Abstract
The identification and development of efficient catalysts made of non-precious materials for oxygen reduction reaction (ORR) are essential for the successful operation of a wide range of energy devices. This study provides evidence that earth-abundant nanoparticles of transition metals encapsulated in a nitrogen-doped carbon shell (M@N–C, M=Fe, Co, Ni, Cu or Fe alloys) are promising catalysts in acidic solutions. By density functional theory calculations and experimental validations, we quantitatively propose a method of tuning the ORR activity of M@N–C by controlling the nitrogen-doping level, the thickness of the N–C shells and binary alloying. FeCo@N–C/KB was chosen as the best ORR catalyst because of its onset and half-wave potentials of 0.92 and 0.74 V vs a reversible hydrogen electrode (RHE), respectively, and its excellent durability. Furthermore, FeCo@N–C/KB possesses a high activity for the hydrogen evolution reaction (HER; −0.24 V vs RHE at −10 mA cm−2), thus demonstrating that it is a good bi-functional ORR and HER catalyst in acidic media. Transition metals encapsulated in a nitrogen-doped carbon shell show high activity as bi-functional catalysts in acidic solutions. Platinum catalysts currently used for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) — both crucial processes in fuel-cells — are expensive and often unstable in acidic media. Now, Takeo Ohsaka and colleagues in Japan and Korea investigate the performance of transition metals and their alloys as the core component of core-shell catalysts for ORR and HER from both theoretical and experimental perspectives. Iron-core catalysts are more active in ORR than copper, cobalt or nickel equivalents. Iron-cobalt (FeCo) alloys coated in the nitrogen-doped carbon shell are highly active for ORR and HER and show high durability in acidic solutions. In general, a thin carbon shell and the presence of graphitic nitrogen sites are advantageous for high catalytic performance. FeCo@N—C nanoparticle as a bi-functional catalyst in both oxygen reduction reaction and hydrogen evolution reaction.
Highlights
Oxygen electrochemistry has a central role in a wide range of devices used for the generation of energy and fuels
ORR has notoriously been known to be sluggish in most electrochemical systems harvesting oxygen to produce renewable energies, hindering the design of highly efficient ORR catalysts
Based on the deconvolution analysis, we identified that pyrrolic N is dominant over a variety of other functional N groups
Summary
Oxygen electrochemistry has a central role in a wide range of devices used for the generation of energy and fuels. Metallic nanoparticles encapsulated in N-doped carbon shells (M@N–Cshells), such as Fe3C@N–C or Fe@Pod,[19,20,21,22] demonstrate superior ORR activity and electrochemical stability in acidic media. This concept of encapsulation has been proposed to simultaneously overcome the conventional limits on the two exclusive properties of electrochemical stability[23] and ORR activity. We extensively used density functional theory calculations and experimental characterizations to understand how the nitrogen-doping level, alloying element and shell thickness of the M@N–C catalyst influence ORR catalysis in a 0.5 M H2SO4 acidic solution. We calculated and experimentally analyzed the catalytic performance of the structures in the hydrogen evolution reaction (HER) to evaluate the possibility of using them as bi-functional catalysts in acidic media
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