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Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution
Journal Pre-proof Pulse electrodeposition of nickel selenide nanostructure as a binder-free and highefficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution S. Esmailzadeh, T. Shahrabi, Gh Barati Darband, Y. Yaghoubinezhad PII:
S0013-4686(19)32421-1
DOI:
https://doi.org/10.1016/j.electacta.2019.135549
Reference:
EA 135549
To appear in:
Electrochimica Acta
Received Date: 17 September 2019 Revised Date:
24 November 2019
Accepted Date: 17 December 2019
Please cite this article as: S. Esmailzadeh, T. Shahrabi, G.B. Darband, Y. Yaghoubinezhad, Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135549. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution S. Esmailzadeh a, T. Shahrabia, *, Gh. Barati Darband a, Y. Yaghoubinezhad b a
Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran
b
Department of Materials Engineering, Birjand University of Technology, P.O. Box: 97175569314, Birjand, Iran *Email: [email protected] ( T. Shahrabi)
Abstract Herein, we report a pulse potential electrodeposition method (PPE) as a fast and one-step route for the production of effective and binder-free Ni-Se catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution. The results demonstrated that the fabricated electrocatalysts by PPE show more desirable electrocatalytic performance than constant potential electrodeposition method (CPE). The electrodeposited NiSe nanostructures with large electrochemically active surface area of 2965 cm2 on the nickel foam (NF) at the frequency of 0.01 Hz exhibited high electrocatalytic activity for HER and OER. It delivers the current density of 10 mA/cm2 at the overpotential of 65 mV for HER and 100 mA/cm2 at 306 mV for OER in 1 M KOH. In addition, the Tafel slope for HER and OER was 89 and 61 mV/dec respectively. Morphological stability of NiSe nanostructure catalyst leads to an increase the electrocatalytic stability during long-term electrolysis. Keywords:
1
Pulse
potential
electrodeposition,
NiSe
nanostructure,
Hydrogen
evolution
reaction,
electrocatalytic activity. 1. Introduction In recent years limited resources of fossil fuels followed by the fast growth of human society and environmental issues such as global warming have led to the development of renewable energy productions. Hydrogen is a clean, secure and carbon-free energy that can be replaced instead of fossil fuels. The other merit of using this fuel is the renewable property so water is the only product of combustion of hydrogen [1, 2]. Steams reforming of fossil fuels, gasification of coal and petroleum coke and water electrolysis are examples of different methods for the production of H2. However, water-splitting technology is one of the most efficient methods for production of hydrogen gas which is produced from aqueous solutions. During the water electrolysis in water splitting process, hydrogen (HER) and oxygen (OER) evolution reactions are being produced at cathode and anode respectively [2, 3]. Although electrochemical water splitting is an easy and low-cost way for hydrogen production but the required overpotential in practice, is much higher than the theoretical amount of voltage calculated from the thermodynamic relationships (1.23 V vs. RHE) [2, 4]. In order to reduce the water splitting over-potential, the water electrolysis system requires materials called electro-catalysts which facilitate electron transfer for HER and OER. In spite of good performance of precious metals based catalysts such as Pt group metals [5] for HER and IrO2 [6] and RuO2 [7] for OER, but due to their limited availability and high price, the widespread use of them has been prohibited [8]. Therefore the scientists try to synthesis the non-precious catalysts as a low cost, earth-rich and high-efficient in water electrolysis [9-11]. Some examples of non-noble catalysts include non-oxide catalysts such as transition metal selenides [12], phosphides [13], carbides [14], and sulfides [15] for HER and 2
transition metals oxides, hydroxides and oxyhydroxide for OER [2, 16]. Using bifunctional electrocatalyst for both HER and OER in the water splitting can play a key role in simplifying the electrolysis system and lowering the costs. Accordingly, it will be highly attractive to make bifunctional electrocatalysts for water splitting in alkaline media [12, 17]. Based on the recent reports, some of the electrodes that were used as bifunctional electrocatalysts are such as iron (Fe)-doped nickel phosphide (Ni2P) nanosheets [18], Hierarchical NiCoP nanocone arrays [19], phase-pure pentlandite Ni4.3Co4.7S8 [20]. Among different non-noble metals, selenide element has distinct characters such as high metallic property, large relative radius and small ionization energy which might cause catalysts formed by metallic selenides to possess unique intrinsic catalytic activity. Therefore, nickel selenide with different stoichiometry could be selected as a bifunctional electrocatalyst that has been extensively used in recent few years [21-24]. Compared with the different approaches for the synthesis of non-noble metal catalysts, electrodeposition is preferred for its simplicity, low cost, and non-using any conductive agent or binder between catalyst materials and substrate [25-27]. Zhu et al. [28] investigated the effect of deposition potential on the phases of nanostructured nickel selenide electrodeposited on the Ni foam and their electrocatalytic activities for HER and OER. They reported that the pure NiSe2, NiSe, and Ni3Se2 were obtained at the electrodeposition potentials of −0.35, −0.46, and −0.60 V (vs. RHE) respectively. Gao et al. [29] electrodeposited NiSe on the Ni foam and studied electrocatalytic activities in both water reduction and oxidation. They showed in a two-electrode water electrolyzer with NiSe as a bifunctional electrocatalyst for anode and cathode, a high water splitting current density of 20 mA/cm2 is achieved at cell voltage around 1.5 V. Recently, pulse plating as a type of electrodeposition method through the presence of many parameters for
3
optimization offers superior control on the structure (micro or nano), morphology and the thickness of film [30-33]. To the best of our knowledge, pulsed electrodeposition of nickel selenide as an electroactive coating for HER and OER has not been considered in the literature. In the present work, pulse potential electrodeposition (PPE) was used for the fabrication of nickel selenide nanosheets as an electrocatalytic platform for HER and OER application on the nickel foam (NF). Moreover, for comparison, the electrocatalyst of nickel selenide was fabricated by conventional constant potential electrodeposition (CPE).
The characterization of products and investigation of
electrocatalytic activity were performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), linear sweep voltammetry (LSV), Tafel polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronopotentiometry in 1 M KOH.
2. Experimental 2.1 Materials The materials including nickel chloride (NiCl2.6H2O), selenide dioxide (SeO2), lithium chloride (LiCl), hydrochloric acid (HCl), potassium hydroxide (KOH) and nickel foam were bought from Merck Company and used for this study without further purification. 2.2 Electrodeposition of nickel selenide electrodes At first, nickel foams as substrate were cut in the square shape with a surface area of 1 cm2, and then they were cleaned ultrasonically in 1M HCl, ethanol and water for 15, 2 and 2 min, respectively. Electroplating of nickel selenide electrode was performed by PPE and CPE modes by potentiostat (Autolab potentiostat/galvanostat 302 N device). Electrodeposition bath consisted of 65 mM NiCl2.6H2O and 35 mM SeO2 as ion sources and 20 mM LiCl as a conductive agent of 4
the electrolyte which were dissolved in deionized water. The electrodeposition was carried out at room temperature and pH 3.5 (adjusted by NaOH). During the PPE and CPE, the Pt, saturated calomel electrode (SCE) and NF were used as a counter, reference, and working electrodes, respectively. In PPE, a square wave potential with the range of 0 to -0.8 V (vs. SCE) was applied at frequencies of 0.01, 0.1, 1, and 10 Hz that are denoted in this article as S-0.01, S-0.1, S-1, and S-10, respectively. The schematic E-t curves of PPE and CPE technique are presented in Fig S1. The time of electrodeposition for PPE at different frequency was constant 900s (sums of tons) with a duty cycle of 0.5. For comparison, nickel selenide coating was electrodeposited by CPE at a constant potential of -0.8 V (vs. SCE) for 900s. The operating conditions in PPE and CPE of nickel selenide films are given in Table 1. In both PPE and CPE, All samples after electrodeposition were rinsed with deionized water and dried in the oven at 60 °C for 4 h. 2.3 Microstructural characterization Surface morphology and chemical composition of the electrodes was studied using SEM equipped by energy-dispersive X-ray spectroscopy (EDS) detector (TESCAN VEGA3). In order to study the phase formation, the XRD spectrum was collected by Philips X'Pert Diffractometer (using Cu-Kα at an accelerating voltage of 40 kV and a current of 40mA). 2.4 Electrochemical characterization Electrochemical tests were carried out by potentiostat (Autolab potentiostat/galvanostat 302 N device) in 1 M KOH using a three-electrode setup. LSV technique was done for obtaining polarization curves for hydrogen evolution and oxygen evolution with a scan rate of 5 mV/s without rotation of the working electrode or creation of turbulence in the electrolyte at room temperature. EIS measurements were performed at overpotentials of 100, 200 and 300 mV (VS. RHE) from an initial frequency of 100 kHz to a final frequency of 10 mHz, with the amplitude of 5
10
mV.
The
electrocatalytic
stability
of
electrodes
was
evaluated
by
CV
and
chronopotentiometry. The CV scanning was done for all of the specimens in the potential range of 0 to 350 mV (vs. RHE) with a scan rate of 100 mV/s for 1000 cycles. The chronopotentiometry test was carried out at 50 mA/cm2 for 20 h. Moreover, measured potentials vs. SCE are thoroughly converted to the reversible hydrogen electrode (RHE) using the Nernst equation [26] (Eq. 1). E (vs. RHE) = E (vs. SCE) + ESCE (0.242 V) + 0.0591 pH
(1)
Besides, for correcting the iR drop in the solution, the value of iRs (Rs was gained from EIS test) was subtracted from all potentials. 3. Results and discussion 3.1 Structural and morphological characterization After the electrodeposition, the colure of nickel foam transformed from silvery to dark black for all frequencies which indicating the deposition of coating on the surface of NF. XRD patterns of the samples resulting from PPE are shown in Fig 1. For all diffraction patterns, strong diffractions at around 44.60 and 51.93 correspond to NF that arising from the underlying substrate. Diffraction patterns of all samples exhibited the standard pattern of NiSe. Three main peaks at around 33.59, 44.60 and 51 belong to the (101), (102) and (110) planes that are wellmatched with NiSe (03-065-6014) [28, 34]. Therefore the formation of NiSe coating on the NF surface for the samples at the different frequencies was confirmed. The SEM images of samples with high and low magnification along accompanied with their EDS analysis are illustrated in parts of a, b, c, d and e of Fig 2. To better illustrate the effect of pulse potential on the structure of the catalyst, Fig 2.a shows the SEM images of the electrode surface obtained at the direct voltage of -0.8 V (vs. SCE). As clearly seen, the application of
6
pulse potential has led to a significant change in the structure especially at low frequencies. Applying a constant voltage for the electrodeposition caused that the nickel selenide particles nucleate on the surface and gradually grow lateral and vertically with the increase in the deposition time leading to the formation of a dense structure with an almost smooth surface morphology which is not favorable for improving the catalytic activity [31]. While in the PPE condition especially for samples of S-0.01 (Fig 2.b) and S-0.1 (Fig 2.c) the surface morphology of NiSe catalyst is completely different. As can be observed in Fig 2.b the catalyst which was produced at the frequency of 0.01 Hz has two different structures (a dual structure) [35] [30]. It consists of particles in different sizes (micro and nano) that have been covered by nanosheets. The thickness of the nanosheets is in the range of 5-20 nm. Fig 2.c shows the structure of NiSe catalyst that has been produced at 0.1 Hz. It is composed of nanoparticles with a diameter of less than 100 nm. By increasing the frequency of pulse to 1, as seen in Fig 2.d the same structures can be observed like S-0.01. The difference is that the height of sheets covering the surface decreased and also the density of nanoparticles became less because the ton and toff are not enough to growth of nucleation and recover the surface concentration, so the surface area is greatly decreased [30]. Increasing the pulse frequency to 10, led to the production of NiSe catalyst with an approximate smooth surface and dispersed particles in the whole surface. Cracks appeared on the surface is likely due to the high thickness followed by stress accumulated in the film (Fig 2.e) [31]. At the frequency of 10 Hz in which the time of each pulse is lower than the other frequencies, two-dimensional nucleation (2D) is occurred but there is probably not enough time in each potential pulse for three dimensional nucleation (3D). Therefore, during the electrodeposition at 10 Hz, nuclei of NiSe nucleate on the surface of substrate at the beginning of process and formed NiSe layers and grow by lateral spreading of discrete layers and finally a
7
catalyst with an approximate smooth surface is produced [36, 37]. The distinct morphology of the samples produced in the PPE conditions compared with the CPE is attributed to periodic applying cathodic potential. In the method of direct pulse electrodeposition, during the toff, diffusion layer which has been depleted of reactants in ton, is recovered by immigration of ions in the electrolyte to the near the surface of electrode. By applying the cathodic potential each time for an exact duration (ton), new nucleation occurs on the film surface and restricts the growth of before nucleations [27, 38, 39]. Therefore as be expected the samples of S-0.01, which produced at pulse conditions with enough time for the 2D and 3D nucleation and growth of new nucleation displayed the dual structure with high surface area and electrocatalytic active sites [31, 36]. According to the EDS analysis in the part f of Fig. 2 it can be seen that the composition of sample of S-0.01 consists of Ni and Se as the precursor elements of catalyst. according to this analysis, the reason of high oxygen percentage of the sample can be related to the drastically reduction of H+ during the ton near the electrode surface and formation of hydroxide species as a result of increasing the local pH [30]. The analysis of the mapping element for S-0.01 is illustrated in Fig. 2f. it depicts the uniform distribution of elements Ni and Se over the produced film which indicates the contribution of these elements in the formation of electrode film. 3.2 Hydrogen evolution activity Electrocatalytic performance of the PPE samples for HER was studied using LSV measurement in 1 M KOH. Fig. 3a shows the LSV curves of different samples and also nickel foam for comparison. The overpotential required for generating the current density of 10, 20 and 100 mA/cm2 was selected as a criterion for evaluating the electrocatalytic activity for HER. According to the LSVs, it is clear that all prepared samples by PPE have better electrocatalytic activity than the NF and even the sample produced by CPE. Among the NiSe electrocatalysts 8
prepared at different frequencies, the over-potential required at a given current density for S-0.01 is the lowest. This sample affords 10, 20 and 100 mA/cm2 for HER at the overpotentials of 65, 87 and 138 mV, respectively. The indeed amount of 138 mV overpotential for generating the current density of 100 mA/cm2 depicts that S-0.01 has a much better electrocatalytic performance in comparison with Ni-Se electrocatalysts produced by constant potential electrodeposition and also by different methods (Table 2). The overpotentials required for all samples and nickel foam to attain the current density of 10, 20 and 100 mA/cm2 are presented in Fig. 3c. A comparison of the different values of over-potentials at a constant current density demonstrates that by decreasing the pulse frequency, the overpotential required for receiving a current density is reduced. Therefore, it means that the electrocatalysts produced at the lower frequency show better catalytic behavior than the higher pulse frequency. According to the SEM images (Fig. 2), it can be attributed to the type of morphology and consequently increase the surface area and formation of many electrocatalytic active sites [26]. Tafel slope is a standard approach for explaining the kinetics and mechanism of hydrogen or oxygen evolution. It is calculated through the LSV curves and fitting their linear parts by the Eq. 2 (Tafel equation) [47]: η = b log (j) + a
(2)
Where b is Tafel slope, j is the current density, η is the overpotential and a is a parameter related to the exchange current density with the name of cathodic intercept. According to the Tafel equation, we can say the lower the overpotential, the smaller the slope and consequently more kinetic for hydrogen evolution and generation of the same current density. Fig. 3b exhibited that the Tafel slopes obtained using LSVs for S-0.01, S-0.1, S-1, and S-10 are 89, 91, 116 and 119 mV/dec respectively which is lower than the CPE sample (139 mV/dec). Also the S-0.01 has the 9
lowest Tafel slope among the others. It displays that the rate of the hydrogen production is fast on the surface of S-0.01 catalyst and the overall kinetic of HER reaction is controlled by the Volmer mechanism [41]. The electrochemically active surface area (ECSA) and the turnover frequency (TOF) are two parameters that have a direct relationship with electrocatalytic activity. Therefore, in order to explain in detail, the better electrocatalytic activity of S-0.01, the parameters of ECSA and TOF for all PPE samples were investigated. For calculation the ECSA for each catalyst, the CV curves were taken in a Non-Faradaic regime at various scan rates (Fig. 4). After estimation of charging current (ic), the double-layer capacitance (Cdl) was calculated on the basis of Eq. 3 (Fig. 4e), ic = νCdl
(3)
The measured values of Cdl for each electrode were converted to the ECSA after dividing the Cdl by 20 µF as a specific capacitance for a uniform atomic surface [2, 3]. The double-layer capacitance along with the calculated ECSA values for PPE samples are shown in Table 3. It can be observed that the values of Cdl and obtained ECSA have very good agreement with the SEM surface morphology (Fig. 2). As we expected for S-0.01 with the dual structure which provides a high surface area with plentiful electrocatalytic active sites, the values of Cdl and ECSA is 59.3 mF/cm2 and 2965 respectively which is much more than the others. As stated, another important parameter that is in principle an exact measurement of inherent electrocatalytic activity is TOF. It represents the rate of hydrogen mole evolution per unit time from the surface of catalyst. Thus there are different methods for calculation of TOF on the basis of the method used for determining the number of active surface sites [48]. Here in TOF was calculated for all PPE samples according to the method presented by Jaramillo et al. [38, 49] and the results are indicated in Fig. 4f. TOF curves show that the S-0.01 and S-0.1 have higher intrinsic
10
electrocatalytic activity compared with the S-1 and S-10. It confirms the better electrocatalytic activity of catalysts obtained at low frequencies. The superior electrocatalytic performance of S0.01than the electrodes produced by different methods with the same active surface area can be attributed to the non-using of the binder in the coating. Binder or conductive agent is usually used in the most synthesis methods of the catalysts which creates a dead zone between the catalyst and the substrate. It may cause blocking the active sites and increasing ohmic resistance and finally decrease the electrocatalytic activity while in the electrodeposition method, the binder is not required and the coating is deposited on the substrate directly and without intermediary [4, 50]. One of the effective factors on the electrocatalytic activity is the creation of resistance to the ionic and charge transport in the electrolyte during the electrochemical measurement. It can be as a barrier during the water electrolysis. EIS is a technique for analyzing the interface properties of the electrode and kinetics of HER on the surface of the catalysts [11]. To know the interface behavior of the catalysts and the kinetic of HER, the EIS test was done and the Nyquist plots in overpotentials of 100, 200 and 300 mV vs. RHE were recorded and shown in Fig. 5. As can be observed, for all samples the Nyquist plots consist of two overlapped semicircles. Therefore an equivalent circuit with two-time constants can be discerned within the studied frequency range [41]. The zoom-in images in Fig. 5 and the results of Table 4 show that the size of the second semicircle at lower frequencies is changed in different over-potentials while the size of first semicircle is almost constant. Therefore it can be concluded that the second semicircle is probably related to the kinetics of hydrogen production that is increased by increasing the overpotentials [3, 50, 51]. According to the electrical equivalent circuit presented in Fig. 5e Rs is the solution resistance, R1 and CPE1 are the charge transfer resistance and constant phase element
11
related to the porosity of electrode’s surface respectively and R2 and CPE2 are resistance and constant phase element related to the kinetic of the reduction reaction of hydrogen on the surface of the electrode [3]. Table 4 shows that the values of R2 for S-0.01 in three over-potentials of 100, 200 and 300 mV are 1.58, 0.73 and 0.5 Ω. Cm2, respectively. Comparatively they are the lowest in comparison of the other PPE samples and even lower than other reported electrocatalysts. So S-0.01 is a more favorable catalyst with superior electrical conductance for overcoming the HER reaction barrier during the water splitting. Electrocatalytic stability is a major factor for practical application of an electrocatalyst in strongly alkaline solutions. It is necessary for a catalyst with excellent catalytic activity to have high electrocatalytic stability too. There are different methods for investigating the electrocatalytic stability [12]. The electrocatalytic stability of S-0.01, using the CV scanning and also at the constant current density of 50 mA/cm2 by chronopotentiometry was evaluated. Fig. 6a shows the LSV curve of asprepared S-0.01and after 1000 cycles CV scanning. As seen the LSV curves before and after 1000 cycles are completely coincident which indicates the very high electrocatalytic stability of S-0.01 [45]. According to Fig. 6b, the required over-potential of S-0.01 for generating of the current density of 50 mA/cm2 is about 1.45 (V vs.RHE). This overpotential remains almost constant over the duration of 20 h. During the electrolysis, the corrosion of electrolyte and the detachment of gas bubbles from the catalyst surface can damage the surface of the catalyst. Therefore morphological stability is an important factor for proving the electrocatalytic activity [26]. Comparison of the SEM images of S-0.01 at the beginning and the end of the chronopotentiometry which inserted in Fig. 6b shows that the morphology and topology of catalyst have been unchanged during the water electrolysis. It depicts the high long term electrocatalytic stability of S-0.01 during the water splitting for 20 h. The similarity of the
12
chemical composition of S-0.01 before (Fig. 2b) and after chronopotentiometry test (Fig. 6c) confirms the high stability of S-0.01. Using the electrochemical deposition and direct deposition of the catalyst on the substrate is the main reason for high adhesion of NiSe catalyst to the NF substrate and ultimately its high stability during the electrolysis. 3.3. Oxygen evolution activity In order to simplify the system of overall water splitting and reduce costs, it is highly attractive that the catalyst to have the desirable electrocatalytic activity for HER as well as good electrocatalytic properties for OER [52]. In accordance with the results reported in recent years, the selenide based catalysts have shown that they can be promising candidates for both HER and OER [11]. Therefore, the electrocatalytic activity of PPE samples for OER was studied in 1 M KOH. Like before, at first, the catalytic performance of samples using LSV curves (Fig. 7a) was investigated. In the OER LSVs curves of Ni-Se films, there is an obvious oxidation peak. For example for S-0.01, the intense of oxidation peak is at 1.44 V (vs. RHE) before the onset of the oxygen evolution. It is attributed to the oxidation of Ni (II) to Ni (III) on the surface of the electrode, according to the Eq. 4. + → +
(4)
The obtained Ni (III) species in the form of NiSe(OH) act as the active sites on the surface of the electrode for OER [34, 53]. The over-potentials required for achieving the current densities of 100, 150 and 200 mA/cm2 for all samples were compared together (Table 5). The magnitude of overpotential for attaining the current density of 100 mA/cm2 using the LSV curves obtained by scanning potential from high to the low potential that are shown in Fig. S2 is 306, 328, 344 and 342 mV for S-0.01, S-0.1, S-1, and S-10, respectively. The results illustrate that the samples produced by the pulse electrodeposition, in addition to having the high electrocatalytic activity
13
for HER compared to CPE sample, they also show good catalytic performance for OER. Also among the PPE samples, S-0.01 showed the more desirable catalytic activity than the others for OER like HER especially for generating of high current density. The main reason of this behavior can be attributed of applying the voltage periodically and having enough time in each period for the growth of new nucleation’s and thus increase the catalytic active surface area and creation of the plentiful sites for the reaction of anions and cations [30]. The OER kinetics of PPE samples was evaluated by corresponding Tafel curves that shown in Fig. 7b The smaller Tafel slope of S-0.01 (61 mV/dec) implies the more OER kinetic than the S-0.1 (87 mV/dec), S1 (93 mV/dec) and S-10 (125 mV/dec). Fig. 7c shows the LSV curve of S-0.01 before and after CV scanning. Negligible change of LSV curve after 500 cycles CV implying the high electrocatalytic stability of S-0.01 for OER [34]. 4. Conclusion In summary, the NiSe nanostructures were successfully prepared using pulse potential electrodeposition on the NF substrate at room temperature by controlling the pulse frequency. The electrocatalytic activity of produced electrodes with different structures was investigated for HER and OER in alkaline solution. The NiSe electrode with dual structure was produced at 0.01 Hz and showed excellent electrocatalytic activity for HER and OER requiring 65 and 285 mV for attaining the current density of 10 and 100 mA/cm2. The results showed that increasing the pulse frequency cause creating an electrode with lower surface area and followed by decreasing the electrocatalytic performance. The superior electrocatalytic activity and stability of S-0.01than the Ni-Se catalysts produced by the constant potential deposition and other synthesis methods attributes to the 3D dual structure, large active surface area, binder-free and high adhesion of coating to the substrate. The main reasons for creating an electrode by the pulse
14
electrodeposition method, in comparison with others are small time of electrodeposition, controlling the structure of the electrode by alternating the variables such as pulse frequency, simplifying and low cost. References [1] D. Zhang, J. Li, J. Luo, P. Xu, L. Wei, D. Zhou, W. Xu, D. Yuan, Ni3S2 nanowires grown on nickel foam as an efficient bifunctional electrocatalyst for water splitting with greatly practical prospects, Nanotechnology, 29 (2018) 245402. [2] S. Anantharaj, S.R. Ede, K. Sakthikumar, K. Karthick, S. Mishra, S. Kundu, Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: a review, ACS Catalysis, 6 (2016) 8069-8097. [3] G.B. Darband, M. Aliofkhazraei, A.S. Rouhaghdam, M. Kiani, Three-dimensional Ni-Co alloy hierarchical nanostructure as efficient non-noble-metal electrocatalyst for hydrogen evolution reaction, Applied Surface Science, 465 (2019) 846-862. [4] C. Xia, Q. Jiang, C. Zhao, M.N. Hedhili, H.N. Alshareef, Selenide‐Based Electrocatalysts and Scaffolds for Water Oxidation Applications, Advanced Materials, 28 (2016) 77-85. [5] L. Zhang, K. Xiong, S. Chen, L. Li, Z. Deng, Z. Wei, In situ growth of ruthenium oxidenickel oxide nanorod arrays on nickel foam as a binder-free integrated cathode for hydrogen evolution, Journal of Power Sources, 274 (2015) 114-120. [6] R. Kötz, RuO2/IrO2 electrocatalysts for anodic O2 evolution, Electrochimica Acta, 29 (1984) 1607-1612. [7] A.T. Marshall, R.G. Haverkamp, Electrocatalytic activity of IrO2–RuO2 supported on Sbdoped SnO2 nanoparticles, Electrochimica Acta, 55 (2010) 1978-1984. [8] K.C. Majhi, P. Karfa, R. Madhuri, Bimetallic transition metal chalcogenide nanowire array: An effective catalyst for overall water splitting, Electrochimica Acta, (2019). [9] N.K. Chaudhari, H. Jin, B. Kim, K. Lee, Nanostructured materials on 3D nickel foam as electrocatalysts for water splitting, Nanoscale, 9 (2017) 12231-12247. [10] S. Dutta, A. Indra, Y. Feng, T. Song, U. Paik, Self-supported nickel iron layered double hydroxide-nickel selenide electrocatalyst for superior water splitting activity, ACS applied materials & interfaces, 9 (2017) 33766-33774. [11] A. Sivanantham, S. Shanmugam, Nickel selenide supported on nickel foam as an efficient and durable non-precious electrocatalyst for the alkaline water electrolysis, Applied Catalysis B: Environmental, 203 (2017) 485-493. [12] J. Zhang, Y. Wang, C. Zhang, H. Gao, L. Lv, L. Han, Z. Zhang, Self-supported porous NiSe2 nanowrinkles as efficient bifunctional electrocatalysts for overall water splitting, ACS Sustainable Chemistry & Engineering, 6 (2017) 2231-2239. [13] J. Ren, Z. Hu, C. Chen, Y. Liu, Z. Yuan, Integrated Ni2P nanosheet arrays on threedimensional Ni foam for highly efficient water reduction and oxidation, Journal of energy chemistry, 26 (2017) 1196-1202. [14] C. Lu, D. Tranca, J. Zhang, F.n. Rodrı́guez Hernández, Y. Su, X. Zhuang, F. Zhang, G. Seifert, X. Feng, Molybdenum carbide-embedded nitrogen-doped porous carbon nanosheets as electrocatalysts for water splitting in alkaline media, ACS nano, 11 (2017) 3933-3942.
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[30] A.M. Sakita, R. Della Noce, E. Vallés, A.V. Benedetti, Pulse electrodeposition of CoFe thin films covered with layered double hydroxides as a fast route to prepare enhanced catalysts for oxygen evolution reaction, Applied Surface Science, 434 (2018) 1153-1160. [31] Y.-H. Lee, Y.-H. Yun, V.H.V. Quy, S.-H. Kang, H. Kim, E. Vijayakumar, K.-S. Ahn, Preparation of nickel selenide by pulsed-voltage electrodeposition and its application as a highlyefficient electrocatalyst at counter electrodes of quantum-dot sensitized solar cells, Electrochimica Acta, 296 (2019) 364-371. [32] F. Ye, C. Xu, G. Liu, M. Yuan, Z. Wang, X. Du, J.K. Lee, Effect of pulse electrodeposition parameters on electrocatalytic the activity of methanol oxidation and morphology of Pt/C catalyst for direct methanol fuel cells, Energy conversion and management, 160 (2018) 85-92. [33] X. Li, G. Guan, X. Du, A.D. Jagadale, J. Cao, X. Hao, X. Ma, A. Abudula, Homogeneous nanosheet Co 3 O 4 film prepared by novel unipolar pulse electro-deposition method for electrochemical water splitting, RSC Advances, 5 (2015) 76026-76031. [34] X. Li, G.-Q. Han, Y.-R. Liu, B. Dong, X. Shang, W.-H. Hu, Y.-M. Chai, Y.-Q. Liu, C.-G. Liu, In situ grown pyramid structures of nickel diselenides dependent on oxidized nickel foam as efficient electrocatalyst for oxygen evolution reaction, Electrochimica Acta, 205 (2016) 77-84. [35] H. Wan, J. Liu, Y. Ruan, L. Lv, L. Peng, X. Ji, L. Miao, J. Jiang, Hierarchical Configuration of NiCo2S4 Nanotube@ Ni–Mn Layered Double Hydroxide Arrays/Three-Dimensional Graphene Sponge as Electrode Materials for High-Capacitance Supercapacitors, ACS Appl. Mater. Interfaces, 7 (2015) 15840. [36] G.B. Darband, M. Aliofkhazraei, A. Dolati, A.S. Rouhaghdam, Electrocrystallization of Ni nanocones from chloride-based bath using crystal modifier by electrochemical methods, Journal of Alloys and Compounds, (2019) 152843. [37] M. Rezaei, S.H. Tabaian, D.F. Haghshenas, Nucleation and growth of Pd nanoparticles during electrocrystallization on pencil graphite, Electrochimica acta, 59 (2012) 360-366. [38] Z. Chen, D. Cummins, B.N. Reinecke, E. Clark, M.K. Sunkara, T.F. Jaramillo, Core–shell MoO3–MoS2 nanowires for hydrogen evolution: a functional design for electrocatalytic materials, Nano letters, 11 (2011) 4168-4175. [39] M. Fang, W. Tang, C. Yu, L. Xia, Z. Xia, Q. Wang, Z. Luo, Performance of Ni-Rich Bimetallic Phosphides on Simultaneous Quinoline Hydrodenitrogenation and Dibenzothiophene Hydrodesulfurization Fuel Process, Fuel Process. Technol., 129 (2015) 236. [40] Z. Pu, Y. Luo, A.M. Asiri, X. Sun, Efficient electrochemical water splitting catalyzed by electrodeposited nickel diselenide nanoparticles based film, ACS applied materials & interfaces, 8 (2016) 4718-4723. [41] J. Yu, Y. Tian, F. Zhou, M. Zhang, R. Chen, Q. Liu, J. Liu, C.-Y. Xu, J. Wang, Metallic and superhydrophilic nickel cobalt diselenide nanosheets electrodeposited on carbon cloth as a bifunctional electrocatalyst, Journal of Materials Chemistry A, 6 (2018) 17353-17360. [42] Z. Zhang, Y. Liu, L. Ren, H. Zhang, Z. Huang, X. Qi, X. Wei, J. Zhong, Three-dimensionalnetworked Ni-Co-Se nanosheet/nanowire arrays on carbon cloth: a flexible electrode for efficient hydrogen evolution, Electrochimica Acta, 200 (2016) 142-151. [43] T. Wang, D. Gao, W. Xiao, P. Xi, D. Xue, J. Wang, Transition-metal-doped NiSe 2 nanosheets towards efficient hydrogen evolution reactions, Nano Research, 11 (2018) 60516061. [44] P.F. Liu, L. Zhang, L.R. Zheng, H.G. Yang, Surface engineering of nickel selenide for an enhanced intrinsic overall water splitting ability, Materials Chemistry Frontiers, 2 (2018) 17251731. 17
[45] H. Zhou, F. Yu, Y. Liu, J. Sun, Z. Zhu, R. He, J. Bao, W.A. Goddard, S. Chen, Z. Ren, Outstanding hydrogen evolution reaction catalyzed by porous nickel diselenide electrocatalysts, Energy & Environmental Science, 10 (2017) 1487-1492. [46] C. Tang, N. Cheng, Z. Pu, W. Xing, X. Sun, NiSe nanowire film supported on nickel foam: an efficient and stable 3D bifunctional electrode for full water splitting, Angewandte Chemie International Edition, 54 (2015) 9351-9355. [47] A.T. Swesi, J. Masud, M. Nath, Nickel selenide as a high-efficiency catalyst for oxygen evolution reaction, Energy & Environmental Science, 9 (2016) 1771-1782. [48] T. Zhang, Y. Zhu, J.Y. Lee, Unconventional noble metal-free catalysts for oxygen evolution in aqueous systems, Journal of Materials Chemistry A, 6 (2018) 8147-8158. [49] J. Kibsgaard, T.F. Jaramillo, Molybdenum phosphosulfide: an active, acid‐stable, earth‐abundant catalyst for the hydrogen evolution reaction, Angewandte Chemie International Edition, 53 (2014) 14433-14437. [50] G.B. Darband, M. Aliofkhazraei, A.S. Rouhaghdam, Facile Electrodeposition of Ternary Ni-Fe-Co Alloy Nanostructure as a Binder Free, Cost-effective and Durable Electrocatalyst for High-Performance Overall Water Splitting, Journal of Colloid and Interface Science, (2019). [51] G.B. Darband, M. Aliofkhazraei, A.S. Rouhaghdam, Three-dimensional porous Ni-CNT composite nanocones as high performance electrocatalysts for hydrogen evolution reaction, Journal of Electroanalytical Chemistry, 829 (2018) 194-207. [52] Y. Yan, B.Y. Xia, B. Zhao, X. Wang, A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochemical water splitting, Journal of Materials Chemistry A, 4 (2016) 17587-17603. [53] C.-W. Tung, Y.-Y. Hsu, Y.-P. Shen, Y. Zheng, T.-S. Chan, H.-S. Sheu, Y.-C. Cheng, H.M. Chen, Reversible adapting layer produces robust single-crystal electrocatalyst for oxygen evolution, Nature communications, 6 (2015) 8106.
18
Figure Captions Figure 1: XRD patterns of PPE samples. Figure 2: SEM images with the different magnification of (a) CPE at -0.8 V (vs. SCE), (b) S0.01, (c) S-0.1, (d) S-1, (e) S-10 and (f) EDS elemental mapping analysis of S-0.01 and corresponding EDS analysis. Figure 3: HER performance of Ni-Se catalysts in 1 M KOH, (a) LSV curves of CPP, CPE and NF, (b) corresponding Tafel curves, (c) comparison of over potential required at current density of 10, 20 and 100 mA/cm2 for different samples. Figure 4: CV curves at the scan rates of 5 to 100 mV/dec for CPP samples, (a) S-0.01, (b) S-0.1, (c) S-1, (d) S-10, (e) scan rate dependences of the capacitive current (ia-ic) at average potential in the selected range, (f) polarization curves normalized to the active surface area. Figure 5: Nyquist plots of (a) S-1, (b) S-10, (c) S-0.1, (d) S-0.01 at different over potentials and (e) Equivalent electrical circuit of PPE samples Figure 6: (a) LSV curves of S-0.01 before and after 1000 cycles CV scanning, (b) chronopotentiometry stability of S-0.01 at 50 mA/cm2for 20 h accompanied with SEM images of the sample before and after of stability test and (c) EDS analysis of S-0.01 after stability. Figure 7: OER performance of Ni-Se catalysts in 1 M KOH, (a) LSV curves of CPP and CPE, (b) corresponding Tafel curves and (c) LSV curve of S-0.01 before and after 500 cycles CV scanning.
19
Table Captions Table 1: Operating condition for electrodeposition of nickel selenide films. Table 2: Comparison of HER performance of S-0.01 and other Ni-Se catalysts in 1 M KOH. Table 3: Calculated parameters of Cdl and ECSA from CV curves for PPE samples. Table 4: The impedance parameters extracted from Nyquist plots in Fig. 5. Table 5: Overpotentials required for OER at different current densities for PPE and CPE samples.
20
Catalysts NiSe/NF
Electrolyte 1M KOH
NiSe/NF NiSe2/Ti Co0.13Ni0.87Se2/Ti NiCoSe2/CC Ni1/3Co2/3Se2 Ni3Se2/NF NiSe/NF NiSe2/NF Ni0.8Fe0.2Se2/CFC
1M KOH 1M KOH 1M KOH 1M KOH 1M KOH 1M KOH
NiSe-PANI
0.5 M H2SO4 1M KOH 1M KOH 1 M KOH
i (mA/cm2) 10 20 100 100 10 100 10 10 10 10 10 10
(mV) η 65 87 138 190 96 64 112.7 131 104 153 170 64
10 20 100 10
120 152 96 166
b (mV/dec) 89
References This work
76.6 82 63 65 40.1 93 99 148 43
[29] [40] [25] [41] [42] [28]
[43] [44]
92.3 [12]
NiSe2/NF
H-NiSe2/NF
NiSe/NF
1 M KOH
1 M KOH
10
107
100
153
10
96
42.6
[45]
120
[46]
S-0.01
S-0.1
S-1
S-10
Cdl (mF/cm2)
59.3
46.8
22
25.6
ECSA
2965
2340
1100
1230
Sample
Catalysts
(mV
Rs
CPE1
R1
(Ω.cm2)
(Fsn-
(Ω.cm2)
vs.
1
n1
R2
CPE2
(Ω.cm2)
(Fsn-
/cm2)
1
n2
/cm2)
RHE) S-0.01
S-0.1
S-1
S-10
100
1.35
0.13
0.03
0.8335
1.58
0.1
0.85
200
1.37
0.05
0.05
0.88
0.73
0.075
0.9
300
1.39
0.04
0.02
0.9
0.5
0.083
0.85
100
1.55
0.05
0.023
0.9
5.63
0.055
0.84
200
1.56
0.05
0.018
0.91
1.04
0.06
0.87
300
1.54
0.05
0.018
0.91
0.565
0.053
0.89
100
1.86
0.14
0.05
0.733
7.25
0.043
0.83
200
1.88
0.08
0.04
0.8
1.22
0.057
0.86
300
1.39
0.05
0.025
0.85
0.47
0.078
0.87
100
1.037
0.04
0.07
0.85
4.75
0.073
0.85
200
1.043
0.035
0.092
0.84
1
0.07
0.89
300
1.043
0.032
0.078
0.84
0.57
0.07
0.87
Overpotential, η/ mV
Catalysts 100 mA/cm2
150 mA/cm2
200 mA/cm2
S-1
344
365
386
S-10
342
363
386
S-0.1
328
343
357
S-0.01
306
318
330
CPE
342
372
405
Catalysts
Pulse
Ton (s)
Toff (s)
Cycle numbers
frequency (Hz)
Deposition time (s)
S-10
10
0.05
0.05
18000
900
S-1
1
0.5
0.5
1800
900
S-0.1
0.1
5
5
180
900
S-0.01
0.01
50
50
18
900
CPE at -0.8 V
-
-
-
-
900
Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution S. Esmailzadeh a, T. Shahrabia, *, Gh. Barati Darband a, Y. Yaghoubinezhad b a
Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran
b
Department of Materials Engineering, Birjand University of Technology, P.O. Box: 97175569314, Birjand, Iran *Email: [email protected] ( T.S. Farahani)
Author Contribution Statement S. Esmailzadeh: Methodology, Validation, Investigation, Writing - Original Draft. T. Shahrabi: Conceptualization, Methodology, Resources, Review & Editing, Supervision. Gh. Barati Darband: Validation, Writing - Original Draft, Supervision, Review & Editing. Y. Yaghoubinezhad: Supervision, Review & Editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0013-4686(19)32421-1
DOI:
https://doi.org/10.1016/j.electacta.2019.135549
Reference:
EA 135549
To appear in:
Electrochimica Acta
Received Date: 17 September 2019 Revised Date:
24 November 2019
Accepted Date: 17 December 2019
Please cite this article as: S. Esmailzadeh, T. Shahrabi, G.B. Darband, Y. Yaghoubinezhad, Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135549. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution S. Esmailzadeh a, T. Shahrabia, *, Gh. Barati Darband a, Y. Yaghoubinezhad b a
Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran
b
Department of Materials Engineering, Birjand University of Technology, P.O. Box: 97175569314, Birjand, Iran *Email: [email protected] ( T. Shahrabi)
Abstract Herein, we report a pulse potential electrodeposition method (PPE) as a fast and one-step route for the production of effective and binder-free Ni-Se catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution. The results demonstrated that the fabricated electrocatalysts by PPE show more desirable electrocatalytic performance than constant potential electrodeposition method (CPE). The electrodeposited NiSe nanostructures with large electrochemically active surface area of 2965 cm2 on the nickel foam (NF) at the frequency of 0.01 Hz exhibited high electrocatalytic activity for HER and OER. It delivers the current density of 10 mA/cm2 at the overpotential of 65 mV for HER and 100 mA/cm2 at 306 mV for OER in 1 M KOH. In addition, the Tafel slope for HER and OER was 89 and 61 mV/dec respectively. Morphological stability of NiSe nanostructure catalyst leads to an increase the electrocatalytic stability during long-term electrolysis. Keywords:
1
Pulse
potential
electrodeposition,
NiSe
nanostructure,
Hydrogen
evolution
reaction,
electrocatalytic activity. 1. Introduction In recent years limited resources of fossil fuels followed by the fast growth of human society and environmental issues such as global warming have led to the development of renewable energy productions. Hydrogen is a clean, secure and carbon-free energy that can be replaced instead of fossil fuels. The other merit of using this fuel is the renewable property so water is the only product of combustion of hydrogen [1, 2]. Steams reforming of fossil fuels, gasification of coal and petroleum coke and water electrolysis are examples of different methods for the production of H2. However, water-splitting technology is one of the most efficient methods for production of hydrogen gas which is produced from aqueous solutions. During the water electrolysis in water splitting process, hydrogen (HER) and oxygen (OER) evolution reactions are being produced at cathode and anode respectively [2, 3]. Although electrochemical water splitting is an easy and low-cost way for hydrogen production but the required overpotential in practice, is much higher than the theoretical amount of voltage calculated from the thermodynamic relationships (1.23 V vs. RHE) [2, 4]. In order to reduce the water splitting over-potential, the water electrolysis system requires materials called electro-catalysts which facilitate electron transfer for HER and OER. In spite of good performance of precious metals based catalysts such as Pt group metals [5] for HER and IrO2 [6] and RuO2 [7] for OER, but due to their limited availability and high price, the widespread use of them has been prohibited [8]. Therefore the scientists try to synthesis the non-precious catalysts as a low cost, earth-rich and high-efficient in water electrolysis [9-11]. Some examples of non-noble catalysts include non-oxide catalysts such as transition metal selenides [12], phosphides [13], carbides [14], and sulfides [15] for HER and 2
transition metals oxides, hydroxides and oxyhydroxide for OER [2, 16]. Using bifunctional electrocatalyst for both HER and OER in the water splitting can play a key role in simplifying the electrolysis system and lowering the costs. Accordingly, it will be highly attractive to make bifunctional electrocatalysts for water splitting in alkaline media [12, 17]. Based on the recent reports, some of the electrodes that were used as bifunctional electrocatalysts are such as iron (Fe)-doped nickel phosphide (Ni2P) nanosheets [18], Hierarchical NiCoP nanocone arrays [19], phase-pure pentlandite Ni4.3Co4.7S8 [20]. Among different non-noble metals, selenide element has distinct characters such as high metallic property, large relative radius and small ionization energy which might cause catalysts formed by metallic selenides to possess unique intrinsic catalytic activity. Therefore, nickel selenide with different stoichiometry could be selected as a bifunctional electrocatalyst that has been extensively used in recent few years [21-24]. Compared with the different approaches for the synthesis of non-noble metal catalysts, electrodeposition is preferred for its simplicity, low cost, and non-using any conductive agent or binder between catalyst materials and substrate [25-27]. Zhu et al. [28] investigated the effect of deposition potential on the phases of nanostructured nickel selenide electrodeposited on the Ni foam and their electrocatalytic activities for HER and OER. They reported that the pure NiSe2, NiSe, and Ni3Se2 were obtained at the electrodeposition potentials of −0.35, −0.46, and −0.60 V (vs. RHE) respectively. Gao et al. [29] electrodeposited NiSe on the Ni foam and studied electrocatalytic activities in both water reduction and oxidation. They showed in a two-electrode water electrolyzer with NiSe as a bifunctional electrocatalyst for anode and cathode, a high water splitting current density of 20 mA/cm2 is achieved at cell voltage around 1.5 V. Recently, pulse plating as a type of electrodeposition method through the presence of many parameters for
3
optimization offers superior control on the structure (micro or nano), morphology and the thickness of film [30-33]. To the best of our knowledge, pulsed electrodeposition of nickel selenide as an electroactive coating for HER and OER has not been considered in the literature. In the present work, pulse potential electrodeposition (PPE) was used for the fabrication of nickel selenide nanosheets as an electrocatalytic platform for HER and OER application on the nickel foam (NF). Moreover, for comparison, the electrocatalyst of nickel selenide was fabricated by conventional constant potential electrodeposition (CPE).
The characterization of products and investigation of
electrocatalytic activity were performed by scanning electron microscopy (SEM), X-ray diffraction (XRD), linear sweep voltammetry (LSV), Tafel polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and chronopotentiometry in 1 M KOH.
2. Experimental 2.1 Materials The materials including nickel chloride (NiCl2.6H2O), selenide dioxide (SeO2), lithium chloride (LiCl), hydrochloric acid (HCl), potassium hydroxide (KOH) and nickel foam were bought from Merck Company and used for this study without further purification. 2.2 Electrodeposition of nickel selenide electrodes At first, nickel foams as substrate were cut in the square shape with a surface area of 1 cm2, and then they were cleaned ultrasonically in 1M HCl, ethanol and water for 15, 2 and 2 min, respectively. Electroplating of nickel selenide electrode was performed by PPE and CPE modes by potentiostat (Autolab potentiostat/galvanostat 302 N device). Electrodeposition bath consisted of 65 mM NiCl2.6H2O and 35 mM SeO2 as ion sources and 20 mM LiCl as a conductive agent of 4
the electrolyte which were dissolved in deionized water. The electrodeposition was carried out at room temperature and pH 3.5 (adjusted by NaOH). During the PPE and CPE, the Pt, saturated calomel electrode (SCE) and NF were used as a counter, reference, and working electrodes, respectively. In PPE, a square wave potential with the range of 0 to -0.8 V (vs. SCE) was applied at frequencies of 0.01, 0.1, 1, and 10 Hz that are denoted in this article as S-0.01, S-0.1, S-1, and S-10, respectively. The schematic E-t curves of PPE and CPE technique are presented in Fig S1. The time of electrodeposition for PPE at different frequency was constant 900s (sums of tons) with a duty cycle of 0.5. For comparison, nickel selenide coating was electrodeposited by CPE at a constant potential of -0.8 V (vs. SCE) for 900s. The operating conditions in PPE and CPE of nickel selenide films are given in Table 1. In both PPE and CPE, All samples after electrodeposition were rinsed with deionized water and dried in the oven at 60 °C for 4 h. 2.3 Microstructural characterization Surface morphology and chemical composition of the electrodes was studied using SEM equipped by energy-dispersive X-ray spectroscopy (EDS) detector (TESCAN VEGA3). In order to study the phase formation, the XRD spectrum was collected by Philips X'Pert Diffractometer (using Cu-Kα at an accelerating voltage of 40 kV and a current of 40mA). 2.4 Electrochemical characterization Electrochemical tests were carried out by potentiostat (Autolab potentiostat/galvanostat 302 N device) in 1 M KOH using a three-electrode setup. LSV technique was done for obtaining polarization curves for hydrogen evolution and oxygen evolution with a scan rate of 5 mV/s without rotation of the working electrode or creation of turbulence in the electrolyte at room temperature. EIS measurements were performed at overpotentials of 100, 200 and 300 mV (VS. RHE) from an initial frequency of 100 kHz to a final frequency of 10 mHz, with the amplitude of 5
10
mV.
The
electrocatalytic
stability
of
electrodes
was
evaluated
by
CV
and
chronopotentiometry. The CV scanning was done for all of the specimens in the potential range of 0 to 350 mV (vs. RHE) with a scan rate of 100 mV/s for 1000 cycles. The chronopotentiometry test was carried out at 50 mA/cm2 for 20 h. Moreover, measured potentials vs. SCE are thoroughly converted to the reversible hydrogen electrode (RHE) using the Nernst equation [26] (Eq. 1). E (vs. RHE) = E (vs. SCE) + ESCE (0.242 V) + 0.0591 pH
(1)
Besides, for correcting the iR drop in the solution, the value of iRs (Rs was gained from EIS test) was subtracted from all potentials. 3. Results and discussion 3.1 Structural and morphological characterization After the electrodeposition, the colure of nickel foam transformed from silvery to dark black for all frequencies which indicating the deposition of coating on the surface of NF. XRD patterns of the samples resulting from PPE are shown in Fig 1. For all diffraction patterns, strong diffractions at around 44.60 and 51.93 correspond to NF that arising from the underlying substrate. Diffraction patterns of all samples exhibited the standard pattern of NiSe. Three main peaks at around 33.59, 44.60 and 51 belong to the (101), (102) and (110) planes that are wellmatched with NiSe (03-065-6014) [28, 34]. Therefore the formation of NiSe coating on the NF surface for the samples at the different frequencies was confirmed. The SEM images of samples with high and low magnification along accompanied with their EDS analysis are illustrated in parts of a, b, c, d and e of Fig 2. To better illustrate the effect of pulse potential on the structure of the catalyst, Fig 2.a shows the SEM images of the electrode surface obtained at the direct voltage of -0.8 V (vs. SCE). As clearly seen, the application of
6
pulse potential has led to a significant change in the structure especially at low frequencies. Applying a constant voltage for the electrodeposition caused that the nickel selenide particles nucleate on the surface and gradually grow lateral and vertically with the increase in the deposition time leading to the formation of a dense structure with an almost smooth surface morphology which is not favorable for improving the catalytic activity [31]. While in the PPE condition especially for samples of S-0.01 (Fig 2.b) and S-0.1 (Fig 2.c) the surface morphology of NiSe catalyst is completely different. As can be observed in Fig 2.b the catalyst which was produced at the frequency of 0.01 Hz has two different structures (a dual structure) [35] [30]. It consists of particles in different sizes (micro and nano) that have been covered by nanosheets. The thickness of the nanosheets is in the range of 5-20 nm. Fig 2.c shows the structure of NiSe catalyst that has been produced at 0.1 Hz. It is composed of nanoparticles with a diameter of less than 100 nm. By increasing the frequency of pulse to 1, as seen in Fig 2.d the same structures can be observed like S-0.01. The difference is that the height of sheets covering the surface decreased and also the density of nanoparticles became less because the ton and toff are not enough to growth of nucleation and recover the surface concentration, so the surface area is greatly decreased [30]. Increasing the pulse frequency to 10, led to the production of NiSe catalyst with an approximate smooth surface and dispersed particles in the whole surface. Cracks appeared on the surface is likely due to the high thickness followed by stress accumulated in the film (Fig 2.e) [31]. At the frequency of 10 Hz in which the time of each pulse is lower than the other frequencies, two-dimensional nucleation (2D) is occurred but there is probably not enough time in each potential pulse for three dimensional nucleation (3D). Therefore, during the electrodeposition at 10 Hz, nuclei of NiSe nucleate on the surface of substrate at the beginning of process and formed NiSe layers and grow by lateral spreading of discrete layers and finally a
7
catalyst with an approximate smooth surface is produced [36, 37]. The distinct morphology of the samples produced in the PPE conditions compared with the CPE is attributed to periodic applying cathodic potential. In the method of direct pulse electrodeposition, during the toff, diffusion layer which has been depleted of reactants in ton, is recovered by immigration of ions in the electrolyte to the near the surface of electrode. By applying the cathodic potential each time for an exact duration (ton), new nucleation occurs on the film surface and restricts the growth of before nucleations [27, 38, 39]. Therefore as be expected the samples of S-0.01, which produced at pulse conditions with enough time for the 2D and 3D nucleation and growth of new nucleation displayed the dual structure with high surface area and electrocatalytic active sites [31, 36]. According to the EDS analysis in the part f of Fig. 2 it can be seen that the composition of sample of S-0.01 consists of Ni and Se as the precursor elements of catalyst. according to this analysis, the reason of high oxygen percentage of the sample can be related to the drastically reduction of H+ during the ton near the electrode surface and formation of hydroxide species as a result of increasing the local pH [30]. The analysis of the mapping element for S-0.01 is illustrated in Fig. 2f. it depicts the uniform distribution of elements Ni and Se over the produced film which indicates the contribution of these elements in the formation of electrode film. 3.2 Hydrogen evolution activity Electrocatalytic performance of the PPE samples for HER was studied using LSV measurement in 1 M KOH. Fig. 3a shows the LSV curves of different samples and also nickel foam for comparison. The overpotential required for generating the current density of 10, 20 and 100 mA/cm2 was selected as a criterion for evaluating the electrocatalytic activity for HER. According to the LSVs, it is clear that all prepared samples by PPE have better electrocatalytic activity than the NF and even the sample produced by CPE. Among the NiSe electrocatalysts 8
prepared at different frequencies, the over-potential required at a given current density for S-0.01 is the lowest. This sample affords 10, 20 and 100 mA/cm2 for HER at the overpotentials of 65, 87 and 138 mV, respectively. The indeed amount of 138 mV overpotential for generating the current density of 100 mA/cm2 depicts that S-0.01 has a much better electrocatalytic performance in comparison with Ni-Se electrocatalysts produced by constant potential electrodeposition and also by different methods (Table 2). The overpotentials required for all samples and nickel foam to attain the current density of 10, 20 and 100 mA/cm2 are presented in Fig. 3c. A comparison of the different values of over-potentials at a constant current density demonstrates that by decreasing the pulse frequency, the overpotential required for receiving a current density is reduced. Therefore, it means that the electrocatalysts produced at the lower frequency show better catalytic behavior than the higher pulse frequency. According to the SEM images (Fig. 2), it can be attributed to the type of morphology and consequently increase the surface area and formation of many electrocatalytic active sites [26]. Tafel slope is a standard approach for explaining the kinetics and mechanism of hydrogen or oxygen evolution. It is calculated through the LSV curves and fitting their linear parts by the Eq. 2 (Tafel equation) [47]: η = b log (j) + a
(2)
Where b is Tafel slope, j is the current density, η is the overpotential and a is a parameter related to the exchange current density with the name of cathodic intercept. According to the Tafel equation, we can say the lower the overpotential, the smaller the slope and consequently more kinetic for hydrogen evolution and generation of the same current density. Fig. 3b exhibited that the Tafel slopes obtained using LSVs for S-0.01, S-0.1, S-1, and S-10 are 89, 91, 116 and 119 mV/dec respectively which is lower than the CPE sample (139 mV/dec). Also the S-0.01 has the 9
lowest Tafel slope among the others. It displays that the rate of the hydrogen production is fast on the surface of S-0.01 catalyst and the overall kinetic of HER reaction is controlled by the Volmer mechanism [41]. The electrochemically active surface area (ECSA) and the turnover frequency (TOF) are two parameters that have a direct relationship with electrocatalytic activity. Therefore, in order to explain in detail, the better electrocatalytic activity of S-0.01, the parameters of ECSA and TOF for all PPE samples were investigated. For calculation the ECSA for each catalyst, the CV curves were taken in a Non-Faradaic regime at various scan rates (Fig. 4). After estimation of charging current (ic), the double-layer capacitance (Cdl) was calculated on the basis of Eq. 3 (Fig. 4e), ic = νCdl
(3)
The measured values of Cdl for each electrode were converted to the ECSA after dividing the Cdl by 20 µF as a specific capacitance for a uniform atomic surface [2, 3]. The double-layer capacitance along with the calculated ECSA values for PPE samples are shown in Table 3. It can be observed that the values of Cdl and obtained ECSA have very good agreement with the SEM surface morphology (Fig. 2). As we expected for S-0.01 with the dual structure which provides a high surface area with plentiful electrocatalytic active sites, the values of Cdl and ECSA is 59.3 mF/cm2 and 2965 respectively which is much more than the others. As stated, another important parameter that is in principle an exact measurement of inherent electrocatalytic activity is TOF. It represents the rate of hydrogen mole evolution per unit time from the surface of catalyst. Thus there are different methods for calculation of TOF on the basis of the method used for determining the number of active surface sites [48]. Here in TOF was calculated for all PPE samples according to the method presented by Jaramillo et al. [38, 49] and the results are indicated in Fig. 4f. TOF curves show that the S-0.01 and S-0.1 have higher intrinsic
10
electrocatalytic activity compared with the S-1 and S-10. It confirms the better electrocatalytic activity of catalysts obtained at low frequencies. The superior electrocatalytic performance of S0.01than the electrodes produced by different methods with the same active surface area can be attributed to the non-using of the binder in the coating. Binder or conductive agent is usually used in the most synthesis methods of the catalysts which creates a dead zone between the catalyst and the substrate. It may cause blocking the active sites and increasing ohmic resistance and finally decrease the electrocatalytic activity while in the electrodeposition method, the binder is not required and the coating is deposited on the substrate directly and without intermediary [4, 50]. One of the effective factors on the electrocatalytic activity is the creation of resistance to the ionic and charge transport in the electrolyte during the electrochemical measurement. It can be as a barrier during the water electrolysis. EIS is a technique for analyzing the interface properties of the electrode and kinetics of HER on the surface of the catalysts [11]. To know the interface behavior of the catalysts and the kinetic of HER, the EIS test was done and the Nyquist plots in overpotentials of 100, 200 and 300 mV vs. RHE were recorded and shown in Fig. 5. As can be observed, for all samples the Nyquist plots consist of two overlapped semicircles. Therefore an equivalent circuit with two-time constants can be discerned within the studied frequency range [41]. The zoom-in images in Fig. 5 and the results of Table 4 show that the size of the second semicircle at lower frequencies is changed in different over-potentials while the size of first semicircle is almost constant. Therefore it can be concluded that the second semicircle is probably related to the kinetics of hydrogen production that is increased by increasing the overpotentials [3, 50, 51]. According to the electrical equivalent circuit presented in Fig. 5e Rs is the solution resistance, R1 and CPE1 are the charge transfer resistance and constant phase element
11
related to the porosity of electrode’s surface respectively and R2 and CPE2 are resistance and constant phase element related to the kinetic of the reduction reaction of hydrogen on the surface of the electrode [3]. Table 4 shows that the values of R2 for S-0.01 in three over-potentials of 100, 200 and 300 mV are 1.58, 0.73 and 0.5 Ω. Cm2, respectively. Comparatively they are the lowest in comparison of the other PPE samples and even lower than other reported electrocatalysts. So S-0.01 is a more favorable catalyst with superior electrical conductance for overcoming the HER reaction barrier during the water splitting. Electrocatalytic stability is a major factor for practical application of an electrocatalyst in strongly alkaline solutions. It is necessary for a catalyst with excellent catalytic activity to have high electrocatalytic stability too. There are different methods for investigating the electrocatalytic stability [12]. The electrocatalytic stability of S-0.01, using the CV scanning and also at the constant current density of 50 mA/cm2 by chronopotentiometry was evaluated. Fig. 6a shows the LSV curve of asprepared S-0.01and after 1000 cycles CV scanning. As seen the LSV curves before and after 1000 cycles are completely coincident which indicates the very high electrocatalytic stability of S-0.01 [45]. According to Fig. 6b, the required over-potential of S-0.01 for generating of the current density of 50 mA/cm2 is about 1.45 (V vs.RHE). This overpotential remains almost constant over the duration of 20 h. During the electrolysis, the corrosion of electrolyte and the detachment of gas bubbles from the catalyst surface can damage the surface of the catalyst. Therefore morphological stability is an important factor for proving the electrocatalytic activity [26]. Comparison of the SEM images of S-0.01 at the beginning and the end of the chronopotentiometry which inserted in Fig. 6b shows that the morphology and topology of catalyst have been unchanged during the water electrolysis. It depicts the high long term electrocatalytic stability of S-0.01 during the water splitting for 20 h. The similarity of the
12
chemical composition of S-0.01 before (Fig. 2b) and after chronopotentiometry test (Fig. 6c) confirms the high stability of S-0.01. Using the electrochemical deposition and direct deposition of the catalyst on the substrate is the main reason for high adhesion of NiSe catalyst to the NF substrate and ultimately its high stability during the electrolysis. 3.3. Oxygen evolution activity In order to simplify the system of overall water splitting and reduce costs, it is highly attractive that the catalyst to have the desirable electrocatalytic activity for HER as well as good electrocatalytic properties for OER [52]. In accordance with the results reported in recent years, the selenide based catalysts have shown that they can be promising candidates for both HER and OER [11]. Therefore, the electrocatalytic activity of PPE samples for OER was studied in 1 M KOH. Like before, at first, the catalytic performance of samples using LSV curves (Fig. 7a) was investigated. In the OER LSVs curves of Ni-Se films, there is an obvious oxidation peak. For example for S-0.01, the intense of oxidation peak is at 1.44 V (vs. RHE) before the onset of the oxygen evolution. It is attributed to the oxidation of Ni (II) to Ni (III) on the surface of the electrode, according to the Eq. 4. + → +
(4)
The obtained Ni (III) species in the form of NiSe(OH) act as the active sites on the surface of the electrode for OER [34, 53]. The over-potentials required for achieving the current densities of 100, 150 and 200 mA/cm2 for all samples were compared together (Table 5). The magnitude of overpotential for attaining the current density of 100 mA/cm2 using the LSV curves obtained by scanning potential from high to the low potential that are shown in Fig. S2 is 306, 328, 344 and 342 mV for S-0.01, S-0.1, S-1, and S-10, respectively. The results illustrate that the samples produced by the pulse electrodeposition, in addition to having the high electrocatalytic activity
13
for HER compared to CPE sample, they also show good catalytic performance for OER. Also among the PPE samples, S-0.01 showed the more desirable catalytic activity than the others for OER like HER especially for generating of high current density. The main reason of this behavior can be attributed of applying the voltage periodically and having enough time in each period for the growth of new nucleation’s and thus increase the catalytic active surface area and creation of the plentiful sites for the reaction of anions and cations [30]. The OER kinetics of PPE samples was evaluated by corresponding Tafel curves that shown in Fig. 7b The smaller Tafel slope of S-0.01 (61 mV/dec) implies the more OER kinetic than the S-0.1 (87 mV/dec), S1 (93 mV/dec) and S-10 (125 mV/dec). Fig. 7c shows the LSV curve of S-0.01 before and after CV scanning. Negligible change of LSV curve after 500 cycles CV implying the high electrocatalytic stability of S-0.01 for OER [34]. 4. Conclusion In summary, the NiSe nanostructures were successfully prepared using pulse potential electrodeposition on the NF substrate at room temperature by controlling the pulse frequency. The electrocatalytic activity of produced electrodes with different structures was investigated for HER and OER in alkaline solution. The NiSe electrode with dual structure was produced at 0.01 Hz and showed excellent electrocatalytic activity for HER and OER requiring 65 and 285 mV for attaining the current density of 10 and 100 mA/cm2. The results showed that increasing the pulse frequency cause creating an electrode with lower surface area and followed by decreasing the electrocatalytic performance. The superior electrocatalytic activity and stability of S-0.01than the Ni-Se catalysts produced by the constant potential deposition and other synthesis methods attributes to the 3D dual structure, large active surface area, binder-free and high adhesion of coating to the substrate. The main reasons for creating an electrode by the pulse
14
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18
Figure Captions Figure 1: XRD patterns of PPE samples. Figure 2: SEM images with the different magnification of (a) CPE at -0.8 V (vs. SCE), (b) S0.01, (c) S-0.1, (d) S-1, (e) S-10 and (f) EDS elemental mapping analysis of S-0.01 and corresponding EDS analysis. Figure 3: HER performance of Ni-Se catalysts in 1 M KOH, (a) LSV curves of CPP, CPE and NF, (b) corresponding Tafel curves, (c) comparison of over potential required at current density of 10, 20 and 100 mA/cm2 for different samples. Figure 4: CV curves at the scan rates of 5 to 100 mV/dec for CPP samples, (a) S-0.01, (b) S-0.1, (c) S-1, (d) S-10, (e) scan rate dependences of the capacitive current (ia-ic) at average potential in the selected range, (f) polarization curves normalized to the active surface area. Figure 5: Nyquist plots of (a) S-1, (b) S-10, (c) S-0.1, (d) S-0.01 at different over potentials and (e) Equivalent electrical circuit of PPE samples Figure 6: (a) LSV curves of S-0.01 before and after 1000 cycles CV scanning, (b) chronopotentiometry stability of S-0.01 at 50 mA/cm2for 20 h accompanied with SEM images of the sample before and after of stability test and (c) EDS analysis of S-0.01 after stability. Figure 7: OER performance of Ni-Se catalysts in 1 M KOH, (a) LSV curves of CPP and CPE, (b) corresponding Tafel curves and (c) LSV curve of S-0.01 before and after 500 cycles CV scanning.
19
Table Captions Table 1: Operating condition for electrodeposition of nickel selenide films. Table 2: Comparison of HER performance of S-0.01 and other Ni-Se catalysts in 1 M KOH. Table 3: Calculated parameters of Cdl and ECSA from CV curves for PPE samples. Table 4: The impedance parameters extracted from Nyquist plots in Fig. 5. Table 5: Overpotentials required for OER at different current densities for PPE and CPE samples.
20
Catalysts NiSe/NF
Electrolyte 1M KOH
NiSe/NF NiSe2/Ti Co0.13Ni0.87Se2/Ti NiCoSe2/CC Ni1/3Co2/3Se2 Ni3Se2/NF NiSe/NF NiSe2/NF Ni0.8Fe0.2Se2/CFC
1M KOH 1M KOH 1M KOH 1M KOH 1M KOH 1M KOH
NiSe-PANI
0.5 M H2SO4 1M KOH 1M KOH 1 M KOH
i (mA/cm2) 10 20 100 100 10 100 10 10 10 10 10 10
(mV) η 65 87 138 190 96 64 112.7 131 104 153 170 64
10 20 100 10
120 152 96 166
b (mV/dec) 89
References This work
76.6 82 63 65 40.1 93 99 148 43
[29] [40] [25] [41] [42] [28]
[43] [44]
92.3 [12]
NiSe2/NF
H-NiSe2/NF
NiSe/NF
1 M KOH
1 M KOH
10
107
100
153
10
96
42.6
[45]
120
[46]
S-0.01
S-0.1
S-1
S-10
Cdl (mF/cm2)
59.3
46.8
22
25.6
ECSA
2965
2340
1100
1230
Sample
Catalysts
(mV
Rs
CPE1
R1
(Ω.cm2)
(Fsn-
(Ω.cm2)
vs.
1
n1
R2
CPE2
(Ω.cm2)
(Fsn-
/cm2)
1
n2
/cm2)
RHE) S-0.01
S-0.1
S-1
S-10
100
1.35
0.13
0.03
0.8335
1.58
0.1
0.85
200
1.37
0.05
0.05
0.88
0.73
0.075
0.9
300
1.39
0.04
0.02
0.9
0.5
0.083
0.85
100
1.55
0.05
0.023
0.9
5.63
0.055
0.84
200
1.56
0.05
0.018
0.91
1.04
0.06
0.87
300
1.54
0.05
0.018
0.91
0.565
0.053
0.89
100
1.86
0.14
0.05
0.733
7.25
0.043
0.83
200
1.88
0.08
0.04
0.8
1.22
0.057
0.86
300
1.39
0.05
0.025
0.85
0.47
0.078
0.87
100
1.037
0.04
0.07
0.85
4.75
0.073
0.85
200
1.043
0.035
0.092
0.84
1
0.07
0.89
300
1.043
0.032
0.078
0.84
0.57
0.07
0.87
Overpotential, η/ mV
Catalysts 100 mA/cm2
150 mA/cm2
200 mA/cm2
S-1
344
365
386
S-10
342
363
386
S-0.1
328
343
357
S-0.01
306
318
330
CPE
342
372
405
Catalysts
Pulse
Ton (s)
Toff (s)
Cycle numbers
frequency (Hz)
Deposition time (s)
S-10
10
0.05
0.05
18000
900
S-1
1
0.5
0.5
1800
900
S-0.1
0.1
5
5
180
900
S-0.01
0.01
50
50
18
900
CPE at -0.8 V
-
-
-
-
900
Pulse electrodeposition of nickel selenide nanostructure as a binder-free and high-efficient catalyst for both electrocatalytic hydrogen and oxygen evolution reactions in alkaline solution S. Esmailzadeh a, T. Shahrabia, *, Gh. Barati Darband a, Y. Yaghoubinezhad b a
Department of Materials Engineering, Faculty of Engineering, Tarbiat Modares University, P.O. Box: 14115-143, Tehran, Iran
b
Department of Materials Engineering, Birjand University of Technology, P.O. Box: 97175569314, Birjand, Iran *Email: [email protected] ( T.S. Farahani)
Author Contribution Statement S. Esmailzadeh: Methodology, Validation, Investigation, Writing - Original Draft. T. Shahrabi: Conceptualization, Methodology, Resources, Review & Editing, Supervision. Gh. Barati Darband: Validation, Writing - Original Draft, Supervision, Review & Editing. Y. Yaghoubinezhad: Supervision, Review & Editing.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: