Exploring the Function of Nitrogen-Doped Carbon Shell on Ordered and Disordered PtCo Alloy Catalysts in Oxygen Reduction Reaction By X-Ray Absorption Spectroscopy
Authors: Gao Yunfei , Uchiyama Tomoki , Yamamoto Kentaro , Matsunaga Toshiyuki , Teranishi Toshiharu , Sato Ryota , Imai Hideto , Sakurai Yoshiharu , Tsuji Yoichiro , Uchimoto Yoshiharu 
Published: November 2022

The development of proton exchange membrane fuel cells (PEMFCs) has received increasing attention for its potential to get rid of dependence on fossil fuels and curb carbon emissions[1]. However, the application of PEMFC has been limited by the consumption of platinum due to the sluggish kinetics of the cathode reaction and limiting platinum reserves. So far, forming Pt-M (M = non-precious metal) alloying phase catalysts has been regarded as an efficient method to improve the activity and decrease the cost of platinum. However, the introduction of the non-precious metal content will decrease the stability of the catalysts by accelerating the dissolution of metal atoms and agglomeration of the nanoparticles[2]. In this research, the nitrogen-doped carbon coating structure was applied to protect the PtCo nanoparticles. And both ordered (I-PtCo@CNx) and disordered (D-PtCo@CNx) PtCo alloy phase core was synthesized to further explore the role of nitrogen-doped carbon shell in improving the activity and stability. To study the electronic statement and structure changes of the catalysts, both in-situ and ex-situ X-ray absorption spectroscopy (XAS) was applied at BL36XU at SPring-8.
The synthesis of order PtCo catalysts, generally, requires a high temperature to overcome the energy barrier of forming the ordering phase, which will cause severe Ostwald ripening and get larger nanoparticles[3]. However, in the participation of nitrogen-doped carbon shells, the particle size could be limited to less than 5 nm. The I-PtCo@CNx exhibited high mass activity and specific activity at 1.08 A mgPt
-1 and 1.51 mA cmPt
-2, respectively. The XAS and X-ray photoelectron spectroscopy (XPS) analysis showed the decreased electronic density on the I-PtCo@CNx compared with I-PtCo catalysts, which suggested the increase of the activity might be originated from the interaction from the nitrogen-doped carbon shell. Furthermore, the accelerate durability tests (ADTs) were applied to measure the stability of the catalysts. In the rotating disk electrode and fuel cell condition, after 30,000 cycles ADTs, the activity of I-PtCo@CNx showed less decrease compared with other samples. The EXAFS analysis confirmed the high stability of I-PtCo@CNx might come from the less oxygen species generation during the ADTs, especially for the Co, and the confined structure benefits to maintaining the structure. In addition, the operando XAS analysis of the membrane electrode assemblies (MEA) samples also showed the high stability of I-PtCo@CNx after polarization at 1.1 V for 1 h.

Figure 1. The Pt L3 edge and Co K edge FT-EXAFS spectra of disordered-PtCo, ordered-PtCo, and ordered-PtCo@CNx in different ADTs cycles at 80 °C in MEA operation condition.

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).

Reference
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[3] Yang C.; Wang L.; Yin P.; Liu J.; Chen M.; Yan Q.; et al. Science
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Figure 1