The Study Of Improving Cathode Biofilm Growth And Methane Production In MEC By Surface Modification And Voltage Control

As the world economy developed rapidly,energy resource shortage and environmental pollution have become two critical factors that obstruct the sustainable development of the world.Microbial electrolysis cells(MECs)are capable of converting biomass into methane,which can be easily stored and utilized as a fuel.This conversion process can effectively reduce CO2 emission and meet the energy demand.In order to improve the performance of MEC,this study investigated two fundamental factors that influence the cathode biofilm of MEC and its methane productivity,namely the external voltage and the cathode surface property.The external voltage impacts on methane productivity and biofilm of MECs were investigated by applying constant voltage or switching voltage.When applying under constant voltage,0.7 V was the optimal operating condition,with the maximum current density of 45.7 A/m3,methane production rate of 0.349 m3/m3·d,and energy recovery of 82.0%,compared to 0.5 V and 0.9 V.The cathode biofilm was thick with dominant microbial species of long bacilli.When the applied voltage was switched between 0.5 V and 0.9 V for multiple times,the coulombic efficiency(CE)was increased,while the methane content increased from 44.1%to 93.0%,the methane production rate improved from 0.25 m3/m3·d to 0.41 m3/m3·d,and the hydrogen content reduced from 50.4%to 0.20%at 0.9 V.This indicates that the voltage switch process can effectively improve the methane content at 0.9 V.SEM observation revealed that the diversity of microorganism was increased,with the ability of survival at both high voltage and low voltage.The different types of biocathodes were also studied under 1.2 V to test the adaptability of high voltage and ability of reducing CO2 to CH4 directly.When the applied voltage was increased to 1.2 V,it was found that the cathode biofilm operated under 0.7 V performed the best adaptability,the highest methane recovery and energy recovery with a methane production rate of 1.32 m3/m3·d.When using CO2 as the solo carbon source for cathode,it was found that MEC operated at low voltage(0.5 V)was able to produce methane at a rate of 0.054 m3/m3·d,which was 6 times the rate of MEC under 0.9 V.This indicates that low voltage is suitable for cultivating methanogens that capable of directly converting CO2 to methane,while high voltage is more suitable for the growth of hydrogenotrophic methanogens.The impacts of polymer and carbon powder modification on surface of cathodes were also investigated respectively.When modified with polyvinylidene fluoride(PVDF),the electric current and methane production rate decreased slightly,due to the non-conductive nature of PVDF.There was no significant difference in microbial adsorption before and after modification.However,surface modification with activated carbon has shown positive effects on the pefromance of the MEC system,with COD removal rate and CE both improved.Methane productivity and energy recovery were increased by 42.9%and 35%,respectively.Electrochemical analysis indicates that the electrical capacity of the electrode together with the activated surface area were the main reasons for the change in the performance of MEC.In order the further investigate the effect of capacity,two types of carbon powders with distinct structures,multiwalled carbon nanotubes(MWCNTs)and single-walled carbon nanohorns(SWCNHs),were loaded on carbon cloth to obtain the cathodes with different capacities.The results revealed that SWCNHs contributed to a higher COD removal rate,a higher CE,and a higher methane yield.In contrast,MWCNT modified anode showed an inferior performance,with an energy recovery rate of only 41%that of SWCNH modified anode.This indicates that the key factor influencing MEC may be the surface structure of cathode instead of its capacity,and in addition,SWCNH has a better biocompatibility for methanogens.When applying PVDF to air-cathode of microbial fuel cells(MFCs),it demonstrated a positive effect on the catalyst performance of microbial fuel cell(MFC);in particular,when 65μl 2%PVDF was used to modify the cathode of MFC,the system achieved the best stability and highest power,with the maximal power density of 27.4 W/m3.