Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... 687–698 EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH) 2 ELECTRODEPOSITED ON NICKEL FOAM AS HIGH-PERFORMANCE SUPERCAPACITORS WITH APPLIED POTENTIAL AND Co(NO 3 ) 2 CONCENTRATION RAZVOJ ELEKTRO-KEMI^NIH LASTNOSTI ELEKTRI^NO NANE[ENEGA Co(OH) 2 NA NIKLJEVO PENO Z UPORABO ELEKTRI^NEGA POTENCIALA IN KONCENTRACIJE Co(NO 3 ) 2 KOT VISOKOKVALITETNEGA SUPER-KONDENZATORJA Zhongjie Lu, Jun Li * , Wanli Jia, Yunqiang Jiang, Yongfei Juan School of Materials Engineering, Shanghai University of Engineering Science, Shanghai, 201620, China Prejem rokopisa – received: 2018-12-26; sprejem za objavo – accepted for publication: 2019-04-11 doi:10.17222/mit.2018.276 Co(OH)2/NF electrodes with a high specific capacitance and an excellent cyclic stability were successfully prepared by directly electrodepositing Co(OH)2 on nickel foam (NF). The synthesis and nucleation mechanisms of Co(OH)2 were revealed. The evolution in morphology and electrochemical performance with deposited potential and Co(NO3)2 concentration were investigated. The mechanism in terms of the increase in specific capacitance after 1000 cycles was explored. The results indicated that Co(OH)2 was nucleated by instantaneous nucleation, and deposited by a single electrochemical reaction NO3 – +7H 2O+8e – NH4 + + 10OH – followed by a chemical reaction Co 2+ + 2OH – Co(OH) 2 . A high potential accompanied with a high Co(NO3)2 concentration accelerated the growth of the Co(OH)2, causing a decrease in the specific surface area of Co(OH)2. The highest specific surface area was obtained in the Co(OH)2/NF electrode prepared at –0.65 V in a 0.1 mol·L –1 Co(NO3)2 solution, at which a highest specific capacitance was measured by CV (836 F·g –1 ) and galvanostatic charge/discharge (725 F·g –1 ). Its specific capacitance was drastically increased from 836 to 1116 F·g –1 when it was immersed into a KOH solution and underwent 1000 cycles, which resulted from the removal of the incorporated Co(NO3)2 into the active substances (increasing the active sites) and the transformation of the -Co(OH)2 to -Co(OH)2 (enhancing the electrode conductivity). Keywords: electrochemical deposition, -Co(OH) 2 , nucleation mechanisms, potential, concentration, supercapacitors Avtorji v prispevku opisujejo uspe{no sintezo Co(OH)2/NF elektrod z visoko specifi~no kapaciteto in odli~no cikli~no stabilnostjo. Le-to so izvedli z direktnim elektri~nim nanosom oz. depozicijo Co(OH)2 na nikljevo peno (NF). Razvili so sintezo in odkrili mehanizme nukleacije oz. tvorbe kali Co(OH)2. Raziskali so njen razvoj in morfologijo nanosa ter njene elektrokemijske lastnosti z depozicijskim potencialom in koncentracijo Co(NO3)2. U~inke pove~anja specifi~ne kapacitete so raziskovali po 1000 ciklih. Rezultati ka`ejo, da se je nukleacija Co(OH)2 zgodila s t.i. spontano nukleacijo in da je do depozicije pri{lo z naslednjo elektrokemi~no reakcijo: NO3 – +7H 2O+8e – NH4 + + 10OH – , kateri je sledila kemi~na reakcija Co 2+ + 2OH – Co(OH) 2 . Visok potencial, zdru`en z visoko koncentracijo Co(NO3)2 je pospe{il rast Co(OH)2, kar je zmanj{alo presek specifi~ne povr{ine Co(OH)2. Le-ta je bil najve~ji pri elektrodi Co(OH)2/NF, pripravljeni pri –0.65 V in v raztopini 0.1 mol·L –1 Co(NO3)2. Pri teh pogojih so izmerili najvi{jo specifi~no kapaciteto nanosa (836 F·g –1 ) s cikli~no voltametrijo (CV) in galvanostati~no naelektritev/razelektritev (725 F·g –1 ). Njegova specifi~na kapaciteta je drasti~no narasla z 836 F·g –1 na 1116 F·g –1 , ko je bil potopljen v raztopino KOH in izpostavljen 1000 ciklom, kar je povzro~ilo spremembo vgrajenega Co(NO3)2 v aktivno substanco, zaradi pove~anja aktivnih polo`ajev, in pretvorbo -Co(OH)2 v -Co(OH)2 (pove~anje prevodnosti elektrode). Klju~ne besede: elektrokemi~na depozicija, -Co(OH)2, mehanizmi nukleacije, potencial, koncentracija, superkondenzatorji 1 INTRODUCTION The shortage of energy and the environmental pollution are increasingly serious, which is accelerating the development of renewable and clean energy conver- sion and storage systems. Supercapacitors, as a new type of energy-storage device that can make up for the traditional storage devices in the energy/power density, have attracted considerable attention. 1,2 The electrode material is regarded as a decisive factor responsible for the electrochemical performance of supercapacitors. Cobalt hydroxide (Co(OH) 2 ) has become a research hot spot due to its high theoretical specific capacitance of up to 3460 F·g –1 , controllable layer spacing/layered struc- ture and high electrochemical reaction activity. 3–8 A large number of investigations into the synthesis of Co(OH) 2 electrode by different methods (such as the hydrothermal method, 4,7,9 chemical deposition method, 7 vapor deposi- tion process 11 and electrochemical deposition 11–13 ) have been carried out. L. B. Kong et al. 4 synthesized Co(OH) 2 nanoflakes by the hydrothermal method with cobalt chlo- ride hydrate solution and NaOH as the reactants. Co(OH) 2 powder (80 %) was mixed with acetylene black (7.5 %), conducting graphite (7.5 %) and tetrafluoro- Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 687 UDK 66.087:669.24:669.058.6 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(5)687(2019) *Corresponding author's e-mail: jacob_lijun@sues.edu.cn ethylene (5 %). The electrode was fabricated by pressing the resulting paste on a nickel gauze with a load of 10 MPa. The specific capacitance and specific energy of the electrode reached 72.4 F·g –1 and 92.7 Wh·kg –1 within the potential range of 0–1.6 V, respectively. It demonstrated a good cycling performance with a capacitance attenu- ation of 6.8 % over 1000 cycles. Y. F. Tang et al. 6 syn- thesized monodispersed -Co(OH) 2 nanowires by a facile hydrothermal process with a hexadecyltrimethyl ammonium bromide (CTAB) as a soft template, the morphology and size of which were controlled by regulating the CTAB content and the reaction time. Then, 80 % b-Co(OH) 2 nanowires, 15 % acetylene black and 5 % polyvinylidene difluoride (PVDF) were mixed in the ethanol to form a homogeneous slurry. The slurry was coated on the pre-cleaned Pt meshwork and dried in vacuum at 80 °C to obtain the electrode. The results of the electrochemical tests indicated that the specific capacitance of the electrode was 358 F·g –1 at a current density of 0.5 A·g –1 . J. P. Cheng et al. 14 prepared the Co(OH) 2 electrode by pressing a mixture of Co(OH) 2 prepared by the hydrothermal method, acetylene black and polyvinylidene fluoride (PVDF) in the weight ratio of 8:1:1. Its specific capacitance reached 153 F·g –1 at a scanning rate of 5 mV·s –1 .Z.A.Huetal. 15 synthesized the four -Co(OH) 2 powders with different anions salts (CoCl 2 ·6H 2 O, Co(NO 3 ) 2 ·6H 2 O, CoSO 4 ·7H 2 O, and Co(CH 3 COO) 2 ·4H 2 O), polyethylene glycol and NH 3 ·H 2 O by a chemical precipitation route. The electrodes were also prepared by pressing mixtures containing as-pre- pared powder, acetylene black, conducting graphite and polytetrafluoroethylene (PTFE) binder (75:10:10:5 in weight ratio) onto a nickel foam. The specific capaci- tances of the samples with intercalated anion chloride, nitrate, acetate, and sulfate were (697, 638, 526, and 420) F·g –1 , respectively. Up to now, Co(OH) 2 is mainly synthesized in the form of a powder, which cannot be used directly as electrodes. The preparation of the electrode usually includes three processes, corresponding to the synthesis of Co(OH) 2 powder, the mixing of Co(OH) 2 , conductive materials (carbon black or acetylene black) and binders (polytetrafluoroethylene, polyvinylidene fluoride) in certain proportions, electrode assembly. 4,6,12,15 . The whole route is comparatively complicated, which can result in the low reproducibility of the electrode performance. Moreover, the electrode conductivity is weakened, result- ing from the additives. The electrodeposition method can effectively solve the above shortcomings, by which the electrode can be directly prepared by depositing the active substances on different substrates. Moreover, its precise controllability, ease of operation, high deposition rate, good repeatability and low cost also accelerate its application in the electrode preparation. Deng et al 13 synthesized Co(OH) 2 petal nanometer flakes in a Co(NO 3 ) 2 solution by electrochemical deposition with carbon-fiber paper (CFP) as the base. Then, a-Co(OH) 2 was transformed into b-Co(OH) 2 in the KOH solution. The specific capacitance measured was 800 F·g –1 at a current density of 2 A·g –1 , and 95.7 % of the initial value was remained after 8000 cycles. C. M. Zhao et al. 16 deposited graphene on foamed nickel by plasma vapor deposition, and then electrodeposited Co(OH) 2 nano- sheets on the graphene/foamed nickel substrate in an aqueous solution containing 0.1 mol·L –1 Co(NO 3 ) 2 . The specific capacitance of the composite electrode material was 693.8 F·g –1 at a current density of 2 A·g –1 .A tt h e same time, the residual capacitance of the electrode was 91.9 % after 3000 cycles of charge and discharge under a high current density of 40 A·g –1 .M.Lietal. 17 firstly syn- thesized the precursors of Ni/Si–MCPs by electrodepo- siting Ni on silicon microchannel plates (MCPs) in the solution of NiCl 2 ·6H 2 O and H 3 BO 3 , then further prepared the Co(OH) 2 /Ni/Si-MCPs electrode by continuous elec- trochemical deposition in the Co(NO 3 ) 2 ·6H 2 O solution. The electrode exhibited a specific capacitance of 697.56 F·g –1 at a current density of 2 mA·cm –2 and a retention rate of 91.20 % after 2500 cycles. It is well known that the electrochemical perform- ance of an electrode is closely related to its morphology and structure when the electrode material is deter- mined. 13,18 The morphology and structure of the electrode material prepared by electrochemical depo- sition mainly depend on the deposition potential 21 , temperature 19,20 , time 21 and electrolyte. 15 At present, research mainly focuses on the electrochemical perform- ance of the electrode produced under the fixed con- ditions, or its change resulting from the variation in a certain factor. There are few reports about the effect of the interaction between the two even more factors on the electrochemical performance. Moreover, the electro- deposition mechanism of Co(OH) 2 is less studied. In this study, Co(OH) 2 was electrodeposited on nickel foam to form the electrodes in the solution containing NaNO 3 and Co(NO 3 ) 2 . The deposited process/nucleation mechanisms of Co(OH) 2 were revealed. The evolution in morphology of Co(OH) 2 with the change in applied potential –0.60 V, –0.65 V, –0.70 V, –0.75 V, –0.80 V, –0.85 V, –0.90 V, –0.95 V and Co(NO 3 ) 2 concentration (0.1 mol·L –1 , 0.9 mol·L –1 ) was investigated in detail. The electrochemical performance of those electrodes depo- sited at different potentials/Co(NO 3 ) 2 concentrations were evaluated by means of cyclic voltammetry (CV) and galvanostatic charge/discharge methods. Finally, the suitable potential and Co(NO 3 ) 2 concentration were con- firmed, at which the electrode exhibited a high specific capacitance and an excellent cycling stability. In parti- cular, the mechanism about the increase in specific capacitance of the electrode after 1000 cycles was explored in depth. Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... 688 Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 2 EXPERIMENTAL PART 2.1 The synthesis/nucleation mechanisms of Co(OH) 2 Cobalt nitrate [Co(NO 3 ) 2· 6H 2 O] (Sinopharm Chemi- cal Reagent Co., Ltd) and sodium nitrate (NaNO 3 ) (Shanghai Titan Scientific Co., Ltd) with analytical grade were selected to prepare two aqueous solutions containing 0.075 mol·L –1 NaNO 3 and 0.1 mol·L –1 /0.9 mol·L –1 Co(NO 3 ) 2 . The investigations were carried out on an electrochemical workstation system (CHI 6082D, CH Instruments Ins, Shanghai, China) with a conventional three-electrode system at room temperature. Nickel foam with a size of 20 mm × 20 mm × 0.5 mm (Shanxi Power Source Battery Materials Co., Ltd., Taiyuan, Shanxi, China) and graphite with a size of 25 mm × 20 mm × 1 mm (Beijing Electrical Carbon Factory, Beijing, China) were used as the working electrode and counter electrode, respectively. They were degreased with an acetone solution in an ultrasonic bath, and then treated in 1 mol·L –1 HCl solution and deionized water for 10 min at room temperature in turn. Then, the two electrodes with a height of 10 mm were immersed into the solutions, between which a 10 mm distance was controlled. A saturated calomel electrode (SCE) was selected as the reference electrode. The synthesis process of Co(OH) 2 was investigated by the cyclic voltammetry (CV) test. The potential was swept from0Vto–1V ,andincreased to0Vatascann- ing rate of 5 mV·s –1 . The nucleation mechanism of Co(OH) 2 was revealed by the chrono-amperometry test with the applied potentials of –0.60 V, –0.65 V, –0.70 V and –0.80 V and –0.95 V for 30 min. 2.2 The electrodeposition of Co(OH) 2 The electrodeposition of Co(OH) 2 was carried out in the two solutions with 0.075 mol·L –1 NaNO 3 and 0.1 mol·L –1 /0.9 mol·L –1 Co(NO 3 ) 2 . Different potentials (vs. SCE: –0.60 V, –0.65 V, –0.70 V, –0.75 V, –0.80 V, –0.85 V, –0.90 V and –0.95 V) were applied for the elec- trodeposition with a time of 30 min. Prior to the depo- sition, nickel foam was dried at room temperature, then weighed using a Sartorius SQP electronic balance (0.01 mg, Sartorius, Beijing, China). After the deposition, the nickel form covered with Co(OH) 2 was washed three times in succession in deionized water for 10 min. Then, it was dried at room temperature, and weighed again to obtain the weight of the deposit by subtracting the weight of nickel foam. 2.3 The tests for electrochemical performance of Co(OH) 2 electrodes The electrochemical performance of the Co(OH) 2 electrodes was evaluated by CV and galvanostatic charge/discharge tests. A 2 mol·L –1 KOH aqueous solu- tion was selected as the supporting electrolyte. The scanning potential of the CV was ranged from –0.2 V to 0.5 V at a scanning rate of 5 mV·s –1 . Galvanostatic charge/discharge was performed in the potential range of –0.1 V to 0.45 V at a current density of 4 A·g –1 . 2.4 The characterization of deposited Co(OH) 2 Prior to the electrochemical tests, the evolution in morphology of the deposits prepared at the applied potentials of –0.60 V/–0.65 V/–0.95 V in the solutions with 0.1 mol·L –1 and 0.9 mol·L –1 Co(NO 3 ) 2 was inves- tigated in detail by a HITACHI S-4800 scanning electron microscope (SEM, Hitachi Limited, Tokyo, Japan). After the electrochemical tests, the morphology of the deposit prepared at –0.65 V in the solution with 0.1 mol·L –1 Co(NO 3 ) 2 was checked again by SEM. Before and after the electrochemical tests, the changes in the chemical state of the Co in the deposit prepared at –0.65 V in the solution with 0.1 mol·L –1 Co(NO 3 ) 2 were identified by an X-ray photo-electron spectroscope (XPS; ESCALAB 250Xi, Thermo Fisher Scientific, Massachusetts, USA). The narrow-scan spectra were acquired with a Kratos Axis Ultra spectrometer using a monochromatic Al–K source. The energy step of 0.1 eV was selected for the spectra. The binding-energy scale was calibrated accord- ing to the C 1s peak (at 781 eV) for the adventitious Co on the analyzed sample surface. The lattice fringes of the deposits for the XPS analyses were obtained by a transmission electron microscope (TEM; JEM-2100F, JEOL, Tokyo, Japan). 3 RESULTS AND DISCUSSION 3.1 The synthesis/nucleation mechanisms of Co(OH) 2 Figure 1 indicates the cyclic voltammogram (CV) curves recorded on nickel foam in a solution consisting of 0.1 mol·L –1 Co(NO 3 ) 2 at a scanning rate of 5 mV·s –1 . When the potential is higher than –0.60 V during the negative-going scan, the current approximately tends to zero, indicating that almost no ions participate in the electrode reactions. However, the current is rapidly enhanced to about 0.035 A with the potential decreased Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 689 Figure 1: CV curves recorded on nickel foam in a solution consisting of 0.1 mol⋅L –1 Co(NO 3 ) 2 at a scanning rate of 5 mV·s –1 from –0.60 V to –1.0 V. An obvious inflexion can be observed clearly at about –0.65 V (as marked with a rectangle), indicating that Co(OH) 2 is synthesized by a one-step electrochemical reaction at about –0.65 V. 17 The synthesis process may include two reactions, corres- ponding to an electrochemical reaction and a chemical deposition reaction in Equations (1) and (2): 22,23 NO 3 –+7H 2 O+8e – NH 4 + + 10OH – (1) Co 2+ + 2OH– Co(OH) 2 (2) Chrono-amperometry was further applied to explore the nucleation mechanism of Co(OH) 2 at five different potentials (–0.60 V, -0.65 V, –0.70 V, –0.80 V, –0.95 V). As shown in Figure 2, a transient current is generated when different potentials are applied to the electrodes, which results from the double-layer charging. However, the current presents different changes with prolonging the time at different potentials. The current is rapidly decreased by increasing the time and finally goes to zero when the potential is –0.60 V, indicating that almost no Co(OH) 2 nucleates and grows on the electrode. This is consistent with the result obtained in the CV tests. When the applied potential is reduced to –0.65 V, the current is decreased to –2.55 mA in a short time of about 15 s, then rises rapidly and tends to a stable value of about –3.95 mA after 600 s. The rapid rise in the current is mainly related to the significant increase in the electrode surface area resulting from the nucleation and growth of the Co(OH) 2 . Therefore, the short time (about 15 s) can be defined as the nucleation incubation period. Accom- panied by the continuous growth of the nuclei, they may be connected with each other, resulting in the formation of deposits with a comparatively stable surface morphol- ogy. Correspondingly, the current begins to stabilize after reaching a maximum value of about –4.4 mA, meaning that Co(OH) 2 is deposited on the electrode surface at a constant rate. The change in current with time at –0.70 V is very similar to that at –0.65 V. Only two subtle differences are observed, corresponding to a shorter nucleation incubation period (11 s) and a higher deposition rate. The phenomenon is attributed to the increase in the driving force for the nucleation and growth of Co(OH) 2 when the potential is increased from –0.65 V to –0.70 V. When the potential is further increased to –0.80 V and –0.95 V, the nucleation incu- bation period almost disappears, indicating that the nucleation is almost instantaneous. The current presents a significant decline trend after it reaches a maximum value, which is significantly deviated from the change trend presented in the curves obtained at –0.65 V and –0.70 V. It can be concluded that the electrode reactions are gradually controlled by the diffusion of the active ions. When a high potential is exerted on the electrode, a large number of active ions around the electrode are violently consumed for the nucleation and growth of Co(OH) 2 , resulting in active ions poor after about 300 s. The diffusion of ions towards the electrode surface gradually becomes the rate-controlling step of the elec- trode reactions. When the diffused ions cannot timely compensate for those consumed by the reactions, the current is gradually decreased with the time. Com- paratively speaking, the decline trend is prominent at –0.95 V due to more serious consumption of the active ions at a high potential. Instantaneous nucleation and progressive nucleation are two important nucleation processes. In the instant- aneous nucleation process, the nuclei are formed almost simultaneously. The growth of the nuclei will be predominant during the subsequent deposition. In the progressive nucleation process, the formed nuclei grow during the deposition, accompanied with which the new nuclei are constantly formed on the substrate surface or the deposit surface. The crystallinity of the deposit obtained by instantaneous nucleation is higher than that by continuous nucleation. 24 Hills has derived two equations describing the relationship between current and time to determine the nucleation process on the electrode surface. For instantaneous nucleation: It zFN DC M t () () () // / / = 0 32 12 12 12 2 π (3) For progressive nucleation: It zFK N DC M t () () () // / / = 22 0 32 12 12 32 n π (4) where z is the valence number, N 0 is the initial nucle- ation number, D is the diffusion coefficient for the depo- sited ions (cm 2 ·s –1 ), C is the bulk concentration of the deposited ions (mol·cm –3 ), M is the atomic weight of the deposits (g·mol –1 ), is the density of the deposits (g·cm –3 ), K n is the nucleation constant, I(t) is the corres- ponding current when time is t (A), and t is the polariza- tion time (s). The above equations show that the current is linear with t 1/2 and t 3/2 , respectively, when the Co(OH) 2 is Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... 690 Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 Figure 2: Chrono-amperograms recorded on nickel foam in a solution consisting of 0.1 mol·L –1 Co(NO 3 ) 2 formed by the instantaneous nucleation mode and the progressive nucleation mode. The experimental data from the rising portion in Figure 2 were collected to investigate the nucleation mechanism. As shown in Figure 3, I is in a good linear relation with t 1/2 and t 3/2 . Owing to some parameters (such as N 0 a n dD )i n Eqations (3) and (4) cannot be determined precisely, the theoretical curve cannot also be drawn clearly. There- fore, the nucleation mechanism is difficult to be deter- mined accurately by comparing the theoretical curves and experimental curves. The other model deduced by Scharifker-Hills can solve the problem mentioned above due to some para- meters crossed out efficiently by normalizing I and t to the peak current I m and the corresponding peak time t m . For instantaneous nucleation: {} I It t tt mm m ⎛ ⎝ ⎜ ⎞ ⎠ ⎟=− − 19542 1 12564 2 . / exp . ( / ) [] (5) For progressive nucleation: {} I It t tt m m m ⎛ ⎝ ⎜ ⎞ ⎠ ⎟=− − 12254 1 23367 2 2 . / exp . ( / ) [] (6) There is a very short nucleation period (t 0 ) observed in Figure 2. Prior to the nucleation, which can interfere with the theoretical curve. Therefore, t and t m are repre- sented by t’=t – t 0 and t m ’=t m – t 0 in order to obtain the correct normalization curve. The (I/I m ) 2 – t/t m curves ob- tained at different potentials and two theoretical curves are plotted and compared (Figure 4). Comparatively speaking, the experimental fitting curves better coincide with the theoretical curve of instantaneous nucleation. It can be concluded that the instantaneous nucleation domi- nates the deposition of Co(OH) 2 . 3.2 Morphological evolution of Co(OH) 2 with poten- tials and Co(NO 3 ) 2 concentrations The morphologies of the deposits are observed by SEM to reveal the effect of deposition potential and con- centration on the morphological evolution. Figure 5a to 5c presents the morphologies of the deposits prepared at the applied potentials of –0.60 V, –0.65 V, –0.95 V in the solution with 0.1 mol·L –1 Co(NO 3 ) 2 . It can be found that their surface morphology changes significantly with the deposition potential. When a potential of –0.60 V is applied, there are almost no deposits adhering to the nickel foam surface due to initial grains and boundaries clearly observed. It indicates that –0.6 V is insufficient to drive the nucleation and growth of Co(OH) 2 , which is completely in accordance with the result obtained in CV. When the potential moves to a more negative value of –0.65 V, the surface of the nickel foam is obviously covered with a thin layer of deposits with a honey- comb-like porous structure. The deposition rate is further enhanced at a higher potential of –0.95 V, resulting in an obvious increase in the thickness of the deposit. However, some gaps with a width of 5 ìm can be clearly observed. The above result demonstrates that a high potential can accelerate the deposition; however, it can also deteriorate the quality of the deposit. When the Co(NO 3 ) 2 concentration is evaluated to 0.9 mol·L –1 , the deposit can be obtained on the nickel foam surface at the applied potential of –0.60 V (Figure 5a), the mor- phology of which is very similar to that at –0.65 V in 0.1 mol·L –1 Co(NO 3 ) 2 solution. Some fine cracks are formed on the deposit surface when the potential reaches Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 691 Figure 4: Experimental and theoretical curves between (I/I m ) 2 and t/t m Figure 3: I as a function of t 1/2 and t 3/2 at various potentials ranging from –0.65 V to –0.95 V –0.65 V (Figure 5b). Moreover, some coarse particles of the deposit protrude out of the surface due to the overgrowth of the partial zones. Those particles cluster to form coarser particles with irregular shape, which are mainly distributed around the edge of the limbs of the nickel foam. This should be attributed to the edge effect. With further increasing the potential to –0.95 V, the deposit becomes denser and thicker, and tiny cracks are also transformed into coarse gaps (Figure 5c). SEM images with a higher magnification are ob- tained to present more morphological characteristics of the deposits (Figure 6). No deposited layer was ob- viously formed at –0.60 V when the Co(NO 3 ) 2 con- centration is 0.1 mol·L –1 , besides a small amount of suspected deposits particles (Figure 6a). A layer of loose and porous deposits completely covers the surface of nickel foam at –0.65 V (Figure 6b). The image located at the right left corner of the figure clearly reveals that the irregular petal-like nanosheets with a thickness of about 25.6 nm, preferentially grow on the comparatively leveled surface of the deposit, which connect with each other to form the honeycomb-like structure. When the potential of –0.95 V is applied, a stronger driving force accelerates the growth of these nanosheets in three- dimensional directions (Figure 6c). Moreover, new nanosheets grow up on the flat surface of the deposit, resulting in the surface split into many fine pores with a smaller size. The layer also becomes more compact due to the growth and interconnection among the numerous nanosheets, which may cause the reduction in specific surface area of the deposit. It is clear that Co(OH) 2 can be deposited at a very low potential (–0.60 V) in a high concentration solution (Figure 6a); however, it is hardly visible in –0.60 V in a low concentration solution (Figure 6a). The morphology in the majority of depo- sited surface is very similar to that shown in Figure 6b. Comparatively speaking, the former is compacter than the latter. Moreover, a considerable number of fine/ coarse flower-like particles with size from 0.27 to 2.7 μm protrude in local areas of the deposit surface. With further increasing the potential to –0.65 and –0.95 V, more and more nanosheets cluster together and form a compact deposit. Especially at –0.95 V, the volume fraction of pores in the deposit layer is greatly reduced. It is proved that a high potential efficiently promotes the growth of nanosheets; however, it is not beneficial to the increase in specific surface area due to the overgrowth of nanosheets. Based on the morphological evolution, it is easy to deduce that the deposit with the maximum spe- cific surface area can be synthesized at a suitable combi- nation of potential (–0.65 V) and Co(NO 3 ) 2 concen- tration (0.1 mol·L –1 ). The morphological evolution of the deposit can be intuitively described as followed. Many nuclei are formed instantaneously by the instantaneous nucleation mode. Accompanied with the growth of those nuclei, the grain boundaries are formed and crosslinked around the grains, resulting in the formation of a thin deposited layer. Then the growth of the grains is inhibited along the two-dimensional direction parallel to the nickel foam surface, the vertical growth becomes dominant. The grain boundary with high energy grows preferentially with the prolonged time when compared with the grains. Some irregular nano-walls sprout along the grain boun- daries, which separate the deposit surface into numerous grids with different sizes. Nano-walls gradually grow Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... 692 Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 Figure 6: High-magnification SEM images of deposits prepared at different deposition potentials and Co(NO 3 ) 2 concentrations: a) –0.60 V 0.1 mol·L –1 , b) –0.65 V, 0.1 mol·L –1 , c) –0.95 V 0.1 mol·L –1 ,A) –0.60 V 0.9 mol·L –1 , B) –0.65 V 0.9 mol·L –1 , C) –0.95 V 0.9 mol·L –1 Figure 5: Low-magnification SEM images of the deposits prepared at different potentials and Co(NO 3 ) 2 concentrations: a) –0.60 V 0.1 mol·L –1 , b) –0.65 V 0.1 mol·L –1 , c) –0.95 V 0.1 mol·L –1 into nanosheets, which protrude into the solution. Due to the weak conductivity of Co(OH) 2 , the resistance bet- ween the protruding nanosheets and the substrate will be gradually increased, the growth of nanosheets are correspondingly slowed down. When a stronger driving force is applied resulting from a higher potential or a higher Co(NO 3 ) 2 concentration, a portion of zones with high growing rate on the flat surface of deposit can protrude out of the surface and grow into some fine parti- cles. Those particles may grow and connect into new nano-walls, which will be transformed into new nano- sheets. Moreover, some new growth sites may be formed resulting from the interconnection of nanosheets, which further grow into fine or coarse flower-like particles. Correspondingly, a compacter deposit layer is formed. 3.3 Electrochemical performance Figure 7 shows cyclic voltametry (CV) curves of Co(OH) 2 /NF prepared in the (0.1, 0.5, 0.7 and 0.9) mol·L –1 Co(NO 3 ) 2 solutions with different potentials, which are recorded at a scanning rate of 5 mV·s –1 in a 2 mol·L –1 KOH solution. It is clear that the CV profiles are significantly deviated from the ideal rectangular shape. Moreover, two redox peaks can also be observed, indicating that the electrodes exhibit typical pseudo- capacitance behavior, namely the capacitance mainly originates from the oxidation/reduction reactions. For the first redox peak (Figure 7a), the oxidation peak is located at 0.025 V, and the corresponding reduction peak can be observed at –0.045 V. Two reactions are followed: Co(OH) 2 +OH – e – CoOOH + H 2 O (Oxidation reaction) (7) CoOOH + H 2 O+e – Co(OH) 2 +OH – (Reduction reaction) (8) For the second redox peak, the oxidation and reduc- tion peaks are formed at 0.24 V and 0.175 V, res- pectively. The corresponding reactions are followed: CoOOH + OH – e – CoO 2 +H 2 O (Oxidation reaction) (9) CoO 2 +H 2 O+e – CoOOH + OH – (Reduction reaction) (10) The whole reaction process can be clearly described throughout the sweeping. When the potential reached to 0.025 V, the Co(OH) 2 deposited on the nickel will be transformed to CoOOH. Along with the increase to 0.24 V in potential, CoOOH is further oxidized into CoO 2 . During the reverse sweeping, CoO 2 is reduced into CoOOH and Co(OH) 2 . Both Co(OH) 2 and CoOOH have the same cobalt structure and large interlayer spacing, which is very beneficial for H + to intercalation and deintercalation during the charge-discharge. 13 As a result, the cycling performance and stability of the Co(OH) 2 /NF electrode can be improved. The result was verified by research. 25,26 Figure 7b clear inspection reveals that the integrated area of CV profiles present the incline tendency with increasing the potential in the (0.1, 0.5, 0.7 and 0.9) mol·L –1 Co(NO 3 ) 2 solutions, meaning that the high deposited potential does not favor to the improvement in capacitance of the electrodes. Besides the potential, the increase in Co(NO 3 ) 2 concentration is not also beneficial to the enhancement in capacitance due to the integrated area of CV profile in 0.1 mol·L –1 Co(NO 3 ) 2 solution larger than that in (0.5, 0.7 and 0.9) mol·L –1 Co(NO 3 ) 2 solution at any given deposited potential. The specific capacitance is a very important index characterizing the power storage capacity in a unit mass of active substances, which is related not only to the capacitance, but also to the mass. The effect of potential and Co(NO 3 ) 2 concentration on the mass of the deposited substances is investigated. It is clear that more substances can be deposited in a high-concentration Co(OH) 2 solution at a given potential. Considering the changes in the mass of deposits and the integrated area of CV curves with potential and Co(NO 3 ) 2 concentration, it is easy to draw a conclusion that the high specific capacitance can be obtained in an electrode prepared in a low Co(NO 3 ) 2 concentration solution with a low applied potential. b) different concentration of Co(NO 3 ) 2 solution The specific capacitance of the Co(OH) 2 can be calculated according to the following equation: C iV SVm m d = ×× × ∫ 2 Δ (11) where C m (F·g –1 ) is the specific capacitance measured from CV tests, iV d ∫ is the integrated area of the CV curves, DV (V) represents the potential drop during dis- charge, S (V·s –1 ) is the scan rate and m (mg) is the mass of the active material on the electrode. The calculated results are shown in Table 1. The specific capacitance reaches 836 F·g –1 for Co(OH) 2 depo- sited at –0.65 V in a 0.1 mol·L –1 Co(NO 3 ) 2 solution. The value is gradually decreased by applying the more negative potential, finally is reduced to 496 F·g –1 at –0.95 V (only about 59.3 % that at –0.65 V). When the concentration is increased to 0.9 mol·L –1 , a similar phenomenon is also observed. The value of Co(OH) 2 deposited at –0.95 V is only about 44.4 % that at –0.60 V. Moreover, the specific capacitance value of Co(OH) 2 deposited in the 0.9 mol·L –1 Co(NO 3 ) 2 solution is 62.1–80.9 % that in the 0.1 mol·L –1 Co(NO 3 ) 2 solution at the same potential, which is well in accordance with the above analyses. The galvanostatic charge/discharge tests were also applied to calculate the specific capacitance of Co(OH) 2 (Figure 9). The specific capacitance can be calculated by the following Equation (12): C it Vm m = × Δ Δ (12) Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 693 where I is the applied constant current (A), m is the mass of Co(OH) 2 , ÄV is the potential window during cycling and t is the time of the cycle (s). The results are shown in Table 1, which present the completely same change with Co(NO 3 ) 2 concentration and applied potential. The specific capacitance is reduced from 725 F·g –1 to 315 F·g –1 with decreasing the potential from –0.65 V to –0.95 V for Co(OH) 2 depo- sited in the 0.1 mol·L –1 Co(NO 3 ) 2 solution. The value is only drastically reduced from 567 F·g –1 to 130 F·g –1 when the potential is decreased from –0.60 V to –0.95 V for Co(OH) 2 deposited in the 0.9 mol·L –1 Co(NO 3 ) 2 solu- tion. The value is reduced about 32–59 % for Co(OH) 2 deposited in the 0.1 mol·L –1 Co(NO 3 ) 2 solution when compared that in the 0.9 mol·L –1 Co(NO 3 ) 2 solution. The change in specific capacitance of the electrodes prepared at different potentials in the solutions with different concentrations of Co(NO 3 ) 2 is closely related to the evolution in the morphology of the deposits. As analyzed above, the overgrowth of the deposits resulting from a high concentration Co(NO 3 ) 2 and a high applied potential will significantly reduce their specific area, which means that the unit mass of deposits with less active sites can undergo insufficient and incomplete redox reactions, and provide fewer tunnels and a stronger resistance to the transportation of ions and electrons. Subsequently, the improvement in the potential and Co(NO 3 ) 2 concentration does not contribute to the im- provement in the specific capacitance. Table 1: Specific capacitance measured by CV and galvanostatic charge/discharge tests Concentration Potential CV Galvanostatic charge/discharge 0.1 mol⋅L –1 –0.65 V 836 F·g –1 725 F·g –1 –0.75 V 742 F·g –1 482 F·g –1 –0.85 V 523 F·g –1 349 F·g –1 –0.95 V 496 F·g –1 315 F·g –1 0.9 mol⋅L –1 –0.60 V 694 F·g –1 567 F·g –1 –0.65 V 593 F·g –1 494 F·g –1 –0.75 V 509 F·g –1 326 F·g –1 –0.85 V 423 F·g –1 221 F·g –1 –0.95 V 308 F·g –1 130 F·g –1 The cycle stability is another important parameter evaluating the electrochemical performance of the super- capacitors, which is used to characterize the performance attenuation of continuous charge and discharge under the same condition. The cycle stability of the Co(OH) 2 /NF Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... 694 Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 Figure 8: Galvanostatic charge/discharge curves recorded on the Co(OH) 2 /NF electrodes at a current density of 4 A·g –1 :a) 0.1 mol·L –1 Co(NO 3 ) 2 solution Figure 7: Cyclic voltammetry (CV) curves recorded on the Co(OH) 2 /NF electrodes at a scanning rate of 5 mV·s –1 . a) 0.1 mol·L –1 Co(NO 3 ) 2 solution, b) different concentration of Co(NO 3 ) 2 solution electrode was examined in 2 mol·L –1 KOH electrolyte at a scanning rate of 5 mV·s –1 . As shown in Figure 9, the specific capacitance presents the gradual rising tendency with the increase in the cycle number, and a high value of 1116 F·g –1 (133.65 % of initial value) is finally retained after 1000 cycles, which indicates that the electrode presents an excellent capacity retention and is not subject to a serious degradation during the repetitive and continuous charge and discharge. The phenomenon about the increase in specific capacitance with prolong- ing the cycle time has been reported previously for other electrodes. Z. P. Feng et al. 27 synthesized MnO 2 multi- layer nanosheets clusters were prepared via electroche- mical deposition route, which has about 133 % increase in the specific capacitance after 1000 cycles. X. H. Lu 28 et al. 30 synthesized large-area manganese oxide nanorod arrays (MONRAs) and herringbones (MOHBs) on F-doped SnO 2 coated glass (FTO) substrates by a electrochemical method. The specific capacitance from 411.4 F·g –1 to 507 F·g –1 after 1500 cycles. Z. Gao et al. 29 synthesized neighboring sheets separate graphene nano- sheet/ Ni 2+ /Al 3+ layered double-hydroxide (GNS/LDH) composite for supercapacitor material by a hydrothermal method. The composite exhibits a maximum specific capacitance of 781.5 F·g –1 and an excellent cycle life with an increase of the specific capacitance of 38.07 % after 50 cycle tests. Even after 200 cycle tests, the in- crease of the capacitance is 22.56 % compared with the initial capacitance. M. J. Pang et al. 30 prepared the porous structures mixed-phase CoO/Co 3 O 4 nanocompo- sites by a facile ethanol-assisted solvothermal approach. The CoO/Co 3 O 4 electrode exhibits a high capacitance of 451 F·g –1 at a current density of 1 A·g –1 and superior cycle stability (108 % retention after 5000 cycles). According to previous researches, 27–33 the phenomenon of increasing the specific capacitance is mainly attributed to more active sites in the active substance activated during the cycle. Besides that, further elaboration of its mechanism in this study, the structural transformation may be the other essential factor causing the significant rising in the specific capacitance in this study, which is validated by comparing the difference in morphology and structure of the deposited active substances before and after long-term cycling. Figure 10 demonstrates SEM images of the active substances deposited in the 0.1 mol·L –1 Co(NO 3 ) 2 solution with the applied potential of –0.65 V before and after the electrochemical tests (1000 cycles). The depo- sits are composed of many nanosheets prior to the electrochemical tests (Figure 10a). Some anisotropic nanosheets cluster together to form the interlaced follower-like morphology. The other approximately parallel nanosheets form the loose platelets. After the electrochemical tests, the drastic change in morphology can be clearly observed (Figure 10b). The loose stacked fine sheets are transformed into the dense coarse sheets. Clearly observation reveals that those are composed of several fine sheets, among which tiny gaps remained. The morphological evolution indicates that the KOH electrolyte can accelerate the cluster of nanosheets by compressing the space among them, which is closely associated with the removal of nitrates incorporated into nanosheets. Figure 11 indicates a narrow spectral scan of the Co element performed on the substances prepared Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 695 Figure 9: Cyclic performance of the Co(OH) 2 /NF electrode at a scanning rate of 5 mV·s –1 for 1000 cycles Figure 10: SEM images of the deposits: a) before and b) after the cycle testing in the 0.1 mol·L –1 Co(NO 3 ) 2 solution with the applied potential of –0.65 V before and after the electrochemical tests, which can be used to identify the chemical valence states of Co. Prior to the electrochemical tests, two strong peaks located at 781.00 eV and 796.5 eV can be clearly observed (Figure 11a). The strongest peak is in good agreement with that of Co 2+ in Co(OH) 2 and the other coincides with that of Co 2+ in CoO. A small amount of Co(NO 3 ) 2 can be confirmed due to the two peaks related to Co 2+ in Co(NO 3 ) 2 with comparatively low intensities at 786.850 eV and 803.05 eV, which are incorporated into the Co(OH) 2 during the synthesis. It is clear that the deposited substances are mainly composed of Co(OH) 2 , accompanied with a small amount of CoO and Co(NO 3 ) 2 . Among them, only Co(OH) 2 and CoO are responsible for the charge storage by participating in the redox reactions occurring on the electrode surface. The incorporated Co(NO 3 ) 2 into the active substances seriously reduces the efficient active sites and deterio- rates the electrochemical performance. However, the XPS result indicates that Co(NO 3 ) 2 incorporated into the nanosheets has been removed completely after the electrochemical tests (Figure 11b). It can be concluded that OH – will infiltrate into the tiny gaps among the sheets and contacts with the nanosheets sufficiently when the electrode is immersed into the KOH electrolyte for the electrochemical tests, which will result in the removal of the interlayer nitrates. Similar results were reported in previous studies. 13 The removal of nitrates can be regarded as an essential factor causing the improvement in specific capacitance due to the increase in active sites. The other factor may be associated with the structural change of active substances. Figure 12a shows the high-resolution TEM image of the as-prepared deposits in the 0.1 mol·L –1 Co(NO 3 ) 2 solution with the applied potential of –0.65 V. It is clear that the synthesized deposit possesses poor crystallinity due to its disordered lattice fringe, which can be identified as a-Co(OH) 2 , as reported in the other investigations. 13,22,34,35 The transformation in phase constituent should occur when the deposit is subject to the electrochemical tests owing to an ordered lattice fringe observed clearly (Figure 12b). A d-spacing of 0.237 nm is indexed, which is in accordance with that of b-Co(OH) 2 (JCPDS Z. LU et al.: EVOLUTION IN ELECTROCHEMICAL PERFORMANCE OF Co(OH)2 ELECTRODEPOSITED ... 696 Materiali in tehnologije / Materials and technology 53 (2019) 5, 687–698 Figure 12: HRTEM images of the deposits: a) before and b) after the cycle testing Figure 11: XPS spectra of Co in the deposits: a) before and b) after the cycle testing card No.30–0443). Compared with the disorder -Co(OH) 2 , the order -Co(OH) 2 exhibits better conduc- tivity, which will enhance the specific capacitance of the electrode. 36 Based on the analyses above, the improve- ment in specific capacitance after 1000 cycles can attributed to an increase in the active sites and the transformation of á-Co(OH) 2 into b-Co(OH) 2 . T. Deng et al. 13 also found that OH – like a scalpel removed NO 3 – from -Co(OH) 2 and transformed -Co(OH) 2 into -Co(OH) 2 . The change made the specific capacitance retain 95.7 % of its initial value (800 F·g –1 at the current of 2 A·g –1 ) after 8000 cycles. 4 CONCLUSIONS 1) Co(OH) 2 nanosheets were electrodeposited on nickel foam by the two reactions including an electro- chemical reaction NO 3 – +7H 2 O+8e – NH 4 + + 10OH – occuring at about –0.6 V (vs. SCE) and a following chemical reaction Co 2 + + 2OH – Co(OH) 2 .Co 2+ + 2OH – Co(OH) 2Co(OH) 2 nuclei were formed by instant- aneous nucleation. 2) The deposited potential and Co(NO 3 ) 2 concen- tration produced significant effects on the morphologies of Co(OH) 2 . With the increase in potential and Co(NO 3 ) 2 concentration, the deposits became more compact and possessed the lower specific area due to their over- growth. The deposit with the highest specific area was obtained at the potential of –0.65 V in the 0.1 mol·L –1 Co(NO 3 ) 2 solution. 3) The specific capacitance of the Co(OH) 2 /NF prepared in the 0.1 mol·L –1 /0.9 mol·L –1 Co(NO 3 ) 2 solution presented a reduction tendency with an increase in the potential. The value of the electrode prepared in the 0.1 mol·L –1 Co(NO 3 ) 2 solution was higher than that in the 0.1 mol·L –1 Co(NO 3 ) 2 solution. The highest value was obtained in the electrode prepared at the potential of –0.65 V in the 0.1 mol·L –1 Co(NO 3 ) 2 solution, which was 836 F·g –1 in CV tests and 725 F·g –1 in galvanostatic charge/discharge tests. 4) The electrode prepared at the potential of –0.65 V in the 0.1 mol·L –1 Co(NO 3 ) 2 solution also exhibited the excellent cyclic stability with 133.65 % of initial capacity retained after 1000 cycles. 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