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Monday, January 6, 2014

Fuel Cell Future Unlikely or Inevitable? Part 3 - H2 from Water

Water as a Hydrogen source 

Rather than extracting H2 from a hydrocarbon, it can be produced by splitting water by a process called Electrolysis.

With electrolysis, electric current can be used to split water into Oxygen and Hydrogen gas:  
2 H2O (+DC current) → O2 + 2 H2

Electrolysis uses two electrodes, an anode and a cathode. During electrolysis these electrodes undergo a chemical reaction. The anode is oxidized and the cathode has electrons stolen from it by a process called reduction. These electrodes need to periodically be replaced.

This process does not create CO2. It does, however, have a high energy cost. The electricity generation method will directly impact the CO2 related to H2 generation. Some reports quote an efficiency of 50 to 70% for alkaline electrolysers. There is research into higher efficiency with the use of proton exchange membranes (PEM) and catalytic technologies that could lead to 95% efficiency, but today electrolysis remains an energy intensive operation. 

The cost of electrode replacement combined with the great energy needed results in only 4% of worldwide H2 being produced by electrolysis today. Because of these costs for electrolysis, steam-Methane reforming remains the industrial method of choice for H2 production.

Electrolysis, however, is only one method of splitting water. Other methods are being researched, here are a couple:

Thermolysis 

At 2500° C water will spontaneously dissociate. This process is called Thermolysis. With the use of catalysts the dissociation temperature can be reduced. Thermolysis occurs at temperatures that are too high for normal piping and equipment and maintaining such an incredibly high temperature is very energy intensive.

Thermochemical Cycle

The Sulfur-iodine cycle is a thermochemical cycle processes for hydrogen production from water. It has an efficiency of approximately 50%. The sulfur and iodine are recovered and can be reused. The cycle requires 950° C temperatures.

There are hybrid methods that use both thermochemical and electricity, such as the Copper–chlorine cycle. It operates at 530° C and has an efficiency of 43 percent.

Ferrosilicon method

Ferrosilicon is used by the military to quickly produce hydrogen for balloons. The chemical reaction uses sodium hydroxide, ferrosilicon, and water. The generator is small enough to fit a truck and requires only a small amount of electric power, the materials are stable and not combustible. The method has been in use since World War I. A heavy steel pressure vessel is filled with sodium hydroxide and ferrosilicon, closed, and a controlled amount of water is added; the dissolving of the hydroxide heats the mixture to about 90° C and starts the reaction; sodium silicate, hydrogen and steam are produced.

Photobiological water splitting
Hydrogen can be produced by algae in a bioreactor. In the late 1990s it was discovered that if the algae is deprived of sulfur, it will switch from the production of oxygen, i.e. normal photosynthesis, to the production of hydrogen. This production is 7–10 percent energy efficiency.


Fermentation

Bacteria and enzymes can be fermented to generate H2. Photofermentation uses light, while dark fermentation, as the name suggests, doesn't need light. The process involves bacteria feeding on hydrocarbons and exhaling hydrogen and CO2. A prototype hydrogen bioreactor using plant waste feedstock is in operation at Welch's grape juice factory in North East, Pennsylvania.

Large Scale Production 

The amount of H2 that would be needed by the world's transportation sector would require a significant increase in H2 production. For H2 to be seen as a clean, renewable fuel, it has to be sourced from water rather than fossil fuels. All of the methods mentioned above are being researched in hopes of finding a better method for large scale H2 production. Unfortunately, today, water sourced H2 remains expensive and energy intensive. This means that natural gas will be the foreseeable fuel stock for H2 production.

Next: Fuel Cell Future Unlikely or Inevitable? Part 4 - Hydrogen Infrastructure