Rajkumar
Patel
*a,
Jung Tae
Park
b,
Madhumita
Patel
c,
Jatis Kumar
Dash
d,
E. Bhoje
Gowd
e,
Rajshekhar
Karpoormath
f,
Amaresh
Mishra
g,
Jeonghun
Kwak
*a and
Jong Hak
Kim
*h
aSchool of Electrical and Computer Engineering, The University of Seoul, Seoul 02504, Korea. E-mail: patelrajku@gmail.com; jkwak82@uos.ac.kr
bDepartment of Chemical Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, South Korea
cDepartment of Chemistry and Nano Science, Ewha Womans University, Seodaemun-gu, Seoul 120-750, South Korea
dDepartment of Physics, SRM University-AP, Amaravati, Guntur-522502, India
eMaterial Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum 695019, Kerala, India
fDepartment of Pharmaceutical Chemistry, College of Health Sciences, University of Kwa Zulu Natal, Durban 4000, South Africa
gSchool of Chemistry, Sambalpur University, Burla, India
hDepartment of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea. E-mail: jonghak@yonsei.ac.kr
First published on 23rd November 2017
Currently, energy storage devices draw considerable attention owing to the growing need for clean energy. The depletion of fossil fuels and the generation of greenhouse gases have led to the development of alternative, environmentally friendly energy storage devices. Supercapacitors with high power densities are excellent devices for energy storage. Although carbon-based materials are widely used in such devices, their non-faradic behavior in electrical double layer capacitors (EDLCs) limits the maximum power density that can be generated. In contrast, the faradaic mechanism of transition metal hydroxides results in better capacitance rates along with good stability during cycling. This review is confined to nickel cobalt layered double hydroxides (NiCo LDHs) classified based on the fabrication of electrodes for application in supercapacitors. We discuss the growth of the active LDH material in situ or ex situ on the current collector and how the synthesis can affect the crystal structure as well as the electrochemical performance of the electrode.
Nickel hydroxide is widely used in rechargeable alkaline batteries because of its good power density, energy density, and proton recyclability.5 However, during the cycling process, there is an increase in the surface area of the nickel hydroxide, leading to poor contact between active particles. This leads to increases in resistance of the electrode reaction; to address this, cobalt oxide can be used.6 The low electrical conductivity of nickel hydroxide (10−5 to 10−9 S cm−1) has limited its electrochemical storage applications. Several studies have described the synthesis of nickel hydroxide for supercapacitor application by using solvothermal and chemical deposition processes.7–14 Nickel hydroxide showed mixed supercapacitor performance. Nickel hydroxides have also been deposited by various methods on modified substrates to enhance the electrochemical performance.15–17 The performance of nickel hydroxide can be enhanced by preparing composites containing various carbon materials.18–28
Cobalt hydroxide is also used for supercapacitor applications because of its high electrical conductivity, but it shows comparatively lower supercapacitive behavior than nickel hydroxide.29 The supercapacitance and cycling stability performance of cobalt hydroxide prepared by different methods have varied widely.30–38 A nanocomposite of cobalt hydroxide and graphene oxide showed good capacitive behavior.39 To combine the properties of nickel and cobalt hydroxides, they can be mixed or doped together at specific ratios.40–42 Cobalt hydroxide has even been intercalated with zinc ions to enhance its electrochemical performance.43
Nickel hydroxide and cobalt hydroxide both show good specific capacitance properties. To improve the overall performance in terms of specific capacitance and charge discharge rate of an electrode, a mixed metal hydroxide seems to be a better proposition than single metal hydroxides. In this review, we discuss how the current collectors are crucial to the performance of electrodes and describe the effects of crystallinity, morphology, surface area, and conductivity of mixed or layered double hydroxide materials based on recent literature (Table 1).
Method | Electrolyte | Specific capacitance (F g−1),a | Morphology/substrate | Reference |
---|---|---|---|---|
a mA h g−1. | ||||
Hydrothermal | 1 M KOH | 1170 | Hexagonal nanosheet | 45 |
Precipitation | 6 M KOH | 1030 | 1 D nanorod | 46 |
Hydrothermal | 2M NaOH | 886.7 | Nanorod | 47 |
Chemical bath deposition | 2 M KOH | 1938 | Nanoflake | 48 |
Hydrothermal | 6 M KOH | 174.3a | Filmy nanoflakes | 49 |
Precipitation | 1 M KOH | 4160 | Nanosheets | 53 |
Electrodeposition | 1 M KOH | 2543 | Nanopetals | 54 |
Hydrothermal | 6 M KOH | 2286.7 | Nanosheets | 61 |
Hydrothermal | 6 M KOH | 1600 | Interconnected flower-like nanostructures | 62 |
Hydrothermal | 6 M KOH | 1170 | Nanosheets | 74 |
Hydrothermal | 2 M KOH | 1803.6 | 3D microflowers | 75 |
Hydrothermal | 6 M KOH | 2164 | Urchin | 76 |
Hydrothermal | 6 M HOH | 369 | Nanohexagons | 77 |
Hydrothermal precipitation | 6 M KOH + 15 g L−1 LiOH | 2193 | Doughnut | 78 |
Precipitation | 1 M KOH | 774 | Crimpled nanosheets | 79 |
Precipitation | 6 M KOH | 2614 | Nanoparticles | 80 |
Precipitation | 3 M KOH | 2548 | Nanosheets | 81 |
Precipitation | — | 2228 | Nanosheets | 82 |
Hydrothermal | 3 M KOH | 1911.1 | Crumpled silk | 83 |
Hydrothermal | 6 M KOH | 1691 | Nanosheets | 84 |
Hydrothermal | 2 M KOH | 811 | Nanosheets | 85 |
Precipitation | 3.5 M KOH | 835 | Sheet-on-sheet | 86 |
Precipitation | 6 M KOH | 1980.7 | 3D flower | 87 |
Chemical bath deposition | 2 M KOH | 1151 | Nanoflake | 88 |
Hydrothermal electrodeposition | 1 M KOH | 1.64 F cm−2 | Nanowire–nanosheet | 89 |
Hydrothermal | 1 M KOH | 2682 | Nanosheets | 91 |
Hydrothermal | 1 M LiOH | 1624 | Nanoflakes | 92 |
Hydrothermal electrodeposition | 3 M KOH | 3.54 C cm−2 | Urchin-like nanosheets | 93 |
Hydrothermal | 1 M KOH | 170a | Nanosheet | 94 |
Hydrothermal | 3 M KOH | 1938 | Nanoflake | 95 |
Hydrothermal | 3 M KOH | 2654.9 | Nanosheets | 96 |
Hydrothermal | 1 M KOH | 2184 | Nanosheets | 98 |
Chemical vapour deposition | 1 M KOH | 502 | Nanorod | 101 |
Chemical bath deposition | 6 M KOH | 1082.6 | Spherical clusters | 102 |
Electrodeposition | 1 M KOH | 0.88 F cm−2 | Nanosheets | 103 |
CVD electrodeposition | 2 M KOH | 2170 | Nanoflake | 104 |
Electrodeposition | 1 M KOH | 2046 | Nanosheets | 105 |
Electrodeposition annealing electrodeposition | 1 M KOH | 5.71 F cm−2 (5.5 mA cm−2) | Nanosheets | 106 |
Electrodeposition | 1 M KOH | 2294 | Nanosheets | 107 |
Electrodeposition | 6 M KOH | 1760 | Nanosheets | 108 |
Electrodeposition | 1 M KOH | 2105 | Nanosheets | 109 |
Electrodeposition | 3 M KOH | 1201 | Nanosheets | 110 |
Electrodeposition | 2 M KOH | 1371 | Nanosheets | 111 |
Electrodeposition | 2 M KOH | 1213 | Nanosheet | 112 |
Precipitation | 2 M KOH | 2360 | Nanodisc | 113 |
Chemical bath deposition | 1 M KOH | 1847 | Nanorod | 114 |
Precipitation | 6 M KOH | 2633 | Nanoflake | 115 |
Electrodeposition | 2 M KOH | 765 F cm−3, (15.3 F cm−2) | Nanosheet | 116 |
Electrodeposition | 1 M KOH | 2442 | Nanosheet | 121 |
Fig. 2 Typical TEM images of the as-prepared Co0.2Ni0.8(OH)2 hexagonal nanosheets at (A) low; (B) middle; (C) high magnification; and (D) HRTEM image. The inset in (D) is the FFT image of the selected area. Reproduced from ref. 45, with permission of Elsevier. |
Fig. 3 SEM images showing the morphology evolution from nanosheets, nanorods to nanoparticles at Ni:Co ratios: (a) 1:0, (b) 1:4, and (c) 0:1. Reproduced from ref. 46, with permission of Elsevier. |
Fig. 4 Low and high magnification SEM images of the nanostructure arrays obtained at different immersion times: (A) and (B) NWA; (C) and (D) NWPA-6 h; (E) and (F) NWPA-12 h; (G) and (H) NWPA-18 h. Reproduced from ref. 47, with permission of The Royal Society of Chemistry. |
Fig. 5 (a) FE-SEM images of (a) carbon cloth (CC), (b and c) conformally coated CoNi0.5LDH nanoflakes and (d) non-conformally coated CoNi0.5LDH nanoflakes. Reproduced from ref. 48, with permission of Elsevier. |
Fig. 6 SEM images of (a) Ni0.74Co0.26(OH)2, (b) Ni0.65Co0.35(OH)2, (c) Ni0.47Co0.53(OH)2, (d) Ni0.34Co0.66(OH)2, and (e) Ni0.28Co0.72(OH)2, respectively. Reproduced from ref. 49, with permission of Elsevier. |
Co(OH)2 + OH− ↔ CoOOH + H2O + e− | (1) |
Ni(OH)2 + OH− ↔ NiOOH + H2O + e− | (2) |
Hu et al. reported that binary hydrous Co–Ni oxides are more suitable for electrochemical supercapacitors than for rechargeable batteries50 which is less supported by some of the recent reports.51,52 But the majority of the reports and reviews mention about the supercapacitor nature of NiCo LDH.1,53–60
However, in the bimetallic hydroxide, the interlayer space is occupied by ethylene glycol, which inhibits the transformation of the α-phase into the β-phase. The absence of ethylene glycol from the interlayer of the monometallic hydroxide results in easy transformation into the β-phase because of which fast deterioration of the cycling stability occurs. The effect of hydrolyzing agents like urea, KOH, and Na2CO3, in the presence of ethylene glycol has been investigated.49 The use of urea leads to the formation of a flower-like morphology, and the presence of ethylene glycol increases the size of the nanoflowers to more than 500 nm. Complexation between metal ions and ammonia as well as high temperature and pressure conditions in an autoclave has been shown to enhance the solubility of the metal hydroxide, resulting in the formation of large metal hydroxide nanoflakes. When the ratio of nickel to cobalt approaches 1:1, dissolution–recrystallization of the metal hydroxide becomes easier, leading to the formation of larger nanoflakes and nanoflowers. Crystallization occurs in the mixed α- and β-phase. Ni0.74Co0.26(OH)2 and Ni0.28Co0.72(OH)2 show the highest surface areas of 162.3 and 122.7 m2 g−1, respectively. The Ni0.28Co0.72(OH)2 electrode has a specific capacitance of 206.7 F g−1 at a scan rate of 1 mV s−1 due to the high electrical conductivity of cobalt, although the surface area is less than that of the Ni0.74Co0.26(OH)2 electrode.
The effect of the surfactant cetyltrimethylammonium bromide (CTAB) on the morphology of NiCo hydroxide has also been studied.74 Addition of CTAB results in the formation of hexagonal nanoplates with smooth surfaces. In the absence of CTAB, the scobinate hexagonal nanoplates have greater surface roughness. Scobinate hexagonal nanoplates have a specific capacitance of 294.32 F g−1 at a current density of 1 A g−1, while that of the smooth nanoplates is 127 F g−1 under the same conditions. This difference is because of the higher surface area of scobinate hexagonal nanoplates (19.32 m2 g−1) than that of smooth nanoplates (12.36 m2 g−1).
The morphologies of NiCo LDHs change with the nickel and cobalt composition, which in turn dictates the electrochemical performance.45 CoNi LDHs crystallize in a hexagonal nanosheet morphology. An increase in the concentration of nickel in LDH composition reduces the size of nanosheets. Co(OH)2, Co0.8Ni0.2(OH)2, Co0.5Ni0.5(OH)2, Co0.2Ni0.8(OH)2, and Ni(OH)2 nanosheets have 1 μm, 400 nm, 200 nm, 100–150 nm, and 160 nm diameters respectively. However, the d-spacing of the (101) lattice plane increases as the cobalt ratio increases in the mixed NiCo hydroxide. The Co0.2Ni0.8(OH)2 electrode has the highest specific capacitance of 1840 F g−1 at a current density of 1 A g−1. The specific capacitance of the electrode is reduced to 1240 F g−1 when the current density increases from 1 to 4 A g−1, which corresponds to a capacitance retention of 67.39%.
Carbonate and nitrate anions intercalated in NiCo LDHs with different nickel and cobalt compositions result in an enhanced interlayer spacing of 9.22 Å.75 The resulting material has a 3D flower morphology comprising nanosheets with a thickness of 30–40 nm. The electrode with high active material loading (6 mg cm−2) has a specific capacitance of 1803 F g−1 at a current density of 1 A g−1 and 40% capacitance retention when the current density is increased to 10 A g−1. For Co7Ni3(OH)2, the capacitance retention is 61.9% under similar conditions, indicating that the higher the cobalt percentage is, the better the rate capability is. However, the electrode with the maximum nickel ratio has the highest capacitance, which might be due to charge hopping between nickel and cobalt ions. The binary hydroxide has binary couples of Co2+/Co3+ and Ni2+/Ni3+ that participate in redox reactions, resulting in better electronic conductivity, which is added by a large surface area of the 3D structure.
Ni(OH)2–Co(OH)2 with a 2:1 molar ratio has an urchin-like hollow microsphere morphology with a diameter of 1.5 μm and needle-like nanorods of 400 nm in length and 20 nm in width (Fig. 7).76 This material crystallizes in a mixed α- and β-phase and has a specific capacitance of 2164 F g−1 at a current density of 1 A g−1 due to its BET surface area of 134 m2 g−1.55 It showed good cycling stability of 56.4% capacitance retention after 500 cycles at 20 A g−1. The Ni(OH)2@Co(OH)2 core–shell structure has a nanohexagon morphology and could be tuned by varying the temperature and concentration of hydrazine monohydrate.77 The material has a specific capacitance of 396 F g−1 at a current density of 0.3 A g−1 due to its high surface area of 23.3 m2 g−1. It shows good cycling stability with 95% capacitance retention after 2500 cycles at a current density of 1 A g−1.
Fig. 7 SEM (a), FESEM (b) and (c), TEM (d) and (e) images, and EDS (f) pattern of the Ni(OH)2–Co(OH)2 composite. |
Nickel hydroxide crystallized in the α-phase was deposited with cobalt hydroxide, which did not change the phase.78 This material has a specific capacitance of 2193 F g−1 at a current density of 2 A g−1 with a good capacitance retention of 63.33% when the current density is increased to 20 A g−1. Nickel hydroxide under similar conditions has 45.7% capacitance retention and good cycling stability with 69.1% capacitance retention after 1000 cycles at a current density of 5 A g−1. These studies revealed that, whether hydroxides of nickel and cobalt are in LDH form or trace of one present in other hydroxide, they act in tandem, which enhances the electrochemical performance of an electrode.
When polyvinyl pyrrolidone (PVP) is used instead of EG in the solution for precipitation of NiCo LDH, the binary hydroxide crystallizes in the α-phase with a porous structure, and the diameters of nanoparticles ranged from 10 to 30 nm.80 The electrode prepared with 3 mg cm−2 mass loading has a BET surface area of 109.8 m2 g−1 with a porosity of 1.5–5 nm; it was easy for solvated ions with diameters of 0.6–0.76 nm to penetrate the inter-gallery spacing. The electrode has a specific capacitance of 2614 F g−1 at a current density of 1 A g−1 with a capacity retention of 85.9%, even at 10 A g−1. The percentage of cobalt between 0.570% and 0.443% gave the optimum capacitance.
Epoxide propylene was used to prepare a NiCo LDH with ammonium persulfate as a hydrolyzing agent.81 This crystallizes in a hydrotalcite-like structure. Sulfate group intercalation in the LDH structure increased the inter-gallery spacing and the material has a nanosheet-like morphology. The specific capacitance of the electrode made up of this material is 2548 F g−1 at a current density of 0.9 A g−1, and it showed a good capacitance retention of 76.9% at a current density of 23.2 A g−1.
NiCo LDHs synthesized by varying the composition ratio, reaction time, and pH of the reactant crystallize in microsphere form with an average size of 2.5 μm. A three-dimensional flower with a hierarchical architecture comprising numerous interconnected nanosheets is observed (Fig. 8).82 Reproducing LDH morphologies is always very difficult when scaling up synthesis; in this case, it remained intact during the scaling-up process. LDH crystallizes in the hydrotalcite α-Ni(OH)2 phase. The strongest (003) diffraction peak is very high and narrow, indicating good crystallinity, with a d-spacing value of 0.89 nm, which is higher than those of α-Ni(OH)2 (0.72 nm) and α-Co(OH)2 (0.46 nm). Intercalation of inorganic anions, such as CO32−, NO3−, and OH−, which functions as electroactive species in the redox reaction, enhanced the d-spacing. The material has a BET surface area of 70.78 m2 g−1, and the effect of the cobalt concentration on the morphology of the NiCo LDH was studied in detail. In the absence of cobalt, the thickness of the nanosheets in the nanoflowers is 50 nm; this decreased to 30 nm as the Ni:Co ratio was increased to 4:6. NiCo LDH with this higher cobalt ratio has a more uniform and porous structure because of the transformation of the crystal structure from β-Ni(OH)2 into the hydrotalcite phase. A further increase in the cobalt ratio (2:8, Ni/Co) resulted in structural collapse because of the poor crystallinity of Co(OH)2. The optimized reaction time and pH are 15 h and 8.72, respectively. Electrodes prepared with 5–8 mg cm−2 of active materials show a specific capacitance of 2228 F g−1 at 1 A g−1, with 32% capacitance retention.
Fig. 8 (a) Typical and (b) partial SEM images of LDH nanosheets based on 3D flower-like hierarchical Ni/Co-LDH microspheres; the inset part in (a) displays the actual sight of a large-scale preparation strategy of the Ni/Co-LDH microsphere material. Reproduced from ref. 82, with the permission of American Chemical Society. |
The composite of LDH and r-GO prepared in the presence of L-ascorbic acid (LAA) resulted in crystallization in the β-phase.84 The presence of LAA lowers the agglomeration of exfoliated r-GO, which enhances the mobility of the electrolyte ions. The NiCo–OH/r-GO electrode has a specific capacitance of 1691 F g−1 at a current density of 0.5 A g−1, with a capacitance retention of 82.6% when the current density is increased to 10 A g−1. The capacitance retention after 1000 charge–discharge cycles was 100% for NiCo–OH/r-GO electrodes, indicating good conductivity. This is attributed to the intimate contact between NiCo–OH and r-GO, which provided good electron mobility, as well as the large interface between the metal hydroxide and r-GO. The CoNi(OH)2/r-GO composite was prepared in the presence of ammonium hydroxide (hydrolyzing agent) and hydrazine.85 A high mass loaded (4.5–5.5 mg cm−2) electrode has a specific capacitance of 1294 F g−1 at a current density of 0.5 A g−1 with a good rate capacitance of 68.8% at a current density of 2 A g−1. When the hydrothermal reaction time increases from 10 to 12 h, the rate capability of the resulting electrode increased to 76% because of the higher crystallinity of the active material. However, specific capacitance decreased with the enhancement of crystallinity. This indicates a trade-off between crystallinity, rate capability, and specific capacitance, indicating that it would be difficult to enhance both specific capacitance and rate capability simultaneously.
The NiCo LDH/CNT composite with 16–17% CNTs has a hydrotalcite-like structure with an enlarged d-spacing of 9.5 Å due to the intercalation of CO32− and SO42− anions in the crystalline lattice.88 Composite nickel cobalt LDH with a 1:2 molar ratio of nickel:cobalt has a specific capacitance of 1151 F g−1 at a current density of 1 A g−1 with a good capacitance retention of 61% at a very high current density of 70 A g−1. The electrode has 77% capacitance retention after 10000 cycles at a current density of 10 A g−1. This might be due to the generation of conducting CoOOH during the charge–discharge process, which increases the overall performance of the electrode. Intercalated SO42− ions enhance the interlayer spacing as well as the conductivity. CNTs enhance the electron mobility and reduce the electrochemical polarization. The presence of the carbon material in the composite LDH electrode enhances electronic conductivity, which results in good capacitance retention and enhances the cycling life of the electrode.
Fig. 9 (a) SEM image of CFP before (inset) and after the growth of NiCo2O4 nanowires. (b) High-magnification SEM image of NiCo2O4 nanowires grown on CFP. (c) TEM image and HRTEM image (inset) of 2 NiCo2O4 nanowires. (d) Diffraction pattern of a NiCo2O4 nanowire. (e) SEM image of a CoDH coating on NiCo2O4 nanowires grown on CFP. (f) TEM image of CoDHs/NiCo2O4 nanowires grown on CFP. Reproduced from ref. 9, with the permission of American Chemical Society. |
NiCo LDHs were deposited on zinc oxide nanoflake (ZnONF) and zinc oxide nanowire (ZnONW) current collectors.92 The porosity of ZnONF was around 25 m2 g−1 which is about 12 times higher than that of ZnONW (1.75 m2 g−1). When the scan rate is increased from 20 to 100 mV s−1, the shape of the redox peak remains intact, indicating the good electronic conductivity of the current collector. The specific capacitance of the nanoflake structure is 1624 F g−1 at a current density of 10 A g−1 which is about 1.6 times higher than the specific capacitance of the nanowire structure and 5 times higher than that of the pristine NiCo LDH. The electrode has a capacitance retention of 71.4% when the current density is increased to 100 A g−1. A nanoflake structure shows good cycling behavior, with 94% capacitance retention after 2000 cycles at a current density of 40 A g−1. The better performance of the nanoflake structure than the nanowire is due to the higher surface area and lower internal resistance. In situ-grown structures show better performance than the electrode prepared with the use of binders.
NiCo LDH nanoflakes were deposited on a NiCo2S4 (NCS) current collector with a nanorod morphology by electrodeposition.93 This electrode has a high capacity of 2.31 C cm−2 at a current density of 0.5 mA cm−2 and a good capacitance retention of 69.9% when the current density is increased to 20 mA cm−2. The electrode has an excellent cycling stability of 78% after 4000 cycles at a scan rate of 50 mV s−1. The advantage of the core–shell structure is an increase in electron mobility at the electrode and plenty of space for deposition of the shell material. Core–shell electrodes made using NiCo LDHs show good electric conductivity, high specific surface area, suitable mesoporosity, and a good electrochemical response.
NiCo LDH prepared in the presence of CTAB as a surfactant crystallizes in the hydrotalcite phase with a nanoplate-array-like morphology.94 The pH of the medium dictates the size of the nanoplates, below 5.5 pH, the thickness of the nanoplate is less than 500 nm and above 5.5 pH the thickness is 500 nm. At the optimized pH of 5.5, the porosity of the sample is 3–5 nm with a BET surface area of 92.4 m2 g−1. The material has the best specific capacity of 178.8 mA h g−1 at a current density of 10 A g−1 with a capacitance retention of 63.78% at a very high current density of 40 A g−1. NiCo LDH that was deposited in the presence of CTAB as a surfactant has an ultrathin-sheet-like morphology and a BET surface area of 84.7 m2 g−1.95 The electrode exhibits good electrochemical performance with a specific capacitance of 2654.9 F g−1 at a current density of 1 A g−1 and a good capacitance retention of 33.69% when the current density is increased to 20 A g−1. It showed 77% capacitance retention after 1500 cycles at a current density of 10 A g−1. NiCo LDH was prepared in the presence of hexamethylenetetramine (HMTA) as a hydrolyzing agent.96 It has a specific capacitance of 1734 F g−1 at a current density of 6 A g−1 and a capacitance retention of 66.1% when the current density is increased to 30 A g−1, which are higher than those reported by Salunkhe et al.97
Zheng et al. prepared a NiCo LDH on a nickel sheet; a nickel/cobalt ratio of 1:2 yielded the optimum performance.98 Porous LDH nanosheets are aligned vertically on the nickel sheet. The resulting electrode has a specific capacitance of 2184 F g−1 at a current density of 1 A g−1 with a good capacitance retention of 68.4% when the current density is increased to 20 A g−1. It too showed good cycling performance (88.5% capacity retention after 2000 cycles at a current density of 1 A g−1).
Ni0.5Co1.5(OH)2CO3 nanowires prepared on a nickel foam substrate and converted to Ni0.5Co1.5(OH)2 by dipping in an alkaline solution for extended period exhibited a nanowire nanoplate (NWNP) morphology and a brucite crystal structure in the β-phase.47 The obtained material has a BET surface area of 24.97 m2 g−1 and a specific capacitance of 928.4 F g−1 at a current density of 5 mA cm−2, as well as good capacity retention (81.1%) when the current density is increased to 50 mA cm−2. The electrode showed 82–85% capacitance retention after 1000 cycles at a current density of 30 mA cm−2. The unusual penetration depth of the electrode is 20 nm, and its thickness is around 10 nm, which is sufficient for full penetration.
NiCo LDH was deposited onto carbon fabric in situ and its properties were compared with the same material prepared ex situ.48 The in situ structure has a nanoflake morphology with a surface area of 212.5 m2 g−1 and a porosity of 3 nm, whereas the ex situ prepared material has a surface area of 136 m2 g−1. The nanoflake material has a specific capacitance of 1938 F g−1 at a current density of 1 A g−1 and a capacity retention of 80% at 50 A g−1 and a good capacitance retention of 95.4% after 3000 cycles at 20 A g−1. The ex situ electrode has a specific capacitance of 1292 F g−1 and lower capacitance retention and cycling stability than the in situ electrode. In other words, the in situ grown electrode shows better electrochemical performance than the electrode prepared with the use of a binder.
Titanium nitride nanotubes (TiN NTA) with length of 100–120 nm were electrodeposited with a NiCo LDH that has a nanosheet-like rippled silk morphology.54 The resulting material has a specific capacitance of 2543 F g−1 at a scan rate of 5 mV s−1 and showed 26% capacitance retention when the scan rate is increased to 500 mV s−1. The electrode showed excellent cycling stability (95.75% capacitance retention after 5000 cycles at a scan rate of 100 mV s−1 in CV tests).
CNTs were grown on a nickel foam substrate and electrodeposited with a NiCo LDH. Both Ni(OH)2 and Co(OH)2 crystallize in the rhombohedral phases, with bands appearing in the (003), (100), and (001) planes.102 An electrode prepared with an active mass of 0.9 mg cm−2 has a specific capacitance of 2170 F g−1 at 1 mA cm−2 and 80.9% capacitance retention when the scan rate is increased to 20 mA cm−2. It has 61.5% retention after 5000 charge discharge cycles at a scan rate of 20 mA cm−2. An electrode without CNTs has a three-fold lower BET surface area than the electrode with CNTs, and the former has a specific capacitance of 1136 F g−1 at a current density of 1 A g−1 with a capacitance retention of 55.9% when the current density is increased to 20 A g−1. Density functional theory (DFT) calculations revealed that Ni(OH)2 shows semiconductor behavior with a band gap of 3.43 eV; this gap reduced to 3.06 eV for Ni0.5Co0.5(OH)2 with an increase in the cobalt concentration, resulting in a better energy storage device.
Another group grew CNTs on nickel foam and electrodeposited NiCo LDH with CNT diameters of 20–250 nm, a length of 10 μm, and density around 16 mg cm−2 (Fig. 10).103 The sp2 carbons of the CNTs resulted in the sp2-C–Ni covalent bond formation, and hence better interaction between the CNTs and the nickel foam. Advantages of this current collector are its high intrinsic electrical conductivity, strong interfacial adhesion, and large specific surface area. It functioned as a good template for the preparation of 3D hierarchical core–shell structures of NiCo LDH@CNT with a thickness around 27 nm. An electrode with an active mass of 8.5 mg cm−2 has a specific capacitance of 2046 F g−1 at a current density of 1 A g−1 with 65.2% capacitance retention when the current density is increased to 15 A g−1. The NiCo LDH electrode without CNTs exhibits similar behavior when the mass loading is 0.54 mg cm−2; when the mass loading is increased to 2.29 mg cm−2, electrochemical performance is reduced to less than half of the original performance. This indicates that the presence of CNTs supports good performance, even with higher mass loading.
Fig. 10 (a) Over-view, (b and c) high magnification FESEM images of the NiCo-LDH@CNT electrode and (d) HRTEM image of the NiCo-LDH@CNT core–shell structure. The inset (d) is the corresponding SAED pattern taken from NiCo-LDH nanoflakes. Reproduced from ref. 105, with permission of The Royal Society of Chemistry. |
NiCo LDH electrodeposited onto nickel foam and annealed at a high temperature for conversion to oxide was used to prepare LDH@NiCo2O4 electrodes.104 In the NiCo2O4@NiCo(OH)2 core–shell nanostructure, mesoporous NiCo2O4 nanosheets with good electrical conductivity provided electron superhighways for charge storage and delivery. The open spaces in this structure can act as ion reservoirs to reduce the diffusion length, resulting in a reduction in transport resistance. Enhancement of intercalation/deintercalation of ions in turn enhanced the utility of the active mass. The NiCo2O4@Co0.33Ni0.67(OH)2 electrode prepared with an active mass of 5.5 mg cm−2 has a specific capacitance of 1045 F g−1 at a current density of 1 A g−1, which is about three times higher than the pristine nickel cobalt oxide. The electrode showed a capacitance retention of 83.7% when the current density is increased to 50 A g−1.
The Ni structured film was deposited on nickel foam grown by electrodeposition, and then NiCo LDH was deposited using the same method.105 The amount of active mass deposited varied from 0.67 to 4.01 mg cm−2. Optimum electrode behavior was obtained with an active mass of 0.67 mg cm−2. The specific capacitance of the electrode is 2294 F g−1 at a current density of 5 A g−1, and it has a capacitance retention of 68% at a very high current density of 100 A g−1. It also showed good cyclability (89.7% capacitance retention after 5000 charge discharge cycles). The specific capacitance of the electrode is reduced to 1560 F g−1 when the active mass of the electrode increased to 2.01 mg cm−2 with a capacitance retention of 37% under similar conditions. The good electrochemical behavior of the electrode is attributed to the absence of aggregation in the NiCo(OH)6 nanosheets, the stable 3D layered structure, and good conductivity of the dual 3D Ni current collectors.
NiCo LDH electrodeposited step-by-step with different compositions of nickel and cobalt precursors was used to obtain a concentration gradient of LDH to avoid aggregation of different phases.106 The nickel-rich inner layer ensures high capacitance and the cobalt-rich outer layer facilitated electron transportation, resulting in considerably increased cycling stability and rate performance. An LDH electrode with an active mass of 0.23 mg cm−2 had two typical pairs of redox peaks in cyclic voltammetry. One pair of redox peaks was obtained from the Ni(OH)2 electrode pair of Ni2+/Ni3+, and two redox pairs of peaks were obtained for the Co(OH)2 electrode pair of Co2+/Co3+ and Co3+/Co4+. The electrode has a specific capacitance of 1760 F g−1 at a current density of 1 A g−1 and a capacitance retention of 62.5% when the current density is increased to 100 A g−1.
NiCo LDH electrodeposited on a carbon textile has a nanosheet thickness of 10–15 nm with a vertical height of 1.2 μm and showed good surface roughness.107 The electrode prepared with an active mass of 0.3 mg cm−2 has a specific capacitance of 2105 F g−1 at a current density of 2 A g−1 and a capacitance retention of 56.5% when the current density is increased to 20 A g−1. The electrode showed good cycling stability (89.5% capacitance retention after 2000 charge discharge cycles at a current density of 10 A g−1). The performance of the electrode remains intact after bending.
NiCo LDH electrodeposited on Ni foam in electrolyte solution containing nitrates of cobalt and nickel at a 2:1 ratio.108 Nanosheet thickness increased with increasing deposition time. The NiCo LDH electrode with an active mass loading of 1.23 mg cm−2 has a specific capacitance of 1767.0 F g−1 at a scan rate of 5 mV s−1.
Nanoflake-structured electrodeposited NiCo LDH has a d-spacing value of 7.67 Å, which is in between the d-spacing values for Ni(OH)2 and Co(OH)2 (7.56 and 7.75 Å, respectively).109 The electrode has a high BET surface area of 355 m2 g−1, a specific capacitance of 1372 F g−1 at a current density of 1 A g−1, and a capacitance retention of 67.7% when the current density is increased to 30 A g−1. NiCo LDH was grown on a stainless-steel substrate by electrodeposition.110 Co0.66Ni0.34 LDH has a specific capacitance of 1313 F g−1 at a scan rate of 5 mV s−1 and 88.6% capacitance retention when the scan rate is increased to 100 mV s−1. The specific capacitance retention was 77% after 10000 CV cycles at a scan rate of 100 mV s−1.
Fig. 11 (a–f) SEM micrographs of CoxNi1−x(OH)2 on the graphene foam at different magnifications. The inset of panel f shows TEM of the CoxNi1−x(OH)2 material. Reproduced from ref. 112, with the permission of American Chemical Society. |
MWCNT paper was prepared by a floating catalyst chemical vapor deposition method (Fig. 12).113 NiCo LDH deposited onto the MWCNT paper has a nanoflake morphology with a thickness of 200 nm and a width of 3 nm. For Ni/Co at a 3/1 mole ratio, resulting in Co0.4Ni0.6(OH)2, uniform deposition was observed. CNTs with a larger diameter are good for uniform LDH deposition. The CNT/Co0.4Ni0.6(OH)2 electrode prepared with an active mass of 9.3 mg cm−2 has a specific capacitance of 2633 F g−1 at a current density of 0.7 A g−1 with a capacitance retention of 63.2% when the current density is increased to 14 A g−1. The specific capacitance of the CNT/Co0.4Ni0.6(OH)2 electrode reaches 107% after 500 charge discharge cycles at a current density of 2 A g−1.
Fig. 12 Optical image of CNT/Co0.4Ni0.6(OH)2 hybrid paper (a); surface morphology of pristine CNT paper (b); and CNT/Co0.4Ni0.6(OH)2 hybrid paper (c, d). Reproduced from ref. 113, with the permission of American Chemical Society. |
Fig. 13 (a) GP foam uniformly covered by NCHPs at low magnification. (b) Smaller vertical NCHPs densely grown on relatively larger GPs (NCHP electrodeposition duration of 0.5 min). (c and d) Close-ups of a GP decorated by a large amount of smaller NCHPs. Reproduced from ref. 114, with permission of John Wiley and Sons. |
Exfoliated graphite foil (FEG) was prepared by an electrochemical exfoliation method, stuck to polyimide tape, and then NiCo LDH was deposited on the foil by chemical bath deposition.121 An electrode with an active mass of 0.44 mg cm−2 has a specific capacitance of 2442 F g−1 at a current density of 1 A g−1 with a capacitance retention of 85.5% at a high current density of 50 A g−1. When the active mass is increased to 1 mg cm−2, the specific capacitance decreases to 2011 F g−1. This is similar to what was observed for an electrode of CoNi(OH)2 integrated graphene foam, with a specific capacitances of 1847 F g−1 at 5 A g−1 and 1268 F g−1 at 60 A g−1 with 68.6% retention at a mass loading of 0.09 mg cm−2.112 NiCo LDH on ZnO nanowires with a mass loading of 0.9 mg cm−2 shows specific capacitances of 1624 F g−1 at a current density of 10 A g−1 and 1331 F g−1 at a current density of 50 A g−1, retention of 82%.92 NiCo LDH nanoflakes grown on ZnO nanowires integrated into carbon cloth with a mass loading of 0.98 g cm−2 show a specific capacitance of 1927 F g−1 at 2 A g−1 and 1546 F g−1 at 30 A g−1, with 81% capacitance retention.122
To improve the electrochemical performance of electrodes, a minimum active mass loading is required. The performance of electrodes needs to be at higher discharge current densities for practical applications. To realize enhanced electrode efficiency, the conductivity of the electrode also needs to be enhanced. It is possible to enhance the rate capability of an electrode either by modifying the current collector or by preparing a composite active material. The cycling stability of the electrode is another important factor that must be considered from an application point of view.
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