cathode bigger than anode

[Scorpion812]

Hey everyone, so i am thinking of getting myself a lead dioxide anode from here: http://store.theamateurchemist.com/lead-dioxide-anode-2-by-3-with-4-stem/

and a titanium cathode from the same store, but they are all bigger than that lead dioxide one. I know its not ideal, but does that matter?

if anyone has some links for suited cathodes, please post them here

 

thx

[WSM]

Hey everyone, so i am thinking of getting myself a lead dioxide anode from here: http://store.theamateurchemist.com/lead-dioxide-anode-2-by-3-with-4-stem/

and a titanium cathode from the same store, but they are all bigger than that lead dioxide one. I know its not ideal, but does that matter?

if anyone has some links for suited cathodes, please post them here thx

 

The description says 2" x 3", but the photo looks more like 2" x 4". I wonder which one is right?

 

Either way, for such a small setup, I don't think oversized cathodes will be a major problem.

 

If you order the LD anode, you may want to order several (for backups or for making a larger cell by sandwiching cathodes and anodes together).

 

Nice find, and good luck.

 

WSM

[Scorpion812]

 

The description says 2" x 3", but the photo looks more like 2" x 4". I wonder which one is right?

 

Either way, for such a small setup, I don't think oversized cathodes will be a major problem.

 

If you order the LD anode, you may want to order several (for backups or for making a larger cell by sandwiching cathodes and anodes together).

 

Nice find, and good luck.

 

WSM

i dont really have the money to get alot of these, how much kclo4 and kclo3 will i be able to produce with those electrodes, in lets say one week in a 3L bucket running on KCl?

[WSM]

i dont really have the money to get alot of these, how much kclo4 and kclo3 will i be able to produce with those electrodes, in lets say one week in a 3L bucket running on KCl?

 

I don't know (every cell is different with myriad variables), but you can set it up and try it to see how it goes. From my experience, use the MMO/titanium for making chlorate and save the LD/titanium for perchlorate production.

 

WSM

Edited September 13, 2017 by WSM

[MadMat]

Unless someone knows otherwise, the surface area of the cathode should be larger than the anode. More surface area = lower current density at the cathode. In electrochemical reactions current density is the driving force behind the reaction. In a chlorate/perchlorate cell, you are attempting to oxidize chloride into chlorate/perchlorate. Oxidation occurs at the anode. Unfortunately, reduction (the opposite of oxidation) can occur at the cathode. So, by reducing the current density at the cathode you can minimize this. There are also catalysts that will reduce reduction at the cathode, namely fluorides or chromates. The type of catalyst used is usually determined by the anode material. With lead dioxide anodes, a fluoride catalyst is recommended and is usually added in the range of ppm.'s (100-200?)

I haven't done enough experimental measurements to know the exact current density (usually measured in amps/square inch) range that is optimal for chlorate or perchlorate production. If you have too much surface area of both anode and cathode in your cell, even with a rather large amount of total current going through the cell, it won't produce any of the desired products. Maybe WSM has more knowledge on the minimal current density needed.

Edited September 15, 2017 by MadMat

[Scorpion812]

Unless someone knows otherwise, the surface area of the cathode should be larger than the anode. More surface area = lower current density at the cathode. In electrochemical reactions current density is the driving force behind the reaction. In a chlorate/perchlorate cell, you are attempting to oxidize chloride into chlorate/perchlorate. Oxidation occurs at the anode. Unfortunately, reduction (the opposite of oxidation) can occur at the cathode. So, by reducing the current density at the cathode you can minimize this. There are also catalysts that will reduce reduction at the cathode, namely fluorides or chromates. The type of catalyst used is usually determined by the anode material. With lead dioxide anodes, a fluoride catalyst is recommended and is usually added in the range of ppm.'s (100-200?)

I haven't done enough experimental measurements to know the exact current density (usually measured in amps/square inch) range that is optimal for chlorate or perchlorate production. If you have too much surface area of both anode and cathode in your cell, even with a rather large amount of total current going through the cell, it won't produce any of the desired products. Maybe WSM has more knowledge on the minimal current density needed.

 

i thought the same before, but i saw nurdrages video about his cell in which he intentionally only used a small Ti strip even though he had humongous mmo anodes. but the anodes i ordered are the same size as the cathode, is that a problem? in an emergency, i could still just use my old Ti stick and add it for more surface area to the cell. I also wanted to use K₂Cr₂O_{7 as additive, but my parents are scared of me getting cancer...} You said that fluorides can also be used with PbO2, is it also MMO compatible? And it isn't carcinogenic, so it seems like a good alternative and i wonder why it hasn't been used before...

[AustralianPyromaniac]

I was just thinking the same thing about the nurd rage video but he still could have been wrong. I think mumbles of WSM will clear this up when they get a chance but I am interested to know as I'm always up for increased efficiency. Certainly my electrodes from the amateur chemist are exactly the same size so I would assume that it makes little difference otherwise it would be a quite wide spread practice considering how easy it is to make a smaller/larger cathode.

 

Regards, AP

[WSM]

Unless someone knows otherwise, the surface area of the cathode should be larger than the anode. More surface area = lower current density at the cathode. In electrochemical reactions current density is the driving force behind the reaction. In a chlorate/perchlorate cell, you are attempting to oxidize chloride into chlorate/perchlorate. Oxidation occurs at the anode. Unfortunately, reduction (the opposite of oxidation) can occur at the cathode. So, by reducing the current density at the cathode you can minimize this. There are also catalysts that will reduce reduction at the cathode, namely fluorides or chromates. The type of catalyst used is usually determined by the anode material. With lead dioxide anodes, a fluoride catalyst is recommended and is usually added in the range of ppm.'s (100-200?)

I haven't done enough experimental measurements to know the exact current density (usually measured in amps/square inch) range that is optimal for chlorate or perchlorate production. If you have too much surface area of both anode and cathode in your cell, even with a rather large amount of total current going through the cell, it won't produce any of the desired products. Maybe WSM has more knowledge on the minimal current density needed.

 

The power (measured in watts) applied to the cell is composed of voltage (electromotive force, somewhat like pressure of electrons) and amperage (intensity, somewhat like volume of electrons). When discussing minimal parts of the power applied, it's my understanding that there's a minimal voltage required to get the system operating properly with the correct oxygen overpotential, but as for the current; once the minimum voltage is met the current determines the speed at which the reaction occurs.

 

It's analogous to filling a pond on a small rise from a lower elevation. First you need to overcome the elevation difference, which the voltage does after about 1.6 volts or so (if I remember correctly), and then the size of the hose used (amperage) determines how fast or slow the pond fills. So more current drives the reaction (plus heat) faster, and less current drives it slower with less heat generated. As to current density, a happy medium is usually sought for (fast enough without too much heat).

 

I hope this helps without muddying the issue.

 

WSM

 

Edit: Internationally, current density is usually measured in Amps per square centimeter. There are about 6.54 cm² per in², so that multiplier works for the conversion. Typically, chlorate is produced at a current density goal of 0.3A/cm², which is just a little less than 2A/in^2, if that helps.

Edited September 16, 2017 by WSM

[WSM]

I was just thinking the same thing about the nurd rage video but he still could have been wrong. I think mumbles of WSM will clear this up when they get a chance but I am interested to know as I'm always up for increased efficiency. Certainly my electrodes from the amateur chemist are exactly the same size so I would assume that it makes little difference otherwise it would be a quite wide spread practice considering how easy it is to make a smaller/larger cathode.

Regards, AP

 

I'm unfamiliar with the video being discussed (or I've forgotten it), but I usually try for equally sized electrodes except that with flat electrodes, I double the cathode area to maximize the anode exposure to the cathodes (I place the flat anode between two cathodes roughly the same size as the anode).

 

The exact size of the electrodes plays a minor role in hobbiest sized cells. In an industrial setting, the electrode size can play a much larger role, where they work to squeeze the highest efficiency out of the system for the greatest return on investment. Heck, we're just glad the thing works; even if it's terribly inefficient.

 

WSM

[WSM]

Unless someone knows otherwise, the surface area of the cathode should be larger than the anode. More surface area = lower current density at the cathode. In electrochemical reactions current density is the driving force behind the reaction. In a chlorate/perchlorate cell, you are attempting to oxidize chloride into chlorate/perchlorate. Oxidation occurs at the anode. Unfortunately, reduction (the opposite of oxidation) can occur at the cathode. So, by reducing the current density at the cathode you can minimize this. There are also catalysts that will reduce reduction at the cathode, namely fluorides or chromates. The type of catalyst used is usually determined by the anode material. With lead dioxide anodes, a fluoride catalyst is recommended and is usually added in the range of ppm.'s (100-200?)

I haven't done enough experimental measurements to know the exact current density (usually measured in amps/square inch) range that is optimal for chlorate or perchlorate production. If you have too much surface area of both anode and cathode in your cell, even with a rather large amount of total current going through the cell, it won't produce any of the desired products. Maybe WSM has more knowledge on the minimal current density needed.

 

The problem with fluorides is that they can quickly destroy titanium electrodes, particularly the anode as there's some electrical protection of the cathode as the system runs. So if we're using MMO or LD on titanium, I'd avoid the use of soluble fluorides as a catalyst. The one place it is vital to use sodium fluoride is running sodium chloride all the way to sodium perchlorate, but this is done using LD with a graphite substrate (GSLD) anode.

 

The common industrial method of catalyzing the run (or rather the process, where a continuous system is the norm), is to use dichromates. As you can imagine, particular attention is called for to avoid contamination of the environment or just the toxic effects to yourself (let alone family members or pets). I've managed to do most of my research without using hexavalent chromium in the cells. If you elect to use them, please take the extra steps to contain the system and keep it safe.

 

WSM

[Scorpion812]

Thanks for clearing this up wsm! That really sucks that i can't use potassium fluoride. Is there any other non toxic additive as a replacement for dichromate? What safetx measure should i take if i have to with it (if i can even get it)

[MadMat]

From what I have read, dichromate is the preferred catalyst when using platinum anodes.

[WSM]

Thanks for clearing this up wsm! That really sucks that i can't use potassium fluoride. (1)Is there any other non toxic additive as a replacement for dichromate? (2)What safetx measure should i take if i have to with it (if i can even get it)

 

(1) Not that I'm aware of, but I don't know everything. Maybe Mumbles is aware of something that may help.

 

(2) The idea of placing the cell in a tub, for a hazardous spill containment, might be a useful first step. Several have suggested and used the cement-mixing trays available from large box home centers, as a containment for a bucket cell. Having a water tight system without leaks is also highly recommended.

 

Having chemicals and materials on hand which can be used to neutralize or reuse the hexavalent chromium compounds for other purposes, is a good idea. Barium or lead chloride solutions can be used to create barium chromate or lead chromate precipitates, which can be removed by filtration. With potassium chromate solution (leftover from treating magnesium with K₂Cr₂O₇₎, I once converted it with copper nitrate solution, to reddish-brown copper chromate which can be roasted in a covered crucible to form the expensive catalyst, black copper chromites.

 

Preparing the work space with a water-tight, disposable covering and possibly a spatter shield and fume hood with filter could prevent any air-borne particles from getting out (see Swede's preparations for plating LD on MMO, in his blogs). Some may feel this is overstating the actual hazards, but keeping hexavalent chromium out of the environment (especially groundwater) is no minor concern. Remember the movie, "Erin Brockovich"? It was based on real events in California, and made her wealthy (and famous).

 

Take dichromate contamination seriously, and work to prevent any sort of spill or leak.

 

WSM

Edited September 17, 2017 by WSM

[WSM]

From what I have read, dichromate is the preferred catalyst when using platinum anodes.

 

If using platinum anodes for making chlorates, that sounds like it might be a good idea, (I'm not sure).

 

For converting sodium chlorate to perchlorate, I don't think it's necessary or even desired. My cell worked fine without it*, and the platinum plating showed no signs of wear.

 

WSM

 

*Edit: I did run the perchlorate cell with a power supply that has constant current (CC) capability and limited the current to 1.5A/cm², because I was concerned about damaging the anode and excess ozone generation. The process does generate ozone, but when limiting the current the amount of ozone produced wasn't overpowering and escaped through the cell vent.

Edited September 22, 2017 by WSM