Mass Transfer And Energy Conversion Characteristics In Bioelectrosynthesis Systems Reducing Carbon Dioxide To Methane

Bioelectrochemical systems（BES）,an innovative energy conversion device coupling high efficient wastewater/gas treatment and energy recovery,have attracted great attention in rencent years.Among them,CH₄-producing BESs are one of the most promising devices for practical applications.However,the long time to form cathode biofilm,low biomass on cathode surface and low performance of BESs largely limit the application of BESs.The above limitations are caused by the unclear characteristic of electron transfer between electrode and microbes,difficulties of biofilm formation on electrode,and the unclear rules of mass transfer from bulk to bioelectrode and reactions in biofilm/solution.Aiming to resolve these problems and improve the performance of BESs,this study is carried out to uncover the rules of electron transfer,biofilm formation,mass transfer and bioelectrochemcial/chemical reactions.The main contents include:（1）the influence of cathode potentials on the startup time,CH₄ production rate,and electron transfer characteristic of biocathodes,and the analysis for the bioelectrochemical characterastics of biocathodes with direct/indirect electron transfer;（2）the model of dynamics for biocathode,the control equation and boundary conditions for mass transfer and conversion,and the influence of CO₂（g）,CO₂（aq）,HCO₃^-,and p H on the performance and stability of biocathodes;（3）the biofilm formation and electrochemical characteristics of the biocathode started up by polarity reversal;（4）an innovative method to exfoliate graphite base and fabricate bioelectrodes with in-situ formed graphene nanosheets,and the influence of graphene nanosheets on the biofilm formation,electron transfer and performance of these electrodes;（5）the global model of BES,the relationships between current density,substrate concentration,p H and operation time,and the influence of applied voltage,substrate concentration,p H and electrode distance on the performance and net energy yield of BES.The major achievements are listed as follows:（1）The startup potential of biocathodes determined the electron transfer pathway between biofilm and electrode.The critical potential of direct and indirect electron transfer was～-0.8 V vs Ag/Ag Cl.The electrons transfered from cathode surface to biofilm directly when the cathode potential was above the critical potential,but transfered from cathode surface to biofilm indirectly when the cathode potential was below it;（2）When a biocathode was in its steady state,the current density might decrease with the increase operating time.Commonly,the decrease of CO₂ concentration during biocathode operation was thought to cause the decrease of current density.However,the model data in our study suggested that it was the increasing p H during biocathode operation limited the activity of cathode biofilm that caused the decrease of current density;（3）Polarity reversal could dramatically short the startup time and meanwhile improved the CH₄ production rate of a biocathode.The startup time of a biocathode started up by polarity reversal was around 18 days,and its CH₄ production rate was 53±10 mmol m^-2 day^-1.However,the startup time of a biocathode started up by a constant potential was around 30 days,and its CH₄ production rate was only 28±5 mmol m^-2day^-1;（4）Graphene nanosheets could be in-situ formed on the surface of graphite by exfoliating a graphite paper（GP）using cyclic voltammetry.After exfoliation,the graphene nanosheets strongly attached atop the graphite base,and this characteristic was very important for the longe-time stable operation of bioelectrodes.When GPs were used to work as bioanodes and biocathodes,respectively,the maxium currents were 2 m A and 0.2 m A,respectively.When exfoliated graphite papers（EGP）were worked as bioanodes and biocathodes,respectively,the maxium currents were 6.4 m A and 1.0 m A,respectively.（5）The maximum net energy yield could be achieved when a 0.85 V voltage was applied to a BES.Based on it,to achieve high performance of BESs,the conditions as follows should be considered:acetate concentration>5 m M,p H belongs 7.2 to 7.6,and electrode distance<4 cm.