Performance Enhancement On Electrochemical Reduction Of CO₂ To Formic Acid Based On Sn Electrodes

CO₂ emission reduction is one of the research hotspots in the energy science.Electrochemical reduction of CO₂ is a way to transform CO₂ gas into organic fuel by electrochemical principle.It is a very promising way of energy storage and CO₂emission reduction due to the advantages of fast reaction rate,constant temperature and atmospheric pressure.In particular,it can make use of intermittent and unstable renewable energy,such as wind energy,solar energy and so on.At present,electrochemical reduction of CO₂ is difficult to be used in practical application owing to low conversion rate of products,serious hydrogen evolution reaction,small current density and poor persistence of electrodes,etc.Therefore,in order to increase the faraday efficiency,the current density and the durability of electrode,the experimental research is carried out in the present study as follows.At first,the nanoscale tin embedded in porous carbon embedded graphene catalyst was synthesized by hydrothermal method and the effects of different temperature treatments on electrochemical performance were also studied.Furthermore,the performance of the fabricated catalyst and the commercial tin catalyst was compared.In order to enhance the conductivity of the catalyst layer,the SnO₂ nanosheets gas diffusion electrode was prepared by hydrothermal method and charactered by physichemical methods.The performance between the electrodes with the same catalyst loading prepared by the hydrothermal and spray methods were compared.In order to increase the specific surface area of the electrode,the Sn/Nano-Cu electrode with nanowires catalyst layer was fabricated by the following method:copper nanowires were grown by oxidation-reduction method on the Cu foil at first,then tin particles were loaded on copper nanowires by electrodeposition.The performance of electrochemical CO₂reduction was compared among the fabricated Sn/Nano-Cu electrode,the electrode prepared by electroplating tin particles directly on copper（the Sn/Cu electrode）and the commercial Sn foil electrode.The major results in this study are listed as follows:1)The result of the experiment on the nanoscale tin embedded in porous carbon catalyst（Sn@C/G）prepared by hydrothermal method indicated that the particle size and electrochemical active area of the catalyst were affected by the heat treatment temperature in the process of preparation.The catalyst yielded the best electrochemical performance at the heat treatment temperature of 550 ^oC.At potential of-1.8 V vs.Ag/AgCl,the maximum Faraday efficiency was 69.3%and the current density was 21.5 mA cm^-2,which were increased by 10.4%and 63%,respectively,compared with the commercial tin catalyst.2)The experiment on the SnO₂ nanosheets gas diffusion electrode prepared by hydrothermal method was performed.It was indicated that the best performance of electrochemical CO₂ reduction was obtained after twice hydrothermal reactions in water bath contained 6.6 mM SnCl₂,150 mM hydrochloric acid,200 mM urea and 3.6 mM mercaptoacetic acid.At potential of-1.8 V vs.Ag/AgCl,the SnO₂ nanosheets gas diffusion electrode reached the maximum faraday efficiency of 61.3%and a current density of 21.8 mA cm^-2 at 0.5 M KHCO₃ solution with a gas flow rate of 25 sccm,which were increased by 7.5%and 20.4%,respectively,compared with the electrode prepared by spray method with the same catalyst loading.The current efficiency can be maintained at the level higher than 30%after testing for 16 hours.3)The experiment on the Sn/Nano-Cu electrode with a nanowires catalyst layer was conducted.It was found that compared with the electrode prepared by electroplating tin particles directly on copper（the Sn/Cu electrode）and the commercial Sn foil electrode,the Sn/Nano-Cu electrode showed an excellent CO₂ electrochemical reduction performance due to the large specific surface area,low charge transfer resistance and strong CO₂^-adsorption capacity.At potential of-1.8 V vs.Ag/AgCl,the Sn/Nano-Cu electrode reached the maximum faraday efficiency of 86.8%and a current density of 38 mA·cm^-2.Moreover,the Sn/Nano-Cu electrodes can still maintain high Faraday efficiency during 25 hours of test.