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Reporting period: 2020-07-01 to 2021-12-31

CO2 reduction reaction (CO2RR) holds great promise for conversion of the green-house gas carbon dioxide into chemical fuels. The absence of catalytic materials demonstrating high performance and high selectivity currently hampers practical demonstration. CO2RR is also limited by the low solubility of CO2 in the electrolyte solution and therefore electrocatalytic reactions in gas phase using gas diffusion electrodes would be preferred. 2D materials have recently emerged as a novel class of electrocatalytic materials thanks to their rich structures and electronic properties. The synthesis of novel 2D catalysts and their implementation into photocatalytic systems would be a major step towards the development of devices for storing solar energy in the form of chemical fuels.
With 2D-4-CO2, I propose to: 1) develop novel class of CO2RR catalysts based on conducting 2D nanosheets and 2) demonstrate photocatalytic conversion of CO2 into chemical fuels using structure engineered gas diffusion electrodes made of 2D conducting catalysts. To reach this goal, the first objective of 2D-4-CO2 is to provide guidelines for the development of novel cutting-edge 2D catalysts towards CO2 conversion into chemical fuel. This will be possible by using a multidisciplinary approach based on 2D materials engineering, advanced methods of characterization and novel designs of gas diffusion electrodes for the reduction of CO2 in gas phase. The second objective is to develop practical photocatalytic systems using van der Waals (vdW) heterostructures for the efficient conversion of CO2 into chemical fuels. vdW heterostructures will consist in rational designs of 2D materials and 2D-like materials deposited by atomic layer deposition in order to achieve highly efficient light conversion and prolonged stability. This project will not only enable a deeper understanding of the CO2RR but it will also provide practical strategies for large-scale application of CO2RR for solar fuel production.

The rapid growth of human population combined to the increase of the energy demand is a central challenge for the decades to come. Taking into account the fast increase of the CO2 concentration in the atmosphere and the search for renewable energy source, the conversion of CO2 to chemical fuels is regarded as a promising strategy to close the carbon cycle and store energy in the form of chemical fuels. One appealing strategy would involve the development of photocatalytic systems for converting CO2 into chemical fuels via the use of solar energy. Photosynthesis is an example of photochemical synthesis, in which CO2 and water are converted to glucose. Efficiency of natural photosynthesis is limited to < 2%. The possibility to convert CO2 into molecules including methane, ethylene or formate holds great potential for storing energy in the form of chemical fuels. The CO2 reduction reaction can generate large variety of added-values molecules, while operating at moderate temperature and pressure. This technology remains however strongly limited by the low activity of the catalytic materials at the surface of the electrodes and the lack of selectivity of the reaction. The development of improved electrocatalytic materials plays also a key role in optimizing the photocatalytic performance notably for reducing the overpotential at the photoelectrodes.
The success of 2D-4-CO2 could therefore open up practical avenues for the development of technology capable of converting CO2 simply by using solar energy. This would lead to industrial developments in Europe both for the realization of systems which perform the electrolysis of CO2 and the photocatalytic conversion of CO2 into valuable products.
Since the beginning of the project, intense efforts have been devoted to improve our understanding of the CO2 reduction reaction. To do this, we have designed and investigated novel materials with controlled facets and surface chemistry in order to explore the structure-performance relationship.
Our study can be divided in 3 main projects:
1) Enhancing the CO2-toTo-CO conversion from 2D silver nanoprisms via superstructure assembly (puACS Nano 2021, 15, 4, 7682–7693)
We identified the edge ribs of Ag quasi-2D nanoprisms as highly active sites for the reduction of CO2 to CO. By using a superstructure design strategy, we prepared self-assembled two-dimensional Ag nanoprisms for maximizing the exposure of active edge ribs. The control of the nanoprisms allow investigating the role of the edges idependently from the other sites available. In particular, we found that the edge ribes exhibit an overpotential of 150 mV compared to 550 mV for the basal plane with the (111) facets. The vertically stacked Ag nanoprisms allow exposure of > 95% of the edge sites, resulting in an enhanced selectivity and activity towards the production of CO from CO2. The Ag superstructures notably demonstrated a selectivity (Faradaic effiency) over 90% for 100 hours together with a ≈ 94% stability at -600 mV versus the reversible hydrogen electrode and a partial energy efficiency for CO production of 70.5%.
2) CO2 to CO conversion using Ni single-atoms supported on ultra-thin carbon nanosheets in a flow electrolyzer
Single atomic catalysts have emerging as next-generation catalysts for improving the performance, while decreasing the use of metal elements. In this context, we designed single atom nickel supported on two-dimensional nitrogen-doped carbon nanosheets for the electrochemical reduction of CO2. Guided by our computational predictions, we prepared and tested this catalyst candidate for the CO2-to-CO conversion reaction. We then explored the correlation between the applied potential and the feeds in electrolyte and CO2 using a ≈ 1 cm2 flow reactor as a model electrolyzer. The catalyst exhibited near-unity (100%) selectivity towards the production of CO with a specific current density as high as 170 mA cm-2. Importantly, our study revealed the influence of feed in electrolytes and CO2 on the catalytic performance in a flow electrolyzer system. We determined the optimum flow rates for the electrolyte and CO2 at 40-80 mL min-1 and 60 mL min-1 respectively. Our investigation have shown that the feed parameters modulate the catalyst behavior in the reactor by tuning the equilibrium of the triple-phase interface between the electrolyte, the CO2 atmosphere and the surface of the electrocatalyst.
3) Improved electrochemical conversion of CO2 to multicarbon products by using molecular doping
Copper (Cu) is one of the few transition metals that can efficiently catalyze the electrolysis of CO2 to multicarbon products (C2+) such as ethylene, ethanol, acetate, propanol8. Because multicarbon products possess higher market values and are more energy concentrated1, intensive efforts have been devoted to improve the reaction selectivity towards the production of C2 and C2+ molecules. We developped an effective method to control the surface oxidation state of bimetallic Ag-Cu electrodes by using functionalization. By combining Auger and X-ray absorption spectroscopies (XAS), we identified that the grafting of aromatic heterocyclic functional groups can efficiently dope the surface of Cu by withdrawing electrons from the metal surface leading to the formation of Cu+ species. Compared to pristine non-functionalized and alkyl functionalized electrodes, the modified electrodes display a clear improvement of the reaction rates and Faradaic efficiency towards the production of C2+ products. When assembled in a membrane electrode assembly flow electrolyzer, the catalyst delivers a selelectivy for C2+ products of 80 ± 1 % and a total C2+ energy efficiency (EE) of 20.3% for the full cell.
We list below the progress beyond the state of the art achieving within the project 2D-4-CO2:
- The direct identification and quantification of the catalytic activity of the edge ribs of Ag nanoprisms for the conversion of CO2 to CO
- The synthesis of the 100% selective catalyst for the conversion of CO2 to CO based on single atomic Ni supported on carbon nanosheets (patent deposited)
- The development of a facile and robust method to dope the surface of Cu electrode using aromatic heterocycles for the conversion of CO2 to ethylene and ethanol (patent deposited)

In future we aim at:
- Exploring further the modification of Cu surface with organic ligands to enhance the selectivity for multicarbon products
- Investigate the role of the CO2 concentration and pressure on the catalytic activity
- Develop photocathodes based on hybrid perovskite for the photocatalytic reduction of CO2
- Combine our best catalysts with the photocathode design to demonstrate direct photo-assisted reduction of CO2