How Does it Work?
In photolytic biological systems, microorganisms—such as green microalgae or cyanobacteria—use sunlight to split water into oxygen and hydrogen ions. The hydrogen ions can be combined through direct or indirect routes and released as hydrogen gas. Challenges for this pathway include low rates of hydrogen production and the fact that splitting water also produces oxygen, which quickly inhibits the hydrogen production reaction and can be a safety issue when mixed with hydrogen in certain concentrations. Researchers are working to develop methods to allow the microbes to produce hydrogen for longer periods of time and to increase the rate of hydrogen production.
Some photosynthetic microbes use sunlight as the driver to break down organic matter, releasing hydrogen. This is known as photofermentative hydrogen production. Some of the major challenges of this pathway include a very low hydrogen production rate and low solar-to-hydrogen efficiency, making it a commercially unviable pathway for hydrogen production at this time.
Researchers are looking at ways to make the microbes better at collecting and using energy to make more available for hydrogen production, and to change their normal biological pathways to increase the rate of hydrogen production.
Why Is This Pathway Being Considered?
In the long term, photobiological production technologies may provide economical hydrogen production from sunlight with low- to net-zero carbon emissions. The algae and bacteria could be grown in water that cannot be used for drinking or for agriculture, and could potentially even use wastewater.
Research Focuses on Overcoming Challenges
Research in photobiological hydrogen has progressed in recent years, though it is still in the early stages. There are a number of common challenges to both photolytic and photofermentative biological hydrogen production. Many of these challenges require further research into basic, fundamental questions, such as that being done in the U.S. Department of Energy’s Office of Science, including:
- Improving the activity of the enzymes that produce the hydrogen, as well as the metabolic pathways needed for the reactions, to increase the hydrogen production rates.
- Developing strains that can efficiently use the sunlight and other inputs to increase the hydrogen yields.
- Developing strains and reactor configurations that can ultimately be used at large scales for commercial hydrogen production.