# Phytochemical-enhanced NiO nanostructures for superior oxygen evolution and asymmetric supercapacitor applications

Asma Hayat, Elmuez Dawi, Aneela Tahira, Muhammad Ali Bhatti, Imran Ali Halepoto, Gulzar Ali, Ihsan Ali Mahar, Irum Naz, Aqeel Ahmed Shah, Abdul Ghaffar Solangi, et al.

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## ► To cite this version:

Asma Hayat, Elmuez Dawi, Aneela Tahira, Muhammad Ali Bhatti, Imran Ali Halepoto, et al. Phytochemical-enhanced NiO nanostructures for superior oxygen evolution and asymmetric supercapacitor applications. Nano Express, 2024, 5 (4), pp.045015. 10.1088/2632-959X/ad9ac0. hal-04966871

HAL Id: hal-04966871

https://hal.univ-lorraine.fr/hal-04966871v1

Submitted on 2 Mar 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Phytochemical-Enhanced NiO Nanostructures for Superior Oxygen Evolution and
Asymmetric Supercapacitor Applications

Asma Hayatª, Elmuez Dawi<sup>d*</sup>, Aneela Tahira<sup>b</sup>, Muhammad Ali Bhatti<sup>c</sup>, Imran Ali Halepoto<sup>i</sup>, Gulzar Ali<sup>a</sup>,
Ihsan Ali Mahar<sup>a</sup>, Irum Naz<sup>a</sup>, Aqeel Ahmed Shah<sup>f</sup>, Abdul Ghaffar Solangi<sup>b</sup>, Umair Aftab<sup>h</sup>, Riyadh H.
Alshammari<sup>c</sup>, Ayman Nafady<sup>c</sup>, Brigitte Vigolo<sup>g</sup>, Zafar Hussain Ibupoto<sup>a*</sup>

<sup>a</sup>Institute of Chemistry, University of Sindh, Jamshoro 76080, Pakistan.

<sup>b</sup>Institute of Chemistry, Shah Abdul Latif University Khairpur Mirs, Sindh, Pakistan.

<sup>c</sup>Centre for Environmental Sciences, University of Sindh Jamshoro, 76080, Sindh, Pakistan.

<sup>d</sup>College of Humanities and Sciences, Department of Mathematics and Sciences, Ajman University, P.O.Box 346, Ajman, UAE.

<sup>e</sup>Chemistry Department, College of Science, King Saud University, Riyadh, 11451, Saudi Arabia.

<sup>f</sup>Wet Chemistry Laboratory, Department of Metallurgical Engineering, NED University of Engineering and Technology, University Road, Karachi 75270, Pakistan.

<sup>g</sup>Université de Lorraine, CNRS, IJL, F-54000 Nancy, France.

<sup>h</sup>Department of Metallurgy and Materials, Mehran University of Engineering and Technology, 7680 Jamshoro, Sindh, Pakistan.

<sup>i</sup>Institute of Physics, University of Sindh, Jamshoro 76080, Pakistan.

*Corresponding author(s): Zafar Hussain Ibupoto, PhD, Email: zaffar.ibhupoto@usindh.edu.pk,
Elmuez Dawi, PhD, Email: e.dawi@ajman.ac.ae

**Abstract**

The development of new energy conversion and storage technologies has contributed to the widespread use of renewable energy. However, new methodologies for electrochemical energy storage systems remain to be developed. This study presents a facile, low-cost, scalable, and environmentally friendly method for the synthesis of nickel oxide (NiO) nanostructures by hydrothermal methods using lotus root extract. The different volumes of lotus root extract were tested on NiO nanostructures (sample 1, sample 2) using 1 mL and 2 mL amounts of the extract, respectively. Therefore, phytochemicals from lotus extract have influenced the surface
morphology, crystal quality, optical band gap, electrical conductivity, and surface active sites of NiO nanostructures. Sample 2 of the NiO nanostructures was found to be highly active for oxygen evolution reaction (OER) and showed an overpotential of 380 mV at 10 mA cm⁻² and a durability of 30 hours at 10 mA cm⁻². Furthermore, sample 2 of NiO has shown specific capacitance of 1503.84 F/g at 2 A/g as well as cycling stability over a period of forty thousand GCD cycles. The percentage specific capacitance retention were highly improved up to 100.6%. An asymmetric energy storage device has been constructed using NiO sample 2 as the anode electrode material, demonstrating excellent specific capacity of about 1113 C/g at 5 A/g. For the asymmetric supercapacitor device, a power density of 20000 W/kg and an energy density of 245 Wh/kg were obtained. In a study of cycling stability for 40000 GCD cycles, it was observed that the asymmetric device retained 96.86% of its specific capacitance. A significant contribution was made to the electrochemical performance of sample 2 of NiO by phytochemicals derived from lotus extract.

**Keywords:** NiO nanostructures, Electrochemical performance, Supercapacitors, Oxygen evolution reaction,

# 1. Introduction

Water splitting is a chemical process that involves two main processes, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These two processes are conducted electrochemically and photochemically [1]. One of these two processes, OER, is a relatively complex reaction with a high energy barrier [2], thus adversely affecting hydrogen production via water splitting. It has been imperatively emphasized that active electrocatalysts must be developed in order to accelerate the kinetics of OER [3, 4]. The researchers have therefore been investigating various electrocatalysts for splitting water in OER half-cells [5-10]. Several OER electrocatalysts are currently being used, which are made from noble metal oxides such as iridium and ruthenium, but they are not suitable for large-scale applications due to their rarity and high cost [11, 12, 13, 14]. It is therefore necessary to develop nonprecious and efficient electrocatalytic materials for the OER process [15]. Apart from the development of renewable energy resources such as hydrogen production from electrochemical water splitting, the development of high performance electrochemical energy storage devices utilizing renewable
energy is also highly necessary [16, 17]. As a result of their high power density, high energy
density, long cyclic stability, considerable sustainability, and eco-friendliness, lithium ion
batteries, and supercapacitors (SCs) are excellent energy storage systems [18-21]. Because of
their high specific power, rapid charging and discharging capabilities, and long-term cycling
stability, SCs have received considerable attention among the energy storage systems. The main
storage devices used in SCs fall into two categories: electric double-layer capacitors (EDLCs)
and pseudo capacitors [22]. In EDLCs, charge is stored electrostatically, whereas in pseudo
capacitors, charge is stored using a redox process. Pseudo capacitors and ELDC energy storage
systems have been designed using transition metal oxides such as RuO₂, NiO, MnO₂, and ZnO
[23], metal sulfides [24], and polyanilines (PANI) [25, 26]. The pseudocapacitors exhibit rapid
reversible redox reactions, and their specific capacitances are 10 to 100 times greater than those
of EDLC energy storage devices [27]. Pseudocapacitors have therefore been studied at a higher
level than ELDCs. Pseudo capacitors require high performance electrode materials in order to
achieve high specific capacitances and power densities. The literature survey regarding water
splitting and SCs indicates that both require highly efficient materials with enhanced electrical
conductivity, abundant surface active sites, stability, and low cost [28-30]. In contrast to the
other electrode materials, NiO has a number of unique and appealing characteristics, such as its
high catalytic activity, abundance of precursors, low cost, good capacitance, and non-noble
characteristics [31-33].

In applications such as supercapacitors and electric energy storage, NiO nanostructures are
ideal due to their surface, structural, and electrochemical properties [34-36]. The above factors
have led to a variety of synthetic routes for NiO, including hydrothermal, co-precipitation, and
sol-gel processes [37]. In spite of their advantages, these methods are relatively expensive,
require toxic chemicals, and are environmentally damaging. As a consequence, environmentally
friendly methods have gained increasing popularity for the synthesis of nanostructures [38].
Plant extracts containing amino acids, nitrogenous bases, reducing sugars, and polyphenols with
antioxidant properties have been studied [39]. As these biomolecules possess the properties of
reducing, capping and stabilizing agents, they have been found to be essential in tuning the
surface, size and stability of various nanostructures. The use of plant extracts in the synthesis of
NiO nanostructures, such as Zea mays leaf silk, Moringa oleifera, and Aloe vera, has
demonstrated improved performance in a variety of electrochemical applications [40-42].
Therefore, extracts of a variety of plant parts have been widely used in the synthesis of materials.

In this study, we investigated the effects of lotus root extract on the synthesis of NiO
nanostructures and their improved electrochemical performance, which is a first of its kind study
in the field. Lotus roots contain a variety of phytochemicals, such as starch, fiber, fat, sugar,
vitamins such as ascorbic acid (vitamin C), vitamins B6, potassium, and minerals such as
calcium, copper, and iron [43]. It has been shown that the phytochemicals found in lotus roots
significantly affect the shape, size, optical band gaps, and charge transport properties of NiO
nanostructures used in the synthesis of NiO nanostructures. These phytochemicals have
properties that reduce, cap, stabilize, and direct the structure.

**2. Method and Material**

**2.1.Chemicals and reagents**

The following chemicals were purchased from Sigma Aldrich Karachi and were used as
received: nickel chloride hexahydrate (98%), aqueous ammonia solution (33%), potassium
chloride (99%), acetone (99.9%), ethanol (99.5%), and 5% Nafion. The desired solutions were
prepared in deionized water. Lotus roots were purchased from a local market in Jamshoro Sindh,
Pakistan, washed with deionized water to remove contaminants, and then dried at room
temperature. As a result, the dried lotus root vegetables were chopped into small pieces and
ground into a powder. Approximately 50 grams of dried powder Lotus root were added to 100
mL of deionized water and stirred at 75 °C for three hours. This was done to obtain the lotus root
extract. After the filtration process, the filtrate of the lotus roots was considered as the lotus
extract, which was kept at 4 °C in the refrigerator before being used for the synthesis of NiO
nanostructures.

**2.2.Synthesis of NiO using lotus root extract by modified hydrothermal method**

According to the literature on the electrochemical aspects of NiO, the material has significant activity towards energy conversion and storage, but does not possess abundant surface active sites or charge storage capacity. The proposed modified hydrothermal method has merits over conventional hydrothermal method such as more ecofriendly, using the green source of reducing, capping, stabilizing and structure directing agents. In this study, the modified hydrothermal
process was used to enhance the electrochemical performance of NiO. NiO nanostructures are
synthesized by a low temperature hydrothermal method in two steps. First, and to begin with,
nickel hydroxide is formed below 100 °C for four to five hours. Accordingly, the presented study
was carried out at 95 °C and for a period of five hours in order to obtain nickel hydroxide
material at a significant rate. The phytochemicals lotus extract having reducing, capping, and
structure directing and stabilizing agents have certain oxygenated terminated groups that could
be used for the rapid kinetics of growth process towards the formation of nickel hydroxide
crystals. Hence, the large amount of precipitates was achieved for the NiO nanostructures
synthesized in the presence of lotus extract compared to the NiO nanostructures prepared without
the use of lotus extract. Second, the transformation of nickel hydroxide into NiO has been carried
out at 500 °C for 5 hours to ensure that the hydroxide phase is completely transformed into metal
oxide. For the preparation of pure NiO, 0.1 M NiCl2.6H2O and 5 mL of ammonia solution were
added to 95 mL of deionized water with a total volume of growth solution of 100 mL. For the
synthesis of NiO using different volume of lotus root extract, two beakers containing 1 mL and 2
mL of lotus root extract along with 0.1 M NiCl2.6H2O and 5 mL of ammonia solution. Then,
total volume of 100 mL of growth solution was made by adding distilled water. The NiO growth
solutions were covered with aluminum sheet in order to avoid the evaporation of water. To
conduct the hydrothermal process, covered growth solutions were placed in an oven for 5 hours
at 95 °C. After completion of the hydrothermal reaction, the growth solutions were filtered and
nickel hydroxide precipitates were collected and dried for overnight. The precipitates of nickel
hydroxide obtained with 1 mL and 2 mL of lotus root extract were labelled as sample 1 and
sample 2 respectively. Later, these precipitates were placed in china made crucible and
combusted in a muffle furnace at 500 °C for 5 hours in order to get NiO oxide nanostructures.
Finally, the grey color NiO products were obtained. In order to examine the crystal structure of
samples, a PAnalytic X'Pert PRO X-ray diffractometer was used with scan parameters of 10 to
80 two theta, 0.02 step, and 0.3 seconds. Scanning electron microscopy was used to study surface
morphology using a Carl Zeiss EVO-18 instrument at an accelerating voltage of 10 kV.
Shimadzu UV-1900 UV-visible spectrophotometer was used to measure the band gap energy
through absorbance spectra from 200-700 nm. Using the KBR pellet method with the Thermo
Nicolet 380 FTIR instrument, a FTIR spectrum in the range 4000-400 cm-1 was used to analyze
the functional groups located on the surface of NiO samples. An overview of the synthesis process for the proposed NiO samples is provided in Scheme 1.

<figure><img src="image_3.png" /><figcaption>**Scheme 1:** Generalized synthesis of NiO nanostructures using lotus extract.</figcaption></figure>

### 2.3. Electrochemical Investigations

For electrochemical analysis, a Versa Potentiostat was used. To measure the OER activity of NiO Versa Potentiostat were used for electrochemical analysis. The OER activity of NiO nanostructures was measured using a three-electrode cell setup, in which glassy carbon electrodes (GCE) were used as working electrodes, the platinum wire was used as counter electrodes, and Ag/AgCl filled with 3M KCl was used as reference electrodes. An Alumina slurry of 0.03m was used in conjunction with silicon paper to clean the GCE.

For the preparation of the catalyst solution, 10mg of NiO material was dissolved in 2 mL of deionized water, then 30 µL of 5% Nafion solution were added and the solution was stirred for 15, 20 minutes. Then, 5µL of material ink was deposited onto GCE with a loading mass of
0.02mg. The preliminary OER electrocatalytic activity was studied in 1M KOH solution as a supporting electrolyte using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS). Prior to LSV at a scan rate of 2 mV/s for preliminary OER characterization involving different NiO nanostructures in 1M KOH aqueous solution, 5 CV cycles were run at a scan rate of 5 mV/s to stabilize the electrode. The inactivity of bare GCE during OER studies was observed compared with NiO nanostructures. Chronoamperometry was used to study NiO durability, while electrochemical impedance spectroscopy (EIS) was employed to investigate charge transport. An EIS study was conducted with sweeping frequencies of 100 kHz to 0.1Hz, an amplitude of 10 mV, and a biasing of the onset potential of the OER. A galvanostatic charge-discharge (GCD) method was employed to study supercapacitors at different current densities and CV modes, using nickel foam as an electrode due to its high electrical conductivity and large surface area. A dip coating method was used to load NiO onto nickel foam, followed by 30 minutes of low temperature sintering at 70 °C. The loading mass of NiO onto nickel foam is 0.5 mg. As the electrocatalytic performance of materials is mainly determined by their catalytic sites and electrical conductivity, we applied 1M KOH as an electrolyte during OER studies. The performance of supercapacitors is highly dependent on the adsorption of hydroxide ions, so we used relatively high concentrations of 3M KOH during supercapacitor studies.

### 3. Results and discussion

#### 3.1. Optical, crystalline structure, morphology, and functional group analysis of as synthesized NiO nanostructures using the lotus root extract

To illustrate electrical conductivity in electrochemical applications, it is essential to investigate the optical properties of semiconducting materials, particularly their optical band gaps. Figure 1a shows the UV-visible absorption spectra of pristine NiO, 1mL and 2mL of lotus root extract-assisted samples 1 and 2, respectively. There were typical band structures observed for NiO between 280 nm and 370 nm, indicating semiconducting properties between the O (2p) valence band and the Ni (3d) valence band [44]. This describes the inter-band π-π* electronic transitions for the proposed NiO nanostructures. Tauc plot estimated a direct energy band gap using Equation 1 [45]:

$$ (\alpha h\nu) = (h\nu - Eg) \qquad (1) $$
Hereby, the absorption coefficient, $\alpha$ indicates absorbance, $h\nu$ describes photon energy, $E_g$ identifies the energy band gap, whereas $n$ illustrates the type of transition process. In Figure 1b, Tauc's plots of pure NiO, sample 1 and sample 2 are shown. For pure NiO, sample 1 and sample 2, the optical band gaps were estimated to be 3.47, 3.04 and 2.69 eV, respectively. The optical band gap values of NiO nanostructures were significantly reduced due to phytochemicals from lotus extracts influencing particle size, shape orientation, and surface properties.

In this study, powder XRD was used to investigate the effect of lotus root extract on the crystalline properties of NiO nanostructures. An XRD reflection of NiO synthesized from lotus root extract of 1 mL and 2 mL, as well as pure NiO, is shown in Figure 2. According to XRD reflections, NiO is the predominant component at two-theta of 37.3°, 43.3°, and 62.8°, 75.6°. The respective Miller indices (111), (200), and (220) confirm the formation of high-quality NiO. It has been found that these observations concerning the diffraction patterns of NiO are in good agreement with those reported in published works [46, 47]. A high degree of agreement was observed between the measured NiO diffraction patterns and the standard JCPDS card no: 01-089-7130, confirming the cubic phase of NiO. Figure 2 shows the NiO materials were free of other impurities. A Scherrer equation was used to calculate the average crystallite size. The corresponding values for pure NiO, sample 1 and sample 2 were 136.62 nm, 70.84 nm and 41.20 nm, respectively. According to the calculated average crystallite size values, the phytochemicals in the lotus extract significantly affected the size of NiO nanostructures through the variation in nucleation rate and its effects on crystal growth during the synthesis process.

SEM has been used to examine the surface morphology of as-prepared materials to determine whether the surface structure is compatible with the material's functionality. As shown in Figure 3a-f, the effect of Lotus root extract on NiO's morphology evolution was examined using SEM analysis at two different magnifications. In the SEM images, NiO is oriented into two-dimensional sheets and clusters, indicating heterogeneous surface morphology of NiO nanostructures. Figures 3a,b illustrate that pure NiO exhibits small leaves with a high level of monodispersity. According to Figures 3c and 3d, NiO prepared with 1 mL of lotus root extract exhibits a loss of homogeneous shape orientation and a slight increase in the formation of clusters. As shown in Figures 3e and 3f, 2 mL of lotus root resulted in fewer leaves, but more increased NiO cluster formation. Because NiO nanoparticles have a variation in surface energy
and ultrafine properties, they can play an important role in electrode applications. Additionally, the phytochemicals contained in lotus extract were found to induce the nonhomogeneous nucleation kinetics of NiO nanostructures. As a result, heterogeneous morphology has been observed [48]. Many of the phytochemicals of lotus extract contain different oxygenated groups which may be bonded to the nickel ions during the growth process in addition to hydroxide ions, thus providing variation in nucleation rate and impacting the final structure development. Figure 3c-f illustrates how these aspects were fully observed for NiO obtained from lotus extract.

An example of the chemical bonding characteristics of various NiO nanostructures is illustrated in Figure 4 by FTIR spectra of pure NiO, sample 1 and sample 2. A typical Ni-O metal oxide bond was confirmed by stretching vibrations in the range of 430-680 cm⁻¹. As shown in Figure 4a, pure NiO has a variety of IR bands including 435, 474, 575, 614, 868, 705, 765, 783, 816, 877, 1023, 1370, 1448, 1599, 2855, 2926, and 3548 cm⁻¹. As a result of the adsorption of hydroxide groups on NiO surfaces, the surfaces had been hydrated, and their presence was detected at 3548 cm⁻¹, as well as a small peak intensity at 1599 cm⁻¹ (Figure 4a). A typical C-H stretching frequency was found at 2855 cm⁻¹ and 2926 cm⁻¹. In a few bands between 1023, 1370, and 1448 cm⁻¹, it is possible to detect aromatic cycles and polyphenol functional groups [49]. According to Figure 4a, some of the IR bands for the hydroxyl groups appeared from the growth precursors, while the C=O groups were observed in the range of 765-877cm⁻¹. Although sample 1 and sample 2 of NiO obtained with lotus extract showed almost the same type of FTIR bands, there was a slight shift in the IR frequency. This indicates that functional groups from the lotus extract react with NiO. An FTIR study has confirmed the presence of NiO nanostructures and strongly supports the identification of the phase by XRD measurements.

### 3.2. Oxygen evolution reaction (OER) performance of various lotus toot extract mediated NiO nanostructures

The lotus root extract improved both the surface properties and the electrical conductivity of NiO, resulting in a favorable OER activity for the sample 2. In 1M KOH alkaline electrolyte, NiO-based electrocatalysts have shown OER catalytic activity by oxidizing Ni(II) to Ni(III) by producing oxyhydroxy intermediates that are extremely conducive to catalysis, providing a large number of surface catalytic sites for the promotion of OER [50]. In Figure 5a, the LSV curves of pure NiO, sample 1 and sample 2 were measured at a scan rate of 2 mV/s in 1M KOH aqueous
solution. The OER activity of the sample was observed at lower overpotentials than that of the sample and pure NiO. Interestingly, pure NiO, sample 1 and sample 2 exhibited overpotentials of 490, 420, and 380 mV at 10 mAcm-2, respectively. In Figure 5b, sample 1 and sample 2 bar graphs are depicted for a better understanding of the estimated overpotential values of pure NiO. Because lotus root extract contains reducing, capping, stabilizing, and structure-directing agents that favorably enhanced NiO active sites and charge transport, the OER kinetics of sample 2 were accelerated at a low overpotential by using different types of reducing agents, capping agents, stabilizing and structure-directing agents. Figure 5c shows the Tafel analysis of the linear region of the LSV curves for pure NiO, sample 1, and sample 2. The Tafel slope of sample 2 was the lowest when compared with that of sample 1 and pure NiO, which indicates that the OER kinetics are favorable and rapid. Accordingly, the Tafel slope values for pure NiO, sample 1 and sample 2 were 291 mVdec⁻¹, 154 mVdec⁻¹, and 112 mVdec⁻¹, respectively. As a matter of fact, Tafel plots can be used to evaluate the reaction kinetics of electrocatalytic materials, with the smaller the Tafel slope value, the faster the reaction kinetics and the higher the performance of the electrocatalyst [51, 52]. To investigate the stability further, LSV curves were used before and after the stability test. It can be seen in Figure 5d that the curves almost overlap. Additionally, the overpotential of sample 2 did not change at 10 mAcm⁻² compared to LSV curves before the stability test, indicating a high degree of durability over 30 hours as shown in Figure 5e. The increased stability and durability of sample 2 can be attributed to phytochemicals that modified the surface compatibility with the GCE surface, resulting in a greater level of NiO adhesion. The enhanced OER kinetics of sample 2 can be attributed to the phytochemicals' dynamic role in increasing the catalytic sites, reducing the particle size, increasing the compatibility of biomass assisted NiO with electrode surfaces due to biomimic effect and decreasing the optical band gap. Because of the unique properties of phytochemicals in the lotus root extract, a NiO based electrocatalyst can be designed to produce hydrogen through electrochemical water splitting. The charge transfer resistance (R<sub>ct</sub>) of pure NiO, sample 1 and sample 2 was determined by EIS Nyquist plots fitted with equivalent circuits as shown in Figure 5f. The equivalent circuit elements were the solution resistance (R<sub>s</sub>), the charge transfer resistance (R<sub>ct</sub>), and the constant phase element (CPE). At the applied OER onset potential of 1.51V vs. RHE, the observed R<sub>ct</sub> value of sample 2 was found 239.3 Ω cm<sup>-2</sup> which was lower than the sample 1 1603 Ω cm<sup>-2</sup> as well as pure NiO 2491 Ω cm<sup>-2</sup>. As a result, the electron transfer rate for sample 2 at the interface
was significantly faster than sample 1 [53]. To understand the enhanced performance of sample 2
in terms of OER, the electrochemical active surface area (ECSA) of sample 2 was calculated.
Based on the existing literature [52], the greater the ECSA value, the more active sites are
exposed, resulting in increased electrocatalytic activity [54]. It has been found that electrical
capacitance double layers (C<sub>dl</sub>) and ECSA exhibit an empirical correlation. The C<sub>dl</sub> has been
achieved by plotting the difference in the current density of the anode and cathode (Δj/2) versus
various scan rates in the given non-faradic portion as shown in Supplementary Figure (S1a-d).
The C<sub>dl</sub> value of sample 2 4.35 mF cm<sup>-2</sup> was found to be higher than bare NF 1.2 F cm<sup>-2</sup>, pure
NiO 1.50 mF cm<sup>-2</sup>, and sample 1 2.28 mF cm<sup>-2</sup> as shown in Supplementary Figure (S1e). Thus,
NiO has the highest ECSA value, which indicates that the active sites are exposed to a greater
extent, making it one of the most active electrocatalytic materials for the OER process.
According to Supplementary Table (S1), the OER activity of sample 2 of NiO was compared to
some of the highly efficient electrocatalytic materials. There is evidence that phytochemicals
have an obvious role to play in enhancing the OER activity of NiO because of their reducing,
capping, stabilizing, and structure directing properties. The sample 2 was synthesized with the
use of phytochemicals from the lotus extract that has influenced on the optical band gap and the
average crystallite size, such aspects of the sample 2 are obviously shown through the UV-
visible and XRD studies, hence they played a vital role in enhancing the OER activity of sample
2. The adsorbed surface groups on the sample 2 from lotus extract also play a role in improving
the electrochemical activity of sample 2. In the present analysis, it was found that the presented
study is simple, low cost, eco-friendly, and has the potential to scale up for the development of
high performance electrocatalytic materials and the provision of green fuel like hydrogen
through the splitting of water. Accordingly, the proposed method of fabricating electrocatalytic
material is capable of showing outstanding OER performance.

3.3.Electrochemical pseudo capacitance performance of various NiO nanostructures

The electrochemical properties of pure NiO, sample 1 and sample 2 were also investigated using
CV and galvanostatic charge-discharge (GCD) methods. The CV curves at various scan rates for
bare nickel foam (NF), pure NiO, sample 1 and sample 2 in 3 M KOH electrolyte aqueous
solution are shown in Figures 6a-d. NiO nanostructures exhibit significant reversible redox
properties with separation potentials of 0.51 V and 0.57 V in different electrode materials [55].
CV curves indicate that the redox peaks capacitance is primarily attributable to the pseudo capacitance characteristics of electrode materials due to their surface faradic properties [56]. The CV analysis of sample 2 indicates that NiO has greater typical battery properties, indicating that it is capable of storing more charge through the Faradic adsorption/desorption process [57]. Further, increasing scan rates were associated with an increase in peak current and a shift to anodic potential, as shown in Figure 6a-d, as well as an increase in peak current and a shift to cathodic potential. As can be seen from the increase in peak current, sample 2 of NiO has a very good capacitance characteristic when compared to all of the electrode materials tested. Additionally, it has been observed that the higher charge storage behavior is observed at low scan rates because electrode materials are used on both the outer and inner surfaces. In contrast, at higher scanning rates, only the surface portions of the electrode material and the process of ionic diffusion are accessible. For better understanding the charge storage performance of sample 2, the comparative CV curves of bare nickel foam, pure NiO, sample 1 and sample 2 at 10 mV/s are shown in Figure 6e. It suggested that the sample 2 have exhibited large area within the CV curves shown in Figure 6e, indicating that the phytochemicals of lotus extract have significantly enhanced the redox aspects of sample 2, hence it has demonstrated the improved specific capacitance. Once again, these aspects are more apparent when the electrode is based on sample 2. Besides CV analysis, GCD analysis was performed on various electrode materials, such as bare NF, pure NiO, sample 1, and sample 2, in 3M KOH electrolyte solution at various current densities, ranging from 1A/g to 5A/g, as shown in Figure 7a-d. In the presence of lower current density, the discharge time of materials was observed for a relatively longer period of time. This was attributed to the pseudo-capacitance behavior of materials [58, 59]. According to Figure 7a-d, sample 2 displayed the largest capacitance values. For sample 2, which has a higher specific capacitance, there is a high degree of coherence in the GCD and CV results. Equation 3 provides the following formula for calculating specific capacitance [60]:

$$C_s = (I \times \Delta t)/(m \times \Delta V) \quad (2)$$

Here $C_s$ indicate the specific capacitance, m signatures are the loading mass of electrode material, potential window as $\Delta V$, current produced as I and $\Delta t$ represents the discharging time, respectively.
As shown in Figure 8a, samples 2, 1, pure NiO, and bare NF exhibit values of 1503.84 F/g, 401.02 F/g, 186.19 F/g, and 35.29 F/g at 2A/g, respectively. Sample 2 has excellent pseudo-capacitance activity due to its large surface area, high adsorption of hydroxide ions, and an abundance of functional groups derived from its organic compounds. To create a large number of active sites and numerous diffusion channels, it is possible to explain its excellent pseudo-capacitance activity. According to Figure 8a, different electrode materials with varying current densities are compared in terms of specific capacitance (C~s~). The correlation between C~s~ and current density can be observed to be inversely proportional. It is highly favorable for ionic diffusion from electrolytic solution to the active surface of electrode materials at low current densities. A higher current density, however, results in poorer ionic diffusion, as described in the reported work [61]. A maximum C~s~ of 1503.84 F/g at 2 A/g for sample 2 of NiO was observed relatively higher than published NiO-based electrode materials [62]. The high electrochemical performance of the synthesized NiO based on lotus root extract has been attributed to the presence of a capping agent and a catalytic agent. Furthermore, it has been shown that functional groups such as carbon and oxygen increase the capacitance of electrode materials for energy storage [63]. Phytochemicals in lotus root extract containing oxygenated and carbonated groups in their structure have been observed to enhance the capacitance performance of NiO in the same way [63]. Energy and power densities were calculated using equations reported in literature [64]. Figure 8a-b illustrates the comparison between power density and energy density at different current densities. As shown in the corresponding columbic efficiency is 98.9 and the percentage retention capacitance is 100.9%. Table 1 summarizes the anode performance obtained with pure NiO, sample 1, and sample 2 prepared with lotus root extract in terms of specific capacitance at various current densities. Nevertheless, the columbic efficiency and parentage capacitance retention were only measured considering the superior performance of this material over pure NiO and sample 1. Both the enhanced capacitance and OER performances of sample 2 could be attributed to crystallite size as estimated from the XRD patterns, revealed the crystallite size dependent electrochemical performance of NiO nanostructures. Furthermore, the supercapacitor performance of lotus extract assisted NiO was compared with the recent literature as given in Supplementary Table (S2). The performance of presented electrode was found to be superior or comparable to many of the recently developed electrode materials in terms of synthesis, specific capacitance, and cycling stability.
<table><caption>**Table 1:** Calculated supercapacitor parameters of pure NiO, Sample 1, sample 2 of NiO prepared with lotus root extract.</caption><thead><tr><th>Sample</th><th>Current Density</th><th>Specific Capacitance (F/g)</th><th>Columbic Efficiency</th><th>Capacitance Retention</th></tr></thead><tbody><tr><td rowspan="5">Pure Sample</td><td>2</td><td>186.19</td><td rowspan="5">--------</td><td rowspan="5">--------</td></tr><tr><td>3</td><td>137.34</td></tr><tr><td>4</td><td>87.98</td></tr><tr><td>5</td><td>75.45</td></tr><tr><td>10</td><td>38.36</td></tr><tr><td rowspan="5">Sample-1 NiO</td><td>2</td><td>401.02</td><td rowspan="5">--------</td><td rowspan="5">--------</td></tr><tr><td>3</td><td>223.27</td></tr><tr><td>4</td><td>145.27</td></tr><tr><td>5</td><td>104.86</td></tr><tr><td>10</td><td>97.19</td></tr><tr><td rowspan="5">Sample-2</td><td>2</td><td>1503.84</td><td rowspan="5">98.9</td><td rowspan="5">100.9</td></tr><tr><td>3</td><td>1350.38</td></tr><tr><td>4</td><td>1154.99</td></tr><tr><td>5</td><td>989.77</td></tr><tr><td>10</td><td>951.41</td></tr></tbody></table>

### 3.4. Asymmetric device applications of NiO nanostructures prepared with lotus root extract

The applicability of NiO nanostructures mediated by lotus root extract as an electrode material for the development of asymmetric supercapacitors (ASC) utilizing NiO nanostructures (sample 2) for two electrode cell set up using NiO and rice husk derived activated carbon (AC) as cathode electrode material. ASC devices are described by the expression NiO LR//AC ASC. It has been shown through the CV cycles at different scan rates that AC has non-Faradic behavior and behaves as an EDLC as shown in Supplementary information Figure (S2). The EDLC stores the charge by electrostatic mechanism. The comparative CV curves of AC and sample 2 at
different potential windows at a scan rate of 10 mV/s are shown in Figure 9a. The selected potential window for the AC was between -1 to 0 V and for the sample 2 was between 0 to 1.6 V vs. Ag/AgCl as shown in Figure 9a.

The sample 2 of NiO as anode material exhibits typical redox properties, resulting in an obvious pseudo-capacitance, hence the current energy storage device store the charge mainly by the redox mechanism. The anode material from sample 2 was assembled into an ASC device with a potential window of 0 to 1.6V. The CV profile of the integrated ASC device at various scan rates is illustrated in Figure 9b. The redox peak was slightly relocated to a potential higher window as shown in Figure 9b, and the scan rates ranged from 10 to 150 mV/s. A direct correlation was found between the redox peak current and the rate of sweeping scans. Due to their capacities and diffusion characteristics, ASC energy storage devices have a variable charge storage capacity. In view of these observations, it appears that the ASC device's charge storage is primarily controlled by the battery type electrode via Faradic reactions. ASC's GCD curves were measured at different current densities ranging from 5A/g to 25A/g, as shown in Figure 9c. GCD profiles have been described as being irregular and asymmetrical, in contrast to EDLC profiles [65-69]. The following is a description of a typical Faradic process that participates in the device:

$$NiO + (OH) \leftrightarrow NiOOH + e^{-} \qquad (3)$$

Due to the fast charge transport and rapid ionic diffusion rate of NiO LR/AC ASC, the specific capacity was calculated to be approximately 1113 C/g at 5A/g based on the GCD curves. The specific capacity varies with current density as shown in Figure 9d. As a result of the presence of C, H, O, and N-like functional groups in lotus root extract, NiO electronic structure and shape were altered, providing an abundance of catalytic sites and numerous ionic diffusion channels. Therefore, it has been found that plant extracts like lotus root can significantly improve the supercapacitor properties of NiO-based anode materials. In Figure 9d, the specific energy and power of the ASC device are illustrated at different current densities. As shown in Figure 9e, at 5 A/g, the highest values were obtained for energy density and power density such as 245 Wh/Kg, and 20000 W/Kg respectively. The energy density of the ASC was found unusual and it could be connected to large current density values using the device characterization. Figure 9f shows the results of the long-term cycling stability of ASC in 40000 GCD cycles at 5A/g, as well as the
columbic efficiency of 93.9 and specific capacitance retention of 96.9 %. As a result of the 40000 GCD cycles, the ASC device retained a specific capacitance of approximately 99.8%, which indicates that the electrode material was activated, resulting in an increased accessibility of the active sites. The columbic efficiency was found to be low because of the mass balancing [70]. Furthermore, the ASC device had a high specific capacitance retention percentage, indicating favorable redox behavior. In this manner, the ASC device exhibits a high specific capacity retention percentage. Due to its modified surface properties and improved electrical efficiency, the ASC device achieved satisfactory columbic efficiency with lotus root extract. In Supplementary Table (S3), the performance of ASC electrodes is compared to the performance of NiO electrodes of various morphologies in KOH electrolyte. As can be seen, the reported materials required multiple steps, require complex methods, involve hazardous chemicals, and require complex procedures in order to be prepared. NiO prepared with Lotus root extract offers a cost-effective, scale-up-friendly, and environmentally friendly route to the development of new generations of functional ASC devices with significant specific capacity. The decrease in the optical band could result the rapid charge transport properties of sample 2 and the surface modification of sample 2 might enabled the significant adsorption of hydroxide ions from the electrolyte thereby these factors played a key role in improving the capacitance properties of sample 2. This The enhanced electrochemical performance of NiO nanostructures prepared with 2 mL of lotus extract (sample 2) indicates that phytochemicals having reducing, capping, stabilizing and structure directing properties have favorable role towards the particle size, shape orientation, crystal structure and surface modification. These aspects of phytochemicals from lotus extract offer a novel green route for the synthesis of next generation of materials for the generation of renewable energy and high performance energy storage systems.

## 4. Conclusions

As a summary, the proposed study highlights the unique and attractive phytochemicals of lotus root extract as potential catalysts and electrode materials based on NiO nanostructures. This is for electrochemical water splitting systems and asymmetric supercapacitors. An analysis of structural studies has revealed that phytochemicals contained in lotus extract significantly altered the surface properties, particle size, and optical band gap during modified hydrothermal methods. Lotus extract phytochemicals have not been reported to have such green properties during the
synthesis of functional NiO nanostructures. NiO sample 2 was found to be an electrocatalytic material with an overpotential of 380 mV at 10 mA cm⁻² for efficient OER activity in alkaline media. Based on the results of the OER measurement, sample 2 of NiO was found to be durable for 30 hours. During the set up of three electrode cells, a specific capacitance of 1503.84 F/g was observed for sample 2 of NiO at a current density of 2 A/g. Using sample 2 of NiO as anode electrode material, an asymmetric device application was studied, and the electrode material had a specific capacity of 1113 C/g at 5 A/g, an energy density of 245 Wh/kg, and a power density of 20000 W/kg. The specific capacitance retention has been observed to be 96.9% after 40000 GCD cycles. Based on these findings, it has been confirmed that sample 2 of NiO has shown significant enhancements in electrochemical performance, thus allowing it to be configured for use in real-time devices. For the development of functional materials, phytochemicals from lotus root extract have demonstrated an alternative source of green reducing, capping, stabilizing, and structure directing agents. Particularly in applications where the catalytic sites, morphology, and particle size of the materials play an important role, such as photoelectrochemical cells, electrolyzers, fuel cells, electrochemical batteries, and supercapacitors.

## Acknowledgement

The authors would like to acknowledge the Higher Education Commission of Pakistan for its partial support of the project (NRPU/8350/8330). The authors would like to acknowledge the partial support of Ajman University, Internal Research Grant No. [DGSR Ref. 2024-IRG-HBS-01]. The authors extend their sincere appreciation to the Researchers Supporting Project number (RSP2024R442), King Saud University, Riyadh, Saudi Arabia, for partial funding of this work.

**Conflicts of Interest:** Authors have no conflict of interest in the presented research work.

**Data Availability Statement:** The authors declare that the data supporting the findings of this study are available within the paper.

## References
1. Chen Q, Gao G, Guo H, Wang S, Wang Q, Fang Y, Hu X, Duan R (2023) Boron-doped polymeric carbon nitride co-modified with carbon-ring and carboxyl for efficient photocatalytic overall water-splitting, Chem Eng J, 470:144199.

2. Shandilya P, Sambyal S, Sharma R, Mandal P, Fang B (2022) Properties, optimized morphologies, and advanced strategies for photocatalytic applications of WO₃ based photocatalysts, J Hazard Mater, 428:128218.

3. Suresh R, Gnanasekaran L, Rajendran S, Soto-Moscoso M (2023) Doped nanomaterials: application in hydrogen production via photocatalytic water splitting, Fuel, 384

4. Kawawaki T, Mori Y, Wakamatsu K, Ozaki S, Kawachi M, Hossain S, Negishi Y (2020) Controlled colloidal metal nanoparticles and nanoclusters: recent applications as cocatalysts for improving photocatalytic water-splitting activity. Journal of Materials Chemistry A. (32):16081-113.

5. Marwat MA, Humayun M, Afridi MW, Zhang H, Abdul Karim MR, Ashtar M, Usman M, Waqar S, Ullah H, Wang C, Luo W (2021) Advanced catalysts for photoelectrochemical water splitting. ACS Applied Energy Materials. (11):12007-31.

6. Jafari S, Shaghaghi Z (2023) CeO₂/CuO/NiO hybrid nanostructures loaded on N-doped reduced graphene oxide nanosheets as an efficient electrocatalyst for water oxidation and non-enzymatic glucose detection. Dalton Transactions. 52(22):7564-80.

7. Zhou Y, Fan HJ (2020) Progress and challenge of amorphous catalysts for electrochemical water splitting. ACS Materials Letters. 3(1):136-47.

8. Shaghaghi Z, Bikas R, Heshmati-Sharabiani Y, Trzybiński D, Woźniak K (2022) Investigation of electrocatalytic activity of a new mononuclear Mn (II) complex for water oxidation in alkaline media. Photosynthesis Research. 154(3):369-81

9. Raveendran A, Chandran M, Dhanusuraman R (2023) A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts. RSC advances. 13(6):3843-76.
10. Zhang K, Wan T, Wang H, Luo Y, Shi Y, Zhang Z, Liu G, Li J (2023) Decorated Oxidation-resistive deficient Titanium oxide nanotube supported NiFe-nanosheets as high-efficiency electrocatalysts for overall water splitting. Journal of Colloid and Interface Science. 645:66-75.

11. Zhao M, Li H, Li W, Li J, Yi L, Hu W, Li CM (2020) Ru-Doping Enhanced Electrocatalysis of Metal–Organic Framework Nanosheets toward Overall Water Splitting. Chemistry–A European Journal. 26(71):17091-6.

12. Luo F, Hu H, Zhao X, Yang Z, Zhang Q, Xu J, Kaneko T, Yoshida Y, Zhu C, Cai W (2020) Robust and stable acidic overall water splitting on Ir single atoms. Nano letters. 20(3):2120-8.

13. Lv F, Feng J, Wang K, Dou Z, Zhang W, Zhou J, Yang C, Luo M, Yang Y, Li Y, Gao P (2018) Iridium–tungsten alloy nanodendrites as pH-universal water-splitting electrocatalysts. ACS central science. 4(9):1244-52.

14. Shaghaghi Z, Jafari S, Mohammad-Rezaei R (2022) The heterostructure of ceria and hybrid transition metal oxides with high electrocatalytic performance for water splitting and enzyme-free glucose detection. Journal of Electroanalytical Chemistry. 915:116369.

15. Gu R, Zhou T, Wang Z, Chen Z, Tao J, Fan Z, Guo L, Liu Y (2023) Mo/P doped NiFeSe as bifunctional electrocatalysts for overall water splitting. International Journal of Hydrogen Energy. 48(71):27642-51.

16. Abdelhameed RM, Al Kiey SA, Wassel AR, El-Shahat M (2021) Silver chromate doped Ti-based metal organic framework: synthesis, characterization, and electrochemical and selective photocatalytic reduction properties. New Journal of Chemistry. 45(21):9526-37.

17. Wang R, Yan X (2014) Superior asymmetric supercapacitor based on Ni–Co oxide nanosheets and carbon nanorods. Sci. Rep. 4, 3712.
18. Al Kiey SA, Hasanin MS (2021) Green and facile synthesis of nickel oxide-porous carbon composite as improved electrochemical electrodes for supercapacitor application from banana peel waste. *Environmental Science and Pollution Research*. (47):66888-900.

19. Khan J, Iqbal MZ, Rubab B, Jamshaid F, Khan A, Shakeel N, Ahmed A, Al-Kahtani AA (2023) Isonicotinic acid-based copper-MOF: An exotic redox propertied electrode material for high energy asymmetric supercapacitor. *Journal of Energy Storage*. 72:108655.

20. Karaman C, Karaman O, Atar N, Yola ML (2021) Tailoring of cobalt phosphide anchored nitrogen and sulfur co-doped three dimensional graphene hybrid: boosted electrocatalytic performance towards hydrogen evolution reaction. *Electrochimica Acta*. 380:138262.

21. Lamba P, Singh P, Singh P, Singh P, Kumar A, Gupta M, Kumar Y (2022) Recent advancements in supercapacitors based on different electrode materials: classifications, synthesis methods and comparative performance. *Journal of Energy Storage*. 48:103871.

22. Lu Z, Zhao H, Luo J, Wang J (2022) Reduced-graphene-oxide-modified self-supported NiSe2 nanospheres on nickel foam as a battery-type electrode material for high-efficiency supercapacitors. *Journal of Physics and Chemistry of Solids*. 163:110593.

23. Ren Z, Li J, Ren Y, Wang S, Qiu Y, Yu J (2016) Large-scale synthesis of hybrid metal oxides through metal redox mechanism for high-performance pseudocapacitors. *Scientific reports*. 6(1):20021.

24. Iqbal MZ, Khan J (2021) Optimization of cobalt-manganese binary sulfide for high performance supercapattery devices. *Electrochimica acta*. 368:137529.

25. Khan J, Khan A, Rubab B, Jamshaid F, Al-Kahtani AA, Dahshan A (2023) Exploring the progression of energy storage toward flexibility: metal-organic framework and conducting polymer aspects. *Applied Materials Today*. 34:101906.
26. Grover S, Kadyan P, Sharma S, Sharma K, Sharma RK (2023) Synthesis and characterization of polyaniline nanotube supported nanocomposite of RuO2 as electrode material for application in supercapacitor device. *Materialia*. 28:101732.

27. Yi TF, Wei TT, Mei J, Zhang W, Zhu Y, Liu YG, Luo S, Liu H, Lu Y, Guo Z (2020) Approaching high-performance supercapacitors via enhancing pseudocapacitive nickel oxide-based materials. *Advanced Sustainable Systems*. (3):1900137.

28. Rodrigues MP, Miguel VM, Germano LD, Córdoba de Torresi SI (2022) Metal oxides as electrocatalysts for water splitting: On plasmon-driven enhanced activity. *Electrochemical Science Advances*. (3):e2100079.

29. da Silva AG, Fernandes CG, Hood ZD, Peng R, Wu Z, Dourado AH, Parreira LS, de Oliveira DC, Camargo PH, de Torresi SI (2020) PdPt-TiO2 nanowires: correlating composition, electronic effects and O-vacancies with activities towards water splitting and oxygen reduction. *Applied Catalysis B: Environmental*. 277:119177.

30. Chang HW, Chen SC, Chen PW, Liu FJ, Tsai YC (2022) Constructing morphologically tunable copper oxide-based nanomaterials on Cu wire with/without the deposition of manganese oxide as bifunctional materials for glucose sensing and supercapacitors. *International Journal of Molecular Sciences*. 23(6):3299.

31. Wei Z, Qiao H, Yang H, Zhang C, Yan X (2009) Characterization of NiO nanoparticles by anodic arc plasma method. *Journal of alloys and compounds*. 479(1-2):855-8.

32. Eder R (2015) Electronic structure of NiO: Antiferromagnetic transition and photoelectron spectra in the ordered phase. *Physical Review B*. 91(24):245146.

33. Xia C, Yanjun X, Ning W (2011) Facile synthesis of NiO nanoflowers and their electrocatalytic performance. *Sensors and Actuators B: Chemical*. 153(2):434-8.

34. Mishra D, Zhou R, Hassan MM, Hu J, Gates I, Mahinpey N, Lu Q (2022) Bitumen and asphaltene derived nanoporous carbon and nickel oxide/carbon composites for supercapacitor electrodes. *Scientific Reports*. 12(1):4095.
35. Zahra T, shahzad Ahmad K, Zequine C, Gupta R, Thomas A, Malik MA, Iram S, ElBadry YA, El-Bahy ZM (2022) Electrochemical trapping of meta-stable NiO consolidated ZnO/PdO by biomimetic provenance for the employment of clean energy generation. Materials Science in Semiconductor Processing. 150:106867.

36. Yu Y, Lee SJ, Theerthagiri J, Lee Y, Choi MY (2022) Architecting the AuPt alloys for hydrazine oxidation as an anolyte in fuel cell: Comparative analysis of hydrazine splitting and water splitting for energy-saving H2 generation. Applied Catalysis B: Environmental. 316:121603.

37. Goel R, Jha R, Ravikant C (2020) Investigating the structural, electrochemical, and optical properties of p-type spherical nickel oxide (NiO) nanoparticles. Journal of Physics and Chemistry of Solids. 144:109488.

38. Jeevanandam J, Kiew SF, Boakye-Ansah S, Lau SY, Barhoum A, Danquah MK (2022) Rodrigues J. Green approaches for the synthesis of metal and metal oxide nanoparticles using microbial and plant extracts. Nanoscale. 14(7):2534-71.

39. Zhong Z, Gong X, Wang L, Bai G, Wei H, Yang W (2020) A facile way for fabrication of silver nanoparticle decorated graphene composites. Materials Chemistry and Physics. 241:122344.

40. Nwanya AC, Ndipingwi MM, Ikpo CO, Obodo RM, Nwanya SC, Botha S, Ezema FI, Iwuoha EI, Maaza M (2020) Zea mays lea silk extract mediated synthesis of nickel oxide nanoparticles as positive electrode material for asymmetric supercabattery. Journal of Alloys and Compounds. 822:153581.

41. Reddy BJ, Vickraman P, Justin AS (2019) Moringa oleifera leaf extract mediated reduced graphene oxide/α-Ni (OH) 2 nanocomposite for asymmetric supercapacitors. Brazilian Journal of Physics. 49:348-59.

42. Samsai T, Praveena S (2016) An analysis of production and processing of Aloe vera. Int. J. Agric. Sci. Res. 6:51-6.
43. https://www.verywellfit.com/lotus-root-nutrition-facts-calories-carbs-and-health-benefits-4772425

44. Mahmoodi NM (2013) Synthesis of amine-functionalized magnetic ferrite nanoparticle and its dye removal ability. Journal of Environmental Engineering. 139(11):1382-90.

45. Devi R, Kumar V, Kumar S, Sisodiya AK, Mishra AK, Jatrana A, Kumar A, Singh P (2023) Development of activated carbon by bio waste material for application in supercapacitor electrodes. Materials Letters. 335:133830.

46. Zhu Y, Cao C, Tao S, Chu W, Wu Z, Li Y (2014) Ultrathin nickel hydroxide and oxide nanosheets: synthesis, characterizations and excellent supercapacitor performances. Scientific reports. 4(1):5787.

47. Rani P, Dahiya R, Bulla M, Devi R, Jeet K, Jatrana A, Kumar V (2023) Hydrothermal-assisted green synthesis of reduced graphene oxide nanosheets (rGO) using lemon (Citrus Limon) peel extract. Materials Today: Proceedings.

48. Goel R, Jha R, Ravikant C (2020) Investigating the structural, electrochemical, and optical properties of p-type spherical nickel oxide (NiO) nanoparticles. Journal of Physics and Chemistry of Solids. 144:109488.

49. Uddin S, Safdar LB, Anwar S, Iqbal J, Laila S, Abbasi BA, Saif MS, Ali M, Rehman A, Basit A, Wang Y (2021) Green synthesis of nickel oxide nanoparticles from Berberis balochistanica stem for investigating bioactivities. Molecules. 26(6):1548.

50. Basharat F, Rana UA, Shahid M, Serwar M (2015) Heat treatment of electrodeposited NiO films for improved catalytic water oxidation. RSC advances. 5(105):86713-22.

51. Jiang J, Jiang H, Chen M, Ma R, Wang X, Wei W, Ai L (2022) Plasmonic MoO₂ coupled with sulfur-incorporated NiMoO₄ as multifunctional heterostructures for solar thermoelectric self-powered urea electrolysis. Applied Surface Science. 600:154116.

52. Wu Y, Sun Z, Wang Y, Yin L, He Z, Zhang Z, Hayat MD, Zang Q, Lian J (2023) Cyclic voltammetric deposition of binder-free Ni-Se film on Ni foams as efficient bifunctional
electrocatalyst for boosting overall urea-water electrolysis. Journal of Alloys and Compounds. 937:168460.

53. Darband GB, Maleki M, Toghraei A, Shanmugam S (2023) Electrodeposition of self-supported transition metal phosphides nanosheets as efficient hydrazine-assisted electrolytic hydrogen production catalyst. International Journal of Hydrogen Energy. 48(11):4253-63.

54. Anantharaj S, Sugime H, Noda S (2021) Why shouldn't double-layer capacitance (Cdl) be always trusted to justify Faradaic electrocatalytic activity differences?. Journal of Electroanalytical Chemistry. 903:115842.

55. Sonavane AC, Inamdar AI, Dalavi DS, Deshmukh HP, Patil PS (2010) Simple and rapid synthesis of NiO/PPy thin films with improved electrochromic performance. Electrochimica Acta. 55(7):2344-51.

56. Hu L, Chen W, Xie X, Liu N, Yang Y, Wu H, Yao Y, Pasta M, Alshareef HN, Cui Y (2011) Symmetrical MnO2-carbon nanotube-textile nanostructures for wearable pseudocapacitors with high mass loading. ACS nano. 5(11):8904-13.

57. Liang M, Zhao M, Wang H, Shen J, Song X (2018) Enhanced cycling stability of hierarchical NiCo 2 S 4@ Ni (OH) 2@ PPy core-shell nanotube arrays for aqueous asymmetric supercapacitors. Journal of Materials Chemistry A. 6(6):2482-93.

58. Xiong S, Jiang S, Wang J, Lin H, Lin M, Weng S, Liu S, Jiao Y, Xu Y, Chen J (2020) A high-performance hybrid supercapacitor with NiO derived NiO@ Ni-MOF composite electrodes. Electrochimica Acta. 340:135956.

59. Raghavendra KV, Sreekanth TV, Ko TJ, Kim J, Yoo K (2021) Facile hydrothermal synthesis of novel ZnWO4/ZnCo2O4 electrode for high-performance supercapacitors. Materials Letters. 287:129296.

60. Iqbal MZ, Khan J, Afzal AM, Aftab S (2021) Exploring the synergetic electrochemical performance of cobalt sulfide/cobalt phosphate composites for supercapattery devices with high-energy and rate capability. Electrochimica Acta. 384:138358.
61. Zhang Y, Shen Y, Xie X, Du W, Kang L, Wang Y, Sun X, Li Z, Wang B (2020) One-step synthesis of the reduced graphene oxide@ NiO composites for supercapacitor electrodes by electrode-assisted plasma electrolysis. *Materials & Design*. 196:109111.

62. Dhas SD, Maldar PS, Patil MD, Nagare AB, Waikar MR, Sonkawade RG, Moholkar AV (2020) Synthesis of NiO nanoparticles for supercapacitor application as an efficient electrode material. *Vacuum*. 181:109646.

63. Gunasekaran SS, Gopalakrishnan A, Subashchandrabose R, Badhulika S (2021) Phytogenic generation of NiO nanoparticles as green-electrode material for high performance asymmetric supercapacitor applications. *Journal of Energy Storage*. 37:102412.

64. Bulla M, Kumar V, Devi R, Kumar S, Sisodiya AK, Dahiya R, Mishra AK (2024) Natural resource-derived NiO nanoparticles via aloe vera for high-performance symmetric supercapacitor. *Scientific Reports*. 14(1):7389.

65. Yin Q, Li D, Zhang J, Zhao Y, Luo J, Shao M, Han J (2020) An all-solid-state fiber-type supercapacitor based on hierarchical Ni/NiO@ CoNi-layered double hydroxide core-shell nanoarrays. *Journal of Alloys and Compounds*. 813:152187.

66. Kummara VG, Rajangam V, Chandu VM, Kummara MR, Kim HJ (2020) Facile synthesis of hierarchical agglomerated cauliflower-like ZnWO4@ NiO nanostructures as an efficient electrode material for high-performance supercapacitor applications. *Materials Letters*. 268:127594.

67. Vijeth H, Ashokkumar SP, Yesappa L, Vandana M, Devendrappa H (2020) Hybrid core-shell nanostructure made of chitosan incorporated polypyrrole nanotubes decorated with NiO for all-solid-state symmetric supercapacitor application. *Electrochimica Acta*. 354:136651.

68. Han D, Jing X, Wang J, Yang P, Song D, Liu J (2012) Porous lanthanum doped NiO microspheres for supercapacitor application. *Journal of electroanalytical chemistry*. 682:37-44.
69. Ren B, Fan M, Liu Q, Wang J, Song D, Bai X (2013) Hollow NiO nanofibers modified by citric acid and the performances as supercapacitor electrode. Electrochimica Acta. 92:197-204.

70. Liangqi J, Kelei Z, Li S, Na Z, Xiao S, Yujuan C, Xiaodong H, Rumeng F, Jianji W (2024) The Mass-Balancing between Positive and Negative Electrodes for Optimizing Energy Density of Supercapacitors, J. Am. Chem. Soc. 146, 14369-14385

<figure><img src="image_4.png" /><figcaption>Figure Captions</figcaption></figure>
**Figure 1:** (a) UV-visible absorbance spectra NiO nanostructures synthesized in the presence of 1 mL and 2 mL lotus root extract (sample 1 and sample 2) and pure NiO, (b) Corresponding optical band analysis using Tauc plots.

<figure><img src="image_5.png" /></figure>

**Figure 2:** XRD reflections of NiO nanostructures synthesized in the presence of 1 mL and 2 mL lotus root extract (sample 1 and sample 2) and pure NiO.
<figure><img src="image_6.png" /><figcaption>**Figure 3:** Typical SEM images at high and low magnifications of (a,b) pure NiO, (c,d) NiO nanostructures synthesized in the presence of 1 mL of lotus extract (sample 1), (e,f) NiO nanostructures using 2 mL of lotus root extract (sample 2).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 4:** Typical FTIR spectra of (a) pure NiO, (b) NiO nanostructures synthesized in the presence of 1 mL and 2 mL of lotus extract (sample 1, sample 2).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 5:** LSV curves of NiO nanostructures synthesized in the presence of 1 mL and 2 mL lotus root extract (sample 1 and sample 2) and pure NiO at 2 mV/s in 1M KOH aqueous solution, (b) Corresponding</figcaption></figure>
bar graph representation of overpotential at 10 mAcm⁻², (c) Tafel slope analysis using Tafel equation onto linear region of LSV curves of XRD reflections of NiO nanostructures synthesized in the presence of 1 mL and 2 mL lotus root extract (sample 1 and sample 2) and pure NiO, (d) LSV curves before and after the durability test for the illustration of stability of sample 2, (e) Chronoamperometry curve for 30 hours at a fixed current density of 10 mAcm⁻², (f) Nyquist plots of XRD reflections of NiO nanostructures synthesized in the presence of 1 mL and 2 mL lotus root extract (sample 1 and sample 2) and pure NiO, using 100 kHz to 0.1Hz, 5 mV amplitude and OER onset potential in 1M KOH solution.

<figure><img src="image_9.png" /><figcaption>**Figure 6:** CV curves at various scan rates in 3M KOH aqueous solution (a) bare nickel foam, (b) pure NiO, (c) 1 mL of lotus extract assisted NiO (sample 1), (d) 2 mL of lotus extract assisted NiO (sample 2), (e) Comparative CV runs of bare nickel foam, pure NiO, sample 1, and sample 2 at 10 mV/s</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7:** GCD curves at various current densities in 3M KOH aqueous solution (a) bare nickel foam, (b) pure NiO, (c) 1 mL of lotus extract assisted NiO (sample 1), (d) 2 mL of lotus extract assisted NiO (sample 2).</figcaption></figure>

<figure><img src="image_11.png" /></figure>
**Figure 8:** (a) Specific capacitance at various current densities in 3M KOH aqueous solution of bare nickel foam, pure NiO, 1 mL of lotus extract assisted NiO (sample 1), 2 mL of lotus extract assisted NiO (sample 2), (b) Energy density and power density in 3M KOH aqueous solution of bare nickel foam, pure NiO, 1 mL of lotus extract assisted NiO (sample 1), 2 mL of lotus extract assisted NiO (sample 2), (c) Corresponding columbic efficiency at 2 A/g, (d) Percentage capacitance retention.

<figure><img src="image_12.png" /></figure>

**Figure 9:** (a) CV curves of AC and sample 2 at 10 mV/s, (b) CV curves at various scan rates in 3M KOH aqueous solution of 2 mL of lotus extract assisted NiO (sample 2) during ASC device fabrication, (c) GCD curves of ASC at various current densities in 3M KOH solution, (d) Specific capacity of ASC at different current densities, (e) Corresponding energy density of ASC, (f) Columbic efficiency of ASC during 40000 GCD cycles, and (g) Percentage capacitance retention of ASC.