Accelerating hydrazine-assisted hydrogen production kinetics with Mn dopant modulated CoS₂ nanowire arrays

Abstract

Electrochemical H₂ production from water splitting is an environmentally sustainable technique but remains a great challenge due to the sluggish anodic oxygen evolution reaction (OER). Replacing the OER with the thermodynamically more favorable electrocatalytic oxidation process is an effective strategy for highly efficient H₂ generation. Herein, Mn-doped CoS₂ has predicted an excellent bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and the hydrazine oxidation reaction (HzOR). With the introduction of Mn, the Gibbs free energy of the adsorbed H* and the potential rate-limiting step (the dehydrogenation of *NH₂NH₂ to *NHNH₂) for the HzOR process of the catalyst can be significantly reduced. As expected, the Mn-CoS₂ catalyst exhibited excellent catalytic activity and robust long-term stability for the HER and HzOR. In detail, the Mn-CoS₂ catalyst only acquired potentials of 46 and 77 mV versus the reversible hydrogen electrode for achieving a current density of 10 mA cm⁻² for the cathodic HER and anodic HzOR, respectively. In addition, the Mn-CoS₂ electrode only needs a cell voltage of 447 mV to output 200 mA cm⁻² in the overall hydrazine splitting system as well as exhibits a robust long-term H₂ production. This work provides theoretical guidance for the design of advanced bifunctional electrocatalysts and promotes high efficiency and energy-saving H₂ production technology.

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Introduction

With the increasing depletion of fossil fuels, serious energy shortage, and environmental problems, clean and renewable energy sources have attracted much attention.^1,2 Hydrogen energy is deemed as one of the most potential alternatives for a sustainable energy system due to its highest energy density (142 MJ kg⁻¹).^3–5 Nevertheless, the current industrial method for H₂ production, natural gas reforming, is an energy-intensive and non-clean process. It consumes considerable amounts of fossil fuel and produces a vast amount of carbon dioxide.⁶ Electrochemical hydrogen (H₂) production with high purity via the hydrogen evolution reaction (HER) on the water-splitting cathodic electrode has been emerging as a sustainable, cost-effective, and viable way.^7–13 Currently, the major obstacles that restricted the development of H₂ production are the lack of high-activity and low-cost electrocatalysts with low overpotential requirements in HER processes.¹⁴ Although H₂ can be obtained based on the state-of-the-art pH-universal Pt-based electrocatalysts, their wide commercial application is severely limited by their scarcity and high cost.^15–17 Therefore, the development of low-cost metal-free or abundant transition metal electrocatalysts toward high-efficient H₂ generation to meet the global hydrogen economy is highly desirable yet key challenging.^15,18

Ongoing research efforts have been devoted to developing cost-effective electrocatalysts with high activity for the HER process, many potential nanomaterials (such as transition metal phosphides,^19–22 sulfides,^23–26 and nitrides^27,28) have been widely put forward and investigated in detail. Among these catalysts, Co-based nanomaterials, especially for the cobalt disulfide (CoS₂), have been regarded as promising HER electrocatalysts due to their high conductivity and unique configuration. Despite achieving significant progress, the HER catalytic activities of Co-based catalysts are also still far less than the Pt-based catalyst. In this regard, more attention has been paid to the modulation of the electronic structure and the optimization of chemical compositions, aiming to adjust the energy band structure as well as the electronic conductivity and thermodynamic H* adsorption/desorption state. The doping strategy with transition metal ions has been proved to play a significant role in optimizing the Gibbs free energy of the adsorbed H (ΔG_H*) for hydrogen adsorption of HER electrocatalysts.^29–35 It has been demonstrated that the HER electrocatalytic activity of Co-based catalysts can be efficiently triggered by varied transition-metal dopants.^36,37 In particular, the volcano plot based on a density functional theory (DFT) revealed that elemental Mn is the best candidate dopant to tune the adsorption behavior of H atoms on adjacent Co atoms, the dopant itself, and consequently the HER activity.³⁸

Additionally, the anodic oxygen evolution reaction (OER) is identified as the bottleneck of water splitting. The OER suffered from a high overpotential and sluggish kinetics, which greatly limit the practical application of H₂ production.³⁹ Therefore, apart from optimizing catalysts, the electrochemical H₂ production can be improved and accelerated by replacing the OER with a thermodynamically more favorable electrocatalytic oxidation process. For instance, the oxidation of hydrazine, 5-hydroxymethylfurfural (HMF),⁴⁰ and urea⁴¹ have been coupled with cathodic H₂O reduction to effectively reduce energy consumption toward high-purity H₂ generation. Among them, hydrazine (N₂H₄) oxidation is the most potential candidate due to its much lower oxidation potential of −0.33 V versus reversible hydrogen electrode (vs. RHE) than that of the OER (1.23 V vs. RHE).^42–44 Moreover, inert N₂ is the anodic oxidation product of N₂H₄, which can well solve the potential security issues of O₂ + H₂ mixture gases in membrane-free overall water splitting. In addition, with the assistance of Earth-abundant electrocatalysts, substituted anodic hydrazine oxidation could reduce the cost and overpotential of the entire water electrolysis, resulting in high-efficiency H₂ production.

Herein, taking the representative CoS₂ as a model electrocatalyst for the HER, we adopted a doping strategy with an Mn atom to accelerate H₂ production with the assistance of anodic N₂H₄ oxidation by rationally modulating the electronic structure. DFT calculations demonstrated that Mn atoms can optimize the Gibbs free energy and enhance the intrinsic activity and tremendously accelerate the kinetic process. Inspired by the theoretical analysis, we successfully prepared Mn-doped CoS₂ nanotubes supported on nickel foam (Mn-CoS₂). As expected, the Mn-CoS₂ electrocatalyst exhibited excellent electrochemical performance with a much lower overpotential of 46 mV at a current density of 10 mA cm⁻¹ and a small Tafel slope of 63.1 mV dec⁻¹ for the HER in 0.1 M KOH aqueous solution. Meanwhile, as for the anodic hydrazine oxidation reaction (HzOR), Mn-CoS₂ only needs a small working potential of 77 mV vs. RHE to generate a current density of 10 mA cm⁻¹.

Experimental methods

Computational details

All the geometric optimization and single-point energy calculations were performed using spin-polarized density-functional theory (DFT) implemented in the DMol³ code. Exchange–correlation functions are taken as generalized gradient approximation (GGA) with Perdew–Burke–Ernzerhof (PBE). The band structure and the corresponding DOS result were conducted by DS-PAW package, and the Device Studio program was used for performing visualization, modeling and simulation^45,46 The double numerical plus polarization (DNP) was chosen as the atomic orbital basis set, and the smearing was set to 0.005 Ha to achieve precise electronic convergence. The convergence tolerances of energy, maximum force, and displacement were set to 2 × 10⁻⁵ Ha, 0.004 Ha Å⁻¹, and 0.005 Å, respectively, and a basis set a cut-off of 4.4 Å was employed to ensure high-quality computational results. The reciprocal space was sampled using a 3 × 3 × 1 k-point grid for geometry optimizations. A 2 × 2 × 1 supercell was used to calculate the electronic property and hydrogen evolution activity. For Mn-doped CoS₂, we used one Mn atom to replace one Co atom. To prevent interaction between two neighboring surfaces, a vacuum slab of 15 Å was employed in the (001) direction of CoS₂ for the calculations of hydrogen evolution activity. The free energy diagram for the HER was obtained by calculating the change of the free energy with a hydrogen atom adsorbed on the surface. The activity of the hydrogen evolution reaction was reflected by Gibb's free energy change (ΔG_H*) and the values of ΔG_H* are obtained using the following formula:

ΔG_H* = ΔE_H* + ΔE_ZPE − TΔS

ΔE_H* = E_surface/H* − E_surface − 1/2E_H2

where the E_surface/H* is the total energy of the surface model, E_surface is the total energy of the surface model with H* adsorption, E_H2 is the energy of a single H₂ molecule isolated in a vacuum. ΔE_ZPE is the zero-point energy change, which can be obtained from vibrational frequency calculation as implemented in DFT. TΔS is estimated to be 0.24 eV to consider the entropy change at room temperature.

The oxidation of hydrazine to nitrogen and hydrogen occurs in the following six consecutive elementary steps:

(1)

* + N₂H₄ → *N₂H₄

(2)

*N₂H₄ → *N₂H₃ + H⁺ + e⁻

(3)

*N₂H₃ → *N₂H₂ + H⁺ + e⁻

(4)

*N₂H₂ → *N₂H + H⁺ + e⁻

(5)

*N₂H → *N₂ + H⁺ + e⁻

(6)

*N₂ → * + N₂

where the asterisk (*) represents the reaction surface of CoS₂(001) and Mn-CoS₂(001). “*N₂H₄”, “N₂H₃”, “N₂H₂”, “N₂H”, and “*N₂” denote the models with the corresponding chemisorbed species residing in the reaction surfaces. Among these six steps, steps (1) and (6) are the adsorption of N₂H₄ and desorption of N₂, respectively. The other four steps involve the generation of one proton and one electron.

Clean nickel foam

Nickel foam with a three-dimensional skeleton was selected as a substrate for the growth of Mn-CoS₂ nanotubes. Before use, nickel foam was carefully cleaned with a dilute hydrochloric acid aqueous solution in an ultrasound bath for several minutes to remove the surface oxide layer. Subsequently, the nickel foam was washed successively with deionized water, acetone, and absolute ethanol. Finally, the cleaned nickel foam was obtained by drying at 60 °C overnight under vacuum conditions.

Synthesis of Mn-CoS₂

Mn-CoS₂ was prepared by a facile hydrothermal reaction and sulfuration method. Typically, Co(NO₃)₂·6H₂O (1.5 mM), NH₄F (3 mM), urea (7.5 mM), and MnCl₂·4H₂O (0.075 mM) were dissolved in 40 mL of deionized water and magnetically stirred for 60 min to form a homogeneous solution. After that, the obtained solution was transferred into a 50 mL Teflon-lined stainless steel autoclave with one piece of clean nickel foam (approximately 1 cm × 2 cm), which was sealed and maintained at 110 °C for 5 h in an oven and was naturally cooled down to room temperature. After cooling to room temperature, the nickel foam with the growth precursor was taken out, washed with water and ethanol, and dried in an oven at 60 °C for 12 h under vacuum conditions.

After that, the as-prepared precursor was put into a ceramic boat and placed in a tube furnace. Another ceramic boat containing 0.5 g of sublimed sulfur was put on the upstream side. After vacuum treatment and filling with high-purity Ar, the furnace was heated to 300 °C for 2 h with a heating rate of 1 °C min⁻¹ with a continuous Ar flow with a flow rate of 100 sccm. After the tube furnace was naturally cooled to ambient temperature, Mn-CoS₂ was obtained. For comparison, CoS₂ was also prepared by a similar approach except without the addition of MnCl₂·4H₂O. The areal loadings of the CoS₂ and Mn-CoS₂ catalyst on the nickel foam were confirmed to be 2.7 and 2.9 mg cm⁻², respectively. The Mn-CoS₂ catalyst with various Mn contents was optimized by adjusting the additive amount of precursor. All the chemicals were directly used after purchase without further purification.

Materials characterization

The crystallographic structure was analyzed by X-ray diffraction (XRD) using an X-ray diffractometer (SmartLab 9 kW) with Cu Kα radiation (λ = 0.154598 nm) at a scan rate of 10° min⁻¹ in the 2θ range from 30° to 70°. The morphology and structure of the prepared sample were characterized using a scanning electron microscope (SEM, Verios 460L) operated at 20 kV and a transmission electron microscope (TEM, Talos F200X) operated at 200 kV equipped with an energy-dispersive X-ray spectrometer (EDS). XPS (ESCALAB 250Xi from Thermo Scientific) was employed for elemental mapping using a monochromatic Al Kα radiation source at a background pressure of 10⁻⁹ Torr. The binding energy calibration was performed by referencing the C 1s main peak at 284.8 eV. Inductively coupled plasma atomic emission spectroscopy (ICP-OES) analysis was recorded on a Thermo iCAP RQ instrument.

Electrochemical measurements

Electrochemical measurements were carried out with a typical three-electrode system by using an electrochemical workstation (CHI Instruments 760E) at room temperature. A graphite rod and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The as-prepared catalysts were indirectly used as working electrodes. 0.1 M KOH and 1 M KOH containing 0.5 M N₂H₄ aqueous solution were used as electrolytes for the HER and HzOR, respectively.

Firstly, the HER and HzOR activity of the catalysts was investigated by linear scan voltammogram (LSV) with a scan rate of 10 mA s⁻¹. For comparison, the commercial noble metal Pt/C catalyst was measured under the same conditions. A stability test was performed using the chronopotentiometry–time and continuous cyclic voltammetry measurement technology in 0.1 M KOH. For hydrazine-assisted water electrolysis, a symmetrical full electrolyzer was assembled by directly using Mn-CoS₂ as the cathode and anode. LSV was measured at a scan rate of 10 mV s⁻¹ in 1.0 M KOH with 0.5 M N₂H₄ or 1 M H₂SO₄ with 0.5 M N₂H₄. For comparison, the commercial Pt/C catalyst coated nickel foam was measured under the same conditions. All the electrocatalytic current density was normalized to the geometric area of the nickel foam (1 × 1 cm²) and all potentials were given versus reversible hydrogen electrode according to the equation: E_RHE = E_Ag/AgCl + 0.0591pH + 0.197.

Electrochemical impedance spectra (EIS) were recorded at an open-circuit potential in a frequency ranging from 100 kHz to 10 mHz with an AC voltage amplitude of 5 mV. To determine the corresponding electrochemical surface area (ECSA), the double-layer capacitance (C_dl) of all the materials was measured in a 0.1 M KOH aqueous solution. A potential range where no apparent Faradaic process occurred was determined first using cyclic voltammetry (CV) with different scan rates of 2, 4, 6, and 8 mV s⁻¹. The value of C_dl was then obtained from the linear curve versus scan rate.

Results and discussion

To clarify the underlying origins between the catalytic activity and the hybrid structure, DFT calculations were applied to predict both HER and HzOR processes. Firstly, the stable (001) surface of the cubic CoS₂ model was built. As shown in Fig. 1a, the structural analysis reveals that CoS₂ has a unique crystal structure, where each Co is coordinated with six S atoms to form a slightly distorted octahedron, and the distorted octahedrons are connected by S–S covalent bonds. In the case of the Mn-CoS₂ catalyst, one Mn atom instead of one Co atom of CoS₂(001) is the theoretical calculation model (Fig. S1†).

Fig. 1 DFT calculation results: (a) crystal structure of CoS₂; (b) the calculated electronic band structure and the corresponding DOS results of Mn-CoS₂; (c) the free energy profiles of HER pathways for CoS₂ and Mn-CoS₂; (d) the charge density difference contour plot of Co–H and Mn–H regions, the red and blue colors represent the accumulation and depletion of electrons, respectively; (e and f) the side-view of the three-dimensional charge density difference of the H-adsorbed configuration at different sites over the catalyst surface, the pink and green region representing charge depletion and accumulation in the space, respectively; (g) the free energy profiles of the HzOR reaction pathways for CoS₂ and Mn-CoS₂.

Firstly, the electronic band structures of CoS₂ and Mn-CoS₂ are investigated. As shown in Fig. 1b, Mn-CoS₂ maintains the metallic nature as pristine CoS₂ (Fig. S2†), which would favor the electron transfer process in the electrochemical catalysis process.⁴⁷ It is well known that the free energy changes of hydrogen adsorption (ΔG_H*) are an effective descriptor for evaluating the HER activity of electrocatalysts. The catalysts with ΔG_H* close to zero are considered promising candidates for the HER. As shown in Fig. 1c, the pristine CoS₂ exhibits a ΔG_H* value of −2.03 eV, indicating the strong adsorption of H* on the CoS₂ surface with a bond length (Co–H) of 1.471 Å (Fig. S3†). After Mn doping, the bond length of Mn–H is extended to 1.552 Å, resulting in the ΔG_H* value of Mn-CoS₂ decreasing to −1.05 eV, which is much close to Pt(111), indicating its excellent thermoneutrality for the HER process.

To further analyze the change in the electron distribution of the Co–H region and Mn–H region for CoS₂ and Mn-CoS₂, the charge density difference (CDD) of the catalyst absorbed H* was studied as shown in Fig. 1d, where the red and blue colors represent the accumulation and depletion of electrons, respectively. As the CDD contour plots shown in Fig. 1d, much more electrons from the Mn atom are transferred to the adsorbed H* in Mn-CoS₂ than that of Co in CoS₂. In detail, the electrons around the H atom in Mn-CoS₂ are much more than that in CoS₂ due to the increased electron transfer, and the Mn site in Mn-CoS₂ is a relatively electron-rich state compared with the Co site in CoS₂, which is probably due to the electronic interaction between Mn and the surrounding atoms (Fig. S4†). These results indicate that the modulation of the electronic structure caused by Mn doping would synergistically promote the water adsorption strength and optimize the adsorption free energy of H*, which can facilitate the electrochemical HER process.³¹

Apart from the investigation of HER activity, the HzOR process over CoS₂ and Mn-CoS₂ surfaces was also explored using first-principles calculations. Firstly, the atomic structure adsorbed N₂H₄ molecule over the Co site of Mn-CoS₂ and Co site of CoS₂ was optimized. As shown in Fig. S5,† Mn-CoS₂ displays the strongest N₂H₄ adsorption with a more negative binding energy of −1.65 eV than that of CoS₂ (−1.48 eV). Furthermore, the charge density difference was also calculated. As the charge density difference contour plot is shown in Fig. 1e and f, it can be seen that much more electrons were transferred from the Mn site to adsorbed N₂H₄ and the charge redistribution was dominantly restricted around the Mn site, proving the strong adsorption of N₂H₄ on Mn-CoS₂. This electron localization behavior of the Mn-CoS₂ catalyst upon Mn doping could contribute to the enhanced catalytic activity.^43,48,49

The HzOR pathway on the catalyst is further studied. As shown in Fig. 1g, it can be found that adsorption of N₂H₄ on the Mn site of the Mn-CoS₂ surface is exothermic by −1.65 eV, lower than that of the Co site of the CoS₂ surface (−1.48 eV), indicating that the electron-deficient Mn species on Mn-CoS₂ would facilitate the adsorption of the N₂H₄ molecule.^50,51 Moreover, it has been revealed that the dehydrogenation process of *NH₂NH₂ to *NHNH₂ is the potential rate-limiting step (PLS) of the HzOR on the CoS₂ and Mn-CoS₂ surfaces, and the PDS barrier of Mn-CoS₂ (0.459 eV) is lower than that of CoS₂ (0.776 eV). Therefore, the Mn-CoS₂ catalyst is preferred for propelling the electrochemical HzOR process than CoS₂.

Motivated by the above promising prediction, we successfully synthesized Mn-doped CoS₂ nanotube supported nickel foam (Mn-CoS₂/NF). Fig. 2a illustrates the synthesis schematics of the Mn-CoS₂/NF. Before the growth of Mn-CoS₂, the commercial NF was cleaned with acid to remove the oxide layer. Then, the cleaned NF serves as the three-dimensional skeleton for the uniform growth of Mn-Co(OH)₂ nanoarrays by a facile hydrothermal synthesis at 200 °C for 6 h. Finally, the Mn-CoS₂ nanotubes were obtained by the sulfuration of Mn-Co(OH)₂ with sublimed sulfur as a S source at 250 °C for 2 h (please see the Experimental section for more details). For comparison, CoS₂ nanoarrays were also fabricated by a similar method except without the addition of a Mn precursor.

Fig. 2 Morphology and structural characterization of Mn-CoS₂: (a) schematic illustration of the synthesized process of Mn-CoS₂; (b–d) SEM images; (e and f) low-magnification TEM images; (g) high-magnification TEM image; (h–k) HAADF-STEM image and the corresponding EDS mapping.

Scanning electron microscopy (SEM) images show that the whole surface of NF is completely covered by interconnected Mn-CoS₂ nanoarrays (Fig. 2b–d). The as-prepared Mn-CoS₂ sample displays a nanotube morphology with a few hundred microns in length and a diameter of about 100 nanometers, as confirmed by transmission electron microscopy (TEM) (Fig. 2e and f). A high-resolution TEM image shows the visible lattice fringes with an equal interplanar distance of 2.72 Å that corresponds to the (200) plane of cubic CoS₂, revealing the high crystallization feature of the Mn-CoS₂ nanotubes (Fig. 2g). Furthermore, this nanotube structure is further explored by the elemental mapping images of the energy-dispersive X-ray spectroscopy (EDS) analysis. As shown in Fig. 2h–k, the Mn signals uniformly overlap with the Co and S signals in the as-prepared Mn-CoS₂ catalyst.

The crystallographic structures of the as-prepared CoS₂ and Mn-CoS₂ catalysts are also characterized by X-ray diffraction (XRD). As the XRD pattern is shown in Fig. 3a, the as-prepared CoS₂ and Mn-CoS₂ exhibit similar XRD diffraction peak positions. In detail, the diffraction peaks at 32.2°, 36.2°, 39.78°, 46.31°, 54.88°, 57.53°, 60.22°, and 62.6°, which correspond to the (200), (210), (211), (220), (311), (222), (023), and (321) planes of cubic CoS₂ (JCPDS no. 89-1492). In detail, it can be observed that the XRD diffraction peaks are shifted to a high angle relative to that of CoS₂ (Fig. S6†), which is caused by the incorporation of Mn atoms with a smaller atomic radius into the lattice of CoS₂ by replacing partial of Co atoms, confirming the successfully doping of Mn atoms into CoS₂.⁴⁴ Importantly, no Mn-related phase could be detected within the XRD detection range, indicating that a Mn-source was completely inserted in the CoS₂ lattice. The Mn-doping level is determined to be 2.1wt% by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Furthermore, X-ray photoelectron spectroscopy (XPS) measurements are employed to investigate the chemical state of the as-prepared catalysts. As shown in Fig. 3b and c, it can be found that CoS₂ and Mn-CoS₂ have similar high-resolution Co 2p and S 2p spectra, revealing that Mn doping does not affect the valence states of Co and S. In detail, as for the Co 2p spectra of Mn-CoS₂, the core-level peaks at binding energies of 778.7 eV, 782.3 eV, 793.6, eV and 798.6 eV correspond to Co³⁺ 2p_3/2, Co²⁺ 2p_3/2, Co³⁺ 2p_1/2, and Co²⁺ 2p_1/2, respectively.^26,52 Concerning the S region of Mn-CoS₂ (Fig. 3c), the signals at 162.6 and 163.9 eV are attributed to the binding energies of S 2p_3/2 and S 2p_1/2, respectively, which belong to the typical metal–sulfur bonds.^26,52 The peak at the binding energy of 168.7 eV could represent the S–O bond arising from surface air exposure.⁵³ Impressively, the Co²⁺ 3d_3/2 doublet of Mn-CoS₂ is shifted by ≈1.6 eV toward a higher binding energy relative to that of pristine CoS₂, whereas S 2p_3/2 is shifted by ≈0.3 eV toward a higher binding energy. These shifts might be attributed to that the Mn dopant can provide more electrons to S than Co after Mn doping. Moreover, the high-resolution Mn 2p XPS spectrum (Fig. 2c) displays two representative XPS characteristic peaks at 643.1 and 653.4 eV corresponding to the Mn 2p_3/2 and Mn 2p_1/2 with an Mn⁴⁺ oxidation state in Mn-CoS₂, respectively. Based on the above XRD and XPS analysis, it can be concluded that the Mn-CoS₂ catalyst was successfully synthesized.

Fig. 3 Structural characterization of the as-prepared CoS₂ and Mn-CoS₂: (a) XRD patterns; high-resolution XPS (b) Co 2p, (c) S 2p, and (d) Mn 2p spectra.

To demonstrate the facilitation of catalytic reactions that arise from Mn-doping, the as-prepared Mn-CoS₂ catalyst was evaluated for the HER with a three-electrode configuration in 0.1 M KOH electrolytes, where Mn-CoS₂ supported on NF was directly used as the working electrode, and the reference electrode and counter electrode were the Ag/AgCl electrode (filled with saturated KCl) and the carbon rod, respectively. Firstly, electrochemical HER catalytic activity was evaluated using linear scan voltammogram (LSV) with a scan rate of 10 mA s⁻¹. As the polarization curves are shown in Fig. 4a, the Mn-CoS₂ catalyst exhibits a negligible onset potential of 7 mV vs. RHE at a current density of −1 mA cm⁻² near the thermodynamic potential of the HER. Moreover, Mn-CoS₂ achieved a much lower overpotential of 46 mV at a current density of −10 mA cm⁻², which is much better than that of pristine CoS₂ (228 mV) and comparable to those of 20 wt% commercial Pt/C catalysts (Fig. 4b), Mn-CoS₂ with different Mn contents (Fig. S7†), and previously reported HER catalysts (Table S1†). Furthermore, Tafel plots were derived from the polarization curves to provide a deeper insight into the detailed mechanism of the HER of the as-prepared Mn-CoS₂ catalyst. As shown in Fig. 4c, a Tafel slope of 63.1 mV dec⁻¹ for the Mn-CoS₂ catalyst is close to that of Pt/C (38.2 mV dec⁻¹).

Fig. 4 HER performance measurements in 0.1 M KOH aqueous solution: (a) polarization curves of Mn-CoS₂ in comparison with CoS₂ and commercial Pt/C (20 wt%); (b) the activity of the Mn-CoS₂ catalyst at a current density of −10 mA cm⁻² with respect to the reference catalysts. (c) Tafel plots derived from the corresponding polarization curves. (d) The time-dependent overpotential curve of the Mn-CoS₂ electrode obtained at a constant current density of −10 mA cm⁻² for 54 h under alkaline conditions.

Furthermore, to gain insights into the interfacial charge transfer kinetics on the catalytic surface during the HER process, electrochemical impedance spectroscopy (EIS) was conducted. Fig. S8† shows the Nyquist plots of the CoS₂ and Mn-CoS₂ electrodes with a frequency from 100 kHz to 10 mHz. It can be found that the Nyquist plots present a typical semicircle in the high-frequency region and a straight line in the low-frequency region profiles, which can be ascribed to the charge-transfer resistance of the electrochemical H⁺ reaction at the electrode–electrolyte interface and the diffusion-controlled impedance, respectively.^54,55 Obviously, the Mn-CoS₂ electrode exhibits an enhanced electron transfer rate and faster catalytic kinetics during the HER process, eventually leading to the enhanced activity for the HER. To explore the enhanced HER activity of Mn-CoS₂ relative to CoS₂, electrochemical double-layer capacitances (C_dl) were measured. As shown in Fig. S9,† the cyclic voltammograms (CVs) of Mn-CoS₂ and CoS₂ at scan rates of 2, 4, 6, and 8 mV s⁻¹, respectively. The Mn-CoS₂ electrode achieved a C_dl value of 82.5 mF cm⁻², which is 4.12 times larger than that of CoS₂ (20.5 mF cm⁻²), implying a higher surface area and more exposed active sites for Mn-CoS₂, which is beneficial for enhancing the HER activity.^56,57

To assess the durability of the Mn-CoS₂ catalyst, the chronopotentiometry–time measurement technology was employed. As shown in Fig. 4d, it can be seen that the Mn-CoS₂ electrode can maintain a high stable overpotential at a current density of −10 mA cm⁻² for 54 hours with a negligible increase (Fig. 3e). Moreover, the long-term cycling stability of Mn-CoS₂ was investigated by performing continuous cyclic voltammetry (CV) at a scan rate of 100 mV s⁻¹ in 0.1 M KOH. As shown in Fig. S10,† the HER polarization curves showed a negligible difference between the curves at the initial and after 1000 cycles, implying the superior stability of Mn-CoS₂ in the long-term electrochemical process. Besides, the Mn-CoS₂ catalyst after the long-term measurement was further characterized by XRD, SEM, and TEM. As shown in Fig. S11,† the crystalline structure and morphology were well preserved after the long-term electrolysis, demonstrating the robust stability of Mn-CoS₂ in alkaline electrolytes.

Next, the electrochemical HzOR performances of the as-prepared catalyst were evaluated in 1.0 M KOH + 0.5 M N₂H₄ aqueous solutions. Meanwhile, the OER performance was also tested in a 1.0 M KOH aqueous solution. Firstly, the HzOR and OER catalytic activity was measured (Fig. 5a and Fig. S12†). As shown, the Mn-CoS₂ electrode exhibits a rapid increase in the HzOR activity in the alkaline electrolyte than that of the acidic one. The comparative polarization curves of the HzOR and OER suggest a dramatic improvement of activity after the addition of N₂H₄, indicating a great thermodynamic advantage of the HzOR. In detail, it only needs a working potential of 77 mV vs. RHE to achieve a current density of 10 mA cm⁻², which is much lower than that of the CoS₂ electrode (102 mV vs. RHE). In particular, the Mn-CoS₂ electrode possesses large current densities of 100 and 200 mA cm⁻² and only requires the potentials of 160 and 188 mV vs. RHE, which is much larger than CoS₂ (216 and 282 mV vs. RHE) and even better than the commercial Pt/C catalyst (161 and 262 mV vs. RHE), indicating that the outstanding HzOR activity of the Mn-CoS₂ electrode makes it a superior HzOR electrocatalyst, which is highly comparable to previously reported HzOR electrocatalysts (Table S2†). Fast charge transfer kinetics is further demonstrated by a small Tafel slope of 47.1 mV dec⁻¹, which is much smaller than that of CoS₂ (58 mV dec⁻¹) and much closer to that of Pt/C (41.5 mV dec⁻¹). In addition, the current density of 10 mA cm⁻² just changes slightly during a long-term test for 40 h (Fig. 5d) and the HzOR polarization curves showed a slight change between the curves at the initial and after 1000 cycles (Fig. S13†), demonstrating the high stability of the Mn-CoS₂ electrode in the HzOR for practical applications.

Fig. 5 HzOR and OER measurements: (a) HzOR polarization curves of Mn-CoS₂ in comparison with CoS₂ and commercial Pt/C (20 wt%) in 1 M KOH + 0.5 M N₂H₄ aqueous solutions and the OER–HzOR polarization curves in 1.0 M KOH aqueous solutions; (b) the activity of the Mn-CoS₂ catalyst at various current densities with respect to the reference catalysts; (c) HzOR Tafel plots derived from the corresponding HzOR polarization curves; (d) the time-dependent working potential curve of the Mn-CoS₂ electrode obtained at a constant current density of 10 mA cm⁻² for 40 h.

Considering the intrinsic activities of the Mn-CoS₂ catalyst, we envisioned that it could realize energy-saving H₂ production by utilizing Mn-CoS₂ as a bifunctional electrocatalyst toward the HER and HzOR. Fig. 6a shows the comparative LSV curves of overall hydrazine splitting (OHzS), in which significantly enhanced energy efficiency can be intuitively seen using hydrazine oxidation-assisted H₂ production. Specifically, the OHzS cell using Mn-CoS₂ as both the anode and cathode only demands 111, 329, and 447 mV to drive current densities of 10, 100, and 200 mA cm⁻², respectively (Fig. 6b), which are much lower than the overall water-splitting theoretical value of 1230 mV. By contrast, CoS₂ (405, 933, and 1227 mV) and commercial Pt/C (86, 319, and 467 mV) catalysts require a higher voltage to reach up to the current densities of 10, 100, and 200 mA cm⁻² (Fig. 6b). Besides, the bifunctional Mn-CoS₂ catalyst in the OHzS system can maintain higher voltage stability at various current densities (10 and 50 mA cm⁻²) during a continuous 24 h test (Fig. 6c). Moreover, the OHzS stability test of the Mn-CoS₂ electrode was also performed for 24 h at a cell potential of 111 mV. As shown in Fig. 6, the current density retention can be determined to be 96.5%. The Mn-CoS₂ electrode is highly comparable to previously reported state-of-the-art OHzS electrocatalysts (Table S3†). These results demonstrate that the Earth-abundant Mn-CoS₂ catalyst as a bifunctional electrode for the HER and HzOR can efficiently save energy for H₂ production by water-assisted electrolysis.

Fig. 6 OHzS measurements: (a) OHzS polarization curves of Mn-CoS₂ in comparison with CoS₂ and commercial Pt/C (20 wt%) in 1 M KOH + 0.5 M N₂H₄ aqueous solutions; (b) the OHzS activity of the catalysts at various current densities; (c) the time-dependent working potential curve of the Mn-CoS₂ electrode for OHzS obtained at various current densities for 24 h.

Conclusions

In summary, we designed and prepared highly active Mn-doped CoS₂ nanotubes supported nickel foam for boosting the electrochemical H₂ evolution reaction by replacing the sluggish OER with a thermodynamically more favorable HzOR. DFT calculations manifest that the incorporation of Mn into the CoS₂ lattice realizes the modulation of the electronic structure and charge distribution on the catalyst, which significantly decrease the hydrogen adsorption Gibbs free energy for the HER and the potential rate-limiting step for the HzOR process. The experimental results demonstrated that the Mn-CoS₂ catalyst exhibited excellent catalytic activity, which can achieve 10 mA cm⁻² at a overpotential of 46 mV vs. RHE for the HER and 77 mV vs. RHE for the anodic HzOR, respectively. In addition, the Mn-CoS₂ electrode only needs a cell voltage of only 447 mV to output 200 mA cm⁻¹ in the OHzS system and exhibits robust long-term H₂ production. It is expected that this work can extend the pathway of the design for high-efficiency electrochemical H₂ evolution catalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22075211, 21601136, 51971157, 62005173, and 51621003), the Zhejiang Provincial Natural Science Foundation of China (LR19B060002), the Research Funds of Institute of Zhejiang University-Quzhou. The Guangdong Province Higher Vocational Colleges & Schools Pearl River Scholar Funded Scheme (2016), the Guangdong Third Generation Semiconductor Engineering Technology Development Center (2020GCZX007), the Science, Technology, and Innovation Commission of Shenzhen Municipality (RCBS20200714114818140), the China Postdoctoral Science Foundation (2019M663118), and the School level Scientific Research Project of Shenzhen Institute of Information Technology (PT2019E002). We are grateful for the HZWTECH for providing computation facilities.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: The optimized structural model, electronic band structure, DOS, charge density difference, adsorption energy, XRD pattern, EIS, CV, ECSA, LSV, and i–t curves. See DOI: https://doi.org/10.1039/d2qi00083k

‡ These authors contributed equally to this work.

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