Electrochemical oxygen evolution reaction (OER) is a key half-reactions in various energy conversion and storage systems, such as rechargeable metal-air batteries, water splitting technique, and nitrogen/carbon dioxide reduction devices [1–6]. To overcome the sluggish four-proton-coupled reaction kinetics and highly oxidative environment of OER, considerable attention is focused on the development of efficient electrocatalysts with expectable activity and stability [7–13]. Up to now, Ir and Ru-based catalysts are the benchmark catalysts for OER, but their scarcity and high cost inevitably limit the large-scale applications [14–17]. Great efforts have been devoted into seeking for low cost and eco-friendly non-precious metal electrocatalysts [18]. Alternatively, pyrite-type nickel sulfides (NiS₂) have emerged as one kind of the promising candidates for alkaline OER due to their high chemical stability and unique structural configuration [19]. The sulfur atoms with high electronegativity can not only modulate the electronic structures of Ni, but also act as proton attachment/detachment sites [20]. Other side, the sulfur atoms in NiS₂ can accelerate the in situ electro-chemical transformation of Ni species into corresponding (oxy) hydroxide active phase, which outperform the pure (oxy) hydroxides in terms of catalytic activity [21–23].

Great efforts have been devoted into further enhancing the OER activity of NiS₂ catalysts, wherein the shape control has been regarded as an effective way due to the follwing reasons: (1) Through designing the geometrical structure of NiS₂, the atom rearrangements may shorten the wideth of forbidden band, making it easier for electrons to jump from valence band to conduction band, thereby enhancing the conductivity of NiS₂ and guaranteeing the electron transport in electro-catalytic process [24]; (2) The remaining cavities in the valence band after electron transition could tune the adsorption strength towards key intermediates, thereby optimizing the reaction pathway [25]; (3) Through tuning the nanostructured morphology, the density of exposed active sites on the surface of NiS₂ will be increased, and the contact area will be extended for electrolyte access [26–28].

Take above considerations, hollow nanocage structures have attracted comparative attentions due to their large surface-area-to-volume ratios and high-density low-coordinated atoms at the edges, which can enable more active site exposure and faster mass/electron transfer pathway than the solid counterparts [29,30]. On this basis, further producing lattice defects (i.e., vacancy defects, dislocations, and grain boundaries) on hollow nanocage structures could provide more unsaturated coordination sites, and optimize the electronic structure to promote catalytic activity [31,32]. However, previously reported methods to synthesize hollow NiS₂ nanocages generally involve high-temperature solid-state methods, which are not only energy-intensive but also easy to form bulk NiS₂ cages with thick and rigid walls (normally with thickness of 16.2–46.5 nm) [33–35]. The too thick walls are not only unfavorable for the actives sites exposure and electrolyte penetration, but also hard to produce lattice defects. Unfortunately, in comparison with the shape control of noble-metal catalyst, the formation of NiS₂ is much more thermodynamical unfavored, together with hardly controllable nucleation and growth kinetics. Still now, the exploration of facial and effective method to fabricate hollow NiS₂ nanocages with ultrathin and rich-defects features still remains a grand challenge.

Herein, we developed an injection rate-mediated method to fabricate ultrathin NiS₂ HNCs with hierarchical walls, high-density lattice defects and abundant grain boundaries (GBs). Then, we systematically understood the underlying formation mechanism for the formation of NiS₂ HNCs, wherein the injection rate determined the concentration of S²⁻ in the steady state and thus controlled the growth pattern. At a slow injection rate, the gradually increased number of S²⁻ in the solution was slow enough to initiate the layer-by-layer stripping of NiCo PBA, but fast enough to sustain the cubic structure, thereby forming the ultrathin NiS₂ HNCs. In contrast, one-shot injection led to the high concentration of S²⁻ at the start of the reaction, thereby forming the NiCo PBA@NiS₂ frames at fast kinetic rate. Benefiting from the ultrathin and hierarchical walls that minimize the mass transport restrictions, the high-density lattice defects and GBs that offer abundant unsaturated reaction sites, the as-prepared NiS₂ HNCs can realize efficient electro-catalysis for OER in terms of the high specific activity, favorable kinetics, superior durability, and rapid charge transfer.

Fig. 1 schematically illustrates two distinct pathways that generate ultrathin NiS₂ HNCs and NiCo PBA@NiS₂ nanoframes when the Na₂S precursor was added dropwise and in one shot, respectively. First, face-centered cubic (fcc)-structured NiCo PBA nanocubes (Ni₃[Co(CN)₆]₂), with transition-metal ions bridged by cyano ligands, were synthesized by a hydrothermal method. Transmission electron microscopy (TEM) image, scanning electron microscopy (SEM) image, and X-ray powder diffraction (XRD) pattern confirm the uniformly cubic shape and high purity phase of Ni₃[Co(CN)₆]₂ with average diameter of 93 nm (Fig. S1 in Supporting information). Then, a facile S²⁻ etching method was used to yield ultrathin NiS₂ HNCs. As the equipment photographs (Fig. S2 in Supporting information), a syringe pump was used to precisely control the injection rate of Na₂S at 4.0 mL/h. At such a slow dropwise rate, the low concentration of S²⁻ in the solution led to the layer-by-layer stripping of NiCo PBA nanocubes, thereby forming the ultrathin NiS₂ nanocages with hierarchically flexible walls. Other side, when the Na₂S was added by one shot, the high concentration of S²⁻ in the solution led to the preferentially etching at the edges/corners of the nanocubes with relatively high surface energy, forming the outer NiS₂ nanoframes. Meanwhile, along with the consuming of S²⁻, the remaining concentration of S²⁻ is too low to etch the total NiCo PBA, leaving the internally residual PBA with eight-corners-cuted cross-like structure, thereby forming the NiCo PBA@NiS₂ nanoframes.

The morphology of the ultrathin NiS₂ HNCs was first analyzed by SEM images (Fig. S3 in Supporting information). The hollow architecture with hierarchical and flexible walls can be discerned unambiguously, signifying that the solid NiCo PBAs nanocubes were all transformed to the hollow-structured NiS₂ nanocages through S²⁻ etching. Large-scale high-resolution TEM (HRTEM) image further unravels the hollow nanocage shape with diameter of 96.2 nm (Fig. 2a). The shell thicknesses measured from the height profile of the randomly selected fragments exhibits an averaging value of ca. 3.2 nm, demonstrating the ultrathin nature of NiS₂ HNCs (Fig. 2b and Fig. S4 in Supporting information). The HRTEM images reveal the clear lattice fringes with spacing of 0.252 nm, which is in accordance with the lattice spacing of standar NiS₂ (200) (Fig. 2c). Meanwhile, as marked by orange dashes, clear drapes could be observed, suggesting that the shell of NiS₂ nanocages is consisted of numerous flexible two-dimensional (2D) nanosheet building blocks. These flexible connections are favourable for enhancing the electron and mass transfer in electro-catalysis [36,37]. Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was then employed to examine the atomic structure of the ultrathin NiS₂ HNCs (Figs. 2d-f and Fig. S5 in Supporting information). Enlarged from the edge of the hierarchical and flexible shell, the NiS₂ nanocrystals exhibit the disordered and anisotropic lattice orientations. A mass of lattice defects, such as atomic kinks and steps, can be observed from the discontinuous fringes regions, as marked by yellow dash box (Fig. 2e). These may be induced by the S²⁻ etching at a very slow kinetics that disrupts the ordered arrangement of Ni and Co atoms in PBA. The further enlarged HAADF-STEM image confirms the existance of rich grain boundaries in NiS₂ HNCs (Fig. 2f). Clear grain boundaries with different atom orientations were catched between two adjacent pieces of NiS₂, indicating that the NiS₂ HNCs are constructed by a crowd of short-range grain boundaries with disordered atom arrangements. The energy-dispersive X-ray (EDX) elemental mapping images reveal a uniform distribution of Ni, S, further confirming the formation of NiS₂ (Fig. 2g). As schematically shown in Fig. 2h, normally, the grain boundary-free NiS₂ exhibites ordered wurtzite structure with Ni²⁺ occupying the octahedral space and S²⁻ occupying the tetrahedral space. In contrast, our NiS₂ with rich grain boundaries indicates highly distorted structures and anisotropic atom orientations (Fig. 2i). Such a unique structure could effectively create more defects, provide accessible active sites, and shorten the mass transport pathway of electrons/ions in electrocatalysis process [38,39]. Other side, by using the same synthetic method expected for adding S²⁻ in one shot, NiCo PBA@NiS₂ frames could be obtained. Large-scale TEM and SEM images indicate the inner 3D cross-like structure with six "arms", together with outer frame-like structure with six "pillars" (Figs. S6a-e in Supporting information). As evidenced by EDX line scan and mapping images, the inner nanocross is composed of NiCo PBA and the outer frame is composed of nickel-based sulfide, pointing to the NiCo PBA@NiS₂ frames structure (Figs. S6f and g in Supporting information).

The crystal structure of as-prepared ultrathin NiS₂ HNCs were unveiled by XRD pattern, which displays a set of peaks corresponding to the cubic NiS₂ (JCPDS No. 88–1709), indicating the completely phase transformation from NiCo PBA to NiS₂ with well crystallinity (Fig. 3a). N₂ adsorption-desorption isotherms were used to examine the porosity and surface area of the ultrathin NiS₂ HNCs. The Brunauer-Emmett-Teller (BET) surface area measurements show that the NiS₂ HNCs possess a large specific surface area of 162.56 m²/g, which could be ascribed to the existence of layer-by-layer stripped ultrathin shells in NiS₂ nanocages (Fig. 3b). Pore diameter distribution exhibits an average pore size of 4.86 nm for the NiS₂ HNCs (Fig. 3c). As revealed the morphology analysis in Fig. 2, numerous atomic steps/kinks and abundant grain boundaries are formed on the surface of NiS₂ nanocages, which may make contribution to the mesopores formation. The surface composition were probed by the X-ray photoelectron spectroscopy (XPS) technology. As the whole XPS survey shown in Fig. 3d, clear characteristic peaks corresponding to Ni and S elements could be detected. Meanwhile, the Ni content was measured to be 18.22 at%, which was similar to the result determined by inductively coupled plasma optical emission spectrometry (ICP-OES, 19.58 at%). For high resolution Ni 2p spectra, the peaks at 870.8 and 852.9 eV are attributed to the oxidation state of Ni 2p_1/2 and Ni 2p_3/2 energy level in NiS₂. Meanwhile, the peaks at 879.4 and 861.4 eV are corresponding to the satellites of Ni 2p_1/2 and Ni 2p_3/2 energy level, respectively (Fig. 3e) [40]. For high resolution S 2p spectra, three distinct peaks located at 161.4, 162.5, and 168.3 eV can be deconvoluted, attributing to the S 2p_3/2 and S 2p_1/2 and the S-O bond (superficial oxidation of NiS₂), respectively (Fig. 3f) [41,42].

For better understanding the formation mechanism of the ultrathin NiS₂ HNCs, the influences of etching agent and NiCo PBA nanocubes were taken into consideration. As the TEM images shown in Fig. S7 (Supporting information), products prepared by using 2 mL of Na₂S solution displayed the solid sphere structure surrounding by a small amount of lamellar nanosheets, indicating the insufficient etching of the NiCo PBA nanocubes. While increasing the volume of Na₂S solution to 6 mL and 8 mL, the products exhibited an irregular shape with randomly dispersed spheres and nanosheets, indicating the uncontrolled etching kinetics when the volume of Na₂S was excessive. Other side, the diameter of NiCo PBA nanocubes was found to influence the formation of ultrathin NiS₂ HNCs. As the TEM images shown in Fig. S8 (Supporting information), products prepared by using the large-sized NiCo PBA nanocubes exhibited the solid structure with rough surfaces, pointing to the insufficient S²⁻ etching on large-sized NiCo PBA nanocubes. When increasing the volume of Na₂S solution to 12 mL, the products exhibited the hollow nanocage structure with diameter of ca. 300 nm, indicating the well transformation from NiCo PBA to NiS₂ nanocages (Fig. S8b). This result indicates that the large-sized NiCo PBA nanocubes could also be used to fabricate the NiS₂ nanocages, but the amount of etching agent should be increased.

Since the injection rate directly influences the growth pattern, TEM images acquired at a wide range of injection rates were revealed to show detailed structural changes. As shown in Fig. S9 (Supporting information), products obtained at a fast injection rate of 6 mL/h exhibited an irregular shape with sphere-like NiCo PBA and randomly dispersed NiS₂ flexible nanosheets, indicating the incomplete etching of NiCo PBA at a too fast injection rate. In contrast, the products obtained at 3 and 2 mL/h exhibited a major hierarchical flower-like structure with hard observation of any cage-like structure. This result implies that further slowing down the injection rate will decrease the NiS₂ stripping rate, thus the hollow cage structure was hard to be maintained (Figs. S9b and c). Therefore, 4 mL/h could be the optimal injection rate to obtain the ultrathin NiS₂ HNCs. To investigate the kinetics-conducted etching mechanism at 4 mL/h, intermediates collected at different reaction time were investigated by TEM images (Figs. 4a-d). TEM images taken at the start of the etching displayed the solid cubic structure corresponding to NiCo PBA (Fig. 4a). While TEM images taken at t = 15 min displayed the cubic NiCo PBA with rounded corners, together with a small amount of lamellar structure around the NiCo PBA, suggesting the occurrence of the S²⁻ etching (Fig. 4b). The TEM images collected at t = 30 min showed an obviously increased number of flexible NiS₂ nanosheets, together with the loss of NiCo PBA cores, indicating the inwards stripping from the outside wall of NiCo PBA nanocubes (Fig. 4c). When the injection time was prolonged to 1 h, the inner NiCo PBA was etched completely, ultimately ripening into ultrathin NiS₂ HNCs with abundant grain boundaries (Fig. 4d). Based on the above experimental results, a simple injection rate-mediated etching mechanism was proposed. As illustrated in Figs. 4e and f, the injection rate determines the number of S²⁻ in the steady state and thus controls the growth pattern. When the S²⁻ was injected at 4 mL/h, the number of S²⁻ in the solution was gradually increasing along with the time proceeding. The continuous supplement of S²⁻ led to a kinetic rate that slow enough to initiate the layer-by-layer stripping of NiCo PBA, but fast enough to sustain the cubic structure, thereby forming the NiS₂ HNCs. In contrast, when the S²⁻ was injected in one-shot, the number of S²⁻ in the solution reached the maximum amount at the start of the reaction, then the S²⁻ were reacted with NiCo PBA and gradually consumed along with the time proceeding. Until the number of S²⁻ in the solution was insufficient to drive the etching, the reduction was terminated, thus forming the NiCo PBA@NiS₂ nanoframe at a relatively fast kinetic rate.

The OER performances of the ultrathin NiS₂ HNCs were thoroughly appraised in 1.0 mol/L KOH electrolyte, using the standard three-electrode system and in comparison with commercial RuO₂. In order to unveil the effects on electrocatalytic performance brought by the ultrathin and hierarchical structure, the catalytic activity of NiCo PBA@NiS₂ frames was taken into comparison. All potentials hereafter have been iR-corrected and referenced to a reversible hydrogen electrode. Fig. 5a presents the linear sweep voltammetry (LSV) curves of all three samples. To reach the current density of 10 mA/cm², the overpotential of NiS₂ HNCs is 255 mV, outperforming that of NiCo PBA@NiS₂ frames (304 mV) and RuO₂ (342 mV). Furthermore, it required 334 mV to provide 30 mA/cm², much lower than that of the NiCo PBA@NiS₂ frames (431 mV) and RuO₂ (561 mV), as the histograms listed in Fig. 5b. Therefore, we could conclude that the ultrathin and rich-GB features impart nickel sulfide higher intrinsic activity for OER. In addition, to reach the overpotential of 300 mV, the current density for NiS₂ HNCs could reach 35.7 A/g, which is much larger than that for NiCo PBA@NiS₂ frame (16.8 A/g) and RuO₂ (10.2 A/g), indicating the highest specific current density per geometric surface area for NiS₂ HNCs (Fig. 5c). Furthermore, as the histogram shown in Fig. S10 (Supporting information), the turnover frequency (TOF) value for NiS₂ HNCs (16.82 s⁻¹) is much higher than that of the NiCo PBA@NiS₂ frames (7.48 s⁻¹) and RuO₂ (4.23 s⁻¹), suggesting the superior OER intrinsic activity of NiS₂ HNCs. Tafel slopes for NiS₂ HNCs, NiCo PBA@NiS₂ frames and RuO₂ were fitted to be 27.44, 36.13, and 88.17 mV/dec, respectively, indicating the fastest OER kinetics for NiS₂ HNCs (Fig. 5d). Moreover, the superior OER performance of NiS₂ HNCs also surpasses most of the reported values in the literatures, as listed in Table S1 (Supporting information). The double-layer capacitances (C_dl) were performed to compare the electrocatalytic active surface areas (ECSAs) of the NiS₂ HNCs and NiCo PBA@NiS₂ frame. Calculated from the cyclic voltammetries at different scan rates (CVs, Fig. S11 in Supporting information), the C_dl for NiS₂ HNCs (22.56 mF/cm²) is much higher than that for NiCo PBA@NiS₂ frame (13.82 mF/cm², Fig. 5e). This result suggests that more active sites are exposed on NiS₂ HNCs, and consequently lead to higher catalytic activity for OER. Electrochemical impedance spectroscopy (EIS) measurements were carried out to evaluate the charge transfer ability (Fig. 5f). The NiS₂ HNCs exhibit a much smaller R_ct value of 155.4 Ω compared with that of NiCo PBA@NiS₂ nanoframe (245.0 Ω), suggesting the faster charge transfer rate and reaction kinetics for NiS₂ HNCs under the same condition.

The electro-catalytic stability of the NiS₂ HNCs was detected by accelerated durability tests (ADTs) and chronoamperometric measurements. The LSV polarization curves for NiS₂ HNCs were overlapped very well before and after 3000 CV cycles (Fig. 5g). In contrast, the LSV polarization curve of commercial RuO₂ catalyst exhibits an obvious decay in both the overpotential and current density after 3000 cycles, which is inferior to that of the NiS₂ HNCs. (Fig. S12 in Supporting information). Moreover, chronoamperometric measurements illustrate that for the NiS₂ HNCs, the current density achieved at a static potential of 1.55 V only decreases about 11.7% after 12 h (Fig. 5h). In contrast, the NiCo PBA@NiS₂ frames and commercial RuO₂ show obvious decay in current density, with decrements of 13.5% and 35.8%, respectively, further confirming the much better OER stability for NiS₂ HNCs (Fig. S13 in Supporting information). In alkaline media-based OER, transition metal sulfide could be transformed into oxyhydroxides/oxide on the surface, which are effective species to boost OER performance [43,44]. As revealed in Fig. S14 (Supporting information), the Ni 2p XPS spectrum for NiS₂ HNCs after OER deliver two characteristic peaks at 855.9 and 873.5 eV, which are assigned to nickel oxide, demonstrating that Ni species were oxidized during the OER. Besides, the signal of S 2p spectra after the OER was hard to be detected, illustrating the leaching and oxidation of S during electrochemical process. Despite the phase transformation, the NiS₂ HNCs still exhibit the ultrathin and hierarchical nanocage-like structure without obvious morphology change, as confirmed by TEM image (Fig. S15 in Supporting information). Overall, above results clearly demonstrate that constructing ultrathin and hierarchical nanocage structure can effectively enhance the OER activity of NiS₂ catalysts, which might be attributed to the following aspects: (1) The hollow and ultrathin NiS₂ nanocages led to atomtic rearrangements, which may enhance the conductivity and improve the charge transfer capability during OER [45,46]; (2) The inter-connected flexible walls could not only minimize mass transport restrictions and facilitate rapid diffusion of reactant molecules, but also provide large contact area for electrolyte access and O₂ release [47–49] (3) The layer-by-layer stripping of NiCo PBA under a slow etching rate led to the formation of abundant lattice and GB defects, which could serve as additional catalytic sites with high intrinsic activity [50].

In summary, we developed a novel approach to preciously controlling the chemical etching kinetics, in-situ transforming NiCo PBA into ultrathin NiS₂ HNCs with rich defects. More importantly, a deep understanding of the underlying formation mechanism for NiS₂ HNCs was provided, wherein the injection rate determined the concentration of S²⁻ in the steady state and thus controlled the growth pattern. Experimental data revealed that such a nanocage-like NiS₂ with ultrathin and hierarchical walls could expose high-density lattice defects and GBs to offer abundant unsaturated reaction sites, and provide interconnected walls to minimize the mass transport restrictions, thereby responsible for optimized intrinsic activity and stability for OER. This work offers new insights into the shape control of non-precious metal-based electrocatalysts, which is promising for developing efficient, cost-effective, and durable electrode materials in energy conversion devices.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This work was financially supported by National Natural Science Foundation of China (Nos. 21902078, 22279062, 22232004, 22072067). The authors are grateful for the supports from National and Local Joint Engineering Research Center of Biomedical Functional Materials and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109940.