Research On Mechanisms Of Formic Acid Production From Cathodic Reduction Of CO₂ On Sn And Bi Enhanced By Anodic Oxidation Reaction

The electrochemical reduction of CO₂ has become a research hotspot in the field of CO₂ utilization due to its advantages of cleanliness and controllability of products.The electrochemical reduction of CO₂ can produce many valuable products.Among them,formic acid,as an important industrial raw material,is widely used in pesticides,leather,pharmaceuticals and other industries,it can also be directly used as fuel for fuel cells.Therefore,research on catalytic reduction of CO₂ to formic acid by Sn cathode（with low toxicity,stable chemical properties and high selectivity of formic acid）has become the focus of many scholars.However,high operating cost（mainly power consumption）has always restricted the practical development of this technology.The power consumption of CO₂ conversion to formic acid is:E_sp=1.19U_cell/（FE）kW·h/kg.Energy saving can be achieved by reducing the operating voltage of U_cell or improving the formic acid Faraday efficiency.In addition,the operating voltage is correlated with the potential difference between the anodic oxidation and cathodic reduction(U_cell=φ_e,a-φ_e,c+η_a+η_c+IR,whereφ_e,a,φ_e,c is anode and cathodic equilibrium potentials,respectively;η_a,η_c is anode and cathodic overpotential,respectively).Therefore,research on anodic oxidation and cathodic reduction were carried out separately:Firstly,the strategy for cathodic reduction of CO₂ enhanced by anodic oxidation was proposed,anodic oxidation potential equilibrium potential and overpotential are reduced,formic acid FE was improved by desigining controllable half-reaction of anodic oxidation;Secondly,the cathodic reduction of CO₂ performance was strengthen by developing low overpotential and high selectivity catalyst for formic acid production,aimed at reducing cathodic reduction overpotential and increasing formic acid FE.The CO₂ conversion efficiency was significantly improved through the above studies.The main research results were as follows:（1）The research revealed the way in which the anodic oxidation reaction involves the cathodic reduction of CO₂,and clarified the mechanism of formic acid production from cathodic reduction of CO₂ enhanced by anodic oxidation.The influence of anode materials oxygen evolution ability and reducing additives on the reduction of CO₂ to formic acid performance showed that half-reaction of anodic oxidation can intervene in the cathodic reduction of CO₂ process by improving the kinetics of anodic oxidation.Improving the kinetics of anodic oxidation could decrease the anodic oxidation equilibrium potentialφ_e,a and overpotentialη_a,thus reducing the operating voltage U_cellell and increasing the CO₂ conversion performance.For example,by adding reductive additive KI to the anolyte,replacing oxygen evolution reaction with low potential oxidation pair（I^-/I₂）,the kinetics of anodic oxidation was significantly improved thus reducing the power consumption for CO₂ conversion.The power consumption of formic acid production was as low as 6.42 kW h/kg,which was only 86.08%of that using half-reaction of oxygen evolution.This research showed that the effect of cathodic reduction of CO₂ can be enhanced by designing controllable anodic oxidation half-reaction,which provides a new research idea for CO₂ conversion with high efficiency and low power consumption.（2）The formic acid faradic effiency and production rate were improved through applying half-reaction of SO₂ oxidation with low oxidation potential,and reaction of SO₂ oxidation also reduced power consumption.The faradic efficiency and production rate for formic acid were as high as 88.6%and 0.144μmol/h·cm²,respectively,which were about 1.48 times and 2.25 times higher than those using half-reaction of oxygen evolution,respectively.This is mainly due to large amount of H⁺is generated in the oxidation of SO₂,providing sufficient hydrogen for formic acid production.In addition,the increasing H⁺accelerates the electron transfer process in the reduction of CO₂.The energy consumption for formic acid production was as low as 5.33 kW·h/kg,which was only 55.29%of that by oxygen evolution half reaction.The half-reaction of SO₂oxidition significantly reduced the power consumption of CO₂ conversion,duing to the improvement of formic acid faradic efficiency enhanced by SO₂ oxidation,on the other hand,it was due to the fact that half reaction of SO₂ oxidation improves the anodic oxidation kinetics and significantly reduces the anode potential.In addition,the technology also achieved 64.75%SO₂ removal rate.The strategy of cathodic reduction of CO₂ enhanced by SO₂ oxidation not only improves the conversion efficiency of CO₂,but also achieved desulfurization,which provides sufficient scientific basis for the design of efficient CO₂ conversion.（3）Bi metal catalysts with low reductive overpotential and high selectivity for formic acid production was successfully designed.The hexagonal flake structure catalyst can significantly reduce the overpotentialη_c,improve the Faraday efficiency FE and production rate for formic acid production,thus enhancing the efficiency of CO₂ reduction to formic acid.For example,under the condition of 0.65 V overpotential,CP-20（catalysts prepared by potentiostatic deposition of 20 C）achieved a nearly 100%formic acid Faraday efficiency at overpential of 0.65 V,and the rate of formic acid production is 92.63μmol/h·cm²;the energy efficiency is as high as 46.79%,and the energy consumption is only 3.64 kW·h/kg.Compared with metal Sn,the formic acid faraday efficiency increased by about 30%⁴5%,while the energy consumption decreased by 51.2%,which significantly enhanced the CO₂ conversion efficiency.The mechanism of hexagonal sheet structure Bi metal material enhancing the efficiency of CO₂ reduction toformic acid is as follows:Firstly,hexagonal sheet structure Bi metal has excellent ability to inhibit hydrogen evolution and improve the electrocatalytic active area of the electrode;Moreover,hexagonal sheet structure can enhance the intrinsic conductivity of Bi metal electrode,accelerate the electron transfer rate,and simultaneously improve the rate of reactant H⁺and product HCOO^-pass thorough over-passivation film,thus improving the kinetics of electrocatalytic reduction of CO₂.Therefore,rapid and highly selective production of formic acid can be achieved at low overpotential conditions.（4）The synergistic enhancement of CO₂ reduction to formic acid by anodic oxidation of SO₂ and Bi cathode was verified.The Faraday efficiency and energy efficiency for formic acid production reached 99.05%and 66.5%respectively,and the energy consumption was reduced to 2.56 kW·h/kg.The synergistic enhancement mechanism of SO₂ oxidation and Bi cathode for formic acid production was revealed:Firstly,the equilibrium potential of SO₂ oxidation half-reaction was reduced by 1.06 V,and the overpotential for SO₂ oxidation was decreased by 0.63 V compared with the oxygen evolution reaction.Secondly,Bi has excellent ability to inhibit hydrogen evolution reaction reducing the voltage drop IR of the electrolytic system;Thirdly,Bi metal material with the regular hexagonal sheet structure can efficiently produce formic acid（FE≈100%）at low overpotential（η_c=0.65 V）.