Incorporating Zinc to Stabilize Indium Sulfide for CO2 Reduction Reaction

By Laura ThomsonNov 3 2021

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.

Results

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 [001] 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 [001] 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.

Methodology

Material Synthesis

The chemicals Indium chloride tetrahydrate (InCl₃·4H₂O), thioacetamide (C₂H₅NS), and Zinc dichloride (ZnCl₂) were used. In₂S₃ and ZnIn₂S₄ were synthesized using the same.

Material Characterizations

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

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.

DFT Calculations

The DFT calculations were carried out by the Vienna ab initio simulation package (VASP) program with projector augmented wave (PAW) method.

Discussion

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.

Journal Reference:

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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