… developing new active electrocatalysts for ORR has recently become a key to boost the practical applications of fuel cells and metal−air batteries. Although platinum (Pt) and its alloys exhibit high activity for ORR, their performance has been overshadowed by the high cost and scarcity of Pt and by the reduced thermal efficiency caused by substantial overpotential for the ORR. Hence, intensive efforts have been devoted to substitute Pt-based catalysts by employing non-precious metal catalysts and preferably metal-free catalysts. … The research of designing new catalysts to reduce the overpotential and understanding the nature of ORR catalytic sites and mechanisms in metal-free catalysts is still in its infancy.

In general, the adsorption of oxygen and formation of superoxide through a one-electron reduction on metal-free catalysts such as N-doped graphene sheets have been suggested as the initial ORR steps, and O₂ adsorption is proposed to be the rate-determining step. Since oxygen is preferred to be adsorbed onto the exposed edges of N-doped graphene rather than the basal planes, it is clear that the edges of N-doped graphene-based catalysts possess high ORR activity while the basal planes remain virtually ORR inactive. Thus, edge-abundant, nitrogen-doped graphene would facilitate the formation of catalytic sites for ORR.

In this regard, unique carbon nanotube−nanoribbon complexes with controllable nitrogen doping have been recently explored via partially unzipping carbon nanotubes and subsequent annealing under NH₃ atmosphere, showing enhanced catalytic activity for ORR. However, in rotating-disk electrode (RDE) polarization studies, their ORR onset potentials and half-wave potentials (E_1/2) are still lower than those of commercially available Pt catalysts. This would result in high overpotentials of fuel cells at practical operating current densities and cause low thermal efficiency. Thus, developing new strategies to engineer efficient metal-free ORR catalysts still remains challenging.

—Gong et al.

Researchers have come to realize that graphene’s potential as a catalyst doesn’t lie along the flat face but along the exposed edges where molecules prefer to interact. The Rice team chemically unzipped carbon nanotubes into ribbons and then collapsed them into porous, three-dimensional aerogels, simultaneously decorating the ribbons’ edges with boron and nitrogen molecules.

First-principles calculations suggested that the resulting excellent electrocatalytic properties originate from the abundant edges of boron- and nitrogen-codoped graphene nanoribbons, which significantly reduce the energy barriers of the rate-determining steps of the ORR reaction.

The key to developing carbon-based catalysts is in the doping process, especially with elements such as nitrogen and boron. The graphitic carbon-boron-nitrogen systems have thrown many surprises in recent years, especially as a viable alternative to platinum-based catalysts.

—Pulickel Ajayan

The Rice process is unique, Ajayan said, because it not only exposes the edges but also provides porous conduits that allow reactants to permeate the material.

Simulations by Rice theoretical physicist Boris Yakobson and his students found that neither boron nor nitrogen doping alone would produce the desired reactions. Testing found that optimized boron/nitrogen aerogels were far better than platinum at avoiding the crossover effect, in which fuel like methanol permeates the polymer electrolyte that separates electrodes and degrades performance. The researchers observed no such effect in 5,000 cycles.

We have demonstrated that optimally doped boron and nitrogen in graphene nanoribbons show excellent ORR electrocatalytic activity, even better than the commercial Pt−C catalysts. The high activity, excellent tolerance to methanol, high durability, and superior high half-wave potential are achieved for optimally doped (10 atom % BN) BNC NR catalysts in comparison to other metal-free catalysts in alkaline solution. The new BNC catalysts could serve as efficient metal-free ORR electrocatalysts for fuel cells and other electro-chemical and catalytic applications.

—Gong et al.

Rice graduate students Yongji Gong and Huilong Fei and postdoctoral researcher Xiaolong Zou are lead authors of the paper. Co-authors are Rice graduate students Gonglan Ye and Zhiwei Peng; Rice alumni Zheng Liu of Nanyang Technical University, Singapore, and Shubin Yang of Beihang University, Beijing; Wu Zhou of Oak Ridge National Laboratory; Jun Lou, an associate professor of materials science and nanoengineering at Rice; and Robert Vajtai, a senior faculty fellow in Rice’s Department of Materials Science and NanoEngineering.

The research was supported by the Welch Foundation; the Air Force Office of Scientific Research; Multidisciplinary University Research Initiative grants from the US Army Research Office, the Air Force Office of Scientific Research and the Office of Naval Research; and the Department of Energy’s Oak Ridge National Laboratory. The researchers utilized the National Science Foundation-supported DAVinCI supercomputer administered by Rice’s Ken Kennedy Institute for Information Technology.

Resources

• Yongji Gong, Huilong Fei, Xiaolong Zou, Wu Zhou, Shubin Yang, Gonglan Ye, Zheng Liu, Zhiwei Peng, Jun Lou, Robert Vajtai, Boris I. Yakobson, James M. Tour, and Pulickel M. Ajayan (2015) “Boron- and Nitrogen-Substituted Graphene Nanoribbons as Efficient Catalysts for Oxygen Reduction Reaction” Chemistry of Materials 27 (4), 1181-1186 doi: 10.1021/cm5037502