Figure 2 (a) Ni 2p, (b) Fe 2p, (c) Co 2p, (d) O 1s, (e) C 1s
and (f) N 1s XPS spectra of NiFeCoCN before and after electrochemical
ageing in a 0.5 M H2SO4 solution.
Figure 3a shows the linear sweep
voltammetry (LSV) curves of NiFeCo,
NiFeCoCN and
IrO2/Ti in a 0.5 M H2SO4solution at a scan rate of 1 mV s-1. NiFeCoCN
electrode has the best electrocatalytic activity for the acidic OER in
comparison with other electrodes.
The overpotentials of the
NiFeCoCN electrode for the acidic OER at current densities of 10 mA
cm-2 and 100 mA cm-2 are only 432 mV
and 567 mV, respectively, which are significantly lower than those of
NiFeCo (526 mV and 664 mV) (see Table S6). The consequence indicates an
obvious improvement in electrocatalytic activity due to the
incorporation of C/N species, which may be attributed to a change of the
electronic structure of active sites and an increase of the coordination
unsaturated metal active sites for the OER. In addition, the NiFeCoCN
electrode achieves a low Tafel slope of 52.4 mV dec-1(see Figure 3b), which is even lower than the value of 58.6 mV
dec-1 of the noble IrO2/Ti electrode.
Electrochemical impedance spectroscopy (EIS) is the preferred technique
to study the kinetic mechanism of porous electrodes for the OER, because
the output is directly affected by the electrode porosity. Figure 3c
shows the EIS of NiFeCo, NiFeCoCN and IrO2/Ti
electrodes. All data are fitted using the equivalent circuit of
(Rs(QdlRct )), whereRs , Qdl andRct are the solution resistance, electric double
layer capacitance and charge transfer resistance in the OER process,
respectively [53, 54]. As shown in Figure 3c and Table S7, theRct value of NiFeCoCN is 0.15 Ω
cm-2, which is much smaller than that of NiFeCo (14.36
Ω cm-2) and similar to that of IrO2/Ti
(0.17 Ω cm-2), which indicates superior
charge-transfer ability and high OER activity in acidic media. Moreover,
NiFeCoCN electrode also exhibits an excellent Faradaic efficiency of
94.7% in a 0.5 M H2SO4 solution (see
Figure S7).
The electrochemical active surface area (ECSA) is proportional to the
catalytic performance and the specific activity of the catalyst can be
obtained by calculating the ratio between the current density
corresponding to a given voltage (IOER ) and ECSA.
The ECSA is usually calculated by using the electric double layer
capacitance value (Cdl ) based on CV data. Figure
3d shows the electric double layer capacitance data calculated based on
Figure S8. As shown in the figure, the NiFeCoCN electrode has a higherCdl value than NiFeCo. The ECSA and specific
activity can be estimated as follows:
ECSA = Cdl /Cs (1)
Specific activity = IOER / ECSA (2)
where Cs is the specific capacitance of the
electrode material and takes the general value of the transition metal
surface oxide: 40 μF cm-2 [55]. After calculation,
the NiFeCoCN electrode obviously has higher ECSA of 105
cm2 and specific activity of 2.7 mA
cm-2 than that of the NiFeCo
(67.5 cm2 and
1.6 mA cm-2), indicating an increase of the intrinsic
activity owing to an incorporation of C/N species (see Table S8).
Stability is another important indicator for evaluating OER catalysts,
especially in acidic electrolytes. Figure 3e shows the stability of the
NiFeCoCN electrode in a 0.5 M
H2SO4 solution. The NiFeCoCN electrode
can stably perform for 10000 seconds at a current density of 100 mA
cm-2, and the oxygen evolution potential tends to
stabilize over time. To further confirm the true stability, the
long-term stability of the NiFeCoCN was determined in a 0.5 M
H2SO4 solution, which could stably
operate for approximately 28 hours at a current density of 100 mA
cm-2 (see
Figure S9), which indicates the excellent stability for the acidic OER.
There is not a change of the phase on the surface of aged sample
compared to initial sample (see Figure S10). However, the increased
binding energy and O content shown in the XPS data indicate that the
sample surface has an oxidation occurred, which is consistent with the
data in Table S3. Figures 3f-h further exhibits the HRTEM images of the
NiFeCoCN electrode before and after 5000 seconds of electrochemical
ageing at a current density of
100 mA cm-2 in 0.5 M
H2SO4solution. As shown in Figures 3g
and h, an amorphous metal oxide layer of approximately 7-10 nm thickness
is formed on the surface of the aged electrode, which is consistent with
the XPS analysis results. The surface and interior of the NiFeCoCN
electrode without electrochemical ageing (see Figure 3f) show the
ordered crystal structure of the γ-Ni solid solution, while the aged
electrode undergoes obvious surface reconstruction. The relative
reference also confirms the amorphous metal oxide layer that has formed
on the outer surface of the electrocatalysts [56-58]. The metal
oxide layer of the active centre for the OER in acidic medium is
generally considered to be reconstructed by the dissolution/deposition
or electromigration mechanism in the OER process [24, 59].