Crystalline porphyrin-based graphdiyne for electrochemical hydrogen and oxygen evolution reactions†

Qingyan Pan‡ , Xinsheng Chen‡ , Hui Liu , Weijin Gan , Naixiu Ding * and Yingjie Zhao *
Engineering Research Center of High Performance Polymer and Molding Technology, Ministry of Education, College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China. E-mail: nxding1717@163.com; yz@qust.edu.cn

First published on 8th April 2021

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Introduction

Exploring efficient, inexpensive, and environmentally friendly electrocatalysts for the hydrogen/oxygen evolution reaction (HER/OER) has attracted great attention.^1,2 In the past few decades, numerous electrocatalysts have been designed and synthesized and are mainly categorized into noble-metal-based^3–8 and carbon-based materials.^9–16 For the noble-metal-based electrocatalysts, they normally exhibit extremely low overpotential and a small Tafel slope, which brings good electrocatalytic performance. However, the disadvantages such as the high manufacturing cost and them suffering from corrosion susceptibility to acidic electrolytes are always obstacles to large-scale commercialization.^2,3 On the other hand, metal-free carbon-based materials have gained much attention and have been considered as a new promising class of electrocatalysts for water splitting due to their tunable chemical/electronic structures and high tolerance to acidic/alkaline conditions.^2,11 But, most of them cannot function both in the HER and OER, and some of them even face the obstacle of low catalytic efficiency (high overpotential and a high Tafel slope).^3,11 Therefore, developing new HER/OER electrocatalysts featuring low-cost, stability and high performance is a long-standing goal.¹¹

Graphdiyne (GDY) is a new carbon allotrope composed of sp and sp² hybridized carbon atoms.¹⁷ As a novel two-dimensional (2D) carbon material, GDY has received widespread attention since the pioneering work reported by Li and co-workers in 2010.¹⁷ Due to the rich sp hybridized carbon, extended 2D π-conjugated plane, and uneven surface charge distribution, GDY has demonstrated intensive application potential in various research studies of traditional and new fields, such as energy storage and conservation,^18–22 catalysis,^23–28 electronic devices^29–34 and other applications.³⁵ Many studies have revealed that GDY or GDY/metal composite materials as electrocatalysts have exhibited excellent catalytic activity and good stability for water splitting or the nitrogen reduction reaction.^36,37 For example, Ni/Fe anchored GDY,³⁸ Fe/Co layered double-hydroxide (LDH) nanosheet array composited GDY,³⁹ and MoS₂ composited GDY.⁴⁰ In addition, GDY analogs containing different functional moieties as metal-free electrocatalysts have also been reported.^41–43 Their interesting characteristics, properties, and applications have also been intensively investigated. The chemical modified and functionalized-GDYs have been considered as enhanced electrocatalysts in the oxygen reduction reaction (ORR).^44,45 For example, triazine-based GDY has been applied as a high-performance supercapacitor electrode and shows a good performance in catalysing the ORR.⁴⁶ Trifluorobenzene-based GDY used as a metal-free catalyst displayed excellent performance for overall water splitting.⁴⁷ Based on the above discussion, these well-known functional units together with the unique diacetylene linkages provide us an ideal approach for the preparation of high-performance electrocatalysts and studying the exact HER/OER mechanisms.

Porphyrins possess a delocalized 18-π electron (for the shortest cyclic path) system with unique photophysical and redox properties, and metal coordination characteristics.^10,48 Porphyrins have been widely used as key molecular building blocks in material chemistry due to their symmetry, substitution, metalation, and complexation, which provide many levers to control their organization.¹⁰ Most importantly, the porphyrin structure can stabilize almost all high-valent and low-valent metal ions,^49,50 which is generally believed as a key intermediate for oxidizing and reducing catalytic cycles.⁵¹ However, as a small molecule, porphyrin possesses poor stability, mass transfer issues and low density of catalytic active sites which seriously hinder the applications as electrocatalysts.⁵² Therefore, porphyrin-based GDY (Por-GDY) with π-conjugated and extended 2D networks may combine the advantages of both GDY and porphyrin and overcome the instability and less catalytic active sites of the small molecule porphyrin compounds. The diacetylene linkages could enhance the electron transfer efficiency, which will result in enhanced electrocatalytic activity.

Herein, we report the targeted synthesis of 2D Por-GDYvia Glaser coupling reaction with the assistance of Cu foil as a template. The Co-loaded Por-GDY (CoPor-GDY) could be achieved by simply treating with CoCl₂ solution in DMF, which was then used as a heterogeneous electrocatalyst for meeting the requirements of the HER and OER. The experimental results revealed that the as-prepared CoPor-GDY exhibited relatively low overpotentials for both the HER and OER in alkaline solution. This work provides a simple method for the rational design of bifunctional/multifunctional electrocatalysts to achieve different catalytic effects.

Results and discussion

The 2D Por-GDY was prepared by an alkyne–alkyne homocoupling reaction through a bottom-up strategy (Scheme 1). The whole procedure is similar to the synthesis approach of GDY. For details, see the ESI.† Briefly, the freshly cleaned Cu foils were directly added into the pyridine solution, and the solution was heated to 80 °C. Then the pyridine solution of the monomer was added into the solution slowly. The system was kept at this temperature for 3 days. And then a uniform dense dark film was observed on the surface of the Cu foils. The low concentration of monomer was employed here to control the reaction rate. The mechanism of the formation of the Por-GDY films was proposed (elaborated in the ESI†). In the beginning, the concentration of Cu(II) ions and monomers is very diluted, so the heterogeneous nucleation and polymerization occurred preferentially on the surface of the Cu foils. The nucleation can minimize the interfacial energy barrier and subsequently lead to the growth of Por-GDY on the surface of the Cu foils. CoPor-GDY was subsequently obtained by the solvothermal method from CoCl₂ and Por-GDY in DMF. The as-synthesized Por-GDY and CoPor-GDY films are stable and insoluble in common organic solvents.

The XPS results reveal the chemical composition and chemical state of the elements existing within Por-GDY and CoPor-GDY. As shown in Fig. 1a, Por-GDY is mainly composed of carbon and nitrogen with an intensity ratio of 7.2:1, which is close to the predicted ratio of 7:1. The atomic content of Co in CoPor-GDY was determined to be 5.45% from the XPS spectrum which is larger than the theoretical value (3.03%).^53,54 The reasons could be the adsorption of the cavities in Por-GDY and the complexation with the alkyne groups.^24,39 Besides, the presence of Co 2p in the spectrum of CoPor-GDY suggests that the Co species have successfully embedded within the porphyrin rings (Fig. 1b).⁵⁵ Fig. S3 (ESI†) presents a high-resolution asymmetric C 1s XPS spectrum of Por-GDY and CoPor-GDY, which results from the overlapping of the signals of several carbon species with close binding energies. The experimental curve is fitted with four subpeaks at 284.5, 285.3, 286.9, and 288.6 eV assigned to the orbital of CC (sp²), CC (sp), CO and C–O, respectively. And the area ratio of the sp/sp² carbon is 2/5, which agrees with the theoretical values. The small amounts of oxygen originate from the adsorption from air, which can also be observed from other carbon materials. Furthermore, Fig. 1c shows a high-resolution N 1s XPS spectrum of CoPor-GDY, which can be deconvoluted into two subpeaks at 398.8 and 400.1 eV, corresponding to the pyrrolic N and Co–N. Compared with that of Por-GDY (Fig. S4, ESI†), N 1s of CoPor-GDY gives peaks located at 398.8 eV (pyrrolic N) and 399.9 eV (pyrrolic N–H), and the area ratio of them is almost 1:1, which agrees with the molecular structure of porphyrin.

The Raman spectra were performed to reveal the chemical structure information of Por-GDY and CoPor-GDY (Fig. 1d). The peaks at 1005, 1070, 1235, 1335, 1457 and 1540 cm⁻¹ confirm the vibration of the porphyrin group in these two materials.⁵⁶ The peak at 2180 cm⁻¹ can be assigned to the vibration of the conjugated diyne links (–CC–CC–). Interestingly, there is a significant decrease at this peak in CoPor-GDY (red curve in Fig. 1d), which indicated a possible interaction of the Co and acetylene groups.⁵⁷ Moreover, the FTIR spectra further verified the structure of the as-prepared Por-GDY (Fig. S5, ESI†). In the UV–vis absorption spectra, there is a significant red shift at the maximum absorption wavelength of Por-GDY and CoPor-GDY, compared with that of compound 1, which indicated the extension of π-conjugation in Por-GDY (Fig. S6, ESI†).

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to confirm the morphology of the as-obtained materials. Fig. 2a–d revealed the compact film morphologies of the as-obtained Por-GDY and CoPor-GDY. Furthermore, Fig. 2e–g displayed the EDS elemental mapping results; carbon, nitrogen and Co are uniformly distributed in the nanosheets of CoPor-GDY. As shown in high-resolution TEM (Fig. S7, ESI†), there are no metal particles observed on the Por-GDY plane, which indicates the high dispersion of Co. The deprotonated porphyrins with the proper size can easily coordinate many transition metal ions to form a four-coordinated structure (metal–N₄). Additionally, the sp hybridization of –CC– in Por-GDY enables the arbitrary angle rotation of π/π* perpendicular to the axis, making it possible to point toward metal atoms and, thereby, chelate a single metal atom.^38,58 These two characteristics make the Co ultra-uniform and highly dispersed in the 2D plane of Por-GDY.

In order to acquire the accurate structure information of the obtained Por-GDY, we then performed a selected area electron diffraction (SAED) experiment from the TEM measurements. As shown in Fig. 3, the SAED patterns of both Por-GDY and CoPor-GDY reveal square lattices with the same d-spacing of 0.94 nm (Fig. 3a and b). The SAED pattern of CoPor-GDY is similar to that of Por-GDY, which indicated the same structures of them. To determine the 2D structure corresponding to the quadrilateral SAED patterns, we built a model with AB stacking mode which has the same square periodicity as Por-GDY (Fig. 3d). The d-spacing calculated from the simulated diffraction pattern is 0.97 nm (Fig. 3c), which is highly consistent with the experimental value. It is evident that the structure of Por-GDY matches with the AB stacking mode. The thermal treatment in the DMF solution with CoCl₂ doesn’t influence the stacking structure. Thus, both Por-GDY and CoPor-GDY were confirmed with AB manner stacking (Fig. 3d and Table S1, ESI†).

The electrocatalytic properties of Por-GDY and CoPor-GDY for the HER and OER were then determined using a typical three-electrode electrochemical system. Por-GDY and CoPor-GDY were loaded on carbon cloth (CC) under the same conditions, which were used directly as working electrodes. The electrocatalytic performance of CC was also investigated as a control experiment. The polarization curves were recorded by linear sweep voltammetry (LSV) tests with a scan rate of 5 mV s⁻¹ in 1.0 M KOH solution to get an overpotential at the current density of 10 mA cm⁻², which is commonly used to evaluate the electrocatalytic performance of catalysts. As shown in Fig. 4a, CoPor-GDY exhibited an onset potential of 115 mV and a low overpotential of 308 mV versus the reversible hydrogen electrode (RHE), which is significantly lower than those of the pure CC (724 mV) and Por-GDY (570 mV). The Tafel plot is used to evaluate the kinetics of the electrocatalytic reaction, and the smaller Tafel slope signifies the faster reaction kinetics. The results suggest that the Tafel slope of CoPor-GDY is about 68 mV dec⁻¹, which is much smaller than that of Por-GDY (234 mV dec⁻¹) and the pure CC (288 mV dec⁻¹) (Fig. 4b). More interestingly, the overpotential of CoPor-GDY is comparable to or even lower than those of the reported state-of-the-art porous polymer-based catalysts (Fig. 4c). At a constant applied potential of 320 mV, CoPor-GDY shows approximately 96% retention of current density even after 7 h (Fig. S8a, ESI†), suggesting the good long-term stability of CoPor-GDY. Furthermore, we also investigated the OER performance of CoPor-GDY, as shown in Fig. 4d–f and Fig. S8b (ESI†). CoPor-GDY shows an onset potential of 196 mV at the current density of 1.0 mA cm⁻² and an overpotential of 400 mV at the current density of 10 mA cm⁻². CoPor-GDY displays a Tafel slope of 129 mV dec⁻¹, which is much smaller than those of Por-GDY (475 mV dec⁻¹) and the pure CC (248 mV dec⁻¹), as shown in Fig. 4e. The stability of the OER performance was determined in 1.0 M KOH; there is only a very slight decrease observed in the current density (Fig. S8b, ESI†). In comparison with some reported non-noble metal-based catalysts, metal-free porous material catalysts, and even their composites, CoPor-GDY exhibited a preferable lower overpotential in the OER (Fig. 4f). All these results indicate that the introduction of Co species in Por-GDY has enhanced its performance for the HER and OER in alkaline solution. Meanwhile, the electrocatalytic performances of CoPor-GDY in acidic and neutral solutions are not as good as in alkaline solution conditions (Fig. S9, ESI†).

The catalytic kinetics of the as-prepared catalysts were assessed by electrochemical impedance spectroscopy (EIS) analysis (Fig. S10a, ESI†). The typical Nyquist plots of CoPor-GDY and Por-GDY at an open-circuit voltage are shown in Fig. 4g. The recorded impedance data were fitted by using a circuit model (Fig. S10b, ESI†). CoPor-GDY has the smallest solution resistance (R_s = 2.0 Ω) and charge-transfer resistance (R_ct = 375.9 Ω) compared to those of Por-GDY (R_s = 2.3 Ω and R_ct = 1266 Ω) and CC (R_s = 2.6 Ω and R_ct = 1690 Ω), indicating the faster charge-transfer efficiency which helps to improve the catalytic performance. To further investigate the differences of the electrocatalytic activity of CoPor-GDY, Por-GDY and pure CC, the electrochemically active surface area (ECSA) values of these three samples were measured via electrochemical double-layer capacitors (C_dl) by cyclic voltammetry experiments, with different scan rates of 20, 40, 60, 80 and 100 mV s⁻¹. The current density at 0.17 V vs. RHE is plotted as a function of the scan rate (Fig. 4h). It can be seen that the C_dl value of CoPor-GDY (5.0 mF cm⁻²) is higher than those of Por-GDY (3.2 mF cm⁻²) and pure CC (3.9 mF cm⁻²). The results indicate that CoPor-GDY possesses more active sites for the redox process, which results in enhanced electrocatalytic HER and OER performances. All these performances are closely related to the structures of the catalysts. Here, both the exposed axial unoccupied positions of the Co atom and the square planar Co–N₄ moieties in CoPor-GDY are ideal electrocatalytic active centers.^51,59 As authenticated in many previous reports, the subtle metal atoms–C(sp) bonding in CoPor-GDY could be understood as strong (sp)-d orbital charge overlaps, which were also considered to be helpful for conductivity and catalytic activity.^38,58,60 Meanwhile, the acetylenic linkages of the Por-GDY structure will enhance the electron transfer efficiency between the Co-porphyrin units, which is undoubtedly able to further improve the electrocatalytic activity of CoPor-GDY. In addition, the extended 2D porous plane is conducive to the escape of small molecular products and charge transfer in the plane during the catalytic process. In addition, the 3D structure of CC can increase the specific surface area of the CoPor-GDY based electrodes, suggesting more catalytic active centers than on the plane substrates. We propose that the porous structure and conductivity of the materials as well as the Co–N₄ moieties as active centers in CoPor-GDY are beneficial to enhance the mass transfer of reactants and products, which is very important for high catalytic activity. In addition, the wettability measurements indicate that CoPor-GDY has the best hydrophilicity which ensures the good contact between the aqueous solution and the catalyst surface. This facilitates the catalytic reaction for H₂ or O₂ regeneration (Fig. S11, ESI†).

Conclusions

In summary, we designed and synthesized a novel porphyrin-unit-based graphdiyne analog with Co-coordinated loaded porphyrin-graphdiyne, which possesses a highly conjugated and porous structure in a 2D plane. The unique distribution of pores and metal coordination ability of porphyrins provided numerous Co-species active centers endowing CoPor-GDY with enhanced electrocatalytic performance in the HER and OER. The commercial CC also provided a robust 3D porous conductive substrate and can further improve the electrocatalytic activity because of the fast and facile molecules/ions/gases transportation. CoPor-GDY loaded on CC used as a working electrode for the electrocatalytic reaction in alkaline solution exhibited an overpotential of 308 mV and a Tafel slope of 68 mV dec⁻¹ for the HER and an overpotential of 400 mV and a Tafel slope of 129 mV dec⁻¹ for the OER. The metal loaded Por-GDY is comparable to or even better than lots of porous polymer-based electrocatalysts. The ternary system formed by transition-metal, GDY analogue and carbon cloth explores a promising way to construct high-performance electrocatalysts in the HER and OER.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of Shandong Province (No. ZR2020ZD38).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis procedures, electrochemical measurments,1H-NMR spectra, MS, HR-TEM, etc. See DOI: 10.1039/d1qm00285f

‡ These authors contributed equally to this work.

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