Sulfur codoping enables efficient oxygen electroreduction on FeCo alloy encapsulated in N-Doped carbon nanotubes

Highlights: • S- and N-codoped peapod-like CNTs are prepared with Prussian blue analogues. • FeCo alloy nanoparticles are encapsulated in these N,S-CNTs. • S codoping enhances the ORR activity of FeCo alloy in N-doped only CNTs. • Intensified charge transfer from FeCo to S,N-CNTs accounts for the higher ORR activity. Sulfur- and nitrogen-codoped peapod-like carbon nanotubes are facilely prepared via pyrolyzing Fe- and Co-containing Prussian blue analogues supported on trithiocyanuric acid. Combined studies with X-ray diffraction, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy measurements reveal the encapsulation of metallic FeCo alloy in such carbon nanotubes with a wall thickness of ca. 2.60 nm. Electrochemical measurements show that sulfur codoping with nitrogen dramatically enhances the catalytic activity towards oxygen reduction reaction compared with that of FeCo alloy encapsulated in N-doped only carbon nanotubes, and the sample prepared at a pyrolysis temperature of 800 °C is the best one among the series, exhibiting a more positive half-wave potential of +0.838 V, nearly 100% enhancement in kinetic current, a higher operation stability and stronger immunity to the negative impacts of fuel crossover than commercial Pt/C catalysts. The remarkable improvement of catalytic activity is ascribed to the intensified charge transfer from encapsulated FeCo alloy nanoparticles to thin walls of CNTs upon the additional sulfur doping besides nitrogen. The present results highlight the importance of deliberate doping and structuring in the development of more efficient catalysts based on metal nanoparticles encapsulated in CNTs for high-performance electrochemical energy devices.