To further reveal the OER activity mechanism, XPS spectra were recorded and the N 1s, C 1s, O 1s, Co 2p binding energies were deconvoluted to gain chemical composition in the bonding and species types. Only TJU-104–900 surface showed exposed N content, and it had significantly higher C, less O and Co content confirmed by the XPS results and the intensity in full XPS spectra (Table S3 and
Fig. 6b). Energy-dispersive X−ray (EDX) spectroscopy was further conducted to demonstrate a comparable superficial composition as the XPS of all the TJU-104 derived samples (Table S3). With the pyrolysis temperature increasing the element compositions varied considerably which may be due to the distinct changes of the morphologies and that only the surface compositions were detected by XPS and EDX. The dramatic decline of the nitrogen content could be attributed to its sacrificial function as reducing agent to Co
2+ metal ions during the pyrolysis. It was consistent with that hierarchical structural TJU-104–900 had a manifestation of N-doped graphitic carbons encapsulating Co nanoparticles. The exposed N content doped in the graphitic carbon matrix influenced the electronic environment of TJU-104–900, favoring the high OER activity. On the other hand, Co–N
x with 7.5% content was discovered in TJU-104–900 that could serve as additional active sites for OER activity (
Fig. 6c and Table S4 in Supporting information). Moreover, pyridinic N (398.7 eV) and graphitic N (401.4 eV) dominate the composition with the high content (32.6%, 43.5%, respectively) [
52], which could facilitate the interfacial electron transfer and improve the electrical conductivity to obtain superior OER performance [
33]. The Co 2p binding energy of TJU-104–900 further confirmed the existence of CoN
x (
Fig. 6d and Table S4), and Co
0 (778.7 eV, 3.7%), Co
3+ of Co
3O
4 (779.6 eV, 794.8 eV, 27.8%) and Co
2+ of CoO (782.5 eV, 796.3 eV, 26.0%) which can serve as rich active sites for enhancing OER activity [
53]. All TJU-104 derived samples showed the sp
2 C=C, sp
3 C‒C, C‒O/C‒N, C=O and O‒C=O bonding [
54], where the sp
2 C=C and sp
3 C‒C could improve the electrical conductivity and the C‒O/C‒N, C=O, O‒C=O could facilitate the surface hydrophilicity benefiting the aqueous OER process (
Fig. 6e and Table S4) [
55]. In view of O 1s binding energies, lattice oxygen (CoO
x), ‒OH, and O‒C were found in all the samples (
Fig. 6f and Table S4) [
56]. Notably, H
2O/C=O was only detected in TJU-104–900, which was ascribed to its hydrophilic structure to adsorb water (H
2O) on the produced C=O functional groups at 900 ℃ [
57]. In brief, by assembling our ultrahigh N-containing ligand, the MOFs could develop a hierarchical structure derivative,
i.e., N-doped graphitic carbons encapsulating nano-cobalt particles, with abundant active components (
e.g., C‒N and C‒O groups, specific Co‒N
x coordination) driving outstanding OER activity [
58]. Meanwhile, in the XPS spectrum of Co after the OER measurement (Fig. S22 in Supporting information), the peaks of Co 2p
3/2 and Co 2p
1/2 apparently shifted to higher binding energies indicating the cobalt active sites might transform to active CoOOH species, which could further facilitate the OER activity [
59]. The XRD of the TJU-104–900 after OER implementations was explored to find a new phase (CoO
3) generation (Fig. S22).