Water electrolyzers

Water electrolysis is often presented as the inverse reaction to that which takes place in a fuel cell (or vice versa). This description is not incorrect, but it tends to overlook the numerous thermodynamic and technological differences which exist between a water electrolyzer and a fuel cell. In addition, water electrolysis is often confused with hydrofysis. Yet these are totally different chemical reactions. Hydrolysis consists of using a reaction with water (hterally) to decompose a chemical substance, whereas water electrolysis consists of using an electrical current and heat to split water irrto Itydrogen and ojgrgert H2O + electricity + heat H2+ /2O2. 

Water electrolysis is one possible solution for producing hydrogen (and oxygen), which is not cirrrently considered a natural resoitrce, although it is known to exist in the natural state at the bottom of the oceans, and the existence of terrestrial sources 

Water electrolysis requires deionized water, electricity and heat. 

As regards the water, the overall balance in terms of environmental impact is potentially slight because its reuse in a fuel cell or in a hydrogen engine restores the same quantity of water as that used by electrolysis. If the sites of production and 

3 Announced in April 2013 by the New Energies Department of the French Petroleum Institute (IFPEN). The potential of these terrestrial sources of natural hydrogen remains to be evaluated, but could be very great. 

---

Plzak, V. Advanced Electrochemical Hydrogen Technologies Water Electrolyzers and Fuel Cells 26... 

Membranes in chlor-alkali electrolysis require highly pure brine feed the water used in membrane water electrolyzers must also be rather pure. 

More than 400 industrial water electrolyzers were in operation by the beginning of the nineteenth century. In 1939, the first large water electrolysis plant of 10,000 Nm3 H2/h capacity went into operation and in 1948, Zdansky/Lonza built the first pressurized industrial electrolyzer [1],... 

Simplified process diagram of an alkaline water electrolyzer. 

Depending on the size, type, and condition of an electrolysis plant, the energy requirement to produce 1 Nm3 of H2 lies in the range 4-6 kWh. With a hydrogen HHV of 3.5 kWh/Nm3, the efficiency of water electrolyzers lies in the range 58-87%. 

The development goals for water electrolyzers are oriented toward increasing the efficiency and reducing the cost. Most of the issues to be addressed are not specific to intermittent power operation. The efficiency of water electrolysis increases with the increasing... 

Commercial water electrolyzers cover a wide range of hydrogen production rates from 0.001 to 750 Nm3/h. Small hydrogen generators are intended for laboratory use, where hydrogen is often used as a carrier in analytical instruments, whereas large units are used in different fields of the chemical industry. 

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. 

Membrane water electrolyzers, platinum and platinum metal cathodes, 40 122 p-Menthane, 20 281... 

Advanced Electrochemical ffydrogen Technologies Water Electrolyzers and Euel Cells Plzak,V. Rohland, B. Wendt, H. 26... 

Also, discussions of a number of applications of Nafion are not included in this document and are, at most, mentioned within the context of a particular study of fundamental properties. A number of these systems are simply proposed rather than in actual commercial applications. Membranes in fuel cells, electrochemical energy storage systems, chlor-alkali cells, water electrolyzers, Donnan dialysis cells, elec-trochromic devices, and sensors, including ion selective electrodes, and the use of these membranes as a strong acid catalyst can be found in the above-mentioned reviews. 

Later, Hinatsu et al. studied the uptake of water, from the liquid and vapor states at various temperatures, in acid form Nafion 117 and 125, and Aciplex and Flemion membranes, although the latter two similar products will not be discussed here. These studies were motivated by a concern over the deleterious effects, involving either overly dry or overly wet membranes, on electrical conductivity within the context of polymer electrolyte fuel cells and polymer electrolyte water electrolyzers. 

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.

Oxygen, a highly useful gas, is the only by-product. Water electrolyzers today satisfy approximately 3.9% of the world s... 

Water electrolyzer units typically consist of several cells or electrodes arranged in two basic configurations, tank type operated in unipolar configuration, or filter press type operated in bipolar configuration. Tbe most common configuration, see Fig. 2.1, is the unipolar tank type where each electrode has only one polarity and all the electrodes of the same polarity are connected in parallel. The anodes and cathodes are alternately connected, with the... 

The reversible potential for the sulfur dioxide electrolysis is only 0.17 V, less than 10% that of water electrolysis (minimum of 1.23V at 298K and 1 bar) [65,69]. However corrosion problems in the electrolysis step are severe due to the presence of high concentration (about 50%) sulfuric acid. The overall thermal efficiency of the process, considering both thermal and electrical energy input derived from the same heat source, is estimated as 48.8% [116]. However, in terms of economics and process complexity the hybrid cycles face tough competition from advanced water electrolyzers. 

---