Ammonia Oxidation Activity Enhancement of Platinum with N-Doped Graphene and Tungsten Carbide Support
Authors: Siddharth Kumar , Shao Minhua 
Published: December 2020

Ammonia electrooxidation reaction (AOR) involving multistep electron-proton transfer is a significant reaction for fuel cells, hydrogen production and understanding nitrogen-cycle. Platinum has been established as the best electrocatalyst for AOR in aqueous alkaline media. 1 In previous studies, it has been observed that nitrogen-doped graphene 2 (NDG) and tungsten carbide 3 (WC) as support could help in increasing activity for some other electrochemical reactions.

In this study, Pt/NDG and Pt/WC/NDG were synthesized by a wet chemistry method 4 and their activities were compared to commercial Pt/C. Structure and morphology of the obtained electrocatalysts were examined by TEM, XPS and XRD. Moreover, Ir atoms were deposited on the surface of electrocatalyst Pt/WC/NDG via a modified self-deposition process. 5 LSV and chronoamperometry methods were applied to illustrate the activity and stability of electrocatalysts, respectively. LSVs of electrocatalysts in 0.1M KOH + 0.1M NH3 aqueous solution at a scan rate of 50 mV s-1 are elucidated in Figure 1.

It was observed that Pt/NDG exhibited a specific activity of 0.472 mA cm-2 which was 44% higher than commercial Pt/C, thus establishing NDG as a more efficient support for AOR than carbon. Furthermore, WC as support also has an impact on AOR activity (30 % increase in activity in comparison to Pt/NDG) with Pt/WC/NDG having a specific activity of 0.61 mA cm-2. Further surface modification with Ir results in the best electrocatalytic activity for AOR with Pt-Ir/WC/NDG (specific activity 0.94 mA cm-2) having almost thrice the current density of commercial Pt/C. Pt-Ir/WC/NDG also demonstrated the lowest onset potential of 0.48 V vs RHE. It was also found that Pt/WC/NDG delineated the best stability among the catalysts, thus highlighting the role of WC in improving stability for AOR.

This work adds insights regarding the role of NDG and WC as efficient supports along with significant impact of Ir surface modification on AOR activity of Pt.

Figure 1. LSVs of electrocatalysts in 0.1M KOH + 0.1M NH3 aqueous solution at 50 mV s-1

References:

[1] Zhong, C., Hu, W.B. and Cheng, Y.F., 2013. Recent advances in electrocatalysts for electro-oxidation of ammonia. Journal of Materials Chemistry A, 1(10), pp.3216-3238.

[2] Song, H., Yang, L., Tang, Y., Yan, D., Liu, C. and Luo, S., 2015. Three‐Dimensional Nitrogen‐Doped Reduced Graphene Oxide–Carbon Nanotubes Architecture Supporting Ultrafine Palladium Nanoparticles for Highly Efficient Methanol Electrooxidation. Chemistry–A European Journal, 21(46), pp.16631-16638.

[3] Shao, M., Merzougui, B., Shoemaker, K., Stolar, L., Protsailo, L., Mellinger, Z.J., Hsu, I.J. and Chen, J.G., 2011. Tungsten carbide modified high surface area carbon as fuel cell catalyst support. Journal of Power Sources, 196(18), pp.7426-7434.

[4] Zhang, C., Sha, J., Fei, H., Liu, M., Yazdi, S., Zhang, J., Zhong, Q., Zou, X., Zhao, N., Yu, H. and Jiang, Z., 2017. Single-atomic ruthenium catalytic sites on nitrogen-doped graphene for oxygen reduction reaction in acidic medium. Acs Nano, 11(7), pp.6930-6941.

[5] Sulaiman, J.E., Zhu, S., Xing, Z., Chang, Q. and Shao, M., 2017. Pt–Ni octahedra as electrocatalysts for the ethanol electro-oxidation reaction. ACS Catalysis, 7(8), pp.5134-5141.

Figure 1