Electrochemical and spectroscopic studies of copper oxide modified electrodes for CO2 reduction

The global carbon balance has changed substantially with major increases in atmospheric CO2 levels by anthropogenic emission, causing growing concerns about global warming and extreme climate issues. This is one of the crucial global challenges in the 21st century. Electrochemical reduction of CO2 is a promising technology to convert CO2 to chemicals and fuels. There are various candidate materials for CO2 reduction, but only copper and copper oxide catalysts are effective for the reduction of CO2 to hydrocarbons and organic substances. This research aims to explore CuO nanoparticle materials for CO2 reduction by electrochemical and spectroscopic studies. Different techniques are used in this research. The electrochemical behaviour of CuO was monitored by cyclic voltammetry. The oxidation states and proportion of different components of copper were investigated by ex situ XPS and Raman spectroscopy. A novel in situ spectroelectrochemical experiment was designed using electrocatalysis and FTIR technique to study adsorption of solution species onto the CuO surface. Two types of buffer solutions with five pH values, from pH 4 to pH 11, were introduced to investigate the appropriate pH conditions for the CO2 reduction processes. Based on the present experimental data and further analysis, it reveals the electrochemical behaviour of CuO catalyst, including the reduction reaction equations, proportion of different oxidation states and adsorption species at different applied potentials. The pH 4 solutions shows the most suppression of current magnitude in cyclic voltammograms in CO2 saturated conditions. CuO was reduced to Cu2O and Cu at cathodic potential, then oxidised to CuO in pH 4 and 7 solutions, while oxidised to Cu(OH)2 in higher pH solutions after application of one cyclic potential. The lower pH conditions show better stability of CuO catalyst after multi-cyclic sweeps. Therefore, the pH 7, 8 and 9 solutions are shown to be suitable pH conditions for CO2 reduction on CuO catalyst. The major liquid phase product of CO2 electrochemical reduction obtained in our study was formic acid. Combining our research results, a reaction pathway is proposed. Adsorbed CO2- is the first reduction step and carboxyl is an important intermediate to form formic acid. Finally, several future areas of research are proposed to improve the understanding of the CO2 reduction processes on CuO catalyst. Copper oxide materials of different oxidation states and mixed phases can be used to clarify the active sites. Gas phase products and quantitative liquid phase products can be measured to study the reduction rate and efficiency.