Mass Transfer And Energy Conversion Characteristics And Performance Enhancement In A Novel Hybrid Microbial-Photoelectrochemical Artificial Photosynthetic System

Artificial photosynthetic system is a new energy conversion device that can directly use solar energy to convert carbon dioxide into valuable chemical fuels.This technology has received extensive attention in recent years.However,artificial photosynthetic system,mainly using inorganic catalysts to reduce carbon dioxide,faces considerable challenges,including large overpotential,low Faradaic efficiency,and poor selectivity for carbon-based chemical fuels synthesis.To address the abovementioned issues,hybrid microbial photoelectrochemical system,which uses microorganism as the catalyst to reduce CO₂ to chemical fuels,was proposed.But the hybrid microbial photoelectrochemical system converts CO₂ to chemical fuels via a“two step”process,which means that the traditional artificial photosynthetic system first splits water to produce hydrogen,and then the microorganism utilizes the produced hydrogen to convert CO₂ to chemical fuels.The performance of the hybrid system is limited by the low solubility and slow mass transfer rate in water of hydrogen as the electron mediator.Integrating a biocathode with a direct electron transfer pathway with a photoanode,this study constructed a novel hybrid microbial photoelectrochemical artificial photosynthesis system,which reduced CO₂ to chemical fuels by a“one step”process.Systematic experimental and theoretical research for the novel hybrid system had been carried out.Firstly,the photoresponse characteristics and methane production performance of the hybrid microbial photoelectrochemical system were studied.Meanwhile,the steady global theoretical model of the hybrid microbial photoelectrochemical system was constructed.The concentration distribution characteristic of the reactants and products on the anode and cathode side was studied,and the influence of the operating parameters on the performance of the hybrid system was analyzed.Next,in terms of photoanode and biocathode,the performance improvement of the hybrid system was studied.Based on the optimization of photoanode,a novel visible-light responsive hybrid microbial photoelectrochemical system was proposed.For the optimization of biocathode,the effect of operating conditions on the biocathode with a direct electron transfer pathway was studied to obtain the optimal operating conditions.Finally,to address the issue of the external bias in the hybrid microbial photoelectrochemical system,a self-biased hybrid microbial photoelectrochemical system was proposed.The main achievements are as follows:（1）A novel hybrid microbial photoelectrochemical system was proposed,and the hybrid system can convert carbon dioxide into chemical fuels using solar energy as the sole energy input.The Faraday efficiency of the hybrid system for methane production was up to 96%,which was the highest value in the field of artificial photosynthesis for CO₂ reduction.The solar-to-chemicals conversion efficiency was around 0.1%.Meanwhile,the biocathode and photoanode in the hybrid system showed good stability.（2）A steady global theoretical model of the hybrid microbial photoelectrochemical system was constructed,and the substance distribution in the concentration boundary layer and the biofilm on the anode and cathode side was obtained.In the photoanode,from anode bulk solution to anode surface,the OH^-concentration and pH value showed a trend of linear decrease;In the biocathode,from cathode bulk solution to cathode biofilm,the HCO₃^-,H⁺,CO₂（aq）concentration showed a trend of non-linear decrease.The decrease rate of HCO₃^-concentration increased gradually,while the decrease rate of H⁺and CO₂（aq）concentration decreased gradually.On the other hand,the CO₃^2-concentration and pH value showed a trend of non-linear increase.The increase rate of CO₃^2-concentration decreased gradually,while the increase rate of pH value increased gradually.In order to achieve higher performance,the optimal values of the initial OH^-concentration on the anode side and the initial HCO₃^-concentration on the cathode side are 0.2 mol/L and 0.06 mol/L,respectively.（3）A visible-light responsive hybrid microbial photoelectrochemical system was proposed.Owing to the construction of TiO₂/CdS composite photoanode,the absorption wavelength of the photoanode was extended from 410 nm to 550 nm,resulting in a higher solar-to-chemicals conversion efficiency of 0.62%,which was 6.2 times higher than that of the TiO₂ photoanode-based hybrid system.Meanwhile,the research found that the solar-to-chemicals conversion efficiency of the visible-light responsive hybrid system first increased and then decreased with the increase of light intensity.When the light intensity was 25 mW/cm²,the solar-to-chemicals conversion efficiency reached up to the highest value,1.28%.（4）Operating parameters including temperature,inorganic carbon source（dissolved CO₂ and gaseous CO₂）and initial pH value have an obvious effect on the biocathode for CO₂ reduction to CH₄.The optimal temperature,initial HCO₃^-concentration and initial pH value are 35°C,4 g/L and pH 7.5,respectively.Meanwhile,CO₂ bubbles can effectively promote the transfer of CO₂ and H⁺into the biofilm,which can improve the performance of the biocathode.Besides,when a bipolar membrane was used as the ion exchange membrane,the hybrid system exhibited the best stability.（5）A self-biased hybrid microbial photoelectrochemical system integrating a microbial fuel cell was proposed.The self-biased hybrid system can realize the conversion of carbon dioxide to chemical fuels using solar energy without external bias,and the Faraday efficiency of the self-biased hybrid system for methane production is up to 95.2±1.3%.In addition,the self-biased hybrid system can also effectively treat organic wastewater and degrade azo dye wastewater.