Bidirectional catalysts with dual-atom dynamic d-band centre modulation and support self-reconstruction for de/hydrogenation in MgH_2/Mg

Domain: biology
Published: 2026-03-16
Editorial outcome: major_revision

Abstract
High-capacity solid-state hydrogen storage using MgH2 requires high de/hydrogenation temperatures and exhibits sluggish kinetics. Although catalytic modification of MgH2 has been extensively studied, existing catalysts largely focus on unidirectional optimisation, failing to simultaneously and efficiently optimise the reversible hydrogen-storage properties of the MgH2/Mg system. Herein, we rationally design and construct a heteronuclear dual-atom catalyst, NiCo@TiO2. In this system, Ni and Co bidirectionally modulate the d-band centres, enabling synergistic and complementary catalysis. Specifically, Ni serves as the primary active site for Mg–H bond cleavage during dehydrogenation, facilitated by Co-induced d-band centre downshift. Conversely, Co acts as the primary active site for H2 dissociation during hydrogenation via Ni-triggered d-band centre upshift. Simultaneously, self-reconstruction of titanium species and oxygen vacancies, coupled with strong metal-support interactions (Ni/Co–TiO2), accelerate interfacial electron transfer and inhibit metal atom migration. This synergy significantly enhances both reaction kinetics and cycling stability, showing great promise for large-scale hydrogen-storage applications.

Introduction
Hydrogen is an indispensable carbon-neutral energy carrier1–5, and its large-scale practical application mainly depends on two key factors: the safety of storage technologies and the enhancement of hydrogen storage density6–8. Solid-state hydrogen storage using magnesium hydride (MgH2) exhibits enhanced safety and high gravimetric density (7.6 wt.%); however, its practical deployment remains constrained by its sluggish de/hydrogenation kinetics requiring considerably high activation energies (dehydrogenation Ea > 160.0 kJ·mol−1, hydrogenation Ea > 87.3 kJ·mol−1)9–11. Several strategies, including alloying, nanoengineering, and catalyst addition, have been employed to overcome these challenges12–14. Reducing the kinetic barriers of de/hydrogenation in MgH2/Mg, particularly lowering the reaction temperatures and increasing the reaction rates, via catalytic engineering is the predominant strategy15–18.

Catalytic materials based on transition metals (e.g., Ni, Co, and Ti) and their oxides (e.g., TiO2) can considerably enhance the de/hydrogenation properties of MgH2/Mg owing to their unique d-orbital electronic structures14–22. Guan et al. developed a MgNi@Ti–MgO catalyst and incorporated it into MgH2 composites, reducing the initial dehydrogenation temperature to 172°C and leading to a dehydrogenation rate of 2.72 wt.%·min−1 H2 at 280°C20. AlMatrouk et al. calculated and revealed that Co-doped MgH2 shows considerably increased adsorption energy of H atoms on Mg surfaces compared to that of Ni atoms, lowering the energy barrier for hydrogen absorption21. However, the catalytic performance of these materials requires further optimisation to meet the stringent operating temperature constraints and reaction kinetic requirements for practical hydrogen fuel cell systems.

Single-atom catalysts (SACs), which are characterised by their maximal atomic utilisation, unique electronic structures, and precisely tuneable active sites, have demonstrated remarkable potential in different catalytic systems23–29. Previous work demonstrated that incorporating a specific single-atom catalyst into MgH2 significantly lowered the apparent activation energy (Ea) for both dehydrogenation and hydrogenation processes to 64.35 and 35.17 kJ·mol−1 H2, respectively30. However, the inherent mono-functionality of such catalysts often leads to an asymmetric catalytic profile27,28,31 which often means that catalysts excelling at dehydrogenation may perform suboptimally for hydrogenation. This kinetic disparity in the de/hydrogenation processes necessitates a rational design of bifunctional catalytic architectures. Heteronuclear dual-atom catalysts (DACs) can simultaneously and efficiently mediate two reaction pathways, and the synergistic effect of electron coupling and orbital hybridisation between bimetallic atoms may further improve the catalytic efficiency32–37. Given the established efficacy of Ni-based catalysts in promoting the dehydrogenation of MgH230, the selection of a complementary catalyst to optimise hydrogenation is critical.

Herein, we synthesised heteronuclear NiCo@TiO2 DACs using a hot molten salt method, leveraging complementary metal synergies to enable the bidirectional regulation of the de/hydrogenation processes in MgH2/Mg systems. Ni and Co atoms synergistically form active centres with complementary roles on MgH2/Mg. During dehydrogenation, Ni serves as the principal site for Mg–H bond cleavage, while Co modulates Ni’s d-band centre to reduce Mg–H bond energy. Conversely, during hydrogenation, Co dominates H2 dissociation, with Ni optimising Co’s d-band centre to facilitate hydrogen adsorption. Concurrently, self-reconstruction of multivalent titanium species (Tix+) and oxygen vacancies (OV) within the TiO2 support and strong metal-support interactions (Ni/Co–TiO2) effectively promote interfacial electron transfer and suppress metal atom migration, remarkably enhancing reaction kinetics and cycling stability. Specifically, the ball-milled NiCo@TiO2–MgH2 composite achieved a low initial dehydrogenation temperature (168°C), with an accelerated initial hydrogen desorption rate of 3.02 wt.%·min−1 at 275°C, while enabling efficient room-temperature hydrogen absorption (3.79 wt.% within 30 min at 30°C). The system exhibited outstanding kinetics, showing low Ea for desorption (Ea,de = 53.54 kJ·mol−1) and absorption (Ea,ab = 16.71 kJ·mol−1), and maintained 97.6% capacity retention over 100 de/hydrogenation cycles. This work enhanced hydrogen storage performances in MgH2-based systems while proposing a universal design framework for reversible chemical processes.

Results
Theoretical design of catalysts
To overcome the kinetic limitations of the de/hydrogenation processes in MgH2/Mg systems, a dual-atom catalytic strategy was employed. The selection of Ni-Co dual-atom sites was motivated by their complementary electronic configurations: Ni’s d8 orbital occupancy enables moderate hydrogen binding strength, facilitating H2 release, while Co’s high-spin d7 state promotes low-barrier H2 dissociation via enhanced σ* orbital back-donation38–42. Concurrently, TiO2 was selected as the support owing to its intrinsic defect-tunable nature (facile oxygen vacancy formation) and reversible redox activity43–45. Moreover, the synergistic metal-metal interactions (Ni–Co) and strong metal-support interaction (SMSI) between reducible oxide supports (TiO2) and loaded metals (Ni/Co) under elevated temperatures can effectively facilitate interfacial charge transfer and enhance cycling stability during the de/hydrogenation cycles of MgH2/Mg systems46–49.

Building upon the preceding literature analysis on atom and support, we further perform theoretical calculations to evaluate the catalytic superiority of the NiCo@TiO2 DACs for hydrogen storage in the MgH2/Mg system, thereby guiding subsequent experimental studies. The Vienna Ab initio Simulation Package was used to develop theoretical models of five dehydrogenation systems: pure MgH2, TiO2–MgH2, Ni@TiO2–MgH2, Co@TiO2–MgH2, and NiCo@TiO2–MgH2 (Supplementary Fig. 1). The theoretical models utilized the anatase TiO2 (101) surface, which is the most stable and predominantly exposed facet, as confirmed by subsequent experimental characterization. Having established the rationality of the model and the stability of the catalyst via the binding and migration energy barriers of Ni/Co atoms on the TiO2 support (Supplementary Fig. 2; also confirmed by the following experiment characterizations), we further employed Density functional theory (DFT) calculations to systematically investigate the electronic coupling between Ni and Co atoms during the de/hydrogenation processes of MgH2/Mg systems. And all calculations were performed under these well-established and rigorously tested settings (Supplementary Fig. 3 and Table 1).

The dehydrogenation process in MgH2/Mg systems was quantitatively evaluated by studying Mg–H bond-strength variations in different catalytic environments. Crystal orbital Hamilton population (COHP) analysis revealed that the combination of different catalysts with MgH2 lead to the weakening of bonding orbitals near the Fermi level of MgH2 (Supplementary Fig. 4). The order of the integrated COHP (-ICOHP) values was as follows: NiCo@TiO2–MgH2 (0.22 eV) < Ni@TiO2–MgH2 (0.41 eV) < Co@TiO2–MgH2 (0.45 eV) < TiO2–MgH2 (0.91 eV) < pure MgH2 (0.95 eV); lower -ICOHP values indicate weaker Mg–H bonding interactions (Fig. 1a). The SACs lead to significant bond weakening, reducing the Mg–H bond strength by 56.84% (Ni@TiO2) and 52.63% (Co@TiO2) compared to that in pure MgH2. Furthermore, the dual-atom system achieved a 76.84% bond-strength reduction through non-linear synergy, suggesting that the Ni/Co dual sites worked together to significantly decrease the Mg–H bond energy, enhancing the dehydrogenation performance of MgH2. In addition, to compare the catalytic effects of the OV-rich and Tix+-rich TiO2 support with those of Ni/Co atoms on MgH2, the -ICOHP values of pure MgH2, TiO2–MgH2, TiO2–OV–MgH2 and NiCo@TiO2–MgH2 were further compared, revealing the predominant catalytic role of Ni/Co atoms in governing dehydrogenation kinetics (Supplementary Fig. 5).

To investigate the catalytic mechanism of NiCo@TiO2 DACs in MgH2 dehydrogenation, a partial density of states (PDOS) analysis was performed. The strong electronic coupling between Ni and Co atoms in NiCo@TiO2 is illustrated in Fig. 1b and c, illustrating that Ni 3d and Co 3d exhibited substantial density of states (DOS) values within the energy range from −4 to 2 eV. The d-band centre of Ni in NiCo@TiO2 shifted downwards to −1.806 eV compared to −1.363 eV in Ni@TiO2. Conversely, the d-band centre of Co shifted upwards from −1.907 eV (Co@TiO2) to −1.774 eV (NiCo@TiO2). The dynamic d-band centre modulation enabled Ni and Co atoms to function as complementary active sites for de/hydrogenation, respectively, suggesting a possibility for achieving bidirectional catalysis in MgH2/Mg systems.

The PDOS of the s orbitals of the H atoms, as well as those of the s and p orbitals of the Mg atoms near the Fermi level decreased (Supplementary Fig. 6), suggesting a decrease in the orbital energy of the H and Mg atoms, which indicated a weakening of the Mg–H bond strength. The PDOS peaks of H and Mg in NiCo@TiO2–MgH2 exhibited the lowest overlap and intensity, which is in good agreement with the COHP analysis results, indicating that this composite had the lowest Mg–H bond strength.

Next, the dissociation of hydrogen molecules was systematically investigated under various catalytic conditions to evaluate the effect of the catalysts on hydrogen absorption. As illustrated in Fig. 1d, the introduction of the SACs and DACs considerably reduced the energy barrier for hydrogen dissociation. Comparative analysis revealed that Co@TiO2–Mg had a lower hydrogen dissociation energy barrier (ΔE = 0.384 eV) compared to Ni@TiO2–Mg (ΔE = 0.415 eV), indicating that Co@TiO2 showed an enhanced capability to destabilise H–H bonds and facilitate the dissociation of H2 into atomic hydrogen. NiCo@TiO2–Mg demonstrated the lowest energy barrier for hydrogen dissociation (ΔE = 0.193 eV), which was attributed to the synergistic interaction between Ni and Co. This synergistic effect increased the kinetics of H2 dissociation and substantially enhanced the overall hydrogen absorption performance of the MgH2/Mg hydrogen storage system.

Collectively, the synergy of Ni–Co triggers a downward shift in the d-band centre of Ni and an upward shift in Co. Specifically, the downward shift of the d-band centre of Ni reduced the orbital overlap between the Ni 3d and H 1s orbitals, reducing the Ni–H adsorption strength. This electronic modification facilitated hydrogen atom diffusion throughout the catalytic system. In contrast, the upward shift of the d-band centre of Co strengthened the interactions between the Co 3d orbitals and H2 molecules. This optimised the binding strength of H2 molecules adsorbed on the Co sites and promoted H2 dissociation by enhancing the hybridisation of Co 3d with H2 σ* orbitals, allowing efficient H migration to the Mg surface for MgH2 formation. Thus, Ni and Co atoms in NiCo@TiO2 DACs can achieve efficient bidirectional catalysis in MgH2/Mg systems, facilitating the rational design and experimental development of DACs.

Synthesis and microstructure of NiCo@TiO2 DACs
Driven by DFT calculations revealing the significant enhancement of MgH2/Mg hydrogen storage performance by the DACs, we strategically designed and synthesised the dual-atom Ni and Co supported on TiO2 with three precisely controlled Ni/Co atomic ratios (7:3, 1:1, and 3:7) via a hot molten salt method. The successful incorporation and precise quantification of Ni and Co in the catalysts were verified by inductively coupled plasma mass spectrometry (ICP-MS). The Ni/Co atomic ratios were controlled by adjusting the precursor concentrations (Supplementary Table 2) and were measured to be 2.13, 0.96, and 0.45, demonstrating the tunability of the synthesis method. NiCo@TiO2 (Ni/Co atomic ratio determined by ICP-MS = 0.96) exhibited the best catalytic performance in enhancing the hydrogen storage properties of MgH2, as evidenced by comprehensive evaluation. Thus, it was selected for subsequent in-depth characterisation.

The phase structure of the synthesised sample was characterised by X-ray diffraction (XRD). The results indicated three strong diffraction peaks at 2θ = 25.3°, 38.2°, and 48.2°, which highly matched the standard spectra of the TiO2 anatase phase (Fig. 2a). No diffraction peaks attributed to Ni and Co were detected in the XRD pattern, which preliminarily suggested that Ni and Co coexisted in an amorphous phase or a single-atom form.

To directly reveal the dispersion state of Ni and Co on TiO2, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used. The catalyst exhibited uniform morphology with particle sizes of 40–60 nm (Fig. 2b and Supplementary Fig. 7). High-resolution transmission electron microscopy (HRTEM) analysis revealed a lattice spacing of 0.3520 nm (Fig. 2c), corresponding to the (101) plane of anatase TiO2. A detailed selected area electron diffraction (SAED) pattern showed ordered diffraction rings with d-spacings of 0.3520 and 0.1892 nm (Fig. 2d), corresponding to the (101) and (200) planes of anatase TiO2, respectively. These results were in good agreement with the XRD patterns, revealing the core structural characteristics of NiCo@TiO2, i.e., the main body is a highly crystallised TiO2 anatase phase.

The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Fig. 2e) revealed bright spots uniformly distributed across the TiO2 substrate. Given the higher atomic masses of Ni and Co compared to those of Ti, these bright spots were attributed to the dual-atom sites (marked by green circles) and a few single-atom sites. Statistical analysis of the measured atomic spacing yielded an average Ni-Co distance of 0.247 nm (Fig. 2f and Supplementary Fig. 8). Moreover, the Ni/Co atomic intensities possess higher intensity than the Ti atomic intensity based on the HAADF-STEM analysis of the NiCo@TiO2 DACs (Fig. 2g). Further, the HAADF-STEM via electron energy-loss spectroscopy (EELS) clearly reveals the presence of Co and Ni, confirming the Co−Ni dual-atom pairs in NiCo@TiO2 DACs (Fig. 2h). The energy dispersive spectroscopy (EDS) elemental mapping (Fig. 2i) confirmed the presence of both metals, suggesting the successful incorporation of Ni and Co species with high dispersion on the TiO2 support. Thus, the synthesised catalyst consisted of mono-dispersed Ni and Co atoms uniformly distributed on the TiO2 substrate. For reference and comparison, the microstructures of corresponding Ni@TiO2 and Co@TiO2 SACs are provided in the Supplementary Information (Supplementary Figs. 9 and 10).

Further, X-ray absorption spectroscopy was employed to precisely determine the local geometric structures of Ni and Co at the atomic level. Figure 3a and b shows the Ni and Co K-edge X-ray absorption near-edge structure (XANES) spectra for NiCo@TiO2, alongside the corresponding reference spectra from metal foils (Ni, Co), oxides (NiO, CoO, Co3O4), and single-atom catalysts (Ni@TiO2, Co@TiO2). Specifically, the Ni K-edge XANES spectrum of NiCo@TiO2 was considerably different from those of the Ni foil and NiO, which further reflected the unique state of Ni in the catalyst. Moreover, the absorption edge energy of NiCo@TiO2 was higher than that of the Ni foil and Ni@TiO2 and lower than that of NiO, indicating that the valency of Ni species in NiCo@TiO2 was between 0 and +2, slightly higher than that of Ni species in Ni@TiO2. According to the Co K-edge XANES spectrum of NiCo@TiO2, the valency of Co species in NiCo@TiO2 is higher than that of Co@TiO2 (Fig. 3b). The observed intermediate valence states of Ni and Co originate from the strong metal-support interaction (SMSI) with the TiO2 support. This interaction causes partial charge transfer, resulting in electronic structures that are intermediate between their metallic and oxidized states, which are distinct from the mere physical mixture25,50–53.

More detailed atomic structure information was obtained using the k2-weighted Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) of Ni and Co K-edge in the R space. The characteristic peaks of the Ni–Ni bond in the Ni foil and Co–Co bonds in the Co foil were located at 2.16 and 2.18 Å, respectively (Fig. 3c and d), which were absent in the NiCo@TiO2, proving that Ni and Co atoms achieved atomic dispersion on TiO2 rather than forming metal clusters or particles. Moreover, the peaks of Ni and Co in the R space of NiCo@TiO2 at 1.75 and 1.53 Å were attributed to the first coordination shell of Ni–O and Co–O bonds (Fig. 3c and 3d), respectively.

To further analyse the coordination states of Ni and Co, their wavelet-transformed (WT) K-edge EXAFS oscillations were studied. As shown in Fig. 3e and f, the strongest oscillations of NiCo@TiO2 appeared around the wave vector of 5 Å−1, which was close to the wave vectors pertaining to the strongest oscillations of NiO and CoO. The backscattered atoms bonded with Ni and Co were O atoms. To further identify the fine coordination configurations of Ni and Co in different samples, quantitative least squares EXAFS curve-fitting analyses were conducted (Fig. 3g, h and Supplementary Figs. 11 and 12). The analysis revealed distinct coordination environments in NiCo@TiO2, with Ni–O and Co–O coordination numbers of 4.9 and 5.1, respectively (Supplementary Table 3). The Ni–O coordination number in NiCo@TiO2 (4.9) was considerably higher than that in Ni@TiO2 SACs (4.3). Conversely, the Co–O coordination number (5.1) was lower than that in Co@TiO2 SACs (5.4). Based on the characterisation results, the coordination environment of isolated Ni and Co atoms on the TiO2 support can be determined (Fig. 3g, h). This altered coordination condition suggested potential electronic interactions between Ni and Co, which may have a positive effect on promoting efficient electron transfer during the de/hydrogenation process of MgH2/Mg systems.

These characterisations demonstrated the atomic dispersion of Ni and Co in NiCo@TiO2. This atomic-level structural elucidation provides a fundamental basis for understanding the enhanced catalytic properties and exploring the potential applications of this dual-atom catalyst system.

Fabrication and dispersion of NiCo@TiO2 DACs on MgH2
Through systematic optimisation of ball-milling parameters, the as-synthesised NiCo@TiO2 DACs were uniformly dispersed on MgH2 surfaces, forming homogeneous composites. XRD analysis confirmed the β-MgH2 phase in composites, with characteristic peaks at 27.9°, 35.7°, and 39.85° corresponding to the (110), (111), and (200) crystal planes, respectively (Supplementary Fig. 13). The distinct broadening of the MgH2 peak indicated its reduced crystallite size (based on Scherrer analysis) and lattice strain induced by mechanical milling, which are structural modifications known to enhance hydrogen diffusion kinetics. The low-intensity peak at 25.3° corresponded to the (101) plane of the TiO2 anatase phase.

SEM images and corresponding EDS mapping reveal that the surface of MgH2 particles (∼8 μm) is uniformly decorated with NiCo@TiO2 DACs (Supplementary Fig. 14). Furthermore, the spatial distribution of NiCo@TiO2 DACs and their coherent boundary with MgH2 were explored using TEM at various scales (Fig. 4a, b). HRTEM images also revealed close interfacial contact between the catalysts and the MgH2 matrix (Fig. 4c). The measured interplanar spacings of 0.2512 and 0.1890 nm corresponded to the (101) plane of β-MgH2 and (200) plane of TiO2 anatase, respectively. This nanoscale integration created coherent phase boundaries that likely generated preferential pathways for hydrogen transport54,55. SAED patterns (Fig. 4d) further confirmed the nature of the composite, displaying concentric rings indexed to the TiO2 (101), MgH2 (101), MgO (200), and TiO2 (200) planes. The presence of MgO (d = 0.2110 nm) may be attributed to surface oxidation during the test.

The elemental distribution was studied based on atomic-scale characterisation using HAADF-STEM (Fig. 4e) coupled with EDS mapping (Fig. 4f). Isolated bright spots with Z-contrast differences confirmed the atomic dispersion of Ni/Co species within the TiO2 lattice. Dual-atom sites (marked by green circles) and a few single-atom sites were observed, indicating that Ni and Co atoms still maintained the atomic-level dispersed state in the composite sample. Elemental mapping results indicated a homogeneous spatial distribution of Ti, Ni, Co, O, and Mg throughout the composite. Thus, atomically dispersed Ni–Co dual-metal sites anchored on the TiO2 support were uniformly distributed within the MgH2 matrix, providing abundant interfaces for catalysing hydrogen storage.

The comprehensive characterisation revealed the successful fabrication of structurally integrated NiCo@TiO2–MgH2 composites. The precisely designed interface architecture, featuring a reduced MgH2 crystallite size and atomically dispersed transition metals on TiO2, provides an ideal platform for catalysing hydrogen storage and investigating mechanisms in subsequent research.

Hydrogen storage property of MgH2/Mg with various catalysts
To systematically investigate the catalytic effect of NiCo@TiO2 DACs on the hydrogen desorption properties of MgH2, we conducted temperature-programmed desorption (TPD) measurements (Fig. 5a). The 15 wt.%-NiCo@TiO2–MgH2 composite exhibited a considerably low initial desorption temperature of 168°C, representing a substantial reduction of 212°C compared to that of pure MgH2, and a hydrogen storage capacity of 6.49 wt.%. The catalytic efficiency of the 15 wt.%-NiCo@TiO2–MgH2 composite surpassed those of Ni@TiO2 and Co@TiO2 SACs under identical conditions. Mass spectrometry analysis revealed that the dehydrogenation peak temperature of 15 wt.%-NiCo@TiO2–MgH2 was 220°C (Fig. 5a inset).

Additionally, previous studies have shown that V2O5 and Nb2O5 exhibit good catalytic performance for MgH256–60. Therefore, we used V2O5 and Nb2O5 as supports to prepare the dual-atom catalysts NiCo@V2O5 and NiCo@Nb2O5, which were then combined with MgH2 using the same method. Compared with 15 wt.%-NiCo@TiO2–MgH2, both 15 wt.%-NiCo@V2O5–MgH2 and 15 wt.%-NiCo@Nb2O5–MgH2 have higher dehydrogenation temperatures (Supplementary Fig. 15), demonstrating the advantage of using TiO2 as the support. Furthermore, we have discovered that the oxygen vacancy (OV) concentration of TiO2 support exhibits a volcano-like dependency on the catalytic performance in the MgH2/Mg systems (Supplementary Fig. 16). This peak performance could be attributed to an optimal density of active sites, which provides a high concentration of vacancies for hydrogen adsorption and dissociation while maintaining their dispersion and stability61–64. Accordingly, the preparation of DACs can involve maintaining the OV concentration within an optimal range.

To further elucidate the effect of the atomic composition on the catalytic activity, DACs with different Ni/Co ratios (Ni0.3Co0.7@TiO2, NiCo@TiO2, and Ni0.7Co0.3@TiO2) were designed, and their TPD profiles were evaluated (Supplementary Fig. 17). Among them, NiCo@TiO2–MgH2 demonstrated superior dehydrogenation performance. Ni0.3Co0.7@TiO2–MgH2 and Ni0.7Co0.3@TiO2–MgH2 exhibited enhanced hydrogen storage properties compared to their monatomic counterparts, highlighting the synergistic catalytic effect between the two metals. MgH2 with different NiCo@TiO2 concentrations (10, 15, and 20 wt.%) was prepared to optimise the catalyst loading through ball milling. As shown in Supplementary Fig. 18, the 15 wt.%-NiCo@TiO2–MgH2 was the optimal catalyst for hydrogen storage applications, as it demonstrated the optimal dehydrogenation temperature, rate, and capacity.

The isothermal desorption properties of 15 wt.%-NiCo@TiO2–MgH2 were further investigated at 275°C, 250°C, 225°C and 200°C (Fig. 5b). At 275°C, 15 wt.%-NiCo@TiO2–MgH2 released over 5.91 wt.% H2 within 4 min, exhibiting excellent dehydrogenation kinetics. As the temperature decreased, the kinetics of dehydrogenation slightly decreased. Even at 200 °C, 15 wt.%-NiCo@TiO2–MgH2 still released 2.31 wt.% H2 in 20 min and eventually released > 4.42 wt.% of H2 within 180 min (Supplementary Fig. 19). The observation of a slight sigmoidal shape in isothermal dehydrogenation curves, particularly at lower temperatures, is characteristic of a reaction controlled by the nucleation and growth of Mg65–67. The brief induction period arises from the energy barrier to form critical Mg nuclei, which is rapidly overcome at elevated temperatures, thereby leading to near-linear kinetics. Moreover, SEM images of the dehydrogenated 15 wt.%-NiCo@TiO2–MgH2 sample at 275°C reveal a uniform distribution of the NiCo@TiO2 DACs on the Mg surface (Supplementary Fig. 20), confirming excellent structural stability.

Figure 5c shows the isothermal hydrogenation kinetics of 15 wt.%-NiCo@TiO2–Mg, i.e., dehydrogenated 15 wt.%-NiCo@TiO2–MgH2, under a hydrogen pressure of 40 bar. The 15 wt.%-NiCo@TiO2–Mg composite absorbed 4.67 wt.% at 50°C within 30 min. Based on the superior hydrogenation kinetics, the hydrogenation kinetics at room temperature (30 °C) were further tested, revealing that 15 wt.%-NiCo@TiO2–Mg absorbed 3.79 wt.% and 5.47 wt.% H2 within 30 and 600 min (Supplementary Fig. 21a), respectively, demonstrating excellent hydrogen absorption kinetics. To obtain faster hydrogen adsorption and higher capacity, we investigated the hydrogenation behaviour of 15 wt.%-NiCo@TiO2–Mg at higher temperatures, which absorbed 6.32 wt.% H2 within 20 s at 250°C (Supplementary Fig. 21b), demonstrating a high hydrogen absorption rate and near-saturation capacity.

The desorption/absorption apparent Ea is a crucial parameter for characterizing the de/hydrogenation kinetics. The dehydrogenation kinetics were evaluated by applying nine distinct kinetic models (Supplementary Table 4). According to the fitting results (Supplementary Fig. 22), the R3 model (three-dimensional phase boundary) exhibited the best agreement with the experimental data at 275°C, as indicated by a slope nearest to unity. The applicability of the R3 model suggests that the dehydrogenation process is controlled by a three-dimensional growth of Mg nuclei at the phase interface, indicating that NiCo@TiO2 DACs could increase the nucleation sites of Mg by promoting the diffusion of hydrogen68,69. The desorption Ea was derived from the linear fitting of lnk and 1000/T (Fig. 5d), yielding a value of 53.54 kJ·mol−1, which corresponds to a 66.5% reduction compared to pure MgH2. Similarly, the Ea for hydrogenation was calculated to be 16.71 kJ·mol−1, representing a decrease of 72.2%.

The thermodynamics were further probed through Pressure-Composition-Temperature measurement (Supplementary Fig. 23). The derived dehydrogenation enthalpy (ΔH) of 71.67 kJ·mol−1 H2 is lower than that of pure MgH2 (76.41 kJ·mol−1 H2), confirming that the NiCo@TiO2 DACs not only improve the kinetics but also decrease thermodynamics stability of the hydrogen storage process in MgH2/Mg systems, which is consistent with our DFT calculations.

To evaluate the practical application potential of 15 wt.%-NiCo@TiO2–MgH2, its reversible cycle performance was investigated in depth. Isothermal H2 absorption and desorption were conducted at 280°C (Fig. 5e). Even after 100 cycles, the catalysed MgH2/Mg retained 97.6% of its capacity, demonstrating excellent cycle stability. The SEM and HAADF-STEM characterisations combined with the corresponding EDS elemental mapping of the 100-cycled sample revealed that the atomic-scale distribution of individual metal atoms and the dispersion of the NiCo@TiO2 DACs on the MgH2 surface remained highly uniform (Supplementary Figs. 24–25). The excellent stability of the sample can be attributed to the enhanced interfacial electron transfer enabled by the metal–metal (Ni–Co) interactions, while the strong metal–support (Ni/Co–TiO2) interactions effectively suppress metal atom migration47,70. Furthermore, the XRD patterns revealed negligible peak shift between the pristine and the 100-cycled sample, indicating excellent structural consistency. However, the pattern of the 100-cycled sample showed intensified peaks and a reduced full width at half maximum for MgH2 (Supplementary Fig. 26), suggesting increased crystallinity and grain coarsening during cycling, which may account for the observed slight capacity degradation. Overall, this structural integrity strongly supports the robust reversibility and exceptional cyclic stability of the NiCo@TiO2 DAC-catalysed MgH2/Mg systems, underscoring its practical suitability for hydrogen storage applications.

To comprehensively evaluate the superior catalytic performance of the NiCo@TiO2 DACs in the MgH2 hydrogen storage system, we systematically compared their catalytic effects with those of nano-catalysts (NiNP@TiO2, CoNP@TiO2, and NiCoNP@TiO2) and SACs (Ni@TiO2 and Co@TiO2) under identical conditions. Unlike those of the SACs and DACs, the XRD patterns of the nano-catalysts exhibited distinct peaks corresponding to metallic Ni and Co phases (Supplementary Fig. 27), confirming the successful deposition of Ni/Co nanoparticles on the TiO2 support. This was confirmed by the presence of characteristic lattice fringes associated with Ni and Co in the HRTEM images (Supplementary Figs. 28–30), confirming nanoparticle formation.

The TPD analysis of the composites of MgH2 with different catalysts (Fig. 5a and Supplementary Fig. 31) revealed that the composites corresponding to nano-catalysts exhibited high dehydrogenation temperatures and sluggish desorption kinetics compared to the SACs and DACs, highlighting the key role of atomic-scale catalytic design in optimising the hydrogen release properties. Furthermore, the Ni-based catalysts (NiNP@TiO2 and Ni@TiO2) led to substantially lower peak desorption temperatures for MgH2 than Co-based catalysts (CoNP@TiO2 and Co@TiO2), indicating more pronounced improvements in the hydrogen release properties (Fig. 5f). The Ni–Co synergistic effect (NiCoNP@TiO2 and NiCo@TiO2) can further reduce the initial and peak dehydrogenation temperatures of MgH2.

Subsequently, we conducted a comprehensive comparative analysis of the average de/hydrogenation rates and the corresponding Ea of MgH2 under isothermal conditions for six distinct catalytic systems (Supplementary Figs. 32–36). Similarly, the Ni-based catalyst substantially enhanced the dehydrogenation process in MgH2/Mg, whereas the Co-based catalyst exhibited better performance in hydrogen absorption. Furthermore, the Ni–Co synergistic interactions simultaneously improved the hydrogen absorption and desorption processes in MgH2/Mg. The NiCo@TiO2 DACs–catalysed MgH2/Mg achieved the fastest reaction kinetics and the lowest Ea barrier. The kinetic analysis revealed that 15 wt.%-NiCo@TiO2–MgH2 achieved a remarkable hydrogen desorption rate (3.02 wt.%·min−1) at 275°C and a near-room-temperature hydrogen absorption rate (0.73 wt.%·min−1) at 50°C (Fig. 5g). The lowest Ea for the hydrogen absorption and desorption in MgH2/Mg under six catalytic conditions further highlight the exceptional capability of NiCo@TiO2 DACs in promoting the Mg–H bonds breaking and reducing the H2 dissociation barrier (Fig. 5h), indicating their superiority in hydrogen storage applications (Supplementary Table 5).

Previous reports have indicated that the elements Pd/Ru exhibit extremely excellent catalytic effects on the MgH2/Mg systems56,71,72. Given these established findings and the promising catalytic performance of Ni and Co observed in our study, we conducted additional TPD experiments to directly compare the hydrogen desorption behaviour of the NiCo@TiO2 DACs with several reference catalysts. These included single-atom catalysts (Ru@TiO2, Pd@TiO2) and other dual-atom systems (such as RuCo@TiO2, RuNi@TiO2, PdNi@TiO2, PdRu@TiO2, PdCo@TiO2). All catalysts were synthesized and tested under identical conditions. The TPD curves (Supplementary Fig. 37) clearly demonstrate that MgH2 catalysed by NiCo@TiO2 DACs achieves the lowest initial and peak desorption temperatures among all tested SACs and DACs. Furthermore, compared to systems reported in previous studies (Supplementary Table 6), 15 wt.%-NiCo@TiO2–MgH2 exhibits superior performance in terms of reduced dehydrogenation temperature, enhanced dehydrogenation rate, as well as decreased activation energy and enthalpy. This exceptional performance highlights that the synergy in NiCo@TiO2 DACs is not merely a generic diatomic effect, but rather a unique electronic cooperation between Ni/Co atoms and TiO2 support.

Catalysis mechanism in NiCo@TiO2–MgH2 systems
Building upon the DFT-calculated mechanism insights in the first section, experimental characterisation was employed to further study the catalytic mechanism of NiCo@TiO2 DACs for hydrogen storage. XPS characterisation reveals a systematic decrease in the value of OL (oxygen lattice)/OV in all samples from 85/15 (pristine TiO2) to 76/24 (NiCo@TiO2), 34/66 (ball-milled composite) and 27/73 (dehydrogenated sample), indicating the gradual increase in the OV concentrations (Fig. 6a). The OV content in the hydrogenation sample decreased compared to that in the dehydrogenated sample, returning to the level observed in the ball-milled sample, which reveals the reversible evolution and self-reconstruction of the catalyst during de/hydrogenation cycles.

These quantitative results also demonstrate significant variations in polyvalent Tix+ species among different samples. The detailed deconvolution of Ti 2p spectra revealed that the change in the Ti valence state is strongly correlated with the OV concentration (Fig. 6b and Supplementary Table 7). Complementary XRD analysis (Supplementary Fig. 26) of the first dehydrogenation products showed only the peaks of the Mg and TiO2 composites. This disappearance of the TiO2 phase confirms the Tix+ and OV evolutions. Moreover, it can be clearly observed that introducing dispersed atoms and using ball milling effectively increases the OV content, as indicated by the XPS results. The increased OV serves as an active site, improving electron transport, whereas the Tix+ species act as crucial intermediates during the electron transfer between Mg2+ and H−73–75.

To elucidate the electronic synergy between the Ni/Co dual-atom sites and the TiO2 support at the nanoscale, atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) were used to quantitatively map the surface potential distribution in pure TiO2, Co@TiO2, Ni@TiO2, and NiCo@TiO2. High-resolution AFM images (Fig. 6c) show TiO2 nanoparticles (diameter = 40–60 nm) with homogeneous dispersion, while KPFM mapping (Fig. 6d) shows pronounced surface potential difference at the nanoscale. Statistical analysis of surface potentials demonstrated that the substantial potential contrast of NiCo@TiO2 DACs (median value = 34.47 mV), representing 71.07% and 105.79% enhancements over Ni@TiO2 SACs (median value = 20.15 mV) and Co@TiO2 SACs (median value = 16.75 mV), respectively (Fig. 6e). The higher surface potential difference in NiCo@TiO2 indicates a lower work function. According to the definition of work function (Eq. 1), the reduced work function corresponds to an upward shift of the Fermi level, enhancing their electron-donating capacity.

Φ = EV − EF, (1)

where EV is the vacuum level (EV = 0) and EF is the Fermi level. Furthermore, the elevated Fermi level reduces the desorption energy barrier of H− in MgH2 and accelerates the dehydrogenation kinetics via a Tix+-mediated electron-transfer pathway, ultimately enhancing the catalytic activity of NiCo@TiO2 DACs18. Thus, the NiCo@TiO2 DACs’ superior catalytic performance compared to SACs can be attributed to the optimised electronic structure of NiCo@TiO2 DACs, which facilitates the charge transfer during the de/hydrogenation cycles of MgH2.

Based on the above experimental and computational results, a comprehensive mechanism for the de/hydrogenation processes was proposed (Fig. 6f). The core of the hydrogen desorption process is breaking the Mg–H bonds. The dehydrogenation process is elucidated in the following points. (1) The introduction of Ni and Co atoms into the MgH2/Mg hydrogen storage system disrupts the Mg–H bonding orbitals, generating anti-bonding states near the Fermi level, weakening the Mg–H bond energy. (2) Ni exhibits superior catalytic activity over Co, and Ni serves as the primary active site. Driven by Co-induced electronic coupling, the downward shift of Ni’s d-band centre precisely modulates Mg–H bond weakening, concertedly reducing bond dissociation energy and facilitating Mg nucleation. (3) The TiO2 support, which is present as polyvalent Tix+ in NiCo@TiO2 DACs, serves as an effective electron-transfer medium due to its mixed-valence characteristics, which were confirmed by the transformation of most high-valent Ti to low-valent Ti after dehydrogenation. (4) The synergistic effect of Ni/Co dual-site considerably enhances the electron-transfer capability of TiO2, facilitating the migration of electrons from H− to Tix+ and then to Mg2+, lowering the energy for breaking the Mg–H bonds and promoting Mg nucleation.

H2 molecule dissociation is the main factor for hydrogen absorption process. The hydrogenation process can be elucidated in the following steps. (1) The synergistic coupling of Ni and Co atoms enables dynamic modulation of the d-band centre. Specifically, Ni induces an upward shift in the d-band centre of Co atoms to enhance hydrogen adsorption. (2) The increased d-band centre of Co enables efficient H2 dissociation and enhanced hydrogen atom diffusion, establishing Co as the primary active site for H2 activation within the Mg hydrogenation system. (3) The TiO2 support maintains a polyvalent valence state (predominantly in low oxidation states) in the dehydrogenation product, functioning as an electron-transfer mediator in the process of hydrogenation to enable catalytic electron cycling, which was confirmed by the reversible transformation of Ti from a low oxidation state to a high oxidation state, indicating the self-reconstruction of the support during de/hydrogenation cycling. (4) The Ni/Co dual-site exhibits a synergistic effect that markedly enhances the electron-transfer capability of TiO2, promoting efficient electron migration from Mg to Tix+ and subsequently to H2.

In summary, the superior catalytic performance of NiCo@TiO2 DACs is attributed to a concerted mechanism orchestrated by Ni/Co atoms and the TiO2 support. (1) Dual-atom synergy: The Ni-Co heteronuclear dual-atom catalyst features complementary electronic and orbital properties, which enhance de/hydrogenation by optimising the d-band centres of both metals and lowering energy barriers for Mg–H bond cleavage and H2 dissociation. During dehydrogenation, Ni acts as the principal site for Mg–H bond cleavage, while Co modulates the d-band centre of Ni to further weaken the Mg–H bond energy. In hydrogenation, Co serves as the dominant site for H–H bond dissociation, whereas Ni optimises the d-band centre of Co, shifting it upwards to facilitate hydrogen adsorption and dissociation. (2) TiO2 support function: TiO2 support undergoes self-reconstruction throughout the reversible de/hydrogenation processes, contributing to the catalytic functionality and stability. (3) Metal–support interaction: Dual-atom Ni/Co promotes the generation of OV and polyvalent Ti in the TiO2 support, OV as the active site accelerates the electron transport, and the Ti as an intermediate for electron transport, passing the electron and presenting valence change, which avoids the high bond energy caused by the direct breaking of the Mg-H bond. Concurrently, the interface formed between isolated Ni/Co atoms and Tix+ species creates atomically dispersed Ni/Co–O–Tix+ active centres, which further enhance catalytic activity.

Methods
The detailed experimental procedures and characterization methods are provided in the Supplementary Information.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript from Jinlong Jin, Jiyue Zhang, Jingjing Zhang, Xiaowei Chen, Heyi Qian, Bohua Jia, Jinghua Liu, Baoxin Han, Wentao Wang, Xiaojun Yan,, Jianglan Shui, and Jianmei Huang reports on the use of heteronuclear dual-atom catalysts (DAC) of Ni and Co as a catalysts for the reversible hydrogenation and dehydrogenation of Mg/MgH2. Specifically, they study the way Ni1Co1@TiO2 serves to improve performance metrices of Mg/MgH2 hydrogenation/dehydrogenation, reducing the temperatures required for dehydrogenation. The materials platform was also demonstrated to be robust, showing minimal degradation after 100 cycles. Furthermore, extensive characterisation has been provided. The findings of this manuscript are certainly relevant and of broad interest. However, there are some outstanding questions that would need to be addressed before this reviewer can recommend for publication.\n\nMajor comments\n1) Part of the mechanism for put forth by the authors invoke that the support (TiO2) plays a salient role as an electron-transfer mediator. However, it is the opinion of the reviewer that there is little experimental data in the manuscript to support such claims. If analogous experiment could be performed on other oxides, or a comparison of performance as a function of the degree of oxygen vacancies, that would significantly bolster the claims put forth by the authors.\n2) What is the actual stoichiometry of the Mg and H that forms in the MgH2, and also how much Hx is remaining after desorption? What are the origins of incomplete conversion of Mg to MgH2?\n3) How do the authors ensure that heat transfer throughout the material is rapid? Are there any possibilities of sluggish thermal conductivities limiting desorption or even hydrogenation?\n\nMinor comments\n1) From the K-edge XANES spectra, the authors invoke that the valency of Ni species in Ni1CO1@TiO2 is between 0 and +2, whereas the Co species is between +2 and +3. It would be helpful if there was more explanation with regards to what this observation actually means. In the case of Ni, does this invoke that there are bimodal distributions of discreet Ni0 and Ni2+, or some electronically coupling of Ni0 with Ni2+ such that they are indistinguishable, or even the existence of a Ni1+ species (however unlikely that is).\n2) Are there any Ni-Co bonds that formed in Ni1CO1@TiO2?\n3) Why do the isothermal dehydrogenation traces (Fig. 5b, supplementary Fig. 17-19) all seem to have a lag time before desorption happens (i.e. the trace follows a slight S like curvature)?\n4) Control experiments in the absence of both Ni or Co (just Mg@TiO2) should be provided.\n5) Control experiments in the absence of Mg (just Ni1Co1@TiO2) should be provided.\n6) Some formatting errors (e.g. axis cut off in supplementary Fig. 17) need to be addressed", "reviewer_2": "I have carefully reviewed the manuscript titled \"Bidirectional Catalysts with Dual-Atom Dynamic d-Band Centre Modulation and Support Self-Reconstruction for Reversible De/Hydrogenation in MgH₂/Mg.\" While the authors present an intriguing catalyst design concept and promising preliminary results, I find that the manuscript suffers from several significant scientific deficiencies in both experimental and theoretical aspects.\nThese issues critically undermine the validity of the claimed conclusions and the reliability of the proposed catalytic mechanism. I therefore recommend rejection of the manuscript in its current form. My main concerns are outlined below:\n1. The reported dehydrogenation activation energy (Ea = 53.5 kJ/mol) is unusually low compared to literature reports with similar onset temperatures. For instance, the work by Guan et al. (Adv. Mater. 2025) achieved an onset temperature of 172 °C with an Ea of 81 kJ/mol—close in temperature, yet significantly higher in energy barrier. The authors provide no rigorous kinetic fitting data, no error analysis, and no alternative model comparisons to justify the credibility of their reported activation energy. This weakens the central claim of exceptional catalytic performance.\n2. Despite using synchrotron-based techniques (e.g., XANES, EXAFS), the manuscript fails to clearly define the structure of the Ni1Co1@TiO2/MgH2 composite. The precise identity and spatial location of the dual-atom catalyst (DAC) within the composite remains vague. Furthermore, the structural model used for DFT calculations does not appear to be sufficiently supported by experimental data such as atomic-resolution imaging, coordination analysis, or local environment validation.\n3. The stability and rationality of the theoretical models are not adequately addressed. Key assumptions—such as the adsorption stability of isolated Ni and Co atoms on TiO2, and their survival during high-temperature processing—are neither experimentally confirmed nor computationally validated via, e.g., binding energy or diffusion barrier calculations. Without such justification, the physical realism of the DFT models is questionable.\n4. The authors attribute the dehydrogenation activity to Ni and the hydrogenation activity to Co, based largely on band-center arguments. However, no experimental results (e.g., selective poisoning, site-specific spectroscopy, or reaction kinetics with isolated monometallic analogs) convincingly prove this assignment. In particular, the superior performance of the bimetallic catalyst might well result from Ni–Co dual-site synergy, rather than a simple bifunctional division. This uncertainty casts doubt on the validity of the mechanistic model and the rationality of the catalyst design logic.\n5. Crucial experimental and computational parameters are either missing or insufficiently described. These include DFT convergence criteria, energy cutoff and k-point details, ball-milling protocol specifics, and structural characterisation settings (e.g., EXAFS fitting ranges, model validation criteria). The lack of such information compromises reproducibility and raises concerns regarding the technical rigor of the study.", "reviewer_3": "This study proposes a dual-atom catalyst coupled with a support self-reconstruction strategy to enhance the hydrogen storage properties of the Mg/MgH2 system. The research topic holds potential value. The heteronuclear dual-atom catalyst (DAC) Ni1Co1@TiO2 substantially reduce the initial dehydrogenation temperature of MgH2 to 168 °C and achieve the highest catalytic performances reported for MgH2-based materials (room-temperature hydrogen uptake 3.79 wt.% within 30 min). However, to some extent, I don't recommend it with the current version to be published in the Nature Communications. Specific comments are given below:\n1. The proposed “bidirectional catalysts with dual-atom” in this work are essentially a conceptual derivative of bimetallic catalysts (e.g., Pd-Ni), demonstrating substantial conceptual overlap with the PdNi biatomic clusters catalyst (Adv. Mater., 2023) reported by Zhang Yao's group at Southeast University. The proposed strategy of “Ni and Co bidirectionally modulate d-band centres for complementary catalysis” represents the dynamic modulation of the d-band center in bimetallic systems. This demonstrates overlap in innovative concepts with the work demonstrating that “PdNi nanoclusters with an optimal d-band center dominate catalytic activity” (Adv. Mater., 2023). Consequently, the innovation of this work requires further elucidation.\n2. Through density functional theory (DFT) calculations, the dual-atom catalyst has achieved a 76.84% reduction in Mg-H bond energy via synergistic effects. Does this correspond to an improvement in the experimental hydrogen desorption enthalpy? Relevant PCT curve data has not yet been observed in the manuscript.\n3. Why do changes in the d-band centers of Ni and Co in the Ni1Co1@TiO2 catalyst enable Ni and Co atoms to function as distinct active sites for dehydrogenation and hydrogenation reactions, respectively? Is there experimental confirmation that Ni and Co atoms with distinct d-band centers exhibit differences in hydrogen sorption performance?\n4. The Ni1Co1@TiO2 catalyst enhances the hydrogen storage kinetics of Mg/MgH2. Specifically, which rate-limiting step in the kinetics is improved by the diatomic catalyst in this work, and how does it distinguish itself from other single-atom or diatomic catalysts?\n5. The manuscript claims that the Ni1Co1@TiO2 catalyst endows Mg/MgH2 with exceptional hydrogen storage performance. However, it fails to visually demonstrate concrete performance levels compared to other reported catalysts.\n6. The main text states that the cycling stability of the material was tested at 280°C, which differs from the testing temperature (290°C) specified in the experimental details of the supporting documents. What is the reason for this discrepancy?\n7. Certain variables should be italicized, such as the activation energy Ea"}, "author_response": "Response Letter\nDear reviewers:\nWe are very grateful for your constructive comments and suggestions for our\nmanuscript entitled “Bidirectional Catalysts with Dual-Atom Dynamic d-Band Centre\nModulation and Support Self-Reconstruction for Reversible De/Hydrogenation in\nMgH /Mg” (ID: NCOMMS-25-53103). Your comments are very valuable and helpful\n2\nfor improving our manuscript. In the following, the responses to all the comments are\nprovided point to point.\nWe have tried our best to make all the revisions clear, and the amendments are\nhighlighted in yellow in the revised manuscript and Supporting Information. We hope\nthat the revised manuscript can satisfy the requirement for publication.\nThank you again for your valuable comments and suggestions. We look forward to\nhearing from you.\nYours sincerely,\nJianmei Huang\nAssociate Professor\nSchool of Energy and Power Engineering\nBeihang University\nThe manuscript from Jinlong Jin, Jiyue Zhang, Jingjing Zhang, Xiaowei Chen, Heyi\nQian, Bohua Jia, Jinghua Liu, Baoxin Han, Wentao Wang, Xiaojun Yan,, Jianglan Shui,\nand Jianmei Huang reports on the use of heteronuclear dual-atom catalysts (DAC) of\nNi and Co as a catalysts for the reversible hydrogenation and dehydrogenation of\nMg/MgH . Specifically, they study the way Ni Co @TiO serves to improve\n2 1 1 2\nperformance metrices of Mg/MgH hydrogenation/dehydrogenation, reducing the\n2\ntemperatures required for dehydrogenation. The materials platform was also\ndemonstrated to be robust, showing minimal degradation after 100 cycles. Furthermore,\nextensive characterisation has been provided. The findings of this manuscript are\ncertainly relevant and of broad interest. However, there are some outstanding questions\nthat would need to be addressed before this reviewer can recommend for publication.\nResponse: We appreciate your positive feedback and valuable comments on our\nmanuscript. And we have thoroughly reviewed all the suggestions and have revised the\nmanuscript to incorporate them. Our responses to each comment are provided below.\nMajor comments\n1) Part of the mechanism for put forth by the authors invoke that the support (TiO )\n2\nplays a salient role as an electron-transfer mediator. However, it is the opinion of the\nreviewer that there is little experimental data in the manuscript to support such claims.\nIf analogous experiment could be performed on other oxides, or a comparison of\nperformance as a function of the degree of oxygen vacancies, that would significantly\nbolster the claims put forth by the authors.\nResponse: Thank you very much for your constructive suggestions. To highlight the\ncrucial role of the TiO support in the catalytic process of the MgH /Mg system, we\n2 2\nfirst evaluated the effect of bimetallic atoms supported on different oxides on the\ndehydrogenation performance of MgH . Previous studies have indicated that transition\n2\nmetal oxides such as V O and Nb O also exhibit excellent catalytic effects on the\n2 5 2 5\nMgH /Mg systems[1-6]. Therefore, following the same hot molten salt method used to\n2\nprepare Ni Co @TiO DACs, we synthesized Ni Co @V O DACs and\n1 1 2 1 1 2 5\n1 1 2 5 2\nball milling process to obtain 15 wt.%-Ni Co @V O –MgH and 15 wt.%\n1 1 2 5 2\nNi Co @Nb O –MgH (the detailed preparation procedure is provided in the\n1 1 2 5 2\nSupplementary Information). Temperature-programmed desorption (TPD) results\n(Figure R1) revealed that the initial dehydrogenation temperature (189 °C) and the peak\ndehydrogenation temperature (263 °C) of 15 wt.%-Ni Co @V O –MgH were higher\n1 1 2 5 2\nthan those of 15 wt.%-Ni Co @TiO –MgH (which exhibited initial and peak\n1 1 2 2\ndehydrogenation temperatures of 163 °C and 220 °C, respectively). Similarly, the TPD\nprofile of 15 wt.%-Ni Co @Nb O –MgH consistently demonstrated higher\n1 1 2 5 2\ndehydrogenation temperatures, with an onset at 170 °C and a peak at 228 °C, further\naffirming the superior catalytic efficacy of the TiO -supported catalyst.\nof the MgH /Mg system, we modulated the oxygen vacancy (O ) concentration within\nthe TiO carrier. Specifically, TiO with a high content of oxygen vacancies was\n2 2\nsynthesized by high-temperature calcination of commercial nano-TiO in the presence\n2\nof the reducing agent NaBH , namely TiO -O (O /O 75/25) and TiO -O\n4 2 V1 L V= 2 V2\n(O /O 68/32), respectively, as analyzed by XPS analysis (Figure R2a). Using the same\nL V=\nsynthesis route, we further prepared 15 wt.%-TiO -O -MgH and 15 wt.%-TiO -O -\nMgH composites (detailed procedures are provided in the Supplementary Information).\n2\ndesorption performance compared to both 15 wt.%-TiO -MgH and 15 wt.%-TiO -O -\n2 2 2 V2\nMgH , suggesting a volcano-type relationship between oxygen vacancy content and\n2\ncatalytic activity in the MgH /Mg systems (Figure R2b). This peak performance could\n2\nbe attributed to an optimal density of active sites, which provides a high concentration\nof vacancies for hydrogen dissociation and adsorption while maintaining their\ndispersion and stability. Accordingly, the preparation of DACs can involve keeping the\nO concentration within an optimal range. Given the similar oxygen vacancy content\nV\nbetween TiO -O (O /O =75/25) and Ni Co @TiO (O /O =76/24), the significantly\nenhanced hydrogen storage performance of 15 wt.%-Ni Co @TiO -MgH over 15\n1 1 2 2\nwt.%-TiO -O -MgH further underscores the pivotal role of Ni/Co dual-atom sites in\nthe catalytic process mediated by Ni Co @TiO DACs.\n1 1 2\nFigure R2. (a) XPS spectra of O 1s of TiO with different concentrations of oxygen vacancies.\n2\n(b)TPD curves of MgH catalyzed by TiO with different concentrations of oxygen vacancies.\n2 2\nTherefore, based on the experimental results presented above, we conclude that\nthe oxygen vacancies play a significant role in regulating the dehydrogenation\nproperties of the MgH /Mg systems. The corresponding contents and Figures R1, R2\n2\nhave been added to the revised main text and Supplementary Information. We sincerely\nappreciate your valuable comments, which have significantly improved the quality of\nour manuscript.\n4\n2) What is the actual stoichiometry of the Mg and H that forms in the MgH , and also\n2\nhow much Hx is remaining after desorption? What are the origins of incomplete\nconversion of Mg to MgH ?\ncapacity of MgH is approximately 7.659 wt.%, corresponding to the theoretical\n2\nstoichiometric ratio Mg: H = 1: 2. In this study, the catalyst accounts for 15% of the\nmass ratio in the Ni Co @TiO –MgH composite. Therefore, the theoretical\n1 1 2 2\ndehydrogenation capacity of the 15 wt.%-Ni Co @TiO –MgH composite is calculated\n1 1 2 2\nto be approximately 6.510 wt.%, considering that the pure catalyst itself does not\nrelease hydrogen. As shown in Figure R3, the TPD results of the 15 wt.%-\nNi Co @TiO –MgH composite show an actual hydrogen release of approximately\n1 1 2 2\n6.485 wt.%. This corresponds to an effective stoichiometry of Mg: H ≈ 1: 1.992.\nFigure R3. TPD curves of Ni Co @TiO and 15 wt.%-Ni Co @TiO -MgH .\n1 1 2 1 1 2 2\nWe further evaluated the isothermal dehydrogenation performance of the 15 wt.%-\nNi Co @TiO –MgH composite at 300 °C. Within 120 minutes, a hydrogen desorption\n1 1 2 2\ncapacity of 6.484 wt.% was achieved (Figure R4a). Subsequent XRD analysis of the\ndesorbed sample revealed no detectable diffraction peaks corresponding to MgH\n2\n(Figure R4b), indicating nearly complete hydrogen release with negligible residual\nhydride (H ).\nX\nThe apparent incomplete conversion of Mg to MgH , as inferred from the\n2\nisothermal hydrogenation curve, is likely due to the relatively low temperature and short\n5\nduration applied during the experiment. These conditions were intentionally selected to\navoid an excessively rapid hydrogenation rate, which would complicate accurate\ncalculation of the hydrogen absorption activation energy. Moreover, hydrogen\nabsorption performance at near-room temperature is highly relevant to the practical\napplication of MgH /Mg systems. To further investigate this issue, we conducted\n2\nadditional isothermal hydrogenation experiments at a higher temperature (250 °C).\nUnder these conditions, 15 wt.%-Ni Co @TiO –MgH achieved a hydrogen absorption\n1 1 2 2\ncapacity of 6.480 wt.% (Figure R5), corresponding to a nearly complete conversion to\nMgH . This result demonstrates the excellent reversible de/hydrogenation performance\n2\nof the MgH /Mg systems.\n1 1 2 2\n(b) XRD pattern of 15 wt.%-Ni Co @TiO –MgH and Dehy-Ni Co @TiO –MgH (the asterisk *\n1 1 2 2 1 1 2 2\nassigned as TiO ).\nx\nFigure R5. Hydrogenation curves of 15 wt.%-Ni Co @TiO –MgH at 250℃.\n1 1 2 2\nNi Co @TiO –MgH at 250 °C in the revised Supplementary Information. Thank you\n1 1 2 2\nvery much for your time and thoughtful consideration of our manuscript.\n3) How do the authors ensure that heat transfer throughout the material is rapid? Are\nthere any possibilities of sluggish thermal conductivities limiting desorption or even\nhydrogenation?\nResponse: Thank you for your valuable comments and insightful feedback regarding\nthe heat transfer aspects of our study. We greatly appreciate your thorough review and\nhave addressed your concerns below.\nTo mitigate potential heat transfer limitations, the MgH particles were first\n2\nsubjected to intensive ball milling to reduce them to micron-scale sizes. This treatment\nsignificantly shortens the diffusion paths for both heat and mass transfer[7, 8], thereby\nenhancing the overall kinetics of the de/hydrogenation reaction. Furthermore, we\nuniformly anchored Ni/Co bimetallic atoms onto a nano-TiO support. This nano-\n2\nengineering approach not only offers abundant active sites but also establishes a highly\nconductive network within the MgH matrix. Given that both TiO (~8 W·m-1·K-1)[9, 10]\n2 2\nand the metallic atoms (Ni: ~70 W·m-1·K-1; Co: ~100 W·m-1·K-1)[11-13] possess thermal\nconductivities substantially higher than that of pure MgH (~1.3 W·m-1·K-1)[14], this\n2\nconfiguration greatly promotes efficient heat dissemination. Additionally, the ball\nmilling process ensures intimate contact between MgH and the Ni Co @TiO DACs,\n2 1 1 2\nwhich further reduces interfacial thermal resistance.\nMeanwhile, in the de/hydrogenation tests, we used a relatively small sample\nquantity (70~100 mg), which could ensure that the materials lie flat at the bottom of the\nsample chamber, thus ensuring efficient thermal contact and minimizing heat transfer\ninhibition (high thermal resistance) caused by excessive sample loading. Moreover, our\nexperimental system is equipped with the thermocouple that can directly monitor the\ntemperature of the sample (T ) (Figure R6).\nS\nFor the dehydrogenation process, the measured hydrogen capacity of the 15 wt.%-\nNi Co @TiO –MgH composite is approximately 6.485 wt.%, which closely matches\n1 1 2 2\n7\nthe theoretical value of 6.510 wt.%. Moreover, the material exhibits excellent\nreversibility over multiple dehydrogenation and hydrogenation cycles. This high\nreaction completeness indicates that heat is efficiently supplied to the reaction front,\nfacilitating full conversion. Regarding the hydrogenation process, the high intrinsic\nthermal conductivity of pure Mg (~156 W·m⁻1·K⁻1)[15], combined with its fast\nabsorption kinetics, effectively eliminates heat transfer limitations, thereby ensuring\nrapid and complete hydrogen uptake.\nFigure R6. De/hydrogenation testing systems.\nPrior studies on Mg-based systems have established that, following the\nintroduction of an effective catalyst, the reaction rate is primarily limited by the\nhydrogen diffusion barrier rather than by heat transfer constraints[8, 16]. Our catalyst\ndesign effectively alleviates these barriers.\nIn summary, through rational nanoscale material design, reasonable sample\nquantity and filling method, as well as the proper placement of thermocouples within\nour testing system, we have mitigated the heat transfer limitations at this scale in the\npresent system, thereby achieving optimal performance. The favorable reaction kinetics\nand complete reaction conversion observed in our composites confirm that the\nde/hydrogenation processes are not constrained by slow heat conduction.\nWe hope that this clarification satisfactorily addresses your questions. Thank you\nonce again for your constructive input.\n1) From the K-edge XANES spectra, the authors invoke that the valency of Ni species\nin Ni CO @TiO is between 0 and +2, whereas the Co species is between +2 and +3. It\n1 1 2\nwould be helpful if there was more explanation with regards to what this observation\nactually means. In the case of Ni, does this invoke that there are bimodal distributions\nof discreet Ni0 and Ni2+, or some electronically coupling of Ni0 with Ni2+ such that they\nare indistinguishable, or even the existence of a Ni1+ species (however unlikely that is).\nResponse: We sincerely thank your insightful question. The valency state of the\ncatalytic species is a key point for understanding the reaction mechanism, and we\nappreciate the opportunity to clarify this in more depth. We have considered the\npossibilities raised and provided our interpretation.\nWe have carefully evaluated each scenario: (i) Bimodal distribution of discrete Ni0\nand Ni2+ particles: This scenario is less likely. A physical mixture of separate metallic\nNi0 and NiO (Ni2+) particles would typically exhibit a XANES spectrum with distinct\nfeatures of both phases[17], or at least a more pronounced shoulder in the white-line\nregion, rather than the single, smooth absorption edge we observed. Furthermore, our\nsynthesis method (hot molten salt method) promotes the formation of DACs rather than\na physical mixture[18, 19]. (ii) Existence of a stable Ni1+ species: As you pointed out, this\nis highly unlikely under our experimental conditions. Ni1+ is notoriously unstable and\nis not a common oxidation state in solid-state catalysts, especially without specific\nstabilizing ligands. (iii) Electronic coupling between Ni and the support leading to an\nintermediate valence state: We find this to be the most plausible explanation. The\nobserved spectrum is consistent with the formation of Ni atoms in strong electronic\ninteraction with the TiO support—a phenomenon associated with strong metal-support\n2\ninteraction (SMSI)[20-23]. In such a configuration, electron density can be partially\ntransferred from the metallic Ni0 to the TiO , resulting in Ni atoms with a partial\n2\npositive charge. The XANES spectrum indicates that the electronic state of Ni in\nNi Co @TiO DACs is an intermediate species, with its spectral features representing\n1 1 2\n9\na weighted average of the Ni foil and NiO references. A similar electronic coupling\neffect between Co2+ and Co3+ species accounts for the intermediate oxidation state of\nCo.\nWe have expanded the discussion in the revised manuscript to elaborate on the\ncharge transfer involving both Ni and Co, which we propose as the most plausible\ninterpretation of the observed XANES spectra. We believe this added discussion\nsignificantly strengthens the manuscript by providing deeper insight into the electronic\nstructure of the bimetallic atoms. Thank you again for raising this crucial point.\n2) Are there any Ni-Co bonds that formed in Ni CO @TiO ?\n1 1 2\nResponse: Thank you for raising this important question. Based on the EXAFS fitting\nresults, we did not observe a noticeable Ni–Co scattering path, which suggests the\nabsence of direct Ni–Co bonds within the detection limit of the technique in our\nNi Co @TiO material. This may be attributed to the formation of highly dispersed\n1 1 2\nbimetallic atoms in which Ni and Co may coexist in proximity without forming direct\nmetal-metal bonds, possibly mediated by the TiO support structure. Thank you again\n2\nfor your insightful comment.\n3) Why do the isothermal dehydrogenation traces (Fig. 5b, supplementary Fig. 17-19)\nall seem to have a lag time before desorption happens (i.e. the trace follows a slight S\nlike curvature)?\nResponse: Thank you for this careful observation. As you pointed out, the isothermal\ndehydrogenation curves exhibit a slight sigmoidal (S-shaped) curvature, particularly at\nlower temperatures, which indicates a brief lag time or induction period prior to\nsustained hydrogen release. This behavior is characteristic of a dehydrogenation\nprocess governed by the nucleation and growth of Mg, which is observed in metal\nhydride systems such as MgH [3, 24-31]. The initial decomposition of MgH requires the\n2 2\nformation of stable Mg nuclei within the hydride matrix[8, 16, 32]. This nucleation step\nhas an inherent energy barrier, leading to a delayed onset of rapid desorption at lower\ntemperatures. Once critical nuclei are formed, dehydrogenation accelerates\nautocatalytically, resulting in the characteristic S-shaped curve. At higher temperatures,\nsufficient thermal energy is provided to overcome the nucleation barrier almost\n10\ninstantaneously. As a result, the induction period becomes very short or undetectable,\nand the desorption curve follows near-linear kinetics, obscuring the sigmoidal behavior.\nWe have included a discussion of this mechanism in the revised manuscript to\nimprove clarity. Thank you once again for your thorough review and valuable comment.\n4) Control experiments in the absence of both Ni or Co (just Mg@TiO ) should be\n2\nprovided.\nResponse: Thank you for this valuable suggestion. In response, we prepared a control\nsample of 15 wt.% TiO –MgH using the same prepared procedure and obtained its\n2 2\nTPD profile (Figure R7). The addition of TiO reduced the initial dehydrogenation\n2\ntemperature of MgH to 269 °C, demonstrating its catalytic role in facilitating hydrogen\n2\ndesorption. However, compared to the 15 wt.%-Ni Co @TiO –MgH sample, 15 wt.%-\n1 1 2 2\nTiO –MgH exhibits a noticeably higher dehydrogenation temperature and slower\n2 2\ndesorption kinetics. These results clearly demonstrate the superior catalytic effect of the\nbimetallic Ni–Co sites in enhancing the dehydrogenation performance of the MgH /Mg\n2\nsystem.\nFigure R7. TPD curves of MgH catalyzed by pure TiO , SACs, and DACs.\n2 2\nWe have included these comparative data in the revised manuscript as Fig. 5a. We\nsincerely appreciate your comment, which has helped us to further strengthen the\nmechanistic discussion and experimental validation in our paper.\n5) Control experiments in the absence of Mg (just Ni Co @TiO ) should be provided.\n1 1 2\nexperiments using the Ni Co @TiO catalyst alone (without MgH ) under the same\n1 1 2 2\ntesting conditions. The TPD analysis showed a negligible hydrogen release signal in\nthe temperature range studied, thereby confirming its negligible contribution as a direct\nhydrogen source in the desorption behavior of the Ni Co @TiO –MgH composites\n1 1 2 2\n(Figure R8). This result highlights that the dehydrogenation performance originates\nexclusively from the MgH phase, and that the role of the Ni Co @TiO DACs is to\n2 1 1 2\nenhance the hydrogen storage performance of MgH /Mg systems.\n1 1 2 1 1 2 2\nThe corresponding TPD curve has been included in Fig. 5a of the revised\nmanuscript. We are grateful for your comment, which has helped further clarify the\norigin of the dehydrogenation behavior and strengthen the discussion of the catalytic\nmechanism.\n6) Some formatting errors (e.g. axis cut off in supplementary Fig. 17) need to be\naddressed.\nResponse: Thank you for pointing this out. We apologize for the oversight in the\nformatting of Supplementary Fig. 17. The figure has been carefully revised to ensure\nthe axes are fully displayed and clearly legible. We have also taken this opportunity to\nreview all supplementary figures to prevent similar formatting issues.\nThe updated supplementary materials have been included with the revised\n12\nsubmission. We greatly appreciate your careful review and valuable feedback, which\nhave helped improve the clarity and presentation of our work.\nI have carefully reviewed the manuscript titled \"Bidirectional Catalysts with Dual-\nAtom Dynamic d-Band Centre Modulation and Support Self-Reconstruction for\nReversible De/Hydrogenation in MgH₂/Mg.\" While the authors present an intriguing\ncatalyst design concept and promising preliminary results, I find that the manuscript\nsuffers from several significant scientific deficiencies in both experimental and\ntheoretical aspects.\nThese issues critically undermine the validity of the claimed conclusions and the\nreliability of the proposed catalytic mechanism. I therefore recommend rejection of the\nmanuscript in its current form. My main concerns are outlined below:\nResponse: We sincerely appreciate the time and effort you have dedicated to reviewing\nour manuscript and providing constructive comments. Your feedback has been\ninvaluable in improving our work. We have thoroughly revised and supplemented the\nmanuscript according to all the suggestions to better present our research findings.\nBelow is our point-by-point response to your concerns.\n1. The reported dehydrogenation activation energy (Ea = 53.5 kJ/mol) is unusually low\ncompared to literature reports with similar onset temperatures. For instance, the work\nby Guan et al. (Adv. Mater. 2025) achieved an onset temperature of 172 °C with an Ea\nof 81 kJ/mol—close in temperature, yet significantly higher in energy barrier. The\nauthors provide no rigorous kinetic fitting data, no error analysis, and no alternative\nmodel comparisons to justify the credibility of their reported activation energy. This\nweakens the central claim of exceptional catalytic performance.\nResponse: We sincerely thank you for this critical comment and for providing the\nrelevant comparison to the work by Guan et al. (Adv. Mater. 2025). In response, we\napplied nine established isothermal kinetic models to the 15wt.% Ni Co @TiO -\n1 1 2\nMgH /Mg composite (Table R1)[33, 34]. For each model, the experimental values (t/t )\n2 0.5\nwere plotted against the corresponding theoretical values (t/t ) . As shown in\nexp 0.5 theo\nFigure R9a, the R3 model (three-dimensional phase-boundary-controlled reaction)\ndemonstrated the optimal fit for dehydrogenation at 275 °C, with a slope nearest to 1.\nadditional validation data provided in Figures R9b and R9c. Moreover, we have also\ncalculated the de/hydrogenation activation energy for 15wt.%-Ni Co @TiO -\n1 1 2\nMgH /Mg using the R2 kinetic analysis model (Figure R10), consistent with the\n2\napproach adopted in your referenced study (Adv. Mater. 2025 by Guan et al.). Although\nthe activation energy for hydrogen desorption derived from the R2 model is somewhat\nhigher than that obtained from the R3 model, the values from both models remain\nconsistently low. While the activation energy for hydrogen absorption remained almost\nunchanged. Considering that the R3 model demonstrated a better goodness-of-fit,\nevidenced by its slope and R2 value closer to 1, we ultimately reported the kinetic\nparameters derived from the R3 analysis in our article. Therefore, the above evidence\ndemonstrates the rationality and accuracy of our activation energy calculation model\nand the data results.\nTable R1. Common solid-state rate expressions for different reaction models.\nSymbol Model References Sharp's expression\nD1 One-dimensional diffusion α2 0.2500(t/t )\n0.5\nD2 Two-dimensional diffusion α+(1-α)ln(1-α) 0.1534(t/t )\n0.5\nThree-dimension diffusion\nD3 [1-(1-α)1/3]2 0.0426(t/t )\n0.5\n(Jander equation)\nThree-dimension diffusion\nD4 (1-2α/3)-(1-α)2/3 0.0367(t/t )\n0.5\n(Ginstling-Braunsshtein equation)\nF1 First-order reaction -ln(1-α) -0.6931(t/t )\n0.5\nR2 Two-dimension phase boundary 1-(1-α)1/2 0.2929(t/t )\n0.5\nR3 Three-dimension phase boundary 1-(1-α)1/3 0.2063(t/t )\n0.5\nA2 Avarami-Erofe'ev [-ln(1-α)]1/2 0.8326(t/t )\n0.5\nA3 Avarami-Erofe'ev [-ln(1-α)]1/3 0.8850(t/t )\n0.5\n1 1 2 2\nCalculated activation energies of the hydrogen desorption (Ea ) and absorption (Ea ) of 15 wt.%-\nde ab\nNi Co @TiO –MgH through the R3 kinetic analysis model. (c) The R3 model fitting of 15wt.%\n1 1 2 2\nNi Co @TiO -MgH /Mg composite at different temperatures.\n1 1 2 2\nThe catalytic performance is macroscopically evidenced by a lowered initial\ndehydrogenation temperature, a decreased peak dehydrogenation temperature, and an\naccelerated dehydrogenation rate. Fundamentally, these improvements are quantified\nby the apparent activation energy, which serves as a comprehensive kinetic parameter\nreflecting the energy barrier of the entire dehydrogenation process, rather than a single\ncharacteristic temperature. In this study, the Ni Co @TiO DACs not only reduce the\n1 1 2\nonset temperature but also significantly lower the dehydrogenation peak temperature\nand accelerate the rate of hydrogen desorption, enhancing the overall reaction\nkinetics[24, 35-49]. Compared to the reference system (Adv. Mater. 2025 by Guan et al.),\nthe 15wt.%-Ni Co @TiO -MgH /Mg composite exhibits lower initial and peak\n1 1 2 2\ndehydrogenation temperature, as well as faster dehydrogenation rate. These\nimprovements collectively correspond to a significantly lower dehydrogenation\nactivation energy.\nFigure R10. (a) Calculated activation energies of the hydrogen desorption (Ea ) and absorption\nde\n16\n(Ea ) of 15 wt.%-Ni Co @TiO –MgH through R2 kinetic analysis model. (b) The R2 model fitting\nab 1 1 2 2\nof 15wt.%-Ni Co @TiO -MgH /Mg composite at different temperatures.\n1 1 2 2\nTo ensure the accuracy and rigor of the activation energy calculations for all\nhydrogen storage systems discussed in this work (in both the main text and the\nSupplementary Information), we carefully evaluated kinetic models for each system.\nWe believe the additional data and analyses provided have thoroughly addressed your\nconcerns regarding the credibility of the reported activation energy. Thank you again\nfor prompting us to strengthen this part of our work.\n2. Despite using synchrotron-based techniques (e.g., XANES, EXAFS), the manuscript\nfails to clearly define the structure of the Ni Co @TiO /MgH composite. The precise\n1 1 2 2\nidentity and spatial location of the dual-atom catalyst (DAC) within the composite\nremains vague. Furthermore, the structural model used for DFT calculations does not\nappear to be sufficiently supported by experimental data such as atomic-resolution\nimaging, coordination analysis, or local environment validation.\nResponse: Thank you for your valuable suggestions. In our originally submitted\nmanuscript, we primarily focused on elucidating the structural and coordination\nenvironment of the Ni Co @TiO DACs and the Ni Co @TiO –MgH composite\n1 1 2 1 1 2 2\nthrough characterization techniques, including high-angle annular dark-field scanning\ntransmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy\n(XAS). As shown in Figure 2 of the original manuscript, atomic-resolution HAADF-\nSTEM imaging coupled with EDS mapping clearly demonstrates the homogeneous\ndistribution of isolated Ni and Co atoms on the TiO support, with no indication of\n2\nnanoparticles or clusters of Ni and Co formation. Further structural insight into the\nNi Co @TiO –MgH composite is provided by HRTEM and EDS mapping in Figure 4\n1 1 2 2\nof the original manuscript, which reveals that the Ni Co @TiO DACs, with particle\n1 1 2\nsizes ranging from 40 to 60 nm, are uniformly dispersed on the MgH matrix. Bright\n2\natomic-scale contrasts attributable to Ni/Co atoms are evident in the HAADF images.\nAdditionally, Figure 3 and the corresponding discussion in Section 2.2 present the X-\nray absorption fine structure (XAFS) spectra collected at the Ni and Co K-edges,\nincluding both XANES and EXAFS regions. The EXAFS fitting results unequivocally\n17\nconfirm the absence of metallic Ni–Ni or Co–Co bonds and further indicate oxygen\ncoordination for the isolated Ni/Co atoms. After 100 cycles, the HAADF-STEM and\nEDS-mapping images as provided in the Supplementary Information revealed that the\natomic-scale distribution of individual metal atoms. These findings are fully consistent\nwith the structural model employed in our DFT simulations.\nFigure R11. SEM images and EDS mapping of the Ni Co @TiO DACs.\n1 1 2\nTo further elucidate the microstructure of the Ni Co @TiO –MgH composite,\n1 1 2 2\nSEM characterization was performed on three samples: as-synthesized Ni Co @TiO\n1 1 2\nDACs, the Ni Co @TiO -MgH composite, the Ni Co @TiO -MgH composite after\n1 1 2 2 1 1 2 2\ndehydrogenation, and the Ni Co @TiO -MgH composite after 100 de/hydrogenation\n1 1 2 2\ncycles. As shown in Figure R11, the Ni Co @TiO DACs consist of spherical\n1 1 2\nnanoparticles with diameters ranging from 40 to 60 nm. EDS mapping confirms the\nhomogeneous distribution of Ni and Co, albeit in relatively low amounts compared to\nthe predominant Ti and O elements. After ball-milling with MgH , the composite\n2\nmorphology is presented in Figure R12. The MgH particles (∼8 μm in size) are\n2\nuniformly decorated with Ni Co @TiO nanoparticles. After the sample was\n1 1 2\ndehydrogenated for the first time, the Ni Co @TiO DACs remained dispersed on the\n1 1 2\nMg matrix (Figure R13). Moreover, even after 100 cycles, the SEM images and\ncorresponding EDS mapping of the cycled composite indicate that the Ni Co @TiO\n1 1 2\nnanoparticles remain well-dispersed on the MgH matrix (Figure R14). This\n2\nobservation demonstrates the high structural and thermal stability of the Ni Co @TiO –\n1 1 2\nMgH composite system under repeated cycling conditions. Notably, the SEM\n2\ncharacterization of all three samples is in excellent agreement with the complementary\nFigure R12. SEM images and EDS mapping of the Ni Co @TiO -MgH .\n1 1 2 2\nFigure R13. SEM images and EDS mapping of the Ni Co @TiO -MgH composite after\n1 1 2 2\ndehydrogenation.\n1 1 2 2\nde/hydrogenation cycles.\nWe recognized that the structural evidence for the Ni Co @TiO -MgH composite\n1 1 2 2\nmay not have been fully highlighted in previous discussions. The additional figures and\nanalyses now provided are intended to fully resolve your concerns for the structure of\nthe Ni Co @TiO -MgH composite and the DFT models. Thank you again for your\n1 1 2 2\nvaluable comments.\n3. The stability and rationality of the theoretical models are not adequately addressed.\nKey assumptions—such as the adsorption stability of isolated Ni and Co atoms on TiO ,\n2\nand their survival during high-temperature processing—are neither experimentally\nconfirmed nor computationally validated via, e.g., binding energy or diffusion barrier\ncalculations. Without such justification, the physical realism of the DFT models is\nquestionable.\nResponse: Thank you for raising this critical point regarding the stability of our DFT\nmodels. In fact, this aspect was carefully considered during our initial model\nconstruction, and the excellent stability was further corroborated by post-recycling\n20\ncharacterizations discussed in our original Supplementary Information (as presented in\nSupplementary Fig.11 in the original Supplementary Information and Supplementary\nFig.20 in the revised Supplementary Information). We recognized that these key points\nmay not have been sufficiently emphasized, leading to your concerns. In response, we\nhave now thoroughly revisited this issue by consolidating supporting evidence from\nboth experimental and theoretical aspects. Specifically, we have: (1) Reinforced the\nexperimental evidence already included in the original manuscript, which demonstrates\nthe structural integrity of the single atoms after long-term reaction; (2) Conducted\nadditional DFT calculations to quantitatively evaluate the binding energies and\ndiffusion barriers, further confirming the thermodynamic stability of Ni/Co atoms on\nTiO .\n2\n(1) Experimental Evidence for Structural Stability.\nThe exceptional stability of the isolated Ni and Co atoms is strongly supported by\nmultiple lines of experimental evidence:\n(i) High-temperature synthesis: The Ni Co @TiO DACs were synthesized via a\n1 1 2\nmolten salt method at 500 °C. This high-temperature synthetic process inherently\nensures the thermal stability of the formed single-atom sites.\n(ii) Superior and stable catalytic performance: The Ni Co @TiO DACs maintain\n1 1 2\nexcellent performance over 100 de/hydrogenation cycles, which involve repeated high-\ntemperature reactions. This long-term cycling stability provides strong evidence that\nthe isolated Ni and Co atoms are effectively stabilized against sintering, agglomeration,\nand deactivation under harsh operational conditions.\n(iii) Direct characterization: The atomic dispersion of Ni and Co on the\nNi Co @TiO catalyst was confirmed by HAADF-STEM, XAS, and EDS mapping.\n1 1 2\nCrucially, we performed identical characterization (HAADF-STEM and EDS mapping)\non the sample after 100 cycles (15wt.%-Ni Co @TiO -MgH -100th). The results\n1 1 2 2\nclearly show that the single atoms remain atomically dispersed without the formation\nof nanoparticles or clusters. These post-reaction characterizations offer direct visual and\nspectroscopic evidence for the remarkable stability of the single-atom sites throughout\n21\nthe entire process of ball-milling and repeated high-temperature de/hydrogenation\ncycles.\n(2) Computational Validation (Newly Added).\nFigure R15. (a) Binding energies and (b) diffusion barriers of Co @TiO , Ni @TiO , and\n1 2 1 2\nNi Co @TiO .\n1 1 2\nDuring the initial modeling, we evaluated various theoretical models with different\natomic spacings and bonding modes. The configuration with the lowest total energy\nwas selected for further calculations, as it represents the most thermodynamically stable\nand thus the most probable structure to form under realistic reaction conditions. To\nfurther validate the rationality of the density functional theory model, we conducted\nadditional DFT calculations to quantitatively assess the stability of the single atoms.\nThe binding energies (E ) of isolated Ni and Co atoms at their most stable adsorption\nb\nsites on the TiO surface were calculated (Figure R15a). The obtained E values are\n2 b\nsignificantly negative (E (Ni @TiO ) = -3.043 eV; E (Co @TiO ) = -2.870 eV; E\nb 1 2 b 1 2 b\n(Ni Co @TiO ) = -5.249 eV), confirming a strong metal–support interaction that\n1 1 2\neffectively anchors the atoms and suppresses their migration. Notably, Ni Co @TiO\n1 1 2\nexhibits the most negative binding energy, indicating the strongest binding and highest\nstability among the three systems. Furthermore, we calculated the diffusion energy\nbarriers for individual Ni/Co atoms migrating between two stable adsorption sites on\nthe TiO surface (Figure R15b). The high diffusion barriers (e.g., 3.475 eV for\n1 2 1 2 1 1 2\ndiffusion and aggregation of single atoms are kinetically hindered. This kinetic\n22\nstabilization effectively prevents the formation of metal clusters (even if\nthermodynamically favored), thereby explaining the excellent structural stability of the\nsingle-atom configurations.\nIn summary, the combination of experimental performance, direct post-reaction\ncharacterization, and additional computational validation robustly confirms the stability\nand rationality of our models. We have revised the manuscript accordingly to\nincorporate these critical discussions and data.\n4. The authors attribute the dehydrogenation activity to Ni and the hydrogenation\nactivity to Co, based largely on band-center arguments. However, no experimental\nresults (e.g., selective poisoning, site-specific spectroscopy, or reaction kinetics with\nisolated monometallic analogs) convincingly prove this assignment. In particular, the\nsuperior performance of the bimetallic catalyst might well result from Ni–Co dual-site\nsynergy, rather than a simple bifunctional division. This uncertainty casts doubt on the\nvalidity of the mechanistic model and the rationality of the catalyst design logic.\nResponse: In our manuscript, we did not assign the dehydrogenation and\nhydrogenation contributions exclusively to Ni and Co atoms, respectively. Instead,\nthrough comprehensive experimental characterization, controlled comparisons, and\nDFT calculations, we aimed to identify which element plays a more prominent role in\neach process. Furthermore, we explicitly emphasized the synergistic interaction\nbetween Ni and Co atoms, rather than proposing a rigid separation of functions. The\nreference to d-band center theory serves as a plausible mechanistic interpretation\nconsistent with the experimental evidence presented.\nIn fact, we conducted extensive control experiments—including tests on the\nenhancement of hydrogen storage performance in MgH /Mg systems using\n2\ncorresponding single-atom catalysts and nano-catalysts—which are provided in the\noriginal manuscript and the Supplementary Information (refer to Section 2.4–2.5, Figs.\n5–6 in the main text, and Supplementary Figs. 1–4 and 16–19). These results\nconsistently demonstrate the synergistic cooperation between Ni and Co sites. We\nrecognized that this discussion may not have been sufficiently emphasized in our\n23\noriginal submission and that the relevant figures in the Supplementary Information may\nnot have received adequate attention, which affected your reading experience. To\nfacilitate your review, the results and discussions of these comparative experiments are\nnow included in Sections 2.4–2.5 and Figs. 5–6 in the main text of revised manuscript,\nas well as Supplementary Figs. 1–5 and 25–30 in the revised Supplementary\nInformation.\nFigure R16. (a) Dehydrogenation curves of 15wt.%-Co @TiO –MgH at different temperatures. (b)\n1 2 2\nHydrogenation curves of 15wt.%-Co @TiO –MgH at different temperatures. (c) Nine kinetic\n1 2 2\nanalysis models at 300°C of 15 wt.%-Co @TiO –MgH . (d) Calculated hydrogen desorption and\n1 2 2\nabsorption activation energies of 15wt.%-Co @TiO –MgH by R2 kinetic analysis model.\n1 2 2\nHerein, we once again presented the detailed reaction kinetics studies for MgH\n2\ncatalyzed by both Ni @TiO SACs and Co @TiO SACs (Figures R16 and R17). The\n1 2 1 2\nkey experimental finding is that Ni @TiO SACs exhibit dehydrogenation kinetics\n1 2\nsignificantly superior to those of Co @TiO SACs, though inferior to the Ni Co @TiO\n1 2 1 1 2\nDACs. Conversely, Co @TiO SACs demonstrate hydrogenation kinetics that are\n1 2\nmarkedly better than those of Ni @TiO SACs, yet still not as performant as the\n1 2\nNi Co @TiO DACs. This clear disparity in the catalytic propensity of the single-atom\n1 1 2\ncatalysts provides strong experimental support for our assignment: Ni sites are more\nactive for dehydrogenation, while Co sites are more active for hydrogenation, and\nNi/Co sites lead to a synergistic enhancement of the hydrogen storage performance in\nMgH /Mg systems. To further support our findings, we test the catalytic functions of\nkinetics of 15 wt.% Ni NP@TiO –MgH , 15 wt.% Co NP@TiO –MgH , and 15 wt.%\n2 2 2 2\nNiCo NP@TiO –MgH (NP represents nanoparticles) in Figures R18–R20. The kinetic\n2 2\nresults of these samples consistently indicate that Ni is more effective in enhancing the\ndehydrogenation performance of the MgH /Mg systems, while Co plays a more active\n2\nrole in improving hydrogenation. Furthermore, the synergistic effect between Ni and\nCo significantly enhances both the hydrogen absorption and desorption kinetics of the\nMgH /Mg systems (Table R2). The above results were originally provided in the\nrevised version as Supplementary Figs. 26–28.\nFigure R17. (a) Dehydrogenation curves of 15wt.%-Ni @TiO –MgH at different temperatures. (b)\n1 2 2\nHydrogenation curves of 15wt.%-Ni @TiO –MgH at different temperatures. (c) Nine kinetic\n1 2 2\nanalysis models at 300°C of 15 wt.%-Ni @TiO –MgH . (d) Calculated hydrogen desorption and\n1 2 2\nabsorption activation energies of 15wt.%-Ni @TiO –MgH by R3 kinetic analysis model.\n1 2 2\nThe superior performance of Ni Co @TiO DACs is a result of Ni-Co dual-site\n1 1 2\nsynergy, not merely the sum of two independent functions. Our concept of \"role\n25\ndivision\" originates from the observed catalytic specificity of the individual metals in\nthe experiment: Ni-based catalysts (e.g., Ni @TiO SACs and Ni NP@TiO ) excel in\n1 2 2\nthe dehydrogenation of MgH , while Co-based catalysts (e.g., Co @TiO SACs and Co\n2 1 2\nNP@TiO ) show superior activity in the hydrogenation of Mg, particularly at near-\n2\nroom temperatures. When Ni and Co are combined, as in Ni Co @TiO DACs or NiCo\n1 1 2\nNP@TiO , this intrinsic division of labor triggers a synergistic enhancement. The\n2\nresulting bidirectional catalysis leads to performance in both hydrogenation and\ndehydrogenation that surpasses that of the single-metal counterparts. The above\nexperimental results also match our characterization and theoretical calculation results,\nconfirming the complementary and synergistic catalytic roles of Ni and Co atoms in the\nMgH /Mg systems. Therefore, we further provided an appropriate explanation for this\n2\nphenomenon through the d-band center theory. In summary, an integrated approach of\ncomparative experiments, characterization, and DFT calculations strongly confirmed\nthe rationality of the catalyst design logic and the working mechanism.\nFigure R18. (a) Dehydrogenation curves of 15wt.%-Co NP@TiO –MgH at different temperatures.\n2 2\n(b) Hydrogenation curves of 15wt.%-Co NP@TiO –MgH at different temperatures. (c) Nine\n2 2\nkinetic analysis models at 300°C of 15 wt.%-Co NP@TiO –MgH . (d) Calculated hydrogen\n2 2\ndesorption and absorption activation energies of 15wt.%-Co NP@TiO –MgH by A2 kinetic\n2 2\nanalysis model.\n2 2\n(b) Hydrogenation curves of 15wt.%-Ni NP@TiO –MgH at different temperatures. (c) Nine kinetic\n2 2\nanalysis models at 300°C of 15 wt.%-Ni NP@TiO –MgH . (d) Calculated hydrogen desorption and\n2 2\nabsorption activation energies of 15wt.%-Ni NP@TiO –MgH by R2 kinetic analysis model.\n2 2\nFigure R20. (a) Dehydrogenation curves of 15wt.%-NiCo NP@TiO –MgH at different\n2 2\ntemperatures. (b) Hydrogenation curves of 15wt.%-NiCo NP@TiO –MgH at different\n2 2\ntemperatures. (c) Nine kinetic analysis models at 300°C of 15 wt.%-NiCo NP@TiO –MgH . (d)\n2 2\nCalculated hydrogen desorption and absorption activation energies of 15wt.%-NiCo NP@TiO –\n2\nComposite Ea (kJ/mol) Ea (kJ/mol)\nde ab\nNi @TiO –MgH 61.77 35.25\n1 2 2\nCo @TiO –MgH 86.63 30.51\n1 2 2\nNi Co @TiO –MgH 53.54 16.71\n1 1 2 2\nNi NP@TiO –MgH 80.73 35.00\n2 2\nCo NP@TiO –MgH 93.78 31.68\n2 2\nNiCo NP@TiO –MgH 68.34 29.68\n2 2\nWe have revised the discussion in the manuscript to make this point of synergy\nunequivocally clear and to avoid any potential misinterpretation of a simple\nbifunctional mechanism.\n5. Crucial experimental and computational parameters are either missing or\ninsufficiently described. These include DFT convergence criteria, energy cutoff and k-\npoint details, ball-milling protocol specifics, and structural characterisation settings\n(e.g., EXAFS fitting ranges, model validation criteria). The lack of such information\ncompromises reproducibility and raises concerns regarding the technical rigor of the\nstudy.\nResponse: Thank you for raising this critical point regarding the completeness of our\nmethodological description. In our original submission, certain methodological details\nwere included in the Supplementary Information. We deemed these procedures to be a\nmature methodology and, consequently, omitted a comprehensive elaboration. In direct\nresponse to your comment, we have now conducted a thorough review and have\nsignificantly expanded the Experimental Section within the Supplementary Information\nto include all crucial parameters necessary for reproducibility.\nThese additions can now be found in the revised Supplementary Information: (i)\nThe complete set of DFT convergence criteria (for energy, force, and displacement), the\nenergy cutoff, and k-point meshes used for all calculations. (ii) A detailed, step-by-step\ndescription of the ball-milling protocol, including equipment specifications, exact ball-\n28\nto-powder ratios, milling speed, duration, and atmosphere. (iii) Full structural\ncharacterization details, including the exact EXAFS fitting ranges (k-range and R-\nrange), the structural models used, and the validation criteria (e.g., R-factor values).\nWe are grateful for your meticulous feedback, which has been invaluable in\nenhancing the technical rigor and reproducibility of our work.\nThis study proposes a dual-atom catalyst coupled with a support self-reconstruction\nstrategy to enhance the hydrogen storage properties of the Mg/MgH system. The\n2\nresearch topic holds potential value. The heteronuclear dual-atom catalyst (DAC)\nNi Co @TiO substantially reduce the initial dehydrogenation temperature of MgH to\n1 1 2 2\n168 °C and achieve the highest catalytic performances reported for MgH -based\n2\nmaterials (room-temperature hydrogen uptake 3.79 wt.% within 30 min). However, to\nsome extent, I don't recommend it with the current version to be published in the Nature\nCommunications. Specific comments are given below:\n1. The proposed “bidirectional catalysts with dual-atom” in this work are essentially a\nconceptual derivative of bimetallic catalysts (e.g., Pd-Ni), demonstrating substantial\nconceptual overlap with the PdNi biatomic clusters catalyst (Adv. Mater., 2023)\nreported by Zhang Yao's group at Southeast University. The proposed strategy of “Ni\nand Co bidirectionally modulate d-band centres for complementary catalysis”\nrepresents the dynamic modulation of the d-band center in bimetallic systems. This\ndemonstrates overlap in innovative concepts with the work demonstrating that “PdNi\nnanoclusters with an optimal d-band center dominate catalytic activity” (Adv. Mater.,\n2023). Consequently, the innovation of this work requires further elucidation.\nResponse: Thank you for your suggestion. While it is true that both our study and the\nwork by Zhang Yao’s group (Adv. Mater., 2023) explore bimetallic systems involving\ntransition metals and address d-band center modulation, there are notable distinctions.\nSpecifically, our research includes several key innovations as below.\n(1) Divergence in Catalyst Design Concepts.\nThe design for the Ni Co @TiO DACs is fundamentally rooted in addressing the\n1 1 2\nintrinsic challenges of reversible hydrogen storage in MgH /Mg, namely the high\n2\nenergy barriers for de/hydrogenation, as well as the need for long-term structural\nstability under repeated high-temperature cycling. Unlike the PdNi bilayer metallene\napproach reported by Zhang Yao’s group (Adv. Mater., 2023), which relies on in situ\n30\nreconstruction into mixed clusters and single atoms, our strategy is based on a rationally\npre-designed bidirectional catalytic architecture. We strategically selected Ni and Co\natoms to address the distinct reaction barriers in dehydrogenation and hydrogenation of\nMgH /Mg systems in a targeted way, and employed the TiO support that not only\n2 2\nstabilizes the metal atoms but also actively participates in the catalytic process through\nstrong electronic interactions.\n(i) The design concept of dual-metal atoms. The work by Zhang Yao’s group\n(Adv. Mater., 2023) relied on post-synthesis reconstruction to form a mixture of clusters\nand single atoms from PdNi metallene. This approach is effective in lowering the initial\ndehydrogenation temperature; however, the PdNi metallene catalyst lacks the precise\nelectronic and spatial control in comparison with our pre-designed dual-atom\narchitecture. Additionally, the metal components in the above-reported work were not\ntailored to the specific challenges of hydrogen release and uptake, respectively. The\ncore design principle of our work originates from the understanding that\nde/hydrogenation in MgH involves different steps: Mg–H bond breaking during\n2\ndesorption and H–H dissociation during absorption. To address these dual challenges\nsimultaneously, we selected Ni and Co as a heteronuclear pair based on their\ncomplementary electronic structures. The synergy between Ni and Co atoms induces a\nbidirectional shift in their d-band centers. Specifically, the downward shift of the d-\nband center of Ni reduced the orbital overlap between the Ni 3d and H 1s orbitals,\nreducing the adsorption strength of Ni/H atoms and further facilitating H atoms\ndiffusion throughout the catalytic system. In contrast, the upward shift of the d-band\ncenter of Co strengthened the interactions between the Co 3d orbitals and H molecules.\nH dissociation by enhancing the hybridization of Co 3d with H σ* orbitals, allowing\n2 2\nefficient H migration to the Mg surface for MgH formation. As a result, the Ni and Co\n2\natoms in Ni Co @TiO DACs have different emphases during the reaction process.\n1 1 2\nSpecifically, Ni and Co atoms in Ni Co @TiO DACs can act as the primary active site\n1 1 2\nfor dehydrogenation and hydrogenation, respectively, thereby achieving efficient\n31\nbidirectional catalysis in MgH /Mg systems. Therefore, the behavior of the Pd and Ni\n2\nelements in the PdNi metallene proposed by Zhang Yao's group (Adv. Mater., 2023) is\ndistinct from our findings, as no analogous role was identified.\n(ii) The design concept and advantages of the support. The PdNi metallene\nsystem reported by Zhang Yao’s group (Adv. Mater., 2023) lacks a supporting substrate,\ncontributing to its relatively limited cycling stability. Furthermore, its reliance on the\nnoble metal Pd raises material costs, and the formation of Mg–Pd intermetallic phases\nduring cycling can induce irreversible capacity loss. In contrast, our Ni Co @TiO\n1 1 2\nDACs design intentionally uses non-noble metals and stabilizes them atomically on a\nrobust TiO support. This approach effectively mitigates the above issues, enabling\n2\nhigh-capacity retention and long-term cyclic stability. The choice of TiO as the support\n2\nis critical and deliberate. TiO is not only thermally stable under repeated\n2\nde/hydrogenation cycles but also possesses a defect-tolerant structure rich in oxygen\nvacancies (O ) and multivalent Ti species (Tix+), which serve as electron mediators and\nV\nenhance interfacial charge transfer. The strong metal–support interaction (SMSI)\nbetween Ni/Co and TiO further inhibits atomic migration and aggregation, ensuring\n2\nthe durability of the atomic dispersion—a key advantage over the in situ reconstructed\nPdNi system, which may suffer from inconsistent active site distribution and gradual\ncoarsening.\n(2) Differences in Synthesis Methods.\nFrom the perspective of synthesis, the differences between the two further reflect\ntheir distinct design philosophies. We employed a high-temperature molten salt method\nto synthesize Ni Co @TiO DACs in a single step. The key advantage of this approach\n1 1 2\nlies in the use of a molten salt medium to create a high-temperature, high-ion-strength\nenvironment that strongly promotes the interaction between the Ni/Co precursors and\nthe TiO support. This enables the direct incorporation and stable anchoring of metal\n2\natoms into the lattice defects or surface sites of the support, thereby establishing a strong\nmetal–support interaction (SMSI) during the initial synthesis stage. This robust\ninteraction is crucial for inhibiting the migration and agglomeration of metal atoms and\n32\nensuring the high-temperature stability of the catalyst.\nIn contrast, the PdNi metallene reported in the literature was prepared via a wet-\nchemical method (hydrothermal synthesis) to first form a predefined two-dimensional\nlayered structure. However, as a catalyst precursor, this structure is not the final active\nform. During subsequent ball-milling with MgH and catalytic cycling, this delicate\n2\nstructure is disrupted and reconstructed into a mixture of clusters. Thus, the preparation\nprocess essentially involves building a metastable precursor, followed by a\n“deconstruction-reconstruction” pathway to generate active sites. The formation of\nthese active sites is somewhat uncontrollable and stochastic in nature.\n(3) Enhanced Catalytic Performance.\nCompared to the published catalysts, the catalytic performance of Ni Co @TiO\n1 1 2\nDACs exhibits better catalytic performance and extremely high stability. The peak\ndehydrogenation temperature of the Ni Co @TiO -MgH system is about 50℃ earlier\n1 1 2 2\nthan the published work reported by Zhang Yao’s group (Adv. Mater., 2023).\nMeanwhile, Ni Co @TiO -MgH can achieve isothermal hydrogenation at room\n1 1 2 2\ntemperature (30℃). In addition, the published work has a capacity attenuation rate of\n2.1% after 10 cycles of hydrogen absorption and desorption, while the Ni Co @TiO -\n1 1 2\nMgH has almost no capacity loss after 10 cycles. It is worth emphasizing that even\n2\nafter 100 cycles, the Ni Co @TiO -MgH still maintains 97.6% capacity, showing the\n1 1 2 2\nextraordinary stability.\n(4) Divergence in Catalytic Mechanism.\nThe d-band center model is a well-established theoretical framework in catalysis\nscience, widely applied across electrocatalysis, thermo-catalysis, and hydrogen storage\nsystems[49-53]. However, the innovation of our work lies not in the mere application of\nthis theory, but in how we achieve and validate a dynamic, bidirectional modulation of\nthe d-band centers via heteronuclear dual-atom sites (Ni and Co) and a self-\nreconstructing TiO support—an approach that has not been previously reported in\nthe Ni Co @TiO DACs feature atomically dispersed Ni and Co sites that play\n1 1 2\n33\ncomplementary and dynamically regulated roles in a bidirectional manner. Specifically,\nduring dehydrogenation, Ni serves as the main active site for cleaving Mg–H bonds,\nwhile Co induces a downward shift in the d-band center of Ni, reducing the adsorption\nstrength of Ni/H atoms and facilitating hydrogen desorption. Conversely, during\nhydrogenation, Co becomes the dominant site for H dissociation, with Ni inducing an\n2\nupward shift in the d-band center of Co, thereby enhancing H adsorption and\n2\ndissociation.\nFurthermore, the direct experimental evidence through XPS characterizations\nshows that the TiO support undergoes reversible self-reconstruction during\n2\nde/hydrogenation cycles, involving the evolution or decay of multivalent Tix+ species\nand O . This process enhances electron transfer between MgH /Mg and the DACs,\nV 2\nstabilizes the dispersed Ni and Co atoms, and contributes significantly to the\nreversibility and cycling stability of the system. Such support-driven dynamic evolution\nwas not observed in the PdNi metallene system, which primarily employed ball-milled\nclusters and single atoms without emphasis on support reactivity.\nTo quantitatively validate the dynamic d-band center modulation, we conducted\nDFT calculations that predictively revealed the shifts in the d-band centers of Ni and\nCo across reaction steps. For instance, the d-band center of Ni shifts from –1.363 eV in\nNi @TiO SACs to –1.806 eV in Ni Co @TiO DACs, favoring dehydrogenation,\n1 2 1 1 2\nwhile that of Co shifts from –1.907 eV in Co @TiO SACs to –1.774 eV in\n1 2\nNi Co @TiO DACs, promoting hydrogenation. This dual-atom synergy enables a truly\n1 1 2\nbidirectional catalytic function, representing a substantial advance over conventional\nunidirectional catalysts.\nWe believe that the d-band center theory serves as a universal principle in catalysis,\nand its reuse does not diminish novelty when extended in meaningful ways. Our work\ndemonstrates dynamic and reversible d-band center shifts mediated by dual-atom\ninteractions, bidirectional functionality within a single catalytic system, and coupling\nwith support self-reconstruction for enhanced kinetics and stability—conceptual and\npractical advancements beyond the static cluster-based mechanism described by Zhang\nIn summary, the Ni Co @TiO DACs introduce a novel paradigm for the design\n1 1 2\nof bidirectional catalysts in reversible hydrogen storage, leveraging dynamic d-band\ncenter modulation and support self-reconstruction. These features distinguish our\nsystem both mechanistically and performance-wise from previously reported catalysts,\nand we believe these insights open new avenues for the rational design of multi-atom\ncatalysts in energy storage applications.\nThank you once again for your perceptive feedback, which has allowed us to better\narticulate the novel contributions of our work.\n2. Through density functional theory (DFT) calculations, the dual-atom catalyst has\nachieved a 76.84% reduction in Mg-H bond energy via synergistic effects. Does this\ncorrespond to an improvement in the experimental hydrogen desorption enthalpy?\nRelevant PCT curve data has not yet been observed in the manuscript.\nResponse: Thank you for raising this important question regarding the correlation\nbetween the density functional theory (DFT) calculations and experimental results.\nWe greatly appreciate the opportunity to clarify this relationship.\nAs indicated by the DFT calculations, the Ni Co @TiO DACs facilitate a 76.84%\n1 1 2\nreduction in Mg–H bond energy. To verify whether this theoretical reduction\ncorresponds to a decrease in the experimental hydrogen desorption enthalpy (ΔH), we\nhave performed Pressure-Composition-Temperature (PCT) measurements on the 15 wt.%\nNi Co @TiO –MgH composite. The Van’t Hoff plot derived from the PCT data\n1 1 2 2\n(Figure R21) confirms a significant reduction in the experimental desorption enthalpy,\nyielding a value of 71.67 kJ/mol H markedly lower than that of pure MgH (76.41\n2 2\nkJ/mol H ). These results qualitatively align with the DFT predictions, demonstrating\n2\nthat the Ni Co @TiO DACs effectively weaken the Mg–H bond and reduce the\n1 1 2\ndehydrogenation thermodynamics stability of the MgH /Mg systems.\nbetween the Mg–H bond energy and ΔH. We sincerely thank you for suggesting this\nessential experiment, which has significantly strengthened the support for our proposed\n35\nmechanism.\nFigure R21. (a) PCT curves of 15 wt.%-Ni Co @TiO –MgH for hydrogen desorption at 350℃,\n1 1 2 2\n375℃, and 400℃. (b) Van't Hoff plots of 15 wt.%-Ni Co @TiO –MgH . (c) PCT curves of pure\n1 1 2 2\nMgH for hydrogen desorption at 350℃, 375℃, and 400℃. (d) Van't Hoff plots of pure MgH .\n2 2\n3. Why do changes in the d-band centers of Ni and Co in the Ni Co @TiO catalyst\n1 1 2\nenable Ni and Co atoms to function as distinct active sites for dehydrogenation and\nhydrogenation reactions, respectively? Is there experimental confirmation that Ni and\nCo atoms with distinct d-band centers exhibit differences in hydrogen sorption\nperformance?\nResponse: Thank you for raising the question regarding the electronic structure and\nexperimental validation of the Ni Co @TiO DACs. The d-band center shift in\n1 1 2\ntransition metals directly correlates with their catalytic activity, selectivity, and\nstability[54]. An upward shift corresponds to a higher average energy of d-orbitals near\nthe Fermi level, which strengthens the interactions between the d-orbitals of the catalyst\nand the anti-bonding orbitals of the adsorbate, thereby enhancing the reactant\n36\nadsorption [55-57]. In contrast, a downward shift weakens the adsorption strength\nbetween the catalyst and adsorbate[58, 59]. This electronic structure can be modulated\nthrough alloying or support effects such as strong metal–support interactions[20-23].\nWhile an elevated d-band center generally promotes catalytic activity by improving\nreactant adsorption and activation, excessively strong adsorption may inhibit product\ndesorption and lower overall efficiency[60]. In our study, the shift in the d-band centers\nof Ni and Co plays a critical role in their distinct catalytic behaviors. Specifically, the\ndownshift of the Ni d-band center weakens the binding strength to H atoms, favoring\ndesorption and thus enhancing dehydrogenation kinetics. Conversely, the upshift of the\nCo d-band center strengthens hydrogen adsorption, promoting H–H bond dissociation\nand improving hydrogenation performance.\nTable R3. Comparison of hydrogen storage properties of MgH catalyzed by SACs and DACs.\n1 2 2 1 2 2 1 1 2 2\nT (°C) 200 172 163\ninitial\nT (°C) 229 252 220\npeak\nR (wt.%/min) @275°C 1.96 1.15 3.02\nde\nR (wt.%/min) @50°C 0 0.42 0.73\nab\nEa (kJ/mol) 61.77 86.63 53.54\nde\nEa (kJ/mol) 35.25 30.51 16.71\nab\nMoreover, there are distinct d-band centers of Ni atoms in Ni Co @TiO DACs\n1 1 2\nand Ni @TiO SACs, as well as in the d-band centers of Co atoms in Ni Co @TiO\n1 2 1 1 2\nDACs and Co @TiO SACs. And the difference in hydrogen sorption performance is\n1 2\nclearly demonstrated through comparative kinetics. The Ni @TiO SACs show\n1 2\nsuperior catalytic performance compared to Co @TiO SACs during the process of\n1 2\ndehydrogenation in MgH /Mg systems, whereas Co @TiO SACs exhibit better\n2 1 2\nperformance than Ni @TiO SACs during the hydrogenation process (Table R3). Most\n1 2\nimportantly, the Ni Co @TiO DACs outperform both corresponding Ni @TiO SACs\n1 1 2 1 2\nand Co @TiO SACs in the reversible reactions, confirming the cooperative role of the\n1 2\nNi/Co atoms. The results, supported by our DFT calculations, provide compelling\n37\nevidence that the d-band center modulation of Ni and Co leads to their distinct and\nsynergistic catalytic functions.\nWe have further emphasized these points in the revised manuscript with expanded\ndiscussion as prompted by your valuable comments.\n4. The Ni Co @TiO catalyst enhances the hydrogen storage kinetics of Mg/MgH .\n1 1 2 2\nSpecifically, which rate-limiting step in the kinetics is improved by the diatomic\ncatalyst in this work, and how does it distinguish itself from other single-atom or\ndiatomic catalysts?\nResponse: Thank you for your insightful question. We will address this question by the\nfollowing two points: first, which rate-limiting step in the kinetics is improved by\nNi Co @TiO DACs; and second, their unique advantages over other SACs and DACs.\n1 1 2\n(1) The analysis of the rate-limiting step in Ni Co @TiO –MgH systems.\n1 1 2 2\nThe mechanism of hydrogen absorption/desorption is generally described in five\nsteps. Taking hydrogen absorption as an example: (i) H molecules approach and are\n2\nphysisorbed on the Mg surface; (ii) H molecules dissociate into H atoms, which are\n2\nthen chemisorbed; (iii) H atoms penetrate beneath the surface; (iv) H atoms diffuse\ninward, forming a solid solution (α-phase) of H in Mg before any ordered hydride\nstructure develops; (v) once a critical concentration is reached, the α-phase transforms\ninto the thermodynamically stable hydride phase (β-MgH )[8, 16]. The relevant studies\n2\nhave revealed that the rate-determining step of the hydrogenation process is the\nnucleation-growth of MgH , which is determined by the hydrogen diffusion rate in Mg\n2\nlattice[32]. And the desorption process is essentially the reverse of absorption. The\nhydrogen desorption of MgH is mainly determined by the breakage of Mg–H bonds,\n2\ndiffusion of hydrogen atoms in the bulk phase, and recombination of hydrogen atoms[8].\nTo clarify the kinetic mechanism in the 15 wt.%-Ni Co @TiO –MgH composite,\n1 1 2 2\nwe applied several solid-state kinetic models to identify the actual rate-determining step\nduring dehydrogenation. Nine classical kinetic models were employed (Table R4),\nrepresenting different potential rate-limiting mechanisms, such as diffusion-controlled,\nnucleation-controlled, and phase-boundary-controlled processes [33, 34]. Among these,\n38\nthe R3 model (three-dimensional phase boundary movement) provided the best fit for\nthe dehydrogenation data at 275 °C (Figure R22a). This was corroborated by the R2 and\nslope nearest to 1 in the R3 fitting (Figure R22b). The applicability of the R3 model\nsuggests that the dehydrogenation process is controlled by a three-dimensional growth\nof Mg nuclei at the phase interface, indicating that Ni Co @TiO DACs could increase\n1 1 2\nthe nucleation sites of Mg by promoting the diffusion of hydrogen[24, 61-63]. The DFT\ncalculations reveal that the Ni Co @TiO DACs provide abundant active sites that\n1 1 2\nsignificantly reduce the Mg–H bond energy and the energy barrier for H dissociation.\n1 1 2\nintroduces numerous defects and grain boundaries, thereby refining the MgH particle\n2\nsize [64-68]. These microstructural modifications create fast diffusion pathways for\nhydrogen, substantially enhancing diffusion kinetics[8].\nFigure R22. (a) Nine kinetic analysis models at 275 °C of 15 wt.%-Ni Co @TiO –MgH . (b) The\n1 1 2 2\nR3 model fitting of 15 wt.%-Ni Co @TiO –MgH /Mg composite at different temperatures.\n1 1 2 2\nTable R4. Common solid-state rate expressions for different reaction models.\nSymbol Model References Sharp's expression\nD1 One-dimensional diffusion α2 0.2500(t/t )\n0.5\nD2 Two-dimensional diffusion α+(1-α)ln(1-α) 0.1534(t/t )\n0.5\nThree-dimension diffusion\nD3 [1-(1-α)1/3]2 0.0426(t/t )\n0.5\n(Jander equation)\nD4 Three-dimension diffusion (1-2α/3)-(1-α)2/3 0.0367(t/t )\n0.5\n39\n(Ginstling-Braunsshtein equation)\nF1 First-order reaction -ln(1-α) -0.6931(t/t )\n0.5\nR2 Two-dimension phase boundary 1-(1-α)1/2 0.2929(t/t )\n0.5\nR3 Three-dimension phase boundary 1-(1-α)1/3 0.2063(t/t )\n0.5\nA2 Avarami-Erofe'ev [-ln(1-α)]1/2 0.8326(t/t )\n0.5\nA3 Avarami-Erofe'ev [-ln(1-α)]1/3 0.8850(t/t )\n0.5\n(2) The unique advantages of Ni Co @TiO over other SACs and DACs.\n1 1 2\nPrevious reports have indicated that the elements Pd and Ru exhibit extremely\nexcellent catalytic effects on the MgH /Mg systems[31, 49, 69]. Given these established\n2\nfindings and the promising catalytic performance of Ni and Co observed in our study,\nwe conducted additional TPD experiments to directly compare the hydrogen desorption\nbehavior of the Ni Co @TiO DACs with several reference catalysts. These included\n1 1 2\nsingle-atom catalysts (Ru @TiO , Pd @TiO ) and other dual-atom systems (such as\n1 2 1 2\nRu Co @TiO , Ru Ni @TiO , Pd Ni @TiO , Pd Ru @TiO , Pd Co @TiO ). All\n1 1 2 1 1 2 1 1 2 1 1 2 1 1 2\ncatalysts were synthesized and tested under identical conditions. The TPD curves\n(Figure R23) clearly demonstrate that MgH catalyzed by Ni Co @TiO DACs\n2 1 1 2\nachieves the lowest initial and peak desorption temperatures among all tested SACs and\nDACs. This exceptional performance highlights that the synergy in Ni Co @TiO\n1 1 2\nDACs is not merely a generic diatomic effect, but rather a unique electronic cooperation\nbetween Ni and Co that optimally tunes the d-band center and facilitates hydrogen\ndesorption and absorption. The superiority of Ni-Co diatomic pairs stems from a\nbalanced and complementary interaction that simultaneously promotes hydrogen\ndesorption (Ni serves as the dominant site for weakening the Mg–H bonds energy) and\nhydrogen absorption (Co serves as the dominant site for H–H bonds dissociation),\nsignificantly lowering the energy barrier for de/hydrogenation reactions.\nWe have added these new experimental comparisons and the corresponding\ndiscussion in the revised manuscript and Supplementary Information. Thank you again\nfor prompting us to perform these meaningful comparisons, which better illustrate the\n40\nuniqueness and catalytic advantage of the Ni Co @TiO DACs.\n1 1 2\nFigure R23. TPD curves of MgH catalyzed by different SACs and DACs\n2\n5. The manuscript claims that the Ni Co @TiO catalyst endows Mg/MgH with\n1 1 2 2\nexceptional hydrogen storage performance. However, it fails to visually demonstrate\nconcrete performance levels compared to other reported catalysts.\nResponse: Thank you for your valuable suggestion. We agree that a direct visual\ncomparison is essential for contextualizing the performance of our catalysts. In\nresponse to your comment, we have included Table R5 that provides a comprehensive\ncomparison of our Ni Co @TiO DACs with catalysts reported in recent literature. This\n1 1 2\ntable lists key performance metrics, including the onset temperature, rate, activation\nenergy, and enthalpy of the dehydrogenation process. The data presented in this table\nclearly demonstrates that our Ni Co @TiO DACs present one of the most effective\n1 1 2\ncatalysts reported for MgH /Mg systems, particularly in terms of its low activation\n2\nenergy and remarkable dehydrogenation rate.\nTable R5. Comparison of hydrogen storage properties of MgH with different catalysts.\nonset de de\nThis\nNi Co @TiO 163 3.02@275°C 53.54 71.65\n1 1 2\nwork\nTiV-MgO 202 2.39@280°C 96.20 — [35]\nTiNb-MgO 208 2.28@280°C 96.80 — [35]\nTiO 180 2.12@300°C 106.70 — [70]\n2\n4 18 55\nTiNb O 178 0.21@250°C 100.40 75.20 [38]\n2 7\nNiTiO 235 0.64@300°C 74.00 — [39]\n2 4\nNb CT — 1.20@300°C 79.73 — [41]\nH-V O 185 1.20@275°C 84.55 73.40 [3]\n2 5\nVTiMn 244 0.35@270°C 90.50 72.80 [42]\nMg Ni Y 200 1.20@320°C 65.80 63.50 [43]\n80 15 5\nTiMgVNi — 0.13@300°C 94.40 — [44]\nK Ti O 200 0.48@300°C 53.10 73.91 [46]\n2 6 13\nNiCl 300 0.60@350°C 102.6 — [47]\nMg Ni@Ti-\n2\n172 2.72@280°C 81.10 75.00 [24]\nMgO\nWe believe this comparison provides direct evidence for the superior catalytic\neffect of the Ni Co @TiO DACs in the MgH /Mg systems over other reported\n1 1 2 2\ncatalysts. And we have placed the comparative discussion of previous studies in the\nmanuscript and the accompanying table in the Supplementary Information. We greatly\nappreciate your important improvement suggestion.\n6. The main text states that the cycling stability of the material was tested at 280°C,\nwhich differs from the testing temperature (290°C) specified in the experimental details\nof the supporting documents. What is the reason for this discrepancy?\nResponse: We sincerely apologize for the oversight in the Supplementary Information,\nwhich may have confused your review. As correctly stated in the main text, all cyclic\nstability measurements were performed at 280 °C. We have provided the temperature\n42\nprofile recorded during the cycling process. As shown in Figure R24, the temperature\nof 100 de/hydrogenation cycles remained at approximately 280°C. The observed\ntemperature fluctuations are attributed not only to the rapid heating and cooling\nassociated with the H evacuation and admission steps, but also to the intrinsic heat of\n2\nreactions: exothermic hydrogenation process and endothermic dehydrogenation\nprocess.\nWe have carefully corrected this error in the revised Supplementary Information.\nWe greatly appreciate your meticulous reading and valuable feedback, which has been\ninstrumental in enhancing the accuracy and overall quality of our manuscript.\nFigure R24. Temperature profile during 100 de/hydrogenation cycles.\n7. Certain variables should be italicized, such as the activation energy Ea.\nResponse: Thank you for your valuable suggestion. We sincerely apologize for the\noversight in formatting. The variable Ea (activation energy) has now been correctly\nitalicized throughout the manuscript. 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Jiang, X., et al., Enhancing the Electrocatalytic Oxidation of 5-\nHydroxymethylfurfural Through Cascade Structure Tuning for Highly Stable\nBiomass Upgrading. Nano-Micro Letters, 2024. 16(1): p. 275.\n58. Li, Z. and C. Yu, Chapter 32 - Atomically dispersed catalysts: recent progress\nin energy and environmental fields, in Nanostructured Materials, Z. Li and C.\nYu, Editors. 2024, Elsevier. p. 509-531.\n59. Zhang, Y., et al., D-band center optimization of iron carbide via Cr substitution\nfor enhanced alkaline hydrogen evolution. Materials Today Energy, 2022. 29: p.\n101133.\n60. Chen, Z., Z. Liu, and X. Xu, Dynamic evolution of the active center driven by\n49\nhemilabile coordination in Cu/CeO single-atom catalyst. Nature\n61. Lin, W., et al., Extreme high reversible capacity with over 8.0 wt% and excellent\nhydrogen storage properties of MgH combined with LiBH and Li AlH .\n2 4 3 6\nJournal of Energy Chemistry, 2020. 50: p. 296-306.\n62. Long, S., et al., Hydrogen storage properties of MgH modified by efficient\n3 2 8\n126901.\n63. Shao, J., et al., Low-Temperature Reversible Hydrogen Storage Properties of\nLiBH : A Synergetic Effect of Nanoconfinement and Nanocatalysis. The\n64. Huot, J., et al., Structural study and hydrogen sorption kinetics of ball-milled\nmagnesium hydride. Journal of Alloys and Compounds, 1999. 293-295: p. 495-\n500.\n65. Chen, H., et al., Insight into the energy conversion and structural evolution of\nmagnesium hydride during high-energy ball milling for its controllable\nsynthesis. Journal of Alloys and Compounds, 2020. 836: p. 155312.\n66. Czujko, T., E.E. Oleszek, and M. Szot New Aspects of MgH Morphological\n2\nand Structural Changes during High-Energy Ball Milling. Materials, 2020. 13.\n67. Kudiyarov, V.N., R.R. Elman, and N.E. Kurdyumov The Effect of High-Energy\nBall Milling Conditions on Microstructure and Hydrogen Desorption Properties\nof Magnesium Hydride and Single-Walled Carbon Nanotubes. Metals, 2021. 11.\n68. Hong, S.-H., H.-J. Kim, and M.Y. Song, Rate enhancement of hydrogen\ngeneration through the reaction of magnesium hydride with water by MgO\naddition and ball milling. Journal of Industrial and Engineering Chemistry, 2012.\n18(1): p. 405-408.\n69. Jia, B., et al., Electronic Structure Modulation of Nb O by Ru Single Atoms\n2 5\nEnabling Efficient Hydrogen Storage of Magnesium Hydrides. Angewandte\nChemie International Edition, 2025. 64(33): p. e202511139.\n50\n70. Ren, L., et al., Oxygen Vacancy-Rich 2D TiO Nanosheets: A Bridge Toward\n51"}, {"round": 2, "reviewer_comments": {"reviewer_1": "We thank the authors for their revisions. The reviewer has no further questions, is now satisfied with the manuscript and recommend it for publication in its current form.", "reviewer_2": "After thoroughly reviewing the revised manuscript titled “Bidirectional Catalysts with Dual-Atom Dynamic d-Band Centre Modulation and Support Self-Reconstruction for Reversible De/Hydrogenation in MgH2/Mg” (ID: NCOMMS-25-53103), I regret to inform you that I must recommend its rejection for publication. Despite the authors' extensive revisions, many fundamental issues remain unaddressed, and the manuscript still largely fails to meet the required scientific rigor and innovation standards for publication in Nature Communications. Below, I outline the major reasons for my rejection recommendation:\n1. The authors' work presents a dual-atom catalyst (DAC) based on Ni-Co for MgH2/Mg systems, which, in itself, lacks sufficient novelty. As Reviewer #3 pointed out, the concept of bidirectional catalysts with dynamic modulation of the d-band centers bears substantial similarity to existing works, particularly the PdNi biatomic clusters catalyst published by Zhang Yao's group in Adv. Mater. (2023). The authors have failed to convincingly distinguish their approach from this earlier work, which fundamentally limits the originality of the study. Moreover, while the dual-atom synergy may be a valuable concept, the authors have not provided a robust enough theoretical or experimental framework to elevate the impact of their proposed approach beyond what is already known in the field.\n2. A crucial issue with this manuscript is the insufficient characterization of the structural features of the Ni1Co1@TiO2 catalyst. The authors claim atomic dispersion of Ni and Co on TiO2 support but fail to demonstrate this with clear evidence. Specifically, the manuscript lacks atomic-resolution imaging or detailed local environment validation (e.g., atomic-scale TEM imaging, site-specific spectroscopic evidence) to substantiate the claims. The XANES and EXAFS data presented are insufficient to resolve the coordination environment, particularly of the Ni and Co species, making the structural model unconvincing. Furthermore, there is no adequate confirmation of the coordination structure via experimental validation. Without such crucial structural characterizations, particularly direct evidence of the coordination of Ni and Co atoms, the proposed model remains speculative. The DFT calculations are also unsupported by sufficient experimental data, leaving the validity of these calculations in serious doubt.\n3. A major issue with the revised manuscript is the insufficient structural characterization of the supplementary catalyst.\n4. The authors claim a reduction in the Mg-H bond energy by 76.84% via DFT calculations, but they fail to provide a comprehensive validation of the DFT parameters and convergence criteria. There is no mention of energy cutoff, k-point settings, or convergence tolerances, which are fundamental to ensuring the reliability of such calculations. Moreover, the authors have not performed a rigorous energy convergence test, making the computational results unreliable and undermining the credibility of the theoretical claims. The absence of key computational details further compromises the reproducibility and accuracy of the results presented.\n5. The proposed catalytic mechanism, which attributes dehydrogenation activity to Ni and the hydrogenation activity to Co based on d-band center modulation, remains speculative. The authors have not provided compelling experimental proof, such as selective poisoning experiments, reaction kinetics with isolated metal species, or site-specific spectroscopic evidence to justify this bifunctional assignment. While the authors claim synergy between Ni and Co, the experimental results fail to conclusively demonstrate this, as there are no clear differences in hydrogen sorption performance directly attributable to the d-band shifts. In fact, the experimental evidence fails to convincingly support the notion that the Ni-Co DACs improve performance significantly beyond what is achievable with other single-atom or bimetallic catalysts. The lack of a thorough mechanistic study to substantiate this claim calls into question the validity of their conclusions.\n6. There are inconsistencies in the experimental details regarding the testing temperatures for the cycling stability experiments. In the main text, the cycling stability is tested at 280°C, while the experimental section in the supplementary information mentions 290 °C. Such discrepancies should be addressed to ensure the accuracy and reproducibility of the study.", "reviewer_3": "The response letter has provided detailed answers to the raised questions, particularly offering a comprehensive comparison and innovative explanation regarding the differences from PdNi catalysts, which has given me a deeper understanding of the manuscript’s content. As for the current version of the manuscript, the following suggestions are offered.\n1. The selection of the exposed surface of the support is a critical parameter in constructing the DFT computational model. It is recommended that the authors briefly state the rationale for selecting the specific crystal plane of the anatase TiO2 support for constructing the interface.\n2. Whether for single-atom catalysts or dual-atom catalysts, the authors employed the molten salt method. What was the reason for this? What are the advantages of this synthesis method?\n3. Given that the composite exhibits excellent hydrogenation properties at room temperature, how much hydrogen capacity can be achieved by hydrogen absorption for a sufficiently long duration?\n4. The composite demonstrates remarkable stability over 100 cycles of de/hydrogenation, with only a slight capacity loss observed. Please discuss the excellent stability and the possible reasons for the slight capacity loss."}, "author_response": "Response Letter\nDear reviewers:\nWe are very grateful for your constructive comments and suggestions for our\nmanuscript entitled “Bidirectional Catalysts with Dual-Atom Dynamic d-Band Centre\nModulation and Support Self-Reconstruction for Reversible De/Hydrogenation in\nMgH /Mg” (ID: NCOMMS-25-53103A-Z). Your comments are very valuable and\n2\nhelpful for improving our manuscript. In the following, the responses to all the\ncomments are provided point-to-point.\nWe have tried our best to make all the revisions clear, and the amendments are\nhighlighted in yellow in the revised manuscript and Supplementary Information. We\nhope that the revised manuscript meets the requirements for publication.\nThank you again for your valuable comments and suggestions. We look forward\nto hearing from you.\nYours sincerely,\nJianmei Huang\nAssociate Professor\nSchool of Energy and Power Engineering\nBeihang University\nWe thank the authors for their revisions. The reviewer has no further questions, is now\nsatisfied with the manuscript and recommend it for publication in its current form.\nResponse: We are sincerely grateful for your professional review and valuable\nfeedback regarding our manuscript. Your detailed comments and insightful suggestions\nhave greatly contributed to our manuscript, leading to a substantial enhancement in its\nquality.\nAfter thoroughly reviewing the revised manuscript titled “Bidirectional Catalysts with\nDual-Atom Dynamic d-Band Centre Modulation and Support Self-Reconstruction for\nReversible De/Hydrogenation in MgH /Mg” (ID: NCOMMS-25-53103), I regret to\n2\ninform you that I must recommend its rejection for publication. Despite the authors'\nextensive revisions, many fundamental issues remain unaddressed, and the manuscript\nstill largely fails to meet the required scientific rigor and innovation standards for\npublication in Nature Communications. Below, I outline the major reasons for my\nrejection recommendation:\n1. The authors' work presents a dual-atom catalyst (DAC) based on Ni-Co for MgH /Mg\n2\nsystems, which, in itself, lacks sufficient novelty. As Reviewer #3 pointed out, the\nconcept of bidirectional catalysts with dynamic modulation of the d-band centers bears\nsubstantial similarity to existing works, particularly the PdNi biatomic clusters catalyst\npublished by Zhang Yao's group in Adv. Mater. (2023). The authors have failed to\nconvincingly distinguish their approach from this earlier work, which fundamentally\nlimits the originality of the study. Moreover, while the dual-atom synergy may be a\nvaluable concept, the authors have not provided a robust enough theoretical or\nexperimental framework to elevate the impact of their proposed approach beyond what\nis already known in the field.\nResponse: Thank you for your review. While it is true that both our study and the work\nby Zhang Yao’s group (Adv. Mater., 2023) explore bimetallic systems involving\ntransition metals and address d-band center modulation 1, there are notable distinctions.\nSpecifically, our research includes several key innovations as below.\n(1) Divergence in Catalyst Design Concepts.\nThe design for the Ni Co @TiO DACs is fundamentally rooted in addressing the\n1 1 2\nintrinsic challenges of reversible hydrogen storage in MgH /Mg, namely the high\n2\nenergy barriers for de/hydrogenation, as well as the need for long-term structural\nstability under repeated high-temperature cycling. Unlike the PdNi bilayer metallene\napproach reported by Zhang Yao’s group (Adv. Mater., 2023), which relies on in situ\n3\nreconstruction into mixed clusters and single atoms, our strategy is based on a rationally\npre-designed bidirectional catalytic architecture. We strategically selected Ni and Co\natoms to address the distinct reaction barriers in dehydrogenation and hydrogenation of\nMgH /Mg systems in a targeted way, and employed the TiO support that not only\n2 2\nstabilizes the metal atoms but also actively participates in the catalytic process through\nstrong electronic interactions.\n(i) The design concept of dual-metal atoms. The work by Zhang Yao’s group\n(Adv. Mater., 2023) relied on post-synthesis reconstruction to form a mixture of clusters\nand single atoms from PdNi metallene. This approach is effective in lowering the initial\ndehydrogenation temperature; however, the PdNi metallene catalyst lacks the precise\nelectronic and spatial control in comparison with our pre-designed dual-atom\narchitecture. Additionally, the metal components in the above-reported work were not\ntailored to the specific challenges of hydrogen release and uptake, respectively. The\ncore design principle of our work originates from the understanding that\nde/hydrogenation in MgH involves different steps: Mg–H bond breaking during\n2\ndesorption and H–H dissociation during absorption. To address these dual challenges\nsimultaneously, we selected Ni and Co as a heteronuclear pair based on their\ncomplementary electronic structures. The synergy between Ni and Co atoms induces a\nbidirectional shift in their d-band centers. Specifically, the downward shift of the d-\nband center of Ni reduced the orbital overlap between the Ni 3d and H 1s orbitals,\nreducing the adsorption strength of Ni/H atoms and further facilitating H atoms\ndiffusion throughout the catalytic system. In contrast, the upward shift of the d-band\ncenter of Co strengthened the interactions between the Co 3d orbitals and H molecules.\n2\npromoted H dissociation by enhancing the hybridization of Co 3d with H σ* orbitals,\n2 2\nallowing efficient H migration to the Mg surface for MgH formation. As a result, the\n1 1 2\nprocess. Specifically, Ni and Co atoms in Ni Co @TiO DACs can act as the primary\n1 1 2\nactive site for dehydrogenation and hydrogenation, respectively, thereby achieving\n4\nefficient bidirectional catalysis in MgH /Mg systems. Therefore, the behavior of the Pd\n2\nand Ni elements in the PdNi metallene proposed by Zhang Yao's group (Adv. Mater.,\n2023) is distinct from our findings, as no analogous role was identified.\n(ii) The design concept and advantages of the support. The PdNi metallene\nsystem reported by Zhang Yao’s group (Adv. Mater., 2023) lacks a supporting substrate,\ncontributing to its relatively limited cycling stability. Furthermore, its reliance on the\nnoble metal Pd raises material costs, and the formation of Mg–Pd intermetallic phases\nduring cycling can induce irreversible capacity loss. In contrast, our Ni Co @TiO\n1 1 2\nDACs design intentionally uses non-noble metals and stabilizes them atomically on a\nrobust TiO support. This approach effectively mitigates the above issues, enabling\n2\nhigh-capacity retention and long-term cyclic stability. The choice of TiO as the support\n2\nis critical and deliberate. TiO is not only thermally stable under repeated\n2\nde/hydrogenation cycles but also possesses a defect-tolerant structure rich in oxygen\nvacancies (O ) and multivalent Ti species (Tix+), which serve as electron mediators and\nV\nenhance interfacial charge transfer. The strong metal–support interaction (SMSI)\nbetween Ni/Co and TiO further inhibits atomic migration and aggregation, ensuring\n2\nthe durability of the atomic dispersion—a key advantage over the in situ reconstructed\nPdNi system, which may suffer from inconsistent active site distribution and gradual\ncoarsening.\n(2) Differences in Synthesis Methods.\nFrom the perspective of synthesis, the differences between the two further reflect\ntheir distinct design philosophies. We employed a high-temperature molten salt method\nto synthesize Ni Co @TiO DACs in a single step. The key advantage of this approach\n1 1 2\nlies in the use of a molten salt medium to create a high-temperature, high-ion-strength\nenvironment that strongly promotes the interaction between the Ni/Co precursors and\nthe TiO support. This enables the direct incorporation and stable anchoring of metal\n2\natoms into the lattice defects or surface sites of the support, thereby establishing a strong\nmetal–support interaction (SMSI) during the initial synthesis stage. This robust\ninteraction is crucial for inhibiting the migration and agglomeration of metal atoms and\n5\nensuring the high-temperature stability of the catalyst.\nIn contrast, the PdNi metallene reported in the literature was prepared via a wet-\nchemical method (hydrothermal synthesis) to first form a predefined two-dimensional\nlayered structure. However, as a catalyst precursor, this structure is not the final active\nform. During subsequent ball-milling with MgH and catalytic cycling, this delicate\n2\nstructure is disrupted and reconstructed into a mixture of clusters. Thus, the preparation\nprocess essentially involves building a metastable precursor, followed by a\n“deconstruction-reconstruction” pathway to generate active sites. The formation of\nthese active sites is somewhat uncontrollable and stochastic in nature.\n(3) Enhanced Catalytic Performance.\nCompared to the published catalysts, the catalytic performance of Ni Co @TiO\n1 1 2\nDACs exhibits better catalytic performance and extremely high stability. The peak\ndehydrogenation temperature of the Ni Co @TiO -MgH system is about 50℃ earlier\n1 1 2 2\nthan the published work reported by Zhang Yao’s group (Adv. Mater., 2023).\nMeanwhile, Ni Co @TiO -MgH can achieve isothermal hydrogenation at room\n1 1 2 2\ntemperature (30℃). In addition, the published work has a capacity attenuation rate of\n2.1% after 10 cycles of hydrogen absorption and desorption, while the Ni Co @TiO -\n1 1 2\nMgH has almost no capacity loss after 10 cycles. It is worth emphasizing that even\n2\nafter 100 cycles, the Ni Co @TiO -MgH still maintains 97.6% capacity, showing the\n1 1 2 2\nextraordinary stability.\n(4) Divergence in Catalytic Mechanism.\nThe d-band center model is a well-established theoretical framework in catalysis\nscience, widely applied across electrocatalysis, thermo-catalysis, and hydrogen storage\nsystems1-5. However, the innovation of our work lies not in the mere application of\nthis theory, but in how we achieve and validate a dynamic, bidirectional modulation of\nthe d-band centers via heteronuclear dual-atom sites (Ni and Co) and a self-reconstructing\nTiO support—an approach that has not been previously reported in MgH hydrogen\n2 2\nstorage systems. In contrast to the PdNi system reported by Zhang Yao’s group (Adv.\nMater., 2023), which relies on relatively static PdNi alloy clusters, the Ni Co @TiO\n1 1 2\ndynamically regulated roles in a bidirectional manner. Specifically, during\ndehydrogenation, Ni serves as the main active site for cleaving Mg–H bonds, while Co\ninduces a downward shift in the d-band center of Ni, reducing the adsorption strength\nof Ni/H atoms and facilitating hydrogen desorption. Conversely, during hydrogenation,\nCo becomes the dominant site for H dissociation, with Ni inducing an upward shift in\n2\nthe d-band center of Co, thereby enhancing H adsorption and dissociation.\nshows that the TiO support undergoes reversible self-reconstruction during\n2\nde/hydrogenation cycles, involving the evolution of multivalent Tix+ species and O .\nV\nThis process enhances electron transfer between MgH /Mg and the DACs, stabilizes\n2\nthe dispersed Ni and Co atoms, and contributes significantly to the reversibility and\ncycling stability of the system. Such support-driven dynamic evolution was not\nobserved in the PdNi metallene system, which primarily employed ball-milled clusters\nand single atoms without emphasis on support reactivity.\nTo quantitatively validate the dynamic d-band center modulation, we conducted\nDFT calculations that predictively revealed the shifts in the d-band centers of Ni and\nCo across reaction steps. For instance, the d-band center of Ni shifts from –1.363 eV in\nNi @TiO SACs to –1.806 eV in Ni Co @TiO DACs, favoring dehydrogenation,\n1 2 1 1 2\nwhile that of Co shifts from –1.907 eV in Co @TiO SACs to –1.774 eV in\n1 2\nNi Co @TiO DACs, promoting hydrogenation. This dual-atom synergy enables a truly\n1 1 2\nbidirectional catalytic function, representing a substantial advance over conventional\nunidirectional catalysts.\nWe believe that the d-band center theory serves as a universal principle in catalysis,\nand its reuse does not diminish novelty when extended in meaningful ways. Our work\ndemonstrates dynamic d-band center shifts mediated by dual-atom interactions,\nbidirectional functionality within a single catalytic system, and coupling with support\nself-reconstruction for enhanced kinetics and stability—conceptual and practical\nadvancements beyond the static cluster-based mechanism described by Zhang Yao’s\n7\ngroup (Adv. Mater., 2023).\nIn summary, the Ni Co @TiO DACs introduce a novel paradigm for the design\n1 1 2\nof bidirectional catalysts in reversible hydrogen storage, leveraging dynamic d-band\ncenter modulation and support self-reconstruction. These features distinguish our\nsystem both mechanistically and performance-wise from previously reported catalysts,\nand we believe these insights open new avenues for the rational design of multi-atom\ncatalysts in energy storage applications.\nThank you once again for your perceptive feedback, which has allowed us to better\narticulate the novel contributions of our work.\n2. A crucial issue with this manuscript is the insufficient characterization of the\nstructural features of the Ni Co @TiO catalyst. The authors claim atomic dispersion\n1 1 2\nof Ni and Co on TiO support but fail to demonstrate this with clear evidence.\nenvironment validation (e.g., atomic-scale TEM imaging, site-specific spectroscopic\nevidence) to substantiate the claims. The XANES and EXAFS data presented are\ninsufficient to resolve the coordination environment, particularly of the Ni and Co\nspecies, making the structural model unconvincing. Furthermore, there is no adequate\nconfirmation of the coordination structure via experimental validation. Without such\ncrucial structural characterizations, particularly direct evidence of the coordination of\nNi and Co atoms, the proposed model remains speculative. The DFT calculations are\nalso unsupported by sufficient experimental data, leaving the validity of these\ncalculations in serious doubt.\nResponse: Thank you for your comments. In our previous manuscript, we have\nprovided clear evidence through atomic-resolution imaging (high-angle annular dark-\nfield scanning transmission electron microscopy) and detailed local environment\nverification (X-ray absorption fine structure) to confirm that the atomic distribution of\nNi and Co atoms on TiO support is uniform. To provide further insight into the\n2\nstructural and coordination characteristics of the Ni Co @TiO DACs, we performed\n1 1 2\nadditional electron energy-loss spectroscopy (EELS) characterization on the DACs and\n8\nobtained X-ray absorption spectroscopy (XAS) data for the Co @TiO SACs.\n1 2\nFurthermore, we present a schematic diagram of the proposed coordination\nenvironments for Ni and Co atoms on the TiO support. Accordingly, a detailed analysis\n2\nintegrating these new findings with previous results was carried out.\nFigure R1. (a) HRTEM pattern of Ni 1 Co 1 @TiO 2. (b) Analysis of the distance profile of two\nadjacent atoms of Ni/Co in Ni 1 Co 1 @TiO 2 DACs. (c) Analysis of the intensity profile of two\nadjacent atoms of Ni 1 Co 1 @TiO 2 . (d) 3D atomic resolution map of the metal-atom sites. (e)\nAtomic-resolution HAADF-STEM image with the corresponding EELS spectra of the atomic\npair in NiCo @TiO DACs.\n1 2\nFirstly, atomic-resolution high-angle annular dark-field scanning transmission\nelectron microscopy (HAADF-STEM) images clearly reveal uniformly distributed\nisolated bright dots on the TiO support (Figure R1a). Owing to the pronounced Z-\n2\ncontrast between Ni/Co and Ti/O, these dots can be confidently assigned to individually\ndispersed Ni and Co atoms. Statistical analysis of the measured atomic spacing yielded\nan average Ni-Co distance of 0.247 nm (Figures R1b and R2). Moreover, the HAADF-\nSTEM image of the Ni Co @TiO DACs showed that the Ni/Co atomic intensity\n1 1 2\n9\npossesses significantly higher intensity than the Ti atomic intensity (Figures R1c-d).\nFurther, the HAADF-STEM via EELS clearly shows the dual-atom pair contains one\nCo atom and one Ni atom, strongly confirming the Co−Ni dual-atom pairs formation in\nNi Co @TiO DACs (Figure R1e). Extensive imaging analysis found no evidence of\n1 1 2\nmetal clusters or nanoparticles, further confirming the atomic dispersion of the metal\nspecies.\nFigure R2. The atomic spacing between Ni and Co in Ni 1 Co 1 @TiO 2 DACs.\nSecondly, XAS characterisation was employed to precisely determine the local\ngeometric structures of Ni and Co at the atomic level. Figures R3a and R3b show the\nNi and Co K-edge X-ray absorption near-edge structure (XANES) spectra for\nNi Co @TiO , alongside the corresponding reference spectra from metal foils (Ni, Co),\n1 1 2\noxides (NiO, CoO, Co O ), and single-atom catalysts (Ni @TiO , Co @TiO ).\n2 3 1 2 1 2\nSpecifically, the Ni K-edge XANES spectrum of Ni Co @TiO was considerably\n1 1 2\ndifferent from those of the Ni foil and NiO, which further reflected the unique state of\nNi in the catalyst. Moreover, the absorption edge energy of Ni Co @TiO was higher\n1 1 2\n10\nthan that of the Ni foil and Ni @TiO and lower than that of NiO,indicating that the\n1 2\nvalency of Ni species in Ni Co @TiO was between 0 and +2, slightly higher than that\n1 1 2\nof Ni species in Ni @TiO . According to the Co K-edge XANES spectrum of\n1 2\nNi Co @TiO , the valency of Co species in Ni Co @TiO is higher than that of\n1 1 2 1 1 2\nCo @TiO (Figure R3b).\n1 2\nFigure R3. Atomic local structure characterisation of Ni Co @TiO . (a) Ni K-edge XANES\n1 1 2\nspectra. (b) Co K-edge XANES spectra. (c) Ni K-edge EXAFS spectra in the R space. (d) Co K-\nedge EXAFS spectra in the R space. (e) WT-EXAFS images of Ni Co @TiO , Ni @TiO , NiO and\n1 1 2 1 2\nthe Ni foil. (f) WT-EXAFS images of Ni Co @TiO , Co @TiO , CoO, and the Co foil. (g) FT-\n1 1 2 1 2\nEXAFS fitting curve in the R space of Ni Co @TiO at the Ni K-edge. (h) FT-EXAFS fitting curve\n1 1 2\nin the R space of Ni Co @TiO at the Co K-edge.\n1 1 2\nMore detailed atomic structure information was obtained using the k2-weighted\nFourier transform (FT) extended X-ray absorption fine structure (EXAFS) of Ni and\nCo K-edge in the R space. The characteristic peaks of the Ni–Ni bond in the Ni foil and\nCo–Co bonds in the Co foil were located at 2.16 and 2.18 Å, respectively (Figure R3c\nand R3d), which were absent in the Ni Co @TiO , proving that Ni and Co atoms\n1 1 2\n11\nachieved atomic dispersion on TiO rather than forming metal clusters or particles.\n(XAFS) at both the Ni and Co K-edges was conducted (Figure R3e and R3f). The WT-\nEXAFS contours exhibit intensity maxima around 5 Å-1, which are characteristic of\nmetal–oxygen coordination and clearly distinguishable from the metal–metal scattering\nsignals present in Ni and Co foil references. This offers direct confirmation that Ni and\nCo atoms are primarily coordinated to oxygen, with no metallic bonding.\nTo identify the fine coordination configurations of Ni and Co in different samples,\nquantitative least squares EXAFS curve-fitting analyses were conducted (Figures R3g\nand R3h). The analysis revealed distinct coordination environments in Ni Co @TiO ,\n1 1 2\nwith Ni–O and Co–O coordination numbers of 4.9 and 5.1, respectively (Table R1).\nThe Ni–O coordination number in Ni Co @TiO (4.9) was considerably higher than\n1 1 2\nthat in Ni @TiO SACs (4.3). Conversely, the Co–O coordination number (5.1) was\n1 2\nlower than that in Co @TiO SACs (5.4). Based on the characterization results, the\n1 2\ncoordination structures of isolated Ni and Co atoms on the TiO support can be\n2\ndetermined (see insets, Figures R3g and R3h). Specifically, the Ni and Co atoms are\nrespectively anchored at two adjacent oxygen vacancy sites, substituting for Ti atoms,\neach forming a five-coordinate structure (i.e., a coordination number of 5 with oxygen,\ndenoted as M–O). Furthermore, charge transfer occurs between the anchored Ni/Co\natoms and the Ti sites of the support via the bridging oxygen (M–O–Ti), which has a\npositive effect on the de/hydrogenation process of MgH /Mg systems.\nSample Shell R(Å) CN σ2(Å2) R factor\nNi–O 2.05±0.01 4.9±0.3 0.009±0.001 0.012\nNi Co @TiO\n1 1 2\nCo–O 2.03±0.01 5.1±0.4 0.013±0.002 0.014\nNi @TiO Ni–O 2.03±0.01 4.3±0.5 0.009±0.002 0.016\n1 2\nCo @TiO Co–O 2.00±0.03 5.4±1.3 0.008±0.001 0.020\n1 2\nMoreover, the absence of distinct diffraction peaks attributable to Ni or Co species\nin the XRD patterns, combined with the lack of corresponding lattice fringes in HRTEM\n12\nimages, collectively indicates that both metals exist as atomically dispersed sites rather\nthan in the form of crystalline nanoclusters.\nIn summary, these experimental findings robustly confirm the atomic-scale\ndispersion of Ni and Co atoms and their coordination environments, thereby\nestablishing a solid structural basis for the accompanying DFT calculations and the\nproposed catalytic model.\n3. A major issue with this manuscript is the insufficient structural characterization of\nthe supplementary catalyst.\nResponse: Thank you for your suggestions. In the manuscript and Supplementary\nInformation, we have provided comprehensive TEM and HRTEM characterization for\nsome reference catalysts, including Ni Co @TiO DACs, as well as the nanoparticle\n1 1 2\ncatalysts: Ni NP@TiO , Co NP@TiO , and NiCo NP@TiO . To address the need for\n2 2 2\nfurther structural characterization of Ni @TiO SACs and Co @TiO SACs, we\n1 2 1 2\nperformed a comprehensive electron microscopy analysis, including TEM, HRTEM,\nHAADF-STEM, and EDS-mapping. The results are presented in detail as follows.\nFigure R4. (a) TEM image under 50nm. (b) TEM image under 20nm. (c) HRTEM image of Ni\nNP@TiO .\nNP@TiO .\nand (e) EDS mapping of Ni @TiO SACs.\n1 2\nAs shown in Figures R4–6, the nanoparticle catalysts (Ni NP@TiO , Co NP@TiO ,\n2 2\nand NiCo NP@TiO ) exhibit clear lattice fringes corresponding to metallic Ni or Co\n2\ncrystals, confirming the presence of nanoparticles. Furthermore, detailed structural\ncharacterization was performed on the Ni @TiO and Co @TiO SACs. TEM images\n1 2 1 2\nreveal that both materials consist of particles with diameters of approximately 50 nm\n14\n(Figures R7a and R8a). HRTEM and selected area electron diffraction (SAED) analyses\nconfirm that these particles are TiO (Figures R7b–c and R8b–c), with no lattice fringes\n2\nor diffraction rings corresponding to Ni or Co species being detected, thereby ruling\nout the formation of metallic Ni/Co nanoparticles or crystalline phases. In contrast,\nHAADF-STEM imaging shows distinct bright spots attributable to individual Ni or Co\natoms (Figures R7d and R8d). This atomic dispersion is further corroborated by energy\ndispersive spectroscopy (EDS) elemental mapping (Figures R7e and R8e), which\ndemonstrates the homogeneous distribution of Ni/Co across the TiO support. This stark\n2\ncontrast visually demonstrates the fundamental structural difference between\natomically dispersed catalysts and nanoparticle-based catalysts.\nFigure R8. (a) TEM image, (b) HRTEM image, (c) SAED diffraction pattern, (d) HAADF image,\nand (e) EDS mapping of Co @TiO SACs.\n1 2\nThese systematic microscopy results provide a solid structural foundation for\ncomparing the catalytic behaviors of atomic and nanoparticulate systems. We believe\nthese additional data sufficiently address the concern regarding the structural\ncharacterization of the supplementary catalysts.\n15\n4. The authors claim a reduction in the Mg-H bond energy by 76.84% via DFT\ncalculations, but they fail to provide a comprehensive validation of the DFT parameters\nand convergence criteria. There is no mention of energy cutoff, k-point settings, or\nconvergence tolerances, which are fundamental to ensuring the reliability of such\ncalculations. Moreover, the authors have not performed a rigorous energy convergence\ntest, making the computational results unreliable and undermining the credibility of the\ntheoretical claims. The absence of key computational details further compromises the\nreproducibility and accuracy of the results presented.\nResponse: Thank you for your comments regarding the completeness of the DFT\ncomputational parameters and convergence validation. We would like to clarify that\nthese details were indeed included in the Supplementary Information section of the\nmanuscript. As stated, all DFT calculations were performed using the VASP code with\nthe PAW pseudopotential and the PBE form of the GGA. The plane-wave cut-off\nenergy was set to 400 eV, and the Brillouin zone was sampled using a Monkhorst–Pack\n2×2×1 k-point mesh with a K-spacing of 0.05 Å⁻1, which ensures a good balance\nbetween computational accuracy and efficiency. The total energy and force\nconvergence tolerance were set to 1×10⁻6 eV and 0.02 eV/Å, respectively. In addition,\nGaussian smearing (0.2 eV) was used for partial occupancies, and the D2 dispersion\ncorrection was applied to account for van der Waals interactions.\nAll calculations were performed under these well-established and rigorously tested\nsettings, which have been widely validated in previous DFT studies of MgH -related\n2\nsystems. To further verify the reliability of the calculations, we performed systematic\nconvergence tests on key computational parameters with respect to the total energy.\nFirst, the plane-wave cutoff energy was evaluated (Figure R9a). The results indicate\nthat the total energy of the system gradually converges as the cutoff energy increases\nfrom 320 eV to 480 eV. When the cutoff energy is raised from 400 eV to 420 eV, the\nchange in energy per atom is only about 0.0012 eV, suggesting that a cutoff energy of\n400 eV is sufficient to meet the accuracy requirements. Subsequently, the influence of\nk-point sampling density on the total energy was examined (Figure R9b). For k-point\n16\nspacings ranging from 0.02 to 0.05 Å-1, the variation in total energy remains below\n0.001 eV/atom. Therefore, a k-point spacing of 0.05 Å-1 was selected to ensure high\naccuracy while maintaining computational efficiency. Additionally, we assessed the\neffect of the energy convergence criterion (1×10-4 eV, 1×10-5 eV, 1×10-6 eV, 1×10-7 eV,\nand 1×10-8 eV) and the force convergence threshold on the results (Figure R9c and\nTable R2). The results show that the variation in the total energy of the system is\nnegligible, and adjustments in the force convergence criterion have almost no impact\non the final convergence behavior. Hence, the chosen convergence criteria of 1×10-6 eV\nfor energy and 0.02 eV/Å for forces are deemed appropriate. Finally, tests on the\nGaussian smearing parameter (0.05, 0.10, 0.15, and 0.20 eV) reveal that its influence\non the total energy is less than 1 meV/atom (Figure R9d), further supporting the\nreasonableness of selecting 0.2 eV. By definition, the y-axis values (Energy\ndifference/Per atom) in Figure R9 are calculated as the difference between consecutive\nmeasurements. Thus, the initial measurement in the DFT process has no preceding\nvalue to compare with and therefore does not appear on this plot. In summary, the above\nparameter settings ensure stable total energies and reliable interface structures,\nproviding a solid foundation for subsequent simulations.\nFigure R9. The values of Energy differences/Per atom under different (a) cut-off energies, (b) K-\nspacing and k-point mesh, (c) energy convergence, (d) Gaussian smearing.\ntolerance.\nForce convergence tolerance (eV/Å) Energy (eV) Energy differences/Per atom\n-0.01 -872.748642 —\n-0.02 -872.748642 0\n-0.03 -872.748642 0\nWe have provided the key computational details in the Supplementary Information,\nwhich are sufficient to demonstrate the reproducibility and accuracy of the results\npresented. We greatly appreciate your thoughtful comments, which helped us improve\nthe clarity and completeness of our methodological description.\n5. The proposed catalytic mechanism, which attributes dehydrogenation activity to Ni\nand hydrogenation activity to Co based on d-band center modulation, remains\nspeculative. The authors have not provided compelling experimental proof, such as\nselective poisoning experiments, reaction kinetics with isolated metal species, or site-\nspecific spectroscopic evidence to justify this bifunctional assignment. While the\nauthors claim synergy between Ni and Co, the experimental results fail to conclusively\ndemonstrate this, as there are no clear differences in hydrogen sorption performance\ndirectly attributable to the d-band shifts. In fact, the experimental evidence fails to\nconvincingly support the notion that the Ni-Co DACs improve performance\nsignificantly beyond what is achievable with other single-atom or bimetallic catalysts.\nThe lack of a thorough mechanistic study to substantiate this claim calls into question\nthe validity of their conclusions.\nResponse: Thank you for your comments. In both the main text and the Supporting\nInformation, we have provided compelling experimental proof, such as reaction\nkinetics with isolated metal species and site-specific spectroscopic evidence, to justify\nthis bifunctional assignment. Moreover, the Supplementary Information provides\ncomparative catalytic data, clearly demonstrating that the Ni Co @TiO DACs deliver\n1 1 2\nsignificantly enhanced hydrogen storage performance in the MgH /Mg system over a\n2\nrange of single-atom and bimetallic references. And we further elaborated on the\npotential correlation between the shifts in the d-band centers of Ni/Co and the\n18\noptimization of the de/hydrogenation performance of MgH /Mg systems. A detailed\n2\nexplanation is provided as follows.\n(1) Reaction kinetics with isolated metal species\nHerein, we once again presented the detailed reaction kinetics studies for MgH\n2\ncatalyzed by both Ni @TiO SACs and Co @TiO SACs (Figures R10 and R11). The\n1 2 1 2\nkey experimental finding is that Ni @TiO SACs exhibit dehydrogenation kinetics\n1 2\nsignificantly superior to those of Co @TiO SACs, though inferior to the Ni Co @TiO\n1 2 1 1 2\nDACs. Conversely, Co @TiO SACs demonstrate hydrogenation kinetics that are\n1 2\nmarkedly better than those of Ni @TiO SACs, yet still not as performant as the\n1 2\nNi Co @TiO DACs. This clear disparity in the catalytic propensity of the single-atom\n1 1 2\ncatalysts provides strong experimental support for our assignment: Ni sites are more\nactive for dehydrogenation, while Co sites are more active for hydrogenation, and\nNi/Co sites lead to a synergistic enhancement of the hydrogen storage performance in\nMgH /Mg systems.\n1 2 2\nHydrogenation curves of 15wt.%-Co @TiO –MgH at different temperatures. (c) Nine kinetic\n1 2 2\nanalysis models at 300°C of 15 wt.%-Co @TiO –MgH . (d) Calculated hydrogen desorption and\n1 2 2\nabsorption activation energies of 15wt.%-Co @TiO –MgH by R2 kinetic analysis model.\n1 2 2\n1 2 2\nHydrogenation curves of 15wt.%-Ni @TiO –MgH at different temperatures. (c) Nine kinetic\n1 2 2\nanalysis models at 300°C of 15 wt.%-Ni @TiO –MgH . (d) Calculated hydrogen desorption and\n1 2 2\nabsorption activation energies of 15wt.%-Ni @TiO –MgH by R3 kinetic analysis model.\n1 2 2\n(2) Site-specific spectroscopic evidence\nDirect evidence for the atomic dispersion and heteronuclear pairing of Ni and Co\nwas obtained through atomic-resolution high-angle annular dark-field scanning\ntransmission electron microscopy (HAADF-STEM) characterization. As shown in\nFigure R12a, uniformly distributed bright dots are clearly visible on the TiO support.\nconfidently assigned to individually dispersed Ni and Co atoms. Statistical analysis of\nthe measured atomic spacing yielded an average Ni-Co distance of 0.247 nm (Figure\nR12b). Moreover, the HAADF-STEM image of the Ni Co @TiO DACs showed that\n1 1 2\nthe Ni/Co atomic intensity possesses significantly higher intensity than the Ti atomic\nintensity (Figures R12c-d). Further, the HAADF-STEM via electron energy-loss\nspectroscopy (EELS) clearly shows the dual-atom pair contains one Co atom and one\nNi atom, strongly confirming the Co−Ni dual-atom pairs formation in Ni Co @TiO\n1 1 2\nDACs (Figure R12e). Extensive imaging analysis found no evidence of metal clusters\n20\nor nanoparticles, further confirming the atomic dispersion of the metal species.\nFigure R12. (a) HRTEM pattern of Ni 1 Co 1 @TiO 2. (b) Analysis of the distance profile of two\nadjacent atoms of Ni/Co in Ni 1 Co 1 @TiO 2 DACs. (c) Analysis of the intensity profile of two\nadjacent atoms of Ni 1 Co 1 @TiO 2 . (d) 3D atomic resolution map of the metal-atom sites. (e)\nAtomic-resolution HAADF-STEM image with the corresponding EELS spectra of the atomic\npair in NiCo @TiO DACs.\n1 2\nFurther, X-ray absorption spectroscopy was used to accurately identify the local\ngeometric structures of Ni and Co at the atomic level. Figure R13a shows the Ni K-\nedge X-ray absorption near-edge structure (XANES) spectra of Ni Co @TiO along\n1 1 2\nwith those of a Ni foil, NiO, and Ni @TiO as references. The XANES spectrum of\n1 2\nNi Co @TiO was considerably different from those of the Ni foil and NiO, which\n1 1 2\nfurther reflected the unique state of Ni in the catalyst. Moreover, the absorption edge\nenergy of Ni Co @TiO was higher than that of the Ni foil and Ni @TiO and lower\n1 1 2 1 2\nthan that of NiO,indicating that the valency of Ni species in Ni Co @TiO was between\n1 1 2\n0 and +2, slightly higher than that of Ni species in Ni @TiO . According to the Co K-\n1 2\nedge XANES spectrum of Ni Co @TiO , the valency of Co species in Ni Co @TiO\n1 1 2 1 1 2\nis higher than that of Co @TiO (Figure R13b). The observed intermediate valence\n1 2\nstates of Ni and Co originate from the strong metal-support interaction (SMSI) with the\nTiO support. This interaction causes partial charge transfer, resulting in electronic\n2\n21\nstructures that are intermediate between their metallic and oxidized states, distinct from\na mere physical mixture.\nFigure R13. Atomic local structure characterisation of Ni Co @TiO . a, Ni K-edge XANES\n1 1 2\nspectra. b, Co K-edge XANES spectra. c, Ni K-edge EXAFS spectra in the R space. d, Co K-edge\nEXAFS spectra in the R space. e, WT-EXAFS images of Ni Co @TiO , Ni @TiO , NiO and the Ni\n1 1 2 1 2\nfoil. f, WT-EXAFS images of Ni Co @TiO , Co @TiO , CoO, and the Co foil. g, FT-EXAFS fitting\n1 1 2 1 2\ncurve in the R space of Ni Co @TiO at the Ni K-edge. h, FT-EXAFS fitting curve in the R space\n1 1 2\nof Ni Co @TiO at the Co K-edge.\n1 1 2\nMore detailed atomic structure information was obtained using the k2-weighted\nFourier transform (FT) extended X-ray absorption fine structure (EXAFS) of Ni and\nCo K-edge in the R space. The characteristic peaks of the Ni–Ni bond in the Ni foil and\nCo–Co bonds in the Co foil were located at 2.16 and 2.18 Å, respectively (Figures R13c\nand R13d), which were absent in the Ni Co @TiO , proving that Ni and Co atoms\n1 1 2\n22\nachieved atomic dispersion on TiO rather than forming metal clusters or particles.\n1 1 2\nwere attributed to the first coordination shell of the Ni–O and Co–O bonds, respectively.\nTo further analyse the coordination states of Ni and Co, their wavelet-transformed\n(WT) K-edge EXAFS oscillations were studied. As shown in Figures R13e and R13f,\nthe strongest oscillations of Ni Co @TiO appeared around the wave vector of 5 Å−1,\n1 1 2\nwhich was close to the wave vectors pertaining to the strongest oscillations of NiO and\nCoO. The backscattered atoms bonded with Ni and Co were O atoms. To further\nidentify the fine coordination configurations of Ni and Co in different samples,\nquantitative least squares EXAFS curve-fitting analyses were conducted (Figure R13g\nand R13h). The analysis revealed distinct coordination environments in Ni Co @TiO ,\n1 1 2\nwith Ni–O and Co–O coordination numbers of 4.9 and 5.1, respectively. The Ni–O\ncoordination number (4.9) in Ni Co @TiO was considerably higher than that in\n1 1 2\nNi @TiO SACs (4.3). Conversely, the Co–O coordination number (5.1) was lower\n1 2\nthan that in Co @TiO SACs (5.4). This altered coordination condition suggested\n1 2\npotential electronic interactions between Ni and Co, which may have a positive effect\non promoting efficient electron transfer during the de/hydrogenation process of\nMgH /Mg systems.\n2\n(3) Comparison between Ni Co @TiO DACs and other SACs/DACs\n1 1 2\nPrevious reports have indicated that the elements Pd and Ru exhibit extremely\nexcellent catalytic effects on the MgH /Mg systems. Given these established findings\n2\nand the promising catalytic performance of Ni and Co observed in our study, we\nconducted additional TPD experiments to directly compare the hydrogen desorption\nbehavior of the Ni Co @TiO DACs with several reference catalysts. These included\n1 1 2\nsingle-atom catalysts (Ru @TiO , Pd @TiO ) and other dual-atom systems (such as\n1 2 1 2\nRu Co @TiO , Ru Ni @TiO , Pd Ni @TiO , Pd Ru @TiO , Pd Co @TiO ). All\n1 1 2 1 1 2 1 1 2 1 1 2 1 1 2\ncatalysts were synthesized and tested under identical conditions. The TPD curves\n(Figure R14) clearly demonstrate that MgH catalyzed by Ni Co @TiO DACs\n2 1 1 2\nachieves the lowest initial and peak desorption temperatures among all tested SACs and\nDACs. This exceptional performance highlights that the synergy in Ni Co @TiO\n1 1 2\nDACs is not merely a generic diatomic effect, but rather a unique electronic cooperation\nbetween Ni and Co that optimally tunes the d-band center and facilitates hydrogen\n23\ndesorption and absorption. Specifically, the balanced and complementary interaction of\nNi/Co dual-atom pairs could simultaneously promote hydrogen desorption (Ni serves\nas the dominant site for weakening the Mg–H bonds energy) and hydrogen absorption\n(Co serves as the dominant site for H–H bonds dissociation), significantly lowering the\nenergy barrier for de/hydrogenation reactions.\nFigure R14. TPD curves of MgH catalyzed by different SACs and DACs\n2\nof the MgH /Mg system, we evaluated the effect of bimetallic atoms supported on\n2\ndifferent oxides on the dehydrogenation performance of MgH . Previous studies have\n2\nshown that transition metal oxides, such as V O and Nb O , also exhibit excellent\n2 5 2 5\ncatalytic effects on the MgH /Mg systems. Therefore, following the same hot molten\n2\nsalt method used to prepare Ni Co @TiO DACs, we synthesized Ni Co @V O DACs\n1 1 2 1 1 2 5\nand Ni Co @Nb O DACs. Subsequently, we composited them with MgH via the\n1 1 2 5 2\nsame ball milling process to obtain 15 wt.%-Ni Co @V O –MgH and 15 wt.%\n1 1 2 5 2\nNi Co @Nb O –MgH (the detailed preparation procedure is provided in the\n1 1 2 5 2\nSupplementary Information). Temperature-programmed desorption (TPD) results\n(Figure R15) revealed that the initial dehydrogenation temperature (189 °C) and the\npeak dehydrogenation temperature (263 °C) of 15 wt.%-Ni Co @V O –MgH were\n1 1 2 5 2\nhigher than those of 15 wt.%-Ni Co @TiO –MgH (which exhibited initial and peak\n1 1 2 2\ndehydrogenation temperatures of 163 °C and 220 °C, respectively). Similarly, the TPD\nprofile of 15 wt.%-Ni Co @Nb O –MgH consistently demonstrated higher\n1 1 2 5 2\n24\ndehydrogenation temperatures, with an onset at 170 °C and a peak at 228 °C, further\naffirming the superior catalytic efficacy of the TiO -supported catalyst.\n2\n(4) Correlation between shifts in Ni/Co d-band centers and de/hydrogenation\nperformance of MgH /Mg systems\nas a powerful computational descriptor for rationalizing and explaining experimental\ntrends in adsorption and catalytic activity. Its core premise is that the position of a\nmetal's d-band center relative to the Fermi level dictates the bond strength between the\nadsorbate and the metal. According to this model, an up-shifted d-band center\nstrengthens the bonding because the resulting anti-bonding states are shifted above the\nFermi level and remain largely unoccupied. This stronger interaction generally\nfacilitates dissociative adsorption (e.g., H bond cleavage) by lowering the activation\n2\nenergy. Conversely, a down-shifted d-band center weakens the adsorption, facilitating\ndesorption processes (e.g., weakening the Mg–H bond energy).\nIn this work, DFT calculations reveal that the d-band center of Ni in Ni Co @TiO\n1 1 2\nDACs is down-shifted compared to Ni @TiO SACs, while that of Co is up-shifted\n1 2\ncompared to Co @TiO SACs. This unique electronic cooperation optimally tunes the\n1 2\nd-band center of the entire active site, providing a direct and rational explanation for\nthe clear differences in hydrogen sorption performance observed experimentally. The\ndown-shifted d-band of Ni correlates with its role in weakening the Mg-H bond. The\n25\nup-shifted d-band of Co correlates with its enhanced ability to adsorb and activate\nhydrogen (H ), facilitating the H-H bond dissociation. The synergy is manifested in the\n2\ndual functionality of DACs, which excel at both adsorption and desorption processes,\noutperforming the single-function SACs. As revealed by experimental results and\ntheoretical calculations, the unique electronic structure of the Ni-Co pair results in a\nlower overall energy barrier for the reversible de/hydrogenation process. Therefore, the\ncomputational d-band center analysis provides a crucial link, connecting the observed\nexperimental synergy to its underlying electronic-structure mechanism.\nIn summary, based on the reaction kinetics of isolated metal species, site-specific\nspectroscopic evidence, comparative experiments with other single- and dual-atom\ncatalysts, and the in-depth analysis of the d-band center mechanism, this comprehensive\nstudy confirms the reliability of the proposed catalytic mechanism. Thank you again\nfor your comments, and we hope that the above evidence can allay your concerns.\n6. There are inconsistencies in the experimental details regarding the testing\ntemperatures for the cycling stability experiments. In the main text, the cycling stability\nis tested at 280°C, while the experimental section in the supplementary information\nmentions 290 °C. Such discrepancies should be addressed to ensure the accuracy and\nreproducibility of the study.\nResponse: Thank you for your comments. We sincerely apologize for this oversight.\nDuring the first peer review, Reviewer 3 had already raised this issue and we have\npromptly corrected it in the last revised Supplementary Information, as well as\nhighlighted it in yellow.\nAs correctly stated in the main text, all cyclic stability measurements were\nperformed at 280 °C. We have provided the temperature profile recorded during the\ncycling process. As shown in Figure R16, the temperature of 100 de/hydrogenation\ncycles remained at approximately 280°C. The observed temperature fluctuations are\nattributed not only to the rapid heating and cooling associated with the H evacuation\n2\nand admission steps, but also to the intrinsic heat of reactions: exothermic\nhydrogenation process and endothermic dehydrogenation process.\nThe response letter has provided detailed answers to the raised questions, particularly\noffering a comprehensive comparison and innovative explanation regarding the\ndifferences from PdNi catalysts, which has given me a deeper understanding of the\nmanuscript’s content. As for the current version of the manuscript, the following\nsuggestions are offered.\nResponse: We appreciate your positive feedback and valuable comments on our\nmanuscript. And we have thoroughly reviewed all the suggestions and have revised the\nmanuscript to incorporate them. Our responses to each comment are provided below.\n1. The selection of the exposed surface of the support is a critical parameter in\nconstructing the DFT computational model. It is recommended that the authors briefly\nstate the rationale for selecting the specific crystal plane of the anatase TiO support for\n2\nconstructing the interface.\nResponse: We sincerely appreciate your insightful comment regarding the\ncomputational model construction. The anatase TiO (101) surface was selected for our\n(101) surface is widely recognized as the most thermodynamically stable and\ndominantly exposed facet in anatase TiO nanocrystals, primarily due to its lowest\n2\nsurface energy, which promotes its predominant presence in realistically synthesized\nmaterials6-10. This theoretical foundation is directly supported by our experimental\nHRTEM characterization (Figure R17), where the measured lattice fringes\nunequivocally correspond to the (101) planes, confirming that this facet is abundantly\nexposed in our as-synthesized Ni Co @TiO dual-atom catalyst. Therefore,\n1 1 2\nconstructing our interface model based on the (101) surface ensures that the\ncomputational simulations are directly representative of the actual catalyst's surface\nstructure and interface chemistry. This alignment between the model and the real\nmaterial is crucial for obtaining physically meaningful and reliable insights into the\ncatalytic mechanism. We fully agree that this rationale enhances the manuscript's rigor,\nand we have incorporated it into the revised manuscript. Thank you again for this\n28\nconstructive comment.\nFigure R17. (a) TEM pattern of Ni Co @TiO DACs. (b) HRTEM pattern of Ni Co @TiO DACs.\n1 1 2 1 1 2\n(c) SAED pattern of Ni Co @TiO DACs. (d) TEM image of 15 wt.%-Ni Co @TiO –MgH at the\n1 1 2 1 1 2 2\n20-nm scale. (e) HRTEM image of 15 wt.%-Ni Co @TiO –MgH . (f) SAED pattern of 15 wt.%-\n1 1 2 2\nNi Co @TiO –MgH .\n1 1 2 2\n2. Whether for single-atom catalysts or dual-atom catalysts, the authors employed the\nmolten salt method. What was the reason for this? What are the advantages of this\nsynthesis method?\nResponse: Thank you for this insightful question regarding our choice of synthesis\nmethod. This method employs a eutectic mixture of salts, such as LiCl and KCl, which\nmelts at high temperatures to create a liquid-phase reaction environment. This molten\nsalt medium acts as a high-temperature solvent that effectively disperses the metal\nprecursors and promotes their interaction with the TiO support, while simultaneously\n2\ninhibiting the mobility and aggregation of metal atoms into nanoparticles. The confined\nspace within the molten salt matrix and the strong electrostatic forces from the ionic\nliquid environment help to anchor the metal atoms firmly onto the support surface,\nthereby enabling the formation of stable atomic sites.\nThe advantages of this synthetic strategy are multifaceted. Primarily, the high-\ntemperature conditions and the intrinsic ionic nature of the molten salt medium actively\npromote strong metal-support interactions (SMSI), which are crucial for modulating the\nelectronic structure of the active sites and enhancing the overall structural stability of\n29\nthe catalyst. Secondly, the high synthesis temperature (500°C) inherently ensures the\nformation of thermally robust single-atom sites, thereby directly contributing to the\nexceptional thermal stability observed in the resulting catalysts. Furthermore, the\nmethod offers remarkable versatility and scalability, as demonstrated by our successful\nsynthesis of a wide range of single-atom and dual-atom catalysts. Therefore, the hot\nmolten salt method was instrumental in achieving the atomically dispersed active sites\nand the outstanding thermal stability that are central to the high performance\ndemonstrated in our study.\n3. Given that the composite exhibits excellent hydrogenation properties at room\ntemperature, how much hydrogen capacity can be achieved by hydrogen absorption for\na sufficiently long duration?\nResponse: Thank you for raising this pertinent question regarding the ultimate room-\ntemperature hydrogen storage capacity of our composite material. We have conducted\nadditional experiments to address this point specifically. Under a hydrogen pressure of\n1 1 2\nachieves a highly promising hydrogen capacity of 5.47 wt.% after an absorption period\nof 10 hours (Figure R18). The ability to reach the hydrogen capacity of 5.47 wt.% at\nroom temperature underscores the exceptional effectiveness of the Ni Co @TiO dual-\n1 1 2\natom catalyst in drastically enhancing the kinetics of hydrogenation under ambient\nconditions. This result convincingly demonstrates that our catalytically enhanced\ncomposite can attain a high hydrogen capacity at room temperature, highlighting its\nsignificant potential for real-world applications.\nFigure R18. Hydrogenation curve of 15wt.%-Ni 1 Co 1 @TiO 2 -MgH 2 at 30℃ for 10 hours.\n30\n4. The composite demonstrates remarkable stability over 100 cycles of\nde/hydrogenation, with only a slight capacity loss observed. Please discuss the excellent\nstability and the possible reasons for the slight capacity loss.\nResponse: Thank you for your insightful comment. Below, we elaborate on the\nexceptional stability of the material over 100 de/hydrogenation cycles and the reasons\nfor the slight capacity loss.\nThe outstanding cycling stability of the 15wt.%-Ni Co @TiO –MgH /Mg\n1 1 2 2\ncomposite (with a capacity retention of 97.6% after 100 cycles) is primarily attributed\nto several synergistic mechanisms. Firstly, the strong metal–support interaction (SMSI)\nbetween the Ni/Co dual atoms and the TiO support effectively suppresses the migration\n2\nand agglomeration of metal atoms during cycling. The HAADF-STEM and EDS\nmapping of 15wt.%-Ni Co @TiO –MgH /Mg after 100 cycles show that Ni and Co\n1 1 2 2\natoms remain highly uniformly dispersed at the atomic level. Secondly, the TiO\n2\nsupport exhibits self-reconstruction behavior during cycling, with reversible evolution\nin the concentration of oxygen vacancies (O ) and multivalent Ti species, dynamically\nV\nmaintaining the active sites. Furthermore, the heteronuclear Ni–Co dual-atom sites\nretain their coordination environment and electronic synergy throughout the cycling\nprocess, forming structurally stable and atomically dispersed Ni/Co–O–Ti active\ncenters. Meanwhile, the close interfacial contact between the catalyst and the MgH /Mg\n2\nmatrix ensures stable and efficient hydrogen transport pathways.\nRegarding the observed slight capacity loss (~2.4%), we believe it mainly stems\nfrom the grain coarsening of MgH during long-term cycling. The post-cycling XRD\n2\npatterns show sharpened MgH diffraction peaks and reduced full width at half\n2\nmaximum (Figure R19), indicating increased crystallinity and grain growth. This\nreduces the specific surface area and active interfaces, slightly impeding hydrogen\ndiffusion and nucleation kinetics.\nIn summary, the excellent cycling stability of the composite is mainly due to the\nstrong metal–support interaction, the self-reconstruction capability of the support, and\nthe atomic-level structural stability of the dual-atom catalyst. The slight capacity decay\n31\nis primarily related to the grain coarsening of the MgH /Mg matrix. We have added a\n2\ndiscussion on the excellent stability and the reasons for the capacity loss in the revised\nmanuscript. Thank you again for your comment.\nFigure R19. XRD pattern of 15wt.%-Ni Co @TiO -MgH after dehydrogenation.\n1 1 2 2\nUnlock Record-Low Onset Dehydrogenation Temperature for Bulk-MgH . Adv.\n2\nto CO in Zn GeO nanorods. Chem. Eng. J. 468, 143569 (2023).\n2 4\nfor Catalytic Therapy. J. Am. Chem. Soc. 146, 10023-10031 (2024).\nEndows (Ni Fe ) P Nanosheets with Efficient Oxygen Evolution Catalysis.\nx 1–x 2\nACS Catal. 10, 9086-9097 (2020).\nmigration and utilization of reactive oxygen species through tailoring the d-band\nin metal organic frameworks for water purification. Appl. Catal. B: Environ.\n328, 122507 (2023).\nHydrogen Evolution. Chem. Select 10, e04106 (2025).\n2\nphotocatalytic coupling of methane. Nat. Commun. 15, 4453 (2024).\n2\nreactive facets. Nature 453, 638-641 (2008).\nreactive {001} facets of TiO -based composites: synthesis, formation\n2\nmechanism and characterization. Nanoscale 6, 1946-2008 (2014).\nDerived Platform Compounds by Surface Engineering of Titanium Oxide.\nChem 6, 3038-3053 (2020).\n33"}, {"round": 3, "reviewer_comments": {"reviewer_4": "This work introduces an innovative design concept centered on a heteronuclear Ni1Co1@TiO2 dual-atom catalyst, which achieves bidirectional modulation of d-band centers. Through the successful preparation of well-dispersed atomic active sites, the catalyst synergistically regulates both the dehydrogenation and hydrogenation kinetics in MgH2/Mg. Furthermore, combined theoretical and experimental analyses clearly elucidate the underlying mechanism. After a careful review of the manuscript and Response Letter to Reviewers, I agree with the opinions expressed by Reviewers 1 and 3. After systematic revisions, the authors have significantly enhanced the quality of the manuscript. Therefore, I recommend this paper for publication in Nature Communications after following minor revisions.\n1. How did the authors distinguish between Co and Ni in Figure 2g, and how was the distance between them determined?\n2. How was the EELS measurement performed to obtain Figure 2i? It is necessary to include the relevant experimental details in the Method section."}, "author_response": "Research Article, ID: NCOMMS-25-53103B\nTitle: Bidirectional Catalysts with Dual-Atom Dynamic d-Band\nCentre Modulation and Support Self-Reconstruction for Reversible\nDe/Hydrogenation in MgH /Mg\nReviewer #3 (Remarks to the Author):\nThe Revised Manuscript solves the reviewer's questions and suggestions on the original\nmanuscript. I recommend its acceptance without further revisions.\nResponse: We are sincerely grateful for your professional review and valuable\nfeedback regarding our manuscript. Your detailed comments and insightful suggestions\nhave significantly contributed to the improvement of our manuscript, resulting in a\nsubstantial enhancement in its quality.\nReviewer #4 (Remarks to the Author):\nThis work introduces an innovative design concept centered on a heteronuclear\nNi Co @TiO dual-atom catalyst, which achieves bidirectional modulation of d-band\n1 1 2\ncenters. Through the successful preparation of well-dispersed atomic active sites, the\ncatalyst synergistically regulates both the dehydrogenation and hydrogenation kinetics\nin MgH /Mg. Furthermore, combined theoretical and experimental analyses clearly\n2\nelucidate the underlying mechanism. After a careful review of the manuscript and\nResponse Letter to Reviewers, I agree with the opinions expressed by Reviewers 1 and\n3. After systematic revisions, the authors have significantly enhanced the quality of the\nmanuscript. Therefore, I recommend this paper for publication in Nature\nCommunications after following minor revisions.\nResponse: We appreciate your positive feedback and valuable comments on our\nmanuscript. And we have thoroughly reviewed all the suggestions and have revised the\nmanuscript to incorporate them. Our responses to each comment are provided below.\n1. How did the authors distinguish between Co and Ni in Figure 2g, and how was the\ndistance between them determined?\nResponse: We sincerely appreciate your insightful questions. In Figure 2g, the\nidentification of Co and Ni atoms primarily relied on the atomic number contrast (Z-\ncontrast) inherent to High-Angle Annular Dark-Field Scanning Transmission Electron\nMicroscopy (HAADF-STEM) imaging. In this mode, the signal intensity is\napproximately proportional to the square of the atomic number (≈ Z2). Given that Ni\n(Z=28) has a higher atomic number than Co (Z=27), Ni atoms are expected to exhibit\ngreater brightness in HAADF-STEM images. Accordingly, in Figure 2g, the atomic\nsites with higher intensity were attributed to Ni, while those with lower intensity were\nassigned to Co.\nThe distances were determined by direct measurement of the interatomic spacings\nin the HAADF-STEM images. Specifically, the interatomic distance was determined\nthrough the following procedure. (1) Atomic Position Extraction: The atomic\ncoordinates in the HAADF-STEM image (Figure 2e) were calibrated using\nDigitalMicrograph software to pinpoint the center of each atomic column. (2) Distance\nCalculation and Statistical Analysis: The distances between multiple identified Ni-Co\natomic pairs were measured (Supplementary Fig. 8 in Supplementary Information).\nFurther, statistical analysis of these measurements yielded an average Ni-Co distance\nof 0.247 nm (Figure 2f in revised Manuscript).\nWe hope this clarification adequately addresses your query.\n2. How was the EELS measurement performed to obtain Figure 2i? It is necessary to\ninclude the relevant experimental details in the Method section.\nResponse: Thank you for raising this important point regarding the detailed description\nof the EELS measurements. The experimental details have been updated in the Methods\nsection (see Supplementary Information). The key points are summarized below for\nyour convenience.\nElectron energy-loss spectroscopy (EELS) measurements were primarily\nperformed in conjunction with HAADF-STEM imaging, using an FEI Spectra 300\nTEM operated at 200 kV with a spot size of 80.1 pm. The convergence and collection\nangles were set to approximately 23.5 mrad and 50–200 mrad, respectively. High-angle\nannular dark-field (HAADF) Z-contrast images were acquired from the same region\nusing an annular detector covering an angular range of 75–210 mrad. Elemental\nidentification was based on energy-resolved spectroscopy using a Gatan GIF\nContinuum system, which provided an energy resolution of 0.6 eV.\nFigure R1. (a) Atomic-resolution HAADF-STEM image. (b) EELS spectrum collected from the\nregion of the marked atomic pair in the HAADF-STEM image.\nSpecifically, HAADF-STEM imaging was conducted first, enabling the\nidentification and selection of a distinct bright pair of monodisperse atoms (marked in\na green square frame, Figure R1a) for EELS point acquisition. The acquired spectra\nexhibited distinct Co and Ni signals, thereby confirming the atomic pair as a Co–Ni\nheteronuclear structure (Figure R1b). Additionally, the layout of Figure 2 in the\nmanuscript has been adjusted to better present the data and improve the clarity of the\nvisual results.\nThese experimental and methodological details are essential for reproducibility\nand are now documented in the revised Supplementary Information. Thank you for your\nvaluable suggestion.\nResearch Article, ID: NCOMMS-25-53103C\nTitle: Bidirectional Catalysts with Dual-Atom Dynamic d-Band\nCentre Modulation and Support Self-Reconstruction for Reversible\nDe/Hydrogenation in MgH /Mg\nReviewer #4 (Remarks to the Author):\nThe author's response has clarified my doubts, and I suggest accepting the manuscript.\nResponse: We are sincerely grateful for your professional review and valuable\nfeedback regarding our manuscript. Your detailed comments and insightful suggestions\nhave significantly contributed to the improvement of our manuscript, resulting in a\nsubstantial enhancement in its quality."}, {"round": 4, "reviewer_comments": {"reviewer_4": "The author's response has clarified my doubts, and I suggest accepting the manuscript."}, "author_response": "Research Article, ID: NCOMMS-25-53103C\nTitle: Bidirectional Catalysts with Dual-Atom Dynamic d-Band\nCentre Modulation and Support Self-Reconstruction for Reversible\nDe/Hydrogenation in MgH /Mg\nReviewer #4 (Remarks to the Author):\nThe author's response has clarified my doubts, and I suggest accepting the manuscript.\nResponse: We are sincerely grateful for your professional review and valuable\nfeedback regarding our manuscript. Your detailed comments and insightful suggestions\nhave significantly contributed to the improvement of our manuscript, resulting in a\nsubstantial enhancement in its quality."}]

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