Vertically aligned BCN nanotubes as efficient metal-free electrocatalysts for the oxygen reduction reaction: a synergetic effect by co-doping with boron and nitrogen.

The oxygen reduction reaction (ORR) is an important process in many fields, including energy conversion (fuel cells, metal–air batteries), corrosion, and biosensing. For fuel cells, the cathodic oxygen reduction is a major factor limiting their performance. The ORR can proceed either through a four-electron process to directly combine oxygen with electrons and protons into water as the end product, or a less efficient two-step, two-electron pathway involving the formation of hydroperoxide ions as intermediate. Oxygen reduction also occurs, albeit too slowly to be of any practical significance, in the absence of an ORR catalyst on the cathode. Platinum nanoparticles have long been regarded as the best catalyst for the ORR and are still commonly used in fuel cells due to their relatively low overpotential and high current density with respect to other commercial catalysts. However, the ORR kinetics on the Pt-based electrode is sluggish, and the Pt electrocatalyst still suffers frommultiple drawbacks, such as susceptibility to fuel crossover from the anode, deactivation by CO, and poor stability under electrochemical conditions. In addition, the high cost of Pt and its limited natural reserves are the major barriers to mass-market fuel cells for commercial applications. Recently, considerable efforts have been made to develop advanced electrocatalysts for reducing or replacing Pt-based electrodes in fuel cells. In particular, certain nitrogendoped carbon nanomaterials (e.g., carbon nanotubes (CNTs), graphene, porous carbon) were demonstrated to act as effective metal-free ORR electrocatalysts free from CO poisoning and crossover effect and having better long-term operational stability than commercially available Pt-based electrodes. The enhanced catalytic activity of these metal-free nitrogen-doped carbon nanomaterials toward ORR could be attributed to the electron-accepting ability of nitrogen species, which creates net positive charges on adjacent carbon atoms to facilitate oxygen adsorption for ORR with low overpotential. The well-defined high surface area and intertube spacing for improved electrokinetics, as well as the good electrical and mechanical properties associated with vertically aligned N-doped carbon nanotubes (VA-NCNTs) provide additional benefits to the metal-free nanotube ORR electrode in achieving record electrocatalytic performance. More recently, Yang et al. reported borondoped carbon nanotubes (BCNTs) as ORR electrocatalysts with improved activities relative to undoped CNTs. On the basis of experimental analyses and theoretical calculations, they concluded that the B atoms in the BCNT lattice are positively charged and act as the active sites for ORR. In contrast to all-carbon nanotubes carbon nanotubes containing both B and N atoms (BCN nanotubes), either in an aligned or nonaligned form, are bandgap-tunable by means of their chemical composition. Unlike CNTs, the bandgap of BCN nanotubes is independent of the diameter and chirality. This unique structure–property relationship makes BCN nanotubes attractive candidates for potential uses in many areas where CNTs have been exploited. In particular, the superb thermal stability and chemically tunable bandgap of BCN nanotubes provide tremendous opportunities to tune nanotube electronic properties for their use as an efficient metal-free ORR electrode, even at elevated temperatures. Here we report, for the first time, metal-free ORR catalysts based on vertically aligned BCN (VA-BCN) nanotubes containing both B and N atoms and exploit possible synergetic effects of co-doping with B and N on the ORR activities by comparison with vertically aligned N-doped carbon nanotubes (VA-NCNTs) and vertically aligned Bdoped carbon nanotubes (VA-BCNTs). While synthesis of VA-CNTs have been widely reported, there is much less discussion in the literature on the synthesis of VA-BCN nanotubes, most probably due to technical difficulties. In most of the previous studies, ternary compounds (e.g., ferrocene, melamine, boron oxide) were used as precursors for nanotube synthesis by metalcatalyzed (e.g., Ni) thermal chemical vapor deposition (CVD), with and without plasma enhancement. In the [*] Dr. S. Wang , Dr. E. Iyyamperumal , Dr. Y. Xue, Dr. D. Yu, Prof. L. Dai Department of Macromolecular Science and Engineering Case Western Reserve University 10900 Euclid Avenue, Cleveland, Ohio 44106 (USA) E-mail: [email protected]