Hydrogen as an energy source

Science fiction or not?

• In Jules Vern novel (1874)

Mysterious Island

, a shipwrecked engineer speculates about what will be used as a fuel when the world’s coal supply has been used up.

“Water,” the engineer declares, “I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light”

Questions

• Is this simply science fiction, or is it energetically and economically feasible to break water into its component elements?

• Can hydrogen really serve as a useful fuel? • What does it have to do with Solar energy and/or Nuclear fusion?

To answer the question … lets begin with the basics

• How much energy is produced or released when H 2 is burned?

H 2(g) + ½ O 2 (g) -> H 2 O (g) Can we calculate it?

(Updated see ppt notes)

H C N O I S F Cl Br Single Bonds H C N 432 411 346 386 305 459 358 363 272 565 485 428 327 362 285 295 213 O S F 167 201 142 226 283 190 284 155 313 218 255 249 201 217 249 201 278 Cl Br I 240 216 190 208 175 149 C=C C=C N=N N=N 602 835 418 942 Multiple Bonds C=N N=O O=O C=O 615 607 494 799 C=O 1072 Bond Energies are given in kJ/mole

Hydrogen

• Clearly, hydrogen has the potential of being a powerful energy source!!! • IN FACT, Hydrogen has the highest heat of combustion of ANY known SUBSTANCE. • Used in rockets, space shuttle, small batteries, etc – High energy, low emission = TANTILIZING for fuel in cars and other

Sources of hydrogen

• Where can we get hydrogen?

– Reaction of metal with Acid • On that Day I demoed Sodium Metal and Water – Na(s) + H 2 O(l) -> NaOH + H 2 • More commonly (a metal w/ Sulfuric acid) – Zn(s) + H 2 SO 4 – Electrolysis -> H 2 (g) + ZnSO 4 • Demoed the bubbling that occurs at an electrode surface. Remember the light bulb.

– H 2 O (l) -> H 2 + ½ O 2 (reverse reaction of combustion) » Same amount of energy NEEDS TO BE INPUTED – Discussed feasibility

Sources of hydrogen (Cont’d)

• Hot steam over pure carbon (coke) – Input 131 kJ/mol – H 2 O + C(s) -> H 2 (g) + CO(g) –

CO is carbon MONOXIDE!!

• Hot steam over methane (natural gas) – Input 165 kJ/mol – H 2 O + CH 4 – CO 2 -> 4 H 2 (g) + CO = Greenhouse gas 2 (g)

How can we store it?

• If we succeed in producing H 2 still faced with the questions: – How do we store it?

– How do we transport it cheaply, we are • 1 gram occupies about 12 liters – If stored as gaseous state, large heavy walled containers will be necessary! This eliminates the benefits of H 2 as a fuel (high energy/low mass ratio) • Liquefied at -250 o C – A lot of energy would be required!!

2 methods that I am aware of

• Adsorption of H 2 • Use of Li metal.

onto activated carbon – Li is highly reactive metal – Reaction of H 2 to about 4-5 ml.

gas can reduce the volume of 12 liter Here is the chemistry: Li(s) + ½ H 2 (g) -> LiH(s) Q:

Ok … how do we get it back when we need it

A: Drop Lithium hydride (LiH) into water LiH + H 2 O -> H 2 + LiOH

Produces Hydrogen gas

Fuel Cells

Same reaction without a flame!!

Conductive Wire H 2 gas Proton Exchange Membrane (PEM) ½ O 2 oxygen gas

Hydrogen

• Consider Hydrogen for a minute Hydrogen subatomic make-up 1 proton and 1 electron e 1 proton

The proton moves through the

e -

PEM and leaves an electron behind

e -

As the hydrogen travels It is light and will travel upwards

e 1 proton Proton can be designated H +

H 2 2e -

Fuel cell cont’d

Electrons moving through a wire =

ELECTRICITY

Proton or H + Proton or H + ½ O 2 oxygen gas Proton Exchange Membrane (PEM)

Reactions occuring

2e Electrons moving through a wire =

ELECTRICITY

Proton or H + Proton or H + H 2

H 2 -> 2H + + 2e -

Proton Exchange Membrane (PEM) ½ O 2 oxygen gas

2H + + 2e + ½ O 2 (g) ->H 2 O

Reactions of Fuel cells

Sum the reactions

+ H 2 -> 2H + + 2e 2H + + 2e + ½ O 2 (g) ->H 2 O H 2 + ½ O 2 (g) ->H 2 O

Conclusion: Same bonds broken and formed as in combustion reaction!!!

Reactions are equivalent Thus Energies are equivalent: E = -239 kJ/mol FUEL CELLS – ENERGY WITHOUT A FLAME

Consequences?

Consequences

• No flame = no generation of NO x ’s – Less NO x = Less pollutants • more efficient than combustion (70% vs. 30%)