In a proposed direct methanol fuelcell electric- power-generating system, the fuel cells would consume neat methanol, in contradistinction to the dilute aqueous methanol solutions consumed in prior direct methanol fuel-cell systems. The design concept of the proposed fuel-cell system takes advantage of (1) electro-osmotic drag and diffusion processes to manage the flows of hydrogen and water between the anode and the cathode and (2) evaporative cooling for regulating temperature. The design concept provides for supplying enough water to the anodes to enable the use of neat methanol while ensuring conservation of water for the whole fuel-cell system. By rendering unnecessary some of the auxiliary components and subsystems needed in other direct methanol fuel-cell systems for redistributing water, diluting methanol, and regulating temperature, this fuel-cell design would make it possible to construct a more compact, less massive, more energy-efficient fuel-cell system.
In a typical prior direct methanol fuelcell system, neat methanol is stored in a container and then diluted with water to a concentration between 2 and 3 percent before it is introduced into the fuel-cell stack. Water for dilution is gathered from the cathode side of the fuel-cell stack. The fuel solution is recirculated, the fuel solution entering the anodes is monitored by use of a methanol sensor, and, in response to the sensor reading, methanol is added to the solution as needed to maintain the required concentration. The collection of water, the dilution of methanol, the control of concentration, and the circulation of the fuel solution entail the use of several pumps and control subsystems, and substantial electrical energy is consumed in operating the pumps and control subsystems. These auxiliary components and subsystems typically contribute about half of the overall volume and mass and at least half of the parasitic energy consumption of the system.
The figure schematically depicts the transport processes involved in the operation of a direct methanol fuel cell (whether of prior or proposed design). Methanol is oxidized to protons and carbon dioxide at the anode, and oxygen is reduced to water at the cathode. As protons migrate from the anode to the cathode through a proton-conducting membrane that is part of a membrane/electrode assembly, water is transported along with them by electro-osmotic drag: in other words, water molecules associated with the protons are dragged along with the protons. Air flowing over the cathode evaporates some of the water. However, some of the water tends to diffuse back toward the anode because the concentration of water at the cathode exceeds the concentration of water in the methanolwater solution at the anode (this diffusion is hereafter denoted "back diffusion"). Water is consumed at the anode by the oxidation of methanol, and water is produced at the cathode by reduction of oxygen.