Electrooxidation of Methanol

A single cell in a DMFC has a maximum thermodynamic voltage of 1.18V at 25°C. The reacdons that take place in a DMFC are given below  

Understanding the nature of this overpotential is a key feature to the development of better fuel-cell catalysts, and also involves understanding the reactivity of intermediates in methanol oxidation. Spectroscopic studies have shown that the electrooxidation of CH3OH (Eq. 9.10) on Pt is thought to follow a dual path mechanism at sufficiently high potentials that involves both indirect and direct pathways (Fig. 9.4). The indirect path, which proceeds through the formation of CO, is shown in the center of Fig. 9.4 

Therefore, a catalyst for methanol oxidation should be able to (a) dissociate the C-H bond and (b) facilitate the reaction of the resulting residue with some O containing species, especially CO, to form On a pure Pt electrode, which is known to be a good cat- 

The second process (b) is crucial and requires dissociation of water, which is the oxygen donor for the reaction. On a pure Pt electrode, sufficient interaction of water with the catalyst surface is only possible at potentials above 0.4-0.45V w. RHE. 

on pure Pt, methanol oxidation to CO2 cannot begin below 0.45V. However, the adsorbate layer does not exhibit good reactivity below at least 0.7V, i.e. at potentials without technological interest. Several binary and ternary catalysts have been proposed for methanol oxidation, most of them based on modifications of Pt with some other metal including Ru, Mo, Rh, Os, Sn, Ni, Zr, Mo, W, Ti, and Ir. 

---

From the results obtained with in situ reflectance spectroscopy and on-line analytical methods, investigators at Universite de Poitiers proposed a complete mechanism for the electrooxidation of methanol at a platinum electrode. The first step of the electrooxidation reaction is the dissociative adsorption of methanol, leading to several species according to the following equations ... 

The crucial aspect is thus to determine the fate of the ( CHO), species. Possible mechanisms for its oxidative removal are schematically shown in Fig. 9. From this scheme, it appears that the desorption of the formyl species can follow different pathways through competitive reactions. This schematic illustrates the main problems and challenges in improving the kinetics of the electrooxidation of methanol. On a pure platinum surface, step (21) is spontaneously favored, since the formation of adsorbed CO is a fast process, even at low potentials. Thus, the coverage... 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

Since oxidation of methanol is an electrocatalytic reaction with different adsorption steps, interactions of the adsorbed species with the metallic surface are important. Using platinum single-crystal electrodes, it has been proven that the electrooxidation of methanol is a surface-sensitive reaction. The initial activity of the Pt(llO) plane is much higher than that of the other low-index planes, but the poisoning phenomenon is so rapid that it causes a fast decrease in the current densities. The... 

The effects of dispersion of the electrocatalyst and of particle size on the kinetics of electrooxidation of methanol have been the subject of numerous studies because of the utilization of carbon support in DMFC anodes. The main objective is to determine the optimum size of the platinum anode particles in order to increase the effectiveness factor of platinum. Such a size effect, which is widely recognized in the case of the reduction of oxygen, is still a subject of discussion for the oxidation of methanol. According to some investigators, an optimum of 2 nm for the platinum particle size exists, but studying particle sizes up to 1.4 nm, other authors observed no size effect. According to a recent study, the rate of oxidation of methanol remains constant for particles greater than 4.5 nm, but decreases with size for smaller particles (up to 2.2 nm). 

The use of adatoms of foreign metals obtained by imderpotential deposition on the platinum surface is another convenient method for investigating the effect of a promoter on the electrocatalytic properties of platinum. However, the effect of adatoms in this case has been shown to be not as effective for electrooxidation of methanol as for the oxidation of other organic molecules such as formic acid adatoms of tin, however, showed a positive effect on the rate of methanol oxidation. ... 

The mechanism of electrooxidation of methanol is now nearly well understood. From the considerable effort made during the past 20 years, it is now possible to propose electrocatalysts with acceptable activities for DMFCs, even though further improvement is still necessary. Despite considerable research efforts, R-Ru alloys are the only acceptable catalysts for the electrooxidation of methanol at low anode potentials. Two questions still remain unanswered ... 

The extensive state of knowledge of the electrooxidation of methanol, as presented in this section, offers prospects of tailoring new multimetallic... 

Borkowska Z, T3rmosiak-Ziehnska A, Shul G. 2004a. Electrooxidation of methanol on polycrystaUine and single crystal gold electrodes. Electrochim Acta 49 1209-1220. 

Lamy C, Leger JM, Clavilier J. 1982. Structural effects in the electrooxidation of methanol in alkahne medium. Comparison of platinum single crystal and polycrystalline electrodes. J Electroanal Chem 135 321-328. 

Yajima T, Uchida H, Watanabe M. 2004. In-situ ATR-FTIR spectroscopic study of electrooxidation of methanol and adsorbed CO at Pt-Ru alloy. J Phys Chem B 108 2654-2659. 

Dubau L, Coutanceau C, Gamier E, Leger JM, Lamy C. 2003a. Electrooxidation of methanol at platinum-mthenium catalysts prepared from colloidal precursors Atomic composition and temperature effects. J Appl Electrochem 33 419-429. 

Watanabe M, Uchida M, Motoo S. 1987. Preparation of highly dispersed Pt-I-Ru alloy clusters and the activity for the electrooxidation of methanol. J Electroanal Chem 229 395-406. 

Figure 13.7 Potential-step electrooxidation of methanol on a Pt/Vulcan thin-film electrode (7 ixgptcm , geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M CH3OH upon stepping the potential from 0.16 to 0.6 V (electrol3de flow rate 5 p,L s at room temperature). (a) Solid line, faradaic current transients dashed line, partial current for CH3OH oxidation to CO2 dotted line, difference between the net faradaic current and that for CO2 formation.

Shukla AK, Ravikumar MK, Roy A, Barman SR, Sarma DD, Arico AS, Antonucci V, Pino L, Giordano N. 1994. Electrooxidation of methanol in sulfuric-acid electrolyte on platinized-carbon electrodes with several functional-group characteristics. J Electrochem Soc 141 1517-1522. 

Micro)titer plate and indicator (dye, fluorescent, etc.) 1. Catalytic electrooxidation of methanol + pH indicator3 2. Catalytic hydrosilylation of alkenes and imines + dyeb... 

The electrooxidation of methanol has attracted tremendous attention over the last decades due to its potential use as the anode reaction in direct methanol fuel cells (DMFCs). A large body of literature exists and has been periodically reviewed [130,131,156], [173-199]. Unlike for formic acid, a generally accepted consensus on the specific mechanistic pathways of methanol electrooxidation is still elusive. 

Figure 6.33. Trends in Pt surface-area normalized electrochemical activity of various ternary alloy compositions with respect to the electrooxidation of methanol. Activity gains are seen in the order Pt, PtRu, PtRuNi, and PtRuCo (adapted from [69]).

Rotating disk electrode (cont.) diffusion coefficient, 1141 diffusion layer in, 1234 disk current in, 1141 ECE reactions determination by. 1144 electrooxidation of methanol, 1139 kinematic viscosity, 1141,1234 intermediate radicals, determination of. 1139. 

The electrodes in the direct methanol fuel cell (DMFC) (i.e. the anode for oxidising the fuel and the cathode for the reduction of oxygen) are based on finely divided Pt dispersed onto a porous carbon support, and the electrooxidation of methanol at a polycrystalline Pt electrode as a model for the DMFC has been the subject of numerous electrochemical studies dating back to the early years ot the 20th century. In this particular section, the discussion is restricted to the identity of the species that result from the chemisorption of methanol at Pt in acid electrolyte. This is principally because (i) the identity of the catalytic poison formed during the chemisorption of methanol has been a source of controversy for many years, and (ii) the advent of in situ IR culminated in this controversy being resolved. 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

Table 6.1. Physical parameters obtained from impedance spectroscopy of the electrooxidation of methanol [46], (Reprinted with permission from Journal of Physical Chemistry B 2001 105(5) 1012-25. 2001 American Chemical Society.)...

Liao S, Linkov V, and Petrik L. Electrooxidation of methanol over a membrane-based electrode and effect of tungsten and molybdenum on the activity. Appl CatalA General 2002 235 149-155. 

Many redox reactions by colloidal nanoparticles have been reported. Three of the most-studied reactions are (1) the catalyzed electron transfer between ferricyanide and thiosulfate [8,19-21], (2) the catalytic reduction of fluorescent dyes by sodium borohydride [22, 23], and (3) the catalytic reduction of organic compounds (e.g., nitro-aryls [9] and alcohols [24]). These reactions have been studied extensively because they are easy to follow spectroscopically allowing for straightforward measurement of reaction kinetics. The third set of reactions has enormous industrial significance, where nitro compounds are reduced to their less toxic nitrate or amine counterparts [25, 26] and the electrooxidation of methanol is utilized for methanol fuel cells [27, 28]. 

T. Frelink, W. Visscher, J.A.R. Vanveen, Particle size effect of carbon-supported platinum catalysts for the electrooxidation of methanol. J. Electroanal. Chem. 1995, 382(1-2), 65-72. 

The DMFC operates, as its name suggests, by direct, complete electrooxidation of methanol to CO2 at the cell anode. The methanol anode is coupled in the DMFC with an air cathode, completing a cell schematically shown in Fig. 50. In the majority of recent development efforts, the DMFC has been based on a protonconducting polymeric membrane. The uniqueness of this type of fuel cell is the direct anodic oxidation of a carbonaceous fuel. Such a direct electrochemical conversion process of liquid fuel and air to electric power at low temperatures can provide a basis for a very simple fuel-cell system. 

---