Interdigital MnO2/PEDOT Alternating Stacked Microelectrodes for High‐Performance On‐Chip Microsupercapacitor and Humidity Sensing

For microelectronic devices, the on‐chip microsupercapacitors with facile construction and high performance, are attracting researchers' prior consideration due to their high compatibility with modern microsystems. Herein, we proposed interchanging interdigital Au‐/MnO2/polyethylene dioxythiophene stacked microsupercapacitor based on a microfabrication process followed by successive electrochemical deposition. The stacked configuration of two pseudocapacitive active microelectrodes meritoriously leads to an enhanced contact area between MnO2 and the conductive and electroactive layer of polyethylene dioxythiophene, hence providing excellent electron transport and diffusion pathways of electrolyte ions, resulting in increased pseudocapacitance of MnO2 and polyethylene dioxythiophene. The stacked quasi‐solid‐state microsupercapacitors delivered the maximum specific capacitance of 43 mF cm−2 (211.9 F cm−3), an energy density of 3.8 μWh cm−2 (at a voltage window of 0.8 V) and 5.1 μWh cm−2 (at a voltage window of 1.0 V) with excellent rate capability (96.6% at 2 mA cm−2) and cycling performance of 85.3% retention of initial capacitance after 10 000 consecutive cycles at a current density of 5 mA cm−2, higher than those of ever reported polyethylene dioxythiophene and MnO2‐based planar microsupercapacitors. Benefiting from the favorable morphology, bilayer microsupercapacitor is utilized as a flexible humidity sensor with a response/relaxation time superior to those of some commercially available integrated microsensors. This strategy will be of significance in developing high‐performance on‐chip integrated microsupercapacitors/microsensors at low cost and environment‐friendly routes.


Introduction
[3] However, the integration of high energy and power devices compatible with microelectronic circuits is thought-provoking and hinders the miniaturization of the whole system.Development in microfabrication techniques has empowered on-chip interdigital microsupercapacitors (MSCs) which are higher compatible compared with sandwiched-like structures.For the MSCs, high specific energy, high power density, long cycling life, and high rate capability with fine patterns are the most anticipated performance vital for the current electronic applications. [4]Being Earth-abundant, carbon-based nanostructured materials and metals embedded carbon materials have been widely used for electric double layer (EDL) MSCs. [5,6]owever, the low specific capacitance (0.1-13 mF cm −2 ) and power density, hightemperature processing, and the scalability hitches in the fabrication procedure limit the practicability of these devices.To overcome these challenges, the utilization of transition metal oxides (TMOs)/dichalcogenides like MnO 2 , [7] RuO 2 , [8] NiOOH, [9] MoO 3 , [10] MoS 2 , [11] and TaS 2 [12] are highly promising because of their higher specific capacitances due to reversible faradaic reactions compared with carbonous materials with double-layer charge storage behavior.The specific capacitance of MSC is directly associated with the effective surface area of the electrode material manageable to the electrolyte ions for high utilization of the active material. [4,13,14][17] However, the high electrical/ionic resistivity of MnO 2 (1-10 MΩ cm −1 ) and the high impedance related to oxides intercalation redox reactions significantly limit the construction of MnO 2 -based planar MSCs with robust electrochemical performance, For microelectronic devices, the on-chip microsupercapacitors with facile construction and high performance, are attracting researchers' prior consideration due to their high compatibility with modern microsystems.Herein, we proposed interchanging interdigital Au-/MnO 2 /polyethylene dioxythiophene stacked microsupercapacitor based on a microfabrication process followed by successive electrochemical deposition.The stacked configuration of two pseudocapacitive active microelectrodes meritoriously leads to an enhanced contact area between MnO 2 and the conductive and electroactive layer of polyethylene dioxythiophene, hence providing excellent electron transport and diffusion pathways of electrolyte ions, resulting in increased pseudocapacitance of MnO 2 and polyethylene dioxythiophene.The stacked quasi-solid-state microsupercapacitors delivered the maximum specific capacitance of 43 mF cm −2 (211.9F cm −3 ), an energy density of 3.8 μWh cm −2 (at a voltage window of 0.8 V) and 5.1 μWh cm −2 (at a voltage window of 1.0 V) with excellent rate capability (96.6% at 2 mA cm −2 ) and cycling performance of 85.3% retention of initial capacitance after 10 000 consecutive cycles at a current density of 5 mA cm −2 , higher than those of ever reported polyethylene dioxythiophene and MnO 2 -based planar microsupercapacitors. Benefiting from the favorable morphology, bilayer microsupercapacitor is utilized as a flexible humidity sensor with a response/ relaxation time superior to those of some commercially available integrated microsensors.This strategy will be of significance in developing highperformance on-chip integrated microsupercapacitors/microsensors at low cost and environment-friendly routes.
particularly where high mass loading is employed for enhanced areal specific capacitance/energy. [18]Also, growing stable nanostructures of MnO 2 with low resistance and capacitive behavior on micropatterned metal current collectors is quite challenging.[21] Conducting polymers (CPs)-based pseudocapacitive materials have the potential to store charge by EDL as well as by faradaic reactions inside the polymer matrix, which make them promising electroactive materials for charge storage applications.Polyethylene dioxythiophene (PEDOT) is a pseudocapacitive conducting polymer obtained by chemical/electrochemical polymerization of 3,4-ethylene dioxythiophene (EDOT) monomer.[27][28][29] However, the specific capacitance/energy of PEDOT-based MSCs requires further enhancement.The main drawback of CPs is their excessive growth along the interspace between micro fingers resulting in nonlinear patterns for the prolonged deposition time.This impacts the accuracy in the areal/volumetric capacitance of the device in addition to the fine pattern with high stability.One of the strategies is coupling PEDOT with nanostructured materials, like carbonous materials and TMOs to enhance their effective area within the micropattern, and maximize the approachability of electrolyte ions to penetrate deep inside the functional material. [30,31]Coupling PEDOT with V 2 O 5 has been shown exclusively beneficial for boosting the electrochemical cycling performance of the electrodes. [32]Similarly, for sensors, the sensitivity of the sensing layer and electrodes is directly associated with the surface-to-volume ratio of the sensing layer in addition to electrodes, that is, the sensing capability can be boosted with increasing specific surface area (SSA).As a result, porous sensing PEDOT with nanostructured materials is advantageous for capacitive humidity sensors in terms of high sensitivity.The growth of 1D interwoven nanowires network of transition metal nitrides, such as capacitive TiN-nanowires (NWs) has been discovered as an effective strategy to boost the electrochemical performance of CPs for free-standing microelectrodes. [30]However, for the on-chip planar devices, the highvoltage growth of such NWs is unmanageable.The facile growth of stable pseudocapacitive NWs of active material on planar microcurrent collectors can effectively boost the electrochemical performance of CPs based on-chip MSCs.
Herein, we rationally constructed the prototype alternating stacked MnO 2 @PEDOT (MPMP)-based quasi-solid-state MSC (MPMP MSC) by low potential electrochemical deposition of uniform MnO 2 nanowires (Mn NWs) on Cr/Au microelectrodes, followed by electrochemical polymerization of PEDOT layer on it.The bare Mn NWs electrodes showed a pseudocapacitive behavior with the cyclic voltammetry (CV) profile of an almost rectangular shape.The uniform nucleation of MnO 2 NWs within the polymer chains enhanced the conductivity and strength of the stacked microelectrodes by interweaving the polymer chains and facilitating a high active surface area.As-constructed MSC delivered a high areal capacitance of 43 mF cm −2 , an energy density of 3.8 μWh cm −2 (0.8 V voltage window) and 5.1 μWh cm −2 (1.0 V voltage window) with excellent rate capability (96.6% at 2 mA cm −2 ) and cycling stability of 85.3% after 10 000 cycles (at a current density of 10 mA cm −2 ), which is superior to previously reported PEDOT and MnO 2 based on-chip planar MSCs.The short polymerization time prevents the excessive spreading of polymer/metal (active material) to construct a fine pattern with accurate areal capacitance.Furthermore, a bilayer MP MSC is utilized as a MS with excellent response/relaxation time of 1.26 and 4.3 s respectively within the high humidity range from 14% to 71%, which is superior to recently reported sensors and even better than commercially available EMD-4000B integrated sensor.

Results and Discussion
Figure 2a shows the scanning electron microscope (SEM) image of low potential deposited MnO 2 NWs on the surface of the Cr/Au micropattern current collector.The uniformly distributed conducting NWs are favorable for ion diffusion as well as electron transportation pathways in the device.The inset of Figure 1a shows the SEM and optical microscopic images of MnO 2 NWs deposited on Cr/Au micropatterns.Similarly, Figure S1a,b, Supporting Information shows the optical images of full and half micropatterns coated with MnO 2 NWs, respectively.After electrochemical polymerization, the whole patterned NWs are fully covered with a highly porous PEDOT layer (Figure 2b).The microchannels in the PEDOT film are favorable for the electrolyte ions to penetrate deep inside the active material to boost the electrochemical performance of microelectrodes.The second layer of MnO 2 NWs deposited on PEDOT is interconnected and comparatively low dense (Figure 2c).The final PEDOT layer displays a bit rough surface with micro-fringes on its top, revealing a highly porous network (Figure 2d).The insets of Figure 1d and Figure S1c, Supporting Information show the optical microscopic images of a single array of the microelectrode justifying the uniform alternating stack geometry of the electrodes.The height of the stacked microelectrodes was deliberated by the surface profiler and the average was about 2.01 μm (Figure S1d, Supporting Information).The comparison of MnO 2 nanostructures deposited at 0.55 and 0.8 V is displayed in Figure S2, Supporting Information.To further analyze the uniform deposition and distribution of the constituent elements (Mn, S, C, and O), energy-dispersive X-ray spectra (EDS) mapping was conducted and the images reveal the precise and uniform distribution of active material on the current collectors (Figure 2e).The elemental mapping of pristine MnO 2 NWs is displayed in Figure S3, Supporting Information indicating the presence of Mn, O, and S elements in the film.The MnO 2 NWs from the acetate solution deposited on the PEDOT layer at a higher potential (0.8 V) results in the growth of uneven Mn spheres randomly distributed on the surface of the polymer layer (Figure S4a, Supporting Information).At low deposition potential, the uniform MnO 2 NWs layer on PEDOT film is displayed in Figure S4c, Supporting Information.The anionic surfactant lowers the deposition potential of Mn from 0.8 to 0.55 V, which helps the uniform nucleation of MnO 2 NWs on Cr/Au substrate as well as PEDOT film.At a higher potential (0.8 V), the current versus time is the sinusoidal waveform, where the current above the mean line causes the deposition of uneven Mn spheres (bumps) on the substrate.For the same solution at a lower voltage (below 0.5 V), no deposition of Mn occurred on either substrate.The optimal structure, conductivity, and stability of the film are achieved at a deposition potential of 0.55 V and are used for further fabrication of MSCs.The current versus time profile at different deposition potentials is displayed in Figure 2f.
One of the major issues with CPs films is their linear growth along the edges of the micropatterns, increasing the effective area of every micro-finger leading toward low precession in the areal capacitance of Energy Environ.Mater.2024, 7, e12546 the device.At a constant potential of 0.9 V with a deposition time of 15 min, about an increase of 20% is observed in the width of a single interdigital finger (Figure S4d-left, Supporting Information).Contrary to this, the alternating stacked microelectrodes with a short deposition time (Mn = 100 s, PEDOT = 200 s each layer) have no prominent spreading along the interspace of the fingers (Figure S4d-right, Supporting Information), leading toward the development of fine micropatterned MSCs with high areal and volumetric capacitances.For the whole device, there is a big difference between the area of the micro-current collectors and micropatterned electrodes.The advantage of MnO 2 NWs on bulk MnO 2 is its high stability, electrical and ionic conductivity, and porous structure with pseudocapacitive charge storage.Further, the nanowires offer a high specific surface area for the growth of highly porous PEDOT with high electrolyte ion diffusion to expose the active material to electrolyte ions, and to boost the pseudocapacitance of both the polymer and metal oxide NWs.To further analyze the nature of MnO 2 NWs, transmission electron microscopy (TEM) analysis was performed and the results are displayed in Fig- ure 2g.Low-crystalline MnO 2 NWs possess a diameter from 15 to 25 nm and a length of about 100 to 200 nm.The high-resolution TEM image in Figure 2h shows that the MnO 2 is low-crystalline.The observed lattice spacing of 0.22 nm displayed in the inset corresponds to the (211) crystal plane of α-MnO 2 (JCPDS # 65-5787).As apparent in X-ray diffraction (XRD) analysis in Figure 2i, other than the signal from the silicon substrate, similar peaks are perceived for both MnO 2 and MnO 2 @PEDOT samples, corresponding well with the standard data for MnO 2 (JCPDS # 65-5787). [15]Additionally, Raman spectroscopy was performed to study the coating of the PEDOT layer on the Mn network in addition to molecular arrangement and bonding aspects of MnO 2 -PEDOT and pristine MnO 2 .As shown in Figure S5a, Supporting Information and Figure 5b, there is no prominent peak shift or visible Mn peak in Raman spectra of MP, and MPMP electrodes, and all the peaks are matching well with the PEDOT peaks revealing that the top surface of Mn NWs electrode is completely coated with a polymer layer.The intensity of the distinctive and intense Raman peak situated at 1436 cm −1 symbolizing to the symmetrical stretching mode of PEDOT C-C, demonstrating the increase in oxidation state as the layers of PEDOT is added on Mn.The Raman shift at 650 cm −1 for the MnO 2 sample is credited to the Mn-O stretching vibration belonging to the [MnO 6 ] group in MnO 2 (Figure S5c,d, Supporting Information).After covering with PEDOT, the signals of MnO 2 were diminished radically and typical peaks of PEDOT become prominent, approving the deposition of the PEDOT on the surface of the MnO 2 .For the PEDOT layer, a comparatively low-intensity peak at 1569 cm −1 resembles the anti-symmetric stretching of carbon atoms (C=C).Likewise, the two peaks found at 1366 and 1271 cm −1 denote the occurrence of C-C inter-ring stretching, while 446 cm −1 corresponds to SO 2 bending, which defends the accumulation of sulfate anions (from sulfuric acid and sodium dodecyl sulfate (SDS)) by PEDOT during the process of electrochemical polymerization.
X-ray photoelectron spectroscopy (XPS) was further performed to characterize the MnO 2 NWs and MnO 2 -PEDOT microelectrodes.MnO 2 has a substantial contracted Mn 2p 3/2 peak compared with MnO or Mn 2 O 3 (Figure 3a).Moreover, it has a distinguishable profile at the top of the Mn 2p 3/2 peak.MnO 2 possesses a satellite peak (~646 eV), which is not prominent for both Mn 2 O 3 and MnO.Moreover, Mn 3s spectra reveal the oxidation state of Mn.The Mn 3s peak has two multiplet split components due to the coupling of non-ionized 3s electrons with 3d valence electrons.The oxidation state is determined by the amplitude of peak splitting.The reported ΔE for MnO (Mn 2+ ) is 6.0 eV, Mn 2 O 3 (Mn 3+ ) ≥5.3 eV, and MnO 2 (Mn 4+ ) 4.7 eV. [33]The energy separation of 4.8 eV in the Mn 3s spectrum reveals that Mn 4+ is the leading valence state in the MnO 2 -PEDOT microelectrode (Figure 3b). [34]The cyclic voltammetry deposition curves and Raman spectra are displayed in Figure S6a,b, Supporting Information.For MP microelectrodes, intensities of Mn 2p and Mn 3s peaks are not considered due to the uniform polymerization of CP on MnO 2 NWs (Figure S6c,d, Supporting Information).The S 2p spectra of Mn just show the low-intensity S 2p peak at 168.0 eV which is due to SO 4 from SDS (Figure 3c).Contrary to the pristine MnO 2 NWs, the Mn-PEDOT microelectrode demonstrated distinct S signals (Figure 3d), again demonstrating the polymerization of EDOT on MnO 2 .Moreover, the two distinct peaks positioned at 164.06 and 162.21 eV present in the S 2p spectrum are due to S 2p 1/2 and S 2p 3/2 of sulfur atoms in PEDOT (Figure 3d). [35]Figure S7a, Supporting Information shows the XPS survey of MnO 2 NWs and MP electrodes.The presence of Mn 3p and Mn 3s in MnO 2 -PEDOT reveals that the PEDOT is uniformly deposited on the MnO 2 NWs.Similarly, low-intensity peaks of Mn 2p and Mn 2s decreases and that of the Mn-OH bond is increased whereas the peak intensity of O-Mn-O is decreased prominently.This will arise due to the absorption of OH ions from the adsorbed water from the PEDOT surface.The peak located at 533.4 eV is attributed to the polyethylene present in PEDOT, whereas the peak positioned at 533.6 eV is attributed to the C-OH bond.The peak at 532.4 eV is associated with C=O bonding (Figure 3f).The comparison of O 1s spectra of PEDOT deposited on Mn is enriched in oxygen and OH content with high peak intensities and a shift toward high binding energy which leads toward the high conductivity of the material (Figure S7b, Supporting Information).The atomic percentages of constituent elements are displayed in Table S1, Supporting Information.The oxygen content in MP MSC microelectrodes reached 42.2% from 25.6% in P MSC.The deposition of MnO 2 resulted in an almost 17% increment in oxygen contents in the microelectrode.Figure S8a,b, Supporting Information defines the C 1s XPS spectra of MnO 2 NWs and MP microelectrodes, respectively.PEDOT has a chemical bonding of carbon atoms with oxygen and sulfur atoms.The intense peak at 284.5 eV relates to sp 3 and sp 2 carboncarbon bonding, while the tail peaks at 285.9 and 288.4 eV are assigned to C-S and C-O bonding, respectively.The presence of carbon peaks in Mn NWs microelectrode is due to the carbon contamination from the air with some associated C-O and C=O structures (285.5-288.5 eV), however, depending on the nature of the material, this is not always a good reference despite its wide use.

Electrochemical Performance of MSCs
To better understand the electrochemical performance of the MPMP MSC, CV, and galvanostatic charge-discharge (GCD) tests are conducted and the results are compared with M MSC, P MSC, and MP MSC. Figure 4a shows the CV curves of M, P, MP, and MPMP MSCs at a constant scan rate of 10 mV s −1 within the operating voltage window from 0 to 0.8 V.All the MSCs display rectangular shape CV curves with different output currents.The P MSC CV curves show some polarization at a low scan rate, signifying that electrolyte ions cannot penetrate deeply inside the bulk structures of PEDOT deposited on Cr/Au.For M MSC, the CV (scan rate from 10 to 50 mV s −1 ) and GCD (current density from 0.05 to 1 mA cm −2 ) curves are displayed in Figure S9, Supporting Information.The quasi-rectangular shape CV curves manifest that tuning the morphology and crystallinity of Mn nanostructures highly impacts the charge storage kinetics (from irreversible redox to pseudocapacitive) of Mn nanostructures with an improvement in the rate performance to make a stable Mn electrode as well as intermediate to facilitate the growth of favorable structures of CPs for electron and ion diffusion pathways.
The M MSC delivered an areal specific capacitance of 312.5 μF cm −2 , with excellent rate capability.In addition, the M MSC delivered a satisfactory cycling performance retaining 67.2% of the initial capacitance after 5000 cycles at a high scan rate.During the first 500 cycles, the Energy Environ.Mater.2024, 7, e12546 capacitance dropped abruptly, and then there is a gradual decrease up to 5000 cycles (Figure S10, Supporting Information).The areal capacitance of the P MSC obtained from the GCD curve is found to be 9.8 mF cm −2 (deposition time: 900 s).Compared with P MSC, MP MSC displayed a large area in CV curves and give high output current, ultimately a high areal capacitance of 23.3 mF cm −2 .The increased capacitance is contributed to the high specific surface area of the PEDOT deposited on MnO 2 NWs which allow fast diffusion of electrolyte inside the active material.The channels formed in the polymer layer deposited on PEDOT facilitate the gel electrolyte to penetrate individual NSs to fully utilize the active material.Figure 4b shows the GCD curves of all four MSCs at a constant current density of 0.1 mA cm −2 .The MPMP MSC displayed the longest charge/discharge time among the other MSCs.The GCD results of other MSCs are also consistent with the results from CV curves and display triangular-type CD curves.To understand the ion transport and charge transport resistance of the MSCs, electrochemical impedance spectroscopy (EIS) test is performed within the frequency range from 0.01 to 100 000 Hz at open circuit potential.Figure 4c shows the Nyquist plots of M, P, MP, and MPMP MSCs.The inset expresses the enlarged view in the high-frequency region for fabricated MSCs.The M MSC being the thin layer offer less resistance.The equivalent series resistance (ESR) values of MPMP-MSC are lower than that of MP-MSC.The low ESR is contributed from the porous alternating stacked layered architecture of the MPMP MSC electrodes with low ion diffusion resistance and high electrical conductivity of the embedded MnO 2 NWs layer between PEDOT layers.Moreover, at the low-frequency zone, the deviation of the slope away from the vertical axis for P MSC is because of the low ionic conductivity owing to the trifling pores present in the electrodeposited PEDOT film, which encourages amplified resistance to the permeation of electrolyte ions in the active material.From the EIS curves, the real (C 0 ) and imaginary (C″) parts of the capacitances of MP and MPMP MSCs are displayed in Figure S11, Supporting Information.The MPMP MSC displays a time constant τ ○ of 2s comparable with MP MSC (τ ○ = 1.7 s) with almost twice the capacitance.To further explore the high reversibility of MPMP MSC, the CV was conducted at diverse scan rates from 2 to 100 mV s −1 (Figure 4d).The CV current increases linearly with the scan rate with minor changes in CV shape.Even at a high scan rate of 100 mV s −1 , the rectangular shape of the CV reveals that electrolyte ions can transport easily within this stacked design and the device possess high reversibility.GCD curves were recorded at ascending current densities to further analyze the capacitance levels of the MSCs since GCD data represent the most adaptive and exact technique in characterizing charge storage devices. [36]or MPMP MSC the CD curves are displayed in Figure 4e.Even at a very low current density of 0.05 mA cm −2 , the triangular CD curves with negligible hysteresis reveal that the alternating stacked network favors ion diffusion pathways and exhibits high electronic conductivity to extract the electrons through current collectors to output circuits.On increasing the current density to 2 mA cm −2 , there is a negligible iR drop of about 0.025 V, which again shows the high ionic and electronic conductivity of the stacked layered materials.The increase in iR drop with current density is displayed in Figure S12, Supporting Information.The areal and volumetric capacitance of MPMP MSC acquired from the discharge curve at a current density of 0.05 mA cm −2 is 43 mF cm −2 and 211.9 F cm −3 , respectively.
One of the main issues with quasi-solid-state MSCs is their low rate performance at a comparatively high current density, because of the small pores or nonporous active material.To compare the rate performance of the fabricated MSCs the areal capacitance was intrigued as a function of current density and scan rate.The increase in current density results in a linear decrease in discharge time and hence the capacitance.The nanowires mediate the porous PEDOT film with holes to penetrate the electrolyte ions deep inside the active material and to the current collectors.The MPMP, MP, P, and M MSCs maintained a rate performance of 94.5%, 92.2%, 87.2%, and 90%, respectively at a current density of 2 mA cm −2 (Figure 4f).Analogous results are achieved by plotting areal capacitance versus scan rate for the fabricated MSCs (Figure 5a).The CV curves at different scan rates in Figure 5b also confirm the excellent rate performance of the alternating stacked device.As both of the active materials exhibit pseudocapacitive charge storage behavior, that is, electric double-layer capacitance (EDLC) and reversible redox reaction, it is important to calculate the capacitive and diffusion-controlled contribution of the active materials.Figure 5c shows the capacitive and diffusioncontrolled contribution of the microelectrodes at a very low scan rate of 2 mV s −1 .The MSC holds 93.3% capacitive dominant behavior with only 6.4% diffusion-controlled contribution in total charge storage.At a high scan rate of 10 mV s −1 , a 96% capacitance dominant characteristic is obtained (Figure 5d).The CV curves at different scan rates are displayed in Figure S13, Supporting Information.To know the stability of the MPMP-MSC for an extended voltage we further cycled the MSC at a scan rate of 50 mV s −1 up to 1.4 V (Figure 5e).At a low scan rate and low current density, the MPMP MSC delivered low energy efficiency for extended voltage.The reason for low energy efficiency is attributed to another redox reaction within the extended voltage above 1 V. Figure 5f shows the CV curves of MPMP MSC (1.0 V voltage widow) at diverse scan rates from 5 to 100 mV s −1 .At a low scan rate of 5 mV s −1 , the MSC achieved an areal capacitance of 41 mF cm −2 and retained 88% at a high scan rate of 100 mV s −1 which is comparable with the values obtained at a voltage window of 0.8 V.The GCD curves in Figure S14a, Supporting Information are symmetric.At low current density, there are redox peaks within the voltage limit from 0.9 to 1 V.The MSC delivered an areal capacitance of 41.7 mF cm −2 at a scan rate of 5 mV s −1 with a good rate performance of 91.7% at 100 mV s −1 (Figure S14b, Supporting Information).
One of the most significant electrochemical tests for polymerbased MSCs is their long-term cycling stability.MPMP MSC is cycled for 10 000 GCD cycles at a high current density of 5 mA cm −2 in polyvinyl alcohol (PVA)/lithium chloride (LiCl) gel electrolyte.The MSC retained 85.3% of the preliminary capacitance after 10 000 cycles with a Coulombic efficiency of 99.1% (Figure 6a).The inset of Figure 6a displays the first and last two CD cycles.Additionally, the EIS is performed after 10 000 GCD cycles, and the impedance comparison before and after the stability test is displayed in Figure S15, Supporting Information.The series resistance (Rs) and charge transfer resistance (Rct) are achieved by simulating Nyquist plots before and after cycling.Before cycling, the Rs and Rct values of MPMS MSC are 67.7 and 2.6 Ω, respectively.After 10 000 GCD cycles, the Rs value increased to 77.8 Ω, whereas the Rct value changed negligibly (with a 0.4 Ω increment).The stable Rct values after cycling indicated that the stacked porous microelectrodes expedite the fast transmission of electrolyte ions, and are advantageous to the long-term stability of the MSC.Further, the MPMP MSC is also cycled for 10 000 cycles at 1.0 V by keeping the other parameters unchanged.The MSC displayed 82% of the initial performance after 10 000 CD cycles (Figure S16a, Supporting Information).During the cycling performance, the Coulombic efficiency is increased from 92.95% to 98.5%, which shows the safe operation of the device up to 1.0 V.
Cells are coupled in series or parallel to suit the energy and power requirements of portable electronic gadgets.Two cells were coupled in parallel and in series to verify the MPMP MSCs' high power and energy densities.The CD curves of a single MSC, two MSCs in series and parallel are demonstrated in Figure 6b.For two cells in parallel, the CD time is doubled compared with a single MSC, while in parallel combination the combined potential of two MSCs is two times of a single MSC.Similarly, in a CV cycle, connecting two MSCs in parallel doubles the resultant current, while MSCs connected in series operate stably up to 1.6 V (Figure 6c).Finally, we used GCD curves to determine the areal and volumetric (vol.)energy and power densities of our MSCs, which we then equated to different types of MSCs in a Ragone plot (Figure 6d and Figure S17, Supporting Information).A variety of key characteristics, including volumetric capacitance, energy density, and power density, must be considered when using an MSC in practice.The MPMP MSC delivered an energy density of 3.8 μWh cm −2 (2.42 Wh cm −3 ) at a voltage window of 0.8 V and 5.1 μWh cm −2 (3 Wh cm −3 ) (at a voltage window of 1.0 V), and a maximum power density of 0.8 mW cm −2 at an energy density of 3.6 μWh cm −2 .[43] Moreover, Table S1, Supporting Information displays the performance comparison of our MSCs with recently reported planar MSCs.

Humidity Sensing Characteristics of MP MSC/MSs
The working principle of a capacitive or resistive humidity sensor is the change in current or resistance with the adsorbed water vapor.More adsorbed vapors result in high electrical conductivity and ultimately high current (in the case of capacitive) or low resistance (in resistive sensors).The highly porous film with excess pores is beneficial for the adsorption/desorption of water vapors for the high-response capacitive sensor.MnNWs with electronic conductivity act as templates for the growth of porous polymer layer and facilitates ion and electron diffusion.For the construction of interdigital MP MSs, we designed the Cr/Au current collector on polyethylene terephthalate (PET) substrate with four interdigital fingers on each side (eight fingers in total) to make it compatible with devices, by keeping other protocols the same as for the preparation of MSCs.For the deposition of active material, the time duration for MnO 2 NWs and PEDOT was reduced to 20 and 40 s respectively to make a highly stable thin layer.For the polymer electrolyte preparation except for reducing the content of LiCl to 1/4th of the previous concentration, the other parameters were kept the same.The high concentration of LiCl makes unstable Li crystals on the film's surface after getting the electrolyte completely dry.The prepared electrolyte (one drop from a 5 μm pipette) was dropped on MP/Cr/Au micropatterns and was spun coated at 4000 rpm to deposit a thin film of polymer electrolyte on the active patterns.The sensor was dried at 50 °C for 6 h to evaporate the excess water.Breathing (exhalation) close to sensors put together caused the humidity.The humidity is altered and monitored by sensors by optimizing the exhale timing and intensity.
The electro-analytic performance of the sensors was determined using chronoamperometry (CA) at a constant potential, and the change in electric current was observed at various relative humidity (RH) values ranging from 13% to 71% (digits after decimal removed).The following equation was used to calculate the sensor sensitivity.
where ▵I (I − I ○ ) is a change in current at a given humidity level in solid-state electrolyte and LiCl solution (reference), respectively.The CA curve in Figure 7a reveals that when the relative humidity increases there is a prominent increase in response current and ultimately a decrease in resistance.The linear fitting curve is presented in Figure S18a, Supporting Information (R 2 = 0.977).The sensing behavior of the MP sensor is ionic and depends mainly on the highly porous as well as conductive network with enhanced SSA of the film, with superior sensitivity.The conductive Mn NWs coated with CP facilitate the adsorption/desorption of polymer electrolyte ions to/from deep inside the electrodes and transfer the electronic current to the outer circuit for fast detection.From 13% to 71% RH range the MP-HS absorbs moisture and reveals a direct and profound response to the humidity variations in a semilogarithmic scale with 2.6%/RH% sensitivity, which is higher than many reported works and even superior to EMD-4000B commercially available sensors.One of the most vital characteristics of HS is its response and recovery time.Usually, the sensors after detection, take much time to come back to their original/mean position, that is, the adsorption and desorption of the electrolyte ions within the material are not quick.This can be optimized by adjusting the pore size comparable to the size of electrolyte ions to make fast adsorption/desorption of electrolyte ions for a fast response and recovery time.Conferring to the quick change in atmosphere humidity (RH ~13-71%) the response and relaxation time of the MP sensor are displayed in Figure 6b.A quick response of 1.26 s was achieved for the MP sensor, faster than many previously reported sensors based on other nanomaterials like graphene/ZrO 2 , [44] PEDOT: PSS, [45] silica/PEDOT, [46] PBObzT 2 , [47] ITO ink, [48] and commercially available integrated EMD-4000B sensor.The adsorption/desorption curves, representative sensitivity response, and optical photograph of EMD-4000B are displayed in Figure 7d-f.The MP MS is also tested to monitor the strength of respiration and normal breathing.Figure S18b, Supporting Information displays the breathing test in relaxed conditions.The uniform variation in sinusoidal current with inhalation and exhalation justifies the accuracy of the as-designed sensor.Based on the achieved results, it is concluded that MP MSs exhibit high sensitivity to humidity in a wide range with low hysteresis, owing to the coupling of copolymers (PEDOT and PVA) with cross-linked nanonetwork of Mn NWs with high SSA.

Conclusion
In summary, we constructed an alternating stacked MnO 2 /PEDOT// MnO 2 /PEDOT MSC (MPMP-MSC), and a bilayer (MnO 2 /PEDOT) micro moisture sensor MSs by electrochemical deposition of MnO 2 nanowires (MnO 2 NWs) on Cr/Au micropatterns followed by the electrochemical polymerization of highly porous PEDOT on it.The fabricated MSC delivered the maximum areal capacitance as high as 43 mF cm −2 (at a current density of 0.05 mA cm −2 ) with 96.6%rate capability and a high areal energy density of 5.1 μWh cm −2 (voltage window: 1.0 V) and 3.8 μWh cm −2 (voltage window: 0.8 V) within the micropatterned footprint.In addition, the MSC showed an excellent cycling performance retaining 85.3% of the initial capacitance after 10 000 charge/discharge cycles at a high current density of 5 mA cm −2 .Notably, the MP MSC possesses a great sensitivity to humidity with a response and relaxation time of 1.26 and 4.2 s, respectively.This strategy is capable of designing highperformance MSCs and MSs with a simple and environmentally friendly route.

Experimental Section
All the chemicals used were of analytical level and used without further treatment.Sinopharm Chemical provided H 2 SO 4 (98%), and EDOT was purchased from Sigma Aldrich.Sodium dodecyl sulfate, PVA, LiCl, and manganese acetate (C 4 H 6 MnO 4 ) were purchased from Alpha Chem.Preparation of Cr/Au micropatterned substrate: Figure 1 illustrates the scheme for the construction of MSCs/MSs.For device construction, a silicon substrate with a 200 nm SiO 2 layer was sliced into pieces (1 × 1.2 cm) and washed with isopropyl alcohol, ethanol, acetone, and deionized (DI) water in a sequential procedure, followed by 30 min of drying at 105 °C on a hot plate.The PR1-9000A photoresist (Futurrex, Inc.Co., Ltd) was spin-coated evenly on pre-cleaned substrates for 40 s at 5000 rpm, then prebaked for 10 min at 100 °C.
To obtain tiny interdigital micropatterns of the photoresist, photolithography and development were used.Lastly, Cr/Au (10/100 nm) fine micropatterned electrodes were obtained via the thermal evaporation process and a lift-off process in acetone.For MSs, the Si/SiO 2 substrate is replaced with PET by keeping all other parameters/steps as for MSCs.Electrolytes preparation: For the preparation of polymer electrolyte, 5 mM 98.7% concentrated H 2 SO 4 was mixed with 30 mL DI water.Afterward, 10 mM EDOT (155 μL) and 10 mM (0.05 g) SDS were mixed into this solution and kept stirring for an additional 2 h.For the MnO 2 electrolyte, 0.015 g of C 4 H 6 MnO 4 was mixed in 20 mL of DI and kept stirring till the solution turns colorless.Then, 5 mM (0.025 g) SDS was added and the solution was further stirred for 1 h.The anionic surfactant lowers the deposition potential to obtain highly stable and uniform Mn NWs.For the gel electrolyte, 2 g LiCl was mixed in 20 mL of deionized water by uninterrupted stirring for half an hour at room temperature.The temperature of the solution increased to 85 °C and 2 g of PVA was slowly added to the mixture and kept stirring for a further 3 h until the solution become clear.The polymer electrolyte for the humidity sensor is prepared under the same conditions except for lowering the quantity of LiCl from 2 g to 0.5 g/20 mL.Finally, the gel electrolyte was stored in a sealed container and let cool down to room temperature.Fabrication of M, P, MP, and MPMP-MSCs: We synthesized four different kinds of MSCs, namely MnO 2 /Cr/Au MSC (M-MSC), PEOT-coated Cr/Au MSC (P-MSC), PEDOT-coated MnO 2 MSC (MP-MSC), and alternately stacked MnO 2 // PEDOT MSC (MPMP-MSC) by electrochemical deposition followed by electrochemical polymerization of polymer.The fabrication of PEDOT-coated MnO 2 MSC (MP-MSC) was performed in two-step electrochemical deposition.First MnO 2 NWs were grown on Cr/Au micropattern at a constant potential of 0.55 V by the three-electrode system for 100 s, from the as-synthesized electrolyte mentioned in the above section.The anionic surfactant SDS lowers the deposition potential up to 0.55 V, which is favorable for the growth of uniform NWs on the micropatterned current collectors.The Cr/Au micropatterns were employed as a working electrode, Pt and Ag/AgCl as counter and reference electrodes, respectively.In the next step, electrochemical polymerization of PEDOT was performed on MnO 2 NWs at a fixed potential of 0.9 V for 200 s and after rinsing with DI water and drying in a vacuum oven overnight, the MP-MSC was prepared for further characterization.For the preparation of MPMP-MSC, the first and second steps were repeated at the same conditions.For assessment, a PEOT MSC with a long deposition time was also organized by applying electrochemical deposition from the same electrolyte on Cr/Au current collectors for an extended time by keeping the other parameters constant.Materials and electrochemical characterization: To examine the apparent morphology and assembly of the composite, a JSM-7100F field emission SEM at 20 kV was used.Transmission electron microscopy images were collected using a TEM-1400-Plus electron microscope.Raman spectra were recorded using a Renishaw RM-1000 laser Raman microscopy system.X-ray photoelectron spectroscopy spectra were scanned using a VG Multilab 2000.To ensure the thicknesses of microelectrodes, a Bruker DektakTX surface profiler was used.Energy-dispersive X-ray spectra were collected utilizing an Oxford IE250 system.All electrochemical tests were conducted using a CHI-760 electrochemical workstation, while EIS and cycling performance were measured using an Autolab PGSTAT302N.

Figure 1 .
Figure 1.a-f) Schematic view of microfabrication process of alternating stacked microsupercapacitor and microsensors.

Figure 2 .
Figure 2. Morphology of the microelectrodes.Scanning electron microscope (SEM) images of a) MnO 2 @Au (the inset shows the optical microscopic and SEM images of microelectrodes), b) MP microsupercapacitor (MSC), c) MPM electrode, and d) MPMP MSC.e) SEM image and corresponding elemental mapping of constituent elements, carbon (red), sulfur (green), manganese (orange), and oxygen (golden) in the stacked microelectrodes.f) Current-time profiles of electrodeposited Mn NWs at different potentials for a constant time.TEM images of MnO 2 NWs on Au, g) low magnification and h) high magnification (inset shows SAED patterns.Scale bar, 2 nm −1 ).i) XRD patterns of MnO 2 NWs deposited on Au and PEDOT.

Figure 3 .
Figure 3. X-ray photoelectron spectroscopy spectra of a, b) Mn 2p and core-level Mn 3s deconvolution of MnO 2 NWs.c, d) The S 2p core-level photoelectron spectra of MnO 2 and MnO 2 -PEDOT.e, f) Oxygen deconvolution spectra of M and MP microelectrode, respectively.

Figure 4 .
Figure 4. Electrochemical performance of as-constructed microsupercapacitors (MSCs).a) CV curves of M, P, MP, and MPMP MSCs at a constant scan rate of 10 mV s −1 .b) GCD curves of M, P, MP, and MPMP MSCs at a constant current density of 0.1 mA cm −2 .c) EIS spectra of M, P, MP, and MPMP MSCs (the inset is the zoom view at high-frequency region).d) CV curves at different scan rates and e) GCD curves of MPMP MSC at different current densities.f) Areal capacitance as a function of current density for MSCs.

Figure 5 .
Figure 5. a) For constructed microsupercapacitors (MSCs), areal capacitance as a function of scan rate.b) MPMP MSC areal capacitance at varied scan speeds ranging from 2 to 100 mV s −1 .c) MPMP MSC contribution to capacitive and diffusion control at a scan rate of 2 mV s −1 .d) MPMP MSC contribution to capacitive and diffusion control as a function of scan rate.e) Voltage stability test of MPMP MSC in PVA/LiCl electrolyte with enlarged voltage window up to 1.4 V (scan rate: 50 mV s −1 ).f) MPMP MSC CV curves at various scan rates (5-100 mV s −1 ) with an extended voltage of 1 V.

Figure 6 .
Figure 6.a) CD cycling performance up to 10 000 cycles of microsupercapacitor (MSC) at a high current density of 5 mA cm −2 (the inset shows the first [black] and last [red] two cycles).b) GCD curves of single (black), two parallel-connected (red), and two series-connected (blue) MPMP MSCs, the inset shows the optical photographs of two MSCs on a glass substrate.c) CV curves of single (black), two parallel-connected (red), and two series-connected (blue) MPMP MSCs, the inset shows the optical photographs of two MSCs on a substrate.d) The Ragone plot displays the energy and power densities of our devices and recently reported MSCs.

Figure 7 .
Figure 7. Sensing behavior of MP microsupercapacitor (MSC) and EMD-4000B sensors.a) Real-time dynamic response curves of MP MSC with PEO/PVA (1:1, 0.01 M LiCl) as integrated sensitive film-based humidity sensor within the sensing capabilities in RH 13-71% and b) rapid response (1.26 s) of the MP sensor at 71% RH. c) Digital photograph of the MP sensor.d) In varied humid environments, real-time dynamic response curves of the EMD-4000B sensor from a commercially available digital clock sensor (HTC-2).e) The real-time dynamic response curves.f) Rapid response and g) corresponding digital photograph.