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WayneL
science forum beginner

Joined: 03 May 2005
Posts: 33

Posted: Sat Mar 25, 2006 5:23 pm    Post subject: 1.2v limit of the double-layer

The figure of 1.2v seems to be use as the max an aqueous double-layer
capacitor can operate.
Is this voltage electrode material dependent or is it mainly based on the
electrolyte e.g. water?
If it is electrode dependent then I would expect this voltage to also vary.
Or is this the activation energy require to decompose water?

Any ref. to a paper kindly welcome.

WayneL
Dieter Britz
science forum beginner

Joined: 04 May 2005
Posts: 45

Posted: Mon Mar 27, 2006 2:10 pm    Post subject: Re: 1.2v limit of the double-layer

WayneL wrote:

 Quote: The figure of 1.2v seems to be use as the max an aqueous double-layer capacitor can operate. Is this voltage electrode material dependent or is it mainly based on the electrolyte e.g. water? If it is electrode dependent then I would expect this voltage to also vary. Or is this the activation energy require to decompose water? Any ref. to a paper kindly welcome.

You don't need a reference. Look up the delta_G for the
decomposition of water, and then translate that into delta-E.
When electrolysing water, this is roughly the voltage needed
to break up the water, and any voltage over that then goes
into ohmic heating.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet
Peter Lowrie
science forum beginner

Joined: 14 Jun 2005
Posts: 17

Posted: Tue Mar 28, 2006 5:28 am    Post subject: Re: 1.2v limit of the double-layer

Dieter Britz wrote:

 Quote: WayneL wrote: The figure of 1.2v seems to be use as the max an aqueous double-layer capacitor can operate. Is this voltage electrode material dependent or is it mainly based on the electrolyte e.g. water? If it is electrode dependent then I would expect this voltage to also vary. Or is this the activation energy require to decompose water? Any ref. to a paper kindly welcome. You don't need a reference. Look up the delta_G for the decomposition of water, and then translate that into delta-E. When electrolysing water, this is roughly the voltage needed to break up the water, and any voltage over that then goes into ohmic heating.

....And on the other hand, at low voltage Ohms law appears to break as the
circuit becomes endothermic and the Wein Effect takes over.

--
Regards,
Peter.
http://www.pelicom.net.nz
Lasse Murtomäki
science forum beginner

Joined: 17 Jun 2005
Posts: 35

Posted: Tue Mar 28, 2006 7:28 am    Post subject: Re: 1.2v limit of the double-layer

Overvoltage plays a crusial role, and the thermodynamic value does not tell
the whole truth. As we know, e.g. mercury has a very high activation
overvoltage for hydrogen evolution. In electrolytic processes, oxygen
evolution at the anode makes a significant contribution to the cell voltage.

--
Dr. Lasse Murtomäki
Helsinki University of Technology
lasse.murtomaki@tkk.fi

 Quote: You don't need a reference. Look up the delta_G for the decomposition of water, and then translate that into delta-E. When electrolysing water, this is roughly the voltage needed to break up the water, and any voltage over that then goes into ohmic heating.
WayneL
science forum beginner

Joined: 03 May 2005
Posts: 33

Posted: Tue Mar 28, 2006 8:31 pm    Post subject: Re: 1.2v limit of the double-layer

Thanks Dieter for the help, again.

E= delta_G/-nF

from CRC delta_G = -237,000Jmol-1

:. -237,000/-2x96458

= 1.2285v

WayneL

"Dieter Britz" <britz@chem.au.dk> wrote in message
news:e08v80\$jtq\$1@news.net.uni-c.dk...
 Quote: WayneL wrote: The figure of 1.2v seems to be use as the max an aqueous double-layer capacitor can operate. Is this voltage electrode material dependent or is it mainly based on the electrolyte e.g. water? If it is electrode dependent then I would expect this voltage to also vary. Or is this the activation energy require to decompose water? Any ref. to a paper kindly welcome. You don't need a reference. Look up the delta_G for the decomposition of water, and then translate that into delta-E. When electrolysing water, this is roughly the voltage needed to break up the water, and any voltage over that then goes into ohmic heating. -- Dieter Britz, Kemisk Institut, Aarhus Universitet
Günter Semrau
science forum beginner

Joined: 29 Mar 2006
Posts: 1

Posted: Wed Mar 29, 2006 9:56 am    Post subject: Re: 1.2v limit of the double-layer

WayneL schrieb:
 Quote: Thanks Dieter for the help, again. E= delta_G/-nF from CRC delta_G = -237,000Jmol-1 :. -237,000/-2x96458 = 1.2285v WayneL "Dieter Britz" wrote in message news:e08v80\$jtq\$1@news.net.uni-c.dk... WayneL wrote: The figure of 1.2v seems to be use as the max an aqueous double-layer capacitor can operate. Is this voltage electrode material dependent or is it mainly based on the electrolyte e.g. water? If it is electrode dependent then I would expect this voltage to also vary. Or is this the activation energy require to decompose water? Any ref. to a paper kindly welcome. You don't need a reference. Look up the delta_G for the decomposition of water, and then translate that into delta-E. When electrolysing water, this is roughly the voltage needed to break up the water, and any voltage over that then goes into ohmic heating. -- Dieter Britz, Kemisk Institut, Aarhus Universitet This is a correct calculation for the theoretical value for the

decomposion of water. But you have to take into consideration the
overvoltage which is material dependent.
Guenter
Dieter Britz
science forum beginner

Joined: 04 May 2005
Posts: 45

Posted: Thu Mar 30, 2006 6:42 am    Post subject: Re: 1.2v limit of the double-layer

GÃ¼nter Semrau wrote:

 Quote: WayneL schrieb: Thanks Dieter for the help, again. E= delta_G/-nF from CRC delta_G = -237,000Jmol-1 :. -237,000/-2x96458 = 1.2285v WayneL "Dieter Britz" wrote in message news:e08v80\$jtq\$1@news.net.uni-c.dk... WayneL wrote: The figure of 1.2v seems to be use as the max an aqueous double-layer capacitor can operate. Is this voltage electrode material dependent or is it mainly based on the electrolyte e.g. water? If it is electrode dependent then I would expect this voltage to also vary. Or is this the activation energy require to decompose water? Any ref. to a paper kindly welcome. You don't need a reference. Look up the delta_G for the decomposition of water, and then translate that into delta-E. When electrolysing water, this is roughly the voltage needed to break up the water, and any voltage over that then goes into ohmic heating. -- Dieter Britz, Kemisk Institut, Aarhus Universitet This is a correct calculation for the theoretical value for the decomposion of water. But you have to take into consideration the overvoltage which is material dependent. Guenter

The original question was about the maximum voltage the
capacitor formed by the double layer can take. Up to 1.2 V,
there is hardly any current, but above that, water will be
electrolysed, and the capacitor will be leaking. How strongly,
depends on the electrode material (Butler-Volmer relations).
Pt would presumably be worst, and Hg best. But in all cases,
more than 1.2 V will make it leaky. Obviously, in another
solvent that voltage will be different.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet
Peter Lowrie
science forum beginner

Joined: 14 Jun 2005
Posts: 17

Posted: Fri Mar 31, 2006 12:29 pm    Post subject: Re: 1.2v limit of the double-layer

 Quote: This is a correct calculation for the theoretical value for the decomposion of water. But you have to take into consideration the overvoltage which is material dependent. Guenter

....In this case then the electrolysis voltage will have to be added to the
cell overvoltage. ie. Cell voltage in a charged state say 2 volts. 2v +
1.2285 = 3.2285V will be the necessary input voltage. We are getting
slightly off the orginal question though.

--
Regards,
Peter.
http://www.pelicom.net.nz
nagy@anl.gov
science forum beginner

Joined: 18 Jul 2005
Posts: 16

Posted: Mon Apr 03, 2006 7:56 pm    Post subject: Re: 1.2v limit of the double-layer

See an article on "electrochemical capacitors" in the Electrochemistry
Encyclopedia

http://electrochem.cwru.edu/ed/encycl/

Good luck: Z.N.

WayneL wrote:
 Quote: The figure of 1.2v seems to be use as the max an aqueous double-layer capacitor can operate. Is this voltage electrode material dependent or is it mainly based on the electrolyte e.g. water? If it is electrode dependent then I would expect this voltage to also vary. Or is this the activation energy require to decompose water? Any ref. to a paper kindly welcome. WayneL

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