Enhanced electrocatalytic Activity of NiCoP/Ni₅P₄@NiFe-LDH catalysts via Ir doping and oxygen vacancy engineering

INTRODUCTION

Driven by the twin pressures of global energy scarcity and environmental degradation, the development of clean, renewable energy technologies has emerged as a critical imperative^[1,2]. Among the promising candidates, hydrogen stands out as a carbon-neutral energy carrier with exceptionally high energy density^[3,4]. Electrocatalytic water splitting offers a sustainable and efficient pathway for hydrogen production that can be seamlessly integrated with renewable electricity sources^[5]. While noble-metal catalysts - particularly Pt for the hydrogen evolution reaction (HER) and RuO₂/IrO₂ for the oxygen evolution reaction (OER) - demonstrate benchmark performance, their prohibitive cost and scarcity severely constrain widespread commercial deployment^[6]. These challenges underscore the urgent need to develop earth-abundant, cost-effective electrocatalysts that combine high catalytic activity with robust long-term stability^[7,8].

Transition metal phosphides (TMPs) have attracted considerable interest as electrocatalysts owing to their high activity, stability, and low cost^[9]. Filled d-orbitals flexible redox behavior and high electron-transfer capability, lowering activation barriers and thereby enhancing hydrogen production efficiency^[10]. Numerous synthetic routes have been reported, including direct reactions between red phosphorus and transition metals, electrolysis in molten metal salts, reactions of metal halides with PH₃ or Na₃P, decomposition of organometallic precursors, and reduction of metal phosphates^[11-15]. For example, Liu et al. prepared the NiCoP/Co_xP catalyst on nickel foam (NF) via electrodeposition followed by chemical vapor deposition (CVD), achieving an HER overpotential of 290 mV at 10 mA cm^-2 and operating stably for 500 h at 10 mA cm^-2 in seawater with excellent corrosion resistance^[16]. Wu et al. produced a core-shell CoP_x@FeOOH/NF catalyst using a three-step hydrothermal-phosphidation-electrodeposition method, which necessitated 190 and 248 mV to achieve 100 and 500 mA cm^-2, respectively, while demonstrating exceptional durability for over 80 h in 1.0 M KOH seawater^[17]. Despite these advances, pristine TMPs suffer from two critical limitations that hinder their practical application: (i) severe surface oxidation and phosphorus leaching under OER conditions, leading to gradual deactivation, and (ii) insufficient active site density and poor mass transport due to inevitable nanoparticle agglomeration. To overcome these challenges, constructing hierarchical composite architectures by integrating TMPs with layered double hydroxides (LDHs) has emerged as a promising strategy. LDHs play multifunctional roles: their 2D layered structure serves as a physical barrier against phosphorus leaching, tunable interlayer spacing facilitates ion diffusion, and abundant surface hydroxyl groups provide additional active sites for OER via a lattice-oxygen-mediated mechanism. Recent studies have demonstrated that strategic compositional engineering of LDH-based composites can significantly enhance OER performance^[18]. For instance, Li et al. reported that atomically dispersed Ru doped into iron-nickel LDH forms an asymmetric Ru-Fe dipole that creates electron-deficient active sites, substantially accelerating OER kinetics and achieving an overpotential of only 230 mV at 10 mA cm^-2 in 1 M KOH ^[19]. Similarly, Abdelghafar et al. demonstrated that cation nonstoichiometry in perovskite oxides can induce the formation of nanocomposites with strongly interacting phases, promoting a lattice-oxygen-participating OER pathway and achieving enhanced electrocatalytic performance through synergistic interfacial effects^[20]. However, conventional physical mixing or weak interfacial contact in TMP/LDH composites often results in limited charge transfer efficiency across the heterojunction, compromising the synergistic potential. To rationally enhance interfacial coupling, precise electronic modulation through selective doping becomes essential. Among various dopants, Ir stands out - despite its high cost - because its unique 5d electronic configuration and moderate electronegativity enable exceptional charge redistribution capability. Critically, atomically dispersed Ir³⁺ can simultaneously stabilize both the TMP core through robust Ir-P bonds and the LDH shell via Ir-O-Fe bridges, forming an asymmetric Ir-Fe dipole that accelerates interfacial electron transfer.

In this work, core-shell structured NiCoP/Ni₅P₄ composites were subsequently coated with NiFe-LDH. Ir incorporation and NaBH₄-induced oxygen vacancy formation were further introduced to modulate the electronic structure and increase the density of exposed active sites. The material exhibited outstanding HER and OER performance in 1 M KOH solution, as well as excellent ammonia oxidation reaction (AOR) performance in 1 M KOH + NH₃·H₂O solution, demonstrating remarkable advantages including low overpotentials, low Tafel slopes, large electrochemical capacitance, and exceptional long-term cycling stability. Furthermore, it displayed superior electrocatalytic performance for efficient overall water splitting, achieving excellent long-term durability.

EXPERIMENTAL

RESULTS AND DISCUSSION

Figure 1A presents the XRD pattern of the synthesized material. The characteristic peaks observed at 2θ values of 40.78°, 44.72°, 47.66°, and 55.29° are attributed to the (111), (201), (210), and (211) planes of hexagonal NiCoP/Ni₅P₄, respectively (JCPDS PDF No.71-2336). Additionally, the peaks at 2θ = 31.55°, 41.49°, 49.34°, and 52.26° correspond well to the (201), (211), (302), and (106) planes (JCPDS PDF No.18-0336). The peaks at 2θ = 19.98° and 22.21° are consistent with the (330) and (332) planes (JCPDS PDF No. 25-0608), respectively^[21,22]. XPS was further performed to investigate the composition and elemental states of NiCoP/Ni₅P₄@NiFe-LDH-1 and Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V. Supplementary Figure 1 presents a comparison of the survey spectra for both materials, which clearly reveals the characteristic peaks of Ir, confirming the successful incorporation of Ir into the material. Supplementary Figure 2 shows the elemental composition of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V, revealing the presence of Ni, Co, P, Ir, O, and C as the main elements, with contents of 11.47%, 3.03%, 3.2%, 3.12%, 47.77%, and 25.81%, respectively. Figure 1B-E illustrates the XPS spectra of the NiCoP/Ni₅P₄@NiFe-LDH-1 and Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V samples. Characterization of the NiCoP/Ni₅P₄@NiFe-LDH-1 material was shown, revealing that the Ni 2p spectrum consists of peaks at approximately 856.3 and 873.5 eV, corresponding to the Ni²⁺ 2p_3/2 and 2p_1/2 states, respectively [Figure 1B]. The peaks for Ni³⁺ 2p_3/2 and 2p_1/2 appear at binding energies of 858.2 and 875.5 eV, respectively, accompanied by two satellite peaks centered at 861.8 and 880 eV^[23]. Furthermore, Figure 1C shows the Co 2p spectrum, where peaks at 782.98 and 797.88 eV are associated with Co²⁺ 2p_3/2 and 2p_1/2, while those at 780.1 and 795.78 eV correspond to Co³⁺ 2p_3/2and 2p_1/2. The peaks at 786.1 and 803.2 eV are indexed as satellite peaks^[24]. In Figure 1D, two characteristic peaks at 714.82 and 725.84 eV, with a spin energy separation of 13.02 eV, are ascribed to Fe 2p_3/2 and Fe 2p_1/2, characteristic of Fe³⁺. The peaks at binding energies of 717.16 and 728.30 eV are satellite peaks of Fe 2p_3/2 and Fe 2p_1/2, while those at 712.30 and 722.40 eV correspond to Fe^2+[25]. The peaks at 707.10 and 721.20 eV are attributed to the Fe-P bond formed by the reaction between Fe and P^[26]. In the P 2p XPS spectrum [Supplementary Figure 3A], a prominent peak at 134.6 eV, attributed to P-O, is observed, primarily resulting from the formation of metal phosphides upon exposure to air. The peaks at 129.5 and 130.4 eV correspond to P 2p_3/2 and P 2p_1/2, respectively^[27]. As shown in Supplementary Figure 3B, the peaks at 531.1 and 532.1 eV originate from the splitting of the O 1s peak. The peak at 531.1 eV is associated with oxygen vacancies^[28], while the peak at 532.1 eV corresponds to adsorbed oxygen^[29]. These peaks are attributed to metal hydroxides and metal carbonates, respectively. XPS analysis was performed to examine the structure after Ir doping. The spectral features of Ni, Co, P, O, and Fe elements remained essentially consistent with those of the pristine NiCoP/Ni₅P₄@NiFe-LDH material. As shown in Figure 1E, spin-orbit doublet peaks were observed at 62.40 and 65.30 eV, corresponding to Ir 4f_7/2 and Ir 4f_5/2, respectively, confirming the presence of Ir in the trivalent state. These results further validate the successful incorporation of Ir into the material. Electron paramagnetic resonance (EPR) spectroscopy provides direct evidence for oxygen vacancies and their concentration modulation in the catalysts. As shown in Figure 1F, all samples display a prominent symmetric signal at g = 2.003, indicating that the unpaired electrons are highly localized at oxygen-deficient sites with minimal spin-orbit coupling. It confirms that NaBH₄ reduction effectively introduces a higher concentration of surface-exposed oxygen vacancies.

Figure 1. Structural characterization of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V. (A) XRD. (B) XPS of Ni 2p; (C) Co 2p; (D) Fe 2p; (E) Ir 4f; (F) EPR of the samples. EPR: Electron paramagnetic resonance; XRD: X-ray diffraction; LDH: layered double hydroxide; XPS: X-ray photoelectron spectroscopy.

SEM was utilized to clarify the microstructural properties of the materials. Figure 2A illustrates that the SEM images of NiCoP/Ni₅P₄@NiFe-LDH-1 exhibit a sheet-like shape on the material’s surface. Following Ir incorporation, the modified NiCoP/Ni₅P₄@NiFe-LDH-1 displays a unique porous structure, as demonstrated by the SEM images in Figure 2B. This porous structure is anticipated to reveal numerous active sites during catalytic reactions, therefore improving catalytic efficacy. SEM images of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V [Figure 2C] demonstrate that NaBH₄ treatment causes a significant morphological alteration, leading to a hierarchical spheroidal structure formed by interconnected nanosheets. This shape augments the density of available active sites, hence improving catalytic performance while maintaining intrinsic activity throughout extended cycling. The microstructural characteristics support the enhanced catalytic efficacy of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V described below.

Figure 2. Morphology of the prepared samples. (A) SEM image of NiCoP/Ni₅P₄@NiFe-LDH-1; (B) SEM image of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1; (C) SEM image of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V; (D) TEM image of Ir-NiCoP/Ni5P4@NiFe-LDH-1-O_V; (E) HRTEM image; (F) SAED pattern of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V; (G) Elemental mapping of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V. SEM: Scanning electron microscopy; LDH: layered double hydroxide; TEM: transmission electron microscopy; HRTEM: high-resolution TEM; SAED: selected area electron diffraction.

Supplementary Figure 4, Figure 2D and E show a low-magnification TEM image and high-resolution TEM (HRTEM) images of the as-prepared NiCoP/Ni₅P₄@NiFe-LDH-1 and Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V. For NiCoP/Ni₅P₄@NiFe-LDH-1 [Supplementary Figure 4], lattice spacings of 0.2268 nm and 0.1658 nm are indexed to the Ni₅P₄ (210) and NiCoP (211) planes, respectively, indicating the presence of both phases and high sample purity^[30]. In the Ir-doped, NaBH₄-treated sample [Figure 2D and E], the corresponding spacings are 0.2248 nm for Ni₅P₄ (210) and 0.1603 nm for NiCoP (211), representing changes of 0.0020 and 0.0055 nm, respectively, relative to NiCoP/Ni₅P₄@NiFe-LDH-1. These observations, consistent with XRD results, further substantiate successful Ir incorporation. The selected area electron diffraction (SAED) pattern further reveals distinct diffraction rings attributable to NiCoP, which can be indexed to the lattice spacings of the (210), (111), (201), and (211) crystallographic planes [Figure 2F]. To further confirm the elemental distribution in the synthesized Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V-1, energy-dispersive X-ray spectroscopy (EDS) mapping was performed [Figure 2G]. The maps reveal uniform distributions of Ni, Co, P, Fe, and Ir across the material, supporting successful synthesis and homogeneous incorporation of all constituent elements.

The electrocatalytic performance of all synthesized samples was evaluated in a three-electrode configuration using an alkaline electrolyte (1 M KOH). Figure 3A presents the linear sweep voltammetry (LSV) curves for HER at a scan rate of 5 mV s^-1. The results demonstrate that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V samples exhibit a significantly lower overpotential compared to all other catalysts, indicating superior HER activity. A systematic comparison of overpotentials at the various current densities (-10, -50, and -100 mA cm^-2) is presented in Supplementary Figure 5. Among all catalysts, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V exhibits superior performance with the lowest overpotentials of 52.7, 106.7, and 154.7 mV, respectively. This represents a notable enhancement compared to Ir-NiCoP/Ni₅P₄@NiFe-LDH-1 (62.7, 127.7, and 182.7 mV) and substantially surpasses all reference materials. The marked improvement following NaBH₄ reduction validates the efficacy of oxygen vacancy introduction as a strategy for enhancing electrocatalytic performance^[31]. Tafel slopes were extracted from the LSV curves [Figure 3B]. The values are 96.77, 110.39, 96.18, 92.70, and 74.09 mV dec^-1. Among these, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V shows the smallest slope, indicating faster HER kinetics^[32]. Double-layer capacitances (C_dl) were extracted from the linear dependence of capacitive current on scan rate [Figure 3C], yielding values of 0.0112, 0.0251, 0.0068, 0.0331, 0.0306, and 0.0487 mF cm^-2 for the above catalysts. Notably, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V exhibits the highest C_dl, indicating a substantially enlarged electrochemically active surface area (ECSA). This enhanced ECSA is attributed to the synergistic effects of Ir doping and oxygen vacancy engineering via NaBH₄ treatment, which collectively increase both the density and accessibility of catalytically active sites for HER.

Figure 3. HER and OER performance of the samples in 1 M KOH. (A) LSV curves in HER; (B) Tafel plots in HER; (C) C_dl value; (D) LSV curves in OER; (E) Tafel plots in OER; (F) C_dl value; (G) Bode diagram; (H) Chronoamperometric stability tests (HER); (I) Chronoamperometric stability tests (OER). LSV: Linear sweep voltammetry.

The OER activity of the as-synthesized materials was evaluated in a three-electrode electrochemical cell. The electrocatalytic performance was investigated in 1 M KOH electrolyte at a scan rate of 2 mV s^-1. Figure 3D presents LSV curves of the materials. Among all the materials examined, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V exhibited the lowest overpotential, demonstrating superior OER activity compared to the other catalysts. To provide a more intuitive comparison, Supplementary Figure 6 displays the overpotentials required to achieve current densities of 10, 50, and 100 mA cm^-2. Among all tested materials, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V exhibited the lowest overpotentials of 197.3, 220.3, and 232.3 mV at 10, 50, and 100 mA cm^-2, respectively. These values are substantially lower than those of the other five materials, confirming its superior OER catalytic performance. Tafel analysis [Figure 3E] yields slopes of 35.1, 35.59, 33.58, 42.04, 35.32, and 29.57 mV dec^-1 for the as-prepared NiCoP/Ni₅P₄, NiCoP/Ni₅P₄@NiFe-LDH-0.5, NiCoP/Ni₅P₄@NiFe-LDH-1, NiCoP/Ni₅P₄@NiFe-LDH-1.5, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1, and Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V, respectively, implying the most favorable apparent kinetics. C_dl derived from cyclic voltammetry [Figure 3F] are 0.455, 0.509, 0.479, 0.429, 0.351, and 0.697 mF cm^-2 for the same series, indicating that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V shows the largest electrochemically active surface area and the highest density of accessible sites. Figure 3G displays the Bode plot of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V, where the phase angle progressively decreases as the applied potential increases from 0.4 V to 0.503 V, indicating enhanced electron transfer kinetics. Particularly at higher potentials (0.503 V), the significantly reduced phase angle indicates accelerated electron transfer, which facilitates the catalytic reaction process. To comprehensively evaluate the electrocatalytic performance, radar charts were constructed [Supplementary Figure 7], comparing the overpotentials at 10 mA cm^-2, Tafel slopes, and C_dl for both HER and OER across all materials. The radar plot clearly demonstrates that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V outperforms the other five catalysts across all evaluated metrics. Electrochemical impedance spectroscopy [Supplementary Figure 8] further shows the smallest semicircle and a Nyquist plot closest to the origin for Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V, indicating the lowest charge-transfer resistance and the most rapid charge transport, consistent with its lower OER overpotentials. Finally, constant potential durability tests for both HER and OER [Figure 3H and I] show that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V maintains superior activity for approximately 28 h compared to NiCoP/Ni₅P₄, confirming its robust operational stability. To further demonstrate the material’s competitiveness, its properties were compared with those of commercial and recently reported state-of-the-art catalysts [Supplementary Table 1], highlighting its excellent performance.

To elucidate the surface evolution during catalysis, XPS spectra were acquired for Ir-NiCoP/Ni₅P₄@NiFe-LDH-O_V-1 before and after prolonged OER operation in an alkaline electrolyte. The spectral changes are consistent with OER-induced surface reconstruction, revealing substantial alterations in the near-surface chemical environment and the in situ generation of catalytically active sites that are absent in the as-prepared material^[33,34]. As shown in Figure 4A, NaBH₄ pretreatment has a negligible effect on the intrinsic electronic states: the binding energies of the constituent elements remain essentially unchanged, indicating that the composition and stability are preserved. Following OER cycling, significant binding energy shifts are observed across multiple metal centers. The Ni³⁺ 2p_1/2 peak shifts by 2.1 eV toward lower binding energy^[34], while the Co²⁺ 2p_1/2 peak exhibits a more pronounced 2.8 eV negative shift [Figure 4B]. In contrast, Fe displays negligible variation [Figure 4C], suggesting relative chemical stability under OER conditions^[35]. Notably, the Ir 4f_7/2 component undergoes a 0.30 eV downshift [Figure 4D]. These systematic negative binding energy shifts for Ni, Co, and Ir provide compelling evidence of extensive surface reconstruction accompanied by charge redistribution during electrochemical activation^[36]. The distinct electronic perturbations, particularly involving Ir, underscore its pivotal role in modulating the local electronic structure and thereby enhancing the electrode’s electrocatalytic performance. As shown in Figure 4E and F, the O 1s and P 2p spectra exhibit no discernible changes, indicating that the oxygen- and phosphorus-related chemical environments remain largely unchanged during the process. The binding energies of metal centers (Ni, Co, Ir) exhibit pronounced negative shifts, whereas the O 1s and P 2p peak positions remain virtually unchanged. The negative shift of metal 2p peaks is attributed to the decreased oxidation states of metal centers during the OER process. The invariant O 1s peak position arises from the dynamic equilibrium established among lattice oxygen, surface hydroxyl groups, and adsorbed oxygen species. More significantly, the stability of P 2p peaks substantiates the effective protective role of the outer NiFe-LDH layer in shielding the phosphide core from electrochemical oxidation. From the TEM images, it is clear that after electrochemical cycling [Figure 4G], the Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V sample retains its nanosheet morphology, indicating high structural stability of the catalyst. As shown in the HRTEM images [Figure 4H], lattice fringes with spacings of 0.1908 and 0.2260 nm can be indexed to the (211) plane of NiCoP and the (210) plane of Ni₅P₄, respectively, revealing a pronounced increase in lattice spacing after cycling.

Figure 4. Structural characterization of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V before and after electrochemical cycling. (A) XPS of Ni 2p; (B) Co 2p; (C) Fe 2p; (D) Ir 4f; (E) O 1s; (F) P 2p; (G) TEM image of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V after electrochemical cycling; (H) HRTEM image. LDH: Layered double hydroxide; XPS: X-ray photoelectron spectroscopy; TEM: transmission electron microscopy; HRTEM: high-resolution TEM.

To evaluate the performance of the synthesized catalyst, a two-electrode overall water splitting (OWS) cell was assembled, using the as-prepared material as both anode and cathode [Figure 5A]. The LSV curves [Figure 5B] show that current densities of 10 and 50 mA cm^-2 are achieved at cell voltages of 1.35 and 1.74 V, respectively. After cycling, the required voltages increased only slightly to 1.42 and 1.80 V, indicating minimal performance decay. The overall cell architecture remains intact and the OWS activity is largely preserved. Faradaic efficiency (FE) was quantified by collecting H₂ and O₂ generated during electrolysis in a symmetric Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V. Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V cell using the water-displacement method. The gas volume-time plots [Figure 5C] show an H₂:O₂ volume ratio of ~2:1, in excellent agreement with water splitting stoichiometry, indicating an FE approaching 100%. Continuous electrolysis for 100 h further demonstrates the durability of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V [Figure 5D]. The post-cycling SEM inset reveals slight surface roughening, consistent with reconstruction during long-term electrolysis.

Figure 5. Overall water splitting performance of samples and Faraday efficiency test. (A) Schematic of the overall water splitting device; (B) LSV curves; (C) Faradaic efficiency curve; (D) Chronoamperometric stability tests. LSV: Linear sweep voltammetry; LDH: layered double hydroxide.

To explore this potential, the performance of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V was evaluated in an ammoniacal alkaline electrolyte (1 M KOH + NH₃·H₂O). This medium, which combines KOH with aqueous ammonia, provides a favorable environment for AOR while maintaining high ionic conductivity. Regarding selectivity, under the operating conditions employed in this work, ammonia molecules are preferentially oxidized at the catalyst surface in the alkaline ammonia solution, making AOR the dominant anodic reaction rather than OER. Furthermore, the theoretical potential of AOR is only -0.06 V vs. RHE (under alkaline conditions), which is significantly lower than that of OER (1.23 V vs. RHE), thereby substantially reducing the overall cell voltage and improving energy efficiency^[36,37]. Electrochemical testing in this electrolyte enables a comprehensive assessment of catalytic activity, stability, and selectivity, generating essential data to guide optimization of ammonia-enabled hydrogen production. In summary, the combination of low thermodynamic energy demand, environmental compatibility, and high hydrogen-storage capacity makes ammonia electrolysis a compelling research direction. With continued advances in catalyst design and electrolyte engineering, ammonia electrolysis could assume a pivotal role in the hydrogen value chain, accelerating the hydrogen economy and offering practical pathways to address hydrogen storage and transport.

For HER evaluation, Figure 6A presents the LSV curves of all materials. The catalysts (Ir-NiCoP/Ni₅P₄@NiFe-LDH-1 and Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V) exhibit significantly lower overpotentials compared to the other materials. Among these, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V demonstrates superior performance compared with Ir-NiCoP/Ni₅P₄@NiFe-LDH-1, consistent with results obtained from alkaline water electrolysis, further confirming the beneficial effect of NaBH₄ treatment on enhancing electrocatalytic activity. Supplementary Figure 9 provides a quantitative comparison of overpotentials at current densities of -10, -50, and -100 mA cm^-2. The baseline materials show relatively high overpotentials. In contrast, Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V achieves remarkably low overpotentials of 72.7, 132.7, and 188.7 mV at -10, -50, and -100 mA cm^-2, respectively. These values represent further improvements compared to Ir-NiCoP/Ni₅P₄@NiFe-LDH-1 (76.7, 138.7, and 188.7 mV), unambiguously demonstrating that NaBH₄ treatment substantially enhances the catalytic performance. To further probe reaction kinetics, Tafel slopes were derived from Tafel plots constructed from the LSV data [Figure 6B]. Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V shows a Tafel slope of 96.44 mV dec^-1, smaller than those of NiCoP/Ni₅P₄ (128.55 mV dec^-1), NiCoP/Ni₅P₄@NiFe-LDH-0.5 (138.95 mV dec^-1), NiCoP/Ni₅P₄@NiFe-LDH-1 (130.85 mV dec^-1), and NiCoP/Ni₅P₄@NiFe-LDH-1.5 (131.79 mV dec^-1). Compared with Ir-NiCoP/Ni₅P₄@NiFe-LDH-1, however, the O_V-1 sample exhibits a slightly higher slope, possibly reflecting electrolyte-specific effects in ammoniacal media. Capacitive measurements corroborate these trends: C_dl of Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V is 0.187 mF cm^-2 [Figure 6C], exceeding those of NiCoP/Ni₅P₄ (0.045 mF cm^-2), NiCoP/Ni₅P₄@NiFe-LDH-0.5 (0.062 mF cm^-2), NiCoP/Ni₅P₄@NiFe-LDH-1 (0.016 mF cm^-2), NiCoP/[email protected] (0.055 mF cm^-2), and Ir-NiCoP/Ni₅P₄@NiFe-LDH-1 (0.155 mF cm^-2). The larger C_dl implies a greater ECSA and a higher density of accessible active sites for Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V, consistent with enhanced apparent kinetics. The AOR performance of the samples was further investigated under identical conditions, with results presented in Figure 6D-F. Figure 6D displays the LSV curves for all materials, revealing that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V exhibits significantly lower overpotentials compared to the other five catalysts. A detailed quantitative analysis is shown in Supplementary Figure 10. Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V achieves overpotentials of 101.3, 148.3, and 197.3 mV at current densities of 10, 50, and 100 mA cm^-2, respectively. These values demonstrate superior performance compared to Ir-NiCoP/Ni₅P₄@NiFe-LDH-1 (101.3, 168.3, and 215.3 mV at the same current densities), with particularly notable improvements at higher current densities. When benchmarked against the remaining materials - NiCoP/Ni₅P₄ (111.3, 193.3, and 229.3 mV), NiCoP/Ni₅P₄@NiFe-LDH-0.5 (131.3, 197.3, and 231.3 mV), NiCoP/Ni₅P₄@NiFe-LDH-1 (123.3, 216.3, and 235.3 mV), and NiCoP/Ni₅P₄@NiFe-LDH-1.5 (117.3, 196.3, and 237.3 mV) - Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V consistently shows substantially reduced overpotentials across all tested current densities. Corresponding Tafel slopes [Figure 6E] are 118.72, 106.49, 128.28, 111.04, 101.48 mV dec^-1 for the sequence of samples, indicating more favorable apparent kinetics for O_V-1 even after replacing KOH with NH₃·H₂O. From cyclic voltammetry measurements, the C_dl [Figure 6F] are 0.489, 0.291, 0.421, 0.257, 0.564, and 0.657 mF cm^-2, respectively, with O_V-1 exhibiting the largest value. This result is consistent with a larger electrochemically active surface area and a higher density of accessible active sites after Ir incorporation and NaBH₄ treatment. Electrochemical impedance spectroscopy (EIS) data are shown in Figure 6G. The semicircle diameter observed in the high-frequency region represents the charge transfer resistance (R_ct) at the electrode-electrolyte interface. The notably smaller semicircle for Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V indicates markedly reduced R_ct, demonstrating accelerated interfacial charge transfer kinetics and enhanced electronic conductivity that underpin its outstanding electrocatalytic activity. To comprehensively assess the electrocatalytic performance, radar charts comparing overpotentials at 10 mA cm^-2, Tafel slopes, and C_dl for both HER and AOR were constructed [Supplementary Figure 11]. These plots clearly demonstrate that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V consistently outperforms the other five materials across all evaluated metrics. Finally, constant potential durability tests for HER and AOR in the ammoniacal electrolyte [Figure 6H and I] show that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V maintains superior activity for approximately 28 h relative to NiCoP/Ni₅P₄, demonstrating robust operational stability.

Figure 6. HER and AOR performance of the samples in 1 M KOH + NH₃·H₂O. (A) LSV curves, (B) Tafel plots, and (C) C_dl value of HER; (D) LSV curves, (E) Tafel plots, (F) C_dl value, and (G) EIS curves of AOR; (H) Chronoamperometric stability tests of HER; (I) Chronoamperometric stability tests of AOR. HER: Hydrogen evolution reaction; AOR: ammonia oxidation reaction; LSV: linear sweep voltammetry; EIS: electrochemical impedance spectroscopy.

Further analysis shows that Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V exhibits excellent activity for water electrolysis. Its engineered architecture and optimally distributed active sites accelerate charge-transfer kinetics while lowering the overpotential. Consequently, water splitting proceeds efficiently with reduced energy input, and the catalyst maintains stability during prolonged operation. Together, these results establish Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V as a high-performance water-electrolysis catalyst with broad application potential. The combination of high energy conversion efficiency and durability provides a robust technological basis for hydrogen production by water splitting in the clean energy sector. This work not only provides an experimental foundation for developing efficient, low-cost electrocatalysts, but also suggests new avenues for the sustainable growth of the hydrogen energy value chain.

CONCLUSION

In summary, we successfully prepared Ir-doped NiCoP/Ni₅P₄@NiFe-LDH composite materials through a facile two-step hydrothermal method. Following NaBH₄ treatment, the Ir-NiCoP/Ni₅P₄@NiFe-LDH-1-O_V material achieved 10 mA cm^-2 at an overpotential of 52.7 mV for HER in 1 M KOH electrolyte. The catalyst retained its activity after 28 h of continuous cycling, indicating robust durability. In the two-electrode electrolyzer configuration, the catalyst achieved a remarkably low cell voltage for overall water splitting and maintained stable operation for 100 h without performance degradation. Furthermore, in NH₃·H₂O + 1 M KOH electrolyte, the catalyst exhibited an exceptionally low overpotential of only 72.7 mV at a current density of 10 mA cm^-2. This work provides valuable insights into the rational design of advanced multifunctional electrocatalysts.