Electrocatalytic splitting of water via hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is widely considered to be a feasible method of producing high-purity hydrogen fuel from aqueous solutions [1-3]. Nevertheless, both HER and OER require active and durable electrocatalysts to lower their overpotential and overcome sluggish kinetics [4]. To date, Pt has been regarded as the best HER electrocatalyst, while IrO₂ and RuO₂ are the most active electrocatalysts for OER [5]. Unfortunately, the scarcity and extremely high cost of these noble metals severely limit their widespread application. To this end, enormous efforts have been committed to exploring earth-abundant and inexpensive electrocatalysts, especially bifunctional electrocatalysts, for overall water splitting. These include alloys of various three-dimensional (3D) transition metals (Mn, Fe, Co, Ni, etc.) [6] and their oxides/hydroxides [7-14], sulfides [15-20], selenides [21], phosphides [22], nitrides [23], as well as carbides [17, 24, 25]. Among these, transition metal sulfides (TMSs), especially CoNi sulfides, have received considerable attention as they exhibit distinctive bifunctional electrocatalytic properties owing to their hydrogenase-like catalytic mechanism [26]. Liu et al. [27] proposed a water electrolyzer, which utilized a NiCo₂S₄ nanowire array on carbon cloth (NiCo₂S₄ NA/CC) as a bifunctional electrocatalyst in an alkaline medium, and could obtain 10 mA cm^–2 at a cell voltage of 1.68 V. Subsequently, Fang et al.[28] reported that an alkaline electrolyzer based on Ni_2.3%–CoS₂/CC only needed a cell voltage of 1.66 V to deliver 10 mA cm^–2. Although recent years have witnessed remarkable progress in this field of research, developing CoNi sulfides with favorable configurations for extraordinary bifunctional electrocatalytic performance remains a practical challenge.

Notably, 3D materials with unique nanostructures have been reported as the most promising electrocatalysts for application in electrochemical reactions due to their large surface area, fast electron transport, and abundance of active sites exposed to the electrolyte [29]. Currently, the majority of TMS-based catalysts are 1D nanowires [19, 27, 28] and 2D nanosheets [26]. However, these materials tend to aggregate and suffer from low electron transmission capacity, which impedes their electrocatalytic activity. Therefore, one possible solution is to develop TMS-based catalysts with distinct 3D nanostructures to obtain potential electrocatalysts for bifunctional water splitting.

Herein, we report our research on novel 3D lily-like CoNi₂S₄ directly grown on Ni foam (CoNi₂S₄/Ni). Benefiting from the active Ni^б+ and S^б– species, large active surface area, mesoporous structure, and enhanced charge transfer capacity, the 3D lily-like CoNi₂S₄/Ni demonstrated excellent catalytic performance and stability toward both HER and OER in basic electrolytes. An alkaline electrolyzer assembled using this versatile CoNi₂S₄/Ni electrode exhibited remarkable activity for overall water splitting with 10 mA cm^–2 at a cell voltage of only 1.56 V.

Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, CH₃CSNH₂ (Thioacetamide, TAA), CON₂H₄ (urea), and NH₄F were purchased from Aladdin. The Ni foam was purchased from Kunshan Electronic Limited Corporation. No additional purification step was performed for all the reagents.

The Ni foam (3 cm × 3 cm) was immersed in 3 mol L^–1 HCl solution by sonication and absolute ethanol for 15 min each. Next, 0.175 g of Co(NO₃)₂·6H₂O, 0.873 g of Ni(NO₃)₂·6H₂O, 1.10 g of urea, and 0.10 g of NH₄F were adequately dissolved in a mixture of 30 mL of deionized water and 20 mL of absolute ethanol under agitated stirring. Next, the homogeneous solution was transferred into a Teflon-lined autoclave (80 mL) with the clean Ni foam in it. The autoclave was sealed and heated at 120 ℃ for 6 h. After it had cooled to ambient temperature, the CoNi–precursor/Ni was taken out and washed with deionized water thoroughly, and then dried at 60 ℃ for 2 h.

The as-prepared CoNi–precursor/Ni was added to 50 mL deionized water with 0.376 g TAA, and the mixture was stirred to form a homogeneous solution. The resulting solution was transferred into an autoclave and heated at 160 ℃ for 4 h. Upon cooling to room temperature spontaneously, the as-made material was washed thrice with deionized water and ethanol to remove impurities on the surface, and then dried at 60 ℃ overnight. The mass loading of CoNi₂S₄/Ni is 3.8 mg cm^–2.

The X-ray diffraction (XRD) pattern was obtained from a MSAL-XD2 X-ray diffractometer with Cu Ka radiation (λ = 1.5406 Å). Scanning electron microscopy (SEM) images were obtained from a Philips SEM-XL30S microscope operated at 15 kV. High-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) coupled with an energy-dispersive X-ray spectroscopy (EDS) analyzer was conducted at an accelerating voltage of 200 kV. Inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out on PerkinElmer Optima 2000DV. Nitrogen sorption isotherm was tested by a Micromeritics TriStar 3000 analyzer at –196 ℃. The X-ray photoelectron spectroscopy (XPS) measurements were analyzed by an ESCALab 250.

Electrochemical measurements were performed in a standard three-electrode system in which the as-prepared samples, Pt foil, and Hg/HgO electrodes served as the working, counter, and reference electrodes, respectively. The electrolyte was 1.0 mol L^–1 KOH solution. All potentials were converted via the Nernst equation (E_RHE = E_Hg/Hg2Cl2 + (0.059 pH + 0.242) V). The polarization curves were obtained by linear sweep voltammetry (LSV) at a scan rate of 1 mV s^–1. The overpotentials (η) were computed according to the equation (η = E_RHE – 1.23 V). Electrochemical impedance spectroscopy (EIS) measurements were analyzed in the frequency range of 10 kHz to 10 mHz with an amplitude of 5 mV. Chronoamperometric tests were conducted at given potentials.

As schematically illustrated in Fig. 1, the 3D lily-like CoNi₂S₄/Ni was synthesized via sulfuration of the CoNi–precursor/Ni. Similar to a previous report [15], the XRD patterns of CoNi₂S₄/Ni and CoNi–precursor/Ni were tested using their powders scraped down from the Ni foam. The XRD pattern of the CoNi–precursor in Fig. 2(a) reveals that the peaks at 11.6°, 23.3°, 34.9°, 39.5°, 46.9°, 60.9°, and 62.3° are derived from (Ni_6.10Co_2.90)(OH)_18.27(CO₃)_1.315·6.7H₂O (JCPDS no. 33–0429), while the other peaks at 17.5° and 36.5° correspond to Co(CO₃)_0.5(OH)·0.11H₂O) (JCPDS no. 48–0083). Therefore, we name this mixture precursor as CoNi–C–Co–CHH. After sulfuration, the diffraction peaks of CoNi–C–Co–CHH are not observed. The emerging peaks located at 31.5°, 38.2°, 47.2°, 50.3°, and 55.0° can be clearly attributed to the (311), (400), (422), (511), and (440) planes of cubic CoNi₂S₄ (JCPDS no. 24–0334), respectively. The broad diffraction peaks of CoNi₂S₄ indicate that its crystallinity is not high. Moreover, the ICP analysis reveals that the atomic ratio of Co:Ni is 1.0:2.2 (Table S1).

To observe the morphology of the as-prepared samples, SEM measurements were carried out. It can be seen in Fig. 2(b) that CoNi–C–Co–CHH/Ni exhibits a lily-like morphology with smooth "petals". Fig. S1(a) reveals that the lily-like CoNi–C–Co–CHH/Ni completely covers the surface of the Ni foam, and the average diameter of the "lily" is about 6.0 µm. After sulfuration, the lily-like morphology of CoNi₂S₄ is well retained (Fig. S1(b)). However, the surface of the "petals" becomes rough (Fig. 2(c)). This might be ascribed to the numerous pores that were formed on the surface of CoNi₂S₄ (Fig. 2(d)). This result can be further probed by the N₂ sorption measurement. As displayed in Fig. S2(a), the pseudo-type Ⅳ isotherm with H1 hysteresis loop can be observed, indicating the typical mesoporous feature of lily-like CoNi₂S₄. The pore size of lily-like CoNi₂S₄ is mainly about 28 nm (see inset in Fig. S2(a)). Note that the abundant porous structure will provide more active sites for contact and improve mass diffusion/transport for water splitting [30, 31]. The HRTEM image (Fig. 2(e)) displays a well-resolved lattice fringe spacing of 0.17 nm, which is assigned to the (440) plane of CoNi₂S₄. Some amorphous area can also be observed in the HRTEM image, which is consistent with the XRD results. The elemental mapping images illustrate the uniform distribution of Co, Ni, and S in CoNi₂S₄ (Fig. 2(f)).

The XPS spectra of CoNi₂S₄ were also characterized to explore its valence and chemical state. As shown in Fig. S2(b), the Co 2p region was fitted with two spin-orbit doublets. One is at 778.3 and 793.3 eV, and the other is at 781.6 and 798.3 eV. The spin-orbit splitting values of Co 2p_1/2 and Co 2p_3/2 for the two spin-orbit doublets are severally 15.0 and 16.7 eV, which correspond to Co³⁺ and Co²⁺, respectively [19]. The atomic ratio of Co³⁺/Co²⁺ is 0.3:1.0, which is obtained by comparing the fitted area [30]. Fig. S2(c) shows the Ni 2p region. The peaks at 856.1 eV for Ni 2p_3/2 and 874.0 eV for Ni 2p_1/2 are ascribed to Ni²⁺, while the other two peaks at 858.2 eV for Ni 2p_3/2 and 876.7 eV for Ni 2p_1/2 are related to Ni³⁺ [26, 27]. The atomic ratio of Ni³⁺/Ni²⁺ is 0.98:1.0. Note that the peak at 853.2 eV is attributed to Ni^δ+, which has a positive shift of 1.0 eV with respect to that of metallic Ni (852.2 eV) [32]. Correspondingly, the peak at 162.6 eV is attributed to the negatively charged S^δ– (Fig. S2(d)) [33]. The above mentioned positive and negative shifts of Ni^δ+ and S^б– are mainly ascribed to the partial transfer of the electron density of Ni to S in CoNi₂S₄. The peak at 161.4 eV corresponds to S 2p_3/2 and is ascribed to S^2–. Moreover, the peak at 169.4 eV is most likely due to the oxidized species of S with a high oxidation state, which is well in accordance with the literature [26].

The HER performance of CoNi₂S₄/Ni was estimated by LSV characterization in 1.0 mol L^–1 KOH solution at a scan rate of 1 mV s^–1. For comparison, bare Ni foam, CoNi–C–Co–CHH/Ni, and 50 wt% Pt/C loading on Ni foam (50 wt% Pt/C/Ni [32], loading 3.8 mg cm^–2) were also tested. Fig. 3(a) displays the polarization curves of these electrocatalysts. The overpotential at a current density of 10 mA cm^–2 (η₁₀) can be employed to evaluate the HER catalytic activity of the electrocatalysts. CoNi–C–Co–CHH/Ni possesses low activity for HER with η₁₀ of 200 mV. As expected, the catalytic performance of CoNi₂S₄/Ni improves sharply at η₁₀ of 54 mV, which is just larger than that (25 mV) of 50 wt% Pt/C/Ni. The η₁₀ of 54 mV of CoNi₂S₄/Ni is also comparable to those of the reported electrocatalysts, as shown in Fig. 3(b), indicating that the as-prepared CoNi₂S₄/Ni electrode is one of the most active electrocatalysts for HER [5, 17-19, 22, 26, 34-36].

To elucidate the HER mechanism of the as-synthesized materials, Tafel plots were determined according to the Tafel equation (η = blogj + a, where a is a constant, b is the Tafel slope, and j is the current density.). It can be seen from Fig. 3(c) that the Tafel slope values of CoNi₂S₄/Ni, CoNi–C–Co–CHH/Ni, 50 wt% Pt/C/Ni, and bare Ni foam are 129.0, 178.3, 29.4, and 229.7 mV dec^–1, respectively. The lower Tafel slope value of CoNi₂S₄/Ni with respect to that of CoNi–C–Co–CHH/Ni implies more favorable catalytic kinetics. According to previous reports [19, 32], this value of CoNi₂S₄/Ni (129.0 mV dec^–1) indicates that it follows the Volmer (H₂O + e^– → H_ads + OH^–)–Heyrovsky (H₂O + H_ads + e^– → H₂ + OH^–) pathway. Moreover, the Ni^б+ and S^б– species present in CoNi₂S₄/Ni (XPS results in Fig. S2) demonstrate that its HER catalytic mechanism is similar to that of hydrogenases [32, 33, 37]. Therefore, the high activity of CoNi₂S₄/Ni for HER intrinsically benefits from the Ni^б+ and S^б– species severally acting as hydride-acceptor and proton-acceptor centers, which can assist in the decomposition of captured H₂O to OH^– species and adsorbed H atoms (H_ads) in the Volmer step, and then accelerate the detachment of OH^– and transformation of H_ads to yield H₂ in the Heyrovsky step.

To further understand the efficient catalytic performance of CoNi₂S₄/Ni for HER, the electrochemically active surface area (ECSA) and EIS measurements were obtained. As shown in Figs. S3(a) and (b), the cyclic voltammetry (CV) curves were measured in the region of 0–0.05 V with scan rates in the range 6–14 mV s^–1. The values of electrochemical double-layer capacitance (C_dl) can be computed by using the above CV curves. It is that the linear slope of capacitive currents versus scan rates is equal to 2C_dl. Moreover, C_dl is proportional to ECSA. As shown in Fig. S3(c), the C_dl of CoNi₂S₄/Ni (61.0 mF cm^–2) is about fourfold higher than that (15.6 mF cm^–2) of CoNi–C–Co–CHH/Ni, revealing that CoNi₂S₄/Ni has exposed more catalytically active sites for HER. Fig. S3(d) shows the Nyquist plots of EIS of CoNi₂S₄/Ni and CoNi–C–Co–CHH/Ni. The smaller charge transfer resistance (R_ct, 0.82 Ω) of CoNi₂S₄/Ni with respect to that (4.86 Ω) of CoNi–C–Co–CHH/Ni suggests fast electron transport in the HER catalytic process.

The above experimental results indicate that the efficient catalytic activity of CoNi₂S₄/Ni for HER can be rationally interpreted as follows (1) CoNi₂S₄/Ni has active centers similar to those of effective hydrogenase. The Ni^б+ and S^б– species in CoNi₂S₄/Ni can be the hydride-acceptors and proton-acceptors to accelerate the HER processes. (2) The rough surface of CoNi₂S₄/Ni possesses a large ECSA to expose more effective active sites. The 3D lily-like morphology and mesoporous characterization of CoNi₂S₄ facilitate contact between the electrolyte and the active sites, thereby improving the utilization efficiency of the active sites. (3) The CoNi₂S₄ directly supported on Ni foam without using the binder endows the electrode with low contact resistance and efficient charge transportation in HER electrolysis.

Stability is another key parameter on the basis of which we can evaluate the performance of electrocatalysts in practical applications. We, therefore, probed the stability of the CoNi₂S₄/Ni electrode using chronoamperometric studies under different overpotential values (Fig. 3(d)). No obvious degradation in the current density was observed, indicating satisfactory durability of CoNi₂S₄/Ni.

Besides the catalytic activity for HER, the catalytic performance of CoNi₂S₄/Ni for OER was assessed. The CoNi–C–Co–CHH/Ni, RuO₂ loading on Ni foam (RuO₂/Ni [23], loading 3.8 mg cm^–2), and bare Ni foam were also investigated. The overpotential at a current density of 100 mA cm^–2 (η₁₀₀) was used to evaluate the OER electrocatalytic activity of the as-prepared samples, because of the large oxidation peaks of the CoNi₂S₄/Ni result in an inaccurate analysis of the onset potential. As displayed in Fig. 4(a), the η₁₀₀ of 328 mV for CoNi₂S₄/Ni undoubtedly outperforms that (545 mV) of CoNi–C–Co–CHH/Ni. Surprisingly, the η₁₀₀ of 328 mV for CoNi₂S₄/Ni is also lower than that (376 mV) of RuO₂/Ni and even those of most of the other recently reported OER electrocatalysts (Fig. 4(b)) [18, 19, 22, 27, 28, 38-42]. Moreover, the lower Tafel slope value of CoNi₂S₄/Ni (124.6 mV dec^–1) compared to that of CoNi–C–Co–CHH/Ni (145.2 mV dec^–1) demonstrates positive OER catalytic kinetics (Fig. 4(c)). The chronoamperometric curve of CoNi₂S₄/Ni in Fig. 4(d) shows an ignorable decline in current density, revealing its good durability. Notably, the first increase in current density in the chronoamperometric curve is ascribed to the oxidation of Ni and Co in the OER catalytic process (Fig. S4). In addition, CoNi₂S₄/Ni exhibits a similar polarization curve between the initial state and after 10 h of OER electrolysis (see the inset of Fig. 4(d)).

Given the excellent electrocatalytic performance of CoNi₂S₄/Ni toward both HER and OER, it may be utilized as a potential anode and cathode for overall water splitting in a two-electrode system. Fig. 5(a) shows that the CoNi₂S₄/Ni couple offers a current density of 10 mA cm^–2 at a cell voltage of only 1.56 V. Although this value is larger than that (1.53 V) of the Ni_xCo_3–xS₄/Ni₃S₂/NF couple [34], it is smaller than that of most other bifunctional electrocatalysts reported including Co₉S₈/WS₂ (1.65 V) [5], Co_6.25Fe_18.75Ni₇₅O_x (1.583 V) [7], Ni/NiTiO₃ (1.63 V) [8], CoNC@MoS₂/CNFs (1.62 V) [17], NiCo₂S₄ NW/NF (1.63 V) [19], NiCo₂S₄NA/CC (1.68 V) [27], Ni_2.3%–CoS₂/CC (1.66 V) [28], and so on (Fig. 5(b)). Moreover, the CoNi₂S₄/Ni couple also exhibits good stability and retains the current density even after continually operating for 10 h in a basic medium, indicating its efficient water splitting performance (see inset in Fig. 5(a)).

In summary, 3D lily-like CoNi₂S₄ has been successfully synthesized on Ni foam via sulfuration of lily-like CoNi–C–Co–CHH/Ni. Notably, lily-like CoNi₂S₄/Ni exhibits excellent catalytic activity and stability for both HER and OER under alkaline conditions. When CoNi₂S₄/Ni is utilized as a bifunctional electrocatalyst for overall water splitting in 1.0 mol L^–1 KOH solution, the couple obtained affords a current density of 10 mA cm^–2 at 1.56 V, which is superior to those of most bifunctional electrocatalysts reported in the literature. The remarkable electrocatalytic activity of this 3D lily-like CoNi₂S/Ni can be mainly attributed to the charged nature of the metals and sulfur in this material, increased surface roughness along with more exposed active sites, mesoporous structure, and accessible pathways for mass diffusion/transport, thus making this material a promising bifunctional electrocatalyst for water splitting.