Water electrolysis is one of the simplest methods used for hydrogen production. It has the advantage of being able to produce hydrogen using only renewable energy. To expand the use of water electrolysis, it is mandatory to reduce energy consumption, cost, and maintenance of current electrolyzers, and, on the other hand, to increase their efficiency, durability, and safety. In this study, modern technologies for hydrogen production by water electrolysis have been investigated. In this article, the electrochemical fundamentals of alkaline water electrolysis are explained and the main process constraints (e.g., electrical, reaction, and transport) are analyzed. The historical background of water electrolysis is described, different technologies are compared, and main research needs for the development of water electrolysis technologies are discussed.

Os filmes “passivos” com propriedades protectoras à superfície de muitos metais e ligas metálicas são responsáveis pelo fenómeno da passividade. Esta contribui significativamente para a tecnologia actual (por exemplo, aços inox, dispositivos semicondutores). Neste artigo revêm-se os modelos de passivação e descrevem-se as técnicas experimentais mais utilizadas para o seu estudo.

The passive films present on some, but not all, metals and alloys have special characteristics that enable them to provide superior corrosion resistant metal surfaces. These protective “passive” films are responsible for the phenomenon of passivity, which has significantly contributed to modern technology (for example, development of stainless steels, semiconductor device technology). In this paper, the theory and basic experimental techniques used to investigate the passivation phenomenon are considered.

Discutem-se processos de obtenção de hidrogénio. Em particular, considera-se a produção electrolítica de hidrogénio em meio aquoso, à escala industrial, e à custa de energia eléctrica renovável (solar, vento, marés, etc.). Fazem-se ainda algumas previsões acerca do armazenamento, transporte e possíveis aplicações do hidrogénio electrolítico.

Possible means of producing hydrogen are discussed. Emphasis is given on the electrolytic hydrogen production from water electrolysis, at large scale, via the use of renewable electricity (solar, wind, tidal, etc.). Its storage, transport and possible end-uses are also considered.

The electrochemical behaviour of zinc has been extensively studied in alkaline and acid media, but only a few studies have been reported in neutral solutions, particularly in deaerated media. Zinc passivation in neutral medium and the effect of the ClO4- ion on the nucleation and growth of the passive layer is studied in this paper by a transient technique at different electrolyte concentrations and applied potentials. ZnO growth rate was shown to decrease with increasing electrolyte concentration. Moreover, passive layer growth occurred followed by pitting nucleation and growth. Film growth and pit nucleation are explained by means of the Macdonald and Engell-Stolica models.

Platina, paládio, ouro, cádmio, níquel e nióbio imersos em soluções alcalinas de borohidreto de sódio são caracterizados por respostas de eléctrodo que permitem monitorizar o conteúdo em borohidreto presente num dado meio aquoso. Descreve-se uma técnica de titulação potenciométrica relativamente simples e precisa que pode ser aplicada para a análise de soluções de borohidreto.

Platinum, palladium, gold, cadmium, nickel and niobium immersed in sodium borohydride alkaline solutions are characterised by electrode responses that allow monitoring the borohydride present in aqueous media. A simple and relatively precise potentiometric titration technique is described and applied for the analysis of borohydride solutions.

A eletrocatálise da reação de redução do oxigênio por manganato de cálcio e lantânio, (La0.5Ca0.5MnO3) (LCM), foi estudada por voltametria cíclica usando-se a técnica do eletrodo rotativo de disco e anel (RRDE) em meio alcalino. Os resultados do disco-anel e outros parâmetros cinéticos indicaram que, a baixas sobretensões, a redução do oxigênio ocorre por adsorção química. Para sobretensões mais elevadas foi observada a formação de peróxido de hidrogênio (HO2- neste caso) no eletrocatalisador. A densidade de corrente de permuta aparente, determinada para a redução do oxigênio no LCM foi de 5 <FONT FACE=Symbol>´</FONT> 10-8 A cm-2, sendo o correspondente coeficiente de Tafel de 0,115 V por década. Discute-se e sugere-se um mecanismo para a reação de eletro-redução do oxigênio.

The electrocatalysis of the oxygen reduction reaction by lanthanum calcium manganate, (La0.5Ca0.5MnO3) (LCM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium. From the ring-disc data and other kinetic parameters it has been assumed that the oxygen reduction occurs by dissociative chemisorption at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2- in this case) has been observed on this electrocatalyst. The apparent exchange current density value for oxygen reduction on LCM has been found to be 5 <FONT FACE=Symbol>´</FONT> 10-8 A cm-2, while the corresponding Tafel slope is 0.115 V per decade. The possible reaction mechanism for electroreduction of oxygen on this oxide catalyst has been discussed.