Palladium Nanoparticles Supported on Nitrogen-Doped Graphene Nanosheets as a Highly Active Electrocatalyst for Oxygen Reduction Reaction

In this study, the oxygen reduction reaction (ORR) on Pd nanoparticle-decorated nitrogen-doped graphene (Pd/NG) catalyst has been investigated in acid and alkaline solutions using the rotating disk electrode (RDE) method.1 Nanoparticles of palladium supported on high surface area carbon have been explored as electrodes for PEMFCs. Pd is a promising alternative to the costly Pt for fuel cell applications. Due to its large surface area and the high electrical conductivity graphene is being used more frequently for electrochemical applications.2 Graphene exhibits several advantages over carbon nanotubes, such as lower cost, larger surface area and more simple preparation. Very important advantage is that graphene synthesized from graphite powder does not contain metallic impurities, which can alter electrochemical properties of carbon nanotubes. Doping graphene with nitrogen atoms is known to result in tunable chemical and physical properties, for example, N-doping improves electrical conductivity of materials, accelerates the growth of nanoparticles on graphene substrate and enhances the catalytic activity of composite materials. It was found recently, that single Pd atoms embedded into N-doped graphene are promising candidates for use in fuel cell cathodes for ORR.3 N-doped graphene was synthesized by pyrolysis of graphite oxide in the presence of dicyandiamide as nitrogen precursor.4NG was further modified with Pd nanoparticles (PdNPs) prepared by borohydride reduction. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were utilized to study the size and distribution of Pd nanoparticles on substrate surface and the type of nitrogen groups present in the N-doped graphene nanosheets. TEM indicated a good dispersion of Pd nanoparticles on the substrate surface and the average size of the PdNPs was 2.6±0.7 nm (Fig. 1a). The XPS survey spectra showed O1, N1s, C1s and Pd3d peaks for Pd/NG sample (Fig. 1b). Four different types of nitrogen species can be seen in inset N1s spectra. Namely, pyridinic-N at binding energy of 397.8 eV prevails on the surface, peak shoulder between 399 and 400 eV corresponds to pyrrolic-N and graphitic-N and smaller peak at ca. 402.5 eV is due to the presence of pyridine-N-oxide functionalities on the surface.1 The RDE results showed remarkable electrocatalytic activity of Pd/NG catalysts towards the ORR. The prepared Pd/NG materials catalyse a 4-electron reduction of oxygen. At low overpotentials the Tafel slope for oxygen reduction is close to −60 mV dec−1 and at higher current densities the slope is approximately −120 mV dec−1. The Pd/NG composite showed excellent ORR performance in alkaline media (Fig. 2) and is a promising material to be used as a cathode catalyst for alkaline membrane fuel cells. This work not only offers a low-cost and high performance alternative for Pt/C catalysts in fuel cells, but also opens the door towards fabrication of varying types of metal cluster/graphene hybrids that will have wide application in electrocatalysis and clean energy fields. References 1. K. Jukk, N. Kongi, L. Matisen, T. Kallio, K. Kontturi, and K. Tammeveski (manuscript submitted). 2. D.A.C. Brownson and C.E. Banks, Phys. Chem. Chem. Phys., 14, 8264, 2012. 3. M. Kaukonen, A.V. Krasheninnikov, E. Kauppinen, and R.M. Nieminen, ACS Catal., 3, 159,2013. 4. M. Vikkisk, I. Kruusenberg, U. Joost, E. Shulga, I. Kink, and K. Tammeveski, Appl. Catal. B: Environ., 147, 369, 2014.