BCN graphene as efficient metal-free electrocatalyst for the oxygen reduction reaction.

The cathodic oxygen reduction reaction (ORR) is an important process in fuel cells and metal–air batteries. Although Pt-based electrocatalysts have been commonly used in commercial fuel cells owing to their relatively low overpotential and high current density, they still suffer from serious intermediate tolerance, anode crossover, sluggish kinetics, and poor stability in an electrochemical environment. This, together with the high cost of Pt and its limited nature reserves, has prompted the extensive search for alternative low-cost and high-performance ORR electrocatalysts. In this context, carbon-based metal-free ORR electrocatalysts have generated a great deal of interest owing to their low-cost, high electrocatalytic activity and selectivity, and excellent durability. Of particular interest, we have previously prepared vertically aligned nitrogendoped carbon nanotubes (VA-NCNTs) as ORR electrocatalysts, which are free from anode crossover and CO poisoning and show a threefold higher catalytic activity and better durability than the commercial Pt/C catalyst. Quantum mechanics calculations indicate that the enhanced catalytic activity of VA-NCNTs toward ORR can be attributed to the electron-accepting ability of the nitrogen species, which creates net positive charges on the CNT surface to enhance oxygen adsorption and to readily attract electrons from the anode for facilitating the ORR. Uncovering this new ORR mechanism in nitrogen-doped carbon nanotube electrodes is significant as the same principle could be applied to the development of various other metal-free efficient ORR catalysts for fuel-cell applications and even beyond fuel cells. Indeed, recent intensive research efforts in developing metal-free ORR electrocatalysts have led to a large variety of carbon-based metal-free ORR electrocatalysts, including heteroatom (N, B, or P)-doped carbon nanotubes, graphene, and graphite. More recently, we have successfully synthesized vertically aligned carbon nanotubes co-doped with N and B (VA-BCN) and demonstrated a significantly improved electrocatalytic activity toward the ORR, with respect to CNTs doped with either N or B, only due to a synergetic effect arising from the N and B co-doping. However, most of the reported carbon-based ORR electrocatalysts (particularly, heteroatom-doped nanotubes and graphene) were produced by chemical vapor-deposition (CVD) processes involving vacuum-based elaborate and careful fabrication, which are often too tedious and too expensive for mass production. As demonstrated in this study, therefore, it is of great importance to develop a facile approach to BCN graphene as low-cost and efficient ORR electrocatalysts. The recent availability of solution-exfoliated graphite oxide (GO) allows the mass production of graphene and derivatives by conventional physicochemical treatments of GO. Herein, we have developed a facile approach to metalfree BCN graphene of tunable B/N co-doping levels as efficient ORR electrocatalysts simply by thermal annealing GO in the presence of boric acid and ammonia. The resultant BCN graphene was shown to have superior electrocatalytic activities to the commercial Pt/C electrocatalyst (C2-20, 20% platinum on Vulcan XC-72R; E-TEK). First-principles calculations were performed to explain the high catalytic capability of the BCN graphene. This newly developed method can thus provide simple but efficient and versatile approaches to low-cost mass production of BCN graphene as efficient metal-free ORR electrocatalysts for fuel cells and other applications. Figure 1 shows XPS survey spectra for three BCN graphene samples of different chemical compositions, along with the mixture of GO and boric acid (B-GO) starting material as reference. Comparing with the B-GO starting mixture, the salient feature for BCN graphene samples is the appearance of a pronounced XPS N1s peak, indicating the incorporation of N in addition to B into GO by thermal annealing in the presence of boric acid and ammonia. Like VA-BCN nanotubes, the presence of a O1s peak in the BCN graphene samples is due to the incorporation of [*] Dr. S. Wang, Prof. L. Dai Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University 10900 Euclid Avenue, Cleveland, OH 44106 (USA) E-mail: [email protected]