Reactant supply cathode

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

Besides their catalytic role, electrodes collect (anode) or supply (cathode) the electrons involved in the electrochemical reactions, and should consist of materials of high electrical conductivity. Continuous electron supply (or removal) is necessary for the electrochemical reactions to proceed, resulting in a constant electron flow from the anode to the cathode. At the same time, the electrolyte, by transporting reactants in the form of ionic species, completes the cell circuit. The electro-combustion of hydrogen sustains a difference in the chemical potentials of the electro-active species (conducting ions) between both electrodes, which is the... 

The enzymes being highly selective, such electrodes were used in two types of fuel cells (i) fuel cells with cathode and anode compartments separated by an ion-exchange membrane, and with an individual reactant supply or (ii) fuel cells without separator, where the reactants are added as a noixture. 

The most basic description of a fuel cell is that of an electrochemical cell that has reactants supplied from an external source. An electrochemical cell consists of two electrodes, an anode and a cathode, to which reactants (oxidant and reductant, normally referred to as the fuel) are supplied and then react... 

For all practical purposes, it can be implied that the fuel cell is operated in a cmrent-driven mode. When the reactant supply system fails to supply a sufficient flow of hydrogen to the anode and oxygen to the cathode, side reactions involving water or functional and stmctural materials of the fuel cell electrodes are likely to occur. 

Shukla et al. (2002) compared the efficiency of a conventional DMFC with that of an analogous cell whose anode was fed by an aqueous methanol solution mixed with air. An increase in the anode and cell performance was explained by a higher liquid saturation in the anode diffusion layer and faster removal of CO2 from the anode (in these experiments the influence of a mixed-reactant supply on cathode performance was not investigated). 

The coplanar fuel ceU design is used primarily for fuel cells with a mixed-reactant supply. In this design both selective electrodes (anodes and cathodes) are situated on the same surface of the electrolyte (ion-conducting membrane, matrix filled with liquid electrolyte, or solid electrolyte). This surface also contacts the reactant mixture. Such an electrolyte is said to be single-faced. This is in contrast to the conventional MEAs used for almost all varieties of fuel cells, in which the electrolyte is dual-faced, contacting the anode and the fuel on one side and the cathode and the oxidizer on the other side. 

All in all, the fuel cell principle explains how an electrostatic potential gradient or electromotive force is created and maintained by controlling the unequal composition of feed components. The current flowing through the load is uniquely determined by the coupled and balanced rates of reactant supply through diffusion media, rates of anode and cathode reactions at electrodes, and electron and proton fluxes through their respective conduction media. 

Fig. 4.1 I-shaped microfluidic biofuel ceU with mixed reactant supply of an oxygen-saturated glucose solution. The highlighted reaction zone (a) is magnified to illustrate the electrode configuration (a ), with the biocathode located upstream from the bioanode, and the growth of the oxygen concentration boundary layer formed on the cathode (a")- Reproduced with permission from Togo et al. [24]. Copyright Elsevier (2008)...

The auxiliary units thus contain feed and exhaust piping for reactant, coolant piping, pumps to flow coolant or liquid fuels, and fans, compressors, and blowers for the gaseous reactant supply or exhaust. If air is used as feed gas to the cathode side of the fuel cell, it needs to be filtered for particulates before it is sent to the blower or compressor. 

The sensing element operation is based on the fact that rates of the electrochemical reaction at the electrodes are significantly higher than the reactant supply rate. The electrode reactions in this case lead to the emergence of a reactant concentration gradient. Figure 2a shows the distribution of/ and/ in the stationary electrolyte. Because of the electrode reactions, triiodide concentration maximizes at the anodes and minimizes at the cathodes, while iodide concentration maximizes at the cathodes and minimizes at the anodes. Since the ion concentrations shown in... 

From these two examples, which as will be seen subsequently, present a very oversimplified picture of the actual situation, it is evident that macroheterogeneities can lead to localised attack by forming a large cathode/small anode corrosion cell. For localised attack to proceed, an ample and continuous supply of the electron acceptor (dissolved oxygen in the example, but other species such as the ion and Cu can act in a similar manner) must be present at the cathode surface, and the anodic reaction must not be stifled by the formation of protective films of corrosion products. In general, localised attack is more prevalent in near-neutral solutions in which dissolved oxygen is the cathode reactant thus in a strongly acid solution the millscale would be removed by reductive dissolution see Section 11.2) and attack would become uniform. 

When corrosion occurs, if the cathodic reactant is in plentiful supply, it can be shown both theoretically and practically that the cathodic kinetics are semi-logarithmic, as shown in Fig. 10.4. The rate of the cathodic reaction is governed by the rate at which electrical charge can be transferred at the metal surface. Such a process responds to changes in electrode potential giving rise to the semi-logarithmic behaviour. 

Limiting currents are usually associated with cathodic reactions (e.g., in metal deposition), although anodic reactions are by no means excluded. Whenever the supply of a dissolved species from the solution to the electrode surface becomes the rate-limiting factor, limiting-current phenomena may be observed. Anodic limiting currents can be obtained, for example, in the oxidation of ferrous to ferric ion, or ferro- to ferricyanide ion (El). Diffusion of H20 limits 02 evolution in fused NaOH (A2). In these examples the limiting current is caused by depletion of the reactant species at the anode. 

Fuel cells are electrochemical systems that convert the energy of a fuel directly into electric power. The design of a fuel cell is based on the key components an anode, to which the fuel is supplied a cathode, to which the oxidant is supplied and an electrolyte, which permits the flow of ions (but no electrons and reactants) from anode to cathode. The net chemical reaction is exactly the same as if the fuel was burned, but by spatially separating the reactants, the fuel cell intercepts the stream of electrons that spontaneously flow from the reducer (fuel) to the oxidant (oxygen) and diverts it for use in an external circuit. 

Figure 2. Representation of (A, top) an electrochemical capacitor (supercapacitor), illustrating the energy storage in the electric double layers at the electrode—electrolyte interfaces, and (B, bottom) a fuel cell showing the continuous supply of reactants (hydrogen at the anode and oxygen at the cathode) and redox reactions in the cell.

However, in the cell the membrane hydration is affected by generic fuel cell processes, including the supply of humidified reactant gases to the electrodes, electroosmotic drag of water from anode to cathode, backtransport of water in the membrane, and production of water at the cathode. It is, therefore, generally important to consider the internal membrane water balance self-consistently and relate it to the membrane microstructure. 

The quantitative treatment of electrolysis was developed primarily by Faraday. He observed that the mass of prodnct formed (or reactant consnmed) at an electrode is proportional to both the amount of electricity transferred at the electrode and the molar mass of the substance in question. For example, in the electrolysis of molten NaCl, the cathode reaction tells us that one Na atom is produced when one Na ion accepts an electron from the electrode. To reduce 1 mole of Na ions, we must supply Avogadro s number (6.02 X 10 ) of electrons to the cathode. On the other hand, the stoichiometry of the anode reaction shows that oxidation of two Cl ions yields one chlorine molecule. Therefore, the formation of 1 mole of CI2 results in the transfer of 2 moles of electrons from the Cl ions to the anode. Similarly, it takes 2 moles of electrons to reduce 1 mole of Mg + ions and 3 moles of electrons to reduce 1 mole of Al " ions ... 

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