Three-dimensional electrode structure

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. 

Chabi, S., Peng, C., Hu, D., Zhu, Y, 2014. Ideal three-dimensional electrode structures for electrochemical energy storage. Adv. Mater. 26,2440-2445. 

Experiments on model electrodes of fractal geometry were found to be in accordance with the theoretical prediction (Pajkossy and Nyikos, 1989). The effect of surface roughness and porosity is of great importance, and several impedance models were derived to explain the effect of three-dimensional electrode structures, as reviewed by de Levie (1967). 

Two hundred years were required before the molecular structure of the double layer could be included in electrochemical models. The time spent to include the surface structure or the structure of three-dimensional electrodes at a molecular level should be shortened in order to transform electrochemistry into a more predictive science that is able to solve the important technological or biological problems we have, such as the storage and transformation of energy and the operation of the nervous system, that in a large part can be addressed by our work as electrochemists. 

Three-dimensional electrode nanoarchitectures exhibit unique structural features, in the guise of amplified surface area and the extensive intermingling of electrode and electrolyte phases over small length scales. The physical consequences of this type of electrode architecture have already been discussed, and the key components include (i) minimized solid-state transport distances (ii) effective mass transport of necessary electroreactants to the large surface-to-volume electrode and (iii) magnified surface—and surface defect—character of the electrochemical behavior. This new terrain demands a more deliberate evaluation of the electrochemical properties inherent therein. 

Three-dimensional electrode arrays have been fabricated using two very different micromachining methods. One approach, named carbon MEMS or C-MEMS, is based on the pyrolysis of photoresists. The use of photoresist as the precursor material is a key consideration, since photolithography can be used to pattern these materials into appropriate structures. The second approach involves the micromachining of silicon molds that are then filled with electrode material. Construction of both anode and cathode electrode arrays has been demonstrated using these microfabrication methods. 

Future work will focus on real three-dimensional electrodes that may slowly penetrate the superficial layer of the retina. We hope to improve the spatial selectivity of a stimulator structure and to lower the energy consumption during stimulation, when the microelectrode is in close proximity to the somata of the ganglion cells. A possible design of this structure is shown in Fig. 27. It demonstrates the design potentials that microfabrication of polymer based microstructure offer. 

This review considers what we believe to be a suitable method to solve a range of electrochemical related problems in science and engineering, i.e., Adomian decomposition. The method is applied to several problems related to the analysis of three dimensional electrodes.4,5 The typical structure of three dimensional electrodes is shown schematically in Figure 1, in terms of two types of electrode. Figure la, is appropriate for electrodes connected by an electrolyte as typically used in synthesis or in batteries, while Figure lb is for electrodes as used in fuel cells, e.g., polymer electrolyte fuel cells (PEMFC). In general the models are concerned with determining the concentration and potential (and current) distributions in the structure. 

Figure 1. The typical structure of three-dimensional electrodes.

However, if only planar electrodes were used, much smaller currents would be observed per geometric (or external) unit area of electrode material than are in fact obtained. Electrochemical converters would in fact have no practical uses. Thus, in all actual electrochemical converters, the electrodes are three-dimensional, porous structures (e.g., of graphite), whose pores usually contain the catalyst material (e.g., platinum) to and from which electric charge transfer occurs. 

Cells with three-dimensional electrodes providing enlarged specific electrode area and improved mass transport due to the specific fluid dynamics inside the three-dimensional structure are, for example, the porous flow-through cell [68], the RETEC (RETEC is a trademark of ELTECH Systems Inc., Cardon, Ohio) cell [15], the packed-bed cell [69-71],... 

In all actual electrochemical converters, the electrodes are three-dimensional, porous structures, the pores of which contain the catalyst material to and from which electric charge transfer occurs.14... 

Multiple platinum electrodes for recording the neural signals were placed on both the inner and outer surfaces of the sheath. The three-dimensional sheath structures were created by thermoforming from flat surface micromachined microchannels. Solid microwires were used as molds. 

UV-cross-linked photoresist stiuctures. The substrate is then pyrolyzed at high temperature (>900 °C) to form the three-dimensional carbon electrodes. These electrodes have similar properties to glassy carbon, but with lower oxygen content. They are particularly useful for three-dimensional electrodes because they can be fabricated with high aspect ratios (up to 20 1), and tall structures can be fabricated easily and reproducibly. 

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