Dopant Effects of Gd3+ on the Electrochemical ... - ACS Publications

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C: Energy Conversion and Storage; Energy and Charge Transport

Dopant Effects of Gd3+ on the Electrochemical Pseudocapacitive Characteristics of Electroactive Mesoporous NiO Electrodes for Supercapacitors Ganesan Boopathi, G. G. Karthikeyan, S.M. Jaimohan, Arumugam Pandurangan, and Ana LF de Barros J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11643 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

Dopant

Effects

Characteristics

of of

Gd3+

on

Electroactive

the

Electrochemical

Mesoporous

NiO

Pseudocapacitive Electrodes

for

Supercapacitors G. Boopathi a, G.G. Karthikeyan b, S.M. Jaimohan c, A. Pandurangan b,* A.L.F. de Barros a a

Laboratory of Experimental and Applied Physics, Centro Federal de Educação Tecnológica

Celso Suckow da Fonseca, Rio de Janeiro, Brazil. b

Chemistry Department, Anna University, Chennai-600025, India.

c

Advanced Materials Laboratory, Central Leather Research Institute, Chennai-600020, India.

*

Corresponding author’s E-mail id: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Undoped and gadolinium doped nanostructued mesoporous materials, such as NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O were synthesized by a facile hydrothermal route using urea as the hydrolysis controlling agent and studied for supercapacitor applications. The thermal-stability of the synthesized samples was identified by TGA. The phase structure of the as-synthesized and calcined materials was characterized by using powder XRD. The average crystallite size of the oxide materials was found to be in the range of 8.2–11.3 nm. FTIR revealed the metal–oxygen bond in the compounds. The analyzed morphological phenomenon of the prepared samples confirms the mesoporous flake-like shape. The N2 adsorption/desorption isotherms were performed to examine the surface area and pore-size distribution. The elemental composition and charge states analyses were obtained by EDX and XPS, respectively. Cyclic voltammetry, galvanostatic charge/discharge and EIS measurements were applied in an aqueous electrolyte to investigate the electrochemical performances of the active electrodes. Among the four electrodes, Ni0.98Gd0.02O exhibits the highest surface redox reactivity and shows optimum high specific capacitance of 1190 F/g at a current density of 2 A/g. The cycling lifespan of Ni0.98Gd0.02O with capacitance retention of 81.43% was inspected over 3000 cycles at a current density of 3 A/g. Keywords: Hydrothermal route, mesoporous NiO, nanoflakes, electrochemical behaviors, supercapacitors, impedance spectroscopy.

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Introduction Among the many existing capacitive energy storage devices1-3, supercapacitors (ultracapacitors) have attracted fabulous interests due to their major characteristics such as rapid charge/discharge rates4, long cycling lifespan (>105 cycles), much higher power density5, safe operation for the time-dependent power requirements of modern electronic devices and low maintenance cost6 in comparison with batteries and even specific capacities that are much greater than those of conventional electrolytic/dielectric capacitors. Electrochemical capacitive devices (devices involve capacitive energy storage mechanism) are used as memory back-up devices or back-up supplies to protect against power disruption and peak-power sources in electric vehicles7. In general, on the basis of the electric charge storage mechanisms, electrochemical supercapacitors can be classified as electric double-layer capacitors (EDLCs - storing energy by means of ion adsorption) and pseudocapacitors (fast surface redox/Faradaic reactions)8. The enhancement on the specific capacity of the active electrode materials for supercapacitors is still a vast challenge since supercapacitors having relatively lower energy density compared with batteries9. The performance of supercapacitors is chiefly dependent on the structures and properties of electroactive electrode materials10. There already numerous materials being found as the electroactive electrode materials in supercapacitive applications and those are broadly classified into three classes: conducting polymers, porous/high surface area carbon-based materials (carbon fibers, templated-carbon, activated-carbon, CNTs, carbon monoliths and graphene) and transition metal hydroxides/oxides11-12. Hydrous RuO2 exhibits an ideal pseudocapacitive performance, has earlier been highly acclaimed to worth of high pseudocapacitance values ((theoretical: 1300–2200 F/g)13; 3



(experimental value in acidic electrolyte: 1580 F/g)14). However, Ru-based aqueous electrochemical capacitors are toxic, expensive and I-V voltage window restrict their commercial applications15. To rectify these issues, the interest of researchers has almost diverted to search for alternative and inexpensive electroactive transition metal oxide materials. Such type of materials provide remarkable capacitive and better reversible characteristics with long cycle life stability, which can be prepared from reproducible, reliable and cheaper precursors with simple techniques. A number of other transition metal oxides have previously been examined for their electrochemical pseudocapacitive performances, which include TiO2, V2O5, MnO2, Fe3O4, Co3O4, NiO, CuO, SnO2 and Bi2O3 materials16. Among the other transition metal oxides, a ptype stable and wide band-gap oxide semiconductor, NiO with nanoscale-range is a versatile candidate. It has been the subject to considerable attention due to its actual theoretical specific capacitance value (2584 F/g), good capacitance retention, cheaper in cost, natural abundance, high specific surface area and large bulk conductivity for supercapacitive applications17-20. Whereas, it’s practical specific capacitance value is much lower than the theoretical value in real applications due to the poor electrical conductivity, crystallinity and surface redox reversibility21. However, functional characteristic NiO material that finds uses in diverse fields such as gas sensors22, catalysts23, magnetic storage devices24 and electrochemical capacitors25. In recent years, supercapacitors have received much attention with the development of electric car industry26. To propose superior pseudocapacitive performance of electroactive materials, it is desirable to intend the morphology as well as geometry with entire utilization of surface area and 4


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also well defined porous systems27, which influence in movement of ions rapidly between the electroactive materials and electrolyte to promote redox reactions. In previous reports elsewhere, the authors referred that the nanostructure with different morphologies of NiO such as nanoparticles, nanoflowers, nanosheets and nanowires were utilized to enhance the accessible surface area of the electrode materials28. Current innovations in nanoscience and nanotechnology have offered many opportunities in synthesizing enhanced surface properties-based nanostructured electroactive materials for supercapacitor applications. It is a crucial task to decrease resistivity and enhance conductivity of NiO electrode. Herein, utilization of metallic/non-metallic dopant (gadolinium) substitution into the lattice planes of transition metal oxides to improve the electrochemical performances which may exhibit further novel physical and chemical behaviors is well known29. So far, we have found that to the best of our knowledge, no report on Gd3+ doping into NiO-based system for utilization for electrochemical energy storage applications even though many reports are available on metallic ions (Li3+, Co3+, La3+, Y3+ and Ce3+) substitution with NiO system. The characteristics gadolinium (Gd3+) ion is distinct from all other rare earth ions because it is well known that more stability exists in its half-filled electronic configuration and having 7f-electrons30. Many physical and chemical preparation techniques have been explored previously to fabricate NiO to achieve the requirements. There are several techniques to synthesize α-Ni(OH)2, β-Ni(OH)2 or mixed phases of α- and β-Ni(OH)2 samples in the presence of urea (pH remains relatively low throughout the entire reaction), such as homogeneous-precipitation31, hydrothermal32, conventional-reflux and microwave assisted homogeneous-precipitation33 techniques. Among them, a facile hydrothermal technique is advantageous to synthesis NiO 5



nanoflakes due to low external energy consumption, inexpensive precursors and equip requirement. Here, we synthesized undoped and Gd doped mixed phases of α- and β-nickel hydroxide samples under urea hydrolysis controlling agent34 by supporting a facile hydrothermal technique. The further calcination led to form undoped and Gd doped single-phase NiO nanoflakes from its nickel hydroxide precursors. Thus, the present work elucidates that this hydrothermal technique is well suited for the preparation of well crystalline NiO nanoflakes with promising behaviors for electrochemical energy storage devices such as supercapacitor applications. The purpose of this work is to demonstrate the dopant effects of Gd3+ with different concentration on the pseudocapacitive performances of electroactive mesoporous NiO electrodes and on the utilization of which was examined by structural, morphological, elemental, cyclic voltammetric, galvanostatic charge/discharge and electrochemical impedance spectroscopic analyses. Experimental procedures Materials used In the present work, all the chemicals were analytical reagent grade obtained from commercial sources in Brazil and used without further purification. The synthesis process of undoped and Gd doped NiO-based samples is illustrated as possible formation mechanism later in results and discussion section. Synthesis of undoped and Gd3+ doped mesoporous NiO nanoflakes

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Undoped and Gd doped NiO nanoflakes were synthesized by hydrothermal treatment. In this present work, Gd was doped into NiO sites by 2, 5 and 8%, respectively. The abbreviations Gd2 wt.%, Gd5 wt.% and Gd8 wt.% are used as Gd2, Gd5 and Gd8, respectively, further in the manuscript. The aqueous solution containing a known amount of precursor materials such as nickel chloride hexahydrate (NiCl2.6H2O) and gadolinium nitrate hexahydrate (Gd(NO3)3.6H2O) were dissolved in deionized water, followed by drop wise addition of 0.33 M urea (CH4N2O). The solution was continuously stirred for about 30 min at 60 ºC and then transferred it into 150 mL Teflon-lined stainless steel autoclave. The capacity of the solution was assumed to be 66.66% to 150 mL stainless steel autoclave. The stainless steel autoclave was sealed and treated at 180 ºC in a furnace for 12 h. The stainless steel autoclave setup cools down to room temperature, the metal hydroxide solid precipitate with green color was washed thoroughly with deionized water repeatedly and finally with an appropriate amount of ethanol, continued to centrifugation at 2000 rpm for 5 min. The pH was maintained at neutral level throughout the entire experiment done. The as-synthesized products were dried at 80 ºC in a hot plate for about 12 h and ground manually in an agate mortar for 30 min. The resultant greenish products were heated gradually at the rate of 10 ºC/min from room temperature to 400 ºC and calcined for 2 h in air atmosphere with an electronic furnace in order to obtain Gd doped NiO nanocrystalline products. The color of the products became black colored. Again, each calcined sample was ground to obtain a fine powder form for further characterizations and measurements. All these experimental procedures were similar to prepare the pure NiO nanocrystalline material excluding the addition of doping content (Gd). Physical characterization techniques

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To test the thermal behavior of the synthesized products, thermogravimetric analysis (TGA) was carried out in a flux of N2 gas at a heat ramp rate of 10 ºC/min by using TGA Q50 thermal analyzer. Phase analysis of the as-synthesized and calcined products were performed by using X-ray diffractometry (XRD) (Rigaku Mini Flex-II Desktop X-ray Diffractometer, CuKα source: 1.5418 Å) at a sampling step of 0.02° with a scanning speed of 4 °/min in the range of 10°–80°. To examine the quality and chemical composition of the as-synthesized and calcined products, Perkin Elmer Spectrum RX I Fourier transform infrared (FTIR) spectrophotometer was used in the range of 4000–400 cm-1. The surface area and pore-size distribution were obtained on a Quanta Chrome Nova-1000 system by N2 adsorption/desorption isotherms at 77.4 K. The specific surface area was estimated by the Brunauer–Emmett–Teller (BET) method and pore-size distribution and total pore-volume were obtained by a Barrett–Joyner–Halenda (BJH) study. The morphological and compositional studies were inspected using high-resolution scanning electron microscope (HRSEM FEI Quanta FEG 200) equipped with an energy-dispersive X-ray spectroscope (HRSEM/EDX). High-resolution transmission electron microscope (HRTEM, JEOL JEM 2100) was used to provide morphological, topographical and crystalline information, operated at an accelerating voltage of 200 kV. For TEM measurement, the sample preparation included a copper grid coated with an amorphous carbon membrane was placed onto a filter, the samples were dispersed in ethanol and casted a drop over copper grid by using a micropipette. Xray photoelectron spectra (XPS) were recorded on Omicron NanoTechnology (Germany) spectrometer. For XPS measurement, a sample was under ultrahigh vacuum, where sample was bombarded with X-ray photons (0–1300 eV) where electrons were ejected from a sample after interacting with these high energy photons. Survey scan were performed with step size of 0.5 eV and pass energy of 50 eV. High resolution multiplex spectra were collected with scan time of 0.2 8


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sec and lower pass energy of 20 eV with step size of 0.03 eV. Take-off angle of the photoelectron was set to be 54.7°. The spectra were corrected with respect to C (1s) binding energy value of 284.6 eV. In situ experiments in the UHV chamber were performed with monochromatic Al (Kα) gun with X-ray energy of 1486.7 eV, operated at 300 W (operating voltage: 15 kV, current flux: 20 mA). All the HRXPS spectra were analyzed using CASA-XPS software. Working electrodes preparation and electrochemical measurements In practice, the working electrodes were fabricated by mixing the active material (8.5 wt.%), acetylene black (1 wt.%) and polyvinylidene difluoride (PVDF) (0.5 wt.%) in a mass ratio of 85:10:5% with few drops of N, N-Dimethylformamide (HCON(CH3)2), then ground at room temperature for about 60 min to form a homogeneous slurry. Then the resulting slurry was coated Doctor’s blade route on one side of a pretreated carbon-felt (exposed geometric area: 1x1 cm2, thickness: 0.325 mm). The slurry coated carbon-felt was introduced to heat treatment at 80ºC for 12 h in hot air oven in order to remove the organic solvents present in the electrode. The mass loading of the active materials onto each carbon-felt was found to be 1.0 (Undoped NiO), 1.4 (Ni0.98Gd0.02O), 1.4 (Ni0.95Gd0.05O) and 2.0 (Ni0.92Gd0.08O) mg/cm2. Electrochemical measurements were carried out using a three-electrode compartment. The carbon-felt coated with undoped NiO and Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O served as working electrode. A platinum wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The 6 M KOH solution was used as electrolyte. The cyclic voltammogram (CV) and electrochemical impedance spectroscopic (EIS) measurements were done on Autolab PGSTAT302F electrochemical workstation (Metrohm Autolab B.V., The Netherlands). The 9



cyclic voltammogram tests of undoped and Gd doped NiO electrodes were carried out between 0.0 and 0.42 V (vs. Ag/AgCl) at various scan rates and the EIS measurements were done in the frequency range from 100 kHz to 0.1 Hz with an alternating current (AC) amplitude of 10 mV. Galvanostatic charge/discharge (GCD) curves of undoped and Gd doped NiO electrodes were measured in a potential range of 0 to 0.41 V and 0 to 0.42 V, respectively, at different current densities. The electrochemical capacitive behavior of bare current collector carbon-felt was also studied and compared to mass loaded carbon-felt electrodes. All the applied potentials were referred to the reference electrode. Results and discussion Investigation of thermal behaviors The thermal behavior of the as-synthesized products was investigated by TGA. Thermogravimetric analysis was conducted in N2 atmosphere to examine the conversion process during calcination. Figure 1 depicts the typical TGA profiles for undoped (blue colored line) and Gd5 doped (red colored line) nickel hydroxide products from room temperature to 600 ºC. It indicates that both the products undergone a similar process while the decomposition temperatures and weight losses were different. These uncalcined products possess both adsorbed and intercalated H2O molecules. TG analysis of aforementioned products involved three discrete stages. The first stage took place at 286 and 256 ºC with a gradual weight loss of around 1–2.74 and 1–12% for undoped and Gd5 doped nickel hydroxide products, respectively, which belongs to the removal of chemically adsorbed and intercalated H2O molecules. Up to this first stage, both the products still contain NO3– intercalation. The second stage is of the thermal decomposition of hydroxide products into oxides as well as the removal of carbonate and nitrate 10


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anions35-36 where the major sudden weight loss about 22.47% was observed in the case of undoped product (286–376 ºC), whereas 16.73% was obtained for Gd5 doped product (256–364 ºC). Above 376 (undoped/blue color) and 364 ºC (Gd doped/red color), a possible third stage was observed after the decomposition reaction. This third stage may have represented a 100% completion of decomposition process, or it may have approached the decomposition of a minor fraction of various non-stoichiometric, unstable nickel oxides formed during the first decomposition stage to a stable NiO37. In third stage of transformation, weight loss about 2.44 and 4.72% was obtained for undoped and Gd5 doped products. Additionally, there are no major weight loss monitored above 376 (undoped/blue color) and 364 ºC (Gd doped/red color), which signifies the absence of structural changes and secondary phase. All these results were confirmed by the following XRD and FTIR results. The calcination process has an influence on the increase of crystallinity of the synthesized products and this maybe the prime reason for calcination at 400 ºC for 2 h was chosen in this study. Phase evaluation and lattice strain analysis A thorough review report on the different routes of preparation of various forms of nickel hydroxides has been already published by Oswald and Asper38. In general, nickel hydroxide materials contradict in forming well shaped and large crystals. In addition, a well crystallized Ni(OH)2 is not the primary yield during its precipitation from aqueous salt solutions. According to the experimental preparative conditions, the yields may be either hydroxyl salts or poorly crystallized α-type hydroxides can be obtained as primary yields. Later, β-Ni(OH)2 phase can be obtained by ageing of such compounds39, however often it also contains adsorbed foreign ions

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and water. The scheme of formation of polymorphs nickel hydroxide phases was reported clearly in Bode’s diagram40.

β -Ni(OH) 2 ⇌ β -NiOOH

↑

տ

↓

α -Ni(OH) 2 ⇌ γ-NiOOH Figure S1(a, b, c and d) shows the wide-angle XRD profiles of the undoped nickel hydroxide, Gd2, Gd5 and Gd8 doped nickel hydroxide products, respectively. These XRD profiles correspond to different mixed phases of nickel hydroxide structures. All the reflections observed in XRD profiles shown in Fig. S1(a, b, c and d) were consistent with the pattern of nickel hydroxide hydrate (α-Ni(OH)2.(0.75)H2O) (ICDD # 22-0444), nickel oxide hydroxides (βNi(OH)2 (ICDD # 14-0117), β-NiOOH (ICDD # 06-0141), γ-NiOOH (ICDD # 06-0075), Ni2O3H (ICDD # 40-1179), NiOOH (ICDD # 27-0956) and pure nickel (Ni) (ICDD # 89-7128) phases. The dominant peaks could be indexed to the (001), (110), (111), (002), (101), (103), (011), (200), (241), (300) and (0014) lattice planes, respectively. Figure 2(a, b, c and d) represents the XRD profiles of the undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O nanocrystalline products, respectively. All the peak profiles are readily indexed to well-crystallized pure NiO, agreed with the standard ICDD no. for NiO (ICDD # 00-022-1189)41. As per the earlier reported ICDD no., the XRD profiles of the oxide samples have five similar diffraction peaks which could be indexed to the (003), (012), (104), (015) and (006) lattice planes of hexagonal geometry of NiO with space group of R-3m (166). The XRD peaks of all oxide samples are identical to each other in Fig. 2. The diffraction peak width of undoped NiO seems to be small and sharp while Gd doped NiO

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samples depict the broad diffraction peaks due to lattice strain taken place. The dislocations and/or excess in volume of grain boundaries lead a path to lattice strain in nanocrystalline samples42. The width of each diffraction profile is a merged contribution of both sample and instrumental. Corresponding to each diffraction peak, the corrected instrumental line-broadening 2 was estimated via the expression43 β hkl = [( β hkl )2measured − ( β hkl )instrumental ](1/ 2) . In Williamson–Hall (W–

H) method, it was proposed that the line-broadening due to a contribution of crystallite size and strain as a function of diffraction angle and can be rewritten as the following Kλ   + (4ε tan θ hkl ) , θ hkl  D cos  

expression, β hkl = β D + βε = 

where D represents the average crystallite size,

ε represents the root-mean square value of micro-strain, βD represents crystallite size contribution and βε represents strain induced broadening. The average crystallite size (D) contribution of the nanocrystallites was calculated by Debye-Scherrer's Eqn. 1:

D =

K λ β D cos θ hkl

(Eqn. 1)

where, K is the crystallite shape constant (0.9 for sphere), θhkl is the Bragg angle, λ is the wavelength of X–rays (1.5418 Å) used and βD (corrected instrumental line-broadening) is full width at half maximum (FWHM) of the diffracted peaks in radians. The average crystallite size was found to be in the range of 11.3 nm for undoped NiO, 8.2 nm for Ni0.98Gd0.02O, 8.2 nm for Ni0.95Gd0.05O and 8.6 nm for Ni0.92Gd0.08O. The crystallographic lattice parameters obtained on the nanocrystalline oxide samples are listed in Table 1.

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The change noticed in lattice parameters with Gd concentration may also be due to the defects formation. Similarly, lattice strain contribution was estimated by the expression, βε = 4εtanθhkl. The micro-strain graph plotted with the values of βhklcosθhkl as a function of 4sinθhkl is shown in Fig. S2(a, b, c and d). Figure S3 depicts the different Gd concentration along with the variation of micro-strain values. This graph reveals that the lattice strain increases with increase of Gd concentration. The crystallite size (Dε) obtained from strain induced diffraction profiles, to be in the range of 11.5 nm for undoped NiO, 11.7 nm for Ni0.98Gd0.02O, 12.3 nm for Ni0.95Gd0.05O and 13.4 nm for Ni0.92Gd0.08O are also listed in Table 1. In all the doped NiO samples, with increased Gd3+ ions concentration, the NiO peaks illustrate a slight angle shift on (012) plane (see supplementary Fig. S4) as compared to undoped NiO sample. This is due to lattice strain arising from the substitution of smaller Ni2+ (ionic radius: 0.083 nm) by larger Gd3+ (ionic radius: 0.107 nm) in NiO lattices44. However, no peaks for gadolinium oxide or any other impurities were detected, confirms the purity of NiO product. This structurally modified Gd doped NiO samples have a considerable account for the enhanced electrochemical behaviors. In general, the stoichiometric NiO nanostructures originating from Ni2+ and O2− collisions could be formed under higher temperature i.e., greater than 300 °C45. Therefore, the calcination temperature of as-synthesized samples was performed at 400 °C, for sufficient electrostatically neutral. FTIR analysis For further support to the XRD and TGA results, the quality and chemical composition of the as-synthesized and calcined products were examined by FTIR spectrophotometer in the range of 4000–400 cm–1 at room temperature and the results are shown in Fig. S5 and Fig. 3, 14


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respectively. Figure S5(a, b, c and d) shows the FTIR spectra obtained on undoped nickel hydroxide, Gd2, Gd5 and Gd8 doped nickel hydroxide products, respectively. For the assynthesized undoped hydroxide product (Fig. S5(a)), a sharp and intense free hydroxyl band centered at 3634.5 cm–1 corresponding to non-hydrogen bonds νOH which is a characteristic peak of β-Ni(OH)2 phase46, whereas a broad vibration band observed between 3000 and 3640 cm–1, is assigned to O–H stretching vibration present in rest of the Gd doped hydroxide products which indicates that the ultrafine powders may tend to physically absorbed moisture from atmosphere47. As shown in Fig. S5, the peaks observed at 1535.5, 1472.8, 1403 and 1078 cm–1 are attributed to the various vibrational modes of the carbonate groups originating from the adsorption of atmospheric CO2. A sharp and strong band at 849.6 cm–1 is ascribed to the presence of δCO3 and δNO348. These peaks were observed significantly even in oxide products. A broad peak obtained at 669.6 cm–1 which corresponds to the C–H stretching, confirms the presence of residual urea in the hydroxide products. It is worth noting down that Ni–O stretching vibrations νNiO appear at 492.4 and 464.7 cm–1. Figure 3(a, b, c and d) represents the FTIR spectra obtained on undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O nanocrystalline products, respectively. Absorbance peaks might shift to higher or lower wave numbers, appearance of new infrared peaks owing to the adsorbate or splitting of original bands. A shift to higher frequencies indicates an increase in bond strength while lower frequencies indicates bond weakening49. Some additional peak shift was obtained in oxide products to lower frequencies, while comparing with the hydroxide products. In principle, the presence of hydroxide stretching vibrations around ~3440 cm–1 should not be obtained on all the oxide products after the thermal treatment at 400 ºC for 2 h. However, the –OH peak is still observed and those are shown in Fig. 3. These observed bands derive due to 15



the fact that the calcined products tend to physically absorb the water molecules during pellet preparation for IR measurement. A decrease in peak intensity was found at 1504.8, 1384.5 and 842.7 cm–1 in oxide products, are ascribed to the existence of carbonate and/or NO3– ions seem to be assisted plates or sheets with nanoscale dimensions in the growth of oxide nanostructured products, as explained elsewhere in earlier reports50. The strongest peaks in the range of 400– 1000 cm−1 are contribution of face-centered cubic phase Ni–O51. As denoted in aforementioned reference, the peaks were observed at 574.4 and 424.7 cm−1 are obviously attributed to Ni–O stretching vibrations. It is clearly seen from Fig. S5 and Fig. 3, the crystalline phase has changed after thermal treatment. Thus, this analysis confirms that XRD and FTIR results are in good agreement. Microstructural analysis and elemental observations The HRSEM micrographs reveal that the external morphologies as well as dimensions of nickel hydroxide and oxide products at different magnifications are shown in Fig. S6 and Fig. 4, respectively. In an earlier report, it was stated that in a certain hydrothermal condition, nickel hydroxides were easily formed in nano-regime because of its intrinsic lamellar structure52-53. Figure S6 exhibits HRSEM micrographs of (a) undoped and (b) Gd2, (c) Gd5 and (d) Gd8 doped nickel hydroxide samples, respectively. It can be seen that some group of nickel hydroxide tiny particles aggregated together with each other heterogeneously in lower degree and formed as some irregular, triangle and hexagonal shaped aggregated/agglomerated larger flake-like structures. It is in agreement with aforementioned report52-53. We also observed that no individual well-defined particles present in all hydroxide products (Fig. S6). Surprisingly, both undoped and Gd doped nickel hydroxides result in similar morphologies and dimensions. 16


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Pore generation observed in oxides due to the thermal decomposition of nickel hydroxides can be confirmed by HRSEM and HRTEM micrographs. Figure 4 represents the HRSEM images of (a) undoped NiO (b) Ni0.98Gd0.02O, (c) Ni0.95Gd0.05O and (d) Ni0.92Gd0.08O products, respectively, calcined at 400 ºC for 2 h, which nearly retained very similar structures to those of as-synthesized products but with noticeably new porous channels. The growth mechanism (discussed later in this paper) of the nanoflakes of undoped and Gd doped NiO follows nickel hydroxide products. It is very hard to hypothesize the accurate mechanism of complex structures of NiO formation. The formation mechanism has not been understood clearly due to different parameters such as crystal-face attraction, hydrophobic interactions, hydrogen bonding, intrinsic crystal contraction, electrostatic and dipolar fields and Van der Waals forces contributing during the formation of the products54. In our study, the growth of the crystals seems to proceed through aggregation rather than by classical ion-by-ion mechanism55. It can be seen that the nanoflake structures with some defined edges (triangle and hexagonal shapes) that remind hexagonal crystallites present in oxide samples. This relatively plenty of loosely packed mesoporous channels nevertheless feature firm connections between nanocrystallites as shown in the HRSEM micrographs (see inset images), which lead to improved electrolyte permeability enhanced electron pathways, thus potentially improving the pseudocapacitance50 feature of the electrode materials. Increasing in the magnification (all inset images in Fig. 4) allows for better differentiation of the morphology of oxide samples. It is worth to consider that Gd-doping has not changed the morphology in as-synthesized samples whereas post heat treatment may be the key factor which created mesoporous channels on the surface of all the oxide samples. Further, for elemental microanalysis, we analyzed only lower (Gd2) and higher (Gd8) concentrated samples with EDX spectra. Figures 4(e) and 4(f) display the EDX microanalysis 17



spectra of the Ni0.98Gd0.02O and Ni0.92Gd0.08O nanoflakes, respectively. EDX spectra of both the samples clearly display peaks correspond to Ni, Gd and O elements and no other impurities detected in the samples. The elemental composition results obtained on nanoflake materials by EDX microanalysis is presented in inset images of Fig. 4(e) and 4(f), respectively. These results confirmed that the appropriate elements exist in both oxide samples. Figure 5 demonstrates a typical elemental mapping based on EDX of the sample, (a) HRSEM image, the corresponding elemental mapping of (b) Ni, (c) Gd and (d) O elements within Gd2 doped NiO network, confirming the spatial distribution of the elements. Obviously, it can be seen that the Gd contents are uniformly distributed over the entire Gd2 doped NiO architectures. The observed features with HRSEM analysis were not well resolved, so as to better resolution and comparison, both undoped and Gd2 doped NiO samples alone has been inspected under TEM tool. Figures 6(a and b) and 7(a and b) illustrate TEM micrographs of undoped NiO and Ni0.98Gd0.02O products, respectively, obtained at different magnifications. The average crystallite sizes of these undoped NiO and Ni0.98Gd0.02O nanoflakes were found to be in the range of around 11 nm (Inset of Fig. 6c)) and 8 nm (Inset of Fig. 7a), respectively, which are consistently agreed with the XRD results. These TEM images reveal that nano scaled particles seem to be united and aggregated forming a flake-like morphology. The morphological analysis using TEM is well supported with HRSEM analysis. HRTEM measurements have been carried out for depth structural view of both undoped NiO and Ni0.98Gd0.02O products. HRTEM micrograph (Fig. 6(c)) of NiO illustrates the well defined lattice fringes which suggest the highly polycrystalline nature consisting of nanoparticle clusters. The distances of the two neighboring planes are about 0.2402 nm and 0.2073 nm, which are consistent with the interplanar separation of the (003) and (012) planes in hexagonal NiO. Also the corresponding SAED pattern obtained 18


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for undoped NiO is shown in Fig. 6(d) and the diffracted rings were indexed to (003), (012), (104), (015) and (006) planes which correspond to hexagonal NiO phase. The authentic SAED patterns are produced in supplementary information Fig. S7. Similarly, the interplanar distances obtained are about 0.2445 nm and 0.1492 nm, which correspond to the (003) and (104) planes in hexagonal Ni0.98Gd0.02O (Fig. 7(c)). The SAED pattern for Ni0.98Gd0.02O nanoflakes is presented in Fig. 7(d) and the diffracted rings could be indexed to (003), (012), (104), (015) and (006) planes. These results agreeing with the XRD profiles obtained for calcined oxide samples. Surface area, pore-size and pore-volume analysis Figure 8 demonstrates the N2 adsorption/desorption isotherms of the undoped NiO (blueline), Ni0.98Gd0.02O (red-line), Ni0.95Gd0.05O (green-line) and Ni0.92Gd0.08O (pink-line) nanoflakes. The obtained BET surface area and total pore-volume are summarized in Table 2. According to the International Union of Pure and Applied Chemistry (IUPAC) categorization, all branches of isotherm can be classified as type-IV with H4-type hysteresis loop56. The type-IV isotherm is attributed to mesoporous (2 nm < pore-size < 50 nm) materials. This hysteresis loop suggests that the tiny nanosized particles generate interparticle porosity of mesoporous sizes that produce capillary condensation as a secondary process, which starts at about P/P0 = 0.4 and expends almost to P/P0 = 1, concerning a complete mesopores filling. This is indicating the existence of mesoporous structures present in all oxide samples. The estimated BET surface area of undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O are 71.679, 96.976, 96.493 and 56.084 m2/g, respectively. The pore-size distribution curve is displayed in inset of Fig. 8. The pore-size distribution of undoped NiO and Ni0.92Gd0.08O samples is narrow with a single-modal obtained at 4.9 and 4.08 nm, respectively. Such these small pores are apparently generated from pore19



opening when the removal of physically absorbed H2O or intercalated H2O between nickel hydroxide layers. Whereas, a large bimodal pore-size distribution centered at 5.5 and 7.07 nm for Ni0.98Gd0.02O sample and at 3.4 and 4.07 nm for Ni0.95Gd0.05O sample due to expansion of pores. The total pore-volumes of undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O are 0.166, 0.248, 0.200 and 0.117 cm3/g, respectively. By comparison, the BET surface area and average pore-size of Ni0.98Gd0.02O are higher than that of all other samples. The outcome of these BET and BJH results with the different surface area and the average pore-size characteristics can lead to differences in ion transport and diffusion behaviors during redox reactions, resulting in different electrochemical performances57. Plausible growth mechanism of flake-like nickel hydroxide and oxide The plausible formation mechanism represented in Scheme 1, illustrating the morphology obtained when the mixture of the metal salts and urea were treated under a hydrothermal treatment followed by calcination. On the basis of the investigations36 on urea which significantly played a major role in the formation of nickel hydroxide product. In the above report, it was stated that NH3 and CO2 were produced around 70 °C after the decomposition of urea and then the free anions OH– and CO32– generated. The byproducts obtained by urea during the hydrothermal treatment were also confirmed by FTIR results. However, urea provided steady OH– ions and perceived that a colloid is formed from solution by association of the other dissolved reagents or hydroxides during the experiment. The species nucleated in colloid to form tiny single crystals58 that are the unit cells (primary building blocks) of the final products. These tiny single crystals then grow in different dimensions by chemical reaction at their surface. During the continuous chemical reaction these tiny single crystals self-aggregate with one 20


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another into flake-like structures. The existence of anions OH– and CO32– groups, electrostatic and hydrogen bonds might be the prime driving forces for self-aggregation. Thus, hundreds of self-aggregated multi-phases nickel hydroxide flake-like structures could be synthesized in the presence of urea under the hydrothermal conditions. The calcination process engages the incorporation of MOH (M′OH) species to form M–O–M or M–O–M′ bonds within the resultant metal oxide species, where M and M′ represent Ni and Gd elements, respectively. In addition, the post calcination treatment could be influenced the non-porous hydroxide materials into porous oxide materials. Thus, the mesoporous undoped and Gd doped NiO nanoflakes were produced in our attempt. Therefore, this facile hydrothermal route not only accelerates the nucleation but also enhances the process of nanosized crystal growth when compared to other conventional methods. XPS analysis XPS technique was used to estimate the binding energy, to analyze the chemical composition and the surface electronic states of all the bonded elements present in the undoped and Gd doped NiO nanoflakes. To verify the existence of Gd into the NiO sites, the XPS measurement on Ni0.92Gd0.08O (higher doping concentration) samples was carried out except two other (2 and 5) concentrations. As shown in Fig. 9(a and b), the wide-scan survey spectrum of both samples, undoped and Gd doped NiO nanoflakes confirms the presence of Ni, O, C and Gd, which is in accordance with the EDX results accompanied by SEM measurement. The selection rules predict that the highest splitting lies in the lowest binding energy tail and the lowest splitting lies in the highest binding energy tail. The binding energy values for Ni, O and Gd elements were calibrated by using standard C (1s) transition at 284.6 eV as reference peak. The 21



charge corrections were obtained to 0.476 eV for undoped NiO and 0.5 eV for Ni0.92Gd0.08O products. Ni (2p) transition in undoped NiO and Ni0.92Gd0.08O products are deconvulated into five and six peaks, respectively. The C (1s) spectra of both samples undoped NiO (Fig. 9(c)(i)) and Ni0.92Gd0.08O (Fig. 9(c)(ii)), the peaks obtained at (284.6 and 284.61) eV, (285.9 and 285.9) eV and (288.53 and 288.48) eV could be assigned to non-oxygenated C–C/C=C bonds of amorphous carbon, C–O and C=O bonds, respectively. The above discussed carbon may derive from the carbon-tape used in sample preparation. The high-resolution Ni (2p) core-level spectrum shown in Fig. 9(d) can be attributed to Ni 2p3/2 (850–868 eV) and Ni 2p1/2 (870–885 eV) spin-orbit levels, which reveal that Ni ion is in +2 valence state. In Fig. 9(d)(i and ii), the observed peaks of Ni 2p3/2 and Ni 2p1/2 states located at 853.53 and 872.5 eV, respectively, had an energy separation of 18.97 eV, which are in well agreement with the references59. On deconvulating the Ni 2p3/2 transition peaks at 853.53eV in both samples, it was found that Gd doped NiO sample showed a peak shift towards higher binding energy (from 853.53 to 853.75 eV) which indicated the substitutional doping of Gd3+ resulting in lattice deficiency when Gd ion incorporates into NiO crystal sites (Fig. 9(d)(i and ii)). Similar reports on deconvulated peaks have been observed in NiO samples where substitutional doping of lanthanum ions60. As the ionic radius of dopant is the most crucial factor, which can strongly influence the ability of the dopant to enter into NiO crystal lattice. In a metal oxide system, if the ionic radius of the substitutional doping metal ions matches that of the oxide metal ion, then the substitutional doping metal ion will substitute itself in the lattice during the substitutional doping reactive process. However, if the ionic radius of the dopant is either smaller or bigger in size than that of Ni2+ like in the present study, then the dopant substituting for Ni2+ would produce crystal lattice deformation61. Therefore, when a few Gd3+ ions are dispersed into the lattice of NiO, a 22


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charge discrepancy and imperfections in the NiO lattice can occur, which may further enhance the electrochemical activity of the electrode. The stronger peaks at (853.53 and 872.5) eV and (853.68 and 872.3) eV correspond to Ni2+ in Ni–O bonds, whereas the weaker peak observed at 855.31 and 855.24 eV correspond to Ni3+ in Ni–OH bonds62 shown in Fig. 9(d)(i and ii), respectively. The Ni–OH bonds mainly arise from nickel hydroxides such as α-Ni(OH)2 and βNi(OH)2 and from higher valence nickel oxides such as β-NiOOH, γ-NiOOH, Ni2O3H and NiOOH63. Meanwhile, the corresponding satellite positions are observed at (861 and 878.95) eV and (860.8 and 879) eV shown in Fig. 9(d)(i and ii), respectively. The satellite peaks may occur from extrinsic energy-loss or intrinsic energy-loss processes64, conduction band interaction (CBI)65 and shake-up interaction66. Extrinsic energy-loss satellites arise due to plasmon losses67. Plasmon satellite peaks are associated with conducting materials. Conduction band interaction is another type of intrinsic energy-loss process, which generally can be observed exclusively for semiconducting or conducting materials. As the reference68 reported, the energy separation (∆E) of the satellite peaks from the main peak was in the range of 3–15 eV. The energy separation for 2p spectra in the transition metals is greater than 3 eV which corresponds to shake-up interactions whereas it is less than 3 eV results from multiplet splitting. The intense satellite peaks in the transition metals confirm that those property correlates with paramagnetic nature. There is a clear characteristic indicative of that another weak shake-up interaction observed in the case of Ni0.92Gd0.08O product at 865.7 eV. Figure 9(e)(i and ii) shows the deconvolution of O (1s) core level spectrum with three peaks, which belongs to undoped NiO and Ni0.92Gd0.08O, respectively. In general, metal oxide peaks occur at a considerably different binding energy compared to most other oxygen species. In O (1s) spectrum, the distinct peak observed at 528.94 23



eV for undoped NiO (Fig. 9(e)(i)) and 529.03 eV for Ni0.92Gd0.08O (Fig. 9(e)(ii)) indicates the presence of oxygen bonded between Ni–O–Ni sites69, which are consistent with Ni 2p3/2 transition. In the case of Ni0.92Gd0.08O product, O (1s) peak at 530.47 eV became broader while compared to that of the undoped NiO (530.44 eV) product. Compared with that of undoped NiO, the XPS spectrum of Ni0.92Gd0.08O mesoporous nanoflakes exhibits a relatively high O 1s peak. The peaks observed at 530.44 and 530.47 eV are attributed to the existence of the shoulder peaks derive from defective sites at the surface of undoped NiO and Ni0.92Gd0.08O products. A tiny tail obtained at 532.18 and 533.18 eV, is ascribed to chemically and physically bound water molecules at the surface of undoped NiO and Ni0.92Gd0.08O products70. This observation is also consistent with aforementioned Ni 2p3/2 transition. The peak positions for Gd (3d) emission line are shown in Fig. 9(f). The binding energy values of spin-orbit doublet Gd 3d5/2 and Gd 3d3/2 peaks were found to be 1187.86 eV and 1220.99 eV. However, the standard elemental binding energy values of Gd 3d5/2 and Gd 3d3/2 are 1186 and 1218 eV, respectively. It is clearly observed that the binding energy values of Gd (3d) peaks were shifted to the higher binding energy tail. This spin-orbit splitting with an energy difference of 32 eV or above (in this work: 33.13 eV) which explores that the Gd characterizes its Gd3+ oxidation state71. The corresponding peaks of Gd3+, which confirms the presence of Gd3+ in the products. These results together with XRD, FTIR and EDX analyses demonstrate an advantage of the hydrothermal synthesis route in producing high-purity samples. Electrochemical pseudocapacitive performances

24


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The detailed pseudocapacitive performances of the nanocrystalline oxides were studied by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements in a three-electrode compartment with 6 M KOH aqueous solution. Figure 10(a) shows the CV curves of bare carbon-felt (black colored line), undoped NiO (green colored line), Ni0.98Gd0.02O (blue colored line), Ni0.95Gd0.05O (red colored line) and Ni0.92Gd0.08O (pink colored line) working electrodes at a scan rate of 10 mV/s, respectively. As indicated from Fig. 10(a), CV curves of all the four working electrodes are in symmetrical nature. Figure 10(a) clearly demonstrates the comparative CV curves obtained on all the four working electrodes along with blank carbon-felt current collector electrode. The current response for the blank carbon-felt electrode is comparatively weak, is negligible in comparison with that of all the other working electrodes. Evidently, the CV trend of bare carbon-felt seems to be far different with the four other working electrodes. As the specific capacitance is quite directly proportional to the area of the CV profile, the role of the blank carbon-felt to the actual specific capacitance of the working electrode materials is just ignored in the discussion section. It is well known that when the concentration of doping ions increases, the surface barrier becomes higher and the space charge region becomes narrower30. The electron-hole pairs within the region are efficiently separated via the large electric field before recombination. The formation of Ni–O–Gd reduces the transition of NiO phase and blocks the Ni–O species at the interface with NiO domains stabilizing them, thus preventing the agglomeration of NiO nanoparticles. Consequently, the addition of Gd3+ ions in the NiO lattices suppresses little the growth of the particles compared to undoped sample but doping doesn’t change the morphology. However, the transformation of hydroxides into oxides by post calcination process which results in the formation of nanoporous structure. Such this mesoporous structure facilitates the electronic and ionic transport during the 25



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charge/discharge reactions and thus makes complete utilization of the electroactive material contribution to the overall capacitance. The CV profile shape of all the aforementioned electrodes is entirely distinct to that of the electric double-layer capacitors (EDLCs), representing the two sturdy redox peaks are evidently noticeable in all the CV curves, which could be ascribed to the surface Faradaic reactions from Ni2+ to Ni3+ by oxidation (charging) and from Ni3+ to Ni2+ by reduction (discharging)72. The following Eqn. 2 expresses redox reaction which takes place during the CV measurement: charge NiO + zOH

−

⇌

zNiOOH + (1 - z ) NiO + ze − discharge

(Eqn. 2)

where, z represents a fraction of Ni sites involves in the surface Faradaic redox reaction. In general, the transfer processes of OH– are slow and thereby they are unable to respond to the rapid change in potential (or change in electron transport) in time during the fast reduction and oxidation sweeps. Accordingly, a fast scan rate directs to a rapid depletion of OH– during oxidation or over-saturation of OH– during reduction73. The ions transfer rate and the interfacial reaction kinetics are not efficient enough at higher scan rate in which redox reaction undergoes limited diffusion. At fast charging/discharging rates, the components of the surface of the electrode inaccessible while the depletion or over-saturation of OH– occurs at the electrolyte/electrode interface. Thus, impeding the charge-transfer process and promoting increase in resistance74. In contrast, at slow charging/discharging rates, electron transport may be coordinated with OH–, resulting in high pseudocapacitance/lower resistance. As analyzed by CV 26


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measurement among four working electrode materials, Ni0.98Gd0.02O electrode exhibits a great deal with specific capacitance than the other three electrode materials, an analogous trend can also be seen in GCD profiles in Fig. 10(b), it may be due to the doping effect of Gd into NiO electrode material. The doping process may improve the electron conduction path and sequentially facilitate the charge-transfer during the redox reaction process. The symmetrical redox reaction peaks with respect to NiO patterns designate the good reversibility nature of the redox processes where commonly ions are charged and discharged continuously through electrolyte and the electrolyte being diffused into the electrode materials, inducing polarization as well as Ohmic resistance (internal resistance) during the redox reaction process. The specific capacitance is graphically determined by integrating the area under the I–V curve and then dividing it by the scan rate (v, V/s), the potential window (Va – Vc) applied and the mass (w, g) of electroactive electrode materials using the Eqn. 3:

V

c 1 C= I VdV w v ( ∆V ) V∫a

(Eqn. 3)

where, ∆V (V) is the applied potential window and I (A) is anodic or cathodic current. The estimated specific capacitance values of all these electrode materials from CV profiles are summarized in a Table S1(i) produced in supporting information. Figure S8 depicts the CV curves of (a) bare carbon-felt, (b) undoped NiO, (c) Ni0.98Gd0.02O, (d) Ni0.95Gd0.05O and (e) Ni0.92Gd0.08O working electrode materials each at scan rates between 2 and 100 mV/s, respectively. No significant changes observed in trend of the CV profiles of all working electrodes except the bare carbon-felt. A tiny anodic-shift on the

27



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oxidation peak potential and a cathodic-shift on the reduction peak potential have been observed with increased scan rate, due to the limitation of ion diffusion rate in electrolyte/electrode interface so as to persuade electronic neutralization. Accordingly, the symmetrical redox peak area increased for Ni0.98Gd0.02O electrode material compared to that of undoped NiO and also to higher order doped NiO electrodes observed at a scan rate of 10 mV/s as shown in Fig. 10(a). As mentioned early in the combined CV profiles of bare carbon-felt (Fig. S8(a)), the CV trend seems to be far different with the four other working electrodes. Conversely, Fig. S8(b–d) Ni0.95Gd0.05O show comparable specific capacitance values at slow scan rates up to 20 mV/s. While, at fast scan rates above 20 mV/s, the specific capacitance values of Fig. S8(c) Ni0.98Gd0.02O is apparently larger than undoped NiO (Fig. S8(b)) and Ni0.95Gd0.05O (Fig. S8(d)) electrode materials. It is a contradict connection observed between the increment of voltage drop (iR loss) and an insufficient use of the active material, the current density and the total amount of charge stored, involved in the redox reaction under the fast scan condition. The electronic kinetics of the working electrodes at such a high scan rate is assigned to the porous system. It is obvious that Ni0.92Gd0.08O (Fig. S8(e)) working electrode reveals the lowest specific capacitance, suggesting a relatively weak oxidation/reduction reaction could occur in the electrode material. In Eqn. 2, z denotes only a fraction of the Ni sites which are involved in the surface redox reaction (i.e., all the Ni sites are oxidized/reduced reversibly when

z equals to 1.0). The value of z can be estimated from the specific capacitance of the undoped NiO and Ni0.98Gd0.02O nanoflakes by using the following Eqn. 4:

z=

CM∆V F

(Eqn. 4) 28


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where, C is the specific capacitance and M represents the molecular weight (g/mol.) of the electroactive electrode materials, respectively, ∆V denotes the potential window (0.0 to 0.42 V) applied and F is the Faraday constant (96487 C/mol). For the comparative analysis from Fig. S8, the obtained z values at the scan rate of 2, 5, 10, 20, 30, 40, 50 and 100 mV/s are 12, 9.9, 8.7, 7.7, 7.2, 6.7, 6.4 and 5% for undoped NiO and 18.4, 16.8, 15, 12.5, 10.8, 9.4, 8.3 and 6.2% for Ni0.98Gd0.02O nanoflakes, respectively. Therefore, the most obtained 12 and 18.4% of undoped NiO and Ni0.98Gd0.02O nanoflake sites contribute in the surface redox reaction at a sweep rate of 2 mV/s, which are appreciably higher than the earlier reports on pure NiO nanostructured material75. The GCD test was performed to further enumerate the specific capacitance of the working electrode materials. The charge/discharge curves of undoped NiO (black color line), Ni0.98Gd0.02O (blue color line), Ni0.95Gd0.05O (red color line) and Ni0.92Gd0.08O (green color line) working electrode materials at a current density of 2 A/g are shown in Fig. 10(b). The precise specific capacitance values of undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O working electrode materials are estimated from the GCD curves according to Ruoff’s Eqn. 5, and tabulated in Table S1(ii) (see supporting information).

C=

where,

I  dV  ∗m   dt 

( dV / dt ) = (Vmax − (1 / 2)Vmax ) / (T2 − T1 )

(Eqn. 5)

, I (A/g) is the discharge current, (T2 –T1) is the

discharge time for which the maximum potential (Vmax) (V) reaches at its half value (1/2Vmax), and m (g) is the mass of the active electrode material. The GCD curves demonstrated that a 29



longer discharge time is mandatory for Ni0.98Gd0.02O, which corresponds to a high specific capacitance of 1190 F/g at 2 A/g. In contrast, Ni0.92Gd0.08O electrode illustrates a rapid discharge progression with a low specific capacitance of 200 F/g at same current density. Meanwhile, the specific capacitances of undoped NiO and Ni0.95Gd0.05O electrode materials result between the high and low regions. The specific capacitance for Ni0.98Gd0.02O was still as high as 140 F/g even at a high current density of 20 A/g. The direct conducting pathway presumably for ions and electrons via the porous architecture facilitates the ionic and electronic transport during the redox reactions. The interfacial distance between the electrolyte and the electrode is considerably larger, which results in higher diffusion rates and significantly faster charge/discharge kinetics. Figure S9 displays the GCD profiles of (a) bare carbon-felt, (b) undoped NiO, (c) Ni0.98Gd0.02O, (d) Ni0.95Gd0.05O and (e) Ni0.92Gd0.08O working electrodes each at different current densities between 2 and 20 A/g, respectively. As seen in the CV curves of bare carbon-felt, the GCD trend of bare carbon-felt is not similar and the quantity of current contribution is very little to the working electrode materials. Surprisingly, great specific capacitance values of 1190, 759, 646.8, 527, 436.4, 204.2, 196 and 140 F/g at different current densities of 2, 3, 4, 5, 6, 10, 15 and 20 A/g, respectively, were obtained for Ni0.98Gd0.02O (Fig. S9(c)) electrode. From Fig. S10, moderately the specific capacitance values as 934, 481, 335.4, 260, 212, 102.4, 65.6 and 44 F/g for Ni0.95Gd0.05O (Fig. S9(d)) electrode and 780, 396, 312, 250, 222.4, 146, 142 and 104 F/g for undoped NiO (Fig. S9(b)) electrode, both were obtained at current densities between 2 and 20 A/g, respectively. Whereas, at the same current densities, comparatively low specific capacitance values 200, 106, 76.2, 60.6, 50.2, 20.2, 13 and 8.8 F/g were attained for Ni0.92Gd0.08O (Fig. S9(e)) electrode material.

30


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For comparison to the outstanding performances of undoped NiO and Ni0.98Gd0.02O electrode materials are superior to the other earlier reported undoped and metal ions doped NiObased supercapacitors listed in Table 3 such as pure NiO nanowires and NiO/MnO2 nanocomposites with the specific capacitance values of 307 and 528 F/g at 1 mV/s, respectively76, pure NiO and La:NiO nanospheres with the specific capacitance values of 127 and 253 F/g at 5 mA/cm2, respectively60, pure NiO and Cu:NiO nanoparticles with the specific capacitance values of 294 and 559 F/g at 0.3 A/g, respectively77 and pure NiO and Ag:NiO nanospheres with the specific capacitance values of 121 and 345 F/g at 1 A/g, respectively78. In this present work, surprisingly enhanced discharge specific capacitance value (1190 F/g) of the mesoporous Ni0.98Gd0.02O is achieved compared to the undoped NiO electrode (780 F/g), indicating that the mesoporous Ni0.98Gd0.02O electrode provides not only a new lane for the excellent contact of Ni0.98Gd0.02O nanoflakes with KOH electrolyte releasing the capacitance, but also proffer more electrochemical redox reactions. These nonlinear behaviors of the GCD profiles further supports the CV measurements that the charge separation in redox processes contribute to the specific capacitance, a feature of pseudocapacitor. Cyclic life stability study The cycling life stability test of the electrode material is another crucial probe obligatory to practical application-based performances. The cycling life stability was inspected by repeating GCD processes of the undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O electrodes at a current density of 3 A/g for about 3000 cycles and it is displayed in Fig. 11(a). As expected, our Ni0.98Gd0.02O nanoflakes exhibit outstanding life stability with capacitance retention of 81.43% even after running for 3000 cycles even than other electrodes, undoped NiO (76.5%), 31



Ni0.95Gd0.05O (78.56%) and Ni0.92Gd0.08O (74.74%). The fact attributed to the mesoporous flakelike structures undergoes slight degradation or structural modification76 during the continuous insertion or extraction of OH− ions into or from the working electrodes. The specific capacitance nearly 620 F/g at a current density of 3 A/g for Ni0.98Gd0.02O was obtained even after 3000 cycles. A gradual decrease in the specific capacitances during the GCD test demonstrates that the retention from the cyclic test is within the voltage window of 0.0 to 0.42 V. The typical GCD profiles of the first 10 cycles of Ni0.98Gd0.02O are shown as inset picture of Fig. 11(a). There is no significant change in morphology of the working electrode found after 3000 cycles tested and displayed in Figure 11(b). The morphology observation is still retained its original appearance. The charge-transfer efficiency in a system to facilitate an electrochemical reaction is called the Coulombic efficiency (η) and its maximum value implies the obligation of least energy to complete the redox reaction and compose the process high feasible. The Coulombic efficiency can be estimated by the following Eqn. 6:

η=

TD × 100 TC

(Eqn. 6)

where, TC and TD are representing the charging and discharging time, respectively. The η value for Ni0.98Gd0.02O electrode material is found at 1st and 3000th cycle tests and the corresponding estimated values are 91.39 and 90.79%, respectively. A little difference found in between these η values is due to the higher kinetic reversibility of the electrode material which is consistent with the CV and GCD results. We believe that the interaction of all these factors directs to the enhanced supercapacitor behaviors of the Ni0.98Gd0.02O electrode. The larger specific capacitance value for Ni0.98Gd0.02O nanoflakes may be derived from the crystal defect after Gd has substituted Ni atom in the lattice sites. Figure S10 represents that the comparative specific 32


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capacity values obtained from GCD results against three different doping concentrations, from which we can confirm the lower weight percentage (at 2 wt.%) provided the high efficiency. According to the reference30, this work clearly revealing that the higher doping concentration of Gd3+ with the host NiO electrode restricts the enhancement on the electrochemical performance of the working electrode materials. However, the lowest doping concentration of Gd3+ with NiO could increase the specific capacitances rather than the undoped NiO electrode. As a result, the Ni0.98Gd0.02O electroactive material with good reversibility and long-term stability is a suitable electrode for pseudocapacitive applications. Electrochemical impedance spectroscopy: Nyquist plot EIS has been recognized as one of the primary techniques used to examine the interfacial charge transport phenomena in the prepared electrode materials. EIS measurements were carried out for an applied open circuit potential (OCP) in the frequency range of 100 kHz to 0.1 Hz at ambient condition. Figure 12 shows the typical Nyquist plots (real part, Z′ vs. imaginary part, Z′′) of prepared electrode materials reveals two semicircle behavior, which confirms the existence of two interfacial resistances. The enlarged view of higher frequency semicircle is shown as inset of Fig. 12. Intersect of these semicircles on the real axis of impedance provides charge transport resistances at every interfacial. The high frequency semicircle intercept on real axis provides charge transport resistance (Rct1) at electrode material/platinum interface and the intersection of mid frequency semicircle contributes a total charge transport resistances (Rct2) within the grain interfacial of prepared materials. The obtained semicircles are fitted with an equivalent circuit model [R(QR)(Q(RW))] (shown in Table 4) and the fit parameters are summarized in Table 4. This circuit was modeled as a series resistance (Rs) which is serially connected with two circuit 33



model containing a parallelly connected constant phase element (CPE) of double-layer electric capacitor (Qdl) with a charge-transfer resistor (Rct). Here Rs is the solution series resistance and Cµ is double-layer chemical capacitance of prepared electrode materials. The appearance of an inclined straight-line in the low frequency region often belongs to the phenomena of electrolyte ionic-diffusion resistances during charging/discharging process in the active electrode material, which is generally called as Warburg resistance (Ws)77. It is noteworthy from Table 4 that obtained Rs has no considerable variation for all the prepared materials owing to the usage of common electrode (carbon-felt) and electrolyte solution (KOH). The charge transport resistance Rct2 at grain interior interface shows decreased resistance for Ni0.98Gd0.02O and Ni0.95Gd0.05O compared to other samples which confirms the efficient electron transportation occurred in Ni0.98Gd0.02O and Ni0.95Gd0.05O, respectively. The higher surface area and pore volume tend to increase the interfacial contact between the electrolyte and electrode materials and reduced the Rct1 to some extent. The increased electron transportation at electrode material/electrolyte interface further enhances the electron transport in grain interior and it is responsible for the reduced Rct2 in Ni0.98Gd0.02O and Ni0.95Gd0.05O electrodes. In addition, the chemical capacitance of Ni0.98Gd0.02O and Ni0.95Gd0.05O electrode gets decreased owing to its dominant electron transport kinetics rather than charge recombination. Even though, undoped NiO and Ni0.92Gd0.08O resulted in high chemical capacitance, its charge transport resistances is high due to its reduced surface volume. Further, the Nyquist plots at low frequency region are almost linear, which corresponds to Ws depicted as diffusive resistance of OH− ions of electrolyte within the mesopores of NiO-based electrode. In the present study, the Warburg diffusion resistance of electrolyte is found to be low in Ni0.98Gd0.02O and Ni0.95Gd0.05O-based

34


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electrodes which could enhance the diffusivity of anion resident in electrolyte and leads to increasing the specific capacitance of the active electrode. Conclusions In our present work, we have demonstrated a facile hydrothermal route to synthesize successfully the undoped and Gd3+ doped mesoporous NiO nanoflake-like electrode materials using urea. This approach has been adopted to synthesize mesoporous NiO-based samples with improved electrochemical capacitive behaviors. The phase analysis reveals that the calcined NiO-based products are in hexagonal structure. The synthesis procedures of hydrothermal route include calcination process are found to be significant influence on the surface morphology and porosity formation of the NiO-based samples. HRSEM and HRTEM microstructural studies of the calcined oxide samples at 400 °C for 2 h, demonstrate mesoporous nanoflakes structure. Herein, self-aggregation is supposed to be the crucial factor for the formation of flake-like morphologies. The incorporation of Gd into NiO lattice sites can prevent the growth of the particles and the mesoporous structures obtain in a proper calcination temperature. The large surface area and narrow pore-size distribution evaluated from the N2 adsorption/desorption implied that the samples consisting of mesoporous NiO nanoflakes. The galvanostatic charge/discharge measurement represents a higher pseudocapacitance and a better reversibility for the mesoporous Ni0.98Gd0.02O electrode rather than other (undoped NiO, Ni0.95Gd0.05O and Ni0.92Gd0.08O) electrodes. The mesoporous Ni0.98Gd0.02O nanoflake structures act as an “ion buffering reservoir” to facilitate OH– ion mobility and exhibit a greater specific capacitance value of 1190 F/g at a current density of 2 A/g. In cyclic lifespan stability test, the Coulombic efficiency of Ni0.98Gd0.02O material estimated at 1st and 3000th cycles of GCD are found to be 35



91.39 and 90.79%, respectively. No morphology change is found on working electrodes after 3000 cycles tested. The EIS analysis illustrates the charge-storage mechanism is hindered by the composed electronic/ionic-resistance of the 6M KOH aqueous electrolyte, intrinsic-resistance of the NiO-based electrodes and the contact-resistance at the surface of electroactive NiO-based material and current collector interface. The technique employed in this study shows that lower (2 wt.%) concentration of Gd doped mesoporous NiO flake-like structures with improved electrochemical behaviors may be used for high-performance pseudocapacitor applications. This facile approach for synthesizing Gd:NiO with novel morphology and superior electrode behaviors suggests that it could be an high promising prospective technique to fabricate other nanostructured mesoporous electroactive electrode materials. Supplementary Information (SI) XRD patterns of as-synthesized nickel hydroxides; the W-H analysis; the variations of micro-strain along with the different Gd concentrations; Gd doping induced peak shift on (012) plane; FTIR profiles of as-synthesized nickel hydroxides; SEM images of as-synthesized nickel hydroxide flake-like; the authentic SAED pattern of undoped NiO and Ni0.98Gd0.02O nanoflakes; the summary of specific capacitance values of all the oxide samples from CV and GCD profiles; individual CV and GCD profile of bare carbon-felt, undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O. Acknowledgements One of the authors Dr. G. Boopathi gratefully acknowledges his thanks to National Postdoctoral Program (PNPD)-Coordination of Improvement of Higher Education Personnel

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(CAPES), Brazilian Government, for the financial support and Dr. Natalia for her help in TGA measurement; Dr. K. Ashok Kumar and Mr. N.S. Palani for their support during the electrochemical measurements and discussion. Mr. G.G. Karthikeyan wishes to thank to Department of Science and Technology (DST-Nanomission Project No. SR/NM/NS-02/2011), Govt. of India, for the funding assitance in partial contributions of this work. References (1)

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Figures 105

100

I (2.74%)

Weight loss (%)

95

286ºC

Undoped Gd5 wt.%

256ºC

90 I (12%)

II (22.47%)

85 80 II (16.73%)

376ºC III (2.44%)

75 70 III (4.72%) 364ºC 65 100 200 300 400 Temperature (ºC)

500

600

(015) (006)

(104)

(d)

(012)

(003)

Figure 1. TG profile of as-synthesized undoped and Gd5 doped samples.

Intensity (arbit. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 58

(c) (b) (a) ICDD # 00-022-1189

10

20

30

40 50 2θ (º)

60

70

80

Figure 2. XRD patterns of (a, b, c and d) calcined single-phase NiO samples at 400 °C for 2h: (a) undoped, (b) Gd2, (c) Gd5 and (d) Gd8 doped samples.

46


574.4

842.7 424.7

(c)

1504.8 1384.5

(d) Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3443.3

Page 47 of 58

(b) (a)

4000

3500

3000

2500

2000

1500 −1

1000

500

Wavenumber (cm )

Figure 3. FTIR patterns of (a, b, c and d) calcined nickel oxide samples at 400 ºC for 2h: (a) undoped, (b) Gd2, (c) Gd5 and (d) Gd8 doped samples.

47



(a)

(b)

(c)

(d)

(e)

Page 48 of 58

(f)

Figure 4. SEM images of calcined single-phase of (a) undoped NiO, (b) Ni0.98Gd0.02O, (c) Ni0.95Gd0.05O and (d) Ni0.92Gd0.08O nanoflakes at 400 °C for 2h; The inset demonstrates HRSEM images of all four respective oxide samples; EDX microanalysis spectrum of (e) Ni0.98Gd0.02O and (f) Ni0.92Gd0.08O nanoflakes. 48


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(a)

(b)

(c)

(d)

Figure 5. (a) FESEM image, the corresponding elemental mapping of (b) Ni, (c) Gd and (d) O elements within Gd doped NiO nanoflakes. The inset demonstrates elemental mapping of Gd2:NiO nanoflakes.

49



(a)

(b)

(c)

(d)

Page 50 of 58

(003) (012) (104) (015) (006)

Figure 6. HRTEM images of calcined (a) low-, (b) high-magnification, (c) lattice fringes and (d) SAED pattern of undoped NiO nanoflakes at 400 °C for 2h. The inset HRTEM images were taken from different areas of the samples.

50


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(b)

(a)

7.89nm

(c)

(d)

(003) (012) (104) (015) (006)

Figure 7. HRTEM images of calcined (a) low-, (b) high-magnification, (c) lattice fringes and (d) SAED pattern of Ni0.98Gd0.02O nanoflakes at 400 °C for 2h. The inset (a) HRTEM image was taken from different areas of the sample and (b) EDX microanalysis spectrum.

51



180 3

160 140 120 100 80

Undoped Gd2 Gd5 Gd8

Undoped NiO Gd2 wt.% Gd5 wt.% Gd8 wt.%

0.035

Pore volume (cm /g)

3

Volume adsorbed (cm /g at STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 52 of 58

0.030 0.025 0.020 0.015 0.010 0.005 0.000 0

5

10

15

20

25

30

35

Pore diameter (nm)

60 40

Adsorption

20

Desorption 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Figure 8. N2 adsorption/desorption isotherms of the undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O nanoflakes. The inset illustrates the pore-size distribution plot.

Scheme 1. General plausible formation mechanism of both undoped and Gd doped nickel hydroxide/oxide samples.

52


Page 53 of 58

Intensity (CPS)

Intensity (CPS) 0

Ni (3p) Ni (3s)

1000 0

Ni (1p) LMM Ni (1p) LMM Ni (1p) LMM Ni (2p3/2) Ni (2p1/2) O KLL

Intensity (CPS)

(b)

855

Ni (3p) Ni (3s) Gd (4d) C (1s)

800

870

Satellite

1200

885

1230

Ni (2p)

880

1000

875

1220


2p1/2

600

Satellite

400

865

Binding Energy (eV)

860

1210

Binding Energy (eV)

Satellite

O (1s)

Gd8 wt.%:NiO

200

(d) (ii)

(i)

850

Gd (3d)

1200

Binding Energy (eV)

(1220.99) Gd (3d3/2)

2p3/2

Undoped NiO

800

C (1s)

290

(f)

1190

(1187.86) Gd (3d5/2)

O (1s) 600

289

O (1s)

Intensity (CPS)

Intensity (CPS)

C (1s) 400

288

Binding Energy (eV)

287 C=O

200

C-O

(a)

(c)

(ii)

286

Binding Energy (eV)

C-C

285

Defective sites

Ni-OH

Intensity (CPS)

284

Ni-O Ni-O

(i)

283

(e)

(ii)

(i)

Binding Energy (eV)

527 528 529 530 531 532 533 534 535 536 1180

53

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46


Ni (1p) LMM Ni (1p) LMM Ni (1p) LMM Ni (2p3/2) Ni (2p1/2) O KLL Ni (2s) Gd (3d5/2) Gd (3d3/2)


Carbon-felt Undoped NiO Gd2:NiO Gd5:NiO Gd8:NiO

(a)

0.012 0.008

at 10 mV/s

0.004 0.000

-0.004

at 2 A/g Undoped NiO Gd2:NiO Gd5:NiO Gd8:NiO

(b)

0.5

E (V) vs. (Ag/AgCl)

Current density (A/mg)

Figure 9. XPS wide-scan survey spectra of (a) undoped NiO and (b) Ni0.92Gd0.08O, (c) XPS deconvolve C (1s) spectra of (i) undoped NiO and (ii) Ni0.92Gd0.08O, (d) XPS deconvolve Ni (2p) spectra of (i) undoped NiO and (ii) Ni0.92Gd0.08O, (e) XPS deconvolve Ni (2p) spectra of (i) undoped NiO and (ii) Ni0.92Gd0.08O and (f) XPS deconvolve Gd (3d) spectrum of Ni0.92Gd0.08O.

0.4 0.3 0.2 0.1

-0.008

0.0 0.1

0.2

0.3

E (V) vs. (Ag/AgCl)

700

Undoped NiO Gd2:NiO Gd5:NiO Gd8:NiO

(c)

600

0

0.4

500 400 300 200 100

100

200

300

400

500

600

Time (s)

1400

Undoped NiO Gd2:NiO Gd5:NiO Gd8:NiO

(d)

1200

Specific capacitance (F/g)

0.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 54 of 58

1000 800 600 400 200 0

0

20

40

60

80

100

2

4

Scan rate (mV/s)

6

8

10

12

14

16

18

20

Current density (A/g)

Figure 10. (a) The CV profiles of bare carbon-felt, undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O electrodes at a scan rate of 10 mV/s. (b) The GCD profiles of undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O electrodes at a current density of 2 A/g. (c and d) The specific capacitance values vs. scan rates and current densities, respectively.

54


1500 at 3A/g 1000 500 0 0.5 -500 -1000 0.4 -1500 0.3 -2000 -2500 0.2 -3000 0.1 -3500 -4000 0.0 0 200 -4500 -5000 0 500

(a)

(b) st

1 10 cycles

E (V) vs. (Ag/AgCl)

Undoped Gd2 Gd5 Gd8 400 600 800 1000 1200 1400 1600

Time (s)

1000

1500

2000

2500

Cycle number

3000

Figure 11. (a) The specific capacitance values of working electrodes as a function of cycle number. The inset displays 10 cyclic charge/discharge curves of Ni0.98Gd0.02O working electrode at a current density of 3A/g. (b) Morphology of Ni0.98Gd0.02O after 3000 cycles tested.

4

160

3 -Z'' (Ω )

180

140

-Z'' (Ω)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 55 of 58

2

120

1

100

0

1

2

80

3 4 Z' (Ω )

5

6

Undoped Fit

60

Gd2

40

Gd5

20

Gd8

Fit Fit Fit

0 0

20

40

60

80

Z' (Ω)

100

120

140

Figure 12. EIS profile of undoped NiO, Ni0.98Gd0.02O, Ni0.95Gd0.05O and Ni0.92Gd0.08O nanoflakes at frequency ranges from 100 kHz to 0.1 Hz. The inset displays the magnified picture of the respective EIS profiles.

55



Page 56 of 58

Tables Table 1. The list of the crystallographic lattice parameters obtained for the nanocrystalline oxide samples.

Sample Undoped NiO Ni0.98Gd0.02O Ni0.95Gd0.05O Ni0.92Gd0.08O

Crystallographic lattice parameters Unit cell Crystal a (Å)

b (Å)

c (Å)

Vol. (Å )

structure

D (nm)

2.9619 2.9597 2.9503 2.9473

2.9619 2.9597 2.9503 2.9473

7.2710 7.2652 7.2600 7.2579

55.05 55.25 55.09 54.96

Hexagonal Hexagonal Hexagonal Hexagonal

11.3 8.2 8.2 8.6

3

Strain (ε)

Dε (nm)

0.00426 0.00434 0.00455 0.00467

11.5 11.7 12.3 13.4

Table 2. Surface area, pore diameter and total pore-volume summary of undoped and Gd doped NiO samples. Samples

Surface area (m2/g)

Total porevolume (cm3/g)

Undoped NiO

71.679

0.166

Ni0.98Gd0.02O

96.976

0.248

Ni0.95Gd0.05O

96.493

0.200

Ni0.92Gd0.08O

56.084

0.117

56


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Table 3. A comparative outstanding performances summary of our undoped NiO and Ni0.98Gd0.02O electrodes with earlier reported undoped and metal ions doped NiO-based electrodes. Active material

Route

Morphology

Pure NiO

Hydrothermal

Nanowire

NiO/MnO2 composite Pure NiO La:NiO Pure NiO Cu:NiO Pure NiO

Hydrothermal

Nanowire/ nanosheet Nanosphere Nanosphere Nanoparticle Nanoparticle Nanosphere

Hydrothermal Hydrothermal Combustion Combustion Microemulsion

Conductive substrate Graphite paper Graphite paper Ni foam Ni foam Ni foam Ni foam Stainlesssteel grid

LiOH (1M)

Specific capacitance (F/g) 307

Current density/ Scan rate 1 mV/s

LiOH (1M)

528

1 mV/s

KOH (5M) KOH (5M) KOH (6M) KOH (6M) KOH (1M)

127 253 294 559 121

5 mA/cm2 5 mA/cm2 0.3 A/g 0.3 A/g 1 A/g

Electrolyte

Ag:NiO

Microemulsion

Nanosphere

Stainlesssteel grid

KOH (6M)

345

1 A/g

Pure NiO Gd2:NiO

Hydrothermal Hydrothermal

Nanoflake Nanoflake

Carbon-felt Carbon-felt

KOH (6M) KOH (6M)

780 1190

2 A/g 2 A/g

Ref. 76

60 77 78

Our work

Table 4. Electrochemical parameters of undoped and Gd doped NiO-based electrodes.

Equivalent circuit model

Samples

Rs (Ω)

Rct1 (Ω)

Rct2 (Ω)

C (µF)

Ws

χ2×10–4

Undoped NiO

0.51

3.08

74.52

5.791

0.02579

7.84

Ni0.98Gd0.02O

0.69

0.57

46.18

4.197

0.01376

8.19

Ni0.95Gd0.05O

0.95

0.94

62.32

5.027

0.00641

8.28

Ni0.92Gd0.08O

1.09

5.20

143.91

7.119

0.03136

8.06

57



TOC Graphic

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Dopant Effects of Gd3+ on the Electrochemical ... - ACS Publications

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