Open Access Article
José Miguel Doña-Rodríguez
a,
Raúl Quesada-Cabrera
a,
Douglas R. MacFarlane
b,
Alexandr N. Simonov
*b and
Luis Miguel Azofra
*a
aInstituto de Estudios Ambientales y Recursos Naturales (i-UNAT), Universidad de Las Palmas de Gran Canaria (ULPGC), Campus de Tafira, 35017 Las Palmas de Gran Canaria, Spain. E-mail: luismiguel.azofra@ulpgc.es
bSchool of Chemistry, Monash University, Clayton, Victoria 3800, Australia. E-mail: alexandr.simonov@monash.edu
First published on 9th April 2026
Activation of N2 with metallic lithium is a key step of the lithium redox-mediated nitrogen reduction reaction (Li-NRR) – a process that can enable practical ammonia electrosynthesis under mild conditions. Our present DFT study revealed that N2 capture and N
N bond cleavage is substantially enhanced on and within Li0 nanoparticles as compared to bulk slabs. This behaviour is not limited to single-molecule adsorption but is also observed during the successive capture of additional N2 molecules. The results presented herein might assist in the design of higher performance Li-NRR systems.
In the Li-NRR, the electrochemically generated metallic lithium mediates N2 cleavage with the formation of lithium nitride via a dissociative mechanism, which demonstrates a range of distinctive features8–10 as compared to the mechanisms of N2 activation proposed for the direct electrocatalytic NRR.11–18 Next, the nitrogen atoms incorporated into the Li3N lattice are converted into NH3 through successive protonation by a suitable proton carrier.19,20 A similar mechanism has recently been reported for calcium21 and magnesium.22
At a theoretical level, this behaviour has been modelled in metal slabs, in the first instance for the N2 cleavage mechanism (Fig. 1),8–10 but also in subsequent protonation steps.23,24 These previous studies established that: (i) end-on adsorption of N2 on Li0 is non-spontaneous; (ii) side-on adsorption, by contrast, is spontaneous, overcoming the physisorption regime, as N2 becomes inserted into the surface structure of the Li0 slabs, with binding free energies characteristic of chemisorption or capture events; (iii) N2 can diffuse into the inner layers of the Li0 bulk; and (iv) the activation free-energy barriers for N2 cleavage on the Li0 surface or within the Li0 bulk are accessible at room temperature.
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| Fig. 1 Summary of the main chemical events observed during the N2 capture and cleavage on and within the bcc-Li0 slabs.8,9 | ||
However, metal slabs represent restricted structures, in contrast to nanoparticles which exhibit diverse structural features, like different facets as well as edges and corners. These features are likely relevant to the real state of the electroplated Li0 intermediate during the Li-NRR, since electrodeposition of lithium metal typically produces highly heterogeneous nanostructured aggregates rather than well-defined crystalline surfaces.25–27 To investigate this, we conducted a density functional theory (DFT) study (pseudopotential plane-wave method, RPBE28 + D329,30 level of theory, VASP,31–34 see SI, for computational details) of a series of structures that were modelled by spherically projecting the body-centred cubic (bcc) Li0 unit cell with radii ranging from 1 to 6 Å. This resulted in a series of initial models: the Li1 atomic dot (1 Å radius), the Li2 (2 Å), Li6 (3 Å), Li15 (4 Å) and Li27 (5 Å) sub-nanoclusters, and the Li51 (6 Å) nanoparticle (Fig. 2).
In our previous study on the interaction of N2 on and within the bcc-Li0 slabs, we computed binding free energies for the (001) and (011) flat surfaces, amongst others.8,9 We found that end-on N2 adsorption exhibited non-spontaneous values of 0.22 eV in both cases, whereas side-on adsorption was spontaneous, with values of −0.46 and −0.66 eV, respectively. The computational protocol used in that study differs from the one employed herein in the calculation of vibrational frequencies only, where atomic positions of the metal were frozen in the previously published work.8,9 While slabs can be considered constrained structures due to their periodicity, this assumption does not hold for nanostructures. Consequently, when recalculating the side-on N2 binding on the (001) and (011) facets without structural constraints in the frequency calculation, the values changed slightly to −0.35 and −0.55 eV, respectively. By applying a consistent methodology, comparison of the N2-adsorption results computed here for our sub-nanoclusters and nanoparticle models with those for metal slabs was therefore straightforward.
Interestingly, both Li1 and Li2 – the simplest models examined herein – exhibited highly non-spontaneous N2 binding free energies (ΔGb). For instance, the linear end-on minima (labelled [1] in both cases in Fig. 3) showed ΔGb values of 0.36 and 0.34 eV, respectively, along with minimal N
N bond elongations of 0.03 and 0.01 Å, compared with the nitrogen–nitrogen distance of 1.12 Å for N2(g) obtained using this computational protocol. These energy values were similar or even less favourable than those others considered herein, specifically minimum [2] for Li1 of side-on type (0.54 eV), and minima [2] and [3] for Li2 of angular end-on (0.31 eV) and side-on (0.55 eV) types.
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Fig. 3 *N2 minima obtained for the Li1 atomic dot and the Li2, Li6, Li15, and Li27 sub-nanoclusters. N N distances (in Å) are shown in blue. Binding free energies (in eV) are indicated in green for spontaneous and red for non-spontaneous values. For the Li15 and Li27 models, Cartesian axes (a, b, c, corresponding to OX, OY, and OZ, respectively) are also shown as a reference for orientation, facilitating comparison with the symmetric structures presented in Fig. 2. | ||
In principle, these results qualitatively follow the trends established before for the Li0 slabs, where the side-on adsorption was energetically more favourable than end-on.8,9 However, it was somewhat surprising to find that the interactions between N2 and these Li0 structures were so endergonic, given that both Li1 and Li2 represent single-site models. Indeed, single-site models have demonstrated superior substrate-interaction capabilities for many other systems. For example, this was shown for the end-on interaction of N2 with iron slabs35 and with a single iron atom,36 where the binding energy in the latter case increases by nearly a factor of seven, although we acknowledge that, in this model, the single-site iron atom was not isolated but supported on a MoS2 sheet. However, in the case of Li0, energetically favourable N2–metal interactions seem to demonstrate an opposite trend.
Li6 formally represents a cluster-type structure of a square-based bipyramid. In this case, we observe binding events that deviate from the general trend in N2-philicity exhibited by Li0 slabs. Four minima were identified for this model: the linear end-on minimum, labelled [1], was slightly physisorbed with a spontaneous ΔGb (ca. –0.1 eV), whereas ΔGb values were endergonic for minima [2]–[4], despite [2] and [3] being side-on (the former interacting with three Li centres and the latter with two) (Fig. 3).
Note that two binding free energy values are reported for all lithium models, except Li1 and Li2, considered herein because, unlike metallic slabs, they undergo distortions relative to their original structures. The first ΔGb value refers to the free energy difference between the minimum in question and the perfectly symmetric cluster. The second ΔGb was calculated by re-optimising the cluster structure after removal of the adsorbed N2 molecule. The difference between the two values provides a measure of the energy associated with cluster structure deformation. It should be noted that the deformation energy discussed herein does not necessarily reflect stability of Li0 nanostructures during Li-NRR. A proper assessment of this aspect would require a separate investigation involving additional species participating in the reaction.
For the Li6 cluster, this difference was negligible, meaning almost no deformation with respect to the perfectly symmetrical cluster of this type (Fig. 3). In contrast, the change was pronounced for the Li15 model, where the cluster deformation energies were, in most cases, around –0.6 eV; that is, the distorted structures were more stable than the initial perfectly symmetric cluster. From the perspective of formal analysis of N2 stabilisation, it is the second ΔGb value that is of major interest.
For Li15, the binding free energies of N2 were similar to those computed for the slabs, with values of −0.53, −0.52, and –0.53 eV for the first three minima, which: (i) are of side-on type; (ii) correspond to molecules embedded within the cluster surface; and (iii) show that superficial interactions lead to the decreased ΔGb values (Fig. 3). We also observed that the captured N2 molecules in these more stable configurations were activated, with elongated N
N bonds to approximately 1.3 Å. Although the ΔGb values for these three most stable minima were similar, their electronic energies, i.e., not including enthalpic and entropy contributions, differed by more than 0.1 eV (≈9.65 kJ mol−1), confirming that they represent distinct structures on the potential energy surface (PES).
The binding free energy results for the Li27 model further emphasised the intensification of the N2–Li0 interactions within atom clusters as compared to the slab models. We identified a series of side-on and embedded N2 minima, labelled [1]–[5], whereas minima [6]–[9] correspond to mixed ([6]), angular ([7]), and linear end-on ([8]–[9]) types (Fig. 3). For the former (side-on), ΔGb was exergonic, while for the latter (mixed and end-on) it was not. This behaviour, although somewhat expected, would not be particularly noteworthy if the N2 binding free energy values did not significantly exceed the reference values obtained for slabs (−0.55 eV). In these clusters, the interactions reached as much as −0.9 eV, indicating that N2 capture was strongly enhanced, accompanied by substantial activation of the molecule. For instance, for side-on minima [1]–[5], N
N bond lengths (rN
N) elongated up to 1.28–1.36 Å (Fig. 3).
To explore if this improved N2 fixation could be maintained, and to what extent, at the nano- rather than sub-nanometre scale discussed above, we further studied the Li51 nanoparticle model. The intensity of N2 capture in Li51 followed a pattern similar to that observed in the Li27 cluster: side-on and embedded N2 structures on the nanoparticle surface exhibited ΔGb values ranging from −0.88 eV for the most stable to –0.60 eV for the least stable, whereas end-on interactions remained endergonic. This is exemplified in Fig. 4a for the three most stable N2 minima identified for the Li51 model, with the remaining minima provided in Fig. S2. Notably, the N
N bond elongation was slightly greater than that observed for the corresponding minima in the Li27 model, and distortion energies were somewhat higher, indicative of enhanced stability in the non-symmetric Li51 nanoparticle compared with the symmetric structure (cf. data in Fig. 3 and 4a). A correlation between ΔGb and the number of lithium atoms interacting with N2 was noted for *N2 minima in both the Li27 and Li51 models (Fig. S5), consistent with our observation that binding free energy became stronger as the number of the Li⋯N bonds increased.
In light of these results obtained for the larger nanostructures, Li0 appears to exhibit a pronounced affinity towards N2. Nevertheless, further investigations including explicit solvent molecules will be required to evaluate possible competitive adsorption effects under more realistic conditions.
Further, we sought to explore fundamental aspects of the N2 cleavage mediated by the lithium metal nanoparticle by addressing two questions: (i) what is the energy cost (activation barrier) associated with cleaving the *N2 molecule into N adatoms (*N) during the formation of lithium nitride; and (ii) is capture of a second N2 molecule also an energetically favourable process?
Addressing the first question, the computed activation energies for the lithium-mediated dinitrogen dissociation process were low to moderate, that is, with values less than 1 eV. Specifically, we performed calculations to locate the transition states (TSs) for N2 cleavage of the six most stable side-on N2 minima in the Li51 model, obtaining values ranging from 0.46 eV for the lowest barrier to 0.64 eV for the highest (Table S3 and Fig. S3). Remarkably, all activation barriers were smaller, in absolute terms, than the energy released upon N2 chemisorption. This is exemplified in Fig. 4b for the minimum [1] of the Li51 model. In this case, the * + N2(g) → *N2 physical process exhibited a free energy drop of −0.88 eV, as discussed previously, and an activation barrier of 0.61 eV relative to this state. Additionally, the chemisorbed N2 molecule showed a bond elongation of 0.28 Å (rN
N = 1.40 Å), while the nitrogen–nitrogen distance in the TS was 2.15 Å.
To assess the kinetic accessibility of the embedded N2-in-Li0 configurations, a constrained approach scan was performed for the Li51 model in which the distance between the central Li atom and N2 was progressively reduced while relaxing the remaining degrees of freedom (see Fig. S6). The resulting profile reveals a very small electronic energy barrier of 0.14 eV for N2 penetration into the subsurface region, followed by stabilisation of the embedded state. This indicates that subsurface incorporation is kinetically accessible within the capabilities of the static DFT approach employed herein.
To explore the capability of a lithium nanoparticle to activate multiple dinitrogen molecules, a second N2 was randomly placed at various positions onto the *N2 minimum [1] of the Li51 model. The capture of an additional N2 molecule was found to be spontaneous, with structural distortion energies being practically negligible. In more detail, Fig. 4c shows six of the nineteen minima identified for this case, while the remaining structures are provided in Fig. S4. Notably, for all these minima, the second N2 molecule adopted a side-on configuration in the most stable structures (shown in green in Fig. 4c), with spontaneous ΔGb values generally comparable to those computed for the first captured N2 molecule (blue in Fig. 4c). Although the N
N bond in the second N2 molecule was also elongated, the magnitude of this effect was less pronounced. By contrast, for all these minima, the bond elongation of the first captured N2 molecule was maintained or even enhanced, with N
N distances exceeding 1.4 Å in some cases (Fig. S4).
Based on these results, Li0 nanoparticles can be considered exceptionally effective N2 capturers, even more effective than the slab models. To further support this conclusion, we performed NVT [substance (N), volume (V), temperature (T); canonical ensemble] molecular dynamics (MD) simulations starting from an initial configuration in which thirty-six N2 molecules were positioned in an end-on orientation, pointing towards each of the surface lithium atoms of the Li51 nanoparticle and placed at a distance of 3 Å. This setup established unfavourable initial conditions, namely, N2 molecules at distances corresponding to non-adsorptive interactions and in an end-on orientation, which, as confirmed by our calculations, were non-spontaneous. If the Li51 nanoparticle indeed behaves as a highly potent N2 capturer, the MD simulation could be expected to evolve towards spontaneous adsorption and activation.
The results in this regard were highly instructive. On the picosecond timescale, a progressive insertion of numerous N2 molecules onto and within the nanoparticle was observed (Fig. 5). As the MD simulation progressed, convergence of the centres of mass of the N2 molecules with those of the Li0 atoms was found, indicating the point of interaction between dinitrogen and the lithium nanoparticle surface. Specifically, a substantial number of N2 molecules were captured within inner layers of Li51 after approximately 1.5 ps of the total 4 ps simulation. Throughout this evolution, up to nearly 4 ps, N2 capture by the Li0 nanoparticle was accompanied by a decrease in the electronic energy (shown in dark red in Fig. 5), indicating that these adsorption events were energetically stabilising. Fig. 5 also includes selected snapshots along the MD trajectory, visually illustrating the progressive N2 capture behaviour of the lithium nanoparticle.
The findings discussed above provide new insights into the chemistry governing the first critical step of the Li-NRR, viz. activation of dinitrogen. In the future, it will be important to investigate how the cleavage of a sufficiently large number of captured N2 molecules ultimately leads to nitridation of the Li0 nanoparticles. Simultaneous capture of multiple N2 molecules by the Li0 nanoparticles may introduce electronic interactions between adsorbates, which could influence the activation of subsequent N2 molecules. Of an even higher interest is the location of the nitride formation, viz. at the surface or in the bulk of the particles, which might result in very different scenarios. Indeed, formation of a lithium nitride surface layer may potentially passivate Li0 and suppress subsequent N2 capture. On the contrary, formation of the lithium nitride in the bulk, which might be possible owing to the N2 diffusion through Li0 demonstrated herein and previously,8–10 might allow for the most effective utilisation of the electrochemically generated lithium metal. A detailed assessment of these effects would require a more extensive exploration of the system and is left for future investigation.
N bond and accessible activation barriers for cleavage, and that multiple N2 molecules can be captured successively. NVT molecular dynamics simulations indicate that the highly energetically favourable capture events take place on a picosecond scale, with N2 molecules progressively inserted into the nanoparticle's inner layers. These findings highlight the unique mechanistic role of Li0 nanostructures in the lithium redox-mediated nitrogen reduction reaction and suggest that such nanoparticles may serve as highly efficient platforms for ammonia synthesis under mild electrochemical conditions. Overall, this study provides fundamental insights into the interplay between size, structure, and reactivity against N2 of lithium metal, opening new avenues for the design of Li-NRR catalysts.
Supplementary information (SI): methodological aspects, full computational details. See DOI: https://doi.org/10.1039/d6cy00192k.
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