Carbon dioxide electrolysis

[Clea]

I am trying to eletrochemically reduce CO2 into HCOO-/HCOOH at a rotating Sn cathode.

I used a 1M KOH anolyte and CH3OH with a lil bit of water(as a proton donor) catholyte. I also added in KCl to improve conductivity.

 

Did preelectrolysis and bubbled N2 into the catholyte for an hour plus minus. Did a pretty good job at reducing graph noise and ghost peaks. Then I pumped in CO2 for another hour. I used a pretty low flowrate though, as the electrolyte will spit out if I set it any higher. The bubble size and flowrate are not unlike the ones produced in a fish tank

 

The Sn cathode I used is Sn plated on glassy carbon. The quality is pretty bad as I only deposited it from a 4M HCl + 0.01M SnCl2.2H20 catholyte and 0.5M H2SO4 anolyte. It corrodes very very much faster than I'd like it to and I have to keep redepositing it every now and then.

 

Ran linear sweep voltammetry at 0 to -2 V vs Ag/AgCl but I couldn't get any reduction curve other than for H2 evolution which kicks off at -1.5 V. I am at a loss. I've tried

1. Drastically increasing the rotating speed

2. Switching out the Ag/AgCl reference electrode with another one.

3. Repeating the work many times

all for naught.

 

It is ironic because when I used a water catholyte, it worked just fine.

Can you guys help brainstorm of other things I can try?

Thanks

Now thinking back, I am starting to suspect it's the KCl salt. The KCl didn't really dissolve in my methanol. But then I dissolved the same amount of salt in another catholyte with 50% water content and nothing happened to that one either.

Edited July 8, 2014 by Clea

[Enthalpy]

Not helping you, but for my personal information:

- Why do you add KCl? I understand the conductivity may increase, but this would be wasted current, which only moves unwanted species. Or?

- What's special with Sn plated on carbon? Sn foil is available and cheap, so how does plating improve?

[Clea]

I suppose that competing cations will not be reduced unless they have a greater standard electrode potential than the cation I'm interested in reducing.Am not particularly concerned about the product obtained in the anode side unless it is rate limiting (which I don't know yet btw). Haven't gave much thought on the role of the KCl other than simply filing it under 'to increase conductivity,' The electrolyte should have enough ions to carry charge to the anode/cathode to balance the charge on both sides so the cathode doesn't get a backlog of electrons on it, and the anode, excess holes.

 

Actually I only have a glassy carbon and a Pt rotating tip, so I was making do with what I've got. Which is fine really, because the process corrodes Sn fairly quick and I'd end up with a damaged expensive rotating Sn tip. Commercial Sn foils should have some impurities such as Pb etc which may poison or interfere with my process.

Edited July 13, 2014 by Clea