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Re: KARPEN PILE «
Reply #440 on: August 12, 2014, 05:31:54 AM »
Quote BTW Im not going to do any tests if they require electrolysis, these will of course show hydrogen overpotentials!
I cant modify the original message, so I'll keep pasting the original, with additions.
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Air Karpen Pd black on Pt vs Glassy Carbon in 1N Sulfuric Acid.
I've just unshorted the cell, after 18 hours and will report hourly emf of cell. I'll keep modifying this post rather than creating new one, so keep coming back here for the updates.
negative value means C is more negative than the Pd/Pt
Time EMF
0 - 0mV
1hr -1mV
2hrs -8mV
3hrs -12mV
Voltage does change very slowly. Decided to end experiment here, now passing UHP hydrogen through exacltly same setup. Cell is shorted out
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Hydrogen Karpen, same setup as above, but flushed with constant UHP quality hydrogen
Notes: Pd/Pt electrode in 1N H2SO4 now becomes essentially the well known NHE electrode. Normal hydrogen electrode found in all text books. Very fast response / kinetics. Large exchange current . In other words, any stray oxygen gas has very little effect on its voltage as it is swamped by the fast hydrogen/proton reaction. We need to focus on the other electrode to explain strange behavior. Forget about this electrode!
As soon as hydrogen was introduced, the voltage immediately rose to a healthy +800mV. The carbon becoming the cathode, with reduction occuring at its surface. This is very easily explained by remembering that any residual oxygen has little effect on the Pt electrode but will give rise to a mixed potential at the Carbon. The hydrogen/proton kinetics are very slow at the carbon, so its potential will be a mixture of the oxygen redox and the hydrogen redox potentials, the onw with faster kinetics will dominate the reading.
Anyone repeating these experiments must remember that oxygen, or oxides on the surface will fool you into believing you have a huge potential.
After a few hours of shorting out the terminals, to reduce any oxygen reaching the Carbon, the short was removed and the voltage monitored, here are the results as measured with a 10^12 ohm electrochemistry style voltmeter.
Time mV
30 secs +26
1 min +40
5 Min +96
10 min +118
40min +137
70 min +147
This is much slower than when the cell is shorted immediately after the hydrogen is first passed, an then unshorted. In this case the voltage rose to +800mV in 15 seconds!
eighteen hours of shorting out later, and here are the results
Time mV
30s +8mV
1 min 13 mV
12 mins +70mV
50 min +122
1hr 50m 134
2h:50 min 141
4h:20 min 146
5hrs +146
Conclusion, after equilibrating, cell seems to show a potential difference of 146mV, with the carbon electrode taking on a positive voltage.
Unless the cell is sealed completely, it is impossible to know if a trace of oxygen from the hydrogen which is being constantly bubbled through the cell has any influence on this voltage.
less than 0.1uA flows when shorted.
I then deliberately use a 6v cell and placed it across the electrodes for 3 seconds , visibly producing hydrogen at he carbon and oxygen at the Pt.
The voltages across the electrodes are now -1200mV, I'm planning to see its recovery