RGO as an effective nano-electrocatalyst toward electrochemical hydrogen evolution reaction and methanol oxidation in two settings for fuel cell application

Accepted Manuscript Fe3O4@MoS2/RGO as an effective nano-electrocatalyst toward electrochemical hydrogen evolution reaction and methanol oxidation in two settings for fuel cell application Mohammad Bagher Askari, Amirkhosro Beheshti-Marnani, Majid Seifi, Seyed Mohammad Rozati, Parisa Salarizadeh PII: DOI: Reference:

S0021-9797(18)31333-X https://doi.org/10.1016/j.jcis.2018.11.019 YJCIS 24284

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

14 September 2018 1 November 2018 7 November 2018

Please cite this article as: M. Bagher Askari, A. Beheshti-Marnani, M. Seifi, S. Mohammad Rozati, P. Salarizadeh, Fe3O4@MoS2/RGO as an effective nano-electrocatalyst toward electrochemical hydrogen evolution reaction and methanol oxidation in two settings for fuel cell application, Journal of Colloid and Interface Science (2018), doi: https://doi.org/10.1016/j.jcis.2018.11.019

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Fe3O4@MoS2/RGO as an effective nano-electrocatalyst toward electrochemical hydrogen evolution reaction and methanol oxidation in two settings for fuel cell application Mohammad Bagher Askari,a,d1 Amirkhosro Beheshti-Marnani,b2 Majid Seifi,a3 Seyed Mohammad Rozati,a4 Parisa Salarizadehc5 a

Department of Physics, Faculty of Science, University of Guilan, P.O. Box 41335-1914, Rasht,

Iran b

Department of Chemistry, Payame Noor University (PNU), P.O. Box 19395-4697, Tehran, Iran

c

High-Temperature Fuel Cell Department, Vali-e-Asr University of Rafsanjan, Rafsanjan

1599637111, Iran d

Department of Physics, Payame Noor University (PNU), P.O.Box:19395-3697, Tehran, Iran

Abstract A three-component nano-electrocatalyst, magnetite coated molybdenum disulfide hybridized with reduced graphene oxide (Fe3O4@MoS2/RGO), is synthesized by a two-step hydrothermal method. This catalyst is applied as an effective substitution for the platinum catalyst in methanol oxidation and hydrogen evolution reactions. Cyclic voltammetry, chronoamperometry, and linear

All authors take responsibility for this paper. Corresponding authors: 1 [email protected] (Mohammad Bagher Askari) 2 [email protected] (Amirkhosro Beheshti-Marnani) 3 [email protected] (Majid Seifi) 4 [email protected] (Seyed Mohammad Rozati) 5 [email protected] (Parisa Salarizadeh)

sweep voltammetry are used to evaluate the performance of the electrocatalyst in acidic and basic media. The results of methanol oxidation reaction on the hybridized nano-electrocatalyst showed good electrocatalytic properties with considerable diffusion currents. This fact is confirmed by the Tafel plots and the calculated kinetic parameters of electron transfer. Fe3O4@MoS2/RGO showed an anodic transfer coefficient and exchange current of 0.464 and 4.80×10-8, respectively that are higher than Fe3O4/RGO. The presence of the porous MoS2 in catalyst has a key effect on supplying electroactive sites for electron transfer. Also, the high actual surface area obtained for the hybridized nano-electrocatalyst (A=0.0295 cm2). The maximum power density of 35.03 mW cm-2 obtained for a single cell containing the prepared hybridized catalyst as the anode which shows a competitive feature of the synthetic catalyst compared to other reports. Furthermore, the synthetic catalyst shows the low-value overpotential of 108 mV and Tafel slope of 48 mV dec-1 during the hydrogen evolution process in acidic media. This is attributed to the synergistic effect between Fe3O4 and MoS2 and also increase the electron transfer rate due to adding conductive RGO to the catalyst. The results show that the synthetic nanocatalyst can have promising applications for hydrogen evolution and methanol oxidation reactions. Keywords: Direct methanol fuel cell, hydrogen evolution reaction, reduced graphene oxide, molybdenum disulfide, iron oxide. 1 Introduction Today, using renewable energy sources has been considered owing to the concerns regarding global warming, population growth, rapid consumption of global fossil resources, and environmental damage caused by the burning fossil fuels [1] . However, hydrogen having all the characteristics of a suitable and clean energy [2] can play an important role as an alternative to

fossil fuels applicable to fuel cell utilization [3]. Various fuels, such as hydrogen, methanol, glycerol, ethanol, natural gas, and even gasoline and diesel are being used in fuel cells [4, 5]. Among the alcohols, methanol seems to be more appropriate due to its simplicity and the small number of carbon atoms [6]. The other important factor could be the low current density of this type of fuel in comparison to hydrogen fuel [7]. In addition, direct methanol fuel cell (DMFC) requires expensive metal catalysts such as platinum or platinum alloys, palladium, and etc [8, 9]. The main drawback of methanol fuel cell is the slow kinetics of methanol oxidation reaction (MOR) in the anode in which there is still ongoing research [10]. Therefore, many reports have been done on the modification of platinum-based catalysts and the development of alternative catalysts to reduce the kinetic obstacles [11]. Besides, some considerable efforts toward electrocatalytic oxidation of methanol have been performed in an alkaline solution [12]. Platinum has been known as an important, expensive, and scarce catalyst for hydrogen energy production at very low overpotential [13, 14]. Compounds such as MoS2 and WS2 were previously proposed by some researchers as suitable substitutes for platinum compounds [15, 16]. However, for better results on electrocatalytic activity, some modifications should be applied to these compounds. In this way, several works have already been conducted to increase the number of active sites in the mentioned compounds. For example, vertical alignment and aligned layer synthesis, synthesis of nanoflakes and clusters, and recently using the laser for creating the quantum dot compounds have been developed [17-20]. Some modifications such as compositing or hybridizing transition metal dichalcogenides with carbonaceous compounds, and introducing metals could facilitate the electron transfer to electrocatalytic active locations [21]. Molybdenum disulfide, MoS2, is one of the considerable compounds most used as a semiconductor with a direct band gap equal to 1.8 eV [22] . However, the main challenge is the modification and promotion of the electrocatalytic

activity of MoS2. Additionally, MoS2 is a nanomaterial with wonderful properties such as large surfaces area and electrocatalytic active sites [23, 24]. Poor conductivity is one of the disadvantages of MoS2 as an electrocatalyst. This problem can be overcome by its hybridizing with carbonaceous materials [25]. The use of materials based on MoS2 composited or hybridized with expensive nanoparticles of palladium, platinum and gold have also been reported in MOR [26]. Most reports suggest the use of nanocatalysts for methanol oxidation in alkaline or acidic solution. The catalytic properties of Fe3O4@MoS2 and its hybrids with carbonaceous materials have been studied including the use of CNTs/MoS2/Fe3O4 in supercapacitors [27] , as well as the use of Fe3O4 @MoS2 nanocomposite as the photo-Fenton-like catalyst [28]. However, based on our knowledge it seems that the use of Fe3O4@MoS2 hybridized with reduced graphene oxide (RGO) has not been reported in methanol oxidation process. In modern electrochemistry, investigation of inexpensive catalysts for methanol electrooxidation and hydrogen production is still the most interesting fields of study. In this regard, in the current work, we synthesized Fe3O4@MoS2/RGO hybrid as a new catalyst for methanol electrooxidation in alkaline and acidic media. It is be noted that in other work a composite including Fe3O4 and MoS2 was studied for hydrogen production. The result showed a good potential for hydrogen evolution reaction (HER) by Tafel slope of 52 mV dec -1. We evaluate the performance and mechanism of Fe3O4@MoS2 with adding RGO (Fe3O4@MoS2/RGO) for HER in addition to methanol oxidation studies. The electrocatalytic properties of prepared Fe3O4 @MoS2/RGO were investigated

by cyclic

voltammetry (CV),

linear

sweep

voltammetry (LSV),

and

chronoamperometry techniques for MOR. Also, the single cell performance of catalyst as the anode in DMFC was examined. The results showed that this catalyst has a good activity for

MOR in both the acidic and alkaline settings and also effective hydrogen evolution property by Tafel slope of 48 mV dec-1. 2 Experimental 2. 1 Chemicals and instrumentals Nafion 117 solution dissolved in a mixture of water and short chain aliphatic alcohols were purchased from Sigma Aldrich. Other pure chemicals such as sodium molybdate (Na 2MoO4), thioacetamide (C2H5NS), iron (III) chloride hexahydrate (FeCl3.6H2O), iron (II) sulfate heptahydrate (FeSO4.7H2O), and ammonia solution (NH4OH) were prepared from Merck. A potentiostat/galvanostat (Origaflex 500, French) and a glassy carbon electrode (GCE) were used for electrochemical measurements. The x-ray diffraction (XRD) patterns were recorded on a PW1800 Philips, Netherlands, using a Cu Kα source (λ=1.541A˚). The energy-dispersive X-ray (EDX) were performed using the Hitachi SU3500. Field emission scanning electron microscopy (FESEM) and transmission electron microscope (TEM) were used to investigate size and morphology of catalysts. 2. 2 Synthesis of Fe3O4@MoS2/RGO nanocatalyst A modified Hummer’s method was applied for the synthesis of graphene oxide (GO) [29]. In this method, graphite powder was used as a pristine material. Firstly, a quantity of 2 g of graphite powder (mesh of 400) was added and stirred in 50 mL of concentrated sulfuric acid followed by the addition of 2 g of sodium nitrate. After 1 h, the temperature decreased to 0 °C by using a water-ice bath. Afterward, 7.3 g of potassium permanganate was gradually added and the mixture kept at 0 °C for 2 h. Then the reaction temperature was elevated to 35 °C being stirred for an extra 2 h at this temperature. In the next step, 46 mL of double-distilled water was added

to the reaction vessel and then stirred at 90 °C for 30 min. The reaction was stopped by adding 140 mL of double-distilled water as well as 16 mL of 30 % H2O2. The resulting suspension was ultrasonicated for 30 min at 35 kHz. The produced GO was washed with 3 % HCl and then with deionized water for 3-4 times. Finally, the resulting solid material was dried in a vacuum oven at 40 °C for 24 h. In order to convert GO to RGO, the GO suspension in deionized water was sonicated for 45 min. Then, it was transferred to the dialyzed bag for 20 h in order to separate the agglomerated particles. A mixture of 10 mL of 25 % ammonia solution and 10 mL of hydrazine hydrate 50 % w/w added to 1 mg/mL of GO suspension. The solution was stirred during reflux in 85 ºC for 1 h and then filtered, washed, and dried in 60 ºC vacuum oven. To make sure about this part of the experiment, XRD, FESEM, and HRTEM images were obtained from the synthetic RGO. The MoS2 and Fe3O4 nanoparticles were prepared according to the literature [30, 31]. In a typical synthesis, 0.5 g of MoS2, 0.3 g Fe3O4, and 0.1 g RGO were mixed in 50 mL of deionized water and stirred for 1 h at room temperature. Thereafter, it was introduced into a 100 mL Teflon-lined stainless steel autoclave (100 mL) and placed in a 240 °C oven for 24 h, followed by cooling to room temperature overnight. The resulting product was washed several times with deionized water and ethanol by centrifuging and dried at 40 °C vacuum oven for 24 h. Eventually, a black powder of Fe3O4@MoS2/RGO obtained and the successful synthesis of the product was confirmed by XRD, Raman spectroscopy, and EDX techniques. The size and surface morphology of the synthesized nanocomposite were investigated by FESEM and TEM. 2. 3 Modification of electrode by Fe3O4@MoS2/RGO A quantity of 0.01 g of Fe3O4@MoS2/RGO was dispersed in 1 mL of 5% Nafion solution and then ultrasonicated in a water bath for 20 min. Then 10 μl of the suspension was dropped on a

GCE with 1 mm diameter and dried at room temperature. This electrode was used as a working electrode. For modification of electrode by Fe3O4/rGO, the same procedure was done. Ag/AgCl and Pt electrodes were used as reference and counter electrodes, respectively. Methanol oxidation in various concentrations of alkaline and acidic media was studied in 0.1 M NaOH and 0.1 M H2SO4, respectively as supporting electrolytes. 3 Results and discussion 3. 1 Material characterization The XRD patterns of synthesized materials were shown in Fig. 1a. Four peaks observed at 14.3°, 33.3°, 39.6°, and 58.4° and the weaker one at 49.8° confirmed the multi-crystallinity and purity of the synthesized MoS2 (Fig.1b). These findings were also confirmed by MoS 2 card (JCPDS card No.37-1492). The characteristic peaks and the crystalline structure of Fe 3O4 were in full agreement with the JCPD card No.19-0619. The characteristics of Fe3O4@MoS2 nanocatalyst by referring to the result of the XRD pattern of MoS 2 and Fe3O4 were matched well with JCPDS cards No.37-1492 and No.19-0629 for MoS2 and Fe3O4, respectively. The Raman spectra of Fe3O4, MoS2, and MoS2@Fe3O4 nanocatalysts were shown in Fig. 1b. The Raman peaks were observed at 217 cm-1, 282 cm-1, 395 cm-1, and 596 cm-1 corresponded to vibration modes of Fe3O4 nanoparticles. Also, in the Raman spectrum of MoS 2 nanoparticles, two strong peaks were observed at 383 cm-1 and 408 cm-1 which confirms its crystalline structure. However, MoS2@Fe3O4 peaks were slightly shifted to 392 cm-1 and 417 cm-1 which are in line with the results obtained by Liu et al [32].

Fig. 1. (a) The XRD pattern of RGO, MoS2, Fe3O4, and Fe3O4@MoS2, (b) the Raman spectra of Fe3O4, MoS2, and MoS2@Fe3O4 and (c) EDX elemental analysis of MoS2, Fe3O4, and MoS2@Fe3O4/RGO (d) EDX mapping related to Fe3O4@MoS2/RGO catalyst.

Moreover, EDX elemental analysis was used to identify the constituents of the Fe 3O4, MoS2, and MoS2@Fe3O4/RGO nanocatalyst. The presence of iron, molybdenum, carbon, oxygen and sulfur elements was confirmed in the structure of nanostructure, as shown in Fig. 1c. EDX mappings related to Fe3O4@MoS2/RGO catalyst were shown in Fig. 1d. The presence and distribution of Mo, S, Fe, and O elements in catalyst were confirmed.

3.2 Morphology and size of the prepared materials FESEM and HRTEM images were obtained in order to observe the nanoparticles and the morphology of the composites. In Fig. 2a and b, FESEM and HRTEM micrographs of the synthetic RGO confirmed the existence of nanosheets which were somewhat transparent in the HRTEM image. FESEM for prepared MoS2 also obviously showed a high degree of porosity (Fig. 2c). Fig. 2d and 2e show the FESEM images of Fe 3O4 and Fe3O4@MoS2 composites, respectively that clearly showed porous MoS2 coated by Fe3O4. Fig. 2f and 2g related to FESEM of Fe3O4@MoS2/RGO and HRTEM of Fe3O4/rGO, respectively plainly showed the dispersion of Fe3O4@MoS2 and Fe3O4 into the reduced graphene nanosheets. HRTEM of Fe3O4@MoS2/RGO (Fig. 2h) showed the folded edges of MoS2 porous nanoparticles dispersed into it and stacked on reduced graphene nanosheets but were not seen in Fe3O4/RGO case (Fig. 2g).

Fig. 2. FESEM and HRTEM images of RGO (a, b), FESEM images of porous MoS2 nanoparticles (c), ultrafine Fe3O4 beads (d), MoS2@Fe3O4 (e) (porous MoS2 marked in image by red circles), and Fe3O4@MoS2/RGO (f), and HRTEM images of Fe3O4/RGO (g) and Fe3O4@MoS2/RGO (h) (folded edges of the MoS2 marked in image by a red circle. 3. 3 Electrochemical investigations of methanol oxidation reaction 3. 3. 1 Studies in the alkaline media Fe3O4@MoS2/RGO modified GCE was applied in different concentrations of methanol in an alkaline solution. As can be seen in Fig. 3a, cyclic voltammograms showed reversible peaks for methanol oxidation, as 0.3 M methanol had the highest peak currents at 0.37 V for anodic currents, respectively, so it was selected for all experiments. Also, the pair of anodic and cathodic peak currents related to 0.5 M methanol was reduced significantly. Herein, it is supposed that oxidation products loaded on the electrode surface could be an obstacle for charge transferring after a critical concentration in alkaline solution [33]. There was not any notable peak for the modified electrode in a blank solution except for trivial non-faradic current. For acquiring the best scan rate, various experiments were done in 0.3 M of methanol in an alkaline solution for Fe3O4@MoS2/RGO modified electrode (Fig. 3b). The best scan rate equal to 100 mV s-1 was selected for future tests. As the scan rate increased, the faradic and non-faradic currents gradually increased, and the peak potentials shifted towards more positive and more negative values for anodic and cathodic currents, respectively.

Fig. 3. (a) CVs of Fe3O4@MoS2/RGO modified GCE in various concentrations of methanol in 0.1 M of NaOH and (b) CVs of Fe3O4@MoS2/RGO modified GCE with different scan rates in 0.3 M methanol and 0.1 M of NaOH. Electron transfer mechanism for the MOR of at the surface of the Fe 3O4@MoS2/RGO catalyst in alkaline media is proposed as follows: (1) (2) (3)

(4) In the first step, methanol adsorbed on the catalyst surface. In this step, RGO and MoS 2 supply more surface area and edge sites for adsorption of methanol molecules [34, 35]. In the second step, the absorbed methanol was decomposed to carbonaceous materials such as CH2OH, CHOH, COH and CO (Eq. 2). Hydroxide ions are also adsorbed on the catalyst surface (Eq. 3) that can help to oxidation of adsorbed CO and thus regenerate the active sites of catalyst (Eq. 4) [36]. It is be noted that OH- ions adsorb on Fe3+ and Mo4+ trap centers [37]. Therefore, the synergic effect of Fe3O4 and MoS2 increase OHads and improve catalyst activity and stability. The electrocatalytic activity and stability of Fe3O4@MoS2/RGO catalyst can be explained as follow: 1- The use of RGO in the composition of catalyst prevent from aggregation and agglomeration of MoS2 and Fe3O4 nanoparticles. 2- In presence of RGO, the available active sites of MoS2 increased. 3- RGO is known as a conductive material and thus can enhance the charge transfer rate. 4- RGO provides a higher surface area for methanol absorption and oxidation thus the rate of methanol electro-oxidation reaction increases. 5- Furthermore, the

porosity of MoS2 have a positive effect on methanol adsorption. 6- The synergistic effect of Fe3O4 and MoS2 lead to MOR activity enhancement of catalyst. Generally, the synergic effect of all of the above items leads to the enhancement of catalytic and stability of the catalyst. For evaluation of the electrochemical activity and electro-catalyst stability, 10 cyclic voltammograms were investigated in 0.3 M methanol solution in the presence of 0.1 M NaOH and the results shown in Fig. 4a. With an increase in the number of cycles up to 7 cycles, the peak current density increased, probably due to the activation of the catalyst sites. But after the 7th cycle, peak current density remained almost constant, which is due to the saturation of electrocatalytic active sites. Also, after the 10th cycle, the peak height is dropped very slightly, which is probably due to the blocking of electro-catalyst sites with intermediates and byproducts caused by the methanol oxidation. The results show the stability of the electrocatalysts. Crossing points of I-V curves (IPPs) observed as marked in Fig. 4a, indicate the accessibility of the electroactive Fe3O4 in the conductive region of the electrode surface [38].

Fig. 4. (a) 10 CVs results of Fe3O4@MoS2/GO modified GCE in 0.3 M methanol and 0.1 M NaOH, (b) CVs of different modified electrodes in 0.3 M methanol and 0.1 M NaOH, inset: 10 CV results of Fe3O4/RGO modified GCE in 0.3 M methanol and 0.1 M NaOH (scan rate was 100 mV s-1).

For demonstrating the role of MoS2 phase existing in the synthetic nanocomposite, the CV curves were obtained as shown in Fig. 4b. As it is clear, there was just a weak peak in the absence of MoS2 for Fe3O4/RGO modified GCE at 0.3 M methanol in alkaline media. As known, transition metal dichalcogenides as catalysts with abundant edges and defects have poor conductivity when used solely [39]. Herein, porous MoS2 (Fig. 5b-c) composited with Fe3O4 showed a brilliant effect for methanol oxidation and reduction process. Continuously, 10 cyclic voltammograms obtained for Fe3O4/RGO modified GCE electrode also did not show any impressive change during the cycles (inset of Fig. 4b).

The effect of MoS2 in electrocatalysis was being described by a few investigators [40, 41] that can depend on acidic Lewis positions. In turn, Fe existing in anionic positions of the Fe3O4 simultaneously facilitates the adsorption of methanol and desorption of oxidation products in our two-component nanocatalyst. It is clear that binding methanol to the catalyst needs the existence of many proximate sites. On the other hand, adding the second transition metal (in form of MoS2) leads oxygen to be easily bound to the carbonaceous mediator in CO 2 production applications [42]. The influence of the presence of iron compounds on platinum nanoparticles has been much studied before. There is no doubt that the existence of iron in the catalyst modifies the electronic behavior of composite due to the low electronegativity of Fe [43]. Compared to molybdenum and carbon solely, compositing molybdenum and metal oxides like

Fe3O4 leads to a better activity and stability in alkaline solutions. During the methanol oxidation, oxygen species (OHads) are formed more effectively on the surface of metal oxides [44, 45]. Through this manner, MoS2 supplies a suitable support for the combination of oxygen with Co or Co-like compounds to forming CO2, and simultaneously the active sites on the surface get more accessible for methanol oxidation [46]. On the other side, the 3D structure formed in Fe3O4@MoS2/RGO can be responsible for the better electrocatalytic activity. Connected channels in the composite are shown in Fig. 2e permitting the small methanol molecules to freely transport. Additionally, the relative largeness of the channels inhibits effectively the occlusion of liquids that can increase the methanol permeability and consequently limits the concentration polarization [43]. The most important kinetic parameters, i.e. exchange current (io) and transfer coefficient (β) which depend on the charge transfer process, are demonstrated by Tafel plots extracted from Eq(1) [47]. Eq (1) Where η is overpotential related to Tafel region and i is the corresponding current. By the way, a and b are respectively dependent on β (anodic transfer coefficient) and

0

(exchange current)

calculated through the below equations:

0

(2)

Eq

Eq (3)

The transferred electrons, n, along with the suggested mechanism for methanol oxidation in alkaline solution was determined to equal to 6 electrons [44].

For calculating the parameters, modified GCEs with Fe3O4@MoS2/RGO and Fe3O4/RGO were individually inserted in 0.3 M methanol and alkaline solution, and an LSV technique with the low scan rate of 20 mV S-1 was applied. The results of LSVs are presented in Fig. 5a.

The obtained results from Tafel plots are presented in Table 1 which shows the impressive enhancement in kinetic parameters of Fe3O4@MoS2/RGO modified GCE. The key effect of porous MoS2 on supplying electroactive sites for electron transfer was confirmed electrokinetically.

Table. 1. Kinetic parameters based on Tafel plots obtained from Fe3O4 @MoS2/RGO and Fe3O4/RGO modified GCE.

Modified electrode

i0 (A)

Β

Fe3O4@MoS2/RGO

4.80×10-8

0.464

Fe3O4/RGO

2.34×10-17

0.289

Chronoamperometry as another valid method was applied for the investigation of the modified electrodes. The current–time plots were obtained by setting the electrode at potential equal to 900 mV versus SCE for 10 seconds in various concentrations of methanol in an alkaline solution. As can be seen in Fig. 6.b, exponential chronoamperograms could imply the manner that was

controlled by the diffusion process [47]. Moreover, with attention to “Cottrell equation” (Eq4), the diffusion coefficient of methanol in the aqueous solution (D0) and also the actual surface area (A) of the modified electrode will be obtained as follows:

i  nfAD0 C0 1 / 2t 1 / 2 1/ 2

Eq

(4)

Here, n is the number of transferred electrons (6 electrons), f, stands for Faraday constant, A is the symbol of the actual surface area of the modified electrode, D 0 is diffusion coefficient of methanol (cm2.S-1), C0 is the concentration of methanol (mol cm3), and t is the time in seconds. First, the plot of i (µA) versus t-1/2 showed straight lines with different slopes based on the Cottrell equation. The linearity of the lines primarily assured the dominance of the diffusion control manner for methanol (inset in Fig. 5b). Next, by plotting the obtained slopes versus methanol concentrations, a new slope (nfAD01/2C0

-1/2

) was determined. Finally, by simultaneous

solving, two slope equations, A and D0, were calculated as 0.0295 cm2 and 6.40×10-5 cm2 s-1, respectively. As compared to the geometrical area of bare GCE (0.00785 cm2), it was found that the porosity of MoS2 was so effective in enhancing the surface area which was seen before in corresponding FESEM image (Fig. 2c). Besides, the obtained value for D 0 was close to the values reported for methanol in the literature [48].

Fig. 5. (a) LSV voltammograms of Fe3O4 @MoS2/RGO and Fe3O4/RGO modified GCEs in 0.3 M methanol and 0.1 M NaOH with scan rate equal to 20 mV s-1, inset: Tafel plots resulted from LSVs, (b) Chronoamperograms obtained for Fe3O4@MoS2/RGO modified GCE in various concentrations of methanol in alkaline solution, inset: (top inset) dependence of current (µA) vs t1/2 for different concentrations of methanol that shows a linear dependence, (low inset) the obtained slopes from the previous section vs various concentrations of methanol (mM). 3.2.3 Studies in the acidic media For evaluating the response of Fe3O4@MoS2/RGO modified GC electrode in acidic media, firstly, the most suitable methanol concentration and scan rate were selected. The response of

Fe3O4@MoS2/RGO modified GCE in various concentrations of methanol (0.05, 0.1, 0.3, and 0.5 M), in the presence of 0.1 M H2SO4 solution was investigated at a scan rate of 60 mV s-1, as shown in Fig. 6a. It can be seen from Fig. 6a, anodic (oxidation) peak is found to be around 0.6 mV in all the methanol concentrations, and the highest anodic peak is for 0.1 M. It is noteworthy to say that the anodic peak in methanol concentrations higher than 0.1 M starts to decrease which may be due to the avoidance of charge transfer when the methanol oxidation by-products reaches over critical concentration. To achieve the most suitable scan rate, we change the scan rate by keeping the methanol concentration constant (0.1 M). Fig. 6b shows the response of Fe 3O4 @MoS2/RGO modified GCE at different scan rates. As can be inferred from Fig. 6b, the Faradaic current of methanol oxidation has increased with the increase in the scan rate, and along with it, the non-Faradaic current has also risen and led to the broadening of the oxidation anodic peaks. This figure reveals that the anodic peak at the scan rate of 80 mV s -1 is the most appropriate peak for continuing the process of surveying the Fe3O4@MoS2/RGO composite potential for methanol oxidation in acidic media. At greater scan rates, there is not adequate time for the oxidation reaction of active species. As it is widely known, the capability of the fuel cell system in both alkaline and acidic media is worthy [48]. In order to evaluate methanol oxidation as the base of DMFC in acidic solution, modified GCE by Fe3O4 @MoS2/RGO was applied in 0.1 M H2SO4, and cyclic voltammograms were obtained (Fig. 6c) indicating that the increase in the number of cycles serves to bolster the oxidation and reduction peaks growth until the 8th cycle, after that, the peak current remained constant and stable. It was utterly converse about the alkaline media.

Previously, some reports have considered DMFC in acidic media. However, what is common in most cases is that the use of platinum compounds in the structure of catalysts applied in acidic media is apparently unavoidable [49, 50]. Within current work, while the porous MoS2 supported by Fe3O4 maintains the oxidative state of Fe (Fe+3) in acidic media, the vicinity of Fe+3 and porous MoS2 causes facilitates the removal of oxygen species that developed during methanol oxidation. On the other hand, dispersion of Fe3O4@MoS2 in reduced graphene nanosheets could help increase catalysis behavior of composite in acidic medium [51]. Electron transfer mechanism for the MOR at the surface of the Fe3O4@MoS2/RGO catalyst in acidic media is proposed as follows: (5) (6) (7)

(8)

Fig. 6. (a) CVs of Fe3O4@MoS2/RGO modified GCE in various concentrations of methanol in 0.1 M of H2SO4, (b) CVs of Fe3O4@MoS2/RGO modified GCE with different scan rates in 0.3 M methanol and 0.1 M of H2SO4, and (c) 10 cycles of Fe3O4@MoS2/GO modified GCE in 0.3 M methanol and 0.1 M H2SO4. 2. 4 Investigation of hydrogen evolution reaction Transition metal dichalcogenides such as MoS 2 and WS2 can have platinum-like electrochemical properties in HER process. These materials have shown promise results at different morphologies and pHs [52, 53]. The number of active sites and electrical conductivity have an important role on the electrocatalytic activity of the catalyst for HER. MoS 2 have been considered as a good electrocatalyst toward HER. The studies have been shown that the unsaturated sulfur atoms on the edge sites of MoS 2 are as active sites for HER. This sites capture H atoms and so improve the HER reaction [37]. However, the low conductivity of MoS2 is the main challenge. To overcome this limitation, dispersion of MoS2 in carbonaceous materials [54] and doping of conducting metals [55] are the methods for its conductivity and activity enhancement in HER process. Dispersion of MoS 2 in carbonaceous materials also prevents from MoS 2 agglomeration, as a result, increase the number of exposed edge sites in the catalyst and specific surface area. Furthermore, it is confirmed that Fe3O4 and Fe2O3 nanoparticles can have a synergistic effect with MoS 2 on HER process. There is a close relationship between electrochemical HER and DMFC performance in acidic media through the equation:

Which describes methanol reforming HER. The modified electrode was investigated based on the following mechanism [56] 1. Discharge step or “Volmer reaction”:

In which the Tafel slope is measured by: Eq (5) 2. Electrochemical desorption step or “Heyrovsky reaction”:

Eq (6) 3. Recombination step or “Tafel reaction”:

Eq (7) For investigating the performance of HER process, definite values of Fe 3O4 @MoS2/RGO and Fe3O4/RGO were individually deposited on the surface of the electrode, and LSV voltammograms were obtained in 0.5 M H2SO4 calibrated versus reversible hydrogen electrode (RHE), with a 20 mV s-1 scan rate. Fig. 7a shows the LSV results that revealed a lower overpotential (108 mV) in hydrogen evolution for Fe3O4@MoS2/RGO modified GCE with Tafel slope of 48 mV dec-1.

Improvement of HER electrocatalytic activity can be attributed to; 1- The synergistic effect between Fe3O4 and MoS2. 2- The decline of the Gibbs free energy of the adsorbed H atoms on the edge sites of MoS2 by Fe atoms [57]. 3- Increase the electron transfer rate due to hybridizing Fe3O4@MoS2 by conductive RGO. 4- Create the large surface area and stability of catalyst by incorporating of RGO [58-62]. The obtained data indicated that the rate-limiting step of desorption step as a “VolmerHeyrovsky” mechanism for HER by Fe3O4 @MoS2/RGO modified GCE was dominated. The importance of MoS2 in decreasing the overpotential of hydrogen polarization has been revealed. Overall, it has been concluded that Fe3O4 @MoS2/RGO is a promising nanocatalyst for DMFC in various mediators.

Fig. 7. LSVs of Fe3O4@MoS2/RGO and Fe3O4/RGO modified GCE in 0.5 M H2SO4, inset: Tafel plot related to Fe3O4@MoS2/RGO modified GCE for HER (a) and chronopotentiometery curves and the amount of hydrogen evolved during 600 s (inset) (b). The amount of hydrogen evolved from modified GCE with Fe3O4/RGO and Fe3O4@MoS2/RGO were measured by applying chronopotentiometery in a constant current of 200 mA cm-2 for 600

s. The evolved hydrogen gas was collected in an inverted burette. The chronopotentiometery curves and hydrogen volumes were shown in Fig. 7b. The volume of produced hydrogen obtained 7 ml and 16 ml for Fe3O4/RGO and Fe3O4@MoS2/RGO, respectively. The hydrogen volumes are consistent with the LSV curves. The enhancement in electrocatalytic efficiency of the Fe3O4@MoS2/RGO compared to Fe3O4/RGO may be assigned to the unsaturated sulfur atoms on the edge sites of MoS2. A literature survey on the electrochemical properties (overpotential and Tafel slope) of the asprepared electrocatalyst and other researches was shown in Table 2. Results indicate that the obtained overpotential and Tafel slope for HER at Fe3O4@MoS2/RGO is better than or comparable with other researches. Table 2. Comparison of the electrochemical properties of the as-prepared catalyst with other researches.

Composite

Fe3O4@Mo S2/RGO

MoS2@Fe3O4

GQDsMoS2

MoS2/CNT-G

O- MoS2/G

Ni-MoS/C

Tafel slope (mV dec-1) Over potential (mV) References

48

52

43

100

51

85.3

108

110

140

140

120

130150

This work

[57]

[63]

[64]

[65]

[66]

3. 5 Effect of temperature in alkaline and acidic setting The effect of temperature on the MOR activity of Fe3O4 @MoS2/rGO electro-catalyst was examined in 0.1 M NaOH and 0.1 M H2SO4 0.1 M methanol at different temperatures. The LSV results were shown in Fig. 8a and b. According to the results, the anodic peak current density

enhanced with temperature, which is attributed to the decrease in charge transfer resistance at the interface of electrolyte and electrode, decline in diffusion phenomena and the increase in OH adsorption that increase CO oxidation rate [39]. Also, there is a linear correlation between anodic peak current density and temperature in the range of 290 K and 310 K as shown in the inset in Fig. 8.

Fig. 8. LSV curves of Fe3O4@MoS2/rGO catalyst in 0.1 M methanol with a scanning rate of 50 mV s-1 at different temperatures (a) and the plot of the anodic peak current density on the Fe3O4@MoS2/rGO catalyst versus temperature (b).

3.6 Single cell performance For investigation of electro-catalyst performance in the single cell, the membrane electrode assembly (MEA) based on Nafion 117 membrane was prepared. The anode catalyst ink was prepared by mixing the suitable amount of Fe3O4@MoS2/RGO catalyst with Nafion binder solution, isopropyl alcohol, and deionized water. The catalyst ink was painted onto carbon cloth to give a Fe3O4@MoS2/RGO loading 4 mg cm–2. The cathode catalyst was Pt/C on carbon cloth with a Pt loading of 2 mg cm–2. The MEA was fabricated by hot pressing of Nafion membrane with electrodes at 125 °C and pressure of 9 MPa for 90 seconds. The DMFC cell was operated at 80 °C. Different concentrations of methanol (0.05 M, 0.1 M, 0.3 M, and 0.5 M) was supplied to the anode at a flow rate of 2 mL min–1 and the flow rate of oxygen gas was 200 mL min–1. Fig. 9 shows the polarization curves of the as-prepared electro-catalyst in a DMFC operating at 80 °C and different concentration of methanol. As shown in Fig. 9, the OCV values in low concentrations of methanol (0.05-0.3 M) were very similar, about 0.67 V, indicating that the OCV attenuation induced by the MORs at the cathode electrode was negligible at the low methanol concentration [5, 67]. However, OCV value decreased to 0.46 V when methanol concentration increased to 0.5 M. similar results were observed by other authors that show methanol crossover by increasing methanol concentration. Maximum power density values in 0.05 M, 0.1 M, 0.3 M and 0.5 M obtained 25.68 mW cm-2, 31.77 mW cm-2, 35.03 mW cm-2 and

16.02 mW cm-2 respectively. These results show that the Fe3O4 @MoS2/RGO catalyst has the potential for use in DMFC.

Fig. 9. Polarization curves and power density of as-prepared electro-catalyst in a DMFC operating at 80 °C and different concentration of methanol. 4. Conclusion Pristine MoS2 was synthesized by hydrothermal method and then Fe3O4 @MoS2/RGO was prepared by a two-step hydrothermal process. The synthetic materials were characterized by XRD, Raman spectroscopy, EDX, FESEM, and HRTEM, and the performance of such twocomponent nanocomposite for methanol oxidation in both alkaline and acidic settings were

investigated. The key role of porous MoS2 in electrocatalysis was represented and discussed by a suggested mechanism. The results obtained from Tafel plots in the alkaline solution for Fe3O4@MoS2/RGO modified GCE showed significant values for kinetic parameters, i0 and β, equal to 4.80×10-8 A and 0.464, respectively. Chronoamperometry gave the values of A and D 0 equal to 0.0295 cm2 and 6.40×10-5 cm2 S-1, respectively that represented a provision of more active sites by the nanocomposite, and also suggested a diffusion process manner for methanol oxidation. In acidic media, a Tafel slope equal to 39 mV dec -1 was concluded from LSVs, proofed a “Volmer-Heyrovsky” mechanism for HER by Fe3O4@MoS2/RGO modified GCE. Also, the nanocomposite showed stability in both alkaline and acidic settings but in the opposite direction. Finally, Single-cell studies showed appropriate mass transport and power density values that could suggest the synthetic nanocomposite as an anode material for methanol oxidation in DMFC.

All authors contributed to this research work equally. We have no conflict of Interest to declare

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Graphical abstract