Journal of Alloys and Compounds · 2019-05-25 · General synthesis of NiCo alloy nanochain arrays with thin oxide coating: a highly efficient bifunctional electrocatalyst for overall

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Journal of Alloys and Compounds 797 (2019) 1216e1223

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

General synthesis of NiCo alloy nanochain arrays with thin oxidecoating: a highly efficient bifunctional electrocatalyst for overall watersplitting

Ben Zhang a, Xuming Zhang a, *, Yong Wei a, Lu Xia a, Chaoran Pi a, Hao Song a, b,Yang Zheng a, Biao Gao a, b, **, Jijiang Fu a, Paul K. Chu b

a The State Key Laboratory of Refractories and Metallurgy and Institute of Advanced Materials and Nanotechnology, Wuhan University of Science andTechnology, Wuhan 430081, Chinab Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

a r t i c l e i n f o

Article history:Received 12 February 2019Received in revised form1 May 2019Accepted 3 May 2019Available online 16 May 2019

Keywords:Overall water splittingBifunctional electrocatalystNiCo alloy nanochain arraysCore-shell structureSynergistic effect

* Corresponding author.** Corresponding author. The State Key Laboratory oand Institute of Advanced Materials and NanotechnScience and Technology, Wuhan 430081, China.

E-mail addresses: [email protected] (X. Z(B. Gao).

https://doi.org/10.1016/j.jallcom.2019.05.0360925-8388/© 2019 Elsevier B.V. All rights reserved.

a b s t r a c t

Bifunctional electrocatalysts that are cost effective and naturally abundant and have high electroactivitiesin both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline elec-trolytes are very desirable. Herein, we describe a general technique to fabricate thin oxide layer coatedNiCo alloy nanochain arrays (NCAs) on Ni foam by a hydrothermal reaction and subsequent low-temperature reduction. The hybrid NiCo-300 electrocatalyst shows excellent bifunctional HER and OERactivities in overall water electrolysis in an alkaline medium. It shows lower overportential and betterelectrocatalytic kinetics than pristine NiCo2O4 nanorod arrays (NRAs) and bare NiCo alloy, reaching10mA cm�2 and 100mA cm�2 at low overpotentials of 156mV and 245mV in HER and 320mV and390mV in OER, respectively. The thin oxide layer on the NiCo-300 surface produces synergistic effects toenhance the HER and OER activities. In a two-electrode system, the symmetrical electrolyzer needs a cellvoltage of 1.688 V for a 10mA cm�2 water-splitting current and exhibits good long-term stability. Theresults reveal a facile way to fabricate nanostructured transition metal alloys with thin oxide coatings ashighly efficient bifunctional electrocatalysts promising for overall water electrolysis.

© 2019 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogen is a clean energy source and promising as a substitutefor fossil fuel in the future. Water electrolysis is an ideal approachfor mass production of hydrogen on account of the harmlessbyproducts, abundant natural resource, and increased electricalpower from sustainable wind and solar energy. The hydrogenevolution reaction (HER) on the cathode and oxygen evolution re-action (OER) on the anode are the two key processes in overallwater electrolysis. Platinum (Pt) and iridium (Ir) group metals areconsidered to be the state-of-art electrocatalysts, but large-scalecommercialization is hindered by the high cost and scarcity [1e3].

f Refractories and Metallurgyology, Wuhan University of

hang), [email protected]

Recently, water-splitting catalysts composed of first-row tran-sitionmetals such as nickel (Ni), cobalt (Co), molybdenum (Mo) andtheir alloys, oxides, phosphides, carbides and sulfides have beenstudied as single functional or bifunctional catalysts because of theredox chemistry, earth abundance and low cost [4e8]. Amongthem, transition metal alloy based electrocatalysts with both OERand HER activities in alkaline solutions are attractive due to thehigh electroactivity and corrosion resistance in aqueous alkali[9,10]. Moreover, alloy catalysts possess the enhanced ability ofregulating the hydrogen adsorption energy in HER in comparisonwith monometallic catalysts as a result of the surface electroninteraction [11e13]. For example, uniform bimetallic Pt75Co25shows enhanced HER and oxygen reduction reaction (ORR) thanthe blank Pt catalyst as a result of the modification of the electronicstructures of Pt [14]. Jia et al. reported Co2Ni alloy anchored onnitrogen-doped carbon nanotubes, which exhibits a low over-potential of 100mV to drive current density of 10mA cm�2 inalkaline solution [15]. Darband et al. prepared Ni-Co alloy nano-cones by electrodeposition method and showed low h10 of �107

B. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 1216e1223 1217

mV in 1.0M KOH solution [16]. In HER, water molecules on themetal surface can be dissociated and charged to form adsorbedhydrogen, the formation energy of Ni-H or Co-H is still less thanthat of Pt-H [17]. On the other hand, the corresponding oxide ex-hibits higher OER activity due to the variable valence states, but theconductivity and HER activity are still poor [18e20]. Recently, thereare many research works focused on the metal/oxide hetero-structures. The catalyst with metal/oxide interfaces could stronglyenhance not only the stability of themetal cluster or even the singleatoms, but also the catalytic performance by strong electron in-teractions [9,21], such as yolk-shell Co/CoO nanoparticles [22],dendritic core-shell Ni@Ni(Fe)OOHmetal/metal oxyhydroxide [23],and CuCo@CuCoOx heterostructured nanowire [24]. To form thecompact oxide shell with proper thickness on metal, oxidativecalcination and dealloying methods were commonly used [9,18,25],however, the oxidation reaction of nanomaterials is fast and hard tocontrol. Beside the coupling effect of metal oxides and metal alloys,the construction of their architecture also plays a crucial role inboosting the electrochemical performance by facilitating not onlydiffusion of active species or gas bubbles and electron transfer, butalso the surface reactions. One-dimensional (1D) core/shell metal/oxide nanowires (NWs) prepared on conductive substrates arepromising architectures. The open space between NWs facilitatesdiffusion of the active species and guarantees that each NWanchored on a conductive substrate participates in the reaction,moreover, electrons generated from the surface and interface canmove swiftly via the 1D paths and the electrochemical cells do not

Fig. 1. (a) Schematic illustration of the fabrication of NiCo NCAs

require binders [24,26,27].Herein, we proposed a facile way to fabricate core/shell nano-

chain arrays (NCAs) composed of the NiCo core and thin CoNiO2shell on Ni foam by one-step low-temperature reduction ofNiCo2O4 nanorod arrays (NRAs) in H2/Ar. The thin CoNiO2 layerwith a thickness of 3.5 nm on the NiCo alloy nanochain exhibitssynergistic effects in both HER and OER in an alkaline medium,which serves as a low cost and high active bifunctional electro-catalyst with long-term stability. The result shows that HER andOER current densities of 10mA cm�2 are achieved at an over-potential of 156mV and 320mV in 1.0M KOH, respectively. Byusing the same materials as both the anode and cathode, a currentdensity of 10mA cm�2 at a cell voltage of 1.688 V and outstandingstability are accomplished suggesting large potential in overallwater splitting.

2. Experimental details

2.1. Materials synthesis

2.1.1. Materials and preparation of NiCo2O4 nanorod arrays on Nifoam

All the chemicals and solvents were analytical reagent grade andused as received without further purification. The NiCo2O4 NRAswere prepared on Ni foam hydrothermally [28]. 1.185 g of nickelchloride hexahydrate (NiCl2$6H2O), 2.37 g of cobalt chloride hexa-hydrate (CoCl2$6H2O), and 0.9 g of urea were dissolved in 30ml of

; SEM images of (b) NiCo2O4 NRAs and (c) NiCo alloy NCAs.

Fig. 2. (a) XRD patterns of the NiCo2O4 NRAs before and after reduction at differenttemperature; High-resolution XPS spectra of (b) Co 2p and (c) Ni 2p of NiCo-300 andNiCo2O4.

B. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 1216e12231218

deionized water and stirred to form a homogeneous pink solution.Before the hydrothermal reaction, the Ni foam (3� 4 cm2) wasdegreased in acetone for 5min and immersed in 1M hydrochloricacid (HCl) for 30min to remove the surface oxide. The residualhydrochloric acid and organic reagents were then removed byrinsing with deionized water and ethanol three times before dryingat the room temperature in air. The pre-treated pure Ni foam andsolution were transferred to a 50ml Teflon-lined autoclave, sealed,and heated to 120 �C for 6 h. After cooling, the product was washedultrasonically to remove the residues and the NiCo2O4 NRAs wereobtained by calcining in air at 400 �C for 3 h.

2.1.2. Preparation of NiCo alloy nanochain arrays on Ni foamThe NiCo2O4 NRAs on Ni foam were annealed in H2/Ar (5 vol%

H2) at a flowing rate of 100 sccm (standard cubic centimeter perminute) at 200 �C, 300 �C, 400 �C, and 500 �C for 3 h. The sampleswere designated as NiCo-200, NiCo-300, NiCo-400, and NiCo-500,respectively. For comparison, the bare Co NCAs on Ni foam wereprepared hydrothermally without adding NiCl2$6H2O and by sub-sequent annealing in H2/Ar at 300 �C for 3 h.

2.2. Materials characterization

Themorphology and structure of the samples were examined byfield-emission scanning electron microscopy (FE-SEM, FEI Nova450 Nano), high-resolution transmission electron microscopy (HR-TEM, FEI Titan G2-300), energy-dispersive X-ray spectroscopy (EDS,Oxford), X-ray diffraction (XRD, Philips X'Pert), and X-ray photo-electron spectroscopy (XPS, ESCAlab250).

2.3. Electrochemical measurements

The HER and OER activities of the catalysts were assessed usinga standard three-electrode cell in 1.0M KOH and CHI 760E bipo-tentiostat (CH Instruments, Inc.). A graphite sheet (2� 3 cm2) andsaturated calomel electrode (SCE) were the counter electrode andreference electrode, respectively, and the materials (1� 1 cm2)constituted the working electrode. The catalyst loading was about4.0mg cm�2 calculated by weighting the Ni foam before and afterthe hydrothermal reaction. In the measurements, the SCE referenceelectrode was calibrated with respect to the reversible hydrogenelectrode (RHE) as ERHE¼ ESCE þ 0.242 (SCE) þ 0.059pH ¼ ESCE þ 1.068. The polarization curves were acquired by linearsweep voltammetry (LSV) at a scanning rate of 2 mV s�1 and thelong-term stability was evaluated by chronopotentiometry per-formed at current densities of 100mA cm�2 for HER, 50mA cm�2

for OER, and 10mA cm�2 for overall water splitting. Cyclic vol-tammetry (CV) was carried out at a scanning rate of 100mV s�1 andelectrochemical impedance spectroscopy (EIS) was performed at anoverpotential of 132mV (�1.2 V vs. SCE) in HER and 338mV (0.5 Vvs. SCE) in OER from 100 kHz to 0.1 Hz with an AC perturbation of5mV. All the potentials in the polarization curves and Tafel plotswere corrected by iR correction using Rs determined from EIS.

3. Results and discussion

The pristine NRAs are prepared by a hydrothermal methodfollowed by calcination at 400 �C in air. Fig. 1a shows the schematicstructure and morphology of NiCo2O4 NRAs before and after ther-mal treatment in H2/Ar ambient. Fig. 1b depicts that the Ni foam iscovered uniformly by ordered NRAs with a rough surface and200 nm in diameter. The TEM image in Fig. S1 discloses that thenanorods are made of small nanoparticles tens of nanometers insize. After thermal reduction in H2/Ar, Fig. 1c shows that the or-dered NRAs are converted into nanochains with a smooth surface.

The nanochains arrays adhere onto the conductive substratestrongly and the large space facilitates gas release from the surfaceactive sites [29]. With increasing annealing temperature, thethickness of nanochain increases from tens to several hundrednanometers (Fig. S2) possibly due to sintering of nanoparticles.

The crystalline structure of the samples collected ultrasonically

B. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 1216e1223 1219

from the Ni foam before and after the thermal treatment is char-acterized by X-ray diffraction (XRD) as shown in Fig. 2a. The XRDpattern of the pristine nanorod matches that of spinel NiCo2O4(JCPDS No: 20-0781) [28]. After annealing in H2/Ar, the crystalphase shows no apparent change until the temperature increases to300 �C. Three typical XRD peaks at 44.40�, 51.69�, and 76.02�

assigned to Co (JCPDS No: 15-0806) and Ni (JCPDS No: 04-0850) canbe observed from NiCo-300 and NiCo-400, corresponding to the(111), (200), and (220) planes of the metallic Co and Ni FCC struc-ture (Fig. S3) [13,30,31]. The diffraction peaks associated to CoNiO2(JCPDS No: 10-0188) at 37.5�, 41.8�, and 61.7� also emerge fromNiCo-300 [25]. The low transformation temperature from NiCo2O4nanorods to NiCo alloy can be attributed to the small size of theNiCo2O4 nanoparticles lowering the reduction temperature.Moreover, the similar crystal phase of metallic Ni and Co favorsformation of the alloy phase. At a high reduction temperature of400 �C, the CoNiO2 phase disappears, suggesting that the surfaceoxide is converted to the metallic phase. The XPS Ni 2p and Co 2p ofNiCo-300 and NiCo2O4 spectra are presented in Fig. 2b and c. Withregard to NiCo2O4, the peaks appear at 779.5 eV and 781.2 eV for

Fig. 3. (a) TEM and (b) HR-TEM images of NiCo-300; (c) Dark-field TEM image a

Co3þ and Co2þ and 855.5 eV and 857.2 eV for Ni2þ and Ni3þ,respectively, consistent with previous results [32,33]. After thermalreduction, peaks at 778.2 eV and 852.9 eV in the Co 2p and Ni 2pspectra, respectively, arise from metallic Co and Ni [34]. There aretwo oxide states of Ni2þ and Co2þ at 781.2 eV and 855.7 eV in NiCo-300 in addition to the metallic Ni0 and Co0, confirming that theremaining oxide containing low chemical valence transition metalis related to CoNiO2.

Fig. 3 shows the transmission electron microscopy (TEM) imageand elemental maps of NiCo-300. Fig. 3a discloses that the nano-chains with a diameter of about 100 nm and an obvious coatedlayer are intertwined and it is thinner than that of the pristineNiCo2O4 NRAs because of shrinking (Fig. 3a). The correspondinglattice fringes of NiCo-300 are shown in Fig. 3b. The spacing of0.208 nm in the core structure between two neighboring fringes isclose to the d-spacing of the (111) plane of metallic Co (0.206 nm,JCPDS No: 15-0806) and Ni (0.204 nm, JCPDS No: 04-0850) [13,31].Besides, the lattice spacing of 0.242 nm in the shell corresponds tothe (111) plane of CoNiO2 suggesting that the core/shell structure iscomposed of the NiCo alloy core and 3.5 nm thickness CoNiO2 oxide

nd corresponding EDS elemental maps of Co, Ni, and O; (d) EDS line scans.

B. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 1216e12231220

shell. Fig. 3c shows the elemental maps obtained by dark-field HR-TEM and EDS revealing that Ni, Co, and O are uniformly distributedin the nanochains. The EDS line scans in Fig. 3d show that theconcentrations of Ni and Co are quite high and Ni/Co ratio is about1:1.5 (Fig. S4). The concentration of O is about 4.4% on the surface.These results indicate that the NiCo alloy in NiCo-300 is coated by athin CoNiO2 layer and expected to have goodHER and OER activitiesdue to the Mott-Schottky interface, where the electrons show shorttransfer length and high transfer efficiency [24,35].

The properties of the NiCo alloy NCAs on Ni foam as a bifunc-tional HER and OER catalyst are assessed in an alkaline solution(1.0M KOH) using a three-electrode configuration. The exposedarea on the working electrode is 1� 1 cm2 and no extra conductivesubstrate or organic binder is required. In general, the overpotentialrequired for HER under basic conditions is related to adsorption anddesorption of water molecules and dissociation of water molecules[36,37]. Fig. 4a and Fig. S5 show the linear sweep voltammetry

Fig. 4. Polarization curves of the electrocatalysts for (a) HER and (b) OER; (c and d) Tafel plotin 1.0M KOH.

(LSV) polarization curves which suggest that NiCo-300 deliversexcellent electrocatalytic HER performance in the alkaline elec-trolyte. Low overpotentials of 156mV and 245mV are needed forcurrent densities of 10mA cm�2 and 100mA cm�2, respectively,which are much better than those of the bare Ni foam, NiCo2O4NRAs, single Co NCAs (Fig. S6 and Fig. S7) and bare NiCo alloy (NiCo-400). The double-layer capacitance at the solideliquid interfacethat is proportional to the effective electrochemical surface isdetermined by cyclic voltammetry in the non-Faradaic capacitancecurrent range. NiCo-300 shows the largest CV area and capacitanceof 24.57mF cm�2, which is larger than the other samples and 4times larger than that of the NiCo2O4 nanorods (6.07mF cm�2)(Fig. S8). The HER performance is better than that of most transitionmetal and oxide electrocatalysts such as Co-CoO/N-rGo and Ni-NiO/N-rGo (170mV at 10mA cm�2 and 140mV at 10mA cm�2) [34],NiFe/NC alloy hybrids (230mV at 10mA cm�2) [38], CoOx@CN(232mV at 10mA cm�2) [39], Co-embedded nitrogen-rich carbon

s; (e and f) Electrocatalytic stability of NiCo-300. All the measurements was carried out

B. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 1216e1223 1221

nanotubes (370mV at 10mA cm�2) [40], NiO@NF-6 (310mV at10mA cm�2) [41], as well as other HER catalysts listed in Table S1.The excellent electrocatalytic properties can be attributed to thesynergic/effects between the oxide shell and NiCo core [42]. TheTafel slope of NiCo-300 in HER (82.7mV dec�1) is less than those ofthe pristine NiCo2O4 (135mV dec�1), NiCo-200 (108mV dec�1),and NiCo-400 (85.2mV dec�1) and comparable to those of mostreported oxide and metal alloy electrocatalysts (Table S1), sug-gesting that the thin oxide coating enhances the HER kinetics(Fig. 4c). Electrochemical impedance spectroscopy (EIS) and simu-lation with an equivalent circuit are performed in 1.0M KOH asshown in Fig. 5a. The charge-transfer resistance (Rct) of NiCo-300 inHER is about 3.5U and better than that of other specimens (24.5Ufor NiCo-400, 29.5U for NiCo-500, and 30U for NiCo-600) orpristine NiCo2O4 (37.4U), confirming that the fast HER kineticsstems from the synergistic effects between the thin oxide shell andNiCo core.

LSV measurement is performed in a more positive potentialregion to evaluate the electrocatalytic OER performance in 1.0MKOH. Transition metal oxides have been reported to be stable andefficient OER catalysts [26,43]. The polarization curve of NiCo-200reveals similar OER activity as NiCo2O4 nanorods. However, thepolarization curve of NiCo-300 shifts to lower potentials comparedto NiCo2O4 nanorods showing overpotentials of 320mV at10mA cm�2 and 390mV at 100mA cm�2 in Fig. 4b. After thermalreduction at high annealing temperature, the overpotential ofNiCo-400 increases to 340mV at a current density of 10mA cm�2

and more positive at higher annealing temperature. It is note-worthy that there is an obvious anodic peak at ~1.4 V (vs. RHE) inthe polarization curve of the NiCo2O4 nanorods and NiCo-200 inFig. 4b, which might due to the oxidation of Ni (II) to Ni (III) and Co(III) to Co (IV) as reported in the literature [44]. The oxidation peakdecreases when the annealing temperature is above 300 �C due toreduction of oxide. The reduced oxide affects OER (Fig. S5) sug-gesting that the OER kinetics is related to the oxide layer. The Tafelslopes in Fig. 4d are used to evaluate the kinetics in adsorption ofthe reactant (OH�) and desorption of the product (O2) on thecatalyst surface. The Tafel slope of NiCo-300 is 69.4mV dec�1 that issmaller than those of NiCo2O4 (92.2mV dec�1), NiCo-200 (76.1mVdec�1), NiCo-400 (72.8mV dec�1), and Ni foam (82.6mV dec�1). Itis smaller than those of many reported transition metal and oxideelectrocatalysts (Table S2). The Nyquist plots indicate that the Rctvalues of NiCo2O4 NRAs and NiCo-300 are about 3.2U, whichbecome larger after annealing at a high temperature confirmingthat the good OER kinetics stems from the surface oxide (Fig. 5b).

Fig. 5. Nyquist plots of different electrocatalysts measured from 100 kHz to 0.1 Hz with an ASCE) and (b) at overpotential of 328mV for OER (0.5 V vs. SCE).

The optimal HER and OER activities are observed from NiCo-300. InHER (cathode: 2H2Oþ2e�/H2þ2OH�), the NiCo alloy core pro-vides not only efficient electron transfer pathways, but also a cat-alytic surface for conversion of H2O into H2 due to the unfilledd orbitals of the transition metal. The CoNiO2 shell with strongelectrostatic affinity due to the positively charged Ni2þ and Co2þ

species can enhance adsorption of H2O at the interface. The alloysites facilitate dissociation of H2O and H adsorption thus enhancingthe HER catalytic activity by the synergistic effects at the interfaceof oxide/alloy [35,39,45]. However, the pure oxide surface is notactive in HER due to the lack of H adsorption sites, and the amountof OH� species occupying the sites on pure transition metal resultin worse HER activity. In OER (anode: 2OH�/H2Oþ1/2O2þ2e�),the Ni-Co based oxide is an excellent OER catalyst due to the redoxreactions, and the NiCo core can improve the electrical conductivityof the oxide [26,34,46]. Hence, the core/shell structure composed ofthe thin oxide layer coated NiCo alloy core delivers both enhancedHER and OER performance.

The HER and OER stability is crucial to water splitting and thedurability of NiCo-300 is evaluated by continuous CV sweeps in1.0M KOH at a scanning rate of 100mV s�1. The polarization curveshows negligible decay in both HER and OER. The electrocatalyticexperiments performed for over 15 h at a current density of100mA cm�2 for HER and 50mA cm�2 for OER confirm the gooddurability of NiCo-300 (Fig. 4e and f) and good structural integrity isobserved form the NiCo-300 electrode after the long-term test(Fig. S9). The overall water splitting performance of the NiCo-300electrocatalyst as both the anode and cathode (NiCo-300jjNiCo-300) is assessed on a two-electrode system in 1.0M KOH. As shownin Fig. 6a, NiCo-300jjNiCo-300 shows 10mA cm�2 at just 1.688 Vand it is comparable with those of NiCo2S4 NA/CCjjNiCo2S4 NA/CC(1.68 V) [47], NiFe LDH/Ni foamjjNiFe LDH/Ni foam (1.70 V) [36],hollow microcuboid NiCo2O4jjNiCo2O4 (1.65 V) [33], and betterthan commercial electrolyzers (1.8 V-2.0 V) [48]. The long-termelectrochemical stability of NiCo-300jjNiCo-300 is also studied(Fig. 6b). The applied potential is very stable at about 1.7 V for over12 h at a current density of 10mA cm�2.

4. Conclusions

NiCo NCAs with a 3.5 nm thick oxide coating are produced byone-step thermal reduction of NiCo2O4 NRAs on Ni foam at lowtemperature. The core shell structure of NiCo-300 electrocatalystshows excellent bifunctional HER and OER activities and overallwater splitting properties in an alkaline medium, delivering low

C perturbation of 5mV in 1.0M KOH, (a) at overpotential of 132mV for HER (�1.2 V vs.

Fig. 6. (a) LSV of the two-electrode configuration of NiCo-300jjNiCo-300 with the inset showing the optical photograph of the NiCo-300 electrode and electrolyzer; (b) Chro-nopotentiometric results for water splitting electrolysis with NiCo-300jjNiCo-300 at a current density of 10mA cm�2 in 1.0M KOH.

B. Zhang et al. / Journal of Alloys and Compounds 797 (2019) 1216e12231222

overportential and high electrocatalytic kinetics than pristineNiCo2O4 NRAs and bare NiCo alloy. Current densities of 10mA cm�2

and 100mA cm�2 are observed at overpotential of 156mV and245mV in HER and 320mV and 390mV in OER. The thin oxidelayer on the NiCo-300 surface provides synergistic effects to boostthe HER and OER activities. The two-electrode alkaline electrolyzeronly requires a cell voltage of 1.688 V to deliver a water splittingcurrent of 10mA cm�2. Our results reveal the large potential ofhybrid structure consisting of transition metal and oxide as low-cost, stable and efficient bifunctional catalysts in overall watersplitting.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This work was financially supported by National Natural ScienceFoundation of China (Nos. 31500783, 51572100, 51504171 and61434001), Major project of Technology Innovation of HubeiProvince (2018AAA011), HUST Key Interdisciplinary Team Project(2016JCTD101), Fundamental Research Funds for the Central Uni-versities (HUST: 2015QN071), and Wuhan Yellow Crane TalentsProgram, China, and City University of Hong Kong StrategicResearch Grant (No. 7005105).

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jallcom.2019.05.036.

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General synthesis of NiCo alloy nanochain arrays with thin

oxide coating: a highly efficient bifunctional electrocatalyst

for overall water splitting

Ben Zhang a, Xuming Zhang a, *, Yong Wei a, Lu Xia a, Chaoran Pi a, Hao Song a, b,

Yang Zheng a, Biao Gao a, b, *, Jijiang Fu a, Paul K. Chu b

a The State Key Laboratory of Refractories and Metallurgy and Institute of Advanced

Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan

430081, China.

b Department of Physics and Department of Materials Science and Engineering, City

University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

*Corresponding authors:

E-mail: [email protected] (X.M. Zhang), [email protected] (B. Gao)

Fig. S1 TEM image of the precursor NiCo2O4 NR

Fig. S2 SEM images of the NiCo alloy NCAs obtained under the different reduction

temperature of (a) 400 °C, (b) 500 °C, (c) 600 °C

Fig. S3 The magnified XRD pattern of NiCo-300 in the range from 43o to 77o

Fig. S4 EDS element content figure of NiCo-300 through TEM

Fig. S5 LSV of the these electrocatalysts toward (a) HER and (b) OER in 1.0 M KOH

Fig. S6 SEM images of (a) Co3O4 NRAs and (b) Co nanoparticle NCAs, (c)

Corresponding XRD partterns

Fig. S7 LSV curves of NiCo-300, Co3O4 and Co electrocatalyst toward (a) HER and

(b) OER at a scanning rate of 2 mV s-1 in 1.0 M KOH

Fig. S8 Electrochemically active surface areas (ECSA) determined by the double layer

capacitance (CDL) measurements from cyclic voltammetry (CV) in 1.0 M KOH from

the (a) NiCo-300, (b) NiCo-400, (c) NiCo-500, (d) NiCo-600, (e) NiCo2O4 catalysts at

scanning rates between 10 and 120 mV s-1. (f) The difference of current densities

between anodic and cathode at 0.92 V vs. SCE as a function of scan rate fitted to a

linear regression for various sample.

Fig. S9 SEM images of the NiCo-300 alloy chain arrays after long-term stability: (a)

and (b) HER, and (c) and (d) OER at a scanning rate of 100 mV s-1 in 1.0 M KOH

Table S1. Comparison of the recently reported highly active HER catalysts in various

kinds of solution. While, the NF, CC, Ti represent the Ni foam, carbon cloth and Ti

plate substrate loaded with the catalyst, respectively.

Catalysts

Electrolyte

Mass

Loading

(mg/cm2)

Overpotential

at 10 mA/cm2

(vs. RHE)

Tafel slope

(mV/dec)

Reference

NiCo-300 1 M KOH 4(NF) 156 mV 82.7 This work

CoNi@NC 0.1 M H2SO4 1.6 142 mV 104 [1]

FeCo 1 M KOH 0.32 211 mV 77 [2]

Ni0.9Fe0.1/NC 1 M KOH 0.2 230 mV 111 [3]

Ni-Et-OAm6 1 M NaOH 0.35 180 mV 111 [4]

Co@N–C 1 M KOH 4.5 210 mV 108 [5]

3DOM/m Ni 1 M NaOH 0.25 170 mV 52 [6]

Co-NRCNTs 1 M KOH 0.28 370 mV - [7]

Co/CoOx 1 M KOH 0.25 204 mV 42 [8]

NiO@NF-6 1 M KOH 3.5(NF) 310 mV 231 [9]

CoOx@CN 1 M KOH 0.12 232 mV - [10]

NiO NRs-m-Ov 1 M KOH 0.216(CF) 113 mV 100 [11]

Ni-NiO/N-rGO 1 M KOH 0.21 260 mV 67 [12]

Co-CoO/N-rGO 1 M KOH 1(NF) 170 mV 51 [12]

NiCo2S4 1 M KOH 4(CC) 200 mV 141 [13]

Co3O4 NCs 1 M KOH 0.35(CC) 380 mV 116 [14]

NiS film 1 M KOH 43(NF) 152 mV 83 [15]

Ni3S2 1 M KOH 1.6(NF) 223 mV - [16]

Table S2. Comparison of the recently reported highly active OER catalysts in various

kinds of solution.

Catalysts

Electrolyte

Mass

Loading

(mg/cm2)

Overpotential

at 10 mA/cm2

(vs. RHE)

Tafel slope

(mV/dec)

Reference

NiCo-300 1 M KOH 4(NF) 320 mV 69.4 This work

Co3O4 NCs 1 M KOH 0.35(CC) 322 mV 101 [14]

Co3O4/SWNTs 1 M KOH 0.05 590 mV 104 [17]

Co/CoOx 1 M KOH 0.25 380 mV 99 [8]

Ni0.9Fe0.1/NC 1 M KOH 0.2 330 mV 45 [3]

Co@N–C 1 M KOH 4.5 400 mV - [5]

NiCo2S4 1 M KOH 4(CC) 270 mV 89 [13]

ZnxCo3–xO4 1 M KOH 1(Ti) 320 mV 51 [18]

N-CG-CoO 1 M KOH 0.7 340 mV 71 [19]

NiCo2O4 1 M KOH 0.28 380 mV 50 [20]

NiCo2O4 NAs 1 M KOH 0.3(Ti) 370 mV 60 [21]

NiCo2O4/G 0.1 M KOH 0.4 440 mV 164 [22]

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