Electrochemically Synthesized Nanoflowers to Nanosphere-Like

metals

Article Electrochemically Synthesized Nanoﬂowers to Nanosphere-Like NiCuSe2 Thin Films for Eﬃcient Supercapacitor Application

Surendra K. Shinde 1, Dae-Young Kim 1, Vinayak G. Parale 2, Hyung-Ho Park 2 and Hemraj M. Yadav 3,* 1 Department of Biological and Environmental Science, College of Life Science and Biotechnology, Dongguk University, 32 Dongguk-ro, Biomedical Campus, Ilsandong-gu, Siksa-dong, Gyeonggi-do, Goyang-si 10326, Korea; [email protected] (S.K.S.); [email protected] (D.-Y.K.) 2 Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea; [email protected] (V.G.P.); [email protected] (H.-H.P.) 3 Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Korea * Correspondence: [email protected] or [email protected]; Tel.: +82-02-2290-1869

Received: 16 November 2020; Accepted: 17 December 2020; Published: 21 December 2020

Abstract: Developing efficient electrochemically active nanostructures from Earth-abundant elements has gained significant interest in recent years. Among different transition metals, nickel and copper are abundant electrocatalysts for energy-storage applications. Nickel–copper selenide (NiCuSe2) nanostructures were prepared on a stainless-steel mesh with a cost-effective, simple, and versatile electrodeposition method for supercapacitor applications. The change effect in the bath concentration of nickel and copper altered the structural and electrochemical properties of NiCuSe2 electrode. X-ray diffraction (XRD) patterns confirmed the pure phase of ternary NiCuSe2 thin films with a cubic crystal structure. The surface morphology of NiCuSe2 was tuned by nickel and copper from spherical porous nanoflowers, nanoplates, nanocubes, and nanosphere-like nanostructures deposited on the stainless-steel mesh. The electrochemical performance of the electrodeposited NiCuSe2 was investigated in alkaline 1 M KOH electrolyte. The synergetic effect of bimetallic nickel and copper 1 1 with the selenide electrode showed superior specific capacity of about 42.46 mAh g− at 10 mV s− along with reasonable cycling stability.

Keywords: nickel–copper selenide; electrodeposition; nanostructures; supercapacitor; stainless-steel mesh; nanoﬂakes

1. Introduction The rapid depletion of fossil fuels and rising pollution have led to high demand for alternative energy-harvesting and -storage systems [1]. Supercapacitors are an advanced type of electrochemical energy-storage technology [2,3], which is used in various electronic devices, electric vehicles, solar cells, and wearable devices. Supercapacitors have advantages such as high power density, high energy density, long life cycle, and low cost of manufacturing and maintenance [4,5]. Recently, porous carbons, conducting polymers, and several transition-metal compounds composed of oxides, nitrides, sulfides, and selenides have been utilized as electrodes for electrochemical-energy storage applications [6–9]. The poor electrical conductivity of transition metal oxides/hydroxides and the lower stability of sulfides limit their electrochemical performance [10]. Among these materials, transition-metal selenides are promising electrode materials for supercapacitors, oxygen evolution, and other applications because of their good electrical conductivity, higher values of theoretical specific capacity, and structural and optoelectronic properties [11–13].

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Metal selenides, made up of Cu, Bi, Sn, etc., have low band-gap energy and are applied in photoelectrochemical cells for solar-energy harvesting systems and electrocatalytic applications [13,14]. Similarly, binary metal selenides were synthesized and are devoted to improving electrocatalytic activity. Recently, bimetallic Cu–Co selenide nanowires showed improvement in the electrochemical properties of supercapacitors [15]. Tang et al. fabricated NiSe nanowires on nickel foam via a two-step 1 1 hydrothermal method, which exhibited capacitance of 1790 F g− at 5 A g− [16]. Zhang et al. reported 1 1 stable SnSe nanosheets having energy-storage capacity (228 F g− at 0.5 A g− ) using a high-temperature refluxing method [17]. Furthermore, Chen et al. fabricated a Ni–Co selenide supercapacitor by a 1 1 hydrothermal method, exhibiting a specific capacity of 535 C g− at 1 A g− [18]. These methods involve complex processes and are not binder-free electrode-preparation approaches. Binder-containing electrodes also have poor electrical conductivity and cause an increased dead surface during electrochemical studies [10]. Transition-metal selenides are produced by various methods, including chemical bath deposition [19], evaporation, and electrodeposition [20]. Among these methods, electrodeposition has advantages such as low cost, ease to control nanostructure growth, simple, and ecofriendly. Appropriate metals for bimetallic selenides are nickel (1.43 107 S m 1) and copper (5.96 107 S m 1) × − × − because of their high electrical conductivity compared to that of other metals, which might assist to preserve a higher electrical conductivity for selenium (1 10 3 S m 1). × − − In this study, we report an electrodeposition method for the preparation of nickel–copper selenide nanostructures. Some reports were on the preparation of ternary NiCuSe2 via other chemical methods. However, less attention has been paid towards electrodeposition and supercapacitor applications. We developed various NiCuSe2-coated nanostructures on stainless-steel mesh by a potentiostatic mode of the electrodeposition method. The effect of nickel and copper bath concentration with a constant concentration of selenide on the structural and electrochemical properties was studied. The prepared electrodes were used for the measurements of supercapacitive properties in the 1 M KOH alkaline electrolyte.

2. Experiment Details

2.1. Chemicals and Materials Nickel sulfate hexahydrate (NiSO 6H O), copper sulfate anhydrous (CuSO ), and selenium 4· 2 4 dioxide (SeO2) were procured from Sigma-Aldrich, Seoul, Korea. Potassium hydroxide (KOH) was purchased from Daejung, Gyeonggi-do, Korea. All chemicals were of analytical grade and used as received.

2.2. Synthesis of NiCuSe2 Thin Films

NiCuSe2 nanostructures were deposited by an electrodeposition method on the stainless-steel mesh. The electrochemical bath contained an aqueous solution of nickel sulfate hexahydrate, copper sulfate, and selenium dioxide in a separate beaker. Deposition was carried out on a cleaned stainless-steel mesh at room temperature with a potentiostatic mode at 0.75 V for 120 s constant for all samples. − The molarity eﬀect of the nickel and copper precursor on the electrochemical properties was investigated. The molarity of nickel and copper was varied, while selenium molarity was kept constant throughout the experiment. The four samples were obtained with nickel concentrations of 0.0025, 0.0050, 0.0075, and 0.01 M, and copper concentrations of 0.011, 0.008, 0.004, and 0.015 M, and denoted as S-1, S-2, S-3, and S-4, respectively.

2.3. Characterizations Field-emission scanning electron micrographs (FE-SEM) were recorded on an JEOL microscope (JEOL JSM-7100, Tokyo, Japan). Powder X-ray diffractograms (XRD) were recorded with a D8 advanced diffractometer system (Bruker, Billerica, MA, USA) equipped with a Cu–Kα radiation source (40 kV, Metals 2020, 10, 1698 3 of 8 Metals 2020, 10, x FOR PEER REVIEW 3 of 8 source40 mA). (40 X-ray kV, photoelectron40 mA). X-ray spectra photoelectron (XPS) were spectra collected (XPS) with were an collected ESCALABMK with an II X-rayESCALABMK photoelectron II X- rayspectrometer photoelectron (Thermo spectrometer Fisher Scientific, (Thermo Waltham, Fisher Sc MA,ientific, USA) Waltham, equipped MA, with USA) an Mg–K equippedα X-ray with source. an Mg–KElectrochemicalα X-ray source. analysis Electrochemical was performed analysis using awas VersaSTAT performed 3 Potentiostat using a VersaSTAT Galvanostat 3 workstationPotentiostat Galvanostat(Princeton Applied workstation Research, (Princeton Princeton, Applied NJ, USA).Research, Princeton, NJ, USA).

2.4.2.4. Electrochemical Electrochemical Measurements Measurements Cyclic-voltammetryCyclic-voltammetry (CV) (CV) and and galvanostatic-ch galvanostatic-chargearge/discharge/discharge (GCD) (GCD) measurements were conductedconducted on on a a VersaSTAT VersaSTAT 3 3 electrochemical electrochemical workst workstationation using using a typical a typical three-electrode three-electrode system system at roomat room temperature. temperature. Stainless-steel Stainless-steel mesh mesh was was used used as asworking working electrode, electrode, while while a aPt Pt wire wire and and a saturatedsaturated AgAg/AgCl/AgCl electrode electrode served served as the counteras the andcounter reference and electrode, reference respectively. electrode, Electrochemical respectively. Electrochemicalcharacteristics were characteristics studied in were aqueous studied 1 M in KOH aqueous solution, 1 M andKOH the solution, geometric and area the ofgeometric the working area ofelectrode the working was one electrode unit cm was2 for one all unit electrodes. cm2 for all electrodes.

3.3. Results Results and and Discussion Discussion

3.1.3.1. NiCuSe NiCuSe22 CharacterizationCharacterization

FigureFigure 11 presentspresents thethe XRDXRD patternspatterns ofof allall NiCuSeNiCuSe2 samplessamples prepared prepared by by the the potentiostatic potentiostatic mode mode ofof the electrodeposition method. A A strong strong diffractio diffractionn peak peak of the stainless-steel mesh was observed atat 43.54 °◦ for all NiCuSe2 samples.samples. Diffraction Diffraction peaks peaks of of NiCuSe NiCuSe22 atat 36.42 36.42°◦ andand 50.66 50.66 were consistent with the (211), (311), and (332) planes of cubic NiCuSe 2 (JCPDS(JCPDS 00-018-0888). 00-018-0888). The The diffraction diffraction peaks peaks of of CuCu at 50.66 °◦ andand 74.4 74.4°◦ overlappedoverlapped with with the the peak peak of of Ni, Ni, and and matched matched with with the the (200) (200) and (220) planes ofof cubic cubic Cu Cu (JCPDS (JCPDS 00-004-0836) 00-004-0836) and and cubic cubic NiCuSe NiCuSe2 (JCPDS2 (JCPDS 00-006-0507). 00-006-0507). The Thepeak peak at 36.42 at 36.42° was◦ onlywas observedonly observed for S-1 for due S-1 to due the toformat the formationion of defects of defects with the with higher the higherconcentration concentration of copper of copper[21]. These [21]. resultsThese results confirmed confirmed the formation the formation of pure of ternary pure ternary phase phase of the ofNiCuSe the NiCuSe2. The2 intensity. The intensity of the ofnickel the nickel peak waspeak linearly was linearly increased, increased, while whilethe intensity the intensity of the of st theeel steelpeak peakwas linearly was linearly decreased decreased from fromS-1 to S-1 S-4. to TheS-4. intensity The intensity of nickel of nickel and and copper copper peaks peaks was was alte alteredred with with the the concentration concentration variation variation experiment experiment without any shift in peak positions. The The crystallite crystallite sizes sizes of of the the samples were were calculated using the ScherrerScherrer equation: 31.47, 31.47, 32.62, 32.62, 32.74, 32.74, and and 33.85 33.85 nm nm fo forr samples samples S-1, S-2, S-3, and S-4, respectively.

Figure 1. XRDXRD patterns patterns of of all all samples. samples.

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FE-SEM images for all samples with different magnifications are shown in Figure 2. NiCuSe2 nanostructuresFE-SEM images were forclearly all samples uniformly with deposited different on magnifications the stainless-steel are shown mesh. in Sample Figure 2S-1. NiCuSe showed2 nanostructureshighly porous werespongy clearly nanoflake-like uniformly deposited nanostructur on thees stainless-steeldeposited on mesh. the stainless-steel Sample S-1 showed mesh. highlyAfter porouschanging spongy bath concentration, nanoflake-like interconnected nanostructures nanofl depositedake-like on the nanostructures stainless-steel grew mesh. on After the surface changing of baththe steel concentration, mesh. This interconnected type of surface nanoflake-like morphology nanostructuresprovided more grewsurface on area the surface during of electrochemical the steel mesh. Thistesting. type Further, of surface by changing morphology the providedbath conc moreentration surface of Ni area and during Cu, the electrochemical surface of NiCuSe testing.2 showed Further, a byregular changing deposition the bath of concentrationa nanocubic-like of Ni nanostructure and Cu, the surfacedeveloping of NiCuSe on the2 showedsurface of a regularthe stainless-steel deposition ofmesh. a nanocubic-like However, at nanostructure a higher bath developing concentration on the of surfaceNi and ofCu the ions, stainless-steel there was mesh.a drastic However, change at in a highersurface bath morphology concentration from ofthe Ni cubic and Cuto the ions, nanosphe there wasre. a Spongy drastic changenanoflake, in surface interconnected morphology nanoflakes, from the cubicnanocubic, to the nanosphere.and nanosphere-like Spongy nanoflake,morphology interconnected was observed nanoflakes, in all samples, nanocubic, while and the nanosphere-like size, thickness, morphologyand length of was the observed nanostructures in all samples, was altered while with the size,the change thickness, in electrolyte and length ofbath the concentration nanostructures of was Cu alteredand Ni withions thefor changeternary in NiCuSe electrolyte2 [22–24]. bath concentrationNanoflake length of Cu for and samp Ni ionsle S-2 for was ternary slightly NiCuSe higher2 [22 than–24]. Nanoflakethat of the lengthother samples, for sample while S-2 wasnanosphere slightly size higher for than sample that S-4 of thewas other comparatively samples, while higher nanosphere than that sizeof other for samplesamples S-4 [25,26]. was comparatively higher than that of other samples [25,26].

Figure 2. FE-SEM images of Samples ( a–c) S-1, (d–f) S-2, (g–i) S-3, and (j–l) S-4.

XPS analysis was performed to further study the chemical environment of NiCuSe2. The representative XPS spectrum of sample S-4 is depicted in Figure 3. Figure 3a shows the survey

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XPS analysis was performed to further study the chemical environment of NiCuSe . Metals 2020, 10, x FOR PEER REVIEW 5 of 28 The representative XPS spectrum of sample S-4 is depicted in Figure3. Figure3a shows the survey spectrum of of S-4, S-4, which which exhibited exhibited peaks peaks of ofnickel nickel,, copper, copper, selenide, selenide, carbon, carbon, and andoxygen oxygen without without any otherany other impurities. impurities. Peaks Peaks of C 1s of and C 1s O and1s were O 1s detec wereted detected for the forsample the sampledue to exposure due to exposure to air (Figure to air 3b,c).(Figure The3b,c). spectrum The spectrum of Cu of2p Cuconfirmed 2p conﬁrmed that the that two the main two main peaks peaks (952.21 (952.21 and and932.35 932.35 eV) eV) could could be be assigned for Cu 2p and Cu 2p , respectively (Figure3d). Similarly, peaks at 855.61 and 873.27 eV assigned for Cu 2p1/2 1and/2 Cu 2p3/2,3 respectively/2 (Figure 3d). Similarly, peaks at 855.61 and 873.27 eV appeared due to Ni 2p and Ni 2p , respectively. The two corresponding satellite peaks were appeared due to Ni 2p11/2/2 and Ni 2p33/2/2, respectively. The two corresponding satellite peaks were observed at binding energies of 879.30 and 860.99 eV (Figure 3 3e).e). SeleniumSelenium (Figure(Figure3 3d)d) showed showed peaks peaks at 53.49 and 54.38 eV due to 3d and 3d , respectively (Figure3f) [27]. at 53.49 and 54.38 eV due to 3d5/25/2 and 3d3/23/,2 respectively (Figure 3f) [27].

Figure 3. XPS of sample S-4. (a) SurveySurvey spectrum.spectrum. CoreCore levelslevels of of ( b(b)) C C1s, 1s, ((cc)) OO1s, 1s, ((d)) CuCu2p, 2p, ( e) NiNi2p, 2p, and ( f) Se3d. Se 3d.

3.2. Electrochemical Electrochemical Study Study The electrochemical measurement ofof the all samp samplesles was performed in 1 M KOH KOH solution solution against against Hg/HgOHg/HgO reference electrode at a potential window window of of 0–0.5 0–0.5 V. V. Figure 44aa showsshows thethe CV,CV, andand thethe insetinset shows thethe specificspecific capacity capacity of of all all four four samples samples in a in potential a potential range range of 0–0.5 of V0–0.5 (vs Hg V /(vsHgO) Hg/HgO) at a constant at a 1 constantscan rate scan of 10 rate mV sof− .10 The mV CV s− curves1. The CV of S-1, curves S-2, andof S-1, S-3 withS-2, and difference S-3 with scan difference rate are depicted scan rate in are the depictedFigure S1a–c. in the A Figure well-defined S1a–c. redoxA well-defined peak was observedredox peak for was S-4 comparedobserved for to thatS-4 ofcompared the other to samples. that of theThe other specific samples. capacity The of all specific samples capacity was calculated of all samples from this was CV calculated profile. The from specific this capacityCV profile. (Cs) The was 1 R estimated from Cs mAhg = I(V)dv/mv 3.6. The specific capacity values for samples S-1 to specific capacity (Cs) was estimated− from 𝐶 𝑚𝐴ℎ𝑔 1 𝐼𝑉𝑑𝑣⁄ 𝑚𝑣 . 3.6 The specific capacity 1× valuesS-4 were for 4.30, samples 10.84, S-1 22.37, to S-4 and were 42.46 4.30, mAh 10.84, g −22.37,, respectively. and 42.46 mAh The performanceg−1, respectively. of NiCuSe The performance2 could be ofoptimized NiCuSe2 by could changing be optimized the electrolyte by changing bath concentration the electrolyt ofe bath Ni and concentration Cu ions. The of optimal Ni and concentrationCu ions. The optimalof Ni and concentration Cu ions in the of electrolyte Ni and Cu bath ions wasin the responsible electrolyte for bath the was higher responsible capacity valuesfor the ofhigher S-4, as capacity shown valuesin the Figure of S-4,4b. as Figure shown4c showsin the theFigure specific 4b. capacityFigure 4c of shows the optimized the specific NiCuSe capacity2 electrode of the with optimized various 1 NiCuSescan rates,2 electrode in the range with ofvarious 10–100 scan mV rates, s− , respectively. in the range Theof 10–100 obtained mV values s−1, respectively. of specific capacityThe obtained were 1 1 values42.46, 37.94,of specific 27.36, capacity 21.01, 17.03, were and42.46, 13.88 37.94, mAh 27.36, g− 21.01,for scan 17.03, rates and 10, 13.88 20, 40,mAh 60, g 80,−1 for and scan 100 rates mV s10,− , 20,respectively. 40, 60, 80, and Owning 100 mV to thes−1, respectively. excellent supercapacitor Owning to the performance excellent supercapacitor and higher area performance of CV for and S-4, higherthe sample area was of selectedCV for asS-4, a representativethe sample was to demonstrate selected as the a electrochemicalrepresentative properties.to demonstrate Figure the4d electrochemicalshows the GCD curveproperties. of S-4 Figure with di 4dfferent shows current the GC densities.D curve Specific of S-4 capacitywith different that was current estimated densities. from 1 2 Specificthe GCD capacity curve of that S-4 was 17.36estimated mAh fr gom− at the 0.25 GCD mA curve cm− .of GCD S-4 was curves 17.36 at dimAhfferent g−1 currentat 0.25 mA densities cm−2. GCDwere alsocurves mainly at different symmetrical, current revealing densities the goodwere electrochemicalalso mainly symmetrical, reversibility revealing of S-4, as the shown good in electrochemicalFigure4d. reversibility of S-4, as shown in Figure 4d. The EIS of all samples is shown in Figure 5. The diffusion resistance of electrolyte ions can be seen in the low-frequency region. The straight line in the low-frequency region shows the low diffusion resistance of ions from the KOH solution to the electrode interface. The cycling stability of S-4 up to 1000 cycles was investigated with GCD at 1 mA cm−2 (as shown in Figure S1d). Sample S-4

Metals 2020, 10, x FOR PEER REVIEW 6 of 8 maintainedMetals 2020, 10 ,reasonable 1698 cycling stability, and was slightly decreased in capacitance after 100 cycles6 of 8 due to the irreversible reaction of the material.

Metals 2020, 10, x FOR PEER REVIEW 6 of 8

maintained reasonable cycling stability, and was slightly decreased in capacitance after 100 cycles due to the irreversible reaction of the material.

FigureFigure 4. 4. ((aa)) Cyclic-voltammetry Cyclic-voltammetry (CV) (CV) curves, curves, inset inset shows shows sp specificecific capacity capacity for for samples samples S-1, S-1, S-2, S-2, S-3, S-3, −1 1 −1 1 andand S-4 S-4 at at 10 10 mV mV s s−. (.(b)b CV) CV of ofS-4 S-4 with with respect respect to todifferent different scan scan rates rates in the in the range range 10–100 10–100 mV mV s . s(−c) . Specific(c) Specific capacity capacity of S-4 of with S-4 with different different scan scanrates. rates. (d) Galvanostatic (d) Galvanostatic charge/discharge charge/discharge (GCD) (GCD) of S-4 ofwith S-4 differentwith diff erentcurrent current densities. densities.

The EIS of all samples is shown in Figure5. The di ﬀusion resistance of electrolyte ions can be seen in the low-frequency region. The straight line in the low-frequency region shows the low diﬀ usion resistanceFigure of4. ions(a) Cyclic-voltammetry from the KOH solution (CV) curves, to the inset electrode shows interface.specific capacity The cycling for samples stability S-1, S-2, of S-4S-3, up to −1 2 −1 1000and cycles S-4was at 10 investigated mV s . (b) CV with of S-4 GCD with at respect 1 mA cm to −different(as shown scan inrates Figure in the S1d). range Sample 10–100 S-4mV maintaineds . (c) reasonableSpecific cyclingcapacity stability,of S-4 with and different was scan slightly rates. decreased (d) Galvanostatic in capacitance charge/discharge after 100 (GCD) cycles of S-4 due with to the irreversibledifferent reaction current densities. of the material.

Figure 5. Nyquist plots of samples S-1 to S-4.

4. Conclusions Variation in the electrolyte bath concentration of nickel and copper improved the electrochemical performance of the nickel–copper selenide electrode. The electrochemical study indicated a superior performance for S-4, with a 42.46 mAh g−1 at 10 mV s−1 in 1 M KOH electrolyte. Reasonable cycling stability due to the stable structure on the stainless-steel mesh substrate was also Figure 5. NyquistNyquist plots plots of of samples samples S-1 S-1 to to S-4.

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4. Conclusions Variation in the electrolyte bath concentration of nickel and copper improved the electrochemical performance of the nickel–copper selenide electrode. The electrochemical study indicated a superior 1 1 performance for S-4, with a 42.46 mAh g− at 10 mV s− in 1 M KOH electrolyte. Reasonable cycling stability due to the stable structure on the stainless-steel mesh substrate was also observed. These results proved that the nickel–copper selenide with an optimal concentration of nickel and copper in the electrolyte bath may be a useful candidate for next-generation hybrid supercapacitors and electrocatalytic applications.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-4701/10/12/1698/s1, 1 Figure S1: (a–c) CV curves of the S-1, S-2 and S-3 samples at a different scan rate from 10–100 mV s− in the potential range 0 to 0.5 V with 1 M KOH electrolytes, respectively, (d) cycling stability with respect to the number of cycles. Author Contributions: Conceptualization, methodology, writing—review and editing, S.K.S. and H.M.Y.; formal analysis, V.G.P. and H.-H.P.; supervision, D.-Y.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: H.M.Y. and S.K.S. are thankful to the Dongguk University for supporting research in 2018-22. V.G.P. and H.-H.P. would like to thank that this work was supported by the National Research Foundation of Korea (NRF), grant funded by the Korean government (MSIT; No. 2020R1A5A1019131). Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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