Dynamic restructuring of nickel sulfides for electrocatalytic hydrogen evolution reaction

Domain: earth_science
Published: 2024-06-24
Editorial outcome: major_revision

Abstract
Transition metal chalcogenides have been identified as low-cost and efficient electrocatalysts to promote the hydrogen evolution reaction in alkaline media. However, the identification of active sites and the underlying catalytic mechanism remain elusive. In this work, we employ operando X-ray absorption spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy to elucidate that NiS undergoes an in-situ phase transition to an intimately mixed phase of Ni3S2 and NiO, generating highly active synergistic dual sites at the Ni3S2/NiO interface. The interfacial Ni is the active site for water dissociation and OH* adsorption while the interfacial S acts as the active site for H* adsorption and H2 evolution. Accordingly, the in-situ formation of Ni3S2/NiO interfaces enables NiS electrocatalysts to achieve an overpotential of only 95 ± 8 mV at a current density of 10 mA cm−2. Our work highlighted that the chemistry of transition metal chalcogenides is highly dynamic, and a careful control of the working conditions may lead to the in-situ formation of catalytic species that boost their catalytic performance.

Introduction
Electrolysis of water powered by renewable electricity to produce green hydrogen is widely considered as a promising pathway to a global clean and sustainable energy future1,2. The key to enable this technology is the development of low-cost and highly efficient electrocatalysts to accelerate the water splitting reactions, i.e., oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)3. In particular, the design of high-performance electrocatalysts for HER in alkaline conditions has received intensive attention, because the alkaline water electrolysis is the most commonly used route in the industry, and the alkaline HER is also a key step in the chlor-alkali process4,5. However, the alkaline HER still suffers from sluggish reaction kinetics compared to that in acidic solution, due to the additional water dissociation step (Volmer step: H2O + M* + e− → M*H + OH−) in order to supply the absorbed H* intermediate for subsequent H2 generation6. The water dissociation has a higher activation barrier and is considered to be the rate determining step that limits the overall reaction6,7. Even the state-of-the-art Pt-based catalysts show about two orders of magnitude lower HER activity in alkaline than in acidic media3,8,9. Significant progress has been made to improve the HER activity of Pt and other noble metals in alkaline environments by constructing dual active site heterostructures, e.g., Pt-Ni(OH)2, in which the Ni(OH)2 functions as a promotor to accelerate the water dissociation10. Improving the activity of noble metal-based catalysts with a single active site has been challenging mainly due to the scaling relationships in the formation energy of intermediates, which affects not only the HER but also the OER and the oxygen reduction reaction (ORR)11,12. Dual active site catalysts provide alternative reaction mechanisms that may break such scaling relationships, offering more chance for achieving better catalytic performance, compared with single active site catalysts12,13.

Nevertheless, the scarcity and high cost of Pt group materials restrict their large-scale application14. In this regard, earth-abundant transition metal (TM)-based chalcogenides, phosphides, nitrides, and carbides have been recently emerging as cost-effective alkaline HER electrocatalysts15–21. Many TM-based catalysts exhibit comparable alkaline HER activity as Pt group materials22. In addition, doping, defect, and strain engineering have been further explored to modulate their electronic structure to enhance the activity and stability23–27. Nickel sulfides such as NiS, NiS2, and Ni3S2 have drawn significant attention because of their high catalytic activity, as well as easy and scalable methodologies for the preparation28–30. Despite this rapid development of alkaline HER catalysts, the underlying reaction mechanism and the active sites for the alkaline HER, which are different from the well-documented acidic HER, are still under considerable debate6,7,31. Furthermore, it has been demonstrated that TM-based compounds catalysts for the OER undergo structural or compositional reconstruction, or even transform into a new phase under OER working condition32–36. On the other hand, the structural and chemical conversion of catalysts under HER conditions has been less explored, and it has been even assumed HER catalysts are stable during the HER process37. However, recent works indicated that some transition metal chalcogenides may indeed undergo a phase transition under alkaline HER working condition38–40. Therefore, real-time monitoring of the catalytic processes and identifying the real active sites under HER operation conditions are of vital importance to uncover the catalytic mechanism.

In this study, we present an in-depth elucidation of the phase conversion process based on operando X-ray absorption spectroscopy (XAS) and operando Raman spectroscopy under HER working conditions. We found that NiS electrodes undergo a significant HER performance enhancement due to an in-situ transformation into a mixed phase of Ni3S2 and NiO, which is identified to be the actual active catalyst phase for alkaline HER. The ex-situ spectroscopy and theoretical investigations further reveal that the interfacial Ni (O-Ni-S) of Ni3S2/NiO provides the active sites to accelerate water dissociation while the interfacial S (O-Ni-S) facilitates the adsorption of hydrogen intermediates and its subsequent association to form molecular hydrogen. As a result, the in-situ formed Ni3S2/NiO exhibits a superior HER activity, requiring a low overpotential of 95 ± 8 mV to reach a current density of 10 mA cm−2 in our performance tests.

Results
Phase transition of NiS to Ni3S2/NiO induces enhanced HER activity
The NiS samples were synthesized on carbon paper using one-step hydrothermal method (Supplementary Fig. 1a). Details of the synthesis and characterization are provided in the Methods section. X-ray diffraction (XRD) pattern shown in Fig. 1a and X-ray photoelectron spectroscopy (XPS) of Ni 2p core level in Fig. 1b suggests that the as-synthesized samples are characteristic of the hexagonal α-phase of NiS28. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images in Supplementary Fig. 1 show that the samples consist of NiS particles with a size of ~200 nm uniformly coating on the carbon paper substrate.

The HER activity was evaluated in 1M KOH using a three-electrode system (details in the Methods section). Cyclic voltammetry (CV) measurements were carried out in order to adjust the conditions for the evaluation of the HER catalytic activity (Supplementary Fig. 2a, b). In the first cycle of as-synthesized samples, there is a sharp onset of the current density for the HER at around −0.30 V vs. RHE, which continues to increase steadily as the sample is further polarized to negative potentials. After that sharp onset of the current density, the CV profile is reproducible in the second cycle, with an upper limit of the potential at 0.05 V vs. RHE. However, as the upper limit of the potential is extended to 0.40 V vs. RHE in the 3rd and 4th cycles, there is an oxidation wave with a peak at +0.27 V vs. RHE and a 15% increase of the current density when the sample is polarized back to negative potentials. Remarkably, the current density keeps on increasing as the sample is subject to chronoamperometry at −1.00 V vs. RHE (Supplementary Fig. 2c). Linear sweep voltammetry (LSV) of the sample after the chronoamperometry measurement shows that the current density increases 25% compared with the pristine sample, and there is also a clear shift of the onset to lower overpotential (Supplementary Fig. 2d). These electrochemical studies clearly demonstrate that in-situ modifications of NiS electrodes lead to a significantly enhanced HER catalytic activity.

On these bases, we carried out a more detailed study of the chronoamperometric conditions of NiS electrodes on their HER catalytic activity. Figure 1c shows the LSV curves of the as-synthesized NiS, as well as NiS samples after 10, 20, 30, 40, and 50 minute chronoamperometry measurements at −1.00 V vs. RHE, labeled as NiS-10, NiS-20, NiS-30, NiS-40, and NiS-50, respectively. In order to avoid deviations caused by ohmic drops, the potential was adjusted by an iR factor (Supplementary Fig. 3a, b). The as-synthesized NiS initially exhibits a low HER activity, but the HER activity gradually increases with the chronoamperometry measurement time until it is conducted for 40 minutes. A similar trend of HER activity is also observed when the current density is normalized by electrochemical active surface area (ECSA) (Supplementary Figs. 4 and 5). As shown in Fig. 1c, the NiS-40 sample exhibits a high HER catalytic activity, requiring an overpotential of 95 ± 8 mV to reach a current density of 10 mA cm−2 (acquisition of error bar can be seen in Supplementary Fig. 3c.). Further electrochemical characterizations showed that the Tafel slope and charge transfer resistance (Supplementary Fig. 6a, b) decrease in the order of NiS > NiS-10 > NiS-20 > NiS-30 > NiS-50 > NiS-40, indicating that the performance enhancement after the chronoamperometric treatments results from an intrinsic faster HER kinetics. Compared with previously reported HER catalysts, electrochemically treated NiS electrocatalysts exhibit competitive HER activity in terms of overpotentials at 10 mA cm−2 and Tafel slope (Supplementary Fig. 7 and Supplementary Table 1). Additionally, the catalytic activity of the NiS-40 sample is very stable, showing no overpotential increase over 25 h of continuous operation at a current density of 200 mA cm−2 in 1M KOH (Supplementary Fig. 6c).

A series of ex-situ characterizations of the highly active samples after performance tests were conducted. The as-synthesized NiS is converted into a mixed phase of Ni3S2 and NiO after HER measurement. Figure 1a shows an XRD pattern of the NiS-40 sample, from which a pattern characteristic of Ni3S2 can be identified, suggesting a full-phase conversion of NiS to crystalline Ni3S241,42. The XPS in the Ni 2p region for the NiS-40 sample exhibits a pronounced spectral feature possibly arising from NiO (Fig. 1b). Additionally, the Ni K-edge XAS of sample NiS-40 (Supplementary Fig. 8) can be fitted by a linear combination of spectra of Ni3S2 and NiO, which further confirms the formation of Ni3S2 and NiO. We further characterized the NiS-40 sample after 25-h chronopotentiometric measurement at a current density of 200 mA cm-2, the XRD, XPS and XAS confirm the HER-post catalysts are present in the form of Ni3S2 and NiO (Supplementary Fig. 9). Spherical Aberration Corrected Transmission Electron Microscope (SAC-TEM) was used to further examine the microstructure of the HER-post catalysts. As shown in Fig. 1d, the large-area TEM image reveals numerous small NiO clusters anchored onto the large Ni3S2. The HR-TEM image exhibits a distinct interface that partitions the image into two regions (Fig. 1e). By analyzing the FFT images corresponding to these two regions, the cluster region has an interplanar spacing of 0.21 nm corresponding well to the (200) lattice plane of NiO, while the other region has an interplanar spacing of 0.30 nm which matches well with (–110) lattice plane of Ni3S2. More importantly, the elemental mapping shows the preferential enrichment of oxygen elements on the small cluster (Fig. 1f), which strongly reveals the small cluster is assigned to NiO. It should be noted that the formed NiO may exist in the form of a very small cluster, which is hard to determine by XRD patterns. To investigate the influence of the ratio of Ni3S2 to NiO on the catalytic activity, NiS samples received by 1, 5, 10, and 50 LSVs in the range from 0.00 V to –1.00 V vs. RHE were prepared. The Ni K-edge XAS of these samples, shown in Supplementary Figs. 10a–d, consists of a linear combination of spectra of Ni3S2 and NiO, which indicates that a full transformation of NiS occurs after a single LSV. However, the NiO to Ni3S2 ratio increases upon increasing the number of LSVs. We extracted the NiO to Ni3S2 ratio based on the XAS spectra (Supplementary Figs. 10a–d and Supplementary Table 2) and found that the HER activity increases with the amount of NiO (Supplementary Fig. 10e), which clearly indicates the beneficial effect of the Ni oxide species on the HER process.

Phase transition dynamics revealed by operando XAS and operando Raman spectroscopy
An operando XAS spectroscopy was employed to gain detailed insights into the NiS → Ni3S2/NiO transition process, and details of the experimental method are provided in the Methods section. Figure 2a shows a three-dimensional (3D) plot of the Ni K-edge XAS of the NiS electrode as a function of the applied potentials during cyclic CV measurement (Fig. 2b). Note that two important changes in the spectral profile occur at applied voltages of −0.27 V and +0.27 V vs. RHE, dividing the 3D plot into three regions as I (applied voltage from +1.00 V to −0.27 V), II (−0.27 V to −1.50 V and back to +0.27 V) and III (from +0.27 V to 1.00 V). Selected Ni K-edge XAS spectra at specific applied voltages within regions I, II and III are shown in Fig. 2c. In region I, with applied voltage from +1.00 V to −0.27 V, the spectrum of the sample remains as that of the NiS phase. The transition from region I to region II at −0.27 V can be identified as the phase transition NiS → Ni3S2, based on spectral fitting and comparisons with reference samples of NiS and Ni3S2 (region I and II in Fig. 2c). The associated reduction of the formal oxidation state of Ni2+ in NiS to Ni1.33+ in Ni3S2 can be observed as a cathodic wave peak in the CV profile just at the onset of the HER current shown in Fig. 2b (see the inset), clearly showing that the HER catalytic activity is associated with the formation of Ni3S2. After the phase transition, the Ni3S2 phase is stable under HER conditions. However, with a more negative voltage applied (region II in Fig. 2a), a continuous decrease of the spectral intensity of the white line of the Ni K-edge XAS spectra suggests a further reduction of the oxidation state of Ni. Consistently, the fitting of Fourier Transform moduli of spectra in region II revealed a decrease of the average coordination number of Ni coordinated by S (Fig. 2d, Supplementary Figs. 11 and 12, and Supplementary Tables 3 and 4, and the simulations procedure in Supplementary Note 1), suggesting the formation of S vacancies, i.e., Ni3S2−δ. As the sample is polarized from negative back to positive potentials, the transition from II to III at +0.27 V can be associated with an oxidation process observed in the CV profile shown in Fig. 2b, consistent with a constant increase of the white line of the spectrum as the sample is further polarized (region III in Fig. 2a). The spectrum can be fitted with a linear combination of Ni3S2 and an increasing contribution of NiO (region III in Fig. 2c), and details of the fitting are discussed in Supplementary Notes 2–5. Based on this analysis, we estimate that the NiO contribution at +0.50 V is about 13%, which further increases to 22% when the sample potential reaches +1.00 V (Fig. 2c and Supplementary Table 5). Thus, the coordination of Ni by O occurs gradually in region III (Fig. 2d), probably caused by the filling of the sulfur vacancies in Ni3S2−δ with water or OH− ions, leading to an intimately mixed phase of Ni3S2 and NiO, which is further verified by the Wavelet transform (WT) analysis of extended X-ray absorption fine structure (EXAFS) and Fourier-transformed k3-weighted EXAFS at applied voltage of +1.00 V in region III (Supplementary Fig. 13). The corresponding results of fitting at +1.00 V were shown in Supplementary Table 4, and the Ni-O show a bond length of ∼2.00 Å. Highly crystalline NiO has a characteristic Ni-O distances around 2.09 Å; however, the short-range order in NiO reveals that the Ni-O distances for amorphous and/or nanocrystalline NiO may decrease to 2.00–2.06 Å43,44. This result further confirms the formed NiO is in the form of a very small cluster.

The formation of mixed-phase of Ni3S2 and NiO was further verified by an operando Raman spectroscopy study (the details of the experiment in the Methods section). The top Raman spectrum in Fig. 2e can be assigned to pristine NiS, with characteristic bands appearing at Raman shifts 281 cm−1 and 473 cm−1. As the sample was polarized to negative potentials, the characteristic peaks assigned to NiS disappear, while the characteristic peaks assigned to Ni3S2 appear at 221, 303, 322, and 349 cm−1 (marked as a, b, c, and d, respectively), indicating the complete phase transition from NiS to Ni3S245. In addition, the newly emerged peaks e, f, g, h, and i located at 453, 493, 600, 800, and 1060 cm−1 can be assigned to Ni-O46–49, suggesting the formation of NiO under HER conditions. Raman spectra features associated with NiS, Ni3S2, and NiO are very well resolved so that even small trace concentrations of NiO species can be detected.

Based on the analysis of the operando XAS and operando Raman spectroscopy data, we propose a three-step mechanism for the phase transition of NiS → Ni3S2/NiO under alkaline HER conditions, as schematically shown in Fig. 2f50. The electrochemical reduction of NiS to Ni3S2 at −0.27 V vs. RHE can be driven by the thermodynamic instability of NiS under HER conditions (step 1). As more negative potentials are set to reach higher HER current densities, S leaching leads to the formation of S vacancies in Ni3S2−δ (step 2). As the sample is polarized to more positive potentials, substantial oxidation occurs at +0.27 V vs. RHE, and S vacancies are occupied by the oxygen species from the media (water/OH− ions), leading to the formation of an intimately mixed phase of Ni3S2/NiO (step 3).

The stability of NiS, Ni3S2, and NiO under different conditions relevant to the HER catalytic performance was further investigated by operando XAS spectroscopy. Figure 3a shows operando XAS at Ni K-edge of the NiS electrode in chronoamperometry for 30 minutes at consecutively applied potentials: 0.00 V, −1.00 V, 0.50 V, and back to −1.00 V (all potentials are vs. RHE). As expected, the sample retains its pristine NiS structure at 0.00 V and converts to Ni3S2 when polarized to −1.00 V (Fig. 3a), due to the electrochemical reduction at −0.27 V as discussed above. Then, as the sample is polarized to +0.50 V, the white line of 8350 eV assigned to Ni-O increases noticeably (the inset in Fig. 3a) and the spectrum can be fitted with a linear combination of 90% of Ni3S2 features and 10% of NiO spectral features (Supplementary Fig. 14a and Supplementary Table 6), due to the bulk oxidation that occurs at +0.27 V as previously discussed. When the sample is brought back to HER conditions at −1.00 V, the content of NiO seems to decrease but still exist (the inset in Fig. 3a) as the spectrum can be fitted with a linear combination of 94% of Ni3S2 spectral features and 6% of NiO spectral features after 30 minutes under the reaction (Supplementary Fig. 14b and Supplementary Table 6). In addition, Fig. 3b shows operando Raman spectra of a NiS electrode in chronoamperometry at −0.30 V vs. RHE for 30 minutes, further confirming the existence of NiO under the HER process. In order to demonstrate the long-term stability of the Ni3S2/NiO mixed phase, we extended the operando Raman experiment at an applied potential of −0.30 V vs. RHE to 25 h. The results demonstrate Ni3S2 and NiO remain stable under HER conditions over an extended period of time (Supplementary Fig. 15).

Insight into the HER mechanism
The HER in alkaline media mainly involves the step for water dissociation to form H* and OH*, i.e., Volmer step, followed by the coupling of H* to form H2 in the Heyrovsky and Tafel steps. The Volmer step has been shown to be the rate-determining step for alkaline HER6,7. Therefore, to identify the active sites formed after the electrochemical treatment of the samples, we carried out a study for the dissociative adsorption of water on NiS and NiS-40 (Ni3S2/NiO) catalysts by means of near-ambient-pressure (NAP)-XPS (experimental details in the Methods section). NAP-XPS is a commonly used spectroscopic technique to study the dissociative adsorption of water on catalysts in a humid environment51,52. Figure 4b shows the XPS spectra of NiS and NiS-40 (Ni3S2/NiO) in the O 1s region in the presence of 0.5 mbar water vapor. Oxygen related species including lattice oxygen in NiO (lattice O), adsorbed OH*, adsorbed H2O* and vapor H2O (H2Ovap) can be identified, and their relative amounts can be quantified by fitting the peak area (Supplementary Fig. 16 and Supplementary Table 7). In particular, the adsorbed OH* originates from the dissociative H2O adsorption, and therefore can be used as a measure of the degree of water dissociation on the catalyst surface, i.e., the higher the value, the better the ability to dissociate water. As shown in Fig. 4c, the ratio (Rdis) of OH* to (H2O* + OH*) in NiS-40 (Ni3S2/NiO) is much higher than that in NiS in the presence of different water vapor pressure, suggesting a stronger ability of dissociative adsorption of water on the NiS-40 (Ni3S2/NiO). In addition, the surface chemistry of the catalyst was further investigated by monitoring changes in the Ni 2p and S 2p core levels upon water exposure. For the NiS-40 (Ni3S2/NiO) sample, water exposure induces a pronounced change in the Ni 2p3/2 and S 2p. As shown in Fig. 4d, compared with the Ni 2p3/2 spectrum in vacuum, the spectral intensity associated with Ni-O increases significantly upon water exposure, which may result from the preferential adsorption of OH* at Ni sites. On the other hand, for S 2p shown in Fig. 4e, after water exposure, a shoulder feature appears at the lower binding energy side, which may result from the reduction of sulfur ions when H* adsorbs at S sites in Ni3S2. In contrast, for NiS (Supplementary Fig. 17), there is no obvious change in Ni 2p and S 2p after water exposure, which suggests that there is much less OH* and H* intermediates generated on NiS surfaces, in accordance with the weak water dissociation ability of NiS observed for this sample.

Considering the involvement of Ni and S sites in the water dissociation process, the influence of each site on the alkaline HER was investigated by a systematic poison experiment (experimental details in the Methods section). Specifically, we used thiocyanate ions (SCN−) and dodecanethiol to selectively block the Ni and S sites, respectively53–55. Figure 4a shows the change in current density at −0.15 V vs. RHE before and after blocking Ni and S sites in as-synthesized NiS and NiS-40 (Ni3S2/NiO). The change in current density is a measure of the activity of a specific site; essentially, the greater the change after blocking a site, the higher the catalytic activity of the site in the non-poisoned state. For as-synthesized NiS, there is only a 0.5 mA cm–2 decrease in the HER current density after blocking Ni sites, while 3.8 mA cm–2 after blocking S sites; suggesting that S sites are the active sites. On the other hand, for the NiS-40 (Ni3S2/NiO) sample, it can be seen that the change is 2.0 mA cm–2 after blocking Ni sites, and 9.5 mA cm–2 after blocking S sites. This result indicates that both the Ni and S sites are the active sites in Ni3S2/NiO.

The above experiments indicate the synergistic effect of Ni3S2 and NiO in the transformed phase leading to the enhanced HER activity, in which the interfacial Ni sites play a crucial role in accelerating the rate determining step of water dissociation and producing H* intermediates while the interfacial S facilitates H* adsorption to generate H2.

In order to support our experimental mechanistic studies for the enhanced HER activity after the formation of the mixed phase of Ni3S2/NiO, we performed theoretical calculations based on density functional theory (DFT) (computational method in the Methods section). In our model, we employ a heterostructure of Ni3S2 and NiO with O sites covered by H (marked as Ni3S2/NiO). We also construct models for individual Ni3S2 and NiO for comparison. Schematics and details of the models are shown in Supplementary Fig. 18, and the atomic coordinates of the optimized models are supplied in Supplementary Data 1. We used crystalline NiO with an FCC crystal structure in our model because our thermodynamic analyses suggest that the stable state of NiO is crystalline (see Supplementary Fig. 19, and corresponding atomic coordinates are supplied in Supplementary Data 1). The Ni3S2/NiO model with O sites covered by H can be rationalized because the adsorption of H at the unsaturated oxygen sites of NiO is spontaneous and very strong, as indicated by the large values of ΔGH* of up to –0.86 eV shown in Supplementary Fig. 20. We carried out our DFT mechanistic study based on comparisons of these three modes, drawing conclusion from the trends we found rather than from individual values.

Using these models, we calculated the Gibbs energy change toward HER for the Ni3S2/NiO heterostructure and the corresponding individual phases, and the results are graphically shown in Fig. 5a. For individual Ni3S2, the energy barrier for the water dissociation step is high (0.42 eV), which is mainly due to the weak adsorption of OH* on Ni sites (Supplementary Fig. 21). Similarly, the water dissociation energy for individual NiO catalyst is as high as 0.51 eV, due to the weak bonding strength between Ni and adsorption H*. On the other hand, for the Ni3S2/NiO heterostructure, by taking complementary advantages of stronger adsorption of OH* at Ni sites at the NiO side and stronger adsorption of H* at S sites at the Ni3S2 side, the energy barrier for water dissociation can be dramatically reduced to 0.11 eV. Furthermore, Ni3S2/NiO heterostructures also facilitate the coupling of H* to produce H2. The adsorption energy of H* on S sites at the Ni3S2/NiO heterostructure is −0.18 eV, which is closer to the optimal value (0 eV) for coupling of H* to form H2, compared with the corresponding value (−0.31 eV) for individual Ni3S2 (Fig. 5a). Interestingly, our DFT results reveal that a strong electronic interaction between Ni3S2 and NiO at the interface provide favorable energetics for coupling of H* to form H2. Figure 5b shows the density of states (DOS) of NiO and Ni3S2. NiO has low DOS at the Fermi level, and Ni3S2 has a metallic state, showing a high density of occupied electrons around the Fermi level. Therefore, electrons can easily transfer from Ni3S2 to the empty state of NiO. As shown in Fig. 5c, the charge differential analysis for the Ni3S2/NiO heterostructure clearly reveals that electrons transfer from S sites of the Ni3S2 side to the NiO side. The depletion of electrons at S sites of the Ni3S2 side can tune the electronic states of S atoms further weaken the adsorption energy of H* from –0.31 eV to –0.18 eV, which provides favorable energetics for coupling of H* to form H2. Therefore, as shown in the schematic model in Fig. 5d, our experimental and theoretical mechanistic study results clearly reveal that the in-situ formed Ni3S2/NiO hetero-interface provides dual active sites to promote water dissociation, and the strong electronic interaction between Ni3S2 and NiO interface also modifies the electronic state of interfacial S sites and thus provide optimized energetics for coupling of H* to form H2.

In summary, an operando XAS and operando Raman were employed to investigate the dynamic restructuring of NiS under alkaline HER conditions. It was found that NiS was electrochemically reduced to Ni3S2, which subsequently oxidized with water and OH– ions from the electrolyte, leading to the formation of a highly active Ni3S2/NiO hetero-interface. The combination of operando spectroscopy and theoretical investigations further unveiled that the Ni3S2/NiO hetero-interface would function as the actual active sites, i.e., the interfacial Ni sites provide the active sites to accelerate water dissociation while the interfacial S sites facilitate the adsorption of H* intermediates and its subsequent association to form H2. Our work shows that the chemistry of transition metal chalcogenides is highly dynamic, and careful control of the working conditions may lead to the in-situ formation of catalytic species that boot their catalytic performance.

Methods
Synthesis of NiS samples
Carbon paper was used as the substrate for NiS. Carbon paper was consecutively washed with HNO3 (1M) and deionized water under sonication for 15 minutes in each solution to thoroughly remove impurities and improve hydrophilicity. 0.2448 g nickel acetate (99 wt%, Alfa Aesar) and 0.2300 g thioacetamide (99 wt%, Alfa Aesar) were dissolved in 35 mL deionized water. After stirring, the mixture was poured into a 50 ml autoclave with a piece of carbon paper (0.5 × 1.5 cm²). The growth was carried out at 120 °C in an electric oven for 12 h. After the autoclave cooled down naturally to room temperature, the samples were removed, washed with deionized water, and then dried in a vacuum oven at room temperature.

Preparation of electrolyte and electrode
1M KOH can be obtained by dissolving 32.0058 g of KOH (85 wt%, Sinopharm Chemical Reagent) in 500 ml of deionized water. Store it in a plastic bottle. We usually prepare it freshly as needed. The pH value was 14.00 ± 0.03 measured by pH meter (details in the Supplementary Table 8 and Supplementary Fig. 22). As shown in Supplementary Fig. 1a, a portion of the carbon paper with 0.5 × 1.0 cm2 of geometric dimensions is covered by the NiS sample, while another portion with 0.5 × 0.5 cm2 of geometric dimensions is left bare for connection to the glassy carbon electrode. Clamping the exposed portion of the carbon paper with the glassy carbon electrode forms the working electrode.

Material characterizations
The crystal structure was analyzed using X-ray diffraction (XRD) employing Cu Kα radiation. Surface morphology was assessed using a ZEISS Sigma field emission scanning electron microscope (SEM). For analysis with transmission electron microscopy (TEM) and spherical aberration-corrected transmission electron microscopy (SAC-TEM), samples were suspended in absolute ethanol and subsequently deposited onto a Cu grid. High-resolution X-ray photoelectron spectroscopy (XPS) measurements were conducted using a monochromatic Al Kα1 X-ray source (hν=1486.6 eV) equipped with a SPECS PHOIBOS 150 electron energy analyzer, achieving a total energy resolution of 0.50 eV. The binding energy was calibrated using the C 1s peak34.

Electrochemical measurements
All electrochemical measurements were carried out using a three-electrode configuration controlled by a CHI 750E electrochemical workstation, consisting of an as-prepared sample on carbon paper as working electrode, Hg/HgO (1M KOH) as the reference electrode and graphite rod as the counter electrode (Supplementary Fig. 23). The reference electrode was calibrated by comparing it with the standard Hg/HgO (1M KOH) purchased from Tianjin Aida Co., LTD. The potentials reported in this work were normalized versus the RHE using ERHE = EHg/HgO + 0.059 pH + 0.098 V. We conducted all HER measurements in 1M KOH electrolyte. Prior to measurements, all fresh electrolytes were bubbled with pure nitrogen for 30 minutes.

Linear sweep voltammetry (LSV) was employed to obtained polarization curves by sweeping the potential from 0.00 V to –1.00 V vs. RHE at a scan rate of 5 mV s−1. The electrochemical impedance spectroscopy (EIS) was performed in the same configuration at –0.20 V vs. RHE applied potential over frequency range from 100 kHz to 0.1 Hz at the amplitude of the sinusoidal voltage of 5 mV.

To correct the ohmic drop, the measured potentials were calibrated using equation Ecor = E - iR, where Ecor was corrected potentials, E was measured potential, i was current and R was the contact resistance determined either through iR measurement or by analyzing EIS curves. In this work, 85% iR was conducted. The polarization curves were replotted as overpotential (η) versus log current (log j) to get Tafel plots for assessing the HER kinetics of investigated catalysts.

Capacitance measurements in the potential region of no faradaic process at different scan rates of 20, 40, 60, 80, 100, and 120 mV s–1 can be used to determine electrochemical active surface area (ECSA).

Operando electrochemical X-ray absorption experiment
Operando XAS measurements were carried out on NiS samples grown on carbon paper as the working electrode at the CLAESS beamline of the ALBA synchrotron in Spain, using a specially designed in-house electrochemical cell setup (see Supplementary Fig. 24). A platinum wire served as the counter electrode, and an Ag/AgCl (3M KCl) electrode was used as the reference electrode in a 1M KOH solution. These electrochemical experiments were done in a computer-controlled electrochemical workstation. Spectra at the Ni K-edge were continuously collected as the samples were subject to cyclic voltammetry (CV) from the open circuit potential to –1.50 V versus RHE and scan rate of 0.5 mV s−1. Spectra acquisition was performed using a Si (311) monochromator that offered an incident energy resolution of 0.3 eV. Appropriate angle and coating for the collimating and focusing mirrors were selected to ensure harmonic rejection. The measurements were done in fluorescence mode, and both NiO pellets and Ni foils were employed for energy calibration. XAS data processing was conducted using the ATHENA software package56.

Operando Raman experiment
Raman spectra were acquired using a Jobin-Yvon Horiba Xplora confocal Raman system. All experiments were done with an excitation wavelength of 638 nm and a ×50 microscope objective with a numerical aperture of 0.55. Laser power was maintained at ~1.5 mW. Operando experiments took place in a K006 Raman cell (see Supplementary Fig. 25), and Raman spectra were recorded at various applied potentials.

Near-ambient-pressure photoemission spectra experiment
Near-ambient-pressure photoemission spectra (NAP-XPS) were recorded on a lab-based spectrometer (SPECS GmbH, Berlin) using a monochromated Al Kα source (hν=1486.6 eV) operating at 50 W. The analyzer is a SPECS PHOIBOS 150 NAP, a 180° hemispherical energy analyzer with a 150 mm mean radius. The entrance to the analyzer is a nozzle with a 300 μm diameter orifice. The analyzer is run in fixed analyzer transmission (FAT) mode. The pass energy was set to 40 eV for survey scans and 20 eV for high-resolution regions. Water vapor dosing was carried out via a leak valve, Ni 2p, S 2p, and O 1s NAP-XPS at all pressures were recorded after at least 15 minutes of pressure stabilization.

Poisoning experiment
In order to explore what is the active site during the HER process, the influence of thiocyanate ion (SCN–) and dodecanethiol on the HER activity of investigated catalysts was evaluated by adding 10 mM SCN– and 10 mM dodecanethiol in the electrolyte respectively, where SCN– and dodecanethiol are known to poison the Ni sites and S sites, respectively53–55.

Density functional theory calculations
For the density functional theory calculations (DFT), Perdew-Burke-Ernzerhof (PBE) method is used to obtain exchange correlation functional57, and projector-augmented wave (PAW) pseudopotentials are employed58,59. Hubbard-U and DFT-D3 (B-J) are also introduced in the calculations to correct the electron correlation. The whole DFT calculations are implemented in the Vienna Ab initio Simulation Package (VASP) code60,61. The value of U-J term was set as 4.0 eV for Ni cations62,63. A Γ-centered Monkhorst-Pack grid with a 2 × 2 × 1 k-point mesh was used. The wave functions of valence electrons were expanded using a plane wave basis set with a cut-off energy of 500 eV. The convergence criterion was set as 10−6 eV between for electronic optimization. For the structural optimization loops, the maximum force on each atom was less than −0.01 eV/Å64. All calculations were spin-polarized.

Gibbs free energy calculations
The steps involved in HER in alkaline media are listed in Eqs. (1)–(3), as shown below:
Volmer: H2O(l) + 2M* → M*OH + M*H (1)
M*OH + M*H + e− → M* + M*H + OH− (2)
Heyrovsky: M*H + H2O + e− → M* + H2 + OH− = Tafel: M*H → M* + 1/2H2 (3)
Here, we applied a method developed previously for modeling the thermochemistry of electrochemical reactions based on density functional calculations64. Specifically, the Gibbs free energy of reactions, served as the criterion to determine the spontaneity of reactions, can be acquired based on DFT with corrections including entropic (TS) and zero-point energy (ZPE) contributions by the following equation:
ΔG = ΔEDFT + ZPE − TΔS (4)
Here, the EDFT represents the DFT energy computed for the respective systems, ZPE refers to the change in zero-point energy determined from the vibrational frequencies, and ΔS is the change in the entropy. For Gibbs free energy calculations, the free energy of a proton (GH+) is replaced by half of a hydrogen molecule in gaseous H2 (1/2GH2) for every step of electrochemical reactions3. The entropies of corresponding species were obtained from the vibrational frequencies. For gas molecules such as H2, the value of entropy can be described in the sum of the translational, vibrational, and rotational sections. Furthermore, the translational and rotational entropic values were not considered due to negligible contributions. The entropy is obtained by the following equation:
S(T) = Σi 3N1 [R ln(1 − e−hv i / kBT) + NA T hv i / kBT (1 − e−hv i / kBT)] (5)
where R denotes the gas constant, k is the Boltzmann constant, h represents Plank’s constant, NA corresponds to Avogadro’s number, vi signifies the frequency and N is the number of adsorbed atoms. Additionally, for frequency calculations, all substrate atoms were fixed and only adducts-related reaction intermediates are calculated, and the contributions from the catalyst phase to ZPE and ΔS are not involved in the Gibbs free energy of those reactions. The accuracy of calculations is provided in Supplementary Note 6.

Data availability
Additional data supporting this study’s findings are available from the corresponding author upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This report is regarding in situ formed NiOx/Ni3S2 for HER and their operando study into the active sites and phase transitions. There is a lack of novelty for a high IF journal like Nat Comm since related works have already been demonstrated for other similar HER catalysts like NiS2, CoS2, NiSe2, as pointed out in ref 37 to 39. In my opinion, this work does not meet the high novelty standards of Nat Comm. Recommend a more specialized journal like JMCA.", "reviewer_2": "The paper deals with a study of NiS-based electrocatalyst for the hydrogen evolution reaction (HER) in alkaline media.\nThe authors propose that during operation, NiS transforms into a mixture of Ni3S2 and NiOx phases, the interface of which provides active centers of HER activity.\nThe phase conversion process was followed mainly using operando Ni K-edge X-ray absorption spectroscopy (XAS). Several other experimental techniques as XRD, SEM, TEM, XPS, Raman spectroscopy were used as well. The theoretical support was provided by DFT calculations.\nThe obtained results are interesting and are based on several nice experiments. However, there are many points that should be clarified and/or explained in more detail. Some conclusions are not supported by the results. Therefore, I cannot recommend the manuscript for publication in the present form.\nThe authors should consider the following points:\n1) It is not clear how were obtained the reference samples Ni3S2 and NiOx/NiO?\n2) What is the meaning of NiOx phase? What is the difference between NiO and NiOx phases?\n3) The linear combination analysis of the Ni K-edge XAS spectra based on reference data for Ni3S2 and NiO compounds should be preceded by the principal component analysed to prove the number of phases present in the sample. The use of the crystalline NiO phase to simulate the contribution from some NiOx phase should be reasoned.\n4) The Ni K-edge XANES data in Fig. 2(c) (region II and III) are rather poor in features and short in the energy range, also the fits in Figs. 2(c) and S6 (dashed lines) are not ideal. Does it mean that other phase or phases can be present in the sample? Can authors perform the same analysis using the Ni K-edge EXAFS spectra in larger energy or k-space?\n5) The description of the EXAFS simulations/fits is fully missing. This makes unclear the origin of data in Tables S3 and S4. The description of the simulation procedure and the approximations used should be given in the Supplemental Materials.\n6) The reliability of the fits is also not clear. Some values reported in the two tables look unphysical. In particular, all Ni-O distances are very short being about 1.7 Å (a longer distance of 2.01 Å is reported in the text!), whereas the Ni-O bond length in bulk NiO is significantly longer being about 2.09 Å.\n7) The fitting results should be provided (in the Supplemental Materials) for data reported in Fig. 2(d).\n8) The Ni K-edge EXAFS spectra should be additionally reported in Fig. 3, and the Ni-O and Ni-S contributions should be shown separately. Currently, the contribution from the Ni-S bonds is not convincing – it could be negligible.\n9) The comparison between the experimental and fitted EXAFS spectra should be provided in separate figures for the results reported in Tables S3 and S4 in the Supplemental Materials as well as in Fig. 3(b).\n10) The values of coordination numbers (CNs) should contain errors. The meaning of asterisks should be explained in the captions of Tables. The small values of the Ni-O CNs do not convince for the occurrence of the NiO phase.\n11) It looks like Fig. 3(d) contains wrong labels for the spectra: the blue line, probably, corresponds to NiO. One can recommend using the same colors in Fig. 3(b) and (d).\n12) The Raman data (Fig. 2e) are available in a short-range, so only two bands around 320 and 480 cm-1 are visible in the catalyst. Their assignment cannot be unique. Also, the band at 560 cm-1 discussed in the text and attributed to NiOx phase is not present in the Raman spectra. Therefore, the Raman data should be re-analysed. Comparison with the literature data should be done.\n13) It is difficult to judge the accuracy of DFT calculations since no comparison with any reference data is provided. It is not clear how well the structural and electronic properties are reproduced, in particular, taking into account that the PBE functional is not hybrid. The position of the Fermi level should be given in Fig. 5(d).", "reviewer_3": "Report on \"In situ formed NiOx/Ni3S2 Nano interfaces Boost High Hydrogen Evolution Reaction Activity in Alkaline Conditions\" by X. Ding, et. al\nThe authors report the results of a comprehensive experimental and theoretical study exploring the structural transformations that occurring within NiS and which accelerate the hydrogen evolution reaction (HER) under alkaline conditions. This is an important area of work with much interest in the formation of H2 as an energy storage medium through the electrolysis of water using renewable energy sources. The results presented by the authors suggest that NiS undergoes a phase transformation during the reaction to form an NiOx/Ni3S2 phase, which then acts as a catalyst for the rate-limited step in the HER under alkaline conditions. This is an important observations because understanding the structure on which a reaction occurs is key for the design of improved catalysts. As such, this manuscript will be of broad interest and importance. I recommend publication after the following points have been adequately addressed. Note that given that my expertise lies in theoretical chemistry, I have only provided comments related to the calculations report by the authors.\n1. The theoretical methods section in the supporting information do not adequately describe key parts of the calculations. In particular, the authors should describe what kinds of pseudopotentials were used (I assume PAWs, but it isn't stated), what planewave cutoff was used for the density and/or augmentation charges, and the k-point grid used in the calculation. In addition, the authors should outline how zero-point energies and entropies were obtained. I assume through phonon calculations, but it is not stated. It is also important to state whether such calculations (if they were phonon calculations) were performed for the whole system or just in the region near the reactive site.\n2. In the first reaction step in Figure 3a, the authors show S leaching from the material and indicate that oxygen should fill the vacancy. However, the middle image int hat figure states that the material is Ni3S2 and shows no oxygen. So, how does oxygen factor in? I found this pretty confusing.\n3. In Figure 5c, the authors show the structure of NiOx/Ni3S2 and suggest a mechanism. The mechanism relies on water binding through the oxygen atom to a nickel atom that is bonded to an oxygen and a sulfur. Based on the structure provided, it's not clear that a site facilitate this process is present and the images provided in the supporting information (Figures S9 - S11) don't really support this mechanism, either. It would be helpful if the authors indicated more accurately in the main text how this reaction occurs. In addition, the authors should provide the coordinates of all calculated structure to the supporting information so others can easily reproduce the results.\n4. The mechanism outlined in the bottom of Figure 5c involves a proton transferring to sulfur. It seems that transfer to oxygen would be preferred. Perhaps I missed the explanation of why this is the case. However, it is important to explain why the oxygen atom isn't protonated, since most people would intuitively expect that to be preferred."}, "author_response": "RESPONSE TO REVIEWERAZ COMMENTS\n20th July 2022\nDear Editor,\nR] Y\\j]kk Ydd j]na]o]jk{comments point-by-point in the following.\nReferee: 1\nComments to the Author:\nThis report is regarding in situ formed NiO /Ni S for HER and their operando study into\nx 3 2\nthe active sites and phase transitions. There is a lack of novelty for a high IF journal like\nNat Comm since related works have already been demonstrated for other similar HER\ncatalysts like NiS , CoS , NiSe , as pointed out in ref 37 to 39. In my opinion, this work\n2 2 2\ndoes not meet the high novelty standards of Nat Comm. Recommend a more specialized\njournal like JMCA.\nResponsex We understand the view of referee 1 that the dynamic nature of metal sulfides\nchemistry has already been demostrated, and therefore, transitions under an\nelectrochemical reaction are rather expected. However, the observation and mechanistic\nelucidation of the phase transition of NiS to Ni S is only a part of the work we report in\n3 2\nour paper. The identification of the yj]Ydz active sites under HER operation conditions\nand the HER mechanism of the transformed NiS electrodes are the most important\ndiscoveries and define the novelty of our work. We found the HER activity does not only\ndepend on the conversion of NiS to Ni S , but also on the formation of surface NiO. We\n3 2\nshow how the interaction of active sites on NiO and Ni S may lead to the observed high\n3 2\ncatalytic performance.\nWe strongly believe these finding are appealing to the broad readership of Nature\nCommunications because, (1) we have shown how the dynamic chemistry of metal\n1\nsulfides, which are materials with multiple catalytic applications, can be controlled to\npromote the formation of highly active catalytic species; (2) we describe a very interesting\ndual-site catalytic system, and this type of systems provide alternative reaction\nmechanisms that may break the scaling relationships in the formation energy of\nintermediates that often limits the catalysis of not only the HER but also the OER and the\nORR, offering more chances for achieving better catalytic performance compared with\nsingle active site catalysts; (3) our experimental methodology, combining in-situ and\noperando spectroscopy, experimental mechanistic studies and DFT calculations may be\napplied for the study of many other challenging metal chalcogenide systems.\nWe have included remarks on the significance of our findings highlighted in green in the\nintroduction and in the discussion, linking them to other works that can benefit from our\nobservations.\nComments to the Author:\nThe paper deals with a study of NiS-based electrocatalyst for the hydrogen evolution\nreaction (HER) in alkaline media.\nThe authors propose that during operation, NiS transforms into a mixture of Ni S and\n3 2\nNiO phases, the interface of which provides active centers of HER activity.\nx\nThe phase conversion process was followed mainly using operando Ni K-edge X-ray\nabsorption spectroscopy (XAS). Several other experimental techniques as XRD, SEM,\nTEM, XPS, Raman spectroscopy were used as well. The theoretical support was provided\nby DFT calculations.\nThe obtained results are interesting and are based on several nice experiments. However,\nthere are many points that should be clarified and/or explained in more detail. Some\nconclusions are not supported by the results. Therefore, I cannot recommend the\nmanuscript for publication in the present form.\nThe authors should consider the following points:\nResponse x We thank referee 2 for his/her positive comments on our work. We are\nappreciated for the helpful suggestions/comments for improving our manuscript.\nFollowings are our responses to the comments.\n[The comments are shown in italic; responses are in black; all revisions in manuscript\nand supplementary information are highlighted in yellow.]\nComment 1. It is not clear how were obtained the reference samples Ni S and NiOx/NiO?\n3 2\nResponse 1 x The reference sample for NiO was a commercial powder. In order to get\nthe Ni K-edge XAS reference of NiO, proper amount of NiO sample was weighted, mixed\nwith cellulose and then pressed into a 13 mm pellet for XAS measurement in transmission\nmode. For Ni S reference, it was a crystal powder synthesized by ourselves (the synthesis\n3 2\nprocedure and the characterization are shown below). In order to get the Ni K-edge XAS\nreference of Ni S , proper amount of Ni S powder was weighted, mixed with cellulose,\n3 2 3 2\nand then pressed into a 13 mm pellet for XAS measurement in transmission mode. The\nspectrum was aligned with our data by a comparison with a Ni foil. (We add this\ndesciptions in Supplementary Note 2 in page 6 of the revised Supplementary Information)\n3 2\nnickel and sulfur at 800 ç for 6 hours in silica tube sealed under vacuum. The XRD\npatterns shown in Fig. R1a confirm that as-synthesized Ni S powder has high crystalline\n3 2\nquality. The shapes and positions of Ni 2p and S 2p XPS spectra shown in Figs. R1b-c\nare also the same to previous report of Ni S 1. As shown in Fig. R1d, the Ni K-edge\n3 2\nspectrum for our Ni S reference also agree well with the spectrum from the Farrel-Lytle\n3 2\ndatabase (http://ixs.iit.edu/database/data/Farrel_Lytle_data/), which allows us to confidently\nuse the Ni S reference for our analysis. (We add this desciptions in Supplementary Note\n3 2\n1 in page 5 of the revised Supplementary Information.)\na\nPDF#85-1802\n10 20 30 40 50 60 70 80\nFig. R1. (a) XRD patterns of the as-synthesized Ni S crystal powder and the standard\n3 2\ncard PDF#85-1802. (b) Ni 2p XPS spectra and (c) S 2p XPS for as-synthesized Ni S . (d)\n3 2\nNi K-edge XAS spectra of as-synthesized Ni S and Ni S spectra from the Farrel-Lytle\n3 2 3 2\ndatabase (http://ixs.iit.edu/database/data/Farrel_Lytle_data/).\nReference:\n0- >m[cd]q+=-J-&Rgg\\k+M-Ad][ljg[`]ea[YdYf\\SLNklm\\a]kg^l`]kmj^Y[]gpa\\Ylagf\ng^kqfl`]la[`]Yrd]ogg\\al] 'Ja N (-@)7\\X) ;XQO`^[OTQY) ,++464,471 '0880(-\n2 1\nComment 2. KTM` U_ `TQ YQMZUZS [R DUEd \\TM_Q6 KTM` U_ `TQ PURRQ^QZOQ NQ`cQQZ DUE\nMZPDUEd\\TM_Q_6\nResponse 2 x We used l`] l]je yJaK z+ Z][Ymk] o] do observe the formation of nickel\nx\noxide, but we are not able to determine the crystal structure or the phase of the nickel\n4\n).u.a(\nytisnetnI\nNi3S2\n880 870 860 850\n2u (degree)\n).u.a(\nytisnetnI\nNi3S2 Ni 2p XPS 2 p\n3/2\nsat. 2 p 1/2 sat.\nsat.\nsat.\nBinding energy (eV)\n168 166 164 162 160\n).u.a(\nytisnetnI\nNi3S2 S 2p XPS\n2 p\n3/2\n2 p\n1/2\n8200 8400 8600 8800 9000\nBinding energy (eV)\nnoitprosba\ndezilamroN\nb\nc d\nNi K edge XAS\nNi3S2 database\nNi3S2\nPhoton energy (eV)\ncomponent analysed to prove the number of phases present in the sample. The use of the\ncrystalline NiO phase to simulate the contribution from some NiOx phase should be\nreasoned.\nResponse 3 x Principal component analysis (PCA) was performed using a set of 6 Ni-K\nedge spectra measured during the positive sweep potential. Despite PCA is a\nmathematical tool that cannot explain which phases are present in a certain sample, it can\ngive an estimation of the number of phases present. (We added the PCA in Supplementary\nNote 3 in page 7 of the revised Supplementary Information.)\n0.9\n0.6\n0.3\n0.0\n-0.3\n-0.6\n-0.9\n-1.2\n8320 8340 8360 8380 8400 8420\nFig. R3. Principal component analysis (PCA) of the XAS spectra measured during the\npositive sweep potential at six different values. The first component (mean) was\nmultiplied by 0.5 for a better comparison.\nIn our case, as shown in Fig. R3, we can see that most of the spectrum is reproduced by\ncomp. 1 (mean spectrum). Additionally, comp. 2 is necessary to explain the different\nspectroscopic features observed. Comp. 3 is not giving a relevant improvement of the fit\nand the cumulative variance is 0.999 after considering only the first two components. The\nother components are mainly associated with noise. On these bases, we think that the use\nof two references, those of Ni S and NiO, is reasonable to explain the spectral features\n3 2\nin our systems. (We added the PCA in Supplementary Note 3 in page 7 of the revised\nSupplementary Information.)\nBased on PCA, the use of two references is reasonable to explain the spectral features in\nour system. We have followed reason to believe the two references should be Ni S and\n3 2\nNiO for linear combination fitting (LCF). (We added the description in Supplementary\nNote 4 in page 7 of the revised Supplementary Information.)\n6\nstnenopmoc\nACP\nComp. 6\nComp. 5\nComp. 4\nComp. 3\nComp. 2\nComp. 1\nx 0.5\nEnergy (eV)\n32 40 48 56\n2u (degree)\nFig. R4. (a) The Ni K-edge XAS spectra at specific applied voltages of +1.00 V (1 V_N\nmeans +1.00 V in negative sweep of CV), -1.50 V (-1.50 V_N means -1.50 V in negative\nsweep of CV), and +0.50 V, +1.00 V (+0.50 V_P and +1.00 V_P mean +0.50 V and +1.00\nV in positive sweep of CV) in region I, II and III respectively, and their linear combination\nof Ni S and NiO. (b) XRD pattern for NiS after HER measurement. The pattern was\n3 2\nassigned to Ni S . (c) Ni K-edge XAS of amorphous NiO (a-NiO NSs) and crystal NiO\n3 2\n(c-NiO NSs). It was extracted from previous report by R. Li et al.2. It shows the XANES\nfeatures of amorphous NiO are very similar to those of crystalline NiO.\nNi K-edge XAS during a cycle of CV give the adequate reasons for using Ni S and NiO\n3 2\nas reference. As shown region II in Fig. R4a, similarity of the measured spectra with Ni S\n3 2\nmakes straightforward its use as a reference for the linear combination fitting. When it\ngoes into region III in Fig. R4a, NiO was a reasonable reference because the increase of\nthe whiteline of 8350 eV is a typical feature of NiO and the slightly shift of the edge\ntowards higher energy indicates the presence of a more electronegative ion. Meanwhile,\nthe XRD of NiS after HER measurement corresponds to Ni S (Fig. R4b), which indicate\n3 2\nit also contains Ni S in region III. Moreover, the first shell suffers an appreciable\n3 2\ncontraction for specific potentials due to the Ni-O bond formation (Supplementary Figs.\n12, 14 and Supplementary Table 4 in supplementary information). We also detect the\nformation of NiO by other characterizations (e.g., Raman spectroscopy, XPS and TEM,\n7\n).u.a(\nytisnetnI\nXRD after HER\n*\n* Carbon paper\nno NiO pattern\nNi3S2PDF#85-1802\n*\n8300 8350 8400 8450 8500\n)stinu\n.bra(\nnoitprosba\ndezilamroN\na b\nNiS I NiS reference\n-1.5 V_N\nII Ni3S2-h\nNi3S2 reference\n0.5 V_P c\nNiO/Ni3S2 Fit\nIII 13% NiO\n87% Ni3S2\nNiO/Ni3S2 Fit\n22% NiO\n78% Ni3S2\nPhoton energy (eV)\nmentioned in main text). As shown in Fig. R4b, because no XRD pattern was observed\nfor NiO, we think the NiO should be in form of small amorphous clusters or thin layers.\nHowever, the XANES features of amorphous NiO are very similar to those of crystalline\nNiO. As reported by R. Li et al.2, the XANES for amorphous NiO and crystalline NiO\nare quite similar (Fig. R4c). For all these reasons, we think that a relevant description of\nthe system may be obtained using Ni S and NiO references. (We added the reasons of\n3 2\nusing Ni S and NiO references in Supplementary Note 4 in page 8, 9 of the revised\n3 2\nSupplementary Information.)\nReference:\n1- Ha+M-Q`MX)N`gjl,jYf_]gj\\]jafYegjh`gmkfa[c]dgpa\\]fYfgk`]]lk]fYZd]kk]d][lan]\nYf\\ ]^^a[a]fl ]d][ljg[`]ea[Yd `q\\jg_]f h]jgpa\\] hjg\\m[lagf- 9QXX GQ\\[^`_ FTe_UOMX\nHOUQZOQ-+0//677 '1/11(-\nComment 4. The Ni K-edge XANES data in Fig. 2(c) (region II and III) are rather poor\nin features and short in the energy range, also the fits in Figs. 2(c) and S6 (dashed lines)\nare not ideal. Does it mean that other phase or phases can be present in the sample? Can\nauthors perform the same analysis using the Ni K-edge EXAFS spectra in larger energy\nor k-space?\nResponse 4 x As motioned response 3 to comment, based on the principal component\nanalysis (PCA)that we introduce in the revised version of the paper, we think that the use\nof two references, Ni S and NiO, is reasonable to explain the spectral features in our\n3 2\nsystem. However, LCFs in k space may not be of good quality because shrinks/dilation\nof bond distances can not be taken into account in a LCF, resulting in the non-ideality of\nthe fittings. It is also important to keep in mind that spectra are taken in a dynamic system\nthat might be changing during the acquisition of each spectrum. We analysed spectra at\npotentials at which the system is more stable (right after each change in the system);\nhowever minor changes might still be happening, adding to deviations from ideality when\ntrying to fit the data.\nWe have extended the LCFs in the revised manuscript, yet focused the discussion on the\nXANES region of the spectra. We have also perfomred a detailed analysis of the Ni K-\nedge EXAFS in the k-space. See Supplementary Figs. 13, 14, and associated\nSupplementary Tables 3, 4 in page 18-20 of the revised Supplementary Information.\nunclear the origin of data in Tables S3 and S4. The description of the simulation\nprocedure and the approximations used should be given in the Supplemental Materials.\nResponse 5xWe have reworked on the EXAFS fits and added the following description\nfor the simulation procedure in Supplementary Note 5 in page 9 of the revised\nSupplementary Information, and more description for Supplementary Table 3 (Table R1)\nand Supplementary Table 4 (Table R2) in Supplementary Discussion 2, 3 in page 19, 20\nof the revised Supplementary Information.\nyThe EXAFS data were analysed in the k-range 3-11 Å-1 and the Fourier Transform\nperformed using a Hanning window. The data were fitted in the R-range 1-2.6 Å using a\nmultiple k-weight fitting. The fits were performed using the most intense scattering paths\nNi-S and Ni-Ni associated to Ni S and NiS. When there were evidences of the presence\n3 2\nof Ni-O bonds, the single scattering path of NiO was added. We weighted the Ni-S and\nNi-O by introducing a parameter x. The amplitude of Ni-S and Ni-O scattering paths was\nx and 1-x, respectively. We left x as a free parameter of the fit.z\nTable R1Operando EXAFS fitting parameters at the Ni-K edge of NiS at different\napplied voltages during the stage of negative sweep of CV. (S 2 = 0.8)\n0\n6QQNMIHVPNTEKI\n?ETL 7= @$X% ,$X,% @(JEGTPR\n$Cvs)@:8%\n&+)** Ja,N 4-2w/-7 1-233w/-/07 /-//4w/-//2\n/-//2\n$RIKMPO;%\nJa,Ja 1-//) 1-6/w/-/2 /-//8w/-//3\n&*)/* Ja,N 5w0 1-241w/-/06 /-//8w/-//2\n/-//6\n$RIKMPO;%\nJa,Ja 1-//) 1-57w/-/2 /-//8w/-//3\n*)** Ja,N 5w0 1-24w/-/1 /-//7w/-//2\n/-//2\n$RIKMPO;%\nJa,Ja 1-//) 1-57w/-/1 /-//4w/-//2\n(*)/* Ja,N 3-2w/-7 1-160w/-/06 /-//6w/-//1\n/-//1\n$RIKMPO;;%\nJa,Ja 3-//) 1-40w/-/1 /-//6w/-//3\n(+)** Ja,N 3-2w/-8 1-17w/-/1 /-//4w/-//0\n/-//4\n$RIKMPO;;%\nJa,Ja 3-//) 1-4/4w/-/04 /-/0/w/-//1\n(+)/* Ja,N 2-2w/-6 1-15w/-/1 /-//4w/-//1\n/-//2\n$RIKMPO;;%\nJa,Ja 3-//) 1-4/6w/-/02 /-//6w/-//1\nCN: coordination number. R: bond distance. a2: Debye-Waller factors. R factor:\ngoodness of fit. The asterisks (*) mean that parameter has been kept fixed during the\nfitting process.\nthe negative sweep potential, see Table R1 (Supplementary Table 3), the results indicate\nthe phase change from NiS to Ni S . At positive potentials, the Ni-S coordination is\n3 2\nconsistent with 6 and the bond distance of Ni-S is around 2.35 Å. The contribution from\nNi-Ni scattering is also observed around 2.68 Å. These values are in good agreement with\nthe structure of NiS. At negative potentials, the Ni-S coordination number is consistent\nwith 4 and the bond length is around 2.27 Å, in agreement with the structure of Ni S .\n3 2\nThe Ni-Ni shell is also 4-coordinated and around 2.51 Å, similar to Ni S .z\n3 2\nTable R2. Operando EXAFS fitting parameters at the Ni-K edge of NiS in different\napplied voltage during the stage of positive sweep of CV. (S 2 = 0.8)\n0\n6QQNMIHVPNTEKI\n?ETL 7= @$X% ,$X,% @(JEGTPR\n$Cvs)@:8%\nJa,N 2-2w/-5 1-151w/-/02 /-//4w/-//2\n(+)/*\n/-//2\n$RIKMPO;;%\nJa,Ja 3-//) 1-4/w/-/0 /-//6w/-//0\nJa,N 1-8w/-7 1-151w/-/05 /-//4w/-//3\n(+)**\n/-//4\n$RIKMPO;;%\nJa,Ja 3-//) 1-4/1w/-/01 /-//6w/-//0\nJa,N 1-8w/-5 1-147w/-/01 /-//5w/-//0\n(*)/*\n/-//1\n$RIKMPO;;%\nJa,Ja 3-//) 1-384w/-//7 /-//6w/-//0\nJa,N 2-/w/-5 1-147w/-/06 /-//5w/-//2\n*)**\n/-//5\n$RIKMPO;;%\nJa,Ja 3-//) 1-4/w/-/0 /-//5w/-//0\nJa,N 3'p</-73w/-0/( 1-156w/-/04 /-//4w/-//0\n&*)/*\nJa,Ja 3-//) 1-4/8w/-/04 /-/0/w/-//1 /-//8\n$RIKMPO;;;%\nJa,K 5'+(W5*)+/( 1-/2w/-/4 /-//4w/-//0\nJa,N 3'p</-63w/-0/( 1-175w/-/03 /-//4w/-//1\n&+)**\nJa,Ja 3-//) 1-40w/-//1 /-/02w/-//1 /-/00\n$RIKMPO;;;%\nJa,K 5'+(W5*),/( 1-//w/-/54 /-//4w/-//1\nCN: coordination number. R: bond distance. a2: Debye-Waller factors. R factor:\ngoodness of fit. The asterisks (*) mean that parameter has been kept fixed during the\nfitting process.\nresults for the positive sweep potential, see Table R2 (Supplementary Table 4), indicate\n1\nthat at more negative potentials the local structure is similar to that of Ni S with a slightly\n3 2\nlower coordination number of Ni-S compared to the crystallographic data, which, as\ndiscussed in the manuscript, might indicated the presence of S vacancies. The Ni-S bond\ndistance is around 2.26 Å, which is in agreement with that of Ni S . At positive potentials,\n3 2\nthere is a clear shrink of the bond length associated to the partial formation of Ni-O bonds\nat around 2.00 Å. The weight of the Ni-O bond compared to Ni-S is around 15 % and 25%\nat +0.50 and +1.00 V vs. RHE respectively (see region III in Table R2), in agreement with\nthe values (13% and 22%) obtained from the linear combination fitting of XANES spectra,\nas shown in Fig. R5.z\nNiO/Ni3S2\nFit\n13% NiO\n87% Ni3S2\n8300 8350 8400 8450 8500\nFig. R5. The Ni K-edge XAS spectra at specific applied voltages of (a) +0.50 and (b)\n+1.00 V vs. RHE (+0.50 V_P and +1.00 V_P mean +0.50 and +1.00 V in positive sweep\nof CV) in III, extracted from Fig. 2c, and their linear combination of Ni S and NiO.\n3 2\nComment 6. The reliability of the fits is also not clear. Some values reported in the two\ntables look unphysical. In particular, all Ni-O distances are very short being about 1.7 Å\n(a longer distance of 2.01 Å is reported in the text!), whereas the Ni-O bond length in\nbulk NiO is significantly longer being about 2.09 Å.\nResponse 6 Y Thanks to the referee for this comment. We realize that in our previous\nmanuscript, the including of Ni-O path in region I and II was not justified. We have\nremoved the Ni-O path in region I and II, only include Ni-O path in region III. As we\nmentioned in response 5, we reworked on the fitting and the reliability of the fitting are\nvery good. The updated results are shown in Supplementary Table 3, 4 in page 19, 20 of\nthe Supplementary Information (i.e., Table R1 and R2 in Response 5).\nWe made a mistake trying to fit in traces of NiO spectral features in spectra of the sample\nduring the negative sweep of the CV. This indeed led to unreliable results in the reported\n11\nnoitprosba\ndezilamroN\n+0.50 V_P\nIII NiO/Ni3S2\nFit\n22% NiO\n78% Ni3S2\n8300 8350 8400 8450 8500\nPhoton energy (eV)\nnoitprosba\ndezilamroN\na b\n+1.00 V_P\nIII\nPhoton energy (eV)\nTable R3. Operando EXAFS fitting parameters at the Ni-K edge of NiS at +0.50 and\n+1.00 V vs. RHE during positive sweep of CV. (S 2 = 0.8)\n0\n6QQNMIHVPNTEKI\n?ETL 7= @$X% ,$X,% @(JEGTPR\n$Cvs)@:8%\nJa,N 3'p</-73w/-0/( 1-156w/-/04 /-//4w/-//0\n&*)/*\nJa,Ja 3-//) 1-4/8w/-/04 /-/0/w/-//1 /-//8\n$RIKMPO;;;%\nJa,K 5'+(W5*)+/( 1-/2w/-/4 /-//4w/-//0\nJa,N 3'p</-63w/-0/( 1-175w/-/03 /-//4w/-//1\n&+)**\nJa,Ja 3-//) 1-40w/-//1 /-/02w/-//1 /-/00\n$RMKMPO;;;%\nJa,K 5'+(W5*),/( 1-//w/-/54 /-//4w/-//1\nCN: coordination number. R: bond distance. a2: Debye-Waller factors. R factor:\ngoodness of fit. The asterisks (*) mean that parameter has been kept fixed during the\nfitting process.\nAs the reviewer mentioned, highly crystalline NiO has a characteristic Ni-O distances\naround 2.09 Å; however, studies of short-range order in amorphous nickel oxide have\nshown significant increases of Ni-O distance upon increasing the crystallinity of the\nsystem. For instance, R. Li et al. found the Ni-O distance increases from 2.06 Å to 2.10\nÅ form an amorphous to a crystalline system2. Also, Y. R. Denny et al. found the Ni-O\ndistance increases from 1.984 Å for amorphous NiO to 2.084 Å for crystalline phase3.\nTherefore, the values obtained for the Ni-O distance are in agreement with the typical\ndistances for amorphous and/or nanocrystalline NiO (in the range 2.00-2.06 Å). This\nfurther confirms the amorphous structural nature of the NiO formed under the\nelectrocatalytic reaction.\nWe included the statement in page 9 of the revised manuscript. yHighly crystalline NiO\nhas a characteristic Ni-O distances around 2.09 Å; however, studies of short-range order\nin amorphous NiO have shown that the Ni-O distances for amorphous and/or\nnanocrystalline NiO may decrease to 2.00-2.06 Å.2,3. This further confirms the nickel\noxide formed in our work is in form of amorphous NiO.z\nReferences:\n1- Ha+M-Q`MX)N`gjl,jYf_]gj\\]jafYegjh`gmkfa[c]dgpa\\]fYfgk`]]lk]fYZd]kk]d][lan]\nYf\\ ]^^a[a]fl ]d][ljg[`]ea[Yd `q\\jg_]f h]jgpa\\] hjg\\m[lagf- 9QXX GQ\\[^`_ FTe_UOMX\nHOUQZOQ-+0//677 '1/11(-\n2- @]ffq+T-M-Q`MX)JaG]\\_]S=BNYfYdqkakg^JaKl`af^adeoal`emdlahd]k[Yll]jaf_\nl`]gjq-Ha^R)?Z`Q^RMOQ7ZMX) .0+886,888'1/03(-\ndata reported in Fig. 2(d).\nResponse 7 x Thanks for this suggestion. As metioned in response 5, we have reworked\non EXAFS fitting. Fig. 2d has been updated, and the fitting results are summarized in\nSupplementary Tables 3, 4 in page 19-20 of the revised Supplementary Information.\nComment 8. The Ni K-edge EXAFS spectra should be additionally reported in Fig. 3,\nand the Ni-O and Ni-S contributions should be shown separately. Currently, the\ncontribution from the Ni-S bonds is not convincing i it could be negligible.\nResponse 8 xFig. 3a in previous manuscript has been moved to Fig. 2f in the revised\nmanuscript. Figs. 3b-d were moved to Supplementary Fig. 12 in page 17 of the revised\nSupplementary Information.\nThe Ni K-edge FT-EXAFS shown in Fig. R7a (Supplementary Fig. 12a) was obtained by\nthe Fourier transformation of Ni K-edge EXAFS spectra at +1.00 V in region III in Fig.\n2c. The Ni-O and Ni-S paths have been shown separatedly in FT-EXAFS (Fig. R7a, i.e.,\nSupplementary Fig. 12a) and Wavelet transform (Fig. R7d, i.e., Supplementary Fig. 12d).\nWe introduced the Ni-S and Ni-O contributions for spectra where we had a clear evidence\nof the presence of Ni-O (PCA analysis and LCFs metioned in response 3). As it can be\nseen (please see the following Table R4), the presence of Ni-O is not negligible for two\npotentials and its weight around 15% and 25%, in agreement with the results from LCF\nof XANES spectra (please see the following Fig. R8).\nthe PCA analysis, we use Ni-O, Ni-S and Ni-Ni scattering paths to fit the data, and the\nresults of fitting was reliable because of the reasonable parameter as shown in Table R4\n(Supplementary Table 4). Because these O, S and Ni backscatting are difficult to\ndistinguish in first shell as shown in Fig. R7a (Supplementary Fig. 12a), we use wavelet\ntransform to resolve them along k. Fig. R7c (Supplementary Fig. 12c) shows the overview\nwavelet in the interval 3 Å-1#k#11 Å-1 with Morlet parameters ‹=10, ‡=1, and a signal\nweighting of k3. Only one main peak A in Fig. R7c (Supplementary Fig. 12c) is in\nagreement well with the FT-EXAFS in Fig. R7a (Supplementary Fig. 12a). By optimizing\nparameters ‹=1.6, ‡=1 in the interval 5 Å-1#k#8 Å-1, the resolution along k is improved\nand the one main peak is resolved to three peaks at k=5.9, 6.6 and 7.4 Å-1, respectively,\nthereby revealing the presence of Ni-O, Ni-S and Ni-Ni in the first shell (see in Figure\nR7d, i.e., Supplementary Fig. 12d).z\nNiO/Ni3S2\nFit\n13% NiO\n87% Ni3S2\n8300 8350 8400 8450 8500\nFig. R8. The Ni K-edge XAS spectra at specific applied voltages of (a) +0.50 and (b)\n+1.00 V vs. RHE (+0.50 V_P and +1.00 V_P mean +0.50 and +1.00 V in positive sweep\nof CV) in III, extracted from Fig. 2c, and their linear combination of Ni S and NiO.\nThe\n3 2\ncontent of NiO is 02%Yf\\11%at +0.50 and +1.00 V vs. RHE respectively, that is similar\nto the results obtained by EXAFS fitting as shown in Table R4.\nWe conducted additional operando XAS to ensure NiO still exists during HER conditions.\nThe analysis of operando XAS in Fig. 2 support the presence of NiO at positive applied\nvoltages in region III in Figs. 2a, c. This experiment is very important to support the HER\nmechanism, so we added Fig. R9 as Fig. 3 in the revised manuscript and added new\ncontents in page 10 of the revised manuscript.\n16\nnoitprosba\ndezilamroN\n+0.50 V_P\nIII NiO/Ni3S2\nFit\n22% NiO\n78% Ni3S2\n8300 8350 8400 8450 8500\nPhoton energy (eV)\nnoitprosba\ndezilamroN\na b\n+1.00 V_P\nIII\nPhoton energy (eV)\n15\n12\n9\n6\n3\n0\n0 1 2 3 4 5\nFig. R10. The operando FT-EXAFS and their fitting of NiS at different applied voltages\n((a) +1.00, (b) +0.50, (c) 0.00, (d) -0.50, (e) -1.00 and (f) -1.50 V vs. RHE) during the\nstage of negative sweep of CV. The fitting results are shown in Table R1 (Supplementary\nTable 3).\nFig. R11. The operando FT-EXAFS and their fitting of NiS at different applied voltages\n((a) -1.50, (b) -1.00, (c) -0.50, (d) 0.00, (e) +0.50 and (f) +1.00 V vs. RHE) during the\nstage of positive sweep of CV. The fitting results are shown in Table R2 (Supplementary\nTable 4).\n18\n)3k*)k(g(TF\nR factor = 0.003 15\n12\n+1.00 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\nR factor = 0.007 15\n12\n+0.50 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\nR factor = 0.003\n+0.00 V\nFit\nR +$e$(Å)\n15\n12\n9\n6\n3\n0\n0 1 2 3 4 5\n)3k*)k(g(TF\nR factor = 0.002 15\n12\n-0.50 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\nR factor = 0.005 15\n12\n-1.00 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\na b c\nd e f\nR factor = 0.003\n-1.50 V\nFit\nR +$e$(Å)\n15\n12\n9\n6\n3\n0\n0 1 2 3 4 5\n)3k*)k(g(TF\nR factor = 0.003 15\n12\n-1.50 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\nR factor = 0.005 15\n12\n-1.00 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\nR factor = 0.002\n-0.50 V\nFit\nR +$e$(Å)\n15\n12\n9\n6\n3\n0\n0 1 2 3 4 5\n)3k*)k(g(TF\nR factor = 0.006 15\n12\n0.00 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\nR factor = 0.009 15\n12\n+0.50 V 9\nFit 6\n3\n0\n0 1 2 3 4 5\nR +$e$(Å)\n)3k*)k(g(TF\na b c\nd e f\nR factor = 0.011\n+1.00 V\nFit\nR +$e$(Å)\nComment 10. The values of coordination numbers (CNs) should contain errors. The\nmeaning of asterisks should be explained in the captions of Tables. The small values of\nthe Ni-O CNs do not convince for the occurrence of the NiO phase.\nResponse 10xThanks for the observation.As described in response 6,we have removed\nthe NiO spectral contribution to the fit of spectra in region I and II in Fig. 2c, and only\nincluded NiO in region III. The refitted results have been presented in response 5, 6.\nBeside this, the errors have been introduced in all the CNs used as free parameters of the\nfit. The asterisks mean that parameter has been kept fixed during the fitting process. This\nis the case for the Ni-Ni distance.\nWe have introduced the following statement in Supplementary Note 5 in page 9 of the\nrevised Supplementary Information: yConsidering that most of the changes occurs in the\nfirst shell, the CN of Ni-S have been used as a free parameter, while for the Ni-Ni we\nused a fixed CN according to the crystallographic data. The use of a free coordination\nnumber for the Ni-Ni gave a very high correlation with the coordination number of Ni-S\nand made the estimation impossible because the two contributions (Ni-S and Ni-Ni) are\nenclosed in a single shell and cannot be separated with a limited k-rangez\nComment 11. It looks like Fig. 3(d) contains wrong labels for the spectra: the blue line,\nprobably, corresponds to NiO. One can recommend using the same colors in Fig. 3(b)\nand (d).\nResponse 11 x Thanks for the careful observation and very good suggestions. Figures\nhave been updated as suggested. In the revised manuscript, this Fig. 3d has been moved\nto page 17 of the Supplementary Information and now it is labelled as Supplementary Fig.\n12b.\nComment 12. The Raman data (Fig. 2e) are available in a short-range, so only two\nbands around 320 and 480 cm-1 are visible in the catalyst. Their assignment cannot be\nunique. Also, the band at 560 cm-1 discussed in the text and attributed to NiOx phase is\nnot present in the Raman spectra. Therefore, the Raman data should be re-analysed.\nComparison with the literature data should be done.\ncomparison with any reference data is provided. It is not clear how well the structural\nand electronic properties are reproduced, in particular, taking into account that the PBE\nfunctional is not hybrid. The position of the Fermi level should be given in Fig. 5(d).\nResponse 13x Thanks for your helpful suggestions. The accuracy of our DFT calculation\nfor the structural and electronic properties of the materials were actually justified by\ncomparing with results from literature. R]Yj]kgjjqo]\\a\\f{lhjgna\\]\\]lYad]\\akmkkagfk-\nIn the revised manuscript, we provide the following descriptions for justification of the\naccuracy of DFT calculations in Supplementary Note 6 in page 9-11 of the revised\nSupplementary Information. We also added the Fermi level (set to zero) in Fig. 5e in\nrevised manuscript, i.e., Fig. 5d in previous manuscript.\nThe crystal structures and electronic structures of NiS and Ni S have been previously\n3 2\nreported by experiments and theory9-13. Therefore, the crystal structure, lattice parameters,\natomic bond distances and angles of NiS and Ni S are optimized according to the\n3 2\nliterature and compared to the results of previous works9,10, as listed Table R5. The good\nagreement between the optimized lattice parameters and the experimental results suggests\nour computational methodology is reliable to describe the crystal structure of NiS and\nNi S .\n3 2\nTable R5. The crystal parameters, internal atomic distances, and angles of NiS and Ni S .\n3 2\n?gehmlYlagfYdj]kmdlk Aph]jae]flYdj]kmdlk8+0/\n'Y+Z+[( '2-34+2-34+4-08( '2-33+2-33+4-24(\n' +‰+Š( '8/-/+8/-/+01/-/( '8/-/+8/-/+01/-/(\nJaN Ja,N 1-27 1-28\n'L52.ee[( Ja,Ja 1-5/ 1-57\n;N,Ja,N '76-/+82-/+07/-/( '77-1+80-7+07/-/(\n;Ja,Ja,Ja '5/-/+8/-/+01/-/+07/-/( '5/-/+8/-/+01/-/+07/-/(\n'Y+Z+[( '3-/7+3-/7+3-/7( '3-/5+3-/5+3-/5(\n' +‰+Š( '78-3+78-3+78-3( '78-5+78-5+78-5(\nJa 2 N 1 Ja,N '1-15+1-17( '1-14+1-18(\n'M21D( Ja,Ja 1-43 1-42\n;N,Ja,N 0//-5+0/2-3+016-7 '0//-7+016-2(\n;Ja,Ja,Ja 5/-/ '5/-/+88-/+0/7-0+037-6(\nboth NiS and and Ni S have metallic states11-13.Our DFT calculated electronic structures\n3 2\nfor NiS (Fig. R13a) and Ni S (Fig. R13b) also indicate the metallic state for both\n3 2\nmaterials. Furthermore, the calculated density of state (DOS) also indicates that there are\nsubstatial Ni 3d state contributing to the DOS around the Fermi level, also in good\nagreement with the literatures11-13. Therefore, our calculation methods can reproduce well\nthe structural and electronic properties of NiS and Ni S\n3 2.\n20\n15\n10\n5\n0\n-8 -6 -4 -2 0 2\nFig. R13. Calculated total and partial density of states of samples: (a) NiS and (b) Ni S .\n3 2\nWe also calculated the Gibbs free energies of H adsorption on pristine Ni S and\n3 2\ncompared the data with other published work14, which was performed by using PBE\nfunctional. Our results are well consistent with the reference data14. The usage of this\nmethods (GGA-PBE) in our work show very good performance in understanding the\nreaction mechanism and activity trends observed in experiments. Therefore, the accuracy\nof DFT calculations used in our work is reliable.\nReferences:\n8- I[R`Yf+ @- >-+ IYj]rag+ I-+ M]e]acY+ F- L- & @]jfa]j+ L- @- Lj]kkmj],l]eh]jYlmj]\nh`Yk]\\aY_jYe Yf\\[jqklYd kljm[lmj]g^ JaN- FTe_) GQb)8/+1441,1444'0861(-\n0/-Q]jk`afaf+ =- @-+ N]danYfgn+ A- J-+ CmdqY]nY+ M- E- & N]d{e]fkcac`+ J- E- O`]jeYd\nAphYfkagfg^Ja N afJa N ,Ja=ddgqk- ?Z[^S)CM`Q^).++771,776'1//4(-\n2 1\n00- RYf_+F-,D-+?`]f_+U-+>j]\\Yk+F-,H-&Ham+I-Ad][ljgfa[Yf\\naZjYlagfYdhjgh]jla]k\ng^ fa[c]d kmd^a\\)k ^jge ^ajkl hjaf[ahd]k- @) 9TQY) FTe_) +,1+ 1036/4.1036/0,\n1036/4.1036/7'1//6(-\n01-Gjak`fYcmeYj+N-M-+N`Yfl`a+J-&NYjeY+@-@-Ad][ljgfa[kljm[lmj]g^eadd]jal]JaN-\nFTe_) GQb)8 00+0040/4 '1//1(-\n02-Hm+ U- R-+ Gd]af+ >- I- & Naf_`+ @- F- Ad][ljgfa[ kljm[lmj] g^ `]Yrd]ogg\\al] Ja N -\n2 1\nFTe_) GQb)859[ZPQZ_) CM``Q^ /.+02431,02434 '0885(-\n03-B]f_+ H-,H- Q` MX) Da_`,Ef\\]p ^Y[]l]\\ Ja N fYfgk`]]l YjjYqk Yk `a_`dq Y[lan] Yf\\n2 1\nmdljYklYZd] ]d][ljg[YlYdqklk ^gj oYl]j khdallaf_- @) 7Y) 9TQY) H[O) +-1+ 03/12,03/15\n'1/04(-\n22\n).u.a(\netats\nfo\nytisneD\nTotal Total\nNi-d Ni-d\n10\n5\n0\n-8 -6 -4 -2 0 2\nEnergy (eV)\n).u.a(\nsetats\nfo\nytisneD\na b\nNiS E F\nEnergy (eV)\nReferee: 3\nComments to the Author:\nReport on \"In situ formed NiOx/Ni S Nano interfaces Boost High Hydrogen Evolution\n3 2\nReaction Activity in Alkaline Conditions\" by X. Ding, et. al\nThe authors report the results of a comprehensive experimental and theoretical study\nexploring the structural transformations that occurring within NiS and which accelerate\nthe hydrogen evolution reaction (HER) under alkaline conditions. This is an important\narea of work with much interest in the formation of H as an energy storage medium\n2\nthrough the electrolysis of water using renewable energy sources. The results presented\nby the authors suggest that NiS undergoes a phase transformation during the reaction to\nform an NiOx/Ni S phase, which then acts as a catalyst for the rate-limited step in the\n3 2\nHER under alkaline conditions. This is an important observation because understanding\nthe structure on which a reaction occurs is key for the design of improved catalysts. As\nsuch, this manuscript will be of broad interest and importance. I recommend publication\nafter the following points have been adequately addressed. Note that given that my\nexpertise lies in theoretical chemistry,\nI have only provided comments related to the calculations report by the authors.\nResponse Y Thanks to referee 3 for his/her positive feedback on our work. We are\nappreciated for the helpful suggestions/comments for improving our manuscript.\nFollowings are our responses to comments.\n[The comments are shown in italic; responses are in black; all revisions in manuscript\nand supplementary information are highlighted in dark green.]\nComment 1. The theoretical methods section in the supporting information do not\nadequately describe key parts of the calculations. In particular, the authors should\ndescribe what kinds of pseudopotentials were used (I assume PAWs, but it isn't stated),\nwhat planewave cutoff was used for the density and/or augmentation charges, and the k-\npoint grid used in the calculation. In addition, the authors should outline how zero-point\nenergies and entropies were obtained. I assume through phonon calculations, but it is not\nstated. It is also important to state whether such calculations (if they were phonon\ncalculations) were performed for the whole system or just in the region near the reactive\nsite.\nthe calculation methods. We have added methods for the calculations in Supplementary\nMethod 7 in page 4, 5 of the revised Supplementary Information, as follows.\nyThe generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE)\nexchange-correlation functional15 and a 450-eV cutoff for plane-wave basis set were\nemployed to perform all the density functional theory (DFT) computations of the\nmaterials within the frame of Vienna ab initio simulation package (VASP)16,17. The\nprojector-augmented plane wave (PAW) was used to describe the electron-ion\ninteractions18,19. 5×5×1 Monkhorst-Pack grid k-point were employed for geometric\noptimization of the slab surface. The convergence threshold was set to 10-5 eV and 0.02\neV/Å for energy and force, respectively. We opened the spin-polarization during the\ncalculations to obtain accurate results of the total energy and density of states (DOS). The\n(110) surface of NiS and ( 10) surface of Ni S with ~15 Å vacuum were simulated as\n9t 3 2\nl`] [YlYdqla[ afl]j^Y[]- QYf \\]j RYYdk afl]jY[lagfk o]j] [gfka\\]j]\\ mkaf_ l`] Cjaee]{k\nD3-type of the semiempirical method because of long-range interaction present between\nthe system and gas molecules.z\nyBased on computational hydrogen electrode (CHE) model20, the Gibbs free energies\nwere calculated by\nhG = hE + hE ƒIhH\nZPE\nwhere hE is the adsorption energy of the adsorbed intermediates, and hE and hS are\nZPE\nthe difference in the zero-point energy and the change in entropy before and after\nadsorption of various intermediates, respectively.The zero-point energies and entropies\nof the reaction species were calculated from the vibrational frequencies. For gas phase\nmolecule, the entropy term can be expressed as the sum of the translational, rotational,\nand vibrational contributions, whereas for adsorbates the translational and rotational\nentropy were not taken into account due to negligible contributions. Specifically, the\nvibration frequency of hydrogen adsorption is 3211.45 cm-1, which is not sensitive to the\nadsorption sites. The entropy is obtained based on the equation:\nNX\nP\n]`r\nEWJK^ I\nP]`r@_f[\nGyHz w x}vF$%{8vI _f[ |u ~\nH 8vI\nP]`r@_f[\n^QL\n24\nwhere R stands for the universal gas constant, k is the Boltzmann constant, h ak LdYfc{k\nB\nconstant, N ak =ng_Y\\jg{k fmeZ]j+ v represents the frequency and N is the number of\nA i\nadsorbed atoms. In addition, during these frequency computations, all atoms of substrate\nwere rigidly constrained so that no additional degrees of freedom from catalysts are\nintroduced into the reaction system. Therefore, such calculations were performed just for\nthe reaction intermediates, and the contributions from the catalysts to ˆE and ˆS are\nZPE\nneglected.z\nReference:\n04-L]j\\]o+ F- L-+ >mjc]+ G- & Ajfr]j`g^+ I- C]f]jYdar]\\ _jY\\a]fl YhhjgpaeYlagf eY\\]\nkaehd]-FTe_) GQb)BQ``) 11+2754,2757'0885(-\n05-Gj]kk]+ C- & DY^f]j+ F-=Z afalag egd][mdYj\\qfYea[k ^gj daima\\ e]lYdk- FTe_) GQb) 8\n.1+447,450'0882(-\n06-Gj]kk]+C-&DY^f]j+F-=Zafalagegd][mdYj,\\qfYea[kkaemdYlagfg^l`]daima\\,e]lYd,\n,Yegjh`gmk,k]ea[gf\\m[lgj ljYfkalagf af _]jeYfame- FTe_) GQb) 8 .3+ 03140,03158\n'0883(-\n07->du[`d+ L- A- Ljgb][lgj Ym_e]fl]\\,oYn] e]l`g\\- FTe_) GQb) 8 /*+ 06842,06868\n'0883(-\n08-Gj]kk]+C-&FgmZ]jl+@-BjgemdljYkg^lhk]m\\ghgl]flaYdklgl`]hjgb][lgjYm_e]fl]\\,\noYn]e]l`g\\- FTe_) GQb) 8 /3+0647,0664'0888(-\n1/-IYf+ E- ?- Q` MX) Pfan]jkYdalq af gpq_]f ]ngdmlagf ]d][ljg[YlYdqkak gf gpa\\) kmj^Y[]k-\n9TQY9M`9TQY-+0048,0054'1/00(-\nComment 2. In the first reaction step in Figure 3a, the authors show S leaching from the\nmaterial and indicate that oxygen should fill the vacancy. However, the middle image in\nthat figure states that the material is Ni S and shows no oxygen. So, how does oxygen\n3 2\nfactor in? I found this pretty confusing.\nResponse 2 x Thank you for this helpful comment and sorry for the confusion. In the\nrevised manuscript, we have changed Fig. 3a to Fig. 2f, and maked some modifications.\nWe went back to synchtrotron and re-performed the operando XAS experiment. Details\nof the new results are provided in Fig. R14a-b and Supplementary Table 3, 4. We found\nsubstantial oxidation of Ni occur at applied voltage of above +0.27 V vs. RHE during\npositive sweep of CV. Therefore, there is only S leaching in the first reaction step, and\noxygen filling mainly occurs during positive sweep. Therefore, we revised the schematic\nillustration for the phase transition shown in Fig. R14c (Fig. 2f), and added the following\ndescription in page 9, 10 in the revised manuscript.\nDetailed calculation methods and structural optimization are provided in Supplementary\nMethod 7 and Supplementary Figs. 18-20. (1) Step I: It is well known that in alkaline\nHER, the dissociation of water molecules is the sluggish step21,22. Hence, we first consider\nadsorption energies of H O molecules at different atomic sites of NiO/Ni S surface. As\n2 3 2\nshown in Fig. 5b (Fig. R15b), the H O molecules has a larger adsorption energy of 0.93\n2\neV at the interfacial Ni site which is bonded to an oxygen and a sulphur (denoted as O-\nNi-S) than that (0.72 eV) atthe Ni site which is bonded to two oxygen atoms (denoted as\nO-Ni-O), indicating a favourable adsorption of H O molecule at the interfacial Ni site (O-\nFig. R16. The calculated free energy diagrams of H O molecule dissociation and their\n2\ncorresponding atomic configurations on different sites of NiO/Ni S . One site is the\n3 2\ninterfacial Ni site that is bonded to an oxygen and a sulfur atom (defined as O-Ni-S), the\nother is Ni site that is bonded to two oxygen atoms (defined as O-Ni-O).\n(2) Step II and III: Fig. 5c (Fig. R16 i.e., Supplementary Fig. 18) shows the free energy\ndiagrams for H O dissociation into OH* and H* at the interfacial Ni site (O-Ni-S) and O-\nwater dissociation, while the energy barrier is 2.70 eV for the O-Ni-O site. This indicates\n28\nthe interfacial Ni (O-Ni-S) provides optimal site for water dissociation, leading to a faster\nproton supply for the following H generation.\natomic configurations on S sites, Ni sites and O sites of NiO/Ni S . We also calculated\n3 2\nthe free energy of H adsorption on other sites as show in Supplementary Fig. 20.\n(3) Step IV, V and VI: Furthermore, Fig. 5d (Fig. R17, i.e., Supplementary Fig. 19) shows\nl`] [Yd[mdYl]\\ ˆG at different sites of NiO/Ni S , and their corresponding atomic\nH* 3 2\nconfigurations of H adsorption are shown in supplementary Fig. 19. The calculated results\nj]n]Yd l`Yl l`] ˆG value at the S site (0.08 eV) is closer to the thermal neutral position\nH*\n(zero) in comparison with that at the Ni site (0.27 eV) and O site (-1.21 eV), implying an\noptimal H* adsorption and desorption at S site, agreeing well with S site as active site for\nhigh HER activity.\nBased on above discussions, it is clear that the interfacial Ni sites (O-Ni-S) of NiO/Ni S )\n3 2\nfacilitate the water dissociation and nearby S sites of Ni S facilitate hydrogen generation.\n3 2\nWe have illustrated this process and reaction energetics in Fig. 5a-d in the revised\nH*\nsites of NiO/Ni S , and their corresponding atomic configurations of H* adsorption are\n3 2\nshown in Supplementary Fig. 19. O`] [Yd[mdYl]\\ j]kmdlk j]n]Yd l`Yl l`] ˆG value at S\nH*\nsite (0.08 eV) is close to the thermal neutral position (zero) in comparison with that at Ni\nsite (0.27 eV) and O site (-1.21 eV), implying optimal H* adsorption and desorption at S\nsite, agreeing well with S site as active site for high HER activity.\nAs shown in Fig. R19 (Fig. 5d in revised manuscript and Supplementary Fig. 19), Our\nresults also indicate that although H* prefer to adsorb on the O site, the desorption of H*\nfrom O is difficult because of the too strong adsorption of H* at O site (ˆG = -1.21 eV).\nH*\nTherefore, in Fig. 5a (Fig. R18), we show the adsorption of H* (marked in blue) on O\nsite, but no cyclic process to generate H .\natomic configurations on S sites, Ni sites and O sites of NiO/Ni S . The value of ˆC (-\n3 2 H*\n1.21 eV) of H* on O sites is too negative, so the adsorption of O to H* is too strong that\nH* [Yf{l \\]kgjhl ^jge K ^gjj]d]Ykaf_D .\n2\n31"}, {"round": 2, "reviewer_comments": {"reviewer_2": "The revised version of the manuscript was significantly improved, however, several important questions are still open.\nThe detailed description of the EXAFS simulations/fits is fully missing which undermines the credibility of the data obtained.\n1) It is not clear what software and how was used for the data analysis. What approximations were employed? Did the authors test the theory used in the fits on any reference samples?\n2) The fitting results in Figs. S13 and S14 show only the modulus of the Fourier transforms (FTs) thus the information on the phase is fully missing. The imaginary part of FTs should be shown or, even better, the plots of EXAFS spectra in k-space should be additionally presented.\n3) The same is true for the result of the fit reported in Fig. S12. In this case (and in Fig. S14 (e,f)), the authors should also prove that the three-component fit is statistically significant.\n4) Finally, the too-short Ni-O distances reported may be due to (1) the harmonic model being used in the case of strong disorder or (2) the “inaccurate” choice of the kinetic energy origin (E0) for the photoelectron wavenumber k. Please, check the data analysis.", "reviewer_4": "The work describes the experimental finding of a good HER catalysts under alkaline conditions, which are in situ reconstructed from NiS. As a theoretician, I mainly checked the theoretical part of the work. However, I am not satisfied with the computation results, which are unclear on both the model and the kinetics. This part is not scientifically sound.\n1. The catalyst structure. There is very limited experimental evidences on the atomic structure of the catalyst. It appears for quite a certain the catalyst is amorphous. The theoretical part utilizes a rather simple layer-by-layer crystalline model, which is obviously not consistent with experiment. There is even no thermodynamics analysis on the stability of the catalyst structure from DFT.\n2. The water dissociation barrier is at least 1.07 eV, which is too high for reaction to occur at the ambient condition.\n3. The H adsorption energy on three sites are evaluated, which shows that H on O site can be very stable. Therefore, this is a good indication that the current model is not right. Even the H coverage has not been explicitly explored.\nFrom my point of view, the theoretical part of the work does not add new insights, but create more confusions on the exact catalyst structure and on the reaction mechanism."}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #2 (Remarks to the Author):\nThe revised version of the manuscript was significantly improved; however, several\nimportant questions are still open. The detailed description of the EXAFS simulations/fits\nis fully missing which undermines the credibility of the data obtained.\nResponse: Thanks a lot for Reviewer #2’s positive comment. According to your kind\nsuggestion, we have further revised our manuscript and provided the detailed description\nof the EXAFS simulations/fits. Hopefully, the current version can be satisfied.\nQ1. It is not clear what software and how was used for the data analysis. What\napproximations were employed? Did the authors test the theory used in the fits on any\nreference samples?\nResponse: Thanks a lot for Reviewer #2’s questions. The software utilized for EXAFS\nanalysis is ARTEMIS from the Demeter package. The crystallographic information files\n(CIFs) for NiS and Ni S were employed in the FEFF 6 calculation to derive the scattering\n3 2\npaths. The software employs the path expansion approximation, treating the EXAFS\nspectrum as the sum of contributions from scattering geometries involving two or more\natoms in a cluster surrounding the absorbing atom (Ravel et al., Journal of Physics:\nConference Series 2013, 430, 012006).\nNote that the data were normalized through background subtraction using the ATHENA\nsoftware within the Demeter package before the EXAFS analysis. As shown in\nSupplementary Note 1, “EXAFS simulations procedure,” (Page 23 and 24 in\n6MIIE?F?GL;JQ1G@HJF;LCHG#%LB?/9-06 >;L;O?J?;G;EQR?>CGLB?D&J;GA? H@*T))S\\Y%\nand the Fourier Transform was executed using a Hanning window. The data were fitted\nin the R-range of 1–2.6 Å using a multiple k-weight fitting. The fits employed the most\nintense single scattering paths, Ni-S and Ni-Ni, associated with NiS and Ni S phases, in\n3 2\naccordance with the results obtained from XRD, Raman, and XANES investigations. The\namplitude reduction factor S 2 was determined from a reference Ni foil, and its value of\n0\n0.8 was fixed for all scattering paths. This procedure is also widely adopted in the analysis\nof EXAFS data, particularly when experiments are conducted under the same conditions.\nRegarding the model for fitting the data, we would like to clarify as following: 1) only\nthe most intense single scattering paths were utilized, 2) the coordination number of the\nNi-Ni shell was fixed based on crystallographic data of NiS or Ni S , while the\n3 2\ncoordination number of the first shell was treated as a free parameter. This approach was\nnecessary because, when leaving the coordination number of the second shell as a free\nparameter in the fit, a high correlation was observed with the Ni-S shell. The contributions\nof Ni-S and Ni-Ni are included in a single shell and cannot be distinguished within a\nECFCL?> D&J;GA?' .HGN?JK?EQ% 5 ;G> Z H@ LB? @CJKL ;G> K?=HG> KB?EE O?J? LJ?;L?> ;K\nindependent parameters.\nRegarding the analysis of reference samples, a good agreement was achieved for the\nreference NiS, as confirmed by the values in Supplementary Table 2 (potential 0.00 in\nregion I corresponding to the NiS sample) on Page 13 in Supplementary Information.\nWe have added the above information and please see them in Supplementary Note 1 on\nPage 23 and 24 in the Supplementary Information.\nQ2. The fitting results in Figs. S13 and S14 show only the modulus of the Fourier\ntransforms (FTs) thus the information on the phase is fully missing. The imaginary part\nof FTs should be shown or, even better, the plots of EXAFS spectra in k-space should be\nadditionally presented.\nResponse: Thanks a lot for Reviewer #2’s kind suggestion. We have provided the\nimaginary parts of the Fourier Transforms (FTs) and the extended X-ray Absorption Fine\nStructure (EXAFS) spectra in k-space. As shown in Fig. R1, a distinct shift in the\nimaginary part is observed when transforming NiS into Ni S , which may be attributed to\n3 2\nthe shorter Ni-S bond distance in Ni S along with a reduced coordination number (CN)\n3 2\nof the Ni-S. The analysis of the imaginary part clearly indicates a significant\ntransformation of the initial NiS compound at positive potential (Fig. R2).\nFig. R1 (Supplementary Fig. 8 in the revised Supplementary Information) | The\noperando FT-EXAFS, imaginary part of FT-EXAFS, EXAFS in k-space and their fitting\nof NiS at different applied voltages: (a) +1.00, (b) +0.50, (c) 0.00, (d# X('+(% \"e# X)'((\nand (f#X)'+(8vs. RHE during the stage of negative sweep of cyclic voltammetry (CV).\nFig. R2 (Supplementary Fig. 9 in the revised Supplementary Information) | The\noperando FT-EXAFS, imaginary part of FT-EXAFS, EXAFS in k-space and their fitting\nof NiS at different applied voltages: (a# X)'+(% \"b# X)'((% \"c# X('+(% \"d) 0.00, (e) +0.50\nand (f) +1.00 V vs. RHE during the stage of positive sweep of CV.\nWe have updated the Supplementary Figs. 13 and Fig. 14 and re-named them as\nSupplementary Fig. 8 and Fig 9 on Page 12 and 14 in the revised Supplementary\nInformation.\nQ3. The same is true for the result of the fit reported in Fig. S12. In this case (and in Fig.\nS14 (e, f)), the authors should also prove that the three-component fit is statistically\nsignificant.\nResponse: Thanks a a lot for Reviewer #2’s comment. In our previous XANES linear\ncombination fitting (LCF), where NiO served as a reference for the oxidized portion (Fig.\nR3 and Table R1), we obtained a good model incorporating 13% or 22% of NiO in\naddition to Ni S . This model effectively represented the XANES spectra at +0.50 and\n3 2\n+1.00 V during the positive sweep of CV, indicating the presence of Ni-O compounds\n(we assumed to be in the form of NiO).\nFig. R3 (Fig. 2c in the revised manuscript) | The Ni K-edge XAS spectra at specific\n;IIEC?> NHEL;A?K H@ $)'(( 8 \") 8:2 F?;GK $)'(( 8 CG G?A;LCN? KO??I H@ .8#% X)'+( 8\n\"X)'+(8:2F?;GKX)'+(8CGG?A;LCN?KO??IH@.8#%;G>$( '+(8%$)'((8\"$( '+(8:4\nand +1.00 V_P mean +0.50 V and +1.00 V in positive sweep of CV) in region I, II and\nIII respectively.\nMoving on to the EXAFS fit using the three-component model (Ni-O, Ni-S, and Ni-Ni),\nfixing the weights of the Ni-O shell (13% or 22%) and the Ni-S shell (87% or 78%) based\non XANES LCF would result in a statistically significant improvement in the fit. While\nintroducing new variables often leads to an improved fit, we employed the F-test to\ndemonstrate the statistical significance of the improvement. The F-test considers the\ndegrees of freedom in the fit (i.e., the number of independent points minus variables) and\nthe reduced chi-squared of the fit. In our analysis, the three-component model yielded a\nprobability of approximately 90% for the +1.00V sample in Table R2, indicating that it\nsignificantly outperformed the two-component model for the data.\nFurthermore, it should be noted that when the presence of distinct compounds (10% or\nmore) is evident in XANES spectra, this information should be extractable through the\nanalysis of the EXAFS spectra. This is particularly relevant for compounds with markedly\ndifferent bond lengths, such as Ni-O and Ni-S.\nTable R1 (Supplementary Table 4 in the revised Supplementary Information) |\nComposition change for NiS during operando XAS at +0.50 and +1.00 V vs. RHE.\nApplied potential\nNiO(%) Ni S (%) R-factor\n(V vs. RHE) 3 2\n+0.50 13±0.3 87±1.7 0.0006716\n+1.00 22±0.5 78±1.6 0.0004868\nTable R2 (Supplementary Table 3 in the revised Supplementary Information) |\nOperando EXAFS fitting parameters at the Ni-K edge of NiS in different applied voltages\nduring the stage of positive sweep of CV. (S 2 = 0.8)\nPath CN R (Å) 2 (Å2) R-factor\n(V vs. RHE)\nNi-S 3.3±0.6 2.262±0.013 0.005±0.003\nX1.50\n0.003\n(region II) Ni-Ni 4.00* 2.50±0.01 0.007±0.001\nNi-S 2.9±0.8 2.262±0.016 0.005±0.004\nX1.00\n0.005\n(region II) Ni-Ni 4.00* 2.502±0.012 0.007±0.001\nNi-S 2.9±0.6 2.258±0.012 0.006±0.001\nX0.50\n0.002\n(region II) Ni-Ni 4.00* 2.495±0.008 0.007±0.001\nNi-S 3.0±0.6 2.258±0.017 0.006±0.003\n0.00\n0.006\n(region II) Ni-Ni 4.00* 2.50±0.01 0.006±0.001\nNi-S 4 (x=0.87) 2.267±0.015 0.005±0.001\n+0.50\nNi-Ni 4.00* 2.509±0.015 0.010±0.002 0.009\n(region III)\nNi-O 6 (1Xx=0.13) 2.03±0.05 0.005±0.001\nNi-S 4 (x=0.78) 2.286±0.014 0.005±0.002\n+1.00\nNi-Ni 4.00* 2.51±0.002 0.013±0.002 0.011\n(region III)\nNi-O 6(1Xx=0.22) 2.00±0.065 0.005±0.002\nCN: coordination number. R: bond distance. \"2: Debye-Waller factors. R factor:\ngoodness of fit. The asterisks (*) mean that parameter is fixed during the fitting process.\n7B?UPV;G> U)XPVJ?IJ?K?GL LB?O?CABL H@2C&6 I;LB;G>2C&3I;LB%J?KI?=LCN?EQ'\nQ4. Finally, the too-short Ni-O distances reported may be due to (1) the harmonic model\nbeing used in the case of strong disorder or (2) the “inaccurate” choice of the kinetic\nenergy origin (E ) for the photoelectron wavenumber k. Please, check the data analysis.\nthe EXAFS signals and the Morlet transform are depicted on a phase-uncorrected scale.\nConsequently, the shells appear at length values considerably shorter than the accurate\nvalues obtained from the fit, which undergo phase correction. The Ni-O bond distances\nderived from the fit are measured at 2.03±0.05 Å and 2.00±0.07 Å (Table R2). These\nvalues agree very well with typical Ni-O bond distances (Anspoks et al., Solid State\nCommun. 2010, 150, 2270; Singh et al., Solid State Commun. 2017, 259, 40; Zhao et al.,\nMaterials Advances 2021, 2, 4667).\nReviewer #4 (Remarks to the Author):\nThe work describes the experimental finding of a good HER catalysts under alkaline\nconditions, which are in situ reconstructed from NiS. As a theoretician, I mainly checked\nthe theoretical part of the work. However, I am not satisfied with the computation results,\nwhich are unclear on both the model and the kinetics. This part is not scientifically sound.\nResponse: Thanks a lot for Reviewer #4’s comments. According to your suggestion,\nwe have re-constructed a more realistic model and conducted DFT calculations to reveal\nmore insight in our system. Accordingly, we have thoroughly revised our manuscript. We\nbelieve that the revised version could be satisfied.\nQ1. The catalyst structure. There is very limited experimental evidence on the atomic\nstructure of the catalyst. It appears for quite a certain the catalyst is amorphous. The\ntheoretical part utilizes a rather simple layer-by-layer crystalline model, which is\nobviously not consistent with experiment. There is even no thermodynamics analysis on\nthe stability of the catalyst structure from DFT.\nResponse: We appreciate Reviewer #4’s insightful question. According to the\ncharacteristic pattern of XRD, the Ni S is determined to be crystalline. While for NiO,\n3 2\nthe combination of XRD, XPS, XAS and Raman results are hard to determine whether\nthe NiO is crystalline or amorphous. Nevertheless, according to your suggestion, we can\nroughly determine the stability of NiO via a thermodynamics analysis. We have\nperformed ab initio molecular dynamics (AIMD) to calculate the most likely stable\nstructure of NiO. AIMD calculation of supercellof NiO (4 × 4 × 4) under the temperature\nof 3000 K (melting points 2260.15 K) govern by Nose-Hoover thermostat for 50 ps was\nperformed to obtain an amorphous structure (Fig. R4a) (Zhao et al., Phys. Rev. B 2005,\n71, 085107; Lee et al., Phys. Rev. Lett. 2011, 107, 145702). Afterward, the amorphous\nstructure was optimized under system temperature from 1000 K to 3000 K for 50 ps to\nsimulate annealing process. Once the temperature was decreased from 1000 K to 300 K\n(room temperature), it can be found that the amorphous structure would finally convert\ninto face-centered cubic (FCC) structure (Fig. R4b) (Qiao et al., J. Am. Chem. Soc. 2018,\n140, 12256; Branicio et al., Phys. Rev. Mater. 2018, 2, 043401). Therefore, the employed\ncrystalline NiO model would be reasonable. Note that the final NiO model via AIMD and\nDFT simulation underwent a geometrical optimizations process in terms of lattice\nparameters and ions position.\nWe have added the above discussion and please see the Note below Supplementary Fig.\n15 on page 21–22 in the revised Supplementary Information.\nFig. R4 (Supplementary Fig. 15 in the revised Supplementary Information) | (\na)\nAIMD Nose-Hoover under 3000 K for 50 ps to obtain an amorphous structure of NiO.\n(b) AIMD Nose-Hoover from 1000 K to 300 K to simulation annealing with NVT\nensemble for 50 ps. The result indicates the stable state of NiO is crystalline.\nQ2. The water dissociation barrier is at least 1.07 eV, which is too high for reaction to\noccur at the ambient condition.\nResponse: We greatly appreciate Reviewer #4’s constructive comment. In the revised\nmanuscript, we have re-built the hetrostructure of Ni S and NiO with O sites covered by\n3 2\nH (marked as Ni S /NiO) as a new model and re-calcualted the energy barrier. As shown\n3 2\nin Fig. R5, the energy barrier for water dissociation at the interfacial Ni-S sites of\nNi S /NiO is 0.11 eV, which is suitable for the occurrence of reaction at the ambient\n3 2\ncondition. Note that energy barrier of 0.11 eV is much lower than those for water\ndissociation on either individual Ni S (0.42 eV) or individual NiO (0.51 eV), suggesting\n3 2\nthat the facile water dissociation on the mixed phase of Ni S /NiO.\n3 2\nFig. R5 (Fig. 5a in the revised manuscript) | Gibbs free energy diagrams of alkaline\nHER pathway at Ni-Ni sites of NiO, Ni-S sites of Ni S and the interfacial Ni-S sites of\n3 2\nNi S /NiO.\n3 2\nWe have also added the above discussion and please see it on Pages 13–15 in the revised\nmanuscript.\nQ3. The H adsorption energy on three sites are evaluated, which shows that H on O site\ncan be very stable. Therefore, this is a good indication that the current model is not right.\nEven the H coverage has not been explicitly explored. From my point of view, the\ntheoretical part of the work does not add new insights, but create more confusions on the\nexact catalyst structure and on the reaction mechanism.\nResponse: Thanks a lot for Reviewer #4’s critical comment. Acoording to your\nsuggestion, we have employed a heterostructure of Ni S and NiO with O sites covered\n3 2\nby H as a new model (marked as Ni S /NiO) for DFT calculations (Fig. R6).\n3 2\nFig. R6 (Supplementary Fig. 14 in the revised Supplementary Information) | The\noptimized structure models for theoretical calculations. (a) Ni S (marked as Ni S ); (b)\n3 2 3 2\nNiO with the O sites covered by H (marked as NiO); (c) Ni heterostructure slab with the\nO sites covered by H (marked as Ni S /NiO).\n3 2\nThe new Ni S /NiO modle with O sites covered by H can be constructed due to the fact\n3 2\nthat the adsorption of H on surface unsaturated oxygen sites for both Ni S /NiO and NiO\n3 2\nCK KIHGL;G?HMK ;G>LHHKLJHGA%;K J?N?;E?><QLB? =;E=ME;L?>N;EM?K H@[G in Fig. R7.\nH*\nFig. R7 (Supplementary Fig. 16 in the revised Supplementary Information) | The\ncalculated Gibbs free energy diagrams of H* adsorption at O sites of Ni S /NiO and NiO.\n3 2\nThe results indicate the adsorption of H on surface unsaturated oxygen sites for both\nNi S /NiO and NiO is spontaneous and too strong.\n3 2\nBased on this new model, we have also calculated the energy barrier ofwater dissociation\nand the adsorption energy of H for pure Ni S , heterostructured Ni S /NiO and pure NiO\n3 2 3 2\ncatalysts. As shown in Fig. R8, the energy barrier for water dissociation at the interfacial\nNi-S sites of Ni S /NiO is 0.11 eV, which is much lower than those for water dissociation\n3 2\nat single Ni S (0.42 eV) and single NiO (0.51 eV), suggesting that the formed\n3 2\nheterostructure is favorable to the dissociation H O molecule. In addition, the adsorption\n2\nenergy of H* on interfacial S sites for Ni S /NiO CK X('), ?8% OBC=B CK =EHK?J LH LB?\n3 2\nHILCF;E N;EM? \"( ?8# CG =HFI;JCKHG OCLB LBHK? N;EM?K \"X('*) ?8 @HJ 2C S and 0.39 eV\n3 2\nfor NiO. These theoretical calculation results further verified that the formed\nherterostructure can indeed enhance the catalytic activity during HER process, which is\nconsistent with the electrochemical measurements.\nWe have added the above discussion and please see it on Pages 13–15 in the revised\nmanuscript.\nFig. R8. (Fig. 5a in the revised manuscript) | Gibbs free energy diagrams of alkaline\nHER pathway at Ni-Ni sites of NiO, Ni-S sites of Ni S and the interfacial Ni-S sites of\n3 2\nNi S /NiO.\n3 2"}, {"round": 3, "reviewer_comments": {"reviewer_2": "The revised version of the manuscript was improved addressing all questions. Therefore, I can recommend its acceptance for publication.", "reviewer_5": "The manuscript presented in-situ formed Ni3S2/NiO interfaces during HER process in alkaline conditions. The process and relative mechanism of the transiformation of NiS to Ni3S2/NiO were discussed clearly. However, in the present form the manuscript is not suitable for publication in Nature Communications. Some important issues are suggested to be addressed carefully.\n1). Based on the operando XAS spectroscopy results, the formation of NiO occured at positive potential (+0.27 V vs. RHE), and its amount of the NiO was increased with an increased the positive potential. Some issues were caused by the experimental results.\nFirst, at the positive potential, the HER current density was too small, and so the transformation did not occur in the HER potential region.\nSecond, based on the phase transiformation the catalysts could be considered to be pre-catalysts. However, as the experimental results shown (Fig.3a and b), “When the sample is brought back to )'"}, "author_response": "RESPONSE TO REVIEWERS’ COMMENTS\nReviewer #2 (Remarks to the Author):\nThe revised version of the manuscript was improved addressing all questions. Therefore, I can\nrecommend its acceptance for publication.\nResponse: We greatly appreciate Reviewer #2 for the positive feedback and recommendation\nfor publication.\nReviewer#5 (Remarks to the Author):\nThe manuscript presented in-situ formed Ni S /NiO interfaces during HER process in alkaline\n3 2\nconditions. The process and relative mechanism of the transformation of NiS to Ni S /NiO were\n3 2\ndiscussed clearly. However, in the present form the manuscript is not suitable for publication\nin Nature Communications. Some important issues are suggested to be addressed carefully.\nResponse: Thanks a lot for Reviewer #5’s very helpful comments. According to your\nsuggestion, we have further revised manuscript and provided the longer operando experiment\nas well as the characterization of HER-post catalyst. Hopefully, the current version can be\nsatisfied.\nQ1. Based on the operando XAS spectroscopy results, the formation of NiO occured at positive\npotential (+0.27 V vs. RHE), and its amount of the NiO was increased with an increased the\npositive potential. Some issues were caused by the experimental results.\nResponse: We appreciate Reviewer #5’s insightful question. Although the formation of NiO\noccurred at positive potential (+0.27 V vs. RHE) based on the operando XAS spectroscopy\nresults (Fig. 2a), the formed NiO remains present at negative potential where the HER can take\nplace. We also performed long-time operando Raman experiments, and the results provide\nfurther evidence of the existence of NiO at negative potential. We believe the new experimental\ninvestigations and the revised manuscript could clarify the concerning raised by Reviewer #5.\nQ1.1. First, at the positive potential, the HER current density was too small, and so the\ntransformation did not occur in the HER potential region.\nResponse: Thanks a lot for Reviewer #5’s comments. The formation of NiO at positive\npotential may be responsible for the increased catalytic activity of the electrode. We have\nperformed additional operando Raman experiment at different applied potential to confirm the\noccurrence of the transformation in the HER potential region. As depicted in Fig. R1, a notable\npresence of NiO is observed at an operating potential of 0.4 V vs. RHE, consistent with the\nresults of operando XAS presented in Fig. 2a of the main text. Followed by operating at 0.1, 0,\n?$#%\"?$#&+6.?$#'*vs. RHE, respectively, the NiO is reduced, but its content quickly levels\noff at a certain magnitude. Within these applied potential regions, the HER current density\nvaries from negligible level to several hundred milliamperes per square centimeter. This\nobservation suggests that a transformation from NiS to Ni S /NiO would occur within the HER\n3 2\npotential region.\nFig. R1 | The operando Raman spectra were acquired at different operating potential of 0.4,\n$#%\"$#$\"?$#%\"?$#&+6. ?$#'*vs. RHE.\nQ1.2. Second, based on the phase transformation the catalysts could be considered to be pre-\ncatalysts. However, as the experimental results shown (Fig.3a and b), “When the sample is\nbrought back to HER conditions at ?1.00 V, the content of NiO seems to decrease but still exist\n(the inset in Fig. 3a) as the spectrum can be fitted with a linear combination of 94% of Ni S\n3 2\nspectral features and 6% of NiO spectral features after 30 minutes under the reaction”.\nTherefore, when Ni S /NiO interfaces were applied for HER at negative potentials, the formed\n3 2\nNiO would be disappeared especially at operation of HER for a long time.\nResponse: Thanks a lot for Reviewer #5’s insightful comments. The amount of bulk NiO may\ndecrease, but it will not disappear. To confirm the existence of NiO, we have conducted long-\ntime operando)+5+6:8/-;9+5/+:<9/5/6;:#(</;7;2//>-/::3=/1/6/9+;37670,<,,4/:+;?%\nV vs. RHE, which significantly impedes the acquisition of reliable Raman spectra, we opted to\n-744/-; ;2/ )+5+6 :8/-;9+ +; + -76:;+6; +8843/. 87;/6;3+4 70 ?$#' * vs. RHE. under HER\ncondition. The electrodes already show a high HER current density of above 300 mA cm?&\nat this potential. As shown in Fig. R2, during 12 hours, the Raman spectra show obvious NiO\ncharacteristic feature. Even after conducting 25-hour measurement, the obvious NiO\ncharacteristic feature can still be observed, further evidencing that the NiO will not disappear.\nWe added more discussion on pages 11-12 in the revised manuscript and added the Fig. R2 on\npages 20 in the revised Supplementary Information.\nFig. R2 (Supplementary Fig. 13 in the revised Supplementary Information) | Operando\nRaman spectra of NiS electrode in CA measurement for 25 hours at a constant applied potential\n70?$#'*vs. RHE.\nQ1.3.Third, the main findings and discussion in the work were based on the transformation of\nNi S /NiO interfaces, including the formation process and the enhanced HER mechanism of\n3 2\nthe interfaces. However, at a large HER current density, the content of NiO was gradually\ndecreased and possibly disappeared for a long-term operation. Therefore, the enhanced\nmechanism may be different from that proposed in this work when the HER operated for a\nlong-term period (such as the increased content of the S vacancies).\nResponse: Thanks a lot for Reviewer #5’s critical question. As shown in Figure R2, the long-\ntime operando Raman investigations confirm the existence of the NiO and Ni S under HER\n3 2\nconditions. On this point, the NiO may be responsible for the improvement of catalytic\nperformance. The enhancement of HER performances by appearance of NiO has also been\nreported (Gong et al., Nat. Commun. 2014, 5, 4695; Subbaraman et al., Science 2011, 334,\n1256; Huang et al., ACS Energy Lett. 2019, 4, 3002). The theoretical calculations in our study\nfurther elucidate the positive influence of NiO on water dissociation and Ni S on facilitating\n3 2\nH* coupling, thereby addressing the enhanced HER activity at the interfaces of Ni S /NiO.\n3 2\nTherefore, we believe the NiO phase play an important role for the enhanced HER activity.\nQ1.4.Fourth, “Additionally, the catalytic activity of the NiS-40 sample is very stable, showing\nno overpotential increase over 25 hours of continuous operation at a current density of 200\nmA cm-2 in 1 M KOH (Supplementary Fig. 5c).” Based on the comment mentioned above, the\nstructure of the HER-post catalysts is suggested to be characterized.\nResponse: We have conducted more post characterization of the electrode after conducting 25-\nhour chronopotentiometric measurement at a current density of 200 mA cm?&. As shown in Fig.\nR3a and R3b, the XRD patterns of HER-post catalyst confirm that the pristine NiS transforms\ninto Ni S , while the Ni 2p XPS spectra show obvious NiO feature. The Ni K-edge XAS\n3 2 3/2\nspectra further show the NiO characteristic feature located at 8350 eV (Fig. R3c), and it can be\nfitted by using NiO and Ni S reference well (Fig. R3d). The above characterizations strongly\n3 2\nevidence that the HER-post catalysts exist in form of Ni S /NiO. In addition, the spherical\n3 2\naberration corrected transmission electron microscope (SAC-TEM) was also employed to\nconfirm the existence of Ni S /NiO, which will be discussed in the following response.\n3 2\nWe have added the above discussion and please see it on Pages 7 in the revised manuscript and\nadded the Fig R3 on Pages 11 in the revised Supplementary Information.\nFig. R3 (Supplementary Fig. 7 in the revised Supplementary Information) | The\ncharacterization of the HER-post catalyst after conducting 25-hour chronopotentiometric\nmeasurement at a current density of 200 mA cm-2: (a) XRD pattern; (b) Ni 2p XPS spectra;\n3/2\n(c) Ni K-edge XAS spectra; (d) the Ni K-edge XAS of HER-post catalyst consists of a linear\ncombination of spectra of Ni S and NiO reference.\n3 2\nQ2. Compared to the most reported catalyst, the HER activity of the Ni S /NiO interfaces (10\n3 2\nmA cm-2@95 mV) in this work had no obvious advantages. In fact, the overpotentials of most\nreported catalysts in the alkaline conditions were lower than that of Pt/C.\nResponse: Thanks a lot for Reviewer #5’s comment. We agree that the HER activity in our\nwork may have no obvious advantage in comparison with the state-of-the-art catalyst.\nNevertheless, the main contribution of our work lies in employing operando XAS and Raman\nspectroscopy to reveal the dynamic phase transition of NiS, a phenomenon not previously\nidentified in the literature. Furthermore, the combination of NAP-XPS and theoretical\ninvestigations further unveiled that the Ni S /NiO hetero-interface would function as the actual\n3 2\nactive sites to promote water dissociation, and the strong electronic interaction in the interface\nprovide optimized energetics for coupling of H* to form H . Therefore, the most valuable\n2\naspect of our work is the study of the mechanism though which transition metal chalcogenides\nreach high HER catalytic performance. Our work inspired that the chemistry of transition metal\nchalcogenides is highly dynamic, and a careful control of the working conditions may lead to\nthe in-situ formation of catalytic species that boot their catalytic performance. In addition, our\nexperimental methodology, combining in-situ spectroscopies, experimental mechanistic\nstudies and DFT calculations, which may be applied for the study of many other challenging\nmetal chalcogenide systems.\nTo further highlight the importance of our work, we have changed our manuscript title to\n‘Dynamic restructuring of nickel sulfides for electrocatalytic hydrogen evolution reaction’.\nQ3. Based on the HRTEM image (Fig. 1d), the identification of NiO region was lack of\nevidence.\nResponse: Thanks a lot for Reviewer #5’s comments. The spherical aberration corrected\ntransmission electron microscope (SAC-TEM) with higher resolution was used to characterize\nthe microstructure of the NiS-40 sample after conducting 25-hour chronopotentiometric\nmeasurement at a current density of 200 mA cm?&. As shown in Fig. R4, the large-area TEM\nimage reveals numerous small NiO clusters anchored onto Ni S . The HR-TEM image exhibits\n3 2\na distinct interface that partitions the image into two regions. By analyzing the FFT images\n(inset) corresponding to these two regions, the cluster region has an interplanar spacing of 0.21\nnm corresponding well to the (200) lattice plane of NiO, while the other region has an\ninterplanar spacing of 0.30 nm which matches well with (-110) lattice plane of Ni S . More\n3 2\nimportantly, elemental mapping shows the preferential enrichment of oxygen elements on the\nsmall cluster, which strongly reveals the small cluster is assigned to be NiO. The SAC-TEM\nobservation results are consistent with the results by the XRD, XPS, operando XAS and\noperando Raman investigations.\nWe have added the discussion and please see it on Pages 7-8 in the revised manuscript and\nupdated the Fig. 1d.\nFig. R4 (Fig. 1d in the revised manuscript) | The SAC-TEM image, high resolution TEM\nimage (inset are the FFT of the both region) and the corresponding elemental mapping of the\nNiS-40 sample after conducting 25-hour chronopotentiometric measurement at a current\ndensity of 200 mA cm-2.\nReviewer #5 (Remarks to the Author):\nThe revised version of the manuscript was improved addressing all questions.I recommend its\nacceptance for publication."}]

Decision letter