Integrated free-standing WS₂ 3D-printed carbon supercapacitor with solid state electrolyte

ABSTRACT

There is a huge need for energy storage devices due to the depletion of natural gas and the increasing requirement for portable electronic gadgets. Fused deposition modeling (FDM) 3D-printing has drawn tremendous interest for the fabrication of batteries and supercapacitors (SCs) due to its tabletop manufacturing technique, bespoke design, fast prototyping and user-friendly process. However, there are fewer available conductive filaments for FDM printing that are ideal from an energy storage standpoint. 2D transition metal dichalcogenide WS₂ has been discovered to be a favourable material for electrochemical energy storage. As a result, in this work, we modified a carbon electrode that was 3D-printed by incorporating WS₂ in order to enhance the capacitive performance of the SC electrode. The WS₂-coated 3D-printed carbon electrode (WS₂/3D-PCE) exhibits 2.8 times higher specific capacitance than the 3D-printed carbon electrode at 50 mV s⁻¹. A solid-state symmetric supercapacitor (SS-SC) was fabricated with WS₂/3D-PCE and polyvinyl alcohol (PVA)/Li₂SO₄ as gel electrolytes. Such modified 3D-PCE opens up the opportunities to design any custom-shaped electrode with tailored properties and pave a route for future research that will lead to more electrochemical devices for portable electronics.

KEYWORDS:

• 2D materials
• transition metal dichalcogenides
• additive manufacturing
• energy storage

Introduction

There has been a fast evolution toward designing and manufacturing complex structures using additive manufacturing or 3D printing techniques in recent years [1, 2]. These open the possibility of fabricating inexpensive 3D components for various applications with ease [3]. Advancements in the wearable and portable electronics industry have intensified the need for flexible and miniaturised electrochemical energy storage (EES) devices [4, 5]. Besides scalability and size, superior power density, excellent energy density, and charging–discharging capability are essential considerations for EES design [6–8]. In this regard, SCs are gaining popularity as an energy storage technology due to their good energy density, high power density, and excellent cyclic stability compared to Li-ion batteries [9–11].

In order to accommodate in complex 3D electrical devices as power sources, SC parts, such as the anode, cathode, current collectors, and separators, should be tailored to a preferred structure and dimension [12, 13]. The advancement of 3D printing processes opens up new avenues for tackling this significant difficulty for supercapacitors. Recently, researchers explored various 3D-printing technologies to fabricate 3D electrodes for SC applications [12–18]. FDM, one of several 3D-printing methods, has been identified as a viable method for the manufacturing of SCs owing to its affordable and simple fabrication process [1, 19]. Despite drawbacks, such as lower resolution than other technologies, its rapid printing speed, versatility in material options and ease of usage make it appropriate for applications, particularly in fields where these factors play a crucial role. In the FDM system, the 3D structure of the desired item is built layer by layer via extruding the filament through a nozzle [20]. Thermoplastic materials, especially polylactic acid (PLA) or acrylonitrile–butadiene–styrene (ABS) polymers, are often used to make the filament used in FDM printing [21, 22]. Foster et al. fabricated SC electrodes utilising a commercially available graphene-containing PLA filament. This filament has ∼92 wt% PLA and ∼8 wt% graphene and obtained a specific capacity of 15.8 mA h g⁻¹ [23]. However, the ease with which such a conductive filament opens up new possibilities for manufacturing electrode materials with the required design. Our group has recently conducted investigations to build electrodes for various electrochemical and sensing applications utilising commercial conductive graphene/PLA filament [24–30]. One of the main challenges while using conductive graphene/PLA filament is their high polymer content affecting the electrochemical activity of the electrode. Deposition of electrochemically effective substance on the 3D-printed electrode can hasten the adoption of the FDM technology for the construction of energy storage devices [31, 32].

Separately, 2D transition metal dichalcogenide, namely WS_2, due to its unique electrical and structural features, has garnered interest as a possible material for energy storage devices [33–36]. The layers of a multilayer WS₂ are held together with van der Waals forces, allowing for facile ion intercalation. The tungsten also exhibits intriguing pseudocapacitive behaviour owing to its broad variety of valence states. Furthermore, the theoretical capacitance of WS₂ material is high (433 mA h g⁻¹) [37, 38]. The benefits of WS₂ as a prospective electrode material for supercapacitors include its low price, large-scale availability, and the possibility of synthesis by low-cost vacuum-free procedures such as hydrothermal method, chemical exfoliation, and so on [39–41]. Electrodeposition has been proven to be an easier and more inexpensive procedure than most synthesis techniques that need intricate routes and expensive vacuum technology. Furthermore, compared to exfoliation and solution phase procedures, electrodeposition creates nano-to-micrometre films quickly and with high command over film deposition. Nonetheless, a WS₂ deposition might be an excellent pseudocapacitive electrode material for efficient SC design. Pseudocapacitance and overall capacitive performance will be improved with a fine layer deposition of WS₂ since the electrochemical energy storage process of SC is primarily restricted to the surface of the electrode.

Herein, we have designed a scalable and low-cost technique to fabricate a supercapacitor by ambient temperature electrodeposition of WS₂ on chemically activated 3D-PCE. The WS₂ electrodeposited 3D-PCE functions as a stand-alone electrode, eliminating the requirement for an external current collector. Thus, a low weight-to-volume ratio is attained by removing an extra component from SC. WS₂ deposition on the surface of activated 3D-PCE improves the overall performance of the 3D electrode by pseudocapacitive reactions. Furthermore, SS-SCs have been constructed using PVA/Li₂SO₄ gel electrolyte. The solid-state electrolyte guarantees the secure usage of SS-SC in electrical devices, avoiding the electrolyte leakage issue that usually happens with liquid electrolytes. In a nutshell, our work illuminates a different path for ambient-temperature WS₂ deposition on tailored free-standing 3D-PCE for supercapacitor electrodes.

Experimental methods

Materials

All materials used were of analytical grades such as ammonium tetrathiotungstate (NH₄)₂WS₄, potassium chloride (KCl), lithium sulfate anhydrous (Li₂SO₄), and polyvinyl alcohol (MW 85,000–124,000 g mol⁻¹) acquired from Sigma Aldrich, Germany. Sodium hydroxide (NaOH) was procured from Penta chemicals unlimited, Czech Republic. The conductive graphene/polylactic acid (PLA) filament (Blackmagic®) was acquired from Graphene Laboratories Inc, USA.

3D printing and activation of the electrodes

Autodesk Fusion 360 software was used to design the electrodes for the 3D printing. The electrodes were designed in the shape of a circular lollipop on a stick, with a 6 mm diameter circular disc electrode and a 15 × 2 mm rectangular stick attached to it as shown in Figure S7a. The printed parts had a thickness of 0.5 mm. This file is then sliced using PrusaSlicer 2.6.1 software keeping infill at 15%. A layer height of 0.2 mm was set for the initial layer and 0.1 mm for the consecutive layers. The default setting of the gyroid fill pattern was used keeping the top fill and bottom fill pattern monotonic. The sliced electrode file was used for the fused filament fabrication with conductive carbon/PLA filament. The electrodes were 3D-printed using a Prusa i3 MK3 printer (Prusa Research, Czech Republic) equipped with an Olsson Ruby ruby-tipped 0.6 mm nozzle (3DVerkstan, Sweden). The printing temperature was set at 215 °C for the nozzle and 60 °C for the bed. The 3D-printed carbon/PLA electrodes were then activated chemically by dipping for 5 h in a 4 M NaOH solution. Then electrochemical activation was carried out in a phosphate buffer solution (PBS) having pH 7.2 utilising a Pt wire as the counter electrode, and a constant potential of 2 V vs Ag/AgCl was applied for 150 s. Such activated electrodes are then denoted as 3D-PCE.

Preparation of WS₂/3D-printed carbon electrodes

The electrodeposition of WS₂ was carried out using previously optimised parameters from our group [42]. For the electrodeposition, 10 mM of (NH₄)₂WS₄ solution was prepared in 0.1 M KCl solution. Electrodeposition of WS₂ on 3D-PCE was carried out by running CV from −1.3 to +1.1 V at 50 mV s^–1 for 50 cycles. The WS₂-coated 3D-PCE is represented as WS₂/3D-PCE in the subsequent sections. An average mass deposition of ∼0.23 mg cm⁻² of WS₂ was obtained through gravimetric measurement.

Fabrication of solid-state supercapacitor cell

A PVA/Li₂SO₄ gel electrolyte was used to assemble two identical electrodes to create the SS-SC cell. There is no need for an external separator since the gel electrolyte acts as both an electrolyte and a separator in the cell. The preparation of the PVA/Li₂SO₄ gel electrolyte was carried out using an altered technique, as outlined in the literature [29, 43]. The preparation process of the PVA/Li₂SO₄ gel electrolyte can be summarised as follows: A clear PVA solution was prepared by dissolving 1 g of PVA in 5 ml of water heating at 90 °C. Then, 4 mL of 1 M Li₂SO₄ was gradually added drop by drop. The solution was then cooled to ambient temperature and placed in a glass vial. The resulting gel was utilised for making SS-SCs.

Materials characterisation

Electrochemical and materials characterisations were carried out for 3D-PCE and WS₂/3D-PCE. Scanning Electron Microscope (SEM, FEI VERIOS 460L) was used to visualise the surface of the electrodes. Energy-dispersive X-ray (EDX) was analyzed using a detector (EDAX SDD Octane Super) coupled with the SEM for the elemental mapping and chemical composition applying an electron beam accelerating voltage of 18 kV. AXIS Supra instrument (Kratos Analytical, Japan) was utilised for X-ray photoelectron spectroscopy (XPS) measurements using a monochromatic Al Kα (1486.7 eV) excitation source used to examine the bonding and atomic composition of the electrodes. The following spectra obtained from the XPS were analyzed using Casa XPS software. The Shirley algorithm was used for background correction of the XPS data.

Electrochemical performances were analyzed using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) using a potentiostat (Metrohm Autolab, Netherland) employing NOVA 2.1.5 software in a 20 mL cell at room temperature with Ag/AgCl (1 M KCl) and Pt as a reference and counter electrodes, respectively. The electrochemical tests were focused on the WS₂-coated active surface of the printed electrodes. The electrochemical performance calculations such as capacitances, energy density, and power density have been determined using equations (S1-S5).

Results and discussion

The cost of producing energy storage units is a significant contributor to the overall cost of electronic devices. 3D printing has emerged as a low-cost and appealing option for prototyping, but a major challenge is the reduced electrochemical activity of 3D-printed electrodes due to high levels of polymer. The fabrication route of the WS₂/3D-PCE symmetric supercapacitor is illustrated in Figure 1. In this study, we addressed this issue by treating the 3D-printed electrodes in a NaOH solution and through electrochemical activation. The activation of the electrodes removed the surface PLA without disturbing the original form, as shown in the insight image of Figure 2a. Along with the PLA removal, the overall mass of the electrode is reduced by ∼20%.

Figure 1. Schematic diagrams of fabrication of electrochemically coated WS₂ modified 3D-printed carbon electrode for fabrication of symmetric supercapacitors.

Figure 2. Scanning electron microscopy (SEM) image of (a, b) blank 3D-PCE (inset: digital image of the electrode after activation; scale 5 mm), (c, d) WS₂/3D-PCE, (e) SEM-energy dispersive X-ray (EDX) spectroscopy mapping area and (f–j) elemental mapping of W, S, O, C and overlay respectively of WS₂/3D-PCE, respectively.

The SEM images of the activated electrode, as depicted in Figure 2(a,b) reveals the morphology of carbon in the 3D-PCE, which has diameters of range from 70 to 130 nm, with lengths ranging between a few micrometres. The electrochemical deposition of WS₂ deposition was carried out by following the previous optimisation published by our group [42]. The corresponding CV of WS₂ deposition is depicted in Figure S2b. Morphological analysis of 50 cycles of WS₂ deposited 3D-PCE is shown in Figure 2. After electrodeposition, WS₂ flakes were observed around the nanocarbon fiber, as shown in SEM image Figure 2(c,d). The elemental composition of the electrodeposited WS₂ electrode was acquired from EDX spectroscopy analysis. The atomic percentage of the elements was attained from the EDX spectra point analysis shown in Figure S1. The atomic ratio of W and S was measured using EDX point analysis and observed to be ∼1:2. Furthermore, trace amounts of Ti and O were also observed in the WS₂/3D-PCE. Ti is obtained from the intrinsic impurity of TiO₂ in the nanocarbon filament [29, 44]. The WS₂ deposition on the nanocarbon electrode is endorsed from the EDX mapping of W and S, as shown in Figure 2(f and g), respectively. Figure 2(h–j) depict the mapping of EDX mapping O, C and overlay of individual elements on 3D-PCE, respectively.

Furthermore, high-resolution XPS analysis of the electrodes was conducted to confirm the bonding environment and chemical composition. For in-depth analysis of the bonding state, high-resolution XPS spectra of individual elements were carried out. Deconvolution of W 4f shows the peaks at 31.80 and 33.92 eV corresponding to the W⁴⁺ 4f_7/2 and W⁴⁺ 4f_5/2, respectively as shown in Figure 3(a) [45]. Besides, the peaks at 33.50 and 35.70 eV correspond to the W⁶⁺ 4f_7/2 and W⁶⁺ 4f_5/2, respectively. The deconvoluted peak at 37.60 eV is attributed to the W 3p_3/2 [46]. The deconvoluted high-resolution spectrum of S 2p is shown in Figure 3(b). The peaks obtained at 163.64 and 164.80 eV have a spin–orbit peak separation of 1.16 eV attributed to S^2- 2p_3/2 and S^2- 2p_1/2, respectively. From the deconvoluted O 1s spectra, four peaks were obtained at 526.9, 528.4, 530.2, 531.4 and 533.3 eV (Figure S4). The peak at 528.4 and 530.2 eV corresponds to the W–O and W = O bonds, respectively due to the lattice oxygen [47]. Other peaks are attributed to the C = O, C–OH and Ti–O bonds which are originated from the 3D-PCE [48–51]. The oxidation states of W (W⁴⁺ and W⁶⁺), S and O support the formation of WS₂ along with small quantity of WO₃.

Figure 3. X-ray photoelectron spectroscopy (XPS). High-resolution core-level spectra of (a) W 4f and (b) S 2p of WS₂/3D-PCE.

Electrochemical measurements were performed in 1 M Li₂SO₄ electrolyte in a three-electrode setup to explore the electrochemical behaviour of the 3D-PCE and WS₂/3D-PCE. The electrodes required no additional current collector for electrochemical testing. The bare electrode exhibits strong current–potential responsiveness, with increasing scan rate scan speeds from 10 to 100 mV s⁻¹, increasing current density as depicted in Figure S2a. The comparison of CV curves between 3D-PCE and WS₂/3D-PCE at a scan rate of 50 mV s⁻¹ reveals the difference in current density after the deposition of WS₂, as presented in Figure 4(a). The current–voltage area at 50 mV s⁻¹, which is connected to the stored charge, increases after WS₂ deposition compared to 3D-PCE. Figure 4(b) shows that the specific capacitance increased by a factor of 2.8 times for the WS₂/3D-PCE at the scan rate of 50 mV s⁻¹. As given in Figure 4(c), the CV was performed for WS₂/3D-PCE in the potential window from −0.2 to +0.8 V at scan rates from 10 to 100 mV s⁻¹. The CV patterns correspond to the Type A (pseudocapacitive) behaviour classification within the categorisation of SC electrodes [52, 53]. The charge–discharge kinetics (diffusion-controlled and capacitive process) can be elucidated from the CV using the power law equation, i.e. [54–58]. (1) $i=a{v}^b$(1) where, i denotes peak current, v represents scan rate, a is a constant and b is the signifies power-law exponent, which varies in the range of 0.5–1. When b = 0.5, it defines the kinetics is governed by diffusion-controlled pseudocapacitive process, while b = 1 states that the kinetics is dominated by surface-controlled capacitive process. For convenience, Equation (1)(1) $i=a{v}^b$(1) is rearranged to Equation (2)(2) $\log i=\log a+\mathrm{b}\log v$(2) , here the value of b is determined by the slope of log i against log v. (2) $\log i=\log a+\mathrm{b}\log v$(2) The plot of log i against log ν for WS₂/3D-PCE is depicted in Figure S3. The value of b determined from the slope of the graph is 0.74. This shows that the reaction kinetics of WS₂/3D-PCE is not constrained to the surface of the electrodes but also influenced by the diffusion process. According to the CV investigations of the 3D-PCE and WS₂/3D-PCE, the WS₂ coating significantly boosts the current density. The fast faradic redox reaction occurs at the electrode surface by the Li⁺ adsorption and desorption is stated by Equation 3(3) $\mathrm{W}{\mathrm{S}}_{2(\mathrm{surface})}+\mathrm{L}{\mathrm{i}}^{+}+{\mathrm{e}}^{-}\leftrightarrow ({\mathrm{WS}}_2^{-}-\mathrm{L}{\mathrm{i}}^{+}{)}_{(\mathrm{surface})}$(3) [59, 60]. (3) $\mathrm{W}{\mathrm{S}}_{2(\mathrm{surface})}+\mathrm{L}{\mathrm{i}}^{+}+{\mathrm{e}}^{-}\leftrightarrow ({\mathrm{WS}}_2^{-}-\mathrm{L}{\mathrm{i}}^{+}{)}_{(\mathrm{surface})}$(3) Furthermore, the GCD analysis of WS₂/3D-PCE was performed at the potential window of −0.2 to +0.8 V at different current densities, as shown in Figure 4(d). The charge–discharge curve exhibits similar behaviour, consistent with the CV results. At constant current densities of 0.178, 0.255 and 0.764 mA cm⁻², the electrode exhibits specific capacitances of 30.62, 6.40, and 4.58 mF cm⁻², respectively. The capacitance was increased with a decrease in current density. This is due to the sluggish ion adsorption–desorption process at the lower current density, which allows for greater access to the active region of the WS₂/3D-PCE and boosts capacitance [61, 62]. At higher current densities, however, the diffusion and migration of electrolyte ions are restricted. While the presence of inherent metal oxide impurity in Blackmagic® filament can contribute to the electrochemical performance of 3D-PCE [24, 63, 64]. This influence of TiO₂ impurity is present in both 3D-PCE and WS₂/3D-PCE which evens out. So, the performance enhancement after WS₂ deposition can be solely attributed to the presence of WS₂.

Figure 4. Electrochemical study in the three-electrode system (a) comparison of WS₂/3D-PCE and 3D-PCE at 50 mV s⁻¹, (b) specific capacitance comparison, (c) cyclic voltammogram of WS₂/3D-PCE at different scan rates, (d) galvanostatic charge-discharge curves of WS₂/3D-PCE at different current densities 0.178, 0.255 and 0.764 mA cm⁻² which is labelled as I, II, and III, respectively.

The improvement in charge storage properties of WS₂/3D-PCE can be correlated with the conductivity and electrochemical reactivity. EIS was carried out at 0.3 V in the frequency range from 100 kHz to 10 mHz. From the Nyquist plots (Figure S5a) of 3D-PCE and WS₂/3D-PCE it is observed that upon WS₂ deposition equivalent series resistance (R_s) has decreased from 76 to 49 Ω. The R_s consists of the inherent resistance of the electrode material, solution resistance, interfacial resistance between electrode and electrolyte, and electrode and current collector. The decrease of R_s after WS₂ deposition on 3D-PCE can be attributed to the 2D structure and inherent semiconductor nature of WS₂ which lowered the inherent resistance of WS₂/3D-PCE [65]. Consequently, it is leading to faster electron and ion transfer mobility that improves the overall charge/discharge kinetics.

Considering these encouraging results of the WS₂/3D-PCE electrode, a free-standing SS-SC cell is designed with two similar WS₂/3D-PCEs and PVA/Li₂SO₄ gel electrolytes, as described in the experimental section. It is important to note that no external current collector was employed in the fabrication of the SS-SC cell. CV and GCD were measured to assess the electrochemical performance of the SS-SC cell. Figure 5(a) shows the CV at various scan rates ranging from 0 to 1 V. The current density raises with the increase of scan rate from 10 to 100 mV s⁻¹, demonstrating the good capacitive behaviour by the solid-state device.

Figure 5. Electrochemical study in two electrode system. (a) Cyclic voltammogram of WS₂/3D-PCE solid-state symmetric supercapacitor at different scan rates (b) galvanostatic charge-discharge (GCD) curves of WS₂/3D-PCE solid-state symmetric supercapacitor at different current densities 56.62, 169.87, and 283.09 µA cm⁻², denoted as I, II, and III, respectively. GCD curves of (c) series and (d) parallel connection of two and three-cells at the current density of 0.11 mA cm⁻², respectively.

Figure 5(b) depicts the GCD study at various current densities. The GCD curves show a symmetrical charge–discharge profile is observed. From the discharge curve, an IR drop of 0.13 V is found at the current density of 56.62 µA cm⁻². The SS-SC exhibits the areal capacitances of 2.66, 2.04, and 1.42 mF cm⁻² and areal energy density of 0.37, 0.28, and 0.20 µWh cm⁻² and areal power density 28.31, 84.94, and 141.56 µW cm⁻² at the current densities of 56.62, 169.87, and 283.09 µA cm⁻², respectively. The cyclic stability of the cell is an important evaluating factor in the lifetime assessment of supercapacitors [66]. The cyclic stability test was carried out at 226.50 µA cm⁻² that revealed excellent capacitance retention of 98% of its initial capacitance of 1.12 mF cm⁻² even after 40,000 charge–discharge cycles (Figure S6).

In real-world applications, a single supercapacitor cannot provide all the energy and power required. To determine if multiple supercapacitors can meet practical requirements, the supercapacitors are connected either in series or in parallel. Figure 5(c, d) depicts that two and three cells are connected in parallel and series, respectively. The two and three series-connected cells attain potential difference of 2 and 3 V, respectively at the current density of 56.62 µA cm⁻². Thus, the series connection of supercapacitors is an effective method to provide the energy demand for higher output voltage. These series-connected cells can deliver enough potential difference to power LEDs and small portable devices. Additionally, a parallel connection of three supercapacitors showed three times longer output than a single supercapacitor cell operating at the same current density. The 3D printed structure facilitates easy assembling of the series and parallel connected cells in the same foot-print area as shown in the inset of Figure 5(c,d). Thus, the serial and parallel-connected 3D-printed cells can be used for obtaining the desired operating voltage and current.

Conclusion

In this work, an easy and scalable manufacturing of free-standing electrodes of WS₂ electrodeposited 3D-printed carbon electrodes are fabricated for supercapacitor application. The WS₂ deposition on a 3D-printed carbon electrode improves the capacitive performance of 3D-printed carbon electrode by several folds. With the customised electrode, solid-state supercapacitor cells can be prepared without the need for an additional current collector or separator, which results in higher capacitive performance. Towards future direction, various electrochemically active transition metal dichalcogenides can be deposited onto the surface of the 3D-printed carbon electrode, allowing for the easy production of supercapacitor cells of any desired shape to meet the growing demand in the modern electronics market.

Conflicts of interest

There are no conflicts to declare.

Authors contribution

S. M. carried out materials synthesis and fabrication, characterisations, and analysis. K. G. conceptualised the project and mentored in fabrications, characterisations and analysing data. S. M. wrote the first draft and K. G reviewed and edited. M. P. supervised the project. All authors contributed to the writing.

Acknowledgements

The work was supported by ERDF/ESF project TECHSCALE (No. CZ.02.01.01/00/22_008/0004587). This research was co-funded by the European Union under the REFRESH – Research Excellence For REgion Sustainability and High-tech Industries project number CZ.10.03.01/00/22_003/0000048 via the Operational Programme Just Transition. CzechNanoLab project LM2018110 funded by MEYS CR is gratefully acknowledged for the financial support of the measurements/sample fabrication at CEITEC Nano Research Infrastructure.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

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