Features - September 2015

The Alchemy of Today: Electrochemical Carbon Conversion

Alchemy: the word is evocative of a rich history, equal parts magic, philosophy, and strange science.  For many of us, it conjures an image of ancient laboratories, with men, half wizard and half mad scientist, brooding over crucibles and alembics in pursuit of the Philosopher’s Stone―a relic that would unlock the secrets of the Elixir of Life and the transmutation of metals.

Gold has been valued in many societies throughout the ages. Now, gold nanoparticles can be used to catalyze electrochemical reactions.Gold has been valued in many societies throughout the ages. Now, gold nanoparticles can be used to catalyze electrochemical reactions.

Although the ancient pursuit of transforming base metals into precious alternatives has proved fruitless, alchemy has actually played an important role in the development of modern day science. Alchemy, though based more in philosophy and desire than in empirical process, helped establish the beginning of chemistry, encouraging the study of the composition, structure, and properties of matter, and developing many of the rudimentary apparatuses used in the infancy of the field.

But while alchemists never succeeded in discovering the secrets that they sought, and never mastered the conversion of lead into gold, modern science has unlocked discoveries just as precious―the conversion of carbon dioxide (CO2) into other, valuable chemicals.

The “magic” of our modern world relies on vast quantities of energy, and the demand keeps rising. But energy has to come from somewhere—roughly 80 percent of the total energy consumption of the world still comes from fossil fuels. And fossil fuels, when harnessed for energy production, are the largest contributing source of CO2 on the planet. But with an energy infrastructure that’s built on fossil fuels, the future of energy must rely on developing, not just sources of alternative energy, but ways to minimize the environmental impact of carbon producing power generation.

While carbon sequestration and storage is a burgeoning field, another exciting alternative for carbon control is the electrochemical conversion of CO2 into industrially valuable chemicals and fuels, like carbon monoxide (CO).  Doug Kauffman, a research scientist at the National Energy Technology Laboratory (NETL), has spent the last several years of his career researching and streamlining this process. “The idea behind electrochemical conversion is, essentially, that you apply electrical energy to a material so that it can convert CO2 into other useable chemicals. In our case we’ve been able to convert CO2 and water into carbon monoxide and hydrogen—which can then be converted into a variety of other products. Basically making industrial precursors that can be used in their base state, or further refined into other products, like methanol.”

To cause this conversion, Doug and the other members of his research team, have used a material that may have titillated their ancient, alchemist forbearers: gold (Au). The goal of electrochemical conversion is to break CO2’s molecular bonds to transform it more useful chemicals. The process involves exposing the CO2 to a catalyst, while powering the conversion through the application of an energy source. The catalyst being used by the NETL team is Au25, a very special form of gold.

A model of the structure of the Au25 gold nanoparticleA model of the structure of the Au25 gold nanoparticle.

“This form of gold is extremely active; it’s a special type of nanoparticle that only has twenty five gold atoms.” Doug explained, “And that arrangement of atoms gives it really specific chemical and structural properties, so it’s very reactive and it’s very stable. In this case, size definitely plays a role because the nanoparticle contains a large fraction of surface atoms that are accessible to incoming reactants =. And that number, 25 atoms, has special properties that also make it very resistant to degradation, so the catalyst can operate for extended periods of time without losing its chemical reactivity.”

The gold nanoparticle with 25 atoms falls into a category that chemists call a “magic number cluster,” possessing an atomic structure that proves exceptionally stable and durable. As a result, Au25 is an ideal choice for a catalyst and, better yet, the nanoparticle is highly effective at CO2 conversion. “Almost every electron that we put into the conversion system was used for chemistry.” Doug said, “It wasn’t wasting electrons through heat generation or another types of energy loss that can reduce a system’s efficiency. With a lot of catalysts―let’s say you put a hundred electrons into the system, maybe only fifty of them participate in the chemical reaction, the other half get lost somewhere. But this catalyst, every electron that we put into the system, every bit of electricity, was used in the chemical reaction to create products.”

But beyond the efficiency of the catalyst, many electrochemical conversion systems face another kind of challenge: powering the reaction. CO2 is a very stable molecule, and transforming it requires a lot of energy. If that energy comes from fossil fuel derived electricity then the CO2 conversion technology may produce more CO2 than it transforms. But the system being developed by Doug and his research team has developed a way to overcome that problem.

Solar cells are electrical devices that converts the energy of light directly into electricity via photovoltaic effect. Modern solar cells could power electrochemical carbon conversion.Solar cells are electrical devices that converts the energy of light directly into electricity via photovoltaic effect. Modern solar cells could power electrochemical carbon conversion.

“One of our most exciting results is that we show 'carbon-negative' CO2 conversion,” Doug said. “We do this by using renewable energy sources that don’t produce additional CO2 emissions. In our lab, we used consumer-grade solar cells from local hobby shops just to demonstrate the concept was possible. Data from our bench-scale reactor then allowed us to estimate the performance of much larger systems, and it should be possible to convert metric tonnes of CO2 per day using currently-available renewable energy sources. There are definitely some challenges associated with building a larger reactor, but our estimates suggest that current technology is absolutely sufficient to power large-scale CO2 conversion systems.”

Taking a technology from the laboratory bench to the commercial market is always difficult, and the NETL system hasn’t yet started to tackle the challenges of scale-up. However, Doug believes that the potential of the technology could lead to successful transfer. “This is a really promising field―there are a lot of talented people working in it right now. Of all the technologies that we’ve looked at, it looks like this is best suited for scaling up. We’ve showed that it works really well, and we’re trying to garner interest in scaling-up efforts. We’ll continue working on it, and other people will continue working on it, and we’ll come up with some really efficient and promising systems.”