## 2 Hydrogen as a Transportation Fuel  
Hydrogen is the simplest form of all molecules; it has the lowest energy content by volume, but it has the highest energy content of any fuel by weight. It is available in the atmosphere as gas and in water as liquid. Due to the high energy content of hydrogen, it is employed as a fuel in applications such as FCs and rockets. Hydrogen creates zero harmful emissions, which is one of the most significant drawbacks of fossil fuels, and the heating value of hydrogen is three times higher than that of petroleum. There is a hefty production cost involved with hydrogen, since it is a manmade fuel, costing about three times more than petroleum refining [14]. Substantial amounts of research are dedicated to creating an efficient and sustainable way to produce hydrogen and applications for hydrogen in transportation engines. Automobile manufacturers such as Honda, Toyota, and Hyundai have started to manufacture fuel cell vehicles (FCVs) with hydrogen as fuel. These FCVs are currently available in North America, Asia, and Europe, and have primarily been bought by early adopters. The current consumers--early adopters--are primarily highly educated people, high income families, those with larger household units, those willing to change their lifestyle, and those with other similar attributes [15]. As of June 2018, there have been over 6500 FCVs sold to consumers. California is the leading market for FCVs, with nearly 3000 FCVs being delivered there out of the 5233 vehicles sold globally due to the state housing the largest network of hydrogen refueling stations in the world and automakers selling the vehicles there. Presently, several automakers are promoting FCVs to consumers, which are often compared to BEVs. Both BEVs and FCVs offer zero tailpipe emissions, the ability to be fueled using renewable and sustainable energy sources, and use electric motors. The most notable differences between FCVs and BEVs are the driving range and the refueling style. FCVs have above a 300-mile driving range and can be refueled in less than 10 min at a hydrogen refueling station, which is more comparable to a traditional ICE fossil fuel powered vehicle. Hydrogen has greater potential for use as a fuel in the future. It is estimated that, by the year 2030, the cost of fuel cells will be competitive with ICEs based on the technological improvements being made and the increased availability [14]. One of the main hurdles that mass hydrogen use faces is more efficient storage. Because of the low density of hydrogen, it cannot be stored as easily as traditional fossil fuels. Hydrogen requires compression, cooling, or a combination of them. The most favorable method of hydrogen storage is physical containment, specifically in compressed tanks, because they are readily available. All composites (Type IV) are primarily used, and sometimes metal lined composites (Type III) are used. The fill time of these tanks is competitive with fossil fuels when the hydrogen is pre-cooled. Cost is the main setback for the wide scale use of compressed hydrogen (CH\({}_{2}\)) tanks, because the material and the assembly are expensive. Another potential setback is the public's concern of using such high pressure (70 MPa) storage tanks in vehicles [16]. An alternative to traditional CH\({}_{2}\) tanks that is still being researched is a tank with an internal skeleton, which is a complex design of struts in tension within the tank to resist the forces of the compressed gas. Liquid hydrogen (LH\({}_{2}\)) storage has improved significantly, achieving the best specific mass (15%) of any other automotive hydrogen storage system [17]. Energy efficiency is decreased when liquid hydrogen is used. Boil off is an area that needs improvement before LH\({}_{2}\) systems can be widely accepted. A promising alternative design is a cryo-compressed tank in which hydrogen is highly compressed at cryogenic temperatures. More studies must be done on this method to determine long term durability and public acceptance of the system.