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Boosting Energy Efficiency of Nickel Cobaltite via Interfacial Engineering in Hierarchical Supercapacitor Electrode Long Chen, Liwen Mu, Tuo Ji, and Jiahua Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07475 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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The Journal of Physical Chemistry

Boosting Energy Efficiency of Nickel Cobaltite via Interfacial Engineering in Hierarchical Supercapacitor Electrode

Long Chen, Liwen Mu, Tuo Ji and Jiahua Zhu* Intelligent Composites Laboratory Department of Chemical and Biomolecular Engineering The University of Akron, Akron, OH 44325 USA

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ABSTRACT Hybrid electrodes with electroactive components on conductive substrates have been demonstrated effective strategy to achieve high energy and power density in supercapacitor. However, the mismatch of interface property could be a huge hurdle to further improve energy storage performance and long term stability. In this work, an interfacial metal seeding approach has been developed targeting to strengthen the interfacial interaction between electroactive NiCo2O4 nanostructure and carbon substrate as well as to promote electron transfer across interface. By implanting low-concentration nickel (Ni) nanoparticles at the interface, the electrochemical capacitance of NiCo2O4 was boosted up to 2367 F/g at current density of 1 A/g in a symmetric two-electrode configuration, which is about two times higher than the capacitance obtained from the electrode without metal seeds. The Ni seeds also contribute to an excellent cycling retention of >96% after 5000 cycles, where only 65% capacitance was retained in electrode without Ni seeds. A synergistic contribution of promoted interfacial interaction, reduced internal resistance, enlarged surface area and mesoporous NiCo2O4 nanorod structure leads to a boosted energy efficiency of NiCo2O4 in this study. A comparative study on different metal nanoparticles (nickel, cobalt and iron) reveal that not only metal species but also particle concentration plays significant roles in determining the energy storage property of the hierarchical NiCo2O4/carbon electrodes.


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1. INTRODUCTION The continuous increasing demands of upgrading energy and power density of supercapacitors has inspired active research efforts in developing advanced electrode materials. Hybrid nanocomposite electrodes have attracted tremendous interests in recent years to develop high energy and power density units, which synergistically integrate various advantageous functions of different materials into one unit.1-6 Electrochemical active metal oxides (or conductive polymers) and conductive carbon support seems one of the most promising combinations to achieve unprecedented energy and power densities in supercapacitor.7 Carbon has been well recognized as promising electrode material in double layer capacitors, which physically stores electrical energy on electrode surface. Therefore, high surface area, good electrical conductivity and chemical stability are the key ingredients to ensure good energy storage performance. Tremendous promising carbon materials have been developed in past decades such as graphene,8 carbon nanotube,9 hierarchical porous carbon,10 carbon aerogels11 and carbon nanofibers.12 The specific capacitance of electric double layer capacitors is typically lower than 300 F/g due to the intrinsic limitations of surface area and electrical conductivity. To boost energy density of supercapacitors, pseudocapacitive materials such as metal oxides13-16 and conductive polymers17-18 are often used since these materials are able to react with electrolyte and generate large current density during charge/discharge. Metal oxides such as RuO2, MnO2, Co3O4, NiO and NiCo2O4 have been demonstrated promising pseudocapacitive electrode materials because of their intrinsic large specific capacitance. Among these metal oxides, NiCo2O4 received significant attention due to its better electrochemical activity and electrical conductivity than other metal oxide materials including NiO and Co3O4.19-21 Even though NiCo2O4 itself could be used as electrode,22 most of the existing research tends to grow 3 ACS Paragon Plus Environment


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NiCo2O4 directly on a conductive substrate, such as nickel foam23 and titanium foil.24 However, besides the extra cost, metal substrates are mostly not stable in acidic electrolyte environment, which restricts their applications in certain circumstances. The high electrical conductivity and excellent chemical inertness of carbon qualify itself a suitable substrate to support the growth of NiCo2O4. Previous reported work have successfully grown NiCo2O4 nanostructures onto various carbon supports such as carbon nanofiber,25 carbon textiles,26 graphene27 and enhanced electrochemical performance has been observed. However, a common issue of these hybrid structures is their weak interface interaction caused by the intrinsic hydrophilic nature of metal oxides and hydrophobic nature of carbon surface, which significantly affects the overall electrode performance and long term stability. One of the major concerns of pseudocapacitive electrode materials is the poor cycling retention28 since volume expansion and shrinkage would occur during charge/discharge process that induces severe micro-cracking at NiCo2O4–carbon interface and cuts off the pathway of electron transportation. Different interfacial techniques have been successfully applied in a variety of energy storage devices, such as supercapacitors, solar cells and batteries, to improve the energy capacity and cycling retention. For example, by taking advantage of the electrostatic interaction and interfacial conjugation between conducting polymers and carbon nanotubes (CNTs),29 electrical conductivity of the composites is improved and thus enhanced capacitance was achieved in polyaniline,30 polypyrrole31 and poly[3, 4-ethylene-dioxythiophene].32 Huang et al.

33

employed

TiO2 coated metallic silver nanowires (AgNWs@TiO2) as electron conductor in mesoporous photoanode of a dye-sensitized solar cells, which improves the energy conversion efficiency from 4.68 to 5.31% due to the reduced TiO2/dye/electrolyte interfacial charge transfer impedance. Haynes and coworkers 34 found that a coating layer of electroactive molecules on top of cuprous


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oxide films increased the photocurrent density by an order of magnitude in non-aqueous photocells. Xia and coworkers

35

introduced graphene as an interfacial layer between current

collector and Si nanowires anode in lithium-ion batteries. Graphene interfacial layer lessened the stress accumulated by volumetric mismatch and inhibited interfacial reactions accelerating the fatigue of Si anodes. As a result, specific charge capacity retention increased by 2.7 times. Metal oxide-carbon represents a series of advanced materials with promising capability in a wide range of energy devices, while effective interfacial techniques in metal oxide/carbon hybrid electrodes are rarely reported. In this work, a metal seeding layer has been artificially introduced at the interface between electroactive NiCo2O4 and carbon fabric support. The embedded metal nanoparticles would serve as a transitional layer that strengthen the interfacial bonding as well as promote the charge transfer efficiency across the interface. Different metal nanoparticles (nickel, cobalt and iron) of different concentrations are embedded at the interface. The effects of particle species, size and concentration on subsequent NiCo2O4 nanostructure growth and microstructure evolution of the hybrid electrodes are systematically investigated. The metal seeds effect on the electrochemical energy storage property and long term cycling stability were evaluated and compared with literature reported results. 2. EXPERIMENTAL 2.1. Materials Cotton fabric was obtained from commercially available T-shirt made of 100% cotton. Nickel (II) nitrate hexahydrate (>98%), iron (III) nitrate nonahydrate (>98%), cobalt(II) nitrate hexahydrate (>98%) and urea were purchased from Sigma Aldrich. Ethanol was purchased from Decon Labs. Potassium hydroxide was purchased from EMD Chemicals. Cotton fabric was



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washed with acetone to remove possible organic contaminants before use. All other chemicals were used without further purification. 2.2. Planting metal nanoparticle interface in carbon fabric (CF) A facile two-step process, soaking and annealing, was applied to prepare the composites. Specifically, 4.0 g cotton fabrics were firstly soaked in 40 mL nickel nitrite precursor solution of different molar concentrations (0.1, 0.5 and 1.0 M) overnight and the soaked cotton fabrics were carefully blotted and dried at room temperature. The dried cotton fabrics were then heated from room temperature to 800 °C with a heating rate of 5 °C min-1 and kept at 800 oC for 2 hours in nitrogen atmosphere. As a control, cotton fabric was annealed following the same temperature profile but without the soaking process and the sample was named CF. Annealed samples presoaked with 0.1, 0.5 and 1.0 M nickel nitrite solutions were denoted as CF-Ni01, CF-Ni05 and CF-Ni10, respectively. To study the effect of metal species, same molar concentration of iron nitrate and cobalt nitrate solutions were prepared and followed the same procedure to embed iron and cobalt nanoparticles in CF. 2.3. NiCo2O4 growth on CF and metal nanoparticles embedded CF NiCo2O4 growth on substrate follows a wet chemical method modified from previously reported literature 36. Typically, 0.5 mL of 1.0 M Ni(NO3)2, 1.0 mL of 1.0 M Co(NO3)2 and 6.0 mmol urea were added to a mixture of ethanol (10.0 mL) and DI water (10.0 mL) to form a pink solution. Then, 0.4 g pure CF or CF-Nixx (xx=01, 05 or 10) were immersed in the prepared pink solution and reacted in a sealed vial at 85 oC for 12 hours. Afterwards, the products were washed by DI water in ultrasonication bath to remove the loosely attached products and dried at 60 oC in oven. Finally, the products were annealed in air at 300 oC for 2 hours to obtain crystallized NiCo2O4. Products obtained from this process were denoted as CF-G, CF-Ni01-G, CF-Ni05-G 6 ACS Paragon Plus Environment

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and CF-Ni10-G (G represents the grown NiCo2O4). The morphology evolution of NiCo2O4 on CF, CF-Ni01 and CF-Ni10 was monitored by SEM at different reaction times of 1, 3, 5, 8 and 12 hours. The same NiCo2O4 growth process was applied to other CF based substrates embedded with iron and cobalt nanoparticles and following products were collected (CF-Fe01-G, CF-Fe05G, CF-Fe10-G, CF-Co01-G, CF-Co05-G and CF-Co10-G). To understand the role of Ni nanoparticles in NiCo2O4 growth, CF-Nixx (xx=01, 05 or 10) were treated by 1.0 M HCl for 12 hours to remove the metal nanoparticles. The treated CF was washed with DI water for a few times and then used as substrate to grow NiCo2O4 following the same procedure. 2.4. Characterization The material morphology was characterized by scanning electron microscopy (SEM, JEOL-7401). NiCo2O4 morphology and crystalline structure were further characterized by transmission electron microscopy (FEI scanning TEM) and high resolution TEM (HRTEM, FEI Tecnai G2 F20 ST TEM/STEM & EDAX energy dispersive x-ray spectrometer). The loadings of embedded metal nanoparticles and subsequent grown NiCo2O4 nanostructure were determined by thermogravimetric analysis (TGA, TA instrument Q500) in air atmosphere up to 800 °C with a heating rate of 10 °C /min. The crystalline structure of CF and CF-Nixx (xx=01, 05 and 10) was examined by X-ray diffraction (XRD, Rigaku, Ultima IV). Brunaure–Emmet–Teller (BET) surface area analysis of samples was performed using a TriStar II 3020 surface analyzer (Micromeritics Instrument Corp., USA) by N2 adsorption–desorption isotherms. The pore size distribution curves were calculated from the adsorption branch of the isotherm. The electrochemical performance of the prepared materials was evaluated on a VersaSTAT 4 electrochemical workstation (Princeton Applied Research) using a two-electrode method. After embedding metal nanoparticles and growing NiCo2O4, the processed fabric 7 ACS Paragon Plus Environment


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remains a whole piece. Two identical pieces of circular-shaped electrode materials were cut from the fabric and soaked in 1.0 M KOH electrolyte for 24 hours. The total mass of two electrodes is 2.20, 2.75, 3.02 and 4.41 mg for CF-G, CF-Ni01-G, CF-Ni05-G and CF-Ni10-G, respectively. Then, the electrodes were assembled into a coin-cell current collector (CR2430) with symmetric configuration, where the two electrodes were separated by an insulating porous media (Whatman filter paper). The cyclic voltammograms (CV) were recorded at different scanning rates of 2, 5, 10, 20 and 50 mV/s in the potential range of 0-1.0 V, charge-discharge tests were conducted at different current densities of 1.0, 2.0, 5.0 and 10.0 A/g. Electrochemical impedance spectroscopy (EIS) was performed using a sinusoidal signal with mean voltage of 0 V and amplitude of 10 mV over a frequency range of 1,000,000 to 0.01 Hz. Cycling tests were performed with up to 5,000 cycles, current density used in the cycling test was 20, 30, 20 and 5 A/g for CF-G, CF-Ni01-G, CF-Ni05-G and CF-Ni10-G, respectively. 3.

RESULTS AND DISCUSSION

3.1. Engineering interface and NiCo2O4 structure control

Scheme 1. Schematic representation of NiCo2O4 growth on pure CF and Ni seeded CF. 8 ACS Paragon Plus Environment

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The interfacial nanoparticle incorporation and subsequent NiCo2O4 structure growth are illustrated in Scheme 1. Initially, the cotton fabrics are immersed in nickel nitrate solution of different concentrations. Further thermal treatment in N2 atmosphere converts cotton fabric to carbon fabric and at the same time decomposes nickel nitrite to form Ni nanoparticles. Depends on the particle size and distribution, these nanoparticles would serve as heterogeneous sites to affect the NiCo2O4 growth process and thus structural morphology. The preferential NiCo2O4 growth from these heterogeneous sites stems from the energetically favorable epitaxial crystal growth, which has been used to control the nanostructure of noble metals in previous studies.37 Low concentration of Ni seeds allows one-dimensional free growth of NiCo2O4 towards perpendicular direction and effectively prevents the cross-growth of neighboring structures. At large Ni seeding density, the cross-growth becomes dominant and as a result two dimensional NiCo2O4 nanosheets will be formed.

Figure 1. SEM images of (a) CF, (b) CF-Ni01, (c) CF-Ni05, (d) CF-Ni10 and (e) their corresponding XRD patterns (I: CF, II: CF-Ni01, III: CF-Ni05 and IV: CF-Ni10).



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The surface morphology of CF with and without embedded Ni nanoparticles is shown in Figure 1. Compared to the relatively smooth surface of pure CF in Figure 1(a), nanoparticles were clearly seen in the composites, Figure 1(b-d). It seems that the particle size and distribution quality largely depend on the initial nickel precursor concentration. The average particle size in CF-Ni01 and CF-Ni05 is similar, 35.2 ± 9.0 and 36.5 ± 8.1 nm, which increased to 45.1 ± 8.6 nm in CF-Ni10. Besides the embedded small nanoparticles, larger particles of ~150 nm also appear on CF-Ni05 and CF-Ni10 surface which is induced by agglomeration with the existence of excess nucleus at high precursor concentrations. To confirm the crystalline structure of the nanoparticles, XRD spectrum were collected as shown in Figure 1(e). A broad peak centered at ~25o indicates the amorphous nature of CF. After embedding Ni nanoparticles, a sharp peak at 2θ=26.4° is observed implying the formation of graphitic carbon (002) (PDF#41-1487).38 Meanwhile, two additional peaks appeared at 2θ=44.6 and 52.0o are indexed to (111) and (200) crystal planes of FCC nickel (PDF#70-1849).39 Thermal graphitization of carbon is typically achieved at high temperature, i.e. above 2500 oC.40 With the assistance of transitional metal catalysts like Fe, Co and Ni,38, 41 graphitization can be realized at lower temperature of 800 oC or even lower.42 In this work, the embedded Ni nanoparticles are responsible for the catalytic conversion of carbon from amorphous to crystalline structure. Two major merits could be expected from this Ni embedding process. Ni nanoparticles create convex-shaped rougher texture and the particle itself could serve as active sites, both of which are beneficial for subsequent NiCo2O4 nucleation and growth. The other is the graphitized carbon possessing higher electric conductivity and chemical resistance that ensures a good electron transfer during charge and discharge.


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Figure 2. SEM surface morphology of (a) CF-G, (b) CF-Ni01-G, (c) CF-Ni03-G, (d) CF-Ni10-G. (e) TEM and (f) HRTEM of NiCo2O4 nanorod in CF-Ni01-G, (g) TEM and (h) HRTEM of NiCo2O4 nanosheet in CF-Ni10-G.

The above pure CF and Ni embedded CF were then used as substrates to grow NiCo2O4 nanostructures and their surface morphology is characterized, Figure 2(a-d). Without Ni nanoparticles at the NiCo2O4/CF interface, NiCo2O4 nanofibers were formed with diameter of ~20 nm and length of ~500 nm, Figure 2(a). The NiCo2O4 morphology changes to nanorod and nanosheet after introducing Ni nanoparticles and the morphology depends on the particle loading at the interface, Figure 2(b-d). Specifically, low loading of Ni leads to well-defined NiCo2O4 nanorod structure while nanosheet was observed in CF-Ni05-G and CF-Ni10-G as increasing Ni loadings. The thickness of NiCo2O4 layer for NF-G, NF-Ni01-G, NF-Ni05-G and NF-Ni10-G was measured as 340, 460, 665 and 830 nm, respectively, Figure S1. The NiCo2O4 nanorod and nanosheet structures in CF-Ni01-G and CF-Ni10-G were further characterized by TEM and HRTEM, Figure 2(e-h). The nanorod is constructed by small particle-shaped nanocrystals with average diameter of 10 nm. Meanwhile, the assembling of these nanocrystals leaves great number of mesopores in between with typical size around 5 nm, which exposes largest possible 11 ACS Paragon Plus Environment


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surface area of NiCo2O4 for chemical reactions, Figure 2(e). Focusing on one single nanorod, the lattice fringes of NiCo2O4 can be clearly identified as 0.25, 0.29 and 0.47 nm corresponding to the (311), (220) and (111) crystal planes of NiCo2O4, 43 Figure 2(f). The XRD spectrum of CFNi01-G also confirmed the formation of NiCo2O4, Figure S2. Figure 2(g) shows the NiCo2O4 nanosheet in CF-Ni10-G and the lattice spacing of 0.25 nm in Figure 2(h) is indexed to its (311) crystal plane. To determine the loading of nickel nanoparticles and NiCo2O4, TGA analysis was performed on CF-Nixx and CF-Nixx-G in air up to 800 oC, Figure S3. Based on the residue percentage of CF-Nixx and CF-Nixx-G, the NiCo2O4 loading can be calculated as 11.6, 4.6, 10.9 and 13.1 wt% in CF-G, CF-Ni01-G, CF-Ni05-G and CF-Ni10-G, respectively. 3.2. Microstructure evolution

Figure 3. N2 adsorption-desorption isotherm of (a) CF-Nixx and (b) CF-Nixx-G and pore size distribution of (c) CF-Nixx and (d) CF-Nixx-G. 12 ACS Paragon Plus Environment

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The interfacial Ni seeding and subsequent NiCo2O4 growth not only change the material composition but also microstructure, which is characterized by nitrogen adsorption-desorption isotherms as shown in Figure 3(a&b). The specific surface area, average pore size and pore volume are summarized in Table 1. The pore size distribution is calculated by Barrett-JoynerHalenda (BJH) method from the adsorption branch. The relatively large N2 uptake quantity at relatively low pressure of CF is primarily attributed to the existence of dominating micropores.44 This low pressure uptake becomes smaller as increasing Ni loading and meanwhile type IV hysteresis loops appear in all nanocomposites indicating the formation of mesopores.44 The BET surface area is calculated to be 396, 486, 264 and 179 m2/g for CF, CF-Ni01, CF-Ni05 and CFNi10, respectively. It is worth mentioning that only ~11% of surface area is composed of external surface in CF, while the external surface percentage increases gradually with increasing Ni nanoparticle loading and reaches up to 40% in CF-Ni10. The mesopore generation is supported and explained in our previous work.38 The formation of larger mesopores is further evidenced by the upshift of the BJH pore size distribution curves in large pore size range, Figure 3(c&d). As expected, the average pore size and pore volume increase simultaneously with increasing Ni loading due to the larger portion of mesopores in the composites. Previous studies have demonstrated the active role of pore size in contributing electrochemical capacitance.45-46 Therefore, it is reasonable to expect a better electrochemical property of the electrodes with Ni seeds.



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Table 1. Summarized BET results of CF-Nixx and CF-Nixx-G. Total surface area Sample

Surface area composition (%)

Pore size

Pore volume*

(m2/g)

Internal

External

(nm)

(cm3/g)

CF

396

89

11

3.4

0.03

CF-Ni01

486

81

19

4.1

0.07

CF-Ni05

264

73

27

5.0

0.08

CF-Ni10

179

60

40

5.4

0.09

CF-G

406

74

26

4.8

0.12

CF-Ni01-G

531

75

25

4.4

0.12

CF-Ni05-G

342

55

45

5.2

0.17

CF-Ni10-G

209

24

76

5.3

0.19

*BJH adsorption cumulative pore volume of pores between 1.7 and 300 nm diameter.

After further growth of active NiCo2O4, the specific surface area of all the CF-Nixx-G is increased comparing to their corresponding CF-Nixx. Only 26% of the total surface area is composed of external surface area in CF-G, while it goes up to 45 and 76% in CF-Ni05-G and CF-Ni10-G, respectively. From the SEM and TEM results, all NiCo2O4 exists in either nanorod or nanosheet structure. These well-defined nanostructures contribute to the enhancement of the total and external surface area. Based on the results summarized in Table 1, a derivation method is used to estimate the specific surface area of the NiCo2O4. Assuming the total specific surface area (SAtotal) is composed of two parts, one from substrate (SAsubstrate, here carbon fabric together with Ni nanoparticles is considered as substrate) and the other from the active NiCo2O4 nanostructure (SANiCo2O4). The SAtotal could be considered the summation of the area contributed by these two parts times their respective mass fraction in the composites, Eq (1): = ∗ + ∗

(1)


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where Xsubstrate and XNiCo2O4 are the mass fraction of substrate and NiCo2O4, respectively. Following Eq (1), the specific surface area of NiCo2O4 can be calculated as 483, 1473, 982 and 407 m2/g in CF-G, CF-Ni01-G, CF-Ni05-G, and CF-Ni10-G. Obviously, NiCo2O4 in CF-Ni01-G acquires the highest surface area among the samples probably due to its mesoporous nanorod structure as revealed in Figure 2Figure 1(e). Overall, the enlarged surface area will expose more active sites for chemical reactions and bigger pore size will promote faster ionic electrolyte diffusion, both of which are beneficial to improve the energy storage performance. 3.3. NiCo2O4 growth mechanism exploration

Figure 4. NiCo2O4 growth on (a) CF, (b) CF-Ni01 and (c) CF-Ni10 with different reaction times of one, three, five and eight hours. 1: one hour, 2: three hours, 3: five hours and 4: eight hours. 15 ACS Paragon Plus Environment


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Before characterizing electrochemical capacitance of the hierarchical electrodes, it is really important to understand the role of the interfacial Ni seeds in NiCo2O4 growth. In this study, the NiCo2O4 growth on CF, CF-Ni01 and CF-Ni10 was monitored at different reaction times of 1, 3, 5 and 8 hours, Figure 4. After first hour, a few NiCo2O4 thin sheets were observed on CF surface. Differently in CF-Ni01 and CF-Ni10, the larger Ni nanoparticles seems been wrapped with a thin layer of NiCo2O4. With reaction proceeds to 3 and 5 hours, the thin pieces in CF-G grows into larger and thicker ones. The curved-up edge area implies a loose adherence and weak interaction at the NiCo2O4-CF interface. On the low Ni-dosed CF, the sheet structure becomes obvious and they are preferred to grow on the pre-existing Ni nanoparticles. A compact packing of NiCo2O4 on Ni nanoparticles was clearly observed in CF-Ni01-G, inset figure of Figure 4(b2), which ensures a good interfacial interaction and thus better electrochemical performance. It is worth mentioning that the surface morphology change of CF-Ni10-G is not distinguishable in the first 3 hours but small sheets of similar Ni particle size is formed. After 5 hours, the surface is covered by densely distributed nanosheets. When the reaction proceeds to 8 hours, nanofiber is still not formed yet on pure CF, Figure 4(a4), while nanorods and nanosheet were clearly seen on CF-Ni01 and CF-Ni10 respectively, Figure 4(b4&c4). These results provide solid evidence of the vital role of Ni nanoparticles in NiCo2O4 growth and structure control. Embedding Ni nanoparticles not only changes the surface composition of the carbon fabric, but also the microscale surface texture by generating mesopores in bulk and at surface. Based on the results above, it is difficult to exclude the effect of mesoporous structure on NiCo2O4 morphology control. To explore the surface texture effect, Ni nanoparticles in CF-Nixx (xx=01, 05 and 10) were firstly etched out by reacting with 1.0 M HCl for 12 hours. Most of the Ni particles can be removed from this process as evidenced by TGA results in Figure S4. The


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etched CF were then used as substrates to grow NiCo2O4 using the same reaction condition. Different from the nanofiber morphology obtained on neat CF, NiCo2O4 nanorods were observed in the etched porous CF, Figure S5. These results clearly reveal that carbon surface texture also influences the NiCo2O4 growth. However, nanosheet morphology was not observed indicating the dominating role of Ni seeds in NiCo2O4 morphology control. Additional experiments were taken to study the precursor concentration effect on NiCo2O4 morphology, Figure S6. Results indicate that NiCo2O4 morphology is less affected by precursor concentration rather than embedded Ni nanoparticles. 3.4. Electrochemical energy storage evaluation

Figure 5. CV curves of (a) CF-G, (b) CF-Ni01-G, (c) CF-Ni05-G and (d) CF-Ni10-G.

Currently, two dominating methods are used to evaluate the electrochemical performance of supercapacitor electrode materials, i.e. two-electrode and three-electrode methods. Twoelectrode method mimics the unit cell configuration, the testing results is more close to the 17 ACS Paragon Plus Environment


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performance of a packaged cell in real application. Although three-electrode is very effective in analyzing faradic reactions and voltages, the heightened sensitivity of the three-electrode configuration can lead to large errors when projecting the energy storage capacity of an electrode material.47 In this study, two identical electrode pieces were assembled into a two-electrode cell for electrochemical tests. An initial screening of CF, CF-Ni01 and CF-Ni01-G electrodes reveals the superior energy storage performance after implanting Ni nanoparticles and growing NiCo2O4 nanostructures, Figure S7. The CV curves scanned at different scanning rates within the potential range of 0~1.0 V are shown in Figure . It’s well known that the specific capacitance of supercapacitor is proportional to the circled area of CV curve. Thus, it can be speculated that the specific capacitance of CF-Nixx-G is in the order of CF-Ni01-G > CF-Ni05-G> CF-Ni10-G, which is further confirmed by following charge-discharge tests. CV curves scanned at different potential windows are also provided in Figure S8.

Figure 6. Charge-discharge of (a) CF-G, (b) CF-Ni01-G, (c) CF-Ni05-G and (d) CF-Ni10-G. 18 ACS Paragon Plus Environment

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The galvanic charge-discharge results of CF-Nixx-G are obtained at different current densities ranging from 1 to 10 A/g, Figure . The specific capacitances are calculated based on the charge-discharge results using the following Eqn (2):

= ∙ ⁄ 2"

where Csp is the specific capacitance (F/g), I is the discharge current (A), dV(t)/dt is the slope of the discharge curve, and m is the total mass of active material NiCo2O4 in both electrodes.

Figure 7. (a) Capacitance vs. current density, (b) electrochemical impedance spectra, (c) cyclic stability, (d) Ragone plots of CF-Nixx-G and performance comparison with literature reported results. The numbers from 51-68 represents the reference number. 19 ACS Paragon Plus Environment


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The rate performance of the electrodes at different current densities is summarized in Figure (a). The capacitance of CF-G is 856, 823, 788 and 745 F/g at the current density of 1, 2, 5 and 10 A/g, which increases to 2367, 2346, 2268 and 2173 F/g after implanting small amount of Ni nanoparticles at the interface (CF-Ni01-G). Increasing the Ni particle density leads to a significant capacitance drop to about 1100 and 400 F/g for CF-Ni05-G and CF-Ni10-G at the current density of 1.0 A/g. It is worth mentioning that excellent rate performance was observed in all these electrodes. Specifically, 87, 92, 89 and 77% of the capacitance was remained in CF-G, CF-Ni01-G, CF-Ni05-G and CF-Ni10-G electrode with increasing current density from 1 to 10 A/g. Even though large capacitance of 2681F/g in NiCo2O4 has been reported in previous work, most of the tests if not all are conducted by using three-electrode method.48-49 In this study, NiCo2O4 in CF-Ni01-G can also achieve an ultrahigh capacitance of 2367 F/g by using symmetric two-electrode method, demonstrating its outstanding energy efficiency as supercapacitor electrode. Electrochemical impedance spectroscopy (EIS) was carried out in order to better understand the interfacial Ni seed effect on electrochemical performance, Figure 7(b). The impedance spectra of CF-G and CF-Nixx-G are composed of one semicircle at high frequency, followed by a linear part for CF-Nixx-G and non-linear part for CF-G at low frequency. The semicircle at high frequency indicates the contribution of the composite interfacial impedance. The larger radius of the semicircle, the higher interfacial impedance. Obviously, CF-Ni01-G acquires the smallest radius and thus the lowest interfacial impedance. An equivalent circuit model in Figure S9 is proposed to quantify the internal resistance including the solution resistance and charge transfer resistance. The equivalent circuit is composed of a double layer capacitance (CDL) in parallel with a series combination of a charge transfer resistance (Rct). The 20 ACS Paragon Plus Environment

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equivalent circuit is then completed by adding this composite interfacial impedance in series with a solution resistance (Rs) and a faradic capacitance (CF). The fitting results for each parameter are summarized in Table S1. The Rs decreases from 1.44 Ω in CF-G to 0.5 Ω in CFNi01-G and Rs increases with further increasing Ni seed concentrations in CF-Ni05-G and CFNi10-G. Similar change of Rct was also observed, Table S1. CF-Ni01-G exhibits the lowest Rs and Rct among all the electrodes revealing the lowest charge transfer resistance at the electrode/electrolyte interface and thus the highest electrochemical capacitance. The electrochemical cycling stability of CF-G and CF-Nixx-G were evaluated for up to 5000 cycles, as shown in Figure (c). For CF-G, the capacitance dropped slowly to about 90% of initial capacitance after the first 2000 cycles and then further decreased to 65% in the following 3000 cycles. While for other CF-Nixx-G electrodes with interfacial Ni seeds, at least 90% of initial capacitance was remained after 5000 cycles. The interfacial bonding between NiCo2O4 and CF is relatively weak in CF-G. The continuous volume change of NiCo2O4 during chargedischarge would loosen the interfacial interaction, impede the effective charge transfer from NiCo2O4 to CF and thus lead to poor electrochemical stability. After embedding the interfacial Ni seeds, the interfacial bonding and stability could be significantly strengthened.

Comparing

the stability of electrodes with different NiCo2O4 morphology, we can see that nanorod structure in CF-Ni01-G shows superior electrochemical stability than nanosheet structure in CF-Ni05-G and CF-Ni10-G. The comparison of specific capacitance, rate performance and capacity retention of the electrodes prepared in this work and reported from other literature are summarized in Table 2.



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Table 2. Electrochemical performance comparison with reported literature. Nanostructures

Specific capacitance

Rate performance

Capacity retention

Reference

CF-Ni01-G

2367 F/g (1 A g−1)

92% (10 A g−1)

96% (5000 cycles)

This work

CF-Ni10-G

1136 F/g (1 A g−1)

89% (10 A g−1)

89% (5000 cycles)

This work

NiCo2O4 nanoplates

294 F/g (1 A g−1)

48% (10 A g−1)

90% (2200 cycles)

50

Nickel–cobalt nanosheets

506 F/g (1 A g−1)

40% (10 A g−1)

94% (2000 cycles)

51

NiCo2O4-RGO

662 F/g (1 A g−1)

53% (16 A g−1)

52% (4000 cycles)

52

NiCo2O4 sub-microspheres

678 F/g (1 A g−1)

80% (20 A g−1)

87% (3500 cycles)

53

NiCo2O4@RGO

737 F/g (1 A g−1)

50% (10 A g−1)

94% (3000 cycles)

27

NiCo2O4@NiCo2O4

660 F/g (1 A g−1)

71% (20 A g−1)

66% (4000 cycles)

54

NiCo2O4 crystals

222 F/g (1 A g−1)

84% (3.5 A g−1)

96 % (600 cycles)

55

CNT/NiCo2O4 core/shell

694 F/g (1 A g−1)

82% (20 A g−1)

91% (1500 cycles)

55

NiCo2O4/CF

2658 F/g (2 A g-1)

70% (20A g-1)

80% (3000 cycles)

56

NiCo2O4 nanosheets

2010 F/g (2 A g-1)

72% (20 A g-1)

94 %(2300 cycles)

23

The energy density (E) of CF-Nixx-G is further calculated using following Eq (3):

# = $+, %& '"('/*

(3)

where t is the discharge time (hours), V(t) is the voltage (V) at discharge time t. The power density (P) can be calculated from Eq (4): - = #/'

(4)

Ragone plots with fabricated electrodes in this work and literature reported results are provided in Figure (d). The highest energy density achieves 60 Wh/kg at power density of 4 kW/kg in CF-Ni01-G, which is much higher than other electrodes with nanosheet structured NiCo2O4 and literature reported NiCo2O4 electrodes, Figure 7(d).57-74


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Figure 8. The electrochemical capacitance of NiCo2O4 on interfacial engineered electrodes with Ni, Co and Fe seeds at different concentrations.

After realizing the positive contrition of Ni nanoparticles at the interface, two more similar nanoparticles, i.e. cobalt and iron, are also studied to evaluate their effectiveness in improving energy storage performance. The morphology of the electrodes and their corresponding specific capacitance are summarized in Figure 8. With increasing Co and Fe seeds concentration, a similar morphology transition of NiCo2O4 from nanorods to nanosheets and gradual capacitance decrease were observed. Mixed nanorods and nanosheets were observed in Co seeded electrodes and obvious larger portion of nanosheets are higher concentration of Co seeds. Compared to the electrodes with Ni seeds at the interface, the capacitance of the electrodes with Co and Fe seeds is significantly lower probably due to the formation of nanosheets. Even though nanorods were formed on Fe seeded sample at the lowest seed concentration, its relatively lower capacitance compared to Ni and Co seeded electrodes could be



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attributed to the compact sheet structure under the nanorods that prevents an effective ion transport across the electrolyte/electrode interface. The underlying mechanism is not clear yet still under investigation, but one of the possible reasons might be the interface mismatch of the NiCo2O4 crystalline structure with Co and Fe seeds. 4. CONCLUSIONS In this work, different concentrations of metal (Ni, Co and Fe) nanoparticles have been inserted into the interface NiCo2O4/CF hybrid electrodes. The concentration of metal nanoparticles on carbon substrate affects the NiCo2O4 structures growth, i.e. low Ni particle density leads to the formation of NiCo2O4 nanorods and high density favors nanosheet formation. Embedding low density Ni nanoparticles at electrode interface successfully boosted the capacitance of NiCo2O4 up to 2367 F/g at current density of 1 A/g in a symmetric two-electrode configuration, which is significantly higher than the one of 856 F/g obtained from the electrode without metal seeds. The boosted capacitance is attributed to two major factors. One is the mesoporous NiCo2O4 nanorods morphology that exposes maximum active surface area to participate electrochemical reactions and the other is the decreased internal resistance that facilitates electron transfer across electrolyte/electrode and NiCo2O4/CF interfaces. In addition, NiCo2O4 with mesoporous nanorod structure show excellent cycling stability with >96% capacitance retention after 5000 cycles, which apparently outperforms electrodes without Ni seeds (65%) or with higher density Ni seeds (~90%). Being assisted by the interfacial Ni seeds, the enlarged surface area, reduced internal resistance and promoted interfacial interaction between NiCo2O4 and CF substrate synergistically contribute to the enhancement of energy storage property and cycling stability. A comparative study reveal that embedding cobalt and iron seeds could effectively control the NiCo2O4 morphology, but not helpful to improve the 24 ACS Paragon Plus Environment

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energy storage performance. In sum, the interfacial metal seeding approach would serve as an alternative method to boost energy capacity and cycling retention of existing hybrid electrodes. However, understanding of the roles of different metal seeds on different electroactive components requires more investigation in the future. ASSOCIATED CONTENT Supporting Information XRD spectrum of CF-Ni01-G, TGA of CF-Nixx and CF-Nixx-G, cross sectional images of CFNixx-G, TGA of CF-Nixx after acid washing, SEM images of NiCo2O4 grown on carbon support after etching out metal nanoparticles, SEM and TGA of CF-G grown in different concentrations of NiCo2O4 precursor solutions. CV of CF, CF-Ni01 and CF-Ni01-G, CV curves at different potential window of CF-Ni01-G, equivalent circuit model for EIS results and summary of EIS modeling results. AUTHOR INFORMATION Corresponding Author * J. Zhu. E-Mail: [email protected]. Tel: 1-(330)972-6859. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Acknowledgement is also made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research (#55570-DNI10). Partial support from the start-up fund of University of Akron is also acknowledged. HRTEM was performed at the Liquid 25 ACS Paragon Plus Environment


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