| Since the 21st century,the mankind has been confronted with ever-increasing energy shortage and environmental pollution that lead to a great and urgent demand of clean,efficient and sustainable energy sources.As a green and efficient electrochemical energy conversion device,fuel cells（FCs）can directly convert the chemical energy into electrical energy and have become one of the most valuable power sources.In this discipline,because of the simplified mechanism and portability,membraneless direct liquid fuel cells（membraneless-DLFCs）have stimulated wide interests of researchers.Compared with proton exchange membrane fuel cells（PEMFCs）,membraneless-DLFCs exhibit the following advantages:1)the elimination of the proton exchange membrane,with sigfinicantly reduced costs of cell;2)the operation with liquid fuel,with higher energy density compared with gas fuel;3)the safety and stability of operation,with the easy to store,non-toxic liquid fuel.Currently,carbon-based non-noble metal air cathodes are commonly used in membraneless-DLFCs to reduce the cost and fabrication process of the cells.However,the poor oxygen reduction reaction（ORR）catalytic performance of carbon-based non-noble metal air cathodes limits the application of membraneless-DLFCs.Thus,enhancing the ORR catalytic performance of cathodes becomes the breakthrough point to solve this problem.Currently,the ORR performance enhancement of carbon-based air cathodes could be achieved mainly from two aspects:1)the synthesis of carbon-based non-noble metal catalyst with high ORR catalytic performance;2)the enhancement of the internal pore structure of the cathode to expand the ORR triple-phase interface in the catalyst layer.However,the current studies still suffer from the following problems:1)the characterization results of catalyst ORR performance are widely varied;2)the morphology of carbon supports differ in many ways,and the influence of carbon supports microstructure on catalyst ORR performance is controversial;3)Many other problems,concerning the carbon-based air cathode,remain unsolved,such as the high cost,the low conductivity,the complex fabrication process,and the insufficient inner ORR triple-phase interface,etc.To address the above issues,this study first investigates the effects of different catalyst loading and different degree of coffee ring effect on the catalyst ORR performance measurement and optimizes the method of rotating ring disk electrode（RRDE）measurement to obtain a reliable catalyst ORR performance.Secondly,the research investigates the effects of carbon support physicochemical properties,such as morphology,pore distribution and oxygen-containing functional groups,on the performance of carbon-based ORR catalysts.Subsequently,a non-noble metal carbon-based catalyst with high ORR performance is synthesized in this study by coal tar soot.In the light of above studies,the research further tunes the carbon support microstructure by changing the agar hydrothermal concentration and charbonizaiton temperature to improve the ORR performance of the carbon-derived catalyst.Finally,the research constructs a monolithic agar carbon-based ORR air cathode to reduce the cost and simplify the fabrication process of the air cathode.The structural dimensions of the monolithic air cathode are further optimized to extend the ORR triple-phase interface of the catalyst layer,which result in a marked increase in the power density of the fuel cell.Based on the above work,the main achievements are as follows:（1）In the RRDE measurement,the catalyst layer cannot completely cover the electrode surface at low catalyst loading.In contrast,high catalyst loading will cause the central catalyst layer to be over-thick,resulting in a significant increase in the mass transport resistance inside the catalyst film.In both cases,the ORR performance data of the catalyst is underestimated.The optimal catalyst loading range is 16.2-24.2μg Ptcm-2.（2）The coffee ring effect causes uneven Nafion/Pt ratio distribution in the catalyst film,resulting in a high Nafion concentration near the edge.A high Nafion concentration hinders oxygen transport inside the catalyst film and poisons the Pt nanoparticles,thus reducing the ORR performance of the catalyst and leading to an underestimation of catalyst ORR performance.Furthermore,the coffee ring patterned catalyst film causes a lower RRDE collection efficiency,leading to an overestimation of the electron transfer number and underestimation of hydrogen peroxide yield.（3）By comparison with the relationships between carbon support morphology,oxygen-containing functional groups,pore distribution and ORR performance,this study finds that spherical structure,rich oxygen-containing functional groups,and high electrical conduction of carbon support give the catalyst a well-developed meso-/macro-pores structure,an excellent Fe-N doping ability,and efficient electron transfer ability,respectively.These features favor the ORR triple-phase interface formation in the catalyst layer and improve fuel cell power.（4）In the light of the above findings,this study synthesizes a high ORR performance catalyst based on the industrial by-product coal tar soot.The research finds that the coal tar soot catalyst obtains a high ORR performance due to its uniform size,well-developed internal pore structure,and rich oxygen-containing functional groups.The air-cathode DFFC and MFC using as-prepared catalyst achieved high-power density outputs of 30.1 m W cm-2and 2026±160 m W m-2,respectively.（5）This study synthesizes a spherical carbon-based non-noble metal catalyst with uniform particle size by hydrothermal-carbonization-solvothermal doping using agar as the precursor.The research finds that the agar carbon catalyst obtained the highest ORR electrochemical performance with an agar solution concentration of 60 mg m L-1 and carbonization temperature of 900℃.Using the as-prepared catalyst,the air cathode DFFC achieved a power density output of 24.5 m W cm-2,slightly higher than other samples.（6）This study prepares a binder-free monolithic carbon-based air cathode using agar as the precursor.By adjusting the structural dimension of the air cathode,the research finds that a thicker cathode will increase the oxygen transport distance.A thinner cathode will result in an electrolyte-flooding of the cathode’s internal pores.Both cases increase the oxygen transport resistance within the cathode and reduce the power density of the membraneless-DLFCs.The results indicated that the as-prepared air cathode（Agar C@Fe-1.5）had a maximum power density of 10.22 m W cm-3,and cathode structural dimension adjusting can effectively extend the ORR triple-phase interface in the cathode. |
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