New Developments Could Create Fully Integrated Solar-Driven CO2-Reduction Devices

Researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have harnessed the power of photosynthesis to transform carbon dioxide into alcohols and fuels at efficiencies far greater than plants. The achievement indicates a noteworthy milestone in the effort to move toward sustainable sources of fuel.

A number of systems have successfully reduced carbon dioxide to fuel and chemical precursors, such as carbon monoxide or syngas, which is actually a mix of hydrogen and carbon monoxide. This recent work is described in a study published in the Energy and Environmental Science journal. It is the first to successfully show the approach of going from carbon dioxide directly to target products, namely ethylene and ethanol, at energy conversion efficiencies rivalling natural counterparts.

The Researchers did this by optimizing every component of a photovoltaic-electrochemical system to lower voltage loss and to produce new materials when existing ones were not enough.

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.

Joel Ager, Study Principal Investigator and a Berkeley Lab Scientist with joint appointments in the Materials Sciences and the Chemical Sciences divisions

That sun-to-fuel path is one of the key goals of the Joint Center for Artificial Photosynthesis (JCAP), a DOE Energy Innovation Hub established in 2010 to make solar fuel research move forward. The study was carried out at JCAP’s Berkeley Lab campus.

Managing the efficient splitting of water in the photosynthesis process was the primary focus of JCAP research. Besides widely achieving that task using several types of devices, JCAP Scientists carrying out solar-driven carbon dioxide reduction began setting their sights on attaining efficiencies comparable to those shown for water splitting, considered by many to be the next big challenge in artificial photosynthesis.

Another research team at Berkeley Lab is dealing with this challenge by focusing on a specific component in a photovoltaic-electrochemical system. A new catalyst that can achieve carbon dioxide to multicarbon conversion using record-low inputs of energy is explained in detail in a new study published on September 18^th, 2017.

Not just for noon

For this JCAP study, Researchers built a complete system to operate at different times of day, not just at a light energy level of 1-sun illumination, which is the same as the peak of brightness at high noon on a sunny day. They changed the brightness of the light source to prove that the system remained efficient even in low-light conditions.

The Researchers achieved solar conversion efficiencies of 3 to 4% for 0.35 to 1-sun illumination, when they connected the electrodes to silicon photovoltaic cells. When the configuration was changed to a high-performance, tandem solar cell coupled in tandem produced conversion efficiency to hydrocarbons and oxygenates exceeding 5% at 1-sun illumination.

“We did a little dance in the lab when we reached 5 percent,” said Ager, who also holds an appointment as an Adjunct Professor at UC Berkeley’s Materials Science and Engineering Department.

A copper-silver nanocoral cathode is one of the new components developed by the Researchers. These components reduce the carbon dioxide to hydrocarbons and oxygenates, and an iridium oxide nanotube anode, that oxidizes the water and produces oxygen.

“The nice feature of the nanocoral is that, like plants, it can make the target products over a wide range of conditions and it is very stable,” said Ager.

The materials were characterized at the National Center for Electron Microscopy at the Molecular Foundry, a DOE Office of Science User Facility at Berkeley Lab. The results were useful in making the Researchers understand how the metals functioned in the bimetallic cathode. Explicitly, they learned that silver helps in the reduction of carbon dioxide to carbon monoxide, while the copper picks up from there to lower carbon monoxide further to alcohols and hydrocarbons.

Seeking better, low-energy breakups

As carbon dioxide is a stubbornly stable molecule, usually a significant input of energy is required to break it up.

“Reducing CO₂ to a hydrocarbon end product like ethanol or ethylene can take up to 5 volts, start to finish,” said Study Lead Author Gurudayal, Postdoctoral Fellow at Berkeley Lab. “Our system reduced that by half while maintaining the selectivity of products.”

Remarkably, the electrodes worked 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,” said Gurudayal. “It is very difficult to find an anode that works in neutral conditions.”

The Investigators personalized the anode by growing the iridium oxide nanotubes on a zinc oxide surface to produce a more uniform surface area in order to support the chemical reactions even better.

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.

Frances Houle, Berkeley Lab Chemist and JCAP Deputy Director for Science and Research Integration

Other Co-authors on the research are James Bullock, a Berkeley Lab Postdoctoral Researcher in Materials Sciences, who was active in engineering the photovoltaic and electrolysis cell pairing of the system. Bullock works in the lab of research Co-author Ali Javey, Berkeley Lab Senior Faculty Scientist and a UC Berkeley Professor of Electrical Engineering and Computer Sciences.

DOE Office of Science has supported this work.