M<sub>3</sub>C (M: Fe, Co, Ni) Nanocrystals Encased in Graphene Nanoribbons: An Active and Stable Bifunctional Electrocatalyst for Oxygen Reduction and Hydrogen Evolution Reactions
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Abstract
Transition metal carbide nanocrystalline M<sub>3</sub>C (M: Fe, Co, Ni) encapsulated in graphitic shells supported with vertically aligned graphene nanoribbons (VA-GNRs) are synthesized through a hot filament chemical vapor deposition (HF-CVD) method. The process is based on the direct reaction between iron group metals (Fe, Co, Ni) and carbon source, which are facilely get high purity carbide nanocrystals (NCs) and avoid any other impurity at relatively low temperature. The M<sub>3</sub>C-GNRs exhibit superior enhanced electrocatalystic activity for oxygen reduction reaction (ORR), including low Tafel slope (39, 41, and 45 mV dec<sup>–1</sup> for Fe<sub>3</sub>C-GNRs, Co<sub>3</sub>C-GNRs, and Ni<sub>3</sub>C-GNRs, respectively), positive onset potential (∼0.8 V), high electron transfer number (∼4), and long-term stability (no obvious drop after 20 000 s test). The M<sub>3</sub>C-GNRs catalyst also exhibits remarkable hydrogen evolution reaction (HER) activity with a large cathodic current density of 166.6, 79.6, and 116.4 mA cm<sup>–2</sup> at an overpotential of 200 mV, low onset overpotential of 32, 41, and 35 mV, small Tafel slope of 46, 57, and 54 mV dec<sup>–1</sup> for Fe<sub>3</sub>C-GNRs, Co<sub>3</sub>C-GNRs, and Ni<sub>3</sub>C-GNRs, respectively, as well as an excellent stability in acidic media