Diffusion overpotentials

At higher current densities, the primary electron transfer rate is usually no longer limiting instead, limitations arise tluough the slow transport of reactants from the solution to the electrode surface or, conversely, the slow transport of the product away from the electrode (diffusion overpotential) or tluough the inability of chemical reactions coupled to the electron transfer step to keep pace (reaction overpotential). 

Diffusion overpotential. When high current densities j exist at electrodes (at the boundary to the electrolyte), an impoverishment of the reacting substances is possible. In this case the reaction kinetics are determined only by diffusion processes through this zone, the so-called Nernst layer. Without dealing with the derivation in detail, the following formula is obtained for the diffusion overpotential that occurs (with as the maximum current density) ... 

In order to obtain a definite breakthrough of current across an electrode, a potential in excess of its equilibrium potential must be applied any such excess potential is called an overpotential. If it concerns an ideal polarizable electrode, i.e., an electrode whose surface acts as an ideal catalyst in the electrolytic process, then the overpotential can be considered merely as a diffusion overpotential (nD) and yields (cf., Section 3.1) a real diffusion current. Often, however, the electrode surface is not ideal, which means that the purely chemical reaction concerned has a free enthalpy barrier especially at low current density, where the ion diffusion control of the electrolytic conversion becomes less pronounced, the thermal activation energy (AG°) plays an appreciable role, so that, once the activated complex is reached at the maximum of the enthalpy barrier, only a fraction a (the transfer coefficient) of the electrical energy difference nF(E ml - E ) = nFtjt is used for conversion. 

Mass transport processes are involved in the overall reaction. In these processes the substances consumed or formed during the electrode reaction are transported from the bulk solution to the interphase (electrode surface) and from the interphase to the bulk solution. This mass transport takes place by diffusion. Pure diffusion overpotential t]A occurs if the mass transport is the slowest process among the partial processes involved in the overall electrode reaction. In this case diffusion is the rate-determining step. 

See also - diffusion overpotential, -> diffusion time, -> fractals in electrochemistry, -> mass transport overpotential, -> mass transport processes. 

Diffusion overpotential — The diffusion overpotential means the extra voltage which could compensate the difference between bulk concentration and surface concentration, and is called -> concentration overpotential [i]. As a performance of industrial electrolysis or batteries, it has been used along with -> activation overpotential and - ohmic overpotential. It not only varies compli-catedly with cell configuration, current, applied voltage, and electrolysis time but also cannot be separated from activation and ohmic overpotentials. 

Overpotential — is the deviation of the - electrode potential from its equilibrium value required to cause a given -> current density to flow through the electrode. This notion is widely applied to the qualitative characteristic of electrode activity in various reactions, namely low overpotential means high activity, and high overpotential means low activity (it is assumed that the values of overpotential are compared for some fixed current density and solution composition). See also - activation overpotential, -> crystallization overpotential, - diffusion overpotential, -> reaction overpotential. 

Often it is necessary to transport excess water from the cathode side to the anode side, where it is used either for humidification of the hydrogen stream in the PEFC or to dilute the methanol fuel. (In order to increase energy and power density, methanol, while being used in solution, will usually be stored as the pure liquid.) Water management needs additional aggregates or devices. This adds to the cost of the fuel cell system and further reduces its efficiency. All these transport limitations give rise to diffusion overpotentials which lead to the rapid breakdown of the cell at high current densities in Fig. 2. 

Once the ohmic overpotential, 0hmic w> has been eliminated or computed via current interruption, one is left with the activation and concentration (or diffusion) overpotentials only. The activation overpotential, rjac,w, is due to slow charge-transfer... 

The thickness distribution of electrodeposits depends on the current distribution over the cathode, which determines the local current density on the surface. The current distribution is determined by the geometrical characteristics of the electrodes and the cell, the polarization at the electrode surface, and the mass transfer in the electrolyte. The primary current distribution depends only on the current and resistance of the electrolyte on the path from anode to cathode. The reaction overpotential (activation overpotential) and the concentration overpotential (diffusion overpotential) are neglected. The secondary... 

Fifi. 3.13 Diffusion overpotentials as a function of current density. 

By combining the Nernst equation with the expressions for charge-transfer overpotential (r CT) and diffusion overpotential (r D), equations can be written for the total experimental polarization behavior, E(iex ox) and E(iex red), of a single half-cell reaction ... 

These relations are based on the fact that the potential loss due to charge-transfer reactions are negligible (the activation overpotential approaches zero). The diffusion overpotential is usually negative during cathodic processes and positive during anodic processes. The effects of concentration polarization are usually pronounced at high current densities, when a reaction species at the interface is consumed at a faster rate than it can be... 

Eq. (5.9) expresses an exponential dependence of the current on the diffusion overpotential... 

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