Figure 4 Electrocatalytic measurements. (a) LSV curves of the
NiFeCo, NiFeCoCN and IrO2/Ti electrodes in a 1 M KOH
solution with a scan rate of 5 mV s-1. (b) TheiR s-corrected Tafel plots of all as-prepared
electrodes derived from the curves of Figure 4a. (c) Nyquist plots of
the EIS of all as-prepared electrodes measured at an OER voltage of
1.503 (vs . RHE) over a frequency range of 0.01 Hz-100 kHz in a 1
M KOH solution. The insets are the equivalent circuit ofRs (QdlRct )
and the plots in the high frequency region. (d)Cdl of NiFeCo and NiFeCoCN in a 1 M KOH solution.
(e) Galvanostatic tests of the electrodes at constant current densities
of 10-200 mA cm-2 in a 1 M KOH solution.
To further insight the intrinsic activity, DFT calculations were
employed to study the process of modifying the electronic structure of
the active sites by the C/N in the amorphous oxidation layer of NiFeCo
and NiFeCoCN. In acidic media, the following steps can describe the
four-electron OER pathways:
H2O + ∗ → ∗OH + e− +
H+ (3)
∗OH → ∗O + H+ +
e− (4)
∗O + H2O → ∗OOH +
H+ + e− (5)
∗OOH →O2 + H+ +
e− (6)
Figure 5a and Figure S11 show the models of intermediates (OH *, O * and
OOH *) adsorbed on the Fe site on the surface of NiFeCoCN MEA. The Gibbs
free energies for each step of the reactions were calculated (0 and 1.23
V vs. RHE) (see Figure 5b, Table S9 and S10). The catalytic performance
of OER is determined rate-determining step (RDS). At 1.23 V, *O
formation (*OH→*O) with the NiFeCo MEA was the rate determining step
(RDS) with a free energy difference of 0.768 eV. After introducing C/N,
the formation energy of *O on the NiFeCoCN MEA decreased to 0.45 eV.
According to previous studies, the theoretical overpotential of OER can
be determined by the adsorption free energy of *O with respect to *OH,
referred to as ΔG (*O)-ΔG (*OH) [14]. In other words, the NiFeCoCN
MEA need a lower overpotential to drive water oxidation. The projected
density of states (PDOS) of Fe s,
Fe p and Fe d orbitals is calculated to insight the electronic structure
(see Figure 5c). The total density of states (TDOS) of the Fe site in
NiFeCoCN MEA is mainly contributed by d orbital, and the d-band center
relative to Fermi level is ‒1.82 eV. For NiFeCo MEA, the d-band center
relative to Fermi level is ‒1.57 eV. It means that the d-band center of
Fe in NiFeCoCN MEA shifts far away from Fermi level. The adjustment in d
orbital of Fe caused by involving
C/N leads to the weakened
adsorption ability of the oxygen-containing intermediates, facilitating
the reduction of energy barrier. In addition, Figure 5d shows the Bader
charge computations of the model. Initially, the metal atoms (Ni, Fe,
Co) in the NiFeCoCN MEA model transfer charges to O atom, respectively.
When N atom replaces O atom in the NiFeCoCN MEA model, the charge
transfer from metal atoms to N atom becomes less, and the Fe atom
changes most significantly (1.24 e-→1.08
e-). This phenomenon of less charges transfer at metal
sites will repeat when C atom replace O atom. The result shows that
electrons tend to be enriched at metal sites dominated by Fe after
introducing C/N. This difference directly represents the weakened
adsorption of O on the electron-rich metal sites over amorphous
oxidation layer of NiFeCoCN MEA, facilitating the lower energy barrier
of RDS and thus has excellent OER activity in acidic media.