Strengthening Mechanism Study On Electrochemical CO₂ Reduction To Formate Over Tin Cathode Based On SP₃^2- Anodic Oxidation

The combustion of fossil fuels emits a large amount of carbon dioxide?CO₂?into the atmosphere,which causes global warming.At the same time,the depletion of fossil energy directly endangers the energy security of the world.Electrochemical CO₂ reduction to formate can reduce the concentration of CO₂ in the atmosphere and cope with potential energy crisis.It has both environmental and economic benefits.However,the high cost of catalysts and high energy consumption restrict the development and application of this technology.Although most of the researches have focused on the improvement of cathode,it is limited that merey improve the cathode because OER?oxygen evolution reaction?with high overpotential is often used as the anode semi-reaction of CO₂ electroreduction,which makes the overpotential difference between cathode and anode electrode reactions very large.However,the effect of anodic oxidation on the electroreduction of CO₂ and its mechanism has not been studied yet.Based on that,starting from the anodic reaction of CO₂electroreduction and taking SO₃^2-anodic oxidation as the core,this thesis proposes the strategy of enhancing CO₂ electroreduction to formate by employing the SO₃^2-oxidation as the anodic reduction of CO₂ cathodic reduction.Firstly,compared the electro-oxidation performance of SO₃^2-with OER,and the enhancement effect of SO₃^2-oxidation was investigated by applying it to the anodic compartment of electroreduction CO₂.On this basis,the factors affecting the enhancement were studied.Finally,based on SO₃^2-anodic oxidation,the strengthening mechanism of electrochemical CO₂ reduction to formate over Tin cathode and the rate control steps in the electrolysis process were tested and discussed,which would provid a theoretical basis for the efficient,energy-saving reduction and optimization of CO₂ reduction by anodic oxidation enhancement.The main results of this study are as follows:?1?The electro-oxidation properties of electrolyte solutions with SO₃^2-oxidation or OER reaction were tested.The results show that SO₃^2-oxidation has lower electrode potential than the conventional OER,and can expand the effective reaction area of the electrode and accelerate the oxidation reaction rate.The electrolyte with SO₃^2-oxidation was used as the anolyte for CO₂ reduction and its effect on CO₂reduction on tin cathode was tested.The results of potentiostatic reduction under continuous mode show that SO₃^2-oxidation can greatly reduce the energy consumption of CO₂ reduction and improve the Faraday efficiency and energy efficiency of formate.I^-oxidation was also tested by electrochemical method and used as anolyte of CO₂ electroreduction under galvanostatic mode.The reduction results show that it has similar characteristics with SO₃^2-oxidation and has the same enhancement effect on CO₂ electroreduction,which confirms the feasibility of using anodic semi-reaction with excellent oxidation performance to enhance CO₂ reduction efficiency.?2?In the process of CO₂ reduction which enhanced by SO₃^2-anodic oxidation,the change of cathode and anode was studied.It is found that the enhancement effect was mainly caused by the effect of proton and electron migration between cathode and anode,and their increase enlarged the effective current density of CO₂ reduction.On this basis,the effects of anode flow rate and cathode electrode potential on the SO₃^2--oxidation-enhanced CO₂ reduction were studied to optimize the operation conditions.The results show that the flow rate of SO₃^2-anode solution was negatively correlated with the reduction efficiency of the formate production and positively correlated with energy consumption.20 mL/min/cm² was the optimum anode flow rate for the system.The electrode potential was positively correlated with the reduction efficiency.When the electrode potential was-1.9 vs.SCE/V,the enhancement effect is most prominent.?3?The electrochemical kinetics and mechanism of formate production from CO₂electroreduction enhanced by SO₃^2-anodic oxidation were analyzed.The results show that the application of SO₃^2-oxidation did not affect the pathway of CO₂ cathodic reduction.Its effect on the cathode was mainly to enhance CO₂ reduction by weakening the competitiveness of side reactions.?4?By accelerating cathodic reduction rate to clarify at the mutual restriction between SO₃^2-oxidation and CO₂ reduction.Based on the recorded data of electrode potential and the previous discussion of anodic mass transfer,it is inferred that when the electrode potential is constant,the reaction rate of the electrolytic cell is controlled by the rate of both cathode and anode simultaneously,and the control step is determined by the critical point of bipolar rate equilibrium.When the mass transfer rate is lower than the critical point,the reaction rate is controlled by SO₃^2-anode oxidation rate,and when higher,the reduction rate is controlled by cathode CO₂reduction.According to the cost and benefit estimation of formate production by the system that enhanced by SO₃^2-anodic oxidation under the continuous mode,it is concluded that this strategy can significantly reduce the power consumption cost and increase the product benefit.