Electrochemical fuel cell reactor

Principles of electrochemical engineering, fuel cell reactors, and electrocatalytic reactor design... 

In a ceramic electrochemical reactor oxygen activation is physically separate from substrate activation. Activation of dioxygen occurs at a cathodic surface, oxygen traverses the electrolyte as oxide ions, the ions react with a substrate on the anodic face, and the electrons return to complete the circuit. Two types of electrochemical membrane reactors are under study which differ in the route by which the electrons travel from anode to cathode these are illustrated schematically in Figure 1. In the shorted fuel cell reactor the electrons return through an external circuit, from which electrical power can be extracted. Electroceramic membranes allow the electrons to make their way from anode to cathode via electrically conductive phases within the membrane proper. 

A fuel cell is an electrochemical reactor with an anodic compartment for the fuel oxidation giving a proton and a cathodic compartment for the reaction of the proton with oxygen. Two scientific problems must be solved finding a low-cost efficient catalyst and finding a membrane for the separation of anodic and cathodic compartments. The membrane is a poly electrolyte allowing the transfer of hydrated proton but being barrier for the gases. 

II. Ease of electrical connection Here the main problem is that of efficient electrical current collection, ideally with only two electrical leads entering the reactor and without an excessive number of interconnects, as in fuel cells. This is because the competitor of an electrochemically promoted chemical reactor is not a fuel cell but a classical chemical reactor. The main breakthrough here is the recent discovery of bipolar or wireless NEMCA,8 11 i.e. electrochemical promotion induced on catalyst films deposited on a solid electrolyte but not directly connected to an electronic conductor (wire). 

The implications of this discovery for electrochemical promotion are quite significant since it shows that, at least in principle, the design of an electroche-mically promoted reactor can become much simpler than that of a fuel cell. 

Figure B.l. (Top) Typical reactor designs used in electrochemical promotion studies singlechamber design (left) and fuel cell type design (right). (Bottom) Typical apparatus for electrochemical promotion studies using a three-pellet single chamber reactor.

Now let the reasons for this insistence on the study of the steady state of an electrode reaction be expounded. The main reason is simply that electrochemical reactions are useful at the steady state. It is most desirable for the current density in a reactor carrying out some organic synthesis to remain constant over hours or days whde the synthesis is going on. It would not be desirable in a fuel cell producing power to run a car if the rate of the electrochemical reaction in it—hence the power output and thus the speed of the car—varied out of control of the driver. 

Fig. 7.190. Diagram of the reactor for partial oxidation of benzene to phenol during 02-H2 fuel cell reactions. (Reprinted from K. Otsuka, M. Kunieda, and H. Yamagata, J. Electrochem. Soc. 139 2382,1992, Fig. 1. Reproduced by permission of the Electrochemical Society, Inc.)...

There are numerous applications that depend on chemically reacting flow in a channel, many of which can be represented accurately using boundary-layer approximations. One important set of applications is chemical vapor deposition in a channel reactor (e.g., Figs. 1.5, 5.1, or 5.6), where both gas-phase and surface chemistry are usually important. Fuel cells often have channels that distribute the fuel and air to the electrochemically active surfaces (e.g., Fig. 1.6). While the flow rates and channel dimensions may be sufficiently small to justify plug-flow models, large systems may require boundary-layer models to represent spatial variations across the channel width. A great variety of catalyst systems use... 

Different materials can be used as oxidants in fuel cells. Yet it is desirable, if possible, to use 02 from air at one electrode in every earthbound fuel cell because this avoids the necessity of carrying a second fuel for the cathodic reaction. Hence, the cathodic reduction of 02 has a special importance in electrochemical reactors. The overall reaction in acid solution (see Sec. 7.10) is... 

Combustion reactions often cause extensive exergy loss. Exeigy calculations show that the entropy production can cause the loss of considerable potential work due to a reaction. An electrochemical membrane reactor or a fuel cell could reduce exergy loss considerably. 

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