines biomass upgrading, glycerol oxidation (GEOR) for glycolic acid (GCA) production with bicarbonate electrolysis. GCA serves as a monomer for bio-plastics, Poly(lactic-co-glycolic acid) (PLGA), but its current production relies on fossil fuels. In this work, to achieve sustainable GCA production via GEOR, we introduced Au catalyst design that incorporates NiO support and multi-walled carbon nanotubes (MWCNTs) to enhance the selectivity of GCA production through a synergy effect. We then successfully demonstrated that the addition of 4 wt% MWCNTs on Au-NiO catalyst leads to a remarkable achievement of 36% GCA selectivity and 28% glycerol conversion at 0.9 V (V vs. RHE) and 2 h in 0.1 M glycerol and 1 M KOH for GEOR. This showed that a 4.6-fold increase in glycerol conversion and a twofold increase in GCA production compared to the conventional Au-CB catalyst. Additionally, by controlling MWCNT ratio of Au-NiO catalyst, it showed that the amount of Au-NiO catalyst required to achieve 10% glycerol conversion at 0.9 V (V vs. RHE) was reduced by 20 wt%. We conducted operando raman analysis to gain mechanistic insights. Our findings suggested that during GEOR, the NiO support undergoes a transformation into Ni(OH)₂, and introduction of MWCNTs on NiO promotes formation of Ni(OH)₂. Additionally, our electrochemical impedance spectroscopy (EIS) indicated that MWCNTs improve the electrical conductivity of NiO, which could facilitate Ni(OH)₂ formation. Therefore, we demonstrated that the reaction between adsorbed glycerol on Au and Ni-OH transformed on the NiO support, resulting in enhanced conversion of glycerol to GCA. Finally, by combining bicarbonate electrolysis, we achieved an H₂/CO ratio of about 2.34 through bicarbonate electrolysis and a GCA selectivity of 26% through GEOR at 200 mA/cm^-2. This accomplishment confirms the successful development of an efficient system, proficiently yielding valuable compounds at both electrodes.