Operando Photo-Electrochemical Catalysts Synchrotron Studies

Abstract

The attempts to develop efficient methods of solar energy conversion into chemical fuel are ongoing amid climate changes associated with global warming. Photo-electrocatalytic (PEC) water splitting and CO₂ reduction reactions show high potential to tackle this challenge. However, the development of economically feasible solutions of PEC solar energy conversion requires novel efficient and stable earth-abundant nanostructured materials. The latter are hardly available without detailed understanding of the local atomic and electronic structure dynamics and mechanisms of the processes occurring during chemical reactions on the catalyst-electrolyte interface. This review considers recent efforts to study photo-electrocatalytic reactions using in situ and operando synchrotron spectroscopies. Particular attention is paid to the operando reaction mechanisms, which were established using X-ray Absorption (XAS) and X-ray Photoelectron (XPS) Spectroscopies. Operando cells that are needed to perform such experiments on synchrotron are covered. Classical and modern theoretical approaches to extract structural information from X-ray Absorption Near-Edge Structure (XANES) spectra are discussed.

Keywords: CO2 reduction; PEC cells; XANES; artificial intelligence; nanostructured materials; operando; photo-electrochemistry; synchrotron; water splitting.

Figure 1

Number of publications per year in accordance with “Photoelectrocatalyst” and “Photoelectrocatalyst + synchrotron” inquiries. The data were taken from Dimensions^® database. The data clearly demonstrate step-like growing interest of synchrotron application on PEC systems in the last 3 years.

Figure 2

(a) The scheme of the PEC setup; (b) FEXRAV (second cycle) of Ir at 11,221 eV in 1 M HK₂PO₄ kept in dark (dark blue, solid line) or illuminated with 410 nm LED (light blue, dashed line). Scan rate was set at 2 mV s⁻¹. The reported absorption is normalized to µ = 1 at an applied potential of 1 V_RHE; (c) Differential (light-dark) XANES spectra for different applied potential on α-Fe₂O₃/IrO_x photoanodes in in 1 M HK₂PO₄. Reproduced with permission from Minguzzi et al. (2017). Copyright Royal Society of Chemistry [62].

Figure 3

(a) Mo K-edge XANES spectra of the MoOx/Pt electrode at various potentials (0.1m KClO4, pH 1.8, 298 K) along with MoO₂ and MoO₃ references; (b) FT-EXAFS for MoOx/Pt at various potentials along with MoO2 (The open symbols represent experimental data, and the full lines indicate spherical wave theory). (c) Theoretical Mo K-edge XANES spectra of the dimeric and trimeric motifs in comparison with experimental XANES taken at −0.15 VRHE (d) Pt L3-edge HERFD-XANES spectra of MoOx/Pt on GC under potential control for electrolysis under O₂ saturation (0.1 m KClO4, pH 1.8, 298 K). The spectrum obtained from bare Pt is included for comparison. Reproduced with permission from Garcia-Esparza et al. (2017). Copyright John Wiley and Sons [65].

Figure 4

In situ NEXAFS data collected for NiB_i/BiVO₄/Au/Si₃N₄ photoanode tested under different conditions about Ni L_2,3 edges (a) and O K edge (b). The test sequence is OCP, 1.15 V_RHE, 1.45 V_RHE, 1.75 V_RHE, and 2.05 V_RHE, first in the dark and then under illumination. Reproduced with permission from Xi et al. (2019). Copyright American Chemical Society [83].

Figure 5

(a) XANES spectra, (b) k³-weighted k-space EXAFS spectra, and (c) Fourier-transformed (FT) EXAFS spectra in R-space of the samples. Red—CoO_x surface modified BiVO₄, blue—1 mol % Ni-doped CoO_x surface modified BiVO₄. (d) Schematic band diagram of the hole transport through the bulk n type BiVO₄ and p type cobalt-containing surface layer. Reproduced with permission from Liu et al. (2016). Copyright American Chemical Society [88].

Figure 6

(a–d) O K-edge STXM images and optical density images. (e–h) polarization-dependent O K-edge XANES spectra of bare ZnO NW and ZnO/Fe₂O₃ core–shell NW with E vector perpendicular or parallel to c-axis. Reproduced with permission from Lu et al. (2020). Copyright Elsevier [94].

Figure 7

(a) Schematic surface engineered ZnO NW. In situ Zn K-edge (b) and Fe L_2,3-edge (c) XAS collected in dark and illuminated conditions. Reproduced with permission from Lu et al. (2020). Copyright Elsevier [94].

Figure 8

Schematic (a) 2D and (b) 3D illustration of the cell composed by vacuum VAC and two He filled (He-1, He-2) chambers which allow simultaneous soft XAS detection in transmission and fluorescence mode. For PEC experiments LEDs source can be located instead of fluorescence GaAs detectors (see panel (b)) and 200 nm Al foil is placed in front of the transmission GaAs detector. Reproduced with permission from Schwanke et al. (2016). Copyright International Union of Crystallography [84].

Figure 9

Schematic design representation of the proposed spectroelectrochemical cells: (a) type A and (b) type B. Reproduced with permission from Achili et al. (2016). Copyright International Union of Crystallography [76].

Figure 10

Schematic representation of a pump-and-probe XAS experiment at synchrotron. Reproduced with permission from Baran et al. (2016). Copyright Elsevier [102].

Figure 11

(a) Fe L-edge and (b) Ti L-edge XAS of Ti-doped hematite nanostructured films under dark and light conditions. Reproduced with permission from Lin et al. (2020). Copyright Elsevier [106].

Figure 12

The list of approaches and available software for XANES simulations. Reproduced with permission from Guda et al. (2019). Copyright Elsevier [126].