Fig. 2. SEM image of (a) NC and (b-c) CoFe-CoxN@NC, (d) TEM image, (e-f) HRTEM image and (g) elemental mappings of Co, Fe, C, N and O of CoFe-CoxN@NC
Raman spectroscopy was conducted to study the effect of pyrolysis temperatures on the composition of CoFe-CoxN@NC, as shown in Fig. S6a. The D-band at 1340 cm−1 and the G-band at 1590 cm−1 represented the defect degree and graphitic degree of carbon, respectively. The degree of defects and graphitization could be measured by the intensity ratio of ID/IG. As the pyrolysis temperature increased from 500 to 800°C, the ID/IGincreased from 0.92 to 1.12; when further increased the pyrolysis temperature, the ID/IG decreased. The high degree of defect structure would lead to the increase of the adsorption capacity while the high degree of graphite structure would lead to the increase of electrical conductivity of the carbon material. The results suggested that CoFe-CoxN@NC pyrolysis at 700°C had the most suitable degree of defects and graphitization, thus had relative high adsorption capacity and high electrical conductivity, which was beneficial to promote the OER electrocatalytic performance. The porous character of CoFe-CoxN@NC was investigated by the N2 adsorption/desorption curve. There was an obvious hysteresis loop, which was a typical feature of the type IV isotherm adsorption curve, indicating that CoFe-CoxN@NC had the characteristics of hierarchical porous structure. Its specific surface area was 195.9 m2·g−1 and the pore volume was 0.27 cm3·g−1. The macro- and mesoporous structure of the carbon material facilitated the adsorption transfer of OH- reactants and O2 products in OER, and the large specific surface area facilitated the contact of the active sites.
The chemical composition and valence state of CoFe-CoxN@NC were determined by XPS in Fig. 3 and Fig. S8. The XPS survey spectrum revealed the presence of Co, Fe, O, N and C elements, which was consistent with the results of elemental mappings by TEM-EDX. The C 1 S spectrum confirmed the existence of C−C (284.8 eV), C−N (285.4 eV), C−O (286.6 eV), C=O (288.7 eV) and O−C=O (290.8 eV) in CoFe-CoxN@NC56 , C-N bond indicated that nitrogen was successfully doped into the carbon skeleton, while the existence of C=O and O−C=O implied substantial oxygen-containing groups decorated on the surface which can create defects in the carbon matrix. The N 1 s spectrum at the peaks of 398.6, 400.1, 401.1 and 403 eV corresponded to pyridine N (38.53%), pyrrole N (17.99%), graphite N (33.45%) and pyridine N oxide (10.04%), respectively 57,58, the abundant pyridinic N can coordinate with metal atoms through metal–N–C structure to optimize the local electronic structure, graphitic N was expected to improve the diffusion-limiting current, both benefiting the electrochemical process. In addition, the surface element content of CoFe-CoxN@NC with various urea dosages and the relative contents of the four nitrogen atoms were listed in Tab. S1 and S2, respectively. The hydrophilicity of carbon materials could be improved by the surface functional groups to balance the adsorption and desorption of intermediates and products during OER performance. As CoFe alloy in CoFe-Co5.47N@NC catalyst was encapsulated by the graphitic carbon layer, the spectral intensity of Fe 2p and Co 2p spectra in CoFe-CoxN@NC catalyst was relatively weak, as shown in Fig. 3c-d. The Fe 2p spectrum of Fe@NC and the Co 2p spectrum of Co@NC was shown in Fig. S8. For the Fe 2p spectrum, the first double peak was 711.5 and 724.2 eV, and the second double peak was 713.9 and 727.3 eV, designated as Fe2+ and Fe3+, respectively59,60, and the peaks near 719.1 eV and 735.2 eV were classified as the satellite peaks of Fe 2p3/2 and Fe 2p1/2, respectively. Similarly, Co 2p spectrum showed the zero-valence state (778.3 and 795.2 eV) and the ionic state (780.0 and 797.4 eV) Co with satellite peaks (785.6 and 803.4eV), which are derived from CoFe alloy and CoxN species, respectively.