Page 175 - Read Online

Li et al. Energy Mater 2023;3:300021 https://dx.doi.org/10.20517/energymater.2023.09 Page 7 of 16

investigate the structural properties of the prepared samples, Raman spectroscopy was performed on them.
-1
As depicted in Supplementary Figure 1B, NiFe-LDH exhibits two distinct peaks at 462 and 542 cm , which
[48]
is consistent with previously reported results in the literature . The low-intensity peak of FePc component
at FePc/Se@NiFe and FePc/Se@Ni in the Raman spectrum confirms better dispersity [Figure 1B]. And the
obvious high intensity Raman peak of FePc/Se@Fe is similar to that of FePc, demonstrating that FePc
molecules mainly aggregate on the surface of Se@Fe . Obviously, FePc/Se@NiFe and FePc/Se@Ni display a
[49]
low-intensity characteristic peak of FePc that can indicate that Se@NiFe or Se@Ni may be connected with
FePc by π-π conjugation interaction . The above results can also be attributed to the fact that NiFe-LDH
[42]
and Ni-LDH have a laminar structure that can adsorb and accommodate part of the FePc, which in turn
helps to promote π-π interaction. In addition, it can be observed that, for Se@NiFe and FePc/Se@NiFe
-1
samples, the representative Raman peaks lied on 125, 197, and 550, 670 cm belong to Ni Se and
4
3
NiFe(OH) . However, FePc/Se@Ni exhibits the Ni Se and Ni(OH) phase. As reported, the amorphous
X
4
3
X
NiFe(OH) and Ni(OH) phases are the catalytically active phases in the course of OER [50,51] . The
X
X
enhancement of activity is contributed to their ability to efficiently accelerate the rate-determining step.
Besides, the existence of FePc can obtain both graphitic carbon (G-band) and disorder carbon (D-band)
with high electrical conductivity and modified electronic structure in the electrocatalytic process. As
observed from Figure 1C and D, the SSA and porous volumes of FePc/Se@NiFe were estimated to be
22.6 m g and 0.16 cm g , as confirmed by the BET method, which is smaller than NiFe-LDH and FePc
3
-1
2
-1
precursors but larger than other selenides [Supplementary Figure 1C and D]. Due to the large SSA and pore
volume of FePc/Se@NiFe, it is beneficial for interfacial electrons and mass transfer, and catalytic activity is
improved.
Next, the surface chemical composition of FePc/Se@NiFe and FePc samples, as well as the intrinsic
interaction between electronic structure and electrocatalytic performance, were evaluated by XPS tests. As
demonstrated in Supplementary Figure 2, in the XPS study spectrum for FePc/Se@NiFe, the co-existence of
these signals including C, N, O, Se, Ni, and Fe were found, whereas the Se and Ni signals were absent in
FePc. The percentage of surface element atoms is displayed in the insertion table. All data were calibrated
with 284.8 eV of C 1s. Compared to FePc, the high-resolution C1s XPS spectral peak of FePc/Se@NiFe
[Figure 2A], in order to express strong π-π interactions, moves in the direction of low coupling energy.
Typically, the coupling energy reduction exhibits an electronic shielding effect, enhanced by increasing the
electron density . As described in Figure 2B, the N 1s XPS spectra of FePc/Se@NiFe were deconvoluted
[52]
into pyridinic-N/C-N and pyrrolic-N/Fe-N. The abundance of pyridinic-N and Fe-N species in the FePc/
Se@NiFe catalyst can serve as real active sites, which are beneficial for ORR electrocatalytic activity . The
[53]
N 1s peaks of the FePc are at 399.0 and 400.6 eV. Undoubtedly, the N 1s in the FePc/Se@NiFe shifted to the
lower coupling energy and the Fe 2p XPS spectra for FePc/Se@NiFe corresponds to the FePc shifted to the
higher energy [Figure 2C]. These results indicate that there is an electron donation from the centered Fe
ions to the neighboring N atoms, alleviating the significant association with intermediates containing N and
O, resulting the approval of the release of N-site O 2 [54,55] . Furthermore, the two peaks found at 707.2 and
3/2
1/2
720.1 eV for FePc/Se@NiFe were ascribed to zero-valence metallic Fe 2p and Fe 2p . The occurrence of
Fe may be due to the reduction of iron ions by hydrazine hydrate with strong reducibility during the
0
hydrothermal process. Three peaks, which correspond to the Ni-Se, Ni 2p , and shakeup satellite peaks,
3/2
were deconvoluted from the elevated Ni 2p XPS spectrum of FePc/Se@NiFe at energies of 853.5, 856.2,
3/2
and 861.9 eV, respectively [Figure 2D]. The Ni-Se bonding further confirms the existence of Ni Se in the
4
3
FePc/Se@NiFe sample. In Figure 2E, the high-resolution Se 3d spectrum able could be separated into three
peaks. The strong peaks at 54.7 eV from Se 3d and 55.7 eV from Se 3d can be ascribed to Ni-Se banding
3/2
5/2
and zero-valence Se moiety, which agrees well with earlier findings. The strong peaks at 59.3 eV can be
ascribed to the SeO due to the oxidation of the sample surface. The formation of SeO can accelerate the
X
X
charge transfer as well as the reaction kinetics of the OER reaction process . The deconvoluted O 1s
[56]