A Study on the catalytic properties of Cu electrode for CO2 Reduction

Abstract

Electrochemical carbon dioxide reduction (ECR) mimics the dark reaction of photosynthesis. The ECR has been thought as one of solutions to relieve the current energy and environmental problems by converting atmospheric CO2 into chemical fuels. In the ECR, catalysts where reduction occurs are the key components. It is known that copper is the only metal electrode that can produce hydrocarbons from CO2. Then it necessary to understand catalytic properties and developing a highly efficient and selective electrocatalyst, Cu is examined. By examination of surface reaction at Cu and electronic structures of Cu-Ag & Cu-Ni alloy electrodes, the ways to achieve selective and efficient CO2 reduction are discussed. Also, hypothesis about correlation between electrocatalytic property and electronic structure of materials is proposed for the further study. Firstly, controlling selectivity of hydrocarbons is studied by performing electroreduction of carbon dioxide on Cu electrodes in hydrogen atmosphere. Injection of gaseous H2 declined overall activity of electrochemical reactions (total charges: 67.5 ? 56.5C). Even though electrolytic H2 generation was suppressed, current efficiency of hydrocarbons was not enhanced, rather maintained (FE = 42.9 ? 43.2%). However, production of methane (CH4) decreased, while that of ethylene (C2H4) increased. This is due to changes in surface coverage of surface hydrogen (Hads) and methylene (CH2). Thus, one can control the selectivity of CH4 and C2H4 by handling an atmosphere of CO2 reaction. Secondly, the alloy electrodes of Cu-Ag & Cu-Ni were prepared and applied for electroreduction of CO2. Alloying tunes electronic structure and redistributes electrons in the metal. This was shown in changes current efficiency of products from CO2 ? limitation of H2 generation (Cu-Ag: 43 ? 6%, Cu-Ni: 43 ? 23%), and enhanced production of hydrocarbons (Cu-Ag: 54 ? 94%, Cu-Ni: 54 ? 74%).UV photoemission spectroscopy examined electronic structures of the electrodes being attributed to the changes of catalytic activity. It is demonstrated that the valence band of the electrodes is upshifted to the Fermi level, which in turn results in remarkable increase of the density of states. Thus, highly efficient and selective production of hydrocarbons from CO2 would be achieved by tuning the electronic structure of an electrode. Finally, correlation between a major product and electronic structure of metal electrodes is demonstrated. They are categorized into two groups ? H2 producing metal group and CO2 reducing metal group. They have the difference in electronic configuration. For H2 producing metals, their d orbitals are partially filled with electrons, while CO2 converting metals have filled d orbitals. And the DOS from p band has considerable value near the Fermi level. The possible explanations are given for the hypothesis. By tuning the electronic structures of metals, one can achieve a highly selective and efficient electrocatalyst for CO2 conversion. It is anticipated that this thesis helps understand catalytic properties of materials for electroreduction of carbon dioxide and contributes to developing a better electrocatalysts.