Hydrogen production in a combined electrochemical system: anode process

Abstract

Various methods of hydrogen production are known: traditional (for example, electrolysis of water and conversion of hydrocarbons) and combined thermochemical methods. The method of obtaining hydrogen by electrolysis of aqueous solutions of hydroxides of alkali metals is the most energy-intensive one, though considered one of the most promising in the European Union. The purpose of this work is the scientific substantiation of the electrochemical production of hydrogen with reduced energy consumption in a combined, open mass transfer system, the composition of the catholyte, and the concentration of its components to ensure the conditions for reducing the energy consumption for the hydrogen release. To reduce the energy consumption for hydrogen production in the combined electrochemical method, the anode on which oxygen is released in an acidic medium is replaced by a soluble anode with an equilibrium potential more negative than the potential of oxygen release. Such a soluble anode can be iron with a standard potential of –0.44 V. At the same time, the decomposition voltage in this system was equal to 0.41 V compared to 1.23 V in the case of traditional electrolysis of water. The overvoltage of iron dissolution in a chloride medium is several tens of millivolts, and the potential difference between the anode and the cathode when hydrogen is released can be much smaller than during the usual decomposition of water. The Pourbaix diagram and possible products of the electrochemical dissolution of iron were considered. The process of iron dissolution was studied in a 1 mol·L–1 solution based on Na2SO4 in the addition of NaCl with concentrations up to about 50 g·L–1. The cathode was platinum, the anode was the St3 iron electrode According to the data of cyclic voltammetry, it was established that the maximum current density of iron dissolution increases with an increase in the concentration of sodium chloride in the electrolyte. The dynamics of changes in the potential values of the onset passivation (the Flade potential) and complete passivation (activation potential) depending on the concentration of sodium chloride were also established. It is established that with an increase in NaCl concentration up to 50 g·L–1, the Flade potential is shifted shifts towards anode by 0.8 V. At average chlorine concentrations of ~10 g·L–1, intense current fluctuations are observed instead of passivation. The maximum of dissolution iron anode current density 700 mA·cm–2 was achieved in the Na2SO4 solution with the addition of NaCl in the amount of 50 g·L–1. Keywords: hydrogen, electrolysis, cathode, soluble iron anode, hydrogen energy.