Plasma etching enabling the fast reconstruction of pre-catalysts into defective metal oxyhydroxides with high spin state and activated lattice oxygen for efficient oxygen evolution

Abstract

Transition metal-based catalytic materials are promising pre-catalysts for oxygen evolution reaction (OER), during which the in situ reconstructed metal oxyhydroxides are real active sites. However, a majority of documented pre-catalysts exhibit sluggish reconstruction dynamics, leading to in-complete reconstruction and consequently poor OER activity. Herein, exemplified by Hoffman-type coordination polymer (NiFe-Ni PBA), plasma etching is employed to create cation-anion dual vacancies (Niv and CNv) to promote the rapid and deep reconstruction of NiFe-Ni PBA into defective NiOOH/FeOOH (P-NiOOH/FeOOH) during the activation process. Langmuir probe diagnostics and structural characterizations of NiFe-Ni PBA before and after plasma etching evidence that Niv and CNv are predominantly generated by the bombardment of high-energy ions, whereas elemental nickel will be produced when electron energy exceeds a critical threshold. Density functional theory (DFT) calculations, in situ Raman spectra, and Laviron analysis reveal that the abundant vacancies in plasma-etched NiFe-Ni PBA effectively lower the reconstruction reaction barrier and promote the accumulation of OH− ions during the reconstruction process, enabling faster reconstruction kinetics. As expected, the P-NiOOH/FeOOH exhibits enhanced OER activity with a low overpotential of 220 mV at 10 mA cm−2 and a small Tafel slope of 29.82 mV dec−1 in 1 M KOH. Magnetic test, differential electrochemical mass spectrometry measurement, and DFT calculations illustrate that the improved OER activity can be attributed to the high spin state, optimized d-band center of metal ions, rich oxygen vacancies, and more activated lattice oxygen in P-NiOOH/FeOOH. Moreover, the P-NiOOH/FeOOH also displays splendid catalytic stability up to 850 h.