Structural defects on converted bismuth oxide nanotubes enable highly active electrocatalysis of carbon dioxide reduction

Abstract

Formic acid (or formate) is suggested to be one of the most economically viable products from electrochemical carbon dioxide reduction. However, its commercial viability hinges on the development of highly active and selective electrocatalysts. Here we report that structural defects have a profound positive impact on the electrocatalytic performance of bismuth. Bismuth oxide double-walled nanotubes with fragmented surface are prepared as a template, and are cathodically converted to defective bismuth nanotubes. This converted electrocatalyst enables carbon dioxide reduction to formate with excellent activity, selectivity and stability. Most significantly, its current density reaches ~288 mA cm^-2 at -0.61 V versus reversible hydrogen electrode within a flow cell reactor under ambient conditions. Using density functional theory calculations, the excellent activity and selectivity are rationalized as the outcome of abundant defective bismuth sites that stabilize the *OCHO intermediate. Furthermore, this electrocatalyst is coupled with silicon photocathodes and achieves high-performance photoelectrochemical carbon dioxide reduction.

Fig. 1

Structural characterizations of bismuth oxide nanotubes. a XRD pattern, b SEM image, and c, d STEM-HAADF images of Bi₂O₃ NTs. e, f High-resolution images of the enclosed areas in (d). g–i Simultaneously acquired g STEM-BF image, h STEM-HAADF image, and i corresponding FFT pattern of a Bi₂O₃ nanotube (NT). j Overlay of the FFT filtered fringes with the original STEM-BF image in (g); the fringes in red are constructed by filtering the FFT spots highlighted in red in (i), and the green fringes are constructed from the green spots. k Schematic illustration of the structure of Bi₂O₃ NTs; the black spheres represent the crystalline inner walls, and the red and purple spheres represent the fragmented outer walls. Scale bar, 100 nm (b); 10 nm (c, d); 5 nm (e–g)

Fig. 2

Electrochemical carbon dioxide reduction measurements in the H-type cell. a Polarization curves of nanotube-derived Bi (NTD-Bi) in CO₂ or N₂-saturated electrolytes. b Chronoamperometric responses in CO₂-saturated electrolyte at different potentials as indicated. c Potential-dependent Faradaic efficiencies of HCOO⁻, CO, and H₂. d Formate partial current density derived from (b) and (c). e Long-term amperometric stability and corresponding selectivity change at −0.82 V. f Operando Bi L-edge XANES spectra of Bi₂O₃ nanotubes (NTs) at OCV and NTD-Bi at −0.24 V in comparison with Bi or Bi₂O₃ standards; inset plot is the partially enlarged spectra. g, h Fitting results of the operando EXAFS spectra to (g) R space and (h) K space

Fig. 3

Electrochemical carbon dioxide reduction measurements in the flow cell. a Schematic illustration of the flow cell configuration. b Polarization curves of nanotube-derived Bi (NTD-Bi) in 1 M KHCO₃ or 1 M KOH. c Chronoamperometric responses at a few different potentials in the two electrolytes. d Long-term amperometric stability in the two electrolytes. e Comparison of our results with previous data in terms of current density and Faradaic efficiency; corresponding reference numbers are included in brackets

Fig. 4

Theoretical calculations of reaction pathway on ideal and defective Bi(001) surfaces. a Optimized geometric structures of *OCHO adsorbed on ideal and three defective Bi(001) surfaces as indicated; the pink, gray, red, and green spheres represent Bi, C, O, and H atoms, respectively. b Free-energy profiles for formate production on ideal and defective surfaces. c Corresponding simulated CO₂RR polarization curves

Fig. 5

Photoelectrochemical carbon dioxide reduction on Si/Bi photocathode. a Schematic illustration of the structure of the Si/Bi photocathode and its working mechanism for PEC CO₂RR. b, c SEM images of Bi₂O₃ nanotube (NT)-loaded Si nanowire array photocathode. d Polarization curves of Si/Bi under dark or 0.5 sun illumination. e Potential-dependent Faradic efficiencies for HCOO⁻, CO, and H₂ from PEC. f Change of total current density and formate Faradaic efficiency at ‒0.4 V with time. Scale bar, 5 μm (b); 1 μm (c)