Thermochemical Hydrogen Production

When water is heated to above 2500 degrees C, it separates into oxygen and hydrogen in a process known as thermolysis. However, at such high temperatures, it is difficult to prevent the oxygen and hydrogen from recombining to form water. Instead, thermochemical water-splitting cycles can lower the temperature and help separate oxygen and hydrogen products to produce pure hydrogen gas. These cycles can improve the efficiency of hydrogen production from 30% for conventional electrolysis to around 50% efficiency.

There are several different thermochemical water-splitting cycles being developed, but one of the most promising ones so far is the sulfur-iodine (S-I) cycle. In this cycle, sulfur dioxide (SO2) and iodine (I2) are feed into the cycle as chemical catalyst. A catalyst lowers the activation energy of a reaction without being used up by the reaction.

There are three steps in the S-I cycle:

Step 1:

I2 + SO2 + 2H2O = 2HI + H2SO4

The reaction is run at 120 degrees C. After this first step, the hy Sulfur Thermochemical Cycles drogen iodide and sulfuric acid can be separated, usually by distallation, which ultimately allows the separation of the hydrogen gas and oxygen gas.

Step 2: Generation of oxygen and regeneration of SO2

H2SO4 = H2O + SO2 + 1/2 O2

This reaction is run at 850 degrees C.

Step 3: Generation of hydrogen and regeneration of I2

2HI = H2 + I2

This reaction is run at 450 degrees C.

Thus, these reactions can reduce the high temperature demands of the thermolysis of water for the production of hydrogen gas and can provide a mechanism for the separation of oxygen and hydrogen products to prevent recombination.

Similar to the S-I Cycle, the sulfur-hybrid thermochemical cycle splits water into hydrogen and oxygen using chemical catalysts. However, instead of using iodine to separate hydrogen and oxygen, hydrogen is generated by the electrolysis of sulfur dioxide and water:

This electrolysis reaction can be run at 80 degrees C and is much more efficient than direct electrolysis of water. More over, the process decreases the complexity and capital costs of hydrogen production compared to the S-I cycle since the S-H process does not require complicated distillation to separate the products.

Bromine is in the same chemical family as iodine, but holds its electrons closer to its nucleus due to its smaller size. As a result, electrolysis is needed to separate hydrogen bromide into hydrogen and bromine.

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Introduction

Production

Transportation

Storage

Atmospheric Effects

Government Involvement

Conclusions