Abstract
Temperature can govern morphologies, structures and properties of products from synthesis in solution. A reaction in solution at low temperature may result in different materials than at higher temperature due to thermodynamics and kinetics of nuclei formation. Here, we report a low-temperature solution synthesis of atomically dispersed cobalt in a catalyst with superior performance. By using a water/alcohol mixed solvent with low freezing point, liquid-phase reduction of a cobalt precursor with hydrazine hydrate is realized at −60 °C. A higher energy barrier and a sluggish nucleation rate are achieved to suppress nuclei formation; thus atomically dispersed cobalt is successfully obtained in a catalyst for oxygen reduction with electrochemical performance superior to that of a Pt/C catalyst. Furthermore, the atomically dispersed cobalt catalyst is applied in a microbial fuel cell to obtain a high maximum power density (2550 ± 60 mW m−2) and no current drop upon operation for 820 h.
Highlights
Temperature can govern morphologies, structures and properties of products from synthesis in solution
The reaction products were stabilized by mixing the solution with N-doped mesoporous carbon (NMC) without increasing the solution temperature back to room temperature (RT), and the Co atomic species were absorbed on substrates at low temperatures by filter drying the solvents
With a direct atomic-level synthesis in solution at −60 °C by suppressing nucleation, we demonstrated that chemically reduced Co species in solution coordinated with pyridinic–N on NMC substrates and got converted into a more active Co–Nx sites embedded into the carbon skeleton by thermal activation[20,28,29]
Summary
Temperature can govern morphologies, structures and properties of products from synthesis in solution. A higher energy barrier and a sluggish nucleation rate are achieved to suppress nuclei formation; atomically dispersed cobalt is successfully obtained in a catalyst for oxygen reduction with electrochemical performance superior to that of a Pt/C catalyst. In principle, if we conduct the solution reaction at a relatively low temperature sufficient for the occurrence of the reaction (see Supplementary Note 1), but with the significantly increased nucleation barrier ΔG1 and critical nucleus size (Supplementary Fig. 1), the ability to regulate the nuclei formation should be attained which results in the formation of atomically dispersed products in solution. The suppressive nuclei formation benefits from the decreased reduction rate and nucleation rate in an ultra-low temperature solution ( shown in Supplementary Note 1), providing opportunities for solution-phase synthesis at the very initial precipitation process. We explore the low-temperature solution chemistry to synthesize high-performance (ORR) electrocatalysts composed with atomically dispersed nonprecious metals
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