Activity and Selectivity of a Nickel Carboxyethylphosphonate 2D-MOF for the Electrochemical Conversion of CO2 to Syngas

The increasing concentration of CO2 in the atmosphere is alarming for modern society and the reduction of CO2 into valuable products would be an ideal and elegant solution. Metal-organic frameworks (MOFs), constructed from organic linkers interconnected with metal (oxide) nodes, with high porosity and large surface area, are an emerging class of electrocatalysts for the reduction of CO2. Herein, we report the synthesis of a two-dimensional nickel metal–organic framework (2D Ni-MOF) with 3-phosphopropionic acid and 4,4-bypyridine as ligands and Ni2+ as the metal node. The electrocatalytic activity of this material was explored for the electrochemical CO2 reduction reaction (CO2RR). The measured onset potential for the CO2 reduction activity was -0.51 V vs RHE, which is 70 mV more positive than the HER in Ar-saturated electrolyte, demonstrating potential for CO2 reduction. Notably, at 1.01 V (vs RHE), the CV profile showed a current–mass activity of 9.5 mA/mg in CO2-saturated 0.1 M carbonate buffer solution, suggesting that the catalyst is active towards CO2 reduction. Long term electrolysis at a CO2/CO potential of -0.8 V (vs RHE) yielded a gaseous mixture of CO and H2 at an average production ratio of 30:70%, respectively. When exploring the electronic structure of the 2D Ni-MOF, we identified a lower limiting potential for hydrogen reduction (1.22 eV) compared to CO2 reduction (1.75 eV). This suggests a more energetically favourable production of H2. This Ni-MOF catalyst retained its crystallinity and morphology after electrolysis as confirmed by powder XRD, HRTEM, FT-IR and Raman spectroscopy, indicating promising stability of the catalyst.