Study On CO₂ Reduction In A Microchannel Electrochemical Reactor

The large-scale application of electrochemical CO2 reduction can not only effectively alleviate the environmental problems caused by the increasing carbon dioxide,but also effectively solve the problem of difficult storage of renewable electrical energy.In addition to the structure,composition and morphology of the catalyst,the activity,selectivity and stability of the electrochemical reduction of CO2 are also affected by the external environment.Due to the low solubility of CO2 in the aqueous solution,when the reaction rate is large,the reactivity is easily limited by the mass transfer of CO2.Therefore,the development of a high-efficiency electrochemical device to enhance the mass transfer of CO2 is the key to improve the activity of CO2 reduction at high reaction rates.Microchannel reactors have efficient heat and mass transfer performance,and are safe to operate and easy to scale up,and show excellent performance in many fields,but there are few reports in the field of electrochemical reduction of CO2.Therefore,from the perspective of developing a high-efficiency carbon dioxide electrocatalytic device and enhancing the mass transfer of carbon dioxide,the microchannel technology is applied to the field of electrochemical CO2 reduction,and two microchannel electrochemical devices have been developed and their electrocatalytic performances were investigated,and the main results are summarized as follows:(1)Designing a microchannel electrochemical reactor for CO2 reduction.A porous oxide-derived Cu foam electrode was prepared by electrodeposition,thermal oxidation,and electrochemical reduction,which was placed in the middle of a cylindrical cation exchange membrane to establish the annular microchannel electrochemical reactor.The results showed that the oxide-derived Cu foam had a high electrochemical surface area and exhibited a high current density under a low overpotential.Furthermore,comparing the electrocatalytic performance in the microchannel reactor and the traditional H-type cell,it revealed that the microchannel had a better mass transfer performance and could significantly improve the selectivity of CO2 reduction.More importantly,it also explored the effects of annulus width,gas-liquid ratio on the electrochemical reduction of CO2,and found that the maximum Faradaic efficiency was obtained at a gas liquid ratio of 2:1 in the microchannel reactor with an channel width of 1.0 mm,which was ascribed to the stable Taylor flow formed under this condition.(2)Optimizing the microchannel electrochemical reactor and developing a high-efficiency catalyst for CO2RR.To further save costs,a single cell microchannel electrochemical reactor was designed.In order to improve the Faradaic efficiency and current density of CO,a porous Ag foam electrode with high active surface area was prepared by electrodeposition method.The results demonstrated that the Ag foam electrode was an efficient catalyst for electrochemical reduction of CO2to CO,and the CO Faradaic efficiency is up to 81.0%at the current density of 20.0 mA/cm2 in the microchannel electrochemical reduction.In addition,according to the results of the electrocatalytic tests under different gas-liquid ratio conditions,it found that the flow regime in the microchannel electrochemical reactor had a significant effect on the activity of CO2RR.The maximum Faradaic efficiency was obtained at a gas liquid ratio of 2:1,which was ascribed to the formation of more gas-liquid-solid three-phase interfaces under this condition.The stability test showed that the Ag foam electrode had an excellent stability performance.After 6 hours of electrolysis,the Faradaic efficiency of CO was almost unchanged.(3)Expanding the universality of the microchannel electrochemical reactor device,and further reduce the cost and improve the activity of CO2 reduction.A porous Zn foam catalyst was synthesized by electrodeposition method,which was used as an electrode for CO2RR in a microchannel electrochemical reactor.It showed that the porous Zn foam surface was covered with many flowers flake-shaped nano-Zn,the lateral size of the flake was about 2.0 μm.Electrochemical tests showed that,compared with the traditional H-type cell,the Zn foam electrode exhibited a better electrocatalytic activity of CO in the microchannel electrochemical reactor,and the maximum CO Faradaic efficiency of 92.5%was achieved at a current density of 25.0 mA/cm2.By changing the gas-liquid ratio,it found that the catalytic activity of the Zn foam electrode was also related to the flow pattern in the microchannel,and the effect of the surrounding environment of the catalyst on the activity of CO2 reduction was verified again.(4)Investigating the effect of mass transfer on the CO2RR in the microchannel,conducting a "scale-up" experiment of the microchannel electrochemical reactor,and exploring the feasibility of future large-scale application.Firstly,the concentration of CO2 in microchannels was measured by chemical titration.It demonstrated that the CO2 saturated-0.1M KHCO3 solution could be easily obtained in a microchannel with a length of 5.0 m.Electrochemical tests showed that the CO Faradaic efficiency increases with increasing CO2 concentration in the electrolyte when the current density was above 15.0 mA/cm2,which indicated that the reaction was affected by mass transfer of CO2.By increasing the length of the microchannel in series,the contact time of CO2 and the electrocatalyst can be extended,and the conversion of CO2 can be increased.The "scale-up" experiment showed that the microchannel electrochemical reactor could be amplified by increasing the number of the annular microchannel,and the CO Faradaic efficiency were all maintained at 95%at a current density of 7.5 mA/cm2.Furthermore,the pressure loss was measured in the microchannel,and found that only 75 kPa pressure loss was achieved in the microchannel with a diameter of 1.0 mm and a length of 5.0 m when the gas liquid flow rates were 20.0 mL/min and 10.0 mL/min,respectively.Therefore,the microchannel electrochemical reactor is an excellent device for commercial application of CO2 reduction in the future.