Inspired by earlier researchers, scientists have recently incorporated zinc (Zn) into indium sulfide (In2S3). They found that the synthesis allows this incorporation and enhances the stability of the resultant catalyst (ZnIn2S4). The research has been published in Nature Communications.
A fascinating means to decrease carbon dioxide (CO2) emissions and achieve carbon neutrality is the electrosynthesis of value-added fuels using CO2 as a feedstock. For the past 10 years, many active and selective catalysts have been studied for CO2 reduction reaction (CO2RR), and it is seen that CO and formate may be the only products that can achieve the industrialization trend of CO2RR in the near future.
Image Credit: J.M. Image Factory/Shutterstock.com
Previous studies revealed that metals such as mercury, lead, bismuth (Bi), indium (In), tin, and cadmium, can convert CO2 to formate, however, most of these metals suffer from unsatisfactory selectivity or issues with toxicity.
This article details the incorporation of zinc (Zn) into indium sulfide (In2S3) synthesis which helps tune its phase and structure, improving the long-term stability of the resultant catalyst (ZnIn2S4). The catalyst morphology remains almost unchanged.
Synthesis and Characterizations of Catalysts
Indium sulfide as a catalyst fascinated the researchers as S-doped In was demonstrated to be effective in catalyzing CO2RR to formate. The presence of S allows facile activation of H2O to form adsorbed H*, which reacts with absorbed CO2 to give HCOO* intermediates.
Results from earlier studies encouraged scientists to analyze the capability of indium sulfide instead of S-doped In in mediating CO2 to formate. Indium sulfide was produced hydrothermally by the reaction of InCl3·4H2O and C2H5NS in deionized water.
Figure 1. Physical characterization of ZnIn2S4. a, b SEM images of the ZnIn2S4 catalyst. The right panel in b shows the crystal structure of ZnIn2S4. Scale bars, 5 μm (a) and 1 μm (b). c STEM-EDX elemental mapping of ZnIn2S4, exhibiting a uniform spatial distribution of Zn (red), In (green), and S (yellow), respectively. Scale bar, 1 μm. d, e Atomic-resolution Z-contrast images of ZnIn2S4 along  zone axis. Scale bars, 1 nm (d) and 0.5 nm (e). f The corresponding FFT pattern of (d). g The line intensity profile acquired along the yellow arrow in (d). h Atomic model of ZnIn2S4 along  zone axis. i–k XRD patterns (i), UPS spectra (j), and BET surface area analysis (k) of ZnIn2S4 and In2S3, respectively. Image Credit: Chi, et al., 2021.
Zn was incorporated into indium sulfide to enhance the stability of high-rate CO2RR. After synthesis, hexagonal-structured ZnIn2S4 (Figure 1i) microflowers were obtained. The thickness of the nanosheets obtained was ~8.69 nm for ZnIn2S4 and 9.32 nm for In2S3 as revealed through atomic force microscopy (AFM) measurements.
Energy-dispersive X-ray (EDX) spectrum elemental mapping showed a uniform spatial distribution of Zn, In, and S (Figure 1c).
High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was employed to analyze the detailed atomic structure of the ZnIn2S4. The fast Fourier transform (FFT) (Figure 1f) results show that Zn incorporation alters the coordination environment of indium sulfide and favors the electronic structure and catalytic properties.
Ultraviolet photoelectron spectroscopy (UPS) (Figure 1j) revealed a superior electronic property by the incorporation of the Zn element.
CO2RR Performances in a Flow Cell
The CO2RR properties of ZnIn2S4 and In2S3 catalysts were analyzed in a flow cell. Figure 2a shows the linear sweep voltammetry curves and Figure 2b depicts the Faradaic efficiency (FE) for formate.
Figure 2. CO2RR performances. a, b The linear sweep voltammetry curves (a) and potential-dependent Faradaic efficiencies for products (b) on ZnIn2S4 and In2S3. c, d Partial current density (c) and half-cell PCE (d) for CO2-to-formate conversion on ZnIn2S4 and In2S3. e, f Comparison of formate partial current densities and FEs (e), and formate production rates (f) for various catalysts reported under KHCO3 environments. g Stability test of the ZnIn2S4 and In2S3 at 300 mA cm−2. The electrolyte was occasionally replaced by new 1 M KHCO3 solution (red arrows) to recover the ionic concentration and conductivity of the anolyte. The error bars represent the standard deviation of three independent measurements. Image Credit: Chi, et al., 2021.
Density functional theory (DFT) was employed to get insights into the CO2RR properties of the catalyst. The results showed that the S sites of ZnIn2S4 allow much smaller hydrogen adsorption free energy of 370 meV.
Comprehensive Stability Study
Figure 2g depicts the major finding that the CO2RR stability of indium sulfide can be remarkably improved by the incorporation of Zn.
Figure 3 depicts the multiple characterization techniques employed to analyze the structural evolution of ZnIn2S4 and In2S3 catalysts during CO2 electrolysis.
Figure 3. Structural stability of ZnIn2S4. a, b XRD patterns of ZnIn2S4 (a) and In2S3 (b) after CO2 electrolysis under various current densities for 10 min. c Corresponding SEM images of ZnIn2S4 (above) and In2S3 (bottom). Scale bars, 1 μm (above) and 500 nm (bottom). d–g Raman spectra of ZnIn2S4 (d) and In2S3 (e), and S 2p XPS spectra of ZnIn2S4 (f) and In2S3 (g) after CO2 electrolysis for various times at 300 mA cm−2. h STEM-EDX elemental mappings of ZnIn2S4 (scale bar: 1 μm) and In2S3 (scale bar: 600 nm) after running for 60 h and 8 h at 300 mA cm−2, respectively. i SEM-EDX measurements of the remained sulfur in catalysts after running for various times at 300 mA cm−2. The error bars represent the standard deviation of three independent measurements. j TEM (above, scale bars: 50 nm) and SAED patterns (down, scale bars: 5 1/nm) of ZnIn2S4 catalyst after CO2 electrolysis for various times at 300 mA cm−2. Image Credit: Chi, et al., 2021.
The amount of S left-back in ZnIn2S4 and In2S3 were quantified with SEM-EDX (Figure 3i) and it was noted that ZnS catalyst performed stably at high current densities. This is because of the strong interaction between Zn and S.
Stability Enhancement Mechanism
The obtained results indicate that the stability degradation of In2S3 is attributed to S leaching and that Zn as a stabilizer hinders S leaching.
Figure 4. Enhanced covalency in ZnIn2S4. a, b Differential charge density (a) and projection on the (110) plane (b). c ELF of ZnIn2S4. d, e Differential charge density (d) and projection on the (011) plane (e). f ELF of In2S3. The azure and yellow clouds represent electron density depressions and accumulations, respectively. g–i COHPs for In−S bonding (g) and Zn−S bonding (h) of ZnIn2S4, as well as In−S bonding (i) of In2S3. Image Credit: Chi, et al., 2021.
The calculated results show that the bond breaking between In(Zn) and S in ZnIn2S4 is kinetically cumbersome elaborating the negligible S dissolution and thus exceptional long-term stability of the ZnIn2S4 catalyst.
The chemicals Indium chloride tetrahydrate (InCl3·4H2O), thioacetamide (C2H5NS), and Zinc dichloride (ZnCl2) were used. In2S3 and ZnIn2S4 were synthesized using the same.
Characterization of the synthesized materials was carried out using XRD, STEM, HRTEM, SAED, and EDX elemental mapping.
Preparation of CO2RR Electrodes
The catalyst ink was produced by ultrasonic dispersion and the resultant ink was spread uniformly on the gas diffusion layer providing the prepared electrode with a catalyst loading of ~1.0 mg cm−2.
Electrochemical measurements were carried out in a flow cell and gaseous CO2 (99.999%) was passed through the gas chamber. The CO2 electrolysis lasted for 10 minutes.
CO2RR Products Analysis
The gas product analysis was carried out with gas chromatography equipped with a thermal conductivity detector (TCD) to quantify H2 concentration and a flame ionization detector (FID) to analyze the CO content.
Quantification of formate products was carried out with the help of 1H NMR spectra measured with a Bruker 400 MHz spectrometer.
The DFT calculations were carried out by the Vienna ab initio simulation package (VASP) program with projector augmented wave (PAW) method.
The article depicts a long-term formate electrosynthesis from CO2. The increased catalyst stability is due to the increase of In−S covalency, which hinders sulfur dissolution during CO2RR. The CO2-to-formate conversion was rapid and selective. The observations are anticipated to promote the creation of effective catalysts for commercial-scale electrosynthesis of formate.
Chi, L.-P., Niu, Z.-Z., Zhang, X.-L., Yang, P.-P., Liao, J., Gao, F.-Y., Wu, Z.-Z., Tang, K.-B., Gao, M.-R. (2021) Stabilizing indium sulfide for CO2 electroreduction to formate at high rate by zinc incorporation. Nature Communications, 12(5835). Available at: doi.org/10.1038/s41467-021-26124-y.
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