PEM electrolysers

Electrolysers can be classified into three basic kinds, based on the type of electrolyte used alkaline water electrolysers, membrane electrolysers (PEM electrolysers) and high-temperature electrolysers. Alkaline water electrolysis, the oldest and, therefore, most widely used technology, is described in more detail below. Figure 10.6 shows a diagram of an alkaline water electrolyser. 

Hydrogen production Development of a PEM electrolyser with an efficiency rating of greater than 90%, and the development of high performance cell technologies. 

PEM. The effort on PEM is mostly at the NTNU in Trondheim. The activity presently concentrates on two projects. The major objectives are two-fold develop alternative catalysts or processes reducing the use of platinum as a catalyst and fundamental research on reversible PEM/electrolyser. 

The best results are obtained with the PEM electrolyser current density of 1 200 A/m2 under cell voltage of 1 V corresponding to a hydrogen production of 500 NL.fr1.nr2. The results from both electrolysers show sulphur deposition at the cathode. This chemical reduction consumes electrons at the expense of hydrogen production causing sulphur to poison the catalyst and modify the membrane conductivity. 

Some of the common electrolysers are Alkaline Electrolysers, Polymer Electrolyte Membrane (PEM) Electrolysers (also known as Proton Exchange Membrane electrolysers), and Steam Electrolysers. In alkaline electrolysers a liquid electrolyte, such as a 25% potassium hydroxide solution, is used. At... 

Another type of electrolyser uses polymer membranes to both support the electrolysis reaction and to separate the gases. Efficiency factors for PEM electrolysers are predicted to reach 94%, but this is only theoretical in 2002. These electrolysers are best suited for small plants that have a variable output of hydrogen173. 

The modem alkaline and PEM electrolysers are characterised by productivity from few tens litres to several hundreds cubic metres per hour at output pressure 1 to 50 atm. Some electrolysis plants can produce up to 4 tons of hydrogen under pressure of 7 atm per a day. The typical current density and efficiency are of 1.6 A/cm2 and 60-75% (power consumption 4-5 kW-h/Nm3 H2), respectively. Nowadays, high operating pressure, up to 200 atm, is used to increase the efficiency of electrolysers. Also, the electrodes covered by noble metals and advanced electrolytes are applied. In the future it is possible to increase the efficiency of the electrolysers up to 80-90% [4-6]. 

The main drawback of the alkali electrolysers is that they require rather stable operation conditions, i.e. stationary operation mode, being loaded by 20-100% of their nominal productivity. Otherwise, the service life of the electrolyser is significantly reduced. The PEM electrolysers are less sensitive to the variations in their load, but they are 5-7 times more expensive and require high purity of the supplied water for normal operation. 

With relatively high cost, low capacity, poor efficiency and short lifetimes, PEM electrolysers currently available are not as mature as alkaline electrolysers. It is expected that the performance of PEM electrolysers can be improved significantly by additional work in materials development and cell-stack design (Grigoriev et al., 2006). 

High-pressure alkaline electrolyser PEM electrolyser SOEC... 

Numerical models for electrochemical process performance assessment or dimensioning generally assume uniform properties or one-dimensional property variations. For example, plug flow with axial dispersion is usually assumed within fdter-press electrolysers [1], whereas a Darcy flow model is commonly used within the gas diffusion layer of PEM electrolysers and fuel cells [2],... 

Figure 3. Photograph of the commercial-type PEM electrolyser on the left and the USC research electrolyser installed in the test facility on the right...

Figure 4. Polarisation Curve for Commercial-type PEM Electrolyser.

Sheridan, E, Thomassen, M., Mokkelbost, T., Lind, A. The development of a supported Iridium catalyst for oxygen evolution in PEM electrolysers. In 61st Annual Meeting of the International Society of Electrochemistry, hitemational Society of Electrochemistry, Nice (2010)... 

Due to the intermittent character of renewable power sources, it may be necessary for short time periods to sustain a power input significantly larger than that conventionally managed by the PEM electrolyser. This issue is also occurring in the case of fuel cell application in the automotive field [2]. Under some circumstances, the electrochemical device operates at an efficiency lower than that designed for normal operating... 

This chapter reviews characteristics and performance of modified sulfonic acid-based membranes, particularly composite membranes including inorganic fillers and short-side chain perfluorosulfonic membranes for intermediate temperature applications. The characteristics of these systems for operation in direct alcohol fuel cells, in polymer membrane (PEM) electrolyser and automotive PEM fuel cells are analyzed. 

The main perspective for PEM electrolysers is a suitable integration with renewable power... 

Fig. 2.19 Terminal voltage vs. current density curves for a PEM electrolyser equipped with Nafion-Si02 (3 %) composite membrane at different temperatures...

The composite PFSA-Si02 membrane for PEM electrolysers shows promising properties for high temperature operation allowing to achieve significantly higher performances with respect to a bare commercial Nafion. This effect... 

In general, PFSA membranes are characterized by excellent performance, electrochemical stability, suitable mechanical properties, and allow rapid startup. However, it appears necessary to ameliorate the PFSA membranes and ionomers to improve the operating efficiency of membrane-electrode assemblies of PEM electrolysers at practical current densities useful to reduce capital costs. PESA membranes used in electrochemical devices are essentially based on Nafion however, several alternative PSFA membranes with shorter pendant side chain have been developed by Dow, 3 M, Gore, Asahi Glass, Solvay Specialty Polymers, etc.. 

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