Iron Oxide Nanosheets and Pulse-Electrodeposited NiâCoâS

Research Article www.acsami.org

Iron Oxide Nanosheets and Pulse-Electrodeposited Ni−Co−S Nanoflake Arrays for High-Performance Charge Storage Hadi Khani and David O. Wipf* Department of Chemistry, Mississippi State University, Mississippi State Mississippi 39762, United States S Supporting Information *

ABSTRACT: Nanostructured nickel cobalt sulfide (Ni4.5Co4.5S8) has been prepared through a single-step pulse-electrodeposition method. Iron oxide nanosheets at hollow graphite shells (Fe3O4@g-shells) were prepared from graphite-coated iron carbide/α-Fe (g-Fe3C/Fe) in a two-step annealing/ electrochemical cycling process. Electrochemical characterization of the Ni4.5Co4.5S8 and g-Fe3C/Fe materials showed that both have high specific capacities (206 mAh g−1 and 147 mAh g−1 at 1 A g−1) and excellent rate capabilities (∼95% and ∼83% retention at 20 A g−1, respectively). To demonstrate the advantageous pairing of these high rate materials, a full-cell battery with supercapacitor-like power behavior was assembled with Ni4.5Co4.5S8 and g-Fe3C/Fe as the positive and negative electrodes, respectively. The (Ni4.5Co4.5S8//g-Fe3C/Fe) device could be reversibly operated in a 0.0−1.6 V potential window, delivering an impressive specific energy of 89 Wh kg−1 at 1.1 kW kg−1 and a remarkable rate performance of 61 Wh kg−1 at a very high specific power of 38.5 kW kg−1. Additionally, long-term cycling demonstrated that the asymmetric full cell assembly retained 91% of its initial specific capacity after 2500 cycles at 40 A g−1. The performance features of this device are among the best for iron oxide/hydroxide and bimetallic sulfide based energy storage devices to date, thereby giving insight into design principles for the next generation high-energy-density devices. KEYWORDS: iron carbide, iron oxide, cobalt−nickel sulfide, supercapacitor, pulse electrodeposition, alkaline rechargeable battery

1. INTRODUCTION The ever-growing global demand for nonfossil-fuel energy sources has stimulated an intensive search for multifunctional energy storage materials capable of delivering high energy with high power density and cyclability. Applications for these highperformance materials include rechargeable batteries and supercapacitors.1−3 This work aims to provide high-performance positive and negative battery-type electrode materials with dual functionality use in both aqueous rechargeable batteries and supercapacitors from a safe and environment-friendly perspective. Supercapacitors have the high power densities, fast charge− discharge rates and extended cycle life that are increasingly required for contemporary applications. However, increased implementation in portable electronic device (e.g., smartphones, tablets, notebook PCs and camcorders) and hybrid electric/plug-in-hybrid (HEV/PHEV) vehicles will require further improvements in supercapacitor specific energy without sacrificing their inherent high power density and excellent cyclability and also maintaining a low-cost and environmentally friendly production process. Supercapacitors are classified as either electric double-layer capacitors (EDLCs) or pseudocapacitors depending on their charge-storage mechanism. In EDLCs, charge is stored through rapid adsorption−desorption of electrolyte ions on high-surface area electrode materials, whereas pseudocapacitors store charge via fast and reversible © 2017 American Chemical Society

Faradaic reactions near the surface of metal oxides (e.g., RuO2, MnOx).4,5 Recently a new type of dual-ion batteries has been introduced as a new category of pseudocapacitors in which the charge is stored via an anion (e.g., PF6−) intercalation into graphite.6,7 Although carbon-based EDLCs exhibit high specific power (>10 kW kg−1) and excellent cycle life (exceeding 10 000 cycles), they have low specific energy (∼5−10 Wh kg−1) because of the limited capacitance of carbon-based electrodes.4,8 The specific capacitance of pseudocapacitive-type electrodes is superior to that of carbon-based EDLCs but their specific energies (30 Wh kg−1) are much inferior to lithium-ion batteries (∼120−200 Wh kg−1). Current supercapacitors cannot, therefore, fulfill the energy requirements of future electrically powered hybrid electric/plug-in-hybrid (HEV/PHEV) vehicles.9,10 Additionally, the high material cost of RuO2 and the poor ionic (10−13 S cm−1) and electronic (10−5−10−6 S cm−1) conductivity of pristine MnO2,11 two of the most widely used pseudocapacitive materials, have placed commercial and performance barriers to widespread commercial application. Research has thus focused on designing new materials to enhance the specific energy of supercapacitors Received: September 10, 2016 Accepted: January 13, 2017 Published: January 13, 2017 6967

DOI: 10.1021/acsami.6b11498 ACS Appl. Mater. Interfaces 2017, 9, 6967−6978

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ACS Applied Materials & Interfaces

sional iron oxide nanosheets offer a notable improvement in rate capability and cycle life of Fe3O4-based negative electrodes. An alternate approach is to recognize that the combination of the complementary voltage range, high capacity, and high-rate charge−discharge capability of the negative and positive electrodes produces an aqueous battery with a high energy storage capacity combined with supercapacitor-like power handling. Reconsideration of safer, aqueous based energy storage devices is increasing since lithium-ion batteries (LIBs), despite their notable merits and widespread applications, retain formidable and intrinsic safety issues. The aqueous full-cell device given here is a step to this requirement as it offers a specific energy comparable with NiMH batteries, having a power density needed by electric vehicles, and cycle lifetime comparable with LIBs but with overall greater safety. With these considerations in mind, we report the fabrication of a positive, battery-type electrode material based on the electrodeposition of nanoflake Ni4.5Co4.5S8 from a dilute solution of metal ions and thiourea onto carbon fiber cloth via a one-step square-wave pulsed potential technique. The Ni4.5Co4.5S8 nanoflakes display a high surface area, low electrical resistivity, and are rich in redox sites, making it a suitable material for high performance asymmetric electrochemical supercapacitor positive electrodes. A suitable battery-type electrode material for the negative electrode is based on Fe3O4 nanosheets at hollow graphite shells (Fe3O4@g-shell). This material is derived from g-Fe3 C/Fe nanoparticle precursors synthesized from aqueous iron nitrate and citric acid. The high retained specific capacities of the Fe3O4@g-shell (147 and 122 mAh g−1 at 1 and 20 A g−1, respectively) and Ni4.5Co4.5S8 (206 and 194 mAh g−1 at 1 and 20 A g−1, respectively) materials demonstrate their high rate performance. The full cell device was reversibly cycled in the potential window of 0.0−1.6 V to give an excellent specific capacity of 80 mAh g−1 at 1 A g−1 and superior rate performance with a specific energy of 89 at 1.1 kW kg−1, while retaining a specific energy of 61 Wh kg−1 at 38.5 kW kg−1 (less than 25% loss).

while maintaining the device’s high power and exceptional charge−discharge cyclability. The specific energy (E, Wh kg−1) of a supercapacitor device can be enhanced by increasing its specific capacitance (C, F g−1) or terminal voltage.12 One path to this end is pairing a battery-type Faradaic electrode with a capacitive/pseudocapacitive electrode to form an asymmetric (hybrid) supercapacitor device. Thanks to the high theoretical capacity of the batterytype Faradaic electrode (an increase in C) and the resulting complementary voltage of adding two dissimilar electrodes (an increase in V), the resulting asymmetric (hybrid) supercapacitors are noted for their high energy/power density and improved rate capability compared to EDLCs and pseudocapacitors.13 Many examples of high-performance positive-battery-type electrode materials have been developed; these include conductive polymers (polyaniline, poly(phenylenevinylene), polypyrrole, etc.),14 metal oxides/hydroxides (Co(OH)2, Ni(OH)2, MoO3, etc.),15 metal nitride/carbide (VN, Ti3C2, etc.),16,17 which have a specific capacitance of about 300−1000 F g−1 in a three-electrode configuration. However, these materials have either low conductivity or poor electrochemical stability, a fact that has largely limited their widespread applications in supercapacitors. To overcome this limitation, ternary nickel cobalt sulfides, Ni−Co−S, have been recently recognized to be a suitable material for high performance energy storage applications.10 It is reported that Ni−Co−S possess a wider variety of redox reactions and higher conductivities than the corresponding binary nickel and cobalt sulfides and ∼100 times higher electric conductivity than nickel−cobalt-oxides.18,19 Stimulated by the remarkable electrochemical performance of ternary nickel cobalt sulfide, we developed, for the first time, a facile one-step pulse-electrodeposition method for the synthesis of pentlandite NixCo9−xS8. The resulting interconnected nanoflake structure has a notable enhancement on both capacitance and rate capability over the corresponding binary pentlandite Co9S8 nanoflake structure. In contrast to the large effort to develop positive electrode materials, negative electrodes are still predominately carbonaceous, despite their limited specific energy. Recently, negative electrode materials based on the transition metal oxides or sulfides, such as Fe2O3@PPy,20 V3S5@3DGH,21 In2O3@ SWNT,22 MoO3@rGO,23 and Co3O4@RuO2, have been suggested.24 Among them, Fe3O4 has drawn the most attention because of its high theoretical specific capacity of ∼346.5 mAh g−1 (for Fe(III)/Fe(0)), wide negative potential window (−1.2 to 0 V vs Ag/AgCl), low cost, low toxicity, and high conductivity (σ = 2 × 104 S m−1).25,26 However, practical application of Fe3O4-based supercapacitor electrodes is limited by poor cycle lifetime, which is mainly associated with Fe3O4 particle aggregation during electrochemical cycling. To address this issue, a few recent reports have described uniformly distributing Fe3O4 on graphene sheets, thus preventing the aggregation of Fe3O4 particles during cycling.27−29 On the basis of these reports, we designed a simple, economical, and ecofriendly synthesis of graphite-coated Fe3C/α-Fe nanoparticles (g-Fe3C/Fe) that are subsequently electrochemically transformed to two-dimensional iron oxide nanosheets at hollow graphite shells (Fe3O4@g-shells). Because of the shortened ion path and high density of exposed active sites at the surface of two-dimensional nanosheets during an electrochemical charge-storage process,30,31 the derived two-dimen-

2. EXPERIMENTAL SECTION 2.1. Preparation of Negative Electrode. Graphite-coated iron/ iron carbide (g-Fe3C/Fe) nanoparticles were synthesized via a twostep sol−gel/heating procedure from an aqueous solution of Fe(III)citric acid complex (Figure S1). All chemicals were reagent grade and used without further purification. Typically, citric acid (5.76 g, 0.03 mol) and iron nitrate nonahydrate (2.4 g, 0.01 mol) were dissolved in 10 mL of distilled−deionized (DDI) water (18.2 MΩ-cm) and sonicated to obtain a homogeneous solution. The solution was continuously stirred at 90 °C to form a gel. Heating the gel in an oven for 12 h at 140 °C formed the highly porous foam product, Fe-CA140. Pyrolysis of this foam in a quartz tube at 700 °C (10 °C min−1 heating rate) for 2 h under UHP Ar atmosphere (flow rate of 50 mL min−1) and cooling naturally to room temperature produced product Fe-CA-700, which contained the g-Fe3C/Fe) nanoparticles. This was ground with an agate mortar and pestle to form a fine powder (∼2 μm). A suspension (slurry) of electrode material was prepared by mixing the powdered Fe-CA-700 powder, carbon black, and polyvinylidene difluoride (PVDF) at a mass ratio of 75:20:5 in N-methyl-2pyrrolidone (NMP). The suspension was mildly sonicated (Branson ultrasonic bath 1510) for 2 h followed by heavy sonication using a probe-type sonicator (Branson Sonifier 250, 2 mm tip diameter) at 40 W for 15 min in an ice bath. Ni foam current collectors (1 cm × 1 cm, Shanghai Winfay Metal & Plastic Manufacturing Co.) were sequentially degreased in hot acetone, cleaned in hydrochloric acid solution (3 M) under mild sonication for 10 min, washed in DDI 6968


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performance calculations. The calculation of specific energy and power is based on the following equations:

water, and rinsed in absolute ethanol. The Ni foam was dried in a vacuum oven at ambient temperature. The slurry was drop cast onto the clean Ni foam and dried at room temperature for 1 h. The electrode was then calendared from an initial 1.0 mm to a final 0.25 mm thickness and dried in the vacuum oven at 80 °C for 12 h before use. 2.2. Preparation of Positive Electrode. Ni−Co−S nanoflakes were electrochemically deposited from a dilute solution of metal ions and thiourea (ion complexing agent and sulfur source) onto woven carbon fiber cloth (CFC) (1 cm × 1 cm, ELATHydrophilic Plain Cloth) current collectors via a one-step square-wave pulsed potential technique. Prior to electrodeposition, the CFC was cleaned by sonicating in hot acetone (10 min) and heat-treating (600 °C for 10 min). The pulse potentiostatic electrodeposition of Ni−Co−S nanoflakes was carried out using a Solarton electrochemical workstation (Solartron Analytical 1470E Cell Test System) with a CFC (working electrode), Pt mesh (counter electrode), and silver/silver chloride (Ag/AgCl) (reference electrode) cell configuration. The aqueous electrodeposition bath was composed of 0.8 M thiourea and a NiCl2/CoCl2 mixture (12 mM total concentration) with molar ratios of 0.5, 1.0, 2.0, 2.5, and 3.0. The resulting samples were labeled Ni− Co−S-1, Ni−Co−S-2, Ni−Co−S-3, Ni−Co−S-4, and Ni−Co−S-5, respectively. Two more samples containing 0.8 M thiourea and 12 mM NiCl2 or 12 mM CoCl2 were made for the electrodeposition of Ni−S and Co−S samples, respectively. The optimal electrodeposition conditions used 15 repetitions of 50 s at −1.2 V and 50 s at 0.2 V. The obtained CFC-electrodeposited Ni−Co−S samples were rinsed in distilled water, dried under flowing N2, and further dried in vacuum oven at 70 °C for 10 h. The mass load of Ni−Co−S nanoflakes onto CFC substrates was about 0.65 mg cm−2. Exact mass loadings were determined by mass difference of the carbon cloth electrodes before and after electrodeposition. 2.3. Electrochemical Tests. Both three-electrode (half-cell) and two-electrode (full-cell) configurations were used in electrochemical measurements. The electrochemical performance of the cells was characterized by cyclic voltammetry (CV), galvanostatic charge− discharge (CC) tests, and electrochemical impedance spectroscopy (EIS) using a Solartron Analytical electrochemical workstation. Threeelectrode cell measurements of the CFC-electrodeposited Ni−Co−S nanoflakes or Ni foam-supported Fe-CA-700 used a Pt mesh auxiliary and a double-junction saturated silver/silver chloride (Ag/AgCl) reference electrode, respectively. The electrolyte was aqueous 4 M KOH. For all two-electrode cell measurements, the full-cell device was assembled into a coin-type cell with CFC-electrodeposited Ni−Co−S4 nanoflakes as the positive electrode, Ni foam-supported Fe-CA-700 as the negative electrode, cellulose filter paper as the separator, and 4 M KOH aqueous solution as the electrolyte (Figure S2). Cycling tests were carried out at room temperature for 5000 cycles at a current density of 40 A g−1 over the potential range of 0 to 1.6 V. EIS measurements used an AC perturbation of 5 mV at frequencies of 10 kHz to 0.01 Hz, while polarizing the electrode at the midpoint of the charging plateau potential. The specific capacities (Cs) were calculated from the charge− discharge curves according to the equation

Cs =

I Δt m

∫

E = I V (t ) dt

P=

E Δt

(4) −1

where E is the specific energy (J g ), I is the discharge current density (A g−1), V is the discharge voltage (V), P is the specific power (W g−1), and Δt refers to the total discharge time (s). The specific capacitance (Cp) is calculated from the following equation by substituting the specific energy obtained from eq 3

Cp =

2E ΔV 2

(5)

where Cp, E, and ΔV represent the specific capacitance (F g−1), specific energy (J g−1), and the working potential of the cell (V). It is worth mentioning here that battery-type Faradaic electrodes, which are the focus of this work, often display flat discharge plateaus during charge/discharge tests under constant current, and this should be distinguished from that EDLCs and pseudocapacitors exhibiting a linear dependence of the charge stored with the width of the potential window.5 2.4. Material Characterization. The morphology, structure, and composition of the as-obtained samples were characterized by fieldemission scanning electron microscopy (FESEM, Zeiss SUPRA 40), transmission electron microscopy (TEM, JEOL 2100 200 kV), energy dispersive spectrometer (EDX), X-ray diffraction (XRD, Rigaku SmartLab, Cu Kα radiation, λ = 1.5406 Å), and X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha). Thermogravimetric analysis (TGA, Mettler Toledo) was conducted at a temperature range of 30−700 °C under Ar flow with a heating rate of 5 °C min−1. Elemental analyses were conducted with a CHN Elemental Analyzer (model NA 1500 NC; Carlo Erba, Milan, Italy) and an inductively coupled plasma mass spectrometer (ICP-MS, ELAN DRC II, PerkinElmer, Inc.).

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of the Positive Electrode Material. High-quality ultrathin macroporous Ni4.5Co4.5S8 nanoflake arrays on CFC were synthesized via a one-step square-wave pulse potential technique from a dilute solution of Co and Ni salts with thiourea as an ion complexing agent and sulfur source. Since nickel-foam substrate is electrochemically active within a positive potential window (0−0.5 V vs Ag/AgCl reference electrode) in KOH electrolyte, it will add an extra capacitance to the Ni−Co−S samples.33 Thus, carbon-fiber cloth (CFC) was selected as an alternate substrate and current collector due to its good electrical conductivity and high surface area. The pulse electrodeposition technique was chosen due to its superior control over electrochemical parameters compared to alternative electrodeposition methods.8,34,35 Furthermore, the method allowed for additional control over the growth of Ni−Co−S nanoflakes by adjusting the concentrations and pulse electrodeposition parameters. Seven metal sulfide samples (Ni−S, Co−S, Ni− Co−S-1, Ni−Co−S-2, Ni−Co−S-3, Ni−Co−S-4, and Ni−Co− S-5) were synthesized and investigated. The EDX elemental maps of the Ni−Co−S-4 nanoflakes on the CFC substrate show a homogeneous distribution of Co, Ni, and S (Figure S3). Table S1 lists the EDX data for the Co:Ni atomic ratio of the five CFC-electrodeposited Ni−Co−S samples. Each Ni−Co−S composition measurement was made in triplicate at 15 different spots (∼10 μm × 10 μm). Table S1 also shows ICP-MS analysis of the five samples, which matches the nickel, cobalt,

(1)

where Cs is the specific capacity (mAh g−1) of a single electrode or cell, I is the charge−discharge current (mA), Δt is the discharge time (h), and m represents the mass of active materials (g) on a single electrode or the total mass of active materials on the two electrodes of the assembled full cell device. In order to balance the charge between two electrodes in full-cell device (q+ = q−), the mass load is balanced as32

m+ C = − m− C+

(3)

(2)

where m is the mass of active material (g) and Cs (mAh g−1) is the specific capacity of electrodes. The total weight (M = m− + m+) of the active materials on both electrodes were considered for the full-cell 6969


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distributed throughout the lattice in groups of eight which lie at the corners of subcubes of edge length 0.250 nm. Of the 32 octahedral holes, 28 are pseudo-octahedral, distorted, and empty, whereas four are truly octahedral and each contains a Co atom in a formally zero oxidation state. Thus, the solid attains the overall stoichiometry Co9S8.38 With this in mind, we believe that doped Ni atoms must maintain the same charge as Co2+ to form a pentlandite-type cobalt−nickel-sulfide structure. To gain more information on the chemical bonding state and composition of the as-synthesized cobalt−nickel−sulfur coating, X-ray photoelectron spectroscopy (XPS) measurements were conducted and the results are presented in Figure S4. By using a Gaussian fitting method, the Co 2p high-resolution spectra can be fitted to a spin−orbit doublet and two shakeup satellites (Sat.). In the Co 2p XPS spectrum (Figure S4), the absence of a notable peak within 779−800 eV indicates that Co3+ does not exist in the as-prepared Ni−Co−S-4 sample. However, the spin−orbit doublet pair corresponds to the low energy (Co2+ 2p3/2) and high energy band (Co2+ 2p1/2) centered at 781.45 and 797.35 eV, respectively.39,40 This pair of doublet peaks for Co 2p is well matched with octahedrally coordinated Co2+ 2p XPS peak of the pentlandite cobalt− nickel-sulfide structure.41 Moreover, the energy difference between Co 2p3/2 (781.38 eV) and Co 2p1/2 (797.28 eV) is 15.9 eV: well matched with the spin−orbit splitting value of Co(II) 2p3/2-Co(II) 2p1/2.39 In the Ni 2p XPS spectrum (Figure S4), the fitted Ni 2p peaks shows two shakeup satellites and one spin−orbit doublet at 856.21 and 873.95 eV, characteristic of Ni2+(2p3/2) and Ni2+(2p1/2), respectively.40−43 In the S 2p spectrum (Figure S4), the peaks at 162.2 and 163.8 eV with a shakeup satellite at 169.5 are characteristics of S2−(2p3/2) and S2−(2p1/2), respectively, which are typical of metal−sulfur bonds,19,43,44 while the binding energy at 168.3 eV can be attributed to the surface-adsorbed sulfur species with high oxidation state such as SO42− and HSO4−,45−47 apparently originated from the electrochemical overoxidation of thiourea.48 According to the XPS analysis, the near-surface of the Ni− Co−S-4 sample has a composition of Co2+, Ni2+, and S2− which is in good agreement with the proposed pentlandite Ni4.5Co4.5S8 structure. Figure 2 shows the SEM images of Ni−Co−S-4 and Co−S samples at different magnifications. As shown in this figure, the CFC substrates appear to be uniformly coated by the Ni−Co− S-4 and Co−S films, which consist of well-defined, randomly interconnected vertical nanoflake arrays. It was observed that increasing the Ni composition in samples reduced the nanoflake size and intersheet spacing and increased the porosity. Note the notably smaller nanoflakes in Ni−Co−S-4 than in Co−S (cf. Figures 2b and 2e). The interconnected Ni− Co−S-4 nanoflakes are typically several hundred nanometers in height with a thickness of about 10 nm and having a hierarchical porous structure with micro- to nanopores (Figure S5) that will enhance electrolyte and electron transport, reducing resistance. Different morphology and microstructures were observed when changing the pulse duration and number of cycles during electrodeposition. Varying the pulse durations from 0.5 to 100 s showed that a pulse duration of 50 s over 15 cycles gave the highest specific charge storage capacity when tested by CV. Shorter pulse durations produced nanoflakes with large intersheet gaps and low porosity coating the CFC substrate (Figure S6a). Longer pulse durations produced a nearly filled pore space between nanoflakes (Figure S6b). The effect of the

and sulfur atomic ratios observed by EDX. The results of elemental analysis reveal that electrodeposition baths containing Co:Ni molar ratios of 2:1, 1:1, 1:2, 1:2.5, and 1:3, result in samples with measured Co:Ni ratios of 4.02, 2.07, 1.25, 1.01, and 0.80, respectively. On this basis, it is proposed that electrodeposition baths with Co:Ni molar ratios of 2:1, 1:1, 1:2, 1:2.5, and 1:3 produce Ni−Co−S nanoflakes with respective compositions of Ni 1.8 Co 7.2 S 9, Ni 3.0 Co 6.0S 9 , Ni4.0 Co 5.0 S 9 , Ni4.5Co4.5S9, and Ni5.0Co4.0S9. The relevant chemical reactions involved in the electrodeposition of Ni−Co−S sample can be expressed as follows: 8TU + x Ni 2 + + (9 − x)Co2 + + 18e− ⇌ NixCo(9 − x)S8 + 8CN− + 8NH4 +

(6)

where TU is thiourea and x = 0, 1.8, 3.0, 4.0, 4.5, and 5.0. It was observed that the charge storage capacity of NixCo(9−x)S8 nanoflakes was increased compared to Co9S8 and increased with increasing Ni composition; showing a maximum capacity for a Co:Ni atomic ratio around 1. Therefore, this sample, Ni− Co−S-4, having the Ni4.5Co4.5S8 structure was selected as the optimized material for the positive electrode of the full-device fabrication in this study. Figure 1a shows the XRD pattern for the Ni−Co−S-4 sample with a 1:1 Co:Ni atomic ratio. Nearly all the diffraction

Figure 1. XRD patterns and the standard JCPDS cards for the (a) Ni− Co−S-4 sample and JCPDS 00-019-0364 (cobalt pentlandite, Co9S8), (b) Fe-CA-450 sample and JCPDS card 01-075-0449 (Fe3O4), and (c) Fe-CA-700 sample and JCPDS cards 98-000-0170 (cohenite, Fe3C) and 03-065-4899 (α-Fe).

peaks in the XRD pattern of Ni−Co−S-4 sample are well indexed with the standard peaks for Co9S8 (cobalt pentlandite, Co9S8, JCPDS card 00-019-0364). Since the ionic radii of Ni and Co are very similar, Ni ions can partially replace the Co ions while maintaining the Co9S8 phase. As a result, the bimetallic Ni4.5Co4.5S8 sample has XRD patterns almost identical to those of the Co9S8 phase. The close similarities in XRD patterns has been observed in several previously reported bimetallic sulfides and is closely matched with their JCPDS cards.36,37 The conventional unit cell of the cobalt pentlandite (Co9S8) has the formula Co36S32 and contains a cubic close-packed arrangement of sulfide ions containing 64 tetrahedral holes and 32 octahedral holes.38 Of the 64 tetrahedral holes, 32 are occupied by Co2+ ions, these being 6970


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Figure 2. SEM characterization of Ni−Co−S-4 (a-d) and Co−S (e-f) nanoflakes.

to mass losses of 29%, 29%, and 15%, respectively. To elucidate the structural composition of the material in each of the large mass loss regions of the TGA curve, representative samples were prepared. Typically, Fe-CA-140 was heated to each target temperature (250, 450, and 700 °C) for 2 h, then cooled to room temperature. The obtained samples were then referred to as Fe-CA-250, Fe-CA-450, and Fe-CA-700 according to the target pyrolysis temperature. The initial mass loss (3%) from 20 to 155 °C is due to the vaporization of free water, possibly adsorbed during the storage and handling of the hygroscopic Fe-CA-140 sample. The first steep section of the TGA curve, between 155 and 320 °C, is considered to be due to the removal of Fe-coordinated water molecules and decomposition of terminal carboxylic acid groups to CO2. The latter was confirmed via FTIR analysis of samples heat treated at 150 and 250 °C. The characteristic carboxylic acid stretches centered at 3000 cm−1 (broad, O−H) and 1706 cm−1 (C = O) for Fe-CA150 completely disappear after heating to 250 °C. The second steep section of TGA curve, between 320 and 565 °C, can be assigned to further decomposition of the carbonaceous matrix to form CO2 or CO gas.49 The XRD patterns of Fe-CA-450 (Figure 1b) are well indexed with the standard peaks for iron oxide (Fe3O4, JCPDS card 01-075-0449). This temperature regime of the TGA curve can therefore be considered as the nucleation step; where the transition from an iron-rich carbonaceous matrix to an iron-oxide-nanoparticle amorphous-carbon composite rapidly occurs. The TEM images in Figure 3a and b show well-dispersed Fe3O4 nanoparticles in an amorphous carbon-rich matrix, which later acts as a reactive

number of electrodeposition cycles was tested with a 50 s pulse duration time. When fewer cycles were applied, the nanoflakes showed large intersheet gaps on the CFC substrate (Figure S6c). With extended cycling the pores between nanoflakes become partially filled in with additional material (Figure S6d) resulting in decreased performance. The optimal number was found to be 15 cycles by evaluating the specific charge storage capacity of the materials. 3.2. Structural Characterization of Negative Electrode. As a starting material for the negative supercapacitor electrode, graphite-coated iron/iron carbide (g-Fe3C/Fe) nanoparticles have been synthesized via a facile two-step heating procedure from an aqueous solution of Fe(III)-citric acid complex. Gravimetric analysis shows that heating the Fe(III)-citric acid complex solution (as described in Experimental Section) at 140 °C for 12 h produces a mass loss of 56.7 ± 0.8%, which is determined to arise from the vaporization of uncoordinated water and elimination of nitrate. This mass loss mechanism is in accordance with elemental analysis listed in Table S2. In our initial investigation, we observed that the pyrolysis of the as-prepared foam (Fe-CA-140) in an argon atmosphere at 700 °C produced a black, highly magnetic powder sample. A powder XRD pattern of this product showed peaks characteristic of Fe3C and α-Fe (Figure 1c). To investigate the mechanism of formation of the iron/iron carbide from the Fe-CA-140, the foam was subjected to TGA analysis in an argon atmosphere. The TGA curve (Figure S7) shows three major mass-loss steps at 155, 320, and 565 °C, corresponding 6971


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Figure 3. HRTEM images of Fe-CA-450 (a−c) and Fe-CA-700 (d−f) samples.

bright graphite shells displays clear lattice fringes with a spacing of 0.34 nm, which matches well to the (200) plane of graphite. The overall conclusion is that product Fe-CA-700 is composed of graphite-coated iron/iron carbide (g-Fe3C/Fe) nanoparticles. 3.3. Electrochemical Characterization of Negative Electrode. Figure 4a shows the first seven consecutive cyclic voltammograms (CV) of Fe-CA-700 in 4 M KOH electrolyte. The broad oxidation peak at −0.52 V is attributed to the oxidation of the Fe3C/α-Fe to Fe3O4. This peak decreases on successive scans, and it absent by the seventh cycle. A background CV (not shown) of bare Ni foam in this potential region shows no significant current. This result indicates that Fe3C/α-Fe oxidation is irreversible in alkaline media and that Fe3C/α-Fe is consumed during the first seven cycles. The reduction and oxidation peaks appearing at −1.22 V and −0.97 V are assigned to the Fe2+ and Fe3+ redox conversion according to mechanism 7:52

template for carbothermal reduction of the Fe3O4 nanoparticles to Fe/Fe3C at 565 °C. As can be seen in Figure 3c, the lattice fringe value of d = 0.485 nm matches well to the (111) orientation of Fe3O4. The final transformation from iron oxide to the iron carbide occurs remarkably quickly, with complete disappearance of the crystalline oxide peaks at 565 °C. This crystallographic transformation coincides with a sharp mass loss of 15% between 565 and 700 °C on the TGA curve. As confirmed by previous reports,49−51 heating the iron oxide embedded in carbon/oxygen-rich matrix above 565 °C brings about the emission of CO2 and CO, suggesting that the iron oxide nanoparticles react with the surrounding carbon-rich matrix to form Fe3C nanoparticles. The observed XRD patterns of product Fe-CA-700 (Figure 1c), are in good agreement with the standard peaks for iron carbide (cohenite, Fe3C, JCPDS card 98-000-0170), and α-Fe (JCPDS card 03-065-4899), implying the material contains both Fe3C and α-Fe within a well-crystallized structure. The XRD peaks around 44.6° and 65.0° are assigned to the diffraction plane of α-Fe. Figure 3d-e shows the TEM images for Fe-CA-700 sample. The Fe3C/Fe nanoparticles are darker in contrast to the surrounding graphitic carbon sheets because of their higher atomic number. The TEM images of Fe-CA-700 show that the products contain spheroidal nanoparticles, which have diameter distributions between 10 and 100 nm and are separated by hollow graphitic carbon eggshell-like nanoparticles. Closer observations reveal that the Fe3C/Fe nanoparticles are entirely encapsulated by the continuous graphitic carbon shells, forming g-Fe3C/Fe core− shell nanoparticles. The HRTEM image (Figure 3f) of the

Fe3O4 + 4H 2O + 2e− ⇌ 3Fe(OH)2 + 2OH−

(7)

The peak values for Fe3O4/Fe(OH)2 increase to a constant value by seven cycles and maintain this value over prolonged cycling periods. The conclusion is that a reversible redox reaction of Fe3O4 to Fe(OH)2 takes place after the initial oxidation of Fe3C/α-Fe to Fe3O4. TEM imaging supports this and shows that upon cycling g-Fe3C/Fe is converted into Fe3O4 and hollow graphitic carbon shells (Figure 5a−e). The Fe3O4 particles appear to be in the form of thin sheets 6972


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0.485 nm, d = 0.295 nm, and d = 0.251 nm in the nanosheets match well with the (111), (220), and (311) orientations of Fe3O4, respectively, corresponding to a cubic inverse spinel structure (Figure 5c−e). CV curves of the negative electrodes at scan rates from 2 to 50 mV s−1 are displayed in Figure 4b. An increase in peak separation with scan rates indicates a kinetically limited reaction, likely due to charge transport. Plots of the peak currents (Ipa and Ipc) versus sweep rate (v) and v1/2 (Figure S8) indicate that the negative electrode reaction is based on diffusional transport. The Ipa and Ipc versus v1/2 plots exhibit a linear relationship, whereas the Ipa and Ipc versus v plots display a nonlinear behavior. Generally, Ipa and Ipc versus v1/2 plots are linear for a kinetically uncomplicated planar diffusion controlled redox reaction while Ipa and Ipc versus v plots are linear for an adsorption process.53 Galvanostatic charge−discharge measurements were conducted to quantify the capacitive performance of the Fe3O4 electrode (derived from the g-Fe3C/Fe in Fe-CA-700). Figure 6a shows the representative plots of the charge−discharge curves in a potential window of −1.1 to 0 V (vs Ag/AgCl) at current densities from 1 to 20 A g−1. The charge−discharge curves are nonlinear, implying a Faradaic redox mechanism, in good agreement with the CV curves and show 97% Coulombic efficiency indicating good electrochemical reversibility. The specific capacity during the galvanostatic charge−discharge process was calculated (eq 1) and was found to be 147, 142, 128, and 122 mAh g−1 at discharge current density of 1, 5, 10, and 20 A g−1, respectively. The material also shows an excellent capacity retention of 96, 87, and 83% at current densities of 5, 10, and 20 A g−1, respectively, when compared with the 1 A g−1 results. This demonstrates that the g-Fe3C/Fe derived Fe3O4 electrode has potential application for high-rate energy storage devices. 3.4. Electrochemical Characterization of Positive Electrode. The electrochemical properties of the nanoflake Ni−Co−S-4 as a positive electrode was investigated in a three-

Figure 4. (a) CV curves of the initial seven cycles for Fe-CA-700 electrode at a scan rate of 10 mV s−1. (b) CV curves of the Fe-CA-700 electrode at different scan rates in 4 M KOH solution.

(nanosheets) several atomic layers in thickness and with average diameters of 20 nm. The lattice fringe values of d =

Figure 5. HRTEM images of KOH-cycled Fe-CA-700 samples (a−e). 6973


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Figure 6. (a) Galvanostatic charge−discharge curves of the Fe-CA-700 electrode at various current densities. (b) CV curves of the Ni−Co−S-4 based electrode at different scan rates. (c) CV curves of Co−S, Ni−S, and Ni−Co−S with different Ni:Co molar ratios at a scan rate of 10 mV s−1. (d) Galvanostatic charge−discharge curves of the optimized Ni−Co−S-4 nanoflakes at various current densities.

prepared with various pulse electrodeposition parameters including pulse duration and number of cycles are depicted in Figure S10. These results support the microscopic data, indicating that electrodes prepared using 15 cycles and 50 s pulse durations gives the best charge storage capacity. Galvanostatic charge−discharge measurements were carried out in a three-electrode cell at current densities from 1 to 20 A g−1 (Figure 6d). A calculated specific capacity of 206, 204, 203, and 194 mAh g−1 was found at current densities of 1, 5, 10, and 20 A g−1, respectively. Consistent with the CV results, the plateaus in the charge/discharge curves indicate the existence of Faradaic processes. The high Coulombic efficiency of 99% indicates the superior reversible redox reaction at Ni−Co−S-4 electrodes. 94.6% of the charge capacity was retained when increasing the current density from 1 to 20 A g−1 (206 mAh g−1 vs 194 mAh g−1), suggesting high rate capability. The excellent electrochemical performance of the proposed positive electrode can be attributed to several factors: first, the ultrathin and mesoporous characteristics of Ni−Co−S-4 nanosheets gives a very high surface area, providing numerous electroactive sites for redox reactions. Second, the open space between these layers could serve as a reservoir for ions, and enhance electrolyte penetration within the electrode. In addition, the good intrinsic electrical conductivity and adhesion to the CFC substrate of the Ni−Co−S-4 nanosheets reduces Ohmic drop. In prior studies of nickel−cobalt-sulfide material, good rate performance was only achieved in carefully designed composites containing doped carbons or graphene as a

electrode cell with 4 M KOH aqueous electrolyte. As shown in Figure 6b, the CV curves obtained under different scan rates show a pair of redox peaks, which are assigned to the reactions between the Ni−Co−S-4 and the alkaline electrolyte, according to eq 8:54 Ni4.5Co4.5S8 + 9OH− ⇌ Ni4.5Co4.5S8(OH)9 + 9e−

(8)

Plots of Ipa and Ipc versus v1/2 are linear (Figure S9), which indicates a diffusion limited (in OH−) reaction. The electrochemical behavior of Ni−Co−S nanoflakes synthesized in different bath compositions/were investigated in alkaline electrolyte. At a scan rate of 10 mV s−1, the separation of the anodic and cathodic peaks increases as the Ni/Co atomic ratio increases from 0 to 1:1 (Figure 6c). Since this increase in peak separation is accompanied by an increase in peak current, we ascribe this to Ohmic drop in the three electrode cell (see Supporting Information data below). In addition, the charge storage capacity is optimized at 1:1 Ni/Co atomic ratio (i.e., Ni−Co−S-4) and this composition demonstrates the best energy storage performance compared to the other Ni−Co−S compositions. Our results agree with previously reported electrochemical behavior of cobalt−nickel sulfides, which indicate superior charge storage capacity of Co− Ni−S material when the Ni/Co atomic ratio is close to 1:1.44,55,56 As discussed above, the morphology of Ni−Co−S-4 electrode is highly correlated to the applied electrodeposition parameters. The CV behavior of Ni−Co−S-4 electrodes 6974


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Figure 7. (a) Nyquist plot of the Ni−Co−S-4 electrode, Fe-CA-700 electrode, and full-cell device. Solid lines indicate fitted curves. (b) CV curves of the full-cell device at various scan rates. (c) Galvanostatic charge−discharge curves of the full-cell device at various current densities. (d) Cycling performance and Coulombic efficiency of the full-cell device.

conductive substrate and active component.57,58 Therefore, the significant enhancement using the pulse-electrodeposition technique described here is evident.45 3.5. Electrochemical Characterization of Ni−Co/Fe Cell. The complementary voltage range and good electrochemical performance of the Ni−Co−S-4 nanoflake and gFe3C/Fe-derived Fe3O4 electrode suggested that a twoelectrode full-cell energy storage device would show superior behavior. To this end, a two-electrode system using Ni−Co−S4 nanoflakes as the positive electrode and g-Fe3C/Fe derived Fe3O4 composite as the negative electrode was constructed and investigated within a potential window of 0.0 to 1.6 V in 4 M KOH aqueous electrolyte. Figure 7a shows Nyquist plots of each positive and negative electrode in a three-electrode systems and full-cell device fabricated in a two-electrode system. For these EIS data, a bias potential was set at the midpoint of the charging plateau, 0.22 and −0.85 for the positive and negative material and 1.28 V for the two-electrode device. At high frequencies, the intercept at the real axis (Z′) represents a combined resistance (Rs), which includes the ionic resistance of electrolyte, intrinsic resistance of active material, and contact resistance at the interface of active material/current collector. These data were fit to equivalent circuits to determine Rs and Rct values (see Figure S11). The Rs values for the positive, negative, and full-capacitor electrodes are 0.91, 1.71, and 3.15 Ω, respectively. The different Rs values for the negative and positive electrode indicates the higher resistance of carbon fiber (positive substrate) than nickel foam (negative substrate) since the other conditions were the same.

The semicircle observed at higher frequency (lower left) arises from the presence of the charge-transfer resistance (Rct) at the electrode/electrolyte interface and the diameter of this semicircle indicates the magnitude of the interfacial charge transfer resistance associated with the Faradaic reactions (Figure S11). The small Rct values for the negative (0.28 Ω) electrodes indicates fast charge-transfer kinetics at the electrodes. However, the nearly negligible semicircle observed for the positive electrode suggests that much more rapid chargetransfer is taking place at the interface of Ni−Co−S-4 nanoflakes due to their high conductivity and accessible surface. Rct of the full capacitor (0.29 Ω) indicates that it is limited by the negative electrode. Figure 7b exhibits CV curves for the full-cell device, assembled with a total mass loading of M = 1.68 mg cm−2 for the two electrodes in 4 M KOH aqueous electrolyte at different scan rates. The specific capacity of the full-cell device (Cs,cell) based on the total mass of the active materials in the two electrodes is calculated from the CC curves (Figure 7c) according to eq 1. The Cs,cell values of the fabricated cell are calculated to be 80, 76, 72, and 67 mAh g−1 (corresponding to 250, 236, 225, and 200 F g−1) at current densities 1, 5, 10, and 20 A g−1, respectively. The device demonstrates an 83% retention of the capacity at 20 A g−1 compared to the capacity at 1 A g−1 (80 mAh g−1 versus 67 mAh g−1), which indicates the excellent rate capability of the full-cell device for high-rate energy storage applications. The decrease in specific capacity as the discharge current density increases may be explained by a limitation from the diffusion and migration of electrolyte ions 6975


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ACS Applied Materials & Interfaces OH− within the electrode. As shown in Figure 7c, CC curves consist of two charge and discharge voltage plateaus corresponding to the pair of strong redox peaks revealed in the CV curves of the full-cell device. This illustrates that both the negative and positive electrode materials exhibit a typical Faradaic behavior consistent with a battery, whose reversible capacity mainly originates from a reversible Faradaic reaction related to Fe3O4/Fe(OH)2 and Ni4.5Co4.5S8/Ni4.5Co4.5S8(OH)9 redox couple. Considering the complementary electrochemical redox regions of the positive and negative electrode materials, a general charge/discharge mechanism can be described for the full asymmetric device. Figure S12, illustrates this concept by superimposing the three-electrode cyclic voltammograms of the positive Ni−Co−S-4 nanoflakes and negative g-Fe3C/Fe derived Fe3O4 nanosheet electrodes. During a charging cycle the negative electrode material can be reduced according to eq 7 (section 3.3) while the positive electrode material is oxidized according eq 8. Likewise, the discharge of the full cell results in the spontaneous reoxidation of the negative electrode and rereduction of the positive electrode resulting in a 1.6 V cycling potential. Additionally, the cycling performance of the full Ni−Co//Fe asymmetric device was determined at a very high current density of 40 A g−1 (Figure 7d), and the results demonstrate that the full device has excellent cycling stability, maintaining 91% and 82% of its initial capacity after 2500 and 5000 cycles, respectively. Furthermore, the full-cell device still maintains a good electrochemical reversibility with 98% Coulombic efficiency even after 5000 cycles (Figure 7d). The loss of capacity after extended cycles was seen to be due to the increase in wettability of the graphite-coated Fe3C/α-Federived iron-oxide nanosheets leading to their dispersion into electrolyte upon cycling. Comparing TEM images (Figure S13) of graphite-coated Fe3C/α-Fe-derived iron-oxide nanosheets after 10 and 5000 cycles shows an aggregation of iron-oxide nanosheets, suggesting a process of dissolution and recrystallization of Fe3O4/Fe(OH)2 with cycling. This is also a likely reason for the loss of capacitance with extended cycling. However, the achieved retention rates at such a high charge− discharge rate are better than those reported KOH-based devices based on iron hydroxide/oxide, as the negative electrode, such as C-PGF//Ni(OH)2/CNTs (78% retention after 2000 cycles)29 and f-Ni/Fe cell (89.1% retention after 1000 cycles).59 Figure 8 demonstrates the relationship between the power and specific energy for our full-cell device and literature devices. The full device is able to achieve an excellent specific energy of 89 Wh kg−1 at the high specific power of 1.1 kW kg−1 and impressive retention of 84 Wh kg−1 at 5.5 kW kg−1 (5.6% decay), 80 Wh kg−1 at 11.1 kW kg−1 (10.1% decay), 71 Wh kg−1 at 21.3 kW kg−1 (20.2% decay), and 61 Wh kg−1 at 38.5 kW kg−1 (31.5% decay). The results show better high energy/power performances than many recent reported values in the literature, such as MnO2//Fe2O3@PPy (2 Wh kg−1 at 19.14 kW kg−1),20 Ni1.5Co1.5S4//rGO (37.6 Wh kg−1 at 0.775 kW kg−1),44 NiCo2S4@NCF//OMC@NCF (45.5 Wh kg−1 at 0.512 kW kg−1),57 Ni−Co−S@G//PCNS (43.3 Wh kg−1 at 0.800 kW kg−1),58 NiCoS//graphene (60 Wh kg−1 at 1.8 kW kg−1),45 GF-CoMoO4//GF-CNT@Fe2O3 (41.1 Wh kg−1 at 11.2 kW kg−1),60 NiCo2S4//G/CSs (42.3 Wh kg−1 at 0.476 kW kg−1),61 CoO@PPy//AC (43.5 Wh kg−1 at 0.0088 kW kg−1),62 NiCo2S4@PPy-50/NF//AC (34.62 Wh/kg at 0.120 kW/kg),63 and Ni(OH)2-MnO2//RGO (54.0 Wh/kg at 0.392 kW/kg),64 Fe3O4@carbon nanosheet//CPY (18.3 Wh/kg at 0.351 kW/

Figure 8. Ragone plot of the proposed full-cell device compared with most recent reported full-cell devices.

kg).65 If we consider the mass-loading of active material (1.68 mg cm−2) and inactive components of the cell, including Nifoam (28.1 mg cm−2) and carbon-cloth (22.7 mg cm−2) current collectors, separator (8.9 mg cm−2), and KOH electrolyte (100 mg), the specific energy of the full-cell actual battery as constructed is 0.93 Wh kg−1 at a specific power of 11.4 W kg−1 and 0.63 Wh kg−1 at 400 W kg−1.

4. CONCLUSION In summary, a straightforward method to fabricate high quality Ni4.5Co4.5S8 nanoflakes on CFC electrodes via a pulse step electrodeposition method has been demonstrated to obtain a high performance positive electrode material for asymmetric supercapacitors. In addition, a facile two-step heating/electrochemical cycling route to obtain an Fe3O4 nanosheet/graphite shell composite, as suitable negative electrode materials for a high performance asymmetric supercapacitors have been achieved. When examined as a positive and negative material for charge storage applications in a three-electrode configuration, the graphite-coated Fe3C/α-Fe-derived Fe3O4/Fe(OH)2 and Ni4.5Co4.5S8/Ni4.5Co4.5S8(OH)9 redox couples show typical Faradaic redox properties consistent with battery-type materials and exhibits an excellent rate performance. Indeed, such high-rate capability electrodes are of great interest when coupled with a capacitive or pseudocapacitive electrode (i.e., exhibiting a linear dependence of the charge stored with the width of the potential window) to design asymmetric (hybrid) supercapacitors devices with improved rate capability. Combining the high performance metrics of the individual materials into a full device has resulted in a battery with supercapacitor-like power behavior that delivers a promising specific energy of 89 Wh kg−1, a specific power of 38.5 kW kg−1 as well as a capacity retention of 82% and Coulombic efficiency of 98% over 5000 cycles. Such materials, therefore, show promise for use in high performance energy storage applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11498. Step-by-step representative preparation of graphitecoated Fe3C/α-Fe nanoparticles; schematic diagram of 6976


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the full-cell; TEM image and EDX mapping images of Ni−Co−S-4 sample; quantitative elemental analysis of NixCo(9−x)S8 samples by EDX and ICP-MS; XPS spectra of Co 2p, Ni 2p, and S 2p for Ni−Co−S-4 nanoflake arrays; high-resolution SEM images of the electrodeposited Ni−Co−S-4 electrode; SEM images of the electrodeposited Ni−Co−S electrodes produced at different pulse electrodeposition parameters; elemental analysis of the iron nitrate/citric acid mixture quenched at various temperatures; TGA curves of the Fe-CA-140 sample; the anodic and cathodic peak current density relationship with scan rate and square root of scan rate for negative and positive electrodes; electrochemical performance of the Ni−Co−S nanosheets with different composition; Nyquist plots of the Ni−Co−S-4 electrode, Fe-CA-700 electrode, and full-cell device at the highfrequency region; cyclic voltammetry of the Ni−Co−S-4 and Fe-CA-700 electrodes; and TEM images of the negative electrode after extended cycling (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tell.: +1 662 325 7608. Fax: +1 662 325 1618. E-mail: wipf@ ra.msstate.edu. ORCID

David O. Wipf: 0000-0003-2365-1175 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Department of Chemistry, Mississippi State University for financial support of this work and also thank Timothy Dowell in the Department of Chemistry, Mississippi State University for preparing the graphical abstract.

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REFERENCES

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