Using Three Variable State Equations to Gain Insight into PEM Fuel Cell Membrane Thermodynamics

A PEM fuel cell is an electrochemical system dependent on multiple variables such as temperature, voltage, current, pressure, volume, and chemical potential. Thermodynamically, parts of this system are seen through simpler mechanical equations of state developed for subsets of the more complex system (a simple compressible substance, for instance), providing a piecewise understanding of the whole. In this research, a method using equations of state with three variables is proposed to arrive at the thermodynamic relationships in a more comprehensive way. As a result of three-variable equations of state, four additional equations of convenience are gained to complement enthalpy, Gibbs, and Helmholtz functions, providing additional relationships that contain a mix of mechanical and electrical physical quantities. The proposed approach is a more comprehensive path of gaining insight into a closer thermodynamic representation of a functioning fuel cell to in turn improve the understanding and modeling of this important electrochemical system.