Methanol Oxidation Electrocatalysis

Free Power Secrets

Making Your Own Fuel

Get Instant Access

The direct electro-oxidation of methanol is attractive from a number of standpoints. The direct conversion of an oxygenated hydrocarbon fuel alleviates the need for a fuel processor system producing hydrogen-rich gas. Methanol is a liquid fuel and as such has a high energy density per unit volume. It can be produced from both fossil fuels and biomass (compare Section 5.8.2). Although the thermodynamic potential for the full electro-oxidation of methanol in acid electrolytes is close to that of hydrogen oxidation, the overall reaction is much more demanding due the multi-electron transfer to carbon dioxide. The half reaction is:

In general, overpotentials on the best catalyst surfaces are high (>200 mV) at PEMFC operating temperatures. This reflects the need to dehydrogenate and insert oxygen into the adsorbed methanol fragment, as well removing six electrons from each molecule. Since the transfer of these electrons does not occur simultaneously, but in a step-wise manner, this can give rise to the formation of surface adsorbed species, which act as poisons for subsequent methanol adsorption and oxidation.

Clean Pt surfaces initially show very high activity for methanol oxidation, but these very rapidly decay in current on the formation of strongly bound intermediates. These intermediates are only removed on going to high overpotentials where they are oxidized. The identification of these intermediates has been the subject of much experimentation and discussion. In recent years, it been concluded that CO is the most widely found methanol residue. As such, its removal at low potentials has much overlap with the area of CO tolerance in reformate PEMFCs.

The generally accepted mechanism of methanol oxidation on Pt catalysts proceeds with the electrosorp-tion of methanol, followed by proton and electron stripping. On low index planes of Pt, these initial steps are considered to be rate determining. Further, removal of protons gives rise to bound CO. Water is coadsorbed at sites adjacent to the bound CO, and oxygen transfer occurs to give carbon dioxide, which desorbs from the catalyst surface. At potentials below ca. 450 mV, the surface of Pt becomes poisoned with a near-monolayer coverage of CO, and further adsorption of water or methanol cannot occur. Hence, the methanol oxidation rate drops to an insignificant level (Jarvi and Stuve, 1998).

The development of advanced Pt-based catalysts has focused on the addition of a secondary component (e.g., Ru, Sn, W, Re) that is able to provide an adsorption site capable of forming OHads species at low

Half Cell Specific Activity Data for DMFC Anode Materials

Half Cell Specific Activity Data for DMFC Anode Materials

FIGURE 6.9 Half-cell comparison of Pt vs. PtRu catalysts for MeOH oxidation.

potentials adjacent to poisoned Pt sites (Wasmus and Kuver, 1999). This adsorption site is also less effective at adsorbing methanol itself. The OHads is then able to react with the bound CO to produce CO2 and free sites for further methanol adsorption.

For promoters such as Ru, stable methanol oxidation currents occur at significantly lower potentials (<250 mV) to Pt, indicating the Ru is capable of the formation of OHads without itself being poisoned by CO. Indeed, at present, the most active catalysts are based on platinum ruthenium alloys. Figure 6.9 shows a comparison between the methanol electro-oxidation activity of carbon-supported Pt and PtRu at 80°C in H2SO4 electrolyte. As can be seen, the PtRu catalyst shows oxidation activity at significantly lower overpotentials.

The acid direct methanol fuel cell (DMFC) was pioneered by Shell Research in the U.K. during the 1960s and 1970s (compare also Section 2.6). After recognizing the issues with operating with platinum anode catalysts, the Shell researchers attempted to develop more active catalysts. They found that only PtRu and PtRh gave effective performances. Subsequently, PtRu black electrodes were developed at a loading of 10 mgcm-2. A review of Shell's efforts in this area has recently been published (McNicol et al., 1999).

In general, the current anode catalysts of choice for DMFC are unsupported PtRu blacks. The use of blacks offers a high concentration of active sites adjacent to the membrane and is considered necessary for high performance. However, unsupported catalysts can suffer from relatively poor surface areas when compared to carbon blacks and are generally used at relatively high electrode loadings (Dinh et al., 2000).

Recently, it has been demonstrated that carbon-supported PtRu catalysts can give equivalent performances to PtRu blacks at much lower electrode loadings, allowing effective operation at 1 mgPtcm-2 anode loading on air at low stoichiometries (Baldauf and Preidel, 1999). Figure 6.10 shows a comparison of carbon-supported PtRu and unsupported PtRu black anodes when tested as DMFC anodes in MEAs. A comparison of resistance-corrected anode performances shows identical performances of the two anodes, although the PtRu/C anode was at half the Pt loading. The carbon-supported PtRu MEA actually shows better overall performance due to differences in MeOH crossover.

Various studies of the Pt:Ru ratio have been carried out, and, despite well-characterized bulk PtRu alloy studies indicating that a Pt66Ru33 surface is optimal for MeOH oxidation at 60 to 80°C, in general 50:50 ratios are found to be give the best activity with high-surface-area unsupported catalysts (Chu and Gilman, 1996; Takasu et al., 2000). There has been much debate over the nature of high-surface-area catalysts and the optimum surface composition necessary for high activity. It has been recognized that oxidized Ru plays a key role in promoting the activity of Pt for MeOH activity. However, much of the debate has centered on whether PtRu alloys are necessary or whether mixtures of Pt and oxidized Ru (as hydrous oxides) are sufficient. Given that PtRu alloys offer the ability to intimately mix Pt and Ru sites, and that the onset of MeOH oxidation occurs at potentials where Ru is in an oxidized form, it is likely that alloys are the preferred surface.

Direct Methanol Anode Performance at 90 C, 49cm2 Single Cell Hardware

Cathode Pt black4mgcm2, Nafion 117Membrane 0.75M Methanol/Air stoich. 4.5/5, Cathode Pressure 30psi

Direct Methanol Anode Performance at 90 C, 49cm2 Single Cell Hardware

Cathode Pt black4mgcm2, Nafion 117Membrane 0.75M Methanol/Air stoich. 4.5/5, Cathode Pressure 30psi

100 200 300 400 500 600 700 800 Current Density (mA/cm-2)

FIGURE 6.10 Comparison of unsupported and carbon-supported PtRu anodes for MeOH/air single cell performances.

100 200 300 400 500 600 700 800 Current Density (mA/cm-2)

FIGURE 6.10 Comparison of unsupported and carbon-supported PtRu anodes for MeOH/air single cell performances.

A recent report suggested that Ru hydrous oxides play both a promotional role and a proton transport role, as they have been shown to act as proton conductors (Rolison et al., 1999; Long et al., 2000). Therefore, it has been suggested that Ru aids the transport of protons from the catalyst layer to the membrane, thus enhancing activity. However, recent in situ EXAFS results at low potentials demonstrate that Ru is generally present as metallic Ru in PtRu catalysts, indicating that bulk Ru hydrous oxides do not play a significant role in MeOH oxidation catalysis (Russell et al., 2001).

Given that PtRu materials for methanol oxidation have remained the catalysts of choice for over 30 years, much work is currently targeted at the identification of superior catalyst formulations. The development of combinatorial methods for electrocatalyst discovery has started with methanol oxidation catalysts. Initial work has focused on ternary and quartenary formulations based on PtRu alloys. Exploring the Pt-Ru-Os-Ir composition space has led to the identification of a Pt47Ru29Os20Ir4 composition, which showed significantly better MeOH oxidation performance than a number of other formulations including Pt50Ru50. The study showed that MeOH oxidation activity appeared very sensitive to composition, with a Pt56Ru20Os20Ir4 formulation performing poorly compared to Pt50Ru50 (Gurau et al., 1998). More recent work using combinatorial methods has identified several ternary and quaternary formulations, again based on PtRu compositions, which in screening experiments appear to have superior activity to PtRu. However, no single cell performances have yet been reported (Gorer, 2000a, b, and c).

Was this article helpful?

0 0
Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment