In this study, we provide electrochemical and X-ray fluorescence evidence of ruthenium crossover in direct methanol fuel cells using a state-of-the-art Pt-Ru alloy catalyst at the anode. We find ruthenium susceptible to leaching out from the highly active Pt-Ru black catalyst, crossing the proton-conducting Nafion membrane and redepositing at the Pt cathode on the opposite side of the fuel cell. After first detecting this phenomenon in a direct methanol fuel cell (DMFC) stack with a history of cell-voltage reversal, we have since observed ruthenium crossover under virtually all DMFC operating conditions, from single cell break-in (humidification) to stack life testing. The degree of cathode contamination by ruthenium species (of chemical form yet unknown) depends on, among other factors, the DMFC anode potential and the cell operating time. Once deposited at the cathode, ruthenium inhibits oxygen reduction kinetics and the catalyst’s ability to handle methanol crossover. Depending on the degree of cathode contamination, the overall effect of ruthenium crossover on cell performance may be from as little as ∼40 normalmV up to 200 mV. © 2004 American Institute of Physics. All rights reserved.

Electrochemical impedance spectroscopy ͑EIS͒ was used as a diagnostics tool for direct methanol fuel cell ͑DMFC͒ single cells and stacks, capable of separating individual contributions to the overall polarization of fuel cells under load. Anode impedance spectra were interpreted assuming porous electrode model and a reaction mechanism involving one adsorbed intermediate. No evidence of methanol transport limitations was found at the anode under operating conditions tailored for portable applications of DMFCs. Anode experiments revealed substantial poisoning of the electrode by methanol-derived surface species ͑CO͒ and negative order of methanol oxidation in methanol concentration. Because of the clear indication of the presence of adsorbed species at the cathode, the cathode process was assumed to be a combination of oxygen reduction reaction and methanol oxidation occurring in parallel. The porosity and oxygen transport effects were included for the cathode in order to adequately describe the impedance spectra. In addition to divulging methanol crossover, the cathode spectra provided an indication of nonequipotentiality of the cathode, flooding of the cathode backing, flooding/dry-out of the cathode catalyst layer, and hindering of oxygen transport in the cathode backing by crossover methanol. The ability of EIS to reveal these phenomena proved to be highly useful in the identification of performance issues in individual cells of a six-cell DMFC stack.High fuel energy content and no need for the fuel reforming make the direct methanol fuel cell ͑DMFC͒ a highly promising power source, especially for portable electronics applications, including mobile phones, laptop computers, battery chargers, small auxiliary power units, etc. 1-5 The last decade has brought significant progress in the fundamental DMFC research, especially in the anode and cathode electrocatalysis, membrane-electrode structure optimization, and cell design. [6][7][8] The research in these areas has proven crucial to the recent acceleration in DMFC hardware development, stack prototyping, and system integration. 9,10 At the same time, however, there has been a growing need for new and powerful diagnostic techniques, capable of providing more comprehensive information on the performance of DMFCs and their individual components.Until recently, DMFC diagnostics has been practically limited to the use of various dc electrochemical techniques, such as potential and current step. The information generated by these techniques is usually only about the sum of various cell polarizations, which is difficult or impossible to break down into individual polarization contributions. Contrary to the dc techniques, electrochemical impedance spectroscopy ͑EIS͒ is based on the resonance with individual processes occurring in the electrochemical system and, if used skillfully, it can provide insight into these processes. In the EIS experiment, a small sinusoidal electrical perturbation is applied to a linearly responding system under study on top of a bias polarizati...

This paper addresses the performance loss due to oxidation of a platinum cathode catalyst in continuous operation of the direct methanol fuel cell. Saturation oxide coverage is reached within 2 h at oxidizing potentials of the cathode in the absence of gas-phase oxygen. This rate of platinum oxide formation is too fast to fully explain the slow rate of cathode performance degradation. There is an indication that oxide reconstruction may be responsible for long-term cathode performance degradation beyond the initial 2 h. Cathode performance loss in the oxygen reduction reaction is fully recoverable via reduction of platinum oxide formed on the surface. Because the methanol anode is a relatively easily polarizable electrode, it cannot serve as a counter electrode for lowering the cathode potential with an external voltage source. However, the oxidized Pt can be reduced to its catalytic form by air-starving the cathode and consuming the remaining oxygen in the oxidation of crossover methanol. The subsequent depletion of oxygen in the cathode plenum leads to a potential drop below the level required for complete reduction of surface Pt oxides ͑hydroxides͒.

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A method of measuring error-free potentials of individual electrodes directly in the polymer electrolyte fuel cell and under a full range of load is introduced. Successful measurement is accomplished by placing a reference electrode in ionic contact with the active layer of a fuel cell electrode of interest, that is, away from the electrolyte membrane. Half-cell measurements with the proposed reference electrode configuration are compared with measurements in two other fuel-cell reference electrode arrangements to illustrate the conditions necessary for directly achieving correct measurement of potentials (overpotentials). Functioning of the fuel cell reference electrodes is explained through modeling of electrical potential and reactant distribution in the working fuel cell.