December 08, 2019, 06:53:48 PM
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Topic: Am I misunderstanding the Butler-Volmer equation?  (Read 506 times)

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Offline Heysitsisaac

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Am I misunderstanding the Butler-Volmer equation?
« on: September 01, 2019, 06:21:34 PM »
Hello everyone, I looked through several subforums and wasn't totally sure which this kind of discussion belonged in. I have no problem moving the topic of course if someone would like to direct me to a more appropriate board.

A friend of mine and myself constructed a rig to experimentally confirm a model we designed in Mathcad. The experiment is based on the Butler-Volmer equation. In simple terms, the experiment consists of a bucket attached to a hose which is connected to a chamber containing two electrodes. The electrodes are connected to a power supply and when provided voltage they split the supplied H20 from the bucket into separate H and OH dominant solutions. The solutions are separated into two different receptacles which provides both acidic and basic effluent streams.

This is similar to how commercial electrolyzers work. The problem we are running into is the mathcad model doesn't match our results in the experiment - namely our current density appears to be much larger than expected experimentally as compared with the Butler-Volmer equation.

I can provide the equations that we are using if a reply requires that but I was curious if anyone had any interest, or if there is some well known or obvious discrepancy that we have not considered.

The electrodes are platinized titanium. We are using tap water, with added salt (NaCl).

Thank you!

Offline Borek

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Re: Am I misunderstanding the Butler-Volmer equation?
« Reply #1 on: September 02, 2019, 03:23:20 AM »
In general - no idea.

Are you sure you are not producing chlorine? I would start with NaOH, not NaCl.
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Offline Enthalpy

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Re: Am I misunderstanding the Butler-Volmer equation?
« Reply #2 on: September 03, 2019, 06:36:17 AM »
What shall be "H and OH dominant solution"? Even in my former microelectronics lab, we could never produce water so pure that both H+ and OH- were the most abundant ions. That would happen in pure water, something extremely difficult to obtain.

And: NaCl electrolysis uses to produce hypochlorite, or with precautions chlorine, not oxygen.

I can't tell how often Butler-Volmer applies, how extraordinary conditions are necessary, nor in which voltage range it has a chance to work. When I see voltages like kT/q, it just rings a bell about purities, offset of the zero-current voltage, and so on.

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