Phenolic hydroxyl-functionalized covalent organic frameworks for efficient hydrogen evolution in alkaline media

Developing economical and highly active metal-free electrocatalysts for the hydrogen evolution reaction (HER) is crucial for advancing sustainable hydrogen energy through water electrolysis. Although alkaline water electrolysis holds great potential as a scalable technology, the process is hindered by two major challenges: the sluggish water dissociation step and the limited intrinsic activity of metal-free catalysts. Here, we report TFPheMePh, a nitrogen-containing, hydroxyl-functionalized covalent organic framework (COF) synthesized via polycondensation, yielding a crystalline, porous material with a high BET surface area of 1309 m 2 g -1. To maximise catalyst utilisation and facilitate efficient electron transport, the synthesized COF was drop-casted onto a three-dimensional dynamic hydrogen bubble template copper foam (DHBT-Cuf). Its interconnected porous architecture provides a highly conductive scaffold, enhanced electrolyte accessibility, and abundant exposed active sites, thereby facilitating water activation and HER. The resulting COF/DHBT-Cuf electrode delivers efficient HER activity in 1 M KOH, achieving an overpotential of 285 mV at 10 mA cm -2 with a Tafel slope of 263 mV dec −1. This performance arises from the synergy of nitrogen-rich active sites, phenolic groups, the electron-donating inductive effect of methyl substituents, a high-surface-area microporous channel network, and an extended π-conjugated backbone, collectively promoting charge transport, reactant accessibility, and catalytic kinetics. This work highlights the potential of the rational design of COFs integrated with three-dimensional conductive architectures as efficient metal-free electrocatalysts for alkaline HER and provides valuable insights into the development of advanced organic framework-based catalysts for sustainable green hydrogen production.

The global demand for eco-friendly and sustainable energy sources has been growing in recent years. This is primarily due to the depletion of fossil fuels and rising concerns over greenhouse gases and climate change (Turner, 2004; Majumder et al., 2023; Arumugam et al., 2024; Emadian et al., 2025). Among the several alternative energy carriers, molecular hydrogen (H 2) has emerged as a promising contender due to its high gravimetric energy density (∼142 MJ kg -1), no carbon emissions during usage, along with wide-ranging applications in fuel cells, water electrolysers, other industrial processes, and energy storage systems (Majumder et al., 2024; Vazhayil et al., 2024). The most promising and scalable route of hydrogen production is considered to be the electrochemical, photochemical and photoelectrochemical water splitting (Das et al., 2022b; Jahangir et al., 2025; Majumder et al., 2025b; 2025a; 2026b; 2026a). It is a process of conversion of water into H 2 and O 2 using electrical energy ideally derived from renewable sources (Zou and Zhang, 2015).

During water electrolysis, the HER occurs at the cathode via a two-electron process. It can proceed through two distinct pathways depending upon (Das et al., 2022b) the pH of the electrolyte. In alkaline medium, HER begins with Volmer step (H 2 O + e − → H ads + OH −), followed by either the Tafel step (2H ads → H 2) or the Heyrovsky step (H ads + H 2 O + e − → H 2 + OH −), depending upon the surface coverage of H ads. (Nørskov et al., 2005; Danilovic et al., 2012). As compared to HER in acidic conditions, the alkaline one is kinetically more sluggish due to extra energy required for water dissociation before hydrogen adsorption. Despite its slower kinetics, alkaline HER is the predominant method for industrial-scale green hydrogen production. Its notable advantage is that earth-abundant and low-cost catalysts such as Fe, Co, Ni and carbon-based materials can be used, unlike acidic media, which require expensive platinum-group metals (AlAqad et al., 2023). Moreover, the widely adopted and commercially used electrolysers are alkaline, using inexpensive KOH or NaOH electrolytes and non-fluorinated separators. It underscores the necessity for highly efficient and robust catalysts for alkaline HER that can (1 efficiently promote water dissociation and 2) favor H-H bond formation (Wang X. et al., 2020; Kou et al., 2021).

The benchmark catalysts for HER are Pt or Pt-based catalysts due to their near-optimal free energy of hydrogen adsorption (ΔG H* ≈ 0 eV) (Vesborg et al., 2015; Zhang et al., 2026). However, their high cost, scarcity, and vulnerability to deactivation after usage pose serious challenges to their widespread application (Greeley et al., 2006). Consequently, there has been extensive research going on to develop earth-abundant alternatives like transition metal-based catalysts, their phosphides, carbides or single-atom catalysts (Jaramillo et al., 2007; Liu et al., 2015; Yadav et al., 2021a; Yadav et al., 2021b; Zhang et al., 2024; Chen et al., 2025). Despite of significant advancements, many of these catalysts have expensive or toxic metals. Also, their performance generally remains inferior to that of Pt/C (Zhu et al., 2024). This has motivated an emerging approach to develop metal-free electrocatalysts that leverage heteroatom-induced modulation in electronic configuration to activate HER (Zheng et al., 2014; Cai et al., 2024).

In metal-free electrocatalysts, COF is a class of crystalline, porous and polymeric materials formed by organic building blocks connected by covalent bonds such as imine, triazine, or boronate linkages (Ding and Wang, 2013; Wu et al., 2025; Kumar et al., 2026). Their unique properties include high surface area, tunable pore size, geometry, and excellent chemical and thermal stability. These features make them promising candidates for catalysis, sensing, gas storage, and energy applications (Côté et al., 2005; Diercks and Yaghi, 2017). In the context of electrocatalysis, compared with amorphous carbon-based materials, COFs have the advantage that they can be precisely designed to engineer the chemical environment of the active sites according to the needs of the reaction (Alsudairy et al., 2023).

Several studies have shown the potential of COF-based materials as HER electrocatalysts. The heteroatoms in the COF are responsible for activity, such as in phosphorous-doped triazine framework (PCTFs), where the isolated N and P-heteroatoms distributed over the high-surface-area network play synergistically to improve HER activity (Jena et al., 2023). Moreover, an imine-linked perylene-porphyrin COF presented the significance of π-conjugation for efficient charge transfer in COFs for excellent HER activity (Samanta et al., 2024). A study on crown-ether-functionalized COFs is being designed to coordinate alkali metal cations from the electrolyte. This leads to local polarization that enhances both HER and OER activity in alkaline media (Pal et al., 2025). All these literature studies present that the COF-based electrocatalysts are influenced by the interplay of heteroatoms and their distribution, π-conjugation, and framework-electrolyte interactions. For alkaline conditions, these design parameters remain largely unexplored.

The present work lies in the rational integration of a metal-free, hydroxyl-functionalized COF with a three-dimensional DHBT copper foam for alkaline HER. In contrast to studies focusing primarily on extended conjugation, the present work specifically examines the influence of hydroxyl-containing functionality by comparison with the structurally related hydroxyl-free TFBMePh framework. Furthermore, coupling the COF with the highly conductive and porous DHBT copper foam creates an organic-metallic hybrid electrode architecture that facilitates electron transport, electrolyte penetration, and exposure of catalytically accessible sites. These results provide a structure-activity perspective for the design of functionalized metal-free COFs for alkaline hydrogen evolution This configuration enhances the electronic conductivity and favors efficient mass and charge transport due to its extensive π-conjugated backbone. When TFPheMePh, was utilised as a metal-free HER electrocatalyst in alkaline media, it demonstrated competitive overpotential. This positions it among the performing metal-free COF-based HER catalysts. The catalytic activity of TFPheMePh compared with TFBMePh further highlights the beneficial role of the hydroxyl-functionalized framework, where the more hydrophilic surface and proton-donating ability of the phenolic -OH groups likely facilitate the catalytic process. Mechanistic analysis based on the Tafel slope indicates a moderately high water dissociation kinetics may be facilitated by the collective features of high surface area bearing COF with Cuf, hydroxyl functionality and amine-imine linkage within the extended framework. This study contributes to a deeper understanding of the structure-activity relationships of N-containing -OH functional COFs and offers a rational design strategy for the development of next-generation metal-free electrocatalysts for alkaline hydrogen production.

Chemicals

Phenol (98%), hexamethylenetetramine (hexamine, 99%), 2,5-dimethyl- p -phenylenediamine (DMePh), 1,3,5-triformylbenzene (TFB), hydrochloric acid (HCl, 37 wt%), trifluoroacetic acid (TFA), toluene, ethanol, N,N-dimethylformamide (DMF), and 1,4-dioxane were purchased from Sigma-Aldrich, Merck, and TCI Chemicals. Unless otherwise stated, all reagents and solvents were of analytical grade and used as received without further purification. 2,4,6-triformylphenol (TFPh) was synthesised following the literature-reported procedure (Li et al., 2026).

Synthesis of TFPheMePh

The COF was synthesised via a solvothermal condensation approach. 2,4,6-triformylphenol (TFPh) and 2,5-dimethyl-p-phenylenediamine (DMePh) were combined in a 2:3 M ratio in a Schlenk tube. A binary solvent mixture of 1,4-dioxane and mesitylene (3:1 v/v, 3 mL total) was added, followed by dropwise addition of 0.2 mL of acetic acid (AcOH, 6 M) as the catalyst. The reaction mixture was degassed by three successive freeze-pump-thaw cycles to remove dissolved gases, after which the tube was sealed and maintained at 120 °C in an oil bath under same conditions for 72 h. Upon cooling to room temperature, the precipitate was collected by filtration and washed sequentially with ethanol (EtOH), N, N-dimethylformamide (DMF), tetrahydrofuran (THF), and acetone to remove unreacted monomers and oligomeric byproducts. The purified solid was dried under vacuum at 110 °C, affording TFPheMePh in 74% isolated yield (Scheme 1). The resulting framework incorporates phenolic (-OH) and imine (-C=N-) functional groups, which are capable of tautomerization between the imine-enol, as governed by the interplay between the C=N linkage and the adjacent hydroxyl groups of TFPh (Supplementary Figure S1 in Supplementary Material (SI)). The as synthesized COF was dropcasted on copper foam (Cuf) synthesized using dynamic hydrogen bubble templated method. (Das et al., 2022a), which was also used as the current collector. The mass loading of TFPheMePh on the Cuf electrode was approximately 0.5 mg cm -2. For comparison, a hydroxyl-free analogue, TFBMePh, was synthesized as a control sample following the previously reported procedure. The PXRD pattern of the synthesized TFBMePh closely matched the reported diffraction pattern, confirming its successful synthesis and phase purity (Supplementary Figure S2 in SI). (Brown et al., 2023; Xu et al., 2026)

Electrochemical measurements

The electrocatalytic performance of the as-synthesized TFPheMePh COF toward the HER was evaluated using a conventional three-electrode electrochemical cell under ambient conditions. The TFPheMePh COF-modified copper foam (TFPheMePh_Cuf) served as the working electrode (WE), an Ag/AgCl (3 M KCl) electrode was employed as the reference electrode (RE), and a graphite rod was used as the counter electrode (CE). All electrochemical measurements were carried out in 1.0 M KOH electrolyte. For comparison, the HER activities of bare copper foam (Cuf) and were also investigated under identical experimental conditions.

All measured potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation: where 0.210 V is the standard potential of the Ag/AgCl (3 M KCl) reference electrode at 25 °C. Unless otherwise stated, iR compensation was not applied to any of the electrochemical data. Linear sweep voltammetry (LSV) was performed at a scan rate of 10 mV s -1 to evaluate the HER activity of the catalysts. The Tafel slopes were determined from polarization curves recorded at a slower scan rate of 1 mV s -1 to minimize capacitive current contributions. All current densities were normalized to the geometric surface area of the working electrode. The long-term electrochemical durability of the as-synthesized TFPheMePh_Cuf catalyst was assessed using chronoamperometry (CA) at a constant applied potential. Repeated LSV measurements (n = 3) yielded an average overpotential of 286.3 ± 2.5 mV at 10 mA cm -2, demonstrating good repeatability of the electrochemical response.

Physical characterisations

The as-synthesized COF was structurally characterized by powder X-ray diffraction (PXRD), and the corresponding crystallographic information is presented in Figure 1a. The COF crystallized in a triclinic crystal system with the P1 space group. The optimized unit cell parameters were determined to be a = 22.6454 Å, b = 22.7242 Å, c = 3.5801 Å, with lattice angles of α = 72.8809°, β = 103.9143°, and γ = 123.6341°, corresponding to a unit cell volume of 1458.98 Å 3. The PXRD pattern exhibited characteristic diffraction peaks at 2θ = 4.7°, 6.6°, and 26.7°, which are indexed to the (100), (110), and (001) crystal planes, respectively, confirming the successful formation of the highly ordered crystalline framework (Figure 1b). In addition, the broad diffraction feature observed in the 25–30° region is attributed to the π-π stacking interactions between adjacent two-dimensional COF layers, indicating the presence of an ordered layered architecture. The good agreement between the experimental and simulated PXRD patterns, further supported by Pawley refinement (inset, Rp = 4.07% and Rwp = 3.15%), validates the proposed crystal structure. The complete crystallographic information, including the atomic fractional coordinates of the structural model, has now been provided in Supplementary Table S1 of the Supplementary Material. Furthermore, the PXRD pattern of the TFPheMePh/Cuf electrode was recorded after drop-casting the COF onto copper foam, as shown in Figure 2a. The diffraction pattern exhibits the characteristic reflections of metallic Cu at 2θ = 43.3°, 50.4°, and 74.1° for Cu (111), (200), and (220), respectively (Das et al., 2019), while the enlarged inset distinctly shows the characteristic diffraction peaks of the TFPheMePh COF, confirming its successful deposition and preservation of the crystalline framework on the conductive Cu support. Confirming that the crystalline framework remains intact after deposition onto the conductive copper foam. These findings confirm the successful synthesis of a highly crystalline COF with well-defined periodicity and layered stacking, making it a promising material for electrocatalytic applications.

The successful formation of the TFPheMePh was confirmed by Fourier-transform infrared (FTIR) spectroscopy through comparison of the spectra of the precursor monomers, TFPh and DMePh, with that of the synthesized COF (Figure 2b). The FTIR spectrum of DMePh displays characteristic primary amine (-NH 2) stretching vibrations in the 3300–3500 cm -1 region, whereas TFPh exhibits a broad phenolic -OH stretching band centred around 3200–3400 cm -1 together with a characteristic aldehyde C=O stretching vibration near 1640–1660 cm -1 (Kandambeth et al., 2012). Upon condensation, the characteristic -NH 2 and aldehyde C=O bands disappear, indicating the complete consumption of the amino and aldehyde functional groups during framework construction. Simultaneously, the appearance of a new absorption band at approximately 1615–1625 cm -1, assigned to the conjugated C=N/C=C stretching vibration of the imine framework, confirms the successful covalent linkage between the two building blocks. In addition, the absorption band around 1260–1290 cm -1 corresponds to C-N stretching, further supporting the formation of the extended nitrogen-containing network (Wang H. et al., 2020). The broad band retained in the 3200–3400 cm -1 region originates from the phenolic -OH groups and imine. These spectral changes unequivocally verify the successful synthesis of a chemically stable, highly conjugated N-containing phenolic -OH functionalized COF. The coexistence of abundant nitrogen sites and phenolic hydroxyl groups is expected to promote water adsorption and dissociation under alkaline conditions, while the extended π-conjugated framework facilitates efficient charge transport, collectively contributing to the enhanced HER performance.

The porosity of the synthesized TFPheMePh COF was investigated by N 2 adsorption-desorption measurements at 77 K (Figure 3). The adsorption isotherm exhibits a characteristic Type-I profile with a steep nitrogen uptake at low relative pressures (P/P 0 < 0.1), confirming the predominance of a microporous framework (Figure 3a). A slight increase in nitrogen uptake at higher relative pressures is attributed to interparticle voids arising from the aggregation of COF crystallites. The Brunauer-Emmett-Teller (BET) specific surface area of the TFPheMePh COF was determined to be 1309 m 2 g -1, demonstrating the successful formation of a highly porous framework. The pore-size distribution, calculated using the non-local density functional theory (NLDFT) method, reveals a narrow micropore distribution centred at approximately 1.4 nm (Figure 3b), which is consistent with the designed periodic pore architecture of the COF (Bhanja et al., 2018). Such a high surface area combined with uniform microporous channels is expected to expose abundant active sites and facilitate electrolyte diffusion, thereby contributing to the enhanced HER performance. The hydroxyl-free control COF (TFBMePh) was also characterized by N 2 adsorption-desorption measurements (Supplementary Figure S3). Similar to TFPheMePh, the control sample exhibited a Type-I adsorption isotherm, confirming its microporous nature. The BET surface area was determined to be 1350 m 2 g -1, which is slightly higher than that of TFPheMePh (1309 m 2 g -1). Despite their comparable surface areas and porous structures, the inferior HER activity of TFBMePh demonstrates that the enhanced electrocatalytic performance of TFPheMePh primarily originates from the incorporation of phenolic -OH functionalities, rather than differences in surface area or porosity alone.

The optical properties of the synthesized TFPheMePh were investigated using UV-Vis spectroscopy (UV-Vis), as shown in Figure 4a. The COF exhibits broad absorption extending from the ultraviolet to the visible region, indicating the formation of an extended π-conjugated framework. Such broad visible-light absorption reflects the enhanced electronic delocalization arising from the imine linkage and nitrogen-containing aromatic backbone. The optical band gap, estimated from the corresponding Tauc plot (Figure 4b), was found to be 1.7 eV, suggesting a narrow-band-gap semiconducting nature. The relatively low band gap facilitates electron excitation and charge transport within the framework, which is beneficial for accelerating electron-transfer kinetics during HER. While in the case of TFBMePh, the band gap derived from the corresponding Tauc plot (Supplementary Figure S4 in SI) was 2.6 eV, responsible for its lower electron and charge transport, consequently, slow HER activity.

The morphology of the synthesized COF was further examined by transmission electron microscopy (TEM) (Figure 4c). The TEM image reveals the formation of thin, interconnected layered nanosheets with a stacked morphology, which is consistent with the two-dimensional layered structure predicted from the crystallographic model. Such ultrathin layered architecture provides abundant exposed active sites and short diffusion pathways for electrolyte penetration and charge transport, thereby promoting efficient electrocatalytic reactions. The elemental composition and distribution were further confirmed by energy-dispersive X-ray spectroscopy (EDS) elemental mapping (Figure 4d). The mapping images demonstrate the homogeneous distribution of carbon (C), nitrogen (N), and oxygen (O) throughout the framework without any observable phase segregation, confirming the successful incorporation of the designed heteroatoms into the COF architecture. The uniform distribution of nitrogen and oxygen functionalities is expected to generate abundant catalytically active sites for water adsorption and dissociation, while the extended conjugated carbon framework provides efficient electron transport pathways. These structural and electronic characteristics collectively contribute to the enhanced alkaline HER activity of the TFPheMePh.

Electrochemical activity

The electrocatalytic HER performance of the synthesised TFPheMePh COF was evaluated in 1.0 M KOH using a conventional three-electrode configuration and compared with the hydroxyl-free control (TFBMePh), TFPheMePh_GC (on glassy carbon) and bare copper foam (Cuf). The corresponding polarization curves are shown in Figure 5a. Among the investigated samples, TFPheMePh exhibited the highest HER activity, requiring an overpotential of 285 mV to achieve a current density of 10 mA cm -2, which is substantially lower than those of TFBMePh and bare Cuf. The enhanced catalytic performance demonstrates that the incorporation of phenolic -OH functionalities into the COF framework plays a decisive role in promoting alkaline HER. In alkaline media, the HER is generally limited by the sluggish *Volmer step (H 2 O + e − → H + OH −) *, where efficient water adsorption and dissociation are essential for rapid hydrogen generation. The phenolic -OH groups present in TFPheMePh are capable of establishing hydrogen-bonding interactions with water molecules, thereby facilitating water adsorption and lowering the activation barrier for O-H bond cleavage. In addition, the neighbouring nitrogen atoms within the imine framework serve as electron-rich catalytic centers for hydrogen intermediate adsorption, while the electron-donating (+I) effect of the methyl substituent further modulates the electronic structure, collectively accelerating the HER kinetics. The reaction kinetics were further analyzed using Tafel plots (Figure 5b). The TFPheMePh catalyst exhibited a lower Tafel slope (263 mV dec −1) than the hydroxyl-free analogue and bare Cuf, indicating relatively faster HER kinetics and improved charge-transfer characteristics. Although the Tafel slope remains higher than that of noble-metal catalysts, the significant improvement over the control sample confirms the beneficial contribution of the phenolic OH groups in facilitating the water dissociation step, which is the rate-determining process under alkaline conditions.

To investigate the electrochemically active surface area (ECSA), cyclic voltammograms (CV) were recorded within the non-faradaic potential region at different scan rates (Figure 5c). The corresponding plot of capacitive current density (Δ j) versus scan rate is presented in Figure 5d. The double-layer capacitance (C dl) of the TFPheMePh catalyst was calculated to be 10.8 mF cm -2, indicating a relatively large electrochemically accessible surface area, resulting in more active sites (Bhardwaj et al., 2025). To better evaluate the contribution of the DHBT copper foam support, we have additionally determined the of bare Cuf under identical conditions, which was found to be 5.2 mF cm -2 (Supplementary Figure S5). Thus, TFPheMePh_Cuf exhibits approximately a two-fold higher than bare Cuf, indicating an increase in the electrochemically accessible interfacial area after incorporation of the COF. The improved activity is discussed as arising from the combined effects of increased electrochemically accessible surface area, the chemical environment provided by the COF, and the conductive DHBT copper foam support.

The high C dl value is attributed to the synergistic effect of the highly porous COF framework, large BET surface area (1309 m 2 g -1), interconnected microporous channels, and the uniform dispersion of the COF on the three-dimensional DHBT copper foam. The larger ECSA provides a greater number of accessible catalytic sites for water adsorption and hydrogen evolution.

The long-term electrochemical stability of the TFPheMePh catalyst was evaluated by chronoamperometry (CA) at a constant applied potential in 1.0 M KOH, as shown in Figure 6a. The current density remained nearly constant throughout the 20 h continuous electrolysis with only negligible fluctuations (76% current retention), demonstrating the sustained operational stability and structural robustness of the catalyst under alkaline HER conditions. The stable current response indicates that the active sites remain accessible during prolonged operation and that the catalyst maintains its electrochemical integrity without noticeable degradation. LSV polarization curves were recorded before and after the durability test (Figure 6b). The two polarization curves exhibit only a slight deviation after continuous electrolysis, indicating minimal loss in catalytic activity. The small shift in the HER polarization curve suggests that the TFPheMePh catalyst retains its intrinsic electrocatalytic performance. The comparative table summarizing the HER performance of TFPheMePh_Cuf together with representative metal-free COF-based and related organic framework electrocatalysts reported in the literature in Supplementary Table S2, Supplementary Material. Electrochemical impedance spectroscopy (EIS) was further employed to investigate the interfacial charge-transfer behavior of TFPheMePh_Cuf under HER conditions. As shown in Supplementary Figure S6 Supplementary Material, the Nyquist plot exhibits a semicircular response in the high-to-intermediate frequency region followed by an inclined low-frequency tail. The EIS yielded a solution resistance R s of 2.3 Ω and a charge-transfer resistance (R ct) of 0.8 Ω. The relatively small R ct indicates favorable electron transfer across the electrode/electrolyte interface. This behavior can be attributed to the combined effects of the extended conjugated framework and the highly conductive three-dimensional DHBT copper-foam substrate, which together provide efficient pathways for interfacial electron transport. The inclined response observed at lower frequencies further suggests a contribution from mass-transport processes during HER. The exchange current density was determined by extrapolating the linear Tafel region to zero overpotential. Based on the Tafel relationship, an exchange current density of approximately 0.8 mA cm -2 was obtained for TFPheMePh_Cuf (Supplementary Figure S7). To further evaluate the structural stability of TFPheMePh_Cuf after prolonged HER operation, post-HER PXRD analysis was performed (Supplementary Figure S8). Compared with the pristine electrode, the post-HER PXRD pattern shows a noticeable attenuation of the characteristic low-angle diffraction features of the COF, indicating a partial loss of long-range crystallinity during extended alkaline electrolysis. Nevertheless, no distinct new crystalline phases are observed within the investigated diffraction range. Together with the post-catalysis FTIR and FESEM analyses (Supplementary Figures S9 and S10), these results suggest that although some structural reorganization and occurs during prolonged HER operation, the chemical framework and overall morphology remain largely retained. Under prolonged operation, continuous and vigorous H 2 bubble evolution progressively results minor dislodges the COF from the electrode surface, and the detached material settles at the bottom of the cell.

In the post-catalysis electrode, the characteristic diffraction peaks of the COF are still observed; however, the peak intensities at higher angles are significantly reduced. This may be attributed to the increased relative contribution of the Cuf, resulting from the partial leaching of the COF catalyst during the long-term durability test. Importantly, the PXRD pattern of the solid collected from the bottom of the electrocatalytic cell shows an identical diffraction pattern to that of the as-synthesized TFPheMePh (Supplementary Figure S11), confirming the structural integrity of the COF rather than chemical degradation. These observations are consistent with a partial loss of the COF overlayer during prolonged electrolysis, which exposes a greater fraction of the underlying Cuf surface and consequently contributes to the gradual decrease in current density observed in the chronoamperometric durability test. Importantly, the COF reflections are retained in both samples, with no new phases or loss of the characteristic diffraction features, indicating that the framework remains structurally intact. These observations demonstrate that the change in performance during extended electrolysis originates from minor mechanical leaching of the catalyst from the electrode, and not from intrinsic chemical degradation of the COF itself.

The improved HER activity of TFPheMePh cannot be explained solely by its high surface area, since the hydroxyl-free control COF (TFBMePh) possesses a comparable BET surface area. Instead, the electrocatalytic performance originates from the synergistic integration of structural and chemical factors, including (i) abundant nitrogen active sites, (ii) phenolic -OH functionalities can act as internal proton donors as well as hydrophilicity enhancement for HER in alkaline medium (Das et al., 2026) (iii) the electron-donating effect of the methyl substituent, (iv) an extended π-conjugated imine framework that facilitates charge transport, and (v) the conductive three-dimensional DHBT copper foam support, which enhances electron transport and electrolyte accessibility. Consequently, these features collectively accelerate the water dissociation and improve the overall HER kinetics in alkaline media.

In summary, a metal-free, hydroxyl-functionalized TFPheMePh COF was successfully developed and integrated with three-dimensional DHBT copper foam for alkaline HER. The COF exhibits a crystalline and highly porous structure with a BET surface area of 1309 m 2 g -1 and a narrow optical band gap of 1.7 eV. The TFPheMePh_Cuf electrode delivered an overpotential of approximately 285 mV at 10 mA cm -2 in 1.0 M KOH. Comparison with the hydroxyl-free TFBMePh analogue and bare Cuf suggests that the oxygen-containing functionality of the COF, together with its nitrogen-containing conjugated framework and the conductive porous copper-foam support, contributes to the improved HER response. The Catalyst’s C dl value of 10.8 mF cm -2, compared with 5.2 mF cm -2 for bare Cuf, further indicates increased electrochemically accessible interfacial area upon COF incorporation. Moreover, the electrode retained approximately 76% of its initial current after 20 h of continuous HER operation. Post-HER PXRD, FTIR, and FESEM analyses further provided insight into the structural and morphological evolution of the catalyst after prolonged electrolysis. Overall, this study demonstrates a practical strategy for combining chemically functionalized metal-free COFs with three-dimensional conductive supports and provides useful structure-activity insights for the development of organic-framework-based electrocatalysts for alkaline hydrogen evolution.