Facile preparation and characterization of metal phosphate for supercapacitor

In this study, nickel cobalt phosphate ((Ni,Co)(PO4)3) has been developed as a positive electrode in supercapacitors. (Ni,Co)(PO4)3 is synthesized through a phosphorization and carbonization method using NiCo glycerate as a precursor combined with triethyl phosphate (TEP), subsequently an annealing process at 600°C under air conditions. The choice of solvent like hexanol has a significant influences on the morphology of nickel cobalt phosphate ((Ni,Co)(PO4)3), leading to the formation of cracker-like structures. Additionally, the resulting product exhibits an amorphous phase, indicating the absence of a well-defined crystalline arrangement. The electrochemical performance evaluation shows the peak from oxidation and reduction reactions at scan rate 5 mVs−1 until 100 mVs−1. Following that the specific capacitance reaches 743 Fg−1 at current density 1 Ag−1.


Introduction
The advancements in energy storage in devices remain a global focus to meet the demand for efficient energy storage and environmentally friendly.These devices include fuel cells, batteries, as well as supercapacitors, among others.The differences among them are distinguished through factors such as energy density, power density, conductivity, efficiency, current density, and voltage window, which are illustrated in the Ragone plot [1,2].The aim of developing energy storage in these devices is to overcome the ongoing rise in fuel prices, pollution impacts, and global warming.As an alternative, supercapacitors are chosen due to their ability to provide high power density of up to 10 2 Wkg -1 , rapid charge-discharge cycles, operational temperature flexibility, safety, and long-term stability.Supercapacitors consist of various components including a conductive substrate, separator, electrolyte, and electrodes.Recently, there has been a significant focus on the development of electrode materials, as they effectively improve the performance of supercapacitors.Typically, these materials come from metallic sulfides, oxides, and phosphates [3][4][5][6].
Bimetallic phosphate is a substance composed of two distinct transition metal elements that is thought to perform better than phosphate containing only one transition metal element in supercapacitor performance.This merit is attributed to its large surface area, wide channels and cavities, as well as high electrical conductivity [7].This material is naturally abundant, economically viable, and environmentally friendly [8].For instance, non-spherical hollow cobalt-nickel phosphate nanocages prepared by Xiao and co-woerkers revealed a specific capacitance of 1616 Fg -1 at current density of 1 Ag -1 [9].Cao et al. also found that low-temperature produced NiCo phosphate had a specific capacitance of 1592 Fg -1 at current density of 1 Ag -1 [10].These findings are related to the ability for fast charging and discharging, a characteristic highly desirable in supercapacitor development.On the other hand, shape and size morphology also influence ion transfer related to active sites.Research by Chen and co-workers on ZnO nanoparticles synthesized using a mixture of hexanol and water as solvents showed an enhancement in the nucleation and growth rate of ZnO nanoparticles [11].Besides its role in nanoparticle nucleation and growth, hexanol also possesses the ability to reduce graphene oxide during the growth of CoFe 2 O 4 -rGO hybrids [12].Findings by Schmutzler and their team indicated that the formation of ellipsoidal micelles in Cetyltrimethylammonium bromide (CTAB) became more elongated and experienced increased solvation due to the addition of hexanol [13].
In this paper, we utilized hexanol as a solvent to fabricate NiCo phosphate using a simple solvothermal method.We found that hexanol has an effect on the shape and size morphology of the bimetallic nickel-cobalt structure.Our main objective was to synthesize NiCo phosphate with unique morphology by blending ethanol and hexanol as solvents.Through this approach, this research presents a straightforward and environmentally friendly method for preparing bimetallic phosphate with affordable costs as an efficient positive (anode) electrode.

Preparation of NiCo glycerate precursors
The precursor of NiCo glycerate was prepared using a solvothermal method based on a previous study [14].Dissolve 40 mL of propanol at a 1:1 rasio of nickel(II) nitrate hexahydrate and cobalt(II) nitrate hexahydrate with a total of 0.5 mmol.Subsequently, before the solution transferring it into Teflon-lined stainless-steel autoclave added 8 mL of glycerol.The mixtured solution was then heated to a temperature of 180°C for 16 h.The resulting precipitate was filtered through centrifugation, washed, and dried at a temperature of 60°C overnight.

Fabrication of NiCo-Hexa particles
In this step, dissolve a mixture solution of 20 mL ethanol and hexanol (in this case, with a ratio of ethanol/hexanol at 3:1) at 0.03 g of NiCo glycerate powder.The solution is subjected to sonication to ensure well-dispersion, following added 800 µL of triethyl phosphate (TEP).Before the mixture solution transferring into Teflon-lined stainless-steel autoclave and heated to a temperature of 180°C for 16 h, stirred for 2 h.After that the resulting precipitate is filtered using centrifugation, washed, and then dried at a temperature of 60°C overnight.Following this, the powder obtained from NiCo-TEP is subjected to calcination at 600°C for 2 h under air conditions.The calcined products is NiCo phosphated labelled as (Ni,Co)(PO 4 ) 3 .

Characterization and Electrochemical Performance
The (Ni,Co)(PO 4 ) 3 product was characterized using a Hitachi SU-8000 Scanning Electron Microscope (SEM) instrument, operated at 5 kV for analysis the morphology.The composition and phase analysis of the samples were conducted using the Bruker D8 Advanced X-Ray Diffraction Spectroscopy (XRD) with Cu Kα radiation (λ=1.54Å).Electrochemical performance was investigated applying a typical three-electrode system and the CHI 660E electrochemical workstation.A platinum wire was used as the counter electrode and a silver/silver chloride (Ag/AgCl) electrode was used as reference electrode.As for the working electrode, disverse of the (Ni,Co)(PO 4 ) 3 sample product in a mixture of 950 µL of water/2-propanol and 50 µL of Nafion solution, was utilized.A carbon cloth with dimensions of 1 cm x 1 cm served as the current collector, and an electrolyte was used 2 M KOH solution.

Result and Discussion
Figure 1a shows monodispersed NiCo glycerate particles with a uniform spherical shape, averaging 400-500nm in size, as previously observed [14,15].The spherical morphology is attributed to the influence of NiCo glycerate particles combined with triethyl phosphate (TEP), with additional hexanol used as a solvent to prepare NiCo-Hexa, as seen in Figure 1b.NiCo-Hexa undergoes a transformation from 1D particles to uniform spherical 2D nanoplates.Figure 1c represents NiCo phosphate resulting from the calcination at 600°C for 2 h under air of NiCo-Hexa, exhibiting a cracker-like structure with small pores.During NiCo phosphate synthesis, the hexanol solvent effectively controls the interaction between Ni 2+ and Co 2+ metal ions, so adjustable size and shape of NiCo phosphate [13,16].In contrast to the report by Septiani, et al.NiCo phosphate when employed with an ethanol solution exhibits nanoplates chain-like structure [17].
Based on XRD analysis as depicted in Figure 1d. the NiCo-Hexa calcination product reveals no crystalline peaks are detected, indicating its amorphous nature.Previous research, Septiani et al. revealed that NiCo phosphate product after calcination at 600℃ displayed a weak peak intensity at index (214) plane [17].Nevertheless, C. Li et al. demonstrate the peak intensity correlation with reference JCPDS 71 2336 shows the crystals in NiCo phosphate after calcination at 300℃ [3].This is to expected of influenced by the presence of hexanol solution during synthesis and significant mass loss when nickel cobalt is heated at temperature of approximately 400°C [17][18][19].The three-electrode setup was used to test the characteristics of the material itself for electrochemical performance.In electrolyte of 2 M KOH, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements have been conducted.Figure 2a illustrates the CV graph of the NiCo phosphate electrode, ranging from a scan rate of 5 mVs -1 -100 mVs -1 within a potential window between 0 and 0.5 V.The figure reveals that at a scan rate of 5 mVs -1 , anodic peaks appear around 0.2 V, while cathodic peaks are observed at 0.35 V.With increasing scan rate, both anodic and cathodic peaks transition into negative and positive potentials, respectively.This phenomenon is attributed to oxidation and reduction reactions, indicating pseudocapacitive faradaic behavior.The shifting is also influenced by elevated electric polarization and potential kinetic irreversibility on the electrode surface [3,7,18].
The galvanostatic charge-discharge (GCD) measurements of the NiCo phosphate electrode were conducted at a potential range of 0 until 0.45 V and current densities ranging from 1 -10 Ag -1 , as depicted in Figure 2b.At a current density of 1 Ag -1 , 0.25 V (charge step) and 0.45 V (discharge step), a chargedischarge cycle occurred.In connection with the reversible redox reaction occurring on the surface of the (Ni,Co)(PO 4 ) 3 , the charge-discharge displays a triangular shape.From the GCD graphs, the specific capacitance of the (Ni,Co)(PO 4 ) 3 electrode can be calculated based on the discharge time.The obtained calculations were 743, 667, 593, 543, 492, and 460 Fg -1 at current densities of 1, 2, 4, 6, 8, and 10 Ag -1 , respectively.It can be observed that the current density increases with decreasing specific capacitance.This phenomenon is attributed to the fact that at lower current densities, more OH -ions reach the active sites of electrode due to diffusion time controlled by OH -ions, which requires a relatively longer time [20,21].

Conclusion
In summary, nickel-cobalt phosphate (Ni,Co)(PO 4 ) 3 with a morphology crackers-like structure has been synthesized using hexanol as a solvent with two-step solvothermal method, exhibiting promising electrochemical performance.At a current density of 1 Ag -1 , the specific capacitance can reach 743 Fg - 1 , while at 10 Ag -1 , the specific capacitance can achieve 460 Fg -1 , both within a 2 M KOH electrolyte.These findings highlight the possibility of using active materials as positive (anode) electrodes in supercapacitors.

Acknowledgment
This work was supported by the Group and Research Center for Nanosciences and Nanotechnology, Institut Teknologi Bandung, and the Ministry of Education, Culture, Research and Technology Indonesia.

References [1]
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