Rationally designed NiMn LDH@NiCo₂O₄ core–shell structures for high energy density supercapacitor and enzyme-free glucose sensor

Cobalt nitrate hexahydrate [Co(NO₃)₂·6H₂O], Nickelnitrate hexahydrate [Ni(NO₃)₂·6H₂O], Urea [CO(NH)₂], Nickel chloride hexahydrate [NiCl₂·6H₂O], Manganese Chloride Tetrahydrate [MnCl₂·4H₂O], Hexamethylenetetramine [C₆H₁₂N₄, HMT], Iron (Ⅲ) nitrate nonahydrate [Fe(NO₃)₃·9H₂O], Carbon cloth (CC). All chemicals used above were of analytical grade.

The NiCo₂O₄/CC was prepared by hydrothermal method, 1 mmol Ni(NO₃)₂·6H₂O, 2 mmol Co(NO₃)₂·6H₂O and 12 mmol CO(NH₂)₂ were dissolved in 80 ml deionized (DI) water by stirring, then the mixture and a piece of CC were transferred to a 100 ml Teflon-lined stainless autoclave, which heated at 120 °C for 6 h. Then the NiCo₂O₄/CC was taken out, rinsed with DI water for several times and heated at a temperature rise rate of 5 °C min⁻¹ in N₂ flow (350 °C for 2 h).

In a typical synthesis, 3 mmol NiCl₂·6H₂O, 1 mmol MnCl₂·4H₂O and 5 mmol HMT were dissolved in 70 ml DI water by stirring. The as-obtained NiCo₂O₄/CC arrays with the solution above were transferred to a 100 ml Teflon-lined stainless autoclave and heated at 90 °C for 8 h. The product was collected and rinsed with DI water, then dried at 60 °C overnight. The estimated loadings of NiMn LDH@NiCo₂O₄ on carbon cloth is 0.98 mg cm⁻².

3 mmol Ni(NO₃)₂·6H₂O, 1 mmol Fe(NO₃)₃·9H₂O and 20 mmol CO(NH₂)₂ were dissolved in 50 ml DI water and stirred, then transferred into an autoclave with the NiCo₂O₄/CC and heated at 120 °C for 8 h. Finally, the product was rinsed with DI, dried at 60 °C overnight. The estimated loadings of NiFe LDH@NiCo₂O₄ on carbon cloth is 1.02 mg cm⁻².

Electrochemical testing for supercapacitors: the electrochemical performance was studied using the Electrochemical Workstation (CHI 760E, Chen Hua, Shanghai). In a three-electrode system, the prepared material was employed as the working electrode, and the standard mercury/mercury oxide (Hg/HgO) and platinum electrode were used as the reference and counter electrode, respectively. The electrolyte used for galvanostatic charge–discharge (GCD), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis was 2 M KOH. For the frequency range of EIS was 0.01–100 kHz with the amplitude potential of 0.005 V, and the measured potential was consistent with the open circuit voltage.

The specific capacitance can be calculated by the formula:

$$\begin{eqnarray}&&C=\displaystyle \frac{I\times {\rm{\Delta }}t}{A\times {\rm{\Delta }}V},\end{eqnarray} \tag{ 1 }$$

where I and Δt is the current (mA) and time (s) during discharge process, respectively, A is active area of electrode (1 cm²), and ΔV is voltage range. When A is replaced by m (the mass of active material), the gravimetric capacitance is obtained.

In ASCs, the loading capacity of the active material on the electrodes can be calculated by the following:

$$\begin{eqnarray}&&\displaystyle \frac{{m}_{+}}{{m}_{-}}=\displaystyle \frac{{C}_{-}\times {\rm{\Delta }}{V}_{-}}{{C}_{+}\times {\rm{\Delta }}{V}_{+}},\end{eqnarray} \tag{ 2 }$$

where m₊ and m₋, ΔV₊ and ΔV₋, C₊ and C₋ represent the mass of active substance loaded by positive and negative electrodes, potential ranges and specific capacitances, respectively. The load of the positive electrode was 0.98 mg before assembling. According to formula (2), the calculated theoretical load of the negative electrode should be 2.72 mg, and the actual load was 2.68 mg for assembling to achieve the charge balance.

The formulas for calculating energy density and power density are as follows:

$$\begin{eqnarray}&&E=\displaystyle \frac{C\times {\left({\rm{\Delta }}V\right)}^{2}}{2\times 3.6},\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&P=\displaystyle \frac{3600\times E}{{\rm{\Delta }}t},\end{eqnarray} \tag{ 4 }$$

where C is specific capacitance of ASCs, ΔV (V) and Δt (s) are the voltage ranges and discharge time, respectively.

Electrochemical test for glucose sensor: All related electrochemical tests for glucose sensor (CV and i–t) were performed in the three-electrode system using the same electrodes as above, except that the electrolyte concentration was changed to 1.5 M KOH.

The structure and morphology characterization of the electrodes were analyzed by field-emission scanning electron microscope (FESEM, Hitachi S4800, Japan) and transmission electron microscope (TEM, Hitachi H7650, Japan). The x-ray diffraction (XRD, Bruker D8 Discover) measurements with a Cu–Kα radiation from 5° to 80° (2θ range), x-ray photoelectron spectroscopy (XPS, Axis Ultra HAS with Al K radiation) were used to study the crystal structure and composition of all electrodes.

Schematic diagram for the fabrication process of NiMn LDH@NiCo₂O₄/CC electrode is shown in figure 1. Firstly, NiCo LDH nanowires (NWs) are grown directly on a carbonized hydrophilic carbon cloth (CC) by hydrothermal treatment in deionized water containing Ni²⁺, Co²⁺ and CO(NH)₂. CC as a conductive substrate provides support for the in situ growth of NiCo LDH NWs and greatly improves the conductivity of electrodes. Secondly, the roughness of the NiCo LDH NWs increases after further calcination. Finally, the NiMn LDH@NiCo₂O₄/CC electrode is fabricated by hydrothermally loading NiMn LDH nanosheets on NiCo₂O₄ NWs. The unique core–shell structure of the electrode provides an abundant surface area for the redox reaction and exposes more active sites, which enables the active substances on the electrode fully contact the electrolyte.

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As shown in figures 2(a), (b), NiCo₂O₄ grow uniformly and densely on carbon cloth fibers, and the length of the nanowires present approximately 2 μm and the diameter is 40 nm. This preparation method leaves many gaps and provides space for the subsequent growth of NiMn LDH. In figures 2(c), (d), NiMn LDH is directly synthesized on CC by hydrothermal method for comparative electrode. The NiMn LDH nanostructures are honeycombed and completely wrapped on the carbon fibers. Figures 2(e), (f) displays the morphology of NiMn LDH@NiCo₂O₄/CC at different magnifications, the NiMn LDH nanosheets are grown uniformly on the NiCo₂O₄ NWs. In addition, the nanosheets are cross-connected to build a 3D network structure with a high open space. In this way, the core–shell structure with rich surface area can provide abundant channels for chemical reactions, achieve sufficient contact between electrodes and electrolytes, and facilitate the transfer of ions and electrons. Figures 2(g), (k) shows the SEM and the EDS images of the NiMn LDH@NiCo₂O₄/CC. The existence and the uniform distribution of Ni, Co, Mn and O elements in the core–shell structure are further confirmed. Figures S1(a), (b) (available online at stacks.iop.org/NANO/32/505710/mmedia) also displays two-dimensional sheets of NiMn LDH shell structures are distributed around NiCo₂O₄ NWs, which indicates the successful preparation of electrode materials.

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As shown in the figure 3(a), the accurate crystal phase and crystalline structure of the prepared NiMn LDH@NiCo₂O₄/CC electrode were characterized by XRD. According to the analysis of the JCPDS No. 73-1702 card and NiCo₂O₄/CC peaks, it is clearly shown that there are obvious characteristic diffraction peaks (denoted by '♥') at 36.8°, 44.5° and 62.8°, corresponding to the (311), (400) and (440) planes of NiCo₂O₄ phases [37], respectively. Furthermore, the peaks (denoted by '♦') at 11.5°, 22.7°, 34.6°, 59.9° are indexed to (003), (006), (012) and (110) planes of the NiMn LDH (JCPDS No. 38-0715) [38, 39]. Finally, the above specific diffraction peaks of NiMn LDH@NiCo₂O₄/CC are all shown in the XRD curves, indicating that the electrodes with NiCo₂O₄ as core and NiMn LDH as shell are prepared successfully. It is noteworthy that the NiMn LDH peak in the XRD pattern is wider and has a lower intensity because of the poor crystallinity of LDH and the less NiMn LDH nanosheets grown on the carbon cloth. To further comprehend the chemical state of the electrode, XPS was used for quantitative and qualitative analysis of the sample, as shown in the figure 3(b). The full scan spectrum reveals that the electrode contained Ni, Co, Mn, O, C and N elements. In figure 3(c), the spectrum of Ni 2p displays two satellite peaks considered as 'sat', in company with two spin–orbit peaks at 873.2 (Ni 2p_1/2) and 855.5 eV (Ni 2p_3/2) [40], both of which can be split into two sub-peaks corresponding to the Ni²⁺ and Ni³⁺ oxidation states, respectively [41]. Figure 3(d) further illustrates the spectrum of Co 2p, in which the two peaks located at 796.7 (Co 2p_1/2) and 781.2 eV (Co 2p_3/2) are originated from the presence of Co²⁺ and Co³⁺ [42]. As shown in figure 3(e), two satellites appear at 654.1 and 643.3 eV, which are corresponded to Mn 2p_1/2 and Mn 2p_3/2 of the trivalent Mn³⁺ oxidation state of NiMn LDH [43]. For the O1s XPS spectrum shown in figure 3(f), the three peaks at 532.5 (O3), 531.3 (O2) and 530.7 (O1) eV are assigned to typical metallic oxygen, hypoxic defects and surface adsorption H₂O defects, respectively [43]. In summary, the conclusions obtained from the XPS spectrum are consistent with the results of XRD and SEM, demonstrating that the NiMn LDH@NiCo₂O₄/CC electrode material with core–shell structure has been successfully synthesized.

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Electrochemical tests were carried out by further evaluating the electrodes. Figure 4(a) shows the integral area of the NiMn LDH@NiCo₂O₄/CC electrode presents the largest at 10 mV s⁻¹, revealing the specific capacitance is the highest among three electrodes. Moreover, the oxidation and reduction peaks of the electrode is obvious and symmetrical, which suggests that the NiMn LDH@NiCo₂O₄/CC electrode possesses a higher electrochemical activity and outstanding energy storage capability. Figure 4(b) displays the GCD curves of NiCo₂O₄/CC, NiMn LDH/CC and NiMn LDH@NiCo₂O₄/CC at 5 mA cm⁻². The discharge time of them are 35, 120 and 270 s, respectively, illustrating the NiMn LDH@NiCo₂O₄/CC electrode presents a higher specific capacitance [10]. Figure 4(c) displays the CV curves of the NiMn LDH@NiCo₂O₄/CC electrode at different scan rates (5, 10, 30, 50, 70 and 80 mV s⁻¹). Obviously, all CV curves have roughly the same trend and symmetric redox peaks, demonstrating that the capacitance is derived from a rapid Faraday reaction [44]. The anode peak and the cathode peak move in the positive and negative directions at various scan rates owing to the better reversibility of the electrode, and which is also influenced by the increase of internal diffusion resistance and slight polarization phenomena. Figure 4(d) illustrates that the GCD curves of the NiMn LDH@NiCo₂O₄/CC electrode at different current densities (1–20 mA cm⁻²) under voltage window of 0–0.5 V. Each curve possesses an obvious platform, and the charge and discharge time is also identical. It is indicated that the electrode shows high redox reaction reversibility and Coulomb efficiency. The corresponding area specific capacitance is calculated to be 3.09, 2.98, 2.71, 2.56, and 2.40 F cm⁻² at 1, 2, 5, 10 and 20 mA cm⁻² (3153, 1520, 553, 261 and 122 F g⁻¹ at 1, 2, 5, 10 and 20 A g⁻¹). The specific capacitance of the electrode decreases with the enhancement of current density because of insufficient chemical reaction under high current. As shown in figure 4(e), the optimized electrode still maintains outstanding specific capacitance of 76.22% from 1 to 20 mA cm⁻². It can be observed that the capacitance performance of the NiMn LDH@NiCo₂O₄/CC electrode is significantly superior than that of NiCo₂O₄/CC and NiMn LDH/CC, which is attributed to the distinctive core–shell structure. The surface roughness of calcined NiCo₂O₄ NWs increases, and which assists NiMn LDH nanosheets in growing more oriented and firmly on the nanowires without aggregation. The combination of different compositions and structures can produce synergistic effects and increase functionality. Therefore, the core–shell structure behaves better than unitary structure [45].

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To further explain the ion diffusion kinetics of the NiMn LDH@NiCo₂O₄/CC electrode, the EIS test was performed, as shown in figure 4(f). The EIS curve is mainly divided into two parts [46]: the high frequency area and the low frequency area represented by a semicircle and the diagonal line. The slope of the curve in low frequency region represented by NiMn LDH@NiCo₂O₄/CC is the highest, revealing the smallest ion diffusion resistance (R_w) and the strongest diffusion ability, reflecting OH⁻ can transport and exchange electrons freely near the interface of the electrode [47]. The response of the high frequency area of EIS can be evaluated from the partial enlarged graph of the curve. It is obvious that the intercept between the curve and the real axis is very small, which means that the interface resistance (R_s) between the electrolyte and the electrode is low, illustrating that the core–shell structure is beneficial to the transport of ions and increase the active area of chemical reactions. The NiMn LDH@NiCo₂O₄/CC electrode maintains outstanding capacitance retention and low resistance owing to the superior electrical conductivity of carbon cloth substrate and the synergistic behavior of the core–shell structure.

Generally, the CV curve of the electrode reflects the capacitance behavior and the diffusion control process. In order to further clarify the charge storage mechanism of the electrodes, the CV tests are used to calculate the relationship between different scan rates and peak current shown in formula (5) [48], which can be applied to analyze the capacitance behavior of the electrodes during the charge and discharge processes

$$\begin{eqnarray}&&i=a{v}^{b}.\end{eqnarray} \tag{ 5 }$$

As shown in the formula (5), i (mA) and v (mV s⁻¹) mean the different peak current values and scan rates, respectively. Where a and b are both adjustable coefficients. The value of slope b is obtained by fitting the straight line according to the above formula. The range of b value is mainly divided into 0.5, 0.5–1 and ≥1, which represents the battery property, coexist battery and pseudo-capacitance properties, and pseudo-capacitance property, respectively. Figure 5(a) shows that the electrode possesses dual properties of battery and pseudo-capacitance. It also means the electrode presents both diffusion and capacitance control during charging and discharging [49].

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According to formula (5), the contribution rate of pseudo-capacitance to charge storage is calculated as follow:

$$\begin{eqnarray}&&i\left(V\right)={k}_{1}v+{k}_{2}{v}^{1/2}.\end{eqnarray} \tag{ 6 }$$

In this formula, i (V, mA) represents the current value corresponding to a specific voltage at various scan rates (v, mV s⁻¹), while k₁ and k₂ are adjusted parameters. For the current value corresponding to each voltage, i(V)/v^1/2 and v^1/2 in formula (6) can be fitted to calculate the corresponding value of k₁ . Figure 5(b) depicts the contribution curves of the pseudo-capacitance of the three electrodes, and which are calculated from a number of k₁ v values. Obviously, the capacitance contribution enhances as the scan rate increases because the diffusion-controlled current presents a slower time response. The transfer and storage of charges on the electrode surface is easier to proceed with the scan rate increases due to the surface adsorption. As shown in figures 5(c)–(e), the contribution to area specific capacitance of the NiMn LDH@NiCo₂O₄/CC (32.88%) electrode is larger than NiMn LDH/CC (23.67%) and NiCo₂O₄/CC (19.71%) at 5 mV s⁻¹, confirming the superiority of the NiMn LDH@NiCo₂O₄ core–shell structure. The analysis of the capacitance contribution rate verifies that the core–shell structure of 2D nanosheets uniformly grown on 1D nanowires can effectively increase the specific surface area, which provide abundant active centers for the reaction and improve the electron transport efficiency.

To evaluate the electrochemical performance of the NiMn LDH@NiCo₂O₄/CC electrode, an ASC with NiMn LDH@NiCo₂O₄/CC as the positive electrode and NiFe LDH@NiCo₂O₄/CC as the negative electrode were assembled. As shown in figure 6(a), the CV curve of NiFe LDH@NiCo₂O₄/CC is similar to a rectangular structure compared with that of NiMn LDH@NiCo₂O₄/CC, which means that the negative electrode possesses the performance of electric double-layer capacitance. And the GCD profile of the NiFe LDH@NiCo₂O₄/CC electrode is shown in figure S2. The gravimetric capacitance of NiFe LDH@NiCo₂O₄/CC electrode is calculated, which are 812, 378, 146, 70 and 34 F g⁻¹ at 1, 2, 5, 10 and 20 A g⁻¹. In figure 6(b), the assembled ASC is tested by CV curves under different voltages to determine the broadened voltage window at 10 mV s⁻¹. Polarization of CV curve could be observed when the voltage is greater than 1.4 V, therefore the voltage window is selected to 0–1.4 V. Figure 6(c) displays the CV tests of ASC at 10–100 mV s⁻¹, and the shape does not change a lot as the scan rate increases, revealing the outstanding fast charging and discharging performance of ASC. In figure 6(d), it shows that the GCD curves of the equilateral triangle shape, demonstrating that ASC presents typical double-layer capacitor characteristics. The gravimetric capacitance of ASC are 174, 167, 152, 147, 100, and 89 F g⁻¹ at 0.5, 1, 1.5, 2, 4, and 5 A g⁻¹. When the current is increased 10 times from 0.5 A g⁻¹ to 5 A g⁻¹, it can still maintain a rate capability of 51.15% (figure S3). As can be seen in figure 6(e), the power and energy density are respectively calculated. It shows that the energy density of the electrode reaches the maximum of 47.74 Wh kg⁻¹ at the power density of 175 W kg⁻¹, which is more excellent than other supercapacitors [29, 50–53], confirming that the ASC shows a broad practical application prospect. Figure 6(f) shows the capacitance retention rate can reach 93.48% after 6000 cycles at 10 mA cm⁻², which displays better cycle stability. As shown in Video 1, the ASC can drive a small fan to rotate after charging, indicating the prepared NiMn LDH@NiCo₂O₄/CC electrode shows application potential for the future research of energy storage devices.

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The electrolyte concentration shows an crucial influence on the performance of different electrodes [54]. Figures 7(a), (b) displays the CV measurements of NiMn LDH@NiCo₂O₄/CC with different concentrations of KOH solution (adding 1 mM glucose) at 10 mV s⁻¹. The current response of the electrode to 1 mM glucose increases with the gradual increase of the electrolyte concentration (0.5–1.5 M). The current response is the largest when the electrolyte concentration is 1.5 M, indicating that the appropriate OH⁻ concentration is beneficial to improve the catalytic efficiency and promote the occurrence of redox reactions. However, the current response of the electrode to 1 mM glucose decreases when the electrolyte concentration reaches 2 M, revealing that the high concentration of OH⁻ would hinder the catalytic activity of the electrode to glucose. Therefore, the optimum concentration of KOH is 1.5 M for glucose oxidation.

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Figures 8(a), (b) displays that the CV curves of the electrode with 1 mM glucose in 1.5 M KOH. It is clearly seen that the current peaks show a regular positive shift with increasing scan rate (5–70 mV s⁻¹). From the linear fitting results of peak current, the surface control process affects the electrochemical reaction [55]. Figure 8(c) displays the CV curves of different electrodes in the absence and the presence of 1 mM glucose in 1.5 M KOH. It is observed that the NiMn LDH@NiCo₂O₄/CC exhibits a superior current density and integral area than other electrodes. The current density of the anode peak increases due to the presence of glucose, illustrating that the electrode is beneficial to the oxidation of glucose. To further prove the electrochemical response of electrodes to glucose, it is tested with 0–8 mM glucose added in 1.5 M KOH. Figure 8(d) shows the peak current of the anode increases regularly with the enhancement of glucose concentration and the correlation diagramas is shown in figure S4, showing that the NiMn LDH@NiCo₂O₄/CC electrode has good electrochemical activity to glucose. The unique core–shell structure provides more active area and facilates the glucose oxidation [56, 57], and the reactions are summarized by the following chemical equations (M = Ni, Co, Mn):

$$\begin{eqnarray}&&{{\rm{NiCo}}}_{2}{{\rm{O}}}_{4}+{{\rm{OH}}}^{-}+{{\rm{H}}}_{2}{\rm{O}}\to {\rm{NiOOH}}+2{\rm{CoOOH}}+{{\rm{e}}}^{-},\end{eqnarray} \tag{ 7 }$$

$$\begin{eqnarray}&&{\rm{M}}{\left({\rm{OH}}\right)}_{2}+{{\rm{OH}}}^{-}\to {\rm{MOOH}}+{{\rm{H}}}_{2}{\rm{O}}+{{\rm{e}}}^{-},\end{eqnarray} \tag{ 8 }$$

$$\begin{eqnarray}&&{\rm{MOOH}}+2{\rm{glucose}}\to {\rm{M}}{\left({\rm{OH}}\right)}_{2}+2{\rm{gluconolactone}}.\end{eqnarray} \tag{ 9 }$$

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The sensitivity displays a vital standard to judge the performance of sensors. As depicted in figures 9(a), (b), the optimal working voltage of the electrode is selected through i–t test and linear fitting. It can be observed that the current response of the NiMn LDH@NiCo₂O₄/CC electrode increases with the enhancement of the working voltage. When the voltage is +0.45 V, there is a relatively high current response and a stable baseline, and the degree of fit (R²) of the linear fit is large. Therefore, +0.45 V is selected as the optimal working voltage. Figure 9(c) shows the i–t response of different electrodes to 1 mM glucose solution. Obviously, the NiMn LDH@NiCo₂O₄/CC electrode displays the highest current response toward glucose. Figure 9(d) displays the linear fitting relationship between glucose concentration and time for different electrodes. The NiMn LDH@NiCo₂O₄/CC electrode has a better fitting. The results indicate that the electrode shows outstanding sensing performance, which is attributed to the core–shell structure exposing more active sites and promoting the occurrence of catalytic reactions.

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Figure 10(a) shows the i–t curve of continuously adding of glucose with different concentrations to 1.5 M KOH solution within 3000 s. It can be seen the current density begin to respond significantly when the glucose concentration approaches 2 μM from the partial enlarged detail. The current response is stable and slow at low concentrations, while the response becomes faster at higher glucose concentrations. However, when the concentration is close to 8 mM, the baseline fluctuates greatly and the linear trend begins to deviate, indicating that the glucose concentration is high. As shown in figure 10(b), the electrode shows different linear correlation in the range of 2 μM⁻³ mM and 4–8 mM with sensitivities of 2139 and 861 μA mM⁻¹ cm⁻². And the minimum detection limit can be calculated as follow formula: LOD = 3S_b/K_l (S/N = 3). Figure 10(c) shows that the current responds quickly within 5 s. At the same time, the anti-interference performance of the electrode is evaluated. The concentration of each interference species (UA, Cys, AA, DA, KCl, NaCl, Suc, Lac, Fru) is 0.1 mM. In the anti-interference test process, glucose and interfering substances are added to the electrolyte sequentially [58, 59]. Figure 10(d) displays the current response is rapidly amplified when 0.1 mM glucose is added in 1.5 M KOH. However, the current density presents no response or minimal response when other interferents are added, and the current density rises rapidly when glucose is added again, revealing that the electrode behaves good anti-interference performance. In conclusion, the electrode shows higher sensitivity, lower detection limit and better anti-interference ability, which provides a new strategy for the research of glucose sensor in the future.

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The reproducibility of the NiMn LDH@NiCo₂O₄/CC electrodes were evaluated by six fabricated electrodes, as shown in figure 11(a). The relative standard deviation (RSD) is calculated as 2.49%. In figure S5, the sensitivities of six different electrodes (0.1 mM glucose is added every 100 s) are tested, and the RSD value is only 3.95%, indicating that the electrode has good reproducibility. In order to measure the long-time stability of the electrodes, the oxidation peak current of the electrode with 1 mM glucose is measured every 5 d at 35 mV s⁻¹. Figure 11(b) shows the NiMn LDH@NiCo₂O₄/CC electrode can still maintain a current density of 89.87% after 30 d, demonstrating that the electrode material has superior reproducibility and stability. To evaluate the mechanical stability of the core–shell heterostructure, the SEM images of the electrode after glucose sensing are collected. As shown in figures S6(a), (b), the fibers on the surface of the electrode after glucose sensing are intact without breaking, and the morphology of the core–shell heterostructure has not collapsed. In addition, the CV curves have no change significantly before and after the electrode is bent (figures S6(c), (d)), revealing that it has good mechanical stability. In general, compared with other similar electrodes in table 1, the NiMn LDH@NiCo₂O₄/CC electrode exhibits higher sensitivity, larger detection range and minimum detection limit towards glucose oxidation. The results show that the core–shell structure electrode represents outstanding advantages when applied to glucose sensors.

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The core–shell heterostructure can improve the charge transfer ability of the redox reaction, and the synergistic effect of the multi-element metal is beneficial to improve the conductivity when it is used as the electrode of the supercapacitor [65]. In addition, since glucose sensing is mainly controlled by the diffusion process, the rich specific surface area and carrier density of the heterostructure are critical to the sensitivity of the sensor. The excellent selectivity of the electrode is attributed to the electrostatic repulsion of the heterostructure and interference. In general, the heterostructure provides a higher diffusion area for the reaction [16]. When it is applied to supercapacitors and glucose sensing, it can increase the diffusion rate of reactants and cause faster charge transfer.