Elsevier

Electrochimica Acta

Volume 56, Issue 24, 1 October 2011, Pages 8509-8518
Electrochimica Acta

Pt–Ru nanoparticles supported on functionalized carbon as electrocatalysts for the methanol oxidation

https://doi.org/10.1016/j.electacta.2011.07.039Get rights and content

Abstract

Platinum–ruthenium alloy electrocatalysts, for methanol oxidation reaction, were prepared on carbons thermally treated in helium atmosphere or chemically functionalized in H2O2, or in HNO3 + H2SO4 or in HNO3 solutions. The functionalized carbon that is produced using acid solutions contains more surface oxygenated functional groups than carbon treated with H2O2 solution or HeTT. The XRD/HR-TEM analysis have showed the existence of a higher alloying degree for Pt–Ru electrocatalysts supported on functionalized carbon, which present superior electrocatalytic performance, assessed by cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy, as compared to electrocatalysts on unfunctionalized carbon. It also was found that Pt–Ru alloy electrocatalysts on functionalized carbon improve the reaction rate compared to Pt–Ru on carbons treated with H2O2 solution and thermally. A mechanism is discussed, where oxygenated groups generated from acid functionalization of carbon and adsorbed on Pt–Ru electrocatalysts are considered to enhance the electrocatalytic activity of the methanol oxidation reaction.

Highlights

► The functionalized carbon using acid solutions contains surface oxygenated groups. ► Uniform dispersion of PtRu nanoparticles on the carbon surface was achieved. ► Physical analysis showed the formation of PtRu alloy catalysts on functionalized carbon. ► PtRu alloy catalysts on functionalized carbon enhanced the methanol oxidation rate.

Introduction

A direct methanol fuel cell (DMFC) based on a polymer electrolyte membrane is attractive for transport and portable applications [1], [2]. The fuel is cheap, widely available and can be handled and distributed easily. It could be directly supplied to the anode, then, the hydrogen ions (protons) migrate through the electrolyte membrane to the cathode, electrons move through an external circuit, and thus, the oxygen reduction occurs at the cathode [1], [3]. The steps and total reaction are:Anode: CH3OH(l) + H2O(aq)  CO2(g) + 6H+ + 6eCathode: (3/2)O2(g) + 6H+ + 6e  3H2O(l)Overall reaction: CH3OH(l) + (3/2)O2(g)  CO2(g) + 2H2O(l)

However, several problems still prohibit their practical uses, such as [2]: (i) the contamination of the cathode by migration of methanol through the electrolyte membrane, (ii) the high cost, (iii) the low electrocatalytic activity and the durability of the electrocatalysts impregnated and lastly (iv) the poisoning of the platinum electrocatalysts that are used for methanol oxidation in the anodic electrode.

Concerning the last point, the use of platinum–ruthenium alloys electrocatalysts can increase the current densities and to avoid the formation of carbon monoxide (CO) on the electrocatalysts (poisoning) [3]. Ru forms oxygenated species at lower potential than Pt and its presence in the electrocatalysts promotes oxidation of CO into CO2 by the bifunctional mechanism and/or a “ligand effect” [4], [5], [6], [7], [8].

To effectively use these metals as electrocatalysts, they have to be well dispersed in small particles on a carbon support [9]. Although carbon is an excellent electronic conductor it is a very poor proton conductor. This is mainly because carbon is hydrophobic. However, on the carbon surface some hydrophilic groups like carbonyl, carboxyl, phenolic, quinone and lactone groups can be inserted [10]. These groups are normally introduced in the carbon by various oxidation treatments (e.g., nitric acid, hydrogen peroxide, hypochlorite and others) [10], [11], [12].

Several material supports, such as black carbon, mesoporous carbon, carbon nanofibers and carbon nanotubes have been researched and used for the manufacture of commercially available metal electrocatalysts, as reported in the literature [9], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. However, the nature of the carbon structures modified with oxygen surface groups (functionalized carbon) and their interaction with the metal are not completely established.

The role of oxygenated groups on the formation of the dispersed Pt/C [16], [22], [23], [24] and Pt–Ru/C [25], [26], [27], [28] electrocatalysts has been investigated. Carmo et al. synthesized Pt–Ru electrocatalysts by the impregnation method and subsequent alcohol reduction on both as received carbon and functionalized carbon with H2O2 [27] and HNO3 [28] solutions. The authors found that the Pt–Ru electrocatalysts supported on functionalized carbon are more homogeneously distributed than other studied materials and the electrochemical results showed higher activity for the Pt–Ru/C. This latter effect was attributed to better nanoparticles distribution/utilization on functionalized carbon.

In the same material support but using other reduction method (ethylene glycol), the effect of the carbon treatment on the stability of Pt/C electrocatalysts was investigated by Chen et al. [23]. The authors showed that after oxidative treatments with H2O2 and HNO3 solutions, the carbon became rich in oxygen-containing functional groups. They observed also that oxidative treatments of the carbon increased the interaction between the metal particle and the support, and that resulted in an improved electrochemical stability of Pt/C electrocatalysts. They showed that the Pt/C electrocatalyst prepared on the H2O2 solution treated carbon exhibited a higher stability than that prepared on the HNO3 solution treated carbon. In other reference, using colloidal method with a solution of NaHSO3 to obtain a colourless soluble intermediate of platinum and ruthenium, which was then oxidized with H2O2, Pt–Ru/C electrocatalysts showed also better CO tolerance and superior methanol oxidation [25].

In other type of carbon specimen (Korea Black Carbon), the effect of the chemical treatment on the electrochemical behaviour of Pt/C electrocatalysts was studied [24]. The authors showed that the size and the loading level of Pt metal clusters were dependent on the surface characteristics of the carbon and the electrocatalytic activity of the Pt electrocatalysts was enhanced when the black carbon was treated by basic or neutral agents, while the activity decayed for the acid-treated black carbon supported Pt [29].

The ordered mesoporous carbon (OMC) treated with HNO3 solution for preparation of Pt/C and Pt–Ru/C electrocatalysts for the carbon monoxide and methanol oxidation reactions was also investigated [17], [18]. In this work, before deposition of the metals, the carbon was functionalized with the purpose to generate oxygenated groups for anchoring the Pt and Pt–Ru nanoparticles by the formic acid and borohydride reduction methods. The authors observed that CO stripping occurs at more negative potentials than for Vulcan XC-72R, and the best results for both methods were achieved with OMC functionalized with concentrated nitric acid for 0.5 h. Both Pt and Pt–Ru/OMC electrocatalysts presented better electrocatalytic activity towards CO and methanol oxidation.

In our previous studies [30], platinum electrocatalyst supported on functionalized carbon for methanol oxidation was prepared and characterized physically and electrochemically. The results showed that functionalized carbon using H2SO4 + HNO3 and HNO3 solutions contains more surface oxygenated functional groups than untreated carbon (Vulcan XC-72R), carbon treated with H2O2 solution or carbon thermally treated in helium atmosphere. In addition, the electrochemical results showed that the Pt electrocatalyst prepared on those functionalized carbons presented a significant improvement of the electrocatalytic activity due to the synergistic effect of carboxyl (–COOH), hydroxyl (–OH) and carbonyl (2 bonds on the lefthand sideCdouble bondO) groups on functionalized carbon and Pt metal nanoparticles.

Section snippets

Preparation of Pt–Ru electrocatalysts on functionalized carbon

Commercial carbon Vulcan XC-72R (Cabot), used as primary support material, was modified by chemical treatments (functionalized carbon) in order to create surface reactive groups. Then, the chemical treatment was done as follows: (i) H2O2 (30%, v/v) solution stirred at 60 °C for 10 h; (ii) 3 mol L−1 HNO3 solution and (iii) H2SO4 + HNO3 (1:1) solution in both cases at 80 °C for 12 h, followed by washing in hot water [30]. Previously, the carbon was kept in contact with the chemicals in an ultrasonic bath

Characterization of the electrocatalysts

The EDX compositions of Pt–Ru electrocatalysts on support functionalized carbon are shown in Table 1. It can be seen that the average EDX compositions of electrocatalysts are near the nominal value (Pt:Ru, 85:15). In a previous paper [35] it was mentioned that it was not possible to anchor more than 25 at.% of Ru on Pt–Ru-supported materials using formic acid as reducing agent. One possible explanation for this is the formation of a complex between Ru and formic acid which partially prevents its

Conclusions

Platinum–ruthenium alloy electrocatalyst supported on carbon (Vulcan XC-72R) treated with H2SO4 + HNO3, HNO3 and H2O2 solution, and carbon thermally treated in helium atmosphere for methanol oxidation was prepared and characterized physically and electrochemically. The functionalized carbons using acid solutions contain more surface oxygenated functional groups than carbons treated with H2O2 solution or HeTT. In addition, uniform dispersion of Pt–Ru nanoparticles on the surface of carbons by acid

Acknowledgments

The authors acknowledge Dr. Andrei Salak (Universidade de Aveiro) for performing the XRD measurements and Dr. A.P.S. Dias (ICEMS/IST) for the useful discussions. J.R.C. Salgado acknowledges the financial support of Portuguese Foundation for Science and Technology (FCT, Ciência 2008). Thanks are also due to the Synchrotron Light Brazilian Laboratory (LNLS, Campinas, SP, Brazil) for assisting with the HR-TEM experiments.

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