Hydrogen Fuel as a Clean Alternative to Gasoline in Transportation

Will Clark

Abstract: Carbon emissions from the transportation industry are a serious global concern. While electric cars are a potential solution, current limitations in range and charge time need to be improved. Hydrogen fuel could fix some of the flaws of electric cars. Hydrogen as a fuel source has been considered a replacement for gasoline due to the benign nature of water emissions. This paper evaluates the two different methods of harnessing hydrogen fuel, the chemistry of each method, and the overall viability of hydrogen fuel.

Why hydrogen fuel is being researched:

Gasoline from transportation contributes to roughly 14% of global carbon emissions.12 The electric vehicle (EV) push in the transportation industry to reduced CO2 emissions from internal combustion is widespread, but electric cars are not perfect. A long-range 2021 Tesla Model 3 has an estimated range of 315 miles. Recharge time for the Tesla Model 3 at a supercharging station, which provides 250 kW of maximum draw, is roughly 32 minutes to reach 80% capacity and double that to reach 100% capacity1. These factors make EV’s inconvenient on road trips and difficult to properly implement in the industrial and transportation sectors. Hydrogen fuel cells have been considered as an alternative, due to their shorter refueling times and higher range. There are currently two methods used for Hydrogen fuel.

Hydrogen fuel cells in EV’s:

One method of harnessing hydrogen energy is through hydrogen fuel cells. Hydrogen is pumped out of a tank into a fuel cell, where a platinum catalyst on the anode side separates hydrogen into protons and electrons. This process generates an electrical current while only emitting water and unused oxygen, which is needed to complete the reaction. (see Figure 1)

Figure 1: a basic hydrogen fuel cell generating electricity 3 Platinum is expensive; however, that can be sidestepped with Molten Carbonate Fuel Cells (MCFC), which is the most complex fuel cell reaction available commercially. (See Figure 2 below.)

Figure 2: an MCFC. the reaction is like that of a standard fuel cell, but it is mediated by CO2 produced by the electrolyte. 4

MCFCs generate much more heat. The additional heat from the reaction in the fuel cell breaks down the electrolyte into CO2. The electrolyte is made of alkali metal carbonates.4 2H2 + 2CO32- à 2H2O + 2CO2 + 4eThe CO2 from this reaction is sent back to the cathode side and reacts with O2 to regenerate carbonate ions in the electrolyte. This happens based on this equation:4 O2 + 2CO2 + 4e- à 2CO32The reaction is similar to the basic fuel cell in Figure 1 but is mediated by carbonate ions from the electrolyte. This reaction also cannot be poisoned by carbon monoxide impurities, which instead form additional hydrogen through a shift reaction.4 CO + H2O à CO2 + H2 MCFC reactions do not generate any CO2 emissions as all CO2 is kept within a closed loop. These types of fuel cells are very efficient but not small enough to be adapted into personal vehicles, which currently use the simple hydrogen fuel cell design.4 Further study is needed on MCFCs to evaluate their full potential in consumer EVs. However, they could be implemented in large industrial vehicles. Standard fuel cells are the more viable method for consumer transportation, where they have already proved their viability. Hydrogen-fueled EVs like the Toyota Mirai have much more range than a standard EV, with the Mirai having an estimated range of 402 miles.5

Hydrogen-fueled Internal combustion engines (HICE):

HICEs are much more chemically simple. Much like a standard internal combustion engine (ICE), HICEs drive a piston with the following combustion reaction. 2H2 + O2 à 2H2O A typical engine cannot use hydrogen without significant problems. Hydrogen has very different properties than gasoline, and since it is gaseous at ambient temperature, it displaces more volume. See figure 3.

Figure 3: displacement comparisons of different engine configurations 6

Hydrogen has a stoichiometric air to fuel ratio of 34:1, which means a larger engine displacement and therefore larger engine block is needed to compete with the horsepower of a gasoline engine. Piston engines also have hot spots that can prematurely ignite the hydrogen. Engine knocking, or when a piston fires out of time, is also a problem with HICEs due to the volatility of hydrogen.

Some people propose using the Wankel Rotary Engine (WRE) in HICEs because it seems more naturally suited to hydrogens' unique properties. WRE’s are engines that are driven by triangle-shaped rotors that are put in a tapered oval and spin as opposed to pistons placed in an engine block that go back and forth. See figure 4.

Figure 4: A Wankel Rotary Engine (WRE) 7 WRE’s run cooler than orthodox ICEs, while also having a higher power per displacement volume, which means a WRE would benefit from a higher displacement requirement.7 Both orthodox ICEs and WREs have been tested with hydrogen fuel and need serious redesign to adapt properly to hydrogen. Companies like Toyota see hydrogen fuel as an alternative for automotive enthusiasts and have begun the development of a hydrogen-powered V8.8

Costs, Context, and Conclusion:

Hydrogen fuel has proven itself to be a viable solution to the problems that afflict EVs, but are also face significant challenges. Primarily, the issue with hydrogen fuel lies in the increased storage volume needed for hydrogen gas. Additionally, because of the volatility of hydrogen, storage tanks need to be carefully designed and built, which is a process that generates a lot of carbon emissions. At present, Hydrogen-fueled EVs are responsible for 15 kg of CO2 emissions per every 100km driven over a 150,000 km lifetime.9 This number accounts for the emissions created when producing a hydrogen tank and the emissions that come from the production and transport of hydrogen fuel. Internal combustion engines, however, are responsible for 1.1 kg of CO2 per 100km driven.10 While total emissions may eventually be balanced out if the fuel cell vehicle is driven over 150,000 km, ICEs are less immediately harmful. Reducing total CO2 production costs for hydrogen-based vehicles is vital to their success. It would help to look into carbon fiber recycling to reduce the production emissions of a hydrogen tank. Hydrogen production that does not emit CO2 is also a vital path to pursue.

Hydrogen-based internal combustion is far less viable for two reasons. First, hydrogen combustion still produces emissions in the form of nitrogen oxide.11 This is because of the nitrogen that is naturally present in the earth's atmosphere, which is converted into various nitrogen oxides as a byproduct of combustion. Tank capacity also follows the same limits as hydrogen fuel cells, the difference being that hydrogen combustion is far less efficient than the reaction that occurs in hydrogen fuel cells, meaning decreased mileage and higher fuel costs.11 Unless emissions can be reduced, HICEs are not practical to apply in transportation.

Hydrogen fuel cell-based vehicles are primarily limited by cost. The Toyota Mirai costs approximately $50,000 MSRP,5 which will not be competitive with future electric cars. If the cost can be reduced and if access to hydrogen fuel can be expanded, hydrogen-based vehicles may become practical. Attention must also be paid to how the hydrogen fuel is sourced, as not all hydrogen produced is produced in a clean manner. Electric cars technology is advancing much more quickly, and is more promising. Hydrogen fuel is likely better applied in the industrial sector where electricity access may not be an option.