Piezoelectric activation of dual lattice-oxygen mechanism through OH^− Grotthuss transport in water electrolysis‏

Domain: biology
Published: 2026-03-23
Editorial outcome: minor_revision

Abstract
The realization of multi-energy water oxidation systems is impeded by the challenge of integrating multiple energy inputs. Here, we overcome this limitation via ultrasonic pre-treatment of the electrolyte, which triggers a mechano-electrochemical coupling effect through piezoelectric polarization. This process promotes a Grotthuss-type OH- state that weakens O-H bonds and increases the interfacial OH- concentration, thereby influencing the electrochemical reconstruction of Ni(OH) to NiOOH and modifying water electrolysis pathways. These changes enhance Ni-O covalency and synergistically activate two low-energy water oxidation pathways on NiOOH involving lattice oxygen: one couples lattice oxygen with adsorbed oxygen, while the other facilitates direct lattice oxygen-oxygen coupling. Both routes bypass the high-energy *OOH intermediate typical of the conventional adsorbate evolution mechanism (*OH → *O → *OOH → O ), with the latter also avoiding *O adsorption entirely. Notably, just one minute of ultrasonic stimulation reduces the overpotential by 222 mV at 100 mA cm-2. This pulsed-energy strategy thus offers an efficient and scalable approach to realizing multi-energy-enhanced water splitting.

Introduction
Hydrogen energy, celebrated for its clean combustion, high energy density, and sustainability, emerges as a pivotal solution to global energy and environmental crises1–3. Among its production routes, green hydrogen via renewable-powered water electrolysis is particularly compelling, yet its efficiency is severely plagued by the sluggish kinetics of the oxygen evolution reaction (OER)4–6. The OER’s four-electron transfer process entails substantial energy barriers, manifesting as large overpotentials that diminish overall electrolysis efficiency7. Thus, accelerating electron transfer and lowering reaction barriers remain critical goals for advancing high-performance electrocatalysts.

Catalyst electronic states are widely recognized as the cornerstone governing OER kinetics8,9. Traditional modulation strategies, such as elemental doping10,11 and defect engineering12,13, primarily focus on static optimization of the catalyst's intrinsic response to the applied electric field, often neglecting energy coupling effects. In contrast, dynamic external energy-coupled electrocatalysis offers a promising alternative. This approach leverages ubiquitous energy fields (e.g., thermal, magnetic, photonic) to drive in situ electronic state reconstruction, enhancing OER activity while utilizing readily available energy sources14–17. Several coupling strategies exemplify this concept, such as light-electricity coupling by utilizing semiconductor-liquid junctions18, magnetism-electricity coupling by employing magnetism-driven spin ordering under high-intensity magnetic fields19, heat-electricity coupling by thermal strain20, thermally stimulated spin switching14, thermally suppressed charge disproportionation15, and thermal-stimulated magnetic phase transitions17,21. Crucially, electrochemical energy conversion is fundamentally driven by the chemical potential gradient of electrons, the primary energy carriers. Therefore, efficient energy coupling necessitates that simultaneous applied energy fields act synergistically to alter the system's electronic chemical potential, defining the energy conversion pathway. However, simultaneous multi-energy input remains fundamentally challenged by the inherent difficulty in achieving spatiotemporal control, which typically induces interfacial energy dissipation, thereby degrading cooperative effects and impeding practical integration of multi-energy water oxidation systems.

This work addresses this critical limitation by deploying a discontinuous piezoelectric polarization of electrolyte via 1 min ultrasonic stimulation, which simultaneously disrupt hydrogen-bonding networks to reduce OH- hydration numbers and generate rapid solution microflow that propagates these perturbations throughout the electrolyte. The generated metastable undercoordinated OH⁻ species persist beyond the ultrasonic pulse duration due to the small energy difference between polarized and equilibrium configurations22,23. This fundamental perturbation enables a transition from conventional vehicular diffusion, where hypercoordinated OH⁻ ions migrate with their hydration shells24, to rapid Grotthuss-type OH⁻ transport, facilitated by undercoordinated species22,25,26 (Fig. 1). Importantly, this shift in transport mechanism triggers in situ electronic reconstruction of the catalyst during electrochemical oxidation, activating dual low-energy-barrier lattice oxygen mechanisms on NiOOH. By lowering reaction energy barriers through these synergistic effects, our strategy enables efficient mechano-electrocatalytic water splitting exhibiting a 222 mV decrease in overpotential at 100 mA cm-2 after 1 min piezoelectric polarization of electrolyte. This transient piezoelectric-electrocatalytic coupling strategy enables efficient water splitting via pulsed activation instead of continuous energy input, offering a highly efficient and scalable approach to multi-energy coupled water splitting.

Results
Piezoelectric-driven OH- Grotthuss state
To generate a piezoelectric polarization field, we prepared a porous polyvinylidene fluoride (PVDF)/BaTiO (BTO) 3 composite film on a nylon mesh substrate using a simple impregnation method (Supplementary Figs. 1a-g). This design combines PVDF's flexibility with BTO's high piezoelectricity27 and utilizes a porous architecture to enhance electrolyte contact and OH- modulation. Scanning electron microscopy (SEM) observation reveals a well-defined porous architecture with uniformly distributed micron- and nanoscale pores in the PVDF/BTO film (Fig. 2a, Supplementary Fig. 1f). Energy dispersive X-ray spectroscopy (EDS) mapping confirms the homogeneous dispersion of BTO nanoparticles within the matrix (Supplementary Fig. 1h). Phase composition analysis by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) verify the coexistence of the piezoelectric β-phase PVDF and BTO in the PVDF/BTO film (Supplementary Figs. 1i, j)28,29.

Compared to the dense PVDF film, which shows a piezoelectric current density of -0.36 ± 0.14 μA/cm2, the porous PVDF film exhibits a significantly higher value of -0.23 ± 0.03 mA/cm2. This enhancement underscores the important role of the hierarchical porous structure in promoting effective polarization (Supplementary Fig. 3a-c). Among the composite films, the porous PVDF/BTO film with 20 wt% BTO (denoted PVDF/20BTO) delivers the highest piezoelectric current density of -0.65 ± 0.07 mA/cm2, which can be attributed to the synergistic coupling between the PVDF matrix and BTO fillers (Fig. 2, Supplementary Fig. 3b-d). Further assessment of piezoelectric performance via piezoresponse force microscopy reveals that the 20 wt% BTO composite achieves a d value of 29.9 ± 0.65 pm/V, marking a 39.7% improvement 33 over PVDF (d = 21.4 ± 0.23 pm/V) (Fig. 2c, Supplementary Fig. 3e). To evaluate the polarization effect of the 33 PVDF/BTO/nylon mesh substrate film on the KOH electrolyte, the OER performance on NiOOH was examined. The results indicate that a pore size of 42 μm in the nylon mesh substrate is optimal (Supplementary Fig. 3f), suggesting that the substrate's pore structure influences the piezoelectric polarization within the electrolyte. This optimal pore size represents a balance between maximizing the electrolyte contact area and preserving polarization effects, unlike larger pores (212 μm), which reduce contact area, or smaller pores (17 μm), which may impede the piezoelectric response.

The shift from vehicular to Grotthuss OH- diffusion requires converting hydrated OH- ions from hypercoordinated to undercoordinated states22. To investigate how the PVDF/BTO piezoelectric field induces this structural transition, we analyzed the 1 M KOH electrolyte (pH = 13.98 ± 0.01) using Raman spectroscopy under piezoelectric polarization. As shown in Fig. 2d, the piezoelectric field induces a distinct blue shift in the broad O-H stretching band (2900-3800 cm⁻¹)30, signifying significant hydrogen bond weakening. Deconvolution of the broad O-H stretching band reveals three distinct components corresponding to specific water coordination states31: A strong tetrahedral hydrogen bonding at 3200 cm-1 for fully coordinated H O, partial hydrogen bonding at 3450 cm- 2 1 for partially coordinated H O, and weak hydrogen bonding at 3600 cm-1 for low-coordinated H O. The latter two 2 2 states, partially and low-coordinated H O, represent the undercoordinated species essential for the Grotthuss OH- 2 transport mechanism. Quantitative analysis confirms piezoelectric polarization drives a profound restructuring of the aqueous network (Fig. 2e): the fully coordinated H O population plummets from 58.6% to 27.3%, while partially 2 coordinated H O surges from 27.6% to 57.7%, and low-coordinated H O increases from 13.8% to 15.0%. This marked 2 2 redistribution unambiguously demonstrates piezoelectric-field-induced generation of undercoordinated water configurations, establishing a favorable environment for Grotthuss diffusion. Molecular dynamics (MD) simulations under a mimicking alternating electric field (±15.68 V/mm, 53 kHz) corroborates this finding (Supplementary Fig. 4).

Analysis of the radial distribution function g(r), for the first hydration shell of OH- reveals that the electric field reduces its coordination number (CN). Quantitatively, the CN for the first hydration shell decreases from ~5.5 to ~4.1 under the alternating electric field stimulation, which is determined by integrating g(r) up to the position of its first minimum (R ) following the first peak in the g(r) (Fig. 2f)22. This decrease directly confirms the electric-field- min induced creation of undercoordinated OH- .

We employed a NiOOH electrode to investigate the influence of undercoordinated OH- on the microenvironment at the NiOOH/electrolyte interface. In situ electrochemical Raman spectroscopy (Supplementary Fig. 5) reveals that across all OER potentials, the NiOOH surface consistently exhibits higher proportions of low-coordinated water molecules, as indicated by Raman bands at 3450 cm-1 and 3600 cm-1, compared to the non-polarized KOH electrolyte. Complementary in situ ATR-SEIRAS measurements (Figs. 2g and h) show a significant enhancement in the intensity of the characteristic peak at 3660 cm-1, assigned to the O-H stretching vibration of OH-, directly confirming an increased surface concentration of OH-. The kinetic advantages of this optimized interfacial microenvironment were further demonstrated by methanol oxidation reaction (MOR) experiments (Fig. 2i). Since MOR competitively consumes adsorbed *OH intermediates on the catalyst surface, the MOR current density is proportional to the surface coverage of *OH32. The charge associated with methanol oxidation, quantified by the integral charge difference between linear sweep voltammetry (LSV) curves in KOH with and without methanol, was substantially larger in the polarized electrolyte (integral charge difference: 32.4 C) than in the non-polarized electrolyte (integral charge difference: 14.9 C). This indicates a higher surface coverage of *OH intermediates in the polarized electrolyte, confirming an elevated local OH- concentration that promotes *OH formation. Collectively, these findings demonstrate that piezoelectric polarization restructures the hydrogen-bonding network of interfacial water, favoring low-coordinated OH- configurations and facilitating efficient Grotthuss-type transport of OH-.

Effects of OH- Grotthuss transport on OER
To assess the impact of polarized electrolyte on OER kinetics, we performed LSV scanning of NiOOH electrode at catalytic state (Fig. 3a). To facilitate electrolyte polarization, the PVDF/BTO/nylon mesh composite film was positioned parallel to the working electrode, with an approximate 2 mm gap maintained at the fixed end by a nylon spacer (see the inset in Fig. 3a). Prior to OER measurements, the system was sonicated in the KOH electrolyte for 1 min as illustrated in Supplementary Fig. 6. Control experiments show that ultrasonic treatment of KOH electrolyte without piezoelectric film induces negligible changes in OER activity (Supplementary Figs. 7a and b). Remarkably, piezoelectric polarization of the KOH electrolyte dramatically enhances the OER activity of NiOOH, reducing the overpotential at 100 mA cm-2 by 222.4 mV (from 599.7 ± 10.5 mV to 377.3 ± 7.3 mV, Supplementary Fig. 7c). This enhancement is further reflected at a current density of 387.6 mA cm-2 at 1.65 V, a 32-fold increase over the non-polarized state and 3.4 times greater than that of commercial RuO (113.1 mA cm-2 under 10 mg cm-2 loading on Ni 2 foam), highlighting its high activity (Fig. 3a). The high average Faradaic efficiency of 95.5% for oxygen evolution (Supplementary Fig. 7d) validates that the observed current enhancement is predominantly attributed to the OER activity boosted by the polarized electrolyte.

The intrinsic activity of NiOOH was significantly enhanced in polarizing KOH electrolyte by PVDF/BTO film, as evidenced by ECSA-normalized LSV curves showing a 29.3-fold increase in current density at 1.65 V (1.7 mA cm-2 vs. 0.058 mA cm-2, Fig. 3b). This improvement was further corroborated at the active-site level by turnover frequency (TOF) value determined by referencing to Ni3+ active sites, which revealed a 7.3-fold rise at 1.65 V (56.7 ± 12.6 s-1 vs. 7.8 ± 0.1 s-1, Supplementary Fig. 7f), confirming that the polarized electrolyte enhances the activity of individual active sites. The reaction kinetics were also accelerated, as indicated by a notable decrease in the Tafel slope from 154.2 to 74.5 mV dec-1 (Fig. 3c). Further mechanistic insight was provided by electrochemical impedance spectroscopy (EIS) at 1.6 V (Figs. 3d, e and Supplementary Figs. 8a, b), which showed reductions in both solution resistance (R : 1.5 → 1.3 Ω) and charge-transfer resistance (R : 16.8 → 8.9 Ω). These results suggest improved s ct OH- mobility and a lower interfacial energy barrier during OER. Crucially, flow-cell experiments provided strong validation: unlike the non-polarized electrolyte, the OER performance showed no improvement under forced convection when the electrolyte was polarized (Supplementary Figs. 8c and d). This lack of enhancement is consistent with the intrinsically high mass transport properties of the polarized electrolyte itself. Additionally, double-layer capacitance (C ) measurements (Supplementary Fig. 9) show that NiOOH in polarized KOH exhibits a dl higher C (9.1 mF cm-2) than in pristine KOH (8.3 mF cm-2), corresponding to larger ECSA values (227.5 vs. 207.5 cm2). dl This confirms electrolyte restructuring not only optimizes ion transport but also exposes additional active sites, both critical for OER enhancement.

The broad applicability of this strategy is demonstrated across various catalysts and electrolytes. For instance, NiFe layered double hydroxide (LDH) in 1 M KOH exhibited a 2.3-fold increase in current density at 1.6 V under polarization, rising from 124.9 to 284.5 mA cm-2 (Supplementary Figs. 10a and b). Similarly, for NiOOH in 1 M phosphate-buffered saline (PBS, pH = 7.4 ± 0.1), the current density at 1.5 V increased from 2.26 mA cm-2 in the non-polarized electrolyte to 0.60 mA cm-2 under piezoelectric polarization, corresponding to a 3.8-fold enhancement (Supplementary Figs. 10c and d). Raman spectroscopy further revealed a blue shift in the O-H stretching band (2900 - 3800 cm-1) and an increase in low-coordinated water molecules in the polarized PBS (Supplementary Figs. 10e and f), suggesting a weakening of hydrogen bonds that likely facilitates the water dissociation step, a critical process in neutral-media OER. The long-term durability of OER on NiOOH in a polarized electrolyte was evaluated through cyclic voltammetry (CV) tests, which demonstrated high stability over 3000 cycles along with a slight performance enhancement (Supplementary Figs. 11a and b). Remarkably, a one-minute ultrasonic treatment enabled NiOOH to sustain high activity for 24 h, maintaining a 2.9-fold increase in current density compared to that in non-polarized KOH (Fig. 3f). This prolonged stability of the polarized electrolyte is attributed to the small energy difference between fully and partially coordinated OH⁻ states22, which stabilizes a metastable Grotthuss-type OH- transport23. After 24 h of continuous OER operation, a 1-minute piezoelectric reactivation restored 88.7% of the initial activity. The observed average per-cycle current density decay of 7.1% stems from intrinsic catalyst degradation rather than ultrasonic-induced damage, as verified by the consistent performance after repeated reactivation at 5-min intervals (Supplementary Figs. 11c and d). Indeed, comparing the behavior of NiOOH in non-polarized (AEM pathway) versus externally polarized (LOM/DOM pathway) electrolytes highlights the intrinsic instability of the latter. This instability stems primarily from the frequent evolution and slow regeneration of lattice oxygen, which accelerates the dissolution of active species. Such degradation can be alleviated by accelerating lattice oxygen regeneration, for instance, through the use of oxyphilic alloys that enhance the capture of oxygen intermediates and facilitate rapid recovery of oxygen vacancies33. To assess long-term performance, we conducted prolonged operation with periodic piezoelectric reactivation. Even after 100 h with periodic reactivation, the system retained a 2.7-fold current density enhancement, 61.2 mA cm-2 at 1.7 V versus 22.9 mA cm-2 in non-polarized KOH (Fig. 3f, Supplementary Fig. 7e). This reproducible regeneration confirms the system’s robust sustainability, demonstrating that periodic piezoelectric modulation can be effectively reapplied to maintain high activity over prolonged durations.

Enhanced Ni-O covalency of NiOOH in piezoelectric-polarized electrolyte
To understand the enhanced OER activity of NiOOH in polarized electrolytes, we systematically investigated its electronic and structural changes. X-ray photoelectron spectroscopy (XPS) analysis shows a slight increase in the Ni 2p binding energy from 855.4 eV to 855.5 eV (Supplementary Fig. 12a), suggesting stabilization of a higher Ni 3/2 oxidation state under polarization. The absorption edge energy of Ni K-edge X-ray absorption near-edge structure (XANES) spectra (Fig. 4a) measured at half height for NiOOH under polarization was 8344.04 eV, exceeding values for NiO (8343.6 eV) and NiOOH in the non-polarized electrolyte (8343.73 eV). This positive shift further supports the presence of Ni in an oxidation state above +3, consistent with highly oxidized nickel species34. The formation of highly oxidized nickel species can be attributed to the reconstruction of the NiOOH electrode during the OER, a process facilitated by undercoordinated OH- species due to the piezoelectric-field-triggered weakening of the *O-H bond (Figs. 2g and h). This is corroborated by the Fourier-transformed extended X-ray absorption fine structure (EXAFS) spectra (Fig. 4b and Supplementary Table 1). For NiOOH in a polarized electrolyte, the peak observed between 1.0 and 2.0 Å corresponds to scattering from Ni-O atoms. Notably, the Ni-O bond length and coordination number for NiOOH in a non-polarized electrolyte (2.04 Å, 5.85) are significantly larger than those in a polarized electrolyte (1.91 Å, 5.08). This marked contraction in bond length and reduction in coordination number confirm the presence of a highly oxidized nickel state during the reconstruction of NiOOH under OER conditions.

The high-resolution transmission electron microscopy (HRTEM) lattice image suggests the contraction of (002) lattice spacing for NiOOH in polarized electrolyte (2.41 Å → 2.26 Å, Supplementary Fig. 12 b). The elevated Ni valence and contraction of the Ni-O bond are key characteristics of enhanced Ni-O covalence. The increased orbital overlap (covalency) is further demonstrated by in situ Raman spectroscopy. As shown in Supplementary Fig. 13a, in non-polarized 1 M KOH, the Raman peaks shift from 455 cm-1 and 539 cm-1 at 1.3 V, corresponding to a mixed state of Ni(OH) and NiOOH35, to 472 cm-1 (assigned to the E bending mode, δ ) and 556 cm-1 (assigned to the A 2 g Ni-O 1g stretching mode, ν ) of NiOOH at 1.5 V36, indicating the reconstruction of Ni(OH) to NiOOH. In these modes, Ni-O 2 oxygen atoms vibrate perpendicular to the oxygen plane in A and within the plane in E 36. In contrast, under the 1g g same potential (1.5 V), the Raman bands of NiOOH formed in the polarized electrolyte shift to higher wavenumbers, 476 cm-1 for the E mode and 562 cm-1 for the A mode (Fig. 4c, Supplementary Fig. 13b). Given that Raman g 1g wavenumber correlates positively with electron cloud overlap, and greater overlap results in a higher vibrational frequency, this upshift reflects enhanced Ni-O covalency in NiOOH formed under polarized conditions. Moreover, the higher δ /ν intensity ratio observed for NiOOH in the polarized electrolyte (Fig. 4d) suggests a looser Ni-O Ni-O structure, which is associated with a high Ni oxidation state37-40. Importantly, a reduced white-line intensity in the XANES spectra of NiOOH in the polarized electrolyte relative to NiO and NiOOH in the non-polarized electrolyte (Fig. 4a) indicates structure distortion of the NiO octahedra in polarized KOH at higher-valence state of Ni species41, 6 along with increased Ni 3d - O 2p orbital hybridization (i.e., greater M-O bond covalency)42,43. Consistently, the O 1s XPS peak shifts to a binding energy 0.2 eV higher (Supplementary Fig. 13c), signifying reduced electron density on oxygen and corroborating the enhanced covalency inferred from XANES.

Density functional theory (DFT) calculations were performed to gain insight into the increased M-O covalency. According to the phase and bond length from the XRD and EXAFS results (Supplementary Fig. 14 and Supplementary Table 1), the β-NiOOH structures with (in polarized electrolyte) and without Ni-O bond contraction (in non-polarized electrolyte) were created. Within these models, which are simplifications of the complex catalyst surface under operational conditions, as shown by the projected density of states (PDOS) in Fig. 4e, NiOOH formed in polarized KOH exhibits significantly enhanced overlap between Ni 3d and O 2p orbitals near the Fermi level, indicating strengthened Ni-O covalency44. This intensified covalent interaction promotes electron delocalization, as reflected by the broader and smoother PDOS peaks45, which facilitates charge distribution across the Ni-O network and thereby reduces charge-transfer barriers. The strengthening of Ni-O covalency is quantitatively supported by crystal orbital Hamiltonian population (COHP) analysis, which yields a more negative integrated COHP (ICOHP) value of -1.6 for the NiOOH in polarized electrolyte compared to -1.19 for the NiOOH in non-polarized electrolyte (Fig. 4f). Notably, an increased density of unoccupied states is observed in the oxygen non-bonding (O ) region just above the Fermi level. This feature is essential for activating lattice oxygen (O ) and enhancing NB lat its donor character46, thereby predicting a lattice oxygen-mediated OER pathway on NiOOH in polarized electrolyte.

Lattice oxygen-mediated oxygen evolution in NiOOH in piezoelectric-polarized electrolyte
Based on this electronic foundation conducive to O activation via enhanced Ni-O covalency40, we elucidated the lat OER mechanism in the piezoelectric-polarized electrolyte via 18O-labeled differential electrochemical mass spectrometry (DEMS). In piezoelectric-polarized 18O-labeled KOH, NiOOH generates 36O (18O18O coupling), 34O 2 2 (16O18O coupling), and 32O (16O16O coupling) (Fig. 5a, b, Supplementary Fig. 15). Quantitatively, the ratio of 36O 2 2 (I /I + I + I ) exhibits a dramatic increase to 13.6%, which is 9.07 times higher than the value of 1.5% observed in 36 36 34 32 the non-polarized KOH. Concurrently, the ratio of 34O (I /I + I + I ) rises to 54.2%, a 5.89-fold enhancement over 2 34 36 34 32 the 9.2% in the control. These results unambiguously demonstrate that OER in the polarized electrolyte follows the lattice oxygen mechanism (LOM, involving direct coupling of O with adsorbed oxygen, O -*O, yielding 34O ) and lat lat 2 diatomic oxygen mechanism (DOM, involving O -O coupling, yielding 36O )47, bypassing high-energy adsorbed lat lat 2 intermediates (*O/*OOH)48. In contrast, OER in non-polarized electrolyte mainly follows the typical AEM involving oxygen-containing intermediates (*OH, *OO, *OOH). In situ ATR-SEIREAS spectra further confirm LOM by capturing O-O intermediates (Fig. 5c), a hallmark of this mechanism. Specifically, NiOOH in the piezoelectric-polarized electrolyte exhibits O-O stretching (ν(O-O), 1176.7 cm-1) and bending (δ(O-O), 1125.6 cm-1) vibrations, confirming direct O-O coupling as the OER pathway. In contrast, NiOOH in non-polarized KOH shows only an *OOH band at 1209.6 cm-1 (Supplementary Fig. 16)49, a signature of the AEM pathway. The LOM/DOM routes are further supported by the upshift of *O-H ATR-SEIRAS bands (3000 - 3600 cm-1) at higher potentials (Fig. 2g), arising from enhanced lattice oxygen reactivity under these conditions. Additionally, selective suppression of OER activity upon addition of TMA+ (a known O-O- scavenger) provides further evidence for LOM/DOM (Fig. 5d)50. Moreover, the OER kinetics in the piezoelectric-polarized system exhibit a more pronounced pH dependence than the non-polarized system (Fig. 5e), a hallmark of LOM/DOM pathways where proton-coupled electron transfer steps can be decoupled51,52.

DFT calculations of the free energy on the (100) facet of β-NiOOH without Ni-O bond contraction (in non-polarized electrolyte) reveal that the deprotonation steps exhibit the highest energy barriers for each reaction pathway: 2.15 eV for *OH → *O(AEM pathway), 2.29 eV for O + O H → O -O (DOM pathway), and 2.22 eV for lat lat lat lat O H + *OH → O + *OH (LOM pathway), respectively (Supplementary Fig. 17). These results clearly indicate that lat lat AEM is the most favorable pathway on NiOOH in a non-polarized electrolyte. In contrast, under polarized electrolyte conditions, the rate-determining steps (RDS) shift to: *OOH → O deprotonation (2.35 eV, AEM), O H + 2 lat O H → O + O H deprotonation (1.91 eV, DOM), and O + O H → O H + *OH hydroxyl ion adsorption (2.19 eV, lat lat lat 2 lat lat LOM), as summarized in Fig. 5f. Notably, the LOM and DOM pathways collectively exhibit more favorable kinetics on NiOOH under polarization, underscoring their essential role in achieving high OER activity. Taken together, we therefore conclude that the applied piezoelectric polarization induce Grotthuss-type OH- transport, which not only increases the local OH- concentration at the electrode surface but also modulates the electronic state of NiOOH during catalyst reconstruction. These changes weaken O-H bonds and strengthen Ni-O covalency within the Ni-O-Ni motifs. The intensified Ni-O covalency facilitates electron delocalization across the metal-oxygen network, concurrently shifting the lower Hubbard band (LHB) downward and the O 2p non-bonding band upward toward the Fermi level53,54 (Fig. 1), thereby modifying the electronic structure of the active phase. This electronic reconfiguration activates O , enabling two low-barrier pathways: direct O -*O coupling and O -O coupling. lat lat lat lat

Methods
Materials
Polyvinylidene fluoride (PVDF, M ≈ 543000) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China). w KOH (99%), N,N-Dimethylformamide (DMF, 99.5%) and acetone (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tetramethylammonium hydroxide (TMAOH, 10% in water), RuO (99.9%), Nafion (5 wt%), BaTiO (BTO, 99.9%), 2 3 deionized water and ethanol (99.5%) were sourced from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). Nickel acetate tetrahydrate (Ni(CH COO)·4HO, 99%) was provided by Shanghai Runze Chemical Technology Co., Ltd. (Shanghai, China). 3 2 2 Phosphate-buffered saline (PBS, pH = 7.4 ± 0.1) was obtained from Changde Bickman Instrument Co., Ltd. (Hunan, China). HCl (36.0%- 38.0%) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Acetone (99%) was obtained from Nanjing Wanqing Chemical Glassware Instrument Co., Ltd. (Jiangsu, China). Nylon mesh substrates were supplied by Taizhou Lifei Screen Mesh Co., Ltd. (Zhejiang, China). Nickel foam (NF, 99.99%) was acquired from Kunshan Shengshi Jingxin New Material Co., Ltd. (Jiangsu, China), and cut into pieces with dimensions of 1 x 2 x 0.15 cm3. Prior to use, the NF pieces were sequentially washed three times with acetone and deionized water, etched in 3 M HCl for 15 min, and then thoroughly rinsed with deionized water and ethanol under ultrasonication for 15 min each. Deionized water was employed as the solvent in all experiments. All reagents were of analytical grade and used as received without further purification.

Electrode preparation
NiOOH electrode: NiOOH was synthesized on nickel foam (NF) with dimensions of 1 x 1 x 0.15 cm3 via electrochemical oxidation in a 1 M KOH solution using a three-electrode system, where NF served as the working electrode, a Pt sheet as the counter electrode, and Hg/HgO as the reference electrode. Surface activation was achieved by performing 10 CV cycles between 0.7 and 2.1 V (vs. Reversible hydrogen electrode) at a scan rate of 50 mV s-1, ensuring the generation of NiOOH layer on the NF. For material characterization (XRD, HRTEM, XAFS), to avoid distractions of NF substrate, NiOOH was electrodeposited on carbon cloth (1 x 2 cm2) to eliminate interference from the NF substrate. Electrodeposition was carried out at a constant potential of -3.5 V for 600 s in a 0.025 mol L-1 Ni(CHCOO) 3 2 electrolyte solution (100 mL), using a Ag/AgCl (saturated KCl) reference electrode and a Pt plate counter electrode. The as-deposited sample was subsequently activated by 10 cyclic voltammetry (CV) cycles between 0.7 and 2.1 V (vs. Reversible hydrogen electrode) at a scan rate of 50 mV s-1, ending at 2.1 V to form NiOOH. This activation step was performed in both polarized and non-polarized KOH electrolytes.

NiFe LDH electrode: To remove the surface oxide layer, the carbon paper was first ultrasonically cleaned in 1 M HCl solution for 30 min. It was then rinsed sequentially with anhydrous ethanol and deionized water several times, followed by drying at 70 oC. Ni Fe -layered double hydroxide (LDH) electrocatalysts were subsequently electrodeposited onto a 1 x 1 cm2 section of the 1 x 2 0.64 0.36 cm2 treated carbon paper. The electrodeposition solution was prepared by mixing 100 mM aqueous solutions of Ni(NO ) 3 2 ·6H O (Sigma- 2 Aldrich, AR, 203874) and Fe(NO)·9HO (Macklin, AR, I809395) at a Ni:Fe molar ratio of 2:1. Electrodeposition was performed using a 3 3 2 constant potential system (CS1002, China) in a two-electrode configuration with a Pt sheet as the counter electrode. A constant current density of -20 mA cm-2 was applied for 600 s, resulting in catalyst deposition over a geometric area of 1 x 1 cm2. The mass loading of the NiFe LDH catalyst was determined to be 4.1 mg cm⁻² by the gravimetric method, calculated from the mass difference of the electrode before and after deposition following drying at 60 °C.

RuO electrode: The RuO catalyst ink was prepared by homogeneously dispersing 10 mg of RuO powder in a mixture of 450 μL 2 2 2 ethanol and 50 μL Nafion solution under 2 h of sonication. Subsequently, the resulting ink was drop-cast onto a pre-cleaned NF substrate with dimensions of 1 x 2 x 0.15 cm3. After drying in air, the NF/RuO electrode was obtained with a catalyst loading of 2 approximately 10 mg cm⁻².

Piezoelectric film preparation
Dense PVDF film preparation: A dense PVDF film was prepared by dissolving 0.5 g of PVDF powder in 5 mL of DMF under magnetic stirring at 70 oC to obtain a transparent solution. The solution was then cast onto a glass substrate, dried at room temperature (25 oC), and peeled off to form a free-standing dense film.

Porous PVDF film preparation: A porous PVDF film was fabricated on a nylon mesh substrate. First, a nylon mesh with a pore size of 42 μm was cut into pieces (approximately 1 x 2 cm2) and cleaned by sequential rinsing with acetone and deionized water (three times each). Subsequently, 0.5 g of PVDF powder was dissolved in 5 mL of DMF at 70 oC to form a precursor solution. The cleaned mesh was immersed in the solution, withdrawn, and immediately sprayed with a coagulation bath composed of water and ethanol (2:1 mass ratio) to induce phase separation. The resulting film was dried at room temperature (25 oC) to yield the porous PVDF film.

Porous PVDF/BTO composite film preparation: Porous PVDF/BTO composite films were prepared by incorporating BTO powder at mass fractions of 0, 10, 20, and 40 wt% into the PVDF solution (0.5 g PVDF in 5 mL DMF), followed by homogenization using a vortex mixer. Nylon meshes with pore sizes of 212, 42, and 17 μm were cut and cleaned as described above, then impregnated with the corresponding PVDF/BTO solution. The impregnated meshes were sprayed with the same water/ethanol coagulation bath (2:1 mass ratio) and dried at room temperature (25 oC), following the procedure used for the porous PVDF film. A specific PVDF/BTO/nylon mesh composite film was fabricated with a PVDF/BTO loading area of 1 x 2 cm2 and a loading density of 5.8 mg/cm2.

Characterizations
The crystal structures were characterized by X-ray diffraction (XRD, Rigaku SmartLab) using Cu Kα radiation (λ = 0.15406 nm). Fourier-transform infrared (FT-IR) spectroscopy was performed on a PerkinElmer spectrometer (Agilent Technologies) to analyze the molecular composition and structure. The surface morphologies of the electrode and piezoelectric porous films were examined using field-emission scanning electron microscopy (FE-SEM, Zeiss ULTRA-55). The lattice fringes of NiOOH under piezoelectric-polarized and non-polarized conditions in KOH were observed by transmission electron microscopy (TEM, Thermo Fisher Scientific Talos F200X). The chemical states and composition of NiOOH were determined by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). A Horiba LabRAM HR800 Raman spectrometer equipped with a 532 nm He-Ne laser and a 50x objective (NA = 0.55) was employed to investigate the water structure in 1 M KOH (60 mL, pH = 13.98 ± 0.01) and PBS (60 mL, pH = 7.4 ± 0.1) electrolytes, as well as the chemical composition of NiOOH electrodes. In-situ Raman spectra were collected in a customized spectroelectrochemical cell that allowed for simultaneous electrochemical control and spectral acquisition, using a Pt wire as the counter electrode and an Ag/Agcl electrode as the reference (Supplementary Fig. 19a). The Raman frequency was calibrated with a silicon wafer prior to each measurement. The Ag/AgCl reference electrode was calibrated in situ against a reversible hydrogen electrode (RHE) in 1 M KOH. The calibration was performed via a slow cyclic voltammetry scan (1 mV s⁻¹) around the hydrogen equilibrium potential using a Pt wire as the working and counter electrode. The average potential at the zero-current point was used for potential conversion, demonstrating a stability of ± 2 mV. The conversion from the measured potential versus the saturated Ag/AgCl reference electrode (E ) to the Ag/AgCl RHE scale (E ) was performed using the Nernst equation:E = E + 0.197 + 0.0592 x pH. Here, the constant 0.197 V is the standard RHE RHE Ag/AgCl potential of the saturated Ag/AgCl reference electrode relative to the standard hydrogen electrode at 25 °C. The piezoelectric properties of the porous films were assessed by piezoresponse force microscopy (PFM, Bruker Dimension Icon). The piezoelectric coefficient d was calculated using the equation of d = (A - A )/(V - V ), where (V, A) represents a point on the butterfly curve and 33 33 0 0 (V , A ) denotes the intersection point of the curve. 0 0

In situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS, Supplementary Fig. 19b) was conducted to investigate the interfacial water structure in 1 M KOH (50 mL, pH = 13.98 ± 0.01) electrolytes, as well as the chemical composition of NiOOH electrodes, in a customized spectroelectrochemical cell using a Pt wire as the counter electrode and an Hg/HgO electrode as the reference. Spectra were acquired on a Bruker VERTEX 70v FTIR spectrometer with a liquid nitrogen-cooled MCT detector at a resolution of 4 cm-1, accumulating at least 128 scans. Differential electrochemical mass spectrometry (DEMS) analyses (Supplementary Fig. 19c) were performed on a Linglu QAS 100 system with 1 M KOH (20 mL, pH = 13.98 ± 0.01) electrolyte and NiOOH electrodes, employing a porous polytetrafluoroethylene (PTFE) membrane (pore size: 0.22 μm; thickness: 100-150 μm) as the interface between the electrolyte and the mass spectrometer. The oxygen evolved during electrolysis was quantified using an Agilent 7890B gas chromatograph equipped with a thermal conductivity detector (TCD). X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were carried out in transmission mode at the XAFS beamline of the Shanghai Synchrotron Radiation Facility (SSRF). Data processing and analysis were performed using the Athena, Artemis, and IFEFFIT software packages. EXAFS data were extracted by standard procedures, with post-edge background subtraction via cubic spline fitting and normalization to the absorption edge.

Electrochemical tests
All electrochemical experiments were carried out with an Autolab electrochemical workstation (Metrohm) and a glass three-electrode cell, using platinum electrode as the counter electrode, Hg/HgO electrode as the reference electrode, and the aforementioned NiOOH as working electrode, and data was acquired using the instrument 's proprietary software. The Hg/HgO reference electrode was calibrated before and after measurements against a reversible hydrogen electrode (RHE) in 1 M KOH electrolyte, using a Pt wire as the working and counter electrode. The calibration involved a slow cyclic voltammetry (CV) scan at 1 mV s-1 near the open-circuit potential to ensure quasi-equilibrium conditions. The average potential at the zero-current point was taken as the corrected value, yielding a stability within ± 2 mV over 30 minutes. The electrolyte was a 1 M KOH aqueous solution (pH = 13.98 ± 0.01), prepared by dissolving 2.81 g of KOH pellets in deionized water and diluted to a final volume of 50 mL. The solution was stored in a sealed plastic bottle at room temperature (25 oC) prior to use. To polarize the electrolyte, a PVDF/BTO/nylon mesh composite film, with a PVDF/BTO loading area of 1 x 2 cm2 and a loading density of 5.8 mg/cm2, was immersed in 50 mL of electrolyte and subjected to 1 min of ultrasound treatment. During OER measurements, the composite film was aligned parallel to the NiOOH electrode, with a 2 mm gap maintained using a nylon spacer. The effective contact area between the composite film and the electrolyte was 1 x 1 cm2. It is noteworthy that, owing to the rapid diffusion induced by ultrasound, the composite film can be positioned anywhere within the electrolyte to achieve uniform polarization. The electrolyte temperature is maintained at 25 °C during the OER tests by circulating water. The piezoelectric current characteristics of the composite film samples were collected by using the above three-electrode system. First, silver paste is coated on one side of the composite piezoelectric film as the working electrode. The coating of the silver paste is to enhance the conductivity of the film. The electrolyte was 1 M KOH, and the change in current density was measured under intermittent ultrasonic treatment. All measured potentials are reported versus the RHE. The conversion from the measured potential versus the Hg/HgO reference electrode (E ) to the RHE scale (E ) was performed using the following equation: E = E + 0.098 + 0.0592 x pH. Hg/HgO RHE RHE Hg/HgO Here, the constant 0.098 V represents the potential of Hg/HgO reference electrode (1 M KOH) relative to the standard hydrogen electrode at 25 °C. The pH value of 1 M KOH was experimentally determined to be 13.98 at room temperature (25 °C).

Linear scan voltammetry (LSV) of NiOOH electrodes was performed at a scan rate of 5 mV s-1. All measurements were subjected to 85% iR compensation, except for the pH-dependent measurements and TMAOH tests. The turnover frequency (TOF) was calculated using the equation TOF = (j × S) / (4 × n × F), where j is the current density at 1.65 V, S is the electrode area (1 × 1 cm2), n is the moles of active sites, and F is the Faraday constant. The active site number (n) was determined from the Ni2+/Ni3+ redox charge (Q ) obtained Redox by CV, using n = Q / (z × F), where z is the number of electrons transferred per metal atom in the redox couple (z = 1). Redox

Electrochemically active surface areas (ECSA) were measured by cyclic voltammetry (CV) in the potential window from 1 to 1.15 V under scan rates of 20, 50, 70, 100, 130, 150, and 200 mV s−1 on the Autolab workstation. The current density at 1.075 V was used for the calculation of ECSA. The electrical double-layer capacitance (C ) was calculated by the equation C = (j-j)/2ν, where j and j are the dl dl a c a c anodic current density and cathodic current density, respectively, and ν is the scan rate. Thus, C is the slope of the linear relationship dl between (j-j)/2 and scan rates. The ECSA can be calculated by ECSA = C /C , where C is the specific capacitance of an ideal flat dl dl,Ref dl,Ref surface of the catalyst. Here, we use the average value of 0.04 mF cm-2 for C in alkaline solutions without taking the used material dl,Ref and measurement conditions into account. Accelerated durability testing involved 3000 cycles of cyclic voltammetry (CV) scanning at 50 mV/s between 1.2 and 1.6 V on the Autolab workstation prior to subsequent LSV measurement. Long-term stability tests were conducted by chronoamperometry at a constant potential of 1.7 V. To simulate periodic piezoelectric activation, the electrolyte was sonicated for 1 min every 24 h. Faraday efficiency (FE) for O₂ production was calculated as FE = V /V , where V experimental theoretical experimental is the measured O₂ volume from a single measurement, and V = (Q x V ) / (4F), with Q being the passed charge, F the Faraday theoretical m constant (96485 C mol⁻¹), V the molar gas volume (24.5 L mol-1 at 298 K and 101 kPa), and the factor 4 representing the electron m number per O molecule. Electrochemical impedance spectroscopy (EIS) was performed at 1.6 V (5 mV amplitude, 100 kHz-1 Hz). Fitting 2 the data to the equivalent circuit in Fig. 3d provided the solution resistance, which was used for iR correction, and the charge transfer resistance. The OER mechanism was examined using in situ differential electrochemical mass spectrometry (DEMS), with the corresponding electrochemical cell illustrated in Supplementary Fig. 19c. Prior to DEMS measurements, the NiOOH electrodes underwent 18O labeling via 20 CV cycles (1.1 - 2.1 V vs. Reversible hydrogen electrode, 20 mV/s) in H18O-enriched piezoelectric-polarized 2 and non-polarized 1 M KOH electrolyte, followed by thorough rinsing with H16O and N drying. For DEMS analysis, LSV (1.1 - 2.1 V vs. 2 2 Reversible hydrogen electrode, 5 mV/s, 6 cycles) was performed in H16O-based electrolyte while monitoring oxygen isotopologues. 2 The modified NiOOH electrode received additional pretreatment (1 min ultrasonication in 1 M KOH) before testing. All measurements were conducted under N purging (0.8 mL/min) to eliminate dissolved oxygen interference.

Molecular dynamics (MD) simulations
To investigate the solvation structure of hydroxide ions (OH⁻), we performed all-atom MD simulations using the general AMBER force field55 with the RESP charges56 using the GROMACS 2023.6 package57 (Supplementary Data 1). The system comprised one OH⁻ ion solvated in 16133 water molecules, with charge neutrality maintained by 308 K+ counterions, representing an equilibrated aqueous system at 298 K. The simulation protocol began with energy minimization using the steepest descent algorithm, converging when the maximum force fell below 100 kJ/mol·nm. This was followed by a 10 ns production run in the isothermal-isobaric (NPT) ensemble at 298 K. Temperature regulation was achieved with the Nosé-Hoover thermostat58, while three-dimensional periodic boundary conditions were implemented to minimize finite-size effects. For electrostatic interactions, we employed the Particle Mesh Ewald (PME) method59,60 with a real-space cutoff of 1 nm and a relative tolerance of 1×10-6 in reciprocal space. Van der Waals interactions were similarly truncated at 1.0 nm. The LINCS algorithm61 constrained all bonds involving hydrogen atoms, enabling a 2 fs integration time step using the leap-frog integrator62. The radial distribution function, g(r), was calculated to characterize the local density of water oxygen atoms as a function of distance from the OH- oxygen atom. It is defined as g(𝑟) = ρ(r)/ρ by the ratio of the local density 0 at a distance r, ρ(r), to the average bulk density of the system, ρ . The coordination number (CN) for the first hydration shell was then 0 determined by integrating g(r) up to the position of the first minimum (RRmin ) following the first peak in the g(r) profile, using the equation of CN =4π𝜌 ∫ 𝑅𝑚𝑖𝑛 𝑔(𝑟)𝑟2𝑑𝑟. o 0

Density functional theory (DFT) calculations
All the density functional theory (DFT) calculations (Supplementary Data 1) were performed with the Vienna Ab-initio Simulation Package (VASP)63. The exchange-correlation functional was treated by the generalized gradient approximation with the Perdew-Burke-Ernzerh (GGA-PBE) functional64. The plane wave cutoff energies were set to 480 eV. To describe the electron correlation of Ni 3d states, DFT plus on-site repulsion U (DFT + U) method was used (U = 5.5 eV) base on the previous investigation of optimized U value in nickel (oxy)hydroxide65. The spin-polarization was considered to identify the true ground state electronic configuration. The DFT-D2 method proposed by Grimme was adopted to describe the van der Waals interactions.

The structure of NiOOH in non-polarized KOH was modeled by deprotonating β-NiOOH66 to achieve an average Ni oxidation state of +3.566. To mimic the polarized state, the model was refined based on EXAFS data (Fig. 4b): a 6.37% compressive strain was applied to account for the shortened Ni-O bond (from 2.04 to 1.91 Å), and an oxygen vacancy was introduced to match the decreased Ni-O coordination number (from 5.85 to 5.08). The structure optimization was based on an energy tolerance of 1 x 10-5 eV and a force tolerance of 0.02 eV Å-1. The (100) facet of β-NiOOH were constructed by a 1 x 3 super cell with four Ni-O layers composed of a 20 Å vacuum space to prevent interactions between adjacent surfaces. The structure optimization was performed by fixing the bottom two layers to mimic the bulk region and allowing the top two layers to relax to mimic the surface. We sampled the irreducible Brillouin zone by a set of 2 x 2 x 1 k-points. The COHP analysis was performed with the Lobster 5.1.1 code767,68. The pair-wise interatomic interaction strength was computed by integrating the COHP up to the Fermi level (ICOHP). The free energy G is calculated by the equation of G = E + ZPE - T x S, where E , ZPE, T and S are the total energy, zero-point energy, temperature and entropy of DFT DFT intermediates, respectively, and T is the temperature (set to 298.15 K). The calculated equations used for the AEM, LOM, and DOM pathways on NiOOH are detailed below.

For AEM:
ΔG = (E + 0.5 × E – E – E) + (ZPE + 0.5 × ZPE – ZPE – ZPE ) – T × (S + 0.5 × S – S – S )
*OH *OH H2 H2O * *OH H2 H2O * *OH H2 H2O * 
ΔG = (E + E – E – E) + (ZPE + ZPE – ZPE – ZPE) – T × (S + S – S – S )
*O *O H2 H2O * *O H2 H2O * *O H2 H2O * 
ΔG = (E + 1.5 × E – 2 × E – E) + (ZPE + 1.5 × ZPE – 2 × ZPE – ZPE) – 
*OOH *OOH H2 H2O * *OOH H2 H2O * 
T × (S + 1.5 × S – 2 × S – S )
*OOH H2 H2O * 

For LOM:
ΔG = (E + 0.5 × E – E ) + (ZPE + 0.5 × ZPE – ZPE ) – T × (S + 0.5 × S – S )
*O-Olat *O-Olat+Vo H2 Olat+*OH *O-Olat+Vo H2 Olat+*OH *O-Olat+Vo H2 Olat+*OH 
ΔG = (E + E + 0.5 × E – E – E ) + (ZPE + ZPE + 0.5 × ZPE – ZPE – ZPE ) – T × (S + S + 0.5 × S – S 
OlatH OlatH O2 H2 H2O *O-Olat OlatH O2 H2 H2O *O-Olat OlatH O2 H2 H2O 
– S )
*O-Olat 
ΔG = (E + 0.5 × E – E – E ) + (ZPE + 0.5 × ZPE – ZPE – ZPE ) – 
Olat + *OH Olat + *OH H2 H2O OlatH Olat + *OH H2 H2O OlatH 
T × (S + 0.5 × S – S – S )
Olat + *OH H2 H2O OlatH 

For DOM:
ΔG = (E + 0.5 × E – E ) + (ZPE + 0.5 × ZPE – ZPE ) – T × (S + 0.5 × S – S )
Olar + -OlatH Olar + -OlatH H2 OlarH + -OlatH Olar + -OlatH H2 OlarH + -OlatH Olar + -OlatH H2 OlarH + -OlatH 
ΔG = (E + 0.5 × E – E ) + (ZPE + 0.5 × ZPE – ZPE ) – T × (S + 0.5 × S – S )
Olat-Olat Olat-Olat H2 Olat + OlatH Olat-Olat H2 Olat + OlatH Olat-Olat H2 Olat + OlatH 
ΔG = (E + E + 0.5 × E – E – E ) + (ZPE + ZPE + 0.5 × ZPE – ZPE – ZPE ) – T × (S + S +
OlatH + Vo OlatH + Vo O2 H2 H2O Olat-Olat OlatH + Vo O2 H2 H2O Olat-Olat OlatH + Vo O2 
0.5 × S – S – S )
H2 H2O Olat-Olat

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, by direct ultrasound-induced piezoelectric polarization, the authors established a mechano-electrochemical\ncoupling effect for appilication in multi-energy coupled water splitting, which can achieve rapid Grotthuss-type OH⁻ transport\nand activate dual low-energy-barrier lattice oxygen mechanism with increased OER activity. The structural characteristics,\npiezoelectric polarization effects, electrocatalytic performance and catalytic mechanisms were well explained by\ncorresponding measurements and comparison. So, this work can be accepted by Nature Communications after major\nrevision. Some comments are listed as follows:\n1) The author mentioned the reconstruction of Ni(OH)2 into its active state, which enhanced the Ni-O covalency and\nfacilitated the synergistic activation of dual-site lattice oxygen coupling. However, the optimization of Ni-O covalency lacks\nrelevant characterization and computational validation. Moreover, the research of the DOM mechanism is superficial,\nnecessitating further in-depth investigation. This includes comparative strucuture characterization before and after the OER\nprocess, the appearance of Ni-O-O-M vibrational peaks, pH dependence and DFT calculations, (J. Am. Chem. Soc. 2024,\n146, 10, 6846–6855; J. Am. Chem. Soc. 2024, 146, 48, 33276–33287).\n2) The piezoelectric effect will provide surrounding electric fields (J. Am. Chem. Soc. 2025, 147, 8289−8299) and may\nenhance electrochemical current, making the current in LSV tests not the real OER catalytic performance. The authors can\ntest the Faradaic efficiency.\n3) The CV stability over 3000 cycles of Ni(OH)₂/PVDF/BTO should be provided. And the commercial RuO2 performance\nshould be compared, The ECSA/TOF-normalized LSV curves should also be compared.\n4) The catalytic performance of this work is inferior to those of reported literature and even commercial catalysts. The authors\nmay extend the universality of the mechano-electrochemical coupling effect to different electrode and electrolyte\nenvironments. For instance, replacing Ni(OH)2 with a typical NiFe LDH catalyst or in neutral electrolytic conditions.\n5) The authors emphasize the optimization of the microenvironment at the electrode-electrolyte interface driven by electrode\npolarization, particularly the disturbance of OH ion concentration, distribution, transport, and hydrogen bond coordination\nnetwork. Therefore, the relavant characterizations and calculations of the electrochemical microenvironment are required.\nFor example, local pH testing and measurements of the OH- ions conductivity (Nat Energy 8, 264–272 (2023); Angew.\nChem. Int. Ed. 2025, 64, e202505924), molecular dynamics calculations and finite element simulations (Adv. Mater. 2025,\n37, 2418527; Nat Commun 16, 7959 (2025)) are needed.\n6) The author has provided almost no structural characterization of the catalyst Ni(OH)2, such as XRD, SEM, and TEM,\nwhich need to be supplied.\n7) The dual-site lattice oxygen mechanism theoretically exhibits poorer stability, primarily due to the insufficient filling of\nlattice oxygen, which can lead to structural collapse and dissolution of metal ions. This instability is evident when comparing\nthe author's results (Figure 3f) to the commonly reported stability of alkaline OER catalysts. Consequently, the author should\nelucidate the reasons behind the stability issues and propose potential solutions.", "reviewer_2": "This paper reports a strategy for ultrasound-induced piezoelectric polarization to drive electrolytes into a rapid OH- Grothuss\ndiffusion state. The author claims that this altered OH- diffusion state achieves enhanced OH- enrichment on the catalyst\nsurface, thereby enhancing the catalyst's performance. Although this work seems impressive in some respects, some of the\ncore arguments contain several key flaws.\n1)Under the electrolyte condition of 1 M KOH, considering that the area around the electrode is inherently filled with a high\nconcentration of OH-, and the PVDF/BTO is not closely attached to the electrode (leaving a gap of approximately 2 mm),\\ the\nsignificant performance improvement brought about by the long-distance OH- diffusion in the electrolyte is astonishing but\nquestionable. Furthermore, it is obviously more practical to focus on the impact under near-neutral/neutral conditions.\n2)All the electrochemical tests conducted by the author were carried out in fixed cells, lacking tests under the condition that\nthe electrolyte maintains a circulating flow in practical applications. Can this Grotthuss diffusion achieve a similar effect in a\nflowing cell?\n3)The author proposed a strategy that uses Grotthuss diffusion to enhance OH transport on the catalyst surface as the\nfundamental reason for the superior performance of the material. However, they failed to provide convincing evidence to\nprove the enhanced enrichment of OH on the surface of the catalyst. The characterization methods used, such as in-situ\nATR-SEIRAS and EIS techniques themselves, cannot achieve quantitative comparison of OH coverage on the catalyst\nsurface. There is insufficient direct and indirect evidence to support this conclusion, and more techniques, such as Methanol\nmolecular probe experiment, local pH test, and in-situ EIS, are needed to verify this hypothesis.\n4)As a novel means of regulating OER performance through external stimuli, authors should add clear schematic diagrams\n(such as the specific structure of the assembled three-electrode) to enhance the readability of the manuscript.\n5)Statistical analysis is required to ensure the reliability of the data.\n6)The author needs to explain whether iR correction was performed when using LSV data.", "reviewer_3": "In this work, the authors established a discontinuous piezoelectric polarization of electrolyte via ultrasonic to increase the\nperformance of OER. By integrating PVDF+BTO with Ni(OH)2 electrode (2 mm gap), the piezoelectric field achieves dual\nregulation: (1) electrolyte modulation, and (2) in situ catalyst activation, which yields a 5.7-fold current density enhancement,\nmaintaining 2.9-fold enhancement after 24 h operation with 88.7% activity recovery via 1 min reactivation, and preserving\n2.7-fold current density after 100 h stability test operation. The method is interesting and effective. However, the evidence of\nthis work and practical application of this idea seems not enough. It can be considered after following points are addressed.\n1. Please confirm whether PVDF+BTO and PVDF/BTO in the figure refer to the same material. If so, please ensure\nconsistency between the figure and the manuscript.\n2. If possible, could authors add some theoretical calculation for the mechanism?\n3. As authors claimed that the “porous pure PVDF itself exhibited a substantially stronger piezoelectric effect (current density\n-0.9 μA/cm2, Supplementary Fig. 3c) than its non-porous counterpart”, have the authors investigated the effect of the porous\nstructure (e.g., pore size) on the polarization behavior?\n4. Please provide the detailed preparation procedure, optimal image, SEM image of the nylon mesh after the pre-treatment\nprocess, and SEM cross-sectional image of the Ni(OH)2/PVDF/BTO electrode used in the OER test.\n5. In the V–T curve (stability test), the initial position of the curve does not match that of the LSV curve (lower current density\nat same potential). Could the authors explain the reason for this discrepancy?\n6. In practical applications, how do the authors introduce the ultrasonic field to induce the piezoelectric polarization effect?\nAnd how to place a PVDF/BTO (2 mm) near electrode in the water electrolyzers?\n7. For the mechanism exploration, the in-situ Raman showed that Ni experienced lower oxidation potential with the\npolarized KOH, which means Ni is easier to be oxidized. However, in the LSV curves, Ni shows a higher oxidation peak,\nwhich means it is harder to be activated. These results are self-contradictory. The author should re-examine the\nexperimental results and provide reasonable explanations; otherwise, the proposed theory will not be convincing.\n8. For the different percentage of BTO, 10% and 20% showed similar current (Supplementary Fig 3b), why the performance\nof OER is very different between these two? Also should re-check and provide the explanations.\n9. For DEMS test, did authors calibrate the gas amount? How authors quantify the specific percentage of different products?\nThe authors should provide detailed description of this. Otherwise, the conclusion is not convincing.\n10. Please re-draw all the figures in manuscript, some borders are missing, some font/size are not same."}, "author_response": "Response to the reviewer’s comments\nReviewer #1 (Remarks to the Author):\nIn this work, by direct ultrasound-induced piezoelectric polarization, the authors established a\nmechano-electrochemical coupling effect for appilication in multi-energy coupled water\nsplitting, which can achieve rapid Grotthuss-type OH⁻ transport and activate dual\nlow-energy-barrier lattice oxygen mechanism with increased OER activity. The structural\ncharacteristics, piezoelectric polarization effects, electrocatalytic performance and catalytic\nmechanisms were well explained by corresponding measurements and comparison. So, this\nwork can be accepted by Nature Communications after major revision. Some comments are\nlisted as follows:\nComment 1: The author mentioned the reconstruction of Ni(OH) into its active state, which\n2\nenhanced the Ni-O covalency and facilitated the synergistic activation of dual-site lattice\noxygen coupling. However, the optimization of Ni-O covalency lacks relevant characterization\nand computational validation. Moreover, the research of the DOM mechanism is superficial,\nnecessitating further in-depth investigation. This includes comparative strucuture\ncharacterization before and after the OER process, the appearance of Ni-O-O-M vibrational\npeaks, pH dependence and DFT calculations, (J. Am. Chem. Soc. 2024, 146, 10, 6846–6855; J.\nAm. Chem. Soc. 2024, 146, 48, 33276–33287).\nReply: We thank the reviewer for your insightful comment. We will answer your questions\nin two parts.\nPart 1: The characterizations of optimized Ni-O covalency. To demonstrate the enhanced\nNi-O covalency, we performed the XANES, EXAFS, HRTEM, XPS, in situ Raman and DFT\ncalculations (with PDOS and COHP analyses) to confirm the higher Ni oxidation states,\ncontraction and strengthening of the Ni-O bond, and the increased Ni3d-O2p orbital overlap (J.\nAm. Chem. Soc. 2024, 146, 6846; J. Am. Chem. Soc. 2025, 147, 20716).\nAccording to the XANES, EXAFS, HRTEM, and XPS results, we add the following\ndescription into the main text to describe the Ni-O covalency:\nTo understand the enhanced OER activity of NiOOH in polarized electrolytes, we\nsystematically investigated its electronic and structural changes. X-ray photoelectron\nspectroscopy (XPS) analysis shows a slight increase in the Ni 2p binding energy from 855.4 eV\n3/2\nto 855.5 eV (Supplementary Fig. 11a), suggesting stabilization of a higher Ni oxidation state under\npolarization. The absorption edge energy of Ni K-edge X-ray absorption near-edge structure\n(XANES) spectra (Fig. 4a) measured at half height for NiOOH under polarization was 8344.04 eV,\nexceeding values for NiO (8343.6 eV) and NiOOH in the non-polarized electrolyte (8343.73 eV).\nThis positive shift further supports the presence of Ni in an oxidation state above +3, consistent\nwith highly oxidized nickel species34. The formation of highly oxidized nickel species can be\nattributed to the reconstruction of the NiOOH electrode during the OER, a process facilitated by\nundercoordinated OH- species due to the piezoelectric-field-triggered weakening of the *O-H\nbond (Figs. 2g and h). This is corroborated by the Fourier-transformed extended X-ray\nabsorption fine structure (EXAFS) spectra (Fig. 4b and Supplementary Table 1). For NiOOH in a\npolarized electrolyte, the peak observed between 1.0 and 2.0 Å corresponds to scattering from\nNi-O atoms. Notably, the Ni-O bond length and coordination number for NiOOH in a non-polarized\nelectrolyte (2.04 Å, 5.85) are significantly larger than those in a polarized electrolyte (1.91 Å, 5.08).\nThis marked contraction in bond length and reduction in coordination number confirm the\npresence of a highly oxidized nickel state during the reconstruction of NiOOH under OER\nconditions.\nThe high-resolution transmission electron microscopy (HRTEM) lattice image suggests the\ncontraction of (002) lattice spacing for NiOOH in polarized electrolyte (2.41 Å → 2.26 Å,\nSupplementary Fig. 11 b). The elevated Ni valence and contraction of the Ni-O bond are key\ncharacteristics of enhanced Ni-O covalence.\nImportantly, a reduced white-line intensity in the XANES spectra of NiOOH in the polarized\nelectrolyte relative to NiO and NiOOH in the non-polarized electrolyte (Fig. 4a) indicates structure\ndistortion of the NiO octahedra in polarized KOH at higher-valence state of Ni species41, along\n6\nwith increased Ni→3d - O→2p orbital hybridization (i.e., greater M-O bond covalency)42,43.\nConsistently, the O→1s XPS peak shifts to a binding energy 0.2→eV higher (Supplementary Fig. 12c),\nsignifying reduced electron density on oxygen and corroborating the enhanced covalency\ninferred from XANES.\nSupplementary Fig. 11 | a, XPS spectra of Ni 2p for the NiOOH in non-polarized and polarized KOH. b, HRTEM\nlattice images of NiOOH in non-polarized and polarized KOH.\nFig. 4 | Enhanced Ni-O covalency of NiOOH in piezoelectric-polarized electrolyte. a, Ni-K edge X-ray\nabsorption near-edge structure (XANES) and b, extended X-ray absorption fine structure (EXFAS) spectra of\nNi foil, NiO, and NiOOH in non-polarized and piezoelectric-polarized KOH.\nSupplementary Table. 1 EXAFS fitting parameters at the Ni K-edge for Ni foil, NiO, and NiOOH in non-polarized\nand piezoelectric-polarized KOH (Ѕ 2=0.85)\nσ2c ΔE0 d\nR factor\n(Å2) (eV)\nNi foil Ni-Ni 12* 2.47±0.01 0.0062 6.7±0.5 0.001\nNi-O 6* 2.07+0.03 0.0089\nNiO -3.9±0.5 0.002\nNi-Ni 12* 2.95+0.02 0.0085\nNi-O 5.85±0.4 2.04±0.08 0.0082\nNiOOH in non-polarized KOH 2.0±0.3 0.011\nNi-Ni 5.94±0.3 3.11±0.07 0.0098\nNi-O 5.08±0.1 1.91±0.01 0.0104\nNiOOH in polarized KOH -3.2±0.2 0.008\nNi-Ni 5.58±0.2 3.06±0.03 0.0089\naCN: coordination numbers; bR: bond distance; cσ2: Debye-Waller factors; d ΔE : the inner potential correction.\nspectroscopy were estimated as CN±20%; R ± 1%; σ2 ± 20%.\nAccording to the in situ Raman and DFT calculations (PDOS and COHP), we add the\nfollowing description into the main text to show the Ni-O covalency:\nAs shown in Supplementary Fig. 12a, in non-polarized 1→M KOH, the Raman peaks shift from\n455→cm-1 and 539→cm-1 at 1.3→V, corresponding to a mixed state of Ni(OH) and NiOOH35, to\n2\n472→cm-1 (assigned to the E bending mode, δ ) and 556→cm-1 (assigned to the A stretching\ng Ni-O 1g\nmode, ν ) of NiOOH at 1.5→V36, indicating the reconstruction of Ni(OH) to NiOOH. In these\nNi-O 2\nmodes, oxygen atoms vibrate perpendicular to the oxygen plane in A and within the plane in\n1g\nE 36. In contrast, under the same potential (1.5→V), the Raman bands of NiOOH formed in the\ng\npolarized electrolyte shift to higher wavenumbers, 476→cm-1 for the E mode and 562→cm-1 for the\ng\nA mode (Fig. 4c, Supplementary Fig. 12b). Given that Raman wavenumber correlates positively\n1g\nwith electron cloud overlap, and greater overlap results in a higher vibrational frequency, this\nupshift reflects enhanced Ni-O covalency in NiOOH formed under polarized conditions. Moreover,\nthe higher δ /ν intensity ratio observed for NiOOH in the polarized electrolyte (Fig. 4d)\nNi-O Ni-O\nsuggests a looser structure, which is associated with a high Ni oxidation state 37-40. Importantly, a\nreduced white-line intensity in the XANES spectra of NiOOH in the polarized electrolyte relative\nto NiO and NiOOH in the non-polarized electrolyte (Fig. 4a) indicates structure distortion of the\nNiO octahedra in polarized KOH at higher-valence state of Ni species41, along with increased\nXPS peak shifts to a binding energy 0.2→eV higher (Supplementary Fig. 12c), signifying reduced\nelectron density on oxygen and corroborating the enhanced covalency inferred from XANES.\nSupplementary Fig. 12 | a, b, In situ Raman spectra of NiOOH in non-polarized (a) and polarized (b) 1 M KOH\nelectrolyte. c, XPS spectra of O 1s for the NiOOH in non-polarized and polarized KOH.\nDensity functional theory (DFT) calculations provide fundamental insight into the increased\nM-O covalency. According to the phase and bond length from the XRD and EXAFS results\n(Supplementary Fig. 13 and Supplementary Table 1), the β-NiOOH structures with (in polarized\nelectrolyte) and without Ni-O bond contraction (in non-polarized electrolyte) were created. As\nshown by the projected density of states (PDOS) in Fig. 4e, NiOOH formed in polarized KOH\nexhibits significantly enhanced overlap between Ni 3d and O 2p orbitals near the Fermi level,\ndirectly indicating strengthened Ni-O covalency44. This intensified covalent interaction promotes\nelectron delocalization, as reflected by the broader and smoother PDOS peaks45, which facilitates\ncharge distribution across the Ni-O network and thereby reduces charge-transfer barriers. The\nstrengthening of Ni-O covalency is quantitatively supported by crystal orbital Hamiltonian\npopulation (COHP) analysis, which yields a more negative integrated COHP (ICOHP) value of -1.6\nfor the NiOOH in polarized electrolyte compared to -1.19 for the NiOOH in non-polarized\nelectrolyte (Fig. 4f). Notably, an increased density of unoccupied states is observed in the oxygen\nnon-bonding (O ) region just above the Fermi level. This feature is essential for activating lattice\nNB\noxygen (O ) and enhancing its donor character46, thereby predicting a lattice oxygen-mediated\nlat\nOER pathway on NiOOH in polarized electrolyte.\nFig. 4 | Enhanced Ni-O covalency of NiOOH in piezoelectric-polarized electrolyte. c, In situ Raman spectra of\nNiOOH in piezoelectric-polarized KOH. d, Intensity ratio (I ) of in situ Raman spectra of NiOOH in\nδ/ν\nnon-polarized and piezoelectric-polarized KOH. e, PDOS band centers of the O 2p and Ni 3d orbitals in NiOOH\ninon-polarized and piezoelectric-polarized KOH. f, Crystal orbital Hamilton population (COHP).\nIn summary, the evidence from XPS, XANES, EXAFS, HRTEM, in situ Raman, and DFT\ncalculations consistently converges to demonstrate the optimized Ni-O covalency in our\nsystem.\nPart 2: LOM/DOM mechanism investigation. To investigate the LOM/DOM mechanism,\nwe have conducted DEMS with isotopic labeling, in situ ATR-SEIRAS, in situ Raman, TMA⁺ and\npH-dependence tests, and the DFT calculations of AEM, LOM, and DOM reaction pathways.\nWe add the following description into the main text to confirm the LOM/DOM\nmechanism:\nBased on this electronic foundation conducive to O activation via enhanced Ni-O\nlat\ncovalency40, we elucidated the OER mechanism in the piezoelectric-polarized electrolyte via\n18O-labeled differential electrochemical mass spectrometry (DEMS). In piezoelectric-polarized\n18O-labeled KOH, NiOOH generates 36O (18O18O coupling), 34O (16O18O coupling), and 32O (16O16O\n2 2 2\ncoupling) (Fig. 5a, b, Supplementary Fig. 14). Quantitatively, the ratio of 36O (I /I + I + I )\n2 36 36 34 32\nexhibits a dramatic increase to 13.6%, which is 9.07 times higher than the value of 1.5% observed\nin the non-polarized KOH. Concurrently, the ratio of 34O (I /I + I + I ) rises to 54.2%, a 5.89-fold\n2 34 36 34 32\nenhancement over the 9.2% in the control.\nThe LOM/DOM routes are further supported by the upshift of *O-H ATR-SEIRAS bands (3000\n- 3600 cm-1) at higher potentials (Fig. 2g), arising from enhanced lattice oxygen reactivity under\nthese conditions. Additionally, selective suppression of OER activity upon addition of TMA+ (a\nknown O-O- scavenger) provides further evidence for LOM/DOM (Fig. 5d)50. Moreover, the OER\nkinetics in the piezoelectric-polarized system exhibit a more pronounced pH dependence than\nthe non-polarized system (Fig. 5e), a hallmark of LOM/DOM pathways where proton-coupled\nelectron transfer steps can be decoupled51,52.\nDFT calculations of the free energy on the (100) facet of β-NiOOH without Ni-O bond\ncontraction (in non-polarized electrolyte) reveal that the deprotonation steps exhibit the highest\nenergy barriers for each reaction pathway: 2.15→eV for *OH → *O(AEM pathway), 2.29→eV for O +\nlat\nO H→ O -O (DOM pathway), and 2.22→eV for O H + *OH→ O + *OH (LOM pathway),\nlat lat lat lat lat\nrespectively (Supplementary Fig.→16). These results clearly indicate that AEM is the most\nfavorable pathway on NiOOH in a non-polarized electrolyte. In contrast, under polarized\nelectrolyte conditions, the rate-determining steps (RDS) shift to: *OOH → O deprotonation\n2\n(2.35→eV, AEM), O H + O H→ O + O Hdeprotonation (1.91→eV, DOM), and O + O H → O H +\nlat lat lat lat 2 lat lat\n*OH hydroxyl ion adsorption (2.19→eV, LOM), as summarized in Fig.→5f. Notably, the LOM and\nDOM pathways collectively exhibit more favorable kinetics on NiOOH under polarization,\nunderscoring their essential role in achieving superior OER activity. Taken together, we therefore\nconclude that the applied piezoelectric polarization induces Grotthuss-type OH- transport, which\nnot only increases the local OH- concentration at the electrode surface but also modulates the\nelectronic state of NiOOH during catalyst reconstruction. These changes weaken O-H bonds and\nstrengthen Ni-O covalency within the Ni-O-Ni motifs. The intensified Ni-O covalency facilitates\nelectron delocalization across the metal-oxygen network, concurrently shifting the lower\nHubbard band (LHB) downward and the O→2p non-bonding band upward toward the Fermi\nlevel53,54 (Fig.→1), thereby modifying the electronic structure of the active phase. This electronic\nreconfiguration activates O , enabling two low-barrier pathways: direct O -*O coupling and\nlat lat\nO -O coupling.\nlat lat\nFig. 5 | OER mechanism on NiOOH in piezoelectric-polarized KOH. a, DEMS signals of the 36O, 34O and 32O\n2 2 2\nproducts for 18O-labeled NiOOHin polarized KOH electrolyte. b, The ratio of 36O (I /I + I + I ), 34O (I /I +\n2 36 36 34 32 2 34 36\nI + I ), and 32O (I /I + I + I ) calculated from Fig. 5a. c, In situ ATR-SEIRAS spectra of NiOOH in polarized\n34 32 2 32 36 34 32\nKOH at varying potentials (OCP - 1.7 V). d, LSV curves of NiOOH in non-polarized and polarized 1 M KOH with\nand without 1 M TMAOH electrolyte. e, pH-dependent LSV curves of NiOOH in non-polarized and polarized\nKOH. f, Gibbs free energy diagram for the AEM, LOM, and DOM OER pathways on NiOOH in polarized KOH,\nwith the rate-determining step.\nSupplementary Fig. 15 | In situ ATR-SEIRAS spectra of NiOOH in non-polarized KOH at varying potentials (OCP\n- 1.7 V).\nComment 2: The piezoelectric effect will provide surrounding electric fields (J. Am. Chem. Soc.\n2025, 147, 8289-8299) and may enhance electrochemical current, making the current in LSV\ntests not the real OER catalytic performance. The authors can test the Faradaic efficiency.\nReply: We thank for your valuable comment. As suggested, we measured the Faradaic\nefficiency and add the following description into the main text: The high average Faradaic\nefficiency of 95.5% for oxygen evolution (Supplementary Fig. 6c) validates that the observed\ncurrent enhancement is predominantly attributed to the OER activity boosted by the polarized\nelectrolyte.\nSupplementary Fig. 6 | c, The OER Faraday efficiency of NiOOH in polarized KOH based on the measured\namount of evolved O.\nthe commercial RuO performance should be compared, The ECSA/TOF-normalized LSV\n2\ncurves should also be compared.\nReply: We thank you for your valuable suggestions. We have now conducted the\nadditional experiments as recommended, and the results, which have been incorporated into\nthe revised manuscript.\n(1) Long-term CV stability: As shown in Supplementary Fig. 10a, NiOOH in the polarized\nKOH electrolyte exhibits high stability over 3000 CV cycles, with even a slight performance\nimprovement observed.\nSupplementary Fig. 10 | a, 85% iR-compensated LSV curves of NiOOH in polarized KOH (1 M) before and after\n3000-cycle CV stability test.\n(2) Comparison with commercial RuO₂: The LSV curves in Fig. 3a demonstrate that our\nNiOOH in piezoelectric-polarized KOH achieves a current density of 387.6 mA cm-2 at 1.65 V,\nwhich is 3.4 times greater than that of commercial RuO (113.1 mA cm-2 under loading of 10 mg\n2\ncm-2 on Ni foam), highlighting its exceptional activity.\nFig. 3 | a, 85% iR-compensated LSV curves for NiOOH in non-polarized and polarized KOH (1 M) and\ncommercial RuO in pristine 1 M KOH. The piezoelectric PVDF/20BTO film was positioned parallel to the\n2\nworking electrode (cell setup photograph was shown in inset). b, ECSA-normalized LSV curves.\n(3) ECSA-normalized LSV curves and TOF calculations: The intrinsic activity of NiOOH was\nsignificantly enhanced in polarizing KOH electrolyte by PVDF/20BTO film, as evidenced by\nECSA-normalized LSV curves showing a 29.3-fold increase in current density at 1.65 V (1.7 mA\ncm-2 vs. 0.058 mA cm-2, Fig. 3b). This improvement was further corroborated at the active-site\nlevel by TOF value determined by referencing to Ni3+ active sites, which revealed a 7.6-fold rise\nat 1.65 V (64.2 s-1 vs. 8.4 s-1, Supplementary Fig. 6d), confirming that the polarized electrolyte\nenhances the activity of individual active sites.\nSupplementary Fig. 6 | d, TOFs of NiOOH in non-polarzied and polarized KOH at 1.65 V.\nComment 4: The catalytic performance of this work is inferior to those of reported literature\nand even commercial catalysts. The authors may extend the universality of the\nmechano-electrochemical coupling effect to different electrode and electrolyte environments.\nFor instance, replacing Ni(OH) with a typical NiFe LDH catalyst or in neutral electrolytic\n2\nconditions.\nReply: We sincerely thank you for the constructive suggestion regarding the universality of\nthe mechano-electrochemical coupling effect. Following this advice, we have conducted\nadditional experiments to demonstrate the broad applicability of our strategy across different\ncatalytic materials and electrolyte environments.\nFor instance, NiFe LDH in 1 M KOH exhibited a 2.3-fold increase in current density at 1.6 V\nunder polarization, rising from 124.9 to 284.5 mA cm-2 (Supplementary Fig. 9a). Similarly, for\nNiOOH in 1 M phosphate-buffered saline (PBS, pH 7), the current density at 1.5 Vincreased\nfrom 2.26 mA cm-2 in the non-polarized electrolyte to 0.60 mA cm-2 under piezoelectric\npolarization, corresponding to a 3.8-fold enhancement (Supplementary Fig. 9b). Raman\nspectroscopy further revealed a blue shift in the O-H stretching band (2900 - 3800 cm-1) and\nan increase in low-coordinated water molecules in the polarized PBS (Supplementary Figs. 9c,\nd), suggesting a weakening of hydrogen bonds that likely facilitates the water dissociation\nstep, a critical process in neutral-media OER.\nSupplementary Fig. 9 | Broad applicability across diverse catalysts and electrolyte systems. a, b, LSV curves\nof NiFe LDH in 1 M KOH (a) and NiOOH in 1 M phosphate-buffered saline (PBS, b) under non-polarized and\npolarized electrolyte conditions, with 85% iR-compensation. c, Raman spectra of polarized and non-polarized\nneutral PBS (1 M). d, Quantified intensities of water coordination states from Raman spectra in\nSupplementary Fig. 9c.\nFurthermore, the LSV curves in Fig. 3a demonstrate that our NiOOH in polarized KOH\nachieves a current density of 387.6 mA cm-2 at 1.65 V, which is 3.4 times greater than that of\ncommercial RuO (113.1 mA cm-2 under loading of 10 mg cm-2 on Ni foam), highlighting its\n2\nexceptional activity. These new findings confirm that the mechano-electrochemical coupling\neffect is an effective route to enhance water oxidation kinetics.\nFig. 3 | a, 85% iR-compensated LSV curves for NiOOH in non-polarized and polarized KOH (1 M) and\ncommercial RuO in pristine 1 M KOH.\nelectrode-electrolyte interface driven by electrode polarization, particularly the disturbance\nof OH ion concentration, distribution, transport, and hydrogen bond coordination network.\nTherefore, the relavant characterizations and calculations of the electrochemical\nmicroenvironment are required. For example, local pH testing and measurements of the OH-\nions conductivity (Nat Energy 8, 264–272 (2023); Angew. Chem. Int. Ed. 2025, 64, e202505924),\nmolecular dynamics calculations and finite element simulations (Adv. Mater. 2025, 37, 2418527;\nNat Commun 16, 7959 (2025)) are needed.\nReply: We thank you for your suggestions and provide the references. We have\ncarefully read these articles and we agree that characterizing the electrochemical\nmicroenvironment is crucial to substantiate our proposed mechanism. In response, we have\naddressed this point by integrating original evidence (EIS, Raman, ATR-SEIRAS) with new\ncharacterizations (MD simulations, MOR probing) to systematically map the\nmicroenvironment from the bulk electrolyte to the interface.\nEvidence to show undercoordinated OH- for Grotthuss transformation. The shift from\nvehicular to Grotthuss OH- diffusion requires converting hydrated OH- ions from\nhypercoordinated to undercoordinated states22. To investigate how the PVDF/BTO\npiezoelectric field induces this structural transition, we analyzed the 1 M KOH electrolyte\nusing Raman spectroscopy under piezoelectric polarization. As shown in Fig. 2d, the\npiezoelectric field induces a distinct blue shift in the broad O-H stretching band (2900-3800\ncm⁻¹)30, signifying significant hydrogen bond weakening. Quantitative analysis confirms\npiezoelectric polarization drives a profound restructuring of the aqueous network (Fig. 2e):\nthe fully coordinated H O population plummets from 58.6% to 27.3%, while partially\n2\ncoordinated H O surges from 27.6% to 57.7%, and low-coordinated H O increases from 13.8% to\n2 2\n15.0%. This marked redistribution unambiguously demonstrates piezoelectric-field-induced\ngeneration of undercoordinated water configurations, establishing a favorable environment\nfor Grotthuss diffusion.\nFig. 2 | Piezoelectric polarization of electrolyte on porous PVDF/BTO film (20 wt% BTO, 42 μm nylon\nsubstrate). d, Raman spectra of polarized (upper panel) and non-polarized (bottom panel) KOH. The inset\nreveals a blue shift in the O-H stretching band, demonstrating the weakening of hydrogen bonds following\npolarization. e, Quantified intensities of water coordination states from Raman spectra in Fig. 2d. f, Radial\ndistribution function, g(r), of water oxygen atoms (O ) around the OH- oxygen (O -), and their\nw OH\ncorresponding integration numbers of water molecules for the first shell around OH- in polarized and\nnon-polarized KOH. The coordination numbers (CN) were obtained by integrating g(r) up to the position of\nthe first minimum (R ) following the first peak in the g(r) profile. g, h, In situ attenuated total\nmin\nreflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) spectra for NiOOH under open\ncircuit potential (OCP) to 1.7 Vin polarized (g) and non-polarized KOH (h). i, Linear sweep voltammetry (LSV)\ncurves of NiOOH with 85% iR compensation in polarized and non-polarized KOH, and with the addition of 1 M\nCHOH.\nalternating electric field (±15.68 V/mm, 53 kHz) corroborates this finding (Supplementary Fig.\n3). Analysis of the radial distribution function g(r), for the first hydration shell of OH- reveals\nthat the electric field reduces its coordination number (CN). Quantitatively, the CN for the\nfirst hydration shell decreases from ~5.5 to ~4.1 under the alternating electric field stimulation,\nwhich is determined by integrating g(r) up to the position of its first minimum (R ) following\nmin\nthe first peak in the g(r) (Fig. 2f)22. This decrease directly confirms the electric-field-induced\ncreation of undercoordinated OH- .\nEnhanced bulk and interfacial ionic conductivity and local OH- concentration. At the bulk\nlevel, EIS measurements show a decrease in solution resistance from 1.5 to 1.3 Ω in the\npolarized KOH electrolyte (Figs. 3d and e), indicating improved overall ionic conductivity. To\nprobe the interfacial microenvironment, we employed in situ electrochemical Raman\nspectroscopy (Supplementary Fig. 4) and ATR-SEIRAS. The Raman results show that across all\nOER potentials, the NiOOH surface consistently exhibits higher proportions of\nlow-coordinated water molecules, as indicated by Raman bands at 3450 cm-1 and 3600 cm-1,\ncompared to the non-polarized KOH electrolyte. Complementary in situ ATR-SEIRAS\nmeasurements (Figs. 2g and h) show a significant enhancement in the intensity of the\ncharacteristic peak at 3660 cm-1, assigned to the O-H stretching vibration of OH-, directly\nconfirming an increased surface concentration of OH-. The kinetic advantages of this\noptimized interfacial microenvironment were further demonstrated by methanol oxidation\nreaction (MOR) experiments (Fig. 2i). Since MOR competitively consumes adsorbed *OH\nintermediates on the catalyst surface, the MOR current density is proportional to the surface\ncoverage of *OH32. The charge associated with methanol oxidation, quantified by the integral\ncharge difference between linear sweep voltammetry (LSV) curves in KOH with and without\nmethanol, was substantially larger in the polarized electrolyte (integral charge difference:\n32.4 C) than in the non-polarized electrolyte (integral charge difference: 14.9 C). This indicates\na higher surface coverage of *OH intermediates in the polarized electrolyte, confirming an\nelevated local OH- concentration that promotes *OH formation. Collectively, these findings\ndemonstrate that piezoelectric polarization restructures the hydrogen-bonding network of\ninterfacial water, favoring low-coordinated OH- configurations and facilitating efficient\nGrotthuss-type transport of OH-.\nFig. 3 | d, Nyquist plots of NiOOH measured at 1.6 V (Inset shows equivalent circuit for impedance fitting) in\nnon-polarized and polarized KOH (1 M). e, Solution resistance (R) and charge transfer resistance (R ).\ns ct\nSupplementary Fig. 4 | Probing the effects of piezoelectric polarization on interfacial water molecules. a, In\nsitu Raman spectra of NiOOH in polarized and non-polarized KOH using PVDF/20BTO film. b, c, In situ Raman\nspectra and the corresponding intensities of specific water peaks for NiOOH in non-polarized (b) and\npolarized (c) KOH. d, Proportions of different water species on NiOOH derived from the data in\nSupplementary Fig. 4c. e, Proportion of low-coordinated water at varied applied potentials on NiOOH in\npolarized and non-polarized KOH.\nComment 6: The author has provided almost no structural characterization of the catalyst\nNi(OH) , such as XRD, SEM, and TEM, which need to be supplied.\nincluded comprehensive structural characterization of the catalyst.\nXRD and SEM: XRD analysis confirms that both NiOOH samples from the polarized and\nnon-polarized KOH consist mainly of β-Ni(OH) (JCPDS #14-0117) and β-NiOOH (JCPDS #06-0141)\n2\nphases. SEM analysis shows that the NiOOH layer in situ grew on Ni foam.\nSupplementary Fig. 13 | XRD patterns and SEM images of NiOOH in non-polarized and polarized KOH. a, The\nXRD pattern confirms the coexistence of β-Ni(OH) (JCPDS #14-0117) and β-NiOOH (JCPDS #06-0141) phases. b,\n2\nc, SEM images for NiOOH layer grew on nickel foam in non-polarized (b) and polarized (c) KOH electrolyte.\nHRTEM: HRTEM lattice images visualize the atomic-scale structural changes\n(Supplementary Fig. 11b), and the interplanar spacings of the (002) planes in β-NiOOH\ndecrease from 2.41 to 2.26 Å, validating the compressive strain.\nNon-polarized KOH\n(002) 2.41Å\n2nm\nCont2r ancmtion\nPolarized KOH\n(002) 2.26Å\n2nm\nSupplementary Fig. 11 | b, HRTEM lattice imag2e nsm of NiOOH in non-polarized and polarized KOH.\nAdditionally, EXAFS analysis quantitatively shows a significant contraction of the Ni-O\nbond length from 2.04 to 1.91 Å. (Fig. 4b, Supplementary Table 1). These combined structural\ncharacterizations consistently demonstrate the key structural evolution of the material.\nFig. 4 | b EXFAS spectra of Ni foil, NiO, and NiOOH in non-polarized and piezoelectric-polarized KOH.\nSupplementary Table. 1 EXAFS fitting parameters at the Ni K-edge for Ni foil, NiO, and NiOOH in non-polarized\nand piezoelectric-polarized KOH (Ѕ 2=0.85)\nσ2c ΔE0 d\nR factor\n(Å2) (eV)\nNi foil Ni-Ni 12* 2.47±0.01 0.0062 6.7±0.5 0.001\nNi-O 6* 2.07+0.03 0.0089\nNiO -3.9±0.5 0.002\nNi-Ni 12* 2.95+0.02 0.0085\nNi-O 5.85±0.4 2.04±0.08 0.0082\nNiOOH in non-polarized KOH 2.0±0.3 0.011\nNi-Ni 5.94±0.3 3.11±0.07 0.0098\nNi-O 5.08±0.1 1.91±0.01 0.0104\nNiOOH in polarized KOH -3.2±0.2 0.008\nNi-Ni 5.58±0.2 3.06±0.03 0.0089\naCN: coordination numbers; bR: bond distance; cσ2: Debye-Waller factors; d ΔE : the inner potential correction.\nspectroscopy were estimated as CN±20%; R ± 1%; σ2 ± 20%.\nComment 7: The dual-site lattice oxygen mechanism theoretically exhibits poorer stability,\nprimarily due to the insufficient filling of lattice oxygen, which can lead to structural collapse\nand dissolution of metal ions. This instability is evident when comparing the author's results\n(Figure 3f) to the commonly reported stability of alkaline OER catalysts. Consequently, the\nauthor should elucidate the reasons behind the stability issues and propose potential\nsolutions.\nReply: We thank the reviewer for raising this critical point regarding the stability\nconcerns often associated with the lattice oxygen mechanism. We fully agree that the\ninstability arising from irreversible lattice oxygen loss is a fundamental challenge for\nLOM-pathway catalysts.\nOur results indicate that piezoelectric polarization enhances the local OH⁻ concentration\nat the NiOOH/electrolyte interface, as directly supported by the decrease in solution\nresistance (from 1.5 to 1.3 Ω, Figs. 3d and e) and MOR probe experiments (Fig. 2i). This\nenhanced OH⁻ environment facilitates lattice oxygen replenishment, thereby mitigating\nstructural degradation. This mechanism contributes to the improved stability observed in our\ncatalyst, which exhibits no degradation after 3000 CV cycles and slow decay during a 100-h\nchronoamperometry test (Fig. 3f and Supplementary Fig. 10a).\nFig. 3 | d, Nyquist plots of NiOOH measured at 1.6 V (Inset shows equivalent circuit for impedance fitting) in\nnon-polarized and polarized KOH (1 M). e, Solution resistance (R) and charge transfer resistance (R ). f,\ns ct\nElectrochemical stability tests of NiOOH in non-polarized and polarized KOH.\nFig. 2 | i, Linear sweep voltammetry (LSV) curves of NiOOH with 85% iR compensation in polarized and\nnon-polarized KOH, and with the addition of 1 M CH OH.\n3000-cycle CV stability test.\nAlthough our study demonstrates that piezoelectric polarization of the electrolyte\ninduces a Grotthuss-type OH- state, thereby increasing the OH- concentration at the\nelectrode/electrolyte interface and facilitating lattice oxygen repair, which to some extent\nenhances the stability of the LOM/DOM mechanism. However, comparing the behavior of\nNiOOH in non-polarized (AEM pathway) versus externally polarized (LOM/DOM pathway)\nelectrolytes highlights the intrinsic instability of the latter. This instability stems primarily from\nthe frequent evolution and slow regeneration of lattice oxygen, which accelerates the\ndissolution of active species. Such degradation can be alleviated by accelerating lattice\noxygen regeneration, for instance, through the use of oxyphilic alloys that enhance the\ncapture of oxygen intermediates and facilitate rapid recovery of oxygen vacancies (Angew.\nChem. Int. Ed. 64, e202413250 (2025)).\nThe corresponding description was added into the main text: Indeed, comparing the\nbehavior of NiOOH in non-polarized (AEM pathway) versus externally polarized (LOM/DOM\npathway) electrolytes highlights the intrinsic instability of the latter. This instability stems\nprimarily from the frequent evolution and slow regeneration of lattice oxygen, which accelerates\nthe dissolution of active species. Such degradation can be alleviated by accelerating lattice\noxygen regeneration, for instance, through the use of oxyphilic alloys that enhance the capture\nof oxygen intermediates and facilitate rapid recovery of oxygen vacancies33.\nReviewer #2 (Remarks to the Author):\nThis paper reports a strategy for ultrasound-induced piezoelectric polarization to drive\nelectrolytes into a rapid OH- Grothuss diffusion state. The author claims that this altered OH-\ndiffusion state achieves enhanced OH- enrichment on the catalyst surface, thereby enhancing\nthe catalyst's performance. Although this work seems impressive in some respects, some of\nthe core arguments contain several key flaws.\nComment 1: Under the electrolyte condition of 1 M KOH, considering that the area around the\nelectrode is inherently filled with a high concentration of OH-, and the PVDF/BTO is not closely\nattached to the electrode (leaving a gap of approximately 2 mm), the significant performance\nimprovement brought about by the long-distance OH- diffusion in the electrolyte is\nastonishing but questionable. Furthermore, it is obviously more practical to focus on the\nimpact under near-neutral/neutral conditions.\nReply: We thank the reviewer for raising these important points regarding the\nmechanism and practical applicability of our approach. In our experiments, a piece of\nPVDF/BTO composite film with a loading area of 1 x 2 cm2 and a loading density of 5.8 mg/cm2\nwas deposited on a nylon mesh substrate. This composite film was used to polarize 50 mL of\nelectrolyte under ultrasound treatment for 1 min, as confirmed by Raman spectroscopy. To\nfacilitate polarization during the OER measurements, the PVDF/BTO/nylon mesh composite\nwas positioned parallel to the NiOOH electrode, with a gap of approximately 2 mm\nmaintained by a nylon spacer. The assembly was sonicated in KOH electrolyte for 1 min prior\nto the OER tests. Notably, in practical applications, the composite film does not need to be\nfixed in a specific location within the electrolyte. Due to the rapid diffusion induced by\nultrasound, effective polarization can be achieved regardless of the film’s position in the\nelectrolyte.\nTo address applicability in neutral media, we evaluated our system in phosphate-buffered\nsaline (PBS, 1 M), and we add the following description into the main text: For NiOOH in 1 M\nphosphate-buffered saline (PBS, pH 7), the current density at 1.5 Vincreased from 2.26 mA cm-2 in\nthe non-polarized electrolyte to 0.60 mA cm-2 under piezoelectric polarization, corresponding to\na 3.8-fold enhancement (Supplementary Fig. 9b). Raman spectroscopy further revealed a blue\nshift in the O-H stretching band (2900 - 3800 cm-1) and an increase in low-coordinated water\nmolecules in the polarized PBS (Supplementary Figs. 9c, d), suggesting a weakening of hydrogen\nbonds that likely facilitates the water dissociation step, a critical process in neutral-media OER.\nSupplementary Fig. 9 | b, LSV curves of NiOOH in 1 M PBS under non-polarized and polarized electrolyte\nconditions, with 85% iR-compensation. c, Raman spectra of polarized and non-polarized neutral PBS (1 M). d,\nQuantified intensities of water coordination states from Raman spectra in Supplementary Fig. 9c.\nComment 2: All the electrochemical tests conducted by the author were carried out in fixed\ncells, lacking tests under the condition that the electrolyte maintains a circulating flow in\npractical applications. Can this Grotthuss diffusion achieve a similar effect in a flowing cell?\nReply: We thank the reviewer for raising this important point about flow conditions. As\nsuggested, circulating electrolyte experiment was carried out and we add the following\ndescription into the main text: Crucially, flow-cell experiments provided strong validation: unlike\nthe non-polarized electrolyte, the OER performance showed no improvement under forced\nconvection when the electrolyte was polarized (Supplementary Fig. 7c). This lack of\nenhancement is consistent with the intrinsically superior mass transport properties of the\npolarized electrolyte itself.\nSupplementary Fig. 7 | c, LSV curves of NiOOH under static (polarized/non-polarized) and circulating\n(polarized/non-polarized) electrolyte conditions.\nComment 3: The author proposed a strategy that uses Grotthuss diffusion to enhance OH\ntransport on the catalyst surface as the fundamental reason for the superior performance of\nthe material. However, they failed to provide convincing evidence to prove the enhanced\nenrichment of OH on the surface of the catalyst. The characterization methods used, such as\nin-situ ATR-SEIRAS and EIS techniques themselves, cannot achieve quantitative comparison of\nOH coverage on the catalyst surface. There is insufficient direct and indirect evidence to\nsupport this conclusion, and more techniques, such as Methanol molecular probe experiment,\nlocal pH test, and in-situ EIS, are needed to verify this hypothesis.\nReply: We thank the reviewer for this critical comment regarding the direct evidence for\nOH- enrichment. As suggested, we have addressed this concern by incorporating three key\nexperiments: 1) Quantitative methanol oxidation probe tests showing the enhanced OH-\ncoverage; and 2) Flow-cell experiments demonstrating that the OER performance under\npolarized electrolyte shows no improvement under forced convection, supporting the\nexistence of efficient surface OH- transport via Grotthuss-type diffusion; 3) The solution\nresistance measured by EIS to show the accelerated OH- transport.\nThus, we add the following description in the main text: The kinetic advantages of this\noptimized interfacial microenvironment were further demonstrated by methanol oxidation\nreaction (MOR) experiments (Fig. 2i). Since MOR competitively consumes adsorbed *OH\nintermediates on the catalyst surface, the MOR current density is proportional to the surface\ncoverage of *OH32. The charge associated with methanol oxidation, quantified by the integral\ncharge difference between linear sweep voltammetry (LSV) curves in KOH with and without\nmethanol, was substantially larger in the polarized electrolyte (integral charge difference: 32.4 C)\nthan in the non-polarized electrolyte (integral charge difference: 14.9 C). This indicates a higher\nsurface coverage of *OH intermediates in the polarized electrolyte, confirming an elevated local\nOH- concentration that promotes *OH formation.\nFig. 2 | i, Linear sweep voltammetry (LSV) curves of NiOOH with 85% iR compensation in polarized and\nnon-polarized KOH, and with the addition of 1 M CHOH.\nelectrolyte, the OER performance showed no improvement under forced convection when the\nelectrolyte was polarized (Supplementary Fig. 7c). This lack of enhancement is consistent with\nthe intrinsically superior mass transport properties of the polarized electrolyte itself.\nSupplementary Fig. 7 | c, LSV curves of NiOOH under static (polarized/non-polarized) and circulating\n(polarized/non-polarized) electrolyte conditions.\nFurther mechanistic insight was provided by electrochemical impedance spectroscopy (EIS)\nat 1.6 V (Figs. 3d, e and Supplementary Figs. 7a, b), which showed reductions in both solution\nresistance (R : 1.5 → 1.3 Ω) and charge-transfer resistance (R : 16.8 → 8.9 Ω). These results suggest\ns ct\nimproved OH- mobility and a lower interfacial energy barrier during OER.\nFig. 3 | Enhanced OER performance in the piezoelectric-polarized electrolyte. d, Nyquist plots of NiOOH\nmeasured at 1.6 V (Inset shows equivalent circuit for impedance fitting) in non-polarized and polarized KOH (1\nM). e, Solution resistance (R) and charge transfer resistance (R ).\ns ct\nComment 4: As a novel means of regulating OER performance through external stimuli,\nauthors should add clear schematic diagrams (such as the specific structure of the assembled\nthree-electrode) to enhance the readability of the manuscript.\nReply: We thank the reviewer for this valuable suggestion. As recommended, we have\nadded schematic diagrams and actual photographs (inset of Fig. 3a and Supplementary Fig. 5)\nof the experimental setup, including the specific structure of the assembled three-electrode\ncell with the piezoelectric film, to the manuscript. These additions are intended to\nsignificantly enhance the readability and clarity of our work.\nTo provide clear experimental context, we have added the following description to the\nmain text: To facilitate electrolyte polarization, the PVDF/BTO/nylon mesh composite film was\npositioned parallel to the working electrode, with an approximate 2 mm gap maintained at the\nfixed end by a nylon spacer (see the inset in Fig. 3a). Prior to OER measurements, the system was\nsonicated in the KOH electrolyte for 1 min as illustrated in Supplementary Fig. 5.\nFig. 3 | Enhanced OER performance in the piezoelectric-polarized electrolyte. a, 85% iR-compensated LSV\ncurves for NiOOH in non-polarized and polarized KOH (1 M) and commercial RuO in pristine 1 M KOH. The\n2\npiezoelectric PVDF/20BTO film was positioned parallel to the working electrode (cell setup photograph was\nshown in inset).\nSupplementary Fig. 5 | Illustration of the experimental procedure. The KOH electrolyte is first polarized by\napplying ultrasound to the PVDF/20BTO film for 1 min. Subsequently, electrochemical measurements are\nperformed on the NiOOH electrode immersed in the pre-polarized electrolyte.\nComment 5: Statistical analysis is required to ensure the reliability of the data.\nReply: We thank the reviewer for highlighting the importance of statistical analysis. In\nresponse, we have conducted multiple independent replicates for key experiments to ensure\ndata reliability. The reproducibility of the piezoelectric effect was first confirmed by\nrepeatedly measuring the piezoelectric currents for porous PVDF films with varying BTO\ncontent (0-40 wt%) under ultrasonic treatment (Supplementary Fig. 2c).\nSupplementary Fig. 2 | c, Average piezoelectric current density of dense and porous PVDF films with varying\nBTO content (0-40 wt%) in 1 M KOH.\nMost importantly, to statistically validate the core claim of enhanced OER performance,\nat least three independent samples were used to compare the OER performance of NiOOH in\npiezoelectric-polarized KOH versus non-polarized KOH. The results consistently confirm that\npiezoelectric polarization of the KOH electrolyte dramatically enhances the OER activity of\nNiOOH, reducing the overpotential at 100 mA cm⁻² by 222.4 mV (from 599.7 ± 10.5 mV to 377.3\n± 7.3 mV; Supplementary Fig. 6b). These repeated experiments support the reproducibility\nand statistical significance of our conclusions.\nSupplementary Fig. 6 | b, Average overpotential at 100 mA cm-2 of NiOOH in polarized and non-polarized KOH\nwith error bars based on at least three independent measurements.\nComment 6: The author needs to explain whether iR correction was performed when using\nLSV data.\nReply: We thank the reviewer for this comment. In response, the revised manuscript now\nexplicitly states that the principle of iR compensation (85%) has been applied to ensure the\naccuracy of the electrochemical data presented. The uncompensated solution resistance was\ndetermined for each condition prior to the correction. The presented polarization curves now\naccurately reflect the intrinsic catalytic activity, free from significant solution resistance\neffects.\nReviewer #3 (Remarks to the Author):\nIn this work, the authors established a discontinuous piezoelectric polarization of electrolyte\nvia ultrasonic to increase the performance of OER. By integrating PVDF+BTO with Ni(OH)\n2\nelectrode (2 mm gap), the piezoelectric field achieves dual regulation: (1) electrolyte\nmodulation, and (2) in situ catalyst activation, which yields a 5.7-fold current density\nenhancement, maintaining 2.9-fold enhancement after 24 h operation with 88.7% activity\nrecovery via 1 min reactivation, and preserving 2.7-fold current density after 100 h stability test\noperation. The method is interesting and effective. However, the evidence of this work and\npractical application of this idea seems not enough. It can be considered after following\npoints are addressed.\nComment 1: Please confirm whether PVDF+BTO and PVDF/BTO in the figure refer to the same\nmaterial. If so, please ensure consistency between the figure and the manuscript.\nReply: We thank the reviewer for pointing out this inconsistency. Yes, “PVDF+BTO” and\n“PVDF/BTO” refer to the same composite material. We have now revised the manuscript and\nfigures to use the consistent notation “PVDF/BTO” throughout.\nComment 2: If possible, could authors add some theoretical calculation for the mechanism?\nReply: We sincerely thank the reviewer for this insightful suggestion. To provide a robust\ntheoretical foundation for our proposed mechanism, we have conducted comprehensive\nmolecular dynamics (MD) and density functional theory (DFT) calculations. The results\nstrongly corroborate our experimental findings and offer multi-scale insights into the\nenhanced OER kinetics under piezoelectric polarization.\nMD Simulations: Elucidating the electrolyte microenvironment. We add the following\ndescription into the main text: Molecular dynamics (MD) simulations under a mimicking\nalternating electric field (±15.68 V/mm, 53 kHz) corroborates this finding (Supplementary Fig. 3).\nAnalysis of the radial distribution function g(r), for the first hydration shell of OH- reveals that\nthe electric field reduces its coordination number (CN). Quantitatively, the CN for the first\nhydration shell decreases from ~5.5 to ~4.1 under the alternating electric field stimulation, which\nis determined by integrating g(r) up to the position of its first minimum (R ) following the first\nmin\npeak in the g(r) (Fig. 2f)22. This decrease directly confirms the electric-field-induced creation of\nundercoordinated OH-.\nSupplementary Fig. 3 | Piezoelectric voltage and corresponding electrolyte simulations. a, Piezoelectric\nvoltage output of the PVDF/20BTO film under 53 kHz ultrasonic excitation. b, c, Molecular dynamics\nsimulation snapshots of the KOH electrolyte with (b) and without (c) an applied alternating electric field of\n±15.7 V/mm at 53 kHz, which mimics the field generated by the piezoelectric film. Atomic color code: K⁺\n(green); O (red) and H (white) of OH⁻. Water molecules are shown as thin sticks (O: red, H: white).\nFig. 2 | f, Radial distribution function, g(r), of water oxygen atoms (O ) around the OH- oxygen (O -), and\nw OH\ntheir corresponding integration numbers of water molecules for the first shell around OH- in polarized and\nnon-polarized KOH. The coordination numbers (CN) were obtained by integrating g(r) up to the position of\nthe first minimum (R ) following the first peak in the g(r) profile.\nmin\nDFT calculations: Unraveling the electronic structure origin. We add the following\ndescription into the main text: Density functional theory (DFT) calculations provide\nfundamental insight into the increased M-O covalency. According to the phase and bond length\nfrom the XRD and EXAFS results (Supplementary Fig. 13 and Supplementary Table 1), the β-NiOOH\nstructures with (in polarized electrolyte) and without Ni-O bond contraction (in non-polarized\nelectrolyte) were created. As shown by the projected density of states (PDOS) in Fig. 4e, NiOOH\nformed in polarized KOH exhibits significantly enhanced overlap between Ni 3d and O 2p orbitals\nnear the Fermi level, directly indicating strengthened Ni-O covalency44. This intensified covalent\ninteraction promotes electron delocalization, as reflected by the broader and smoother PDOS\npeaks45, which facilitates charge distribution across the Ni-O network and thereby reduces\ncharge-transfer barriers. The strengthening of Ni-O covalency is quantitatively supported by\ncrystal orbital Hamiltonian population (COHP) analysis, which yields a more negative integrated\nCOHP (ICOHP) value of -1.6 for the NiOOH in polarized electrolyte compared to -1.19 for the NiOOH\nin non-polarized electrolyte (Fig. 4f). Notably, an increased density of unoccupied states is\nobserved in the oxygen non-bonding (O ) region just above the Fermi level. This feature is\nNB\nessential for activating lattice oxygen (O ) and enhancing its donor character46, thereby\nlat\npredicting a lattice oxygen-mediated OER pathway on NiOOH in polarized electrolyte.\nSupplementary Fig. 13 | XRD patterns of NiOOH in non-polarized and polarized KOH. The XRD pattern\nconfirms the coexistence of β-Ni(OH) (JCPDS #14-0117) and β-NiOOH (JCPDS #06-0141) phases.\nand piezoelectric-polarized KOH (Ѕ 2=0.85)\nσ2c ΔE0 d\nR factor\n(Å2) (eV)\nNi foil Ni-Ni 12* 2.47±0.01 0.0062 6.7±0.5 0.001\nNi-O 6* 2.07+0.03 0.0089\nNiO -3.9±0.5 0.002\nNi-Ni 12* 2.95+0.02 0.0085\nNi-O 5.85±0.4 2.04±0.08 0.0082\nNiOOH in non-polarized KOH 2.0±0.3 0.011\nNi-Ni 5.94±0.3 3.11±0.07 0.0098\nNi-O 5.08±0.1 1.91±0.01 0.0104\nNiOOH in polarized KOH -3.2±0.2 0.008\nNi-Ni 5.58±0.2 3.06±0.03 0.0089\naCN: coordination numbers; bR: bond distance; cσ2: Debye-Waller factors; d ΔE : the inner potential correction.\nspectroscopy were estimated as CN±20%; R ± 1%; σ2 ± 20%.\nFig. 4 | e, PDOS band centers of the O 2p and Ni 3d orbitals in NiOOH in non-polarized and\npiezoelectric-polarized KOH. f, Crystal orbital Hamilton population (COHP).\nDFT Calculations: Unraveling the OER mechanism. We add the following description into\nthe main text: DFT calculations of the free energy on the (100) facet of β-NiOOH without Ni-O\nbond contraction (in non-polarized electrolyte) reveal that the deprotonation steps exhibit the\nhighest energy barriers for each reaction pathway: 2.15→eV for *OH → *O(AEM pathway), 2.29→eV\nfor O + O H → O -O (DOM pathway), and 2.22→eV for O H + *OH → O + *OH (LOM pathway),\nlat lat lat lat lat lat\nrespectively (Supplementary Fig.→16). These results clearly indicate that AEM is the most\nfavorable pathway on NiOOH in a non-polarized electrolyte. In contrast, under polarized\nelectrolyte conditions, the rate-determining steps (RDS) shift to: *OOH → O deprotonation\n2\n(2.35→eV, AEM), O H + O H → O + O Hdeprotonation (1.91→eV, DOM), and O + O H → O H +\nlat lat lat lat 2 lat lat\n*OH hydroxyl ion adsorption (2.19→eV, LOM), as summarized in Fig.→5f. Notably, the LOM and\nDOM pathways collectively exhibit more favorable kinetics on NiOOH under polarization,\nunderscoring their essential role in achieving superior OER activity. Taken together, we therefore\nconclude that the applied piezoelectric polarization induces Grotthuss-type OH- transport, which\nnot only increases the local OH- concentration at the electrode surface but also modulates the\nelectronic state of NiOOH during catalyst reconstruction. These changes weaken O-H bonds and\nstrengthen Ni-O covalency within the Ni-O-H motifs. The intensified Ni-O covalency facilitates\nelectron delocalization across the metal-oxygen network, concurrently shifting the lower\nHubbard band (LHB) downward and the O→2p non-bonding band upward toward the Fermi\nlevel53,54 (Fig.→1), thereby modifying the electronic structure of the active phase. This electronic\nreconfiguration activates O , enabling two low-barrier pathways: direct O -*O coupling and\nlat lat\nO -O coupling.\nlat lat\nSupplementary Fig. 16 | Gibbs free energy diagram for the AEM, LOM, and DOM OER pathways on NiOOH in\nnon-polarized KOH.\nFig. 5 | f, Gibbs free energy diagram for the AEM, LOM, and DOM OER pathways on NiOOH in polarized KOH,\nwith the rate-determining step.\nIn summary, these theoretical calculations provide a multi-faceted mechanistic\nunderstanding, from the modified electrolyte structure to the activated catalyst electronic\nstates and the favored reaction pathway, strongly supporting our experimental hypothesis.\nComment 3: As authors claimed that the “porous pure PVDF itself exhibited a substantially\nstronger piezoelectric effect (current density -0.9 μA/cm2, Supplementary Fig. 3c) than its\nnon-porous counterpart”, have the authors investigated the effect of the porous structure\n(e.g., pore size) on the polarization behavior?\nReply: Thank you for this important question. In our study, a PVDF/BTO/nylon mesh\nsubstrate composite film was used to produce the piezoelectric effect. Therefore, we\nconsider to optimize the piezoelectric effect of PVDF/BTO/nylon mesh composite film. The\nporous structures of PVDF/20BTO/nylon mesh film was described in details by SEM\nobservations.\nSupplementary Fig. 1 | Fabrication and characterization of porous PVDF/BTO piezoelectric films. a,\nIllustration of the fabrication process and a macroscopic photo of a resultant PVDF/BTO porous film. b-d, SEM\nimages of the nylon mesh substrates with varying feature sizes (212, 42, and 17 μm). e-g, SEM images of the\nPVDF/20BTO films obtained using the substrates in Supplementary Fig. 1b-d.\n%)\n1\n2\n3\n0\n0\n0\n0\n0 1000200030004000\nSize (nm)\nFig. 2 | Piezoelectric polarization of electrolyte on porous PVDF/BTO film (20 wt% BTO, 42 μm nylon\nsubstrate). a, SEM image of PVDF/20BTO composite film. Inset shows the higher-magnification view and the\npore size distribution.\nSupplementary Fig. 2 | f, LSV curves of NiOOH in polarized KOH, using PVDF/20BTO on nylon meshes with\nvarying pore sizes (212, 42, and 17 μm).\nAccording to these results, we added the following content into main text: Compared to\nthe dense PVDF film, which shows a piezoelectric current density of -0.36 ± 0.14 μA/cm2, the porous\nPVDF film exhibits a significantly higher value of -0.23 ± 0.03 mA/cm2. This enhancement underscores\nthe important role of the hierarchical porous structure in promoting effective polarization\n(Supplementary Fig. 2a-c). Among the composite films, the porous PVDF/BTO film with 20 wt% BTO\n(denoted PVDF/20BTO) delivers the highest piezoelectric current density of -0.65 ± 0.07 mA/cm2,\nwhich can be attributed to the synergistic coupling between the PVDF matrix and BTO fillers (Fig. 2,\nSupplementary Fig. 2b-d). Further assessment of piezoelectric performance via piezoresponse force\nmicroscopy reveals that the 20 wt% BTO composite achieves a d value of 29.9 ± 0.65 pm/V, marking a\n33\n39.7% improvement over PVDF (d = 21.4 ± 0.23 pm/V) (Fig. 2c, Supplementary Fig. 2e). To evaluate the\n33\npolarization effect of the PVDF/BTO/nylon mesh substrate film on the KOH electrolyte, the OER\nperformance on NiOOH was examined. The results indicate that a pore size of 42 μm in the nylon\nmesh substrate is optimal (Supplementary Fig. 2f), suggesting that the substrate's pore structure\ninfluences the piezoelectric polarization within the electrolyte. This optimal pore size represents a\nbalance between maximizing the electrolyte contact area and preserving polarization effects, unlike\nlarger pores (212 μm), which reduce contact area, or smaller pores (17 μm), which may impede the\npiezoelectric response.\nComment 4: Please provide the detailed preparation procedure, optimal image, SEM image of\nthe nylon mesh after the pre-treatment process, and SEM cross-sectional image of the\nNi(OH) /PVDF/BTO electrode used in the OER test.\nPreparation Procedure and Optical and SEM Images of pretreated Nylon mesh. As\nsuggested, we add the following description into the main text: To generate a piezoelectric\npolarization field, we prepared a porous polyvinylidene fluoride (PVDF)/BaTiO (BTO) composite\n3\nfilm on a nylon mesh substrate using a simple impregnation method (Supplementary Figs. 1a-g).\nPiezoelectric film preparation was added to the Methods section: Dense PVDF film preparation:\nA dense PVDF film was prepared by dissolving 0.5 g of PVDF powder in 5 mL of DMF under\nmagnetic stirring at 70→oC to obtain a transparent solution. The solution was then cast onto a\nglass substrate, dried at room temperature, and peeled off to form a free-standing dense film.\nPorous PVDF film preparation: A porous PVDF film was fabricated on a nylon mesh substrate.\nFirst, a nylon mesh with a pore size of 42 μm was cut into pieces (approximately 1 x 2 cm2) and\ncleaned by sequential rinsing with acetone and deionized water (three times each). Subsequently,\n0.5 g of PVDF powder was dissolved in 5 mL of DMF at 70 oC to form a precursor solution. The\ncleaned mesh was immersed in the solution, withdrawn, and immediately sprayed with a\ncoagulation bath composed of water and ethanol (2:1 mass ratio) to induce phase separation.\nThe resulting film was dried at room temperature to yield the porous PVDF film.\nPorous PVDF/BTO composite film preparation: Porous PVDF/BTO composite films were\nprepared by incorporating BTO powder at mass fractions of 0, 10, 20, and 40 wt% into the PVDF\nsolution (0.5 g PVDF in 5 mL DMF), followed by homogenization using a vortex mixer. Nylon\nmeshes with pore sizes of 212, 42, and 17 μm were cut and cleaned as described above, then\nimpregnated with the corresponding PVDF/BTO solution. The impregnated meshes were sprayed\nwith the same water/ethanol coagulation bath (2:1 mass ratio) and dried at room temperature,\nfollowing the procedure used for the porous PVDF film. A specific PVDF/BTO/nylon mesh\ncomposite film was fabricated with a PVDF/BTO loading area of 1 x 2 cm2 and a loading density of\n5.8 mg/cm2.\nSupplementary Fig. 1 | Fabrication and characterization of porous PVDF/BTO piezoelectric films. a,\nIllustration of the fabrication process and a macroscopic photo of a resultant PVDF/BTO porous film. b-d, SEM\nimages of the nylon mesh substrates with varying feature sizes (212, 42, and 17 μm). e-g, SEM images of the\nPVDF/20BTO films obtained using the substrates in Supplementary Fig. 1b-d.\n%)\n1\n2\n3\n0\n0\n0\n0\n0 1000200030004000\nSize (nm)\nFig. 2 | Piezoelectric polarization of electrolyte on porous PVDF/BTO film (20 wt% BTO, 42 μm nylon\nsubstrate). a, SEM image of PVDF/20BTO composite film. Inset shows the higher-magnification view and the\npore size distribution.\nSEM cross-sectional image of NiOOH/PVDF/BTO electrode: We appreciate this request\nfor clarification. In our cell design, was positioned parallel to the working electrode, with an\napproximate 2 mm gap maintained at the fixed end by a nylon spacer (see the inset in Fig. 3a).\nTo enhance clarity, we have added more detailed descriptions in the main text: To facilitate\nelectrolyte polarization, the PVDF/BTO film was positioned parallel to the working electrode,\nwith an approximate 2 mm gap maintained at the fixed end by a nylon spacer (see the inset in Fig.\n3a).\nFig. 3 | Enhanced OER performance in the piezoelectric-polarized electrolyte. a, 85% iR-compensated LSV\ncurves for NiOOH in non-polarized and polarized KOH (1 M) and commercial RuO in pristine 1 M KOH. The\n2\npiezoelectric PVDF/20BTO film was positioned parallel to the working electrode (cell setup photograph was\nshown in inset).\nComment 5: In the V-T curve (stability test), the initial position of the curve does not match\nthat of the LSV curve (lower current density at same potential). Could the authors explain the\nreason for this discrepancy?\nReply: We thank the reviewer for this insightful comment. The observed difference in\ncurrent density at 1.7 V between the LSV and the chronoamperometry (i–t) curves originates\nfrom the distinct measurement modes employed. In the LSV curve recorded at a scan rate of\n5 mV s-1, the measured current includes contributions from the charging current, the oxygen\nevolution reaction (OER) current, and non-steady-state currents such as those arising from\nthe redox processes of catalytic centers. In contrast, the i-t stability test reflects the\nsteady-state OER current after the system has stabilized. Therefore, it is generally observed\nthat the current density in the LSV is higher than that under steady-state i-t conditions.\nSupplementary Fig. 10| d, LSV curves of NiOOH in non-polarized and polarized KOH (1 M) without additional\niR-compensation at a scan rate of 5 mV s-1. and Fig. 3 | f, Electrochemical stability tests of NiOOH in\nnon-polarized and polarized KOH at 1.7 V, without iR compensation.\nComment 6: In practical applications, how do the authors introduce the ultrasonic field to\ninduce the piezoelectric polarization effect? And how to place a PVDF/BTO (2 mm) near\nelectrode in the water electrolyzers?\nReply: Thank you for this insightful comment, which points directly toward the real-world\napplication of our concept. Our proposed solution for scalable application is to house the\npiezoelectric component within an external module integrated directly into the electrolyte\nflow path of a flow-type electrolyzer, as illustrated in Supplementary Fig. 17. This design\nallows a pulsed ultrasonic field to polarize the electrolyte before it enters the reaction\nchamber, effectively decoupling the polarization process from the electrolysis reaction for a\nmore robust and scalable system.\nSupplementary Fig. 17 | A schematic diagram depicting an industrial application of mechano-electrochemical\ncoupling water electrolysis: a flow-type electrolyzer integrated with an external piezoelectric module for\nelectrolyte polarization.\nIn our study, to facilitate electrolyte polarization, the PVDF/BTO/nylon mesh composite\nfilm was positioned parallel to the working electrode, with an approximate 2 mm gap\nmaintained at the fixed end by a nylon spacer (see the inset in Fig. 3a). It is noteworthy that,\nowing to the rapid diffusion induced by ultrasound, the composite film can be positioned\nanywhere within the electrolyte to achieve uniform polarization.\nFig. 3 | Enhanced OER performance in the piezoelectric-polarized electrolyte. a, 85% iR-compensated LSV\ncurves for NiOOH in non-polarized and polarized KOH (1 M) and commercial RuO in pristine 1 M KOH. The\n2\npiezoelectric PVDF/20BTO film was positioned parallel to the working electrode (cell setup photograph was\nshown in inset).\nSupplementary Fig. 5 | Illustration of the experimental procedure. The KOH electrolyte is first polarized by\napplying ultrasound to the PVDF/20BTO film for 1 min. Subsequently, electrochemical measurements are\nperformed on the NiOOH electrode immersed in the pre-polarized electrolyte.\nComment 7: For the mechanism exploration, the in-situ Raman showed that Ni experienced\nlower oxidation potential with the polarized KOH, which means Ni is easier to be oxidized.\nHowever, in the LSV curves, Ni shows a higher oxidation peak, which means it is harder to be\nactivated. These results are self-contradictory. The author should re-examine the\nexperimental results and provide reasonable explanations; otherwise, the proposed theory\nwill not be convincing.\nReply: We thank the reviewer for this important observation. The results are not\ncontradictory but complementary, reflecting different aspects of the oxidation process under\npolarization. The in-situ Raman shows a lower onset potential for NiOOH formation,\nindicating that polarization thermodynamically favors the initial oxidation. The higher LSV\noxidation peak reflects a greater extent of synchronous surface conversion driven by\npolarization. The intensified current results from promoted Ni(OH) →NiOOH transformation,\n2\nnot increased difficulty.\nTogether, the two observations are consistent: the polarized electrolyte (1) lowers the\ninitiation potential for the formation of the active phase (as shown by in situ Raman,\nSupplementary Figs. 12a, b), and (2) enhances the rate and extent of the surface\ntransformation across the electrode (resulting in a sharper, more intense LSV peak, Fig. 3a).\nThis coherent picture strongly supports our theory that piezoelectric polarization facilitates\nthe generation of the critical NiOOH species, thereby enhancing the OER activity.\nSupplementary Fig. 12 | a, b, In situ Raman spectra of NiOOH in non-polarized (a) and polarized (b) 1 M KOH\nelectrolyte.\nFig. 3 | a, 85% iR-compensated LSV curves for NiOOH in non-polarized and polarized KOH (1 M) and\ncommercial RuO in pristine 1 M KOH.\n(Supplementary Fig 3b), why the performance of OER is very different between these two?\nAlso should re-check and provide the explanations.\nReply: We thank the reviewer for this insightful observation. While the piezoelectric\ncurrent densities for the 10 wt% and 20 wt% BTO samples are relatively close, our repeated\nmeasurements confirm a statistically significant difference between them (Supplementary Fig.\n3d). Specifically, the PVDF/20BTO composite consistently generated a higher piezoelectric\ncurrent density (-0.65 ± 0.07 mA/cm²) compared to the PVDF/10BTO control (-0.51 ± 0.07\nmA/cm²). This quantifiable enhancement in field strength, though modest, may have crossed\na critical threshold to qualitatively alter the interfacial polarization of the electrolyte. This\noptimized microenvironment at the catalyst surface could lead to a disproportionate boost in\nOER performance by more effectively concentrating reactants or lowering activation barriers.\nSupplementary Fig. 2 | c, Average piezoelectric current density of dense and porous PVDF films\nwith varying BTO content (0-40 wt%) in 1 M KOH.\nComment 9: For DEMS test, did authors calibrate the gas amount? How authors quantify the\nspecific percentage of different products? The authors should provide detailed description of\nthis. Otherwise, the conclusion is not convincing.\nReply: We thank the reviewer for this important question regarding the DEMS\nquantification. The principle of DEMS quantification is that the mass spectrometer measures\nthe ion current for specific mass-to-charge ratios (m/z) unique to each gas. The detected ion\ncurrent signal is proportional to the partial pressure (concentration) of the gas in the chamber.\nIn our analysis, the relative product distribution was determined by comparing the integrated\nsignal intensities (i.e., the area under the ion current versus time curve) for the characteristic\nm/z values of the different oxygen isotopologues (e.g., m/z = 32 for ¹⁶O₂ and m/z = 36 for ¹⁸O₂).\nThis approach of comparing the ratios of these integrated values is a well-established\nsemi-quantitative method in the DEMS field for determining relative product selectivity [J. Am.\nChem. Soc. 2024, 146, 33276−33287].\nTo enable a direct comparison of the relative product distribution between samples,\nthe integrated intensity for each isotopologue (I , I , I ) was normalized to the total oxygen\n36 34 32\nevolution signal (I + I + I ) for that specific measurement, and we add the following\n32 34 36\ndescriptions into the main text: Based on this electronic foundation conducive to O activation\nlat\nvia enhanced Ni-O covalency40, we elucidated the OER mechanism in the piezoelectric-polarized\nelectrolyte via 18O-labeled differential electrochemical mass spectrometry (DEMS). In\npiezoelectric-polarized 18O-labeled KOH, NiOOH generates 36O (18O18O coupling), 34O (16O18O\n2 2\ncoupling), and 32O (16O16O coupling) (Fig. 5a, b, Supplementary Fig. 14). Quantitatively, the ratio\n2\nof 36O (I /I + I + I ) exhibits a dramatic increase to 13.6%, which is 9.07 times higher than the\n2 36 36 34 32\nvalue of 1.5% observed in the non-polarized KOH. Concurrently, the ratio of 34O (I /I + I + I )\n2 34 36 34 32\nrises to 54.2%, a 5.89-fold enhancement over the 9.2% in the control. These results unambiguously\ndemonstrate that OER in the polarized electrolyte follows the lattice oxygen mechanism (LOM,\ninvolving direct coupling of O with adsorbed oxygen, O -*O, yielding 34O ) and diatomic oxygen\nlat lat 2\nmechanism (DOM, involving O -O coupling, yielding 36O )47, bypassing high-energy adsorbed\nlat lat 2\nintermediates (*O/*OOH)48. In contrast, OER in non-polarized electrolyte mainly follows the\ntypical AEM involving oxygen-containing intermediates (*OH, *OO, *OOH).\nFig. 5 | a, DEMS signals of the 36O, 34O and 32O products for 18O-labeled NiOOHin polarized KOH electrolyte.\n2 2 2\nb, The ratio of 36O (I /I + I + I ), 34O (I /I + I + I ), and 32O (I /I + I + I ) calculated from Fig. 5a.\n2 36 36 34 32 2 34 36 34 32 2 32 36 34 32\na b\na b\nSupplementary Fig. 14 | Evidence to show OER mechanism. a, b, DEMS signals of the 36O, 34O and 32O\n2 2 2\nproducts for 18O-labeled NiOOH in non-polarized (a) and polarized (b) KOH.\nComment 10: Please re-draw all the figures in manuscript, some borders are missing, some\nfont/size are not same.\nReply: We thank the reviewer for pointing this out. All figures have been revised\naccordingly to ensure consistency and are now included in the resubmitted manuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The response and corresponding revision are good, thus this manuscript can be accepted as is.", "reviewer_2": "The manuscript was recommended for acceptance.", "reviewer_3": "The author addressed comments properly. The paper is more rigorous now."}, "author_response": ""}]

Decision letter