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.