Fig.5. (a) the LSV curve of CoFe-CoxN@NC for OER before
and after the 80000 s cycle at a constant current density of 10
mA·cm−2, (b) the chronopotentiometri measurement of
recorded at a constant current density of 10 mA·cm−2for CoFe-CoxN@NC
3.3 Electrocatalytic
mechanism
In order to provide insights into
the electrocatalytic mechanism of CoFe-CoxN@NC for OER,
DFT calculation was employed to investigate and compare the OER
catalytic performances of four catalyst models including naked FeCo
alloy, FeCo alloy encapsulated by carbon layer (CoFe@C), CoFe alloy
encapsulated by nitrogen-doped carbon layer (CoFe@NC), and the
as-prepared CoFe-CoxN@NC. Generally, OER undergoes a
four-electron step pathway in a basic electrolyte showed as Fig. 6a. The
Gibbs free energy profiles for four catalysts were depicted in Fig. 6b,
and the rate-determining step (RDS) with the value of the Gibbs free
energy barrier was highlighted, where a fairly high Gibbs free energy
barrier of 1.71 eV was obtained at 1.23 V for OER over naked CoFe alloy.
The reason for this mainly attributed to the strong absorption (a Gibbs
free energy barrier of -1.93 eV) of the active oxygen on the surface of
CoFe alloy to form an excessively stable *O intermediate for OER. The
charge density differences presented in Fig. S13 showed that charges
were almost transferred onto the surface of oxygen atom, further
revealing the enhanced electrostatic interaction between oxygen and CoFe
alloy. In such case, more energies were required to overcome the barrier
from *O to *OOH, resulting in the high overpotential for OER over CoFe
alloy. Theoretically, CoFe alloy encapsulated by graphitic carbon layer
can prevent it from the direct contact with reactive oxygen species,
however, the interfacial distance between CoFe alloy and graphitic
carbon layer would affect the stability and charge transfer. It can be
seen from Fig. 6c that the total energy of the CoFe@C heterojunction
catalyst was lowest when the interfacial distance reached 4Å.
Nevertheless, the electrons mainly accumulated in its interfacial space
at its most stable configuration based on the charge density difference
showed in Fig. 6c inserted images, making the active site to *OH pathway
as the RDS with the high energy barrier of 1.56 eV. Interestingly,
doping nitrogen in the graphitic carbon layer can redistribute the
charge density without sacrificing its stability. The electrons
transferred from CoFe alloy can be well distributed on the
nitrogen-doped graphitic carbon layer as described in Fig. 6d-e,
promoting the OER process with a lower energy barrier of 0.68 eV at RDS
from *O to *OOH. Actually, in this study, in situ nitrogen-doping
pyrolysis process can form a specific heterojunction
CoFe-CoxN@NC with three phases, nitrogen-doped
Co5.47N-connected CoFe alloy. The CoxN
as a linker can induce the uniform distribution of charge density as
shown in Fig. 6f based on the DFT simulation, which could optimize the
adsorption/desorption strength of the OER intermediates and products. As
a result, a lowest limiting energy barrier of 0.43 eV can be achieved by
DFT calculation at 1.23 V for OER over CoFe-CoxN@NC
(Fig. 6b) in comparison with other used catalysts, in consistent with
the trend of experimental results.