At first it tasted just like water from the tap. Lukewarm, not hot, and with a slight metallic zing - the only signs the liquid had, seconds earlier, spurted from an outlet underneath a car.
But water is the only emission from a process of electrolysis undertaken in hydrogen cars, an emerging technology which could fuel as many as 1.6 million vehicles in Britain by 2030, according to the Centre for Future Studies.
And it’s so clean, manufacturers boast, you can enjoy a sip.
Fuel cells in the cars generate power by using hydrogen in the tank and oxygen from outside, with the only resulting emission being H2O.
And it’s a development that, alongside other cleaner fuel tech like battery electric cars, can’t come soon enough.
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The problem of poor air quality is the largest environmental risk to public health in the UK, costing the country up to £2.7 billion in lost productivity in 2012.
And it’s lethal too. Bad air is linked to as many as 40,000 early deaths a year.
The issue was highlighted by the government last month when it announced petrol and diesel vehicles would be banned from Britain’s roads by 2040.
While some way off yet, car manufacturers will see the 2040 deadline as an opportunity.
Toyota’s Mirai hydrogen fuel cell car is one example already on UK roads, with the 28 or so currently registered here owned by businesses, local government and taxi companies.
Cost is one reason for the low numbers of registrations - at £66,000 the model isn’t cheap. But low public perception and misgivings about mileage factor in too. One of the first questions many people have is: where can you refuel?
It’s something Mirai driver and self-confessed petrolhead Theo Ellis thought when he got the opportunity to get behind the wheel of the car 18 months ago.
“Depending on where you are in the country, refuelling isn’t a problem. Firstly you can get up to 300 miles from a tank, which is a lot for most people.
“In London there are five filling stations, one for west, east, and north, one at Heathrow and then Cobham services on the M25.”
The government announced in March a new £23 million fund to accelerate the take up of hydrogen vehicles. It sees them as complimenting battery electric cars.
“Hydrogen fuel cell electric vehicles can play a vital role... to help us cut harmful emissions,” Transport Minister John Hayes said at the time.
The fuel has found supporters from motoring commentators too, with ‘Grand Tour’ presenter James May praising its potential.
“What hydrogen is in effect is a very useful form of battery,” May told journalists last year. “You get so much out of a tank of hydrogen compared with a battery that it is very appealing.”
“Hydrogen is available and it’s being used. I’ve done over 40,000 miles. This actually works,” an enthused Ellis added.
“I’ve driven so many different cars over the past thirty years and this is one of the most practical cars I’ve come across.
“What I like most about it is the stability and assurance it gives you as you drive it. Regardless of weather or type of road, it copes really well.
“The aerodynamics are perfect. You can feel it when you drive it.”
On a recent drive through central London, the Mirai glided through the hustle and bustle of city traffic almost silently.
Its interior cabin defended from exterior noise by insulation, with the absence of the vibrations and sounds of a piston engine immediately apparent.
Ellis, who uses the car both at home and for his work at private hire firm greentomatocars, has clocked up 40,000 miles - which Toyota thinks could make him one of the world’s most experienced hydrogen drivers.
The car’s design is striking, with its air intake outlets designed to dominate the front end of the model, boxed out for impact. There’s an ever so slight Batmobile look about them.
The 47-year-old added: “People take notice of the car, they stop and take pictures.
“The other day a cyclist flagged me down on the Euston Road to ask if it was the real thing.”
Some questions remain, however.
The cost of filling up is somewhat difficult to gauge. Ellis’s lease deal means that his costs are paid by Toyota.
Previous estimates from hydrogen suppliers have shown the cost of a full tank to be around £50 - with suggestions this could fall further as demand increases.
And safety is often never far from mind when the word hydrogen comes up in conversation.
The explosive potential of hydrogen was proved in spectacular fashion with the doomed-Nazi craft Hindenburg, which crashed to the ground in a fireball in front of assembled media in 1937.
The disaster claimed 36 lives and inspired a blockbuster Hollywood film, linking hydrogen and fatal fire in the minds of millions.
Toyota went back to its historic weaving technology for inspiration in protecting its car’s hydrogen fuel tanks. Tight weaves act as a barrier to insulate the tanks from damage.
And to prove the point, the containers have been dropped from great heights and shot at point-blank range by researchers.
“I find it safer than petrol or diesel - this is air not a solid state material,” Ellis said when asked if he had any misgivings. “If something happens it just goes into the atmosphere, it doesn’t stay around like oil.”
The issue means hydrogen car drivers need to pay for bespoke insurance - at a premium.
And the energy required transfer the hydrogen gas into the fuel cell technology would need to be from renewable sources to truly help solve the air pollution problem.
As of now, experts see hydrogen as having a part to play in the UK’s efforts to reduce to zero its carbon emissions.
And while the Mirai makes a practical private hire car, it might not yet suit the needs of the average family outside London.
Nonetheless, the fuel is already being used for public transport in Japan, and a whole fleet of cars at the Tokyo 2020 Olympics will run on the gas.
“It does feel like I’m doing something completely new,” Ellis said. “That’s exciting.”
Hydrogen cars in numbers
1.6 million - the number of hydrogen cars on Britain’s roads by 2030, according to the Centre for Future Studies
£66,000 - approximate cost of a Toyota Mirai
£50 - estimated cost of a full tank of hydrogen
3 - minutes taken to fill up the Mirai at a specialist filling station