A flexible solid-state asymmetric supercapacitor device comprising cobalt hydroxide and biomass-derived porous carbon

Abstract

Development in the field of alternative and renewable energy sources is becoming necessary considering the current energy demands of the growing technologies. The main challenge associated with the produced energy is to store it for future use, such that it can be used when needed. Supercapacitors are among the electrochemical energy storage systems that provides higher power density, faster charging–discharging, high specific capacitance (C_S), and long cycling life. Herein, the fabrication of a flexible solid-state asymmetric supercapacitor (ASC) device is reported, where Co(OH)₂ hollow spheres and biomass-derived porous carbon (PC) are the cathode and anode, respectively. Co(OH)₂ is a highly redox active material, whereas PC is an electric double-layer capacitive (EDLC) material. In this device, aqueous KOH solution (electrolyte) encapsulated in PVA gel (separator) was used to bind the electrodes. This Co(OH)₂//PC ASC device exhibited a high C_S of 260 F g⁻¹ (at 2 A g⁻¹). It retained ∼91% of the initial C_S value (at 6 A g⁻¹) till ∼5000 cycles. Electrochemical impedance spectroscopy (EIS) study confirmed low internal resistance (0.95 Ω) and charge transfer resistance (1.41 Ω) values of Co(OH)₂//PC. These results indicate that the high electron transfer process in the electrode–electrolyte interface during the electrochemical reaction, which is responsible for the excellent performance of this ASC device. The high-performance Co(OH)₂//PC ASC device exhibited an energy density of 76.7 W h kg⁻¹ at a power density of 1416.9 W kg⁻¹. To demonstrate its practical use, LED lights were illuminated using this Co(OH)₂//PC ASC device.

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1. Introduction

The world is now implementing a low-emission development strategy.^1,2 This strategy mainly focuses on the security of forests, the environment, and climate change. The decline in long-term greenhouse gas emissions and improved resilience to climate change could be effective measures for this implementation. This leads to a transition from the use of fossil fuels to renewable and sustainable energy conversion and storage technologies.³ The important role of energy storage and conversion devices is to store electricity during spare time at a low cost and deliver it later. Among the various developed electrochemical energy storage technologies (batteries, capacitors, fuel cells, supercapacitors, etc.),^4,5 supercapacitors have attracted widespread attention from researchers. Supercapacitors can quickly store and deliver energy by offering higher current in a short time than batteries and exhibit higher energy density in comparison to traditional capacitors, which enables them to provide high power density and moderate energy density to bridge the gap between both batteries and parallel plate capacitors.^5–7 Supercapacitors are mainly used in many fields, such as portable electronics, electric or hybrid vehicles, uninterruptible power supplies, heavy loading trucks, cranes, and military (i.e., laser weapons and missile launch systems).⁸ The advance of supercapacitors primarily depends on synthesized electrode materials, their charge storage mechanism (capacitive or diffusive), electrolytes, binders, and separators. Further, supercapacitors can be divided into two categories: (i) symmetric supercapacitors and (ii) asymmetric supercapacitors (ASCs). By preparing the anode and cathode using the same material with the same mass loading, a symmetric supercapacitor device can be fabricated. Symmetric supercapacitor devices deliver a lower energy density because of their smaller voltage window, which limits their use in practical applications.^8,9 One way to enhance the energy density of a supercapacitor can be an assembly of asymmetric supercapacitor (ASC) devices using two different materials to prepare an anode and cathode, which increases the voltage window and, consequently, the energy density. Nevertheless, with the substantial advancement made in the development of ASC devices, increasing interest in flexible and wearable energy storage systems has led to the preparation of flexible ASC devices.

Co(OH)₂ is an inexpensive, earth-abundance material and possesses excellent redox activity in electrochemical reactions.^10,11 Co(OH)₂ has two polymorphs: α-Co(OH)₂ and β-Co(OH)₂. The α-Co(OH)₂ structure contains intercalated molecules (i.e., H₂O, NO₃) inside the interlayers, which is why it has a higher interlayer spacing than the β-Co(OH)₂.¹² High interlayer distance and good electronic conductivity of α-Co(OH)₂ deliver higher electrochemical activity compared to β-Co(OH)₂. However, the nanostructure of cobalt hydroxide suffers from agglomeration, poor electronic conductivity, and slow ion diffusion kinetics, which can lead to a low reversible capacity when used as high-performance electrodes.^13,14 Different morphologies of the metal hydroxides show different electrochemical activities.^15,16 Innovative nanostructures, such as hollow, core–shell, and hollow–core–shell designs, must be explored to achieve enhanced electrochemical performance because of their high surface-to-volume ratios, large surface areas, excellent permeabilities, and abundant surface-active sites. These attributes are crucial for achieving high energy and power densities in electrochemical supercapacitors.¹³ For instance, utilizing nanospheres instead of nanosheets can significantly increase the material's specific surface area and reduce the “dead volume” that occurs when internal sheet materials are unable to interact with the external environment due to accumulation.¹⁷

In this work, a hollow sphere-like structure formed by a combination of flakes of Co(OH)₂ was synthesized to develop a solid-state asymmetric supercapacitor (ASC) device. Besides, biomass-derived porous carbon was synthesized using tea waste (after brewing the tea leaves) as precursors. The Co(OH)₂ showed pseudocapacitive redox activity and was utilized as cathode material, and the EDLC behavior of PC established its role as anode material for the fabrication of the ASC device. The anode material of the ASC device also plays an important role. Another key component in the equipped device was the use of polymeric gel to act as a solid-state electrolyte, which made the device flexible. In particular, aqueous gel electrolytes are the most studied because they combine the advantages of both aqueous and solid electrolytes.^18,19 Here, polyvinyl alcohol (PVA) was selected to entrench a 3 M KOH aqueous electrolyte into a gel because of its high solubility in water, low toxicity, and low cost. The electrochemical performances of a flexible solid-state Co(OH)₂//PC ASC device were examined using a two-electrode system (2E-cell), and parameters such as specific capacitance (C_S) value, energy and power densities, flexibility, and stability were calculated. The device exhibited a high C_S value of 260 F g⁻¹ at a current density of 2 A g⁻¹. It delivered a higher energy density of 76.7 W h kg⁻¹ at a power density of 1416.9 W kg⁻¹, which is comparable to and superior to many already reported Co(OH)₂-based asymmetric supercapacitor devices.^12,20–23 The cyclic stability of the device after ∼5000 galvanostatic charge–discharge (GCD) cycles was 91% observed. A series of 16 red LEDs were illuminated with the prepared solid-state ASC device. The present work elaborates on the successful synthesis of the nanomaterials and nanocomposites, the excellent electrochemical activity of synthesized materials, the assembly of a solid-state ASC device, the high performance of the assembled device, its flexibility, stability, and its practical application on a lab scale demonstration.

2 Experimental

2.1 Synthesis of materials

2.1.1 Synthesis of the cathode material (Co(OH)₂). A hollow-sphere structured Co(OH)₂ was synthesized using a one-step hydrothermal process. First, 4 mM of Co(NO₃)₂·6H₂O in 30 mL of ethanol and 8 mM of urea in 30 mL of ethanol were mixed together. Then, 12 mg of trisodium citrate (Na₃C₆H₅O₇) was added to the prepared solution and stirred for 30 min. After that, the clear solution was transferred to a Teflon-lined stainless steel autoclave and heated at 150 °C for 24 h. The brown precipitate of Co(OH)₂ was obtained after centrifugation, washed repeatedly, and dried at 60 °C.

2.1.2 Preparation of the anode using porous carbon derived from tea waste. Porous carbon was synthesized from a bio-mass (tea waste) using the KOH activation method. In the first step, 2 gm of tea waste was treated with 50 mL of conc. H₂SO₄ at 200 °C for 24 h. Then, the obtained black product (precursor) was washed with distilled water and then dried. In the second step, the precursor was mixed with an equivalent amount of KOH and then calcined at 900 °C for 3 h in a nitrogen atmosphere. Then, it was washed with distilled water and dried for further use, and the obtained product was porous carbon (PC).²⁴

2.2 Electrochemical measurements

2.2.1 Three-electrode cell setup and measurements. Three-electrode cell (3E-cell) configuration was used to study the electrochemical properties of synthesized Co(OH)₂ and PC. The Hg/HgO electrode was used as a reference electrode, and a Pt wire was used as a counter electrode. The working electrode was fabricated by preparing a slurry of synthesized material, PVDF in NMP, and acetylene black in a ratio of 80 : 10 : 10. A paste was coated on a Ni foam with a mass of ∼2 mg. From this 3E-cell setup, important parameters of the electrode materials, such as capacitance, voltage window, and charge transfer resistance, were measured by employing cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests. EIS measurements were conducted in the frequency range of 0.01–10000 Hz with an alternating current amplitude of 0.05 V.

2.2.2. Two-electrode cell setup (asymmetric supercapacitor device) and measurements. An asymmetric cell was assembled with Co(OH)₂ (cathode) and PC (anode), Whatman filter paper as a separator, and 3 M KOH as an electrolyte. The cathode-to-anode mass ratio was determined from the charge balance equation (eqn S6†) (m⁺/m⁻ = 1.28). Here, the cathode was prepared using ∼1.56 mg of Co(OH)₂, and for the anode, ∼2 mg of PC was used. The operating voltage window of 0 to 1.5 V was determined from the 3E-cell measurements of the cathode and anode. A solid-state supercapacitor device was constructed using the same cathode and anode materials as the asymmetric cell. In this case, a polymer gel (PVA) containing 3 M KOH was used, which acted as both a separator and an electrolyte. All the equations used to calculate the different parameters of the supercapacitor device are presented in the ESI.†

3 Results and discussion

3.1 Structural and morphological analysis

Fig. 1a presents a powder XRD pattern of Co(OH)₂, in which the diffraction peaks at 2θ values of 11.1, 22.2, 33.4, 37.7, and 59.5° corresponding to (003), (006), (100), (102), and (110) planes were observed [JCPDS card no.: 46-0605]. The (003) plane obtained d-values of 8.03 Å and showed higher intensity than other planes, which suggested that the α-Co(OH)₂ was formed with a greater number of interlayer spacers, such as H₂O and NO₃⁻, and a more ordered structure in the direction of the (003) plane.²⁵ XRD pattern of porous carbon showed an amorphous nature with two humps between 2θ values of 21–34 and 39–48° (Fig. 1b).²⁴

Fig. 1 XRD pattern of (a) Co(OH)₂ and (b) porous carbon. (c) Raman spectra and (d) FTIR spectra of Co(OH)₂.

Raman spectroscopy analysis of the Co(OH)₂ is displayed in Fig. 1c. The spectra showed two vibrational bands at 450 and 510 cm⁻¹, corresponding to A_1g and A_2u, respectively. The observed Raman shift at 450 cm⁻¹ was due to the OCoO vibrational mode.^26,27 An intense peak was observed at 510 cm⁻¹, which was assigned to the Co–O–H mode.²⁸ Fourier transform infrared spectroscopy (FTIR) of Co(OH)₂ is presented in Fig. 1d. The bands at 3615, 1480, 1382, 1352, 1050, and 640 cm⁻¹ were obtained. A broad peak observed at 3615 cm⁻¹ corresponds to the stretching vibration of the O–H groups.²⁹ A sharp peak appeared at 1382 cm⁻¹, which was the characteristic peak of nitro groups and indicated the presence of intercalated NO₃⁻ ions in the cobalt hydroxide.^30,31 The peaks appearing at 1480, 1352, and 1050 cm⁻¹ were ascribed to the intercalated carbonate ions in the α-Co(OH)₂.^30,32 A peak observed at 640 cm⁻¹ was assigned to Co–O–H vibration.³¹

A field emission scanning electron microscopy (FESEM) study showed a hollow sphere-like morphology of the synthesized Co(OH)₂ within the diameter of 7–9 μm (Fig. 2a). The magnified image of the Co(OH)₂ hollow sphere depicted its hollowness (Fig. 2b), which was formed due to the assemblage of ∼0.22 μm long petals. The elemental color mapping of Co(OH)₂ displayed the presence of Co and O, as shown in Fig. 2c–e. The energy dispersive X-ray spectroscopy (EDS) study of the Co(OH)₂ is shown in Fig. S1,† in which the Co and O were observed. A highly porous matrix was observed in the FESEM micrograph of the synthesized PC, and it is presented in Fig. 2f. High-resolution transmission electron microscopy (HRTEM) presented in Fig. 2g shows the structural features of Co(OH)₂ in more detail. The lattice fringe spacings of 0.38, 0.23, and 0.19 nm attributed to the (006), (105), and (108) planes of Co(OH)₂ were observed in the HRTEM micrograph (Fig. 2h) [JCPDS card no.: 46-0605].^33,34 The selected area electron diffraction (SAED) pattern of the Co(OH)₂ hollow-sphere showed well-ordered diffraction rings corresponding to (006), (100), (105), and (110) planes (Fig. 2i), which coincide with XRD pattern and also confirmed its polycrystalline structure.

Fig. 2 (a and b) FESEM images of Co(OH)₂. EDS elemental color mapping of Co(OH)₂: (c) overlay, (d) O, and (e) Co. (f) FESEM image of porous carbon. (g and h) HRTEM images and (i) SAED pattern of Co(OH)₂.

X-ray photoelectron spectroscopy (XPS) analysis of the Co(OH)₂ was carried out to assess the chemical states of the elements. The XPS survey spectrum of the Co(OH)₂ is shown in Fig. 3a, which indicates the peak appearance of Co 2p and O 1s at binding energies of 531 and 782 eV, respectively. The high-resolution Co 2p spectrum (Fig. 3b) was fitted with two main peaks corresponding to spin–orbit doublets of Co 2p_3/2 (781.07 eV), Co 2p_1/2 (796.7 eV), and their satellite (sat) peaks (785.6 and 802.9 eV), which confirmed the presence of Co²⁺ state.^35,36 The O 1s spectrum was deconvoluted to two peaks at binding energies of 530.8 and 532.26 eV (Fig. 3c), assigning to metal–oxygen bond and oxygen-containing functional groups, respectively.³⁶

Fig. 3 XPS spectra of Co(OH)₂: (a) survey spectrum, (b) Co 2p, and (c) O 1s.

The results from XRD, Raman, FTIR, FESEM, EDS, HRTEM, SAED, and XPS analyses confirmed the formation of Co(OH)₂ and PC. Scheme 1 demonstrates the preparation route of the Co(OH)₂ and porous carbon. The Co(OH)₂ preparation could have plausibly followed the three reaction steps during the hydrothermal process as hydrolysis of urea (eqn (1)), formation of Co(II)-citrate complex (eqn (2)), and nucleation of flowerlike Co(OH)₂ via substitution of [Co₂(C₆H₄O₇)₂]⁴⁻ complex (eqn (3)).^37,38

NH₂CONH₂ + 3H₂O → 2NH₄⁺ + OH⁻ + HCO₃⁻ (1)

Co²⁺ + citrate → [Co₂(C₆H₄O₇)₂]⁴⁻ (2)

[Co₂(C₆H₄O₇)₂]⁴⁻ + OH⁻ → Co(OH)₂ (3)

Scheme 1 Graphical illustration of the synthesis routes for (a) Co(OH)₂ and (b) porous carbon.

The detailed formation protocol for the PC is elucidated in our previous work.²⁴ Briefly, during the reaction, the carbon source (i.e. tea waste) first reacts with solid KOH, and intercalation of potassium occurs in the carbon matrix.³⁹ After washing, the removal of potassium leads to the creation of pores. Plausible reaction steps are described as eqn (4)–(8).²⁴

2C + 6KOH → 2K₂CO₃ + 2K + 3H₂ (4)

2K₂CO₃ → K₂O + CO₂ (5)

K₂CO₃ + 2C → 2K + 3CO (6)

CO₂ + C → 2CO (7)

C + K₂O → 2K + CO (8)

3.2 Electrochemical property evaluation

3.2.1 Three-electrode cell study of synthesized materials. Assessment of the electrochemical properties of the prepared Co(OH)₂ and PC were performed by conducting CV, GCD, and EIS measurements using a three-electrode (3-E) cell and 3 M KOH as the electrolyte. The working electrode was constructed by coating the active electrode materials on 1.5 cm × 1.5 cm Ni foam. A stable CV profile was achieved for Co(OH)₂ and PC in the potential range from 0 to 0.6 V and from −1 to 0 V, respectively. The CV profile of Co(OH)₂ showed two redox peaks at ∼0.44/0.29 V (Fig. 4a) at a 10 mV s⁻¹ scan rate, which can be attributed to its faradaic redox reactions at the electrode/electrolyte interface (eqn (9) and (10)):^40,41

Co(OH)₂ + OH⁻ ↔ CoOOH + H₂O + e⁻ (9)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (10)

Fig. 4 (a) CV curves and (b) GCD curves of Co(OH)₂. (c) CV curves and (d) GCD curves of PC in 3 M KOH.

When the scan rate was increased from 10 to 100 mV s⁻¹, the oxidation and reduction peaks shifted towards the forward and backward sides. These findings suggest the diffusion-controlled behavior of the redox reaction. At high scan rates, the electrode material could not efficiently access the electrolyte ions. The Randles–Sevcik plot exhibited a linear connection for the oxidation and reduction peak currents and the square root of the scan rate (Fig. S2†).

The charging and discharging properties of the Co(OH)₂ electrode were analyzed through galvanostatic charge–discharge (GCD) measurement (Fig. 4b). These measurements were conducted in a voltage range of 0–0.5 V. A specific capacitance value of 801 F g⁻¹ at a current density of 1 A g⁻¹ was attained for the Co(OH)₂ electrode. CV and GCD measurements of PC were performed in the potential window ranging from −1 to 0 V in 3 M KOH. Electrochemical impedance spectroscopy (EIS) of Co(OH)₂ was assessed in the frequency range of 0.01–10 000 Hz. Fig. S3† displays the Nyquist plot of Co(OH)₂. EIS measurements were performed to determine the values of charge-transfer resistance (R_CT) and internal resistance (R_S) of the Co(OH)₂ electrode in the aqueous KOH electrolyte during the electrochemical process, and a Nyquist plot was generated (Z′ vs. Z′′) (Fig. S3†). R_S and R_CT values were determined by fitting the data with an equivalent Randle's circuit (inset of Fig. S3†). In the Nyquist plot, the appearance of a semicircle in the high-frequency region indicates the presence of R_CT. R_CT originates from the resistance at the electrode–electrolyte interface and can be estimated from the diameter of the semicircle.^42,43 R_S signifies the combination of the ionic resistance of the electrolyte, the intrinsic resistance of the electrode material, and the contact resistance between the electrode and the current collector.⁴² R_S can be determined from the intercept of the semicircle on the x-axis (Z′) at the high-frequency region. The R_S and R_CT values of Co(OH)₂ were 0.38 and 1.98 Ω, respectively.

Fig. 4c and d present the CV and GCD of PC, respectively. The quasi-rectangular cyclic voltammogram and linear galvanostatic charging–discharging curves indicated its electric double-layer capacitive (EDLC) characteristics. PC attained a specific capacitance of 516 F g⁻¹ at a current density of 1 A g⁻¹. This PC was used as the anode material in the assembled asymmetric supercapacitor (ASC) device. The highly porous structure of PC promotes rapid electrolyte ion diffusion and accelerates ion transport within the device, which could contribute to the improved performance of the device.

3.2.2 Asymmetric supercapacitor device study. A two-electrode (2-E) asymmetric cell was constructed with Co(OH)₂ as the cathode, tea-waste-derived PC as the anode, and a filter paper (as a separator) soaked in 3 M KOH as the electrolyte. This 2-E cell can be designated as Co(OH)₂//PC, and its operational potential can be extended up to 1.5 V (Fig. 5a). Beyond that potential, the CV profile showed slight distortion caused by the thermodynamic breakdown of water (Fig. 5b). Fig. 5c shows the CV profiles of the ASC cell in the potential range of 0–1.5 V, depicting good stability even at high scan rates. Besides, the ASC cell also showed stable GCD curves at different current densities (2 to 12 A g⁻¹) (Fig. 5d), which was congruous with its CV observations. These analyses indicated that the ASC cell possesses great electrochemical reversibility. At the current densities of 2 and 12 A g⁻¹, the ASC cell exhibited a C_S value of 225.3 F g⁻¹ and 200 F g⁻¹, respectively. Fig. 5e presents the derived Ragone plot of the ASC cell from its energy and power densities, which demonstrated that it has the capability of supplying an energy density of 57.4 W h kg⁻¹ at a power density of 1222.7 W kg⁻¹. The EIS measurements revealed that it possesses R_S and R_CT values of 0.6 and 1.06 Ω, respectively. The obtained Nyquist plot is presented in Fig. 5f.

Fig. 5 (a) CV curves of Co(OH)₂ (0–0.6 V), PC (−1–0 V), and Co(OH)₂//PC ASC devices (0–1.5 V) at a scan rate of 30 mV s⁻¹. (b) CV curves with different potential window, (c) CV curves with different scan rates, (d) GCD profile with different current densities, (e) Ragone plot, and (f) Nyquist plot (inset: Nyquist plot in the high-frequency region and fitted circuit) of the Co(OH)₂//PC ASC device.

The successful fabrication of the ASC cell encouraged us to construct a solid-state supercapacitor device to demonstrate the potentiality of the synthesized materials in modern portable electronic devices. Here, both the cathode and anode were kept the same as in the previous ASC cell, but the filter paper was replaced by solidified 3 M KOH encapsulated PVA gel. In this case, PVA gel acted as the separator, and aqueous KOH was the electrolyte. This also acts as the electrolyte in the fabricated solid-state supercapacitor device. The as-constructed device is graphically portrayed in Fig. 6a. Then, to analyze the performance of the device, different parameters, such as specific capacitance (C_S), energy and power densities, and coulombic efficiency, were evaluated. Fig. 6b presents the CV curves of this device at scan rates varying from 10 to 100 mV s⁻¹, which displays the retention of the initial shape with the increase in scan rates. This depicted the occurrence of reversible electrochemical processes in the device. The GCD curves of the device measured at numerous current densities are shown in Fig. 6c, and the observed C_S value at 2 A g⁻¹ was 260 F g⁻¹. The device also exhibited a coulombic efficiency of ∼100% (Fig. S4†) at 6 A g⁻¹; hence, a stability test was conducted at this current density. After ∼5000 cycles of constant charging and discharging processes, the device retained ∼91% of its initial C_S value, demonstrating a long lasting shelf life (Fig. 6d). Fig. S5† displays the XRD and FESEM analyses of the Co(OH)₂. After ∼5000 GCD cycles, no significant changes were observed in the structural properties and morphology of the electrode material.

Fig. 6 (a) Graphical sketch of a solid-state Co(OH)₂//PC ASC device. (b) CV scans and (c) GCD curves of the solid-state Co(OH)₂//PC ASC device. (d) Cycling stability test (inset: first GCD 10 cycles) of the device.

The Ragone plot (Fig. 7a) was derived from the resultant energy and power density values and compared with a few of the values reported on Co(OH)₂ and/or porous carbon-based ASC devices. The energy density of the solid-state Co(OH)₂//PC device was found to be 76.7 W h kg⁻¹ at a power density of 1416.9 W kg⁻¹. These resultant values were comparable and, in a few cases, superior to the values found in the literature (Table 1). The R_S and R_CT values of the solid-state ASC were determined from EIS measurements (Nyquist plot and equivalent circuit as inset presented in Fig. 7b). R_S and R_CT values were 0.95 and 1.41 Ω, respectively. Equivalent circuit resistance (ESR) was calculated by summing R_S and R_CT values, The low value of ESR of this device contributes to its high power density.⁸

Fig. 7 (a) Ragone plot of the solid-state Co(OH)₂//PC ASC device with a comparison of reported research on Co(OH)₂-based supercapacitors, (b) Nyquist plot, and (c) CV test with different angles of the solid-state ASC device. (d) LED light illuminated by the solid-state ASC device.

Table 1 Comparison of the fabricated Co(OH)₂//PC ASC device (this work) with some of the already reported Co(OH)₂-based two-electrode supercapacitors

Sr. No.	Material	Electrolyte	Potential (V)	Energy density (W h kg^−1)	Power density (W kg^−1)	Retention (cycles)	Ref.
1	Co(OH)2/GNS//AC/CFP	1 M KOH	1.5	19.3	187.5	109% (20 000)	44
2	Ni1–Co2–S/Co(OH)2//AC	PVA/KOH	1.6	48.8	800	88% (10 000)	45
3	Co(OH)2@PPy//CNTMN	PVA/KOH	1.6	10.53@mW h cm^−3	0.53@W cm^−3	89% (5000)	46
4	Co(OH)2 NAs@Ni//CNTs@Ni	2 M KOH	1.5	19.05	112.5	∼96% (7000)	47
5	CF–Ni(OH)2//CF–CNT	PVA–KOH	1.4	41.1	1400	98% (3000)	48
6	CF–Co(OH)xCO3//CF–CNT	PVA–KOH	1.4	33.45	1400	97.6% (3000)	49
7	MnS/Ni3S2@Co(OH)2//AC	1 M KOH	1.5	68.6	750	91.2% (10 000)	50
8	A-ZCo@NF//AC@GF	3 M KOH	1.5	365.5@μW h cm^−2	752.5@μW cm^−2	80.86% (10 000)	14
9	Co(OH)2/rGO//AC	PVA/KOH	1.4	28	1754	89% (4000)	20
10	ACH//ACH	2 M KOH	—	6.85	601.46	95% (5000)	51
11	Co(OH)2//Co(OH)2	1 M KOH	1.2	3.96	42 000	81% (5000)	21
12	AC//Co(OH)2 buds/Ni foam	6 M KOH	1.6	20.3	90.6	69.3% (1000)	10
13	β-Co(OH)2//AC	6 M KOH	1.6	37.3	800	>90% (5000)	22
14	β-Co(OH)2//rGO	6 M KOH	1.4	20.05	1220	93% (10 000)	12
15	Co(OH)2@PANI//AC	6 M KOH	1.4	31.2	1050	74.8% (5000)	52
16	Co(OH)2/fCNT//fCNT	PVA : KOH	1.4	3.8	7000	84% (5000)	53
17	CF/Cu(OH)2/Co(OH)2//CF/AC	PVA–KOH	1.5	3.5@mW h cm^−2	1.87@mW cm^−2	81% (12 000)	54
18	Co2(CO3)(OH)2NDs/NiCo(CO3)(OH)2//graphene	1 M KOH	—	35.5	2555.6	71.3% (10 000)	55
19	MgCo2O4@α-Co(OH)2/NF-2//AC	PVA–KOH	1.6	65.94	773.19	91.7% (10 000)	56
20	AC//AC/MB-400/Co(OH)2	3 M KOH	1.6	18	400	80.05% (5000)	57
21	Cu(OH)2@ZnNi–CoCH//AC	PVA–KOH	1.5	34.75	375	86.54% (6000)	35
22	Co(OH)2@NiMoS4//AC	3 M KOH	2.0	59.5	1000	100% (5000)	11
23	β-Co(OH)2//AC	6 M KOH	1.2	10	300	69.6% (5000)	23
24	Co3O4/Co(OH)2//AC	2 M KOH	1.5	25.6	939	84.5% (5000)	58
25	Mn–Co(OH)2/CoS//AC	2 M KOH	1.65	62.1	410	164.1% (30 000)	59
26	Co(OH)2//PC	KOH + PVA	1.5	76.7	1416.9	91% (5000)	This work

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To establish the flexibility of the device, CV measurement was performed when bending it at different angles (Fig. 7c). Almost identical CV profiles were observed for the different bending shapes of this device, which illustrated its remarkable flexibility. The as-fabricated device effectively lit up a panel of red LEDs, which validated its real-life applicability (Fig. 7d).

4. Conclusions

In conclusion, the Co(OH)₂ hollow sphere was synthesized using a single-step hydrothermal process. Biomass-derived (i.e., from the tea waste) porous carbon was synthesized using the KOH activation method, which showed a highly porous matrix. Co(OH)₂ showed redox activity in an alkaline medium with a high C_S value of 801 F g⁻¹ at a current density of 1 A g⁻¹. Moreover, porous carbon exhibited an EDLC nature and C_S value of 516 F g⁻¹ at a current density of 1 A g⁻¹. An asymmetric supercapacitor device was assembled using Co(OH)₂ as a cathode and PC as an anode. First, the filter paper soaked in KOH was used as an electrolyte/separator to test the ASC device, which verified its good activity as an ASC device. Then, a gel electrolyte with PVA and KOH was prepared to fabricate a flexible solid-state Co(OH)₂//PC ASC device. The prepared PVA + KOH gel electrolyte acted as an electrolyte and a separator. The solid-state Co(OH)₂//PC ASC device performed well and showed a high C_S value of 260 F g⁻¹ at a current density of 2 A g⁻¹. It retained ∼91% of the initial C_S value (at 6 A g⁻¹) till ∼5000 cycles. The high-performance Co(OH)₂//PC ASC device exhibited an energy density of 76.7 W h kg⁻¹ at a power density of 1416.9 W kg⁻¹. Thus, this study presents an improved performance of cobalt hydroxide and porous carbon for asymmetric supercapacitor devices.

Data availability

All relevant data are available from the corresponding authors upon reasonable request.

Author contributions

Rajeshvari Samatbhai Karmur and Debika Gogoi: conceptualization, investigation, validation, methodology, formal analysis, data curation, writing – original draft, writing – review & editing, visualization. Manash R. Das: XPS analysis data collection. Narendra Nath Ghosh: resources, funding acquisition, supervision, data curation, writing – review & editing.

Conflicts of interest

The authors disclose that they have no conflicting financial interests.

Acknowledgements

N. N. G. acknowledges the Cross-Disciplinary Research Framework (CDRF) scheme BITS Pilani, India (Grant: C1/23/121) for providing financial support. The R. S. K. is thankful to CSIR-HRDG, India, for awarding a senior research fellowship (File No. 09/0919(11330)/2021-EMR-I). The authors acknowledge the Central Sophisticated Instrumentation Facility (CSIF), BITS Pilani K. K. Birla Goa Campus, for providing Raman spectroscopy, FESEM, and EDX facilities.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra05106h

‡ Rajeshvari Samatbhai Karmur and Debika Gogoi contributed equally to this paper.

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