Construction And Performance Enhancement Of Monolithic Carbon Cathode For Direct Liquid Fuel Cells

Nowadays,with the rapid development of society and economy,people’s demand for the utilization of energy has increased rapidly,making the shortage of energy reserves become a social problem.At the same time,with the massive exploitation and use of fossil energy,the global environmental pollution is becoming more and more serious.Therefore,facing the great challenge of these double pressure,it is extremely urgent to find and develop clean and environmental-friendly energy.Direct liquid fuel cell(DLFC),as an environment-friendly energy conversion device,which can directly convert the chemical energy of liquid fuel into electric energy,attracts great attention of many scholars.The oxygen reduction reaction(ORR)occurs in the cathode of DLFCs.However,due to the sluggish kinetics of ORR,which limits the performance of the cathode and the large-scale practical application of DLFCs.Therefore,how to construct air cathodes with high-performance is the key factor to improve the performance of DLFCs.At present,the commonly used ORR catalyst for air cathode is platinum based precious metal catalyst.This kind of catalyst has good ORR performance,which can ensure that oxygen reduction reaction is a four-electron reaction process.But precious metal catalysts often suffer from some multiple drawbacks,including high price,scarce resources,fuel crossover effects and easy to be impurity poisoning,it cannot be applied on a large scale,which make them hard to apply on a large scale.Many studies have shown that the heteroatom-doped carbon catalysts have become the focus of attention as an alternative to platinum-based precious metal catalysts due to their advantages such as good pore structure,simple preparation method,cheap and large abundance.However,whether it is precious metal catalyst or carbon catalyst,this kind of catalyst is generally powder state,and need to be prepared into ink,mixed with binder evenly,then the cathode is obtained through spraying or hot pressing and other methods to load it on the carbon cloth or carbon paper.Moreover,the binder is non-conductive and may cover the surface of the catalyst,which not only increase the resistance of the catalytic layer but also hinder the contact between oxygen and catalyst.It reduces the mass transfer efficiency of oxygen and affects the ORR process.This further degrades the overall performance of the cathode.In view of the existing problems and shortages of the air cathode of DLFCs.From two aspects of catalyst and electrode structure,based on electrocatalysis and transport enhancement of air cathode,the performance of ORR catalyst,transport characteristics of monolithic cathode were studied.The main research contents include:(1)Using agar powder as raw material and iron phthalocyanine as doping source,high performance ORR carbon catalysts with carbon microsphere morphology were prepared by solvothermal after low temperature hydrothermal and high temperature carbonization.The effects of different carbonization temperatures on the physical and chemical properties such as pore distribution and ORR activity of the catalysts were discussed.(2)The method by direct carbonization of agar powder to construct the monolithic carbon air cathode was proposed.The influence of the concentration of doping source on the doping efficiency and pore distribution during the preparation of cathode was discussed.The performance of different DFFCs obtained by different cathodes was analyzed.(3)On the premise of the constructed monolithic carbon air cathode,the pore structure inside the electrode was enriched and the cathode performance was enhanced by activation of zinc chloride solution.Change the concentration of the zinc chloride solution,the effects on the electrode morphology,pore distribution,doping efficiency,transmission characteristics and the performance of DFFCs were studied.The main conclusions of the study are as follows:1)High performance ORR carbon catalyst was successfully prepared using agar powder as the precursor through the method of low temperature hydrothermal,high temperature carbonization and Fe-N doping.The catalyst obtained not only has carbon microsphere structure and abundant pore structure,but also has a large number of Fe-N active sites.Catalyst prepared at different carbonization temperature has different ORR performance,and HTCS-900 at carbonization temperature of 900℃has the best performance.The effects of carbonization temperature on different catalysts are as follows:HTCS-900 has the largest number of mesopores and the highest doping content of Fe-N active site.By comparing the performance of HTCS-900 and commercial Pt/C catalyst in 0.1 M KOH,the ORR process of HTCS-900 catalyst is four-electron pathway,and the half-wave potential,Tafel slope and kinetic current density of HTCS-900catalyst are better than those of Pt/C catalyst.In addition,HTCS-900 has better stability and ORR selective catalysis than commercial Pt/C.2)A non-noble metal monolithic carbon air cathode with binder-free and self-supporting properties was prepared by using agar powder powder as the precursor by invert-mold carbonization and solvothermal method with iron and nitrogen doping.The doping efficiency of the electrode was improved by adjusting the concentration of iron phthalocyanine in the doping process.Among them,the electrode doped with 1.0mg/m L iron phthalocyanine has the best performance.3)The pore structure of carbon air cathode was enriched by means of activation of zinc chloride solution.The doping efficiency and mass transfer characteristics of the cathode were further enhanced,so that the ORR performance of the cathode was improved,and the DFFCs assembled by the cathode has a better maximum power density output.Through SEM test,mercury injection test,nitrogen adsorption and desorption test,XPS test and ICP test,the influence of different concentrations of zinc chloride on the pore formation of the electrode was analyzed.More abundant pore structure inside the electrode was obtained by appropriate improving the zinc chloride concentration,which not only provides enough gas supply channels,but also provides more effective doping area.However,too high concentration of zinc chloride will cause the internal structure of the electrode to collapse,destroy the integrity of the electrode structure,and reduce the performance of the electrode.The performance of the DFFCs assembled with different cathode obtained with different concentrations of zinc chloride was tested.The DFFCs assembled by the cathode with the concentration of 0.5 M zinc chloride showed the best maximum power density output of 16 m W/cm~2,showing a good application prospect.