Finding sustainable forms of energy is a key component in the race to mitigate the worst effects of climate change. Fortunately, solar power has recently surpassed natural gas as the cheapest source of energy (1). Not only is solar energy affordable, but it is also renewable, carbon-neutral, and abundant: 173,000 terawatts of solar energy are hitting Earth at any given moment – far exceeding global energy consumption (2). However, solar energy is only reliable as long as the sun is shining. How can we produce energy sustainably at night or when it is cloudy?
Enter electrolyzers, a device that uses electricity to split water into hydrogen and oxygen to create hydrogen fuel. Hydrogen fuel is light and energy-dense, which means it can be easily stored until needed. Given the abundance and low cost of solar energy, any excess electricity produced when the sun is shining can be converted to hydrogen fuel in a carbon neutral process using an electrolyzer. The hydrogen fuel can then be used to generate electricity using a fuel cell, which involves a process opposite from the one that occurs in electrolyzers. With this electrolyzer-fuel cell system, excess electricity harvested from the sun during the day can be stored for use when the sun is not shining.
An electrolyzer consists of an anode and a cathode, with a separator called a membrane sandwiched in the middle. There are many types of electrolyzers based on different chemical reactions, but one of the most common is the proton exchange membrane (PEM) electrolyzer. In a PEM electrolyzer, water is fed to the anode, where it is split to create protons (hydrogen ions), oxygen, and free electrons. An external source of electricity – ideally solar or wind – provides the energy needed to drive these reactions. Applying electricity causes the anode to become positively charged and the cathode to become negatively charged. Negatively-charged oxygen atoms in the water are so attracted to the positively-charged anode that the anode removes electrons from the water, weakening the hydrogen bonds holding the water atoms together and causing the atoms to split. The oxygen is released to the air, the protons move across the membrane to the cathode, and the electrons go around the non-conductive membrane to the cathode. At the cathode, the protons and electrons, which are oppositely charged and thus attracted to each other, combine to form hydrogen gas which is a lower-energy state (3). This hydrogen fuel is the end product. To obtain usable energy from it, we generate electricity using a fuel cell. A PEM fuel cell is the opposite of a PEM electrolyzer, separating hydrogen gas into protons and electrons. The electrons flow around the membrane, creating a stream of electricity that can be harvested, while protons move across the membrane and combine with oxygen to create water as the only byproduct (4).
Though the processes involved in PEM electrolyzers may be key to our transition to renewables, they are not widely used today due to their cost. It currently costs around $5 to produce a kilogram of hydrogen by electrolysis, which contains roughly the same amount of energy as a gallon of gasoline (5). One contributing factor to this problem is the catalyst, which lowers the amount of electricity needed to drive the electrochemical reactions to attainable quantities. The most effective catalysts today consist of rare metals such as platinum and iridium layered on either side of the membrane, and obtaining these metals is both expensive and environmentally unsustainable. Nonetheless, solutions are being developed such as using more abundant metals like cobalt (6). The U.S. Department of Energy is aiming to decrease the price of hydrogen to $1 per kilogram by 2032, which will increase hydrogen use by at least five times (7). Ongoing advances in cell material, hydrogen fuel efficiency, electrolyzer design will lead us ever closer to this goal and the widespread use of hydrogen fuel.
References
Hydrogen Production: Electrolysis. Department of Energy Office of Energy Efficiency &
Renewable Energy. https://www.energy.gov/eere/fuelcells/hydrogen-production-electrolysis
Hydrogen’s Role in Transportation. Department of Energy Office of Energy Efficiency & Renewable Energy. https://www.energy.gov/eere/vehicles/articles/hydrogens-role-transportation#:~:text=One%20kg%20of%20hydrogen%20contains,on%20a%20gallon%20of%20gasoline.
Hydrogen Shot. Department of Energy Office of Energy Efficiency & Renewable Energy. https://www.energy.gov/eere/fuelcells/hydrogen-shot.
Penn, I. (2020, July 6). The next energy battle: renewables vs. natural gas. The New York Times. Retrieved July 23, 2024, from https://www.nytimes.com/2020/07/06/business/energy-environment/renewable-energy-natural-gas.html.
Phiddian, E. (2022, February 15). Taking the rarest of metals out of hydrogen electrolysis. Cosmos Magazine. https://cosmosmagazine.com/science/chemistry/hydrogen-electrolysis-precious-metals-catalyst/
Phongsavan, P. (2015, August 10). Energy On a Sphere. National Oceanic and Atmospheric Administration. https://sos.noaa.gov/catalog/live-programs/energy-on-a-sphere/#description-key-points
Types of Fuel Cells. Department of Energy Office of Energy Efficiency & Renewable Energy. https://www.energy.gov/eere/fuelcells/types-fuel-cells