Electroreductionof greenhouse gas CO2 into valuable chemical feedstocksdriven by renewable electricity is a mild and controllable manner to simultaneously mitigate the greenhouse effect and store the intermittent renewable energy sources. However, the commercial application of the electrochemical CO2 reduction (CO2RR) is restricted by the high activation energy of activate thermodynamically stable CO2 molecule, low selectivity of products, and the competition of the hydrogen evolution reaction (HER) in aqueous solutions. Therefore, it is imperative to rational design of highly-efficient electrocatalysts with high activity and selectivity for theCO2RR. Bismuth (Bi), as a promising nontoxic metal electrocatalyst, has been proven to enhance HCOOH selectivity and suppress the HER in the CO2RR process. These Bi-based electrocatalysts are often derived from ionic compound precursorsto form special Bi nanostructures. However, there are still several disputes about whether the Bi0 or Bi3+ should be the active centre. On the other hand, the structure transformation mechanisms of Bi-based compounds during the CO2RR are still lack of detailed exposition. Furthermore, no credible in-situ Raman evidence of intermediates during the CO2RR has been provided in bismuth-related nanostructure systems.
Figure 1. Preparation and morphology characterization of Bi@Bi2O3-NDs and Bi-NFs.
Recently, Professor Zhong Jin’s group reported the in-situ structure refactoring fromBi@Bi2O3 nanodendrites (Bi@Bi2O3-NDs) to Bi nanoflowers (Bi-NFs) (Figure 1) with a greatly improved Faradaic efficiency of 92.3% for formate production. Moreover, this work revealed a two-step in-situ reconstruction process of the Bi-based catalyst in electrocatalysis with Bi2O2CO3 as the intermediate. Furthermore, through in-situ Raman spectroscopy, the selective and stable adsorption of the *OCHO intermediate was confirmed on Bi0 active centre, which is a key factor to achieve high selectivity.
Figure 2. Step-by-step exploration of the in-situ reconstruction process from Bi@ Bi2O3-NDs to Bi-NFs.
During CO2RR in KHCO3 solution, Bi-based catalysts often undergo the in-situreconstruction, but the specific mechanism is still unclear. To explore this process, Professor Zhong Jin’s groupelectroreduced Bi@Bi2O3-NDs in different solutions, and found that reconstruction only occurred in KHCO3 solutions, with no obvious surface morphology change in KCl and KOH solutions. Moreover, the Raman signal intensity of Bi2O3only decreased obviously after reduction in KHCO3 solution (Figure 2a). These results prove that HCO3- plays a significant role in the in-situreconstruction process. To explore the concrete function of HCO3-, a control sample was produced by vertically immerging the Bi@Bi2O3-NDs into 4 mL of 0.1 M KHCO3 solution for 2 h without any other disturbances. SEM images showed that a large number of smaller nanosheets formed on the surface of nanodendrites (Figure 2b and 2c).
Figure 3. XPS spectra, XRD patterns and wettability of Cu foil substrate, Bi@Bi2O3-NDs, Bi-NFs and intermediate Bi2O2CO3.
Based on EDX, XPS and XRD results, the composition of these nanosheets was finally determined to be Bi2O2CO3 (Figure 2 and Figure 3). Accrodingly, the in-situ electroreduction process can be divided into two steps. At first, the Bi2O3 layer on Bi@Bi2O3-NDs react with HCO3- to form Bi2O2CO3 nanosheets. The Bi2O2CO3 was then reduced under a negative potential to form active Bi0. The active Bi0 was not stable and could be suddenly re-oxidized in air according to the XPS spectra (Figure 3a). After several cycles of the reactions above, the Bi@Bi2O3-NDs fully transformed to Bi-NFs.
Figure 4. Electrochemical properties of Bi-NFs and other comparison samples.
By comparing the electrochemical properties of bare copper foil, Bi@Bi2O3-NDs and Bi-NFs, it can be determined that Bi-NFs have lower impedance, larger electrochemical active area and more active sites after in-situ reconstruction, thus greatly improving the selectivity of electrocatalytic CO2RR to formate (Figure 4).
Figure 5. Stability tests and in-situ Raman intermediate characterization of Bi-NFs.
The catalytic stability of Bi-NFs electrocatalyst was evaluated by long-term CO2RR tests (Figure 5a). The Bi@Bi2O3-NDs were prepared as the precursor to in-situ form Bi-NFs in the first cycle of 4 h. As shown in Figure 5a, at first, the Bi@Bi2O3-NDs needed to be reduced to remove the Bi2O3 layer and the transform to Bi-NFs, so the FEformate was only 78.6% in the activation cycle. After complete transformation to Bi-NFs for 4 h, the FEformate remained at approximately 92% in the next three cycles. Meanwhile, the nanoflower structure of Bi-NFs could be well maintained after the catalytic stability test (Figure 5b). In addition, through in-situ Raman spectroscopic measurements, it was clear that the stabilization of the *OCHO intermediate played a significant role in CO2 reduction (Figure 5c). In conclusion, extensive surface reconstruction of Bi@Bi2O3-NDs rendered the realization of tailored Bi-NFs electrocatalyst that maximized the number of exposed active sites and active component (Bi0 species), which is beneficial for the CO2RR towards HCOOH product, owing to the preferential selective adsorption and stabilization of *OCHO intermediates.
This work was published in Advanced Functional Materials with the title “In Situ Structure Refactoring of Bismuth Nanoflowers for Highly Selective Electrochemical Reduction of CO2 to Formate” (https://doi.org/10.1002/adfm.202301984). The first authors of the paper are the Ph.D. candidates, Songyuan Yang and Minghang Jiang, from Nanjing University. This work is supported by the National Key R&D Program of China, the National Natural Science Foundation of China, the Fundamental Research Funds for the Central Universities of China, the Scientific and Technological Innovation Special Fund for Carbon Peak and Carbon Neutrality of Jiangsu Province, the Nanjing International Collaboration Research Program, the Suzhou Gusu Leading Talent Program of Science and Technology Innovation and Entrepreneurship in Wujiang District, and Natural Science Foundation of Sichuan Province.