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C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction Yousuf Ali 1, Van-Toan Nguyen 1, Ngoc-Anh Nguyen, Sangho Shin, Ho-Suk Choi* Department of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon, 34134, South Korea
article info
abstract
Article history:
Development of highly efficient electrocatalysts to produce hydrogen has been a sig-
Received 21 December 2018
nificant topic over the past few decades. Currently, the platinum metal group shows the
Received in revised form
best catalytic performance for the hydrogen evolution reaction (HER), but the high cost
28 January 2019
and low abundance of these materials limit their wider application. Therefore, we syn-
Accepted 31 January 2019
thesized transition-metal-based NiCoS along with carbon dots (C-dots) as a structure-
Available online xxx
directing agent by a hydrothermal method. We also synthesized sulfur-doped NiCo, where the sulfur enhances the conductivity of the catalysts. Herein, the synthesis
Keywords:
temperatures were changed in the range from 120 to 240 C. Among all, NiCoS synthe-
NiCo alloy
sized at 150 C shows the best HER performance capabilities. In more detail, NiCoS
S-doping
prepared at this temperature exhibits an onset potential of 96 mV and an overpotential of
Carbon dots
232 mV. Especially, as-prepared NiCoS nanoflower subjects to long-term stability over
Nanoflower
20 h at a current density of 10 mA/cm2, making it a promising low-cost candidate for
Transition metal
hydrogen production.
Hydrogen evolution reaction
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Global warming is currently a major problem and has devastating effects on animals and plants. Greenhouse gases such as CO2, N2O, O3, CFCs, and CH4 are mainly responsible for global warming. Humans rely heavily on fossil fuels such as coal, oil, and natural gas in daily life. As a result, greenhouse gas emissions have occurred and the average earth temperature is continuously increasing. Reconciling this threat and the energy needs of large human populations, renewable
energy sources such as solar, wind, and hydropower can replace fossil fuels. However, the intermittent nature of renewable energy sources is one of the key drawbacks hindered the wider use of these sources, leading not to undertake as sources of continues energy supply. According to this aspect, several technologies based on energy generation and storage have been developed [1,2], in which the energy can be stored in thermal, electrochemical or chemical processes derived by a renewable source [3]. Among various existing technologies, hydrogen (H2) production has significantly received attention because it is a clean fuel and energy carrier
* Corresponding author. E-mail address: [email protected] (H.-S. Choi). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.01.297 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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[4]. Electrochemical electrolysis and photo-electrochemical electrolysis of water represent the most promising hydrogen production strategies [5]. However, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) still remain challenging due to their kinetically sluggish in nature [6e8]. Recently, the platinum (Pt) metal group is considered to be the most efficient electrocatalysts for HER applications in both acidic and basic media, while the high cost and scarcity of these metals severely preclude their large-scale use for H2 production [9e13]. Therefore, it is necessary to investigate low-cost, earth-abundant and highly active electrocatalysts to replace the Pt group and to expedite HER kinetics for constructing the next generation for water-splitting applications. Over the past decades, numerous efforts have been dedicated to develop active, stable and low-cost catalysts for HER applications that can be worked efficiently under harsh conditions such as acidic [14e16] and basic [17e19] solutions. They include transition-metal alloys [20], dichalcogenides [21], nitrides [22], carbides [23], phosphides [24e26], and sulfides [27]. Among them, transition-metal dichalcogenides (MX2, M ¼ Ni, Co, Fe, and X ¼ S, Se) have drawn considerable attention, due to their low price, high earth abundance and environmentally benign nature [28]. In particular, nickel-cobalt sulfide [29,30] and nickel disulfide (NiS2) [31] exhibit intrinsically conductive properties, making them highly attractive as electrocatalysts for hydrogen production. Although several approaches improve the electrocatalytic activity to some extent, the achievements are limited to a certain specified level. Nanostructures with complex morphology, such as nanosponge, nanocone, nanoniddle, nanosheet, and nanoball increase their specific surface areas, thus increasing the active sites of catalysts [32]. Similarly, the nanostructure grown on a conductive substrate also facilitates charge transport by creating a charge transfer pathways [33]. Constructing heterostructures with different metallic oxides and sulfides by adjusting the surface electronic structures improve electrocatalytic activity [34]. Both sul fur-doped NiS2 and CoS2 show electrochemical potential for the HER and sulfur-doped bimetallic NiCo with a pyrite-type structure reveals superior catalytic activity compared to monometallic Ni and Co sulfides. This occurs due to the synergistic effects between surface-unoccupied Ni and Co active sites [35]. Meanwhile, the HER performance can be significantly affected by elemental ratios between nickel-cobalt alloy and sulfur elements, such as the Ni-doping amount in the final composition material. Regarding HER performance, the electrocatalytic activity of the bimetal catalyst (Ni0.1Co0.9S2) with doping 10% of Ni atoms exhibits the significantly higher than that of bare CoS2 [36]. However, certain characteristics of these materials have yet to be investigated extensively, especially their electrochemical activity and durability levels in different environments, such as in acidic and alkaline media. Considering the lower activity and instability of transition metal-based catalysts in an acidic medium, a new strategy has to be urgently developed to produce active and stable catalysts from earth-abundant elements. In this work, we report a facile synthetic strategy for the fabrication of non-noble, nickel cobalt sulfide as an electrocatalyst for the HER. The synthesis was performed through a two-step hydrothermal method in which NiCo precursor was
initially synthesized along with C-dots and then transformed into corresponding NiCoS nanoflower through a thermal treatment with thiourea. Interestingly, the C-dots act as a structure-directing agent and produce a nanoflowerlike structure with a high specific surface area. A detailed investigation of the effects of the temperature and sulfur dose was carried out through characterization analyses coupled with HER electrocatalytic performance tests, showing the relationships between the structural properties and these variables.
Experimental Materials Nickel(II) chloride hexahydrate (NiCl2.6H2O, 99.9% trace metal basic), cobalt(II) chloride hexahydrate (CoCl2.6H2O, 99.999% trace metal basic), thiourea (CH4N2S, 99.0%), sodium borohydride (NaBH4, 98.0%) and ethanol were purchased from SigmaAldrich (USA). The commercial Pt/C catalyst used here (20 wt% Pt) was obtained from Alfa Aesar (USA). The nitrogen gas was purchased from Yonhap LPG (Korea). The Nafion solution (5 wt %, Ion Power, LIQUion 1100) was purchased from Ion-Power (USA). The glassy carbon (diameter: 3 mm; area: ~0.071 cm2) used here was purchased from ALS Co., Ltd. (Japan).
Synthesis of the NiCo nanoflower In a typical procedure, NiCo nanoflowers were synthesized from nickel chloride hexahydrate and cobalt chloride hexahydrate, as shown in Fig. 1. Firstly, the nickel chloride hexahydrate, cobalt chloride hexahydrate (5:5) and 24 mg of C-dots were dissolved in 50 ml of DI water and sonicated for at least 30 min to create a fine solution. This solution served as the precursor solution. Second, 226.98 mg of NaBH4 was dispersed to 50 ml of DI water separately and sonicated for 30 min to prepare a homogeneous solution. In the next step, the precursor solution was added to a three-opening round-bottomed flask and the reducing agent was added dropwise in a continuous magnetic stirring condition at a homogeneous temperature of 70 C. After the completion of the reaction, black precipitates formed at the bottom of the flask. The precipitates were then washed with DI water and ethanol using a centrifuge at 13,000 rpm to remove unnecessary chemicals. The final nanoparticles were dried in a vacuum drier at 60 C for 24 h.
Synthesis of NiCoS To synthesize the NiCoS nanoflowers, 10 mg of NiCo nanoflowers was added to 50 ml of ethanol. Additionally, 228.36 mg of (NH2)2CS (thiourea) was added to the solution and then sonicated for 30 min to create a fine solution. The solution was then transferred to a 100 ml Teflon-lined autoclave and hydrothermally kept at different temperature (120, 150, 180, 210, 240 C) for 24 h to complete the reaction properly. After cooling to room temperature naturally, the products were collected by centrifugation, washed several times with water, and dried at 60 C for 24 h under vacuum.
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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Fig. 1 e Schematic diagram of the NiCoS nanocatalyst synthesis process.
Characterization of the NiCoS nanoflower The crystal structure of NiCoS-series nanoflowers was measured by using a powder X-ray diffractometer (XRD, D8 Discover, Bruker AXS, Germany) equipped with a Cu Ka radiation (l ¼ 1.5406 A). The morphologies of NiCoS-series nanoflowers were conducted by scanning electron microscopy (SEM, 0.2e30 kV, Merlin Compact) and transmission electron microscopy (TEM, 200 kV, Tecnai F30 ST, United States). The NiCoS nanoflowers were sonicated in methanol until the formation of a homogeneous solution before dripping onto the copper grid. The surface compositions and elemental state of as-synthesized nanoparticle were detected by X-ray photoelectron spectroscopy (XPS) (Thermo-Fisher Scientific, Waltham, MA, USA) apparatus with Al as the excitation source.
with a perturbation voltage amplitude of 10 mV. The EIS spectra were analyzed and fitted using the Z-view software. All potentials reported are converted to the RHE.
Results and discussion Structural analysis In order to confirm the crystalline structure, the assynthesized NiCoS nanoflowers were characterized by XRD
Electrochemical measurements The electrochemical measurements were conducted in a three-electrode system with an electrochemical workstation (rotating ring-disk electrode) and in an IVIUM potentiostat at room temperature. A Pt-wire, an Ag/AgCl (in a saturated KCl solution) electrode and a glassy carbon electrode (0.071 cm2) were used as the counter, reference, and working electrodes, respectively. The HER activities of the synthesized catalysts were investigated with linear sweep voltammetry (LSV) at a scan rate of 5 mV/s in 0.5 M H2SO4 solutions. The electrolyte solution was typically purged with ultrapure N2 for 30 min before the electrochemical measurement. The catalyst ink was prepared by blending the catalyst powder (2.0 mg) in a 0.5 ml stock solution of isopropyl alcohol (IPA)/H2O/Nafion (20.0/80.0/ 0.04 ml). After sonication for 30 min, 3 mL of the catalyst ink was deposited onto the glassy carbon disk using a micropipette. The Nafion solution improved adhesion to the glassy carbon electrode. Electrochemical impedance spectroscopy (EIS) was performed at 0.3 V using a frequency range from 0.1 to 105 Hz
Fig. 2 e XRD analysis of NiCoS at different temperatures with similar and smaller peaks that indicate the amorphous structure of the samples. The red, green and black baselines indicate the peaks of the cubic NiCoS, rhombohedral NiS, and carbon, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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(Fig. 2). The diffraction peaks are quite weak owing to the ultrathin nanosheet building blocks, as found by characterization of the morphology. For the XRD patterns of the NiCoS nanoflowers, all of the characteristic diffraction peaks located at 2q values of 20.5 , 22.6 , 26.9 , 31.8 , 38.5 , 51.2 , and 55.5 could be readily indexed to the (002), (101), (220), (311), (400), (511) and (440) planes [37,38]. Herein, the (220), (311), (400), (511) and (440) planes reflect the cubic phase of NiCoS (JCPDS card No. 20-0782). Similarly, (101) represents the rhombohedral NiS and (002) symbolizes the plane for carbon. It can be concluded that nearly all of the NiCo precursors transformed into the NiCoS phases after sulfurization, as there are no major impurity phases in the XRD pattern. Therefore, the synthesized NiCoS nanoflowers are of high purity and show a polycrystalline structure. The morphological properties of the NiCo and NiCoS were investigated by SEM characterization, and typical images are displayed in Fig. 3, Fig. S1 and Fig. S2. Fig. S1(a) shows an SEM image of as-synthesized NiCo, which has a 3D-nanoflowerlike structure. This figure clearly reveals the surface of the nanoflower, constructed by randomly oriented nanosheets that intersect each other to form a 3D-network structure with numerous unoccupied spaces. However, the morphology of NiCo without C-dots appears as a nano-cluster instead of nanoflower (as shown in Fig. S2). Therefore, the formation of such a structure is caused by the presence of C-dots. This type of structure provides a high specific surface area for the electrochemical reaction to occur. Fig. 3(aee) shows the SEM images of NiCoS, synthesized at different temperatures. The effects of the synthesis temperature are apparent in the SEM images of NiCoS. At a temperature of 120 C, as shown in Fig. 3(a), the sulfur content does not properly cover the nanoflower and agglomeration occurs due to the insufficient
reaction condition at this low temperature. Due to the agglomeration, NiCoS-120 shows a less active surface area. With regard to the NiCoS-180, 210 and 240 C samples, the morphologies also change with the temperature (Fig. 3(cee)). It is easily conceivable that the crystallinity of NiCoS increases with the temperature and the nanosheets disappear at a higher temperature. Therefore, the surface-to-volume ratio was abated with an increase in the temperature, showing low catalytic activity. On the other hand, the NiCoS-150 C sample (Fig. 3(b)) retained most of its flower-like structure after the sulfurization process. Similarly, the sulfur content was properly distributed throughout the surface. Furthermore, the Cdots act as a protective layer of the nanoparticles while sulfur content enhancing electrical conductivity [39]. A detailed investigation of the morphological properties was investigated with an HRTEM analysis. The TEM image of the NiCoS-150 C nanoflowers (Fig. S1b) clearly reveals the existence of a nanoflower-like complex structure. The inset in the figure shows the presence of a nanosheet-like appearance. Similarly, the enlarged image of the NiCoS-150 sample in Fig. 3(f) shows a sulfur-coated surface with a highly porous structure. It also depicts the apparently smaller size (mostly 15e20 nm) of the particles. Furthermore, the uniform distribution of the numerous inter-particle porous channels throughout the nanosheet can be observed from the TEM image taken under high magnification. The high-resolution HRTEM image in Fig. 3(g) reveals the lattice fringes on the nanosheet edge with inter-planar distances of 3.34, 2.81 and 2.08 A corresponding to the (220), (311) and (400) planes of cubic NiCoS, which is in accord with the results of the XRD analysis [40]. This observation proves that nearly all NiCo precursors were turned into the NiCoS phase after sulfurization at 150 C. In addition, the composition of the NiCoS
Fig. 3 e SEM images of various NiCoS nanoflowers: (a) NiCoS-120, (b) NiCoS-150, (c) NiCoS-180, (d) NiCoS-210, and (e) NiCoS are matched to (400), 240 and TEM image of (f) NiCoS-150 nanoflower catalyst. (g) Lattice spacings of 2.08, 2.81, and 3.34 A (311), and (220), respectively. Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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nanoflowers was measured by EDS based on selected-area diffraction. Figs. S3, S4, and S5 confirm the presence of all C, S, Co and Ni elements in NiCoS-150, NiCoS-180 and NiCoS-120, respectively. While Fig. S6 displays the presence of only Ni, Co and S elements and the absence of C-dots. The elemental compositions are shown in Tables S1, S2, S3 and S4 in the inset of each figure. The amount of sulfur content in NiCoS-150 is highest compared to other samples. The 3D-networked and porous NiCoS-150 nanoflowers are considered to highly benefit for the electrocatalysis operation mainly due to the ultrathin and porous NiCoS nanosheets, which provide a large number of active sites to expedite the HER activity. The plentiful pores and void spaces between the nanosheets, making it easier to diffuse and penetrate the ions and electrons during the electrochemical operation [41]. As a result, the NiCoS nanoflowers provide a superior electrocatalytic activity during electrochemical operation by creating easy electron transfer routes. In addition, the existence of Ni, Co, S and C elements were confirmed by further monitoring the XPS result, as shown in the survey spectrum (Fig. S7). The other peaks come from the silicon substrate and some carbons. The Ni 2p, Co 2p, and S 2p were deconvoluted by the XPSPEAK41 software. Fig. 4(a) exhibits a spin-orbit doublet and two satellite peaks of the Ni 2p region. The main peaks at 889.98 and 857.2 eV correspond to Ni 2p1/2 and Ni 2p3/2, respectively [42]. In Fig. 4(b), the Co 2p spectrum depicts the two main peaks of Co 2p1/2 and 2p3/2 located at 811.98 and 782.62 eV along with two satellite peaks [29,32,40]. Fig. 4(c) shows the presence of carbon with CeC, CeO and C]O bonds at 284, 285.5 and 288.4 eV. In the S 2p region, as shown in Fig. 4(d), the peaks centered at 168.84 and 169.2 eV are due to S2 and sulfur ions with a satellite peak [29,32,40]. This result
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suggests that the surface of the catalyst still retains some oxygen-containing functional groups under the hydrothermal conditions. Especially, those oxygen-containing functional groups are responsible for anchoring S atoms on the surface of NiCo alloy structure and strengthening the structural stability of hybrid materials.
Electrocatalytic performance of NiCoS The as-synthesized nanocatalysts were then introduced to measure the electrocatalytic HER activity in 0.5 M of H2SO4 electrolyte with a scan rate of 10 mV/s. Linear sweep voltammetry (LSV) measurements were conducted to estimate the HER performance of the NiCoS nanocatalysts synthesized at different temperatures, with control experiments conducted as well. The electrochemical performance of commercial platinum carbon and bare NiCo was also assessed to compare various catalysts. The LSV polarization curve of NiCoS-150 C exhibited an onset potential of only 90 mV, which is much smaller than those of NiCoS-120 (202 mV), NiCoS-180 (194 mV), NiCoS-210 (191 mV), NiCoS-240 (225 mV) and bare NiCo (242 mV) (Fig. 5(a)). This result suggests that the surface of NiCoS-150 is much more active for the HER. Furthermore, the overpotential of NiCoS-150 approached 232 mV, remarkably lower than those of NiCoS-120 (381 mV), NiCoS-180 (322 mV), NiCoS-210 (356 mV), NiCoS-240 (371 mV) and bare NiCo (433 mV) at a cathodic current density of 10 mA/ cm2, whereas commercial platinum carbon exhibits a value of only 63 mV, much smaller than the catalyst synthesized here. Nevertheless, due to the level of earth-abundancy of the material, this result is comparable to that of commercial platinum. This result also can be compared to other sulfur-doped
Fig. 4 e High-resolution XPS spectra of NiCoS-150 of (a) Ni 2p (b) Co 2p, (c) C 1s and (d) S 2p. Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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Fig. 5 e (a) Linear sweep voltammograms of various NiCoS nanoflowers at different temperatures at a scan rate of 10 mV/s, (b) Tafel slope, (c) linear fitting shows the differences of the capacitive current density against scan rates of various NiCoS nanoflowers. (d) Electrochemical impedance spectroscopy of NiCoS at ¡0.30 V.
nanocatalysts, as shown in Table.S5. At a low temperature, NiCoS shows low catalytic activity due to the agglomeration of nanocatalysts. Similarly, at the higher temperature, the nanosheets of nanoflower disappear and at the same time, crystallinity arises, providing a lower surface area. Therefore, the catalyst shows a lower catalytic activity. The optimization curve showed in Fig. S8 represents the temperature effect on the electrocatalytic performance, proving the superiority of the NiCoS-150 nanoflowers. The effect of the sulfur dose was also studied while varying the amount of thiourea from 1 to 12 mM at a fixed temperature of 150 C. The electrolytic performance is shown in Fig. S9. Furthermore, it is clear from the optimization curve in Fig. S10 that 3 mM of sulfur produces higher catalytic activity comparing the other samples. This occurred because 3 mM is synthetically sufficient to disperse homogeneously on NiCo nanoflowers. Furthermore, this homogeneity ensures the proper interaction between elements that helps to create easy routes for electrons to transfer. However, at higher amounts of dose, the density of sulfur on NiCo also increases, reducing the interaction with NiCo and reducing the number of active sites as well. As a result, it shows lower catalytic activity. Similarly, small sulfur dose cannot create enough routes for electrons to transfer, also showing lower catalytic activity. Therefore, it can be concluded that 3 mM of thiourea is the optimal dose amount for NiCoS synthesis. Especially, we further measured the HER performance of NiCoS-150 without C-dot to compare with NiCoS-150 nanoflowers (as shown in Fig. S11(a) and (b)). The catalytic activity and stability of NiCoS-150 nanoflower were
better than those of NiCoS-150 without C-dot. This result proves that C-dot has a role to enhance not only HER electrocatalytic activity but also improve the stability of NiCoS-150 nanoflowers. The Tafel plot and the fitting curves of various catalysts are displayed in Fig. 5(b). The Tafel equation h ¼ a þ b log j (where j is the current density and b is the Tafel slope) was used to obtain Tafel slope b, which is related to the inherent properties of the catalyst and which indicates the ratedetermining step in the entire HER process. The Tafel slopes of NiCoS-120, NiCoS-150, NiCoS-180, NiCoS-210, NiCoS-240, and NiCo were calculated as 138.89, 80.17, 85.2, 84.86, 91.84 and 210.39 mV/dec, respectively, proving that the 3D-network NiCoS-150 reveals the highest catalytic activity compared to other samples. The Tafel slope of c-Pt/c was also measured (45.77 mV/dec) for comparison. According to a previous report [43], the mechanism of the HER process is the Volmer-Heyrovsky mechanism, which consists of the rapid discharge of a proton and the slow combination of the discharged proton with an additional proton following the reaction of Had þ H3Oþ þ e / H2 þ H2O. The relatively low Tafel slope for NiCoS-150 implies a faster reaction rate compared to the other cases, leading to a rapid increment of hydrogen evolution reaction. Although NiCoS150 shows a higher Tafel slope than commercial Pt/c, the 3D nanoflower structure of the NiCoS-150 catalysts nonetheless have great potential to use in practical HER applications owing to their low cost and relatively high levels of electrochemical activity.
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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Table 1 e LSV, Tafel slope, and EIS parameters after fitting Nyquist plots and Cdl obtained from NiCoS synthesized at different temperatures. Electrocatalysts NiCoS-120 NiCoS-150 NiCoS-180 NiCoS-210 NiCoS-240
Overpotential (mV)
Tafel slope (mV/dec)
Rct (U)
Cdl (mF)
ECSA (mFcm2)
382 232 310 358 378
138.89 80.17 85.2 84.86 91.84
394.7 66.3 104.6 650.5 950.2
0.481 0.929 0.660 0.314 0.251
13.74 26.54 18.85 8.97 7.17
The HER performance depends strongly on the electrochemically active surface area (ECSA) because the higher ECSA provides more active site [44,45]. The double-layer capacitance (Cdl) at the interface of solid/liquid layers was monitored to evaluate the ECSA of these samples [44]. The CV curves were conducted in the potential sweep of 0.0e0.3 V to show the region of the electrical double layers on the surfaces of the electrodes (Fig. S12). Fig. 5(c) shows the ECSA outcomes of NiCoS-120, NiCoS-150, NiCoS-180, NiCoS-210 and NiCoS240 C, which were calculated as 13.74, 26.54, 18.85, 8.97 and 7.17 mF/cm2 respectively. These results indicate that the NiCoS-150 nanoflowers have a much higher ECSA than the other catalysts, suggesting that the better HER activity of NiCoS-150 may be attributed to its higher ECSA and a larger number of active sites. The HER activity of as-synthesized NiCoS at a different temperature is also evidenced by the EIS measurements. Nyquist plots during the HER operation at 0.30 V vs RHE in acidic (0.5 M H2SO4) electrolytes are shown in Fig. 5(d). In all cases, one semicircle was observed, and the corresponding model of the electrical equivalent circuit is shown in the inset of Fig. 5(d). Rs, Rct, and Cdl indicate the electrolyte resistance, charge transfer resistance, and double-layer capacitance, respectively. The lower Rct value means, the greater the possibility of transferring the charge through the double layer. Therefore, the catalyst will perform better for HER. Table 1 shows the values of the parameters obtained by fitting the
impedance spectra. Obviously, the Rct value of the NiCoS-150 sample is 63.3 U, which is the lowest charge transfer resistance among all of the synthesized catalysts. The Rct values obtained for NiCoS-120, NiCos-180, NiCoS-210 and NiCoS240 C were 394.7 U, 104.6 U, 650.5 U and 950.2 U, respectively. Hence, the NiCoS-150 sample shows the best charge transfer which aid for accelerating the electrocatalytic activity, resulting in the higher HER performance of NiCoS-150 compared to others. These results are well consistent with the voltammetric outcomes. Chronopotentiometric measurements were carried out on NiCoS-150 nanoflowers based on the HER activity at a constant current density of 10 mA/cm2, lasting a long time, as shown in Fig. 6. Remarkably, NiCoS-150 exhibited a constant overpotential for approximately 20 h. However, the overpotential starts to increase from 232 mV after 20 h, implying the increase of the catalytic stability of the NiCoS-150 nanoflower toward the HER. Similarly, the inset of Fig. 6 shows hydrogen bubbling, which indicates the generation of hydrogen. The C-dots act as a protective layer and sulfur content increase the conductivity of the nanostructure, confirming the high architectural stability and catalytic activity. As a result, the NiCoS-150 nanoflower depicts much higher catalytic performance than the other catalysts. Furthermore, the main reasons for the higher catalytic activity of NiCoS-150 are as follows. First, at 150 C, the transition-metal chalcogenides possess coordination structures, which act as the active centers in the effective hydrogen evolution reaction [46]. The charge characteristics of the metals and sulfur in the metal sulfides operate as a pair of hydride/proton acceptors in hydrogenase and its counterparts [47,48]. Secondly, NiCoS-150 shows much better electrical conductivity than the other samples (see the EIS data in Fig. 6(d)), which accelerate the electron transport, leading to boost the HER activity. Finally, the NiCoS-150 nanoflowers show the best hierarchical and porous structures compared to the others. Therefore, the electrolyte can easily penetrate and migrate to the inner/outer surfaces of the NiCoS-150 nanoflowers, providing short pathways for the transport of electrons and consequently much more unoccupied active sites for the HER.
Conclusion
Fig. 6 e Long-term chronopotentiometric tests of NiCoS150 carried out for HER at a current density of 10 mA/cm2 for 20 h.
In summary, we have rationally synthesized NiCoS/C-dot, 3D-nanoflowers as an excellent and non-noble HER electrocatalyst through the sulfurization of NiCo at 150 C. The HER activity of the NiCoS-150 nanoflower was superior to those of the others because the porous nanoflower-like structure provides a large number of exposed active sites, leading to
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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accelerating the transport of electrons and ions. Therefore, NiCoS-150 reveals a great enhancement of catalytic performance in an acidic solution compared to NiCoS synthesized at 120,180, 210 and 240 C. Moreover, bare NiCo shows almost no catalytic activity during the HER. On the other hand, NiCoS-150 showed a small overpotential of 232 mV required to reach a current density of 10 mA/cm2, as well as a Tafel slope of 80.17 mV/dec and long-term stability of up to 20 h. Finally, it can be said that the high activity and stability of NiCoS synthesized at 150 C makes this nanoflower as a promising low-cost and stable electrocatalyst for hydrogen evolution reaction.
[9]
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Acknowledgment This research was supported by a National Research Foundation (NRF) grant (NRF-2017R1A2B2001911; NRF-2017R1A4A1015360) funded by the Ministry of Science and ICT, Republic of Korea.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2019.01.297.
[13]
[14]
[15]
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Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
Available online at www.sciencedirect.com
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Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction Yousuf Ali 1, Van-Toan Nguyen 1, Ngoc-Anh Nguyen, Sangho Shin, Ho-Suk Choi* Department of Chemical Engineering and Applied Chemistry, Chungnam National University, Daejeon, 34134, South Korea
article info
abstract
Article history:
Development of highly efficient electrocatalysts to produce hydrogen has been a sig-
Received 21 December 2018
nificant topic over the past few decades. Currently, the platinum metal group shows the
Received in revised form
best catalytic performance for the hydrogen evolution reaction (HER), but the high cost
28 January 2019
and low abundance of these materials limit their wider application. Therefore, we syn-
Accepted 31 January 2019
thesized transition-metal-based NiCoS along with carbon dots (C-dots) as a structure-
Available online xxx
directing agent by a hydrothermal method. We also synthesized sulfur-doped NiCo, where the sulfur enhances the conductivity of the catalysts. Herein, the synthesis
Keywords:
temperatures were changed in the range from 120 to 240 C. Among all, NiCoS synthe-
NiCo alloy
sized at 150 C shows the best HER performance capabilities. In more detail, NiCoS
S-doping
prepared at this temperature exhibits an onset potential of 96 mV and an overpotential of
Carbon dots
232 mV. Especially, as-prepared NiCoS nanoflower subjects to long-term stability over
Nanoflower
20 h at a current density of 10 mA/cm2, making it a promising low-cost candidate for
Transition metal
hydrogen production.
Hydrogen evolution reaction
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Global warming is currently a major problem and has devastating effects on animals and plants. Greenhouse gases such as CO2, N2O, O3, CFCs, and CH4 are mainly responsible for global warming. Humans rely heavily on fossil fuels such as coal, oil, and natural gas in daily life. As a result, greenhouse gas emissions have occurred and the average earth temperature is continuously increasing. Reconciling this threat and the energy needs of large human populations, renewable
energy sources such as solar, wind, and hydropower can replace fossil fuels. However, the intermittent nature of renewable energy sources is one of the key drawbacks hindered the wider use of these sources, leading not to undertake as sources of continues energy supply. According to this aspect, several technologies based on energy generation and storage have been developed [1,2], in which the energy can be stored in thermal, electrochemical or chemical processes derived by a renewable source [3]. Among various existing technologies, hydrogen (H2) production has significantly received attention because it is a clean fuel and energy carrier
* Corresponding author. E-mail address: [email protected] (H.-S. Choi). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.01.297 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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[4]. Electrochemical electrolysis and photo-electrochemical electrolysis of water represent the most promising hydrogen production strategies [5]. However, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) still remain challenging due to their kinetically sluggish in nature [6e8]. Recently, the platinum (Pt) metal group is considered to be the most efficient electrocatalysts for HER applications in both acidic and basic media, while the high cost and scarcity of these metals severely preclude their large-scale use for H2 production [9e13]. Therefore, it is necessary to investigate low-cost, earth-abundant and highly active electrocatalysts to replace the Pt group and to expedite HER kinetics for constructing the next generation for water-splitting applications. Over the past decades, numerous efforts have been dedicated to develop active, stable and low-cost catalysts for HER applications that can be worked efficiently under harsh conditions such as acidic [14e16] and basic [17e19] solutions. They include transition-metal alloys [20], dichalcogenides [21], nitrides [22], carbides [23], phosphides [24e26], and sulfides [27]. Among them, transition-metal dichalcogenides (MX2, M ¼ Ni, Co, Fe, and X ¼ S, Se) have drawn considerable attention, due to their low price, high earth abundance and environmentally benign nature [28]. In particular, nickel-cobalt sulfide [29,30] and nickel disulfide (NiS2) [31] exhibit intrinsically conductive properties, making them highly attractive as electrocatalysts for hydrogen production. Although several approaches improve the electrocatalytic activity to some extent, the achievements are limited to a certain specified level. Nanostructures with complex morphology, such as nanosponge, nanocone, nanoniddle, nanosheet, and nanoball increase their specific surface areas, thus increasing the active sites of catalysts [32]. Similarly, the nanostructure grown on a conductive substrate also facilitates charge transport by creating a charge transfer pathways [33]. Constructing heterostructures with different metallic oxides and sulfides by adjusting the surface electronic structures improve electrocatalytic activity [34]. Both sul fur-doped NiS2 and CoS2 show electrochemical potential for the HER and sulfur-doped bimetallic NiCo with a pyrite-type structure reveals superior catalytic activity compared to monometallic Ni and Co sulfides. This occurs due to the synergistic effects between surface-unoccupied Ni and Co active sites [35]. Meanwhile, the HER performance can be significantly affected by elemental ratios between nickel-cobalt alloy and sulfur elements, such as the Ni-doping amount in the final composition material. Regarding HER performance, the electrocatalytic activity of the bimetal catalyst (Ni0.1Co0.9S2) with doping 10% of Ni atoms exhibits the significantly higher than that of bare CoS2 [36]. However, certain characteristics of these materials have yet to be investigated extensively, especially their electrochemical activity and durability levels in different environments, such as in acidic and alkaline media. Considering the lower activity and instability of transition metal-based catalysts in an acidic medium, a new strategy has to be urgently developed to produce active and stable catalysts from earth-abundant elements. In this work, we report a facile synthetic strategy for the fabrication of non-noble, nickel cobalt sulfide as an electrocatalyst for the HER. The synthesis was performed through a two-step hydrothermal method in which NiCo precursor was
initially synthesized along with C-dots and then transformed into corresponding NiCoS nanoflower through a thermal treatment with thiourea. Interestingly, the C-dots act as a structure-directing agent and produce a nanoflowerlike structure with a high specific surface area. A detailed investigation of the effects of the temperature and sulfur dose was carried out through characterization analyses coupled with HER electrocatalytic performance tests, showing the relationships between the structural properties and these variables.
Experimental Materials Nickel(II) chloride hexahydrate (NiCl2.6H2O, 99.9% trace metal basic), cobalt(II) chloride hexahydrate (CoCl2.6H2O, 99.999% trace metal basic), thiourea (CH4N2S, 99.0%), sodium borohydride (NaBH4, 98.0%) and ethanol were purchased from SigmaAldrich (USA). The commercial Pt/C catalyst used here (20 wt% Pt) was obtained from Alfa Aesar (USA). The nitrogen gas was purchased from Yonhap LPG (Korea). The Nafion solution (5 wt %, Ion Power, LIQUion 1100) was purchased from Ion-Power (USA). The glassy carbon (diameter: 3 mm; area: ~0.071 cm2) used here was purchased from ALS Co., Ltd. (Japan).
Synthesis of the NiCo nanoflower In a typical procedure, NiCo nanoflowers were synthesized from nickel chloride hexahydrate and cobalt chloride hexahydrate, as shown in Fig. 1. Firstly, the nickel chloride hexahydrate, cobalt chloride hexahydrate (5:5) and 24 mg of C-dots were dissolved in 50 ml of DI water and sonicated for at least 30 min to create a fine solution. This solution served as the precursor solution. Second, 226.98 mg of NaBH4 was dispersed to 50 ml of DI water separately and sonicated for 30 min to prepare a homogeneous solution. In the next step, the precursor solution was added to a three-opening round-bottomed flask and the reducing agent was added dropwise in a continuous magnetic stirring condition at a homogeneous temperature of 70 C. After the completion of the reaction, black precipitates formed at the bottom of the flask. The precipitates were then washed with DI water and ethanol using a centrifuge at 13,000 rpm to remove unnecessary chemicals. The final nanoparticles were dried in a vacuum drier at 60 C for 24 h.
Synthesis of NiCoS To synthesize the NiCoS nanoflowers, 10 mg of NiCo nanoflowers was added to 50 ml of ethanol. Additionally, 228.36 mg of (NH2)2CS (thiourea) was added to the solution and then sonicated for 30 min to create a fine solution. The solution was then transferred to a 100 ml Teflon-lined autoclave and hydrothermally kept at different temperature (120, 150, 180, 210, 240 C) for 24 h to complete the reaction properly. After cooling to room temperature naturally, the products were collected by centrifugation, washed several times with water, and dried at 60 C for 24 h under vacuum.
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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Fig. 1 e Schematic diagram of the NiCoS nanocatalyst synthesis process.
Characterization of the NiCoS nanoflower The crystal structure of NiCoS-series nanoflowers was measured by using a powder X-ray diffractometer (XRD, D8 Discover, Bruker AXS, Germany) equipped with a Cu Ka radiation (l ¼ 1.5406 A). The morphologies of NiCoS-series nanoflowers were conducted by scanning electron microscopy (SEM, 0.2e30 kV, Merlin Compact) and transmission electron microscopy (TEM, 200 kV, Tecnai F30 ST, United States). The NiCoS nanoflowers were sonicated in methanol until the formation of a homogeneous solution before dripping onto the copper grid. The surface compositions and elemental state of as-synthesized nanoparticle were detected by X-ray photoelectron spectroscopy (XPS) (Thermo-Fisher Scientific, Waltham, MA, USA) apparatus with Al as the excitation source.
with a perturbation voltage amplitude of 10 mV. The EIS spectra were analyzed and fitted using the Z-view software. All potentials reported are converted to the RHE.
Results and discussion Structural analysis In order to confirm the crystalline structure, the assynthesized NiCoS nanoflowers were characterized by XRD
Electrochemical measurements The electrochemical measurements were conducted in a three-electrode system with an electrochemical workstation (rotating ring-disk electrode) and in an IVIUM potentiostat at room temperature. A Pt-wire, an Ag/AgCl (in a saturated KCl solution) electrode and a glassy carbon electrode (0.071 cm2) were used as the counter, reference, and working electrodes, respectively. The HER activities of the synthesized catalysts were investigated with linear sweep voltammetry (LSV) at a scan rate of 5 mV/s in 0.5 M H2SO4 solutions. The electrolyte solution was typically purged with ultrapure N2 for 30 min before the electrochemical measurement. The catalyst ink was prepared by blending the catalyst powder (2.0 mg) in a 0.5 ml stock solution of isopropyl alcohol (IPA)/H2O/Nafion (20.0/80.0/ 0.04 ml). After sonication for 30 min, 3 mL of the catalyst ink was deposited onto the glassy carbon disk using a micropipette. The Nafion solution improved adhesion to the glassy carbon electrode. Electrochemical impedance spectroscopy (EIS) was performed at 0.3 V using a frequency range from 0.1 to 105 Hz
Fig. 2 e XRD analysis of NiCoS at different temperatures with similar and smaller peaks that indicate the amorphous structure of the samples. The red, green and black baselines indicate the peaks of the cubic NiCoS, rhombohedral NiS, and carbon, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article).
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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(Fig. 2). The diffraction peaks are quite weak owing to the ultrathin nanosheet building blocks, as found by characterization of the morphology. For the XRD patterns of the NiCoS nanoflowers, all of the characteristic diffraction peaks located at 2q values of 20.5 , 22.6 , 26.9 , 31.8 , 38.5 , 51.2 , and 55.5 could be readily indexed to the (002), (101), (220), (311), (400), (511) and (440) planes [37,38]. Herein, the (220), (311), (400), (511) and (440) planes reflect the cubic phase of NiCoS (JCPDS card No. 20-0782). Similarly, (101) represents the rhombohedral NiS and (002) symbolizes the plane for carbon. It can be concluded that nearly all of the NiCo precursors transformed into the NiCoS phases after sulfurization, as there are no major impurity phases in the XRD pattern. Therefore, the synthesized NiCoS nanoflowers are of high purity and show a polycrystalline structure. The morphological properties of the NiCo and NiCoS were investigated by SEM characterization, and typical images are displayed in Fig. 3, Fig. S1 and Fig. S2. Fig. S1(a) shows an SEM image of as-synthesized NiCo, which has a 3D-nanoflowerlike structure. This figure clearly reveals the surface of the nanoflower, constructed by randomly oriented nanosheets that intersect each other to form a 3D-network structure with numerous unoccupied spaces. However, the morphology of NiCo without C-dots appears as a nano-cluster instead of nanoflower (as shown in Fig. S2). Therefore, the formation of such a structure is caused by the presence of C-dots. This type of structure provides a high specific surface area for the electrochemical reaction to occur. Fig. 3(aee) shows the SEM images of NiCoS, synthesized at different temperatures. The effects of the synthesis temperature are apparent in the SEM images of NiCoS. At a temperature of 120 C, as shown in Fig. 3(a), the sulfur content does not properly cover the nanoflower and agglomeration occurs due to the insufficient
reaction condition at this low temperature. Due to the agglomeration, NiCoS-120 shows a less active surface area. With regard to the NiCoS-180, 210 and 240 C samples, the morphologies also change with the temperature (Fig. 3(cee)). It is easily conceivable that the crystallinity of NiCoS increases with the temperature and the nanosheets disappear at a higher temperature. Therefore, the surface-to-volume ratio was abated with an increase in the temperature, showing low catalytic activity. On the other hand, the NiCoS-150 C sample (Fig. 3(b)) retained most of its flower-like structure after the sulfurization process. Similarly, the sulfur content was properly distributed throughout the surface. Furthermore, the Cdots act as a protective layer of the nanoparticles while sulfur content enhancing electrical conductivity [39]. A detailed investigation of the morphological properties was investigated with an HRTEM analysis. The TEM image of the NiCoS-150 C nanoflowers (Fig. S1b) clearly reveals the existence of a nanoflower-like complex structure. The inset in the figure shows the presence of a nanosheet-like appearance. Similarly, the enlarged image of the NiCoS-150 sample in Fig. 3(f) shows a sulfur-coated surface with a highly porous structure. It also depicts the apparently smaller size (mostly 15e20 nm) of the particles. Furthermore, the uniform distribution of the numerous inter-particle porous channels throughout the nanosheet can be observed from the TEM image taken under high magnification. The high-resolution HRTEM image in Fig. 3(g) reveals the lattice fringes on the nanosheet edge with inter-planar distances of 3.34, 2.81 and 2.08 A corresponding to the (220), (311) and (400) planes of cubic NiCoS, which is in accord with the results of the XRD analysis [40]. This observation proves that nearly all NiCo precursors were turned into the NiCoS phase after sulfurization at 150 C. In addition, the composition of the NiCoS
Fig. 3 e SEM images of various NiCoS nanoflowers: (a) NiCoS-120, (b) NiCoS-150, (c) NiCoS-180, (d) NiCoS-210, and (e) NiCoS are matched to (400), 240 and TEM image of (f) NiCoS-150 nanoflower catalyst. (g) Lattice spacings of 2.08, 2.81, and 3.34 A (311), and (220), respectively. Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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nanoflowers was measured by EDS based on selected-area diffraction. Figs. S3, S4, and S5 confirm the presence of all C, S, Co and Ni elements in NiCoS-150, NiCoS-180 and NiCoS-120, respectively. While Fig. S6 displays the presence of only Ni, Co and S elements and the absence of C-dots. The elemental compositions are shown in Tables S1, S2, S3 and S4 in the inset of each figure. The amount of sulfur content in NiCoS-150 is highest compared to other samples. The 3D-networked and porous NiCoS-150 nanoflowers are considered to highly benefit for the electrocatalysis operation mainly due to the ultrathin and porous NiCoS nanosheets, which provide a large number of active sites to expedite the HER activity. The plentiful pores and void spaces between the nanosheets, making it easier to diffuse and penetrate the ions and electrons during the electrochemical operation [41]. As a result, the NiCoS nanoflowers provide a superior electrocatalytic activity during electrochemical operation by creating easy electron transfer routes. In addition, the existence of Ni, Co, S and C elements were confirmed by further monitoring the XPS result, as shown in the survey spectrum (Fig. S7). The other peaks come from the silicon substrate and some carbons. The Ni 2p, Co 2p, and S 2p were deconvoluted by the XPSPEAK41 software. Fig. 4(a) exhibits a spin-orbit doublet and two satellite peaks of the Ni 2p region. The main peaks at 889.98 and 857.2 eV correspond to Ni 2p1/2 and Ni 2p3/2, respectively [42]. In Fig. 4(b), the Co 2p spectrum depicts the two main peaks of Co 2p1/2 and 2p3/2 located at 811.98 and 782.62 eV along with two satellite peaks [29,32,40]. Fig. 4(c) shows the presence of carbon with CeC, CeO and C]O bonds at 284, 285.5 and 288.4 eV. In the S 2p region, as shown in Fig. 4(d), the peaks centered at 168.84 and 169.2 eV are due to S2 and sulfur ions with a satellite peak [29,32,40]. This result
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suggests that the surface of the catalyst still retains some oxygen-containing functional groups under the hydrothermal conditions. Especially, those oxygen-containing functional groups are responsible for anchoring S atoms on the surface of NiCo alloy structure and strengthening the structural stability of hybrid materials.
Electrocatalytic performance of NiCoS The as-synthesized nanocatalysts were then introduced to measure the electrocatalytic HER activity in 0.5 M of H2SO4 electrolyte with a scan rate of 10 mV/s. Linear sweep voltammetry (LSV) measurements were conducted to estimate the HER performance of the NiCoS nanocatalysts synthesized at different temperatures, with control experiments conducted as well. The electrochemical performance of commercial platinum carbon and bare NiCo was also assessed to compare various catalysts. The LSV polarization curve of NiCoS-150 C exhibited an onset potential of only 90 mV, which is much smaller than those of NiCoS-120 (202 mV), NiCoS-180 (194 mV), NiCoS-210 (191 mV), NiCoS-240 (225 mV) and bare NiCo (242 mV) (Fig. 5(a)). This result suggests that the surface of NiCoS-150 is much more active for the HER. Furthermore, the overpotential of NiCoS-150 approached 232 mV, remarkably lower than those of NiCoS-120 (381 mV), NiCoS-180 (322 mV), NiCoS-210 (356 mV), NiCoS-240 (371 mV) and bare NiCo (433 mV) at a cathodic current density of 10 mA/ cm2, whereas commercial platinum carbon exhibits a value of only 63 mV, much smaller than the catalyst synthesized here. Nevertheless, due to the level of earth-abundancy of the material, this result is comparable to that of commercial platinum. This result also can be compared to other sulfur-doped
Fig. 4 e High-resolution XPS spectra of NiCoS-150 of (a) Ni 2p (b) Co 2p, (c) C 1s and (d) S 2p. Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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Fig. 5 e (a) Linear sweep voltammograms of various NiCoS nanoflowers at different temperatures at a scan rate of 10 mV/s, (b) Tafel slope, (c) linear fitting shows the differences of the capacitive current density against scan rates of various NiCoS nanoflowers. (d) Electrochemical impedance spectroscopy of NiCoS at ¡0.30 V.
nanocatalysts, as shown in Table.S5. At a low temperature, NiCoS shows low catalytic activity due to the agglomeration of nanocatalysts. Similarly, at the higher temperature, the nanosheets of nanoflower disappear and at the same time, crystallinity arises, providing a lower surface area. Therefore, the catalyst shows a lower catalytic activity. The optimization curve showed in Fig. S8 represents the temperature effect on the electrocatalytic performance, proving the superiority of the NiCoS-150 nanoflowers. The effect of the sulfur dose was also studied while varying the amount of thiourea from 1 to 12 mM at a fixed temperature of 150 C. The electrolytic performance is shown in Fig. S9. Furthermore, it is clear from the optimization curve in Fig. S10 that 3 mM of sulfur produces higher catalytic activity comparing the other samples. This occurred because 3 mM is synthetically sufficient to disperse homogeneously on NiCo nanoflowers. Furthermore, this homogeneity ensures the proper interaction between elements that helps to create easy routes for electrons to transfer. However, at higher amounts of dose, the density of sulfur on NiCo also increases, reducing the interaction with NiCo and reducing the number of active sites as well. As a result, it shows lower catalytic activity. Similarly, small sulfur dose cannot create enough routes for electrons to transfer, also showing lower catalytic activity. Therefore, it can be concluded that 3 mM of thiourea is the optimal dose amount for NiCoS synthesis. Especially, we further measured the HER performance of NiCoS-150 without C-dot to compare with NiCoS-150 nanoflowers (as shown in Fig. S11(a) and (b)). The catalytic activity and stability of NiCoS-150 nanoflower were
better than those of NiCoS-150 without C-dot. This result proves that C-dot has a role to enhance not only HER electrocatalytic activity but also improve the stability of NiCoS-150 nanoflowers. The Tafel plot and the fitting curves of various catalysts are displayed in Fig. 5(b). The Tafel equation h ¼ a þ b log j (where j is the current density and b is the Tafel slope) was used to obtain Tafel slope b, which is related to the inherent properties of the catalyst and which indicates the ratedetermining step in the entire HER process. The Tafel slopes of NiCoS-120, NiCoS-150, NiCoS-180, NiCoS-210, NiCoS-240, and NiCo were calculated as 138.89, 80.17, 85.2, 84.86, 91.84 and 210.39 mV/dec, respectively, proving that the 3D-network NiCoS-150 reveals the highest catalytic activity compared to other samples. The Tafel slope of c-Pt/c was also measured (45.77 mV/dec) for comparison. According to a previous report [43], the mechanism of the HER process is the Volmer-Heyrovsky mechanism, which consists of the rapid discharge of a proton and the slow combination of the discharged proton with an additional proton following the reaction of Had þ H3Oþ þ e / H2 þ H2O. The relatively low Tafel slope for NiCoS-150 implies a faster reaction rate compared to the other cases, leading to a rapid increment of hydrogen evolution reaction. Although NiCoS150 shows a higher Tafel slope than commercial Pt/c, the 3D nanoflower structure of the NiCoS-150 catalysts nonetheless have great potential to use in practical HER applications owing to their low cost and relatively high levels of electrochemical activity.
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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Table 1 e LSV, Tafel slope, and EIS parameters after fitting Nyquist plots and Cdl obtained from NiCoS synthesized at different temperatures. Electrocatalysts NiCoS-120 NiCoS-150 NiCoS-180 NiCoS-210 NiCoS-240
Overpotential (mV)
Tafel slope (mV/dec)
Rct (U)
Cdl (mF)
ECSA (mFcm2)
382 232 310 358 378
138.89 80.17 85.2 84.86 91.84
394.7 66.3 104.6 650.5 950.2
0.481 0.929 0.660 0.314 0.251
13.74 26.54 18.85 8.97 7.17
The HER performance depends strongly on the electrochemically active surface area (ECSA) because the higher ECSA provides more active site [44,45]. The double-layer capacitance (Cdl) at the interface of solid/liquid layers was monitored to evaluate the ECSA of these samples [44]. The CV curves were conducted in the potential sweep of 0.0e0.3 V to show the region of the electrical double layers on the surfaces of the electrodes (Fig. S12). Fig. 5(c) shows the ECSA outcomes of NiCoS-120, NiCoS-150, NiCoS-180, NiCoS-210 and NiCoS240 C, which were calculated as 13.74, 26.54, 18.85, 8.97 and 7.17 mF/cm2 respectively. These results indicate that the NiCoS-150 nanoflowers have a much higher ECSA than the other catalysts, suggesting that the better HER activity of NiCoS-150 may be attributed to its higher ECSA and a larger number of active sites. The HER activity of as-synthesized NiCoS at a different temperature is also evidenced by the EIS measurements. Nyquist plots during the HER operation at 0.30 V vs RHE in acidic (0.5 M H2SO4) electrolytes are shown in Fig. 5(d). In all cases, one semicircle was observed, and the corresponding model of the electrical equivalent circuit is shown in the inset of Fig. 5(d). Rs, Rct, and Cdl indicate the electrolyte resistance, charge transfer resistance, and double-layer capacitance, respectively. The lower Rct value means, the greater the possibility of transferring the charge through the double layer. Therefore, the catalyst will perform better for HER. Table 1 shows the values of the parameters obtained by fitting the
impedance spectra. Obviously, the Rct value of the NiCoS-150 sample is 63.3 U, which is the lowest charge transfer resistance among all of the synthesized catalysts. The Rct values obtained for NiCoS-120, NiCos-180, NiCoS-210 and NiCoS240 C were 394.7 U, 104.6 U, 650.5 U and 950.2 U, respectively. Hence, the NiCoS-150 sample shows the best charge transfer which aid for accelerating the electrocatalytic activity, resulting in the higher HER performance of NiCoS-150 compared to others. These results are well consistent with the voltammetric outcomes. Chronopotentiometric measurements were carried out on NiCoS-150 nanoflowers based on the HER activity at a constant current density of 10 mA/cm2, lasting a long time, as shown in Fig. 6. Remarkably, NiCoS-150 exhibited a constant overpotential for approximately 20 h. However, the overpotential starts to increase from 232 mV after 20 h, implying the increase of the catalytic stability of the NiCoS-150 nanoflower toward the HER. Similarly, the inset of Fig. 6 shows hydrogen bubbling, which indicates the generation of hydrogen. The C-dots act as a protective layer and sulfur content increase the conductivity of the nanostructure, confirming the high architectural stability and catalytic activity. As a result, the NiCoS-150 nanoflower depicts much higher catalytic performance than the other catalysts. Furthermore, the main reasons for the higher catalytic activity of NiCoS-150 are as follows. First, at 150 C, the transition-metal chalcogenides possess coordination structures, which act as the active centers in the effective hydrogen evolution reaction [46]. The charge characteristics of the metals and sulfur in the metal sulfides operate as a pair of hydride/proton acceptors in hydrogenase and its counterparts [47,48]. Secondly, NiCoS-150 shows much better electrical conductivity than the other samples (see the EIS data in Fig. 6(d)), which accelerate the electron transport, leading to boost the HER activity. Finally, the NiCoS-150 nanoflowers show the best hierarchical and porous structures compared to the others. Therefore, the electrolyte can easily penetrate and migrate to the inner/outer surfaces of the NiCoS-150 nanoflowers, providing short pathways for the transport of electrons and consequently much more unoccupied active sites for the HER.
Conclusion
Fig. 6 e Long-term chronopotentiometric tests of NiCoS150 carried out for HER at a current density of 10 mA/cm2 for 20 h.
In summary, we have rationally synthesized NiCoS/C-dot, 3D-nanoflowers as an excellent and non-noble HER electrocatalyst through the sulfurization of NiCo at 150 C. The HER activity of the NiCoS-150 nanoflower was superior to those of the others because the porous nanoflower-like structure provides a large number of exposed active sites, leading to
Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297
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accelerating the transport of electrons and ions. Therefore, NiCoS-150 reveals a great enhancement of catalytic performance in an acidic solution compared to NiCoS synthesized at 120,180, 210 and 240 C. Moreover, bare NiCo shows almost no catalytic activity during the HER. On the other hand, NiCoS-150 showed a small overpotential of 232 mV required to reach a current density of 10 mA/cm2, as well as a Tafel slope of 80.17 mV/dec and long-term stability of up to 20 h. Finally, it can be said that the high activity and stability of NiCoS synthesized at 150 C makes this nanoflower as a promising low-cost and stable electrocatalyst for hydrogen evolution reaction.
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Acknowledgment This research was supported by a National Research Foundation (NRF) grant (NRF-2017R1A2B2001911; NRF-2017R1A4A1015360) funded by the Ministry of Science and ICT, Republic of Korea.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2019.01.297.
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Please cite this article as: Ali Y et al., Transition-metal-based NiCoS/C-dot nanoflower as a stable electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.297