Unitized reversible fuel cells

On the H2 side of the plant, the main inventions involve the fuel cells. They include the idea of using the "dual-function" electrolyzer and fuel cell combination units, which I call reversible fuel cells (RFCs) (Figure 4.2). This way, in the electrolyzer mode, the RFCs convert electricity into H2/ and in the fuel cell mode, they generate electricity from the H2 in storage. 

United Technologies Fuel Cells is engaged in DMFC development, in competition with Ballard/Johnson Matthey. It is a part in the project by Renault to develop the Scenic vehicle fuel cell. Neither for its PEFC, nor for its DMFC (and MCFC), does UTC Fuel Cells offer product-coloured illustrations. Moreover, its literature or listed web site does not deal with the cell voltage reversal problem, mentioned in Ballard patents above in connection with fuel cell bus operation. Accordingly it is not possible for the author to portray the UTC Fuel Cells scheme of things. 

Clearly, the CRR is a far more reversible reaction than ORR and the performance is excellent (0.5 W/cm ). In fact, the exchange current density of CRR and HOR (Vilekar et al., 2010) are of a similar order. Consequently, the H2—CI2 unitized regenerative fuel cell (URFC) is an excellent energy storage device as discussed later. 

A.H., Molter, T.M., and Smith, W.F. (1999) Reversible (unitized) PEM fuel cell devices. Fuel Cell Bull., 11, 6-11. 

The literature uses the terminology URFC (Unitized Regenerative Fuel Cell) for a solution with a single core, and RFC (Regenerative/Reversible Fuel Cell) for a solution with two cores. 

Fujiwara et al. (2011) showed the possibility of preparing reversible air electrodes that can be used in metal-air storage batteries or unitized regenerative fuel cells. To reduce the impact of atmospheric carbon dioxide the reversible air electrodes were integrated with a polymer anion-exchange membrane which was placed between the cathode s catalytic layer (with Pt and Pt-Ir catalysts) and the alkaline solution. The membrane with anion permselectivity presumably inhibited the permeation of COj cations to the air electrode and thus suppressed precipitation of carbonates in pores of the air electrode. 

Much progress in reducing the actual amounts of noble metals needed in fuel cells per unit area was made in the 1990s. (Srinivasan, 1993) and it is probable that some of this technology could be used in water electrolyzers that perform reactions reverse to that in H2-O2 fuel cells. 

Figure 3.52. Efficiency of a reversible PEM fuel cell as a function of the amount (at. % or mol %) of Ir in the form of IrOj relative to Pt in the positive electrode catalyst, for fuel cell electricity production (EC) or for water electrolysis (WE). Also the product of the two efficiencies relevant for storage cycles is shown. The catalyst is otherwise similar to that of Fig. 3.51, with PTFE and Nation channels. (From T. loroi, K. Ya-suda, Z. Siroma, N. Fujiwara, Y. Miyazaki (2002). Thin film electrocatalyst layer for unitized regenerative polymer electrolyte fuel cell. J. Power Sources 112, 583-587. Used by permission from Elsevier. See also loroi et al. (2004).

Ledjeff, K., Mahlendorf, F., Peinecke, V., Heinzel, A. Development of electrode membrane units for the reversible solid polymer fuel-cell (Rspfc). Electrochim. Acta 40,315-319 (1995)... 

The H30 -P7P-Al203 of the 6N3-type was used in a reversible steam electrolysis/fuel cell unit and the results are shown in Fig. 33.9. The efficiency at the high temperatures (200-300 °C) is substantially below theoretical due to the dehydration of the electrolyte. As this is reversible, a high-pressure steam cell unit was constructed and initial trials showed a substantial improvement in steam-electrolysis/fuel-cell voltage/current-density characteristics. 

Reversed-flow gas chromatography (RF-GC) has been successfully used to characterize solid catalysts under conditions compatible with the operation of real catalysts. RF-GC is not limited to chromatographic separation since RF-GC is accompanied by suitable mathematical analysis of the chromatographic data, the simultaneous determination of various physicochemical parameters is possible. Thus, various catalytic processes related to the operation of fuel cell units such as steam reforming, catal3dic partial oxidation, autothermal reforming, as well as water-gas shift (WGS) reaction and selective CO oxidation can be studied. 

We leave to specialists the task to describe the desired new approaches in the preparation of zeolites and mesoporous materials. Much work remains to be done for the scale-up of fabrication of MCM and similar materials. The manufacture of specially structured catalysts for very compact devices, like on-board fuel cells and automobile reforming units, is certainly able to bring much information valuable for the fabrication of many specific catalysts in environmental protection, especially for reverse-flow reactors. The demands of photocatalysis also impose constraints. 

PEM fuel cells, with appropriate catalysts (e.g., Pt for the H2 electrode and Pt—Ir for the O2 electrode), can be operated as unitized (reversible) regenerative fuel cells (URFCs) (Figure 15.22), i.e., they can be used for storing electricity produced by renewable, but intermittent, resources such as solar cells and windmills, much like a... 

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