New insight into the electrodeposition of NiCo layered double hydroxide and its capacitive evaluation

Journal Pre-proof New insight into the electrodeposition of NiCo layered double hydroxide and its capacitive evaluation You Wang, Zhoulan Yin, Guochun Yan, Zhixing Wang, Xinhai Li, Huajun Guo, Jiexi Wang PII:

S0013-4686(20)30126-2

DOI:

https://doi.org/10.1016/j.electacta.2020.135734

Reference:

EA 135734

To appear in:

Electrochimica Acta

Received Date: 16 October 2019 Revised Date:

15 January 2020

Accepted Date: 17 January 2020

Please cite this article as: Y. Wang, Z. Yin, G. Yan, Z. Wang, X. Li, H. Guo, J. Wang, New insight into the electrodeposition of NiCo layered double hydroxide and its capacitive evaluation, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135734. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

CRediT Author Statement You Wang: Data curation, Writing-Original draft preparation. Zhoulan Yin: Writing- Reviewing and Editing. Guochun Yan: Conceptualization, Methodology. Zhixing Wang: Validation,Funding acquisition. Xinhai Li: Project administration, Investigation .Huajun Guo: Software,Resources. Jiexi Wang: Visualization, Formal analysis.

New insight into the electrodeposition of NiCo layered double hydroxide and its capacitive evaluation You Wanga,b, Zhoulan Yina, Guochun Yana*, Zhixing Wanga, Xinhai Lia, Huajun Guoa, Jiexi Wanga a

School of Metallurgy and Environment & College of Chemistry and Chemical Engineering,

Central South University, Changsha 410083, P.R. China b

College of Materials and Chemical Engineering , Hezhou University, Hezhou 542899, P.R.

China

* Corresponding author, email address: [email protected]; Abstract: Although the peculiarities of electrodeposition in the preparation of metal hydroxides is quite attractive, adequate interpretation of this process still lacks. Here, a novel interpretation of the electrodeposition of nickel cobalt double layered hydroxides (NiCo-LDHs) is proposed. Different from the traditional strategies implemented in the pure nitrate or nitrate-assisted electrolytic baths, the sulfate solution with thiourea (TU) additive is used. Benefited from the chelating and inducing effects of TU, honeycomb-shaped NiCo-LDH nanosheets can be successfully electrodeposited on nickel foam (NF). When performed as free-standing electrodes, these integrated electrodes show a robust capacitive performance. Specifically, NC2S12-15/NF (deposition for 15 min) electrode can deliver remarkable electrochemical properties, such as large capacity (1198 F g-1 at 1 A g-1), excellent rate performance (1000 F g-1 at 100 A g-1) and good cycling stability. Additionally, the 1

NC2S12-15/NF//activated carbon (AC) hybrid capacitor (HC) can sustain extraordinary cycle life (88.3% retention for 10000 cycles) and high energy density (29.1 Wh kg-1), further demonstrating the qualified capacitive behavior of NC2S12-15/NF electrode. This work provides a new insight into the electrodeposition of metal LDHs based on the design of the electrolyte solution. Key words: NiCo-layered double hydroxides; nanosheets; electrodeposition; free-standing; hybrid capacitor 1 Introduction As one kind of energy storage and conversion devices, electrochemical capacitors (ECs), also called supercapacitors, have drawn increasing attention from both industry and academia owing to their high power densities and excellent cycling stabilities[1-5]. Despite these, their low energy densities substantially impede their scalable applications [6, 7]. To alleviate this situation, developing electrode materials with high capacitances/capacities tends to be of great importance and challenge since they are highly responsible for the energy densities of ECs [7-9]. It is well known that electrode materials usually play decisive role in various energy storage systems in terms of capacities, working potentials and cycling performance [10-17]. Hence, selecting the electrode materials is pretty critical for designing ECs with high energy density. Therein, three sorts of electrode materials are available for ECs: (i) capacitive ones, such as carbon-based materials [18-20]; (ii) pesudocapacitive ones, such as RuO2 [21], MnO2 [22, 23], Nb2O5 [24] and so on [25]; (iii) battery-type ones, e.g. Ni, Co oxides [26-29], sulfides [9, 30-36] selenides[37] 2

and hydroxides [38-41]. Among them, the battery-type materials have evoked great attention recently due to their high capacities/capacitances. They have been extensively explored as electrode materials for hybrid capacitors (HCs) that can deliver much higher energy density than conventional electrochemical double layer capacitors [7]. As battery-type materials, transition metal layered double hydroxides, especially NiCo-layered double hydroxides (NiCo-LDHs), formulated as [A2+1-xA3+x (OH)2]x+[By-x/y·mH2O]x-, where A2+and A3+ represent bivalent and trivalent metal cations, By- refers to the intercalated anions (such as CO32-, NO3-, SO42- ,et al) [42, 43], have attracted considerable attention due to their unique layered structures that facilitate charge storage [44-47]. Moreover, many of free-standing electrodes with ultrathin nanosheets [48-52], nanowires [53], microflowers [54] and hierarchical petals [5] shaped NiCo-LDHs as active materials have been successfully fabricated, which show superior electrochemical behaviors because of removing of the low conductive additives [55]. However, the morphologies and production costs of LDHs are tightly associated with their preparation processes. Accordingly, it is vital to choose a suitable synthetic procedure for fabricating desired NiCo-LDHs. So far, hydrothermal [42, 50, 56-59], solvothermal [48, 59, 60], chemical bath deposition [61, 62] and electrodeposition [5, 49, 51, 55, 63] are the commonly used routines for preparing NiCo-LDHs-based free-standing electrodes. Compared with other processes, electrodeposition takes advantages of time-saving, flexible, controllable and environmental friendliness, exhibiting abroad prospects in material production especially in large-scale production [55, 64, 65]. However, the ever 3

reported electrodepositions of NiCo-LDHs were nearly realized in nitrate solutions [65] rather than in sulfate solutions. In contrast, metallic nickel cobalt rather than hydroxides were deposited in the baths of sulfate and chloride electrolytes [66-70]. Thus, obviously, the solution composition, especially the anions play a key role in the electrodeposition of NiCo-LDHs, which should be thermodynamically related to the standard reduction potentials (φθ) of various ions in the deposition bath (Table S1). Specifically, in the nitrate bath, NO3- obtains the electrons prior to H+, Ni2+ and Co2+ to produce OH- ions, leading to the formation of LDHs. While in the sulfate and chloride solutions, the metal ions are prone to be reduced into metals firstly considering the hydrogen evolution overpotential. Besides, most publications researching the electrodepsotion of NiCo-LDHs have stressed too much on the morphology and electrochemical performance evaluations but the profound understandings of the deposition mechanism. Hence, it is challengeable to accomplish the electrodeposition of NiCo-LDH from sulfate electrolyte solution and to probe the underlying mechanism. Herein, we have demonstrated the electrodeposition of NiCo-LDH on NF from sulfate bath containing TU additive. Particularly, this attempt has been conducted based on the following theoretical considerations: (1) Thermodynamically, the addition of TU can inhibit the metal deposition by chelating effect which can lower down the reduction potentials of Ni2+ and Co2+ ions [71, 72]; (2) TU may introduce some sulfur atoms into LDHs [73], so as to boost the electrochemical performance of the LDHs [74]. Specifically, the deposition mechanism of NiCo-LDHs on NF is 4

particularly illustrated. Additionally, the time-resolved electrochemical performance and the topological evolution of the as-prepared NC2S12-x/NF electrodes were investigated. As expected, their notable electrochemical behaviors perfectly confirm their eligible qualities for capacitor application. The HCs consists of NC2S12-15/NF cathode and AC anode can deliver excellent cycling performance and high energy density. 2 Experimental 2.1 Materials The reagents are all analytical grade, which was used without purification. All the aqueous solutions are prepared by using de-ionized water. NF was ultrasonically washed in 1 mol L-1 HCl solution, ethanol and deionized water for 15 min, respectively. 2.2 Electrodes preparation The free-standing NC2S12-x/NF electrodes were prepared via a simple electrodeposition process. In detail, the electrodeposition experiments were conducted in a three electrode cell consisted of NF working electrode (WE), platinum plate counter electrode (CE) and Ag/AgCl (3 mol L-1 KCl) reference electrode (RE). The electrolyte solution contained 0.005 mol L-1 NiSO4·6H2O, 0.01 mol L-1 CoSO4·6H2O and 0.06 mol L-1 TU. All the electrodepositions were performed on a Chenhua electrochemical workstation (CHI660A) under -1.0 V (vs. Ag/AgCl) at 25 °C. The deposits were washed and dried before electrochemical and mass measurements. Here, a series of NiCo-LDH/NF free-standing electrodes, noted as NC2S12-x/NF were 5

prepared. The stoichiometric numbers of NC2S12-x/NF represent the molar ratio of NiSO4, CoSO4 and TU, while x hints the deposition time (min). Meanwhile, NC2S0-15/NF and NC2S0-N-15/NF electrodes were also prepared from pure sulfate and nitrate electrolyte solutions under the same deposition condition as NC2S12-15/NF electrode. The mass of the deposits were measured to be 0.27, 0.47, 0.69, 0.85, 1.03, 1.74, 0.47 and 0.79 mg cm-2 for NC2S12-5/NF, NC2S12-10/NF, NC2S12-15/NF, NC2S12-20/NF, NC2S12-30/NF, NC2S12-30/NF, NC2S12-60/NF, NC2S0-15/NF

and

NC2S0-N-15/NF

electrodes,

respectively.

(SHIMADU,

AUW120D) 2.3 Fabrication of hybrid capacitors The NC2S12-15/NF//AC HC was fabricated with NC2S12-15/NF cathode, AC anode and KOH solution (2 mol L-1). The AC anode was prepared by coating a paste consists of AC, super p, PTFE in a mass ratio of 85:10:5 onto a NF sheet (1cm2). Then the anode was dried and pressed under 20 Mpa for 10 min. The mass ratio of cathode to anode was calculated according to the charge balance equation (1) [38]:
  ∆ =
  ∆

(1)

where m+, m-, C+, C-, ∆V+ and ∆V- are the active mass, specific capacitances (SCs), potential windows of cathode and anode, respectively. The SCs here were calculated from CV curves according to formula (2) [40]:  =

2 ∆


(2)

where Cm, S, v, ∆ and m mean the SC, CV integrated area, potential scan rate, potential window and electrode active mass, respectively. 6

2.4 Material characterizations The structure, morphology, and valence of the materials were characterized by X-ray diffraction (XRD, Rint-2000, Rigaku), scanning electron microscopy (SEM, Sirion 200), transmission electron microscopy (TEM, Tecnai G12), X-ray photoelectron spectroscopy (XPS, PHI5600, PerkineElmer) and energy dispersive X-ray spectroscopy (EDS) respectively. 2.5 Electrochemical measurements The NC2S12-x/NF electrodes were performed in a three electrode cell in a 2 mol L-1 KOH solution, coupled with a platinum plate (CE) and Hg/HgO (RE). The galvanostatic charge-discharge (GCD) and cyclic tests of NC2S12-x/NF electrodes and HC were all performed on a Land battery tester. The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were conducted on CHI660A electrochemical workstation. The SCs of the electrodes and HC were computed from the discharge curves based on formula (3) [45]:  =

∆ ∆

(3)

where Cm is the SC (F g-1), I is the current density (A g-1), ∆ is the discharge time (s) and ∆ is the potential window. The energy density (E, Wh kg-1) and power density (P, W kg-1) of the HC were calculated by equations (4) and (5) [56]: 1  ∆  2   = ∆

=

(4) (5)

Where  (F g-1), ∆ (V) and ∆ (s) are the SC, potential window and discharge time of the device, respectively. And the  is obtained from equation (3). 7

3 Results and discussions 3.1 Deposition mechanism of NC2S12 electrodes Fig. 1 exhibits the schematic illustration of the electrodeposition mechanism of NC2S12 LDHs in a three electrode cell where NF, Pt plate and Ag/AgCl electrode work as WE, CE and RE, respectively. Unlike the deposition conducted in a pure sulfate bath as shown in the upper section of Fig.1, which only contains metal deposition reactions. The deposition in our modified sulfate solution (adding TU additive) as shown in the underneath part of Fig.1 involves at least four reactions to obtain the final product is NiCo-LDHs. Firstly, Ni2+ and Co2+ cations are prone to chelate with TU molecules as expressed by reaction (a), which is the key factor determining the deposition of NC2S12 LDHs. Since it can reduce the activities of these cations by forming stable complex M(TU)n2+ (M=Ni or Co), thermodynamically lowers down the deposition potentials of Ni and Co according to Nernst equation, thus inhibits the metal deposition [72]. Secondly, the hydrolysis of TU as depicted by reaction (b) can release H2S and NH3 molecules that can further produce S2- and OHions, promoting the formation of NC2S12 LDHs. Thirdly, the electrochemical reduction of H2O, as hinted by reaction (c), can generate hydrogen gas and OH-, is dominantly responsible for the deposition of LDHs. Fourthly, the Ni2+, Co2+ react with OH- to form the NC LDHs as described by reaction (d). Inevitably, some sulfides are simultaneously deposited that has been proved to be beneficial for the conductivity of transition metal hydroxide [75]. Meanwhile, some SO42- ions may intercalate into the Van der Waals galleries of NCsS12, which enlarge the diffusion channels of electrolyte ions. More importantly, the potentiostatic current-time transient curve of 8

electrodepositing NC2S12 is also presented in Fig. S1a for profound investigation of the deposition mechanism. As shown in Fig. S1a, the curve can be divided into three zones: (i) the first zone ranging from 0 to a=0.768 s, in which the current sharply declines to a minimum value, corresponds to the charging of electrical double layer resulting in electroadsorption of electroactive substances on NF surface [76]; (ii) The second zone ranging from a to b, in which the current dramatically increases from a minimum to a maximum value, indicates the kinetics-limited and mixed-controlled nucleation and growth processes of NC2S12 [76]; (iii) the third zone ranging from b to 160 s, corresponds to the growth process from particulate nuclei to nanosheets, which is characteristic of diffusion-controlled kinetics [76]. To get more insight into the nucleation of NC2S12-LDHs, the non-dimensional current-time curves of our experimental result together with the theoretical progressive and instantaneous nucleation for 2D and 3D models are presented in Fig. S1b. Apparently, the experimental curve is much closer to the 3D instantaneous one, manifesting that NC2S12 LDHs follow the 3D instantaneous nucleation model [77]. To emphasize the influence of electrolyte composition on the electrodeposition kinetics, the current-time transients curve of the NC2S0 and NC2S0-N electrodes are also exhibited in Figs. S1c and d. Obviously, the NC2S0 and NC2S0-N systems show continuous increasing current in the given time, suggesting that the deposition has not been under diffusion control yet due to the fast mass transfer in these electrolyte solutions without TU. But in the NC2S12 electrolyte solution, the fast migration of Ni2+ and Co2+ ions is substantially inhibited by the strong chelation effect of TU, thus this case is diffusion 9

controlled. Additionally, the inducing times determining the difficulties of nucleation for these systems are also shown in Fig. S1d. As can be seen from Fig. S1, the NC2S12 system exhibits the longest inducing time (tc=0.768 s) compared with the NC2S0 (ta=0.112 s) and NC2S0-N (tb=0.464 s) systems, implicating the slowest nucleation rate for it. Besides, for NC2S0, it displays faster kinetics than NC2S0-N due to the direct metal deposition, in which multiple reactions are included. M  + TU ↔ MTU M = Ni, Co

(a)

NH  CS + H O ↔ H S + 2NH& + CO

(b)

2H O + 2e ↔ H + 2OH 

(c)

 Ni + 2Co +2 − 2)OH  + )S  + *SO + ↔ NiCo OH, S, SO+ - (d)

3.2. Material characterization The morphological evolution of NC2S12 electrode during elecrtodeposition is exhibited in Fig. 2. For tracing the nucleation and growth processes of NC2S12 LDHs, six observation points, namely 5, 10, 20, 40, 80 and 160 s are chosen, which are located in different zones of potentiostatic transient curve as labeled in Fig. S1a. As shown in (a1, a2), (b1, b2) and (c1, c2) which are located in the nucleation zone, numerous dispersed tiny spherical and quasi-spherical nuclei can be explicitly observed on the surface of NF after merely 5 s deposition. Then they become bigger and bigger with time elapses. These observations powerfully prove the above theoretical analyses. When the deposition time is lengthened into the third zone, although only subtle change can be observed in (d1, d2) but significant change arises at 80 s. Specifically, the previously observed spherical nuclei have transformed into 10

crumpled rare nanosheets (Fig. 2e1, e2) which become denser and thicker after 160 s deposition (seen in Fig. 2f1, f2). That is to say that from b to 80 s, the deposition undergoes a diffusion-controlled nuclei growth process which then changes to a diffusion-controlled nanosheets growth process after 80 s. In short, considering the above theoretical analyses and SEM observations, we can get a clear electrodeposition mechanism of NC2S12 LDHs, which can be summarized as below: (1) When a negative potential is imposed on NF electrode, an electrochemical double layer is firstly build due to the electrosorptoion of MTU ions, H2O and TU on the surface of NF. In this case, the distribution of MTU ions is greatly influenced by the large-size of TU molecules, which may result in the odd dispersion of NC2S12 nuclei. (2) The nucleation favored by the electrochemical reduction of water molecules and reversible release of Ni and Co ions from the MTU complexes takes place. It is worth noting that this process should follow a 3D instantaneous nucleation model. Afterwards, with the number of nuclei increases, the adjacent nuclei tends to crash into each other under electro-drive. The nuclei gradually grow bigger and bigger by this way. (3) Because of the continuous mutual collision, the tiny nuclei gradually merge into nanosheet-like deposits, which can be further proved by TEM image. To emphasize the critical role of TU in fabricating NC2S12/NF electrode, two electrodes, namely NC2S0/NF and NC2S0-N/NF were prepared simultaneously from pure sulfate and nitrate baths that have the same total concentration of anions and the 11

same molar ratio of nickel to cobalt, respectively. It can be found from the optical images in Fig. S2 that NC2S12/NF electrode exhibits similar light green color as NC2S0-N/NF electrode that has been well proved to be NiCo-LDHs[38]. But its color looks darker than that of the latter one due to the partial vulcanization. Nevertheless, NC2S0/NF electrode appears nearly the same silver color as NF substrate, implying the metallic nature of its deposits. These scenarios illustrate that TU contributed significantly to the electrodeposition of NiCo-LDH in the sulfate solution. Moreover, the SEM images are obtained to reveal the significant influence of the electrolyte constituent on the morphologies of the deposits. As shown in Fig. S3a and d, in the case of NC2S0, only some irregular island-like metal or alloy grains are sporadically deposited, hinting its characteristic of island nucleation [78]. Furthermore, the EDS spectrum of NC2S0/NF electrode as shown in Fig. S4 suggests it contains only Ni and Co elements, strongly supporting our previous hypothesis. For NC2S0-N/NF electrode, as seen in Fig. S3b and e, some dense and chaotic nanoflakes were observed, suggesting low-oriented growth of the deposits. Excitingly, vertically oriented nanosheets grown on the NF surface for NC2S12/NF electrode after introducing TU as shown in Fig. S3c and f. These observations demonstrate that TU plays a role of growth controlling agent in the fabrication of NC2S12. To carefully explore the microstructure evolution of NC2S12 deposits during electrodeposition, the SEM images of NC2S12-x/NF electrodes (x=5, 10, 15, 20, 30 and 60 (min) with different magnifications are displayed in Fig. 3. Different from those electrodes prepared under very short deposition time as shown in Fig. 2, the 12

nanosheets deposited appear porous interlaced honeycomb-like microstructures. In addition, as increasing the deposition duration from 5 min to 60 min, the thicknesses of the nanosheets gradually increase and the voids among the nanosheets become wider and then get narrower. In short, these porous honeycomb-like nanosheets can provide abundant surface areas for energy storage reactions and rapid mass transportation highways. As a result, the derived NC2S12-5/NF electrode exhibits great potential as promising electrode material for high performance ECs. To illustrate the deposition effect, the elemental distribution of NC2S12-15/NF electrode is also presented by EDS images as shown in Fig. S5. Obviously, every element distributes evenly in the as-obtained material. As can be seen from Fig. 4a, the TEM image of NC2S12-15 nanosheet demonstrates that many nanoparticles with diameters of 100 nm gather together to form dendrites that tightly stick to the bulk nanosheet. More importantly, numerous nanosize voids are formed among these nanoparticles and dendrites, which is favorable for the permeation of electrolyte and the enlargement of electro-active surface. Additionally, the dendrites formation phenomenon further verifies the collision hypothesis discussed in the mechanism section. Meanwhile, the HRTEM presented in Fig. 4b nicely illustrates the poor crystallinity of these nanosheets through some visible semicircular lattice stripes. As shown in Fig. 4c, the d-spacing of these lattice stripes is 0.36 nm, agrees well with the (200) planes of Co3(SO4)2(OH)2·H2O (PDF 22-0227). And some remarkable lattice defects marked by dash white circles can be clearly observed, which may be associated with the 13

intercalations of SO42- ions and H2O molecules. The corresponding SAED pattern (Fig. 4d) displays multiple diffraction rings, indicative of polycrystalline nature of NC2S12 sample. Definitely, the (023) planes (white) of Co3(SO4)2(OH)2·H2O, (400) and (117) planes (yellow) of Ni(SO4)0.3(OH)1.4 (PDF 41-1424) can be well distinguished, suggesting that NC2S12 is a mixture of nickel and cobalt hydroxides with some SO42- ions and H2O molecules. To identify the component of NC2S12 deposit more accurately, the XRD patterns of NC2S12, NC2S12/NF and NF are compared in Fig. 5a. It is obvious that nearly no distinguishable diffraction peaks indexed to Ni(OH)2 and Co(OH)2 phases can be identified for the NC2S12 and NC2S12/NF samples except for some noisy peaks, implying the poor crystallinity of the as-deposited NC2S12. Moreover, from the XRD pattern of NC2S12, the peaks of Ni and Co metals cannot be observed as well, indicating the absence of metal deposition suggested in the mechanism discussions. Fig. 5b exhibits the XPS survey spectrum of NC2S12 deposit. From which clear signals of Ni, Co and O can be observed, demonstrating the successful preparation of NiCo-LDHs. Besides, the weak signal of S is also found, proving our speculation of partial vulcanization and the insertion of SO42-. The high-resolution XPS spectra of Co, Ni, S and O are given in Fig. 5c-f. As shown in Fig.5c, the spectra of Co2p3/2 and Co2p1/2 can be fitted into two separate peaks with two satellites at 785.9 eV and 803.1 eV, respectively, hinting the existence of Co2+ and Co3+. In detail, the peaks at 780.9 eV and 796.8 eV correspond to Co3+, while those at 783.2 eV and 799.5 eV correspond to Co2+ [59, 79]. Analogously, the Ni2p3/2 and Ni2p1/2 spectra (Fig. 5d) can 14

also be assigned to Ni2+ and Ni3+. Specifically, the peaks located at 855.9 eV and 873.4 eV are indexed to Ni2+ and those located at 857.9 eV and 875.4 eV are indexed to Ni3+ [45, 59]. The spectrum of S2p (Fig. 5e) can be indexed to S2- (162.3 eV for S2p3/2 and 163.6 eV for S2p1/2) and SO42- (168.5 eV), respectively, further demonstrating the possible vulcanization during electrodeposition of NC2S12 [33, 80]. Finally, the O1s spectrum can be fitted into four peaks that centered at 530.6 eV (O1), 531.2 eV (O2), 532.3 eV (O3) and 532.9 eV (O4), respectively, which probably correspond to O atoms in Ni-O, Co-O, O-H and H2O structures, respectively[26, 59, 81]. In short, the XPS results illustrate that NiCo-LDHs deprived here are well accordance with its general formula. 2.2 Electrochemical evaluations The electrochemical properties of the NC2S12-x/NF electrodes are investigated systematically as shown in Fig. 6a-e. It can be seen from Fig. 6a that the CV curves display one pair of evident redox peaks in the potential window of 0-0.6 V (vs. Hg/HgO), corresponding to the reversible conversion between Ni2+/Ni3+ and Co2+/Co3+/Co4+ couples, which can be expressed as reactions (1-3) [5, 61]: NiOH + OH  − e → NiOOH+H O

(1)

CoOH + OH  − e → CoOOH+H O

(2)

CoOOH + OH  − e → CoO + H O

(3)

The good symmetric CV curves indicate good reversibility of these electrodes, which is ascribed to their favorable electrochemical resistances endowed by their porous nanostructures. Additionally, the potential gaps between the anodic and 15

cathodic peaks tend to increase with the deposition time, implicating the increment of the polarity stemmed from the thickening of nanosheets. More importantly, the CV curve of pristine NF appears nearly a straight line, indicating its omissible capacitance contribution compared with the other electrodes [59]. For comparison, the CV curves of the NC2S0-15/NF, NC2S12-15/NF and NC2S0-N-15/NF electrodes are also provided in Fig. S6a. It is clear that the NC2S0-15/NF electrode shows much smaller CV loop and higher redox potentials than the other two electrodes, indicating its lowest capacitive response (maybe come from nickel/cobalt oxides on the surface)[59]. While the NC2S12-15/NF and NC2S0-N-15/NF electrodes exhibit analogous CV shape, displaying similar electrochemical behavior. This particular founding further proves the different natures of the deposits derived from different electrodeposition baths. Fig. 6b shows the initial GCD profiles of NC2S12-x/NF electrodes under current density of 1 A g-1. Obviously, every profile presents one pair of charge/discharge voltage plateaus, which is in well accordance with the redox peaks in their CV curves. However, the NC2S12-15/NF electrode shows the minimal potential gap, illustrating its good conductivity. Their discharge specific capacitances (DSCs) at various current densities are exhibited in Fig. 6c and Table S2. When increasing the current density from 1 A g-1 to 10 A g-1, the DSCs decline from 1174, 1082, 1198, 1146, 1162, and 1176 F g-1 to 980, 880, 1060, 1060, 1020 and 1060 F g-1 for NC2S12-x/NF electrodes with x=5, 10, 15, 20, 30 and 60, respectively, corresponding to 83.4%, 81.3%, 88.5%, 92.5%, 87.8% and 90.1% capacitance retentions, respectively. The outstanding rate 16

performance of the NC2S12-x/NF electrodes should be ascribed to their unique nanosheet-like microstructures enabling fast electron and ion transportations. Excitingly, as shown in Fig. 6f, a high capacitance of 1000 F g-1 can be still retained even at 100 A g-1 for NC2S12-15/NF electrode, demonstrating its excellent power-output capability. Moreover, its GCD curves under different current densities preserve well on their shapes except for slight polarities at extreme high current densities as shown in Fig. 7e-f, further indicating its superior rate performance. Meanwhile, the GCD curves and rate performance of the NC2S0-15/NF and NC2S0-N-15/NF electrodes are also displayed in Fig. S6b-c. Apparently, the former shows completely different GCD curve compared with the latter that shows similar GCD curve with NC2S12-15/NF electrode. This observation frankly hints the NiCo-LDH feature of the NC2S12 deposit. However, the NC2S0-N-15/NF electrode exhibits inferior rate response (78.3% retention) in contrast to the NC2S12-x/NF electrodes. Besides, the NC2S0-15/NF electrode delivers a capacitance of merely 230 F g-1 at 1 A g-1, suggesting that it is not NiCo-LDH. The cycling stabilities of the above mentioned electrodes at 5 A g-1are presented in Fig. 5d and Fig. S6d. The NC2S12-15/NF electrode displays the best stability among these NiCo-LDH/NF electrodes as it can keep 81.6% of the initial DSC after 2000 cycles, superior to 52.2%, 71.4%, 67.8%, 77.8% and 63.9% for the NC2S12-5, NC2S12-10, NC2S12-20, NC2S12-30 and NC2S12-60, respectively. Additionally, this result outperforms those of the NC2S0-15/NF and NC2S0-N-15/NF electrodes (Fig. S6d), as well as that of our previously reported Ni(OH)2@Co(OH)2 17

heterostructured

free-standing

electrode[40].

Moreover,

the

as-prepared

NC2S12-15/NF electrode shows good competitiveness as compared with some recently reported NiCo-LDH free-standing electrodes as listed in Table S3. The SEM images and XRD pattern of the cycled NC2S12-15/NF electrode are also characterized as shown in Fig. S7. It is clear that the micromorphology and crystal structure of the cycled electrode keep nearly the same as those of the fresh electrode, further indicating its good cycling performance. Undoubtedly, its robust electrochemical properties ought to be related to its porous honeycomb-like morphology which can provide plentiful reaction sites, short transfer paths for electrons and ions. Additionally, the strong attachment of the deposit on NF substrate ensures the good electronic passage and diminishes the loss of the active materials during cycling test. The experimental and calculated Nyquist plots (NPs) of the NC2S12-x/NF electrodes are provided in Fig. 6e. Through these NPs the kinetic information of these electrodes can be easily acquired by fitting the NPs according to a proper equivalent electric circuit (EEC) as shown in Fig. S8. In the EEC, the element Rs represents the series internal resistances of the electrolyte, active material and current collector, as well as the contact resistances among them, corresponding to the intercept of the Z′ axis in the high frequency range of the NP. And the parallel elements Rct//C indicated by a semicircle in the middle-high frequency range of the NP are related to the charge transfer resistance and interface capacitance across the electrolyte/electrode interface. Rw which corresponds to the sloped line in the low frequency regime of the NP is 18

assigned to the diffusion resistance (Warburg impedance) [82]. According to the fitting results in Table S4, the NC2S12-15/NF electrode possesses the lowest Rct of 0.025 Ω cm2, compared to those of 0.073, 0.295, 0.414, 0.095 and 0.038 Ω cm2 for the NC2S12-x/NF electrodes with x=5, 10, 20, 30 and 60, respectively, suggesting that it has the fastest charge transfer rate. However, from the small Rct values shown in Table S4, we can deduce that all of these electrodes should possess favorable charge transfer capabilities which are benefited from their special nanosheet-like morphologies. Furthermore, Fig. 7a exhibits the CV curves of NC2S12-15/NF electrode at various sweeping rates. Obviously, with increasing the sweeping rate from 5 to 100 mV s-1, the redox peaks increase proportionally, revealing its good rate performance. The distinct redox peaks also indicate the battery-like behavior of this electrode, which is similar with the other NC2S12-x/NF electrodes as shown in Fig. 6a [53]. Besides, the kinetic characteristics of the energy storage in the NC2S12-15/NF free-standing electrode are also comprehensively investigated by plotting the peak currents to the squares of sweeping rates (v1/2) (Fig. 7b) derived from the data in Fig. 7a. A good linear relationship can be observed for both the oxidation and reduction processes of this electrode, demonstrating that its charge storage process is diffusion controlled [53, 59]. More importantly, as suggested by Trasatti, et al, two types of capacitance contribution should be considered for the faradic electrodes, namely capacitive (charge transfer controlled) and diffusion-controlled contributions. Among them, the capacitive capacitance is proportional to the potential scan rate (v), while the diffusion-controlled one is proportional to v-1/2. So the total capacitance can be 19

expressed as the sum of these two types of capacitances as depicted by the equation: / =  + 0 = 123 + const 8/ , where Ct , Cc, Cd, and v are the total specific capacitance, capacitive specific capacitance, diffusion-controlled specific capacitance and scan rate, respectively [25]. Fig. 7c shows the dependence of the DSCs of NC2S12-15/NF electrode on v-1/2. From which Cc=690.3 F g-1 can be obtained by extrapolating the linear fitting line to v-1/2=0. Eventually, the capacitive contribution accounts for 66.1%, 70.0%, 77.1%, 83.1% and 96.9% at 5, 10, 30, 50 and 100 mV s-1, respectively. It is evident that the intercalation of ions into LDH becomes slower and shallower as increasing the scan rate, resulting in the increment of surface capacitive contribution. What is more, a hybrid capacitor consists of NC2S12-15/NF cathode and AC anode was assembled. The electrochemical performance of this HC was evaluated for its practical application. The theoretical mass ratio of the cathode to the anode was determined to be ~0.2. But the practical mass ratio is 0.26. Fig. 8a exhibits the CV curves of the cathode and anode within different working potential windows at 10 mV s-1, which is used to determine the optimized potential window of the HC and to calculate the coupling capacitances of the cathode and anode. Fig. 8b exhibits the CV curves of the as assembled NC2S1215/NF//AC HC at 10 mV s-1 within various working potential windows (WPW) from 1.0 to 1.6 V, respectively. As increasing the WPW from 1.0 V to 1.6 V, the device maintains similar shapes of the CV curves. No evident oxygen evolution is observed even up to 1.6 V, indicating that the safe WPW can be extended to 1.6 V. Fig.S9 shows the GCD curves of the NC2S12-15/NF//AC 20

HC operated at various WPWs at 1 A g-1. As seen, the areas of the GCD curves increase largely when the WPW increases from 1.0 V to 1.6 V, implying the increment of the DSCs with the augment of the WPWs. Moreover, as seen in Fig. 8c, the CV curves with a WPW of 1.6 V can preserve a stable near-rectangular shape with increasing the potential sweeping rate from 5 to 100 mV s-1, showing good capacitive performance. Meanwhile, the current densities and areas of the CV loops increase proportionally with increasing of the scan rates, demonstrating superior rate performance. Fig. 8d gives the GCD profiles of this device at different current densities (based on the total mass of the active materials of the cathode and anode) at a relative low current density of 1 A g-1. One evident potential plateau emerges at ~1.55 V in the charge branch, which should be ascribed to the oxygen evolution reaction. However, the potential plateau disappears at the higher current densities, such as 2, 4 and 8 A g-1, which may be due to the increment of the oxygen evolution overpotential. Prominently, the good linearity of the discharge curves indicates the good capacitive behavior of the device. The DSCs computed from the discharge curves according to formula (3) are 81.8, 61.3, 54.3 and 48.7 F g-1 at 1, 2, 4 and 8 A g-1, respectively, corresponding to a capacitance retention of 59.5% when the current density increases from 1 to 8 A g-1. This results demonstrates that HC exhibits good rate performance. Additionally, the cycle life of the HC was also evaluated as shown in Fig. 8e. Nearly 88.3% of its initial capacitance can be preserved after 10000 successive cycles at 2 A g-1, with no evident change observed in the GCD profiles of the 1st and 10000th cycles (inset in Fig. 8e), revealing outstanding cycling stability of 21

this HC. Moreover, the Ragone plot (energy density vs. power density) of this HC is also presented in Fig. 8f. In detail, a high energy density of 29.1 Wh kg-1 can be obtained at a power density of 903.1 W kg-1, and an energy density of 17.3 Wh kg-1 can still be retained even at a high power density of 5661.8 W kg-1. As compared with some

recently

reported

NiCo-based

HCs

(Table

S5),

the

as-fabricated

NC2S12-15/NF//AC HC still behaves comparable or competitive electrochemical performance, showing its promising applicability. Additionally, the self-discharge behavior of the HC is also evaluated by the voltage holding test as shown in Fig. S10. It can be observed that the voltage of the HC declines quickly from 1.6 V to 0.26 V in the beginning, then the voltage continues to decrease to 0.01 V during the first 2 h. This result suggests that this HC is still far from the commercial applications and lots of efforts are needed to improve its performance. 4. Conclusions Herein, we have demonstrated the feasibility of electrochemical deposition of NiCo-LDH on NF from a sulfate solution in presence of TU additive. Surprisingly, the favorable honeycomb-like NiCo-LDH nanosheets can be successfully deposited on NF under this condition. As illustrated in the deposition process, TU plays a key role in the nucleation and growth of NC2S12 nanosheet due to its strong complex capability. Benefited from the unique nanoscale morphologies of the deposits and the robust electronic contact between the deposits and the substrate, the as-prepared NC2S12-x/NF free-standing electrodes can deliver good capacitive behaviors. Moreover, the NC2S12-15/NF electrode exhibited the best electrochemical 22

performance. Besides, the NC2S12-15/NF//AC hybrid capacitor can perform superior cycling stability (88.3% retention for 10000 cycles) and high energy density of 29.1 Wh kg-1, exhibiting promising application potential. In all, this work provides a new insight into the electrodeposition of nickel cobalt based hydroxides in view of programming the electrolyte solution composition. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (51674295). We also thank the Advanced Research Center of CSU for performing the HRTEM examination. References [1] M. Winter, R.J. Brodd, Chem. Rev., 104 (2004) 4245-4269. [2] L. Yu, G.Z. Chen, J. Power Sources, 326 (2016) 604-612. [3] M.R. Lukatskaya, B. Dunn, Y. Gogotsi, Nat. Commun., 7 (2016) 12647. [4] S.D. Liu, S.C. Lee, U. Patil, I. Shackery, S. Kang, K. Zhang, J.H. Park, K.Y. Chung, S.C. Jun, J. Mater. Chem. A, 5 (2017) 1043-1049. [5] G.P. Xiong, P.G. He, D.N. Wang, Q.Q. Zhang, T.F. Chen, T.S. Fisher, Adv. Funct. Mater., 26 (2016) 5460-5470. [6] F. Wang, X. Wu, X. Yuan, Z. Liu, Y. Zhang, L. Fu, Y. Zhu, Q. Zhou, Y. Wu, W. Huang, Chem. Soc. Rev., 46 (2017) 6816-6854. [7] Y. Wang, Y. Song, Y. Xia, Chem. Soc. Rev., 45 (2016) 5925-5950. [8] Q. Li, H. Yao, F. Liu, Z. Gao, Y. Yang, Electrochim. Acta, 321 (2019) 134682. [9] J. Zhao, Z. Li, T. Shen, X. Yuan, G. Qiu, Q. Jiang, Y. Lin, G. Song, A. Meng, Q. Li, J. Mater. Chem. A, 7 (2019) 7918-7931. [10] J. Zhao, Z. Wang, J. Wang, H. Guo, X. Li, W. Gui, N. Chen, G. Yan, Energy Technol., 10.1002/ente.201800361 (2018). [11] X. Ge, X. Li, Z. Wang, H. Guo, G. Yan, X. Wu, J. Wang, Chem. Eng. J., 357 (2019) 458-462. [12] W. Pan, W. Peng, G. Yan, H. Guo, Z. Wang, X. Li, W. Gui, J. Wang, N. Chen, Energy Technol., 10.1002/ente.201800253 (2018). [13] G. Li, Z. Yin, H. Guo, Z. Wang, G. Yan, Z. Yang, Y. Liu, X. Ji, J. Wang, Adv. Energy Mater., 9 (2019) 1802878. [14] T. Li, X. Li, Z. Wang, H. Guo, Y. Li, J. Wang, J. Mater. Chem. A, 5 (2017) 13469-13474. [15] J. Chen, L. Li, L. Wu, Q. Yao, H. Yang, Z. Liu, L. Xia, Z. Chen, J. Duan, S. 23

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26

Figures

Fig.1 Schematic illustration of electrodepositing NiCo-LDHs on NF.

Fig.2 SEM images of the NC2S12-5s /NF (a1, a2), NC2S12-10s /NF (b1, b2), NC2S12-20s /NF (c1, c2), NC2S12-40s /NF (d1, d2), NC2S12-80s /NF (e1, e2) and NC2S12-160s /NF (f1, f2) electrodes.

Fig.3 SEM images of the NC2S12-5/NF (a1-a3), NC2S12-10/NF (b1-b3), NC2S12-15/NF (c1-c3),

NC2S12-20/NF (d1-d3), NC2S12-30/NF (e1-e3) and NC2S12-60/NF (f1-f3) electrodes.

Fig.4 (a) TEM image, (b) HRTEM image of Ni/Co LDH nanosheet scratched from the NC2S12-15/NF electrode, (c) inverse FFT image of the area labeled by the yellow square in (b), (d) SAED image of the above mentioned nanosheet.

Fig.5 (a) XRD patterns of the NC2S12, NC2S12/NF and NF, respectively. (b) Survey XPS spectrum of the NC2S12-15/NF electrode. (c)-(f) High-resolution XPS spectra of Co2p (c), Ni2p (d), S2p (e) and O1s (f), respectively.

Fig.6 (a) CV curves of the NC2S12-x/NF electrodes and NF substrate at 10 mV s-1, (b) initial GCD profiles at 1 A g-1, (c) the specific capacitances as a function of the current densities, (d) cycling stabilities at 5 A g-1 of the NC2S12-x/NF electrodes, (e) Nyquist plots of the NC2S12-x electrodes, the hollow symbols and the line-solid-symbols represent the experimental ones and the calculated ones according to EEC in Fig.S7 (inset is the magnified plots), (f) DSCs of the NC2S12-15/NF electrode as a function of the current densities from 1 to 100 A g-1.

Fig.7 (a) CV curves at various scan rates from 5 to 100 mV s-1, (b)the oxidation and reduction peak currents as a function of v1/2, (c) the plot of specific capacitances versus v1/2, (d) capacitance contribution percentages of the capacitive contribution (yellow) and the diffusion contribution (red) as a function of scan rates, (e) GCD profiles at different current densities from 1 to 100 A g-1, (f) enlarged GCD curves at high current densities of 20, 30, 50 and 100 A g-1 for the NC2S12-15/NF electrode.

Fig.8 (a) CV curves of the NC2S12-15 cathode and AC anode at 10 mV s-1, (b) CV curves at 10 mV s-1 within different potential ranges, (c) CV curves at different scan rates, (d) GCD curves at different current densities, (e) cycling performance and (f) Ragone plot of the NC2S12-15/NF//AC hybrid capacitor. Inset in Fig.8 (e) exhibits the GCD curves of the 1st (black) and 10000th (red) cycles for the hybrid capacitor, respectively.

Declaration of interests ✔The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: