A Novel Ternary Pseudocapacitive Electrode with Synergistic Contributions

Ternary electrodes represent an exciting strategy to substantially enhance the performance of supercapacitors beyond what binary electrodes offer. However, the interplays among ternary constituents and their possible synergies remain poorly understood. This study investigates a ternary electrode design, wherein ferric oxyhydroxide (FeOOH) and magnesium dioxide (MnO2) are co‐deposited onto carbon nanotube (CNT) mats. The results reveal that the ternary electrode demonstrates a 33.3% reduction in internal resistance and a 59% increase in areal capacitance compared to its binary MnO2/CNT counterpart. Furthermore, the ternary electrode achieves a 25% increase in capacitance compared to the combined capacitances of separate MnO2/CNT and FeOOH/CNT electrodes. These findings demonstrate that combining FeOOH and MnO2 can synergistically enhance electrical conductivity and pseudocapacitance beyond their binary counterparts. This design yields a surface capacitance exceeding 3500 mF cm−2 at an active material loading up to 15 mg cm−2. By pairing with a binary FeOOH/CNT electrode, the resulting asymmetric supercapacitor exhibits an operational voltage window to 1.6 V. To demonstrate the potential of the new design, ternary electrodes are integrated into wearable and structural supercapacitors. The new synergistic ternary electrode approach provides a promising avenue for enhancing energy storage capacitance and expanding the scope of energy storage structure applications.


Introduction
[3] Moreover, the implementation of effective energy storage solutions is critical to the successful integration of renewable energy sources within the aviation sector. [4]For instance, Flygildi, an Icelandic drone start-up, has recently crafted an unmanned aerial vehicle that mimics bird flight with the integration of a structural energy storage system as its power source.The system turns carbon fiber body parts into a supplementary battery, where the wing armature can store energy without the need for additional batteries. [5]Structural supercapacitors are particularly promising in powerintensive situations like emergency aircraft doors, [6] drills in space stations, [7] and battery management in electrical vehicles. [8]However, a significant limitation to their widespread adoption is the relatively low energy density.To address this impediment, pseudocapacitive materials have been utilized, given their potential to considerably boost energy density.Examples include oxidized black phosphorus nanosheets coupled with graphene, which have shown a remarkable pseudocapacitive mechanism that simultaneously bolsters capacitance and cycling stability. [9]Another recent example is the use of a solvated-ion-intercalated hydrothermal strategy for manufacturing Molybdenum Disufide (MoS 2 ) film electrodes. [10]mong various pseudocaptive materials, manganese dioxide (MnO 2 ) provides a high capacitance of 1380 F g −1 [11] in theory, and it is also cost-effective, abundant, and has low toxicity.However, compared with electric double-layer capacitor electrodes, many transition metal oxides (TMOs), such as MnO 2, suffer from lower stability, [12,13] poorer electrical conductivity (i.e., 10 −7 to 10, −6 [14] ) and lower ion diffusion rates, [15] which have hindered their practical applications in powering electronic devices. [16] promising strategy to tackle the above challenges is to develop ternary electrodes, which consist of three distinct energy storage phases.Specifically, in supercapacitors, a ternary electrode can comprise three different materials: an electrically conducting substrate [17] along with two pseudocapacitive materials (e.g., transition metal oxide like MnO 2 or layered double hydroxides (LDH)). [18]Compared to binary electrodes, which incorporate only one pseudocapacitive material onto a conductive substrate, ternary electrodes introduce two pseudocapacitive materials that are strategically organized to produce enhanced capacitive contributions.This design increases the density of redox reaction sites and offers a favorable morphology that facilitates faster electron/ion transfer kinetics.These attributes lead to more seamless interface interactions, harnessing the collective advantages and minimizing the individual drawbacks. [17]For example, Zhang and his team reported a ternary composite electrode, referred to as graphite felt@vanadium oxide@polyindole (GVP-3), which was made of graphite felt substrates, vanadium pentoxide, and polyindole.This composite electrode displayed an impressive capacitance of 2254 mF per square centimeter.This performance distinctly surpassed that of its binary counterparts: graphite felt@vanadium oxide (GF@V 2 O 5 ) recorded a capacitance of 1431 mF cm −2 , while graphite felt@polyindole (GF@PIn) managed 647 mF cm −2 , thereby underscoring the enhanced electrochemical attributes of the ternary composite over binary ones. [19]owever, managing and simultaneously optimizing for electrochemical efficiency in ternary electrodes made of nanocomposites [20] has inherent difficulties, with the fabrication of such multi-component hybrid electrode materials typically involving complex and time-consuming procedures.For instance, Xu et al. fabricated graphene/polyaniline/MnO 2 ternary supercapacitor electrodes [21] using a sequential electrodeposition method that may be impractical for large-scale productions.Huang et al. fabricated conductive yarns modified with reduced graphene oxide (rGO) that were further adorned with a hierarchical arrangement of MnO 2 and polypyrrole.This intricate design was achieved through the integration of the twist-bundle-drawing technique with hydrothermal and electrodeposition methods. [22]Furthermore, achieving optimal performance in ternary electrodes can be challenging due to the complex electrode structure without resorting to large quantities of active materials.For instance, although some work reported outstanding specific capacitance (e.g., 3346.6 F g −1 ), the mass loading of ternary electrodes remains low (i.e., 1 mg). [23]Therefore, it remains a major challenge to develop novel material combinations that can synergistically enhance the capacitance performance of supercapacitors, while also streamlining the manufacturing processes to be suitable for large-scale production.
In this work, we propose a facile electrodeposition method for the fabrication of a ternary electrode consisting of pseudocapacitive MnO 2 and FeOOH on carbon nanotube (CNT) mats.The method involves a novel co-deposition method to graft high loading of MnO 2 and FeOOH onto the CNT mat substrates.It's important to highlight that substrates are crucial for the performance of supercapacitors.Carbon substrates are frequently chosen because of their robust mechanical properties, expansive surface area, and excellent conductivity.Among various substrate options, CNT mats emerge as an outstanding candidate due to their exceptional conductivity and robust mechanical properties, qualifying them as an ideal substrate. [24]Iron-based materials are increasingly recognized as promising candidates for supercapacitors.This is attributed to their multiple valence states, specifically Fe, Fe 2+ , and Fe 3+ , their stable and expansive operating windows, and their natural abundance. [25]To achieve this, an acidic precursor solution containing Fe 3+ and Mn 2+ ions is utilized, serving multiple functions.First, the precursor involved in the procedure slightly oxidized the surface of the CNTs, resulting in the creation of more reactive locations for subsequent deposition.By exploiting the difference of the standard reduction potential between Fe 3+ /Fe 2+ and MnO 2 /Mn 2+ , [26] the surplus Fe 3+ ions react with MnO 2 leading to further incorporation of FeOOH into the MnO 2 layer at elevated temperature.The incorporation of the FeOOH component enhances the electrode conductivity, compensating for the lower conductivity limitations of MnO 2 , and also providing additional pseudocapacitance. [27]This synergistic effect significantly enhances the electrochemical performance of the nanocomposite electrode, denoted as MnO 2 /FeOOH/CNT (MFC), leading to an impressive mass loading of active materials (i.e., greater than 15 mg cm −2 ) and surface capacitance that exceeds 3500 mF/cm 2 (equivalent to 438 F cm −3 ).This study introduces the concept of the "synergistic ratio" to quantify the level of synergy between different active materials of FeOOH and MnO 2 .This ratio is determined by dividing the capacitance of the ternary system by the sum of the energy densities of the two binary systems (i.e., MnO 2 /CNT and FeOOH/CNT).Results suggest a promising synergistic ratio of 1.25 between MnO 2 and FeOOH when combined with CNT.To enhance the energy density of supercapacitors, we paired a FeOOH/CNT (FC) electrode with the MFC electrode.In this setup, the FC electrode serves as the negative electrode, facilitating the formation of an asymmetric supercapacitor.This design not only broadens the voltage window but also results in a supercapacitor boasting a substantially enhanced energy density, all while preserving its high-power efficiency.Furthermore, we demonstrate the satisfactory performance of the asymmetric supercapacitors with two different solid electrolytes: i) a PVA-based gel electrolyte suitable for wearable applications and ii) a polyvinylidene fluoride (PVDF)/ionic liquid-based electrolyte capable of operating across a wide temperature range.

Microstructures and Properties of Electrodes
The morphologies of the FeOOH/CNT electrode, the MnO 2 /CNT electrode, and the MFC electrodes are shown in Figure 1. Figure 1a provides a visual illustration of the morphologies of the FeOOH/CNT electrode, where the active material FeOOH encases the underlying CNT mat.The average diameter of the FeOOH-coated CNT bundles measures ≈200 nm, significantly larger than that of the uncoated CNT bundles.Despite the presence of the coating, individual CNT bundles are clearly discernible, indicating that without the manganese acetate precursor, the electrodeposition yielded a limited mass loading of FeOOH.By contrast, the MnO 2 /CNT electrode fabricated without FeCl 3 in the precursor shows a classic morphology, as shown in Figure 1b.The MnO 2 particle assembly exhibits a rough, heterogeneous morphology.These particles form a porous matrix through a dense agglomeration of flaky, angular grains, with pore diameters ranging from 200 to 500 nm.Distinct from the FeOOH/CNT and the MnO 2 /CNT electrodes, the MFC electrodes feature nanoflower structures, as depicted in Figure 1c.These nanoflowers, comprised of numerous nanoscale petals substantially enhance the surface-to-volume ratio.The nanoflowers not only offer various active sites but also promote improved electrolytic ion diffusion, facilitating a more effective material utilization during electrochemical processes, which is reflected in an increased charge storage capacity and accelerated chargedischarge kinetics.In terms of mass loading of active materials, the MnO 2 /CNT electrodes contain 8 to 10 mg cm −2 of MnO 2 .In contrast, the MFC electrodes demonstrate a notably higher mass loading of 15 mg cm −2 .This contrast is visually evident in the inset of Figure 1c, showing that both MnO 2 and FeOOH are extensively deposited on the surface and within the interior of the CNT mat, as depicted in Figure 1b, where only a few uncoated CNTs remain discernible.The increased volumetric utilization in MFC electrodes, coupled with a substantially higher specific surface area, results in a significantly greater number of active sites.These attributes contribute to the superior energy storage performance of the MFC electrodes.
X-ray powder diffraction (XRD) patterns for the pristine CNT, MnO 2 /CNT, FeOOH/CNT, and the MFC electrodes are depicted in Figure 1d.Diffraction characteristic peaks assigned to Fe, MnO 2 , FeOOH, and CNT are denoted as diamond, triangle, pentagon, and hollow circle, respectively.Accordingly, peaks to assigned (100), (101), (102), and (110) planes are observed in both MFC and MnO 2 /CNT, indicating the presence of -MnO 2 [28] and the fact that the formation of FeOOH does not alter the crystallinity of MnO 2 .Furthermore, in the spectrum of MFC, characteristic peaks of FeOOH were observed around 28°.It is im-portant to note that due to the partial overlap of several characteristic peaks between FeOOH and MnO 2 , the XRD spectrum of the MFC sample appears slightly broader compared to that of MnO 2 /CNT.To further elucidate this issue, XPS characterization was also performed.Table S1 (Supporting Information) in Supporting Information details the XPS atomic percentages and binding energy positions determined at two locations on the MFC electrodes for the elements detected.The elemental ratio within the electrode is ≈1:2 for the Fe:Mn oxides, suggesting the coexistence of MnO 2 and FeOOH (Figure S1, Supporting Information).Figure 2a-c shows the Fe 2p, Mn 2p, and O 1s photoelectron spectra from one location on the electrode surface, identifying Fe 2p is present in the 3 + oxidation state, confirmed by the peaks at 710.8 and 724.3 eV for the 2p 3/2 and 2p 1/2 , peaks respectively, accompanied by the satellite peak at 718.7 eV, which is a fingerprint for the 3+ state. [29]The Mn 2p 3/2 peak shows a distinct shoulder and peak fitting is consistent with Mn present in two oxidation states, 641.0 and 642.5 eV for the 2+ and 4+ oxidation states, respectively.The corresponding O 1s spectrum reveals peaks at 529.9 eV, 531.3, and 532.3 eV, corresponding to the iron and manganese oxide, hydroxide, and acetate residue, respectively.A plot of the experimentally determined binding energy shifts for the Fe 2p and Mn 2p peaks compared to the literature in Figure 2d supports the oxidation state assignments for the MFC electrodes close to the surface area.The binding energies and shake-up satellite position for the Fe and Mn oxides are very similar to previously reported results [29] for Fe-Mn mixed oxide prepared by microemulsion methods where the Fe-Mn phases  [29,[45][46][47] are distributed homogeneously.The Mn-Fe interactions are expected to be enhanced as the homogeneity increases and is typically observed with oxide particles in the nanometer range.
To substantiate the co-existence of FeOOH and MnO 2 on CNTs, transmission electron microscopy energy dispersive Xray (TEM EDX) spectroscopy was performed, with the results shown in subsequent figures.In Figure 3a, the EDX mapping region shows a layer of active materials along with a single CNT. Figure 3c-g demonstrates the simultaneous presence of Mn, Fe, and O within the same region, thus confirming the co-existence of Fe and Mn oxides.Figure 3e reveals regions corresponding to the Fe nanoparticles (FeNPs), notably brighter, indicating their role as catalysts in the CNT manufacturing process.This observation is reinforced by the findings in Figure S2 (Supporting Information), which depicts similar features of FeNPs in samples containing only CNTs, as illustrated in Figure 3e.Importantly, our analysis of data obtained from XPS (Figure 2) and XANES (Figure 4) reveals an oxidation state of Fe exceeding +2, thereby confirming the absence of significant metallic Fe within the coating.Therefore, the contribution of these FeNPs to the overall Fe mapping within the coating area is negligible.
Further experiments were performed to highlight the distinct capacitance profiles of the different electrodes, thereby reinforcing the unique benefits of the co-deposition method.The data presented in Figure S3 (Supporting Information) show that the ternary electrodes fabricated through the co-deposition of MnO 2 and FeOOH, outperform the binary electrodes comprising solely MnO 2 or FeOOH, as well as electrodes produced via sequential deposition methods (where either MnO 2 or FeOOH is deposited first, followed by the other).This enhanced performance strongly suggests a more effective charge storage mech-anism in the co-deposited electrodes than in the single-material counterparts.
The enhanced co-deposition of MnO 2 and FeOOH can be attributed to the employment of a high-current electrodeposition technique.Unlike the lower currents (< 1 mA cm −2 ) and shorter deposition durations typically reported in the literature, [30] an exceptionally high current density (10 mA cm −2 ) was employed in this study.The potential voltage used for the electrodeposition of MnO 2 on CNT electrode is ≈0.9 V versus Ag/AgCl while a voltage ≈1.5 V was employed for the electrodeposition of MnO 2 /FeOOH onto CNT mats.The use of such high current density, coupled with the precursor solution, may partially oxidize the underlying CNT mats and create additional reaction sites.As illustrated in Figure S4 (Supporting Information), the Raman spectra presented in the figure distinctly show the characteristic D-band and G-band peaks associated with CNT mats, both in their pristine form and after etching with FeCl 3 .The G-band, indicative of graphitic domains, appears ≈1580 cm −1 , whereas the D-band, signaling structural irregularities within the carbon, is observed ≈1350 cm −1 .The D-to-G intensity ratio for the unmodified CNT mat is 0.28.In contrast, after immersion in FeCl 3 at elevated temperatures, the D-to-G intensity ratio increases significantly to 0.61.This substantial increase in the D-to-G ratio reflects an increase in the disorder and defect concentration in the carbon network, indicating that an oxidization process has occurred.Moreover, the FeCl 3 treatment likely introduces defects or compromises the structural integrity of the CNTs, thereby increasing their resistance.Four-point probe resistance measurements indicate that the initial sheet resistance of the untreated CNT mat was 0.074 Ω per square.Subsequent immersion of the CNT mat in a FeCl 3 solution for 40 min at 60 °C, the sheet resistance increased significantly to 13.63 Ω per square for the FeCl 3 -treated CNT sample.
The formation of FeOOH is corroborated by X-ray diffraction (XRD) analysis, which confirms the energetic stability of the phase FeOOH in the newly developed MFC, aligning with previous findings. [31]To gain further insight into the composition of the synthesized MFC electrode, X-ray Absorption Near Edge Structure (XANES) data were obtained and the results are presented in Figure 4.
In Figure 4a, the spectrum of the MFC electrode and MnO 2 /CNT electrode synthesized in this work are compared with other reference spectrums (i.e., MnO, MnO 2 , MnO 3 , Mn 3 O 4 .)Therefore, the oxidation state of manganese (Mn) can be deduced from the linear regression analysis of the oxidation states of these Mn oxides.Figure 4b suggests that the average Mn oxidation state of Mn in the newly synthesized MFC electrode lies between +3 and +4. [32]Compared to the oxidation state of the MnO 2 /CNT electrode, which is 3.4, the MFC electrode displays a marginally lower oxidation state.This indicates a possible interaction between the Fe and Mn elements within the MFC electrodes, which could be a major mechanism responsible for the enhanced electrochemical performance.In a similar manner, the oxidation state of iron (Fe) was evaluated by comparing the XANES spectra of the MFC electrode against those of standard references of FeO, Fe 3 O 4 , Fe 2 O 3 , and FeOOH.The results show that the normalized absorption spectrum of the Fe element in MFC is similar to that of FeOOH.The regression analysis reveals that the oxidation state of Fe in the crafted MFC electrode is ≈2.7, verifying the findings from XPS.Additionally, a linear combination fitting analysis was utilized and illustrated in Figure S5 (Supporting Information), demonstrating that the Mn content in the MFC electrode is ≈70% MnO 2 and 30% Mn 3 O 4 .Similarly, the Fe content in the MFC electrode is represented by a mixture of FeOOH and Fe 3 O 4 , with the FeOOH component being the dominant contributor (exhibiting a coefficient value of 0.8) to the resultant spectra.
In summary, the findings from XANES, XPS, and XRD together provide a comprehensive characterization of MFC electrodes, both at the surface and within the bulk.XANES analysis reveals a dominant +3 oxidation state for iron in the bulk of the MFC electrode, with traces of a lower oxidation state suggestive of Fe 3 O 4 -a hybrid of Fe 2 O 3 and FeO.This observation is consistent with XPS results, which offer a detailed examination of the electrode's surface, probing the outermost 5 nm and identifying Fe 2 O 3 , potentially formed through the dehydration of hydroxides under ultrahigh vacuum conditions.On the other hand, XRD, tailored to identify long-range atomic order in crystalline materials, reveals minimal iron signals, indicating that the electrode predominantly possesses an amorphous structure or lacks significant crystalline order.
By integrating the insights garnered from these analytical techniques, it is concluded that the MFC electrode is characterized by non-standard Fe oxides and displays localized atomic level alterations, with MnO 2 being the primary phase.These findings reveal a complex interaction between Fe and Mn, which is crucial for imparting the electrode with its unique electrochemical properties.

Electrochemical Performance of MFC
Figure 5 illustrates the electrochemical performance of various electrode materials, and all test results are measured in 1 m Na 2 SO 4 electrolyte at room temperature unless otherwise specified.
According to Figure 5a,b, the areal capacitance of FeOOH/CNT calculated via both CV and galvanic charge-discharge methods are both ˂100 mF cm −2 .Compared with MnO 2 /CNT electrodes without FeOOH, the benefit of FeOOH and high mass loading was observed in both CV and GCD curves, which might be due to the subsequent reasons.First, the multivalent Fe ion can change its redox valence as described below, providing extra pseudo capacitance to the electrode: Further, the presence of FeOOH (i.e., 10 −5 S cm −1 ) [33] in addition to MnO 2 increased the electrical conductivity of the ternary electrode, above that of MnO 2 alone (i.e., 10 −6 S cm −1 ). [34]Nyquist plot shows that the internal resistance of MFC is 5 Ω, significantly lower than the resistance of 7.5 Ω for the MnO 2 /CNT electrode, a reduction of 33.3% (Figure S6, Supporting Information).Furthermore, we have conducted the in-plane four-point probe and through-thickness resistivity measurements on the MFC, MnO 2 /CNT, and pristine CNT electrodes.When comparing the resistance values after coating CNT with MnO 2 and FeOOH/MnO 2 , it is observed that the through-thickness resistance of the CNT substrate increases from 1.4 to 241.3 Ω with a MnO 2 coating but is substantially lower at 8.6 Ω when coated with FeOOH/MnO 2 (i.e., the MFC electrode), indicating a more effective conductivity improvement with the ternary composition.Similarly, the in-plane resistance of the CNT increases from 1.24 Ω/square to 396 Ω/square with MnO 2 coating, significantly reduced to 220 Ω/square when coated with FeOOH/MnO 2 .This observation suggests that the MFC electrode possesses superior electrical conductivity compared to the MnO 2 /CNT electrode.This enhanced conductivity can be attributed to the atomic-level interaction between Fe and MnO 2 , as evidenced by the XANES findings as one significant advantage of this interaction is the potential enhancement of both ionic and electronic conduction.These results highlight the synergistic effect of FeOOH and MnO 2 in enhancing the electrical properties of CNT-based electrodes, making the FeOOH/MnO 2 (MFC) a more conducive and efficient material compared to MnO 2 -coated CNT.
To identify the optimum improvement in the electrochemical performance of MFC electrodes, the concentration ratio of the  [20,[35][36][37] precursor FeCl 3 to Mn(AC) 2 was varied and the effect on the capacitance was investigated, as shown in Figure 5c, where the optimal areal capacitance was achieved at around 3500 mF cm −2 (equivalent to 438 F cm −3 ) when the FeCl 3 concentration reached 0.05 m, far outperforming the MnO 2 /CNT electrode made with the same deposition time.Table S2 (Supporting Information) provides a more comprehensive comparison of the MFC electrodes with other ternary electrodes reported previously.To quantify the synergistic effect between FeOOH and MnO 2 , a synergistic ratio S is used here as follows, where C FMC refers to the total capacitance of the ternary electrodes (MnO 2 /FeOOH/CNT), m (MnO2) and m (FeOOH) refers to the mass loading of the corresponding electrodes, c S,MC and c S,FC denote the specific capacitance values of binary electrodes of MnO 2 /CNT and FeOOH/CNT, respectively.In essence, this equation provides a method to quantify the level of synergy in the ternary system that utilizes identical amounts of active materials as present in the binary electrodes.A value of S MCF >1 would suggest a positive synergistic effect, meaning the ternary system has a higher capacitance than the sum of the binary systems.Conversely, a value of S MCF < 1 would suggest that there is no synergistic effect, and the ternary system performs worse than the sum of the binary systems.The respective weights of MnO 2 and FeOOH in the MFC electrode can be calculated based on the atomic weight percentage ratio (i.e., Fe:Mn = 1:2) obtained from energy dispersive spectroscopy (EDS) mapping (Figure S1, Supporting Information).Accordingly, the synergistic ratio of the ternary electrode is determined to be 1.25±0.05,indicating a beneficial synergistic effect within the ternary system, specifically the MFC electrode.This means that the ternary system exhibits a 25% higher capacitance than the anticipated performance of two separate binary electrodes of MnO 2 /CNT and FeOOH/CNT.In the absence of FeCl 3 precursor, a high mass loading of MnO 2 typically occurs, particularly in a neutral or alkaline environment.However, with the inclusion of FeCl 3 , the co-deposition of FeOOH and MnO 2 takes place, resulting in FeOOH that provides additional pseudocapacitance.It is worth noting that while FeOOH enhances the overall capacitance, the primary contribution still stems from MnO 2 rather than FeOOH.The addition of FeCl 3 leads to a decrease in the pH value of the electrolyte solution.Excessive concentrations of FeCl 3 may hinder the formation of MnO 2 , thereby diminishing the performance as FeCl 3 concentration increases.Thus, the optimal concentrations of FeCl 3 are important for maximizing the performance of the supercapacitors.
We further investigated the influence of deposition temperature on capacitance.(Figure S7, Supporting Information) Increasing temperature from room temperature to 60 °C improves capacitance, which could be attributed to the fact that higher energy provided from higher temperature facilitates the formation of additional nucleation sites.However, the performance dropped back when reaching an excessive temperature of 80 °C.This could be attributed to the formation of -MnO 2 , which has a denser crystal structure that is less favorable for ion migration. [28]ence, for the rest of this work, MFC with optimized capacitance will be used, which was achieved at 60 °C with 0.05 m FeCl 3 precursor concentration.The corresponding electrochemical performance is shown in Figure 5d,e.The electrode displays specific areal capacitances from 690 (tested under 10 mV s −1 scan rate) to 89 (tested under 200 mV s −1 scan rate) mF cm −2 .Notably, at more gradual scan rates, the electrode showcases nearly rectangular cyclic voltammetry (CV) curves, hinting at a capacitive behavior along with exceptional electrical conductivity within this scan rate domain.This behavior is attributed to the facilitated diffusion of Na ions into and out of the MFC electrode during these slower scan rates.However, with an increase in the scan rate, the CV outcomes begin to deviate from the ideal rectangular shape.This shift underscores the hampered diffusion rate of Na ions, which detrimentally affects the rate performance.Benefiting from the dual pseudocapacitance mechanisms, the chargedischarge curves show high specific areal capacitance that can be calculated at 3510, 2672, 2299, 2013, and 1796 for the current density ranging from 1 to 5 mA cm −2 , respectively.][37] The reference average values for these capacitance categories were 456 F g −1 , 1330 F cm −2 , and 462 F cm −3 , respectively.Our investigation revealed that the MFC electrode exhibited capacitance values of 233 F g −1 for gravimetric capacitance, 3500 F cm −2 for areal capacitance, and 438 F cm −3 for volumetric capacitance.The results from our study indicated that the MFC electrode demonstrated promising performance in terms of areal capacitance, significantly surpassing the reference average value.Nonetheless, it showed a marginally reduced gravimetric capacitance value relative to the reference, which might be due to the substantial amount of active materials loaded.However, the volumetric capacitance value of the MFC electrode was found to be near the reference average.

Asymmetric Supercapacitor Using MFC and FC Electrodes
For asymmetric supercapacitors, the working voltage window E can be described by the following equation: [7] where ΔE 1 and ΔE 2 denote the surface electrode potentials of the positive and negative electrodes, w a and w  their corresponding work functions.N A and F are Avogadro's number and constant, respectively.Accordingly, the widened voltage window can be attributed to the large difference between the work functions of Mn-and Fe-based oxides. [38]As a result, the MFC electrode can be paired with a FeOOH/CNT electrode to significantly increase the working voltage window.Compared with the symmetric supercapacitor design where the voltage window is 0.8 V, the asymmetric supercapacitor design can double the voltage window to 1.6 V. Therefore, this asymmetric design offers significantly higher energy density up to three times higher than a symmetric supercapacitor made of two identical MFC electrodes.
For an asymmetric supercapacitor with a positive electrode made of MnO 2 /FeOOH/CNT and a negative electrode made of FeOOH/CNT, the optimum design for the highest energy density corresponds to the matching of the charges, [39] i.e., This means that mass loadings of the two electrodes are related to each other via the following relationship, where m, c and ΔV, and Q denote the mass of the active material, the specific capacitance, the working potential range for an individual electrode, and the total charge on an electrode at its peak voltage, respectively, and the subscripts "+" and "−" indicate the positive and negative electrodes, respectively.The mass loadings of the electrodes can be tailored by controlling the deposition current and time.The above relationship assumes that the specific energy densities of the electrodes are insensitive to the mass loadings of the active materials.
The XRD spectrum of the FC electrode is shown in Figure 6a, where the characteristic peaks of iron oxide hydroxide are labeled, and the morphology of FC can be found in Figure S8 (Supporting Information).The electrochemical performance of the FC electrode obtained at optimal deposition conditions is shown in Figure 6b,c.The electrode functioned over an extensive potential range from −0.8 to 0 V, showcasing the highest areal capacitances measured at 1, 2, 3, 4, and 5 mA cm −2 current densities were calculated at 897, 666, 567, 517, and 486 mF cm −2 , respectively.
An MFC||FC asymmetric supercapacitor was assembled using a gel electrolyte.Figure 6d,e presents the CV and GCD results of the asymmetric supercapacitor consisting of an MFC cathode, FC anode, and PVA/LiCl gel electrolyte.Under 1 mA cm −2 current density, the corresponding areal capacitance achieved the highest areal capacitance at 450 mF cm −2 at a high voltage window of 1.6 V. Accordingly, the widened voltage window can be attributed to the large difference between the work functions of Mn-and Febased oxides. [38]Compared with the symmetric supercapacitor design where the voltage window is 0.8 V, an extra 0.8 V voltage is achieved from employing the asymmetric supercapacitor design.As shown in Figure 6f, the asymmetric supercapacitor demonstrates superior long-term stability compared to its symmetric counterparts.This improvement can be attributed to the lower internal resistance of MFC and the formation of effective and enduring ion transport channels between MFC and FeOOH.After undergoing 10000 cycles of charging and discharging, the asymmetric supercapacitor retains ≈70% of its initial capacitance, surprising the retention ratio of 55% observed for the symmetric supercapacitor.

Asymmetric Supercapacitor with Wide Operating Temperature and Voltage Range
In addition to the PVA-based gel electrolyte, asymmetric supercapacitors using MFC and FC electrodes can also be made with a solid electrolyte of dual-phase microstructure made of PVDF and ionic liquid to achieve a wider operating temperature and voltage range.Figure 7a illustrates the surface morphology of a PVDF-ionic liquid solid electrolyte.A dual-phase microstructure is observed, where the voided areas indicate the formation of discrete ionic liquid droplets dispersed in the PVDF polymer matrix as indicated by the white contour.
The PVA-based supercapacitor exhibits superior electrochemical behavior than the PVDF-based supercapacitor.The volumetric energy density reached 14.40 mWh cm −3 at a power density of 72 mW cm −3 and 10.5 mWh cm −3 at 360 mW cm −3 for the PVA-based device.The PVDF-based electrolyte exhibits a range of volumetric energy densities from 0.52 to 0.72 mWh cm −3 with the corresponding power density from 125 to 25 mW cm −3 .Figure 7d exhibits the Nyquist plot derived from electrochemical impedance spectroscopy measurements, with a frequency range from 0.1 Hz to 100 kHz and a 5 mV sinusoidal signal, and the equivalent circuit is presented in the inset.The equivalent series resistance of an asymmetric supercapacitor using PVAbased electrolytes was found to be 15 Ω, as determined by the real component while the ESR of a PVDF-based device was calculated to be 23 Ω.In the low-frequency region, both curves closely approach a vertical alignment, signaling the favorable capacitive behavior exhibited.To further investigate the performance of PVDF-based devices under various temperatures, electrochemical characterization was conducted after keeping the supercapacitor device at different temperatures for 20 min.Figure 7e presents the impedance of supercapacitors employing PVDFbased and PVA-based electrolytes across temperatures ranging from 0 to 110 °C.The equivalent series resistance (ESR) can be estimated based on the x-axis intercept values.It is worth noting that the ESR results for supercapacitors with PVA-based electrolytes are not available at 0 and 110 °C due to the limited operating temperature range of the aqueous electrolyte.The curves in Figure 7f show the capacitance values for supercapacitors with PVDF and PVA-based electrolytes measured under different temperatures.In both cases, better performance is achieved with increased temperature, which is attributed to the enhanced ionic conductivity of the electrolyte and the decrease in ESR.For instance, at a current density level of 1 mA cm −2 , the capacitance of PVDF-based supercapacitors increased from approximately 40 to 80 mF cm −2 when the temperature rose to 110 °C.Furthermore, the PVDF-based electrolytes offer the advantage of enhanced mechanical properties.Specifically, the tensile strength and Young's modulus of the PVDF-based supercapacitor were measured to be 27 and 475 MPa respectively, as detailed in Figure S9 (Supporting Information).These values are notably higher than those of devices that employed the PVA-based electrolytes (i.e., 8 and 242 MPa).The water uptake ratio, degree of swelling, and crosslinking density of the as-prepared PVDF samples were measured to be 18.9%, 6.25%, and 19%, respectively.Conversely, supercapacitors employing PVA-based electrolytes exhibit higher capacitance, as PVA-based gel electrolytes can offer higher ionic conductivity compared to PVDF/IL electrolytes.

Conclusion
We have developed and demonstrated a novel method to construct a ternary electrode by co-depositing MnO 2 and FeOOH onto carbon nanotube mats, MnO 2 /FeOOH (MFC).The addition of FeOOH to MnO 2 not only improves its electrical conductivity but also provides additional pseudocapacitance as well.This configuration not only reduces internal resistance by 33.3% but also enhances areal and specific capacitance by 59% and 16.5%, respectively, compared to standard MnO 2 /CNT electrodes.Compared to two distinct binary electrodes containing either MnO 2 or FeOOH individually, the ternary electrode with the same quantity of active materials demonstrated a notable synergy with a ratio of 1.25, indicating a collaborative enhancement that exceeded the combined performance of the two binary systems by 25%.Furthermore, when paired with a FeOOH/CNT binary electrode, this ternary system broadened the voltage window to 1.6 V.The resulting asymmetric supercapacitors exhibited high energy storage performance in three electrolyte systems, including an aqueous electrolyte, a PVA-based gel electrolyte for wearable applications, and an ionic liquid-based electrolyte for widetemperature range operations.When the PVA-based electrolyte was used, the volumetric energy density of the asymmetric supercapacitor reached 10.5 mWh cm −3 at 360 mW cm −3 .By contrast, the PVDF-based asymmetric supercapacitors exhibited ≈200% higher Young's modulus than that of PVA-based devices as well as a wide operating range from 0 to 110 °C, although the energy and power density decreased by 95% and 65%, respectively.The findings from this investigation underscore that the ternary electrodes devised in this study present a viable pathway to substantially augment the energy density of asymmetric supercapacitors, enhancing their potential for wearable technologies and advanced energy-storage architectures.

Experimental Section
Fabrication of Electrodes-MC Binary Electrode: For comparison, benchmark MnO 2 /CNT electrodes, denoted as MC electrodes, were made by performing constant current electrodeposition (10 mA cm −2 ) of MnO 2 on CNT mats at 60 °C using a manganese acetate precursor solution, i.e., 0.1 m (CH 3 CO 2 ) 2 Mn.
Fabrication of Electrodes-FC Binary Electrode: The FeOOH/CNT electrode, denoted as the FC electrode, was made by depositing FeOOH onto CNT mats via a static potential method at 1.2 V in 20 mm FeCl 2 solution at room temperature and elevated temperatures for various times using the same three-electrode configuration described above, followed by water rinsing and oven drying at 60 °C for 12 h.

Fabrication of Electrodes-MFC Ternary Electrode:
The MFC electrode was made by co-depositing FeOOH and MnO 2 onto CNT mats by electrodeposition at various temperatures (25, 40, 60, and 80 °C) (Figure S7, Supporting Information).The co-deposition procedure was conducted using a three-electrode configuration at a constant current density (10 mA cm −2 ).The CNT mat, a graphite electrode, and an Ag/AgCl electrode were used as working, counter, and reference electrodes, respectively.To study the effect of Fe concentration on the performance of the MFC ternary electrode, the co-deposition was performed in a solution containing 0.1 (CH 3 CO 2 ) 2 Mn, 0.1 m Na 2 SO 4 , and FeCl 3 of varying concentrations.The resultant MFC electrodes were cleaned with deionized water and dried afterward.
Electrolytes: Characterizations of the electrodes' performance were carried out in an aqueous electrolyte consisting of 1 m Na 2 SO 4 , which was prepared by dissolving 14.2 g Na 2 SO 4 in 100 mL deionized water.Two solid electrolytes were synthesized to demonstrate flexible and structural supercapacitors.
PVA Gel Electrolyte: The gel electrolyte was formulated in the following manner: 6 g of polyvinyl alcohol (PVA) was initially dissolved in 60 mL of distilled water at 85 °C while stirring, followed by the addition of 13 g of LiCl and the solution was kept under stirring until a homogeneous solution was formed.
Characterization-MFC Electrode: Electrochemical characterization of the MFC electrodes is performed using Bio-Logic VPS potentiostat (VPS-300).X-ray photoelectron spectroscopy (XPS) analyses were conducted using a Kratos Nova spectrometer.This device utilized a monochromatized Al K 1 X-ray source with an energy level set at 1486.6 eV.During operation, the instrument maintained a power output of 150 W and adopted pass energies of 160 eV for broad-spectrum surveys and 20 eV for focused region-specific spectral investigations.All recorded spectra were chargecorrected by setting the C 1s peak from adventitious carbon to 285.0 eV.Quantification was performed using Kratos XPS elemental sensitivity data, post the Shirley background subtraction.XRD profiles of electrodes were obtained by Empyrean1 multi-functional research X-ray diffractometer.
X-ray Absorption Near Edge Structure (XANES) results were obtained at the Australian Synchrotron, part of the Australian Nuclear Science and Technology Organisation (ANSTO).Raman spectroscopy (Renishaw in-Via) with a laser wavelength of 514 nm was used to characterize the MFC electrodes.
Characterization-PVDF/IL Electrolyte: The PVDF/IL electrolyte samples were immersed in acetone for 8 h to facilitate the extraction of the IL.Following this, the electrolyte was dried in an oven for 8 h.Subsequently, a platinum coating was applied, resulting in a thickness of ≈20 nm.
Water uptake and swelling ratio: For the water uptake and swelling test analysis, PVDF-based solid electrolyte samples were immersed in deionized water and left undisturbed.After 24 h, they were re-weighed and their thickness was re-measured for analysis.Specifically, the following equations are employed to calculate the water uptake W and swelling ratio: S = S wet − S dry S dry (10)   where W wet and W dry are the weights of the wet and dry samples.Parameters S wet and S dry denote the thickness of the wet and dry membranes, respectively.Cross-linking degree: A solvent extraction method was utilized to determine the degree of crosslinking.As-prepared PVDF-based solid electrolyte sample was immersed in DMF solution for 3 h.Afterward, the sample was dried and weighed.The degree of cross-linking C was determined from the fraction of the PVDF-based solid electrolyte sample, which it was calculated by using the following equation: [44] C = W 1 W 0 × 100 (11)   where W 0 is the original weight of the dried membrane and W 1 is the weight of the sample after solvent extraction.

Figure 2 .
Figure 2. XPS spectra for MFC electrodes.a) Fe 2p, b) Mn 2p, and c) O 1s peaks and curve fitting with d) the corresponding experimental binding energy values for Mn 2p and Fe 2p peaks compared to the literature as a function of oxidation state.[29,[45][46][47]

Figure 3 .
Figure 3. TEM and EDX mapping characterization of MnO 2 /FeOOH/CNT (MFC) ternary electrode.a) morphology of the MFC electrode; b) EDX mapping area; c-g) combined overlap and individual element mapping images: manganese d), iron e), oxygen f), and carbon g).

Figure 4 .
Figure 4. XANES characterization of the MFC electrode.a) K edge absorption spectrum of Mn; b) oxidization states of Mn; c) K edge absorption spectrum of Fe; d) oxidization state of Fe in MFC electrodes.

Figure 5 .
Figure 5. Electrochemical performance comparison for CNT, MnO 2 /CNT, and MFC electrodes.a) CV curves.b) Charge-discharge curves.c) Influence of FeCl 3 concentration ratio on areal capacitance for the same deposition time.Electrochemical performance of d) CV, and e) charge-discharge curves obtained on MFC electrodes.f) Boxplot comparison of volumetric, areal, and gravimetric capacitance of the MFC electrode with other ternary electrodes reported previously (red stars indicate values of the MFC electrode).[20,[35][36][37]

Figure 6 .
Figure 6.Electrochemical performance of the negative electrode and the asymmetric supercapacitor.a) XRD spectrum of FC electrodes; b) CV curves obtained under 10-200 mV s −1 scan rate and c) GCD results of electrochemical performance of FC electrodes; d) CV results measured at different scan rates (10-200 mV s −1 ), and e) GCD results measured at 1-5 mA cm −2 current density of asymmetric supercapacitors; f) Cyclic stability test of the asymmetric supercapacitor and the comparison with the MnO 2 only counterparts.