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Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/nanoenergy

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RAPID COMMUNICATION

Tweaking the composition of NiMoZn alloyelectrocatalyst for enhanced hydrogenevolution reaction performance

Xueqin Wanga,b, Ren Sub,n, Hüsnü Aslanb, Jakob Kibsgaardb,Stefan Wendtb, Linghui Menga, Mingdong Dongb, Yudong Huanga,n,Flemming Besenbacherb

aSchool of Chemical Engineering and Technology, Harbin Institute of Technology, 150001 Harbin, ChinabInterdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark

Received 17 August 2014; received in revised form 23 November 2014; accepted 11 December 2014

KEYWORDSElectrocatalysis;Water splitting;Hydrogen evolutionreaction;

Zn concentration;Composition effect

0.1016/j.nanoen.2ublished by Elsevi

thors.: rensu@inano.au.(Y. Huang).

ticle as: X. Wang,nce, Nano Energy

AbstractNickel-based alloy catalysts are promising candidates to replace noble metal materials forelectrocatalytic hydrogen evolution reaction (HER), but the performance of the alloy systems stillneeds to be improved to effectively substitute noble metal catalysts, such as platinum. Here wereport a detailed study of the controlled deposition of NiMoZn ternary alloy electrocatalysts andwe subsequently investigate the influence of chemical composition on the HER reactivity. Wehave identified the essential role of Zn for boosting the intrinsic electrocatalytic performance andrevealed the optimum Zn concentration in the alloy. While the low concentration of Zn facilitatesthe charge transfer process, massive Zn deposition in the alloy results in a strong inhibition ofproton adsorption on the surface of the electrocatalysts. The NiMoZn alloy electrocatalyst with aZn concentration of 1–3 at% shows an extremely high activity for HER, which is significantly higherthan that of most non-noble metal electrocatalysts. Moreover, the alloy with 2 at% of Zn exhibitsan excellent stability in acid conditions, featuring the NiMoZn alloy a promising candidate toreplace noble metal electrocatalyst for water splitting process.& 2014 Published by Elsevier Ltd.

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014.12.007er Ltd.

dk (R. Su),

et al., Tweaking the composition(2014), http://dx.doi.org/10.1016

Introduction

Efficient hydrogen production techniques have been investi-gated for decades as the demand for green energy increases[1–4]. Compared to the conventional H2 production through

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stream-reforming of fossil fuels at high temperatures andpressures, the environmental friendly H2 production by watersplitting has attracted great attention [5–7]. Depending onthe input energy sources, the water splitting process can besubdivided into electrocatalytic- [8,9] and photocatalytic[10,11] water splitting. Electrocatalytic water splitting hasbeen considered to be one of the most promising approachesdue to its relative high efficiency and long-term stability[12,13]. Alkaline water electrolysis has become the maturetechnology to produce hydrogen for decades, but drawbacksin high ohmic resistance and low operating pressures aredifficult to overcome [14]. The proton exchange membrane(PEM) water electrolysis mitigates these problems [15], butthe acidic nature of this membrane needs Pt or Pt-basedalloys as catalysts, which limit the large scale application ofthe water electrolysis due to the scarcity and high cost of thenoble metals. Since the electrocatalytic water splittingprocess can be considered as the sum of oxygen evolutionreaction (OER) and hydrogen evolution reaction (HER), designand synthesis of high performance electrocatalyst materialsfor both reactions using earth-abundant elements is crucial inthe development of large-scale water splitting.

The HER process using a metallic electrode (M) in acidicelectrolyte can be described by either the Volmer–Heyr-ovsky mechanism or the Volmer–Tafel mechanism [16–18], asdepicted in Eqs. (1)–(3):

MþHþ þe�2M�Hads Volmer reactionð Þ ð1Þ

M�HadsþHþ þe�2H2þM Heyrovsky reactionð Þ ð2Þ

2M�Hads2H2þ2M Tafel reactionð Þ ð3ÞM–Hads represents the adsorbed H atom on the surface of themetal electrode, which obviously plays a key role in HERregardless of the reaction mechanisms. Therefore the bindingenergy of the H atom (ΔGH) to the surface of the HER catalystdetermines the reactivity of material. In order to predict theideal HER catalyst, density functional theory (DFT) calcula-tions have been performed on numerous materials rangingfrom transition metals to their alloy compounds [6], sulfides[19], phosphides [20], carbides [21], enzymes [22]. It is clearthat the noble metals (i.e., Pt, Ir) occupy the top of thevolcano curve [23,24] and present optimized ΔGH, which showpremier activities for HER. Nevertheless, it also suggests thatimproving the ΔGH of the elements not at the top of thevolcano plot may lead to enhance HER performances.

Recently, great efforts have been undertaken to find highlyefficient, earth abundant HER catalysts, including alloys,metal sulfides and phosphides (i.e., MoS2 [25], Ni2P [26],Co–P [27], WS2 [28], FeS2 and NiS2 [29]). Nickel based HERcatalysts has been widely studied due to their low cost andrelative high efficiency [9,30–36]. Various synthesis methodshave been employed to increase the effective surface area topromote the HER performance [36–39]. Raney Ni basedmaterials, which are prepared by selective leaching of e.g.Al or Zn from an alloy, show large surface area and arepromising candidates for large scale HER operation [32,37].However, the variations in morphologies barely alter the ΔGH

thus the intrinsic reactivity remains unchanged. Therefore,optimization of the ΔGH to reduce the overpotential of HERhas been considered to be a more promising approach toimprove the electrocatalytic performance of Ni-based

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.101

materials. Alloying Ni with other transition metals (e.g.,Au, W, Zr) has been widely applied to tune the ΔGH thus theelectrocatalytic activity to reduce the overpotential of HER[40–44]. Metikoš-Hukovic et al. [43] found out that theaddition of Zr in the NiZr alloy results in a rapid increase inthe density of states of the Ni 3d orbitals at the Fermi level,which consequently resulted in a weaker bond of M–Hads anda higher activity for HER. A similar synergistic effect was alsoobserved in the NiW electrocatalyst [44], where a 10 at% of Win the alloy demonstrates the highest intrinsic electrocataly-tic reactivity that could be attributed to the change of theelectronic structure of Ni upon alloying.

The NiMo alloy HER electrocatalyst systems has been inves-tigated [9,30,31,35,36,45–49] since it has been reported thatthe introduction of Mo into Ni can modify the electron densitystates of the d orbitals thus change the ΔGH on the metalsurface [35,36]. Experimental results have confirmed thisprediction and the influence of the chemical composition ofNiMo alloy catalysts has been studied under various operatingconditions [9,30,47]. It has been reported that a synergisticeffect of Ni and Mo concentration for the optimum perfor-mance, and a 1:1 ratio of Ni:Mo showed the highest activity forHER under alkaline conditions [30]. However, other optimizedcomposition of NiMo alloy has also been reported to yield theoptimum HER reactivity in alkaline solution [47], indicating acomplicated composition effect on the intrinsic activity of Ni-based alloy for HER. In addition, Zn has been considered to be adecent element for the alloying with Ni to improve the HERreactivity. While the addition of Zn in the alloy can alter the dorbital structure of Ni, the selective dissolution of Zn from thealloys also facilitate the generation of a relative high surfacearea with increased surface roughness that is beneficial to theHER performance [50–52]. Therefore it is expected that theternary NiMoZn alloy catalysts to exhibit excellent activity forHER [33] although the stability remains unclear. However,precise control of the composition during a one-step prepara-tion of the ternary alloy to obtain a high HER reactivity ischallenging and few studies have addressed the influence ofcompositions on the physical properties of the alloy electro-catalyst (i.e., morphology, particle size, and chemical states).It has also rarely been reported the homogeneity of the co-deposited alloy nanoparticles (NPs) within the as-synthesizedelectrocatalysts. Moreover, the correlation between the physi-cal properties of the material and HER reactivity remainslargely unknown due to the lack of well-defined systems, whichhinders the development of next generation alloy catalystmaterials for HER. Furthermore, the stability of Ni-basedelectrocatalysts in acidic conditions is also a key parameter toconsider [53,54]. Therefore, a precise controlled ternary alloysystem is desired to investigate the effect of individualcomponent and their oxidation states on the intrinsic activityand long-term stability for HER.

Here we have synthesized a series of NiMoZn alloy catalystsby electrodeposition method with precision control of thealloy compositions. The concentrations of Zn and Ni within thealloys have been tuned independently by varying the electro-lyte composition systematically. We studied the influence ofthe chemical composition on the morphology, particle size,and the oxidation states of each component of the alloyelectrocatalyst. We have further investigated the effect of thechemical composition on the HER reactivity and the ratedetermining steps.

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Table 1 Electrolyte compositi Q8on for the synthesis ofNiMoZn alloy NPs with various chemical compositions. Theconcentrations of Na4P2O7 and NaHCO3 in the electrolytewere 0.13 M and 0.89 M respectively for all samples.

Sample Electrolyte composition

[Ni2+](mM)

[Mo6+](mM)

[Zn2+](mM)

Zn series a

NiMoZn0 40 20 0NiMoZn1 40 20 0.1NiMoZn2 40 20 0.2NiMoZn3 40 20 0.3NiMoZn5 40 20 0.6NiMoZn6 40 20 1NiMoZn15 40 20 3NiMoZn26 40 20 5

Ni series a

Ni38MoZn 20 20 0.1Ni44MoZn 30 20 0.1Ni48MoZn 40 20 0.1Ni49MoZn 50 20 0.1

aThe NiMoZn alloy catalysts were labeled by the atomicconcentrations of Zn (for Zn series) and Ni (for Ni series) asdetermined by XPS.

3Enhanced hydrogen evolution reaction performance

Experimental details

Sample preparation

The high loading NiMoZn alloy NPs were deposited onto a Tifoil substrate (0.1� 10� 10 mm3, 99.6+%, Advent, UK) byelectrochemical deposition method. Prior to deposition, theTi foil was polished by silicon carbide papers (220#-320#-500#-800#-1200#). The polished foil was degreased bysonication in acetone, ethanol, and deionized (DI) water for10 min respectively and dried in a nitrogen stream. A two-electrode system was used for the electrodeposition of thealloy NPs under a constant current mode (i=120 mA/cm2 andvoltage at�4 V) for a period of 10 min. A Pt wire was used asthe counter electrode (CE). After deposition, the as-preparedsamples were rinsed using DI water thoroughly and driedunder nitrogen stream subsequently. Note that chemicalanalysis by X-ray photoelectron spectroscopy (XPS) confirmedthat all as-prepared electrodes were free of trace Pt,indicating the observed HER activity was solely correlatedto the NiMoZn catalyst rather than the Pt impurities (FigureS1 in Supporting information). The electrolyte was preparedusing nickel (II) chloride hexahydrate (NiCl2∙6H2O, 98%),sodium molybdate dehydrate (Na2MoO4∙2H2O, 99+%), anhy-drous zinc chloride (ZnCl2, 97%), tetrabasic sodium pyropho-sphate (Na4P2O7, 99%) and sodium bicarbonate (NaHCO3,99%) [33]. All chemicals were purchased from Sigma-Aldrichand used directly without any treatment. Two series ofNiMoZn catalysts NPs with various chemical compositionswere synthesized by tuning the Zn2+ and Ni2+ concentrationsin the electrolytes independently, as listed in Table 1. Prior tothe deposition, formic acid (18.5 mM) was added immedi-ately to assist the co-deposition of the metals.

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Physical properties characterization

The morphology and quantitative nanomechanical propertiesof the as-deposited NiMoZn alloy NPs were characterizedusing a Multimode VIII AFM (Bruker, Santa Barbara, CA, USA)under ambient conditions (temperature: 21 1C and humidity:53%). Peak force tapping was performed using rectangularsilicon nitride probes (nominal resonance frequency:525 kHz, nominal spring constant: 200 N/m, and tip radius:8 nm; TAP525A, Bruker, USA). Topography, stiffness andadhesion images were acquired simultaneously. The linearscanning rate was set to 0.5–0.8 Hz with a scan resolution of512 per line. The imaging force and other scan parameterswere optimized to achieve the highest possible resolutionalso to avoid damaging the sample or wearing the tip.Collected images were further processed using the SPIPsoftware (Image Metrology ApS, Lyngby, Denmark). Thesamples for AFM imaging were deposited on finely polishedTi disks (root mean squared roughness, RRMS=1 nm) using thesame electrolyte recipes but the deposition time was 15 sunder a constant current mode (i=20 mA/cm2 and voltageat�3 V). The chemical composition of the alloy NPs and theoxidation states of the containing elements were analyzed byX-ray photoelectron spectroscopy (XPS, AXIS Ultra, Kratos)equipped with a monochromated Al Kα X-ray source andoperated at 10 kV and 15 mA. The pass energies and scanrates were 160 eV, 10 eV/s and 40 eV, 0.2 eV/s for recording

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.1016

survey and high resolution scans, respectively. The adventi-tious carbon spectrum (C 1s=284.8 eV) was used for calibra-tion of the XPS data.

Electrochemical characterizations

All electrochemical measurements were carried out underdeaerated conditions, which were achieved by continuouspurging of a 0.5 M H2SO4 electrolyte at room temperaturewith Ar gas (99.998%). The NiMoZn alloy electrocatalyst, aPt mesh (99.99%, 25� 25 mm2, Advent), and a Ag|AgClelectrode (3 M KCl, Metrohm) were used as the workingelectrode (WE), counter electrode (CE), and referenceelectrode (RE), respectively. Cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS) analysis wereperformed in a three-electrode configuration using a poten-tiostat (AUTOLAB PGSTAT 30). Prior to HER testing, The alloyelectrocatalysts were activated by cycling the potential (0to �0.4 V vs. Ag|AgCl) at a scan rate of 50 mV/s until the CVsweeps stabilized, and subsequently the HER performancewas recorded with a scan rate of 5 mV/s. Acceleratedstability test of the specified NiMoZn alloy electrocatalystwas performed by cycling the sample for 1000 sweeps (0 to�0.4 V vs. Ag|AgCl, 100 mV/s) after stabilization. Again,XPS analysis confirmed that the tested electrode was free oftrace Pt, indicating the observed HER activity was solelycorrelated to the NiMoZn catalyst rather than the Ptimpurities (Figure S1 in Supporting information). The EISanalysis was conducted using a 10 mV amplitude AC signalover a frequency range from 100 kHz to 0.01 Hz. Theelectrochemical capacitance was measured to determinethe surface area of the alloy catalysts. For each sample, the

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potential was swept from �0.2 to �0.05 V (vs. Ag|AgCl) atfive different scan rates (10, 20, 40, 80, and 160 mV/s).Detailed procedure can be found elsewhere [55]. The REwas calibrated to the reversible hydrogen potential (RHE)using platinum mesh for WE and a platinum wire for CE in0.5 M H2SO4 electrolyte under saturated H2 conditions. Thiscalibration resulted in a shift of �0.232 V versus the RHE.All potentials were re-calculated and cited vs. the RHE.

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Results and discussion

We first investigated the influence of chemical composition onthe micromorphology of the NiMoZn alloy NPs of the as-prepared AFM samples, as representatively illustrated inFigure 1(a)–(c). The NiMoZn alloy NPs were deposited ontothe Ti substrate uniformly regardless of the electrolyterecipes. However, the surface coverage of the NPs increasedas increasing of [Zn] in the alloy NPs. We also observed theformation of agglomerations containing few NPs along withindividual NPs, and these individual NPs are somehow larger inNiMoZn0 (Figure 1(a) compared to that of within NiMoZn1(Figure 1(b) and NiMoZn15 (Figure 1(c) samples. This phenom-enon suggests that the addition of Zn2+ influenced thedeposition mechanisms of the alloy NPs, as the standardreduction potential of Zn2+ (Zn2+-Zn0(s), �0.76 V) is sig-nificantly negative compare to that of Ni2+ (Ni2+-Ni0(s),-0.25 V) and Mo2+ (Mo4+-Mo0(s), �0.15 V). Statistical ana-lysis of the particle size and height of the samples suggeststhat the NiMoZn alloy NPs are disc shaped with a height of�12 nm and diameters of�60 nm, as shown in Figure 1(d) and(e), respectively. Furthermore, we performed nanomechanicalproperties mapping of the corresponding samples to deter-mine the chemical composition of individual NPs (Figure 1(a)–(c). It is obvious that the NPs in each sample showedhomogeneous values of stiffness and adhesion that presenteda distinct contrast to the Ti substrate, indicating a homo-geneous co-deposition of Ni, Mo, and Zn within individual NPs.Moreover, quantitative analysis showed that both stiffness andadhesion of the NPs decreased following the increasing of [Zn]in the alloy (17 GPa, 6 GPa and 5.4 GPa) and (22.6 nN,19.78 nN and 19.41 nN), which revealed a successful deposi-tion of NiMoZn alloy NPs with different compositions. It shouldbe noted that the morphology and mechanical properties weresimilar for both NiMoZn1 and NiMoZn15 alloys, allowing us adirect comparison of electrochemical performance as a func-tion of the chemical composition of the alloy electrocatalyst.

We have further analyzed the chemical composition of thehigh loading alloy NPs by XPS, as shown in Figure 2(a) and (b).The survey spectra suggested that all alloy NPs contained Ni,Mo, and Zn, indicating the successful co-deposition of thethree elements into the alloy NPs. The presence of C on thesurfaces can be ascribed to adventitious contaminants. Todemonstrate the electrolyte effect on the formation ofNiMoZn electrocatalyst, the compositions of the alloy NPsas a function of [Ni2+] and [Zn2+] in the electrolyte wereplotted in Figure 2(c) and (d), respectively. Whilst increasingthe [Ni2+] in the electrolyte showed a negligible effect onthe deposition of Zn, an exponential increase and decreasetendency for [Ni] and [Mo], respectively, in the alloy wereobserved (Figure 2(c)). Further increase of [Ni2+] in theelectrolyte resulted in a 1:1 ratio deposition of Ni and Mo in

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.101

the alloy NPs. Interestingly, a linear increase of [Zn] in thealloy was observed following the increasing of [Zn2+] in theelectrolyte, and more significant exponential changes of theconcentrations of Ni and Mo (Figure 2(d)) were noticed. Theresult suggested that the addition of Zn2+ in the electrolytepromotes the deposition of Ni but strongly inhibits thedeposition of Mo.

The influence of chemical compositions on the oxidationstates of individual elements were demonstrated by the high-resolution spectra, as shown in Figure (3) and Fig. S2 inSupporting information. The oxidation states of Zn remainedto be metallic independent of the [Ni] and [Zn] concentra-tions in the alloy NPs, as indicated by the Zn 2p3/2 peaks at1021.6 eV (Figure 3(a) and (b)) [56]. Deconvolution of the Ni2p3/2 spectra suggested that the coexistence of Ni0

(�852.6 eV) and Ni2+ species [Ni(OH)2 (�856.6 eV) andNiO (�853.6 eV)] in all samples [57] (Figure 3(c) and (d)),whereas Mo0 (�227.5 eV), Mo4+ (� 228.7 eV), Mo5+

(�230.7 eV), and Mo6+ (�232.1 eV) species were observedfor all alloy NPs [58] (Figure 3(e) and (f)). Further analysisshowed that the concentrations of both Ni and Mo speciesremained constants following the increase of [Ni] in the alloy(Figure 3(g) and Table S1 in Supporting information). Whilstthe concentration of Ni show negligible effect on theoxidation states of all elements, the increment of Zninfluence the chemical states of Ni and Mo significantly, asdepicted in Figure 3(d), (f), and (h), respectively. Metallic Niwas the dominant species in the alloy at low [Zn], and the Ni0

species gradually converted to Ni2+ species [59] followingthe increasing of [Zn] in the alloy. Similarly, the Mo 3d peaksalso shifted to higher binding energies as the [Zn] increased.The Mo6+ became the dominant species and the rest Mospecies were diminished at the highest Zn concentration.Since the induced deposition of Mo0 has been reported to becorrelated to the presence and concentration of Ni0 species[60,61], the reduction of [Mo0] in the alloy may associate tothe insufficient amount of Ni0 species in the alloy catalystsprepared at high [Zn2+] in the electrolyte.

Since the [Zn] showed huge impact on the chemicalcomposition of the alloy NPs, it might also alter the activesurface area of the electrocatalyst and subsequently influ-ence the intrinsic reactivity. We performed electrochemicalcapacitance measurements to determine the active surfacearea of the high loading NiMoZn alloy NPs, as demonstrated inFigures 4 and S3 in Supporting information. Figure 4(a)representatively illustrates the CV sweeps recorded at dif-ferent scan rates that were used for the determination of thenon-faradic capacitive current, which was generated due tothe double layer charging during cycling the potential. As thedouble layer charging current (jc) is proportional to theelectrochemically active surface area (Aechem) and the scanrate (v) [55,62], the Aechem can be derived by comparing theslope to that of a standard reference of NiMoZn alloy catalystwith known surface area. However, such reference is difficultto prepare in practice therefore we normalized the geo-metric surface area (Anor) of all alloy electrocatalysts to thatof the NiMoZn0 sample using the slopes derived from Figure 4(b) and (c) and summarized in below. It was obvious that theactive surface area remained constant at low Zn concentra-tions ([Zn]o5 at%) and further increase of [Zn] to 15 at%resulted in a gradual increase of the Anor, which may be dueto the roughening of the surface by selective dissolution of Zn

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Figure 1 (a)–(c) Representative AFM height images and corresponding mechanical mappings of the as-synthesized AFM samples ofNiMoZn0, NiMoZn1 and NiMoZn15, respectively. (d) Particle height distribution of the samples derived from the AFM height images.The height profiles were deconvoluted into two peaks, where the first one can be assigned to the Ti substrate and was subtractedand indexed as 0. The second peak indicates the height profile of the NiMoZn NPs. Dashed lines are Gaussian fit to the raw data.(e) Particle size distribution of the samples derived from the AFM height images. Solid lines are guide lines.

5Enhanced hydrogen evolution reaction performance

in the alloy catalyst. Surprisingly, further increase of [Zn] to26 at% leads to a dramatic decrease of the Anor, which can beassociated to the stripping-off of the alloy particles from theTi surface that originated from the excessive dissolution of Zn(See Figure S4 in Supporting information). On the other hand,the Ni series samples that were characterized by similar Znconcentrations present a constant active surface area, againindicating that the increase of the active surface area aremainly resulting from the selective dissolution of Zn in thealloy NPs.

The influence of composition on the HER reactivity of thehigh loading NiMoZn alloy NPs was demonstrated in Figure 5. Allthe CV sweeps were corrected for uncompensated ohmicpotential drop (iR) losses. It was clear that the addition of Znaltered the HER performance of the alloy NPs significantly(Figure 5(a)), where an enhanced current density was observedat low Zn concentrations (o3 at%), and further increase of Znconcentration in the alloy NPs resulted in a dramatic drop ofthe current density and a larger overpotential. An optimizedperformance was reached at lower Zn concentrations (1–3 at%),which yielded a current density of �20 mA/cm2 at an over-potential of 46 mV (vs. RHE). Interestingly, for the Ni series NPs(Figure 5(b)), all NiMoZn alloy electrocatalysts exhibited similarCV curves with an onset for HER at approximately �20 mV andthe current density of all electrocatalysts reached approxi-mately �20 mA/cm2 at an overpotential of 40 mV, indicating a

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.1016

negligible effect of Ni on the HER performance. The optimizedNiMoZn NPs demonstrates an overpotential for a current densityof 20 mA/cm2 that is smaller than that of most recentlydeveloped non-noble metal electrocatalytsts ranging from Ni–Mo [49], Co–P [27], Mo–P [63], Ni2P [26], and MoS2 [64]. To get abetter understanding of the composition effect, we havecompared the normalized HER performance using the Anor asa function of [Zn] and [Ni], as depicted in the current densityplots at 50 mV of overpotential (see Figure 5(c) and (d)),respectively. For comparison, the performance of a pure Nisample that was prepared via a similar method (Figure S5 inSupporting information) was presented as well. Apparently, aprecision control of Zn was crucial in achieving excellent HERperformance. The presence of few atomic percent of Zn withinthe alloy NPs showed enhanced reactivity compared to that ofpure Ni and the NiMo alloy. The HER performance of the NiMoZnNPs experienced an exponential decay at elevated Zn concen-tration, which was found to be closely related to the oxidationof metallic Ni and Mo to Ni2+ and Mo6+ at higher Znconcentrations (see Figure 3(f) and (h)), respectively. Incomparison, increasing of Ni concentration resulted in anegligible improvement in HER reactivity (Figure 5(d)). Further-more, the Tafel plots of NiMoZn alloy catalysts with lower Znconcentration (o3 at%) yielded slopes of �40 mV/decade,which suggests the rate determining step (RDS) can be assignedto the charge transfer process compared to that of proton

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Figure 2 (a) and (b) XPS survey spectra of high loading NiMoZnalloy catalysts prepared with different [Ni2+] and [Zn2+]concentrations, respectively. (c) and (d) NiMoZn alloy NPscompositions as a function of [Ni2+] and [Zn2+] within the bulkelectrolyte, respectively.

Figure 3 (a)–(f) Zn 2p, Ni 2p, and Mo 3d high resolution XPSspectra of the as-synthesized high loading NiMoZn alloy NPs.Colored dashed lines represented the fitting results of repre-sentative spectra. Black dashed lines indicate the bindingenergies of specified oxidation states. (g) and (h) Oxidationstates of Ni and Mo as a function of [Ni] and [Zn] in the alloyNPs, respectively. Mon+ indicates the sum of Mo4+, Mo5+, Mo6+

species.

X. Wang et al.6

adsorption as the RDS for pure Ni (�120 mV/decade), as shownin Figure S6 in Supporting information [9,65–67].

To facilitate direct comparison of the Ni-based alloy HERcatalysts with literatures, we have tested the HER perfor-mance of NiMoZn0 and NiMoZn2 sample that has been studiedby AFM to estimate the turnover frequencies (TOF), as shownin Figure 5(e) and Figure S7 in Supporting information. TheTOF was enhanced by the addition of Zn in the NiMo alloyNPs. Since the AFM sample revealed a particle density of�150/mm2 with a mean particle size of �60 nm (Figure 1),we estimated that a TOF of �9500 s�1 per NiMoZn2 NP canbe achieved at overpotential of 100 mV vs. RHE. Noticeably,this corresponding to a TOF of �0.22 s�1 per surface atom(at �100 mV vs. RHE), which is �4 times better than that ofreported value of Ni6Mo4 film and even slightly better thanthat of commercially available NiMo alloy [49].

We have further evaluated the durability of an high loading,activated NiMoZn2 electrocatalyst deposited on Ti foil bymeans of accelerated stability test [7] and representativelyplotted the cathodic sweeps of the initial cycle (1 cycle), after100 and 1000 cycles, as shown in Figure 5(f). During this test,the material has experienced both extreme (30 mA/cm2) andnormal working conditions (10 mA/cm2), providing compre-hensive diagnosis of the durability. In addition, such stabilitytest can also represent the water electrolysis process poweredby intermittent renewable energies (e.g., photovoltaics). A

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.101

slightly enhanced performance was observed during the first100 cycles and then the samples remained stable for the restperiod of the accelerated stability test. Noticeably, theNiMoZn2 electrocatalyst provided an impressive current den-sity of �20 mA/cm2 at a bias as low as �40 mV (vs. RHE),which is equivalent to a solar water splitting device that yieldsa efficiency of �25% for solar-to-H2 conversion [68]. Thecorresponding electrochemical impedance spectra (�70 mV

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Figure 4 Electrochemical capacitance measurements for the calculation of surface area of high loading NiMoZn alloy catalysts.(a) Representative CV sweeps measured at different scan rates of the NiMoZn0 sample in the potential region that no faradicprocesses were observed for the determination of capacitive current. (b) and (c) The capacitive current measured at 0.15 V (vs.RHE) as a function of scan rate for Zn series and Ni series electrocatalysts, respectively. The derived slopes (k) and normalizedsurface area (Anor) to NiMoZn0 were listed.

7Enhanced hydrogen evolution reaction performance

vs. RHE) was shown in the inset of Figure 5(f). The diameter ofthe semicircles remained unchanged after 1000 cycles, whichreconfirmed the durability of the NiMoZn2 alloy NPs (seeFigure S9 in Supporting information).

We have further performed the EIS analysis at �95 mV (vs.RHE) to investigate the composition effect of the high loadingalloy NPs on the kinetics of HER, as shown in Figure 6. In general,the performance of HER electrocatalysts was correlated to theoverall polarization resistance that can be estimated by thediameters of the semicircles. For the Zn series electrocatalysts, aminimized polarization resistance was observed at low [Zn] (1–3 at%) and the overall polarization resistance increased drasti-cally at higher Zn concentrations (Figure 6(a) and inset). For theNi series alloy NPs, the increment of [Ni] within the alloy slightlyreduced the overall polarization resistance.

All impedance spectra were characterized by a two-semicircle model, suggesting that the HER process wasconsisted of two elemental steps [36]. In order to extractthe composition effect on the two elemental steps, theimpedance responses were fitted using the followingequivalent circuit [9,25,36,69,70]:

LRΩðRHCPEHÞ ðRLCPELÞ ð4Þwhere L and RΩ correspond to the inductance and ohmicresistance that originated from the external circuit, respec-tively. CPEH, CPEL and RH, RL denote the constant phaseelements and electrode polarization resistances at high andlow frequencies, respectively. All fitted results were pre-sented in Table S2 in the Supporting information.

The influence of chemical composition on the HER kineticswas studied using the derived time constants, τH and τL, fromthe corresponding capacitance and resistance at high- andlow- frequency semicircles, respectively, as presented inFigure 6(c) and (d). For the Zn series NPs (Figure 6(c)), itwas clear that the elemental step, which corresponded tothe high-frequency semicircle, were characterized by

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.1016

relative small time constants (τH) compared to that of thelow-frequency elemental step (τL). At lower Zn concentra-tions (o3 at%), τH decreased along with the increasing of Znconcentration, which means an improvement of the HERkinetics. However, the value of τL stayed unchanged at lowerZn concentrations. Since the high-frequency and low-frequency semicircles have been postulated to be corre-sponded to the charge transfer process and the protonadsorption process [9,36,70], it suggested that the presenceof small amount of Zn in the alloy NPs improved the HERkinetics by promotion of the charge transfer process, which isconsistent with the results derived from Tafel slopes. How-ever, by further increasing of the Zn concentration, both τHand τL increased significantly, which means a inhibition ofboth charge transfer process and proton absorption processat higher Zn concentration. Similarly, the influence of Ni onthe HER kinetics was demonstrated in Figure 6(d). Whilst τLremained to be constant, τH decreased slightly following theincreasing of [Ni] within the alloy NPs, indicating the increaseof Ni concentration in the electrocatalysts improved the HERkinetics by promoting the charge transfer process.

Conclusions

We have synthesized NiMoZn ternary alloy NPs as electrocata-lysts for HER by an electrochemical deposition method. Thechemical composition of the alloy NPs have been preciselycontrolled via fine tuning the electrolyte composition. Whileadjusting the [Ni] in the alloy showed a negligible effect on theoxidation states of other components, the addition of Zn withinthe alloy promoted the conversion of metallic Ni and Mo to highvalence states significantly. The AFM imaging suggested thathomogeneous alloy NPs have been deposited on the substrateuniformly, and the addition of Zn accelerated the depositionand aggregation of the NPs. We have further explored the

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Figure 5 (a) and (b) Representative CV curves (iR-dropcorrected) of high loading NiMoZn alloy NPs with different Znand Ni concentrations, respectively. The current densities werenormalized to the geometric surface area of each sample.(c) and (d) The influence of [Zn] and [Ni] on the current densityat �50 mV vs. RHE. All current densities were normalized toAnor in Figure 4. The dots represent experimental data and thelines (solid and dashed) are guidelines. The performance of apure Ni NPs (black dot in (c)) was presented for comparison.(e) TOF of the NiMoZn0 and NiMoZn2 AFM sample derived fromthe corresponding cathodic-going CV sweeps. (f) Acceleratedstability test of the NiMoZn2 NPs showing the iR-drop correctedcathodic sweeps for the initial cycle (1 cycle), and after 100and 1000 cycles. The inset depicts the corresponding Nyquistplots at �70 mV vs. RHE. The ohmic resistances of all spectraare similar and have been subtracted for comparison.

Figure 6 (a)–(b) Comparison of the Nyquist plots for high loadingNiMoZn alloy catalysts with different Zn and Ni concentration atan overpotential of 95 mV (vs. RHE). Inset plots were theimpedance spectra of samples with high concentrations of Zn.The colored dots and the corresponding solid lines were experi-mental data and fitted results, respectively. The inductance (L)and ohmic resistance (RΩ) of the impedance spectra have beensubtracted for better comparison purpose. (c) and (d) Timeconstants of the high frequency response (τH) and low frequencyresponse (τL) as a function of Zn and Ni concentration within theNiMoZn alloy NPs. Solid lines were guidelines.

X. Wang et al.8

influence of chemical composition on the HER performance ofthe NiMoZn NPs and revealed that a precise control of Zn in thealloy was essential in achieving the optimum TOF for HER.Specifically, the alloy NPs with a [Zn] of 1–3 at% presented a 10-and 1.5- fold enhanced current density compared to that ofpure Ni and NiMo alloy NPs, respectively. Tafel slopes andimpedance spectra analysis indicated that the charge transferprocess was the rate determining step for NiMoZn alloyelectrocatalysts with low Zn concentration. The presence ofZn promoted the charge transfer process at low concentrations,but inhibited the surface adsorption of proton at high concen-trations. Nevertheless, the optimized NiMoZn alloy electroca-talyst exhibited excellent stability in acid condition, whichappoints this noble-metal-free alloy NPs a promising candidatefor large-scale water splitting applications.

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.101

Acknowledgment

We would like to thank Dr. Gilbère Mannie at iNANO forfruitful discussions on the XPS fitting methods. Financialsupports by the Center for Materials Crystallography (DanishNational Research Foundation, DNRF93), the iNANO Centerthrough the Danish Strategic Research Council, the ChineseScholarship Council (CSC), Haldor Topsøe A/S, and theCarlsberg Foundation are gratefully acknowledged.

Appendix A. Supporting information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2014.12.007.

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Xueqin Wang received her M.Sc degree inChemistry from the Northeast Petroleum,China. In 2010, she started her Ph.D. inProf. Yudong Huang's group at the HarbinInstitute of Technology, China. In 2012, shejoined the group of Flemming Besenbacherat the Aarhus University as a joint Ph.D.student sponsored by China ScholarshipCouncil (CSC) for two years. Her researchinterests include synthesis of nanomaterials

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for water splitting hydrogen evolution and TiO2-based photocata-lysts for renewable energy and water purification.

Ren Su received his Ph.D. degree at theInterdisciplinary Nanoscience Centre (iNANO),Aarhus University, Denmark in 2012 and con-tinued as a postdoctoral researcher under thesupervision of Prof. Flemming Besenbacher.He is currently working at Syngaschem BV andSynfuelsChina as a research scientist. Hisresearch interests focus on application andfundamental understanding of novel photo-electrocatalyst materials for sustainable

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hydrogen production, CO2 reduction, and organic synthesis.

Hüsnü Aslan obtained his M.Sc in AppliedPhysics from the Sabanci University, Turkey.In 2013, he started his Ph.D. in Bio-SPMgroup at iNANO, Aarhus University, Den-mark. Currently, he focuses on the devel-opment of scanning probe microscopytechniques and its relative applications formonitoring biomolecular activity and study-ing functional materials.

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Jakob Kibsgaard is currently an AssociatedStaff Scientist at the SUNCAT Center forInterface Science and Catalysis, SLACNational Accelerator Laboratory, USA. Hereceived his Ph.D. in Physics and Nanosciencefrom Aarhus University, Denmark. Followinghis Ph.D. he was a visiting scholar and later anEngineering Research Associate at Depart-ment of Chemical Engineering, Stanford Uni-versity, USA. His current research interest

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includes understanding and engineering surface and bulk propertiesof materials to improve the efficiency of chemical transformations inrenewable energy and heterogeneous catalysis.

Stefan Wendt studied Physics before he didhis Ph.D. research at the Fritz Haber Insti-tute of the Max Planck Society in Berlin. Heobtained his Ph.D. degree at the FreieUniversity in 2002. He then conducted post-doctoral work for two years at CollegeStation, Texas, in the group of Wayne Good-man. In 2004 he moved to Aarhus, Denmark,where he joined the group of FlemmingBesenbacher. His research is primarily

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focused on the surface science of metal oxides using scanningtunneling microscopy, temperature-programmed desorption andphotoelectron emission spectroscopy. In 2011, Stefan Wendtbecame Senior Researcher at the Interdisciplinary NanoscienceCenter (iNANO).

Please cite this article as: X. Wang, et al., Tweaking the compositionreaction performance, Nano Energy (2014), http://dx.doi.org/10.101

Linghui Meng received his Ph.D. degree inMaterials Science in 2003 from the HarbinInstitute of Technology (HIT), China. From1997–1999, he worked at the Chiba Instituteof Technology as a visiting scholar. From1999–2004, he was an Associate Professor atthe Department of Applied Chemistry ofHIT. Now he is a Full Professor and theAssociate Dean of the School of ChemicalEngineering and Technology at HIT. His

current research interests cover the surface and the interface ofpolymer composites and nanomaterials for renewable energy.

Mingdong Dong obtained his Ph.D. degree inApplied Physics in 2006 from the AarhusUniversity (AU), Denmark. From 2006–2009,he worked at the Rowland Institute at Har-vard of Harvard University, USA as a post-doctoral fellow. Since 2009, he was anAssistant Professor, and later an AssociateProfessor at the Interdisciplinary NanoscienceCenter of AU. His research focuses on boththe implementation and further development

of a novel scanning probe microscope technique to study molecularself-assembly for biological applications and new functional materials.

Yudong Huang received his Ph.D. degree inMaterials Science in 1993 from the HarbinInstitute of Technology (HIT), China. Since1993 he worked at the Department ofApplied Chemistry of HIT as a Lecturer,Associate Professor and then a Full profes-sor. From 2001–2002, he visited Akita Pre-fectural University as a Guest Professor.Since 2008 he is the Full Professor and theDean of the School of Chemical Engineering

and Technology. His current research interests cover the surfaceand interface of polymer matrix composites and functional poly-mers, the synthesis and applications of the photocatalytic andphotoelectrocatalytic nanomaterials.

Flemming Besenbacher received his doc-toral degree in Natural Sciences from theAarhus University (AU), Denmark. Since1996 he is a full-time professor at theDepartment of Physics of AU. From 2002–2012, he was the Director of the Interdisci-plinary Nanoscience Center (iNANO) at AU.His current research interests covernanoscience, nanotechnology, nanocataly-sis, scanning tunneling microscopy, atomic

force microscopy, nucleation and growth of nanoclusters, interac-tion of hydrogen with defects in metals, hydrogen storage, andphotocatalyitc nanomaterials and biosensors.

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