Muhammad Yameen Solangi

Umair Aftab

Aneela Tahira

Abdul Hanan

Monica Montecchi

Luca Pasquali

Matteo H I Tonezzer

Raffaello Mazzaro

Vittorio Morandi

Abdul Jaleel Laghari

Ayman Nafady

Muhammad Ishaq Abro

Melanie Emo

Brigitte Vigolo

Elmuez Dawi

Elfatih Mustafa

Zafar Hussain

Dr M A Kazi

In-situ growth of nonstoichiometric CrO 0.87 and Co 3 O 4 hybrid system for the enhanced electrocatalytic water splitting in alkaline media

Keywords: Nonstoichiometric growth, CrO 0.87 and Co 3 O 4 hybrid material, oxygen evolution reaction

The development of electrocatalysts for electrochemical water splitting has received considerable attention in response to the growing demand for renewable energy sources and environmental concerns. In this study, a simple hydrothermal growth approach was developed for the in-situ growth of non-stoichiometric CrO 0.87 and Co 3 O 4 hybrid materials. It is apparent that the morphology of the prepared material shows a heterogeneous aggregate of irregularly shaped nanoparticles. Both CrO 0.87 and Co 3 O 4 have cubic crystal structures. Its chemical composition was governed by the presence of Co, Cr, and O as its main constituents. For understanding the role CrO 0.87 plays in the half-cell oxygen evolution reaction (OER) in alkaline conditions, CrO 0.87 was optimized into Co 3 O 4 nanostructures. The hybrid material with the highest concentration of CrO 0.87 was found to be highly efficient at driving OER reactions at 255 mV and 20 mA cm -2 .

The optimized material demonstrated excellent durability for 45 hours and a Tafel slope of 56 mV dec -1 . Several factors may explain the outstanding performance of CrO 0.87 and Co 3 O 4 hybrid materials, including multiple metallic oxidation states, tailored surface properties, fast charge transport, and surface defects. An alternative method is proposed for the preparation of new generations of electrocatalysts for the conversion and storage of energy.

Introduction

Global warming, the loss of natural energy resources, the growth of civilizations, and the industrialization of the world have all contributed to the energy crisis that has been recognized as a major problem for the survival of life on earth [1][2][3]. Fossil fuel production is considered a perilous issue because of the depletion of fossil resources and the generation of toxic pollutants harmful to the earth's ecosystem [4,5]. As a solution to these issues, the world is demanding renewable energy that has the characteristics of green fuels and produces no pollution of the environment [6,7]. Based on this perspective, hydrogen energy represents an attractive energy source for sustainable energy production and the rapid development of fuel cell technology. In comparison to other renewable energy sources, hydrogen energy consists of hydrogen gas fuel which carries the greatest density of energy per unit mass [8][9][START_REF] Zhiznin | Economics of secondary renewable energy sources with hydrogen generation[END_REF][START_REF] Schönauer | Hydrogen in future energy systems: Social acceptance of the technology and its large-scale infrastructure[END_REF]. A method of electrochemical water splitting followed by two reactions, namely the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). [START_REF] Kumbhar | Recent advances in water-splitting electrocatalysts based on manganese oxide[END_REF][START_REF] Liu | Activation strategies of water-splitting electrocatalysts[END_REF][START_REF] Tahira | Electrochemical water splitting based on metal oxide composite nanostructures[END_REF][START_REF] Wang | Hydrogen production from water electrolysis: role of catalysts[END_REF][START_REF] Hanan | Co2FeO4@rGO composite: Towards trifunctional water splitting in alkaline media[END_REF]. The generation of hydrogen energy from water splitting is potentially cost-effective, if highly efficient nonprecious electrocatalysts are developed in the near future, as precious metal electrocatalysts have been used to date. However, the lack of efficient non-noble electrocatalysts renders hydrogen production impossible [START_REF] Li | An electrochemically neutralized energy-assisted low-cost acid-alkaline electrolyzer for energy-saving electrolysis hydrogen generation[END_REF][START_REF] Suen | Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives[END_REF][START_REF] Jiang | Ionic-Liquid-Assisted One-Step Synthesis of CoO Nanosheets as Electrocatalysts for Oxygen Evolution Reaction[END_REF].

The main challenge for hydrogen energy production from water splitting is the slow kinetics of the OER reaction as it occurs by the four electron transfer mechanism [START_REF] Yuan | A review on non-noble metal based electrocatalysis for the oxygen evolution reaction[END_REF][START_REF] Younis | Noble metal-free two dimensional carbonbased electrocatalysts for water splitting[END_REF][START_REF] Hanan | PdO@ CoSe 2 composites: efficient electrocatalysts for water oxidation in alkaline media[END_REF]. Thus, noble metal based electrocatalysts such as iridium oxide (IrO 2 ) and ruthenium oxide (RuO 2 ) have been utilized as catalysts in OER reactions, but their high cost and limited availability have limited their application to large-scale water splitting [START_REF] Guo | RuO2/Co3O4 Nanocubes based on Ru ions impregnation into prussian blue precursor for oxygen evolution[END_REF][START_REF] Liu | MOF-derived RuO2/Co3O4 heterojunctions as highly efficient bifunctional electrocatalysts for HER and OER in alkaline solutions[END_REF][START_REF] Reier | Electrocatalytic Oxygen Evolution Reaction (OER) on Ru, Ir, and Pt Catalysts: A Comparative Study of Nanoparticles and Bulk Materials[END_REF]. In light of these circumstances, researchers are focusing on the development of nonprecious electrocatalysts based on transition metal compounds in light of their abundance, low cost of fabrication, and significant effectiveness for the OER reaction [START_REF] Song | Transition Metal Oxides as Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Solutions: An Application-Inspired Renaissance[END_REF][START_REF] Goswami | Transition metal oxide nanocatalysts for oxygen reduction reaction[END_REF][START_REF] Song | MOFs fertilized transition-metallic single-atom electrocatalysts for highly-efficient oxygen reduction: Spreading the synthesis strategies and advanced identification[END_REF][START_REF] Wang | Earth-abundant transition-metalbased bifunctional catalysts for overall electrochemical water splitting: A review[END_REF]. Several transition metal-based catalysts have been investigated, including metal phosphide, metal selenide, metal sulfide, bimetallic sulfide, metal oxide, bimetallic oxide, and trimetallic oxide [START_REF] Ibupoto | NiCo2O4 nanostructures loaded onto pencil graphite rod: An advanced composite material for oxygen evolution reaction[END_REF][START_REF] Tahira | Facile deposition of palladium oxide (PdO) nanoparticles on CoNi2S4microstructures towards enhanced oxygen evolution reaction[END_REF][START_REF] Aftab | Two step synthesis of TiO2-Co3O4 composite for efficient oxygen evolution reaction[END_REF][START_REF] Shuai | Hierarchical NiCo2S4 nanosheets grown on graphene to catalyze the oxygen evolution reaction[END_REF][START_REF] Kaneti | Self-templated fabrication of hierarchical hollow manganese-cobalt phosphide yolk-shell spheres for enhanced oxygen evolution reaction[END_REF][START_REF] Song | Highly efficient oxygen evolution and stable water splitting by coupling NiFe LDH with metal phosphides[END_REF][START_REF] Li | Facile preparation of CoSe2 nano-vesicle derived from ZIF-67 and their application for efficient water oxidation[END_REF][START_REF] Zhang | Heterointerface engineered NiFe(OH)x/Ni3S2 electrocatalysts to overcome the scaling relationship for ultrahigh-current-density water oxidation[END_REF][START_REF] Hanan | PdO@CoSe2 composites: efficient electrocatalysts for water oxidation in alkaline media[END_REF]. As a result of their enhanced electrocatalytic properties, metal oxides (Co, Ni, Mn, Mg, and Fe, for example) tend to be preferred among these catalysts [START_REF] Solangi | An efficient palladium oxide nanoparticles@Co3O4 nanocomposite with low chemisorbed species for enhanced oxygen evolution reaction[END_REF][START_REF] Biesinger | Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Cr, Mn, Fe, Co and[END_REF][START_REF] Bai | A nitrogen-doped nano carbon dodecahedron with Co@Co3O4 implants as a bi-functional electrocatalyst for efficient overall water splitting[END_REF][START_REF] Gong | Controlled synthesis of bifunctional particle-like Mo/Mn-NixSy/NF electrocatalyst for highly efficient overall water splitting[END_REF][START_REF] Wu | The cobalt carbide/bimetallic CoFe phosphide dispersed on carbon nanospheres as advanced bifunctional electrocatalysts for the ORR, OER, and rechargeable Zn-air batteries[END_REF]. The cobalt based electrocatalyst including Co 3 O 4 [START_REF] Mehboob | Nanoscale cobalt-oxide electrocatalyst for efficient oxygen evolution reactions in alkaline electrolyte[END_REF] , Co 3 O 4 -P [START_REF] Zhu | Cobalt Oxide Supported on Phosphorus-Doped g-C3N4 as an Efficient Electrocatalyst for Oxygen Evolution Reaction[END_REF], Ni-Co 3 O 4 [START_REF] Bhatti | Facile doping of nickel into Co 3 O 4 nanostructures to make them efficient for catalyzing the oxygen evolution reaction[END_REF], Mg-Co 3 O 4 [START_REF] Hanan Samo | MAGNESIUM DOPED COBALT-OXIDE COMPOSITE FOR ACTIVE OXYGEN EVOLUTION REACTION[END_REF], CdO-Co 3 O 4 [START_REF] Hanan | CdO/Co 3 O 4 Nanocomposite as an Efficient Electrocatalyst for Oxygen Evolution Reaction in Alkaline Media[END_REF], Fe 3 O 4 /Co 3 O 4 [START_REF] Bhatti | An Efficient and Functional Fe 3 O 4 /Co 3 O 4 Composite for Oxygen Evolution Reaction[END_REF], and MgO@Co 3 O 4 [START_REF] Laghari | MgO as promoter for electrocatalytic activities of Co3O4-MgO composite via abundant oxygen vacancies and Co2+ ions towards oxygen evolution reaction[END_REF] etc. are widely reported owing to their significant corrosion resistance in alkaline media, high active sites and cost effective fabrication process [START_REF] Flores | Electrocatalytic oxygen evolution reaction of hierarchical micro/nanostructured mixed transition cobalt oxide in alkaline medium[END_REF][START_REF] Badruzzaman | Recent advances in cobalt based heterogeneous catalysts for oxygen evolution reaction[END_REF]. This nanostructured catalyst, however, exhibits a large overpotential for OER reactions and consumes a significant amount of potential during the reaction. Thus, it is highly imperative to develop a new generation of electrocatalysts based on Co 3 O 4 that can fully satisfy the requirements for electrocatalytic properties for water splitting. In our review of the literature, we find that there has been no study or effort made to develop a non-stoichiometric hybrid electrocatalyst for efficient OER half-cell water splitting based on chromium oxide (CrO 0.87 ) and In the present study, the hybrid material was used as an efficient OER electrocatalyst that drove the reaction with a low overpotential and was highly durable at different densities of current.

Experimental work

Synthesis of CrO 0.87 and Co 3 O 4 hybrid material

Hybrid materials are typically synthesized in the following manner: Hydrothermal treatment of cobalt chloride hexahydrate and urea in 100 mL of deionized water was used to prepare the cobalt precursors. For the in situ growth of CrO 0.87 and Co 3 O 4 hybrid material, 0.1 g, 0.2 g, and 0.3 g of potassium chromate were mixed with the cobalt precursors and labeled CCR-0.1, CCR-0.2, and CCR-0.3, respectively. Potassium chromate was mechanically stirred for 30 minutes in a highly acidic growth solution. Therefore, we adjusted the pH of growth solutions to 7.5 by adding 0.2 M NaOH. The growth solutions were covered with a thin aluminum sheet to prevent spoilage. The hydrothermal process was conducted in an electric oven at 95⁰ C for five hours. In the event that the growth time and cooling to room temperature have been completed, the prepared samples will be removed from the electric oven.

A fine powder of the grown material was collected through filter paper in a China dish and washed several times with deionized water. The samples were dried in an electric oven at 65⁰ C for four hours, and we were able to obtain a hybrid material based on cobalt hydroxide.

Additionally, the samples were thermally burned in an electric furnace at 500°C for 5 hours to convert the hydroxide phase into the oxide phase. Having completed the calcination process and reached room temperature, samples were removed from the electric furnace. In this regard, we were able to achieve a number of CrO 0.87 and Co 3 O 4 hybrid materials, which were then used for a variety of characterizations, such as structural and functional studies. It was also possible to prepare pristine samples of CrO 0.87 and Co 3 O 4 by the same method in the absence of potassium chromate in order to understand the role of the desired hypothesis. Schematic 1 illustrates the growth process as a stepwise process.

Physical Characterization CrO 0.87 and Co 3 O 4 hybrid materials

ZEISS Gemini SEM 500 equipped with a field emission gun was used to perform scanning electron microscopy (SEM). The X-ray diffractograms were recorded through Bruker D8 Advance diffractometer with source CuKα radiation having a wavelength of 1.54050 Å, operated at 45 mA and 45 kV, for the determination of phase purity and crystal quality of material. FTIR-PerkinElmer Spectrum Two was used to perform Fourier transform infrared spectroscopy to validate the interaction between CrO 0.87 and Co 3 O 4 in the 400 cm-1 to 4000 cm -1 region.

Half-cell OER measurements on the CrO 0.87 and Co 3 O 4

Measurements of the OER were carried out using the Potentiostat Model VERSASTAT4-500, which consists of a silver-silver chloride (Ag/AgCl) reference electrode, a platinum (Pt) wire counter electrode, and a glassy carbon electrode (GCE) as a working electrode. Initially, catalyst ink solutions of different samples of CrO 0.87 and Co 3 O 4 were prepared by mixing 5 mg of each sample in 1 mL of deionized water accompanied by the addition of 20 µL of Nafion (5 wt.%).

GCEs were cleaned properly by polishing silicon paper and alumina slurry. The ink was then modified by drop casting with the prepared catalyst ink. An oxygen evolution reaction (OER) was analyzed using linear sweep voltammetry (LSV) in 1.0 M KOH electrolyte at a scan rate of 1 mV/s. An electrochemical active surface area (ECSA) was calculated based on cyclic voltammetry (CV) of electrocatalysts recorded for non-Faradic regions at different scan rates. In order to determine the charge transport characteristics of various catalytic materials, electrochemical impedance spectroscopy (EIS) was applied at onset OER potential, sinusoidal potential of 10mV, and frequency range of 100 kHz to 0.1 Hz. Using the Nernst equation, the experimental potential measured against (Ag/AgCl) was converted into the reversible hydrogen electrode potential (RHE).

Results and discussion

Crystal quality, morphology, and surface chemical composition characterizations

A powder XRD method was used to obtain XRD results for pristine Co 3 O 4 , CCR-0.1, CCR-0. TEM micrographs of pristine Co 3 O 4 at different magnifications are shown in Fig. 3(a-c). It is characteristic of a pure sample that the nanoparticles that comprise the microstructure are heterogeneous in size. Additionally, the morphology of the nanoparticles has changed from a spherical shape to a rectangle. The size distribution is quite heterogeneous, with smaller nanoparticles having a diameter of 30 nm and larger particles having a diameter of >100 nm.

TEM micrographs of CCR-0.3 at various magnifications are shown in Fig. 4(a-c). In the sample, there appears to be a large number of aggregates with a diameter of several microns. The induction of CrO 0.87 into Co 3 O 4 shows a high degree of variation and irregular morphology, probably the result of many nanoparticles merging. The size distribution can therefore be difficult to evaluate precisely for this reason. Further, some aggregates with a needle-like shape are dispersed in an irregular arrangement of the microaggregates.

Fig. 3 (a-c Based on the HR-TEM image at high magnification, pristine Co 3 O 4 is confirmed to have a crystalline structure as shown in Fig. 3(d). The crystal lattice of the particle is similar to that of Co 3 O 4 with a spinel-like structure. Specifically, the crystal lattice does not differ when moving from a smaller to a larger nanoparticle, proving the same composition in both cases.

HR-TEM, however, has also validated the crystallinity of the CCR-0.3 material as shown in Fig. 4(d). This crystal structure resembles that of Co 3 O 4 , which has a spinel-like structure. In particular, the sample's surface is quite regular with large crystals. In most cases, the addition of CrO 0.87 to the preparation step does not affect the crystalline phase, but rather the morphology. As with other transition metals, it is not straightforward to interpret Co 2p spectra, due to peak asymmetries, shake-up and loss structures, multiplet splitting, and, in this case, charging effects.

Micrographs of

Based on literature reports [START_REF] Xu | Atomic heterointerface engineering overcomes the activity limitation of electrocatalysts and promises highly-efficient alkaline water splitting[END_REF][START_REF] Zhang | Continuous oxygen vacancy engineering of the Co3O4 layer for an enhanced alkaline electrocatalytic hydrogen evolution reaction[END_REF][START_REF] Zhang | Dual-defective Co3O4 nanoarrays enrich target intermediates and promise high-efficient overall water splitting[END_REF], a qualitative approach was adopted in order to minimize the uncertainty in peak assignment. As shown in Fig. 5A-B, the spectra of cobalt oxide powders were measured. In the spectrum, there are two prominent peaks, one located at about 780 eV (Co2p3/2), and the other at about 796 eV (Co2p1/2). In the high binding energy region of the major peak, a weak satellite structure is observed, which is commonly used to identify Co 3 O 4 spinel structures [START_REF] Gautier | Characterisation by X-ray photoelectron spectroscopy of thin MnxCo3-xO4(1≥x≥0) spinel films prepared by low-temperature spray pyrolysis[END_REF][START_REF] Carson Mhnamal | Epitaxial growth of Co3O4 on CoO(100)[END_REF]. Fig. 5c shows that Cr 2p 3/2 and Cr 2p 1/2 deconvolved into CrO 2 as indicated by the peak in binding energy at 576.0 (585.7) eV. [START_REF] Heinig | Fabrication of epitaxial CrO2 nanostructures directly on MgO (100) by pulsed laser deposition[END_REF][START_REF] Li | Influence of strain on the magnetic properties of epitaxial (100) chromium dioxide (CrO2) films[END_REF]. The comparison between the two samples, based on the overall lineshape observation, indicates a similar surface chemical composition for the two samples. (410 mV), CCR-0.1 (310 mV) and CCR-0.2 (270 mV), CCR-0.3 had the lowest overpotential of 255 mV. According to these results, CrO 0.87 impurities added to Co 3 O 4 nanostructures lead to the formation of vacancies in the nanostructure. Therefore, a reduction in the overpotential of the electrocatalyst and significant catalytic activity have been observed. However, it was observed during experimental results (Fig. S7) that the addition of CrO 0.87 impurities in high concentration such as CCR-0.4 leads to deterioration of its reaction kinetics and decrease its OER performance.

In Table S1, the CCR-0.3 catalyst outperforms other electrocatalysts reported. In terms of Tafel value and overpotential, CCR-0.3 demonstrated superior or equal performance to numerous recently reported electrocatalysts. These improvements in the activity of CCR-0.3 hybrid material could be explained by surface vacancies, abundant metallic chemical states, surface alteration, and tunable morphology as described by XRD, SEM, HRTEM, and XPS investigations.

The Tafel plot values were calculated from the linear region of the LSV curves and they provided information about OER kinetics. Pristine Co 3 O 4 , CCR-0.1, CCR-0.2, and CCR-0.3 have been determined to have Tafel values of 97 mV/dec, 72 mV/dec, 64 mV/dec, and 56 mV/dec, respectively as shown in Fig. (6c). Among the lowest Tafel values of CCR-0.3 (56 mV/dec), 56 mV/dec is representative of OER kinetics and serves as a confirmation of the possibility of swift OER reactions on the surface of the CCR-0.3 hybrid material. , Having a Tafel value that is similar to that of a noble metal catalyst (RuO 2 ), the newly designed in situ nonstoichiometric CCR-0.3 is likely to achieve practical and promising performance [START_REF] Wasalathanthri | Electrodeposited Copper-Cobalt-Phosphide: A Stable Bifunctional Catalyst for Both Hydrogen and Oxygen Evolution Reactions[END_REF][START_REF] Lu | Interfacing Manganese Oxide and Cobalt in Porous Graphitic Carbon Polyhedrons Boosts Oxygen Electrocatalysis for Zn-Air Batteries[END_REF][START_REF] Cai | Simple and cost effective fabrication of 3D porous core-shell Ni nanochains@NiFe layered double hydroxide nanosheet bifunctional electrocatalysts for overall water splitting[END_REF]. A generalized four-step process is followed by the transfer of four electrons in the OER process as described below.

Mechanism

Step No.

01.

02.

03.

04.

The theoretical Tafel values of OER kinetic steps were 120 mV/dec (step.1), 60 mV/dec (step.2), 40 mV/dec (step.3), and 15 mV/dec (step.4). [START_REF] Aftab | The chemically reduced CuO-Co 3 O 4 composite as a highly efficient electrocatalyst for oxygen evolution reaction in alkaline media[END_REF]. An electrocatalyst prepared in this case yielded a Tafel value of 56 mV/dec, indicating that step 2 is the rate-limiting step. From the reaction kinetic of OER mechanism, it is suggested that the MOH + OH ->MO -+H 2 O is responsible reaction taking place due to doping of CrO 0.8 into Co 3 O 4 nanostructured matrix that facilitates the oxygen vacancies to promote the reaction kinetics of OER activity.

In order to determine the double layer capacitance (Cdl) and electrochemical active surface area (ECSA) of the electrocatalysts, cyclic voltammetry (CV) was used. mF.cm -2 , 13.8 mF.cm -2 and 20.6 mF.cm -2 , respectively. Moreover, the electrochemical active surface area of the catalysts prepared was also determined by determining their C dl values. A summary of the computed ECSA values and the OER performance of each sample is shown in OER Table .1. As can be observed, the CCR-0.3 sample exhibits the highest active surface area of approximately 515 cm 2 , making it an ideal candidate for advanced electrocatalytical applications.

To eliminate the effect of greater electrochemical surface area (ECSA) on OER performance, the LSV curves are normalized by ECSA [START_REF] Huang | Highly Efficient Oxygen Evolution Reaction Enabled by Phosphorus Doping of the Fe Electronic Structure in Iron-Nickel Selenide Nanosheets[END_REF]. Fig. S8 results show that CCR-0.3 exhibits significantly higher OER activity than CCR-0.2, CCR-0.1 and pristine Co 3 O 4 samples, demonstrating that the improved OER activity is due to both an increased ECSA and the catalyst's improved intrinsic activity as a result of its optimized electronic structure. By contrasting the actual performance of catalysts, per-site turnover frequency (TOF) is used to better explain the intrinsic process for the improvement of OER activity. While computing the TOF, it is assumed that all metal atoms are active sites [START_REF] Anantharaj | The Significance of Properly Reporting Turnover Frequency in Electrocatalysis Research[END_REF][START_REF] Wu | Non-Noble-Metal-Based Electrocatalysts toward the Oxygen Evolution Reaction[END_REF]. The TOF value of corresponding catalyst can be seen in Fig. S9, was determined at 255 mV overpotential. As can be seen, the CCR-0.3 catalyst has exhibited significantly higher current density and TOF values (0.000909 mA.cm -1 , 0.37x10 -2 s -1 ) than CCR-0.2 (0.000620 mA.cm -1 , 0.25x10 -2 s -1 ) and pristine Co 3 O 4 (0.000075 mA.cm -1 , 0.03x10 -2 s -1 ) suggesting that the CrO 0.8 dopants significantly influence the coordination environment and symmetry of Co sites, thereby significantly improving the intrinsic activity of active Co sites. Previous research often shown that this heteroatom doping approach greatly boosted TOF values [START_REF] Xu | Trends in activity for the oxygen evolution reaction on transition metal (M = Fe, Co, Ni) phosphide pre-catalysts[END_REF][START_REF] Zhang | Metal Atom-Doped Co3O4 Hierarchical Nanoplates for Electrocatalytic Oxygen Evolution[END_REF].

The stability test was conducted by measuring the LSV before and after the durability test. The stability of the CCR-0.3 catalyst is illustrated in Fig. 6(e), and the polarization curve was almost the same as it was before, but a slight change was observed at the end of the curve due to small degradation of the catalyst. Nevertheless, the durability of CCR-0.3 electrocatalyst was tested by chronopotentiometric technique at different current densities (20 mA/cm 2 , 40 mA/cm 2 , 60 mA/cm 2 ) for 45 hours. It is evident from Fig. 6(f) that the potential of the catalyst did not decrease over a 45-hour period, and the variation in current densities did not affect its performance. It follows that the proposed electrocatalyst is capable of maintaining its functionality over an extended period of time. In order to determine the charge transfer resistance (Rct) between electrode and electrolyte, electrochemical impedance spectroscopy (EIS) was performed. Data were analyzed using Z view software using equivalent circuits as described in Fig. 7

Conclusions

The synthesis of (CrO 0.8 @Co 3 O 4 ) nano-electrocatalyst was carried out by co-precipitation. The among all prepared electrocatalysts. Moreover, the electrocatalyst proposed was stable at a variety of current densities and durable for more than 45 hours, which makes it suitable for industrial applications. Additionally, this catalyst has a low charge transport resistance (58.2) and a high electrochemical active surface area (515 cm 2 ), which makes it easier to facilitate the movement of charge carriers at the interface. Based on these accumulated data, it can be concluded that the addition of CrO 0.8 to Co 3 O 4 enhances its catalytic properties by modifying its morphological attributes, which is useful for a variety of applications.

Co 3 O 4 .

 34 Under alkaline conditions, CrO 0.87 has abundant catalytic sites and chemical states that could support the creation of surface vacancies, and enhance the performance of Co 3 O 4 with respect to active OER. As part of this study, we utilized a simple and low-cost approach for developing in situ heterogeneous electrocatalysts based on CrO 0.87 and Co 3 O 4 using a hydrothermal method. Different analytical techniques were employed to study the morphology, surface chemical composition, and crystal quality of the hybrid CrO 0.87 and Co 3 O 4 material. XRD and XPS tests demonstrated that a hybrid material based on CrO 0.87 and Co 3 O 4 had been successfully formed.

  2, and CCR-0.3. The recorded diffractograms are shown in Fig.1. At scanning angles of 30.99°, 36.51°, 38.20°, 44.40°, 55.13°, 58.79°, 64.60°, 73.37°, the diffraction patterns of pure Co 3 O 4 correspond to crystal planes (220), (311), (222), (400), (422), (511) and (440), respectively. According to the XRD patterns, Co3O4 exhibits a cubic crystalline phase (JCPDS card no. 01-080-1544). The composite materials CCR-0.1, CCR-0.2, and CCR-0.3 have shown diffraction peaks at 2 theta angles of 38.56 o , 44.83 o , and 65.26 o , respectively, in the crystallographic planes (111), (200), and (220) of the nonstoichiometric phase of chromic CrO 0.87 . These patterns confirm FTC card number 01-078-0722, which indicates cubic crystalline CrO 0.87 [53, 54]. From XRD analysis, it was noticed that the increasing concentration of CrO 0.87 in composite decreased the intensity of major peak (311) of Co 3 O 4 as shown in Fig. 1. It was evident that Co 3 O 4 reflections were observed in the hybrid material, indicating that Co 3 O 4 and CrO 0.87 materials were incorporated in situ in a non-stoichiometric manner. According to the XRD analysis, the two theta angle has shifted significantly towards a lower angle, as shown in the figure to the right; Fig. 1. A large chromate ion in potassium chromate has produced a stress on crystal growth, resulting in a shift in the 2 theta angle. It is possible that this shift could lead to defects in the structure of the hybrid CrO 0.87 and Co 3 O 4 material that may be exploited for lowering the overpotential of OERs. XRD measurements of the prepared materials revealed no other impurities. Additionally, we performed FTIR experiments on the prepared CrO 0.87 and Co 3 O 4hybrid materials to support the XRD results; Fig.S1. FTIR analysis of all the samples revealed similar inorganic peaks due to the presence of potassium bromide (KBr) used for making the pallets for each sample.[START_REF] Galli | Superparamagnetic iron oxide nanoparticles functionalized by peptide nucleic acids[END_REF][START_REF] Banerjee | Melanin from the Nitrogen-Fixing Bacterium Azotobacter chroococcum: A Spectroscopic Characterization[END_REF][START_REF] Kricheldorf | Macrocycles 27: Cyclic aliphatic polyesters of isosorbide[END_REF]. However, there was only a significant difference observed between O-Co-O and Co-O metal oxide peaks at 667 cm -1 and 577 cm -1 , respectively[START_REF] Laghari | Surface modification of Co3O4 nanostructures using wide range of natural compounds from rotten apple juice for the efficient oxygen evolution reaction[END_REF][START_REF] Yameen Solangi | MnO2@ Co3O4 nanocomposite based electrocatalyst for effective oxygen evolution reaction[END_REF].FTIR studies indicated that higher concentrations of potassium chromate resulted in a decrease in metal oxide peaks, which indicates in situ growth of CrO 0.87 and Co 3 O 4.

Fig. 1 XFig. 2

 12 Fig. 1 X-ray diffractograms of pristine Co 3 O 4 , CCR-0.1, CCR-0.2 and CCR-0.3 and their relative shift in the two theta (right hand side).

)

  Fig.3 (a-c) TEM micrographs of pristine Co 3 O 4 at different magnifications (d) HR-TEM micrograph and FFT in the inset reporting the crystal pattern.

Fig. 4 (

 4 Fig. 4 (a-c) TEM micrographs of hybrid CrO 0.87 into Co 3 O 4 (CCR-0.3) at different magnifications (d) HR-TEM micrograph and FFT in the inset reporting the crystal pattern.

  Co 3 O 4 were taken using HAADF and STEM techniques at various magnifications as shown in Fig. S3. According to these images, both the morphology and the distribution of dimensions are in agreement with what was observed on TEM micrographs. While HAADF -STEM micrographs of CCR-0.3 confirmed what was observed in the TEM micrographs, making more visible the more condensed crystalline phase with respect to the pristine Co 3 O 4 as seen in Fig. S4. Fig. S5 shows the position analysis of the pristine sample by EDS. The atomic percentage ratio between O and Co in the highlighted area (orange circle) is exactly what was expected with a value of 1.3. Furthermore, considering other specific points, the ratio does not change, indicating homogeneity in the composition. EDS mapping of the sample Cr doped is demonstrating that the Co content of the nanostructure is perfectly comparable with the signals of O and Cr, with their distributions having a similar morphology to those observed by STEM-HAADF micrographs, indicating a homogeneous composition; Fig. S6. Despite the increase in the O signal, probably caused by the presence of Cr in the form of oxide, the atomic percentages of the different chemical components do not permit confirmation of the crystal phase analysis. The atomic ratio of O to Co is 2.8 on average (normally, this ratio is 1.33 for Co 3 O 4 ). Taking into account the different points on the EDS map, the ratio of Cr/Co is approximately 0.27 on average.

Fig. 5 . 3 . 2 .

 532 Fig. 5. (A) XPS survey scans for pure Co 3 O 4 powder (solid line) and Cr-doped sample (dotted line). Photoemission peaks, reported toward binding energy, are labelled: Co2p and Cr2p structures are visible, together with C1s and O1s partially due to surface contamination, (B) Comparison of XPS Co2p spectra reported towards binding energy of pure Co 3 O 4 (solid line) and the Cr doped Co 3 O 4 (dotted line), (C)

  Fig. S10 shows the CV curves for various prepared samples analyzed at various scan rates including 30 mV/sec, 50 mV/sec, 70 mV/sec, 90 mV/sec, 110 mV/sec, and 130 mV/sec in the non-faradic region. The Cdl values were derived from CV data by plotting the linear fit slope of current densities at different scan rates as depicted in Fig. 6(d). Pristine Co 3 O 4 exhibited the lowest capacitance value of 3.7 mF.cm -2 , while CCR-0.1, CCR-0.2 and CCR-0.3 showed ascending capacitance values of 17.1

Fig. 6

 6 Fig.6 Electrochemical analysis (a) LSV curves of various electrocatalyst (b) Overpotential histogram (c) Tafel values plot (d) Double layer capacitance C dl values plot (e) Stability curve of CCR-0.3 and (f) Chrono-potentiometric durability of CCR-0.3 at various current densities.

Fig. 7

 7 Fig.7 Electrochemical impedance spectroscopic analysis of Co 3 O 4 pristine, CCR-0.1, CCR-0.2 and CCR-0.3 (a) Nyquist (b) Bode-1 and (c) Bode-2 plot.

  (a) based on the results of the EIS test in 1.0 M KOH solution. According to the Nyquist plot of the prepared electrocatalyst shown in Fig.

(

  CCR-0.3) electrocatalyst was found to be an optimized electrocatalyst. XRD results confirmed the existence of a cubic phase of CrO 0.8 and Co 3 O 4 . Based on electrochemical investigations, CCR-0.3 exhibits the highest OER activity due to its low overpotential (255mV), near Tafel values (56 mV/dec) to bobel catalyst, and higher double layer capacitance values (20.6 µF.cm -2 )

Fig. S1 Fourier

 S1 Fig. S1 Fourier Transform Infrared Spectroscopy of Co 3 O 4 pristine, CCR-0.1, CCR-0.2 and CCR-0.3

Fig. S3

 S3 Fig. S3 HAADF -STEM micrographs of Co 3 O 4 pristine at different magnifications.

Fig. S4

 S4 Fig. S4 HAADF -STEM micrographs of CCR-0.3 at different magnifications.

Fig. S5

 S5 Fig. S5 STEM-EDS analysis of the pristine sample Co 3 O 4 .

Fig. S6

 S6 Fig. S6 STEM-EDS mapping of the CCR-0.3 displaying Cr as doping.

Fig. S7

 S7 Fig. S7 LSV curves of various electrocatalyst along with high concentration (CCR-0.4).

Fig. S8

 S8 Fig. S8 ECSA-normalized LSV curves (Inset: a bar chart of the ECSA)

Fig. S10

 S10 Fig. S10 Cyclic voltametric curves at various scan rate of different catalyst for computation of C dl values.

2.1. Materials Potassium

  chromate (K₂ CrO₄ ), cobalt chloride hexahydrate (CoCl 2 .6H 2 O), urea (CH₄ N₂ O), Nafion (5 wt%), 20% RuO 2 /C, sodium hydroxide (NaOH) and potassium hydroxide (KOH) were received from Sigma Aldrich, Karachi, Sindh Pakistan. The chemical components were of high analytical grade and utilized without any further preprocessing. Growth and electrolytic solutions were prepared in deionized water.

Table 1 :

 1 Data from the EIS were used to calculate double layer capacitance values (CPE dl ) that are mentioned in the summarized features Table.1. It can be observed that the CCR-0.3 catalyst has the highest CPE dl value. Based on these data, CrO 0.87 is effective against Co 3 O 4 nanostructure that facilitates the movement of charges at the interface between electrode and electrolyte. Fig.7(b and c) illustrates bode plots 1 and 2. Bode plot-1 gives information about the catalyst's gain at a frequency level ranging from 10 6 Hz to 10 -1 Hz. There are orderly gain features of 46.15 o , Therefore, CrO 0.8 @ Co 3 O 4 electrocatalyst is the most promising candidate for practical OER applications. Summarized OER features of presented catalysts.

	7(a), all samples have similar solution resistances (R S ) since the test was conducted at a similar
	electrolyte concentration, i.e., 1.0M KOH. Furthermore, pristine Co 3 O 4 has a higher estimated
	value of R ct of 470 and the most effective catalyst (CCR-0.3) has a lower estimate of Rct of 58.2.
	38.13 o , 31.46 o and 21.11 o in pristine Co 3 O 4 , CCR-0.1, CCR-0.2 and CCR-0.3. In particular, the
	CCR-0.3 electrocatalyst outperforms the other three catalysts in terms of gain characteristics.
	However, CCR-0.3 (CrO 0.8 @Co 3 O 4 ) showed a lower maximum oscillation frequency than other

electrocatalysts in the bode plot-2. Consequently, a longer electron recombination lifetime was achieved, which may have contributed to a higher adsorption of active species at the catalyst interface.

Acknowledgments

We also extend our sincere appreciation to the Researchers Supporting Project Number (RSP2023R79) at King Saud University, Riyadh, Saudi Arabia. B. V. thank the platform "Microscopies, Microprobes and Metallography (3 M)" (Institut Jean Lamour, IJL, Nancy, France) for access to SEM facilities and F. Alnjiman for his valuable help. We also acknowledge partial funding of the Ajman University, Grant ID: DGSR ref. 2022-IRG-HBS-5

Conflict of Interest

Authors declare no competing interests in the resented research work

Supporting Information

In-situ growth of nonstoichiometric CrO 0.


Shusheel Kumar

Aneela Tahira

Adeel Liaquat Bhatti

Muhammad Ali Bhatti

Riaz Hussain

Nek Muhammad Shaikh

Muhammad Yameen Solangi

Ayman Nafady

Mélanie Emo

Brigitte Vigolo

Antonia Infantes-Molina

Alberto Vomiero
email: alberto.vomiero@ltu.se

Zafar Hussain

Transforming NiCo 2 O 4 nanorods into nanoparticles using citrus lemon juice enhancing electrochemical properties for asymmetric supercapacitor and water oxidation

Keywords: Citrus lemon juice, oxygenated groups, NiCo 2 O 4 material, asymmetric supercapacitor, oxygen evolution reaction

Recently, the nanostructured nickel-cobalt bimetallic oxide (NiCo 2 O 4 ) material with high electrochemical activity has received an intensive attention. Beside this, biomass assisted synthesis of NiCo 2 O 4 is getting popularity due to advantageous features such as low cost, simple, minimum use of toxic chemicals, environment and ecofriendly The electrochemical activity of spinel NiCo 2 O 4 is associated to mixed metal oxidation states. Therefore, large attention is paid on the crystal quality, morphology and tunable surface chemistry of NiCo 2 O 4 nanostructures. In this study, we have used citrus lemon juice consisting variety of chemical compounds having properties of stabilizing agent, capping agent and chelating agent. Moreover, the presence of several acidic nature chemical compounds in citrus lemon juice have changed the pH of growth solution and consequently we observed the surface modified and structural changes which were found to be very effective for the development of energy conversion an energy storage systems.

These naturally occurring compounds in citrus lemon juice have played a dynamic role in transforming the one dimensional (1D) nanorod morphology of NiCo 2 O 4 to small and well packed zero dimensional (0D) nanoparticles. Hence, the prepared of NiCo 2 O 4 nanostructures are exhibiting new surface-oriented nanoparticles, high defects on the surface (especially oxygen vacancies), sufficient ionic diffusion and reaction of electrolytic ions, enhanced electrical conductivity, and favorable reaction kinetics rate at the interface. The electrocatalytic properties of NiCo 2 O 4 nanostructures were studied for the evaluation of oxygen evolution reaction (OER) at low overpotential of 250 mV for10 mAcm -2 , Tafel slope of 98 mVdec -1 , and durability of 40 h. Moreover, the asymmetric supercapacitor was investigated and obtained results indicated a high specific capacitance of (Cs) of 1519.19 Fg -1 , and energy density of 33.08 Wh kg -1 at 0.8 Ag -1 .

The enhanced electrochemical performance could be attributed to the favorable structural changes, surface modification, and surface crystal facets exposure due to the use of citrus lemon juice. The proposed method of transformation of 1D nanorod to 0D nanoparticles could be used for the design of new generation of efficient electrocatalyst materials for energy storage and conversion uses.

Introduction

Currently, the supercapacitors are found highly effective electrochemical energy storage devices, hence significant advancement has been made in this field [1][2][3]. Supercapacitors have advantageous features including high energy density, long cycling durability, high power density and extended spectrum of operational temperature conditions in comparison to the conventional capacitors and regular electrochemical batteries [4][5][6]. Generally, there are two classes of supercapacitors described by charge-discharge behavior. First type is the electrochemical double layer capacitors (EDLCs), which stores charges on the basis of electrostatic approach on the active sites of electrode material on the basis of reversible adsorption of electrolytic ions. Second type is the pseudo-capacitors or redox supercapacitors, which stores the charges using fast and reversible redox processes [6]. For this reason, the preparation of unique architectures of nanostructured materials are highly desirable for the development of energy storage devices like supercapacitors (SCs). The use of spinel nickel-cobalt bimetallic oxide (NiCo 2 O 4 ) can be seen as a strong candidate for oxygen evolution reaction (OER) and SCs due to its favorable variety of mixed oxidation states, fast charge transfer of electron and an excellent electrical conductivity [7][8][9]. It has been shown that the activity of NiCo 2 O 4 material is highly depending on well oriented shape structures towards OER and SCs such as nanoparticles [10,11], microcuboids [12], hollow spheres [13], octahedrons [14], and nanocages [15]. The combustion of fossil fuels inherently produce large amount of greenhouse gases causing global warming effect ultimately polluted the environment [16,17]. Hence, the handling of challenges of clean energy, and its storage are taken seriously by developing new functional materials. For the purpose of clean energy, various technologies are used such as hydropower, solar energy, airstream and biomass based fuel productions [18][19][20]. The water dissociation into hydrogen gas does not take place without the input of energy due to strong H-O chemical bonding, therefore theoretically a minimum voltage of 1.23 V is needed for the dissociation of water. Such a high energy is difficult to manage from the existing renewable energy sources or fossil fuels; therefore the synthesis of numerous electrocatalysts has been taken into consideration to lower the energy barrier for the water splitting [13][14][15]. Typically, the OER and HER reaction process take place at the anode and the cathode, respectively [21].

Among both reactions, OER is very challenging as it is accompanied by the transfer of four electrons and considered as bottleneck challenge for the exploitation of efficient hydrogen production from water splitting [22][23][24][25][26][27]. For this purpose, the most efficient electrocatalysts are noble metals, like Ru/Ir/Pt/Pd. However, their large scale utilization is limited due to high scarcity and cost [21]. To overcome this, low-cost and earth abundant electrocatalysts have been investigated belonging to transition metal-based materials [28][29][30][31][32].

The NiCo 2 O 4 material nanostructures are widely used for OER owing to its high catalytic activity and electrical conductivity [33][34][35]. However, the performance of NiCo 2 O 4 material is limited by the low density of catalytic sites and sluggish charge transfer rate at the interface of electrode and electrolyte. Furthermore, it has been shown that the electrochemical properties of NiCo 2 O 4 are highly dependent on the shape, structure and desirable chemical composition [36,37]. For this reason, an enhanced electrochemical activity of nanoplate like NiCo 2 O 4 was observed [38]. Various factors effect on the morphological transformation of nanostructured material and electrochemical properties such as pH, temperature, surfactant, and synthetic routes have extensively studied [39]. Hence, it is a big challenge to prepare the different morphologies of NiCo 2 O 4 for improved electrochemical performances.

The use of new synthetic methodologies demonstrated effective to increase the performance of NiCo 2 O 4 nanostructures [40,41]. The use of biomass for tuning the catalytic activity of NiCo 2 O 4 nanostructured material is considered extensively these days, hence different biomasses have been used during the synthesis [40,41]. Previous studies have used different biomasses consisting phytochemicals and they have modified the surface properties of NiCo 2 O 4 nanostructures very effectively [40,41]. Previously, the utilization of ascorbic acid and citric acid from citrus lemon juice has been carried out on the ZnO material and its phase transformation, morphology and optical properties [42]. The citrus lemon juice exhibits wide range of unique chemical compounds as could be seen from Supplementary Scheme 1 [43], and their molecular structure was drawn by king draw software. The scientific details of citrus lemons as tracheophyta (phylum), magnoliopsida (class), sapindales (order), rutaceae (family), and citrus limon/limonum (species). In this study, the use of citrus lemon was employed to alter the pH of growth solution in addition to provision of capping, reducing and stabilizing agents during the preparation of NiCo 2 O 4 nanostructured material. Moreover, there is less attention paid about the citrus lemon juice to create surface oxygen vacancies, high Ni and Co oxidation states ratios, structural transformation, creating high surface active sites, high compatibility of NiCo 2 O 4 nanostructured material with electrode, and these aspects of material design have not been studied for the development of energy conversion and storage systems in the existing literature.

Importantly, low cost, and simple method for the transformation of 1D nanorod to 0D nanoparticles bring an advancement in the field of nanoscience for the development of future generation of functional nanostructures for a wide range of applications. Therefore, keeping in view the natural product chemistry of citrus lemon juice, particularly citric acid, malic acid, and ascorbic acid, we have utilized them together for the first time to enhance the electrochemical performance of NiCo 2 O 4 nanostructured material. Furthermore, the concept of terminal oxygenated groups through the use of variety of citrus lemon juice ingredients opens the new gateway for the synthesis of surface modified nanostructured materials with advanced functional properties. For this purpose, we have studied these types of aspects of citrus lemon juice on the NiCo 2 O 4 nanostructured material towards efficient energy storage and OER applications for the first time.

In this research work, we have used a variety of oxygenated sites of various natural compounds of citrus lemon juice for the hydrothermal preparation of short range and well packed nanoparticles of NiCo 2 O 4 .

Experimental Section

Preparation of NiCo 2 O 4 nanostructured material using citrus lemon juice

Nickel chloride hexahydrate, cobalt chloride hexahydrate, potassium hydroxide, ruthenium oxide (RuO 2 ), alumina paste of (0.3µM) and urea were of analytical grade and they were purchased from Sigma Aldrich Karachi, Sindh Pakistan. Fresh citrus lemon fruit was purchased from local market of Jamshoro, Sindh Pakistan. Prior to the synthesis of NiCo 2 O 4 nanostructured material, the lemon juice was collected from the cleaned lemon fruit using Philips juicer machine. The preparation of NiCo 2 O 4 nanostructured material was followed by two steps. First hydrothermal process was carried out to prepare bimetallic hydroxide phase, then thermal combustion was done in air to transform hydroxide phase into bimetallic oxide. A typical synthesis was initiated as follows: first, three beakers were taken and containing three main precursors concentrations of 0.015M nickel chloride hexahydrate, 0.1M cobalt chloride hexahydrate, and 0.1M urea were mixed in 100 mL of the deionized water. Two beakers were containing main precursors of nickel salt, cobalt salt and urea and successive addition of 0.5 mL, 1 mL and 2 mL of lemon juice was performed. They were labeled as sample 1, sample 2 and sample 3 respectively. The fourth beaker was containing the growth precursors of nickel salt, cobalt salt and urea and it was labeled as pure sample of NiCo 2 O 4 nanostructured material. The primary use of urea was to add certain value of hydroxyl ions in the growth solution to favor the formation of hydroxide phase of nickel-cobalt. Moreover, urea provides the hydroxyl ions through the release of ammonia which further reacts with the water and gives out hydroxyl ions for binding with metallic ions coming from their respective salt precursors. Whereas the use of citrus lemon was to change the pH of the growth solution in addition to supply capping, reducing and stabilizing agents during the preparation of NiCo 2 O 4 nanostructured material. The relative acidic nature of several compounds like ascorbic acid, malic acid and citric acid existing in lemon juice can alter the pH of the solution. However, the relative amount of those acidic nature ingredients of lemon juice in the 1 mL and 2 mL samples could be very low but their presence in the growth solution has slightly changed the pH of growth solution. The pH of growth solution without the use of lemon was about 6.72 and the pH of the growth solution with the use of 0.5mL and 1mL of lemon juice was about 6.2 and 5.8 respectively. Whereas pH of growth solution with the use of 2 mL of lemon juice was found about 3.2. The significant variation in the pH of growth solutions with 0.5mL, 1mL and 2 mL of lemon juice could play a dynamic role to alter the structure and surface properties of the nanostructured material. Four beakers with growth solutions were sealed very tightly with aluminum foil and placed in electric oven for 5 hours at 95 o C. After the growth process, the hydroxide product of nickel-cobalt was collected onto filter paper and washed with the deionized water. Later, the product was dried and allowed for the thermal combustion treatment in air for 5 hours at 500 o C. At the end, we were successful to get a black product of NiCo 2 O 4 nanostructured material including sample 1, sample 2, sample 3 and pure sample. The characterization of various NiCo 2 O 4 nanostructured materials was done with respect to morphology, crystal arrays, chemical composition and surface defects using various analytical techniques. SEM was used to study morphology of NiCo 2 O 4 nanostructures at applied voltage of 2 kV with a ZEISS Gemini SEM 500 equipped with a field emission gun.

Powder X-ray diffraction was employed to identify the phase and purity of NiCo 2 O 4 nanostructured material under the experimental conditions of X-rays with a wavelength of wavelength λ Kα = 1.5406 Å coming from Cu anode at applied potential of 45 kV and 45 mA current. Transmission and scanning transmission electron microscopy (TEM and STEM, respectively) observations were performed on a JEOL JEM -ARM 200F Cold FEG microscope working at 200 kV and equipped with a probe corrector (Cs). Chemical analyses were done with energy dispersive X-ray spectroscopy (EDX) (SDD, Jeol DRY SD 30 GV). X-ray photoelectron spectroscopy (XPS) with X-ray source from Al K α at 30 eV for the surface chemical composition analysis. The measured binding energies of as prepared NiCo 2 O 4 nanostructured materials were fitted with the reference binding energy of C1s peak at 284.6 eV.

Electrochemical characterization of surface modified NiCo 2 O 4 nanostructured material

We have tested different NiCo 2 O 4 samples towards supercapacitor and OER half-cell water splitting applications in aqueous solution of KOH electrolyte. The supercapacitor investigations were performed with three electrode cell set up in 3.0M KOH. The reference electrode was based on calomel electrode (Hg/HgO), and graphite rod as counter electrode. The working electrode was glassy carbon electrode (GCE) with a diameter of 3.0 mm diameter and it was used for the deposition of NiCo 2 O 4 nanostructured materials using drop casting approach. Before modification, the GCE was cleaned with the paste of 0.3µM alumina powder slurry and rubbed by silicon paper; consequently it was washed several times with the deionized water. The supercapacitor characterization was done with chronopotentiometry (CP) by measuring galvanostatic charge-discharge (GCD) curves at various current densities for the estimation of specific capacitance (C s ) F/g, cycling stability and energy density. The calculations of supercapacitor application are shown calculation (S1).

Furthermore, OER studies were performed on the different NiCo 2 O 4 nanostructures in 1.M KOH.

The catalyst ink of NiCo 2 O 4 nanostructures was prepared by dispersing 5 mg of each sample in 4 mL of the deionized water and 100µL of 5% Nafion. The homogenous dispersion was obtained using sonication process for 10 min. Then, 5µL of each catalyst ink with approximately mass of (0.2mg) was deposited onto cleaned GCE using drop casting method and dried with the blow of air. The OER half-cell investigations were studied by different electrochemical modes like cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS). Prior to LSV measurement, three CV cycles were done at a scan rate of 10 mV/s for the measurement of stable LSV curves for OER process at a scan rate of 2 mV/s. The EIS measurement was used to measure the kinetics of charge transfer at the interface of modified electrode and electrolyte under the measurement conditions of sweeping frequency range from 100 kHz to 0.1 Hz using an amplitude of 5 mV and OER onset potential. The CV curves in the non-Faradic region at various scan rates were measured to find the amount of active surface area using electrochemical active surface area (ECSA)

Results and discussion

Physical characterization of as prepared different NiCo 2 O 4 nanostructures

Morphology and structure of the synthesized NiCo 2 O 4 based nanocatalysts were studied by SEM and XRD, respectively (Figure 1). Figure 1a shows the typical shape orientation of pristine NiCo 2 O 4 nanostructures prepared by hydrothermal process which is characterized by nanorod like morphology with a length of several microns and average diameter in the range of 300-500 nm. The morphology of nanorod is assembled by the nanoparticles with a dimension of below 100 nm as shown in Figure 1a, insert. Furthermore, the morphology of citrus lemon juice assisted NiCo 2 O 4 nanostructures with volume of 1 mL (sample 2 Figure 1b) was found with different morphology from pure sample of NiCo 2 O 4 nanostructured material. For both of them, 1D nanorod morphology was transformed into well packed small size 0D nanoparticles. The size of small size nanoparticles could be below 100 nm, which is a typical a nanostructured phase of as prepared NiCo 2 O 4 nanostructures. The growth process in the presence of citrus lemon juice can be proposed in terms of oxygenated terminal of variety natural compounds present in the citrus lemon juice. These natural product molecules have capability of stabilizing agent, capping agent, and chelating agent which played an important role towards the transformation of 1D nanorod morphology into 0D nanoparticles. Moreover, the pH of was changed growth solution due to the presence of ascorbic acid, citric acid, malic acids and also their stabilizing agent, capping agent and chelating agent properties resulted the 0D nanoparticles as can be seen from the Figures 1b.

Thus, the compact nature of nanoparticles with uniform electron transfer during the electrochemical reaction can bring obvious role towards the durability of material for long term applications. The citrus lemon juice is naturally rich with acidic compounds, high density of terminated oxygen groups in each molecule for transforming the one dimensional (1D) structure into zero dimensional (0D) structure with typical nanoscale features. The significant variation in the pH of growth solutions with 0.5 mL, 1 mL and 2 mL of lemon juice could play a dynamic role to alter the structure and surface properties of the nanostructured material. From acidic nature of ascorbic acid, malic acid and citric acid, we hereby describe that they have shown influence on the pH of growth solution, consequently the structural transformation and surface modifications were significantly observed. However, the primary use of citrus lemon juice was to alter the pH of growth solution and to induce the significant effect on the structure and surface properties of material. We have also noticed that the relative amount of nanostructured material formation was low when 2 mL of citrus lemon juice was used because of the decreased amount of hydroxyl ions which has slowed down the growth kinetics. From SEM observations, the presence of lemon juice alters significantly the morphology (Figs. 1a and1b, with and without lemon juice respectively). The rod-like shape is completely lost when lemon juice is used for the catalyst preparation and the outcome catalysts have a powdered disordered structure. Tiny particles are still detectable, as confirmed by XRD analysis of crystallite size. 1. It is obvious that 1 mL and 2 mL of lemon juice did not bring substantial influence on the average crystallite size of pure NiCo 2 O 4 nanostructures and 1 mL (sample 2) and 2 mL of lemon juice (sample 3). A slight increase on average crystallite size is recorded in sample 3. To have deeper insight on the chemical states of metallic ions and surface vacancies about the pure and citrus lemon juice assisted NiCo 2 O 4 samples, XPS analysis was performed as shown in Figure 2a. The photoelectron contributions for Co 2p 3/2 signal in the pure NiCo 2 O 4 sample were observed at 779.5 (Co 3+ ), 781.4 (Co 2+ ), 784.2 (Satellite) and 788.9 (Satellite) eV which is in good agreement with the reported works [44][45][46]. Similarly, the Ni 2p 3/2 contributions were located at different binding energies such as 854.0 (Ni 2+ ), 855.7 (Ni 3+ ), 857.0 (Ni-OH) and 861.1 (satellite) eV [47]. The relative ratio of Ni 2+ /Ni 3+ ions and Co 2+ /Co 3+ ions in the pure sample was found to be 1.13 and 0.44 respectively. For the citrus lemon juice assisted NiCo 2 O 4 nanostructures, the highly resolved spectrum of Co 2p 3/2 was also fitted, the corresponding peaks of Co 3+ and Co 2+ were found at binding energies of 779. For better view of structural information, various TEM images were shown (Figure S3), suggesting the effect of citrus lemon juice on the morphology compared to the material prepared without the citrus lemon juice. The TEM analysis confirms the results from SEM: addition of citrus lemon juice inhibits the formation of a rod-shaped structure of self-assembled nanoparticles, and originates a homogeneous distribution of tiny nanoparticles, roughly hierarchically clustered in sub-micrometer sized spheres. Also, the d-spacing were calculated by using HRTEM micrographs of citrus lemon juice assisted juice NiCo 2 O 4 and pure NiCo 2 O 4 as shown in Figure S4. and corresponding X-maps of cobalt (e,i), nickel (f,j) and oxygen (g,k). shown in Figure 3(a-f). The CV characterization described the behavior of three materials with pronounced redox behavior at various scan rates ranging from 10-60 mV/s as shown in Figure 3a-c. The area of each curve was enhanced with increasing scan rate. Moreover, the shape of CV curves did not alter with increasing scan rate, suggesting the prepared materials could deliver superior electrochemical performance. The capacitance from CV was observed higher for sample 2 compared to the sample 3 and pure NiCo 2 O 4 nanostructures at low scan rate. Beside this, GCD curves for these materials were measured at different current densities such as 0.8 to 0.94 Ag -1 and their corresponding electrochemical behavior is shown in Figure 3d-f. It is obvious that the sample 2 is exhibiting the longest discharge time in comparison to sample 3 and pure NiCo 2 O 4 nanostructures as enclosed in Figure 3e. Furthermore, the specific capacitance (C s ) values were calculated at 0.8 Ag -1 for the sample 2, sample 3 and pure NiCo 2 O 4 nanostructures which were found in the order of 358 Fg -1 , 163 Fg -1 , and 117 Fg -1 respectively as shown in Figure 4a. The cyclic stability was also measured at 0.8 Ag -1 for three samples and corresponding retention efficiency of C s were found in the order 100-94%, 108-83%, 90-61%, for sample 2, sample 3 and pure sample, respectively as show in Figure 4b. After 900 cycles, the retention capacitance was found highest for the sample 1. It is verified that the sample 2 exhibited enhanced cyclic stability with highest retention; hence it could be considered as an optimized electrode material for the development of real energy storage devices. The columbic efficiency for the sample 2, sample 3 and pure sample was observed in the order of 70%, 64%, and 43% respectively as shown in For three electrode supercapacitor application, the cyclic stability for the sample 1 is either equal or higher to many of the recently reported electrode materials as given in Supplementary Table (S3). Many of the materials reported in the Supplementary Table (S3) are complex in nature and involving hybrid systems which apparently have excellent performance. However our presented material synthesis approach is facile, low cost, ecofriendly and environment friendly and it has comparable of specific capacitance to the previous studies, hence offering an alternative material with promising green synthetic strategy for the scale up synthesis of nanostructured materials.

Electrochemical performance evaluation of

The citrus lemon juice has changed the pH of growth solution and it has influenced on the structural transformation, surface modification and the exposed the favorable crystal facets for the enhanced electrochemical performance. The performance of our material can be enhanced compare to the existing literature on the supercapacitor application by high loading of proposed material onto large surface area electrode. Despite the use of low current density of 0.8 Ag -1 and narrow potential window, we have obtained the considerable high energy density and specific capacitance using newly prepared NiCo 2 O 4 nanostructures (sample 2).

Before to the development of two electrode asymmetric supercapacitor device, we have evaluated the supercapacitor properties of activated carbon using three electrode cell configuration and the measured performance is enclosed in Supplementary Figure (S5). First GCD curves were obtained at various current densities of 0.8, 0.85, 0.88 and 0.94 Ag -1 , then specific capacitance of 247 F/g and energy density of 5.47 Wh/Kg were calculated as shown in Supplementary (S5). The analyzed results if GCD curves indicated the poor performance of activated carbon (AC). The CV curves of (NiCo 2 O 4 Sample 2) and for AC for the potential range of -0.2 to 1.5 V are shown in Supplementary Figure (S5) and it was assumed that the operating potential window of (NiCo 2 O 4 Sample 2//AC ASC) could be enlarged to 1.45 V and the potential window was selected according to published work [50][51][52][53][54]. Further increasing the potential range could cause the H 2 /O 2 evolution reaction as previously reported [50,51]. Hence, the optimum operational (S6). The capacitance retention for the 900 cycles was found around 64-52%, which is still far better than many of the reported works on bimetallic oxides in the literature as shown in Supplementary Figure (S6). The specific capacitance C s at 0.8Ag -1 was calculated as 1519 Fg -1 even at a 0.94Ag -1 specific capacitance was found high like 1459 Fg -1 as shown in Supplementary Figure (S6). The estimated energy density of 33.08 Wh kg -1 is relatively higher than the reported works with a power density of 633 W/kg, confirming the significant advancement in the field of ASC devices as shown in Supplementary Figure (S6). The columbic efficiency of 100-84% was also an excellent for the presented ASC system as shown in Supplementary Figure (S6) (38.4 Wh kg -1 at a power density of 794.5 W kg -1 ) [58]. For the simplicity, the observed performance of ASC is provided in the Table (S3). was observed compared to the sample 2. The linear region of LSV polarization curves was used to assess the reaction kinetics and the estimated Tafel plots are given in Figure 5b. Significantly, the Tafel slope of NiCo 2 O 4 nanostructures grown with 1 mL was as low as 98 mVdec -1 which is lower compare to pure and 2 mL assisted NiCo 2 O 4 nanostructured materials (105 mVdec -1 , and 112 mVdec -1 respectively). The Tafel slope of sample 1 was estimated about 107 mVdec -1 . The Tafel analysis revealed that the NiCo 2 O 4 nanostructures grown with 1 mL of citrus lemon juice has speedy OER kinetics due to favorable catalytic surface properties. Stability of 1 mL assisted NiCo 2 O 4 nanomaterial was also studied in 1.0M KOH using LSV polarization curves before and after the durability measurements as shown in Figure 5c. It was noticed that the durability test for 40 hour did not alter the onset potential, overpotential and current density, suggesting the high stability and compatibility of material with the GCE. Durability is an important parameter to examine the activity of nonprecious catalyst; hence chronopotentiometry was used to describe the electro catalytic durability of NiCo 2 O 4 nanostructures prepared with 1 mL of citrus lemon juice as shown in Figure 5d. There was negligible overpotential drop at constant 20 mAcm -2 for the period of 40 hours, confirming the durability of material for a significant period of OER operation. The NiCo 2 O 4 nanostructures have been found stable under 1.0M KOH alkaline conditions and they are strongly supported by the reported works elsewhere. It has been observed that the electrochemical active surface area (ECSA) is directly connected to the double layer capacitance (Cdl) of an electrocatalyst [59], and known as a crucial parameter to assess the amount of active sites on the surface of catalyst and they have to be involved in the electrochemical reaction. For this purpose, cyclic voltammetry (CV) curves were measured with non-Faradic regions to calculate the Cdl as shown in Supplementary Figure (S7). The Cdl values were obtained by plotting the difference of anodic and cathodic current densities at 0.2V versus RHE against different scan rates [60]. The Cdl is considered as a half value of slope achieved after linear fitting and ECSA could be measured by using equation of ECSA= Cdl/C s . The C s is a constant value related to nature of electrode material surface which is found to be 40 mFcm -2 in 1.0M KOH as previously reported [61]. The measured values of ECSA for three nanostructures of NiCo 2 O 4 nanostructures (sample 2, sample 3) and pure sample were in the order, 15.2 µFcm -2 , 8.2 µFcm -2 , and 6.3 µFcm -2 respectively. The ECSA calculations revealed that the NiCo 2 O 4 nanostructures (sample 2) with highest value is describing the predominant contributing factor towards faster OER activity. Furthermore, the charge transfer resistance rate was also evaluated for the NiCo 2 O 4 nanostructures prepared with and without citrus lemon juice during OER operation using electrochemical impedance spectroscopy (EIS) as shown in Figure 5e The OER performance of NiCo 2 O 4 nanostructures (sample 2) prepared with citrus lemon juice was compared with the previously reported electrocatalyst as given in Table S6. It is shown that activity of NiCo 2 O 4 nanostructures (sample 2) is superior to many of the catalyst in terms of low overpotential of 250 mV at the same current density and 1.0M KOH aqueous solution, confirming the dynamic role of natural molecules from citrus lemon juice on the enhanced activity towards water splitting. Importantly, the synthetic strategy of presented work is green, used minimum toxic chemicals, low cost, scale up for the synthesis of large quantity of material, ecofriendly and environment friendly. Furthermore, the obtained electrochemical performance is shown bar graph for the better understanding and view as enclosed in Supplementary Figure S8 (a-d). We report the obtained results in these bar graphs for OER overpotential at 10 and 50 mAcm -2 , Tafel values, specific capacitance (C s ) and energy density, respectively, for the pristine and citrus lemon juice assisted NiCo 2 O 4 nanostructures (sample 1, sample 2, sample 3) and RuO 2 . For better view, the OER of ( sample 1, sample 2, sample 3) and capacitance performance of the pure and citrus lemon juice assisted NiCo 2 O 4 nanostructures (sample 2, sample 3) are given in Table 2. It is clear that the sample 2 has relatively better electrochemical activity compare to pure and sample 2. Here: m±, Cs±, and ΔV± describes the mass, specific capacitance C s , and potential range of NiCo 2 O 4 sample 2 ( + ) and AC (-) electrodes, respectively. The estimated total mass of m+/m-was about 0.17 mg in NiCo 2 O 4 sample 2//AC ASC. For the calculation of C s , and energy density and power density the total mass was calculated as per the reported work [3].

The practical aspects of NiCo

Oxygen evolution reaction performance evaluation of various citrus lemon juice assisted

C s represents specific capacitance, I current, discharge time , potential change , loaded mass as .

Energy density was calculated as: E d is energy, C s as specific capacitance, potential variation

The power density was determined by [2]:

Here P d is the power density, C s is the specific capacitance and the discharge time is

Scheme 1 :

 1 Scheme 1: Stepwise preparation of NiCo 2 O 4 nanostructured material using lemon juice and their electrochemical testing set up

  The crystalline phase was studied by using powderXRD technique and recorded diffraction patterns of different samples including sample 1, sample 2 and sample 3 are shown in Figure 1d. The diffraction patterns of sample 1 prepared with 0.5mL of citrus lemon juice are shown Supplementary Figure (S1). Various diffraction patterns of three samples of NiCo 2 O 4 nanostructures showed crystal planes such as 111, 220, 311, 222, 400, 422, 511, 531, 442, 533, 620 and 622 and they were well matched the cubic phase of NiCo 2 O 4 . The measured diffraction patterns of NiCo 2 O 4 nanostructures were in good agreement with the standard JCPDS card (01-080-1542). Interestingly, the increasing volume of citrus lemon juice has reduced the intensity of many of the diffraction peaks due to variation in the pH of growth solution, suggesting the dynamic role of citrus lemon juice about the alteration of shape of NiCo 2 O 4 nanostructured material. The sample 2 suggests preferential growth along the 400 direction due its strong reflection intensity and offered favorable crystal orientation for high exposure of catalytic sites and swift charge transfer towards the electrochemical applications. This is the point of difference between sample 2 and pure NiCo 2 O 4 nanostructures and sample 3. The XRD analysis confirmed only the spinel structure with cubic phase of NiCo 2 O 4 nanostructures and the preparation of high quality nanostructured material. Moreover, an enhanced peak for 620 crystal plane of NiCo 2 O 4 nanostructures in case of (sample 3) is indicating the preferential growth along this direction and it might change the crystal geometry of NiCo 2 O 4 nanostructures that could not favor the electrocatalytic activity. The average crystallite size was calculated by Scherrer equation and the calculated values are given in Table

Figure 1 :

 1 Figure 1: Distinctive SEM images at different magnifications of (a) pristine NiCo 2 O 4 nanostructures, (b) NiCo 2 O 4 nanostructures prepared with 1 mL (sample 2) and (c) XRD patterns of pristine NiCo 2 O 4 nanostructures, (b, c) NiCo 2 O 4 nanostructures prepared with 1 mL (sample 2), and 2 mL of lemon juice (sample 3).

  3, and 781.1 eV, respectively, as shown in Figure 2 [48]. For the Ni 2p 3/2 signal, the fitting showed the contributions at various binding energies 854.0 (Ni 2+ ), 855.7 (Ni 3+ ), 856.1 (Ni-OH) and 860.8 (satellite). The relative ratio of Ni 2+ /Ni 3+ ions and Co 2+ /Co 3+ ions for citrus lemon juice assisted NiCo 2 O 4 sample was approximately 1.18 and 0.66, respectively. It is indicating a greater proportion of reduced species on the surface of citrus lemon juice assisted NiCo 2 O 4 sample compare to pure NiCo 2 O 4 sample. Moreover, Ni/Co ratio pure NiCo 2 O 4 sample was changed from 1.02 to 1.37 in case of NiCo 2 O 4 sample grown with citrus lemon juice. In this regard, the XPS analysis revealed that pure NiCo 2 O 4 sample and citrus lemon juice assisted NiCo 2 O 4 sample reveal the coexistence of Ni 2+ /Ni 3+ ions and Co 2+ /Co 3+ ions on their surface and the presented results are strongly supported by the reported work [49]. Finally, the O 1s spectra for the pure NiCo 2 O 4 sample and citrus lemon juice assisted NiCo 2 O 4 sample show three essential contributions at ca. 529.5 eV, 531.15 eV, 532.6 eV due to O Lat , O Sur , O Che , respectively. O Lat refers to lattice oxygen in the metal-oxygen structure, O Sur is usually assigned to surface defects and oxygen vacancies, andO Che is assigned to chemisorbed oxygen as revealed by previous studies[44]. By quantifying the different oxygen species in both samples, the O Sur /O Lat ratio changes from 0.52 to 0.76 from pure to citrus lemon juice based sample, indicating that there is a greater proportion of surface species in the latter that could be playing an important role in the OER reaction.The morphology, crystalline and chemical composition investigations of pristine and citrus lemon juice assisted NiCo 2 O 4 samples were performed through scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDXS), high resolution transmission electron microscopy (HRTEM) and selective area electron diffraction (SAED). The pristine NiCo 2 O 4 sample is mainly composed of particles of diameter between 20 and 80 nm assembled in nanorod-like structure (Figures2b, 2d), while citrus lemon juice assisted NiCo 2 O 4 is composed of aggregates of nanoparticles of less than 20 nm (Figures2c, 2h). Similar morphological features were observed during SEM analysis. The aggregation of nanoparticles (sample 2) could be ascribed to the collective attraction between nanoparticles through chemical interactions or Vander Waals forces. EDXS experiments confirm the presence of Ni, Co, and O for both samples, without any impurities (Figures2 d-k). SAED patterns of pure NiCo 2 O 4 and citrus lemon juice assisted NiCo 2 O 4 (FigureS2) present rings at 4.7, 2.9, 2.4 and 2.0 Å, corresponding to the Fd-3m cubic structure of NiCo 2 O 4 (a = 8.1 Å), as found on XRD experiments. The overall observations on both samples suggest that citrus lemon juice has mainly a role in transforming 1D nanorod of pure NiCo 2 O 4 into 0D nanoparticles.

Figure 2 :

 2 Figure 2: Ni 2p 3/2 , Co 2p 3/2 and O 1s spectra of pure NiCo 2 O 4 and NiCo 2 O 4 nanostructures grown with citrus lemon juice with 1 mL (Sample 2) (a). HRTEM micrographs with corresponding Fast Fourier Transform (FFT) (insert of Figures) of pure NiCo 2 O 4 (b) and citrus lemon juice assisted grown NiCo 2 O 4 nanostructures (c). STEM High Angle Annular Dark Field (HAADF) of pure NiCo 2 O 4 (d) and citrus lemon juice assisted grown NiCo 2 O 4 nanostructures (h)

  NiCo 2 O 4 nanostructures for energy storage applications Three newly prepared samples of NiCo 2 O 4 nanostructures including such as (sample 1, sample 2, Sample 3) and pure NiCo 2 O 4 nanostructures as reference material to understand the role of citrus lemon juice towards tuning the electrocatalytic behavior of materials. The electrochemical performance was evaluated towards the development of SCs. The prepared nanostructured NiCo 2 O 4 materials were characterized by cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) curves as shown in Figure 3. Both CV and GCD curves were measured with three electrode cell set up under electrolytic environment of 3.0M KOH aqueous solution as

Figure 4c .

 4c Figure 4c. The energy density was also estimated at 0.8 Ag -1 for sample 2, sample 3 and pure sample of NiCo 2 O 4 nanostructures in the series of 7.96 Wh kg -1 , 3.57 Wh kg -1 , and 2.47 Wh kg -1 respectively. The power densities were also calculated for the sample 2, sample 3 and pure NiCo 2 O 4 nanostructures with corresponding values of 160.00 KW/Kg, 158.80 KW/Kg, and 156.00 KW/Kg respectively as shown in Figure 4d-f. The evaluated electrochemical activity of as prepared NiCo 2 O 4 nanostructures (sample 2), (sample 2), and pure NiCo 2 O 4 nanostructures for the SCs is shown in Table (S3).

Figure 3 :Figure 4 :

 34 Figure 3: (a-c) Various CV curves at different scan rates for NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3), pure NiCo 2 O 4 nanostructure, in 3.0M KOH aqueous electrolytic solution. (d-f)

  2 O 4 nanostructures (sample 2), was also studied by developing asymmetric supercapacitor (ASC) using anode of NiCo 2 O 4 nanostructures (sample 2), and activated carbon as cathode electrode in aqueous 3.0M KOH solution. The ASC was represented by (NiCo 2 O 4 Sample 2//AC ASC).

  potential window was kept approximately to 1.45 V to examine the electrochemical activity of asymmetric device (NiCo 2 O 4 Sample 2//AC ASC) and the corresponding CV curves are enclosed in Supplementary Figure (S6). The CV results have revealed the overall phenomenon of capacitance of (NiCo 2 O 4 Sample 2//AC ASC) was extracted from double-layer capacitance and battery like aspects of newly prepared spinel bimetallic oxide material. From CV curves, it is clear that there is no change of shape of CV, indicating the swift charge-discharge features of (NiCo 2 O 4 Sample 2//AC ASC) device. The corresponding GCD curves of (NiCo 2 O 4 Sample 2//AC ASC) were also measured as shown in Supplementary Figure (S6) for the current density range of 0.8 to 0.94 Ag -1 and derived different calculation values of ASC characterization are enclosed in Supplementary Figure

  . A power density of 633 W/kg is reflecting that the (NiCo 2 O 4 Sample 2//AC ASC) can maintain the energy density of 33.1 Wh kg -1 , suggesting that these values are still impressive compared to the already existing NiCo 2 O 4 based ASCs systems such as composite system of NiCo 2 O 4 @MnO 2 hybrid//AC (an energy density of 37.8 Wh kg -1 at power density 187.5 W kg -1 ) [52], N-doped carbon-coated NiCo 2 O 4 //AC (29.4 Wh kg -1 at the power density of 349 W kg -1 ) [55], HDC@NiCo 2 O 4 @PPy//HDC@NiCo 2 O 4 @PPy (17.5 Wh kg -1 at the power density of 500 W kg -1 ) [50], NiCo 2 O 4 microspheres//AC (45.3 Wh kg -1 at the power density of 533.3 W kg -1 ) [56], NiCo 2 O 4 /NGN/CNTs//NGN/CNTs (42.7 Wh kg -1 at the power density of 775 W kg -1 ) [51], and NiCo 2 O 4 @Co-Fe LDH//AC (28.94 Wh kg -1 at the power density of 950 W kg -1 ) [52]. Beside NiCo 2 O 4 based ASCs devices, the previously reported ASCs based on other materials have either inferior or equal performance to the presented (NiCo 2 O 4 Sample 2//AC ASC) such as CuS hollow micro flowers//AC (15.97 Wh kg -1 at a power density of 185.4 W kg -1 ) [57], and Ni 3 Se 2 3D HMNN//AC

NiCo 2 O 4 nanostructures

 4 Electrochemical performance of NiCo 2 O 4 nanostructures was studied for OER half-cell water splitting in 1.0M KOH aqueous solution using three electrode cell configuration. For this characterization, LSV was used for the pure NiCo 2 O 4 and lemon juice assisted NiCo 2 O 4 nanostructures (sample1, sample 2, sample 3) at sweeping scan rate of 2 mV/s and corresponding iR corrected polarization curves are enclosed in Figure 5a. The performance of these newly prepared materials of NiCo 2 O 4 was also compared with the reference noble metal (RuO 2 ) electrocatalyst as shown in Figure 5a. The overpotential for NiCo 2 O 4 nanostructures grown with 1 mL of citrus lemon juice was found to be 250 mV at 10 mAcm -2 , which is smaller than pure and 2mL citrus lemon juice assisted NiCo 2 O 4 nanostructures. An overpotential of 270 mV at 10 mAcm -2 was recorded for the pure and 2 mL assisted NiCo 2 O 4 nanostructures as shows in Figure 5a. Whereas the sample 1 has shown relatively better performance compare to the pure NiCo 2 O 4 nanostructures with a low overpotential of 280 mV at 10 mAcm -2 . However, 1 mL of citrus lemon juice drastically changed the OER activity of NiCo 2 O 4 nanostructures with lower overpotential compare to sample 1, sample 2 and pure NiCo 2 O 4 nanostructures. The pure NiCo 2 O 4 nanostructures have shown the limiting performance owing to its low density of active sites and poor electron transfer capacity. An addition of 2 mL of citrus lemon juice supplied high concentration of reducing agents which significantly enabled the surface with limited number of active sites and charge transport of the material, hence poor performance NiCo 2 O 4 nanostructures

  . Bode plots obtained from EIS data at different sweeping frequencies are shown in Figure 5f. The Nyquist plot with higher frequency region has shown the information about the porosity of electrode film whereas in the low frequency region the sub circuit is associated with the charge transfer resistance [62]. The fitted circuit has provided the charge transfer resistance values for three NiCo 2 O 4 nanostructures (sample 2, sample 3 and pure) are in order 165 Ω, 547 Ω and 968 Ω respectively and these values are provided in the Table (S6). The charge transfer resistance of low value for the sample 2, further supported its OER performance in 1.0M KOH aqueous solution. The XRD results of sample 2 has revealed that the preferred crystal orientation along the 400 crystal plane with high intensity has shown desirable crystal orientation for the exposure of high surface catalytic sites, consequently fast charge transport has been shown at the interface of electrode and electrolyte. The HRTEM study has shown that NiCo 2 O 4 (sample 2) is composed of nanoparticles of less than 20 nm and exhibited a large surface area. Therefore, a low nanoscale dimension of less than 20 nm for the sample 2 offered frequent contact with electrolyte, thus efficient electrochemical performance was observed. The SAED patterns of lemon assisted NiCo 2 O 4 (sample 2) presents rings at 2.4 and 2.0 Å, indicating small lattice rings and they could be further effectively driven the electrochemical activity of material towards supercapacitor and half-cell OER applications. The improved performance of proposed electrocatalyst from fundamental point of view could be explained in terms of shape transformation of material, variation in the crystal orientation, high surface oxygen vacancies, high active sites, fast charge transfer rate at the interface of electrode and electrolyte favored the electrocatalytic kinetics, high Ni/Co ratio of 1.37 in case of citrus lemon juice assisted NiCo 2 O 4 sample , and biomimetic features of citrus lemon juice assisted NiCo 2 O 4 sample supported the high compatibility with glassy carbon electrode. The present study was aimed to understand the fundamental role of structural transformation, creation of defects on the surface, and to alter the crystal orientation during growth process, therefore they together insightfully enhanced the electrochemical performance of NiCo 2 O 4 nanostructures. These fundamental features of prepared NiCo 2 O 4 nanostructures of presented study bring an advancement in the development of new generation of electrocatalysts for the energy conversion and storage systems. Furthermore, an enhanced SC and electro catalytic performances of NiCo 2 O 4 nanostructures (sample 2) were contributed from unique fabrication aspects, such as short range nanoparticles resulted from the unique chemistry of citrus lemon juice has exposed the material at large extent to the electrolytic molecules. The use of biomass packed the nanoparticles avoided the volume changes, hence maintained the mechanical strength, demonstrating the significant cycling stability during SC and OER processes.

Figure 5 :

 5 Figure 5: (a) LSV polarization curves at 2 mV/s for pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3) and RuO 2 in 1.0M KOH aqueous solution, (b) Corresponding Tafel analysis of pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3) and RuO 2 , (c) stability of NiCo 2 O 4 (sample 3) (d) durability for 40 hours at 20 mAcm -2 , (e) Electrochemical impedance spectroscopy (EIS) Nyquist plots of pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3) for frequency range of 100 kHz to 0.1 Hz at an amplitude of 5 mV and onset potential of OER, inset shows the fitted equivalent circuit (f) Corresponded Bode Plots of pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3) from same EIS

  Asymmetric supercapacitor was fabricated using two electrodes set up involving positive electrode NiCo 2 O 4 nanostructures prepared with 2 mL of lemon juice and negative electrode of activated carbon (AC) in 3.0M KOH electrolytic solution. Commercial activated carbon was used for the development of negative electrode during two electrode configuration for the asymmetric device system. The slurry of activated carbon was prepared by dispersing 17 mg of Ac into 3:1 volume of ethanol and the deionized water. In this mixture 100µL of 5% Nafion was also added. The well dispersed slurry was achieved by sonication for 10 min. The anode of NiCo 2 O 4 and cathode of AC was developed onto GCE respectively. The asymmetric supercapacitor was described by following configuration of NiCo 2 O 4 sample 2//AC ASC. The balancing of voltammetric charges for the prepared NiCo 2 O 4 sample 2 and AC was done with calculation of masses of two electrode by following relationship as previously reported Q+ = Qequation [2]. m+ m-= C s -ΔV-C s +ΔV+.

  Scheme (S1):Chemical compounds in the lemon juice.

Figure S1 :

 S1 Figure S1: XRD diffraction patterns of sample 1 prepared with 0.5 mL of citrus lemon juice

Figure S2 :Figure S4 :

 S2S4 Figure S2: SAED patterns of pure NiCo 2 O 4 and lemon assisted NiCo 2 O 4 nanostructures

Figure S7 :

 S7 Figure S7: Different CV curves at variosu scan rates in 1.0M KOH for (a) pristine NiCo 2 O 4 nanostructure, (b) NiCo 2 O 4 (sample 2), (c) NiCo 2 O 4 (sample 3), (d) caluclated ECSA from non-Faradic region of estimated CV curevs for pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3).

Figure S8 :

 S8 Figure S8: Bar graph view (a) overpotential at 10 and 50 mAcm -2 for RuO 2 , pure NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3), (b) Tafel slope for RuO 2 , pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3), (c) Energy density of supercapacitor for pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2), NiCo 2 O 4 (sample 3), (d) specific capacitance (Cs) for pristine NiCo 2 O 4 nanostructure, NiCo 2 O 4 (sample 2) and (sample 2).

Table 2 :

 2 Overall performance comparison of the pristine and citrus lemon juice assisted NiCo 2 O 4 nanostructures (sample 1, sample 2).For this reason, we have noticed an excellent OER reaction at an overpotential of 250mV at 10 mAcm -2 with a durability of 40 hours under alkaline conditions. Furthermore, we have also studied the energy storage aspects of NiCo 2 O 4 material for the development of asymmetric supercapacitor and observed values of specific capacitance (C s ) of 1519.19 Fg -1 , and energy density of 33.08 Wh kg -1 are highly meritorious. The extended spectrum of electrochemical properties of NiCo 2 O 4 material, associated with our low cost and facile approach enabled it highly favorable for the development of high performance energy conversion and storage systems in near future.The calculations of Specific Capacitance C s and energy density was done according to the published work[1] using the following mathematical relations:

	Sample ID	Over	Tafel Slope	Charge Transfer	Specific	Energy	Power Density
		potential	(mVdec -1 )	Resistance (Rct)	Capacitance	Density	(W/kg)
		(mV)@10		(Ω)	(F/g) @ 0.8	(Wh/kg)	@ 0.8 A/g
		mAcm -2			A/g	@ 0.8 A/g
	Pure	300	112	968	117	2.47	156
	(NiCo 2 O 4 )
	Sample 1	280	107	-	-	-	-
	Sample 2	250	98	165	358	7.95	160
	Sample 3	280	105	547	163	3.57	159
	4. Conclusions
	In summary, we have used natural resources of chemical compounds with stabilizing agent,
	capping agent and chelating properties from citrus lemon juice for the preparation of 0D
	nanoparticles of NiCo 2 O 4 using hydrothermal. The structure, chemical composition and
	crystalline studies were carried out by wide range of analytical techniques. We have observed
	that 1D nanorod morphology of NiCo 2 O 4 was successfully transformed into 0D nanoparticles
	due to change in pH of growth solution, capping agent, stabilizing agent and chelating agent
	properties of citric acid, malic acid and ascorbic acid during growth process. At the same time,

these natural products have tailored surface properties, enhanced electron communication, fast ionic diffusion of electrolyte, swift charge transfer at the interface of electrode and electrolyte. d represents the power density; E is the energy and discharge time.

Table S1 :Table S2 :

 S1S2 Calculated average crystallite size of pure NiCo 2 O 4 and sample 1, sample 2 and sample 3.

	Sample ID	Height	FWHM	Crystalline size (nm)
	Pure NiCo 2 O 4	44.61	0.26	14
	Sample 1	36.84	0.28	16
	Sample 2	45.03	0.25	14
	Sample 3	44.06	0.12	18

Table S3 :

 S3 The calculated various supercapacitor indicators pure NiCo 2 O 4 nanostructure (sample 2), NiCo 2 O 4 (sample 3)

		Current	Specific	Energy	Power	Columbic	Capacitance
	Samples	Density	Capacitance	Density	Density	Efficiency	Retention
		(Ag -1 )	(Fg -1 )	(Wh kg -1 )	(W kg -1 )	(%)	(%)
		0.8	358	8	160
	Sample 2	0.85	284	6	167	70%	100-94%
		0.882	126	3	176
		0.94	118	3	188
		0.8	163	4	159
		0.85	154	3	169
	Sample 3	0.882	149	3	175	64%	108-83%
		0.94	60	1	183
		0.8	117	2	156
		0.85	113	2	166
	Pure (NiCo 2 O 4 )	0.882	112	2	171	43%	90-61%
		0.94	109	2	183

Table S4 :

 S4 Supercapacitor performance evaluation of as synthesized NiCo 2 O 4 nanostructure with lemon juice (sample 2) with reported supercapacitor based on NiCo 2 O 4

	Material	Specific Capacitance (Fg -1 )	Current Density (Fg -1 )	Potential Window	Energy (Wh kg -1 ) Density	Power (W kg -1 ) Density	Reference
	NCO@MWCNT	374 F/g	2 A/g	-0.5 2.2V	to	95	3964	4
	MWCNTs	84 F/g	0.6 A/g	-0.5 2.2V	to	21	6237	4
	NCO//MWCNT	157 F/g	0.6 A/g	-0.5 2.2V	to	40	2816	4
	NCO@MWCNT//MWCNT	242 F/g	0.6 A/g	-0.5 2.2V	to	61	2837	4
	NiCoF	50.0	1 A/g	0 to 1 V	-	-	5
	NiCuF	44	1 A/g	0 to 1 V	-	-	5
	NC6	1294.25	10 A/g	0.4		-	-	6
	NC10	687.20	10 A/g	0.4		-	-	6
	Ni-Co-O-1	568	20 A g	0 to 0.5 V	19.72	5000	7
	Ni-Co-O-2	532	20 Ag	-		-	-	7

NiCo 2 O 4 (Lemon juice) 358 0.8 A/g 0 to 0.4 V 7.96 160 This Work

Acknowledgments

The authors extend their sincere appreciations to Researchers Supporting Project number (RSP2023R79), King Saud University, Riyadh, Saudi Arabia, for partial funding of this work. B.V. and E.M would like to also thank the platform "Microscopies, Microprobes and Metallography (3M)" at the Institut Jean Lamour (IJL, Nancy, France) for TEM and SEM facilities. A.V. acknowledges the Kempe Foundation and the Knut och Alice Wallenberg Foundation for financial support.

Author contribution

Shusheel Kumar