1. Field of the Invention
The invention relates generally to fuel cell systems, and more particularly, the invention relates to methods and apparatuses for controlling fuel flow and sensing fuel concentration in a fuel cell system.
2. Background of the Invention
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel depending upon the materials chosen for the components of the cell and the intended application for which the fuel cell will provide electric power.
Fuel cell systems may be divided into “reformer based” systems (which make up the majority of currently available fuel cells), in which fuel is processed to improve fuel cell system performance before it is introduced into the fuel cell, and “direct oxidation” systems in which the fuel is fed directly into the fuel cell without internal processing.
Because of their ability to provide sustained electrical energy, fuel cells have increasingly been considered as a power source for smaller devices including consumer electronics such as portable computers and mobile phones. Accordingly, designs for both reformer based and direct oxidation fuel cells have been investigated for use in portable electronic devices. Reformer based systems are not generally considered a viable power source for small devices due to size and technical complexity of present fuel reformers.
Thus, significant research has focused on designing direct oxidation fuel cell systems for small applications, and in particular, direct systems using carbonaceous fuels including but not limited to methanol ethanol and aqueous solutions thereof. One example of a direct oxidation fuel cell system is a direct methanol fuel cell system. A direct methanol fuel cell power system is advantageous for providing power for smaller applications since methanol has a high energy content, thus providing a compact means of storing energy, can be stored and handled with relative ease, and because the reactions necessary to generate electricity occur under ambient conditions.
DMFC power systems are also particularly advantageous since they are environmentally friendly. The chemical reaction in a DMFC power system yields only carbon dioxide and water as by products (in addition to the electricity produced). Moreover, a constant supply of methanol and oxygen (preferably from ambient air) can continuously generate electrical energy to maintain a continuous, specific power output. Thus, mobile phones, portable computers, and other portable electronic devices can be powered for extended periods of time while substantially reducing or eliminating at least some of the environmental hazards and costs associated with recycling and disposal of alkaline, Ni—MH and Li-Ion batteries.
The electrochemical reaction in a DMFC power system is a conversion of methanol and water to CO2 and water. More specifically, in a DMFC, methanol, preferably in an aqueous solution, is introduced to the anode face of a protonically-conductive, electronically non-conductive membrane in the presence of a catalyst. When the fuel contacts the catalyst, hydrogen atoms from the fuel are separated from the other components of the fuel molecule. Upon closing of a circuit connecting a flow field plate of the anode chamber to a flow field plate of the cathode chamber through an external electrical load, the protons and electrons from the hydrogen atoms are separated, resulting in the protons passing through the membrane electrolyte and the electrons traveling through an external load. The protons and electrons then combine in the cathode chamber with oxygen producing water. Within the anode chamber, the carbon component of the fuel is converted by combination with water into CO2, generating additional protons and electrons.
The specific electrochemical processes in a DMFC are:                Anode Reaction: CH3OH+H2O=CO2+6H++6e        Cathode Reaction: 3/2O2+6H++6e=2H2O        Net Reaction: CH3OH+3/2O2=CO2+H2O        
The methanol in a DMFC is preferably used in an aqueous solution to reduce the effect of “methanol crossover”. Methanol crossover is a phenomenon whereby methanol molecules pass from the anode side of the membrane electrolyte, through the membrane electrolyte, to the cathode side without generating electricity. Heat is also generated when the “crossed over” methanol is oxidized in the cathode chamber. Methanol crossover occurs because present membrane electrolytes are permeable (to some degree) to methanol and water. One method of reducing methanol crossover is to introduce the methanol in an aqueous solution, thus providing the fuel cell with little more methanol than is required, minimizing crossover without depriving the fuel cell of the necessary fuel.
One of the problems with using DMFC power systems in portable power applications is the lack of a low-cost, effective method for controlling the concentration of methanol fuel in the fuel mixture. Specifically, a problem exists in keeping the proper ratio of fuel to water delivered to the anode chamber in DMFC power systems. For example, if the methanol concentration on the anode face of the membrane electrolyte is too high, then methanol crossover is likely to occur. Similarly, when a fuel cell is too hot, it may encourage excess methanol crossover. Methanol crossover not only wastes fuel, and increases the heat of the fuel cell, but can contribute to cathode flooding, which compromises the performance of the fuel cell.
Because saturation of the cathode prevents the energy producing reactions from proceeding, excess water on the cathode side of the membrane can lead to an increase in methanol concentration at the anode. The increased concentration of methanol may then lead to additional methanol crossover resulting in decreased efficiency, a waste of methanol, and the generation of unwanted heat.
According, the suitability of DMFC power systems for powering portable devices and consumer electronics is dependent upon the development of systems and methods for controlling the amount of fuel concentration in the fuel mixture of a direct methanol fuel cell.
Moreover, it is desirable to utilize physical properties of materials and mechanisms to control the behavior of a fuel cell (i.e., passive components/systems), to reduce product costs and system complexity.