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Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER
international journal of hydrogen energy xxx (xxxx) xxx
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Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER Qi Qiu 1, Tao Wang 1, Linhai Jing, Kun Huang*, Dabin Qin** Key Laboratory of Chemical Synthesis and Pollution Control of Sichuan Province, School of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637002, China
highlights
graphical abstract
Two 3D porous MOFs 1 and 2 contained
metal
clusters
have
been facilely constructed. A mixed-metal Co/Cu-MOF 3 has been further designed and fabricated for electrocatalytic OER with good performances. Co/Cu-MOF
3
enhanced
HER
shows
markedly
activity
when
compared with that of either 1 or 2.
article info
abstract
Article history:
Tetra-carboxylic acid based 3D porous MOFs 1 and 2, named {[Co2.5(L)]$5H2O}n and 0.5 (L
¼
4,40 -di(ethoxy)biphenyl-3,30 ,5,50 -tetra-(phenyl-4-carboxylic
Received 27 November 2019
{[Cu(L)]$2H2O}n
Received in revised form
bearing metal clusters have been facilely constructed by hydrothermal synthesis. Struc-
acid)),
28 January 2020
tural studies indicate that 1 presents a 3-nodal (4, 4, 8)-connected topology with the point
Accepted 6 February 2020
notation of {44.62}2{48.67.813}, while 2 shows a uninodal (4, 4)-connected network with point
Available online xxx
symbol of {44.62}. The pristine MOFs are directly utilized for electrocatalysis and poor HER activities are obtained in alkaline solution, which promote the further design and fabri-
Keywords:
cation of a mixed-metal Co/Cu-MOF (3). As expected, 3 shows significantly improved
Tetra-carboxylic acid
performances for HER with overpotential of 391 mV (10 mA cm2 current density), low
MOF
Tafel slope of 94 mV dec1 and long-term operation stability (14 h). More importantly, the
Catalysis
direct utilization of 3 for accelerating OER also presents a fascinating performance in
HER
overpotential at 10 mA cm2 current and durability. The above electrocatalytic perfor-
OER
mance of pristine 3 can be ascribed to the result of hybridizing strategy for constructing
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Huang), [email protected] (D. Qin). 1 The authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2020.02.033 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
2
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MOFs under hydrothermal procedure, which may favorably produces synergistic effect and more open metal sites. This work provides in-depth understanding of hybrid pristine MOFs for electrocatalysis. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasing energy demand and growing environmental problems caused an urgent demand for searching clean alternatives as energy carriers alternative to fossil fuels [1,2]. The sustainable and efficient energy conversion and storage systems, including water-splitting devices, renewable fuel cells and metal-air batteries have attracted great attention and wide interests all over the word [3e5]. Among them, water electrolysis has emerged as an attractive strategy for production of clean hydrogen fuel [6,7]. As one of the most challenging tasks of modern electrochemistry, anodic oxygen evolution is identified as the bottleneck of water splitting because it requires a multi-electron transfer process building up of very high potentials [8]. To this end, considerable recent efforts have been devoted to design and develop earthabundant electrocatalysts for the oxygen evolution reaction (OER), for example metal oxides [9e11], perovskites [12], sulfides [13] and phosphides [14]. Nerveless, up to date, IrO2 and RuO2 still take the main roles of benchmark OER catalysts due to their high catalytic natures, though they are not suitable for large-scale applications owing to their valuableness and scarcity [15,16]. On the other hand, efficient electrocatalysts for the cathodic hydrogen evolution reaction (HER) are also required and several kinds of such catalysts have been welldocumented, for instance, Mo-derived materials [17], metal phosphides [18,19] and MOF-based photocatalytic system [20e34]. However, Pt and Pt-based materials are still considered to be the promising electrocatalysts for HER in spite of their valuableness and insufficiency [35]. As stated above, it is of great significance to explore non-precious and efficient OER/HER electrocatalysts with the aim of solving the above problem and realize the practicable applications. By comparison, bifunctional materials for HER and OER share the advantages including simplifying catalyst design and preparation, preventing cross-contamination, etc. [36] Though several such bifunctional electrocatalysts have recently been reported, their development remains in infancy stage due to the above mentioned disadvantages in OER and HER [37e44]. Therefore, low-cost and high-efficiency bifunctional electrocatalysts for HER and OER are highly desired. Metal-organic frameworks (MOFs) have gradually emerged as a novel type of promising materials since 1999 because of their several favorable structural properties such as porosity, structural tunability and diversity [45e48], which allow their extensive applications in conventional catalysis, gas storage/ separation, biomedicine/drug delivery system and luminescence sensors [49e51]. Notably, the utilization of highlyordered pristine MOFs for individually electrocatalytic HER/ OER has gradually become an active topic in recent years
[52e65]. Nevertheless, several factors including low conductivity, low chemical stability in acid/alkali medium, ligands blocked active metal sites, etc., hinder their development because these factors usually caused poor electrocatalytic performances [66e68]. To address those problems, some pristine MOFs have been designed with special architectures to accelerate the HER/OER process and lowering the overpotential, such as metal cluster unit [56], ultrathin nanosheet [57,58], mixed-metal active sites [59e64] and modulating electronic structure [65]. Among them, the strategy of introduction of non-noble mixed-metal cores exhibits intriguing performances due to the probably synergistic effect and rich active sites producing by multi-components surface. So far, a limited number of pristine MOFs can be directly used as bifunctional materials for HER and OER [58,63]. As stated above, it is obviously that most bulk MOFs show unsatisfied electrocatalytic activities, which hinders the direct use of MOFs for electrocatalysis. Though there are some examples of preparations for metal mixed MOFs, limited investigations have been conducted to understand the effect of hybrid metal in MOFs structures on electrocatalytic activities. Hence, the development of pristine MOFs for bifunctional electrocatalysts is in the initial stage. Meanwhile, it is also noted most reported MOFs to date have been tailored in ultrathin nanosheets or powder form, and little endeavors have been made to tune the electrocatalytic centres of bulk MOFs especial those with 3D architectures. Therefore, we developed two examples of 3D porous MOFs (1 and 2) with novel metal clusters [Co5(COO)6] and [Cu2(COO)4] respectively. The unsatisfied HER activities of 1 and 2 promoted the further design and preparation of mixed-metal Co/Cu-MOF (3), which delivered significantly improved HER activity when compared to that of either 1 or 2, with an overpotential of 391 mV for 10 mA cm2 and a Tafel slope of 94 mV dec1. Besides, 3 can also be directly used as an OER catalyst with attractive performance, requiring overpotential of 395 mV for producing 10 mA cm2 current density and delivering a Tafel slope of 94 mV dec1, which are superior to Co3O4 and Pt/C. We anticipate the present work could stimulate the extensive development of bifunctional MOFs for efficient HER and OER electrocatalysts.
Experimental Materials and equipment The tetracarboxyl ligand L was synthesized according to the literature [47]. Co3O4 were purchased from Aladdin Reagent Company (Shanghai, China). All other chemical reagents and
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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solvents were of analytical grade and used directly. IR spectra were recorded on a Nicolet 6700 FTIR spectrometer scanning between 4000 and 400 cm1, using KBr pellets. Powder X-ray diffraction (PXRD) data were obtained on a Rigaku Dmax/Ultima IV diffractometer using Cu Ka radiation (l ¼ 0.15418 nm). Thermogravimetric analyses (TGA) were conducted by using a simultaneous thermal analyzer (Netzsch STA 449 F3) in argon atmosphere. N2 adsorption isotherms at 77 K were performed on an automated gas sorption analyzer (Autosorb-IQ, Quantachrome). Brunauer-Emmett-Teller (BET) method was used to evaluate the specific surface area and pore size distribution. The microstructural images of samples were obtained from a JSM-6510 scanning electron microscope (SEM, JEOL) or Fieldemission scanning electron microscopy (FESEM, Hitachi S4800) coupled with an energy dispersive X-ray spectrometry (EDS). The surface compositions of obtained MOFs were collected on an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi) with an Al Ka irradiation. The contents of Co and Cu in the mixed MOFs were evaluated via inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. Electrochemical behaviors were examined on a CHI 660E electrochemical workstation (CH Instrumental Inc.).
Synthesis of {[Co2.5(L)]·5H2O}n (1) A mixture of L (0.02 mmol, 14.1 mg) and Co(NO3)2$6H2O (0.12 mmol, 34.9 mg) in a solution of CH3CN/H2O (14 mL, v/v ¼ 1/1) was heated at 160 C for 4 days a in Teflon-lined steel bomb. Then red crystals were obtained with a yield of 51.0% (on the basis of L). Anal. Calcd. for C44H38Co2$5O15 (%): C, 55.34; H, 3.98. Found (%): C, 55.82; H, 4.02. IR (KBr, cm1): 3416 (s), 2920 (m), 2851 (m), 1610 (s), 1221 (w), 1079 (w), 957 (w), 788 (w), 716 (w).
Synthesis of 0.5{[Cu(L)]·2H2O}n (2) The target compound was synthesized using the similar procedure to that of 1, except for the use of CuCl2 (0.23 mmol, 40.4 mg) as metal source. Finally, blue crystals were obtained with a yield of 57.4% (on the basis of L). Anal. Calcd. for C20H13CuO6 (%): C, 58.11; H, 3.14. Found (%): C, 58.32; H, 3.22. IR (KBr, cm1): 3417 (s), 1611 (m), 1545 (m), 1405 (s), 785 (w), 714 (w), 515 (w).
Synthesis of Co/Cu-MOF (3) The compound was synthesized according to the above mentioned procedure, except for the use of mixed-metal salts Co(NO3)2,6H2O and CuCl2 (0.12 mmol and 0.12 mmol) as metal sources. The samples were also prepared by using other ratios of mixed-metal salts (nCoðNO3 Þ2 ,6H2 O =nCuCl2 ¼ 1=3; 2=3). IR (KBr, cm1) for 3: 3416 (s), 2920 (m), 2851 (m), 1610 (s), 1221 (w), 1079 (w), 957 (w), 788 (w), 716 (w).
Preparation of working electrode Catalyst powder (5 mg) was dispersed in water-ethanol (5 mL, v/v ¼ 4/1) containing Nafion solution (10 mL, 5 wt% of water), which were sonicated for 30 min to form a homogeneous
3
suspension. Subsequently, the as-prepared sample (5 mL) was drop-coast on the newly polished glassy carbon electrode (GCE, geometric area: 0.07 cm2; loading density: 0.5 mg cm2), which was slowly dried under N2 flow to obtain the modified electrode.
Electrochemical measurements All electrochemical measurements were carried out on a CHI660E electrochemical workstation using a three-electrode configuration in 1 M KOH electrolyte, in which a platinum wire was served as counter electrode and an Ag/AgCl electrode were employed for reference electrode. Working electrode was obtained as above mentioned method. The catalysts were electrochemically preconditioned with 10 cyclic voltammetric scans to reach a steady state before LSV tests. All measured potentials were converted to the reversible hydrogen electrode (RHE) potential based on equation E (RHE) ¼ E (Ag/AgCl) þ 0.059 pH þ 0.197 V. Electrochemical impedance spectra (EIS) were recorded at the open circuit potential in potentiostatic mode in a frequency range from 100 kHz to 0.1 Hz with an AC amplitude of 5 mV. The charging current versus the scan rate was directly proportional to Cdl, and the electrochemical active surface area could be evaluated from the slope.
X-ray crystallography The X-ray single crystal data of 1 and 2 were obtained from a Bruker SMART APEX-II CCD X-ray diffractometer equipped with a graphite monochromatic Mo Ka radiation (l ¼ 0.71073 A) at 273 K. Crystal structures were determined by direct methods with further full matrix Least-squares refinement (SHELXL-97). The non-hydrogen atoms were refined with anisotropic displacement parameters and all the hydrogen atom positions were placed in idealized positions with a riding model. The corresponding data of 1 and 2 were provided in Table S1 and Table S2. The CCDC deposition numbers for 1 and 2 were assigned as 1845905 and 1845907, respectively.
Results and discussion The structure of ligand 4,4′-di(ethoxy)biphenyl-3,3′,5,5′tetra-(phenyl-4-carboxylic acid (L)) was determined by NMR spectra in Fig. S1. MOFs 1, 2 and 3 were prepared via the solvothermal synthesis method. Interestingly, the ethyl groups in the organic ligand have dropped off during the preparation of 2, which may be favorably for improving its porosity. As seen in Fig. S2, IR spectra exhibit CeH (Et-) stretching vibrations of 1 locate in the region of 2975e2845 cm1, while no such stretching vibrations of 2 can be observed, demonstrating the disappearance of ethyl group in MOF 2. Fig. S3 presents the powder X-ray diffraction (PXRD) data of 1 and 2, which suggests that peak positions of all samples are in excellent agreement with those of simulated patterns, demonstrating the phase purity of synthesized samples.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 1 e (a) Representation of the coordination environment of the Co2þ center. (b) The butterfly-shaped metallic core SBU. (c) 3D framework of 1. (d) Topology network of 1.
Crystal structure of [Co2.5(L)]·4H2O}n (1) Single-crystal X-ray diffraction analyses reveal that 1 crystallizes in orthorhombic system with C12/c1 space group and two coordination modes of Co2þ can be observed in the asymmetrical unit. Fig. 1a shows Co2 coordinated with six oxygen atoms from six L ligands, while Co1 and Co3 are connected to four oxygen atoms from four L ligands and two oxygen atoms from two coordination water molecules, respectively. Then each Co2 is linked to two Co1 ions and two Co3 ions via six oxygen atoms of carboxylic acid to form a metal cluster [Co5(COO)6] containing four quadrangles frameworks. The metal cluster is further connected with organic ligands to form a butterfly-shaped metallic core as secondary building units (SBUs), in which the organic ligands occupy a wing tip position while the two coordinated O atoms were presented as antennae (Fig. 1b). Then the SBUs is connected by the backbone of eight-connected L ligands to generate a supramolecular building block (SBB) with numerous cavities (17.6197 (12) A 12.4107 (16) A), which fabricates a complicated three-dimensional (3D) framework (Fig. 1c). Topologically, structure of 1 can be simplified as a 3€ lfli notation nodal (4, 4, 8)-connected framework with the Scha of {44.62}2{48.67.813} as depicted in Fig. 1d.
Crystal structure of 0.5{[Cu(L)]·2H2O}n (2) Single-crystal X-ray diffraction analyses indicate 2 crystallizes in a monoclinic space group C12/m1. A paddlewheel [Cu2(COO)4] cluster (SBU) linked to four organic ligands and coordinated to two water molecules can be observed in the
structure, in which the water molecules may be easily removed to form unsaturated open metal sites (Fig. 2a). The critical bond distances in the SBU is measured as 2.607 A for Cu…Cu and 1.961 A for CueOCO, respectively, which are consistent with that reported in literatures [47,69]. Then [Cu2(COO)4] cluster is further linked by organic ligands to construct a highly porous framework with 3D interconnected channels along the [001] and [010] directions (Fig. 2b and c). Topologically, 2 can be rationalized as an uninodal (4, 4)connected network with point symbol {44.62} in the case that [Cu2(COO)4] cluster is considered to be 6-connector (Fig. 2d).
Characterization of morphology and structure The microstructures and morphologies of as-synthesized MOFs were examined through SEM. As shown in Fig. 3 and Fig. S4, accumulation mode is observed in crystals of 1e3 via low-magnified SEM images. The lumps are composed by a large amount of irregular bulks, which can be observed with rough surface and unequal sizes in high-magnification SEM images. Overall, crystals of 3 exhibits relatively regular shape compared to that of 1 and 2, however, they all present as bulk crystals. As depicted in Fig. 3d, the EDS analyses of 3 reveal that Co and Cu as well as C and O elements coexist in the sample. And the four elements exhibit a homogeneous distribution in the irregular bulks (Fig. 3e). Furthermore, EDS analysis of distinct regions from the same sample (3) was performed to assess the metal distribution at the micrometer scale (Fig. S5). The data also confirms the presence of the two metallic elements (Co and Cu). Besides, a nonnegligible degree of deviation can be observed between the two regions when
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 2 e (a) Coordination environment of Cu2þ ions in 2. (b) 2D network of 2. (c) 3D structure of 2. (d) Topology graph of 2.
EDS mapping were adopted to assess metal distribution of different regions (Table S3), suggesting the presence of all the metals expected in samples but with different distributions of the metals in each crystal, which was similar to that described in previous report [70]. In addition, the ICP-AES analysis demonstrates the Co/Cu molar ratio in sample 3 is about 3.42/ 1, demonstrating the successful preparation of Co/Cu-MOF. The electronic structure and surface composition of Co/CuMOF were analyzed by XPS. Fig. 4a shows the presence of obvious signals of C, O, Co and Cu elements in Co/Cu-MOF sample. As displayed in Fig. 4b, the high-resolution XPS spectrum of C 1s is clearly observed, which can be deconvoluted into three peaks centered at 288.85 eV, 286.69 eV and
284.75 eV, corresponding to CeO, O]CeO and CeC, respectively. As can be seen from Fig. 4c, three deconvoluted peaks in high-resolution XPS spectrum of O 1s can be observed, correlating to function groups of CeO (533.27 eV), O]CeO (531.82 eV), and CoeO (530.86 eV). Fig. 4d shows the highresolution XPS spectrum of Co 2p with binding energies of Co 2p 3/2 and Co 2p 1/2 located at 781.95 and 797.82 eV, respectively, which keep agreement with the features of Co(II) [71]. Fig. 4e shows the deconvolution of the high-resolution Cu 2p spectrum results in two peaks (934.39 and 954.22 eV), which confirms the existence of Cu(II). Besides, distinct satellite peaks are observed in the spectra of Co 2p and Cu 2p at roughly 6 eV above their principal peaks [72,73], further
Fig. 3 e (aec) FESEM (d) EDS spectrum and (e) EDS mapping images of 3.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 4 e XPS spectra of 3. (a) Survey, (b) C1s, (c) O1s, (d) Co2p, (e) Cu2p.
demonstrating the co-existence of Co(II) and Cu (II) species in Co/Cu-MOF. Furthermore, XPS analyses for 1 and 2 were also investigated and the data were provided in Figs. S6eS7, which confirmed the existence of Co(II) in 1 and Cu(II) in 2.
TGA and N2 adsorption TGA for all samples were carried out to evaluate the stability of the MOFs in the temperature range of 30e600 C (Fig. S8). The thermogravimetric curves show that 1 loses a weight of 6.6% (calcd. 6.6%) from 30 to 160 C owing to the residual water in crystals, and loses 4.8% weight (calcd 4.4%) further corresponding to two coordinated water molecules in temperature range from 160 to 430 C, then begins to collapse. While 2 loses 4.4% (calcd. 4.1%) of its weigh from 60 to 180 C due to the departure of two coordinated molecules, following with no obvious loss of weight from 180 to 310 C, which decomposes beyond 310 C. Sample 3 exhibits obvious decomposition around 340 C. The results indicate the excellent stabilities of samples 1e3. N2 adsorption-desorption isotherms for 1 and 2 were recorded at 77 K as depicted in Fig. S9. Accordingly, the Brunauer-Emmett-Teller (BET) surface areas and pore volumes are calculated to be 39.869 cm2 g1 and 0.039 cm3 g1 for 1, 59.019 cm2 g1 and 0.066 cm3 g1 for 2, respectively, indicating their structural porosities (Fig. S10).
Electrocatalytic performance for HER Electrocatalytic HER behaviors of as-prepared samples were investigated in KOH solution (1 M) through a standard threeelectrode system at a scan rate of 5 mV s1. The polarization profiles were acquired from LSV measurements. As shown in Fig. 5a, 1 and 2 exhibit catalytic HER activities with high onset potentials of 400 and 411 mV, and overpotentials of 567 and
585 mV at current density of 10 mA cm2 (J10) respectively, showing inefficient electrocatalytic performances. To obtain more satisfied catalytic activity, a mixed-metal Co/Cu-MOF (3) was further prepared and its electrochemical measurements deliver low onset potential of 300 mV, overpotential of 391 mV at J10 value, and Tafel slope of 94 mV dec1 (Fig. 5a and b). In addition, the values of charge-transfer resistance (Rct) corresponded to the electrocatalytic kinetics of those catalysts were further evaluated by calculating the semicircle diameter in the Nyquist slope. The results show 3 presents the lowest value of Rct, suggesting an enhanced charge transfer process and a faster reaction rate (Fig. 5c). The above results clearly confirm the enhanced HER activity of 3 compared to that of 1 and 2. For further comparison, the electrocatalytic properties of mechanical mixed samples (50 wt% 1 and 50 wt% 2, named sample 4) were also examined under the same conditions but poor activity for HER can be observed (Fig. 5aec). As depicted above, it is suggested that the hydrothermal method is crucial for improving their HER activity during the samples preparation, which may produce more exposed metal sites, larger surface area and higher electrical conductivity in comparison with those of mechanical grinding samples [74]. To better understand the effect of Co/Cu content on HER activity, altered molar ratios of Co/Cu nitrate in hydrothermal procedures were implemented to produce MOFs with different Co/Cu content and their electrocatalytic properties were also investigated. As presented in Fig. S11, their electrocatalytic performances were evaluated by LSV measurements, Tafel slopes and charge-transfer resistance. The results indicate the HER activity enhanced significantly with the increase of Co/Cu molar ratios from 0.51 to 3.42 and the best activity for HER can be obtained when the Co/Cu molar ratio reaches 3.42 (MOF 3) (Table S4). It is hypothesized the synergistic effect associated with the mixed-metal interfaces and metal ratios should be
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 5 e (a) HER polarization curves. (b) Tafel slopes. (c) Nyquist plots of 1e4. (d) Estimated Cdl of 1e4. (e) Time-dependent current density curve of 3 at a fixed potential of 380 mV. (f) pH-dependent tests. 3: Co/Cu-MOF; 4: a mixture of 50 wt% 1 and 50 wt% 2.
ascribed to the enhanced catalytic activity, which may benefit from more open metal sites as well as more effective electron transport channels by the mixed Co/Cu ions doped mode [59e64]. To get a full knowledge of the enhanced HER activity observed in mixed-metal Co/Cu-MOF, electrochemical surface areas (ECSA) were conducted and analyzed through electrochemical double-layer capacitances (Cdl) in a non-Faradaic potential window (0.2 to 0.1 vs RHE) by using the cyclic voltammograms at varying scan rates from 10 to 50 mV s1. As depicted in Fig. 5d and Fig. S12, the plots of scan rate vs current density are observed with good linear relationships. According to the data, the Cdl value for 3 is calculated to be 7.08 mF cm2, which is the largest among all tested samples, suggesting the largest surface area of 3 during the HER process [8]. I-t profiles show a negligible drop of 3 in catalytic current density for continuous test of 14 h (remaining 90.5%), further demonstrating their excellent durability (Fig. 5e). Additionally, pH-dependent HER activity of 3 was examined in alkaline solutions at different pH values. As illustrated in Fig. 5f, the HER activity of 3 enhances with the increase of pH values, suggesting the HER performance is pH-dependent and favors more alkaline KOH solution, which is similar to our previous works [55].
Electrocatalytic activities for OER Electrochemical behaviors of 1e3 for OER were conducted in O2-saturated 1 M KOH solution using a standard threeelectrode system. By comparison, the OER activities of blank GCE, commercial IrO2 and Co3O4 (with same mass loading) were subsequently evaluated under the identical conditions for the reason Co3O4 and IrO2 are considered to be excellent OER catalysts in alkaline medium [75]. As depicted in Fig. 6a, 1 and 3 exhibits onset overpotentials (320 mV/339 mV)
comparable to those of Co3O4 (346 mV) and IrO2 (334 mV), presenting high catalytic OER activities among all the samples. Meanwhile, the required overpotentials of 1 and 3 to reach the current density of 10 mA cm2 are 380 mV and 395 mV respectively, which have obvious advantage when compared with that of IrO2 (412 mV) and some Co-based organic/inorganic catalysts as depicted in Table S5. Fig. 6b shows that the Tafel slope values of 1 and 3 (49 and 94 mV dec1) are much lower even compared to that of IrO2 (85 mV dec1) and Co3O4 (132 mV dec1), suggesting favorable kinetics of 1 and 3 for electrochemical OER process. The poor performances of commercial IrO2 and Co3O4 towards OER are analogues to those reported in literatures [76e78]. It should be noted that compound 2 exhibits the poorest activity for catalytic OER (onset overpotential: 443 mV, overpotential at 10 mA cm2: 731 mV), as a result, the relatively poor OER performance of 3 compared to 1 may be ascribed to the hybrid of 2. Obviously, the blank GCE exhibits negligible catalytic activity. Additionally, the electrochemical impedance spectroscopy (EIS) measurements were implemented to study the electrode kinetics at the overpotential of 1.55 V (Fig. 6c). The results show the charge transfer resistance (Rct) value 3 according to the semicircle at high frequency region compares favorably to that of IrO2, suggesting a faster electron transfer of them during the electrochemical OER. Moreover, the ECSA of 1e3 were evaluated from the calculation of Cdl values in a non-Faradaic potential window (1.18e1.32 vs RHE) by using the cyclic voltammograms at varying scan rates from 10 to 50 mV s1 (Fig. S13). The plots of scan rate vs current density are provided in Fig. 6d and good linear relationships can be observed with respective Cdl values for 1e3 proportional to ECSA of electrocatalyst of 83.5, 0.488 and 44.3 mF cm2, suggesting more exposed active sites and stabilities of 1 and 3 during OER process [13,76].
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 6 e (a) OER polarization profiles; (b) Tafel slopes of samples according to polarization curves; (c) Nyquist slopes for samples, inset: partial enlarged view; (d) Estimated Cdl of 1e3.
In addition, multi-current processes of samples 1 and 3 were examined with an increment of 10 mA cm2 per 200 s at current density ranging from 10 to 80 mA cm2 (Fig. 7). The sharp increases of potentials is elicited by increasing the current density but immediately remains almost unchanged, indicating their outstanding stabilities as well as good mass transportation, conductivity and mechanical robustness, which can also be demonstrated by chronopotentiometric durability tests under constant potential of 1.729 V and 1.886 V, 1.666 V and 1.770 V for 1 and 3 respectively as shown
in Fig. S14. Subsequently, potentiostatic electrolysis was further conducted in 1 M KOH so as to estimate stabilities of 1 and 3 during OER process (Fig. 8a and b). I-t profiles show a negligible drop of 3 in catalytic current density for continuous test of 14 h, further demonstrating their excellent durability. Furthermore, time-dependent current density curve of 3 was investigated in 1 M KOH. The result indicates the current density has a decrease of about 18.2% in comparison with the initial current density after 14 h test, which may be ascribed to a slight detachment of catalysts from
Fig. 7 e Multistep current chronopotentiometric curves of (a) 1 and (b) 3 in 1 M KOH with an increment of 10 mA cm¡2 per 200 s in the current density range of 10e80 mA cm¡2.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 8 e Chronopotentiometric durability experiment of (a) 1 and (b) 3 at a constant potential of 1.595 V and 1.635 V in 1 M KOH; OER polarization profiles of (c) 1 and (d) 3 in KOH solutions under different pH.
electrode, demonstrating a good durability of 3 (Fig. 8b). After that, the sample were analyzed by XPS, and the results demonstrate the existence of Co(II) and Cu(II) (Fig. S15). Meanwhile, the XRD pattern furtherly shows its unaltered crystal structure (Fig. S16). The above facts clearly demonstrate the stability of MOF 3 during eletrocatalytic process. Moreover, the pH-dependent OER activities of 1 and 3 were examined in alkaline solutions at different pH values. As shown in Fig. 8c and d, the electrocatalytic OER activities of 1 and 3 get significantly improved with the increase of pH values, suggesting the improving OER performance is favorably to a KOH solution with higher pH values, which are in a good agreement with literatures [53,54,71]. To deeply understand the stabilities of MOFs in experimental conditions, PXRD profiles of 1 and 3 immersed in 1 M KOH were recorded under different time and pH conditions, respectively. As shown in Figs. S17eS18, PXRD profiles of 1 and 3 have no obvious changes after the above test, revealing their retained crystallinity, which further demonstrates their good stabilities.
Conclusion In conclusion, two new MOFs 1 and 2 with 3D porous structures have been prepared and well characterized. 1 presents a butterfly-shaped metallic core secondary building units [Co5(COO)6], while 2 forms a paddlewheel like Cu2(COO)4 cluster. Electrocatalytic tests show both 1 and 2 present poor
HER activities. As a result, a mixed-metal Co/Cu-MOF (3) has been further prepared for HER with overpotential of 391 mV (10 mA cm2) and Tafel slope of 94 mV dec1, which delivers enhanced electrocatalytic performances compared to that of 1 and 2. Meanwhile, both 1 and 3 exhibit attractive performances in OER activities with respective overpotentials of 380 mV and 395 mV (10 mA cm2), Tafel slope of 49 mV dec1 and 94 mV dec1, and long operation stability in alkaline conditions, which can compare favorably with that of some reported OER benchmarks. The results indicate that 3 shows great potential as high-performance bifunctional electrocatalyst for HER and OER, which may promote the extensive exploits of pristine MOFs as electrocatalysts.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21671159), Meritocracy Research Funds of China West Normal University (17YC030) and Doctoral Scientific Research Start-up Foundation of China West Normal University (18Q022).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.02.033.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Available online at www.sciencedirect.com
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Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER Qi Qiu 1, Tao Wang 1, Linhai Jing, Kun Huang*, Dabin Qin** Key Laboratory of Chemical Synthesis and Pollution Control of Sichuan Province, School of Chemistry and Chemical Engineering, China West Normal University, Nanchong, 637002, China
highlights
graphical abstract
Two 3D porous MOFs 1 and 2 contained
metal
clusters
have
been facilely constructed. A mixed-metal Co/Cu-MOF 3 has been further designed and fabricated for electrocatalytic OER with good performances. Co/Cu-MOF
3
enhanced
HER
shows
markedly
activity
when
compared with that of either 1 or 2.
article info
abstract
Article history:
Tetra-carboxylic acid based 3D porous MOFs 1 and 2, named {[Co2.5(L)]$5H2O}n and 0.5 (L
¼
4,40 -di(ethoxy)biphenyl-3,30 ,5,50 -tetra-(phenyl-4-carboxylic
Received 27 November 2019
{[Cu(L)]$2H2O}n
Received in revised form
bearing metal clusters have been facilely constructed by hydrothermal synthesis. Struc-
acid)),
28 January 2020
tural studies indicate that 1 presents a 3-nodal (4, 4, 8)-connected topology with the point
Accepted 6 February 2020
notation of {44.62}2{48.67.813}, while 2 shows a uninodal (4, 4)-connected network with point
Available online xxx
symbol of {44.62}. The pristine MOFs are directly utilized for electrocatalysis and poor HER activities are obtained in alkaline solution, which promote the further design and fabri-
Keywords:
cation of a mixed-metal Co/Cu-MOF (3). As expected, 3 shows significantly improved
Tetra-carboxylic acid
performances for HER with overpotential of 391 mV (10 mA cm2 current density), low
MOF
Tafel slope of 94 mV dec1 and long-term operation stability (14 h). More importantly, the
Catalysis
direct utilization of 3 for accelerating OER also presents a fascinating performance in
HER
overpotential at 10 mA cm2 current and durability. The above electrocatalytic perfor-
OER
mance of pristine 3 can be ascribed to the result of hybridizing strategy for constructing
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Huang), [email protected] (D. Qin). 1 The authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2020.02.033 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
2
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MOFs under hydrothermal procedure, which may favorably produces synergistic effect and more open metal sites. This work provides in-depth understanding of hybrid pristine MOFs for electrocatalysis. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasing energy demand and growing environmental problems caused an urgent demand for searching clean alternatives as energy carriers alternative to fossil fuels [1,2]. The sustainable and efficient energy conversion and storage systems, including water-splitting devices, renewable fuel cells and metal-air batteries have attracted great attention and wide interests all over the word [3e5]. Among them, water electrolysis has emerged as an attractive strategy for production of clean hydrogen fuel [6,7]. As one of the most challenging tasks of modern electrochemistry, anodic oxygen evolution is identified as the bottleneck of water splitting because it requires a multi-electron transfer process building up of very high potentials [8]. To this end, considerable recent efforts have been devoted to design and develop earthabundant electrocatalysts for the oxygen evolution reaction (OER), for example metal oxides [9e11], perovskites [12], sulfides [13] and phosphides [14]. Nerveless, up to date, IrO2 and RuO2 still take the main roles of benchmark OER catalysts due to their high catalytic natures, though they are not suitable for large-scale applications owing to their valuableness and scarcity [15,16]. On the other hand, efficient electrocatalysts for the cathodic hydrogen evolution reaction (HER) are also required and several kinds of such catalysts have been welldocumented, for instance, Mo-derived materials [17], metal phosphides [18,19] and MOF-based photocatalytic system [20e34]. However, Pt and Pt-based materials are still considered to be the promising electrocatalysts for HER in spite of their valuableness and insufficiency [35]. As stated above, it is of great significance to explore non-precious and efficient OER/HER electrocatalysts with the aim of solving the above problem and realize the practicable applications. By comparison, bifunctional materials for HER and OER share the advantages including simplifying catalyst design and preparation, preventing cross-contamination, etc. [36] Though several such bifunctional electrocatalysts have recently been reported, their development remains in infancy stage due to the above mentioned disadvantages in OER and HER [37e44]. Therefore, low-cost and high-efficiency bifunctional electrocatalysts for HER and OER are highly desired. Metal-organic frameworks (MOFs) have gradually emerged as a novel type of promising materials since 1999 because of their several favorable structural properties such as porosity, structural tunability and diversity [45e48], which allow their extensive applications in conventional catalysis, gas storage/ separation, biomedicine/drug delivery system and luminescence sensors [49e51]. Notably, the utilization of highlyordered pristine MOFs for individually electrocatalytic HER/ OER has gradually become an active topic in recent years
[52e65]. Nevertheless, several factors including low conductivity, low chemical stability in acid/alkali medium, ligands blocked active metal sites, etc., hinder their development because these factors usually caused poor electrocatalytic performances [66e68]. To address those problems, some pristine MOFs have been designed with special architectures to accelerate the HER/OER process and lowering the overpotential, such as metal cluster unit [56], ultrathin nanosheet [57,58], mixed-metal active sites [59e64] and modulating electronic structure [65]. Among them, the strategy of introduction of non-noble mixed-metal cores exhibits intriguing performances due to the probably synergistic effect and rich active sites producing by multi-components surface. So far, a limited number of pristine MOFs can be directly used as bifunctional materials for HER and OER [58,63]. As stated above, it is obviously that most bulk MOFs show unsatisfied electrocatalytic activities, which hinders the direct use of MOFs for electrocatalysis. Though there are some examples of preparations for metal mixed MOFs, limited investigations have been conducted to understand the effect of hybrid metal in MOFs structures on electrocatalytic activities. Hence, the development of pristine MOFs for bifunctional electrocatalysts is in the initial stage. Meanwhile, it is also noted most reported MOFs to date have been tailored in ultrathin nanosheets or powder form, and little endeavors have been made to tune the electrocatalytic centres of bulk MOFs especial those with 3D architectures. Therefore, we developed two examples of 3D porous MOFs (1 and 2) with novel metal clusters [Co5(COO)6] and [Cu2(COO)4] respectively. The unsatisfied HER activities of 1 and 2 promoted the further design and preparation of mixed-metal Co/Cu-MOF (3), which delivered significantly improved HER activity when compared to that of either 1 or 2, with an overpotential of 391 mV for 10 mA cm2 and a Tafel slope of 94 mV dec1. Besides, 3 can also be directly used as an OER catalyst with attractive performance, requiring overpotential of 395 mV for producing 10 mA cm2 current density and delivering a Tafel slope of 94 mV dec1, which are superior to Co3O4 and Pt/C. We anticipate the present work could stimulate the extensive development of bifunctional MOFs for efficient HER and OER electrocatalysts.
Experimental Materials and equipment The tetracarboxyl ligand L was synthesized according to the literature [47]. Co3O4 were purchased from Aladdin Reagent Company (Shanghai, China). All other chemical reagents and
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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solvents were of analytical grade and used directly. IR spectra were recorded on a Nicolet 6700 FTIR spectrometer scanning between 4000 and 400 cm1, using KBr pellets. Powder X-ray diffraction (PXRD) data were obtained on a Rigaku Dmax/Ultima IV diffractometer using Cu Ka radiation (l ¼ 0.15418 nm). Thermogravimetric analyses (TGA) were conducted by using a simultaneous thermal analyzer (Netzsch STA 449 F3) in argon atmosphere. N2 adsorption isotherms at 77 K were performed on an automated gas sorption analyzer (Autosorb-IQ, Quantachrome). Brunauer-Emmett-Teller (BET) method was used to evaluate the specific surface area and pore size distribution. The microstructural images of samples were obtained from a JSM-6510 scanning electron microscope (SEM, JEOL) or Fieldemission scanning electron microscopy (FESEM, Hitachi S4800) coupled with an energy dispersive X-ray spectrometry (EDS). The surface compositions of obtained MOFs were collected on an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi) with an Al Ka irradiation. The contents of Co and Cu in the mixed MOFs were evaluated via inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis. Electrochemical behaviors were examined on a CHI 660E electrochemical workstation (CH Instrumental Inc.).
Synthesis of {[Co2.5(L)]·5H2O}n (1) A mixture of L (0.02 mmol, 14.1 mg) and Co(NO3)2$6H2O (0.12 mmol, 34.9 mg) in a solution of CH3CN/H2O (14 mL, v/v ¼ 1/1) was heated at 160 C for 4 days a in Teflon-lined steel bomb. Then red crystals were obtained with a yield of 51.0% (on the basis of L). Anal. Calcd. for C44H38Co2$5O15 (%): C, 55.34; H, 3.98. Found (%): C, 55.82; H, 4.02. IR (KBr, cm1): 3416 (s), 2920 (m), 2851 (m), 1610 (s), 1221 (w), 1079 (w), 957 (w), 788 (w), 716 (w).
Synthesis of 0.5{[Cu(L)]·2H2O}n (2) The target compound was synthesized using the similar procedure to that of 1, except for the use of CuCl2 (0.23 mmol, 40.4 mg) as metal source. Finally, blue crystals were obtained with a yield of 57.4% (on the basis of L). Anal. Calcd. for C20H13CuO6 (%): C, 58.11; H, 3.14. Found (%): C, 58.32; H, 3.22. IR (KBr, cm1): 3417 (s), 1611 (m), 1545 (m), 1405 (s), 785 (w), 714 (w), 515 (w).
Synthesis of Co/Cu-MOF (3) The compound was synthesized according to the above mentioned procedure, except for the use of mixed-metal salts Co(NO3)2,6H2O and CuCl2 (0.12 mmol and 0.12 mmol) as metal sources. The samples were also prepared by using other ratios of mixed-metal salts (nCoðNO3 Þ2 ,6H2 O =nCuCl2 ¼ 1=3; 2=3). IR (KBr, cm1) for 3: 3416 (s), 2920 (m), 2851 (m), 1610 (s), 1221 (w), 1079 (w), 957 (w), 788 (w), 716 (w).
Preparation of working electrode Catalyst powder (5 mg) was dispersed in water-ethanol (5 mL, v/v ¼ 4/1) containing Nafion solution (10 mL, 5 wt% of water), which were sonicated for 30 min to form a homogeneous
3
suspension. Subsequently, the as-prepared sample (5 mL) was drop-coast on the newly polished glassy carbon electrode (GCE, geometric area: 0.07 cm2; loading density: 0.5 mg cm2), which was slowly dried under N2 flow to obtain the modified electrode.
Electrochemical measurements All electrochemical measurements were carried out on a CHI660E electrochemical workstation using a three-electrode configuration in 1 M KOH electrolyte, in which a platinum wire was served as counter electrode and an Ag/AgCl electrode were employed for reference electrode. Working electrode was obtained as above mentioned method. The catalysts were electrochemically preconditioned with 10 cyclic voltammetric scans to reach a steady state before LSV tests. All measured potentials were converted to the reversible hydrogen electrode (RHE) potential based on equation E (RHE) ¼ E (Ag/AgCl) þ 0.059 pH þ 0.197 V. Electrochemical impedance spectra (EIS) were recorded at the open circuit potential in potentiostatic mode in a frequency range from 100 kHz to 0.1 Hz with an AC amplitude of 5 mV. The charging current versus the scan rate was directly proportional to Cdl, and the electrochemical active surface area could be evaluated from the slope.
X-ray crystallography The X-ray single crystal data of 1 and 2 were obtained from a Bruker SMART APEX-II CCD X-ray diffractometer equipped with a graphite monochromatic Mo Ka radiation (l ¼ 0.71073 A) at 273 K. Crystal structures were determined by direct methods with further full matrix Least-squares refinement (SHELXL-97). The non-hydrogen atoms were refined with anisotropic displacement parameters and all the hydrogen atom positions were placed in idealized positions with a riding model. The corresponding data of 1 and 2 were provided in Table S1 and Table S2. The CCDC deposition numbers for 1 and 2 were assigned as 1845905 and 1845907, respectively.
Results and discussion The structure of ligand 4,4′-di(ethoxy)biphenyl-3,3′,5,5′tetra-(phenyl-4-carboxylic acid (L)) was determined by NMR spectra in Fig. S1. MOFs 1, 2 and 3 were prepared via the solvothermal synthesis method. Interestingly, the ethyl groups in the organic ligand have dropped off during the preparation of 2, which may be favorably for improving its porosity. As seen in Fig. S2, IR spectra exhibit CeH (Et-) stretching vibrations of 1 locate in the region of 2975e2845 cm1, while no such stretching vibrations of 2 can be observed, demonstrating the disappearance of ethyl group in MOF 2. Fig. S3 presents the powder X-ray diffraction (PXRD) data of 1 and 2, which suggests that peak positions of all samples are in excellent agreement with those of simulated patterns, demonstrating the phase purity of synthesized samples.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 1 e (a) Representation of the coordination environment of the Co2þ center. (b) The butterfly-shaped metallic core SBU. (c) 3D framework of 1. (d) Topology network of 1.
Crystal structure of [Co2.5(L)]·4H2O}n (1) Single-crystal X-ray diffraction analyses reveal that 1 crystallizes in orthorhombic system with C12/c1 space group and two coordination modes of Co2þ can be observed in the asymmetrical unit. Fig. 1a shows Co2 coordinated with six oxygen atoms from six L ligands, while Co1 and Co3 are connected to four oxygen atoms from four L ligands and two oxygen atoms from two coordination water molecules, respectively. Then each Co2 is linked to two Co1 ions and two Co3 ions via six oxygen atoms of carboxylic acid to form a metal cluster [Co5(COO)6] containing four quadrangles frameworks. The metal cluster is further connected with organic ligands to form a butterfly-shaped metallic core as secondary building units (SBUs), in which the organic ligands occupy a wing tip position while the two coordinated O atoms were presented as antennae (Fig. 1b). Then the SBUs is connected by the backbone of eight-connected L ligands to generate a supramolecular building block (SBB) with numerous cavities (17.6197 (12) A 12.4107 (16) A), which fabricates a complicated three-dimensional (3D) framework (Fig. 1c). Topologically, structure of 1 can be simplified as a 3€ lfli notation nodal (4, 4, 8)-connected framework with the Scha of {44.62}2{48.67.813} as depicted in Fig. 1d.
Crystal structure of 0.5{[Cu(L)]·2H2O}n (2) Single-crystal X-ray diffraction analyses indicate 2 crystallizes in a monoclinic space group C12/m1. A paddlewheel [Cu2(COO)4] cluster (SBU) linked to four organic ligands and coordinated to two water molecules can be observed in the
structure, in which the water molecules may be easily removed to form unsaturated open metal sites (Fig. 2a). The critical bond distances in the SBU is measured as 2.607 A for Cu…Cu and 1.961 A for CueOCO, respectively, which are consistent with that reported in literatures [47,69]. Then [Cu2(COO)4] cluster is further linked by organic ligands to construct a highly porous framework with 3D interconnected channels along the [001] and [010] directions (Fig. 2b and c). Topologically, 2 can be rationalized as an uninodal (4, 4)connected network with point symbol {44.62} in the case that [Cu2(COO)4] cluster is considered to be 6-connector (Fig. 2d).
Characterization of morphology and structure The microstructures and morphologies of as-synthesized MOFs were examined through SEM. As shown in Fig. 3 and Fig. S4, accumulation mode is observed in crystals of 1e3 via low-magnified SEM images. The lumps are composed by a large amount of irregular bulks, which can be observed with rough surface and unequal sizes in high-magnification SEM images. Overall, crystals of 3 exhibits relatively regular shape compared to that of 1 and 2, however, they all present as bulk crystals. As depicted in Fig. 3d, the EDS analyses of 3 reveal that Co and Cu as well as C and O elements coexist in the sample. And the four elements exhibit a homogeneous distribution in the irregular bulks (Fig. 3e). Furthermore, EDS analysis of distinct regions from the same sample (3) was performed to assess the metal distribution at the micrometer scale (Fig. S5). The data also confirms the presence of the two metallic elements (Co and Cu). Besides, a nonnegligible degree of deviation can be observed between the two regions when
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Fig. 2 e (a) Coordination environment of Cu2þ ions in 2. (b) 2D network of 2. (c) 3D structure of 2. (d) Topology graph of 2.
EDS mapping were adopted to assess metal distribution of different regions (Table S3), suggesting the presence of all the metals expected in samples but with different distributions of the metals in each crystal, which was similar to that described in previous report [70]. In addition, the ICP-AES analysis demonstrates the Co/Cu molar ratio in sample 3 is about 3.42/ 1, demonstrating the successful preparation of Co/Cu-MOF. The electronic structure and surface composition of Co/CuMOF were analyzed by XPS. Fig. 4a shows the presence of obvious signals of C, O, Co and Cu elements in Co/Cu-MOF sample. As displayed in Fig. 4b, the high-resolution XPS spectrum of C 1s is clearly observed, which can be deconvoluted into three peaks centered at 288.85 eV, 286.69 eV and
284.75 eV, corresponding to CeO, O]CeO and CeC, respectively. As can be seen from Fig. 4c, three deconvoluted peaks in high-resolution XPS spectrum of O 1s can be observed, correlating to function groups of CeO (533.27 eV), O]CeO (531.82 eV), and CoeO (530.86 eV). Fig. 4d shows the highresolution XPS spectrum of Co 2p with binding energies of Co 2p 3/2 and Co 2p 1/2 located at 781.95 and 797.82 eV, respectively, which keep agreement with the features of Co(II) [71]. Fig. 4e shows the deconvolution of the high-resolution Cu 2p spectrum results in two peaks (934.39 and 954.22 eV), which confirms the existence of Cu(II). Besides, distinct satellite peaks are observed in the spectra of Co 2p and Cu 2p at roughly 6 eV above their principal peaks [72,73], further
Fig. 3 e (aec) FESEM (d) EDS spectrum and (e) EDS mapping images of 3.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 4 e XPS spectra of 3. (a) Survey, (b) C1s, (c) O1s, (d) Co2p, (e) Cu2p.
demonstrating the co-existence of Co(II) and Cu (II) species in Co/Cu-MOF. Furthermore, XPS analyses for 1 and 2 were also investigated and the data were provided in Figs. S6eS7, which confirmed the existence of Co(II) in 1 and Cu(II) in 2.
TGA and N2 adsorption TGA for all samples were carried out to evaluate the stability of the MOFs in the temperature range of 30e600 C (Fig. S8). The thermogravimetric curves show that 1 loses a weight of 6.6% (calcd. 6.6%) from 30 to 160 C owing to the residual water in crystals, and loses 4.8% weight (calcd 4.4%) further corresponding to two coordinated water molecules in temperature range from 160 to 430 C, then begins to collapse. While 2 loses 4.4% (calcd. 4.1%) of its weigh from 60 to 180 C due to the departure of two coordinated molecules, following with no obvious loss of weight from 180 to 310 C, which decomposes beyond 310 C. Sample 3 exhibits obvious decomposition around 340 C. The results indicate the excellent stabilities of samples 1e3. N2 adsorption-desorption isotherms for 1 and 2 were recorded at 77 K as depicted in Fig. S9. Accordingly, the Brunauer-Emmett-Teller (BET) surface areas and pore volumes are calculated to be 39.869 cm2 g1 and 0.039 cm3 g1 for 1, 59.019 cm2 g1 and 0.066 cm3 g1 for 2, respectively, indicating their structural porosities (Fig. S10).
Electrocatalytic performance for HER Electrocatalytic HER behaviors of as-prepared samples were investigated in KOH solution (1 M) through a standard threeelectrode system at a scan rate of 5 mV s1. The polarization profiles were acquired from LSV measurements. As shown in Fig. 5a, 1 and 2 exhibit catalytic HER activities with high onset potentials of 400 and 411 mV, and overpotentials of 567 and
585 mV at current density of 10 mA cm2 (J10) respectively, showing inefficient electrocatalytic performances. To obtain more satisfied catalytic activity, a mixed-metal Co/Cu-MOF (3) was further prepared and its electrochemical measurements deliver low onset potential of 300 mV, overpotential of 391 mV at J10 value, and Tafel slope of 94 mV dec1 (Fig. 5a and b). In addition, the values of charge-transfer resistance (Rct) corresponded to the electrocatalytic kinetics of those catalysts were further evaluated by calculating the semicircle diameter in the Nyquist slope. The results show 3 presents the lowest value of Rct, suggesting an enhanced charge transfer process and a faster reaction rate (Fig. 5c). The above results clearly confirm the enhanced HER activity of 3 compared to that of 1 and 2. For further comparison, the electrocatalytic properties of mechanical mixed samples (50 wt% 1 and 50 wt% 2, named sample 4) were also examined under the same conditions but poor activity for HER can be observed (Fig. 5aec). As depicted above, it is suggested that the hydrothermal method is crucial for improving their HER activity during the samples preparation, which may produce more exposed metal sites, larger surface area and higher electrical conductivity in comparison with those of mechanical grinding samples [74]. To better understand the effect of Co/Cu content on HER activity, altered molar ratios of Co/Cu nitrate in hydrothermal procedures were implemented to produce MOFs with different Co/Cu content and their electrocatalytic properties were also investigated. As presented in Fig. S11, their electrocatalytic performances were evaluated by LSV measurements, Tafel slopes and charge-transfer resistance. The results indicate the HER activity enhanced significantly with the increase of Co/Cu molar ratios from 0.51 to 3.42 and the best activity for HER can be obtained when the Co/Cu molar ratio reaches 3.42 (MOF 3) (Table S4). It is hypothesized the synergistic effect associated with the mixed-metal interfaces and metal ratios should be
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 5 e (a) HER polarization curves. (b) Tafel slopes. (c) Nyquist plots of 1e4. (d) Estimated Cdl of 1e4. (e) Time-dependent current density curve of 3 at a fixed potential of 380 mV. (f) pH-dependent tests. 3: Co/Cu-MOF; 4: a mixture of 50 wt% 1 and 50 wt% 2.
ascribed to the enhanced catalytic activity, which may benefit from more open metal sites as well as more effective electron transport channels by the mixed Co/Cu ions doped mode [59e64]. To get a full knowledge of the enhanced HER activity observed in mixed-metal Co/Cu-MOF, electrochemical surface areas (ECSA) were conducted and analyzed through electrochemical double-layer capacitances (Cdl) in a non-Faradaic potential window (0.2 to 0.1 vs RHE) by using the cyclic voltammograms at varying scan rates from 10 to 50 mV s1. As depicted in Fig. 5d and Fig. S12, the plots of scan rate vs current density are observed with good linear relationships. According to the data, the Cdl value for 3 is calculated to be 7.08 mF cm2, which is the largest among all tested samples, suggesting the largest surface area of 3 during the HER process [8]. I-t profiles show a negligible drop of 3 in catalytic current density for continuous test of 14 h (remaining 90.5%), further demonstrating their excellent durability (Fig. 5e). Additionally, pH-dependent HER activity of 3 was examined in alkaline solutions at different pH values. As illustrated in Fig. 5f, the HER activity of 3 enhances with the increase of pH values, suggesting the HER performance is pH-dependent and favors more alkaline KOH solution, which is similar to our previous works [55].
Electrocatalytic activities for OER Electrochemical behaviors of 1e3 for OER were conducted in O2-saturated 1 M KOH solution using a standard threeelectrode system. By comparison, the OER activities of blank GCE, commercial IrO2 and Co3O4 (with same mass loading) were subsequently evaluated under the identical conditions for the reason Co3O4 and IrO2 are considered to be excellent OER catalysts in alkaline medium [75]. As depicted in Fig. 6a, 1 and 3 exhibits onset overpotentials (320 mV/339 mV)
comparable to those of Co3O4 (346 mV) and IrO2 (334 mV), presenting high catalytic OER activities among all the samples. Meanwhile, the required overpotentials of 1 and 3 to reach the current density of 10 mA cm2 are 380 mV and 395 mV respectively, which have obvious advantage when compared with that of IrO2 (412 mV) and some Co-based organic/inorganic catalysts as depicted in Table S5. Fig. 6b shows that the Tafel slope values of 1 and 3 (49 and 94 mV dec1) are much lower even compared to that of IrO2 (85 mV dec1) and Co3O4 (132 mV dec1), suggesting favorable kinetics of 1 and 3 for electrochemical OER process. The poor performances of commercial IrO2 and Co3O4 towards OER are analogues to those reported in literatures [76e78]. It should be noted that compound 2 exhibits the poorest activity for catalytic OER (onset overpotential: 443 mV, overpotential at 10 mA cm2: 731 mV), as a result, the relatively poor OER performance of 3 compared to 1 may be ascribed to the hybrid of 2. Obviously, the blank GCE exhibits negligible catalytic activity. Additionally, the electrochemical impedance spectroscopy (EIS) measurements were implemented to study the electrode kinetics at the overpotential of 1.55 V (Fig. 6c). The results show the charge transfer resistance (Rct) value 3 according to the semicircle at high frequency region compares favorably to that of IrO2, suggesting a faster electron transfer of them during the electrochemical OER. Moreover, the ECSA of 1e3 were evaluated from the calculation of Cdl values in a non-Faradaic potential window (1.18e1.32 vs RHE) by using the cyclic voltammograms at varying scan rates from 10 to 50 mV s1 (Fig. S13). The plots of scan rate vs current density are provided in Fig. 6d and good linear relationships can be observed with respective Cdl values for 1e3 proportional to ECSA of electrocatalyst of 83.5, 0.488 and 44.3 mF cm2, suggesting more exposed active sites and stabilities of 1 and 3 during OER process [13,76].
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 6 e (a) OER polarization profiles; (b) Tafel slopes of samples according to polarization curves; (c) Nyquist slopes for samples, inset: partial enlarged view; (d) Estimated Cdl of 1e3.
In addition, multi-current processes of samples 1 and 3 were examined with an increment of 10 mA cm2 per 200 s at current density ranging from 10 to 80 mA cm2 (Fig. 7). The sharp increases of potentials is elicited by increasing the current density but immediately remains almost unchanged, indicating their outstanding stabilities as well as good mass transportation, conductivity and mechanical robustness, which can also be demonstrated by chronopotentiometric durability tests under constant potential of 1.729 V and 1.886 V, 1.666 V and 1.770 V for 1 and 3 respectively as shown
in Fig. S14. Subsequently, potentiostatic electrolysis was further conducted in 1 M KOH so as to estimate stabilities of 1 and 3 during OER process (Fig. 8a and b). I-t profiles show a negligible drop of 3 in catalytic current density for continuous test of 14 h, further demonstrating their excellent durability. Furthermore, time-dependent current density curve of 3 was investigated in 1 M KOH. The result indicates the current density has a decrease of about 18.2% in comparison with the initial current density after 14 h test, which may be ascribed to a slight detachment of catalysts from
Fig. 7 e Multistep current chronopotentiometric curves of (a) 1 and (b) 3 in 1 M KOH with an increment of 10 mA cm¡2 per 200 s in the current density range of 10e80 mA cm¡2.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Fig. 8 e Chronopotentiometric durability experiment of (a) 1 and (b) 3 at a constant potential of 1.595 V and 1.635 V in 1 M KOH; OER polarization profiles of (c) 1 and (d) 3 in KOH solutions under different pH.
electrode, demonstrating a good durability of 3 (Fig. 8b). After that, the sample were analyzed by XPS, and the results demonstrate the existence of Co(II) and Cu(II) (Fig. S15). Meanwhile, the XRD pattern furtherly shows its unaltered crystal structure (Fig. S16). The above facts clearly demonstrate the stability of MOF 3 during eletrocatalytic process. Moreover, the pH-dependent OER activities of 1 and 3 were examined in alkaline solutions at different pH values. As shown in Fig. 8c and d, the electrocatalytic OER activities of 1 and 3 get significantly improved with the increase of pH values, suggesting the improving OER performance is favorably to a KOH solution with higher pH values, which are in a good agreement with literatures [53,54,71]. To deeply understand the stabilities of MOFs in experimental conditions, PXRD profiles of 1 and 3 immersed in 1 M KOH were recorded under different time and pH conditions, respectively. As shown in Figs. S17eS18, PXRD profiles of 1 and 3 have no obvious changes after the above test, revealing their retained crystallinity, which further demonstrates their good stabilities.
Conclusion In conclusion, two new MOFs 1 and 2 with 3D porous structures have been prepared and well characterized. 1 presents a butterfly-shaped metallic core secondary building units [Co5(COO)6], while 2 forms a paddlewheel like Cu2(COO)4 cluster. Electrocatalytic tests show both 1 and 2 present poor
HER activities. As a result, a mixed-metal Co/Cu-MOF (3) has been further prepared for HER with overpotential of 391 mV (10 mA cm2) and Tafel slope of 94 mV dec1, which delivers enhanced electrocatalytic performances compared to that of 1 and 2. Meanwhile, both 1 and 3 exhibit attractive performances in OER activities with respective overpotentials of 380 mV and 395 mV (10 mA cm2), Tafel slope of 49 mV dec1 and 94 mV dec1, and long operation stability in alkaline conditions, which can compare favorably with that of some reported OER benchmarks. The results indicate that 3 shows great potential as high-performance bifunctional electrocatalyst for HER and OER, which may promote the extensive exploits of pristine MOFs as electrocatalysts.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21671159), Meritocracy Research Funds of China West Normal University (17YC030) and Doctoral Scientific Research Start-up Foundation of China West Normal University (18Q022).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.02.033.
Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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Please cite this article as: Qiu Q et al., Tetra-carboxylic acid based metal-organic framework as a high-performance bifunctional electrocatalyst for HER and OER, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.02.033
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