Principles of Direct Methanol Fuel Cells for Portable and Micro Power

A direct methanol fuel cell (DMFC) is an electrochemical cell that generates electricity based on the oxidation of methanol and reduction of oxygen. An aqueous methanol solution of low molarity acts as the reducing agent that traverses the anode flow field. Once inside the flow channel, the aqueous solution diffuses through the backing layer, comprised of carbon cloth or carbon paper. The backing layer collects the current generated by the oxidation of aqueous methanol and transports it laterally to ribs in the current collector plate. The global oxidation reaction occurring at the platinumruthenium catalyst of the anode is given by:$CH_3 OH + H_2 O \to CO_2 + 6H^ + + 6e^ -$

The carbon dioxide generated from the oxidation reaction emerges from the anode backing layer as bubbles and is removed via the flowing aqueous methanol solution.

Air is fed to the flow field on the cathode side. The oxygen in the air combines with the electrons and protons at the platinum catalyst sites to form water. The reduction reaction taking place on the cathode is given by:$3/2O_2 + 6H^ + + 6e^ - \to 3H_2 O$

These two electrochemical reactions are combined to form an overall cell reaction as:$CH_3 OH + 3/2O_2 \to CO_2 + 2H_2 O$

As expected, a DMFC exhibits lower power densities than that of a H₂/air PsEFC. However, the DMFC has the advantages of easier fuel storage, no need for humidification, and simpler design. Thus, DMFC is presently considered a leading contender for portable power application. To compete with lithium-ion batteries, the first and foremost property of a portable DMFC system must be higher energy density in Wh/L. This requirement entails overcoming four key technical challenges: (1) low rate of methanol oxidation kinetics on the anode, (2) methanol crossover through the polymer membrane, (3) water management, and (4) heat management.