Synthesis and Electrochemical Characterization of Activated Porous Carbon Derived from Walnut Shells as an Electrode Material for Symmetric Supercapacitor Application ^†

Abstract

One of the greatest options to address the growing need for hybrid energy storage systems is a supercapacitor with high specific capacitance, high power density, and more charge and discharge cycles. The valorization of walnut shells, a bio waste, into an activated biocarbon electrode material for the symmetric electric double-layer supercapacitor (EDLC), has been carried out. The valorization method comprises of two-steps for the synthesis of activated biocarbon which are thermal carbonization and ZnCl₂ chemical activation of walnut shells at 700 °C. The sample has good long-term stability and a specific capacitance of 50 Fg⁻¹ @1 Ag⁻¹, making it an excellent supercapacitor electrode material. So, the symmetric electric double-layer capacitor’s (EDLC) promising electrode material was found to be porous AC samples made from walnut shells.

1. Introduction

Rapid climate change and the depletion of fossil fuels necessitate the use of sustainable and renewable energy sources. In order to address these challenges, a variety of clean, sustainable sources—such as solar, wind, geothermal energy, etc.—have been created. However, in order to store and effectively use the energy that has been harvested, renewable energies must require effective energy storage technologies like batteries and supercapacitors [1,2]. Supercapacitors have more benefits than batteries and traditional dielectric capacitors, such as high power density, a long cycle life, a large working temperature range, and improved operation safety, but they have lower energy density than batteries [3]. Supercapacitors are commonly used in various sectors, including transportation (e.g., electric vehicles, buses, trains), consumer electronics, renewable energy systems, industrial machinery, and grid-level energy storage. They are continuously being researched and developed to improve their performance and expand their applications further. Supercapacitors can be divided into two categories based on the electrochemical process that drives their operation: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. In EDLCs, charge transfer occurs on the surface of carbon-based electrode material and yields high power density [4,5,6,7]. On the other hand in the case of pseudocapacitors, charge adsorption (redox reaction) occurs on the surface of metal oxide or conducting polymer-based electrodes and yields high energy density [8,9]. The electrode material is, therefore, a supercapacitor’s most important component because it has an impact on its overall performance [10,11]. Supercapacitors can be used on a large scale for commercial purposes if the electrode material is carefully chosen and the preparation steps are quick and inexpensive. As a result, there is a lot of work being put into finding novel, inexpensive electrode materials.

As electrode materials in supercapacitors, carbon-containing substances such as activated carbons (ACs), carbon aerogels, carbon nanotubes (CNTs), carbide-derived carbons, graphene, and others are frequently employed since they are affordable, have great electrical conductivity, and are broadly accessible [12]. Due to its widespread availability, low cost, and straightforward manufacturing process, activated carbon generated from biomass has attracted a lot of attention lately. As a result, more energy storage and conversion devices are using activated carbon made from biowaste as an electrode material [13]. In most cases, physical or chemical activation methods have been used to activate the carbon generated from biowaste. Pre-carbonized carbon (PCC) was activated chemically to create the perfect carbon structure, whereas ortho-phosphoric acid, KOH, NaOH, ZnCl₂, and other chemicals were used as activating agents [14].

Numerous areas in the world’s temperate zones are home to natural walnut trees. They are valued for their priceless timber and wholesome nuts and are members of the Juglans genus [15]. There are multiple species of walnut trees that may be found on several continents, and in India, the northern states of Jammu and Kashmir, Himachal Pradesh, and Uttarakhand are where walnut trees are mostly planted. These regions benefit from a temperate climate with chilly winters that are ideal for walnut farming. The walnut shell was discarded as waste and is still lying around. In this study, activated carbon was made utilising walnut shells as a precursor material, and the electrodes’ capacity for energy storage was tested in accordance with the waste-into-wealth strategy.

As a result, we present a simple ZnCl₂ activation procedure to produce biomass carbon materials based on walnut shells (WS) for symmetrical supercapacitors. The surface structure of the carbon series generated from a walnut shell, such as its surface area, morphology, and pore size, would be affected by the activation temperature. Additionally, there is a link between surface structure and electrochemical performance for SCs, showing that carbon materials generated from WS with outstanding surface structure make attractive biomass electrode materials [9]. This research gives new insight into the potential uses of carbon materials derived from biomass in clean energy systems.

2. Materials and Methods

2.1. Chemicals Used

Walnut shells were bought from D’mart (supermarket) near to National Institute of Technology, Warangal, Telangana-506004, India. Nickel mesh (0.2 mm), acetone (FINAR, extrapure), HCl acid (FINAR, extrapure), carbon black (Alfa Aesar, 100% compressed), 1-methyl-2-pyrrolidone (NMP)(Merck), Poly (vinylidene difluoride) (PVDF), and ZnCl₂ (extrapure). All chemicals were bought from Taranath chemicals (Hanumakonda, Telangana, India) and used as received.

2.2. Characterization Techniques

X-Ray Diffraction (XRD) analysis: XRD characterization was carried out to find the crystallinity or amorphous nature of the sample and for the confirmation of the synthesized compound, using model name X-pert powder made by PANalytical with characteristic Cu Kα (λ = 1.54 Å) radiation and scintillation detector. Scan rate was fixed at 10.6° min⁻¹ with step size of 0.008° s⁻¹, and 2θ angle range between 20° and 80°.

Surface morphology and EDX analysis: Scanning Electron Microscope (SEM) manufactured by TESCAN, model name VEGA₃ LMU was used to analyse surface morphology and EDX along with mapping of walnut shell-derived activated carbon (AC-w). Tungsten heated cathode as a source of electron gun and resolution of 1 um in the secondary electron mode at a working distance of 5 mm. Before putting the sample for SEM images, a thin coating of gold (gold sputtering) was performed, in case of non-conducting or less for better conductivity and resolution. After that, AC-w was placed in a sample holder and ready for SEM analysis.

Fourier transform infrared (FTIR) analysis: It was performed to identify the organic, polymeric materials and surface functional groups present in the AC-w sample. The analysis was conducted by Bruker’s Alpha II model. In the wave number range 4000 to 500 cm⁻¹ the spectral data was being recorded for our sample.

Ultraviolet–visible (UV–visible) spectroscopy: This refers to absorption or reflectance spectroscopy in the UV and visible regions of the electromagnetic spectrum. We can calculate the band gap of a sample corresponding to its absorption or reflectance spectra.

Electrochemical measurements: All the electrochemical measurements of AC-w as an electrode material were measured by Electrochem’s Origalys 500 in a three-electrode setup.

2.3. Synthesis of Walnut Shell Derived Activated Carbon (AC-w)

Initially, a sufficient number of walnut shells were ground to fine powder using a blender. After that, the walnut powder was washed with de-ionised (DI) water, filtered, and dried in a vacuum oven at 90 °C overnight. Then, fully dried and free from any contamination walnut powder was filled in a boat and pressed to remove air trapped in the sample. The boat loaded with powder was transferred to a tube furnace and heated to 350 °C in an inert atmosphere (Argon gas flow @10 sccm), to avoid unnecessary oxidation and contamination, for 2.5 h. The obtained black powder was mentioned as walnut shell-derived pre-carbonized carbon (PCC-w).

PCC-w powder was mixed with ZnCl₂ until uniform texture in the weight ratio of 1:4. Again, the mixed powder was filled in a boat, pressed, and transferred to the tube furnace. This time it is heated to 700 °C, an optimized temperature from the previous literature, to remove organic impurities and activate the carbon in an inert atmosphere with the same Ar gas flow for 3 h. Finally, the obtained sample was rinsed in 10% HCL acid to remove ash and then washed with DI water several times. Dried the final sample and mentioned as walnut shell-derived activated carbon (AC-w). A schematic representation of the preparation of AC-w from walnut shells is shown in Figure 1.

2.4. Electrode Preparation and Supercapacitor Cell (Three-Electrode Setup) Measurements

The three-electrode setup requires three different electrodes (working electrode, reference electrode, and counter/auxiliary electrode) for supercapacitor cell measurements. AC-w (as active material), PVDF (as a binder), and carbon black (conducting material) were mixed properly in the weight ratio of 8:1:1 (self-optimization for the synthesized sample) using mortar and pestle. NMP is added to the mixed sample as a solvent to make the slurry (mixed sample paste). A nickel foam (2 cm × 1 cm × 0.2 mm) was degreased with acetone and etched with HCl for some time to enhance the roughness of the surface which tends to expose more surface area for slurry to be adsorbed. After that, it was rinsed with DI water and the prepared slurry was coated/pasted on the nickel foam on a particular (1 cm × 1 cm) area. Mass loading depends on the slurry’s viscosity and nickel foam dimensions. Then, the nickel foam was transferred to a vacuum oven for drying and removing extra solvent overnight at 90 °C. This obtained nickel foam will work as a working electrode, platinum wire as a counter/auxiliary, and Ag/AgCl as reference one in the three-electrode setup.

Origalys 500 potentiostat was used for electrochemical measurements, which included cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). Then, 1 M NaOH was used as an electrolyte for all the measurements.

3. Results and Discussion

3.1. Characterizations and Figures

In Figure 2, small peaks at 27° and 28.4° related to 100/101 and 002 planes, respectively, show the existence of graphitic carbon. These peaks are sharp and a little bit shifted to a higher angle due to the presence of zinc oxide (ZnO). Sharp peaks and peaks shifting towards higher angles confirm the crystallinity of graphitic carbon and the shrinking of interplanar spacing, respectively. Further, the presence of micro-graphitic carbon can increase the electrical conductivity required for electrochemical applications [16,17]. Other sharp peaks match with the ICDD #96-900-4182 which clearly reveals the presence of crystalline ZnO. ZnCl₂ used for chemical activation as an activating agent was the source for ZnO also. ZnO has a corrosion-resistant property which will prevent electrode degradation. Other properties of ZnO include good transparency and high electron mobility, which favours the engineering of transparent and efficient electrodes for supercapacitors.

The SEM images of AC-w (Figure 3a–d) show the flake-like surface morphology and well-built porous structure which help to enhance the efficiency by storing more charges. It also helps in rapid ion diffusion. Figure 3c is a backscattered electron detector (BSE) image, in which a brighter region shows heavy metals. In addition, the EDX analysis of AC-w confirms the presence of C, Zn, Si, O, and Al elements (Figure 3e).

Peaks between 600 to 1400 cm⁻¹ represent the presence of alkanes and alkenes groups (hydrogen adjacent with single and double bonds). The peak at 1117 cm⁻¹ shows stretching of C-O bonding (Figure 4a). Thus, the FTIR analysis confirms that AC-w samples have a greater number of carbon bonding and oxygen functionalities, which are favourable for EDLC behaviour and conductivity of WC-w [18,19].

The band gap is found to be 0.92 eV, calculated using the Kubelka Munk function, shown in the UV–visible graph (Figure 4b). The low band gap of synthesized AC-w favours low resistance and high conductivity.

The tilted elliptical shape of CV (Figure 5a) shows the EDLC behaviour (because of no oxidation or reduction peak) of the AC-w as electrode material [20]. In the voltage window from −500 to 0 mV, it shows a specific capacitance of 15.3 Fg⁻¹, while in −1 V to 0 V it shows a maximum specific capacitance of 50 Fg⁻¹. Figure 5b shows GCD curves at different current densities. A specific capacitance of 2.5 and 50 Fg⁻¹ is given by the AC-w electrode at 0.1 and 1 Ag⁻¹, respectively. After that, we calculated the energy density and power density using the above-mentioned formulas. Symmetric electric double layer capacitor showed a max. energy density of 25 Whkg⁻¹ and power density of 10 kWkg⁻¹. Figure 5c is the Nyquist plot obtained from the EIS data analysis, and after that equivalent circuit is obtained by fitting the observed data [7,21]. We fitted the Nyquist plot close to the most relevant equivalent circuit and it showed a minimal equivalent resistance of 2.20 Ω.

3.2. Mathematical Calculations

We can calculate different electrochemical parameters of the prepared electrode in the three-electrode cell setup from electrochemical measurements (CV, GCD) using the following equations:

C_s = I∆t/m∆V (for GCD)

(1)

or

C_s = area under I–V graph/ms∆V (for CV)

(2)

E_d = ½ × 3.6C_s∆V²

(3)

P_d = 3600 × E_d/∆t

(4)

where C_s is specific capacitance

• I is current supplied during GCD
• ∆t is the discharging time in GCD
• m is mass loading on current collector (nickel mesh)
• ∆V is voltage window (V₂ − V₁)
• s is the scan rate
• E_d is energy density
• P_d is power density.

4. Conclusions

As a waste-into-wealth strategy, walnut shell-derived activated carbon was effectively generated from the biowaste with an optimal activation temperature by chemical activation method. The as-prepared AC-w samples were evaluated using a variety of spectrochemical and analytical instruments, which proved the sample’s crystallinity, porousness, and availability of oxygen functionalities that allowed for the simple diffusion of electrolyte ions and conductivity. CV of the symmetric supercapacitor confirms the EDLC behaviour and, hence, it gives a high-power density and specific capacitance of 50 Fg⁻¹ @ 1 Ag⁻¹. Owing to the favourable and most required properties as mentioned above in this paragraph, this can improve ion diffusion and lower device charge transfer resistance. All of these findings demonstrate the wide access of AC-w sample materials for symmetric EDLCs.