U.S. Department of Energy - Energy Efficiency and Renewable Energy
Fuel Cell Technologies Office – Hydrogen Production
A promising option for hydrogen production from renewable resources is electrolysis, in which electricity is used to dissociate water into hydrogen and oxygen. Photo courtesy of the Schatz Energy Research Center, Humboldt State University
Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer. Electrolyzers can be small, appliance-size equipment and well-suited for small-scale distributed hydrogen production. Research is also under way to examine larger-scale electrolysis that could be tied directly to renewable or other non-greenhouse gas emitting electricity production. Hydrogen production at a wind farm generating electricity is an example of this.
Hydrogen produced via electrolysis can result in zero greenhouse gas emissions, depending on the source of the electricity used. The source of the required electricity—including its cost and efficiency, as well as emissions resulting from electricity generation—must be considered when evaluating the benefits of hydrogen production via electrolysis. In many regions of the country, today's power grid is not ideal for providing the electricity required for electrolysis because of the greenhouse gases released and the amount of energy required to generate electricity. Hydrogen production via electrolysis is being pursued for renewable (wind) and nuclear options. These pathways result in virtually zero GHG emissions and criteria pollutants.
How does it work?
Like fuel cells, electrolyzers consist of an anode and a cathode separated by an electrolyte. Different electrolyzers function in slightly different ways.
In a polymer electrolyte membrane (PEM) electrolyzer, the electrolyte is a solid specialty plastic material.
Water reacts at the anode to form oxygen and positively charged hydrogen ions (protons).
The electrons flow through an external circuit and the hydrogen ions selectively move across the PEM to the cathode.
At the cathode, hydrogen ions combine with electrons from the external circuit to form hydrogen gas.
Anode Reaction: 2H2O → O2 + 4H+ + 4e-
Cathode Reaction: 4H+ + 4e- → 2H2
Alkaline electrolyzers are similar to PEM electrolyzers but use an alkaline solution (of sodium or potassium hydroxide) that acts as the electrolyte. These electrolyzers have been commercially available for many years.
Solid Oxide Electrolyzers
Solid oxide electrolyzers, which use a solid ceramic material as the electrolyte that selectively transmits negatively charged oxygen ions at elevated temperatures, generate hydrogen in a slightly different way.
Water at the cathode combines with electrons from the external circuit to form hydrogen gas and negatively charged oxygen ions.
The oxygen ions pass through the membrane and react at the anode to form oxygen gas and give up the electrons to the external circuit.
Solid oxide electrolyzers must operate at temperatures high enough for the solid oxide membranes to function properly (about 500°C–800°C, compared to PEM electrolyzers, which operate at 80°C–100°C, and alkaline electrolyzers, which operate at 100°C—150°C). The solid oxide electrolyzers can effectively use heat available at these elevated temperatures (from various sources, including nuclear energy) to decrease the amount of electrical energy needed to produce hydrogen from water.
Why is this technology being researched?
Potential for synergy with renewable energy power generation.
For example, though the cost of wind power has continued to drop, the inherent variability of wind is an impediment to the effective use of wind power. Hydrogen production via electrolysis may offer opportunities for synergy with variable power generation, which is characteristic of some renewable energy technologies. Hydrogen fuel and electric power generation could be integrated at a wind farm, allowing flexibility to shift production to best match resource availability with system operational needs and market factors.
No greenhouse gas emissions.
Hydrogen produced via electrolysis can result in zero or near-zero greenhouse gas emissions, depending on the source of the electricity used (emissions resulting from electricity generation must be considered when evaluating the environmental benefits of electrolytic hydrogen production technologies). Nuclear high-temperature electrolysis and renewable electrolysis, using renewable sources of electricity such as wind turbines, will result in near-zero greenhouse gas emissions.
It is important to note...
Today's electricity grid is not ideal to provide the electricity required for electrolysis because of the greenhouse gas emissions and energy-intensive nature of the electricity generation technologies used. Electricity generation using renewable or nuclear energy technologies, separate from the grid, is a possible option for hydrogen production via electrolysis.
DOE and others continue efforts to bring down the cost of renewable-based electricity production and develop more efficient coal-based electricity production with carbon sequestration. Wind-based electricity production, for example, is growing rapidly in the U.S. and globally.
Research focuses on overcoming challenges
Reducing the capital cost of the electrolyzer and improving energy efficiency.
Integrating compression into the electrolyzer and avoiding the cost of a separate compressor needed to increase pressure for hydrogen storage.
For more information, refer to the Electric Power Research Institute presentation about home hydrogen electrolyzers and the hydrogen market.