The electrochemically-driven approach for converting CO2 into fuels and other chemicals has provided an attractive means of capturing the excess carbon in the atmosphere and also providing an alternative source of storage of the renewable energy. Nonetheless, it remains a challenge to maintain catalyst stability and yet achieve high selectivity for the desired products. A recent work published in Energy & Environmental Science sheds light on how one can be able to influence selectivity using pulsed electrochemistry on bimetallic Cu-Zn catalysts.
"climate change mitigation"
Electrochemical Reduction of CO2: Plus Points and Troubling Issues
Taking serious steps and measures to combat greenhouse gas emissions is vital for the survival of the planet, and turning CO2 into fuels and chemicals is one way of achieving this while producing asked for and more useful products in the process. A space characteristic of copper catalysts has been proven to produce a variety of carbon products like ethylene and ethanol. However, poor selectivity and stability performance have seemed common.
Tandoh reported the performance enhancement of Cu catalysts by the incorporation of metal additives like zinc. The understanding of the Cu-Zn system is of practical relevance as the materials selected fall within the low-cost range and also are reasonably abundant. However, especially when it comes to pharmaceutical industry effective catalytic reactions, certain properties of metal ions in organic solvents are still somewhat mysterious for researchers in practice.
A New Approach: Pulsed Electrochemistry
Scientists at the Fritz Haber Institute in Berlin, leading a research team, went about the study and optimization of Cu-Zn catalysts in a different way. Instead of applying a potential that would remain constant, they adapted pulsed electrochemistry to the application – a CO2 reduction potential immediately followed by a more ‘anodic’ positive potential and tension related to working adsorbates and reagents.
Such a pulsed approach can also result in changing the oxidation state and the surface structure of the catalytic material. What’s more, this mode of operation may have much more pronounced product selectivity than the corresponding static conditions.
"CO2 TO BIOFUEL"
Cutting-Edge Characterization Techniques
In order to understand the sequence of processes taking place during the pulsed operation, the pulse structure was accompanied by a number of sophisticated methods of investigation:
- Operando X-ray absorption spectroscopy (XAS) for tracking the oxidation state and local atomic structure changes.
- High-energy X-ray diffraction (HE-XRD) for control of the crystalline phases’ texture.
- Surface-enhanced Raman spectroscopy (SERS) for the identification of the adsorbed species on the catalyst surface.
Most importantly, these experiments were performed with sub-second time resolution, enabling them to monitor the movement of elements within the current pulses.
Key Findings: Tuning Selectivity Through Pulsing
The study brought up a number of several important observations: The anodic pulse potential could be altered to the point of dramatically adjusting product selectivity. For instance, a thoughtful anodic potential at 0 V vs RHE significantly increased ethanol selectivity in comparison to static conditions.
- The increase in selectivity for ethanol was directly related to the production of the ZnO species and the adsorption of hydroxide ions in the course of the pulsing.
- At more positive anodic potentials, preference was towards hydrogen and CO indicating an over-oxidation and restructuring of the catalyst.
- Complex machine learning analysis of XAS data revealed oscillating zinc environment (for example zinc oxides, alloys) formation during pulsing, which are very significant for the trends in selectivities.
Mechanistic Insights
The authors suggest that the pulsed strategy enables the Cu/Zn interface and oxide formation to be controlled accurately. The results at suitable conditions are:
- Development of zinc rich phases which are conducive for producing C2+ products
- Increased adsorption of hydroxides which favors C-C coupling
- Decreasing the parasitic hydrogen evolution reaction
It is noteworthy that the pulsed technique has also enhanced the efficiency of the catalyst as an alternative to static high potential process on pure copper catalysts.
Outlook and Future Directions
This work illustrates of tuning CO2 reduction selectivity by pulsed electrochemistry. The mechanistic knowledge attained from the detailed operando characterization should be useful in improving the design of the catalysts.
Future work could explore more:
- Improving the selectivity even further by optimizing pulse parameters (timing, waveform) for instance
- Applying the concept to other bimetallic systems
- Increasing the application of the pulsed method
Conclusion
By using pulsed electrochemistry and advanced operando analysis together, this study advanced our knowledge of Cu-Zn based catalysts for CO2 reduction. The possibility of real-time modification of the structure and the surface of the catalyst introduces new prospects for the
development of this promising technology. As we look to make advances in energy storage and generating chemicals, it will be important to have such developments as the design of the next generation of CO2 electroreduction catalysts.
