Hydrogen.

From wikipedia, where I do all of my 'original' 'research'.

Hydrogen costs...

Natural Gas at Small Scale
Uses steam reformation. Requires 15.9 million cubic feet (450,000 m³) of gas, which, if produced by small 500 kg/day reformers at the point of dispensing (i.e., the filling station), would equate to 777,000 reformers costing $1 trillion dollars and producing 150 million tons of hydrogen gas annually. Obviates the need for distribution infrastructure dedicated to hydrogen. $3.00 per GGE (Gallons of Gasoline Equivalent)

Nuclear
Provides energy for electrolysis of water. Would require 240,000 tons of unenriched uranium — that's 2,000 600-megawatt power plants, which would cost $840 billion, or about $2.50 per GGE.[25]

Solar
Provides energy for electrolysis of water. Would require 2,500 kWh of sun per square meter, 113 million 40-kilowatt systems, which would cost $22 trillion, or about $9.50 per GGE.

Wind
Provides energy for electrolysis of water. At 7 meters per second average wind speed, it would require 1 million 2-MW wind turbines, which would cost $3 trillion dollars, or about $3.00 per GGE.

Biomass
Gasification plants would produce gas with steam reformation. 1.5 billion tons of dry biomass, 3,300 plants which would require 113.4 million acres (460,000 km²) of farm to produce the biomass. $565 billion dollars in cast, or about $1.90 per GGE

Coal
FutureGen plants use coal gasification then steam reformation. Requires 1 billion tons of coal or about 1,000 275-megawatt plants with a cost of about $500 billion, or about $1 per GGE.