The Research Of Anodic Reactions Of Carbon Electrooxidation In Direct Carbon Fuel Cell

Electric power as a most widely used and convenient secondary energy,has become an indispensable element for the development of mankind society.Electricity production is heavily depended on supply of the conventional coal-fired plants and this condition will be remained for some time in the future.The current issue of utilizing coal for power generation in coal-fired plants is the low efficiency?below 40%?determined by the thermal engines,which causes large loss of energy and emission of carbon dioxide.Therefore,it becomes very important to meet the needs for sustainable energy supply by seeking new technologies for the conversion of coal to electricity efficiently.The direct carbon fuel cell?DCFC?allows the direct conversion of the chemical energy stored in solid carbon materials into electrical energy via an electrochemical route instead of combustion.It provides a promising means for the coal power generation in an efficient and clean way.However,the factors,such as sluggish kinetics of carbon electrooxidation,the unknown effect of some impurities on carbon electrooxidation,and high operating temperature,limit the direct application of DCFC.To solve these problems,this thesis carried out studies on improved anodic reaction activity,effect of impurity in coal on anode reaction activity,and carbon fuel cell operated at low temperature.The transition metal oxide?V₂O₅,MoO₃,and CuO?were investigated as catalysts,which was introduced into carbonate electrolyte for enhancing the electrode reaction of carbon electrooxidation by utilizing the variable valence state of these oxides.At 750 ^oC,the onset oxidation potential of carbon was shifted from-0.85 V to-0.87 V,-0.80 V to-0.85 V,and-0.83 V to-0.89 V by V₂O₅,MoO₃,and CuO,respectively.At-0.4 V,the performance of carbon electrooxidation was increased from 101.4 mA cm^-2 to 172.9 mA cm^-2,93.6 mA cm^-2to 135.6 mA cm^-2,and 62.0 mA cm^-2 to 168.7 mA cm^-2 by V₂O₅,MoO₃,and CuO,respectively.The steady-state test results proved the catalysis process of V₂O₅,MoO₃,and CuO was continuous and stable.Accordingly,the mechanism of improved reactions was proposed and demonstrated by the cyclic voltammetry with gold electrode.Different methods were employed to prepare CeO₂ as the catalysts for the graphite electrooxidation in the molten alkali carbonate.Ceria and graphite powders were first homogeneously mixed and then pressed into electrodes for enhanced catalysis behavior.It was found that the Sol-Gel prepared CeO₂ had the best catalytic effect certified by the electrochemical measurements.The performance of graphite electrooxidation was remarkably improved by the addition of CeO₂.The current density at-0.4 V of the graphite electrooxidation with 50 wt.%ceria reached 196 mA cm^-2 at 750 ^oC,which was considerably higher than that of pure graphite electrode(117 mA cm^-2).The activation energy of graphite electrooxidation was effectively reduced by ceria from 42.8 kJ mol^-1 to 14.9 kJ mol^-1.So in this respect,the role of ceria was similar to a catalyst.The SEM and XRD revealed that the morphology and average crystallite size of ceria was changed after the reaction.Cyclic voltammetry of ceria electrode verified the electrochemical redox reaction of ceria.Thermogravimetry testified the carbothermic reaction between ceria and graphite.Then the reaction mechanism relative to the ceria catalytic effect was explored.Sulfur and some of its inorganic compounds?CaSO₄,K₂SO₃,K₂S,FeS₂?were added in molten carbonate and their effect on graphite electrooxidation was investigated.At high operating temperature,Ca in CaSO₄ could improve the Boudouard reaction,which added pathways for carbon oxidation.As a result,the anodic reaction activity was improved.Meanwhile,the other four sulfur compounds would electrochemically oxidize in the anodic reaction process with the increase of sulfur valence state.Moreover,the electrooxidation of sulfur compounds were parallel reactions of carbon electrooxidaiton and had no impact on the electrooxidation process of graphite.Accordingly,a small amount of sulfur compounds in molten carbonate can be utilized as fuels for direct carbon fuel cell.The phosphomolybdic acid was investigated as a mediator adding into anodic solution to achieve the indirect carbon electrooxidation at low temperature.It was found that the reaction activity of coconut derived activated carbon was higher than that of coal derived activated carbon and coal by investigating the chemical reaction between phosphomolybdic acid and carbon materials.The reaction mechanism of indirect carbon electrooxidation was explored by cyclic voltammetry,indicating that hexavalent Mo in phosphomolybdic oxidized the carbon to form pentavalent Mo,then followed by an anodic reaction electrooxidized back to hexavalent Mo and releasing an electron obtained from carbon materials.This process achieved the indirect oxidation of carbon at low temperature.The effect of sunlight on enhancing chemical reaction between phosphomolybdic acid and carbon materials was discovered from the experiments.The facilitations of sunlight for reactions came from two ways.One was the thermal effect of sunlight,which increased in reaction temperature and improved the reaction rate.The other was photocatalysis of phosphomolybdic acid,absorbing the sunlight energy and changing it into the chemical energy.The full cell test with phosphomolybdic acid demonstrated the concept of low temperature carbon fuel cell is feasibility,which has provided an approach of carbon fuel cell operating at ambient temperature.