ABSTRACT
In 1899, Walter Hermann Nernst observed the current of oxygen molecules through dense ceramics, when somewhat heated. The current of oxygen was similar to the current of electrons in metals under an electrical potential difference. The partial pressure of oxygen played the role of electrical potential. Some years later, he discussed this with Albert Einstein, and this resulted in the Nernst-Einstein formula:
j RT
L nF
P
= ′ ′′
σ ( )
.ln
(4.1)
Here jO2 is the oxygen ux, F is Faraday’s constant, L is the membrane thickness, n is the charge of the charge carrier (n = 2 for oxygen ions), R is the universal gas constant, T is the absolute temperature, ′PO2 is the oxygen partial pressure at the feed surface of the membrane, ′′PO2 is the oxygen partial pressure at the permeate surface of the membrane, and si represents the material conductivity. This expression clearly identies the natural logarithm of the oxygen partial pressure ratio as the driving force for the oxygen ux.
