Oxygen Defect Engineering Promotes Synergy Between Adsorbate Evolution and Single Lattice Oxygen Mechanisms of OER in Transition Metal‐Based (oxy)Hydroxide

Abstract The oxygen evolution reaction (OER) activity of transition metal (TM)‐based (oxy)hydroxide is dominated by the number and nature of surface active sites, which are generally considered to be TM atoms occupying less than half of surface sites, with most being inactive oxygen atoms. Herein, based on an in situ competing growth strategy of bimetallic ions and OH− ions, a facile one‐step method is proposed to modulate oxygen defects in NiFe‐layered double hydroxide (NiFe‐LDH)/FeOOH heterostructure, which may trigger the single lattice oxygen mechanism (sLOM). Interestingly, by only varying the addition of H2O2, one can simultaneously regulate the concentration of oxygen defects, the valence of metal sites, and the ratio of components. The proper oxygen defects promote synergy between the adsorbate evolution mechanism (AEM, metal redox chemistry) and sLOM (oxygen redox chemistry) of OER in NiFe‐based (oxy)hydroxide, practically maximizing the use of surface TM and oxygen atoms as active sites. Consequently, the optimal NiFe‐LDH/FeOOH heterostructure outperforms the reported non‐noble OER catalysts in electrocatalytic activity, with an overpotential of 177 mV to deliver a current density of 20 mA cm−2 and high stability. The novel strategy exemplifies a facile and versatile approach to designing highly active TM‐LDH‐based OER electrocatalysts for energy and environmental applications.

electrolyte.And both sides of every working electrode are estimated.The linear sweep voltammetry (LSV) polarization curves were measured at a rate of 2 mV s -1 .The staircase cyclic voltammetry (SCV) curves were tested with the potential increment of 4 mV.All polarization curves were corrected by 85% iR compensation for ohmic losses arising from active materials, substrate, and solution resistances.The electrochemical impedance spectroscopy (EIS) measurements are carried out in the frequency range from 1 MHz to 0.05 Hz.The chronopotentiometry was performed with a constant current density of 20, 100, and 1000 mA cm -2 in an O2-saturated 1.0 M KOH solution.
The Tafel slope is calculated according to the equation: η = b × log |j| + a, where η is the overpotential (V), j is the current density (mA cm -2 ), and b is the Tafel slope (mV dec -1 ).The Cdl was calculated by taking CV measurements at varied scan rates of 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV/s.The value of Cdl was the linear slope obtained from plotting Δj = ja − jc against the scan rates (ja and jc are anode and cathode current density, respectively).All the potentials were calibrated to the reversible hydrogen electrode (vs.RHE), and the corresponding equation is ERHE = EAg/AgCl + 1.022 V.
18 O-labeling experiment: NiFe-LDH/FeOOH with 0 and 1 mL H2O2 were labeled with 18 O isotopes by potentiostatic reaction for 30 min in KOH solution with H2 18 O.
The 18 O-labeled catalysts were then rinsed several times with H2 16 O to remove the remaining H2 18 O.DEMS measurements: DEMS measurements were carried out by a QAS 100 device (Linglu Instruments, Shanghai).The NiFe-LDH/FeOOH with 18 O-labeling, Pt sheet, and Ag/AgCl electrode in saturated KCl solution were used as working, counter, and reference electrode, respectively.CV measurement was performed in KOH solution with H2 16 O with a scan rate of 5 mV/s.Meanwhile, the gas products of different molecular weights were monitored by mass spectroscopy in real time.

Theoretical Calculations
The Spin-polarized DFT calculations were performed on Vienna ab initio Simulation package (VASP) [1,2] with projector augmented wave (PAW) [3,4] pseudopotential.The exchange-correlation interaction was described by Perdew-Burke-Ernzerhof (PBE) functional within the generalized gradient approximation (GGA). [5]The kinetic energy cut-off was set to 500 eV, and the dispersion corrections in Grimme's scheme (DFT-D3) was utilized to treat the long-range van der Waals interactions. [6,7]The Brillouin zone was sampled by Gamma-centered k-point with 1 × 1 × 1, [8] and the convergence criterion of force and energy were set to 0.01 eV Å −1 and 10 −5 eV, respectively.To better describe the localized 3d orbital, the effective U value of Ni and Fe atoms were set to 2.56 and 5.2 eV, respectively. [9]e Gibbs free energy are calculated to investigate the OER performances.For the AEM pathway in an alkaline electrolyte, the four-electron reactions can be described as: where "*" represents the adsorption sites, which are generally the exposed metal sites.
The Gibbs free energy of each step can be calculated as: where GO2 is the Gibbs free energies of O2 which can be calculated as GO2

Growth Mechanism of NiFe-LDH/FeOOH Heterostructure
Under hydrothermal conditions, urea is slowly hydrolyzed to produce NH4 + and CO3 2-(Equation 1), which would further be hydrolyzed to generate H + (Equation 2) and OH -(Equation 3) ions, respectively.The produced OH -ions will coprecipitate with Fe 3+ ions in the solution to form FeOOH (Equation 4); meanwhile, the produced H + ions will etch NF to generate Ni 2+ ions (Equation 5).Since the urea hydrolysis is relatively weak, only small amount of Ni 2+ ions are generated from the NF etching and will coprecipitate with Fe 3+ ions to form NiFe-LDH lobulate nanosheets (Equation 6).As H2O2 is added into the reaction solution, the Fenton-like and Fenton reactions take place (Equation 7, 8), resulting in large quantities of hydroxyl radicals (*OH) and OH -ions.The strongly oxidative *OH would accelerate the etching of NF to produce more Ni 2+ ions and enhance the localized alkaline environment near the NF surface (Equation 9).As a result, more Ni 2+ and Fe 3+ ions will coprecipitate with OH -ions to form abundant NiFe-LDH (Equation 6), besides Fe 3+ ions bond with OH -ions forming FeOOH (Equation 4).This competing interaction between metal ions (Ni 2+ and Fe 3+ ) with OH -ions will lead to the formation of NiFe-LDH/FeOOH heterostructure accompanying with oxygen defects.

Figure S3 .
Figure S3.a) Lattice fringe image of NiFe-LDH and b) line scan image indicated by the lattice fringe in (a).c) Lattice fringe image of FeOOH and d) line scan image indicated by the lattice fringe in (c).

Figure
Figure S9.a) LSV curves and b) corresponding Tafel plots of samples synthesized with different H2O2 addition in 1.0 M KOH.

Figure S10 .
Figure S10.Time-dependent OER performance of the samples.

Figure S13 .
Figure S13.An equivalent circuit for the electrode/electrolyte interface.

Figure S17 .
Figure S17.The a) top and b) side view of FeOOH.The Fe, O, and H atoms are represented by brown, red, and pink balls, respectively.The active sites for AEM and LOM are highlighted by blue and black circles, respectively.c) The Gibbs free energy diagram of AEM and LOM pathways on FeOOH, where the arrows show the RDS and η are corresponding overpotentials.The adsorption intermediates of d) AEM pathways (*OH, *O and *OOH) and e) LOM pathways (*OOH, * OV and *OH) for FeOOH.

Figure S18 .
Figure S18.The a) top and b) side view of NiFe-LDH without and with oxygen vacancies.The Ni, Fe, O, and H atoms are represented by gray, brown, red, and pink balls, respectively.The active sites for AEM and LOM are highlighted by blue and black circles, respectively.

Table S1 .
Comparison of electrocatalytic OER performances of NiFe-LDH/FeOOH with other reported catalysts in 1.0 M KOH.η100 corresponds to the overpotential at current density of 100 mA cm -2 .(Note: *: Value calculated from curves shown in the respective reference.NF: nickel foam.)