Theoretical study of a CuCo dual-atom catalyst for nitrogen fixation

Abstract

Developing low-cost catalysts for highly efficient nitrogen reduction reaction (NRR) in industrial applications is a great challenge. Dual-atom catalysts (DACs) have also aroused scientific interest as potential NRR catalysts due to their possible higher activity and atom utilization than single atom catalysts. Using density functional theory, we have investigated the NRR performances of heteronuclear CuCo DACs with different coordination configurations. Comparisons with the possible Cu or Co SACs and homonuclear dual-atom catalysts for the NRR performance were also made. We find that O-CuCo-N DAC has superior NRR performance, where the NRR easily takes place through the alternating pathway from the side-on N₂ adsorption mode and the potential limiting step is the first hydrogenation step (*NN* → *NHN*) with a Gibbs free energy change of 0.55 eV. The good activity of O-CuCo-N DAC benefits from the regulation provided by Cu and the coordination environment. In addition, O-CuCo-N DAC also exhibits good selectivity and durability.

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1 Introduction

Reduction of dinitrogen (N₂) to ammonia (NH₃) is of critical importance to sustain life on earth, since ammonia is an essential commodity chemical for agriculture and the chemical industry. Currently, ammonia production is still dominated by the traditional Haber–Bosch process,^1,2 which is emission and energy intensive due to the high working temperature and pressure. The electrochemical nitrogen reduction reaction (NRR, N₂ + 6H⁺ + 6e⁻ → 2NH₃), as performed by the nitrogenase enzymes in bacteria, is a promising alternative because it can be powered by clean energy and operated under ambient conditions, and does not cause CO₂ emissions.^3–6 The main challenge of the NRR lies in finding an electrocatalyst with high stability, activity and selectivity. To date, different types of NRR catalysts such as noble metals,^7–11 two dimensional materials,^12–14 oxides,^15,16 nitrides^17,18 and sulfides^19–21 have been explored. However, their selectivity is generally low because of competition with the hydrogen evolution reaction (HER) and thus they cannot achieve a high faradaic efficiency. Recently, single-atom catalysts (SACs) with atomically distributed metal active centers have emerged as a new frontier in catalysis and aroused widespread attention since they can theoretically realize the largest metal atom utilization, a higher catalytic activity and selectivity.^22,23 Various single metal atoms anchored on different systems have been extensively investigated for their catalytic performance.^24–29 SACs also exhibit excellent potential for N₂ fixation.^30–34 Nitrogen or oxygen-doped carbon materials, which are earth-abundant and cost-effective, have a great advantage as substrates to support single atoms. High faradaic efficiency and NH₃ yield rate have been reported in experiments from single atom dispersed, N- or O-doped carbon materials for ambient ammonia synthesis.^35,36 Also, other kinds of single atoms, anchored on doped graphene were proposed for the NRR from theoretical computations.³⁷ In those models, the single atoms were normally coordinated purely by either N or O atoms.³⁸ However, based on experiments, assuming nearby O and N dopants in a carbon material is also reasonable.

Inspired by enzymes containing metal pair, dual metal atoms catalysts (DACs), including both homonuclear and heteronuclear DACs, have also been developed to boost the activity of SACs by providing diverse active sites and synergistic interactions between adjacent atoms for the oxygen reduction reaction (ORR)^39,40 and NRR.^41–44 Fe and Mo metal dimers situ-anchored on defect-rich graphene layer (FeMoN_xC), and isolated Fe, Mo atoms anchored by hierarchical N doped carbon nanotubes (FeMo/NC) were reported that they could achieve selective electroreduction of N₂ to NH₃ and a stronger catalytic activity than its single-metal counterparts.^41,42 Wang et al. reported that the synergy of Fe and Cu multi-atom clusters on graphitic carbon nitride provides greatly improved NRR performance, with nearly doubled NH₃ yield and Faradaic efficiency of up to 34% when compared to the single-metal counterparts.⁴³ Effects of adjacent single-TM atoms on Fe anchored C₂N SACs,⁴⁴ as well as dual metal atoms anchored on graphite carbon nitride were theoretically investigated.⁴⁵ Nevertheless, a deeper understanding of the synergistic mechanism is imperative to bridge the gap between theory and experiments. It is noted that Cu is an earth-abundant non-noble metal element, Cu-based catalysts have attracted much attention due to their facile preparation in NRR and Co-based catalysts have also been widely investigated to be excellent NRR catalysts.^46–48 We speculate that the coexistence of dual metal atoms Cu and Co, as well as the O and N coordination, could lead to a synergistic regulation on the NRR intermediates. Also, carbon-based catalyst with diatomic Fe–Co sites in which the Fe and Co atoms are respectively coordinated to N and O atoms (as witnessed by HAADF-STEM and XAS) have been recently successfully synthesized for electrocatalytic oxygen reduction and evolution in experiments,⁴⁹ which proves that DACs with N and O coordination are experimentally feasible.

Herein, we chose Cu and Co as the two hetero metal atoms of our dual-atom catalysts,^50–53 as well as N and O doped graphene as the support, to study its NRR performance. In our work, the NRR activities of heteronuclear CuCo DACs with various N and O coordination structures were investigated and compared with the Cu(Co) single atom catalysts and homonuclear CuCu(CoCo) dual atom catalysts. Our results show that an O-CuCo-N DAC has the lowest NRR reaction energy barrier (0.55 eV) and exhibits the best NRR performance. We also reveal that the Cu–O₃ moiety efficiently regulates the d orbital energy levels of Co in the Co–N₃ moiety and optimizes the adsorption energy of N containing intermediates on CuCo DAC.

2 Computational methods

All the computations were performed within spin-polarized density functional theory (DFT) methods using the Vienna Ab initio Simulation Package (VASP).^54–56 Though, transition metals with partially filled d orbitals were involved as active center, negligible differences (<0.05 eV) between DFT and DFT+U method were reported for the free energies.⁵⁷ Therefore, the electron–electron exchange–correlation interaction was described within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof functional through out.⁵⁸ The van der Waals interactions were accounted for by using the Grimme custom method for DFT-D3.⁵⁹ The projector augmented wave (PAW) method has been used to describe the inert core electrons.^60,61 A cut-off energy of 450 eV was employed for the expansion of the wave functions. A vacuum gap of 15 Å was imposed to avoid interactions between periodic images. Since the solvation-induced stabilization of reaction intermediates in the NRR is within 0.2 eV (affecting the limiting potential for the NRR by about 0.1 eV),⁶² the effects of solvation were not taken into account. The different configurations were constructed in a 72-atom graphene layer, where 4 carbon atoms were removed to achieve a cavity and anchor the Cu and Co atoms. The coordination of Cu and Co was realized by substitution of C atoms with O and N atoms. Therefore, the basic model was a periodic graphene structure containing 62C atoms, one Cu atom, one Co atom, three O and three N atoms. We relaxed the structures until the total energy changes within 1 × 10⁻⁴ eV per atom and the Hellmann–Feynman force on each atomic site was less than 0.02 eV Å⁻¹. A 2 × 2 × 1 Monkhorst–Pack k-point sampling was used for Brillouin-zone integration throughout.⁶³ The formation energy of the anchored dual atom system was calculated according to the equationwhere E_HAC is the total energy of the supercell with the hetero atoms anchored, E_host is the total energy of the supercell without the metal atoms, n_i is the number of atoms type i, introduced into the supercell when forming the configuration, and μ_i is the chemical potential of that atom in its standard metallic states.

Six coupled proton and electron (H⁺/e⁻) transfer steps are involved in the NRR process. In each step, the coupled H⁺/e⁻ pair transfers from solution to the adsorbed species on the surface of the catalyst. The Gibbs free energy change (ΔG) of each step was calculated by employing the standard hydrogen electrode (SHE) model proposed by Norskov et al., where the chemical potential of the H⁺/e⁻ pair was considered as half of the energy of H₂ gas molecule.⁶⁴ This way, the ΔG can be calculated as

ΔG = ΔE + ΔE_ZPE − TΔS + ΔG_U + ΔG_pH (2)

where ΔE is the reaction energy which can be obtained directly from the DFT calculations, ΔE_ZPE and ΔS represent the difference in zero point energy and entropy,⁶⁵ which were computed by considering only the vibrational frequencies. The frequencies and entropies of the molecules in the gas phase were taken from the NIST database. The temperature T was set to be 298.15 K. ΔG_U is the free energy contribution related to the applied electrode potential. ΔG_pH is the concentration dependent correction of the H⁺ free energy and can be obtained by the formula ΔG_pH = k_BT × pH × ln10, where the Boltzmann constant is denoted as k_B and the pH is set to zero. The potential-limiting step in the NRR processes possess the most positive ΔG (expressed as ΔG_max) and the overpotential (h) is calculated by the definition:

η = U_equilibrium − (−ΔG_max/e), (3)

where the U_equilibrium is the equilibrium potential of NRR and set to be −0.16 V.⁶⁶

3 Results and discussion

3.1 Heteronuclear CuCo DACs

3.1.1 Stabilities. In the NRR process, besides the metal species, the coordination of the metal center also has a great impact on the adsorption, activation, and conversion of N₂. According to experimental observation, Cu and Co are likely to be coordinated with N or O. While, theoretically, there should be many configurations (C³₆ = 120 in total), we can not consider all of them and just chose those typical as the experimental indicated.⁴² Therefore, four different coordination structures of heteronuclear DACs were constructed, as shown in Fig. 1. Table 1 shows the formation energies of these, with N-CuCo-N having the lowest value, while for O-CuCo-O it is so high that this configuration is unlikely to form. Then we investigated the stability of DACs by calculating the binding energies (E_b) of the hetero atoms with doped graphene, and compared with their corresponding cohesive energies (E_c) of Cu and Co metals. For a stable catalyst, it is vital that the metal atoms should not diffuse and aggregate. The definitions of E_b and E_c are:

E_C = E(M_bulk)/N − E(M) (5)

where the E(M@G) is the energy of DAC, E(M), E(M_bulk) and N represent the energy of the single atom in vacuum, the energy of the bulk crystal unit cell of the corresponding metal, and the number of atoms in the unit cell, respectively.

Fig. 1 Coordination structures of O-CuCo-O, O-CuCo-N, N-CuCo-O, N-CuCo-N. Brown spheres: C, red spheres: O, orange spheres: Cu, pink spheres: Co, blue spheres: N.

Table 1 Cohesive energy (E_c), binding energy (E_b) and formation energy (E_f) of HACs, all in eV

	Ec (cal.)	Ec (exp.)
Cu	−3.70	−3.5
Co	−5.27	−4.4

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	Eb	Ef
O-CuCo-O	−0.25	4.72
O-CuCo-N	−5.59	3.10
N-CuCo-O	−5.37	3.33
N-CuCo-N	−10.89	2.96

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The binding of metal atoms with N and O doped graphene is thermodynamically more favorable if E_b is more negative than E_c. Our results show that except for O-CuCo-O, all the other 3 configurations have a much more negative binding energy than the cohesive energy of Cu and Co in bulk metal, which means that Cu and Co atoms can anchor on doped graphene stably without aggregation. O-CuCo-O, with E_b being −0.25 eV, was proved to be thermodynamically unstable under 500 K. Therefore, further investigations were made only on the other three configurations in following.

3.1.2 N₂ conversion into NH₃. The adsorption of N₂ is an essential prerequisite of NRR, so we first calculated the adsorption properties of N₂.

For the O-CuCo-N configuration, we considered three possibilities: adsorption on top of the Cu site, on top of the Co site and on the CuCo site. The structural optimization showed that, upon adsorption of N₂ on the Cu site, the structure will break down due to the weak bonding between Cu and O, but N₂ can be easily adsorbed on the CuCo site with end-on and side-on adsorption mode, as shown in Fig. 2. The adsorption energy in the side-on mode is −0.57 eV and in the end-on mode −0.50 eV, the difference being negligible.

Fig. 2 Adsorption modes of N₂ on the Cu-Co site with end-on (left) and side-on mode (right). Brown spheres: C, red spheres: O, orange spheres: Cu, pink spheres: Co, blue spheres: N.

In addition, the charge density difference in Fig. 3 clearly show that Cu and Co donate electrons to the empty π* orbitals of N₂, while, the loss of electrons in the N–N σ bond indicates that the N₂ is indeed activated. The Bader charge analysis also shows that, after adsorption, N₂ gains 0.41 e and 0.53 e from the substrate with end-on and side-on adsorption mode, respectively. Consequently, the bond lengths of N₂ are elongated from 1.12 Å in the free gas state to 1.15 Å and 1.18 Å for end-on and side-on adsorption mode, respectively. These results suggest that the side-on adsorbed N₂ is better activated.

Fig. 3 Charge density difference (δρ = ρ_A+B − ρ_A − ρ_B) of adsorbed N₂ with end-on (left) and side-on mode (right) on O-CuCo-N DAC. Yellow stands for electron accumulation and cyan for electron loss, the isosurface value is 0.004 e/a₀².

We calculated first the possible nitrogen reduction reaction pathway starting from the side-on mode (red lines in Fig. 4.) As can be seen, the first H⁺/e⁻ pair prefers to attack the N atom that is connected to Cu. The energy change of this step is 0.55 eV, and induces a N–N bond length elongation from 1.18 Å to 1.27 Å. In the second step, *NHHN* or *NNH₂ can form through the enzymatic, by H⁺/e⁻ alternatively attacking the N atom connected with Co or through an alternative pathway by H⁺/e⁻ attacking the same N atom. The results show that to form *NHHN* and *NNH₂, it needs 0.05 eV and 0.95 eV respectively. Therefore, the enzymatic pathway from *NHNH* is further considered. In the third step, *NH₂HN* is formed with an uphill energy of 0.17 eV, and the bond length is correspondingly further elongated from 1.33 Å in *NHHN* to 1.46 Å. In the fourth step, the N–N bond breaks and forms *NH₂ + *NH₂ on the Cu and Co atoms with an energy release of 0.99 eV. The next two steps to form*NH₃ are still downhill in energy by 0.90 eV and 0.02 eV. Desorption free energy of NH₃ is calculated to be 0.42 eV, indicating the excellent durability of O-CuCo-N DAC.

Fig. 4 Free energy diagram along the reaction path of NRR proceeded on O-CuCo-N starting with side-on and end-on configuration.

Next, we considered the distal reaction pathway from the end-on mode. As shown in Fig. 4 (blue lines), the Gibbs free energy change of the first hydrogenation step is 0.79 eV, which is 0.24 eV higher than that from the side-on mode. From *N₂H to *N₂H₂, it only needs an energy input of 0.06 eV. The potential limiting step is from *N₂H₂ to *N₂H₃ with an energy change of 0.98 eV. The following two steps are downhill in energy, whereas, the last step to form *NH₃ and the final desorption of NH₃ are endothermic processes where an energy investment of 0.44 eV and 0.46 eV is needed, respectively.

Therefore, for O-CuCo-N DAC the NRR prefers to take place along the enzymatic pathway from the initial side-on N₂ adsorption mode and the potential-limiting step is the first hydrogenation step of N₂ with overpotential η being 0.39 V.

For the N-CuCo-O and N-CuCo-N configuration, we found that after relaxation N₂ will finally bond to the O/N coordinated Co atom with end-on adsorption mode (see Fig. 5). The adsorption of N₂ is strong in the N-CuCo-O configuration, while much weaker in N-CuCo-N, with the adsorption energy calculated to be -0.94 eV and −0.13 eV, respectively. For N-CuCo-O, the first hydrogenation step of N₂, which is also the potential determining step, needs an input energy of 1.41 eV (Fig. S1†). This is much higher than 0.79 eV/0.55 eV in O-CuCo-N configuration. Also in N-CuCo-N, it takes 1.50 eV to be first hydrogenated. Therefore, we may safely draw the conclusion that for the heteronuclear CuCo DACs considered herein, the O-CuCo-N configuration is the best one given the formation energy, stability, and NRR activities.

Fig. 5 Adsorption mode of N₂ on Co site of N-CuCo-O and N-CuCo-N.

3.2 Homonuclear DACs

3.2.1 Stabilities. Since homonuclear DACs also has a big chance to form during the experimental fabrication, we investigated the NRR properties of the following possible DACs: O-CoCo-O, O-CoCo-N, N-CoCo-N, O-CuCu-O, O-CuCu-N, N-CuCu-N. Results of geometry optimization suggest that O-CuCu-O is structurally not stable because of the weak bonding between Cu and O. The formation and binding energies listed in Table 2 indicate that O-CoCo-O has a relatively weak binding (E_b being −1.34 eV) between CoCo and the carbon support, as well as a high formation energy. All the other configurations are strongly bonded and have a formation energy less than 4 eV. Considering that O-CoCo-O is most likely not stable, we carried out the molecule dynamics simulation. The results show that the energy fluctuates and the structure also breaks down in 3000 fs under 1000 K, but under 500 K, the structure is thermodynamically stable (see Fig. 6). We subsequently studied their NRR properties.

Table 2 Cohesive energy (E_c), binding energy (E_b) and formation energy (E_f) of DACs, all in eV

	Ec (cal.)	Ec (exp.)
Cu	−3.70	−3.5
Co	−5.27	−4.4

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	Eb	Ef
O-CoCo-O	−1.34	5.26
O-CoCo-N	−6.65	3.67
N-CoCo-N	−12.12	3.36
O-CuCu-N	−4.73	2.34
N-CuCu-N	−9.36	2.86

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Fig. 6 The energy and structure of O-CoCo-O vs. number of steps in 5000 fs.

3.2.2 N₂ conversion into NH₃. For O-CoCo-O, our results show that N₂ can be strongly adsorbed with both side-on and end-on mode, and the first hydrogenation step takes 0.47 and 0.48 eV, respectively, which are even lower than that in O-CuCo-N. Therefore, we continue to study the following reaction steps through the advantageous distal pathway from end-on mode and enzymatic pathway from the side-on mode. As shown in Fig. 7, it needs a high energy input of 1.17 eV for *N₂H₂ to form *N₂H₃ and it takes 1.32 eV for the *NH₃ desorption through the distal pathway. While, through the enzymatic pathway, the highest hydrogenation barrier takes places in forming *NH₃NH₃* with an uphill energy of 0.54 eV, which is comparable with the limiting potential in O-CuCo-N. Whereas, the desorption of *NH₃ requires a high energy of 1.01 eV, indicating a relatively poor durability.

Fig. 7 Free energy diagram along the reaction path of NRR proceeded on O-CoCo-O starting with side-on and end-on configuration.

As can be seen from Table 3, for O-CoCo-N, ΔG(N₂→N₂H) is lower from the side-on N₂ adsorption mode (0.75 eV) than from the end-on N₂ adsorption mode (1.29 eV). For N-CoCo-N, ΔG(N₂→N₂H) is 0.61 eV from side-on N₂ adsorption mode. While, for O-CuCu-N, N₂ prefers to adsorb with end-on mode on Cu site connected to O, but with a much higher ΔG(N₂→N₂H) of 1.48 eV. As for N-CuCu-N, N₂ cannot be effectively activated as the bond length (1.116 Å) elongation is negligible, also the first hydrogenation step needs an energy of more than 2.00 eV.

Table 3 Calculated adsorption energy of N₂ , N–N bond length, Gibbs free energy change for the first hydrogenation step and maximum Gibbs free energy change (corresponds to potential determining step, PDS) of NRR in different DACs. All energy in eV

Heteronuclear DACs	ΔG(N2)	N–N bond length	ΔG(N2 → N2H)	ΔGmax (PDS)
O-CuCo-N (side-on)	−0.50	1.185	0.55	0.55
O-CuCo-N (end-on)	−0.57	1.147	0.79	0.98
N-CuCo-O (end-on Co site)	−0.94	1.142	1.41	1.41
N-CuCo-N (end-on Co site)	−0.13	1.136	1.50	—

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Homonuclear DACs	ΔG(N2)	N–N bond length	ΔG(N2 → N2H)	ΔGmax (PDS)
O-CoCo-O (side-on)	−1.58	1.210	0.47	1.01
O-CoCo-O (end-on)	−1.24	1.170	0.48	1.32
O-CoCo-N (side-on)	−0.21	1.186	0.75	—
O-CoCo-N (end-on)	−0.67	1.143	1.29	—
N-CoCo-N (side-on)	−0.22	1.182	0.61	—
O-CuCu-N (end-on)	−0.22	1.129	1.48	—
N-CuCu-N (side-on)	−0.23	1.116	2.03	—

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As summarized in Table 3, while O-CoCo-O is the only homonuclear DAC that has comparable ΔG(N₂→N₂H) with O-CuCo-N, it is not as good as O-CuCo-N DAC since the structure is relatively not stable enough and desorption of NH₃ is difficult to achieve. Heteronuclear O-CuCo-N DAC exhibits the best NNR activity due to the lowest ΔG_max (PDS) of 0.55 eV, which is also lower than the reported results in different references, as shown in Table 4.

Table 4 Comparisons of the maximum Gibbs free energy change of our work with data reported in difference references

	O-CuCo-N^our work	FeMoNxC^41	Fe2Cu@C3N4^43	Fe–Fe@C2N^44	Fe–Mo@C2N^44
ΔGmax (eV)	0.55	0.91	0.58	0.76	0.68

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3.3 Comparisons with SACs

Earlier, N coordinated single atom catalysts have been widely investigated. In reality, dual-atom and single-atom active sites will coexist to a large extent. Therefore, we compared the NRR activities of DACs and possible SACs: CuN₄, CoN₄, CuO₄ and CoO₄.

We analyzed the stabilities of SACs and found that both Cu and Co have strong binding with N coordinations, which are more negative than the cohesive energies of Cu and Co in bulk. In contrast, Cu and Co have a pretty weak binding with O coordinations, as can be seen from Table 5. Though both CuO₄ and CoO₄ can strongly bond with N₂, as the adsorption energies being −0.69 eV and −1.08 eV, respectively, the weak binding indicates that Cu and Co may not stay disperse as single atoms but prefer to get aggregated. Inversely, though CuN₄ and CoN₄ are stable in binding, the adsorption of N₂ is very weak in both the end-on and side-on mode. Most importantly, the corresponding Gibbs free energy changes from *N₂ to *N₂H are all very large in both O and N coordinated SACs, e.g. the lowest ΔG(N₂→N₂H) is 0.93 eV in CoO₄ from the side-on adsorption mode. Therefore, Cu SACs and Co SACs are not in the same league as DACs in NRR catalytic performance.

Table 5 Adsorption energies (s denotes side-on configuration), binding energies and corresponding cohesive energies of N and O coordinated SACs (energy in eV)

SACs	Eb	Gads(N2)	ΔG(N2→N2H)
CuN4	−5.28	0.37/0.37(s)	2.13/2.16
CoN4	−7.65	0.29/0.38(s)	1.17/1.17
CuO4	−0.28	−0.69/−0.28(s)	1.47/1.08
CoO4	−0.92	−1.08/−0.66(s)	1.18/0.93

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3.4 Competition with HER

As has been discussed above, the heteronuclear O-CuCo-N is the most promising highly efficient NRR electrocatalyst. To assure that a good NRR performance can indeed be achieved in O-CuCo-N DAC, we have also investigated the hydrogen evolution reaction (HER), as it is the major competing reaction that affects the faradaic efficiency of the nitrogen reduction reaction. The ΔG of HER was calculated to be 0.92 eV in O-CuCo-N, which is much higher than that of NRR. Theoretically, the faradaic efficiency of NRR can be estimated according to the Boltzmann distribution:⁶⁷
where δ is the Gibbs free energy difference between HER and NRR limiting-potential step, k_B is the Boltzmann constant, and T is the room temperature which was set to be 298.15 K. The O-CuCo-N achieves a high theoretical faradaic efficiency of 100%, demonstrating a high NRR selectivity over HER.

3.5 Activity mechanisms

For NRR catalysts, transition metal atoms are always chosen as the active center. N₂ adsorption and activation are usually manipulated by the mechanism of electron acceptance-donation between d orbitals of the transition atoms and the frontier molecular orbital of N₂.⁶⁸ From the projected density of states (PDOS) in Fig. 8, it can be seen that before N₂ adsorption, the O-CuCo-N configuration has a large spin polarization mainly contributed by Co and the Co spin-down d orbital is much closer to the N₂ empty π* orbital than Cu, which means that Co will contribute most to the N₂ activation, i.e., the Cu atom may take the role as electrons bank. After N₂ adsorption, the spin polarization of Co is weakened, and some d–π* orbitals are partially occupied due to the electrons transferred from Co and Cu d states.

Fig. 8 Density of states of N₂ with end-on (left) and side-on adsorption mode (right) on O-CuCo-N DAC.

To deepen our insight into the origin of the adsorption behaviour of N₂, we made a calculation on the spin moment of Fe and Co. We found that N₂ prefers to adsorb with end-on mode on the metal site which is more highly spin-polarized, namely the transition metal Co. The side-on N₂ adsorption mode can be achieved as the two metal atoms are both spin polarized.

Compared with the homonuclear DACs, we found that the existence of Cu can regulate the spin polarization of Co. The magnetic moment of Co in O-CoCo-N is 1.208 μB (O–Co) and 0.552 μB (Co–N), respectively, leading to N₂ end-on adsorption on O coordinated Co site. Whereas, the magnetic moment of Co in O-CuCo-N changes to 1.326 μB (Co–N) due to regulation of Cu (0.202 μB), leading to the N₂ end-on adsorption site change to N coordinated Co in O-CuCo-N. Since both Cu and Co are spin polarized in O-CoCo-N and O-CuCo-N, N₂ side-on adsorption can also be realized. Also, the coordination structure of metal atom has an effect on its spin polarization. Compared with O-CuCo-N (Co: 1.326 μB, Cu: 0.202 μB), the spin polarization of N coordinated Cu in N-CuCo-N is negligible (Co: 0.275 μB, Cu: 0.009 μB), resulting in only an end-on N₂ adsorption mode.

In addition, the d orbital energy level center (ε_d) of the metal atom is of vital role in the charge transfer between N₂ and the catalysts. We found that ε_d of Cu is much lower in energy than that of Co, Co is the main electron donor, as proved in Fig. 8, PDOS of O-CuCo-N. Coexistence with Cu could alter the d orbital energy level of Co. As shown in Table 6, the spin-down ε_d of Co in O-CoCo-N lies at −0.779 eV, while the presence of Cu pulls the ε_d of Co in O-CuCo-N up to −0.356 eV, which will facilitate the charge transfer between N₂ and DACs. On the other hand, the coordination environment has an effect on the ε_d too. The ε_d of Co in O-CuCo-N is 0.438 eV higher than ε_d of Co in N-CuCo-O. The PDOS of Co-3d in O-CuCo-N, O-CoCo-N, N-CuCo-O and N-CuCo-N as displayed in Fig. S2,† also indicates the effects of coordination on Co.

Table 6 d-Orbital energy level centers of Co in various configurations (all referenced to Fermi level E_F = 0 eV)

Configuration		O-CuCo-N	O-CoCo-N	N-CuCo-O
εd (eV)	Spin-up	−1.635	−1.596	−0.901
	Spin-dw	−0.356	−0.779	−0.794

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We summarized the adsorption energies, N–N bond length after adsorption and Gibbs free energy change of the first hydrogenation step ΔG(*N₂→*N₂H) (as shown in Table 4) and found that there exists an inverse relationship between the N–N bond length of *N₂ and ΔG(N₂→N₂H). The longer the N–N bond length is, the lower ΔG(N₂→N₂H) will be. This phenomenon was observed in both heteronuclear, homonuclear DACs and SACs, as shown in Fig. 9 and S3.† The activation of the inert N₂ can be reflected by the bond length elongation, which is determined by the charge transfer between N₂ and metal active sites. Therefore, the more charge is transferred to N₂, the better will the inert N₂ be activated, and the easier will it be hydrogenated. However, such relationship does not exist between the adsorption energy of *N₂ and the N–N bond length.

Fig. 9 The relationship between N–N bond length of adsorbed *N₂ and Gibbs free energy change ΔG from *N₂ to *N₂H on different DACs and SACs.

3.6 Practical application of O-CuCo-N as NRR electrocatalyst

For practical application, O-CuCo-N should have good structural stability, electric conductivity and durability. Structural stability has been discussed when screening out the possible candidates. As shown in Fig. 8 and S2,† O-CuCo-N is metallic, endowing it excellent electrical conductivity. Since nitrogen atoms are contained in O-CuCo-N electrocatalyst, under the drive of certain potential the electrocatalyst may decompose and these nitrogen atoms could become the source of the formation of NH₄⁺. Therefore, thermodynamics calculations were performed to evaluate the possibility of substrate decomposition, which can be expressed as O₃CuCoN₃ + 3(H⁺ + e⁻) → O₃CuCoN₂ + NH₃(g). Accordingly, the free energy change for the decomposition reaction can be calculated following ΔG_d = E(O₃CuCoN₂) + G(NH₃) − E(O₃CuCoN₃) − 3G(H⁺ + e⁻). The required potential to drive the decomposition and then NH₃ release can be obtained by U_d = −ΔG_d/3e. Our calculated decomposition potential is −0.85 V, which is lower than the limiting potential of NRR (−0.39 V). Therefore, O-CuCo-N could serve as efficient NRR with high stability and durability.

4 Conclusions

In this paper, the potential for N₂ fixation of the heteronuclear CuCo dual-atom electrocatalyst anchored on doped graphene, was investigated by DFT calculations. Our computations on various coordination structures revealed that the heteronuclear O-CuCo-N DAC is a promising candidate due to its relatively low formation energy, strong structural stability and high NRR activity, where NRR easily take place though the alternating pathway from the side-on N₂ adsorption mode, since the N≡N bond can easily break under the pulling force of two metal atoms. The potential limiting step is the first hydrogenation (*NN*→*NHN*) step with a pretty low Gibbs free energy change of 0.55 eV. We also made comparisons with the possible Cu or Co single atom catalysts and homonuclear (CuCu and CoCo) dual-atom catalysts, which were found not good as heteronuclear O-CuCo-N DAC in NRR activity. In addition, O-CuCo-N also exhibits good selectivity and durability. The mechanism study reveals that Cu and the coordination structure could regulate the spin polarization and the d orbital energy level center of Co, adjusting the adsorption of intermediates, leading to the superior NRR performance of O-CuCo-N DAC. The discovery of the distinct interaction between dual metal atom sites that can regulate their electronic spin states provides a powerful strategy for guiding the design of highly efficient DACs or SACs with high loading in the future. We hope that our studies could motivate more experimental research on CuCo as an NRR electrocatalyst.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from the Zhejiang Provincial Natural Science Foundation (ZCLY24A0401) and National Natural Science Foundation of China under Grant No. 61804154. The support from the Hefei advanced computing center for theoretical calculations is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra06077f

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