The following is a brief treatment of the thermodynamics governing the methanol oxidation reaction of a DMFC. Also, the impact of surface kinetics on the practical efficiency of the cell are presented. Some intriguing reports suggesting a new general direction for CO-tolerant catalyst development are cited [14,15,16].

Thermodynamic Optimum

When an organic fuel is used, essentially as a hydrogen source in a fuel cell, the expectation is that the fuel will be completely oxidized to carbon dioxide. For methanol, this is summarized thermodynamically [17] in terms of the reduction potentials as

CO2 + 6H++ 6e ^ CH3OHW+ H2O E0 = 0.016V (9.1)

While methanol is oxidized at the anode, oxygen is reduced at the cathode: O2 + 4H ++ 4e ^ 2H2O E0 = 1.229V (9.2)

The net cell reaction is

CH3OHW + 1.5O2 ^ 2H2O + CO2 (9.3)

where the standard cell potential (electromotive force, emf) is E0e„ = 1.229 — 0.016 = 1.213 V For a six-electron process (n = 6), the standard free energy is AG0 = — nFE0eU = -702.2 kJ/mol for methanol. With a molecular mass, M, of 0.03204 kg/mol, the theoretical specific energy for methanol is W = — AG0/(M x 3600s/hr) = 6.088kWh/kg; because the density of methanol is 0.7914 kg/l, this corresponds to an energy density of 4.818kWh/l. The standard enthalpy [17], AH0 = -726kJ/mol, is similar to AG0, consistent with a small entropy term.

Formally, the complete oxidation of methanol can be viewed thermodynam­ically as a series of two-electron/two-proton oxidation steps. The reduction poten­tials for this sequence in acid are given as [18]:

HCHO(aq) + 2H + + 2e ^ CH3OHw E0 = 0.232V (9.4)

HCOOH(aq) + 2H + + 2e ^ HCHO(aq) + H2O E0 = 0.034F (9.5)

H2CO3(aq) + 2H+ + 2e ^ HCOOH(aq) + H2O E0 = -0.166F (9.6)

where the oxidation of formic acid is reported to carbonic acid, consistent with the solubility of carbon dioxide and its equilibrium with carbonate [19].

CO2(g) ^ CO2W Kco2 = 0.034 (9.7)

CO2(aq) + H2O ^ HCO — + H+aq) pK = 6.36 (9.8)

The acidity constants for carbonic acid are pKa1 = 6.352 and pKa2 = 10.329; for formic acid, pKa = 3.745. Note that these reaction steps embed information about the complexities of the solution chemistry in the fuel cell as reaction products build and local pH changes. Note also, that reactions are reported in acid because practical DMFCs are usually run under acidic conditions. Under basic conditions, the formation of insoluble carbonates dramatically complicates the design of plant and limits applicability as electrolytes must be replaced as carbonate levels build.

For species in solution, the standard potentials (reactions 4 to 6) are such that thermodynamically, the oxidation of methanol proceeds cleanly and sequentially from alcohol to aldehyde to acid to CO2/carbonic acid with approximately 200 mV separating each successive two proton/two electron transfer. The specific energy and energy density of methanol are high. Thus, thermodynamically, the expectation is that methanol is an excellent fuel for a direct reformation fuel cell. However, the thermodynamics do not capture the complexity of the surface reactions that dictate the fate of methanol in a direct reformation fuel cell.

Realities of Surface Kinetics

The kinetic limitations of DMFCs have been well reviewed in detail from several different perspectives in recent years [17,20,21]; an early and thorough review is provided by Parsons and VanderNoot [22]. For effective utilization of methanol as a fuel, the catalyst must provide a good surface for adsorption of methanol and its sequential breakdown to carbon dioxide/carbonate through loss of paired protons and electrons. Under acidic conditions, this has largely restricted practical catalysts to platinum and its alloys and bimetallics. Methanol will adsorb to platinum and platinum serves as an excellent electron transfer catalyst. The difficulty is that platinum passivates as carbon monoxide by­product accumulates and adsorbs to the platinum surface. To oxidize carbon monoxide to carbon dioxide/carbonic acid, oxygenated species such as water must adsorb to the catalyst surface. Because platinum is not strongly

CO,

SCHEME 9.1 Reaction pathways for methanol oxidation.

hydrophilic, platinum bimetallics and alloys formed with more hydrophilic metals such as ruthenium are typically used to facilitate CO oxidation.

Consider the mechanistic constraints for oxidation of methanol. As in equa­tion 1, the complete oxidation of methanol to carbon dioxide proceeds by a six — proton, six-electron process. The mechanism presented in Scheme 9.1 outlines the basic route by which methanol is fully oxidized. The loss of paired protons and electrons is noted for each step. To account for all six electrons, recognize that the adsorption of water to the catalyst surface also generates an electron and proton. For a catalyst metal site, M,

M + H2O ^ M — OH + H + + e

Following the notation from Ref. [21], methanol first adsorbs to liberate one electron and one proton.

CH3OH + Pt ^ Pt — CH2OHads + H+ + e (9.10)

This is followed by two steps to form the formyl intermediate, — CHO.

Pt — CH2OH„* + Pt ^ Pt2CHOH + H+ + e

Pt2CHOH ^ Pt + Pt — CHO + H+ + e

On clean platinum surfaces, these oxidations proceed smoothly to provide two electrons and two protons. Consider Scheme 9.1. The weakly adsorbed -CHO is a point at which the oxidation mechanism breaks into two paths. One path yields adsorbed CO and the other adsorbed COOH. Adsorbed COOH is generated by reaction of — CHO and an adjacent M — OH to yield one proton and one electron and form weakly adsorbed — COOH. Adsorbed CO is generated by the direct oxidation of — CHO by one proton and one electron to form strongly adsorbed CO. Basic kinetic arguments would favor the strongly adsorbed CO over the weakly adsorbed — COOH because first, the oxidation of — CHO to — CO is direct and does not require an adjacent second species, M — OH, and second, because — CO is strongly bound and — COOH is weakly bound.

It should be pointed out that there is an alternative branch point in the oxidation process in which adsorbed — CHOH undergoes a one-electron and a one-proton oxidation to form adsorbed — COH.

Pt2CHOH + Pt ^ Pt3COH + H+ + e

The adsorbed — COH can then either undergo one-proton/one-electron oxida­tion to adsorbed — CO or react with an adjacent M — OH to form HCOOH in solution. Neither process leads to the efficient oxidation to carbon dioxide/car — bonic acid.

To the extent the platinum surface is passivated by CO, the reaction is terminated. Thus, the design of a system for the efficient and complete oxidation of methanol can be approached in two ways.

The first approach is to circumvent the formation of adsorbed CO by favoring the formation of — COOH. Experimentally, this is done by enhancing the proba­bility that — CHO is adjacent to an oxygen source, M — OH, by using bimetallics and alloys of platinum where M is more hydrophilic than platinum. There are questions of stability and cost associated with these catalysts although they have been shown to enhance conversion efficiency. But, based on the relative strengths of the adsorbates — CO and — COOH and the need for an additional catalyst site (M — OH), this approach poses some challenges.

The second approach is to consider why — CO is so difficult to oxidize; that is, why does CO adsorb so strongly. Thermodynamically, the oxidation of CO to CO2 in solution occurs at low potential [18].

CO2 + 2H+ + 2e ^ CO + H2O (1) in E0 = -0.106F (9.14)

But, the oxidation of CO on platinum in acidic solution occurs 600 to 700 mV positive of this value; Pt-Ru alloys are shown to oxidize CO at 200 to 300 mV lower overpotential than Pt [23]. The oxidation of adsorbed CO is strongly disfavored. There are two ways to think about overcoming this large overpotential. One is to design better catalysts. One common approach has been through the bifunctional mechanism where the bimetallic catalyst is designed to place Pt — CO adjacent to an oxygen source through M — OH. The other approach would rely on a paradigm shift in how the oxidation of -CO is viewed at a more fundamental level; better understanding could lead to better catalysts [14,15,16].

The above discussion is provided in a very general manner. Many factors significantly impact the catalytic efficiency of the conversion of methanol to carbon dioxide/carbonic acid. This includes surface structure, catalyst size, and catalyst crystal face as well as the history of the cell, the current coverage of CO, the pH, and the time since the start of the cell.