Electrochemical reactions possess energy barriers which must be overcome by the reacting species. This energy barrier is called the 'activation energy' and results in activation overpotential, which are irreversible losses (heat) in the system. Activation energy is due to the transfer of charges between the electronic and the ionic conductors. The activation overpotential is the extra potential necessary to overcome the energy barrier of the rate-determining step of the reaction to a value such that the electrode reaction proceeds at a desired rate.23 The anode (Eq. 17) and cathode (Eq. 18) activation overpotentials, ^a and nc, represent irreversible losses of the PEM stack and dominate the overall overpotential at low current densities:
where aa and ac are the anode and cathode electron transfer coefficients respectively, ia,o and ic,o are the anode and cathode exchange current densities (A cm-2) respectively and i is the current density of the stack (A cm-2). The electron transfer coefficient is a measure of the symmetry of the activation energy barrier and can range from zero to unity.22
The higher the exchange current density the easier it is for reaction to continue when current is supplied to the stack. The cathode exchange current density is thus not the limiting parameter of the activation overpotential term and is often ignored. The current density (i) normalizes the stack current (I) to the active area of the cell.
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