This graph shows system energy densities. By “system”, we are taking into account typical efficiencies of the systems that these chemicals are used in. Note that the closer a system reaction is to the upper right hand corner of the graph, the better the overall energy density. A direct methanol fuel cell (DMFC) can exhibit higher efficiency and comparable energy density, but problems like cross over tend to kill its ability to be a legitimate reactant. An ICE typically has poor efficiency (around 35%), and is mathematically limited to around 48%. This cannot compare with fuel cells that operate typically in the 60% to 90% range. Please note that a hydrogen fuel cell has excellent gravimetric energy density, but volumetrically is poor; it must be stored in high pressure tanks that are unsuitable in a vehicle.