Introduction

Electrochemical reduction of CO₂ has attracted researchers’ attention as it has the potential to utilize the abundant greenhouse gas in the Earth’s atmosphere and store intermittent energy from solar panels and wind turbines in chemical bonds. Many metals show activities in reducing CO₂ in aqueous phase. However, with the competition of the hydrogen evolution reaction, the selectivity is poor towards valued chemicals. Also, the high overpotential needed to drive the reactions causes low efficiencies that inhibits the practical applications.

CO₂ cycle

Many metals show CO2₂ reduction activity

Current Knowledge and Focus

By using Bismuth (Bi) as catalysts and ionic liquids (ILs) as electrolytes under electrochemical conditions:

• CO₂ was reduced to CO instead of formate.
• The CO₂ reduction overpotential was reduced.
• The selectivity was enhanced since the HER was compressed.

The underlying mechanism remains elusive due to the multi-step kinetics and the unclear interactions between ILs and key intermediates

CO₂ Reduction in ILs

• Exhibit an almost complete conversion of CO₂ to CO (96.1 % FE_CO), and a mass activity for CO evolution (MA_CO) of 15.6 mA mg−1 at -2.0 V vs Ag/AgCl (~ 0.3 V overpotential).

Reaction Pathways of CO₂ Reduction in ILs

Determining Energetics

• Total reaction from CO₂ to CO

• Using thermodynamic cycle to determine reaction barriers

Modeling Solid-Electrolyte Interfaces

Calculation Details

• VASP package
• PBE functionals
• 3 by 3 by 3 unit cell
• Monkhorst-Pack mesh of 4 by 4 by 1
• VASPsol hybrid solvation
• Constant potential calculation

Electrostatic potential plot

• Tuning the number of electrons in the system to change the Fermi level

Electrolyte-dependent Energy Pathways

• Compared with aqueous phase condition, free formation energies of key intermediates are lowered in non-aqueous phase, and CO is produced.

Conclusions

• Constant potential calculations can capture selectivity trends from aqueous phase and IL non-aqueous phase.
• By employing ILs as co-catalysts on Bi surfaces, thermodynamic barriers are reduced since the stabilization of *CO2 and *COOH by hydrogen bonding, and CO is obtained as the final product.

References

• Zhang, Zhiyong, Miaofang Chi, Gabriel M. Veith, Pengfei Zhang, Daniel A. Lutterman, Joel Rosenthal, Steven H. Overbury, Sheng Dai, and Huiyuan Zhu. 2016. “Rational Design of Bi Nanoparticles for Efficient Electrochemical CO2 Reduction: The Elucidation of Size and Surface Condition Effects.” ACS Catalysis 6 (9): 6255–64.
• Han, Na, Yu Wang, Hui Yang, Jun Deng, Jinghua Wu, Yafei Li, and Yanguang Li. 2018. “Ultrathin Bismuth Nanosheets from in Situ Topotactic Transformation for Selective Electrocatalytic CO2 Reduction to Formate.” Nature Communications 9 (1): 1320.
• Urushihara, Makoto, Karen Chan, Chuan Shi, and Jens K. Nørskov. 2015. “Theoretical Study of EMIM+ Adsorption on Silver Electrode Surfaces.” Journal of Physical Chemistry C 119 (34): 20023–29.
• Pillai, Hemanth Somarajan, and Hongliang Xin. 2019. “New Insights into Electrochemical Ammonia Oxidation on Pt(100) from First Principles.” Industrial & Engineering Chemistry Research. https://doi.org/10.1021/acs.iecr.9b01471.

Tianyou Mou

Chemical Engineer and Data Scientist

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