One-Pot Microwave-Assisted Synthesis of Graphene-Supported PtCoM (M = Mn, Ru, Mo) Catalysts for Low-Temperature Fuel Cells

: In this study, one-pot microwave-assisted synthesis was used to fabricate the graphene (GR)-supported PtCoM catalysts where M = Mn, Ru, and Mo. The catalysts with the molar ratios of metals Pt:Co:Mn, Pt:Co:Ru, and Pt:Co:Mo equal to 1:3:1, 1:2:2, and 7:2:1, respectively, were prepared. Catalysts were characterized using Transmission Electron Microscopy (TEM). The electrocatalytic activity of the GR-supported PtCoMn, PtCoRu, and PtCoMo catalysts was evaluated toward methanol oxidation in an alkaline medium employing cyclic voltammetry and chrono-techniques. The most efﬁcient electrochemical characteristics demonstrated the PtCoMn/GR catalyst with a current density value of 144.5 mA cm − 2 , which was up to 4.8 times higher than that at the PtCoRu(1:2:2)/GR, PtCoMo(7:2:1)/GR, and bare Pt/GR catalysts. showed enhanced electrocatalytic activity toward methanol oxidation than the pure Pt/GR catalyst. However, the highest current density, activity, and stability toward methanol oxidation demonstrated the PtCoMn(1:3:1)/GR catalyst. The results suggest that the enhanced electrocatalytic activity of investigated ternary PtCoM/GR (M = Mn, Ru, Mo) catalysts might be related to the synergistic effect between metals in the catalyst composition.


Introduction
Among the different fuel cells (FCs) developed so far, direct methanol fuel cells (DMFCs) are considered one of the most promising clean energy sources, especially for portable power supply [1,2]. They receive significant attention due to a whole range of benefits, including their superior specific energy, economical price, and physical state (liquid form at ambient conditions) that unambiguously determine the system's operational safety. The main reactions occurring in the alkaline DMFCs are the oxidation of methanol (MOR) at the anode and the reduction of oxygen (ORR) at the cathode according to the following Equations (1) and (2), respectively [3]: Anode reaction: CH 3 Cathode reaction: 3/2O 2 + 3H 2 Overall reaction: CH 3 OH + 3/2O 2 → CO 2 + 2H 2 O.
However, the successful worldwide commercialization of DMFCs is related mainly to the Pt or Pt-based electrocatalysts that are expensive and generate considerable manufacturing costs. One more drawback is the relatively low resistance of such catalysts toward carbon monoxide adsorption (CO ads ). Developing novel catalysts that substitute Pt in low-temperature fuel cell anodes and have low CO tolerance is vital. To reduce the use of precious metals, several metal catalysts have been investigated. The addition of a second or third metal and the formation of di-or trimetallic catalysts have been found to be an effective way to significantly increase the resistance of the catalyst to CO poisoning while increasing electrocatalytic activity and stability. Therefore, bimetallic noble systems such as Pt-Ru [4][5][6][7][8][9][10][11][12][13][14], Pt-Au [15,16], and Pt-Pd [10,14,[17][18][19][20] have been developed for the oxidation of methanol. However, as was mentioned above, to reduce the amount of noble metal in the catalysts, Pt alloying with one or two non-noble (transition) metals, such as Co [14,[21][22][23][24][25][26][27][28][29][30][31], Ni [14,21,24,27,[32][33][34], Cu [24,[35][36][37][38], Fe [39], Mn [40], Sn [10,40], and Zn [40] or W [41] was studied. Although cobalt was observed as one of the added metals to increase the activity of Pt catalysts, the addition of a third metal to the catalyst showed better catalytic properties of trimetallic catalysts due to the formation of metal alloys or a synergistic effect between the metals in the catalysts as well as for creating of cheaper catalysts.
There are some methods described as suitable for the preparation of binary or ternary PtM alloys. Shi [42]. Li also investigated the activity of PtCu alloy for methanol oxidation reaction, where the catalyst was prepared from relevant salts aqueous solutions, and synthesis was executed in a Teflon autoclave at a temperature of 200 • C for 1 h [37]. However, heating in traditional autoclaves takes a lot of time, making catalyst formation not as easy as possible.
In recent years, microwave irradiation heating became more attractive for the formation of catalysts for fuel cells. In a microwave reactor, some liquids and solids transform electromagnetic radiation into heating. Many solvents such as water, alcohols, acetone, acetic acid, etc., are heated rapidly when microwave irradiation is used [43][44][45][46][47][48]. There are many advantages of using microwave heating instead of traditional heating. These include shorter reaction time, high efficiency, product purity, low cost, and the reduction of hazardous substances, which is very important nowadays [43,48]. Additionally, nanostructures with smaller sizes, narrower size distributions, and a higher degree of crystallization could be obtained under microwave heating than those in conventional oil-bath heating. By changing various experimental parameters, such as the concentration of metallic salt, the solvent, the reaction temperature, time, etc., the morphologies and sizes of nanostructures could be controlled. Çögenli et al. prepared PtM (M = Pd, Ru, Sn) bimetallic catalysts using microwave irradiation heating at 800 W for 120 s [10]. Kepenienė et al. investigated PtCo catalysts with different Pt:Co molar ratios deposited on a graphene substrate, using microwave irradiation heating method at a temperature of 170 • C for 30 min [23]. Kilmonis et al. synthesized the graphene-supported PtW efficient catalysts by the microwave irradiation method [41]. One-pot synthesis simplifies the catalysts' preparation method compared to the synthesis, which requires multiple steps [12,13,23,34]. Moreover, all of the above-mentioned binary PtM alloys exhibited higher electrocatalytic activity toward MOR than bare Pt/C. However, various ternary or even quarternary PtM alloys have also emerged in the literature as effective MOR electro-catalysts [13][14][15]. Great attention has been attributed to the investigation of multi-component systems, such as PtNiCu [42], PtNiCo [49], PtCoRh [50], FePtCu [51], FePtSn [52], RuMPt (M = Fe, Co, Ni, and Cu) [53], PtRuFeCo [54], and PtRuCuW [55] as efficient catalysts for MOR. It should be noted that generally, the catalysts mentioned above allow reducing the amount of noble metal and lowering the catalysts' overall cost. They demonstrated enhanced stability, higher resistance to catalyst poisoning, and markedly improved catalytic activity for MOR compared to those characteristics of monometallic catalysts. Zhao [50]. It was found that mass activity values for PtCoRh were two times higher than those for Pt/C catalyst.
In this study, we investigated ternary PtCoM (M = Mn, Ru, Mo) catalysts based on recent research on multi-component systems. Catalysts of this composition have not been extensively described. A one-pot microwave-assisted synthesis was used to prepare the GR-supported PtCoM (M = Mn, Ru, Mo) nanoparticle catalysts with different Pt:Co:M molar ratios as electrocatalysts for the oxidation of methanol in an alkaline medium. The prepared PtCoM/GR catalysts were characterized using Transmission Electron Microscopy (TEM), X-ray Energy Dispersive Analysis (EDX), and Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). The electrocatalytic activity of the graphene-supported PtCoM catalysts toward the oxidation of methanol in an alkaline medium was investigated using cyclic voltammetry and chrono-techniques.

Results
According to the synthesis conditions, using a one-pot microwave synthesis by microwave-assisted heating of Pt(IV), Co(II), Mn(II), Ru(III), and Mo(VI) salts in an ethylene glycol solution, the PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts were formed. The XRD patterns of the synthesized catalysts are shown in Figure 1. A sharp peak at 26.5 • observed for investigated catalysts can be attributed to the graphite structure (002) plane of the supports. For PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts, the peaks corresponding to polycrystalline Pt and other metals cannot be clearly discerned, because the catalysts are amorphous, and PtCoMn, PtCoRu, and PtCoMo nanoparticles are too small, which results in the broadening of the small peaks. The practical composition of synthesized catalysts was determined by ICP-OES and is given in Table 1. It was found that Pt:Co:Mn, Pt:Co:Ru, and Pt:Co:Mo molar ratios in the prepared PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts are 1:3:1, 1:2:2, and 7:2:1, respectively. It was determined that Pt loadings in the PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts were 103.8, 183.2, and 83.8 µg cm −2 , respectively. The ESA values of Pt in the catalysts were determined from CVs recorded in the 0.5 M H 2 SO 4 solution and calculating the charge associated with the hydrogen adsorption (220 µC cm −2 ) and are given in Table 1. The TEM examination was used to assess the size of reduced Pt nanoparticles in the investigated catalysts. Figure 2 shows TEM images and the corresponding particle size distribution of PtCoMn   The EDX analysis showed that all metals in the prepared catalysts are well-distributed on graphene's surface. The images of metals distribution and amount of metal wt % from the mass spectrum of investigated catalysts are shown in Figure 3.    Table 1). The ESA of Pt is 2.2, 2.9, and 3.1 times higher at PtCoMn/GR than those at Pt/GR, PtCoRu/GR, and PtCoMo/GR catalysts, respectively (Table 1). The electrocatalytic activity of the graphene-supported PtCoMn, PtCoRu, and Pt-CoMo catalysts toward the oxidation of methanol in an alkaline medium was investigated using cyclic voltammetry. Figure 5a-c shows long-term CVs for PtCoMn(1:3:1)/GR, Pt-CoRu(1:2:2)/GR, and PtCoMo(7:2:1)/GR catalysts recorded in a 1 M CH 3 OH + 1 M NaOH solution at a sweep rate of 50 mV s −1 . As evident, in the forward sweep, anodic peaks A are observed at 0.2 V. They may be related to the direct oxidation of methanol in an alkaline medium. Anodic peaks B were detected at approximately −0.4 V for all the investigated catalysts in the reverse sweep. They may be attributed to the removal of the incompletely oxidized carbonaceous species formed in the forward sweep. During long-term cycling, the methanol oxidation current density values (anodic peak A) recorded at the investigated PtCoMn(1:3:1)/GR catalyst at first slightly are decreased and then stabilized (Figure 5a). The other way is for the PtCoRu(1:2:2)/GR and PtCoMo (7:2:1)/GR catalysts (Figure 5b,c). Current density values are rising and then stabilized. Figure 5d shows positive-potential going scans of the Pt/GR, PtCoMn(1:3:1)/GR, PtCoRu(1:2:2)/GR, and PtCoMo(7:2:1)/GR catalysts. The highest electrocatalytic activity for methanol oxidation reaction is noted for the PtCoMn(1:3:1)/GR catalyst (Figure 5d).
Furthermore, all ternary PtCoM/GR catalysts show more negative onset potential of MOR than the Pt/GR catalyst.
As well as methanol oxidation current densities are ca. 4.8, 1.5, and 2.8 times higher at ternary PtCoMn(1:3:1)/GR, PtCoRu(1:2:2)/GR, and PtCoMo(7:2:1)/GR catalysts than those at the bare Pt/GR catalyst. The comparison of current densities of our prepared catalysts with various similar catalysts described in the literature is given in Table 2.  Figure 6 presents the specific (a) and mass (b) activities of the methanol oxidation reaction on Pt/GR and ternary PtCoM/GR catalysts. Methanol oxidation current densities were normalized by electrochemically active surface areas for each catalyst to represent the catalysts' specific activity. As seen from the data obtained, the highest specific activity has PtCoMo(7:2:1)/GR. It is approximately two times higher than that at PtCoMn(1:3:1)/GR and PtCoRu(1:2:2)/GR catalysts and ca. four times higher than that at Pt/GR (Figure 6a). To obtain the mass activity data, methanol oxidation current densities at the potential values of peak A were normalized by the Pt loadings for the PtCoMn/GR, PtCoRu/GR, PtCoMo/GR, and Pt/GR catalysts (Figure 6b). The highest mass activity for methanol oxidation was calculated for PtCoMn(1:3:1)/GR and PtCoMo(7:2:1)/GR catalysts, and it was ca. 3 times higher than that at Pt/GR and PtCoRu(1:2:2)/GR catalysts (Figure 6b). Those catalysts also show significantly higher mass activity for methanol oxidation over reported similar materials. Figure 7 presents the Tafel plots for methanol oxidation derived from the data in Figure 5d. Tafel plots can be divided into two linear regions for Pt/GR, PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts. In the low potential range, a linear region of the Tafel plots on Pt/GR, PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts was found, giving the Tafel slopes of 159.9, 157.4, 239.6, and 152.1 mV dec −1 , respectively (Figure 7).
The similar slopes indicate that the reaction mechanism is the same and corresponded to methanol adsorption and dehydrogenation [63]. Notably, PtCoMn/GR, PtCoRu/GR, and PtCoMo/GR catalysts are more active than Pt/GR. The PtCoMn/GR possess the highest output current density among the four electrocatalysts tested in this study under an identical polarization overpotential E = −0.5 V (Figure 7).  The linear region at high potential is related to oxidative removal of CO-like species [63]. At high potentials, the Tafel slopes of 360.3 mV dec −1 for PtCoMn/GR, 358.7 mV dec −1 for PtCoRu/GR, and 252.0 mV dec −1 for PtCoMo/GR were determined. A sharp change of the Tafel slope shows that removing poisonous species becomes the rate-determining process in the methanol oxidation reaction. The difference in the Tafel slopes at low and high potential ranges indicates a possible change of reaction mechanism or at least a change of rate-determining steps at different potential ranges [64].
Slope values of PtCoMn/GR and PtCoRu/GR are greater than that of PtCoMo/GR at high potential, indicating that the step of removing poisonous species on PtCoMn/GR and PtCoRu/GR is slower than on PtCoMo/GR. Further, the electrochemical stability of the PtCoM/GR and Pt/GR catalysts for methanol oxidation was investigated using chrono-techniques. Figure 8 shows the CA data obtained at the PtCoMn(1:3:1)/GR, PtCoRu(1:2:2)/GR, and PtCoMo(7:2:1)/GR catalysts recorded at a constant potential of −0.2 V in a 1 M CH 3 OH + 1 M NaOH solution at a temperature of 25 ºC for 1800 s. As seen in Figure 8, the investigated catalysts show current decay for the methanol oxidation reaction. However, after approximately 600 s, the current density stabilizes. At the end of the experimental period (t = 1800 s), the current densities recorded at the ternary catalysts are ca. 14, 2, and 8 mA cm −2 for PtCoMn(1:3:1)/GR, PtCoRu(1:2:2)/GR, and PtCoMo(7:2:1)/GR, respectively, and 5.5 mA cm −2 for Pt/GR (Figure 8).

Synthesis of Graphene-Supported PtCoM (M = Mn, Mo, Ru) Nanoparticle Catalysts
For the one-pot synthesis of PtCoM/GR (M = Ru, Co, Mn) nanoparticle catalysts, reaction mixtures containing constant amounts of H 2 PtCl 6 and CoCl 2 , and required amounts of RuCl 3 , Na 2 MoO 4, and MnCl 2 were used. Graphene with a purity of 97% and specific surface area of 60 m 2 g −1 ) was used as a substrate. The composition of reaction mixtures was as follows: The synthesis of the catalysts was carried out in a Monowave 300 (Anton Paar, Graz, Austria) reactor at a temperature of 170 • C for 30 min. Microwave output power was 750 W. After preparation, the synthesized catalysts were washed with acetone, filtered, and dried in a vacuum oven at 80 • C for two h. Pt/GR was prepared at the same conditions, but the synthesis duration was shortened to 30 s to prepare Pt particles with approximately similar ones in the created PtCoM/GR catalysts.

Characterization of Catalysts
The shape and size of catalyst particles were examined using a Transmission Electron Microscope Tecnai G2 F20 X-TWIN (FEI, Eindhoven, Netherlands) equipped with an EDAX spectrometer with an r-TEM detector. For microscopic examinations, 10 mg of sample was first sonicated in 1 mL of ethanol for 1 h and then deposited on the Cu grid covered with a continuous carbon film.
XRD patterns of studied powders were measured using an X-ray diffractometer D2 PHASER (Bruker, Karlsruhe, Germany). The measurements were conducted in the 2θ range 10-90 • .
The Pt, Co, Mn, Mo, and Ru metal loadings were estimated using an ICP optical emission spectrometer Optima700DV (Perkin Elmer, Waltham, MA, USA).
The distribution of elements in the catalyst was analyzed using a scanning electron microscope TM4000Plus with an AZetecOne detector (Hitachi, Tokyo, Japan).

Electrochemical Measurements
All electrochemical measurements were performed with a Zennium electrochemical workstation (ZAHNER-Elektrik GmbH & Co.KG, Kronach, Germany). A three-electrode electrochemical cell was used for measurements. The working electrode was a glassy carbon electrode coated with a thin layer of Nafion-impregnated Pt/GR and PtCoM/GR catalysts. The Pt sheet was used as a counter electrode and Ag/AgCl as a reference electrode. The catalyst layer was obtained according to the following steps: at first, the required amounts of Pt/GR and PtCoM/GR catalysts were dispersed ultrasonically for 1 h in a solution containing 0.25 µL of 5 wt % Nafion and 0.75 µL deionized H 2 O to produce a homogeneous slurry. Then, 5 µL of the prepared suspension mixture was pipetted onto the polished surface of a glassy carbon electrode with a geometric area of a 0.07 cm 2 and dried in the air for 12 h.
Cyclic voltammograms (CVs) were recorded at Pt/GR and PtCoM/GR in a deaerated 1 M CH 3 OH + 1 M NaOH solution at a potential sweep rate of 50 mV s −1 . The presented current densities are normalized with respect to the geometric area of catalysts.
The electrochemically active surface areas (ESAs) of Pt in the prepared catalysts were determined from the CVs of the PtCoM/GR and Pt/GR catalysts recorded in a deaerated 0.5 M H 2 SO 4 solution at a potential sweep rate of 50 mV s −1 by calculating the charge associated with hydrogen adsorption (220 µC cm −2 ) [65]. The ESA values of Pt in the prepared catalysts were calculated by the following Equation (4): S ESA (cm 2 ) = Q (µC)/Q H (220 µC cm −2 ).
The stability of the PtCoM/GR (M = Mn, Mo, Ru) and Pt/GR catalysts was examined using chrono-techniques. Chronoamperometric curves (CA) were recorded in 1 M CH 3 OH + 1 M NaOH solution at a constant potential value of 0.2 V for 30 min. Chronopotentiometric curves (CP) were recorded at a constant current density of 2 mA cm −2 for 30 min in the same solution.
The methanol oxidation current density values were normalized by the electrochemically active surface areas of Pt and Pt loadings for each catalyst, respectively, to evaluate the specific activity (mA cm −2 ) and mass activity (mA mg Pt −1 ) of the PtCoM/GR and Pt/GR catalysts.

Conclusions
The graphene-supported PtCoMn, PtCoRu, and PtCoMo catalysts were prepared using one-pot microwave-assisted synthesis with the molar ratios of metals Pt:Co:Mn, Pt:Co:Ru, and Pt:Co:Mo equal to 1:3:1, 1:2:2, and 7:2:1, respectively. It has been found that the Pt nanoparticles with an average size of approximately 2.5 nm were deposited in the PtCoMn(1:3:1)/GR, PtCoRu(1:2:2)/GR, and PtCoMo(7:2:1)/GR catalysts. In addition, all metals in the prepared catalysts were uniformly distributed on the surface of graphene. All investigated ternary catalysts showed enhanced electrocatalytic activity toward methanol oxidation than the pure Pt/GR catalyst. However, the highest current density, activity, and stability toward methanol oxidation demonstrated the PtCoMn(1:3:1)/GR catalyst. The results suggest that the enhanced electrocatalytic activity of investigated ternary PtCoM/GR (M = Mn, Ru, Mo) catalysts might be related to the synergistic effect between metals in the catalyst composition.