Electrolytic cell stack

Ffig. 5J Module voltage as a function of lime for a solid-polymer electrolyte cell stack (cf. Fig. 5.7)l Each module consists of 14 cathodes, each of area 0.093 m operating at 1.075 A cm and 55 C. 

A possible problem of sealing the electrolyte path is found in the Foreman and Veatch cell. This can be avoided by placing the cells in a vessel. The best known example of this is the Beck and Guthke cell shown in Figure 8 (74). The cell consists of a stack of circular bipolar electrodes in which the electrolyte is fed to the center and flows radially out. Synthesis experience using this cell at BASF has been described (76). This cell exhibits problems of current by-pass at the inner and outer edge of the disk cells. Where this has become a serious problem, insulator edges have been fitted. The cell stack has parallel electrolyte flow however, it is not readily adaptable to divided cell operation. 

Both share more or less the same merits but also the same disadvantages. The beneficial properties are high OCV (2.12 and 1.85 V respectively) flexibility in design (because the active chemicals are mainly stored in tanks outside the (usually bipolar) cell stack) no problems with zinc deposition in the charging cycle because it works under nearly ideal conditions (perfect mass transport by electrolyte convection, carbon substrates [52]) self-discharge by chemical attack of the acid on the deposited zinc may be ignored because the stack runs dry in the standby mode and use of relatively cheap construction materials (polymers) and reactants. 

Figure 6.18 Schematic diagram of a fuel cell stack using a stabilized zirconia electrolyte.

FIGURE 2.29 Change of cell voltage versus time for an electrolyte-supported cell stack at 850°C when 207 ppm H2S was introduced into a fuel mixture 24.8% H2/40% CO/35.7% N2. (From Trembly, J.P. et al., J. Power Sources, 158 263-273, 2006. Copyright by Elsevier, reproduced with permission.)... 

Extensive work has been reported on the deposition of individual cell layers and of full anode-electrolyte-cathode fuel cells on metallic interconnect substrates, much of it by VPS, with no sintering or other post-deposition heat treatments required [112]. However, so far relatively thick YSZ electrolytes, approximately 25 to 35 pm, have been needed to provide sufficient gas tightness [108, 114], so further optimization of the process is required to produce thinner, gas-tight electrolytes. Peak power densities of 300 mW/cm2 have been reported at 750°C for APS single cells [114], with four-cell stacks exhibiting power densities of approximately 200 mW/cm2 at 800°C [55],... 

Summarizing progress in the field thus far, the book describes current materials, future advances in materials, and significant technical problems that remain unresolved. The first three chapters explore materials for the electrochemical cell electrolytes, anodes, and cathodes. The next two chapters discuss interconnects and sealants, which are two supporting components of the fuel cell stack. The final chapter addresses the various issues involved in materials processing for SOFC applications, such as the microstructure of the component layers and the processing methods used to fabricate the microstructure. 

Other important parts of the cell are 1) the structure for distributing the reactant gases across the electrode surface and which serves as mechanical support, shown as ribs in Figure 1-4, 2) electrolyte reservoirs for liquid electrolyte cells to replenish electrolyte lost over life, and 3) current collectors (not shown) that provide a path for the current between the electrodes and the separator of flat plate cells. Other arrangements of gas flow and current flow are used in fuel cell stack designs, and are mentioned in Sections 3 through 8 for the various type cells. 

There has been an accelerated interest in polymer electrolyte fuel cells within the last few years, which has led to improvements in both cost and performance. Development has reached the point where motive power applications appear achievable at an acceptable cost for commercial markets. Noticeable accomplishments in the technology, which have been published, have been made at Ballard Power Systems. PEFC operation at ambient pressure has been validated for over 25,000 hours with a six-cell stack without forced air flow, humidification, or active cooling (17). Complete fuel cell systems have been demonstrated for a number of transportation applications including public transit buses and passenger automobiles. Recent development has focused on cost reduction and high volume manufacture for the catalyst, membranes, and bipolar plates. 

Figure 26. Schematic of a polymer electrolyte membrane (PEM) fuel cell. The fuel cell stacks operate at 30—180 °C with 30—60% efficiency. Fuel options include pure hydrogen, methanol, natural gas, and gasoline.

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