Discovery Of Methanol Electrooxidation Catalysts By Combinatorial Analysis

T.E. Mallouk*1, E. Reddington1, C. Pu2, K. L. Ley2, and E.S. Smotkin*2 'Deparmcnt of Chemistry The Pennsylvania State University, University Park, PA 16802 and department of Chcmical Engineering Illinois Institute of Technology , Chicago, IL 60616

Hydrogen fuel cclls are likely to become a major energy source in the next century, but they are not ideal for all applications. A safe alternative fuel with a high energy density will be necessary for transportation and mobile applications. Direct mcthanol-air fuel cells (DMFCs) arc an attractive alternative to hydrogen fuel cclls bccause of the high energy density and low cost of methanol as a fuel [1], However, in order for DMFCs to become commercially viable, better clcctrocatalysts for the anode reaction need to be developed.

In a DMFC, methanol and water undergo a six electron oxidation [2] at the anode to produce carbon dioxide and protons (1),

CH3OH(aq) + H20(1)----> 6H*(aq) +C02(g) +6e", E° =+0.04 V vs. NHE (1).

Platinum is the most efficient single metal catalyst for this reaction. The generally accepted reaction mechanism [3], (2)-(4),

COldj + OH^ ---> C02(g) + H*(aq) + e (4), involves adsorption and partial oxidation of methanol on the surface of the electrode (2). Adsorbed OH from hydrolysis of water (3) reacts with the adsorbed CO to produce carbon dioxide and a proton. At low overpotentials, the slow step in the sequence is (3). Platinum electrodes suffer from many drawbacks, such as expense, low current density, and rapid formation of a CO poisoning layer.

It has been shown that a ruthenium/platinum alloy electrochemically oxidizes methanol more efficiently than pure platinum [4], Ru improves the kinetics of the slow step of the reaction (3) by dissociating water at a lower ovcrpotential than Pt. Alloying ruthenium with platinum also improves the catalyst resistance to CO poisoning. In addition to ruthenium, many alloys of Pt and different metals have been tested for catalytic ability, including Ag [5], Au [5], and Pd [6]. Ad-atoms of different clements[7], such as Bi [8], Sn [9], and Ru [10], on Pt have also been tested for catalytic ability. In general, these modifications only slightly improve the kinetics of electrochemical oxidation of methanol, if at all. The best known catalysts at present are Pt(50)/Ru(50) (mole ratio) [11] and a recently developed Pt/Ru/Os ternary composition [12],

There is at present no way to calculate the chcmical composition of different metals that will afford the best catalyst for this reaction, although knowledge of phase equilibria and heuristic bond strength/activity relationships do provide some guidance in the search for effective ternary compositions [12], In the abscnce of a more general and predictive model, an Edisonian approach to the problem is appealing. Combinatorial methods, which have been used extensively in bio-organic systems, provide a way to screen a very large number of multi-componcnt catalysts simultaneously. Binary systems of platinum and other metals have been well studied, but with one exception [12], ternary and higher systems have not been investigated, and offer a ripe area for combinatorial discovery.

There are only a few examples in the literature of combinatorial approaches to materials discovery. Schultz and co-workcrs searched for magnetoresistive [13] and superconducting [14] properties of different mixtures of metal oxides, and Natan and co-workcrs [15] developed a way to deposit colloidal gold and silver on a substrate with varying densities. The area is currently undergoing rapid growth, as combinatorial libraries of thermal catalysts and other materials are now being prepared. However, there has been to our knowledge no previous application of combinatorial methods to problems in electrocatalysis.

This combinatorial technique involves generating an array of electrodes with varying metal compositions, on a conductive substrate. Generally, to test the effectiveness of an electrocatalyst, one measures the current as a function of potential. This would be a time-consuming, serial process for arrays containing many elcctrocatalyst compositions, and would not have the combinatorial advantage of parallel screening. Instead, an indirect measurement was used which allows optical imaging of the activity of an array of electrocatalysts. The clectrochcmical oxidation of methanol (or any other organic molecule) involves the generation of protons at the elcctrode. Therefore, the local pH in the diffusion layer at the electrode surface drops considerably. When the potential of an array electrode is swept slowly from cathodic to anodic potential, the best catalyst compositions (those that oxidize methanol at lowest ovetpotcntials) generate acid first. A solution phase, fluorescent pH indicator can then be used to detect catalysts that most efficiently oxidize methanol.

We report here a proof of concept of the application of combinatorial methods, to the discovery of optimized ternary catalysts for methanol electro-oxidation. Toray carbon paper was used as a substrate for the elcctrode arrays because it is electrically conductive, but not catalytic. It is composed of thin carbon fibers that help to hold the catalyst on its surface. These substrates were cut into triangular shapes, and solutions of metal salts were applied with a microliter syringe such that each spot contained the appropriate molar ratio of three different metals. The salts were then rcduccd to zero-valcnt metals by addition of a sodium borohydride solution [16]. After rinsing and drying, a ternary array of three metals has been prepared, with a simple map (much like a ternary phase diagram) describing the chcmical composition of each spot. Ternary combinations of five transition elements (Pi, Ru, Os, Rh, Pd) were studied. When the metal salts were reduced individually by borohydride, only platinum showed a powder x-ray diffraction pattern. The individual particles produced in this way arc therefore either amorphous, or the size of the crystalline domains is too small to give an x-ray pattern. The individual metals were also characterized by comparing their cyclic voliammograms (CVs) in sulfuric acid to crystalline samples. All CVs of the amorphous metals appeared similar to that of the microcrystalline metals, exccpt for Os. An element map performed by SEM/EDAX shows that the three metals are mixed intimately on the 200 nm scale.

The oxidation of methanol produces protons according to reaction (1). At the concentrations and current densities used in this study, the local pH around the electrode is calculatcd to drop considerably (to about -0.5 or 0.5) with efficient catalysis. A fluorescent pH indicator that is not fluorescent in base, but strongly fluorescent in acid is the ideal molecular probe for this reaction.

Efficient catalysts were detected experimentally as follows. The electrode array was electrically contacted and, the contacts were insulated with cpoxy resin. The triangular array was then immersed, face-up, into an aqueous solution of methanol, electrolyte, and fluorescent indicator. The electrode array is the working clectrode in a typical three electrode cell, with a platinum countcr electrode and an SCE reference clectrode. The pH of the indicator solution was adjusted to bo slightly higher than the indicator pKa, i.e.,

the iniial state is non-fluoresccnt. A hand-held UV lamp was used to generate UV (354 nm) light. A single potential sweep was conducted, starling at cathodic and ending at anodic potentials, where all the electrodes in the array oxidize methanol. Efficient catalysts arc those that cause the solution dircctly above the electrode to fluoresce at lowest overpotcntial, and were dctccted by visual inspection. The chcmical composition of the most efficient catalyst in an array was known from its location, and larger individual electrodes of the same composition were then prepared. The i-V characteristics of these individual electrodes were then compared with those of Pt(50)/Ru(50), and apparent heterogeneous rate consiants were determined by rotating disk vollammetry.

Initial results using acridine as the indicator were quite promising. Acridine, pKa = 5.0, fluoresces a weak violet in base, and fluoresces bright green in acid. Quinine, pKa = 5.5,

N acridine also fluoresces brightly in acidic solutions. Electrode arrays that each contained 15 different ternary compositions were prepared from the five metals studied. The binary "edges" of each ternary phase diagram were eliminated in order to allow more thorough evaluation of ternary compositions. Assessment of the possible ternary compositions of Pt, Ru, Os, Rh, and Pd required 9 arrays and a total of 135 unique compositions. Some compositions of Pt/Os/Rh oxidized methanol very effectively at low overpotentials. Individual elcctrodes were prepared for the most promising compositions, and the i-V curves were then compared with those of Pt(50)/Ru(50). At high pH, the Pt(60)/Os(25)/Rh(15) catalyst (Figure 1) oxidizes methanol more efficiently than the

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