Journal Pre-proof Conductive and porous ZIF-67/PEDOT hybrid composite as superior electrode for allsolid-state symmetrical supercapacitors Vishal Shrivastav, Shashank Sundriyal, Ashwinder Kaur, Umesh K. Tiwari, Sunita Mishra, Akash Deep PII:
To appear in:
Journal of Alloys and Compounds
Received Date: 10 February 2020 Revised Date:
5 June 2020
Accepted Date: 6 June 2020
Please cite this article as: V. Shrivastav, S. Sundriyal, A. Kaur, U.K. Tiwari, S. Mishra, A. Deep, Conductive and porous ZIF-67/PEDOT hybrid composite as superior electrode for all-solid-state symmetrical supercapacitors, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2020.155992. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Author CRediT Statement Vishal Shrivastav: Experimental, Characterizations, Original draft preparation; Shashank Sundriyal: Experimental protocol, Device fabrication, Original draft preparation; Aswinder Kaur: Characterizations, Material optimization; Sunita Mishra: Supervision, Data interpretation, Writing-Reviewing; Umesh K. Tiwari: Supervision, Data interpretation, WritingReviewing; Akash Deep: Conceptualization, Supervision, Writing-Reviewing and Editing
Conductive and porous ZIF-67/PEDOT hybrid composite as superior electrode for all-solid-state symmetrical supercapacitors
Vishal Shrivastav1,2,$, Shashank Sundriyal1,2,$,#, Ashwinder Kaur1,3, Umesh K. Tiwari1,2, Sunita Mishra1,2, Akash Deep1,2*
CSIR-Central Scientific Instrument Organisation (CSIR-CSIO), Chandigarh 160030, India;
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad- 201002, India;
Department of Physics, Punjabi University, Patiala 147002, India
*Correspondence: [email protected]
(A. Deep), Tel: +91-172-2672236 #
Present Address: CSIR-National Physical Laboratory (CSIR-NPL), New Delhi 110012, India
Both authors contributed equally
ABSTRACT The application of porous and conductive composite materials in energy storage devices has received great attention in the recent years. Porous materials, such as metal-organic frameworks (MOFs), possess features like large specific surface area, mesoporous structures, and multiple reaction sites that make them attractive for energy storage devices. Further, the integration of MOFs with suitable conducting agents is a promising strategy to introduce additional conductive pathways in them, which facilitates fast charge transfer. In the present work, we report synthesis and supercapacitor application of a ZIF-67/PEDOT heterostructure composite. The prepared ZIF-67/PEDOT composite, when analysed for electrochemical performance in three-electrode systems, has delivered a specific capacitance of 106.8 F/g at a current density of 1 A/g. This value is significantly better than that attaintable with only 1
pristine ZIF-67 electrode (34.75 F/g at 1 A/g). Extending the study, two ZIF-67/PEDOT electrodes have been used to assemble an all-solid-state symmetrical supercapacitor, which delivers excellent values of energy density (~11 Wh/kg) and power density (200 W/kg). Hence, this work paves the way for the potential application of ZIF-67/PEDOT composite in the development of next-generation supercapacitors. Keywords: ZIF-67; PEDOT; Symmetrical supercapacitor; Energy storage; Hybrid materials
1. Introduction The demand for clean and alternate energy has rapidly increased in the last decades. This has also catalyzed the development of efficient energy storage devices, e.g., rechargeable batteries and supercapacitors [1-3]. Supercapacitors can yield higher specific capacitance than the conventional capacitor, while also delivering moderate levels of energy and power densities. They are known for better cyclic stability than the batteries [2, 4]. Supercapacitors function primarily on two types of charge storage mechanisms: (i) adsorption of charge (called electric double layer capacitance (EDLC)), and (ii) redox reactions associated with structural and chemical changes (pseudocapacitacne). Generally, the supercapacitor electrodes are fabricated by using carbonaceous materials, metal oxides, sulfide, and conducting polymers. Carbon based electrodes bear high specific surface area to allow efficient storage of electrolyte ions in their porous structure [5, 6]. The oxides, sulfide, and conducting polymers based electrodes work on the principle of faradic reactions at the electrode surface and store the energy via pseudocapacitance [7-9]. The EDLC based electrode materials provide high power density with rapid exchange of adsorbed electrolyte ions, whereas the pseudocapacitive electrode materials deliver high energy density through the exchange of electrons in oxidation/reduction steps [10, 11]. Nonetheless, carbon based electrodes do not show high energy density while oxides / sulphides face a limitation of low
conductivity [12, 13]. Therefore, the development of newer electrode materials for supercapacitors, particularly in the context of addressing the issues of low energy density and poor cyclic life, is a matter of crucial importance. Hence, there is a need to engineer novel electrode materials evincing high surface area and enhanced conductivity. The nature and morphology of the electrode materials are known to greatly influence the superpacitor parameters like energy and power densities [14, 15]. For example, some special morphologies
intercalation/deintercalation rate of ions during the charge storage process . Researchers have established the relatioship between a material’s morphology and the electrochemical performances of the energy storage devices [17, 18]. At times, cumbersome synthesis conditions are needed to obtain desired morphologies [19, 20]. The morphology of polymers is rather easy to control by selecting suitable surfactant, temperature, or reaction time [21, 22]. The use of conducting polymers in the energy storage devices has gained popularity in the recent decades [23-26]. They offer some important benefits (e.g., flexible morphology, high electrical conductivity, and reversible faradaic reactions at the electrode surface) over the oxide and carbon based electrodes [27-29]. Conducting polymers also aid in fast electron transport and efficient charge storage [30, 31]. Among the various conducting polymers, PEDOT (Poly(3,4-ethylene dioxythiophene) is an interesting candidate for the energy storage applications owing to its flexibility and high electrical conductivity (e.g., up to 3400 S cm-1) . PEDOT has been used in photovoltaic cells as well as in integrated solar cell and supercapacitor devices [33, 34]. PEDOT does not possess a high specific surface area, which is a limitation with respect to its supercapacitor application. Metal-organic frameworks (MOFs) are a class of coordination polymer with numerous unique material features, such as high specific surface area, hierarchical pore size distribution, accessible porosity, and post-synthetic modifications [35, 36]. MOFs can be
synthesized with wide ranges of pore size distribution, morphologies, and reactivities . They can also be integrated with other nanomaterials and polymers to realize novel hybrid composites . MOF composites have also been explored as electrodes/electrolyte in the energy storage devices . Zeolitic imidazole frameworks (ZIFs) are one of the most investigated MOF subclass materials due to their stability, successful commercialization, hierarchal pore size distribution, and large specific surface area . ZIFs have been reported for both electro- and photocatalytic applications [41, 42]. Despite above mentioned desirable material features, ZIFs are not electrically conducting. To overcome this limitations, they have been integrated with different other nanomaterials and nanopolymers . For instance, ZIF-8 crystals, integrated with carbon nanotubes (CNTs) or titanium dioxide
nanoparticles, have been found with augmented features, e.g., dielectric permittivity and photocatalytic and antimicrobial activity [44, 45]. ZIF-polymer composites, endowed with large specific surface area, electrical conductivity, and redox activity, can facilitate the development of highly stable and efficient supercapacitors. In one of our previous works, ZIF-67 was mixed with reduced graphene oxide (rGO) and the resulting composite found to deliver a good electrochemical performance in the presence of a redox active electrolyte . The present work embodies the synthesis of ZIF-67/PEDOT composite and, for the first time, we investigate it as a supercapacitor electrode. The addition of PEDOT has enhanced the conductivity of pristine ZIF-67 while maintaining its basic features of porosity and large specific surface area. The ZIF-67/PEDOT composite based supercapacitor has yielded a high specific capacitance of 107 F/g at a current density of 1 A/g, which is much better than ZIF-67 alone (35 F/g). The composite material has also been used to fabricate an all-solid-state symmetrical supercapacitor, which showed high values of energy (~11 Wh/kg) and power densities (~200
W/kg). This device could maintain a high cyclic stability of 93% after 4000 charge-discharge cycles.
2. Experimental Section 2.1. Materials All the chemicals used for the synthesis of materials and the preparathion of electrodes were of analytical grade purity and used without further purification. Cobalt nitrate hexahydrate, 2-methyl imidazole, and PEDOT ((Poly(3,4-ethylene dioxythiophene)) were purchased from Sigma, India. The solvents, including ethanol and N-methyl-2-pyrrolidone (NMP) were purchased from Himedia, India.
2.2. Synthesis of ZIF-67/PEDOT composite 1 mL of PEDOT was dispersed in 20 mL of deionized (DI) water (ultrasonication treatment of 30 minutes). It was followed by the addition of 5.5 g of 2-methylimidazole (organic linker). A separately prepared cobalt nitrate hexahydrate solution (450 mg in 3 mL DI water) was then added to the above PEDOT-linker mixture and these contents were stirred for 6 hours at room temperature (RT, 25±2ºC). After the completion of the reaction, the ZIF67/PEDOT composite was recovered by centrifugation (10,000 rpm, 20 minutes). The obtained product was washed with DI water (twice) and methanol (once) before drying it under vacuum conditions (80 ºC, 12 hours). It should be noted here that the above taken ratio of ZIF-67 and PEDOT was pre-optimized in some prilimimary experiments. A lower content of PEDOT (e.g., 0.5 mL) in the composite was not helpful to attain a desirable conductivity (Fig. S1), whereas a higher PEDOT ratio (e.g., 1.5 mL) caused the deformation of the ZIF-67 morphology (Fig. S1). Therefore, the ZIF-67/PEDOT composite, containing 1 mL PEDOT, was used for all further studies.
2.3. Synthesis of ZIF-67 ZIF-67 was also prepared in the absence of PEDOT for some comparative studies. The synthesis steps were similar as mentioned in section 2.2 but without any addition of PEDOT.
2.4. Preparation of polymer gel electrolyte A polymer gel electrolyte (PVA/1M H2SO4) (PVA = polyvinyl alcohol) was prepared by dissolving 1.5 gm of PVA in 10 mL of deionized (DI) water. The solution was heated and stirred (95ºC) to obtain a clear gel . The gel was then cooled down to room temperature (RT, 25±2ºC), followed by the addition of DI (to make up to 10 mL) and 10 mL of 2 M H2SO4. This gel is denoted as PVA/1 M H2SO4 polymer gel electrolyte.
2.5. Characterizations N2 adsorption-desorption measurements were carried out on a Belsorp Max system from Microtac to calculate the specific surface area and pore volume. XRD patterns were collected on an X-ray diffractometer (XRD, Bruker, D8 Advance, λ = 1.54 Å). The morphological and EDS (Energy-dispersive X-ray spectroscopy) analyses were carried out with a field-emission scanning electron microscope (FE-SEM, Hitachi SU8010). Fourier-transform infrared spectroscopy (FTIR) studies were made on a Nicolet iS10 system. Raman spectra were collected on a Renishaw (Invia) system using a laser excitation wavelength of 532 nm. X-ray photoelectron spectra (XPS) spectra were collected on a Physical Electronics (PHI 5000 VersaProbe III) system. Transmission electron microscopy (TEM) was carried out with a HRTEM (Jeol/JEM 2100). Current-voltage (I-V) characteristics were studied with a Keithley 4200-SCS semiconductor characterization system.
2.6. Electrochemical measurements All the electrochemical studies were performed using a PGSTAT302N electrochemical workstation from Autolab (Metrohm). The working electrodes were prepared by coating (drop-casting) the graphite foils with a slurry of the active material (i.e. ZIF-67/PEDOT or ZIF-67). This slurry (solvent = NMP) was prepared by mixing the active material with carbon black and PVDF (polyvinylidene difluoride) in a w/w ratio of 8:1:1. The effective area of each of the above-prepared electrode was 1 × 1 cm2 with an active material weight loading of ~ 1 mg/cm2. The electrodes were dried for 24 h at 70°C in a vacuum oven. To study the capacitance values, charge-discharge capability, and the involved mechanisms, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves have been recorded within a potential window of 0-1 V. Other important parameters like series resistance and charge transfer resistance were evaluated by performing the electrochemical impedance spectroscopy (EIS) studies within a frequency band of 0.1 Hz-100 kHz (AC amplitude = 10 mV). In three-electrode studies, a platinum wire and an Ag/AgCl electrode were used as counter and reference electrode, respectively. 1 M H2SO4 was used as an aqueous electrolyte. The symmetrical supercapacitor device was prepared with two electrodes (2 cm × 2 cm) loaded with equal amounts of ZIF-67/PEDOT. The preparation of such electrodes has been mentioned in the preceding paragraph. The PVA/1M H2SO4 electrolyte was spreaded over the two electrodes, which were then sandwiched. The device was sealed with a Teflon tape to protect the contents from evaporation under ambient conditions. The device was allowed to settle for 4 hours before using it for further studies.
3. Results and discussion 3.1. Characterization of the material
ZIF-67/PEDOT composite was synthesized in one-step stirring approach. The material was characterized by different techniques to confirm its successful formation and investigate important morphological and structural properties. Fig. 1a contains the XRD patterns of ZIF67 and ZIF-67/PEDOT. All the observed peaks are in good agreement with previous reports . The typical 2θ peaks of ZIF-67 have been observed at 7.3° (011), 10.3° (002), 12.6° (112), 14.6° (022), 16.3° (013), 17.9° (222), 22° (114), and 24.42° (233). These peaks are also present in the XRD pattern of the ZIF-67/PEDOT composite, confirming that ZIF-67 did not lose its primary features. An additional peak at 26.5° corresponds to the presence of PEDOT in the composite sample . Therefore, the XRD analysis has confirmed the successful formation of the ZIF-67/PEDOT composite. Furthermore, sharp XRD peaks also suggest high crystallinity of the prepared materials. Raman spectroscopy (Fig. 1b) studies are useful to ascertain the inelastic scattering of light from C=C and C-C bonds. In the composite sample, Raman peaks at 1307 and 1391 cm1
, resolved after deconvolution, correspond to the C-C bond stretching and interring
stretching, respectively (also refer to Fig. S2 of SI). Other two peaks, centered at 1431 and 1506 cm-1, are due to the C=C symmetrical and asymmetrical vibrations . All the above four vibration bands observed in the composite sample confirmed the presence of PEDOT. Further, the peaks at 473, 520, 678, and 1097 cm-1 in both the samples (ZIF-67/PEDOT and ZIF-67) are associated with different vibrational modes arisen as a result of cobalt-nitrogen interaction in ZIF-67 . Hence, Raman spectroscopy studies have provided useful evidences about successful combination of ZIF-67 and PEDOT in the prepared composite. The presence of functional groups on ZIF-67/PEDOT and ZIF-67 crystals has been invesigated with the FTIR spectroscopy (Fig. 1c). The vibrational bands in the range of 6001500 cm-1 are attributed to the presence of 2-methyimidazole molecules. A band around 1420 cm-1 in the ZIF-67 sample refers to the stretching vibrational mode of C=N in the 2-
methimazole structure . The intensity of the above particular band in the composite sample is found reduced as a result of combination of PEDOT polymer.
Fig. 1. Characterization results of ZIF-67 and ZIF-67/PEDOT samples: (a) XRD patterns; (b) Raman spectra; (c) FTIR spectra; (d) XPS spectra of ZIF-67/PEDOT (survey scan)
The interaction between ZIF-67 and PEDOT in the ZIF-67/PEDOT composite sample has been investigated by the XPS analysis. The survey scan is given in Fig. 1d. The C1s peak is associated with the presence of C in both ZIF-67 (aliphatic rings of imidazole) and PEDOT counterparts. Co2p and N1s peaks are reasoned to the presence of ZIF-67, as metal nodes got connected with the imidazole structure . The O1s and S2p peaks could be assigned to the presence of PEDOT in the structure [52, 53]. High-resolution XPS spectra have been deconvoluted to further investigate the nature of interaction between ZIF-67 and PEDOT 9
(Fig. 2). The deconvoluted spectra of Co2p splits into the 2p1/2 (796.6 eV) and 2p3/2 (781.2 eV) bands due to the spin-orbital coupling (Fig. 2a). There is a shifting in the characteristic peak of metallic cobalt (Co2p3/2) from 779.3 eV to 781.2 eV, suggesting its presence in Co2+ state (Co=N) . Further, the presence of two satellite peaks at 802.3 eV and 785.5 eV indicate the presence of Co in 3+ state. The Co3+ state should be the result of surface interaction of Co with the oxygen atoms, that changed its oxidation state from Co2+ to Co3+ . The deconvoluted high-resolution spectra of C1s, N1s, and O1s are also provided (Fig. 2b-d). The above XPS data are vital indicators to confirm the interaction of ZIF-67 with PEDOT through the binding between Co and O atoms.
Fig. 2. High-Resolution XPS spectra of ZIF-67/PEDOT for Co2p, C1s, N1s, and O1s peaks
Fig. 3 (a, b) shows the FESEM images of the ZIF-67/PEDOT composite. The FESEM image of ZIF-67 crystals is provided in SI (Fig. S3). ZIF-67 crystals are visibly dispersed over the PEDOT layers (Fig. 3a and 3b). Almost uniformly sized polyhedral shaped ZIF-67
crystals, with an average lateral dimension in a range of few hundred nanometers, can be observed (Fig. 3b). The ZIF-67 crystals are well connected with the PEDOT polymer, which ensured the creation of highly conductive pathways for electron transfer. An irregularity in the polyhedral shape of the ZIF-67 crystals in the composite could be attributed to the copresence of PEDOT, which is likely to inhibit the formation of large and regularly shaped crystals. The presence of different atoms, such as nitrogen, carbon, and metals can influence the double layer and faradic contributions during the energy storage processes [56-58]. Therefore, the composite sample was analyzed for its elemental composition by the EDS analysis. The percentage of different atoms in the sample is given in Fig. S4 of SI. Concerning the supercapacitor performance, a material’s specific surface area and distribution of pores are crucial parameters that can greatly affect the specific capacitance. Therefore, the specific surface area and pore size distribution of the ZIF-67/PEDOT composite have been studied with N2 adsorption-desorption studies. These curves resemble a Type IV isotherm behavior (Fig. 3c). Though, the curves initially seemed to follow a typical type I isotherm but a small hysteresis should classify them as type IV isotherms . Based on these results, the BET surface area of the composite has been estimated to be 1926 m2/g. A steep increase in the adsorption curve at low relative pressure values, observed due to the formation of first adsorbed layer of the gas molecules, is indicative of a microporous nature of the material . Parellely, there is an evidence about the presence of mesopores as well. BJH (Barrett, Joyner, Halenda) plot has also supported the above points (Fig. 3d). The average pore size is determined to be 1.26 nm. Additionally, the hysteresis in BJH plot suggests the presence of mesopores in the material. ZIF-67 is known for its high surface area owing to the distribution of both micro and mesopores. Such a hierarchal distribution of both types of pores in the composite material should facilitate better transport of electrolyte ions during the supercapacitor operation.
Fig. 3. Different characteristics of ZIF-67/PEDOT composites: (a & b) FESEM images; (c) N2 adsorption-desorption isotherms; (d) BJH pore size distribution curve.
The TEM images of the ZIF-67/PEDOT composite are shown in Fig. 4. A polyhedron shaped morphology of the ZIF-67 crystals can be observed (Fig. 4a). As seen in different magnified TEM images, the ZIF-67 crystals with a lateral dimension of around 100-500 nm are found well attached with PEDOT polymer. Fig. 4 (as well as Fig. 3) also reveals that the ZIF-67 crystals have assembled over the PEDOT thin films and are linked together. In the absence of PEDOT, the ZIF-67 particles do not show such connectivity (Fig. S3). The above TEM investigation confirmed the successful formation of the desired composite material.
Fig. 4. TEM images of ZIF-67/PEDOT composite at different magnifications.
3.2. Electrochemical study of ZIF-67/PEDOT composite in three-electrode system The electrochemical performance of the ZIF-67/PEDOT composite electrode has been first investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) studies. The CV curves at different scan rates (5 to 100 mV/s) are presented in Fig. 5a. These studies were carried out within a potential scan range of 0-0.5 V and taking 1 M H2SO4 as an aqueous electrolyte. The area under the CV curves has been considered to compute the values of specific capacitance (refer to eq. S1 of SI). The CV of the pristine ZIF-67 electrode has also been carried out for the purpose of comparison (Fig. 5b). The ZIF-67/PEDOT electrode manifests a much larger CV curve area than the bare ZIF-67 electrode, which is an evidence of a higher specific capacitance of the composite electrode. At a scan rate of 5 mV/s, the ZIF67/PEDOT electrode delivers a specific capacitance of 107 F/g, compared to 35 F/g from the bare ZIF-67 electrode. A better specific capacitance of the composite electrode can be attributed to the fast-electronic transport resulting from the presence of PEDOT which created excellent charge transfer pathways in the material. The composite inherits the favorable features of both the counterparts, i.e., a high specific surface area of ZIF-67 and an 13
efficient electron transport property of PEDOT. This synergistic effect translated into an excellent electrochemical performance in terms of specific capacitance and conductivity. As the CV curves also reveal, the charge storage capability of the electrode decreases with the increasing scan rates. The composite electrode shows a specific capacitance of 107 F/g at a scan rate of 5 mV/s, which decreases to around 50 F/g at 100 mV/s. The variation of specific capacitance with the scan rate is depicted in Fig. 5c. The decrease can be explained by the fact that at the fast scan rates the electrolyte ions would not have enough time to completely or uniformly access the pores of the electrode material. The fast scan rates are detrimental to the electrolyte adsorption and also cause partial redox reaction at the electrode surface. Both the above factors can be reasoned for a decrease in specific capacitance. The practical usefulness of the ZIF-67/PEDOT composites as a supercapacitor electrode material was further investigated through GCD studies. Fig. 5d shows the GCD curves for the composite electrode within a potential window of 0-0.5 V and at current densities varying from 0.5 to 10 A/g. The GCD data are used to calculate the values of specific capacitance according to eq. S2 of SI. Based on these evaluations, a specific capacitance of 107 F/g is achieved at a current density of 1 A/g. The GCD curves also highlight a low voltage drop and a large discharging time (Fig. 5d). The GCD characteristics of the bare ZIF-67 electrode are shown in Fig. 5e. The bare ZIF-67 electrode exhibits a lower discharging time, suggesting an attenuated specific capacitance (35 F/g at 1 A/g). Again, a low specific capacitance of the bare ZIF-67 electrode can be explained by its relatively poor electron transport characteristics. The variation of specific capacitance as a function of current density is summarized in Fig. 5f. The increasing current density results in a decreasing specific capacitance because at such conditions the electrolyte ions do not get enough time to access all the pores of the electrode material.
Fig. 5. (a) CV curves for ZIF-67/PEDOT electrode; (b) Comparative CV curves for ZIF-67 and ZIF-67/PEDOT electrodes at a scan rate of 10 mV/s; (c) Variation of specific capacitance of composite electrode as a function of CV scan rate; (d) GCD curves for ZIF-67/PEDOT electrode; (e) Comparative GCD curves of ZIF-67/PEDOT and bare ZIF-67; (f) Variation of specific capacitance of composite electrode as a function of current density
As explained earlier also, the high specific capacitance of the ZIF-67/PEDOT composite electrode is explained by an efficient electron transfer (due to PEDOT) and a high specific surface area to store the electrolyte ions (due to ZIF-67). The charge storage capability of the material is also influenced by the favorable distribution of pores. The conductivity and the interfacial resistance of the electrode material also need to be investigated to assess its robustness in practical devices. Ideal electrode materials should possess good electrical conductivity and low charge transfer resistance. The electrodes with low conductivity (or high resistance) render slow charge transfer and a large amount of energy might be lost as heat. Therefore, the electrochemical impedance spectroscopy (EIS) studies on the ZIF-67/PEDOT and ZIF-67 electrodes have been conducted to determine their
ESR (equivalent series resistance) and Rct (charge transfer resistance) characteristics. The corresponding Nyquist plots are shown in Fig. 6a. The equivalent circuit applied for the calculation of the above parameters is shown in the inset of Fig. 6a. Nyquist plot consists of three frequency regions, i.e., low, medium, and high. The high-frequency region consists of a semicircle, whose diameter is taken as a measure of Rct. The low-frequency region consists of a straight line with some value of slope. In the case of ZIF-67/PEDOT, no apparant tsemicircle is found in the high-frequency region and consequently it has a negligible value of charge transfer resistance (0.5 Ω cm-2). The ESR value (x-axis intercept) is estimated to be 1.75 Ω cm-2. A straight line in the low-frequency region highlights a low relaxation time of the material. In comparison, the bare ZIF-67 exhibits a larger ESR (7.23 Ω cm-2) value with some slope in the low-frequency region. The magnified high-frequency region is shown in Fig. S5. Based on this study, it can be mentioned that the bare ZIF-67 electrode could not address the energy loss and subsequently delivers an inferior specific capacitance. The distribution of micro and mesopores and the presence of conductive pathways in the ZIF67/PEDOT composite proved to be the major advantageous factors in achieving an efficient electrochemical performance. The cycle stability test is an important parameter to determine the durability of a supercapacitor system. The ZIF-67/PEDOT composite electrode has been investigated for its cyclic stability at a high current density of 20 A/g (Fig. 6b). The system exhibits excellent capacitance retention, e.g., 85% after the 2000 charging /discharging cycles.
Fig. 6. Nyquist plots for ZIF-67/PEDOT (black) and bare ZIF-67 (red) electrodes with data fitting (inset: equivalent circuit); (b) Cyclic stability of ZIF-67/PEDOT electrode.
3.3. Electrochemical performance of solid-state symmetrical ZIF-67/PEDOT//ZIF67/PEDOT supercapacitor device The assessment of the actual practicality of a new supercapacitor electrode can be made by investigating its performance in a prototype device, e.g., two-electrode symmetrical supercapacitor. The three-electrode studies have concluded that the ZIF-67/PEDOT composite can be explored for the development of an efficient supercapacitor. This material possesses a desirable level of high specific capacitance. In comparison, ZIF-67 alone yields an inferior performance. Therefore, we further investigated the performance of the ZIF67/PEDOT electrode in a symmetrical supercapacitor. An all-solid-state two-electrode symmetrical supercapacitor was assembled. A polymer gel of PVA/1 M H2SO4 was used as the electrolyte. The specific capacitance of the device was calculated as per the expressions given in eq. S3 and S4 of SI. The CV curve for the above device has been collected for different potential windows, maintaining a constant scan rate (Fig. 7a). This study was carried out to evaluate the operating voltage window of the device. The device works commendably well within a potential window of 0-1.6 V. Beyond 1.6 V, a sharp increase in 17
the current is observed as some gases (O2/H2) are evolved as a result of decomposition of electrolyte. The decomposition of electrolyte and the evolution of gases can cause critical problems, such as unstable cycle life and low coulombic efficiency. Therefore, the device was further operated within a voltage window of 0-1.6 V. The CV and GCD curves recorded for the device are presented in Fig. 7b and 7c. The CV curves, obtained at various scan rates, are observed to form rectangular shapes at high scan rates. This indicates that at high scan rates the charge is stored primarily via the EDLC process. This is caused by the incomplete or slow Faradic reactions at the electrode surface. Slower scan rates (e.g., 5 mV/s) favor more prominent Faradaic reactions as the electrolyte ions experience favourable kinetics to interact with the active material (Fig. S6, SI). Therefore at slow scan rates, CV curves deviate from the rectangular shape due to the increased Faradaic activity at the electrode surface.
Fig. 7. Electrochemical performance of ZIF-67/PEDOT//ZIF-67/PEDOT symmetrical supercapacitor device: (a) Evaluation of potential window at various voltage range keeping a
scan rate of 50 mV/s; (b) CV curves at different scan rates; (c) GCD curves at different current densities; (d) Nyquist plot of the device in presence of PVA/1M H2SO4 gel electrolyte (inset- Equivalent circuit and the high-frequency region)
The GCD features of the symmetrical device are shown in Fig. 7c. The transition from the EDLC to pseudocapacitive behavior can be observed as these curves assume a linear discharging pattern upon reducing the current density from 10 A/g to 1 A/g. The device delivers a specific capacitance of 123 F/g at a current density of 0.5 A/g. The specific capacitance decreases to 22 F/g with an increase in the current density (10 A/g). The GCD investigations also suggest the device to bear only a low voltage drop. This feature is realized due to the fast charge transfer and low resistance characteristics of the material. The variations in the specific capacitance of the assembled device as a function of CV scan rates and current density are shown in Fig. S7a and S7b of SI. The EIS characteristics of the device have also been studied to evaluate the ESR and Rct values (Fig. 7d). The ESR value for the device is computed to be 2.5 Ω/cm2. The value of Rct (0.48 Ω m-2) is almost negligible and similar to that obtained with the three-electrode system. Both these favorable parameters contributed to a higher specific capacitance and a fast response time of the device. The practical significance of energy storage devices is also assessed by two other critical parameters, i.e., energy density and power density. Supercapacitors can be operated for a high energy density at the cost of power density. The device can be operated with optimized power outputs as per the need of the particular application. Ragone plot of supercapacitors helps in determining the suitable operating conditions. It provides a relationship between energy density and power density. Fig. 8a shows Ragone plot for the ZIF-67/PEDOT//ZIF67/PEDOT supercapacitor device. The device has delivered high values of energy and power densities, e.g., 11 Wh/kg and 200 W/kg, respectively. The mathematical expressions for the
above estimations are mentioned as eq. S5 and eq. S6 (SI). These values are much better than achieved previously with other MOFs and their derived structures (Table S1) [61-65].
Fig. 8. Performance features of ZIF-67/PEDOT//ZIF-67/PEDOT supercapacitor device: (a) Ragone plot (results from previously published reports also indicated) and a photograph of the device with LED glow; (b) Cycle stability of the device (Electrolyte = PVA/1M H2SO4 gel electrolyte)
The cyclic stability of the device was also tested to determine its long-term performance capability. The device functions with excellent capacitance retention (93%) as studied for 4000 charging/discharging cycles at a high current density of 20 A/g (Fig. 8b). It may be highlighted here that the cyclic stability of the symmetrical ZIF-67/PEDOT//ZIF-67/PEDOT supercapacitor device is much better compared to what we observed during the threeelectrode based cyclic stability study with the ZIF-67/PEDOT electrode. The stability test for the three-electrode system was investigated in an aqueous electrolyte. The concentration of electrolyte solution might have fluctuated during the 2000 charging-discharging cycles under the continuous operation due to factors like surface reactions and water evaporation. The symmetrical system contains a solid-state gel electrolyte (PVA/1 M H2SO4), which can retain the primary properties of the electrolyte even during the long-duration stability tests.
4. Conclusions In this research work, a simple one-pot synthesis of the ZIF-67/PEDOT composite material has been reported, which is then explored as a potential supercapacitor electrode material. ZIF-67/PEDOT material has yielded improved conductive pathways compared to pristine ZIF-67. In itself, ZIF-67 possesses a high specific surface area which should be useful to allow the efficient diffusion of electrolyte ions. However, ZIF-67 alone does not bear conductive pathways which becomes a hindrance to facilitate the transfer of charge to the current collector. PEDOT is conducting but its application would also not yield a desirable electrochemical performance because of a low surface area and poor porosity. The structural and morphological characterizations have provided enough evidences about the successful combination of ZIF-67 with PEDOT. The surface area investigations have revealed that the composite assumes electrical conduction properties, yet maintains a large surface area and favorable distribution of micro- and mesopores. The application of ZIF-67 and PEDOT together as a composite material has combined the material features, such as high surface area, porosity, and electrical conductivity. Both the two and three-electrode studies have proven that the addition of PEDOT in ZIF-67 is extremely useful to attain superior electrochemical performance than the bare ZIF-67 material. A symmetrical supercapacitor device has delivered a high energy density of 11 Wh/kg along with a power density of 200 W/kg. It also has a long cycle life. The ZIF-67/PEDOT composite based supercapacitor device is an attractive energy storage device.
Authors’ Credit Statement Vishal Shrivastav performed material synthesis and electrochemical studies. Shashank Sundriyal planned the experimental protocols and fabricated the device. Vishal Shrivastav and Shashank Sundriyal also prepared the first draft of the manuscript. Ashwinder Kaur
contributed to material characterization and ratio optimization. Sunita Mishra and Umesh K. Tiwari corrected the manuscript and contributed to data interpretation. Akash Deep supervised and led the research work, interpreted data, and edited the manuscript.
Acknowledgments Vishal Shrivastav and Shashank Sundriyal thank the University Grant Commission (UGC, India) for their research fellowship. The financial support from CSIR projects MLP029 and MLP-023 is gratefully acknowledged. We also thank Director, CSIR-CSIO Chandigarh for providing research facilities.
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PEDOT has been incorporated in ZIF-67 to prepare a material with enhanced conductivity and large specific surface area.
ZIF-67/PEDOT has been explored as a supercapacitor electrode material.
An assembled symmetrical supercapacitor device operates within a wide potential window of 0-1.6 V and delivers high energy (11 Wh/kg) and power (200 W/kg) densities. Supercapacitor device has a long cycle life (93% capacity retention even after 4000 chargedischarge cycles).
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: