In an attempt to shed light on the inherent causes for the unusually excellent OER performance, we tracked the active phase evolution of the CoNiCuFe-PBA catalyst during the OER process and compared it with that of CoNiCuFe-Oxide via in situ electrochemical Raman spectroscopy (Figure ). As shown in Figure , the peak at 681 cm -1 can be attributed to Fe-O species in the initial state of CoNiCuFe-PBA. The Fe-O peaks with initial high intensity disappeared when the potential increased to 1.4 V, while the two well-resolved M III -O vibration bands appeared at ~460 and 540 cm -1 , which correspond to the M-O bending (IB) and stretching (IS) vibrations of MOOH, respectively. We thereby confirmed potential-dependent MOOH as the active state, which possesses a great number of unsaturated coordination sites to optimize the absorption of *OH and the deprotonation of *OOH. The Figure Raman spectra indicate that the CoNiCuFe-PBA catalyst on the electrode surface transformed into the active phase structures (CoOOH and/or NiOOH) after 1.4 V, where the divalent metal ions were oxidized to trivalent ones during OER measurement . In the process of potential reduction, the signals of M III -O bands gradually weakened but did not vanish, suggesting that the active phase transformation was irreversible and active intermediate species were retained. Turning to the CoNiCuFe-Oxide in Figure , there was less active phase transformation observed, and the weak Fe-O peak of CoNiCuFe-Oxide turns broad and almost disappears when the potential decline to 1.3 V.