2D Covalent Metals: A New Materials Domain of Electrochemical CO2

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Letter

2D Covalent Metals: A New Materials Domain of Electrochemical CO Conversion with Broken Scaling Relationship 2

Hyeyoung Shin, Yoonhoo Ha, and Hyungjun Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01876 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 28, 2016

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The Journal of Physical Chemistry Letters

2D Covalent Metals: A New Materials Domain of Electrochemical CO2 Conversion with Broken Scaling Relationship Hyeyoung Shin, Yoonhoo Ha, and Hyungjun Kim* Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-Ro, Yuseong-Gu, Daejeon 34141, Korea

AUTHOR INFORMATION *Correspondence to H.K. ([email protected])

ACS Paragon Plus Environment

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ABSTRACT

Towards a sustainable carbon cycle, electrochemical conversion of CO2 into valuable fuels has drawn much attention. However, sluggish kinetics and a substantial overpotential, originating from the strong correlation between the adsorption energies of intermediates and products, are key obstacles of electrochemical CO2 conversion. Here we show that twodimensional (2D) covalent metals with a zero band-gap can overcome the intrinsic limitation of conventional metals and metal alloys and thereby substantially decrease the overpotential for CO2 reduction because of their covalent characteristics. From first-principles-based highthroughput screening results on 61 2D covalent materials, we find that the strong correlation between the adsorption energies of COOH and CO can be entirely broken. This leads to the computational design of CO2-to-CO and CO2-to-CH4 conversion catalysts in addition to hydrogen-evolution-reaction catalysts. Towards efficient electrochemical catalysts for CO2 reduction, this work suggests a new materials domain having two contradictory properties in single material; covalent nature and electrical conductance.

TOC GRAPHICS


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As an emerging technology to mitigate the anthropogenic climate change, electro-catalytic reduction of carbon dioxide (CO2) has been attracting much attention.1 In particular, the catalytic conversion of CO2 to carbon-based fuels and useful chemical feedstock is considered to be a critical goal to recycle the environmentally threatening chemical CO2 and thereby eventually establish a sustainable carbon economy. Toward this, many metal catalysts have been investigated as electro-catalysts for CO2 reduction. As examples of these, Cu has been highlighted as a metal yielding CH4 from CO2;2-4 Ag and Au convert CO2 into CO;2, 5-7 and Sn, Hg, and Pb convert CO2 into HCOOH.2 However, the catalytic activities of these metals are still insufficient for its practical applications,8 requiring further development. Fundamental origin of the limited catalytic activity is attributed to the strong correlation among the adsorption energies of intermediates and the final product, usually referred to as a scaling relationship.9-15 For the case of CO2 reduction to CO, requiring a sequential 2ereduction steps, one needs to modify the catalyst surface to strongly bind the COOH intermediate to expedite the sluggish kinetics of the first electron transfer to the chemically stable CO2. On the other hand, for the facile desorption of the final product as well as for affordable kinetics of the second electron transfer step, CO binding should not be too substantial. However, usually the COOH binding energy and CO binding energy of common metals are interdependent forming a strong positive correlation line, hampering the modulation of individual binding energies, which is essential to achieve a sufficiently high catalytic activity for CO2 reduction.10, 12 Imposing covalent character to the catalytic surface via active center design is suggested as a promising strategy to break the scaling relationship.10 Bearing in mind that the COOH has an unpaired electron while CO has a lone-pair of electrons, the active center that is functionalized to form a covalent bond can effectively stabilize the COOH intermediate, significantly lowering the overpotential of the electro-reduction of CO2. However, the


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realization of this strategy is confronted with an intrinsic limitation; most covalent materials are insulators that cannot be used in electrochemical applications. Two dimensional (2D) materials or layered materials are often able to conduct electrons even through the covalent bond network of nonmetallic constituents (e.g., graphene/graphite versus diamond). As such, 2D covalent metals are conceived as a new materials domain of electro-catalytic materials, which can potentially provide extraordinary electro-catalytic properties overcoming the intrinsic limitation of the conventional metallic catalysts. Their large surface areas add more merit for the utilization of 2D materials as electrocatalysts. For example, albeit they are usually semiconductors, transition metal dichalcogenides (TMD) such as MoS2 are widely discussed as promising electrolysis catalysts for hydrogen evolution reaction (HER)13,14 and CO2 reduction.16-18 With regard to the above, we herein investigate the electrochemical activities of 2D covalent metals, particularly aiming for the development of efficient CO2 reduction catalysts. We performed first-principles high-throughput catalyst screening for the 61 2D covalent metals listed in Table S1. We chose 39 candidate conducting materials of TMDs and nonTMDs from the recently established list from data mining among crystal structures experimentally reported in the International Crystallographic Structural Database (ICSD)19 and included an additional 22 conducting TMDs and several 2D oxides.20 Using the Vienna Ab-initio Simulation Package (VASP),21 we performed spin-polarized density functional theory (DFT) calculations using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional22 and plane-wave basis sets. A plane-wave energy cutoff was set as 450 eV, and the projector augmented wave (PAW) method23 was used to describe the core electrons as well as the interaction between valence electrons and the ion (further simulation details are fully described in the Supporting Information).


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DFT calculated COOH and CO binding energies (∆EbCOOH and ∆EbCO) of 2D covalent metals show virtually no mutual correlation (Figure 1a), which is in stark contrast to the

∆EbCOOH and ∆EbCO of Cu, Ag, Au, and Zn forming a strong linear correlation line as reported in previous literature10. Except for several outliers, which are either non-TMDs (IYGa, AlCl3, PFeLi, LiFeAs, FeSe, Ag2ReCl6) or TMDs with early transition metal cores (YS2, ScS2 (T), ScSe2 (T), ScTe2 (H)), interestingly, most of the 2D covalent metals show nearly zero CO binding affinity (less than -0.2 eV), although the COOH binding affinity can be modulated over the considered materials from -4.12 eV to -0.29 eV. This suggests that the characteristics of the chemical bond of the adsorbate with the 2D covalent metals is entirely different from that with the conventional transition metal surface; the transition metal-adsorbate bond can be described by means of the interaction between the d-band of the metal and adsorbate orbitals,24 but the 2D covalent metal-adsorbate bond can be understood by means of the covalent bond character of the surface covering nonmetallic or metalloid elements favoring the adsorbate with an unpaired electron while disfavoring the one with a lone pair of electrons. Density of states (DOS) analyses, shown in Figure 1b, provide further details about the chemical nature of the adsorbate-surface bond. As representatively shown for the IrTe2 system, COOH adsorption onto the surface leads to the formation of a localized state (appeared as a single peak at -12.95 eV), which is identified as the carbon-chalcogen σbonding state as shown in its real-space visualization (right bottom of Figure 1b). The angular momentum projection of DOS (PDOS) characterizes that the σ-bond is particularly manifested by the interaction of the carbon pz orbital of COOH with the s-band state of chalcogens. This is in contrast to the case of CO adsorption onto the conventional metal catalyst surface; there is no noticeable change in the DOS compared with the DOS of the bare


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surface (Figure S1a), and real-space visualization of the chalcogen s-band shows no electronic coupling with the CO adsorbate molecule (Figure S1b). Participation of the chalcogen s-band in the formation of the adsorbate-surface bond suggests the possibility of employing the location of the s-band center as a descriptor for predicting ∆EbCOOH. We thus explore the correlation of ∆EbCOOH with the location of the chalcogen s-band center (Figure 1c). Notably, the s-band centers of sulfides and selenides show a distinct linear correlation with ∆EbCOOH (R2 = 0.82 and R2 = 0.87, respectively), but the s-band centers of tellurides show a weak correlation with ∆EbCOOH (R2 = 0.21; presumably due to the partial metallic character of the tellurium). This suggests that the engineering of the

s-band center would be helpful in tuning the adsorbate binding energy to the chalcogenide surface, analogous to the d-band center engineering of conventional transition metal catalysts. However, involvement of both the covalent bond nature and partial metallic character adds complications, and therefore, merely a single descriptor-based screening may hardly be available. Full DFT calculations become the only suitable way for full understanding and quantitative prediction of the adsorbate binding energies for 2D covalent metals. Based on the capability of the large modulation of ∆EbCOOH with little variance of ∆EbCO, which is advantageous for CO2 electrochemical conversion, we now computationally screen the 2D covalent metals for designing electrochemical catalysts for CO2 reduction. In our previous study10, we found that the elementary steps of CO2 reduction to CO can be expressed as follows: CO2(aq) + [H+ + e-] → *COOH(aq)

(R1)

*COOH(aq) + [H+ + e-] → *CO(aq) + H2O(aq)

(R2)

*CO(aq) → CO(g)

(R3)

and the reaction free energy (∆Gº) of each step can be estimated as ∆Gº(R1) = 2.02 + U +

∆GbCOOH, ∆Gº(R2) = -1.07 + U + ∆GbCO - ∆GbCOOH, and ∆Gº(R3) = -0.50 - ∆GbCO in eVs,


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respectively. Here, ∆GbCOOH and ∆GbCO are the binding free energy quantities of COOH and CO, and U is the applied bias potential voltage versus a reversible hydrogen electrode (RHE). This leads the theoretical definition of the threshold potential for the CO2 reduction into CO to be the reduction potential for the CO2 electrochemical reduction reaction; EºCO2ER = MAX(∆Gº(R1), ∆Gº(R2)) when ∆Gº(R3) < 0, and thus, CO is spontaneously released from the catalyst surface as a final product.10 Using the same computational procedure10 including the entropic contribution and solvation effect (by means of diagonalizing the partial Hessian of the adsorbate and implicit solvation model25, respectively), we calculated EºCO2ER of 2D covalent metals. Figure 2 shows the catalysts demonstrating lower overpotentials than Au (showing the lowest overpotential among metal catalysts) for CO2 conversion into CO, which are listed as (1) IrTe2(T) with EºCO2ER = -0.58 VRHE, (2) RhTe2(T) with EºCO2ER = -0.61 VRHE, (3) PFeLi with

EºCO2ER = -0.65 VRHE, and (4) TiS2(T) with EºCO2ER = -0.69 VRHE, decreasing the overpotential by up to 230 meV compared with Au (calculated EºCO2ER = -0.71 VRHE). We note that some materials including CrS2(T), NbS2(H), ScSe2(T), ScTe2(T), and VS2(H) are excluded from the list due to their moderate catalytic activity of the HER, which can compete with CO2 reduction (more detailed discussion about HER will follow). We further find that the CO affinities to the surfaces of LiFeAs, FeSe, Ag2ReCl6, ScS2(T), and ScTe2(H) are substantial as ∆GbCO = -0.65 eV, -0.87 eV, -1.00 eV, -1.51 eV, and -2.10 eV, respectively, and thus, spontaneous desorption of CO is not expected. We thus consider further reduction steps of CO eventually yielding CH4 similar to for Cu metal. The reaction free energies of further reduction steps completing the 8e- reaction are calculated. To determine the reaction path for each material, we investigate all possible intermediates similar to the previous work done for Cu system.26 The identified reaction paths are shown in Figure S2. EºCO2ER is determined based on the elementary step requiring the most unfavorable


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energetics. This leads LiFeAs and ScS2(T) to show lower EºCO2ER values of -0.55 VRHE and 0.62 VRHE, respectively, indicating decreased overpotential by up to approximately 420 meV for CH4 formation compared with the Cu catalyst (calculated EºCO2ER = -0.97 VRHE). On the other hand, HER is the major side reaction competing with CO2 reduction in an aqueous environment, which should be suppressed to achieve a high Faradaic efficiency. Fundamentally, however, the existence of another strong scaling relationship between the COOH binding affinity and H binding affinity limits high selectivity toward CO2 reduction being obtained. ∆EbCOOH should be tuned to increase for enhanced CO2 reduction catalytic activity, which concomitantly increases the H binding affinity elevating the HER catalytic activity in most cases. For 2D covalent metals, unfortunately, we still observe the existence of a linear correlation between ∆EbCOOH and ∆Eb1/2H2, as shown in Figure 3a (∆Eb1/2H2 is the H binding energy defined using 1/2H2(g) as a reference state), similar to existing transition metals. Considering the chemical nature of the adsorbates of COOH and H, the observed correlation is not surprising; both intermediates have strong radical characters that can be simultaneously stabilized using the covalent character of the catalyst surface, which is unlikely in the case of COOH versus CO. Despite the well-defined overall correlation, however, we find that several materials show a deviation from the correlation line between ∆EbCOOH and ∆Eb1/2H2, particularly toward the stronger COOH binding side (right bottom side from the correlation line in the enlarged region of Figure 3a (right)). Such a deviation is advantageous in obtaining a high Faradaic efficiency for CO2 reduction, which is mostly observed for either non-TMDs or tellurides. From analysis on the locations of the chalcogen s-band centers (Figure S3), we further elucidated that the s-band centers of tellurides show very weak correlation with ∆Eb1/2H2 (R2 = 0.57) unlike sulfides and selenides (R2 = 0.85), similar to the case of ∆EbCOOH. It is of note that the weak correlation of adsorbate binding energies with any single physical variable such


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as the s-band center helps to perturb the scaling relation. Bearing in mind that the surface chalcogen components are having the same valence configuration27-28, we conclude that the mixed nature of covalent and metallic bonding of the tellurides is the key leading to a perturbed correlation between ∆EbCOOH and ∆Eb1/2H2. This is beneficial for CO2 reduction with a high activity and selectivity. To quantitatively determine the HER activity and then categorize materials for different catalytic applications of CO2 reduction and HER, we calculated the H binding free energies of ∆Gb1/2H2, yielding the theoretical reduction potential of EºHER = -|∆Gb1/2H2|, where DFT calculated ∆Gb1/2H2 values were empirically shifted by 0.23 eV to correct the well-known overbinding tendency of the PBE functional (more details are described in the Supporting

Information). We then used the calculated HER activity of Au (EºHER = -0.51 VRHE) as a criterion for the choice of poor HER catalysts (and thus suitable for the application of CO2 reduction), considering that Au is one of the well-known poorly performing HER catalysts and thereby shows a high Faradaic efficiency for CO production (87.1%)2 when it is used for a CO2 reduction application. This has excluded CrS2(T), NbS2(H), ScSe2(T), ScTe2(T), and VS2(H) from the list of CO2-reducing electro-catalysts with higher catalytic activities than Au (Figure 3b), albeit their low overpotential for CO2 reduction. Using the calculated HER activity of MoS2 (EºHER = -0.29 VRHE) as a selection criterion, we next find five promising electrocatalysts for the application of HER, which include CrS2(T), NbS2(H), SbSiNi, Zn2In2S5 and ScSe2(T), as shown in the HER volcano plot of Figure 3c. They show lower HER overpotentials than the already-known good catalyst MoS2 as well as lower HER overpotentials than Pt for Zn2In2S5 and ScSe2(T) catalysts. For the successful implementation of the 2D covalent metals for the applications of electrocatalysts, it is essential to maintain the material stability during electrochemical operations in


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the solution phase. To assess the electrochemical stability, we calculated the electrochemical stability window (ESW) of the considered candidate materials. For the calculation of ESW, we estimated the required potentials for oxidation/reduction leaching of the candidate materials with respect to SHE (details are in the Supporting Information). As shown in Figure 4, calculated operation potentials for CO2 reduction or H2 evolution are in between the reduction/oxidation leaching potential of the catalyst materials. However, it should also be noted that the stability of the candidate materials should be further investigated in-depth since there could exist a number of other decomposition pathways (e.g. decomposition via vacancy formation and selective dissolution). In summary, it has been discussed that an intrinsic mechanistic limitation of transition metal catalysts exists for achieving a breakthrough in developing electro-catalysts for CO2 reduction due to the existence of the strong scaling relationship between the COOH and CO binding energies. From first-principles-based high-throughput screening of 2D covalent metals, we demonstrated that the covalent nature of the catalyst surface of 2D materials specifically stabilizes COOH while keeping nearly zero affinity for CO, breaking the preexisting scaling relationship between the COOH and CO binding energies of metals. Based on calculated CO2 reduction activities and HER activities, we suggest a list of promising CO2 reduction catalysts (with a high activity as well as a high selectivity) and a list of promising HER catalysts, which have not been explored in the field to our knowledge. We anticipate that our study will be a starting point for expanding the domain of materials for CO2 reduction electro-catalysts into a new realm of 2D covalent metals, which can suggest a strategy for the material optimization to overcome the intrinsic limitations of metals or metallic alloys.


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Figure 1. COOH and CO binding characteristics towards 2D covalent metals. (a) Mutual correlation between the binding energies of COOH and CO on the catalyst surface (IYGa and AgCl3 are not shown because those data points are out of range). Most of the 2D covalent metals enable the specific stabilization of COOH over CO, which is in contrast to conventional transition metals. For the calculations of conventional transition metals, we employed the closest-packed faces; (111) for the FCC structure (for Au, Ag, and Cu) and (0001) for the HCP structure (for Zn). (b) Density of states of bare IrTe2 (left) and COOH-adsorbed IrTe2 (IrTe2-COOH, right) are shown as representative cases, and the real space visualization of the Te-COOH σ-bonding state shows a hybridization of the pz orbital of COOH and s orbital of the chalcogen. (c) Correlation of the COOH binding energy (∆EbCOOH) of 2D covalent metals with the center of the s-band of chalcogenides (Ecenter of s-bandX). The ∆EbCOOH values of sulfides and selenides show well-defined correlations with the Ecenter of s-bandX in contrast to the ∆EbCOOH values of tellurides.


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Figure 2. High-throughput catalyst screening of 2D covalent metals for CO2 reduction. (a) Using density functional binding free energies, theoretical reduction potentials of CO2 (vs. RHE) into CO (left) or CH4 (right) are calculated. From the screening of 61 2D covalent metals, IrTe2, RhTe2, PFeLi, and TiS2 show lower overpotentials than Au for CO production, and LiFeAs and ScS2 show lower overpotentials than Cu for CH4 production. (b) Atomistic structures of the candidate materials are displayed.


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Figure 3. H binding energies and hydrogen evolution reaction (HER) activities of 2D covalent metals. (a) Mutual correlation between the binding energies of COOH and H on the catalyst surface (IYGa and AgCl3 are not shown because those data points are out of range). Both 2D covalent metals and conventional transition metals show a relatively significant correlation. However, part of the entire correlation line (in the gray-shaded region) enlarged on the right shows that several 2D covalent metals (mostly non-TMDs or tellurides) show a non-negligible deviation from the correlation toward a stronger COOH binding but with a weaker H binding, which is advantageous for selective CO2 reduction. (b) Comparison of the theoretical reduction potentials for CO2 reduction (E0CO2ER) and HER (E0HER) with all of the reduction potentials referenced to VRHE. 2D materials in the right-bottom side (blue shaded regime) show a moderate HER activity and are excluded from the candidate list for CO2 reduction. (c) Volcano plot of HER shows five 2D materials (ScSe2, Zn2In2S5, SbSiNi, NbS2(H), and CrS2) having higher activities than MoS2 (and sometimes even higher than Pt).


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Figure 4. Calculated electrochemical stability window (ESW) of the candidate 2D covalent metals. ESW was estimated using calculated electrochemical potentials required for the reduction/oxidation leaching of the materials, ∆E0Reduction/∆E0Oxidation (blue/red). The calculated operation potential for CO2 reduction or H2 evolution (black) is located in between the ECW.


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ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.XX. Computational details; Calculation of the reduction potential for the hydrogen evolution reaction (HER); Calculation of the decomposition energy; Supplementary figures and tables.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program (2013M3A6B1078884) on Center for Hybrid

Interface Materials (HIM) and C1 Gas Refinery Program

(2016M3D3A1A01913256) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning.


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2D Covalent Metals: A New Materials Domain of Electrochemical CO2

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