chief3 - 15-12-2014 at 20:48
In the following thread there is discussion about a flow-battery, where I propose a hypothetical type that might work by electrolyzing Na2SO4 to NaOH
and H2SO4 ...
https://www.sciencemadness.org/whisper/viewthread.php?tid=51...
So there would be 2 cells, separated by a diaphragma of maybe clay, or for testing, by some rough pottery ...
==> Originally there would be a Na2SO4-Solution ... ,
==> a voltage would be applied for charging ... ,
===> at the anode there would be generated H2SO4 ... ,
===> and at the kathode there would be generated Na, which would instantly react to NaOH ... .
Now the kathode may be made of stainless steel , but what about the anode ?
Which materials would stand it for a long time as an anode ... and would at the same time remain conductive ?
==> Carbon-rods ?
==> Lead ?
==> What other metals ? (preferredly cheap and available ...)
Most metals probably just would dissolve, and the few ones which wouldn't probably will develope a layer of quite non-conducting sulfate ... ...
===================
What do you think ? 
j_sum1 - 15-12-2014 at 21:51
Graphite erodes.
Lead dioxide is a possibility.
Platinum (as usual) is the best.
markx - 16-12-2014 at 00:35
From cheaply available options you are left with either graphite if erosion and the resulting sludge is no objection to your goal or with metallic
lead if you do not wish to deal with the graphite dust in the solution.
Metallic lead will form a PbO2 layer on the surface in situ when subjected to anodic potentials in sulfate or sulfuric acid conditions. This layer
will form an inert anode that works acceptably well for electrolysis of sulfuric acid solutions (or Na sulfate for that matter).
What you are basically conducting in this cell is electrolysis of water for the most part:
On the anode you oxidize water : H2O-> (2H+) + O2 - 2e
Oxygen is released as bubbles and 2 electrons stripped into the circuit. Also 2 protons remain in the solution turning it acidic. Since you have
sulfate ions present we view this anodic solution as sulfuric acid.
On the cathode you reduce water: 2H2O-> (2OH-) + H2 + 2e
Hydrogen is released and OH- ions turn the solution alkaline. Since you have Na+ ions present we view this as a solution of sodium hydroxide. Gaseous
hydrogen is released and two electrons from the circuit are bound to the OH-. Sodium will not form on the cathode in solutions containing water.
Simply because it is energetically easier to reduce the hydrogen into its free state compared to sodium.
If a membrane is not used to separate the anodic and cathodic compartments then the H+ and OH- will recombine into H2O again and the cycle repeats
until no more water is present.
Well, this is a very simplified way of describing the system without any bells and whistles of the electrochemical finesse. But perhaps is serves as a
guidance to the inner workings of the reaction.
For the flow battery application why would you start with Na2SO4 solution? Start with H2SO4 and NaOH solutions. What you do in a flow battery is
basically separate the charge transfer part of the H2SO4 and NaOH recombination into the outside electrical circuit connected to the battery and force
it to do meaningful work in the process.
The direct recombination of NaOH and H2SO4 solutions basically yields to:
(H+) + (OH-)-> H2O
In a flow battery the same summary reaction is conducted by forcing charges through the outside electrical circuit....simply put it is the reverse
electrolysis of Na2SO4 solution in this particular case. The hydroxide and acid solutions have the potential to recombine into the lower energetic
state of sulfate, the membrane prevents direct recombination and hence the system pumps charges through the outside circuitry to conduct reverse
electrolysis on itself at the expense of energy that can be released by the recombination of components in the system(with losses at every stage).
Basic principle of any battery or fuel cell for that matter. Hope that made any sense.
[Edited on 16-12-2014 by markx]
chief3 - 21-12-2014 at 19:21
Aha, thanks ... ; so Pb is possible ... and graphite erodes in sulfate-cells too ... , not only in chlorine-containing chemistrys ... ... - I wasn't
sure about that ... ...