High-energy X-ray irradiation-induced functionalization of Ni(OH)₂ for enhanced supercapacitor electrodes

Structural and chemical characterization

The structural integrity and surface modifications induced by high-energy X-ray irradiation were first investigated using X-ray diffraction (XRD) analysis. Figure 1a presents the XRD patterns of the pristine Ni(OH)₂ and the f-Ni(OH)₂ samples. The phase identification of both pristine and f-Ni(OH)₂ samples was conducted by comparing the recorded XRD patterns with the standard reference file from the Joint Committee on Powder Diffraction Standards (JCPDS Card No. 14–0117), corresponding to the β-Ni(OH)₂ phase^48,49. The major diffraction peaks observed at 2θ ≈ 18.7°, 33.1°, 38.5°, 52.1°, and 59.3° can be indexed to the (001), (100), (101), (102), and (110) crystallographic planes of hexagonal β-Ni(OH)₂, respectively^24,48,50. In addition to the major reflections, weaker high-angle peaks are observed near 2θ ≈ 71.6° and 77.5°, corresponding to the (103) and (201)/(203) planes of β-Ni(OH)₂, respectively, consistent with JCPDS Card No. 14–0117⁵¹. Although these reflections are of low intensity, their presence further confirms the preservation of the long-range crystalline order after X-ray irradiation. These reflections are in good agreement with the standard β-phase, confirming the retention of the layered hydroxide structure after high-energy X-ray irradiation.

Notably, a broad and low-intensity feature was observed near 2θ ≈ 15° in both pristine and irradiated samples. This peak does not correspond to a standard β-phase reflection but can be attributed to the (003) plane of α-Ni(OH)₂, as indexed in JCPDS Card No. 38–0715^52,53. In fully hydrated α-Ni(OH)₂, this basal reflection typically appears at lower angles (~ 11.3°–12.2°). However, it has been reported to shift toward higher angles (13°–15°) upon partial dehydration, interlayer collapse, or environmental exposure^53,54,55. Given that both samples exhibit this feature regardless of irradiation, it is likely related to structural disorder or minor interstratified α-like domains inherent in the pristine Ni(OH)₂ material, possibly induced during ambient storage. The slightly increased intensity in the irradiated sample may reflect additional surface rearrangement induced by high-energy exposure, although no sharp α-phase peaks are observed. These findings suggest that while the dominant phase remains β-Ni(OH)₂, a small fraction of structurally disordered or dehydrated α-Ni(OH)₂-like regions may coexist, giving rise to the broad feature at ~ 15°. The main β-phase reflections remain sharp and unchanged, confirming that the irradiation primarily induces surface-level modifications without compromising the crystallographic integrity of the bulk structure. The absence of new diffraction peaks further confirms that the functionalization process selectively alters the surface chemistry while maintaining the bulk phase integrity of Ni(OH)₂.

These structural modifications at the surface are further supported by Raman spectroscopy results shown in Fig. 1b. Both pristine and irradiated Ni(OH)₂ samples exhibit a strong Raman band around 2922 cm⁻¹, attributed to the stretching vibration of hydroxyl (OH⁻) groups associated with the Ni(OH)₂ lattice. In addition to the common features, a new peak emerged near 2850 cm⁻¹ in the f-Ni(OH)₂ spectrum, which is absent in the pristine sample²⁴. This new vibrational mode is indicative of local chemical rearrangements, potentially corresponding to newly formed hydroxyl-related configurations or surface defects induced by X-ray interaction. The appearance of this feature suggests that X-ray irradiation successfully introduced additional active sites at the material surface, which can facilitate improved redox reactions during electrochemical cycling.

Further insights into the chemical bonding environment were obtained via FTIR spectroscopy, as shown in Fig. 2. Both Ni(OH)₂ and f-Ni(OH)₂ samples displayed characteristic absorption bands at 635 cm⁻¹, 1360 cm⁻¹, and 1600 cm⁻¹, corresponding to Ni–O stretching vibrations and O–H bending modes. These observations confirm that the primary chemical structure of Ni(OH)₂ was maintained following irradiation^56,57. Notably, in the high wavenumber region, a shift in the hydroxyl stretching vibration band from 3283 cm⁻¹ in pristine Ni(OH)₂ to approximately 3307 cm⁻¹ in f-Ni(OH)₂ was recorded. This shift indicates a change in the hydrogen bonding network and suggests modifications in the surface chemical environment, likely arising from the interaction of high-energy X-ray with surface hydroxyl groups^56,57. Such modifications are expected to enhance ionic transport and improve pseudocapacitive behavior by creating more favorable redox-active sites.

Ni(OH)₂ is readily converted into NiOOH or its intermediate phase, Ni–O–OH, particularly under electroactive environments or oxidative conditions. This transformation commonly occurs during the β-Ni(OH)₂ → γ-NiOOH phase transition and can be triggered by exposure to high-energy oxidative stimuli such as X-ray irradiation. The formation of Ni–O–OH species is highly probable via mechanisms including Compton scattering and the photoelectric effect initiated by incident X-ray. Such high-energy radiation can induce deprotonation of hydroxide ions, promoting the formation of oxygen-bridged nickel oxyhydroxide species (Ni–O–OH). The spectral differences observed in FTIR and Raman analyses provide evidence supporting this transformation. Accordingly, the following surface reaction may occur post-irradiation:

Ni(OH)₂ (:underrightarrow{high:energy:X-ray:}) Ni-O-OH + H⁺ + e⁻

This process results in the formation of an electroactive surface, which can significantly enhance charge storage capability. The generation of Ni–O–OH species introduces additional redox-active sites, thereby facilitating faradaic reactions that contribute to improved pseudocapacitive behavior. As a result, this structural transformation may lead to increased specific capacitance and enhanced overall electrochemical performance of the electrode material, making it particularly attractive for advanced supercapacitor applications.

FTIR spectra of pristine Ni(OH)₂ and high-energy X-ray irradiated f-Ni(OH)₂ samples.

ICP-MS analysis was conducted to examine potential changes in the isotopic composition after irradiation (Table 1). The major isotopic distribution of nickel (Ni-58, Ni-60, Ni-61, and Ni-62) remained stable, confirming that the irradiation process did not induce significant bulk transmutation or atomic disintegration. However, trace amounts of radioactive isotopes such as Ni-59 and Ni-63 were significantly reduced after irradiation, indicating a purification effect. Although these isotopes are present at extremely low levels and do not directly contribute to electrochemical performance, their depletion enhances the material’s safety and stability profile for potential energy storage applications. After irradiation, a significant reduction in the isotopes Ni-59 and Ni-63 has been observed. This phenomenon is likely not due to isotope decay, but rather attributed to surface ion exchange processes or the desorption of these isotopes from the surface due to their relatively low stability.

Collectively, these structural and chemical characterizations demonstrate that high-energy X-ray irradiation effectively functionalized the surface of Ni(OH)₂ while preserving its bulk crystalline framework. The introduced surface defects, modified hydroxyl groups, and enhanced active sites are anticipated to contribute significantly to the improved electrochemical behavior observed in the subsequent analyses.

Evaluation of electrochemical characterizations

The electrochemical behavior of pristine and X-ray irradiated Ni(OH)₂ electrodes was investigated by CV analysis using a three-electrode system over a potential window from − 1.0 V to + 1.0 V at various scan rates ranging from 1 to 100 mV·s⁻¹. The resulting voltammograms, presented in Fig. 3, reveal profound differences between the pristine and irradiated electrodes, clearly reflecting the beneficial effects of X-ray surface functionalization. The CV curves of the pristine Ni(OH)₂ electrode (Fig. 3a) exhibit relatively narrow profiles with distorted shapes, particularly at lower scan rates. The current response is modest, and the capacitive behavior appears limited, suggesting sluggish electrochemical kinetics and poor ionic accessibility to active sites. As the scan rate increases, the current density declines sharply, and significant distortion of the CV curves is observed, indicative of diffusion-limited charge storage and slow faradaic processes. These results highlight the intrinsic limitations of pristine Ni(OH)₂, including low electrical conductivity and limited redox-active surface areas, which restrict its charge storage performance. In contrast, the f-Ni(OH)₂ electrode (Fig. 3b) demonstrates a substantially enhanced electrochemical response. At all scan rates, the f-Ni(OH)₂ voltammograms show significantly higher current densities compared to the pristine sample, with broader and more symmetric curves. The f-Ni(OH)₂ electrode maintains a well-defined capacitive response even at high scan rates such as 100 mV·s⁻¹, reflecting excellent rate capability and efficient ion diffusion within the electrode structure. The improved current retention with increasing scan rate suggests a lower internal resistance and enhanced charge transfer kinetics, both of which are critical for high-power energy storage applications. The calculated specific capacitance values from CV voltammograms (C_{s, CV}; mF.cm⁻²) were tabulated in Table 2. At a scan rate of 1 mV·s⁻¹, the f-Ni(OH)₂ electrode achieves a specific areal capacitance of 671.2 mF·cm⁻², representing an approximately 41% enhancement over the pristine Ni(OH)₂ electrode, which exhibits 474.7 mF·cm⁻² under identical conditions. This significant improvement is attributed to the structural and chemical modifications induced by high-energy X-ray irradiation. The X-ray treatment is believed to create defect-rich regions, increased surface roughness, and localized chemical disorder, thereby increasing the density of electrochemically active sites. These modifications facilitate improved electrolyte penetration, faster redox reactions, and enhanced ion adsorption-desorption dynamics at the electrode–electrolyte interface.

Mechanistically, the improvement in electrochemical performance can be explained by the interactions induced during X-ray irradiation. High-energy photons primarily interact with the Ni(OH)₂ structure through Compton scattering, leading to localized energy deposition that causes bond reorganization, surface atom displacement, and generation of shallow traps. These newly created surface states act as active centers for faradaic reactions, enhancing pseudocapacitive behavior. The emergence of new vibrational bands around 2850 cm⁻¹ in the Raman spectra of f-Ni(OH)₂ further confirms the formation of chemically modified surface groups that can participate in charge storage processes. Additionally, shifts observed in FTIR spectra indicate changes in hydrogen bonding and hydroxyl environments, which likely improve the proton-coupled electron transfer mechanisms.

The difference between the two electrodes becomes even more pronounced at higher scan rates. While the pristine Ni(OH)₂ electrode exhibits a sharp decrease in current density with increasing scan rate highlighting ion transport limitations, the f-Ni(OH)₂ electrode retains a substantial portion of its current response. This behavior suggests that the irradiated surface supports faster ion diffusion and more efficient electron transport, minimizing resistive losses during rapid cycling. The reduced peak separation and smoother current-voltage profiles of f-Ni(OH)₂ indicate lower polarization and superior electrochemical reversibility, both of which are critical for high-performance supercapacitor applications. All together, the CV analysis demonstrates that high-energy X-ray irradiation transforms the electrochemical behavior of Ni(OH)₂ by creating a surface that combines high pseudocapacitive activity with excellent rate capability. The resulting f-Ni(OH)₂ electrodes achieve enhanced areal capacitance, faster redox kinetics, and improved structural stability, positioning them as strong candidates for integration into advanced microsupercapacitor and high-power energy storage systems.

CV voltammograms of (a) pristine Ni(OH)₂ and (b) high energy X-ray irradiated f-Ni(OH)₂ electrodes in a three-electrode electrochemical cell in 1.0 M Na₂SO₄ electrolyte.

To further evaluate the charge storage performance and internal resistance characteristics of pristine and high energy X-ray irradiated Ni(OH)₂ electrodes, GCD and EIS analysis were conducted. GCD curves were recorded over a potential window of − 1.0 V to + 1.0 V at current densities ranging from 1.0 to 5.0 mA·cm⁻², and the resulting voltage–time profiles are shown in Fig. 4a and b for pristine Ni(OH)₂ and f-Ni(OH)₂ electrodes, respectively. The GCD curves of the pristine Ni(OH)₂ electrode (Fig. 4a) exhibit non-linear voltage profiles, with significant IR drops and shorter discharge times across all current densities. These characteristics are indicative of high internal resistance, sluggish charge propagation, and limited capacitive behavior, particularly under elevated current loads. As the current density increases, the discharge time decreases rapidly, confirming that the electrode cannot maintain stable redox reactions under fast cycling conditions. The poor rate performance and cycle stability can be attributed to low ionic diffusivity, limited redox-active surface area, and inefficient electron transport pathways inherent to the bulk Ni(OH)₂ structure. On the other hand, the GCD curves of the f-Ni(OH)₂ electrode (Fig. 4b) show significantly extended discharge durations and more symmetric charge–discharge profiles, especially at lower current densities. At 1 mA·cm⁻², the f-Ni(OH)₂ electrode delivers a discharge time of approximately 7000 s, compared to ~ 4500 s for the pristine counterpart, reflecting a substantially higher areal capacitance. Even at high current densities of 5 mA·cm⁻², the irradiated electrode maintains a relatively flat voltage plateau and reduced IR drop, suggesting improved electrochemical reversibility and lower resistance. The specific capacitance values calculated from the GCD curves (C_{s, GCD;} mF.cm⁻²) (Table 2) reveal that the f-Ni(OH)₂ electrode consistently outperforms the pristine Ni(OH)₂ at all current densities. At 1.0 mA·cm⁻², the f-Ni(OH)₂ electrode achieves a specific capacitance of 1253.5 mF·cm⁻², representing a ~ 25% improvement over the pristine sample (1002.5 mF·cm⁻²), and retains higher capacitance even at 5.0 mA·cm⁻² (89.0 vs. 57.4 mF·cm⁻²), indicating superior rate capability and efficient charge storage under high current conditions. This enhancement is attributed to the increased surface reactivity, reduced resistance, and improved ion-accessibility imparted by high-energy X-ray irradiation. The enhanced GCD behavior of the irradiated electrode is strongly correlated with the surface-level modifications induced by X-ray exposure. At an energy level of 15 MV, the dominant radiation–matter interaction mechanism is Compton scattering, which disrupts surface bonds, generates structural defects, and partially amorphizes the electrode surface without compromising bulk crystallinity. These physical modifications result in a more disordered, defect-rich interface with improved electrolyte wettability and a higher density of electrochemically active sites. Additionally, the appearance of new hydroxyl-related vibrational bands, as confirmed by Raman and FTIR spectroscopy, supports the formation of favorable redox environments that contribute to the observed pseudocapacitive behavior.

GCD curves at varying current densities (1–5 mA·cm⁻²) for (a) pristine Ni(OH)₂ and (b) high energy X-ray irradiated f-Ni(OH)₂ electrodes, and corresponding Nyquist plots from EIS analysis for (c) pristine and irradiated electrodes in 1.0 M Na₂SO₄ electrolyte.

Electrochemical impedance spectroscopy further elucidates the impact of irradiation on charge transfer and internal resistance. The Nyquist plots shown in Figs. 4c compare the impedance response of the pristine Ni(OH)₂ and f-Ni(OH)₂ electrodes, respectively. In the high-frequency region, the intercept of the real axis represents the solution resistance (R_s), while the semicircle diameter reflects the charge transfer resistance (R_ct). As a result of fitting the EIS spectra using the Randles equivalent circuit mode, the Rs value of the Ni(OH)₂ electrode was observed to be 18.4 Ω, whereas this value decreased to 13.0 Ω for the f-Ni(OH)₂ electrode. Furthermore, while the R_ct value for the Ni(OH)₂ electrode increased for the f-Ni(OH)₂ electrode. This increase can be attributed to the presence of high-energy surface defects, amorphous domains, or oxidized passive regions formed during the irradiation process, which may introduce additional resistance to electron transfer. Indeed, the functionalization process enhanced the interaction between electrolyte ions and the electrode surface, thereby increasing the pseudocapacitance. However, the structural distortions or atomic-level changes that occurred during this process likely imposed limitations on the electron transport pathways, ultimately slowing down the charge transfer kinetics. In other words, while the functionalization improved the capacitive performance, it also introduced complexity to the system’s electrochemical kinetics, thereby explaining the apparent inconsistency between the EIS and CV/GCD results. This discrepancy highlights that charge transfer and ion diffusion can be affected independently, and that each electrochemical characterization technique reflects a different aspect of the overall energy storage process.

ICP-MS data indicate a marked reduction in trace radioactive isotopes such as Ni-59 and Ni-63 following X-ray irradiation. Although present in extremely low concentrations, these isotopes may contribute to lattice strain or electronic inhomogeneity under operating conditions. Their depletion suggests that X-ray treatment leads to a purification of the electrochemically active surface, which may reduce defect-related recombination or non-productive redox sites, thereby improving overall electrode stability and response. Moreover, while the concentration of Ni isotopes such as Ni-58, Ni-60, Ni-61, and Ni-62 remains stable, the radiation-induced excitation of surface-bound ions could alter local electron density, coordination environments, or valence states, all of which play a role in modulating electrochemical behavior. These modifications, although not directly reliant on isotopic transmutation, reflect the fine balance between atomic structure and charge storage efficiency a relationship that X-ray irradiation uniquely enables by driving targeted surface functionalization without the introduction of foreign elements. The GCD and EIS analyses confirm that high-energy X-ray irradiation is an effective and scalable tool for enhancing the electrochemical performance of Ni(OH)₂ electrodes. By simultaneously reducing internal resistance, increasing active site density, and stabilizing redox behavior, the irradiated electrodes demonstrate the characteristics necessary for application in long-cycle, high-rate, and microscale supercapacitor devices.

One of the most significant challenges facing Ni(OH)₂-based electrode materials is their inherently poor long-term cyclic stability, primarily caused by irreversible structural degradation, volumetric changes, and loss of electroactive sites during repeated redox cycling. In previously reported studies, pristine Ni(OH)₂ electrodes typically exhibit capacity retention values in the range of 60–85% after 3000–5000 cycles, especially under high current densities or potential scan rates (Asaithambi et al. 2023). These limitations have significantly hindered their commercial viability in durable supercapacitor applications. As a result of 5000 CV cycles conducted at a scan rate of 500 mV·s⁻¹, the pristine Ni(OH)₂ electrode exhibited a capacitive retention of 96.3%. In contrast, the X-ray irradiated f-Ni(OH)₂ electrode in this study exhibits outstanding electrochemical stability, retaining 99.2% of its initial capacitance after 5,000 consecutive CV cycles at a potential scan rate of 500 mV·s⁻¹ (Fig. 5). The near-overlapping CV curves of the 1 st and 5000th cycles indicate negligible structural degradation, high coulombic efficiency, and remarkable redox reversibility. This superior performance can be attributed to the X-ray-induced formation of a defect-rich yet structurally robust surface that accommodates repeated ion insertion/extraction with minimal phase disruption. Moreover, the increased chemical homogeneity and the possible purification of surface-active sites evidenced by isotope depletion (e.g., Ni-59, Ni-63) may contribute to the enhanced durability and suppression of side reactions. In all measurements, the initial and final CV curves of both electrodes nearly overlap, indicating excellent structural integrity. However, the differences observed between the Ni(OH)₂ and f-Ni(OH)₂ electrodes at low scan rates largely disappeared at this high scan rate. This can be explained by the insufficient time available for redox reactions to fully occur at such a rapid scan rate of 500 mV·s⁻¹. These findings place f-Ni(OH)₂ among the most stable Ni-based hydroxide electrodes reported to date, and position high-energy X-ray irradiation as a transformative strategy for engineering ultra-stable pseudocapacitive materials.

Cycling performance of (a) pristine Ni(OH)₂ and (b) f-Ni(OH)₂ electrodes over 5,000 concequtive CV cycles at 500 mV.s⁻¹.

Table 3 presents a comparative summary of various nickel hydroxide-based electrode systems with different modifications and electrolytes, highlighting their specific capacitance, cycle stability, and performance metrics. Among the entries, f-Ni(OH)₂ demonstrates a remarkable enhancement in electrochemical performance, especially when compared to pristine Ni(OH)₂ and several reported advanced composites. Specifically, the f-Ni(OH)₂ electrode exhibits a high areal capacitance of 1253.5 mF·cm⁻² (GCD) in 1.0 M Na₂SO₄, representing a ~ 25% improvement over the unmodified Ni(OH)₂ (1002.5 mF·cm⁻²). This enhancement can be directly attributed to the formation of electroactive Ni–O–OH species induced by X-ray irradiation, which introduces additional redox-active sites and promotes improved ion diffusion and charge storage behavior. Notably, this was achieved without any compositional additives or hybridization strategies, setting it apart from most other systems in the literature that rely on carbon-based composites or MOF-derived architectures. While other reported systems, such as Ni(OH)₂@ZIF-67 and Ni(OH)₂/Ni foam structures, demonstrate higher gravimetric capacitance values (up to 1950 F·g⁻¹), these results are typically obtained in alkaline media (KOH) and involve complex fabrication processes and hybrid structures. In contrast, this study use a neutral electrolyte (Na₂SO₄) ensuring better biocompatibility and stability, which is advantageous for real-world, environmentally friendly applications. In terms of cycling performance, the f-Ni(OH)₂ electrode also exhibits excellent stability with 99.2% capacitance retention over 5000 cycles, outperforming or matching other entries such as the Ni(OH)₂/Carbon Nanotube composite (92%) and the Ni(OH)₂@ZIF-67 hybrid (94.8%).