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].