Large-Size Graphene Tube Catalysts for Bifuncational Oxygen Reduction and Oxygen Evolution Electrocatalysis in Alkaline Media

The oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are one pair of the most technologically important reactions for a variety of electrochemical energy storage and conversion technologies (e.g., fuel cells, metal-air batteries, and water splitting). It is important to design and develop advanced nonprecious-metal catalysts capable of facilitating these sluggish reactions at sufficient rates for practical applications. In our recent efforts,^1-7 cost-effective nitrogen-doped graphene nanocomposites have been prepared via the graphitization of nitrogen-containing polymers or compounds, combined with transition metals (Co, Ni or Fe). Among others, a new type of N-doped carbon tubes with large diameters (up to 500 nm) and relatively thin walls (less than 10 layers), which we call graphene tubes (GTs), were prepared via a scalable graphitization process of inexpensive dicyandiamide. Due to the high ratio of diameter-to-wall thickness, much increased surface area was obtained compared to conventional multi-walled carbon nanotubes (MWNTs). An effective strategy for tuning the size of large-diameter nitrogen-doped graphene tubes (GT) from 50 to 200 nm was developed by varying the transition metals used to catalyze the graphitization. This effort demonstrates optimal manipulation of morphology and surface area, thereby providing an effective approach to further improving the performance of nonprecious metal catalysts.

Furthermore, aiming to improve the activity and stability of the state-of-the-art Pt catalysts, the ORR active GT is used as a matrix to disperse Pt nanoparticles in order to build a unique hybrid Pt cathode catalyst. This is the first demonstration of the integration of a nonprecious metal catalyst with Pt catalyst in a nanocomposite. Relative to traditional Pt/C catalyst, much improved activity and stability were achieved. This work provides a new hybrid concept to design and synthesis novel cathode catalysts for fuel cells.

Apart from dramatically enhanced ORR activity, the large-size graphene tubes demonstrated excellent OER activity comparable to state-of-the-art Ir black in alkaline media. Importantly, unlike traditional carbon blacks and 2D reduced graphene oxides, the graphene tubes can be remain stable in harsh electrochemical environments (potentials up to 1.9 V) in alkaline media. This provide a great opportunity to develop bifunctional oxygen catalysts for reversible electrochemical energy applications such as reversible alkaline media and metal-air batteries.

References

1. G. Wu, K.L. More, C.M. Johnston, P. Zelenay, Science, 332, 443, 2011.
2. G. Wu, P. Zelenay, Acc. Chem. Res., 46, 1878, 2013.
3. G. Wu, N.H. Mack, W. Gao, S. Ma, J. Han, J.K. Baldwin, P. Zelenay. ACS Nano. 6, 9764, 2012.
4. Q. Li, P. Xu, W. Gao, S. Ma, R. Cao, J. Cho, H.L. Wang, G Wu, Adv. Mater., 26, 1378,2014.
5. Q Li, H Pan, D Higgins, R Cao, G Zhang, H Lv, K Wu, J Cho, G Wu, Small 11, 1443, 2015.
6. Q. Li, R. Cao, J. Cho, G. Wu, Adv. Energy Mater., 4, 1301415,2014.
7. X Wang, Q Li, H Pan, Y Lin, Y Ke, H Sheng, MT Swihart, G Wu, Nanoscale 7, 20290, 2015.