Design of Metal Structure Encapsulated in N-Doped Carbon Layers As Tunable Catalyst for Electrochemical Applications
Authors: Ohsaka Takeo , Noh Seung Hyo , Seo Min Ho , Kang Joonhee , Okajima Takeyoshi , Han Byungchan , Matsumoto Futoshi 
Published: February 2020

Polymer electrolyte membrane fuel cells (PEMFCs) have been widely recognized as a promising energy conversion device owing to its high power density, environmentally clean products, low working temperature, and so on. On the other hand, the several issues such as a high material cost, a low activity, and a weak durability have impeded the commercialization of fuel cell applications. In general, these issues are directly correlated to the utilization of Pt catalyst, which is one of the noble metals and of the most broadly utilized catalyst. Hence, a number of studies of fuel cell catalysts have been concentrated on reducing the amount of Pt, for instance, by alloying with 3d-metals such as Pt-Co, Pt-Ni, and Pt-Cu. Stamenkovic et al. (1) and Doyle et al.(2) insisted, for example, that Pt3Ni (111) can improve its ORR activity about 10 times compared to pure Pt (111). To synthesize the Pt-alloy catalyst, however, an amount of expensive Pt metal in Pt-3d binary or ternary alloys is still consumed in spite of its high oxygen reduction reaction (ORR) activity.

Recently, the studies on a non-precious catalyst increase rapidly due to its low material cost and a variety of applications such as oxygen reduction reaction (ORR) and metal-air batteries in cathode catalysts. Representatively, nitrogen doped graphene (N-Gr) has been considered as a proper candidate material to substitute Pt catalyst. Several researches have insisted that the performance of N-Gr is superior to that of single Pt catalyst in alkaline media (3). In addition, Zelenay et al. reported that the ORR activity of polyaniline-Fe carbon composite (PANI-Fe-C) is similar to that of Pt in acidic solution(4). Dodelet et al. insisted that the enhanced activity is based on the effect of 3d-metals (5).

The commercialization of non-precious metal catalyst, however, is not enough to replace Pt-based catalysts due to its lack long-term stability and performance for ORR. Furthermore, a reaction mechanism or an active site of the catalyst has not been explained clearly until now. This study suggests that metal encapsulated in nitrogen doped carbon (M@N-C) catalyst could be a novel approach in acidic fuel cell by utilizing density functional theory (DFT) calculations and experimental results. As an important factor to tune the binding between a reaction intermediate and catalyst, the following three effects were mainly discussed: 1) nitrogen doped level (or position), 2) thickness effect of carbon layer and 3) alloying effects.

The main purpose of this study is to provide the guideline for the design of N-doped carbon coated metal structure.

References

1. Stamenkovic, V. R. et al. Trends in electrocatalysis on extended and nanoscale Pt-bimetallic alloy surfaces. Nat. Mater.
6, 241-247 (2007).

2. Doyle, M., Rajendran, G., Vielstich, W., Gasteiger, H. A. & Lamm, A. Handbook of Fuel Cells Fundamentals, Technology and Applications. Vol. 3 (Wiley, 2003).

3. Geng, D. et al. High oxygen-reduction activity and durability of nitrogen-doped graphene. Energy Environ. Sci.
4, 760-764 (2011).

4. Wu, G. & Zelenay, P. Nanostructured nonprecious metal catalysts for oxygen reduction reaction. Acc. Chem. Res.
46, 1878-1889 (2013).

5. Lefèvre, M., Proietti, E., Jaouen, F. & Dodelet, J.-P. Iron-based catalysts with improved oxygen reduction activity in polymer electrolyte fuel cells. Science
324, 71-74 (2009).

Figure 1