Electrochemical Catalytic Properties of Pt/FeSnO(OH)5 towards Methanol Oxidation①

The Pt/FeSnO(OH)5has been prepared by depositing Pt nanoparticles on the synthesized FeSnO(OH)5nanoboxes and demonstrates excellent catalytic activity towards methanol oxidation reaction as an electrode catalyst in DMFCs.The Pt/FeSnO(OH)5catalyst exhibits a higher mass activity (1182.35 mA/mgPt) compared with the Pt/C (594.57 mA/mgPt) catalyst.The result shows that the as-prepared Pt/FeSnO(OH)5has a great application prospect as a high-performance electrocatalyst in DMFCs.

1 INTRODUCTION

Direct methanol fuel cells (DMFCs) with Pt as the catalyst feasibly convert the chemical energy stored in methanol directly into electric energy, which have shown potential applications such as electric vehicles and portable electronic devices due to its attractive features including low operating temperature, easy refueling, high energy density and simplicity of system among the different types of fuel cells[1-3].However, some serious shortcomings need to be overcome before commercialization of DMFCs, including the high cost of noble-metal, the decay of catalytic activity of Pt catalyst, the low catalytic activity due to the slow methanol oxidation reaction kinetics and the low stability of the catalysts in acidic media[4].

Deferent methods are employed to overcome these defects.One of them is to switch the working media from acidic solution to alkaline solution,which possesses apparent advantages, such as enormously enhanced methanol oxidation reaction kinetics, lower overpotential for oxygen reduction reaction and more choices for catalysts towards MOR[5,6].Another method is to fabricate composite catalysts by combining Pt with supports, which show higher electro-catalytic activity and platinum utilization efficiency compared with unsupported catalysts because of their large surface area and high dispersion of Pt on the supports[7].Nowadays the carbonaceous materials are commonly used as the electrocatalyst supports of commercial fuel cells for their high conductivity and large surface areas.However, carbon corrosion is a hard problem for all carbon supports[8].Therefore, it is significant to search for non-carbonaceous supports.

As for the decay of catalytic activity of the Pt catalyst, this problem can be owed to the following reasons including CO poisoning of Pt during the methanol oxidation reaction (MOR), the weak interactions between Pt and support materials, the low intrinsic activity of Pt, the exfoliation of Pt element and electrochemical corrosion of the support materials[9].The CO species, the oxidation intermediates of MOR, adsorbed on the surface of Pt nanoparticles would lead to very low power densities and the loss of electrochemically active surface areas (ECSAs) by hampering further adsorption of methanol[10], so it is necessary to remove CO from the surface of platinum at a relatively negative potential.To solve the CO-poisoning problem, one common strategy is to combine Pt with other non-precious transition metals such as Ru, Fe, Co,Sn and Zn[11-15]to form Pt alloy or metal oxides like TiO2, CeO2, V2O5and WO3[16-19]to fabricate Pt-based catalysts, which would improve the catalytic activity and durability as well as lower the cost of Pt-based catalysts.The Pt-based catalysts combined with metals or metal oxides own a better CO resistance via the bifunctional mechanism and the electronic effect[20-24].According to the bifunctional mechanism model, the supporting materials can effectively activate H2O to form oxygen-containing species of OH adspecies (OHads), resulting in the oxidation of neighboring CO adspecies (COads) into CO2at a relatively negative potential, thus alleviating the CO poisoning effect and providing more active Pt sites for methanol oxidation.The electronic effect is a result of the modification of electronic structure of the Pt surface, which weakens the CO?Pt bonding and intermediate adsorptive strength for Pt, thereby improving the kinetics of methanol and CO oxidation.

Among the supports, the stannate hydroxides have caught the attention of researchers.MSn(OH)6(M =Co, Cu, Fe, Mg, Mn, Zn), which are a kind of special perovskite-structural materials, have been used as photocatalysts and electrode materials for Li-ion batteries[25-27].Furthermore, the researchers have reported that the CoSn(OH)6supported Pt exhibited a high electro-catalytic activity, good CO resistant ability and catalytic stability towards methanol oxidation in alkaline solution[28].However,the catalytic mechanism of stannate hydroxide supported catalyst has not been deeply discussed,and fabrication of higher performance electrocatalyst towards MOR is still a challenge.

In this paper, we synthesized hollow FeSnO(OH)5nanocubes as the support of Pt catalyst for methanol electro-oxidation.L-ascorbic acid was used as the soft reductant to prepare Pt/FeSnO(OH)5.It was found that the activity and stability toward MOR of Pt/FeSnO(OH)5was improved more effectively compared with Pt/C (Vulcan XC-72).The CO-stripping data also confirmed the enhanced electro-catalytic performance of Pt/FeSnO(OH)5as an anodic catalyst.

2 EXPERIMENTAL

2.1 Preparation

All chemical reagents were used as received without further purification.Stannic chloride pentahydrate (SnCl4·5H2O, AR), iron(II) sulfate heptahydrate (FeSO4·7H2O), sodium hydroxide (NaOH,AR), L-ascorbic acid and methanol (CH3OH, AR)were purchased from Sinopharm Chemical Reagent Co., Ltd (China).Chloroplatinic acid hexahydrate(H2PtCl6·6H2O, AR) were purchased from Aladdin Reagent.Nafion solution 5% (Dupont) and carbon vulcan XC-72 (Cabot) were used as received.

In a typical synthesis, a solution of SnCl4·5H2O in deionized water (DI water) (0.5 M, 5 mL) was added to a solution of FeSO4·7H2O (0.5 M, 5mL) at room temperature with vigorous agitation, and a solution of NaOH (2 M, 10 mL) was added to the mixture slowly, which was stirred for 6 hours in a beaker at 60℃.The synthesized FeSnO(OH)5was collected by centrifugation and washed several times with DI water, and dried under vacuum at 60 ℃ for 6 h.Afterwards, 0.3 g prepared FeSnO(OH)5was added to 45 ml water, and then 15 ml HCl solution (1.0 M)was dropped into the suspension, stirring for 2.5 h at room temperature.The product was washed with DI water and absolute alcohol for several times, and dried in vacuum oven at 60 ℃ for 6 h to obtain FeSnO(OH)5nanoboxes.

The complex catalyst was prepared by a sonochemical reaction in the L-ascorbic acid.Firstly, 0.5 ml H2PtCl6·6H2O (0.019 M) was added into 10 mL ice water rapidly under a strong agitation.Then, 10 mL L-ascorbic acid ice-water solution (0.1 M) was dropped slowly into the above mixture.0.005 g prepared FeSnO(OH)5was dropped into the above pale-yellow solution and stirred for 10 minutes.Subsequently, the solution was treated in an ultrasonic cleaning instrument for 1 h and then was deposited for 24 h.The obtained product was washed with DI water and absolute alcohol for several times and dried at 60 ℃ for 6 h in a vacuum oven, which was denoted as Pt/FeSnO(OH)5.For comparison, the Pt/C (Vulcan XC-72) electrocatalyst was synthesized using the carbon vulcan XC-72 as the precursor following the same procedure.

2.2 Characterization

The X-ray diffraction (XRD) measurements of the powder samples were performed in the reflection mode (CuKα radiation, l = 1.5418 ?) on a Rigaku Ultima III X-ray diffractometer.The field emission scanning electron microscopy (FESEM) images were obtained by Hitachi S4800 field emission scanning electron microscopy.The field emission transmission electron microscopy (FETEM) images were obtained by FEI Tecnai G2 F20 S-TWIN with a field emission gun operated at 200 kV.The X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALab250-XI electron spectrometer from VG Scientific using a 300 W AlKα radiation.The base pressure was about 3 ×10-9mbar and the binding energies were corrected by adjusting the binding energy of the C1s peak to 284.8 eV from adventitious carbon.

2.3 Electrochemical measurement

The electrochemical measurements were performed on a CHI-660D electrochemical workstation with a conventional three-electrode cell.The catalyst ink was prepared by dispersing 5 mg prepared nanocomposite in a mixture containing 1 ml ethanol and 0.025 mL 5% Nafion solution under ultrasonication for 30 min.A glassy carbon electrode (3 mm in diameter) was used as the working electrode,which was carefully polished with a diamond pad/0.3 μm polishing suspension and rinsed with DI water and ethanol.After dropping 5 μL of the catalyst ink onto the electrode surface, the electrode was dried in air.A Pt wire and an Ag/AgCl electrode were used as the counter electrode and the reference electrode, respectively.The electrochemical impedance spectroscopy (EIS) was measured in a mixture of N2-purged 1 M methanol and 1 M KOH under open-circuit conduction.The EIS tests were conducted by sweeping the frequency from 100 KHz to 1 Hz under open circuit potential with 5 mV of amplitude.For the measurement of hydrogen adsorption/desorption reaction, the potential was cycled between –1 and 0.4 V at 50 mV/s in N2-purged 1 M KOH solution.The electrocatalytic properties for methanol oxidation of the catalysts were measured in a mixture of 1 M methanol and 1 M KOH.The chronoamperometry (CA) was recorded at –0.2 V for 3600 s in a mixture of 1 M methanol and 1 M KOH.

The electrocatalytic activity for CO-stripping was obtained through the following steps: (i) CO gas was bubbled into a N2saturated 1 M KOH aqueous solution for 10 min; (ii) then N2was bubbled to remove the dissolved CO in the electrolyte, while the CO molecules adsorbed on the Pt surface were not affected by this treatment; (iii) finally, the cyclic voltammetric (CV) measurements were carried out in a N2saturated 1 M KOH over the potential range from –1 to +0.4 V at a scan rate of 50 mV/s.The ECSA derived from the CO-stripping was calculated using the following equation[29].

where QCOis the measured charge for the CO stripping and WPtis the mass of Pt.The value 420 represents the charge density required to oxidize a monolayer of CO on Pt.

3 RESULTS AND DISCUSSION

3.1 Structure and morphology

As shown in the XRD profiles (Fig.1a), all peaks of the prepared FeSnO(OH)5can be indexed to the diffractions of FeSnO(OH)5(JCPDS 74-1745), indicating there is no other phase.However, four new broadened peaks located at 39.8°, 46.2°, 67.5° and 81.3° appear in the pattern of the synthesized Pt/FeSnO(OH)5, corresponding to the diffractions of Pt (1 1 1), (2 0 0), (2 2 0) and (3 3 1) planes of the face-centered cubic (fcc) Pt (JCPDS 87-0640),respectively, indicating the target complex has been prepared.As shown in Fig.1b, the Pt/C is also indexed to the cubic Pt phase (JCPDS No.87-0640).

Fig.1.XRD patterns of (a) the prepared FeSnO(OH)5, Pt/FeSnO(OH)5 and (b) Pt/C

The XPS measurements were used to explore the electronic states and surface composition of the catalysts.As shown in Fig.2a, the two peaks corresponding to the Pt 4f7/2and Pt 4f5/2states with a 3.3 eV spacing and a 3:4 atomic ratio can be found[30,31].For Pt/FeSnO(OH)5, the most intense doublet (at 71.00 and 74.32 eV) is the signature of metal Pt.The second and weaker doublet (at 72.40 and 75.70 eV)with the binding energy at 1.4 eV higher than Pt(0)can be attributed to the Pt(II) oxidation state (PtO and Pt(OH)2-like species)[32,33].It is notable that Pt

in +4 oxidation state is present in Pt/C.Table 1 summarizes the relative intensities of Pt0, Pt2+and Pt4+in the catalysts, which can be estimated from their peak surface area.There is a significant difference between the relative intensities of Pt0in the catalysts.The chemical state of Pt is an important factor on the electrochemical activity.There are reports that metallic Pt is a superior catalyst to Pt in the +4 oxidation state, and Pt0has better electrocatalytic activity toward methanol electro-oxidation in comparison with Pt2+and Pt4+[34,35].

Table 1.Atomic % of Different Valenced Pt for Different Catalysts

As marked by the dashed lines in Fig.2a, the binding energy of Pt 4f7/2in the Pt/FeSnO(OH)5(71.0 eV) is negatively shifted almost 0.6 eV compared with the Pt/C (71.6 eV), which implies that the electronic structure of Pt was modified by the hydroxide support because of an enhanced interaction between the Pt and the support material,indicating a transfer of electrons from FeSnO(OH)5to Pt[36,37].The shift is mainly caused by the electronegativity difference between the transition element and Pt, leading to the charge transfer from the more electropositive element such as Fe to Pt[38,39].This notion can be further supported by a positive shift of the Fe 2p peaks shown in Fig.2b.

Fig.2.(a) Pt 4f XPS spectra of Pt/FeSnO(OH)5 and Pt/C; (b) Fe 2p XPS spectra of Pt/FeSnO(OH)5

As shown in Fig.2b, the Fe 2p XPS spectrum of Pt/FeSnO(OH)5is split into two parts, namely Fe 2p3/2and Fe 2p1/2, with an atomic ratio of about 2/1.Each part consists of a main peak and a “shake-up”satellite[40].The peaks at 712.4.0 eV (2p3/2) and 726.3 eV (2p1/2) are attributed to Fe3+species, while the second pair of peaks observed at 711.0 eV (2p3/2)and 725.0 eV (2p1/2) are related to Fe2+species[41].The shake-up satellite peaks at 734.0 eV (2p1/2) and 729.9 eV (2p1/2) confirm the species, respectively[42].Thus, there are mixed valence states of Fe3+/Fe2+in Pt/FeSnO(OH)5for the binding-energies of Fe 2p in Pt/FeSnO(OH)5to be positively shifted.

The SEM and TEM images of the samples are displayed in Figs.3 and 4, respectively.It can be clearly seen from Fig.3a that the FeSnO(OH)5crystals are nanocubes with the size of about 200～500 nm.After etching, the morphology of FeSn-O(OH)5is maintained as shown in Fig.3b.However,it can be found from the TEM image (Figs.3a and 3b) that the FeSnO(OH)5nanocubes have been etched into hollow nanoboxes after being treated in the acid solution.Figs.3c and 3d show Pt particles have been dispersed on the FeSnO(OH)5nanoboxes and the carbon (Vulcan XC-72).The corresponding TEM images are displayed in Figs.4c and 4d,respectively, showing both Pt based complexes have been successfully synthesized.As shown in the corresponding selected area electron diffraction(SAED) pattern inserted in Fig.4c, the Pt-based catalysts possess the Pt fcc structure.The high-resolution TEM (HRTEM) image (Fig.4d) of Pt/FeSnO(OH)5exhibits the lattice fringes with the interplanar distance of 0.225 nm, corresponding to the (111) plane of the cubic Pt, and the average size of Pt nanoparticles in Pt/ FeSnO(OH)5is about 4 nm,while the Pt particles on the carbon shown in Fig.4f have a similar size and the lattice fringes of 0.226 nm, which can be also attributed to the (111) plane of the cubic Pt.Additionally, the element com-positions measured by EDX analysis (shown in Figs.4g and 4h) are in good matchup with the Pt/FeSnO(OH)5and Pt/C.

Fig.3.SEM images of (a) FeSnO(OH)5, (b) FeSnO(OH)5 after etching, (c) Pt/FeSnO(OH)5 and (d) Pt/C, respectively

Fig.4.TEM images of (a) FeSnO(OH)5, (b) FeSnO(OH)5 after etching and (c) Pt/FeSnO(OH)5 with the corresponding SAED patterns inserted and (e) Pt/C, respectively; HRTEM images of (d) Pt/FeSnO(OH)5 and (f) Pt/C, respectively; EDX patterns of (g) Pt/FeSnO(OH)5 and (h) Pt/C, respectively

3.2 Electrochemical measurement

Fig.5a presents the CV curves of the prepared samples, which has three typical regions described as the hydrogen region, the double layer region and the oxygen region.Their electrochemically active surface areas (ECSAs) are determined from the charge of the hydrogen adsorption-desorption (HAD)signatures, which are related to the dispersion and nanoparticle sizes of Pt.The ECSA value is estimated according to the following equation[43]:

where [Pt] represents the platinum loaded in the electrode (g/cm2), QHis the charge for hydrogen desorption (mC/cm2), and 0.21 represents the charge required to oxidize a monolayer of adsorbed hydrogen on bright Pt (mC/cm2).The ECSAs for the catalysts determined by hydrogen desorption peaks are listed in Table 2.The ECSAs derived from the CO-stripping of these samples show similar values in Table 2, proving the validity of the ECSA data.The calculated ECSAHADvalues for Pt/FeSnO(OH)5and Pt/C are about 8.364 and 24.464 m2/gPt, respectively.

In the CV curves for both catalysts, two peaks are observed.The more positive current peak in the forward scan (If) is ascribed to the electro-oxidation of methanol, while the anodic peak in the backward scan (Ib) is attributed to the removal of incompletely oxidized carbonaceous species mainly composed of CO species formed during the forward scan[44].Fig.5b displays the CV curves normalized by the loading mass of Pt on the electrode for different catalysts.As shown in Fig.5b, although the ECSAHADof Pt/FeSnO(OH)5is lower, its mass activity (1182.35 mA/mgPt) is obviously higher than that of Pt/C(594.57 mA/mgPt).The current densities normalized by ECSAHADare also compared in Fig.5c, showing the specific activity of Pt/C is 1.76 mA/cm2, which is much lower than that of Pt/FeSnO(OH)5(14.30 mA/cm2).These results indicate the excellent electrocatalytic activity of Pt/FeSnO(OH)5toward MOR.

To compare the CO-resistance ability of the catalysts, the CO stripping experiment was carried out.Fig.5d shows the CO stripping voltammograms for different catalysts.In the first positive scan, CO adsorbed on the electrode surface limited the presence of hydrogen oxidation peaks, and the adsorbed CO was oxidized at more positive potentials subsequently.On the second positive scan, the reappearance of hydrogen peaks at negative potentials indicates the freedom of dissolved CO on the electrode surface[45].The onset potential and peak potential for the CO oxidation and ECSA estimate using the CO-stripping curves are listed in Table 2.The onset potential of Pt/FeSnO(OH)5catalyst is 59 mV more negative than that of the commercial Pt/C catalyst.The positive peak potential for CO oxidation on the Pt/FeSnO(OH)5(–0.338 V) is shifted negatively compared with the Pt/C electrode (–0.282 V).These results significantly indicate the favorable role of FeSnO(OH)5for CO-tolerance, which is in accordance with the mass activity in Fig.5b.

Table 2.Results of CO Stripping with the Prepared Catalysts and ECSA from H Adsorption-desorption

Fig.5.(a) CV curves of the catalysts; (b) mass-normalized CV curves and (c) ECSA-normalized CV curves of the catalysts; (d) electrochemical CO-stripping curves of the catalysts

Fig.6a depicts the Nyquist plot of EIS for the electrodes modified with Pt/FeSnO(OH)5and Pt/C.Both catalysts show a typical characteristic semicircle at the high frequency region.The semicircle in the high frequency region is taken as a measure of the charge transfer resistance (Rct) between the aqueous solution and the modified electrode[46],showing that the Rctof Pt/FeSnO(OH)5is lower than that of Pt/C, suggesting the faster kinetics of methanol oxidation and the higher electrocatalytic activity of Pt/FeSnO(OH)5compared with Pt/C[47,48].

Fig.6b shows the CA curves of Pt-based catalysts in a solution of 1 M KOH with 1 M methanol for 3600 s at –0.2 V vs.Ag/AgCl.Both catalysts showed an initial faster decay, which is attributed to a double layer capacitance effect[49].After the initial significant drop period, the current decreased slowly because the MOR byproducts such as COads,CH3OHadsand CHOadswere adsorbed on the active surface of the catalysts[50].Obviously, the current density on the Pt/FeSnO(OH)5catalyst is the highest during the 1 h measurement, displaying its excellent electrocatalytic activity.The better stability of Pt/C may be attributed to the stronger binding energy between Pt and the carbon compared with Pt/FeSnO(OH)5, which can be proved by the XPS analysis.

Fig.6.(a) Nyquist plot of EIS of the catalysts.(b) CA curves of the catalysts

The different performance of MOR between the Pt/FeSnO(OH)5and Pt/C can be explained by the following factors.The first factor is the different interaction between the Pt particles and the transition metal of the support.As shown in the XPS curves(Fig.2a), the binding energy of the 4f7/2in Pt/FeSnO(OH)5is negatively shifted 0.6 eV compared with Pt/C, indicating a stronger interaction between Pt and FeSnO(OH)5.The increase of electron charge transfer from the transition metal to Pt atom is the major factor for the weakening of CO?Pt bonding and intermediate adsorptive strength for Pt, leading to the enhancement of electrochemical performance[51,52].The second factor is based on the bifunctional mechanism of the support.The OHadsis formed at lower potential on Sn sites than on the Pt sites, thus CO and CO-like intermediates could be oxidized at low potential, resulting in the better electrochemical activity for Pt/FeSnO(OH)5compared with Pt/C[53].Thirdly, as shown in Table 1, the atomic percentage of Pt0in Pt/FeSnO(OH)5is higher than that in Pt/C, which is also responsible for the better electrocatalytic activity.The metallic Pt in zero oxidation state is beneficial to the electrocatalytic activity towards methanol electro-oxidation in comparison with Pt2+and Pt4+[34].Furthermore, the higher amount of metallic Pt in zero oxidation state in Pt/FeSnO(OH)5proved by XPS and the better electronic conductivity of Pt/FeSnO(OH)5confirmed by the EIS measurement are both in favor of the MOR performance.

4 CONCLUSION

In conclusion, FeSnO(OH)5nanoboxes have been synthesized and deposited with Pt nanoparticles as an electrode catalyst in DMFCs.The catalytic performance of the prepared Pt/FeSnO(OH)5toward MOR has been evaluated and compared with the commercial carbon supported Pt.The XRD, XPS, SEM,TEM and electrochemical experiments have been employed to explore the relationships between the crystal structure and the electrochemical properties.The characterizations show that the prepared Pt/FeSnO(OH)5catalyst obtains enhanced performance toward MOR compared with Pt/C, which can be attributed to the lower interaction between Pt and the FeSnO(OH)5support, the bifunctional effect of FeSnO(OH)5, the higher atomic percentage of Pt0in FeSnO(OH)5and the better electronic conductivity of FeSnO(OH)5.The study has revealed the effect of support on the electrochemical catalytic activity and shows that the Pt/FeSnO(OH)5is a promising anode catalyst in DMFCs.

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