Formation of conformal NiO underlayer on carbon for strong metal-support interactions effects on electrocatalytic performance of supported Pd nanoparticles

Enhancing the performance of noble metal electrocatalysts is regarded as an urgent issue for the commercialization of fuel cells in the near future. In this study, we demonstrate that the electrocatalytic performance of Pd nanoparticles (NPs) supported on carbon can be enhanced by introducing NiO coating on carbon. We further demonstrate that a uniform thin layer of NiO coating on carbon support can be readily achieved by using a sonochemical reaction (ultrasound-assisted polyol synthesis, UPS) method. NiO-modified carbon (NiO/C) supports were synthesized by two different methods, atomic layer deposition (ALD) and UPS methods, and Pd NPs were formed on them. Compared with Pd NPs on carbon support, Pd NPs on NiO/C supports showed 3-5 fold enhanced electrocatalytic performance for formic acid oxidation and oxygen reduction reactions. Between the two NiO/C supports, the one synthesized by UPS method outperforms the one by ALD by up to 2 times. Detailed structural analysis data show that the NiO/C synthesized by UPS is composed of uniform and continuous thin (0.42 nm) NiO coating on the external surface of carbon while the NiO/C by ALD has thicker (1.13 nm) NiO islands to form patch-like coating on the carbon surface. The enhanced electrocatalytic performance of Pd NPs on NiO/C supports can be explained by the change of the electronic structure of strong metal-support interaction between Pd and NiO and the bifunctional mechanism enabled by the NiO surface around Pd NPs.