| As one of the most essential components for direct formate fuel cell（DFFC）,the cathode has become the bottleneck for further DFFC performance enhancement because of the sluggish kinetics and high overpotential of oxygen reduction reaction（ORR）.Currently,noble metal catalysts（like Pt/C）are usually used as benchmark ORR catalysts because of their high catalytic ORR activity.Further,the catalyst loading in the cathode is always higher than that in the anode to match the rapid reaction in the anode,leading to a high cost of cathode accounting for the majority of the whole DFFC.In addition,noble metal catalysts usually suffer from scarce reserves,prohibitive capital costs,poor reaction selectivity,and catalytic instability,hampering their large-scale practical application.Thus,developing highly effective and stable non-noble metal catalysts to replace noble metal catalysts has become the focus of research.At present,among the numerous non-noble metal catalysts,transition element-doped carbon catalysts have attracted much attention because of their inherent economic,natural abundance,high catalytic efficiency and activity for ORR.Among them,the Fe-N-C catalyst has further been considered as the most potential catalyst for marketization application due to its extremely high catalytic activity.Although the most used pyrolysis or organic synthesis methods for Fe-N-C catalyst can be facilely conducted,they often result in catalyst aggregation,considerably reducing the exposure of active surface area and leading to maldistribution of ORR active sites,lowering down the stability and reproducibility of the catalyst.In addition,the lack of systematic design and regulation of the internal pores in the catalyst layer is disadvantaged for the formation of highly efficient three-phase interfaces,limiting the further improvement of DFFC performance.Aiming at the low density and exposure of active site in Fe-N-C catalyst,this research focused on boosting exogenous heteroatomic precursor into the carbon support to promote active site formation,regulating the porous structure of carbon catalyst to enhance active site exposure,and constructing porous monolithic air cathode to optimize reaction interface to enhance the performance of Fe-N-C catalyst and the relevant carbonaceous air cathode.The main research contents include:（1）Using traditional carbon black as the support to synthesize Fe-N-C catalyst by solvothermal method.Disclosing the influence of solutions with different solid/liquid interfacial energy on exogenous heteroatomic precursor transfer inside the support and the changes of pores in carbon material.Then,disclosing the reactant concentration and transfer in the cathodes fabricated by different catalysts before and after heteroatomic doping by simulation.（2）Synthesizing high-performance Fe-N-C catalyst based on porous hollow carbon spheres.Revealing the influence of water/alcohol ratio on solution infiltration inside the carbon support and the formation of Fe-N active sites.（3）Synthesizing monosaccharide-derived carbonaceous catalyst assembly with the linked skeleton nanostructure.Revealing the effects of microstructure characteristics on the ORR performance,ion transfer coefficient,and fuel cell performance.（4）Fabricating self-surpported and monolithic air-breathing cathode based on natural biocarbon and evaluating the effects of porous structure inside the cathode on the fuel cell performance.The major achievements are as follows:（1）To solve the problem of low density of active site inside carbonaceous catalysts,we regulated the solid/liquid interfacial energy between the exogenous heteroatomic precursor containing solution and carbon support to improve the infiltration of solution into the carbon support,forming an extended mass transfer channel network for exogenous heteroatomic precursor inside the carbon support and realizing active site targeted deposition inside meso-and macropores.The onset and half-wave potentials were effectively enhanced to 0.937 V and 0.895 V（vs.RHE）,which were 10 m V and 49m V higher than those of commercial Pt/C.The relevant membraneless DFFC fabricated by the cathode with the carbonaceous catalyst delivered a power density of 16.06±0.31m W cm-2,achieving 18.09%of improvement when compared with that of commercial Pt/C.（2）To solve the problem of low exposure of active site inside carbonaceous catalysts,we synthesized hollow carbon sphere with mesoporous shell and changed the ratio of water/ethanol of solution to realize the regulation of solid/liquid interfacial energy between the solution and carbon support,boosting the infiltration of the exogenous heteroatomic precursor containing solution into the carbon support and the formation of ORR active sites.The meseporous hollow structure was beneficial for the exposure of active sites and the transfer of reactant.Thus,the onset and half-wave potentials of the hollow carbon sphere-derived catalyst reached 0.953 V and 0.901 V（vs.RHE）,which were 26 m V and 55 m V higher than those of commercial Pt/C.The relevant membraneless DFFC fabricated by the cathode with the carbonaceous catalyst delivered a power density of 20.17±0.31 m W cm-2,achieving 48.31%of improvement when compared with that of commercial Pt/C.（3）Basing on the above experiments,we proposed to synthesize a linked carbon support with high surface area and numerous meso-and macropores by hydrothermal.We adjusted the concentration of hydrothermal precursor and surface properties of hydrothermal intermediates of carbon to realize the regulation of structure for the carbon support.The high specific surface area provided numerous deposition sites for active founctional groups and the high content of meso-and macropores from the linked skeleton nanostructure was beneficial for the exposure of active sites and mass transfer.Thus,the onset and half-wave potentials of linked carbonaceous catalyst reached 0.955V and 0.902 V（vs.RHE）,which were 28 m V and 56 m V higher than those of commercial Pt/C.The relevant membraneless DFFC fabricated by the cathode with the carbonaceous catalyst delivered a power density of 26.26±0.96 m W cm-2,achieving93.09%of improvement when compared with that of commercial Pt/C.（4）To solve the problem of complex layered structure of the traditional cathode and low stability caused by binder used in cathode fabrication,we proposed to construct a self-supported and monolithic air-breathing cathode by the natural porous bamboo internode.KOH activation and pyrolysis with（NH4）3PO4 increased the number and connectivity of pores and the electrical conductivity of the cathode.Solvothermal treatment introduced high-content ORR active sites.The monolithic structure simultaneously containing gas diffusion layer and catalyst layer decreased the complex of layered structure of traditional cathode,and the self-supported structure realized the exclusion of binder.Thus,the membraneless DFFC based on the self-supported and monolithic air-breathing cathode exhibited a high power density(10.01±0.16 m W cm-3),superior energy conversion efficiency（coulombic efficiency of 92.32%）,and satisfied stability（110 h）.It was preposed to decrease the solid/liquid interfacial energy between the exogenous heteroatomic precursor-contained solution and carbon support to improve the infiltration of precursor into the carbon support by solution,improving the formation of active sites.Then,the structure of carbon support was regulated by constructing hollow sphere with mesoporous shell and linked skeleton structure with mass meso-and macropores,which improved the density and exposure of active sites,enhanced the ORR catalytic activity,accelerated the mass transfer of reactant/ion,and increased power output of the membraneless DFFC.Based on the strategy of doping enhancement and porous structure regulation,a self-supported monolithic cathode was proposed to solve the problem of complexcity of layered structure of traditional cathode and low stability from using polymer binder,achieving satisfactory power output,energy conversion efficiency,and stability of the fuel cell.This work effectively promoted the practical application of this kind of direct liquid fuel cell. |
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