If we grasp water content inside stack and control it in appropriate quantities during a run, we can decrease water content at shut-down and energy and gas consumption which is required for purge. However, water content inside stack cannot be measured directly while operation. Therefore, a new sensing method is desired that water content is calculated from parameters available in car.

Accordingly, we focused on relation between oxygen diffusion resistance and water content and between oxygen diffusion resistance and two different frequencies impedance and suggested a detecting method to find water content from oxygen diffusion resistance. In this method, it was assumed that oxygen diffusion resistance in catalyst layer can be divided into two components: *R _{ion}* caused by oxygen gas transport in solid and

*R*caused by vacant space in catalyst. We obtained oxygen diffusion resistance by calculating them from 200 Hz and 20 Hz two-frequency impedance. If two-frequency superimposed impedance becomes obvious, we can estimate water content in stack. In this matter, we established detecting method to seek water content during driving from impedance measurement.

_{void}First of all, we obtained oxygen diffusion resistance at cathode catalyst layer. Oxygen diffusion resistance *R _{total}* was assumed to be the sum of gas diffusion resistance due to an obstacle of liquid water,

*R*and diffusion resistance concerned with oxygen transport inside ionomer,

_{void}*R*.

_{ion}*R*can be calculated from a property of matter. We assumed an electrical equivalent circuit and Cole-Cole plot profile and found the diameter of the plot through 20 Hz and 200 Hz two-frequency superimposed impedance measurement geometrically. The diameter of Cole-Cole plot stands for mass transfer resistance, and this equals to the sum of resistance components of activation and concentration overpotential,

_{ion}*R*+

_{act}*R*. Concentration overpotential resistance was in terms of partial oxygen pressure

_{dif}*P*and oxygen diffusion resistance

_{O2}*R*, and

_{total}*R*was dissolved by concentration overpotential

_{total}*η*calculated with overpotential separation based on Butler-Volmer equation and the diameter

_{dif}*R*+

_{act}*R*.

_{dif}*R*could be taken

_{void}*R*from

_{ion}*R*.

_{total}Moreover, we operated cell with measuring impedance for a certain time and measured water content after the operation on a plurality of various conditions. 20 Hz and 200 Hz two-frequency superimposed impedance was analyzed by FFT. Water content was found out by comparing stack’s weight before and after removal of cell fastening.

As a result, oxygen diffusion resistance increased in both cases of light and heavy water content from the relation between water content and *R _{total}* calculated by impedance measurement on a plurality of different conditions. When water content is heavy and flooding is supposed to occur, liquid water blocks gas transport inside voids, and

*R*increases. Conversely, it is considered that

_{void}*R*increases with a drop of water content inside ionomer when relative humidity is low and drying-up is supposed to occur. Therefore,

_{ion}*R*which is the sum of

_{total}*R*and

_{void}*R*showed such a downward profile against the variation of water content.

_{ion}Moreover, *R _{void}* increased simply as water content increased. As mentioned above, it was considered that gas transport was blocked by liquid water in large quantities, and it was verified that

*R*is an indicator of oxygen gas behavior due to liquid water inside voids at catalyst layer. Because this increment of

_{void}*R*for water content corresponds our approximate expression, we concluded that water content in cell can be found out uniquely from

_{void}*R*measured by impedance spectroscopy.

_{void}