Ultra-small ReS2 nanoparticles hybridized with rGO as cathode and anode catalysts towards hydrogen evolution reaction and methanol electro-oxidation for DMFC in acidic and alkaline media

Synthetic Metals 256 (2019) 116131

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Ultra-small ReS2 nanoparticles hybridized with rGO as cathode and anode catalysts towards hydrogen evolution reaction and methanol electrooxidation for DMFC in acidic and alkaline media

T

Mohammad Bagher Askaria,c, Parisa Salarizadehb,

⁎

a

Department of Physics, Faculty of Science, University of Guilan, P.O. Box: 41335-1914, Rasht, Iran High-Temperature Fuel Cell Research Department, Vali-e-Asr University of Rafsanjan, Rafsanjan, P.O. Box: 1599637111, Iran c Department of Physics, Payame Noor University (PNU), P.O. Box: 19395-3697, Tehran, Iran b

ARTICLE INFO

ABSTRACT

Keywords: ReS2 rGO Hydrogen evolution reaction Methanol oxidation DMFC

Cost-effective non-noble metal electrocatalysts are of crucial significance to the successful use of direct methanol fuel cells (DMFCs) and hydrogen generation. Rhenium sulfide (ReS2) is a semiconducting transition metal dichalcogenide with very week interconnect between its layers. Herein, we report ReS2/reduced graphene oxide nanocomposite (ReS2/rGO) for advanced dual-applications towards methanol oxidation reaction (MOR) and hydrogen evolution reaction (HER) in acidic and alkaline medium. For MOR, ReS2/rGO catalyst displays 198 μA cm–2 in alkaline and 38 μA cm–2 in acidic media. Also, the ReS2/rGO reveals a maximum power density of 38.6 mW cm−2 and 14.5 mW cm−2 at 60 °C in alkaline and acidic media, respectively. For HER, the Tafel slope of 67 mV dec–1 and 92 mV dec–1 and overpotential of 148 mV and 296 mV at the current density of 100 mA cm−2 obtain in acidic and alkaline media, respectively. Also, ReS2/rGO shows the double-layer capacitance of 1200 μF cm-2 and charge transfer resistance of 99 Ω. These excellent performances are assigned to the separated thin layers of ReS2 that increase specific surface area and edge sites of the catalyst and the presence of rGO, which increases the conductivity of catalyst.

1. Introduction Platinum (Pt) is an efficient catalyst for many applications such as hydrogen evolution reaction (HER) and methanol oxidation reaction (MOR) [1–5]. Both the HER and MOR are very important for the enlargement of future green-energy devices. Electrocatalytic HER is a pathway without greenhouse-gas emissions for hydrogen generation in large scales, and MOR is an anodic reaction in direct methanol fuel cells, which considerably manages fuel cell performance [6]. Although Pt is an efficient catalyst for both HER and MOR, its high price and scarcity prevent from its application at large scales. Therefore, the development of inexpensive and active catalysts is essential to practical applications. In recent years, two-dimensional material such as transition metal chalcogenides (TMDs) and graphene have been noticed and considered for many energy applications such as, Li-ion batteries [7], electrocatalytic and photo-electrocatalytic water splitting [8,9], solar cells [10], MOR [11], and HER [12]. It should be noted that the surface engineering, heterostructures, hierarchical structural, and chalcogen

⁎

substitutions of TMDs are efficient on their electrocatalytic properties [13]. TMDs such as MoS2 [14,15], WS2 [16], WSe2 [17], MoSe2 [18], and ReS2 [19] are promising due to the presence of many catalytically active sites on their surfaces, which can be exposed for adsorption of methanol and hydrogen atoms [13]. The very week interconnect (Van der Waals bonds) between TMD layers makes the layers to be available as single-layers, not bulk crystals [20]. ReS2 is a unique member in the semiconductor TMDs family that it has been recently considered by researchers. Recently ReS2 has been raised as a rival for MoS2. The very week interconnect between its layers makes its superior catalytic features than other TMDs and causes ReS2 to access as single-layer. Furthermore, the separated thin layers of ReS2 increase specific surface area and exposed edge sites of the catalyst. Moreover, 1 T distorted crystal structure of ReS2 with a triclinic system makes larger interlayer space and weaker interlayer coupling, which supply more exposed edge sites for proton permeation [21,22]. On the other hand, ReS2 is as a promising electrode material with different applications [23]. The discrepant activities of ReS2 in lithium-ion batteries [24] and photoelectrochemical hydrogen evolution [25] have

Corresponding author. E-mail address: [email protected] (P. Salarizadeh).

https://doi.org/10.1016/j.synthmet.2019.116131 Received 12 May 2019; Received in revised form 23 July 2019; Accepted 31 July 2019 Available online 23 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.

Synthetic Metals 256 (2019) 116131

M.B. Askari and P. Salarizadeh

already been widely investigated. But, its electrochemical properties for MOR and HER in alkaline and acidic media, especially in the cases of incorporating them with rGO, has not been reported. RGO is a twodimensional material with good electrical conductivity and large surface area, which can improve the conductivity and catalytic properties of the catalyst. Furthermore, the presence of oxygen-containing functional groups on its edges and low cost is promising for its application in energy production [26,27]. So, it’s expected when rGO combine with ReS2, ReS2 dispersion is improved, and more edge sites are available. In current work, we demonstrate the HER and MOR performance of ReS2 hybridized with rGO (ReS2/rGO). ReS2 and ReS2/rGO were synthesized by hydrothermal method. Ultra-small ReS2 with porous structure and lattice spacing of about 0.2 nm were observed from TEM images ReS2/rGO showed the Tafel slope of 67 mV dec–1 and 92 mV dec–1 and overpotential of 148 mV and 296 mV at the current density of 100 mA cm−2 in acidic and alkaline media, respectively which is comparable to that of other TMD catalysts for HER. Also, the MOR results and DMFC performance reported in this study suggest the potential for the possible application of ReS2/rGO as DMFC anodes.

2.4. Electrochemical investigations

2. Experimental

To the investigation of the methanol oxidation process, 0.002 g ReS2/rGO was dispersed in a mixture of Nafion solution (5%), water, and isopropyl alcohol (10 ml), and 5 μl suspension placed on the surface of the GCE. Cyclic voltammetry (CV) and chronoamperometry were applied to evaluate the electrochemical behavior of ReS2/rGO catalyst for methanol oxidation in acidic and alkaline media with a three-electrode system including Ag/AgCl, Pt wire, and ReS2/rGO/GCE on a potentiostat/galvanostat (Origaflex 500, French). Also, electrochemical impedance spectroscopy (EIS) was carried out at a voltage amplitude of 0.05 V and a frequency range of 0.1 Hz–100 kHz on an Autolab potentiostat/galvanostat 302 N. The diameter of GCE was 1 mm, and the loading of the catalyst was 0.13 mg cm−2 for all electrochemical investigations. Similarly, for evaluation of the hydrogen evolution process, 0.002 g ReS2/rGO was dispersed in a mixture of Nafion solution (5%), water, and isopropyl alcohol (10 ml), and 5 μl of the suspension placed on the surface of the GCE. LSV and chronopotentiometry analysis were applied in acidic and alkaline media in a three-electrode system. Also, for comparison, GCE was modified with Pt/C (20 wt%) similar to ReS2/ rGO/GCE.

2.1. Materials

2.5. DMFC performance

Ammonium perrhenate (NH4ReO4), hydroxylamine hydrochloride (HONH2·HCl), thioacetamide (C2H5NS), ammonia solution (25%), and hydrazine hydrate (50%) were provided from Merck. The Nafion solution (5 wt%) was purchased from Sigma-Aldrich.

The electrocatalyst performance of the catalyst was evaluated in a single cell. 30 mg ReS2/rGO (ratio of rGO to ReS2 is 1:5) catalyst was mixed with Nafion solution (as a binder), isopropyl alcohol, and DI water to prepare anode catalyst ink. The catalyst ink was coated on carbon cloth to provide a ReS2 loading of 4 mg cm–2. The cathode catalyst obtained by the same method by the painting of Pt/C on carbon cloth to take a Pt loading of 2 mg cm–2. Nafion membrane was hotpressed between two electrodes at 125 °C and 9 MPa for 90 s to the preparation of the membrane electrode assembly. The DMFC test was investigated at 80 °C in optimum methanol concentrations in alkaline and acidic media, 0.3 M and 0.5 M, respectively. Methanol and oxygen were supplied with a flow rate of 2 ml min–1 and 200 ml min–1, respectively.

2.2. Synthesis of ReS2, rGO, and ReS2/rGO ReS2 was synthesized by hydrothermal method. 322 mg NH4ReO4, 250 mg HONH2·HCl, and 420 mg C2H5NS were dissolved in 20 ml deionized (DI) water by magnetic stirring for 1 h. The solution was transferred to a 100 ml reactor and heated at 240 °C for 24 h. The black powder was purified with DI water and ethanol for three times then it was dried in a vacuum oven at 40 °C for 12 h [28]. GO was synthesized by hummer method according to our previous works [29]. The GO suspension was prepared by dispersion of GO in DI water by ultrasonic for 45 min. The agglomerated particles were separated from the suspension by a dialyzed bag. Then, 10 ml of ammonia solution and 10 ml of hydrazine hydrate were added to 1 mg/ml of GO suspension. The reaction was continued by reflux in 85 °C for 1 h to obtain rGO. Then, rGO was filtered, purified, and dried at 60 °C in an oven. To synthesize of ReS2/rGO catalyst, 0.5 g ReS2 powder, and 0.1 g rGO (ratio of rGO to ReS2 is 1:5) were added to 50 ml DI water by stirring for 2 h. The solution was moved into a 100 ml reactor and heated at 240 °C for 24 h. The black powder was washed with DI water and ethanol for three times then it was dried in a vacuum oven at 40 °C for 24 h.

3. Result and discussion 3.1. Characterization Crystal structure of ReS2/rGO is studied by XRD and related XRD patterns indicated in Fig. 1a. The observed peaks in 2theta of 57.8°, 44.6°, 32.7°, and 14.5° confirm the synthesis of ReS2, which is attributed to (100), (002), (300), and (122), respectively that is in agreement with the card number of JCPDS 82-1379. The synthesized rGO shows a peak in 2theta of 20-30°, which overlapped with ReS2 peak. Also, Raman spectra of ReS2/rGO, rGO, and ReS2 are shown in Fig. 1b, c, and d. The observed peaks at 211.4 cm−1 and 161.7 cm−1 are assigned to out-of-plane (Eg) and in-plane (Ag) of ReS2 (Fig. 1b, d). Furthermore, the G band and D band of graphene can be seen at 1596 cm−1, which prove the synthesis of ReS2/rGO (Fig.1b, c). XPS measurement is employed to observe the elemental composition and bonding formations of the ReS2/rGO composite. The XPS scan of ReS2/rGO composites is shown in Fig. 2a. The XPS C 1s spectra of rGO afford two peaks at 284.9 eV and 286.2 eV, which is related to the CeC and CeO bonding energies, respectively as it is revealed in Fig. 2b. The bonding configurations of Re are shown in Fig. 2c. Two characteristic peaks related to the 4f7/2 and 4f5/2 level peaks of Re4+ are located at 42.3 eV and 44.7 eV, respectively. Also, the bonding configurations of S 2p signals centered at 2p binding energies of 162.6 eV and 163.7 eV correspond to the 2p3/2 and 2p1/2 of S2−, respectively, as can be seen in Fig. 2d. The results are in agreement with the reported data for rGO and ReS2 [30].

2.3. Apparatus The x-ray diffraction (XRD, PW1800 Philips) spectroscopy was accomplished for investigation of the crystal structure of the catalyst. Raman spectroscopy was used to confirm the successful synthesis of ReS2/rGO. Field-emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to investigate the size and morphology of catalysts. The energy-dispersive X-ray spectroscopy (EDX) was performed using the Hitachi SU3500. X-ray photoelectron spectroscopy (XPS, Thermo Scientific KAlpha) was used to investigate the chemistry of the catalyst surface. 2

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Fig. 1. The XRD pattern of ReS2/rGO (a) and Raman spectra of ReS2/rGO (b), rGO (c) and ReS2.

Fig. 2. XPS survey spectrum (a) and high resolution XPS spectra of C 1s (b), Re 4f (c), and S 2p (d) of ReS2/rGO. 3

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Fig. 3. FESEM images (a, c, and e) and HRTEM images (b, d, and f) of ReS2, rGO, and ReS2/rGO respectively and EDX mapping (g) and analysis (inset g) of ReS2/rGO and ReS2, respectively.

3.2. Morphology and size

FESEM and TEM micrographs of rGO nanosheets are shown in Fig. 3c and d, respectively. Plate morphology and small thickness of rGO are specified. FESEM image of ReS2/rGO is indicated in Fig. 3e. As it is clear, ReS2 nanoparticles are homogenously placed on the surface of rGO, which the related HRTEM (Fig. 3f) also confirm the presence of ReS2 on rGO and its uniformity. The ReS2 is shown with red circles in the TEM image. Furthermore, the synthesis of ReS2/rGO is proved by EDX analysis and mapping as it is shown in Fig. 3g. Uniform distribution of Re and S elements on rGO surface have been shown in EDX

High surface area and porosity are one of the most important characteristics of a catalyst for MOR and HER. FESEM and TEM are accomplished for investigation of the size and morphology of nanoparticles. FESEM and TEM images of ReS2 are shown in Fig. 3a and b, respectively. As can be seen, the porosity and high surface area due to the presence of pores in its structure is clear in Fig. 3a. Also, the lattice space of ReS2 obtain about 0.2 nm from the HRTEM image, Fig. 3b. The 4

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faradic region is calculated by measuring capacitive current density (JC) according to the literature [31]. By plotting of JC versus scan rate, the Cdl obtain by half of the slope (Fig. 4c). The calculated Cdl for ReS2/rGO was 1200 μF cm−2, which is higher than that of ReS2 (700 μF cm−2) and the previously reported values for MoS2/rGO (870 μF cm−2) [32]. The ECSA is calculated from the Cdl (ECSA = Cdl/Cs), where Cs is a specific capacitance of a smooth surface (assumed 40 μF cm−2 per cm2ECSA). The ECSA for ReS2/rGO is 30 cm2ECSA, which is higher than that of ReS2 (17.5 cm2ECSA), indicating higher catalytic activity. The good dispersion of ReS2, the high specific surface area of rGO, and its excellent conductivity are some reasons for this improvement. 3.3.1. MOR in the alkaline media Electrochemical investigations of ReS2/rGO catalyst at the presence of methanol and 0.1 M NaOH are shown in Fig. 5. To optimize the methanol concentration, the electrode is cycled at different methanol concentrations (0.1, 0.3, 0.5 and 0.7 M), and the sweeping rate of 30 mV s−1 as it is shown in Fig. 5a. The methanol oxidation peak, anodic peak, is observed at the potential of 200 mV, and shows a current density of 75 μA at 0.1 M methanol. With increasing of methanol concentration up to 0.5 M, a significant rise is observed in peak height with maximum peak current density of 110 μA. However, in methanol concentrations above 0.5 M, the current density is decreased, as shown in 0.7 M methanol. This is likely assigned to the prevention of charge transfer due to by-products of methanol oxidation after a critical concentration. Therefore, 0.5 M methanol is selected as the optimum concentration. For the optimization of the sweep rate, the ReS2/rGO/GCE is cycled at different scan rates (10–100 mV s−1) and a methanol concentration of 0.5 M. As can be observed from Fig. 5b, as the scanning rate increases, the peak height of methanol oxidation increases, and the anode peaks expands. At the scan rate of 80 mV s−1, a current density of 198 μA is observed. But, at the scan rates of higher than 80 mV s−1, the current density is slightly decreased, which attributed to the insufficient time for the oxidation of some active compounds. Consequently, 80 mV s−1 is selected as optimum scan rate. To validate optimum methanol concentration (0.5 M) in alkaline media, the cyclic voltammetry tests are performed at different methanol concentrations with the scan rate of 80 mV s−1 (Fig. 5c). The anodic peaks are observed at all of them, and the highest peak was revealed at 0.5 M methanol that confirms the cyclic voltammetry results of Fig. 5a. The effect of scan rate is observed from the comparison of Fig. 5a,c. The current density at the scan rate of 30 mV s−1 and 80 mV s−1 obtain 110 μA and 198 μA, respectively. The stability of ReS2/rGO catalyst at the presence of 0.5 M methanol and 0.1 M NaOH is evaluated by ten cycles at the sweep rate of 80 mV s−1 (Fig. 5d). With increasing of cycles, the current density enhances and reaches 305 μA at the 6th cycle, and it is constant after that. The increase is likely due to the activation of ReS2/rGO catalyst, and the fixing is because of the saturation of electrocatalytic active sites and the blocking of electro-catalyst with intermediates and byproducts. The oxidation mechanism in alkaline media is proposed as follow:

Fig. 4. Cyclic voltammograms of ReS2/rGO/GCE (a) and GCE and GCE/rGO (b) at the presence of 0.1 M NaOH and 0.1 M H2SO4, and the capacitance current density of ReS2 and ReS2/rGO versus scan rates (c).

mapping and successfully synthesize of ReS2 can be seen from EDX analysis (Fig. 3g, inset).

ReS2/ rGO + CH3 OH 3.3. Electrochemical investigations in MOR

ReS2/ rGO

ReS2/ rGO

CH3 OHads + 4OH

CH3 OHads ReS2/ rGO

(1)

(CO)ads + 4H2 O+ 4e (2)

The cyclic voltammograms of ReS2/rGO/GCE at the presence of 0.1 M NaOH and 0.1 M H2SO4 are shown in Fig. 4a. As can be seen, no oxidation peak is observed in the absence of methanol. For comparison, the voltammograms of GCE and GCE/rGO in alkaline and acidic media are indicated, as we expected, no oxidation peak is seen (Fig. 4b). The electrochemical surface area (ECSA) of ReS2 and ReS2/rGO catalyst is evaluated by cyclic voltammetry in 0.3 M methanol and 0.5 M NaOH at different scan rates. Since the electrochemical doublelayer capacitance (Cdl) have linearly proportional to ECSA, it can compare the relative surface area of ReS2/rGO with ReS2. Cdl in a non-

ReS2/ rGO + OH ReS2/ rGO

ReS2/ rGO

COads + ReS2/ rGO

OHads + e OHads + OH

(3)

ReS2/ rGO + CO2

+ H2 O+ e (4) 3.3.2. MOR in the acidic media For the investigation of the electrochemical behavior of ReS2/rGO 5

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Fig. 5. CV curves of ReS2/rGO/GCE at different methanol concentrations with the scan rate of 30 mV s−1 (a), different sweeping rates in 0.5 M methanol (b), and different methanol concentrations with the scan rate of 80 mV s−1 (c) and the stability of ReS2/rGO catalyst at the presence of 0.5 M methanol (d). All tests were accomplished at the presence of 0.1 M NaOH.

catalyst for methanol oxidation in acidic media, optimization of methanol concentration and scan rate were performed similarly to the alkaline media. Cyclic voltammograms of ReS2/rGO/GCE at different methanol concentrations and the presence of 0.1 M H2SO4 in the scan rate of 30 mV s−1 are shown in Fig. 6a. The methanol oxidation peak is detected at the potential of 500 mV and shows a current density of 9 μA at 0.1 M methanol. With increasing of methanol concentration up to 0.3 M, an increase is observed, and the peak height reaches to the maximum current density of 14 μA. However, in methanol concentrations above 0.3 M, the current density declines so that the current density reaches 8 μA in 0.7 M methanol. This decrease is attributed to the absorption of byproducts on the catalyst surface, which prevents from charge transfer. Therefore, 0.3 M methanol is selected as the optimum concentration in acidic media. For determination of the optimum sweep rate, the ReS2/rGO/GCE is evaluated at different scan rates while methanol concentration is fixed at 0.3 M. The related voltammograms are shown in Fig. 6b. With the increase of scan rate from 10 mV s−1 to 70 mV s−1, the current density enhances from 14 μA to 37 μA. However, the anodic peak height declines at scan rates higher than 70 mV s−1 due to the insufficient time for the oxidation of some active compounds. For validation of optimum methanol concentration (0.3 M) in acidic media, the cyclic voltammograms of the catalyst were investigated in various concentrations at a constant scan rate of 70 mV s−1. The results show that the maximum current density of 38 μA at 0.3 M methanol and the minimum current density of 19 μA at 0.7 M methanol. It confirms the correct choice of 0.3 M methanol as an optimum concentration in acidic media.

The stability of ReS2/rGO catalyst in acidic media is evaluated by 10-cycles voltammetry test in 0.3 M methanol at the scan rate of 70 mV s−1, as shown in Fig. 6d. The peak height increases from 38 μA in the 1 st cycle to 59 μA in the 4th cycle, and after that, it is fixed. With increasing of the number of cycles, the catalyst sites are activated, and after that, the active sites are saturated or block with intermediates. The proposed mechanism for methanol oxidation in acidic is as follow:

ReS2/ rGO + CH3 OH ReS2/ rGO

CH3 OHads

ReS2/ rGO + H2 O ReS2/ rGO

ReS2/ rGO

CH3 OHads

ReS2/ rGO

ReS2/ rGO

COads + ReS2/ rGO

(CO)ads + 4H+ + 4e

OHads + H+ + e OHads

+ H+ + e

(5) (6) (7)

ReS2/rGO O+ CO2 (8)

In addition to cyclic voltammetry (10 cycles), the durability of the catalyst is evaluated by chronoamperometry technique at a constant potential of 0.5 V for 2500s. As shown in Fig. 7a, in the initial time, the current density decline due to the formation of oxidation intermediates and double-layer capacitance [33]. After that, the curves slowly decay, owing to CO adsorption on the surface of the catalyst [34]. ReS2/rGO preserves 64% of the initial current density value in alkaline media, indicating excellent tolerance of the catalyst against oxidation intermediates. In acidic media, the oxidation intermediates strongly bind on the catalyst surface, causing decrement in electro-oxidation kinetics. The obtained current density retention of 64% in alkaline media is higher than that of the other catalysts, as reported in the literature [33,34]. EIS test is carried out in 0.5 M methanol in alkaline and acidic 6

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Fig. 6. CV curves of ReS2/rGO/GCE at different methanol concentrations with the scan rate of 30 mV s−1 (a), different sweeping rates in 0.3 M methanol (b), and different methanol concentrations with the scan rate of 70 mV s−1 (c) and the stability of ReS2/rGO catalyst at the presence of 0.3 M methanol (d). All tests were performed at the presence of 0.1 M H2SO4.

media. The Nyquist plots of ReS2/rGO indicate one semicircular and Warburg, as shown in Fig. 7b. The plots are fitted with the equivalent circuit with ZView, as shown in the inset of Fig.7b. The equivalent circuit is consist of charge transfer resistance (Rct), solution resistance (Rs), constant phase element (CPE), and Warburg region (W). The charge transfer resistance (Rct), the diameter of a semicircle, obtain 99 Ω and 240 Ω for alkaline and acidic media, respectively. The higher Rct in acidic media is attributed to the strong binding of oxidation intermediates on the catalyst surface. However, in alkaline media, the active species are bounded on the active sites of the catalyst, enhancing the

kinetics of methanol oxidation. In alkaline media, ReS2/rGO has more resistance to oxidation intermediates such as CO. Also, the density functional theory (DFT) studies in the literature have supported these reports [35]. Sharma et al. reported that the incorporating of rGO to catalyst increase its dispersion and significantly increases its CO tolerance. Moreover, the length of the Warburg line in alkaline media is lower than that of in acidic media, indicating faster diffusion of the electrolyte ions towards the electrode. Comparison of ReS2/rGO catalyst performance in acidic and alkaline media, is shown in Table 1. It is clear that the catalyst is more

Fig. 7. Chronoamperograms of ReS2/rGO catalyst at 0.5 V vs. Ag/AgCl in 0.5 M methanol (a) and EIS of ReS2/rGO in acidic and alkaline media (b). 7

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Table 1 Comparison of catalyst performance in acidic and alkaline media. ReS2/rGO

Peak current density (μA cm−2)

Minimum current density (μA cm−2)

Maximum current density in the 10th cycle (μA cm−2)

Overvoltage (mV)

Optimum concentration (M)

Optimum scan rate (mV s−1)

Alkaline Acidic

198 38

130 19

305 59

200 500

0.5 0.3

80 70

3.4. Effect of temperature The effect of temperature on the activity of ReS2/rGO catalyst for methanol oxidation process was investigated in 0.1 M NaOH and 0.1 M H2SO4 at the presence of 0.1 M methanol at different temperatures. According to Fig. 8, in both electrolytes, the anodic peak current density improve with increasing temperature with a linear correlation (inset of Fig. 8a and b). These results are mainly assigned to the decrease of charge transfer resistance at the interface of electrolyte and catalyst [36]. For more investigation of kinetic and mechanism of MOR, the activation energy is calculated from Arrhenius plots, ln (Ip) vs. (1/T). The Arrhenius plots are indicated in Fig. 8c. The results show the activation energy for methanol oxidation on ReS2/rGO, in alkaline (16.8 kJ mol−1) and acidic media (17.2 kJ mol−1), is lower than that of reported in other literature [31,37]. The low value of activation energy for MOR on ReS2/rGO is considered to be due to the providing higher active surface area by rGO as well as the unsaturated sulfur atoms on the edge sites of ReS2. The activation energy for ReS2/rGO is close to Pt/C (13–17 kJ mol−1) [38], and it is lower than PtRu/C (Ea values between 35–70 kJ mol−1) reported in other reported work [39,40]. 3.5. DMFC performance The potential-current density and power density-current density curves of ReS2/rGO catalyst in optimum concentrations of methanol in acidic (0.3 M) and alkaline media (0.5 M) are investigated, as shown in Fig. 9. The cell performance, describes as maximum power density, enhances in 0.1 M NaOH solution (38.6 mW cm−2) than that of in 0.1 M H2SO4 solution (14.5 mW cm−2). The result reveals a maximum current density of 174 mA cm−2 and 60 mA cm−2 at 0.2 V for alkaline and acidic media, respectively. Hence, ReS2/rGO could be an inexpensive alternative catalyst to the commercial Pt–Ru/C catalyst for DMFC application. ReS2 is cheaper than Pt, and rGO can be produced in large scales by chemical reduction of GO [34].

Fig. 8. LSV curves of ReS2/rGO catalyst in 0.3 M methanol and 0.1 M H2SO4 with a scanning rate of 70 mV s−1 (a) and 0.5 M methanol and 0.1 M H2SO4 with a scanning rate of 80 mV s−1 (b) at different temperatures and the logarithm of Ip versus 1/T (c). Insets show the linear plots of the anodic peak current density versus temperature.

efficient in alkaline media than acidic media. Although the catalyst efficiency in acidic media is lower, ReS2/rGO function in both media is a significant point.

Fig. 9. Polarization curves of ReS2/rGO catalyst in optimum concentrations of methanol in acidic (0.3 M) and alkaline media (0.5 M). 8

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3.6. Electrochemical investigations in HER

2H+ (aq) + 2e

TMDs have platinum-like electrochemical properties for both MOR and HER process. MoS2 nanoparticles have been introduced as a great TMD electrocatalyst toward HER in other researches [41–43]. The unsaturated sulfur atoms on the edge sites of the MoS2 act as active sites for HER, which capture H atoms and so improve the HER [44] weak electrostatic forces between ReS2 layers facilitate its synthesis with a few layers. Thus it can provide a high surface area and the active catalytic sites for HER. That’s why we also reviewed the electrochemical properties of ReS2 for HER. Since the electrical conductivity has a principal role on the electrocatalytic activity of the catalyst, rGO as a conductive material was added to ReS2. In other literature, rGO, carbon nanotube, and hollow carbon were hybridized with TMDs and applied for MOR and HER [45–47]. Also, the dispersion of ReS2 nanoparticles in rGO can prevent their agglomeration. Also, the DFT studies on the HER activity of MoS2 and MoS2/reduced graphene oxide (rGO) [48,49] showed the electronic coupling of MoS2 with rGO makes an evident change in the Mo-edges shape and increases charge transfer. Thus, the theoretical results prove the role of carbonaceous materials on HER activity of TMDs. LSV is accomplished with the scan rate of 20 mV s−1 and calibrated versus reversible hydrogen electrode (RHE). Fig. 10a shows the LSV curves of ReS2/rGO in 0.5 M H2SO4 and 0.5 M NaOH. ReS2/rGO/GCE shows an onset overpotential of −72 mV and −190 mV in acidic and alkaline media, respectively. The higher overpotential in alkaline media can be assigned to the low efficiency of water dissociation on the catalyst surface resulting in slow kinetics of HER and high energy consumption [50]. HER proceeds through the reduction of protons through a half-reaction. This reaction in acidic and alkaline media is as follows:

2H2 O + 2e

H2 (g ) (Acidic electrolyte) H2 (g ) + 2HO (aq) (Alkaline electrolyte)

(9) (10)

Hydrogen evolution process in acidic media accomplished from three-step as follows:

H3 O+ + e Hads + H3

Hads + H2 O (Volmer) O+

Hads + Hads

+e

H2 + H2 O (Heyrovsky)

(11) (12) (13)

H2 (Tafel)

According to various studies, hydrogen evolution process is different from acidic media and involves three-step mechanism under alkaline medium as follows:

H2 O + e

H * + HO

H2 O + e + H *

H* + H*

(Volmer)

H2 + HO

H2 (Tafel)

(Heyrovsky)

(14) (15) (16)

Therefore, similarly to acidic media, HER can follow Volmer-Heyrovsky pathway or Volmer-Tafel recombination pathway. However, in alkaline media HeOeH covalent bond should be broken to adsorb H* species. All these reactions depend on inherent surface chemistry and electronic structure of catalyst [51]. Also, the overpotential of the HER at the ReS2/rGO electrode at 100 mA cm–2 decreases from 296 mV in 0.5 M NaOH to 148 mV in 0.5 M H2SO4, and the Tafel slope decreases from 92 to 67 mV dec–1, correspondingly (Fig. 10b). The performance of the ReS2/rGO catalyst is compared with other Pt-free catalysts, as shown in Table 2. As can be seen, the performance of ReS2/rGO catalyst surpasses that of most of

Fig. 10. LSVs of ReS2/rGO/GCE in 0.5 M H2SO4 and 0.5 M NaOH (a), related Tafel plots for HER (b), chronopotentiometry curves (c), and the amount of hydrogen evolved during 2500s (d). 9

Synthetic Metals 256 (2019) 116131

M.B. Askari and P. Salarizadeh

Table 2 The electrochemical properties of some TMDs and comparison with the as-prepared catalyst. ReS2/rGOa

ReS2/rGOb

MoS2 nanomesh

MoS2/CNT-G

O- MoS2/G

Ni-Mo-S/C

Tafel slope (mV dec ) Over potential (mV) Ref.

67 40 This work

92 90 This work

46 160 [20]

100 140 [52]

51 120 [53]

85.3 130-150 [54]

Composite

Cu2MoS4/MoSe2

MoS2@Fe3O4

GQDsc-MoS2

WS2/MoS2/ rGO

GD-WS2 2D-NHd

WS2–CoS2@CPe

Tafel slope (mV dec−1) Over potential (mV) Ref.

74.4 166 [55]

52 110 [56]

43 140 [57]

54.1 275 [58]

54 140 [59]

270 245f [60]

Composite −1

a b c d e f

In acidic media. In alkaline media. Graphene quantum dots. Graphdiyne-WS2-2D-nanohybrid. Carbon paper. In current density of 100 mA cm−2.

the previously reported catalysts, or it is comparable with others. The electronic coupling of ReS2 with rGO increases charge transfer and enhances the electrochemical properties of the catalyst. The amount of hydrogen evolved from ReS2/rGO/GCE in acidic and alkaline media are measured using chronopotentiometry in a constant current of 200 mA cm−2 for 2500s. The evolved hydrogen gas is collected in an inverted burette. The chronopotentiometry curves and hydrogen evolved histograms are shown in Fig. 10c, d. The volume of produced hydrogen obtain 12 ml and 6.5 ml in acidic and alkaline media, respectively. The hydrogen volumes are in close agreement with the LSV curves. The unsaturated sulfur atoms on the edge sites of ReS2, few-layers of ReS2, and the presence of rGO, which increase electric conductivity of catalyst are reasons for good electrocatalytic efficiency of ReS2/rGO.

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4. Conclusion In the current work, we synthesize rhenium sulfide/reduced graphene oxide nanocomposite (ReS2/rGO) for advanced dual-applications towards MOR and HER in the acidic and alkaline medium by hydrothermal method. For MOR, ReS2/rGO catalyst displays 198 μA cm–2 in alkaline and 38 μA cm–2 in acidic media. The activation energy for methanol oxidation on ReS2/rGO, in alkaline (16.8 kJ mol−1) and acidic media (17.2 kJ mol−1), is close to Pt/C (13–17 kJ mol−1) [38] and it is lower than that of reported in other literature [31,37]. The low value of activation energy is considered to be due to the providing higher active surface area by rGO as well as the unsaturated sulfur atoms on the edge sites of ReS2. For HER, the Tafel slope of 67 mV dec–1 and 92 mV dec–1 and overpotential of 148 mV and 296 mV at the current density of 100 mA cm−2 obtain in acidic and alkaline media, respectively which is close to those of Pt/C (0 mV and 30 mV dec−1) [61]. The good performance of the as-prepared catalyst is assigned to the separated thin layers of ReS2 with the high edge sites and the presence of rGO, which increases the conductivity of catalyst. Also, the ReS2/rGO reveals a maximum power density of 38.6 mW cm−2 and 14.5 mW cm−2 at 60 °C in alkaline and acidic media, respectively. Hence, the asprepared ReS2/rGO may be applied in the DMFC and in HER. In the future work, we will study the photo-electrocatalytic HER performance of ReS2/rGO catalyst and other 2D materials. Acknowledgments The authors are grateful to acknowledge the support of the fuel cell research laboratory of the Vali-e-Asr University of Rafsanjan.

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