Since children can generate hydrogen for science fairs using nine-volt batteries, it should be easy for adults to generate it to fill their fuel cell cars at home, right? Not so fast. For very rich people with a taste for needlessly convoluted solutions it might make sense, but not for anyone else.
Let’s break this down.
Can you generate hydrogen at home? Yes, it’s possible to generate hydrogen in a science fair kind of way by electrolysing water. A liter of water will get you about 111 grams of hydrogen if you can capture it all. You would probably need one of these industrial electrolysis units to actually get pure enough hydrogen for your car. That’s one parking space in your garage gone.
A kilogram of hydrogen is the fuel cell car equivalent to a gallon of gas. The Mirai holds five kilograms. Getting enough hydrogen would require electrolyzing 45 liters or about 12 gallons of water to get enough hydrogen to fill the tank. That’s very reasonable. It would take about 167 KWH of electricity for the basic electrolysis so it would cost about $20 USD at 12 cents per KWH (the US average).
So far so good.
The volume is a problem though. Those five kilograms of hydrogen as a room temperature gas would have a volume of 6,175 liters. That’s about 6 cubic meters or about 212 cubic feet. You would need a big honking hydrogen storage tank attached to your science fair project. There goes another parking spot.
Then you would need to get it into your car. That requires both compressing it and cooling it. That’s an entire process in and of itself with its own set of machinery and automated controls.
Filling a Toyota Mirai tank to get full range requires H70 compression which is 700 bar or 70 MPa. A bar is a unit of pressure that equates to air at sea level, so you are looking at 700 atmospheres of pressure, which is quite a bit above most home compressors. They tend to tap out around 14 atmospheres.
There’s there’s the small pair of problems of hydrogen molecules both being incredibly tiny and highly flammable. The first part means that you have to manufacture all of this equipment to incredibly tight tolerances. Home compressors need not apply because they work with that incredibly thick substance we call air. The second means that you need to have negative pressure back ups and exhausts to the outside built into the system as well or there’s a good chance of having a fair amount of flammable gas in your garage.
A Toyota Mirai needs to know exactly what pressure and temperature of hydrogen it is getting so it can safely receive the right amount of hydrogen. This requires computer interlocks, otherwise you would have troubles up to and including blowing a gasket on 700 atmospheres of pressure and filling your garage with flammable hydrogen. You’d end up with something that looks like this. There goes a third parking spot in your garage.
The pumping, cooling and computer chunks of all of this are why H70 hydrogen fueling stations cost a minimum of $500,000 USD. And they don’t make the hydrogen, they get it delivered.
So could you generate hydrogen at home to fill your car?
Sure. If you have about a million US to spend on it, a three-car garage just for the hydrogen processing facilities and it’s far enough way from the house and neighbors that the sound of pumps capable of creating 700 atmospheres of pressure doesn’t cause problems. If you are rich enough to consider this, you can probably afford a few acres. Maybe you don’t even mind walking a couple of minutes through your grounds to get to your isolated garage.
Or you could get a just get a battery electric car and spend a couple of hundred bucks to have an outlet installed for it to plug into. And since the biggest electric car battery available is 100 KWH with about the same range as the Mirai (and much better performance), it would only cost about $12 to fill it up.