Carbon capture in mineral form
Khai Trung Le talks to Alison Parkin and Michael North about a new form of carbon capture that may not only help capture more than 820 million tonnes of CO2 but also produce hydrogen in a low-energy process.
A proposed carbon capture system uses seawater, recycled aluminium and renewable electricity to transform captured CO2 into a safe mineral, in this instance dawsonite.
Researchers from the University of York, UK, propose that this could help capture more than 820 million tonnes of unwanted CO2 from the atmosphere, based on current annual production and recycling rates.
The capturing system, described in the paper, Capacitance-Assisted Sustainable Electrochemical Carbon Dioxide Mineralisation, published in ChemSusChem, uses an electrochemical cell comprising a dual-component graphite and aluminium anode, a hydrogen-producing cathode and an aqueous sodium chloride electrolyte. The graphite acts as a supercapacitive reagent concentrator, pumping CO2 from the atmosphere into the seawater as hydrogen carbonate. The anodes generate cations as they oxidise, which react with the hydrogen carbonate to produce mineralised CO2.
Michael North, Professor of Chemistry at the University of York, told Materials World, ‘What’s unique about the process is the use of the anode derived from not one material but two, and using, for the first time, graphite, which has interesting properties including its ability to act as a supercapacitor. This allows the anode to concentrate CO2 around its surface and convert it with the aluminium, which acts as a sacrificial anode.’
The paper notes that any Earth crust-abundant metal would be appropriate, with the team conducting a ‘small amount’ of experiments with iron. North said, ‘We found you get more efficient CO2 capture into iron, but its different oxidation states complicate the electrochemistry.’ Aluminium was chosen as the most abundant metal in the Earth’s crust, with the hope of promoting further recycling of it.
Additionally, the cell produces hydrogen as a by-product, further differing the York method from other CO2 electrochemistry processes. North explained, ‘They reduce CO2 to something like methanol or methane because they’re useful fuels, but the process needs vast amounts of hydrogen. We’re unique in going the other way, generating hydrogen. It can, in principle, be separated and used in energy production, used in hydrogen fuel cells or fed into other CO2 electrochemical processes.’
God’s own country
The cell is a first-generation model designed for flexibility and proof of concept and created with the Yorkshire environment specifically in mind. Dr Alison Parkin, Research Lecturer at the University of York, noted that the seawater was collected off the coast of Whitby, UK, waste foil aluminium was locally acquired and the solar energy was generated from a north-facing window, rather than east.
While the mineralisation process is far from optimised, North was determined to begin exploring it with a CO2 level that mimics typically available waste. ‘We’re not starting with 100% CO2. We decided from the outset that we wanted to use a gas that was 95% nitrogen and 5% CO2. Obviously what comes out of a cement factory or power station is a very dilute stream, and we’ve demonstrated we can use that directly.’ Proving the concept without using peak efficiency was important for the team, and North added that the entire process was open for optimisation. ‘Now we are getting an understanding of the system, we can look into new cell designs, energy efficiencies and even seawater from different places.’
Other priorities of optimisation include the speed of mineralisation. Parkin said, ‘At the moment, we think it is limited by the current passing through – we’ve tested 10mA running through the system, and have experiments looking into how much faster we can go by putting higher currents or apply current to large-scale units.’ Increasing the surface area of the electrode may also help speed up the reaction. Regardless, North stressed that while CO2 mineralisation typically occurs in a timescale of thousands of years, the York process completes within 24–48 hours.
The team describe the electrochemical cell as being at technology readiness level 3, having experimental proof of concept, and are looking for commercial partners to support further research.
To read Capacitance-Assisted Sustainable Electrochemical Carbon Dioxide Mineralisation, visit bit.ly/2mEjwd