Fuel cells are clean, sustainable energy conversion devices for power generation, and they most commonly use platinum as the electrocatalyst.[1] However, Pt-based catalysts suffer from very limited reserves, high cost, and inactivation by CO poisoning; these are major obstacles that fuel cells have to overcome for commercialization. [1][2][3][4][5][6] Thus, exploring nonprecious metal or even metal-free catalysts to rival platinum in activity and durability is absolutely crucial, with a potentially revolutionary impact on fuel-cell technologies. Very recently, metal-free PEDOT [6] and nitrogen-doped carbon nanotubes (NCNTs) [7,8] have shown a striking electrocatalytic performance for the oxygen reduction reaction (ORR). These breakthroughs have activated an exciting field for exploring the advanced metal-free electrocatalysts and understanding the related mechanism.As one of the most important carbon nanostructures, carbon-based nanotubes have been widely studied as the support of electrocatalysts for fuel cells in recent years. [9][10][11][12] Recent progress involving doping carbon nanotubes (CNTs) with electron-rich nitrogen to transform CNTs into superb metal-free electrocatalysts for the ORR [7,8] has motivated our curiosity to examine the corresponding performance of its counterpart by doping CNTs with electron-deficient boron. Intuitively, the adsorption of O 2 on boron dopant should be quite easy owing to the large difference of electronegativity between boron and oxygen, which is the precondition for the subsequent O 2 dissociation. In this study, BCNTs with tunable boron content of 0-2.24 atom % were synthesized. The ORR onset and peak potentials shift positively and the current density increases noticeably with increasing boron content, indicating a strong dependence of the ORR performance on boron content. Moreover, the origin of the electrocatalytic activity of BCNTs including the role of the boron dopant has been revealed by density functional theory (DFT) calculations. The experimental and theoretical results provide a new strategy to explore carbon-based metal-free electrocatalysts that are significant to the development of fuel cells.Using chemical vapor deposition (CVD) with benzene, triphenylborane (TPB), and ferrocene as precursors and catalyst, BCNTs were synthesized with tunable boron content of 0-2.24 at % by using different TPB concentrations. BCNTs with boron content of 0.86, 1.33, and 2.24 at %, as determined by X-ray photoelectron spectroscopy (XPS), were denoted as B 1 CNTs, B 2 CNTs, and B 3 CNTs, respectively (Supporting Information, S1

Two kinds of boron and nitrogen co-doped carbon nanotubes (CNTs) dominated by bonded or separated B and N are intentionally prepared, which present distinct oxygen reduction reaction (ORR) performances. The experimental and theoretical results indicate that the bonded case cannot, while the separated one can, turn the inert CNTs into ORR electrocatalysts. This progress demonstrates the crucial role of the doping microstructure on ORR performance, which is of significance in exploring the advanced C-based metal-free electrocatalysts.

No abstract

While the carbon-based metal-free electrocatalysts for oxygen reduction reaction (ORR) have experienced great progress in recent years, the fundamental issue on the origin of ORR activity is yet far from being clarified. To date, the ORR activities of these electrocatalysts are usually attributed to different dopants, while the contribution of intrinsic carbon defects has been little touched. Herein, we report the high ORR activity of the defective carbon nanocages, which is better than that of the B-doped carbon nanotubes and comparable to that of the N-doped carbon nanostructures. Density functional theory (DFT) calculations indicate that pentagon and zigzag edge defects are responsible for the high ORR activity. The mutually corroborated experimental and theoretical results reveal the significant contribution of the intrinsic carbon defects to ORR activity, which is crucial for understanding the ORR origin and exploring the advanced carbon-based metal-free electrocatalysts.

The synergism of large surface area, multiscale porous structure, and good conductivity endows hierarchical carbon nanocages with high-level supercapacitive performances. Further nitrogen doping greatly improves the hydrophilicity, which boosts the supercapacitive performances to an ultrahigh specific capacitance of up to 313 F g(-1) at 1 A g(-1).