Palladium-Cobalt Bimetallic Nanoparticles Supported on Nitrogen-Doped Graphene as Efficient Electrocatalyst for Oxygen Reduction

The sluggish kinetics of oxygen reduction reaction (ORR) and the high cost of traditional Pt-based catalyst prompt us to develop highly efficient alternatives with low cost. In this paper, nitrogen-doped graphene (NG)-supported palladium (Pd) and cobalt (Co) catalysts were prepared by the improved Hummers methods assisted with metal ion adsorption and thermal reduction processes, and subsequently were used as electrocatalysts for ORR in alkaline solution. The X-ray diffraction analysis illustrated that the (111), (200), and (220) crystal planes of PdCo/NG shifted in the high-angle direction relative to Pd/NG, mainly due to the simultaneous loading of Pd and Co on NG. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements further confirmed that metallic Pd and Co nanoparticles were simultaneously loaded on NG and uniformly distributed. The Pd and Co nanoparticles were 3-30 nm and 2-15 nm in size, respectively. In particular, unlike the Co nanoparticles encapsulated by several carbon layers in Co/NG, no carbon layers were detected outside of Co nanoparticles in PdCo/NG. This is probably due to the existence of Pd that prevents the carbon agglomeration around Co. Electrochemical measurements suggested that all catalysts manifested catalytic activity for ORR, and ORR catalyzed by them obeys the first-order reaction kinetics with respect to the concentration of O-2 in the solution. The PdCo/NG has the most positive peak potential (-0.198 V versus Ag/AgCl) of oxygen reduction in the cyclic voltammetry (CV) curves and most positive onset potential (-0.056 V versus Ag/AgCl) in the linear sweep voltammetry (LSV) curves, illuminating its excellent ORR activity relative to the catalysts loaded with Pd or Co alone. The ORR catalyzed by metal-free NG mainly goes through a 2e(-) pathway, while a mixed 2e(-) and 4e(-) pathway dominated the ORR catalyzed by Pd/NG, Co/NG, and PdCo/NG. Compared with Pd/NG- and Co/NG-catalyzed ORR, the electron transfer number stabilized at 3.2 without fluctuation with the potential negative shift, and the KouteckATIN SMALL LETTER Y WITH ACUTE-Levich (K-L) plots have better linearity and parallelism for PdCo/NG. These indicated that ORR steps are more concise and stable on PdCo/NG than on other samples. Such conspicuous features of PdCo/NG are probably ascribed to the simultaneous loading of Pd and Co nanoparticles. In particular, replacing partial Pd with Co not only reduces the cost, but also improves the activity of the electrocatalysts. This work developed a bimetallic-supported catalyst with low cost and highlighted the feasibility of replacing partial Pd with Co supported on carbons as ORR electrocatalyst, laying the foundation for large-scale application of non-Pt catalysts.