Microwave Assisted Synthesis of PtxRu1−x-NiTiO3: A Novel Hybrid Nanostructured Electrocatalyst for Direct Methanol Oxidation

In the present investigation, we have explored the preparation of PtxRu1-x-NiTiO3 hybrid electrocatalysts for the application of methanol oxidization. The XRD and Raman analysis of PtxRu1-x-NiTiO3 confirmed the rhombohedral crystal structure of NiTiO3 and cubic-structured Pt(Ru) nanoparticles. TEM analysis signifies that Pt(Ru) nanoparticles is decorated on the surface of the NiTiO3. The mean size of the Pt(Ru) nanoparticles calculated from the TEM profile is around 2 nm which corroborates with the crystallite size estimated from XRD. The XPS analysis confirms the presence of metallic Pt along with its oxidized components (Pt2+ and Pt4+). The cyclic voltammetry analysis evidence that Pt0.5Ru0.5-NiTiO3 has shown better activity in methanol oxidation compared with the other compositions of PtxRu1−x-NiTiO3 along with the maximum current density of 65.41 mA cm−2 with less carbon poisoning. Chronoamperometry and polarization studies represent the stability and electrochemical activity of the Pt0.5Ru0.5-NiTiO3 electrocatalysts towards methanol oxidation. Based on the research carried out in this work, it is concluded that the PtxRu1−x-NiTiO3 would be a promising candidate as an electrocatalyst for methanol oxidation in direct methanol fuel cells.

Research on energy materials is a need of the hour as the demand for energy increases every day.Particularly, alternate energy resources are required to be used in place of conventional energy resources such as fossil fuel-based ones.Direct alcohol fuel cells (DAFC) are considered as an efficient alternate power source due to the fact that alcohol oxidation delivers much higher energy density than pure hydrogen.Compared to Li-ion batteries, the energy density of DAFCs is much higher, of the order of 4000 to 7000 W h l −1 .Compared to hydrogen, alcohol possesses a very high energy density 1 and this energy density is comparable to those of hydrocarbons and gasoline, which have an energy density of 9800 W h l −1 . 2 The energy density of these alcohols takes a close place to the other fuel materials and this is due to the elemental composition, molecular structure configuration and number of electrons released by a molecule for a complete fuel electrooxidation.Due to this, alcohol is a very potential candidate as an alternate energy source.Another great advantage of alcohol is that it can be produced from natural resources through fermentation. 3Even compared to hydrogen-based fuel cells, alcohol has higher reversible energy efficiency.Comparing other alcohols as fuel, methanol is a highly volumetric and gravimetric energy density possessing liquid fuel.Being liquid, it is also easy to carry and transport it and therefore, it is preferred these days.
Therefore, DAFC has received great attention to address the current energy demand and protect the environment from greenhouse gas.Though DAFCs have many advantages, the main challenge is to choose the right electrocatalyst for the methanol oxidation reaction (MOR) to occur and it is the crucial part of the direct methanol fuel cell (DMFC).One of the electrocatalysts, Pt is known to be the best electrocatalyst for MOR and till now no other electrocatalyst material was found to replace it.It can do both reduction and oxidation reactions in the cell.Use of Pt in MOR causes two important issues; (i) the cost of the material goes high and hence the device and (ii) CO poisoning.To overcome these two issues, it is important to replace fully or partially the Pt electrocatalyst.Many attempts were made to modify the Pt with Pt alloying with other metals such as Ru, Rh, Co, Sn etc. 4 It is significant to develop a proper electrocatalytic material for the application in DMFC.The electrocatalyst should be free from CO poisoning and less expensive compared to the currently commercially used Pt electrode.When support materials such as metal oxides are used along with Pt, then the amount of Pt used as an electrocatalyst can be reduced and therefore, it would be affordable.At the same time, the electrocatalytic activity of the metal-oxide hybrid electrocatalyst should be on par with Pt.
Many attempts have been made in the literature 1,2,5 to identify appropriate electrocatalyst materials.Instead of Pt electrocatalyst, other noble metals such as Pd-Sn have been used for alcohol electrooxidation in an alkaline medium 6 Nanostructured Pd-Sn catalysts for alcohol electro-oxidation in alkaline medium.In this case, pure and three combinations of Pd-Sn were used for the methanol, ethanol, isopropanol, ethanol, glycerol, and ethylene glycol oxidation.Recently, core-shell nanoparticles of Pt-Ru along with carbon was used as a CO-tolerant electrocatalyst. 7It makes difficult to use methanol oxidation due to its slow kinetics with the presently available electrocatalysts.Therefore, the development of an efficient and low-cost catalyst for the electrooxidation of methanol and this task is taken up in this work.
In this aspect, an attempt is made to prepare the nanoparticles of NiTiO 3 supported PtRu electrocatalyst for DMFC applications.It is known that Ru is also a very good catalyst, and therefore, it is made an alloy with Pt particles.Pt along with Ru alloyed had been investigated by many researchers for the oxidation reaction of methanol. 8The behaviour of platinized-platinum and platinumruthenium electrodes in methanol solutions was studied.Though the enhancement of the oxidation was not very well explained in the earlier literature it was well described by Watanabe and Motoo as due to the adsorption of OH radicals on Ru surface followed by their oxidation. 9he hybrid structure PtRu-NiTiO 3 contains NiTiO 3 as base material whose surface is decorated with the PtRu metal nanoparticles.Preparation of these types of hybrids is quite challenging, and the microwave-assisted polyol method was used in this work for the synthesis.They were prepared with four different compositions of Pt to Ru ratio in Pt x Ru 1−x -NiTiO 3 .The XRD and Raman analysis of Pt x Ru 1−x -NiTiO 3 confirmed the rhombohedral crystal structure of NiTiO 3 and cubic-structured Pt(Ru) nanoparticles.TEM analysis signifies that Pt(Ru) nanoparticles is decorated on the surface of the NiTiO 3 .XPS analysis confirms the presence of metallic Pt along z E-mail: thangaduraip.nst@pondiuni.edu.in;thangadurai.p@gmail.comECS Advances, 2024 3 014503 with its oxidized components (Pt 2+ and Pt 4+ ).The cyclic voltammetry analysis evidence that Pt 0.5 Ru 0.5 -NiTiO 3 has shown better activity in methanol oxidation compared with the composition of Pt x Ru 1-x -NiTiO 3 along with the maximum current density of 65.41 mA cm −2 with less carbon poisoning.It concludes that Pt x Ru 1-x -NiTiO 3 would be a promising candidate as an electrocatalyst for methanol oxidation in DMFC.These results are presented and discussed in this paper.

Experimental
Preparation of Pt x Ru 1-x -NiTiO 3 .-Theheterostructured Pt x Ru 1-x -NiTiO 3 electrocatalyst was prepared in nanostructured form as reported elsewhere. 10In a typical synthesis, using precursor materials such as Ni(NO 3 ) 2 .9H 2 O (Himedia Chemicals, India) and Ti(OC 4 H 9 ) 4 (Sigma-Aldrich), first, the NiTiO 3 nanoparticles were prepared by Sol-Gel method.The citric acid (Thermofisher) and ethylene glycol (Himedia chemicals, India) were used as a chelating agent and solvent, respectively.The Pt(Ru) nanoparticles decorated NiTiO 3 heterostructure (Pt x Ru 1-x -NiTiO 3 ) was prepared by a microwave-assisted polyol method 10 using chloroplatinic acid as a Pt source, and ruthenium trichloride trihydrate (RuCl 3 .3H 2 O) as a Ru source.In this case, ethylene glycol was used as solvent and NaOH for the maintenance of pH.In the synthesis, 10 wt% of the Sol-Gel prepared NiTiO 3 nanoparticles was taken, then added to the prepared mixture of 0.2 M RuCl 3 .3H 2 O and Chloroplatinic acid and stirred for 30 min.Later, NaOH was added to it with 0.8 M concentration to maintain the pH at 9. The whole solution was heated in a microwave (LG-MH-6549QS, power 100%) for 40 s, and then the reactant mixture was cooled down to room temperature.The final solution was centrifuged to get the Pt x Ru 1-x -NiTiO 3 product, which was further dried at 100 °C to remove water and unreacted content from it, and to get the final samples.By varying Ru and Pt concentrations, four samples of Pt x Ru 1-x -NiTiO 3 were prepared and they are Pt 0.1 Ru 0.9 -NiTiO 3 , Pt 0.3 Ru 0.7 -NiTiO 3 , Pt 0.4 Ru 0.6 -NiTiO 3 and Pt 0.5 Ru 0.5 -NiTiO 3 .The samples Pt x Ru 1-x -NiTiO 3 may also be denoted as either Pt x Ru 1-x -NiTiO 3 or Pt x Ru 1-x -NT in this work.
Characterization of materials.-Structuralphase analysis of Pt x Ru 1-x -NiTiO 3 nanostructured electrocatalyst materials was carried out by X-ray diffraction (XRD) in a Rigaku Ultima-IV powder X-ray diffractometer working at an operating voltage of 40 kV and 30 mA in a 2θ range from 20 to 80°.The Cu-K α1 line, filtered with a quartz crystal monochromator, was used to acquire the XRD data in a Bragg-Brentanno geometry.The optical absorption behaviour of the samples was studied by UV-vis absorption spectroscopy (Lambda 650 (Perkin Elmer) spectrometer).Raman spectroscopic analysis was done in a Renishaw In-via system using a laser excitation wavelength of 785 nm.Fourier transform infrared (FTIR) spectra were recorded in transmission mode (Thermo Nicolet-6700) with KBr as a reference.A scanning electron microscope (JEOL Model JSM−6390LV) equipped with X-ray energy dispersive spectroscopy (XEDS) detector (JEOL Model JED-2300) was used for microstructure analysis and composition analysis, respectively.Transmission electron microscopy (TEM) was done in JEOL 3010 TEM microscope.Electrochemical measurements were performed in the Electrochemical Workstation (Potentiostat Model No: Solartron 1287) at room temperature (25  °C).Electrochemical characterization was done by acquiring cyclic voltammograms, chronoamperometry and steady-state polarization measurements in a standard three-electrode geometry.The working electrode was prepared using these electrocatalysts on a 3 mm glassy carbon electrode, using the same method as reported earlier. 10

Results and Discussion
X-ray diffraction studies of Pt x Ru 1-x -NiTiO 3 .-Figure 1 presents the powder X-ray diffraction patterns of Pt 0.1 Ru 0.9 -NiTiO 3 , Pt 0.3 Ru 0.7 -NiTiO 3 , Pt 0.4 Ru 0.6 -NiTiO 3 and Pt 0.5 Ru 0.5 -NiTiO 3 .All the diffraction patterns are found to contain a superposition of two This implies that the size of Pt(Ru) nanoparticles must be much lower compared to that of the NiTiO 3 particles.This particular peak (111) of Pt(Ru) is merged with other reflections and therefore deconvoluted using the pseudo-Voigt function to obtain the FWHM of 6.43 (in 2θ) and Bragg angle of 39.3 (in 2θ) of that peak.Using these peak parameters in the Scherrer formula, particle size of Pt(Ru) particles is estimated to be 1.8 nm and the average particle size of NiTiO 3 particle is obtained to be 29 nm.
UV-vis absorption spectroscopy studies of Pt(Ru)-NiTiO 3 .-Itis essential to understand the optical absorption of Pt x Ru 1−x -NiTiO 3 because this material has photoinduced application such as photocatalyst and as electrocatalysts.For the latter application, the absorption edge must be known.The UV-vis absorption spectrum of pure NiTiO 3 was already published by our research group elsewhere 11 however, for reference, that spectrum is given in Fig. S2 of the supplementary information along with its discussion.Therefore, UV-vis absorption spectroscopy of Pt x Ru 1-x -NiTiO 3 was done, and the corresponding optical absorption spectra are presented in Fig. 2. Two strong absorption peaks are observed at 314 nm and 437 nm.The absorption peak at 437 nm is attributed to the transition of charge transfer from the O 2 −2p valence band to the Ti 4+ −3d conduction band in the NiTiO 3 . 12As far as NiTiO 3 is concerned, three absorption peaks are reported in the literature, at 448 nm, 502 nm and 743 nm, and they were attributed to Ni: d-d transitions. 3The broad peak at 314 nm is looking broad and should be attributed to the Pt(Ru) metal particles induced absorption.The broadness is attributed to the nanocrystalline nature of these Pt(Ru) particles because of their small size (∼2 nm) in all the cases.When the particle is small, more surface bound defects are possible and probably increased the width of the spectrum.
Raman scattering results.-SinceRaman spectroscopy provides a highly local information and sensitive to structural features, it is used to understand the structural features of Pt x Ru 1−x -NiTiO 3 .The acquired Raman spectra for Pt x Ru 1-x -NiTiO 3 are shown in Fig. 3a.All these Raman spectra were acquired with the laser excitation wavelength of 785 nm.As seen in Fig. 3a, ten Raman modes are observed from the Pt x Ru 1-x -NiTiO 3 materials. 4,13The NiTiO 3 crystal system belongs to the R-3 space group and for this symmetry, it can have 10 Raman active modes such as 5A g + 5E g and eight IR active modes such as 4A u + 4E u . 4From the Raman spectra shown in Fig. 3, there are nine Raman modes observed and they are located at 226, 242, 286, 344, 391, 464, 604, 704 and 729 cm −1 , respectively.The mode at 185 cm −1 is not visible as the spectral acquisition was  done in the range from 200 to 900 cm −1 .Also, the 729 cm −1 Raman mode is merged with the highest intensity peak at 704 cm −1 .Therefore, only eight peaks are marked in the spectra shown in Fig. 3a.To visualize the 729 cm −1 mode, the particular Raman band at 704 cm −1 , typically of Pt 0.5 Ru 0.5 -NiTiO 3 , is deconvoluted using a Lorentzian function and the deconvoluted Raman spectra are presented in Fig. 3b, where the two Raman modes are clearly visible at 704 and 729 cm −1 .These modes have been assigned to the respective Raman modes by comparing the Raman spectra of NiTiO 3 reported already in the literature. 14The E g modes have been observed at 226, 242, 286 and 344 cm −1 and also at 464 and 604 cm −1 .The two E g modes at 226 and 246 cm −1 can be attributed to the asymmetric breathing vibration of the oxygen octahedra, and a twist of oxygen octahedra due to vibrations of the Ni and Ti atoms parallel to the XY plane, respectively.The modes at 246, 392 and 455 cm −1 are assigned to the vibrations of Ti atoms along the Z axis (246 cm −1 ), breathing-like stretching of the Ti-centred oxygen octahedra, and both are A g modes.Thus, the Raman analysis showed the crystalline structure and formation of NiTiO 3 in the Pt x Ru 1-x -NiTiO 3 nanostructured electrocatalyst through the expression of their vibrational modes.
Microstructure analysis.-Themicrostructure analysis of Pt x Ru 1-x -NiTiO 3 , is carried out by TEM. Figure 4 presents the bright field and the respective dark field TEM micrographs for the Pt 0.5 Ru 0.5 -NT nanostructured electrocatalysts acquired from two different locations.Figures 4a and 4b are bright and dark field images, respectively acquired from the same location.Figure 4a shows a big size particle, in which if we look more carefully on the surface, small dot-like features can be seen.The big size particle is nothing but a NiTiO 3 nanoparticle, whereas the small dots-like particles are the Pt(Ru) nanoparticles decorating the NiTiO 3 surface.Thus, it is clear that the Pt(Ru) metal nanoparticles decorate the surface of the base material NiTiO 3 , which is a hybrid electrocatalyst material that is the objective of this work.In order to complement this bright field image, the dark field image is also taken from the same region and shown in Fig. 4b.On comparison, it is obviously seen that the smaller particles look bright due to Bragg diffraction occurring in the crystal planes oriented with respect to the electron beam direction, confirming all these Pt(Ru) particles are crystalline in nature.For better understanding, one more set of dark and bright field images are shown in Figs.4c and 4d, respectively.Careful comparison between bright and dark field images provides a good spatial correlation of the particles in each case.
Further, it is necessary to understand the size of the Pt(Ru) nanoparticles because the size determines the active surface area and the catalytic activity of the electrocatalyst Pt x Ru 1-x -NiTiO 3 .The particle size analysis was performed by loading the TEM image in the image J (freeware) software, and individual particles were measured after appropriate calibration of the length scale.About 100 particles were accounted and a size distribution histogram is drawn as shown in the insert of Fig. 4d.The histogram is fitted with the log-normal distribution function to get the average particle size of Pt(Ru) to be 2.5 nm.It is to be noted that the particle size obtained from the XRD analysis (1.8 nm) matches well with the same obtained from the TEM analysis.This shows that each of the Pt (Ru) nanoparticles is formed like a single crystallite.This is also the reason why the whole particle is lighted up uniformly in the dark field images (Figs.4b and 4c).Similar microstructure data acquired from one another region of the same sample is presented in Fig. S3 of the supplementary information file, wherein, the average particle size of the Pt(Ru) particles is obtained to be 1.9 nm, from the particle size histogram analysis and this size lies much closer to the one obtained from the XRD results.In addition, TEM analysis was done for the other sample (Pt 0.1 Ru 0.9 -NiTiO 3 ) and the relevant bright field TEM images are presented in Fig. S4 of the supplementary information.In this case also, the Pt(Ru) particles are seen on the surface of NiTiO 3 .
Elemental composition analysis.-Chemicalcomposition of the Pt x Ru 1-x -NiTiO 3 electrocatalysts is determined by XEDS analysis equipped with TEM microscopy.The obtained XEDS spectra for Pt 0.5 Ru 0.5 -NiTiO 3 and Pt 0.1 Ru 0.9 -NiTiO 3 are presented in Figs.5a  and 5b, respectively.The XEDS spectra show the presence of Ni, Ti and O as they are the major constituents of the Pt x Ru 1-x -NiTiO 3 samples.However, Pt and Ru are also seen in both samples implying that the Pt(Ru) alloy particles are formed clearly.In addition to the constituent elements of the electrocatalysts investigated, the peaks corresponding to Copper and Carbon are also observed.The carbon and copper peaks are originated from the TEM grid that was used for the TEM measurements.
X-ray photoelectron spectroscopy analysis.-Additionally,Xray photoelectron spectroscopy (XPS) was carried out on Pt x Ru 1-x -NiTiO 3 to better understand not only the chemical information but also the oxidation states of the constituent elements.Since the XPS of the NiTiO 3 component in this heterostructured electrocatalyst has already been reported by us in our earlier work. 10This work discusses the XPS studies of Pt and Ru components only.Typical core level high-resolution XPS spectra for the Pt and Ru elements acquired from Pt 0.5 Ru 0.5 -NiTiO 3 are presented in Figs.6a  and 6b, respectively.The spectrum of Pt shows a doublet peak at 71.4 and 74.7 eV corresponding to 4f 7/2 and 4f 5/2 states, respectively.The reported values of the Pt 4f 7/2 and 4f 5/2 peaks are 71.2 and 74.5 eV, respectively for the metallic form of Pt. 15 The binding energy for the PtO with Pt 2+ state for the 4f 7/2 is reported to be 72.3 eV and none for the 4f 5/2 . 16The PtO 2 has the binding energy of 74.1 and 77.4 eV, respectively, for 4f 7/2 and 4f 5/2 states, respectively. 17When PtO 2 is found on metallic Pt, these 4f 7/2 and 4f 5/2 components were reported to be found at 71.2 and 74.5 eV, respectively. 17he peaks of the core levels 4f 7/2 and 4f 5/2 are further deconvoluted into six peak components as shown in Fig. 6a.The Pt-4f 7/2 peak has two components with peak values of 71.4 and 72.4 eV.It should be noted that the fitting was carried out by taking into account of the peak height and FWHM of the relevant peaks.In comparison with the literature, 18 the peak assignments were made.The 4f 7/2 peak at 71.4 and 4f 7/2 peak at 74.73 eV are assigned to metallic Pt°of Pt in PtRu-NiTiO 3 .Similarly, the peaks at 72.4 eV  The other two peaks at 77.8 and 79.9 eV are attributed to the Pt 4+ state of Pt in PtRu-NiTiO 3 .When looking at the positions of Pt 2+ , it matches with the PtO 2 on Pt component reported in the literature, 18,19 and therefore, it can be concluded that the Pt is slightly oxidized on their surface, which is quite expected.Thus, the XPS analysis clearly shows the presence of metallic Pt along with its oxidized components such as Pt 2+ and Pt 4+ .
As far as the Ru-3P 3/2 peak is concerned, it is further deconvoluted into four peaks with their maxima at 458.3, 459.4,463.9 and 465.5 eV.In the literature, metallic Ru°is reported to have a binding energy at 461.70 eV, whereas for the Ru 4+ (i.e., RuO 2 ) the binding energy is reported to be at 463.20 eV 12,20,21 for the 3P 3/2 peak.The peak assignment for Ru is made based on the literature. 22The peak at 458.3 eV is assigned to Ru°state and the 463.9 eV peak is assigned to the 4+ state of Ru and its corresponding satellite peak is observed at 465.4 eV.The peak at 459.5 eV can be attributed to the 2 + state of Ru.Therefore, it can be concluded that the Ru exists not only in the metallic state but also present with 2+ and 4+ oxidation states in coherence with oxidation states of Pt.
Electrochemical studies of Pt x Ru x-1 -NiTiO 3 .-Theelectrocatalytic activity of the Pt(Ru)-modified NiTiO 3 supported electrocatalyst was investigated in a standard three-electrode system by using a modified glassy carbon electrode as the working electrode, Pt disc with the surface area ∼1.6 cm 2 as a counter electrode and Ag/AgCl as a reference electrode.The modified GCE-Pt x Ru 1-x -NiTiO 3 /C was kept immersed in the electrolyte solution for 15 min to attain an equilibrium state, and then pure nitrogen gas was purged into the electrolyte solution before the experiments were recorded, to ensure no residual oxygen present in the electrode-electrolyte solution interface.The electrolyte 0.5 mol l −1 H 2 SO 4 with 0.5 mol l −1 of CH 3 OH) was used to evaluate the electrocatalyst towards methanol oxidation.All the electrochemical studies were carried out at room temperature (25 °C).
The cyclic voltammetry (CV) studies were performed for all the Pt x Ru 1-x -NiTiO 3 electrocatalysts in the presence of 0.5 m l −1 sulphuric acid alone and this particular measurement is a control experiment for the real cyclic voltammogram measured with the presence of methanol in the electrolyte.The cyclic voltammogram for the Pt x Ru 1-x -NiTiO 3 with H 2 SO 4 only as an electrolyte is presented in Fig. 7.All the CV data in the Fig. 7 were acquired in the applied voltage window of −0.2 to 1.0 V at the scan rate of 25 mV s −1 .The CV graphs for all the compositions of Pt and Ru in NiTiO 3 (from Figs. 7a to 7d) show no oxidation peaks when measured in the presence of only sulphuric acid.However, in the applied voltage window range of −0.01 to 0.04 V, a feature observed here can be attributed to the normal behaviour due to hydrogen adsorption-desorption.This is a typical characteristic of Pt-based electrocatalysts towards hydrogen adsorption-desorption.The electrochemical surface area (ECSA) was estimated by integrating the area of the hydrogen under-potential adsorption region from the CV curves that are presented in Fig. 7. 23 Figure 8 shows the cyclic voltammograms of the Pt x Ru 1-x -NiTiO 3 electrocatalyst measured in the presence of 0.5 m l −1 methanol with 0.5 m l −1 sulphuric acid as an electrolyte.The CV graphs in Figs.8a-8(d) represent Pt 0.1 Ru 0.9 -NT, Pt 0.3 Ru 0.7 -NT, Pt 0.4 Ru 0.6 -NT and Pt 0.5 Ru 0.5 -NT, respectively.It is obviously seen from the cyclic voltammetry curves that there are two oxidation peaks observed in all compositions of Pt and Ru in Pt x Ru 1-x -NiTiO 3 .One is observed during the forward sweep and the other one is in the reverse sweep.The forward oxidation peak is attributed to the methanol oxidation whereas the reverse oxidation peak is ascribed to the oxidation of residual molecules formed during the forward methanol oxidation.Comparing Figs. 7 and 8, first of all, it is evident that the Pt x Ru 1-x -NiTiO 3 have participated in methanol oxidation because Fig. 7 does not show any oxidation where no methanol was added.However, in the presence of methanol, a welldefined oxidation peak is observed in Fig. 8.This implies the active participation of Pt x Ru 1-x -NiTiO 3 in methanol oxidation.Peak current values for the forward and reverse oxidations are marked as I f and I b , respectively in Fig. 8.The methanol oxidation peaks are used to obtain the current density from their respective peak current values, and these current densities for the forward and backward oxidations are listed in Table I.The current density for forward oxidation varies from 65.4 to 31.8 mA cm −2 , and the same for backward oxidation varies from 43.5 to 17.6 mA cm −2 .Among the four samples, the Pt 0.5 Ru 0.5 -NT exhibits the highest current density over the other electrocatalysts, which implies that the Pt to Rh ratio of 1:1 shows more activity towards methanol oxidation.Next higher current density of 51.4 mA cm −2 is shown by Pt 0.4 Ru 0.6 -NT sample; this implies that with decreasing Pt content, clearly, the current density goes down.The other aspect to consider in this case is the CO poisoning shown by these electrocatalysts.To understand CO poisoning, the indicator is the ratio of forward oxidation current density to the same of backward oxidation, i.e., I f /I b . 24The current density ratio I f /I b was calculated and presented in Table I.The I f /I b ratio is found to be high to be 2.2 for Pt 0.4 Ru 0.6 -NT, followed by 1.9 for Pt 0.3 Ru 0.7 -NT, 1.8 for Pt 0.1 Ru 0.9 -NT and 1.5 for Pt 0.5 Ru 0.5 -NT.It is known that a higher value of I f /I b implies that the material is highly CO poison-tolerant and vice-versa.It can be seen that Pt 0.4 Ru 0.6 shows the maximum tolerance for CO poisoning and the Pt 0.5 Ru 0.5 shows the least tolerance compared to other electrocatalysts.When the absolute current density is considered, Pt 0.5 Ru 0.5 possesses the maximum value (65.4 mA cm −2 ) compared to any other electrocatalysts.The highest current density is attributed to the maximum content of Pt in the electrocatalyst, and the trend in the current density just follows the Pt content.Whereas, the CO poisoning tolerance is better when the Ru content is higher.Therefore, this electrocatalyst's performance is a trade-off between Pt and Ru contents based on CO poisoning tolerance and delivered current density.Based on these, the Pt 0.4 Ru 0.6 is found to be the optimum ratio to provide a reasonably good current density and the best CO poisoning tolerance.It was reported that the CO poisoning Table I.Electrochemical parameters for the methanol oxidation by Pt x Ru 1-x -NiTiO 3 in the presence of 0.5 M H 2 SO 4 + 0.5 M CH 3 OH electrolyte.These parameters were obtained from the CV curves of the respective samples.

Sample code
Current density I f (mA/cm effect could be reduced by adding Ru with Pt (relative quantity of Ru with respect to Pt) to the electrocatalyst. 25,26As discussed in the literature, the following chemical reaction 25,26 is given in Eq. 1, the CO tolerance has been increased due to the bifunctional effect of Ru and Pt, In order to have better visibility, a comparison of the CV curves of all the electrocatalysts with and without methanol is made in Figs.9a and 9b, respectively.In the first case, only H 2 SO 4 was used as an electrolyte, whereas in the latter case, methanol was also included with the electrolyte.As discussed earlier, no oxidation peaks are shown by Pt x Ru 1-x -NT in Fig. 9a and the oxidation peaks seen in Fig. 9b imply that Pt x Ru 1-x -NT oxidises methanol very well.The discussion given above holds good for Figs.9a-9c.As discussed earlier, when the Ru content in the Pt x Ru 1-x -NT is increased it enhances the removal of CO from the active sites due to bifunctional effects. 27The variation in current density for the methanol oxidation shown in Fig. 9c clearly demonstrates the increase in current density with increasing concentration of Pt in Pt x Ru 1-x -NT electrocatalyst.
In order to make sure that the methanol oxidation is better done by the Pt x Ru 1-x -NT electrocatalyst, a blank CV experiment was done just with Pt/C by keeping all the other experimental parameters the same.The cyclic voltammetry graph of this experiment presented in Fig. 10 displays the forward oxidation potential at 0.68 V and a maximum current density of 12 mA cm −2 .Similarly, the backward oxidation potential is registered at 0.49 V with a maximum current density of 13.7 mA cm −2 .Analysis of figure (a) indicates that the current density of the forward oxidation reaction is relatively low compared to the backward oxidation reaction, suggesting that the forward reactions did not entirely oxidize the methanol molecules.
As a result, more CO molecules were produced, which underwent further oxidation in the backward reactions.Also, this current density (12 mA cm −2 ) is very much low when compared to the maximum current density obtained with Pt x Ru 1-x -NT electrocatalyst (65.4 mA cm −2 ) and this clearly ensures the contribution to methanol oxidation by the combined effect of Pt-Ru.To further investigate the durability, stability and CO poising of the Pt x Ru 1-x -NT electrocatalyst under constant operation conditions, a chronoamperometry experiment was carried out, and the results are presented in Fig. 9d.The operating conditions for this experiment were the following: 0.5 m l −1 H 2 SO 4 with 0.5 m l −1 methanol was the electrolyte, the constant applied potential was 50 mV, and the total time of the experiment was 1800 s.In all the samples, initially within 150 s, the current has dropped down rapidly followed by a slow decay to reach the limiting current density. 28The sudden drop of the current density at the initial stage implies that the poisoning of the catalyst by the highly probable intermediate's molecules such as CH 2 OH, CHO, and CO occurred during the methanol oxidation reaction. 29Among the four electrocatalysts, the Pt 0.5 Ru 0.5 -NiTiO 3 electrocatalyst shows better stability for methanol oxidation compared to the other electrocatalysts.
The electrochemical active surface area (ECSA) for all the samples was calculated by measuring the charges collected in the hydrogen adsorption/desorption region from the electroactive Pt electrode surface.The experiments were carried out with 0.5 m l −1 H 2 SO 4 at a scan rate of 25 mV s −1 .The ECSA can be determined from the coulombic charge for the hydrogen adsorption(Q H ) from the Eq. 2. 30

=
where W Pt represents the platinum loading mass in grams(g), Q is the charge for hydrogen adsorption/desorption (mC), and 0.21 implies the charge required to oxidize a monolayer of H 2 on Pt (mC cm −2 ).The calculated results are presented in Table I.It shows that the electrochemical surface area value of the Pt 0.5 Ru 0.5 -NT is 154 m 2 g −1 , which is much higher than the value of other electrocatalysts.The higher value of ECSA infers the higher methanol oxidation activity of the Pt x Ru 1-x -NT electrocatalysts.In the presence of only electrolyte (0.5 m l −1 H 2 SO 4 ), the hydrogen adsorption peaks from the active platinum surface could not be seen clearly, which could be due to the initial nucleation and growth stage on Pt electrocatalyst.The enhanced electrocatalytic activity of the electrocatalysts is not only due to the improved ECSA but also due to the metal oxide support materials. 31he polarization resistance experiment was done for the Pt x Ru 1-x -NT in the presence of 0.5 m l −1 methanol and 0.5 m l −1 sulphuric acid electrolyte, to find the polarization resistance of the electrocatalysts for methanol oxidation.The measured polarization graphs are presented in Fig. 11a I. Figure 11b shows the polarization curves fitted using the Tafel equation for two electrocatalysts.It is known that when the polarization resistance of electrocatalyst less, it will show high electrocatalytic activity in methanol oxidation.In line with that, the Pt 0.5 Ru 0.5 -NT exhibits good CO tolerance in methanol oxidation by having the lowest Tafel value of 161.0 mV dec −1 .
In order to compare the electron transfer resistance of the methanol oxidation reaction, the electrochemical impedance spectra (EIS) of the electrocatalysts were acquired.The EIS were acquired in the presence of 0.5 m l −1 H 2 SO 4, and 0.5 m l −1 CH 3 OH and the respective Nyquist plots for all the electrocatalysts are presented in Fig. 11c.All the plots are featured with a semi-circle in the lowfrequency region 32 whereas the low frequency region shows a continuous line.The appropriate equivalent model was used to analyse the impedance spectra and it was modelled by using suitable elements such as R1, CPE1, CPE2, R2 and Ws that are solution resistance, constant phase elements, charge transfer resistance and Warburg elements, respectively; the Ws implies a diffusion process. 33The obtained electrochemical impedance data are listed in Table II.The electrochemical impedance of Pt 0.5 Ru 0.5 -NT shows a low charge transfer resistance during the methanol oxidation, compared to other electrocatalysts Pt 0.1 Ru 0.9 -NT, Pt 0.3 Ru 0.7 -NT, and Pt 0.4 Ru 0.6 -NT.The numerical values of the charge transfer resistance for the Pt 0.5 Ru 0.5 -NT, Pt 0.4 Ru 0.6 -NT, Pt 0.3 Ru 0.7 -NT and Pt 0.1 Ru 0.9 -NT are 35.54,45.5, 59.61 and 42.82 Ω cm −2 , respectively.It is evident that the Pt 0.5 Ru 0.5 -NT electrocatalyst owns the lowest charge transfer resistance of 35.54 Ω cm −2 , which could be the major reason for its best electrocatalytic activity over the others.

Summary and Conclusions
This work has dealt with the preparation of Pt x Ru 1-x -NiTiO 3 hybrid electrocatalysts in nanostructured form for four different relative compositions of Pt to Ru in them.The NiTiO 3 was prepared by Sol-Gel method and used in the following polyol method to decorate its surface with Pt(Ru) metal nanoparticles.The preparation was complete  and successful, and the product was obtained with a Pt(Ru) nanoparticles decorated NiTiO 3 .The XRD and Raman analysis confirmed the rhombohedral crystal structure of NiTiO 3 and cubic-structured Pt(Ru) nanoparticles.Combined XRD and TEM analysis showed that 1.8 nm Pt(Ru) nanoparticles decorated on the surface of the NiTiO 3 .XEDS combined with XPS showed the constituent elements present in Pt(Ru) metal nanoparticles and the chemical oxidation states.The Pt(Ru) was found mostly to be in the metallic state, however, it was also having slightly double and quadruple oxidized states.The cyclic voltammetry analysis showed an efficient participation of Pt x Ru 1-x -NiTiO 3 of all the compositions in methanol oxidation and they performed well as a good electrocatalyst.However, the Pt 0.5 Ru 0.5 -NiTiO 3 has shown better activity in methanol oxidation compared to the Pt 0.1 Ru 0.9 -NiTiO 3, Pt 0.3 Ru 0.7 -NiTiO 3 and Pt 0.4 Ru 0.6 -NiTiO 3 .The maximum current density obtained for Pt 0.5 Ru 0.5 -NiTiO 3 was 65.41 mA cm −2 with less carbon poisoning.It is also due to the lowest charge transfer resistance, and the highest electrochemical surface area of the Pt 0.5 Ru 0.5 -NiTiO 3 over the other electrocatalysts.Chronoamperometry and polarization studies showed the stability and electrochemical activity of these Pt x Ru 1-x -NiTiO 3 electrocatalysts towards methanol oxidation.In summary, the NiTiO 3 can be used as a support materials with Pt(Ru) as electrocatalyst with less CO poisoning and the quantity of Pt is also reduced by replacing a part of it with Ru.And therefore, the Pt x Ru 1-x -NiTiO 3 would be a promising candidate, as electrocatalyst, for methanol oxidation in direct methanol fuel cells.

Figure 1 .
Figure 1.The X-ray diffraction patterns of Pt x Ru 1−x -NiTiO 3 with different concentration ratios of Pt to Ru.The standard ICDD data corresponding to the rhombohedral phase (#33-960) and cubic Pt phase (#04-0802) are presented as stick patterns at the bottom for comparison.

Figure 2 .
Figure 2. The UV-vis absorption spectra for Pt x Ru 1−x -NiTiO 3 electrocatalysts with different concentration ratios of Pt to Ru.

Figure 3 .
Figure 3. (a) Raman spectra of Pt x Ru 1-x -NiTiO 3 with different concentration ratios of Pt to Ru.(b) The deconvoluted 704 cm −1 band of Pt 0.5 Ru 0.5 -NiTiO 3 into two Raman bands using the Lorentzian function.

Figure 4 .
Figure 4. TEM micrographs of Pt 0.5 Ru 0.5 -NiTiO 3 .(a) Bright field image, (b) dark field image.(c) and (d) are the dark and its respective bright-field image acquired from another location.Insert of (d) is the particle size histogram of the Pt(Ru) nanoparticles that decorates the NiTiO 3 surface.

Figure 9 .
Figure 9.Comparison of cyclic voltammograms of Pt x Ru 1-x -NT.(a) CV curves acquired in the presence of 0.5 mol l −1 H 2 SO 4 (b) CV curves acquired in the presence of 0.5 mol l −1 H 2 SO 4 and 0.5 mol l −1 methanol, at a scan rate, 25 mV s −1 .(c) Variation of peak current density versus concentration of Pt in Pt x Ru 1-x -NT electrocatalyst.(d) Chronoamperometry curves of Pt x Ru 1-x -NT measured at the constant applied potential of 50 mV for 1800 s.

Figure 10 .
Figure 10.The cyclic voltammograms of Pt/C acquired in the presence of 0.5 mol l −1 H 2 SO 4 and 0.5 mol l −1 methanol, at a scan rate, 25 mV s −1 .
that are fitted with the Tafel function and the corresponding Tafel plots fitted with the most linear part of the acquired polarization are presented in Fig. 11b.The polarization resistance values obtained from the fitted Tafel slope for Pt 0.1 Ru 0.9 -NT, Pt 0.3 Ru 0.7 -NT, Pt 0.4 Ru 0.6 -NT, and Pt 0.5 Ru 0.5 -NT are 268, 180.0, 165.7 and, 161.0 mV dec −1 , respectively.They are listed in Table

Figure 11 .
Figure 11.(a) Polarization curves of Pt x Ru 1-x -NT acquired at 5 mV s −1 scan rate, (b) Tafel fitted polarization curves for two selected electrocatalysts.(c) The electrochemical impedance (Nyquist) plots of Pt x Ru 1-x -NT electrocatalyst with the equivalent circuit used to fit them shown in its insert.