Mo-Triggered amorphous Ni₃S₂ nanosheets as efficient and durable electrocatalysts for water splitting

Abstract

Exploring efficient non-noble materials as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) bifunctional electrocatalysts is of great importance for overall water splitting. Herein, we report a low-temperature and rapid synthesis of Mo-triggered amorphous Ni₃S₂ nanosheets as such dual-function electrocatalysts for the first time by a simple solid-phase melting strategy. It is found that Mo engineering can not only dramatically enhance the adsorption ability of Ni active sites to the active intermediates of HER, but also generate more targeted intermediates for OER. The resulting a-Mo–Ni₃S₂ catalysts demonstrate exceptionally high HER/OER activity and stability in alkaline media, outperforming the baseline commercial noble-metal (Pt, IrO₂ and RuO₂) and other reported advanced electrocatalysts to date. A two-electrode electrolyzer assembled using the a-Mo–Ni₃S₂ electrocatalysts can afford a current density of 1000 mA cm⁻² at a voltage of only 1.97 V which is stable for over 300 h. This work provides a feasible tactic to develop efficient and durable bifunctional electrocatalysts by engineering on surfaces and nanostructures.

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Introduction

Producing hydrogen by water electrolysis can effectively integrate renewable energy such as solar energy and wind energy to truly realize sustainable utilization of clean energy.^1–4 The development of this technique has been greatly limited by the high overpotential of the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). Although noble metal catalysts (e.g. Pt, IrO₂ and RuO₂) can remarkably reduce reaction overpotentials, low abundance and high cost severely restrict their large-scale application.^5,6 Therefore, it is indispensable to exploit highly active and low-cost non-precious metal electrocatalysts. And meanwhile, developing dual-function electrocatalysts for both HER/OER half-reactions is of even greater importance to further facilitate water splitting.^7–9 It has been reported that the rich Ni–S and Ni–Ni bonds of Ni₃S₂ can not only dramatically improve the key OER active intermediate (OOH*) adsorption ability in alkaline media, but also accelerate the charge transfer in the bulk.^10,11 These advantages render it an excellent OER electrocatalyst. However, the corresponding Ni active site has weak adsorption ability to key intermediates (OH⁻) of HER, resulting in sluggish kinetics of the rate-determining Volmer step (H₂O + e⁻ + * → H* + OH⁻) in the hydrogen evolution process.^12,13

Balancing the ability of the active sites to adsorb/desorb the key intermediate (OH⁻) for HER by tailoring the surface electronic structure of catalysts has been recognized as the most effective tactic to boost HER kinetics in alkaline media.^14,15 Both theoretical and experimental studies have found that introducing free electron-enriched metal heteroatoms can increase the density of states around the Femi level of electrocatalysts and promote the separation of positive and negative charges at the interface.^16,17 The finding is very favorable for exposing more active sites and regulating their adsorption ability to the key intermediates.¹⁸ Nevertheless, metal doping methods generally involve a high temperature treatment, easily causing precipitation and agglomeration of heteroatoms.^19,20 On the other hand, compared with crystalline materials, amorphous structures are rich in unsaturated chemical bonds and defect sites, which can provide additional active sites and further improve the adsorption of active intermediates at the electrode/electrolyte interface.^21,22 However, it is very difficult to simultaneously implement metal doping and amorphization of electrocatalysts.

Herein, we report a low-temperature and rapid synthesis of Mo-doped amorphous Ni₃S₂ electrocatalysts for the first time by means of a solid-phase melting strategy. The crystal structure and surface chemical state analyses indicate that Mo doping converts Ni₃S₂ from heazlewoodite into an amorphous phase, which significantly enhances the adsorption ability of Ni active sites to the active intermediates of HER. Meanwhile, it also promotes the generation of more targeted intermediates of OER. The resulting a-Mo–Ni₃S₂ catalysts demonstrate exceptionally high HER/OER activity and stability in alkaline media, outperforming the baseline commercial noble-metal (Pt, IrO₂ and RuO₂) and other advanced catalysts reported to date. Specifically, the a-Mo–Ni₃S₂ catalysts exhibit ultralow overpotentials (η) of 74 and 165 mV at 10 and 100 mA cm⁻² for HER. The turnover frequency (TOF) is 6.39 s⁻¹ at an overpotential of 150 mV, which is about four times higher than that of crystalline Ni₃S₂. Meanwhile, an overpotential of 276 mV at 100 mA cm⁻² is attained for OER. A two-electrode electrolyzer assembled using the a-Mo–Ni₃S₂ electrocatalysts can afford a current density of 1000 mA cm⁻² at a voltage of only 1.97 V which is stable for over 300 h.

Results and discussion

Mo-Doped amorphous Ni₃S₂ electrocatalysts have been synthesized on nickel foam by a low-temperature and rapid solid-phase melting strategy, in which the Ni foam was directly embedded in the precursor powders (CH₃CSNH₂ and Na₂MoO₄) at 220 °C for 40 minutes. The digital photographs (Fig. S1, ESI†) display an obvious colour change from gray to black. It is found that Mo content in products plays an important role in generating the amorphous structure, which can be easily controlled by changing the amount of Na₂MoO₄. Fig. 1a is the X-ray diffraction (XRD) patterns of the Ni₃S₂ samples with different Mo contents based on the inductively coupled plasma mass spectrometry (ICP-MS) results. Without the addition of Mo, the products show strong diffractive peaks of crystalline Ni₃S₂ (labelled as c-Ni₃S₂) with a bulk morphology (Fig. S2, ESI†). These diffractive peaks gradually weaken with the increase of Mo content and then totally disappear until the Mo content reaches 12.5 at%, implying the formation of the amorphous structure (labelled as a-Mo–Ni₃S₂). The fact has been further verified by the XRD pattern of the corresponding powder sample scraped from the Ni foam substrate (Fig. 1b). The a-Mo–Ni₃S₂ sample also seems to show a bulk morphology with a layer-like nanostructure (Fig. S3, ESI†), which is firmly covered on the Ni foam surface. The transmission electron microscopy (TEM) image in Fig. S4 (ESI†) gives a large nanosheet. The two-dimensional (2D) feature is expected to expose more electrocatalytic active sites. The high-resolution TEM image (Fig. 1c) shows the absence of a crystal lattice fringe, further confirming the amorphous feature, which has also been evidenced by the diffused rings in the selected area electron diffraction (SAED) pattern. Fig. 1d gives the homogeneous Ni, Mo and S elemental distribution in a-Mo–Ni₃S₂ nanosheets.

Fig. 1 XRD patterns of (a) the Mo-doped Ni₃S₂ products with different Mo contents and (b) the resultant a-Mo–Ni₃S₂ nanosheets, (c) high-resolution TEM image (inset showing the corresponding SAED pattern) and (d) TEM-EDS mapping of the a-Mo–Ni₃S₂ nanosheets.

The chemical state and composition of the a-Mo–Ni₃S₂ nanosheets have been characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. In Fig. 2a, the Ni 2p_3/2 region can be fitted into three characteristic peaks at 855.9, 853.9 and 853.1 eV corresponding to Ni–OH, Ni–O and Ni–S bonds, respectively,^23,24 and a satellite peak at 861.1 eV. For the S 2p region (Fig. 2b), the two peaks at 163.5 and 164.7 eV are attributed to S–Ni bonds while another two peaks at 161.8 and 164.3 eV belong to S–Mo bonds.^25,26 A weak S–O bond appears at 168.3 eV, mainly due to the adsorption of oxygen species in air.²⁷ In Fig. 2c, the S 2s region consists of two peaks at 227.9 and 228.4 eV corresponding to S–Mo and S–Ni bonds. The Mo 3d region shows two peaks at 232.5 and 235.7 eV, revealing that Mo is in the Mo⁶⁺ oxidation state.²⁸ Moreover, the Raman spectrum (Fig. 2d) exhibits peaks at 272.4, 342.9, 480.0 and 557.8 cm⁻¹ corresponding to the Ni–S bond while 723.8, 818.9, 886.4 and 945.8 cm⁻¹ for the formation of the Ni–Mo bond.^29,30 These results indicate that Mo atoms have been successfully doped into Ni₃S₂ by the way of chemical bonding.

Fig. 2 XPS spectra of (a) Ni 2p_3/2, (b) S 2p, (c) Mo 3d/S 2s regions, and (d) Raman spectrum of the a-Mo–Ni₃S₂ nanosheets.

The a-Mo–Ni₃S₂ nanosheets are firstly evaluated as HER electrocatalysts in alkaline media by a standard three-electrode system with a saturated Ag/AgCl reference electrode and a graphite counter electrode, respectively. As shown in Fig. 3a, they exhibit ultralow overpotentials of 74 and 165 mV to obtain 10 and 100 mA cm⁻², respectively, which are much lower than those of c-Ni₃S₂ (η₁₀ = 176, η₁₀₀ = 280 mV) and Pt sheet (η₁₀ = 85, η₁₀₀ = 204 mV). Such low overpotentials outperform the baseline commercial noble metals (Pt, IrO₂ and RuO₂). In a very recent work, Zhang et al. reported an advanced Ni_1−xCo_xSe₂ mesoporous nanosheet network electrocatalyst by topological transformation and subsequent acid etching, which required a relatively high overpotential of 85 mV to generate 10 mA cm⁻².³¹ Some representative works in the literature are summarized in Table S1 (ESI†). The Tafel slope (Fig. 3b) of a-Mo–Ni₃S₂ is 54 mV dec⁻¹, which is more than twice as low as that of c-Ni₃S₂ (112 mV dec⁻¹) and close to that of the commercial Pt/C electrocatalyst, indicating reinforced HER kinetics dominated by the Volmer–Heyrovsky mechanism. This feature can also be evidenced by the higher exchange current density (j₀) of a-Mo–Ni₃S₂ (1.16 mA cm⁻²) in comparison to c-Ni₃S₂ (0.28 mA cm⁻²), as shown in Fig. 3c. Furthermore, the a-Mo–Ni₃S₂ electrocatalysts give a markedly lower charge-transfer resistance (∼6.0 Ω) than the c-Ni₃S₂ (∼30.0 Ω) (Fig. 3d), also indicating the improved HER kinetics. The turnover frequency (TOF) of a-Mo–Ni₃S₂ catalysts is calculated to be 6.39 s⁻¹ according to a CV method (Fig. S5, ESI†),³² four times higher than that of c-Ni₃S₂ (1.56 s⁻¹), as shown in Fig. 3e, exhibiting the enhanced intrinsic HER activity. In addition to catalytic activity, stability is another factor to be considered for sustainable energy conversion. Fig. 3f is the chronopotentiometry (CP) plots of a-Mo–Ni₃S₂ catalysts, demonstrating excellent stability with negligible potential change even after 36 h CP measurement at 10, 50 and 100 mA cm⁻², respectively. The amorphous a-Mo–Ni₃S₂ electrocatalyst has been well-maintained after CP evaluation (Fig. S6, ESI†), implying the outstanding durability for HER.

Fig. 3 (a) HER polarization curves and (b) Tafel plots of the a-Mo–Ni₃S₂, the c-Ni₃S₂, Pt foil and 20% Pt/C electrocatalysts; (c) the calculated exchange current density, (d) Nyquist plots and (e) turnover frequency curves of the a-Mo–Ni₃S₂ and c-Ni₃S₂ electrocatalysts; (f) chronopotentiometry curves of the a-Mo–Ni₃S₂ electrocatalyst.

In order to clarify the effect of Mo engineering on hydrogen generation, the electrochemically active surface area (ECSA) and roughness factor (RF) are firstly provided based on their double-layer specific capacitance (Fig. S7 and S8, ESI†). The ECSA and RF of our a-Mo–Ni₃S₂ catalysts are 319 cm² and 1595, as shown in Fig. 4a, nearly six times higher than those of c-Ni₃S₂ (56 cm² and 280). These data indicate that the apparent active sites of the a-Mo–Ni₃S₂ electrocatalysts have been significantly increased. To investigate the underlying factors in detail, the high-resolution XPS spectra are employed to study the electronic structural change of the a-Mo–Ni₃S₂ catalysts after HER with the c-Ni₃S₂ sample as a control. As shown in Fig. 4b, the binding energies of Ni–OH and Ni–S bonds are more positive with large bias of 0.2 and 0.3 eV, respectively, compared to the c-Ni₃S₂ electrocatalysts in the Ni 2p_3/2 region. The Ni–OH content is also greatly improved to 92% after HER by integrating the respective peak areas, two times higher than the case of c-Ni₃S₂. The phenomena imply the electron-deficient characteristics of the a-Mo–Ni₃S₂ catalysts, which have been recognized to help the improvement of Ni sites adsorption ability to targeted active intermediates (OH⁻) during hydrogen generation.^33,34 It is noted that the Mo atoms contribute negligible catalytic activity because of the unchanged Mo⁶⁺ oxidation state (Fig. S9, ESI†) that is inactive for HER.³⁵ To elucidate the role of Mo, Ni₃S₂ products with different Mo contents after HER are investigated by high-resolution XPS spectra (Fig. 4c). The Mo content in all samples is estimated by ICP-MS, which is in good agreement with the calculated results of the respective peak area. It can be observed that all samples show a Mo⁶⁺ oxidation state. Impressively, the characteristic peaks of Ni–OH bonds in the Ni 2p_3/2 region (Fig. S10, ESI†) have been gradually enhanced with increasing Mo content until 12.5 at%, as shown in Fig. 4d. This result indicates that Mo atoms can promote the formation of Ni–OH bonds. The maximized Ni–OH content is reported to accelerate HER kinetics.¹⁴ Direct evidence is provided by their HER polarization curves (Fig. S11, ESI†), where the sample with 12.5 at% Mo exhibits the best HER activity.

Fig. 4 (a) Electrochemically active surface area and roughness factor, (b) XPS spectra of the Ni 2p region after HER of the a-Mo–Ni₃S₂ and c-Ni₃S₂ electrocatalysts; (c) XPS spectra of the Mo 3d region and (d) the Ni–OH content of the Mo-doped Ni₃S₂ products with various Mo contents after HER.

As aforementioned, Mo engineered Ni₃S₂ catalysts greatly enhanced the HER activity without sacrificing their excellent OER performance. As shown in Fig. 5a, a small overpotential of only 276 mV is required to attain 100 mA cm⁻², much lower than those of c-Ni₃S₂ (318 mV) and the commercial IrO₂ and RuO₂ catalysts. The corresponding Tafel slope (Fig. S12, ESI†) is 61 mV dec⁻¹, closer to that of RuO₂ than c-Ni₃S₂. The enhanced OER kinetics can also be verified by its higher exchange current density (j₀) of 1.39 × 10⁻⁴ mA cm⁻² than c-Ni₃S₂ (2.20 × 10⁻⁵ mA cm⁻²). The a-Mo–Ni₃S₂ electrocatalysts can be stable for over 12 h CP at 100 mA cm⁻² (Fig. S13, ESI†) without any change in morphology and crystal phase (Fig. S14, ESI†). The improved OER performance is attributed to the introduction of Mo atoms, which can increase the OER active intermediate OOH* content from 4% of c-Ni₃S₂ to 10% of a-Mo–Ni₃S₂ according to the XPS results after OER (Fig. S15, ESI†).³⁶ We then assembled a two-electrode alkaline electrolyzer using a-Mo–Ni₃S₂ as both anode and cathode, delivering current densities of 50 and 100 mA cm⁻² at small voltages of 1.61 and 1.67 V, respectively, as shown in Fig. 5b. Even for achieving an ultrahigh current density of 1000 mA cm⁻², only a voltage of 1.97 V is required with strong bubble evolution. To highlight the outstanding bifunctional electrocatalytic activity for water splitting, we also assembled c-Ni₃S₂//c-Ni₃S₂ and Pt/C//IrO₂ as controls. They required higher voltages to obtain the same current density. In addition, the a-Mo–Ni₃S₂ catalyst displays nearly 100% Faradaic efficiency calculated by comparing the generated H₂ and O₂ volumes with theoretical values (Fig. 5c). It also possesses high stability for water splitting (Fig. 5d) with a very slight voltage decrease for 300 h CP measurement at 1000 mA cm⁻². The water splitting ability of the a-Mo–Ni₃S₂ electrocatalysts is among the best reported in the literature, showing their fascinating potential for practical application.

Fig. 5 (a) The OER polarization curves of the a-Mo–Ni₃S₂, c-Ni₃S₂, commercial IrO₂ and RuO₂ electrocatalysts, and (b) their corresponding overall water splitting polarization curves (inset of cell voltage comparison), (c) Faradaic efficiency and (d) chronopotentiometry curve of the a-Mo–Ni₃S₂ electrocatalysts.

Conclusions

In summary, we demonstrated a low-temperature and rapid synthesis of Mo-triggered amorphous Ni₃S₂ electrocatalysts for the first time by a solid-phase melting strategy. The resulting a-Mo–Ni₃S₂ catalysts possess exceptionally high activity and stability for both HER and OER in alkaline media, outperforming the commercial noble-metal benchmarks (Pt, IrO₂ and RuO₂) and other advanced Ni-based electrocatalysts in the literature. Prominently, the a-Mo–Ni₃S₂ catalysts require ultralow HER overpotentials of 74 and 165 mV to obtain 10 and 100 mA cm⁻² with a four times higher TOF of 6.39 s⁻¹ at η = 150 mV in comparison to crystalline Ni₃S₂. In addition, a small OER overpotential of 276 mV at 100 mA cm⁻² is attained. A two-electrode electrolyzer assembled using the a-Mo–Ni₃S₂ electrocatalysts can achieve a current density of 1000 mA cm⁻² at only 1.97 V which is stable for over 300 h. Such fascinating catalytic performances are mainly attributed to the enhanced adsorption ability of Ni active sites to key intermediates of HER as well as the enrichment of OER targeted intermediates caused by Mo engineering. This finding verifies the feasibility in design of highly efficient non-noble electrocatalysts by engineering on surfaces and nanostructures.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51672082, 91534122 and 91534202), the Social Development Program of Shanghai (17DZ1200900), the Shanghai Scientific and Technological Innovation Project (18JC1410600), the Program for Shanghai Youth Top-notch Talent, and the Fundamental Research Funds for the Central Universities (222201718002).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qm00178b

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