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)

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 -> H 2 (g) + ZnSO 4 –Electrolysis 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 -> 4 H 2 (g) + CO 2 (g) –CO 2 = Greenhouse gas

How can we store it? If we succeed in producing H 2 cheaply, we are still faced with the questions: –How do we store it? –How do we transport it 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 onto activated carbon Use of Li metal. –Li is highly reactive metal –Reaction of H 2 gas can reduce the volume of 12 liter to about 4-5 ml. 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!! Proton Exchange Membrane (PEM) Conductive Wire H 2 gas ½ O 2 oxygen gas

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

e-e- As the hydrogen travels It is light and will travel upwards e-e- e-e- The proton moves through the PEM and leaves an electron behind Proton can be designated H +

Fuel cell cont’d Proton Exchange Membrane (PEM) ½ O 2 oxygen gasH2H2 2e - Electrons moving through a wire = ELECTRICITY Proton or H +

Reactions occuring Proton Exchange Membrane (PEM) ½ O 2 oxygen gasH2H2 2e - Electrons moving through a wire = ELECTRICITY Proton or H + H 2 -> 2H + + 2e - 2H + + 2e - + ½ O 2 (g) ->H 2 O

Reactions of Fuel cells 2H + + 2e - + ½ O 2 (g) ->H 2 O H 2 -> 2H + + 2e - + 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 Sum the reactions Consequences?

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