The role of Mn in widening the potential window of solid solution derived electrodes for aqueous supercapacitors

Abstract

The interaction mechanism of the multi-element oxide electrodes remains a challenge, despite the fact that their synergy can not only improve the electronic structure, but also broaden the working potential window to some extent. Herein, the simple NiMn alloy system was taken up as a model to investigate the origin of the wide voltage window. Compared with conventional electrodes, this sandwich-type nanoporous-NiO(MnO_x)@Ni electrode derived from a NiMn alloy could exhibit a wide working potential window of 1.5 V in aqueous electrolyte. The facilitated successive redox reactions between NiO and MnO_x throughout the entire potential window strongly promote the capacitance extension without water decomposition, despite the fact that the cycling lifespan was limited by chemical dissolution of MnO_x. The assembled quasi-solid-state supercapacitor can overcome the irreversible conversion and exhibit a remarkable lifespan of 5000 cycles at 10 A cm⁻³ (91% retention) and high energy density (37.67 mW h cm⁻³ at 0.2 A cm⁻³). This work provides new perceptions for the design of multi-element collaborative energy storage electrodes.

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1 Introduction

Supercapacitors have drawn a lot of attention in current energy storage devices, considering their exceptional charge/discharge, remarkable power density and superior lifespan.^1–4 However, the growing practical demand of supercapacitors is hindered by their lower energy density. Advanced electrode materials, mainly represented by transition metal oxides (TMOs), have a more crucial role in contributing to devices with higher energy density.^5–7 Currently, NiO and its derivatives have become attractive materials among TMOs due to their high theoretical capacitance and other great characteristics.^8,9 However, their practical application is limited by their low conductivity and narrow effective potential windows. According to E = 0.5CU², the specific capacitance (C) and the operating voltage (U) are two key parameters to determine the energy density (E).^10,11 To date, many strategies are centered on complicated structure and composition electrode design to enhance the specific capacitance, which limits their scale up.^12–14 The design of asymmetric supercapacitors (ASCs) is the primary approach to broaden the operating voltage, which are typically assembled with low capacity carbon materials and require additional capacity matching.^15–17 Consequently, how to further enhance the energy density of NiO-based symmetric pseudocapacitors through a simple and practical way has become more imperative.

The stable working potential window of the NiO electrode is limited by the redox reaction and water splitting in general. Actually, neither an extra redox reaction nor the hydrogen evolution reaction obviously occurs in the range of −1.1–0 V (vs. Ag/AgCl), resulting in a distinct potential platform.^18,19 When assembled into a symmetric supercapacitor (SSC), the operating voltage of the device can be extended up to 1.6 V, but it exhibits poor capacitance at lower potentials due to the platform.²⁰ Guo et al. found that MnO-based electrodes can provide a redox reaction at such a low potential which could provide a possibility to match the platform.²¹ Some research studies and our previous studies have found that NiMnO-based electrodes derived from solid solution alloys could display this special electrochemical performance and facilitate the capacitance enhancement.^22–24 Nevertheless, just chemical synthesis or doping cannot achieve continuous redox reactions.^25,26 The reason why this kind of electrode can exhibit such a special electrochemical performance and its energy storage mechanism are still unknown.

Herein, the role of residual Mn in widening the potential window was investigated by using the model of an np-NiO(MnO_x)@Ni electrode derived from a simple NiMn alloy. It is found that the mixed valence of Mn substance provides additional redox reactions to ideally match the NiO platform and suppress hydrogen evolution. Benefiting from the consecutive conversion of MnO_x and NiO in the whole effective voltage range, this electrode obtained a conspicuous capacitance enhancement with a wide potential window (1.5 V). Due to the high activity and chemical conversion of MnO_x substance in aqueous electrolyte, the long-term cycle testing would meet an irreversible capacitance decline. Impressively, by confining the influence of MnO_x dissolution through the application of gel polymer electrolyte (GPE) with less free water, the symmetric quasi-solid-state SSC exhibited desirable lifespan performance (91% after 5000 cycles) with a high volumetric energy density of 37.67 mW h cm⁻³. Our research contributes to a valuable understanding for designing multiple element synergy electrodes in electrochemical energy storage and conversion.

2 Results and discussion

The morphology of np-Ni precursors after chemical dealloying for different times was analysed by SEM as illustrated in Fig. 1a–c. With the steps of selective dissolution and surface diffusion,^27–29 the typical bicontinuous nanopores and ligament network gradually increased while the Mn content decreased gradually. The pores in the np-Ni-1 precursor could not be clearly observed because of small pore size under shallow corrosion, while the nanopore sizes expand to about 10 nm in the np-Ni-5 precursor. The details of element content in np-Ni precursors are shown in Table S1.† The residual Mn content in the np-Ni-5 precursor (16.90 at%) exceeds that achieved through electrochemical dealloying (np-Ni-ED) for the same time (11.02 at%), indicating that electrochemical dealloying can accelerate the diffusion and dissolution of Mn atoms. At the same time, the specific surface areas of these precursors are also observed in comparison to demonstrate the formation of a three-dimensional bicontinuous structure (Fig. S1 and Table S2†). The np-Ni-X samples gradually exhibited an increase in specific surface area (from 3.387 m² g⁻¹ to 63.048 m² g⁻¹). The specific surface area of the np-Ni-ED sample is slightly smaller (33.647 m² g⁻¹). After electrochemical polarization, the Mn atoms inside the ligament would diffuse into the surface and form the Mn-rich oxide layer (Table S3†).

Fig. 1 SEM images of (a) np-Ni-1, (b) np-Ni-3 and (c) np-Ni-5; (d) SAED pattern of np-NiO(MnO_x)@Ni-5; (e) HRTEM image of np-NiO(MnO_x)@Ni-5; (f) XPS full spectra of the as-prepared np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni; (g) relative surface element content in np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni; (h) high-resolution XPS spectra of Mn 3s and 2p; (i) changes in the valence state of the Mn element.

Since the morphologies of the oxide layer are similar, we take np-NiO(MnO_x)@Ni-5 as a characteristic example. Based on the XRD pattern analysis (Fig. S2†), the presence of the FCC-Ni phase (PDF #87-0712) and (NiO)_0.75(MnO)_0.25 crystal structure (PDF #78-0425) can be observed. However, it should be noted that these peaks exhibit a broadening half width, indicating the relatively inferior crystallinity. Moreover, four clear rings can be attributed to the (111) and (200) planes for the Ni matrix, and the (111) and (220) planes for the (NiO)_0.75(MnO)_0.25 phase from np-NiO(MnO_x)@Ni-5 in the selected area electron diffraction (SAED) pattern (Fig. 1d), which are consistent with XRD results. The micromorphology images captured by SEM reveal that a large number of nanosheets are formed and uniformly distributed on the np-Ni surface and internal ligaments after the electrochemical polarization treatment. These nanosheets would grow into thin layered oxides through a CV activation step (see Fig. S3a–d†). The high-resolution TEM (HRTEM) image in Fig. 1e indicates that the thin layered oxides are made up of a large number of crystallization and amorphous regions. The lattice spacings of 0.177 nm and 0.210 nm are assigned to the Ni (200) plane and (NiO)_0.75(MnO)_0.25 (200) plane, respectively. Notably, the lattice fringes in the crystallization region have apparent lattice distortion due to the solid solution effect with Mn atoms. Besides, the uniform distribution of Ni, Mn and O elements confirms the presence of the solid solution oxides (NiO and MnO_x) based on TEM-EDS images (Fig. S3f†).

In order to investigate any distinction between np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni, the differences in surface element content and valence state were further elucidated by XPS. The full XPS spectra (Fig. 1f) evidence the presence of the Ni, Mn and O elements. After semi-quantitative analysis, the relative content of Mn on the surface continues to decrease with the progress of dealloying (Fig. 1g), which shows the same trends as np-Ni precursors. Further analyses demonstrate that the ΔE (the difference in binding energy) of Mn 3s for the samples gradually reduces from 5.36 eV to 4.98 eV with the ratio of Mn(IV) (Fig. S4†) gradually increasing to 0.498 in Mn 2p in np-NiO(MnO_x)@Ni-5 samples,^30,31 which demonstrates that MnO_x mainly exists in the form of Mn(III) species in np-NiO@Ni and np-NiO(MnO_x)@Ni-1, while the proportion of Mn(IV) species increases gradually in np-NiO(MnO_x)@Ni-3 and np-NiO(MnO_x)@Ni-5 (Fig. 1h–i). The presence of a large amount of mixed valence Mn(III/IV) species in np-NiO(MnO_x)@Ni-5 provides the basis for subsequent continuous valence changes and capacitance enhancement.³² Since transition metals have incompletely occupied electron orbitals (3d, 4d, and 5d…) and lone pair electrons, the electron paramagnetic resonance (EPR) measurement can detect their valence states.³³ Generally, Mn⁴⁺ and Mn²⁺ with 3d³ and 3d⁵ electronic configurations containing unpaired electrons, respectively, can be measured with obvious EPR signals, in which Mn²⁺ ions with a high spin state show a typical sextet ESR pattern. In contrast, Mn³⁺ with a low spin state without a lone pair of electrons (3d⁴) generally shows no ESR signal.³⁴ As shown in Fig. S5,† the np-NiO(MnO_x)@Ni-5 electrode shows an obvious Mn⁴⁺ single intense ESR signal, which may be attributed to the Mn⁴⁺ in the low-symmetry crystal field (Mn⁴⁺-lscf).³⁵ This is consistent with previous XPS findings, which indicate that MnO_x exhibits a higher mixed valence state content of the Mn(IV/III) component, thereby rendering it more active. But in the np-NiO@Ni electrode, there is no obvious Mn⁴⁺ signal in the same region (the other signal may be affected by the magnetic properties of Ni).³⁶ The spectra of other elements in np-NiO(MnO_x)@Ni-5 are shown in Fig. S6.† The Ni 2p can be divided into two sets of peaks made up of Ni(II) (855.2 eV and 872.8 eV) and Ni(III) (856.3 eV and 873.9 eV).^37,38 The O 1s can be decomposed into three peaks formed by metal oxides (M–O–M, 529.6 eV), metal hydroxides (M–OH, 531.0 eV) and adsorbed water (H₂O, 531.9 eV), respectively.³⁹ The above results illustrate that np-NiO(MnO_x)@Ni-5 exhibits mixed valence state hybrid Ni–Mn oxides/oxyhydroxides.

The electrochemical performance of np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni electrodes was studied to evaluate the role of Mn in electrochemical contribution. CV curves were obtained to discover any difference in the redox reactions (Fig. 2a), and the effect of Mn content on the current response is directly displayed in Fig. 2b. The current response is divided into two regions, and the highest peak is recorded for analysis. The positive potential region is mainly controlled by NiO while the negative region is dominated by MnO_x. As the degree of dealloying increases, there is a clear enhancement in the current response in np-NiO(MnO_x)@Ni-X and np-NiO@Ni electrodes. np-NiO(MnO_x)@Ni-1 shows an extremely low response similar to that of the original Ni₃₀Mn₇₀ alloy (Fig. S7†) due to the shallow corrosion. Additionally, in np-NiO(MnO_x)@Ni-3 and np-NiO(MnO_x)@Ni-5, the redox peaks become progressively more pronounced and the current response gradually increased thanks to the contribution of MnO_x (more details in Fig. S8†). However, it is clearly observed that the np-NiO@Ni obtained via electrochemical dealloying displayed enhanced redox peaks associated with NiO but the redox reactions of MnO_x disappeared because of the lower Mn content. Obviously, the expansion of the effective working potential window and capacitance enhancement of the np-NiO(MnO_x)@Ni-5 electrode are associated with the introduction of Mn, especially the mixed Mn(III/IV) species. The EIS properties of different electrodes were also tested as shown in Fig. 2c. The Nyquist plot shows that the slope in the low frequency region gradually increases as the degree of dealloying increases. In the high frequency region, the charge transfer resistance (R_ct) reflects the charge transfer process.^40,41 According to the EIS results, np-NiO(MnO_x)@Ni-5 exhibits a more rapid ion diffusion rate and a smaller R_ct (0.136 Ω), indicating that this electrode has a better kinetic process. Moreover, the best specific capacitance (Fig. 2d) achieved by np-NiO(MnO_x)@Ni-5 is due to the multiple redox reactions of MnO_x and NiO, and these result in the electrode having the best energy storage performance.

Fig. 2 (a) CV curves and (b) current response of np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni; (c) Nyquist diagrams of np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni; (d) the calculated specific capacitance of np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni; (e) CV curves of np-NiO(MnO_x)@Ni-5 at different scan rates; (f) dQ/dV plots of np-NiO(MnO_x)@Ni-5 by CV analysis between −1.05 V and 0.65 V.

As shown in Fig. 2e, the redox peaks of np-NiO(MnO_x)@Ni-5 exhibit a slight positive and negative shift but maintain their distinct shape as the scan rate increases, indicating rapid redox reactions. The peaks in the dQ/dV (differential capacitance; = current/scan rate) curves are considered to be due to the process of material transformation into a stable new phase.⁴² In the dQ/dV plot (Fig. 2f), four main peaks can be found. While one pair of peaks corresponds to the typical NiO/NiOOH redox reaction (peak 1 and peak 4), the other pair of peaks (peak 2 and peak 3) is associated with the multi-step redox evolution of MnO_x.^43,44 The red curve with evident peaks, which may be mainly associated with Mn(II)–Mn(III)–Mn(IV) and Ni(II)–Ni(III) redox reactions, indicates the battery behavior dominant charge-storage mechanism at 3 mV s⁻¹. At 30 mV s⁻¹, the intensities of all the redox peaks in the blue curve reduced, which is connected with the capacitive behavior.

To further verify its mechanism, the redox process of the np-NiO(MnO_x)@Ni-5 electrode was recorded by ex situ XRD and in situ Raman spectroscopy. The comparison of XRD diffraction peaks of the electrode at different potentials is shown in Fig. 3a. During the charging process, the peaks of the Ni matrix weakened gradually (almost disappeared at the state of full charge), while the intensity of (NiO)_0.75(MnO)_0.25 strengthened due to the action of MnO_x (especially at −0.3 V). At the full charged state, a transition occurred between oxides and oxyhydroxides and the formation of NiOOH can be observed (yellow area). The opposite process occurred during the discharging process and then returned to the original state. The crystal structural transformation was further elucidated by in situ Raman spectra (Fig. 3b). At a potential close to full charge (∼0.5 V), the Raman spectra showed two typical peaks at around 472 cm⁻¹ and 553 cm⁻¹ attributed to the Ni–O vibrations of NiOOH.^45,46 The intensity of the metal–O peak (∼560 cm⁻¹ range) affected by NiO and MnO_x gradually weakened in the process of discharging to lower potential, while the stretching band increased when charging close to positive potential.^47,48 The alteration in peak intensity at negative potential may be related to the transformation of MnO_x, as NiO remains constant in this negative region. Fig. 3c–f show the XPS spectra of the Ni 2p, Mn 2p, Mn 3s and O 1s at fully charged and discharged states. The fitting results for the Ni 2p spectra indicate a significant change in the valence state of Ni, with the ratio of Ni(II) changing from 0.32 (charged) to 0.65 (discharged) during the redox process (Fig. 3g). Similarly in Mn 2p spectra, when at the fully charged state, the curve can be divided into Mn(III) substance (at 642.1 eV and 653.8 eV) and Mn(IV) substance (at 643.1 eV and 654.8 eV). The curve at the fully discharged state can be decomposed into Mn(II) substance (at 641.3 eV and 653.0 eV) and Mn(IV) substance (at 642.7 eV and 654.4 eV). The change was further confirmed using the Mn 3s spectra (Fig. 3h), which showed an increase in binding energy difference from 4.44 eV (charged) to 5.19 eV (discharged). These results suggest that Mn oxidized into Mn(IV)/Mn(III) during charging, and was partially reduced to Mn(IV)/Mn(II) on discharging. In the O 1s spectrum, the ratio of M–O–M/M–OH reduced from 0.78 (charged) to 0.28 (discharged), which is due to the transition between M–O and M–OOH.

Fig. 3 (a) Ex situ XRD patterns of np-NiO(MnO_x)@Ni; (b) in situ Raman spectra of np-NiO(MnO_x)@Ni; XPS spectra of charge and discharge states: (c) Ni 2p, (d) Mn 2p, (e) Mn 3s and (f) O 1s; the valence state of (g) Ni and (h) Mn in XPS spectra.

The investigation of stored charge contribution is imperative to understand the reaction kinetics during the electrochemical process. For most electrodes, the contribution of stored charge originates from the diffusion-controlled battery behavior reaction and surface-controlled capacitive reaction. By employing the following eqn (1), the respective contributions can be accurately calculated:

log (i) = log(a) + b log(v) (1)

where i is the peak current of the anode and cathode, v is the scan rate, and a and b are adjustment values. Particularly, it can be regarded as a surface-controlled capacitive reaction if b equals 1. When b equals 0.5, the reaction is attributed to a diffusion-controlled battery behavior reaction.⁴⁹ Based on eqn (1), the calculated values of b corresponding to different peaks are between 0.5 and 1 and correspond to 0.54, 0.78, 0.67 and 0.69 for peak 1 to peak 4, respectively, as shown in Fig. 4a. Thus, the redox reactions occurring on the electrode are dominated by both the surface-controlled process and diffusion-controlled process. The subsequent eqn (2) can accurately ascertain the proportion of charge contribution between the surface-controlled and diffusion-controlled processes.⁵⁰

i = k₁v + k₂v^1/2 (2)

where k₁ and k₂ are two adjustment values, k₁v and k₂v^1/2 present the current response originating from the surface-controlled and diffusion-controlled process, respectively. As shown in Fig. 4b and c, the contribution of the surface-controlled process accounts for 32% of the total stored charges at 3 mV s⁻¹. With an increase in scan rate, the contribution of the surface-controlled capacitive process gradually increases to 85%. These results illustrate the fact that the battery behavior provides a substantial and additional amount of charge-storage capacity at lower scan rates.

Fig. 4 (a) Logarithmic relationship between peak current densities and scan rates of np-NiO(MnO_x)@Ni-5; (b) contribution ratios of the surface-controlled capacitive process and diffusion-controlled battery behavior at different scan rates for np-NiO(MnO_x)@Ni-5; (c) surface-controlled process in np-NiO(MnO_x)@Ni-5 at 3 mV s⁻¹; (d) long-term cycling stability of np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni; (e) schematic diagram of Mn dissolution in KOH electrolyte; (f) changes in Ni and Mn content in the electrolyte every 1000 cycles during the whole long-term cycle test.

The cycling stability of np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5) and np-NiO@Ni electrodes was measured by the full charge/discharge process at a current density of 10 A cm⁻³. As shown in Fig. 4d, the capacitance of the np-NiO@Ni electrode downs rapidly at first and then inclines to be stable at ∼52 F cm⁻³. Although the np-NiO(MnO_x)@Ni-1 electrode has a very steady stability, it suffers from a low level capacitance (∼23 F cm⁻³). The stability of the np-NiO(MnO_x)@Ni-3 electrode decreases rapidly after more than 1000 cycles and then shows a slow decay. Compared with others, np-NiO(MnO_x)@Ni-5 exhibits an adequate performance and its capacitance still remains nearly 86.4 F cm⁻³ until the end of cycling although a steady decrease was observed. The poor performance of long-term cycle stability may be related to the instability of the Mn element in the aqueous electrolyte and cause the failure of long-term storage capability.^42,43,51 The chemical dissolution of MnO_x substance could be attributed as a primary factor in electrode degradation (Fig. 4e). Through the examination of the variations in Ni and Mn content in KOH electrolyte every 1000 cycles, it is convenient to analyze the changes that lead to the failure during the cycling process. Throughout the long cycling process, the Ni concentration in the solution remained relatively stable (Fig. 4f). However, the concentration of the Mn element showed the most significant loss in the first 1000 cycles and continuously dissolved in the electrolyte. This is also the reason for the fast decrease in capacitance at the beginning of the long cycling process. Additionally, the electrochemical tests and XPS analyses were also conducted on the electrode after long-term cycling, as depicted in Fig. S9.† This shows that the CV curves display a reduction at lower potential after 2000 and 5000 cycles, related to the redox of MnO_x. Noteworthily, after 5000 long cycles, Mn(IV) is the mainly existing valence state of the Mn element, whereas Ni primarily persisted as Ni(II).

Based on the above results, it can be inferred that in the long-term cycle process, Mn(III) would gradually precipitate from the thin layered oxides and continuously dissolve from MnO_x species. During the redox process, MnO₂ is progressively reduced to MnO, leading to both dissolution of Mn²⁺ ions and chemical dissolution of the intermediate Mn(III) substance. Mn²⁺ ions will continuously dissolve out, and subsequently diffuses in the electrolyte and causes permanent capacitance loss.⁵¹ The newly deposited MnO_2(s) will accumulate and cause an increase in valence on the electrode surface, which is consistent with the above XPS results.

The assumed dissolution reaction mechanism for Mn can be represented by using the following eqn (3):

2MnOOH_(S) → MnO_2(S) + Mn_(aq)²⁺ + 2OH_(aq)⁻ (3)

Therefore, inhibiting the dissolution reaction of Mn is promising to improve the long-term cycling stability of the electrodes. It has been reported that GPE with limited free water can slow down the dissolution and side reactions of active substances, helping to improve the reversible capacity and stability of the electrode.^52–54 Thus, for practical application, we use PVA–KOH gel electrolyte as a model to assemble quasi-solid-state SSCs (as shown in Fig. S10b†). Since the electrode has a double-side active substance, we can assemble laminated devices easily as needed. As shown in Fig. 5a, due to the fact that there are few free water molecules and a large amount of polar hydroxyl groups (˙OH) in the PVA gel, it can effectively combine with dissolved Mn²⁺ ions through coordination and exhibit a strong interaction.^55,56 To a certain extent, this obviously slows the chemical dissolution of Mn substance and thus improves the device cycle life. The capacitance retention rate can reach 91% after 5000 cycles (refer to Fig. 5b). The CV curves of a single device were measured ranging from 3 to 40 mV s⁻¹, as shown in Fig. 5c. It is worth noting that a single device retains a wide operating voltage window of 1.5 V without polarization and shows a good capacitance at lower potential, which is very effective in improving the energy density of the device. Fig. 5d and S11a† depict the GCD curves of a single SSC device. These isosceles triangle curves demonstrate the good coulombic efficiency and outstanding capacitive behavior. Fig. 5e and f show CV and GCD curves of a laminated SSC device. The CV curves are similar to those of a single device but the discharge times get improved, indicating the advantages of lamination. The thickness of each type of device is also shown in Fig. S10c and d.† The volumetric specific capacitance of the SSC devices calculated from the corresponding GCD curves is presented in Fig. 5g. This shows that a single SSC device has an excellent specific capacitance of 138.36 F cm⁻³ at a current density of 0.2 A cm⁻³, and still retains 52.71 F cm⁻³ at ten times the current density (2 A cm⁻³), suggesting the remarkable performance of this device. The laminated device exhibits a great improvement in capacitance and also has an excellent specific capacitance of 83.83 F cm⁻³ at a current density of 2 A cm⁻³.

Fig. 5 (a) Schematic diagram of the quasi-solid-state device and Mn ions in PVA–KOH GPE; (b) long-term cycling performance of a single SSC device; (c) CV curves of a single SSC device at different scan rates; (d) GCD curves of a single SSC device at different current densities; (e) CV curves of the laminated SSC at different scan rates; (f) GCD curves of a laminated SSC device at different current densities; (g) volumetric specific capacitance of a single SSC device and a laminated SSC device; (h) GCD curves in both positive and negative directions; (i) OCP of a single SSC device; (j) GCD curves of series and parallel planar SSCs; (k) Ragone plots for a comparison with other reported SSCs.

The characteristic of nonpolarity of a single device is studied by charging and discharging between the positive and negative operating voltages (see Fig. 5h). The orange curve is the charge and discharge in the positive direction, while the green curve is equivalent to the charge and discharge in the negative direction. Remarkably, even during repeated charging and discharging cycles in both directions, the shape of GCD curves remains consistently triangular, providing substantial evidence for its nonpolar nature. At the same time, thanks to the application of PVA–KOH GPE,⁵⁷ the self-discharge performance of the single device exhibited a remarkable practicality compared to other types of supercapacitors (refer to Table S4†), as it took over 89 000 seconds (∼25 h) for the open circuit potential (OCP) to drop to 50% of its initial value (Fig. 5i).To further evaluate the uniformity and efficiency of the supercapacitors, series and parallel connections were used (Fig. 5j). When two devices were connected in a series connection, the GCD curve demonstrated a discharge time similar to that of a single device, while achieving an operating voltage over 2.8 V. When connecting the two devices in parallel, the discharge time and output current can increase without compromising overall capacitive response. In addition, by connecting the two devices in series, a little yellow LED lamp can be successfully lit. As presented in Fig. 5k, based on the total volume of the device, a single SSC device delivered a high volumetric energy density of 37.67 mW h cm⁻³ at a power density of 57.59 mW cm⁻³, surpassing that of most of the reported supercapacitors, including NiCo₂O₄//Fe foam,⁵⁸ MnO₂//S–MoO_3−x,⁵⁹ ZnFe₂O₄-NCF//NiCo₂O₄-NCF,⁶⁰ Nb-CMO₄-C_xS_yNC//rGO/Ti₃C₂T_x,⁶¹ Ni–Co (oxy)hydroxides//AC,⁶² Ni(OH)₂ nanowire//carbon fiber,⁶³ Ni/MnO₂-FP//Ni/AC-FP,⁶⁴ MnO₂/PEDOT-PSS//OMC,⁶⁵ Cu–CuO@CoFe-LDH//Cu-AC,⁶⁶ Ni-doped MnO₂@CC⁶⁷ and so on. As the power density increases, the energy density decreases to some extent, but can still retain 8.07 mW h cm⁻³ at an outstanding power density of 3253.68 mW cm⁻³. Therefore, the nonpolar quasi-solid-state device exhibits remarkable volumetric energy and power densities, thus indicating significant prospects in the field of electronics.

3 Conclusions

This work successfully investigated the energy storage mechanism of free-standing np-NiO(MnO_x)@Ni composite electrodes with high activity and a wide potential window. The effect of Mn content and valence state is strongly connected with the process of dealloying. Only the coexistence of NiO and mixed valence Mn(III/IV) substance can stimulate continues redox reactions and exhibit the synergetic effect to build an outstanding capacitance. Particularly, the MnO_x substance provides a continuous valence transfer between Mn(IV)/(III) and Mn(III)/(II), which primarily contributes to the remarkable capacitance in the negative potential region of NiO. However, the instability of MnO_x in the alkaline electrolyte has an unfavourable impact on the lifespan. GPE with limited free water can slow down the dissolution and side reactions of Mn ions, and thus improve the reversible capacity and stability of the electrode. The quasi-solid-state device assembled with PVA–KOH gels exhibits an excellent lifespan (more than 90% after 5000 cycles) and high energy density (37.67 mW h cm⁻³), as well as excellent self-discharge performance (over 89 000 s for a self-discharge rate from zero to 50%). This study will provide a new view on the design and application of novel electrodes with multiple element synergy for energy storage.

4 Experimental

4.1 Materials

Ammonium sulfate ((NH₄)₂SO₄, 99%, AR, Aladdin), potassium hydroxide (KOH, 95%, AR, MERYER), and PVA (M_w: 89 000–98 000, 99% hydrolysed, Sigma-Aldrich). All the chemicals were of analytical grade and used without further purification.

4.2 Synthesis of np-NiO(MnO_x)@Ni-X electrodes

Ni₃₀Mn₇₀/Ni foil, obtained by casting and rolling processes,⁶⁸ was dealloyed for 1 h, 3 h and 5 h (no more big bubbles were generated) in 1 M (NH₄)₂SO₄ at 50 °C by free chemical etching to obtain np-Ni-X (X = 1, 3, and 5) precursors. The precursors were rinsed several times with deionized (DI) water and ethanol. Then the np-Ni precursor was oxidized through electrochemical polarization to prepare np-NiO(MnO_x)@Ni-X (X = 1, 3, and 5). The Pt foil and Ag/AgCl electrode were used as the counter electrode and reference electrode, respectively. The electrochemical polarization treatment was performed at 0.9 V, 0.8 V and 0.6 V for 60 s in 1 M KOH by multi-potential steps. The counterpart np-NiO@Ni was prepared based on the np-Ni-ED precursor through an electrochemical dealloying method using a three-electrode system for 5 h and polarization process, similar to our previous report.⁶⁹

4.3 Synthesis of PVA–KOH GPE

To prepare the PVA–KOH GPE, 3 g of PVA was dissolved in 25 mL of DI water at 90 °C and stirred until the solution became a colloidal liquid. Then, 3 g of KOH was dissolved in 5 mL of DI water and added to the previous PVA gel. After several minutes of mixing, the PVA–KOH gel electrolyte was obtained.

4.4 Fabrication of quasi-solid-state SSC devices

The quasi-solid-state device was assembled with np-NiO(MnO_x)@Ni-5 electrodes and PVA–KOH gel. First, immerse the prepared electrode in the PVA–KOH gel for about 3 min. Then, stack the electrodes face to face and press together. At last, the whole supercapacitor was packaged in a plastic film. The total volume of a single quasi-solid-state device was ∼52 mm³ [20 mm (L) × 10 mm (W) × 0.26 mm (H)].

4.5 Microstructure characterization

Phase identification was performed by X-ray diffraction (XRD; D8 advance), using a Cu Kα source in the range of 10° to 90°. The micromorphological characteristics and elemental composition of the electrodes were investigated by using a scanning electron microscope (SEM; S-4800) equipped with an X-ray energy spectrometer (EDS). More detailed structural characterization was carried out and crystallographic structures of the electrodes were characterized by transmission electron microscopy (TEM; JEM-F200). The surface valence state of the electrodes was characterized by X-ray photoelectron spectroscopy (XPS) using an Axis Supra. A Raman spectrometer (Ostec-ArtTool; Ramos S120) with a 532 nm laser was used in the in situ Raman study. Inductively coupled plasma-mass spectrometry (ICP-MS; Aglient 7800) was used for multi-element analysis in KOH electrolytes.

4.6 Electrochemical measurements

All electrode measurements were performed on a three-electrode system at room temperature, with Pt foil, a Ag/AgCl electrode and 1 M KOH used as the counter electrode, reference electrode and electrolyte, respectively. The tests on SSCs were performed using a two-electrode system. The cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) tests were carried out on an IVIUM workstation. Electrochemical impedance spectroscopy (EIS) was carried out at open circuit potential (OCP) at an amplitude of 5 mV with the frequency from 10⁻² Hz to 10⁵ Hz by using a Shanghai Chenhua workstation. Long-term cycle testing of the electrodes and device was performed on a LANHE D350A.

Author contributions

Ziying Shi: investigation, writing – original draft, validation, visualization. Enzuo Liu: software, visualization. Biao Chen: investigation, methodology, supervision. Junwei Sha: formal analysis, investigation. Lihua Qian: funding acquisition, project administration. Zhijia Zhang: funding acquisition, project administration. Xiaopeng Han: resources. Wenbin Hu: resources. Chunnian He: resources, supervision. Naiqin Zhao: resources, supervision. Jianli Kang: conceptualization, funding acquisition, methodology, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge the financial support by the National Natural Science Foundation of China (Grant No. 52071232, Grant No. 52171156 and Grant No. 52075369).

Notes and references

1. G. Y. Zhang, S. H. Jiang, I. D. Kim, F. Wang, J. Lee, L. Chen, S. J. He, J. Q. Han, J. Ahn and J. Y. Cheong, ACS Nano, 2023, 17, 8866–8898.
2. J. Sun, B. Luo and H. Li, Adv. Energy Sustainability Res., 2022, 3, 2100191.
3. M. Pershaanaa, S. Bashir, S. Ramesh and K. Ramesh, J. Energy Storage, 2022, 50, 104599.
4. R. Liu, Z. L. Wang, K. Fukuda and T. Someya, Nat. Rev. Mater., 2022, 7, 870–886.
5. J. Gao, K. Wang, J. Cao, M. Zhang, F. Lin, M. Ling, M. Wang, C. Liang and J. Chen, Small, 2023, 19, 2302786.
6. Y. Zhang, X. Jing, X.-H. Yan, H.-L. Gao, K.-Z. Gao, Y. Cao, S. Hu and Y.-y. Zhang, Coord. Chem. Rev., 2024, 499, 215494.
7. J. Liang, C. Jiang and W. Wu, Appl. Phys. Rev., 2021, 8, 021319.
8. C. Huang, S. Lv, A. Gao, J. Ling, F. Yi, J. Hao, M. Wang, Z. Luo and D. Shu, Chem. Eng. J., 2022, 431, 134083.
9. R. S. Kate, S. A. Khalate and R. J. Deokate, J. Alloys Compd., 2018, 734, 89–111.
10. D. Pandey, K. S. Kumar and J. Thomas, Prog. Mater. Sci., 2024, 141, 101219.
11. G. Tang, J. Liang and W. Wu, Adv. Funct. Mater., 2023, 34, 2310399.
12. H. Fan, J. Song, L. Bai, Y. Wang, Y. Jin, S. Liu, X. Xie, W. Zheng and W. Liu, Energy Storage Mater., 2023, 55, 84–93.
13. Z. H. Zhang, S. S. Sun, Z. H. Xu and S. G. Yin, Small, 2023, 19, e2302479.
14. J. Kang, Y. Xue, J. Yang, Q. Hu, Q. Zhang, L. Gu, A. Selloni, L. M. Liu and L. Guo, J. Am. Chem. Soc., 2022, 144, 8969–8976.
15. Y. Shao, M. F. El-Kady, J. Sun, Y. Li, Q. Zhang, M. Zhu, H. Wang, B. Dunn and R. B. Kaner, Chem. Rev., 2018, 118, 9233–9280.
16. G. H. Zhang, J. Hu, Y. Nie, Y. L. Zhao, L. Wang, Y. Z. Li, H. Z. Liu, L. Z. Tang, X. N. Zhang, D. Li, L. Sun and H. G. Duan, Adv. Funct. Mater., 2021, 31, 2100290.
17. J. Zhan, G. R. Li, Q. H. Gu, H. Wu, L. W. Su and L. B. Wang, ACS Appl. Mater. Interfaces, 2021, 13, 52519–52529.
18. M. Vukovi, J. Appl. Electrochem., 1994, 24, 878–882.
19. A. Seghiouer, J. Chevalet, A. Barhoun and F. Lantelme, J. Electroanal. Chem., 1998, 442, 113–123.
20. Y. Yang, L. Li, G. D. Ruan, H. L. Fei, C. S. Xiang, X. J. Fan and J. M. Tour, ACS Nano, 2014, 8, 9622–9628.
21. W. Guo, X. Guo, L. Yang, T. Wang, M. Zhang, G. Duan, X. Liu and Y. Li, Polymer, 2021, 235, 124276.
22. A. Thomas, A. Kumar, G. Perumal, R. K. Sharma, V. Manivasagam, K. Popat, A. Ayyagari, A. Yu, S. Tripathi, E. Buck, B. Gwalani, M. Bhogra and H. S. Arora, ACS Appl. Mater. Interfaces, 2023, 15, 5086–5098.
23. S. Zhang, J. Liu, L. Wang, Z. Zhang, J. Kang, P. Liu and W. Wang, Chem. Eng. J., 2021, 405, 126632.
24. J. Kang, A. Hirata, L. Chen, S. Zhu, T. Fujita and M. Chen, Angew Chem. Int. Ed. Engl., 2015, 54, 8100–8104.
25. M. Huang, Y. Lin, H. Huang, X. Fan, K. Shi, Z. Yang and W. Zhang, Electrochim. Acta, 2021, 383, 138353.
26. Q. Ren, R. Wang, H. Wang, J. Key, D. J. L. Brett, S. Ji, S. Yin and P. Kang Shen, J. Mater. Chem. A, 2016, 4, 7591–7595.
27. Q. Chen, Chem. Mater., 2022, 34, 10237–10248.
28. H. Meng, Q. Ran, T.-Y. Dai, H. Shi, S.-P. Zeng, Y.-F. Zhu, Z. Wen, W. Zhang, X.-Y. Lang, W.-T. Zheng and Q. Jiang, Nano-Micro Lett., 2022, 14, 128.
29. Y.-Q. Li, H. Shi, S.-B. Wang, Y.-T. Zhou, Z. Wen, X.-Y. Lang and Q. Jiang, Nat. Commun., 2019, 10, 4292.
30. S. Zhu, Y. C. Wang, J. S. Zhang, J. Sheng, F. Yang, M. Wang, J. F. Ni, H. Jiang and Y. Li, Energy Environ. Mater., 2023, 6, 12382.
31. E. S. Ilton, J. E. Post, P. J. Heaney, F. T. Ling and S. N. Kerisit, Appl. Surf. Sci., 2016, 366, 475–485.
32. M.-K. Song, S. Cheng, H. Chen, W. Qin, K.-W. Nam, S. Xu, X.-Q. Yang, A. Bongiorno, J. Lee, J. Bai, T. A. Tyson, J. Cho and M. Liu, Nano Lett., 2012, 12, 3483–3490.
33. H. Tian, L. Zeng, Y. Huang, Z. Ma, G. Meng, L. Peng, C. Chen, X. Cui and J. Shi, Nano-Micro Lett., 2020, 12, 161.
34. L. Tang, C. Wang, S. Tian, Z. Zhang, Y. Yu, D. Song and Z. Zhang, Anal. Chem., 2022, 94, 16189–16195.
35. R. Stoyanova, M. Gorova and E. Zhecheva, J. Phys. Chem. Solids, 2000, 61, 609–614.
36. J. Zhu, F. Cannizzaro, L. Liu, H. Zhang, N. Kosinov, I. A. W. Filot, J. Rabeah, A. Brückner and E. J. M. Hensen, ACS Catal., 2021, 11, 11371–11384.
37. C. Liang, P. Zou, A. Nairan, Y. Zhang, J. Liu, K. Liu, S. Hu, F. Kang, H. J. Fan and C. Yang, Energy Environ. Sci., 2020, 13, 86–95.
38. P. Zou, J. Li, Y. Zhang, C. Liang, C. Yang and H. J. Fan, Nano Energy, 2018, 51, 349–357.
39. G. Y. Shi, T. Tano, D. A. Tryk, M. Yamaguchi, A. Iiyama, M. Uchida, K. Iida, C. Arata, S. Watanabe and K. Kakinuma, ACS Catal., 2022, 12, 14209–14219.
40. A. C. Lazanas and M. I. Prodromidis, ACS Meas. Sci. Au, 2023, 3, 162–193.
41. S.-B. Wang, Q. Ran, W.-B. Wan, H. Shi, S.-P. Zeng, Z. Wen, X.-Y. Lang and Q. Jiang, J. Mater. Sci. Technol., 2022, 120, 159–166.
42. Y.-T. Weng, H.-A. Pan, R.-C. Lee, T.-Y. Huang, Y. Chu, J.-F. Lee, H.-S. Sheu and N.-L. Wu, Adv. Energy Mater., 2015, 5, 1500772.
43. T. Xing, M. Sun, S. Guo, O. Harris, Y. Zhong, L. Tang, S. Zhang, M. Yang and H. Xia, J. Power Sources, 2021, 495, 229801.
44. B. Messaoudi, S. Joiret, M. Keddam and H. Takenouti, Electrochim. Acta, 2001, 46, 2487–2498.
45. C. Huang, Y. Huang, C. Liu, Y. Yu and B. Zhang, Angew. Chem., 2019, 131, 12142–12145.
46. Y. Huang, X. Chong, C. Liu, Y. Liang and B. Zhang, Angew. Chem., Int. Ed., 2018, 57, 13163–13166.
47. S. Bernardini, F. Bellatreccia, G. Della Ventura and A. Sodo, Geostand. Geoanal. Res., 2020, 45, 223–244.
48. N. Mironova-Ulmane, A. Kuzmin, I. Steins, J. Grabis, I. Sildos and M. Pärs, J. Phys.: Conf. Ser., 2007, 93, 012039.
49. X. Yang and A. L. Rogach, Adv. Energy Mater., 2019, 9, 1900747.
50. X. Pu, D. Zhao, C. Fu, Z. Chen, S. Cao, C. Wang and Y. Cao, Angew Chem. Int. Ed. Engl., 2021, 60, 21310–21318.
51. W. Guo, C. Yu, S. Li, Z. Wang, J. Yu, H. Huang and J. Qiu, Nano Energy, 2019, 57, 459–472.
52. Y. Lin, M. Zhang, Y. Hu, S. Zhang, Z. Xu, T. Feng, H. Zhou and M. Wu, Chem. Eng. J., 2023, 472, 145136.
53. S. Tian, T. Hwang, S. Malakpour Estalaki, Y. Tian, L. Zhou, T. Milazzo, S. Moon, S. Wu, R. Jian, K. Balkus, T. Luo, K. Cho and G. Xiong, Adv. Energy Mater., 2023, 13, 2300782.
54. J. Liang and W. Wu, Appl. Phys. Rev., 2024, 11, 011301.
55. M. Zhu, J. Wu, W. H. Zhong, J. Lan, G. Sui and X. Yang, Adv. Energy Mater., 2018, 8, 1702561.
56. S. Tian, T. Hwang, Y. Tian, Y. Zhou, L. Zhou, T. Milazzo, S. Moon, S. Malakpour Estalaki, S. Wu, R. Jian, K. Balkus, T. Luo, K. Cho and G. Xiong, ACS Nano, 2023, 17, 14930–14942.
57. W. Shang, W. Yu, X. Xiao, Y. Ma, Y. He, Z. Zhao and P. Tan, Adv. Powder Technol., 2023, 2, 100075.
58. C. Zhang, L. Hou, W. Yang, S. Du, B. Jiang, H. Bai, Z. Li, C. Wang, F. Yang and Y. Li, Chem. Eng. J., 2023, 467, 143410.
59. S. Liu, C. Xu, H. Yang, G. Qian, S. Hua, J. Liu, X. Zheng and X. Lu, Small, 2020, 16, 1905778.
60. B. H. Xiao, R. T. Lin, K. Xiao and Z. Q. Liu, J. Power Sources, 2022, 530, 231307.
61. A. M. Patil, S. Moon, Y. Seo, S. B. Roy, A. A. Jadhav, D. P. Dubal, K. Kang and S. C. Jun, Small, 2023, 19, e2205491.
62. X. Ren, M. Li, L. Qiu, X. Guo, F. Tian, G. Han, W. Yang and Y. Yu, J. Mater. Chem. A, 2023, 11, 5754–5765.
63. X. Dong, Z. Guo, Y. Song, M. Hou, J. Wang, Y. Wang and Y. Xia, Adv. Funct. Mater., 2014, 24, 3405–3412.
64. L. Zhang, P. Zhu, F. Zhou, W. Zeng, H. Su, G. Li, J. Gao, R. Sun and C.-p. Wong, ACS Nano, 2015, 10, 1273–1282.
65. X. Cheng, J. Zhang, J. Ren, N. Liu, P. Chen, Y. Zhang, J. Deng, Y. Wang and H. Peng, J. Phys. Chem. C, 2016, 120, 9685–9691.
66. Z. Li, M. Shao, L. Zhou, R. Zhang, C. Zhang, J. Han, M. Wei, D. G. Evans and X. Duan, Nano Energy, 2016, 20, 294–304.
67. R. Zhong, M. Xu, N. Fu, R. Liu, A. A. Zhou, X. Wang and Z. Yang, Electrochim. Acta, 2020, 348, 136209.
68. H. J. Qiu, J. L. Kang, P. Liu, A. Hirata, T. Fujita and M. W. Chen, J. Power Sources, 2014, 247, 896–905.
69. J. Kang, A. Hirata, H. J. Qiu, L. Chen, X. Ge, T. Fujita and M. Chen, Adv. Mater., 2014, 26, 269–272.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta00979g

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