Fuel cells eCMRs

The chemistry of electrocatalytic reactions at either electrode and the operating temperature of a fuel cell eCMR are determined mainly by the nature of the chargecarrying ion and the EM. To ftiUy appreciate the broad potential of the fuel cell eCMR, it is useful to start with a generic description. 

In general, there are two types of EMs, cation-exchange and anion-exchange, resulting in two classes of fuel cell eCMRs (1) cation-exchange eCMR, and (2)... 

Figure 15,5 An electrical analog of fuel cell eCMR internals including an ideal voltage source and internal resistances.

The Wacker anode catalytic cycle, the anode OR, and the cathode and overall cell reactions in this fuel cell eCMR are described below ... 

Figure 15.9 Schematic of a glycerol AEM-based fuel cell eCMR for cogeneration of electricity...

Figure 15.11 Fuel cell eCMR performance for several H2/unsaturated organic acids in a Nafion 117-based MEA with coimnercial ETEK electrodes.

To promote selective H2O2 synthesis in a fuel cell eCMR, rather than the ORR to water, the catalyst of choice, e.g., Pt—Hg (Siahrostami et al., 2013) or carbon (Assumpcao et al., 2011), must not be effective for the 4-electron deep ORR, which proceeds via the 0—0 bond scission (Figure 15.13). Further, the formed H2O2 needs to be washed away from the surface promptly with an aqueous solvent to suppress successive reduction or decomposition of the H2O2 (Figure 15.13). In fact, acidic, basic and deionized water have been used for such H2O2 recovery (Yamanaka, Onizawa, Takenaka, Otsuka, 2003). 

HT SE membrane fuel cell eCMRs (Garagounis et al., 2011 Sundmacher et al., 2005) are also of substantial interest for electrochemical co-generation because at these temperatures many industrial raw materials may be used directiy, as in the... 

Enriching oxygen from air or pumping oxygen is of interest in a number of applications, including medical, for which fuel cell ECMRs may be attractive (Figure 15.29). In such an eCMR O2 pump, one would preferably (due to its lower overpotential... 

Electrocatalytic membrane reactors (eCMRs) for fuel cell and other applications... 

A common example of an eCMR for generating power is the H2—O2 PEM fuel cell ... 

The alternate possibility in a fuel cell cCMR is the formation of anions at the cathode, their diffusion across the electrolyte from the cathode to the anode, and finally their consmnption at the anode. For such an anion-exchange eCMR, when the... 

Figure 15.1 A cation-exchange eCMR as a fuel cell with a vehicle molecule (Q) ferrying the charge across the electrolyte from anode to cathode and undergoing internal or external recycle.

The simplest elementary ions, considering the case of the H2—O2 fuel cell (similar logic may be applied to other eCMRs), are H+ and However, other more complex ions are often formed from these elementary ions and a vehicle molecule (Q) that ferries the elementary ion across the electrolyte and then returns for the next payload, e.g., water molecule, i.e., (Q) = (H2O), which forms H30 ion at the anode as the ion-carrier species, i.e., H+ + (H2O) <= H30+. As another example, an oxygen anion may form the hydroxyl ion with water ((Q) = (H2O)) in the cathode of an AFC, i.e., + (H20) 20H . On the other hand, in an molten-carbonate fuel cell... 

The electrocatalysts chosen for the eCMR depend on the nature of the reactants and especially the operating temperature. Thus, LT to medium temperature fuel cells (<250 °C) involve precious metal catalysts (often Pt and its alloys), while HT fuel cells use cheaper transition metals (e.g., Ni) and ceramics. The interplay between temperature and overpotential, i.e., potential losses, in the most common fuel cells is schematically illustrated in Figure 15.3. In general, fuel cells operating at HT have lower overpotentials, because temperature and potential are complementary factors in promoting electrode kinetics and thermodynamics. 

Finally, although not discussed here any further, the eCMR can also serve as a sensor (e.g., for H2) operating at OCV, i.e., with no external current. Thus, Figure 15.4 illustrates the power and broad utility of the eCMR, power generation in the conventional fuel cell being only one of the many possibilities. 

It is useful to discuss the performance characteristics of an eCMR in terms of a polarization plot to help understand its various modes of operation depicted in Figure 15.4. This analysis follows our earlier approach (Choi et al., 2004 Thampan et al., 2001 Vilekar Datta, 2010), in which we consider the membrane electrode assembly (MEA) for a fuel cell cCMR as consisting of the live layers, an electrical analog of which is shown in Figure 15.5. 

Although derived for a fuel cell, the model above applies to all the eCMR modi describedinEigure 15.4,i.e.,for both/ > 0 (power generation mode) and for / < 0(elec-trolysis mode). Of course, the model parameters for each specihe case are different. 

The eCMR hydrogen separator/compressor based on the PEM fuel cell, as shown in Figure 15.26, can be more efficient at the small scale when compared with mechanical compressors (Ibeh, Gardner, Teman, 2007 Onda, Ichihara, Nagahama, Minamoto, Araki, 2007). 

Although at the present time, besides fuel cells, only electrolytic mode applications of eCMR for the production of metals and inorganic chemicals are mature, it is clear from the above discussion that eCMRs have a much broader potential. Additional... 

In conventional reactors or in CMRs, thus, only thermal control is possible, while in the eCMR, both temperature and electrical potential is available to control the reaction kinetics and its thermodynamics. Consequently, many endergonic reactions (AG > 0) can be driven by supplying external electric power, available, for example, from a wind turbine or a solar cell. In this manner, excess renewable energy when available could be converted into a useful chemical or stored in a fuel molecule, e.g., H2, to be used later for producing electricity as needed, e.g., in a PEM electrolyzer, which is the reverse of Eqn (15.1). 

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