Electrochemical reactor electrocatalyst

Fig. 31. Typical channel flow electrochemical reactors (CER) with flow-by (a, b) and packed-bed (c) working electrodes. The control volume (d) includes the active electrocatalyst area (67). A, reactant B, C, products E, electrolyte ce, counter electrode we, working electrode. (Reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

The determination of the real surface area of the electrocatalysts is an important factor for the calculation of the important parameters in the electrochemical reactors. It has been noticed that the real surface area determined by the electrochemical methods depends on the method used and on the experimental conditions. The STM and similar techniques are quite expensive for this single purpose. It is possible to determine the real surface area by means of different electrochemical methods in the aqueous and non-aqueous solutions in the presence of a non-adsorbing electrolyte. The values of the roughness factor using the methods based on the Gouy-Chapman theory are dependent on the diffuse layer thickness via the electrolyte concentration or the solvent dielectric constant. In general, the methods for the determination of the real area are based on either the mass transfer processes under diffusion control, or the adsorption processes at the surface or the measurements of the differential capacitance in the double layer region [56],... 

From the electrochemical engineering point of view, the electrocatalyst design depends on the purpose of the electrochemical reactor, gas electrosynthesis, organic synthesis, batteries or supercapacitors, metal electrodeposition, and the fuel cells. 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

On the other hand, the selectivity of the electrochemical deposition of the metal on the substrate must be 100% of the current efficiency, with no interference from the other metal deposition processes. Therefore, the potential distribution needs to be presented for any serious electrochemical reactor study and the electrocatalyst selection problem. The major problem of current distribution depends on the type of the process that controls the entire reaction rate, such as charge transfer, ohmic contributions, or mass transport to or from the electrode. Many parameters have to be evaluated in the course of an electrochemical process to obtain the desired uniform potential and current distributions. One of the conditions that has to be fulfilled is the continuity equation for the current density vector, j ... 

FIGURE 13.4 Current distribution for the primary approach in an electrochemical reactor of two parallel plate electrode geometries. Black and gray rectangles are the electrocatalyst and insulator surfaces, respectively. 

Sometimes it is not possible or convenient to lower the values of Wa, so we have to play with all the adjustable parameters of Equation 13.37. In the case of the electrocatalyst and for an electrochemical reactor scale-up problem, Wa has to be of an appreciable magnitude that is, we can change the electrode size (not geometry) but Wa should remain constant. The problem with real electrochemical reactors is that concentration gradients (especially for long-time uses) are inevitable, and thus the shape of the /, // vs. x/L plot is useful (Figure 13.6). 

Most of the electrochemical reactors fail due to different attacks on the electrocatalysts, where the anodes are attacked faster than the cathodes (electrochemical corrosion, mechanical fissures due to electrodissolution, or bubble formation and evolutions, etc.) [43]. In new technologies, the use of the anode, membrane, or cathode assemblies solves this problem. In the case of the solid polymer electrolytes, the anode and the cathode catalysts are integrated to the membrane promoting the mechanical and electrochemical stability of the device [44,45]. This new technology replaces the problem of the diaphragm-based electrochemical industry that was established in the beginning of the twentieth century [46]. 

The optimization of an electrochemical reactor calls for a full description of the process to accomplish the specific objective. The problem of the optimization of the electrocatalyst is of real importance in most of the recently developed technical electrodes that were prepared without detailed studies. It must be borne in mind that the strong experimental conditions in which the large electrical currents and large ionic forces of the electrolytes prevail change the morphology and the composition of the catalyst. 

The fluid hydrodynamics and geometric configuration of the electrochemical reactor are key to understand the mixed processes that occur in a system. Though the specific geometry of the electrocatalysts is important, the mass transfer can be determined solely by fluid hydrodynamics [1],... 

This part of the chapter deals with the effects of viscosity on an electrolyte flowing in the electrochemical reactor in two dimensions. The boundary layers appear on the surface of bodies in viscous flow because the fluid seems to stick to the electrocatalyst s surface. As we have described above, right at the surface, the flow has zero speed, and this fluid transfers a linear momentum to the adjacent layers through the action of dynamic viscosity. Therefore, a thin fluid... 

The importance of electrocatalysts as the core of an electrochemical reactor gave us a whole new comprehension using introductive concepts and personal experiences. The main success of this... 

Were it possible to make highly diverse electrocatalysts which had specificity introduced, i.e., introduction of artificial enzymes or the active parts of enzymes, the possibility of electrochemical reactors, in... 

The Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT) works on design, construction and testing of PEM components, electrochemical reactions/reactors, electrocatalysts for PEMs, hydrogen production by catalytic processes and water splitting. 

In order to get answers to these questions, the ability to better characterize catalysts and electrocatalysts in situ under actual reactor or cell operating conditions (i.e., operando conditions) with element specificity and surface sensitivity is crucial. However, there are very few techniques that lend themselves to the rigorous requirements in electrochemical and in particular fuel cell studies (Fig. 1). With respect to structure, in-situ X-ray diffraction (XRD) could be the method of choice, but it has severe limitations for very small particles. Fourier transform infra red (FTTR), " and optical sum frequency generation (SFG) directly reveal the adsorption sites of such probe molecules as CO," but cannot provide much information on the adsorption of 0 and OH. To follow both structure and adsorbates at once (i.e., with extended X-ray absorption fine stmcture (EXAFS) and X-ray absorption near edge stmc-ture (XANES), respectively), only X-ray absorption spectroscopy (XAS) has proven to be an appropriate technique. This statement is supported by the comparatively large number of in situ XAS studies that have been published during the last decade. 16,17,18,19,20,21,22,23,24,25 highly Versatile, since in situ measme-... 

The electrochemical in situ hydrogen generation route is an alternative route that eliminates the need for an external hydrogen source for HDS. The process involves parallel reactors for batch operation. For continuous operation, parallel reactors can also be employed. In both cases, the residence time of the reactor is governed by the deployment of optimal surface area of the electrocatalysts in relation to reactor configuration. A simplified PFD of such a process for batch operation is shown in Fig. 2. 

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