Bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play predominant role in improving electrochemical performance of reversible fuel cell (RFC) and metal-air batteries (MRB).^1-3 In addition, the cost of these electrocatalysts largely determine economic feasibility and market penetration of these renewable energy technologies. Although precious metals such as Pt, Ru and Ir have been used as bifunctional electrocatalysts, their high cost and poor long-term stability has restricted the commercial feasibility of these devices. On the other hand, transition metal oxides (such as carbon supported MnO2) including perovskite oxides (such as carbon supported BSCF, PBCO, SNCF) have appreciable catalytic activity, they suffer from poor surface area, electronic conductivity and carbon support corrosion in high potential of OER regime. Nanocarbon-based catalysts have many advantages over others due to their low cost, excellent electrical conductivity, high surface area, and easy functionalization. However, they typically cannot withstand the highly oxidative OER environment. In our recent works, we found that the electrochemical stability of carbon is highly depended on their morphology, structure and surrounding electronic environment. For instance, amorphous carbon black was found to be highly unstable, whereas highly graphitized carbon nanotubes were very stable during accelerated stress test (AST). Subsequently, in a parallel effort we found that the presence of transition metals (Fe, Co, Ni, Mn) and their alloys can significantly improve the stability of the as synthesized transition metal derived and nitrogen (N) doped graphene tubes. As a result, we were successful to synthesize ultralarge sized N doped graphene tubes (N-GTs) (>500 nm) decorated with FeCoNi alloy particles as a highly active and stable bifunctional electrocatalyst (N-GT(FeCoNi)). To present our more recent work, I will also show some electrochemical and related physical characterization highlighting tremendous increase in the stability and corrosion resistant as a result of the incorporation of multivalent Mn cation with variable oxidation states as the fourth dopant transition metal in the resulting highly graphitized N co-doped nanocarbon structure. The cost effective, easily scalable, and environmentally benign synthesis method along with impressive overall bifunctional performance favors the novel nanocarbon, i.e., graphene tubes’ application as a practical bifunctional electrocatalyst for alkaline membrane fuel cells and metal air batteries.

References

1. Osgood, H.; Devaguptapu, S. V.; Xu, H.; Cho, J. P.; Wu, G., Transition Metal (Fe, Co, Ni, and Mn) Oxides for Oxygen Reduction and Evolution Bifunctional Catalysts in Alkaline Media. Nano Today 2016, 11, 601–625.

2. Gupta, S.; Qiao, L.; Zhao, S.; Lin, Y.; Vamsi, D. S.; Xu, H.; Wang, X.; Swihart, M.; Wu, G., Highly Active and Stable Graphene Tubes Decorated with Feconi Alloy Nanoparticles Via a Template-Free Graphitization for Bifunctional Oxygen Reduction and Evolution. Advanced Energy Materials 2016, 6, 1601198.

3. Gupta, S.; Kellogg, W.; Xu, H.; Liu, X.; Cho, J.; Wu, G., Bifunctional Perovskite Oxide Catalysts for Oxygen Reduction and Evolution in Alkaline Media. Chemistry - An Asian Journal 2016, 11, 10–21.

4. Liu, X.; Liu, W.; Ko, M.; Chae, S.; Park, S.; Casimir, A.; Wu, G.; Cho, J., Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts. Advanced Functional Materials 2015, 25, 5799–5808.

5. Liu, X.; Park, M.; Kim, M. G.; Gupta, S.; Wu, G.; Cho, J., Integrating Nico Alloys with Their Oxides as Efficient Bifunctional Cathode Catalysts for Rechargeable Zinc-Air Batteries. Angew. Chem.-Int. Edit. 2015, 54, 9654–9658.