Let's analyse how Aluminium would work out as hydrogen for transport:
AssumptionsDensity of Gasoline = 737.22 kg.m
^{3} = 2.7907 kg/gallon
Efficiency of Internal Combustion Engine (ICE) = 35%
Efficiency of H
_{2} Fuel Cell (FC) = 60%
 Substance   Energy Density 
1.  Hydrogen Gas   120 MJ/kg 
2.  Ethanol   23.4 MJ/kg 
3.  Gasoline   45 MJ/kg 
4.  Diesel   45 MJ/kg 


 Vehicle Type   Petrol Mileage   ICE Consumption   FC Consumption 
1.  Passenger Car   22.4 Miles/Gallon   2.769 kJ/Mile   1.615 kJ/Mile 
2.  Vans, Pickups, SUVs   16.2 Miles/Gallon   3.828 kJ/Mile   2.233 kJ/Mile 
3.  Trucks   06.7 Miles/Gallon   9.259 kJ/Mile   5.402 kJ/Mile 
4.  All Vehicles   17.1 Miles/Gallon   3.627 kJ/Mile   2.116 kJ/Mile 


 On a basis of 1000 miles 
 Vehicle Type   H_{2} Required   H_{2}O Required   Al Required   Gasoline Required 
1.  Passenger Car   0.0135 kg   0.1215 kg   0.1214 kg   124.6 kg 
2.  Vans, Pickups, SUVs   0.0186 kg   0.1674 kg   0.1673 kg   172.3 kg 
3.  Trucks   0.0450 kg   0.4050 kg   0.4047 kg   416.5 kg 
4.  All Vehicles   0.0176 kg   0.1584 kg   0.1583 kg   163.2 kg 
There is storage space available for 158g of water and 158g of Aluminium. Their total space storage would be so much less than the space required for 163.2 kg of gasoline (58 gallons).
Based on the price peak (US$3000 / 25 tons) in the above chart, the price of aluminium for topping up an average vehicle is $0.02. However, if one to take in account of the effect of supply and demand of Aluminium for transport, I would not be surprised that Aluminium price would grow to at least 5070X its current price. Taking in account of logistics, tax and profit, one might expect the the price increment of $100200/kg. A more realistic price for Aluminium at "petrol" kioks would be $24 per 1000 miles, compared to $1.50/gallon for petrol.
But there are definitely challenges in implementing this technology.
1. The availability of Gallium to store Aluminium in the form of a liquid alloy
2. Will the liquid alloy freeze in winter?
3. Controlling hydrogen production rate at the microgram level in a car
4. Separating Al
_{2}O
_{3} from the Gallium alloy
5. Can Aluminium be regenerated via electrolysis in liquid Gallium?
Reference:
1. Energy Density Data:
http://hypertextbook.com/facts/2003/2. Transport Consumption (USA, 2004):
http://www.eia.doe.gov/emeu/aer/txt/ptb0208.html