Biodegradable MoN_x@Mo-foil electrodes for human-friendly supercapacitors

Abstract

With the advancement in the field of biomedical research, there is a growing demand for biodegradable electronic devices. Biodegradable supercapacitors (SCs) have emerged as an ideal solution for mitigating the risks associated with secondary surgeries, reducing patient discomfort, and promoting environmental sustainability. In this study, MoN_x@Mo-foil was prepared as an active material for biodegradable supercapacitors through high-temperature and nitridation processes. The composite electrode exhibited superior electrochemical performance in both aqueous and solid-state electrolytes. In the case of the solid-state electrolyte, the MoN_x@Mo-foil composite electrode-based device demonstrated excellent cycling stability and electrochemical performance. Additionally, the composite electrode exhibited rapid and complete biodegradability in a 3% H₂O₂ solution. Through detailed experimental analysis and performance testing, we verified the potential application of the MoN_x@Mo-foil composite electrode in biodegradable supercapacitors. This work provides a new choice of degradable material for developing biomedical electronic devices.

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Introduction

In recent years, the increasing severity of the aging population issue and the continuous development in the field of biomedicine have led to a steady growth in demand for energy storage devices, implantable medical devices, and other electronic components.^1–3 These devices can be used for monitoring physiological parameters, drug delivery, etc.^4–7 This trend has spurred the development of biodegradable biomedical devices, presenting new challenges for the flexibility, portability, biocompatibility, and degradability of energy storage devices in temperature-dependent environments.^8,9 However, providing a stable and efficient energy supply for these devices has become a significant challenge for scientists. Flexible supercapacitors have attracted considerable interest owing to their impressive characteristics such as elevated energy density, fast charge–discharge efficiency, and robust cycling stability.^10,11 However, the energy density of supercapacitors is lower than that of batteries.¹² Recently, new kinds of batteries have emerged to compensate for the long life with high energy density, such as divalent and trivalent ion batteries.^13,14 Furthermore, dual-ion batteries and supercapacitors are also proposed to boost the energy and power densities of the devices.^15,16 Researchers are dedicated to selecting biodegradable materials to fabricate biodegradable supercapacitors, serving as novel bioenergy storage devices for implantable electronic devices within organisms. This innovation provides a solution to the energy supply challenge for bio-devices.^17,18 After completing the energy supply tasks, these biodegradable electronic devices can autonomously degrade inside the biological body, thereby avoiding the risk of secondary surgeries, enhancing safety, and being environmentally friendly, representing a technology in line with the concept of sustainable development.^19–22 Le et al.²³ reported a CsPbBr₃ electrode composed of CsPbBr₃ nanocrystals synthesized via a hot injection method, exhibiting a specific capacitance of 528 mF g⁻¹. In a symmetric supercapacitor fabricated using TBAPF₆ as the electrolyte, the specific capacitance reached 119.8 mF g⁻¹, indicating its potential as an electrode material in supercapacitors. However, it has been noted that the charge/discharge time is short, and the potential window is narrow, leaving room for further improvement in power density and energy density. On the other hand, He et al.²⁴ reported the continuous preparation of hollow poly(3,4-ethylenedioxythiophene):polystyrene sulfonate thin-walled fibers (HPFs) via coaxial wet-spinning. These HPF electrodes exhibited a specific capacitance of 115.2 mF cm⁻² at a current density of 0.3 mA cm⁻². Moreover, at a power density of 0.112 mW cm⁻², the energy density of the HPFs reached 9 μW h cm⁻², demonstrating promising prospects for application in smart wearable devices. In addition, Li et al.²⁵ reported a symmetric supercapacitor using biomass-derived activated carbon as electrodes, which can operate under alkaline acidic electrolyte, with stable operating voltage and high cycling performance, and the energy density is able to reach 36.9 W h kg⁻¹, which is an excellent aqueous supercapacitor. Compared to previously reported works, the MoN_x@Mo-foil electrode exhibits certain advantages while maintaining considerable energy density and power density. It achieves a balance between cycling stability and stability. Furthermore, it demonstrates superior biocompatibility as it can undergo stable degradation in H₂O₂ solution.

Flexible solid-state biodegradable supercapacitors represent the primary research focus in this direction currently. Their extensibility, portability, biocompatibility, and degradability make them applicable to a wide range of biomedical applications, including wearable electronic devices, implantable medical devices, and various other biologically related electronic applications.^17,26 In the ongoing quest to find suitable electrode materials for biodegradable supercapacitors, molybdenum, and its compounds have demonstrated a range of superior properties, including excellent conductivity, chemical stability, and mechanical strength. Furthermore, molybdenum, as a soluble metal, can be metabolized, decomposed, or excreted by organisms under certain conditions, rendering it an excellent biocompatible material.^27,28 Consequently, molybdenum-based materials emerge as ideal candidate electrodes for biodegradable supercapacitors.

In this study, we employed pure molybdenum foil as the substrate. Following cleaning with dilute hydrochloric acid and alcohol, the foil underwent high-temperature annealing in pure ammonia gas.^29,30 This process facilitated the uniform growth of molybdenum nitride nanosheet arrays on the surface, providing more attachment sites for electrolyte ions, thereby enhancing their adsorption capacity and electrochemical performance. The MoN_x@Mo-foil composite electrode was characterized using techniques such as XRD, XPS, and SEM.

1 M sodium chloride (NaCl) aqueous solution and PVA/NaCl gel were used as electrolytes. We assembled both aqueous hybrid supercapacitors and flexible solid-state supercapacitors for electrochemical testing. Compared to the original molybdenum foil electrode, the MoN_x@Mo-foil composite electrode exhibited significantly improved electrochemical performance. In the NaCl aqueous electrolyte, MoN_x@Mo-foil demonstrated superior electrochemical energy storage efficiency and discharge time. After undergoing 5000 cycles at a current density of 10 A g⁻¹, it exhibited a capacitance retention of 88.55%, showcasing its robustness and longevity in electrochemical performance.

In solid-state supercapacitors employing the PVA/NaCl electrolyte, the MoN_x@Mo-foil electrode demonstrated exceptional electrochemical performance, exhibiting remarkable energy and power density. Even after 5000 cycles, it sustained a capacitance retention rate of 77.51%, underscoring its enduring stability and efficacy.

Finally, to validate biocompatibility, biodegradation experiments were conducted in a 3% concentration of H₂O₂ solution. Both the MoN_x@Mo-foil composite electrode and the MoN_x@Mo//PVA/NaCl//MoN_x@Mo assembly were found to completely degrade in the 3% H₂O₂ solution, demonstrating excellent biodegradability.

Fig. 1a illustrates the synthesis procedure of the MoN_x@Mo-foil electrode. Initially, the molybdenum foil undergoes ultrasonic cleaning in ethanol and dilute hydrochloric acid, followed by rinsing with DI water to ensure cleanliness. Subsequently, the cleaned foils are subjected to annealing at 500 °C in a tubular furnace under an ammonia gas atmosphere for 3 hours, with a gradual heating rate of 3 °C min⁻¹, and then naturally cooled to room temperature. This meticulous process leads to the formation of a layer comprising MoN_x nanosheets on the surface of the molybdenum foil, a structural transformation confirmed through scanning electron microscopy analysis for surface alterations. Fig. 1b illustrates the surface morphology of the original molybdenum foil, which exhibits a smooth metallic surface. After high-temperature annealing under an ammonia gas atmosphere, a layer of smooth nanosheet arrays uniformly grew on the surface, forming the MoN_x@Mo-foil composite electrode (Fig. 1c). Fig. 1d provides SEM images at higher resolutions, exposing the uniform sizes and distributions of MoN_x nanoparticles, indicative of consistent morphological characteristics within the electrode material.

Fig. 1 (a) Schematic diagram of the molybdenum nitride synthesis process. (b) SEM image of the original Mo-foil. (c) SEM image of MoN_x@Mo foil. (d) High-magnification SEM image of MoN_x@Mo foil.

X-ray diffraction (XRD) was performed to examine the crystal structures of both the pristine Mo foil and the MoN_x@Mo-foil, as depicted in Fig. 2a. The diffraction peak observed at 58.4° is attributed to the (200) crystal plane of metallic Mo, as per the PDF card, and the peak observed at 37° can be attributed to the (110) crystal plane of Mo₅N₆, delineating the presence of specific structural arrangements within the MoN_x@Mo-foil. Additionally, peaks observed at 47°, 59°, 76°, and 79° correspond to the (200), (220), (311), and (222) crystal planes of Mo₂N, respectively, according to the PDF card. These findings suggest the formation of a composite material comprising various molybdenum-based nitrides on the surface of the original molybdenum foil after high-temperature annealing under an ammonia gas atmosphere. For more comprehensive analysis, X-ray photoelectron spectroscopy (XPS) was utilized to explore the distribution of valence states in MoN_x@Mo-foil. As depicted in Fig. 2b, the XPS survey spectrum of MoN_x@Mo-foil validates the existence of Mo, O, N, and C elements, underscoring the elemental composition of the material under investigation. Fig. 2c displays the high-resolution XPS spectrum analysis of Mo 3d, delineating peaks observed at 235.65 eV, indicative of Mo⁴⁺, along with peaks detected at 233.69 eV and 230.9 eV, corresponding to Mo⁶⁺ and Mo^δ+ states, respectively, and a sub-peak at 229.13 eV corresponding to Mo⁴⁺. The high-resolution XPS spectrum of N-1s (depicted in Fig. 2d) reveals a peak at 398.44 eV, associated with the Mo–N bond, along with a peak at 395.55 eV representing N 1s, and another peak observed at 392.79 eV attributed to Mo 3p. Additionally, the XPS spectrum of C 1s illustrates peaks at 284.6 eV and 286.1 eV, corresponding to C–C and C=C bonds, respectively, accompanied by a sub-peak at 288.7 eV attributed to the C=O bond. Through the comprehensive analysis of the X-ray photoelectron spectroscopy (XPS) spectra, the nitrogen content in the MoN_x@Mo-foil electrode obtained after annealing was determined, as depicted in Fig. S1 (ESI†). XPS spectra were acquired for four elements: Mo, N, O, and C. Upon normalization, the nitrogen content was found to be approximately 32.77%, while the Mo content was approximately 10.74%. This finding corroborates the formation of a thin and dense MoN_x layer on the surface of the original molybdenum foil after ammonia annealing and provides insight into the nitrogen content within it. After BET testing, the specific surface area and pore information of the pristine molybdenum foil electrode and MoN_x@Mo-foil electrode were obtained. The experimental data are, respectively, presented in Fig. S2 and S3 (ESI†). Following annealing, the specific surface area of the MoN_x@Mo-foil electrode increased from 2.31 m² g⁻¹ to 2.39 m² g⁻¹, while the pore diameter expanded from 31.00 Å to 3308.26 Å. The augmented specific surface area and pore size enable the MoN_x@Mo-foil electrode to offer more insertion/deinsertion sites for cations during the electrochemical process, resulting in enhanced electron transfer rates and improved cycling stability.

Fig. 2 (a) XRD analysis of the Mo foil, MoN_x@Mo foil, and labeled PDF cards. (b) XPS full spectrum of the Mo foil and MoN_x@Mo foil. (c) XPS spectra of Mo-3d. (d) XPS spectra of N-1s.

To evaluate the electrochemical performance of the MoN_x@Mo-foil in three electrode system. The three-electrode setup comprised an Ag/AgCl electrode as the reference electrode and a platinum foil with an area of approximately 2 cm² was employed as the counter electrode, operating within a defined voltage window of −1.0 to 0 V. Fig. 3a and b depicts the cyclic voltammetry (CV) curves obtained for both the original molybdenum foil and the MoN_x@Mo-foil electrode. The analysis reveals that the MoN_x@Mo-foil electrode exhibits a significantly larger area under the CV curve compared to the original molybdenum foil electrode across various scan rates. This observation suggests a marked enhancement in electrochemical performance with the integration of MoN_x onto the Mo-foil substrate. Furthermore, as the scan rate increases, the morphology of the CV curve for the MoN_x@Mo-foil electrode remains stable and maintains a characteristic rectangular shape.³¹ This behavior indicates consistent double-layer electrochemical behavior, underscoring the electrode's stability and suitability for high-performance energy storage applications.

Fig. 3 (a) CV curve at scan rates increasing from 1 mV s⁻¹ to 75 mV s⁻¹ for Mo foil. (b) CV curve at scan rates increasing from 1 mV s⁻¹ to 75 mV s⁻¹ for MoN_x@Mo foil. (c) and (d) Capacitive and diffusion–controlled storage contributions of Mo foil and MoN_x@Mo foil.

Moreover, the charge storage mechanisms of both electrodes were meticulously calculated (as depicted in Fig. 3c and d). At a scan rate of 10 mV s⁻¹, the original molybdenum foil electrode exhibited approximately 20.26% charge storage through the capacitive process, with the remaining 79.74% stored via the diffusion process. In contrast, the capacitive charge storage of the MoN_x@Mo-foil composite electrode notably increased to 35.15%, representing a substantial enhancement.

Furthermore, as the scan rate escalated from 1 mV s⁻¹ to 75 mV s⁻¹, the capacitive charge storage capacity of the MoN_x@Mo-foil electrode exhibited a remarkable surge from 14.63% to 59.75%, surpassing the original molybdenum foil electrode, which increased from 7.43% to 41.03%. This observation underscores the outstanding charge storage capability of the MoN_x@Mo-foil composite electrode, primarily characterized by capacitive charge storage, particularly at elevated scan rates.

Additionally, constant current charge–discharge tests (GCD) were performed on both electrodes within a potential window of −1.0 V to 0 V. As shown in Fig. 4a, the discharge time of the MoN_x@Mo-foil electrode at a current density of 0.3 A g⁻¹ is significantly longer compared to that of the original molybdenum foil electrode, indicating superior electrochemical performance, consistent with the conclusion drawn from the CV curves. Fig. 4b and c showcases the galvanostatic charge–discharge (GCD) curves of the original molybdenum foil and MoN_x@Mo-foil electrodes under varying current densities. Notably, the MoN_x@Mo-foil electrode exhibits prolonged discharge times compared to the original molybdenum foil electrode across different current densities, indicative of stable electrochemical performance. The specific capacitance of both electrodes was computed based on the discharge time obtained from the GCD curves (Fig. 4d). At a current density of 0.1 A g⁻¹, the MoN_x@Mo-foil electrode achieved a notable capacitance of 27 F cm⁻², significantly surpassing that of the original molybdenum foil electrode. Subsequently, as the current density increased to 2.0 A g⁻¹, the capacitance of the MoN_x@Mo-foil electrode decreased to 8 F cm⁻², with a capacitance retention rate of approximately 24.49%. This marks a substantial improvement compared to the 10.58% capacitance retention rate of the original molybdenum foil. Furthermore, Fig. 4e illustrates the cycling stability test conducted on both electrode materials. Following 5000 cycles, the MoN_x@Mo-foil electrode demonstrated an outstanding capacitance retention rate of 88.55%, surpassing the 82.64% capacitance retention rate of the original molybdenum foil electrode, thus underscoring its superior capacitance retention characteristics.

Fig. 4 (a) Comparison of the GCD curves of the two electrodes at 0.3 A g⁻¹. (b) GCD curves of Mo-foil at different current densities. (c) GCD curves of MoN_x@Mo foil at different current densities. (d) Specific capacitance is calculated from discharge time and current density. (e) Comparison of the capacitance retention of the two electrodes during 5000 cycles.

To assess the practical performance of the solid-state flexible biodegradable supercapacitor (MoN_x@Mo//NaCl/PVA//MoN_x@Mo-SBSC), we assembled the device using two identical MoN_x@Mo-foil electrodes and a NaCl/PVA solid-state electrolyte, as depicted in Fig. 5a. The capacitor structure comprises two MoN_x@Mo-foil composite electrodes of identical size, separated by a blend of 0.9% NaCl and PVA gel, which also serves as a binder. CV tests were conducted at varying scan rates (1–20 mV s⁻¹) within a voltage range of −0.5 to 0.9 V using an electrochemical workstation, as depicted in Fig. 5b. The increasing area under the CV curve with scan rate signifies the robust electrochemical performance of the assembled solid-state supercapacitor. Furthermore, Fig. 5c exhibits the GCD curves of the solid-state flexible biodegradable supercapacitor at different current densities (0.1–2.0 A g⁻¹). The symmetric distribution of the GCD curve shapes indicates excellent Coulombic efficiency. As shown in Fig. 5d, the specific capacitance was computed based on the discharge time, and even at a current density of 2 A g⁻¹, it retained a capacitance retention rate of 37.2%, underscoring its remarkable performance. Moreover, cycling stability tests were conducted on the solid-state flexible biodegradable supercapacitor (Fig. 5e). After 5000 charge–discharge cycles, it retained 77.51% of its initial capacitance, demonstrating commendable electrochemical performance. Compared to previously published results for biodegradable supercapacitors, the MoN_x@Mo-foil electrode exhibits certain advantages while maintaining considerable energy density and power density. It achieves a balance between cycling stability and stability. Furthermore, it demonstrates superior biocompatibility as it can undergo stable degradation in H₂O₂ solution, which has an objective prospect in the field of implantable electronic devices and other fields.

Fig. 5 (a) Schematic diagram of a solid-state supercapacitor. (b) CV curve at scan rates increasing from 1 mV s⁻¹ to 75 mV s⁻¹ for the device. (c) GCD curves at current densities ranging from 0.1 A g⁻¹ to 2 A g⁻¹ for the device. (d) Specific capacitance calculated from discharge time and current density. (e) The device capacitance retention over 5000 cycles.

To verify their biodegradability, biodegradation experiments were conducted on the MoN_x@Mo-foil electrode and the MoN_x@Mo//NaCl/PVA//MoN_x@Mo-SBSC. The samples were immersed in a 3% H₂O₂ solution, and changes in their appearance in the solution were observed using an optical camera and an optical microscope. Fig. 6a and b illustrate the degradation process of the MoN_x@Mo-foil electrode. Initially, the surface was smooth upon immersion in the solution. After 2 hours, the electrode began to dissolve, showing wrinkling and cracking on the surface. Throughout 2 to 6 hours, the electrode gradually dissolved, with cracks on the surface contracting inward, and the overall area gradually decreasing. Partial debris started to detach after 8 hours, and after 10 hours, internal fractures appeared, gradually dispersing into particles. By 12 hours, the entire electrode had dissolved into particles and gradually disappeared. Upon further magnification with an optical microscope, the dissolved electrode was observed as fine particles and flake-like structures dispersed in the solution. The scanning electron microscopy (SEM) characterization of the MoN_x@Mo-foil electrode after 6 hours of degradation, as depicted in Fig. 6b, reveals that compared to the dense array of MoN_x nanosheets observed in Fig. 1, the nanosheet array on the electrode surface has largely disappeared after degradation, with some debris adhering to the surface. Additionally, the molybdenum foil substrate has fractured, forming multiple cracks, consistent with observations under optical microscopy. Fig. 6c displays the degradation process of the MoN_x@Mo//NaCl/PVA//MoN_x@Mo-SBSC. Apart from the fixed area, both MoN_x@Mo-foil composite electrodes lost their metallic luster after 2 hours of degradation, deforming and shrinking gradually after 8 hours. By 12 hours, the structure of the supercapacitor started to disintegrate. After 24 hours, most of the electrode had dissolved, and after 36 hours, complete dissolution was observed. Only the membrane, which is harmless to the organism, remained intact and could not degrade. It is mainly organic material, which can be naturally expelled from the body's circulatory system.

Fig. 6 (a) Optical microscope images of the MoN_x@Mo foil electrode in H₂O₂ solution (3%) at various stages of the biodegradation process. (b) Optical microscopy images of the MoN_x@Mo foil electrode in H₂O₂ solution (3%) at different stages of the biodegradation process and the scanning images of the surface after 6 h of degradation. (c) Optical microscope images of the biodegradable process at various stages for the MoN_x@Mo-foil//NaCl-PVA//MoN_x@Mo-foil electrode in H₂O₂ solution (3%).

Conclusion

In summary, we synthesized a MoN_x@Mo-foil electrode through annealing under an ammonia atmosphere, resulting in a composite electrode with a unique morphology characterized by uniform growth of MoN_x nanoplates on the surface of the molybdenum foil. XRD and XPS confirmed the formation of molybdenum-based nitride on the electrode surface, indicating the successful synthesis of the sample. Electrochemical tests revealed the improved performance of the MoN_x@Mo-foil electrode compared to the original molybdenum foil, with significant enhancements in charge storage mechanisms, specific capacitance, and cycling stability. Additionally, the solid-state flexible and biodegradable supercapacitor assembled using the MoN_x@Mo-foil electrode exhibited robust electrochemical performance and good capacitance retention properties. Biodegradation experiments confirmed the biodegradability of the MoN_x@Mo-foil electrode and supercapacitor, highlighting their potential for environmentally friendly applications. Overall, the synthesized MoN_x@Mo-foil electrode demonstrates enhanced electrochemical performance and biodegradability, making it promising for various energy storage and biodegradable electronic applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Research Fund for International Young Scientists of the National Natural Science Foundation of China (NSFC) under grant no. 52250410342 and a Scientific research start-up grant for Youth Researchers at Lanzhou University (China). This work was funded by the Researchers Supporting Project Number (RSP2024R441), King Saud University, Riyadh, Saudi Arabia.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tb00649f

‡ These authors contributed equally to this work.

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