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# Polyoxometalate Imide-Linked Molecules, Covalent Organic Polymers, and Frameworks: Dimensionality Effects on Supercapacitors Performance

Daria Nowicka, Verónica Montes-García, Aleksandra Sikora, Violetta Patroniak, Adam Gorczyński,* Dawid Pakulski,* and Artur Ciesielski*

Efficient and durable energy storage materials are essential to meet the increasing demand for renewable energy technologies. However, existing materials often encounter trade-offs among energy density, power density, and cycling stability. To overcome these limitations, a 2D imide-linked polyoxometalate-covalent organic framework (i-POCOF) is introduced. This hybrid material combines the redox activity of polyoxometalates (POMs) with the structural adaptability of covalent organic frameworks (COFs). Complementarily, 0D imide-functionalized POM-molecule and 1D POM-polymer systems are investigated, enabling a systematic evaluation of how dimensionality affects their physicochemical properties and electrochemical performance. By increasing dimensionality, the hybrids exhibit improved surface area ranging from 107 m² g⁻¹ for 0D to 257 m² g⁻¹ for 2D, and optimized porosity (average pore size from 1.9 nm for 0D to 3.7 nm for 2D), resulting in enhanced ion diffusion and charge transport. In particular, 2D i-POCOF exhibits remarkable electrochemical performance, achieving a specific capacitance of 132 F g⁻¹, energy density of 73.3 Wh kg⁻¹, and power density of 0.9 kW kg⁻¹, with only 6% capacitance loss after 5000 cycles. These findings highlight the potential of POM hybrids as high-performance and stable energy storage hybrids, providing a promising pathway to overcome current limitations in electrode materials.

D. Nowicka, A. Sikora, V. Patroniak, A. Gorczyński
Faculty of Chemistry
Adam Mickiewicz University
Uniwersytetu Poznańskiego 8, Poznań 61–614, Poland
E-mail: adam.gorczynski@amu.edu.pl

V. Montes-García, D. Pakulski, A. Ciesielski
Center for Advanced Technologies
Adam Mickiewicz University
Uniwersytetu Poznańskiego 10, Poznań 61–614, Poland
E-mail: dawid.pakulski@amu.edu.pl; ciesielski@unistra.fr

V. Montes-García, A. Ciesielski
Université de Strasbourg, CNRS
ISIS UMR7006
8 allée Gaspard Monge, Strasbourg F-67000, France

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ID The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adsu.202500356
© 2025 The Author(s). Advanced Sustainable Systems published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/adsu.202500356

## 1. Introduction

The ongoing energy crisis has underscored the pressing need for a paradigm shift in energy systems to mitigate geopolitical dependencies and fortify energy infrastructure. Achieving EU climate and energy targets necessitates a profound transformation focused on integrating diverse renewable energy sources. This transition requires innovative, highly efficient energy storage solutions capable of balancing supply and demand across varying time scales.[1,2] Energy storage systems (ESS) have emerged as a cornerstone in this effort, providing flexibility, mitigating price fluctuations, empowering consumers, and accelerating the electrification of key sectors such as buildings and transportation. Technologies like rechargeable batteries and supercapacitors (SCs) play a crucial role in grid balancing, system integration, and decarbonization, paving the way for a more sustainable energy landscape.[3–6]

Among these, organic cathode materials (OCMs) stand out due to their high resource sustainability, structural versatility, high theoretical energy density, and potential cost-effectiveness.[7,8] These advantages position OCMs as viable alternatives to critical raw materials (CRMs) such as natural graphite, cobalt, or lithium, making them an optimal choice for advancing next-generation electrodes in energy storage systems.[9–13]

Among various OCMs, covalent organic frameworks (COFs) represent an expanding class of crystalline organic polymers known for their multitude of advantageous features as components in ESS. These include sustainability, precisely tunable chemical structures, high chemical, and thermal stability (depending on the linkage type), remarkable porosity, and large surface areas.[9,14–24] COFs can be synthesized with a variety of linkage types, such as imide,[25–30] amide,[31–34] imine,[22,28,35,36] or b-ketoenamines,[22,37–39] among others. Imide-based COFs offer several superior features compared to other linkage types, including exceptional stability, enhanced structural integrity, high thermal resistance, and reduced vulnerability to hydrolysis.[25,26,32] For example, a benzimidazole-linked arylimide-based COF incorporating chemically robust imide bonds exhibits excellent
electrochemical durability, retaining 93.61% of its capacitance after 5000 cycles in a supercapacitor device,[40] and a polyimide-COF anode in a lithium-ion battery sustained a capacity of 450 mAh g⁻¹ after 800 cycles.[41] Such performances underscore the potential of imide-linked architectures for long-term and high-performance energy storage applications. However, pristine COFs often suffer from limited intrinsic electrical conductivity, often below 10⁻⁸ S m⁻¹,[42⁻⁴⁴] and a lack of sufficient redox-active sites, which can restrict their electrochemical performance unless chemically modified or combined with other active components.

Polyoxometalates (POMs), a class of molecular metal-oxo clusters, with the general formula $[X_x M_m O_y]^{n-}$, where X represents the heteroatom, M denotes an early transition metal (TM) ion in its highest oxidation state ($W^{6+}$, $Mo^{6+}$, $V^{5+}$), offer complementary properties. POMs are categorized into various structural types, including Anderson–Evans, Lindqvist, Keggin, Wells–Dawson, Silverton, and Preyssler, among others.[45–47] Among these, tris(hydroxymethyl)aminomethane (TRIS)-functionalized Anderson-type POMs[47,48] are widely recognized and extensively used as building blocks for the construction of discrete or net-worked materials, with applications spanning energy,[22,49–55] catalysis,[56–58] biology,[59,60] and magnetism.[61] Renowned for their rich and reversible multi-electron redox activity, high charge density, and structural versatility, POMs are ideal candidates for high-capacity charge storage. Yet, their practical application is hindered by poor electronic conductivity, high solubility in polar electrolytes, and a tendency to aggregate or leach from electrode matrices during cycling.

To address these challenges, hybridizing COFs with POMs has emerged as a compelling strategy.[62] By embedding redox-active POM clusters into the porous and chemically robust COF framework–via covalent or coordination bonding–it is possible to synergistically combine the advantages of both components.[62] The COF acts as a stable, porous scaffold that enables effective confinement and spatial organization of POMs, ensuring their retention during cycling and facilitating ion diffusion. Meanwhile, the POMs contribute strong pseudocapacitive behavior through fast and reversible redox reactions, compensating for the typically limited redox activity of the COF matrix. Although POMs and COFs generally exhibit poor electrical conductivity, this limitation can be effectively mitigated through the incorporation of a small amount of conductive additives, such as carbon black, without compromising the structural integrity of the hybrid. This design paradigm offers a promising pathway toward the development of high-performance, resource-efficient super-capacitor electrodes aligned with sustainability goals. Despite their promise in energy storage, POCOFs (POM–COF hybrids) face significant challenges related to synthesis, structural control, and material compatibility. Achieving high crystallinity and uniform pore architecture is particularly difficult, especially when incorporating bulky and rigid POM clusters, which can disrupt the orderly assembly of the COF framework. These complications are exacerbated in 3D architectures, where interpenetration and low porosity are common. Additionally, controlling the dimensionality and homogeneous dispersion of hydrophilic POMs within typically hydrophobic COF matrices remains a critical hurdle, as limited synthetic methodologies exist to precisely integrate these components. Ensuring strong and stable chemical bonding

–whether covalent or coordinative–is essential to prevent POM leaching or phase separation, particularly during electrochemical cycling. COFs themselves can be fragile during activation, and their dynamic covalent bonds are prone to hydrolytic degradation unless robust linkages are employed. Overcoming these obstacles requires more durable bonding strategies, environmentally benign synthesis protocols, and improved control over framework assembly to fully unlock the potential of POCOFs in practical applications. To date, only seven different POCOFs have been reported, six of which are based on imine linkages.[34,63–66] Recently, some of us reported the POCOFs synthesis through a one-step solvothermal approach, integrating an amino-functionalized Anderson-type POM with a tri-aldehyde-based unit.[22] By incorporating hydroxyl groups into the benzene-1,3,5-tricarbaldehyde, the POCOF exhibited a keto-enol tautomerization, resulting in a high surface area (347 m² g⁻¹) and excellent electrochemical performance, including specific capacitance of 125 F g⁻¹ and remarkable cyclability (94% capacitance retention after 5000 cycles).[22]

Despite the superior properties of imide-based COFs, to the best of our knowledge, no imide-based POCOFs have been reported to date. Incorporating imide linkages into POCOFs offers a promising strategy to overcome key limitations such as structural fragility, poor crystallinity, and POM leaching. First, imide linkages offer exceptional chemical and thermal stability, ensuring framework integrity under operational conditions. More importantly, these bonds create highly conjugated systems that facilitate efficient charge delocalization across the framework, a critical feature for electrochemical applications.[67–69] The rigid planar geometry of imide linkages promotes π-stacking interactions that enhance both structural order and electronic communication between POM nodes. Compared to alternative linkage chemistries (such as imines or boronate esters), imides provide superior redox stability while maintaining sufficient flexibility for structural tuning through careful linker selection. The electron-withdrawing nature of the imide group also helps modulate the electronic structure of the organic component, optimizing its interaction with the POM redox centers. These combined properties make imide-based linkages uniquely positioned to create robust, electrochemically active porous frameworks.

In this study, we report the first 2D imide-based POCOF (i-POCOF) utilizing Anderson-type POMs as the inorganic building block and 2,3,6,7,10,11-hexaimidotriphenylene (MTA) as the organic linker, to address the limitations of current energy storage materials. Specifically, we employed the $MnMo_6$ Anderson POM ($^{n}$Bu$_4$N)$_3$[$MnMo_6O_{18}$((OCH$_2$)$_3$CNH$_2$)$_2$], featuring terminal amino groups for covalent functionalization. Additionally, this POM was hybridized with pyromellitic dianhydride (PTA) or 1,2,4,5-benzenetetracarboxylic dianhydride (PMDA) to form 0D imide-functionalized POM-molecule and 1D POM-polymer structures, respectively, enabling a systematic evaluation of how dimensionality affects their physicochemical properties and electrochemical performance (Figure 1; Scheme S1, Supporting Information). By combining the redox activity of Anderson-type POMs with the structural versatility, high porosity and stability imparted by the organic linker, these hybrids offer an innovative approach to overcoming challenges of the pristine counterparts such as low conductivity, limited ion accessibility, and insufficient cycling stability. Through careful dimensional
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Schematic pathway for the synthesis of POM hybrids with various dimensionalities. The synthesis includes a 0D discrete molecule (POM-PTA), a 1D polymer (POM-PMDA), and a 2D i-POCOF (POM-MTA).</figcaption></figure>

tuning, we explore the correlation between structural properties such as surface area, porosity, and charge transport pathways and electrochemical performance. This work not only highlights the unique advantages of dimensionality-controlled hybrids but also provides a comprehensive framework for optimizing the design of high-performance electrode materials for energy storage applications.

## 2. Results and Discussion

The synthesis and optimization of the POM hybrids begin with the fabrication of the well-established polyoxometalate $[(nBu_4N)_4(Mo_8O_{26})]$ via the reaction of sodium molybdate dihydrate with tetrabutylammonium bromide in an acidic medium (see Supporting Information for details). Subsequently, this polyoxometalate is modified using tris(hydroxymethyl)aminomethane (TRIS) and $Mn^{111}$ as the central heteroatom, following the procedure described by Marcoux et al.[70] The resulting TRIS-functionalized Anderson-type POM, $^{[n}(Bu_4N)_3[MnMo_6O_{18}((OCH_2)_3CNH_2)_2]$, referred to as [NH_2-MnMo_6-NH_2], features two terminal amino groups that act as anchor points (i.e., C_2 knot) for subsequent functionalization, serving as one of the building blocks for POM hybrids synthesis.

To optimize the synthesis of the POM hybrids, the initial focus is on the simplest form: a discrete molecule formed through the condensation of [NH₂-MnMo₆-NH₂] POM with PTA (i.e., 0D POM-PTA). After optimizing the 0D system, these established conditions are extended to fabricate 1D POM-polymer and 2D i-POCOF using components derived from PTA. For the synthesis, a solution of [NH₂-MnMo₆-NH₂] POM and PTA in a 1:2 molar ratio (POM:PTA) is prepared in a solvent mixture of mesitylene, N-Methyl-2-pyrrolidone (NMP), and isoquinoline in 1:1:0.1 molar ratio. The specific mesitylene/NMP/isoquinoline mixture (1:1:0.1 ratio) is carefully selected after extensive optimization to enable the formation of well-ordered crystalline products, and we would like to elaborate on its critical functions. This particular solvent combination creates a balanced chemical environment

where each component plays a distinct yet complementary role. Mesitylene provides a high-boiling, non-polar medium that facilitates slow and controlled crystallization, while NMP ensures adequate solubility for both the POM clusters and organic linkers throughout the reaction process. The carefully controlled addition of isoquinoline (10 vol%) serves as a mild base to promote imide bond formation while simultaneously modulating reaction kinetics to prevent rapid, disordered precipitation. Our systematic screening reveals that this exact ratio represents a delicate equilibrium point. Reducing the isoquinoline content results in incomplete imidation, while increasing it leads to the loss of crystallinity through amorphous aggregation. Alternative tested solvent systems, such as DMF/o-DCB mixtures, consistently fail to produce the desired crystalline frameworks. The success of our chosen ratio stems from its ability to simultaneously maintain component solubility, provide appropriate basicity for the condensation reaction, control framework assembly kinetics, and preserve the structural integrity of the POM building blocks.

The optimization process focuses on reaction temperature, a critical parameter influencing the chemistry of both POMs and bis-imides.[30,71] Reactions are conducted at 80, 120, and 160 °C, and structural characteristics are analyzed using Fourier-transform infrared spectroscopy (FTIR), as presented in Figure S1a (Supporting Information). In the FTIR spectrum of POM, the band at ≈1380 cm⁻¹ corresponds to C—N stretching vibration. Indicators of successful condensation between POM and PTA include the emergence of the $C=O_{(imide)}$ broadband at 1683 cm⁻¹ (POM-PTA_80), 1651 cm⁻¹ (POM-PTA_120), or 1656 cm⁻¹ (POM-PTA_160), accompanied by the disappearance of two $C=O_{(carbonyl anhydride)}$ bands at 1761 and 1854 cm⁻¹ from PTA. Additionally, C—O stretching vibrations of PTA are observed in the 1050–1070 cm⁻¹ range in POM-PTA, while the stability of the POM building block is confirmed by the persistence of $Mo=O$ band, in the range of 900–960 cm⁻¹, and $Mo=O—Mo$ band, ≈660 cm⁻¹.

At 80 °C, the $C=O_{(imide)}$ band is scarcely visible, indicating insufficient thermal energy for the reaction (Figure S1a,
<figure><img src="image_4.png" /><figcaption>**Figure 2.** a) FTIR spectra and b) PXRD patterns of POM, 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA. and b) PXRD patterns of POM, 0D POM-PTA, 1D POM-PMDA and 2D POM-MTA.</figcaption></figure>

Supporting Information, blue spectrum). Increasing the temperature to 120 °C results in a stronger C=O<sub>(imide)</sub> band at lower wavenumbers compared to the carbonyl anhydride<sup>[72]</sup> (Figure S1a, Supporting Information, green spectrum). However, further increasing of temperature to 160 °C does not significantly alter the intensity of the C=O<sub>(imide)</sub> band (Figure S1a, Supporting Information, purple spectrum), establishing 120 °C as the optimal temperature for further optimization. The reaction time is then optimized by conducting the synthesis at 120 °C for three (72 h) or seven days (168 h). FTIR analysis (Figure S1b, Supporting Information) reveals that three days is the optimal reaction time, as prolonged reaction times reduce the intensity of the C=O<sub>(imide)</sub> band, likely due to partial decomposition of the bis-imide into the corresponding amides and/or ammonium carboxylate salts.<sup>[30]</sup> Finally, the synthetic protocol is assessed using two distinct methods (A)—a conventional solvothermal approach; and (B)—a sealed-tube method. The sealed-tube method diminishes the intensity of the hybrid 0D POM-PTA C=O<sub>(imide)</sub> band (Figure S1c, Supporting Information, green spectrum), suggesting it compromises the integrity of the bis-imide moieties in the POM hybrid.<sup>[30]</sup> Thus, the optimal synthetic conditions involve conducting the solvothermal method at 120 °C for three days, yielding a discrete 0D POM-PTA with an 84.6% efficiency. Subsequent experiments employ these optimized conditions.

To confirm the successful synthesis of 0D POM-PTA, Heteronuclear Multiple Bond Correlation (HMBC) analysis is performed in deuterated acetonitrile (d<sub>3</sub>-MeCN), and the correlation spectrum is shown in Figure S2 (Supporting Information). The proton signals at δ 0.97, 1.36, 1.61, and 3.10 ppm can be assigned to the protons H<sub>A</sub>, H<sub>B</sub>, H<sub>C</sub>, and H<sub>D</sub>, respectively, originating from the tetrabutylammonium ion (TBA<sup>+</sup>). These signals correlate with carbons C<sub>A</sub>, C<sub>B</sub>, C<sub>C</sub>, and C<sub>D</sub>, which are attributed to carbons from TBA<sup>+</sup> and observed at δ 13.77, 20.30, 24.30, and 59.29 ppm, respectively. The aryl protons of 0D POM-PTA are located at δ 7.48 ppm and 8.27 ppm and correspond to the H<sub>1</sub> and H<sub>2</sub> protons, respectively. The H<sub>2</sub> proton exhibits a stronger correlation with carbon C<sub>2</sub> at δ 131.00 ppm but is also related to carbons C<sub>1</sub> and C<sub>5</sub> observed at δ 133.66 and 169.74 ppm, respectively. Importantly, the H<sub>1</sub> proton only correlates with the C<sub>1</sub> carbon. Noteworthy, the presence of a carbon C<sub>4</sub> signal located at δ 178.70 ppm confirms the presence of an O=C-N imide group which is related to the aliphatic H<sub>3</sub> proton located at δ 2.73 ppm. The H<sub>3</sub> aliphatic proton is also correlated with the C<sub>3</sub> aliphatic carbon located at δ 49.73 ppm. The observed correlations between protons and carbons indicate the successful formation of

0D POM-PTA. The ¹H NMR and ¹³C NMR spectra of POM-PTA are shown in Supporting Information (Figures S3 and S4, Supporting Information).

Two POM hybrids of distinct dimensionalities, incorporating PTA-derived components and Anderson-Evans polyoxometalate, are synthesized under the optimized synthetic conditions. Condensation reactions between [NH<sub>2</sub>-MnMo<sub>6</sub>-NH<sub>2</sub>] POM with PMDA and MTA result in the formation of the 1D POM-PMDA (i.e., polymer) and 2D POM-MTA (i.e., i-POCOF), respectively. These reactions employ molar ratios of 1:1 (POM:PMDA) and 3:2 (POM:MTA), consistent with stoichiometric requirements dictated by the anhydride functional groups. The FTIR spectra of the POM hybrids (Figure 2a; Figures S5 and S6, Supporting Information) corroborate successful condensation. In agreement with the findings for POM-PTA (Figure S1, Supporting Information), the carbonyl anhydrides bands of PMDA and MTA at 1846, 1766 cm<sup>-1</sup> (PMDA, Figure S5, Supporting Information) and broad band centered at 1721 cm<sup>-1</sup> (MTA, Figure S6, Supporting Information) are absent in the spectra of the corresponding POM hybrids, indicating complete reaction between the two building units. The condensation between [NH<sub>2</sub>-MnMo<sub>6</sub>-NH<sub>2</sub>] POM and PMDA or MTA leads to the formation of C=O<sub>(imide)</sub> broad band at 1596 cm<sup>-1</sup> (1D POM-PMDA), and broad band at 1603 cm<sup>-1</sup> for 2D POM-MTA. The bands observed at 1380 cm<sup>-1</sup> for POM-PTA, 1381 cm<sup>-1</sup> for POM-PMDA, and 1354 cm<sup>-1</sup> for POM-MTA are attributed to the C-N stretching vibrations of aromatic amide (imide) bonds, confirming successful covalent linkage formation. Additionally, the characteristic vibrational modes of the pristine POM-namely, the Mo=O-Mo stretching ≈660 cm<sup>-1</sup> and the terminal Mo=O stretching near 910 cm<sup>-1</sup>-are clearly retained in all POM hybrids, indicating that the POM structural integrity is preserved throughout the synthesis. Powder X-ray diffraction (PXRD) patterns of the pristine POM and POM hybrids (Figure 2b) further validate the structural transformations.<sup>[70,64]</sup> The pristine POM exhibits a sharp peak at 2θ = 7.9°, corresponding to the 001 plane, along with an additional peak at 7.1°, characteristic of Anderson-type POMs.<sup>[22,64]</sup> In the hybrid materials, the main peak shifts slightly to 2θ = 8.1° for 0D POM-PTA, 2θ = 8.3° for 1D POM-PMDA and 2θ = 8.5° for 2D POM-MTA. Additionally, new peaks appear in the hybrids, in particular at 2θ = 9.7° and 10.3°. These new peaks are in agreement with other imide-based covalent networks<sup>[27,28,73]</sup> as well as with other reported POCOF structures.<sup>[22,63,64]</sup> Unlike pure COFs, the 001 plane in POM hybrids appears at 2θ above 5°, reflecting the influence of the large size and negative surface charge
<figure><img src="image_5.png" /><figcaption>**Figure 3.** SEM images of a) POM, b) 0D POM-PTA, c) 1D POM-PMDA and d) 2D POM-MTA.</figcaption></figure>

of the POM subunits. This phenomenon is consistent with other reported POCOFs.[22]

To elucidate the chemical composition of the hybrid materials, X-ray photoelectron spectroscopy (XPS) analyses are then performed (Figures S7–S11, Supporting Information). The XPS survey spectra of pristine POM and the POM hybrids (Figure S7, Supporting Information) confirm the presence of Mo, C, N, O and Mn in all investigated compounds, providing strong evidence for the stable integration of the inorganic POM units within the organic frameworks. High-resolution spectra of C1s and N1s regions for pristine POM and the POM hybrids are shown in Figures S8 and S9 (Supporting Information). The C1s spectra reveal two peaks for pristine POM and three peaks for the POM hybrids (Figure S8, Supporting Information), consistent with previous studies.[22] The peaks at ≈284.7 and ≈286 eV corresponded to carbon-carbon (C—C, C=C) and carbon-heteroatom (C—N, C—O) bonds, respectively, and can be observed in both pristine POM and in the POM hybrids. Notably, in the hybrids, a new peak emerges at 287.8 (0D POM-PTA), 287.9 (1D POM-PMDA) and 288 eV (2D POM-MTA) with a slight shift depending on the structural dimensionality. This peak corresponds to the imide N—C=O bond, confirming the formation of the hybrid structures. The N1s spectrum (Figure S9, Supporting Information) for pristine POM provides evidence of peaks at 398.4 and 402.2 eV, which can be attributed to primary amines of POM and quaternary amines of tetraethylammonium counterions, respectively. Differently, for the N1s spectra for 0D POM-PTA, 1D POM-PMDA and 2D POM-MTA, the peak corresponding to the quaternary amine C—N bond shifts toward higher binding energies (Figure S9, Supporting Information). It is also worth noting the disappearance of the peak originating from the N—H bond with the simultaneous appearance of the peak originating from the nitrogen-carbon-oxygen bond (N—C=O) and

appearing at 401, 401, and 401.3 eV for 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA, respectively. These observations further confirm the successful formation of imide bonds in the POM hybrids.

Scanning electron microscopy (SEM) analysis reveals significant morphological differences between the pristine POM and the POM hybrids (Figure 3). The pristine POM (Figure 3a) exhibits elongated, flat structures. In contrast, 0D POM-PTA (Figure 3b) displays a sponge-like porous morphology. Similarly, 1D POM-PMDA (Figure 3c) exhibits sponge-like structures with additional geometric shapes. The 2D POM-MTA (Figure 3d) differs markedly, consisting predominantly of spherical structures of ≈300 nm in diameter. While molecular topology establishes the fundamental connectivity pattern, the final particle morphology arises from a complex interplay of supramolecular assembly processes during crystallization, competitive growth kinetics along different crystallographic directions, solvent-mediated stabilization of specific crystal faces, and interfacial interactions during nucleation. The spherical morphology of 2D POM-MTA, for instance, reflects the isotropic growth conditions under our synthesis protocol, where the high connectivity of MTA promotes rapid nucleation over directional growth. This contrasts with typical 2D COFs that form under conditions favoring anisotropic growth. Similarly, the 1D POM-PMDA chains assemble into irregular aggregates rather than elongated fibers, suggesting that interchain interactions and packing effects outweigh the intrinsic molecular anisotropy.

The porosity of POM, 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA is evaluated by acquiring nitrogen adsorption-desorption isotherms at 77 K (Figure 4a). The observed progression in the calculated Brunauer–Emmett–Teller (BET) surface areas—from 44 m² g⁻¹ for pristine POM to 257 m² g⁻¹ for 2D POM-MTA—can be understood through fundamental structural
<figure><img src="image_6.png" /><figcaption>**Figure 4.** a) BET surface area and b) pore size distribution of POM, 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA.</figcaption></figure>

considerations. The modest surface area of the pristine POM (44 ± 3 m² g⁻¹) is characteristic of non-porous molecular materials, where surface area is limited to external crystal faces and interparticle voids. This aligns well with established literature reports for unsupported POM clusters in the solid state.[74] The transformation to extended frameworks through linker incorporation introduces intrinsic porosity, as clearly evidenced by the gas sorption data (Figure 4). The systematic increase in surface area with dimensionality reflects several key structural developments: The 0D POM-PTA system (107 ± 5 m² g⁻¹) represents an intermediate case where limited dimensionality extension occurs. The 1D POM-PMDA chains (163 ± 7 m² g⁻¹) create more continuous pore channels, while the 2D POM-MTA framework (257 ± 9 m² g⁻¹) benefits from both in-plane porosity and reduced framework interpenetration due to its planar connectivity. This dimensional progression is further supported by the parallel evolution of pore volumes (0.18 → 0.31 → 0.49 cm³ g⁻¹) and the emergence of defined mesoporosity (Figure 4b). The pristine POM and 0D POM-PTA show average pore sizes of 1.5 and 1.9 nm, consistent with a largely microporous character. In contrast, the 1D and 2D hybrids exhibit broader mesoporous features with average pore sizes of 3.3 and 3.7 nm, respectively, as determined by the Barrett-Joyner-Halenda (BJH) method. Type-IV isotherms observed in all cases support this assessment. The observed progression in average pore sizes—from 1.5 nm in pristine POM to 3.7 nm in 2D POM-MTA—primarily reflects the systematic increase in framework dimensionality enabled by our choice of organic linkers. However, this correlation is not governed solely by topology. Rather, it emerges from an intricate interplay between several linker-dependent factors that collectively dictate the final pore architecture. The molecular characteristics of each anhydride linker critically influence the resulting porosity. PTA, as a compact monofunctional linker, terminates framework growth and yields discrete 0D clusters where pore dimensions are constrained by the POM's inherent size. In contrast, the linear connectivity of PMDA facilitates 1D chain propagation, creating more extended channels where pore size expands due to both the linker's increased length and the reduced packing density of these linear arrays. The most dramatic pore enlargement occurs with MTA, where its trifunctionality and rigid planar geometry promote the formation of 2D networks with well-defined, interconnected mesopores. Here, the combination of higher connectivity and steric demand leads to more open frameworks with reduced interpenetration. We acknowledge that synthetic parameters—including crystallization kinetics, solvent effects, and template interactions—may introduce additional vari-

ability in pore sizes. However, our systematic characterization (through PXRD, and gas adsorption) confirms that the dominant trend correlates with the designed dimensionality progression.

Furthermore, the porosity achieved in this series of POM hybrids surpasses that of many reported POM-organic hybrids. For example, Zhang et al. synthesized a POM-organic framework using planar tetra-aldehyde linkers, which exhibited a surface area of 134 m² g⁻¹.[65] This comparison underscores the effectiveness of dimensional control and monomer selection in achieving high porosity and structural openness in the i-POCOF.

The thermal stability of pristine POM, 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA is assessed by thermogravimetric analysis (TGA) (Figure S12, Supporting Information). The thermograms of POM hybrids demonstrate a noteworthy degree of thermal stability, with Td10 (thermal decomposition of 10% weight) values spanning a range of 305–340 °C.

## 2.1. Electrochemical Characterization

The electrochemical properties of the POM-based materials are first investigated in a three-electrode configuration using both aqueous (H₂SO₄) and ionic liquid (EMIMCl) electrolytes to understand the influence of the medium on charge storage behavior (Figures S13–S17, Supporting Information). In H₂SO₄, the materials exhibit b-values close to 0.5–0.6, indicative of diffusion-controlled faradaic processes. In contrast, in EMIMCl, the b-values range from 0.83 to 0.96, suggesting a dominant capacitive contribution and faster surface-controlled kinetics. Based on these observations, all subsequent electrochemical measurements are performed in a symmetric two-electrode configuration using 1-ethyl-3-methylimidazolium chloride (EMIMCl) as the electrolyte.

Employing EMIMCl as an electrolyte offers numerous benefits, such as exceptional chemical, thermal, and electrochemical stability, minimal volatility, non-flammability, and resilience across a wide potential range. **Figure 5** displays the cyclic voltammetry (CV) curves of POM (Figure 5a), 0D POM-PTA (Figure 5b), 1D POM-PMDA (Figure 5c), and 2D POM-MTA (Figure 5d) between 0–1.8 V for POM and 0–2 V for the POM hybrids at scan rates 2–50 mV s⁻¹. The CV profiles of POM, 0D POM-PTA, and 1D POM-PMDA display a nearly rectangular shape, while the CV profile of 2D POM-MTA displays multiple redox peaks. This suggests differences in the charge storage mechanisms between the pristine POM and the POM hybrids.
<figure><img src="image_7.png" /><figcaption>**Figure 5.** CV at various scan rates of a) POM, b) 0D POM-PTA, c) 1D POM-PMDA, and d) 2D POM-MTA.</figcaption></figure>

The average *b*-value for POM, 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA is 0.95, 0.91, 0.85, and 0.74, respectively (Figure S18, Supporting Information). These values indicate that charge storage in the POM electrodes is predominantly ruled by a capacitive contribution and partial diffusion-controlled contribution. Differently, POM hybrids possess a hybrid nature in their charge storage mechanism, with a contribution of both capacitive and diffusion-controlled processes. Notably, the diffusion-controlled mechanism becomes progressively more significant with the increase in dimensionality in the POM hybrids. This observation aligns with the CV profiles of the 2D POM-MTA, where multiple redox peaks are evident, reflecting the increasing role of diffusion-controlled processes in the hybrid charge storage mechanism.

To further deconvolute the charge storage mechanism and quantify the relative contributions of capacitive and diffusion-controlled processes, we applied the equation: $i = k_1v + k_2v^{1/2}$. Where $k_1v$ and $k_2v^{1/2}$ represent the capacitive and diffusion-limited effects, respectively. The resulting capacitive and diffusion-controlled contributions at various scan rates are shown in Figure S19 (Supporting Information) for all samples. At low scan rates (2 mV s⁻¹), the capacitive contribution ranges from 63% to 79%, increasing steadily with scan rate and reaching up to 90–95% at 50 mV s⁻¹. This behavior highlights the dominance of capacitive processes under fast scan conditions, especially in the pristine POM and low-dimensional hybrids.

Interestingly, a clear trend emerges with increasing structural dimensionality: the diffusion-controlled contribution becomes progressively more significant from 0D to 2D POM hybrids. This can be attributed to the increase in intrinsic porosity and internal free volume within the higher-dimensional frameworks, which facilitates better penetration of electrolyte ions into the bulk of the material. As a result, redox reactions are no longer confined to the surface but occur more extensively throughout the material's interior. This is particularly evident for the 2D POM-MTA, which maintains the highest diffusion-controlled fraction across scan rates, in line with its distinct redox peaks and lower *b*-value observed in CV analysis. These findings confirm that the dimen-

sional POM hybrids exhibit a synergistic charge storage mechanism, where enhanced porosity and structural complexity enable both fast capacitive response and deeper diffusion-driven redox activity—a favorable combination for high-performance energy storage applications.

**Figure 6** shows the galvanostatic charge–discharge (GCD) curves between 0–1.8 V for pristine POM and 0–2 V for the POM hybrids at current densities between 0.5 and 2 A g⁻¹. The GCD profiles at 0.5 A g⁻¹ of POM, 0D POM-PTA, 1D POM-PMDA, and 2D POM-MTA within the investigated potential windows exhibit a distorted triangular shape consistent with their hybrid nature. As the GCD curves of electrodes are not linear, the specific capacitances are determined from the CV curves (Figure 5a–d), with the highest value of 132 F g⁻¹ achieved for 2D POM-MTA at the scan rate of 2 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) data are analyzed using Nyquist plots (Figure S20, Supporting Information), and the experimental results are accurately fitted to the specified circuit model (Figure S21, Supporting Information). All the POM hybrid-based electrodes demonstrate low charge transfer resistance ($R_{ct}$) (below 100 Ω), indicating that they facilitate efficient and rapid transfer of charge (electrons or ions) between the electrode surface and the electrolyte or external circuit (Table S1, Supporting Information). The pristine POM exhibits a comparatively higher $R_{ct}$ than the POM hybrids due to its isolated molecular nature, which lacks structural connectivity and accessible pathways for charge transfer. In contrast, the incorporation of COF linkers in the hybrids introduces extended porous architectures that facilitate ion transport and improve interfacial contact with the electrolyte. Notably, the $R_{ct}$ decreases progressively with increasing dimensionality of the POM hybrids. While 0D POM-PTA remains largely discrete, 1D POM-PMDA introduces linear connectivity, allowing improved ion accessibility. The 2D i-POCOF displays the lowest $R_{ct}$, which we attribute primarily to their continuous, planar morphology that enhances electrolyte penetration, increases surface area, and provides highly accessible redox-active sites. Although this trend might suggest improved charge transport, electronic conductivity measurements confirm that all hybrids remain intrinsically
<figure><img src="image_8.png" /><figcaption>**Figure 6.** GCD profiles at various current densities of a) POM, b) 0D POM-PTA, c) 1D POM-PMDA, and d) 2D POM-MTA. e) cycling stability test of POM and POM hybrids at 1 A g⁻¹.</figcaption></figure>

insulating ($\sigma < 10^{-7}$ S cm⁻¹). Therefore, the reduction in Rct across the series is dominated by enhanced ion diffusion and interfacial effects rather than bulk electronic conductivity.

The dimensionality of the POM hybrids plays a critical role in determining their physicochemical properties and electrochemical performance. Aggregates of discrete molecules 0D POM-PTA have a confined, particle-like structure, resulting in low surface area (107 m² g⁻¹) and limited porosity (average pore size 1.9 nm). These factors restrict ion accessibility and increase charge transfer resistance, leading to lower capacitance (54 F g⁻¹). 1D POM-polymer demonstrates a linear arrangement, offering moderate improvements in surface area (163 m² g⁻¹) and porosity (average pore size 3.3 nm) compared to their 0D counterparts. This leads to a reduced charge transfer resistance, enhanced ion diffusion, and charge storage capacity, with specific capacitance reaching 90 F g⁻¹. 2D i-POCOF achieves the highest surface area (257 m² g⁻¹) and mesoporous structure (average pore size 3.7 nm), which not only facilitates efficient ion diffusion and charge transport but also significantly lowers charge transfer resistance. This dimensionality results in the best electrochemical performance, with a specific capacitance of 132 F g⁻¹. These findings underscore that increasing dimensionality enhances the structural and electrochemical properties of the POM hybrids.

Finally, to assess the electrochemical stability of the POM and POM hybrids, GCD cycling is performed at a current density

of 1 A g⁻¹ (Figure 6e). After 5000 cycles, the capacitance loss is below 6%, 12%, 28%, and 41% for 2D POM-MTA, 1D POM-PMDA, 0D POM-PTA, and POM, respectively, highlighting the enhanced chemical stability of the POM hybrids compared to the pristine POM starting material. This data indicates a correlation between chemical stability and the dimensionality of the hybrids; specifically, 2D POM-MTA exhibits greater stability than its lower-dimensional counterparts. The dimensionality of these materials plays a crucial role in their ability to maintain structural integrity under prolonged cycling conditions. Finally, energy density and power density are calculated to evaluate the practical applications of energy storage materials. Remarkably, due to their hybrid nature, 2D POM-MTA-based electrodes (Table S2, Supporting Information) display an outstanding maximum energy density (73.3 Wh kg⁻¹) and high power density (1 kW kg⁻¹). These metrics surpass the state-of-the-art benchmarks for all previously reported POM hybrids, COFs, metal-organic frameworks (MOFs), and carbon-based electrode materials.

To evaluate the long-term stability of the POM-based electrodes, post-cycling characterization is carried out using XPS and SEM. These analyses aim to assess the chemical state of the electroactive components and the structural integrity of the electrodes after prolonged electrochemical operation (Figures S22–S24, Supporting Information).
XPS analysis focuses specifically on the Mo 3d and Mn 2p regions, which provide direct insight into the oxidation states of the metal centers in the Anderson-type POM clusters. The C 1s and N 1s regions are not included in this analysis, as their signals are significantly affected by the presence of the PTFE binder and residual EMIMCl electrolyte in polycarbonate solvent. Despite careful washing, these components remain partially adsorbed on the surface and interfere with accurate interpretation. In contrast, the Mo and Mn signals remain clean and unambiguous, allowing reliable conclusions to be drawn about the chemical stability of the active material. As shown in Figures S22 and S23 (Supporting Information), all samples exhibit sharp and well-defined doublets consistent with Mo⁶⁺ (Mo 3d₅/₂ at ≈232.5 eV and Mo 3d₃/₂ at ≈235.6 eV) and Mn³⁺ (Mn 2p₃/₂ at ≈641.9 eV and Mn 2p₁/₂ at ≈653.5 eV), with no detectable peak shifts, broadening, or satellite features. These results confirm that the POM units retain their original oxidation states and are not chemically degraded by the cycling process. Although some decay in capacitance is observed during long-term cycling, this is not attributed to chemical changes in the active POM species. Instead, it is likely caused by secondary factors such as partial loss of electroactive surface area due to pore blockage, reduced ion mobility in the ionic liquid electrolyte, or minor interfacial effects related to the binder or residual electrolyte. In systems using EMIMCl, limited ion transport and slow electrolyte dynamics can also contribute to performance fading over time. Nevertheless, the chemical stability of the active redox centers is preserved, as confirmed by the XPS data.

SEM imaging (Figure S24, Supporting Information) is used to
examine the electrode morphology before and after cycling. The
electrodes maintain a uniform and cohesive structure, with no
signs of delamination, severe cracking, or agglomeration. Some
morphological differences are observed compared to the pristine
powder forms, which is expected due to the inclusion of PTFE
binder, Super P conductive carbon, and mechanical grinding dur-
ing ink formulation. Despite these processing steps, the hybrid
materials remain well-distributed and adhere effectively to the
current collector throughout cycling. These results demonstrate
that the POM-based hybrid electrodes exhibit excellent chemi-
cal and structural stability under extended electrochemical op-
eration, supporting their suitability for long-term energy storage
applications.

**3. Conclusion**

In this study, we report the synthesis of an imide-linked polyoxometalate-covalent organic framework (i-POCOF) along with a 0D discrete system and a 1D polymer to systematically evaluate the influence of dimensionality on material properties. Using Anderson-type polyoxometalates as redox-active building blocks and carefully selected anhydride linkers, we demonstrate a clear structure–property–performance relationship. The dimensional evolution from 0D to 2D leads to progressive enhancements in surface area, porosity, and charge storage capabilities. Nitrogen sorption analysis reveals that the BET surface area increases from 107 m² g⁻¹ (0D POM-PTA) to 163 m² g⁻¹ (1D POM-PMDA) and 257 m² g⁻¹ (2D POM-MTA). Pore size distributions confirm the transition from microporous to mesoporous structures, facilitating improved electrolyte ion diffusion. Among all

materials, the 2D POM-MTA hybrid exhibits the most optimized
porosity and electrochemical behavior, delivering a high specific
capacitance of 132 F g⁻¹, energy density of 73.3 Wh kg⁻¹, and
power density of 0.9 kW kg⁻¹, along with excellent capacitance
retention of 94% after 5000 cycles. These performance gains are
attributed to the extended 2D framework, which enables efficient
confinement of POM clusters, enhanced accessibility of redox-
active sites, and stable charge transport pathways. The use of
chemically robust imide linkages further ensures hydrolytic and
thermal stability, critical for long-term operation. XPS and SEM
analyses confirm that both chemical composition and morphol-
ogy are preserved after cycling. Overall, this work underscores
the advantages of dimensionality control in hybrid frameworks
and establishes imide-linked POCOFs as a promising platform
for the development of durable and high-performance electrode
materials. Future efforts will focus on improving electronic con-
ductivity through composite strategies and expanding this design
concept to broader families of redox-active molecular and frame-
work components.

**4. Experimental Section**

**Materials:** 1-Ethyl-3-methylimidazolium chloride, propylene carbonate, separators Whatman glass microfiber filters, poly(tetrafluoroethylene) (PTFE), 1-methyl-2-pyrrolidinone (NMP), mesitylene, isoquinoline, sodium molybdate dihydrate, tetrabutylammonium bromide,манганese(III) acetate dihydrate, tris(hydroxymethyl)aminomethane (TRIS), phthalic anhydride (PTA), pyromellitic dianhydride (PMDA) and all solvents for synthesis and NMR studies were purchased from Sigma-Aldrich. ("Bu₄N)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (MnMo₆) and mellitic trianhydride (MTA) were prepared according to previously reported procedures.[70,75] Conductive Carbon Black Super P (H30253) was acquired from Alfa Aesar and carbon AvCarb P75 substrate was purchased from FuelCellStore.

**Characterization:** Infrared spectra were recorded in the range from 400 to 4000 cm⁻¹ using a Fourier-transform infrared (FTIR) IFS 66/s Bruker spectrometer. Electrospray ionization mass spectroscopy (ESI-MS) was performed using a Mas ZQ Water spectrometer. Solution nuclear magnetic resonance (NMR) spectra were measured in CD₃CN and were acquired using Spektrometru Bruker Avance 600 MHz, for ¹H NMR operating at 600 MHz and for ¹³C NMR at 151 MHz. Heteronuclear multiple bond coherence (HMBC) spectra were acquired in the range of 0–10 ppm for ¹H NMR and 240–0 ppm for ¹³C NMR. Solid NMR (ss-NMR) spectra were recorded using Bruker Avance III HD spectrometer coupled with an 11.7 T wide bore superconducting magnet operating at 125.76 MHz 13C Larmor frequency. Powder X-ray diffraction (PXRD) patterns were recorded using Bruker AXS D8 Advance diffractometer. X-ray photoelectron spectroscopy (XPS) analyses were performed on a Thermo Scientific Kα X-ray photoelectron spectrometer. The measurements were recorded with an Al anode as the X-ray source (X-ray radiation of 1486 eV) and a basic chamber pressure of ≈10⁻⁹ mbar. Scanning electron microscopy (SEM) was performed on a FEI Dual Beam 235 with the accelerating voltage of 30 keV incident beam energy. Before SEM imaging, the surface of the studied compounds was sputtered with a thin layer of gold.

**Synthesis of POM** ("Bu₄N)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂]: POM was prepared according to a reported procedure by Marcoux et al.[70] In a 250 mL two-necked flask ("Bu₄N)₄[Mo₆O₂₆] (2.50 g, 1.31 mmol), (HOCH₂)₃CNH₂ (0.46 g, 3.80 mmol), Mn(OAc)₃·2H₂O (0.48 g, 1.79 mmol) were added and were dissolved in acetonitrile (75 mL) under magnetic stirring. The mixture was heated at 85 °C under stirring for 24 h. After cooling down, the reaction mixture was centrifuged and the precipitate was discarded. The large, green crystals were obtained by vial-to-vial diffusion of diethyl ether. Yield: 1.36 g (55.2% based on ("Bu₄N)₄[Mo₆O₂₆]).
IR (KBr) v = 3461 (br, m), 2967 (m), 2937 (sh), 2876 (m), 1639 (w), 1483 (m), 1375 (w), 1037 (s), 942 (sh), 916 (s), 669 (br, s), 560 (w), 464 (w), 408 cm⁻¹ (w); ESIMS (+) m/z (%): 243 (100) [Bu₄N]⁺; ESIMS (-) m/z (%): 707 (70) [(Bu₄N)(MnMo₆O₁₈((OCH₂)₃CNH₂)₂)]²⁻.

**Synthesis of POM-PTA:** ("Bu₄N)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (50.00 mg, 0.026 mmol), and phthalic anhydride (PTA) (7.90 mg, 0.052 mmol) were dissolved in a mixture of NMP (1 mL), mesitylene (1 mL) and isoquinoline (1 mL). Then, to obtain a homogenous dispersion, the mixture was sonicated for 5 min. Next, the tube was frozen at 77 K (liquid N₂ bath) and degassed for three cycles. The reaction mixture was heated at 120 °C for 3 days. The product was precipitated with Et₂O, centrifuged, washed with Et₂O and the dark brown solid was dried under vacuum. Yield: 47.30 mg (84.6% based on polyoxometalate).

¹H NMR (600 MHz, Acetonitrile-d₃, δ): 8.27 (d, 4H, Ar H), 7.48 (t, 4H, Ar H), 3.10 (t, 67H; CH₂), 2.73 (s, 4H; CH₂), 1.61 (p, 67H; CH₂), 1.36 (h, 68H; CH₂), 0.97 (t, 100H; CH₃); ¹³C NMR (151 MHz, Acetonitrile-d₃, δ): 178.70 (C = O (C₄)), 169.74 (C₅), 136.49 (C₆), 133.66 (C₁), 131.00 (C₂), 59.29 (C₃), 49.73 (C₃), 24.30 (C₄), 20.30(C₈), 13.77 (C₆); IR (KBr) v = 3428 (m), 3306 (m), 3225 (m), 3126 (m), 2968 (m), 2923 (m), 2870 (m), 1659 (w), 1563 (w), 1473 (m), 1431 (w), 1384 (w), 1058 (s), 1027 (s), 958 (sh), 803 (sh), 737 (br, s), 567 (w), 473 cm⁻¹ (w).

**Synthesis of POM-PMDA:** ("Bu₄N)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (50.00 mg, 0.026 mmol), and pyromellitic dianhydride (PMDA) (5.79 mg, 0.026 mmol) were dissolved in a mixture of NMP (1 mL), mesitylene (1 mL) and isoquinoline (1 mL). Then, to obtain a homogenous dispersion, the mixture was sonicated for 5 min. Next, the tube was frozen at 77 K (liquid N₂ bath) and degassed via three cycles. The reaction mixture was heated at 120 °C for 3 days, the product was precipitated with Et₂O, centrifuged, washed with Et₂O and the dark brown solid was dried under vacuum. Yield: 48.10 mg (57.7% based on polyoxometalate).

IR (KBr) v = 3400 (m), 2969 (m), 2928 (m), 2876 (m), 1588 (m), 1483 (w), 1426 (m), 1379 (w), 1049 (s), 955 (s), 912 (sh), 798 (br, s), 651 cm⁻¹ (w).

**Synthesis of POM-MTA:** ("Bu₄N)₃[MnMo₆O₁₈((OCH₂)₃CNH₂)₂] (50.00 mg, 0.026 mmol), and mellitic trianhydride (MTA) (5.10 mg, 0.017 mmol) were dissolved in a mixture of NMP (1 mL), mesitylene (1 mL) and isoquinoline (1 mL). Then, to obtain a homogenous dispersion, the mixture was sonicated for 5 min. Next, the tube was frozen at 77 K (liquid N₂ bath) and degassed via three cycles. The reaction mixture was heated at 120 °C for 3 days, the product was precipitated with Et₂O, centrifuged, washed with Et₂O and the dark brown solid was dried under vacuum. Yield: 40.80 mg (34.6% based on polyoxometalate).

IR (KBr) v = 3377 (br, m), 2961 (m), 2881 (m), 1714 (w), 1596 (m), 1473 (w), 1379 (m), 1064 (s), 958 (s), 917 (s), 807 (s), 673 cm⁻¹ (s).

**Electrode Preparation—POM-Based Electrodes Preparation:** The POM-based electrodes were fabricated by mixing the active material (POM, POM-PTA, POM-PMDA, or POM-MTA) (80 wt.%, 16 mg), Super p (10 wt.%, 2 mg), and PTFE (10 wt.%, 2 mg) in an agate mortar, along with several drops of NMP to form a homogeneous paste. This paste was then dropped and cast onto a carbon paper electrode with a diameter of 1.6 cm and subsequently dried under vacuum at 80 °C overnight. The mass loading of POM, POM-PTA, POM-PMDA, and POM-MTA in each electrode was ≈1 mg cm⁻².

**Electrode Preparation—Activated Carbon Electrode Preparation:** These electrodes were fabricated by mixing the active material (activated carbon) (90 wt.%, 18 mg), and PVDF (10 wt.%, 2 mg) in an agate mortar, along with several drops of NMP to form a homogeneous paste. This paste was then drop-cast onto a carbon paper electrode with a diameter of 1.2 cm and subsequently dried under vacuum at 80 °C overnight. The mass loading of each electrode was ≈1 mg cm⁻².

**Electrode Preparation—Three-Electrode Configuration:**

1) Aqueous Electrolyte: 1 M H₂SO₄ aqueous solution served as an electrolyte, AC as a counter electrode, Ag/AgCl as a reference electrode, and POM-based electrodes as working electrodes.

2) Ionic Liquid Electrolyte: 1-ethyl-3-methylimidazolium chloride in propylene carbonate served as an electrolyte, AC as counter and pseudo-reference electrode, and POM-based electrodes as working electrodes.

**Electrode Preparation—Two-Electrode Configuration:** Two POM-based electrodes were assembled into CR2032 stainless steel coin cells, using a porous cellulose membrane as the separator and 1 M 1-ethyl-3-methylimidazolium chloride in propylene carbonate as the electrolyte.

**Electrochemical Measurements:** Electrochemical measurements, including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) measurements, and electrochemical impedance spectroscopy (EIS) were performed at room temperature using an EC-LAB VMP3 (BioLogic Science Instrument). CV curves were obtained at scan rates of 2–50 mV s⁻¹. The voltage window for the three-electrode systems in the H₂SO₄ electrolyte was -0.2 to + 1 V. The voltage window for the three-electrode systems in 1-ethyl-3-methylimidazolium chloride in propylene carbonate electrolyte was -0.4 to + 0.4 V. The voltage window in the two-electrode configuration was from 0 to 1.8 V (POM) or 0 to 2 V (POM-PTA, POM-PMDA, POM-MTA). EIS measurements were recorded within a frequency range of 0.01 Hz–1 MHz. GCD measurements were performed within the same voltage window using current densities ranging from 0.5 to 2 A g⁻¹. Cycloability tests were conducted through GCD at a current density of 1 A g⁻¹ for 5000 cycles, within the same potential window.

**Electrochemical Calculations:** The specific capacitance was calculated by means of CV measurements using the following Equation (1):

$$C = \frac{\int f \cdot dV}{m \cdot \nu \cdot \Delta V} \qquad (1)$$

where, I is the current, v is the scan rate (V/s), ΔV (V) is the voltage window and m is the mass of active material in a single electrode.

The energy density of the device was obtained from the following Equation (2):

$$E = \frac{l}{2 \cdot m} \cdot \int_{0}^{t} V \Delta t \qquad (2)$$

where E is specific energy (Wh kg⁻¹), l is the applied current (A), and ∫₀^t V Δt is the integral area of the GCD curve.

The power density of the device was calculated from the following Equation (3):

$$P = \frac{E}{\Delta t} \times 3600 \qquad (3)$$

where P is power density (W kg⁻¹) and Δt is the discharge time (s).

**Calculation of Electrochemical Active Surface Area (ECSA):** The ECSA was calculated using the equation:

$$ECSA = \frac{C_{dl}}{C_{s}} \qquad (4)$$

where C₉₁₁ represents the double-layer capacitance and C₅ represents the specific capacitance of a flat surface.

## Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

## Acknowledgements

This work was supported by the National Science Centre, Poland (grant no. 2022/47/D/ST5/00261; 2020/39/D/ST4/01182 and 2022/45/N/ST4/00632). Daria Nowicka is a scholarship holder of the Adam Mickiewicz University in Poznan Foundation for the academic year 2023/2024.
**Conflict of Interest**

The authors declare no conflict of interest.

**Author Contributions**

D.P., A.G., and A.C. conceived the experiments and designed the study. D.N. and A.S. performed all synthetic parts and the physicochemical characterization. D.P. and V.M.-G. performed the electrochemical analysis including kinetic study, CV, GCD, and EIS. All authors discussed the results and contributed to the interpretation of data. D.P., A.G., V.P., V.M-G, and A.C. co-wrote the paper with input from all co-authors.

**Data Availability Statement**

The data that support the findings of this study are available from the corresponding author upon reasonable request.

**Keywords**

covalent organic frameworks, hybrid materials, polyoxometalates, supercapacitors

Received: March 23, 2025

Revised: May 21, 2025

Published online: May 30, 2025

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