*Electrocatalysis and Electrocatalysts for a Cleaner Environment - Fundamentals…*

**Figure 2.**

*HRTEM images with their respective histograms for PtRu/MWCNT prepared through the impregnation, polyol, modified polyol, and microwave-assisted modified polyol methods.*

chronoamperometry (CA), and cyclic voltammetry (CV), and performed on an autolab electrochemical workstation (PGSTAT128N, Eco Chemie, the Netherlands). CA tests were carried out for the electrocatalytic stability of the PtRu/MWCNT catalysts for the methanol electro-oxidation. The CA was carried out for 30 minutes. CV evaluations were carried out at 30 mV/s covering a potential window from 0.2 to 1.2 V vs. Ag/AgCl. Perchloric acid was used as the electrolyte. Inert nitrogen gas was used to deaerate the solutions. To obtain a homogeneous catalyst layer, a stock solution was first prepared by mixing 20 ml of isopropanol, 79.6 ml of ultra-pure

*Investigation of Synthesis Methods for Improved Platinum-Ruthenium Nanoparticles… DOI: http://dx.doi.org/10.5772/intechopen.104541*

water, and 0.4 ml of 5wt% Nafion solution in a 100-ml volumetric flask. Thereafter, 10 mg of the catalyst powder was measured into a 10-ml vial and 5 ml of stock solution was added, mixed thoroughly, and sonicated for 60 minutes in an ultrasonicator. A measured volume of this mixture was dropped on top of the glassy carbon disc and then dried to form the desired catalyst layer.

Electrochemical activities of the prepared catalysts in a 0.5 M HClO4 solution were firstly examined by CV. From the CV of the prepared electrocatalysts, the adsorption peaks for the different catalysts were observed. The peak area of the adsorption peak of the electrocatalysts in the CV was used to determine the electroactive surface area of the catalysts using the equation 2 [22]:

$$ECSA = \frac{Q}{210 \mu \text{C/}cm2.m.Ag} \tag{3}$$

where *Q* is the charge from the adsorption peak in Coulomb taking within the negative potential region of �0.2 to 0.08 V in the forward scan, as shown in **Figure 3**, *m* is the working electrode Pt loading in mg cm�<sup>2</sup> , *Ag* is the geometric surface area of the electrode (5 mm in diameter, Ag = 0.196 cm2 ) and 210 μCcm�<sup>2</sup> is the value for the charge of full coverage for a clean polycrystalline Pt monolayer [23].

The obtained ECSA values were 4.15 � 103 cm2 /g for PtRu/MWCNT microwaveassisted modified polyol, 3.2 � 103 cm2 /g for PtRu/MWCNT modified polyol, 0.35 � 103 cm2 /g for PtRu/MWCNT polyol and 0.28 � 102 m2 /g for PtRu/MWCNT impregnation. The higher ECSA value of PtRu/MWCNT prepared through the microwave-assisted modified polyol method can be attributed to its lower particle size value, hence a higher surface area compared to the other electrocatalysts.

#### **2.4 Methanol oxidation**

The electrocatalytic activity of the PtRu/MWCNT series catalysts towards the methanol oxidation was investigated using the CV technique in 0.5 M HClO4 with 2 M methanol at a scan rate of 30 mVs�<sup>1</sup> , as shown in **Figure 4**.

#### **Figure 3.**

*Cyclic voltammograms of PtRu/MWCNT electrocatalysts in N2-saturated 0.5 M perchloric acid HClO4 at a scan rate of 30 mV.s*�*<sup>1</sup> .*

#### **Figure 4.**

*Cyclic voltammograms of PtRu/MWCNT electrocatalysts in N2-saturated 0.5 M perchloric acid HClO4 and 2 M methanol at a scan rate of 30 mV.s<sup>1</sup> .*


*[a] Onset potential, [b] forward anodic peak potential at 30 mVs<sup>1</sup> , [c] forward anodic peak current density at 30 mVs<sup>1</sup> .*

#### **Table 2.**

*Comparison of the electrocatalytic activity of the catalysts for methanol oxidation.*

The electrocatalytic activity towards methanol oxidation is summarized in **Table 2**. By comparing the characteristics of the CVs, the change in catalyst preparation methods leading to varying compositions of Pt and Ru in the metal alloys was found to substantially enhance the catalytic activity for methanol electro-oxidation. First, the onset potentials (a measure of catalytic activity) of methanol oxidation for the PtRu/MWCNT prepared through the microwave-assisted modified polyol method and PtRu/MWCNT showed relatively lower values than that of PtRu/ MWCNT electrocatalysts prepared through the impregnation, polyol and modified polyol methods. The positions of the onset potentials follow the order of PtRu/ MWCNT microwave-assisted modified polyol < PtRu/MWCNT modified polyol < PtRu/MWCNT polyol < PtRu/MWCNT impregnation. Second, the forward peak current densities (measure of the maximum catalyst performance) of the PtRu/ MWCNT catalysts took the order PtRu/MWCNT microwave-assisted modified polyol > PtRu/MWCNT impregnation > PtRu/MWCNT modified polyol > PtRu/ MWCNT polyol. Therefore, PtRu/MWCNT prepared through the microwaveassisted modified polyol method exhibited the most prominent electrochemical performance in terms of the highest forward peak current density and the lowest onset potential, followed by PtRu/MWCNT prepared through the modified polyol method.

*Investigation of Synthesis Methods for Improved Platinum-Ruthenium Nanoparticles… DOI: http://dx.doi.org/10.5772/intechopen.104541*

It was found that PtRu/MWCNT produced through the microwave-assisted modified polyol method of Pt: Ru ratio close to 1:1 outperformed all other PtRu/ MWCNT electrocatalysts produced through other synthesis methods in methanol electro-oxidation reaction evident from the current density of 0.190 mA/cm<sup>2</sup> .

Electro-oxidation of methanol to form CO2 can be via dual path mechanisms consisting of non-CO and adsorbed CO reactive intermediates [24]:

$$\text{Pt(CH}\_3\text{OH)}\text{ads} + \text{H}\_2\text{O} \rightarrow \text{CO}\_2 + \text{6H} + + \text{6e} \tag{4}$$

$$\text{Pt(CH}\_3\text{OH)}\text{ads} \rightarrow \text{Pt(CO)}\text{ads} + 4\text{H} + + 4\text{e} \tag{5}$$

The non-CO reaction pathway is preferred for methanol oxidation for which it does not involve CO, a poison for Pt metal. The adsorbed CO reaction pathway often presents, however, in which the intermediates via (CO)ads are mostly in the form of linearly bonded CO, that is, Pt = C = O [25]. Interaction of this complex on the catalyst surface leads to CO poisoning. The presence of Ru in the bimetallic catalyst assists in the oxidation of CO through chemisorbed -OH on the Ru sites [26]:

$$\text{Ru} + \text{H}\_2\text{O} \rightarrow \text{Ru} - \text{OH} + \text{H} + + \text{e} \tag{6}$$

$$\text{Ru OH} + \text{Pt(CO)ads} \rightarrow \text{Ru} + \text{Pt} + \text{CO2} + \text{H} + + \text{e} \tag{7}$$

In this way, the poisoned Pt is regenerated and can again participate in the oxidation of methanol. Due to the single species of CO and OH on Pt and Ru, respectively, the best results can be obtained when the Pt to Ru atomic ratio is 1:1 [27] (**Figure 5**).

The EIS technique was used to investigate the catalytic reaction kinetics for the methanol oxidation on the anodic PtRu/MWCNT electrocatalysts surfaces. The charge transfer resistance (Rct) values using equivalent circuit fitting were 5.985, 8.926, 4.061, and 6.184 kΩ for PtRu/MWCNT modified polyol, PtRu/MWCNT polyol, PtRu/MWCNT impregnation, and PtRu/MWCNT microwave-assisted modified polyol, respectively, indicating that PtRu/MWCNT prepared through the

#### **Figure 5.**

*Electrochemical impedance curves of methanol oxidation on PtRu/MWCNT electrocatalysts prepared through different synthesis methods in N2-saturated 0.5 M HClO4 and 0.2 M methanol.*

#### **Figure 6.**

*Chronoamperometry curves of methanol oxidation on PtRu/MWCNT electrocatalysts in 0.5 M HClO4 and 2.0 M CH3OH.*

impregnation method exhibited the best kinetics towards the methanol electrooxidation with the least resistance to flow of electric current. PtRu/MWCNT prepared through the modified polyol method also showed promising kinetics with an Rct value of 5.985 kΩ.

The stability of the electrocatalysts is extremely important for their real applications in direct methanol fuel cells. **Figure 6** shows the CA of PtRu electrocatalysts on MWCNT support in N2-saturated 0.5 M HClO4 with 2.0 M methanol. This was to test the stability of the different catalysts after 1800 seconds. As observed at the start of the CA curve, the current density decreases sharply with time (I proportional to t1/2). The decreasing rate with time may characterize the inhibition of the electrodes by the methanol oxidation reaction products. When comparing the prepared catalysts, PtRu/MWCNT catalyst prepared through the polyol method performed better, followed by PtRu/MWCNT modified polyol. PtRu/MWCNT prepared by microwaving also showed better stability with higher current density than PtRu/MWCNT prepared through the impregnation method.

#### **3. Conclusion**

In this study, PtRu, supported by MWCNT, was successfully fabricated using the impregnation, polyol, modified polyol, and microwave-assisted modified polyol catalyst preparation methods. The synthesized electrocatalysts had crystalline sizes of 1.95–7.11 nm and average particle sizes of 1.87–6.90 nm, determined using XRD and HRTEM, respectively. The PtRu alloy phase is pronounced for the prepared electrocatalysts according to XRD analysis. It is found that the PtRu/MWCNT electrocatalyst produced through the microwave-assisted modified polyol method and PtRu/MWCNT modified polyol showed enhanced electrocatalytic activity towards methanol oxidation compared to other PtRu electrocatalysts on MWCNT support. Furthermore, the microwave-assisted prepared PtRu/MWCNT electrocatalyst had the largest current density for methanol oxidation compared to other electrocatalysts. This can be attributed to it having the smallest particle size and being the most active toward anode oxidation reaction. From the EIS, it was

### *Investigation of Synthesis Methods for Improved Platinum-Ruthenium Nanoparticles… DOI: http://dx.doi.org/10.5772/intechopen.104541*

concluded that the PtRu/MWCNT electrocatalysts produced through the impregnation method exhibited a faster electrochemical reaction kinetics than both PtRu/MWCNT electrocatalysts produced through the polyol and modified polyol methods. Microwave-assisted modified polyol method PtRu electrocatalysts had the highest ECSA values compared to all other PtRu catalysts on MWCNT support, followed by PtRu/MWCNT produced by the modified polyol method. This was as a result of their smaller crystalline particle sizes of 1.95 and 4.33 nm, respectively. Polyol method synthesized PtRu/MWCNT was found to be the most stable electrocatalyst, followed by PtRu/MWCNT produced through the modified polyol method, as revealed by the chronoamperometry tests.

Based on all the results acquired in this investigation, it was concluded that the microwave-assisted modified polyol process of catalyst preparation method produced the best PtRu electrocatalyst on MWCNT to support the improved catalytic activity.
