Gradient electrodes

Fig. 1 Top view of gradient electrodes on a glass substrate. The height of the electrodes is 55 nm. The electrode gap is not true to scale and the gradient (5-50 pm) has been exaggerated for clarity. The dotted line shows the axis along which the SFM images were taken. Reprinted with permission from Macromolecules [20]. Copyright 2008 American Chemical Society...

Prasanna M, Cho EA, Kim HI, Oh IH, Lim TH, Hong SA. Performance of proton-exchange membrane fuel cells using the catalyst-gradient electrode technique. J Power Sources 2007 166 53-8. 

Figure 23.30. Comparison of the cumulative carbon corrosion following a 24-h 1.2 V potentiostatic hold at 80 °C in 1 M H2SO4 for two commercial carbons COl, C02 and one heat-treated carbon C03, and platinum and Pt/Co alloy catalysts on these carbons [95]. (Reprinted from Journal of Power Sources, 166(1), Prasanna M, Cho EA, Kim H-J, Oh I-H, Lim T-H, Hong S-A, Performance of proton-exchange membrane fuel cells using the catalyst-gradient electrode technique, 18-25, 2007, with permission from Elsevier.)...

Migration is the movement of ions due to a potential gradient. In an electrochemical cell the external electric field at the electrode/solution interface due to the drop in electrical potential between the two phases exerts an electrostatic force on the charged species present in the interfacial region, thus inducing movement of ions to or from the electrode. The magnitude is proportional to the concentration of the ion, the electric field and the ionic mobility. 

O, a large current is detected, which decays steadily with time. The change in potential from will initiate the very rapid reduction of all the oxidized species at the electrode surface and consequently of all the electroactive species diffrising to the surface. It is effectively an instruction to the electrode to instantaneously change the concentration of O at its surface from the bulk value to zero. The chemical change will lead to concentration gradients, which will decrease with time, ultimately to zero, as the diffrision-layer thickness increases. At time t = 0, on the other hand, dc-Jdx) r. will tend to infinity. The linearity of a plot of i versus r... 

In Figure 6.4, the two electrodes are marked as cathode and anode, arising from the application of an external voltage between them. Before any discharge occurs, the electric-field gradient between the electrodes is uniform and is simply the applied voltage divided by the their separation distance, as shown in Figure 6.7. 

If the electrodes are moved closer together, the positive column begins to shorten as it moves through the Faraday dark space because the ions and electrons within it have a shorter distance through which to diffuse. Near the cathode, however, the electric-field gradient becomes steeper and electrons from the cathode are accelerated more quickly. Thus atom excitation through collision with electrons occurs nearer and nearer to the cathode, and the cathode glow moves down toward the electrode. 

Particularly in mass spectrometry, where discharges are used to enhance or produce ions from sample materials, mostly coronas, plasmas, and arcs are used. The gas pressure is normally atmospheric, and the electrodes are arranged to give nonuniform electric fields. Usually, coronas and plasmas are struck between electrodes that are not of similar shapes, complicating any description of the discharge because the resulting electric-field gradients are not uniform between the electrodes. 

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