The electrochemical reduction of CO2 to value-added products

Abstract

Carbon dioxide is the fourth most abundant gas in the atmosphere making up 0.035 %. Carbon dioxide prevents heat from escaping into the outer space and is used by plants to make oxygen. The levels of carbon dioxide have significantly increased since the start of the industrialisation era and the effects thereof is felt on a global scale. The aim of this project is to electrochemically transform the inert CO2 molecule into value added products such as formic acid, carbon monoxide, methanol, methane etc., as an energy storage media and reduce its global footprint. Use of water electrolysis as a medium for production of hydrogen ion (green hydrogen route) has been employed. IrO2:TaC 70:30 wt. % was selected as a suitable anode electrocatalyst and In2O3 enriched with 0.15 wt. % PTFE as the cathode electrocatalyst under investigation. The electrocatalysts synthesized were characterised using scanning electron microscopy and powder X-ray diffraction techniques. The IrO2:TaC 70:30 wt. % proved to be a good system for the water electrolysis in accordance to our previous studies and with literature1,2 Catalyst inks were prepared to deposit the electrocatalysts via spray coating methods thereby making 5 sets of catalyst coated membranes. The anode electrocatalyst IrO2:TaC 70:30 wt % was deposited directly onto the Nafion® membrane while the cathode electrocatalyst PTFE infused In2O3 onto the carbon paper gas diffusion layer. Humidified gaseous CO2 as well as studies were also performed along with the usual aqueous CO2 which eliminates solubility issues and the need of electrolytes. Linear sweep voltammetry for ECR revealed onset potentials ranging from -1.10 V to -1.49 V which corroborated with literature while showing some changes with each membrane, accompanied with high current densities and low ohmic resistance. Tafel slopes revealed mechanistic changes over potentials applied indicating a possible change in product formation with increasing potentials. Chronoamperometry experiments were conducted using applied potentials of -1.6 V, and, -1.8 V, with average current densities reaching up to -99 mA/cm2 in bubbled CO2 and -223.32 mA/cm2 in CO2 humidified in H2SO4. Gas injected gas chromatography was used to analyse gaseous products. Products obtained were carbon monoxide, methane, ethane, and ethylene with the best results yielding a Faradaic efficiency of 4.36 % for ethane formation.