Odorless and invisible, hydrogen is the most abundant element in the universe, and has the potential to provide a clean, possibly an infinite source of energy.
It is already used in oil refining and fertilizer manufacture. But, despite efforts, and a lot of hype, it has never take off as a fuel.
That could be about to change, as the need to tackle climate change becomes more acute. Analysts agree that much of the global economy can be decarbonized in the near term by electrifying things like cars by using renewables to generate power.
But something else will be needed to clean up other carbon-intensive sectors such as aviation, shipping, long-haul trucking and heavy industry like concrete and steel manufacture.
Hydrogen gas does not emit carbon when burnt and has been touted by some as the ideal replacement for fossil fuels in these sectors.
Despite the abundance of hydrogen as an element, its atoms do not exist by themselves. They need to be separated from other elements in order to be used to deliver energy.
This can be done in a number of different ways, each color coded according to emissions produced.
At the dirtier end of scale is brown hydrogen, made from coal. Grey hydrogen is made from natural gas, a process that still creates a lot of carbon waste. The cleaner blue variety is also made from natural gas, but with carbon capture technology that stores the CO2, rather than spilling it out into the atmosphere.
Green hydrogen tops the eco-friendly table. It’s produced by the electrolysis of water using renewable energy.
An electric current produced from wind or solar splits water molecules into hydrogen and oxygen. Green hydrogen is attracting the attention of investors and policymakers as the world scrambles to cut emissions.
But, there is a downside. The cost of green hydrogen remain too high at around $5 per kg. To be competitive with fossil fuels, this figure needs to fall below $1 per kg. According to analysts, this could happen within the next few decades, or even sooner, as agencies like the United States Energy Department increase research and development efforts.
If it does take off, it is estimated that the hydrogen economy could be worth as much as $2.5 trillion by 2050.