The Study of Catalytic Activity and Reaction Mechanism for the Electrochemical Reduction of CO₂on Nanostructured Cu Electrode and Nano-Cu Oxides

【作者】 姜萍；

【导师】 柳建设； 乔锦丽；

【作者基本信息】 东华大学 ， 环境科学， 2014， 硕士

【摘要】 Extra emission of carbon dioxide (CO2) into the atmosphere, induced by human industrial activities, has been considered one of the primary causes in possible global warming due to the greenhouse effect, and also becoming an increasing concern in recent years. To address this issue, using electrochemical reduction to convert CO2to low carbon fuels (methane, methanol, formic acid, and ethylene) represents a value-added approach to the simultaneous generation of alternative fuels and environmental remediation of carbon emissions from the continued use of conventional fuels. Actually, it seems that the electrochemical reduction of CO2would be an ideal energy storage strategy in converting undesired CO2into useful fuels using electricity from renewable sources such as hydro-electrical, solar, wind, tidal/wave, and ocean-thermal energy. However, because of the slow electrode kinetics of CO2electroreduction, large negative overpotential is required which not only causes low energy efficiency but also induces high hydrogen evolution at such negative electrode potentials. Therefore, effective electrocatalysts are highly desired in order to reduce overpotentials.An effective electrocatalyst should be process the following three important factors, i.e., activity, stability and the product selectivity. In this regard, the copper and its oxides have been reported as the unique metal catalysts for CO2electroreduction in aqueous electrolytes, since the component elements are plentiful and environmentally friendly compared to Pb and Hg. However, the current efficiency of Cu was still limited by the relative large overpotential and, the selectivity of the production was also not satisfied. Additionally, the deactivation of Cu electrodes in CO2 reduction was found to be fast in aqueous solutions. Given all these unfavorable attributes, improvement in both the activity and stability of the Cu metal-based electrcatalysts for CO2reduction are definitely essential in terms of practical applications. In fact, the performances of Cu-based electrocatalysts are strongly affected by the synthesis methods and the pretreatment procedures. This is because different methods and the pretreatment procedures could produce different morphologies with different microstructures. It has been reported that the thermal process could produce Cu2O film, which was then subsequently electroreduced into metal Cu surfaces, resulting in a stable catalyst with favorable energy-efficiency in CO2reduction. On the other hand, different types of Cu-oxide electrocatalysts showed different current efficiencies in KHCO3electrolyte, for example, the cuprous oxide (Cu2O) could give the highest efficiency. Although all Cu oxides could be reduced into metal Cu at negative potentials, different Cu oxides with different morphologies might give different surface structures and surface areas, thus leading to different catalytic activity and stability as well as selectivity.In this thesis, by a combined thermal-electroreduction approach, the hybrid Cu nanostructures from CuO-Cu2O oxide films prepared on Cu substrates was demonstrated to afford high catalytic activity for CO2electroreduction. Because the Cu nanostructure contains two surface morphologies of sphericity-nanofibers, it is expected to contribute the Cu electrode high surface area and active phase, which is superior active for CO2electroreduction to previously investigated, according to both onset potential and maximum current density along with the high stability of Cu electrode as well.更多还原

【Abstract】 Extra emission of carbon dioxide (CO2) into the atmosphere, induced by human industrial activities, has been considered one of the primary causes in possible global warming due to the greenhouse effect, and also becoming an increasing concern in recent years. To address this issue, using electrochemical reduction to convert CO2to low carbon fuels (methane, methanol, formic acid, and ethylene) represents a value-added approach to the simultaneous generation of alternative fuels and environmental remediation of carbon emissions from the continued use of conventional fuels. Actually, it seems that the electrochemical reduction of CO2would be an ideal energy storage strategy in converting undesired CO2into useful fuels using electricity from renewable sources such as hydro-electrical, solar, wind, tidal/wave, and ocean-thermal energy. However, because of the slow electrode kinetics of CO2electroreduction, large negative overpotential is required which not only causes low energy efficiency but also induces high hydrogen evolution at such negative electrode potentials. Therefore, effective electrocatalysts are highly desired in order to reduce overpotentials.An effective electrocatalyst should be process the following three important factors, i.e., activity, stability and the product selectivity. In this regard, the copper and its oxides have been reported as the unique metal catalysts for CO2electroreduction in aqueous electrolytes, since the component elements are plentiful and environmentally friendly compared to Pb and Hg. However, the current efficiency of Cu was still limited by the relative large overpotential and, the selectivity of the production was also not satisfied. Additionally, the deactivation of Cu electrodes in CO2 reduction was found to be fast in aqueous solutions. Given all these unfavorable attributes, improvement in both the activity and stability of the Cu metal-based electrcatalysts for CO2reduction are definitely essential in terms of practical applications. In fact, the performances of Cu-based electrocatalysts are strongly affected by the synthesis methods and the pretreatment procedures. This is because different methods and the pretreatment procedures could produce different morphologies with different microstructures. It has been reported that the thermal process could produce Cu2O film, which was then subsequently electroreduced into metal Cu surfaces, resulting in a stable catalyst with favorable energy-efficiency in CO2reduction. On the other hand, different types of Cu-oxide electrocatalysts showed different current efficiencies in KHCO3electrolyte, for example, the cuprous oxide (Cu2O) could give the highest efficiency. Although all Cu oxides could be reduced into metal Cu at negative potentials, different Cu oxides with different morphologies might give different surface structures and surface areas, thus leading to different catalytic activity and stability as well as selectivity.In this thesis, by a combined thermal-electroreduction approach, the hybrid Cu nanostructures from CuO-Cu2O oxide films prepared on Cu substrates was demonstrated to afford high catalytic activity for CO2electroreduction. Because the Cu nanostructure contains two surface morphologies of sphericity-nanofibers, it is expected to contribute the Cu electrode high surface area and active phase, which is superior active for CO2electroreduction to previously investigated, according to both onset potential and maximum current density along with the high stability of Cu electrode as well.更多还原