How a hydrogen fuel cell makes its own electricity instead of storing it

How a hydrogen fuel cell works

The plain answer to how a hydrogen fuel cell works starts with what a fuel cell is, a device that runs a chemical reaction and keeps making electricity for as long as hydrogen and oxygen keep arriving at it, the United States Department of Energy states. Hydrogen gas flows to 1 electrode, called the anode, where a catalyst, a material that speeds up a reaction without being consumed, splits each hydrogen molecule into protons and electrons. A thin sheet called a membrane lets the protons pass through but blocks the electrons, so the electrons are forced out through a wire, and that flow of electrons is the electricity a fuel cell car uses to turn its motor, the Department states. On the far side of the membrane, the protons, the electrons and oxygen from the air combine into water and heat. The Department states plainly that fuel cells work like batteries, but they do not run down or need recharging, and that they keep producing electricity and heat as long as fuel is supplied. Toyota describes this in the release for its 2014 Mirai, saying the car generates its own electricity from hydrogen.

What that means for a driver, refueling time and range

The Department of Energy states that fuel cell cars refuel in a few minutes and can drive more than 300 miles on 1 tank, giving them a fueling experience close to a gasoline car rather than a battery car waiting to recharge. Toyota states its 2014 Mirai took about 3 minutes to refuel and could then drive 650 kilometers on the Japanese JC08 test cycle, a dated example in 1 real car, though it used an older Japanese standard and does not match a current United States range rating. A fuel cell car also carries a small battery of its own, but that battery only recaptures energy from braking and smooths out short bursts of power, the Department of Energy states. It is the size of the hydrogen tank, not the battery, that decides how far the car can go, the opposite of an all electric car, where the battery size sets both.

Why a fuel cell does not run down like a battery

Reading the Department fuel cell fact sheet directly turns up a detail easy to miss. It notes that spare electricity from wind and solar farms can be turned into hydrogen and stored for later use in a fuel cell. That step, turning electricity into hydrogen, happens before hydrogen ever reaches the fuel cell, and it is not something the fuel cell itself does. A fuel cell only runs 1 direction, hydrogen and oxygen combine into water and electricity for as long as fuel keeps arriving, and it never reverses to take on a charge, which is exactly what a battery does and a fuel cell does not.

The environmental case depends on how the hydrogen was made

Efficiency alone does not decide how clean a hydrogen car is. The Department of Energy states that a fuel cell car produces about 50% less carbon dioxide than a gasoline car when its hydrogen was made from natural gas, and about 90% less when the hydrogen was made from renewable electricity instead. That difference is decided before hydrogen ever reaches the car, since the fuel cell inside simply runs on whatever hydrogen it is given.