Methanol oxidation reaction activities

The presence of defects opens new pathways which significantly decrease the thermal stability of the reconstructed Rh(llO) surfaces [39]. Although the activity of electrochemical oxidation of carbon monoxide and methanol electro-oxidation can be increased by increasing surface steps on Pt nanoparticles, the oxygen reduction reaction activity of the 2-nm-sized Pt nanoparticle has been found insensitive to the step area, as shown in Fig. 20.3 [40], in contrast to the methanol oxidation reaction activity. 

The electrocatalytic activity of the nanostructured catalysts was investigated for electrocatalytic reduction of oxygen and oxidation of methanol. Several selected examples are discussed in this section. The results from electrochemical characterization of the oxygen reduction reaction (ORR) are first described. This description is followed by discussion of the results from electrochemical characterization of the methanol oxidation reaction (MOR). 

Since platinum in its pure state, and either alloyed or in mixtures with other metals/metal oxides, (which act as promoters), is among the most active materials for methanol oxidation, much attention has been devoted to the nature of, and mechanism involved in, the methanol oxidation reaction on platinum. As such, platinum has served as a useful model system illustrating the general features of metal electro-oxidation in an aqueous environment. There are many postulated mechanisms for the oxidation of methanol, and detailed descriptions of the same can be found in the literature [55-59] and will not be discussed in the present work, except from the point of view of contributions of IR and STM toward the understanding of the overall picture of electrocatalysis at model electrodes. 

Wieckowski et al. reported the spontaneous deposition Ru adlayers from RuO" solutions on three low-index Pt surfaces. The maximum coverage of Ru on these adlayer is about 20%, and potential must be applied to reduce the Ru adlayer to metallic Ru. The Ru-decorated Pt nanoparticles showed considerable catalytic activity in the methanol- oxidation reaction. We will discuss the catalytic properties of the Ru-decorated Pt nanoparticles in Section V.3. 

The work presented shows that an increase of the electrocatalytic activity can be obtained, if a suitable method for the catalyst synthesis is employed. In this sense, the Alcohol Reduction Method showed a positive effect, probably due to the good particle dispersion at the carbon surface and the suitable particle size distribution that this method produces. For the methanol oxidation results, an increase in the cell potential by PtRu/C electrocatalyst on Vulcan XC72 system was observed compared to the PtRu/C E-TEK formulation. This can be explained due to the better conductivity of this Carbon Suport, enhancing the speed of the electron transference in the Methanol Oxidation Reaction (MOR).These results can also be attributed to the good particle distribution at... 

Recently, utilization of CNFs and CNTs in the CLs of PEMFCs and DMFCs has become an active research area. Numerous studies have focused on the stabilization of Pt and PtRu particles on the surface of CNTs and CNFs (see the review articles [153 and 230] and references therein). Adhesion of Pt to the basal plane of graphite is weak thus various approaches have been proposed to activate CNTs, including oxidation, grafting with various functional groups [153], and wrapping with a polymer [231], in order to stabilize highly dispersed Pt on their surfaces. Pt and Pt-Ru particles supported on CNTs and CNFs were tested in the oxygen reduction reaction and in the methanol oxidation reaction. 

K. Bouzek, KM. MaDgoId, and K. Jiittner, Electrocatalytic activity of platinum modified poly-pyrrole films for the methanol oxidation reaction, J. AppL Electrochem., 31, 501-507 (2001). 

In addition to in situ reduction by the CPs, metal nanoparticles can also be synthesized on the surface of the CPs via a reduction process in the presence of other reducing agents. Yan and co-workers prepared PANI/Pt composite nanofibers by reducing a Pt salt with ethylene glycol on the surface of PANI nanofibers [78]. The diameter of PANI nanofibers was about 60 nm, and that of as-synthesized Pt nanoparticles (PtNPs) was only about 1.8 nm as calculated from X-ray diffraction (XRD) data. The small size of PtNPs on the CP matrix enhanced the electrocatalytic activity for the methanol oxidation reaction. 

Figure 5.9 shows the voltammetric behavior of Pt/C (a) and Pd-Co-Pt/C (b) towards ORR in the presence and absence of methanol. As can be also observed, the presence of 0.5 mol methanol causes a negative shift of 50 mV in the halfwave potential, in contrast to Pt/C for which there is a severe loss of activity. The linear scan voltammograms of the methanol oxidation on all the investigated materials in 0.5 mol H2SO4 + 0.5 mol L CH3OH solution, showed that the current densities of the methanol oxidation reaction on Pd-Co-X alloy catalysts (X = Au, Ag, Pt) diminish to values much lower than for Pt/C catalyst, and the onset of methanol oxidation occurs at more positive potentials, demonstrating the lowered MOR activity of the Pd-Co-Pt alloy catalysts. 

Furthermore, surprisingly, very few works have been reported on the study of materials degradation phenomena in DAFCs. For instance, a model for carbon and Ru corrosion in a DMFC anode under strong methanol depletion has been very recently proposed by Kulikovsky [195]. The model is based on the mathematical description of the current conservation in the membrane. In the methanol-depleted domain, methanol oxidation reaction is substituted by the carbon oxidation (corrosion). This is supposed to dramatically lowering the membrane potential and to create an environment for electrochemical oxidation of Ru. His calculations show that 50-100 mV loss in the cell potential manifests quite a significant (above 50 %) methanol-depleted fraction of the cell active area (Fig. 8.21). 

CrN- and TiN-supported Pt have also been studied for methanol oxidation [69, 87]. The supports were synthesized by ammonolysis of the parent oxide under flows of ammonia using slow ramp rates and high temperatures (>700 C). Surface areas for the materials were 28 m g for TiN and 72 m g for the CrN. Electrocatalytic oxidation studies demonstrated that both supported Pt materials had higher catalytic activity compared to the standard Pt/C materials. This can be attributed to the higher conductivity of the nitride over the carbon as well as a possible synergy between the support and the Pt for the methanol oxidation reaction. Martinez-Huerta and [88] coworkers have also observed enhanced catalytic activity for acidic methanol oxidation when nitrogen is featured into a Ti support. 

Platinum has the highest catalytic activity for the methanol oxidation reaction (MOR) of any of the pure metals both in acid and in alkaline media. An excellent review on the MOR on Pt and Pt-based catalysts in alkaline media was done by Spendelow and Wieckowski [20]. Concisely, the enhanced activity in alkaline media results from the lack of specifically adsorbing spectator ions in alkaline solutions, and the higher coverage of adsorbed OH at low potential, which is required for methanol oxidation [20]. 

However, the important criteria for WC to be implemented in fuel cells are its surface area, phase, and porosity. Ganesan and Lee reported that WC with a surface area of 170 m /g was obtained by thermal method, but the product tuned to be containing more sub-tungsten carbide (W2C) [70]. The latter was used to support Pt catalyst for methanol oxidation reaction. No test was done for ORR. Nevertheless, authors believed that oxide layer formed on carbide support is the key player in promoting alcohol oxidation by providing oxygen species as indicated by the decrease in desorption temperature of CO. In a different study carried out by the same group, mesoporous WC was synthesized and used as a support for Pt [71]. The mesoporosity was introduced by addition of surfactant like cetyltrimethylammonium bromide (CTABr). Catalyst performance was evaluated under identical conditions as previously stated however, no statement has been reported on ORR activity and electrochemical stability in both cases [70, 71]. 

Because of the significant activation losses from both the methanol oxidation reaction (around 0.25 V loss) and the oxygen reduction reaction (around 0.25 V loss), the cell voltage drops to around 0.65 V when a DMFC generates a very small current density. For a working DMFC operated under 100°C, its voltage is normally kept around 0.40 V, which means that the electrical efficiency is about 33% with the remaining 67% as heat. 

There is another fuel cell working under the ambient condition, that is, direct methanol fuel cells (DMFCs). Difference in the PEFCs and DMFCs is their anode fuels (the cathode fuel is oxygen in both cases). In the DMFCs, methanol (CH3OH) is supplied to the anode instead of the hydrogen for the PEFCs and this difference is crucial for their ceU performances. Although the Pt is known to be an active catalyst for both HOR and methanol oxidation reaction (MOR), kinetics of the MOR is much slower than that of the HOR and ORR on the Pt catalyst, which increases anode overpotential and gives an inferior cell performance in the DMFCs as demonstrated in Fig. 1. Therefore, an important research topic is... 

---