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Wherein I tackle the idea of hydrogen fuel cell technology in private vehicles. Buckle up. For the past few years, Hydrogen Fuelcell technology has been making the news in regards to personal transportation. Regardless of how long fuel cell technology has been used in the space industry, the media treated it like a futuristic Godsend for personal vehicles. I recall there being plans drawn up for refueling stations in California, even. But the Bear State is fond of making whatever promise they can to skim as much money from its people. See the High Speed rail fiasco. http://www.bloomberg.com/view/articles/2016-06-28/california-hits-the-brakes-on-high-speed-rail-fiasco When I first heard of hydrogen fuel cells being used in vehicles, I was too young and immature to have much of an opinion. Back then, I was still cruising around downtown and hanging out with Sturgeon to care about motor vehicle trends. Those nights were spent talking trash about HK products, doing blow through rolled up Benjamins, and dozens of questionably legal Polynesian women. I'll be approaching this issue regards to efficiency, safety, production, and storage of many elements. We'll first discuss what hydrogen is. https://en.wikipedia.org/wiki/Hydrogen The Wiki gives a ton of great information. Most of you will know the basics. It's a gas at STP, contains a single valence electron in it's 1s shell (“shell” or cloud of probability derived by shrodinger's blah blah blah Physical Chemistry nonsense, don't make me do that derivation again). It was first artificially made by a guy named Cavendish, and is found naturally as a diatomic molecule. This diatomic molecule also really likes to explode if it gets near an energy source. H2 combustion is well documented, and releases 286 kJ/Mol. In fact, it can undergo combustion at as low as 4% concentration with air. That's low. Hydrogen's low molecular weight makes it the lightest gas around. This was capitalized during the second to last turn of the century, where mighty Zeppelins pushed through the sky like herds of giant sky manatees. http://smhttp.41037.nexcesscdn.net/80153AD/magento/media/catalog/product/cache/1/thumbnail/750x/17f82f742ffe127f42dca9de82fb58b1/m/i/misc156_2.jpg Large enough to fit Colli-man's collection of miss-matched socks Germany loved these guys, and for a while they were all the rage in luxurious travel. Indeed, it was certainly the 20th century now! We had Airships, the Haber Process that was fueling an industrial revolution, and all of physics was completely solved! Thanks, Maxwell! (And then came the ultraviolet catastrophe, but that's another topic.) However, I mentioned above that hydrogen is extremely flammable at even very low concentrations within air. This fact really sunk the airship industry with a certain spectacular disaster. http://www.hipstersofthecoast.com/wp-content/uploads/2013/06/hindinburg-crashing-burning-640x420.jpg German Engineering, or Masonic Zion plot? It's easy to skip over this picture entirely. We've all seen it so many times (Unless you're one of my tutoring students, who look at me like I've got two heads when I mention it. “The hinda-what?”). But, this was the end of an era. Static charges ignited the hydrogen sacks that kept the big rigid frame afloat. And though we could have used Helium, a much more stable gas, the damage was done. No one would step foot near a rigid airship again. (Also our world's supply of Helium is finite and diminishing very very quickly. It would be wasted in airships. But again, another topic another time) Let's get back to the Hydrogen Fuelcell. What exactly is it, and how does it work? The basic model is shown below. https://upload.wikimedia.org/wikipedia/commons/thumb/6/63/Proton_Exchange_Fuel_Cell_Diagram.svg/2000px-Proton_Exchange_Fuel_Cell_Diagram.svg.png This diagram is for a Proton Exchange Fuel Cell. The proton here is simply a hydrogen that's been stripped of its single electron. A fuel cell works by having very special membranes carefully constructed to permit the passage of a positively charged ion, but not the negatively charged electron. This travels through another path, leading to a voltage across the cell. This voltage can be used to power any electrical device. This is an oversimplification of how the device works, but it's a start. The benefits of such a device include the shear efficiency that it can have. When properly insulated and owing to proper low-resistance connections, these devices are pushing out efficiencies twice that of internal combustion engines. Which, despite what many places attempt to sell you, are actually quite thermodynamically efficient. These proton based fuel cells have great cold-start characteristics and energy density. Their outputs can actually be very high. Indeed, these fuel cells are efficient at all power outputs as well. Their efficiency does not vary with flow of fuel source either. Their temperatures can be as low as 80 degrees C. However, usually they are kept above 100 degrees C because steam is far more manageable than liquid water byproduct. So with all of this information, you're probably wondering why haven't we started putting these into all sorts of places. This post is about personal vehicles, however, and I'll get right back to that. No. I disagree completely with them being used in personal vehicles. While I love fuel cells as a power device, their use in personal vehicles is greatly limited. One of the biggest engineering hurdles is the flammability and storage of pure hydrogen. Since hydrogen has such a low molecular weight, to obtain a large enough amount to power a personal vehicle would require a very high pressure container. If you remember back to your Chemistry classes in high school, you may remember the Ideal Gas Equation. Hydrogen is pretty close to an Ideal Gas. As close as you'll get, really. The Ideal Gas Law, in actual use, is only about 84% accurate when used to guess thermodynamic systems. For hydrogen it's much higher. PV=nRT, where n is the number of moles. Keeping everything but Pressure and number of moles the same, to increase the number of moles directly increases the pressure. And H2, having a molecular weight of 2 Grams per Mole, would require a ton of moles to get a decent amount of the gas. A very high pressure container of pure hydrogen gas in a vehicle that routinely travels at 70 mph. Which is statistically guaranteed to be in an accident in its lifespan. The Germans are watching this and going “Nein Nein Nein!” http://lmgtfy.com/?q=how+many+car+crashes+per+day+in+the+US According to this nifty search, over 3,000 people die per day in the US due to vehicular collisions. Ouch. However, this issue is the first to be solved. The introduction of Metal Hydrides have solved the storage issues. Metal Hydrides act as chemical sponges for Hydrogen gas (H2), binding the molecules inside their chemical structure. These metal hydrides are usually used as powders, where the hydrogen is then pushed through to store. To release the hydrogen, the metal hydride must be heated. The rate of diffusion is directly related to the temperature at which the metal hydride is heated, and thus the fuel rate into the fuel cell can be varied by varying the temperature of the metal hydride. Metal hydrides can absorb 2 to 10% H2 usually, but better compounds are being produced to increase the number. https://en.wikipedia.org/wiki/Metal_hydride_fuel_cell This is good, because it gives us a safe way to store hydrogen gas for fuel cells. This is bad, because the fuel delivery rate is much lower, and metal hydride fuel cells are, at their very best, 1/4th as powerful as their PEM brothers. At worst, they are 1/50th. But this is the best we can do in a vehicle. No one wants pressurized hydrogen canisters on the highways. Hell, most of the time you need special clearance and big signs to transport the stuff. And imagine the safety concerns for the EMTs and Paramedics during a car crash. Even if the tank isn't ruptured, no EMT or Paramedic would risk their lives until the wreckage was cleared. When I was going through my EMT training, they made it very clear that it doesn't matter if people are bleeding out in front of you. If you go in while it's still dangerous, you're only being a liability to your fellow EMTs, Firefighters, and police. But let's ignore the low power outputs of these MH Fuel Cells. What other issues do we have? Well, the fuel cell itself must be created using some very interesting techniques and materials. The biggest expense would be the platinum. Other catalysts are needed as well. As well as a very special proton-permeable membrane. To function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect "short circuit" the fuel cell. The membrane must also not allow either gas to pass to the other side of the cell, a problem known as gas crossover. Finally, the membrane must be resistant to the reducing environment at the cathode as well as the harsh oxidative environment at the anode. This system includes electrodes, electrolyte, catalyst, and a porous gas diffusion layer. The rate of reaction will be dependent also on how quickly the water vapor product can diffuse through the porous material and out of the system. A system can have a lowered efficiency if the fuel cell is too dry or too wet. A balance must be met. And while yes, all of these situations can be worked around, it all comes at a heavy price. Currently we are using 30 grams of platinum in vehicle sized PEM fuel cells. This number will be going down once different catalysts are created, but the cost of these vehicles still pushes up to $50,000. The cost will go down, like any technology. I've yet to speak about where we obtain this hydrogen gas from. The easiest way to obtain hydrogen gas is via the electrolysis of water. H20 + An Electric Current → H2 + O2, essentially (it's not balanced, I know this.) But that electric current must be created as well. This usually comes from the electric grid, which is still, depending on the state, a majority coal-burning. Natural Gas reformation is another way to obtain Hydrogen gas, and is the most common way we currently use. It's the cheapest as well. Synthesis gas, a mixture of hydrogen, carbon monoxide, and a small amount of carbon dioxide, is created by reacting natural gas with high-temperature steam. The carbon monoxide is reacted with water to produce additional hydrogen. The other common ways are via fermentation of biofuel stocks (which is a long process without a great yield) or liquid reforming, which is really unfeasible in large quantities. The only way to obtain large amounts of hydrogen is via natural gas reformation, and that's still technically a fossil fuel source. So why were we going with hydrogen fuel cells again? To rid ourselves of dirty, dirty fossil fuel? Well shit. So to sum this up, the only way to safely use hydrogen as a fuel source in a moving vehicle would be by using metal hydrides, which require energy to access the stored hydrogen. This stored hydrogen flow rate is lower than standard PEMs, and results in a lower voltage, which in turn leads to a lower power output for the vehicle. More research and development must be done to find proper catalysts that can be made at a low cost, and production methods must be worked out to create the membranes more cheaply. All of this is held up by our hydrogen production systems. PEM fuel cell technology is awesome and I love it to death in many many situations. But vehicles isn't one of them. I may read about more advances in the near future that would change my opinion completely, but I would be surprised. Below I've added a problem out of my heat and mass transfer book (Incropera, 7th edition).