Sayali Ashok
Patil
*a,
Pallavi Bhaktapralhad
Jagdale
a,
Asif
Iqbal‡
b,
Samim
Reza‡
b,
Mallamma
Jinagi
a,
Parasmani
Rajput
cd,
Amanda
Sfeir
e,
Sébastien
Royer
ef,
Ranjit
Thapa
b,
Akshaya Kumar
Samal
a and
Manav
Saxena
*a
aCentre for Nano and Material Sciences, Jain (Deemed-to-be University), Jain Global Campus, Ramanagara, Bangalore 562112, India. E-mail: s.manav@jainuniversity.ac.in; manavsaxena19@gmail.com; ap.sayali@jainuniversity.ac.in; sayalipatil931997@gmail.com
bDepartment of Physics, SRM University-AP, Andhra Pradesh, 522 240, India
cBeamline Development & Application Section, Bhabha Atomic Research Centre, Mumbai 400085, India
dHomi Bhabha National Institute, Anushaktinagar, Mumbai, 400094, India
eUniversité de Lille, CNRS, Centrale Lille, Université Artois, UMR 8181-UCCS-12 Unité de Catalyse et Chimie du Solide, Lille 59000, France
fUniversité du Littoral Côte d'Opale, Ecole d'Ingénieur du Littoral Côte d'Opale, UCEIV, UR 4492, MREI 1, 189 A, Avenue Maurice Schumann, 59140 Dunkerque, France
First published on 28th April 2025
Advancing energy storage and conversion research on 2D nanostructures hinges on the critical development of bifunctional electrodes capable of effectively catalyzing oxygen evolution reactions and facilitating charge storage applications. Although metal oxide materials have been shown to be promising electrode materials for energy storage and conversion, an easy and reliable synthesis strategy for achieving a 2D morphology to fully utilize their electrochemical potential has not yet been achieved. Herein, we report the synthesis of NiMoO4 self-assembled, ultrathin nanosheets through ionic layer epitaxy with precise control over the Ni:
Mo composition ratio. X-ray absorption spectroscopy reveals a uniform radial distance shift in NiMoO4, indicating the homogeneous distribution of Ni and Mo in equal proportions. The optimized 1
:
1 NiMoO4 nanosheet device exhibits a high areal capacitance of 4.93 mF cm−2 with promising stability (20
000 cycles). Furthermore, the OER activity of ultrathin 1
:
1 NiMoO4 exhibits an overpotential (η10) of 318 mV and a Tafel value of 51 mV dec−1, suggesting fast reaction kinetics. This investigation reveals a promising possibility for developing high-performance electrode materials using 2D metal oxides, thereby achieving high material efficiency.
Bifunctional materials attract more attention due to their unique intrinsic properties compared to those focused on a single function. Thus, engineering such materials at the nanoscale could be an excellent possibility for energy conversion and storage applications. In this context, naturally abundant transition metal oxides (TMOs) with precious metal-free compositions are highly promising due to their remarkable properties, including charge storage capability, the ability to facilitate oxygen evolution from water, high reactivity with metal ions, electrochemical stability, and cost-effectiveness.9 Furthermore, several reports on materials containing more than one cationic component are available due to their enhanced electrical conductivity and robustness resulting from strong and metal correlations. To date, numerous metal oxides containing transition metals as core cationic components like NiCo2O4,10 ZnCo2O4,11 NiMoO4,12 ZnV2O4,13 and ZnO,14 have been effectively employed as bifunctional electrodes in energy storage and conversion devices but always prepared and modified separately.15,16 Besides, their intricate synthesis process and potential trade-offs in performance between the two functions could limit overall device efficiency.
Various research groups have extensively studied NiMoO4 due to its affordability, abundance, chemical stability, and eco-friendly nature. Additionally, the material boasts high electrical conductivity from molybdenum and significant electrochemical activity from nickel, enabling it to exhibit a high specific capacity.17–19 Recently, Sawant et al. developed NiMoO4 based hybrid supercapacitor to study the ion diffusion and transport mechanisms through Density Functional Theory (DFT) analysis. A comprehensive computational approach incorporating structural optimization, self-consistent field calculations, density of states (DOS) analysis, and band structure calculations was employed to achieve a thorough understanding of NiMoO4's electronic characteristics. This integrated theoretical and experimental strategy underscores the potential of NiMoO4 for high-performance energy storage applications.20 These studies focus on synthesizing various nanostructured NiMoO4 and evaluating their electrochemical performance. Nevertheless, few studies have elucidated the relationship between synthesis conditions, phase structure, and the impact of varying metal precursor ratios, which influence the electrochemical properties of NiMoO4. The present study focuses on synthesizing NiMoO4 using various metal precursor ratios to explore how these variations influence the material's physicochemical properties and their correlation with its electrochemical performance. The primary goal is to investigate how different ratios of metal ions affect the phase structure and electrochemical behavior of NiMoO4 in charge storage and conversion applications, ultimately establishing a clear relationship between structure and properties. The robust electronic and atomic interactions within NiMoO4 have significantly enhanced its catalytic performance.21 Furthermore, improving the materials' structure, morphology, and architecture can enhance the exposure of electroactive sites and facilitate mass transport. The quasi-two-dimensional (2D) configuration is an ideal electrode material for energy storage and conversion devices. Its ultra-thin nanoscale thickness and bulk-scale lateral dimensions offer remarkably high surface-to-volume ratios, which improve electrochemical reaction efficiency and boost charge storage capacity by facilitating rapid ion exchange between the electrode and the electrolyte. Additionally, its large surface-to-volume ratio maximizes exposure to active catalytic sites, enhancing interfacial charge transfer and facilitating electrocatalytic reactions.8 Despite the great potential of 2D morphology, achieving a nanometer-thick 2D bimetallic electrode material remains a significant challenge. Recent advancements have shown promising developments in overcoming this challenge using the ionic layer epitaxy (ILE) technique.8,14,22–24 ILE is a kinetically controlled synthesis strategy that utilizes an ionized surfactant monolayer to form an electrical double layer, thereby self-confining crystal growth within a 2D-nanometer regime. Compared to methods like rapid reduction, commonly used for metallic nanosheets, ILE offers superior thickness control down to one-unit cell. Moreover, it boasts versatility in material selection for forming 2D structures. However, due to the use of amphiphilic templates and low-temperature solution growth, ILE nanosheets often exhibit defects and impurities, necessitating post-synthesis treatments for specific applications.25,26
In this study, we synthesized ultrathin quadrangle-shaped nanosheets of bimetallic NiMoO4 with varying Ni/Mo ratios using the interfacial layer epitaxy (ILE) method, serving as a bifunctional electrode material for charge storage and oxygen evolution reactions (OER). The 1:
1 ratio of NiMoO4 nanosheets exhibited a quadrangular shape and an average thickness of ∼17 nm. In symmetric device configuration, nanosheets with a Ni/Mo ratio (1
:
1) exhibited an areal capacitance of 4.93 mF cm−2 at a current density of 0.3 mA cm−2 at ultra-low mass loading. The device exhibits outstanding cyclability after 20
000 subsequent charge–discharge cycles, with no notable change in surface chemical structure. Furthermore, 1
:
1 NiMoO4 on an SS substrate outperformed as an excellent OER electrocatalyst, exhibiting a low overpotential (η) of 318 mV at a 10 mA cm−2 current density, along with a low Tafel value of 51 mV dec−1, and demonstrating excellent operational stability of 13 hours. This work presents a promising strategy for the rational design and synthesis of ultrathin charge storage devices and high-performance electrocatalysts from earth-abundant bimetallic materials.
E(RHE) = E(Ag/AgCl) + 0.059 × pH + 0.197 V − iR | (1) |
η = E(RHE) − 1.236 V. | (2) |
All the potentials reported here were iR-compensated potentials where “i” represents the capacitive current and “R” represents the uncompensated ohmic electrolyte resistance obtained from the Nyquist measurements. The electrochemical impedance spectroscopy was carried out in a frequency region of 100 kHz to 0.1 Hz at an applied potential of 0.589 V (vs. Ag/AgCl) by employing a 5 mV bias to the catalysts. Further, Tafel plots were obtained from the polarization curves, and the Tafel slope was calculated from the following Tafel equation.
η = a + b![]() ![]() | (3) |
Furthermore, atomic force microscopy (AFM) was employed to assess the surface smoothness and thickness of the assembled nanosheets. Two-line profile at different areas shows that the 2D nanosheets self-assembled to give a 3D structure with 17 nm thickness (Fig. 1d). The surfaces of the nanosheets appeared uniformly flat, with a roughness factor (Ra) of 0.9 nm, indicating a smooth and even surface texture throughout. X-ray photoelectron spectroscopy (XPS) was performed to investigate the composition and chemical states of the nanosheets. The XPS survey spectrum illustrated characteristic peaks corresponding to Ni, Mo, O, C, N, and Pt elements. The Pt and C signals originated from the substrate and adventitious carbon/residues, respectively (Fig. S1a†). Further, detailed high-resolution spectra of Ni 2p (Fig. 1e) and Mo 3d (Fig. 1f) exhibited distinct split orbital doublets, specifically 2p3/2 and 2p1/2, occurring at 855 eV and 872.6 eV for Ni, and 3d5/2, 3d3/2 at 231.9 eV and 235.1 eV for Mo, respectively resulting in spin–orbit splitting of Ni2+ and Mo6+, is measured at 17.6 eV and 3.14 eV, respectively. The high-resolution Ni 2p spectrum exhibits deconvoluted peaks at 854.9, 856.1, 872.6, and 874.7 eV, which are attributed to Ni2+ species. Additionally, corresponding shake-up satellite features are observed at 861.6, 866.8, 879.5, and 882.5 eV, further corroborating the presence of Ni2+ oxidation states.27 Meanwhile, Mo 3d spectrum shows that two characteristic peaks were observed at 231.9 eV and 235.1 eV, associated with Mo 3d5/2 and Mo 3d3/2, respectively, indicating that Mo is primarily present in its +6 oxidation state.8 Additionally, the O 1s spectrum can be deconvoluted into three peaks. The primary peak at 530.6 eV is attributed to the lattice oxygen in the metal (Ni/Mo)–oxygen framework. A peak at 531.4 eV corresponds to the oxygen in –OH groups, indicating surface hydroxylation of the compound. The very weak peak at 532.0 eV is related to adsorbed water at or near the surface (Fig. S1b†).8
Further, X-ray Absorption Near-Edge Structure (XANES) measurements were performed to gain deeper insights into the valence states and local coordination environments of the metal atoms in optimized 1:
1 NiMoO4 (Fig. 1g and h). The Ni K-edge XANES spectrum for NiMoO4 exhibits a distinct peak at 8345 eV, indicating the presence of Niδ+ with δ ≥ 2 (Fig. 1g).28 This spectrum exhibits a positive binding energy shift of 7 eV relative to the Ni0 peak at 8333 eV in the Ni foil. The Fourier transform (FT) EXAFS spectra of NiMoO4 show a Ni–O first-shell scattering path at 1.8 Å. Furthermore, a peak at 2.7 Å is associated with Ni–Ni or Ni–Mo bonds in NiMoO4, closely resembling the 2.5 Å Ni–Ni bond length observed in Ni foil (Fig. 1i).28
For the Mo K-edge, the spectrum for NiMoO4 shows a negative shift, placing the binding energy at 19994 eV as compared to the 20
000 eV Mo0 peak of the Mo foil reference (Fig. 1h). The Mo K-edge EXAFS spectrum for NiMoO4 reveals a distinct Ni–Mo bond at 2.3 Å, which is shorter than the Mo–Mo bond distance of 2.4 Å in Mo foil but longer than the Mo–O bond at 1.7 Å, which confirm the formation of structure (Fig. 1j).29 Furthermore, the uniform radial distance shift in NiMoO4 can be attributed to the equal proportions of Ni and Mo. The Mo–O bond distance in NiMoO4 is greater than that of MoO3 due to differences in their coordination environments.
Transmission electron microscopy (TEM) was utilized to investigate the morphology and crystal structure of the nanosheets. The Fig. 2a–d illustrates the edge of a single quadrangular nanosheet, where the exposed edge reveals a layered structure with aligned 2D layers to give a thick 3D structure. Selected area electron diffraction (SAED) patterns obtained from several nanosheets exhibited a broad and diffuse diffraction ring, indicating that the NiMoO4 nanosheets possess a nanocrystalline structure with low crystallinity (Fig. 2e).8 The nanocrystalline structure was revealed by the high-resolution TEM (HRTEM) image, where irregular crystal lattices with sizes of ∼1–3 nm could be observed (Fig. 2f). The absence of well-defined lattice fringes is attributed to the extremely small crystallite size and partial structural disorder, which limit the long-range periodicity necessary for clear fringe formation.8 The elemental mapping of Ni, Mo, and O confirms a uniform distribution of these elements across the surface of the nanosheets, which supports the integrity of the chemical structure (Fig. 2g–j).
The Ni/Mo ratio in bimetallic oxide is a crucial factor in determining its electrochemical properties. Here, various molar ratios of Ni and Mo precursors (1:
3, 1
:
1, and 3
:
1) were used to synthesize NiMoO4 nanosheets with different Ni/Mo ratios. Fig. 3a–f displays the FESEM images of the synthesized NiMoO4 nanosheets at molar ratios of 1
:
3, 1
:
1, and 3
:
1, respectively.
The 1:
3 ratio of NiMoO4 exhibits a mixed morphology of quadrangle nanosheets and nanorods, whereas the 1
:
1 and 3
:
1 ratio of NiMoO4 displays only quadrangle nanosheets. The size of the nanosheet substantially increased with the rise in the molar ratio of Ni to Mo from 1
:
3 to 3
:
1, which is attributed to the higher Ni content in the precursors at the 3
:
1 ratio. Additionally, X-ray diffraction (XRD) experiment was conducted to understand the internal structure of all optimized ratios of NiMoO4 (1
:
3, 1
:
1, and 3
:
1). The XRD patterns for all ratios display similar trends (Fig. 3g–i), with peaks corresponding to the (−111), (−202), (112), (241), (−424), and (−351) planes, which are consistent with the monoclinic NiMoO4 crystalline structure (JCPDS No. 01-086-0361). Moreover, an additional peak indexed to (101) in the 1
:
3 NiMoO4 and (133) in the 3
:
1 NiMoO4 corresponds to molybdenum oxide (JCPDS No. 01-055-0506) and nickel oxide hydroxide (JCPDS No. 01-078-2343), respectively. For comparison, monometallic Mo–O and Ni–O were also synthesized using the same method but without adding the second metal precursor. In the controlled sample Ni–O (Ni/Mo = 1
:
0), three peaks were observed at (100), (211), and (214), corresponding to a mixed phase of nickel hydroxide (JCPDS No. 01-073-1520) and nickel oxide hydroxide (JCPDS No. 01-078-2343). Meanwhile, the XRD pattern of the controlled sample Mo–O (Ni/Mo = 0
:
1) shows peaks assigned to MoO3 (JCPDS No. 33–0664), as shown in Fig. S2.† To understand the relationship between the precursor ratio and the as-synthesized nanosheet composition, inductively coupled plasma-optical emission spectroscopy (ICP-OES) was carried out to measure the atomic contents of NiMoO4 nanosheets synthesized with varying Ni-to-Mo ratios. Using a standard Ni solution at two different concentrations, the atomic percent content of Ni was found to be approximately 34% and 50% for the two NiMoO4 samples (1
:
3 and 1
:
1), respectively, which is close to the reaction concentration. As summarized in Table ST1,† the Ni-to-Mo ratio in the synthesized nanosheets is approximately similar to the precursor ratio. This confirms that precursor concentration adjustments effectively control the elemental composition of the nanosheets. However, for the 3
:
1 precursor ratio, the Ni content was 48%, suggesting that excess nickel was not fully incorporated into the NiMoO4 lattice. To further support this, enthalpy of formation (ΔHf) was calculated for all the ratios, which are 1
:
3 NiMoO4 possess ΔHf = −4.36 eV while 1
:
1 NiMoO4 exhibits ΔHf = −4.00 eV and 3
:
1 NiMoO4 discloses ΔHf = −2.97 eV. This study suggests that the 1
:
3 NiMoO4 structure is the most stable due to its lowest ΔHf, while stability decreases with an increase in Ni amount, ultimately indicating that the formation of 3
:
1 NiMoO4 is the least probable. Furthermore, XRD analysis supports this conclusion, as the diffraction patterns of 3
:
1 NiMoO4 appear similar even with increased nickel concentrations, indicating that NiMoO4 remains the dominant phase despite the excess Ni content. To keep the descriptions consistent, the nanosheets discussed further are referred to by their precursor ratios.
Ni2+ ↔ Ni3+ + e− |
It is important to highlight Mo does not participate in the actual reaction, instead remaining in the MoO42− form as illustrated by the absence of Mo redox peaks in the CV curves as shown in Fig. 4a. To determine the electrochemical performance of the NiMoO4, we have computed areal charge storage for all the electrode materials at a sweep rate of 100 mV s−1. As shown in Fig. 4b, 1:
1 NiMoO4 retains higher areal charge storage of 11.2 mC cm−2 than other optimized electrode materials, revealing that 1
:
1 NiMoO4 has superior electrochemical capabilities. Analogously, GCD analysis was conducted under identical conditions, maintaining a constant current density of 0.6 mA cm−2 within the potential range of 0–0.45 V (vs. Ag/AgCl), as depicted in Fig. 4c. The GCD curves demonstrated similar trends to those seen in the CV curves. The 1
:
1 NiMoO4 electrode outperforms other optimized electrodes in terms of charging and discharging time, indicating that it possesses superior capacitive characteristics. To gain deeper insights into the rate kinetics of the electrode material, we conducted electrochemical impedance spectroscopy (EIS) measurements for all optimized electrode ratios in a frequency range of 0.1 Hz to 100 kHz. The obtained EIS results with an equivalent circuit are depicted in Fig. 4d. In the Nyquist plot, the start of the semicircle indicates the equivalent series resistance (Rs), while the diameter of this semicircle reflects the charge transfer resistance (Rct). The series resistance is the result of the cumulative resistances from the electrolyte ions, the intrinsic resistance of the electrode material, and the ohmic contact resistance at the interfaces between the electrode, electrolyte, and current collector. The Warburg impedance is attributed to the diffusion of ions from the bulk electrolyte to the electrode surface, while the double-layer capacitance arises from the non-faradaic charge accumulation at the electrode–electrolyte interface. Low values of Rs and Rct are typically preferred for facilitating fast and efficient electrochemical reactions. The 1
:
1 NiMoO4 electrode material demonstrated significantly lower Rs (1 Ω) and Rct (0.8 Ω), indicating higher electrical conductivity than other electrode materials. This suggests efficient electrochemical reactions between electrolyte ions and the active electrode material. At low frequencies region reflects the electrolyte ion penetration efficiency into the nanosheets, facilitated by its ultrathin thickness. The steep slope (approximately 45°) in the lower frequency range suggests easy ion penetration through the ultrathin thickness of the nanosheets. Comparative Rct values for all the optimized electrode materials are summarized in Table ST2.†
The excellent electrochemical performance is attributed to the enhanced electrical conductivity of the Mo-based compound, the synergistic effect of transition metals, the 2D nanostructure of the electrode materials, which offer abundant active sites for fast and efficient electrochemical reactions, and the direct deposition of the electrode material onto the current collector improves electrical conductivity and eliminate unnecessary inactive surface areas. At lower concentrations of nickel 1:
3 NiMoO4, a minimal amount of MoO3 is formed, which diminishes the beneficial effect of the Ni–Mo interaction. With an increased nickel concentration (1
:
1 NiMoO4), the electrochemical performance improves due to the high theoretical capacitance of nickel and the presence of a 2D nanosheet structure. However, this trend does not persist at higher nickel concentrations. In the case of 3
:
1 NiMoO4, a minimal amount of non-reactive nickel oxide hydroxide phase forms, which reduces the beneficial effect of the Ni–Mo interaction. Consequently, the electrochemical performance of the material decreases. Computational studies further support this observation, which reveals that the 1
:
1 NiMoO4 ratio exhibits the highest charge storage among the other ratios. Quantum capacitance value for different models (i.e. 1
:
1 NiMoO4, 1
:
3 NiMoO4 and 3
:
1 NiMoO4) with the variations of applied electrode potential (U) from −0.6 V to +0.6 V, is represented in Fig. 4e. The quantum capacitance value for 1
:
1 NiMoO4 varies from a maximum value of 838 μF cm−2 at electrode potential U = −0.6 V to minimum value of 231 μF cm−2 at U = 0 V. On the other hand, the quantum capacitance value for 1
:
3 NiMoO4 varies from a maximum of 437 μF cm−2 at U = −0.6 V to a minimum of 76 μF cm−2 at U = 0 V, and for 3
:
1 NiMoO4 varies from a maximum of 485 μF cm−2 at U = −0.6 to minimum 312 μF cm−2 at U = 0 V. The density of states (DOS) analysis reveals that the 1
:
1 NiMoO4 contains a higher value of electronic states near the Fermi level than the other two models. This can be the reason for the higher supercapacitor performance of 1
:
1 NiMoO4 than the others. Thus, this theoretical analysis sheds light on the possible underlying reason for the equal ratio of Ni and Mo in NiMoO4, which improves the supercapacitor and OER performance.
According to the above results, the 1:
1 NiMoO4 electrode exhibits a superior energy storage capacity compared to other optimized electrode materials. Additional electrochemical measurements were conducted using CV and GCD techniques at various scan rates and current densities to assess its rate capability. Fig. 4f illustrates the CV curves of the 1
:
1 NiMoO4 electrode at various scan rates ranging from 5 to 100 mV s−1. These curves exhibit distinct redox peaks at both the anodic and cathodic sides within the potential window of 0 to 0.45 V/Ag/AgCl at all scan rates, suggesting significant charge storage resulting from Faradaic reactions of the electrode material. Notably, the shape of the CV curves remains the same at both low and high scan rates, indicating excellent electrical conductivity and superior rate capability of the electrode material. Fig. S4† demonstrates a linear correlation between the redox peak current and the square root of the scan rate, suggesting excellent electrochemical reversibility of the material. Fig. 4g depicted the areal charge stored of 31.0, 24.5, 19.4, 15.3, 12.1, 11.2 mC cm−2. CV curves were further studied in depth to understand the contribution of kinetic behavior and surface functionality to the charge storage mechanism of the electrode materials. The correlation between the peak current (i) and the scan rate (v) was examined using a power law:9
i = aνb |
Furthermore, the comparative contributions of surface-controlled and diffusion-controlled mechanisms to the overall current response for each scan rate were quantified using Dunn's equation.22
i = k1ν + k2ν1/2 |
Fig. 4k shows the GCD curves of the 1:
1 NiMoO4 electrode at various current densities, ranging from 0.3 to 1 mA cm−2. The nonlinear behavior observed at all current densities corresponds with the redox peaks observed in the CV curves. The negligible potential drop, even at high current densities, reveals the superior electrical conductivity of the electrode material, ensuring its suitability for achieving high performance. Due to the battery-type behavior of the electrode material, the areal capacity was computed based on the discharge time of the GCD curve. Obtained areal capacity values are 4.1, 3.7, 3.3, 3.1, 2.9, 2.7, 2.5, and 2.3 μA h cm−2 at current densities of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0 mA cm−2, respectively as shown in Fig. 4l. With increasing the current density areal capacity is decreases gradually attributed to a limited time available for electrolyte ions to undergo electrochemical reactions resulting in predominantly surface-based reactions are occurring at higher current densities whereas, at low current density electrolyte ions get enough time to diffuse into the electrode material, allowing the entire electrode to participate in electrochemical reactions. Considering the superior electrochemical properties of 1
:
1 NiMoO4, a symmetric device was assembled to assess its viability at the device level. In a symmetric device, 1
:
1 NiMoO4 is used as both the negative and positive electrodes, separated by Whatman filter paper soaked in 2 M KOH electrolyte. Initially, we determined the optimal working voltage range for the symmetric device by conducting CV measurements in various voltage windows at a consistent scan rate of 100 mV s−1. As illustrated in Fig. 5a, CV curves at all potential windows exhibit oxidation–reduction peaks, indicating the pseudocapacitive nature of the symmetric device. With an increasing potential window, the current response also increases. When widening the potential window, enhancement in the current response is also observed. Nevertheless, by increasing the potential window to 1.0 V, polarization occurred due to oxygen evolution in the electrolyte. Therefore, 0.8 V was chosen as the optimal operational potential window for further electrochemical analysis of a symmetric supercapacitor (NiMoO4-Pt//NiMoO4-Pt). Fig. 5b shows the CV curve at different scan rates from 5 to 100 mV s−1 at a fixed potential window of 0–0.8 V. The shape of the CV curves remains the same at both low and high scan rates, indicating that the fabricated symmetric device exhibits excellent pseudocapacitive characteristics with high reversibility. Like the three-electrode configuration, an increase in the scan rate resulted in a higher current response, indicating favorable I–V characteristics and low internal resistance within the cell. The charge storage mechanism in NiMoO4 is predominantly driven by surface or near-surface faradaic redox reactions involving hydroxide ions (OH−) in the alkaline electrolyte. During charging, OH− ions facilitate the redox transition of Ni2+/Ni3+, leading to the formation of NiOOH, while Mo6+ remains electrochemically inactive within the operational potential window. Due to its stable oxidation state, Mo does not undergo reduction during the reverse scan, instead contributing to improved electronic conductivity and structural stability of the electrode. The electrochemical process involves the intercalation of OH− ions during charge and their subsequent deintercalation during discharge, marking the transition between oxidized and reduced states, as depicted in Fig. 5c.33
Fig. 5d illustrates the GCD curves of the NiMoO4-Pt//NiMoO4-Pt symmetric device at different current densities from 0.3 to 1 mA cm−2. It is observed that all GCD curves exhibit nearly symmetrical shapes, indicating good capacitor behavior along with high coulombic efficiency. The non-linear behavior and longer charging and discharging times of the GCD curve suggest the symmetry device has good energy storage capability. Moreover, there is no potential drop during discharge, even at high current densities, which signifies exceptional electrical conductivity within the electrode and electrolyte, indicating outstanding compatibility. The areal capacitance and capacity values calculated for NiMoO4-Pt//NiMoO4-Pt symmetric device are 4.93 (1.1), 3.25 (0.72), 2.65 (0.58), 1.2 (0.26), 0.96 (0.21), 0.92 mF cm−2 (0.2 μA h cm−2) at current densities of 0.3, 0.7, 1, 4, 8, 10 mA cm−2, respectively as depicted in Fig. 5e. The outstanding charge storage capability observed can be ascribed to the fast redox reactions and the balanced kinetics of both electrodes. Electrochemical impedance spectroscopy (EIS) was employed to assess the carrier transport efficiency of the symmetric device, as depicted in Fig. 5f. The Nyquist plot displayed linear and vertical characteristics, indicating excellent supercapacitor performance. A low equivalent series resistance (ESR) value of 3 Ω corroborates low internal resistance and high-rate capacitive behavior. The absence of a semicircle in the Nyquist plot at the high-frequency region suggests small charge-transfer resistance, likely due to the large surface area, which reduces contact impedance between electrodes and electrolytes. The charge storage and delivery efficiency in the NiMoO4-Pt//NiMoO4-Pt symmetric device was assessed by calculating the areal energy density and power density based on the GCD curve at different applied current densities from 0.3 to 10 mA cm−2 and plotted in the Ragone plot (Fig. 5g). The symmetric device achieves areal energy density of 0.44 μW h cm−2 at areal power density of 0.24 mW cm−2. The NiMoO4-Pt//NiMoO4-Pt symmetric device achieves a high areal power density of 8 mW cm−2 without significantly compromising the areal energy density of 0.08 μW h cm−2. Moreover, the areal energy and power density values of the NiMoO4-Pt//NiMoO4-Pt symmetric device surpass those of reported nanostructured-based symmetric devices, as depicted in Fig. 5g on the Ragone plot.34–41
Further, self-discharge is a common phenomenon in supercapacitors, which results in a gradual decline in voltage and consequently impacts the overall energy density of the device. To evaluate this behavior, the device was charged to 0.8 V, and the open-circuit voltage was monitored over time. As shown in Fig. 5h, the voltage decay profile indicates that the device retained 90% of its initial potential within the first 60 seconds. Even after 1000 seconds under open-circuit conditions, it preserved approximately 50% of its original voltage, demonstrating excellent self-discharge characteristics and suitability for powering various miniaturized electronic applications. To determine the recycling capability and efficiency of the proposed symmetric device for a real-world application, the long-term cyclic stability and coulombic efficiency were assessed by consecutive charge–discharge cycles at a current density of 1 mA cm−2 as shown in Fig. 5i. The NiMoO4-Pt//NiMoO4-Pt symmetric device demonstrates remarkable cyclic stability of 80% of their initial capacity over 20000 cycles, as illustrated in Fig. 5i. During the initial 5000 cycles, an increase in capacity was observed due to the self-activation of the electrode material, resulting from extensive ion intercalation/de-intercalation. Additionally, the improvement in electrode surface wettability during the early charge–discharge process facilitates better electrolyte penetration and enhances the accessibility of electrochemically active sites, thereby contributing to the observed enhancement in capacitance retention.42,43 Furthermore, the electrochemical stability of the device is confirmed by comparing the first and 20
000th GCD cycles, as shown in inset Fig. 5i. The nearly symmetrical shape of the charge–discharge cycles indicates a high coulombic efficiency attributed to the reversible redox reactions of the device. These GCD cycles also indicate a slight capacity loss, which is offset by the device's excellent long-term cyclic stability, extending up to 20
000 cycles. Moreover, the initial coulombic efficiency of the device, which starts at 91%, improves after several GCD cycles and remains stable at 97% even after 20
000 cycles. This enhancement can be attributed to the improved penetrability of the active material and the reduction in electrolyte ionic resistance over the course of the cycles. As a result, this leads to shorter charge times and increased coulombic efficiency.44 FESEM images showed that after 20
000 continuous GCD cycles, the size and shape of the nanosheets had changed slightly without resulting in detachment or peeling (Fig. S5a†). The accumulation of KOH electrolytes on the nanosheet was observed after the electrochemical reactions (Fig. S5b†). The nanosheets' uniform thickness and ultrafine nature are believed to be beneficial for achieving uniform and ultrasmall out-of-plane charge diffusion, potentially enhancing charge distribution uniformity and preventing hot spot formation, which consequently reduces localized nanosheet damage. Further, to gain insight into the charge storage performance, we conducted a comparative analysis of the surface chemical states of NiMoO4 before and after long-term stability testing. Additionally, XPS analysis was carried out after 20
000 cycles (Fig. S6†) and compared with the as-synthesized material to assess the effects of repeated charge–discharge cycles on the chemical composition of the electrode material. No significant peak shift was observed in the XPS survey after 20
000 cycles for Ni 2p, Mo 3d, C 1s, and O 1s, as depicted in Fig. S6a.† The pronounced K 2p peak after 20
000 cycles is attributed to the deposition of KOH electrolyte on the film. The nanosheets predominantly maintain a Ni(II) oxidation state, with 75% of the surface composition confirmed as NiMoO4 after 20
000 cycles (Fig. S6b†). Post-analysis, two new peaks at 858 and 862.8 eV were observed, which are owing to the presence of Ni3+ formed during the ongoing charge–discharge cycles. On the other hand, the Mo 3d spectrum remains unchanged, indicating that Mo retains its +6 oxidation state even after the OER analysis, which confirms the excellent stability of the Mo elements (Fig. S6c†). This suggests the durability and stability of the nanosheets after cycling. This is further supported by the broadening of the O 1s peak after the stability test, where the OH peak slightly shifted to a lower binding energy, and an increase in H2O content was observed, indicating the formation of oxyhydroxide within the nanosheet structure (Fig. S6d†).45
The OER activity was initially measured using iR-corrected (concerning ionic resistance of the electrolyte) linear sweep voltammetry (LSV) under ambient atmospheric conditions at a low scan rate of 5 mV s−1 to reduce the capacitive current. As illustrated in Fig. 6a, the 1:
1 NiMoO4 nanosheets demonstrated the lowest turn-on voltage and the most rapid increase in current density, surpassing both mono and bimetallic nanosheets. This suggests higher catalytic activity compared to other optimized ratios. To attain a geometric current density of 10 mA cm−2, a metric relevant to solar fuel synthesis, 1
:
1 NiMoO4 nanosheets require a considerably lower overpotential of 0.318 V compared to other mono and bimetallic nanosheets. Meanwhile, 1
:
1 NiMoO4 nanosheets exhibited superior current density of 26.1 mA cm−2 at constant bias of 1.6 V vs. RHE surpassing 3
:
1 NiMoO4 (24.7 mA cm−2), 1
:
3 NiMoO4 (15.2 mA cm−2) and RuO2-SS (12.7 mA cm−2). The notably higher current density of the 1
:
1 NiMoO4 catalyst indicates a significantly enhanced oxygen evolution rate compared to the other optimized ratios and benchmark RuO2 catalysts. The considerable difference in current density and overpotential observed with bare SS suggests that there is no significant contribution from the substrate to the OER activity of NiMoO4 nanosheets. Furthermore, we have also compared the OER activity of the 1
:
1 NiMoO4 nanosheets with previously studied catalysts.
The Tafel slope is a fundamental kinetic metric to study the electrocatalytic feature, particularly for the rate-determining step of the rearrangement and deprotonation of OH under low overpotential conditions. Consistent with the LSV, the 1:
1 NiMoO4 exhibited the lowest Tafel slope of 51 mV dec−1, suggesting rapid OER kinetics due to efficient electron transport (Fig. 6b). Fig. 6c illustrates the comparative OER potential needed to attain a catalytic current density of 10 mA cm−2, along with the corresponding Tafel values for the other optimized ratios. EIS was applied to assess the charge transport behavior between NiMoO4 nanosheets and electrolyte at an applied potential of 0.589 V vs. Ag/AgCl in Fig. 6d. Here, 1
:
1 NiMoO4 shows the lowest Rct at 11 Ω, whereas the monometallic nanosheets exhibited the highest Rct values (21 Ω for Ni–O and 32 Ω for Mo–O) (Table ST4†). The EIS results indicate that the 1
:
1 NiMoO4 exhibits a rapid charge-transfer process during the electrochemical reaction, consistent with the similar trend observed in electrocatalysis measurements, characterized by a more positive onset potential and a lower Tafel slope. It's worth mentioning that the Rct of the SS substrate (37 Ω, see Fig. 6d) is comparatively higher than that of all nanosheets, indicating that the catalytic performance comes from the ultrathin layer of nanosheet coating. All the Nyquist plots were well-fitted using an equivalent circuit composed of three components: solution resistance (Rs), charge transfer resistance (Rct) arising between the reference and working electrodes, which reflects the kinetic behavior of the electrode material, and double-layer capacitance (Cdl). The summarized data are presented in Table ST4.†
Another remarkable benefit of ultrathin nanosheet electrocatalysts is their remarkably high ratio of active catalytic surface sites relative to the mass of the electrocatalyst. To illustrate this benefit, electrochemical active surface areas (ECSA) of all optimized ratios and mono metallic electrocatalysts were measured through Cdl derived using CV curves at different scan rates from 5 to 80 mV s−1 in the non-faradaic region (Fig. S7†). A linear correlation is observed when plotting the current density at 0.1 V against the scan rate (Fig. 6e). Fig. 6e shows the Cdl values of all optimized ratios and monometallic electrocatalysts. The ECSA values of electrocatalysts with NiMoO4 ratios of 1:
3, 1
:
1, and 3
:
1 are 2.3, 3.0, and 2.4 cm−2, respectively, suggesting that the quantity of active sites remains similar across different molar ratios of NiMoO4. Furthermore, the ECSA of commercially available RuO2 (22.2 cm−2) was significantly greater than that of the optimized ratios of NiMoO4 (Table ST4†). Nonetheless, despite its higher ECSA, RuO2 exhibited significantly lower OER performance compared to NiMoO4, which might be due to the high conductivity but relatively weak intrinsic OER catalytic activity of RuO2.23 The comparative OER activities in the alkaline electrolyte of electrocatalysts based on NiMoO4 catalysts is summarized in Table ST5.†
Apart from the activity, the operational stability of electrocatalysts is crucial for real-time applications. The stability of the best-performing 1:
1 NiMoO4 nanosheets was analyzed using current density as a function of time in a 0.1 M KOH electrolyte (Fig. 6f). At an overpotential of 318 mV, the initial current density of the NiMoO4 nanosheets was 10 mA cm−2 and gradually decreased to 9.5 mA cm−2 after 13 hours of continuous OER testing, retaining 95% of the initial value. Furthermore, the LSV curves showed a slight decrease in overpotential of 14 mV compared to the initial curve after 13 hours of OER. This long-term stability demonstrated stable nanosheet morphology, even with its ultrasmall thickness. The nanosheets' uniform thickness and ultrafine nature are believed to be beneficial for achieving uniform and ultrasmall out-of-plane charge diffusion, potentially enhancing charge distribution uniformity and preventing hot spot formation, thereby reducing localized damage to the nanosheets. Further, to gain insight into the excellent OER performance, we conducted a comparative analysis of the surface chemical states of NiMoO4 before and after long-term stability testing. Commencing with the survey spectra, no notable changes were seen between the electrocatalyst after 13 hours of continuous OER operation, indicating the NiMoO4 film maintained good stability. As previously noted with NiMoO4 on SS, the additional K 2s and K 2p peaks originate from KOH deposited from the electrolyte solution (Fig. S8a†). Upon magnifying the core-level Ni 2p, high-resolution spectra for both the synthesized and post-OER samples reveal that Ni is present in the +2 oxidation state. In the as-synthesized thin film, Ni exists as 100% Ni(OH)2 before OER, decreasing to 92.9% after OER. The Ni 2p3/2 spectrum was deconvoluted into four peaks with binding energies (BE) at 855.1, 856.6, 861.5, and 865.2 eV before OER analysis, and at 853.3, 855.4, 856, 858.9, 861.2, and 863.3 eV after OER analysis. Additionally, post-stability testing revealed a new peak at 858.9 eV, which is attributed to the formation of Ni3+ during the OER process. The peak at 853.3 eV corresponds to the metallic nickel peak, which arises from the SS substrate due to cracks or detachment of the nanosheet (Fig. S8b†). On the other hand, the Mo 3d spectrum remains unchanged, indicating a +6 oxidation state after the OER analysis, which verifies the superior stability of the Mo elements (Fig. S8c†). To support this, the O 1s peak broadened after stability, and the OH peak is slightly shifted to a lower binding energy, suggesting the formation of oxyhydroxide in the nanosheet structure (Fig. S8d†).
Typically, the substantial performance achieved by the bimetallic metal oxide structure can be attributed to the supramolecular metallic interactions. These interactions result in substantial alterations to the atomic arrangement and local electronic properties of the Ni–Mo catalytic sites. Consequently, these differences lower the energy barriers for intermediate catalytic species, leading to enhanced oxygen evolution reaction (OER) performance of the nanosheets. Specifically, at low nickel concentrations, it may react with Mo and form a superficial Ni–Mo-like structure. Here, Ni sites show significant activity but tend to be unstable. However, the presence of Mo stabilizes these Ni sites, enhancing their catalytic activity and structural integrity. This interaction between their electronic and atomic properties optimizes electrochemical reactions, improving performance. At higher concentrations of nickel (3:
1 NiMoO4), a minimal amount of non-reactive nickel oxide hydroxide phase forms, reducing the positive impact of Ni–Mo interaction and thereby decreasing the electrochemical performance of the electrode material. At the nanoscale level, enhanced interfacial charge transport within the catalyst material and at the solid–liquid interface significantly improves catalytic performance. These findings underscore the crucial role of electron transport and catalytic kinetics in achieving superior activity, particularly evident with controlled nanoscale morphology.
Further, theoretical calculations align well with these results, predicting 1:
1 MiMoO4 shows the best overpotential η = 0.66 eV (Fig. 7b). In contrast, the overpotential for the low Mo amount than Ni is η = 0.76 eV while η = 1.10 eV is obtained for higher Mo amount than Ni. Thus, the trade-off between Ni and Mo can improve the OER activity in Ni-based catalysts.46,47 For 3
:
1 and 1
:
1 NiMoO4, the reaction steps from *OH to *O act as the potential determining step (PDS), while for 1
:
3 NiMoO4, *O to *OOH is found to be the PDS. This result is consistent with the experimental analysis, which indicates that an equal amount of Ni and Mo is beneficial for improving OER activity. The charge analysis on the active site reveals that it contains 8.79|e|, 8.72|e|, and 9.18|e| for the 3
:
1 NiMoO4, 1
:
1 NiMoO4, and 1
:
3 NiMoO4 structures, respectively (Table ST6†). The existence of less charge indicates a low transfer of charges during the adsorption of intermediates, which may be a possible reason for the low overpotential in 1
:
1 NiMoO4. Additionally, the projector density of states study suggests that the d-band centre (dc) on 3
:
1, 1
:
1, and 1
:
3 NiMoO4 is −1.93, −2.34, and −1.79, respectively (see Fig. 7c–h and Table ST6†). This suggests that the dc closer to the Fermi level in the 1
:
3 and 3
:
1 models exhibits a strong adsorption strength of the intermediates and, therefore, possesses a higher overpotential. Alternatively, in the 1
:
1 model, dc lies far from the Fermi level, suggesting weaker binding with intermediates and leading to lower overpotential.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta02450a |
‡ This authors contributed equally. |
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