where R is gas constant (J. K #[ . mol #[ ), T is the absolute temperature (K), Z is the valence of the ionic species, F is the Faraday's constant (C. mol #[ ), and p is oxygen partial pressure in air and fuel. Under the fuel cell mode (figure ), both electrodes are reversible for oxygen and serve as source/sink for oxygen ions while they are blocking sodium ions since none of them contain any sodium; therefore, there is no sodium source. The lower measured voltage before and after reaching equilibration is attributed to the transient transport process of sodium ions before reaching equilibrium measured before time τ. Before time τ, due to the oxygen ion transport, the cathode is positively charged, and due to the electroneutrality to compensate for the generated voltage across the cell, sodium ions start to migrate (via diffusion mechanism) in the opposite direction from the anode to right to neutralize the induced voltage. The time required for diffusion of Na ! ions, is τ.