Sunday, September 20, 2009

Viologen-Mediated Direct Carbohydrate Fuel Cell 2

Dear Dr. Dean Wheeler,

Thank you so much for sending the power point presentation. I thoroughly enjoyed every slide. I was very excited to see that you were able to measure respectable current densities. Several of your slides actually resolved some questions I had had at the time. I especially appreciated the Pourbaix diagram. I just wanted to pass on some more background info that came to mind after going through your power point presentation.In my first email, I described unsuccessfully measuring CO2 evolution by gas chromatography (GC). I understood that CO2 would not exist in significant amounts at basic pH but it was nice to see your potential/pH diagram map out the various phases in this electrochemical system. After seeing your presentation, I had a few additional ideas as possible models to characterize the chemistry better.
When I was playing around with MV and glucose, I was routinely using high-Molar and saturated glucose solutions and a limiting amount of MV. Using Dr. Watt's Schlenk line/manifold, I made a basic, aqueous glucose solution anaerobic and then added an anaerobic solution of MV. The solution immediately turned deep blue. I see that you also discovered that the reaction occurs with both reducing and non-reducing sugars. I think if I had acidified the solution, any carbonate would have been converted to CO2, which I could have then injected into a Hewlett Packard-CG and been able to make a quantitative estimate of the extent of the reaction.

I also noted that you monitored the MV-glucose reaction using a pressurized cell and indirectly calculating the O2 consumption based on pressure differences. A simple variation on this theme would be to measure O2 directly. Dr. Watt had a direct electrochemical dissolved-oxygen probe in the lab. The probe worked by covering the end of the electrochemical probe with a cellulose acetate film with a concentrated KCl solution between the film and the end of the probe. I believe there are several much simpler oxygen concentration probes on the market today.

What I did was created an ambient-pressure reaction cell using two pieces of mortised Plexiglas bolted together which created a central reaction well similar to your reaction apparatus. The oxygen probe fit snuggly into a bore hole drilled into one side of the block; the tip of the probe inserted into the central reservoir. Stop cock vacuum grease was used to make the probe insertion air-tight. Another deep but small-diameter hole was also cut into the Plexiglas. The small-diameter hole was just big enough to insert a long small-gauge syringe needle down into the central reaction reservoir. I found by leaving this deep bore hole open, I could normalize the reaction to atmospheric pressures but the bore hole didn't allow ambient oxygen to enter the reaction system to a significant degree. The bore hole was so deep and so small-gauge, that the diffusion of oxygen from outside was negligible (see figure).

The electrochemical oxygen probe with the cellulose-acetate membrane was very pressure sensitive; so this setup, which operated at atmospheric pressure, allowed me to add and remove small volumes of solution without altering the pressure of the system. And believe it or not, ambient oxygen didn't diffuse into the system. Part of this is because the rate of the diffusion of atmospheric oxygen into and out of solution is porportional to the exposed surface area. In my ambient pressure reaction vessel, the surface area exposed to the atmosphere was only the size of a small gauge needle.

Now that I think about it, Dr. Watt had a glass reaction vessel I found especially blown for the electrochemical oxygen probe. The peace of glassware was composed of inner and outer glass cylinders with 2 communicating ports from the outer to the inner cylinder. One port was sized for the oxygen probe and the second port was sized to fit a rubber stopper. Instead of a rubber stopper I used a cylindrical Plexiglas stopper that fit snug into this second port. This Plexiglas stopper was modified with a small-gauged bore hole drilled lengthwise; providing communication between the inner reaction reservoir and the outside.

This glassware piece had 2 more ports which emanated from the outer cylinder and opened up into the space between in the inner and outer cylinders allowing fluid circulation around the inner reaction reservoir for purposes of temperature control.I used this same cellulose-acetate membrane material used to cover the oxygen probe as a membrane to make a concentrated KCL salt bridge and as the membrane material for my fuel cell. I am not sure if the cellulose-acetate membrane allowed diffusion of MV and glucose molecules or not. I didn't have any Nafion so, it was the best I could come up with.When I used concentrated alkalinized methanol in the reaction, MV was reduced and the solution turned blue. But when the MV was exposed to oxygen again and allowed to oxidize, a yellow-orange species was produced. I am not sure what this species was. It may have been a MV dimer. We never characterized it. The more I reduced and reoxidized MV, the more orange the solution became. This yellow-orange species was not seen with glucose. Using glucose, aqueous MV could be reversibly reduced and reoxidized again and again without degradation.If you want to make doubly reduced MV, you add hexanes or other organic phase over water. You add Zinc powder to an aqueous MV and shake. doubly reduced MV diffuses and is soluble in the organic phase and forms a deep orange color.


David D. Brosnahan MD MS

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