Hydrogen is an attractive fuel or power source because when used as a fuel, mixed and burned with oxygen, it produces only water and presents a carbon neutral footprint. Further, hydrogen can be used in fuel cells or in internal combustion engines and has begun to find use in passenger cars, buses, spacecraft propulsion, and electricity generation. Hydrogen, as a replacement for gasoline, offers the same energy in 2.2 pounds (1 kilogram) as gasoline offers in 6.2 pounds (2.8 kilograms). But hydrogen has a low volumetric energy when stored as a compressed gas and requires larger tanks to achieve the driving ranges of conventional gasoline powered vehicles.
Example of a hydrogen burning rocket.
In the United States, most of the hydrogen produced each year is used for refining petroleum, treating metals, producing fertilizer, and processing foods. However, NASA has used hydrogen since the 1970s to propel space shuttles and other rockets into orbit. As well, hydrogen fuel cells are used to power the shuttle's electrical systems and the water, which is produced as a byproduct of a hydrogen fuel cell, is consumed by the crew as drinking water.
Hydrogen is abundant in the environment, and can be produced from natural gas, nuclear power, biomass, water, and renewable power such as solar and wind. There are four general processes for producing or separating hydrogen: thermal process, electrolytic process, solar-driven process, and biological process.
The thermal process for hydrogen production typically involves steam reforming. About 95 percent of current hydrogen is produced from steam reforming of natural gas. This is a process involving high-temperatures in which steam reacts with a hydrocarbon fuel to produce hydrogen. Many hydrocarbon fuels can be reformed to produce hydrogen. These include:
- Natrual Gas
- Diesel
- Renewable liquid fuels
- Gasified coal
- Gasified biomass
Image of the electrolytic process.
The separation of water into oxygen and hydrogen on the molecular level is done through a process known as the electrolytic process. This process involves the use of a particular machine called an electrolyzer. This process functions like a fuel cell in reverse, in that the electrolyzer does not use the energy of the hydrogen molecule resulting in water, but creates hydrogen from water.
There are a few solar-driven processes for hydrogen production, but each uses light as the agent for separation or production. These processes include photobiological, photoelectrochemical, and solar thermochemical. Photobiological processes use natural photosynthetic activity of bacteria and green algae to produce hydrogen. The photoelectrochemical process uses specialized semiconductors to separate water into hydrogen and oxygen. And a solar thermochemical hydrogen production which uses concentrated solar power to drive water-splitting reactions, often done along with other species such as metal oxides.
Bacterial process to produce hydrogen.
The biological process uses microbes such as bacteria and microalgae and produce hydrogen through biological reactions. In microbial biomass conversion, the microbes break down organic matter like biomass or wastewater to produce hydrogen.
A hydrogen fuel cell is structured similar to a chemical battery, where hydrogen and oxygen are combined to produce electricity, heat, and water. Similar to batteries, a hydrogen fuel cell is composed of an anode, cathode, and an electrolyte membrane. A typical fuel cell passes hydrogen through the anode and oxygen through the cathode. At the anode side, a catalyst splits the hydrogen molecule into electrons and protons. The protons pass through the porous electrolyte membrane while the electrons are forced through a circuit, generating an electric current and excess heat. At the cathod, the protons, electrons and oxygen combine to produce water molecules. A fuel cell can also be combined with a heat and power system to use the cell's waste heat for heating and cooling applications.
Visualization of a hydrogen fuel cell.
Fuel cells can be used to provide power to homes, businesses, hospitals, grocery stores, data centers, and to power cars, buses, trucks, forklifts, and trains. Unlike batteries, hydrogen fuel cells do not need to be periodically recharged like batteries and continue to run so long as hydrogen is supplied without losing charge. Fuel cells can also be stacked and combined in larger systems to produce greater amounts of power and produce power for the power grid.
Hydrogen infrastructure refers to the infrastructure of hydrogen pipeline transport, points of hydrogen production, hydrogen stations for distribution, and the sale of hydrogen fuel. These all represent prerequisites before hydrogen power can be commercialized as an alternative fuel for automotive, transport, and aeronautical fuel source. In mid-2020, there were 43 open retail hydrogen stations in the United States with a further 30 stations in various stages of planning or construction. Most of the existing or planned stations were in California, with one in Hawaii, and 12 planned stations for the northeastern states.
And the United States Department of Energy, in a public-private collaboration with federal agencies, automakers, hydrogen providers, fuel cell developers, national laboratories, and other stakeholders, launched H2HUSA to focus on advancing hydrogen infrastructure and support more transportation energy options for United States consumers.
Hydrogen cars work on the same principles of electric cars, such that they are essentially electric vehicles (in most cases) except that instead of lithium-ion batteries as a fuel source, hydrogen cars use hydrogen fuel cells. And the use of a hydrogen fuel cell in place of a lithium-ion battery offers certain advantages, including the ability to be refueled in about 5 minutes compared to the possible hours that can be necessary to charge a battery. Hydrogen cars also have travel ranges similar to gasoline vehicles, with the smallest range belonging to the Toyota Mirai at 317 miles on a full tank, which can in turn be compared to the 220 mile range of the base model Tesla Model 3.
a view of the working components of a hydrogen fuel-cell car.
The need for a more extensive hydrogen infrastructure has slowed the production and adoption of hydrogen cars, which has in turn slowed the development of the necessary hydrogen infrastructure. Interest in hydrogen powered cars began in 2013, with the statements of multiple automakers and collaboration between automakers, to develop hydrogen cars. This lead to the 2015 introduction of Hyundai's NEXO and Toyota's Mirai, and the 2016 introduction of the Honda Clarity. These three models remain publicly available as hydrogen cars, and in the case of the Honda Clarity, the base model uses either lithium-ion or hydrogen fuel cells as a power source.
Meanwhile, in 2020, Mercedes-Benz, which began research and developing hydrogen fuel-cell powered passenger cars 30 years prior, announced they were ending their hydrogen fuel cell powered passenger car program. In 2013, Mercedes-Benz entered a collaboration with Ford and Nissan to develop hydrogen powered vehicles. Mercedes-Benz produced one production-possible model, the GLC F-Cell, which was used for business promotions and demonstrating the possibility of developing hydrogen cars. However, the GLC F-Cell was never offered for sale to the public due to high manufacturing costs. The same manufacturing costs were part of the reason for ending the development program.
Similar to hydrogen cars, hydrogen planes use hydrogen as a power source. Hydrogen can either be burned in a jet engine or other kind of internal combustion engine, or can be used to power a fuel cell to generate electricity to power a propeller. Besides infrastructure challenges similar to those faced by hydrogen cars, a challenge to the use of hydrogen fuel cells in aircraft comes in the weight required for fuel storage if it is in liquid or gaseous form. For liquid hydrogen the challenge is making lightweight vacuum-insulated tanks that maintain the fuel below the 20 kelvin boiling point. Gas carries a greater weight penalty since the tanks must be built to withstand high pressures of 250 to 350 bar. Also, hydrogen powered planes require greater external surface area to accommodate for the larger hydrogen tanks and there is a need to redesign the aircraft to reduce increased aerodynamic drag.
The ZeroAvia six seater hydrogen powered propeller plane taking off.
In 2008, Boeing built and operated the first aircraft to fly solely on hydrogen power. The fuel cells on the single person plane were supplemented with power from lithium-ion batteries during takeoff and ascent. Four years following this flight, Boeing released the Phantom Eye, a liquid-hydrogen-powered unmanned aerial vehicle designed to fly reconnaissance missions of up to four days at an altitude of 20,000 meters. The company was unsuccessful selling the UAV to the military. Whereas Airbus has publicly stated they will decide by 2025 whether to manufacture hydrogen-powered aircraft. Their decision depends on whether the market can support these airliners.
In October 2020, ZeroAvia, a small startup set on manufacturing 10 to 20 passenger aircraft powered by hydrogen fuel cells by 2023, successfully flew a six-seater aircraft powered by a hydrogen-fuel cell. This marked the first hydrogen-fuel-cell-powered flight for an aircraft classed as commercial size.
Hydrogen use as a power source for heavy transport trucks is similar to the ideas for electric powered heavy transport truck, and faces similar challenges to hydrogen powered cars. The high energy density of hydrogen and the available space in truck cabs also make hydrogen a possible option for heavy-goods transport vehicles. As well, hydrogen power for the industry is seen as a more attractive option compared to lithium-ion electric power because hydrogen has less downtime for fueling compared to the hours of downtime lost to recharging batteries. This downtime can represent lost income for heavy transport trucks. Although hydrogen-powered heavy transport trucks suggest a possible range of 500 miles, better compared to the 300 miles electric powered heavy transport trucks have been quoted at, but nowhere near the 1000 miles traditional diesel engines can achieve.
Concept for a hydrogen powered heavy transport truck.
In the European Union, the adoption of hydrogen fuel cell heavy transport trucks may happen in response to the 2025 emissions limits being placed on the heavy transport industry which requires a 15 percent limit in emissions. These emissions limits are increased again in 2030 to a 30 percent limit in emissions. Also, emerging methods of chemically bonding hydrogen or using ammonia to transport hydrogen present a stable transportation and storage method that does not require the pressurization or cryogenic liquefaction current hydrogen power requires. These methods would also require less energy for transportation and storage.
When stored, hydrogen can be stored physically as either a gas or a liquid. The storage of hydrogen gas typically requires high-pressure tanks (around 250 to 700 bar). Storage of hydrogen as a liquid requires cryogenic temperatures due to the boiling point of hydrogen at one atmosphere pressure being -252.8 degrees Celsius. Hydrogen can also be stored on the surface of solids or within solids, by absorption. The more popular version of hydrogen storage for portable and stationary storage is to store it compressed in gaseous form. The footprint of compressed gas tanks are easier to accommodate than a cryogenic liquid hydrogen storage unit. However, for fuel cell vehicles, which require at least enough hydrogen to provide a driving range of 300 miles or more, the required storage volumes for compressed gas may be higher than can be reasonably accommodated.
There is a push to develop novel hydrogen storage materials with the goal to provide adequate storage to meet the U.S. Department of Energy hydrogen storage targets for onboard light-duty vehicle, material-handling equipment, and portable power applications.
