Enhanced rate of hydrogen production by corrosion of commercial aluminum
Graphical abstract
Introduction
Corrosion of aluminum in alkaline environment leads to hydrogen production by the following general scheme:2Al + 6H2O → 2Al(OH)3 + 3H2
Aluminum can thus be used as a “fuel” to produce energy and store it in the form of hydrogen. This project is justified by the fact that aluminum is one of the most abundant elements in the earth crust (3rd after oxygen and silicon) and it can be recycled by electrolysis, which recovers aluminum metal from Al(OH)3. Hydrogen production by aluminum corrosion is well known and studied for several decades [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]]. Many researchers consider reaction (1) as a parasitic reaction complicating the application of aluminum-air batteries [39,40], however, several other researchers see it as an economical source of hydrogen energy. The present work is inspired by the latter approach.
Recent trends in the study of hydrogen production by aluminum corrosion focus on the control of hydrogen formation rate. Temperature, of course, has a direct effect on hydrogen formation [18] as on most chemical reactions. Beyond this basic effect, other more specific measures have been undertaken to control hydrogen production. Thus sodium aluminate and sodium stannate solutions were found to improve the rate of hydrogen production in comparison with sodium hydroxide solutions, in addition to the fact that the use of sodium metalates is economically more viable [6,7,9,13]. Since the rate of hydrogen production depends on removing of the passivating alumina layer, which is formed on the surface of aluminum, addition of inorganic non-toxic salts in solution promote hydrogen production by preventing alumina formation [12]. The same effect was obtained by covering the metal surface by a mercury-zinc amalgam [15]. Pitting of the aluminum surface in the presence of metal oxides and chlorides had a similar effect by penetrating the passivation layer and thus promoting hydrogen evolution [16,26,30]. In a highly alkaline environment, the formation of alumina is not important since the alkaline salt dissolves alumina through, for example, the following reaction [13]:
Penetration through the alumina layer becomes a challenge at relatively low temperature and low salt concentration and at these conditions the above measures become important. Indeed, promotion of hydrogen evolution in water at relatively low temperature was achieved by using an alumina/graphite core/shell structure [19]. Penetration through the passivation layer was also studied in the presence of Bi, where an aluminum/bismuth microgalvanic cell is formed on the surface of Al causing pitting and thus removal of the passivation layer [31,33]. Therefore, pitting through the passivation layer provides a very important approach for controlling hydrogen production. Another important consideration is the active area presented by aluminum structure towards hydrogen formation. Grain boundaries formed in aluminum alloys become hydrolysis sites and for this reason most aluminum alloys promote higher hydrogen evolution rates than pure aluminum [3,5,38]. This effect has actually been studied in several works [3,5,11,14,17,20,33,[35], [36], [37], [38]] and it is also presently confirmed. Taking into account the above aspects, the most drastic approach for increasing hydrogen production rate by aluminum corrosion is decreasing the size of aluminum particles thus increasing active surface exposed to corrosion. Hydrolysis of too small aluminum particles may even become explosive. Many researchers have adopted ball milling of aluminum to produce small particles. Ball milling is frequently done in the presence of salts which may act as milling balls at the same time forming an active layer on Al particles thus preventing the formation of a passivation layer [4,8,17,[23], [24], [25],32]. Decreasing of the size of aluminum particles has also been the purpose of the present work.
The present study is focused on the use of a common, technical grade commercial aluminum, i.e. Al-6061, which has been studied as a means to produce hydrogen by corrosion in alkaline solutions. Al-6061 is an alloy, therefore it is expected to offer higher hydrogen production rate than pure aluminum and this is confirmed by the present data. In addition, it has been mechanically treated to produce turnings and then pressurized to form porous aluminum pellets. These pellets have produced hydrogen at very high rates thus providing the means for effective on board hydrogen generation. In this respect, simple metal turnings may be economically more interesting than the more sophisticated products of ball milling.
Section snippets
Materials
All materials were of reagent grade and were provided by Sigma-Aldrich unless otherwise specified. Al-6061 was a donation from Alcoa Corp (USA) and high purity aluminum was purchased from Merck. Aluminum pellets were constructed by first making Al-6061 turnings using a lathe and then pressurizing turnings at 25 ton cm−2. 3 mm thick pellets were produced as disks of 20 mm diameter.
In some cases, Al-6061 and pure aluminum slides have also been studied. They were polished before use in order to
Results and discussion
Al-6061 is an alloy containing aluminum at 95.85 to 98.56 wt percentage and several elements, mainly Mg and Si, which, according to the producer, reach the following weight percentages: Mg 0.8–1.2; Si 0.4–0.8; Fe max 0.7; Zn max 0.25; Cu 0.15–0.4; Mn max 0.15; Cr 0.04–0.35 and Ti max 0.15. The presence of the highest percentage elements was verified by EDX analysis (not shown).
Being an alloy, Al-6061 is expected to demonstrate a different behavior than pure aluminum towards corrosion and
Conclusions
This work has shown that it is possible to produce hydrogen at high rates by corrosion of Al-6061 turnings produced with the help of a lathe and then compressed into small porous pellets. It takes one such pellet and a concentrated KOH aqueous solution to produce in a controllable manner a substantial and technologically interesting quantity of hydrogen gas at ambient conditions. Hydrogen production rate by such pellets was much higher than that produced by a solid aluminum slide of the same
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