Hi,
I've encountered a strange phenomenon with a new material I've made. It contains Manganese Oxide (specific phase is not completely characterised yet) on activated carbon.
The substrate is stainless steel and the electrolyte is 2M KCl (although this issue has arisen with Na2SO4 as well). 3 electrode, counter electrode: Platinum wire; Reference: Ag/AgCl
Essentially, when I galvanostatically charge my device, the voltage increases up to around 0.8 V (Vref), after which the voltage simply drops off and sometimes goes all the way to zero, while applying a current. This has happened on various substrates with this material. Impedance looks fine, CVs also look fine, except I notice at the aforementioned voltage (on the reverse sweep), my current density looks a bit reduced (see: fig).
Any idea what the cause may be? Could I avoid this by altering the electrolyte or substrate?
Regards,
Akash
Dear Akash Ratnayaka,
you say: "Impedance looks fine", presumably, measured at Vdc=OCV, polarization value. However, if you could (re)try to measure EIS, near the polarization of interest, e.g. near the polarization value parameter of:
Vdc = 0.8 V (vs Ag/AgCl),
then we might be able to express a helpful guess for your testing SC (supercapacitor) case(s), your SCs' issue(s).
Large activation polarization of the working electrode (if there exist strong specific adsorption of ionic species) may cause this potential loss. Try to decrease the potential range, it may vanish. This drop in potential may also be caused by IR drop across the ESR (at the interface). You may also verify this by narrowing down the potential range to see if the same trend is obtained.
Dear Akash Ratnayaka,
you say: "Impedance looks fine", presumably, measured at Vdc=OCV, polarization value. However, if you could (re)try to measure EIS, near the polarization of interest, e.g. near the polarization value parameter of:
Vdc = 0.8 V (vs Ag/AgCl),
then we might be able to express a helpful guess for your testing SC (supercapacitor) case(s), your SCs' issue(s).
As Ioannis Samaras suggests, remeasure using EIS at 0.75 - 0.8 V vs Ag/AgCl.
Hi Ioannis Samaras ,
thanks for the answer! I'm actually measuring it as I type this (well, doing the CVs first to identify the voltage at this specific electrode I've made) Will report back asap.
Kashif Mairaj Deen
Yes lowering the voltage window eliminates the problem. ESR from charge-discharge and EIS is consistent with my undoped electrode (which does not have the leakage issue). WIll be going EIS as other answers suggested and report back.
Thank you all for the help so far!
So after a few tests, it appears the culprit is the stainless steel substrate. Usually I have no issues with stainless steel in KCl, but it appears the presence of MnO2 is causing the stainless steel to undergo a reaction at 0.4-0.8 (depending on the roughness of the surface). I'll have to switch substrates or electrolytes now.
Thank you all.
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