Electrochemical oxygen evolution and reduction reactions have received great attention due to their importance in several key technologies such as fuel cells, electrolyzers, and metal–air batteries. Here, we present a simple approach to the preparation of cobalt sulfide nanoparticles in situ grown on a nitrogen and sulfur codoped graphene oxide surface. The particle size and phase were controlled by changing the treatment temperature. Cobalt sulfide nanoparticles dispersed on graphene oxide hybrids were successfully prepared by a solid-state thermolysis approach at different temperatures (400, 500, and 600 °C) using cobalt thiourea and graphene oxide. X-ray diffraction studies revealed that hybrids prepared at 400 and 500 °C result in pure CoS2 phase, whereas the hybrid prepared at 600 °C exhibits Co9S8 phase. X-ray photoelectron spectroscopy studies revealed that nitrogen and sulfur simultaneously codoped on the graphene oxide surface, and these sites act to anchor the CoS2 nanoparticles strongly on the GO surface. The strong coupling between CoS2 and N,S-GO was reflected in the improvement of the oxygen electrode potential. CoS2(400)/N,S-GO showed an outstanding oxygen electrode activity with a potential of about 0.82 V against a reversible hydrogen electrode in alkaline medium, which is far better than the performance of precious catalysts such as Pt/C (1.16 V), Ru/C (1.01 V), and Ir/C (0.92 V).

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Non-precious metal catalysts, such as metal-coordinated to nitrogen doped-carbon, have shown reasonable oxygen reduction reaction (ORR) performances in alkaline fuel cells. In this report, we present the development of a highly active, stable and low-cost non-precious metal ORR catalyst by direct synthesis under autogenic-pressure conditions. Transmission electron microscopy studies show highly porous Fe-N-C and Co-N-C structures, which were further confirmed by Brunauer-Emmett-Teller surface area measurements. The surface areas of the Fe-N-C and Co-N-C catalysts were found to be 377.5 and 369.3 m(2) g(-1), respectively. XPS results show the possible existence of N-C and M-Nx structures, which are generally proposed to be the active sites in non-precious metal catalysts. The Fe-N-C electrocatalyst exhibits an ORR half-wave potential 20 mV higher than the reference Pt/C catalyst. The cycling durability test for Fe-N-C over 5000 cycles shows that the half-wave potential lost only 4 mV, whereas the half-wave potential of the Pt/C catalyst lost about 50 mV. The Fe-N-C catalyst exhibited an improved activity and stability compared to the reference Pt/C catalyst and it possesses a direct 4-electron transfer pathway for the ORR process. Further, the Fe-N-C catalyst produces extremely low HO2(-) content, as confirmed by the rotating ring-disk electrode measurements. In the alkaline fuel single cell tests, maximum power densities of 75 and 80 mW cm(-2) were observed for the Fe-N-C and Pt/C cathodes, respectively. Durability studies (100 h) showed that decay of the fuel cell current was more prominent for the Pt/C cathode catalyst compared to the Fe-N-C cathode catalyst. Therefore, the Fe-N-C catalyst appears to be a promising new class of non-precious metal catalysts prepared by an autogenic synthetic method.

Hollow nitrogen-doped carbon spheres (HNCSs) were prepared by a facile method as non-precious catalysts for the oxygen reduction reaction (ORR). The HNCS catalysts exhibited ORR activity comparable with a commercial Pt/C catalyst and superior stability in alkaline electrolyte medium.