Electrocatalytic Activity and Stability of Supported Palladium Nanoparticles

The overall performance of the polymer electrolyte membrane fuel cell (PEMFC) essentially depends on the catalytic activity of its core-part: the membrane electrode assembly (MEA). Palladium (Pd), one of the platinum group elements, is a potential substitute for Pt to be applied as the electrocatalyst, since it has more abundant reserves and a lower price compared to Pt. Pd is stable under extremely corrosive conditions similar to Pt.

Nanosized metal particles perform considerably different as compared to the corresponding bulk materials. Furthermore, some reactions catalyzed by metal particles show a strong particle-size sensitivity. In previous work of our group, particle-size dependent activity of metal particles for electrocatalytic reaction was observed, such as Au nanoparticles supported on HOPG for oxygen reduction reaction (ORR) [1] and Pd nanoislands supported on Au(111) surface for hydrogen evolution reaction (HER) [2]. Beside the particle-size effects, support effects (e.g. ligand effect, strain effect) play important roles on electrocatalytic activity as well [3].

In order to research the particle-size effects and support effects on the activity and stability of Pd nanoparticles, highly oriented pyrolytic graphite (HOPG) and nitrogen doped HOPG (N-HOPG) [4] were used in this work. Due to the weak interaction of HOPG with metal deposits, the substrate-related effects can be eliminated. N-HOPG is rich in electronegative nitrogen functional groups, and the roughness of N-HOPG is much higher than the untreated HOPG before the nitrogen implantation process. Pd nanoparticles were electrochemically deposited by double-pulse electrochemical deposition on both substrates. Pd nanoparticles are located mainly at the defects and step-edges on HOPG surface, but on the N-HOPG particles are distributed homogeneously. For ORR, the smaller particles have a negative particle-size effect. Pd nanoparticles supported on N-HOPG do not show enhanced activity for ORR. For HER, the case is much more difficult for the behavior of hydrogen absorption of Pd. The content of absorbed hydrogen atoms in Pd NPs is influenced by the particle size. The stability of Pd nanoparticles on HOPG is influenced not only by the particle size, but also by the pH value of the electrolyte. Pd particles on N-HOPG show poorer stability as compared to the pure HOPG insofar as they dissolved rapidly in potential cycling measurements.

[1] Brülle, T.; Ju, W.; Niedermayr, P.; Denisenko, A.; Paschos, O.; Schneider, O.; Stimming, U., Molecules 2011, 16 (12), 10059-77.

[2] Pandelov, S.; Stimming, U., Electrochimica Acta 2007, 52 (18), 5548-5555.

[3] T. Bligaard, J.K. Nørskov, Electrochimica Acta, 52 (2007) 5512-5516.

[4] Favaro, M.; Perini, L.; Agnoli, S.; Durante, C.; Granozzi, G.; Gennaro, A., Electrochimica Acta 2013, 88, 477-487.