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

The increasing concentration of CO 2 in the atmosphere is alarming for modern society and the reduction of CO 2 into valuable products would be a unique solution. 1 Metal–organic frameworks (MOFs), constructed from organic linkers interconnected with metal (oxide) nodes, with high porosity and large surface area, have become an emerging class of electrocatalysts for reduction of CO 2 . 2,3 Herein, we report the synthesis and characterization of a two-dimensional nickel metal-organic framework (2D Ni-MOF) with 3-phosphopropionic acid and 4,4-bypyridine as ligands and Ni 2+ as the node (figure 1). The electrocatalytic activity of this material is explored for the electrochemical CO 2 reduction reaction (CO 2 RR). An onset potential for the electrochemical reduction CO 2 activity of -0.51 V vs RHE was observed which is 70 mV more positive than the HER in Ar-saturated electrolyte suggesting an efficient CO 2 reduction. This catalyst showed a high current density of 33.5 mA cm -2 at 1.01 V vs RHE, exhibiting superior performance to most of the MOFs tested so far for CO 2 reduction (figure 2a and b). Long term electrolysis at -0.8 V (vs RHE) yielded a current density of ∼10 mA cm -2 with high selectivity towards the formation of CO and H 2 with an average production ratio of 30:70%, respectively (figure 2c and d). This new 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. Density functional theory (DFT) was employed to calculate adsorption-free energy of the reaction intermediates to identify the active catalytic sites responsible for CO 2 reduction in our 2D Ni-MOF structure. Findings show that the catalyst more readily reduces H* to H 2 (with a limiting potential of 0.48 eV) than CO 2 to COOH* (limiting potential of 0.78 eV). The reaction mechanism upon which this calculation was based begins with the removal of the H 2 O molecule bound to the Ni (II) site, followed by CO 2 adsorption and the two proton-coupled electron transfer steps yielded COOH* and then CO & H 2 O. The H 2 O is more strongly adsorbed to the Ni (II) site than CO which closes the mechanistic cycle. Reference 1. M. J. B. Kabeyi and O. A. Olanrewaju, Frontiers in Energy Research , 2022, 9 . 2. B.-X. Dong, S.-L. Qian, F.-Y. Bu, Y.-C. Wu, L.-G. Feng, Y.-L. Teng, W.-L. Liu and Z.-W. Li, ACS Applied Energy Materials , 2018, 1 , 4662-4669. 3. J.-X. Wu, S.-Z. Hou, X.-D. Zhang, M. Xu, H.-F. Yang, P.-S. Cao and Z.-Y. Gu, Chemical Science , 2019, 10 , 2199-2205. Figure 1