Enhancing hydrogen permeation barrier performance of ErCo_2 magnetic refrigeration material via surface oxide layer formation

Domain: neuroscience
Published: 2026-04-07
Editorial outcome: major_revision

Abstract
The ErCo2 intermetallic compound exhibits a significant magnetocaloric effect at approximately 32 K and has potential applications as a magnetic refrigeration material for hydrogen liquefaction. However, exposure to a hydrogen atmosphere may lead to hydride formation, which weakens the magnetocaloric effect. Thus, preventing hydrogen permeation into ErCo2 is crucial. Herein, we enhance the hydrogen permeation barrier (HPB) performance of ErCo2 particles by using electroless Cu plating followed by oxidation treatment to form a CuO layer with a thickness of a few micrometers. In experiments, ErCo2 particles, with a 1.5- to 5-µm-thick CuO surface layer, exhibited a large magnetic entropy change of 24 J kg⁻¹ K⁻¹ even after exposure to a H2 atmosphere at 1.27 MPa and 296 K for 7 d. Experimental analyses and first-principles calculations revealed the potential of CuO as an HPB material for magnetic refrigeration.

Introduction
Hydrogen liquefaction is crucial for its transportation and storage1-3. Magnetic refrigeration offers a more energy-efficient alternative to conventional gas compression/expansion refrigeration technology4-7, and the efficiency of such refrigeration systems can be further improved using magnetic refrigeration materials with large magnetocaloric effects. Numerous magnetic refrigeration materials with large magnetocaloric effects have been developed, such as DyNi2, ErCo2, ErNi2, Dy1-xErxNi2, HoB2, and HoAl2.

However, the development of magnetic refrigeration materials with a large magnetic entropy change (∆SM) over a broad temperature range (77–20 K) is challenging16-20. A layered composite magnetic refrigerant of HoNi2, DyNi2, and TbNi2 has been developed and obtained an average ∆SM value of 4.7 J kg–1 K–1 under a 2 T field change in a broad temperature range of 7.5–53.4 K21. ErCo2 exhibits a ∆SM > 20 J kg–1 K–1 and an adiabatic temperature change (ΔTad) of ~10 K under a 5 T field change at the Curie temperature of ~32 K9-11, making it suitable for hydrogen liquefaction. Recent strategies of alloying ErCo2 with Ni, Al, or Fe have expanded the available temperature range to 20–77 K22.

The shape optimization of magnetic materials is important for maximizing the heat transfer between the magnetic material and heat exchange fluid within the active magnetic regenerative (AMR) refrigeration system6,7. Magnetic refrigeration materials with a spherical shape (diameter of 200–500 μm) are expected to increase the cooling efficiency of refrigeration systems because such particles can be densely packed in the system, while retaining a certain amount of gaps between them for the flow of heat exchange fluid23. Currently, magnetic refrigeration systems use a helium gas atmosphere rather than a hydrogen gas atmosphere to liquefy hydrogen via helium gas flow6.

The AMR refrigeration system could be simplified and the energy efficiency significantly improved if magnetic refrigeration materials could be placed directly in a hydrogen atmosphere. However, ErCo2 is prone to hydride formation upon hydrogen exposure24-26, leading to particle fragmentation and loss of the magnetocaloric effect. Since the magnetic refrigeration materials must be exposed to a hydrogen atmosphere at room temperature for a few days each year during routine maintenance period of magnetic refrigeration systems, it is imperative to develop ErCo2-based materials with superior hydrogen permeation barrier (HPB) performance that are effective not only at cryogenic temperatures but also at room temperature.

Previous studies have explored HPB coatings for steel materials to prevent hydrogen embrittlement and leakage in nuclear power plants27-29. Oxides such as Cr2O3-SiO2, Al2O3, Y2O3, Er2O3, La2O3, and SiO2 have been investigated owing to their low hydrogen solubility and permeability30-32. They are typically coated via a variety of techniques: hot-dipping, thermal spraying, plasma-spraying, physical vapor deposition (PVD), and chemical vapor deposition. We attempted to use hot-dipping, thermal spraying, and barrel PVD methods to form an Cr2O3 or Al2O3 layer onto the ErCo2 particles. However, particle aggregation is difficult to prevent via thermal spraying, and the coating uniformity is hard to control on the small ErCo2 particles via PVD method.

Compared to these coating methods, formation of CuO layer by copper plating followed by oxidation treatment offers great advantages in terms of cost and efficiency. CuO has not previously been recognized as an HPB material, and this study has demonstrated for the first time that it possesses sufficient HPB properties near room temperature.

Herein, we report our work to synthesize ErCo2 magnetic refrigeration materials with excellent HPB performance using well-established techniques, i.e., preparing ErCo2 particles with diameters of 212–355 µm using an electrode induction melting gas atomization (EIGA) process33-36, forming Cu layer on the ErCo2 particles using electroless plating, and forming CuO layer using the subsequent oxidation treatment. To compare the HPB performance of different oxide layers, we formed a Co(Er) oxide layer through oxidation treatment alone and a CuO layer through a combination of electroless Cu plating and oxidation treatment on the particle surface. The HPB performance of the particles with these layers was evaluated using a Sieverts-type instrument (Fig. S1) at an initial H2 pressure of ~1.27 MPa and 296 K. The effects of the oxide layers on the magnetic properties of ErCo2 particles were evaluated using a Quantum Design SQUID magnetometer. The ErCo2 particles with a Co(Er) oxide layer significantly improved the HPB performance, while particles with a CuO layer further improved it, almost completely blocking hydrogen permeation for 7 d under the abovementioned hydrogen exposure conditions. The CuO layer was analyzed via synchrotron X-ray diffraction (SXRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provided experimental evidence that hydrogen is blocked at the surface. Density functional theory (DFT) calculations revealed that H2 is energetically favored for physisorption on the CuO surface and that its dissociation to H atoms must overcome an energy barrier of >20 kJ/mol, while diffusion of the H atom into CuO requires overcoming an even larger energy barrier (>300 kJ/mol). The DFT calculation results agreed with the experimental results, demonstrating the potential of CuO as an HPB material.

Results
The effects of different oxide layers on the HPB performance of ErCo2 particles were investigated by comparing three types of samples: (1) particles after homogenization at 1123 K for 7 d in an Ar atmosphere (referred to as “as-homogenized”), (2) the homogenized particles after oxidation at 773 K for 0.5 h in air (referred to as “oxidized”), and (3) the homogenized particles after electroless Cu plating and subsequent oxidation at 773 K for 0.5 h in air (referred to as “Cu-plated/oxidized”). Fig. 1a shows the hydrogen pressure changes over time for these samples under an initial hydrogen pressure of ∼1.27 MPa at 296 K, and Fig. 1b shows the corresponding absorbed hydrogen amounts calculated from the pressure changes.

For the as-homogenized ErCo2 particles, the hydrogen pressure rapidly decreased from 1.27 to 1.08 MPa within ∼1 h, corresponding to an absorbed hydrogen amount of ~1.311 mass% of the sample weight—comparable to that observed in typical hydrogen storage alloys37,38. Stereomicroscopy reveal that the ErCo2 particles pulverized after hydrogen exposure (Figs. S2a and S2b), which could be due to the large amount of absorbed hydrogen in ErCo2.

In contrast, the oxidized ErCo2 particles underwent slower hydrogen absorption, achieving a significantly reduced absorbed hydrogen amount. The initial hydrogen pressure of 1.271 MPa started to decrease only after 14 h and reached ~1.245 MPa after 167.5 h, corresponding to an absorbed hydrogen amount of 0.182 mass%. Stereomicroscopy images (Figs. S2c and S2d) showed that most of the particles retained their shape, and only a few were pulverized. These results demonstrate that the oxidation treatment substantially improved the HPB performance of ErCo2 by restricting hydrogen permeation.

Further improvement was observed for the Cu-plated/oxidized particles. The hydrogen pressure (initially 1.272 MPa) remained almost unchanged, decreased slightly after 60 h, and stabilized at 1.255 MPa after 166.75 h. The corresponding absorbed hydrogen amount was 0.117 mass%, indicating even slower absorption than that upon oxidation alone. Stereomicroscopy confirmed that most particles retained their shape after hydrogen exposure (Figs. S2e and S2f). The as-homogenized ErCo2 particles exhibited low HPB performance under 1.27 MPa hydrogen at room temperature, while oxidation significantly improved the HPB properties. Moreover, Cu plating followed by oxidation further improved the HPB performance, effectively preventing hydrogen permeation for over 100 h.

Pressure–composition–temperature (PCT) measurements were performed for the as-homogenized, oxidized, and Cu-plated/oxidized ErCo2 particles (Fig. S3). For the as-homogenized particles, no obvious hydrogen absorption was observed up to 4 MPa at 303 K, whereas the amount of hydrogen absorption reached approximately 1.3 mass% at 333 K. The PCT results are consistent with the hydrogen exposure test results, considering that the PCT measurement was completed in less than an hour (0.57 h) at 303 K and hydrogen absorption commenced in the as-homogenized particles during the hydrogen exposure test (Fig. 1a). In addition, the amount of hydrogen absorption obtained from the PCT measurement at 333 K agrees well with the value of 1.311 mass% obtained from the hydrogen exposure test. The absorbed hydrogen maintained approximately 1 mass% even after reducing the pressure to a low level (<0.005 MPa), suggesting that hydrogen absorbed by ErCo2 does not easily desorb.

For the oxidized particles, no obvious hydrogen absorption was observed at 303 K, and only a small amount of hydrogen absorption (~0.2 mass%) was observed at 333 K (Fig. S3b). For the Cu-plated/oxidized particles, no apparent hydrogen absorption was observed at both 303 and 333 K (Fig. S3c). These PCT measurements are consistent with those of hydrogen exposure test (Fig. 1a and 1b), indicating that the HPB performance was improved by the oxidation and further enhanced by Cu-plated/oxidation treatment.

Furthermore, we performed long-term hydrogen exposure tests at room temperature up to 28 days for evaluating long-term hydrogen permeation stability of oxidized only and Cu-plated/oxidized ErCo2 particles. Particles after different Cu plating time (0.17 and 1.5 h) were used for examining the influence of CuO layer thickness on HPB performance (Fig. S4). For all samples, the decrease in hydrogen pressure throughout the test period was less than 2%, indicating an extremely slow hydrogen absorption rate. This demonstrated that both oxidized and Cu-plated/oxidized ErCo2 exhibit excellent long-term HPB stability. Among these, the Cu-plated/oxidized particles showed slightly superior HPB characteristics compared to the oxidized particles. Furthermore, samples with shorter Cu plating time (0.17 h) exhibited HPB characteristics no less than that with long Cu plating time (1.5 h) (Table S1). As the thickness of CuO layer formed on the sample with shorter Cu plating time (0.17 h) was approximately 1.5 μm (Fig. S5), this result suggests that a 1.5 μⅿ-thick CuO layer can provide a sufficient HPB effect.

The effect of the Cu plating–oxidation treatment on the magnetic properties of ErCo2 was investigated by measuring the M–T curves under varying field up to 5 T for the as-homogenized, oxidized, and Cu-plated/oxidized samples before and after H2 exposure test (Fig. S6) to calculate the magnetic entropy change (∆SM) using Eq. 3. Figs. 1c and 1d show the M–T curves of the as-homogenized, oxidized, and Cu-plated/oxidized particles, measured at 0.01 T before and after hydrogen exposure. In the as-homogenized particles, magnetization sharply increased at ~32 K (the Curie temperature) owing to the first-order ferrimagnetic transition9,10. After hydrogen exposure, this transition was absent, replaced by a rapid magnetization increase below 10 K—suggesting ErCo2 conversion to hydride, which significantly altered the magnetic properties. Conversely, the oxidized and Cu-plated/oxidized particles retained a clear magnetization increase at ~32 K, both before and after hydrogen exposure, confirming that the Cu plating–oxidation treatment blocks hydrogen permeation and preserves the magnetic properties of ErCo2.

Figs. 1f and 1g present ∆SM for the as-homogenized, oxidized, and Cu-plated/oxidized samples under μ0∆H = 5 T. The maximum ∆SM was ~33 J kg⁻¹ K⁻¹ for the as-homogenized particles, ~26 J kg⁻¹ K⁻¹ after the oxidation treatment, and ~24 J kg⁻¹ K⁻¹ after the Cu plating−oxidation treatment. The temperature range of large ∆SM (32–42 K) remained unchanged. After hydrogen exposure, both ∆SM value and temperature range remained stable for the oxidized and Cu-plated/oxidized samples. The reduction in ΔSM is due to the decrease of the weight fraction of ErCo2 main phase in the ErCo2 particle after Cu plating and/or oxidation treatments. As the ΔSM data in the present study are shown in unit of J per K per total mass of ErCo2 particles, its magnitude decreases proportionally to the weight fraction of the ErCo2 phase. From the microstructural analysis results shown in Fig. 3, we obtained an average thickness of each phase by measuring 3–5 particles and evaluated the weight fraction of each phase in the ErCo2 particle: the innermost ErCo2 phase, the Er–Co–O layer, the Co–O layer, and the outermost CuO layer (Fig. S7). The weight fraction of the ErCo2 phase decreased by approximately 29% after Cu plating and oxidation, which roughly corresponds to the 30% decrease in magnetic entropy change. For the oxidized-only sample, we also confirmed that the decrease of the weight fraction of the ErCo2 phase corresponds to the decrease in magnetic entropy via the same process (Fig. S8). These results indicate that while the magnetocaloric effect per unit weight after oxidation or Cu plating–oxidation treatments is smaller, it maintains a high intensity of this effect and effectively prevent hydrogen-induced deterioration of the magnetic properties of ErCo2.

The changes in the crystal structure of the ErCo2 particles during the Cu plating–oxidation treatment was evaluated through SXRD. Fig. 2a presents the SXRD profiles of the as-homogenized, Cu-plated, and Cu-plated/oxidized ErCo2 particles before and after hydrogen exposure. For the as-homogenized particles, most peaks corresponded to the ErCo2 phase (cubic, with space group symmetry Fd3̅m), accompanied by weak peaks indexed to the ErCo3 (hexagonal, R3̅m) and HT- & LT-Er2O3 (Ia3̅ and C2/m) phases. Qualitative analysis using the reference intensity ratio revealed the mass percentages of ErCo2, ErCo3, and Er2O3 (Sum of HT- and LT-phase values) to be 94%, 5.5%, and 0.5%, respectively (Fig. S9). The trace amounts of the Er2O3 phase likely resulted from the formation of a natural surface oxide layer upon air exposure, as confirmed by cross-sectional analyses via SEM and energy-dispersive X-ray spectroscopy (EDS; Fig. S10).

After Cu plating, in addition to the aforementioned phases, trace amounts of the metallic Cu phase (cubic, Fm3̅m) were detected. Following the oxidation of the Cu-plated sample, ErCo2 remained as the main phase, while the metallic Cu phase disappeared and trace amounts of CuO (C2/c) and CoO (cubic, Fm3̅m) were observed. A magnified view of 2θ = 17°–22.5° clearly revealed the presence of a 111 peak after Cu plating and 111 and 111 peaks after oxidation (Fig. 2b). After hydrogen exposure, the SXRD profiles of the Cu-plated/oxidized particles remained unchanged, indicating that the oxide phases persist even after hydrogen exposure at 1.27 MPa and ~296 K. This reveals that CuO is not reduced to its metallic state under these conditions, which is consistent with previous reports suggesting that CuO is difficult to be reduced at room temperature39,40. The unit lattice schematics of the identified phases are shown in Fig. 2c. These results indicate that small amounts of oxides (CuO, CoO, and LT-Er2O3) were formed after Cu plating and the subsequent oxidation; however, the primary phase remained as ErCo2.

The surface and cross-sectional morphologies of the as-homogenized ErCo2 particles were examined through SEM. Fig. 3a shows a secondary electron (SE) image revealing surface irregularities, and Fig. 3b shows a backscattered electron (BSE) image indicating a largely uniform composition, except for some small dark contrast areas. EDS identified these areas as Co-rich regions (Fig. S10), which likely corresponded to the ErCo3 phase, as supported by the SXRD results. EDS line analysis along the particle diameter revealed a thin Er- and O-enriched surface layer, suggesting the presence of a natural Er oxide film (Fig. S10).

ErCo2 particles with Er and Co oxide surface layers were prepared via the oxidation treatment. Fig. 3c shows the SE image and EDS line analysis results of an oxidized particle cross-section, revealing a two-layered structure. The outermost layer (1–2 μm thick) exhibited a dark contrast and consisted mainly of Co oxide. Beneath it, a 5- to 20-μm-thick intermediate layer appeared brighter than the outermost layer but darker than the interior. EDS indicated the presence of Er and Co oxides in this layer, suggesting the partial oxidation of ErCo2. Cracks were observed within some areas of this oxide layer (Fig. 3d), indicating its brittleness and lack of densification.

ErCo2 particles with a CuO surface layer were prepared via the Cu plating–oxidation treatment. To ensure uniform plating, brief pretreatment with dilute hydrochloric acid was conducted before Cu deposition, forming an oxide layer with a larger atomic ratio of Co/Er compared with that of subsurface region (Fig. S11). The dissolution of Er was more pronounced than that of Co, leading to a higher Co concentration on the surface. After Cu plating, a 10 to 15-μm-thick dark contrast layer was formed (Figs. 3e and 3f). EDS identified Cu as the dominant element in the outermost region, with Co, O, and Er in the underlying region within this layer (Fig. 3g). The Co/Er ratio in the underlying region (1.9) was higher than that in the adjacent interior (1.2), likely because of the acid pretreatment.

Subsequent oxidation of the Cu-plated particles led to further structural changes. The subsurface layer formed beneath the surface layer with dark contrast exhibited an intermediate contrast (Figs. 3h and 3i). EDS analysis (Fig. 3j) confirmed that the outermost layer primarily consisted of CuO, whereas the underlying layer contained CoO with minor Er oxides. The subsurface layer contained Co, Er, and O, with an O concentration (32 at%) lower than that in the surface layer (41 at%) but significantly higher than that in the interior (Fig. 3g), indicating the presence of distinct Er and Co oxides.

The surface layer structures of the Cu-plated/oxidized particles were further analyzed through TEM. Fig. 4a presents a high-angle annular dark-field scanning TEM (HAADF-STEM) image with EDS elemental mapping, which reveals a continuous CuO outer layer on top of an underlying Co-rich oxide layer with minor Cu and Er oxides. Fig. 4b shows the TEM image of the selected region, with selected-area electron diffraction (SAED) patterns confirming the crystallinity of CuO and CoO. Fig. 4c presents the high-resolution TEM (HRTEM) image and the fast Fourier transform (FFT) pattern of the CuO layer, indicating well-crystallized CuO(111). These results indicate that a well-crystallized CuO layer with large crystallites forms on the outer surface, whereas the subsurface Co-, Er-, and Cu-oxide layers exhibit some missing regions, which could be due to a weak interface between the subsurface oxide layer and the outmost surface of the CuO layer.

Hydrogen permeation involves adsorption, dissociation, and diffusion41-43. To determine the rate-limiting step for ErCo2 with the CuO surface layer, ToF-SIMS was conducted to evaluate the deuterium distribution in the surface oxide layer after exposure to D2. Fig. 5a shows the ToF-SIMS depth profiles of 167Er, 59Co, 16O, 2H, and 1H/100 in the oxidized ErCo2 particles after D2 exposure. In the outermost ~3 µm, the peak intensity of 59Co was higher, that of 167Er was lower, and that of 16O was higher than that of the deeper layers (~40 µm), suggesting the formation of a Co-enriched oxide layer, consistent with SEM results (Fig. 3). Beneath this layer, up to ~20 µm, the peak intensities of 167Er, 59Co, and 16O were relatively constant, whereas with further increase of the depth, the peak densities of 167Er and 16O decreased, while that of 59Co remained constant, indicating the presence of an Er-rich oxide layer—again consistent with the SEM results (Fig. 3). The calculated D concentration in the CoO layer significantly reduced, reaching nearly zero at the interface of the next layer (Fig. 5b), suggesting that deuterium was primarily blocked by the CoO layer.

Fig. 5c shows the ToF-SIMS depth profiles of 167Er, 59Co, 16O, 63Cu, 2H, and 1H/100 in the Cu-plated/oxidized ErCo2 particles after D2 exposure. The outermost surface exhibited strong signals for 63Cu, 2H, and 1H/100, along with a relatively high 16O intensity. Moreover, the 59Co intensity was at similar levels and the 167Er signal was weak compared to their intensities at deeper layers (~40 μm). With an increase in depth, the intensities of the 63Cu, 2H, and 1H/100 signals rapidly decreased, while those of the 167Er, 59Co, and 16O signals increased, reaching a maximum at ~4 μm. Beyond this depth, the 59Co signal intensity gradually declined, while the 167Er and 16O signal intensities remained constant up to ~20 μm, after which all intensities declined. These findings align with the SXRD, SEM, and TEM results, confirming the formation of a CuO outer layer (several micrometers thick) followed by mixed Cu–Co–Er oxide layers. Fig. 5d shows that the D concentration of the CuO layer significantly decreased and reached zero in the underlying mixed oxide layer. This suggests that the CuO layer prevents hydrogen permeation, with additional blocking from the mixed Cu–Co–Er oxides.

These results indicate that both CoO and CuO have a significant inhibition effect on hydrogen permeation. However, as shown in Fig. 1, the oxidized particles with the Co(Er)O surface layer did not allow sufficient hydrogen permeation blocking. This was likely due to the presence of fine cracks in the Co(Er)O oxide layer formed on some particles (Fig. 3d), which resulted in hydrogen permeation into particles. In contrast, the CuO layer was denser (Fig. 4) and exhibited a stronger hydrogen blocking effect.

DFT calculations were performed to further investigate the hydrogen adsorption, dissociation, and diffusion on the CuO(111) surface. The adsorption energies and optimized geometries were calculated for an H2 molecule at various sites on CuO(111), including three-coordinated O (hereafter denoted as O3) and Cu (as Cu3), four-coordinated O (as O4) and Cu (as Cu4), 11 bridge sites, and 8 hollow sites (Fig. S12a). Five molecular orientations were considered: one perpendicular to the CuO(111) surface two parallel, and two tilted approximately 45° toward the z-axis from the two parallel orientations (Fig. S12c). Initially, H2 was positioned ~1.5 Å from the surface, with an H–H bond length of 0.74 Å. After optimization, the H2 molecule moved away from the surface (2.8–6 Å) (Fig. 6a), with minimal bond length change (0.74–0.75 Å) and low adsorption energies (−2.71–2.36 kJ/mol) (Table S2_1 and S2_2). For example, a H2 molecule initially at the Cu4–Cu4 bridge site (shown as ⑩ Bridge-Cu4-Cu4 in Fig. S12a) and vertical to the bridge direction was set 1.7409 Å from the nearest surface atom (Fig. 6b) and moved to 3.6999 Å from the nearest surface atom after optimization. In contrast, the H–H bond length changed only from 0.7406 to 0.7497 Å (Fig. 6c), and the adsorption energy was only 0.68 kJ/mol (Table S2_1). The results indicate that H2 prefers physisorption and requires an energy barrier for chemisorption and dissociation on the CuO(111) surface.

The activation barriers for H2 dissociation at the optimized sites were calculated for two cases: (i) both H atoms adsorbed onto adjacent O3 atoms (forming two H–O bonds) and (ii) one H atom adsorbed onto O3 and the other onto Cu3 (forming H–O and H–Cu bonds). The activation barriers ranged from 27.7 to 152.4 kJ/mol for H–O bond formation and from 40.4 to 63.8 kJ/mol for H–O and H–Cu bond formation (Fig. S13). The lowest activation energy (27.7 kJ/mol) was found at the Cu4–Cu4 bridge site for H–O bond formation, while 40.4 kJ/mol was the lowest for H–O and H–Cu bond formation. This suggests that H2 dissociation favored H–O bond formation (Fig. 6d). In contrast, the diffusion barrier for H migration from H–Cu to H–O was much lower (13.5 kJ/mol) (Fig. 6e), suggesting that H atoms diffuse easily on the CuO surface. Moreover, the energy barrier for H diffusion from the surface to subsurface was 337.4 kJ/mol (obtained from the difference between the total energy of TS (240.8 kJ/mol) and that of the H–O/H–O absorbed surface state (–96.6 kJ/mol, Fig. 6e), indicating that subsurface diffusion is rate-limiting.

The effects of H2 molecule surface coverage on H2 adsorption structure on CuO (111) surface were examined by setting various surface coverage rates of H2 molecule on CuO(111) surface. The results revealed that H2 molecules moved away from the surface at all surface coverage rates (0.0625–1 ML, Fig. S14). The effect of H-atom surface coverage on H adsorption structure on CuO(111) were also examined by setting various surface coverage rates of H atom on CuO(111) surface. The results revealed that H atoms most readily adsorb onto O3 atoms, followed by Cu3 atoms with the increase of H coverage (Fig. S15). The adsorption energy of a H atom on the CuO (111) surface decreased with the increase of H coverage up to 0.5 ML (Fig. S16). Furthermore, when the H surface coverage exceeds 0.5 ML, additional H atoms can no longer adsorb onto the surface. They detach from the surface and form hydrogen molecules (Figs. S15 c.4 and d.4).

The hydrogen atom solid solubility limit in CuO was evaluated by calculating the solution energies when inserting H atoms sequentially up to 9 atoms into interstitial octahedral sites in a larger slab model comprising 256 atoms (Figs. S17 and S18). The H solubility at ΔG_sol = 0 was determined as 0.01377 at% by extrapolation, indicating an extremely low H solubility in CuO at 298 K and 1.27 MPa (Fig. S19). These DFT results align with the ToF-SIMS findings (Fig. 5d), revealing that the CuO layer effectively blocks hydrogen permeation.

We successfully synthesized an ErCo2 magnetic refrigeration materials with excellent HPB performance at 1.27 MPa and ~296 K through electroless Cu plating followed by oxidation. The CuO layer effectively blocked hydrogen permeation because of its high activation energy for H diffusion (~337 kJ/mol). This low-cost and efficient approach can be applied to other materials that require HPB properties near room temperature.

Methods
Fabrication of ErCo2 spherical particles
A master alloy ingot of ErCo2 was prepared using high-frequency vacuum melting equipment with pure raw materials of Er (99.9%, Nippon Yttrium Co., Ltd.) and Co (99.9%, Sumitomo Metal Mining Co., Ltd.). ErCo2 spherical particles were then fabricated from the ingot using an EIGA process33-35. The ingot, serving as an electrode rod, was inductively melted without a crucible, enabling the production of high-purity spherical particles. After atomization, the particles were sieved, and those with diameters of 212–355 μm were collected and homogenized at 1123 K for seven days in Ar gas atmosphere for subsequent experiments.

Oxidation treatments and electroless Cu plating
Oxidation treatments were conducted on both the as-homogenized and Cu-plated particles using a box-type electric furnace in air. The oxidation conditions were varied within the ranges of 473 to 773 K and 0.1 to 0.5 h. Oxidation at 773 K for 0.5 h was more effective for forming the CuO layer. The samples were heated to 773 K at a rate of ~15 K/min, held at this temperature for 0.5 h, and then slowly cooled to room temperature in the furnace.

Electroless Cu plating was performed using a plating solution with the composition listed in Table S3, at a pH of 12–12.5. The plating was conducted at 248 K for 1.5 h with magnetic stirring. Prior to plating, the ErCo2 particle surfaces were cleaned by immersion in 4 vol% HCl aqueous solution at 298 K for 20 s, followed by rinsing with purified water. This pre-cleaning step was essential for ensuring uniform Cu deposition, as some particles failed to be plated without it (Fig. S20). SEM and EDS analyses revealed that a very thin Er–Co oxide layer existed on the surface of as-homogenized ErCo2 particles, formed during the homogenization heat treatment process or subsequent storage at room temperature in air. This oxide layer is believed to affect the copper plating process and need to be removed by pre-cleaning. The short pre-cleaning time of 20 s was used to avoid unnecessary sample elution (Table S4).

Evaluation test of HPB performance
The HPB performance of ErCo2 particles, both before and after Cu plating, was evaluated using a Sieverts-type instrument (Suzuki Syokan Co., Ltd.). A schematic of the hydrogen exposure system and detailed experimental setup are provided in the Supporting Information (Fig. S1). For each measurement, 0.5 g of particles was used. Hydrogen gas was introduced at an initial pressure of ~1.27 MPa after vacuuming with a rotary pump for at least 15 min. The average relative error of the pressure gauge (PG-50KU-F, KYOWA Electronic Instruments Co., Ltd., Japan) is 0.5%. The calibration sheet of the gauge shows its rate output is 1997 μV/V, and the calibration constant is 0.00245 MPa/ 1μV/V. The pressure change in the sealed cell was monitored for up to seven days at 293–298 K to assess the HPB properties of the particles. We performed the D2 exposure experiment at the same temperature and pressure as that for the hydrogen exposure experiments, and the duration was also very close, i.e., 137.5 h for oxidized particles and 139 h for Cu-plated/oxidized particles.

The amount of hydrogen adsorbed by the ErCo2 sample (Δn, mol) at each time point was calculated using Equation (1), derived from the ideal gas law:
∆𝑛 = (𝑃0𝑉 / 𝑅𝑇0) − (𝑃𝑖𝑉 / 𝑅𝑇𝑖) (1)
where P0 and T0 are the initial hydrogen pressure (Pa) and temperature (K) in the cell, respectively, while Pi and Ti represent the pressure and temperature at the i-th measurement. V is the total volume of the cell, reservoir, and gas pipeline (m³), and R is the molar gas constant (8.314462 J K⁻¹ mol⁻¹).

The hydrogen absorption capacity of the ErCo2 particles was calculated as:
Absorbed hydrogen (mass%) = weight of absorbed hydrogen / weight of ErCo2 × 100 (2)

Characterization
The morphology and composition of ErCo2 particles, before and after Cu plating and oxidation treatments, were analyzed using SEM (JEOL, JSM-7000F, JSM-6500F) and TEM (JEOL, JEM-ARM200F) coupled with X-ray energy-dispersive spectroscopy. Cross-sectional SEM samples were prepared by mechanically polishing the particles affixed to a conductive adhesive. TEM samples were prepared from SEM cross-sections using a focused ion beam technique, with details provided in the Supporting Information (Fig. S21). The crystalline structures of ErCo2 particles at various stages of treatment were analyzed via SXRD (SPring-8, BL02B2). The wavelength of the incident beam (λ) was 0.775980 Å.

To assess the effects of Cu coating and oxidation on the magnetic properties of ErCo2 particles, temperature-dependent magnetization (M–T) measurements were conducted using a Quantum Design SQUID magnetometer. Measurements were taken from 2 to 60 K under applied magnetic fields ranging from 0.001 to 5 T. All the M(T) measurements were performed under field-cooling processes. The temperature sweep rate was 0.6 K per min; the mass of the samples was approximately 3–4 mg. Demagnetization corrections were not performed in this study because it is difficult to evaluate the exact demagnetization factors for a group of the particles. The demagnetization effect does not significantly affect the evaluation of ΔSM for large magnetic fields such as 5 T44. The magnetic entropy change (∆SM) was calculated using the following equation45.
−∆𝑆𝑀 = −𝜇0 ∫ (𝜕𝑀 / 𝜕𝑇) 𝑑𝐻 (3)
where μ0 is permeability of vacuum, H is external magnetic field, T is temperature, and M is magnetization. The measurement error of ΔSM is mainly from the weight error of the particle samples used in the measurements, which is estimated to be approximately 1–2%.

ToF-SIMS analysis of the particles after deuterium exposure test was performed using a time-of-flight secondary ion mass spectrometer (PHI, TRIFT V nanoTOF). A Ga emitter with 30 kV was used to sputter the particle surface. Deuterium (an isotope of hydrogen) was selected because it exhibits diffusion behavior similar to that of hydrogen46,47 and has a detection limit 1–2 orders of magnitude lower than that of hydrogen in ToF-SIMS48,49. D2 also eliminates interference from trace hydrogen originally present in the sample and chamber. The D concentration was derived from the 2H signal by subtracting the background signal of 1H/100 intensity, which was confirmed to appear at the 2H position in all measurements.

DFT calculation
The adsorption energies and the dissociation energy of molecular hydrogen on the CuO (111) surface were calculated using the PHASE0 first-principles program package based on DFT (https://azuma.nims.go.jp/)50. The exchange-correlation energy was described using the generalized gradient approximation (GGA)51-53. A slab model comprising 128 atoms in four layers (4×8 atoms per layer) was used with a vacuum layer of 15 Å to prevent interactions between periodic images. The Brillouin zone was sampled using a 2×2×1 k-point mesh, with cut-off energies of 450 eV for the wave function and 4050 eV for the charge density. The state determination was performed using a convergence criterion of 5×10–6 eV for total energy and a force criterion of 0.01 eV/Å for force action.

The zero-point energy (ZPE) correction was performed by calculating vibration modes using the linear response function of PHASE0 after optimizing the slab model and each adsorption state, and the ZPE energy was calculated as follows:
ZPE = 1/2 ∑𝑛 (ℎ𝛾𝑖) (4)
where 𝛾𝑖 is the frequency of each actual value vibration mode, n is the total numbers of actual value vibration mode, and h is Planck constant (4.135667696×10-15 eV·s). By this method, the ZPE value of free H2 molecule was calculated to be 0.273436 eV, which agrees well with the literature value54.

The adsorption energy of a single molecule on the surface (Eads) was calculated as follows:
𝐸𝑎𝑑𝑠 = (𝐸𝑠𝑙𝑎𝑏+ZPE𝑠𝑙𝑎𝑏)+(𝐸𝐻2+ZPE𝐻2)−(𝐸𝐻2/𝑠𝑙𝑎𝑏+ZPE𝐻2/𝑠𝑙𝑎𝑏) (5)
where 𝐸𝑠𝑙𝑎𝑏 is the total energy of the slab, 𝐸𝐻2 is the total energy of an isolated hydrogen molecule, 𝐸𝐻2/𝑠𝑙𝑎𝑏 is the energy of the slab with adsorbed hydrogen, ZPE𝑠𝑙𝑎𝑏 is the ZPE of the slab, ZPE𝐻2 is the ZPE of a free H2 molecule, and ZPE𝐻2/𝑠𝑙𝑎𝑏 is the ZPE of the slab with adsorbed hydrogen. A larger Eads indicates stronger hydrogen adsorption.

The activation energies for H2 dissociation and atom diffusion on CuO(111) surface were determined using the climbing image nudged elastic band method55. The transition states were determined by interpolating 6 images between the initial and final states. The transition state determination was performed using a convergence criterion of 5×10–6 eV for total energy and a force criterion of 0.01 eV/Å for force action. The k-point mesh was 2×2×1. The cut-off energies were 450 eV for the wave function and 4050 eV for the charge density.

The effects of H2 surface coverage on H2 adsorption structure on CuO (111) surface were examined by setting various surface coverage rates of H2 molecules on CuO(111) surface (0.0625–1 ML) (Fig. S14). In the initial state, the H2 molecules were placed at positions approximately 1.5 Å above each outermost O or Cu atom, with their orientation parallel to the X-axis. The effect of H surface coverage on H adsorption structure on CuO (111) surface were examined by setting various surface coverage rates of H atom on CuO(111) surface (0.0625–1 ML) (Fig. S15). In the initial state, the H atoms were placed at positions approximately 1.5 Å above each outermost O or Cu atom. The slab model and calculation conditions for these surface coverage effects were the same as that for the single H2 molecule on the CuO(111) surface.

The hydrogen atom solid solubility limit in CuO was evaluated by calculating the solution energies as follows when inserting H atoms sequentially up to 9 atoms into interstitial octahedral sites in a larger slab model comprising 256 atoms (Fig. S17). Structural optimization of the slab model was first performed with all Cu and O atoms movable. Subsequently, the total energies of each state with solid solution H atoms were calculated by fixing the Cu and O atoms in the optimized model, and only the H atoms being movable. The Brillouin zone was sampled using a 1×2×3 k-point mesh, with cut-off energies of 450 eV for the wave function and 1800 eV for the charge density. The state determination was performed using a convergence criterion of 1×10–5 eV for total energy and a force criterion of 0.01 eV/Å for force action. The solution energy for solid solution of H atoms into CuO (∆𝐺𝑠𝑜𝑙) was calculated as follows:
∆𝐺𝑠𝑜𝑙 = (𝐸(𝐶𝑢𝑂+𝑛𝐻)+ZPE(𝐶𝑢𝑂+𝑛𝐻))−(𝐸𝐶𝑢𝑂+ZPE𝐶𝑢𝑂)−𝑛/2(𝐸𝐻2+ZPE𝐻2+𝜇𝐻2(𝑇,𝑝)) (6)
where n is the numbers of H atom (n = 1~9), 𝐸(𝐶𝑢𝑂+𝑛𝐻) and ZPE(𝐶𝑢𝑂+𝑛𝐻) are the total energy and ZPE of CuO with solubilized H atoms, 𝐸𝐶𝑢𝑂 and ZPE𝐶𝑢𝑂 are the total energy and ZPE of CuO, 𝐸𝐻2 and ZPE𝐻2 are the total energy and ZPE of free H2 molecule, and 𝜇𝐻2(𝑇,𝑝) is the chemical potential of gaseous H2 contributed by temperature (T) and pressure (p) as follows.
𝜇𝐻2(𝑇,𝑝) = 𝜇𝐻2(𝑇,𝑝°)+𝑘𝐵𝑇𝑙𝑛(𝑝/𝑝°) (7)
The H concentration (𝑐𝐻) was estimated as follows.
𝑐𝐻 ≈ exp (−∆𝐺𝑠𝑜𝑙 / 𝑘𝐵𝑇) (8)
where 𝑘𝐵 is Boltzmann’s constant (1.380649×10–23 J/K), T is absolute temperature (K), p° is the reference pressure, and p is the actual pressure. Moreover, 𝜇𝐻2(𝑇,𝑝°) is the chemical potential of H2 at the reference pressure (1 atm) which were obtained from the NIST standard reference database (https://doi.org/10.18434/T4D303).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The manuscript is generally well written and presents some interesting findings. However, the interpretation of certain results and their correlation with the material properties require more in-depth analysis and clarification. Additionally, the manuscript appears to be narrowly targeted at specialists in magnetocaloric materials for hydrogen liquefaction, rather than the broader, interdisciplinary audience of Nature Communications. After careful evaluation, I do not find the overall quality and novelty of the work sufficient for publication in Nature Communications. Instead, the paper would be more suitable for a specialized journal focused on magnetocaloric materials, pending substantial revision. Below are detailed comments:\n1. Questionable Novelty of the Work\nThe novelty claimed by the authors is not sufficiently justified. In both the introduction and the final paragraph before the Methods section, the authors state: “we demonstrated a novel method for synthesizing ErCo₂ magnetic refrigeration materials with excellent HPB performance.” However, the synthesis method employed — EIGA — is a well-established technique, as acknowledged in the authors’ own references (26–28). Similarly, electroless Cu plating is a well-known method.\nFurthermore, the use of Cu or CuO as hydrogen permeation barriers is not new. For instance: Liu et al. (Corrosion Science 221 (2023) 111314) demonstrated the effectiveness of Cu coatings on carbon steels to reduce hydrogen permeation. And studies involving CuO in combination with other oxides for hydrogen protection of cobalt composite membranes also exist (https://doi.org/10.4028/www.scientific.net/DDF.297-301.483).\nThe mechanism proposed in the manuscript — that Cu-based coatings act as hydrogen barriers — is consistent with prior studies. Therefore, it is unclear how the authors justify the novelty of their approach. Although the results may be of interest to researchers in hydrogen liquefaction, the current level of innovation does not meet the standard of Nature Communications.\n2. Inadequate Evaluation of Magnetocaloric Performance\nThe manuscript provides very limited data on the magnetocaloric properties of the materials. For example:\n• Only M–T curves under a low magnetic field are presented. Why are there no M–T curves under varying fields up to 5 T?\n• Key performance indicators such as thermal hysteresis changes, adiabatic change of temperature, or consideration on how heat exchange is influenced after surface modifications are not discussed.\n• The microstructural data indicate that the protective surface layer is under 10 µm, while the particles are approximately 0.3 mm in diameter. How does such a thin layer lead to a 30% reduction in magnetic entropy change? The authors’ explanations and microstructural evidence do not convincingly support the observed magnetocaloric behavior.\n3. Questionable Microstructural Characterization and Interpretation\nThere are inconsistencies and ambiguities in the microstructural analysis:\n• Lines 180–182 claim that “a 20–30 µm thick intermediate layer appeared brighter than the outermost layer but darker than the interior,” yet this is not evident in Figure 3.\n• The description of the TEM data suggests the formation of “dense CuO with large crystallites,” but the images do not provide sufficient evidence to assess density or crystallite size. Drawing such conclusions from limited local TEM observations is problematic.\n• The authors claim material loss during TEM sample preparation due to brittleness. However, numerous studies have successfully prepared brittle materials for TEM without such loss. The missing regions could be due to a weak interface between the oxide and underlying layers, rather than sample preparation artifacts.\n4. Incomplete and Unclear Hydrogen Diffusion Analysis\nThe methodology for assessing hydrogen diffusion is inadequately described:\n• The D₂ exposure conditions (e.g., duration, temperature, pressure) are not clearly specified. If they differ from the hydrogen exposure conditions, how can the results be directly compared?\n• Why were alternative techniques not employed to quantify hydrogen content and distribution immediately following hydrogen exposure?\n• Figure 1 shows some hydrogen absorption even in the Cu-plated/oxidized sample. However, the correlation between hydrogen absorption data and the D₂ exposure/ToF-SIMS measurements is unclear.\nIn conclusion, the manuscript does not meet the standards for publication in Nature Communications. The novelty is not convincingly demonstrated, key material characterizations are insufficient or inadequately interpreted, and the relevance of some experimental results is not clearly established. I recommend reconsideration for a more specialized journal, contingent upon addressing the points outlined above.", "reviewer_2": "This manuscript contains some interesting results, which may be useful for researchers working in this field. Generally the article is well organized, the authors quite well refer to other literature data.\nIn my opinion, this article is suitable for publication. However, there are some minor attention which must be addressed before the final decision. Below I present some minor comments.\n1. Is ΔSM the only parameter considered when selecting a suitable magnetocaloric material? What about ΔTad ? I believe that the introduction should present the magnetocaloric properties of the studied compound in detail — that is, all available data from the literature, not only ΔSM .\n2. In what type of cooling cycle is the proposed refrigerant intended to be used? It might be worth enriching the introduction with reports on other types of magnetocaloric materials, for example, in layered form. It should be added in the introduction that in various forms magnetocaloric materials as promising materials for helium or hydrogen liquefaction are studied, e.g. as composites, for example pleas take a look at recent work: DOI: 10.1039/d4tc02247e or doi.org/10.1016/j.ijhydene.2024.09.082\n3. Under what conditions was M(T) measured? ZFC or FC?\n4. The temperature sweep rate for the presented M(T) curves should be given. What was the mass of the samples for magnetic measurements ? Were isothermal magnetization curves for the samples studied corrected due to the internal demagnetizing field ?\n5. The presented ΔSM(T) dependencies generally indicate that ΔSM decreased after applying the proposed barrier. This should be clearly emphasized. Moreover, the figure shows that after exposure, ΔSM increases. This fact should be explained. What was the measurement error of ΔSM?\n6. Fig.1a no description of the insert.\nI will make a further decision after responding to the above comments.", "reviewer_3": "In this manuscript titled “Enhancing hydrogen permeation barrier performance of ErCo2 magnetic refrigeration material via surface oxide layer formation”, the authors investigate ErCo2 as a magneto-caloric material for magnetic refrigeration, with potential application in liquid hydrogen production. Due to their small atomic radius, hydrogen atoms can easily diffuse into host materials, leading to property degradation and, in severe cases, hydrogen embrittlement. To address this, the authors explore the hydrogen permeation barrier (HPB) performance of ErCo2 particles by applying electroless copper plating followed by oxidation, resulting in a CuO layer several micrometers thick. This surface modification is shown to effectively suppress hydrogen ingress and maintain the structural integrity of the material. The study addresses an important challenge in the development of hydrogen-compatible magnetic refrigeration materials and provides useful data regarding coating-based hydrogen permeation barrier (HPB) strategies. While the manuscript reports encouraging results, the methods and techniques employed are largely established, and thus the level of technical innovation is limited. Unless these substantive issues are addressed clearly, the manuscript does not reach the level to be considered for publication in Nature Communications.\n1. Why was electroless Cu plating chosen? The authors should explain why coatings with inherently low hydrogen diffusivity (e.g., Al2O3, Y2O3, Er2O3, La2O3, or SiO2) were not used. It is not enough to say that PVD/CVD methods struggle to form uniform HPB layers on sub-500 μm ErCo2 particles, please provide a thorough explanation/justification. Moreover, electroless Ni or Co plating is also feasible.\n2. The authors should conduct further experimental tests on H2 permeation of the Cu-plated/oxidized particles, including pressure–composition–temperature (PCT) curves, intrinsic hydrogen diffusion coefficients, hydrogen permeation flux measurements, and in particular long-term hydrogen permeation stability.\n3. The manuscript does not provide a systematic analysis of how different Cu coating thicknesses and the formation of surface oxides thicknesses influence hydrogen permeation behavior, nor does it address the potential impact on the magnetocaloric effect of ErCo2. Include thickness-dependent investigations to better understand the impact on functional performance.\n4. Why did the authors choose an initial hydrogen pressure of approximately 1.27 MPa, rather than a different or higher value (e.g., 3 MPa)? Is there a specific thermodynamic or kinetic consideration behind this choice?\n5. In Fig. 1a and 1b, the H₂ absorption curves for the oxidized ErCo2 particles and the Cu-plated/oxidized particles do not intersect under the presented exposure times. Could the authors extend the H2 exposure duration to check whether these curves might intersect or partially overlap at longer times? Additionally, would simply oxidizing the ErCo2 surface (without Cu plating) be sufficient to slow hydrogen permeation while causing only a minimal decrease in magnetization and magnetic entropy change?\n6. In lines 102 ~ 104, the authors infer that ErCo2 particles were fully penetrated and completely transformed into hydrides based solely on stereomicroscope observations of particle fragmentation. This conclusion may be premature, as morphological changes alone are insufficient to confirm complete hydrogenation.\n7. Why weren’t the magnetization and magnetic entropy change measured for the oxidized ErCo2 particles or those exposed to H2, as shown in Fig. 1c and 1d? These data would allow a clearer comparison with the as-homogenized and Cu-coated/oxidized samples, and help clarify how surface oxidation affects magnetocaloric performance.\n8. After the Cu plating–oxidation treatment on ErCo2, the magnetic entropy change (ΔSM) decreased significantly from 33 J·kg⁻¹·K⁻¹ to 24 J·kg⁻¹·K⁻¹. What is the underlying reason for this reduction? Furthermore, is it possible to design surface treatments (e.g., coatings or modifications) that can preserve or even enhance the magnetic entropy change while still effectively preventing hydrogen embrittlement?\n9. For the SEM (JEOL JSM-7000F, JSM-6500F) and TEM (JEOL JEM-ARM200F) characterizations coupled with X-ray EDS, could the authors please specify the accelerating voltage and beam current used?\n10. The manuscript mentions that TOF-SIMS analysis was performed using a 30 kV Ga+ ion source for sputtering the surface of ErCo2 particles. However, the primary ion beam current was not reported. Could the authors clarify the beam current used during analysis? This parameter critically affects sputtering rate, signal sensitivity, and surface damage, especially when analyzing non-planar particle surfaces.\n11. In the DFT calculations, the authors did not specify the energy convergence threshold and the force convergence criterion used during the slab model optimization. Similarly, the convergence settings applied in the CI-NEB (climbing image nudged elastic band) calculations were not given. It is recommended that the authors provide these critical computational parameters to ensure the reproducibility and reliability of the results.\n12. In line 187, the statement “forming an initial Co-rich oxide layer (Fig. S5)” seems incorrect, because the oxide layer is more likely Er-rich according to the EDS results shown in Fig. S5c.\n13. In line 230, the sentence “while the 59Co intensity remained at similar levels” appears to be inaccurate. According to Fig. 5c, the ToF-SIMS depth profile of 59Co shows an initial increase followed by a decrease as the analysis progresses into deeper layers. The authors are advised to revise this description to more accurately reflect the observed trend.\n14. In Fig. 5b, there appears to be a typo in the y-axis label — “od” should likely be “of”. The authors are advised to correct this for clarity and accuracy.\n15. In lines 221–223, the statement “Beneath this layer, up to ~20 µm, the peak intensities of 167Er and 16O increased, while that of 59Co decreased” appears to be inaccurate. According to the ToF-SIMS depth profiles shown in Fig. 5b, the intensities of 167Er, 16O, and 59Co seem to remain relatively constant or slightly decrease in the 3–20 μm depth range. Please have the author confirm again.\n16. In lines 234–235, the authors state, “while the 167Er and 16O signal intensities remained constant up to ~20 μm.” However, in Fig. 5c both 67Er and 16O intensities clearly show a gradual decline with increasing depth up to ~20 μm. Could the authors please clarify or correct this description so that it accurately reflects the observed trend?\n17. In Fig. 5d, the normalized D (deuterium) yield for the Cu-plated/oxidized ErCo2 particles shows fluctuations between approximately 15 μm and 26 μm depth, whereas one would expect the D signal to be zero beyond the oxidation layer. Could the authors please explain the origin of these D‐signal fluctuations in that depth range and clarify why D is still detected where it should theoretically be absent?\n18. In the DFT calculations, only the adsorption and dissociation of a single H2 molecule on the CuO(111) surface were considered. However, under experimental conditions multiple H₂ molecules adsorb, dissociate, and diffuse concurrently, leading to coverage‐dependent interactions. Could the authors discuss how multi‐molecule effects (e.g., surface coverage, H–H lateral interactions, and diffusion pathways) might alter the adsorption/dissociation energetics? Would considering cluster models, larger supercells, or molecular dynamics simulations be necessary to capture these effects?\n19. To accurately predict H₂ adsorption and dissociation on CuO(111), zero-point energy corrections should be included.\n20. In the subtitle of Fig. 6a, the phrase “Optimized positions of H₂ molecules at all initial adsorption sites (shown in Fig. S4a)” appears to be incorrect—shouldn’t it refer to Fig. S6a instead? Additionally, the wording “showing that all H₂ molecules moved away from the CuO(111) surface, to a distance exceeding 2.6 Å” seems inconsistent with the data, as the distances actually exceed 2.8 Å. Please verify and correct these details.\n21. Three molecular orientations were considered: one perpendicular to the CuO(111) surface and two parallel, but the tilted side-on orientation was not examined. Could the authors comment on whether this tilted geometry might be energetically favorable, and if so, please provide its adsorption energy and optimized structure.\n22. In lines 312~314, for the statement “For each measurement, 0.5 g of particles was used. Hydrogen gas was introduced at an initial pressure of ~1.27 MPa after vacuuming with a rotary pump for at least 15 minutes,” could the authors clarify whether the samples were first purged with an inert gas to remove residual air? Additionally, what vacuum pressure was reached before introducing hydrogen?\n23. In the caption of Fig. 6, the description for subfigure (c) is missing.\n24. DFT calculations indicate a high barrier for H diffusion into the CuO(111) subsurface. However, the authors could also calculate hydrogen solubility in the CuO layer to show that it is low, further supporting its enhanced resistance to hydrogen embrittlement.", "reviewer_4": "This paper reported the formation of oxide layer on the surface of ErCo2 sphere as hydrogen permeation barrier. By comparing the performance of CuO layer and the in situ generated oxide layer of ErCo2, the authors found that CuO is more efficient for hydrogen permeation protection. This finding may provide some valuable insights for protecting the magnetic refrigeration materials from hydrogen embrittlement. But there are still some questions needed to be further clarified.\n1.Some important content that can emphasize the novelty of this work is suggested to be included as follows\n1)Is the hydrogen permeation issue a key challenge faced by the magnetic refrigeration materials?\n2)Are there any other reports on developing the hydrogen permeation barrier for magnetic refrigeration material? If so, how about their performance?\n3)The background for choosing CuO instead of other materials as the hydrogen permeation barrier should be provided, as there are many materials have been reported as efficient hydrogen permeation barriers, such as ZrO2, Er2O3, ɑ-Al2O3, CrxN, et al. These materials have higher chemical stability in the hydrogen environment than CuO. What is the principle for choosing the alternative hydrogen resistant materials for ErCo2? Except the hydrogen permeation resistance, are there any other characteristics or functional properties required by the barriers?\n2.As the hydrogen permeation barrier performance is evaluated by the hydrogen pressure measured in the equipment. The accuracy of the pressure gauge is very important, as the changes of the hydrogen pressure mentioned in the work are 0.01~0.02 MPa, the type of the pressure gauge used and its Instrument detection limit, the average relative error should be given.\n3.It seems that∆SM was decreased with CuO layer on the sphere surface, is the thickness of CuO layer affect the ∆SM? Can the CuO thickness be controlled by the plating-oxidation method? If thinner the layer, will the ∆SM have a higher value?\n4.The authors have tested the performance of the hydrogen permeation barriers under 1.27 MPa H2 at 296 K for 7 days. Is this pressure, temperature and time agreed with the condition faced by ErCo2 spheres when in practical applications? What is biggest pressure and the longest time for the ErCo2 spheres exposure to H2 toward hydrogen liquefaction? And how about the performance of CuO layer under higher pressures or for expanding times?\n5.It is mentioned that the pre-cleaning step was essential for ensuring uniform Cu deposition. What is the reason? A detailed discussion about this phenomenon is absent. As the quality of the CuO layer is the key to determine its performance, therefore, the growth mechanism of Cu on the sphere surface and the method to improve its quality should be demonstrated in the text.\n6.In DFT calculations, line 274 “suggesting that H atoms diffuse easily on the CuO surface”, this conclusion cannot be easily understand. And how could the authors get the result “the energy barrier for H diffusion from the surface to subsurface was 337.4 kJ/mol”? These results need to be clarified more clearly in order to be understandable for the readers.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #1 (Remarks to the Author):\nThe manuscript is generally well written and presents some interesting findings.\nHowever, the interpretation of certain results and their correlation with the material\nproperties require more in-depth analysis and clarification. Additionally, the\nmanuscript appears to be narrowly targeted at specialists in magnetocaloric materials\nfor hydrogen liquefaction, rather than the broader, interdisciplinary audience of\nNature Communications. After careful evaluation, I do not find the overall quality and\nnovelty of the work sufficient for publication in Nature Communications. Instead, the\npaper would be more suitable for a specialized journal focused on magnetocaloric\nmaterials, pending substantial revision. Below are detailed comments:\n1. Questionable Novelty of the Work\nThe novelty claimed by the authors is not sufficiently justified. In both the\nintroduction and the final paragraph before the Methods section, the authors state:\n“we demonstrated a novel method for synthesizing ErCo₂ magnetic refrigeration\nmaterials with excellent HPB performance.” However, the synthesis method\nemployed — EIGA — is a well-established technique, as acknowledged in the authors’\nown references (26–28). Similarly, electroless Cu plating is a well-known method.\nFurthermore, the use of Cu or CuO as hydrogen permeation barriers is not new. For\ninstance: Liu et al. (Corrosion Science 221 (2023) 111314) demonstrated the\neffectiveness of Cu coatings on carbon steels to reduce hydrogen permeation. And\nstudies involving CuO in combination with other oxides for hydrogen protection of\ncobalt composite membranes also exist\n(https://doi.org/10.4028/www.scientific.net/DDF.297-301.483).\nThe mechanism proposed in the manuscript — that Cu-based coatings act as\nhydrogen barriers — is consistent with prior studies. Therefore, it is unclear how the\nauthors justify the novelty of their approach. Although the results may be of interest\nto researchers in hydrogen liquefaction, the current level of innovation does not meet\nthe standard of Nature Communications.\nResponse: We appreciate reviewer #1’s thorough evaluation of our manuscript. We\nacknowledge that the following statement insufficiently justifies the novelty of our\nwork: “we demonstrated a novel method for synthesizing ErCo₂ magnetic\nrefrigeration materials with excellent HPB performance.” The novelty of our work is\non the synthesis ErCo₂ magnetic refrigeration materials with excellent HPB\nperformance using well-established techniques, i.e., EIGA, Cu plating, and oxidation\ntreatment.\nThe literature cited by reviewer #1 (Corrosion Science 221 (2023) 111314) reported\nthe effectiveness of Cu coatings on carbon steels to reduce hydrogen permeation.\nHowever, we confirmed in our study that the Cu coating alone on magnetic\nrefrigeration materials is not effective to reduce hydrogen permeation. In fact, we\ninvestigated the HPB performance of Cu-plated ErCo without oxidation during the\n2\nsame hydrogen exposure test as for the Cu-plated/oxidized ErCo and found that the\n2 2\nparticles, demonstrating that Cu coating alone is insufficient as a hydrogen\npermeation barrier for magnetic refrigeration materials.\nAs for the use of CuO as a hydrogen permeation barrier: although the literature cited\nby the reviewer #1 reported the hydrogen permeation rate of Al O –CuO–ZnO-based\n2 3\ncobalt composite membrane (https://doi.org/10.4028/www.scientific.net/DDF.297-\n301.483), there was no indication that CuO might be a potential hydrogen barrier\nmaterial. To our knowledge, there is no literature reporting CuO as a hydrogen barrier\nmaterial, either experimentally or through theoretical calculations. Therefore, this\nwork is clearly novel.\nWe revised the related description to clarify more the novelty in the Introduction and\nfinal paragraph.\n(In the Introduction)\n“Compared to these coating methods, formation of CuO layer by copper plating followed\nby oxidation treatment offers great advantages in terms of cost and efficiency. CuO has\nnot previously been recognized as an HPB material, and this study has demonstrated for\nthe first time that it possesses sufficient HPB properties near room temperature.”\n(In the final paragraph)\n“We successfully synthesized an ErCo magnetic refrigeration materials with excellent\noxidation.”\n2. Inadequate Evaluation of Magnetocaloric Performance\nThe manuscript provides very limited data on the magnetocaloric properties of the\nmaterials. For example:\n• Only M–T curves under a low magnetic field are presented. Why are there no M–T\ncurves under varying fields up to 5 T?\nResponse: We measured the M–T curves under varying fields up to 5 T for the as-\nhomogenized, oxidized, and Cu-plated/oxidized samples before and after H\n2\nexposure (Fig. r1) to calculate magnetic entropy change ( ) using Eq. 3. In the\nrevised manuscript, we have cited the following M–T curves under various magnetic fields\n∆𝑆𝑀\nup to 5 T for these samples and compiled them in the Supplemental Information (Fig. S ).\nFig. r1. M–T curves under varying field up to 5 T for the as-homogenized ErCo (a),\n2\noxidized ErCo before H exposure (b) and after H exposure (c), and Cu-\n2 2 2\nplated/oxidized ErCo H exposure (d) and after H exposure (e).\n2 2 2\n• Key performance indicators such as thermal hysteresis changes, adiabatic change\nof temperature, or consideration on how heat exchange is influenced after surface\nmodifications are not discussed.\nResponse: We certainly recognize the importance of evaluating key performance\nindicators beyond magnetic entropy change (ΔS ), as the reviewer pointed out.\nM\nHowever, this manuscript focuses on enhancing the HPB performance of\nmagnetocaloric materials and evaluate the HPB performance of the prepared\nsamples. Therefore, this study focused on the effect of HPB coating on ΔS , the most\nM\nfundamental performance metric for magnetocaloric materials. Evaluating other\nmetrics is considered a promising direction for future research. These points are\nemphasized in the revised manuscript.\nWe also acknowledge the importance of clarifying the impact of the surface\nmodification on heat exchange. We have assessed its impact by measuring the\nthermal diffusivity of ErCo , oxidized ErCo , and Cu-plated/oxidized ErCo plate with\n2 2 2\na size of approximately 6×6×1 mm3at 300 K using a laser flash apparatus (NETZSCH,\nLFA 467HyperFlash). The difference in thermal diffusivities of these samples was less\nthan 2% (Table r1), which falls below the device’s maximum measurement error (3%),\nindicating that surface modification (Cu plating and oxidation treatment) does not\nsignificantly affect the thermal diffusivity of ErCo₂ plates. However, since this device\ncannot measure the thermal diffusivity of micro-spherical samples, directly evaluating\nthe effect of surface modification on the thermal diffusivity of the small ErCo₂\nparticles remains a future challenge.\nTable r1. Thermal diffusivity of ErCo , oxidized ErCo , and Cu-plated/oxidized ErCo\n2 2 2\nplates (The plate size is 6×6×1 mm3 ).\nSample Thermal diffusivity at 300 K (10-6 m2 s-1)\nAs-homogenized ErCo 3.57\n2\n• The microstructural data indicate that the protective surface layer is under 10 µm,\nwhile the particles are approximately 0.3 mm in diameter. How does such a thin layer\nlead to a 30% reduction in magnetic entropy change? The authors’ explanations and\nmicrostructural evidence do not convincingly support the observed magnetocaloric\nbehavior.\nResponse: The reduction in ΔS is due to the decrease of the weight fraction of ErCo\nM 2\nmain phase in the ErCo particle after Cu-plating and/or oxidation treatments. As the\n2\nΔS data in the present study are shown in unit of J per K per total mass of ErCo\nM 2\nparticles, its magnitude decreases proportionally to the weight fraction of the ErCo\n2\nphase. From the microstructural observation data, we obtained an average thickness\nof each phase by measuring 3–5 particles and evaluated the weight fraction of each\nphase in the ErCo particle: the innermost ErCo phase, the Er–Co–O layer, the Co–O\n2 2\nlayer, and the outermost CuO layer (Fig. r2). The results show that the decrease of the\nweight fraction of the ErCo phase is approximately 29% after Cu-plating and\n2\noxidation, which roughly corresponds to the 30% decrease in magnetic entropy\nchange. For the oxidized-only sample, we also confirmed that the decrease of the\nweight fraction of the ErCo phase corresponds to the decrease in magnetic entropy\n2\nvia the same process (Fig. r3). We have added the above explanation in the revised\nmanuscript and Figs. r2 and r3 in the Supporting Information.\n“The reduction in ΔS is due to the decrease of the weight fraction of ErCo main phase in\nM 2\nthe ErCo particle after Cu plating and/or oxidation treatments. As the ΔS data in the present\nstudy are shown in unit of J per K per total mass of ErCo particles, its magnitude decreases\n2\nproportionally to the weight fraction of the ErCo phase. From the microstructural analysis\n2\nresults shown in Fig. 3, we obtained an average thickness of each phase by measuring 3–5\nparticles and evaluated the weight fraction of each phase in the ErCo particle: the innermost\n2\nweight fraction of the ErCo phase decreased by approximately 29% after Cu plating and\n2\noxidation, which roughly corresponds to the 30% decrease in magnetic entropy change. For\nthe oxidized-only sample, we also confirmed that the decrease of the weight fraction of the\nErCo phase corresponds to the decrease in magnetic entropy via the same process (Fig. S8).”\n2\nweight of each layer. For the mass calculation, the density of Er–Co–O layer was\nassumed to be 8 g/cm3 which is less than that of ErCo (10.35 g/cm3) but higher than\n2\nthat of CoO (6.5 g/cm3).\nFig. r3. Schematic cross-section of a Cu-plated/oxidized ErCo particle, and estimated\n2\nweight of each layer. For the mass calculation, the density of Er–Co–O layer was\nassumed to be 8 g/cm3 which is less than that of ErCo (10.35 g/cm3) but higher than\n2\nthat of CoO (6.5 g/cm3).\n3. Questionable Microstructural Characterization and Interpretation\nThere are inconsistencies and ambiguities in the microstructural analysis:\n• Lines 180–182 claim that “a 20–30 µm thick intermediate layer appeared brighter\nthan the outermost layer but darker than the interior,” yet this is not evident in Figure\n3.\nResponse: We appreciate the reviewer for pointing out the typo error. We corrected\nthe thickness of the intermediate layer in the manuscript as follows:\n“a 5- to 20-μm-thick intermediate layer appeared brighter than the outermost layer but\ndarker than the interior.”\n• The description of the TEM data suggests the formation of “dense CuO with large crystallites,” but the images do not provide sufficient evidence to assess density or crystallite size. Drawing such conclusions from limited local TEM observations is problematic.\nResponse: We revised the description of “dense CuO with large crystallites,” to “well-\ncrystalized CuO,” in the text.\n• The authors claim material loss during TEM sample preparation due to brittleness.\nHowever, numerous studies have successfully prepared brittle materials for TEM\nwithout such loss. The missing regions could be due to a weak interface between the\noxide and underlying layers, rather than sample preparation artifacts.\nResponse: We agree that the missing regions could be due to a weak interface\nbetween the oxide and underlying layers. We revised the related description in the\nmanuscript as follows.\n“These results indicate that a well-crystalized CuO layer with large crystallites forms on\nthe outer surface, whereas the subsurface Co-, Er-, and Cu-oxide layers exhibit some\nmissing regions, which could be due to a weak interface between the subsurface oxide\nlayer and the outmost surface of the CuO layer.”\n4. Incomplete and Unclear Hydrogen Diffusion Analysis\nThe methodology for assessing hydrogen diffusion is inadequately described:\n• The D₂ exposure conditions (e.g., duration, temperature, pressure) are not clearly\nspecified. If they differ from the hydrogen exposure conditions, how can the results\nbe directly compared?\nResponse: We performed the D exposure experiment at the same temperature and\n2\npressure as that for the hydrogen exposure experiments, and the duration was also\nvery close, i.e., 137.5 h for oxidized particles and 139 h for Cu-plated/oxidized\nparticles. To describe it more accurately, the original description in the Methods (lines\n316–317) “Similar experiments were conducted using deuterium for subsequent TOF-\nSIMS analysis.” has been amended as follows.\n”We performed the D exposure experiment at the same temperature and pressure as that for\n2\nthe hydrogen exposure experiments, and the duration was also very close, i.e., 137.5 h for\noxidized particles and 139 h for Cu-plated/oxidized particles.”\n• Why were alternative techniques not employed to quantify hydrogen content and\ndistribution immediately following hydrogen exposure?\nResponse: We attempted to quantify the hydrogen content and distribution using\nToF-SIMS after hydrogen exposure test. However, clear results could not be obtained\nprobably due to the lower analysis precision of hydrogen compared to deuterium\nand the difficulty to distinguish between originally existing hydrogen and adsorbed\nhydrogen in the sample.\n• Figure 1 shows some hydrogen absorption even in the Cu-plated/oxidized sample.\nHowever, the correlation between hydrogen absorption data and the D₂\nexposure/ToF-SIMS measurements is unclear.\nResponse: Because quantitative analysis of deuterium was difficult by ToF-SIMS,\nwe\ncould not obtain an accurate correlation between the hydrogen absorption data and\nthe D exposure/ToF-SIMS measurements.\nThis manuscript contains some interesting results, which may be useful for\nresearchers working in this field. Generally the article is well organized, the authors\nquite well refer to other literature data.\nIn my opinion, this article is suitable for publication. However, there are some minor\nattention which must be addressed before the final decision. Below I present some\nminor comments.\n1. Is ΔSM the only parameter considered when selecting a suitable magnetocaloric\nmaterial? What about ΔTad ? I believe that the introduction should present the\nmagnetocaloric properties of the studied compound in detail — that is, all available\ndata from the literature, not only ΔSM .\nResponse: We appreciate reviewer #2’s constructive comments, which helped\nimprove the quality of our manuscript. We acknowledge that the adiabatic\ntemperature change ΔT is also an important parameter for evaluating\nad\nmagnetocaloric materials. According to previous works [H. Wada et al., Cryogenics\n39 (1999) 915-919; J. Alloys Compd. 316 (2001) 245-249.], the ΔT of ErCo is about\nad 2\nreferred to the ΔT data available from the literature as follows.\nad\n“ErCo exhibits a ∆S > 20 J kg–1 K–1 and an adiabatic temperature change (ΔT ) of ~10\nK under a 5 T field change at the Curie temperature of ~32 K9-11, making it suitable for\nhydrogen liquefaction.“\nAdded reference:\n11. Wada, H., Tanabe, Y., Shiga, M., Sugawara, H. & Sato, H. Magnetocaloric effects of Laves\nphase Er(CoNi) compounds. J Alloy Compd 316, 245-249, doi:Doi 10.1016/S0925-\n8388(00)01305-0 (2001).\n2. In what type of cooling cycle is the proposed refrigerant intended to be used? It\nmight be worth enriching the introduction with reports on other types of\nmagnetocaloric materials, for example, in layered form. It should be added in the\nintroduction that in various forms magnetocaloric materials as promising materials\nfor helium or hydrogen liquefaction are studied, e.g. as composites, for example pleas\ntake a look at recent work: DOI: 10.1039/d4tc02247e or\ndoi.org/10.1016/j.ijhydene.2024.09.082\nResponse: The proposed refrigerant is intended to be used in an active magnetic\nregenerator (AMR) cycle. We understand that various forms such as composites could\nbe options for the shape optimization of magnetocaloric materials for hydrogen\nliquefaction via magnetic refrigeration with the AMR cycle. As suggested by the\nreviewer, we have cited recent works in the Introduction and accordingly modified\nthe sentences. Notably, the method for HPB coating proposed in this study may be\napplicable not only to particles but also to other material forms. This point has been\nhighlighted in the revised manuscript.\n“Magnetic refrigeration offers a more energy-efficient alternative to conventional gas\ncompression/expansion refrigeration technology4-7, and the efficiency of such\nrefrigeration systems can be further improved using magnetic refrigeration materials with\nlarge magnetocaloric effects. Numerous magnetic refrigeration materials with large\nmagnetocaloric effects have been developed, such as DrNi 8, ErCo 9-11, ErNi 12, Dr\n2 2 2 1-\nEr Ni 13, HoB 14, and HoAl 15. However, the development of magnetic refrigeration\nx x 2 2 2\nmaterials with a large magnetic entropy change (∆S ) over a broad temperature range\nM\n(77–20 K) is challenging16-20. A layered composite magnetic refrigerant of HoB , DyNi ,\n2 2\nand TbNi has been developed and obtained an average ∆S value of 4.7 J kg–1 K–1 under\na 2 T field change in a broad temperature range of 7.5–53.4 K21.“\nAdded references:\n8. Cwik, J. et al. Magnetocaloric effect in Laves-phase rare-earth compounds with the second-\norder magnetic phase transition: Estimation of the high-field properties. Acta Mater133,\n230-239, doi:10.1016/j.actamat.2017.05.054 (2017).\n12. Llamazares, J. L. S., Ibarra-Gaytán, P., Sánchez-Valdés, C. F., Ríos-Jara, D. & Alvarez-Alonso,\nP. Magnetocaloric effect in ErNi melt-spun ribbons. J Rare Earth 38, 612-616,\ndoi:10.1016/j.jre.2019.07.011 (2020).\npromise for hydrogen liquefaction. J Mater Chem C 12, 14421-14432,\ndoi:10.1039/d4tc02247e (2024).\n14 de Castro, P. B. et al. Machine-learning-guided discovery of the gigantic magnetocaloric\neffect in HoB near the hydrogen liquefaction temperature. Npg Asia Mater12, doi:ARTN\n3510.1038/s41427-020-0214-y (2020).\nto the field-induced change of magnetic entropy in Al compounds (=Pr,Nd,Tb,Dy,Ho,Er).\nPhys Rev B74, doi:ARTN 13441010.1103/PhysrevB.74.134410 (2006).\n3. Under what conditions was M(T) measured? ZFC or FC?\nResponse:All the M(T) measurements were performed under field-cooling processes.\nThis point has been clarified in the Methods section of the revised manuscript.\n“All the M(T) measurements were performed under field-cooling processes.”\n4. The temperature sweep rate for the presented M(T) curves should be given. What\nwas the mass of the samples for magnetic measurements? Were isothermal\nmagnetization curves for the samples studied corrected due to the internal\ndemagnetizing field ?\nResponse:The experimental conditions for the M(T) measurements are as follows: the\ntemperature sweep rate was 0.6 K per min; the mass of the samples was about 3–4\nmg. In addition, demagnetization corrections were not performed in this study\nbecause it is difficult to evaluate the exact demagnetization factors for a group of the\nparticles. Besides, a previous work [e.g., H. N. Bez et al., J. Magn. Magn. Mater. 458\n(2018) 301-309.] has shown that the demagnetization effect does not significantly\naffect the evaluation of ΔS for large magnetic fields such as 5 T, which was the field\nM\nused for this study. These points have been clarified in the experimental section in\nthe revised manuscript as follows.\n“The temperature sweep rate was 0.6 K per min; the mass of the samples was approximately\n3–4 mg. Demagnetization corrections were not performed in this study because it is difficult\nto evaluate the exact demagnetization factors for a group of the particles. The\ndemagnetization effect does not significantly affect the evaluation of ΔS for large magnetic\nM\nfields such as 5 T44.”\nAdded References:\n“44 Bez, H. N., Yibole, H., Pathak, A., Mudryk, Y. & Pecharsky, V. K. Best practices in\nevaluation of the magnetocaloric effect from bulk magnetization measurements. J Magn\nMagn Mater 458, 301-309, doi:10.1016/j.jmmm.2018.03.020 (2018).”\n5. The presented ΔSM(T) dependencies generally indicate that ΔSM decreased after\napplying the proposed barrier. This should be clearly emphasized. Moreover, the\nfigure shows that after exposure, ΔSM increases. This fact should be explained. What\nwas the measurement error of ΔSM?\nResponse: We apologize for the insufficient explanation about the insufficient explanation about the decrease in ΔS\nM\nafter applying the proposed barrier. As mentioned in response to reviewer #1’s\ncomment, this decrease is due to the reduction in the weight fraction of ErCo phase\n2\nin the particle after the barrier coating (Figs. r2 and r3). The explanation has been\nadded in the revised manuscript.\nMeanwhile, the largest measurement error of ΔS in our study is due to the weight\nM\nerror of the particle samples used in the measurements, which is estimated to be\napproximately 1–2%. The difference in ΔS before and after hydrogen exposure falls\nM\nwithin this error range; therefore, it cannot be concluded that ΔS increased after\nM\nexposure. The following description has been added in the experimental section in\nthe revised manuscript.\n“The measurement error of ΔS is mainly from the weight error of the particle samples used\nM\nin the measurements, which is estimated to be approximately 1–2%.”\n6. Fig.1a no description of the insert.\nResponse: We added the following description in the caption of Fig. 1a.\n“Inset shows the hydrogen pressure change during the initial ten hours.”\nI will make a further decision after responding to the above comments.\nReviewer #3 (Remarks to the Author):\nIn this manuscript titled “Enhancing hydrogen permeation barrier performance of\nErCo magnetic refrigeration material via surface oxide layer formation”, the authors\n2\ninvestigate ErCo as a magneto-caloric material for magnetic refrigeration, with\n2\npotential application in liquid hydrogen production. Due to their small atomic radius,\nhydrogen atoms can easily diffuse into host materials, leading to property\ndegradation and, in severe cases, hydrogen embrittlement. To address this, the\nauthors explore the hydrogen permeation barrier (HPB) performance of ErCo\n2\nparticles by applying electroless copper plating followed by oxidation, resulting in a\nCuO layer several micrometers thick. This surface modification is shown to effectively\nsuppress hydrogen ingress and maintain the structural integrity of the material. The\nstudy addresses an important challenge in the development of hydrogen-compatible\nmagnetic refrigeration materials and provides useful data regarding coating-based\nhydrogen permeation barrier (HPB) strategies. While the manuscript reports\nencouraging results, the methods and techniques employed are largely established,\nand thus the level of technical innovation is limited. Unless these substantive issues\nare addressed clearly, the manuscript does not reach the level to be considered for\npublication in Nature Communications.\n1. Why was electroless Cu plating chosen? The authors should explain why coatings\nwith inherently low hydrogen diffusivity (e.g., Al2O3, Y2O3, Er2O3, La2O3, or SiO2)\nwere not used. It is not enough to say that PVD/CVD methods struggle to form\nuniform HPB layers on sub-500 μm ErCo2 particles, please provide a thorough\nexplanation/justification. Moreover, electroless Ni or Co plating is also feasible.\nResponse: We appreciate reviewer #3’s insightful and valuable comments, which\nhelped improve the quality of our manuscript. In fact, we have attempted to use\nmaterials with low hydrogen diffusivity, such as Al O and Cr O , for forming HPB\n2 3 2 3\nlayer on magnetic refrigeration materials. Specifically, we asked a company to form\nCr O layer on ErCo particles via thermal spraying, however, it was proved difficult\n2 3 2\nto prevent particle aggregation and to control the uniformity of the coating layer on\nthe small ErCo particles via thermal spraying. Fig. r4 shows the SEM image of ErCo\n2 2\ncoated with Cr O via thermal spraying, indicating that aggregation of particles\n2 3\noccurred although the Cr O might be formed on the surface.\n2 3\nFig. r4. a. SEM image of ErCo particles coated with Cr O via thermal spraying. b.\n2 2 3\nClose-up of the surface of a single particle. c. EDS elemental mapping of b. d.\nQuantitative analysis results for components in the region of b.\nFurthermore, we commissioned a company specializing in PVD to form an Al₂O₃ layer\nonto the ErCo₂ particles via a barrel PVD method. Specifically, this involves placing\nErCo₂ particles within a multi-faceted barrel and rotating the barrel during the\ncoating process. However, it was found difficult to form Al O layer uniformly on the\n2 3\nsurface of small ErCo particles with significant surface irregularities.\ntreatment offers great advantages in terms of cost and efficiency. CuO has not\npreviously been recognized as an HPB material, and this study has demonstrated for\nthe first time that it possesses sufficient HPB properties at temperatures near room\ntemperature.\nIn addition, regarding the feasibility of electroless Ni or Co plating, we attempted to\ndo electroless Ni plating onto ErCo particles. However, due to the acidity of\n2\ncommonly used Ni plating solution, a large amount of Er dissolved during plating,\ncausing the particles to pulverize. However, should a proper alkaline plating solution\nbe achievable, the utilization of Ni plating may be possible. Further research on this\nmatter is ongoing.\nWe have added the following description in the introduction.\n“We attempted to form an Al₂O₃ layer onto the ErCo₂ particles via a barrel PVD method.\nErCo₂ particles were placed within a multi-faceted barrel, which was rotated during the\ncoating process. However, it was difficult to form Al O layer uniformly on irregularly\n2 3\nsurfaced small ErCo particles. We also attempted to form Cr O layer on ErCo particles\n2 2 3 2\nvia thermal spraying. However, particle aggregation is difficult to prevent and the coating\nuniformity is hard to control on the small ErCo particles via thermal spraying.“\n2\n2. The authors should conduct further experimental tests on H2 permeation of the\nCu-plated/oxidized particles, including pressure–composition–temperature (PCT)\ncurves, intrinsic hydrogen diffusion coefficients, hydrogen permeation flux\nmeasurements, and in particular long-term hydrogen permeation stability.\nResponse: We have performed PCT measurements for the as-homogenized, oxidized,\nand Cu-plated/oxidized ErCo particles. We have also performed the long-term\n2\nhydrogen exposure tests up to 28 days for the oxidized and Cu-plated/oxidized ErCo\n2\nparticles. The following results have been obtained and added in the Results and\nDiscussion section and Supplementary Information.\nThe pressure–composition–temperature (PCT) measurements were performed for\nthe as-homogenized, oxidized, and Cu-plated/oxidized samples (Fig. r5). For the as-\nhomogenized samples, no obvious hydrogen absorption was observed up to 4 MPa\nat 303 K, whereas the amount of hydrogen absorption reached approximately 1.3\nmass% at 333 K. The PCT results are consistent with the hydrogen exposure\nmeasurement results, considering that the PCT measurement process was completed\nin less than an hour (0.57 h) at 303 K and hydrogen absorption commenced in the\nas-homogenized particles during the hydrogen exposure experiments (Fig. 1a). In\naddition, the amount of hydrogen absorption obtained from the PCT measurement\nat 333 K agrees well with the value of 1.311 mass% obtained from the hydrogen\nexposure experiment. Furthermore, it was confirmed that the absorbed hydrogen\nmaintained approximately 1 mass% even after the pressure was reduced to a low\nlevel (<0.005 MPa), indicating that the hydrogen absorbed by ErCo does not desorb\n2\neasily.\nFor the oxidized samples, no obvious hydrogen absorption was observed at 303 K,\nand only a small amount of hydrogen absorption (~0.2 mass%) was observed at 333\nK (Fig. r5b). For the Cu-plated/oxidized samples, no apparent hydrogen absorption\nwas observed at both 303 and 333 K (Fig. r5c). These PCT measurements are\nconsistent with those of hydrogen exposure experiments (Fig. 1a and 1b), indicating\nthat the HPB performance was improved by the oxidation and further enhanced by\nCu-plated/oxidation treatment.\nFig. r5. PCT measurement results of ErCo particles. a. as-homogenized samples; b.\n2\nsamples after oxidization at 773 K for 0.5 h in air (oxidized only); c. samples after Cu-\nplated at 348 K for 0.17 h followed by oxidation at 773 K for 0.5 h in air (Cu-\nplated/oxidized).\nFurthermore, we performed long-term hydrogen exposure experiments at room\ntemperature up to 28 days for evaluating long-term hydrogen permeation stability\nof oxidized and Cu-plated/oxidized ErCo particles. Particles after different Cu-plating\n2\ntime (0.17 and 1.5 h) were used for examining the influence of thickness of CuO layer\non HPB performance (Fig. r6). The initial pressure differed slightly among samples\ndue to variations in cell capacity and piping capacity for each sample, that is, 1.314\nMPa for oxidized ErCo , 1.277 MPa for Cu-plated_0.17h/oxidized ErCo , and 1.276\n2 2"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have addressed all the questions raised before. I would like to recommend the revised manuscript for publication in Nature Communications.", "reviewer_2": "The authors responded appropriately to my comments. Please find attached the manuscript, where I have marked the incorrect names of compounds in red.", "reviewer_3": "In the manuscript titled “Enhancing hydrogen permeation barrier performance of ErCo2 magnetic refrigeration material via surface oxide layer formation,” the authors reveal, through experimental analyses and first-principles calculations, the CuO layer serves as an effective hydrogen permeation barrier, enhancing the hydrogen resistance of ErCo₂ intermetallic compounds for magnetic refrigeration applications. The revised manuscript shows clear improvement after the first-round revision. However, several technical and formatting issues still need to be addressed.\n(1) The “DFT calculation(s)” section contains font size inconsistencies and apparent character encoding errors, likely related to equation formatting.\n(2) The authors should carefully correct these issues and conduct a thorough check of the entire manuscript to ensure consistency in formatting, figure captions, symbols, overall presentation, and figure resolution.", "reviewer_4": "The authors replied the reviewers’comments and carried out additional experiments and calculations. The reviewer agrees for its publication.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_2": "The authors have revised the manuscript in accordance with my suggestions. The work can now proceed to the next stages of production.", "reviewer_3": "The revised manuscript has been improved to the level that can be considered for publication."}, "author_response": ""}]

Decision letter