b Key Lab of Mesoscopic Chemistry MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
Electrochemical capacitors (EC) [1, 2, 3], also named supercapacitor, have attracted increasing attentions because of their higher power density and longer cycling life than secondary batteries. To develop an advanced EC device, an active electrode material with high capacity and rate capability performances is indispensable. Among various electrode materials, Ni(OH)2 has been considered as a potential candidate [4, 5, 6] for EC due to its high theoretic capacity, excellent redox behavior, ease of synthesis, abundant sources, low cost, environmentally friendly and etc. To date, various approaches [7, 8, 9, 10, 11, 12] have been explored to fabricate nanostructured nickel hydroxide, however, the actual rate capability reported for most nanostructures remains unsatisfactory, which mainly caused by the poor electrode conductivity. As a result, in recent years, extensive research has been carried out to improve the electrode conductivity through chemical doping or morphology control of active materials on the nanoscale [13, 14]. It is reported that the electrode conductivity can be improved by chemical doping with other cations such as cobalt (Co) [15, 16], aluminum (Al) , and zinc (Zn) .
Herein, a simple cation-anion (Ni2+ and AlO2-) double hydrolysis, i.e. Ni2+ (aq.) + 2AlO2- (aq.) + 4H2O→Ni(OH)2 (s) + 2Al(OH)3 (s) method was developed for the synthesis of nickel-aluminum (Ni-Al) composite hydroxides. Unlike previous synthesis methods of Ni(OH)2-based materials, it needs neither any adscititious alkali [19, 20] sources nor heat treatment (hydrothermal or microwave irradiation) for producing OH- ions due to the described double hydrolysis is a spontaneous reaction. The obtained composite hydroxides possesses a porous structure, large surface area (121 m2/g), and a homogeneous element distribution. Moreover, the final composite nickel hydroxide exhibits a superior supercapacitive performances (specific capacitance of 1670 F/g and rate capability of 87% from 0.5 A/g to 20 A/g) to doping-free nickel hydroxide (specific capacitance of 1227 F/g and rate capability of 47% from 0.5 A/g to 20 A/g).2. Experimental
Synthesis: Typically, 2.91 g (0.01 mol) Ni(NO3)2·6H2O were dissolved in 500 mL distilled water to obtain the precursor solution. Then, 1.63 g NaAlO2 (0.02 mol) was added to the precursor solution under magnetic stirring for 24 h to undergo a cation-anion double hydrolysis reaction. After double hydrolysis reaction, the obtained precipitate was filtered, washed and dried. The final obtained product was denoted as D-Ni(OH)2 (D: doping). As comparison, a doping-free Ni(OH)2 was prepared by a reported solvothermal method with anhydrous ethanol as solvent, nickel nitrate as precursor. Typically, 0.145 g nickel nitrate was firstly dissolved in 50 mLanhydrous ethanol and then transferred to an autoclave to undergo a solvothermal treatment at 120 ℃ for 24 h. After solvothermal treatment, the obtained precipitate was filtered, washed and dried. The final obtained product by solvothermal method was denoted as DF-Ni(OH)2 (DF: dopingfree).
Characterization: X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert X-ray diffractometer with a Cu Ka radiation (40 kV, 40 mA). Transmission electron micrograph (TEM) and element mapping images were obtained with a JEOL 2100 microscope operated at 200 kV. N2 sorption isotherms were measured using a Micromeritics ASAP2020 analyzer at -196 ℃. Before measurements, all samples were degassed at 200 ℃ for 6 h. The specific surface area is calculated by Brunauer-Emmett-Teller (BET) theory and the pore size distribution is determined by adsorption branch via Barrett-Joyner-Halenda (BJH) method. Xray photoelectron spectrum (XPS) were recorded on a Thermo ESCALAB 250 by using Al Ka radiation (15 kV, 150 W). All binding energies were referenced to the C1s peak at 284.6 eV. The composition of the product is examined by inductively coupled plasma atomic emission spectroscopy (ICP) on Optima 5300DV.
Electrochemical test: Electrochemical measurements were carried out in a 6.0 mol/L KOH aqueous electrolyte at room temperature, using a three-electrode cell with an Hg/HgO reference electrode and a platinum coil counter electrode. The testing electrode was prepared by mixing the obtained product powder, carbon black and polytetrafluorethylene (PTFE) together at a mass ratio of 7:2:1, and dipping the resulting mixture into nickel foam (1 cm × 2 cm, current collector) before being pressed together at 10.0 MPa. The electrochemical performances of samples were determined by cyclic voltammetry (CV) and galvanostatic charge/discharge curves. The mass specific capacitance was calculated by the discharge curve according to the formula of C = (IΔt/mΔV)(F/g), where I is the current density (A), Δt is the discharge time (s), m is the weight of active material, ΔV is the potential window of discharging (V).3. Results and discussion
The structure and the composition of the obtained nickel hydroxides were characterized by X-ray diffraction (XRD) and Xray photoelectron spectroscopy (XPS), respectively. For DFNi( OH)2, the XRD pattern (Fig. 1a—red) shows six well-defined diffraction peaks that can be indexed to (0 0 3), (0 0 6), (1 0 1), (0 1 5), (1 0 2) and (1 1 0) diffractions of Ni(OH)2 (JCPDS: 00-001- 1047). As to D-Ni(OH)2, the diffraction pattern (Fig. 1a—black) shows seven diffraction peaks that can be indexed to the mixed phase of Al(OH)3 and Ni(OH)2, indicative of the feasibility of the presented double hydrolysis method toward the fabrication of Ni- Al composite hydroxides. The XPS survey spectrum (Fig. 1b) reveals that the obtained D-Ni(OH)2 is composed of three elements of Ni, Al and O. The high-resolution XPS spectra of Al2p (Fig. 1c) show that the elements of Alin D-Ni(OH)2 exist in the form of Al(OH)3. The high-resolution XPS spectra of Ni2p of the two hydroxides (Fig. 1d) both show four our peaks around 855, 860, 873 and 878 eV that can be indexed to Ni2p3/2, Ni2p3/2 satellite, Ni2p1/2 and Ni2p1/2 satellite signals of Ni(OH)2, which further confirms the successful preparation of Ni-Al composite hydroxides by the presented double hydrolysis method. The actual elemental composition was confirmed by ICP and the measured molar ration of Ni/Al in the final composite hydroxides is 1:1.86.
|Fig. 1.X-ray diffraction patterns (XRD) and X-ray photoelectron spectroscopy (XPS) analyses. (a) XRD patterns of D-Ni(OH)2 and DF-Ni(OH)2; (b) XPS survey spectrum of DNi(OH)2; (c) Al2p XPS spectrum of D-Ni(OH)2; (d) Ni2p XPS spectrum of D-Ni(OH)2 and DF-Ni(OH)2.|
The morphology of the obtained Ni-Al composite hydroxides was characterized by transmission electron micrograph (TEM) shown in Fig. 2. The TEM image (Fig. 2a) reveals that the final obtained D-Ni(OH)2 possesses a disordered porous structure. Fig. 2b-d shows the element maps of Ni, Al and O, which are all have a similar profile to the selected area in Fig. 2a, suggesting a homogeneous element distribution in D-Ni(OH)2. N2 adsorption- desorption isotherms and corresponding pore size distribution (PSD) curve of D-Ni(OH)2 (shown in Fig. 3) are employed to furtherinvestigate the structure of the final obtained composite hydroxides. The N2 sorption curve of D-Ni(OH)2 is a IV-type isotherms with a hysteresis loop at relative pressure of 0.4-0.6, which indicates the existence of mesoporous structure, consistent with the TEM measurements. The calculated specificsurface area (BET) and the corresponding pore volume are 121 m2 /g and 0.35 cm3/g, respectively. The Barrett-Joyner-Halenda (BJH) pore size distribution curve calculated from the adsorption branches, depicted as the inset of Fig. 3—inset, show thatthe obtained DNi( OH)2 has a wide distributed pore size centered at ∼6.0 nm.
|Fig. 2.TEM image of D-Ni(OH)2 and correspondingelement maps of Ni, Al and O.|
|Fig. 3.Nitrogen sorption isotherms and pore size distribution curve (PSD-inset) of D-Ni(OH)2.|
To find the merit of chemical doping, the electrochemical capacitive (EC) performances of the obtained composite Ni-Al hydroxidescompared to the solvothermal method derived dopingfree nickel hydroxide (surface area of 45 m2/g) were investigated by cyclic voltammetry and galvanostatic charge/discharge curves (GDC). Fig. 4a and b shows the cyclic voltammograms (CVs) of DNi( OH)2 and DF-Ni(OH)2 under various scan rates, which both display a typical pseudocapacitive behavior with two redox peaks around 0.5 V and 0.3 V, ascribed to the Faradaic reaction of Ni(OH)2 + OH-↔NiOOH + H2O + e-. Galvanostatic charge/discharge curves (GDC, Fig. 4c) of D-Ni(OH)2 at different current densities from 0.5 A/g to 20 A/g show deviation from the typical triangular shape of non-Faradaic electric double-layer capacitor (EDLCs), further evidencing the Faradaic characteristics of the charge storage. For D-Ni(OH)2, the capacitances (Fig. 4d, from 0.5 to 20 A/g) calculated from GDC curves are 1670, 1596, 1566, 1512, 1496 and 1465 F/g, the values are higher than those of DF-Ni(OH)2 (maximumcapacitance of 1227 F/g) at every current density. Moreover, from 0.5 A/g to 20 A/g, the D-Ni(OH)2 exhibits a higher capacitance retention of 87% than DF-Ni(OH)2 (capacitance retention of 47%), indicative of an excellent rate capability. The superior EC performances of D-Ni(OH)2 to DF-Ni(OH)2 can be attributed to the chemical dopingby Al cations, improving the electrode conductivity and thus leading to an enhanced EC performance.
|Fig. 4.Electrochemical capacitive performance of D-Ni(OH)2 and DF-Ni(OH)2 in 6.0 mol/L KOH, (a) cyclic voltammogram of D-Ni(OH)2, (b) cyclic voltammogram of the DFNi(OH)2, (c) galvanostatic charge/discharge curves of D-Ni(OH)2 under various current densities, (d) specific capacitance of the two nickel hydroxides under various current densities.|
In order to evaluate the electrochemical cyclic stability of the DNi( OH)2, galvanostatic charge/discharge investigations were performed at a high current density of 10.0 A/g. As shown in Fig. 5, after 2000 cycles, the D-Ni(OH)2 remains a high specific capacitance of 1481 F/g and the retention is about 98%, indicative of an excellent electrochemical cycleability. The superior EC performances D-Ni(OH)2 to DF-Ni(OH)2 may be attributed to chemical doping and porous structure, providing improved electrode conductivity and large surface area (121 m2 /g vs. 45m2/g).
|Fig. 5.Cycle stability test of D-Ni(OH)2 under current density of 10 A/g.|
A cation-anion double hydrolysis method was developed and realized the synthesis of composite Ni-Al hydroxides. The obtained composite hydroxides possesses a porous structure, large surface area (121 m2/g) and homogeneous element distribution. Moreover, the obtained composite nickel hydroxides exhibits a superior supercapacitive performances (specific capacitance of 1670 F/g and rate capability of 87%) to doping-free nickel hydroxide (specific capacitance of 1227 F/g and rate capability of 47%). The current method not only offers an approach to prepare doped nickel hydroxide but also may be extended to the synthesis of other aluminum-based composite hydroxides and mixed metal oxides since the obtained composite hydroxides can be easily transformed to composite metal oxides by heat treatment.Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 20773062, 20773063, 21173119, and 21273109), the Fundamental Research Funds for the Central Universities, and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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