Basic chemical reactions involving metals are out (batteries), cause we've hit the limits of electronegativity with lithium. Mechanical storage (springs, flywheels) have never been that great of an idea because of structural limits, and natural occurring chemicals have reached their limits of energy density and have pollution issues (such as the release of carbon to form CO2).
So what's left? Nuclear power, fusion, electric fields and artificial molecules. I don't see society accepting nuclear powered vehicles any time soon (though it works for submarines, destroyers and aircraft carriers; perhaps commercial shipping will convert as well if they can learn not to leave drunk 3rd mates in charge of steering through tight passages); and fusion research is still indicating that a lot of energy must go in to get any kind of return; so how about electric fields and artificial molecules?
* Artificial Molecules
So if we're thinking about making fuel, why not make something better than hydrogen gas. Carbon isn't the greatest since it produces CO2, so lets look for another core element to use. Hmmm, a lot of the other semi-metals are kind of noxious: Phosphorus, Sulfur, Gallium, Arsenic, Selenium, ..., Antimony, Lead. But there are two that aren't very heavy, and are fairly common: Aluminum and Silicon. Now Aluminum is a metal and probably not as common as sand (Si), so lets pick Si for starters.
So the first strategy would be to create long chains of it, like you do with carbon. Maybe: Si12H26
If you can't make chains of pure silicon, maybe we'll mix some carbon back into the mix, like: C7Si8H32
Playing with chemicals like this is pretty standard stuff. Its looking for fuels specifically that is driving this research. A lot of work is already being done to find a binder for hydrogen (like the DOE's proposed NaBH4). And then there's the rocket team at scaled.com that decided on a synthetic rubber (HTPB) as the fuel in their hybrid rocket motor (though more for reasons of availability and safety than energy density).
This is probably the least SciFi of them all. In the last five to ten years, a new product line has popped into existence: super-caps. These have expanded the storage of electrons by orders of magnitudes (in terms of energy density). You have super-caps that are starting to rival lead acid batteries in terms of energy storage, and don't have any of the drawbacks of batteries: low discharge rates, fatigue, charging side effects, toxic chemicals, etc. Because of their high current cost, they're not replacing batteries in any wide spread way yet, but in some applications their being put in along side batteries for their benefits of fast current drain, and no recharging memory.
The coolest part is that current products are in no way up against any laws of physics, only limits of current manufacturing. The only theoretical limits for capacitors are the size of the electron, the size of the plate metal atoms, and creating enough of a gap. So in theory, with 100V charge in a 1 MF capacitor, you'd have 1.4 MWh's which could be compressed down to a device the size of a coffee cup. In comparison, a tank of gas is around 100kWh. (Thanks to Nils for opening his physics text book at correcting my original numbers.)
1) Its not an energy source (you have to expend energy to get it)
2) Its not very dense
3) Its dangerous.
4) It doesn't matter.
1. Its not an energy source.
Hydrogen doesn't exist around here at ground level by itself. It has to be made. One way to make it is to refine petroleum products, but we're trying to get away from that, so the next best suggestion is electrolyzing water (H2O). Now H2O is the end result after extracting the energy from H2 in a fuel cell or an engine, so really you're just putting the energy in up front so you can get it back later. And that process is never 100% efficient.
Its like Aluminum batteries. Al2O3 exists naturally and the industrial world spends a lot of energy ripping the oxygen off the metal so we can have soda cans and light folding chairs. (As a side note, when people talking about using "spare" capacity from the grid, aluminum refining is actually using most of this extra capacity already.) You can get a lot of energy back out of aluminum by letting it oxidize again in either electric form (batteries), or as heat(think thermite); but you're just cashing out what you put into it before.
There are also some strange nuclear solutions involving thermochemical reactions with water, Iodine and Sulfur; but currently this country seems to be allergic to nuclear power despite some great strides in safety of design and operation (like pebble bed helium reactors).
2. Its not very dense.
All gases have the same number of molecules in a given amount of space. The difference in weights is a matter of how many neutrons and protons a given gas has. H2 is the simplest, and thus lightest gas of all. It also ends up wasting a lot of space for the amount of energy it contains compared to anything else.
In a DOE report, they rated the various fuels in energy density compared with diesel. It went something like this:
Compressed Hydrogen (at 3600 PSI): 6%
NiMH battery: 1.3%
Strangely they didn't list lithium batteries, probably would make compressed hydrogen look even worse.
Basically, their conclusion was that compressed hydrogen would never be practical for cars, and was even a worse idea for things like trucks. (Basically, for a truck, you were replacing an 84 gallon fuel tank, with a giant heavy steel pressure tank four feet in diameter and over twelve feet long. The battery idea wasn't much better: you could get half the needed range with 42,635 pounds of battery, which represented 85% of the weight load of the truck.)
3. Its dangerous.
Dangerous how? The ignition velocity for hydrogen is ten time the velocity for natural gas (and five times that of propane), which means for the same amount of energy, an explosion does ten times the damage.
And because of its density, they're talking about storing it at pressures above 5,000 PSI. A typical welding tank is about a quarter inch thick high strength steel, runs from 1,600-2,400 PSI, and is safety tested for 4,000 PSI; and that amount of pressure is quite dangerous in the event of a rupture. A car with 4 x 5,000 PSI tanks with a rupture would essentially be a car-bomb.
Finally, if burned in an engine, it will actually be a worse contributor towards pollutants such as NOx, because of its high temperature of combustion. Hydrogen burners usually have to use an expensive catalyst in the burner assembly to keep the temperature down. This impacts efficiency as well.
4. It doesn't matter.
The Hydrogen solution is touted for "cars" using a combination of hydrogen fuel, and greater fuel efficiency (since there's not as much power in hydrogen). But lets take a look at the Oil market:
Nasa tried running airplanes on liquid hydrogen, but to make it practical, the equipment size and weight needs to scale down by a factor of over 100. Current research is focused on the factor of five that might make small personal prop planes work (like a Cessna), but even that is facing its own hurdles.
So why tilt at the 5-10% of fuel use going into cars? Its an uphill battle, with no economic advantage. Fuel shortages are coming, and with the corresponding rising prices will come changes in behavior and new solutions.
So finding the green one was a very happy occasion. Of course it had been left on, and the battery was totally dead, and had spilt acid all over the inside. But luckily, the case is machined aluminum, and thus corrosion proof, so it just needs a little cleaning. For the time being, I borrowed another body from a white LED light and am back in business (though I had to wrestle Zak for it).
Sometimes it's the little things in life that bring you joy.
Infinity AA LED light, about $20 retail ($13-16 street), several colors. They also have a new Sonic AAA which I haven't tried.
Arc AAA White LED light, about $25 retail. Used to come in about nine different colors (I originally bought an orange, aqua, and some whites), now only available in white and blue.
The ARC AAA and CMG Infinity.
LED converter for your mini-mag, a three white LED replacement module that fits in a mini-mag(tm) light. About $22.
The LED Museum, more than you'll ever want to know about LEDs and LED flashlights. Featuring the Punishment Zone.
This drives my wife crazy, as she has to clean them up at some point; but I think its great. I live vicariously through my kids.
On the other hand, once in a while someone turns conventional wisdom on its head and does the exact opposite, like in Beauty and the Beast where they take the handsome strong white guy, Gaston, and make him the bad guy. He was a great villain and they got to make fun of being privileged.
Disclaimer: I know people who once worked at Disney Feature Animation.