By Kyle Warner, Illustration by Elaine Lee
One of the most significant sources of greenhouse gas emissions comes from the combustion of hydrocarbon fuels such as gasoline, natural gas, and petroleum. To reduce these emissions, researchers are developing more environmentally friendly, carbon-free alternatives. The most promising of these fuel substitutes is hydrogen, which has the potential to be free of greenhouse gas emissions throughout its life cycle and could be used for heating, vehicular transport, seasonal energy storage, or long-distance energy transmission.
Hydrogen is the most abundant atom in the universe, but is rarely found in its elemental form because it prefers to combine with other elements to form plant matter, water, and natural gas. To obtain the pure form of hydrogen (H2) that can be run through a fuel cell to provide useful energy, hydrogen must first be separated from its attached elements using one of two processes: electrolysis or steam reforming.
Electrolysis refers to the separation of hydrogen from oxygen via running water through an electrolyzer, which consists of an anode, cathode, and electrolyte. The newest technologies have an efficiency of around 80%, meaning that 20% of the initial energy input is wasted through the water-splitting process. One issue with electrolysis is that the energy input is often derived from non-renewable grid sources such as coal, so the emissions saved by the hydrogen fuel are overshadowed by the emissions from its production. However, if the electricity is produced from renewable sources, then electrolysis becomes a hydrogen production method entirely free of greenhouse gas emissions.
Steam reforming, or methane reforming, is the primary method of large-scale hydrogen production, in which hydrogen is separated from methane or natural gas (CH4) through the use of a sequence of two reactions. The first reaction involves mixing natural gas with water and heat to produce carbon monoxide and hydrogen. Because carbon monoxide is a toxic gas, a second reaction known as a water-gas shift reaction is required to convert the carbon monoxide and water into carbon dioxide, hydrogen, and a small amount of excess waste heat. This waste heat is most often recovered and recycled back into the reactor to drive the initial reaction. Although the natural gas needed for this process is less expensive than the electricity required for electrolysis, steam reforming is still at a disadvantage environmentally because carbon dioxide is a primary byproduct. In theory, a carbon capture device paired with a steam reforming reactor would solve this issue, but this design demands further investment and research.
In addition to the high cost of its production, hydrogen’s extreme flammabilityis another significant barrier to large-scale implementation. Although driving a car with a highly combustible fuel under your feet presents valid cause for concern, hydrogen is fourteen times lighter than air and would quickly disperse into the atmosphere if a fuel cell was punctured. Unlike gasoline, hydrogen is also odorless, which makes it difficult to detect a leak. However, the addition of hydrogen sulfide would produce a rotten egg odor in the event of a leak and address some of the above-mentioned safety concerns.
Another drawback to hydrogen fuel is the high cost of storage, transport, and fuel cell catalysts. Because hydrogen is so light and has a large volume under standard temperature and pressure, it must be stored under extremely high pressure (approximately 10,000 psi) in gas form or cryogenically cooled into a liquid to optimize efficiency. Both of these processes are highly energy intensive and require expensive, thick storage tanks. Hydrogen can also experience energy losses in the range of 10-40% during transport by trucks or pipeline, largely due to the evaporation of liquefied hydrogen. Further improvements in the insulation of hydrogen infrastructure and an increase in smaller scale, on-site hydrogen production are needed to eliminate these transport losses. In addition, a fuel cell requires expensive catalysts such as platinum to facilitate the reaction of hydrogen with oxygen, making it a pricier alternative to traditional batteries.
Despite the remaining barriers that prevent widespread adoption of hydrogen, hydrogen represents an incredibly promising fuel alternative with the potential to completely eliminate our dependence on fossil fuels. Ongoing research to reduce the price of electrolysis, steam reforming, hydrogen storage, and fuel cell catalysts is critical if we want hydrogen to become cost-competitive with conventional hydrocarbon fuels. Ideally, electrolysis driven by solar and wind energy would be the primary method of hydrogen production. This carbon-free future is not out of reach, but we must continue to develop current technology if we wish to fully capitalize on the potential of hydrogen fuel.