| The future development of today’s world faces two major problems—the crisis of energy and environmental pollution.Microbial electrosynthesis system（MES）,with biofilm attached to the electrode surface as the catalyst,can convert CO2 into methane while treating wastewater,which attracts great attention in the field of environmental protection and renewable energy.This technology has the advantages in cheap and easily available electron acceptors,high Faradaic efficiency,simple operating conditions,and green and pollution-free reaction processes.As one of the core components,microorganisms with electrochemical activity are attached to the surface to form a biofilm,which can directly receive electrons from the cathode surface to reduce carbon dioxide and produce methane,so the biocathode determines the performance of the system.At present,however,there are few theoretical researches on the characteristics of mass and electron transport of biocathode biofilm,which limits the in-depth understanding of the characteristics of biocathode and the proposal of the corresponding strengthening methods.In order to solve the problem that the electron and mass transfer characteristics of biocathode are still not clear,in this paper the biocathode is taken as the modeling object,the kinetics equation suitable for biocathode is deduced,and a mathematical model of charge conservation and mass transport coupled with biochemical/electrochemical reactions is constructed.This paper studies the effects of different cathode potential,biofilm conductivity and biofilm porosity on the mass transfer and electricity characteristics of biocathode.At the same time,considering the flow in the cathode chamber,the velocity and concentration distribution in the cathode chamber are simulated.The effects of flow rate,substrate concentration and different potentials on the mass transport,microbial electrochemical reaction,and electricity characteristics in the cathode chamber are studied.Finally,the effects of the geometrical size of the electrode surface morphology on the current density of the biocathode and mass transport process are investigated.The main research results of this article are as follows:（1）In this paper,the biocathode is taken as the research object,which is regarded as a porous medium with conductive properties,the kinetic equation（Nernst-Monod equation）of the biocathode is derived,and a steady-state mass transport model coupled with biochemical/electrochemical reactions is constructed.It is found that when the cathode potential is higher than-0.5 V vs.SHE while the cathode potential decreases,the current density in the biofilm increases and the substrate concentration decreases.However,when the cathode potential decreases to-0.5 V vs.SHE,the biofilm consumption The ability of the electron to reduce the substrate is almost saturated;low conductivity(<10-3 S/m)will cause a significant potential difference in the biofilm,which reduces the substrate utilization rate and seriously affects the performance of the biocathode;small porosity makes increased conductivity of the biofilm increases the reaction sites,but it is not conducive to the transport of the substrate;the large porosity is conducive to the transport of substances,but it will reduce the conductivity of the biofilm and the overall reaction rate.When the biofilm porosity is controlled at 0.4,the biocathode can achieve the best current density.（2）Considering that the Nernst-Monod equation is derived under the assumption of equilibrium potential,as the overpotential increases,the difference between the theoretical value and the actual value of the current also increases,which is not conducive to subsequent research.To this end,the Butler-Volmer-Monod equation of the electrode reaction kinetics of the coupled biochemical reaction is deduced.At the same time,the flow problem in the cathode chamber is considered,and a steady-state mass transport model coupled with biochemical/electrochemical reactions is constructed.Compared with the Nernst-Monod equation,the Butler-Volmer-Monod equation fits the experimental data better and is more suitable for describing the biocathode current density curve under different overpotentials.The increase of substrate concentration increases the current density of the biocathode and improves the performance of the cathode,however,the degradation efficiency of the substrate will decrease,which reduces the utilization efficiency of the substrate and is not conducive to the economics of the system.Under the same operating conditions,the increase in flow rate can improve the performance of the cathode,but when the flow rate is higher than 120 m L/h,it has little effect on the distribution of the substrate concentration in the cathode chamber.When the cathode potential was reduced from-0.3 V vs.SHE to-0.5 V vs.SHE,the cathode current density increased from 165 m A/m2 to 594.5 m A/m2.At the same time,the degradation efficiency of the substrate also increases.The continued reduction of the potential has limited influence to increase the current density of the biocathode.（3）The effect of micro-topography of electrode surface on the performance of biocathode was studied.Compared with biocathode without protrusions,the maximum current density of biocathode with array or agglomerated protrusions is significantly improved.For array type protrusions,the height LH of the protrusion is more important than the width LL.When using the same volume of material to change the surface morphology of the electrode,selecting high and narrow protrusions can increase the average overpotential on the electrode surface,and the substrate utilization rate in the biofilm increases,which is more conducive to promote biocathode performance.While the protrusion spacing LDincreases,the density of the protrusions on the electrode surface decreases,thereby reducing the maximum current density of the biocathode.The value of LD is preferably within 100μm.For agglomeration type protrusions,the growth of the protrusion radius LR increases the specific surface area of the electrode surface,enhances the ability of the cathode biofilm to reduce the substrate,and increases the maximum current density of the biocathode. |
