A metal-free perylene–porphyrin based covalent organic framework for electrocatalytic hydrogen evolution

Abstract

A crystalline perylene–porphyrin based covalent organic framework is synthesized via Schiff base [2 + 2] type condensation between 4,4′,4′′,4′′′-(perylene-2,5,8,11-tetrayl) tetrabenzaldehyde (PETA) and 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (PAM) under solvothermal conditions. The porphyrin and perylene units occupy the vertex of a three-dimensional triclinic crystal in an alternate manner. PETA-PAM-COF exhibits permanent microporosity, a reasonably high surface area (about 1400 m² g⁻¹), and promising chemical stability. A conducting perylene bridged channel is created by AA stacking. PETA-PAM-COF has been utilized for the metal-free hydrogen evolution reaction with a low charge-transfer resistance (R_ct) value of 62.22 Ω and a Tafel slope of 122 mV dec⁻¹, demonstrating its potential for practical utilization. PETA-PAM COF showed a current density of 10 mA cm⁻² at an overpotential of 261 mV. Remarkable HER activities are demonstrated by the PETA-PAM-COF catalyst as indicated by its faradaic efficiency (96%) and durability (which retained 93% of its original current density after 1000 cycles). We anticipate that the imine-based COF will not only enhance the structural variety but also the electrochemical behavior of these classes of materials.

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Introduction

Extensive consumption of fossil fuels has caused a global energy crisis,¹ climate change, and environmental pollution, which has prompted us to explore renewable green energy. Fuels such as hydrogen, which have high specific energy density (143 kJ g⁻¹), have been suggested as suitable and eco-friendly alternatives to diminishing fossil fuels. Steam methane reforming, coal gasification, and water splitting are the most cost-effective and widely used processes to produce hydrogen. However, the first two approaches remain dependent on fossil fuels and emit large amounts of CO₂. Reduction of protons to molecular hydrogen electrocatalytically can be a crucial step towards the generation of sustainable energy by a water splitting process known as the hydrogen evolution reaction (HER).^2–4 Platinum (Pt) is the most potent HER catalyst⁵ with outstanding electrocatalytic activity, but its extensive use is limited by economic constraints and limited stability. Over the past decade, various non-platinum transition metal-based electrocatalysts including Rh, Ru, Ir, Fe, Co, Ni, Pd, Cu, Mo, etc. have been developed. The oxides,^6,7 carbides,^8,9 sulfides,^10,11 nitrides,^12,13 phosphides^14,15 selenides,^16,17 and hydroxides^18,19 of these transition metals are economically feasible and more effective compared to Pt/C catalysts. In these green energy systems, reducing overpotential is crucial for increasing energy efficiency and output power. However, most of the metal catalysts either possess high onset overpotential and Tafel slope, or poor stability. Recent research has been focused on developing new covalent organic polymer (COPs) and covalent organic framework (COF) electrocatalysts with metal-free structures. Their structural tunability and regularity, permanent porosity, and ideal building blocks allow them to serve as potential candidates in the applications of renewable energy fields.^20,21

Covalent organic frameworks (COFs),^22–27 crystalline porous materials with a uniform framework and intrinsic porosity, formed by covalent connections between organic building blocks, exhibit excellent and unique properties including gas storage and separation,^28–32 drug delivery^33–35 and molecular catalysis.^36–40 It has been observed that because of the intrinsic stacking between the two nearby layers (π–π stacking), 2D-COFs are widely accepted as more promising than 3D-COFs. As a result of their stacking properties, 2D-COFs provide pronounced mobility^41–43 to serve as charge carriers in the columnar channel, resulting in promising optoelectronic phenomena, conductivity^44–47 and electroactivity.^48–51 Polymeric materials such as graphitic carbon nitride (g-C₃N₄),⁵² carbon nanostructures,^53,54 and phosphorene⁵⁵ based materials have brought a key breakthrough for metal free HER catalysis. However, such active catalysts are usually employed in the photocatalytic HER.⁵⁶ It is a matter of interest that a novel electrocatalyst enabling energy conversion through the HER would be the most beneficial for addressing daily energy demands. We have recently reported porphyrin based porous materials^57,58 having excellent potential for the electrocatalytic HER. Porphyrins, highly conjugated macrocycles with distinct photophysical and electrochemical characteristics, have been exclusively used in the design and development of photo-chemical sensitizer, photocatalyst^59,60 and electrocatalyst^57,58 materials with pronounced catalytic HER. Porphyrin-based porous materials have been shown to exhibit high specific surface area, crystallinity, and strong photo-electric activity. On the other hand, perylene, a polyphenylene containing aromatic compound with distinct charge carrier mobility and excellent electrochemical stability, has been found to be a very promising building block in the last few decades in optoelectronics. However, to the best of our knowledge, perylene–porphyrin based COF has not been reported yet. Therefore, designing a metal-free perylene–porphyrin containing covalent organic framework might be a potential approach to develop a new efficient and robust HER electrocatalyst.

Herein, we investigate the development of a 2D COF named PETA-PAM-COF (Scheme 1) by the homogeneous solution phase solvothermal Schiff base condensation reaction for HER electrocatalysis. We then demonstrate the electrochemical response of a perylene–porphyrin conjugated imine-based covalent organic framework (PETA-PAM-COF) with extensive π conjugation to glean insight into the charge carrier mobility in a metal-free system. The perylene–imine based core promoted π–π stacking, durability, and charge carrier mobility of the metal-free system with significant π conjugation.

Scheme 1 Synthetic scheme of a metal free covalent organic framework (PETA-PAM-COF).

Experimental section

Materials

The reagents were obtained from a commercial source and utilized without any further purification. 5,10,15,20-Tetrakis(4-aminophenyl)porphyrin (PAM) was synthesized using a previously reported procedure.⁶¹ 4,4′,4′′,4′′′-(perylene-2,5,8,11-tetrayl) tetrabenzaldehyde (PETA) was synthesized by a new procedure for better yield. The (1,5-cyclo-octadiene) (methoxy)iridium(I) dimer and 4,4′-di-tert-butyl-2,2′-dipyridyl were purchased from Sigma-Aldrich. Bis(pinacolato)diboron, 4-bromo benzaldehyde, dimethylacetamide, o-dichlorobenzene, etc., were obtained from Spectrochem. The required solvents, acid, and bases were purchased from a local commercial supplier.

Instrumentation

¹H and ¹³C NMR data for the synthesized compounds were collected with a JEOLJNM ECZ400S/L1 (400 MHz) NMR spectrometer. The ¹H-NMR spectrum was collected on a 400 MHz instrument using CDCl₃ which possesses a residual peak at 7.26 ppm. The multiplicities are written as s (singlet), d (doublet), t (triplet), m (multiplet), and br (broad). A Nicolet MAGNA-FT-IR 750 Spectrometer Series II was used to record the Fourier Transform Infrared Spectroscopy (FTIR) data for the synthesized compounds. ¹³C cross-polarization magic angle spinning (CP-MAS) NMR (solid-state) spectra were measured on a 400 MHz Bruker Avance III spectrometer operating at a MAS frequency of 10 kHz. The X-ray Photoelectron Spectroscopy (XPS) spectrum for the carbon and nitrogen contents of the final compound was analyzed using an Omicron nanotech device running at 15 kV and 20 mA. The powder X-ray diffraction (PXRD) pattern was obtained using a Bruker AXS D8 Advanced SWAX diffractometer using Cu K_α (=0.15406 nm) radiation. The volumetric nitrogen adsorption/desorption data at 77 K using nitrogen as an inert gas, the Brunauer–Emmett–Teller (BET) specific surface area, pore volume and pore size distribution were obtained using an Autosorb 1C (Quantachrome, USA). A JEOL JEM 6700F field emission scanning electron microscope was used for determining the morphology of the synthesized compound. High resolution transmission electron microscopy (HRTEM) images were acquired using a JEOL JEM 2010 transmission electron microscope operating at 200 kV. Thermogravimetric analysis (TGA) data have been studied using a TA Instruments TGA Q500. All the electrochemical activities were performed on a CHI Biologic SP150 instrument with a three-electrode setup, in 0.5 M H₂SO₄ electrolyte solution at room temperature. The scan rate maintained for the linear sweep voltammetry (LSV) study is 1 mV s⁻¹. The stability of the catalyst was evaluated between 0.2 V and −0.5 V (vs. RHE) at a scan rate of 100 mVs⁻¹ for 1000 cycles. The frequency range for the impedance measurement is 100 kHz to 10 mHz.

Synthesis of 2,5,8,11-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)perylene (Per-Bpin4) (7)

Perylene (0.252 g, 1.00 mmol), bis(pinacol)diborane (1.04 g, 4.1 mmol), Ir[(cod)OMe]₂ (20 mg, 0.03 mmol), and 4,4′-di-tert-butyl-2,2′-bipyridine (16 mg, 0.06 mmol) were dissolved in cyclohexane (10 mL) in a Schlenk tube under an inert atmosphere. The mixture was subjected to freeze–pump–thaw three times, and the tube was sealed. Then the reaction mixture was stirred at 80 °C for 48 h. The solvent was extracted under reduced pressure and it had cooled to room temperature. The residue was obtained and purified via recrystallization from DCM/MeOH and DCM/hexane, yielding 0.645 g (yield: 85%) of 7 as a yellow solid. ¹H NMR (400 MHz, chloroform-d) δ 8.63 (s, 4H), 8.25 (s, 4H), 1.43 (s, 48H). ¹³C NMR (101 MHz, chloroform-d) δ 137.07, 133.41, 130.51, 126.18, 84.16, 25.09. Mp: >300 °C.

Synthesis of 4,4′,4′′,4′′′-(perylene-2,5,8,11-tetrayl)tetrabenzaldehyde (PETA) (9)

In a Schlenk tube, a mixture of 2,5,8,11-tetrakis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) perylene 7 (0.076 g, 0.1 mmol), 4-bromo benzaldehyde (0.11 g, 0.6 mmol), palladium tetrakis(triphenylphosphine) (0.005 g, 0.005 mmol), and K₂CO₃ (0.37 g, 2 mmol) was placed, in 5 mL of a 1,4-dioxane/water mixture (4 : 1). The mixture was stirred for three days at 90 °C in a nitrogen atmosphere. Then, the mixture was extracted using EtOAc once it came to room temperature. The solvent was removed under reduced pressure and the crude product was isolated.

The orange solid was then purified using column chromatography on silica gel using 50% ethyl acetate/petroleum ether as the eluent to obtain perylene-(2,5,8,11-tetrayl)tetrabenzaldehyde (PETA) 9 as an orange solid (0.06 g, 89%). ¹H NMR (400 MHz, DMSO-d₆) δ 10.14 (s, 4H), 8.95 (s, 4H), 8.39 (s, 4H), 8.28–8.26 (d, J = 8.3 Hz, 8H), 8.13–8.11 (d, J = 8.3 Hz, 8H). Mp: >300 °C.

Synthesis of PETA-PAM-COF

To a Schlenk tube, 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (PAM) (0.1 mmol, 0.067 g), 4,4′,4′′,4′′′-(perylene-2,5,8,11-tetrayl)tetra-benzaldehyde (PETA) (0.1 mmol, 0.061 g), 5 (M) acetic-acid catalyst (0.5 mL) and a dimethylacetamide (DMAc)/o-dichlorobenzene(o-DCB) (3/1 mL) solvent mixture were added, which was then sonicated for an hour. After that, the tube was degassed by three freeze–pump–thaw cycles and flame-sealed under vacuum and heated statically in an oil bath for seven days at 120 °C. After collecting the reddish-black precipitate through filtration, the trapped guest molecules were removed by repeatedly washing it with THF and acetone. The reddish-black powder was collected and vacuum-dried to obtain PETA-PAM-COF.

Catalyst preparation

Mechanical grinding has been used for the preparation of the PETA-PAM-COF catalyst. To make the catalyst ink, 1 mg of the catalyst was dissolved in 1 mL of a solvent mixture (IPA : H₂O = 1 : 1 v/v) and 20 μl of 0.5 weight percent Nafion binder. The Nafion binder (5 wt%) was diluted using isopropanol (IPA). A 10 μl drop of the generated catalyst ink was applied to a 3 mm diameter GC electrode and allowed to air dry for the whole night. Before the catalyst was applied, the GC electrode was cleaned several times with distilled water and polished with a 0.05 μm alumina slurry.

Results and discussion

For the synthesis of PETA-PAM-COF, 4,4′,4′′,4′′′-(porphyrin-5,10,15,20-tetrayl)tetra-aniline (PAM) of D_4h symmetry and 4,4′,4′′,4′′′-(perylene-2,5,8,11-tetrayl)tetra benzaldehyde (PETA) of D_2h symmetry were combined through a solvothermal Schiff base condensation reaction. The synthesized COF is not only insoluble in common organic solvents but also in strong inorganic acids and bases such as aq. H₂SO₄ and NaOH. Fourier transform Infrared (FT-IR) and ¹³C cross-polarized (CP) solid-state MAS NMR spectroscopy reveal the formation of fundamental imine linkage. The FT-IR spectra (Fig. 1a) confirm the existence of imine functionality within the porphyrin and perylene moieties, with an absorption peak at 1600 cm⁻¹, corresponding to the C=N bond stretching. The absence of amine (N–H) stretching frequency at 3343 cm⁻¹ of PAM and (–C=O) stretching frequency for the aldehyde carbonyl at 1704 cm⁻¹ of PETA suggest that all starting materials were consumed. Furthermore, the extensive formation of imine linkage for the synthesized PETA-PAM-COF has been detected from the CP-MAS NMR spectra. Solid state ¹³C CP-MAS NMR (Fig. 1b) shows the resonance signal at about 157 ppm for the carbon of the C=N bond. The peak at 150 ppm indicates the quaternary carbon of the PETA-PAM covalent organic framework. The peak for β-pyrrolic carbon is at around 115 ppm, while the methine carbon peak for the porphyrin unit is found at 120 ppm. The perylene–phenyl moiety of the covalent organic framework is mostly responsible for the largest resonance peak, which is located at 133 ppm to 138 ppm. N₂ adsorption/desorption experiment was performed on PETA-PAM-COF to confirm its permanent porosity. A type I isotherm is observed from the volumetric N₂ adsorption/desorption experiment at 77 K, suggesting the intrinsic microporosity of the as-synthesized COF (Fig. 1c). The total pore volume is 0.57 cm³ g⁻¹ and the Brunauer–Emmett–Teller (BET) surface area is found to be around 1400 m² g⁻¹. According to the nonlocal density functional theory (NLDFT), the pore size distribution shows unimodal porosity at 1.82 nm (Fig. 1c). XPS (Fig. 1d) revealed two broad peaks: one at about 284 eV and one centred at around 399 eV, which are associated with C_1S and N_1S, respectively. Furthermore, the individual XPS spectra for C_1S (Fig. 1f) and N_1S (Fig. 1e) are deconvoluted, providing precise insight into the chemical states of the respective elements. The deconvoluted N_1S spectra, made up of two component peaks (398 and 399 eV), directly reflect the connection between the perylene and porphyrin molecules through the formation of C=N bonds. The first peak (398 eV) is primarily generated by the pyrrolic nitrogen, while the 399 eV peak appears due to the N-atom attached to the Schiff base bond (C=N). To determine the surface morphology of PETA-PAM-COF, SEM and HRTEM studies were conducted. The SEM image (Fig. 2a and b) reveals that PETA-PAM-COF exhibits a hollow-spherical morphology with a diameter of roughly 0.7 μm. Stacking of layers may trigger self-aggregation of the molecule, resulting in hollow spherical morphology,⁶² which is quite evident from the HRTEM image (Fig. 2c). Furthermore, ordered lattice fringes were observed from the HRTEM image (Fig. 2d), indicating its high crystallinity. The fast Fourier transform (FFT) pattern of the selected-area (Fig. 2d) was evaluated (Fig. 2e), suggesting an alignment of the tetragonal pores along the (−5,−4,1) facet of PETA-PAM-COF with a d-spacing of ∼3.73 Å (Fig. 2f). This is in good agreement with the π–π stacking distance of the two neighbouring layers of the COF crystallite (Fig. 3f). Further simulation of the observed (−5,−4,1) facet provided an equivalent Fig. 2f FFT pattern, demonstrating the high crystallinity of COF.

Fig. 1 (a) FTIR spectra of PETA-PAM-COF, (b) ¹³C solid-state CP-MAS NMR of PETA-PAM-COF, (c) N₂ adsorption/desorption isotherm at 77 K with pore size distribution (inset), (d) XPS survey spectra of PETA-PAM-COF, and (e and f) N_1S and C_1S XPS peaks of PETA-PAM-COF.

Fig. 2 (a and b) SEM images of PETA-PAM-COF, (c and d) HRTEM images of PETA-PAM-COF, (e) FFT pattern of the (−5,−4,1) plane, and (f) simulated HRTEM image corresponding to (e).

Fig. 3 (a) Experimental PXRD (red), (b) theoretical AA stacking (black), (c) theoretical AB stacking (blue), (d) Pawley refinement of PXRD, (e and f) front and side views of simulated AA stacking, and (g) front views of slipped AA and (h) AB stacking.

The crystallinity of the synthesized COF was also investigated by the PXRD technique. The PXRD pattern of PETA-PAM-COF displayed three significant peaks. The appearance of the most intense peak at 2θ = 4.78° is attributed to the existence of the (1,–1,0) plane (Fig. 3a) of the regular tetragonal lattice, indicating the crystallinity present in the material. The other peaks at 2θ = 13.77° and 17.85° correspond to (3,−3,0) and (4,1,−1) planes respectively, may be due to plane reflection caused by the interlayer π–π stacking of the synthesized COF. The distinct geometries (D_2h and D_4h) of the monomeric units cause a small topological strain on the molecular assembly. However, long-term static heating of the reaction mixture for 7 days leads to formation of crystalline PETA-PAM-COF. By considering the AA (Fig. 3e), AB (Fig. 3h), and slipped AA (Fig. 3g) stacking, the crystalline structure of PETA-PAM-COF was effectively identified utilizing the density-functional tight-binding (DFTB+) approach along with Lennard-Jones (LJ) dispersion. The observed intensity and peak position of the experimental PXRD (Fig. 3a) data are in accordance with the theoretical PXRD pattern of the AA stacking model (Fig. 3b). Using Pawley refinement, the XRD data generated a pattern which perfectly matched the experimentally observed pattern, obtaining a Pawley refinement factor (R_wp) of 5.77% (Fig. 3d). From the theoretical optimization, the unit cell parameters were evaluated to be a = 26.8849 Å, b = 27.9821 Å, c = 7.65738 Å, α = 90.1737°, β = 89.7679° and γ = 89.7558° with a space group of P₁. Importantly, to determine the energetically superior framework, stacking energies were calculated as well (Table S2 and ESI†). The overall crystal stacking energy of the slipped-AA stacking is found to be around −89.77 kcal mol⁻¹, which is comparatively higher than the stacking energies of the AA (−75.72 kcal mol⁻¹) and AB (−62.34 kcal mol⁻¹) stacking modes. The thermal stability of the aforementioned COF was found up to 500 °C as revealed from the thermogravimetric (TGA) analysis (Fig. S6 and the ESI†). The chemical stability of the synthesized PETA-PAM-COF was studied in both acidic (2 M H₂SO₄) and alkaline (2 M NaOH) media for 3 days. As shown in Fig. S7 and the ESI† the COF is found to be highly stable in both the solutions for 3 days. The solid-state electronic absorption spectrum exhibits wide absorption with four major peaks (Fig. S5 and the ESI†). The maxima centred at 375 nm with two shoulder peaks at 392 nm and 398 nm are ascribed to the porphyrin sorbet band and the peak at 273 nm may be responsible due to the π–π stacking of the two adjacent layers.⁶³ The COF showed a photoluminescence emission maximum at 498 nm.

Electrochemical HER activity

In order to evaluate the HER activities of PETA-PAM-COF, a typical three electrode system has been implemented. In the traditional three electrode setup, the three different electrodes (working, reference and counter electrodes) are placed in an electrolytic cell containing 0.5 (M) H₂SO₄ electrolyte solution. We used a glassy carbon (GC) electrode as the working electrode and platinum electrode and Ag/AgCl (calomel) electrode as the counter electrode and reference electrode respectively. Using linear sweep voltammetry (LSV), the electrochemical activity of the synthesized catalyst was measured in 0.5 (M) H₂SO₄ solution. A rapid increase in the cathodic current was observed in the LSV plot at a certain negative bias. The significant increase in cathodic current indicated the start of the HER on the catalyst-coated electrode surface. Additionally, a similar type of LSV analysis was conducted for the benchmark Pt/C catalyst and for bare glassy carbon (GC), to compare the catalytic activity with that of the developed catalyst (Fig. 4a). A negligible amount of HER activity was recorded due to the low cathodic current of the bare GC electrode. Consequently, a high cathodic current was attained for the catalyst coated GC electrode, implying the catalytic HER activity of PETA-PAM-COF. The overpotential (η) for the HER indicates the electrocatalyst efficiency and thus gives a measure of the extent of polarization obeying the passage of faradaic current generated due to the HER. For a small cathodic bias, a lower overpotential (η) value reflects enhanced catalytic activity towards efficient hydrogen production. The efficiency of HER catalysts is often calculated using the required overpotential (η) to reach a current density of 10 mA cm⁻² (η₁₀). In this work, the synthesized COF showed a current density of 10 mA cm⁻² at an overpotential of 261 mV, which is 203 mV more negative than that of the ideal Pt/C catalyst (58 mV). So, this result suggests that the PETA- PAM electrocatalyst exhibits potential HER activity. With PETA-PAM-COF, the overpotential achieved to produce a 10 mA cm⁻² current density is quite relevant to that of several previously reported HER electrocatalysts (Table S1 and the ESI†).

Fig. 4 (a) LSV plots of PETA-PAM-COF, Pt/C and bare GC in 0.5 (M) H₂SO₄, (b) Tafel plot of PETA-PAM-COF, (c) Nyquist plot of PETA-PAM-COF, (d) faradaic efficiency for H₂ generation using PETA-PAM-COF, (e) LSV plot of PETA-PAM-COF in its 1st and 1000th cycles, and (f) chronoamperometry plot of PETA-PAM-COF.

The Tafel slope, which involves the following three phases on the catalyst surface, provides a clear reflection of the HER process.

Step 1: Volmer: H⁺ + e⁻ → H*

Step 2: Heyrovsky: H* + H⁺ + e⁻ → H₂

Step 3: Tafel: H* + H* → H₂

The first step involves the adsorption of protons on the surface of the porous electrocatalyst by proton coupled electron transfer (Volmer), followed by one step to generate hydrogen. Either the adsorbed H* reacts with another H⁺ (Heyrovsky) or by combination⁵⁷ of two H* to generate H₂ (Tafel). Therefore, the Volmer–Heyrovsky or Volmer–Tafel mechanisms will be followed to generate H₂. The Tafel slope value of 30 mV dec⁻¹ signifies that the rate-determining step (r.d.s.) is Tafel. If the Tafel slope value is 40 mV dec⁻¹ and 120 mV dec⁻¹, it implies that the Heyrovsky and Volmer mechanisms will be the r.d.s., respectively.^64–67 According to the polarization curve, the Tafel slope was linear (Fig. 4b) and measured at around 122 mV dec⁻¹, suggesting that the HER on the PETA-PAM-COF surface is caused by the Volmer–Heyrovsky mechanism. Therefore, it can be demonstrated that the Volmer step is responsible for defining the rate of this mechanism. The COF's imine nitrogen sites (–N=) as well as pyrrolic nitrogen of porphyrin units are thought to be free docking sites for hydrogen evolution. Imine nitrogen atoms and pyrrolic nitrogen undergo protonation in an acidic solution, resulting in a positively charged moiety, while SO₄²⁻ preserves the electroneutrality. In contrast, H* and H⁺ combination⁵⁷ happened due to the assistance of additional nearby imine nitrogen sites, which aided in the process of hydrogen evolution. In addition to the Tafel slope, the exchange current density provides significant insight into the electrode's performance. It could be derived using Tafel plots and is defined as the current density at the equilibrium potential when the cathodic current equals the anodic current. An effective electrocatalyst should have a high exchange current density, in addition to a low overpotential. The synthesized catalyst shows an exchange current density of 3.7 × 10⁻⁴ A cm⁻² exhibiting effective electrode kinetics. To get a deep insight into the electrocatalytic activity of the catalyst, we need to study the interfacial charge-transfer characteristics. The diameter of the semicircle reflected the charge transfer resistance (R_ct) of the catalyst as revealed by the Nyquist plot (Fig. 4c). A lower R_ct value corresponds to faster kinetics on the electrode–electrolyte interface. The PETA-PAM electrocatalyst shows a low charge-transfer resistance (R_ct) value of 62.22 Ω. The extended π-conjugation of perylene–porphyrin based COF is responsible for its excellent charge-transfer capacity. Furthermore, a large surface area offers more interfacial area for proton reduction activity. The stability of a HER catalyst is strongly connected to its long-term efficacy. To investigate the stability of the as-synthesized HER catalyst, 1000 continuous cycles were conducted using the GC electrode coated with PETA-PAM-COF. During the entire 1000 cycles, the synthesized COF retained 93% (Fig. 4e) of its original current density, indicating good stability. PETA-PAM-COF was found to have a 96% (Fig. 4d) faradaic efficiency (FE), indicating that the catalyst is pure enough for electrochemical hydrogen evolution. The long-term stability of PETA-PAM-COF was also studied from its current–time response (Fig. 4f), which reveals that the current response of the electrocatalyst is stable with negligible loss over 10 000 seconds. Therefore, both the voltametric and amperometry studies reveal good electrochemical stability of the synthesized COF.

Conclusions

In summary, we developed a highly conjugated perylene–porphyrin microporous 2D-covalent organic framework based on imine linkage. Despite the topological restriction in molecular geometry, a thermodynamically regulated crystalline framework is formed. The π conjugation between perylene and porphyrin (AA stacking) forms a regulating passage. We have studied the properties of charge carrier mobility and HER catalytic activity of the synthesized PETA-PAM-COF. To the best of our knowledge, the first ever perylene and porphyrin-based COF was used as an electrocatalyst for metal free hydrogen evolution through water splitting reactions. Such COFs show structural robustness even under strongly basic or acidic conditions. This kind of crystalline COF broadens the range of structural diversity and makes them more applicable to understand their electrochemical features.

Data availability

¹H NMR, ¹³C (CP-MAS), PXRD, XPS, FE-SEM and TGA are available in the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Dr A. P. acknowledges DST-SERB for funding through the DSTSERB-SRG project (SRG/2022/000628). MAA is grateful for HPC time from the MMM Hub via membership of the UK's HEC Materials Chemistry Consortium, partially funded by EPSRC (EP/T022213/1). We also thank CIF, NMR facility of BIT Mesra for their help.

Notes and references

1. M. I. Farewell, Science, 2010, 329, 1292–1294.
2. M. Carmo, D. L. Fritz, J. Mergel and D. A. Stolten, Int. J. Hydrogen Energy, 2013, 38, 4901–4934.
3. J. Rossmeisl, Z. W. Qu, H. Zhu, G. J. Kroes and J. K. Nørskov, J. Electroanal. Chem., 2007, 607, 83–89.
4. Y. Yan, B. Xia, Z. Xu and X. Wang, ACS Catal., 2014, 4, 1693–1705.
5. L. Lin, Z. Sun, M. Yuan, J. He, R. Long, H. Li, C. Nan, G. Sun and S. Ma, J. Mater. Chem. A, 2018, 6, 8068–8077.
6. V. Maruthapandian, M. Mathankumar, V. Saraswathy, B. Subramanian and S. Muralidharan, ACS Appl. Mater. Interfaces, 2017, 9, 13132–13141.
7. X. Pan and G. Zhou, Phys. Chem. Chem. Phys., 2016, 18, 21–46.
8. Y. Hu, B. Yu, W. Li, M. Ramadoss and Y. Chen, Nanoscale, 2019, 11, 4876–4884.
9. J. Deng, P. Ren, D. Deng and X. Bao, Angew. Chem., Int. Ed., 2015, 54, 2011–2104.
10. Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296–7299.
11. F. Qi, P. Li, Y. Chen, B. Zheng, J. Liu, J. Zhou, J. He, X. Hao and W. Zhang, Int. J. Hydrogen Energy, 2017, 42, 7811–7819.
12. W. Chen, K. Sacaki, C. Ma, A. I. Frenkel, N. Marinkovic, J. T. Muckerman, Y. Zhu and R. R. Adzic, Angew. Chem., Int. Ed., 2015, 51, 6131–6135.
13. J. Yin, Q. Fan, Y. Li, F. Cheng, P. Zhou, P. Xi and S. Sun, J. Am. Chem. Soc., 2016, 138, 14546–14549.
14. C. Wu, Y. Yang, D. Dong, Y. Zhang and J. Li, Small, 2017, 13, 1602873.
15. S. Li, G. Zhang, X. Tu and J. Li, ChemElectroChem, 2018, 5, 701–707.
16. W. Li, B. Yu, Y. Hu, X. Wang, D. Yang and Y. Chen, ACS Sustainable Chem. Eng., 2019, 7, 4351–4359.
17. B. Yu, X. Wang, F. Qi, B. Zheng, J. He, J. Lin, W. Zhang, Y. Li and Y. Chen, ACS Appl. Mater. Interfaces, 2017, 9, 7154–7159.
18. Y. Wang, H. Zhuo, X. Zhang, X. Dai, K. Yu, C. Luan, L. Yu, Y. Xiao, J. Li, M. Wang and F. Gao, Nano Energy, 2018, 48, 590–599.
19. Y. Wang, Y. Qin, X. Zhang, X. Dai, H. Zhou, C. Luan, Y. Jiang, H. Zhao, H. Wang and H. Huang, Dalton Trans., 2018, 47, 7975–7982.
20. D. Li, C. Li, L. Zhang, H. Li, L. Zhu, D. Yang, Q. Fang, S. Qiu and X. Yao, J. Am. Chem. Soc., 2020, 142, 8104–8108.
21. S. Bhunia, S. K. Das, R. Jana, S. C. Peter, S. Bhattacharya, M. Addicoat, A. Bhaumik and A. Pradhan, ACS Appl. Mater. Interfaces, 2017, 9, 23843–23851.
22. H. Xu, J. Gao and D. Jiang, Nat. Chem., 2015, 7, 905–912.
23. S. Dalapati, M. Addicoat, S. Jin, T. Sakurai, J. Gao, H. Xu, S. Irle, S. Seki and D. Jiang, Nat. Commun., 2015, 6, 7786.
24. Y. Liu, Y. Ma, Y. Zhao, X. Sun, F. Gándara, H. Furukawa, Z. Liu, H. Zhu, C. Zhu, K. Suenaga, P. Oleynikov, A. S. Alshammari, X. Zhang, O. Terasaki and O. M. Yaghi, Science, 2016, 351, 365–369.
25. N. Huang, X. Ding, J. Kim, H. Ihee and D. Jiang, Angew. Chem., Int. Ed., 2015, 54, 8704–8707.
26. S. Kandambeth, B. P. Biswal, H. D. Chaudhari, K. C. Rout, H. S. Kunjattu, S. Mitra, S. Karak, A. R. Das, R. Mukherjee, U. K. Kharul and R. Banerjee, Adv. Mater., 2017, 29, 1603945.
27. Y. Cheng, J. Xin, L. Xiao, X. Wang, X. Zhou, D. Li, B. Gui, J. Sun and C. Wang, J. Am. Chem. Soc., 2023, 145(34), 18737–18741.
28. M. G. Rabbani, A. K. Sekizkardes, Z. Kahveci, T. E. Reich, R. E. Ding and H. M. El-Kaderi, Chem.–Eur. J., 2013, 19, 3324–3328.
29. M. Shan, B. Seoane, E. Rozhko, A. Dikhtiarenko, G. Clet, F. Kapteijn and J. Gascon, Chem.–Eur. J., 2016, 22, 14467–14470.
30. N. Huang, X. Chen, R. Krishna and D. Jiang, Angew. Chem., Int. Ed., 2015, 54, 2986–2990.
31. F. Furukawa and O. M. Yaghi, J. Am. Chem. Soc., 2009, 131, 8875–8883.
32. X. D Li, H. P. Zang, J. T. Wang, J. F. Wang and H. Zhang, J. Mater. Chem. A, 2014, 2, 18554–18561.
33. Q. Fang, J. Wang, S. Gu, R. B. Kaspar, Z. Zhuang, J. Zheng, H. Guo, S. Qiu and Y. Yan, J. Am. Chem. Soc., 2015, 137, 8352–8355.
34. L. Bai, S. Z. F. Phua, W. Q. Lim, A. Jana, Z. Luo, H. P. Tham, L. Zhao, Q. Gao and Y. Zhao, Chem. Commun., 2016, 52, 4128–4131.
35. V. S. Vyas, M. Vishwakarma, I. Moudrakovski, F. Haase, G. Savasci, C. Ochsenfeld, J. P. Spatz and B. V. Lotsch, Adv. Mater., 2016, 28, 8749–8754.
36. X. Shi, Y. Yao, Y. Xu, K. Liu, G. Zhu, L. Chi and G. Lu, ACS Appl. Mater. Interfaces, 2017, 9, 7481–7488.
37. D. B. Shinde, S. Kandambeth, P. Pachfule, R. R. Kumar and R. Banerjee, Chem. Commun., 2015, 51, 310–313.
38. Y. Peng, Z. Hu, Y. Gao, D. Yuan, Z. Kang, Y. Qian, N. Yan and D. Zhao, ChemSusChem, 2015, 8, 3208–3212.
39. H. Xu, X. Chen, J. Gao, J. Lin, M. Addicoat, S. Irle and D. Jiang, Chem. Commun., 2014, 50, 1292–1294.
40. P. Li, X. J. Wang, J. Liu, J. S. Lim, R. Zou and Y. A. Zhao, J. Am. Chem. Soc., 2016, 138, 2142–2145.
41. V. S. Vyas, M. Vishwakarma, I. Moudrakovski, F. Haase, G. Savasci, C. Ochsenfeld, J. P. Spatz and B. V. Lotsch, Adv. Mater., 2016, 28, 8749–8754.
42. S. Wan, F. Gándara, A. Asano, H. Furukawa, A. Saeki, S. K. Dey, L. Liao, M. W. Ambrogio, Y. Y. Botros, X. Duan, S. Seki, J. F. Stoddart and O. M. Yaghi, Chem. Mater., 2011, 23, 4094–4097.
43. X. Ding, J. Guo, X. Feng, Y. Honsho, J. Guo, S. Seki, P. Maitarad, A. Saeki, S. Nagase and D. Jiang, Angew. Chem., Int. Ed., 2011, 50, 1289–1293.
44. T. He, P. Pachfule, H. Wu, Q. Xu and P. Chen, Nat. Rev. Mater., 2016, 1, 16059.
45. H. Yang, S. Zhang, L. Han, Z. Zhang, Z. Xue, J. Gao, Y. Li, C. Huang, Y. Yi, H. Liu and Y. Li, ACS Appl. Mater. Interfaces, 2016, 8, 5366–5375.
46. S. L. Cai, Y. B. Zhang, A. B. Pun, B. He, J. Yang, F. M. Toma, I. D. Sharp, O. M. Yaghi, J. Fan, S. R. Zheng, W. G. Zhang and Y. Liu, Chem. Sci., 2014, 5, 4693–4700.
47. B. Nath, W.-H. Li, J.-H. Huang, G.-E. Wang, Z.-H. Fu, M.-S. Yao and G. Xu, CrystEngComm, 2016, 18, 4259–4263.
48. B. J. Smith, N. Hwang, A. D. Chavez, J. L. Novotney and W. R. Dichtel, Chem. Commun., 2015, 51, 7532–7535.
49. D. D. Medina, V. Werner, F. Auras, R. Tautz, M. Dogru, J. Schuster, S. Linke, M. Döblinger, J. Feldmann, P. Knochel and T. Bein, ACS Nano, 2014, 8, 4042–4052.
50. P. Wang, Q. Wu, L. Han, S. Wang, S. Fang, Z. Zhang and S. Sun, RSC Adv., 2015, 5, 27290–27294.
51. B. C. Patra, S. Khilari, L. Satyanarayana, D. Pradhan and A. Bhaumik, Chem. Commun., 2016, 52, 7592–7595.
52. J. Zhang, J. Chen, Y. Wan, H. Liu, W. Chen, G. Wang and R. Wang, ACS Appl. Mater. Interfaces, 2020, 12, 13805–13812.
53. X. Wang, Y. Fei, J. Chen, Y. Pan, W. Yuan, L. Y. Zhang, C. X. Guo and C. M. Li, Small, 2022, 18, 2103866.
54. X. Fan, X. Wang, W. Yuan and C. M. Li, Sustainable Energy Fuels, 2017, 1, 2172.
55. Y. Cai, J. Gao, S. Chen, Q. Ke, G. Zhang and Y.-W. Zhang, Chem. Mater., 2019, 31, 8948–8956.
56. X. Chu, C. I. Sathish, J. H. Yang, X. Guan, X. Zhang, L. Qiao, K. Domen, S. Wang, A. Vinu and J. Yi, Small, 2023, 19, 2302875.
57. B. C. Patra, S. Khilari, R. N. Manna, S. Mondal, D. Pradhan, A. Pradhan and A. Bhaumik, ACS Catal., 2017, 7, 6120–6127.
58. A. Pradhan and R. N. Manna, ACS Appl. Polym. Mater., 2021, 3, 1376–1384.
59. R. Chen, J. Shi, Y. Ma, G. Lin, X. Lang and C. Wang, Angew. Chem., Int. Ed., 2019, 58, 6430–6434.
60. M. He, Z. Ye, G. Lin, S. Yin, X. Huang, X. Zhou, Y. Yin, B. Gui and C. Wang, Acta Chim. Sin., 2023, 81, 784–792.
61. Z. Xue, P. P. S. Lee, Y. Wang, D. W. J. Kwong, J. Li, J. H. Xin, W.-K. Wong and K. K. L. Cheuk, Tetrahedron, 2011, 67, 6030–6035.
62. S. Kandambeth, V. Venkatesh, D. B. Shinde, S. Kumari, A. Halder, S. Verma and R. Banerjee, Nat. Commun., 2015, 6, 6786.
63. X. Han, H. Kong, T. Chen, J. Gao, Y. Zhao, Y. Sang and G. Hu, Nanomaterials, 2021, 11, 2158.
64. Z. Gruba, M. M. Hukovic and R. Babic, Int. J. Hydrogen Energy, 2013, 38, 4437–4444.
65. D. Huang, J. Lu, S. Li, Y. Luo, C. Zhao, B. Hu, M. Wang and Y. Shen, Langmuir, 2014, 30, 6990–6998.
66. H. Lv, Z. Xi, Z. Chen, S. Guo, Y. Yu, W. Zhu, Q. Li, X. Zhang, M. Pan, G. Lu, S. Mu and S. Sun, J. Am. Chem. Soc., 2015, 137, 5859–5862.
67. N. Pentland, J. O. M. Bockris and E. Sheldon, J. Electrochem. Soc., 1957, 104, 182–194.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4se00829d

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