Hydrogen is a promising energy vector for the future. Among the different methods of its production, the electrolysis of water has attracted great attention because it is a sustainable and renewable chemical technology. Thus, hydrogen represents a suitable energy vector for the storage of intermittent energies. This chapter is devoted to the hydrogen generation by water electrolysis as an important part of both existing and emerging industrial electrochemical processes. It aims to give an insight into the theoretical foundations of the operating principles of different types of electrolyzers. Also, it is developed in this chapter, the thermodynamic and kinetic aspects of the reactions taking place at the electrodes of water electrolysis. The evolution reaction of hydrogen has a rapid kinetics, and thus, the polarization of the cathode is not critical. On the other hand, the evolution reaction of oxygen is characterized by a very slow kinetics and is thus responsible for most of the overvoltage in the electrolysis of water. The most important technologies of water electrolysis are addressed: alkaline electrolysis, proton exchange membrane electrolysis, and solid oxide high-temperature electrolysis.

Azoic dyes are known as toxic contaminants. Methyl orange is one of these dyes, is removed from wastewater principally through biological, chemical or physical processes. This article will describe the removal of the methyl orange from the water by adsorption onto activated carbon. This material is characterized before and after use in adsorption by various techniques. The methyl orange adsorption efficiency was studied varying contact time, the mass of activated carbon, pH, initial solution concentration, and temperature. The adsorption was found fast attainment equilibrium in 30 min with better performance for the sorbent and appears to be enhanced by low pH. Kinetic data for methyl orange was found to follow pseudo-second order model and the Langmuir model better describes the adsorption process. A maximum amount of methyl orange adsorbed was registered at 113.63 mg/g. The influence of temperature on adsorption and the thermodynamic study showed that free energy and the enthalpy exhibit negative and positive values, respectively, which shows that the methyl orange adsorption on the activated carbon is a spontaneous and endothermic process. The desorption/regeneration process was studied by using different percentages of ethanol and distilled water and it appears to be the best desorption to regenerate methyl orange. The use of 90% ethanol gave a rate of adsorbed methyl orange regeneration close to 89%. On other hand, these results indicate the significance of activated carbon as new support for the elimination of the methyl orange.