Berkeley Lab solar-to-fuel system for CO2 to ethanol and ethylene; light-powered production of fuel via artificial photosynthesis
Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have harnessed the power of photosynthesis to convert carbon dioxide into fuels and alcohols at efficiencies far greater than plants. The achievement marks a significant milestone in the effort to move toward sustainable sources of fuel.
Many systems have successfully reduced carbon dioxide to chemical and fuel precursors, such as carbon monoxide or a mix of carbon monoxide and hydrogen known as syngas. This new work, described in a study published in the journal Energy and Environmental Science, is the first to successfully demonstrate the approach of going from carbon dioxide directly to target products—ethanol and ethylene—at energy conversion efficiencies rivaling natural counterparts.
Solar to chemical energy conversion could provide an alternative to mankind's unsustainable use of fossil fuels. One promising approach is the electrochemical reduction of CO2 into chemical products, in particular hydrocarbons and oxygenates which are formed by multi-electron transfer reactions. Here, a nanostructured Cu–Ag bimetallic cathode is utilized to selectively and efficiently facilitate these reactions.—Gurudayal et al.
The researchers did this by optimizing each component of a photovoltaic-electrochemical system to reduce voltage loss, and creating new materials when existing ones did not suffice.
This is an exciting development. As rising atmospheric CO2 levels change Earth’s climate, the need to develop sustainable sources of power has become increasingly urgent. Our work here shows that we have a plausible path to making fuels directly from sunlight.—study principal investigator Joel Ager
That sun-to-fuel path is among the key goals of the Joint Center for Artificial Photosynthesis (JCAP), a DOE Energy Innovation Hub established in 2010 to advance solar fuel research. The study was conducted at JCAP’s Berkeley Lab campus.
The initial focus of JCAP research was tackling the efficient splitting of water in the photosynthesis process. Having largely achieved that task using several types of devices, JCAP scientists doing solar-driven carbon dioxide reduction began setting their sights on achieving efficiencies similar to those demonstrated for water splitting, considered by many to be the next big challenge in artificial photosynthesis.
Another research group at Berkeley Lab is tackling this challenge by focusing on a specific component in a photovoltaic-electrochemical system. In another newly published study, they describe a new catalyst that can achieve carbon dioxide to multicarbon conversion using record-low inputs of energy. (Earlier post.)
For this JCAP study, researchers engineered a complete system to work at different times of day, not just at a light energy level of 1-sun illumination, which is equivalent to the peak of brightness at high noon on a sunny day. They varied the brightness of the light source to show that the system remained efficient even in low light conditions.
When the researchers coupled the electrodes to silicon photovoltaic cells, they achieved solar conversion efficiencies of 3 to 4 percent for 0.35 to 1-sun illumination. Changing the configuration to a high-performance, tandem solar cell connected in tandem yielded a conversion efficiency to hydrocarbons and oxygenates exceeding 5 percent at 1-sun illumination.
Among the new components developed by the researchers are a copper-silver nanocoral cathode, which reduces the carbon dioxide to hydrocarbons and oxygenates, and an iridium oxide nanotube anode, which oxidizes the water and creates oxygen. The nanocoral can make the target products over a wide range of conditions, and it is very stable, said Ager.
The researchers characterized the materials at the National Center for Electron Microscopy at the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab. The results helped them understand how the metals functioned in the bimetallic cathode. Specifically, they learned that silver aids in the reduction of carbon dioxide to carbon monoxide, while the copper picks up from there to reduce carbon monoxide further to hydrocarbons and alcohols.
Because carbon dioxide is a stubbornly stable molecule, breaking it up typically involves a significant input of energy.
Reducing CO2 to a hydrocarbon end product like ethanol or ethylene can take up to 5 volts, start to finish. Our system reduced that by half while maintaining the selectivity of products.—lead author Gurudayal
Notably, the electrodes operated well in water, a neutral pH environment.
Research groups working on anodes mostly do so using alkaline conditions since anodes typically require a high pH environment, which is not ideal for the solubility of CO2. It is very difficult to find an anode that works in neutral conditions.—Gurudayal
The researchers customized the anode by growing the iridium oxide nanotubes on a zinc oxide surface to create a more uniform surface area to better support chemical reactions.
By working through each step so carefully, these researchers demonstrated a level of performance and efficiency that people did not think was possible at this point. This is a big step forward in the design of devices for efficient CO2 reduction and testing of new materials, and it provides a clear framework for the future advancement of fully integrated solar-driven CO2-reduction devices.—Berkeley Lab chemist Frances Houle, JCAP deputy director for Science and Research Integration, who was not part of the study
Other co-authors on the study include James Bullock, a Berkeley Lab postdoctoral researcher in materials sciences, who was instrumental in engineering the system’s photovoltaic and electrolysis cell pairing. Bullock works in the lab of study co-author Ali Javey, Berkeley Lab senior faculty scientist and a UC Berkeley professor of electrical engineering and computer sciences.
This work is supported by the DOE Office of Science.
Gurudayal, James Bullock, Dávid F. Srankó, Clarissa M. Towle, Yanwei Lum, Mark Hettick, M. C. Scott, Ali Javey and Joel Ager (2017) “Efficient solar-driven electrochemical CO2 reduction to hydrocarbons and oxygenates” Energy & Environmental Science doi: 10.1039/C7EE01764B