Synthesis and Characterization of Ternary Pt–Ru–Mo/MC Anode Catalyst on Membraneless Methanol Fuel Cells

Abstract

In the present work, mesoporous carbon-supported Pt–Ru–Mo electrocatalyst for methanol electrooxidation were synthesized by Bonnemann’s method and characterized in terms of their structure, morphology, and composition by using XRD, TEM, and EDX techniques. XRD and EDX results revealed the structural information for alloy catalysts together with their carbon support. TEM measurements revealed a decrease in the mean particle size (5 nm) of the catalysts for the ternary compositions. The structural change was beneficial for the catalytic activity of the compositions. The electrocatalytic activities of Pt34Ru33Mo33/MC, Pt50Ru50/MC, Pt50Mo50/MC, and Pt100/MC catalysts for methanol oxidation in an acid medium were investigated by CV and CA. CA results showed that Pt34Ru33Mo33/MC gives a high current under a steady condition. The single membraneless methanol fuel cell performances of the Pt34Ru33Mo33/MC, Pt50Ru50/MC and Pt50Mo50/MC anode catalysts were evaluated at room temperature. Among the catalysts investigated, the power density obtained for Pt34Ru33Mo33/MC (34.3 mW cm−2) catalyst was higher than that of Pt50Ru50/MC (23.1 mW cm−2) and Pt50Mo50/MC (20.3 mW cm−2) using 1.0 M methanol + 0.5 M H2SO4 as the anode feed and 0.1 M sodium percarbonate + 0.5 M H2SO4 as the cathode feed.

Keywords

Electrocatalysts; Mesoporous carbon; Methanol; Power density; Membraneless methanol

References

1. 0 M methanol + 0.5 M H2SO4 Table 3. CV results of Pt34Ru33Mo33/MC, Pt50Ru50/MC, Pt50Mo50/MC and Pt100/MC electrocatalysts Scan rate 50 mV s−1 ___________________________ Catalyst Positive peak potential Peak current density (mV vs. Ag/AgCl) (mA/cm2) Pt100/MC 787 40.2 Pt50Mo50/MC 785 47.1 Pt50Ru50/MC 786 49.0 Pt34Ru33Mo33/MC 788 68.1 The peak current densities of Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt 50Mo50/MC and Pt 100/MC catalysts are 68.1, 49.0, 47.1 and 40.2 mA/cm 2, respectivel y, showing that the activity of the ternary Pt34Ru33Mo33/MC catalyst is a factor of ~1.5 times higher than that of the Pt/MC catalyst. Table 6.3 summarizes the CV results of Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt50Mo50/MC and Pt 100/MC electrocatalysts including the positive peak potentials and the corresponding peak current densities of MOR. The CV results show that pure Pt 100/MC catalysts do not behave as an appropriate anode for MOR due to its poisoning by strongly adsorbed intermediates such as CO. However, the introduction of Ru and Mo promotes the electrocatalytic activity. CV for methanol oxidation reactions showed that the CO poisoning effect was largely inhibited by Pt34Ru33Mo33/MC electrocatalysts, indicating the ability of Mo to promote either the CO to CO2 oxidation or a weaker adsorption of CO on the Pt34Ru33Mo33/MC catalysts. Chronoamperometry Fig. 5 shows the current densities measured from 0.05 to 1.2 V in 1.0 M methanol+0.5 M H2SO4. The currents decay with time in a parabolic style and reach an apparent steady state within 80s. Fig. 5. CA of Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt50Mo50/MC and Pt100/MC electrocatalysts It can be seen that the current density of methanol electro -oxidation on the Pt34Ru33Mo33/MC catalyst is higher than that on the Pt 50Ru50/MC, Pt 50Mo50/MC and Pt 100/MC catalyst at the same potentials. The activity change for methanol oxidation decreases in the order of Pt34Ru33Mo33/MC > Pt 50Ru50/MC > Pt50Mo50/MC > Pt100/MC, which is in fairly good agreement with our CV results. For the durability test, the chronoamperometric experiments were carried out at 0.05 to 1.2 V for 1750 s in the same conditions. Before each measurement, the solution was purged with high- purity nitrogen gas for at least 30 min to ensure oxygen-free measurements. Single cell performance A single cell performance was tested using Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt 50Mo50/MC and Pt 100/MC electrocatalysts as the anode. Polarization curves and power densities are shown in Fig. 6. Fig. 6. Polarization and power density curves of Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt 50Mo50/MC and Pt100/MC electrocatalysts For each catalyst, the open -circuit voltages (OCV) were different, as would be expected in onset potentials. The OCVs of Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt 50Mo50/MC, are higher than that of Pt 100/MC, 0.53 V, and the order of OCV is exactly same as the onset potentials. The OCV of Pt34Ru33Mo33/MC is the highest value of 0.83 V, which is approximately 0.30 V higher than that of Pt 100/MC. This indicates that Pt 100/MC is more rapidly poiso ned by CO than any other alloy catalyst and that the oxidation of adsorbed CO is enhanced by the second or third metal. In the case of Pt34Ru33Mo33/MC the overall performance is superior to that of the bimetallic electrocatalysts. The maximum power densiti es obtained for Pt34Ru33Mo33/MC, Pt 50Ru50/MC, Pt 50Mo50/MC and Pt 100/MC are 36.5, 30.3, 21.2 and 5.7 mW cm−2, respectively (Table 4). We conclude that the substitution of a small amount of Mo for Ru aids in cleaning surfaces poisoned by CO and provides addi tional reaction sites for methanol oxidation. In membraneless fuel cells, pure Pt/MC catalyst does not behave as a very good anode for methanol electro -oxidation due to its poisoning by strongly adsorbed intermediates such as CO. But the addition of Mo to Pt (Pt – Mo/MC) had a little effect, whereas addition of Mo to Pt –Ru/MC greatly enhanced the electrocatalytic activity. Therefore the results demonstrated that the performance of the developed membraneless fuel cell enhanced profoundly as mentioned in our e arlier study [17, 18] , if the concentration of oxidant in cathodic stream is 10 times larger, and the current density is also increased approximately ten times. Table 4. Summary of performance of single fuel cell tests using (2 mg cm –2 catalyst loading, 4 0 wt% catalyst on carbon) Anode catalysts Open circuit Maximum power Maximum current Voltage (V) density (mW cm–2) density (mA cm–2) Pt100/MC 0.53 5.7 49.7 Pt50Mo50/MC 0.61 21.2 111.5 Pt50Ru50/MC 0.69 30.3 149.8 Pt34Ru33Mo33/MC 0.83 36.5 226.0 Conclusions In present work, the study of methanol oxidation on mesoporous carbon -supported Pt–Ru–Mo ternary nanoparticles has revealed details concerning the activity and stability of the catalysts in membraneless fuel cells. The maximum activity for methanol oxidation was found for the Pt34Ru33Mo33/MC than the Pt50Ru50/MC, Pt 50Mo50/MC and Pt 100/MC. The significantly enhanced catalytic activity for methanol oxidation can be attributed to the high dispersion of ternary catalysts and to Mo acting as a promotion agent. XRD results show the homogenous alloy structure of Pt, Ru and Mo. The TEM images indicated an average size of Pt34Ru33Mo33/MC nanoparticles of 1.5 nm. The atom ratio of Pt, Ru and Mo from EDX analyses is close agreement with the original precursor concentration. The composition of ternary Pt34Ru33Mo33/MC nanoparticles can be conveniently controlled by adjusting the initial metal salt solution and preparation conditions. The electrochemical experiments showed that the Pt34Ru33Mo33/MC nanoparticles have higher catalytic activity than that of the other catalysts. We expect that the MLMFC may be a promising candidate for practical fuel cells to establish a clean and sustainable energy future. Further work is necessary to character ize the catalysts using different surface analysis techniques and to conduct tests of these electrocatalysts in microfluidic membraneless fuel cells. Conflict of interest Authors declare there are no conflicts of interest. References [1] Kalaikathir SPR, Begil a S. 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