Hydrogen Evolution Reaction Activities of Room-Temperature Self-Grown Glycerol-Assisted Nickel Chloride Nanostructures

Abstract

Three-dimensional nanomaterials of desired structural/morphological properties and highly porous with a high specific surface area are important in a variety of applications. In this work, glycerol-mediated self-growth of 3-D dandelion flower-like nickel chloride (NiCl₂) from nickel-foam (NiF) is obtained for the first time using a room-temperature (27 °C) processed wet chemical method for electrocatalysis application. Glycerol-mediated self-grown NiCl₂ flowers demonstrate an excellent electrocatalytic performance towards the hydrogen evolution reaction (HER), which is much superior to the NiF (303 mV) and NiCl₂ electrode prepared without glycerol (208 mV) in the same electrolyte solution. With a Tafel slope of 41 mV dec⁻¹, the NiCl₂ flower electrode confirms improved reaction kinetics as compared to the other two electrodes, i.e., NiF (106 mVdec⁻¹) and NiCl₂ obtained without glycerol (56 mV dec⁻¹). The stability of the glycerol-based NiCl₂ electrode has further been carried out for 2000 cycles with the overpotential diminution of just 8 mV, approving an electrocatalyst potential of glycerol-based NiCl₂ electrode towards HER kinetics. This simple and easy growth process involves nucleation, aggregation, and crystal growth steps for producing NiCl₂ nanostructures for electrocatalytic water splitting application through the HER process.

1. Introduction

The large-scale reliance on fossil fuels, and their predictable reduction, is of serious concern not only for worldwide environmental pollution but also the energy crisis, which has encouraged researchers to look for new renewable and clean energy resources [1,2,3,4]. Hydrogen is a viable alternative to fossil fuels [5,6]. After water electrolysis, high-purity hydrogen evolution reactions (HER) take place at the cathode, while the anode produces oxygen evolution reactions (OER). Both HER and OER kinetics are essential for the overall water-splitting process. Because the hydrogen adsorption free energy on these metal surfaces is thermally neutral, platinum group metals are known to be the best HER catalysts [6,7]. Due to the high cost of platinum, its use as an HER catalyst has been substantially limited [8,9,10,11,12,13,14,15]. The development and synthesis of HER catalysts with high catalytic activity are some of the top issues for researchers across the globe. Versatile nickel-based electrodes, including NiS₂, NiO, NiSe, etc., have widely been envisaged for HER as well as OER electrocatalysts [16,17,18,19,20,21,22]. An electrocatalyzer assembly consisting of a cathode (platinum–carbon) for the HER with zero overpotential and an anode (ruthenium oxide/iridium oxide) for the OER with a 1.23 V overpotential for water splitting (at 10 mA cm⁻² current density) is considered to be a standard measurement [7]. To-date, very few reports describing the use of self-supported Ni-based electrocatalysts in HER studies are known. For example, Zhu et al. revealed an in-situ chemical etching (ICE) approach for obtaining 3D Ni₃S₂ superstructures (needle array and leaf pattern) on a nickel-foam (NF) substrate for probable HER/OER catalyst with the lowest overpotential of 182/340 mV [23]. Shinde et al. have grown promising NiF₂ nanorod arrays directly on a 3D NF via a reduction procedurethat obtained172 mV HER overpotential [24]. According to Wang et al., a NiS/NiF electrode delivered 158 mV overpotential for the HER and 50 mV overpotential for the OER [25]. These processes produce only one form of nanostructure, and their sophisticated customization necessitates the precursor composition ratio variation [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]. If the product is in powder form, several annealing courses are required to remove the binders, which eventually can reduce electrochemical performance by obstructing the porosity of the employed matrix or host materials.

This issue has been resolved in this manuscript to some limit by self-growing dandelion flower-like NiCl₂@NiF nanostructured electrode material at room-temperature (25°C) in the presence of glycerol. The use of glycerol demonstrates a significant impact on the shape, surface area, and electrochemical performance of 3D NiCl₂@NiF nanostructure over pristine NiF and NiCl₂@NiF (obtained without glycerol) electrodes. Glycerol in a precursor solution can aid in the formation of a well-controlled dandelion flower-like nanostructure. We present a simple and scalable self-grown wet chemical, i.e., hydrothermal synthesis method, for obtaining the NiCl₂@NiF electrode, which is intended for water splitting with a low overpotential of 176 mV for HER electrocatalysis with a Tafel slope of 41 mV dec⁻¹ and long-term stability up to 2000 cycles.

2. Results

2.1. Morphology Evolution Studies

The FE-SEM depictions of the NiCl₂@NiF electrodes obtained with and without glycerol are shown in Figure 1. From Figure 1a, several nanorods are interlocked with each other and well-cover the 3D NiF skeleton. The diameters of these nanorods are found to be in the range of 1–3 µm, and the average length is 10 (±4) µm. The compact nanorod structure of this electrode may offer a smaller surface area and hence may seriously impact on its electrochemical performance. Moreover, the ion percolation is complicated with the present form of nanorods. So, it is hopefully possible to control the NiCl₂ nanostructure using glycerol as a surfactant.

As shown in Figure 1b, well-controlled parallel and conical arranged nanorods are grown together to form dandelion flowers on the NiF (see Figure 1(b-1) for a closer view). Some NiF spaces are exposed, suggesting the selective growth of NiCl₂ for obtaining dandelion flower-like architecture from NiF without adding any nickel salt. Compared with Figure 1a, the dandelion flowers are uniformly distributed on the NiF substrate surface, suggestinga decrease in the self-aggregation of NiCl₂, which is totally disappeared (Figure 1(b,b-1)). Vertically inclined nanorods are confirmed in the dandelion flowers. These upright standing nanorods have an average height of 10 (±3) µm with a separation distance of 5–10 µm (see Figure 1(b-2)), which will effectively help the deep penetration of the electrolyte ion for majority access and hassle-free charge/mass transportation. Figure S1 shows the related FE-SEM-assisted EDX elemental mapping measurements of the NiCl₂ electrodes obtained with and without glycerol (a, a-2 and b, b-2). Both Ni and Cl elements are evenly distributed over the NiCl₂ electrode surface (with and without glycerol) in a 1:2 stoichiometric ratio, indicating that the 3D NiF surface has converted to NiCl₂. The TEM image of the NiCl₂ sample obtained with glycerol, as shown in Figure 1c, clearly displays a dandelion flower-like nanostructure, which is constructed by several nanorods. As screened at higher magnification images, these interconnected nanorods are well-distinguished from one another. The HR-TEM image shown in Figure 1c is a TEM image of the NiCl₂ obtained with glycerol where a dandelion flower-like morphology, consistent with the FE-SEM image, is constructed by the nanorods, which is consistent with the above FE-SEM observations as shown in the Figure 1b.

The formation of the NiCl₂ is confirmed by HR-TEM imaging (Figure 1(c,c-1)) as the visible lattice fringes with an interlayer spacing of 0.271 nm, in close agreement with the XRD result (discussed later), are observed. The same part of the nanorods, as seen in the HR-TEM image, does not have clearly reflected lattice fringes, suggesting the presence of an amorphous or non-crystallineNiCl₂. The well-ordered circular arrays of lattice points in the selected-area electron diffraction pattern (inset of Figure 1(c-1)) are noted, indicating the significant single crystallinity of the NiCl₂ nanorods [32]. The above results suggest that the formation process of the NiCl₂ dandelion flowers strongly depends on the use of glycerol in the precursor solution, which has been described as a growth process (see Scheme 1), in which the use of an appropriate amount of glycerol with acetone to form a uniform and transparent mixed solution can act as a micro-emulsion. This can be confirmed to be a Tyndall-effect-free and miscible solvent under thermodynamic equilibrium [40], so it is reasonable to control the self-aggregation of a NiCl₂ dandelion flower in precursor solution through the following reaction steps. The NiF is initially oxidized to create nickel ions on the surface when dipped in a homogeneous solution containing acetone, glycerol, and hydrochloric acid (Equation (1)). Glycerol surfactant and hydrochloric acid are disintegrated in acetone at the same time, yielding glycerol, chlorine, and hydrogen free radical ions (see Equations (2) and (3)). Furthermore, Ni⁺ ions serve as nucleation sites, attracting glycerol surfactant molecules to create an intermediate complex known as Ni[O-CH₂-CH(OH)-CH₂-OH]₃. Furthermore, this intermediate combination transfers electrons to Cl^- ions, converting NiCl₂ primary nanorods on the NiF surface, which serves as a basic mother root for the growth of a dandelion flower NiCl₂ nanostructure (see bottom reaction pathway of schematic 1). As time proceeds, seed nuclei will grow in the vertical direction without aggregation under the control of the glycerol surfactant (see Scheme 1), forming dandelion flower-type NiCl₂, which is summarized in flowing reaction kinetics [32]. On the other hand, in the case of the NiCl₂ sample prepared without glycerol surfactant, the growth of the NiCl₂ nanorods is out of control; hence the formation of scattered and irregular nanorods over the NiF surface (see upper reaction pathway in Scheme 1) is seen. A very similar type of growth mechanism for the Fe₂O₃ nanoparticles obtained in the presence of glycerol has been reported elsewhere [32,40].

$$\mathrm{Ni}-\mathrm{foam} \to { \mathrm{Ni}}^{2+}$$

(1)

$$\left[\mathrm{HO}-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-\mathrm{OH}\right]\leftrightarrow {\left[\mathrm{HO}-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-\mathrm{O}\right]}^{-}+{\mathrm{H}}^{+}$$

(2)

$$\mathrm{HCl}\to {\mathrm{H}}^{+}+{\mathrm{Cl}}^{-}$$

(3)

$${\mathrm{Ni}}^{2+}+3{\left[\mathrm{HO}-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-\mathrm{O}\right]}^{-}\to \mathrm{Ni}{\left[\mathrm{O}-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-\mathrm{OH}\right]}_3$$

(4)

$$\mathrm{Ni}{\left[\mathrm{O}-{\mathrm{CH}}_2-\mathrm{CH}\left(\mathrm{OH}\right)-{\mathrm{CH}}_2-\mathrm{OH}\right]}_3+2{\mathrm{Cl}}^{-}\to { \mathrm{NiCl}}_2$$

(5)

2.2. Structural Elucidation

Figure 2 confirms the X-ray diffraction patterns of the NiCl₂ electrode materials on the NiF obtained with and without glycerol (A and B). Two intensive peaks, marked as symbol ‘∆’, of metallic nickel (see Figure 2a(C)) in all patterns are recognized ((JCPDS card no. 04-0850)). The newly reflected diffraction peaks denoted with a ‘*’ symbol and positioned at a different 2-theta location according to JCPDS card no. 01-072-0044 are assigned to NiCl₂. According to the XRD results, the intensity of the diffraction peak in the glycerol-meditated NiCl₂ sample is higher compared to the non-glycerol-involved sample, showing higher crystallinity in the prior over the latter. The detailed surface optimization of the glycerol-mediated NiCl₂ electrode was obtained using XPS, and the corresponding results are presented in Figure 2b–d. The presence of Ni and Cl elements is confirmed by the survey spectrum (Figure 2b). The deconvoluted Ni 2p high-resolution spectrum (Figure 2c) adduces two major peaks at 856.3 and 874.1 eV for Ni 2p_3/2 and Ni 2P_1/2, respectively, with a spin energy separation distance of 17.8 eV. The satellite peaks at 862.2 and 882.5 eV, which are in good agreement with previous findings, are also evidenced [32].

The prominent peak obtained at 198.56 eV in the Cl high-resolution spectrum indicates that NiCl₂ develops on the NiF skeleton during the self-growth process at ambient temperature. The specific surface area (IV-type isotherm and H₃ hysteresis type), pore-size distribution, and pore surface area estimations of the optimized with glycerol-mediated NiCl₂ electrode determined by N₂ adsorption–desorption measurements are, respectively, 15.98 m²g⁻¹, 7.4 nm, and 47.66 m²g⁻¹ (Figure 2e) [32]. The nature of the adsorption and the desorption branches is assigned to the mesoporous behavior [32].

2.3. HER Activity

Furthermore, NiCl₂ electrocatalysts i.e., prepared with and without glycerol were utilized to study the HER water catalysis activity in 1.0 M KOH electrolyte solution (Figure 3) and compared with pristine NiF.

The polarization curves of the as-prepared with and without glycerol-mediated NiCl₂ electrode materials are shown in Figure 3a. Both electrode materials demonstrate considerable HER activity. As compared to the nanorod-type NiCl₂ (208 mV and pristine NiF (303 mV) electrodes, the ‘with dandelion flower-like NiCl₂ electrode reveals an enhanced electrocatalytic activity with as small as 176 mV overpotential, proving an enhancement in electrocatalytic activity due to the change in morphology caused by the glycerol incorporation of the NiCl₂ electrode for HER. The Tafel slopes for the corresponding electrode materials, as shown in Figure 3b, are lowered from 41 mV dec⁻¹ to 57 mV dec⁻¹ when the HER rate is increased (see supporting information (SI) for more details on the equations used for computation). The Volmer–Heyrovsky mechanism is obeyed on the electrode surface, as evidenced by the tiny Tafel slope of 41 mVdec⁻¹ [39,40,41,42]. As shown in Figure 3c, the electrochemical impedance spectroscopy (EIS) analysis performed for the three aforementioned electrodes from the 100 kHz to 10 kHz frequency range with a potential amplitude of 5 mV adduces clear and visible difference. Compared to the NiCl₂ obtained without glycerol and pristine NiF electrodes, the glycerol-mediated NiCl₂ electrode has low equivalent series resistance (2.62 Ω) and 1.78 Ω charge transfer resistance, the NiCl₂ electrode obtained with glycerol has improved electron mobility and electrocatalytic activity [24,41]. The cyclability test of the dandelion flower-type NiCl₂ electrocatalyst for HER was further examined by successive CV plots in a three-electrode system at a scan rate of 10 mV s⁻¹ for 2000 cycles (Figure 3d). A negligible decrease of 8 mV has been noticed in the 1st and 2000th polarization curves after long-term HER activity scanning, demonstrating its excellent HER catalytic ability with moderate chemical stability. The inset of Figure 3d presents a genuine photograph of the three-electrode system used for the electrocatalytic study, where a clear progression of oxygen bubble evaluation, indicating strong HER activity, is confirmed. This obtained over-potential is comparable with those reported in the literature (

Table 1. The comparative set up showing HER activities of present work with reported data.

Catalysts	Electrolyte	ɳ (mV)	Tafel Slope (mV dec−1)	J (mA cm−2)	Ref.
Ni2P@NSG	1.0 M KOH	110	43	10	[42]
NiFeCo	1.0 M KOH	108	64	10	[43]
NiS/Ni2P/carbon cloth	1.0 M KOH	111	78.1	20	[44]
NiS/NiS2	1.0 M KOH	248	142.3	10	[45]
Fe- NiS2	0.5 M H2SO4	198	42	10	[46]
NiS2	1.0 M KOH	219	157	10	[47]
NiS2/graphite substrate	0.5 M H2SO4	240	41	10	[48]
Te@NiTe2/NiS/acetylene black	1.0 M KOH	101	118	10	[49]
NiTe2	0.5 M H2SO4	560	44	10	[50]
Ni3Te2	1.0 M KOH	304	94.2	10	[51]
NiSSe	1.0 M KOH	154	125	10	[52]
NiF	0.5 M H2SO4	210	1006.6	10	[53]
NiF2	0.1 M KOH	172	47	10	[12]
NiCl2	1.0 M KOH	176	41	10	Current work

). According to the electrocatalysis results, the glycerol-containing NiCl₂ electrode exhibits excellent catalytic activity and stability towards the HER for several reasons (Scheme 2), which are as follows; (i) the porous nano-array structure of dandelion flower with a high surface area results in a greater number of active sites for electrocatalytic activity, (ii) the direct binder-free growth of mesoporous dandelion flower-like NiCl₂ on 3D conductive NiF substrate provides good connectivity with zero dead volume, (iii) the direct deposition of the NiCl₂ dandelion flower on the NiF surface and the close interconnection of these nanorods, forming a network that maintains good structural stability with efficient electron transport, and (iv) the room-temperature NiCl₂ synthesis approach as shown in Scheme 2 clearly depicts the important features of the dandelion flower-type structure for better catalytic performance.

3. Materials and Methods

3.1. Chemicals

NiF, with a pore density of 110 pore size and a mass density of 320 g m⁻², was obtained (Artenano Company Limited, Central, Hong Kong). Concentrated hydrochloric acid (HCl, 37%) was obtained from Daejung Chemicals (Seongnam-si, Republic of Korea). Acetone and anhydrous ethanol standard solutions were purchased from SK Chemicals (Seongnam-si, Republic of Korea). Deionized (DI) water, obtained from (Millipore Darmstadt, Darmstadt, Germany), was used throughout the experiments as a solvent. The NiCl₂ dandelion flowers were fabricated under optimized conditions on NiF at room-temperature. A NiF (2.00 × 6.00 cm²) was ultrasonicated for 20 minin ethanol, acetone, and finally, water to ensure that the NiF surface was thoroughly cleaned and then dried in a vacuum oven at 60 °C for 12 h using a process described elsewhere [32]. The NiF was then placed in a glass falcon tube (25 mL) that was filled with 20 mL acetone and 0.1 M HCl, in addition to a few drops of glycerol, which was stirred continuously for 10 min. The glass falcon tube was then sealed with a lid and left at room-temperature for 4 h. Interestingly, after a few hours, the NiF was changed from a polished silvery texture to a rough texture with a bright green tint (see on-ground experimental digital images displayed in Scheme 1). Finally, the NiF was removed and properly washed with deionized water multiple times before being thoroughly dried in a vacuum at 60 °C for 12 h and labeled as electrode ‘with glycerol’. For comparison, a NiCl₂ sample was obtained under identical conditions but just without the addition of glycerol into the for mentioned solution, which was referred to as ‘without glycerol’.

3.2. Characterizations

The structural interpretation and surface morphology evolution studies for the NiCl₂ electrodes, i.e., with and without glycerol, were carried out using X-ray diffraction (XRD, D8-Discovery Bruker, 40 kV, 40 mA, Cu Kα, λ = 1.5406 Å) patterns and field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800, 15 kV) digital images, equipped with an energy-dispersive X-ray spectroscopy (EDX) measurement unit. Both transmission and high-resolution transmission electron microscopy (HRTEM, JEOL 2100F) images were recorded over the optimized NiCl₂ nanostructured electrode. The X-ray photoelectron spectroscopy (XPS, VG Scientifics ESCALAB250) measurement was carried out to analyze the chemical bonding status of the NiCl₂ nanostructures. The surface area and pore-size distribution for the NiCl₂ nanostructures were obtained from Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) plots.

3.3. HER Confirmation

Both NiCl₂ nanostructure electrocatalysts were envisaged in a three-electrode system (in addition to NiF as a reference) in the presence of a Hg/HgO reference electrode and a platinum counter electrode. A nitrogen purge was used for 10 min before the trials to normalize the system in terms of external and internal oxygen.

3.4. Formulae Used

The formulae used for estimating the overpotential and the Tafel slope are provided in the supporting information.

4. Conclusions

In summary, the glycerol-mediated self-growth of dandelion flower-type NiCl₂ nanostructure is proposed at room-temperature viaa soft chemical approach. After sorting out the phase formation confirmation through the XRD pattern analysis. Change in the dandelion flower-type surface appearance with the 15.98 m²g⁻¹ surface area of the as-obtained NiCl₂ on glycerol loading has been evidenced by the FE-SEM images and BJH curves, respectively. Furthermore, the HER activities of the aforementioned electrodes studied for water-splitting applications demonstrate the feasibility of NiCl₂ as an HER electrode material in the water-splitting process. The NiCl₂ electrode prepared in the presence of glycerol demonstrates an overpotential of 176 mV, while the electrode prepared without glycerol confirms the overpotential of 208 mV, suggesting the impact of surface modification on HER performance. The NiCl₂ dandelion flower-like electrode reveals the superior reaction kinetics with the small Tafel slope of 41 mV dec⁻¹ as compared to that of the nanorod-type NiCl₂ (57 mV dec⁻¹) electrode obtained without glycerol. Moreover, the NiCl₂ dandelion flower electrode is found to be stable even after 2000 operation cycles with a minute decrease of 8 mV in overpotential. The considerable HER activity of the NiCl₂ dandelion flower-type electrode is attributed to its high surface area, flower-like morphology, and low internal resistance due to uncomplicated charge transport.