Balance between FeIV–NiIV synergy and Lattice Oxygen Contribution for Accelerating Water Oxidation

Hydrogen obtained from electrochemical water splitting is the most promising clean energy carrier, which is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). Thus, the development of an efficient OER electrocatalyst using nonprecious 3d transition elements is desirable. Multielement synergistic effect and lattice oxygen oxidation are two well-known mechanisms to enhance the OER activity of catalysts. The latter is generally related to the high valence state of 3d transition elements leading to structural destabilization under the OER condition. We have found that Al doping in nanosheet Ni–Fe hydroxide exhibits 2-fold advantage: (1) a strong enhanced OER activity from 277 mV to 238 mV at 10 mA cm–2 as the Ni valence state increases from Ni3.58+ to Ni3.79+ observed from in situ X-ray absorption spectra. (2) Operational stability is strengthened, while weakness is expected since the increased NiIV content with 3d8L2 (L denotes O 2p hole) would lead to structural instability. This contradiction is attributed to a reduced lattice oxygen contribution to the OER upon Al doping, as verified through in situ Raman spectroscopy, while the enhanced OER activity is interpreted as an enormous gain in exchange energy of FeIV–NiIV, facilitated by their intersite hopping. This study reveals a mechanism of Fe–Ni synergy effect to enhance OER activity and simultaneously to strengthen operational stability by suppressing the contribution of lattice oxygen.

Preparation of Fe-free KOH.2][3] A total of 2.0 g of nickel nitrate salt was dissolved in 5 mL of ultrapure water.Subsequently, 20 mL of 1 M KOH was added to induce the precipitation of nickel hydroxide.The resulting solution was subjected to agitation followed by centrifugation to separate the supernatant.The obtained nickel hydroxide was then subjected to washing using 2 mL of 1 M KOH and 20 mL of ultrapure water in three consecutive dispersing-centrifugation cycles.The resultant nickel hydroxide precipitate was suspended by adding 50 mL of 1 M KOH.After shaking for a minimum of 30 min, the mixture was allowed to settle undisturbed for at least 3 h.The subsequent centrifugation process yielded a purified KOH electrolyte, free from iron impurities.

Physicochemical characterization. Field emission scanning electron microscopy
(SEM) images (Zeiss Merlin) were acquired at a voltage of 5 kV.Transmission electron microscopy (TEM) measurements were conducted using an FEI Tecnai G2 F20 S-TWIN electron microscope.High-resolution (HR) inductively coupled plasma mass spectrometry (ICP-MS) was analyzed on a Nu Attom instrument to detect the concentration of different elements.
Density functional theory (DFT) calculations.For this study, the Vienna ab initio simulation package (VASP) 4,5 was used to perform the DFT calculations in conjunction with projector-augmented wave (PAW) formalism. 6Consequently, The Ni 3d 8 4s 2 , Fe 3d 6 4s 2 , Al 3s 2 3p 1 , and O 2s 2 2p 4 states were treated as valence electrons.The electronic wave functions were expanded in plane waves using an energy cut-off of 500 eV, and the force and energy convergence criteria were set to 0.02 eV Å -1 and 10 -5 eV, respectively.The electron exchange and correlation within the generalized gradient approximation of the Perdew-Burke-Ernzerhof functional were used to optimize the configurations. 7To accurately evaluate the electronic properties of NiO2 (Ni), FeO2 (Fe), Fe-doped NiO2 (Ni0.9-Fe0.1),and Fe-Al-doped NiO2 (Ni0.75-Fe0.1-Al0.1 with 5% cation vacancy), the Hubbard U-model was employed to describe the strong correlation of the localized Ni 3d and Fe 3d states, and the values of Ueff were set to 5.5 and 4.0 eV, respectively, according to a previous study. 8,9 he reciprocal space was sampled using a 5 × 4 × 4 Monkhorst-Pack k-point mesh for DFT + U calculations.The OER steps for AEM, LOV and MLOV routes are depicted as below: AEM: LOV: MLOV:

Figure S1 .
Figure S1.Ratio optimization of the doped Fe in Ni-Fe hydroxide (a) and doped Al in Ni-Fe-Al (b)

Figure S7 .
Figure S7.Stability test at 1000 mA cm -2 of Ni-Fe hydroxide using Ni foam as the working

Figure S9 .
Figure S9.Concentration of dissolved Al ions of Ni-Fe-Al hydroxide during the OER process at

Figure S12 .
Figure S12.(a) Electrochemical performance of Zr doped Ni-Fe hydroxide in 1 M KOH electrolyte

Figure S13 .
Figure S13.(a) Total electron yield (TEY) mode of the O-K edge and (b) Fluorescence yield (FY)

Figure S14 .
Figure S14.(a) TEM characterization of the Ni, Ni-Fe, Ni-Al, and Ni-Fe-Al hydroxides after the

Figure S15 .
Figure S15.SEM characterization of the Ni, Ni-Fe, Ni-Al, and Ni-Fe-Al hydroxides after the OER

Table S1 .
Ratios of different elements in the Ni-Fe, Ni-Al, and Ni-Fe-Al hydroxides before and after the OER process in 1 M KOH electrolyte (at 1.8 V vs. RHE for 2 h) measured by ICP-MS.

Table S2 .
EXAFS simulation parameters for the as-prepared Ni, Ni-Fe, Ni-Al, and Ni-Fe-Al hydroxide catalysts.