Electrochemical Reduction Of Carbon Dioxide To Formate

In nature,the photosynthesis of green plants absorbs light energy,synthesizes carbon dioxide?CO₂?and water?H₂O?into rich organic matter,and simultaneously releases oxygen?O₂?to achieve carbon cycle and oxygen cycle.However,the use of a large amount of fossil fuels has caused a continuous increase in the concentration of CO₂ in the atmosphere since the industrial revolution.It has now exceeded 400 ppm while natural photosynthesis has not increased.In the long term,high concentrations of CO₂ in the atmosphere are bound to cause changes in ecosystems.Therefore,it is necessary to introduce"artificial photosynthesis"to assist this weak link in the natural circulation."Artificial photosynthesis"is the conversion of renewable energy such as solar energy into electrical energy,and then electrochemical conversion of CO₂ and H₂O into carbon-containing compounds?formic acid in this case?and oxygen.Formic acid and oxygen are needed,this thesis mainly studies the electrochemical reduction of CO₂ to formic acid in aqueous system,and the following research is carried out based on Sn-based catalyst:1.Sn-plated Cu foam gas diffusion electrode was made by electroplating tin on copper foam,which was found to have higher specific surface area.With the study of reaction characteristics,a biomimetic electrochemical cell?BEC?based on the diffusion electrode was designed and fabricated by 3D printing technology.The Sn-plated Cu foam electrode matched to BEC well to forces the electrolyte to flow from one electrode to the other at high speed,thus greatly improving the transfer of CO₂,O₂ and other substances in the electrolyte.When the flow rate reaches 400 mL min^-1,the current density of ERCO₂F reaction reaches its maximum value,which is 3.4 times as high as when the electrolyte is stationary.When the electrolysis potential is-1.8 V?vs.Ag/AgCl?,the Space-time yield of formic acid can reach290?mol h^-1 cm^-2,which is more than twice that of undivided electrolytic cell(143?mol h^-1cm^-2,-1.8 V).2.To obtain bimetallic catalyst diffusion electrode,Sn?CH₃COO?₂ and In?CH₃COO?₃were dissolved in dilute CH₃COOH and then the pH was adjusted to 2.50 with concentrated CH₃COOH.The copper foam was used as the working electrode to be constant potential plated in the solution,it was found that different deposition potentials could obtain different Sn/In ratios electrodes.The Sn₄₂In₅₈ electrode obtained at-1.3 V?vs.Ag/AgCl?showed better performance in catalyzing ERCO₂F reaction.The morphology of the electrode is 400 nm crystallites dispersed on the dendritic structure.The Faraday efficiency of the formic acid production is 88%at-1.6 V?vs.Ag/AgCl?,and the Tafel slope is only 159 mV decade^-1.The Space-time yield of ERCO₂F at-1.8 V?vs.Ag/AgCl?is 309?mol h^-1 cm^-2 on Sn₄₂In₅₈electrode.If the electrode was assembled in BEC,STY can reach 468?mol h^-1 cm^-2,which is superior to the Sn-plated Cu foam electrode studied previously,and also has higher energy efficiency in the range of-1.5^-1.8 V.3.The Ni Foam electrode was immersed into 0.2 M aniline and 0.5 M PTSA solution,and the potentiodynamic polarization was carried out at a sweep speed of 25 mV s^-1 between-0.2 and 1.0 V.It was found that after 5 cycles,the PANI nanowires could cover the entire electrode surface.After 8 cycles?PANI/NF?,the diameter of the PANI nanowires was about150 nm.Only the thickness of PANI nanowire layer increases as the number of cycles continues to increase.A complete PANI film?R-PANI/NF?was obtained at-0.4 V?vs.Ag/AgCl?for 10 min in 0.5 M H₂SO₄ solution.The plated tin particles were obtained by electroplating directly on R-PANI/NF with adding MSA to the plating solution.The composite electrode can catalyze the ERCO₂F reaction to reach the maximum FE?94%±1%?and STY(306±14?mol h^-11 cm^-2)at-1.70 V?vs.Ag/AgCl?.4.Through the study of electrolyte for ERCO₂F,it was determined that the optimal cation and anion were K⁺and HCO₃^-,and the optimal electrolyte was 0.35 M KHCO₃.When the potential was lower than-2.0 V,the FE of a mixture of KHCO₃ and KH₂PO₄ in a volume ratio of 10:20?mL?was higher than that in KHCO₃.When MgSO₄ or ZnSO₄ is added to the electrolyte,the metal cation can react with OH^-to affect reaction balance;the current density of ERCO₂F is significantly increased while the FE of formic acid production was kept constant.When ZrO₂ is added,[ZrO?OH?]⁺and OH^-can be produced by hydrolysis of ZrO₂.In the cathode region,it can adsorp CO₂,react with CO₂ to HCO₃^-,excessive HCO₃^-will react with[ZrO?OH?]⁺to obtain ZrO₂ and CO₂ again;in the anode region,it can neutralize H⁺,which can effectively prevent the anode consumption of electrolyte and alleviate the rise of cell voltage.