2009/11/3 Horace Heffner <[email protected]> > Yes, those photos are of spots on the back side. However, I seriously > doubt there are not similar spots on the front side too.
To be sure, one would have to look obviously, but I wouldn't be surprised if there were no or less hot spots on the front. >> So, what is the direction of the deuterium flux through the back surface >> of a Pd cathode close to a plastic window do you think? Out, because >> electrolyte paths to get there are more resistive. > I don't think this is right. I think the interface will be about the > same on both sides. The potential drop through the mesh has little > practical effect. The mesh is made of wires. The front of the wire will receive more current than the back, which is close to the plastic window. > If you look at ordinary cathodes in electrolysis, even > big ones, they evolve hydrogen on both sides. Have you ever seen a > cathode evolve gas on only one side? Sure, they evolve hydrogen on both sides. But if you stick the back side against an insulator, that side will evolve less, because it will receive less current, down to zero current at the points of contact. Now, if the cathode is H/D permeable, the back side will still evolve gas, but that will be hydrogen which will have gone through the lattice, like it does in electrochemical hydrogen generators (electrolytic pressure on one side of a tubular Pd membrane, no electrolytic pressure on the other side) >> This confirms my suggestion, earlier in this thread, that the hot spots >> occur where the deuterium desorbs from Pd deuteride to D2 gas (not >> solvated D Horace), which chemically is an endothermic reaction. > These assertions are highly debatable. It appears you are assuming the > front side looks any different. I suspect so anyway. One way to know would be to put the anode wire between the mylar window and the cathode, and look at the cathode through the anode. Or use Mylar with a thin transparent metallization as the anode. >> Am I the only one to see how significant this is? > It might be significant if it were true. My point is I don't think > it is true in the sense you are implying it is. I don't think the > interface layers differ much in potential on either side of that cathode, > and certainly don't think one side is desorbing more than the other. > Except at cracks or any location where there is no electrolyte, > desorption must occur in reverse through the interface, which is > equilibrium with respect to hydrogen flow. The chemical concentrations > are in equilibrium as well, meaning there is no enthalpy of reaction > because there is no net reaction, except when there have been major > changes in potential such that the Pd loading is in change and loading or > de-loading is in progress. I expect the electrode filmed is pretty much > in equilibrium, when it comes to hydrogen concentrations of all kinds, in > the Pd (i.e. the loading percentage), and in the electrolyte across the > interface, and in terms of flow rates cross the interface, which net to > zero. There is thus no enthalpy of reaction involved, especially on the > front side vs back side. As you certainly know the loading factor (hydrogen content) depends on the current density. If you have more current density on one side, you will have more hydrogen content on that side. And then by diffusion, you will have migration to the other side and desorption. Implacable, it's like trying to inflate a punctured tyre. Michel
