Improved the methanol electrooxidation and carbon monoxide tolerance for direct methanol fuel cells using strontium molybdate

Electrocatalysts possess high methanol electrooxidation (MOR) and carbon monoxide (CO) tolerance abilities to meet application requirements for enhanced performance of direct methanol fuel cells (DMFCs). Higher MOR and CO tolerance activities measured via cyclic voltammetry (CV) are achieved based on a strontium molybdate (SrMoO 4 ) mixed with Vulcan XC-72 carbon and loaded with 20% Pt. The synergistic effect of the bifunctional mechanism inducing the strontium molybdate is benecial for removing CO-like intermediate products on the Pt surface, which leads to more Pt active sites released during MOR. The simultaneous unique structural formation of H x MoO 3 /H y MoO 3 and SrMoO 3 in uncalcined Sr 0.5 Mo 0.5 O 4−δ provides key synergistic effects for the 20%-Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C electrocatalysts, improving DMFCs performance. Results show that the 20%-Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C electrocatalysts exhibit excellent electrocatalytic activity for MOR (with a forward peak current density of 12.56 mA/cm 2 and large electrochemically active surface area (ECSA H ) of 116.53 m 2 /g, the best CO tolerance for electrooxidation of carbon monoxide (largest ECSA CO of 116.53 m 2 /g), and the highest electric conductivity (R ct of 940 Ω∙ cm 2 ). Furthermore, the fabricated DMFC shows excellent long-term electrochemical stability after 1000 cycles and a maximum power density (1.42 mW/cm 2 ) higher than that with commercial 20%-Pt/C (1.27 mW/cm 2 ).


Introduction
Global warming has already created extreme weather globally. In recent years, the European Union energy and climate policy has required energy system transformations to reduce greenhouse gas emissions in 2050 to less than 80% of 1990 levels [1]. The conversion of carbon dioxide produced by feedstock into green methanol [2] is a key strategic factor for renewable energy and creation of a low-carbon economy.
The manufacturing cost of renewable methanol will be gradually reduced by 2030, making it cheaper than coal and natural gas [2]. Methanol (CH 3 OH) is a liquid fuel at room temperature, which makes it less toxic and provides higher octane value [3] than gasoline. It is also much easier to handle and store than pure H 2 , another alternative fuel. Methanol is a key fuel for direct methanol fuel cells (DMFCs). Although DMFCs face di culty when used in transportation power technology compared to well-known hydrogen fuels of polymer electrolyte membrane fuel cells, there is still opportunity for their use in portable power systems. Simultaneously, DMFCs are environmentally friendly when converting chemical energy from liquid methanol fuel to electrical energy as they only generate water and carbon dioxide (CO 2 ) byproducts. Until now, DMFCs have faced many problems such as the crossover of methanol fuel from the anode to cathode electrode [4]. The anode side of the DMFCs undergoes electrooxidation to CO 2 , and the instability of the electrocatalysts occurs due to Pt poisoned by the absorbed carbon monoxide (CO) during methanol electrooxidation reaction (MOR) [5][6], as shown in Eqs. (1)- (3). CO adsorbed strongly on the surface of the Pt causes blockage of the Pt surface, reducing durability [7][8].
CH 3 OH + H 2 O →CO 2 + 6H + + 6e - (1) 2Pt + H 2 → 2Pt-H ads (2) CO 2 + 2Pt-H ads → Pt-CO ads +H 2 O + Pt (3) One method used to maintain the performance of DMFCs includes the addition of a high loading amount of Pt or Pt alloys on carbon as an electrocatalyst to increase CO tolerance [9]. Another method to improve Pt poisoned by CO is to introduce Pt loading on a metal oxide (MO) with carbon as electrocatalyst, which creates a synergistic effect from the bifunctional mechanism of Pt and MO. This synergistic effect enhances CO tolerance, as expressed in Eqs. (4)-(5) [10].
MO + H 2 O → MO-OH ads +H + +e - (4) Pt-CO ads +MO-OH ads → Pt + MO + CO 2 + H + +e - (5) Therefore, many studies have been performed with MOs incorporating Pt to create electrocatalysts that not only enhance MOR activity but also are not susceptible to carbon monoxide poisoning, for example, Pt/CeO 2 -C [11], Pt-Co 3 O 4 [12], Pt/NiO-C [13], Pt/SnO x -C [14], Pt-Ru/Al 2 O 3 -C [15], Pt/WO 3  In the present study, 20% Pt loading on strontium molybdate mixed with carbon black was prepared as the electrocatalyst to successfully improve MOR and CO tolerance in fabricated DMFCs. Strontium molybdate is a scheelite-type complex oxide [27] that consists of Sr 2+ cations and (MoO 4 ) 2− anions. It has been used recently in electrocatalysts to increase hydrogen evolution reaction activity in acid water electrolytes [28] when applied to the electrochemical determination and photocatalytic degradation of diphenylamine [29] and tetracycline [30]. Thrane et al. [31] reported that SrMoO 4 catalysts can be used for the selective oxidation of methanol to formaldehyde. These recent studies have inspired our current investigation.
In this work, the in uence of different calcination temperatures and different amounts of Sr mixed with the Mo contact during SrMoO 4 preparation on MOR and CO tolerance was investigated. Although commercial PtRu-C (E-TEK) catalyst has a higher MOR and more tolerance CO poisoning than commercial Pt/C [32][33], its cost is twice that Pt/C cause more limiting applied in DMFCs. Therefore, the results in this study focus on comparing with commercial Pt/C. The novel developed 20%-Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C electrocatalyst exhibits signi cantly enhanced catalytic activity for MOR and CO tolerance, as well as enhanced DMFCs performance compared with commercial 20%-Pt/C. solution was supplemented to the suspension and stirred for 1 h. The pH of the suspension was adjusted to 11 by the drop-wise addition of 0.5 M sodium hydroxide (NaOH; Showa Chemical Industry Co., Ltd., Japan). The suspension was then heated to 140°C under re ux for 3 h. After the mixture was cooled, the precipitate was centrifuged at 5000 rpm three times for 15 min each. The precipitate was washed with distilled water, placed in a dish, and dried in an oven at 80°C for 24 h.

Material characterization
Raman spectroscopy measurements were performed at room temperature using a Raman spectrometer coupled to a microscope (Unidron, Taiwan) with a diode-pumped solid-state laser with a wavelength of 532 nm at 100 mW in the spectral range between 100 and 1100 cm − 1 . The objective lens of the microscope resulted in a 1.2-mm diameter laser spot. SrMoO 4 structural properties were identi ed using X-ray diffraction (XRD) in a Rigaku (Japan) ultima IV rotating anode diffractometer with a Ni-ltered Cu-Kα radiation source (wavelength of 1.54 Å). The binding energies of different electrocatalyst elements were identi ed using X-ray photoelectron spectroscopy (XPS) with a VG Scienti c ESCALAB 250 (UK) equipped with a dual Al X-ray source operated at 200 W and 15 kV. The beam size of the XPS X-ray source was 650 µm, and a hemispherical analyzer was operated in constant analyzer energy mode during measurements. The base pressure in the XPS analyzing chamber was maintained at 10 − 10 mbar. A nonlinear least-squares curve-tting program with a Gaussian-Lorentzian production function, the Casa XPS program (Casa Software Ltd., UK), used to process XPS data. An adventitious C1s binding energy of 284.9 eV was set as the reference binding energy for charge correction. The morphology of the electrocatalysts was analyzed using scanning electron microscopy (SEM; JEOL JSM-6700F instrument) with energy dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) was performed with a Philips FEI Tecnai G2F30 electron microscope with an acceleration voltage of 300 kV with a 1-25-nm probe size of EDS detector.

Electrochemical measurements
Electrochemical measurements were performed on a computer-controlled CHI 608E electrochemical analysis instrument (CH Instruments, Inc., USA). The electrocatalyst cyclic voltammetry (CV) curves of MOR were measured using a three-electrode cell system at room temperature within a potential range of − 0. in this equation is the charge under the CO oxidation peak, which is related to the following oxidation process converting CO to CO 2 : CO + H 2 O → CO 2 + 2H + + 2e − . The value of 0.420 [mC/m 2 ] corresponds to the charge required to oxidize a monolayer of CO adsorbed on Pt.
Electrochemical impedance spectra (EIS) were obtained at 0.4 V and from 100 kHz to 0.01 Hz in a 0.5 M H 2 SO 4 containing 1 M CH 3 OH mixture de-aerated with ultrapure N 2 gas (4 cc/min) for 30 min. The charge reaction resistances (R ct ) associated with the MOR [36] of the electrocatalysts were assessed by the diameter of the primary semicircle using Nyquist plots of EIS measurements.

Membrane electrode assembly fabrication and singlecell performance testing
The anodic catalyst ink was prepared with 12 mg of varying Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C catalyst content (i.e., 20%, 18%, and 15%) dispersed in 400 µl of ethanol, 100 µl of ethylene glycol, and mixed with 15 µl of 20 wt.% Na on solution with 30 min sonication. The ink was then dropped on a 2.5 cm × 2.5 cm carbon paper (25BC, Hephas energy) and dried at 40°C for 24 h to create the anode electrode. The cathode electrode was prepared using commercial 20%-Pt/C at the same volume ratio as the 20%-Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C catalyst on a 2.5 cm × 2.5 cm carbon paper. A 3 cm × 3 cm Na on® 117 membrane was used as the solid electrolyte. Before applying to the electrodes, the membrane was pretreated by immersion in 5% H 2 O 2 solution at 80°C for 30 min and then rinsed with deionized (DI) water.
Next, the membrane was immersed in 0.5 M sulfuric acid at 80°C for 30 min and nally rinsed with DI water. The treated membrane was stored in a beaker lled with DI water until used.
The membrane electrode assembly (MEA) had a 6.25 cm 2 active area, as shown in Fig. S1(a) and was hot-pressed on both sides between the anode/Na on® lm/cathode at 140°C with a pressure of 50 kg/cm 2 for 3 min. A simple single DMFCs was assembled without a bipolar plate, as shown in Fig. S1(b), to avoid electrocatalysts characteristic interface resistance effects. The DMFCs consisted of an air lled cathode tank, the MEA, and a 35-ml 20% methanol lled anode tank. The DMFCs performance was measured at ambient pressure and room temperature using the potentiostat/galvanostat of a CHI 608E electrochemical analysis instrument. The cell polarization curve was obtained for the electrocatalysts.
The long-term durability of the catalysts was tested by conducting 1000 continuous potential cycles between 0.05 and 1.20 V at 50 mV s − 1 .

Results And Discussion
3.1 Characterization of Pt/different calcination temperature Sr 0.5 Mo 0.5 O 4−δ -C catalysts SrMoO 4 formation of uncalcined to a calcination temperature at 400°C was con rmed by XRD data in  Fig. 2(b). The peaks of Mo 5+ 3d 5/2 and 3d 3/2 were located at 232.7 and 235.6 eV, respectively. The Mo doublets located at 233.8 and 237.2 eV proved that the Mo 6+ chemical state existed [42] in the Sr 0.5 Mo 0.5 O 4−δ . The relative areas of integrated peak intensities of Mo 5+ decreased and, in contrast to that of Mo 6+ increased with calcination temperature increase, as shown in Table 1. The Mo 6+ relative area increased, meaning more SrMoO 4 compound formation with increasing calcination temperature.
In addition, the Pt 4f region displays spin-orbit splitting doublet peaks of 4f 7/2 and 4f 5/2 for Pt/various  The particle shape of uncalcined Sr 0.5 Mo 0.5 O 4−δ was donut-like or ower-like, as indicated by the SEM image in Fig. 3 Fig. 5(a). The hydrogen adsorption and desorption peaks obtained between − 0.2 and 0.1 V are shown in Fig. 5(b). The average areas were used to calculate the ECSA H . The removal of incompletely oxidized carbonaceous species formation is demonstrated by the forward current density peak of potentials [47] during MOR. All electrocatalysts have a similar forward current density peak of potentials, and the oxidation peak potentials in the reverse scan are similar in Fig. 5(a). can oxidize adsorption intermediates (CO or CH z O ads , 0 ≤ z ≤ 4) on Pt, as shown by Eq. (7) [49]. However, H x MoO 3 provides charge transport across the support and can be easily oxidized on the Pt to form H x MoO 3 with less hydrogen (H y MoO 3 ) (0 < y < x < 2), which plays a role as proton acceptor [20]. This can also oxidize adsorption intermediates on Pt, as shown by Eq. (8) [50]. compound in uncalcined Sr 0.5 Mo 0.5 O 4−δ structure is con rmed by Raman data, as shown in Fig. 1(c).
In addition, the charge transfer resistance (R ct ) and Nyquist plots of the various electrocatalysts were tted with the corresponding equivalent circuit, as shown in Fig. 5 The highest electrical conductivity obtained by the Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C was better than that by commercial Pt/C, indicating that the ion transfer rate was faster and leads to the best electrocatalytic activity for MOR.
CO stripping voltammetry for various electrocatalysts is as shown in Fig. 5(d). The peak area appeared in the difference in current density between the 1st cycle and that of the 2nd cycle, indicating the amount of CO stripped off from the Pt surface [51]. The ECSA CO decreased with increasing Sr 0.5 Mo 0.5 O 4−δ calcination temperature ( Pt(CO) ad + SrMoO 4 -OH ad → Pt + SrMoO 4 + CO 2 + H + + e -(9) The highest relative area of Mo 5+ contact was in the uncalcined Sr 0.5 Mo 0.5 O 4−δ , higher than that of 200°C and 400°C, which is attributed to MoO 3 formed from distorted octahedra [54] and indicates the generation of a higher concentration of oxygen vacancies [55]. The generation of oxygen vacancies allows better electron transfer from the catalysts to the Pt [56]. Moreover, the cations in the vicinity of the oxygen vacancies in uncalcined Sr 0.5 Mo 0.5 O 4−δ are reduced, which triggers electronic interactions between reduced cations and Pt atoms. This gives rise to interactions known as strong metal-support interactions [57], which help improve electrocatalytic MOR performance.
In addition, the binding energy peak of Pt 0 is the dominant component of the Pt 4f due to its important role in supplying available active Pt sites for methanol adsorption for MOR. Therefore, the results show that Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C generates the largest relative area of Pt 0 peak, which is 54.   Fig. 6(a). The forward peak potentials on the Ptvarious SrMoO 4 /C catalysts were observed at approximately 0.40-0.43 V in the forward scan.
Furthermore, the CV of MOR, CO tolerance, and EIS of the various catalysts is shown in Fig. 6(b)-(d).
Their forward peak intensities, ECSA H , ECSA CO , and R ct are shown in structures. These electrocatalysts certainly affect Pt morphology when Pt is loaded on the electrocatalysts, as con rmed by XPS. The binding energy of Pt 4f XPS and relative area Pt 0 of Pt/various SrMoO 4 -C are shown in Fig. 7(a) and Table 4. The results showed that the relative area of Pt 0 decreased and oxidized Pt 2+ and Pt 4+ increased with increasing amount of Sr contact. The Pt-Sr 0.5 Mo 0.5 O 4−δ -C electrocatalysts obtained the largest relative area of metallic Pt 0 , higher than commercial Pt/C and with other mole ratios of Sr and Mo precursors, which con rms that Pt/Sr 0.5 Mo 0.5 O 4−δ -C has the best MOR and CO tolerances.
From the Mo 3d XPS, the binding energy of Mo 5+ was positively shifted when Sr contact mole ratio increased from 0.67 to 0.75, as shown in Fig. 7(b). This suggests that more Sr 2+ ions incorporated MoO 4 2− . Therefore, a lower amount of MoO 3 formation is demonstrated by decreasing intensities of the peak between 0.08 and 0.32 V, as shown in Fig. 6(b). In addition, when the Sr contact mole ratio was 0.75, binding energy peaks of Sr 2+ for SrCO 3 formed in Fig. 6(c). This could be the in uence of less metallic Pt 0 formation, causing decreased MOR performance. Furthermore, the Mo 6+ binding energy peaks were negatively shifted when the Mo contact was increased to 0.67. This suggests that less SrMoO 4 production could reduce the synergistic effect of the bifunctional mechanism, resulting in lower MOR and CO tolerances.  Fig. 8(b). Clearly, the current density of both electrocatalysts decayed when the number of cycles increased.

Conclusions
In the present study, strontium molybdate was successfully prepared, mixed with carbon, loaded with 20% Pt, and used as an electrocatalyst for MOR and CO tolerance in a DMFCs. Tetragonal SrMoO 4 structure is the main compound for strontium molybdate. Strontium molybdate prepared with increased calcination temperature and increased Sr and Mo precursor contact ratios (larger than 1 mole ratio) resulted in reduced MOR and CO tolerance. The best MOR and CO tolerances and electrical conductivity were obtained by 20%-Pt/uncalcined Sr 0.5 Mo 0.5 O 4−δ -C, which showed improved DMFCs performance and improved cycling performance after 1000 cycles. This optimum strontium molybdate and contact ratio is