Team captures CO2 with liquid gallium

Researchers in Australia and the United States have proven that gallium in its liquid form can economically capture and convert substantial quantities of carbon dioxide greenhouse gas emissions.

The process, reported in the Oct. 6 edition of Advanced Materials Journal, uses liquid gallium to convert carbon dioxide into oxygen and a high-value solid carbon product that can be sold to manufacturers of batteries, aircraft, or construction materials.

"We see very strong industrial applications with regards to decarbonization. This technology offers an unprecedent(ed) process for capturing and converting CO2 at an exceptionally competitive cost," Junma Tang, first author of the research paper, said.

Another critical element

Gallium is one of the three-dozen minerals recently deemed critical to national security by the United States due to a high reliance on China, which currently accounts for 97% of the world's gallium production, according to the U.S. Geological Survey.

The semiconductor metal has uses in telecommunications, renewable energy, high-tech, and military applications.

Gallium does not occur as a free element in nature but as gallium compounds in trace amounts in the ores for other metals, such as sphalerite (zinc) and bauxite (aluminum).

Silvery-blue in its solid form, gallium appears silvery-white when it melts at about 30 degrees Celsius (85.5 degrees Fahrenheit). It will melt in a person's hands at normal human body temperature.

Since its discovery in France in 1875, gallium has widely been used to make alloys with low melting points.

Imports of gallium metal and gallium-arsenide wafers continue to account for all U.S. consumption of gallium. In 2020, gallium imports decreased by an estimated 20% from 2019 levels, owing to higher import tariffs on gallium from China that began in 2019, according to the USGS. This followed a 300% increase in gallium metal imports from China in 2018 before the tariffs took effect. In 2019, gallium metal imports from China decreased by 97% from those of 2018.

The U.S. has produced no primary (low-purity, unrefined) gallium since 1987. However, globally, miners recover primary gallium as a byproduct of processing bauxite and zinc ores.

The USGS also reported that primary low-purity (99.99% pure) gallium prices in China increased by an estimated 32% in 2020, owing mostly to reduced production. The price for primary low-purity gallium in China increased to US$185 per kilogram in September 2020 from about US$140/kg at year's end 2019. China's primary low-purity gallium production capacity has averaged 600,000 kg per year since 2016, following an expansion from 140,000 kg/yr in 2010. China accounted for more than 80% of worldwide low-purity gallium capacity.

The remaining primary low-purity gallium producers outside of China have restricted output owing to a large surplus of primary gallium that began in 2012, the USGS said. These producers included Japan, the Republic of Korea, and Russia. Germany and Kazakhstan ceased primary production in 2016 and 2013, respectively. In addition, Hungary and Ukraine have ceased primary production in recent years, the survey said.

International development

A 24-member team of scientists, led by Professor Kourosh Kalantar-Zadeh at the University of New South Wales School of Chemical Engineering, and including researchers at the University of California, Los Angeles, North Carolina State University, Royal Melbourne Institute of Technology, University of Melbourne, Queensland University of Technology, and the Australian Synchrotron has discovered a potentially crucial new role for gallium.

This UNSW-led team of scientists demonstrated how low-melting-point metal can be used to efficiently break down carbon dioxide into its constituent parts via a process that dissolves captured CO2 gas into a solvent around the nanoparticles of gallium.

The reactor used for this process also contains nano-sized solid silver rods that are the key to generating the tribo-electrochemical reactions that take place when mechanical energy, such as stirring or mixing, occurs.

A tribo-electrochemical reaction occurs in solid-liquid interfaces due to friction between the two surfaces, with an electric field also created that sparks a chemical reaction.

The reactions break the carbon dioxide into oxygen gas and carbonaceous sheets, which float to the solution's surface due to differences in density. The carbon sheets then can easily be removed from the solution, according to the researchers.

"The nonpolar nature of the liquid gallium interface allows the solid products to instantaneously exfoliate, hence keeping active sites accessible. The solid co-contributor of silver-gallium rods ensure a cyclic sustainable process," wrote Tang.

The technology has the potential to significantly reduce the levels of greenhouse gases in the atmosphere in a variety of ways, according to the UNSW researchers.

They say the process can convert polluting exhaust gases from combustion engines in cars and trucks, and on a much larger scale, immediately capture and convert CO2 emissions from industrial sites.

"We have already scaled this system up to 2.5-liter dimensions, which can deal with around 0.1 liter of CO2 per minute. We've tested that (amount) running continuously for a whole month and the efficiency of the system did not degrade," Tang told reporters.

In the paper, the researchers show a 92% efficiency in converting a metric ton of CO2, using 230 kilowatts per hour of energy. They estimate that this equates to a cost of about $100 per metric ton of CO2.

Commercial applications

To commercialize the process, a startup company called LM Plus was created with the support of UNSW's Knowledge Exchange and seed investment from Uniseed. According to the researchers, Knowledge Exchange is a program that helps transform the university's research discoveries into successful innovations designed to benefit society.

UNSW also became a shareholder of the new company as part of a license agreement.

A "very green" process, the technology also produces a high-value carbonaceous sheet that can be used to make electrodes in batteries or carbon fiber materials in high-performance products like aircraft, racing cars, and luxury vehicles, according to LM Plus director and Uniseed investment manager Paul Butler.

"What we are working towards now is to raise funds to build a larger size proof-of-concept for this system to work within a 40-foot container – the size of a truck trailer – that could ultimately help industrial sites immediately capture any CO2 emissions and convert them," Butler said.

LM Plus hopes to be able to build the larger system within 15 months and is already in talks with potential commercial partners about other ways of implementing the new technology, he added.