Skip to content

Hydrogen

Energy and power · level 7, lesson 3 of 6

A question to start

Mars has ice, and it has sunlight. Could a base use spare sunlight to turn water into hydrogen, and get the energy back weeks later?

The idea

Electricity can split water into hydrogen and oxygen. This is electrolysis:

\[2\text{H}_2\text{O} \rightarrow 2\text{H}_2 + \text{O}_2.\]

The two gases go into tanks. Later, a fuel cell joins them back into water, and the chemical energy comes out as electricity and heat. Nothing is burned, and the only product is water.

electrolyserhydrogenoxygen fuel cellwater goes roundelectricity inelectricity out

Hydrogen holds a lot of energy for its mass: about 142 MJ per kg, or 39 kWh. But:

  • Losses. An electrolyser keeps about 70% of the energy put in; a fuel cell turns about 50 to 60% of the hydrogen's energy back into electricity. The round trip is only about 35 to 40%, against about 90% for a battery.
  • Oxygen. Water is 16 parts oxygen to 2 parts hydrogen by mass, so each kilogram of hydrogen needs 8 kg of oxygen.
The fuel cell's oxygen Mars Earth
Oxygen in the air 0.16% 20.95%
Where a fuel cell gets it Stored in tanks with the hydrogen Taken from the air

Its strength is time. A tank of gas can sit for months, and a bigger store needs only bigger tanks, not more cells. Oxygen can be made on Mars from its carbon dioxide air too: Perseverance's MOXIE experiment made 122 g of oxygen in all, at up to 12 g an hour.

Worked examples

1. Energy in a kilogram. \(142 \times 10^6\) J ÷ \(3.6\times10^6\) J/kWh \(= 39\) kWh.

2. Storing a night. A base needs 123 kWh of electricity. With a 55% fuel cell, the hydrogen must hold \(123/0.55 = 224\) kWh: \(224/39 = 5.7\) kg of hydrogen and \(8 \times 5.7 = 46\) kg of oxygen, split from 52 kg of water.

3. The round trip. \(0.70 \times 0.55 = 0.385\), about 39%. To get 123 kWh back, the panels must put in \(123/0.385 \approx 320\) kWh, more than twice what a battery needs.

Try it

  1. Write the word equation for what a fuel cell does.
  2. A fuel cell uses 2 kg of hydrogen. What mass of water does it make?
  3. An electrolyser is 70% efficient and a fuel cell 50%. What is the round-trip efficiency?
  4. Why might a Mars base still pick hydrogen to get through a storm lasting months?
Answers
  1. Hydrogen + oxygen → water (plus electricity and heat).
  2. 2 kg of hydrogen joins 16 kg of oxygen: 18 kg of water.
  3. \(0.7 \times 0.5 = 35\%\).
  4. It keeps for months, a bigger store needs only bigger tanks, and the same gases give water, oxygen to breathe and rocket fuel.

Common mistake

"Hydrogen is a source of energy." There is almost no free hydrogen to dig up, on Earth or on Mars. Making it takes more energy than it gives back. It is a store, like a battery.

Where it leads