Reaction engineering perspective of PEM fuel cells: The stirred tank reactor as a building block

A summary of the stirred tank proton exchange membrane (PEM) fuel cell reactor findings, the remarkable analogy to the autocatalycity in an exothermic stirred tank reactor, and an extension of the stirred tank PEM fuel cell to approximate the conventional integral reactor was presented. The interplay of proton transport with water activity in the PEM membrane underpinned the observed dynamical phenomena. Water ionized and shielded stationary anions in the membrane, enhancing proton transport by orders of magnitude. Existing fuel cell literature contains extensive reports that PEM fuel cells only operate when sufficient water is present in the membrane. It was possible to operate the PEM fuel cell with dry feeds of hydrogen and oxygen. The role of a critical initial membrane water content for ignition was elucidated. The product water catalyzed the reaction in an autocatalytic manner analogous to the autocatalytic rate acceleration in the exothermic stirred tank reactor. The water balance in the membrane led to multiple steady states in the fuel cell. The membrane functioned like a reservoir for water and needed time to equilibrate to changes in the operating conditions. Although the model reactors were not optimal for reactant conversion, they were specifically designed to measure system parameters, including effective kinetic and transport properties.