The precise impedance measurement system based on the well-known two-probe and four-probe
methods was prepared to measure the impedance and consequent proton conductivity of Nafion
membranes as standard samples, considering not only the number of electrodes and the bridging
methods but also the elimination of electrical error sources for the precise electrical analysis.
The proton conductivities of Nafion membranes are reported and discussed in the present study
with respect to different cell configurations and measurement conditions. The values of proton
conductivities measured using the four-probe method were always higher (2−5 times) than those
measured using the two-probe method at ambient humidity and temperature. The proton
conductivities were also measured under two different humidity conditions: water-vapor state
and liquid-water state. In the water-vapor state (95% relative humidity), completely different
impedance behaviors for the identical Nafion membrane were observed from the Nyquist
impedance plots. All Nyquist plots derived from the two-probe method represented the inductive
reactance derived from various components, such as Pt electrodes, electric conductive leads,
and a potentiometer in the path of current flow rather than the capacitive reactance between
Pt electrodes. The effect of contact resistance between the membrane sample and electrodes on
proton conductivity was also investigated by using the two conductivity-cell configurations. It
was shown that the effect of the contact resistance on the proton conductivity was more severe
in the two-probe measurement, and this factor should be seriously considered in the water-vapor state. Furthermore, the humidity and the temperature effects on the proton conductivity
of the membranes were observed under various measuring conditions and compared with the
two different cell configurations. The four-probe method well reflected the proton conductivity
behavior of Nafion membrane in the wide range of temperature and humidity, as compared
with the two-probe method with reasonable proton conductivity in the low humidity.
A comparison of capacitance at 10 pF was conducted between thirteen participating laboratories from the Asia-Pacific region. Measurements were made between 2004 and 2006. The behaviour of the travelling artefact was consistent with a steady linear drift at a rate of approximately 0.1 µF/F per year. Despite the wide range of capabilities within the region, the results showed good agreement between all but one of the participating laboratories.Main text.
To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).
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