Effect of various electrolytes on the electrochemical properties of Ni(OH)2 nanostructures

Accepted Manuscript Full Length Article Effect of various electrolytes on the electrochemical properties of Ni(OH)2 nanostructures Thongsuk Sichumsaeng, Narong Chanlek, Santi Maensiri PII: DOI: Reference:

S0169-4332(18)30294-0 https://doi.org/10.1016/j.apsusc.2018.01.276 APSUSC 38415

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

15 October 2017 20 January 2018 29 January 2018

Please cite this article as: T. Sichumsaeng, N. Chanlek, S. Maensiri, Effect of various electrolytes on the electrochemical properties of Ni(OH)2 nanostructures, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.01.276

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Effect of various electrolytes on the electrochemical properties of Ni(OH)2 nanostructures Thongsuk Sichumsaeng a *, Narong Chanlek b, Santi Maensiri a, c a

School of Physics, Institute of Science, Suranaree University of Technology, Nakhon

Ratchasima, 30000, Thailand b

c

Synchrotron Light Research Institute, Nakhon Ratchasima, 30000, Thailand SUT CoE on Advanced Functional Materials (SUT-AFM), Suranaree University of

Technology, Nakhon Ratchasima, 30000, Thailand

*Corresponding author: Tel: +66-9-02642196, email: [email protected]

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Graphical abstract

The obtained hexagonal Ni(OH)2 nanoplates from hydrothermal method show the best electrochemical performances in 1M NaOH electrolyte. The highest specific capacitance was calculated to be 447 F/g at the current density of 1 A/g in 1M NaOH electrolyte that was two times higher than that in other electrolytes and the capacitance retention retains about 100% after 1000 cycles.

Highlights 

Hexagonal Ni(OH)2 nanoplates were synthesized by a simple hydrothermal method.



The Ni(OH)2 electrode exhibits excellent performance in 1M NaOH electrolyte.



The capacitance retention of the Ni(OH)2 electrode is maintained at about 100% after 1000 cycles

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Abstract Hexagonal nanoplates of Ni(OH)2 with a thickness of  11 nm were synthesized by a simple hydrothermal method. The formation of Ni(OH)2 phase was confirmed by XRD technique. TEM images revealed a stack of hexagonal nanosheets with average nanoplates size of  62 nm. XPS results confirm the elemental compositions at the surface of Ni(OH)2 which are in good agreement with XRD results. As we know, the properties of electrolyte affect the performance of supercapacitors. Therefore, the electrochemical performance of Ni(OH) 2 electrodes were investigated in various aqueous electrolyte solutions including 1M NaOH, 2M KOH, and 1M KOH mixed with 0.5M Na2SO4. The electrochemical results show that the Ni(OH)2 electrode in 1M NaOH electrolyte reached the highest specific capacitance of 447 F/g at a current density of 1 A/g that was nearly two times higher than that in the 2M KOH electrolyte. This can be attributed to the smaller ionic radius of the Na+ ion and a higher intercalation/deintercalation rate of Na+ ions into the surface of the Ni(OH)2 electrode. In addition, a coulombic efficiency of 94% was found in 1M NaOH at a current density of 10 A/g. The capacitance retention of the Ni(OH)2 electrode in 1M NaOH was maintained at about 100% after 1000 cycles. The present work proves that the best performance of Ni(OH)2 nanoplates was performed in the 1M NaOH electrolyte.

Keywords: Hydrothermal synthesis; Hexagonal Ni(OH)2; Pseudocapacitors; Electrochemical properties

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1. Introduction Electrochemical capacitors (ECs) also known as supercapacitors or ultracapacitors have attracted increasing attention as one of the energy storage devices due to their high power density, long cycle life, and low maintenance cost [1,2] . Basically, ECs can be classified into two types based on the energy storage mechanisms that are electrical double layer capacitors (EDLCs) and

pseudocapacitors. EDLCs

store

energy

based

on

the

reversible

adsorption/desorption of ion at the interface between the electrode materials and the electrolytes [3,4]. On the other hand, pseudocapacitors or redox supercapacitors store energy from a fast and reversible surface or near surface reactions of the electrode materials [5]. At this point, it is worth noting that electrolytes are one of the most important components influencing the performance of ECs. Therefore, the selection of suitable electrolytes is crucial for the improvement in performance of ECs. Among various electrolytes, the aqueous electrolyte is known as an electrolyte with high conductivity which plays an important role in the performance ECs. Because it is inexpensive, non-toxic, and easy to handle in a laboratory, aqueous electrolytes have been extensively used in research and development compared with organic electrolytes [6-8] There are reports on the effects of aqueous electrolytes including the ion size and type and the ion concentration on the electrochemical performance of pseudocapacitive materials [9-11]. Nickel hydroxide (Ni(OH)2) is one of the pseudocapacitive materials which is interested in the electrochemical applications because of its low cost, easy processing, and its high theoretical specific capacitance value of 2082 F/g within the potential window of 0.5 V [12,13]. The electrochemical properties of Ni(OH)2 have been studied in different electrolytes. In 1M NaOH electrolyte, the ultra-thin Ni(OH)2 nanowalls showed the ultra-high specific capacitance value [14] while the Ni(OH)2 based composite electrodes possessed the highest specific capacitance value in 2M KOH electrolyte [15]. Also, the Ni(OH)2 based

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electrode materials reached the highest specific capacitance value in the mixed solution of 1M KOH + 0.5M Na2SO4 [16]. For nanostructured layered nickel hydroxide, the capacitance arises generally from the intercalation/de-intercalation mechanism, which is involved the moving of electrolyte ions into/out the electrode matrix and undergo redox reaction [17,18]. However, there are a few works that reported on the effects of the different electrolytes on the electrochemical properties of Ni(OH)2 [7,19,20]. In this work, a simple hydrothermal synthesis of hexagonal Ni(OH)2 nanoplates and the material characterizations are reported. In particular, the electrochemical properties of the Ni(OH)2 electrodes are studied and discussed in various alkaline electrolytes including 1M NaOH, 2M KOH, and 1M KOH + 0.5M Na2SO4 based on the previous reports. 2. Experiments 2.1 Materials Nickel (II) chloride hexahydrate (> 98%) was purchased from Sigma - Aldrich Co., Ltd. and sodium hydroxide (97%) was obtained from Ajax Finechem Co., Ltd. All the reagents were directly used without further purification. 2.2 Synthesis of hexagonal Ni(OH)2 nanoplates The hexagonal Ni(OH)2 nanoplates were synthesized by a simple hydrothermal method which followed Singh’s method [12]. In a typical process, 0.33M of NiCl2.6H2O was dissolved in 150 mL of DI water. Then, 2M of a precipitating agent NaOH solution was added to the above solution under vigorous stirring for 1 h. In order to remove Na+ and Clions and other impurities, the Ni(OH)2 suspension was washed with DI water several times until pH = 7. After washing, the Ni(OH)2 was directly added to 2.5M NaOH solution under vigorous stirring for 30 min and then the mixture was immediately transferred to a Teflonlined stainless - steel autoclave. The autoclave was sealed and heated at 160 oC for 20h. Finally, the resultant green precipitate of Ni(OH)2 was washed with DI water and ethanol

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several times until the pH became neutral and it was then dried in a vacuum oven at 70 oC overnight. 2.3 Materials characterizations The phase formation of the synthesized Ni(OH) 2 was characterized by X-ray diffraction (XRD) analysis on Bruker D2 using Cu K radiation ( = 0.15406 nm) at a scanning rate of 0.02o/min in the 2 range from 10 to 80o. The morphology and the corresponding electron diffraction patterns (SAED) were revealed by Transmission electron microscopy (TEM, FEI Tecnai G2) with an acceleration voltage of 200 kV. The chemical compositions and oxidation state of the prepared Ni(OH)2 samples were carried out by using X-ray photoelectron spectra (XPS) with PHI Versa Probe II XPS system (ULVAC-PHI) with an Al K radiation (1486.6 eV). A BEL SORP-mini II instrument was used to examine the Brunaure-Emmert-Teller (BET) specific surface area and the pore size distribution was determined by Barrett-Joyner-Halenda (BJH) method. The Ni(OH)2 sample was degassed at 120 oC for 3h before the measurements. In addition, the morphology of the Ni(OH) 2 electrodes before and after 1000 charge-discharge cycles measurement was examined using field emission scanning electron microscope (FE-SEM, AURIGA, Carl Zeiss) coupled with dispersive X-ray spectroscopy (EDS). 2.4 Electrochemical measurements The electrochemical properties of the Ni(OH)2 electrode were performed in a three electrodes system. A working electrode was the prepared Ni(OH)2 on Ni foam substrate. A reference electrode was Ag/AgCl saturated in 3M KCl and a platinum wire was used as a counter electrode. To prepare working electrode, Ni(OH)2 (active material), carbon black (conducting agent), and polyvinylidene difluoride (binder) in the mass ratio of 80 : 10 : 10 were mixed in 200 L of N - methylpyrrolidone (NMP) solution. The resulting slurry was pasted onto 1 x 2 cm2 of Ni foam substrate. The prepared working electrode was dried in a

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vacuum oven at 70 oC overnight and pressed at 5 MPa for 1 min with the obtained mass loading of Ni(OH)2 about 2 mg. To evaluate the electrochemical performance, the cells were connected to a Metrohm Autolab PGSTAT 302N potentiostat/galvanostat system. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were carried out. Three different alkaline electrolytes including 1M NaOH, 2M KOH, and 1M KOH mixed with 0.5M Na2SO4 were selected to study the effects of electrolytes on the electrochemical properties of Ni(OH)2 nanostructures. The prepared Ni(OH)2 electrode was soaked in the electrolytes before the electrochemical measurements were obtained. The specific capacitance (C), energy density (E), and power density (P) of the Ni(OH)2 electrode were calculated by using the equations below [17,21,22]: 𝐼𝑥 𝑡

𝐶𝐺𝐶𝐷

𝑚𝑥 𝑉

𝐸

𝐶𝐺𝐶𝐷 𝑉

𝑃

𝑥𝐸 𝑡

(1) (2) (3)

Where CGCD (F/g) is specific capacitance obtained from GCD technique. I (A) is the discharge current, m(g) is the mass of the active material, t (s) is the discharge time, V (V) is the operating voltage, E (Wh/kg) is the energy density, and P (W/kg) is the power density. Also, the coulombic efficiency () vs. the current densities was calculated by the following equation [23]:



𝑡𝑑 𝑡𝑐

Where td (s) is the discharge time and t c (s) is the charge time. 3. Results and discussion 3.1 Structural and morphology analysis

(4)

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The XRD pattern of the synthesized Ni(OH)2 nanostructures is presented in Fig. 1. All the diffraction peaks can be indexed and well matched with the standard diffraction pattern of hexagonal  - Ni(OH)2 (JCPDF file no. 14 - 0117). The observed diffraction peaks at 2 = 19.29, 33.04, 38.54, 52.15, 59.02, 62.64, 69.33, 70.45, and 72.69 correspond to the lattice planes of (001), (100), (101), (102), (110), (111), (200), (103), and (201), respectively. In addition, the sharp of diffraction peak indicates high crystallinity of Ni(OH)2 nanostructures [24]. Note that no extra peaks were observed, confirming the high purity phase of Ni(OH)2. Fig. 2 (a - d) shows the TEM bright field images of the synthesized Ni(OH)2 nanostructures. As shown in Fig. 2 (a - d), TEM images reveal the hexagonal shape of Ni(OH)2 with an average plate size of about 62 nm. Furthermore, some of the Ni(OH)2 nanoplates were stacked together into a columnar shape with an average thickness of about 11 nm. It can be clearly seen from the high magnification TEM image (Fig. 2 (c)) that an individual hexagonal is thin and the measured interfacial angle is 120. More importantly, a porous structure was also observed on the surface of the Ni(OH)2 nanoplate as can be clearly seen in Fig. 2 (c) which suggests that the generation of pores within the nanoplates could be attributed to the loss of water molecules under hydrothermal conditions and then followed by the formation of nickel-oxygen bond at the interface of the aggregated nanoparticles [12]. These pores are very useful for charge storage applications as they can decrease the ionic diffusion length and also the electrolyte resistance [12]. Fig. 2 (e) shows a spotty ring pattern of the selected area electron diffraction (SAED) image that confirms the polycrystalline Ni(OH)2 and the lattice planes of the Ni(OH)2 phase are in good agreement with the XRD results. In order to further investigate the type of pores observed in the TEM images, the BET technique was used. Fig.3 (a) shows the N2 adsorption/desorption isotherms of the hexagonal Ni(OH)2 nanoplates. According to IUPAC classifications, the N2 isotherm curves can be

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classified as type IV. At the relative pressures ranging from 0.4 to 0.9, the presence of the hysteresis loop was observed, which is characteristic of mesoporous materials [25]. The calculated BET specific surface area from the N2 isotherm was found to be 52.80 m2/g, which is higher than that of a previous study [12]. The higher surface area of Ni(OH)2 nanoplates suggested that to provide a larger contact area with the electrolyte, which is highly beneficial for the delivery of high capacitance for pseudocapacitors [26]. The calculated pore size distribution from the adsorption branch of the nitrogen isotherm by using the Barrett - Joyner - Halenda (BJH) method shows that the mean pore diameter of Ni(OH)2 is centered at 24.49 nm with the mesopore volume of 0.3903 cm3/g as shown in Fig. 3 (b). Therefore, the obtained mean pore diameter indicates the existence of mesopores on the Ni(OH)2 nanoplates. 3.2 Surface analysis The surface characteristics and chemical compositions of Ni(OH)2 nanoplates were investigated using XPS technique. As shown in Fig. 4 (a), a survey spectrum of Ni(OH)2 confirms the presence of C 1s, O 1s, and Ni 2p on the sample surface. The high resolution XPS in Ni 2p and O 1s region are shown in Fig. 4 (b - c). Peak fitting of Ni 2p region shows two main peaks at the binding energy of 855.2 eV (Ni 2p1/2) and 872.8 eV (Ni 2p3/2) with the separation of spin energy around 17.6 eV, indicating the characteristics of Ni(OH)2 phase [27-29]. The O 1s peak was observed at the binding energy of 531.0 eV corresponding to NiOH [30]. This result clearly indicates the formation of Ni(OH) 2. 3.3 Electrochemical properties The selection of suitable electrolytes is very important for electrochemical performance. So, the selected alkaline electrolytes including 1M NaOH, 2M KOH and 1M KOH mixed with 0.5M Na2SO4 were evaluated in a three electrodes system. Fig. 5 (a - e) shows CV curves of the Ni(OH)2 electrodes measured in different aqueous electrolytes. Note

10

that the Ni(OH)2 electrode was also performed in 1M KOH and 0.5M Na2SO4 electrolytes as contrast aqueous electrolytes. The observed redox peaks are presented in all aqueous electrolytes. Except in 0.5M Na2SO4 electrolyte, the CV curves exhibit a quasi-rectangular shape, indicating an ideal capacitive behavior [31]. It was also observed that the redox peaks shift to higher and lower potential with increasing scan rate (mV/s). The shift of redox peaks with increasing scan rate ranging from 2 to 20 mV/s was used to identify the charge storage mechanism of the Ni(OH)2 electrodes in 1M NaOH, 2M KOH, 1M KOH + 0.5M Na2SO4 and 1M KOH electrolytes. The scan rate dependence of CV peak current can be expressed as where i is the CV peak current (A/g), a and b are the adjustable parameters, and v is the scan rate (V/s). The value of b was determined from the slope of the plot of log (i) versus log (v). There are two meanings of b-value. The value of b is 1, indicating the obtained CV current is due to a capacitive mechanism. While the value of b is 0.5, the obtained CV current is due to the intercalation/de-intercalation mechanism [32-34]. As shown in Fig. 6 (a - d), the slopes or the b-values are close to 0.5. Hence, it may be concluded that the Ni(OH) 2 electrodes in 1M NaOH, 2M KOH, 1M KOH + 0.5M Na2SO4 and 1M KOH electrolytes store the charge based on intercalation/de-intercalation mechanism. Fig. 7 (a) shows a comparison of CV curves in different alkaline electrolytes at a scan rate of 2 mV/s. It can be clearly seen that all CV curves show the oxidation and reduction peaks during both the charging and discharging process, indicating a faradic reaction [35]. The redox peaks of Ni(OH)2 electrodes correspond to the reversible redox reaction of Ni2+  Ni3+ + e- [36]. Based on the intercalation/de-intercalation process, the possible charge storage mechanism, of Ni(OH)2 electrode in 1M NaOH aqueous electrolyte is proposed as [17,18,37]

 It can be explained by during the charging process, the Na cation of the electrolyte intercalates into Ni(OH)2 matrix and release one electron. Whereas the discharging process,

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the Na cation of the electrolyte de-intercalate from Ni(OH)2 matrix and diffuse into the electrolyte solution [18]. Importantly, the larger CV curve was observed in 1M NaOH electrolyte, suggesting that it offers a higher capacitance value. It was also observed that when 0.5M Na2SO4 is added to 1M KOH solution, there is an increase in a potential window which leads to an increase in the energy density according to eq. (2). With increasing the concentration from 1M KOH to 2M KOH aqueous electrolyte, the CV curves shift to lower potentials and the area under CV curve is larger because of the increasing in the ionic conductivity in 2M KOH electrolyte. Fig. 7 (b) shows a comparison of discharge curves of the Ni(OH)2 electrodes at a current density of 1 A/g. Non-linear curves are observed, implying that the electrodes store the charge base on redox or intercalation mechanism [38]. Similar to the CV results, a longer time for discharging was observed in the 1M NaOH electrolyte. The calculated specific capacitance for Ni(OH)2 in various electrolytes is presented in Fig.8. As a result, the Ni(OH)2 electrode measured in 1M NaOH electrolyte shows the highest specific capacitance value of 447.03 F/g at a current density of 1 A/g that is nearly two times higher than in other electrolytes. The calculated specific capacitance of various electrolytes is summarized in Table 1. Compared with the electrolyte of 1M NaOH and 1M KOH, the hydrated sphere, the ionic conductivity, and the ionic mobility [39–41] of the aquo-cations have no significant influence on the specific capacitance value of the Ni(OH) 2 electrodes, which higher specific capacitance value is expected in the smaller hydrated sphere with higher ionic conductivity and mobility of K+ aquo-ion. In contrast, the free ionic radius (Na+ < K+) plays an important role on the specific capacitance value of the Ni(OH) 2 electrodes. This can be explained by the relatively easy intercalation/deintercalation of the smaller free ionic radius of Na+ ion (0.95 A) compared with the K+ ion (1.33 A) [6]. Another reason is due to the lowest value of IR drop in 1M NaOH electrolyte as shown in the inset of Fig. 8, which is related to the contact

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resistance between current collector and electrode material, solution resistance and charge transfer resistance [31]. It is also observed that with increasing the current density, the specific capacitance value decreases. This can be explained by the electrolyte ions have enough time to diffuse into the active sites of the electrode material at the lower current density, providing the higher capacitance value while the diffusion time is limited at the higher current density [42]. In a contrary, the lowest specific capacitance value was found in the mixed solution, which is in good agreement with the coulombic efficiency result as shown in Fig. 9. Generally, the coulombic efficiency is higher than 95%, indicating an ideal capacitive behavior [43], [44]. Based on these results, it was confirmed that the highest capacitance value was found in the 1M NaOH electrolyte due to the highest coulombic efficiency of about 94% at a current density of 10A/g while the mixed solution shows the lowest coulombic efficiency of about 60% at the same current density. Not only the free ionic radius of cation electrolyte affects the performance of Ni(OH)2 electrodes, but also the free ionic radius of anions. Therefore, the lowest capacitance values found in the mixed solution may be explained by the larger anions of SO42- with a size of (2.90 A) compared with the size of OH - ions (1.76 A) [6] that slow the ions transportation. To further understand the electrochemical performance of the Ni(OH)2 electrode in different electrolytes, EIS was carried out in the frequency range of 0.1 Hz to 100 kHz at an amplitude of 0.1 V. In general, EIS data are presented as a Nyquist plot or the plot between the imaginary part (-Z) against the real part (Z). As shown in Fig. 10 (a), a Nyquist plot can be divided into three regions. In the high frequency region, the intercept of the real axis indicates the bulk resistance (Rs), which is a combination of pore electrolyte resistance, bulk resistance, and a contact resistance between the current collector and electroactive material [45]. The Rs values were estimated to be 1.80, 1.38, 2.42, and 1.48 Ohm in 1M NaOH, 2M KOH, 1M KOH + 0.5M Na2SO4, and 1M KOH, respectively. Moreover, the semicircle

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curves were observed at the middle frequency. The diameter of the semicircle corresponds to the charge transfer resistance (Rct) caused by Faradic reactions [46]. It can be seen that the smallest diameter of the semicircle was found in 2M KOH electrolyte, indicating that it offers a faster charge transfer, ensuring a high capacitive performance [47]. In the low frequency region, nearly straight lines with an angle along the real axis of about 65 were observed. The slope of the curve shows the Warburg impedance, which represents the electrolytes diffusion in the host materials that is observed in all electrolytes [48]. Fig. 10 (b) shows a bode plots of Ni(OH)2 electrodes in different electrolytes. A bode plot is used to determine the maximum phase angle and frequency response time [49]. It is observed that the Ni(OH)2 electrode measured in 1M NaOH shows the maximum phase angle of about -78 degree, which is close to the ideal capacitive behavior (-90 degree). According to the equation o = 1/fo where o is relaxation time and fo is frequency [50], the higher the frequency the faster of ion response time [51]. The Bode plots also presented peaks in a high frequency region in only 2M KOH and 1M KOH mixed with 0.5M Na2SO4 electrolytes, demonstrating a pseudo charge transfer resistance in the system [52]. To the best of our knowledge, the presence of a charge transfer resistance in 2M KOH and 1M KOH mixed with 0.5M Na2SO4 electrolytes is attributed to a decrease in the electrochemical performance. The long-term stability of the Ni(OH)2 electrode in different electrolytes as shown in Fig. 11 was investigated by repeating the GCD test at a current density of 5 A/g for 1000 cycles. Interestingly, the Ni(OH)2 electrode performance in the 1M NaOH electrolyte shows the greatest cycle stability with increasing capacitance from the initial capacitance and retains about 100% after 1000 cycles. Turning to the synthesis of Ni(OH)2 nanostructures process, the NaOH solution plays an important role as a precipitating agent that is associated with the creation of mesopores on the surface of the hexagonal Ni(OH)2 nanostructures as clearly seen in the TEM image. Moreover, NaOH is frequently used in a chemical activation process in

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order to provide a very high surface area of carbon materials [53,54]. Therefore, the increase in the capacitance retention in 1M NaOH electrolyte upon charging and discharging for 1000 cycles may be come from to the enhancement or development of the pores on the Ni(OH)2 surface that were created in the 1M NaOH electrolyte. Another reason is due to the increasing active species exposed to the electrolyte upon repetitive charge/discharge cycling [55]. However, both the 2M KOH and 1M KOH + 0.5M Na2SO4 electrolytes exhibit poor cyclic stability with loss of about 25% after 1000 cycles. This might be induced by the adhesion loss between the electro - active material and the current collector [56] or due to the phase transformations and/or the dissolution of active materials [57]. To further understand the cyclic stability of the Ni(OH)2 electrodes in different aqueous electrolytes, the morphology of Ni(OH) 2 electrodes with corresponding EDS result before and after 1000 charge-discharge cycles in 1M NaOH, 1M KOH, and 1M KOH + 0.5M Na2SO4 electrolytes were carried out. According to the FE-SEM images, the closely packed of hexagonal Ni(OH)2 nanoplates with providing a porosity on the surface before the electrochemical test was observed (Fig. 12 (a)), which the element compositions of electrode materials including active material Ni(OH) 2, polymer binder (PVDF), and carbon black were confirmed as shown in the EDS results (Fig. 12 (a) right). After 1000 GCD cycles test, the FE-SEM image of 1M KOH shows the cracks on the surface of the Ni(OH) 2 electrode without the detected of the polymer binder (F) element (Fig. 12 (b) right). Besides that, the decrease of the interconnected pore size between the agglomerated nanoplates was observed in the mixed electrolyte (Fig. 12 (c)). Therefore, the decrease of the cyclic stability in 2M KOH, 1M KOH + 0.5M Na2SO4, and 1M KOH electrolyte may be explained from the above results. More importantly, the structural instability from repeating intercalation/deintercalation of Na+ ion into the Ni(OH)2 electrode causes a seriously agglomerated of the

15

Ni(OH)2 nanoplates as clearly seen in the FE-SEM image of 1M NaOH electrolyte after 1000 GCD cycles measurement (Fig. 13 (a)). Furthermore, the pores producing by PVDF with the presence of Ni(OH)2 nanoplates inside the pore were observed (Fig. 13 (b)) and some of the agglomerated nanoplates were covered with thin polymer layer of PVDF ((Fig. 13 (c)). The thin polymer layer found here is very important for the long-term cyclic stability for the Ni(OH)2 electrode that it acted as a glue to connect the electrode materials, including Ni(OH)2 nanoplates and carbon black together, resulting in an adhesion between electrode materials and current collector. The forming of polymer layer was reported to limit the mobility of ions [58]. However, due to the higher content of active material Ni(OH)2 (80 wt. %) there is insufficient PVDF to form a thin layer, which wraps around both Ni(OH)2 nanoplates and carbon black as evidence in the lack of the detected F element in EDS result of Fig. 2 (e). Therefore, the carbon black tends to form as the ionic conducting network [59] that promotes the intercalation/de-intercalation of Na+ ions into the active sites of the electrode material. From the above results, we can conclude that the presence of polymer layer on the surface of the Ni(OH)2 electrode is very important for the long-term cyclic stability. Therefore, the decrease of the cyclic stability in 1M KOH, 2M KOH, and 1M KOH + 0.5M Na2SO4 aqueous electrolytes may be concluded by the absence of polymer layer or the adhesion loss between electrode materials and current collector after the measurement of GCD for 1000 cycles. Moreover, the electrochemical performance of Ni(OH)2 electrode was also determined by the well - known Ragone plot or the plot between energy density and power density. Fig. 14 shows the Ragone plots of Ni(OH)2 electrodes in different electrolytes. The energy density and power density of Ni(OH)2 electrodes were calculated by using eq. 2 and 3, respectively. The maximum energy density was found to be 16.28 Wh/kg at a power density

16

of 256 W/kg in 1M NaOH electrolyte, suggesting that the Ni(OH)2 electrodes are promising candidates for supercapacitor applications.

4. Conclusion We have successfully synthesized hexagonal Ni(OH)2 nanoplates by using a simple hydrothermal method. The obtained mesoporous Ni(OH)2 sample delivers the best electrochemical performance in 1M NaOH electrolyte including high specific capacitance, superior cyclic stability, and high energy density. The possible reasons are summarized as follow: (i) the smaller ionic radius of Na + ion that is easily intercalated/deintercalated into or out the electrode. (ii) the lower the charge transfer resistance the faster the ions transportation. (iii) the increasing or developing of the pores during the charging and discharging process. Therefore, it can be concluded that Ni(OH)2 electrodes are suitable for energy storage applications.

Acknowledgments We gratefully acknowledge all our contributors, including Suranaree University of Technology and SUT-NANOTEC-SLRI joint research facility, Synchrotron Light Research Institute (BL 5.3) (Public Organization), Nakhon Ratchasima, Thailand for XPS facilities. T. Sichumsaeng would like to thank the Science Achievement Scholarship of Thailand (SAST) for the support of her PhD study. This research is financially supported by the SUT CoE on Advanced Functional Materials (SUT-AFM), Suranaree University of Technology.

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Figure and table captions Fig. 1 XRD pattern of the synthesized Ni(OH)2 nanostructures. Fig. 2 (a - d) TEM bright field images, (e) the corresponding SAED diffraction pattern, and (f) the estimated nanoplates size of the synthesized Ni(OH)2 nanostructures. Fig.3 (a) N2 adsorption/desorption isotherm and (b) the pore size distribution of Ni(OH)2 calculated from BET and BJH methods, respectively. Fig. 4 (a) the survey XPS spectrum and the high resolution XPS spectrum of (b) O1s region and (c) Ni 2p region of the Ni(OH)2 nanoplates. Fig. 5 (a - e) CV curves of the Ni(OH)2 electrodes measured in different aqueous electrolytes and scan rate (mV/s). Fig. 6 (a - d) The plot between log (i) vs. log (v) of the Ni(OH)2 electrodes in 1M NaOH, 2M KOH, 1M KOH + 0.5M Na2SO4, and 1M KOH, respectively. Fig. 7 (a) Comparison of CV curves of Ni(OH)2 electrodes measured at scan rate of 2 mV/s and (b) comparison of discharging curves of Ni(OH)2 electrodes measured at current density of 2 A/g in various aqueous electrolytes. Fig. 8 (a) The calculated specific capacitance of Ni(OH)2 electrodes obtained from equ. 1. Fig. 9 The calculated coulombic efficiency of Ni(OH)2 electrodes obtained from equation 4. Fig. 10 (a) Nyquist plots and (b) Bode plots of Ni(OH)2 electrodes measured at the frequency range of 0.1 Hz to 100 kHz. Fig. 11 Comparison of capacitance retention of Ni(OH)2 electrodes measured in different electrolytes at 5 A/g for 1000 cycles. Fig. 12 (left) FE-SEM images of the Ni(OH)2 electrodes (a) before the electrochemical measurements and after 1000 GCD cycles test at current density of 5 A/g in (b) 1M KOH and

25

(c) 1M KOH + 0.5M Na2SO4 electrolytes. (right) The corresponding EDS result of the Ni(OH)2 electrodes before and after the electrochemical test. Fig. 13 (a - c) FE-SEM images with different magnifications of the Ni(OH) 2 electrodes after 1000 GCD cycles test at current density of 5 A/g in 1M NaOH electrolytes and (d – e) the corresponding EDS result of the Ni(OH)2 electrodes in different detected areas. Fig. 14 Ragone plots of Ni(OH)2 electrodes compared in various electrolytes.

26

Fig. 1

27

Fig. 2

28

Fig. 3

29

Fig. 4

30

Fig. 5

31

Fig. 6

32

Fig. 7

33

Fig. 8

34

Fig. 9

35

Fig. 10

36

Fig. 11

37

Fig. 12

38

Fig. 13

39

Fig. 14

40

Table 1

Current density (A/g)

Specific capacitance (F/g) 1M NaOH

2M KOH

1M KOH + 0.5M Na2SO4

1M KOH

1

447.03

246.59

244.33

373.99

2

380.50

212.12

203.29

238.75

3

189.18 171.45

175.75 146.13

182.55

4

347.60 326.19

5

309.73

156.69

120.15

115.34

10

231.07

88.94

10.15

14.80

145.13

41

Table caption Table 1 The summarized data of the calculated specific capacitance of Ni(OH)2 electrodes in various electrolytes.

Current density (A/g)

Specific capacitance (F/g) 1M NaOH

2M KOH

1M KOH + 0.5M Na2SO4

1M KOH

1

447.03

246.59

244.33

373.99

2

380.50

212.12

203.29

238.75

3 4

347.60 326.19

189.18 171.45

175.75 146.13

182.55 145.13

5

309.73

156.69

120.15

115.34

10

231.07

88.94

10.15

14.80

42

Graphical abstract

The obtained hexagonal Ni(OH)2 nanoplates from hydrothermal method show the best electrochemical performances in 1M NaOH electrolyte. The highest specific capacitance was calculated to be 447 F/g at the current density of 1 A/g in 1M NaOH electrolyte that was two times higher than that in other electrolytes and the capacitance retention retains about 100% after 1000 cycles.

43

Highlights 

Hexagonal Ni(OH)2 nanoplates were synthesized by a simple hydrothermal method.



The Ni(OH)2 electrode exhibits excellent performance in 1M NaOH electrolyte.



The capacitance retention of the Ni(OH)2 electrode is maintained at about 100% after 1000 cycles