Alkaline water electrolyzers

Simplified process diagram of an alkaline water electrolyzer. 

Schiller G., Henne R., Mohr R, Peinecke V., High performance electrodes for an advanced intermittently operated 10-kW alkaline water electrolyzer, Int.. Hydrogen Energ., 23(9), 761-765,1998. 

Gandia LM, Oroz R, Ursua A, Sanchis P, Dieguez PM. Renewable hydrogen production performance of an alkaline water electrolyzer working under emulated wind conditions. Energy Fuels. 2007 21 1699-706. 

Recently, much interest has centered on the electrocatalytic behavior of Ni-Mo alloyst (prepared electrolytically or by thermal reduction of oxide or molybdate mixtures) for cathodic H2 evolution in alkaline-water electrolyzers or as cathodes in electrolytic CI2 cells. The h.e.r. at these electrode materials exhibits remarkably low Tafel slopes, in the range 22-26 mV, at elevated temperatures, 363 K. This is one of the reasons for their excellent behavior as electrocatalysts for the h.e.r. However, the Tafel slopes exhibited by these materials depend on temperature in a most unusual way, decreasing with increasing temperature, as do some of the results at NP (Fig. 4). 

Yeo et al have reported an analysis of the conductivity of Nafion in different alkaline electrolytes, based on the correlation of membrane conductivity with water content. The analysis reveals that larger conductivities arise when the membranes are equilibrated with NaOH solutions than with KOH solutions of equal molarity. Also, it is shown that better conductivity can be realized with thinner and lower-EW membranes. These effects have been proven in an alkaline-water electrolyzer and in relation to the conductivity... 

Alkaline water electrolyzers that operate at pressures up to 5 MPa are under development. At present, they are commercially available with a capacity range of 1-120 N-m h h The goals are to optimize the units for energy efficiency and to make them responsive to fluctuating current supply that would occur with... 

In further German projects (HySolar [13], SWB [14], PHOEBUS [15]), various other advanced alkaline water electrolyzers were developed, constructed and tested in conjunction with renewable solar energy. Since then, no new and innovative approaches in alkaline water electrolysis had been funded or pursued in Germany until the profound change that the introduction of the German Renewable Energies Act [2] brought about. 

Hug, W., Divisek, J., Mergel, J., Seeger, W., Steeb, H. High efficient advanced alkaline water electrolyzer for solar operation. Adv. Hydrogen Energy 8, 681-690 (1990)... 

Present electrolyzers normally operate at 60 -90 C at ambient pressures using concentrated KOH electrolytes (3) (solid polymer electrolyte-acid electrolyzers are used for specialized applications (4). ) For both thermodynamic and kinetic reasons, it would be desirable to increase the temperature and pressure of operation to 150-200 C and 30 MPa. A major problem with these new conditions of operation is the lack of availability of a separator to prevent the mixing of product gases. Asbestos is universally used as a separator-diaphragm for conventional alkaline water electrolyzers, but it dissolves above ca. 100 C. Several new... 

Table 8.Rank distribution analysis of the SECversus current density, cell potential, and degree of efficiency in industrial alkaline bipolar water electrolyzers at 80 C electrolyte temperature and 1 bar pressure [21]. The bracketed numbers are observed values.

The first water electrolyzers used the tank design and an alkaline electrolyte.20 These electrolyzers can be configured as unipolar (tank) or bipolar (filter press) designs. In the unipolar design (see Figure 1), electrodes, anodes, and cathodes are alternatively suspended in a tank. In this design, each cell is connected in parallel and the entire system operated at 1.9-2.5 Vac. 

Wendt, H., Hofmann, H., and Plzak, V. Anode and cathode-activation, diaphragm-construction and electrolyzer configuration in advanced alkaline water electrolysis, Int. J. Hydrogen Energy, 9, 297-302 (1984). 

The major part of water electrolyzers operated today use alkaline electrolytes and follow the bipolar design. Figure 21 represents the principle setup. 

The perfluorinated sulfonic acid (Nafion) membranes have found a great variety of electrochemical applications. These include the SPE water electrolyzers, alkaline water elec-trolyzers, " hydrochloric acid electrolyzers, Na2S04... 

A recent study (12) has shown that Nafion is also suitable for use in water electrolyzers with alkaline solution as the supporting electrolyte. The major charge carrier is the alkali metal ion because of the negligible IT " ion concentration in alkaline solution and the Off ion rejection capability of the cation exchange membrane. The current efficiency of the cell is related to the inhibition of the transport of gaseous products across the separator. Thus, the ionic groups of the membrane are not important, in this case, because alkaline solution is the major electrolyte, and the migration of any ionic species across the membrane would not affect the current efficiency of the cell (33). 

The problem of ohmic drops by diaphragms has been studied for a long time. A laboratory scale diaphragm-less water electrolyzer was developed for hydrogen production at large pressures of up to 140 kPa by electrolysis in an alkaline solution. Porous electrodes with a nickel catalyst and a copper cover layer serve as cathodes, whereas nickel sheets are used as anodes. Modular construction of the electrolyzer permits simple combination of its cells into larger units. Thus, up to 20 cells with diskshaped electrodes of 7 cm in diameter were connected in series and provided with electrolyte manifolds, automatic pressure, and electrolyte level control devices. The dimensions of the electrolyte manifolds were optimized based on the calculations of parasitic currents [50],... 

Three main water electrolyzer technologies can be distinguished alkaline technology, proton exchange membrane (PEM) technology and solid oxide (SO) technology. Note that these three technologies present numerous similarities respectively with the fuel cells (AFC, PEMFC and SOFC) discussed in... 

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