RESUMO
By annealing an Fe(III)-coordination compound (Fe-CC), [FeCl3(Hbta)2] (Hbta = benzotriazole) in the presence of a carbon nanotube precursor (PCNT) template, an Fe4N/Fe3N/Fe/CNT heterostructure was successfully synthesized without an extra nitrogen source. The decomposition of the Hbta in Fe-CC under high-temperature annealing can produce carbon sheets and reduced graphene oxide (rGO), and the presence of CNTs can alleviate the stacking of thein situ-generated carbon materials. Meanwhile, iron nitride nanoparticles (NPs) can be anchored on the carbon sheets, and the anchoring effect efficiently prevents the agglomeration of NPs and increases the amount of active catalytic sites for the oxygen evolution reaction (OER). Fe4N/Fe3N/Fe/CNT shows an excellent OER activity with a Tafel slope of 63 mV dec-1as well as overpotentials of 121 (η10) and 275 mV (η100) at 10 and 100 mA cm-2, respectively - far exceeding commercial RuO2and other catalysts. Density functional theory calculations show that the excellent OER performance of Fe4N/Fe3N/Fe/CNT is associated with the Fe4N/Fe3N heterojunction, which can improve the electron conductivity and boost the electron transfer from N to Fe. The Fe4N/Fe3N/Fe/CNT catalyst exhibits long-term OER activity during 100 h of electrolysis at 20 mA cm-2. This is related to the dual coatings of thein situ-generated thin carbon shell and few-layered rGO on the surface of the iron nitride NPs, which can protect the fast leaching of iron nitride during the OER process. Furthermore, the effects of the annealing temperature, the PCNT template and the heating rate on the calcined products were investigated.
RESUMO
Based on a coordination polymer, FeCl2(4,4'-bpy) (4,4'-bpy = 4,4'-bipyridine) and the carbon nanotube (CNT)/NaCl dual template, Fe3N nanoparticles (NPs) were synthesized via chemical thermolysis in the absence of an extra nitrogen source. The decomposition of 4,4'-bpy under high temperature produces thin carbon coating for Fe3N NPs. Also, the CNT template anchors the Fe3N NPs to avoid aggregation. The sample (denoted as Fe3N-C N) exhibits excellent electrocatalytic oxygen evolution reaction (OER) behavior even with a small molar ratio of Fe3N (Fe: 4.9 at. %), which can deliver a current density of 10 mA cm-2 at an overpotential of 218 mV with a Tafel slope of 84 mV dec-1 and long-term OER activity during 60 h electrolysis at 20 mA cm-2. Furthermore, the sample after 20 h electrolysis, denoted as Post-Fe3N-C N (20 h), displays enhanced OER activity with a smaller Tafel slope of 41 mV dec-1 and overpotentials of 195 and 327 mV at 10 and 100 mA cm-2, respectively, which is mainly due to the partial transformation of Fe3N into FeOOH. The OER mechanism is investigated by density functional theory calculations, and it is found that the surface partial oxidation of Fe3N leads to the effective OER electrolysis, which changes the electron density of the superficial atoms and induces the moderate adsorption for the intermediates.
