Water splitting refers to methods by which Water (H2O) is separated into Hydrogen and Oxygen. Hydrogen is proposed as a reactant in a large number of lunar applications, including metal oxide and carbon oxide reduction, which would produce water as a byproduct. Splitting said water to recover the hydrogen during these applications is expected to be an important component of their functions.
One of the simplest ways to reduce water to hydrogen and oxygen is by electrolysis. In the simplest implementation, two electrodes are placed in water containing enough dissolved salts to allow for electrical conduction, and a current is run between the electrodes. This action causes the water molecules to split into hydrogen and oxygen. The oxygen collects at the anode (positively charged or ground connection), and the hydrogen collects at the cathode (the negative connection, where the current enters), where they are pumped out.
This process can be improved in efficiency by careful choice of electrode material and design. Also, performing electrolysis on water at very high temperatures results in less electricity being used, as a portion of the required energy is provided as heat. This approach is currently being researched.
Efficiencies vary widely depending on the exact setup, but efficiencies in the range of 50%-80% appear to be feasible. These efficiencies reflect the conversion of electrical energy into chemical energy, not counting the efficiency of producing the electricity.
Sulfur Iodine Cycle
Another proposed method of water splitting is the the Sulfur Iodine Cycle. In this process, water is mixed with iodine and sulfur dioxide, resulting in hydrogen iodide and sulfuric acid:
The hydrogen iodide evaporates out and is subjected to more intense heat treatment, where it breaks down into hydrogen and iodine:
the result is cooled to separate the iodine and send it back to the first step, and the hydrogen is piped out.
Meanwhile, the sulfuric acid produced in the first step is subjected to very high temperatures, producing oxygen, water, and sulfur dioxide:
The water and sulfur dioxide are condensed out and sent back to the first step, and the oxygen is piped out.
This process has the disadvantage of being significantly more complex when compared with electrolysis, as well as requiring careful selection of apparatus materials due to the high temperature and corrosive environment. However, the sulfur iodine cycle has a high efficiency of around 50%. More importantly, this efficiency represents the conversion of heat to chemical energy directly, no electricity is needed.
If direct heat sources are available in the temperatures needed (from solar concentrators or a nuclear reactor for example), producing hydrogen with the sulfur iodine cycle has potential to be significantly more efficient than electrolysis, as electrolysis efficiency ends up being in the 30-45% range once losses in power generation are included. If, however, electricity is the default source of energy available (from Solar Power Satellites or a lunar electrical grid), electrolysis would likely be the method of choice, as it is simpler and has roughly the same (or higher) efficiencies when using electricity.