Electrode fluid mechanics

Surface tension gradient effects add to the better known phenomena of density-gradient-driven convection, concentration-gradient-driven diffusion and electrical-potential-gradient-driven ion migration, which appear in the existing theory of cells and electrodes. The potential difference of a working cell is affected by all the near electrode effects mentioned here. The experimental and analytical difficulty is to separate the variables. Indeed the fluid mechanical effects stir the electrochemical reaction, and make cause and effect difficult to discern. 

While an understanding of the molecular processes at the fuel cell electrodes requires a quantum mechanical description, the flows through the inlet channels, the gas diffusion layer and across the electrolyte can be described by classical physical theories such as fluid mechanics and diffusion theory. The equivalent of Newton s equations for continuous media is an Eulerian transport equation of the form... 

It will be seen in the next and subsequent chapters that a wide variety of cell geometries (e.g. parallel plates, concentric cylinders, Swiss roll), types of electrode (e.g. plates, beds, porous, expanded metals and gauzes) and flow patterns are used in industrial electrochemistry. In most the flow is too complex to warrant a detailed fluid-mechanical calculation. Rather the normal approach to mass transport in electrolytic cells is to treat the cell as a unified whole and to seek expressions in terms of space-averaged quantities which permit some insight into the mass transport conditions within the cell. 

The fluid mechanics and transport of electrolyte through various electrode and cell architectures... 

Fluid mechanics is the study of the motion of flowing or stirred fluids, usually liquids or gases. In electrochemical technology it has two different applications (1) to describe the movement of electrolyte solutions in a cell, since this will be a principal driving force for mass transport to the electrodes and (2) to ensure the proper design of the pipes, valves and junctions which join the cell to the rest of the plant. Quantitative fluid mechanics is based upon the continuity equations which state that at all points in space, charge, mass, momentum and, for inviscid flow (i.e. fluids where no viscous forces operate) energy must be conserved. This section will deal mainly with the qualitative concepts because of the very complex nature of flow in most electrolysers. 

The main objective of this chapter is to show that transport processes can significantly contribute to the electrochemical kinetics measurements and therefore should be taken into account. Transport processes are based on hydrodynamics (fluid mechanics), which is described with mathematics of the vector analysis. Students should know what the terms vector, gradient, and divergence represent. Solutions of Pick s second law of diffnsion are given as examples of hydrodynamics coupled with electrochemical kinetics. The theory and use of the rotating disk electrode (RDE) are explained. Introdnction to cyclic voltammetry (CV) techniques is also given. 

Petroleum coke is an excellent fuel, and that is its main use, especially for the coke from fluid coking". There are some other markets that have to do with calcined coke electrodes for aluminum production or for all other electrolytic cells, carbons for electro-mechanical equipment, graphite, and pigments. 

An example of a practical dielec trofilter which uses both of the features described, namely, sharp electrodes and dielectric field-warping filler materials, is that described in Fig. 22-34 [H. I. Hall and R. F. Brown, Lubric. Eng., 22, 488 (1966)]) It is intended for use with hydrauhc fluids, fuel oils, lubricating oils, transformer oils, lubricants, and various refineiy streams. Performance data are cited in Fig. 22-35. It must be remarked that in the opinion of Hall and Brown the action of the dielec trofilter was electrostatic and due to free charge on the particles dispersed in the hquids. It is the present authors opinion, however, that both elec trophoresis and dielectrophoresis are operative here but that the dominant mechanism is that of DEP, in wdiich neutral particles are polarized and attracted to the regions of highest field intensity. 

The external set-up of different battery systems is generally simple and differs in principle only little from one system to another. A mechanically stable cell case bears the positive and negative electrodes, which are separated by a membrane and are connected with electron-conducting poles. Ion conduction between the electrodes is guaranteed usually by fluid or gel-like electrolyte [13]. 

Eredictions. A rotating cyhnder within a cyhnder electrode test system as been developed that operates under a defined hydrodynamics relationship (Figs. 25-15 and 25-16). The assumption is that if the rotating electrode operates at a shear stress comparable to that in plant geometry, the mechanism in the plant geometty may be modeled in the laboratory. Once the mechanism is defined, the appropriate relationship between fluid flow rate and corrosion rate in the plant equipment as defined by the mechanism can be used to predict the expected corrosion... 

In the literature we can now find several papers which establish a widely accepted scenario of the benefits and effects of an ultrasound field in an electrochemical process [13-15]. Most of this work has been focused on low frequency and high power ultrasound fields. Its propagation in a fluid such as water is quite complex, where the acoustic streaming and especially the cavitation are the two most important phenomena. In addition, other effects derived from the cavitation such as microjetting and shock waves have been related with other benefits reported for this coupling. For example, shock waves induced in the liquid cause not only an enhanced convective movement of material but also a possible surface damage. Micro jets of liquid, with speeds of up to 100 ms-1, result from the asymmetric collapse of cavitation bubbles at the solid surface [16] and contribute to the enhancement of the mass transport of material to the solid surface of the electrode. Therefore, depassivation [17], reaction mechanism modification [18], surface activation [19], adsorption phenomena decrease [20] and the mass transport enhancement [21] are effects derived from the presence of an ultrasound field on electrode processes. We have only listed the main phenomena referring to the reader to the specific reviews [22, 23] and reference therein. 

Numerous atomization techniques have evolved for the production of metal/alloy powders or as a step in spray forming processes. Atomization of melts may be achieved by a variety of means such as aerodynamic, hydrodynamic, mechanical, ultrasonic, electrostatic, electromagnetic, or pressure effect, or a combination of some of these effects. Some of the atomization techniques have been extensively developed and applied to commercial productions, including (a) two-fluid atomization using gas, water, or oil (i.e., gas atomization, water atomization, oil atomization), (b) vacuum atomization, and (c) rotating electrode atomization. Two-fluid atomization... 

The electrolyte (the bulk solution) is in the inner of the two chambers, and thermostat fluid circulates in the outer chamber, hi the top of the Avesta cell are mounted the salt bridge from the reference electrode, a thermometer, nitrogen gas inlet, counter electrode, and perhaps a mechanical stirrer. 

An optimum relationship between the DL and the flow field channels is a key factor in the overall improvement of fhe fuel cell s performance at both high and low current densities. Currently, flow field designs are typically serpentine, interdigitated, or parallel [207,264]. The FF plate performs several functions If is a current collector, provides mechanical support for the electrodes, provides access channels for the reactants to their respective electrode surfaces and for the removal of producf water, and it prevents mixing of oxidant, fuel, and coolant fluids. 

ISEs are well suited for flow measurements because the instrumentation and signal handling are simple, the measurement is almost independent of the liquid flow-rate, the linear dynamic range is broad, the temperature dependence is not very pronounced and the measurement is selective (the selectivity is, however, a drawback in applications to chromatography). The experimental conditions are readily adjusted and often only consist of ionic strength and pH maintenance. ISEs with solid membranes usually exhibit better performance than liquid membrane electrodes and gas probes, because their response is faster and they are mechanically stronger. The most difficult problem is passivation of the electrodes in some media, for example, biological fluids or surface and waste waters. 

Though there is fluid flow in the bulk of the electrolyte, it is found that there is a layer adjacent to the electrode in which the electrolyte is stationary, or stagnant. Thus the electron acceptors travel by convection from the bulk up to the stagnant layer and then cross the remaining boundary layer by diffusion. This transport by a convection-with-diffusion mechanism has not been taken into account so far. The equations for the time and space variation of concentration [i.e., Eq. (7.178)], for the transition time [Eq. (7.190)], and for the time variation of potential [Eq. (7.192)] have been derived for convection-free conditions, and they break down when convection becomes significant. The first approximation theory given above, therefore, deviates from experiment if the constant current is applied sufficiently long (times on the order of seconds) for convection to be important. 

In nimen-listulaled experimental animals, it has been observed that for certain distribution ratios of chloride in ihc ruminal fluids and hlood plasma, the calculated equilibrium potential Tor chloride is relatively the same as that measured directly with KCI-agar bridges and calomel electrodes However, in many circumstances, the calculated and measured values have been found lo be significantly different, an observation that could only be accounted for by the presence of an active transport mechanism responsible for the movement of chloride out of its equilibrium distribution. 

---