Overpotential defined

In this review, the reference electrode used is defined as a platinum metal electrode exposed to hydrogen at the same temperature and electrolyte (e.g.. Nation) as the solution of interest. With this reference electrode, the electrode overpotential defined in eq 10 is the same as having the reference electrode located next to the reaction site but exposed to the reference conditions (i.e., it carries its own extraneous phases with it). Typical values for the reference conditions are those in the gas channels. If the reference electrode is exposed to the conditions at the reaction site, then a surface overpotential can be defined... 

As COR and OER occur simultaneously in the cathode, their kinetics are particularly important in evaluating carbon-support corrosion. The kinetics of OER is material-specific, dependent on catalyst composition and electrode fabrication.35,37 -39 A number of OER kinetics studies were done on Pt metal electrodes.37-39 However, there is a lack of OER kinetics data on electrodes made of Pt nano-particles dispersed on carbon supports. Figure 2 shows the measured OER current density with respect to the overpotential defined by Eq. (6).35 The 02 concentration was measured at the exit of a 50-cm2 cell using a gas chromatograph (GC). The 02 evolution rate (= 02 concentration x cathode flow rate) was then converted to the OER current density, assuming 4e /02 molecule. Diluted H2 (10%) and a thicker membrane (50 p,m) were used in the measurement to minimize H2 crossover from anode to cathode, because H2 would react with 02 evolved at the cathode and incur inaccuracy in the measured OER current density. Figure 2 indicates that the OER... 

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

Surface overpotential for a given reaction k,t]s = V — Vb,jt-Concentration overpotential defined by equation (5.75). Electrode potential with respect to an unspecified but common reference potential. 

In the derivations of Eq 3.14 and 3.19 for the metal oxidation current density, iox M, and the metal-ion reduction current density, ired M, it was not necessary to restrict the half-cell potential to its equilibrium value. Deviation from E M will occur if the potential of either the metal or the solution is changed, resulting in an overpotential defined in general by Eq 3.1. More specifically, small deviations are associated with charge-transfer polarization, and the overpotential is designated as ... 

Besides the mentioned theoretically founded overpotential relations, linear overpotentials as well as overpotentials defined by a set of (measured) data can be used. 

The overpotential defined by (4.85) is exclusively due to the local variation of concentration at the electrode surface, thus its name concentration overpotential. In certain works (particularly older ones), the term diffusion overpotential is also found— a term that underscores the importance of diffusion at the electrode surface. 

This is the expression of the electrode reaction rate. If we introduce an overpotential defined by [11.72], the current density becomes ... 

Referring to Figure 3.9, the overpotential defined by Equation 7-29 is positive on the anode and negative on the cathode. The cell potential is then a difference between the cathode and anode solids at the two ends of the cell ... 

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