Hierarchically mesoporous carbon spheres coated with a single atomic Fe-N-C layer for balancing activity and mass transfer in fuel cells

Novel cost-effective fuel cells have become more attractive due to the demands for rare and expensive platinum-group metal(PGM)catalysts for mitigating the sluggish kinetics of the oxygen reduction reaction(ORR).The high-cost PGM catalyst in fuel cells can be replaced by earth-abundant transition-metalbased catalysts,that is,an Fe-N-C catalyst,which is considered one of the most promising alternatives.However,the performance of the Fe-N-C catalyst is hindered by the low catalytic activity and poor stability,which is caused by insufficient active sites and the lack of optimization of the triple-phase interface for mass transportation.Herein,a novel Fe–N–C catalyst consisting of mono-dispersed hierarchically mesoporous carbon sphere cores and single Fe atom-dispersed functional shells are presented.The synergistic effect between highly dispersed Fe-active sites and well-organized porous structures yields the combination of high ORR activity and high mass transfer performance.The half-wave potential of the catalyst in 0.1M H_(2)SO_(4) is 0.82 V versus reversible hydrogen electrode,and the peak power density is 812 mW·cm^(−2) in H_(2)–O_(2) fuel cells.Furthermore,it shows superior methanol tolerance,which is almost immune to methanol poisoning and generates up to 162 mW·cm^(−2) power density in direct methanol fuel cells.

Tailoring the Porous Structure of Mono-dispersed Hierarchically Nitrogen-doped Carbon Spheres for Highly Efficient Oxygen Reduction Reaction

The search for a low-cost metal-free cathode material with excellent mass transfer structure and catalytic activity in oxygen reduction reaction(ORR)is one of the most challenging issues in fuel cells.In this work,nitrogen-rich mphenylenediamine is introduced into the synthesis of porous carbon spheres to tune the pore structure and nitrogen-doped active sites.As a result,more pyridinic N and pyrrolic N functional species were observed at the interior and surface of the carbon spheres.The introduction of m-phenylenediamine also regulated the nucleating of precursors,an urchin-like mesoporous surface structure ensures point contact and less agglomeration between each particle was obtained.With optimized proportion of micropores/mesopores and improved nitrogen-contained functional species,the ORR activity can be remarkably improved.The half-wave potential of this catalyst could achieve to 0.81 V(versus RHE)which is only 42 m V lower than commercial Pt/C catalyst.Furthermore,the optimized cathode catalyst achieved a 69 m W cm-2 maximum power density when operated in direct methanol fuel cells at room temperature.