Harmonizing the Electronic Structures of the Adsorbate and Catalysts

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Harmonizing the Electronic Structures of the Adsorbate and Catalysts for Efficient CO2 Reduction An Zhang, Yongxiang Liang, Huiping Li, Xinyu Zhao, Yuliang Chen, Boyan Zhang, Wenguang Zhu, and Jie Zeng Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02782 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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Revised ms #nl-2019-027823

Harmonizing the Electronic Structures of the Adsorbate and Catalysts for Efficient CO2 Reduction An Zhang,†,|| Yongxiang Liang,†,|| Huiping Li,§,|| Xinyu Zhao,† Yuliang Chen,† Boyan Zhang,† Wenguang Zhu,§ and Jie Zeng*,†

†Hefei

National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, National Synchrotron Radiation Laboratory, Key Laboratory of Surface and Interface Chemistry and Energy Catalysis of Anhui Higher Education Institutes, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China §International

Center for Quantum Design of Functional Materials, Hefei National Laboratory for

Physical Sciences at the Microscale, Synergetic Innovation Center of Quantum Information and Quantum Physics, Department of Physics, School of Physical Sciences, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China *To whom correspondence should be addressed. E-mail: [email protected] (JZ) ||These

authors contributed equally.

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Abstract In CO2 electroreduction, the critical bottleneck lies in the CO2 activation which requires high overpotentials. CO2 activation is related to both the electronic structures of catalysts and those of adsorbates, thus an ideal catalyst should match its electronic structures with those of the adsorbate. Here, we harmonized the electronic structures of the adsorbate and Mn-doped In2S3 nanosheets for efficient CO2 reduction. The introduction of Mn dopants into In2S3 nanosheets enhanced both the Faradaic efficiency (FE) for carbonaceous product and current density (j). At -0.9 V vs RHE, Mn-doped In2S3 nanosheets exhibited a remarkable FE of 92% for carbonaceous product at a high j of 20.1 mA cm-2. Mechanistic studies revealed that Mn doping enabled the harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms near the conduction band edge of Mn-doped In2S3 nanosheets during the activation of CO2. Due to the unique electronic structures of the co-adsorbed configurations, Mn-doped In2S3 nanosheets exhibited a lower energy barrier for CO2 activation into HCOO* compared with that over pristine In2S3 nanosheets. Keywords: CO2 electroreduction; CO2 activation; charge-transfer; harmonic electronic structures; orbital overlap Table of Contents

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The ever-increasing energy consumption and the shortage of fossil fuels propell the exploration of sustainable alternatives to long-term energy supply.1-4 Although renewable electricity from wind and solar energy is a promising strategy, the intermittent characteristics of these resources results in high cost for electricity storage.5-9 One appealing approach is the electroreduction of CO2 into value-added fuels which stores the renewable electricity and simultaneously alleviates the rising concentration of CO2. In CO2 electroreduction, the efficiency is largely limited by the high energy barrier of CO2 activation which requires a high overpotential of -1.9 V vs RHE.10-14 Tremendous efforts have been devoted to promoting CO2 activation, including elemental doping, creating surface defects, and engineering surface strain.15-17 For instance, the incorporation of Ni into the N-doped graphene matrix enabled the delocalization of electrons in Ni which induced the spontaneous electron transfer from Ni to the antibonding orbitals of CO2 molecules, thus boosting the activation of CO2.18 Besides, the oxygen vacancy on amorphous InOx nanoribbons was reported to regulate the electron density near the valence band edge, facilitating the electron transfer kinetics for CO2 reduction.19 Moreover, the tensile strain on the surface of Pd icosahedra shifted up the d-band center and thus strengthened the adsorption of CO2 intermediates.20 These approaches mostly focused on modifying the electronic structures of electrocatalysts to promote the activation of CO2. CO2 activation is not only related to the electronic structures of catalysts, but also associated with those of the adsorbates such as CO2, HCOO*, and other intermediates. A plain idea is to utilize the electronegativity of the catalysts and CO2 molecules. Specially, the electron-rich site favors the coordination with the electrophilic C atom in CO2, whereas the electron-deficient site can attract the nucleophilic O atoms or utilize the lone-pair electrons of C=O bonds.21-23 However, the electronic structures involve the band structure, the position of d-band center, the spin states of electrons and so on, which are far more complicated than electronegativity.24-27 Therefore, it is of pivotal importance and remains as a grand challenge to harmonize the electronic structures of the adsorbate and catalysts for efficient CO2 electroreduction. Herein, we harmonized the electronic structures of the adsorbate and atomically thin Mn-doped In2S3 nanosheets (denoted as Mn-In2S3 nanosheets) for efficient CO2 electroreduction. The current density and FE for carbonaceous product exhibited a dramatic increase through the introduction of Mn dopants into In2S3 nanosheets. At -0.9 V vs RHE, the Mn-In2S3 nanosheets achieved a remarkable FE of 92% for carbonaceous product with a current density of 20.1 mA


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cm-2. Mechanistic studies revealed that Mn doping enabled the harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms near the conduction band edge of Mn-In2S3 slab during the process of CO2 activation to HCOO* species. The newly formed states in the gap region are expected to facilitate the charge transfer between Mn-In2S3 catalysts and adsorbed HCOO* intermediates, thus promoting the formation of HCOO* on Mn-In2S3 slab. The atomically thin Mn-In2S3 nanosheets were prepared via a modified thermal decomposition method, as illustrated in Figure 1a. Specifically, InCl3, MnCl2, and Na-DDTC were mixed in methanol solution to get the precipitates, followed by the thermal decomposition in dodecylamine at 260 oC for 150 min. Pristine In2S3 nanosheets were also prepared for comparison through the similar procedure except for the addition of MnCl2. The transmission electron microscopy (TEM) image of Mn-In2S3 nanosheets shows the typical ultrathin nanosheet morphology (Fig. 1b). The thickness of Mn-In2S3 nanosheets was as thin as ~0.6 nm, approximating to 3 atomic layers of pristine In2S3 (Fig. S1).28 As revealed by the high-angle annular dark field scanning TEM (HAADF-STEM) image of Mn-In2S3 nanosheets, the interplanar spacings of two distinct lattice fringes were both 0.19 nm, which was assigned to (440) and (404) planes of β-In2S3 and equal to that of pristine In2S3 nanosheets (Figs. 1c and S2).29 As such, the doping of Mn induced negligible changes in the morphology and phase of In2S3 nanosheets. Figure 1d shows the HAADF-STEM images and corresponding energy dispersive X-ray (EDX) elemental mapping of a freestanding Mn-In2S3 nanosheet. Mn atoms were homogeneously distributed throughout the whole nanosheets. The existence of In, S, and Mn elements in the entire nanosheet was further confirmed by the line-scan profiles and EDX spectrum of Mn-In2S3 nanosheets (Figs. S3 and S4). In addition, the inductively coupled plasma atomic emission spectroscopy result suggests that the content of Mn in Mn-In2S3 nanosheets was 3.2% (Table S1). The structure of Mn-In2S3 nanosheets was determined using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements. As evidenced by XRD patterns (Fig. 1e), pristine In2S3 and Mn-In2S3 nanosheets exhibited similar diffraction peaks, with no extra peaks corresponding to other phases or impurities. The XPS survey spectra show the weak signals of Mn in Mn-In2S3 nanosheets (Fig. S5). The chemical state of Mn in Mn-In2S3 nanosheets was further analyzed by the Mn 2p XPS spectra (Fig. S6). The peaks at 641.5 eV and 646.0 eV were attributed to Mn 2p3/2 and the corresponding satellite peaks, respectively.30 The binding energy 4 ACS Paragon Plus Environment

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of Mn was much higher than that of the reported MnS, suggesting that Mn was likely to substitute In sites in Mn-In2S3 nanosheets. Furthermore, as illustrated by the In 3d and S 2p XPS spectra of pristine In2S3 and Mn-In2S3 nanosheets, the XPS peaks of In and S both showed a shift to lower binding energy upon Mn doping (Figs. S7 and S8). These results were due to the electron transfer from Mn to In and S atoms.31 The catalytic properties of pristine In2S3 and Mn-In2S3 nanosheets for CO2 electroreduction were tested in CO2-saturated 0.1 M KHCO3. As shown in Figure 2a, the geometrical current densities were enhanced with the introduction of Mn into In2S3 nanosheets. At an overpotential of -1.0 V vs RHE, Mn-In2S3 nanosheets exhibited a high current density of 27.2 mA cm-2, which was 1.4 times larger than that (19.8 mA cm-2) of pristine In2S3 nanosheets. For both pristine In2S3 and Mn-In2S3 nanosheets, CO, H2, and formate were the main catalytic products with gaseous product detected by online gas chromatography. Liquid product was quantatively analyzed by 1H NMR spectroscopy (Fig. S9). Figure 2b shows the FE for carbonaceous products of CO and formate, which reflects the efficiency of CO2 electroreduction. Notably, at -0.9 V vs RHE, Mn-In2S3 nanosheets exhibited a higher FE of 92% for carbonaceous product compared with that (66%) of pristine In2S3 nanosheets. Formate was the major product of both pristine In2S3 and Mn-In2S3 nanosheets towards CO2 electroreduction (Fig. 2c). At -0.9 V vs RHE, the maximum FE for formate production on Mn-In2S3 nanosheets reached 86%, which was 1.4-fold higher than that (62%) of pristine In2S3. Notably, such FE value was comparable to that of the state-of-the-art catalysts (Table S2). The FE for CO production exhibited no obvious enhancement upon Mn doping (Fig. S10). The electrochemical surface area (ECSA) values of pristine In2S3 and Mn-In2S3 nanosheets were measured by Pb underpotential deposition method (Fig. 2d).32 The ECSA values of pristine In2S3 and Mn-In2S3 nanosheets were calculated to be 6.1 and 6.8 cm2 mg-1, respectively. The slight increased ECSA was also supported by the measured Cdl values (Fig. S11). We further normalized the geometrical current density by ECSA values (Fig. 2e). At -0.9 V vs RHE, Mn-In2S3 nanosheets exhibited much higher ECSA-normalized current density (6.0 mA cm-2) than that (5.0 mA cm-2) of pristine In2S3 nanosheets. As such, Mn doping promoted the intrinsic activity of In2S3 nanosheets. In addition, Mn-In2S3 nanosheets exhibited negligible decay in current density, while the FE for carbonaceous product remained 90% during a 8-h potentiostatic test (Fig. 2f).


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To explore why Mn doping enhanced the formate production, we analyzed the tafel plots. As shown in Figure 3a, the tafel slopes of both pristine In2S3 and Mn-In2S3 nanosheets were close to 118 mV dec-1, indicating that the activation of CO2 served as the rate-limiting step.33 Generally, the CO2 activation process was initialized with the adsorption of CO2 molecules on the surface of catalysts.34 To compare the capacity of CO2 adsorption, we conducted CO2 adsorption isotherms of pristine In2S3 and Mn-In2S3 nanosheets. The CO2 adsorption capacity of Mn-In2S3 nanosheets was 124 μmol g-1 under 1 atm at 25 oC, which was 1.2 times larger than that (100 μmol g-1) of pristine In2S3 nanosheets (Fig. 3b). As such, Mn doping improved the capacity of CO2 adsorption. To further compare the strength of CO2 adsorption, we conducted CO2 temperature programmed desorption measurements of pristine In2S3 and Mn-In2S3 nanosheets. As shown in Figure 3c, the peak of CO2 desorption from Mn-In2S3 nanosheets was located at 266 oC, higher than that (206 oC) from pristine In2S3 nanosheets, indicating the strengthened adsorption of CO2 via Mn doping. Therefore, Mn doping enhanced both the capacity and strength of CO2 adsorption. The enhanced CO2 adsorption promoted the activation of CO2, because the activation energy is related to adsorption energies by linear scaling based on Brønsted-Evans-Polanyi relationships.35 To decipher the origin of promoted CO2 activation via Mn doping, we explored the electronic structures of pristine In2S3 and Mn-In2S3 nanosheets. Figure 3d shows the ultraviolet-visible-near-infrared diffuse reflection spectra. Compared with pristine In2S3 nanosheets, Mn-In2S3 nanosheets exhibited increased absorption over the long wavelength ranging from 600 to 1000 nm, suggesting the narrowed band gap of Mn-In2S3 nanosheets. This narrowed bandgap contributed to the increased carrier concentration in Mn-In2S3 nanosheets. Figure 3e shows the Mott-Schottky plots of pristine In2S3 and Mn-In2S3 nanosheets. The carrier density of Mn-In2S3 nanosheets was calculated to be 1.23 × 1019 cm-3, which was 5.3 times larger than that of pristine In2S3 nanosheets. The increased electron concentration accounted for high electron conductivity which benefited the electron-transfer efficiency in CO2 electroreduction. The facilitated electron-transfer process was further verified by the Nyquist plots of pristine In2S3 and Mn-In2S3 nanosheets (Fig. 3f). Based on the diameter of semicircular Nyquist plots, the Mn-In2S3 nanosheets exhibited the charge-transfer resistance (RCT) of 3.1 Ω, which was lower than that (7.0 Ω) of pristine In2S3 nanosheets. Therefore, Mn doping narrowed the bandgap and gave rise to high carrier density, resulting in the accelerated charge-transfer process of In2S3 6 ACS Paragon Plus Environment

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nanosheets towards CO2 electroreduction. To further understand the CO2 activation process on pristine In2S3 and Mn-In2S3 nanosheets, the electronic interaction between the adsorbate and catalysts was analyzed via density functional theory (DFT) calculations. Given the main product of the CO2 electroreduction on both pristine In2S3 and Mn-In2S3 nanosheets was formate, we only concern the pathway for formate product from the HCOO* species (Fig. S12). Based on structural optimization, HCOO* intermediates adopted bidentate adsorption on both pristine In2S3 and Mn-In2S3 slabs, where two O atoms in HCOO* were bound to the adjacent two metal atoms (Fig. 4, a and b, Table S3). To illustrate the electronic interaction between the adsorbate and catalysts, the projected density of states (PDOS) of In2S3 and Mn-In2S3 slabs were calculated (Fig. 4, c and d). Upon the adsorption of HCOO* intermediates, In2S3 slab exhibited the main overlaps between the p orbitals of O atoms and d orbitals of In atoms below the Fermi level. In contrast, the new harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms appeared near the conduction band edge of Mn-In2S3 slab. The newly formed states in the gap region are expected to facilitate the charge transfer between Mn-In2S3 catalysts and adsorbed HCOO* intermediates, thus promoting the formation of HCOO* on Mn-In2S3 slab.36 Furthermore, we calculated the Gibbs free energy for each step involved in CO2 electroreduction (Figs. 4e, S13, S14). The CO2 activation process held the highest energy barrier among the four steps over In2S3 or Mn-In2S3 slabs (Table S4). Notably, the introduction of Mn dopants into In2S3 slab significantly reduced the Gibbs free energy in the formation of HCOO* intermediates, such a rate-limiting process, by 0.32 eV. In addition, to elucidate why formate instead of methanol or ethanol serves as the product, we compared the energy barriers between HCOOH* desorption and the further hydrogenation of HCOOH* (Fig. S15). Specially, the reaction of HCOOH* desorption on both In2S3 and Mn-In2S3 slabs was exothermic whereas the further hydrogenation step was endothermic. As such, HCOOH* intermediates were more likely to desorb to formate product instead of further hydrogenation to the deeply reduced product like methanol. Based on the investigations above, we proposed the mechanism of atomically thin Mn-In2S3 nanosheets for CO2 electroreduction reaction. Mn doping harmonized the electronic structures of HCOO* and Mn-In2S3 nanosheets for efficient CO2 activation. Benefiting from this, the current density and FE for formate product obviously enhanced over Mn-In2S3 nanosheets towards CO2 electroreduction. Specially, Mn doping narrowed the bandgap and gave rise to high carrier


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density, resulting in the accelerated charge-transfer process of In2S3 nanosheets. Furthermore, DFT results revealed that Mn doping enabled the harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms near the conduction band edge of Mn-In2S3 slab upon the bidendate adsorption of HCOO* species. The newly formed states in the gap region are expected to facilitate the charge transfer between Mn-In2S3 nanosheets and adsorbed HCOO* intermediates, thus promoting the formation of HCOO* on Mn-In2S3 slab. In conclusion, we harmonized the electronic structures of the adsorbate and atomically thin Mn-In2S3 nanosheets for efficient CO2 electroreduction. The introduction of Mn dopants into In2S3 nanosheets enhanced both the FE for carbonaceous product and current density. Mechanistic studies revealed that Mn doping enabled the harmonic overlaps between the p orbitals of O atoms and d orbitals of Mn atoms near the conduction band edge of Mn-In2S3 slab upon the bidendate adsorption of HCOO* species. Due to the unique electronic structures of the co-adsorbed configurations, Mn-In2S3 nanosheets exhibited a lower energy barrier for CO2 activation into HCOO* compared with that over pristine In2S3 nanosheets. Overall, this work not only developed atomically thin Mn-In2S3 nanosheets as a promising catalyst for CO2 electroreduction, but also opened up a new avenue into the design of efficient electrocatalysts. These findings provide a solid understanding of the process of CO2 activation, thus offering a valuable platform for gaining further insights towards CO2 electroreduction.


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ASSOCIATED CONTENT Supporting Information. Experimental details, AFM images, TEM images, Line-scanning profiles, EDX profiles, ICP results, XPS survey spectra, Mn 2p XPS spectrum, In 3d XPS spectra, S 2p XPS spectra, 1H NMR spectrum, Faradaic efficiencies for H2 and CO, Cyclic voltammetry curves, Optimized configurations of HCOO* intermediate, Top view and side view of the intermediates adsorbed on In2S3 and Mn-In2S3 slabs, Calculated reaction energy profiles for the hydrogenation of HCOOH* to H2COOH*, The calculated energy for different species involved in CO2 electroreduction. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions ||A.Z.,

Y.L., and H.L. contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by NSFC (21573206, 11674299, 11634011), Key Research Program of Frontier Sciences of the CAS (QYZDB-SSW-SLH017), Anhui Provincial Key Scientific and Technological Project (1704a0902013), Major Program of Development Foundation of Hefei Center for Physical Science and Technology (2017FXZY002), the National Key Research and Development Program of China (2017YFA0204904), Anhui Initiative in Quantum Information Technologies, the Strategic Priority Research Program of Chinese Academy of Sciences (XDB30000000), and Fundamental Research Funds for the Central Universities. This work was partially carried out at the USTC Center for Micro and Nanoscale Research and Fabrication.


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Figure 1. (a) Scheme of the synthetic procedure of Mn-In2S3 nanosheets. (b) TEM image of Mn-In2S3 nanosheets. (c) HAADF-STEM image of an individual Mn-In2S3 nanosheet. (d) HAADF-STEM and STEM-EDX elemental mapping images of Mn-In2S3 nanosheet. (e) XRD patterns of pristine In2S3 and Mn-In2S3 nanosheets.


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Figure 2. (a) Geometrical current density over pristine In2S3 and Mn-In2S3 nanosheets at different overpotentials. (b) FE for carbonaceous product (C-product) over pristine In2S3 and Mn-In2S3 nanosheets at different overpotentials. (c) FE for formate over pristine In2S3 and Mn-In2S3 nanosheets at different overpotentials. (d) Pb stripping curves on pristine In2S3 and Mn-In2S3 nanosheets. (e) ECSA-normalized current density over pristine In2S3 and Mn-In2S3 nanosheets. (f) Plots of current density and carbonaceous product versus time over pristine In2S3 and Mn-In2S3 nanosheets at a constant overpotential of -0.9 V vs RHE.


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Figure 3. (a) Tafel plots of pristine In2S3 and Mn-In2S3 nanosheets. (b) CO2 adsorption isotherms of pristine In2S3 and Mn-In2S3 nanosheets at 25 oC. (c) CO2-TPD spectra of pristine In2S3 and Mn-In2S3 nanosheets. (d) UV-vis-NIR diffuse reflection spectra of pristine In2S3 and Mn-In2S3 nanosheets. (e) Mott-Schottky plots of pristine In2S3 and Mn-In2S3 nanosheets. (f) Nyquist plots of pristine In2S3 and Mn-In2S3 nanosheets.


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Figure 4. The optimized adsorption configurations of HCOO* intermediates on the surface of (a) In2S3 slab, and (b) Mn-In2S3 slab. (c) PDOS of the d orbitals of In atoms and p orbitals of O atoms on In2S3 slab. (d) PDOS of the d orbitals of In atoms, d orbitals of Mn atoms, and p orbitals of O atoms on Mn-In2S3 slab. In panels c and d, the top image shows the PDOS before HCOO* adsorption, while the bottom image shows that after HCOO* adsorption. (e) Calculated reaction energy profiles for CO2 electroreduction to form formate on In2S3 and Mn-In2S3 slabs. * represents an adsorption site.


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Harmonizing the Electronic Structures of the Adsorbate and Catalysts

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