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Article

Reduced Graphene Oxide Supported Zinc Tungstate Nanoparticles as Proficient Electro-Catalysts for Hydrogen Evolution Reactions

by
Norah Alhokbany
*,
Tansir Ahamad
,
Saad M. Alshehri
and
Jahangeer Ahmed
*
Department of Chemistry, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(5), 530; https://doi.org/10.3390/catal12050530
Submission received: 5 April 2022 / Revised: 25 April 2022 / Accepted: 7 May 2022 / Published: 9 May 2022
(This article belongs to the Special Issue The Importance of Shape-Tailoring at Nano- and Micro-Levels in Catalytic and Photocatalytic Applications)
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Abstract

:
The nanocomposites of reduced graphene oxide (rGO) supported zinc tungstate nanoparticles (ZnWO4-NPs) receive considerable attention in electro-catalytic hydrogen evolution reactions (HER) and reveal significantly higher electro-catalytic performances than pure ZnWO4-NPs in alkaline media (i.e., 0.5 M KOH electrolyte). The polarization studies show that the ZnWO4-NPs@rGO nanocomposites exhibit low energy loss and good electrode stability during electrochemical reactions for HER. Furthermore, the Tafel slope of ZnWO4-NPs@rGO nanocomposites is found to be approximately 149 mV/dec, which closely agrees with the reported Tafel values of the noble metal electrocatalyst. In contrast, the performance of the ZnWO4-NPs@rGO nanocomposite is found to be approximately 1.5 times higher than that of ZnWO4-NPs in hydrogen production efficiency. Our results emphasize the significance of the nanocomposites with enhanced electro-catalytic activities by lowering the energy loss during electro-catalysis in an alkaline medium.

1. Introduction

Renewable energy is of current research interest among scientists in the energy conversion process. Transition metal oxides have recently been shown to possess potential in electrocatalytic hydrogen evolution reactions (HER). Hydrogen energy has fascinated the world as the most promising clean and renewable energy technology. Hydrogen evolution reaction (HER) via water electrolysis is a profitable strategy in the clean energy conversion process for fuel cell devices [1]. The transformation of solar energy to chemical energy is one of the ways to produce clean energy, but in this paper, we have focused on the electro-catalysis of water to HER [2,3,4]. Electrochemically active and low-cost electrode materials follow the given reaction process at the cathode for HER in alkaline media (2H2O + 2e− → H2 + 2OH−) [5]. The most common electro-catalysts for gas evolution reactions are noble metal-based electrocatalysts. Noble metals such as Pt, Ru, and Ir-based electro-catalysts are also reported as well-organized electro-catalysts in HER [6,7,8,9]. However, the disadvantages of these catalysts are their scarce nature and very high cost. Therefore, it is very important to design low-cost, stable, and well-performed HER electrodes. However, scientists have been working worldwide to develop cost-effective and highly efficient electrode materials of clean energy technologies for commercialization. Recently, carbon-based nanocomposites are advanced materials used as electrocatalyst, photocatalyst, and electrode materials for energy storage devices [10,11,12]. Pt/rGO nanocomposites were reported as superior electro-catalysts over Pt nanoparticles for high HER activity in the acidic medium due to their excellent conductivity and surface area of rGO [13]. The Tafel slopes of Pt/GCE, Pt/rGO/GCE, and Pd/rGO electrodes for HER were reported to be of ~90, ~33, and ~154 mV/dec, respectively [13,14]. Graphene anchored Ag/cobalt ferrite nanoparticles were also reported as efficient electrode materials for electrochemical applications [15]. On the other hand, tungstate [16,17,18,19], molybdate [20,21,22,23], ferrites [24,25], and delafossites [26] materials were also reported as proficient HER/OER/ORR electro-catalysts in clean energy applications. Heteroatoms doped carbon-based materials were recently reported as efficient electrocatalysts in water-splitting reactions [25,27]. Carbon-coated iron nanoparticles were reported as sustainable and non-precious trifunctional electro-catalysts, including HER catalysts in alkaline and acidic media [28]. Sulfide/carbides/nitrides based nanostructured materials as HER electrocatalysts were efficiently used in water electrolysis under acidic or basic media [22,29,30,31]. The hetero-structured materials were also reported as efficient electro-catalysts in water-splitting reactions [32,33,34]. It is noteworthy that nanocomposite materials with rGO show better electrical properties due to the high surface area and good thermal and chemical stability of rGO. Hence, the nanocomposite electrode materials perform well for HER. Hydrothermally synthesized tungstate nanorods have been recently reported as sensors with comprehensive performance [35]. The photo-catalysts have also produced hydrogen via solar water splitting reactions [36,37,38]. Subsequently, one of the key tasks could be an environmentally friendly approach to developing low-cost electrode materials for sustainable and clean energy with desirable performances. This study mainly delivers the conceivable electro-catalytic performance of ZnWO4-NPs@rGO nanocomposites to HER via electro-chemical water-splitting reaction as a clean energy resource for fuel cell applications.

2. Results and Discussion

The electron microscopic studies (FESEM and TEM) were used to recognize the morphology and size of the prepared nanocomposites. The FESEM micrograph reveals that the prepared nanocomposites show well-defined ZnWO4-NPs supported by rGO sheets (Figure 1a). The inset of Figure 1a shows high magnification FESEM micrographs for morphological analysis. The TEM study confirmed that ZnWO4-NPs with an average diameter of ~50 nm are supported by rGO sheets (Figure 1b). The inset of Figure 1b shows the high resolution TEM (HRTEM) micrographs. The HRTEM study shows the <111> plane of the lattice fringes of monoclinic ZnWO4-NPs. Figure 1c shows the XRD patterns of the ZnWO4-NPs and the ZnWO4-NPs@rGO nanocomposites. The XRD patterns demonstrated that the ZnWO4 nanoparticles are recognized in the monoclinic structure, with a crystalline phase of reduced graphene oxide (rGO) appearing at two theta of 26.40°. The XRD patterns were indexed with JCPDS # 15-774 of ZnWO4. Figure 1d shows the N2 adsorption-desorption isotherm of ZnWO4-NPs@rGO nanocomposites. The ZnWO4-NPs@rGO nanocomposites exhibit a type IV adsorption–desorption isotherm [39,40]. The BET surface area of ZnWO4-NPs@rGO was recorded (~118 m2/g) to be approximately nine times higher than pure ZnWO4-NPs, as also reported elsewhere [41]. A high surface area is an important tool in electrochemical hydrogen evolution reactions via water-splitting reactions. Figure 2 shows the selected area FESEM-elemental mapping analysis of the ZnWO4-NPs@rGO nanocomposites. This elemental mapping analysis confirmed the presence of Zn, W, O, and C elements in the nanocomposites, as shown in different colors. XPS was carried out to investigate the chemical states of the elements, i.e., Zn, W, O, and C, in ZnWO4-NPs@rGO nanocomposites. The XPS spectrum of Zn (2p) shows two peaks at 1024.75 eV and 1048.73 eV of Zn2p3/2 and Zn2p1/2, respectively, which confirmed that Zn is present in the Zn2+ chemical state (Figure 3a). The XPS spectrum of W (4f) displays two doublet peaks at 35.09 eV and 37.30 eV that will be represented to W4f7/2 and W4f5/2, respectively, of the W6+ chemical state (Figure 3b). The XPS spectrum of O (1s) is shown in Figure 3c. The deconvoluted peaks confirmed the appearance of two types of oxygen atoms, i.e., one is due to the metal oxide at ~532.8 eV, and other one belongs to the functional group present in the graphene oxide matrix at ~533.7 eV. The XPS spectrum of C (1s) shows a peak at ~286.70 eV, which is deconvoluted into three peaks, i.e., peaks of C=C at ~285 eV, C-C at ~286 eV, and C-O at ~289 eV. The discussed characterization methods are enough to characterize the final products for further studies. We have used the prepared nanocomposites as the electro-catalysts for hydrogen evolution reactions (HERs) in alkaline media.
The electro-catalytic HER performances of the prepared ZnWO4-NPs@rGO nanocomposites have been investigated and also compared with the electro-catalytic performances of pure ZnWO4-NPs. The electro-catalytic HER performances of the ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs were tested in 0.5 M KOH with a three-electrode electrochemical system at room temperature. CV and LSV polarization experiments were conducted at 25 mV/s from 0.0 V to −2.0 V versus Ag/AgCl for HER. Figure 4a shows the CV plots of ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs. The CV studies reveal that both ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs show HER activities in an alkaline medium, but, we have noted that the ZnWO4-NPs@rGO nanocomposites show better HER activity than the pure ZnWO4-NPs, as expected. Figure 4b shows the LSV polarization plots of ZnWO4-NPs@rGO nanocomposites and ZnWO4-NPs. The LSV studies confirm the HER activities of the ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs in the cathodic region and strongly support the CV results. From the CV and LSV results, the current densities were ~19 and ~12 mA/cm2 of the ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs, respectively, at 25 mV/s. It is remarkable that the ZnWO4-NPs@rGO nanocomposites show better electro-catalytic HER efficiency than the ZnWO4-NPs. The onset over-potential of the ZnWO4-NPs@rGO nanocomposites was found to be approximately 205 mV, which is better than that of the pure ZnWO4-NPs (i.e., ~315 mV). The Tafel plots of the ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs are shown in Figure 4c. The Tafel slopes are found to be ~149 mV/dec and ~235 mV/dec of the ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs, respectively, at 25 mV/s. The Tafel slope of the Pt/GCE electrode for HER was reported to be ~90 mV/dec [13]. The Tafel slope of the Pd/rGO electrode was reported to of ~154 mV/dec for HER [14]. The Tafel slopes of the NiMoO4 and ZnMoO4 nanostructured materials were reported to be ~133 mV/dec and ~230 mV/dec for HER in 0.5 M KOH [20,42]. It should be noted that the low onset potentials and low Tafel slope values with good stability indicate that ZnWO4-NPs@rGO nanocomposite is a better HER electro-catalyst than pure ZnWO4-NPs. It is also noteworthy that the Tafel value of the prepared ZnWO4-NPs@rGO nanocomposites is consistent with the reported Tafel value of the expensive Pt-electrocatalyst [13]. The CA experiments also determined the electro-catalytic HER activity and stability of the electrodes containing ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs. The CA studies were investigated at a fixed cathodic potential of –1.65 V for 4 h (Figure 4d). The CA curves of the ZnWO4-NPs@rGO nanocomposites show a higher current density with stability than the pure ZnWO4-NPs for HER in 0.5 M KOH. These electro-catalytic results reveal the stable nature of the electrodes with efficient electro-catalytic HER performance. Therefore, we claim that the ZnWO4-NPs@rGO nanocomposite has the potential to be one of the worthwhile HER electro-catalysts for energy conversion technologies.
The gas generated during the electrolysis was also analyzed with an Agilent 7820A gas chromatograph equipped with a Molesieve GC column (30 m × 0.53 mm × 25 µm) and a thermal conductivity detector thermostatted at 40 °C for the detection of hydrogen (H2). Argon was used as the carrier gas. The potentiostatic cathodic electrolysis was operated by maintaining a catalyst-loaded glassy carbon electrode at −1.65 V for 240 min in 0.5 M KOH solutions. Then, 100 µL aliquots of gas were collected from the headspace of the electrochemical cell over 20 min intervals with a gas-tight Hamilton syringe. The Faradic efficiency of the HER catalysts is defined as the ratio of the amount of experimentally determined H2 to that of the theoretically expected H2 from the reaction. The catalytic activity of ZnWO4-NPs and ZnWO4-NPs@rGO nanocomposite for H2 production was determined at a fixed cathodic potential of −1.65 V for 180 min. As shown in Figure 5a, the H2 production efficiency of both the ZnWO4-NPs and ZnWO4-NPs@rGO nanocomposites is almost liner and increasing with time in the 0.5 M KOH solution and was found to be 95.68 mL/cm2 after 180 min, which is higher than that of ZnWO4-NPs (64.27 mL/cm2). Figure 5b shows the hydrogen production per hour, and the ZnWO4-NPs@rGO nanocomposite shows 31.36 mL/cm2.h, which is about 1.5 times higher than that of pristine ZnWO4-NPs. Furthermore, a 92–93% Faradic efficiency was obtained under alkaline conditions, suggesting the current density is directly related to hydrogen generation.

3. Experimental

ZnWO4-NPs and ZnWO4-NPs@rGO nanocomposites were prepared by following the previous reports [43]. The ZnWO4-NPs have been synthesized using the molten salts method. Zn(NO3)2·6H2O, Na2WO4.2H2O, NaNO3, and KNO3 reagents were taken in motor pastel with a molar ratio of 1:1:40:40, respectively, and homogeneously hand grounded for 30 min. Thereafter, the homogenous mixture was transferred into a crucible and then placed in a muffle furnace at 500 °C for 5 h. The resulting material was washed several times using de-ionized water for the removal of inorganic moieties. The collected white colored powder was dried at 60 ± 5 °C. ZnWO4-NPs@rGO nanocomposites were synthesized by taking commercially available rGO and synthesized ZnWO4-NPs materials in 1:10 weight fractions along with 2.5 mL of ethylene glycol and 17.5 mL of de-ionized water followed by ultra-sonication for 15 min. This suspension was further treated hydrothermally at 120 °C/48 h to obtain the dark grey colored ZnWO4-NPs@rGO nanocomposites. Powder X-ray diffraction (PXRD, Bruker D-8 Advanced Diffractometer), a transmission electron microscope (TEM, JEOL, JSM-2100F, Japan), a field emission scanning electron microscope (FESEM, JEOL, JSM-7600F), and elemental mapping techniques were used to characterize the synthesized nanocomposites. Brunauere–Emmett–Teller (BET) measurements (V-Sorb 2800 Porosimetry Analyser) were conducted to estimate the surface area of the materials. X-ray photoelectron spectroscopy (XPS) data were collected on a PHI5300 spectrometer. Electro-catalytic studies of freshly prepared ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs were investigated with three electrodes configured with a CHI-660E electrochemical workstation using alkaline electrolyte at room temperature. The glassy carbon (0.07 cm2) was used as the working electrode, and an Ag/AgCl electrode was used as a reference electrode. KOH (0.5 M) was used as an electrolyte solution in the electrochemical studies of ZnWO4-NPs@rGO nanocomposites. Thus, we have measured the potential against Ag/AgCl, but the electrode potential can be converted to RHE using the given equation, i.e., E(RHE) = E(Ag/AgCl) + 0.197V + 0.059 × pH. Polarization studies were carried out using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) at a scan rate of 25 mV s−1. The chronoamperometry (CA) method was performed at a constant potential of −1.60 V to study the current stability.

4. Conclusions

ZnWO4-NPs@rGO nanocomposites show better electro-catalytic HER performances than pure ZnWO4-NPs in an alkaline medium (0.5M KOH). The polarization studies confirmed that ZnWO4-NPs@rGO nanocomposites exhibit a lower onset over-potential (~205 mV) and Tafel slope (~149 mV/dec) than pure ZnWO4-NPs in electrochemical water splitting to HER. ZnWO4-NPs@rGO nanocomposite shows approximately 1.5 times higher hydrogen production efficiency than ZnWO4-NPs. Hence, we can conclude that ZnWO4-NPs@rGO nanocomposites could be considered one of the highly effective HER electro-catalysts for clean energy applications.

Author Contributions

N.A., Conceptualization, Data curation, Methodology, Formal analysis, Writing–Original draft preparation; T.A., Conceptualization, Methodology, Validation; S.M.A., Supervision, Resources, Project administration; J.A., Investigation, Validation, Project administration, Writing–review & editing. All authors have read and agreed to the published version of the manuscript.

Funding

Researchers Supporting Project number (RSP-2021/29), King Saud University.

Acknowledgments

The authors extend their sincere appreciation to the Researchers Supporting Project (RSP-2021/29) at the King Saud University, Riyadh, Saudi Arabia.

Conflicts of Interest

We state that the authors have no conflict of interest with this research.

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Figure 1. (a) FESEM (scale bar = 100 nm), (b) TEM, (c) XRD, and (d) N2 adsorption–desorption isotherm of ZnWO4-NPs@rGO nanocomposites. The inset of (a,b) shows high magnification FESEM and HR-TEM images, respectively.
Figure 1. (a) FESEM (scale bar = 100 nm), (b) TEM, (c) XRD, and (d) N2 adsorption–desorption isotherm of ZnWO4-NPs@rGO nanocomposites. The inset of (a,b) shows high magnification FESEM and HR-TEM images, respectively.
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Figure 2. Elemental mapping analysis of the ZnWO4-NPs@rGO nanocomposites.
Figure 2. Elemental mapping analysis of the ZnWO4-NPs@rGO nanocomposites.
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Figure 3. High resolution XPS spectra of (a) Zn 2p, (b) W 4f, (c) O 1s, and (d) C 1s.
Figure 3. High resolution XPS spectra of (a) Zn 2p, (b) W 4f, (c) O 1s, and (d) C 1s.
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Figure 4. (a) CV, (b) LSV, (c) Tafel, and (d) CA curves of ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs for HER in 0.5 M KOH.
Figure 4. (a) CV, (b) LSV, (c) Tafel, and (d) CA curves of ZnWO4-NPs@rGO nanocomposites and pure ZnWO4-NPs for HER in 0.5 M KOH.
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Figure 5. (a,b) Electro-catalytic H2 production efficiency of ZnWO4-NPs@rGO nanocomposites and ZnWO4-NPs.
Figure 5. (a,b) Electro-catalytic H2 production efficiency of ZnWO4-NPs@rGO nanocomposites and ZnWO4-NPs.
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Alhokbany, N.; Ahamad, T.; Alshehri, S.M.; Ahmed, J. Reduced Graphene Oxide Supported Zinc Tungstate Nanoparticles as Proficient Electro-Catalysts for Hydrogen Evolution Reactions. Catalysts 2022, 12, 530. https://doi.org/10.3390/catal12050530

AMA Style

Alhokbany N, Ahamad T, Alshehri SM, Ahmed J. Reduced Graphene Oxide Supported Zinc Tungstate Nanoparticles as Proficient Electro-Catalysts for Hydrogen Evolution Reactions. Catalysts. 2022; 12(5):530. https://doi.org/10.3390/catal12050530

Chicago/Turabian Style

Alhokbany, Norah, Tansir Ahamad, Saad M. Alshehri, and Jahangeer Ahmed. 2022. "Reduced Graphene Oxide Supported Zinc Tungstate Nanoparticles as Proficient Electro-Catalysts for Hydrogen Evolution Reactions" Catalysts 12, no. 5: 530. https://doi.org/10.3390/catal12050530

APA Style

Alhokbany, N., Ahamad, T., Alshehri, S. M., & Ahmed, J. (2022). Reduced Graphene Oxide Supported Zinc Tungstate Nanoparticles as Proficient Electro-Catalysts for Hydrogen Evolution Reactions. Catalysts, 12(5), 530. https://doi.org/10.3390/catal12050530

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