Partially covered electrode

T. Gueshi, K. Tokudam, and H. Matsuda, Voltammetry at partially covered electrodes Part I. Chronopotentiometry and chronoamperometry at model electrodes. J. Electroanal. Chem. 89, 247-260 (1978). 

If (49) and (15) are valid, even for small and overpotentials, all the surface behaves as an active one if jo/jh oo. This means that the application of a partially covered inert substrate with active micro and nanoparticles will be more effective for the cases of fast electrochemical reactions. It is obvious that the above reasoning is valid not only for an inert substrate covered with microparticles, but also for any kind of partially covered electrode. 

In both cases, the current density on the partially covered electrode was 21 mA cm-2. The current density on a completely covered graphite electrode in the nitrate solution was 22 mA cm-2 at an overpotential of 40 mV... 

Another way to determine 9 is based on the measurement of the double layer capacity, whose value is smaller in the presence of an adsorbed inhibitor. As long as the contributions of the covered and uncovered parts of the surface are simply additive, the double layer capacity of a partially covered electrode Ce is equal to ... 

Gueshi T, Tokuda K, Matsuda H (1979) Voltammetry at partially covered electrodes. Part n. Linear potential sweep and cyclic voltammetry. J Electroanal Chem 101 29-38... 

When ion spillover-backspillover is fast, i.e. when A=eAUwR, then even under open-circuit the catalyst electrode surface is partially covered by backspillover O2 ions. Thus it is the removal of such ions via cathodic polarization which causes NEMCA under such conditions. 

Figure 2-31 showed the cyclic voltammogram of the platinum electrode partially covered with COad in 0.5 M sulfuric acid. The potential was held at -(-400 mV during the CO adsorbing period (10 s, 100 CO bubbled) and the COad removing period (10 min., 100% N2 bubbled). After that, the potential was swept in the cathodic direction to +50 mV first and then in the anodic direction to 1500 mV to take the voltammogram. 

Solvent Interactions. Consider an electrode surface originally free of contact-adsorbed ions. The metal is partially covered with solvent molecules, and the ions, beyond the IHP, may or may not be solvated (Fig. 6.90). 

A series of MSFTIR spectra of CO adsorbed on nm-Pt/GC and Ru-modified nm-Pt/GC electrodes are illustrated in Figure 17(b) [48]. The AIREs are manifested in all spectra. We observe two COl bands from spectra c, d, and e one is the COL-Pt band near 2065 cm and another is the COl-Ru band close to 2025 cm The COl-Ru band appeared as a shoulder peak in spectrum b. It can be seen that the intensity of the COL-Pt band progressively decreases and the intensity of the COl-Ru band increases with the increase of the quantity of Ru deposited on the nm-Pt/GC surface. Nevertheless, the COl-Ru band remains discernible in spectrum e for 10 Ru deposition potential cycles, which may indicate that the nm-Pt/GC surface is still partially covered by Ru. The results imply that the deposition of Ru on an nm-Pt/ GC surface is less efficient than the inverse process, i.e., the deposition of Pt on an nm-Ru/GC surface. Similar results have been reported concerning in-situ FTIRS studies of CO adsorption on Ru ad-atom or Ru nanoparticle modified Pt(lll) single-crystal electrodes [76-78], in which a COL-Pt band near 2070 cm and a COl-Ru band around 2010 cm were observed in the spectra. 

Figure 3. Schematic presentation of the cross section of the diffusion layer of a partially covered inert electrode with hemispherical active particles, where rm is the radius of the microelectrodes, 8 is the diffusion layer thickness of the macroelectrode, Cq and C are the bulk and the surface concentrations of reacting ions, respectively, x is the ratio of the distance between the centers of neighboring particles and the particle diameter, and 8 > r. Reprinted from ref.7 with permission of Elsevier.

A mathematical model can be derived under the assumption that the electrochemical process on the microelectrodes inside the diffusion layer of a partially covered inert macroelectrode is under activation control, despite the overall rate being controlled by the diffusion layer of the macroelectrode. The process on the microelectrodes decreases the concentration of the electrochemically active ions on the surfaces of the microelectrodes inside the diffusion layer of the macroelectrode, and the zones of decreased concentration around them overlap, giving way to linear mass transfer to an effectively planar surface.15 Assuming that the surface concentration is the same on the total area of the electrode surface, under steady-state conditions, the current density on the whole electrode surface, j, is given by ... 

Equation (7) is valid for the complete active electrode surface. On the other hand, if the inert substrate is partially covered with the same active material, the polarization curve equation is given by (44).7... 

Figure 12. The physical model of a partially covered inert electrode with active grains and a completely covered inert electrode (a) a graphite electrode completely covered by deposition from the ammonium bath current density on the electrode completely covered with silver was 62.5 mA cm-2 at an overpotential of 120 mV in the nitrate solution magnification 500 x (b) the silver deposit on the graphite electrode after the polarization measurements ended at an overpotential of 120 mV in the nitrate solution magnification 500 x current density on such electrode was 59.4 mA cm-2 at the same overpotential in the nitrate solution. Reprinted from ref.7 with permission of Elsevier.

The current density on the graphite electrode partially covered with silver grains obtained from the nitrate solution by a pulse of an overpotential of 100 mV for 20 ms and by further growth at an overpotential of 40 mV for 30 s (Fig. 14a) is practically the same as the current density on a massive silver electrode at an overpotential of 40 mV The same occurs with a graphite electrode covered with silver grains by a pulse of an overpotential of 500 mV for 5 ms and by further growth at an overpotential of 40 mV for 5 s (Fig. 14b). It can be seen that the deposits depicted in Fig. 14a and this in Fig. 14b,... 

Hence, the procedure described above could be unavoidable for the elucidation of the polarization behavior of an inert electrode partially covered with small active grains, probably with nanoparticles, too. 

On the other hand, due to the overlapping of the nucleation exclusion zones,7,35,36 deposition on the partially covered graphite electrode is an excellent illustration of the above discussion. Namely, the diffusion layer on the inert electrode partially covered with grains of active metal can be formed and diffusion control established in the same way as on an electrode of massive active metal if the deposition process is characterized by a large jo/jh-1 If dendrites are formed on the grains, their tips enter the bulk solution and overall control of the deposition process becomes activation or mixed controlled. 

Naturally, the same effect can be expected if some very fast electrochemical process, other than electrodeposition, occurs on the inert electrode partially covered by dendrites of active catalyst, especially if concentration of reacting ion is low.60 This could be of great importance for the activation of inert substrates for catalytic purposes. 

The gas diffusion layers, one next to the anode and the other next to the cathode, are usually made of a porous carbon paper or carbon cloth, typically 100 pm to 300 pm thick. Fig. 14 shows a porous GDL made of carbon paper, which is partially covered by catalyst layer. The porous nature of the backing layer ensures effective diffusion of feed and product components to and from the electrode on the MEA. The correct balance of hydrophobicity in the backing material, obtained by PTFE treatment, allows the appropriate amount of water vapor to reach the MEA, keeping the membrane humidified while allowing the liquid water produced at the cathode to leave the cell. The permeability of oxygen in the GDL affects the limiting current density of ORR, and thus the performance of PEMFC.[ l... 

The Diffusion Categories Following a classification in a recent review [42], the situation of the microparticle-modified electrode can be described and treated as that of a spatially heterogeneous, or partially blocked, electrode. The solid macroelectrode is partially covered with microparticles, which - for the sake of theoretically... 

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