Current -overpotential three-dimensional electrode

Three dimensional electrode structures are used in several applications, where high current densities are required at relatively low electrode and cell polarisations, e g. water electrolysis and fuel cells. In these applications it is desirable to fully utilize all of the available electrode area in supporting high current densities at low polarisation. However conductivity limitations of three-dimensional electrodes generally cause current and overpotential to be non-uniform in the structure. In addition the reaction rate distribution may also be non-uniform due to the influence of mass transfer.1... 

Keith Scott and Yan-Ping Sun review and discuss three dimensional electrode structures and mathematical models of three dimensional electrode structures in chapter four. Conductivity limitations of these three-dimensional electrodes can cause the current overpotential to be non-uniform in structure. Adomian s Decomposition Method is used to solve model equations and approximate analytical models are obtained. The first three to seven terms of the series in terms of the nonlinearities of the model are generally sufficient to meet the accuracy required in engineering applications. 

The use of three-dimensional electrodes requires that the microkinetic polarization curve of the main reaction, sketched in Fig. 2, shows a potential range of width Arj within this the current density approaches the limiting current density. Thus, the optimal bed depth in the direction of current flow is introduced as a new important design parameter, for which the whole bed is working under limiting current conditions. This means that at each point the local overpotential lies within the Arj range, and the full limiting current density is realized. 

Figure 45 shows a three-dimensional SECV spectrum for a stage II PEVD system at 550°C. This spectrum can be projected to three planes, i.e., the working electrode overpotential - PEVD current plane, the PEVD current - time... 

Besides the activation overpotential, mass transport losses is an important contributor to the overall overpotential loss, especially at high current density. By use of such high-surface-area electrocatalysts, activation overpotential is minimized. But since a three-dimensional reaction zone is essential for the consumption of the fuel-cell gaseous reactants, it is necessary to incorporate the supported electrocatalysts in the porous gas diffusion electrodes, with optimized structures, for aqueous electrolyte fuel-cell applications. The supported electrocatalysts and the structure and composition of the active layer play a significant role in minimizing the mass transport and ohmic limitations, particularly in respect to the former when air is the cathodic reactant. In general, mass transport limitations are predominant in the active layer of the electrode, while ohmic limitations are mainly due to resistance to ionic transport in the electrolyte. For the purposes of this chapter, the focus will be on the role of the supported electrocatalysts in inhibiting both mass transport and ohmic limitations within the porous gas diffusion electrodes, in acid electrolyte fuel cells. These may be summarized as follows ... 

The model presented here is a comprehensive full three-dimensional, non-isothermal, singlephase, steady-state model that resolves coupled transport processes in the membrane, eatalyst layer, gas diffusion eleetrodes and reactant flow channels of a PEM fuel cell. This model accounts for a distributed over potential at the catalyst layer as well as in the membrane and gas diffusion electrodes. The model features an algorithm that allows for a more realistie representation of the loeal activation overpotentials which leads to improved prediction of the local current density distribution. This model also takes into aeeount convection and diffusion of different species in the channels as well as in the porous gas diffusion layer, heat transfer in the solids as well as in the gases, electrochemical reactions and the transport of water through the membrane. 

Dissolution of the oxidant or reductant during a redox reaction can greatly reduce the reversibility of the system. The dissolution can alter an electrode material from an undissolved state to a dissolved one. In an imdissolved state, it exhibits a controlled three-dimensional morphology that is closely linked to the conductive transport pathways of the current collector. When a redox reaction moves the material into a dissolved state, the charge is lost. Then the process becomes heavily diffusion limited and if the electrode was designed for charging based on its undissolved state, the diffusion will likely generate an overpotential on the material and the capacitance will be irreversible. 

Amperometric sensors as well as most electrocatalytic and battery electrodes operate under diffusion-controlled conditions, and thus their response depends on the flux of electroactive solutes (or their transformation products) to the conductive electrode surface, be it a film-coated electrode or the three-dimensional conductive network of porous electrodes. In order to examine the qualitative factors that influence the flux to such electrodes, it is valuable to examine a simple conceptual model. We examine here the diffusion-limited current to a test case planar electrode of area. A, coated by a thin flhn of thickness, d. Diffusion-limited conditions frequently accompany a high overpotential operation such that the concentration of the key analyte is negligible at the surface of the electrode. Under these conditions, the concentration drop across the membrane determines the faradaic current ... 

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