Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Paper
RSC Advances
rotating disk electrode (RDE) and a rotating ring disk electrode distance of black particles was calculated to be 0.21 nm and
(RRDE) were the working electrodes. Cyclic voltammetry (CV), 0.16 nm, respectively, corresponding to the (111) crystal planes
linear-sweep voltammetry (LSV) and chronoamperometry (CA) of the Fe3C phase (JCPDS 01-089-2005) and the (ꢁ222) crystal
were performed on a rotating ring disk electrode instrument planes of the Fe3O4 phase (JCPDS 00-028-0491). The results are
(RRDE-3A, ALS). The catalyst loading was 0.4 mg cmꢁ2. All the consistent with the XRD analysis in Fig. 1f, where 44.7ꢀ corre-
resulting potentials were converted to those of a reversible sponds to Fe3C and 59.6ꢀ corresponds to Fe3O4. Notably, the
hydrogen electrode (RHE) by the following formula:
diffraction peak appearing at 44.7ꢀ in Fig. 1f may be attributed
to the binding of iron and nitrogen. It is speculated that the
presence of iron–nitrogen bonds in the material might provide
active sites for ORR.8 In addition, we could see that both Fe3C
and Fe3O4 particles were encapsulated by the carbon layer,
which might reduce the corrosion of the active reactive site on
the catalyst and have a great effect on improving the electro-
chemical stability of the material.20 The XRD patterns of the
other samples are shown in Fig. S3.† As revealed in Fig. S3,† the
broad peaks at about 26.2ꢀ and 44.3ꢀ were attributed to the (002)
and (101) planes of graphitic carbon (JCPDS 01-075-1621),
respectively. The difference in the Fe–PC pattern (Fig. S3c†) was
the peaks for iron oxides at 35.6ꢀ (JCPDS 01-033-0664) and Fe3C
at 48.4ꢀ (JCPDS 01-035-0772). As shown in Fig. 1f, aer the
addition of a suitable nitrogen source to Fe–N–PC, some of the
iron combined with nitrogen to form the iron nitride compound
phase.
Raman spectroscopy was used to determine the degree of
graphitization of the various catalysts prepared under different
conditions. The Raman spectra are shown in Fig. 1g. All
samples had two signicant characteristic peaks: the D peak at
about 1320 cmꢁ1 corresponding to amorphous carbon and the
G peak at about 1590 cmꢁ1 corresponding to graphitized
carbon. The ratio of the intensity of the D peak to the intensity
of the G peak is an important index for evaluating the degree of
graphitization of the material. It could be seen that with the
introduction of iron and nitrogen, the value of ID/IG gradually
increased, indicating that the doping of iron and nitrogen
caused the formation of more defects.
E(RHE) ¼ E(SCE) + 0.0591 ꢂ pH + 0.241
(1)
The RDE measurements were carried out at various rotating
rates (400–2500 rpm). The electron transfer number (n) was
determined by the Koutecky–Levich equation:17
1
1
1
¼
þ
(2)
j
jk Bu0:5
B ¼ 0.2nF(DO
)
2/3vꢁ1/6CO
(3)
2
2
where j represents the measured current density, jk represents
kinetic current densities, u represents the angular velocity of
the disk, n is the number of electrons transferred per oxygen
molecule, F is the Faraday constant (96 500 C molꢁ1), DO is the
2
diffusion coefficient of O2 in 0.1 M KOH, y is the kinetic viscosity
(0.01 cm2 sꢁ1), and CO is the bulk concentration of O2 (1.2 ꢂ
2
10ꢁ6 mol cmꢁ3).
In this work, the RRDE measurements were carried out in
a 0.1 M KOH and 0.1 M HCIO4 solution under saturated oxygen
conditions with a constant potential of 0.5 V (vs. SCE) on the
ring electrode. The H2O2 yield and the number of electrons
transferred during the ORR reaction were calculated using the
following equations:18,19
2IR=N
% H2O2 ¼ 100 ꢂ
(4)
(5)
ID þ ðIR=NÞ
4ID
n ¼
The elemental composition of these catalysts was further
studied by XPS. The surface survey XPS spectra of these catalysts
are shown in Fig. 2a, and elemental analysis results are shown
in Table 1. It could be seen that iron and nitrogen were
successfully doped into the catalyst.21 Fig. 2b shows the N 1s
spectra of Fe–N–PC, and it could be divided into four peaks at
398.41 eV, 399.6 eV, 400.75 eV and 402.7 eV, which correspond
to pyridinic N, pyrrolic N, graphitic N and oxidized nitrogen,
respectively.22 Notably, the peak at the binding energy of
398.4 eV might also present Fe–N binding due to the small
ID þ ðIR=NÞ
Here, ID is the disk current, IR is the ring electrode current, N is
the acquisition efficiency 0.424, %H2O2 is the hydrogen
peroxide yield during the oxygen reduction reaction, and n is the
number of electrons transferred per oxygen molecule.
Results and discussion
Structure characterization
The scanning electron microscopy (SEM) and transmission difference between the binding energies of Fe–N and
electron microscopy (TEM) images of Fe–N–PC are shown in pyridinic N.23,24 It is generally believed that in addition to
Fig. 1a and b. Moreover, Fig. S1† shows the morphology of nitrogen oxides, pyridinic nitrogen, pyrrolic nitrogen and
soybean straw mixed with MgO before and aer calcination. graphitic nitrogen are active sites for ORR and play an impor-
The morphology of PC, N–PC and Fe–PC are shown in Fig. S2.† tant role in increasing the ORR activity.25,26 Fig. 2c shows the
As seen in the SEM images, all samples were oriented to form high-resolution Fe 2p XPS spectrum of Fe–N–PC, which could
a honeycomb-like porous structure using MgO as the template. be separated into four peaks at about 707.64, 710.8, 720.9 and
From Fig. 1b and c, we found that some carbon-coated black 723.2 eV, corresponding to Fe2+ (2p3/2), Fe3+ (2p3/2), Fe2+ (2p1/2
particles, which may be iron/iron compounds, were present in and Fe3+ (2p1/2), respectively. In addition, the peaks at about
the Fe–N–PC catalyst.
715.7 and 713.5 eV were attributable to the satellites.27 The
)
The HRTEM images of Fe–N–PC are shown in Fig. 1c–e, in gure also suggested the existence of Fe3O4 in the catalyst,
which the crystalline lattice can be observed. The lattice which is consistent with the XRD analysis results.15 The peak at
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 6763–6771 | 6765
Li, Dahuan
