Triple-Phase Boundary
The triple phase boundary (TPB) of a fuel cell is the area of contact between the three phases necessary for electrochemical reactions at the electrode: ion conducting phase, electron conducting phase, and gas phase. A good fuel cell maximizes the TPB area, allowing the reaction to occur in more sites, thus maximizing current flow.
Fuel cells can expand the TPB into a two phase boundary by using a mixed electronic-ionic conductor (MEIC), in which one phase is both the electron and ion conductor. The entire contact between gas phase and MEIC becomes electrochemically active.
Figure 1: TPB Model
