Water Splitting - Chemical Production

Chemical Production

A variety of materials react with water or acids to release hydrogen. Such methods are non-sustainable. In terms of stoichiometry, these methods resemble the steam reforming process. The great difference between such chemical methods and steam reforming (which is also a "chemical method"), is that the necessary reduced metals do not exist naturally and require considerable energy for their production. For example, in the laboratory strong acids react with zinc metal in Kipp's apparatus.

In the presence of sodium hydroxide, aluminium and its alloys react with water to generate hydrogen gas. Unfortunately, due to its energetic inefficiency, aluminium is expensive and usable only for low volume hydrogen generation. Also high amounts of waste heats must be disposed.

Although other metals can perform the same reaction, aluminium is among the most promising materials for future development because it is safer, cheaper and easier to transport than some other hydrogen storage materials like sodium borohydride.

The initial reaction (1) consumes sodium hydroxide and produces both hydrogen gas and an aluminate byproduct. Upon reaching its saturation limit, the aluminate compound decomposes (2) into sodium hydroxide and a crystalline precipitate of aluminum hydroxide. This process is similar to the reactions inside an aluminium battery.

(1) Al + 3 H₂O + NaOH → NaAl(OH)₄ + 1.5 H₂

(2) NaAl(OH)₄ → NaOH + Al(OH)₃

Overall:

Al + 3 H₂O → Al(OH)₃ + 1.5 H₂

In this process, aluminium functions as a compact hydrogen storage material because 1 kg of aluminum can produce up to 0.111 kg of hydrogen (or 11.1%) from water. When employed in a fuel cell, that hydrogen can also produce electricity, recovering half of the water previously consumed. The U.S. Department of Energy has outlined its goals for a compact hydrogen storage device and researchers are trying many approaches, such as by using a combination of aluminum and NaBH₄, to achieve these goals.

Since the oxidation of aluminum is exothermic, these reactions can operate under mild temperatures and pressures, providing a stable and compact source of hydrogen. This chemical reduction process is specially suitable for back-up, remote or marine applications. While the passivation of aluminum would normally slow this reaction considerably, its negative effects can be minimized by changing several experimental parameters such as temperature, alkali concentration, physical form of the aluminum, and solution composition.