The Impact of Nickel Content on Structure and Electrochemical CO2 Reduction Performance of Nickel-Nitrogen-Carbon Catalysts Driven By Zeolitic Imidazolate Frameworks As a Platform

Abstract

The electrochemical conversion of CO2 to chemicals and fuels is a promising strategy to overcome the increase of CO2 concentration in the atmosphere driven by the extensive dependence on fossil fuels as a main global energy source. A significant shortcoming of this technology is the ability to design new active and selective catalysts for CO2reduction (CO2R) reaction. Atomically dispersed metal-nitrogen doped carbon (M-N-C) materials has attracted extensive attention as highly selective and active electrocatalysts for CO2R due to their exceptional geometric structures and electronic properties compared to other materials. M-N-C electrocatalysts are being synthesized by simple chemical methods through pyrolyzing a mixture of metal salt, nitrogen and carbon containing precursors. Among different precursors, metal organic frameworks (MOFs) are being widely used as a platform for driving M-N-C catalysts owing to their well-defined structures that can assist in forming and isolating the atomically dispersed metal sites doped in N-C skeleton. While several M-N-C (where M=Fe, Co, Mn, Ni) catalysts were utilized for CO2R, atomically dispersed Ni-Nx sites were found to be very selective for the CO2R towards CO. However, the influence of the metal concentration impregnated into the prepared structure on the overall performance of M-N-C catalysts in CO2R is still under investigation.Herein, we’ve selected zeolitic imidazolate frameworks (ZIF-8) — a subclass of MOFs — as a platform for synthesizing Ni-N-C electrocatalyst. A systematic study has been performed to investigate the role of Ni concentration integrated into the structure of ZIF-8 during the synthesis on the structure, properties and overall CO2R activity and selectivity of the prepared Ni-N-C catalyst. A series of four Ni concentrations has been investigated in CO2electroreduction system, our findings exhibit that higher Ni impregnation concentration has resulted in forming metallic Ni nanoparticles which are favouring the competing hydrogen evolution reaction (HER), while low Ni content has assisted on the formation of atomically dispersed and isolated Ni active sites which are very selective towards CO in CO2R ~99% Faradaic efficiency at a potential of -0.68 V vs the reversible hydrogen electrode. Through this study, we show how the optimization of the Ni content in the prepared Ni-N-C catalysts has a significant impact on their performance as CO2R electrocatalysts.