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reduced graphene oxide composite: A highly efficient electrocatalyst for hydrogen evolution reaction in alkaline solutions
Accepted Manuscript Title: Binder-free prickly nickel nanostructured/reduced graphene oxide composite: A highly efficient electrocatalyst for hydrogen evolution reaction in alkaline solutions Author: Reza Karimi Shervedani Mostafa Torabi Fatemeh Yaghoobi PII: DOI: Reference:
S0013-4686(17)31095-2 http://dx.doi.org/doi:10.1016/j.electacta.2017.05.099 EA 29534
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
14-9-2016 19-4-2017 15-5-2017
Please cite this article as: R.K. Shervedani, M. Torabi, F. Yaghoobi, Binder-free prickly nickel nanostructured/reduced graphene oxide composite: A highly efficient electrocatalyst for hydrogen evolution reaction in alkaline solutions, Electrochimica Acta (2017), http://dx.doi.org/10.1016/j.electacta.2017.05.099 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Research Highlights-EA-EO16-1684R2
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Research Highlights Efficient & binder-free prickly nickel nanostructured/graphene is made for electrocatalytic HER. Surface analysis showed that PNiNS wrapped in RGONs was pinned into Cu-Nifpl by prickles. Nanocomposite exposed excellent stability & electrocatalytic activity, b=43mV/dec, η20=−57mV. Increased activity comes partially from improved surface roughness & mainly synergetic effect.
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Graphical Abstract-EA-EO16-1684R2
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Manuscript-EA-EO16-1684R2-Revised-Clean Click here to view linked References
Binder-free prickly nickel nanostructured/reduced graphene oxide composite: A highly efficient electrocatalyst for
Reza Karimi Shervedani,* Mostafa Torabi, Fatemeh Yaghoobi
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hydrogen evolution reaction in alkaline solutions
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Department of Chemistry, University of Isfahan, Isfahan 81746-73441, I.R. IRAN.
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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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ABSTRACT Non-precious metal electrocatalysts with high activity towards hydrogen evolution reaction (HER) are desirable regarding renewable energy devices such as fuel cells and water electrolysis.
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However, fabrication of new materials for this purpose remains a main challenge. Here, a binder-free nanocomposite, prickly nickel nanostructured/reduced graphene oxide nanosheets, is
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constructed via electroless-deposition on cupper surface covered with a fresh prelayer of nickel
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(Cu-Nifpl-PNiNS/RGONs) for the first time. Then, the fabricated system is tested successfully for the HER in alkaline solutions. Structure and activity of the composite are characterized
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quantitatively by surface techniques and electrochemical methods. The results show that the hedgehog-like prickly nickel nanostructures wrapped in the RGONs cloth are formed, pinning the
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PNiNS/RGONs into the Cu-Nifpl surface, resulting in exceptional stability and activity for the
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Cu-Nifpl-PNiNS/RGONs system. In effect, the composite has shown excellent structural stability
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against disintegration by ultrasound waves; and electrocatalytic activity towards the HER as 20 = 57 mV, Tafel slope = 43 mV dec1 and j0 = 1.05 mA cm2, quite close to 22 mV, 40 mV dec1 and 5.88 mA cm2, obtained in the same conditions for commercial Pt/C,
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respectively. The remarkable increase in electrocatalytic activity was found to be originated partially from increase in the surface roughness and mainly from synergetic chemical coupling effects between PNiNS and RGONs.
Keywords: Binder-free
nanocomposite;
Prickly nickel nanostructured/graphene;
Hydrogen
evolution reaction; Electrochemical impedance spectroscopy; Synergetic effects
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1. Introduction Hydrogen evolution reaction (HER) has been treated as a prototype reaction regarding electrochemical kinetic and catalytic studies, and as a method of H2 production regarding
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decreasing the cost of hydrogen via reduction of the electrode reaction overpotential (η). The Pt or Pt-based materials [1,2], due to their negligible overpotential and excellent kinetics for the
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HER, have been used traditionally, however, the Pt-group metals are scarce and very expensive,
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and thus, may not be appropriate for large-scale production of hydrogen [3]. Therefore, the electrocatalytic HER has been studied extensively on various electrode materials to reduce the
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cost of production and at the same time achieve a low overpotential at a reasonably acceptable cathodic current density [4-10].
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Among the studied electrode materials like Ni, Fe, Co, Mo, or their compounds [6,11-15], nickel
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is well-known as one of the best non-noble catalysts for the HER [6,9,10,15]. Specially,
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nanostructured Ni materials have shown acceptable activity for this propose [16], although their stability is criticized [17]. Furthermore, nickel nanowires [18-20] and prickly nickel nanowires [21] have been examined to increase the effective surface area of the electrodes and
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promote the HER performance. While, the stability of the electrodes has been significantly improved in some cases, moderate activities have been observed [21], compared with the Pt-based electrode materials [1,2].
Graphene; due to its remarkable physicochemical properties [22,23], is highly promising for application in photocatalysts [24,25], pseudocapacitors [26] and batteries [24,27]. Especially, investigations regarding graphene based-nickel-composites [7,28,29] are important. Recently, graphene nanosheets (GNs) thin films, decorated with metal nanoparticles as Au, Ru, or enzymes,
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have been developed by us [8,30] to catalyze electrochemical reactions [30], and improve performance of the HER [8]. For assembling the modifier system onto the solid surface, where interactions of the system
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components with each other as well as with the solid surface is not strong enough to keep them together, usually a binder is applied [8,12,31] which may block the electrode pores and deactivate
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it [32-34], or a template is used which is tedious to construct it. Such necessities, in turn, limit
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applications of the constructed composite. Therefore, binder- [35,36] and template-free [37] nanocomposites, having the same or better stability and activity, are preferred. These
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requirements besides the observed behavior for the nickel nonmaterial [6,9,21] and our experiences with graphene nanosheets towards catalytic redox reactions [8,30] motivated us to
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fabricate and study binder- and template-free prickly nickel nanostructures/reduced graphene
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oxide nanosheets (PNiNS/RGONs) composite for the HER in alkaline solutions here. The
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behavior of Ni-foam/GNs composite for the HER has been investigated theoretically as well as practically, and interesting results have been reported about cooperative action (synergetic effect) between the Ni and RGO [7], however, to the best of our knowledge, there is no previous report
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regarding preparation and electrocatalytic activity of the prickly nickel nanostructures composited with graphene towards the HER. The current design combines interesting aspects of the PNiNS (physical stability and relatively good electrocatalytic activity) [21,38-40] with those of the RGONs (edge activity and excellent conductivity) [8,22,23,30] to fabricate the nanocomposite system on the electrode base, cupper surface covered with a fresh prelayer of nickel (Cu-Nifpl) [6,9], resulting in the Cu-Nifpl-PNiNS/RGONs electrocatalyst. The fabricated system benefits of cooperative action or synergetic effect between nickel and graphene [7,41], and exposes excellent physical stability,
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electrocatalytic activity and durability towards the HER. Details of the experimental works and the obtained data are described below.
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2. Experimental 2.1. Materials.
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Graphene oxide (GO) was synthesized from graphite powder (Sigma-Aldrich, 1-2 m) according
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to the Hummers’ method [42] partially modified by us (see Supporting Information, Section S1, Fig. S1). Potassium permanganate (KMnO4), sulfuric acid (H2SO4 98%), hydrogen peroxide
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solution (H2O2 30%), nickel sulfate (NiSO4,6H2O), nickel chloride (NiCl2,6H2O), sodium
(N2H4,H2O 99%),
commercial
Pt/C
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hydroxide (NaOH), boric acid (H3BO3), trisodium citrate (Na3C6H5O7), hydrazine hydrate (Pt 20%),
Nafion #117 (5% solution
in aliphatic
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alcohol/water (75:25 v/v) mixture) and other chemicals were of analytical grade obtained from
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commercial sources (Sigma® or Merck®), and used without further purification.
2.2. Synthesis of nanocomposites and modifications of the electrode surface.
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A fresh prelayer of nickel (Nifpl) was electrodeposited onto the clean cylindrical cupper (exposed surface area: 0.237 cm2) and labeled as Cu-Nifpl (see Supporting Information, Section S2.1). Several sets of the Cu-Nifpl substrate were prepared and used as platform for electroless synthesis of PNiNS/RGONs composites. Also, several baths were tested (see Supporting Information, Section S2.2 for details), and finally a well attached layer of the PNiNS/RGONs composite with excellent
physical
stability
was
formed
onto
the
Cu-Nifpl
surface
leading
to
Cu-Nifpl-PNiNS/RGONs electrode based on the bath and conditions designed here. A set of Cu-Nifpl-PNiNS/RGONs electrode was prepared.
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Typically, 500 mg trisodium citrate dehydrate and 6 mg graphene oxide (GO) were added into a 20.0 ml of 75 mM NiCl2 aqueous solution, and the solution was stirred for 30 minutes, then, the whole mixture was sonicated for another 30 min to obtain a homogeneous solution (different
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amount of GO as 3, 6 and 9 mg were tested, and the amount 6 mg was found as the optimized quantity). A 1.0 mL of hydrazine hydrate (99%) was mixed with 9.0 mL of distilled water and
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added to the pervious solution. Then, the solution pH was increased to above 12 using NaOH
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solution. The resulting solution was transferred into a 50 mL flask along with shaking followed by slow heating at ~85 oC on a water bath.
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Immediately, the freshly prepared Cu-Nifpl substrate (as fresh as possible) was immersed into the flask solution for electroless-deposition; and the process was followed by shaking (not magnetic
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stirring!) the solution. After a few seconds a gray rough material appeared and grew on the
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Cu-Nifpl surface. The process was completed within ~40 min, and the solution become
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transparent. The fabricated system, labeled as Cu-Nifpl-PNiNS/RGONs electrode, was removed and washed several times with water to drain out excess surfactant or hydrazine, sonicated in the ultrasonic bath, washed with ethanol, dried under vacuum and used for next steps;
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characterization by surface techniques. Another set of Cu-Nifpl substrate was treated in the same way, but in the absence of GO. This electrode was labeled as Cu-Nifpl-PNiNS. An amount of commercial Pt/C was suspended in 5% solution of Nafion #117 to obtain a suspension solution of 2 mg mL1 Pt/C. The solution was coated onto a clean GC (0.0314 cm2) electrode surface, and dried in argon gas stream. Approximately, the amount of 0.32 mg cm2 Pt/C on the GC surface was found as optimized value. This electrode was labeled as GC-Pt/C.
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Accordingly,
we
had
five
sets
of
the
electrodes,
including
(a) Cu,
(b) Cu-Nifpl,
(c) Cu-Nifpl-PNiNS, (d) Cu-Nifpl-PNiNS/RGONs, and (e) GC-Pt/C to characterize and assess their activities towards the HER. It should be mentioned that in the absence of Ni(II) in the bath
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solution, a layer of the RGONs was formed on the Cu-Nifpl surface, but it was unstable, washed out easily by water, so that practically we could not fabricate a stable Cu-Nifpl-RGONs electrode
2.3. Apparatus, measurements and data analysis.
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2.3.1. Surface analysis.
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to assess its activity for the HER.
Field-emission scanning electron microscopic (FESEM) images and Energy Dispersive X-ray
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microanalysis (EDX) were performed by using FESEM, Hitachi S4160, Cold Field Emission,
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Japan. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a VG Microtech
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Twin anode XR3E2 X-ray source and a concentric hemispherical analyzer operated at a base pressure of 5 1010 mbar using Al K (h 1486.6 eV). The XPS high resolution data were deconvoluted and fitted using PeakFit v4.12 software (235 Walnut St.S7, Framingham, MA
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01702, USA), and the fitting results were plotted. The XRD analysis of the samples was performed with a Bruker D8 Advance powder diffractometer using Ni filtered Cu-K radiation, and analyzed by using PANalytical XPert HighScore software. Adherence testing and physical stability of the catalyst onto the surface, homogenizing the solutions, and removing the physically adsorbed species from the electrode surface were performed by using an ultrasonic bath (FLAC, LBS2: 60 KHz, Italy). The shaking process was performed using a shaker (Nuve, ST402, Turkey).
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2.3.2. Electrochemical measurements and data analysis. The
electrochemical
measurements
were
carried
out
on
a
Potentiostat/Galvanostat,
Autolab 302 N equipped with a frequency response analyzer, and controlled by Nova 1.18
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software (Eco Chemie, Utrecht, the Netherlands).
The HER measurements were carried out at 25 ± 2 oC using a three-electrode glass cell assembly
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including a Ag/AgCl (3 M KCl) electrode as reference, a large Pt plate as counter electrode, and
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the Cu substrate modified with synthesized catalyst (Section 2.2) as working electrode. The cell was filled with ~50 mL of purified 0.1 M NaOH as electrolyte solution, degassed with argon
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(99.99%) for 30 min prior to each experiment. Several LSV were recorded at slow potential scan
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rate, 5.0 mV s1, to achieve the steady-state, then, the last one was recorded for kinetic analysis. The kinetic parameters were extracted for each electrode using the related steady-state Tafel plots
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and linear least square (LLS) approximation method [9].
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The EIS measurements were performed at the steady-state in the frequency range of 10 kHz to 100 mHz. The AC amplitude was set to 5 mV superimposed on DC potentials. The DC potentials were selected in the range of the HER overpotential. Details of the electrochemical experimental
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setup and data analysis are given in our previous reports [9,43].
3. Results and discussion
3.1. Construction and physicochemical characterization of the composite 3.1.1. Construction. The Cu-Nifpl-PNiNS/RGONs nanocomposite was constructed according to the Section 2.2. Some important points should be explained in this case.
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(i) Pretreatment of the Cu substrate and use of a fresh prelayer of Ni on Cu are critical to achieve clean and active surface, resulting in considerable improvement in the stability of PNiNS/RGONs nanocomposite onto the Cu-Nifpl surface.
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(ii) The nature of the surfactant is also important, for example, while the PNiNS/RGONs, i.e. PNiNS wrapped in the RGONs cloth, could not be formed in the presence of TritonX-100 on
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Cu-Nifpl substrate; the desirable nanostructures were formed in the presence of citrate ions.
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(iii) The presence of the binder in the electrode modifying film can decrease accessibility of the electrolyte to the electrode surface active sites, which in turn decreases the electrode
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performance. Thus, the binder-free composites is preferred, if the desired properties of the composite are not diminished, compared with that based on the binder [28,36]. The binder-free
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characteristic is one of the most interesting aspects of this catalyst. While no binder is used in the
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texture of the Cu-Nifpl-PNiNS/RGONs composite, both the stability and the electrocatalytic
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activity (Section 3.2, below) are exceptionally improved. Rarely, both characteristics have been achieved in one electrocatalyst.
(iv) The prepared Cu-Nifpl-PNiNS/RGONs electrode was sonicated in ethanol (few seconds) to
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remove physically adsorbed species and obtain reproducible electrochemical response. Furthermore, the PNiNS/RGONs catalyst attached onto the Cu-Nifpl surface showed excellent structural stability against disintegrations that could potentially occur by ultrasonic waves. This is one of the interesting aspects of this work. Finally, the Cu-Nifpl-PNiNS/RGONs system was washed with ethanol, dried under vacuum and used for surface analysis.
3.1.2. Surface analysis and characterization 3.1.2.1. Characterization by FESEM
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Surface morphology and topographical details of the electrodes modified by synthesized materials are characterized by using FESEM [44]. The FESEM images obtained with different magnifications on the nanocomposite, fabricated
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successfully from the electroless bath in this work (Section 2.2), are presented in Fig. 1. While starfish-like structures of Ni with nano-spikes grown perpendicular on their centers are formed on
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the Cu-Nifpl substrate in the absence of GO (Panels A and B), hedgehog-like prickly nickel
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nanostructures wrapped in the RGONs cloth are formed in the presence of GO (Panels C and D).
and form the Cu-Nifpl-PNiNS/RGONs composite.
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The prickles can pin the PNiNS/RGONs composite onto the freshly prepared Cu-Nifpl surface
The diameter of the nano-spikes (Panel B) varies from several tens to several hundred
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nanometers, and tends to grow vertically. This structure imparts excellent stability to the
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PNiNS/RGONs composite when it is brought onto the Cu-Nifpl surface together with GO. The
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diameter of hedgehog-like PNiNS/RGONs varies, also from ~150 to ~200 nm from which the prickles with diameter varies from a few to ~30 nm, are grown out as revealed from FESEM microimages (Panel D).
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Here Figure 1
Composition of the surface was described by EDX; elemental analysis (Fig. S3 and Table S1) and elemental mapping (Fig. S4) [45]. The presence of oxygen on the surface is a normal behavior, comes from RGONs and the residual nickel oxides. This issue is discussed subsequently. Overall, the obtained results support formation of the PNiNS/RGONs composite onto the Cu-Nifpl surface (during fabrication of the electrodes some precipitate was formed at the
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floor of the cell. This material was also collected and characterized; see Supporting Information, Section S4, Fig. S5).
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3.1.2.2. Characterization by XPS
The XPS is a powerful tool to identify all of the elements in periodical table and determine their
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oxidation state, except hydrogen and helium [46,47]. Therefore, the structure of the fabricated
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composite and the presence of different species on the surface was further traced and approved by the XPS measurements (Fig. 2).
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The XPS survey spectrum obtained on the PNiNS/RGONs surface (Fig. 2A) revealed the
supporting formation of a pure system.
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presence of C1s and O1s peaks, and several peaks related to the Ni, without any other species,
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The high resolution XPS spectra, recorded in the range of 282.5 to 291.5 eV for C (1s) on the
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PNiNS/RGONs composite is presented in Fig. 2B. Carbon atoms in different functional groups, having different oxidation state and apparent charge, are appeared clearly in this range. The spectra revealed several peaks around 284.60, 285.76, 286.85, 288.22, and 289.15 eV, which are
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attributed to the carbon atoms of C=C, C−OH, epoxy, C=O and O−C=O groups, respectively. The intensity of the C=C peak is several times larger than sum of those observed for the other carbon atoms, supporting that most of the hydroxyl, epoxide, carbonyl and carboxyl functional groups are reduced (GO is converted to RGONs) [48] by N2H4 simultaneously during electroless reduction of Ni(II) to Ni, leading to formation of PNiNS/RGONs composite onto Cu-Nifpl surface. The high resolution spectrum obtained in the range of 845 to 890 eV for Ni (2p) on PNiNS/RGONs is shown in Fig. 2C. The peaks appeared around 854.23 and 872.4 eV are
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assigned to Ni (2p3/2) and Ni (2p1/2) energy levels, respectively, which are very close to those reported for Ni in literatures [49,50]. The Ni (2p) XPS spectrum is best fitted with three peak of Ni, Ni(II) and Ni(III), and shakeup satellites (indicated as “Sat”) [50]. The results show that the
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chemical composition of the PNiNS/RGONs contains RGONs, metallic Ni and some surface
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oxides.
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Here Figure 2
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3.1.2.3. Characterization by XRD
The XRD patterns obtained on PNiNS and PNiNS/RGONs composite (Fig. 3) indicate that only
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metallic phase of Ni has been formed via electroless-deposition in both structures. The peaks
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appeared around 44.5o, 51.9o, 76.5o and 92.9o are attributed to the Ni(111), Ni(200), Ni(220) and
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Ni(311) crystalline planes, corresponding to the face-centered cubic (fcc) structure of Ni [51]. In addition, a comparison between the XRD patterns of the PNiNS and PNiNS/RGONs composite shows a wide and relatively small peak labeled as RGONs, which is attributed to the presence of
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reduced graphene oxide in the prepared nanocomposite [23].
Here Figure 3
Overall, the results obtained by FESEM, EDX, XPS, and XRD support formation of the Cu-Nifpl-PNiNS/RGONs composite system. The characterized system was used as the cathode electrode for the HER. The obtained data are analyzed and discussed in the following.
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3.2. Electrocatalytic activity of the nanocomposite for the HER. 3.2.1. Linear sweep voltammetry and steady-state polarization curves. Several sets of Cu-Nifpl electrode substrate were prepared and immediately furnished with the
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PNiNS/RGONs composite or its ancestors (Section 2.2), separately, and tested for the electrocatalytic HER.
(a) Cu,
(b) Cu-Nifpl,
(c) Cu-Nifpl-PNiNS,
(d) the Cu-Nifpl-PNiNS/RGONs
and (e) the
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the
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The steady-state linear sweep voltammograms and polarization curves (Tafel plots) obtained on
GC-Pt/C electrodes are displayed in Fig. 4A, curves a to e. The recorded potentials are corrected
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for ohmic drop (iRs) potential and reported vs. reversible hydrogen electrode (RHE) potential. The data indicate that kinetics of the HER are essentially improved upon modification of the
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Cu-Nifpl electrode by the catalyst layers (Fig. 4A, curves a to e). For example, the values obtained
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for the onset potentials by using asymptotic lines cross are moved to more facile direction (less
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negative values) as (a) 760, (b) 500, (c) 160, (d) 30 and (e) 15 mV. Clearly, the value of 30 mV obtained on Cu-Nifpl-PNiNS/RGONs is very close to 15 mV obtained on GC electrode modified with Pt/C commercial catalyst. Similar tendency is observed for other kinetic
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parameters, analyzed and discussed below.
Here Figure 4
3.2.2. Tafel plots Mechanisms and kinetics of the electrocatalyst are better understood by using steady-state Tafel plots. Typical plots obtained on the modified electrodes are displayed in Fig. 4B. The linear part of the plots is fitted into the Tafel equation, = EiRE0= b log(j0) b log(jk) in which E is
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measured potential, E0 is equilibrium potential for the HER, b is Tafel slope, and j0 and jk are exchange and measured current densities, respectively [52]. The behavior of the Tafel plots at the negative overpotentials (Tafel slop near 40 mV dec1) indicates that the HER proceeds via
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Volmer-Heyrovský mechanism, neglecting the Tafel reaction [53,54]. The kinetic parameters
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obtained from the steady-state Tafel curves are presented in Table 1. One may compare the kinetic behavior of the current catalyst, Cu-Nifpl-PNiNS/RGONs, with that of the GC-Pt/C.
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The Cu-Nifpl-PNiNS/RGONs electrode is characterized by fast kinetics of j0 = 1.05 mA cm2, b = −43.0 mV dec1, and low overpotentials at current densities of 20 mA cm2, η20 = −57 mV,
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which are quite close to those obtained for GC-Pt/C; j0 = 5.88 mA cm2, b = −40.0 mV dec1, and
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η20 = 22 mV at the same conditions, still, the physical stability of the current catalyst, Cu-Nifpl-PNiNS/RGONs, was much better than that of Pt/C. Similar comparison may be
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performed with Cu-Nifpl or Cu-Nifpl-PNiNS modified electrodes. Clearly, the electroactivity of
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Cu-Nifpl-PNiNS/RGONs electrode for the HER is exceptionally improved compared with its ancestors, Cu, Cu-Nifpl, and Cu-Nifpl-PNiNS (Table 1). A
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comparison
was
also
made
between
the
kinetics
of
the
electroless-deposited
Cu-Nifpl-PNiNS/RGONs and electrodeposited Cu-Nifpl-Ni/GO electrodes (see Supporting Information, Fig. S6, curve f).
Here Table 1
3.2.3. Electrochemical impedance spectroscopy The electrocatalytic activity of the constructed nanocomposites towards the HER was more precisely investigated at the electrode/electrolyte interface by the EIS (Figs. 5, Panels A to E).
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Here Figure 5
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The EIS complex plane (Zrea vs. Zim) plots were approximated by using the appropriate
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equivalent circuit models built in the ZView 2.9 software and the CNLS approximation method. After testing several models related to the HER considering of the fit criteria such as
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‘‘number of model parameters” and ‘‘Chi square values’’ [43], the modified CPE model was found to be enough for approximations of the EIS data of the investigated electrodes, except for
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the GC-Pt/C data. The impedance of this electrode was explained by a two-CPE model in which
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the high frequency (small time-constants) semicircle was attributed to the electrode texture (charge transfer through interface formed between GC electrode and commercial Pt/C film), and
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the kinetic parameters were extracted from the low frequency semicircle [54,55]. The CPE model
te
element consists of the charge transfer resistance (Rct) in parallel with the impedance of the electrode/solution double layer, and both in series with Rs, where ZCPE = 1/[Q(j)g], Q is a
Ac ce p
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 61 62 63 64 65
capacitive parameter related to the average double layer capacitance (Cdl) of the electrode; Q = Cdlg[1/Rs 1/Rct]1g, j = (1)1/2, is angular frequency, and g is dispersion parameter. The model parameters are extracted in Table S2 (Supporting Information, Section S6). Some important points should be discussed here: (i) Variations of log(1/Rct) vs. displayed a linear behavior, and the related kinetic parameters were extracted [6,9], (Fig. 6A, also Table 1, right side), supporting those obtained by the Tafel plots. The small differences is normal, however, the EIS data are more accurate, since the contribution of nonfaradaic currents are more accurately measured and separated from the total measured currents by this technique [54].
15
Page 17 of 39
(ii) A plot of Cdl values vs. shows that Cdl is almost invariant for Cu and Cu-Nifpl electrodes (Fig. 6B). However, it changes somewhat for GC-Pt/C and Cu-Nifpl-PNiNS electrodes, and significantly for Cu-Nifpl-PNiNS/RGONs electrode. This behavior is due to effect of the H2
ip t
gas small bubbles trapped into the electrode pores, which is a normal behavior for gas evolving
cr
electrodes as suggested previously [6,54]. Still, the process is highly repeatable; suggesting that the active surface area of the catalyst is maintained unchanged at different applied potentials,
us
which is consistent with durability of the catalyst.
(iii) The results indicate that kinetics of the HER is significantly improved upon modification of
an
the Cu-Nifpl substrate with PNiNS/RGONs composite. For example, a comparison between
intrinsic
activity,
is
M
Cu-Nifpl-PNiNS and Cu-Nifpl-PNiNS/RGONs shows that the j0, which is an indicative of the increased
by
a
factor
of
196.3
as
d
{(j0,[Cu-Nifpl-PNiNS/RGONs])/(j0,[Cu-Nifpl-PNiNS])} = 196.3). Alternatively, by comparison between the Cdl
te
values, one will find that the surface roughness is increased partially. These results imply that the increase in the activity of the PNiNS/RGONs composite is originated partially from increase in the surface roughness and mainly from cooperative action or synergetic chemical coupling
Ac ce p
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 61 62 63 64 65
effects between nickel and graphene [7,41,56], which is in good agreement with our previous findings on nickel electrodes modified with carbon supplied from organic source (L-Lysine) [6,9]. This finding is another interesting aspect of the current work. (iv) The absence of high frequency semicircle from the EIS data (Fig. 5, compare Panels D and E) means that the electron transfer rate is independent of the electrode texture (material film) which in turn is explained based on excellent connection between Cu-Nifpl electrode base and the PNiNS/RGONs film as well as between PNiNS and RGONs.
16
Page 18 of 39
Here Figure 6
The EIS results support those extracted from the Tafel plots; indicating excellent electrocatalytic
ip t
activities for Cu-Nifpl-PNiNS/RGONs electrode, quite close to that of GC-Pt/C electrode and incomparable with its ancestors, Cu, Cu-Nifpl, and Cu-Nifpl-PNiNS (please note that the
cr
Cu-Nifpl-RGONs was not physically stable, Section 2.2, so no kinetics could be obtained for this
us
electrode).
(v) The last point in this section is that the increased activities of Cu-Nifpl-PNiNS/RGONs
an
electrode can be explained based on cooperative action [7] or so called synergetic effect [41] between PNiNS and RGONs. According to the experimental results and the thermodynamic
M
calculations which rely on density functional theory (DFT), it can be concluded that the
d
H-adsorbed intermediately formed on Ni surface spillover onto the RGONs, where the
te
H-adsorbeds recombine quickly and form H2 [7]. These effects can result in a clear PNiNS surface, thus, enhance the kinetics of the HER. The LSV, Tafel plots, and EIS results obtained in the current study suggest a Volmer-Heyrovský route with less contribution of Tafel step for the
Ac ce p
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 61 62 63 64 65
HER. It means that the recombination of H-adsorbeds [7] is also fast here and does not influence on the HER kinetics. Thus, the results obtained here on PNiNS/RGONs, regarding the role of RGONs, are in good agreement with those reported on Ni foam/RGO by Chanda et al. [7].
3.3. Durability and stability The durability and performance of the prepared catalyst was investigated by following the response of the Cu-Nifpl-PNiNS/RGONs electrode to the HER in different ways (see Supporting Information, Section S7, Figs. S7 and S8). In addition, the electrode was subjected to HER at a
17
Page 19 of 39
large current density and high temperature, j = 300 mA cm2 and ~60 oC, in 3 M KOH; and the overpotential was followed. The results show less than 2% change in over a period of ~60
ip t
hours (Fig. 7), indicating a good durability and stability for the electrode.
cr
Here Figure 7
us
Finally, the FESEM obtained on the Cu-Nifpl-PNiNS/RGONs electrode surface before and after long time HER showed no significant changes in the surface morphology (see Supporting
an
Information, Fig. S9). Overall, the results of this section show that the electrode benefits of
M
excellent durability and storage stability towards the HER.
This interesting durability can be related to the pinning of the PNiNS/RGONs composite into the
d
Cu-Nifpl surface via strong bond formed between the freshly and in-situ electrolessly prepared
te
nickel prickles (the hedgehog-like PNiNS/RGONs composite) and the freshly and ex-situ electrochemically prepared active Ni (Nifpl) on the surface of Cu-Nifpl substrate.
Ac ce p
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 61 62 63 64 65
4. Conclusions
An efficient composite catalyst, binder-free prickly nickel nanostructured/reduced graphene oxide, Cu-Nifpl-PNiNS/RGONs, was synthesized for the first time, and tested successfully for electrocatalytic HER in alkaline solutions. Surface analysis of the nanocomposite through FESEM, EDX, XPS and XRD methods show that the prickly nickel nanostructures wrapped in a cloth of the reduced graphene oxides are fabricated and pinned into the Cu-Nifpl surface via nickel prickles, resulting in hedgehog-like structures for the surface of the Cu-Nifpl-PNiNS/RGONs composite. The nanocomposite showed
18
Page 20 of 39
exceptional structural stability against disintegrations that could potentially occur by ultrasonic waves. Quantitative kinetic analysis of the information, obtained for the HER by electrochemical
ip t
methods, revealed excellent electrocatalytic activities; j0 = 1.05 mA cm2, b = −43.0 mV dec1 and η20 = −57.5 mV for the Cu-Nifpl-PNiNS/RGONs composite electrode, quite close to those
cr
obtained for the commercial GC-Pt/C electrode; j0 = 5.88 mA cm2, b = −40.0 mV dec1 and
us
η20 = 22 mV at the same conditions.
an
Precise analysis of the EIS data of Cu-Nifpl-PNiNS/RGONs in comparison with Cu-Nifpl-PNiNS shows that the increased activity (j0) of the Cu-Nifpl-PNiNS/RGONs composite is originated
M
partially from increase in the surface roughness and mainly from cooperative action (synergetic chemical coupling effects) between nickel and graphene.
d
The present study provides new insights into the design and development of nanocomposites for
especially for the HER.
te
electrocatalytic reactions at the electrode/solution interface, energy storage and production,
Ac ce p
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 61 62 63 64 65
Acknowledgment
The authors gratefully acknowledge the University of Isfahan (UI) for financial support and providing research facilities. The Iran National Science Foundation, Vise-Presidency for Science and Technology (INSF/VPST) is acknowledged for supporting and the use of high technology facilities and services in the Iran High-Tech Laboratory Network (IH-TLN).
Supporting Information
19
Page 21 of 39
The XRD data of the graphite and graphene structure, FESEM micrographs displaying the role of Teriton-X100 as surfactant, the EDX spectrum, the TEM images, durability tests, FESEM micrographs of the Cu-Nifpl-PNiNS/RGONs catalyst before and after HER, elemental mapping
ip t
images, and activities of graphene/nickel electrodes prepared via electrodeposition method all are
te
d
M
an
us
cr
given in Supporting Information file.
Ac ce p
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 61 62 63 64 65
20
Page 22 of 39
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ip t cr
Tables
Tafela b/ mV dec1 300 190 140 43 40
(a) Cu (b) Cu-Nifpl (c) Cu-Nifpl-PNiNS (d) Cu-Nifpl-PNiNS/RGONs (e) GC-Pt/C
j0/ A cm2 5.20 106 1.15 105 7.58 105 1.05 103 5.88 103
0.3 0.4 0.5 0.7 0.6
-20/ mV 1100 750 365 57.5 22
b/ mV dec1 250 182 131 48 45
an
Electrode
us
Table 1. Kinetic parameters obtained from steady-states polarization curves (Tafel plots) and the EIS measurements for the HER in 0.1 NaOH at 298 K. EISb
j0/ A cm2 2.93 107 1.24 106 4.05 106 7.95 104 1.95 103
Cdl,/ mF cm-2 0.036 0.040 1.5585 13.707 0.490
k0app/ cm s1 7.60 109 3.22 108 1.05 107 2.06 105 5.06 105
: Extracted from Fig. 4, b: Extracted from Fig. 6, = EiRE0= b log(j0) b log(jk), Q = Cdlg [Rs1 Rct1]1g, k0app = RT(n2F2RctAC)1, A = 0.237 cm2, C = 1.0 104 mol cm3. j0 is apparent current density [j0 = i0/0.237 cm2].
te
d
M
a
Ac ce p
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 61 62 63 64 65
Captions of Figurers Fig. 1.
The
FESEM
images
obtained
on
the
(A and B) PNiNS
and
(C and D) PNiNS/RGONs composite with two different magnifications.
28
Page 30 of 39
Fig. 2.
(A) The survey XPS spectrum, and the high resolution spectrum (B) of C1s and
Fig. 3.
ip t
(C) of Ni2p obtained on PNiNS/RGONs composite.
The XRD patterns obtained on the PNiNS (dotted line) and PNiNS/RGONs (solid
us
Fig. 4.
cr
line) materials.
(A) Steady-state linear sweep voltammograms obtained in 0.1 M NaOH solution at
an
room temperature and potential scan rate of 5 mVs1 on (a) Cu, (b) Cu-Nifpl, (c) Cu-Nifpl-PNiNS, (d) Cu-Nifpl-PNiNS/RGONs, and (e) GC-Pt/C electrodes. (B) The steady-state polarization curves (Tafel plots) extracted from the data of
M
Panel (A). Symbols indicate experimental data and solid lines the fitted results
The EIS complex plane plots obtained for the HER on (A) Cu, (B) Cu-Nifpl,
te
Fig. 5.
d
obtained by using linear least square approximation (LLS) method.
(C) Cu-Nifpl-PNiNS, (D) Cu-Nifpl-PNiNS/RGONs, and (E) commercial GC-Pt/C
Ac ce p
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electrodes at various overpotentials in 0.1 M NaOH. Symbols indicate experimental (Zrea vs. -Zim) data and solid lines the fitted results obtained by approximation of the experimental data into appropriate model (one-CPE for A, B, C, and D; and two-CPE
for E data)
based
on
complex
nonlinear
(CNLS) approximation method and ZView software. Minus
least
square
applied DC
overpotentials () is shown by arrows.
Fig. 6.
(A) Variations of inverse of the charge transfer resistance (1/Rct) as a function of overpotential () extracted
from
Fig. 5
for (a) Cu,
(b) Cu-Nifpl,
(c) Cu-Nifpl-PNiNS, (d) Cu-Nifpl-PNiNS/RGONs, and (e) GC-Pt/C electrodes.
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Symbols indicate the semi-experimental data extracted from Fig. 5 by CNLS method and solid lines the approximated results obtained by LLS method.
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over a period of ~ 60 hours HER in 3 M KOH at 60 0C.
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Durability of the Cu-Nifpl-PNiNS/RGONs electrode obtained at j = 300 mA cm2
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Fig. 7.
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(B) Variations of Cdl as a function of η.
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Figures
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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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Figures 6A and B
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Figure 7
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S0013-4686(17)31095-2 http://dx.doi.org/doi:10.1016/j.electacta.2017.05.099 EA 29534
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
14-9-2016 19-4-2017 15-5-2017
Please cite this article as: R.K. Shervedani, M. Torabi, F. Yaghoobi, Binder-free prickly nickel nanostructured/reduced graphene oxide composite: A highly efficient electrocatalyst for hydrogen evolution reaction in alkaline solutions, Electrochimica Acta (2017), http://dx.doi.org/10.1016/j.electacta.2017.05.099 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Research Highlights-EA-EO16-1684R2
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Research Highlights Efficient & binder-free prickly nickel nanostructured/graphene is made for electrocatalytic HER. Surface analysis showed that PNiNS wrapped in RGONs was pinned into Cu-Nifpl by prickles. Nanocomposite exposed excellent stability & electrocatalytic activity, b=43mV/dec, η20=−57mV. Increased activity comes partially from improved surface roughness & mainly synergetic effect.
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Graphical Abstract-EA-EO16-1684R2
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Manuscript-EA-EO16-1684R2-Revised-Clean Click here to view linked References
Binder-free prickly nickel nanostructured/reduced graphene oxide composite: A highly efficient electrocatalyst for
Reza Karimi Shervedani,* Mostafa Torabi, Fatemeh Yaghoobi
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hydrogen evolution reaction in alkaline solutions
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Department of Chemistry, University of Isfahan, Isfahan 81746-73441, I.R. IRAN.
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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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ABSTRACT Non-precious metal electrocatalysts with high activity towards hydrogen evolution reaction (HER) are desirable regarding renewable energy devices such as fuel cells and water electrolysis.
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However, fabrication of new materials for this purpose remains a main challenge. Here, a binder-free nanocomposite, prickly nickel nanostructured/reduced graphene oxide nanosheets, is
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constructed via electroless-deposition on cupper surface covered with a fresh prelayer of nickel
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(Cu-Nifpl-PNiNS/RGONs) for the first time. Then, the fabricated system is tested successfully for the HER in alkaline solutions. Structure and activity of the composite are characterized
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quantitatively by surface techniques and electrochemical methods. The results show that the hedgehog-like prickly nickel nanostructures wrapped in the RGONs cloth are formed, pinning the
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PNiNS/RGONs into the Cu-Nifpl surface, resulting in exceptional stability and activity for the
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Cu-Nifpl-PNiNS/RGONs system. In effect, the composite has shown excellent structural stability
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against disintegration by ultrasound waves; and electrocatalytic activity towards the HER as 20 = 57 mV, Tafel slope = 43 mV dec1 and j0 = 1.05 mA cm2, quite close to 22 mV, 40 mV dec1 and 5.88 mA cm2, obtained in the same conditions for commercial Pt/C,
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respectively. The remarkable increase in electrocatalytic activity was found to be originated partially from increase in the surface roughness and mainly from synergetic chemical coupling effects between PNiNS and RGONs.
Keywords: Binder-free
nanocomposite;
Prickly nickel nanostructured/graphene;
Hydrogen
evolution reaction; Electrochemical impedance spectroscopy; Synergetic effects
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1. Introduction Hydrogen evolution reaction (HER) has been treated as a prototype reaction regarding electrochemical kinetic and catalytic studies, and as a method of H2 production regarding
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decreasing the cost of hydrogen via reduction of the electrode reaction overpotential (η). The Pt or Pt-based materials [1,2], due to their negligible overpotential and excellent kinetics for the
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HER, have been used traditionally, however, the Pt-group metals are scarce and very expensive,
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and thus, may not be appropriate for large-scale production of hydrogen [3]. Therefore, the electrocatalytic HER has been studied extensively on various electrode materials to reduce the
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cost of production and at the same time achieve a low overpotential at a reasonably acceptable cathodic current density [4-10].
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Among the studied electrode materials like Ni, Fe, Co, Mo, or their compounds [6,11-15], nickel
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is well-known as one of the best non-noble catalysts for the HER [6,9,10,15]. Specially,
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nanostructured Ni materials have shown acceptable activity for this propose [16], although their stability is criticized [17]. Furthermore, nickel nanowires [18-20] and prickly nickel nanowires [21] have been examined to increase the effective surface area of the electrodes and
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promote the HER performance. While, the stability of the electrodes has been significantly improved in some cases, moderate activities have been observed [21], compared with the Pt-based electrode materials [1,2].
Graphene; due to its remarkable physicochemical properties [22,23], is highly promising for application in photocatalysts [24,25], pseudocapacitors [26] and batteries [24,27]. Especially, investigations regarding graphene based-nickel-composites [7,28,29] are important. Recently, graphene nanosheets (GNs) thin films, decorated with metal nanoparticles as Au, Ru, or enzymes,
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have been developed by us [8,30] to catalyze electrochemical reactions [30], and improve performance of the HER [8]. For assembling the modifier system onto the solid surface, where interactions of the system
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components with each other as well as with the solid surface is not strong enough to keep them together, usually a binder is applied [8,12,31] which may block the electrode pores and deactivate
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it [32-34], or a template is used which is tedious to construct it. Such necessities, in turn, limit
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applications of the constructed composite. Therefore, binder- [35,36] and template-free [37] nanocomposites, having the same or better stability and activity, are preferred. These
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requirements besides the observed behavior for the nickel nonmaterial [6,9,21] and our experiences with graphene nanosheets towards catalytic redox reactions [8,30] motivated us to
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fabricate and study binder- and template-free prickly nickel nanostructures/reduced graphene
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oxide nanosheets (PNiNS/RGONs) composite for the HER in alkaline solutions here. The
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behavior of Ni-foam/GNs composite for the HER has been investigated theoretically as well as practically, and interesting results have been reported about cooperative action (synergetic effect) between the Ni and RGO [7], however, to the best of our knowledge, there is no previous report
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regarding preparation and electrocatalytic activity of the prickly nickel nanostructures composited with graphene towards the HER. The current design combines interesting aspects of the PNiNS (physical stability and relatively good electrocatalytic activity) [21,38-40] with those of the RGONs (edge activity and excellent conductivity) [8,22,23,30] to fabricate the nanocomposite system on the electrode base, cupper surface covered with a fresh prelayer of nickel (Cu-Nifpl) [6,9], resulting in the Cu-Nifpl-PNiNS/RGONs electrocatalyst. The fabricated system benefits of cooperative action or synergetic effect between nickel and graphene [7,41], and exposes excellent physical stability,
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electrocatalytic activity and durability towards the HER. Details of the experimental works and the obtained data are described below.
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2. Experimental 2.1. Materials.
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Graphene oxide (GO) was synthesized from graphite powder (Sigma-Aldrich, 1-2 m) according
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to the Hummers’ method [42] partially modified by us (see Supporting Information, Section S1, Fig. S1). Potassium permanganate (KMnO4), sulfuric acid (H2SO4 98%), hydrogen peroxide
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solution (H2O2 30%), nickel sulfate (NiSO4,6H2O), nickel chloride (NiCl2,6H2O), sodium
(N2H4,H2O 99%),
commercial
Pt/C
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hydroxide (NaOH), boric acid (H3BO3), trisodium citrate (Na3C6H5O7), hydrazine hydrate (Pt 20%),
Nafion #117 (5% solution
in aliphatic
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alcohol/water (75:25 v/v) mixture) and other chemicals were of analytical grade obtained from
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commercial sources (Sigma® or Merck®), and used without further purification.
2.2. Synthesis of nanocomposites and modifications of the electrode surface.
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A fresh prelayer of nickel (Nifpl) was electrodeposited onto the clean cylindrical cupper (exposed surface area: 0.237 cm2) and labeled as Cu-Nifpl (see Supporting Information, Section S2.1). Several sets of the Cu-Nifpl substrate were prepared and used as platform for electroless synthesis of PNiNS/RGONs composites. Also, several baths were tested (see Supporting Information, Section S2.2 for details), and finally a well attached layer of the PNiNS/RGONs composite with excellent
physical
stability
was
formed
onto
the
Cu-Nifpl
surface
leading
to
Cu-Nifpl-PNiNS/RGONs electrode based on the bath and conditions designed here. A set of Cu-Nifpl-PNiNS/RGONs electrode was prepared.
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Typically, 500 mg trisodium citrate dehydrate and 6 mg graphene oxide (GO) were added into a 20.0 ml of 75 mM NiCl2 aqueous solution, and the solution was stirred for 30 minutes, then, the whole mixture was sonicated for another 30 min to obtain a homogeneous solution (different
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amount of GO as 3, 6 and 9 mg were tested, and the amount 6 mg was found as the optimized quantity). A 1.0 mL of hydrazine hydrate (99%) was mixed with 9.0 mL of distilled water and
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added to the pervious solution. Then, the solution pH was increased to above 12 using NaOH
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solution. The resulting solution was transferred into a 50 mL flask along with shaking followed by slow heating at ~85 oC on a water bath.
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Immediately, the freshly prepared Cu-Nifpl substrate (as fresh as possible) was immersed into the flask solution for electroless-deposition; and the process was followed by shaking (not magnetic
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stirring!) the solution. After a few seconds a gray rough material appeared and grew on the
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Cu-Nifpl surface. The process was completed within ~40 min, and the solution become
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transparent. The fabricated system, labeled as Cu-Nifpl-PNiNS/RGONs electrode, was removed and washed several times with water to drain out excess surfactant or hydrazine, sonicated in the ultrasonic bath, washed with ethanol, dried under vacuum and used for next steps;
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characterization by surface techniques. Another set of Cu-Nifpl substrate was treated in the same way, but in the absence of GO. This electrode was labeled as Cu-Nifpl-PNiNS. An amount of commercial Pt/C was suspended in 5% solution of Nafion #117 to obtain a suspension solution of 2 mg mL1 Pt/C. The solution was coated onto a clean GC (0.0314 cm2) electrode surface, and dried in argon gas stream. Approximately, the amount of 0.32 mg cm2 Pt/C on the GC surface was found as optimized value. This electrode was labeled as GC-Pt/C.
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Accordingly,
we
had
five
sets
of
the
electrodes,
including
(a) Cu,
(b) Cu-Nifpl,
(c) Cu-Nifpl-PNiNS, (d) Cu-Nifpl-PNiNS/RGONs, and (e) GC-Pt/C to characterize and assess their activities towards the HER. It should be mentioned that in the absence of Ni(II) in the bath
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solution, a layer of the RGONs was formed on the Cu-Nifpl surface, but it was unstable, washed out easily by water, so that practically we could not fabricate a stable Cu-Nifpl-RGONs electrode
2.3. Apparatus, measurements and data analysis.
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2.3.1. Surface analysis.
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to assess its activity for the HER.
Field-emission scanning electron microscopic (FESEM) images and Energy Dispersive X-ray
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microanalysis (EDX) were performed by using FESEM, Hitachi S4160, Cold Field Emission,
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Japan. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a VG Microtech
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Twin anode XR3E2 X-ray source and a concentric hemispherical analyzer operated at a base pressure of 5 1010 mbar using Al K (h 1486.6 eV). The XPS high resolution data were deconvoluted and fitted using PeakFit v4.12 software (235 Walnut St.S7, Framingham, MA
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01702, USA), and the fitting results were plotted. The XRD analysis of the samples was performed with a Bruker D8 Advance powder diffractometer using Ni filtered Cu-K radiation, and analyzed by using PANalytical XPert HighScore software. Adherence testing and physical stability of the catalyst onto the surface, homogenizing the solutions, and removing the physically adsorbed species from the electrode surface were performed by using an ultrasonic bath (FLAC, LBS2: 60 KHz, Italy). The shaking process was performed using a shaker (Nuve, ST402, Turkey).
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2.3.2. Electrochemical measurements and data analysis. The
electrochemical
measurements
were
carried
out
on
a
Potentiostat/Galvanostat,
Autolab 302 N equipped with a frequency response analyzer, and controlled by Nova 1.18
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software (Eco Chemie, Utrecht, the Netherlands).
The HER measurements were carried out at 25 ± 2 oC using a three-electrode glass cell assembly
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including a Ag/AgCl (3 M KCl) electrode as reference, a large Pt plate as counter electrode, and
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the Cu substrate modified with synthesized catalyst (Section 2.2) as working electrode. The cell was filled with ~50 mL of purified 0.1 M NaOH as electrolyte solution, degassed with argon
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(99.99%) for 30 min prior to each experiment. Several LSV were recorded at slow potential scan
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rate, 5.0 mV s1, to achieve the steady-state, then, the last one was recorded for kinetic analysis. The kinetic parameters were extracted for each electrode using the related steady-state Tafel plots
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and linear least square (LLS) approximation method [9].
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The EIS measurements were performed at the steady-state in the frequency range of 10 kHz to 100 mHz. The AC amplitude was set to 5 mV superimposed on DC potentials. The DC potentials were selected in the range of the HER overpotential. Details of the electrochemical experimental
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setup and data analysis are given in our previous reports [9,43].
3. Results and discussion
3.1. Construction and physicochemical characterization of the composite 3.1.1. Construction. The Cu-Nifpl-PNiNS/RGONs nanocomposite was constructed according to the Section 2.2. Some important points should be explained in this case.
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(i) Pretreatment of the Cu substrate and use of a fresh prelayer of Ni on Cu are critical to achieve clean and active surface, resulting in considerable improvement in the stability of PNiNS/RGONs nanocomposite onto the Cu-Nifpl surface.
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(ii) The nature of the surfactant is also important, for example, while the PNiNS/RGONs, i.e. PNiNS wrapped in the RGONs cloth, could not be formed in the presence of TritonX-100 on
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Cu-Nifpl substrate; the desirable nanostructures were formed in the presence of citrate ions.
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(iii) The presence of the binder in the electrode modifying film can decrease accessibility of the electrolyte to the electrode surface active sites, which in turn decreases the electrode
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performance. Thus, the binder-free composites is preferred, if the desired properties of the composite are not diminished, compared with that based on the binder [28,36]. The binder-free
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characteristic is one of the most interesting aspects of this catalyst. While no binder is used in the
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texture of the Cu-Nifpl-PNiNS/RGONs composite, both the stability and the electrocatalytic
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activity (Section 3.2, below) are exceptionally improved. Rarely, both characteristics have been achieved in one electrocatalyst.
(iv) The prepared Cu-Nifpl-PNiNS/RGONs electrode was sonicated in ethanol (few seconds) to
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remove physically adsorbed species and obtain reproducible electrochemical response. Furthermore, the PNiNS/RGONs catalyst attached onto the Cu-Nifpl surface showed excellent structural stability against disintegrations that could potentially occur by ultrasonic waves. This is one of the interesting aspects of this work. Finally, the Cu-Nifpl-PNiNS/RGONs system was washed with ethanol, dried under vacuum and used for surface analysis.
3.1.2. Surface analysis and characterization 3.1.2.1. Characterization by FESEM
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Surface morphology and topographical details of the electrodes modified by synthesized materials are characterized by using FESEM [44]. The FESEM images obtained with different magnifications on the nanocomposite, fabricated
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successfully from the electroless bath in this work (Section 2.2), are presented in Fig. 1. While starfish-like structures of Ni with nano-spikes grown perpendicular on their centers are formed on
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the Cu-Nifpl substrate in the absence of GO (Panels A and B), hedgehog-like prickly nickel
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nanostructures wrapped in the RGONs cloth are formed in the presence of GO (Panels C and D).
and form the Cu-Nifpl-PNiNS/RGONs composite.
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The prickles can pin the PNiNS/RGONs composite onto the freshly prepared Cu-Nifpl surface
The diameter of the nano-spikes (Panel B) varies from several tens to several hundred
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nanometers, and tends to grow vertically. This structure imparts excellent stability to the
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PNiNS/RGONs composite when it is brought onto the Cu-Nifpl surface together with GO. The
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diameter of hedgehog-like PNiNS/RGONs varies, also from ~150 to ~200 nm from which the prickles with diameter varies from a few to ~30 nm, are grown out as revealed from FESEM microimages (Panel D).
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Here Figure 1
Composition of the surface was described by EDX; elemental analysis (Fig. S3 and Table S1) and elemental mapping (Fig. S4) [45]. The presence of oxygen on the surface is a normal behavior, comes from RGONs and the residual nickel oxides. This issue is discussed subsequently. Overall, the obtained results support formation of the PNiNS/RGONs composite onto the Cu-Nifpl surface (during fabrication of the electrodes some precipitate was formed at the
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floor of the cell. This material was also collected and characterized; see Supporting Information, Section S4, Fig. S5).
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3.1.2.2. Characterization by XPS
The XPS is a powerful tool to identify all of the elements in periodical table and determine their
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oxidation state, except hydrogen and helium [46,47]. Therefore, the structure of the fabricated
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composite and the presence of different species on the surface was further traced and approved by the XPS measurements (Fig. 2).
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The XPS survey spectrum obtained on the PNiNS/RGONs surface (Fig. 2A) revealed the
supporting formation of a pure system.
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presence of C1s and O1s peaks, and several peaks related to the Ni, without any other species,
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The high resolution XPS spectra, recorded in the range of 282.5 to 291.5 eV for C (1s) on the
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PNiNS/RGONs composite is presented in Fig. 2B. Carbon atoms in different functional groups, having different oxidation state and apparent charge, are appeared clearly in this range. The spectra revealed several peaks around 284.60, 285.76, 286.85, 288.22, and 289.15 eV, which are
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attributed to the carbon atoms of C=C, C−OH, epoxy, C=O and O−C=O groups, respectively. The intensity of the C=C peak is several times larger than sum of those observed for the other carbon atoms, supporting that most of the hydroxyl, epoxide, carbonyl and carboxyl functional groups are reduced (GO is converted to RGONs) [48] by N2H4 simultaneously during electroless reduction of Ni(II) to Ni, leading to formation of PNiNS/RGONs composite onto Cu-Nifpl surface. The high resolution spectrum obtained in the range of 845 to 890 eV for Ni (2p) on PNiNS/RGONs is shown in Fig. 2C. The peaks appeared around 854.23 and 872.4 eV are
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assigned to Ni (2p3/2) and Ni (2p1/2) energy levels, respectively, which are very close to those reported for Ni in literatures [49,50]. The Ni (2p) XPS spectrum is best fitted with three peak of Ni, Ni(II) and Ni(III), and shakeup satellites (indicated as “Sat”) [50]. The results show that the
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chemical composition of the PNiNS/RGONs contains RGONs, metallic Ni and some surface
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oxides.
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Here Figure 2
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3.1.2.3. Characterization by XRD
The XRD patterns obtained on PNiNS and PNiNS/RGONs composite (Fig. 3) indicate that only
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metallic phase of Ni has been formed via electroless-deposition in both structures. The peaks
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appeared around 44.5o, 51.9o, 76.5o and 92.9o are attributed to the Ni(111), Ni(200), Ni(220) and
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Ni(311) crystalline planes, corresponding to the face-centered cubic (fcc) structure of Ni [51]. In addition, a comparison between the XRD patterns of the PNiNS and PNiNS/RGONs composite shows a wide and relatively small peak labeled as RGONs, which is attributed to the presence of
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reduced graphene oxide in the prepared nanocomposite [23].
Here Figure 3
Overall, the results obtained by FESEM, EDX, XPS, and XRD support formation of the Cu-Nifpl-PNiNS/RGONs composite system. The characterized system was used as the cathode electrode for the HER. The obtained data are analyzed and discussed in the following.
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3.2. Electrocatalytic activity of the nanocomposite for the HER. 3.2.1. Linear sweep voltammetry and steady-state polarization curves. Several sets of Cu-Nifpl electrode substrate were prepared and immediately furnished with the
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PNiNS/RGONs composite or its ancestors (Section 2.2), separately, and tested for the electrocatalytic HER.
(a) Cu,
(b) Cu-Nifpl,
(c) Cu-Nifpl-PNiNS,
(d) the Cu-Nifpl-PNiNS/RGONs
and (e) the
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the
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The steady-state linear sweep voltammograms and polarization curves (Tafel plots) obtained on
GC-Pt/C electrodes are displayed in Fig. 4A, curves a to e. The recorded potentials are corrected
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for ohmic drop (iRs) potential and reported vs. reversible hydrogen electrode (RHE) potential. The data indicate that kinetics of the HER are essentially improved upon modification of the
M
Cu-Nifpl electrode by the catalyst layers (Fig. 4A, curves a to e). For example, the values obtained
d
for the onset potentials by using asymptotic lines cross are moved to more facile direction (less
te
negative values) as (a) 760, (b) 500, (c) 160, (d) 30 and (e) 15 mV. Clearly, the value of 30 mV obtained on Cu-Nifpl-PNiNS/RGONs is very close to 15 mV obtained on GC electrode modified with Pt/C commercial catalyst. Similar tendency is observed for other kinetic
Ac ce p
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 61 62 63 64 65
parameters, analyzed and discussed below.
Here Figure 4
3.2.2. Tafel plots Mechanisms and kinetics of the electrocatalyst are better understood by using steady-state Tafel plots. Typical plots obtained on the modified electrodes are displayed in Fig. 4B. The linear part of the plots is fitted into the Tafel equation, = EiRE0= b log(j0) b log(jk) in which E is
13
Page 15 of 39
measured potential, E0 is equilibrium potential for the HER, b is Tafel slope, and j0 and jk are exchange and measured current densities, respectively [52]. The behavior of the Tafel plots at the negative overpotentials (Tafel slop near 40 mV dec1) indicates that the HER proceeds via
ip t
Volmer-Heyrovský mechanism, neglecting the Tafel reaction [53,54]. The kinetic parameters
cr
obtained from the steady-state Tafel curves are presented in Table 1. One may compare the kinetic behavior of the current catalyst, Cu-Nifpl-PNiNS/RGONs, with that of the GC-Pt/C.
us
The Cu-Nifpl-PNiNS/RGONs electrode is characterized by fast kinetics of j0 = 1.05 mA cm2, b = −43.0 mV dec1, and low overpotentials at current densities of 20 mA cm2, η20 = −57 mV,
an
which are quite close to those obtained for GC-Pt/C; j0 = 5.88 mA cm2, b = −40.0 mV dec1, and
M
η20 = 22 mV at the same conditions, still, the physical stability of the current catalyst, Cu-Nifpl-PNiNS/RGONs, was much better than that of Pt/C. Similar comparison may be
d
performed with Cu-Nifpl or Cu-Nifpl-PNiNS modified electrodes. Clearly, the electroactivity of
te
Cu-Nifpl-PNiNS/RGONs electrode for the HER is exceptionally improved compared with its ancestors, Cu, Cu-Nifpl, and Cu-Nifpl-PNiNS (Table 1). A
Ac ce p
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 61 62 63 64 65
comparison
was
also
made
between
the
kinetics
of
the
electroless-deposited
Cu-Nifpl-PNiNS/RGONs and electrodeposited Cu-Nifpl-Ni/GO electrodes (see Supporting Information, Fig. S6, curve f).
Here Table 1
3.2.3. Electrochemical impedance spectroscopy The electrocatalytic activity of the constructed nanocomposites towards the HER was more precisely investigated at the electrode/electrolyte interface by the EIS (Figs. 5, Panels A to E).
14
Page 16 of 39
Here Figure 5
ip t
The EIS complex plane (Zrea vs. Zim) plots were approximated by using the appropriate
cr
equivalent circuit models built in the ZView 2.9 software and the CNLS approximation method. After testing several models related to the HER considering of the fit criteria such as
us
‘‘number of model parameters” and ‘‘Chi square values’’ [43], the modified CPE model was found to be enough for approximations of the EIS data of the investigated electrodes, except for
an
the GC-Pt/C data. The impedance of this electrode was explained by a two-CPE model in which
M
the high frequency (small time-constants) semicircle was attributed to the electrode texture (charge transfer through interface formed between GC electrode and commercial Pt/C film), and
d
the kinetic parameters were extracted from the low frequency semicircle [54,55]. The CPE model
te
element consists of the charge transfer resistance (Rct) in parallel with the impedance of the electrode/solution double layer, and both in series with Rs, where ZCPE = 1/[Q(j)g], Q is a
Ac ce p
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 61 62 63 64 65
capacitive parameter related to the average double layer capacitance (Cdl) of the electrode; Q = Cdlg[1/Rs 1/Rct]1g, j = (1)1/2, is angular frequency, and g is dispersion parameter. The model parameters are extracted in Table S2 (Supporting Information, Section S6). Some important points should be discussed here: (i) Variations of log(1/Rct) vs. displayed a linear behavior, and the related kinetic parameters were extracted [6,9], (Fig. 6A, also Table 1, right side), supporting those obtained by the Tafel plots. The small differences is normal, however, the EIS data are more accurate, since the contribution of nonfaradaic currents are more accurately measured and separated from the total measured currents by this technique [54].
15
Page 17 of 39
(ii) A plot of Cdl values vs. shows that Cdl is almost invariant for Cu and Cu-Nifpl electrodes (Fig. 6B). However, it changes somewhat for GC-Pt/C and Cu-Nifpl-PNiNS electrodes, and significantly for Cu-Nifpl-PNiNS/RGONs electrode. This behavior is due to effect of the H2
ip t
gas small bubbles trapped into the electrode pores, which is a normal behavior for gas evolving
cr
electrodes as suggested previously [6,54]. Still, the process is highly repeatable; suggesting that the active surface area of the catalyst is maintained unchanged at different applied potentials,
us
which is consistent with durability of the catalyst.
(iii) The results indicate that kinetics of the HER is significantly improved upon modification of
an
the Cu-Nifpl substrate with PNiNS/RGONs composite. For example, a comparison between
intrinsic
activity,
is
M
Cu-Nifpl-PNiNS and Cu-Nifpl-PNiNS/RGONs shows that the j0, which is an indicative of the increased
by
a
factor
of
196.3
as
d
{(j0,[Cu-Nifpl-PNiNS/RGONs])/(j0,[Cu-Nifpl-PNiNS])} = 196.3). Alternatively, by comparison between the Cdl
te
values, one will find that the surface roughness is increased partially. These results imply that the increase in the activity of the PNiNS/RGONs composite is originated partially from increase in the surface roughness and mainly from cooperative action or synergetic chemical coupling
Ac ce p
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 61 62 63 64 65
effects between nickel and graphene [7,41,56], which is in good agreement with our previous findings on nickel electrodes modified with carbon supplied from organic source (L-Lysine) [6,9]. This finding is another interesting aspect of the current work. (iv) The absence of high frequency semicircle from the EIS data (Fig. 5, compare Panels D and E) means that the electron transfer rate is independent of the electrode texture (material film) which in turn is explained based on excellent connection between Cu-Nifpl electrode base and the PNiNS/RGONs film as well as between PNiNS and RGONs.
16
Page 18 of 39
Here Figure 6
The EIS results support those extracted from the Tafel plots; indicating excellent electrocatalytic
ip t
activities for Cu-Nifpl-PNiNS/RGONs electrode, quite close to that of GC-Pt/C electrode and incomparable with its ancestors, Cu, Cu-Nifpl, and Cu-Nifpl-PNiNS (please note that the
cr
Cu-Nifpl-RGONs was not physically stable, Section 2.2, so no kinetics could be obtained for this
us
electrode).
(v) The last point in this section is that the increased activities of Cu-Nifpl-PNiNS/RGONs
an
electrode can be explained based on cooperative action [7] or so called synergetic effect [41] between PNiNS and RGONs. According to the experimental results and the thermodynamic
M
calculations which rely on density functional theory (DFT), it can be concluded that the
d
H-adsorbed intermediately formed on Ni surface spillover onto the RGONs, where the
te
H-adsorbeds recombine quickly and form H2 [7]. These effects can result in a clear PNiNS surface, thus, enhance the kinetics of the HER. The LSV, Tafel plots, and EIS results obtained in the current study suggest a Volmer-Heyrovský route with less contribution of Tafel step for the
Ac ce p
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 61 62 63 64 65
HER. It means that the recombination of H-adsorbeds [7] is also fast here and does not influence on the HER kinetics. Thus, the results obtained here on PNiNS/RGONs, regarding the role of RGONs, are in good agreement with those reported on Ni foam/RGO by Chanda et al. [7].
3.3. Durability and stability The durability and performance of the prepared catalyst was investigated by following the response of the Cu-Nifpl-PNiNS/RGONs electrode to the HER in different ways (see Supporting Information, Section S7, Figs. S7 and S8). In addition, the electrode was subjected to HER at a
17
Page 19 of 39
large current density and high temperature, j = 300 mA cm2 and ~60 oC, in 3 M KOH; and the overpotential was followed. The results show less than 2% change in over a period of ~60
ip t
hours (Fig. 7), indicating a good durability and stability for the electrode.
cr
Here Figure 7
us
Finally, the FESEM obtained on the Cu-Nifpl-PNiNS/RGONs electrode surface before and after long time HER showed no significant changes in the surface morphology (see Supporting
an
Information, Fig. S9). Overall, the results of this section show that the electrode benefits of
M
excellent durability and storage stability towards the HER.
This interesting durability can be related to the pinning of the PNiNS/RGONs composite into the
d
Cu-Nifpl surface via strong bond formed between the freshly and in-situ electrolessly prepared
te
nickel prickles (the hedgehog-like PNiNS/RGONs composite) and the freshly and ex-situ electrochemically prepared active Ni (Nifpl) on the surface of Cu-Nifpl substrate.
Ac ce p
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 61 62 63 64 65
4. Conclusions
An efficient composite catalyst, binder-free prickly nickel nanostructured/reduced graphene oxide, Cu-Nifpl-PNiNS/RGONs, was synthesized for the first time, and tested successfully for electrocatalytic HER in alkaline solutions. Surface analysis of the nanocomposite through FESEM, EDX, XPS and XRD methods show that the prickly nickel nanostructures wrapped in a cloth of the reduced graphene oxides are fabricated and pinned into the Cu-Nifpl surface via nickel prickles, resulting in hedgehog-like structures for the surface of the Cu-Nifpl-PNiNS/RGONs composite. The nanocomposite showed
18
Page 20 of 39
exceptional structural stability against disintegrations that could potentially occur by ultrasonic waves. Quantitative kinetic analysis of the information, obtained for the HER by electrochemical
ip t
methods, revealed excellent electrocatalytic activities; j0 = 1.05 mA cm2, b = −43.0 mV dec1 and η20 = −57.5 mV for the Cu-Nifpl-PNiNS/RGONs composite electrode, quite close to those
cr
obtained for the commercial GC-Pt/C electrode; j0 = 5.88 mA cm2, b = −40.0 mV dec1 and
us
η20 = 22 mV at the same conditions.
an
Precise analysis of the EIS data of Cu-Nifpl-PNiNS/RGONs in comparison with Cu-Nifpl-PNiNS shows that the increased activity (j0) of the Cu-Nifpl-PNiNS/RGONs composite is originated
M
partially from increase in the surface roughness and mainly from cooperative action (synergetic chemical coupling effects) between nickel and graphene.
d
The present study provides new insights into the design and development of nanocomposites for
especially for the HER.
te
electrocatalytic reactions at the electrode/solution interface, energy storage and production,
Ac ce p
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 61 62 63 64 65
Acknowledgment
The authors gratefully acknowledge the University of Isfahan (UI) for financial support and providing research facilities. The Iran National Science Foundation, Vise-Presidency for Science and Technology (INSF/VPST) is acknowledged for supporting and the use of high technology facilities and services in the Iran High-Tech Laboratory Network (IH-TLN).
Supporting Information
19
Page 21 of 39
The XRD data of the graphite and graphene structure, FESEM micrographs displaying the role of Teriton-X100 as surfactant, the EDX spectrum, the TEM images, durability tests, FESEM micrographs of the Cu-Nifpl-PNiNS/RGONs catalyst before and after HER, elemental mapping
ip t
images, and activities of graphene/nickel electrodes prepared via electrodeposition method all are
te
d
M
an
us
cr
given in Supporting Information file.
Ac ce p
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 61 62 63 64 65
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Adsorption, Evolution and Absorption into Metals, Springer, 2002, pp. 1-49. [55] H. Vrubel, T. Moehl, M. Grätzel, X. Hu, Revealing and accelerating slow electron transport in amorphous molybdenum sulphide particles for hydrogen evolution reaction, Chem. Commun. 49 (2013) 8985-8987. [56] R. Mu, Q. Fu, H. Xu, H. Zhang, Y. Huang, Z. Jiang, S. Zhang, D. Tan, X. Bao, Synergetic Effect of Surface and Subsurface Ni Species at Pt-Ni Bimetallic Catalysts for CO Oxidation, J. Am. Chem. Soc. 133 (2011) 1978-1986.
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ip t cr
Tables
Tafela b/ mV dec1 300 190 140 43 40
(a) Cu (b) Cu-Nifpl (c) Cu-Nifpl-PNiNS (d) Cu-Nifpl-PNiNS/RGONs (e) GC-Pt/C
j0/ A cm2 5.20 106 1.15 105 7.58 105 1.05 103 5.88 103
0.3 0.4 0.5 0.7 0.6
-20/ mV 1100 750 365 57.5 22
b/ mV dec1 250 182 131 48 45
an
Electrode
us
Table 1. Kinetic parameters obtained from steady-states polarization curves (Tafel plots) and the EIS measurements for the HER in 0.1 NaOH at 298 K. EISb
j0/ A cm2 2.93 107 1.24 106 4.05 106 7.95 104 1.95 103
Cdl,/ mF cm-2 0.036 0.040 1.5585 13.707 0.490
k0app/ cm s1 7.60 109 3.22 108 1.05 107 2.06 105 5.06 105
: Extracted from Fig. 4, b: Extracted from Fig. 6, = EiRE0= b log(j0) b log(jk), Q = Cdlg [Rs1 Rct1]1g, k0app = RT(n2F2RctAC)1, A = 0.237 cm2, C = 1.0 104 mol cm3. j0 is apparent current density [j0 = i0/0.237 cm2].
te
d
M
a
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Captions of Figurers Fig. 1.
The
FESEM
images
obtained
on
the
(A and B) PNiNS
and
(C and D) PNiNS/RGONs composite with two different magnifications.
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Page 30 of 39
Fig. 2.
(A) The survey XPS spectrum, and the high resolution spectrum (B) of C1s and
Fig. 3.
ip t
(C) of Ni2p obtained on PNiNS/RGONs composite.
The XRD patterns obtained on the PNiNS (dotted line) and PNiNS/RGONs (solid
us
Fig. 4.
cr
line) materials.
(A) Steady-state linear sweep voltammograms obtained in 0.1 M NaOH solution at
an
room temperature and potential scan rate of 5 mVs1 on (a) Cu, (b) Cu-Nifpl, (c) Cu-Nifpl-PNiNS, (d) Cu-Nifpl-PNiNS/RGONs, and (e) GC-Pt/C electrodes. (B) The steady-state polarization curves (Tafel plots) extracted from the data of
M
Panel (A). Symbols indicate experimental data and solid lines the fitted results
The EIS complex plane plots obtained for the HER on (A) Cu, (B) Cu-Nifpl,
te
Fig. 5.
d
obtained by using linear least square approximation (LLS) method.
(C) Cu-Nifpl-PNiNS, (D) Cu-Nifpl-PNiNS/RGONs, and (E) commercial GC-Pt/C
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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 61 62 63 64 65
electrodes at various overpotentials in 0.1 M NaOH. Symbols indicate experimental (Zrea vs. -Zim) data and solid lines the fitted results obtained by approximation of the experimental data into appropriate model (one-CPE for A, B, C, and D; and two-CPE
for E data)
based
on
complex
nonlinear
(CNLS) approximation method and ZView software. Minus
least
square
applied DC
overpotentials () is shown by arrows.
Fig. 6.
(A) Variations of inverse of the charge transfer resistance (1/Rct) as a function of overpotential () extracted
from
Fig. 5
for (a) Cu,
(b) Cu-Nifpl,
(c) Cu-Nifpl-PNiNS, (d) Cu-Nifpl-PNiNS/RGONs, and (e) GC-Pt/C electrodes.
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Symbols indicate the semi-experimental data extracted from Fig. 5 by CNLS method and solid lines the approximated results obtained by LLS method.
d
M
an
us
over a period of ~ 60 hours HER in 3 M KOH at 60 0C.
cr
Durability of the Cu-Nifpl-PNiNS/RGONs electrode obtained at j = 300 mA cm2
te
Fig. 7.
ip t
(B) Variations of Cdl as a function of η.
Ac ce p
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te
d
M
an
us
cr
ip t
Figures
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Figure 1
31
Page 33 of 39
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te
d
M
an
us
cr
ip t
Figure 2
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Figure 3
33
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Figure 4
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Figure 5
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ip t cr us an M d te
Figures 6A and B
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Page 38 of 39
ip t cr us an M d te Ac ce p
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Figure 7
37
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