6. Solid oxide electrolyzer

5. PEM electrolyzer

12 Advances In Hydrogen Generation Technologies

water: it is the electro-osmotic flow.

reject the gas production at the rear of the interpolar space.

• No electrical conductivity to avoid short-circuits;

• Low permeability to oxygen and hydrogen;

• Good chemical stability;

properties of this assembly:

their case.

gas release.

especially its conductivity.

PEM electrolyzers are characterized by their very simple construction and their compactness. The operating principle of electrolysis of water with an electrolyte protons exchange membrane (PEM) is simple. When operating in electrolysis, the water decomposes at the anode into protons and molecular oxygen. The oxygen is evacuated by the water circulation, and the protons migrate to the cathode under the effect of the electric field. There, they are reduced to molecular hydrogen. Each proton carries with it a procession of several molecules of solvation

During the twentieth century, several major innovations have significantly increased the energy and faradic efficiencies of electrolyzers. The concept of zero-gap cells has been developed in order to overcome the disadvantages of the electro-osmotic flow. It consists of pressing porous electrodes against the solid separator in order to reduce the interpolar distance and to

The zero-gap concept with immobilized electrolyte goes even further: it consists of maintaining the electrolyte (acid) in the separator so as to be able to electrolyze the water in the acidic medium while avoiding corrosion problems. Of course, this interesting approach was practically

The membrane thus serves both electrolyte and separator of electrodes and gases. Therefore,

Compared to a liquid electrolyte, we can note some behavioral deference resulting from the

• The anionic charges of the membrane are fixed; there can be no concentration gradient in

• The gas evolution is done by the back of the electrodes, the ohmic drop is not disturbed by the reactions to the electrodes, in return, it is necessary to make laying electrodes to allow

• The nature of the ions also intervenes, but the water content of the membrane, different according to the nature of the ions carried and according to the conditions of preparation and use of the membrane, will condition both its thickness, its mechanical strength, and

limited by the leakage of electrolyte pushed back into the circuit of the electrolyzer.

• High ionic conductivity to promote proton migration and reduce ohmic drop;

• Good mechanical and dimensional stability, especially resistance to pressure;

the membrane must have certain physicochemical properties, such as:

• Good thermal stability, operating temperature up to 80–100C.

Solid oxide fuel cells are electrochemical devices that can operate reversibly in the electrolysis mode. In the solid oxide electrolyzer, water vapor is reduced to H2.

Electrolysis at high temperature allows decreasing the electric consumption because with the increase of the temperature offers an additional part of the global energy; which allows high operational efficiencies in the solid oxide electrolyzer.

The main advantage is that a substantial part of the energy required for the electrolysis process is added in the form of heat, which is much cheaper than electrical energy. In addition, the high temperature promotes the conduction of the electrolyte and accelerates the kinetics of the reaction, reducing the energy loss due to the polarization of the electrode. Thus, the efficiency of the electrolysis at high temperature is higher than that obtained at low temperature. The typical high-temperature electrolyzer can achieve an electrical efficiency of 92% while the electrolyzers at low temperature reach a maximum of 85% efficiency.

The high-temperature system uses oxygen ion conducting ceramics as an electrolyte (ZrO2 stabilized by Y2O3, MgO or CaO). The water is brought to a temperature of 200C to supply steam in the cathode. The electrolysis cell operates at a temperature of 800–1000C, which ensures the conduction of the solid electrolyte. The water vapor is decomposed into hydrogen gases and oxygen ions (O<sup>2</sup>). The oxygen ions are transported through the ceramic solid electrolyte to the anode, where they are oxidized to form gaseous oxygen.
