Two-Dimensional Metastable-Phase Nickel Hexagonal Nanosheets for Highly-Performance Electrochemical Acetone Hydrogenation

Domain: physics
Published: 2025-12-24
Editorial outcome: major_revision

Abstract
Electrocatalytic acetone hydrogenation into high-value alcohols offers a sustainable alternative to thermal methods. However, developing non-noble metal catalysts with superior performance for electrocatalytic acetone hydrogenation remains challenging. Two-dimensional non-noble metal structure with metastable-phase active sites is promising due to its high-energy structure and unique electronic environment. However, its precise synthesis remains a great challenge. In this work, we report a two-dimensional metastable hexagonal close-packed phase nickel nanosheets (hcp Ni NSs) with a space group of P6/mmc. In the electrocatalytic acetone hydrogenation, it achieves 95.0% acetone conversion with 100% isopropanol selectivity. Notably, in the scale-up experiment, the hcp Ni NSs also exhibit 11.51 g g-1 h-1 isopropanol production rate at a current density of −250 mA cm-2. Furthermore, this catalyst is widely applicable to various carbonyl compounds. Mechanism study shows that hcp Ni NSs enable strong acetone adsorption and rapid isopropanol desorption.

Introduction
As the demand for renewable energy and sustainable chemicals continues to increase, finding efficient and environmentally friendly catalysts has become one of the important challenges in today’s chemistry. Selective hydrogenation has been shown to be an economical and environmentally compatible synthetic method for the preparation of various fine chemicals1–6. Acetone catalytic hydrogenation is widely used in the manufacture of fine chemicals and the refining of biomass-derived compounds. So far, a variety of metal catalysts for acetone selective hydrogenation to prepare isopropanol have been reported7–10. However, they are mainly based on traditional thermal catalytic hydrogenation, using explosive hydrogen or other hydrogen donors (e.g., HCOOH, NaBH4) as hydrogen sources11–15, which lead to high energy consumption, high cost and serious safety problems.

Electrocatalytic hydrogenation (ECH) provides a green and environmentally friendly route for catalytic hydrogenation16–19. Yet, it is often accompanied by the more kinetically favourable hydrogen evolution reaction, resulting in unsatisfactory conversion and selectivity of ECH. Therefore, the rational design of catalysts with high activity, selectivity and stability has attracted extensive attention. Nickel (Ni), as the fourth most abundant transition metal on earth, is considered as a potential candidate to replace precious metal catalysts such as Pt and Pd20–24, however, its activity for electrocatalytic hydrogenation remains unsatisfactory. Compared with the stable-phase, two-dimensional (2D) metastable-phase catalysts exhibit high catalytic activity due to their large specific surface area, high-energy structure and unique electronic environment25–31, making them promising candidates to solve this problem.

In this work, we report a two-dimensional metastable hexagonal close-packed (hcp) phase Ni hexagonal nanosheets (NSs) catalyst with a space group of P6/mmc. The hcp Ni NSs exhibit high performance in the electrocatalytic acetone hydrogenation to isopropanol, which has 95.0% acetone conversion (con.) with 100% isopropanol selectivity (sel.), much higher than fcc Ni NPs (con.: 51.0%, sel.: 100%) (Fig. 1). Notably, in the scale-up experiment, the hcp Ni NSs exhibited acetone conversion of 95.9% with isopropanol selectivity of 100% and isopropanol production rate of 11.51 g g−1 h−1 at a current density of −250 mA cm−2. Furthermore, this catalyst remains at around 80% acetone conversion after thirty cycles of testing at −1.32 V vs. reversible hydrogen electrode (RHE) and is widely applicable to various carbonyl compounds.

Results
Synthesis and structural characterizations of hcp Ni hexagonal nanosheets
The hcp Ni NSs was successfully fabricated by a one-pot method using bis (η5-2,4-cyclopentadien-1-yl) nickel as a precursor, glucose as a reducing agent, formaldehyde as a surfactant, and oleylamine as a solvent, as illustrated in Fig. 2a. The phase of hcp Ni NSs was identified by powder X-ray diffraction (XRD). As shown in Fig. 2b, the diffraction peaks of hcp Ni NSs can be clearly attributed to the hcp Ni phase (JCPDS No. 45-1027, P6/mmc), while there is no face-centered cubic (fcc) Ni phase (JCPDS No. 04-0850, Fm-3m)32, indicating the high purity of the metastable hcp phase Ni. Low-magnification transmission electron microscopy (TEM) images showed the sheet-like morphology of hcp Ni NSs (Fig. 2c, f). TEM image shows that hcp Ni NSs has a two-dimensional hexagonal nanosheets morphology with the side length of 21.9 ± 0.5 nm and the diameter of 33.6 ± 0.4 nm (Fig. 2d, e). Moreover, zoom-inside-view TEM image demonstrated the thickness of the two-dimensional hexagonal nanosheets is 6.9 nm (Fig. 2f). The high-resolution transmission electron microscopy (HRTEM) image displays the fringe patterns showing the presence of the (01-10) plane of hcp Ni NSs (Fig. 2g, h, i). Figure 2i and Fig. 2j clearly reveal the crystal structure of hcp Ni NSs. Corresponding Fourier diffractogram (FD) patterns framed by dotted lines further confirm the hexagonal lattice of hcp Ni NSs (the inset of Fig. 2h). The lattice spacing framed by dotted lines in Fig. 2i was measured as 0.23 nm (Fig. 2k), corresponding to the (10-10) and (01-10) crystal planes. Moreover, we also synthesized hcp Ni NPs (JCPDS No. 45-1027, P6/mmc) (Fig. 2b and Supplementary Fig. 1) and fcc Ni NPs (JCPDS No. 04-0850, Fm-3m) (Fig. 2b and Supplementary Fig. 2) as comparative materials.

The synthesis of metastable-phase material is a kinetic-controlled process and the reducibility of different precursors is crucial for controlling the morphology. As shown in Supplementary Figs. 3, 4, when using other nickel precursors such as nickel acetylacetonate (Ni(acac)2), nickel (II) formate (Ni(HCO)2·2H2O), nickel oxalate dihydrate (NiC2O4·2H2O) and bis(triphenylphosphine)nickel(II) bromide (NiBr2(PPh3)2), the crystallinity of hcp Ni was weak and even hcp Ni cannot be synthesized (Supplementary Figs. 3, 4). Therefore, bis(η5-2,4-cyclopentadien-1-yl) nickel was used to synthesize hcp Ni hexagonal nanosheets. Glucose is important to form the hcp Ni. When other sugar compounds such as fructose and ribose were used as reducing agents, the characteristic diffraction peaks of nickel hydroxide (Ni(OH)2) and metastable phase hcp Ni both appeared (Supplementary Fig. 5a, b). In addition, when phenol is used as a reducing agent, only stable phase fcc Ni can be synthesized (Supplementary Fig. 5c). Formaldehyde, as a morphology regulator, releases CO during heating, which can promote the growth of two-dimensional hexagonal nanosheets. When reducing the amount of formaldehyde, the hcp Ni tends to grow toward a nanoparticle morphology (Supplementary Fig. 6). To explore the formation process of hcp Ni NSs specifically, the samples obtained at different reaction times were characterized respectively (Supplementary Figs. 7-8). At reaction time of 15 minutes, the amorphous nanosheets were synthesized (Supplementary Figs. 7a, 8a). After 90 minutes of continuous reaction, the amorphous nanosheets were gradually disappear with hexagonal sheets appear (Supplementary Fig. 7e), and the characteristic diffraction peaks of nickel hydroxide and metastable hcp phase Ni appeared (Supplementary Fig. 8e). When the reaction time was extended to 120 minutes, the characteristic peaks of nickel hydroxide and metastable hcp phase Ni are more obvious (Supplementary Figs. 7f, 8f). When extending the reaction time, the characteristic diffraction peak of nickel hydroxide gradually disappeared. When the reaction time was 240 minutes, only the characteristic XRD peak of metastable hcp phase Ni existed, and the surface of hexagonal sheet was more flat (Supplementary Figs. 7h, 8h).

The electronic and atomic structure of hcp Ni hexagonal nanosheets
To gain insight into the local coordination environment of the catalyst, X-ray absorption near-edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) were further used to conduct in-depth analysis of the electronic structure of the catalyst. Figure 3a, b show the Ni K-edge X-ray absorption near-edge spectroscopy (XANES) spectra of hcp Ni NSs, fcc Ni NPs, while NiO and Ni foil used as the references. The absorption edges at the energy positions are as follows: NiO > hcp Ni NSs > fcc Ni NPs > Ni foil, indicating that hcp Ni NSs with higher valence state33–38. The k3χ(k) function of the Fourier transform of the EXAFS data in the frequency domain (R) space reveals information about the coordination environment (Fig. 3c). In the EXAFS spectrum, the Ni-Ni coordination bonds of Ni foil (green dashed line in Fig. 3c) and fcc Ni NPs (blue solid line in Fig. 3c) are located at ~2.18 Å, and the Ni-Ni coordination bond in NiO (yellow dashed line in Fig. 3c) is located at ~2.59 Å. The peak at ~1.62 Å in NiO belongs to the Ni-O coordination39. The Ni-Ni coordination bond of hcp Ni NSs is ~2.34 Å, which is different from the Ni-Ni bond in Ni foil and fcc Ni NPs due to the different phase structure of hcp Ni NSs from fcc Ni NPs and Ni foil. Wavelet transform (WT) was used to further analyze the Ni K-edge EXAFS oscillation (Fig. 3d–f). The WT contour of hcp Ni NSs showed a maximum intensity at 7.30 Å−1, different from the maximum intensity of fcc Ni NPs (8.0 Å−1) and the maximum intensity of Ni foil (8.1 Å−1), which originated from Ni–Ni coordination. The results showed that hcp Ni NSs had a different coordination environment from fcc Ni NPs and Ni foil. To further highlight the differences between hcp Ni NSs and fcc Ni NPs, we analysed the crystal structure and atomic stacking of hcp Ni NSs along the a, b, and c directions, as shown in Fig. 3g. The results show that the atomic stacking of hcp Ni NSs is a typical ABAB… stacking order along the [100] direction, which is different from the ABCABC… stacking order of the stable fcc structure along the [111] direction.

Electrocatalytic hydrogenation performance of hcp Ni hexagonal nanosheets
The electrocatalytic acetone hydrogenation was chosen as a model reaction to evaluate the catalytic performance of hcp Ni NSs (Fig. 4a–d), while hcp Ni NPs (Fig. 4e–h) and fcc Ni NPs (Fig. 4i–l) were chosen as the references. The reference electrode was calibrated before the electrocatalytic testing (Supplementary Fig. 9). The reaction was carried out in a typical H-type cell by using 0.1 mmol acetone (1a) as the model substrate with 1.0 M KOH as the electrolyte (Supplementary Fig. 10). Figure 4b, f and j show the linear sweep voltammetry (LSV) curves of hcp Ni NSs, hcp Ni NPs and fcc Ni NPs in 1.0 M KOH electrolyte with/without acetone, respectively. The LSV curves are measured based on at least three independent replicates (Supplementary Fig. 11). Obviously, hcp Ni NSs, hcp Ni NPs, and fcc Ni NPs all show enhanced current densities in the presence of acetone, indicating that acetone electrocatalytic reduction prevails over the competing hydrogen evolution reaction (HER) on Ni surfaces. The optimal potential was screened by potential-dependent electrochemical experiments. The acetone conversion reached 95.0 ± 0.35% and the isopropanol selectivity was 100% at −1.32 V vs. RHE (Supplementary Fig. 12).

To further evaluate the performance of the catalyst for electrocatalytic acetone hydrogenation, we performed time-controlled experiments at −1.32 V vs. RHE within 5 h (Supplementary Fig. 13). The results showed that hcp Ni NSs showed the largest acetone conversion in each time period (Fig. 4c). Furthermore, the acetone conversion of hcp Ni NSs and hcp Ni NPs in each time period was higher than that of fcc Ni NPs (Fig. 4c, g, k and Supplementary Fig. 13), indicating that the metastable-phase has better hydrogenation activity than the stable one. The acetone conversion is as follows: hcp Ni NSs > hcp Ni NPs > fcc Ni NPs. Quantitative analysis and verification by hydrogen nuclear magnetic resonance spectroscopy (1H-NMR) show that, after 5 h of acetone electrocatalytic reduction, hcp Ni NSs, hcp Ni NPs and fcc Ni NPs showed 95.0%, 75.1% and 51.0% acetone conversion, respectively, with no related by-products detected (Fig. 4d, h, l). Compared to other reported catalysts, hcp Ni NSs exhibits competitive in electrocatalytic acetone hydrogenation performance (Fig. 4m and Supplementary Table 1). Compared with other commercial catalysts such as Pt/C and Raney Ni, hcp Ni NSs exhibits high acetone conversion (95.0 ± 0.35%) and reaction rate (19 ± 0.07 μmol cm−2 h−1) (Fig. 4n). In addition, hcp Ni NSs exhibits the production rate of 0.23 g g−1 h–1, higher than the stable one and other commercial catalysts (Supplementary Fig. 14). In addition, we investigate the effect of increasing the acetone content in the electrolyte on the acetone conversion and the Faradaic efficiency of isopropanol using hcp Ni NSs at −1.32 V vs. RHE in 5 h (Supplementary Fig. 15). The results show that adding more acetone to the electrolyte inhibits the side reaction HER and improves the Faradaic efficiency of isopropanol, but decreases the acetone conversion. Since we focused on the acetone conversion, we chose 0.1 mmol of acetone.

After electrocatalytic acetone hydrogenation reaction, hcp Ni NSs does not change the crystal phase and morphology, indicating that hcp Ni NSs is relatively stable in the reaction process (Supplementary Fig. 16). Furthermore, in order to explore the Ni valence state changes of hcp Ni NSs catalyst surface after the reaction, we performed X-ray photoelectron spectroscopy (XPS) tests on hcp Ni NSs at open circuit potential (OCP) condition and at −1.32 V vs. RHE for 5 h respectively. As observed in Ni 2p XPS spectra (Supplementary Fig. 17), the peak intensity of Ni-Ni bond (852.9 eV and 870.2 eV) is weakened after the reaction, indicating that the surface of the hcp Ni NSs is slightly oxidized. This may be due to the accumulation of acetone on the nickel surface and hydrogen/hydroxyl groups adsorption on hcp Ni NSs. In addition to the high reaction activity, hcp Ni NSs remains at around 80% acetone conversion after thirty cycles of testing at −1.32 V vs. RHE, As can be seen in Fig. 4o. Furthermore, the TEM image and XRD pattern show that the hexagonal sheet morphology and metastable-phase of hcp Ni NSs largely remain after the stability test (Supplementary Fig. 18).

In order to evaluate the feasibility of the catalyst for industrial application, a scale-up experiment of electrocatalytic acetone hydrogenation was carried out using a flow electrolysis cell (Fig. 4p and Supplementary Fig. 19), it is worth noting that the hcp Ni NSs exhibited 95.9 ± 1.4% acetone conversion, with 100% isopropanol selectivity and 11.51 g g−1 h−1 isopropanol production rate at a current density of −250 mA cm−2 (Fig. 4m, q, r, s). The result of 1H NMR spectrum is shown in Supplementary Fig. 20.

To further characterize the electrocatalytic hydrogenation behaviour of the hcp Ni NSs catalyst, we next explored the substrate range of the ECH reaction, including aliphatic ketones, alicyclic ketones, and aromatic ketones, such as 2-butanone (1b), 2-pentanone (1c), 3-pentanone (1d), 2-hexanone (1e), 3-hexanone (1f), 2-heptanone (1g), 3-heptanone (1h), 4-heptanone (1i), cyclopentanone (1j), cyclohexanone (1k), cycloheptanone (1l), cyclooctenone (1m); acetophenone (1n), 2-indanone (1o), and benzophenone (1p) (Fig. 5). Considering the low solubility of some substrates in 1.0 M KOH/H2O electrolyte, some experiments were conducted on these substrates using water-miscible organic cosolvents to eliminate the solubility limitation on their reactivity. When these ketones (1b–1p) are treated by our electrocatalytic system, the corresponding alcohol products (2b–2p) can be easily obtained. Impressively, when hcp Ni NSs were used as catalysts, the corresponding these alcohol products (2b–2p) could be readily obtained with selectivity of >99%, and the formation of by-products was almost undetectable. The corresponding conversions of aliphatic ketones, alicyclic ketones, and aromatic ketones were 98.2% con. of 1b, 89.5% con. of 1c, 96.4% con. of 1d, 98.0% con. of 1e, 87.0% con. of 1f, 98.0% con. of 1g, 90.6% con. of 1h, 92.0% con. of 1i, 95.0% con. of 1j, 80.3% con. of 1k, 86.9% con. of 1l, 80.0% con. of 1m, 100.0% con. of 1n, 95.0% con. of 1o and 100.0% con. of 1p, respectively (Fig. 5), with all these data being quantified and verified by 1H-NMR (Supplementary Figs. 21-37). The results suggest that hcp Ni NSs show good catalytic universality.

Mechanism exploration of electrocatalytic acetone hydrogenation
Now, it is significant to understand the catalytic mechanism. hcp Ni NSs exhibited the lowest impedance compared to hcp Ni NPs and fcc Ni NPs, which suggests that the efficient mass transfer of hcp Ni NSs (Supplementary Fig. 38). The adsorption capacity of acetone and isopropanol on hcp Ni NSs, hcp Ni NPs and fcc Ni NPs catalysts was studied by attenuated total reflection infrared spectroscopy (ATR-IR). As shown in Fig. 6a, for hcp Ni NSs and Pt/C catalysts, the strong carbonyl (C=O) (~ 1662.8 cm−1) absorption peak of acetone can be observed, indicating that hcp Ni NSs and Pt/C have a strong adsorption capacity for acetone, which is conducive to the hydrogenation reaction, but the competitive HER reaction is prone to occur on the Pt/C surface, resulting in an unsatisfactory acetone conversion. As shown in Supplementary Fig. 39, the LSV curves of Pt/C show that the current density of the electrolyte without acetone (−32.2 mA cm−2) is more negative than that of the electrolyte with acetone (−29.9 mA cm−2) at −1.32 V vs. RHE, which indicates that Pt/C is more likely to undergo HER reaction. The adsorption strength of hcp Ni NPs and fcc Ni NPs for acetone is weak, resulting in low catalyst conversion. Figure 6b clearly reveals the adsorption strength of isopropanol on the catalyst. When using hcp Ni NSs as the catalyst, a weak peak was observed at the hydroxyl group (-OH) (~ 3419.7 cm−1), indicating that the desorption capacity of isopropanol to hcp Ni NSs was very strong. Therefore, the product isopropanol could quickly fall off after formation on the surface of the catalyst. For hcp Ni NPs, fcc Ni NPs and Pt/C, the absorption peak of hydroxyl group (-OH) (~ 3419.7 cm−1) can be seen, indicating that isopropanol has a strong adsorption capacity on these catalysts, which makes it difficult for the isopropanol generated on the hcp Ni NPs, fcc Ni NPs and Pt/C surface to desorb quickly from their surface. This results in an unsatisfactory acetone conversion by hcp Ni NPs, fcc Ni NPs and Pt/C.

The free radical capture experiment was carried out by electron paramagnetic resonance (EPR) using 5,5-dimethyl-1-pyrrolin-n-oxide (DMPO) as a free radical spin catcher. Only DMPO-H* signal was captured in 1 M KOH electrolyte, after adding acetone to the electrolyte, the DMPO-H* signal decreased significantly, suggesting that the surface-adsorbed hydrogen intermediates (H*) were actively consumed in the hydrogenation reaction (Fig. 6c). This provides direct evidence for the involvement of H* species in the electrocatalytic conversion of acetone. In order to explore the power and hydrogen source in the electrocatalytic hydrogenation reaction, the control experiments were carried out. These experiments confirm the necessity of electricity as the driving force and H2O as the sole hydrogen source (Supplementary Fig. 40). The electrocatalytic acetone hydrogenation reaction was carried out by replacing H2O with D2O, and it was found that only deuterated isopropanol was produced by high-resolution mass spectrometry (HR-MS) (Supplementary Fig. 41), thus demonstrating that the H* required for the hydrogenation process can only originate from the in-situ dissociation of H2O40.

To further understand the underlying mechanism of the enhanced activity for hcp Ni, DFT calculations were carried out. The acetone reduction (i.e. hydrogenation) is composed of acetone adsorption, followed by two steps of proton-coupled electron transfer (PCET) processes, and then desorption from active sites accordingly. As shown in Fig. 6d, the formation energy of key intermediate *C3H7O on the (0001) surface of hcp Ni, is slightly lower than that on the (111) surface of fcc Ni (0.18 vs 0.19 eV), combined with the desorption energy of isopropanol on hcp Ni is 0.07 eV lower than that on fcc Ni (Fig. 6e), indicating hydrogenation of acetone is thermodynamically favourable for hcp Ni. Moreover, considering H2O as proton source, its adsorption and dissociation on active sites are also important factors for hydrogenation performance. As shown in Fig. 6f, the dissociation barrier of water molecule on the (0001) surface of hcp Ni is also lower by 0.02 eV than that on the (111) surface of fcc Ni, suggesting faster kinetics for water capture and dissociation over hcp Ni surface. Hence, the overall scenario of hydrogenation of acetone can be emphasized by the synergy of reduced product binding capacity and faster kinetics which is responsible for enhanced performance over hcp Ni catalysts.

To gain deeper insight into the origin of the high catalytic activity of hcp Ni NSs, quasi-in-situ ATR-IR spectroscopy was employed. Specifically, the acetone adsorption intensity on hcp Ni NSs increased markedly within the first 4 minutes of reaction (Fig. 6g, h), which is attributed to their higher specific surface area and the abundance of accessible active sites (Supplementary Figs. 42, 43 and Supplementary Notes 1, 2). In contrast, hcp Ni NPs exhibited a delayed increase in adsorption intensity, becoming significant only after 15 minutes (Fig. 6j, k), likely due to the relatively lower density of active sites accessible on their surface compared to the nanosheets. For fcc Ni NPs, a pronounced enhancement in acetone adsorption was observed only after 20 minutes (Fig. 6m, n), this maybe attributed to the weak adsorption of acetone on the surface of fcc Ni NPs. In addition, the acetone adsorption intensity on the surface of Pt/C increased significantly at around 8 minutes (Supplementary Fig. 44), which maybe attributed to the strong competitive HER reaction during the ECH of Pt/C.

Based on the above results, the possible reaction mechanisms of electrocatalytic acetone hydrogenation with H2O over hcp Ni NSs, hcp Ni NPs and fcc Ni NPs are proposed (Fig. 6i, l, o). Acetone exhibits a high adsorption energy on hcp Ni NSs, enabling its rapid adsorption onto the catalyst surface. Concurrently, water molecules readily dissociate to release H* on the surface, which subsequently participate in the electrocatalytic acetone hydrogenation, accompanied by the evolution of a small amount of hydrogen gas. Owing to the lower desorption energy of isopropanol on hcp Ni NSs compared to that on hcp Ni NPs and fcc Ni NPs, the isopropanol product is rapidly released from the catalyst surface. This facilitates the regeneration of active sites and promotes the continuous adsorption of acetone molecules, thereby sustaining a high catalytic turnover. In contrast, acetone exhibits relatively weak adsorption on fcc Ni NPs, leading to a slower surface reaction rate. Furthermore, the strong desorption energy of isopropanol on fcc Ni NPs results in the accumulation of products on the surface, which blocks active sites and further suppresses acetone conversion.

Methods
Chemicals
Bis (η5-2,4-cyclopentadien-1-yl) nickel (Ni(C5H5)2, 98%), nickel (II) oxalate dihydrate (NiC2O4·2H2O, 98%), ribose (C5H10O5, 99%) deuterium oxide (D2O, AR, 99%) and 2-heptanone (C7H14O, 98%) were purchased from Aladdin Industrial Corporation. Oleylamine (CH3(CH2)7CH=CH(CH2)7CH2NH2, OAm, 68−70%) and nickel (II) acetylacetonate (Ni(acac)2, 96%) were purchased from J&K Scientific Ltd. Nickel (II) formate dihydrate (Ni(HCO2)2·2H2O, 95%) and dibromobis(triphenylphosphine)nickel(II) (NiBr2(PPh3)2, 98%) were purchased from Alfa Aesar. Glucose (C6H12O6, AR), cyclohexane (C6H12, ≥ 99.5%), ethanol (C2H6O, 95.0%), isopropanol (C3H8O, ≥ 98.5%), formaldehyde (CH2O, AR, Shanghai, China), 2-butanone (C4H8O, AR), 3-pentanone (C5H10O, ≥ 98%), cyclohexanone (C6H10O, AR), dichloromethane (CH2Cl2, AR) and potassium hydroxide (KOH, 95.0−98.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Nafion solution (~5 wt%, a mixture of lower aliphatic alcohols and water) was purchased from Sigma-Aldrich. Acetone (C3H6O, AR) was purchased from Chinasun Specialty Products Co., Ltd. (Jiangsu, China). Dimethyl sulfoxide-d6 (DMSO-d6, 99.9%) and Raney Nickel (Ni, AR) were purchased from Energy Chemical. 2-Pentanone (C5H10O, > 99.0%), 3-heptanone (C7H14O, > 98.0%), cyclopentanone (C5H8O, > 99.0%) and benzophenone (C13H10O, > 99.0%) were purchased from TCI. 2-Hexanone (C6H12O, 98%) was purchased from Adamas. Cycloheptanone (C7H12O, 99%), cyclooctanone (C8H14O, 97%) and 2-indanone (C9H8O, 98%) were purchased from Acmec. 3-Hexanone (C6H12O, 98%) and 4-heptanone (C7H14O, 98%) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Acetophenone (C8H8O, 98%) was purchased from Accela ChemBio Co. Ltd. (Shanghai, China). Commercial Pt/C (with noble metal loading of 20 wt%) was purchased from Johnson Matthey (JM). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, ≥ 98%) was purchased from Beijing Solarbio Science & Technology Co., Ltd. All the chemicals were used without further purification. The FAB-PK-130 anion exchange membrane was purchased from Suzhou Yilongsheng Energy Technology Co. Ltd. The de-ionized water (18 MΩ cm−1) used in all experiments was prepared by passing water through an ultra-pure purification system (Aqua Solutions).

Syntheses of hcp Ni NSs
Firstly, 9.4 mg of bis (η5-2,4-cyclopentadien-1-yl) nickel, 60 mg of glucose, 100 μL of formaldehyde, and 5.0 mL of oleylamine (CH3(CH2)7CH=CH(CH2)7CH2NH2, OAm, 68−70%) were mixed and added to a glass bottle (volume 35 mL), which was placed in an ultrasonic machine for ultrasonic treatment for about 2 h. Then, the resulting homogeneous mixture was heated from ambient temperature to 210 °C in an oil bath at the heating rate of 5 °C per minute and maintained for 5 h. After the mixture was cooled to ambient temperature naturally, the resulting product was collected, washed, and centrifuged three times through a cyclohexane/ethanol mixture.

Syntheses of hcp Ni NPs and fcc Ni NPs
Nickel (II) acetylacetonate (Ni(acac)2) (12.8 mg), glucose (60 mg) and OAm (5.0 mL) were added into a glass vial (volume: 35 mL). After the vial had been capped, the mixture was ultrasonicated for around 2 h. The resulting homogeneous mixture was then heated from ambient temperature to 230 °C for 5 h in an oil bath pan. After it was cooled to ambient temperature, the resulting products were collected by centrifugation and washed three times with a cyclohexane/ethanol mixture. The similar procedure was adopted for the preparation of fcc Ni NPs except adding glucose.

Characterizations
Low-magnification transmission electron microscopy (TEM) images were collected on a Hitachi HT7700 transmission electron microscope with the accelerating voltage of 120 kV. The crystal nanostructures were analyzed by an X-ray powder diffraction (XRD, X’Pert-Pro MPD diffractometer) equipped with a Cu Kα radiation source (λ = 0.15406 nm). The high-resolution TEM (HRTEM), energy dispersive X-ray spectroscopy (EDS) and the mapping of were collected at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were collected with an SSI S-Probe XPS spectrometer. XAS data were collected at the TPS44 beamline of the National Synchrotron Radiation Research Center (NSRRC, Hsinchu, Taiwan), respectively. Data were processed according to standard procedures using the Demeter program package (Version 0.9.24). The liquid products were test by 1H NMR (Aligent DD2-600, BRUKER AVANCE NEO 400 MHz and BRUKER AVANCE III HD-400) spectrometer. Attenuated total reflectance infrared (ATR-IR) spectra was recorded on a Bruker Vertex 70 spectrometer in the spectral range of 4000 cm−1 to 600 cm−1. Hydrogen and radicals were investigated via electron paramagnetic resonance (EPR) spectroscopy (JES-X320, Japan). High-Resolution mass spectrometry (HR-MS) was recorded on a Bruker micrOTOF-Q III.

The preparation of catalysts for electrochemical of characterizations
5 mg of catalyst was dispersed in 500 μL ethanol and 10 μL of 5 wt% Nafion by sonication for 30 minutes to form a homogeneous ink. Then the homogeneous catalyst ink was dropped evenly on carbon paper (CP, loading area 1.0 cm2) as the working electrode. The catalysts used in this experiment were all loaded onto 1.0 cm2 carbon paper and used as the working electrode, with a loading of 5 mg cm−2, unless otherwise specified. catalyst loading = m (catalyst amount) / s (area of carbon paper).

Electrochemical characterization
The electrochemical measurements were carried out on CHI 660E workstation. All the experiments were carried out at room temperature. All electrolytes used in the tests were freshly prepared and used immediately. 1.0 M KOH/H2O electrolyte was prepared by dissolving KOH solid powder in deionized water, the pH of the 1 M KOH electrolyte is 13.7 ± 0.3. The electrochemical measurements were carried out in a divided two-compartment electrochemical cell consisting of a working electrode, a carbon rod counter electrode, and a saturated calomel electrode as the reference electrode. The cathode cell (10 mL) and anode cell (10 mL), respectively, containing 1.0 M KOH/H2O electrolyte (7 mL) were separated by an untreated Nafion 211 proton exchange membrane (size: 2 × 2 cm2, thickness: 25 mm). 0.1 mmol acetone dissolving in 1.0 M KOH/H2O electrolyte (7 mL) was added into the cathode cell and stirred to form a homogeneous solution. Then, chronoamperometry was carried out under stirring (750 rpm). After the reactions were finished, the solution at the cathode cell was extracted with dichloromethane and tested using 1H NMR to calculate the acetone conversion (con.) and the isopropanol selectivity (sel.). For scaled-up experiment, The electrolysis was carried out in a flow electrolysis cell with Ni foam with a working area of 1 cm2 as the anode, hcp Ni NSs with a working area of 1 cm2 as the cathode, 1.0 M KOH/H2O electrolyte (10 mL) containing 20 mmol acetone as the catholyte, 1.0 M KOH/H2O electrolyte (10 mL) as the anolyte, and a flow rate of the cathode and anolyte of 235 mL min−1. The cathodic and anodic blocks were separated by an anion exchange membrane. In this work, conversion (con.) is referred to as acetone substrates, and selectivity (sel.) is referred to as isopropanol products. The acetone conversion (con.) and isopropanol selectivity (sel.) were calculated using Eqs. (1)-(2) below.

Con: (%) = n(consumed acetones) / n(initial acetones) × 100% (1)
Sel: (%) = n(obtained isopropanol) / n(consumed acetones) × 100% (2)

The production rate and reaction rate were calculated as follows:
Production Rate (g gcat−1 h−1) = m(obtained isopropanol) / m(mass of the catalyst) × t(reaction time) (3)
Reaction Rate (μmol cm−2 h−1) = n(obtained isopropanol) / s(area of carbon paper) × t(reaction time) (4)

The Faradaic efficiency (FE) was obtained according to the following equation:
FE = 2nF / It × 100% (5)
Where F is the Faraday constant (96485 C mol−1), n is the number of moles of the produced isopropanol, I is the current (A) and t is the reaction time (s).

In 1H NMR spectra, when using DMSO-d6 as the solvent for NMR tests, the peaks at 2.5 ppm and 3.3 ppm were assigned to the hydrogen signal of DMSO-d6 and H2O.

All potentials in this work were referenced to Hg/HgCl2. The reversible hydrogen electrode (RHE) is suitable for electrolytes containing 1.0 M KOH. The reference electrode calibration steps are as follows: Using one Pt wire as the working electrode and the other Pt wire as the counter electrode, LSV tests were performed on a saturated calomel electrode in the highly pure H2-saturated 1.0 M KOH electrolyte at a scan rate of 1 mV s−1. The calibration value of the reference electrode is −1.01 V vs. SCE. All potentials measured were calibrated to RHE using the following equation:
E(RHE, 1.0 M KOH) = E(SCE) + 1.01 V (6)

LSV was recorded without iR correction to obtain the polarization curves. Then, chronoamperometry was carried out at a given constant potential. Electrochemical double-layer (Cdl) was carried out by CV in the potential range of 0.65~0.85 V vs. RHE with a scanning rate of 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mV/s. The Δj/2 obtained at 0.75 V vs. RHE was plotted against the scan rate, and the fitted slope was the Cdl.

EPR experiments for electrochemical hydrogenation of acetone
The hcp Ni NSs working electrode and reference electrode were placed in the cathode cell, and the carbon rod counter electrode was placed in the anode cell, 0.1 mmol acetone dissolving in 1.0 M KOH/H2O electrolyte (7.0 mL) was added into the cathode cell and stirred to form a homogeneous solution. Then, chronoamperometry was carried out at a given constant potential of −1.32 V vs. RHE in 1.0 M KOH/H2O (7.0 mL). Then add 0.1 mmol of 5, 5-dimethyl-1-pyrrolin-n-oxide (DMPO) dissolved in 1.0 M KOH/H2O electrolyte, stirring for 1 minute, and quickly remove it for EPR test.

Quasi-in-situ ATR-IR measurements
First of all, the Quasi-in-situ FTIR spectra testing method is listed as follows. The catalyst ink is dropped onto a glassy carbon electrode and then dried. In a divided two-compartment electrochemical cell, we add 0.1 mmol acetone in 1.0 M KOH/H2O (7 mL) electrolyte and apply the voltage of −1.32 V vs. RHE to the working electrode for different time. Finally, the catalyst on the electrode is taken out for FTIR spectra testing.

Computational details
Ab initio quantum mechanical (QM) calculations were performed using the Vienna Ab initio Simulation Package (VASP), version 5.4.4, employing the projector augmented wave (PAW) method with a plane-wave basis set. The electronic structure was described within the framework of density functional theory (DFT, Supplementary Data 1) using the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional41–43. Dispersion corrections were incorporated via the DFT-D3 method with Becke-Johnson damping44. A plane-wave energy cutoff of 400 eV was used, as benchmark tests indicated that a higher cutoff did not improve accuracy. Integration over the Brillouin zone was performed using a Γ-centered 3 × 3 × 1 Monkhorst-Pack k-point grid. Finer k-point spacing did not yield more accurate predictions. Partial orbital occupancies were treated using the first-order Methfessel-Paxton scheme with a smearing width of 0.2 eV. Dipole moment corrections were applied along the surface normal direction. Self-consistent field (SCF) iterations were considered converged when the change in total energy and eigenvalues between successive steps both fell below 1 × 10−6 eV. Ionic relaxation was carried out using a conjugate-gradient algorithm until all atomic forces were below 0.01 eV/Å. The resulting geometries were confirmed as energy minima through vibrational frequency analysis, ensuring the absence of imaginary frequencies. The Hessian matrix, required for frequency calculations, was obtained via finite-difference displacements of ± 0.04 Å along each Cartesian direction for each ion. In these calculations, only the adsorbed species were displaced while the slab atoms remained fixed. All computations were performed under standard conditions of 298 K and 1 bar. Free energies were calculated using standard statistical mechanical expressions accounting for translational, rotational, vibrational, and electronic contributions45. For surface-adsorbed species, translational and rotational contributions to the free energy were omitted. The partition function under the harmonic oscillator approximation is as follows:
qvib = Πi [1 - e^(-hνi / kBT)]^-1 (7)
where k is a force constant, νi is the vibrational frequency, after substituting the partition function qvib, the internal energy correction is as follows:
Uvib(T) = R Σi [hνi / 2k + 1 / (e^(hνi / kBT) - 1)] (8)
The first term is the contribution of zero-point energy (ZPE), and the second term is the contribution of internal energy correction from 0 K to 298 K. The correction of entropy (S) is as follows:
Svib(T) = R Σi [hνi / kBT * 1 / (e^(hνi / kBT) - 1) - ln(1 - e^(-hνi / kBT))] (9)
Here, the first term exactly cancels with the second term of the internal energy shown above. Therefore, only the second term of the entropy need to be corrected. For adsorbed species, the six degrees of freedom of the translation and rotation are frustrated and considered vibration. Such approximation fails when the vibration is extremely low that has a significant contribution to the correction. To avoid such overestimation, the contribution of frequencies below 50 cm−1 are all considered as 50 cm−1.
Zero-point energy (ZPE) in thermo energy correction is as follows:
EZPE = Σ hνi / 2 (10)
The Gibbs free energy G can be derived as follows:
G = EZPE + E + kBT - TS (11)
The above correction is obtained by using VASPKIT code for a post-processing of the VASP output data46. While the DFT results provide insights into adsorption and reaction pathways, there are many difficulties in accurately simulating the material under catalytic conditions, and it may not fully represent the real catalytic interface under electrochemical conditions, thus having certain limitations.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper investigates two-dimensional metastable hexagonal close-packed nickel nanosheets (hcp Ni NSs) for the\nelectrocatalytic hydrogenation of acetone into isopropanol. This non-noble metal catalyst outperforms traditional nickel\ncatalysts and commercial platinum-carbon catalysts in terms of acetone conversion rate and isopropanol selectivity. In\nscaled-up experiments, its performance remains excellent, and it exhibits versatility for various carbonyl compounds. Studies\nshow that this two-dimensional metastable structure enhances catalytic efficiency by strongly adsorbing acetone and\nrapidly desorbing isopropanol, thereby providing more active sites. However, there are some problems in the structure and certain\naspects of the article, which need to be addressed in the future. I recommend that, the manuscript could be considered for\npublication in Nature Communications after minor revision. Here are some comments.\n1. The article points out that nickel precursors have a significant impact on the morphology of hexagonal close-packed nickel\nnanoparticles, but it does not explain the underlying reasons. It is recommended to discuss more about why hcp Ni NPs can\nbe synthesized.\n2. What is the origin of the increased Ni valence state and the elongated Ni-Ni bond length in Ni NSs, as revealed by\nXANES, and how are these changes correlated with the electrocatalytic performance?\n3. To clarify the active phase that plays a role in the stability test, it is recommended to add characterization analysis before\nand after the test to explore its true identity. Currently, it is speculated that it might be the oxide layer formed on the surface of\nthe hcp Ni NPs, rather than the hcp Ni NPs themselves.\n4. The article points out that the significant increase in the material’s adsorption strength stems from its high specific surface\narea and the abundance of accessible active sites, but lacks supporting data. It is recommended to supplement relevant data\nand clarify the types of active sites.\n5. In electrocatalytic processes involving organic molecules, efficient mass transport is crucial. Therefore, it is recommended\nto supplement relevant data to clarify whether the effective transport originates from the residual oleylamine or whether the\nhcp Ni NPs themselves possess excellent transport capabilities.", "reviewer_2": "This work reports the synthesis of two-dimensional metastable-phase Ni hexagonal nanosheets (hcp Ni NSs) and its\napplication as a catalyst for efficient electrocatalytic acetone hydrogenation to isopropanol. The morphological properties of\nthe catalyst were well characterized and elucidated. In electrochemical tests with H-cell, the paper reports that the hcp Ni\nNSs catalyst exhibited superior performance compared to hcp Ni nanoparticles, fcc Ni nanoparticles, and commercial Pt/C\nand Raney Ni. In a flow-cell configuration, the hcp Ni NSs exhibited acetone conversion of 94.3 %, isopropanol selectivity of\n100 %, and isopropanol production rate of 11.31 g/ gcat*h at -250 mA/ cm2. The work demonstrated that the catalyst was\nalso active for the hydrogenation of various ketone substrates. Moreover, significant efforts were made to explain the\nsuperior activity of hcp Ni NSs catalyst as well as the catalytic mechanism by using quasi-in-situ ATR-IR and DFT\ncalculations.\n• The work is original and significant to the field of electrosynthesis.\nHowever, there are still some concerning points that need to be addressed.\n• The LSV curves of Figure a, b, c are quite similar for three catalysts. The LSV of fcc Ni NPs (Figure 4 c2) is even better\nthan hcp Ni NPs. They all exhibited a selectivity of 100 %. It is illogical that fcc Ni NPs (Figure 4 c) showed the lowest\nconversion.\n• It seems that only single electrochemical measurement data are presented for each catalyst. For a concrete conclusion,\nreproducible data should be presented.\n• Full cell voltage of the flow electrolysis cell measurement should be presented at least in the supplementary information.\n• In the mechanism investigation of ECH, Pt/C should also be investigated as a standard reference sample.\n• The authors should compare their results to previous works on ECH of acetone in acidic media (e.g., J. Electrochem. Soc.\n171 114505). In which much lower overpotentials have been reported at similar current densities. The authors should justify\nwhy they chose to use alkaline media, which leads to significantly higher overpotentials at similar current densities.", "reviewer_3": "In this work, Chen et al. report the development of a new 2D metastable-phase nickel hexagonal nanosheet catalyst for the\nelectrocatalytic hydrogenation of acetone to isopropanol, aiming to achieve the electroreduction of acetone to isopropanol at\nroom temperature using pure non-precious metal catalysts. The topic is very interesting and the study is comprehensive and\nconvincing. Here are some comments to further increase the quality of this work.\n1. For the conversion rate, why is hcp Ni better than fcc Ni? What are the advantages of metastable phase materials over\nstable phase materials?\n2. The authors performed the reaction in 1 M KOH electrolyte, so what about the performance in alkaline electrolytes at other\nconcentrations, such as 0.1 M KOH and 0.5 M KOH?\n3. Generally, Ni is not stable in alkaline media. Will the two-dimensional metastable nickel hexagonal nanosheets undergo\nphase transition or electrochemical reconstruction after electrocatalytic hydrogenation of acetone?\n4. The author performed electrocatalytic hydrogenation of acetone to produce isopropanol at industrial current density and\nachieved an acetone conversion of 94.3%, 100% isopropanol selectivity, and an isopropanol yield of 11.31 g gcat-1 h-1.\nTherefore the 1H NMR spectrum data after the large-scale experiment should be given.\n5. Stability test is one of the important indicators to measure catalyst performance. The stability test over eight cycles is\ninsufficient to demonstrate long-term stability of the catalyst. The authors should extend the stability evaluation to at least 10\ncycles.", "reviewer_4": "The authors present a study on the development of non-precious metal catalysts for electrocatalytic hydrogenation, a key\narea in sustainable chemistry. The reported performance of the hcp Ni nanosheets, particularly the high conversion and\nselectivity for acetone hydrogenation, is exceptional and of considerable interest. However, the manuscript's scientific rigor\nand mechanistic depth require substantial improvement to fully support its strong claims. The work shows great promise and\ncould be considered for publication following major revisions that thoroughly address the critical points outlined below.\ncurrently provides limited insight into the underlying formation mechanism. To elevate the synthetic chemistry contribution of\nthis study, a more detailed discussion is required. The authors should elaborate on the specific roles of the key reagents—\nglucose, formaldehyde, and oleylamine—in directing the growth and stabilizing this unique, thermodynamically unfavorable\nphase and morphology. A more complete mechanistic proposal would significantly enhance the impact and reproducibility of\nthe synthetic methodology.\nerror bars in the performance plots. The data points in Fig 4, particularly the comparisons of conversion and reaction rates\nbetween different catalysts, must be supported by standard deviations derived from at least three independent replicate\nexperiments. This will provide a quantitative measure of the data's variability and strengthen the conclusions drawn from\nthese comparisons.\nclaims of high stability, especially in the context of industrial relevance. To substantiate these claims, the authors should\nprovide more demanding, longer-term stability data (e.g., for 20-50 cycles) or present stability tests conducted at higher,\nmore industrially relevant current densities. This would provide a more convincing assessment of the catalyst's practical\ndurability.\nunderstand the material's durability, an investigation of the catalytically active surface is warranted. The authors are\nencouraged to provide post-reaction XPS analysis. This would be highly informative for probing potential changes in the\nsurface, such as the degree of oxidation, shifts in the Ni valence state, or the accumulation of strongly bound adsorbates,\nwhich are crucial factors that could affect long-term catalytic performance and deactivation pathways."}, "author_response": "We sincerely thank you for the positive and encouraging evaluation of our\nwork, and for recognizing the novelty and significance of our two-dimensional metastable hcp Ni\nnanosheets in the electrocatalytic hydrogenation of acetone and other carbonyl compounds. Here, we\ncarefully modified our manuscript according to your important comments.\n1. The article points out that nickel precursors have a significant impact on the morphology of\nhexagonal close-packed nickel nanoparticles, but it does not explain the underlying reasons. It is\nrecommended to discuss more about why hcp Ni NPs can be synthesized.\n[Author’s Response]: Thank you for the insightful suggestion. The synthesis of metastable-phase\nmaterial is a kinetic-controlled process (Nat. Commun. 2025, 16, 4162) and the reducibility of the\nprecursors is crucial for controlling the morphology (Nat. Nanotechnol. 2024, 19, 1306-1315).\nTherefore, we explored which of these precursors (bis (η5-2,4-cyclopentadien-1-yl) nickel, nickel\nacetylacetonate, nickel (II) formate, nickel oxalate dihydrate and bis (triphenylphosphine) nickel (II)\nbromide) is more easily reduced. As shown in Figure 2b, bis (η5-2,4-cyclopentadien-1-yl) nickel\n(Ni(C H ) ) can be easily reduced to hcp Ni NSs with strong crystallinity. However, as shown in\n5 5 2\nSupplementary Figure 4, when using other nickel precursors such as nickel acetylacetonate\n(Ni(acac) ), nickel (II) formate (Ni(HCO ) ·2H O), nickel oxalate dihydrate (NiC O ·2H O) and bis\n2 2 2 2 2 4 2\n(triphenylphosphine) nickel (II) bromide (NiBr (PPh ) ), the crystallinity of hcp Ni was weak and even\n2 3 2\nhcp Ni cannot be synthesized. Therefore, we chose bis (η5-2,4-cyclopentadien-1-yl) nickel in this work.\nSupplementary Figure 4. XRD patterns of using different Ni precursors: (a) Ni(acac) , (b)\n2 2 2 2 4 2 2 3 2\n[Added content]: [Page 3] “The synthesis of metastable-phase material is a kinetic-controlled process\nand the reducibility of different precursors is crucial for controlling the morphology. As shown in\nSupplementary Figures 3, 4, when using other nickel precursors such as nickel acetylacetonate\n(Ni(acac) ), nickel (II) formate (Ni(HCO ) ·2H O), nickel oxalate dihydrate (NiC O ·2H O) and bis\n2 2 2 2 2 4 2\n(triphenylphosphine) nickel (II) bromide (NiBr (PPh ) ), the crystallinity of hcp Ni was weak and even\n2 3 2\nhcp Ni cannot be synthesized (Supplementary Figures 3, 4). Therefore, bis (η5-2,4-cyclopentadien-\n1-yl) nickel was used to synthesize hcp Ni hexagonal nanosheets.”\n2. What is the origin of the increased Ni valence state and the elongated Ni-Ni bond length in Ni NSs,\nas revealed by XANES, and how are these changes correlated with the electrocatalytic performance?\n[Author’s Response]: Thank you for the valuable question. We answer these questions one by one.\n1. The absorption edge depends on non-local screening, namely neighboring Ni screening reduces with\nincreasing the Ni-Ni distance. Therefore, for hcp Ni NSs, the absorption edge shifts to higher energy\nsince core-hole is poorly screened.\nFrom the perspective of crystal structure, the bond length of hcp Ni NSs is longer than that of fcc Ni\nNPs. For fcc Ni NPs and hcp Ni NSs, each nickel atom is surrounded by 12 nickel atoms. fcc Ni are\nABC stacked, while hcp Ni are AB stacked. Therefore, the inter-nickel atomic distance d of fcc Ni and\nhcp Ni were calculated as follows:\nFor fcc Ni, the distance between the central nickel atom and the surrounding 12 nickel atoms is:\na 3.48Å\nfcc Ni\nd = = = 2.4607 Å\n√2 √2\nFor hcp Ni, the distance between the central nickel atom and the six surrounding nickel atoms is:\nd = a = 2.45 Å\nhcp Ni\nThe central Ni atom is equidistant from the six nickel atoms above and below it, and the central nickel\natom forms a triangular pyramid with the three nickel atoms above and below it. The base of this\ntriangular pyramid is an equilateral triangle with a side length of a and a height of c /2. The\nhcp Ni hcp Ni\ndistance between the vertices of the centroid of the equilateral triangle at the base of the triangular\npyramid is a /√3. Therefore, the side length of the triangular pyramid (that is, the distance from\nhcp Ni\nthe central nickel atom to the nickel atoms above or below) is:\na 2 c 2 2.45 2 4.07 2\nd =√( hcp Ni ) +( hcp Ni ) =√( ) +( ) = 2.4783 Å\n√3 2 √3 2\nTherefore, the average distance between the central nickel atom and the surrounding 12 nickel atoms\nin hcp Ni is:\n2.45+2.4783\nd = = 2.4642 Å > 2.4607 Å\nreduced neighboring Ni screening and shift of absorption edge to higher energy.\n2. According to DFT calculation (Figure 6e), the longer Ni-Ni bond length of hcp Ni NSs leads to\nlower the adsorption energy of acetone (-0.8 eV), lower than that fcc Ni (-0.75 eV). Therefore, acetone\nis more easily adsorbed on hcp Ni surface, lead to the higher catalytic activity.\n3. To clarify the active phase that plays a role in the stability test, it is recommended to add\ncharacterization analysis before and after the test to explore its true identity. Currently, it is speculated\nthat it might be the oxide layer formed on the surface of the hcp Ni NPs, rather than the hcp Ni NPs\nthemselves.\n[Author’s Response]: Thank you for the constructive suggestion. To clarify the origin of the active\nphase of hcp Ni NPs, we carried out more experiments.\n1. We performed X-ray photoelectron spectroscopy (XPS) tests on hcp Ni NPs at open circuit potential\n(OCP) condition and after the reaction, respectively. As shown in Ni 2p XPS spectra (Figure R1a),\nthere are Ni-Ni bonds (852.9 eV and 870.2 eV) and Ni-OH bonds (856.0 eV and 873.5 eV) of hcp Ni\nNPs (Nat. Commun. 2022, 13, 3857; J. Am. Chem. Soc. 2018, 140, 9046-9050). No Ni-O bonds were\ndetected, indicating that there was no oxide layer on the surface of hcp Ni NPs. The existence of Ni-\nOH bonds may be due to the adsorption of hydroxyl groups (-OH) in the KOH electrolyte on the\nsurface of hcp Ni NPs.\n2. In addition, we also tested the electrocatalytic acetone hydrogenation performance of Ni(OH) and\nR1b, it was found that the acetone conversion of Ni(OH) and NiO are 37.1% and 26.2% respectively,\n2\nwhile the acetone conversion using the hcp Ni NSs and hcp Ni NPs reached 95% and 75.1%,\nrespectively.\nThese results suggest that the active phase originates from hcp Ni NPs themselves, rather than from\nthe oxide layer formed on the surface of the hcp Ni NPs.\nFigure R1 (a) Ni 2p XPS spectra of hcp Ni NPs at OCP condition and at -1.32 V vs. RHE for 5 h. (b)\nElectrochemical catalytic hydrogenation of acetone with hcp Ni NSs, hcp Ni NPs, Ni(OH) and NiO\n2\nat -1.32 V vs. RHE for 5 h.\n4. The article points out that the significant increase in the material’s adsorption strength stems from\nits high specific surface area and the abundance of accessible active sites, but lacks supporting data.\nIt is recommended to supplement relevant data and clarify the types of active sites.\n[Author’s Response]: Thank you for the valuable suggestion.\n1. Firstly, the electrochemically active surface area (ECSA) is estimated using the double layer\ncapacitance method at non-Faradaic potential windows. In our previous version, the current is not\nsymmetrical due to the presence of faraday current, which leads to the higher measured double layer\ncapacitance (C ) and ECSA. Therefore, in order to ensure the accuracy of the measurement results,\ndl\nwe re-selected a non-Faradaic potential window for testing to avoid the presence of Faradaic current.\nAs shown in Supplementary Figures 40 and 41, the results show that hcp Ni NSs has a larger ECSA\n(13.25 cm2) than hcp Ni NPs (10.75 cm2) and fcc Ni NPs (8.85 cm2), indicating the largest\nelectrochemically accessible surface area of hcp Ni NSs.\n2. Then, we calculate the numbers of active sites for hcp Ni and fcc Ni respectively. For hcp Ni, since\nits active surface is the (0001) crystal plane, and its unit cell parameters are a = b = 0.245 nm, c= 0.407\nnm, therefore the average active surface atoms per 1 square centimeter of hcp Ni were calculated as\nfollows:\n1 atom atom\nActive sites = =1.924×1015\n(hcp Ni)\n√3\ncm2\n(0.245×0.245× )nm2\n0.348 nm, the average active surface atoms per 1 square centimeter of fcc Ni were calculated as follows:\n8 atom atom\nActive sites = =1.907×1015\n(fcc Ni) (2×√3×0.348×0.348)nm2 cm2\nTherefore, the number of active sites of hcp Ni NSs is 2.549×1016 atom, while the number of active\nsites of hcp Ni NPs and fcc Ni NPs are 2.068×1016 atom and 1.688×1016 atom , respectively,\nindicating that the larger electrochemical specific surface area (ECSA) can provide more active sites\nfor acetone adsorption.\nSupplementary Figure 40. CV curves in double layer region and plots of the charging current density\nas function of scan rate of (a , a ) hcp Ni NSs, (b , b ) hcp Ni NPs and (c , c ) fcc Ni NPs with the\n1 2 1 2 1 2\nscan rates of 20, 40, 60, 80, 100, 120, 140, 160, 180 and 200 mV s-1.\nSupplementary Figure 41. (a) The double-layer capacitance of hcp Ni NSs, hcp Ni NPs and fcc Ni\nNPs catalysts at different potential scan rates. (b) The corresponding relative electrochemically active\nsurface area (ECSA) of hcp Ni NSs, hcp Ni NPs and fcc Ni NPs. hcp Ni NSs has a significantly larger\nECSA (13.25 cm2) than hcp Ni NPs (10.75 cm2) and fcc Ni NPs (8.85 cm2), indicating that it has a\nlarger electrochemically accessible surface area.\n3. According to DFT calculation (Figure 6d), we compared the acetone reduction pathway on hcp Ni\n(0001) surface and fcc Ni (111) surface, where hcp Ni (0001) surface has lower energy barrier than fcc\nNi (111) surface. In addition, we tested the electrocatalytic acetone hydrogenation performance of\nNi(OH) and NiO, and compared the performance with those of hcp Ni NSs and hcp Ni NPs. As shown\n2\nin Figure R2, it was found that the acetone conversion of Ni(OH) and NiO are 37.1% and 26.2%\n2\nrespectively, while the acetone conversion using the hcp Ni NSs and hcp Ni NPs reached 95% and\n75.1%, respectively. Therefore, we consider the active site is the hcp Ni.\nFigure R2 Electrochemical catalytic hydrogenation of acetone with hcp Ni NSs, hcp Ni NPs, Ni(OH)\n2\nand NiO at -1.32 V vs. RHE for 5 h.\n[Added content]: [Page 8] “which is attributed to their higher specific surface area and the abundance\nof accessible active sites (Supplementary Figures 40, 41 and Supplementary Notes 1, 2).”\nSupplementary Note 2. The average active sites calculation.\nFor hcp Ni, since its active surface is the (0001) crystal plane, and its unit cell parameters are a = b =\n0.245 nm, c= 0.407 nm, therefore the average active surface atoms per 1 square centimeter of hcp Ni\nwere calculated as follows:\n1 atom atom\nActive sites = =1.924×1015 (S2)\n(hcp Ni)\n√3\ncm2\n(0.245×0.245× )nm2\n0.348 nm, therefore the average active surface atoms per 1 square centimeter of hcp Ni were calculated\nas follows:\n8 atom atom\nActive sites = =1.907×1015 (S3)\n(fcc Ni) (2×√3×0.348×0.348) nm2 cm2\nThe number of active sites of hcp Ni NSs is 2.549×1016 atom, while the number of active sites of\nhcp Ni NPs and fcc Ni NPs are 2.068×1016 atom and 1.688×1016 atom, respectively, indicating\nthat the larger electrochemical specific surface area (ECSA) can provide more active sites for acetone\nadsorption.\n5. In electrocatalytic processes involving organic molecules, efficient mass transport is crucial.\nTherefore, it is recommended to supplement relevant data to clarify whether the effective transport\noriginates from the residual oleylamine or whether the hcp Ni NPs themselves possess excellent\ntransport capabilities.\n[Author’s Response]: Thank you for the valuable suggestion. To clarify the origin of effective mass\ntransport, more experiments were carried out.\n1. First of all, we conducted the ATR-IR testing of hcp Ni NPs, and the results showed that there was\nno oleylamine residue on the surface of hcp Ni NPs (Figure R3a).\n2. We also synthesized unwashed hcp Ni NPs. The ATR spectrum shows that there was residual\noleylamine on the surface of unwashed hcp Ni NPs (Figure R3a). Then, we tested the electrocatalytic\nhydrogenation of acetone using both unwashed hcp Ni NPs and hcp Ni NPs. As shown in Figure R3b,\nthe acetone conversion using the unwashed hcp Ni NPs was only 35.3 %, while the conversion using\nhcp Ni NPs reached 75.1%.\n3. In addition, electrochemical impedance spectroscopy (EIS) measurements of the unwashed hcp Ni\nNPs and hcp Ni NPs in 1 M KOH electrolyte were performed. As shown in Figure R3c, hcp Ni NPs\nexhibited a lower impedance than the unwashed hcp Ni NPs.\nThese results suggest that the efficient mass transfer originates from the excellent transport capability\nof the hcp Ni NPs catalyst itself, rather than from oleylamine.\nFigure R3. (a) The ATR spectra of oleylamine, unwashed hcp Ni NPs and hcp Ni NPs. (b)\nElectrochemical catalytic hydrogenation of acetone with unwashed hcp Ni NPs and hcp Ni NPs at -\n1.32 V vs. RHE for 5 h. (c) EIS Nyquist plots of unwashed hcp Ni NPs and hcp Ni NPs in 1.0 M KOH\nelectrolyte.\nReviewer #2 (Remarks to the Author):\nThis work reports the synthesis of two-dimensional metastable-phase Ni hexagonal nanosheets (hcp\nNi NSs) and its application as a catalyst for efficient electrocatalytic acetone hydrogenation to\nisopropanol. The morphological properties of the catalyst were well characterized and elucidated. In\nelectrochemical tests with H-cell, the paper reports that the hcp Ni NSs catalyst exhibited superior\nperformance compared to hcp Ni nanoparticles, fcc Ni nanoparticles, and commercial Pt/C and Raney\nNi. In a flow-cell configuration, the hcp Ni NSs exhibited acetone conversion of 94.3 %, isopropanol\nselectivity of 100 %, and isopropanol production rate of 11.31 g/ g h-1 at -250 mA/ cm2. The work\ncat\ndemonstrated that the catalyst was also active for the hydrogenation of various ketone substrates. The\nwork is original and significant to the field of electrosynthesis. However, there are still some concerning points that\nneed to be addressed.\n[Author’s Response]: We sincerely thank you for recognizing the originality and significance of our\nwork. We have carefully considered each of these comments and have addressed them in details in the\nrevised manuscript according to your important comments.\n• The LSV curves of Figure a, b, c are quite similar for three catalysts. The LSV of fcc Ni NPs (Figure\n4 c ) is even better than hcp Ni NPs. They all exhibited a selectivity of 100 %. It is illogical that fcc Ni\n[Author’s Response]: We sincerely thank you for pointing out the inconsistency in the LSV curves\nof hcp Ni NPs (Figure 4b ) and fcc Ni NPs (Figure 4c ) presented in Figure 4. After careful re-\n2 2\nexamination, we found that the LSV curves in Figure 4b and Figure 4c were mistakenly placed in\n2 2\nreverse order. We have now corrected the figure in the revised manuscript.\nIn addition, to strengthen the conclusion, we conducted three repeated experiments on hcp Ni NSs,\nhcp Ni NPs and fcc Ni NPs respectively (Supplementary Figure 10), and the results showed that\nwhen the electrolyte contained acetone, hcp Ni NSs had a larger current density than hcp Ni NPs and\nfcc Ni NPs at -1.32 V vs. RHE (Supplementary Figure 10).\nFigure 4. Electrochemical performance. The linear sweep voltammetry (LSV) curves with or\nwithout acetone on (a ) hcp Ni NSs, (b ) hcp Ni NPs and (c ) fcc Ni NPs.\n2 2 2\nSupplementary Figure 10. The linear sweep voltammetry (LSV) curves with acetone (solid lines)\nand without acetone (dashed lines) of three repeated experiments on (a) hcp Ni NSs, (b) hcp Ni NPs\nand (c) fcc Ni NPs.\n• It seems that only single electrochemical measurement data are presented for each catalyst. For a\nconcrete conclusion, reproducible data should be presented.\n[Author’s Response]: Thank you for the constructive suggestion. To ensure the reliability and\nreproducibility of our electrochemical results, we have conducted three independent measurements for\neach catalyst. The averaged data with standard deviations are now presented in the revised figures, and\nerror bars have been added accordingly (Figure 4a , b , c , d, e, j and Supplementary Figures 11, 12,\n3 3 3\n13).\nFigure 4. Electrochemical performance. Time-dependent acetone conversion and isopropanol\nselectivity at -1.32 V vs. RHE over (a ) hcp Ni NSs, (b ) hcp Ni NPs and (c ) fcc Ni NPs. (d)\n3 3 3\nComparisons of con., sel. and production rate with the reported catalysts. (e) The con., sel. and reaction\nrate of hcp Ni NSs, hcp Ni NPs, fcc Ni NPs, commercial Pt/C and Raney Ni. (j) Con. and sel. of the\nhcp Ni NSs in 1.0 M KOH/H O electrolyte with 20 mmol acetone at the current density of -250 mA\n2\ncm-2.\nSupplementary Figure 11. Potential-dependent conversion (Con.) of acetone and selectivity (Sel.)\nof isopropanol over hcp Ni NSs within 5 h.\nSupplementary Figure 12. Comparison of acetone conversion in different time periods within 5 h at\n-1.32 V vs. RHE over hcp Ni NSs, hcp Ni NPs and fcc Ni NPs.\nSupplementary Figure 13. Comparison of the production rate of hcp Ni NSs, hcp Ni NPs, fcc Ni\nNPs, commercial Pt/C and Raney Ni in a divided two-compartment electrochemical cell.\n• Full cell voltage of the flow electrolysis cell measurement should be presented at least in the\nsupplementary information.\n[Author’s Response]: Thank you for the valuable suggestion. In this work, the flow electrolysis cell\nwe used was a three-electrode system with a reference electrode, where the potential of -1.45 V vs.\nRHE was reached at a current density of -250 mA cm-2 (Figure 4i). No cell voltage is involved. We\nhave added this information in Figure 4i.\n• In the mechanism investigation of ECH, Pt/C should also be investigated as a standard reference\nsample.\n[Author’s Response]: According to your suggestion, more experiments were conducted to explore\nthe mechanism of Pt/C:\n(1) As shown in Supplementary Figure 37, we tested the LSV curves of electrolytes with acetone\nand without acetone when using Pt/C as a catalyst. The results showed that the current density of the\nelectrolyte without acetone (-32.2 mA cm-2) was more negative than that of the electrolyte with acetone\n(-29.9 mA cm-2) at -1.32 V vs. RHE, which indicates that Pt/C is more likely to undergo HER reaction.\nTherefore, the competitive HER reaction is prone to occur on the Pt/C surface, resulting in an\nunsatisfactory acetone conversion (Supplementary Figure 37).\n(2) Then, we conducted a Quasi-in-situ ATR-IR test on Pt/C and compared them with hcp Ni NSs, as\nshown in Figure R4. The acetone adsorption intensity on hcp Ni NSs increased markedly within the\nfirst 4 minutes of reaction (Figure R4a, b), while acetone adsorption intensity on the surface of Pt/C\nincreased significantly around 8 minutes (Figure R4c, d), which may be attributed to the strong\ncompetitive HER reaction during the ECH of Pt/C.\n(3) Figure 6b clearly reveals the adsorption strength of isopropanol on the catalyst. When using hcp\nNi NSs as the catalyst, a weak peak was observed at the hydroxyl group (-OH) (~ 3419.7 cm-1),\nindicating that isopropanol is more prone to desorption from the surface of the hcp Ni NSs. Therefore,\nthe product isopropanol could quickly fall off after formation on the surface of hcp Ni NSs. For Pt/C,\nthe strong absorption peak of hydroxyl group (-OH) (~ 3419.7 cm-1) can be seen, indicating that\nisopropanol has a strong adsorption capacity on the Pt/C catalyst, which makes it difficult for the\nisopropanol to desorb quickly from the surface of Pt/C, resulting in an unsatisfactory acetone\nconversion by Pt/C.\nSupplementary Figure 37. Linear sweep voltammetry (LSV) curves of Pt/C at a scan rate of 10 mV\ns-1 in 1 M KOH electrolyte with or without 0.1 mmol acetone.\nFigure R4. Quasi-in-situ ATR-IR spectroscopy and the corresponding heatmap of normalized in (a)\nfor electrocatalytic acetone hydrogenation of (a, b) hcp Ni NSs and (c, d) Pt/C.\nFigure 6. (b) Infrared (IR) spectra of four catalysts treated with isopropanol (IPA): hcp Ni NSs with\nIPA, hcp Ni NPs with IPA, fcc Ni NPs with IPA and Pt/C with IPA (from top to bottom). Orange\nrectangle highlights the region of the chemical vibrations of -OH. 2350 cm-1 is the characteristic peak\nof CO in air.\n2\n[Added content]: [Page 7] “but the competitive HER reaction is prone to occur on the Pt/C surface,\nresulting in an unsatisfactory acetone conversion. As shown in Supplementary Figure 37, the LSV\ncurves of Pt/C show that the current density of the electrolyte without acetone (-32.2 mA cm-2) is more\nnegative than that of the electrolyte with acetone (-29.9 mA cm-2) at -1.32 V vs. RHE, which indicates\nthat Pt/C is more likely to undergo HER reaction.”\n[Page 7] “For hcp Ni NPs, fcc Ni NPs and Pt/C, the absorption peak of hydroxyl group (-OH) (~\n3419.7 cm-1) can be seen, indicating that isopropanol has a strong adsorption capacity on the these\ncatalysts, which makes it difficult for the isopropanol generated on the hcp Ni NPs, fcc Ni NPs and\nPt/C surface to desorb quickly from their surface. This results in an unsatisfactory acetone conversion\nby hcp Ni NPs, fcc Ni NPs and Pt/C.”\n[Page 9] “In addition, the acetone adsorption intensity on the surface of Pt/C increased significantly at\naround 8 minutes (Supplementary Figures 42), which may be attributed to the strong competitive\nHER reaction during the ECH of Pt/C.”\n• The authors should compare their results to previous works on ECH of acetone in acidic media (e.g.,\nJ. Electrochem. Soc. 171 114505). In which much lower overpotentials have been reported at similar\ncurrent densities. The authors should justify why they chose to use alkaline media, which leads to\nsignificantly higher overpotentials at similar current densities.\n[Author’s Response]: Thank you for the valuable question. We have answered the questions one by\none.\n1. The literature and our work used totally different devices. For this literature (J. Electrochem. Soc.\n2025, 171, 114505), it used a fuel cell system, which uses chemical energy to produce electricity. In\nour work, we used a flow electrolysis cell with a three-electrode system, which uses electricity to\nproduce chemicals. Therefore, it is difficult to compare the overpotential and current density of these\ntwo works.\n2. The advantages of usage of alkaline electrolyte are based on the following reasons: (1) Alkaline\nmedia can help suppress the HER, resulting in higher conversion rates for the electrocatalytic\nhydrogenation reaction (J. Am. Chem. Soc. 2025, 147, 13158-13168). (2) Considering the stability of\nthe catalysts, non-noble metal catalysts generally need to operate in alkaline media. For example,\nnickel-based catalysts need to work under alkaline conditions, enabling long-term stable operation\n(Nat. Commun. 2020, 11, 2720; Nat. Commun. 2014, 5, 4695).\nReviewer #3 (Remarks to the Author):\nIn this work, Chen et al. report the development of a new 2D metastable-phase nickel hexagonal\nnanosheet catalyst for the electrocatalytic hydrogenation of acetone to isopropanol, aiming to achieve\nthe electroreduction of acetone to isopropanol at room temperature using pure non-precious metal\ncatalysts. The topic is very interesting and the study is comprehensive and convincing. Here are some\ncomments to further increase the quality of this work.\n[Author’s Response]: We sincerely thank you for recognizing the novelty, scientific interest, and\ncomprehensive nature of our study. We greatly appreciate your constructive comments and suggestions\nfor further improvement. According to your suggestions, we have made seriously modifications to our\nmanuscript.\n1. For the conversion rate, why is hcp Ni better than fcc Ni? What are the advantages of metastable\nphase materials over stable phase materials?\n[Author’s Response]: Thank you for the valuable question. We answer these questions one by one.\n1. In the electrocatalytic acetone hydrogenation performance test, the higher conversion rate of hcp Ni\nNSs can be attributed to the following reasons: (1) For the electrochemically active surface area\n(ECSA), hcp Ni NSs have a larger ECSA than fcc Ni NPs. (2) hcp Ni NSs preferentially exposes the\nhighly active (0001) crystal plane, which has lower the adsorption energy of acetone (-0.8 eV) than\nthat of fcc Ni (111) surface (-0.75 eV) (Figure 6e). Therefore, acetone is more easily adsorbed on hcp\nNi NSs surface, lead to the higher catalytic activity. (3) For DFT calculation, we compared the acetone\nreduction pathway on hcp Ni (0001) surface and fcc Ni (111) surface (Figure 6d), where hcp Ni NSs\n(0001) surface has lower energy barrier than that of fcc Ni (111) surface. Therefore, hcp Ni NSs has\nhigher acetone conversion than fcc Ni NPs.\n2. The advantages of metastable phase materials are: (1) Compared with stable phase materials,\nmetastable phase materials have larger Gibbs free energy and more unique electronic structure, which\nis benefit for achieving higher catalytic performance (J. Am. Chem. Soc. 2024, 146, 20251-20262;\nChem. Rev. 2023, 123, 9676-9717; Nat. Commun. 2023, 14, 1761; Angew. Chem. Int. Ed. 2023,\ne202218924). (2) In the process of electrocatalytic reaction, metastable phase materials often\naccelerate electron transfer, promote substrate adsorption and optimize intermediates, thus having\nhigher catalytic activity (Adv. Mater. 2024 36, 2308839).\n2. The authors performed the reaction in 1 M KOH electrolyte, so what about the performance in\nalkaline electrolytes at other concentrations, such as 0.1 M KOH and 0.5 M KOH?\n[Author’s Response]: Thank you for the insightful question. Following your suggestions, we\nconducted acetone electrocatalytic hydrogenation tests in 0.1 M KOH and 0.5 M KOH over hcp Ni\nNSs within 5 h at -1.32 V vs. RHE. The results are shown in Figure R5. In 0.1 M KOH electrolyte,\nthe acetone conversion is 58.3 % and the isopropanol selectivity is 100%, while in 0.5 M KOH\nelectrolyte, the acetone conversion is 78.3 % and the isopropanol selectivity is 100%. This may be\nattribute to the higher OH- content in 1.0 M KOH, which makes the electrolyte more conductive and\nultimately makes it easier for electrocatalysis.\nFigure R5. Conversion and selectivity of acetone electrocatalytic hydrogenation in 0.1 M KOH and\n0.5 M KOH electrolyte over hcp Ni NSs within 5 h at -1.32 V vs. RHE.\n3. Generally, Ni is not stable in alkaline media. Will the two-dimensional metastable nickel hexagonal\nnanosheets undergo phase transition or electrochemical reconstruction after electrocatalytic\nhydrogenation of acetone?\n[Author’s Response]: Thank you for the valuable question. We characterized the morphology and\nstructure of the hcp Ni NSs catalyst after the electrocatalytic hydrogenation of acetone. As shown in\nSupplementary Figure 14, after the reaction, the catalyst still maintains the two-dimensional\nmorphology. HRTEM and XRD results show that the catalyst remains the close-packed hexagonal\nphase. All these results show that the hcp Ni NSs catalyst did not undergo phase change and\nelectrochemical reconstruction during the reaction.\nSupplementary Figure 14. (a) HAADF-STEM image, (b) EDS element mapping, (c) HRTEM image,\n(d) enlarged HRTEM image in (c) and (e) XRD pattern of hcp Ni NSs after electrochemical acetone\nhydrogenation.\n4. The author performed electrocatalytic hydrogenation of acetone to produce isopropanol at industrial\ncurrent density and achieved an acetone conversion of 94.3%, 100% isopropanol selectivity, and an\nisopropanol yield of 11.31 g g -1 h-1. Therefore the 1H NMR spectrum data after the large-scale\ncat\nexperiment should be given.\n[Author’s Response]: Thank you for the valuable suggestion. First of all, to ensure the reliability and\nreproducibility of our large-scale experiment result, we have conducted three independent\nmeasurements. The results showed that the averaged acetone conversion was 95.9% and the\nisopropanol selectivity was 100%. In addition, we have added the 1H NMR spectrum of the large-scale\nexperiment, as shown in Supplementary Figure 18.\nSupplementary Figure 18. (a) The 1H-NMR spectrum of electrocatalytic hydrogenation of acetone\nproducts with DCM and DMSO-d , hcp Ni NSs electrode, -250 mA cm-2, RT, 20 h. (b) Enlarge view\n6\nin (a).\n[Added content]: [Page 6] “The result of 1H NMR spectrum is shown in Supplementary Figure 18.”\n5. Stability test is one of the important indicators to measure catalyst performance. The stability test\nover eight cycles is insufficient to demonstrate long-term stability of the catalyst. The authors should\nextend the stability evaluation to at least 10 cycles.\n[Author’s Response]: Thank you for the valuable suggestion. Following your suggestions, we\nperformed a thirty-cycles stability test. As shown in Figure 4f, hcp Ni NSs remains at around 80%\nacetone conversion and 100% isopropanol selectivity after thirty cycles of testing at -1.32 V vs. RHE.\nFigure 4. (f) Catalytic stability of hcp Ni NSs. Reaction conditions: acetone (0.1 mmol), 1.0 M\nKOH/H O electrolyte, -1.32 V vs. RHE, RT, 5 h, catalyst (5.0 mg) and working area: 1 cm2.\n2\n[Added content]: [Page 2] “Furthermore, this catalyst remains at around 80% acetone conversion\nafter thirty cycles of testing at -1.32 V vs. reversible hydrogen electrode (RHE)”\n[Page 6] “As can be seen in Figure 4f, hcp Ni NSs remains at around 80% acetone conversion after\nthirty cycles of testing at -1.32 V vs. RHE.”\nReviewer #4 (Remarks to the Author):\nThe authors present a study on the development of non-precious metal catalysts for electrocatalytic\nhydrogenation, a key area in sustainable chemistry. The reported performance of the hcp Ni nanosheets,\nparticularly the high conversion and selectivity for acetone hydrogenation, is exceptional and of\nconsiderable interest. However, the manuscript's scientific rigor and mechanistic depth require\nsubstantial improvement to fully support its strong claims. The work shows great promise and could\nbe considered for publication following major revisions that thoroughly address the critical points\noutlined below.\n[Author’s Response]: We sincerely thank you for your recognition of our work and appreciate your\nvaluable suggestions for improving the scientific rigor and mechanistic depth. We have carefully\nrevised the manuscript based on your suggestions.\nthe manuscript currently provides limited insight into the underlying formation mechanism. To elevate\nthe synthetic chemistry contribution of this study, a more detailed discussion is required. The authors\nshould elaborate on the specific roles of the key reagents—glucose, formaldehyde, and oleylamine—\nin directing the growth and stabilizing this unique, thermodynamically unfavorable phase and\nmorphology. A more complete mechanistic proposal would significantly enhance the impact and\nreproducibility of the synthetic methodology.\n[Author’s Response]: Thank you for the valuable question. We first discuss the role of key reagents\nfor this synthesis.\n1. Glucose is important to generate the metastable phase nickel (hcp Ni). When other sugar compounds\nsuch as fructose and ribose are used as reducing agents, the characteristic diffraction peaks of nickel\nhydroxide (Ni(OH) ) and metastable phase nickel (hcp Ni) appeared (Supplementary Figure 5a, b).\nsynthesized (Supplementary Figure 5c).\nSupplementary Figure 5. XRD patterns of using different reducing agent: (a) fructose, (b) ribose and\n(c) phenol.\n2. Formaldehyde is important to obtain two-dimensional morphology. During the synthesis process,\nformaldehyde decomposes to release CO, which has been reported to promote the induction of the\ngrowth of two-dimensional hexagonal nanosheets (Nat. Nanotech. 2011, 6, 28-32). As shown in Figure\nR6, in the absence of formaldehyde, the hcp Ni tends to grow toward a nanoparticle morphology.\nFigure R6. (a) TEM image and (b) XRD pattern of synthesizing hcp Ni without adding formaldehyde.\n3. Oleylamine is mainly used as an organic solvent in the synthesis of many metastable-phase materials\n(Nat. Commun. 2023, 14, 1761; Adv. Mater. 2024, 36, 2308839; Angew. Chem. Int. Ed. 2025, 64,\ne202504641). Its good solubility helps to uniformly dissolve and mix nickel precursors, glucose, and\nformaldehyde, ensuring a uniform reaction. In addition, when other organic solvents such as oleic acid\nand dodecylamine were used, no crystalline products can be obtained (Figure R7).\nFigure R7. XRD patterns of using different organic solvents: (a) oleic acid and (b) dodecylamine.\nTherefore, we proposal the mechanism for the synthesis of hcp Ni NSs. During the synthesis of hcp\nNi NSs, glucose plays a major role in forming metastable phase nickel (hcp Ni), while formaldehyde\nis a morphology regulator that mainly controls the growth of two-dimensional hexagonal nanosheets\nof hcp Ni NSs. Oleylamine, as an organic solvent, can well dissolve nickel precursors, glucose and\nformaldehyde.\n[Added content]: [Page 3] “Glucose is important to form the hcp Ni. When other sugar compounds\nsuch as fructose and ribose were used as reducing agents, the characteristic diffraction peaks of nickel\nhydroxide (Ni(OH) ) and metastable phase hcp Ni both appeared (Supplementary Figure 5a, b). In\n2\naddition, when phenol is used as a reducing agent, only stable phase fcc Ni can be synthesized\n(Supplementary Figure 5c). Formaldehyde, as a morphology regulator, releases CO during heating,\nwhich can promote the growth of two-dimensional hexagonal nanosheets. When reducing the amount\nof formaldehyde, the hcp Ni tends to grow toward a nanoparticle morphology (Supplementary Figure\n6).”\nis essential to include error bars in the performance plots. The data points in Fig 4, particularly the\ncomparisons of conversion and reaction rates between different catalysts, must be supported by\nstandard deviations derived from at least three independent replicate experiments. This will provide a\nquantitative measure of the data's variability and strengthen the conclusions drawn from these\ncomparisons.\n[Author’s Response]: Thank you for the valuable suggestion. To ensure the reliability and\nreproducibility of our electrochemical results, we have conducted three independent measurements for\neach catalyst. The averaged data with standard deviations are now presented in the revised figures, and\nerror bars have been added accordingly. The revised figures have been added in Figure 4a , b , c , d,\n3 3 3\ne, j and Supplementary Figures 11, 12, 13.\nFigure 4. Electrochemical performance. Time-dependent acetone conversion and isopropanol\nselectivity at -1.32 V vs. RHE over (a ) hcp Ni NSs, (b ) hcp Ni NPs and (c ) fcc Ni NPs. (d)\n3 3 3\nComparisons of con., sel. and production rate with the reported catalysts. (e) The con., sel. and reaction\nrate of hcp Ni NSs, hcp Ni NPs, fcc Ni NPs, commercial Pt/C and Raney Ni. (j) Con. and sel. of the\nhcp Ni NSs in 1.0 M KOH/H O electrolyte with 20 mmol acetone at the current density of -250 mA\n2\ncm-2.\nSupplementary Figure 11. Potential-dependent conversion (Con.) of acetone and selectivity (Sel.)\nof isopropanol over hcp Ni NSs within 5 h.\nSupplementary Figure 12. Comparison of acetone conversion in different time periods within 5 h at\n-1.32 V vs. RHE over hcp Ni NSs, hcp Ni NPs and fcc Ni NPs.\nSupplementary Figure 13. Comparison of the production rate of hcp Ni NSs, hcp Ni NPs, fcc Ni\nNPs, commercial Pt/C and Raney Ni in a divided two-compartment electrochemical cell.\nto fully support the claims of high stability, especially in the context of industrial relevance. To\nsubstantiate these claims, the authors should provide more demanding, longer-term stability data (e.g.,\nfor 20-50 cycles) or present stability tests conducted at higher, more industrially relevant current\ndensities. This would provide a more convincing assessment of the catalyst's practical durability.\n[Author’s Response]: Thank you for the valuable suggestion. According to your suggestions, we\nperformed a thirty-cycles stability test. As shown in Figure 4f, hcp Ni NSs remains at around 80%\nacetone conversion and 100% isopropanol selectivity after thirty cycles of testing at -1.32 V vs. RHE.\nFigure 4. (f) Catalytic stability of hcp Ni NSs. Reaction conditions: acetone (0.1 mmol), 1.0 M\nKOH/H O electrolyte, -1.32 V vs. RHE, RT, 5 h, catalyst (5.0 mg) and working area: 1 cm2.\n2\n[Added content]: [Page 2] “Furthermore, this catalyst remains at around 80% acetone conversion\nafter thirty cycles of testing at -1.32 V vs. reversible hydrogen electrode (RHE).”\n[Page 6] “As can be seen in Figure 4f, hcp Ni NSs remains at around 80% acetone conversion after\nthirty cycles of testing at -1.32 V vs. RHE.”\nstructure. However, to truly understand the material's durability, an investigation of the catalytically\nactive surface is warranted. The authors are encouraged to provide post-reaction XPS analysis. This\nwould be highly informative for probing potential changes in the surface, such as the degree of\noxidation, shifts in the Ni valence state, or the accumulation of strongly bound adsorbates, which are\ncrucial factors that could affect long-term catalytic performance and deactivation pathways.\n[Author’s Response]: Thank you for the insightful suggestion. According to your suggestion, we\nconducted X-ray photoelectron spectroscopy (XPS) analysis on the hcp Ni NSs at open circuit potential\n(OCP) condition and after the reaction. No Ni-O bonds were detected, indicating that there was no\noxide layer on the surface of hcp Ni NSs (Supplementary Figure 15). As shown in Ni 2p XPS spectra ,\nthe peak intensity of Ni-Ni bonds (852.9 eV and 870.2 eV) is weakened after the reaction, indicating\nthat the surface of the hcp Ni NSs is slightly oxidized (Nat. Commun. 2022, 13, 3857; J. Am. Chem.\nSoc. 2018, 140, 9046-9050). This may be due to the accumulation of acetone on the nickel surface and\nhydrogen/hydroxyl groups adsorption on hcp Ni NSs (Nat. Commun. 2020, 11, 2720).\nSupplementary Figure 15. Ni 2p XPS spectra of hcp Ni NSs at OCP condition and at -1.32 V vs.\nRHE for 5 h.\n[Added content]: [Page 6] “Furthermore, in order to explore the Ni valence state changes of hcp Ni\nNSs catalyst surface after the reaction, we performed X-ray photoelectron spectroscopy (XPS) tests on\nhcp Ni NSs at open circuit potential (OCP) condition and at -1.32 V vs. RHE for 5 h respectively. As\nobserved in Ni 2p XPS spectra (Supplementary Figure 15), the peak intensity of Ni-Ni bond (852.9\neV and 870.2 eV) is weakened after the reaction, indicating that the surface of the hcp Ni NSs is slightly\noxidized. This may be due to the accumulation of acetone on the nickel surface and hydrogen/hydroxyl\ngroups adsorption on hcp Ni NSs.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have well addressed all my concerns and I would like to suggest its acceptance as it is now.", "reviewer_2": "My concerns in the previous version of the manuscript have been addressed in the current version. Therefore, Therefore, I\nrecommend its publication with the current version.", "reviewer_3": "The revised manuscript has adequately addressed the reviewers’ concerns, and I am happy to accept it.", "reviewer_4": "The authors have addressed my questions and comments. The manuscript can now be published as is."}, "author_response": "Dear Reviewers,\nThank you again for your valuable time to constructive comments on our manuscript\ntitled “Two-Dimensional Metastable-Phase Nickel Hexagonal Nanosheets for Highly-\nPerformance Electrochemical Acetone Hydrogenation” (NCOMMS-25-39812A). We\nsincerely appreciate your comments and recommendation for the publication of our\nmanuscript on Nature Communications.\nThe reviewer’ comments are shown in black and our responses are shown in blue.\nReviewer #1 (Remarks to the Author): The authors have well addressed all my concerns\nand I would like to suggest its acceptance as it is now.\nResponse: Thank you for agreeing to publish our manuscript. We also sincerely\nappreciate your comments and suggestions on our work, which have certainly improved\nour manuscript.\nReviewer #2 (Remarks to the Author): My concerns in the previous version of the\nmanuscript have been addressed in the current version. Therefore, Therefore, I\nrecommend its publication with the current version.\nResponse: We thank you for the recommendation for the publication of our manuscript.\nReviewer #3 (Remarks to the Author): The revised manuscript has adequately\naddressed the reviewers’ concerns, and I am happy to accept it.\nResponse: Thank you for your review. I sincerely appreciate your positive feedback\nand recommendation for publication.\nReviewer #4 (Remarks to the Author): The authors have addressed my questions and\ncomments. The manuscript can now be published as is.\nResponse: We are grateful for your supportive comments and are glad that the revised\nmanuscript is now recommended for publication."}]

Decision letter