Mixture Design of NiCoMo Ternary Alloy Nanoparticles Assembled on

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C: Energy Conversion and Storage; Energy and Charge Transport

Mixture Design of NiCoMo Ternary Alloy Nanoparticles Assembled on the Graphene Nanosheets and Decorated with Ru Nanoparticles: A Pt/C Like Kinetics for Hydrogen Evolution Reaction Reza Karimi Shervedani, Mostafa Torabi, and Marzieh Samiei Foroushani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02837 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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The Journal of Physical Chemistry

Mixture Design of NiCoMo Ternary Alloy Nanoparticles Assembled on the Graphene Nanosheets and Decorated with Ru Nanoparticles: A Pt/C-like Kinetics for Hydrogen Evolution Reaction Reza Karimi Shervedani,* Mostafa Torabi, Marzieh Samiei Foroushani Department of Chemistry, University of Isfahan, Isfahan 81746-73441, I.R. IRAN

*

To whom correspondence should be addressed. Tel.: +98-31-37934922. Fax: +98-31-36689732. E-mail address: [email protected] (R. Karimi Shervedani).

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract The development of nanocomposites with high activity and stability to generate hydrogen as a green fuel is an interesting challenge for several industrial requirements. Here, ternary alloy nanoparticles (TANPs) of NiCoMo are fabricated electrochemically on the graphene nanosheets (GNs) as new nanocomposites using a glassy carbon electrode base, then, the prepared electrode systems are decorated with ruthenium nanoparticles. The electrodes are characterized by several surface analysis techniques. The electrocatalytic activity of the electrodes is monitored toward the hydrogen evolution reaction (HER) in alkaline solutions, and the fabrication conditions are optimized systematically based on advanced optimization methods; central composite design (CCD) and simplex-lattice mixture design (SLMD). Initially, the electrodeposition bath parameters are investigated and optimized by CCD. Then, SLMD analysis is utilized to obtain the optimal mole ratio of metal ions using HER kinetic data. The results revealed an optimized mole ratio of 33:27:40 for metal ions of Ni:Co:Mo in electrodeposition bath leading to GC-GNs-Ni0.20Co0.36Mo0.44 electrode structure. A Tafel slope of −45 mV dec−1, j0 of 1.26 mA cm−2, and η50 of −82 mV are obtained at the optimized structure, which are close to −42 mV dec−1, 5.93 mA cm−2, and −48 mV obtained under the same conditions on the GC electrode modified with commercial Pt/C (GC-Pt/C). The electrode is further decorated with Ru NPs through electrodeposition to form GC-GNs-Ni0.15Co0.31Mo0.38/Ru0.16. The results obtained on this electrode showed high physical and electrochemical stability and excellent kinetic performance for the HER, Tafel slope of −38 mV dec−1, j0 of 6.31 mA cm−2, and η50 of −49 mV, which are similar or even better than the results obtained on GC-Pt/C electrode.


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1. INTRODUCTION Hydrogen as a promising energy carrier must be produced in an efficient and sustainable manner due to increasing demand for green and renewable energy.1-4 Although, hydrogen can be generated from water, biomass, or coal, only hydrogen derived from water splitting can provide a secure and green energy system without environmental waste.5-7 Platinum is considered as the best-known catalyst for electrochemical hydrogen evolution reaction (HER), because of its negligible overpotential and excellent kinetics;8 but, the inadequacy and high price of Pt limit mostly its applications.9 Therefore, demanding efforts have been focused on the investigation of non-noble metal alternatives like metal oxides,10 alloys11,12 and composites.13,14 In effect, (i) nickel-based alloys15,16 and (ii) graphene based nickel composites13,14 are interesting non-noble metal materials for electrocatalytic HER in alkaline solutions. (i) Nickel-based alloys 17-21 have been fabricated and applied to decrease the overpotential of HER, among them NiCoMo ternary alloys are especially interesting for HER11,22,23 and other industrial purposes.24 (ii) Graphene; due to its remarkable physicochemical properties, like excellent conductivity, great number of edges in its structure, large surface area and fast heterogeneous electron transfer, is highly promising for different applications in science and energy technology.13,14,25,26 Nickel-based graphene nanocomposites have been recently prepared by us13 and others14 and studied quantitatively for electrocatalytic HER.13 The theoretical studies14 as well as experimental results13 have supported the important role of graphene in HER mechanism. The H-adsorbed atoms can move from intermediate Ni-H sites to graphene, recombine quickly and form H2, resulting in a clear Ni surface in alkaline solutions,14 thus, nickel/graphene nanocomposites modified electrodes have shown large kinetics for HER.13



These results encouraged us to assemble ternary alloy nanoparticles (TANPs) of NiCoMo onto the glassy carbon electrode modified by graphene nanosheets (GC-GNs) and investigate the resulted system, GC-GNs-NiCoMoTANPs, for HER here. While, several papers have dealt with preparation of the NiCoMo ternary alloys,22-24 up to our knowledge, there is a lack of information concerning fabrication of the composite made of NiCoMo nanoparticles on GNs. (iii) Previous studies related to the construction and development of multicomponent catalysts (such as ternary alloys) for HER20,21 have been typically based on traditional onefactor-at-a-time (OFAT) methodology, which is not an efficient method. The OFAT requires more experiments for obtaining the same precision, in effect; the method is time consuming and misses optimal settings of factors. Furthermore, OFAT cannot estimate interactions between effective parameters on the response of the system. These drawbacks can be overcome by employing a systematic optimization of the conditions based on statistical algorithms. Statistical design of experiments (DOE) is a powerful method to optimize chemical processes and execution of informative experiments.27,28 The DOE has various types like central composite design (CCD) families, which have been used to address the experimental factors such as time and concentration, affecting the product properties.29 The mixture designs (MD) is one of the main type of DOE methods, concerning preparation and modification of the mixtures. This method changes mixture composition and explores how such variations will affect the product properties.29 The MD method has been employed effectively to optimize the preparation conditions of the ternary catalysts for oxygen reduction and evolution reactions.30-33 Here, the technique will be employed to optimize the conditions systematically for construction of the NiCoMo ternary alloy on the GNs surface.


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Accordingly, the synthesized composite is applied as working electrode for the HER in the alkaline solution at a constant current density (e.g. j = 50 mA cm−2), while the overpotential needed to achieve this current density (η50) is monitored. Then, the conditions of the synthesis bath are changed and optimized to achieve a composite exhibiting maximum current density for the HER. The surface of the optimized composite electrode, GC-GNs-NiCoMoTANPs, is decorated with Ru electrodeposited nanoparticles,34 leading to GC-GNs-NiCoMoTANPs/Ru. This electrode is again examined for HER. The semi-empirical chemometric results in conjunction with surface analysis data and quantitative kinetics of HER are presented and discussed here. Excellent activity was achieved for HER on GC-GNs-NiCoMoTANPs and GC-GNs-NiCoMo/Ru electrode in comparison with that obtained on GC-Pt/C.

2. MATERIALS AND METHODS 2.1. Reagents and Chemicals Graphene oxide was prepared according to the literature.13,35 nickel(II) chloride hexahydrate (NiCl2.6H2O), ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24.4H2O), cobalt(II) chloride (CoCl2.6H2O), ruthenium(III) chloride hydrate (RuCl3.xH2O), tetrabasic sodium pyrophosphate (Na4P2O7), sodium bicarbonate (NaHCO3), formic acid (CH2O2) and other chemicals were purchased from Sigma-Aldrich and Merck. All solutions were prepared with distilled water.

2.2. Electrode Preparation The NiCoMoTANPs is loaded electrochemically onto the GC-GNs electrode surface. A two-electrode cell system, including the modified GC-GNs and a Pt disc (7.07 cm2) was used



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for the electrodeposition of NiCoMoTANPs. The cell mainly consisted of the following precursors: NiCl2.6H2O, (NH4)6Mo7O24.4H2O and CoCl2.6H2O. The Na4P2O7 (0.13 M) as a complexing agent and the NaHCO3 (0.89 M) as a buffering agent were added to all electrochemical cell solutions.18 Finally, 0.0185 M formic acid was added immediately prior to the deposition, to assist the co-deposition of the metals.18 While the η50, found from the steady-state polarization curves recorded for the HER cell, was monitored as a signal, the conditions of the electrodeposition cell (bath) were studied. In effect, the mole ratio of Ni, Co and Mo ions in the bath was fixed (∼%33 for each one) and the initial electrodeposition parameters (time, current density and total metal ions concentration) were optimized through the CCD method (See Supporting Information, Section S1). Then, at the optimum conditions obtained from the CCD, the mole ratio of the precursors in the electrodepositing bath was optimized by simplex-lattice mixture design (SLMD) (See Supporting Information, Section S2). Finally, the GC-GNs-NiCoMoTANPs electrode surface was additionally modified with Ru nanoparticles (NPs) using 0.2 mM RuCl3 in the 0.1 M H2SO4 solution by electrodeposition at 20 mA cm-2 for 300 s.34 The modified GC-GNs-NiCoMoTANPs/Ru electrodes were used for studying the kinetics of HER activity. For XPS measurements, the screen-printed carbon electrode (SPCE; DS110, Oviedo, Spain) was used as a platform, and modified according to the method explained for the GC electrode.

2.3. Apparatus, Measurements and Data Analysis Field-emission scanning electron microscopic (FESEM) images and Energy Dispersive X-ray microanalysis (EDX) were acquired using a scanning electron microscope (FESEM, Hitachi

S4160,

Cold Field

Emission,

Japan).

X-ray and

Auger photoelectron

spectroscopy (XPS) analysis was carried out on a VG Microtech Twin anode XR3E2 X-ray 6 ACS Paragon Plus Environment

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source and a concentric hemispherical analyzer operated at a base pressure of 5 × 10−10 mbar using Al K (hν ≅ 1486.6 eV). The XPS high resolution data were deconvoluted and fitted using PeakFit v4.12 software, and the fitting results were plotted. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurements were performed by ICP-OES, Perkin Elmer, Optima 7300DV. The HER measurements were performed at 25 ± 2 °C in a three-electrode cell assembly including a Ag/AgCl (3 M KCl) electrode as reference, a Pt plate as counter electrode, and the GC electrode (0.0314 cm2, Azar electrode Co., Urmia, I.R. Iran) modified with prepared composite catalysts as working electrode. All the potentials were corrected and reported vs. SHE. The electrochemical measurements were carried out on the Potentiostat/Galvanostat, Autolab 302N equipped with a frequency response analyzer, and controlled by NOVA 1.10 software (Eco Chemie, Utrecht, the Netherlands). All electrochemical measurements were performed in 0.1 M NaOH degassed with argon (99.99%) for 20 min prior to each experiment and blanketed with the gas during the experiments. The optimal HER activity of TANPs was obtained statistically using response surface methodology (RSM) by Design Expert software (version 7.1.5).

3. RESULT AND DISCUSSION 3.1. Experimental Design and Data Processing The DOE is inherently a multi-objective optimization problem, including maximization the accuracy of the information getting from the experiments, minimization the number of experiments, and in some cases, estimation of some effective experimental factors affecting the response of the system.36



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The RSM based on statistical DOEs as an efficient chemometric method is rational trend toward replacing inefficient OFAT method37 and seeking the optimum conditions for multivariable systems.38,39

3.1.1. Central Composite Design In this study, CCD, the most commonly form of RSM, was utilized to investigate the effect of three independent variables in 20 sets of experiments. Three operating factors, namely, time of electrodeposition (X1), total concentration of metal ions (X2) and current density (X3), were chosen as independent variables that influence on the electrodeposition of TANPs on GNs and consequently, on the HER activity (η50). The responses obtained from experiments proposed by CCD for η50 are given in Table S1. The CCD analysis resulted in a second order equation between independent variables and observed responses as;

2

R = −0.1 − 0.028X1 + 0.020X3 + 0.038X1X2 + 0.057X1X3 + 0.039X2X3 − 0.048X1 − 0.050 2

X2 − 0.064X3

2

(1)

Analysis of variance (ANOVA) was applied for evaluating the adequacy of the model and indicating the factors affecting the response (Table 1). The obtained F-value and P-value of 418.69 and less than 0.0001, respectively, indicate that the model is significant because of a 0.01% chance that the model could occur due to noise (It should be noted that p-values greater than 0.1000 indicate that the model is not significant). The lack-of-fit value of 0.57 implies that it is not significant in comparison with the pure error because of a 74.61% chance could occur due to noise. There is a good predictability for the model due to the insignificant lack of fit and a reasonable agreement between the R2pred, 0.9890, and the R2adj, 8 ACS Paragon Plus Environment

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0.9967 (please note that both F and R2adj values indicate a statistically significant regression model).

Here Table 1

The independent variables including the time (X1), current density (X3), second-order of time (X11), second order of the total concentration (X22), second-order of the current density (X33) and the interactions between time-total concentration (X12), time-current density (X13) and total concentration-current density (X23) are important where the value of p is less than 0.0001 (Table S2). According to the monomial coefficient values of the Equation 1, the importance of the factors follows this order: time (X1)> current density (X3)> total concentration of metal ions (X2). Interactions between these independent variables and their related responses are presented in Figure 1.

Here Figure 1

Optimization of the conditions for electrodepositing of TANPs was performed based on the minimum η50 needed for HER. The optimum point that maximizes the desirability function was identified by using Design Expert software (Table 2). Therefore, the optimal conditions for the electrodeposition of TANPs obtained from CCD are given as time (51 s), total concentration of metal ions (42 mM) and current density (19.30 mA cm−2).

Here Table 2 9 ACS Paragon Plus Environment


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3.1.2. Simplex-Lattice Mixture Design Augmented After finding the optimum conditions for electrodeposition of TANPs according to CCD, the mole ratio of these the metal ions in the electrodeposition bath was optimized by using SLMD to explore the relationship between the proportion of alloy components and their HER activity (η50). Table S3 shows the composition of the electrodeposition bath for each point of SLMD and the corresponding values obtained for η50. The collected results were fitted to full cubic polynomial model, based on least-squares approximation, to find the unknown coefficients in following equation:

R1 = − 0.23713Ni − 0.27160Co − 0.33711Mo + 0.22763NiCo + 0.54362NiMo + 0.52856Co Mo + 1.37258NiCoMo − 0.28988NiMo(Ni − Mo) − 0.26168CoMo(Co − Mo)

(2)

This equation shows the relation between the mole ratio of three metal ions (Co2+, Ni2+ and Mo6+) in the electrodeposition bath and the response of prepared system, η50. ANOVA results for analytical methods, indicating the effects of model terms, are summarized in Table 1. The empirical cubic polynomial model is highly significant regarding F-value of 84.93 and P value less than

Mixture Design of NiCoMo Ternary Alloy Nanoparticles Assembled on

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