A Hybrid Microbial-photoelectrochemical Artificial Photosynthesis System For Carbon Dioxide Reduction To Methane

At present,global energy demand is increasing year by year,and fossil fuels still account for more than 80 percent of all the energy sources employed in the world.Fossil fuels are finite resources,and their indiscriminate utilization causes significant harmful effects on the environment,such as greenhouse gas emissions and water pollution,which is a serious threat to the sustainable development of human beings.Using renewable energy sources as an energy input for the generation of value-added chemicals from CO₂is of environmental and economic importance,which has become one of the main technological challenges of our time.In terms of renewable energy,solar energy has become the most attractive alternative energy because of its inexhaustible and easy access.In recent years,solar energy conversion technology has developed rapidly and attracted widespread attention.Therefore,the artificial photosynthesis system uses solar energy to drive CO₂conversion to produce high value-added chemicals,which will provide a great space for the development of renewable energy and CO₂conversion and utilization,and make a great contribution to the sustainable development of energy and the environment.Artificial photosynthesis system has three important constraints of low energy utilization rate,low product yield,and low Faraday efficiency that must be considered for industrial-scale applications soon.According to the low solar energy utilization and product yield,this paper proposes a composite anode,which can effectively absorb the visible light of solar energy,increasing the solar energy utilization of the coupling system.And silicon cells produce electrical bias at the same time,which promote the charge transfer,promoting the improvement of product yield;To solve the problem of low Faraday efficiency,methanogens can be cultured to reduce carbon dioxide and produce methane efficiently by utilizing the unique advantage of the high selectivity of microorganisms.Using the composite photoanode and the microbial cathode,the hybrid microbial photoelectrochemical system was constructed in this paper,which can achieve an efficient reduction of carbon dioxide to produce methane by only using sunlight as energy input.To further improve the methane production rate and solar energy utilization rate,the silicon nanowire array photocathode modified with copper hydroxide was proposed in this paper,and the hybrid microbial photoelectrochemical system with dual photoelectrodes was constructed.Copper hydroxide（instead of precious metal platinum,etc.）is deposited on the surface of the silicon nanowire array by the precipitation method.The preparation is simple,the cost is low,and the photocathode prepared has high hydrogen production activity.The dual photoelectrode system can further improve the utilization rate of solar energy and the bias voltage between anode and cathode,thus promoting the increase of methane production rate.The research results of this paper are as follows:（1）A composite photoanode composed of solar silicon cell and TiO₂nanowire array is proposed,which can effectively absorb visible light in the solar spectrum.A novel microbial photoelectrochemical system was constructed using a composite photoanode and biocathode,which can effectively reduce carbon dioxide to methane.CH₄was produced at an average rate of 10.7±0.2 L day^-1m^-2with an average faradaic efficiency of 98.5±2.1%.Finally,our proof-of-concept hybrid artificial photosynthetic system yielded a solar-fuel conversion efficiency of around 0.44%,which is more than double the efficiency of global natural photosynthesis.（2）Copper hydroxide was used as a co-catalyst to replace the precious metal platinum and modified silicon nanowire arrays by precipitation method.The preparation of the electrodes is simple and low cost,and the prepared electrode shows good hydrogen production activity.The optimum loading capacity of copper hydroxide is about 0.8 mol%.At-1.0 V vs.Ag/Ag Cl,the current generated by the prepared photoelectrode is about 2.11 A m^-2.The yield of hydrogen is about 488.4±10.2 L day^-1m^-2,which is nearly ten times the size of the unmodified silicon nanowire array electrode.（3）A hybrid microbial photoelectrochemical artificial photosynthesis system based on composite photoanode and photocathode was constructed for the reduction of carbon dioxide to methane.The system can be completely powered by solar energy,using water and carbon dioxide to produce methane.It can run continuously for 12 hours.The methane production rate is 12.2±0.1 L day^-1m^-2and the solar-fuel conversion efficiency is about 0.69%,which are both higher than that of the coupling system using only a single photoelectrode.