1 3
Topics in Current Chemistry (2018) 376:41
3 Challenges and Opportunities of Bimetallic Electrocatalyts for CO 2
Reduction
The literature review described has demonstrated the great potential of bimetallic materials for CO 2 RR. Many bimetallic catalysts have been designed to enhance
CO, hydrocarbon and oxygenate yields. Even though improvement in the selectivity, FE and activity was shown, the catalytic mechanisms have not been thoroughly
revealed. A typical commercial catalyst would require an overpotential of less than
300 mV with a significant current density of higher than 10 mA/cm
2
. The FE should
be larger than 80% [71]. Therefore, the practical application of bimetallic materials for CO 2 RR still faces several challenges: (1) high energy barrier for CO 2 activation, (2) mass transfer limitation for CO 2 gas to catalytic surfaces, (3) the sluggish
kinetics, (4) high separation costs due to a wide range of products, (5) active sites
poisoning from intermediates and impurities, and (6) low reduction selectivity for
multi-carbon species. In most cases it remains unclear the exact reaction pathways
for product formation, especially for the C–C coupling.
One important consideration for designing improved bimetallic CO 2 RR catalysts
is to tune the intermediate binding energy, which can be achieved by selecting the
appropriate bimetallic components with guidance from DFT calculations. Core/shell
structure is a viable way to control the surface strain, with the extent of the strain
being controlled by shell thickness. Designing catalysts with grain boundaries and
surface defects could also enhance CO 2 RR selectivity and activity.
In general, the final CO 2 RR products are controlled by the reaction intermediates. For example, if the target product is CO, a bimetallic catalyst with relatively
low CO binding energy would be preferred in most cases. If methanol or methane is
the target product, the tuned binding energy of CHO* and COH* is critical for high
selectivity.
Enhanced reactant coverage should result in higher CO 2 RR yields. Increasing CO 2 partial pressure near the catalyst surface can increase the activity for CO 2
reduction. Using organic solvents and ionic liquids can also help overcome the low
solubility of CO 2 in water-based electrolytes. Many CO 2 reduction catalysts tested in
organic solvent or with ionic liquid show an increased performance.
However, to successfully evaluate the true active sites and mechanisms of reactivity, catalyst samples must be carefully characterized using a variety of techniques.
The surface and bulk compositions, structure and morphology before and after the
CO 2 RR should be examined. The identification of intermediates formed upon activation and reduction of CO 2 is essential to understanding the reaction mechanism.
In-situ characterization methods such as infrared spectroscopy, ultraviolet–visible
spectroscopy and Raman spectroscopy, as well as synchrotron-based techniques such
as XRD and EXAFS, should be more extensively employed to understand effects of
applied voltage on catalyst morphological changes and reaction mechanisms.
Acknowledgements Authors from Columbia University are partially supported by the US Department
of Energy, Catalysis Program (DE-FG02-13ER16381). Authors at University of Delaware thank the
financial support from the Department of Energy under Award Number DE-FE0029868. The authors
also thank the National Science Foundation Faculty Early Career Development program (Award No.
CBET-1350911).
Reprinted from the journal
121
Topics in Current Chemistry (2018) 376:41
3 Challenges and Opportunities of Bimetallic Electrocatalyts for CO 2
Reduction
The literature review described has demonstrated the great potential of bimetallic materials for CO 2 RR. Many bimetallic catalysts have been designed to enhance
CO, hydrocarbon and oxygenate yields. Even though improvement in the selectivity, FE and activity was shown, the catalytic mechanisms have not been thoroughly
revealed. A typical commercial catalyst would require an overpotential of less than
300 mV with a significant current density of higher than 10 mA/cm
2
. The FE should
be larger than 80% [71]. Therefore, the practical application of bimetallic materials for CO 2 RR still faces several challenges: (1) high energy barrier for CO 2 activation, (2) mass transfer limitation for CO 2 gas to catalytic surfaces, (3) the sluggish
kinetics, (4) high separation costs due to a wide range of products, (5) active sites
poisoning from intermediates and impurities, and (6) low reduction selectivity for
multi-carbon species. In most cases it remains unclear the exact reaction pathways
for product formation, especially for the C–C coupling.
One important consideration for designing improved bimetallic CO 2 RR catalysts
is to tune the intermediate binding energy, which can be achieved by selecting the
appropriate bimetallic components with guidance from DFT calculations. Core/shell
structure is a viable way to control the surface strain, with the extent of the strain
being controlled by shell thickness. Designing catalysts with grain boundaries and
surface defects could also enhance CO 2 RR selectivity and activity.
In general, the final CO 2 RR products are controlled by the reaction intermediates. For example, if the target product is CO, a bimetallic catalyst with relatively
low CO binding energy would be preferred in most cases. If methanol or methane is
the target product, the tuned binding energy of CHO* and COH* is critical for high
selectivity.
Enhanced reactant coverage should result in higher CO 2 RR yields. Increasing CO 2 partial pressure near the catalyst surface can increase the activity for CO 2
reduction. Using organic solvents and ionic liquids can also help overcome the low
solubility of CO 2 in water-based electrolytes. Many CO 2 reduction catalysts tested in
organic solvent or with ionic liquid show an increased performance.
However, to successfully evaluate the true active sites and mechanisms of reactivity, catalyst samples must be carefully characterized using a variety of techniques.
The surface and bulk compositions, structure and morphology before and after the
CO 2 RR should be examined. The identification of intermediates formed upon activation and reduction of CO 2 is essential to understanding the reaction mechanism.
In-situ characterization methods such as infrared spectroscopy, ultraviolet–visible
spectroscopy and Raman spectroscopy, as well as synchrotron-based techniques such
as XRD and EXAFS, should be more extensively employed to understand effects of
applied voltage on catalyst morphological changes and reaction mechanisms.
Acknowledgements Authors from Columbia University are partially supported by the US Department
of Energy, Catalysis Program (DE-FG02-13ER16381). Authors at University of Delaware thank the
financial support from the Department of Energy under Award Number DE-FE0029868. The authors
also thank the National Science Foundation Faculty Early Career Development program (Award No.
CBET-1350911).
Reprinted from the journal
121