A bifunctional two dimensional TM₃(HHTP)₂ monolayer and its variations for oxygen electrode reactions

Abstract

To achieve renewable energy technologies, low-cost electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are required to replace Pt and IrO₂/RuO₂ catalysts. Based on density functional theory, the catalytic activity of TM₃(HHTP)₂ (2,3,6,7,10,11-hexahydroxytriphenylene) monolayer and its variations (TMX₄, where TM = Fe, Co, Ni, X = O, S, Se) for bifunctional ORR/OER have been investigated. The adsorption ability is dominated by the metal center, in the order of Fe > Co > Ni while the ligand shows the minor contribution. Due to the presence of linear relations between the intermediates, the activity of TMX₄ for the ORR/OER follows a dual volcano curve as a function of the OH adsorption strength. Considering the overpotential, CoO₄ and CoS₄ possess superior bifunctional activity, implying their promise as candidates for the oxygen electrode reaction. This systematical work may open new avenues for the development of high-performance non-PGM catalysts for practical applications of ORR and OER.

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1. Introduction

There is growing interest in oxygen electrochemistry as conversions between O₂ and H₂O play important roles in renewable energy technologies, such as the rechargeable air based battery and devices that require two key electrochemical reactions, oxygen reduction (ORR) and oxygen evolution (OER).^1,2 The electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play key roles in such renewable energy devices.³ The current spectrum of catalysts utilized for these fundamental electrochemical reactions are Pt for the ORR and IrO₂ for applications in the OER.^4–6 Their “rare earth” status and associated high cost renders them less than ideal materials for incorporation into commercialization. In addition, the use of two different single function catalysts for the ORR and OER, respectively, makes the air cell significantly more complex as it requires the combination of three electrodes. In this regard, the development of active and affordable bifunctional electrocatalysts remains a challenging task.¹

As alternatives, great efforts have been devoted to the development of the functional carbon based materials with specific atomic configuration where the heteroatom-doped, such as the nonmetallic as well as the nonprecious transition metal elements, would activate the inert C material and further boost the ORR/OER activity.^7–9 Typical example is shown by J. D. Baran et al. that phthalocyanines, porphyrins and their variations with the active sites composed of TMN₄ motif could be acted as the bifunctional catalysts.¹⁰ Furthermore, S. Z. Qiao et al. demonstrate that the TMN₂ embedded in g-C₃N₄ promotes the oxygen electrode reactions.² Based on the mentioned results, the performance is obviously tuned by the selection of TM atom. However, limited investigations have been focused on the influence of the different TM/ligand combination on the activity.

The metal–organic framework (MOF) provide the abundant active sites due to the structural flexibility where the high level of individually coordinated metal and the wide selection of building block.^11,12 Besides the TM coordinated with N ligand, other combinations within MOF have been experimentally synthesized,^13–16 such as the uniform distribution of TMS₄ and TMO₄ motifs,^14,16 which could offer the prototype for the investigation of the TM/ligand effect. On the other hand, inspired by the attractive bifunctional electrocatalysis exhibited by the TM dichalcogenides,^17–20 the active centers consisted by the TM coordinated to the O/S/Se atoms have raised our attention as the ORR/OER electrode.

To classify the effect of the mentioned combinations, the primary consideration is the theoretical model where the TM₃(HHTP)₂ monolayer is selected as the prototype in our investigation.¹⁶ In the regard, DFT calculations are used within the electrochemical framework to analyze the ORR/OER reaction. The TM₃(HHTP)₂ prototype and its variations have been systemically studied to illustrate the critical role of the metal/ligand combination, where the schematic monolayer structures are shown in Fig. 1(a). The corresponding stability of the reaction intermediates is considered, which allows for the evaluation of the free energy and overpotentials. Based on the information, the bifunctional candidates are screened out by a thorough comparison.

Fig. 1 The schematic structure (a) and the corresponding adsorption energy of ORR/OER intermediates (b).

2. Computational method

All calculations are performed within the DFT framework as implemented in DMol³ code.^21,22 The generalized gradient approximation with the Perdew–Burke–Ernzerhof (PBE) functional is employed to describe exchange and correlation effects.²³ The DFT semi-core pseudopots (DSPP) core treat method is implemented for relativistic effects, which replace core electrons by a single effective potential and introduce some degree of relativistic correction into the core.²⁴ The double numerical atomic orbital augmented by a polarization function (DNP) is chosen as the basis set.²¹ A smearing of 0.005 Ha (1 Ha = 27.21 eV) to the orbital occupation is applied to achieve accurate electronic convergence. In order to ensure high-quality results, the real-space global orbital cutoff radius is set as high as 5.2 Å. In the geometry structural optimization, the convergence tolerances of energy, maximum force and displacement are 1.0 × 10⁻⁵ Ha, 0.002 Ha Å⁻¹ and 0.005 Å, respectively. The spin-unrestricted method is used for all calculations. A conductor-like screening model (COSMO) was used to simulate a H₂O solvent environment for all calculations,²⁵ which is a continuum model where the solute molecule forms a cavity within the dielectric continuum. The DMol³/COSMO method has been generalized to periodic boundary cases. The dielectric constant is set as 78.54 for H₂O. Some previous results have shown that this implicit solvation model is an effective method to describe solvation.^15,26 The 15 Å-thick vacuum is added to avoid the artificial interactions between the nanosheet and its images.

In the reaction energy landscape, all ORR/OER intermediates are described as proton/electron (H⁺ + e⁻) transfers.^4,5 The adsorption energy of the corresponding intermediates are calculated by the following,¹⁰

E_ORR(OOH) = E_OOH − E_substrate − E_O2 − (H⁺ + e⁻) (1)

E_ORR(O) = E_O − E_substrate − 1/2E_O2 + H_f(H₂O) (2)

E_ORR(OH) = E_OH − E_substrate − 1/2E_O2 − (H⁺ + e⁻) + H_f(H₂O) (3)

where E_sys, E_substrate, E_H2O, E_O2 and E_H2 are the total energy of the adsorption systems, the TM₃(HITP)₂ monolayer, H₂O molecule, O₂ molecule and H₂ molecule, respectively. E_ORR < 0 corresponds to an exothermic adsorption process.

To study the ORR/OER activity, the Gibbs free energy changes (ΔG) 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 H₂.⁴ The ΔG for every elemental step can be determined as following:

ΔG = ΔE + ΔZPE − TΔS + ΔG_pH + ΔG_U (4)

where ΔE is the electronic energy difference based on DFT calculations, ΔZPE is the change in zero point energy, T is the temperature (equal to 298.15 K here), ΔS is the change in the entropy, and ΔG_pH and ΔG_U 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 ΔZPE − TΔS to ΔE (0.05/0.30/0.35 eV of O*/OH*/OOH*) are used for constructed the ΔG. As OER are reverse process of the ORR, the corresponding ΔG of OER intermediates are calculated in the following equation:

ΔG_OER(M) = ΔG_ORR(M) + 4.92 (5)

The thermodynamic CHE model has been applied to interpret the experimental data and design the novel electrocatalysts for metal, oxides as well as carbon-based materials.^{10,15,27–32} Besides, the present computational method has been applied to illustrate the ORR mechanism of the TM₃(HITP)₂ (HITP = hexaiminotriphenylene) monolayer. Therein, the 2e⁻ mechanism is predicted to be prevalent for Ni₃(HITP)₂ system owing to the insufficient O₂ activation, which is in accordance with the experimental data established by E. M. Miner.³³ Therefore, considered the structural similarity of the TM₃(HITP)₂ and TM₃(HHTP)₂ systems, the reliability of our calculation could be confirmed. However, it should be realized that the material stability under the harsh electrochemical environment has been neglected herein. Despite the bifunctional ORR/OER electrocatalytic candidates have been screened out based on our theoretical trend, further performance needs the experimental confirmation.

3. Results and discussion

The favorable adsorption properties of the ORR/OER intermediates are the prerequisite for the reaction proceeding. The corresponding adsorption energies are tabulated in Table 1. It should be point out that the values do not signify the absolute strength of the intermediates adsorption. As shown, the adsorption energy decreases monotonically with increasing the d-electron in the valence shell of the metal center, which could be accounted by the d-band model.³⁴ That is, the adsorption ability is tuned by the variation of the metal center, following the order of Fe > Co > Ni.^2,15,35 In order to reveal the ligand effect, the corresponding Mulliken charges of the TM active center as well as the ORR intermediates are plotted in Fig. 2 where positive and negative represent charge depletion and accumulation, respectively. As shown in Fig. 2(a), the charge is transferred from the TM atom to the C-based skeleton for the TMO₄ systems without adsorption, indicating the charge depletion of the TM active center for NiO₄, CoO₄ and FeO₄, respectively meanwhile the charge accumulation is found for the other two systems, especially for CoS₄ and CoSe₄. On the other hand, interacted with the ORR intermediates, the phenomenon of the charge depletion for NiO₄, CoO₄ and FeO₄ in combination with the charge accumulation of CoS₄ and CoSe₄ are remained, being similar compared with the un-adsorbed systems. However, for NiS₄, NiSe₄, FeS₄ and FeSe₄, the generally tendency of the charge accumulation before adsorption changed toward to the charge depletion with ORR intermediates adsorption is observed. Herein, the ligand effect of the charge distribution possesses the similar behavior on the certain degrees for the TMS₄ and TMSe₄, as implied by Fig. 2(a). The point is further supported by the partially density of states (PDOS). For CoX₄ as shown in Fig. 3, the appearance of the p-orbital ranged from −3 eV to the Fermi energy is observed for CoS₄ and CoSe₄, which is missed for the CoO₄. Besides, the sharp double d-peaks are located at ∼−1.6 eV and −1.0 eV for the former systems, being different from the d-peaks of the CoO₄, which confirms the variation of the d-orbital as the ligand charged. The similarity is found for NiX₄ and FeX₄, where the detail PDOS plots are shown in Fig. S1 and S2 of the ESI.† As discussed, the charge analysis as well as PDOS morphology demonstrates the complex ligand effect on the subtle electronic structure of the TM active center, being dependent on the TM element selection, which would lead to the variation of the adsorption energy. Taken Co combination as an example, the neglect variation is found for E_ORR(O) as the ligand changes. However, the E_ORR(OOH) and E_ORR(OH) are significantly weakened as the ligand varied from O/S to Se. The situation is completely different for the Ni/Fe combination. As shown in Fig. 2, due to the charge transfer, the ionic bonding between the O-containing species is formed.³⁶ The charge accumulations of the Co active center and the O-containing species are observed, implying the presence of electrostatic repulsion, which is different from the situations between Ni/Fe and the ORR intermediates with the electrostatic attraction.^37,38 It is plausibly accounted for the mentioned weakening phenomenon for CoX₄. On the other hand, it should been noted that the covalent bonds are dominated by the interaction of the O-p orbital and the TM-d orbital, in combination with the minor contribution of the p–p orbital overlap demonstrated in Fig. 3 as well as Fig. S1 and S2.† Herein, both covalent bonds due to the overlap of orbitals and ionic bonds induced by the charge transfer influence the adsorption.³⁶ Due to the difference of the electronic structures, the general trend of the ligand effect is difficultly summarized.

Table 1 The adsorption energy of ORR intermediates (E_ORR) and Gibbs free energies of the ORR/OER intermediates. All results are in units of eV

	EORR			GOER			GORR
	OOH	O	OH	OOH	O	OH	OOH	O	OH
Ni–O	−0.92	−1.03	−3.64	−0.57	−0.98	−3.34	4.35	3.94	1.58
Co–O	−1.39	−2.43	−4.29	−1.04	−2.38	−3.99	3.88	2.54	0.93
Fe–O	−1.70	−3.12	−4.65	−1.35	−3.07	−4.35	3.57	1.85	0.57
Ni–S	−0.67	−1.07	−3.53	−0.32	−1.02	−3.23	4.60	3.90	1.69
Co–S	−1.30	−2.41	−4.23	−0.95	−2.36	−3.93	3.97	2.56	0.99
Fe–S	−1.68	−3.22	−4.52	−1.33	−3.17	−4.22	3.59	1.75	0.70
Ni–Se	−0.73	−1.28	−3.62	−0.38	−1.23	−3.32	4.54	3.69	1.60
Co–Se	−0.91	−2.45	−3.83	−0.56	−2.40	−3.53	4.36	2.52	1.39
Fe–Se	−1.96	−3.37	−4.54	−1.26	−3.32	−4.24	3.66	1.60	0.68

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Fig. 2 The Mulliken charge of the TM active center (a) and the ORR intermediates (b). TM denotes the un-adsorbed TM atom while TM_OOH, TM_O and TM_OH are the OOH, O and OH adsorbed TM atom, respectively.

Fig. 3 The partially density of states of Co₃(HHTP)₂ and its variations. (a) is the un-adsorbed CoX₄. (b), (c) and (d) are OOH, O and OH adsorption systems, respectively.

The data could be plotted as a function of OH adsorption, as shown in Fig. 1(b). From our results, the universal linear relationships between the ORR intermediates are clearly observed, which is in agreement with the previous studies.^10,15,35,39 That is,

E_ORR(OOH) = 1.02E_ORR(OH) + 2.93 (6)

E_ORR(O) = 1.97E_ORR(OH) + 5.79 (7)

Compared with the previous data of the porphyrins analogues,¹⁰ it is found that the slopes of the fitting lines are comparable. It should be noted that the data deviation of CoSe₄ [E_ORR(O) = −2.45 eV, E_ORR(OH) = −3.83 eV] from the scaling relation between E_ORR(O) and E_ORR(OH) is obvious. Such deviation could be observed in the C-based electrocatalysts,^10,15,35 which is caused by the different electron transfer required for ORR intermediates adsorption (formally 2e/1e for O/OH, respectively).¹⁰ Herein, our Mulliken charge analysis shows the consistence with the statement where the adsorbed O possesses the more electrons compared with the adsorbed OH, indicated by the values shown in Fig. 2(b). Furthermore, the intercept of E_ORR(OOH) vs. E_ORR(OH) generally approaches 3.2 eV, regardless the catalytic materials.^10,31,35,39 Herein, it is well-known that the mentioned scaling relations allow the dependence of the ORR activity on the adsorption strength that too strong means the poisoning of the O-containing species whereas too weak implies the insufficient activation ability, both of which is considered as the origin the ORR overpotential.^4,40

To evaluate the activity of the mentioned systems, the OOH associative mechanisms are taken into consideration with the elemental steps R_i listing in the following,^10,35 where asterisks denote active TM sites. Due to the small barrier of proton transfer, which could be ignored at high applied voltages, our attentions are only are focused on the reaction energies.^4,15,35,41 The corresponding free energy value G is analyzed and depicted in Fig. 4. As shown by the following equations, the four-electron ORR pathways in OOH association mechanism can proceed through OOH formation (R₁), O formation (R₂), OH formation (R₃) and H₂O formation (R₄).

O₂(g) + * + (H⁺ + e⁻) → OOH*, (R1)

OOH* + (H⁺ + e⁻) → O* + H₂O(l), (R2)

O* + (H⁺ + e⁻) → OH* + H₂O(l), (R3)

OH* + (H⁺ + e⁻) → 2H₂O(l). (R4)

Fig. 4 The free energy of ORR. ①: O₂ + 4(H⁺ + e⁻); ②: *OOH + 3(H⁺ + e⁻); ③: *O + H₂O + 2(H⁺ + e⁻); ④: *OH + H₂O + (H⁺ + e⁻); ⑤: 2H₂O.

From the figures, for NiO₄ monolayer, the whole elemental steps are exothermic at the potential U of 0 V, indicating the thermodynamic favor. However, as the U is raised to 1.23 V, the situation is changed that (R₁) and (R₂) become endothermic, respectively, implying the ORR reaction would be not proceeded spontaneously. For clearly observation, the ΔG values are gathered in Table 2. Herein, the rate-determining step (RDS) with the largest ΔG_max value could be acted as a measure of the catalyst activity.⁴ The RDS is located at (R₂) with ΔG_max of 0.82 eV for NiO₄ monolayer at 1.23 V. Furthermore, other catalytic materials possess good activity at 0 V without the endothermic reaction steps while the unfavorable thermodynamics are observed at 1.23 V, being similar with NiO₄ monolayer. However, it should be noted that two different RDS could be identified. That is, (R₁) is for NiS₄, NiSe₄, CoS₄, CoSe₄ with the ΔG_max of 0.91, 0.28, 0.85 and 0.67 eV while (R₄) is for CoO₄, FeO₄, FeS₄, FeSe₄ with the ΔG_max of 0.30, 0.66, 0.53 and 0.55 eV, respectively.

Table 2 Gibbs free energies changes of the elemental steps for ORR at the potential U of 0 and 1.23 V

	U = 0 V				U = 1.23 V
	R1	R2	R3	R4	R1	R2	R3	R4
Ni–O	−0.57	−0.41	−2.36	−1.58	0.66	0.82	−1.13	−0.35
Co–O	−1.04	−1.34	−1.61	−0.93	0.19	−0.11	−0.38	0.30
Fe–O	−1.35	−1.72	−1.28	−0.57	−0.12	−0.49	−0.05	0.66
Ni–S	−0.32	−0.71	−2.21	−1.69	0.91	0.52	−0.98	−0.46
Co–S	−0.95	−1.40	−1.57	−0.99	0.28	−0.17	−0.34	0.24
Fe–S	−1.33	−1.84	−1.05	−0.70	−0.10	−0.61	0.18	0.53
Ni–Se	−0.38	−0.85	−2.09	−1.60	0.85	0.38	−0.86	−0.37
Co–Se	−0.56	−1.84	−1.13	−1.39	0.67	−0.61	0.10	−0.16
Fe–Se	−1.26	−2.06	−0.92	−0.68	−0.03	−0.83	0.31	0.55

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Based on the free energy profiles, the highest potential for the feasible thermodynamic ORR steps are obtained and its corresponding overpotential μ_ORR are summarized in Table 3. The ORR activity follows the order of CoS₄(0.28) ≈ CoO₄(0.30) > FeS₄(0.53) ≈ FeSe₄(0.55) > FeO₄(0.66) ≈ CoSe₄(0.67) > NiO₄(0.82) ≈ NiSe₄(0.85) ≈ NiS₄(0.91). From the data, the relative minor dependence of the activity on the coordinated ligand of the Ni and Fe active sites is observed. The corresponding μ_ORR changes are less than 0.10 V and 0.15 V, respectively. However, for Co metal center, the μ_ORR of the CoSe₄ are much higher than those of CoS₄ and CoO₄, indicating the importance of the ligand selection. On the other hand, the effect of the metal center on the ORR activity is obvious. Generally, the inferior/moderate activities are found for the Ni/Fe combination. For the Co active centers, the superior performances of the CoO₄ and CoS₄ are found referred to the current Pt with the μ_ORR of 0.45 V.⁴

Table 3 The overpotential of the intermediates for ORR and OER

	μORR	μOER
Ni–O	0.82	−1.13
Co–O	0.30	−0.38
Fe–O	0.66	−0.49
Ni–S	0.91	−0.98
Co–S	0.28	−0.34
Fe–S	0.53	−0.61
Ni–Se	0.85	−0.86
Co–Se	0.67	−0.61
Fe–Se	0.55	−0.83

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As discussed by the previous reports,^10,15,35,39 the ORR activity depends on the adsorption of the intermediates. Due to the linear relationship between the adsorption of ORR intermediates and the OH, the overpotential μ_ORR as a function of the E_ORR(OH) is described in Fig. 6. As the enhancement of the adsorption ability, the μ_ORR reduces and then increase, demonstrating the classical volcano-shaped activity is found.^2,4,10,35 For NiO₄, NiS₄, NiSe₄ and CoSe₄ with weak adsorption strength, the high μ_ORR originates from the ineffective weakening the O–O coupling. For Fe combination located at the branch of the strong adsorption, the OH poisoning accounts for the increased μ_ORR. Due to the suitable adsorption ability, the CoO₄ and CoS₄ are situated at the apex of the volcano curve. Our results are in accordance with the previous reports that the bond strength should be compromised for the effective ORR catalysts on the basis of Sabatier principle.⁴⁰

Besides the ORR activity, the OER activity is characterized in Fig. 5 where the reversed process of the OOH associative mechanisms is considered. The free energy of OER intermediates are obtained by the eqn (5) and the corresponding data are shown in Table 2. From our results, no OER activity could be found at 1.23 V where the reaction steps are thermodynamically hindered due to the endothermic characterization. The RDS are located at the OH* oxidation (the reserve (R₃)) for NiO₄, NiS₄, NiSe₄, CoO₄, CoS₄ and the OOH* formation from the adsorbed O* (the reserve (R₂)) for CoSe₄, FeO₄, FeS₄, FeSe₄, respectively. As shown, the unfavorable endothermic steps are changed to be exothermic as the potential U increases. The corresponding equilibrium potentials are in the order of NiO₄(2.36) > NiS₄(2.21) > NiSe₄(2.09) ≈ FeSe₄(2.06) > FeS₄(1.84) = CoSe₄(1.84) > FeO₄(1.72) > CoO₄(1.61) ≈ CoS₄(1.57). Herein, for clear observation, the overpotential μ_OER are collected in Table 3 and its E_ORR(OH)-dependence is shown in Fig. 6 where the volcano-curve is roughly observed.^2,10 Too weak or too strong E_ORR(OH) leads to the high μ_OER and then the inferior activity. The CoO₄ and CoS₄ possess the superior activity the minimum μ_OER with the values of 0.38 and 0.34 V, in comparison with the IrO₂ of 0.56 V, respectively.⁵

Fig. 5 The free energy of OER. ①: O₂ + 4(H⁺ + e⁻); ②: *OOH + 3(H⁺ + e⁻); ③: *O + H₂O + 2(H⁺ + e⁻); ④: *OH + H₂O + (H⁺ + e⁻); ⑤: 2H₂O.

Fig. 6 The dual volcano plot of the overpotential as a function of the OH adsorption energy for ORR and OER.

Based on the mentioned results, CoX₄ show the bests activity with the exception of CoSe₄ due to the too weak capture ability of the reaction intermediates. Herein, the CoO₄ and CoS₄ are identified as the high efficient electrocatalysts to replace Pt for ORR and IrO₂/RuO₂ for OER. Besides, FeX₄ provides better activity in comparison with NiX₄. Generally, the reversible ORR/OER activity is mainly relied on the selection of the metal center. The different ligand gives slightly tuned. Herein, our stimulation provides the potential candidates for experimental synthesis. However, the structural stability as well as its conductivity are not concerned and out of our scope. Furthermore, due to the structure-dependence property, the model selection leads to distinct possibility for electrocatalysis application. Therefore, due to the limited work, extending our results to the whole MOF systems is not suitable.

4. Conclusion

Based on density functional theory, ORR/OER activity on a TMX₄ monolayer has been systematically studied. It is found that the combination of the metal center and the ligand affects the ORR/OER bifunctional activity where the classical volcano-curve as a function of E_ORR(OH) is roughly observed. Furthermore, based on the overpotential obtained from the free energy profiles, the ORR activity follows the order of CoS₄ ≈ CoO₄ > FeS₄ ≈ FeSe₄ > FeO₄ ≈ CoSe₄ > NiO₄ ≈ NiSe₄ ≈ NiS₄ while the OER activity follows the order of CoS₄ ≈ CoO₄ > FeO₄ > CoSe₄ = FeS₄ > FeSe₄ ≈ NiSe₄ > NiS₄ > NiO₄, suggesting that CoO₄ and CoS₄ exhibit the superior catalytic activity. These results may serve as guidance for rational material design and synthesis.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the supports from the National Natural Science Foundation of China (No. 21503097, 51631004), the Natural Science Foundation of Jiangsu (No. BK20140518).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra09974f

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