I'm confused about something. A common method for proton conductivity measurement is to use a 4 probe impedance measurement technique in humid environment. In this method, an AC current is applied trough two plate electrodes and the voltage drop is measured between two other wire electrodes and then the conductivity of the materiel is calculated using this information (drop of voltage at specific current and using Ohms law). In the literature it seems this conductivity is the same as proton conductivity for materials which have high electron resistance in dry state. What is the basis for this assumption? How actually protons move through the electrodes at all?
It is easier for me to understand the phenomenon when a reaction is involved. But can anyone explain what's going on when there is only metal electrodes in contact with the material? (by material I mean Nafion or any other ionic conductive material)
Proton exchange in electrolyte membrane has two mechanisms:
1- Proton hopping mechanism or Grotthus mechanism
2- Vehicular mechanism
You can see the following article:
Deluca, N. W., Elabd, Y. A., (2006). Polymer electrolyte membranes for the direct methanol fuel cell: A Review, Journal of Polymer Science: Part B: Polymer Physics, 44, 2201-2213.
Protons do not really move thru the inner electrodes. In the H2 atmosphere and with an acidic electrolyte, the inner (voltage-measuring) electrodes acquire the absolute potential dictated by the Nernst equation for H2 evolution/ oxidation- which would be same for both, provided that they are in the same environment. What is left as the potential difference between the two is the relative potential equal to the ohmic drop between them. In practice, H2 atmosphere is rarely used but AC rather than DC current is applied. This allows the experimenter to filter out the slow DC drift due to the lack of stable Nernst potential, and measure in-phase the ohmic drop accurately.
Hi Hussein, Thanks for the paper. It was useful indeed. But still it does not answer my question. What is the connection between ohmic resistance and proton resistance? How these two are differentiable by Impedance measurement?
Proton with water transfers through Nafion using cation exchange sites. H2 gas also transfers through Nafion. Please refer to the following article. "Gas Crossover and Membrane Degradation in Polymer Electrolyte Fuel Cells”, Minoru Inaba, Taro Kinumoto, Masayuki Kiriake, Ryota Umebayashi, Akimasa Tasaka, and Zempachi Ogumi, Electrochim. Acta, 51(26), pp. 5746－5753 (2006).
Thanks Yuriy for your response. I read it a couple of times to fully understand the thing you are trying to say. A couple of further questions:
1- How we know the inner electrodes only measure the potential related to H2 evolution/oxidation reaction? Why is that so?
2- Doesn't this mean the potential difference between the inner electrodes has to be greater than H2 evolution/oxidation reaction in order for this measurement to be accurate? If the potential difference is below the reaction threshold logically all the potential difference we measure should be ohmic.
3- I though the purpose of using the two middle electrodes is in this way we can have full control over both current and voltage (we can keep the applied current constant and measure the voltage drop at a frequency in which interracial impedance is not major)
Thanks again
Very interesting paper about ionizied water clusters in Nafion and water channels:
Electrochemistry Communications 30 (2013) 5–8
welcome. here are the answers:
1- How we know the inner electrodes only measure the potential related to H2 evolution/oxidation reaction? Why is that so?
The exchange current for H2/2H+ reaction on Pt is so high that it imposes its potential on Pt. What other redox couple can you think of that can interfere with Nafion in H2 atmosphere?
2- Doesn't this mean the potential difference between the inner electrodes has to be greater than H2 evolution/oxidation reaction in order for this measurement to be accurate? If the potential difference is below the reaction threshold logically all the potential difference we measure should be ohmic.
> Not at all. The difference between H2 evolution and H2 oxidation potentials is zero if they are measured with respect to the H2/2H+ equilibrium potential (no DC current flows thru them). In the presence of current going thru the outer electrodes , the Volta potentials of the inner electrodes do not change but the ohmic potential drop develops between them (in the electrolyte and in the gas phase). And this ohmic drop is what you measure in a DC experiment. In an AC experiment you measure this ohmic drop more accurately by eliminating slow drift noise.
3- I though the purpose of using the two middle electrodes is in this way we can have full control over both current and voltage (we can keep the applied current constant and measure the voltage drop at a frequency in which interracial impedance is not major). You can use EITHER AC with 2 electrodes OR DC with 4 electrodes. However, you get the most accurate/precise measurements if you use BOTH AC and 4 probe.
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