Paragenesis of Palladium-Cobalt Nanoparticle in Nitrogen-Rich Carbon Nanotubes as a Bifunctional Electrocatalyst for Hydrogen-Evolution Reaction and Oxygen-Reduction Reaction.

The hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) play important roles in many energy conversion and storage systems. To accelerate the reaction processes, there is a constant need for efficient new catalysts. In the present work, we have developed a facile pyrolysis-based process for the co-synthesis of palladium-cobalt nanoparticles supported on carbon nanotubes (Pd-CoCNTs), which exhibit superior catalytic activity for the HER and enhanced ORR performance. Non-agglomerated Pd nanoparticles of diameters 2-4 nm are uniformly distributed on the surface of CoCNTs, while the inner Co particles are an essential element in forming the framework of the CoCNTs. Compared to the Pd-free N-rich CoCNTs, Pd-CoCNTs have a more defective surface with a larger electrochemically active surface area (ECSA), and show enhanced ORR activity, outstanding methanol tolerance, and long-term stability in alkaline solution. At a low Pd loading of 0.0292 mg cm-2 , the Pd-CoCNTs require overpotentials of 0.024 V and 0.215 V to catalyze the HER and to drive a current density of 50 mA cm-2 in acidic solution, respectively. The palladium nanoparticles on the surface of the CoCNTs are considered to be highly active sites for HER, based on the results of control experiments, and it is easy to adjust the catalytic activity of the Pd-CoCNTs by changing the concentration of Pd therein. The proposed method provides a means of fabricating efficient bifunctional catalysts with controllable low contents of precious metals.