2009/11/4 Horace Heffner <[email protected]>: >> 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.
> I'd like to see a reference on that relationship. I don't think there is a > linear relationship between loading flux and hydrogen evolution or current > density. Then you don't think right ;-) In the case of single side electrolytic loading of Pd membranes (with the other side unloaded), unpublished research I have participated in, based on experimental data from various sources, does show a nice linear relationship between steady state permeated flux through the membrane and current density, for current densities above 10mA/cm^2. The proportionality factor is inversely proportional to thickness, apart from this it only depends on the cathode temperature (increases by a factor 1.6 for every 20°C increase), and on the isotope (permeation for D is higher than that for H). Here are the empirical formulae we obtained: H2: Permeated flux (mL hr^-1 cm^-2) = 4.6 / thickness (µm) * current density (mA cm^-2) * 1.6^((Tcath(°C)-45)/20) D2: Permeated flux (mL hr^-1 cm^-2) = 7.4 / thickness (µm) * current density (mA cm^-2) * 1.6^((Tcath(°C)-45)/20) Now if there are different non null current densites on both sides of a membrane, I speculate that the flux will be proportional to the difference between the front and back side current densities, but I know of no experimental data on this. Michel P.S. your comments below may be correct though, I haven't studied the relationship between permeated flux and overpotential (which I don't think is linear with current density) > Hydrogen evolution in the form of bubbles is almost entirely the > result of electronation, the tunneling of electrons across the interface. > The tunneling probability is exponential in terms of the overpotential. > When hydrogen is removed from the electrolyte side of the interface it > drops the potential there. The solvated hydrogen concentration is dropped > up close to the interface. The path for protons through the interface to > the Pd is more linear in nature (compared to the electrons path in the > opposed direction across the interface) with the overpotential because it > requires a couple short distance proton tunnelings, and two water molecule > rotations which are more linear with overpotential. > > The metal side of the interface is clearly very close to the same potential > everywhere. There ion conduction path to the back side covers many routes, > but in the case of the mesh electrode is primarily through the mesh, a mm or > less. In either case, the back side, or front side, the ion conduction rate > is largely a product of diffusion and not potential gradient, at least in > ordinary electrolytic cells, because most of the potential drop and vastly > more gradient is at the interfaces. The field gradient between electrodes is > so small that concentration gradients have more effect on the diffusion > rates. However, it is unfortunately not clear at all what kinds of > potentials and currents are involved. We are left to guess what is > happening > > As you can see, I have a very difficult time seeing that such a small change > in potential can be responsible for that much "fusion". I may be highly > biased though, especially by my work with anode discharges, which sound and > look very similar, except in the visual range. > > I find it concerning that the sound track on the film sounds very much like > an electrospark experiment. It sounds like cavitation. The noise shows up > when the cathode heats up, just like electrospark. I eventually started > heating up my cells to boiling before starting (I was running a boiloff > protocol) just to get more uniform and faster data. I think the cathode > white dots may in part be steam bubbles. > > Best regards, > > Horace Heffner > http://www.mtaonline.net/~hheffner/ > > > > >
