Journal of the Korean Crystal Growth and Crystal Technology (한국결정성장학회지)

The Korea Association of Crystal Growth (한국결정성장학회)
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Electrocatalytic properties of Te incorporated Ni(OH)2 microcrystals grown on Ni foam

  • Lee, Jung-Il (Department of Materials Science and Engineering, Korea National University of Transportation) ;
  • Oh, Seong Gyun (Department of Materials Science and Engineering, Korea National University of Transportation) ;
  • Kim, Yun Jeong (Department of Materials Science and Engineering, Korea National University of Transportation) ;
  • Park, Seong Ju (Department of Materials Science and Engineering, Korea National University of Transportation) ;
  • Sin, Gyoung Seon (Department of Materials Science and Engineering, Korea National University of Transportation) ;
  • Kim, Ji Hyeon (Department of Materials Science and Engineering, Korea National University of Transportation) ;
  • Ryu, Jeong Ho (Department of Materials Science and Engineering, Korea National University of Transportation)
  • Received : 2021.03.25
  • Accepted : 2021.04.02
  • Published : 2021.04.30
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Abstract

Developing effective and earth-abundant electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is critical for the commercialization of a water splitting system. In particular, the overpotential of the OER is relatively higher than the HER, and thus, it is considered that one of the important methods to enhance the performance of the electrocatalyst is to reduce the overpotential of the OER. We report effects of incorporation of metalloid into Ni(OH)2 microcrystal on electrocatalytic activities. In this study, Te incorporated Ni(OH)2 (��Te-Ni(OH)2) were grown on three-dimensional porous NF by a facile solvothermal method with �� = 1, 3 and 5. Homogeneous microplate structure on the NF was clearly observed for the Ni(OH)2/NF and ��Te-Ni(OH)2/NF samples. However, irregular and collapsed nanostructures were found on the surface of nickel foam when Te precursor ratio is (��) over 3. Electrocatalytic OER properties were analysed by Linear sweep voltammetry (LSV) and Electrochemical impedance spectroscopy (EIS). The amount of Te incorporation used in the electrocatalytic reaction was found to play a crucial role in improving catalytic activity. The optimum Te amount (��) introduced into the Ni(OH)2/NF was discussed with respect to their OER performance.

Keywords

1. Introduction

Water splitting is one of efficient methods to produce clean hydrogen, which can be utilized as an alternative energy for traditional fossil fuels [1,2]. Water electrolysis process contains anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER) [3,4]. The slow kinetic process of OER on the anode always leads to a high overpotential, which would decrease the efficiency of water splitting and result in a slow-going hydrogen production rate on the cathode. Even some noble metal materials such as ruthenium/iridium oxides (RuO2/IrO2) have been proved to be efficient catalysts for OER [5], their large-scale commercialization is limited due to their high cost and low production. Therefore, development of efficient and cheap OER electrocatalysts is extremely necessary for the hydrogen production through water electrolysis [6]. In the past decade, Ni-based catalysts, specially oxides, hydroxides, chalcogenides and others have been applied for OER due to their highly active and low-cost properties [7]. In order to address the corrosion issue under harsh alkaline environment and increase the reactive areas of Ni-based catalysts [8], the coupling of Ni nanoparticles with carbon materials has become an effectiveway [9]. Additionally, heteroatom doping in composite catalyst could ulteriorly modify the electron transfer [10]. However, the increased structural complexity would gain difficulty in mechanism analysis for this kind of catalysts. In this case, in-situ techniques along with other effective characterizations becomes strongly needed to better understand the catalytic roles of different parts and mechanism. Here, we synthesized three-dimensional (3D) porous nickel foam with Ni(OH)2 microcrystals via facile solvothermal route.

In this study, we show that metalloid incorporation into Ni(OH)2 microcrystals can be a promising strategy for designing highly efficient and commercial electrocatalysts for alkaline OER. Tellurium (Te) was selected as a promising metalloid element for modulating the catalytic properties of nickel oxide microstructure, where electrical conductivity and number of active sites can be simultaneously enhanced [11]. To the best of our knowledge, metalloid incorporation for tuning electrocatalytic properties of Ni(OH)2 has rarely been studied despite of its high potential. Encouraged by the above discussion, we introduced Te into the Ni(OH)2 microcrystals grown on three-dimensional porous nickel foam (NF) by a facile solvothermal method. Benefiting from the optimized electronic structure from the introduced Te and the conductive NF as the current collector, the Ni(OH)2 microcrystals on NF displays superior OER performance. Optimum Te amount introduced into the Ni(OH)2 microcrystals was discussed on the electrocatalytic OER performance.

2. Experimental

2.1. Growth of Ni(OH)2 and \(\chi \)Te-Ni(OH)2 nanoplateon nickel foam

Nickel foam (NF) was treated with acetone and then with 3M HCl solution for 10 min using ultrasonication cleaner. The cleaned NF was dried overnight. All the chemicals were purchased from Kojundo chemical and were directly used without further purification. For the synthesis of Ni(OH)2 microcrystal, as-prepared NF was immersed in 20 ml of deionized (DI) water. Then, the resulting solution was transferred to Teflon-lined autoclave and hydrothermally reacted at 180ºC for 5 h. The Ni(OH)2 microcrystals on NF sample was washed several times with DI water. The final Ni(OH)2 microcrystals grown on NF sample (Ni(OH)2/NF) was subsequently dried in vacuum at 60ºC for overnight. Synthesis method for \(\chi \)Te-Ni(OH)2 on NF samples (\(\chi \)Te-Ni(OH)2/NF) were same as that of Ni(OH)2/NF, except that tellurium tetrachloride (TeCl4) solution was added before hydrothermal reaction. The added TeCl4 solution concentration in the \(\chi \)Te-Ni(OH)2 were 1.0, 3.0 and 5.0 × 10-3 mol for the \(\chi \) = 1, 3, 5, respectively.

2.2. Characterizations

Microstructural images of each samples were observed by field emission scanning electron microscopy (FESEM; model S4800; Hitachi) equipped with energy dispersive X-ray (EDX). X-ray diffraction (XRD) was performed using a D/MAX-2500/PC (Rigaku) diffractometer at 40 kV and 100 mA with Cu-K\(\alpha\) radiation (\(\lambda \)=0.15418 nm). The electrochemical properties of catalysts in 1 M KOH were tested using a three-electrode electrochemical cell controlled by an electrochemistry workstation (model Autolab PGSTAT; Metrohm), in which the catalyst grown on NF was used directly as the working electrode [12]. Prior to measurements, the electrolyte (1M KOH, pH 13.7) was purged for about 10 min with O2, and the working electrodes were sealed on all edges with a custom-made acrylate adhesive except for the working surface area of 0.25 cm2.  Graphite rod and Hg|Hg2SO4 were used as the counter and reference electrodes, respectively. A titration vessel of clear glass was used as a testing cell. The distance between working and reference electrodes was close to 1 cm. Linear sweep voltammetry (LSV) was measured with a scan rate of 0.5 mV/s in a range from 1.23 to 1.9 V vs. a reversible hydrogen electrode (RHE). Applied potential values were calibrated against RHE, and all polarization curves were iR-corrected. Electrochemical impedance spectroscopy (EIS) measurement was conducted over the frequency range of 0.1-100 kHz at 1.45 VRHE with a sinusoidal amplitude of 5 mV.

3. Results and Discussion

Ni(OH)2/NF exhibits poor electrical conductivity, which impedes facile charge transfer between the surface of catalyst and the adsorbed reactant. After Te incorporation, electrical properties of \(\chi \)Te-Ni(OH)2/NF can be significantly improved to easily take up electrons in hydroxyl ions, crucially accelerating water oxidation kinetics. Te element can be incorporated into the transition metal that is, a real catalytic active site in Ni(OH)2/NF, where a strong covalent p-d hybridization occurred with a highly polarized local electronic structure significantly accelerating electrocatalytic OER activity of \(\chi \)Te-Ni(OH)2/NF samples [13].

Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF microcrystals were synthesized by direct growth of NF using tellurium chloride via hydrothermal reaction at 180ºC for 5 h. Typical XRD analysis confirmed the formation of the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF crystal structures as shown in Fig. 1. The strong diffraction peaks near 44º and 51º could be assigned to Ni metal in the NF substrate. The XRD patterns revealed the crystal structure and phase purity of the Ni(OH)2/NF microcrystals. Except for the peaks from the NF substrate, all other detectable diffraction peaks at low 2\(\theta \) angles could be indexed to the theophrastite-like \(\beta\)-Ni(OH)2 phase (PDF 00-059-0462). Prominent diffraction peaks for the layered nickel-cobalt hydroxide were observed from at 19º, 33º and 38º. Notably, secondary phase related to Te (Ni2Te3O8) was observed at 31º in the XRD pattern of the 5Te-Ni(OH)2/NF sample, indicating that a very high Te content is not desirable for synthesizing Te-incorporated transition metal hydroxide nanosheets on the NF substrate.

Fig.1.jpg 이미지

Fig. 1. XRD patterns of the (a) Ni(OH)2/NF, (b) Te-Ni(OH)2/NF, (c) 3Te-Ni(OH)2/NF, and (d) 5Te-Ni(OH)2/NF samples.

Figure 2 shows highly magnified FE-SEM images for the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2NF (\(\chi \)=1, 3, 5) microcrystals. Figure 2 represents that large-scale highly interconnected and aligned microplate structure vertically grow on the skeletons of the NF with uniform morphology and dense loading, forming an ordered and 3D network with a highly open and interstitial structure. Homogeneous microplate structure on the 3D macroporous NF was clearly observed for the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF (\(\chi \)= 1, 3) microcrystals. However, when the Te adding amount (\(\chi \)) is over 3, irregular and collapsed microstructure were found on the surface of Ni(OH)2 microcrystals. Also, highly aggregated particle morphology with the large area of bare nickel foam, uncovered by the grown material, was formed for when Te precursor ratio further increased to more than \(\chi \) = 3 as shown in Fig. 2(d).

Fig.2.jpg 이미지

Fig. 2. Highly magnified FE-SEM images for the (a) Ni(OH)2/NF, (b) Te-Ni(OH)2/NF, (c) 3Te-Ni(OH)2/NF and (d) 5Te-Ni(OH)2/NF samples.

The electrocatalytic activities of Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples were tested in alkaline media (1M KOH aqueous solution) using a three-electrode system. Linear sweep voltammetry (LSV) was performed at a scan rate of 0.5 mV/s on the samples. NF substrates were directly used as working electrodes, while a rotating disk electrode (RDE) was employed for testing powder samples such as RuO2. To remove the bubbles generated during measurement, the RDE was continu-ously rotated at 2,000 rpm. All measured potentials were iR-compensated and were then referenced to the reversible hydrogen electrode (RHE). Notably, \(\chi \)Te-Ni(OH)2/NF exhibited enhanced catalytic activity for water oxidation compared to Ni(OH)2/NF sample. The overpotential (\(\eta\)) required to transmit a current density of 10 mA/cm2(\(\eta\)10) is conventionally used as a standard to compare electrocatalytic OER performance [14].

Figure 3 shows LSV curves with a scan rate of 0.5 mV/s for electrocatalytic OER properties of the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples. The \(\eta\)10 substantially decreased when Te was introduced into Ni(OH)2/NF. The 3Te-Ni(OH)2/NF sample required an \(\eta\)10 of 270mV, while \(\eta\)10 for Ni(OH)2/NF was about 295 mV as shown in Fig. 3. Importantly, an overpotential of only 285 mV was needed for 3Te-Ni(OH)2/NF to generate a high current density of 100 mA/cm2, compared to the 330 mV for the Ni(OH)2/NF sample, which can be beneficial for practical electrolysis applications. The amount of Te incorporation used in the reaction was found to play a crucial role in improving catalytic activity. In terms of overpotential, the Te content in 3Te-Ni(OH)2/NF resulted in the best catalytic activity for water oxidation. However, further increasing Te content to 5Te-Ni(OH)2/NF significantly disturbed its catalytic activity, which may be related to secondary phase observed in Fig. 1(d), and irregular microcrystal growth on the nickel foam substrate as shown in Fig. 2(d).

Fig.3.jpg 이미지

Fig. 3. LSV curves at a scan rate of 0.5 mV/s for electrocatalytic OER properties of the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples.

Tafel plots of the samples were derived from the measured LSV curves based on the Tafel equation (\(\eta\)=b×logj+a), where \(\chi \) is the overpotential, j is the current density, and b is the Tafel slope. Tafel slopes of Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF were calculated and represented in Fig. 4. The measured Tafel slopes of the Ni(OH)2/NF was 36.5 and the and \(\chi \)Te-Ni(OH)2/NF were 33.5, 26.9 and 31.4 mV/dec for the \(\chi \) values of 1, 3, and 5, respectively. Among the samples, 3Te-Ni(OH)2/NF exhibited the smallest Tafel slope (26.9 mV/dec) compared to Ni(OH)2/NF (36.5 mV/dec), highlighting the potential use of Te-Ni(OH)2/NF for industrial electrolyzer because a smaller Tafel slope is desirable to reduce power losses [15].

Fig.4.jpg 이미지

Fig. 4. Tafel slopes of Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples derived from the measured LSV curves.

Charge transfer resistance (Rct) between electrocatalysts and electrolytes can be obtained from the semicircle diameter in the high-frequency region of a Nyquist plot (Zʹ vs. -Zʹʹ) measured by electrochemical impedance spectroscopy (EIS). Here, the smaller diameter generally represents the lower Rct. Fig. 5 represents the EIS data of the samples, where \(\chi \)Te-Ni(OH)2/NF sam-ples (6.0~7.5 \(\Omega\)) showed a value of Rct that was obviously lower than that of the Ni(OH)2/NFsample (~8.0 \(\Omega\)). Notably, 3Te-Ni(OH)2/NF exhibited a minimum Rct (~6.0 \(\Omega\)), implying the significant role of Te incorporation for facilitating charge transfer from the catalyst surface to the adsorbed chemical reactants. Further increasing Te content (5Te-Ni(OH)2/NF) increased its charge transfer resistance, which may be related to poorly grown nanosheets on the nickel foam substrate, which is same result with LSV curve and Tafel slope.

Fig.5.jpg 이미지

Fig. 5. EIS data of the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples.

To quantitatively evaluate catalytic activity of Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples, we first estimated the double-layer capacitance (Cdl) using a CV method in the non-Faradaic region where the current generated from electrical double layer charging is essentially related to electrochemical surface area (ECSA). The CV test was conducted in an O2-saturated 1 M KOH solution to estimate the Cdl at non-Faradaic overpotentials. CV measurements were performed at various scan rates (20, 40, 60, 80, 100, and 120 mV/s). The difference in current density between the anodic and cathodic sweeps (Janodic - Jcathodic) at the middle of potential range was plotted as a function of the scan rate, where the slope has a linear relationship with twice the Cdl of the catalyst [16]. The Cdl was calculated to be 40.18 mF for 3Te-Ni(OH)2/NF sample, while Ni(OH)2/NF and 5Te-Ni(OH)2/NF exhibited much lower Cdl values of 17.95 and 31.50 mF/cm2, respectively as shown in Fig. 6.

Fig.6.jpg 이미지

Fig. 6. The current difference between anodic and cathodic sweeps as a function of scan rate. The slope of the fitted line was used to calculate Cdl.

For quantitative analysis of the catalytic activity of the samples, the turnover frequency (TOF) needs to be calculated as TOF = j × A/(4 × F × Ns), where j is the current density (A/cm2) at a specific overpotential, A is the electrode area (cm2), F is the Faraday constant (96,485 C/mol), and Ns is the concentration of active sites (mol/cm2). Cyclic voltammetry (CV) was performed to calculate Ns for the redox reactions at different scan rates in 1.0M KOH electrolyte. The peak currents were plotted verses scan rates where the slope is equal to, Slope =n2F2ANs/4RT, here n is the amount of electrons transferred (here n = 1), F presents Faradic constant, A is the surface area of the electrode, Ns is the surface concentration of active sites (mol/cm2), R and T are the ideal gas constant and the absolute temperature, respectively [11]. The TOF value of 3Te-Ni(OH)2/NF was 0.4 s-1 at an overpotential of 290 mV, which is much higher than those of Ni(OH)2/NF (0.06 s-1) and 5.0Te-Ni(OH)2/NF(0.28 s-1) as shown in Fig. 7, indicating the intrinsically high catalytic activity of 3Te-Ni(OH)2/NF sample.

Fig.7.jpg 이미지

Fig. 7. TOFs of the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples at an overpotential of 290 mV.

4. Summary

In summary, Te-incorporated nickel oxide microcrystals were developed as highly efficient and low-cost electrocatalysts for water oxidation under alkaline conditions. The Ni(OH)2 and \(\chi \)Te-Ni(OH)2 (\(\chi \)=1,3,5) microcrystals were successfully grown on nickel foam (NF) via hydrothermal reaction at 180ºC for 5 h. Homogeneous nanoplate structure on the NF was clearly observed for the Ni(OH)2/NF and \(\chi \)Te-Ni(OH)2/NF samples. When Te ratio (\(\chi \)) is higher over 3, irregular and collapsed nanostructures were found on the surface of NF. The electrocatalyst of 3Te-Ni(OH)2/NF sample provided current densities of 10 and 100 mA/cm2 at overpotentials of only 270 and 285 mV, respectively, with a very small Tafel slope of 26.9 mV/dec in an alkaline medium. Moreover, 3Te-Ni(OH)2/NF exhibited a minimum Rct(~6\(\Omega\)) wiht respect to the Ni(OH)2/NF sample (~8\(\Omega\)), implying the significant role of Te incorporation for facilitating charge transfer from the catalyst surface to the adsorbed chemical reactants. The double layer capacitance Cdl was calculated to be 40.18 mF for 3Te-Ni(OH)2/NF compared to the value of 17.95 for Ni(OH)2/NF. The 3Te-Ni(OH)2/NF sample showed much higher TOF compared to the Ni(OH)2/NF and other \(\chi \)Te-Ni(OH)2 samples. These results demonstrate that the \(\chi \)Te-Ni(OH)2/NF are promising electrocatalysts for water oxidation and optimum Te incorporation content (\(\chi \)) is 3.

Acknowledgement

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2019R1I1A3A0106266212).

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