Hydrogen, H2, is one of many gasses that can be used to produce energy.
It is used in all sorts of industry, and it can also power anything from a child's toy to space rockets, and all manner of vehicles, machinery and gadgets in between.
Assisted bicycles, motorcycles, cars, busses, trams and trucks are all able to use it when they have the correct powertrain. Even drone-style helicopter alternatives in development could use it instead of batteries.
The most efficient use of it as a motive fuel is an on-board process of recombining the hydrogen with oxygen in the air to produce electricity. It's commonly referred to as a fuel cell. Electric motors then power the vehicle and you only have water vapour (H2O) as a vehicle emission.
However, just as it does for biofuels, the way the hydrogen was produced really, really matters. Hydrogen can be produced in several ways, and there is a colour designation system to tell you what the source was.
Black and brown hydrogen are produced after the gasification of black (bituminous) or brown (lignite) coal. However, this method releases a lot of CO2 into the atmosphere.
Grey hydrogen comes from natural gas (methane and ethane) and again, quite a lot of carbon gasses are released for the amount of hydrogen it yields through a process called steam reforming. It is also the most common form of hydrogen production right now.
Blue hydrogen is also produced from fossil fuels, except the carbon is captured and stored or reused somehow.
That's a contentious one though, because there are studies that have concluded that blue hydrogen from natural gas still releases more greenhouse emissions in the form of unintentionally leaked methane (a much more potent greenhouse gas than carbon dioxide) than simply burning coal or natural gas for heat. It's also worth remembering that this leakage of methane occurs with grey hydrogen production.
Another issue of contention is whether the storage of the carbon dioxide is actually permanent.
There are other colours too. Turquoise hydrogen is in the experimental phase, and it involves a process called methane pyrolysis, which results in solid carbon. This would resolve the carbon capture issue with more certainty because it's not gaseous, but there's still the issue of unintentional leakage of the methane required during its extraction.
Another one is pink hydrogen. This involves a process called electrolysis which separates water (H2O) into hydrogen and oxygen, except it uses nuclear power to generate the electricity required.
The one we want is green hydrogen, because it's the only option currently offering a long-term solution. Green hydrogen is also produced via electrolysis from water (H2O), but the electricity has come from renewable sources such as wind and solar.
The bad news is green hydrogen makes up barely any of the world's production at present. The good news is that's supposed to be changing, albeit very slowly.
For example, in October 2021 the NSW and Queensland state governments both made commitments to the development of renewable hydrogen production.
"Only renewable hydrogen, generated through solar and wind, can play a role in our zero emissions future. As one of the sunniest and windiest countries in the world, why would we invest in any other type of hydrogen?" said Climate Council member and energy expert, Dr Madeline Taylor, in response to the NSW government's October 2021 announcement of $3 billion in incentives to produce and distribute renewable hydrogen.
Meanwhile, there are still advocates for blue hydrogen. Their point is, it can help with the transition to a hydrogen economy by providing a lot of hydrogen before green hydrogen production is at the full (and frankly, epic) scale needed.
Maybe we should be satisfied that it would at least try to capture the carbon for now.
However, these advocates are groups with a vested interest in fossil fuel extraction and they're quite happy to ignore that issue of leaked methane as well as the argument that storing the carbon underground may not actually keep it there permanently.