Boron‐Doped Carbon Nanotubes as Metal‐Free Electrocatalysts for the Oxygen Reduction Reaction

Abstract

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

Combining theory and experiment in electrocatalysis: Insights into materials design

“…heteroatom-doped carbons (132)(133)(134). By doping carbon with more electronegative atoms such as nitrogen, a net positive charge is created on adjacent carbon atoms (C + ) which facilitates oxygen adsorption and charge transfer, resulting in enhanced ORR/OER activity (132).…”

“…By doping carbon with more electronegative atoms such as nitrogen, a net positive charge is created on adjacent carbon atoms (C + ) which facilitates oxygen adsorption and charge transfer, resulting in enhanced ORR/OER activity (132). This strategy has also been extended to dopant atoms that are less electronegative than carbon (such as boron), which create similar charge sites (such as B + ) to facilitate the catalytic process (133). To exploit synergistic effects of different dopant atoms, codoping of carbon catalysts has been demonstrated with success as well (134).…”

Carbon-Based Metal-Free Electrocatalysis for Energy Conversion, Energy Storage, and Environmental Protection

“…Unlike N dopants however, B atoms can also act as positively charged sites for ORR because B atoms possess lower electronegativity than C (Fig. 1c) [88]. Similarly, the high electronegativity of F atoms can induce adjacent C polarization to create active sites.…”

CO2 capture and gas separation on boron carbon nanotubes

“…However, many boron carbon materials and boron doped carbon nanotubes have been successfully synthesized and reported. 44,[48][49][50] So that the nanotubes in this study could in principle be synthesized, especially for B 2 CNT(0, n) based on their low strain energies.…”