Species volumetric source terms (-s) in the triple-phase boundaries due to electrochemical reactions for the PEMFC, SOFC, and Electrolysis, respectively, are:

For PEMFC:

(20–72)

(20–73)

(20–74)

For SOFC:

(20–75)

(20–76)

(20–77)

For Electrolysis:

(20–78)

(20–79)

(20–80)

In the above equations, , , and are the molecular mass of water, oxygen and hydrogen, respectively, is the Faraday constant, and 2 and 4 are the numbers of electrons per mole of reactants and products.

Since the total electrical current produced in the cathode and the anode TPBs, respectively, is the same, we have the following equation for current conservation:

(20–81)