Study On Carbon Fuel Generation From Molten Mixed Carbonate Electrolysis Driven By Solar Energy

Resource utilization of carbon dioxide has a broad prospect,and solar energy has been considered the most ideal green energy in the new century.For the purpose of CO₂ resource recovery and solar energy utilization,CO₂ treatment technology will be developed with the solar thermal electrochemical production theory（STEP theory）in which carbon fuel can be produced from CO₂ by molten salt electrolysis.According to the basic principal and theory of STEP process,the systematic theoretical framework is proposed and three subunits will be constructed.That is photo-thermal unit.photo-electric unit and electrochemical unit.Photo-thermal unit utilizes solar thermal effect to provide high temperature environment,photo-electric unit uses photovoltaic effect to provide electricity,and the electrochemical unit with high temperature realizes the transformation from carbon dioxide to carbon fuel which is driven by solar energy.Then the perfect matching coupled principle of energy level,spectrum,potential,efficiency is used to build efficient coupling solar-thermal-electrochemical-energetic molecules system.The system is wholly driven by solar energy and its core is the high temperature electrolysis reaction.In this paper carbon fuel is produced by electrolysis of molten carbonate mixture of Li₂CO₃,Na₂CO₃,K2CO₃ at high temperature and the rapid reaction back to the original carbonate electrolyte occurs while CO₂ is absorbed by intermediate product Li₂O,Na₂O,K2O,then a cycle is completed.The composition-melting point relations,electrochemical properties,electrolysis potential and the decomposition products of Li-Na,Li-K and Li-Na-K mixed carbonate system are studied.And the operating temperature range is widen by tuning the ratio of the mixed molten carbonates.The effect of electrode material,reaction temperature,different current intensity and electrolytic time under same electric quantity on the reaction is investigated.The experimental results show that the required potential decreases with the increasing temperature while increases with the current density.The melting point and electrolysis potential of the Li_0.896Na_0.625K_0.479CO₃ are lower than that of the Li_1.179Na_0.821CO₃ and Li_1.303K_0.697CO₃ which under the same conditions.Then the amorphous carbon is prepared at molten Li_0.896Na_0.625K_0.479CO₃ electrolyte by using coiled Fe cathode and Ni foil anode at 500℃ with a high electrode stability.When the current intensity is 1.0A and the electrolytic time is 1h,the current efficiency reach up to 65.98%.During the experimental research,[Spectrolab GalnP2/GaAs/Ge CPV]photo-electrical unit,[Edmund Fresnel lens]photo-thermal unit,[Fe/Li₂CO₃-Na₂CO₃-K2CO₃/Ni]electrolysis unit are constructed.And the results show that the decomposition potential of carbon dioxide is 2.43V,and the operating current is 0.905A.The maximum power point of the photo-electric unit is（2.348V,0.971 A）.The working point is close to the Pmax.At this time,the photo-electric conversion efficiency of the system is up to 29.2%,and the current efficiency can be higher than 65%.This system is a high active continuable Solar-CO₂-Fuel environmental friendly circulation and provides a new pathway for resource utilization of carbon dioxide.