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  See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/318986761 Hydrothermal Reaction Combined with a PostAnion-Exchange Reaction of Hierarchically Nanostructured NiCo2S4 ....  Article   in  New Journal of Chemistry · August 2017 DOI: 10.1039/C7NJ02379K CITATION 1 READS 78 8 authors , including: Some of the authors of this publication are also working on these related projects: Electrochemistry   View projectSunkara Srinivasa RaoPusan National University 42   PUBLICATIONS   386   CITATIONS   SEE PROFILE Chandu V. V. Muralee GopiPusan National University 56   PUBLICATIONS   465   CITATIONS   SEE PROFILE Araveeti Eswar ReddyPusan National University 20   PUBLICATIONS   46   CITATIONS   SEE PROFILE All content following this page was uploaded by Sunkara Srinivasa Rao on 10 October 2017. The user has requested enhancement of the downloaded file.  This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017  New J. Chem.,  2017,  41 , 10037--10047 |  10037 Cite this: NewJ.Chem.,  2017, 41 , 10037 A hydrothermal reaction combined with a postanion-exchange reaction of hierarchicallynanostructured NiCo 2 S 4  for high-performanceQDSSCs and supercapacitors † S. Srinivasa Rao,  a Ikkurthi Kanaka Durga, a Nagabhushanam Kundakarla, b Dinah Punnoose, a Chandu V. V. M. Gopi,  a Araveeti Eswar Reddy, a M. Jagadeesh c and Hee-Je Kim * a This paper proposes a novel and facile strategy to synthesize hierarchical nanostructured NiCo 2 S 4  via  asimple hydrothermal reaction combined with a post anion-exchange reaction that was used directly asan electrode in supercapacitor and quantum-dot sensitized solar cells. By applying the appropriatematerial concentration, deposition temperature, and time, the NiCo 2 S 4  was densely deposited over theentire surface of the Ni foam with good adhesion. The NiCo 2 S 4  electrode exhibited outstandingelectrochemical performance in both the applications with a high specific capacitance of 1612.95 F g  1 ,energy density of 56 W h kg  1 at 5 A g  1 , good cycling stability, power conversion efficiency of 3.94%,and  J sc  of 12.27 mA cm  2 . The NiCo 2 S 4  electrode exhibited almost double the output values comparedto NiCo 2 O 4  (693.6 F g  1 at 6.66 A g  1 and efficiency of 1.81%) and NiCo 2 Se (1401.4 F g  1 at 10 A g  1 and efficiency of 3.86%), resulting from the excellent electrochemical performance of the NiCo 2 S 4 .Overall the excellent performance of the fabricated electrode was attributed mainly to the highlyelectrocatalytic activity, large surface area and decent conductivity of the hierarchical nanostructures ofthe NiCo 2 S 4 . This study shows that the NiCo 2 S 4  nanostructures can be applied not only in high energydensity fields, but also in high power density and energy harvesting applications. 1. Introduction The detrimental long-term effects of greenhouse gas emissionsinto the atmosphere, industrialization at an enormous pace,increasing global population, and the finite supply of fossilfuels underscore the urgency of exploring renewable energy resources as well as energy storage, generation, and conservationtechnologies. 1,2 In this fast growing world, lightweight, high-performance, environmentally friendly, safe, and long lifespanenergy storage devices are needed urgently for sustainable andrenewable power sources in the modern electronics industry. Untilnow, a number of electrical energy storage (EES) technologies havebeen developed for use in daily life (batteries and fuel cells) and forindustrial applications, such as compressed air, superconducting magnetic, pumped hydro, flywheels, and supercapacitors (SCs). 3 Li-ion batteries can be rechargeable or non-rechargeabledepending on the specific battery chemistry and both can produceelectricity from chemical energy. Typically, Li-ion batteries havelow power density, high energy densities and limited cycle livesbecause of the lack of a fully reversible redox reaction during thecharge and discharge process. 4,5 SCs, which are often referred to as ultracapacitors or electro-chemical capacitors, demonstrate outstanding power density compared to batteries, and have a higher energy density thanconventional dielectric capacitors. In addition, they have a very long cycle life ( 4 1000000 cycles), ease of integration intoelectronics, and simple mode of operation. 6 SCs are consideredto be the ideal storage device and they generate less thermo-chemical heat because of the simpler charge storage mechanismsassociated with them. Owing to the multiple advantages listedabove, these SCs have been used widely in memory back-upsystems,consumerelectronics,energymanagementandindustrialpower, and are expected to be found in more niche markets in thenear future. 7,8 On the other hand, there are certain areas whereSCs have shortcomings or fail to meet various needs, such as low energy density, compared to batteries. Therefore, the central issue a School of Electrical Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, Republic of Korea. E-mail: heeje@pusan.ac.kr; Fax:  + 82 51 513 0212; Tel:  + 82 51 510 2364 b  Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201,USA c  Department of Chemistry, Indian Institute of Technology-Tirupati, Andhra Pradesh, 517506, India †  Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj02379k  Received 3rd July 2017,Accepted 7th August 2017DOI: 10.1039/c7nj02379k rsc.li/njc NJC PAPER    P  u   b   l   i  s   h  e   d  o  n   0   8   A  u  g  u  s   t   2   0   1   7 .   D  o  w  n   l  o  a   d  e   d   b  y   P   U   S   A   N    N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   o  n   0   4   /   1   0   /   2   0   1   7   0   8  :   2   0  :   3   8 . View Article Online View Journal | View Issue  10038  |  New J. Chem.,  2017,  41 , 10037--10047 This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017 in the development of practical SCs is to identify new materialsand fabricate high energy SCs without losing the above-mentionedbenefits. Therefore, there is increasing focus on identifying asingle electrode system for both SC and quantum-dot sensitizedsolar cells (QDSSCs) that would be beneficial for compact electronicdevices and smart energy storage.In general, typical QDSSCs are composed of quantum dot semiconductor materials, such as CdS and CdSe sensitized as aphotoanode, a redox electrolyte consisting of the polysulfide redox couple,andacounterelectrode(CE). 9,10 TheseQDSSCsareattractiveowingtotheirhighabsorptioncoefficient,hot-carriertransfer,largerintrinsic dipole moments leading to charge separation, size-dependent tunable band gap with quantum confinement to suit the solar spectrum, and the generation of multiple electron carriersunder high energy excitation. Although QDSSCs have tremendousadvantages, the power conversion efficiency (PCE) is still behindthat of dye-sensitized solar cells (DSSCs) because of the high chargetransfer resistance between the CE/electrolyte and high electronrecombination occurring at the photoanode/electrolyte. In QDSSCs,the CE plays a vital role because the bifunctional CE collectselectrons from the external circuit and reduction occurs at the CE.Platinum has been a successful candidate for the CE in DSSCbecauseofitshigherelectroncatalyticactivityfortheorganiciodide/triode redox electrolyte. On the other hand, in conjunction with thepolysulfide redox electrolyte, Pt and other metals, such as Au, arenot effective CEs because the physical adsorption of sulfur-containing groups on the surface of Pt limits the surface activity and conductivity of the electrode. 11 To overcome this problem,other efficient CE materials in conjunction of the polysulfideelectrolyte have been investigated to achieve a higher PCE andlow charge transfer resistance.Two charge storage processes occur in SCs: a Faradaic chargetransfer process introduced by Trasatti  et al.  ( i.e.  pseudocapacitorsthat employ electroactive materials of transition metal hydroxides/oxides and their composites) and a non-Faradaic charge storageaccumulation process developed accurately by the Gouy–Chapman–Stern model. 12 Electric double layer capacitors (EDLCs) usegraphene, activated carbon, carbon nanotubes (CNTs), and carbonaerogels to prepare active materials in SCs. Currently pseudo-capacitors have attracted considerable attention owing to theirhigh specific capacitance, enhanced working potential window,and ability to store more charge than EDLCs because of thecontinuous reversible redox reactions in electroactive materials. 13 Until now, numerous materials, including conducting polymers,transition metal compounds, carbon-based materials, and singletransition metal oxides/hydroxides, have been studied extensively inSC and QDSSC applications. Previous studies have shown that electrodes with the appropriate network microstructures andporosity can improve the power density and cycling stability forSCs. Unfortunately, the inferior performance of electrode materials,such as the meager cycling stability in conducting polymers, low specific capacitance in carbon-based materials, high cost of RuO 2 ,and very low electrical conductivity in transition metal oxides, limitstheir practical applications. 14,15 Compared with carbon nanomaterials, the transition-metalsulfides, such as NiS x  and CoS x  have received extra attentiondue to their higher theoretical capacitance and multiple redox reactions in SCs. Unfortunately, poor electrochemical performance was observed due to their low surface area and non-porosity. Inorder toaddressthisproblem,manyresearchershave been working on the synthesis and design of metal sulfides with a higher surfacearea to enhance the performance of SCs. Mixing metal sulfides withlowcostelements(Ni,Mn,FeandSn)areanotherinterestingwayof enhancing the surface area and specific capacitance because theintroduction of mixing can lead to superior electrical conductivity. 15  AmongtheemergingelectrodematerialsforSCs,NiCo 2 O 3 ,NiCo 2 Se,and NiCo 2 S 4  are the most promising electrode materials owing totheir high specific capacitance from the rapid and reversible redox reactions on the active electrode surface. In particular, NiCo 2 S 4  hasattracted recent interest due to its richer redox reactions than thecorresponding binary NiS x  and CoS x , low-cost, natural abundance,and environmental benignity. In addition, it has a major advantageover NiCo 2 O 4  in terms of its higher conductivity. NiCo 2 S 4  powder isgenerally synthesized by using a hydrothermal method and a slurry is coated directly on the substrate, which seriously limits thecapacitive performance due to the increased ‘‘dead surface’’. 16  Aneffective way to improve the utilization of NiCo 2 S 4  is to grow thematerial directly on a conductive substrate to form an integratedelectrode for electrochemical evaluations. NiCo 2 S 4  nanostructurescanbedirectlysynthesizedonaconductivesubstrateusingdifferent growth methods, such as chemical bath deposition, solvothermalmethods, microwave-assisted methods, hydrothermal synthesis,and the electrochemical deposition method (EDM). 17–20  Among these methods, the EDM is relatively attractive because it can grow functional materials through complex 3D masks and be performedat room temperature for a short growth time from water-basedelectrolytes. The EDM is carried out using a three electrode system with a counter electrode ( i.e.  Pt wire) and reference electrodes, suchas the standard calomel and Ag/AgCl electrode. On the other hand,these reference electrodes are quite sensitive and expensive. More-over, the tip of the reference electrode might be broken during theEDM process under increased temperature. 6 Based on the above consideration, this paper reports afacile, cost-effective, and eco-friendly metal source to obtain well-assembled porous NiCo 2 O 4 , NiCo 2 S 4 , and NiCoSe on Nifoam using a simple hydrothermal method followed by thermaltreatment. The as-fabricated NiCo 2 S 4  electrode produced efficient performance in a SC and also good CE performance in a QDSSC. Assembled in QDSSCs, the hierarchical nanostructured electrodeshowed a good PCE, fill factor,  V  oc , and  J  sc  of 3.94%, 0.54, 0.58 V,and 12.27 mA cm  2 , respectively. Moreover, the NiCo 2 S 4  possessedan enhanced specific capacitance of 1612.95 F g   1 at 5 A g   1 andenergy density of 56 W h kg   1 and excellent cycling performance. 2. Experimental methods 2.1 Preparation of the Ni foam substrate Prior to synthesis, the Ni foam substrate (1 cm    1 cm inrectangular shape) was cleaned by sonication in acetone for15 min and immersed into an HCl solution for 15 min toremove the surface oxide layer. Finally, the Ni foam was washed Paper NJC    P  u   b   l   i  s   h  e   d  o  n   0   8   A  u  g  u  s   t   2   0   1   7 .   D  o  w  n   l  o  a   d  e   d   b  y   P   U   S   A   N    N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   o  n   0   4   /   1   0   /   2   0   1   7   0   8  :   2   0  :   3   8 . View Article Online  This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2017  New J. Chem.,  2017,  41 , 10037--10047 |  10039 sequentially in ethanol and deionized (DI) water for 10 mineach, and dried with a hair dryer. 2.2 Fabrication of Hierarchical NiCo 2 O 4  nanowire arrays  All the chemicals used in the experiment were of analyticalgrade and used without any further purification. HierarchicalNiCo 2 O 4 /Ni foam nanowire arrays were synthesized from asimple hydrothermal method, according to similar proceduresreported elsewhere. In a typical procedure, the growth solution was prepared by mixing 8 mmol of CoCl 2  6H 2 O, 4 mmol of NiCl 2  6H 2 O, and 12 mmol of urea in 60 mL of DI water to forma clear pink solution at room temperature. Sequentially, themixed solution was transferred to an 80 mL Teflon-linedstainless steel autoclave with cleaned Ni foam substrates andheated to 120  1 C for 6 h and cooled naturally to room tempera-ture. After the Teflon-lined stainless steel autoclave was cooledto room temperature, the NiCo 2 (CO 3 ) 1.5 (OH) 3  nanowire arrayssupported on the Ni foam were collected, washed several times with DI water and ethanol, and treated thermally at 400  1 C for2 h to convert them to NiCo 2 O 4 . 2.3 Fabrication of NiCo 2 S 4  nanotube arrays The NiCo 2 (CO 3 ) 1.5 (OH) 3  nanowire arrays supported on the Nifoam were used to prepare the NiCo 2 S 4  nanotube arrays by ananion-exchange reaction and used directly as the electrode forsupercapacitors. In a typical procedure, 8 mmol of Na 2 S wasdissolved in 60 mL of DI water and stirred for 20 min. Thesolution was then transferred to an 80 mL Teflon-lined stainlesssteel autoclave, followed by the addition of NiCo 2 (CO 3 ) 1.5 (OH) 3 supported on Ni foam. After cooling naturally to room temperature,the NiCo 2 S 4  nanotube arrays were collected and washed with DI water and ethanol, and the sample was dried at 60  1 C for 10 h. 2.4 Fabrication of NiCo 2 Se nanoparticles TheNiCo 2 SenanoparticlesontheNifoamsubstrateswerepreparedby hydrothermally treating the as-collected NiCo 2 (CO 3 ) 1.5 (OH) 3 nanowire arrays with Se and sodium sulfite. The selenium solution waspreparedbymixingSe(4mmol)andsodiumsulfite(8mmol)in60 mL of DI water. The resulting mixture with NiCo 2 (CO 3 ) 1.5 (OH) 3  was kept in an 80 mL stainless steel autoclave at 120  1 C for 6 h anddried at 60  1 C for 10 h. 2.5 Electrochemical measurements The electrochemical measurements, including electrochemicalimpedance spectroscopy (EIS), cyclic voltammetry (CV), andgalvanic charge–discharge were taken using a BioLogic potentio-stat/galvanostat/EIS analyzer (SP-150, France) at room temperature.EIS was performed in a three-electrode glass cell setup and thefrequency range was 500 kHz to 0.1 Hz at the open-circuit potential with an amplitude of 10 mV. The weight of the active materialon the Ni foam was acquired by measuring the electrode with amicrobalance with an accuracy of 0.01 mg and the weight wasapproximately 3 to 3.5 mg cm  2 for all electrodes. The galvano-static charge–discharge was performed at 25  1 C in 1 M KOH with different current densities, such as 5, 8.3, 11.6, 15, 20 and23.3 A g   1 . The specific capacitance of the electrode wascalculated from the CV and galvanostatic discharge curves using eqn (1) and (2). C   ¼ ð  vcva I  ð v Þ d vmV V  a    V  c ð Þ  (1) where  I  (  A ) is the charge or discharge current,  C   is the specificcapacitance (F g   1 ),  V   is the scan rate of the CV curve (V s  1 ),  m is the mass of the electrode, and ( V  a    V  c ) represents the voltagechange after a full charge or discharge. C   ¼  I  D tm D V   (2) where  D t   (s) is the time for a full charge or discharge,  D V   is thepotential drop (V),  C   is the specific capacitance (F g   1 ), and  m  isthe mass of the electrode. The energy density (  E  ) and powerdensity (  P  ) were calculated from the CV curves using thefollowing well-known formulae: E   ¼  12 C  ð D V  Þ 2    10003600  (3) P   ¼  E t ¼  i  D V  2 m   1000  (4) where  C   is the capacitance (F g   1 ),  D V   is the potential window (V), and  t   is the discharge time (s). 2.6 Material characterization The product was characterized by X-ray diffraction (XRD,D/Maz-2400, Rigaku), X-ray photoelectron spectroscopy (XPS),high-resolution scanning electron microscopy (HR-SEM) andtransmission electron microscopy (TEM) at the Busan KBSI toidentify the microstructure, surface morphology and chemicalcomposition. The powder XRD patterns were obtained using CuK a  radiation at 40 kV and 30 mA. Photocurrent density–voltage(  J  – V  ) and EIS were performed using an ABET technology (USA)solar simulator under 1 Sun illumination (100 mW cm  2 , AM1.5G). Tafel polarization measurements were analyzed using symmetric dummy cells under dark conditions; the detailedmethod for the fabrication of this symmetrical cell is reportedelsewhere. The surface roughness of the prepared products wasexplored using atomic force microscopy (JPK NanoWizard II AFM, JPK Instruments, Berlin, Germany) with a scan rate of 0.8 Hz in contact mode. 3. Results and discussion 3.1 Synthesis and characterization of the active materialelectrodes Fig. 1 presents a schematic diagram of the synthesis processof NiCo 2 O 4 , NiCo 2 S 4 , and NiCo 2 Se on Ni foam  via  a one-stepand/or two-step process. The Ni foam substrate was suitable forgrowing the active material electrodes using a hydrothermalmethod owing to its high conductivity and flexible. First, theNiCo 2 (CO 3 ) 1.5 (OH) 3  nanowire arrays were fabricated on an Nifoam substrate  via  a hydrothermal method. After thermaltreatment of the grown NiCo 2 (CO 3 ) 1.5 (OH) 3  at 400  1 C for 2 h, NJC Paper    P  u   b   l   i  s   h  e   d  o  n   0   8   A  u  g  u  s   t   2   0   1   7 .   D  o  w  n   l  o  a   d  e   d   b  y   P   U   S   A   N    N   A   T   I   O   N   A   L   U   N   I   V   E   R   S   I   T   Y   o  n   0   4   /   1   0   /   2   0   1   7   0   8  :   2   0  :   3   8 . View Article Online
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