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. 2020 Jan 15;5(3):1581–1585. doi: 10.1021/acsomega.9b03564

Oxygen Reduction Reaction on a CuII Complex of 3,5-Diamino-1,2,4-triazole: A DFT Approach

Sarah Fonseca 1, Leandro Moreira de Campos Pinto 1,*
PMCID: PMC6990643  PMID: 32010832

Abstract

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The high costs for producing catalysts for fuel cells combined with low efficiency in oxygen reduction make metal–organic complexes a promising alternative to noble-metal catalysts. The electrochemical activity of Cu-complex-based catalysts has been reported by many authors, but only a few works are devoted to theoretical studies. In this manuscript, we use density functional theory (DFT) calculations to investigate the oxygen reduction reaction (ORR) on a CuII complex of 3,5-diamino-1,2,4-triazole. The determining steps for the associative and dissociative mechanisms are the oxygen adsorption and the oxygen bond cleavage, respectively. The barrier for breaking the O–O bond in the dissociative mechanism was estimated at 0.7 eV.

Introduction

Oxygen reduction reaction (ORR) is the cathodic reaction in energy-converting devices such as fuel cells, which are still limited by the absence of efficient catalysts.1−3 Pt-based catalysts are the most used for ORR because of their electrochemical properties, such as high activity and electrochemical stability and durability in acid media.2 However, the high cost of platinum makes commercial production of fuel cells economically nonviable.4 Metal–organic complexes are a promising alternative to Pt-based catalysts. Gewirth and co-workers showed that a copper complex of 3,5-diamino-1,2,4-triazole supported on carbon black has a very high activity for oxygen reduction.5 Although many works report on the activity of Cu-complex-based catalysts,5−11 there is lack of information from the theoretical point of view.

Computational methods based on density functional theory (DFT) afford a convenient and practical means of investigating the thermodynamics of many processes.12−14 Such methods are commonly used in the study of a great variety of materials, as they do not require the preparation and testing of the large numbers of specimens demanded by experimental investigations.15 Besides, these methods can be considered a crucial complement to experimental techniques in which some information is difficult to access. DFT calculations can be used to introduce useful information about the ORR on a copper-based complex employed as the catalyst material. The energetics associated with the interaction between the species are fundamental in the study of electrocatalysis of the reactions involved in fuel cells, and the density of states (DOS) plots enable the evaluation of orbitals participating in this interaction.

To provide relevant information about oxygen reduction, in this manuscript, we use DFT calculations to investigate the mechanisms for ORR on a CuII complex of 3,5-diamino-1,2,4-triazole (Hdatrz).

Results and Discussion

Geometry and Electronic Structure

A proper understanding of the ORR demands an adequate description of the active sites on the catalytic material, both with regard to the arrangement of atoms and the electronic structure.

The geometry of the [CuII(Hdatrz)·5H2O·2SO4] complex was fully optimized, and the resulting structure is represented in Figure 1A; the complex is pentahydrated and each Cu atom is connected to a sulfate. Water molecules and sulfates are omitted in Figure 1B to provide more clarity. The Cu–Cu distance after optimization is of the order of 3.4 Å, in good agreement with the value reported in ref (5). Computational details are described in Appendix A.

Figure 1.

Figure 1

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(A) [CuII(Hdatrz)·5H2O·2SO4] complex. (B) Water molecules and sulfates are omitted for clarity.

Both species O* and HO* adsorb on one of the Cu atoms with a distance of 2.39 and 2.33 Å, respectively. HOO* adsorbs on a nitrogen atom (Cu–N), 2.78 Å away from it. Also, the molecular species, O2* and H2O*, are somewhat apart from the [CuII(Hdatrz)] complex, 3.54 and 3.84 Å, respectively.

Figure 2 shows the density of states plots projections onto the d orbital of copper and p orbital of nitrogen atoms in the [CuII(Hdatrz)] complex and the p orbital of the oxygen in the adsorbed species. For the projections, we selected the atoms of Cu and N closest to the interacting species, therefore, those with the highest contribution to the DOS. O* strongly interacts with both the d orbital of copper and the p orbital of nitrogen, in which there is a significant change in the structure of these orbitals compared to the pure complex (Figure 2A,C). The shoulder in the range of −1 to 0 eV for the Cu d orbital disappears. Also, for the N p orbital, the structure below −3 eV completely changes. The oxygen 2p orbital in the HO* and HOO* species interacts with the nitrogen 2p of the complex, while no meaningful change in the structure of the copper d orbital is observed. Calculated binding energy of O* is about 5.3 eV, followed by HO* (2.9 eV) and HOO* (0.5 eV). On the other hand, the interaction of molecular species with the complex are weaker, of the order of 0.3 eV.

Figure 2.

Figure 2

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DOS diagrams. (A) [CuII(Hdatrz)] complex, (B) adsorbed HOO*, (C) adsorbed O*, and (D) adsorbed HO*. Orbitals 3d for copper and 2p for oxygen and nitrogen.

ORR Mechanisms

In the present work, we investigated two distinct mechanisms for the ORR: associative and dissociative (this with two possible routes). While in the former, the cleavage of the O–O bond does not occur before hydrogenation; in the latter, the bond breaking is an important condition before the hydrogenation process. Both mechanisms start with the chemisorption of the O2 molecule. In the following, we present the reaction steps for the associative mechanism

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The free energy diagram for the associative mechanism is depicted in Figure 3. We calculate the free energies using the method outlined in Appendix B. Two configurations were considered: under standard conditions (pH = 0, p = 1 bar, T = 298 K, and U = 0) and changing the electrode potential to U = 1.23 V (the maximum operating potential of fuel cells). The pH was chosen to be zero, hence considering the reaction steps are in acid media.

Figure 3.

Figure 3

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ORR free energy diagram for the associative mechanism.

O2 adsorption precedes hydrogenation, and at potential U = 0.0 V, this step is uphill and all other steps are downhill. As shown in the DOS plots (Figure 2B), HOO is weakly adsorbed to the complex (small interaction with the 2p orbital of nitrogen), thus favoring the breaking of the HO–O bond, which leads to water and O* formation (step 3). Despite the strong binding energy of O*, at low potentials, the hydrogenation weakens the interaction with the [CuII(Hdatrz)] complex, making the reaction in step 4 exergonic. The same holds for hydrating HO* (step 5), the binding energy drops to 0.34 eV and water is formed. Considering the high potential of 1.23 V, only the adsorption of O is downhill and all other steps are uphill.

The reaction steps that were considered in our investigation for the dissociative mechanism are shown below

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Alternatively, the dissociative mechanism may occur by a different route, with reactions 11 and 12 replacing reactions 8 and 9, respectively

graphic file with name ao9b03564_m011.jpg 11
graphic file with name ao9b03564_m012.jpg 12

The two possible routes for the dissociative mechanism are displayed in the free energy diagram in Figure 4. Oxygen dissociation takes place before hydrogenation, and for this step, the barrier was estimated at 0.7 eV. Using the zero cell potential, U = 0.0 V, subsequent steps are downhill. The dissociation barrier of the O2 molecule was determined using a series of calculations moving the molecule toward the [CuII(Hdatrz)] complex. The O–O interaction weakens during the approach, and the energy reaches a maximum (transition state). The bond length at the transition state is 1.52 Å (against 1.26 Å in the gas phase). The oxygen is parallel to the [CuII(Hdatrz)] complex, with each oxygen directing to opposite Cu–N bonds. As previously pointed out, hydrogenation weakens the interaction with the [CuII(Hdatrz)] complex at low potentials. At U = 1.23 V and after breaking the oxygen bond, the steps are uphill. Reaction 11 is approximately 0.3 eV more favorable than reaction 8. Through this mechanism, there is no formation of HOO*.

Figure 4.

Figure 4

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ORR free energy diagram for the dissociative mechanism.

Conclusions

Few theoretical works report on the use of copper-based complexes as catalyst materials for ORR. The [CuII(Hdatrz)] complex is presented as an alternative to noble-metal-based catalysts providing good activity at affordable prices. The theoretical method employed in our investigation constitutes a highly useful tool to assist in the elucidation of processes occurring in the electrochemical environment. DFT calculations were performed to describe the geometric properties and the electronic structure of the complex, and its interaction with some species. Our results show that oxygen adsorption is the determining step for the associative mechanism. On the other hand, for the dissociative mechanism, the process should overcome a barrier of the order of 0.7 eV to cleavage the oxygen bond before hydrogenation.

Acknowledgments

The authors would like to thank the Centro Nacional de Processamento de Alto Desempenho em São Paulo (CENAPAD-SP) for the grant of computing time. Financial support by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, grant no. 434138/2018-5) is gratefully acknowledged. S.F. thanks the CNPq for a stipend. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001.

Appendix A. Computer methods

All DFT calculations were carried out using the SIESTA code.16 SIESTA uses a numerical linear combination of atomic orbitals as the basis set and describes the core–electron interaction using norm-conserving pseudopotentials in the Kleinman–Bylander form.17 Exchange and correlation interactions were treated with the PBE18,19 generalized gradient approximation functional. For comparison, the PW91 exchange–correlation functional20,21 was also used for geometry optimization. We considered the PBE functional more suitable and employed it to the rest of the calculations.

First, we performed the geometry optimization of the [CuII(Hdatrz)·5H2O·2SO4] complex and the convergence criterion was achieved when the force on each atom was less than 0.04 eV/Å. To calculate the interaction with O2, HOO, O, HO, and H2O, each species was placed close to the complex and the optimization procedure was repeated with all atoms free to move.

To calculate the barrier to break the O–O bond in the dissociative mechanism, we bring the O2 molecule closer to the [CuII(Hdatrz)] complex, in steps of 0.1 Å, fixing the oxygen atoms on the orthogonal axis only. While approaching the molecule to the complex, the bond length increases and the energy rises to a maximum. After that, the energy decreases and each oxygen atom adsorbs to a site. The nudged elastic band (NEB) method could be also an option to calculate the transition state since both the initial (O2) and final (2O*) states are well-known. We used the NEB method elsewhere.14

Details on the method used to calculate the differences in the free energies presented in this manuscript can be found in the next appendix.

Appendix B. Free Energy Calculation

Here, we summarize the method used for calculating the differences in the free energy for the ORR. The procedure was proposed by Nørskov et al.12

The reference potential is set to be that of the standard hydrogen electrode, and the chemical potential for the reaction H+ + e– can be related to that of 1/2H2 in the gas phase, which is in equilibrium under standard conditions (pH = 0, p = 1 bar, T = 298 K, and U = 0)

graphic file with name ao9b03564_m013.jpg 13

Differences in the free energy, ΔG, are calculated using the equation

graphic file with name ao9b03564_m014.jpg 14

where ΔE is the interaction energy obtained directly from DFT calculations, ΔZPE is the change in the zero-point energy, and the values used in the present study are taken from ref (12).TΔS is the entropic term whose values were obtained from standard tables for gas-phase molecules,22 ΔGpH is the contribution of the H+ concentration to the free energy (in our study we considered pH = 0), and ΔGU is the shift in energy at specific electrode potential for all states involving an electron in the electrode (−eU).

For pH other than zero, one can correct the free energy of H+ ions using the equation

graphic file with name ao9b03564_m015.jpg 15

In eq 15, kB is the Boltzmann constant.

At standard conditions ΔG (pH = 0 and U = 0) = ΔG0, and eq 14 becomes

graphic file with name ao9b03564_m016.jpg 16

Change in the free energy of O2, ΔG0 = 4.92 eV, was derived from the reaction 2H2O → O2 + 2H2 using experimental values. Besides, the entropy for gas-phase water was taken at p = 0.035 bar and T = 298 K, in which this is in equilibrium with liquid water.

The authors declare no competing financial interest.

References

  1. Nie Y.; Li L.; Wei Z. Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem. Soc. Rev. 2015, 44, 2168–2201. 10.1039/c4cs00484a. [DOI] [PubMed] [Google Scholar]
  2. Sui S.; Wang X.; Zhou X.; Su Y.; Riffat S.; Liu C.-j. A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells. J. Mater. Chem. A 2017, 5, 1808–1825. 10.1039/C6TA08580F. [DOI] [Google Scholar]
  3. Chen M.; He Y.; Spendelow J. S.; Wu G. Atomically Dispersed Metal Catalysts for Oxygen Reduction. ACS Energy Lett. 2019, 4, 1619–1633. 10.1021/acsenergylett.9b00804. [DOI] [Google Scholar]
  4. Stacy J.; Regmi Y. N.; Leonard B.; Fan M. The recent progress and future of oxygen reduction reaction catalysis: A review. Renewable Sustainable Energy Rev. 2017, 69, 401–414. 10.1016/j.rser.2016.09.135. [DOI] [Google Scholar]
  5. Thorum M. S.; Yadav J.; Gewirth A. A. Oxygen Reduction Activity of a Copper Complex of 3,5-Diamino-1,2,4-triazole Supported on Carbon Black. Angew. Chem., Int. Ed. 2009, 48, 165–167. 10.1002/anie.200803554. [DOI] [PubMed] [Google Scholar]
  6. Zhang J.; Anson F. C. Electrochemistry of the Cu(II) complex of 4,7-diphenyl-1,10-phenanthrolinedisulfonate adsorbed on graphite electrodes and its behavior as an electrocatalyst for the reduction of O2 and H2O2. J. Electroanal. Chem. 1992, 341, 323–341. 10.1016/0022-0728(92)80491-L. [DOI] [Google Scholar]
  7. Blanford C. F.; Heath R. S.; Armstrong F. A. A stable electrode for high-potential, electrocatalytic O2 reduction based on rational attachment of a blue copper oxidase to a graphite surface. Chem. Commun. 2007, 1710–1712. 10.1039/b703114a. [DOI] [PubMed] [Google Scholar]
  8. Gurusamy T.; Gayathri P.; Mandal S.; Ramanujam K. Redox-Active Copper-Benzotriazole Stacked Multiwalled Carbon Nanotubes for the Oxygen Reduction Reaction. ChemElectroChem 2018, 5, 1837–1847. 10.1002/celc.201800110. [DOI] [Google Scholar]
  9. McCrory C. C. L.; Devadoss A.; Ottenwaelder X.; Lowe R. D.; Stack T. D. P.; Chidsey C. E. D. Electrocatalytic O2 Reduction by Covalently Immobilized Mononuclear Copper(I) Complexes: Evidence for a Binuclear Cu2O2 Intermediate. J. Am. Chem. Soc. 2011, 133, 3696–3699. 10.1021/ja106338h. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Du C.; Gao X.; Chen W. Recent developments in copper-based, non-noble metal electrocatalysts for the oxygen reduction reaction. Chin. J. Catal. 2016, 37, 1049–1061. 10.1016/S1872-2067(15)61059-2. [DOI] [Google Scholar]
  11. Asahi M.; Yamazaki S.-i.; Morimoto Y.; Itoh S.; Ioroi T. Crystal structure and oxygen reduction reaction (ORR) activity of copper(II) complexes of pyridylmethylamine ligands containing a carboxy group. Inorg. Chim. Acta 2018, 471, 91–98. 10.1016/j.ica.2017.10.031. [DOI] [Google Scholar]
  12. Nørskov J. K.; Rossmeisl J.; Logadottir A.; Lindqvist L.; Kitchin J. R.; Bligaard T.; Jónsson H. Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886–17892. 10.1021/jp047349j. [DOI] [Google Scholar]
  13. Pinto L. M. C.; Juárez M. F.; Ângelo A. C. D.; Schmickler W. Some properties of intermetallic compounds of Sn with noble metals relevant for hydrogen electrocatalysis. Electrochim. Acta 2014, 116, 39–43. 10.1016/j.electacta.2013.11.003. [DOI] [Google Scholar]
  14. Pinto L. M. C.; Maia G. Oxygen Adsorption on PdPt/Au(111) – DFT Calculations. J. Phys. Chem. C 2015, 119, 8213–8216. 10.1021/acs.jpcc.5b01358. [DOI] [Google Scholar]
  15. Greeley J.; Nørskov J. K.; Mavrikakis M. Electronic structure and catalysis on metal surfaces. Annu. Rev. Phys. Chem. 2002, 53, 319. 10.1146/annurev.physchem.53.100301.131630. [DOI] [PubMed] [Google Scholar]
  16. Soler J. M.; Artacho E.; Gale J. D.; García A.; Junquera J.; Ordejón P.; Sánchez-Portal D. The SIESTA method for ab initio order-N materials simulation. J. Phys.: Condens. Matter 2002, 14, 2745–2779. 10.1088/0953-8984/14/11/302. [DOI] [Google Scholar]
  17. Kleinman L.; Bylander D. M. Efficacious Form for Model Pseudopotentials. Phys. Rev. Lett. 1982, 48, 1425–1428. 10.1103/PhysRevLett.48.1425. [DOI] [Google Scholar]
  18. Perdew J. P.; Burke K.; Ernzerhof M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865–3868. 10.1103/PhysRevLett.77.3865. [DOI] [PubMed] [Google Scholar]
  19. Perdew J. P.; Burke K.; Ernzerhof M. Errata: Generalized gradient approximation made simple [Phys. Rev. Lett. 1996, 77, 3865 – 3868]. Phys. Rev. Lett. 1997, 78, 1396. 10.1103/PhysRevLett.78.1396. [DOI] [PubMed] [Google Scholar]
  20. Perdew J. P.; Wang Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. B 1992, 45, 13244–13249. 10.1103/PhysRevB.45.13244. [DOI] [PubMed] [Google Scholar]
  21. Perdew J. P.; Chevary J. A.; Vosko S. H.; Jackson K. A.; Pederson M. R.; Singh D. J.; Fiolhais C. Atoms, molecules, solids, and surfaces: Applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B 1992, 46, 6671–6687. 10.1103/PhysRevB.46.6671. [DOI] [PubMed] [Google Scholar]
  22. Atkins P.; Paula J. D.. Physical Chemistry: Thermodynamics, Structure, and Change, 10th ed.; W. H. Freeman and Company: New York, 2014. [Google Scholar]

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