OER is a sluggish 4e^-/4H⁺ transfer reaction with high kinetic barriers, and its catalytic mechanism varies with electrolyte pH^[39-41]. Yan et al.^[42] reported that in alkaline media, hydroxide ions (OH^-) participate in the formation of reaction intermediates following the pathway of M-OH → M-O → M-OOH → O₂ [Figure 2A], whereas water molecules are directly oxidized under acidic conditions. For POM-based hybrid electrocatalysts, the inherent topological structures of POM components and strong interfacial electronic coupling between POMs and supporting substrates jointly regulate the entire OER catalytic pathway, including intermediate adsorption and conversion^[43,44]. POMs with open extended frameworks and abundant polymetallic active sites (Wells-Dawson, Anderson, etc.) exhibit superior structural suitability for OER in composite systems^[45]. Their large cluster configurations not only accommodate the multi-step electron transfer of the four-electron OER process, but also work synergistically with the adjacent substrate phase to optimize the adsorption strength of key intermediates (*O, *OH, *OOH) and promote efficient O-O bond coupling^[46]. Specifically, Wells-Dawson-type POMs (e.g., [P₂Mo₁₈O₆₂]^6-) are composed of two Keggin-derived subunits connected by bridging oxygen atoms^[47]. When integrated with metal oxides, carbon materials, or metal-organic frameworks (MOFs) to form hybrids, the unique dual-subunit structure strengthens intermetallic synergistic effects at the heterointerface, redistributes interfacial electrons, and further reduces the energy barrier of the rate-limiting O-O bond formation step. Meanwhile, the open skeleton of such POMs exposes more accessible active sites in hybrid systems, avoiding the aggregation of individual POM clusters and maintaining stable adsorption and conversion of OER intermediates. The corresponding OER equations under acidic and alkaline conditions are given in:

(1) $$ \begin{equation} \begin{aligned} 2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{O}_{2}+4 \mathrm{H}^{+}+4 \mathrm{e}^{-} \text {(acidity) } \end{aligned} \end{equation} $$

(2) $$ \begin{equation} \begin{aligned} 4 \mathrm{OH}^{-} \rightarrow \mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O}+4 \mathrm{e}^{-} \text {(alkalinity) } \end{aligned} \end{equation} $$

HER is a typical 2e^-/2H⁺ reduction reaction via Volmer-Heyrovsky or Volmer-Tafel mechanisms^[48]. Ahmad et al.^[49] proposed that in acidic electrolytes, H⁺ undergoes direct cathodic reduction, whereas water dissociation acts as the rate-determining step in alkaline media [Figure 2B]. In POM-based hybrid systems, the compact cluster skeletons and strong proton adsorption capability of specific POMs (Keggin, Silverton, etc.) endow the composites with natural structural advantages for the HER^[50]. Unlike pristine POMs, the combination of POM clusters and conductive substrates forms continuous charge-transfer channels throughout the hybrid material. The compact molecular configurations of Keggin-type POMs guarantee favorable proton adsorption and fast charge transfer kinetics at the interface. A representative Keggin-type POM ([PW₁₂O₄₀]^3-) consists of a central {PO₄} tetrahedron encapsulated by twelve {MO₆} octahedra^[51]. Such dense architectures exhibit excellent redox reversibility. When anchored on metal oxides or carbon-based supports, intense electronic coupling between the two components modulates the d-band center of active sites, optimizes the adsorption free energy of adsorbed hydrogen (*H) intermediates following the Sabatier principle, and effectively accelerates the conversion of adsorbed protons into H₂ products. Additionally, POMs serve as "electron sponges" in hybrid systems, reversibly capturing and releasing electrons to supplement charge carriers and thereby further improving overall HER efficiency. The HER pathways under different pH conditions are summarized as:

(3) $$ \begin{equation} \begin{aligned} 2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2} \text { (acidity) } \end{aligned} \end{equation} $$

(4) $$ \begin{equation} \begin{aligned} 2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}+2 \mathrm{OH}^{-} \text {(alkalinity) } \end{aligned} \end{equation} $$

Keggin-type POMs with the general formula [XM₁₂O₄₀]^n- are the most well-known and versatile POM archetype, featuring a central tetrahedral {XO₄} heteroatom template (e.g., P, Si, B) encapsulated by 12 edge- and corner-sharing {MO₆} octahedra built from addenda metals (e.g., W, Mo, V)^[52]. As illustrated in Figure 3, such compact and highly symmetric topology provides numerous opportunities for structural modification^[53-56]. Adjusting central heteroatoms and addendum metals can regulate cluster charge state, structural stability and redox properties; modifying surface ligands optimizes electrolyte solubility and intermediate adsorption behavior; doping with transition metals (Cu²⁺, Fe³⁺, Ni²⁺) creates additional catalytic active sites^[57,58]. Benefiting from flexible compositional tuning, favorable proton transport and abundant reversible redox sites, Keggin-type POMs are ideal candidates for high-efficiency HER, OER and bifunctional OWS electrocatalysis.

Wells-Dawson-type POMs are a prominent POM archetype with the general formula [X₂M_18-nY_nO₆₂]^n- (X = P, As, Si; M = Mo, W; Y = V, Cr, Co; n = 1-3), formed by two fused trilacunary Keggin fragments^[59]. Their rich structural tunability, via central templates, addenda metals, substituted heteroatoms, surface ligands, and counter-cations, enables precise modulation of electronic properties and active sites, making them highly promising for OWS. Their structures can be tailored to simultaneously catalyze HER and OER through optimized proton/electron transfer and intermediate adsorption^[60-62].

Anderson-type POMs, whose general formula is [XM₆O₂₄]^n- (2 ≤ n ≤ 8, M denotes an addenda atom and X a central heteroatom), belong to the fundamental topological frameworks of the POM family^[63] and possess a unique planar hexagonal structure distinct from the aforementioned POM frameworks. Their disc-like skeleton and high structural symmetry endow them with excellent electronic conductivity and abundant redox-active sites^[64,65]. For OWS applications, tuning the central heteroatom, addenda metal atoms, or modifying the surface with organic ligands enables precise optimization of the electronic structure and active-site environment^[66-68]. These modifications regulate the adsorption/desorption behavior of HER and OER intermediates, accelerate charge-transfer kinetics, and enable efficient and stable bifunctional electrocatalytic performance in EWS.

In addition to the aforementioned Keggin, Anderson, and Wells-Dawson, the POM family also includes numerous other classical structural archetypes, including Lindqvist, Waugh, and Silverton clusters derived from Dawson skeletons^[69,70], as well as distinctive structures such as lacunary, sandwich-type, and wheel-shaped POMs. Lindqvist POMs feature cage-like hexanuclear metal-oxo clusters with high symmetry and simple structural configurations^[71-73]; Waugh POMs possess nonanuclear metal-oxo frameworks with unique three-dimensional pore channels^[74,75]; Silverton POMs adopt cage architectures containing twelve coordinating heteroatoms, whose central cavities can accommodate bulky heteroatoms^[76-78]. Moreover, lacunary POMs (e.g., mono-lacunary and tri-lacunary Keggin/Dawson species) serve as versatile building blocks, which can assemble with metal cations and organic chelates to construct various sandwich-type, extended and wheel-shaped POM clusters.

Carbon materials are widely adopted as ideal confined substrates and conductive carriers for POMs, owing to their rich structural diversity and electrical conductivities of 10²-10⁶ S·cm^-1, thereby greatly accelerating electron transfer during EWS. Typical 2D carbon materials including graphene and g-C₃N₄^[79] can anchor POMs via covalent linkage or intercalation confinement. Meanwhile, 3D carbon materials including carbon nanotubes (CNTs)^[80] and microporous carbon^[81] can be integrated with POMs through spatial encapsulation and surface immobilization. Nevertheless, carbon supports tend to undergo structural corrosion under long-term high-potential oxidation conditions, which inevitably compromises long-term catalytic activity. Heteroatom doping of carbon frameworks has therefore been developed to optimize surface electronic configurations and internal charge distributions, thereby markedly enhancing the long-term operational stability of POMs composites^[82].

Graphene oxide (GO) possesses outstanding electrical conductivity, a wide electrochemical potential window, and abundant surface binding sites, and serves as an excellent substrate for immobilizing POM species^[83]. For example, Li et al.^[84] used H₃PMo₁₂O₄₀-PPy/reduced graphene oxide (rGO) as sacrificial precursors to synthesize Mo₂C encapsulated in N/P-codoped carbon (NPC) on N/P-codoped reduced graphene oxide (NPrGO), denoted Mo₂C@NPC/NPrGO [Figure 4A]. Transmission electron microscopy (TEM) showed that PMo₁₂ was evenly distributed on rGO [Figure 4B]. High-resolution TEM (HRTEM) images also showed the unique porous structure of the as-obtained material, where 2-5 nm Mo₂C nanoparticles were densely dispersed on rGO nanosheets [Figure 4C]. The carbon coating successfully prevented the aggregation and excessive growth of Mo₂C NPs. Furthermore, electrochemical tests in 0.5 M H₂SO₄ (three-electrode system, 100 mV s^-1 indicated that Mo₂C@NPC/NPrGO exhibited an onset overpotential of 0 mV and a low overpotential of ~34 mV at 10 mA cm^-2, superior to commercial Pt/C (40 mV) [Figure 4D]. These results demonstrated that rGO greatly facilitated the even dispersion of Mo₂C NPs and PMo₁₂ clusters.

CNTs serve as typical one-dimensional conductive substrates, possessing a large specific surface area, outstanding electrical and thermal conductivity, and excellent mechanical stability. These merits make CNTs ideal hosts for POMs encapsulation and ensure fast multi-electron transfer throughout catalytic processes^[85]. Quirós-Díez et al.^[86] successfully fabricated the hybrid material 1@CNT by immersing carbon nanotubes in polar solvents to induce the assembly of anionic [V₁₀O₂₈]^6- together with Na⁺ and tris(hydroxymethyl)aminomethane (TRIS⁺) cations on nanotube surfaces [Figure 5A]. HRTEM images confirmed that the carbon nanotubes were modified with [V₁₀O₂₈]^6-, Na⁺ and TRIS⁺ both internally and externally [Figure 5B]. The amorphous structure of the as-obtained material provided it with a large specific surface area and high electrochemical activity. Electrochemical tests in acidic media revealed that 1@CNT delivered excellent HER performance with an onset potential of -0.07 V [Figure 5C], approaching the performance of the commercial Pt/C benchmark. For OER, 1@CNT also exhibited a low onset potential (1.45 V) and overpotential (0.34 V) at 10 mA cm^-2 [Figure 5D], which outperformed state-of-the-art Ir/C and IrO₂/C measured under identical acidic conditions. Combined with the anodic oxidation reaction (AOR) performance and corresponding reaction pathways depicted in Figure 5E, the results demonstrated that the [V₁₀O₂₈]^6- moieties on 1@CNT acted as proton sponges to accelerate the OER process. This study identified regulation of crystal interactions through assembly engineering and modulation of counterion electrochemical properties as key strategies for constructing high-performance POM-based electrocatalysts for both HER and OER.

Porous carbon materials are promising supports for POM-based electrocatalysts, featuring a high specific surface area and a 3D interconnected pore structure, excellent conductivity and tunable framework properties^[87,88]. For example, Huang et al.^[89] synthesized WS₂/Co₁S@N/S-codoped porous carbon nanocomposite via a one-step vulcanization-carbonization strategy, using phosphotungstic acid (H₃PW₁₂O₄₀)-encapsulated ZF-67 as precursors [Figure 6A]. Scanning electron microscopy (SEM) and TEM revealed that the High-temperature carbonization of ZIF-67 generated a 3D porous carbon framework containing mesopores ranging from 1-50 nm, providing a confined space to inhibit aggregation of 5-10 nm metal sulfide particles during thermal treatment [Figure 6B and C]. Meanwhile, N/S co-doping enhanced the electron cloud density of the carbon matrix to promote charge transfer. Electrochemical measurements demonstrated that the as-obtained composite delivered remarkable HER and OER activities. It achieved a HER overpotential of 250 mV in 0.5 M H₂SO₄ [Figure 6D] and an OER overpotential of 365 mV at 10 mA cm^-2 in 1 M KOH [Figure 6E]. Mechanistic studies revealed that the superior catalytic activity stemmed from the uniform distribution of Co_1-xS and WS₂ nanoparticles within the heteroatom-doped carbon framework. Specifically, WS₂ derived from [PW₁₂O₄₀]^3- served as an active component for the HER, which significantly increased the density of active sites in the composite. In addition, Cao et al.^[90] synthesized Ni₃N and Co nanoparticles supported on N-doped porous carbon (PW-NiCo-NC) by calcination, based on the favorable matching of pore and cavity dimensions between ZIF-8/67 and Keggin-type H₃PW₁₂O₄₀ [Figure 6F]. A TEM image confirmed that PW-NiCo-NC displayed a typical hollow cage morphology with obvious inner cavities [Figure 6G], in which the active nanoparticles were uniformly dispersed on the carbon cage shells. Electrochemical measurements were carried out using the device shown in Figure 6H. The catalyst achieved superior HER activity in 1 M KOH, with an overpotential of 211 mV and a Tafel slope of 106.9 mV dec^-1 at 10 mA cm^-2. Remarkably, the excellent HER activity was mainly attributed to the combined merits of POMs and porous carbon, in which POMs served as efficient active centers to regulate interfacial electron distribution and enrich catalytic sites, and the porous hollow carbon structure enlarged the active surface area and shortened ion/electron diffusion pathways, thereby fundamentally optimizing the reaction kinetics of EWS.

Transition metal compounds have garnered widespread research attention for EWS owing to their natural abundance, low cost, and tailorable catalytic activity^[91]. Among them, metal oxides^[92], metal sulfides^[93], metal phosphides^[94], and metal carbides^[95] have been widely explored as EWS catalytic materials. These compounds possess abundant surface active sites and adjustable electronic structures, enabling strong interfacial interactions with POMs. As efficient electron reservoirs, POMs can spontaneously modulate interfacial charge distribution, thereby optimizing the adsorption-desorption processes of catalytic intermediates and reinforcing the synergistic effect between heterogeneous phases.

Metal oxides have emerged as an important class of electrocatalytic materials, benefiting from their robust structural durability, distinctive electronic structures, and adjustable valence state characteristics^[96]. However, inherent limitations such as insufficient catalytic activity and ambiguous active site identification persist^[97]. Integrating metal oxides with POMs has emerged as an effective strategy to construct rapid charge transfer interfaces and augment active site density^[98-100]. Cui et al.^[101] synthesized nanoflower-like POM-Fe_0.2Ni_0.8Co₂O₄ heterostructures on nickel foam (NF) via an in-situ hydrothermal strategy [Figure 7A and B]. The introduced tri-vanadium-substituted Keggin-type POM clusters (PMo₉V₃) functioned as electron sponges with abundant reversible redox centers, while the heterogeneous interface between POM and spinel Fe_0.2Ni_0.8Co₂O₄ induced strong electronic modulation and optimized interfacial electron distribution. This unique synergy created numerous undercoordinated active sites, accelerated proton-coupled electron transfer, and facilitated the migration as well as adsorption/desorption of reaction intermediates in the EWS process. Meanwhile, the combination of POM and Fe_0.2Ni_0.8Co₂O₄ effectively increased the surface roughness and electrochemical active surface area (ECSA) of the composite^[52], further exposing accessible active sites and alleviating particle aggregation. Benefiting from these favorable characteristics, the POM-Fe_0.2Ni_0.8Co₂O₄/NF catalyst delivers exceptional bifunctional catalytic performance. At a current density of 10 mA cm^-2, it required overpotentials of only 89 mV for the HER and 259 mV for the OER, respectively [Figure 7C], outperforming the single-component POM and Fe_0.2Ni_0.8Co₂O₄ catalysts as well as commercial noble-metal benchmarks. When integrated into a two-electrode OWS system, the fabricated electrolyzer required a low cell voltage of 1.58 V to attain a current density of 10 mA cm^-2, demonstrating its great potential for practical EWS applications.

Metal sulfides suffer from inherently strong metal-sulfur bonding, which limits OER kinetics and hinders their widespread application in EWS^[102]. However, incorporating POMs units into metal sulfides has proven effective in modulating their physicochemical properties, introducing abundant transition metal active sites and tailoring charge distribution. Gautam et al.^[103] further fabricated a POM-coated Zn-Co sulfide nanowire heterostructure (POM@ZnCoS/NF) on NF via a facile two-step hydrothermal method [Figure 8A]. A TEM image revealed that POM (PW₁₂) nanoparticles were firmly anchored on the surfaces of ZnCoS nanowires [Figure 8B]. This coating roughened the nanowire surfaces, increased the specific surface area, and enhanced electrocatalytic activity. When tested using the setup shown in Figure 8C, the POM@ZnCoS/NF electrode delivered better catalytic performance than commercial Pt/C (for HER) and RuO₂ (for OER) in 1 M KOH. For HER, POM@ZnCoS/NF achieved overpotentials of 170 and 337 mV to reach 10 and 40 mA cm^-2, respectively. LSV measurements were adopted to evaluate its OER activity, and the catalyst only required overpotentials of 200 and 300 mV at 20 and 50 mA cm^-2. Furthermore, Guillen-Soler et al.^[104] constructed a POM/Pd/MoS₂ heterostructure by integrating molybdenum disulfide (MoS₂) flakes, palladium (Pd) nanoparticles, and cobalt-doped POM (Co-POM) [Figure 8D]. The hybrid featured an interwoven fibrous morphology that substantially increased the specific surface area and exposed more active sites [Figure 8E and F]. Benefiting from the synergistic effects among the three components, the heterostructure exhibited HER and OER activity comparable to commercial noble-metal catalysts. Electrochemical tests in 0.1 M NaOH and 0.5 M H₂SO₄ demonstrate onset potentials of 0.10 V (HER) and 1.45 V (OER). Correspondingly, low overpotentials of 0.08 and 0.25 V were obtained at 10 mA cm^-2 for the HER and OER, respectively [Figure 8G and H]. Moreover, the catalyst maintained robust long-term durability, sustaining stable electrocatalytic output over 8 h of continuous operation. These studies confirmed that combining POMs with metal sulfides provided complementary advantages. The unique structural properties of POMs and the distinctive catalytic properties of metal sulfides worked together to greatly boost the OWS performance of the as-obtained bifunctional electrocatalysts.

Metal phosphides exhibit high intrinsic HER activity because phosphorus atoms favor proton adsorption, whereas metal atoms facilitate the adsorption/desorption of hydrogen intermediates^[105]. Additionally, they facilitate rapid electron transfer and enhanced OER kinetics^[106]. However, transition metal phosphides are often fabricated into working electrodes using binders, which may cause mechanical detachment of active species or chemical instability^[107,108]. Owing to the rich surface charge distribution of POMs, their integration with metal phosphides enables controllable formation of electrostatic interactions, hydrogen bonds, or covalent bonds, thereby alleviating the aforementioned issues. Integrating POMs with metal phosphides therefore offers a viable and efficient strategy for the rational design of electrocatalysts with superior activity, long-term stability, and potential for scalable application. Jiao et al.^[109] assembled lamellar precursors from PMo₁₂ clusters and egg white through hydrogen bonds. Subsequent high-temperature phosphorization treatment enabled the in situ formation of N/P/S-tripledoped carbon layers, transforming the precursors into porous sheet-structured MoP@NPSC composites [Figure 9A]. HRTEM images showed that well-crystallized MoP nanoflakes were encapsulated and linked by thin carbon layers, and the distinct lattice spacings of 0.210 and 0.278 nm corresponded to the (101) and (100) crystal planes of MoP, confirming the intimate interfacial contact between diverse crystalline facets [Figure 9B and C]. Owing to its distinctive porous nanosheet architecture, the MoP@NPSC composite delivered markedly improved HER catalytic performance. It achieved a low overpotential of 50 mV at 10 mA cm^-2, which was much lower than that of pristine MoP (89 mV) [Figure 9D]. The results confirmed that POM-derived metal phosphides enabled homogeneous distribution of metal sites and formed intimate interfaces, thereby regulating electronic structures and hydrogen adsorption properties. Zhao et al.^[110] constructed a densely porous phosphide composite (MoP/MoNiP@NPC-800) via high-temperature pyrolysis using [Ni(2,2'-bipy)₃][Mo₆O₁₉] (Ni-POM) and NaH₂PO₂·H₂O as precursors [Figure 9E]. The composite presented a honeycomb-shaped porous aggregate morphology, and TEM characterization revealed that MoP and MoNiP nanoparticles were encapsulated within graphitic carbon layers [Figure 9F]. Owing to the synergistic effect of dual MoP/MoNiP active sites and the stabilizing effect of NPC, the catalyst delivered a low overpotential of 50.4 mV at 10 mA cm^-2 in 1 M KOH [Figure 9G], which was lower than the values reported for most previous MoNi-based HER electrocatalysts. Moreover, electrochemical impedance spectroscopy (EIS) measurements revealed that MoP/MoNiP@NPC-800 exhibited a smaller Nyquist semicircle than the other four catalysts [Figure 9H]. A simplified equivalent circuit was adopted to fit the Nyquist plots. The minimum charge-transfer resistance (R_ct) of 5.88 Ω indicated the excellent electrical conductivity and fast interfacial charge transfer kinetics of MoP/MoNiP@NPC. This study further verified that the introduction of POM units could create additional active sites and thus improve the HER and OER performance of phosphide-based electrocatalysts.

Transition-metal-based layered double hydroxides (LDHs) exhibit outstanding bifunctional electrocatalytic activity for HER and OER in alkaline media, primarily due to their favorable nanoarray architectures^[111,112]. However, inherent drawbacks such as poor electrical conductivity and limited accessible active sites restrict their further development^[113]. Integrating POMs with LDHs has emerged as an effective strategy to address these limitations by constructing synergistic heterostructures. Wang et al.^[114] reported an in situ anchoring approach to immobilize [PCoW₁₁O₃₉]^5- (Co-POM) nanoparticles onto NiFe-LDH nanosheets via hydrothermal treatment, forming Co-POM@LDH composites [Figure 10A]. TEM and HRTEM lattice analyses confirmed the formation of three-dimensional heterostructures [Figure 10B and C]. Electrochemical tests revealed that Co-POM@LDH/NF delivered HER and OER overpotentials of 220 and 226 mV at 10 mA cm^-2 in 1.0 M KOH, respectively [Figure 10D and E]. More notably, the two-electrode electrolyzer assembled with Co-POM@LDH/NF as both the anode and cathode required an ultralow cell voltage of 1.51 V to drive OWS at 10 mA cm^-2 [Figure 10F]. This remarkable performance was attributed to the abundant Co-POM nanoparticles firmly anchored on LDH nanosheets, which formed heterostructures with numerous active sites and large specific surface areas, thereby accelerating electron and mass transport. Furthermore, the introduced Co-POM component could offer abundant active species, including Co²⁺ ions and labile lattice oxygen species, to further boost catalytic reactions. Using a similar approach, Zhang et al.^[115] synthesized phosphotungstic acid (PTA)-intercalated NiFe-LDH (NiFe-LDH-PTA) electrocatalysts on NF via a one-step hydrothermal method [Figure 10G]. SEM images revealed that NiFe-LDH-PTA presented a uniform and ultrathin cross-linked nanosheet structure without any agglomeration [Figure 10H]. Such structural features enabled NiFe-LDH-PTA to achieve the highest OER activity across all catalysts. Its overpotentials reached 220 ± 3, 267 ± 5, and 306 ± 8 mV at 100, 300, and 600 mA cm^-2, far superior to pristine NiFe-LDH (258, 332 and 411 mV) [Figure 10I]. Given its excellent OER performance, an anion-exchange membrane (AEM) electrolyzer using NiFe-LDH-PTA||Pt/C as the electrodes was assembled [Figure 10J], demonstrating great potential for industrial hydrogen generation. Collectively, these works proved that LDHs were ideal carriers for POMs in bifunctional HER/OER catalysis. Electron-rich POMs effectively adsorbed H^*, while electron-deficient LDHs activated H₂O/OH^- and facilitated O-O bond formation with the assistance of POMs. Additionally, LDHs promoted water dissociation and supplied sufficient H^* to accelerate H₂ desorption.

Composites of POMs and metal nanoparticles (MNPs) hold great promise for precisely tailoring charge distribution and engineering active sites, with POM clusters serving as core electronic modulators governing the electrocatalytic properties and reaction kinetics of the hybrid system^[116,117]. Li et al.^[118] synthesized Keggin-type [PW₁₂O₄₀]^3- (PW₁₂)/Ag/graphene composites using Ag(H₃biim)₂ as both a POM-binding ligand and Ag precursor [Figure 11A]. An HRTEM image revealed distinct lattice fringes with an interplanar spacing of 0.234 nm, which corresponded to the (111) crystal plane of face-centered cubic (fcc) silver (Ag) [Figure 11B]. The controlled release of Ag⁺ from the Ag(H₃biim)₂ during N,N-dimethylformamide (DMF) reflux effectively suppressed nanoparticle agglomeration, and the presence of PW₁₂ further promoted the uniform dispersion of Ag nanoparticles on graphene. LSV measurements demonstrated that the PW₁₂/Ag/graphene-a catalyst exhibited an overpotential of 540 mV at 10 mA cm^-2, surpassing pristine graphene, Ag/graphene, and PW₁₂/graphene counterparts [Figure 11C]. This remarkable OER activity stemmed from the synergistic interplay between PW₁₂ and AgNPs, wherein Ag accelerated interfacial electron transfer, whereas the PW₁₂ cluster functioned as a critical electronic regulator to refine the electronic configuration of Ag and promote H₂O activation. Moreover, the multi-oxo bridging skeleton of PW₁₂ offered abundant surface oxygen sites that facilitated O-O bond formation, thus further boosting OER kinetics. Notably, although MNP-decorated POMs were mainly explored for OER catalysis, the intrinsic redox behavior and flexible electronic structure of POMs drove the conversion of MNPs from electron-rich to electron-deficient states, ultimately enhancing their intrinsic catalytic activity. Meanwhile, the graphene support ensured fast electron transport and structural robustness, collectively contributing to the improved electrocatalytic performance.

MOFs have become ideal host materials for constructing POM-MOF hybrids (POMOFs) owing to their inherent merits including tunable porous structures, large specific surface areas and customizable functional sites^[119,120]. The strong host-guest interaction between POM clusters and MOF skeletons effectively regulates electronic distribution and accelerates electron transfer, which optimizes the redox behavior of POMs and produces a pronounced greater-than-additive synergistic effect in EWS. A series of typical POM@MOF electrocatalysts have been investigated for the HER and OER. For instance, Zhang et al.^[121] encapsulated Keggin-type PMo₁₂ clusters into Co-based MOF-74 via a one-step solvothermal strategy to fabricate PMo₁₂@MOF-74 composites [Figure 12A]. The composite exhibited a distinct nanorod morphology, as observed in Figure 12B. The MoC@CoO catalyst derived from PMo₁₂@MOF-74 showed remarkable HER performance [Figure 12C]. EIS further confirmed its rapid charge transfer capability [Figure 12D]. In addition, Zeb et al.^[122] adopted a scalable hydrothermal strategy to synthesize Mo-CuS/NiS/NF on NF using POM-MOF composites as precursors [Figure 12E]. SEM and TEM images verified the successful synthesis of the material, which formed vertically aligned and regularly arranged nanorods on the substrate surface [Figure 12F]. Uniform Mo doping derived from the NiMo₆ precursor efficiently tuned the electronic configuration of the composite. Benefiting from the cooperative interactions generated at bimetallic sulfide heterojunctions, the material provided numerous exposed catalytic sites and drastically lowered the kinetic barrier toward hydrogen evolution. When tested at 10 mA cm^-2, the electrocatalyst achieved overpotentials of 78, 95 and 111 mV in alkaline medium, artificial seawater, and natural seawater, respectively, which surpassed the catalytic activity of NF loaded with commercial 20% Pt/C [Figure 12G]. Density functional theory (DFT) simulations uncovered the alkaline HER pathway over Mo-CuS/NiS/NF and validated that the catalytic reaction followed the Volmer-Heyrovsky route. As visualized in Figure 12H, the complete catalytic mechanism of HER on Mo-CuS/NiS/NF under alkaline conditions is illustrated, demonstrating that Mo-doped active centers and CuS/NiS heterointerfaces cooperated to facilitate successive reaction steps: water capture, H-O bond cleavage, and the subsequent formation and release of H₂.

These studies demonstrated that the outstanding electrocatalytic activity of POMOF hybrid composites stemmed from their porous MOF frameworks. Such structural backbones supplied numerous accessible active sites and facilitated rapid mass transport kinetics. Intense electronic coupling at the heterogeneous interfaces between POM clusters and MOF substrates reshaped the electron distribution of active centers and lowered the kinetic barrier of rate-limiting reaction steps. Meanwhile, boosted surface wettability accelerated electrolyte infiltration and the release of gaseous products. All these structural merits provide useful guidance for the rational fabrication and performance optimization of robust POMOF composite electrocatalysts toward efficient EWS.