v-Tin+1CnT2 MXene Surfaces for Oxygen

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Termination Effects of Pt/v-Ti CT MXene Surfaces for Oxygen Reduction Reaction Catalysis Chi-You Liu, and Elise Y. Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17600 • Publication Date (Web): 12 Dec 2018 Downloaded from http://pubs.acs.org on December 12, 2018

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ACS Applied Materials & Interfaces

Termination Effects of Pt/v-Tin+1CnT2 MXene Surfaces for Oxygen Reduction Reaction Catalysis

Chi-You Liu and Elise Y. Li* Department of Chemistry, National Taiwan Normal University No. 88, Section 4, Tingchow Road, Taipei 116, Taiwan

* Corresponding author E-mail: [email protected] Phone: (886)-2-77346218 Fax: (886)-2-29324249 1

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Abstract Ideal catalysts for the oxygen reduction reaction (ORR) have been searched and researched for decades with the goal to overcome the overpotential problem in proton exchange membrane fuel cells (PEMFC). A recent experimental study reports the application of Pt nanoparticles on the newly discovered 2D material, MXene, with high stability and good performance in ORR. In this work, we simulate the Tin+1CnTx and the Pt decorated Pt/v-Tin+1CnTx (n = 1-3, T = O and/or F) surfaces by first-principles calculations. We focus on the termination effects of MXene, which may be an important factor to enhance the performance of ORR. The properties of different surfaces are clarified by exhaustive computational analyses on the geometries, charges, and their electronic structures. The free energy diagrams as well as the volcano plots for ORR are also calculated. Based on our results, the F-terminated surfaces are predicted to show a better performance for ORR but with a lower stability than the O-terminated counterparts, and the underlying mechanisms are investigated in detail. This study provides a better understanding of the electronic effect induced by the terminators and may inspire realizations of practical MXene systems for ORR catalysis. Keywords: MXene, Termination effects, Oxygen reduction reaction, Single atom catalysis, DFT, VASP 2


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I. Introduction During the operation of proton exchange membrane fuel cells (PEMFC), the overpotential of the oxygen reduction reaction (ORR) is one of the determining factors for cell efficiency. In the past decades, the Pt/C or Pt-based catalyst has been observed to show the highest performance for ORR.1-2 However, the popularization of PEMFC is limited by the high cost of Pt. Many recent literatures focus on reducing or replacing the usage of Pt catalyst by applying Pt-based alloy,1-6 metal oxides,1,7-8 noble-metal-free1,6,9-14 or even metal-free materials.1,15 Compared with pure Pt catalyst, these promising materials show similar to slightly better catalytic capability, with an onset potential and a half-wave potential of ORR in the range of 0.8~1.0 V and 0.7~0.9 V (vs RHE), respectively.1-6,8-9,11-13 Alternatively, the applications of 2D materials are attracting growing attentions in many battery or fuel cell systems due to their unusual optical, mechanical, and electronic properties.9,15-21 In particular, the newly discovered early transition metal carbides/nitrides (MXenes),22 has also been explored for their catalytic potential in ORR.10,23-24 The general formula of MXene is Mn+1XnTx (n = 1-3), in which M represents an early transition metal (usually Ti or Zr), X is carbon and/or nitrogen, and Tx represents the surface terminations (usually O, OH, or F)25-34 controlled by HF etching from the MAX phases (A represents mostly group 13 or 14 elements, e.g. 3



Al).34 With respect to other 2D materials, MXenes can be relatively easily maneuvered by switching the elements M, X, and T to achieve chemical and mechanical stability, as well as to adjust physical properties for different applications. Among all possible permutations and combinations of MXene, the Tin+1CnTx (n = 1-3) is the most commonly used material in several territories, such as NH3 sensor,35 Li-S battery,36-37 ORR,23 CO oxidation,38 CO2 reduction,39 and so on. The properties of the Tin+1CnTx surfaces are largely determined by the identity of Tx (the termination type), but the effect has not been fully investigated due to its high complexity. One literature reported a low overpotential as well as a high stability in ORR when catalyzed by the Pt nanoparticle-decorated Ti3C2X2 (X = OH and F) surface under the acidic environment at room temperature.23 However, it has also been predicted that the terminated fluorine groups may desorb or can be replaced by other elements.26,33 The influence of the terminators on the surfaces has not been explored, which may be an important factor to enhance the performance of ORR. In this work, we simulate the Tin+1CnTx and the Pt/v-Tin+1CnTx (n = 1-3, T = O and/or F) surfaces to investigate their catalytic behavior for ORR by first-principles calculations. To mimic the experimental environment, both the pure and hybrid terminations of Ti3C2T2 are constructed. The active center is constructed by a single Pt atom adsorption for ORR intermediates to adsorb and to react. Single-atom catalysis 4


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has been reported to provide a superior catalytic performance in many systems. 40-46 From the computational point of view, the single atom catalyst represents a simplified, primary model that provides fundamental insights and explanation to the material properties, as is frequently constructed in many theoretical studies,47-52 e.g., single Pt atom catalysis for ORR on doped graphene.16,44 Our detailed density of states (DOS) and Bader charge population analyses indicate a weaker bonding between the surfaces with the F terminators than with O, and we observe a linear variation between the overall ORR performance and the O/F ratio. The free energy diagrams as well as the volcano plots for ORR intermediates are also computed. We believe that the results could provide a microscopic recognition of the experimental surface structure as well as a clearer understanding of the Tin+1CnTx properties for ORR catalysis.

II. Computational Details All simulations are performed using the density functional theory (DFT) in the generalized

gradient-approximation

(GGA),

Perdew-Burke-Ernzerhof

(PBE)53

exchange-correlation functional and the plane-wave method by the Vienna ab initio simulation package (VASP).54-57 The energies and structures of adsorbates and surfaces are fully optimized in the calculations. The projector-augmented-wave method (PAW)58-59 is used with a cutoff energy of 400 eV, and the energy is 5



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converged to within 10-5 eV for the electronic steps. The supercell for the Tin+1CnT2 (n = 1-3, T = O and/or F) surfaces include 4 x 4 and 7 x 5 repeated unit cells. A vacuum distance of at least 15 Å is given to minimize interactions between periodic slabs. The k-point grids60 are set as 5 x 5 x 1 and 5 x 7 x 1 for the smaller and the bigger supercells, respectively. Bader charge population analysis is employed in this study.61-63 The spin is constrained for systems with unpaired electrons by using the parameters ISPIN = 2 and NUPDOWN = 0, 1, or 2. The following equation is used to calculate the adsorption energies (ΔEads) (eq 1): ΔEads = E(sur + adsorbate) - (Esur + Eadsorbate)

(eq1)

where E(sur + adsorbate), Esur, and Eadsorbate, represent the calculated electronic energies of the surface with the adsorbed molecule, the un-adsorbed surface, and the free gas phase molecule, respectively. The ORR is calculated by standard hydrogen electrode (SHE) method.64 The four-electron pathway is considered in the acidic media (eq 2-5): O2(g) + H+ + e- → OOH*

(eq2)

OOH* + H+ + e- → O* + H2O(l)

(eq3)

O* + H+ + e- → OH*

(eq4)

OH* + H+ + e- → H2O(l)

(eq5)

6


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where * represents the adsorbed intermediates on the surfaces. Other calculation details are presented in Supporting Information.

III. Results and discussions Structures of Tin+1CnT2 and Pt/v-Tin+1CnT2 surfaces Figure 1 shows the top and the side view of Tin+1CnT2 (n = 1-3, T = O or F) surfaces. All surfaces are constructed with type I termination of MXene, in which both the top and the bottom terminators (O or F) are added to the hollow sites vertically above the Ti atoms in the inner layer.25-28,65 A careful analysis of the surface structure shows that the distances between the O or F terminators and the surface Ti layers (l) are about 0.90 and 1.24 Å , respectively, regardless of the layer number n in Tin+1CnT2 (Figure 1), indicating more strongly bound O terminators with respect to F. Figure S1 shows the detailed projected density of states (PDOS) analyses of different surfaces. All surfaces are conductors except Ti2CO2, which is semi-conducting with a band gap within 1 eV. The different bonding scheme between the O or F terminators and the surfaces can also be observed in the PDOS overlap pattern. As shown in Figure S1, while the states of F show only a small overlap with those of Ti at around -7 eV, the states of O show a large overlap with those of Ti across the range from -1 to -6 eV. This result is consistent with the observed shorter bond length (l) for the O 7



terminators. The weaker bonding between F and the surfaces can also be confirmed by the Bader charge analysis. Figure 2 shows that the O terminators are more negatively charged on the Tin+1CnO2 surface even though F has a higher electronegativity. On the other hand, the charges of the Ti layers are only slightly affected by the terminators. As a result, the first C layer in Tin+1CnF2 becomes more negatively charged than that in Tin+1CnO2. The longer bonding distance as well as the weaker electrostatic interaction for the F terminators may explain the F desorption and/or replacement phenomenon proposed by various literatures.23,26,33,65-67 This in turn may lead to the surface vacancies and an increased O/F ratio in hybrid termination conditions. We then consider Pt atom adsorption on pristine Tin+1CnO2 and v-Tin+1CnT2 surfaces (v denotes the surface vacancy). The adsorption of Pt atom on pristine Tin+1CnF2 surfaces leads to a spontaneous desorption of an F terminator. We therefore consider the adsorption of Pt on the vacancy site of the v-Tin+1CnT2 surfaces with one O or F terminator removed from the surfaces (Figure S2 (a) and (e)). The adsorbed Pt takes a rare trigonal coordination with respect to the more commonly observed bridge site adsorption.51,68-69 In general, the adsorption energies, ΔEads, are much more negative when Pt is adsorbed at the vacancy site of v-Tin+1CnT2 (ΔEads ~ -4.2 eV) than on the pristine Tin+1CnO2 (ΔEads ~ -2.0 eV), as shown in Table 1. Figure S2 reveals the 8


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Bader charge populations of Pt/v-Tin+1CnT2 surfaces. The adsorptions of Pt show almost no effect on the charges of the terminators. The charges of Pt are in the range of -0.91~-1.04 |e|, which are slightly more negative than the F terminators. Note that the charges of the first C layer become less negative when n increases in Pt/v-Tin+1CnF2. Highly symmetrical d states are observed in PDOS when Pt is adsorbed on the v-Tin+1CnT2 surfaces, as shown in Figure S3. The d states are split into three degenerate sets (dz2 , dyz = dxz, dxy = dx2 −y2 ) by the trigonal coordination between Pt and the three “active” Ti atoms (labeled as Act. Ti in Figure S2(a) and (e)) on surfaces. A general trend can be observed that the d state distribution of Pt becomes broader with increasing n in Pt/v-Tin+1CnF2, indicating a stronger interaction between Pt and the surfaces, which is also consistent with the increasing ΔEads (Table 1). Hybrid terminations of Ti3C2T2 with different O/F ratios (O/F = 32/0, 31/1, 16/16, 1/31, and 0/32) are also constructed to simulate experimental scenarios,33,65,70 and the geometries are shown in Figure 3. The highest and the lowest O/F ratios, “32/0” and “0/32”, correspond to the pristine Ti3C2O2 and Ti3C2F2, respectively. Under the premise of facile F desorption, the adsorption of Pt on the surface vacancy with different O/F ratios (v-Ti3C2T2-O/F) are considered. The terminator site that would subsequently be replaced by Pt is marked by yellow circles in Figure 3, and the 9



adsorption energies (ΔEads) are shown in Table S1. Except for the v-Ti3C2T2-31/1 surface, the Pt adsorption becomes stronger with decreasing O/F ratio, as indicated by the increasing ΔEads. The unfavorable removal of O on the Ti3C2T2-31/1 surface can be expected as F would easily desorb or be replaced by Pt. Figure S4 shows the linear variation of PDOS in Ti3C2T2-O/F and Pt/v-Ti3C2T2-O/F when the O/F ratio gradually decreases. With decreasing O/F ratio, the overlap of the states between T and Ti becomes smaller (Figure S4a-e), and the d states of Pt also shift to a lower value (Figure S4f-j). Oxygen reduction reaction Three types of the ORR intermediates are adsorbed on the Pt/v-Tin+1CnT2 surfaces to form OOH*, O*, and OH*, and their geometries are shown in Figure S5. Due to the special tetrahedral coordination of Pt, the splitting of OOH* (OOH* → O* + OH*) is energetically unfavored and is therefore excluded in this study. All intermediates involve a strong bonding between the Pt and the attached O, and the orientations of *O-OH and *O-H show almost no influence on the total energies. Table S2 lists the Bader charge populations of the adsorbed OOH*, O*, and OH*. All intermediates are more negatively charged when adsorbed on Pt/v-Tin+1CnF2 than on Pt/v-Tin+1CnO2. In general, the overall charge carried by the OOH*, OH* and O* adsorbates are only slightly affected by n. 10


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The Bader charge population difference before and after the adsorption of O* on the Tin+1CnO2 and the Tin+1CnF2 surfaces (Δe) are shown in Figure S6. Both the Pt as well as the first C layer lose electrons (positive Δe) with O* adsorption. The layer number n appears to play an insignificant role. However, the extent of electron transfer from the first C layer to O* almost doubles from 0.19 to 0.36 |e| for O- and F-terminated surfaces, respectively. This can be rationalized since the first C layer on the F-terminated surfaces are already more negatively charged than those on the O-terminated surfaces (Figure 2) prior to the O* adsorption. Note that upon adsorption, the overall native charges transferred to O* are almost the same for all surfaces (Table S2). This means that the deficiency of electrons need to be supplied by Pt, which leads to a stronger electron transfer from Pt to O* on O-terminated surfaces. Scheme 1 summarizes the extent of electron flow among the surfaces, Pt, and the adsorbates. The less strongly bound F and the more negatively charged C layer in the Tin+1CnF2 surface lead to a higher extent of an overall electron transfer from the first C layer to the adsorbates. The slightly more negatively charged intermediate on the Pt/v-Tin+1CnF2 surface might lead to a more facile reduction reaction. The adsorption of OOH*, O*, and OH* on the hybrid Pt/v-Ti3C2T2-O/F surfaces are also considered and the Bader charge populations are shown in Table S3. It is observed that the charge of the intermediate shows a linear variation with respect 11



to the O/F ratio. The enthalpy of ORR intermediates on hybrid Pt/v-Ti3C2T2-O/F surfaces with different O/F ratios, as shown in Figure S7, confirms a clear linear relationship on the O/F ratio. Free energy diagram and the volcano plot Energy corrections including the zero-point energy (ZPE), the entropy contribution (TΔS), and the solvation energy (ΔSol), are computed to properly characterize the overall electrochemical reaction profile of ORR as shown in its free energy diagram at U = 0 and U = 1.23 V on different Pt/v-Tin+1CnT2 surfaces (Figure 4). For surfaces with the same n, the F-terminated surfaces show a lower free energy than the O-terminated systems. Note that a lower free energy does not necessarily guarantee a better performance in ORR since the overpotential has to be determined by the largest free energy difference with the ideal scenario (ΔG = -1.23 eV at U = 0 V) within the four electrochemical steps. The computed energy corrections as well as the overall free energies of ORR on Pt/v-Tin+1CnT2 surfaces are listed in Table S4, with the solvation model illustrated in Figure S8. The volcano plot is calculated to predict the performance of ORR on different surfaces, which is shown in Figure 5 and Figure S9. The predicted theoretical overpotential (ηORR) is defined by the equation: ηORR = 1.23 – min{UORR} (in V), where min{UORR} is the smallest ORR potential in four electrochemical steps, and 12


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1.23 V is the equilibrium potential of ORR. All the surfaces with F terminations show equal to better performance of ORR than those with O terminations, which is consistent with our Bader charge analyses. The lowest overpotential can be achieved by the Pt/v-Ti3C2F2 surface with ηORR = 0.74 V. The limiting step for different surfaces could also be predicted from Figure 5. For most surfaces, the limiting step is the formation of the OOH intermediate (the first proton coupled electron transfer step), except for the Pt/v-Ti3C2F2 and the Pt/v-Ti4C3F2 surfaces. On the other hand, the overall reaction on the Pt/v-Ti3C2F2 and the Pt/v-Ti4C3F2 surfaces is limited by water formation (the fourth proton coupled electron transfer step). This may result in different O2 depletion rate at the initial stage of the reaction for different surfaces. The PDOS diagram is plotted in order to analyze the activity trend between different Pt/v-Ti3C2T2-O/F surfaces. As can be seen in Figure 6, with a decreasing O/F ratio, the d states of Pt shift to a lower energy, but surprisingly, the d states of the first Ti layer shift towards the Fermi level. An increasing d-state distribution of Ti in the range of -1 to +1 eV (orange region in Figure 6) is observed with decreasing O/F ratio. The larger overlap between the d bands of Ti and the states of C for the low O/F ratio surfaces indicates a higher covalency between Ti and C, which in turn leads to higher electron transfer extent from C to Pt via Ti, as have been shown by Figure S6. In general, as the center of the d bands (dcen) moves to a higher energy, the activity for 13



the adsorbates is expected to become higher according to the d-band theory. Although the adsorbates are not directly attached to the Ti atom, our result implies that the adsorbates may be indirectly affected by Ti. VI. Conclusion The Tin+1CnT2 (n = 1-3, T = O or F) and Pt/v-Tin+1CnT2 surfaces are simulated to investigate the effect of surface terminations of MXenes on ORR. The weaker bonding between the surfaces with F terminators than with O is confirmed by the geometries, the Bader charge populations, and the PDOS results. Hybrid terminations are also considered and are found to give a linear variation in terms of the extent of electron transfer. With proper energy corrections, the free energy diagrams as well as the volcano plots for ORR are calculated. Based on our results, the F-terminated surfaces are predicted to show a better performance on ORR than O-terminated surfaces. However, the F termination may suffer from lower stability due to weaker chemical bonding. Our results provide a semi-quantitative analysis on Tin+1CnT2 or Pt/v-Tin+1CnT2 surfaces with different ratio of O/F termination, which might inspire the experimentalists to develop practical MXene systems with improved performance for ORR catalysis.

Supporting Information 14


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Detailed Bader charge populations, PDOS, and Eads data on different surfaces. Our simulation of the solvation model, the calculations of energy corrections and the volcano plots are also demonstrated as well.

Acknowledgments

This research is supported by the Ministry of Science and Technology (MOST) in Taiwan (MOST 106-2113-M-003-010-MY3). We thank the help on computational resources by Center for Cloud Computing in National Taiwan Normal University (NTNU).

ORCID Elise Y. Li: 0000-0003-1206-1110

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Figure 1. Optimized structures of (a) the top view of Tin+1CnO2, (b) Ti2CO2, (c) Ti3C2O2, (d) Ti4C3O2, (e) the top view of Tin+1CnF2, (f) Ti2CF2, (g) Ti3C2F2, and (h) Ti4C3F2. The distances between the terminators and the first Ti layer (l, in Å ) are also shown. The colors of elements Ti, C, O, and F are purple, gray, red, and cyan, respectively.

27



Figure 2. Average Bader charge populations (in |e|/atom) of elements in different layers for (a) Ti2CO2, (b) Ti3C2O2, (c) Ti4C3O2, (d) Ti2CF2, (e) Ti3C2F2, and (f) Ti4C3F2 surfaces. The colors of elements Ti, C, O, and F are purple, gray, red, and cyan, respectively.

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Figure 3. Optimized structures of the side and top views of the (a) Ti3C2O2, (b) Ti3C2T2-31/1, (c) Ti3C2T2-16/16, (d) Ti3C2T2-1/31, and (e) Ti3C2F2 surfaces. The yellow circle on each surface denotes the terminator that is replaced by the Pt atom. The colors of elements Ti, C, O, and F are purple, gray, red, and cyan, respectively.

29



Figure 4. The free energy diagram of ORR intermediates on Pt/v-Tin+1CnT2 (n = 1-3, T = O or F) surfaces. The colored solid and dashed lines correspond to the condition when the potential (U) equals to 0 and 1.23 V, respectively. The black dashed line marks the reference free energy (0 eV).

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Figure 5. The volcano plot for the ORR potentials (UORR, in V) as a function of the free energies of OOH* (ΔGOOH*, in eV) on different surfaces. The blue and purple lines correspond to the first and the fourth proton coupled electron transfer step in ORR, respectively. The predicted theoretical overpotentials (ηORR) are the value between the smallest UORR and the equilibrium potential (1.23 V), as marked in the figure.

31



Figure 6. Partial density of state (PDOS) projection of the Pt/v-Ti3C2T2-O/F surfaces with different O/F ratios: (a) 31/1, (b) 16/16, and (c) 1/31. The center of the d bands (dcen) of the first Ti layer are shown. The states in the range from -1 to +1 eV with respect to the Fermi level are shaded in orange. The units of axes are show in the figure.

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Scheme 1. Schematic electron transfer on different Pt/v-Tin+1CnT2 (n = 1-3, T = O or F) surfaces with ORR intermediates (IM). The solid and dashed arrows represent the stronger and the weaker extent of electron transfer on surfaces.

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Table 1. The adsorption energies of the Pt atom (ΔEads, in eV) on different surfaces. ΔEads (eV)

ΔEads (eV)

v-Ti2CO2

-4.82

Ti2CO2

-2.24

v-Ti2CF2

-5.43

Ti2CF2

N/A

v-Ti3C2O2

-4.27

Ti3C2O2

-2.02

v-Ti3C2F2

-5.59

Ti3C2F2

N/A

v-Ti4C3O2

-4.25

Ti4C3O2

-2.02

v-Ti4C3F2

-5.70

Ti4C3F2

N/A

34


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Table of contents

35


v-Tin+1CnT2 MXene Surfaces for Oxygen

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