Insight into the Catalytic Mechanism of Bimetallic Platinum Copper Core Shell Nanostructures for Nonaqueous Oxygen Evolution Reactions
Authors: Ma Lu , Luo Xiangyi , Kropf Jeremy , Wen Jianguo , Wang Xiaoping , Lee Sungsik , Myers Deborah J , Miller Dean , Wu Tianpin , Lu Jun , Amine Khalil 
Published: February 2020

Rechargeable nonaqueous Li-O2 battery has attracted considerable attention as a promising technique for next-generation energy storage due to its high theoretic specific energy density. However, several challenges have to be addressed before its practical application. One of the most challenging issues is the large overpotentials, especially during charge, which leads to the low round-trip efficiency, low power capability and poor cycle life of Li-O2 battery. During charge, the discharge product Li2O2 decomposes and releases oxygen, i.e. an oxygen evolution reaction (OER). Due to the intrinsic low conductivity of Li2O2 and the multiple electron transfer nature of OER , the sluggish kinetics leads to large voltage polarization. Therefore, designing efficient catalysts that could facilitate this process has become an emerging topic.
Owing to the electronic and geometric structures, bimetallic nanostructured catalysts typically exhibit unusual catalyzing properties. Through the rational compositional and structural design, the catalytic activity can be significantly tuned and improved towards specific reactions. Here, we present a unique Pt–Cu core–shell nanostructure for catalyzing the nonaqueous OER, which exhibited dramatically reduced charging overpotential (<0.2 V) in nonaqueous Li–O2 cells, compared to a typical 1 V overpotential. The structure and the properties of the Pt-Cu catalyst was systematically studied first. The Pt-Cu catalyst was made by a facile wet-impregnation method. The High-Energy X-ray Diffraction (HEXRD) and X-ray absorption near edge structure (XANES) spectra, showed that the catalyst was composed by oxidized Cu, that is Cu(II), and partially oxidized Pt in the whole particle level. The particle sizes of the Pt-Cu catalyst was analyzed by small angle X-ray scattering (SAXS), which showed narrow size distributions in the diameter range of 2-4 nm, in agreement with the size distribution histogram from TEM analysis. To determine the structure in bulk scale, the nearest-neighbor coordination numbers around Cu and Pt atoms were extracted from fitting the extended X-ray absorption fine structure (EXAFS) spectra. The conclusive evidence of the core-shell structure was the distinct inequality of the nearest coordination number NPt-Pt and NCu-Pt, as well as confirmed then by scanning transmission electron microscopy (STEM). The extents of the alloying of Pt and Cu were also calculated from the EXAFS-derived coordination numbers and from the coordination numbers expected for the complete and random alloying. The result indicated that Pt-Cu alloy formed at the interface. Because these as-prepared Pt-Cu bimetallic nanoparticles possess unique features on porous carbon matrix with high specific surface area, they can provide more active sites for the electrochemical reactions. Consequently, catalytic activity for OER is expected to be increased, as demonstrated in the tests with Li-O2 cells. To reveal the catalytic mechanism, the X-ray photoelectron spectroscopy (XPS), which is a surface sensitive technique was employed. The result indicated that the robust catalytic activity can be attributed to the active surface Cu(I) sites which were stabilized by the Pt-Cu alloyed interface. The results of this study have proved that such Pt-Cu core-shell nanostructures are efficient catalysts for non-aqueous OER and our protocol can be generalized to guide the design of other bimetallic catalysts.
REFERENCES
ADDIN EN.REFLIST 1. Ma, L. et al. Insight into the Catalytic Mechanism of Bimetallic Platinum–Copper Core–Shell Nanostructures for Nonaqueous Oxygen Evolution Reactions. Nano Letters(2015).

Figure 1