Two-dimensional iron–tetracyanoquinodimethane (Fe–TCNQ) monolayer: an efficient electrocatalyst for the oxygen reduction reaction

Abstract

Searching for efficient, cheap, and stable non-Pt electrocatalysts for the oxygen reduction reaction (ORR) has been a major challenge for the development of fuel cells. Herein, we systematically investigated the potential of the experimentally synthesized two-dimensional (2D) metal–tetracyanoquinodimethane (M–TCNQ, where M denotes Mn, Fe, and Co) monolayers as novel ORR catalysts by means of density functional theory (DFT) computations. Our results revealed that O₂ molecules can be chemisorbed and efficiently activated on the M–TCNQ monolayers, and the subsequent oxygen reduction can readily proceed via a 4e⁻ pathway. Among the monolayers, the Fe–TCNQ monolayer exhibits the highest catalytic activity with onset potentials of 0.63 and −0.20 V in acidic and alkaline media, respectively. Remarkably, its electrocatalytic performance could be further enhanced by the attachment of axial halogen ligands. Therefore, the Fe–TCNQ monolayer might serve as a promising alternative to Pt-based catalysts for the ORR in fuel cells.

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1. Introduction

Fuel cells have been hotly pursued in the past few decades as novel electrical energy conversion systems.^1–3 However, their large-scale practical application is greatly limited by the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. Although platinum (Pt) is the most effective catalyst for ORR thus far,^4–12 the shortcomings such as high cost, limited supply, and poor durability severely hinder its wide commercialization.¹³ Therefore, the search for inexpensive, efficient and stable non-Pt ORR electrocatalysts is of great importance for fuel cell technology. Some promising alternative catalysts include transition metal macrocyclic compound-based catalysts, metal-free doped carbon materials, and transition metal oxides.^14–37

One class of candidate materials to replace Pt is metal–organic frameworks (MOFs), which consist of metal ions or clusters coordinated to organic molecules to form one-, two-, or three-dimensional porous structures.^38,39 MOFs feature high surface area, controllable porosity and diversity in metal and functional groups.^40–42 These advantages render them very attractive materials for gas storage, gas purification, gas separation, and heterogeneous catalysis.^43–52 Very recently, MOFs have begun to be used for electrocatalytic applications.^53–57 For examples, Lin et al. reported that cobalt porphyrins-based MOFs exhibit outstanding catalytic activity for the CO₂ reduction in water.⁵⁵ Miner et al. demonstrated that Ni₃(HITP)₂ (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) is a very promising electrocatalyst for ORR in alkaline solution.⁵⁷

Despite these pioneering investigations on the MOF-based electrocatalysts, the theoretical studies on their potential as ORR catalysts are scarce.^58,59 We note that, as new 2D MOFs, the experimentally available metal–tetracyanoquinodimethane (M–TCNQ) monolayers have manifested unique electronic, paramagnetic, and photoactive properties.^60–62 To the best of our knowledge, however, their catalytic performance towards ORR has never been reported before, although the related studies may provide valuable insights into the development of novel non-Pt ORR catalysts.

In this work, the adsorption and reduction pathways of O₂ molecules on the Mn–, Fe–, and Co–TCNQ monolayers in both acidic and alkaline environments were systematically studied by means of spin-polarized density functional theory (DFT) computations. Our calculations demonstrated that the Fe–TCNQ monolayer exhibits excellent ORR catalytic activity, and is thus a highly potential candidate for the next-generation low-cost ORR catalysts.

2. Computational methods

All the spin-polarized DFT computations were carried out by employing the PBE functional⁶³ and the double numerical plus polarization (DNP) basis set, as implemented in the DMol³ code.^64,65 The DNP basis set has an accuracy comparable to that of Pople's 6-31 G** basis set.^66,67 The DFT semicore pseudopotential (DSPP) method,⁶⁸ which replaces core electrons by a single effective potential to introduce some degree of relativistic corrections to the core, was used for the transition metals. To accurately describe the long-range electrostatic interactions between the ORR species and catalysts, the PBE-D2 method including Grimme's dispersion correction was adopted.⁶⁹ The self-consistent field (SCF) calculations were performed with a convergence criterion of 10⁻⁶ a.u. on the total energy and electronic computations. The conductor-like screening model (COSMO)⁷⁰ was used to simulate the water solvent environment (dielectric constant: 78.54).

We set the x and y directions parallel and the z direction perpendicular to the planes of M–TCNQ monolayers, and adopted a supercell length of 10 Å in the z direction. The Brillouin zone was sampled with a 5 × 5 × 1 mesh in the geometry optimizations, whereas a 12 × 12 × 1 grid was used for the electronic structure computations. The charge transfer was calculated according to the Hirshfeld method.⁷¹

The adsorption energy (E_ad) of an ORR species on a M–TCNQ monolayer was defined as E_ad = E_{species/M–TCNQ} − E_species − E_M–TCNQ, where E_{species/M–TCNQ}, E_species, E_M–TCNQ are the total energies of the adsorbed system, free ORR species, and bare M–TCNQ monolayer, respectively. With this definition, a negative value indicates an exothermic adsorption.

We evaluated the change in Gibbs free energy (ΔG) for all the ORR steps based on a computational hydrogen electrode (CHE) model suggested by Nøskov et al.:^72–74

ΔG = ΔE + ΔE_ZPE − TΔS + ΔG_pH + ΔG_U

In the equation, ΔE is the reaction energy directly obtained from the DFT calculations. ΔE_ZPE and ΔS are the zero point energy change and entropy change upon adsorption. The system temperature T is 298.15 K. ΔG_pH = 2.303k_BT pH is the correction of the H⁺ free energy by concentration (k_B is the Boltzmann constant, and pH = 0 and 14 in acidic and alkaline conditions, respectively). ΔG_U = −neU, where n is the number of electrons transferred and U is the electrode potential. The entropies and vibrational frequencies of the gas-phase molecules are taken from the NIST database. The vibrational frequencies of the adsorbed ORR species were calculated explicitly to obtain ZPE contribution in the free energy expression, while the M–TCNQ monolayers are fixed by assuming that their vibrations are negligible.

3. Results and discussion

3.1. Structures and properties of the M–TCNQ monolayers

The Fe–TCNQ monolayer was selected as a representative of the M–TCNQ monolayers. Fig. 1 depicts its optimized structure in a 2 × 2 supercell, in which all the atoms are in a plane. The unit cell is rectangular (lattice parameters: 7.03 and 11.35 Å) and contains twelve C atoms, four H atoms, four N atoms, and one Fe atom. In the Fe–TCNQ monolayer, each TCNQ acts as a ligand and binds to the central Fe atom through N atoms, leading to a Fe–N₄ moiety in the unit cell (Fe–N bond lengths: 1.91 Å).

Fig. 1 (a) Optimized structure (in a 2 × 2 supercell) and (b) density of states of the Fe–TCNQ monolayer. The red dotted lines in (a) indicate a unit cell. The purple, gray, blue, and green balls represent Fe, C, N, and H atoms, respectively.

The binding energy of Fe with TCNQ monolayer was computed as high as −7.91 eV, which is much larger than the cohesive energy of bulk Fe, indicating the high stability of the Fe–TCNQ monolayer. The Fe–TCNQ monolayer exhibits a spin-polarized ground state with a magnetic moment of 1.70 μ_B, which is mainly on the Fe atom (1.51 μ_B) with negligible contribution from the adjacent N atoms (0.03 μ_B, Fig. S1 in ESI†). According to the Hirshfeld population analysis, the Fe atom carries a positive charge of +0.21e, much larger than those of other atoms. Since the highly positive atomic charge density or high spin density of the catalyst plays a significant role in facilitating the ORR process,⁷⁵ the Fe atom would be an ideal reactive site to catalyze the ORR. The optimized Mn– and Co–TCNQ nanosheets possess similar planar structures, in which the Mn and Co atoms coordinate with the TCNQ ligands and exhibit large binding energies of −7.13 and −7.89 eV, respectively. In addition, the magnetic moments also mainly localize on the metal atoms (2.91 and 0.78 μ_B for Mn and Co, respectively).

To get a deeper understanding on the electronic structure of the Fe–TCNQ monolayer, we computed its density of states (DOS). As shown in Fig. 1b, there are strong hybridizations between the N 2p orbitals and Fe 3d orbitals in both spin-up and spin-down channels. In particular, there are some high energy levels near the Fermi levels, indicating a high activity of the Fe–TCNQ monolayer toward the adsorbates.

3.2. O₂ adsorption on the M–TCNQ monolayers

To serve as the ORR catalyst, a 2D material should first favor the O₂ adsorption on its surface. To obtain the most stable adsorption configuration, different adsorption sites in each M–TCNQ monolayer were considered with the O₂ molecule assuming an end-on or side-on orientation.

After the geometry optimizations, the most favorable O₂ adsorption site in each M–TCNQ monolayer is found to be the central metal atom, which results are consistent with the aforementioned large positive charges and high spin densities of the metals. O₂ energetically prefers the end-on configuration with a tilted O–O bond, and forms a chemical bond with the metal atom (bond lengths: 1.79, 1.82, and 1.95 Å for the Mn–, Fe–, and, Co–O bonds, respectively, Fig. 2). As a result, each metal atom was pulled out from the basal plane by about 0.30 Å. The O–O bond is elongated from 1.21 to ∼1.29 Å due to the charge transfer of ∼0.20e from the M–TCNQ monolayers to the half-filled O₂–2π* orbitals. Clearly, the O₂ molecule has been effectively activated.

Fig. 2 Optimized structures of the O₂ adsorbed-(a) Mn–, (b) Fe–, and (c) Co–TCNQ monolayers. The unit of bond length is Å.

The O₂ adsorption energies on the Mn–, Fe–, and Co–TCNQ monolayers are −0.94, −0.94, and −0.64 eV, respectively, suggesting that the oxygen molecule is strongly chemisorbed. The corresponding ΔG values are −0.25, −0.24, and 0.04 eV, respectively. This elementary reaction step can be written as: O₂(g) → , where the asterisk denotes an adsorption site. Notably, the adsorbed O₂ molecule is unlikely to dissociate into two separated O atoms at room temperature. For example, the energy barrier for the reaction of → 2O^* on the Fe–TCNQ monolayer is calculated as high as 2.99 eV.

3.3. ORR pathways in an acidic medium

After the O₂ activation, we further studied the ORR steps on the surfaces of the three M–TCNQ monolayers in both acidic and alkaline media.

In an acidic solution, the complete 4e⁻ ORR would occur via the following pathway:

OOH^* + 3(H⁺ + e⁻) → O^* + H₂O + 2(H⁺ + e⁻) (3a)

OOH^* + 3(H⁺ + e⁻) → H₂O₂ + 2(H⁺ + e⁻) (3b)

O^* + H₂O + 2(H⁺ + e⁻) → OH^* + (H⁺ + e⁻) (4)

OH^* + (H⁺ + e⁻) → H₂O (5)

The optimized configurations of various ORR species-adsorbed M–TCNQ monolayers along the above reaction pathway are depicted in Fig. 3, and the corresponding free energy profiles are summarized in Fig. 4.

Fig. 3 Optimized configurations of various ORR species, including OOH, O, OH, H₂O, and H₂O₂, on (a) Mn–, (b) Fe–, and (c) Co–TCNQ monolayers (two views). The unit of bond length is Å.

Fig. 4 Free energy profiles (at different electrode potentials) for the ORR on (a) Mn–, (b) Fe–, and (c) Co–TCNQ monolayers in an acidic medium.

By adsorbing a proton coupled with an electron transfer, the activated O₂ might be first hydrogenated to an OOH group (eqn (2)). This process should be feasible, as indicated by the calculated ΔG values of −0.60, −0.63, and −0.47 eV for the Mn–, Fe– and Co–TCNQ monolayers, respectively (Fig. 4). In the OOH^* configuration, the hydrogen atom binds to the upper O atom, and the O–O bonds are further elongated to 1.47, 1.45, and 1.42 Å for M = Mn, Fe and Co, respectively.

The as-formed OOH* species can be further hydrogenated by reacting with a second proton, which may attack either the pre-hydrogenated O site (O2 in Fig. 3) or the pre-unhydrogenated one (O1 in Fig. 3). Therefore, the ORR can further proceed via two different pathways: a 4e⁻ or a 2e⁻ process. Accordingly, O₂ is reduced to two H₂O molecules or a H₂O₂ molecule.

Providing that the ORR follows the 4e⁻ pathway, the pre-hydrogenated O2 site of the OOH group will adsorb one proton coupled with one electron transfer to generate the first H₂O molecule (eqn (3a)). As a result, the O1 atom will be left on the metal site with O–M bond lengths of 1.64, 1.68, and 1.78 Å for Mn, Fe, and Co, respectively (Fig. 3). The above processes on the Mn–, Fe– and Co–TCNQ monolayers are all downhill in the free energy profiles by 2.44, 1.77, and 1.18 eV, respectively (Fig. 4). Subsequently, the remaining O1 atom interacts with another proton upon one-electron reduction to yield an OH group (eqn (4)), which still resides on the metal site with O–M distances of 1.82, 1.87, and 1.87 Å for Mn, Fe, and Co, respectively. The corresponding Gibbs free energies decrease by 0.76, 1.34, and 1.89 eV, respectively (Fig. 4). Finally, the adsorbed OH group can form a new H₂O molecule by reacting with one proton and undergoing one-electron reduction (eqn (5)). This final step is also downhill in the free energy profiles by 0.83, 0.90, and 1.38 eV on the three electrocatalysts, respectively (Fig. 4). Because of the weak interactions between H₂O and the M–TCNQ monolayers (E_ad is less negative than −0.30 eV), the water molecule can easily escape from the layer surfaces, leading to the recovery of the catalysts for the next cycle.

In the alternative 2e⁻ reduction pathway, the new coming proton interacts with the pre-unhydrogenated O1 site of the OOH group to yield a H₂O₂ molecule (eqn (3b)). The ΔG values of this process are −0.41, −0.38, and −0.82 eV on the Mn–, Fe–, and Co–TCNQ monolayers, respectively, much less than those of the step OOH^* → O^* + H₂O (−2.44, −1.77, and −1.18 eV). Clearly, the ORR on the M–TCNQ monolayers energetically prefers the 4e⁻ reduction pathway rather than the 2e⁻ one. Therefore, the M–TCNQ monolayers may serve as potential ORR catalysts, since it is well known that H₂O₂ could greatly affect the durability of fuel cells and is a highly undesirable ORR product.

Fig. 4 also shows the effect of electrode potential on the ORR processes. At U = 0 V, most of the elementary steps are exothermic and can occur spontaneously with the only exception of the O₂ adsorption on Co–TCNQ nanosheet (ΔG = 0.04 eV). With increased potentials, however, the energy profiles gradually turn to be uphill. On each M–TCNQ monolayer, the reaction of + H⁺ + e⁻ → OOH^*exhibits the smallest ΔG value and is thus the rate-determining step. Remarkably, the maximum potentials at which all the reactions are still exothermic (i.e., the onset potentials) are 0.60, 0.63, and 0.47 V, respectively, for the Mn–, Fe–, and Co–TCNQ nanosheets. Thus, in acidic media, the Fe–TCNQ nanosheet may exhibit the best catalytic performance due to its largest onset potential (0.63 V), which is comparable to those of well-known electrocatalysts such as Pt (0.79 V),⁷⁶ iron–phthalocyanine monolayer (0.68 V),⁵⁹ and N-doped graphitic materials (0.50–0.80 V).⁷⁷

3.4. ORR pathways in an alkaline medium

In an alkaline medium, the complete 4e⁻ ORR can proceed via the following steps:

OOH^* + OH⁻ + H₂O + 3e⁻ → O^* + 2OH⁻ + H₂O + 2e⁻ (8)

O^* + 2OH⁻ + H₂O + 2e⁻ → OH^* + 3OH⁻ + e⁻ (9)

OH^* + 3OH⁻ + e⁻ → 4OH⁻ (10)

In Fig. 5, we present the corresponding free energy profiles at different potentials. The reaction of → OOH^* is uphill at U = 0 V on the three M–TCNQ nanosheets, suggesting that it is the rate-determining step. The onset potential for the Fe–TCNQ monolayer is 0.20 V, which is smaller than that on the Mn–(0.23 V) and Co–TCNQ sheets (0.36 eV). When an ideal electrode potential of 0.40 V is applied at pH = 14, the energy levels of every net coupled proton and electron transfer step are shifted up by 0.40 eV. In an alkaline environment, the free energies of the reactions between O₂ and H₂O (i.e., the rate-determining step) change to 0.63, 0.60, and 0.76 eV on the Mn–, Fe–, and Co–TCNQ monolayers, respectively. Generally, the ΔG value of the rate-determining step can be used as a measure of the whole ORR rate, and smaller ΔG indicates faster ORR process. Hence, in an alkaline medium, the Fe–TCNQ monolayer is a more efficient ORR catalyst than the other two candidates.

Fig. 5 Free energy profiles (at different electrode potentials) for the ORR on (a) Mn–, (b) Fe–, and (c) Co–TCNQ monolayers in an alkaline medium.

3.5. Effect of the axial ligands

Due to their high activity, the central metal atoms in the M–TCNQ monolayers could be readily modulated from 4- to 5-coordination. Then, a question naturally arises: how will the coordination number of metal affect the catalytic performance of a M–TCNQ monolayer? To address this issue, we further studied the ORR on the Fe–TCNQ monolayer functionalized by some common axial ligands (R) such as F, Cl, Br, NO₂ or CN group.

Our computations show that the E_ad values for O₂ are −0.58, −0.58, −0.60, −0.22, and −0.13 eV on the F⁻, Cl⁻, Br⁻, NO₂⁻, and CN–Fe–TCNQ nanosheets, respectively. The less negative values than that on the Fe–TCNQ nanosheet (−0.94 eV) suggest less favorable O₂ adsorption on the R–Fe–TCNQ monolayers. Especially, the NO₂ and CN groups seriously reduce the chemical activity of the Fe–TCNQ monolayer due to their strong electron-withdrawing capability. Thus, the 5-coordination Fe in R–Fe–TCNQ monolayer is less effective in donating electrons to O₂ than the one without axial ligand. Note that the halogen–Fe–TCNQ monolayers may be still feasible for O₂ activation, since their ΔG values for the O₂ adsorption are close to zero (0.08 eV). In contrast, the corresponding ΔG values are 0.42 and 0.55 eV for the NO₂⁻ and CN–Fe–TCNQ monolayers, respectively, indicating that the O₂ activations on them are unlikely.

Since O₂ molecule could be activated on the halogen–Fe–TCNQ nanosheets, we further computed the ΔG values of the subsequent ORR along the 4e⁻ reduction pathway by choosing the Br–Fe–TCNQ monolayer as an example (Fig. S2†). The reduction of to OOH^* is still the rate-determining step and has the ΔG value of about −0.70 eV in an acidic medium, which is slightly more negative than that of the Fe–TCNQ monolayer (−0.63 eV). In contrast, in an alkaline medium, the onset potential on the Br–Fe–TCNQ nanosheet (0.12 V) is smaller than that on the Fe–TCNQ monolayer (0.20 V). Thus, the Br addition could enhance the electrocatalytic performance of the Fe–TCNQ monolayer under both acidic and alkaline conditions, although the corresponding O₂ adsorption is slightly endothermic (ΔG: 0.08 eV). Our results are consistent with the Sabatier principle,^21,78 which states that an ideal catalyst should bind with the reaction species neither too strongly nor too weakly.

4. Conclusions

In summary, our spin-polarized DFT computations demonstrated that the 2D Fe–TCNQ monolayer exhibits superior catalytic performance for the ORR in both acidic and alkaline media. Its onset potentials are 0.63 and 0.20 V in acidic and alkaline media, respectively. Especially, attaching suitable axial ligands such as halogen to the Fe–TCNQ nanosheet would further improve its catalytic activity. Therefore, our theoretical results suggest a new class of quite promising non-Pt ORR catalyst for fuel cells. Recent studies have found that introducing active nonprecious transition metal (e.g., Fe, Co, and Ni) atoms into the organic polymers complexes could lead to electrocatalysts with high ORR activity and durability.^79–81 More recently, the applicability of TCNQ-based nanomaterials has emerged in some important research areas including catalysis, sensing and biology.⁸² We thus believe that the metal–TCNQ monolayers will be employed by experimental peers for ORR applications in the quite near future. We also hope that the current work could inspire more experimental and theoretical studies on exploring the potential of 2D organometallic sheets as single-atom catalysts for ORR.

Acknowledgements

Support by NSFC (21203048 and 21103224), the Natural Science Foundation of Heilongjiang Province (B2015006), and the Excellent Young Foundation of Harbin Normal University (XKYQ201304) are gratefully acknowledged.

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Footnotes

† Electronic supplementary information (ESI) available: Spin charge density of the Fe–TCNQ monolayer, free energy profile for ORR on Br–Fe–TCNQ monolayers in acidic and alkaline media. See DOI: 10.1039/c6ra14339c

‡ Nan Wang and Liyan Feng contributed equally.

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