| After the industrial revolution, the concentration of carbon dioxide (CO2) in the atmosphere increased sharply with the massive burning of fossil fuels. CO2 is a greenhouse gas, and it can affect the growth of plants, bringing about the changes of climate and plant nutrients. In nature, CO2 is consumed by the photosynthesis of plants and the corresponding amount of organic compounds and oxygen is produced in this process, which can complete the carbon cycle and oxygen cycle in the natural world. The increase of CO2 concentration in the atmosphere shows that the rate of carbon cycle in nature is lower than that of burning carbon materials by human beings. Therefore, it is necessary to search for the corresponding industrial process to promote the carbon cycle. This process is the subject of this thesis: artificial photosynthesis.The so-called artificial photosynthesis, is usingthe devices which can convert the solar energy (natural energy) into electrical energy, such as photovoltaic, wind power and tidal power device, to convert solar energy (natural energy) into electrical energy, and then using the electrochemical process to convert CO2 and water into carbonaceous compounds (formic acid in this paper) and oxygen. Since this process uses natural energyproduces formic acid and oxygen which areneeded by human beings and can be converted into electrical energy through direct formic acid fuel cell, and does not produce by-products, it is a kind of energy storage technology and has important significance for human energy industry. Electrochemical reduction of CO2 has simple process and mildly reaction conditions, and this process has abundant raw materials and can be enlarged easily, so we can expect that the process is one of the effective ways for use of CO2 in the future.In this paper, the process of electrochemical reduction of CO2 in aqueous solution has been studied. The effect of Sn based catalysts on the electrochemical reduction of CO2 and the effect of the oxidation of the formic acid on anodehave been studied.We have also discussed how to reduce the energy consumption of the reaction. The main contents and results are as follows:1.In order to study the performance of the carbon materials and their composite with SnO2 on CO2 electrochemical reduction, we have prepared nitrogen doped multiwalled carbon nanotubes (N-MWCNTs) by the methodof chemical vapor deposition, and then SnO2 nanoparticles were decorated on N-MWCNTs by wet chemical method. The morphology and structure of the obtained materials were analyzed by TEM, XRD and XPS. Then the N-MWCNTs and SnO2/N-MWCNTs were modified to the glassy carbon (GC) electrode for electrochemical reduction of CO2. The best potential for electrochemical reduction of CO2 to formate on the SnO2/N-MWCNTs modified GC electrode was -0.9 V, and the faradaic efficiency can reach to 46%.However, the faradaic efficiency for electrochemical reduction of CO2 to formate on the N-MWCNTs modified GC electrode is less than 10%. This shows that the selectivity of SnO2 is higher than that of N-MWCNTsfor electrochemical reduction of CO2 to formate.2.In order to study the effects of Sn and its surface oxide on the electrochemical reduction of CO2 to formate, the Sn electrodes have been treated by heat treatment and surface etching. Firstly, the faraday efficiencies forproducing formateonthe Sn electrode and the treated Sn electrodeswhich have calcined at 120,140,160 and 180? in the airhave been compared, it has been found that the faraday efficiency of the treated Sn electrode is lower than the Sn electrode and it decreases with the increase of calcination temperature. Then, the Sn electrodeswereetched in the HCl solutionsbythe method of cathodic polarization, the oxide layer on their surfaces can be effectively removed by polarization treatment. The faraday efficiency for electrochemical reduction of CO2 to formate on the etched electrodeis greatly reduced to 43%, while the faraday efficiency for producing formate on the Sn electrode without etching treatment is 84%. It shows that the oxide layer on the surface of Sn electrode is benefit to the reduction of CO2. The electrode after etching process and exposing to the air for 24 h and the oxide layer can naturally form on its surface, the faraday efficiencyfor electrochemical reduction of CO2 to formatecan get backto 85%. It has been foundby cyclic voltammetry that the hydrogen evolution reaction on the etched Sn electrode is obviously enhanced, which is the main reason for the decrease of the catalytic activity of the etched Sn electrode toward CO2 reduction.3.The formate generated by electrochemical reduction of CO2 can be consumed onthe anode by its oxidation reaction.in order to retard the oxidation of formic acid on the anode, a Pt electrode coated with a Nafion film (Pt@Nafion) has been obtained by immersing a clean Pt plate into the Nafion solution. It has been found that the Pt@Nafion used as the anode can effectively retard the oxidation of formic acid. With the increase of the electric quantity from 50 C to 500 C, when the Pt@Nafion electrode is used as the anode, the faraday efficiencydecreases slowly from 90.3% to 78.5%; however, when the Pt electrode is used as the anode, the faraday efficiency decreases from 89.2% to 35.3%. The CV study shows that the Pt@Nafion electrode and the Pt electrode had similar oxygen evolution performance, but the Pt@Nafion electrode could effectively retard the oxidation of formate. When the Sn cathode and the Pt@Nafion anode are repeatedly used for electrolysis of 400 C up to 5 times, the faraday efficiency does not significantly change in which only the electrolyte is replaced. This indicates that the Nafion film on Pt anode and the Sn electrode are fairly stable and can be used for a long time.4.We also try to use the proton exchange membrane to separate the H type electrolytic cell for study the electrochemical reduction of CO2, in order to solve the problem of formate oxidation. The study has been found that the voltage between the two poles of H type electrolytic cell in the long time electrolysis process will increase. We have found the reason of the increase of the cell voltage through some experiments. We analysised the solution of the anode region after electrolytic 180 C using titration, and found that concentration of HCO3- reduced from 0.1 mol L-1 to 0.030 mol L-1. For reasons of this phenomenon, we thought that H+is produced from the anode reaction of oxygen evolution reaction. The formed H+ and HCO3- reaction generates water and CO2, the solution of the HCO3- is consumed. The consumption of HCO3- to make the solution conductivity fall and lead to the increase of anode potential,so as to cause the cell voltage also continue to rise.5.Reducing cell voltage is an essential method to reduce the energy consumption of the process of electrochemical reduction of CO2 to formate. For this purpose, we have studied the faraday efficiency and energy efficiency for electrochemical reduction of CO2 to formatewhen using a Sn plate as the cathode, an IrxSnyRuzO2/Ti electrode ora Pt plate as the anode. We have estimated the performances of the Pt electrode and the IrxSnyRuzO2/Ti electrodefor oxygen evolution reaction,and have found that the IrxSnyRuzO2/Ti electrode has more excellent oxygen evolution performance than that of the Pt electrode. It has been found that the maximum faraday efficiencies of the IrxSnyRuzO2/Ti electrode and the Pt electrode are 84.8% and 85.1% respectively, and the valuesare close to each other. However, the energy efficiency is only 35.6% when the Pt electrode is used as the anode, and the energy efficiency of IrxSnyRuzO2/Ti is 42.1%. This is due to the overpotential of the oxygen evolution reaction on the IrxSnyRuzO2/Ti anode is lower than that of the Pt anode, and the reactions can be performed at a lower voltage,whichcan improve the energy efficiency.When the electrochemical reduction of CO2 increased from 50 C to 500 C,the faraday efficiency decreased from 85.3% to 79.6% when the IrxSnyRuzO2/Ti electrode which coated with a layer of Nafion film as the anode. While the faraday efficiency decreased to 24.3% when the IrxSnyRuzO2/Ti electrodewithout coating Nafion film was used as the anode. The oxidation of formic acid on the anode can be effectively reduced by the Nafion film onIrxSnyRuzO2/Ti electrode. |
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