Electrocatalytic Reduction of Carbon Dioxide Using Supported Bimetallic Electrocatalysts by Tuning the Composition and Structural Properties of Metal Nanoparticles

• Main Author: Mulatu Kassie Birhanu
• Other Authors: Bing-Joe Hwang
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/44ndpp

博士 === 國立臺灣科技大學 === 化學工程系 === 107 === Emissions of CO2 from various types of industries, power plants, vehicles and many other factories are increasing and becomes one of the causes of global warming together with chlorofluorocarbons (CFCs), methane, ozone, nitrous oxide and others. Global warming provokes melting of polar ice, the increment of atmospheric temperature, expansion of dissertation and impact of human health. Even though there are different techniques for mitigation of CO2, electrochemical reduction (ECR) of CO2 is the interesting field of study for this work. In the literature part of the thesis, basic ideas about characterizations of bimetallic (Cu-based), principles and mechanisms of electrochemical reduction of CO2 and descriptions to major and supplementary factors that significantly affect the reduction efficiency and selectivity of products are compiled coherently. The catalytic capability of Cu and Cu-based electrocatalysts and the advantages gained from Cu alloy over its monometallic are compared and discussed. The proposed reduction mechanisms, which are studied by several researchers based on experimental and density functional theory (DFT) approaches are compiled and analyzed. Reduction products and the corresponding faradaic efficiency using Cu monometallic and Cu-M bimetallic are identified at specific potential and concentration of electrolyte solutions. The literature part of this thesis addresses, understands of electrochemical reduction mechanisms and products of CO2 on Cu-based bimetallic catalysts and also provides an outlook for designing better bimetallic catalysts to obtain demanded products. Therefore, prior to the experimental work, this thesis provides an outlook for designing better bimetallic electrocatalysts to obtain selective products through ECR of CO2. In the first experimental research approach, bimetallic and monometallic nanoparticles of Au and Cu are prepared for the electrochemical reduction (ECR) of CO2. Each electrocatalyst is supported and coupled with multi-walled carbon nanotubes (MWCNT), which able to enhance the conductivity, electron transfer, activity and the stability of the electrocatalyst. The supported metal nanoparticles (NPs) are loaded and dispersed on hydrophilic carbon fiber paper. The goal of this work is tuning the electronic structure and the surface composition of bimetallic and monometallic nanoparticles, which naturally manipulates the interaction between active sites and intermediate species of CO2. The synergistic effect between Au and Cu alloys and alloying extent with the coupling of MWCNT are the major contributions for enhanced activity and selectivity. Nearly 70-80 mA/cm2 of current was reached at a potential below -1.0 V vs. RHE and approximately up to ~78% of CO selectivity was obtained on the bimetallic composition in addition to other minor reduction products. Generally, faradaic efficiency and activity are significantly enhanced and the onset potential is comparatively lower than that of other metal electrocatalysts. In the second work, bimetallic electrocatalysts of Au and Cu nanoparticles (NPs) are synthesized for electrochemical reduction (ECR) of CO2. The NPs are supported on a functionalized carbon nanotubes (CNT) to increase the stability and conductivity that resulted in high current density. To enhance the activity and selectivity of the reduction products, the preparation process was incorporated with capping agent i.e. Polyvinylpyrrolidone (PVP). Consequently, PVP can tune the synergistic (electronic and geometric) effects including the structural properties of the atoms, inhibit particle overgrowth and prevent the aggregation of NPs due to its steric hindrance effect. This scenario of varying the structural properties like a significant decrease of particle size and atomic distribution of AuCu are manipulated by PVP and results enhanced catalytic rate and selectivity of the reduction process. The role of PVP on the distribution of NPs have been investigated by XAS analysis and proved using computational calculations through the determination of adsorption energy of PVP with Au (111) and Cu (111) on the bimetallic surface. The electrocatalyst prepared with PVP has better activity and selectivity compared to the bimetallic synthesized without PVP. At -0.91 V vs. RHE the faradaic efficiency (FE%) of AuCu/CNT (1.5g PVP) is around 88% towards the formation of CO, but in the absence of PVP resulted in ~71%, indicates the existence of PVP increase the FE% by 24%. Bimetallic composition showed better activity and selectivity compared with its monometallic electrocatalyst using the same amount of PVP. In the other alternative work, bimetallic NPs of Au and Cu are synthesized, which is supported on TiO2-C composites. The electron transfers between TiO2 and metal NPs makes the coupling between the catalyst and supporting material strong and able to enhance the stability of the electrocatalyst during ECR of CO2. The role of carbon is to increase the conductivity and surface area of the electrocatalyst and led to enhance the overall activity of the electrocatalytic reduction of CO2. Characterization of electrocatalysts and supporting materials are performed by XRD, XAS, XPS, SEM, EDS and using other characterization tools to confirm their property and its availability. The performance of the reduction activity has been performed by electrochemical test including determination of reduction products using gas chromatography hyphenated with electrochemical analysis.