Mechanistic Details of CO2 Electroreduction on Ni and Ni{Cu}-YSZ Electrodes Using Operando Spectroscopy

Abstract

Electroreduction of CO2 to fuels using renewable energy can significantly help in reducing emissions and dependence of fossil fuels. Electrochemical reduction of CO2 to hydrocarbon fuels (CHx) is energy inefficient owing to multistep-multielectron transfer process which posses many kinetic limitations. The selective conversion of CO2 to CO is energy efficient. CO as product can be directly used as a fuel or converted to hydrocarbon fuels by using green hydrogen via Fischer-Tropsch reactions.Well known Ni/YSZ electrode architectures have both well-established lifetimes, performance benchmarks and optimised manufacturing protocols when it comes to use as SOFC. Unfortunately, using these electrodes as CO2 reduction electrodes requires use of H2 at the inlet. The reaction proceeds through a reverse water gas shift reaction (RWGS) (CO2 + H2 à CO + H2O) in conjunction with water electrolysis. Most of the CO originates from non-electrochemical RWGS reaction. Use of pure CO2 streams at currents exceeding 240mA/cm2 leads to catastrophic electrode failure. In the literature, it is believed that transformation of Ni metal to NiOx and carbon deposition via Bouduard reaction are the causes of electrode failure in pure CO2. We have adapted this well-known Ni/YSZ electrode by impregnation of Cu into the Ni architecture. The Ni{Cu}/YSZ electrode not only does not deactivate but also shows improved performance in every aspect compared to the pure Ni/YSZ electrode.We have developed a unique setup for operando Raman Spectroscopy and online mass spectroscopy which can be used to study electrode reactions under both steady state and transient conditions. Using this setup, we have shown that Ni-YSZ and Ni{Cu}/YSZ electrodes go through an oxide mediated mechanism of CO2 reduction. Metal oxides such as NiOx and Ni{Cu}Ox are the active catalyst species and not the metals. Upon applying strongly reducing conditions (currents > 240mA/cm2), NiOx reduces to Ni metal which can no longer catalyse the reaction, whereas the oxide on Ni{Cu}Ox is much stronger and does not reduce under even the most reducing of conditions (reducing currents >480mA/cm2). Such an electrode remains active for reducing pure CO2. Supporting studies using SEM, TEM, XPS, operando EIS, TPR and DFT modelling were also carried out. We believe that enabling CO2 reduction Ni/YSZ architecture using Cu impregnation is a game changer as all aspects of electrode manufacturing and device compatibility for Ni/YSZ have been extensively tested and market proven. This will allow for quick adaptation for this electrode in the CO2-SOEC market. Besides this, such a mechanistic study remains unique in the field of solid oxide-based research. Figure 1