Discover
Hydrogen
Published May 27, 2022
If you let hydrogen gas alone for 13 billion years, it will become giraffes, rose bushes, and humans.
—BRIAN SWIMME
During the 26th Conference of the Parties in November of 2021, 40 countries agreed to work together to assure renewable, low-carbon hydrogen is made globally available by 2030.
Hydrogen is widely employed in industrial processes, such as extracting metals from minerals, refining petroleum, fertilizer production, and food production. Its use also extends to fuel, powering fuel cells, rocket thrusters in space shuttles, and various electrical systems on spacecraft.
Energy Basics
Hydrogen is the lightest and most abundant element in the universe. The one attribute that catches the eye of scientists and engineers is its immense heat value, or specific energy (energy per unit mass).
Hydrogen has the potential to be produced domestically, at which each home can contribute to hydrogen production and generate electricity. Theoretically, domestic production of hydrogen allows for the sharing of electricity between homesteads, giving rise to self-generated local electric grids.
Hydrogen is known as an energy carrier, a fuel that can be converted into other forms of energy like electrical and thermal energy.
The amount of energy required to produce hydrogen is the same as the amount of energy hydrogen releases when used in a fuel cell to produce electricity. Therefore, producing hydrogen is a method of energy storage.
Production
Along with its classification as an energy carrier, hydrogen is considered a secondary power source; its production requires primary power sources: natural gas, nuclear radiation, biomass, and renewable energies like solar and wind.
While world leaders at COP26 discussed the goal of reaching zero-emission hydrogen production by 2030, they did not include which methods to advance. The following paragraphs below describe the most effective methods of hydrogen production and their influence on the climate crisis.
Electrolysis
Electrolysis is the process of dissociating water molecules into hydrogen and oxygen with electricity. Performing electrolysis requires an electrolyzer, which can range from a small appliance to large-scale machinery.
Hydrogen production can only be as carbon-free as the energy used to initiate the process. To assure zero-emission hydrogen production, the electricity needed to power an electrolyzer must come from renewable energy sources.
An electrolyzer consists of an anode side and a cathode side. Between these two components resides a polymer electrolyte membrane. On the anode side, water dissociates into oxygen and hydrogen ions, as demonstrated below:
Once water dissociates, the electrons travel through an external circuit that leads to the cathode, while the hydrogen ions permeate through the polymer electrolyte membrane. On the cathode side, the hydrogen ions and electrons combine to form hydrogen gas:
Electrolysis is currently 70-80% efficient. The goal is to reach 82-86% efficiency with hydrogen production via electrolysis by 2030. Meeting this goal would require advanced proton exchange membrane (PEM) electrolyzers.
Steam Reforming
Steam reforming is a process in which steam reacts with methane at temperatures 700°C-1100°C to produce hydrogen. The process occurs in the presence of a catalyst like nickel. The reaction taking place, known as the steam-methane reforming reaction, is shown below:
This process extends to the water-gas shift reaction to produce more hydrogen, in which carbon monoxide, the product of the reaction above, subsequently reacts with excess steam:
Around 95% of all hydrogen is produced from steam reforming with pipeline natural gas, a process 60-75% efficient.
Steam reforming is the preferred method for producing hydrogen because the cost of collecting and distributing methane for steam reforming is much cheaper than the cost of electricity for electrolysis.
As soon as the cost of producing electricity is cheaper than collecting and refining natural gas, electrolysis will be the primary method of producing hydrogen.
Fuel
Under the Energy Policy Act of 1992, hydrogen is listed as an alternative transportation fuel. What makes hydrogen so promising as an alternative fuel is its innocuous nature; the fuel does not release pollutants or contribute to the climate crisis.
Although hydrogen has the highest energy content compared to any transportation fuel by weight, it has the lowest energy content by volume. When used as the source for a fuel cell, hydrogen can be three times more efficient than an internal combustion engine running on gasoline.
Hydrogen gas has the lowest energy content by volume because its intermolecular forces of attraction are weaker compared to other compounds used as a fuel. The intermolecular forces of attraction are so weak because hydrogen gas contains 2 electrons per molecule. As the number of electrons increases, intermolecular forces become stronger. The weak intermolecular forces of hydrogen are also the reason for its low boiling point, its gaseous state at room temperature.
The energy content in 2.2 lbs of hydrogen gas is the same as the energy content in 6.2 lbs of gasoline. To account for its low volumetric energy density, hydrogen gas is stored as a compressed gas to reach the driving range of most gas-powered vehicles.
Fuel Cells
A fuel cell is a device that uses the energy of hydrogen to produce electricity. Hydrogen is the fuel to a fuel cell, as gasoline is to a combustion engine. Hydrogen, though, has three times the specific energy as methane or gasoline.
Carbon compounds such as those within gasoline have always been the most practical energy carriers, due to their ability to be stored as a liquid. The greater the number of carbon atoms—along with electrons—a compound has, the higher its boiling point will be. With so many electrons occupying a given compound in gasoline, the intermolecular forces of attraction are quite strong, granting gasoline a rather high volumetric energy density. Diatomic hydrogen, however, does not have such strong intermolecular forces of attraction like gasoline.
Most fuel cells in our economy use hydrogen as an energy carrier to produce electricity. Hydrogen seeps into the anode side of the fuel cell, and air dispenses into the cathode. On the anode side, platinum oxidizes hydrogen into two protons and two electrons as shown below:
The electrons create a flow of electricity traveling through an exterior circuit, while the protons conduct through a perfluorosulfonic acid membrane. On the cathode side, the protons and electrons reunite and combine with oxygen to form water:
2H+ + 2e– + ½O2 → H2O + heat
The applications for hydrogen fuel cells are limitless, having the capacity to supply electricity to any equipment, from the batteries of laptops to grid-scale power storage.
Climate
Hydrogen fuel is climate neutral. A Fuel Cell Electric Vehicle (FCEV) running on hydrogen will only emit water vapor and heat, which classifies it as a zero-emission vehicle.
You may be wondering though, as am I, the reasoning for manufacturing FCEVs when Battery Electric Vehicles (BEVs) are sufficient. Compared to BEVs, FCEVs have much faster refueling times, and once electrolysis becomes the primary method of production, hydrogen will be exceptionally abundant.
Storage Methods
The storage of hydrogen is an ongoing issue that requires further optimization. The best storage methods in the hydrogen industry aim to maximize energy density. Below are some of the most relevant methods of storage.
Compressed Gas
Storage of hydrogen gas requires high-pressure tanks at 5,000-10,000 pounds per square inch (psi). This method of storage is the most economically feasible in the US.
Cryogenic Tanks
The boiling point of hydrogen at one atmosphere is -252.9°C. Storing hydrogen as a liquid at or below this temperature increases its energy density, however the electricity cryogenic tanks require to liquify hydrogen is extremely intensive.
Metal Hydrides
Hydrogen can be stored on the surface of—or within—a solid, by the process of absorption. This process dissociates diatomic hydrogen gas into its individual atoms, which then fit inside a solid lattice framework.
Absorption may have the most potential in storing greater quantities of hydrogen in smaller volumes at low pressure and room temperature.
Organic Chemical Hydrides
The most interesting method I’ve found to store hydrogen is through reactions of hydrogen-containing compounds in water to form chemical hydrides. Also known as chemical hydrogen storage, hydrogen is stored in both the compound and the water.
Organic chemical hydrides appear to store a greater density of hydrogen atoms than compressed gas and other methods involving hydrides—surface absorption, intermetallic hydrides, and complex hydrides.
The organic chemical hydride method involves converting an aromatic group to a saturated cyclic compound by way of a hydrogenation reaction.