Among the hydrogen-based energy conversion devices, the proton exchange membrane fuel cell (PEMFC) has shown some advantages in high conversion efficiency, no polluted gas generation, and low-temperature operation, which made it possible to be applied in many fields^[1]. To date, Pt/C catalysts are still regarded as one the most promising catalysts for the PEMFC. However, for the real application of platinum, the activity will be influenced by the operation environment, like the specific adsorption of the sulfo group in the Nafion ionomer, which is critical to applying the Pt/C as practical electrocatalysts in PEMFC^[2]. Recently, the two-dimensional material overlayers like nitrogen-doped carbon shells have shown the function to tune the electronic statement of the metal surface, which was considered to originate from the strong p-d coupling between the doped nitrogen and the platinum nanoparticles and could optimize the O binding energy to improve the activity^[3]. In this research, the nitrogen-doped carbon-coated platinum (PTCN) was synthesized by facile thermal treatment from the polydopamine to carbon shells with tunable thickness. To measure the electronic statement change on the catalysts modified with Nafion ionomer during the oxygen reduction reaction (ORR), the Operando X-ray absorption spectroscopy (XAS) was applied at BL36XU in Spring-8.

After being coated with the nitrogen-doped carbon shell, the specific activity of PTCN increased by 1.3 times higher than commercial Pt/C catalysts, which could be originated from the interaction between the nitrogen-doped carbon shell and the platinum surface atom. It could be identified by the changes after coated with a nitrogen-doped carbon shell from the X-ray photoelectron spectroscopy (XPS) and XAS N K-edge results. To explore the protective effect of nitrogen-doped carbon shell against the specific adsorption by Nafion ionomer, different amount of ionomer was modified onto the Pt/C and PTCN catalysts, of which the specific activity showed a 16 % decay on Pt/C, however, almost no changes on PTCN. To illustrate this protection effect, the CO displacement charge test was applied to quantitatively analyze the adsorbed species coverage on the platinum surface, and increased coverage of anion species was observed on the higher amount of ionomer additive for Pt/C, however, it showed slight changes on PTCN varying different ionomer content, which suggested the sulfo-group from ionomer could hardly be absorbed on the platinum surface after coated with nitrogen-doped carbon shell. Furthermore, since the CO will get oxidation on the platinum surface from 0.6 V (vs. RHE), the Operando XAS was applied to measure the electronic statement and structure changes during the reaction potential range (0.5 V – 1.1 V vs. RHE). It also showed less change on the 5d vacancy for PTCN. These results showed potential for using carbon-coated catalysts in solving the specific adsorption problem from Nafion ionomer and promoting the large-scale application for the PEMFCs.

Figure 1. Influence of the I/C ratio on the specific activity of Pt/C and PTCN at 0.90 V vs RHE.

Acknowledgement

This work was supported by the project (JPNP20003) and a NEDO FC-Platform project commissioned by the New Energy and Industrial Technology Development Organization (NEDO). And China Scholarship Council (CSC) was acknowledged for the doctoral scholarship of Yunfei Gao (202006270046).

References:

1. Debe, M. K. Nature 2012, 486 (7401), 43-51.

2. Kodama, K.; Nagai, T.; Kuwaki, A.; Jinnouchi, R.; Morimoto, Y., Nature Nanotech. 2021, 16 (2), 140-147.

3. Gan, T.; Yang, J.; Morris, D.; Chu, X.; Zhang, P.; Zhang, W.; Zou, Y.; Yan, W.; Wei, S.-H.; Liu, G., Commun. 2021, 12 (1), 2741.