Direct dimethyl ether fuel cells

Details Direct dimethyl ether fuel cells Direct dimethyl ether fuel cells

1999-02-24

2004-08-17

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S010000, C429S010000, C429S010000

Reexamination Certificate

active

06777116

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to fuel cells operating directly on fuel streams comprising dimethyl ether in which dimethyl ether is directly oxidized at the anode. In particular, it relates to solid polymer fuel cells operating directly on liquid fuel streams comprising dimethyl ether. The dimethyl ether may serve as the primary fuel or as a component of a mixed fuel.
BACKGROUND OF THE INVENTION
Solid polymer electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. Solid polymer fuel cells operate in a range from about 80° C. to about 200° C. and are particularly preferred for portable and motive applications. Solid polymer fuel cells employ a membrane electrode assembly (“MEA”) which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. The electrocatalyst used may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. This can be accomplished, for example, by depositing it on a porous electrically conductive sheet material, or “electrode substrate”, or on the membrane electrolyte.
Effective sites on the electrocatalyst are accessible to the reactant, are electrically connected to the fuel cell current collectors, and are ionically connected to the fuel cell electrolyte. Electrons, protons, and possibly other species are typically generated at the anode electrocatalyst. The electrolyte is typically a proton conductor, and protons generated at the anode electrocatalyst migrate through the electrolyte to the cathode.
A measure of electrochemical fuel cell performance is the voltage output from the cell for a given current density. Higher performance is associated with a higher voltage output for a given current density or higher current density for a given voltage output. Another measure of fuel cell performance is the Faradaic efficiency, which is the ratio of the actual output current to the total current associated with the consumption of fuel in the fuel cell. For various reasons, fuel can be consumed in fuel cells without generating an output current, such as when an oxygen bleed is used in the fuel stream (for removing carbon monoxide impurity) or when fuel crosses through a membrane electrolyte and reacts on the cathode instead. A higher Faradaic efficiency thus represents a more efficient use of fuel.
A broad range of reactants have been contemplated for use in electrochemical fuel cells and such reactants may be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture.
The choice of fuel may vary depending on the fuel cell application. Preferably, the fuel is relatively reactive electrochemically, inexpensive, easy to handle, and relatively safe for the environment. Hydrogen gas is a preferred fuel since it is electrochemically reactive and the by-products of the fuel cell reaction are simply heat and water. However, hydrogen can be more difficult to store and handle than other fuels or fuel feedstocks, particularly in non-stationary applications (e.g. portable or motive). For this reason, liquid fuels are preferred in many applications. Fuel cell systems employing liquid fuels generally incorporate a reformer to generate hydrogen as required from a liquid feedstock that is easier to store and handle, e.g. methanol. However, the use of a reformer complicates the construction of the system and results in a substantial loss in system efficiency. To avoid using a separate reformer, fuels other than hydrogen may instead be used directly in fuel cells (i.e. supplied unreformed to the fuel cell anodes). Inside the fuel cell, a fuel mixture may be reacted electrochemically (directly oxidized) to generate electricity or instead it may first be reformed in-situ (internally reformed), as in certain high temperature fuel cells (e.g. solid oxide fuel cells). After being internally reformed, the fuel is then electrochemically converted to generate electricity. While such fuel cell systems may employ fuels that are easier to handle than hydrogen, without the need for a separate reformer subsystem, generally hydrogen offers fundamental advantages with regards to performance and the environment. Thus, improvements in these areas are desirable in order for internally reforming and direct oxidation fuel cell systems to compete more favorably to hydrogen-based systems.
A direct methanol fuel cell (DMFC) is a type of direct oxidation fuel cell that has received much attention recently. A DMFC is generally a liquid feed solid polymer fuel cell that operates directly on an aqueous methanol fuel mixture. There is often a problem in DMFCs with substantial crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over then reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, very dilute solutions of methanol (e.g. about 5% methanol in water) are typically used as fuel streams in DMFCs. Unfortunately, such dilute solutions afford only minimal protection against freezing during system shutdown in cold weather conditions, typically down to about −5° C.
In PCT/International Publication No. WO 96/12317 (Application No. PCT/US94/11911), alternative liquid fuels, including dimethoxymethane (DMM), trimethoxymethane (TMM), and trioxane, are suggested for direct use in liquid feed solid polymer fuel cells. Like methanol, these fuels can be oxidized at the fuel cell anode to form carbon dioxide and water at a rate that provides satisfactory fuel cell performance. Methanol appears to be an intermediate product of the oxidation for each of these fuels.
Dimethyl ether (DME) is available in quantity and has been considered as a cleaner alternative fuel for diesel combustion engines. DME is a gas at room temperature and pressure, but it will liquefy at about 5 bar. DME is also highly soluble in water. Aqueous solutions somewhat greater than 1.5 M DME can be prepared at ambient temperature and pressure. DME can be synthesized from natural gas with greater efficiency than methanol and thus it may be preferred over methanol as a fuel or fuel feedstock. DME can be made from methanol via an essentially irreversible reaction, whereas methanol is typically made via a reversible reaction step. Consequently, a higher yield of DME can be produced than methanol. Further, DME is relatively safe, especially compared to other common ethers.
DME has been used as a feedstock in producing reformate streams for use as fuel streams in fuel cell systems by external reforming. For instance, R. A. J. Dams et al. discuss the possibility of using reformed DME for solid polymer fuel cells in “The processing of alcohols, hydrocarbons and ethers to produce hydrogen for a PEMFC for transportation applications”, Proc. Inter. Soc. Energy Convers. Eng. Conf. (1997), 32nd, p 837-842, Society of Automotive Engineers. Various apparatus and methods for reforming DME have been disclosed in the art, for example, in published European Patent No. 0754649 and PCT/International Publication No. WO 96/18573 (Application No. PCT/US95/15628). Furthermore, DME has been used as a fuel stream in solid oxide fuel cells, which typically operate circa 1000° C. In this case, the DME is internally ref

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