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The Effect of Iridium Addition to Platinum on the Alcohol Electrooxidation Activity

Ozlem Sahin, Hilal Kivrak*, Mustafa Karaman, Dilan Atbas

2Chemical Engineering Department, Yüzüncü Yıl University, Van Turkey

Abstract: Pt-Ir@C and Pt@C catalysts were prepared by ethylene glycol method and tested for methanol and ethanol in H₂SO₄ electrolyte. The electrocatalytic activity of these electrocatalysts was investigated using cyclic voltammograms (CVs), linear sweep voltammograms (LSVs), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). Their CVs show that Pt₄₀Ir₆₀@C catalyst present significantly high current of methanol and ethanol oxidation compared to the other catalysts. Moreover, chronoamperometric measurements showed that the steady-state current of Pt₄₀Ir₆₀ catalyst was also higher than other electro-catalysts. EIS analysis shows that the impedances on both the imaginary and real axes are much lower than those of the other catalysts. As a result, electrocatalytic activity measurements indicated that the Pt₄₀Ir₆₀ catalyst was the most active electrode for methanol and ethanol oxidation.

Keywords: Pt-Ir, alcohol, electrooxidation, fuel cells, CV, CA, impedance

Received May 26, 2015; Revised July 24, 2015; Accepted July 27, 2015

Citation: Ozlem Sahin, Hilal Kivrak, Mustafa Karaman, Dilan Atbas, The Effect of Iridium Addition to Platinum on the Alcohol Electrooxidation Activity. American Journal of Materials Science and Engineering, 3 (1). 15-20. January 2015. doi:10.12691/ajmse-3-1-4

1. Introduction

Direct alcohol fuel cells (DAFCs) based on liquid fuels have attracted considerable interest due to the depletion of fossil fuels and the increase in environmental pollution. Direct methanol fuel cells (DMFCs) and direct ethanol fuel cells (DEFCs) are excellent power sources among different types of fuel cells. The main advantage of methanol and ethanol is that they are liquid. Thus, the storage problems when hydrogen is used are solved. Additionally, the power density of the alcohols in terms of energy by volume of fuel is much higher at standard conditions. When comparing methanol and ethanol fuels, the attention is focused on the use of methanol because of its better reaction kinetics. Hence, DMFCs give better performance than DEFCs. However, ethanol has received considerable interest due to several reasons as follows: (i) its low toxicity, (ii) easy production in large amounts as renewable biofuel from the fermentation of biomass, (iii) higher energy density, and (iv) abundant availability [1-5].

Pt is known active and stable metal for alcohol oxidation especially in acidic medium. Nevertheless, Pt is readily poisoned by CO-like intermediates of alcohol electrooxidation. The effective approach is to use Pt alloys to enhance the electrocatalyst activity [6,7,8,9]. Among these Pt alloys, the recent researches have focused on the development of Pt-Sn and Pt-Ru alcohol electrocatalysts due to the effective for CO and alcohol oxidation [10-15]. Tsiakaras et al. conducted a study on Pt-Sn/C catalysts at different atomic ratios by using a single DEFC at 60oC and 90°C. It was reported that Pt-Sn (3:2)/C catalyst revealed the highest MPD at 60oC. On the contrary, the OCV and MPD of Pt-Sn (1:1)/C was smaller than that of Pt-Sn (3:2)/C catalyst, attributed to the fact that platinum active sites of catalysts with a high Sn content could be partly blocked by surface tin or its oxides [16]. The decoration of carbon supported Pt with Sn, the decoration of carbon supported Sn with Pt, and the co-deposition of Pt and Sn on carbon were the different preparation orders to investigate the effect of preparation orders on the formation of oxide phases and on the electrochemical activity [17]. As a result, Sn decorated Pt showed the best performance for ethanol electro-oxidation reaction and XPS data showed that Sn existed as Sn oxides in the Pt-Sn catalyst. Jiang et al. conducted as study on Pt₃Sn/C catalyst prepared by a modified polyol process and treated in O₂, H₂/Ar, and Ar atmosphere, respectively. In consequence, among these treated catalysts, the as-prepared Pt-Sn/C catalyst gave the higher power density, while Ar-treated Pt-Sn/C showed the lower cell performance. This is due to the fact that more zero valence of Sn appeared in Pt-Sn/C-Ar catalyst, while more multi-valence Sn existed in the other catalysts [18,19]. Other binary catalysts Pt-Pd [16 ], Pt-W [16], Pt-Re [20], Pt-Rh [21-26], Pt-Pb [27,28], Pt₃Tex [29], Pt-Sb [30], Pt-CeO2 [31,32,33,34], Pt-ZrO₂ [31,35], Pt-MgO [36], Pt-TiO₂ [37,38,39,40,41], Pt-SiO₂ [42] were investigated for ethanol electro-oxidation reaction. Generally these catalysts presented lower activity than that of Pt and lower than that of Pt-Ru.

Harish and coworkers [10] employed a polyol process activated by microwave irradiation to prepare efficient Pt/C, Ru/C and Pt-Ru/C electrocatalysts. Pt-Ru/C catalyst displayed a high activity towards CO and methanol electrooxidation activity (MOR). Furthermore, the lowest onset potentials and lowest surface poisoning of MOR for Pt-Ru catalysts than those obtained on Pt/C catalysts were observed.

Other binary or ternary/quaternary Pt alloys catalysts have been paid less attention to enhance the performance of Pt catalysts. Ir is unique among them, because Ir has a relatively high oxidation potential of because Ir can react with water to produce Ir–OH species, which will combine with Pt– CO species to remove surface-adsorbed CO at the potential lower than pure Pt. [43].

Herein, Pt-Ir@C and Pt@C electrocatalysts were prepared by polyol method at different weight percentages of Ir. These catalysts prepared at varying Pt: Ir ratios were examined as DAFC catalysts to investigate the effect of Ir ratio.

2. Materials and Methods

Carbon black (Vulcan XC72R) was purchased from Cabot Corp. CH₃OH, CH₃CH₂OH, H₂SO₄, KOH, H₂PtCl₆.6H₂O, and IrCl₃.xH₂O were purchased from Merck and Aldrich. All chemicals were of analytical grade and were used as received.

Pt@C and Pt-Ir@C catalysts were prepared by ethylene glycol reduction method [15]. For Pt/C catalyst, H₂PtCl₆.6H₂O and for Pt-Ir catalysts H₂PtCl₆.6H₂O and IrCl₃.xH₂O were dissolved in 200 mL ethylene glycol solution per gram carbon support. The pH of the solution was fixed at 10 by the addition of KOH solution and carbon support was dispersed in this solution. Consequently, the temperature was increased to 130°C and kept constant at this temperature for 2 h. After refluxing at 130°C for 2 h, the slurry suspension was rapidly cooled down in cold water, filtered, and dried. Pt-Ir@C catalysts were prepared at varying 80:20, 40:60, and 20:80 Pt: Ir atomic ratios. Nominal Pt loadings of all catalysts are 10%.

XRD patterns were measured on a Bruker D 8 Advance X-Ray diffractometer using Cu Kα-ray radiation (k = 1.5405 A°) operating at 30 kV and 15 mA. XRD patterns were recorded between 2h = 10.0–100.0o with 0.05o intervals, 1o data collection velocity in 1 min.

Surface characterization of catalysts for the oxidation states of the surface species X-ray Photoelectron Spectroscopy (XPS) was performed. The X-ray photoelectron spectra was obtained using Mg-Kα (hv = 1253.6 eV) unmonochromatized radiation with SPECS spectrometer. The charging effects were corrected by using the C 1s peak, as reference for all samples at a binding energy (BE) of 284.8 eV.

Electrochemical measurements were carried out in a conventional three-electrode cell with Pt wire as a counter electrode and Ag/AgCl as a reference electrode with a CHI 660E potentiostat. The working electrode was a glassy carbon disk with a diameter of 3.0 mm held in a Teflon cylinder. Cyclic voltammograms (CVs) were recorded between -0.25 V and 1 V with a scan rate of 100 mV s⁻¹ in 0.5 M H₂SO₄ solution on these catalysts. Furthermore, CV and CA techniques were employed to examine the MOR and EOR activities of Pt@C and Pt-Ir@C catalysts. In all experiments, the electrolyte was previously saturated by nitrogen. Before each experiment, the electrode surface was activated in 0.5 M H₂SO₄. For MOR, CVs were recorded between -0.25 V and 1.0 V with a scan rate of 100 mV s⁻¹ in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 25°C. For EOR, CVs were recorded between -0.25 V and 1.0 V with a scan rate of 100 mV s⁻¹ in 0.5 M H₂SO₄ + 1 M C₂H₅OH at 25oC. For MOR, chronoamperomograms (CAs) were recorded in 0.5 M H₂SO₄ + 1 M CH₃OH solution on Pt-Ir@C catalysts. Moreover, CA was performed in 0.5 M H₂SO₄ + 1 M C₂H₅OH solution at 0.6 V for 200 s with 1000 s pulse width and 2 s quiet times.

Electrochemical impedance spectroscopy (EIS) as a dynamic method was used to examine the electrochemical behavior of Pt@C and Pt-Ir@C catalysts. EIS measurements were recorded between 30 kHz and 0.01 Hz at amplitude of 10 mV at the desired electrode potential.

3. Results and Discussion

XRD patterns of Pt-Ir@C catalysts were illustrated in Figure 1, which reveal the structural information for the bulk of catalyst nanoclusters together with the carbon support. The diffraction peak at around 25° was attributed to the (002) plane of the hexagonal structure of Vulcan XC-72R carbon. The diffraction peaks at around 39◦, 46◦, 68° and 81° are due to diffraction of Pt (111), (200), (220) and (311) planes, respectively. For these catalysts, Ir peaks were observed when 40:60 Pt:Ir ratio was reached. It is clear that crystallinity of Pt catalyst decrease by the addition of the Ir due to ensemble size effect [43].

Figure 1. XRD patterns of Pt@C and Pt-Ir@C catalysts

XPS analyses are performed to investigate the chemical nature of these catalysts. High resolution Pt 4f and Ir 4d XPS scans were obtained to investigate the chemical nature of alloys having different compositions. Figure 2 a and Figure 2b shows Pt and Ir XPS spectra together with the spectrum deconvolutions. Curve fitting of the Pt 4f and Ir 4d spectra gives three different types of Pt moieties and two different types of Ir moieties, respectively. Binding energy values for each moiety and the corresponding peak area ratios are summarized in Table 1. According to the results, the films consist of Pt, Ir, PtO, IrO₂ and Pt(OH)₂ states. A rough estimate of atomic mole percentage of each species using relative intensities of XPS peaks reveal that Pt in its elemental state contains around 26 mol% PtO-Pt(OH)2 species. The change of Ir composition in the alloy had little effect on relative amounts of Ir and IrO₂. In the case of Pt/Ir alloys, the oxygen related species tend to composition of Ir in the alloy increases, the amount of such oxygen related species decrease the alloys contain.

Figure 2. a. Pt 4f XPS spectra of Pt@C and Pt-Ir@C catalysts, b. Ir 4d spectra of Pt@C and Pt-Ir@C catalysts

Table 1. Pt 4f and Ir 4 d binding energies of Pt and Pt-Ir electrocatalysts

Figure 3. CVs of Pt@C and Pt-Ir@C catalysts in 0.5 M H₂SO₄ at 100 mVs⁻¹

Figure 4. CVs of Pt@C and Pt-Ir@C catalysts in in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 100 mV s⁻¹

Figure 5. CVs of Pt@C and Pt-Ir@C catalysts in in 0.5 M H₂SO₄ + 1.0 M C₂H₅OH at 100 mV s⁻¹

The cyclic voltammograms (CVs) taken in 0.5 M H₂SO₄ for the Pt@C and Pt-Ir@C catalysts shown in Figure 3. The Pt-Ir electrocatalysts do not have a very defined hydrogen oxidation region (-0.2–0.0 V), as observed for pure platinum, and the currents in the double layer region (0.0–0.4 V) are larger, which are characteristic of bimetallic catalysts. The CVs taken for MOR and EOR measurements on Pt@C and Pt-Ir@C catalysts at a scan rate of 100 mVs⁻¹ were given in Figure 4 and Figure 5, respectively.

Obviously, the peaks on the Pt₄₀-Ir₆₀@C electrodes are much stronger than that on the Pt@C and other Pt-Ir@C catalysts, which suggests the Pt₄₀-Ir₆₀@C catalyst has better catalytic activity. Cyclic voltammograms obtained for MOR and EOR on Pt-Ir@C catalysts, have one anodic peak in the forward scan and one oxidation peak in the backward scan showing similar voltammetric behavior as the Pt catalyst. The presence of Ir in the structure of platinum promotes the electrocatalytic activity of platinum towards MOR and EOR. However, the oxidation currents increase reaches the optimum value up to 40:60 Pt:Ir ratio. The role of iridium addition on MOR and EOR can be attributed to the oxidation of Ir at relatively low positive potentials, assisting the redox process of Pt⁰/Pt²⁺ or Pt2+/Pt⁴⁺ [10-15]. These redox processes may play an important part on MOR. The reverse anodic peak observed in the MOR can be attributed to the oxidative removal of incompletely oxidized carbonaceous species strongly adsorbed on the Pt sites, formed in the forward scan. The maximum current ratios of forward to reverse peaks obtained from the CV curves for MOR and EOR were given in Table 2 and Table 3, respectively. One can see that Pt₄₀-Ir₆₀@C catalyst have high If/Ib ratios that are closely related to the less accumulation of reaction intermediates during the forward scan and shows low reverse anodic peak current during reverse scan for MOR and EOR. If/Ib ratios becomes greater with the increase in the Ir ratio for MOR and EOR, attributed to the presence of Ir in Pt-Ir alloy enhances the catalytic activity and improves the CO conversion to CO₂. Furthermore, If /Ib ratio for MOR was higher than the EOR for catalysts having the Pt: Ir same ratio. Pt₄₀-Ir₆₀@C catalyst showed the best performance which could be attributed to the presence of more iridium oxide species on the nanoparticles surface. Hence, the ratio of forward peak to the reverse peak describes the catalyst tolerance to carbonaceous species accumulation formed on the catalyst during the forward potential scan. The high If /Ib ratio indicated that the addition of Ir to the Pt enhances the CO conversion to CO₂, and less accumulation of carbonaceous species on the catalyst. On the other hand, the high If/Ib ratio indicates excellent MOR during the reverse anodic scan and less accumulation of residues on the catalyst [6,7,8,9].

Table 2. Comparison of electrocatalytic activity of MOR on Pt@C and Pt-Ir@C catalysts

Table 3. Comparison of electrocatalytic activity of EOR on Pt@C and Pt-Ir@C catalysts

The catalytic activities and stability of Pt@C and Pt-Ir@C catalysts have also been investigated by chronoamperometry. Figure 6 and Figure 7 show the current vs time curves of catalysts measured at 0.6 V for MOR and EOR, respectively. The shape of the CA curves obtained in methanol solution is similar to those obtained in ethanol solution. There is a continuous current drop with time for MOR and EOR, which is rapid at the beginning and then followed by a relatively slow decay. The higher initial current means a greater number of active sites available for oxidation. It is demonstrated that the higher catalytic activity and better stability are achieved on the Pt₄₀-Ir₆₀@C catalyst for MOR and EOR.

Figure 6. CAs of Pt@C and Pt-Ir@C catalysts in in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 0.6 V

Figure 7. CAs of Pt@C and Pt-Ir@C catalysts in 0.5 M H₂SO₄ + 1.0 M C₂H₅OH at 0.6V

Figure 8. Nyquist plots of Pt@C and Pt-Ir@C catalysts in 0.5 M H₂SO₄ + 1.0 M CH₃OH

Figure 9. Nyquist plots of Pt@C and Pt-Ir@C catalysts in 0.5 M H₂SO₄ + 1.0 M C₂H₅OH

Electrochemical impedance spectroscopy (EIS) is a powerful technique to control the activity of the catalyst to analyze the electrochemical processes occurring at the electrode interface. Figure 8 and Figure 9 presents a typical electrochemical impedance spectrum in the form of a Nyquist plot of Pt@C and Pt-Ir@C catalysts for MOR and EOR respectively. At initial lower potentials, a large arc appears and the diameter of this arc decreases with potential. The semicircle diameter equals to the electron-transfer resistance, which is affected obviously by the surface modification of the electrode. The initial slow kinetics is caused by COad from alcohol dehydrogenation that blocks further adsorption and dehydrogenation of alcohol. As shown in Figure 7, the electron-transfer resistance of the Pt₄₀-Ir₆₀@C catalyst is less than that of the Pt@C and other Pt-Ir@C catalysts, revealing that iridium oxide species formed can enhance the conductivity of the electrode owing to its faster interfacial charge carrier transfer as a result of higher oxide formation. Usually, conductivity and surface activity are two important factors for a good analytical electrode. Therefore, the Pt₄₀-Ir₆₀@C catalyst possesses excellent performance in MOR and EOR [15].

4. Conclusions

As a conclusion, the study of the preparation, characterization and employment of Pt@C and Pt-Ir@C catalysts has led to the following conclusions and insights:

•  Pt-Ir nanoparticles could be easily prepared from the co-reduction of corresponding platinum and Ir salts by polyol method.

•  Pt₄₀-Ir₆₀@C catalyst is highly efficient catalyst for MOR activity compared to Pt@C and other Pt-Ir@C catalysts.

•  The Ir addition to Pt increases the activity of alcohol electrooxidation until and 40: 60 optimum ratio was reached.

•  One can note that iridium oxide species increased by the addition of Ir at 40: 60 optimum ratio. This iridium oxide species can enhance the conductivity of the electrode owing to its faster interfacial charge carrier transfer as a result of higher oxide formation. Therefore, the Pt₄₀-Ir₆₀@C catalyst possesses excellent performance in MOR and EOR.

Acknowledgement

Authors would like to thank The Administrative Units of the scientific research projects of Selcuk University for the financial support (project no: S.U. 13401011).

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