Despite the good progress in developing doped carbon catalysts for oxygen-reduction reaction (ORR), the current metal-free carbon catalysts are still far from satisfactory for large-scale applications of fuel cell. Developing new metal free doped carbon materials with abundance active sites as well as excellent electron transfer and reactant transport rate towards ORR may be a potential solution. Herein, we develop a novel three-dimensional (3D) sulfur-nitrogen co-doped carbon foams (S-N-CF) with hierarchical pore structures, using a convenient, economical, and scalable method. The experimental results have demonstrated that the obtained 3D S-N-CF exhibited better catalytic activity, longer-term stability and higher methanol tolerance than a commercial Pt/C catalyst. Such excellent performances may be attributed to the synergistic effect, which includes high catalytic sites for ORR provided by high S-N heteroatom loading, excellent reactant transport caused by hierarchical pore structures and high electron transfer rate provided by 3D continuous networks. Our results not only develop a new type of catalysts with excellent electrocatalytic performance by a commercially valid route, but also provide useful information for further clarification of the relationship between the microstructures of metal-free carbon materials and catalyst properties for ORR. More importantly, the idea to design hierarchical pore structures could be applied to other catalytic materials and serve as a general strategy for improving the activity of various ORR catalysts.

A simple, green, scalable one-step approach to obtain porous CNTs (PCNTs) with a high specific surface area and pore volume has been developed involving a mild chemical reaction between CNTs and rare oxygen sourced from nebulized water vapor at high temperatures.

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Despite the good progress in developing carbon catalysts for oxygen reduction reaction (ORR), the current metal-free carbon catalysts are still far from satisfactory for large-scale applications of fuel cell. Developing hollow graphene balls with a selfsupporting structure is considered to be an ideal method to inhibit graphene stacking and improve their catalytic performance. Herein, we fabricated metalfree hollow graphene balls with a self-supporting structure, through using a new strategy that involves direct metal-free catalytic growth from assembly of SiO 2 spheres. To our knowledge, although much researches involving the synthesis of graphene balls have been reported, investigations into the direct metal-free catalytic growth of hollow graphene balls are rare. Furthermore, the electrocatalytic performance shows that the resulting hollow graphene balls have significantly high catalytic activity. More importantly, such catalysts also possess much improved stability and better methanol tolerance in alkaline media during the ORR compared with commercial Pt/ C catalysts. The outstanding performances coupled with an easy and inexpensive preparing method indicated the great potential of the hollow graphene balls with a self-supporting structure in large-scale applications of fuel cell.

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