Abstract
Transition metal (Fe, Co, Ni) complexes on carbon nanomaterials are promising candidates as electrocatalysts towards the oxygen reduction reaction (ORR). In this paper, nitrogen-doped hollow carbon spheres with embedded Co nanoparticles were successfully prepared via a controllable synthesis strategy. The morphology characterization shows that the hollow carbon spheres possess an average diameter of ~150 nm with a narrow size distribution and a shell thickness of ~14.5 nm. The content of N doping ranges from 2.1 to 6.6 at.% depending on the calcination temperature from 900 to 1050 °C. Compared with commercial Pt/C, the Co-containing nitrogen-doped hollow carbon spheres prepared at 900 °C (CoNHCS-900) as an ORR electrocatalyst shows a half-wave potential shift of only ∆E1/2 = 55 mV, but a superior stability of about 90.2% maintenance after 20,000 s in the O2-saturated 0.1 M KOH at a rotating speed of 1600 rpm. This could be ascribed to the synergistic effects of N-containing moieties, Co-Nx species, and Co nanoparticles, which significantly increase the density of active sites and promote the charge transfer during the ORR process.
Highlights
In recent years, fuel cells have been attracting lots of attention due to their high energy conversion efficiency for converting chemical energy into electricity [1,2,3,4,5]
The Co nanoparticles embedded into the hollow carbon spheres, while the carbon spheres with a diameter of about 140 nm show a shell thickness of 14.5 nm
82.5%, In order to investigate the effect of Co, a KSCN poisoning experiment was performed
Summary
Fuel cells have been attracting lots of attention due to their high energy conversion efficiency for converting chemical energy into electricity [1,2,3,4,5]. The energy transfer at fuel cells, is restricted by the intrinsically sluggish kinetics of ORR [6,7] Many catalysts such as Pt/C and Pt alloys are traditionally used to improve the reaction efficiency, the large-scale practical application of them is still difficult due to their high cost, low durability, and scarcity. Lee et al described the synthesis of nitrogen-doped hollow carbon spheres to encapsulate mixed cobalt-nitride (Co-N) and cobalt-oxide (Co-O) nanosized species, which shows enhanced bifunctional oxygen electrochemical activity [27]. Cobalt-nitride nanoparticles have been prepared within nitrogen-doped carbon spheres [28], which were studied as the ORR electrocatalysts in basic solution. We present a successful synthesis of cobalt nanoparticles and Co-Nx species within nitrogen-doped hollow carbon spheres using a soft template method, ammonia decomposition, and impregnation method. The reaction mechanism and active sites were explored and discussed
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