Hydrogen — a colorless gas at normal atmospheric pressure and temperature — has suddenly gone green. And not only is it green, but it might also be a potential solution for our low-carbon future.

No, you haven't missed a fundamental change in Earth's chemistry, and gases aren't really changing color. So then, what is green hydrogen exactly? Let's find out.

Hydrogen: A Clean-Burning Wonderfuel

Hydrogen is extremely abundant on the planet but not in its pure form. It's usually combined with other elements, such as oxygen or carbon, and presents as water (H₂O) or methane (CH₄). However, in its pure form, H₂, it shows great potential as a clean-burning fuel that can help us build a low-carbon future.

Used as fuel, hydrogen is considered a clean energy source and emits only water as a byproduct when used in fuel cells to generate electricity. This makes it extremely valuable in certain situations, like on space missions. It's also a great way to reduce the carbon load in heavy industries, such as heavy transport and aviation. By going green, hydrogen fuel doesn't emit harmful residues during its production or its use.

A Rainbow of Hydrogen Options

What is green hydrogen, though, if it's technically a clear gas? For starters, green is not the only "color" of hydrogen. The color coding refers to the way hydrogen is produced, giving an indication of how clean each manufacturing process is as described by the U.S. Energy Information Administration.

Traditional production methods use coal (brown hydrogen) or natural gas methane or petroleum (gray hydrogen), with the energy used in the process coming from fossil fuels. This pumps out a lot of carbon dioxide as a waste product. By installing carbon capture into the process, the resulting hydrogen can be called blue, and there is less impact on greenhouse gas accumulation.

Hydrogen can also be produced by splitting water during a process called electrolysis, and this is where green hydrogen comes from. An electrical current splits the hydrogen from the oxygen atoms. Other hydrogens in the spectrum include yellow hydrogen (using solar power) and pink hydrogen (powered by nuclear energy).

The U.S. Department of Energy's Alternative Fuels Data Center notes a few more renewable methods under investigation. These include generating hydrogen photobiologically from algae as they consume water in the presence of sunlight, using renewable liquid fuels such as ethanol, and fermentation that acts on sugar-rich feedstocks such as biomass to release hydrogen.

Why Is Green Hydrogen Better?

Since hydrogen as a fuel is presented as an alternative to replace fossil fuels, both its use and production should aim to reduce carbon dioxide emissions as much as possible. Most hydrogen production in the U.S. comes from steam treatment to reform methane gas (steam-methane reforming). Currently, making hydrogen is responsible for more than 2% of global carbon dioxide emissions. With a predicted rise in energy requirements of 25% to 30% by 2040, low-carbon alternatives are definitely needed.

Using the color-coding designations for hydrogen, green is a good way forward since it doesn't contribute to global warming by way of carbon emissions. This is achieved by switching the source of the energy required to generate the electrolysis reaction from fossil fuels. Instead, this hydrogen uses renewable energy sources to power the conversion, which means there are no carbon emissions to deal with, and water is the only byproduct.

What's Holding It Back?

Electrolytic conversion using renewable energy to abolish carbon emissions seems like a simple solution. However, there are a number of obstacles researchers need to overcome before this method of production is adopted widely.

Cost is one major barrier. Electrolysis is an energy-hungry process. This pushes up the cost of the hydrogen produced beyond what's affordable for subsequent energy generation, and to be a realistic replacement for fossil fuels, it needs to be affordable.

Harvesting the hydrogen produced from electrolysis is relatively straightforward, but there are issues around its volatility and, therefore, its safety in handling. Hydrogen as a liquid needs to be stored under pressure for delivery and transportation. Work at the National Renewable Energy Laboratory describes how something as simple as the fuel delivery hose has to undergo extensive testing. Simulating the device in use allows researchers to learn more about flexibility, entanglement and other issues that could compromise safety and delivery.

The Alternative Fuels Data Center also notes that the current infrastructure presents limitations. Most hydrogen is produced near where it is used. Adoption more widely — such as for fuel cells in cars — requires a wider distribution network. This means either pipeline development or expanding a fleet of distribution tankers. Because hydrogen rapidly becomes a gas when pressure and temperature reduce, supply must be closely matched with demand so tanks of the liquefied product don't simply evaporate before use.

Though there are finer points in production and supply that need to be addressed, in the future, alternative fuel sources for our planet will probably include green hydrogen, and perhaps the yellow and pink varieties, too.

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