SPE electrolyzer

More importantly, in the SPE technology gaseous H2 and O2 are liberated on the electrode surface on the side of the solution, thus solving the problem of the solution resistance due to the presence of bubbles. The membrane acts as an electrolyte. At the anode H2O is oxidized to O2 with liberation of H, which migrates through the membrane to the cathode, where it is reduced to H2. In practice, a flow of solution is needed only at the anode to replace water molecules oxidized to O2. However, the solution no longer needs to be conductive since no current passes through it. Actually, SPE electrolyzers are fed with plain water [20]. 

The structure of a SPE cell is shown in Fig. 2.3. The basic unit of a SPE electrolyzer is an electrode membrane electrode (EME) structure that consists of the polymer membrane coated on either side with layers (typically several microns thick) of suitable catalyst materials acting as electrodes [43,49,50], with an electrolyzer module consisting of several such cells connected in series. The polymer membrane is highly acidic and hence acid resistant materials must be used in the structure fabrication noble metals like Pt, Ir, Rh, Ru or their oxides or alloys are generally used as electrode materials. Generally Pt and other noble metal alloys are used as cathodes, and Ir, Ir02, Rh, Pt, Rh-Pt, Pt-Ru etc. are used as anodes [43,46]. The EME is pressed from either side by porous, gas permeable plates that provide support to the EME and ensure... 

Fig. 15.12. Daily variation in electrolytic hydrogen production rate (1), the solar array temperature (2), and radiation power density (3). Single crystalline silicon solar cells, SPE electrolyzer location, Cape Canaveral, Florida. The time scale denotes minutes elapsed from 5 a.m. (Reprinted from Yu. I. Khar-kats, Electrochemical Storage of Solar Energy, in Environmental Oriented Electrochemistry, C. A. C. Sequeira, ed., Fig. 5, p. 477, copyright 1994. Reproduced with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV Amsterdam, The Netherlands.)...

A more important conclusion is reached from Figure 10, which indicates that there would be severe penalties in operating the coal and oil plants intermittently. It also indicates that even electrolysis, which has been promoted as an off-peak power user, would be very expensive if operated below the 20% to 30% plant factor that is associated with "cheap off-peak power. The huge reduction in capital cost promised by the SPE electrolyzer offsets this somewhat, but operation of even an advanced electrolyzer at less than 20% of the time (5 hours/day) is unattractive compared with a dedicated, full-time reformer or gasifier. The low capital cost of steam reforming is again shown to advantage. 

Applications in Other Electrolyzers. Apart from applications in hydrogen-oxygen fuel cells and water electrolyzers which operate without any supporting electrolyte, SPE electrolyzers are also used efficiently with electrolyte solutions, such as HC1 and Na2SCK+. Recently, LaConti, et al (34), have reported the application of the cell with Nafion as SPE for some important electrochemical processes, including electrolysis of water, HC1, Na2S0 +, and brine solution. 

Nafion-315 is currently used in the SPE cell for brine electrolysis. The SPE electrolyzer exhibits a 15-20% energy savings when compared to conventional brine electrolyzers, primarily due to the decrease in ohmic and cathode overvoltages. Figure 5 shows the schematic of the SPE electrolyzer along with a typical membrane electrolyzer. The current distribution across the membrane of an SPE electrolyzer is more uniform than that of a typical brine electrolyzer. 

Fig 4. Daily variation of electrolytic hydrogen production rate (1), the solar array temperature (2) and radiation power density (3). Single crystalline silicon solar cells, SPE electrolyzer, location Cape Canaveral, FA. The time scale denotes minutes elapsed from 5 a.m. [23],... 

Fig. 5, Accumulated annual specific hydrogen production per 1 m of the solar array 1 - for the real plant, 2 - assuming zero matching losses. Single crystalline silicon solar cell SPE electrolyzer location Stuttgart (Germany) the design parameter t = Nj/Ng = 4.3 [23].

During these programs, PME fuel cells and electrolyzers were developed. These first solid membranes were made of sodium polystyrene sulfonate, and would be replaced by Nafion, which was discovered by DuPont in the same decade. Thus, it was in 1966 that the very first solid-polymer electrolyte (SPE) electrolyzer was built by General Electric (GE) for Project GEMINI to produce oxygen on board the spacecraft. 

MEA 25 was then used to test the effects of water flow rate on SPE electrolyzer performance. Data was collected using flow rates between 2 and 4.5 mL/s. Each data point obtained was stable over 300 s during chronocoulometry with a GPES system at a temperature of 23 °C. The cell resistance was 164 mQ. As expected, the current density increased with faster flow rates (Figure 8.7a), and can be simply attributed to improved transport phenomena i.e., optimal movement for ingress of reactants and removal of products. The lowest flow rate with the maximal current density, with this test module, was found to be 3.27mL/s, above which minimal improvement in the current density was observed (Figure 8.7a). 

Further testing with flow rates, and applied potentials of —1.6, —1.7, and -1.8 V, found that the optimum flow rate, between 3.5 and 4.5mL/s, was similar irrespective of the potential applied (Figure 8.7b). Thus further improvement with the SPE electrolyzer performance would need to be optimized using other parameters. 

FIGURE 8.9 Current density for MEA 38 in the SPE electrolyzer test module set up at various temperatures. 

This series of MEAs did not show any improvement over the iridium black catalyst (MEAs 25, 28, and 29) and is either due to the GDL, catalyst loading on the GDL, catalysts dispersion method or the different catalysts used. Iridium oxide catalysts have been shown to outperform iridium black, and Pt/C catalysts in SPE electrolyzers [1,2,7,18,21], and the loadings between the different MEAs were similar, thus the likely difference is due to the use of the Etek GDL. 

MEAs tested so far, had established that use of Ft/C catalysts on the anode reduced the performance of the SPE electrolyzer. In addition flow rate had been optimized, trends with temperature were observed, the effect of using alternative proton conducting membrane on SPE performance, and a suitable carbon-based GDL had been demonstrated. The use of a titanium fiber based GDL was identified as an option to optimize SPE performance via judicious selection of material components that make up the system. MEA 42 was assembled using a proton conducting membrane from GEEC (GEEC 117), commercial iridium oxide catalyst on the anode and titanium fibers GDL on both anode and cathode side. In contrast, MEA 51 was made with a homemade iridium oxide catalyst on the anode, Bekinit GDL on the anode and... 

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