The electrocatalytic activities of NiFe-MOF, CQDs
10@NiFe-MOF, CQDs
10@NiFe-MOF-A, and benchmark IrO
2 catalysts for OER were evaluated in 1 mol/L KOH with a typical three-electrode system (
Fig. 3a). Each catalyst was subjected to hundreds of CVs to remove the unstable components from the electrode surface and produce active substances prior to testing, and the results were shown in Fig. S8 (Supporting information). Both of the CQDs
10@NiFe-MOF and CQDs
10@NiFe-MOF-A exhibited an obvious evolution process, with the gradual increase of Ni
2+/Ni
3+ redox peak intensity and cut-off current density. The difference is that the CQDs
10@NiFe-MOF almost completed activation process after 20 CVs, while CQDs
10@NiFe-MOF-A took almost 50 CVs, meaning more active sites for OER were generated [
23]. As for NiFe-MOF, this process was compressed within 10 CVs, so that the final cut-off current density was not greatly increased. Compared with the IrO
2, the LSV curves without
iR-correction of the NiFe-MOF, the CQDs
10@NiFe-MOF and calcined CQDs
10@NiFe-MOF-A revealed distinct redox peaks at about 1.3 V, which was caused by nickel species in the samples. Among them, the CQDs
10@NiFe-MOF-A exhibited the highest intensity of Ni
2+/Ni
3+ redox peak at 1.3 V (
vs. RHE, inset in
Fig. 3a), suggesting more electrochemically active layers (hydroxide/oxyhydroxide) were generated [
24], which is essential to boost the OER performance. The OER metal redox peak shifted negatively upon the introduction of CQDs into the NiFe-MOF and calcination, indicating the presence of a strong synergistic electronic interaction between the CQDs and Ni/Fe ions [
25,
26]. Therefore, a much better OER performance was achieved by CQDs
10@NiFe-MOF-A, which was demonstrated by the overpotentials (Fig. S9 in Supporting information) and Tafel slope in
Fig. 3b. In Fig. S9, only 289 mV was needed to deliver the current density of 10 mA/cm
2 (
η10 = 289 mV) with ultralow loading mass of 150 μg/cm
2 for CQDs
10@NiFe-MOF-A, and it outperformed that of NiFe-MOF (
η10 = 352 mV) and CQDs
10@NiFe-MOF (
η10 = 319 mV). The CQDs
10@NiFe-MOF-A also exhibited lower overpotentials at higher current densities, ofwhich
η20 = 315 mV and
η50 = 369 mV, surpassed those of NiFe-MOF (
η20 = 383 mV,
η50 = 445 mV), CQDs
10@NiFe-MOF (
η20 = 350 mV,
η50 = 413 mV) and IrO
2 (
η20 = 386 mV,
η50 = 465 mV). Tafel slopes in
Fig. 3b showed the CQDs
10@NiFe-MOF-A possessed a smaller Tafel slope (52.7 mV/dec) than NiFe-MOF (63.1 mV/dec), CQDs
10@NiFe-MOF (58.5 mV/dec) and IrO
2 (68.1 mV/dec), which indicated doping of CQDs and calcination intrinsically accelerated its OER kinetics. The CQDs
10@NiFe-MOF-A also had advantages in electrochemical surface area (ECSA), which was estimated by the double-layer capacitance (
Cdl) in Fig. S10 (Supporting information). The
Cdl valve of CQDs
10@NiFe-MOF-A was 267.6 μF/cm
2 and almost 3.3 times and 1.3 times as large as that of NiFe-MOF (80.5 μF/cm
2) and CQDs
10@NiFe-MOF (201.7 μF/cm
2), respectively, indicating abundance surface active sites were formed by introducing CQDs to NiFe-MOF and calcination, which was beneficial to mass transport during OER. To further evaluate the intrinsical activity of these catalysts, turnover frequency (TOF) was compared, which represented the reaction rate of per active site in unit time.
Fig. 3c revealed that CQDs
10@NiFe-MOF-A exhibited the highest TOF valve at the range of 1.45–1.7 V (
vs. RHE) among the four samples, demonstrating its intrinsically enhanced OER performance. Moreover, the current density was mass-normalized at the overpotential = 300 mV. As shown in Fig. S11 (Supporting information), CQDs
10@NiFe-MOF-A possessed a mass-normalized current density of 91.6 A/g, which was 3.6 times that of IrO
2. These results depicted the significant improvement of intrinsic activity by the incorporation of CQDs and the low temperature calcination. Thus, the CQDs
10@NiFe-MOF-A is comparable even surpasses most of state-of-the-art MOF-based catalysts at the same load (Fig. S12 and Table S1 in Supporting information).