Electrochemical CO₂ Reduction Catalytic Activity and Materials Characterization of Heterogeneous Zinc Carbonaceous Layers Derived from a Zinc(II) Molecular Catalyst

Homogeneous molecular electrocatalysts have shown promise as a highly tailorable platform for designing active sites with high selectivity for target products. However, many such catalysts face durability issues under applied potential, which can cause reductive or oxidative decomposition of the molecule. The resulting deposition of the metal center and possible ligand fragments on the electrode surface as a heterogeneous catalyst convolutes the measured electrochemical activity, leading to uncertainty regarding the role of the molecular catalyst. Herein, the molecular catalyst Zn(DMTH) (DMTH = diacetyl-2-(4-methyl-3-thiosemicarbazonato)-3-(2-pyridinehydrazonato)), which has been reported for the reduction activity of CO2 to formate, was investigated carefully to uncover its role in reductive decomposition to form a heterogeneous zinc carbonaceous layer (HZCL) on the cathode. The deposited film appears to be primarily responsible for the CO2 reduction activity to formate, with the current density and formate faradaic efficiency increasing over time as the layer develops. Interestingly, deposition of the HZCL under CO2 bubbling led to a significantly more formate-selective catalyst layer than the deposition of the HZCL under Ar bubbling. The morphological and chemical nature of the Zn(DMTH)-derived films was studied via several electrochemical and materials characterization techniques to provide insight into the catalyst layer and which aspects may contribute to the observed difference between HZCL film types.