I have fabricated thin films and want to measure the impedance parallel to the material. I suspect the films are conducting protons and electrons based on some Hall effect measurements I have taken. I am a little confused in terms of the electrodes used for these measurements and what one might expect in terms of a Nyquist plot. I am using Pt wire as the WE and CE for one measurement and the clamps that came with our potentiostat (which I assume are stainless steel).
In the case of Pt electrodes; proton conductivity should be measured as long as there is sufficient water and and electrons will also move through the material. Therefore, it isn't blocking to anything, correct? I also do not see any Warburg impedance at this point which makes me wonder if protons are in fact mobile charges in this material. When I use the clamps then I should be blocking electrons and measuring the protons, right? If I just wanted to measure electrons and not ions what electrode material should I use (graphite?) or should I try to make the relative humidity 0% and have dry films?
Any advice on this matter would be appreciative. I apologize if this is confusing. Thank you!
My understanding is that a blocking electrode means that mass (such as ions and molecules) is unable to enter it, but electrons can. I am not aware of the term "non-blocking electrode", but would believe what you meant is electrode that allows mass ingression, such as intercalation of ions in the electrodes in lithium ion batteries. With these in mind, a Pt wire as you described is always a blocking electrode.
However, if you make a redox active or ion conducting coating on the Pt wire, the assembly of "Pt + coating" is then not a blocking electrode. When the coating is sufficiently thick (e.g. > 100 nm), you will likely see a Warburg response on your Nyquist plot.
The Warburg response can also appear when an electron transfer reaction occurs on a blocking electrode when the reaction becomes controlled by diffusion in the liquid electrolyte toward the electrode under appropriate conditions. This can happen even when the electrode reaction is the reduction of proton to hydrogen on a Pt electrode in an acidic aqueous electrolyte.
In summary, the observation of a Warburg response on the Nyquist plot should not be taken as evidence of the electrode being or not being a blocking electrode.
Thank you for your response George Zheng Chen . I was under the impression that Pt is not a blocking electrode for protons due to the reduction of H+ to hydrogen gas at the working electrode as long as there (1) high enough humidity for water to act as the proton source or (2) more protons available in the bulk. Is that not the case? That is why I used stainless steel as my electrodes since the reduction of H+ to hydrogen gas at potentials
Caleb Wehrmann: I would place HER on a Pt electrode in the blocking electrode category, simply because there is no mass ingression into the Pt electrode. The electrode reaction occurs at the Pt/electrolyte interface, whilst all reactants and products are in the liquid electrolyte side, or at least passing through the liquid.
The thermodynamics of electrode reaction is largely determined by the potential, not by the frequency in the EIS experiment. The frequency only affects the kinetics of the reaction. In other words, at a given bias potential (electrode potential), the same electrode reaction occurs, disregarding the frequency. However, the frequency changes the relative rates of the electron transfer and mass transport (diffusion in most cases).
At high frequencies, the forward and backward reactions swaps very fast, tickling only the electron transfer step without causing consumption of too much reactants and hence no diffusion difficulty. Under such conditions, the electrode/electrolyte behaves like a leaky capacitor which is equivalent to a parallel RC circuit, giving rise to the semicircle on the Nyquist plot.
However, at medium or low frequencies, the time for reaction becomes longer, eventually leading to full consumption of reactants at the interface, calling for more supply of reactant by diffusion, which is reflected by the Warburg response on the Nyquist plot.
To add to the above, I would like to mention a principle of electrode classification: polarisable and non-polarisable electrodes. The former is a type of electrodes on which the potential can vary within a given potential range without causing any electron transfer reactions. For example, a Pt electrode in aqueous solution of H2SO4 is polarisable within the potential range of above 1.2 V from HER at the negative end to OER at the positive end. However, in the same potential range, a steel electrode is non-polarisable (or the polarisable potential range is much narrowed) because of iron oxidation (dissolution and passivation) that occurs at potentials less positive than that for OER.
By the way, I shall mention that the high frequency semi-circles shown in your Nyquist plots are very likely an artefact due to the coupling between the potentiostat and your electrochemical cell. The frequencies of the semi-circle are too high for ions to respond, and the shape of the semi-circle is also unusually different from what is expected to occur for a charge transfer reaction at the electrode/electrolyte interface.
Dear Caleb Wehrmann ,
try to use (2+2=) 4-Pt electrodes[1-7] on your films (deposited on glass) and (re)measure, please, some EIS spectra using, initially, a 'medium-to-low' value for Vsource, AC
Dear Ioannis Samaras ,
Thank you for your answer and the linked resources. My concern at this point is the proton reduction at the electrode at low potentials with Pt. If I start at 0 DC voltage and a low AC voltage of ~ 10 mV then I should not measure proton conductivity, correct?
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