Hydrogen generation from water by means of activated aluminum

Abstract

An important part of the hydrogen energy problems is the search of hydrogen sources for feeding hydrogen–air fuel cells. One of the most convenient methods for hydrogen generation is based on oxidation of aluminum by water. In this paper the method of aluminum activation based on the application of gallium alloys (gallams) is suggested.

In this work the activated aluminum powders made of commercially available aluminum wires are investigated. The kinetic parameters of the reaction of activated aluminum with water (hydrogen generation rate and hydrogen yield) depending on the gallam’s amount and composition, powder particle sizes and reaction temperature are studied.

Introduction

The progress of hydrogen energy depends on development of new effective and ecologically sustainable sources of hydrogen. There are many offers for development of such sources. One of the perspective methods is the hydrogen generation from water by means of metals or alloys. The appropriate material for this goal is aluminum because of its properties such as high efficiency (1.24 l/g), availability, environmental safety of the reaction products, safety storage and transportation, and low price. However, oxidation of aluminum by water at ambient conditions is impossible owing to the formation of solid oxide or hydroxide film on the surface. Aluminum without this protective film reacts with water easily and rapidly generating pure hydrogen, which could feed fuel cells, including portable.

There are many ways of aluminum activation described in the literature. One of the early and well known activation methods is the amalgamation [1], [2], [3]. But, through the mercury toxicity such aluminum is unacceptable for application in home devices. Nevertheless, works in this field are carried out today [4], [5]. The maximum hydrogen generation rate for the amalgamated aluminum does not exceed 600 mL/(g min) at temperature 90 °C [6]. Investigation of amalgamation method may be useful only in terms of studying the reaction mechanism of activated aluminum with water.

There are many propositions to use reaction of aluminum oxidation by alkaline aqueous solutions [7], [8], [9], [10], [11]. The alkali dissolves the oxide film and exposes the aluminum surface for the reaction with water. However, the reaction rate is rather low (100–280 mL/(g min) at ambient conditions). The higher rate is reached at higher temperatures and in more concentrated alkaline solutions.

It is possible to generate hydrogen at mild conditions (atmospheric absolute pressure, temperature ≤ 100 °C) [12], [13] using the micron and ultrafine particles having the high specific surface. Nevertheless, there are some shortcomings: reaction starts at the temperatures more than 40 °С; reaction rates are 100–350 mL/(g min) at 95–100 °C; high content of oxide film layer on the aluminum surface (∼20%), which reduces the ratio of active aluminum; high price of ultrafine powders.

A new direction in the development of aluminum activation methods is associated with the use of ultrasonic influence directly in the process of oxidation of the metal with water [13], [14]. The effect of the ultrasonic field allows reduce the stage of induction period; increase the reaction rate and degree of aluminum conversion. Use of ultrasonic activation during the oxidation reaction of aluminum particles in aqueous Ca(OH)₂ saturated solution can further increase the hydrogen generation rate [14]. However, the sources of ultrasound are cumbersome, their work requires additional energy cost and specialized facilities, which limits their use in portable devices, but can be applied in stationary devices for the simultaneous production of hydrogen and aluminum hydroxides.

The hydrothermal oxidation of aluminum allows to rich the high reaction rate and complete hydrogen yield [15], [16]. However, this method is realized at high temperatures (more than 200 °C) and pressures (more than 20 Atm) and requires complex and expensive equipment, preventing its application in mobile device.

The aluminum activation by such metals as gallium, indium, tin, etc. makes it possible to carry out oxidation reaction even at room temperature [17], [18], [19], [20], [21]. The shortcomings of such method are: high content of activated metals (gallium and indium) and respective high price of activated aluminum; rather low reaction rate at room temperature (100–300 mL/(g min)); deterioration of reactivity of aluminum with time.

The mechanochemical treatment of metal in ball mill allows not only to crush the material, but also to change its structure and physicochemical properties [22]. To get the active aluminum it needs the addition of carbon [22], [23], silicon or other metals or complex systems, such as Bi-hydride or Bi-salt (KCl, NaCl, LiCl, MgCl₂, AlCl₃) [19], [24]. The material obtained using mechanochemical processing, allows to eliminate some of the shortcomings described above methods, but also has its own drawbacks: low hydrogen yield (not more than 80%); pollution of hydrogen by methane at oxidation of Al–C system by water; the toxicity of bismuth; rapid deterioration of reaction properties of the material during storage it in the air; the need for prolonged (2–5 h) mechanical treatment of the metal in high-energy mills.

Thus, considering the advantages and disadvantages of the above activating methods, a new method of aluminum activation for hydrogen generation from water was developed [25], [26], [27]. The activation was performed in two steps: manual treatment of aluminum granules with gallam (step 1) [25], [26] and additional trituration in the high-energy ball mill (step 2) [27]. Such activated aluminum powders reacting with water give high rate of hydrogen production – 1000–2500 mL/(g min).

In this paper we investigate activated powders made of commercially available aluminum wires. The kinetic parameters of the reaction of activated aluminum with water (hydrogen generation rate and hydrogen yield) depending on the gallam’s amount and composition, powder particle sizes and reaction temperatures are studied.