Hydrothermal Synthesis of Co-N-C Nanocomposites As High Performance Electrocatalysts for Oxygen Reduction Reaction
Authors: Nan Bo , Lu Zhouguang , Lyu Fucong , Shi Yang , Shi Haohong , Yu Sicen 
Published: February 2020

With the dramatically increased market demands of new energy materials due to the development of a large number of electronic devices and environmental awareness of the awakening, catalysts for oxygen reduction reaction (ORR) which are the most critical parts for high energy capacity and cleaning power sources including fuel cells and metal-air batteries have attracted highly attention and intense researches recently. Despite massive efforts, developing catalysts of high efficiency, long-term durability and excellent methanol tolerance with low cost for ORR is still a big challenge. Pt and Pt–based alloys are still the best choices for catalyzing oxygen reduction process by now. However, their high cost, low abundance, inferior stability and declining activity hinder their large-scale uses in industry. Therefore, selecting alternative materials to replace Pt and improve its performance is of great concern.
Chitosan, as the second most abundant biopolymer in nature, is rich in the outer shell of some arthropod (prawns, crabs, insects) and the inner shell and cartilage of mollusks (squid, cuttlefish) with some attractive properties, such as non-toxic, biocompatibility and safety, especially, its natural nitrogen rich, strong chelation effects, sustainable and environmental friendly and low cost depend that it can be heavy used to synthesize metal and nitrogen co-doped carbon (MNC) nanomaterials , which can provide an enhanced catalytic activities.
In the present work, taking advantage of strong chelation effects between hydroxyl group and metal ions and abundant natural nitrogen of chitosan, we developed a novel Co and N co-doped carbon (Co-N-C) nanocomposites as high performance electrocatalysts for ORR by using a simple and facile hydrothermal method, followed by high-temperature calcination at inert gas atmosphere. This material shows a delectable catalytic activity compared to the commercial 20% Pt/C electrode in the alkaline solution for ORR with a relative positive onset potential of -0.045V (vs. Hg/HgO), a much improved by 16.7% current density than Pt/C (loading ~ 0.4mg/cm2) as well as excellent durability performance and superior methanol tolerance. In addition, the method we used in this research is also suitable for preparing other MNC, which demonstrates that we found a new, straightforward and low-cost way to synthesis a series of catalysts for ORR which are comparable to commercial Pt/C.
Cyclic voltammetry (CV) was employed to test the ORR catalytic activity of Co-N-C-800 and the commercial 20% Pt/C catalyst in saturated O2 alkaline solution (0.1 M KOH) and shown in Figure 1a, an obvious cathodic peak can be found at -0.10 V vs. Hg/HgO which implies excellent ORR performance, and according to figure 1b, we can find that our Co-N-C-800 has a very close onset potential (-0.045V) and much higher current density (16.7% improved) in comparison to that of the commercial Pt/C, respectively, superior to the performances of pure chitosan without Co (C-800), which can confirm that Co-doping is crucial to the outstanding activity performance of Co-N-C-800. Owning to small metal nanocrystals of cobalt can improve the conductivity of materials and charge transfer efficiency between carbon and cobalt and give more active sites1. RDE measurements at various rotating speeds at a scan rate of 10 mV s-1 in O2-saturated solution were carried out for Co-N-C-800 and shown in Figure 1c. The current density shows a typical increase with rotation rate due to the shortened diffusion layer and the linearity of K-L plots for Co-N-C-800 indicates a good four-electron reaction path selectivity with the electron transfer numbers (n) is ≈ 3.9, which is close to that of Pt/C catalyst and further demonstrates an outstanding catalytic performance of this catalyst.
Acknowledgments
This work was financially supported by the Shenzhen Peacock Plan (KQCX20140522150815065), the Natural Science Foundation of Shenzhen (JCYJ20150331101823677) and the Science and Technology Innovation Foundation for the Undergraduates of SUSTC (2015x19 and 2015x12).

Reference

1. Xie, S.; Huang, S.; Wei, W.; Yang, X.; Liu, Y.; Lu, X.; Tong, Y., Chitosan Waste-Derived Co and N Co-doped Carbon Electrocatalyst for Efficient Oxygen Reduction Reaction. ChemElectroChem
2015,
2 (11), 1806-1812.

Figure 1