Potential And Mass Transfer Of CO₂ Hydrogenation With Proton Exchange Membrane Reactor

Nowadays,with the increasing use of fossil fuels and the aggravation of the greenhouse effect,the conversion of CO₂ into useful fossil fuels has received more and more attention,and the hydrogenation of CO₂ is one of the most important ways.The proton exchange membrane reactor?PEMR?has a unique advantage for its ability to produce adsorbed hydrogen atoms in situ on the catalyst layer to achieve the operation at normal temperature and pressure with higher reaction rate.However,with the CO₂ hydrogenation in the PEMR,high cathode potential is needed for the higher activation energy and the current efficiency is very low in the conventional PEMR.A modified PEMR which added a buffer layer circulating buffer solutions between the Nafion membrane and cathode catalyst layer improved the current efficiency of CO₂ hydrigenation greatly,however,there is no detailed mechanisfic investigation.Besides,the problem of CO₂ mass transfer is seldom solved.This work compared the different mechanisms of the regulation of cathodic potential with and without buffer layer and proposed the diffierent regulation mechanism of cathode potential.The potential threshould of CO₂ hydrogenation is around-1.3V in the PEMR with buffer layer.When the catalyst layer is in contact with electrolyte solution,an electric double layer,induced by the electrostatic attraction between protons in the solution and the negatively charged cathode,would be formed near the surface of cathode catalyst and produce significant potential for CO₂ hydrogenation,therefore high hydrogenation rate?i.e.89.8 nmol cm-2s-1 at-1.8V?is achieved.However,the electricdouble layer can't be formed in the conventional PEMR due to the repulsion between the same kinds of charges in the membrane and cathode catalyst.The potential is mainly consumed by the membrane,so the current efficiency is low.In addition to the regulation of cathode potential,the buffer layer can absorb large amounts of H+ that is transferred from the anode,thereby inhibiting the hydrogen evolution on cathode,but the effect is limited during the reaction process.Therefore,the effects of buffer type,concentration,pH value and reaction time on the reaction performance were investigated in this work.It is found that the ability to surpress hydrogenation evolution increases with the decrease of the H+ concentration variation.As the reaction proceeds,the buffer solution was gradually saturated with H+ and pH value decreases to less than 4.0 after 5h period of reaction.As a result,current efficiency of H₂ evolution significantly goes up to around 50% dramatically.The decrease of CO₂ hydrogenation performance is due to the intensification of hydrogen evolution and the product inhibition.After adding the buffer layer,the mass transfer resistant could be increased in a certain extent.So in this paper,some solutions are proposeed to reduce the mass transfer resistance.Increasing CO₂ adsorption on the catalyst surface,by adding tetrahydrofuran in KHCO3 solution or ZIF-L which has a good ability to adsorbe CO₂ in catalyst layer helps to reduce CO₂ mass transfer resistance and therefore increases current efficiency of HCOOH by around 10-15%.The porous Sn/C catalyst prepared by in situ synthesis can improve the CO₂ mass transfer in the catalyst layer,and increase the active site of the catalyst surface,which increases the current efficiency of HCOOH by around 15%-25%.