Composition-performance relationship of NixCuy nanoalloys as hydrogen evolution electrocatalyst

Accepted Manuscript Composition-performance relationship of NixCuy nanoalloys as hydrogen evolution electrocatalyst

Xiao-Di He, Fei Xu, Fang Li, Lu Liu, Yan Wang, Ning Deng, YanWu Zhu, Jian-Bo He PII: DOI: Reference:

S1572-6657(17)30411-3 doi: 10.1016/j.jelechem.2017.05.050 JEAC 3326

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

21 April 2017 26 May 2017 28 May 2017

Please cite this article as: Xiao-Di He, Fei Xu, Fang Li, Lu Liu, Yan Wang, Ning Deng, Yan-Wu Zhu, Jian-Bo He , Composition-performance relationship of NixCuy nanoalloys as hydrogen evolution electrocatalyst, Journal of Electroanalytical Chemistry (2017), doi: 10.1016/j.jelechem.2017.05.050

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ACCEPTED MANUSCRIPT

Composition-performance relationship of NixCuy nanoalloys as hydrogen evolution electrocatalyst Xiao-Di He, Fei Xu, Fang Li, Lu Liu, Yan Wang, Ning Deng, Yan-Wu Zhu, Jian-Bo He*

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Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei

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230009, China

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E–mail address: [email protected] (J.-B. He).

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* Corresponding author. Tel.: +86-551-62904653; fax: +86-551-62901450.

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ACCEPTED MANUSCRIPT ABSTRACT The nickel-copper nanoalloys with tunable composition and morphology were prepared by galvanostatic deposition on copper substrate, in order to investigate the relationship between alloy composition and electrocatalytic activity. Both the composition and morphology of the

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NixCuy nanoalloys are highly dependent on the applied current density. The atomic ratio of Ni to Cu in the alloys changes from 1:9 to 3:1, with the increase of current density from 10 to

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100 mA cm–2. The difference in electrocatalytic activity among these nanoalloys was

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evaluated through the hydrogen evolution reaction (HER) in 1.0 M H2SO4 and 1.0 M KOH.

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The composition-dependence of the electrocatalytic activity of the alloys is more pronounced in 1.0 M H2SO4 than in 1.0 M KOH. By tuning the composition of NixCuy alloys, 13.5 and

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5.7 times increase in exchange current density of the HER was achieved in 1.0 M H2SO4 and 1.0 M KOH, respectively. Meanwhile, 4.5 and 2.0 times decrease in charge transfer resistance

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was observed in the same two media. The best electrocatalytic activity to the HER was

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always achieved on the nanoalloy with a 1:1 atom ratio and a single crystal (111) plane. This favorable nanoalloy is composed of four-level dendritic nanochains. The results demonstrate

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that galvanostatic method can tune not only the composition but also the morphology of nanoalloys, both being important for nanoscale design of industrial electrocatalysts.

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Keywords: NiCu nanoalloy; Dendritic nanochain; Electrocatalysis; Composition tuning; Hydrogen evolution reaction

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ACCEPTED MANUSCRIPT 1. Introduction Production of molecular hydrogen from water electrolysis is an important component of several developing clean-energy technologies because of cleanliness, high efficiency and renewability of hydrogen [1,2]. The main obstacle with hydrogen evolution reaction (HER)

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from water electrolysis is the high energy consumption. Pt-based metals are currently the best catalysts for electrochemical water splitting. Much effort has been devoted to prepare

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inexpensive and highly catalytic materials instead of noble metals for lowering the

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overpotential of the HER [2-4]. As a non-noble metal, nickel has been considered as one of

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the most important materials [5] because of its low cost, good corrosion resistance, and the promising catalytic activity to the HER when combined with other metals [6], nonmetals [7]

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or composites [8].

The efficiency of an electrocatalyst can be improved by enlarging the real surface area or

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increasing the intrinsic catalytic activity by synergistic combination of multi-components

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[9,10]. The approaches to enlarge surface area include the utilization of nanoparticles [11], metal foams [12], porous metal layers [5,13], and catalyst supports (such as carbon-felt [6,14])

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with high specific surface areas, etc. The intrinsic activity of a metal catalyst can be enhanced

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by forming binary and ternary alloys [1,15-17]. For example, the NiCu alloys have intermediate free energies of hydrogen adsorption which favor catalytic activity to the HER [10]. Actually the nickel-based alloys have been widely studied since past decades, such as NiCo [6,13,18-20], NiMo [11,21,22], NiFe [21,23], NiW [21,24], NiZn [13,25], NiBi [26], NiCu [27-31], NiMoCu [9], and CuNiZn [32]. Electrochemical deposition is one of the most convenient and efficient methods for preparing alloy coatings on a substrate under ambient conditions. The alloy coating properties are strongly influenced by deposition techniques and conditions [26]. As for the NiCu alloys, 3

ACCEPTED MANUSCRIPT trisodium citrate (Na3C6H5O7) is generally needed as a complexing agent [12,27-32] to prevent the metal ions from acting as autocatalysts [30]. The deposited NiCu alloys showed various surface morphologies, such as grains or nodules [28-31], foams [12], and compact and smooth coatings [27], depending on the deposition conditions. The atomic ratio of the

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alloy deposits can also be tuned by changing electrolyte composition, applied potential or current density. The NiZn [25] and NiBi [26] alloy coatings with various compositions were

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galvanostatically prepared by changing the concentration ratio of the two metal ions in the

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plating bath. It is possible to have a wide range of compositions for NiCu alloys because the

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pure Ni and Cu have the same face-centered cubic structure with similar lattice parameters (a = 3.523 for Ni and 3.616 for Cu) [27]. Carbon supported Ni1−xCux catalysts with different

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Ni:Cu atomic ratios have been prepared by the incipient wetness impregnation followed by freeze-drying, and it was found that addition of Cu at low fractions significantly improves the

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electrocatalytic activity of Ni in the hydrogen oxidation reaction [33].

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In the present work, the NixCuy nanoalloys with different Ni:Cu atomic ratios (x:y) were galvanostatically prepared by applying different current densities, without the assistance of

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Na3C6H5O7 and any template. The NixCuy nanoalloys showed a series of morphologies

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corresponding to their compositions, including a nanopearl chain dendritic structure in 1:1 atomic ratio. The results demonstrate that galvanostatic method can tune not only the composition but also the morphology of nanoalloys, both being important for nanoscale design of industrial electrocatalysts.

2. Experimental 2.1. Chemicals and apparatus All chemicals used in this work, NiCl2·6H2O, CuSO4·5H2O, H3BO3, H2SO4 and KOH, 4

ACCEPTED MANUSCRIPT are of analytical grade from Sinopharm group. Polycrystalline copper disc (99.9% purity) was used as the substrate for depositing the NixCuy alloys. The geometric area of the copper disc (0.049 cm2) was used to calculate current density, areal capacitance and areal resistance. Doubly-distilled water was used as solvent. High-purity nitrogen was used for deoxygenating

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the electrolyte solutions. Electrochemical experiments, including sample preparation and characterization, were

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performed on a CHI660C workstation (CH Instruments Co., Shanghai, China). The surface

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morphology and composition of the NixCuy alloys were characterized on a SU8020 field-emission scanning electron microscope (FE-SEM, Hitachi, Japan) equipped with an

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energy dispersive X-ray (EDX) analyzer. Transmission electron microscope (TEM)

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micrographs of the samples were recorded on a JEM-2100F field emission transmission electron microscope (JEOL, Japan) with a beam energy of 200 kV.

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2.2. Alloys preparation

The copper substrate was successively polished on the 800 and 2000 grit emery papers, washed in doubly-distilled water and then electrochemically cleaned in 1.0 M H2SO4 at –0.5

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V (vs. RHE, reversible hydrogen electrode) for 120 s. The electrolyte solution for NixCuy

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electrodeposition contained 0.050 M CuSO4, 0.50 M NiCl2 and 0.50 M H3BO3 with a pH of 4.3. The temperature of the solution was kept at 22C without stirring during electrodeposition. The high molar ratio of Ni2+ to Cu2+ (10:1) in the electrolyte was used for compensating the thermodynamically less favorable Ni2+ reduction than that of Cu2+. The H3BO3, which has been used as a buffer agent for the electrodeposition of pure Ni to counteract the generation of hydroxides on the deposit surfaces [34,35], was employed herein instead of Na3C6H5O3 (a complexing agent employed for the electrodeposition of NiCu alloys [12,27-32]). The NixCuy alloys were galvanostatically deposited from on a copper substrate, 5

ACCEPTED MANUSCRIPT with a saturated calomel reference electrode and a platinum wire auxiliary electrode. The composition of the alloys was adjusted by applying different current density for a certain time. The current density values ranged from 10 to 200 mA cm–2. The product of current density and time was set constant at 3.0 C cm–2, in order to deposit the approximately same number of metal atoms (Cu + Ni, ca. 15.5 mol cm–2) in each preparation. For comparison, the pure

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Ni deposit was prepared at a current density of 200 mA cm–2 for 15 s in the precursor solution

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without addition of CuSO4.

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2.3. Electrochemical characterizations

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Potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and chronopotentiometry were used to evaluate the electrocatalytic activity of the nanoalloy

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catalysts to the HER in 1.0 M H2SO4 and 1.0 M KOH. The reference and counter electrodes were the same as in alloy preparation. The potentials in this paper are all reported versus RHE

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in the same electrolyte (1.0 M H2SO4 or 1.0 M KOH) with automatic IR drop compensation. Before each measurement, the electrolyte solution was deaerated for 15 min using high purity N2, and the working electrode was electrochemically cleaned at –0.5 V (vs. RHE) for 120 s to

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reduce the oxide film on the alloy surface. Potentiodynamic polarization experiments were

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conducted at a scan rate of 2.0 mV s–1. EIS measurements were carried out in the frequency range from 0.1 Hz to 100 kHz with AC voltage amplitude of 5 mV. The data were collected with 12 points per decade of frequencies at equal intervals on a logarithmic frequency scale. The measured data were fitted to an appropriate equivalent circuit model using the complex nonlinear least-squares method (CNLS) [36]. All electrochemical characterizations were performed at room temperature (ca. 22 C).

3. Results and discussion

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ACCEPTED MANUSCRIPT 3.1. Morphology and composition Fig. 1 shows the typical FESEM micrographs of the NixCuy samples deposited on the Cu substrates. It is found that the morphology of the samples highly depends on the applied current density At a low current density of 10 mA cm–2, the samples showed a foliated

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structure that was constituted of a central trunk with wide branches on both sides (Fig. 1A). With the increase of the current density, the branches became thinner, denser and more

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symmetrical, and the shapes of both the trunks and branches changed from flaky to catenulate

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(Fig. 1B and 1C). Fig. 1C shows a two-dimensional dendritic structure with multi-level

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branches prepared at 50 mA cm–2. All the central trunks and branches were consisted of pearl-like nanoballs with a diameter of ca. 50–80 nm, which arranged in a line one by one to

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form straight nanoalloy chains. A larger scope of observation at a lower magnification (Fig. 1D) revealed that the dendritic structure included four-level branches, and there were some

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other nanoballs dispersing on the substrate surface. When the current density was increased to

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100 mA cm–2, the samples showed a three-dimensional corn cob-like structure that was still composed of the nanoballs (Fig. 1E). Further, a random stacking of these nanoballs was

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observed from the samples prepared at 200 mA cm–2 (Fig. 1F). In comparison, the application

structure.

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of a current density of 50 mA cm–2 produced the most organized nanopearl chain dendritic

The EDX analysis was used to determine the chemical composition of various NixCuy nanoalloys identified by FESEM. As an example, Fig. 2 shows the EDX spectrum of the sample with the dendritic nanoalloy chains. In all the spectra, Al signals came from the sample tray in EDX analysis and Cl signals from the electrolyte solution in sample preparation. The results indicate that the composition of the nanoalloys also highly depends on the applied current density. As expected, the content of Ni in the alloys was increased

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ACCEPTED MANUSCRIPT significantly with the increase of current density. The atom ratio of Ni to Cu was determined to be 1:9, 1:4, 1:1, 3:1 and 3:1, for the samples prepared at 10, 25, 50, 100 and 200 mA cm–2, respectively. When the current density was set at 100 mA cm–2 or higher, the content of Ni in the alloys is no longer increased with the applied current density. This observation is probably

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because both metal ions were deposited at their diffusion-limited rates when such high current densities were applied.

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The intrinsic structures of these dendritic Ni1Cu1 nanoalloy chains were further depicted

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by HRTEM technique. Fig. 3A displays the representative HRTEM micrograph of the Ni1Cu1 nanostructures, which reveals the dendritic shapes in a panoramic manner. Apparently, it is

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composed of a long central trunk with secondary and tertiary branches. Fig. 3B presents the

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magnified HRTEM micrograph that was randomly taken from the Ni1Cu1 dendrites. The ordered lattice fringes clearly indicate a single crystal nature with excellent crystallinity. The

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distance between the lattice fringes is 0.21 nm (the inset of Fig. 3B), which agrees with

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interplanar spacing of the (111) plane of either Cu (0.21 nm) or Ni (0.20 nm) (JCPDS 65-9743; 01-1260) with a face-centered cubic structure.

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3.2. Potentiodynamic curves

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The potentiodynamic polarization curves of the HER were recorded in 1.0 M H2SO4 and 1.0 M KOH, respectively, using Cu, Ni/Cu and NixCuy/Cu electrodes (Fig. 4). All the NixCuy alloys present a significant depolarization to HER compared with the pure metals Cu and Ni. Whether in acidic or alkaline solution, the Ni1Cu1 alloy shows the lowest overpotentials, the decreases of which at 10 mA cm–2 (10) reached 256, 101, 231 and 117 mV relative to Cu and Ni/Cu in 1.0 M H2SO4 and Cu and Ni/Cu in 1.0 M KOH, respectively. The potentiodynamic curves in semilogarithmic coordinates are shown in the insets of Fig. 4. The slopes of the linear segments (Tafel slope) were found to be around 120 mV dec−1, which is generally 8

ACCEPTED MANUSCRIPT considered as an indication of Volmer reaction step being the rate determining step for HER [1,17]. The kinetic parameters can be derived from Tafel equation written as Eq. (1): ERHE  E  a  b log(| i | )  

2.3RT 2.3RT log i0  log(| i |) F F

(1)

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where ERHE is the potential of reversible hydrogen electrode (–0.052 and –0.820 V vs. SHE in 1.0 M H2SO4 and 1.0 M KOH, respectively, H2 pressure 101.3 kPa, 295 K), a and b are the

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Tafel intercept and slope, i0 is the exchange current density, α is the cathodic charge transfer

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coefficient, and R, T, F have their usual meanings. The resulting kinetic parameters are

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summarized in Table 1. The exchange current density i0 increases firstly and then decreases with increasing Ni content in the alloy materials, whether in acidic or alkaline solution. The

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largest i0 was obtained with the Ni1Cu1 alloy, which indicates the most reversible electron transfer kinetics across the Ni1Cu1|solution interface. By tuning the composition of NixCuy

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alloys, 13.5 times increase in i0 was achieved in 1.0 M H2SO4, but only 5.7 times increase

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was obtained in 1.0 M KOH. Therefore, the dependence of i0 on the alloy composition is more pronounced in 1.0 M H2SO4 than in 1.0 M KOH.

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The i0 values of the Ni1Cu1 alloy in 1.0 M H2SO4 and 1.0 M KOH were 88.3 and 96.2 A cm–2, respectively. These results are higher than the reported values, 1.70 A cm–2 of

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Ni51Cu49 alloy in 6.0 M KOH [30] and 52 A cm2 of Ni55Cu45 alloy in 1 M KOH [31], although the reported values were possibly over-estimated due to the use of 5–25 times higher scan rate than in the present work. Large i0 values in this work might be due to large electrochemically active true surface area due to the dendritic structure of Ni1Cu1 alloy. 3.3. Electrochemical impedance spectroscopy The charge transfer kinetics of HER on various NixCuy surfaces were comparatively

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ACCEPTED MANUSCRIPT investigated in 1.0 M H2SO4 and 1.0 M KOH by EIS measurements. Each of the Nyquist plots in Fig. 5 presented a broadened (distorted) semicircle. The time constants can be estimated from the frequencies (shown in Fig. 5) corresponding to the top of the semicircles. All NixCuy electrodes in two electrolytes yielded a time constant in the order of milliseconds.

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An equivalent circuit (inset of Fig. 5A) is selected for data fitting, where Rs is the solution resistance, Rct is the charge transfer resistance, CPE is a constant phase element in place of

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the double layer capacitance (Cdl), L and RL are the low-frequency inductance and resistance,

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respectively. The CPE was used to account for the non-ideal behavior of the capacitive element due to the inhomogeneity of the electrode surfaces. The elements L and RL are

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considered only for the Nyquist plots with a small loop at the low frequency ends (Fig. 5B for

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various NixCuy alloys). This inductive loop reflects a pseudo-inductive behavior resulting from the absorption of hydrogen and formation of surface hydride, which has been reported

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for the HER on Ni-based materials [37], phytic acid-coated titanium [38] and a nitrogen- and

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sulfur-rich conductive polymer [39].

Fitting the above models results in a good agreement between the experimental and

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simulated data (the solid lines in Fig. 5). The parameters obtained for the equivalent circuit elements are listed in Table 2. Whether in acidic or alkaline solution, the resistance Rct

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decreases firstly and then increases with increasing Ni content in the alloys. All the alloy materials show Rct values much smaller than those on the pure metals Cu and Ni. A minimal Rct was achieved with the Ni1Cu1 alloy, which indicates the fastest electron transfer kinetics across the Ni1Cu1|solution interface. The dependence of Rct on the alloy composition is more pronounced in 1.0 M H2SO4 than in 1.0 M KOH. By tuning the composition of NixCuy alloys, 4.5 times decrease in Rct was achieved in 1.0 M H2SO4, but only 2 times decrease was obtained in 1.0 M KOH. On the other hand, the low frequency inductive loop occurred only

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ACCEPTED MANUSCRIPT on the interfaces between the alloys and 1.0 M KOH solution, which suggests the difficulty in stripping of the surface hydride from the alloy surfaces. Therefore, the desorption resistance of the surface hydride affects the reaction rate of the HER mainly in alkaline media. Table 2 also shows the parameters Y0, n and Cdl, which are the CPE parameter, CPE

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exponent and the double layer capacitance, respectively. The values of Cdl were calculated

Y0  Cdln ( Rs1  Rct1 )1n

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according to the Brug’s equation [40], Eq. (2),

(2)

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The resulting capacitance Cdl in both solutions increases firstly and then decreases with

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increasing Ni content in the alloys. The maximal Cdl was observed with the Ni1Cu4 alloy, instead of Ni1Cu1. The magnitude of Cdl depends on several factors, such as electrode

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material, interface adsorption, applied potential and real interface area. The highest catalytic activity of Ni1Cu1 to HER can be attributed mostly to the intrinsic property of the materials

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and less to the increase in real surface area.

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Romanowski et al. [41] reported the results of the density functional calculations of the H2 dissociation on the NixCu1–x (x = 0, 0.1875, 0.3750, 0.6250, 0.8125, 1) and AgxPd1–x alloys.

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The authors have found that the Ni2 dimers are the active catalytic centers in NixCu1–x and the Cu component plays the role of modifying the distance between the Ni atoms and the

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modified centers fit better to the hydrogen molecule. The results of the calculations suggest that the alloy compositions with roughly equal amounts of nickel and copper, x = 0.375 and x = 0.625 make the best catalysts for the hydrogen dissociation reaction. Accordingly, the reason behind the highest activity of Ni1Cu1 electrocatalyst in HER can be considered that the Ni2 catalytic centers in Ni1Cu1 fit best to the hydrogen molecule. 3.4. Chronopotentiometry

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ACCEPTED MANUSCRIPT The stability of catalysts is extremely important for their practical applications. For example, the CoMo nanoparticles are initially very active in 1.0 M H2SO4, but they are not stable, and quickly dissolved, as do their NiMo counterparts [4]. The short term stability of Cu, Ni/Cu and Ni1Cu1/Cu electrodes was evaluated by recording chronopotentiometric

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transients of the HER at 50 mA cm–2 for 8 h. As expected, the Ni1Cu1/Cu showed a significant depolarization effect on the HER whether in 1.0 M H2SO4 (Fig. 6A) or 1.0 M

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KOH (Fig. 6B), compared with Cu and Ni/Cu. Within the initial 10-30 min period, the three

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electrodes showed a significantly increasing depolarization effect on the HER in 1.0 M H2SO4, but the inverse effect was observed in 1.0 M KOH. The potential plateau of

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plateau became stationary after about 6 h.

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Ni1Cu1/Cu in 1.0 M H2SO4 was very stationary with time, while in 1.0 M KOH, the potential

4. Conclusions

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The use of current density to affect the morphology, composition and catalytic activity of NixCuy nanoalloys is presented. The atomic ratio of Ni to Cu in the alloys changes from 1:9 to 3:1, with the increase of current density from 10 to 100 mA cm–2 or higher. A series of

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morphologies were observed corresponding to the atomic ratios of the alloys. The

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electrocatalytic activity of the NixCuy alloys to the HER decreases firstly and then increases with increasing Ni content. This dependence is more pronounced in 1.0 M H2SO4 than in 1.0 M KOH. By tuning the composition of NixCuy alloys, 13.5 and 5.7 times increase in exchange current density of the HER was achieved in 1.0 M H2SO4 and 1.0 M KOH, respectively. Meanwhile, 4.5 and 2.0 times decrease in charge transfer resistance was observed in the same two media. The best electrocatalytic activity to the HER was always achieved on the nanoalloy with a 1:1 atom ratio and a single crystal (111) plane. This favorable nanoalloy is composed of four-level dendritic nanochains. The highest catalytic 12

ACCEPTED MANUSCRIPT activity of Ni1Cu1 to the HER can be attributed mostly to the intrinsic property of the material and less to the increase in real surface area.

Acknowledgments The authors gratefully acknowledge the financial support from the National Natural

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Science Foundation of China (No. 21576063).

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oxidation of glucose on Ni and NiCu alloy modified glassy carbon electrode, J. Solid State Electrochem. 13 (2009) 1171-1179.

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K. Ngamlerdpokin, N. Tantavichet, Electrodeposition of nickel-copper alloys to use as a cathode for hydrogen evolution in an alkaline media, Int. J. Hydrogen Energy 39

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R. Solmaz, A. Döner, G. Kardaş, Preparation, characterization and application of alkaline leached CuNiZn ternary coatings for long-term electrolysis in alkaline solution, Int. J. Hydrogen Energy 35 (2010) 10045-10049. O.V. Cherstiouk, P.A. Simonov, A.G. Oshchepkov, V.I. Zaikovskii, T.Y. Kardash, A.

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and liquids, J. Electrochem. Soc. 124 (1977) 1022-1030. I. Herraiz-Cardona, E. Ortega, J.G. Antón, V. Pérez-Herranz, Assessment of the

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S. Romanowski, W.M. Bartczak, R. Wesołkowski, Density functional calculations of the hydrogen adsorption on transition metals and their alloys. An application to

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catalysis, Langmuir 15 (1999) 5773-5780.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1. FESEM micrographs of the NixCuy alloys prepared under the deposition conditions: (A) i = 10 mA cm–2, t = 300 s; (B) i = 25 mA cm–2, t = 120 s; (C, D) i = 50 mA cm–2, t = 60 s; (E) i = 100 mA cm–2, t = 30 s; (F) i = 200 mA cm–2, t = 15 s.

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Fig. 2. EDX pattern of the dendritic nanoalloy shown in Fig. 1C.

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Fig. 3. (A) TEM and (B) HRTEM micrographs of the dendritic Ni1Cu1 nanoalloy chains. The

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plane of either Cu or Ni with a face-centered cubic structure.

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Fig. 4. Potentiodynamic curves of the HER at 2.0 mV s–1 in 1.0 M H2SO4 (A) and 1.0 M KOH (B). The insets are the corresponding semilogarithmic plots. Working electrode: (a) Cu,

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(b) Ni1Cu9, (c) Ni1Cu4, (d) Ni1Cu1, (e) Ni3Cu1, (f) Ni. Fig. 5. Experimental (symbols) and fitted (solid lines) data for the HER in 1.0 M H2SO4 at a bias potential of –0.4 V vs. RHE (A) and 1.0 M KOH at –0.3 V vs. RHE (B). Working

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electrode: (a) Cu, (b) Ni1Cu9, (c) Ni1Cu4, (d) Ni1Cu1, (e) Ni3Cu1, (f) Ni; frequency range: 100

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kHz – 0.1 Hz; amplitude: 5 mV. The numerals shown in both panels are the frequencies corresponding to the top of the semicircles. The equivalent circuit is shown in the inset of panel A.

Fig. 6. Galvanostatic curves of the HER at 50 mA cm–2 on Cu (a), Ni/Cu (b) and Ni1Cu1/Cu (c) in 1.0 M H2SO4 (A) and in 1.0 M KOH (B).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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ACCEPTED MANUSCRIPT Table 1 Kinetic parameters derived from the polarization plots shown in Fig. 4. Electrode

b/mV dec–1

α

i0/A cm–2

10/mV

1.0 M H2SO4

Cu

97

0.60

0.045

518

Ni1Cu9

109

0.53

6.55

360

Ni1Cu4

122

0.48

34.8

297

Ni1Cu1

130

0.45

Ni3Cu1

120

0.49

Ni

135

0.43

Cu

137

Ni1Cu9

112

Ni1Cu4

118

Ni1Cu1

116

23.3

309

19.8

363

0.43

4.5

460

0.52

17.0

307

0.50

76.2

252

0.50

96.2

229

109

0.53

37.9

270

117

0.50

10.8

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Ni3Cu1 Ni

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1.0 M KOH

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ACCEPTED MANUSCRIPT Table 2 Model parameters simulated from the experimental EIS data in Fig. 5. The equivalent circuit model is shown in the inset of Fig. 5A. The values in parentheses are the percentage errors in the fitting parameters. RL /Ω cm2

630

22.8 (45)

40.2 (33)

0.309 (2.2)

57.0 (0.8)

0.109 (3.6)

0.85 (0.5)

17.7

Ni1Cu9

0.310 (1.7)

5.85 (1.4)

1.86 (7.0)

0.81 (1.1)

Ni1Cu4

0.358 (1.7)

1.76 (1.9)

4.32 (13)

0.82 (2.2)

1002

Ni1Cu1

0.287 (1.5)

1.30 (1.5)

2.97 (12)

0.84 (1.8)

744

Ni3Cu1

0.298 (1.4)

4.18 (1.1)

1.43 (6.5)

0.84 (1.0)

322

Ni

0.286 (2.4)

7.72 (1.1)

0.247 (8.1)

0.85 (1.0)

45

Cu

0.637 (3.3)

90.5 (0.1)

0.625 (0.1)

0.89 (0.5)

237

5.58 (1.7)

1.18 (0.1)

0.92 (0.4)

0.697 (1.3)

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Ni1Cu4

0.700 (1.3)

3.97 (2.1)

2.51 (0.1)

0.89 (0.5)

1123

7.07 (47)

27.2 (24)

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1.0 M KOH

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Cu

1.0 M H2SO4

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n

Cdl /F cm–2

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L /H cm2

Rs Rct Y0 Electrolyte Electrode /Ω cm2 /Ω cm2 /mS sn cm–2

Ni1Cu1

0.678 (1.3)

2.86 (1.8)

1.62 (11)

0.89 (1.8)

680

15.7 (55)

25.3 (39)

Ni3Cu1

0.638 (1.1)

3.63 (1.7)

1.11 (9.0)

0.90 (1.4)

487

5.45 (41)

28.7 (20)

Ni

0.668 (1.0)

15.3 (0.8)

0.432 (4.3)

0.91 (0.6)

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Graphical abstract

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Highlights

Both composition and morphology of NiCu nanoalloys are tunable with current density.

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Electrocatalytic activity highly depends on composition and morphology.

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Dendritic nanopearl chains in 1:1 atomic ratio show best electrocatalytic activity.

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Difference in activity among alloys is more significant in acidic than basic media.

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