Improved conductivity of a new Co(II)-MOF by assembled acetylene black for efficient hydrogen evolution reaction†

Wei Zhou ^a, Ya-Pan Wu *^a, Xiao Wang ^a, Jun-Wu Tian ^a, Dan-Dan Huang ^a, Jun Zhao ^a, Ya-Qian Lan ^b and Dong-Sheng Li *^a
aCollege of Materials & Chemical Engineering, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang, 443002, China. E-mail: wyapan2008@163.com; lidongsheng1@126.com; Fax: +86 717 6397506
^bSchool of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210046, China

First published on 18th July 2018

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Introduction

The electrocatalytic water splitting reaction has been proven to be an effective strategy for countering the shortfall in energy production.^1,2 Hydrogen is a clean and promising fuel and a potential substitute for petroleum resources. The search for efficient electrocatalysts for the hydrogen evolution reaction (HER) is of great significance.^3–6 Pt or Pt-based materials are the best electrocatalysts for HER. However, due to their high cost and low abundance, they are poorly suited to large-scale application.^7–10 Therefore, the exploration of high-efficiency, low-cost, easily synthesized non-noble metal catalysts is important.

Metal–organic frameworks (MOFs)-based electrocatalysts for HER have been widely explored. Generally, these electrocatalysts can be constructed by (i) using MOFs as precursors to synthesize metal, metal oxide, transition metal phosphide (TMP), and transition metal sulfide (TMS) nanomaterials;^11–13 (ii) exploiting pristine MOFs to catalyze the cracking of water into hydrogen through unsaturated metal atoms and the coupling effect;^14–17 (iii) synthesizing polyoxometalate (POM)-based MOFs to combine the redox nature of the POM moiety and the porosity of a MOF structure;^18–21 or (iv) developing MOFs as a carrier to entrap Pt or Mo-based composites through confinement effects.^22–24 Nevertheless, the poor electrical conductivity of MOFs limits their application in electrocatalysis for HER.²⁵ However, whether being chosen as an intrinsic catalyst or as a catalyst carrier or precursor, they still have the limits of poor conductivity and low active area. Thus, combining the abundant active area of MOFs with superior conductivity could be an effective strategy to attain better electrocatalytic performance.²⁶

In our previous work, we found that the HER activity of 2D layered structure or 3D compact MOFs could be enhanced via the synergistic effect of MOFs and conducting cocatalysts.²⁷ Herein, a new 3D microporous Co(II)-MOF, [Co_1.5(TTAB)_0.5(4,4′-bipy)(H₂O)] (denoted CTGU-9), has been prepared and characterized. This compound features a 3D network with the (4,4,8)-c net topology. CTGU-9 possesses higher intake capacity for CO₂ (21.637 cm³ g⁻¹) than for CH₄. Additionally, by using a facile mechanical grinding method, a series of composites containing CTGU-9 and acetylene black (AB) have been fabricated and examined in the electrocatalytic process for HER. Promisingly, the AB&CTGU-9 (3:4) composites exhibit excellent performance towards HER and demonstrate a low onset potential of 98 mV, an overpotential of 128 mV at 10 mA cm⁻², a small Tafel slope of 87 mV dec⁻¹ and also long-term stability. More importantly, the easy synthesis method of the hybrid MOF-composite materials, which combine abundant active sites and high conductivity, could be a promising strategy to achieve high performance electrochemical materials.

Results and discussion

Crystal structure of CTGU-9

Single-crystal X-ray diffraction analysis shows CTGU-9 is a 3D microporous framework. The asymmetric unit contains one and a half Co²⁺ ions, half of a fully-deprotonated TTAB⁶⁻, one 4,4′-bipy ligand and one coordinated water molecule. As is shown in Fig. 1a, Co1 is ligated by six-coordination by two O atoms (O1, O1#1) of two TTAB⁶⁻ carboxylate groups, two O atoms (O2, O2#1) of coordinated water molecules, and two N atoms (N1, N1#) of two different 4,4′-bipy ligands. Co2 adopts tetrahedral geometry and is coordinated by three O atoms (O4#3, O6 and O6#2) of three TTAB⁶⁻ carboxylate groups, and one N atom (N2) of one 4,4′-dipyridyl molecule. In CTGU-9, the H₆TTAB ligands are completely deprotonated and bound to eight Co^II atoms with a μ₈-bridging coordination mode, forming a 3D framework. Finally, the 4,4′-bipy linkers ensure the necessary metal coordination numbers under the given system as well as stabilizing the 3D network (Fig. S2†) and furnishing the resulting 3D coordination porous networks (Fig. 1b and c). The total accessible volume of the two compounds is ca. 21.5%, which was calculated by using the PLATON program. A topological calculation using the TOPOS software suggests that such a subnet possesses a (4,4,8)-c net topology (Fig. 1d).

Gas sorption properties

CTGU-9 has two dimensional (2D) rectangle shaped channels with dimensions of approximately 13.27 Å × 11.03 Å along the a axis (Fig. 1b) and another channel along the b axis (Fig. S2†). Additionally, the X-ray powder diffraction (PXRD) patterns taken after the gas sorption experiments are shown in Fig. 3 (blue line), revealing the frameworks did not collapse. The BET study (by nitrogen sorption at 77 K) of CTGU-9 shows promisingly high porosity. As shown in Fig. 2b, the experimental results display typical type I behavior, a characteristic of microporous materials. The maximum intake of N₂ at 1 bar for CTGU-9 is 54.162 cm³ g⁻¹ (STP). The BET surface area for CTGU-9 is estimated to be 168.08 m² g⁻¹. The pore-size distributions from the analysis of the N₂ isotherm at 77 K by using the Horvath–Kawazoe (HK) method suggest that the effective pore diameter for desolvated CTGU-9 is 1.9845 nm (Fig. 2). To fully explore the permanent microporosity of the frameworks, the CH₄ and CO₂ adsorption were also tested (Fig. 2c and d). The maximum intake of CO₂ and CH₄ at 1 bar for CTGU-9 is 21.637, 4.021 cm³ g⁻¹ (STP), respectively. We propose that the unique pore structure promotes electrolyte ingress and accommodation of large volume changes.²⁵

FT-IR, PXRD, TEM and thermal analysis

The as-prepared CTGU-9 was also monitored by FTIR spectroscopy, PXRD, and thermogravimetric analysis (TGA). The characteristic absorption bands are mainly attributed to the stretching vibrations of carboxylate groups. As is shown in Fig. S1,† the absorption bands appearing at 3500 cm⁻¹ and 3100 cm⁻¹ are assigned to the O–H stretching vibrations of free lattice water and coordinated water molecules for CTGU-9, respectively. The peaks at 1590 and 1456 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of CO, respectively. Compared with the H₆TTAB carboxyl groups (1716 cm⁻¹), the lower wavenumber of CO implies coordination between carboxyl groups and Co²⁺ ions. The PXRD patterns were recorded to examine the phase purity of CTGU-9 and the change of the structure of the samples before and after electrocatalysis. The experimental patterns are in good agreement with the simulated patterns from crystal diffraction data, indicating the phase purity of the as-synthesized products. Additionally, the PXRD pattern of CTGU-9 after being characterized by linear sweep voltammetry (LSV) for 30 min is also in good agreement with the experimental and synthesized patterns. As is shown in Fig. 3, the AB&CTGU-9 pattern also matches well with the simulated pattern except for a peak at 2θ = 25.5°, which is attributed to the presence of AB. To further investigate the structural stability of CTGU-9, it was imaged by TEM before and after 30 min of use as a water-splitting catalyst (Fig. S6 and S7†). It is clear that the morphology of CTGU-9 after catalysis was not changed significantly. Those results strongly reveal the stability of CTGU-9. The TGA diagram of CTGU-9 was recorded in the temperature range of 30–800 °C (Fig. S3†). The TGA curve of CTGU-9 shows two main steps of weight loss. The first weight loss of 10.05% corresponds to the loss of coordinated water molecules. The second weight loss of 63.89% between 300 and 540 °C is attributed to the loss of the ligand composite (calc. 65.58%). The final residues are composed of metal oxide.

Electrocatalytic properties

The pure CTGU-9 and series of composites fabricated from AB and CTGU-9 were investigated as electrocatalysts for the HER. Note that none of the polarization curves are corrected for IR loss. The electrocatalysts were examined by modifying the samples onto a glassy carbon electrode (GCE) using a three electrode system in 0.5 M H₂SO₄.

The cyclic voltammetry (CV) and LSV polarization curves were obtained to give a preliminary assessment of the materials' electrocatalytic activity. The Tafel slope for commercial 20% Pt/C was 30 mV dec⁻¹, in agreement with values reported in the literature (Fig. 4a).^28–30 As is well known, the unmodified GCE shows poor HER activity. However, the CTGU-9-modified GCE reached a large current density of 10 mA cm⁻² at an overpotential of 424 mV and positive onset potential of 321 mV (Fig. 4a and Table 1). These small values of overpotential and onset potential indicate that CTGU-9 possesses good HER catalytic activity. In order to enhance the conductivity of the pure Co(II)-MOF, for comparison, new composite materials made from CTGU-9 and AB were also studied. As shown in Fig. 4a, the samples with different additive amounts of CTGU-9 show an enhanced electrocatalytic activity. The HER kinetics of the different catalysts was also evaluated by Tafel plots. As the content of AB was increased, the corresponding overpotential and Tafel slope decreased. It was found that the AB&CTGU-9 (3:4) exhibited optimal HER performance among all samples, with a positive onset potential of 98 mV, an overpotential of 128 mV and a small Tafel slope of 87 mV dec⁻¹, the HER reaction process take place via the Volmer–Heyrovsky mechanism (Fig. 4b). Meanwhile, when the amount of AB is 42.8%, the overpotential is minimized (Fig. 4c). The relationship between overpotential and AB additive amount reveals that adding a suitable content of AB can not only improve the conductivity of the AB&MOF composite (in agreement with the electrochemical impedance spectroscopy (EIS) result shown in Fig. S6†) but also enhance the electrocatalytic activity of the pure Co(II)-MOF. Moreover, the current density versus time (i–t) curve for AB&CTGU-9 (3:4) at a current density of 10 mA cm⁻² reveals the catalyst retains long-term stability for at least 21 h (Fig. 4d). Additionally, as is shown in Fig. S5,† the polarization curve of AB&CTGU-9 (3:4) shows very little change versus the original curve even after 2000 cycles, which demonstrates its good stability during the electrocatalytic reaction. To explore the acid durability of the catalysts before and after catalysis, we tested the CTGU-9 samples after LSV for 30 min. The main characteristic peaks after catalysis showed no obvious change. This result also provides evidence for the stability of CTGU-9.

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The electrochemical double-layer capacitance (C_dl) was assessed to explore the electrochemically active surface area, where no apparent faradaic processes occurred.³⁰ CV was carried out at rates varying from 20 to 500 mV S⁻¹ over a potential range of 0.21–0.31 V vs. RHE (Fig. 5). The AB showed a low C_dl of 0.040 mF cm⁻², while the CTGU-9 showed a C_dl of 0.249 mF cm⁻². Meanwhile, the C_dl value of AB&CTGU-9 (3:4) was 0.809 mF cm⁻², which is higher than that of both pure AB and CTGU-9, which reveals that the presence of AB could not only improve the electrical conductivity of CTGU-9 but also reduce resistance to electrolyte ingress and ultimately allow the material to accommodate large volume changes while containing more active sites.

EIS was also performed to analyze the electrocatalytic kinetics on the electrode/electrolyte interface. The Nyquist plots demonstrate that AB&CTGU-9 (3:4) has smaller charge transfer resistance than AB and CTGU-9, which is consistent with the CV and LSV results (Fig. 4a and b). The R(CR)(CR) circuit model includes a solution resistance (Rs), ionic resistance (R₁) caused by the double layer capacitance, and charge transfer resistance (Rct) for the electrochemical reaction. The AB&CTGU-9 (3:4) modified electrode has an Rct of 633 Ω, which is higher than AB (442 Ω) and smaller than CTGU-9 (1335 Ω) (Fig. S4†). All the data strongly confirm that AB&CTGU-9 (3:4) has the fastest electron transfer among all the catalysts during the equivalent HER process.

Finally, a possible mechanism of AB&CTGU-9 for the electrocatalytic HER process is proposed as a combination of two elementary steps, where the first step is the Volmer step: H₃O⁺ + e⁻ → H_ads + H₂O.³¹ As is shown in Fig. S8,† a proton and electron pair (H⁺ + e⁻) is introduced to the catalyst, with H⁺ combining with coordinated water to form H₃O⁺. Subsequently, an adsorbed hydrogen atom (H_ads) is formed. Due to the synergistic effect of AB, the AB&CTGU-9 composite can transport more electrons than CTGU-9 within a given time. The second step is mainly determined by the Volmer–Heyrovsky mechanism. In this situation, the Tafel (2H_ads → H₂) process and Heyrovsky step (H_ads + H₃O⁺ + e⁻ → H₂ + H₂O) occur simultaneously. It is noteworthy that CTGU-9 adopts two types of coordination modes, i.e. the six-coordinated active center and unsaturated coordinated active center, which synergistically catalyze the HER process.

Experimental

Materials and methods

All chemicals were purchased from commercial companies without further purification. The 3,4,5-tricarboxylic-(3′,4′,5′-tricarboxylazophenyl)benzene ligands were purchased from Jinan Camolai Trading Company. Co(NO₃)₂·6H₂O was purchased from Energy Chemical. AB and Nafion were purchased from Alfa Aesar Company. Powder X-ray diffraction (PXRD) was performed on a Rigaku Ultima IV diffractometer (Cu Kα radiation, λ = 1.5406 Å). FT-IR spectra (KBr pellets) were recorded on a Thermo Electron NEXUS 670 FTIR spectrometer in the range of 4000–400 cm⁻¹. Thermogravimetric (TG) curves were recorded on a NETZSCH 449C thermal analyzer with a heating rate of 10 °C min⁻¹ under an air atmosphere.

X-ray crystallography

The diffraction data for CTGU-9 were collected on a Rigaku XtaLAB diffractometer with graphite monochromatic Mo Kα radiation (λ = 0.71073 Å) and were reduced using the program CrystalClear together with an empirical absorption correction. The crystal structure was solved through direct methods and refined based on F² by the full matrix least-squares method using SHELXTL. All non-H atoms were refined anisotropically. The positions of hydrogen atoms attached to carbon atoms were generated geometrically. Crystallographic data and the selected bond lengths and angles for CTGU-9 are summarized in Tables S1 and S2.†

Synthetic procedures

A mixture of Co(NO₃)₂·6H₂O (0.05 mmol, 16.9 mg), 3,4,5-tricarboxyl-(3′,4′,5′-tricarboxylazophenyl)benzene (H₆TTAB, 0.05 mmol, 22 mg), 4,4′-bipyridine (0.05 mmol, 8 mg), 8 mL H₂O, and 0.5 mL NaOH (0.1 mol L⁻¹) was placed in a 25 mL stainless steel autoclave at 140 °C for 4 days, and cooled to room temperature. Purple bulk crystals were obtained and collected after washing with water and ethanol (yield: 52%). Elemental analysis (%): calcd for CTGU-9 (C₁₉H₁₂N₃O₇ Co_1.5): C: 47.28, H: 2.51, N: 8.71; found: C: 47.36, H: 2.65, N: 8.75; FTIR (KBr, cm⁻¹) (Fig. S1 ESI†): 3424 (vs), 2964 (vs), 2916 (m), 2853 (w), 2361 (m), 1618 (s), 1578 (s), 1456 (s), 1419 (s), 1383 (vs), 1045 (m), 800 (w), 723 (m), 662 (w).

Preparation of AB&CTGU-9 composite

The AB powder was boiled and washed in deionized water three times, and then dried in a vacuum oven. Different amounts of AB (10, 20, 30, 40 and 50 mg) were mixed with 40 mg CTGU-9 powder and then ground for 30 min. The as-synthesized samples were fabricated by the above-mentioned process.

Gas sorption measurements

The N₂, CH₄ and CO₂ isotherms were collected using an ASAP 2020 surface area and pore size analyzer in a clean ultra-high-vacuum system. Approximately 200 mg of solvent-exchanged sample was loaded into the sample basket within the adsorption instrument and then degassed under dynamic vacuum at 120 °C for 6 h to obtain the fully desolvated sample. Volumetric N₂, CH₄, and CO₂ sorption measurements were performed over the pressure range 0–1 bar at 77 K for N₂, and at 273 K and 298 K for CH₄ and CO₂.

Electrochemical measurements

Electrochemical measurements were performed with a CHI 660e electrochemical workstation. All measurements were carried out in a three-electrode electrochemical cell at room temperature. A glassy carbon electrode (Ø3 mm) modified with the catalysts was used as the working electrode and a carbon rod as the counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode and the potential was converted to the reversible hydrogen electrode (RHE) via the Nernst equation: E_RHE = E_SCE + (0.059 × pH) + 0.241. The available area of the electrodes for loading the catalyst (CTGU-9 or 20% wt Pt/C) was ∼0.0706 mg cm⁻². Before the electrochemical measurement, the electrolyte (0.5 M H₂SO₄) was degassed by bubbling nitrogen for 30 min. The catalyst ink was prepared by dispersing a certain amount of sample into ethanol/deionized water and sonicated to ensure well-distributed dispersion. Then the catalyst ink was loaded onto a GCE of 3 mm diameter.

Conclusions

In summary, a novel 3D microporous Co(II)-MOF (CTGU-9) has been prepared and characterized. CTGU-9 possesses higher intake capacities for CO₂ than for CH₄. Additionally, introducing AB into the porous Co(II)-MOF not only improves the conductivity of CTGU-9 but also promotes HER performance due to the more efficient charge transport and synergistic interactions between the AB and Co(II)-MOF. The AB&CTGU-9 (3:4) composite possesses the optimal HER activity, with a remarkably low onset potential of 98 mV, an overpotential of 128 mV at 10 mA cm⁻², a small Tafel slope of 89 mV dec⁻¹ and long-term stability in acidic media. The results suggest that this strategy is suitable for preparation of other highly efficient MOF-based catalysts for water splitting.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the NSF of China (No.: 21673127, 21671119, 51572152 and 51502155) and The Fund Program of Cultivating Outstanding Degree Theses of China Three Gorges University (No.: 2018SSPY057).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1583330. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8ce00921j

This journal is © The Royal Society of Chemistry 2018