Carbon-based Oxygen Reduction Reaction Catalysts

Developing high performance and low cost oxygen reduction reaction?ORR?catalyst is the key to the commercialization of fuel cells.Recently,carbon-based ORR catalyst is regarded as the most promising candidate for replacing Pt catalyst due to its high ORR activity,excellent stability,low cost and environmental friendliness.However,the activity of carbon-based ORR catalyst is still inferior to that of Pt-based catalyst.Therefore,it is the urgent and challenging to improve the catalytic activity of carbon based ORR catalysts.In this paper,the preparation method and structure design of ORR catalysts have been carried out in the following four aspects:?1?We report a template-free method to fabricate hollow nitrogen-doped carbon?HNC?by pyrolysis of hollow polyaniline microspheres,which were synthesized by suspension polymerization method.The entire synthesis is simple,environmentally benign,and economic.The oxygen reduction reaction?ORR?activity of the as-prepared HNC catalyst is close to that of the commercial Pt/C catalyst in alkaline media with four electron pathway.Moreover,the HNC shows much better fuel crossover resistance and long-term durability than the commercial Pt/C in alkaline medium.All these features make HNC a potentially promising and suitable substitute for the expensive noble metal catalysts in the next generation alkaline fuel cells.?2?to evaluate the structural effect of wall thickness and specific surface area on the electrocatalytic activity,we have demonstrated the rational design and preparation of hollow micro/mesoporous Fe/N/C catalyst?HM-Fe/N/C?.The wall thickness and specific surface area of HM-Fe/N/C are delicately tailored via simply regulating the Fe content absorbed on PDA precursor.Combined with TEM and N₂ isothermal adsorption desorption test,we found that increasing the Fe³⁺content,the wall of the catalyst decreases from ca.50 nm for HM-N/C-0-800 to ca.5 nm for HM-Fe/N/C-0.8-800 and HM-Fe/N/C-1.0-800,the specific surface area increased from 361.9 m² g^-1 for HM-N/C-0-800 to 1703.5 m² g^-1 nm for HM-Fe/N/C-0.8-800.The as-prepared HM-Fe/N/C-0.8-800 sample with ca.5 nm wall thickness and a super-high surface area(1703.5 m² g^-1)showed highest ORR performance,whose half-wave potential can reach 0.71 V and0.91V in 0.1 M HClO₄ and 0.1 M KOH,respectably.Impedance analysis prove that the outstanding ORR activity of HM-Fe/N/C-0.8-800 is derived from the unique hollow mico/meso-pore structure which is favorable for achieving a high effective surface area and low-resistant diffusion channels for O₂ and electrolyte and thus guarantee quick mass transport during the ORR process.?3?In order to overcome mass loss,low nitrogen-doped efficiency and structure collapse problems during high-temperature pyrolysis,we develop a novel method based on the combination of space-confined pyrolysis and second heat-treatment for the synthesis of Fe/N-doped Carbon?Fe/N/C?ORR catalyst,targeting solving the low yield,insufficient nitrogen-doped level,and nanostructure collapse problems.The silica-confined space is constructed by covering an outer inorganic silica layer onto the surface Fe/Phenanthroline/C?Fe/Phen/C?precursor.Within this silica-confined space,the precursor loss,particle coagulation and structure collapse that are commonly encountered during high-temperature pyrolysis in an open system can be eliminated.Moreover,the porosity and nitrogen-doped configuration can further be tuned by a second heat-treatment process.The resultant Fe/N/C catalyst show excellent ORR activity in alkaline media,better durability and stronger methanol tolerance compared to those of state-of-the-art Pt/C catalyst.?4?Presently,the practical application of Fe/N/C catalysts as replacements of Pt for oxygen reduction reaction?ORR?is still limited by insufficient activity.Herein,we demonstrate a novel design for such catalyst.On one hand,the obtained Fe/N/C-SiO₂-ZnCl₂ catalyst owns high densities of well-exposed active-sites derived from 3D well-balanced macro-,meso-,and microporous structures constructed by adopting ZnCl₂ salt and SiO₂ microspheres as combined templates.On the other hand,simulation reveals that a high loading of catalyst in cathode catalyst layer would not benefit cell performance and fast ORR process occurs only inside a limited thickness of catalyst layer.Particularly,the Fe/N/C-SiO₂-ZnCl₂ shows a maximal output power density as high as 480 mW cm^-2at an ultra-low loading of 0.5 mg cm^-2.This study firstly exhibits that development of catalysts with high-density active sites and construct of ultra-thin catalyst layer are of great significance for improving the performance of fuel cell.