From CO2 to Solid Carbon – Realizing Carbon Dioxide Removal with Liquid Metal-Based Electrocatalysis

The time window, in which global warming can be limited to the 2° target without large-scale carbon dioxide removal (CDR) narrows down quickly. All CDR methods require energy as an input, which translates to land requirements for energy harvesting and potential land-use conflicts.¹(Photo-)electrochemical methods for CDR aim to convert CO₂ to storage products that broaden potential storage reservoirs when compared to direct CO₂ injection.

Here, graphitic carbon from a process where CO2 is split into elemental C and O is a highly promising sink product, enabling straightforward, near-surface long-term storage. While natural photosynthesis can also produce solid, carbon rich products using solar energy, the artificial photosynthesis route promises solar-to-carbon conversion efficiencies at least one order of magnitude higher than natural photosynthesis, which accordingly translates to a significantly reduced land footprint for a given CDR target.² Yet, to realize these efficiencies and make the process scalable, the catalysts and the overall, solar-driven electrochemical process need to be developed.

In our work we aim to deconvolute the reaction mechanism of the GaInSn/Ce – system, which is a liquid metal at ambient conditions and allows electrocatalytic splitting of CO₂ to graphitic carbon.³ Herein, we identify CO as the main intermediate, as well as the addition of alkaline H₂O to the non-aqueous electrolyte solution as beneficial to the carbon yield, likely due to the formation of OH – terminated surface species like Ce(OH)₃. Furthermore, we show the importance of the liquid metal matrix not only as a co-catalyst for CO₂ reduction itself, but also as the reason for an interfacial restructuring and optimization process.⁴

This shows that the electrochemical conversion of CO₂ to graphite on the liquid-metal surface does not only provide stable long-term performance due it’s non-coking behavior but also promises an accessible way for further optimization of its catalytic activity and selectivity. Due to its stable intermediate state (CO) it even offers an alternative approach of breaking down the process of CO₂ and CO reduction in a cascaded process. While this study clearly shows the potential of electrocatalytic CO₂ splitting as a CDR technology, we also identify current bottlenecks on the way to large-scale, competitive implementation.

1: Adam, M., Kleinen, T., May, M.M., Rehfeld, K., Land conversions not climate effects are the dominant indirect consequence of sun-driven CO₂ capture, conversion, and sequestration, Environmental Research Letters, in print (2025)

2: May, M. M. and Rehfeld, K.: ESD Ideas: Photoelectrochemical carbon removal as negative emission technology, Earth System Dynamics, 10, 1–7, 2019

3: Esrafilzadeh, D., Zavabeti, A., Jalili, R. et al. Room temperature CO2 reduction to solid carbon species on liquid metals featuring atomically thin ceria interfaces. Nat Commun 10, 865 (2019)

4: Lörch, D., et al., From CO 2 to Solid Carbon: Reaction Mechanism, Active Species, and Conditioning the Ce-Alloyed GaInSn Catalyst, Journal of Physical Chemistry C 128 (49)