Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis

He, Zuyun; Zhang, Jun; Gong, Zhiheng; Lei, Hang; Zhou, Deng; Zhang, Nian; Mai, Wenjie; Zhao, Shijun; Chen, Yan

doi:10.1038/s41467-022-29875-4

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Published: 21 April 2022

Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis

Nature Communications volume 13, Article number: 2191 (2022) Cite this article

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Abstract

Transition metal oxides or (oxy)hydroxides have been intensively investigated as promising electrocatalysts for energy and environmental applications. Oxygen in the lattice was reported recently to actively participate in surface reactions. Herein, we report a sacrificial template-directed approach to synthesize Mo-doped NiFe (oxy)hydroxide with modulated oxygen activity as an enhanced electrocatalyst towards oxygen evolution reaction (OER). The obtained MoNiFe (oxy)hydroxide displays a high mass activity of 1910 A/g_metal at the overpotential of 300 mV. The combination of density functional theory calculations and advanced spectroscopy techniques suggests that the Mo dopant upshifts the O 2p band and weakens the metal-oxygen bond of NiFe (oxy)hydroxide, facilitating oxygen vacancy formation and shifting the reaction pathway for OER. Our results provide critical insights into the role of lattice oxygen in determining the activity of (oxy)hydroxides and demonstrate tuning oxygen activity as a promising approach for constructing highly active electrocatalysts.

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Introduction

Transition metal oxides or (oxy)hydroxides have been intensively investigated as promising alternative electrocatalysts because of their high catalytic activity, low cost, and good stability. The surface metal sites in these materials are generally considered as the active sites for surface reactions^1,2. Interestingly, recent studies have shown that oxygen in the lattice of metal oxides and (oxy)hydroxides can also participate in surface reactions and may play a critical role in regulating the catalyst activity^3,4,5. For example, Grimaud⁶ and Mefford⁷ reported that lattice oxygen could participate in the oxygen evolution reaction (OER) on the (La,Sr)CoO₃ surface, which was later referred to as lattice oxygen mechanism (LOM). Similar LOM mechanisms were then discovered for the OER on other oxides and (oxy)hydroxides, such as SrCo_1-ySi_yO_3-δ⁸, PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ⁹, Sr(Co_0.8Fe_0.2)_0.7B_0.3O_3-δ¹⁰, CoZn (oxy)hydroxide¹¹, CoAl₂O₄¹², Na_xMn₃O₇¹³ and NiFe (oxy)hydroxide¹⁴. In addition to the OER reaction at room temperature, Hwang et al.¹⁵ reported that the NO oxidation reaction on La_1-xSr_xCoO₃ oxides also strongly depended on the surface oxygen activity, which is defined as the oxygen 2p-band center relative to the Fermi level. Chen et al.¹⁶ tuned the oxygen activity in perovskite ferrite by Co doping, leading to a change in hydrogen oxidation reaction performance at elevated temperatures. All these results demonstrate that modulating lattice oxygen activity is an effective method for improving the activity of transition metal oxide or (oxy)hydroxide electrocatalysts.

OER is regarded as the main bottleneck in many electrochemical energy devices due to its sluggish reaction kinetics⁹. Because of the highly oxidative environment, OER catalysts suffer from effects such as spontaneous dissolution and surface reconstruction during operation, which strongly impact the stability of the devices^17,18. These effects in certain cases were also reported to promote the OER activities^19,20,21,22. For instance, several research groups reported highly active OER catalysts with perovskite oxide^8,23, nitride²⁴, or phoshide²⁵ as the core materials, and with self-reconstructed amorphous phase or (oxy)hydroxides as the active phase on the surface^18,19,26. Jiang et al.²⁷ demonstrated that the leaching of lattice Cl⁻ from cobalt oxychloride (Co₂(OH)₃Cl) during the OER process could trigger the atomic-level unsaturated sites and efficiently boost catalytic activity. While all the pioneering works mentioned above have demonstrated the self-reconstruction or material leaching effects during operation as an effective way to achieve highly active catalysts, the impacts of pre-catalysts on the activity of final catalysts are still lack of investigation.

In this work, we report a sacrificial template-directed approach to synthesize ultrathin NiFe-based (oxy)hydroxide with modulated lattice oxygen activity as highly efficient and stable OER catalysts. MoS₂ nanosheets are used as sacrificial templates to adsorb metal cations to form self-assembled NiFe (oxy)hydroxide on the surface. After removing the MoS₂ sacrificial template by Mo leaching under the OER condition, ultra-thin NiFe (oxy)hydroxides with Mo doping (MoNiFe (oxy)hydroxide) are obtained. The MoNiFe (oxy)hydroxide displays enhanced OER performance, with a high mass activity of 1910 A/g_metal at the overpotential of 300 mV, which is about 60 times higher than that of NiFe (oxy)hydroxide. The combination of synchrotron-based soft X-ray absorption spectroscopy (sXAS), X-ray photoelectron spectroscopy (XPS), in-situ Raman spectroscopy, and density functional theory (DFT) calculation results suggest that the lattice oxygen activity of NiFe (oxy)hydroxide is effectively modulated by Mo doping, resulting in the shift of reaction pathway and a significantly improved intrinsic OER activity. The approach used in this work can be easily adapted for synthesizing (oxy)hydroxide with controlled oxygen activity for other reactions such as H₂O₂ generation and biomass conversion in energy and environmental applications.

Results

Synthesis and characterization of catalysts

Ultrathin NiFe (oxy)hydroxide with Mo doping was synthesized using a sacrificial template-directed synthesis approach, as shown in Fig. 1a. First, MoS₂ nanosheets were grown on carbon cloth substrates by a hydrothermal reaction (Fig. 1b), which were then used as the template to physically adsorb Ni and Fe ions from the solution. After drying the material in air, we obtained a hetero-structured pre-catalyst, which consists of an ultra-thin layer of NiFe layered double hydroxide (LDH) coated on 1 T phase MoS₂ nanosheets (Supplementary Figs. 1, 2). The ratio of Ni and Fe in the LDH can be easily controlled by varying the ion ratio in the solution. The as-synthesized MoS₂/NiFe LDH pre-catalysts were then subjected to cyclic voltammetry (CV) to remove the MoS₂ sacrificial template through the Mo leaching process.

**Fig. 1: Preparation and characterizations of the MoNiFe (oxy)hydroxide.**

The final structure we obtained were ultra-thin (Ni_1-xFe_x) (oxy)hydroxides with Mo doping (MoNiFe-x% (oxy)hydroxide, x = 0%, 5%, 27%, 50%, 85%, 100%) on the carbon fiber substrates. Fig. 1c–h show the characterization results for a representative MoNiFe-27% sample with Ni: Fe = 73: 27. The scanning electron microscopy (SEM) image (Fig. 1c) shows that the (oxy)hydroxide layer uniformly covers the carbon fiber. The atomic force microscopy (AFM) results of the (oxy)hydroxide flake prepared by ultrasonic treatment suggest that the active catalyst was ultrathin with an atomic thickness of 0.8 nm (mono-layer, denoted as 1 L) or 1.5 nm (double-layer, denoted as 2 L) (Fig. 1d). Using inductively coupled plasma-optical emission spectrometry (ICP-OES), the Ni, Fe, and Mo contents of the obtained catalyst were determined to be 48.7, 19.4, and 0.11 μg/cm², respectively, indicating a small concentration (0.1%) of Mo doping in MoNiFe-27% (oxy)hydroxide (Supplementary Fig. 3). The presence of Mo dopant in NiFe (oxy)hydroxide was further confirmed by the aberration-corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) (Supplementary Fig. 4). Because of the low loading mass of MoNiFe (oxy)hydroxide, we could not determine the crystal structure of MoNiFe-27% (oxy)hydroxide using X-ray diffraction measurement (Supplementary Fig. 5). We relied on the transmission electron microscopy (TEM) measurement to confirm the formation of (oxy)hydroxide phase. Figure 1e, f show the TEM images of MoNiFe-27% (oxy)hydroxide flakes. The spacing between two adjacent lattice planes is quantified to be 0.2 nm (Fig. 1f), which is assigned to the (105) plane of oxyhydroxide. The selected area electron diffraction (SAED) pattern of the (oxy)hydroxide in Fig. 1g exhibits clear diffraction rings of (101), (110) plane for Ni-based hydroxide (PDF-#14-0117) and (105), (006) plane for Ni-based oxyhydroxide (PDF-#06-0075). The energy dispersive spectroscopy (EDS) mapping revealed the uniform distribution of O, Fe, Ni, and Mo elements in MoNiFe (oxy)hydroxide (Fig. 1h). MoNiFe-x% (oxy)hydroxide samples with other Fe contents exhibited very similar structure characteristics (Supplementary Fig. 6). All the MoNiFe-x% exhibited similar Mo content (Supplementary Fig. 7).

Oxygen evolution reaction performance evaluation

Having confirmed the successful synthesis of MoNiFe (oxy)hydroxide by using MoS₂ nanosheets as sacrificial templates and Mo sources, the OER activities of the obtained catalysts were further evaluated. To reveal the role of Mo dopant, pure NiFe (oxy)hydroxides with various Fe contents were synthesized for comparison, which were denoted as NiFe-x% (x = 0%, 5%, 27%, 50%, 85%, 100%).

We systematically evaluated the OER activity of MoNiFe (oxy)hydroxide and NiFe (oxy)hydroxide reference samples using a typical three-electrode configuration in 1.0 M KOH at room temperature. The OER activity of MoNiFe (oxy)hydroxide and NiFe (oxy)hydroxide exhibited similar dependence on the Fe contents. As shown in Supplementary Fig. 8-9, the current density firstly increased with Fe contents, but declined after Fe contents further increased. The optimal Fe content was 27% for both MoNiFe and NiFe (oxy)hydroxide samples. For the samples with the same Fe content, the MoNiFe (oxy)hydroxide exhibited a noticeable higher OER activity than the NiFe (oxy)hydroxide reference samples. For clarity, we show the cyclic voltammetry (CV) polarization curves for the Ni, MoNi, Fe, MoFe, NiFe-27%, and MoNiFe-27% (oxy)hydroxides in Fig. 2a. The MoNiFe-27% (oxy)hydroxide delivered an overpotential of 242 mV at the current density of 10 mA/cm², which was much lower than the NiFe-27% did (306 mV). To reach a current density of 100 mA/cm², the MoNiFe-27% (oxy)hydroxide required only 290 mV overpotential. Electrochemical impedance spectra (EIS) and Tafel curves were also measured to evaluate the OER kinetics. The semicircle of EIS curves for the MoNiFe (oxy)hydroxide samples was much smaller than that of NiFe (oxy)hydroxide with the same Fe content (Supplementary Fig. 10), indicating a smaller charge transfer resistance after Mo doping. The Tafel slope for the MoNiFe-27% (oxy)hydroxide was 23 mV/dec, which was the smallest among all samples (Fig. 2c), indicating its fastest OER kinetics.

**Fig. 2: OER performance of the NiFe and MoNiFe (oxy)hydroxides.**

We further quantified the mass activity of the electrocatalysts by normalizing the CV curves using loading mass obtained from ICP-OES results (Supplementary Fig. 11). Consistently, the MoNiFe (oxy)hydroxide showed significantly higher mass activity than that for the NiFe (oxy)hydroxide. Particularly, the MoNiFe-27% (oxy)hydroxide exhibited the highest mass activity among all samples, with a current density of 1910 A/g at the overpotential of 300 mV. Such high mass activity of MoNiFe-27% (oxy)hydroxide is about 60 times higher than that of NiFe-27% (oxy)hydroxide (Fig. 2b). In addition, MoNiFe-27% (oxy)hydroxide also delivered a noticeable lower overpotential and higher mass activity than the benchmark RuO₂ and IrO₂ catalysts (Supplementary Fig. 12, note 1). The high mass activity of MoNiFe (oxy)hydroxide is attributed to the following two reasons: first, the ultra-thin nature of the catalyst layer enables the full exposure of active sites and strongly facilitates the charge transfer process between the conductive substrate and the catalyst; secondly, as will be shown in the later section, Mo doping can effectively modulate the lattice oxygen activity of NiFe (oxy)hydroxide, leading to the strongly enhanced intrinsic OER activity.

The stability of MoNiFe (oxy)hydroxide was evaluated by chronopotentiometry (CP) tests at the current density of 10 mA/cm². As shown in Fig. 2d, the MoNiFe-27% (oxy)hydroxide displayed significantly better stability than the commercial RuO₂ catalyst. The inset figure in Fig. 2d shows the CV curves of MoNiFe (oxy)hydroxide and RuO₂ before and after CP measurement. The decline of OER performance for MoNiFe-27% (oxy)hydroxide was much smaller than that of RuO₂. The structure and composition of MoNiFe (oxy)hydroxide catalyst remain unchanged after the long-time operation, as revealed by SEM, TEM, EDS, XPS, and ICP-OES characterizations (Supplementary Fig. 13-17, note 2). It is reported that the OER stability of (oxy)hydroxide is strongly dependent on its structural charcteristics^28,29. Chen et al.²⁸ reported that the slow diffusion of proton acceptors within interlayer in NiFe hydroxide could lead to a local acidic environment. This can be one primary reason for the performance degradation of multilayer NiFe hydroxide due to the local etching process. The ultra-thin nature of our MoNiFe (oxy)hydroxide can effectively prevent such local etching, and therefore is beneficial for the catalyst to remain stable during operation in alkaline solution.

Elucidation of the OER mechanism

To reveal the mechanism of the high intrinsic OER activity of MoNiFe (oxy)hydroxides, isotope-labeling experiments and DFT calculations were carried out on NiFe-27% and MoNiFe-27% (oxy)hydroxide. For simplicity, the NiFe-27% and MoNiFe-27% (oxy)hydroxide will be referred to as NiFe and MoNiFe (oxy)hydroxide in the following context.

As mentioned above, the OER on NiFe-based (oxy)hydroxides was reported to follow the LOM, in which lattice oxygen directly participates in the OER reactions^14,30. To validate the participation of lattice oxygen in OER for our material systems, the ¹⁸O isotope-labeling experiments were carried out using the procedure described in the experimental section. In-situ differential electrochemical mass spectrometry (DEMS) measurements results on the ¹⁸O-labeled NiFe and MoNiFe (oxy)hydroxide showed the signals of m/z = 32, m/z = 34, and m/z = 36 (Supplementary Fig. 18), suggesting the presence of ¹⁶O₂, ¹⁶O¹⁸O, and ¹⁸O₂ in the gas production^31,32,33. This result implies that both NiFe and MoNiFe (oxy)hydroxide follow the LOM mechanism^14,30. The mass spectrometric cyclic voltammograms (MSCVs) which plot the real-time gas product contents as a function of applied potential can provide direct comparison about the participation of lattice oxygen in OER process. The ¹⁸O-labeled MoNiFe (oxy)hydroxide is with noticeably higher contents of ¹⁶O¹⁸O and ¹⁸O₂ in the reaction product than the ¹⁸O-labeled NiFe (oxy)hydroxide (Fig. 3a–d and Supplementary Fig. 19-20), implying the lattice oxygen of MoNiFe (oxy)hydroxide participated more actively into the OER reaction than that of NiFe (oxy)hydroxide.

**Fig. 3: Evidence of lattice oxygen participating in OER provided by ¹⁸O isotope-labeling experiments.**

In addition to the DEMS measurement, the quasi in-situ Raman spectra were also used to confirm the participation of lattice oxygen in OER. The samples were first activated in electrolyte with ¹⁸O to form ¹⁸O-NiOOH species, and then were subjected to a positive potential (1.65 V vs. RHE) in electrolyte with H₂¹⁶O. The Raman peaks of the samples activated in electrolyte with ¹⁸O (named as ¹⁸O-labelled sample) shifted to lower wavenumber comparing to that of the samples activated in electrolyte with ¹⁶O (named as ¹⁶O-labelled sample), because of the impact of oxygen mass on the vibration mode^2,34(Fig. 3e, f). This result suggests that we successfully labelled both NiFe and MoNiFe samples with ¹⁸O. Then, the ¹⁸O-labelled (oxy)hydroxides were placed in electrolyte with ¹⁶O and were treated by applying a positive potential of 1.65 V (vs. RHE) for different periods of time (1 min to 20 min). The Raman spectra of the obtained samples are shown in Fig. 3e, f. The Raman peak of ¹⁸O-labelled MoNiFe (oxy)hydroxide shifts back to the position for ¹⁶O-labelled MoNiFe (oxy)hydroxide within 1 min of treatment, which is much faster than that for the NiFe (oxy)hydroxide (20 min). This result suggests that while both samples follow the LOM mechanism, the MoNiFe (oxy)hydroxide exhibits much higher rate of oxygen exchange between lattice oxygen and electrolyte. The Raman spectra and DEMS results on the ¹⁸O-labeled samples consistently suggest that Mo doping in NiFe (oxy)hydroxide effectively promots the lattice oxygen to participate in the OER reaction.

In additional to ¹⁸O isotope-labeling experiments, DFT calculations were also carried out to identify the OER mechanism on NiFe and MoNiFe (oxy)hydroxide. Both adsorbate evolution mechanism (AEM) pathway (Supplementary Fig. 21a) and LOM pathway (Fig. 4a) of OER were considered. In the AEM pathway, the Fe sites were found to be the active sites with lower barriers than Ni sites (Supplementary Fig. 21b, c). The deprotonation of *OH in AEM pathway serves as the potential determining step (PDS) for both NiFe and MoNiFe (oxy)hydroxide, with a barrier of 1.05 eV and 0.76 eV, respectively. In the LOM pathway, the (oxy)hydroxides first go through the deprotonation process to form oxyhydroxide (step 1) (Fig. 4a). The exposed lattice oxygen then receives OH⁻ via nucleophilic attack to form *OOH (step 2). After the deprotonation of *OOH (step 3), gaseous O₂ releases from the lattice, and an oxygen vacancy is generated on the surface (step 4). The resulting oxygen vacancy sites are refilled by OH⁻ and the surface is recovered (step 5). The calculated Gibbs free energy diagrams of OER on NiFe and MoNiFe (oxy)hydroxide are displayed in Fig. 4b. For the NiFe (oxy)hydroxide, the desorption of O₂, which was accompanied by the formation of oxygen vacancy, was found to be the PDS with a high energy barrier of 0.75 eV. In contrast, the barrier of oxygen vacancy formation became much smaller after Mo doping, which pushed the PDS on MoNiFe (oxy)hydroxide to the deprotonation of *OOH with a decreased energy barrier of 0.42 eV. It is noted that both the barriers of PDS for NiFe and MoNiFe (oxy)hydroxide in LOM pathway were much lower than that in AEM pathway, suggesting that both the NiFe and MoNiFe (oxy)hydroxide follow the LOM mechanism^14,30. This result is consistent with the results of the ¹⁸O isotope-labeling experiments. The changes in mechanism and PDS derived from Mo doping are quite different from other cation doping reported by previous works^35,36,37.

**Fig. 4: OER mechanism revealed by DFT calculation, pH dependence, and deuterium isotopic labeling experiments.**

The DFT results suggest that Mo doping shifts the PDS from oxygen vacancy formation to the deprotonation of *OOH. Such a transition of PDS from the one involving only lattice oxygen to the one involving surface proton transfer might result in enhanced dependence of the OER activity on the proton activity in solution. Therefore, to confirm such a shift of PDS by Mo doping as revealed by calculations, we evaluated the dependence of OER activity of NiFe and MoNiFe(oxy)hydroxide on proton activity by carrying out pH dependence measurements and deuterium isotopic labeling experiments.

The OER activities of NiFe and MoNiFe (oxy)hydroxide were assessed at different pH conditions (pH = 11.78, 12.75, 12.91, and 13.65) (Fig. 4c,d). Fig. 4d shows the OER current density at 1.52 V (vs. RHE) in log scale as a function of pH, from which the proton reaction orders on RHE scale (ρ^RHE = ∂logi/∂pH) are calculated to be 0.38 and 0.87 for NiFe and MoNiFe (oxy)hydroxide, respectively. The higher ρ^RHE for MoNiFe (oxy)hydroxide implied a stronger pH-dependent OER activity, which might be due to the higher degree of decoupled proton-electron transfer during the PDS step, i.e., the deprotonation of *OOH. (Supplementary note 3, Supplementary Fig. 22-23)^8,9,38. To further prove the impact of proton activity, the OER activity of NiFe and MoNiFe (oxy)hydroxide were also evaluated in the NaOD and NaOH solution. The LSV curves for NiFe and MoNiFe (oxy)hydroxide measured in 1 M NaOH (dissolved in H₂O) and NaOD (dissolved in D₂O) solution are shown in Fig. 4e. To show the kinetic isotope effect (KIE) for NiFe and MoNiFe (oxy)hydroxide clearly, the ratio of current density obtained in NaOH and in NaOD at the given potential^39,40 is plotted in Fig. 4f. MoNiFe (oxy)hydroxide exhibited a noticeably larger KIE value in comparison to NiFe (oxy)hydroxide, suggesting a severe degradation of OER activity in NaOD. This result suggests that proton transfer has a greater impact on the OER process on MoNiFe (oxy)hydroxide than that on NiFe (oxy)hydroxide. The deuterium isotopic experiments performed in NaOH/NaOD with a different concentration of 0.5 M provided consistent results (Supplementary Fig. 24). The large isotopic effect of MoNiFe (oxy)hydroxide suggests that the proton transfer is involved in the PDS. This conclusion is in accord with the DFT calculation results, which show that the PDS step of OER on MoNiFe (oxy)hydroxide is the deprotonation of *OOH (Fig. 4b).

Analysis of lattice oxygen activity

Further insight into the underlying reason for the shift of the reaction pathway was deduced by analyzing the oxygen activity of the NiFe and MoNiFe (oxy)hydroxide based on DFT calculations of electronic structures.

The oxygen activity can be represented by the metal-oxygen bond strength, which was evaluated by calculating the crystal orbital Hamilton populations (COHP) by DFT (Fig. 5a and Supplementary Fig. 25)^26,41. The negative and positive values of -COHP correspond to the anti-bonding (grey area) and bonding state (white area), respectively. The occupied anti-bonding states of Ni and Fe 3d band appeared under Fermi level for both NiFe and MoNiFe (oxy)hydroxide (-COHP peak whose energy level is higher than −2.5 eV, Fig. 5a and Supplementary Fig. 25). Owing to the upshift of the O 2p band, MoNiFe (oxy)hydroxide shows a more significant overlap between the O 2p band and Ni 3d and Fe 3d anti-bonding state under Fermi level than NiFe (oxy)hydroxide. To quantify the metal-oxygen bond strength, the integral of -COHP up to the Fermi level (-IpCOHP_Fermi) of Ni-O (Fe-O) bonding were determined to be 2.41 (1.78) and 1.19 (1.34) for NiFe and MoNiFe (oxy)hydroxide, respectively (Fig. 5b). The lower value of -IpCOHP_Fermi for the MoNiFe indicates that Mo doping results in more electrons filled into the anti-bonding orbitals, leading to the weaker metal-oxygen bond. Such a weakened metal-oxygen bond can facilitate oxygen vacancy formation.

**Fig. 5: Lattice oxygen activity determined by density functional theory (DFT) calculations.**

Secondly, we calculated the O 2p band center (ε_O-2p) position for both NiFe and MoNiFe (oxy)hydroxide. The density of state (DOS) profile of NiFe and MoNiFe (oxy)hydroxide are shown in Fig. 5c. The O 2p band center is determined to be −1.58 eV and −1.40 eV for NiFe and MoNiFe (oxy)hydroxide, respectively (Fig.5c). The O 2p band noticeably shiftes toward the Fermi level after Mo doping into NiFe (oxy)hydroxide. The distance between the O 2p band center to the Fermi level has been frequently employed as a descriptor for oxygen activity^15,42,43. It is reported that O 2p-band center is required to be high enough to guarantee the lattice oxygen to escape from the lattice⁴³. The upshift of the O 2p band results in deeper penetration of Fermi level into the O 2p band, which further facilitates the electron flow away from oxygen sites when an anodic potential is applied, making the lattice oxygen release from the lattice more easily^3,12,43. As a consequence, oxygen with high O 2p band position exhibites facilitated oxygen vacancy formation process and thus promotes the LOM mechanism⁴².

In addition to the O 2p band position, the Mott-Hubbard splitting in d-orbitals was also investigated. For late transition metals, d-orbitals can further split into electron-filled lower Hubbard band (LHB) and empty upper Hubbard band (UHB) due to the strong d-d Coulomb interaction^3,14. The LHB/UHB center is determined by the total metal 3d-orbital distribution below/above E_Fermi in DOS diagrams. The specific positions of LHB and UHB were calculated to be −4.36 eV and 2.01 eV for NiFe (oxy)hydroxide, and −4.67 eV and 2.90 eV for MoNiFe (oxy)hydroxide, respectively. The energy distance between the LHB and UHB band center (U) is also an important parameter governing the lattice oxygen activity^3,14. The U values of NiFe and MoNiFe (oxy)hydroxide were calculated to be 6.38 eV and 7.58 eV, respectively, indicating a stronger d-d Coulomb interaction after Mo doping. Such an enlarged U value gives rise to the downshift of LHB (Fig. 5d). As a result, as anodic potential is applied, the electron removal from oxygen sites is strongly facilitated^11,14. It is noted that the LHB center is located beneath the O 2p band center. Therefore, the downshift of LHB center and upshift of O 2p band center for MoNiFe (oxy)hydroxide leads to a smaller overlap of metal 3d-orbital and oxygen 2p-orbital, which results in the weaker metal-oxygen bond. In addition, the density of states of metal 3d-orbital, especially for Ni 3d-orbital, upshift close to Fermi level. Although such upshift lead to an increased overlap between Ni 3d-orbital and O 2p-orbital in DOS diagrams, the overlap of O 2p - Ni 3d orbital occurs on the anti-bonding states below Fermi level as highlighted in dash circles in Fig. 5c and results in a weaker Ni-O bond, which is consistent with the COHP calculations (Fig. 5b).

The weakened metal-oxygen bond, the upshifted O 2p band relative to Fermi level, and the enlarged U value in the MoNiFe (oxy)hydroxide in comparison with the NiFe (oxy)hydroxide indicated that Mo doping effectively activated the lattice oxygen, thereby promoting the oxygen vacancy formation process^15,42,43. To further confirm such impact of Mo doping, we directly calculated the oxygen vacancy formation energy (E_{f_vac}) using DFT. The E_{f_vac} of MoNiFe (oxy)hydroxide was determined to be 0.56 eV, which is much lower than the 1.51 eV for the NiFe (oxy)hydroxide, as shown in Fig. 5e. This result was also confirmed by the higher content of defective oxygen in MoNiFe (oxy)hydroxide by O 1 s XPS anlysis (Supplementary Fig. 26, note 4). Our DFT calculation further shows that the LOM pathway is still dominant for both NiFe and MoNiFe (oxy)hydroxide when there is oxygen vacancy presence on the surface (Supplementary Fig. 27-30, note 5).

The DFT results above demonstrated that Mo doping in NiFe (oxy)hydroxide effectively enhanced the oxygen activity. In the following section, we further compare the lattice oxygen activity of NiFe and MoNiFe (oxy)hydroxide experimentally by probing the local density of states around the oxygen ligands, the metal oxidation state, and cationic electrochemical redox process using advanced spectroscopy techniques, including synchrotron-based sXAS, XPS, in-situ Raman spectroscopy.

First, the variation of the local density of states around the oxygen ligands of NiFe and MoNiFe (oxy)hydroxide was detected by carrying out O K-edge sXAS measurement with total electron yield (TEY) mode. The O K-edge sXAS spectra consist of two characteristic peaks at ~533.5 eV and ~540 eV, which were assigned to the O 2p - metal 3d hybridization and the O 2p - metal 4sp hybridization^44,45. As shown in Fig. 6a, the intensity of O K-edge decreases after Mo doping, indicating a decrease in unoccupied density of states⁴⁶ and a weakening of 3d/4sp−2p hybridization^44,47. Such decreased intensity in O K-edge spectra, accompanying with the increased intensity of Ni L-edge and Fe L-edge peak for MoNiFe (oxy)hydroxide (Fig. 6b, c), suggests a higher electron density at the O site and a lower electron density at the Ni/Fe sites, a higher ionic metal-oxygen bond^46,48. This result is consistent with the weaker metal-oxygen bond after Mo doping, as revealed by the COHP calculation (Fig. 5b). In addition, the increased electron density on oxygen sites in MoNiFe (oxy)hydroxide might promote the donation of electrons from oxygen as an anodic potential was applied. The O K-edge sXAS result suggests that the Mo doping effectively increased the local density of states around the oxygen ligands, which can potentially give rise to the lattice oxygen activation.

**Fig. 6: Lattice oxygen activity determined by advanced spectroscopy techniques.**

There have been many previous works that demonstrated that the oxygen activity could be probed indirectly by characterizing the metal oxidation state, and a higher oxidation state of the transition metal was normally correlated to an increased oxygen activity^7,14,49. For example, Grimaud et al.⁴⁹ reported that the hybridization of Co-O bonds in perovskites cobaltite increased with cobalt oxidation state, which was correlated to the upshift of O p-band center relative to Fermi level. Mefford et al.⁷ also showed that the d-orbitals of cobalt have a greater overlap with the s, p orbitals of oxygen as the cobalt oxidation state increased, leading to the promoted lattice oxygen activity of perovskite cobaltite. Zhang et al.¹⁴ reported that the formation of Ni⁴⁺ species in Ni-based (oxy)hydroxide can drive holes into oxygen ligands to trigger lattice oxygen activation. In addition, an enlarged U value, a descriptor for enhanced oxygen activity, was reported to be related to the increased valence state of the metal^11,50. Inspired by these pioneering works, we compared the change of oxidation states of Ni and Fe in (oxy)hydroxide after Mo doping.

The Ni L-edge sXAS spectra of NiFe and MoNiFe (oxy)hydroxide are shown in Fig. 6b. The MoNiFe (oxy)hydroxide exhibited higher intensity than that of NiFe (oxy)hydroxide. In addition to intensity changes, the Ni L-edge spectra of MoNiFe (oxy)hydroxide shifted to higher photon energy relative to NiFe (oxy)hydroxide (Supplementary Fig. 31). Both the higher intensity and positive shift of Ni L-edge peak for MoNiFe (oxy)hydroxide than NiFe (oxy)hydroxide suggest an increased number of unoccupied density of states on Ni sites^51,52. Similarly, the Fe L-edge spectra of MoNiFe (oxy)hydroxide exhibited higher intensity than that of the NiFe (oxy)hydroxide, indicating an increased unoccupied density of states on Fe sites (Fig. 6c). The changes in Ni/Fe L-edge spectra derived from the partial electron transfer from Ni/Fe sites to the Mo sites through bridging oxygen (μ-O) in Ni-O-Mo-O-Fe moiety in MoNiFe (oxy)hydroxide (Supplementary Fig. 32, note 6)³⁶, leading to the electron depletion in metal sites and the increment of metal oxidation state. In addition to the sXAS results, the XPS analysis (Fig. 6d, Supplementary Fig. 33-34) provided the same conclusion of higher metal oxidation state in the MoNiFe (oxy)hydroxide with detailed discussion in Supplementary note 7.

As mentioned above, the upshifted O 2p band and enlarged U value in MoNiFe (oxy)hydroxide would promote the lattice oxygen redox chemistry as an anodic potential is applied. Because of the competition of electron donation from oxygen anion and metal cations redox process, the enhanced oxygen reactivity should be reflected on the delayed cationic electrochemical redox process (Supplementary Fig. 38, note 8)³. As shown in Fig. 6e, the Ni²⁺/Ni³⁺ redox peak for MoNiFe (oxy)hydroxide (1.450 V vs. RHE) shifted positively compared to that of NiFe (oxy)hydroxide (1.414 V vs. RHE), indicating that Ni in (oxy)hydroxide required higher positive potential to oxidize after Mo doping. We further carried out in-situ Raman spectroscopy to confirm such a change of Ni²⁺/Ni³⁺ electrochemical redox during OER process (Fig. 6f). Two characteristic peaks of Ni³⁺-O were found on the Raman spectra at 476 and 557 cm⁻¹ when a sufficiently high positive potential was applied. These two peaks corresponded to the E_g bending vibration (δ(Ni-O)) and A_1g stretching vibration (ν(Ni-O)) mode in γ-NiOOH, respectively². The emergence of the Raman peaks of Ni³⁺-O occurred at 0.44 V and 0.48 V (vs. Hg/HgO) for NiFe and MoNiFe (oxy)hydroxide, respectively. These results suggest that the nickel redox process gets delayed after Mo doping due to the facilitated lattice oxygen oxidation, which is consistent with DFT calculation results.

All the DFT calculations and experimental results above consistently suggest that the lattice oxygen activity was strongly enhanced by Mo doping, leading to the facilitated oxygen vacancies formation. Consistently, we observed the reaction barrier of oxygen vacancy formation for the MoNiFe (oxy)hydroxide to be much smaller than that for the NiFe (oxy)hydroxide (Fig. 4b). Consequently, the PDS transforms from oxygen vacancy formation for the NiFe (oxy)hydroxide to the *OOH deprotonation for the MoNiFe (oxy)hydroxide. These results provide critical insight into the role of lattice oxygen in determining the electrocatalytic activity of transition metal (oxy)hydroxide.

Overall water splitting performance

To demonstrate the practical application of MoNiFe (oxy)hydroxide for electrochemical production of hydrogen, a two-electrode electrolytic cell was constructed using MoS₂/NiFe LDH pre-catalyst as both anode and cathode for overall water splitting. During the water splitting process, the anode was transformed from the MoS₂/NiFe LDH pre-catalyst into MoNiFe (oxy)hydroxide, while the MoS₂/NiFe LDH cathode remained unchanged (Supplementary Fig. 39). The final cell structure was denoted as MoS₂/NiFe LDH | MoNiFe in the following context. A reference cell with commercial noble-metal-based catalysts Pt/C and RuO₂ as cathode and anode, respectively, was also tested for comparison (denoted as Pt/C | RuO₂). The polarization curves of the cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes and the reference cell with Pt/C | RuO₂ coupled electrodes for overall water splitting in 1 M KOH electrolyte were shown in Fig. 7a. The electrolytic cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes presented higher overpotential at low current density than the cell with Pt/C | RuO₂ coupled electrodes. Nevertheless, the cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes exhibited a noticeable better performance at high current density. To reach a current density of 100 mA/cm², the cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes only required a voltage of 1.728 V, which was significantly lower than the reference cell with Pt/C | RuO₂ coupled electrodes (1.755 V).

To achieve sufficient hydrogen production rate, the electrolytic cell needs to operate at a high current density. Therefore, the stability of the electrolytic cell was evaluated at a current density of 10 mA/cm² and 100 mA/cm². At a current density of 10 mA/cm², both the cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes and the one with Pt/C | RuO₂ coupled electrodes displayed excellent stability (Supplementary Fig. 40). At a current density of 100 mA/cm², the cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes remained stable during operation, while the cell with Pt/C | RuO₂ coupled electrodes degraded rapidly (Fig. 7b).

Finally, we compared the cell voltage for MoS₂/NiFe LDH | MoNiFe at a high current density of 100 mA/cm² with recently reported noble-metal-free electrocatalysts for overall water splitting (Fig. 7c, Supplementary Table 1). As shown in Fig. 7c, our cell performance at high current density (1.728 V at 100 mA/cm²) is competitive among the noble-metal-free electrocatalysts reported in the literature.

Discussion

In this work, a sacrificial template-directed approach was reported to synthesize ultra-thin NiFe-based (oxy)hydroxide with Mo doping as highly efficient and stable OER catalysts. MoS₂ nanosheets grown by hydrothermal approach were used as templates to adsorb metal cations to form self-assembly NiFe (oxy)hydroxide and served as Mo sources for doping. The obtained MoNiFe (oxy)hydroxide exhibited a high mass activity of 1910 A/g_metal at the overpotential of 300 mV, which is 60 times higher than that of the NiFe (oxy)hydroxide. The electrolytic cell with MoS₂/NiFe LDH | MoNiFe coupled electrodes exhibited good activity and stability for the overall water splitting, which required a low voltage of 1.728 V to achieve a current density of 100 mA/cm². DFT calculation suggested that MoNiFe (oxy)hydroxide exhibited higher lattice oxygen activity, which was represented by the weakened metal-oxygen bond, upshifted O 2p center relative to Fermi level, enlarged U values, and lower oxygen vacancy formation energy. Consistently, synchrotron-based sXAS, XPS, and in-situ Raman measurements demonstrated that the MoNiFe (oxy)hydroxide was with higher local density of states around the oxygen ligands, a higher metal oxidation state, and a delayed cationic electrochemical redox process in comparison with the NiFe (oxy)hydroxide. Such activation of lattice oxygen shifted the potential determining step from oxygen vacancy formation for the NiFe (oxy)hydroxide to the *OOH deprotonation for the MoNiFe (oxy)hydroxide, resulting in strongly enhanced intrinsic OER activity. The methodology used in this work can be easily adapted for constructing other transition metal (oxy)hydroxide with modulated oxygen activity for catalyzing other reactions such as biomass electrooxidation reactions. The mechanistic understanding of the role of lattice oxygen in determining surface reactions can guide the rational design of high-performance photo-, thermal-, or electro-catalysts.

Methods

Synthesis of MoNiFe (oxy)hydroxide

MoS₂ nanosheets were grown on carbon cloths by a hydrothermal method with ammonium molybdate tetrahydrate [(NH₄)₆Mo₇O₂₄·4H₂O] and thiourea (CH₄N₂S) as the precursors. The obtained MoS₂ nanosheets were immersed into a mixed solution of nickel acetate and ferrous sulfate to adsorb Fe and Ni ions onto the surface. After drying in air, the MoS₂/NiFe LDH pre-catalysts were constructed. The MoS₂/NiFe LDH pre-catalysts were subjected to cyclic voltammetry activation in 1 M KOH solution to obtain self-reconstruction Mo doping NiFe (oxy)hydroxide through Mo leaching. The NiFe (oxy)hydroxide reference sample was synthesized by a commonly used wet-chemical method. Further detailed information about NiFe and MoNiFe (oxy)hydroxides synthesis can be found in Supplementary note 9, 10.

Characterizations

The morphologies of samples were characterized by high-resolution field emission scanning electron microscopy (SEM) (SU8010, Hitachi, Japan). The chemical composition was detected by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Agilent 730 series) and X-ray photoelectron spectroscopy (XPS) (Escalab250Xi, Thermo Scientific) with Al anode. High-resolution transmission electron microscopy (HRTEM) images and energy dispersive spectroscopy (EDS) were recorded by a JEM-3200FS microscope. In-situ Raman measurements were performed on a confocal microscopic system (LabRAM HR Evolution, Horiba, France) equipped with a semiconductor laser (λ = 532 nm, Laser Quantum Ltd.). The laser was focused using a 50× objective lens and 600 lines/mm grating. The Raman spectra were collected continuously with a step of 2 mV during linear sweep voltammetry measurement with a scanning rate of 0.1 mV/s. Synchrotron-based soft X-ray absorption spectroscopy (sXAS) was carried out at the BL02B02 station in Shanghai Synchrotron Radiation Facility⁵³.

Electrochemical measurements

The electrochemical measurements were performed in a three-electrode system using a CHI-660E electrochemical station. 1 M KOH aqueous solution was used as the electrolyte, and it was bubbled by O₂ for 30 min prior to OER measurements. The catalyst-loaded carbon cloths acted as the working electrode. The reference electrode and counter electrode were a Ag/AgCl electrode prefilled with saturated KCl aqueous solution and a Pt mesh, respectively. All electrode potentials were given versus the reversible hydrogen electrode (vs. RHE) unless otherwise mentioned. The detailed information about the electrochemical measurements can be found in Supplementary note 11.

¹⁸O-labeling experiment

NiFe and MoNiFe (oxy)hydroxides were labeled with ¹⁸O-isotopes by potentiostatic reaction at 1.65 V (vs. Ag/AgCl) for 30 min in KOH solution with H₂¹⁸O. Afterward, the ¹⁸O-labeled catalysts were rinsed with H₂¹⁶O for serval times to remove the remaining H₂¹⁸O.

DEMS measurements

DEMS measurements were carried out using a QAS 100 device (Linglu Instruments, Shanghai). The NiFe or MoNiFe (oxy)hydroxide with ¹⁸O-labeling, a Ag/AgCl electrode prefilled with saturated KCl aqueous solution, and a Pt mesh were used as working electrode, reference electrode, and counter electrode, respectively. CV measurement was performed in KOH solution with H₂¹⁶O with a scan rate of 5 mV/s. In the meantime, gas products with different molecular weights were detected in real time by mass spectroscopy.

Theoretical calculation

Spin-polarized DFT calculations were performed using the Vienna ab initio simulation package (VASP)⁵⁴. The generalized gradient approximation (GGA) of the Perdue-Burke-Ernzerhof (PBE) version⁵⁵ was used to describe the exchange-correlation interactions. The projector-augmented wave (PAW) method is used to model core-valence electron interactions⁵⁶. The COHP of considered atomic pairs was calculated by the Lobster code^57,58,59,60. The detailed information about the DFT calculation can be found in Supplementary note 12.

Data availability

The data that support the findings of this study are available from https://figshare.com/s/489adcc0875ef42536c8. Source data are provided with this paper.

References

Zhao, T. et al. In situ reconstruction of V-doped Ni₂P pre-catalysts with tunable electronic structures for water oxidation. Adv. Funct. Mater. 31, 2100614 (2021).
Article CAS Google Scholar
Lee, S., Bai, L. & Hu, X. Deciphering iron-dependent activity in oxygen evolution catalyzed by nickel-iron layered double hydroxide. Angew. Chem. Int. Ed. Engl. 59, 8072–8077 (2020).
Article CAS PubMed Google Scholar
Zhang, N., Chai, Y. Lattice oxygen redox chemistry in solid-state electrocatalysts for water oxidation. Energy Environ. Sci. 14, 4647–4671 (2021).
Kuznetsov, D. A. et al. Tailoring lattice oxygen binding in ruthenium pyrochlores to enhance oxygen evolution activity. J. Am. Chem. Soc. 142, 7883–7888 (2020).
Article CAS PubMed Google Scholar
Zhu, Y. et al. Boosting oxygen evolution reaction by creating both metal ion and lattice-oxygen active sites in a complex oxide. Adv. Mater. 32, 1905025 (2020).
Article CAS Google Scholar
Grimaud, A. et al. Activating lattice oxygen redox reactions in metal oxides to catalyse oxygen evolution. Nat. Chem. 9, 457–465 (2017).
Article CAS PubMed Google Scholar
Mefford, J. T. et al. Water electrolysis on La_1-xSr_xCoO_3-δ perovskite electrocatalysts. Nat. Commun. 7, 11053 (2016).
Article ADS CAS PubMed PubMed Central Google Scholar
Pan, Y. et al. Direct evidence of boosted oxygen evolution over perovskite by enhanced lattice oxygen participation. Nat. Commun. 11, 2002 (2020).
Article ADS CAS PubMed PubMed Central Google Scholar
Zhu, Y. et al. Oxygen defect engineering in double perovskite oxides for effective water oxidation. J. Mater. Chem. A 8, 10957–10965 (2020).
Article CAS Google Scholar
She, S. et al. Realizing ultrafast oxygen evolution by introducing proton acceptor into perovskites. Adv. Energy Mater. 9, 1900429 (2019).
Article CAS Google Scholar
Huang, Z.-F. et al. Chemical and structural origin of lattice oxygen oxidation in Co-Zn oxyhydroxide oxygen evolution electrocatalysts. Nat. Energy 4, 329–338 (2019).
Article ADS CAS Google Scholar
Wu, T. et al. Iron-facilitated dynamic active-site generation on spinel CoAl₂O₄ with self-termination of surface reconstruction for water oxidation. Nat. Catal. 2, 763–772 (2019).
Article CAS Google Scholar
Huang, Z.-F. et al. Tuning of lattice oxygen reactivity and scaling relation to construct better oxygen evolution electrocatalyst. Nat. Commun. 12, 3992 (2021).
Article ADS CAS PubMed PubMed Central Google Scholar
Zhang, N. et al. Lattice oxygen activation enabled by high-valence metal sites for enhanced water oxidation. Nat. Commun. 11, 4066 (2020).
Article ADS CAS PubMed PubMed Central Google Scholar
Hwang, J. et al. Regulating oxygen activity of perovskites to promote NO_x oxidation and reduction kinetics. Nat. Catal. 4, 663–673 (2021).
Article CAS Google Scholar
Chen, H., et al. Activating lattice oxygen in perovskite oxide by B site cation doping for modulated stability and activity at elevated temperatures. Adv. Sci. 8, 2102713 (2021).
Xu, Z. J. Transition metal oxides for water oxidation: All about oxyhydroxides? Sci. China Mater. 63, 3–7 (2019).
Article CAS Google Scholar
Jin, S. Are metal chalcogenides, nitrides, and phosphides oxygen evolution catalysts or bifunctional catalysts? ACS Energy Lett. 2, 1937–1938 (2017).
Article CAS Google Scholar
Liu, J. & Guo, L. In situ self-reconstruction inducing amorphous species: A key to electrocatalysis. Matter 4, 2850–2873 (2021).
Article CAS Google Scholar
Liu, X. et al. Comprehensive understandings into complete reconstruction of precatalysts: synthesis, applications, and characterizations. Adv. Mater. 33, 2007344 (2021).
Article CAS Google Scholar
Wang, Y. et al. Anion etching for accessing rapid and deep self-reconstruction of precatalysts for water oxidation. Matter 3, 2124–2137 (2020).
Article Google Scholar
Wang, J. et al. Redirecting dynamic surface restructuring of a layered transition metal oxide catalyst for superior water oxidation. Nat. Catal. 4, 212–222 (2021).
Article CAS Google Scholar
Lopes, P. P. et al. Dynamically stable active sites from surface evolution of perovskite materials during the oxygen evolution reaction. J. Am. Chem. Soc. 143, 2741–2750 (2021).
Article CAS PubMed Google Scholar
Chen, P. et al. Metallic Co₄N porous nanowire arrays activated by surface oxidation as electrocatalysts for the oxygen evolution reaction. Angew. Chem. Int. Ed. Engl. 127, 14923–14927 (2015).
Article Google Scholar
Stern, L.-A., Feng, L., Song, F. & Hu, X. Ni₂P as a Janus catalyst for water splitting: the oxygen evolution activity of Ni₂P nanoparticles. Energy Environ. Sci. 8, 2347–2351 (2015).
Article CAS Google Scholar
Zhao, S. et al. Structural transformation of highly active metal-organic framework electrocatalysts during the oxygen evolution reaction. Nat. Energy 5, 881–890 (2020).
Article ADS CAS Google Scholar
Jiang, H. et al. Tracking structural self-reconstruction and identifying true active sites toward cobalt oxychloride precatalyst of oxygen evolution reaction. Adv. Mater. 31, 1805127 (2019).
Article CAS Google Scholar
Chen, R. et al. Layered structure causes bulk NiFe layered double hydroxide unstable in alkaline oxygen evolution reaction. Adv. Mater. 31, 1903909 (2019).
Article CAS Google Scholar
Tripkovic, V., Hansen, H. A. & Vegge, T. From 3D to 2D Co and Ni oxyhydroxide catalysts: Elucidation of the active site and influence of doping on the oxygen evolution activity. ACS Catal. 7, 8558–8571 (2017).
Article CAS Google Scholar
Ferreira de Araujo, J., Dionigi, F., Merzdorf, T., Oh, H. S. & Strasser, P. Evidence of Mars-Van-Krevelen mechanism in the electrochemical oxygen evolution on Ni-based catalysts. Angew. Chem. Int. Ed. Engl. 60, 14981–14988 (2021).
Article CAS PubMed PubMed Central Google Scholar
Roy, C. et al. Impact of nanoparticle size and lattice oxygen on water oxidation on NiFeO_xH_y. Nat. Catal. 1, 820–829 (2018).
Article CAS Google Scholar
Wen, Y. et al. Stabilizing highly active Ru sites by suppressing lattice oxygen participation in acidic water oxidation. J. Am. Chem. Soc. 143, 6482–6490 (2021).
Article CAS PubMed Google Scholar
Shi, Z. et al. Confined Ir single sites with triggered lattice oxygen redox: Toward boosted and sustained water oxidation catalysis. Joule 5, 2164–2176 (2021).
Article CAS Google Scholar
Lee, S., Banjac, K., Lingenfelder, M. & Hu, X. Oxygen Isotope Labeling Experiments Reveal Different Reaction Sites for the Oxygen Evolution Reaction on Nickel and Nickel Iron Oxides. Angew. Chem. Int. Ed. Engl. 58, 10295–10299 (2019).
Article CAS PubMed PubMed Central Google Scholar
Baker, J. G. et al. The role of aluminum in promoting Ni-Fe-OOH electrocatalysts for the oxygen evolution reaction. ACS Appl. Energy Mater. 2, 3488–3499 (2019).
Article CAS Google Scholar
Jiang, J. et al. Atomic-level insight into super-efficient electrocatalytic oxygen evolution on iron and vanadium co-doped nickel (oxy)hydroxide. Nat. Commun. 9, 2885 (2018).
Article ADS CAS PubMed PubMed Central Google Scholar
Li, P. et al. Tuning electronic structure of NiFe layered double hydroxides with vanadium doping toward high efficient electrocatalytic water oxidation. Adv. Energy Mater. 8, 1703341 (2018).
Article CAS Google Scholar
Zhu, Y. et al. Tuning proton-coupled electron transfer by crystal orientation for efficient water oxidization on double perovskite oxides. Nat. Commun. 11, 4299 (2020).
Article ADS CAS PubMed PubMed Central Google Scholar
Bai, L., Lee, S. & Hu, X. Spectroscopic and electrokinetic evidence for a bifunctional mechanism of the oxygen evolution reaction. Angew. Chem. Int. Ed. Engl. 60, 3095–3103 (2021).
Article CAS PubMed Google Scholar
Tse, E. C. M., Hoang, T. T. H., Varnell, J. A. & Gewirth, A. A. Observation of an inverse kinetic isotope effect in oxygen evolution electrochemistry. ACS Catal. 6, 5706–5714 (2016).
Article CAS Google Scholar
Liu, X., Jiao, Y., Zheng, Y., Jaroniec, M. & Qiao, S.-Z. Building up a picture of the electrocatalytic nitrogen reduction activity of transition metal single-atom catalysts. J. Am. Chem. Soc. 141, 9664–9672 (2019).
Article CAS PubMed Google Scholar
Lee, Y.-L., Kleis, J., Rossmeisl, J., Shao-Horn, Y. & Morgan, D. Prediction of solid oxide fuel cell cathode activity with first-principles descriptors. Energy Environ. Sci. 4, 3966–3970 (2011).
Article CAS Google Scholar
Sun, Y. et al. Covalency competition dominates the water oxidation structure-activity relationship on spinel oxides. Nat. Catal. 3, 554–563 (2020).
Article CAS Google Scholar
Yang, J. et al. Surface-confined fabrication of ultrathin nickel cobalt-layered double hydroxide nanosheets for high-performance supercapacitors. Adv. Funct. Mater. 28, 1803272 (2018).
Article CAS Google Scholar
Rojas, T. C. et al. Preparation, characterization and thermal evolution of oxygen passivated nanocrystalline cobalt. J. Mater. Chem. 9, 1011–1017 (1999).
Article CAS Google Scholar
Liang, Y. et al. Co₃O₄ nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat. Mater. 10, 780–786 (2011).
Article ADS CAS PubMed Google Scholar
Kwon, J.-H. et al. Nanoscale spin-state ordering in LaCoO₃ epitaxial thin films. Chem. Mater. 26, 2496–2501 (2014).
Article CAS Google Scholar
Zhou, J. G. et al. Electronic structure of TiO₂ nanotube arrays from X-ray absorption near edge structure studies. J. Mater. Chem. 19, 6804 (2009).
Article CAS Google Scholar
Grimaud, A. et al. Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 4, 2439 (2013).
Article ADS CAS PubMed Google Scholar
Zaanen, J., Sawatzky, G. A. & Allen, J. W. Band gaps and electronic structure of transition-metal compounds. Phys. Rev. Lett. 55, 418–421 (1985).
Article ADS CAS PubMed Google Scholar
Wang, D. et al. In situ X-ray absorption near-edge structure study of advanced NiFe(OH)_x electrocatalyst on carbon paper for water oxidation. J. Phys. Chem. C. 119, 19573–19583 (2015).
Article CAS Google Scholar
Al Samarai, M. et al. Elucidation of structure-activity correlations in a nickel manganese oxide oxygen evolution reaction catalyst by operando Ni L-edge X-ray absorption spectroscopy and 2p3d resonant inelastic X-ray scattering. ACS Appl. Mater. Interfaces 11, 38595–38605 (2019).
Article CAS PubMed Google Scholar
Ren, G. et al. Photon-in/photon-out endstation for studies of energy materials at beamline 02B02 of Shanghai Synchrotron Radiation Facility. Chin. Phys. B 29, 016101 (2020).
Article ADS CAS Google Scholar
Kresse, G. & Furthmuller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 6, 15–50 (1996).
Article CAS Google Scholar
Perdew, J. P., Burke, K. & Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 77, 3865–3868 (1996).
ADS CAS PubMed Google Scholar
Blochl, P. E. Projector augmented-wave method. Phys. Rev. B Condens. Matter 50, 17953–17979 (1994).
Article ADS CAS PubMed Google Scholar
Dronskowski, R. & Blochl, P. E. Crystal orbital Hamilton populations (COHP). Energy-resolved visualization of chemical bonding in solids based on density-functional calculations. J. Phys. Chem. 97, 8617–8624 (1993).
Article CAS Google Scholar
Maintz, S., Deringer, V. L., Tchougreeff, A. L. & Dronskowski, R. Analytic projection from plane-wave and PAW wavefunctions and application to chemical-bonding analysis in solids. J. Comput. Chem. 34, 2557–2567 (2013).
Article CAS PubMed Google Scholar
Maintz, S., Deringer, V. L., Tchougreeff, A. L. & Dronskowski, R. LOBSTER: A tool to extract chemical bonding from plane-wave based DFT. J. Comput. Chem. 37, 1030–1035 (2016).
Article CAS PubMed PubMed Central Google Scholar
Deringer, V. L., Tchougreeff, A. L. & Dronskowski, R. Crystal orbital Hamilton population (COHP) analysis as projected from plane-wave basis sets. J. Phys. Chem. A 115, 5461–5466 (2011).
Article CAS PubMed Google Scholar
Zeng, Y. et al. Construction of hydroxide pn junction for water splitting electrocatalysis. Appl. Catal. B 292, 120160 (2021).
Article CAS Google Scholar
Gultom, N. S., Abdullah, H., Hsu, C.-N. & Kuo, D.-H. Activating nickel iron layer double hydroxide for alkaline hydrogen evolution reaction and overall water splitting by electrodepositing nickel hydroxide. Chem. Eng. J. 419, 129608 (2021).
Article CAS Google Scholar
Wang, Z. et al. Contemporaneous inverse manipulation of the valence configuration to preferred Co²⁺ and Ni³⁺ for enhanced overall water electrocatalysis. Appl. Catal. B 284, 119725 (2021).
Article CAS Google Scholar
Yang, L., Huang, L., Yao, Y. & Jiao, L. In-situ construction of lattice-matching NiP₂/NiSe₂ heterointerfaces with electron redistribution for boosting overall water splitting. Appl. Catal. B 282, 119584 (2021).
Article CAS Google Scholar
Wu, L. et al. Heterogeneous bimetallic phosphide Ni₂P‐Fe₂P as an efficient bifunctional catalyst for water/seawater splitting. Adv. Funct. Mater. 31, 2006484 (2020).
Article CAS Google Scholar
Wang, Z. et al. A hierarchical carbon nanotube forest supported metal phosphide electrode for efficient overall water splitting. J. Mater. Chem. A 9, 1150–1158 (2021).
Article CAS Google Scholar
Shi, Q. et al. High‐performance trifunctional electrocatalysts based on FeCo/Co₂P hybrid nanoparticles for zinc-air battery and self‐powered overall water splitting. Adv. Energy Mater. 10, 1903854 (2020).
Article CAS Google Scholar
Huang, H. et al. Rapid and energy-efficient microwave pyrolysis for high-yield production of highly-active bifunctional electrocatalysts for water splitting. Energy Environ. Sci. 13, 545–553 (2020).
Article CAS Google Scholar
Zhu, Y. et al. Anionic defect engineering of transition metal oxides for oxygen reduction and evolution reactions. J. Mater. Chem. A 7, 5875–5897 (2019).
Article CAS Google Scholar

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (11975102, Y.C.); the State Key Laboratory of Pulp and Paper Engineering (2020C01, Y.C.); the Guangdong Pearl River Talent Program (2017GC010281, Y.C.). The synchrotron experiments were carried out at Beamline 02B of the Shanghai Synchrotron Radiation Facility, which is supported by ME2 project under contract from the National Natural Science Foundation of China (11227902, N.Z.). S. Zhao acknowledges the support from the City University of Hong Kong (No. 9610425, S.Z.). The computational time provided by the Shanghai Supercomputer Center and the CityU Burgundy Supercomputer is highly acknowledged.

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These authors contributed equally: Zuyun He, Jun Zhang.

Authors and Affiliations

School of Environment and Energy, State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong, 510006, China
Zuyun He, Zhiheng Gong & Yan Chen
Department of Mechanical Engineering, City University of Hong Kong, Hong Kong, China
Jun Zhang & Shijun Zhao
Siyuan Laboratory, Guangdong Provincial Engineering Technology Research Centre of Vacuum Coating Technologies and New Energy Materials, Department of Physics, Jinan University, Guangzhou, Guangdong, 510632, China
Hang Lei & Wenjie Mai
State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai, 200050, China
Deng Zhou & Nian Zhang

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Zuyun He
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Jun Zhang
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Contributions

Z.H. and Y.C. conducted the experiments and analyzed the results. J.Z. and S.Z. are responsible for the DFT calculations. Z.H., Z.G., D.Z., N.Z., and Y.C. are responsible for the testing and analysis of XAS. Z.H., H.L., W.M., and Y.C. are responsible for the in-situ Raman measurement and analysis. Z.H., J.Z., S.Z., and Y.C. planned and designed the project and wrote the manuscript.

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Correspondence to Shijun Zhao or Yan Chen.

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He, Z., Zhang, J., Gong, Z. et al. Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis. Nat Commun 13, 2191 (2022). https://doi.org/10.1038/s41467-022-29875-4

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Received: 12 September 2021
Accepted: 04 April 2022
Published: 21 April 2022
DOI: https://doi.org/10.1038/s41467-022-29875-4

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