Better Ways for Hydrogen Fuel from Water

The two main difficulties preventing us from using hydrogen power for everything we need are storage and production.

At the moment, hydrogen production is energy-intensive and expensive.

Normally, industrial production of hydrogen requires high temperatures, large facilities and an enormous amount of energy.

In fact, it usually comes from fossil fuels like natural gas – and therefore isn’t actually a zero-emission fuel source.

Making the process cheaper, efficient and sustainable would go a long way toward making hydrogen a more commonly used fuel.

An excellent – and abundant – source of hydrogen is water.

Chemically, that requires reversing the reaction in which hydrogen releases energy when combining with other chemicals. That means we have to put energy into a compound, to get the hydrogen out. 

One method involves mixing water with a helpful chemical, a catalyst , to reduce the amount of energy needed to break the connections between hydrogen and oxygen atoms.

There are several promising catalysts for hydrogen generation, including  molybdenum sulfide, graphene and cadmium sulfate . 

Making hydrogen

Hydrogen is the most abundant element in the universe, but it’s rarely available as pure hydrogen. It combines with other elements to form chemicals and compounds, such as organic solvents like methanol, and proteins in the human body.

Its pure form, Hâ‚‚, can be used as a transportable and efficient fuel.

There are several ways to produce hydrogen to be usable as fuel.

Electrolysis uses electricity to split water into hydrogen and oxygen. 

Steam methane reforming starts with methane (four hydrogen atoms bound to a carbon atom) and heats it, separating the hydrogen from the carbon. This energy-intensive method is usually how industries produce hydrogen that is used in things like producing ammonia or the refining of oil.

Photocatalytic water splitting,  with a catalyst, can use light to provide the amount of energy needed to “split” water into hydrogen and oxygen. When exposed to light, a proper mixture of water and a catalyst produces both oxygen and hydrogen. This is very attractive to industry because it then allows us to use water as the source of hydrogen instead of dirty fossil fuels.

Understanding catalysts

Water molecules can be split into hydrogen and oxygen with the addition of energy, but the amount of energy needed would be more than would be generated as a result of the reaction.

A catalyst lowers the amount of energy needed for two compounds to react.

Photocatalysts function only when exposed to light, e.g. titanium dioxide.

With a photocatalyst in the mix, the energy needed to split water drops significantly.

We can make the splitting even more efficient by adding another substance, in a role called co-catalyst .

Co-catalysts in hydrogen generation alter the electronic structure of the reaction, making it more effective at producing hydrogen.

So far, there aren’t any commercialized systems for producing hydrogen this way.

The first promising combination, titanium dioxide and platinum , was discovered in 1972. Platinum, however, is a very expensive metal (well over US$1,000 per ounce). 

Metals like these are so rare in the Earth’s crust that this makes them not suitable for large-scale applications even though there are processes being developed to recycle these materials.

Finding a new catalyst

There are many requirements for a good catalyst, such as being able to be recycled and being able to withstand the heat and pressure involved in the reaction.

Just as crucial is how common the material is, because the most abundant catalysts are the cheapest.

One of the newest and most promising materials is molybdenum sulfide , MoSâ‚‚.

Because it is made up of the elements molybdenum and sulfur – both relatively common on Earth – it is far cheaper than more traditional catalysts, well under a dollar per ounce. It also has the correct electronic properties and other attributes.

Before the late 1990s, researchers had found that molybdenum sulfide was not particularly effective at turning water into hydrogen.

That was because researchers were using thick chunks of the mineral, essentially the form it’s in when mined from the ground.

Today, however, we can use processes like chemical vapor deposition or solution-based processes to create much thinner crystals of MoSâ‚‚ – even down to the thickness of a single molecule – which are vastly more efficient at extracting hydrogen from water.

Source: The Conversation

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Taxonomy

• Photocatalysis
• Catalysts
• Energy
• Water