Revealing the Intrinsic Restructuring of Bi2O3 Nanoparticles into Bi Nanosheets during Electrochemical CO2 Reduction

Abstract

Bismuth is a catalyst material that selectively produces formate during the electrochemical reduction of CO₂. While different synthesis strategies have been employed to create electrocatalysts with better performance, the restructuring of bismuth precatalysts during the reaction has also been previously reported. The mechanism behind the change has, however, remained unclear. Here, we show that Bi₂O₃ nanoparticles supported on Vulcan carbon intrinsically transform into stellated nanosheet aggregates upon exposure to an electrolyte. Liquid cell transmission electron microscopy observations first revealed the gradual restructuring of the nanoparticles into nanosheets in the presence of 0.1 M KHCO₃ without an applied potential. Our experiments also associated the restructuring with solubility of bismuth in the electrolyte. While the consequent agglomerates were stable under moderate negative potentials (-0.3 V_RHE), they dissolved over time at larger negative potentials (-0.4 and -0.5 V_RHE). Operando Raman spectra collected during the reaction showed that under an applied potential, the oxide particles reduced to metallic bismuth, thereby confirming the metal as the working phase for producing formate. These results inform us about the working morphology of these electrocatalysts and their formation and degradation mechanisms.

Keywords: bismuth oxide nanoparticles; carbon dioxide electroreduction; catalyst restructuring; in situ studies; liquid cell transmission electron microscopy; operando Raman spectroscopy.

Figure 1

(a) STEM images of the carbon-supported Bi₂O₃ NPs dispersed on the working electrode of an LC-TEM chip before the assembly at different magnifications, (b) LC-TEM images of the sample with only Milli-Q water flow at open-circuit potential illustrating nanosheet aggregation under this condition, and (c) LC-TEM images that were collected while the samples were in contact with 0.1 M KHCO₃ solution at open-circuit potential showing extended restructuring into nanosheets. The time stamps are in minutes:seconds.

Figure 2

(a) HR-TEM images of the carbon-supported Bi₂O₃ NPs (i,ii) before and (iii,iv) after contact with 1 M KHCO₃ solution and (b) EDX-TEM images of samples on the LC-TEM chip after the CO₂RR reaction in CO₂-saturated 0.1 M KHCO₃ during 1 h at different potentials where (i) is the STEM image and (ii) is the bismuth EDX map.

Figure 3

STEM image sequence collected as 1.0 M KHCO₃ solution was introduced into the LC-TEM cell at an open-circuit potential showing bismuth dissolution. The time stamps are in minutes:seconds.

Figure 4

(i) Initial and (ii) after 1 h LC-TEM images of the electrocatalysts at constant applied potentials of (a) −0.3, (b) −0.4, and (c) −0.5 V_RHE in CO₂-saturated 1 M KHCO₃ solution. The current jumps seen every 15–20 min are due to the pause in the chronoamperometry when the electrolyte-containing syringe is exchanged to maintain the electrolyte flow. At E = −0.5 V_RHE, the current density started to oscillate abruptly from about minute 40 onward. These irregularities in the current may be due to the formation of hydrogen microbubbles, which are not observed in the imaged area but may exist in other parts of the electrode. Red arrows highlight examples where nanosheet dissolution is seen.

Figure 5

Operando Raman spectra of the Bi/C samples collected with a λ = 785 nm laser (a) at different constant potentials and (b) at OCP and at −1.0 V_RHE over time. The spectra are offset in the y-direction for better visualization. (c) Schematic of the operando Raman spectrometer setup.