Proton-coupled pseudocapacitive behavior of a manganese oxide-decorated nitrogen-rich COP for sustainable high-performance energy storage devices

Abstract

A hierarchical hybrid material (Mn₂O₃@COP) with dual charge storage capabilities was created by synthesizing a triazine-based covalent organic polymer (COP) that is rich in nitrogen functionalities and integrating it with Mn₂O₃ nanoparticles. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Brunauer–Emmett–Teller (BET) studies, and X-ray photoelectron spectroscopy (XPS) demonstrated a distinct architecture: Mn₂O₃ nanoparticles were uniformly embedded in a stable, porous COP matrix. Mn₂O₃ loading caused a modest decrease in surface area, but the composite still had the mesoporosity needed for quick ion diffusion. The existence of electroactive nitrogen centers—pyridinic, pyrrolic, and graphitic—and mixed-valence Mn³⁺/Mn⁴⁺ species, which together improve redox kinetics and charge transfer routes, was verified by XPS analysis. An intriguing combination of electric double-layer capacitance (EDLC) and pseudocapacitive behavior was demonstrated by electrochemical experiments in a 1 M H₂SO₄ electrolyte. The Mn₂O₃@COP composite electrode in the three-electrode system had a high specific capacitance of 113 F g⁻¹ at 5 mV s⁻¹ and 69.1 F g⁻¹ at 0.1 A g⁻¹, an energy density of 9.6 Wh kg⁻¹ and a power density of 500 W kg⁻¹ at 0.1 A g⁻¹. The fabricated symmetric supercapacitor device maintained 95% of its capacitance after 10 000 charge–discharge cycles at 0.5 A g⁻¹ and provided a specific capacitance of 16.2 F g⁻¹ and energy density and power density of 2.25 Wh kg⁻¹ and 250 W kg⁻¹ at 0.2 A g⁻¹, respectively. This study offers a viable method for combining the energy density of transition metal oxides with the quick kinetics of conductive organic networks, opening the door to long-lasting, highly effective energy storage devices.

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1 Introduction

The rapid rise in population has led to a significant surge in energy demand across all sectors—from everyday necessities to large-scale industrial operations. Currently, fossil fuels such as coal, natural gas, and oil are major contributors, accounting for nearly one-third of global greenhouse gas emissions. In the 21st century, one of the most critical challenges is ensuring access to cleaner, more reliable energy while simultaneously improving quality of life. This highlights the pressing need to transition toward sustainable, renewable, and eco-friendly energy alternatives. Renewable sources such as solar and wind power offer viable alternatives to the crisis of fossil fuels, and their intermittent nature presents challenges for grid integration. However, to guarantee a steady supply of energy, their intermittent output calls for the creation of advanced energy storage technologies.¹ Various energy storage technologies, such as batteries and supercapacitors, have emerged as key solutions to address these challenges of the global energy crisis.² They bridge the gap between the demand and supply of energy. Since 1995, batteries have emerged as among the greatest energy storage devices that provide high energy density outputs. However, despite their high energy density, they possess low power density, which is their major drawback. In addition to batteries, supercapacitors are advantageous because of their high-power density, rapid charge/discharge rates, and long cycle life, making them ideal for applications such as regenerative breaking, grid stabilization, and backup power.³ Supercapacitors come into play when practical applications call for an energy storage device that can offer the advantages of high energy density and significant power density. They bridge the gap between capacitors and batteries. The electrode is a major part of the supercapacitor and determines the performance of the supercapacitor. Hence, the choice of electrode material plays a crucial role in the performance of a supercapacitor.

A wide range of materials have been explored for supercapacitor applications, and mainly carbon-based materials (activated carbon, carbon nanotubes, graphene, mesoporous carbon, etc.), metal oxides and hydroxides, conducting polymers, and composite materials have been extensively studied. These materials often face limitations such as low energy density, limited specific capacity, and mechanical instability. Composite materials, however, offer a promising alternative by synergistically enhancing conductivity, stability, and capacitance, making them highly effective for next-generation supercapacitors.⁴ Several articles highlighting the above-mentioned materials have been published. Hou et al. reported a hierarchically nanostructured polypyrrole/MXene hybrid hydrogel modified with a lignosulfonate composite as an electrode material for supercapacitor applications. Even though the symmetric device has an energy density of 29.3 Wh kg⁻¹ and a power density of 3200 W kg⁻¹, it faces several challenges, such as aggregation and restacking of the MXene layers, poor mechanical and structural stability of the polypyrrol, and interface control and optimization of the hybrid composition.⁵ Similarly, an Ag@TiO₂ nanotube array composite was used as an anode material for sodium metal battery applications. The device achieved 87% capacity retention after 2000 cycles at 8 C. However, the composite has the major drawback of dendrite formation in sodium metal.⁶ Another report described a Cu-MoO₃ composite as an anode material for a zinc-ion battery. The composite showed a capacity of 254.4 mAh g⁻¹ at 0.4 A g⁻¹. The composite has the major drawback of precisely tuning the Cu concentration to generate an optimal oxygen vacancy density without compromising structural integrity or introducing unwanted phases, which affect the performance of the device.⁷

Despite these composite materials and their drawbacks, covalent organic polymers (COPs) have emerged as promising electrode materials for supercapacitors because of their exceptional chemical and structural properties. As a subclass of crystalline and porous covalent organic frameworks (COFs), COPs consist of organic molecules linked by covalent bonds, offering a highly tunable platform for energy storage applications. Unlike traditional inorganic materials, COPs provide extensive chemical versatility, enabling precise control over their electronic, electrochemical, and structural characteristics. These properties make COPs highly suitable for efficient energy storage, as they have a high surface area, tunable porosity, and excellent structural stability. The ability of these materials to function with redox-active groups further enhances their pseudocapacitive charge storage capabilities, significantly improving the overall energy density of asymmetric supercapacitors. Additionally, their inherent stability and excellent rate capability ensure long-term cycling performance, making them a compelling choice for next-generation energy storage devices.⁸ Tao et al. reported the use of nickel-doped COF (Ni-COF) as an electrode material for an asymmetric supercapacitor; the device exhibited an excellent specific capacitance of 417 F g⁻¹ at 1 A g⁻¹ and high energy and power densities of 130 Wh kg⁻¹ and 839 W kg⁻¹, respectively.⁹ Kang et al. developed an antimony-doped triphenyl stibine-based COF (Sb-COF) composite as an electrode material for asymmetric supercapacitors. The device showed an excellent specific capacity of 78.6 F g⁻¹ at 3 A g⁻¹ with an energy density of 44 Wh kg⁻¹ and a power density of 3024 W kg⁻¹.¹⁰

On the other hand, metal oxides have emerged as promising electrode materials for high-performance supercapacitors owing to their remarkable specific capacitance, favorable electrical conductivity, and superior electrochemical stability. Typically, these materials exhibit higher charge storage capacities per unit mass or volume than other electrode candidates do.¹¹ Prominent examples of metal oxides utilized in energy storage applications include manganese(III) oxide (Mn₂O₃), manganese dioxide (MnO₂), nickel oxide (NiO), cobalt oxide (Co₃O₄), ruthenium dioxide (RuO₂), vanadium pentoxide (V₂O₅), titanium dioxide (TiO₂), molybdenum trioxide (MoO₃), etc. In comparison, Mn-based oxides have garnered much attention because of their affordability, availability, and environmental friendliness. Mn₂O₃ has shown great promise as an electrode material for supercapacitor applications. Many studies have investigated the outstanding physicochemical properties of Mn₂O₃, making it a suitable candidate for electrode applications. Ansari et al. reported the synthesis of porous hollow double-walled Mn₂O₃ nanocubes through a hydrothermal approach, which demonstrated a high specific capacitance of 256.25 F g⁻¹ along with excellent cyclic stability, retaining 90% of the initial capacitance after 3000 charge–discharge cycles.¹² Li et al. synthesized porous Mn₂O₃ nanocubes via a hydrothermal method, achieving a specific capacitance of 191.1 F g⁻¹ at a current density of 0.1 A g⁻¹, along with a rate capability of 58.6%.¹³ Similarly, Xu et al. fabricated porous Mn₂O₃ microsheet arrays, which exhibited a high specific capacitance of 566.6 F g⁻¹ at 0.5 A g⁻¹, which decreased to 294.4 F g⁻¹ when the current density was increased to 20 A g⁻¹, indicating good rate performance.¹⁴ Although Mn₂O₃ has been extensively investigated, there is still a great deal of unrealized potential to increase its capacity and energy storage capacity by means of structural modification, doping strategies, composite formations, etc.¹⁵ Recent studies on Prussian blue analogues (PBAs) for sodium-ion storage emphasize the value of combining transition-metal redox activity with robust, ion-accessible frameworks to boost energy and power densities. Yet, PBAs face issues such as structural degradation, vacancy instability, and sluggish ion transport, addressed through strategies like particle size control, surface functionalization, and framework integration.¹⁶ These principles extend to Mn₂O₃ systems, where incorporating it into a conductive, porous COP matrix can enhance redox accessibility, stabilize morphology, improve electrolyte interaction, and overcome ion diffusion barriers for superior capacitive performance. Tong et al. observed in recent battery management studies, electrochemical performance is often limited by material degradation, including capacity loss, increased resistance, and polarization changes during cycling.¹⁷ Such challenges underscore the importance of frameworks that combine active redox materials with robust structural matrices. In this context, embedding Mn₂O₃ within a covalent organic polymer (COP) offers a pathway to maintain high capacitance, mitigate structural degradation, and ensure stable operation in long-term energy storage applications. Many other recent advances in energy storage highlight the need for electrode materials that combine high performance with long-term stability.¹⁸ In lithium-ion systems, hybrid model- and data-driven approaches have improved performance retention under aging, while optimized structural designs in thermal management have enhanced operational consistency.¹⁹ Translating these principles to material design, embedding Mn₂O₃ within a conductive COP can integrate efficient redox activity with mechanical and structural resilience, offering a robust platform for high-rate capacitive performance and extended cycling life in next-generation hybrid energy storage devices.^20,21

We report a Mn₂O₃@COP composite, where Mn₂O₃ nanoparticles are uniformly embedded within a nitrogen-rich triazine–urea-linked COP matrix. This hybrid structure offers synergistic integration of faradaic and non-faradaic charge storage mechanisms, facilitating rapid ion diffusion, enhancing conductivity, and improving structural integrity during cycling. Unlike previously reported Mn₂O₃-based composites that suffer from poor cycle life or COP systems with limited capacitance, the Mn₂O₃@COP electrode material demonstrates enhanced electrochemical performance, superior rate capability, and remarkable cycling stability. This work introduces a cost-effective and scalable synthesis strategy for hybrid materials, paving the way for their application in high-performance energy storage devices (Fig. 1).

Fig. 1 Graphical abstract of the synthesis, fabrication and electrochemical behavior of the Mn₂O₃@COP composite electrode.

2 Materials and methods

2.1 Materials

Cyanuric chloride (98.5%), urea (98%), and acetone (99%) were purchased from Loba Chemical. Manganese oxide (99%) was purchased from BLD Pharma. Tetrahydrofuran (99.9%) was purchased from Merck.

2.2 Synthesis of COP

COP was synthesized via a previously reported method.²² Cyanuric chloride (0.02 mol, 3.69 g) and urea (0.03 mol, 1.8 g) were thoroughly mixed on a magnetic stirrer in a 100 mL beaker and gradually heated to 140 °C with continuous stirring until a molten mixture was obtained. The reaction was maintained under these conditions until the evolution of HCl ceased, indicating that upon completion of the reaction, the mixture was cooled to room temperature. The resulting foamy white solid was successively washed with water, tetrahydrofuran (THF), and acetone, followed by vacuum drying, yielding the final product with an overall yield of 76% (Fig. 2).

Fig. 2 Schematic representation of the polymerization reaction between cyanuric chloride and urea to synthesize increase the COP.

2.3 Synthesis of the Mn₂O₃@COP composite

COP and Mn₂O₃ were mixed at a weight ratio of 1 : 1 by dispersing them in 30 mL of distilled water and then transferred to a Teflon autoclave vessel. The vessel was placed in a muffle furnace at 150 °C for 16 hours. The resulting dark-colored solid was then filtered and washed thrice with acetone, methanol and multiple rinses with water. Finally, the product was dried, and the obtained product was named the Mn₂O₃@COP composite (Fig. 3).

Fig. 3 Schematic illustration showing the incorporation of Mn₂O₃ nanoparticles into the COP matrix.

2.4 Electrochemical characterization and supercapacitor fabrication

Mn₂O₃@COP was used as the active material; it was mixed with polyvinylidene fluoride (PVDF) and N-methyl pyrrolidone (NMP) at a ratio of 8 : 1 : 1 to form a uniform slurry. This slurry was coated on a 1 cm × 1 cm dimension stainless steel (SS) sheet, which was used as a working electrode along with Ag/AgCl as a reference and platinum foil as a counter electrode. A swagelok-type cell was constructed with Mn₂O₃@COP as both electrodes for the symmetric device (Mn₂O₃@COP//Mn₂O₃@COP). The electrochemical studies for both the three- and two-electrode systems were carried out in a 1 M H₂SO₄ electrolyte solution via an electrochemical workstation (Biologic, SP-50e, France).

2.5 Material characterization

The crystallinity of the as-prepared material was analyzed via X-ray diffraction (XRD) with a Bruker D8 instrument (Bruker, USA) with a Cu Kα radiation source (40 kV, 40 mA). The chemical interactions in the material were identified via Fourier transform infrared (FTIR) spectroscopy (Shimadzu, IRSpirit, Japan). The morphology of the material was determined via field emission scanning electron microscopy (Thermo Fisher Scientific and ZEISS, EVO MA18 with Oxford EDS (X-act), IT700 HR). Energy-dispersive X-ray spectroscopy (integrated with FESEM) was used to analyze the elemental composition of the as-prepared material. X-ray photoelectron spectroscopy was carried out via a ThermoFisher instrument (Nexsa base, K-α). The surface area was determined via Brunauer–Emmett–Teller (BET) analysis (BELSORP mini-X, BELPREP VAC 111, Japan).

3 Results and discussion

3.1 FTIR studies

The FTIR spectra clearly demonstrated the successful synthesis and integration of the Mn₂O₃@COP composite (Fig. 4). In the case of cyanuric chloride, distinct peaks between 700 and 1600 cm⁻¹ highlight its triazine ring structure, with a notable peak at approximately 810 cm⁻¹ attributed to C–Cl stretching and others between 1350 and 1550 cm⁻¹ corresponding to C–N and C=N vibrations.²³ Urea, on the other hand, shows a broad, intense band from 3300 to 3500 cm⁻¹ due to N–H stretching, alongside a strong C=O stretching peak at approximately 1670 cm⁻¹ and N–H bending modes near 1450–1550 cm⁻¹.²⁴ Upon polymerization, the FTIR spectrum of the resulting COP undergoes notable changes, indicating the formation of new covalent bonds. The broad absorption at approximately 3300 cm⁻¹ suggests the presence of N–H groups, potentially from unreacted amines or trace moisture. A sharp peak near 1625–1650 cm⁻¹ confirms the formation of C=N bonds, which is consistent with triazine ring formation through nucleophilic substitution between cyanuric chloride and urea. Additional peaks between 1350 and 1550 cm⁻¹ further support the incorporation of nitrogen-containing linkages.²⁵ Mn₂O₃ shows strong absorption bands in the 500–700 cm⁻¹ region, typical of Mn–O stretching vibrations, confirming its oxide nature.²⁶ For the Mn₂O₃@COP composite, the spectrum reflects features from both components. The broad N–H stretching band and the C=N peak remains visible, indicating that the structural integrity of the COP is maintained. Moreover, the Mn–O bands confirmed that Mn₂O₃ was successfully incorporated into the polymer matrix. Subtle shifts or changes in peak intensity suggest interactions such as hydrogen bonding or weak coordination between the polymer and Mn₂O₃, indicating a well-integrated hybrid structure. Taken together, these spectral features provide strong evidence for the successful formation of both COP and the Mn₂O₃@COP composite.

Fig. 4 FTIR spectra of CC, urea, COP, Mn₂O₃, and Mn₂O₃@COP.

3.2 XRD studies

Comprehensive structural information about the synthesized materials may be obtained from X-ray diffraction (XRD) patterns (Fig. 5). Pure cyanuric chloride (black trace) has a diffraction pattern with distinct, sharp peaks that show that it is crystalline. The (100), (110), (111), (002), (210), (211), (310), and (320) planes are represented by prominent diffraction peaks at 2θ values of approximately 14.0°, 15.3°, 24.5°, 27.2°, 29.6°, 33.0°, 40.6°, and 49.0°, respectively.²⁷ The diffraction pattern of the produced COP material (red trace), on the other hand, is much less intense and broader, with a dominant peak at approximately 27°, which corresponds to the (002) plane and is indicative of the π–π stacking of aromatic layers.²⁸ The broad properties and lack of sharp peaks suggest that the COP material has an amorphous or weak crystalline structure. Since the (211), (222), (400), (332), (440), and (622) planes correspond to the typical reflections at 2θ values of 23.1°, 32.9°, 38.3°, 45.5°, 50.8°, 55.1°, and 65.5°, the diffraction peaks of pure Mn₂O₃ are in good agreement with those of the conventional cubic bixbyite phase (JCPDS no. 41-1442).²⁹ The Mn₂O₃@COP composite retains the crystalline signature of Mn₂O₃ with intact peak locations during inclusion into the COP matrix, suggesting that the structural integrity of Mn₂O₃ is maintained after encapsulation. The (222) and (400) planes in particular show discernible broadening and a slight reduction in peak intensity, which can be ascribed to the nanoscale dispersion of Mn₂O₃ within the amorphous COP network as well as potential surface interactions or partial encapsulation by the polymeric matrix. Successful hybrid synthesis without disturbing the crystalline phase of Mn₂O₃ is suggested by the emergence of (002) and (110) reflections in the composite, which also match the structural characteristics observed in the solitary COP. Overall, the XRD results demonstrate the effective production of the Mn₂O₃@COP composite by confirming the presence of crystalline Mn₂O₃ domains embedded inside the amorphous COP framework.

Fig. 5 XRD patterns of (a) cyanuric chloride, COP, and (b) Mn₂O₃, Mn₂O₃@COP.

3.3 XPS studies

High-resolution X-ray photoelectron spectroscopy (XPS) analysis of the Mn₂O₃@COP composite provides comprehensive insights into its surface chemistry and elemental composition (Fig. 6). The XPS survey spectrum of the Mn₂O₃@COP composite confirms the presence of Mn, O, N, and C. The C1s spectrum reveals multiple chemical environments of carbon. The dominant peak at ∼284.6 eV corresponds to sp² hybridized carbon (C–C/C=C), which is indicative of the conjugated aromatic backbone of the COP and is likely composed of triazine and urea moieties. The second peak at ∼286.0 eV is attributed to carbon bonded to electronegative atoms (C–O/C–N), suggesting the presence of nitrogen-rich functionalities and possible interactions with oxygen introduced during synthesis. The third peak, in the ∼288.5–289.0 eV range, corresponds to oxidized carbon species such as carboxyl (O–C=O) or ester groups,³⁰ potentially arising from surface oxidation or chemisorbed oxygen linked to the Mn₂O₃ interface. These observations confirm the preservation of the COP framework and highlight its chemically active surface. The O1s spectrum further supported the complex surface chemistry of the composite. The main peak at ∼531.2 eV is associated with lattice oxygen (O²⁻) in Mn–O bonds,³¹ confirming the presence of crystalline Mn₂O₃. A secondary peak at ∼532.2 eV is assigned to surface hydroxyl groups or adsorbed water, which are common in hydrated or environmentally exposed materials.³² The low-binding energy peak at ∼529.9 eV is indicative of chemisorbed oxygen or oxygen vacancies³³ (O⁻, O₂²⁻), suggesting the presence of nonstoichiometric regions or defect sites within the Mn₂O₃ structure. This multiplicity of oxygen species points to a surface environment conducive to redox activity and ion transport, which are critical for electrochemical energy storage. The N1s spectrum revealed three distinct nitrogen species within the COP matrix. The peak at ∼398.2 eV corresponds to pyridinic nitrogen,³⁴ which enhances the electron-donating capacity and supports redox behavior. The signal at ∼399.5 eV is assigned to pyrrolic nitrogen,³⁵ which contributes to pseudocapacitance through proton-coupled electron transfer. The peak near ∼400.6 eV corresponds to graphitic nitrogen,³⁶ which improves the electrical conductivity by integrating into the π-conjugated system.³⁷ The presence of these various nitrogen functionalities underscores the chemical richness of the COP framework and its potential role in facilitating both charge storage and transport. Finally, the Mn 2p spectrum reveals two sets of spin–orbit split peaks characteristic of the Mn³⁺ and Mn⁴⁺ oxidation states. The peaks at ∼641.7 eV and ∼653.4 eV are assigned to the Mn 2p_3/2 and Mn 2p_1/2 components of Mn³⁺, respectively,³⁸ in agreement with known values for Mn₂O₃. Moreover, the peaks at ∼643.2 eV and ∼654.9 eV correspond to the Mn 2p_3/2 and 2p_1/2 components of Mn⁴⁺, which is consistent with MnO₂.³⁹ This mixed-valence state of manganese supports the redox-active nature of the composite, facilitating reversible faradaic processes. The coexistence of Mn³⁺/Mn⁴⁺, along with the rich surface chemistry of carbon, oxygen, and nitrogen species, suggests that the Mn₂O₃@COP composite possesses essential attributes for enhanced pseudocapacitive performance, including favorable charge transfer kinetics and high electrochemical activity, making it a promising candidate for advanced energy storage applications.

Fig. 6 XPS spectra of the Mn₂O₃@COP composite: (a) survey spectrum, (b) C1s, (c) O1s, (d) N1s, and (e) Mn 2p.

3.4 FE-SEM studies

SEM images revealed distinct morphological differences between the pristine COP (Fig. 7a) and the Mn₂O₃@COP composite (Fig. 7b). The COP results in a porous and loosely aggregated structure with irregular nanosheet-like features, characteristic of amorphous or weak crystalline polymer networks.⁴⁰ This morphology offers a high surface area, facilitating efficient electrolyte diffusion. Upon incorporation of Mn₂O₃, morphology transforms significantly, as observed in the formation of well-defined spherical particles distributed throughout the COP matrix.⁴¹ These Mn₂O₃ particles, ranging from submicron to micron sizes, appear as dense clusters embedded within the polymer framework. This hierarchical architecture suggests successful nucleation and growth of Mn₂O₃ on the COP surface, likely due to strong physicochemical interactions. The composite structure synergistically combines the high surface area of the COP with the redox-active nature of Mn₂O₃, making it a promising candidate for supercapacitors. A particle size histogram (Fig. S1) was created via SEM image analysis to verify the nanoscale dispersion of Mn₂O₃ in the Mn₂O₃@COP composite. Uniform dispersion is shown by the size distribution plot's unimodal and very narrow distribution. The histogram data indicate that the average particle size of the Mn₂O₃@COP composite is approximately 106.75 nm, which is inside the nanodomain. The nanoscale size supports the excellent embedding and homogenous distribution of Mn₂O₃ particles inside the COP matrix, allowing for increased surface area exposure and possible electrochemical accessibility. Consequently, the size analysis and histogram support the assertion of the effective integration of Mn₂O₃ into the COP framework and its nanoscale dispersion (Fig. S2).

Fig. 7 FE-SEM images of (a) COP and (b) Mn₂O₃@COP.

3.5 EDS studies

Fig. 8a shows the area where the EDS analysis was performed. The corresponding EDS spectrum (Fig. 8b) reveals clear peaks for carbon, nitrogen, oxygen, and manganese—elements that are consistent with both the COP structure and the Mn₂O₃ component. The composition table shows that manganese is present in the highest amount (42.97 wt%), followed by oxygen (33.19 wt%), carbon (17.26 wt%), and nitrogen (6.58 wt%). These values confirm that Mn₂O₃ is well incorporated into the polymer matrix. The presence of carbon and nitrogen indicates that the covalent organic polymer, which includes triazine and urea units, is retained in the final material. Overall, the EDS analysis confirmed the successful formation of the Mn₂O₃@COP composite.

Fig. 8 (a) SEM image with EDS mapping region, (b) EDS spectrum with the inset of the elemental composition table of Mn₂O₃@COP.

3.6 BET and surface area studies

The nitrogen adsorption–desorption isotherms and pore size distribution profiles in Fig. 9 offer detailed insights into the textural characteristics of the pristine COP and the Mn₂O₃@COP composite. The isotherm of the COP (Fig. 9a) shows a classic type III pattern, which is commonly found in mesoporous materials that have slit-shaped pores.⁴² As the relative pressure (p/p_o) increases, nitrogen uptake gradually increases, indicating a well-formed mesoporous structure that supports multilayer adsorption. A noticeable spike in nitrogen adsorption occurs near p/p₀ = 0.9–1.0, suggesting that capillary condensation takes place in larger mesopores or even macropores. This confirms the presence of a hierarchical pore system. The total amount of nitrogen adsorbed reached approximately 14 cm³ g⁻¹ STP, indicating a high surface area. The surface area of the COP was found to be 4.18 m² g⁻¹. The inset graph reinforces these findings by displaying the pore size distribution calculated via the BJH method. The COP shows a narrow distribution, mostly concentrated between 2 and 3 nanometers, which confirms that mesopores are the primary feature of the material. A distinct peak at approximately 2.2 nm indicates a uniform mesoporous structure, which is beneficial for efficient ion transport and better access to the electrolyte—key factors for effective charge storage applications. In contrast, the isotherm for the Mn₂O₃@COP composite shows a marked decrease in nitrogen uptake, with maximum adsorption reaching only approximately 5 cm³ g⁻¹ STP. The surface area of the Mn₂O₃@COP composite was found to be 0.11 m² g⁻¹. This significant decline in the surface area is mainly due to the introduction of Mn₂O₃ nanoparticles into the COP structure. These metal oxide particles likely occupy some of the pore space or obstruct access to certain pores, leading to a decrease in both the accessible surface area and overall porosity. Nevertheless, the isothermal shape continues to indicate mesoporous features, although they are less prominent than those of the pristine COP. The hysteresis loop is still present but less distinct, and the sharp increase in adsorption at high relative pressures (p/p₀) continues to point to the existence of larger mesopores or voids between Mn₂O₃ particles that have clumped together. The inset graph of the pore size distribution again highlights mesopores, primarily in the 2–4 nm range, but with a reduced intensity and a wider spread than those of the pristine COP.

Fig. 9 N₂ adsorption/desorption isotherms of (a) COP and (b) Mn₂O₃@COP [inset: pore size distributions of COP and Mn₂O₃@COP.

3.7 Electrochemical studies

3.7.1 Three-electrode system. The electrochemical performance of the Mn₂O₃@COP composite was thoroughly examined in a three-electrode configuration with 1 M H₂SO₄ as the electrolyte, and the findings are shown in Fig. 10. The CV curves have quasi rectangular profiles with minor redox peaks, indicating that the capacitive behavior is controlled by a mix of electric double-layer capacitance (EDLC) and pseudocapacitance. The triazine-based COP creates a highly porous and chemically stable matrix with several nitrogen-containing sites that aid in electron delocalization and charge storage via EDLC. Furthermore, the triazine units increase the wettability and chemical affinity with the aqueous electrolyte, allowing for efficient ion transport and quick charge propagation.⁴³ The addition of Mn₂O₃ nanoparticles into the COF matrix considerably enhances the pseudocapacitive component of charge storage. These Mn₂O₃ sites conduct reversible redox transformations (Mn³⁺/Mn⁴⁺) in the acidic environment of H₂SO₄, allowing faradaic charge storage beyond the contribution of the pure EDLC of the COF.⁴⁴ This synergy can be seen in the CV curves, where increasing scan rates result in greater current responsiveness while maintaining mild redox characteristics, indicating strong electrochemical reversibility and structural integrity under dynamic ion flow circumstances. The possible redox reactions involving Mn³⁺/Mn⁴⁺ conversion are as follows:

Mn₂O₃ + H₂O ↔ 2MnO₂ + 2H⁺ + 2e⁻

Mn³⁺ ↔ Mn⁴⁺ + e⁻

Fig. 10 Electrochemical behavior of the Mn₂O₃@COP composite in a three-electrode system using a 1 M H₂SO₄ electrolyte: (a) CV, (b) EIS, (c) GCD, and (d) specific capacitance at various current densities.

The Nyquist plot of the EIS data indicated a low equivalent series resistance (R_s), indicating good ionic conductivity enabled by the continuous COF network and the proton-rich environment of 1 M H₂SO₄. The moderate semicircle in the high-frequency region corresponds to the charge transfer resistance (R_ct), implying that the Mn redox processes have rather rapid kinetics. The linear Warburg zone in the low-frequency domain demonstrates advantageous ion diffusion behavior, which is most likely due to the hierarchical porosity and accessible surface area of the COF framework.⁴⁵ The GCD curves confirmed the composite's pseudocapacitive nature, exhibiting nonlinear potential–time profiles, particularly at lower current densities, that are consistent with the CV curves. The specific capacitance values determined from the GCD data indicate a decreasing trend with increasing current density (Fig. S3), from 69.1 F g⁻¹ at 0.1 A g⁻¹ to 30 F g⁻¹ at 1 A g⁻¹, owing to restricted ion access to deep redox-active sites under high-rate circumstances. A similar pattern was found for the CV-derived capacitance values, which decreased from 113 F g⁻¹ at 5 mV s⁻¹ to 98 F g⁻¹ at 100 mV s⁻¹, highlighting the diffusion-limited charge storage dynamics at higher scan rates. The energy density and power density values were found to be 9.6 Wh kg⁻¹ and 500 W kg⁻¹ at 0.1 A g⁻¹, respectively. The choice of 1 M H₂SO₄ as an electrolyte is crucial for improving the electrochemical performance. The strong ionic conductivity and proton availability of H₂SO₄ improve not only the electrolyte/electrode interface kinetics but also the redox activity of Mn₂O₃. The acidic medium stabilizes the Mn³⁺/Mn⁴⁺ redox pair and increases the overall charge transfer efficiency, resulting in better capacitive behavior than that of neutral or alkaline media.⁴⁶ Overall, the triazine COF framework serves as an efficient conductive and porous support, while Mn₂O₃ offers redox activity, and the acidic environment promotes optimum ion dynamics, all of which contribute to the promising electrochemical properties of the Mn₂O₃@COF composite.

3.7.2 Supercapacitor application. Fig. 11 shows the electrochemical assessment of the Mn₂O₃@COP composite-based supercapacitor device, which was evaluated in a symmetric two-electrode configuration with a 1 M H₂SO₄ electrolyte. The CV curves displayed quasi rectangular behavior with small redox peaks at scan speeds ranging from 5 to 100 mV s⁻¹, indicating a mixture of EDLC and pseudocapacitance⁴⁷ similar to that of a three-electrode system. The EIS profile showed a typical Nyquist plot, with a tiny semicircle in the high-frequency area and a straight tail in the low-frequency region, suggesting low charge-transfer resistance and good ion diffusion properties⁴⁸ with an R_ct value of 190 Ω. The GCD curves at various current densities (0.2, 0.5, and 0.8 A g⁻¹) exhibited similar nonlinear potential–time profiles that were present in the three-electrode system, indicating good pseudocapacitive performance and great reversibility.⁴⁹ The specific capacitance decreased as the scan rate increased due to restricted ion diffusion (Fig. S4(a)). Similarly, there was a steady decrease in the specific capacitance with increasing current density (Fig. S4(b)), which is common in capacitive systems due to polarization effects. The specific capacitance values were found to be 35 F g⁻¹ at 5 mV s⁻¹ and 16.2 F g⁻¹ at 0.2 A g⁻¹ from CV and GCD, respectively. Furthermore, the energy density and power density values were calculated to be 2.25 Wh kg⁻¹ and 250 W kg⁻¹, respectively. The cycling stability test revealed excellent long-term durability, with a specific capacitance retention of 95% and a coulombic efficiency of 94% after 10 000 cycles at 0.5 A g⁻¹. This highlights the robustness of the electrode–electrolyte interface and the structural integrity of the composite. These findings highlight the effective electrochemical performance of Mn₂O₃@COP for supercapacitor applications.

Fig. 11 Electrochemical behavior of a supercapacitor device with a Mn₂O₃@COP composite in a symmetric two-electrode system using a 1 M H₂SO₄ electrolyte: (a) CV, (b) EIS, (c) GCD, and (d) cycling stability.

4 Mechanism

The electrochemical energy storage mechanism in the Mn₂O₃@COP composite is governed by a synergistic interplay between the EDLC and pseudocapacitance (Fig. 12). The triazine-based COP forms a porous, nitrogen-rich matrix that facilitates EDLC by offering abundant adsorption sites for electrolyte ions and promoting electron delocalization. This porous structure, along with enhanced surface wettability, allows for efficient ion diffusion and rapid charge–discharge cycles. The incorporation of Mn₂O₃ nanoparticles introduces redox-active sites that undergo reversible faradaic reactions, specifically Mn³⁺/Mn⁴⁺ redox transitions, in an acidic 1 M H₂SO₄ environment. The H⁺ ions actively participate in these redox transitions, enabling proton-coupled electron transfer reactions that enhance the pseudocapacitive behavior of the composite. These coupled processes are evident in the CV profiles, which exhibit quasi rectangular shapes with small redox humps. EIS confirmed efficient ion transport and low resistance within the system owing to both the high ionic conductivity of the acidic medium and the porous COP matrix. The GCD curves further support this hybrid mechanism, showing nonlinear potential–time behavior characteristic of pseudocapacitance. The performance decreases slightly at higher scan rates and current densities because of limited ion diffusion to deep redox-active sites, emphasizing the diffusion-controlled dynamics. Overall, the Mn₂O₃@COP composite achieves enhanced charge storage through a cooperative mechanism involving the EDLC from the COP, reversible redox reactions from Mn₂O₃, and crucial proton participation from the H₂SO₄ electrolyte, all of which contribute to its superior supercapacitor performance.

The low electrochemical capacity observed in the present study can be attributed to factors such as poor electrode–electrolyte interfacial contact, limited intrinsic conductivity of the polymer, and restricted charge transport due to its disordered framework. These issues hinder effective ion diffusion and redox activity. To address these limitations, strategies such as incorporating aqueous-compatible conductive binders (e.g., PEDOT:PSS, PVA, CMC), converting COPs into more crystalline COF structures, and integrating redox-active components such as metal oxides (NiO, V₂O₅) or carbon nanomaterials (graphene, CNTs) are proposed. This work highlights both the challenges and future directions for optimizing COP-based materials in energy storage applications (Table 1).

Fig. 12 Schematic illustration of the energy storage mechanism of the Mn₂O₃@COP composite along with the supercapacitor in the inset.

Table 1 Specific capacitance performance of the Mn₂O₃@COP composite with previously reported materials

Sl. No.	Composite material	Specific capacitance (F g^−1)	Reference
1	CFGCN-3 (triazine based)	147.22	50
2	Porous Mn2O3	102	51
3	Triazine-based CTF	151.3	52
4	IITR-COF-1	182.6	53
5	MnO2-LCF-800	131.28	54
6	Mn2O3@COP	113	Present work
7	Al2O3@DHTA-COF composite	261.5	55
8	TPT@BDA-COF	92.6	56
9	Triazine-CTF/CNO	638	57
10	PDT-COF	104	58

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5 Conclusion

In summary, a triazine-based covalent organic polymer (COP) was effectively synthesized and integrated with Mn₂O₃ to construct a Mn₂O₃@COP composite electrode for high-performance supercapacitors. Comprehensive structural and surface investigations, including FTIR, XRD, SEM, EDS, BET, and XPS, indicated that the hierarchical composite was successfully formed. FTIR and XRD investigations confirmed the presence of Mn₂O₃ and the stability of the COP framework. SEM and EDS confirmed the homogeneous dispersion of Mn₂O₃ nanoparticles inside the porous matrix, whereas BET analysis revealed that the composite preserved mesoporous characteristics suitable for ion transport, despite a decreased surface area owing to Mn₂O₃ loading. XPS research revealed the existence of mixed-valence Mn³⁺/Mn⁴⁺ species and nitrogen functionalities (pyridinic, pyrrolic, graphitic) in the COP, resulting in increased redox activity and charge transfer. Electrochemical investigations in three- and two-electrode configurations with a 1 M H₂SO₄ electrolyte revealed synergistic EDLC and pseudocapacitive behavior. The composite had a high specific capacitance of 113 F g⁻¹ at 5 mV s⁻¹ and 69.1 F g⁻¹ at 0.1 A g⁻¹, an energy density of 9.6 Wh kg⁻¹ and a power density of 500 W kg⁻¹ at 0.1 A g⁻¹. The symmetric supercapacitor had a specific capacitance of 16.2 F g⁻¹ and an energy density and power density of 2.25 Wh kg⁻¹ and 250 W kg⁻¹ at 0.2 A g⁻¹, respectively. Additionally, the device achieved 95% capacitance retention over 10 000 cycles. Thus, the Mn₂O₃@COP composite shows promise as a redox-active electrode material for long-lasting and efficient supercapacitor devices. In addition to its scientific merit, this work directly contributes to the United Nations Sustainable Development Goals: SDG 7 (Affordable and Clean Energy) through the development of high-performance energy storage materials; SDG 9 (Industry, Innovation and Infrastructure) by promoting advanced material innovation for sustainable technologies; and SDG 13 (Climate Action) by supporting low-carbon energy systems and facilitating the transition to clean energy infrastructure.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. See DOI: https://doi.org/10.1039/d5ra04868k.

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