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Article

How to Boost the Activity of the Monolayer Pt Supported on TiC Catalysts for Oxygen Reduction Reaction: A Density Functional Theory Study

by
Hui Zhu
,
Houyi Liu
,
Lei Yang
and
Beibei Xiao
*
School of Energy and Power Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China
*
Author to whom correspondence should be addressed.
Materials 2019, 12(9), 1560; https://doi.org/10.3390/ma12091560
Submission received: 10 April 2019 / Revised: 3 May 2019 / Accepted: 6 May 2019 / Published: 13 May 2019
Browse Figures
Versions Notes

Abstract

:
Developing the optimized electrocatalysts with high Pt utilization as well as the outstanding performance for the oxygen reduction reaction (ORR) has raised great attention. Herein, the effects of the interlayer ZrC, HfC, or TiN and the multilayer Pt shell on the adsorption ability and the catalytic activity of the TiC@Pt core-shell structures are systemically investigated by density functional theory (DFT) calculations. For the sandwich structures, the presence of TiN significantly enhances the adsorption ability of the Pt shell, leading to the deterioration of the activity whilst the negligible influence of the ZrC and HfC insertion results the comparable performance with respect to TiC@Pt1ML. In addition, increasing the thickness of the Pt shell reduces the oxyphilic capacity and then mitigates the OH poisoning. From the free energy plots, the superior activity of TiC@Pt2ML is identified in comparison with 1ML and 3ML Pt shell. Herein, the improved activity with its high Pt atomic utilization makes the potential TiC@Pt2ML electrocatalyst for the future fuel cells.

1. Introduction

Proton exchange membrane fuel cells (PEMFC) have attracted widespread attention due to their high efficient and zero carbon emission for the hydrogen economy [1,2,3,4]. To accelerate the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode, commercial catalysts consist of platinum deposited on a carbon support [5]. However, the high loading of the Pt catalysts results in a major challenge for future commercialization [6]. In this regard, the development of efficient catalysts with reduced Pt content is of great importance.
The core-shell structure with a non-Pt core can significantly improve the Pt utilization, thereby reducing the Pt content and, thus, the cost [7,8]. It as previously revealed that when transition metal (TM) elements acted as the core, such as Pd [9,10], Ru [11], or Ir [12], the ORR activity of the corresponding TM@Pt core-shell was enhanced compared with the commercial Pt/C. However, such a TM core would not be suitable from an economic aspect [13]. On the other hand, titanium carbide (TiC) is a good alternative of Pt due to its similar electronic structure [14], being important in the field of catalysis. As reported, the TiC supported Pt catalysts possess the enhanced performance of the methanol oxidation reaction, hydrogen evolution reaction, and ORR, implying the positive effect of TiC [15,16,17,18]. It is believed that the TiC core could modify the electronic structure of the corresponding Pt shell to boost the ORR performance. Therefore, the core-shell structure consisting of the TiC core and the Pt shell acting as the ORR cathode could be the solution for the future requirements of the PEMFC cathode material.
TiC suffers from stability degradation due to oxide formation during the electrochemical cycles [19]. To settle the issue, increasing the thickness of the Pt shell is a viable strategy to protect the core [20,21,22]. The durability enhancement of the Pd@Pt catalysts with multilayer Pt shells provides the direct evidence [9]. In addition, the sandwich structure created by inserting an interlayer would be another good solution [23], which is easily achieved by the controllable synthesis benefit from the experimental development. As reported, the robust stability of the ZrC and HfC are of great potential to resist the electrochemical corrosion, besides the TiC support, being favorable as support materials in the harsh conditions [24]. Furthermore, the efficient and durable TiN materials are also merged due to the passivation degree by oxygen [25,26]. Therefore, ZrC, HfC, as well as TiN, would be suitable selections to act as the interlayer. Since the different electronic effects caused by the shell thickness and the interlayer would modulate the ORR activity of TiC@Pt [19,27,28,29], the systematic influences of the aforementioned factors on the ORR activity of TiC@Pt core-shell material are as yet untouched, raising our interest.
In the manuscript, density functional theory (DFT) calculations are used within an electrochemical framework to analyze the ORR electrocatalysis on the TiC@Pt core-shell materials and their derivatives. The adsorption behavior of the intermediates is calculated, for the evaluation of the scaling relationship and then thermodynamically free energy. The data provides the fundamental understanding of relationship between the activity of TiC@Pt core-shell materials and the interlayer or the shell thickness and further identify the optimal candidate to guide the experimental progress for top-down material design.

2. The Calculation Details

All calculations are performed within the DFT framework as implemented in DMol3 code [30,31]. The generalized gradient approximation with the Perdew–Burke–Ernzerhof functional (GGA–PBE) is employed to describe exchange and correlation effects [32]. The DFT semi-core pseudopots (DSPP) core treatment is implemented for relativistic effects, which replace core electrons by a single effective potential and introduce some degree of relativistic correction into the core [33]. The double numerical atomic orbital augmented by a polarization function is chosen as the basis set [30]. Herein, the PBE/DNP combination in Dmol3 code has been widely employed for the ORR electrocatalysis [7,8,34,35,36]. Furthermore, these parameters have been used for the TiC@Pt or TiN@Pt system [37]. Our calculation method is consistent with the previous works, indicating the feasibility. A smearing of 0.005 Ha (1 Ha = 27.21 eV) to the orbital occupation is applied to achieve accurate electronic convergence. The spin-unrestricted method is used for all calculations. The minimum energy paths for the ORR were obtained by the LST/QST tools in the DMol3 code.
The TiC@Pt(001) surfaces are modeled as periodically repeated 2 × 2 supercell. A 15 Å-thick vacuum is added along the direction perpendicular to the surface to avoid the artificial interactions between slab and its images. The corresponding structure of TiC@Pt(001) and its derivatives are schematically illustrated in Figure 1. In all of the structure optimization calculations, the atoms in the bottom two layers are fixed while other are fully relaxed.
The adsorption energies Eads(M) are calculated by the following equations:
Eads(M) = EM/slab − (EM + Eslab)
where EM/slab, EM, and Eslab are the energies of the adsorption systems, the ORR intermediates and the catalyst, respectively.
Gibbs free energy changes (∆G) of the ORR elemental steps have been calculated according to the computational hydrogen electrode (CHE) model developed by Nørskov et al. where the chemical potential of proton/electron (H+ + e−) in solution is equal to the half of the chemical potential of a gas-phase H2 [5]. The ∆G for every elemental step can be determined as following:
∆G = ∆E + ∆ZPE − T∆S + ∆GpH + ∆GU
where ∆E is the electronic energy difference based on DFT calculations, ∆ZPE is the change in zero point energy, T is the temperature (equal to 298.15 K here), ∆S is the change in the entropy, and ∆GpH and ∆GU are the free energy contributions due to variation in pH value (pH is set as 0 in acid medium) and electrode potential U, respectively. In order to decrease the calculation consumption, the approximate correction ∆ZPE − T∆S to ∆E (0.05/0.35 eV of O*/OH*) are used for constructed the ∆G [5].

3. Results and Discussion

In order to characterize the adsorption ability, the high-symmetry adsorption sites are considered, including the top, bridge, and hollow sites [7,38]. The favorable adsorption sites are shown in Figure 1 and the corresponding adsorption energies Eads are listed in Table 1. For TiC@Pt1ML, the favorable adsorption site of O2 is the bridge site with the Eads(O2) of −1.83 eV, indicating the efficiency of the O–O activation, in line with the previous work [39]. Similarly, the O and OH are located at the bridge sites with the Eads of −1.27 and −3.44 eV, respectively. The product H2O is suited at the top site and the Eads(H2O) is −0.80 eV, being stronger than the solvation stabilization energy of bulk H2O (about −0.40 eV) [40]. Comparing the data of Eads(O2) and Eads(H2O), the product H2O is readily replaced by the reactant O2 for the next ORR cycle. For the TiC@ZrC@Pt1ML or TiC@HfC@Pt1ML, the difference of the Eads is less than −0.05 eV with respect to the TiC@Pt1ML, indicating the negligible effects of the ZrC or HfC interlayer on the adsorption behavior. However, the binding between the Pt shell and the adsorbates is significantly enhanced by inserting TiN interlayer. Reserving the stable adsorption sites, the corresponding Eads are −2.39, −1.80, −3.82, and −1.07 eV for O2, O, OH and H2O, respectively. Herein, the presence of TiN boost the oxyphilic ability compared with TiC@Pt1ML. Such adsorption variation is reasonable that the interaction between the Pt shell and the substrate of the TiC@TiN@Pt1ML is via the Pt–N bonds, being different from the Pt–C bonds for TiC@Pt1ML. On the other hand, increasing the Pt shell thickness would weaken the ligand effect of the TiC core, changing the adsorption capability [11]. As listed in Table 1, the Eads of O2, OH and H2O are decreased to −1.25, −3.14, and −0.57 eV, while Eads(O) is slightly disturbed with the value of −1.22 eV for the TiC@Pt2ML whilst the corresponding Eads of the TiC@Pt3ML are −1.63, −1.49, −3.31, and −0.70 eV for the O2, O, OH, and H2O adsorption, respectively. That is, the multilayer Pt shell weakens the O2, OH and H2O adsorption besides O affinity referred to 1ML Pt system, with the Eads order of TiC@Pt1ML > TiC@Pt3ML > TiC@Pt2ML. Due to the Eads dependence, it is implied that the ORR activity could be tuned by the interlayer and the Pt thickness. Herein, the Eads of the ORR intermediates as a function of Eads(OH) is established in Figure 2a. As shown, the scaling relationship is clearly observed, in consistence with the previous results [41]. That is:
Eads(O2) = 1.27Eads(OH) + 2.48
Eads(O) = 0.65Eads(OH) + 0.84
Eads(H2O) = 0.56Eads(OH) + 1.13
As is well-known, the adsorption strength is correlated with the d band center of the catalysts according to the d band model where the higher (lower) of the d band center referred to the Fermi energy generally corresponds to the stronger (weaker) adsorption ability [42]. Herein, in order to understand the physical origin of the Eads change, the d partial density of states (PDOS) of the Pt surface is plotted in Figure 2b. As shown in the top panel, the d orbital of the sandwich structures are altered by the different interlayers. Therein, the d bands are almost overlapped for the TiC@ZrC@Pt1ML and TiC@HfC@Pt1ML while the obvious upshift is observed for TiC@TiN@Pt1ML. Quantitatively, the d band centers are calculated and listed in Table 2. The corresponding values are −2.78, −2.80, and −2.40 eV for the mentioned systems, respectively. That is, the enhanced adsorption ability of TiC@TiN@Pt1ML is attributed by the robustness of the d electrons. Conversely, the d band model is unfeasible for the multilayer Pt shell. In the bottom panel of Figure 2b, the d orbital of TiC@Pt2ML and TiC@Pt3ML are obviously moved toward the Fermi energy with respect to TiC@Pt1ML. As the Pt thickness increases from 1ML to 2ML and 3ML, the corresponding d band centers are changed from −2.85 to −1.83 and −2.05 eV, respectively. That is, the d band center follows the order of TiC@Pt2ML > TiC@Pt3ML > TiC@Pt1ML, being contrary against the Eads(OH) tendency. Herein, the higher d band centers is accompanied by the weaker Eads(OH), being deviated from the d band model [43]. In order to explain the abnormal phenomenon, the Mulliken charge is analyzed. As shown in Table 2, the charge transferred from the TiC core to Pt shell is reduced as the thickness increases, indicating Pt shell trends to be electronic neutrality. It implies that the electrostatic repulsion between the multilayer Pt shell and the OH would be lessened. Herein, the charge transformation is unaccountable for the Eads(OH) variation. As previous revealed, the adsorption energy is divided into the interaction energy and the deformation energy where the endothermic latter leads to the energetically loss of the adsorption energy [44]. Therefore, the geometrical factors are considered where the average bond length of the Pt–Pt bonds before and after OH adsorption (Dbef and Daft) are given in Table 2. As shown, no significant change occurs during the OH attachment. However, the Pt–Pt bond underlying the adsorbed OH are elongated with the values of 3.32 and 3.28 Å for TiC@Pt2ML and TiC@Pt3ML, compared with the shortened 2.80 Å for the TiC@Pt1ML, respectively. Plausibly, it is inferred that the deviation from the d band model is attributed from the catalysts deformation [44].
Due to the scaling relationship, the optimization prerequisite of the electrocatalysts is located at the trade-off adsorption ability since too strong leads to the poisoning and too weak implies the insufficient capture [45,46]. To evaluate the activity, the simple O2 dissociation are taken into consideration with the elemental steps listing in the following according to the previous report [38]. Due to the small kinetic barrier of proton transfer, which could be ignored at the high potential [47,48], our attentions are focused on the free energies G based on the computational hydrogen model [5]:
1/2O2 + * → O*
O* + (H+ + e−) → HO*
HO* + (H+ + e−) → H2O + *
Figure 3 describes the reaction process at the potential U of 0 V and 1.23 V, respectively. The corresponding free energies change ΔG are summarized in Table 3 where the positive (negative) ΔG means the endothermic (exothermic) reaction. For TiC@Pt1ML at the potential of 0 V, the O2 dissociation and the OH formation are exothermic processes with the ΔG values of −1.23 and −1.23 eV, respectively. Meanwhile, the H2O formation from OH protonation is energetically balanced with the ΔG of 0 eV. Due to the potential-dependence, at U = 1.23 V, the ΔG of the OH formation and H2O formation are increased to 0 and 1.23 eV, respectively. Thus, the rate-determining step (RDS) of TiC@Pt1ML is located at the H2O formation. Based on the data in Table 3, the similar situation is found for the sandwich structures. The RDS are reserved at the final step of H2O formation with the ΔG of 0.01, 0.05 and 0.37 eV at U = 0 V or 1.24, 1.28, and 1.60 eV at U = 1.23 V for inserting ZrC, HfC, and TiN interlayer, respectively. Herein, no activity improvement is achieved in comparison with TiC@Pt1ML. On the other hand, being different from TiC@Pt1ML, the elemental steps of the TiC@Pt2ML at U = 0 V are energetically downward with the ΔG of −1.18, −0.98, and −0.30 eV for the O2 dissociation, the OH formation and the H2O formation, respectively. At U = 1.23 V, the OH formation and H2O formation are changed to be endothermic and the corresponding ΔG are 0.25 and 0.93 eV, indicating the RDS is reserved as the H2O formation. Analogously, for 3ML Pt shell, the RDS of H2O formation with the ΔG of −0.14 eV at U = 0 V and 1.09 eV at U = 1.23 V are observed. Herein, from the thermodynamic aspect, the promotion effect on the ORR activity is observed for the multilayer Pt shell.
In addition to the thermodynamic analysis, the kinetic barriers of the O2 dissociation mechanism on TiC@Pt2ML are further considered. The reaction pathway of TiC@Pt2ML is plotted in Figure 4. The corresponding reaction barriers Ea and reaction energy Er are tabulated in Table 4. Herein, TiC@Pt1ML and TiC@TiN@Pt1ML are selected as references. For O2 splitting into the O atoms, the Ea of TiC@Pt2ML is 1.28 eV, being slightly higher than 1.05 eV of TiC@Pt1ML and 0.76 eV of TiC@TiN@Pt1ML. The weaker Eads(O2) correlates to the higher Ea of O2 dissociation, indicating the degradation of O2 activation, in consistence with the previous reports [39,49]. Noting that the O2 dissociation on TiC@Pt1ML would be significantly boosted by lowing O2 coverage where the Ea reduces from 0.86 eV to 0.36 eV as the O2 coverage changes from 1/4 ML to 1/9 ML [39]. Therefore, it is reasonably believed that the mentioned phenomenon is occurred on TiC@Pt2ML, implying that the barrier of O2 splitting would be overcome at the room temperature [50]. Furthermore, the similar situation is observed for the OH formation where the unfeasibility of TiC@Pt2ML is identified compared with TiC@Pt1ML and TiC@TiN@Pt1ML. However, the Ea of the H2O formation is 0.91, 1.04, and 1.41 eV for TiC@Pt2ML, TiC@Pt1ML, and TiC@TiN@Pt1ML, respectively. The lower value implies that the OH hydrogenation is speeded by the presence of TiC@Pt2ML. Herein, the kinetic benefit of TiC@Pt2ML is confirmed that the low oxyphilic character avails the OH poisoning, in line with the thermodynamic data [51].
Noting that he experimental verification should be urgently needed to confirm the DFT prediction. Herein, we believe our results realizable due to the following reasons: firstly, CHE model has been successfully applied to interpret the experimental data and design the novel electrocatalysts for metal, oxides as well as carbon-based materials [41,52,53,54,55,56,57,58]; secondly, the development of the synthesis technology leads to the feasibility of the TiC@Pt materials with different core composition as well as shell thickness [17,59,60,61]. Therefore, it is reasonably believed that the DFT candidate of TiC@Pt2ML materials could be experimentally achieved.

4. Conclusions

In this study, DFT calculation is used to investigate the effect of the interlayer and shell thickness on ORR activity. Compared with the TiC@Pt1ML, the comparable adsorption ability was found for TiC@ZrC@Pt1ML and TiC@HfC@Pt1ML whilst the presence of TiN causes a sharp enhancement of the adsorption energy. From the PDOS analysis, the upshifted d band of TiC@TiN@Pt1ML supports the variation of the adsorption behavior. On the other hand, the multiplayer Pt shell generally weakens the oxyphilic affinity with the order of TiC@Pt1ML > TiC@Pt3ML > TiC@Pt2ML. The deviation from the famous d band model is plausibly attributed from the structural deformation. Furthermore, the RDS of the considered systems are identified as the H2O formation. The decrease of the adsorption capacity alleviates the OH poisoning and boosts the ORR activity. Herein, the enhanced activity of the TiC@Pt2ML is confirmed compared with TiC@Pt1ML. The promising ORR performance of the multilayer Pt supported on TiC supplies the theoretical guide for the synthesis.

Author Contributions

B.X. designed the material and wrote the paper. H.Z. carried out the simulation. H.L. and L.Y. entered the discussion. All authors commented on the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number no. 21503097; the Natural Science Foundation of Jiangsu, grant number no. BK20140518.

Acknowledgments

We acknowledge the support of ZhiWen Chen (Department of Materials Science and Engineering, Jilin University).

Conflicts of Interest

The authors declare no competing financial interest.

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Materials 12 01560 g001 550
Figure 1. The catalyst structures and the stable adsorption configurations.
Figure 1. The catalyst structures and the stable adsorption configurations.
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Materials 12 01560 g002 550
Figure 2. (a) The adsorption energy Eads of the ORR intermediates as a function of Eads (OH); and (b) the partial density of states (PDOS) for the Pt surface atoms.
Figure 2. (a) The adsorption energy Eads of the ORR intermediates as a function of Eads (OH); and (b) the partial density of states (PDOS) for the Pt surface atoms.
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Materials 12 01560 g003 550
Figure 3. The free energies at the potential of 0 V (a) and 1.23 V (b). The RDS ΔG are shown in the insets.
Figure 3. The free energies at the potential of 0 V (a) and 1.23 V (b). The RDS ΔG are shown in the insets.
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Materials 12 01560 g004 550
Figure 4. The reaction pathway of the O2 dissociation mechanism on TiC@Pt2ML.
Figure 4. The reaction pathway of the O2 dissociation mechanism on TiC@Pt2ML.
Materials 12 01560 g004
Table
Table 1. The corresponding adsorption energy Eads of possible ORR intermediates.
Table 1. The corresponding adsorption energy Eads of possible ORR intermediates.
Catalyst SystemEads(O2)Eads(O)Eads(OH)Eads(H2O)
TiC@Pt1ML−1.83−1.27−3.44−0.8
TiC@ZrC@Pt1ML−1.92−1.22−3.46−0.84
TiC@HfC@Pt1ML−1.95−1.26−3.49−0.84
TiC@TiN@Pt1ML−2.39−1.80−3.82−1.07
TiC@Pt2ML−1.25−1.22−3.14−0.57
TiC@Pt3ML−1.63−1.49−3.31−0.70
Table
Table 2. The d band center and the Mulliken charge of the Pt surface atom. Dbef and Daft stand for the average bond length of the surficial Pt–Pt bonds before and after OH adsorption while d is the Pt–Pt bond length underlying the OH species.
Table 2. The d band center and the Mulliken charge of the Pt surface atom. Dbef and Daft stand for the average bond length of the surficial Pt–Pt bonds before and after OH adsorption while d is the Pt–Pt bond length underlying the OH species.
Catalyst Systemd Band CenterMulliken ChargeDbefDaftd
TiC@Pt1ML−2.85−0.2253.043.012.80
TiC@ZrC@Pt1ML−2.78−0.2753.063.002.81
TiC@HfC@Pt1ML−2.80−0.2753.063.002.80
TiC@TiN@Pt1ML−2.40−0.1083.073.002.75
TiC@Pt2ML−1.83−0.1073.063.093.32
TiC@Pt3ML−2.05−0.0292.933.073.28
Table
Table 3. The free energy change ΔG at the potential of 0 V and 1.23 V.
Table 3. The free energy change ΔG at the potential of 0 V and 1.23 V.
Catalyst SystemU = 0 VU = 1.23 V
R1R2R3R1R2R3
TiC@Pt1ML−1.23−1.230−1.2301.23
TiC@ZrC@Pt1ML−1.18−1.290.01−1.18−0.061.24
TiC@HfC@Pt1ML−1.22−1.290.05−1.22−0.061.28
TiC@TiN@Pt1ML−1.76−1.070.37−1.760.161.60
TiC@Pt2ML−1.18−0.98−0.30−1.180.250.93
TiC@Pt3ML−1.45−0.87−0.14−1.450.361.09
R1: 1/2O2 + * → O*; R2: O* + (H+ + e−) → HO*; R3: HO* + (H+ + e−) → H2O + *.
Table
Table 4. The reaction barriers Ea and reaction energy Er of the O2 dissociation mechanism.
Table 4. The reaction barriers Ea and reaction energy Er of the O2 dissociation mechanism.
Catalyst SystemO2→2OO + H→OHOH + H→H2O
EaErEaErEaEr
TiC@Pt1ML1.05−0.300.46−1.081.04−0.22
TiC@TiN@Pt1ML0.76−0.700.55−0.901.410.68
TiC@Pt2ML1.28−0.970.69−0.460.910.04

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Zhu, H.; Liu, H.; Yang, L.; Xiao, B. How to Boost the Activity of the Monolayer Pt Supported on TiC Catalysts for Oxygen Reduction Reaction: A Density Functional Theory Study. Materials 2019, 12, 1560. https://doi.org/10.3390/ma12091560

AMA Style

Zhu H, Liu H, Yang L, Xiao B. How to Boost the Activity of the Monolayer Pt Supported on TiC Catalysts for Oxygen Reduction Reaction: A Density Functional Theory Study. Materials. 2019; 12(9):1560. https://doi.org/10.3390/ma12091560

Chicago/Turabian Style

Zhu, Hui, Houyi Liu, Lei Yang, and Beibei Xiao. 2019. "How to Boost the Activity of the Monolayer Pt Supported on TiC Catalysts for Oxygen Reduction Reaction: A Density Functional Theory Study" Materials 12, no. 9: 1560. https://doi.org/10.3390/ma12091560

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