sp‐Carbon Incorporated Conductive Metal‐Organic Framework as Photocathode for Photoelectrochemical Hydrogen Generation

Abstract

Metal‐organic frameworks (MOFs) have attracted increasing interest for broad applications in catalysis and gas separation due to their high porosity. However, the insulating feature and the limited active sites hindered MOFs as photocathode active materials for application in photoelectrocatalytic hydrogen generation. Herein, we develop a layered conductive two‐dimensional conjugated MOF (2D c‐MOF) comprising sp‐carbon active sites based on arylene‐ethynylene macrocycle ligand via CuO₄ linking, named as Cu₃HHAE₂. This sp‐carbon 2D c‐MOF displays apparent semiconducting behavior and broad light absorption till the near‐infrared band (1600 nm). Due to the abundant acetylene units, the Cu₃HHAE₂ could act as the first case of MOF photocathode for photoelectrochemical (PEC) hydrogen generation and presents a record hydrogen‐evolution photocurrent density of ≈260 μA cm⁻² at 0 V vs. reversible hydrogen electrode among the structurally‐defined cocatalyst‐free organic photocathodes.

The first sp‐carbon incorporated 2D c‐MOF (Cu₃HHAE₂) single crystals are achieved based on the hexahydroxyarylene‐ethynyl macrocycle ligand. Owing to the abundant acetylene units, Cu₃HHAE₂ acts as the first example of a MOF photocathode for highly efficient photoelectrochemical (PEC) hydrogen generation.

Metal‐organic frameworks (MOFs) are a class of porous crystalline coordination polymers consisting of metal ions/clusters and organic ligands, ^[1] and have exhibited great potential for the applications in gas storage and separation, catalysis, etc. ^[2] Recent advances reported that MOFs could also be utilized as cocatalysts and photosensitizers to mix with inorganic semiconductor nanomaterials for photoelectrochemical (PEC) hydrogen generation, due to their high porosity, structural diversity and tailorability, and ability to reduce electron‐hole pair recombination. ^[3] However, the insulating feature and the limited active sites in traditional MOFs inevitably restrict their utilization as direct photocathode materials in PEC energy conversion. ^[4]

Generally, the photoelectrocatalysis efficiency is dominated by two fundamental processes: 1) semiconductors that absorb incident photons and generate photoexcited electron‐hole pairs; 2) separation and migration of photogenerated excitons to the semiconductor surface. ^[5] Recently, two‐dimensional conjugated MOFs (2D c‐MOFs) have emerged as a unique class of layer‐stacked materials; they exhibit intrinsic electrical conductivities (up to 10³ S cm⁻¹), tunable band gaps (from metal to semiconductors), and excellent light‐harvesting capabilities in the visible and near‐infrared regions due to the inclusion of planar π‐conjugated ligands and d‐π‐conjugated planar metal nodes. ^[6] Despite the great interest, 2D c‐MOFs as direct photocathodes for PEC hydrogen generation have not been explored so far due to the lack of active sites. The currently reported 2D c‐MOFs are exclusively composed of sp² hybridized carbon atoms that are not effective for assisting photogenerated exciton separation and migration.

Herein, we propose a design strategy and incorporate acetylene units (−C≡C−), known as efficient active sites for PEC hydrogen generation,[ ⁶ⁱ , ⁷ ] into sp²‐carbon hybridized 2D c‐MOFs. Thus, we develop the first sp‐carbon embedded 2D c‐MOF (Cu₃HHAE₂) single crystals via a coordination reaction between a hexahydroxyarylene‐ethynylene macrocycle ligand (HHAE) and Cu²⁺ salt. According to the continuous rotation electron diffraction (cRED) and high‐resolution transmission electron microscopy (HRTEM) analysis, Cu₃HHAE₂ exhibits a fully eclipsed layer stacking structure with an interlayer distance of 0.32 nm. Notably, the Cu₃HHAE₂ features a hierarchical pore structure with a kagome (kgm) geometry comprising hexagonal (2.20 nm) and triangular (0.30 nm) dual pores. The Cu₃HHAE₂ shows a broad light‐harvesting capability with an optical gap of 0.87 eV and possesses the HOMO and LUMO levels of −5.26 eV and −4.39 eV, respectively, which are highly suitable for the PEC hydrogen evolution reaction (HER). Upon integration as a photocathode, the Cu₃HHAE₂ displays a high photocurrent density up to 260 μA cm⁻² at 0 V vs. reversible hydrogen electrode (RHE), which is one of the highest among the cocatalyst‐free organic photocathodes reported to date. By combining DFT calculations and electrochemical operando resonance Raman spectroscopy, we further demonstrate that the C≡C bonds serve as the active sites for hydrogen evolution. This work brings up a new design for unique acetylene functionalized dual‐pore 2D c‐MOF and demonstrates the possibility of 2D c‐MOFs for highly‐efficient photoelectrochemical catalytic conversions.

HHAE ligand was synthesized via acyclic diyne metathesis macrocyclization with a multidentate triphenolsilane‐based Mo^VI carbyne complex as the catalyst, according to the literature. ^[7d] According to the DFT calculation, the incorporation of electron‐deficient acetylene units could reduce the electron density at the metal‐binding site, which would increase the acidity of the coordination group (−OH) and the reversibility of the metal‐ligand bond during the MOF growth. (Figure S1). ^[8] In our optimized conditions, Cu₃HHAE₂ was synthesized under solvothermal conditions by reacting HHAE with CuSO₄⋅5 H₂O in a 3 : 1 (v/v) H₂O and DMF in a glass vial at 85 °C for 12 hours (Figure 1a). Fourier‐transform infrared (FT‐IR) spectroscopy revealed the almost disappearance of the O−H stretching vibration band from the monomer HHAE in Cu₃HHAE₂ (Figure S2), demonstrating the successful coordination polymerization. Scanning electron microscopy (SEM, Figure 1b and Figure S3) showed hexagonal rods with a length of 1–5 μm.

Figure 1.

a) The synthetic scheme of Cu₃HHAE₂. b) SEM image of Cu₃HHAE₂. c) N₂ adsorption/desorption isotherms at 77.3 K and the corresponding pore size distribution in the inset. The high‐resolution XPS scan of d) C 1s range (after etching) of Cu₃HHAE₂ MOF with area percentages of the deconvoluted peaks is given.

The porosity of Cu₃HHAE₂ was investigated by N₂ adsorption isotherms at 77 K. The sample activated by heating at 90 °C under a dynamic vacuum showed a Brunauer–Emmett–Teller (BET) surface area as high as 801 m² g⁻¹ (Figure 1c). The stacked 2D sheets form 1D hexagonal channels with a pore diameter of ≈2.2 nm, which agrees with the average pore size of 2.16 nm obtained by fitting an N₂ adsorption isotherm to Cu₃HHAE₂. And the triangular pores located in the ligand are too small to be detected. X‐ray photoelectron spectra (XPS) evidenced the presence of C, O, and Cu in the Cu₃HHAE₂ sample. The high‐resolution analysis of the Cu 2p range confirmed a dominant distribution of Cu^II in the sample (Figure S4). The C 1s peaks of Cu₃HHAE₂ in Figure 1d can be deconvoluted into four subpeaks of C−C (sp²) at 284.9 eV, C−C (sp) at 285.6 eV, C−O at 286.9 eV and C=O at 288.9 eV, respectively. The typical binding energy values of sp² and sp hybrid carbon atoms in Cu₃HHAE₂ are consistent with other sp‐carbon embedded organic conjugated materials. ^[9] XPS data show that the samples have both sp² and sp hybrid carbon atoms, and the area ratio of the two peaks is close to 2 : 1, consistent with the chemical structure of Cu₃HHAE₂. Furthermore, the nearly 1 : 1 distribution of C−O and C=O is observed in Cu₃HHAE₂, which is consistent with the reported 2D c‐MOFs with [CuO₄] secondary building units (SBUs). ^[10] Thermogravimetric analysis (TGA) showed that Cu₃HHAE₂ started desolvation over 100 °C and likely decomposition above 180 °C (Figure S5).

The continuous rotation electron diffraction (cRED) technique was performed on Cu₃HHAE₂ single crystals to determine its exact structure. ^[11] The cRED data showed hexagonal symmetry for Cu₃HHAE₂, and the structure solution revealed that Cu²⁺ cations possess square‐planar coordination, forming square‐planar [CuO₄] units with two HHAE ligands. Each trigonal HHAE ligand is further surrounded by three [CuO₄] units to form an infinite planar 2D layer with a honeycomb‐like arrangement (a=b=26.54 Å). Cu₃HHAE₂ possessed a short interlayer distance of 3.21±0.02 Å and a perfect π–π stacking between the HHAE ligands (Figure 2a,b, Figure S6). Powder X‐ray diffraction (PXRD) analysis of Cu₃HHAE₂ revealed a crystalline structure with prominent peaks at 2θ=3.8°, 7.6°, and 10.1°, corresponding to the (100), (200), and (210) reflections, respectively (Figure 2c). In addition, the peak at 2θ=27.9° refers to the π–π stacking (001) reflections. The perfect eclipsed stacking structure of Cu₃HHAE₂ was further confirmed by Rietveld refinement against PXRD, which converged with R _p=3.04 %, and R _wp=5.18 % (Figure 2c). The experimental data of PXRD perfectly agrees with the calculated XRD pattern, which confirms the high phase purity of the Cu₃HHAE₂ sample. High‐resolution transmission electron microscopy (HRTEM) of a tiny crystallite oriented close to the (001) zone axis (Figure 2d) allowed direct imaging of the honeycomb‐like porous network with a=b=26.2 Å and γ=120°, which agrees well with the single‐crystal analysis. Furthermore, the experimental elastically filtered selection area electron diffraction also presented a perfect agreement with the simulation using the structural model derived above (Figure 2e). It should be noticed that Cu₃HHAE₂ is the first case of a kagome 2D c‐MOF with a hierarchical dual‐pore structure. The diameters for the hexagonal and triangle pore are determined as 2.2 and 0.3 nm, respectively.

Figure 2.

a) A portion of the crystal structure along the c direction. b) Views parallel to the ab plane. Single‐crystal structure derived from cRED. c) Overlay of the experimental and Rietveld refinement plots of Cu₃HHAE₂, with profile, weighted profile fitting factors R _p=3.04 %, R _wp=5.81 %, respectively. d) HRTEM image of a Cu₃HHAE₂ (inset: FFT). e) Zero‐loss filtered selection area electron diffraction (SAED) pattern of Cu₃HHAE₂ with simulated powder pattern superimposed.

The four‐probe van der Pauw measurements under vacuum indicated the electrical conductivities at 298 K of the powder pellets as 2.40×10⁻⁴ S m⁻¹ for Cu₃HHAE₂ (Figure 3a). As shown in Figure 3b, Cu₃HHAE₂ presents thermally‐activated charge transport according to the variable‐temperature (VT) conductivity measurements from 200 to 320 K with a hopping activation energy of 0.41 eV, which revealed an apparent semiconducting behavior in Cu₃HHAE₂. The Vis‐NIR spectra of Cu₃HHAE₂ present two kinds of broad absorption bands till 1600 nm. The optical gap estimated from the onset of the near‐infrared region and Tauc plots was ≈0.87 eV (Figure 3c and Figure S7), further suggesting the semiconducting property of bulk Cu₃HHAE₂. Figure 3d and Figure S8 show the ultraviolet photoelectron spectra (UPS) of Cu₃HHAE₂, recorded in both the secondary electron cutoff region (SECO) and the low binding energy region of the highest occupied molecular orbital (HOMO) region. ^[12] The Fermi level and HOMO level of Cu₃HHAE₂ are located at −4.60 and −5.26 eV, respectively. Combined with the UV‐derived optical band gap, the LUMO level of Cu₃HHAE₂ was estimated to be −4.39 eV. In comparison to the energy level of the H₂O/H₂ redox pair (−4.44 eV), the higher‐lying LUMO levels suggest that Cu₃HHAE₂ is a potential candidate for the transfer of photoexcited electrons to H⁺ for H₂ evolution.

Figure 3.

a,b) variable‐temperature electrical conductivities of Cu₃HHAE₂. c) Vis‐NIR absorption spectroscopy of Cu₃HHAE₂. d) UPS results of Cu₃HHAE₂; the left image is the low kinetic energy region of UPS spectra, and the right image is the low binding energy region (HOMO).

According to the detailed characterization of the chemical structure and optoelectronic properties, Cu₃HHAE₂ can serve as a promising cocatalyst‐free organic photocathode to potentially enhance the performance of PEC HER due to its broad light‐harvesting ability and the abundant embedded acetylene as the promising active sites to assist the photogenerated exciton migration. To validate this concept, PEC HER measurement was performed using a three‐electrode electrochemical cell with the prepared Cu₃HHAE₂ or HHAE monomer on Cu foam as working electrode, Ag/AgCl (sat.) as reference electrode, and graphite rode as counter electrode in 0.1 M Na₂SO₄ electrolyte. The linear sweep voltammetry curves recorded under dark and solar irradiation (Figure S9) revealed the apparent photocurrent response of these samples, wherein the recorded dark current density originated from the capacitive behavior of the catalysts and Cu foam current collector. Cu₃HHAE₂ cathode exhibited a more significant photocurrent density up to ≈260 μA cm⁻² recorded at 0 V vs. RHE (Figure 4a), which was 2.6 higher than that of HHAE (≈100 μA cm⁻²) and among the state‐of‐art cocatalyst‐free organic photocathodes like graphdiyne, carbon nitride, covalent organic frameworks, polymers, and organic heterojunction, etc. (<100 uA cm⁻², Table S1).[ ^7a , ^7e , ⁷ⁱ , ¹³ ] Of note, no obvious photocurrent was detected for the Cu foam substrate under identical condition, suggesting that the photocurrent of Cu₃HHAE₂ cathode originates from the Cu₃HHAE₂ MOF (Figure 4a).

Figure 4.

a) Photocurrent‐time plots for Cu₃HHAE₂ and HHAE cathodes at 0 V vs. RHE. On: illumination on; off: illumination off. b) IPCE of Cu₃HHAE₂ and HHAE cathodes under AM 1.5G irradiation. c) Potentiostatic electrochemical impedance spectroscopy (PEIS) plots of Cu₃HHAE₂ and HHAE cathodes at 0 V vs. RHE with and without light irradiation. The inset is the enlarged image. d) Carrier‐separation efficiency (η _sep) of Cu₃HHAE₂ and HHAE cathodes under different applied potentials.

Besides, the higher intrinsic PEC performance of Cu₃HHAE₂ was gleaned from its larger peak incident‐photon‐to‐current efficiency (IPCE) value (0.75 % at 480 nm) compared to HHAE (0.28 % at 420 nm) at 0 V vs. RHE (Figure 4b and Figure S10). Next, the electrochemical impedance analysis was carried out to investigate the charge‐transfer resistance of Cu₃HHAE₂. As shown in Figure 4c, all these electrodes showed a reduced charge‐transfer resistance under light illumination, which is associated with the photoinduced charge carriers under light illumination. Due to the lack of in‐plane d‐π conjugated transport channels, the HHAE cathode exhibited a higher charge‐transfer resistance than Cu₃HHAE₂. These results demonstrated the synergistic contribution of conjugated acetylenic structure and efficient charge transfer pathway to boosting the PEC HER performance of Cu₃HHAE₂ photocathode. Accordingly, the carrier‐separation efficiency of Cu₃HHAE₂ was calculated to be 0.09 %, which is higher than HHAE (0.005 %) (Figure 4d). Thus, a 2D conjugated framework with large acetylenic and metal complexes centers afford Cu₃HHAE₂ 2D c‐MOF with superior PEC HER performance. Furthermore, the long‐term durability of Cu₃HHAE₂ was estimated (Figure S11). Cu₃HHAE₂ exhibited good stability as only a slight decrease of the photocurrent density was observed for Cu₃HHAE₂ after more than 32 hours of solar light irradiation at 0 V vs. RHE, which was likely due to the peeling off of the catalyst from Cu foam induced by the hydrogen evolution. The ex situ SEM, elemental mapping, XRD, XPS, FT‐IR, and Raman spectrum analysis after the long‐term test (Figures S2, S11–14) verified the stable chemical states and crystalline structure of Cu₃HHAE₂.

To verify the role of the sp carbons of Cu₃HHAE₂ in PEC HER, operando electrochemical‐Raman spectroscopy was carried out using an excitation laser line of 594 nm to trace the alterations of acetylene bond strength during the PEC HER. The intrinsic Raman signal (at the open circuit potential) of sp carbons in Cu₃HHAE₂ was shown at 2116 cm⁻¹, which is the typical vibration of sp carbons.[ ^7e , ⁷ⁱ ] As the cathodic current density of Cu₃HHAE₂ gradually increased in a potential scan from 0.7 to 0.0 V vs. RHE, the intensity of the C≡C bond vibrational mode at 2116 cm⁻¹ decreased, and a new broadband correspondingly arose at 2044 cm⁻¹ (Figure 5a, Figure S9c). Notably, the band at 2044 cm⁻¹ reversibly disappeared upon the increase of the potential to 0.7 V vs. RHE, thus assigning it to a transient state of C≡C bonds during the PEC HER process. The shift of the transient Raman active vibration from 2116 to 2044 cm⁻¹ indicates a decrease in the bond strength of the C≡C bond induced by external potential and light irradiation. ^[14] This activated state with enhanced electron density in the antibonding orbital is ideally suited to act as a highly active center for proton adsorption enabling subsequent HER process. ^[7g]

Figure 5.

a) Potential‐dependent Raman spectra of Cu₃HHAE₂ in 0.1 M Na₂SO₄ under 594 nm laser excitation (OCP, open circuit potential). Operando resonance Raman spectra. b) Free‐energy diagram for H₂‐evolution via a single‐site reaction pathway: 1, 2, 3, 4, 5, 6 denote different active sites of Cu₃HHAE₂.

Then, we performed the theoretical calculation to further investigate the hydrogen evolution mechanism, including eight different structural sites and their following HER process (Figure 5b). In order to be an ideal catalyst for HER, the absolute value of Gibbs free energy should be close to 0 eV. ^[15] A section of the pore wall consisting of one full ligand and three half SBUs was cut from the single‐crystal structures and further used for calculations. Indeed, site 1 exhibits a lower absolute value of ΔG _H*(0.17 eV) compared to sites 2–4 (ΔG _H*: 0.33, 0.47, and 0.72 eV), indicating fast HER kinetics on sp carbons. Moreover, the Cu (site 5) and the O (site 6) in SBUs display high free energies for H* formation of 1.52 and 2.24 eV, respectively. Thus, the calculation results further support that the −C≡C− bonds in Cu₃HHAE₂ serve as the active sites for PEC HER.

In conclusion, we reported the first acetylene‐containing 2D c‐MOF based on the hexahydroxyarylene‐ethynylene macrocycle ligand. The chemical structure of Cu₃HHAE₂ single crystals was elucidated by cRED at the atomic level. Cu₃HHAE₂ exhibits a kagome (kgm) geometry with unique dual‐pore structures at the atomic scale. The optoelectronic property studies indicated the apparent semiconducting feature of the MOF with the optical band gap of 0.87 eV, largely due to the localized feature of cross‐conjugation acetylene bonds. The conductive MOF embedded with sp‐hybridized carbon displayed an excellent photocurrent density up to 260 μA cm⁻², which is the first case of conductive MOF as the direct cathode material for PEC hydrogen generation. DFT calculations and operando Raman measurements proved that the active sites actually originated from the acetylene units. This work provides a reliable structure‐photoelectrochemical property relationship for a unique sp‐carbon incorporated MOF and will drive the discovery of conductive MOFs for various photoelectrochemical conversion applications.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supporting Information

Y. Lu, H. Zhong, J. Li, A. M. Dominic, Y. Hu, Z. Gao, Y. Jiao, M. Wu, H. Qi, C. Huang, L. J. Wayment, U. Kaiser, E. Spiecker, I. M. Weidinger, W. Zhang, X. Feng, R. Dong, Angew. Chem. Int. Ed. 2022, 61, e202208163; Angew. Chem. 2022, 134, e202208163.

Data Availability Statement

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