Constructing Amorphous‐Crystalline Interfacial Bifunctional Site Island‐Sea Synergy by Morphology Engineering Boosts Alkaline Seawater Hydrogen Evolution

Abstract The development of efficient and durable non‐precious hydrogen evolution reaction (HER) catalysts for scaling up alkaline water/seawater electrolysis is highly desirable but challenging. Amorphous‐crystalline (A‐C) heterostructures have garnered attention due to their unusual atomic arrangements at hetero‐interfaces, highly exposed active sites, and excellent stability. Here, a heterogeneous synthesis strategy for constructing A‐C non‐homogeneous interfacial centers of electrocatalysts on nanocages is presented. Isolated PdCo clusters on nanoscale islands in conjunction with Co3S4 A‐C, functioning as a bifunctional site “island‐sea” synergy, enable the dynamic confinement design of metal active atoms, resulting in excellent HER catalytic activity and durability. The hierarchical structure of hollow porous nanocages and nanoclusters, along with their large surface area and multi‐dimensional A‐C boundaries and defects, provides the catalyst with abundant active centers. Theoretical calculations demonstrate that the combination of PdCo and Co3S4 regulates the redistribution of interface electrons effectively, promoting the sluggish water‐dissociation kinetics at the cluster Co sites. Additionally, PdCo‐Co3S4 heterostructure nanocages exhibit outstanding HER activity in alkaline seawater and long‐term stability for 100 h, which can be powered by commercial silicon solar cells. This finding significantly advances the development of alkaline seawater electrolysis for large‐scale hydrogen production.


Introduction
The energy crisis and environmental challenges rooted in the widespread use of fossil fuels compel us to explore sustainable DOI: 10.1002/advs.202309927 and clean energy. [1]As an ideal clean fuel and energy carrier, hydrogen has recently sparked a new wave of research due to its exceptionally high energy density and zero carbon emissions. [2]Electrochemical water splitting has emerged as the most promising method for high-purity hydrogen production, given its relatively high energy exchange efficiency and absence of greenhouse gas emissions. [3]As freshwater resources become increasingly scarce, the utilization of seawater or brine electrolytes for hydrogen production is of paramount importance. [4]Currently, platinum (Pt) is extensively studied as the most effective catalyst for hydrogen production. [5]owever, the scarcity and high cost of Pt hinder its feasibility for large-scale commercial applications in the field of hydrogen production. [6]Therefore, the quest for low-cost, highly stable, and active non-platinum catalysts for stable hydrogen production in alkaline seawater remains a prominent focus and challenge.
Transition metal chalcogenides (TMCs) have gained significant attention due to their excellent intrinsic activity and the ability to fine-tune their structures and compositions through nanoscale engineering. [7]Among these TMCs, such as MoS 2 , [8] Co 3 S 4 , [9] and Ni 3 S 2 [7b,10] have been extensively studied as electro-catalysts for various applications, particularly in electrochemical water splitting.Particularly, the construction of shell-in-hollow structures with diverse beneficial constituents and features can harness the internal voids of the hollow architecture, exposing abundant active sites, increasing the catalyst-electrolyte interface contact area, and reducing mass/charge transport distances, which offers a promising avenue for accelerating the kinetics of hydrogen evolution reaction (HER). [11]Beyond morphological engineering, atomic-level heterostructure engineering offers an alternative avenue.When two distinct components come into contact to form a heterostructure, spontaneous atomic configuration, and electronic structure reorganization occur in the vicinity of the heterointerface. [12]Through the construction of coupled interfaces and the synergistic effects of heterostructures, electron transfer, active site modulation, and catalytic activity can be effectively tuned. [13]7a] Therefore, there is an urgent need to enhance the HER performance of TMCs through comprehensive control of their structural morphology and heterostructure engineering.Rational design of heterostructures, active sites, optimization of energy adsorption, and acceleration of H 2 O dissociation kinetics are essential for achieving large-scale electrolysis.
Interface engineering is widely employed to enhance catalytic activity by effectively regulating the electronic structure and surface properties of metal chalcogenides. [14]Among them, the amorphous-crystalline (A-C) heterostructure is recognized as a promising electrocatalyst. [15]These materials harness the combined advantages of amorphous and crystalline structures and exhibit unconventional atomic arrangements at heterostructure interfaces. [16]In comparison to crystalline materials, amorphous materials exhibit superior electrocatalytic HER performance due to their abundant active sites, defects, and unsaturated electron configurations, which attributes arise from the disordered atomic structure inherent to amorphous materials. [17]Jin et al. [15b] achieved excellent electrocatalytic performance through phase structure engineering in the optimized Ni-TPA@NiSe/NF heterostructure.Furthermore, the geometric mismatch in the heterointerface region may induce localized lattice strain, consequently altering defect formation energies and migration barriers, leading to the generation of a substantial number of lattice defects.Yang et al. [18] emphasized the coupling of oxygen vacancies with interface engineering, introducing a novel amorphous/crystalline CrO x -Ni 3 N heterostructure, where high-energy lattice defects were confirmed to be favorable active centers for electrochemical reactions.17a] Therefore, it is of paramount importance to establish a simple and versatile method for the fabrication of electrocatalysts with high-density A-C interfaces.
Herein, we present a nanoscale island confinement strategy through in situ ion-exchange-induced amorphous-crystalline heterostructure remodeling, to construct atomic-layer seawater splitting catalysts with high activity and stability.The intricately designed 3D PdCo-Co 3 S 4 nanocages boast a well-defined hierarchical nanocages-nanoclusters structure, a high-stability A-C heterointerface, and accelerated charge/mass transfer capability, exhibiting significantly enhanced HER activity.Benefiting from the abundant morphological structure and numerous A-C heterointerfaces, the PdCo-Co 3 S 4 heterostructure nanocages show a low overpotential of 98 mV to drive −10 mA cm −2 and long-term stability, demonstrating superior performance compared to traditional cobalt sulfide-based HER catalysts.Moreover, we have constructed an alkaline seawater electrolyzer system using solar photovoltaic panels, demonstrating excellent HER activity and long-term stability over 100 h.This work not only paves the way for the synthesis of functional catalysts rich in crystalline and amorphous interfaces, but also offers novel insights into the design of efficient and stable catalysts for alkaline seawater electrolysis.

Synthesis and Structural Characterization
9a] The cation exchange reaction between Co 2+ and Pd 2+ was achieved through a solvothermal reaction.Due to differences in their solubility product constants (k sp ), a natural non-equivalent cation exchange reaction occurred between Co 2+ and Pd 2+ .However, this led to changes in the coordination modes due to chemical valence mismatch, resulting in a porous morphology enriched with defects.Due to the reducing property of L-ascorbic acid, a small amount of free Pd/Co ions will be reduced to monometallic, thus obtaining a small amount of PdCo clusters.The presence of numerous defects and surface PdCo clusters enhanced the efficiency of electron transfer from the catalyst to protons, thereby achieving efficient HER.
In particular, the images captured by field emission scanning electron microscopy (FE-SEM) (Figure 1b) and highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 1c) reveal the distinctive hollow nano-cubic structure of PdCo-Co 3 S 4 .Within the STEM image (Figure 1d), a nanoscale island-like arrangement of PdCo clusters in conjunction with Co 3 S 4 nanocages is evident, showcasing the "moving but not aggregating" design of metal active sites (Figure 2g).The strategy of dual-functional site synergy in the " island-sea synergy" architecture.The strategy involves loading PdCo nanoclusters onto Co 3 S 4 nano-islands and dispersing PdCo nanoclusters, enhancing the beneficial catalytic effect of dispersed PdCo atoms.PdCo atoms remain dispersed in a reducing environment, that is, "mobile but not aggregated", Abundant A-C heterointerfaces and the synergistic effect of nanoclusters significantly improve HER activity.Consequently, this unique multi-tiered architecture facilitates rapid electron and ion transfer.Furthermore, notable atomic defects and lattice distortions are discernible, and the lattice distortion serves to lower energy barriers and augment active edge sites, thus enhancing the electrochemical performance.Moreover, through HAADF-STEM observations (Figure 2e), it was possible to further confirm lattice fringes with spacings of 0.33 and 0.21 nm, which could be attributed to the ( 220) and ( 311) crystal planes of Co 3 S 4 (JCPDS No.73-1703), respectively.Furthermore, within the regions demarcated by white dashed lines, an abundance of interfaces was evident, encompassing amorphous, low-crystalline, and well-crystallized domains.It was also observed that Pd and Co clusters coexisted at heterojunctions, which highly discrete clusters generated a wealth of grain boundaries and heterointerfaces, which, in turn, induced robust interfacial electron transfer properties, facilitated the immersion of electrolyte ions, and provided additional A-C interfacial active sites for the HER.Results from HAADF-STEM imaging combined with energy-dispersive X-ray spectroscopy (STEM-EDX) revealed uniform distribution of Co, Pd, and S elements within the PdCo-Co 3 S 4 nanocages (Figure 1f), and the measured atomic distribution is 51.13%, 3.58%, and 45.29%, respectively (Figure S1, Supporting Information).
In the X-ray diffraction (XRD) of the PdCo-Co 3 S 4 nanocages, all diffraction peaks were attributed to Co 3 S 4 (JCPDS No.73-1703), Pd (JCPDS No. 72-0710), and Co (JCPDS No. 70-2633), indicating the formation of heterogeneous PdCo-Co 3 S 4 composite material (Figure 2a).Furthermore, the increase in the concentration of 2-methylimidazole could potentially alter the coordination environment of the metal centers and provide steric hindrance during the pyrolysis process, leading to potential aggregation, which might result in the growth of Co 3 S 4 and Pd/Co along different directions and an increase in interlayer spacing, consistent with the analysis of different crystal planes observed in STEM.From the results of BET and BJH analyses, it can be observed that PdCo-Co 3 S 4 nanocages exhibit a commendable surface area and a relatively high pore volume, indicating their mesoporous nature (Figure 2b).As is well-known, a higher surface area and mesoporous structure provide an abundance of metal active sites, facilitating electron transfer between the electrolyte and the catalyst, thereby promoting the absorption and dissociation of water molecules. [19]In addition, Raman spectroscopy reveals several peaks at ≈193, 472, 515, and 678 cm −1 , corresponding to the F 2g , E g , F 2g , and A 1g modes of Co 3 S 4 , respectively (Figure 2c; Figure  Utilizing X-ray absorption spectroscopy (XAS) provided essential insights into the electronic and local structural features of the samples.The Co K-edge X-ray absorption near-edge structure (XANES), as depicted in Figure 3a, showcased a higher energy shift for the Co K-edge in PdCo-Co 3 S 4 compared to CoO, yet lower than that of Co 3 O 4 .This indicates that upon the formation of the PdCo-Co 3 S 4 heterostructure, the average valence state of Co ranged between +2 and +3.PdCo-Co 3 S 4 exhibited a Co─S coordination structure akin to Co 3 S 4 , albeit with a slightly reduced white-line intensity, suggesting both the presence of Co 3 S 4 and the interaction between Co 3 S 4 and PdCo.In addition, the Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) of PdCo-Co 3 S 4 in Figure 3b resembles Co 3 S 4 , yet significantly differs from Co foil, Co 3 O 4 , and CoO.Therefore, the peak at 1.62 Å can be attributed to the Co─S bond. [20]Concurrently, employing Co foil as a reference, EXAFS fitting of PdCo-Co 3 S 4 within the 1.0−3.0Å range revealed a coordination number of 3.7 for the Co─S bond, while in Co foil, the coordination number for the Co─Co bond is 12 (Figure 3c; Figure S3 and Table S1, Supporting Information).Furthermore, wavelet transforms were applied to the EXAFS spectra, as they afford a clear elu-cidation of the changes in coordination environments at radial distances and in K-space resolution. [21]Figures 3d-f

Electrocatalytic HER Performance and Insight into the Catalytic Activity
In order to elucidate the excellent electrochemical HER properties arising from the phase engineering, we conducted electrocatalytic performance tests on the prepared materials in a 1.0 m KOH electrolyte.Figure 4a illustrates the polarization (LSV) curves of the samples, where PdCo-Co 3 S 4 exhibits a remarkable HER performance, with its current density rapidly increasing with increasing potential.PdCo-Co 3 S 4 achieves an overpotential of only 96 mV to reach 10 mA cm −2 (Figure 4c), significantly lower than that of Co 3 S 4 (206 mV) and carbon paper (275 mV).Importantly, PdCo-Co 3 S 4 requires only 306 mV to achieve 217 mA cm −2 , surpassing 20% Pt/C, thus demonstrating exceptional electrocatalytic performance.Tafel curves for all electrodes were produced by fitting the LSV curves to further examine the HER kinetics for each electrode (Figure 4b).Among these, PdCo-Co 3 S 4 exhibits the smallest Tafel value (22.23 mV dec −1 ), indicating superior kinetics and faster electron transfer capability compared to Co 3 S 4 (32.56 mV dec −1 ) and carbon paper (48.99 mV dec −1 ).Furthermore, the Tafel slopes of PdCo-Co 3 S 4 are close to 30 mV dec −1 , indicating that the kinetics of the HER process are primarily controlled by the hydrogen evolution step (M + H 2 O + e − → MH ads +OH − , where M represents the catalyst, i.e., the Volmer-Tafel mechanism). [22]Furthermore, the multiphase cluster-crystalline-amorphous defect interface provides additional electrochemical active regions and catalytic active sites, thereby enhancing the kinetics of HER. [23]he charge transfer resistance (R ct ) of PdCo-Co 3 S 4 determined by electrochemical impedance spectroscopy (EIS) (Figure 4d) is significantly lower than that of Co 3 S 4 and bare carbon paper, indicating that the former accelerates charge transfer and exhibits outstanding reaction kinetics.This is advantageous for the binding of electrons and adsorbed hydrogen (H ads ), further enhancing the performance of the HER. [24]Therefore, refining the interface with fully exposed clusters to achieve perfect electronic interactions contributes to improving the intrinsic conductivity and catalytic performance of the material.
To further investigate the intrinsic performance of the prepared PdCo-Co 3 S 4 nanocages, the electrochemical surface area (ECSA) was calculated according to the double-layer capacitance (C dl ).The value of C dl is further fitted by measuring CV curves (Figure S5, Supporting Information) at different scan rates.The measurement of the ECSA, which is directly proportional to the double-layer capacitance (C dl ), was undertaken to ascertain the intrinsic performance of the catalyst (Figure 4e).Notably, PdCo-Co 3 S 4 exhibited the highest ECSA value at 44.91 mF cm −2 .This substantial increase in ECSA, attributed to its significantly larger specific surface area, surpassing that of Co 3 S 4 by more than four-fold, can be attributed to the open and hollow nanocages structure of PdCo-Co 3 S 4 .This unique structural feature contributes to an extended electrocatalyst-electrolyte interface, thereby enhancing the exposure of active sites and promoting heightened electrocatalytic activity. [25]urthermore, the mass activity of PdCo-Co 3 S 4 exhibited remarkable enhancements, surpassing that of Co 3 S 4 by factors of 10.4 and 6.04 at overpotentials of 300 and 500 mV (Figures 4f).This underscores the beneficial influence of the nanocages architecture in augmenting the intrinsic activity of each accessible active site.Impressively, when the specific activity of the samples was normalized by their ECSA to eliminate the influence of active site quantity, PdCo-Co 3 S 4 exhibited substantially superior normalized HER activity compared to other controls (Figure 4g), which demonstrates that the incorporation of Pd-Co heterogeneous clusters and the formation of defects resulting in surface restructuring can profoundly enhance intrinsic activity.
Notably, the outstanding HER performance promoted by Pd-Co heterogeneous clusters into Co 3 S 4 can compete with or even outperform most of the previously reported HER catalysts in 1.0 m KOH alkaline solutions (Figure 4h; Table S2, Supporting Information).Meanwhile, the stability of PdCo-Co 3 S 4 can be maintained by multi-step chronopotentiometry tests at variable current density (Figure 4i).The constructed electrolyzer demonstrated outstanding stability in long-term operation, even at 100 mA cm −2 , the catalytic performance declined only slightly after 20 h of electrolysis (Figures S6, Supporting Information).The SEM, and XRD analyses following stability testing revealed the well-preserved nanocage morphology and structure of PdCo-Co 3 S 4 , demonstrating excellent stability (Figures S7 and S8, Supporting Information).Furthermore, the nanocluster island effect and heterogeneous A-C interface contribute to stabilizing the surface-active species, ensuring efficient HER.The intact A-C heterostructure observed via STEM further underscores the exceptional structural stability of PdCo-Co 3 S 4 (Figure S9, Supporting Information).These findings confirm the outstanding intrinsic catalytic activity and corrosion resistance of the PdCo-Co 3 S 4 electrode for HER in alkaline solutions.

Solar-Driven Alkaline Seawater Hydrogen Evolution Systems
Inspired by the outstanding electrocatalytic performance of the PdCo-Co 3 S 4 electrode in alkaline water, further investigations were conducted to assess its electrocatalytic HER performance in simulated alkaline seawater and natural alkaline seawater electrolytes.Natural seawater (pH 7.8) was collected from the East China Sea (Figure S10, Supporting Information).The HER catalytic performance of the catalyst in both natural seawater and simulated alkaline seawater is depicted in Figure 5a-c.As anticipated, PdCo-Co 3 S 4 exhibited excellent catalytic performance in the complex alkaline seawater electrolyte (Figure 5d).It achieved a low overpotential of 163 mV for hydrogen evolution at 10 mA cm −2 , which is much lower than that of Co 3 S 4 (295 mV), and 20% Pt/C (173 mV) (Figure 5f).This indicates the practical potential of PdCo-Co 3 S 4 for electrolysis based on natural seawater hydrogen production.Furthermore, in the 1.0 m KOH + seawater electrolyte, the electrocatalyst displays some activity degradation due to the burial of active sites and electrode poisoning by pollutants or small insoluble precipitates in natural seawater.
5b] These ions tend to generate precipitates on the electrode surface during the HER process, thereby covering active sites and diminishing the catalyst's performance. [4]Impressively, this configuration remained highly stable (Figure S11, Supporting Information), with no significant activity degradation, even after 100 h of operation at 10 mA cm −2 .Consequently, it exhibits exceptional corrosion resistance in alkaline seawater electrolysis, significantly surpassing commercial Pt/C catalysts (Figure 5h).The slight positive shift can be attributed to surface blockage caused by the formation of irreversible intermediates and bubble attachment, leading to a reduction in the effective contact area between the electrode and the electrolyte. [26]In conclusion, the induction of surface amorphization in electrocatalysts represents a promising strategy for enhancing the exposure of catalytically active sites, elevating intrinsic catalytic activity, and improving electrochemical stability.
Given its exceptional electrocatalytic performance, we constructed a solar-driven seawater electrolysis system (Figure 5g) powered by solar cells to showcase the superior practical utility of PdCo-Co 3 S 4 nanocages under real solar irradiation conditions.This can be attributed to the nanoscale island-like structure of PdCo clusters and the A-C heterostructure of Co 3 S 4 nanocages, which embodies the "moving but not aggregating" design of metal active sites (Figure 5e).These findings indicate that PdCo-Co 3 S 4 nanocages demonstrate outstanding seawater splitting performance, rendering them a highly promising candidate for use in solar energy storage and the production of solar to hydrogen.

Density Functional Theory (DFT) Calculations
The first-principles DFT calculations were carried out to elucidate the origin of the remarkable HER activity of PdCo-Co 3 S 4 heterostructure on the theoretical level.From the obtained Tafel slope, it can be inferred that the adsorption/desorption of water and the adsorption mechanism of hydrogen on the catalyst follow the Volmer-Tafel pathway.Consequently, the free energy was computed according to this reaction pathway (Figure 6a).The adsorption of reaction intermediates on the surfaces of PdCo-Co 3 S 4 and Co 3 S 4 is illustrated in Figures S12 and S13 (Supporting Information).The intrinsic catalytic activity of the catalyst for the HER is typically assessed in terms of the hydrogen adsorptionfree energy (∆G H* ). [27]An ideal catalyst should exhibit an optimal ∆G H* value close to 0 eV, indicating a suitable strength of H adsorption/desorption during the catalytic process. [28]In this study, PdCo-Co 3 S 4 and Co 3 S 4 (220) were employed as models (Figure S14, Supporting Information) to investigate potential adsorption sites for the H* intermediate with various active atoms.The ΔG H* value for Co sites on the clusters was found to be 0.18 eV, which is closer to 0 eV compared to other sites (Figure 6b).Furthermore, the ΔG H* value for Pd sites on PdCo-Co 3 S 4 was −0.33 eV, closer to 0 eV than the ΔG H* value for Co The calculation of water molecule adsorption energy (ΔE H2O ) on the catalyst surface is highly necessary as it directly impacts the rate of catalytic reactions. [29]Generally, the alkaline HER has a two-step process: the Volmer step including the H 2 O adsorption and H 2 O dissociation along with the cleavage of O─H bonds to form H atoms, and the Heyrovsky step or the Tafel step corresponding to the H 2 generation. [30]Thus, we further verify the origin of the activity on PdCo- The key reaction steps of the alkaline HER and the optimal configuration of relevant species on Co 3 S 4 (220) and PdCo-Co 3 S 4 are shown in Figure 6c as well.As discussed in previous reports, [31] the sluggish kinetics of the HER in alkaline solution can be attributed to the initial adsorption and dissociation of water molecules on the catalyst surface.As shown in Figure 6c, the Gibbs free energy change associated with the formation of H─OH intermediates in the water dissociation step (ΔG H─OH ) was found to be energetically favorable for both Co 3 sites and Co sites, suggesting a rate-limiting role, and hence it was applied as an activity descriptor.The ΔG H─OH value of Co 3 S 4 (Co 3 sites) was as high as 0.38 eV, and reduced to −0.02 eV for PdCo-Co 3 S 4 (Co  6e,f).These changes in the d-band centers resulted in reduced adsorption strength for hydrogen intermediates, thus expediting the desorption process during the HER. [32]In summary, the incor-poration of PdCo clusters serves not only as high-activity sites but also as modulators of Co 3 S 4 electronic structure, enhancing electrical conductivity and charge transfer kinetics, lowering reaction barriers, and promoting HER.Furthermore, the analysis of the Electron Localization Function (ELF) provides additional support for the stability of PdCo-Co 3 S 4 , offering insights into the material bonding characteristics. [33]The ELF value between Co and Pd atoms is ≈0.05, indicating a metallic bond characteristic in Co─Pd interactions (Figure 6g).The robust interaction between the PdCo nanoclusters and the Co 3 S 4 surface significantly impedes the migration and aggregation of active species, resulting in improved dispersion and uniformity of PdCo nanoclusters throughout the synthesis process.

Conclusion
In summary, we have employed an ion-exchange strategy to fabricate a novel PdCo cluster-Co 3 S 4 heterogeneous nanocage with distinct amorphous-crystalline phase boundaries for efficient and stable electrocatalytic seawater splitting to produce hydrogen.The optimized PdCo-Co 3 S 4 exhibits an overpotential of 96 mV at a current density of 10 mA cm −2 and a Tafel slope of 22.23 mV dec −1 in 1.0 m KOH, demonstrating excellent catalytic activity and long-term stability in naturally occurring alkaline seawater solutions.These results are achieved through the rich electrocatalytic active centers provided by the A-C heterogeneous interface, enhanced surface permeability, high electron conductivity of PdCo clusters, and the porous 3D structure of the nanocage.The amorphous portion of the material offers a wealth of active sites, including defects and unsaturated coordination sites, while the crystalline segment, distinguished by its high electron conductivity, ensures rapid charge transfer.The distinctive heterointerface formed within the A-C heterostructure effectively mitigates the weaknesses associated with a singular amorphous or crystalline heterostructure, markedly shortening both ion and electron diffusion pathways, thereby enhancing the kinetics of the HER.Theoretical calculations indicate that the constructed A-C heterostructures can significantly reduce the Gibbs free energy, providing a diversified range of active sites, and thereby enhancing HER activity.This study offers a potential solution to the challenges in electrochemical water splitting for HER, contributing to the United Nations Sustainable Development Goal 7: Affordable and Clean Energy.

Experimental Section
Synthesis of the PdCo-Co 3 S 4 Heterostructure Nanocages: To synthesize the hollow nanocages in PdCo-Co 3 S 4 , 0.15 g of Co 3 S 4 nanoboxes was initially dispersed in 50 mL of EtOH, and ultrasonicated for 5 min to yield a homogeneous slurry.Subsequently, 0.1 mm of sodium tetrachloropalladate (Na 2 PdCl 4 ) was introduced into the solution, followed by stirring at 60 °C for 180 min.The reaction mixture was then allowed to cool to room temperature (25 °C), and the resulting precipitate was harvested after several cycles of ethanol washing and centrifugation.Finally, L-ascorbic acid solution (0.2 m) was added and stirred for 30 min.The collected material was subsequently dried at 60 °C.
Electrochemical Measurements: All electrochemical measurements were conducted within a three-electrode cell using an electrochemical workstation CHI 760E.The working electrode consisted of carbon paper (CP) cut to 0.5 × 0.5 cm 2 loaded with catalyst, in which electrocatalyst powder ink was prepared using a mixture of 0.70 mL deionized water, 0.25 mL ethanol, 0.05 mL Nafion solution, and 10 mg catalyst, and then sonicated for 30 min.The ink was then uniformly applied on a CP with a catalyst loading of 1 mg cm −2 , the CP was used as the working electrode, the Pt net as the counter electrode, Hg/HgO as the reference electrode, and 1.0 m saturated KOH aqueous solution was used.All measured potentials were referred to the reversible hydrogen electrode (RHE) using the following equation: E(RHE) = E(Hg/HgO) + 0.059 × pH + 0.098 V, and the current densities (j) were normalized by geometric surface area.The frequency setting range of the EIS test is from 100 kHz to 0.01 Hz.C dl was estimated from the CV method at various scan rates (20, 40, 60, 80, 100, 120 mV s −1 ) in the non-Faraday zone, and C dl was given in the following equation: Δj = j anodic -j cathodic = 2 × v × C dl .All the potential was recorded without iR-correction.

Figure 1 .
Figure 1.Structural characterization of the electrocatalysts.a) Schematic of the synthesis procedure of PdCo-Co 3 S 4 .b) SEM and c) STEM images of PdCo-Co 3 S 4 .d,e) HAADF-STEM image of the amorphous-crystalline heterostructures PdCo clusters-Co 3 S 4 nanocages.f) HAADF image and energydispersive spectroscopy (EDS) element mappings of Pd, Co, and S for the amorphous PdCo-Co 3 S 4 sample.

Figure 2 .
Figure 2. Spectroscopy characterization and chemical states of PdCo-Co3S4.a) XRD patterns of PdCo-Co 3 S 4 nanocages along with the standard PDF cards for Co 3 S 4 .b) Nitrogen adsorption-desorption isotherms and (inset) pore-size distribution and c) Raman spectra of the PdCo-Co 3 S 4 .Highresolution XPS spectra of d) S 2p, e) Co 2p, and f) Pd 3d for the as-prepared catalysts.g) Structural schematic diagram.

Figure 3 .
Figure 3.Chemical state and coordination structure analysis.a) Normalized XANES spectra and b) Fourier transformed EXAFS spectra at Co K-edge of the Co foil, CoO, Co 3 O 4 , Co 3 S 4 , and PdCo-Co 3 S 4 .c) Co K-edge EXAFS fitting curve on PdCo-Co 3 S 4 in R-space.WT-EXAFS of d) PdCo-Co 3 S 4 , e) Co 3 S 4 , and f) Co foil at the Co K-edge.
present the wavelet transform (WT) profiles of the first shell with optimal resolution based on Morlet wavelets for Co foil, Co 3 S 4 , and PdCo-Co 3 S 4 .In the WT contour plot of PdCo-Co 3 S 4 , the peak intensity at 5.60 Å corresponds to the Co─S contribution, akin to that of Co 3 S 4 , yet notably distinct from other contributions (Figure S4, Supporting Information).

Figure 4 .
Figure 4. Evaluation of Electrocatalytic HER activity.a) Polarization (LSV) curves and b) corresponding Tafel plots of PdCo-Co 3 S 4 , Co 3 S 4 , and Pt/C catalysts in 1 m KOH electrolyte.c) Overpotentials of various electrodes at the corresponding current densities; d) EIS Nyquist plots.e) C dl values for PdCo-Co 3 S 4 , and Co 3 S 4 .f) Mass activity for PdCo-Co 3 S 4 , Pt/C, and Co 3 S 4 at overpotentials of 300 and 500 mV.g) ECSA-normalized LSV curves of PdCo-Co 3 S 4 , and Co 3 S 4 .h) Comparison of the HER activities of various Co-based non-noble metal catalysts in 1.0 m KOH alkaline solutions.i) Multi-step chronopotentiometry tests at variable current density.

Figure 5 .
Figure 5. Natural/Simulated alkaline seawater hydrogen evolution.Polarization Curves of a) PdCo-Co 3 S 4 , b) Co 3 S 4 , and c) 20% Pt/C in varied electrolytes during the HER.d) Overpotential requirements for PdCo-Co 3 S 4 at 10, 50, and 100 mA cm −2 in different electrolytes.e) Schematic of the HER steps in basic medium.f) Overpotential requirements for various catalysts at 10 mA cm −2 in different electrolytes.g) Solar-driven water electrolyzer.h) Extended Durability Assessment of a Self-Sustaining Seawater Electrolysis System.
Co 3 S 4 by evaluating the free energies of the H 2 O adsorption, H 2 O dissociation (ΔG OH−H ), and H adsorption (ΔG H ) on the surfaces of the Co 3 S 4 (Co 3 sites), PdCo-Co 3 S 4 (Co 3 sites), PdCo-Co 3 S 4 (Pd sites), and PdCo-Co 3 S 4 (Co sites).In Figure 6d, the Co 3 S 4 (Co 3 sites) has the strongest ΔE H2O value (−0.75 eV), implying that the abundant surface area and mesoporous structure are favorable to the strong adsorption capacity for H 2 O on Co 3 S 4 nanocage surface.The PdCo-Co 3 S 4 (Co sites), containing the distinctive hollow nano-cubic structure of Co 3 S 4 as a promoter for H 2 O adsorption, shows integrated adsorption energy (−0.56 eV), which is superior to the PdCo-Co 3 S 4 (Pd sites) (−0.33 eV) and PdCo-Co 3 S 4 (Co 3 sites) (−0.51 eV).

Figure 6 .
Figure 6.Theoretical investigations.a) Process of hydrogen production on PdCo-Co 3 S 4 .b) Calculated free energy diagram of H adsorption for PdCo-Co 3 S 4 .c) Calculated free energy of HER intermediates at zero potential.d) Calculated H 2 O adsorption energy.e,f) Electronic density of states (DOS), and d band center.g) Electron localization function (ELF) maps.