Fuel

Details

Pros

Cons

Methanol

CH₃OH

Lots of interest into methanol, hydrogen is extracted using the steam reforming process:

CH₃OH + H₂O → CO₂ + 3H₂

Reformers have undergone significant advances in recent years. See section on automobiles.

• Relatively safe
• Storage isn’t too difficult but must ensure that it can’t leak into water.

• Highly flammable (although less than petrol)
• Toxic (highly soluble in water and tasteless hence quite dangerous) but NOT an accumulative toxin – small amounts can be broken down by the body
  

Alkali metal hydrides

such as NaH or CaH2

MH + H₂O → MOH + H₂

is a very simple way to produce hydrogen. “Powerballs” have been marketed, 3 cm spheres of NaH coated with polythene and stored underwater. They’re cut in half as required producing hydrogen.

Some applications in some situations perhaps, but not very practical.

• Decent energy density and specific energy
• Simple.

• Need to dispose of lots of hydroxide solution, and also carry it around in a mobile application.
• Requires more energy to manufacture the hydride than is released by the fuel.

Ammonia – NH₃

Hydrogen is recovered by a catalysed dissociation reaction:

NH₃ → ½ N₂ + 3/2 H₂

which takes place at between 600 and 800 °C. It’s an endothermic reaction,
ΔH = +46.4 kJ mol^-1

• Liquefies at -33 °C
• Very high energy density

• Toxic from 100 ppm (and smelly)
• Requires a lot of energy (and hydrogen) to produce ammonia via the Harber process.
• Product gas is difficult to use in cells other than AFCs due to remnants of ammonium hydroxide

Sodium borohydride solution

NaBH₄ – 30% solution is typical. (dry NaBH₄ ignites spontaneously in air)

The reaction is as follows:

NaBH₄ + 2H₂O → NaBO₂ + 4H₂

It’s an exothermic reaction. The NaBH₄ solution contains everything necessary for the reaction; it’s brought into contact with a catalyst to initiate the dissociation.

This is a very promising fuel and is used in several military applications. Currently the price is prohibitive.

• Hydrogen gas is the only product gas produced, no CO₂ diluting it.
• The Hydrogen is produced with water vapour which is very useful for PEMFCs

• NaBH₄ is very expensive to produce
• The tank can’t really be refilled; the NaBO₂ solution must be emptied and replaced with fresh borohydride.

Liquids

Formula

Percent Hydrogen

Density
(kg L^-1)

Vol to store 1 kg of H₂
(L)

Other details

LH₂

H₂

100

0.07

14

Stored at 22 K

Ammonia

NH₃

17.76

0.67

8.5

Toxic at 100 ppm

Liquid Methane

CH₄

25.1

0.415

9.6

Liquid at 98K

Methanol

CH₃OH

12.5

0.79

10

Ethanol

CH₃CH₂OH

13.0

0.79

9.7

Hydrazine

N₂H₂

12.58

1.011

7.8

Very toxic

30% Sodium-borohydride solution

NaBH₄ + H₂O

6.3

1.06

15

Expensive but very effective