Atomically ordered intermetallic PdZn coupled with Co nanoparticles as a highly dispersed dual catalyst chemically bonded to N-doped carbon for boosting oxygen reduction reaction performance

Abstract

In this work, a dual catalyst composed of atomically ordered intermetallic PdZn and Co nanoparticles was synthesized and chemically bonded to nitrogen-doped carbon by simultaneously modulating the lattice spacing of the Pd nanoparticle guest and the metal centers of the zeolite imidazolate framework (ZIF) host through a one-step heating procedure. Results reveal that the ability of the PdZn alloy to catalyze the oxygen reduction reaction can be regulated by adjusting the lattice spacing of Pd–Pd through the formation of the ordered PdZn, resulting from Zn²⁺ in the ZIF-8 host lattice diffusing into the Pd phase. Meanwhile, the secondary active sites (Co nanoparticles) and N-doped carbon (particularly N-doped carbon nanotubes) carrier are obtained by doping Co²⁺ into the ZIF-8 host lattice, which can further improve the catalytic activity. The characterization results show the existence of Pd–N and Co–N bonds between the dual catalyst and the carrier, which can be regulated by controlling the synthesis conditions, such as the atomic ratios of the Pd guest to Zn in the ZIF-8 host, pyrolysis temperatures, and doping Co²⁺ into the ZIF-8 host lattice. The dual catalyst bonded to N-doped carbon exhibits high ORR catalytic activity with a desirable onset potential (0.916 V), and diffusion-limiting current density (∼5.70 mA cm⁻²), superior to other reported Pd-based catalysts and comparable to commercial Pt/C. Moreover, the chemical bonds between the dual catalyst and the carrier result in the dual catalyst with a small size of 5.9 nm and prolonged duration of electrochemical activity (no obvious change of the half peak potential after 10 000 cyclic voltammetry cycles and 90.6% current retention after a 4 h chronoamperometric test). Thus, the highly dispersed ordered-PdZn/Co on N-doped carbon is a promising substitute to commercial Pt/C for alkaline fuel cells.