e-ISSN 2150-5551

CN 31-2103/TB

ARTICLE https://doi.org/10.1007/s40820-023-01016-6

Boosting Pseudocapacitive Behavior

of Supercapattery Electrodes by Incorporating
Cite as
Nano-Micro Lett. a Schottky Junction for Ultrahigh Energy Density
(2023) 15:62

Received: 27 October 2022 Selvaraj Seenivasan1, Kyu In Shim2, Chaesung Lim3, Thangavel Kavinkumar1,
Accepted: 30 December 2022 Amarnath T. Sivagurunathan1, Jeong Woo Han2,3 *, Do‑Heyoung Kim1 *
Published online: 10 March 2023
© The Author(s) 2023

HIGHLIGHTS

• Incorporation of Schottky Junction increases the pseudocapacitive mechanism at higher current rate.

• The pseudocapacitance behavior of the positive and negative electrodes is balanced to construct a solid-state supercapattery device.

• An energy density of 236.14 Wh ­kg−1 is achieved for solid-state supercapattery device.

ABSTRACT Pseudo-capacitive negative electrodes remain a major

bottleneck in the development of supercapacitor devices with high
energy density because the electric double-layer capacitance of the
negative electrodes does not match the pseudocapacitance of the cor-
responding positive electrodes. In the present study, a strategically
improved Ni-Co-Mo sulfide is demonstrated to be a promising candi-
date for high energy density supercapattery devices due to its sustained
pseudocapacitive charge storage mechanism. The pseudocapacitive
behavior is enhanced when operating under a high current through the
addition of a classical Schottky junction next to the electrode–electrolyte interface using atomic layer deposition. The Schottky junction
accelerates and decelerates the diffusion of ­OH‒/K+ ions during the charging and discharging processes, respectively, to improve the pseu-
docapacitive behavior. The resulting pseudocapacitive negative electrodes exhibits a specific capacity of 2,114 C ­g−1 at 2 A ­g−1 matches
almost that of the positive electrode’s 2,795 C g­ −1 at 3 A ­g−1. As a result, with the equivalent contribution from the positive and negative
electrodes, an energy density of 236.1 Wh ­kg−1 is achieved at a power density of 921.9 W ­kg−1 with a total active mass of 15 mg ­cm−2.
This strategy demonstrates the possibility of producing supercapacitors that adapt well to the supercapattery zone of a Ragone plot and
that are equal to batteries in terms of energy density, thus, offering a route for further advances in electrochemical energy storage and
conversion processes.

KEYWORDS Pseudo-capacitance; Negative electrode; Supercapattery; Atomic layer deposition; Energy density

* Jeong Woo Han, jwhan@postech.ac.kr; Do‑Heyoung Kim, kdhh@chonnam.ac.kr

1
School of Chemical Engineering, Chonnam National University, 77 Yongbong‑Ro, Gwangju 61186, Republic of Korea
2
Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
3
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea

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62 Page 2 of 21 Nano-Micro Lett. (2023) 15:62

1 Introduction graphene, and reduced graphene oxide, few pseudocapaci-

tive transition metal oxides/sulfides have been studied to
Supercapacitors and batteries are the primary candidates match the specific capacity of positive electrodes [16, 17].
that have widespread usage and attract the most interest for Therefore, designing compatible negative electrodes with
further development [1]. Batteries have a larger retail market sustained pseudocapacitive behavior is essential to realize
share owing to their capability of operating for a long dura- the real-time application of a supercapattery device.
tion with high energy density. For example, the global elec- In this study, we designed positive and negative elec-
tric vehicles batteries market (GEVB) is expected to reach a trodes with a large surface area that had a large proportion
valuation of USD 87.2 billion in the year 2027 [2]. Notably, of surface-active sites, as well as a Schottky junction next
supercapacitors push the boundaries by allowing the fusion to the electrode–electrolyte interface through atomic layer
of different electrodes as hybrid, asymmetrical and super- deposition (ALD) to boost the pseudocapacitive behavior
capattery devices, while corresponding metal anodes are [18–20]. The presence of a Schottky junction controls the
mandatorily required for batteries (for example, Li, Na and ion diffusion rate during the charging/discharging process
Zn ion battery) [3–5]. In addition, supercapacitors provide and significantly improves the pseudocapacitive contribution
attractive benefits such as a long cycle life, dependence on to the overall specific capacity. Another unique advantage is
abundant elements, and compatibility with other applica- that ALD-coated thin films are in the 1­ 0–9 g ­cm−2 range, so
tions [6]. they do not increase the total mass, but provide the advan-
A comparison of the classical electrical double layer tages of traditional core–shell structures resulting in high
(EDLC) and pseudocapacitive charge storage reveals that mass activity [21]. The aforementioned strategies proved
the latter is worth greater attention because it is analogous successful as the manufactured supercapattery device dem-
to a battery and thus presents broader opportunities for fur- onstrated an energy density of 236.14 Wh ­kg−1, which fits
ther development [7]. The rapid and reversible surface redox well in the supercapattery zone of the Ragone diagram [22].
reaction in pseudocapacitive charge storage systems results
in higher energy density and wider application prospects.
In the pursuit of both high energy and power density, a new 2 Experimental Section
type of energy storage device configuration called superca-
pattery has emerged [8, 9]. In a conventional supercapattery 2.1 Synthesis of the ­NiCo2S4/NiMo2S4/ALD‑Co3O4
configuration, EDLC and pseudocapacitive electrodes are Positive Electrode
combined to operate at a high energy density, although in a
real-time application, each electrode must be capable of stor- 2.1.1 Synthesis of ­NiCo2S4 Nanoneedles
ing charge via both mechanisms to deliver a high energy out-
put [10–12]. A classical method involves designing a high NiCo2O4 (NCO) nanoneedles were synthesized on Ni foam
surface area electrode using an intrinsically pseudocapaci- (NF) via a hydrothermal method. A piece of NF (2 × 5 ­cm2)
tive material, whereby both the pseudocapacitive contribu- was cleaned by ultrasonication using ethanol and deionized
tion (from the material) and EDLC contribution (owing to (DI) water for several minutes and then dried. The cleaned
the high surface area) can be derived and summed-up as NF was transferred to a Teflon-lined autoclave containing a
overall device capacity [13]. The inadequate specific capac- precursor solution prepared by dissolving 0.388 g of cobalt
ity of negative electrodes to match positive electrodes is one nitrate, 0.3 g of nickel nitrate, 0.3 g of urea, and 0.062 g of
of the remaining bottlenecks for the commercialization of ammonium fluoride in 40 mL of DI water. The hydrothermal
supercapattery devices. The poor pseudocapacitive contri- reaction was conducted at 120 °C for 6 h. Subsequently, the
bution of negative electrodes can limit the overall device obtained NCO on NF was rinsed with DI water and dried at
capacity of the supercapattery device [14]. The research 60 °C. A wet sulfurization process was employed to drive
on pseudocapacitive negative electrodes is limited because the anion exchange reaction (AER) for converting NCO to
of an inadequate choice of materials [15]. Exempting the ­NiCo2S4 (NCS). The sulfurization solution was prepared
numerous EDLC carbon materials such as carbon nanotubes, using 2 g of N ­ a2S in DI water and the AER reaction was

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Nano-Micro Lett. (2023) 15:62 Page 3 of 21 62

completed at 120 °C for 8 h. Finally, the obtained NCS on was rinsed with Ethanol and DI water, and then vacuum
NF was rinsed with Ethanol and DI water, and then vacuum dried at 80 °C for further use.
dried at 80 °C for further use.
2.2.2 Synthesis of NMS/FeO Hollow Cuboids

2.1.2 Synthesis of NCS/NMS Hollow Cuboids Fe2O3 (FeO) thin films were deposited through a homemade
flow-type ALD reactor maintained at 175 °C and 800 mTorr
To synthesize N­ iCo2O4/NiMo2O4 (NCO/NMO), N ­ iMo2O4 [25]. Bis(bis(trimethylsilyl)amide) iron(II) (Fe(btmsa)2—
(NMO) cuboids were synthesized on NF/NCO following Hansol Chemicals, South Korea) and ozone (5% in ­O2) were
a hydrothermal method [23]. The precursor solution was used as the iron precursor and counter reactant, respectively,
prepared by dissolving 0.5 g of ammonium molybdate and in the ALD process. An ALD cycle comprised four steps,
0.465 g of nickel nitrate in 40 mL of DI water. The hydro- i.e., a precursor pulse of 1.5 s, followed by precursor purge
thermal reaction was conducted at 150 °C for 6 h. The wet for 60 s, an ozone pulse of 2 s, and final ozone purge for 60 s.
sulfurization process was employed to drive the AER for The thickness of the FeO shell over NMS substrates was
converting NCO/NMO to N ­ iCo2S4/NiMo2S4 (NCS/NMS). controlled by the number of ALD cycles (150, 300 and 450).
The AER duration was varied from 4 to 10 h to optimize
the hollow structure formation. Finally, the obtained NCS/
2.3 Characterization
NMS on NF was rinsed with Ethanol and DI water, and then
vacuum dried at 80 °C for further use.
The crystallinity of each sample was analyzed using a high-
resolution X-ray diffractometer (XRD; PANalytical) using
a 3D-PIXcel detector and equipped with Cu–Kα radiation
2.1.3 Synthesis of NCS/NMS/ALD‑Co3O4 Hollow at 60 kV and 55 mA. High-resolution X-ray photoelectron
Cuboids spectroscopy (XPS) was employed with Kα radiation and
seven Channeltron detectors. High-resolution scanning elec-
Co3O4 (CoO) thin films were deposited using a homemade tron microscopy (HR-SEM; JEOL JSM-7500F) coupled with
flow-type ALD reactor maintained at 175 °C and 800 mTorr. an energy-dispersive X-ray spectroscopy (EDS) analyzer
Cobalt cyclopentadienyl (Co(Cp) 2-Sigma-Aldrich, USA) with 15 kV acceleration and high-resolution transmission
and ozone (5% in ­O2) were used as the cobalt precursor electron microscopy (HR-TEM, TECNAI G2 F20) were used
and counter reactant, respectively, in the ALD process [24]. to study the structural and morphological properties of the
Argon (99.999%) was used as both the carrier (50 sccm) electrodes.
and purging (250 sccm) gas. An ALD cycle comprised four
steps, namely, a precursor pulse of 1.5 s, followed by pre-
2.4 Electrochemical Measurements
cursor purge for 60 s, an ozone pulse of 2 s, and the final
ozone purge for 60 s. The thickness of the CoO shell over
The electrochemical measurements were conducted in a
NCS/NMS substrates was controlled by the number of ALD
three-electrode system containing 2 M KOH electrolyte,
cycles (100, 200, and 300).
wherein a saturated calomel electrode (SCE) and Pt foil
were used as reference and counter electrodes, respectively.
2.2 Synthesis of the NMS/ALD‑Fe2O3 Negative A WonATech WBCS3000 automatic battery cycler was used
Electrode for the electrochemical experiments. Electrochemical imped-
ance spectroscopy (EIS) measurements were conducted at
2.2.1 Synthesis of NMS Hollow Cuboids frequencies ranging from ­10–2 to ­106 Hz with an amplitude
of 10 mV. The electrochemically active surface area (ECSA)
NMO cuboids were synthesized on cleaned bare NF, as was estimated by calculating the electric double layer capac-
described in the previous section. After rinsing and drying, itance (Cdl = d(∆I)/dν) in the non-Faradaic potential region.
the AER was conducted for 8 h. The obtained NMS on NF Cyclic stability tests were conducted at a constant charging/

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62 Page 4 of 21 Nano-Micro Lett. (2023) 15:62

discharging current density with frequent replacement of the was used for the exchange–correlation energies [27, 28].
electrolyte. The specific capacitance, Cs (F ­g−1), and specific The energy cut-off for the plane-wave basis set was set at
capacity, C (C ­g−1), of the electrodes were calculated from 400 eV, and the Brillouin-zone was sampled using DFT
the galvanostatic charge/discharge (GCD) curves using the Monkhorst–Pack 5 × 5 × 5, 2 × 2 × 1, and 4 × 4 × 1 k-point
following formulae: meshes for the bulk, surface, and charge analysis models,
respectively [29]. For the structural optimization, all atoms
2I ∫ Vdt
Cs = (1) were relaxed using a conjugate-gradient algorithm until
mΔV 2 the difference in the total force was < 0.03 Ev Å−1 with a
convergence criterion of ­10–4 eV for the total energy, while
2I ∫ Vdt
C= (2) spin polarization was also taken into considerations [30].
mΔV For all slab models, termination with a lower surface
where I(A) is the discharge current, ΔV(V) is the potential energy was selected. Equation (6) was used to calculate
range, and m(g) is the mass of the active materials. the surface energy ( γ):
Eslab − nEbulk
γ= (6)
2.5 NCS/NMS/CoO||NMS/FeO Device 2A
where Eslab is the total energy of the slab model, n is the
For the fabrication of the supercapattery device, NCS/NMS/ stoichiometry parameter, Ebulk is the total energy of the bulk
CoO and NMS/FeO electrodes were employed as positive model, and A is the surface area of the slab model. Addition-
and negative electrodes, respectively, with 2 M KOH/PVA ally, vacuum layer spacing of ~ 15 Å was employed.
gel electrolyte. For the constructed energy storage device, In this study, three hybrid electrode models were con-
the mass ratio between the two electrodes was calculated structed: 1) NCS/NMS 2) NCS/NMS/CoO/OH – and 3)
according to the following formula [24]: NMS/FeO/K+. For the NCS/NMS model, NCS(110) was
m+ C− ΔV − selected because its surface energy is lower than that of
= (3) NCS(100) and NCS(111), while NMS(110) was used
m− C+ ΔV +
because the hydrothermal method was used for NCS/NMS
where C− and C+ are the specific capacities, and m− and
synthesis, during which Co atoms were hydrothermally
m+ are the masses of the negative and positive electrodes,
replaced with Mo atoms. To ensure a suitable comparison
respectively. ΔV− and ΔV+ denote the voltage window of
the two electrodes, respectively. The energy density, E (Wh with NCS/NMS/CoO/OH–, NMS(110) was adopted for the
­kg−1), and power density, P (W ­kg−1), of the supercapattery NMS/FeO/K+ model. Moreover, eight layers for the NCS/
device were calculated as follows: NMS surface model were used while bottom four layers
were fixed, so were for NCS/NMS/CoO/OH– model. Simi-
I ∫ Vdt
E= (4) larly, eight layers were used with bottom four layers fixed
3.6
for NMS/FeO/K+.
Fo r NC S / N M S / C o O / O H – a n d N M S / F e O / K + ,
3600E
P= (5) ­Co3O4(110) and ­Fe2O3(0001) were selected as the most
Δt
stable facets for electrode purposes according to litera-
where Δt(s) is the discharge time and I(A ­g−1) is the dis- ture [31, 32]. The VESTA package was used for structural
charge current density.
visualization [33].

2.6 Computational Details

All density functional theory (DFT) calculations were

performed using the Vienna ab Initio Simulation Package
(VASP) [26]. The Perdew–Burke–Ernzerhof (PBE) func-
tional of the generalized gradient approximation (GGA)

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Nano-Micro Lett. (2023) 15:62 Page 5 of 21 62

3 Results and Discussion in the range of 165.00–170.00 eV originated from sur-

face oxidation by air. The high-resolution Mo 3d spec-
3.1 Physical Characterizations of NCS/NMS/CoO trum showed a single peak at 232.26 eV corresponding to
Positive Electrode Mo 3d5/2 and this peak was attributed to the + 3 oxidation
states of Mo atoms (Fig. 1e) [34]. The high-resolution Ni
Figure 1a shows the XRD spectra of the prepared positive 2p spectrum of NCS/NMS clearly showed the dominant
electrodes after the AER. The as-prepared NCS showed ­Ni2+ (855.35 eV) and ­Ni3+ (856.72 eV) states of mixed
distinct peaks of thiospinel ­NiCo2S4 stoichiometry (JCPDS NCS/NMS stoichiometry (Fig. 1f). Upon incorporation of
No. 02-0788) at 31.35°, 38.04°, 49.86°, and 55.41°, which ALD-CoO, NiO was formed beneath the CoO thin layer,
correspond to the (311), (400), (511), and (440) planes as evidenced by the dominant N ­ i2+ (855.54 eV) peak in
of the NCS crystal. Additional peaks of ­N i 3S 2 (JCPDS the NCS/NMS/CoO sample [21]. The positive peak shift
No. 44-1418) and NiS (JCPDS No. 75-0612) stoichiom- of 0.15 and 0.19 eV in the S 2p and S 2s spectra, respec-
etry were also observed. All samples showed the diffrac- tively, originated from the low electron density near the S
tion peaks of Ni foam (JCPDS: 04-0850) in addition to atoms after the incorporation of the CoO thin layer. The Ni
the peaks of the coated material. The NCS/NMS struc- 2p spectrum showed a relatively negative shift of 0.45 eV
ture showed a few more pronounced peaks of ­NiMo2S4 at owing to the high electron density around the Ni atoms
50.15° and 77.75° along with the inherent peaks of NCS in the NMS phase. The electronic equilibrium attained
[34]. This observation suggested that the NCS/NMS struc- by the sharing of the outer shell electrons between the
ture has separate crystals of NCS and NMS, and does not NMS phase and CoO thin layer further generated high
correspond to the Mo-doped stoichiometry of NCS. local electronic interactions, and this presented a freeway
The surface compositions of bare and ALD-CoO-coated for electron transfer. The diminished S 2p, Ni 2p, Mo 3d,
NCS/NMS electrodes were analyzed by XPS. The survey and S 2s peaks observed after ALD-CoO incorporation
spectra of all samples are shown in Fig. S1a, b. The high- indicated that the metal-sulfide structure was wrapped by
resolution Co 2p spectrum of NCS/NMS revealed C ­ o3+ the conformal CoO coating.
(781.61 eV) as the major chemical state corresponding Figure S2 shows that the as-prepared NCS electrodes
to the NCS phase with a small amount of elemental ­Co0 had a well-defined flower structure with multiple nano-
(778.37 eV) that represented the existence of oxygen needles connected to single root. Figure 2a shows that
vacancies (Fig. 1b) [35]. The incorporation of CoO thin the as-prepared NCS/NMS electrodes had cuboidal struc-
layers induced the ­Co2+ (779.97 eV) state and the overall tures with cuboidal walls composed of several NCS and
Co 2p peak intensities increased dramatically owing to NMS nanoparticles. Especially, a broken nano-cuboid in
the enrichment of the surface with Co atoms. However, Fig. 2b illustrates the hollow features of the formed NCS/
the major 3 + state revealed the ­C o 3O 4 spinel structure NMS cuboids. The hollow structures were formed by the
with ­Co2+ ions in tetrahedral interstices and C ­ o3+ ions in wet sulfurization process and have also been observed in
the octahedral interstices of the cubic close-packed lat- various metal-oxide to metal-sulfide transformations[38].
tice of oxide anions. In addition, the disappearance of the Hollow structures are well known for their large surface
elemental ­Co0 peak indicated the oxidation by exposure to area, electrolyte contact, and fast charge transfer kinet-
reactive ozone molecules in the ALD process. As shown in ics. The HR-TEM images in Fig. 2d–f show the hol-
Fig. 1c, both electrodes show two major peaks at 531.12 low nature of the NCS/NMS cuboids, with cuboid and
and 530.40 eV corresponds to oxygen vacancies and M‒ wall thicknesses of ~ 350 and ~ 50 nm, respectively. The
OH bond, respectively [36]. The Co‒O peak (529.51 eV) image of a wall portion at higher magnification is shown
emerged after the deposition of an ultrathin ALD layer. in Fig. 2g, h; it shows NCS crystals with d-spacings of
Figure 1d shows the deconvolution of S 2p peaks with dou- 0.28 and 0.54 nm that correspond to the (311) and (111)
blet peaks at 162.51 and 161.48 eV corresponding to the planes [39]. In addition, NMS crystals with 0.17 nm lat-
­S22− (disulfide) and S
­ 2− (sulfide) ligands, respectively [37]. tice d-spacing were observed [40]. The selected area elec-
The M‒SOx (metal sulfate/metal sulfite) peaks observed tron diffraction (SAED) pattern observed for the cuboidal
wall reveals the polycrystalline nature of NCS through

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Fig. 1  XRD patterns of a NCS/NMS (yellow) and NCS (blue) positive electrodes. Comparison of high-resolution XPS profiles, b Co 2p, c O
1s, d S 2p, e Mo 3d, and f Ni 2p of NCS/NMS, and NCS/NMS/CoO electrodes

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Nano-Micro Lett. (2023) 15:62 Page 7 of 21 62

Fig. 2  a–c HR-SEM images of bare NCS/NMS electrode. HR-TEM images of d–i bare NCS/NMS electrode with diffraction plane analysis and
SAED pattern and j NCS/NMS/CoO (200 cycles), and k NCS/NMS/CoO (300 cycles)

the formation of concentric circles with bright spots. 3.2 Physical Characterizations of NMS/FeO Negative
The major peaks observed by XRD analysis, including Electrode
the (311), (400), and (511) planes, were indexed in the
SAED pattern. Figure 2j shows the conformal coating of As illustrated in Fig. S4, the XRD spectrum of the NMO
the CoO thin layer over the cuboid with a uniform thick- negative electrode showed low-intensity peaks of the NMO
ness of ~ 3.25 nm (200 ALD cycles). An image of the phase (JCPDS No. 16-0291) with additional peaks of M ­ oO3
CoO thin layer at a higher magnification is presented in (JCPDS No. 01-0706) owing to a lack of crystallinity. The
Fig. 2k, showing C­ o3O4 crystals with d-spacings of 0.24 XRD spectrum of the NMS electrode showed clear peaks
and 0.28 nm corresponding to the (311) and (220) planes, of the NMS phase (JCPDS No. 21-1273) at 29.64°, 50.45°,
respectively [41]. Furthermore, the scanning transmis- and 78.06°. Ni foam exposed to the N ­ a2S solution during
sion electron microscopy energy-dispersive spectro- AER and led to the formation of ­Ni3S2 phase. The surface
scopic (STEM-EDS) images in Fig. S3 reveal the homo- composition of the NMS negative electrode was analyzed
geneous distribution of Co, Ni, Mo, S and O elements. by XPS, and the results are shown in Fig. S5. The high-
These results confirm the formation of the hollow NCS/ resolution Ni 2p spectrum of NMS clearly shows the domi-
NMS/CoO core–shell structure with excellent interface nant ­Ni2+ (855.48 eV) state and ­Ni3+ (856.94 eV) state of
properties. NMS stoichiometry (Fig. S5a) [42]. The high-resolution Mo

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3d spectrum shows a single peak at 232.26 eV that corre- as shown in Fig. 3e, f. At high magnification, clear differ-
sponds to Mo 3d5/2 and can be attributed to the + 3 oxida- ences could be seen between Figs. 2c and 3d. While the
tion states of Mo atoms (Fig. S5b). The emergence of S 2s NCS/NMS cuboids mainly composed of nanoparticles, the
peak at 226.66 eV confirmed the successful conversion of NMS electrodes were made up of nanosheets. This dif-
NMO to NMS. Figure S5c shows the deconvolution of S ference is because of the formation of a unique S‒Mo‒S
2p peaks into doublet peaks at 162.79 and 161.77 eV cor- layered structure linked through van der Waals forces [43].
responding to the ­S22− (disulfide) and S
­ 2− ligands, respec- The NMS crystal lattice d-spacing was 0.17 nm (Fig. 3g).
tively [37]. As shown in Fig. S5d, the oxygen vacancy peak The well-ordered bright spots arranged in a straight line
at 531.22 eV was the major peak representing one of the observed in the SAED pattern corresponded to the [101]
surface-active sites of the NMS negative electrode. The Fe facet and indicated that the NMS was highly crystalline
2p spectrum (Fig. S5e) exhibits two significant peaks cen- (Fig. 3h). The perfect circles were indexed to the (100)
tered at 712.15 and 725.65 eV, which correspond to 2p3/2 and and (110) planes of the NMS crystals. Collectively, these
2p1/2 spin orbitals, respectively. results show that the NMS cuboids had both polycrys-
The HR-SEM images of the bare NMS negative elec- talline and single crystalline properties. The STEM-EDS
trodes are shown in Fig. 3a–d. From Identical hollow images in Fig. 3i demonstrate the homogeneous distribu-
cuboid structures were formed after the wet sulfurization tion of Mo, Ni, O, and S elements.
process with a thick wall composed of NMS nanosheets

Fig. 3  a–d HR-SEM images of the bare NMS electrode. e–h HR-TEM images of the bare NMS electrode with diffraction plane analysis and
SAED pattern. i Elemental mapping of the NMS electrode

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Nano-Micro Lett. (2023) 15:62 Page 9 of 21 62

3.3 Electrochemical Performance of the NCS/NMS/ increased the likelihood of intercalation, thus improving the
CoO Positive Electrode pseudocapacitive behavior. Figure 4a shows the comparative
CV curves of NCS, NCS/NMS, and NCS/NMS/CoO elec-
Figure S6 shows the CV plots of the NCS, NCS/NMS and trodes between 0 and 0.5 V at 5 mV ­s−1. All potentials are
NCS/NMS/CoO electrodes at various scan rates in 2 M mentioned relative to the SCE unless specified otherwise.
KOH electrolyte. O ­ H– is the anion with the highest ionic The NCS electrode showed oxidation and reduction peaks at
­ + is
conductivity and the highest mobility in water, while K 0.310 and 0.164 V, respectively, corresponding to the inher-
the cation with the second-highest ionic conductivity after ent pseudocapacitive behavior of ­NiCo2S4 [44]. After the
­H3O+. Furthermore, the smaller size of the ­OH– anions formation of the NCS/NMS structure, the oxidation peaks

Fig. 4  a CV curves of NCS, NCS/NMS, and NCS/NMS/CoO electrodes at 5 mV ­s−1 scan rate. b Estimation of Cdl. c GCD curves of NCS,
NCS/NMS, and NCS/NMS/CoO electrodes at 6 A/g current density. d Schematic of improved wettability by CoO ALD layer. e Comparison
of the specific capacities of NCS, NCS/NMS, and NCS/NMS/CoO electrodes. f Plot of logarithmic scan rate versus logarithmic peak current.
g Calculated contribution of diffusion-controlled charge storage mechanism for NCS/NMS and NCS/NMS/CoO electrodes. h Nyquist plot at
0.25 V and i plot of capacity retention

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shifted positively (0.177 V) and the reduction peak shifted NMS/CoO (10 mg ­cm−2), NCS/NMS (10 mg/cm2), and NCS
negatively (− 0.176 V) owing to the incorporation of the (3 mg ­cm−2) electrodes based on the GCD curves at current
additional ­Mo4+/Mo6+ redox couple in the electrode. The densities ranging from 3 to 18 A ­g−1. When calculating the
electrochemical redox process involves the following reac- specific capacity/capacitance of the active material, the mass
tions [45]: of the current collector was not considered because the NF
was completely covered by the active material. This means
M - S + OH− ↔ M − SOH + ne−
( ) that the contribution of the NF to the charge storage was
M = Ni2+∕3+ , Co2+∕3+ , and Mo4+∕6+ , negligible. The obtained specific capacity was thus exclu-
sively derived from the active material–electrolyte interface.
( )
Co(OH)2 + OH− ↔ CoOOH + H2 O + e− Co2+ and Co3+ , and Specific capacities of 2794.5, 2530.2, and 831.8 C ­g−1 were
( )
CoOOH + OH− ↔ CoO2 + H2 O + e− Co3+ and Co4+ achieved for NCS/NMS/CoO, NCS/NMS, and NCS elec-
trodes at 3.0 A ­g−1, respectively. In terms of specific capaci-
tance, the NCS/NMS/CoO electrode achieved 6210.0 F ­g−1
The opposite shift in redox peaks compared to the bare at 3.0 A g­ −1. These values are a new benchmark for all types
NCS/NMS electrode indicated the additional contribution of transition metal-based positive electrodes for charge stor-
of ­Co2+/Co3+ ­Co3+/Co4+ redox couples in the pseudocapaci- age application (Table S1). The discharge time gradually
tive reaction. The clear difference in the capacitive areas decreased with increasing current density because of inad-
of the curves of NCS/NMS and NCS electrode illustrated equate time for electrolyte ion diffusion, ultimately resulting
the synergic effect of mixed metal sulfides (M‒S) pre- in a lower specific capacity (Fig. S9). However, the incorpo-
sent in the NCS/NMS electrode and the large surface area ration of CoO significantly improved the rate capability of
obtained by the hollow structure. The Cdl values of each NCS/NMS might originate from the improved pseudocapac-
electrode were obtained using the non-faradaic CV curves itive contribution to the overall specific capacity at the high
shown in Fig. S7. The Cdl of the NCS/NMS electrode was current rate. To verify this, we have calculated the induvial
measured to be 54.9 mF ­cm−2, which is approximately 4.5 contribution of EDLC and pseudocapacitive mechanism to
times higher than that of the NCS electrode (Fig. 4b). The the overall specific capacity with and without the CoO shell
capacitive areas of the NCS/NMS electrode curves were fur- layer. By assuming the semi-infinite linear diffusion, the
ther extended with the incorporation of thin CoO films due capacitive contribution (CEDLC) can be calculated using the
to the ~ 3.75-fold increase in ECSA. The contact angle (θ) Trasatti method (Fig. S10) [48, 49]. The calculated diffusion
measurement shown in Fig. S8, shows significant reduction contribution (CD) to the total specific capacity (CT) before
in contact angel before and after the CoO deposition. The and after the incorporation of CoO ALD layer was plot-
CoO incorporation improved the wettability of NCS/NMS ted against the scan rate (Fig. 4g). At a slow scan rate, the
electrodes by exposing the electrolyte inaccessible areas to diffusion-controlled ion intercalation is dominant because
the electrolyte as shown in Fig. 4d. Owing to the ultra-high sufficient time is available to execute the slow ion migration
vacuum (UHV) processing conditions, the metal precursors and intercalation. The EDLC clearly becomes dominant at
could reach the trenches of the electrode surface that could higher scan rates where only ion adsorption on the surface
not be physically reached by the electrolyte which increases can happen with a large current rate. The CoO thin layer
the hydrophilicity of the electrode surface and results in an significantly increases the diffusion process at all scan rates
exponential increase in electrode–electrolyte interactions compared to the bare NCS/NMS electrode. To verify the
[46]. obtained values, the power-law equation was employed:
The GCD curves were obtained in the 0–0.45 V range to
avoid polarization in aqueous electrolyte. The GCD curves i = avb (7)
in Fig. 4c for all electrodes exhibit a nonlinear plateau struc-
ture, indicating pseudocapacitive electrochemical behavior logi = loga + blogv (8)
[47]. The NCS/NMS/CoO electrode had the longest dis- where i is the redox peak current (mA), v is the scan rate
charge time, which indicated its superior energy storage (mV/s), and a and b are constants. The value of slope
capability. Figure 4e shows the specific capacity of the NCS/ (b) has three well defined regions: 0.5 ≥ b refers to the

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Nano-Micro Lett. (2023) 15:62 Page 11 of 21 62

diffusion-controlled region, 1 > b ≥ 0.5 refers to the transi- at 5 mV ­s−1 scan rate. The shape of the CV curves repre-
tion region, and b ≥ 1 refers to the capacitive region [50, 51]. sents the surface redox-mediated pseudocapacitive charge
The plot of log v versus log i (reduction peak from Fig. S6c) storage mechanism with a few layers of the active mate-
is given in Fig. 4f. The slope was determined as b = 0.359, rial at the electrode‒electrolyte interface participating in
which confirmed the dominance of the diffusion-controlled
the redox reaction [47]. The NCS phase was intentionally
pseudocapacitive mechanism throughout the potential win-
removed (from negative electrode) to obtain a nanosheet
dow. This indicates significant reversibility and high ion dif-
fusion rate during a rapid high current rate electrochemical like morphology to boost the pseudocapacitive mechanism
redox reaction. in the negative potential window. The unique structures of
The Nyquist plots for the NCS, NCS/NMS, and NCS/ S‒Mo‒S nanosheets can provide sufficient space between
NMS/CoO electrodes are shown in Fig. 4h. The equivalent the sheets for K­ + ion intercalation/de-intercalation [52].
circuit used for impedance data fitting is shown in Fig. S11a The NMS electrode shows two strong symmetrical redox
and the fitted results are listed in Table S2. The low Rs value peaks at − 0.788 and − 0.408 V corresponding to the M ­ o4+/
6+ 2+ 3+
of the NCS/NMS/CoO electrode compared to the pristine Mo and ­Ni /Ni redox couples present in the electrode
electrode due to electronic interaction achieved between [53]. The peak current and peak intensity of the NMS elec-
NCS/NMS‒CoO interfaces. The plots show that the NCS/ trode increased with the increasing scan rate and without
NMS/CoO electrode had the lowest RCT value compared any deformation of shape resulted superior rate capabil-
to all other electrodes, indicating that the ultra-thin CoO ity. Specifically, the boosted electrochemical performance
greatly improved charge tunneling at the interface. The of the NMS/FeO heterostructure could be attributed to the
cyclic stability of bare and CoO-coated NCS/NMS elec- additional Fe–O active sites (­ Fe2+/Fe3+ and F
­ e3+/Fe4+ redox
trode was tested at 10.8 A ­g−1 for 20,000 charge/discharge couples) and typical pseudo-capacitance behavior of ALD-
cycles in 2 M KOH electrolyte. Only 5.7% capacity loss was Fe2O3 by the following redox reaction [16]:
observed for the NCS/NMS/CoO electrode, whereas NCS/
Fe2 O3 + 2e− + 3H2 O ↔ Fe(OH)2 + 2OH− Fe2+ and Fe3+
( )
NMS lost 16.5% of its initial specific capacity, as shown
in Fig. 4i. This suggests that the CoO layer preserved the
structural integrity of the NCS/NMS electrode throughout The comparative integrated area of NMS compared to
the charging/discharging process. The insignificant change NMO shows the superior charge storage capability of NMS,
in morphology after 20,000 cycles demonstrates the excel- due to the increased surface area of NMS hollow cuboids
lent durability of the NCS/NMS/CoO nanostructures and and the pseudocapacitive nature of charge storage. The
their resistance to volume expansion or agglomeration phe- measured Cdl of NMS (52.4 mF ­cm−2) was approximately
nomena during the charge/discharge processes (Fig. S12a, one order larger than that of NMO (5.2 mF c­ m−2), reveal-
b). The disappearance of the M‒O peak at 529.56 eV and ing that the AER induced larger microroughness (Fig. 5b).
advent of the M‒OH peak at 530.16 eV indicated the con- The advantages of the thin FeO layer are identical to CoO
version metal oxide to corrosion resistive metal hydroxides on NCS/NMS positive electrodes. The estimated pseudo-
and (oxy)hydroxides (Fig. S12c) [21]. The S 2p spectrum capacitive contribution was 88.7% and 43.28% at 2 and
showed slightly weaker M‒SOx peaks, indicating the forma- 100 mV ­s−1, respectively (Figs. 5c and S10). The plot of
tion of M‒OOH phase (Fig. S12d). log v versus log i (peak 1 and peak 2 from Fig. 5a) is given
in Fig. 5d. The slope was determined for peak 1 and peak 2
as b1 = 0.593 and b2 = 0.580, respectively. Both values are
3.4 Electrochemical Performance of the NMS/FeO at the beginning of the transition mode, which confirms the
Negative Electrode dominance of the pseudocapacitive mechanism throughout
the potential window [47].
The CV curves of NMO, NMS, and NMS/FeO negative Figure 5e depicts the GCD curves of the negative elec-
electrodes between − 1.1 and 0.0 V at various scan rates trodes at 3 A g­ −1 in the potential range from ‒1.1 to 0.0 V.
are given in Fig. S6e–h. Figure 5a shows the compara- Notably, the triangle shape of the NMO GCD curve showed
tive CV curves of NMO, NMS, and NMS/FeO electrodes pure EDLC behavior because of the lack of surface-active
sites. The analogous time of charging and discharging with

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62 Page 12 of 21 Nano-Micro Lett. (2023) 15:62

Fig. 5  a CV curves of NMO, NMS, and NMS/FeO electrodes at 5 mV ­s−1 scan rate. b Estimation of Cdl. c Calculated contribution of diffusion-
controlled and EDLC charge storage mechanism for the NMS and NMS/FeO electrodes. d Plot of logarithmic scan rate versus logarithmic peak
current. e GCD curves at 3 A ­g−1 current density. f GCD curve of NMS/FeO electrode at various current density. g Comparison of the specific
capacities of NMO, NMS, and NMS/FeO electrodes. h Nyquist plot at ‒0.50 V and i Capacity retention

a symmetric profile implied high columbic efficiency of the negative electrode materials reported for supercapacitors
NMS and NMS/FeO electrodes. The GCD plot of NMS/FeO (Table S3).
obtained at various current densities is shown in Fig. 5f. Fig- Figure 5h shows the Nyquist plot at an applied poten-
ure 5g shows the specific capacity of the NMO (5 mg ­cm−2), tial of ‒ 0.5 V, which is fitted with the equivalent circuit
NMS (5 mg ­cm−2), and NMS/FeO (5 mg ­cm−2) electrodes shown in Fig. S11a. The fitted results are given in Table S2.
based on the GCD curves at current densities ranging from The smaller series resistance (Rs) and RCT of the NMS/FeO
2 to 20 A ­g−1. The NMS/FeO electrode achieved a specific electrode as compared to those of other negative electrodes
capacity of 2,114 C g­ −1 at 2 A g­ −1, whereas the NMS and reveal the improvement in overall conductivity and inter-
NMO electrodes reached specific capacities of 1,720 and face kinetics, respectively. The ­K+ ion diffusivity was cal-
189 C g­ −1, respectively. The obtained values are a new culated using the Warburg region by plotting the inverse
benchmark compared to both carbon- and inorganic-based square root of angular frequency (ω−1/2) and real impedance

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Nano-Micro Lett. (2023) 15:62 Page 13 of 21 62

(Z´) as shown in Fig. S13. The ionic coefficients of the ALD layer thickness is vital in achieving the maximum
NMS and NMS/FeO electrodes were thus determined to be NA. The thickness of the ALD layer can be precisely con-
0.395 × ­10–8 and 1.259 × ­10–8 ­cm2 ­s−1, respectively [54, 55]. trolled by the number of ALD cycles [19]. The NCS/NMS
It shows that the FeO shell layer provides more diffusion and NMS electrodes were thus coated with CoO and FeO
path for ­K+ ions. layers of different thicknesses, respectively, to optimize the
The long-term cycling stability of the electrodes was interface kinetics (Table S4). The specific capacities of the
evaluated after 20,000 GCD cycles at a current density of NCS/NMS/CoO electrode with 100, 200, and 300 were then
10 A ­g−1. The NMS/FeO electrode displayed 93.8% of its measured (Fig. S16a). At a low current rate, the specific
original specific capacity after 20,000 cycles. The bare NMS capacity was independent of the number of ALD cycles but
electrode retained 83.2% of the specific capacity after 10,000 gradually became dependent as the current rate increased.
cycles (Fig. 5i). The thin layer of FeO minimized the dete- The lower rate capability associated with 100 and 300 cycles
rioration of NMS in the alkaline electrolyte. The observed indicates the presence of a low number of Co–O active sites
insignificant change in morphology indicates the excellent in the first row and long/masked ion diffusion paths to the
durability of the NMS/FeO nanostructures and their resist- M‒S active sites in the second row, respectively. Similar
ance to volume expansion or agglomeration phenomena results were obtained for different ALD cycles with the NMS
during the charge/discharge processes (Fig. S14). The S 2p negative electrode (Fig. S16b). Therefore, the participation
spectrum showed slightly weak 2p3/2 and 2p1/2 peaks and of M‒S active sites is key to maintaining pseudocapacitive
high intensity M‒SOx peaks, indicating the formation of behavior at a high current rate, and the solid–solid interface
M‒OOH phase like NCS/NMS/CoO electrodes (Fig. S2c). (i.e., NCS/NMS-CoO and NMS-FeO) plays a crucial role in
The appearance of the M-OH peak at 530.52 eV indicated enhancing this behavior.
the conversion of M‒S to M‒OH, Fig. S2d. However, the Theoretical calculations for the solid–solid junctions of
significant S 2s and S 2p peaks indicate that Ni‒S and S‒ the positive and negative electrodes were made to define
Mo‒S active sites are available in the electrolyte permeable the origin of the high-rate capability. The more stable and
region near the surface after 20,000 charge/discharge cycles. exposed surfaces of NCS(110), NMS(110), ­Co3O4(110) and
­Fe2O3(0001) were used for these calculations (Figs. S17 and
S18). More details on surface and junction optimization are
3.5 Analysis of Enhanced Pseudo Capacitance Under presented in Figs. S19 and S20. The calculated density of
a High Current Rate states (DOS) for NCS/NMS, NCS/NMS/CoO, NMS, and
NMS/FeO is shown in Figs. S21 and S22. There was a
Both the intercalation and the surface redox pseudoca- greater number of electrons in the Fermi level (Ef) which
pacitance mechanisms depend on the number of surface- reflected the metallic nature of the NMS and NCS/NMS
active sites (NA) at the electrode–electrolyte interface. NA models. Furthermore, the CoO and FeO layers increased the
was calculated using the reduction peak of each electrode DOS of the Ef, indicating more favorable electronic prop-
(Fig. S15). Incorporation of the CoO and FeO layers sig- erties, e.g., conductivity and electron mobility. Metallic
nificantly improved NA (Fig. 6a). The optimized CoO and NCS/NMS and CoO form a classical Schottky junction on
FeO layers were very thin and the aqueous electrolyte was contact because ­Co3O4 is a well-known p-type semiconduc-
able to permeate the transition metal oxides layers, so redox tor. This is a bipolar Schottky junction because the major
reactions occurred at the NCS/NMS-CoO and NMS-FeO charge carriers on both sides are opposites (i.e., electrons
interfaces and more deeply within the material. Therefore, and holes). Electronic equilibrium is achieved by transfer-
NCS/NMS/CoO exhibited M–O (redox form CoOOH) as ring electrons from the NCS/NMS to the valance band of
the first row of active sites and M‒S (redox form M‒SOH) CoO; consequently, a hole depletion layer (i.e., an electron
as the second row. While NMS/FeO also had two rows of accumulation layer) is formed on the CoO side (Fig. 6e)
active sites. Increasing the thickness of the ALD layer can [56]. Similarly, metallic NMS and n-type FeO form a uni-
increase the number of M–O active sites in the first row, polar Schottky junction with electrons as the major charge
but this blocks the diffusion pathways to the M‒S active carriers on both sides (Fig. 6f). At equilibrium, electrons
sites in the second row. Therefore, the optimization of the will flow to the NMS from the FeO conduction band and

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62 Page 14 of 21 Nano-Micro Lett. (2023) 15:62

Fig. 6  a Calculated number of active sites (NA) before and after the incorporation of the ALD layer. b, c Partial density of states for the S 2p
orbital with the NCS/NMS, NCS/NMS/CoO, NMS, and NMS/FeO models. d Comparison of the S p-band center. Band alignment of a e bipolar
NCS/NMS||CoO, and f unipolar NMS||FeO Schottky junction under equilibrium Charge density difference map for a g NCS/NMS||CoO, and h
NMS||FeO Schottky junction at equilibrium

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Nano-Micro Lett. (2023) 15:62 Page 15 of 21 62

form a hole accumulation layer (i.e., an electron depletion and FeO gaining 0.17 e due to the formation of the Schottky
layer) on the FeO side [57]. To verify this, the S p-band junction. The charge distribution at the Schottky junction was
center for NMS and NCS/NMS was calculated before and more significant than at the electrode–electrolyte interface,
after the addition of FeO and CoO layers. For the NCS/ meaning that the Schottky junction was responsible for the
NMS/CoO model, the S p-band center shifted to the right, net current flow across the electrode–electrolyte interface.
indicating electron-donor-like behavior (Fig. 6b). For the Schottky barrier height (ɸSBH) and the voltage barrier (VBi)
NMS/FeO model, S p-band center shifted to the left, indicat- are the two important parameters that control the flow of
ing electron- acceptor-like behavior (Fig. 6c). Therefore, the electrons through the Schottky junction. ɸSBH is a constant
electrical field created in the Schottky junction acts in the energy barrier that limits the further flow of electrons through
opposite direction for the positive and negative electrodes, the Schottky junction once equilibrium is reached. However,
as illustrated by the up and down shift of the S p-band center external alteration of VBi can cause electrons to flow in one
shown in Fig. 6d. This electric field can act as a driving force direction depending on the type of Schottky junction [59].
for the ion diffusion of oppositely charged ions (­ K+/OH–) Under forward bias (i.e., charging), the NCS/NMS-elec-
during the charging and discharging processes [58]. trolyte interface acts as a normal electrical double layer
Bader charge analysis was conducted to quantify the (EDL; Fig. 7a), where the current flow through the interface
charge distribution in the Schottky junction. Two heterojunc- depends only on RCT (which is inversely proportional to the
tions were modeled with ­K+ and ­OH– ions at the edges to applied bias) [60]. At a high current rate, there is no driving
visualize the charge distribution under working conditions for force for the diffusion of ­OH– ions through the bulk, so most
NCS/NMS/CoO/OH– (Fig. 6g) and NMS/FeO/K+ (Fig. 6h). of the charge is stored via EDLC mechanism (Fig. 7c). Simi-
The electron accumulation and depletion areas are shaded larly, under reverse bias (i.e., discharging), OH- is rapidly
in cyan and magenta, respectively, with CoO losing 0.79 e repelled from the EDL by the electrons accumulating on the

Fig. 7  Band alignment for the NCS/NMS-electrolyte interface under a charging and b discharging c Schematic of the charge–discharge curves
with ohmic and Schottky behavior. Band alignment for the NCS/NMS||CoO junction under d charging and e discharging f CV curves for bare
and CoO deposited NCS/NMS positive electrodes at a scan ate of 100 mV s.−1

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62 Page 16 of 21 Nano-Micro Lett. (2023) 15:62

metal side (Fig. 7b). Thus, the likelihood of pseudocapaci- and pseudocapacitive NMS/FeO or activated carbon (AC)
tive mechanism occurring is drastically reduced. as the negative electrode. AC is known for its superior
For the NCS/NMS/CoO electrode, ­VBi and the width EDLC behavior. The active mass of AC negative electrode
of the depletion layer (D) become negligible while charg- was adjusted to be equal to that of the NMS/FeO negative
ing and the ­OH– ions are attracted towards the junction electrode. The 2 M KOH-PVA gel electrolyte was used with
by the holes on the CoO side (Fig. 7d). Therefore, more stainless sheets as current collectors at both ends. PVA was
­OH– ions reach the M‒S active sites. While discharging, selected as the polymer due to its excellent chemical sta-
­VBi and D become larger, limiting the release of ­OH– from bility, water solubility, non-toxicity, and biodegradability.
the bulk NCS/NMS side (Fig. 7e). Due to the slow release PVA is a linear polymer that contains multiple OH- groups
of ­OH– ions from Schottky junction, the discharge time will that absorb a large number of water molecules, thus improv-
be longer than with bare NCS/NMS. Unlike a normal EDL, ing the ionic conductivity of a solid electrolyte [61]. The
­ H– ions during the
the Schottky junction attracts and traps O comparison of CV curves of NCS/NMS/CoO||NMS/FeO
charging and discharging processes, respectively. In theory, and NCS/NMS/CoO||AC devices at 100 mV ­s−1 is shown
this process is not affected by the current rate, but Schottky in Fig. 8b. The smaller EDLC contribution of the AC nega-
junction increases the likelihood of ­OH– diffusion to the tive electrode limited the overall charge storage of the NCS/
M‒S active sites at all current rates, which helps to maintain NMS/CoO||AC device. The CV curves of the NCS/NMS/
the pseudocapacitive behavior at high current rates. The CV CoO||NMS/FeO supercapattery device obtained for the
curves for the NCS/NMS and NCS/NMS/CoO electrodes scan rate range of 2–100 mV ­s−1 is shown in Fig. 8c. It was
at a scan rate of 100 mV ­s−1 are compared in Fig. 7f. The evident that the full operating window of the NCS/NMS/
NCS/NMS/CoO electrode demonstrated negligible deforma- CoO and NMS/FeO electrodes could be used for the solid-
tion compared to the NCS/NMS electrode. The redox peaks state device (1.6 V). The quasi-rectangular shape of the CV
visible after the addition of the CoO layer indicate that the curves even at 100 mV ­s−1 scan rate indicates the sustained
Schottky junction improves the pseudocapacitive behavior pseudocapacitive charge storage mechanism at high current
at a high current rate. The impact of the unipolar Schottky rate operation. The calculated diffusion contribution (CD) to
junction on the NMS/FeO negative electrode is like that of the total specific capacity (CT) of the supercapattery devices
the NCS/NMS/CoO positive electrode. The band alignment with NMS/FeO and AC negative electrodes is plotted against
for NMS/FeO with respect to the charge and discharge con- the scan rate in Fig. 8d. The NMS/FeO negative electrode
ditions is displayed in Fig. S23. The opposite direction of significantly increased the diffusion process at all scan rates
the electric field formed in NMS||FeO junction attracts and compared to the bare AC negative electrode and maintained
traps ­K+ ions during the charging and discharging processes, the battery behavior at ~ 12.86% even at 100 mV ­s−1.
respectively. Figure 8e shows the GCD curves of the NCS/NMS/
CoO||NMS/FeO device at current densities in the range of
3–20 A ­g−1. The nonlinear profile of the charging and dis-
3.6 Balanced Pseudo‑capacitance of NCS/NMS/ charging curves denotes the pseudocapacitive charge storage
CoO||NMS/FeO Supercapattery Device mechanism. The IR drop in the GCD curve was negligible,
suggesting excellent material conductivity and low inter-
As shown in Fig. 8a, the maximum specific capacity of a face resistance in the fabricated device. The NCS/NMS/
practical device can be limited by an unbalanced contribu- CoO||NMS/FeO supercapattery device achieved the highest
tion of the positive and negative electrodes. Incorporating a specific capacity value of 1062.62 C g­ −1 at 2 A g­ −1 as com-
positive pseudocapacitive and a negative EDLC electrode or pared to other devices described in the literature. In contrast,
vice versa in a practical device can compromise its overall the NCS/NMS/CoO||AC device achieved a specific capac-
specific capacity. Therefore, both the positive and negative ity of only 480.12 C ­g−1 at 2 A ­g−1 (Fig. 8f). Such a high
electrode must have the same charge storage mechanism and throughput electrochemical performance can be attributed to
a similar specific capacity range to ensure a balanced con- the balanced contribution of positive and negative electrodes
tribution [14]. Supercapattery devices were fabricated using to support the pseudocapacitive charge storage mechanism.
pseudocapacitive NCS/NMS/CoO as a positive electrode

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Nano-Micro Lett. (2023) 15:62 Page 17 of 21 62

Fig. 8  a Schematic of super capacity variation of the all solid-sate device depends on EDLC and pseudocapacitive negative electrode. b
Comparison of CV curves of NCS/NMS/CoO||NMS/FeO and NCS/NMS/CoO||AC devices at 100 mV ­s−1. c CV curves of the NCS/NMS/
CoO||NMS/FeO device at various scan rates. d Calculated contribution of the diffusion-controlled charge storage mechanism. e GCD curves of
the NCS/NMS/CoO||NMS/FeO device. f Comparison of specific capacity. g Capacity retention, h Ragone plot with the comparison of recent
reports tabulated in Table S4

The cycling stabilities of both supercapattery devices with power densities of 921.94 and 2417.94 W ­k g −1 ,
were estimated by GCD curves obtained at 10 A/g current respectively. These values fall in the supercapattery
density for 20,000 cycles. As shown in Fig. 8g, the NCS/ zone of the Ragone plot and exceed those of all super-
NMS/CoO||AC and NCS/NMS/CoO||NMS/FeO devices capacitors reported in the literature. For comparison,
retained 83.2% and ~ 90.5% of their original specific see Table S5. The EIS analysis results of the fabricated
capacities, respectively. Figure 8h represents a Ragone supercapattery devices are shown in Fig. S24, which was
plot of gravimetric energy and power density calculated fitted with the equivalent circuit in Fig. S11b. The fitted
using the total mass of the active material. The NCS/ results are shown in Table S6. The observed ­RCT for NCS/
NMS/CoO||NMS/FeO and NCS/NMS/CoO||AC devices NMS/CoO||NMS/FeO and NCS/NMS/CoO||AC devices
obtained energy densities of 236.14 and 140.82 Wh ­kg−1 was 7.31 and 17.54 Ω, respectively. The higher Cdl value

13
62 Page 18 of 21 Nano-Micro Lett. (2023) 15:62

without any Warburg component (W) at the low fre- and conversion processes and is suitable for large-scale
quency region of the NCS/NMS/CoO||AC device shows operation.
the dominance of EDLC behavior and poor diffusion of
electrolyte ions in bulk AC [62]. The Warburg component
5 Supporting Information
of the NCS/NMS/CoO||NMS/FeO device suggests strong
electrolyte ion diffusion in the electrode bulk structure.
Additional XRD data, XPS analysis, J–V curves, HR-SEM
The self-discharge of NCS/NMS/CoO||NMS/FeO device
images, and electrochemical analysis results.
is shown in Fig. S25. The device retained 43.75% of its
initial cell voltage up to 48 h at 25 °C. Two NCS/NMS/ Acknowledgements We are grateful to Prof. Hong H. Lee and
CoO||NMS/FeO devices were connected in series with the Prof. Hee Moon for their guidance. This study was financially
total output voltage of 3.2 V to demonstrate their practical supported by the National Research Foundation of Korea (NRF-
2022R1A2C2010803). We would also like to thank the researchers
application prospects. Initially, the two devices connected at the Gwangju Center of the Korea Basic Science Institute (KBSI)
in series were charged for 20 s and connected to different for their assistance with SEM and TEM analyses.
colored light emitting diodes such as red (1.6 V), yel-
low (1.8 V), blue (2.5–3.0 V), and white (3.0 V) during Funding Open access funding provided by Shanghai Jiao Tong
University.
discharge (Fig. S26). The intensity of light emitted at the
initial stage and the slow long-term fading indicated the Open Access This article is licensed under a Creative Commons
high energy density of the fabricated device. Additionally, Attribution 4.0 International License, which permits use, sharing,
a small motor (3 V, 750 mA) was operated by the charged adaptation, distribution and reproduction in any medium or format,
as long as you give appropriate credit to the original author(s) and
device, showing the excellent energy output of the NCS/
the source, provide a link to the Creative Commons licence, and
NMS/CoO||NMS/FeO device. indicate if changes were made. The images or other third party
material in this article are included in the article’s Creative Com-
mons licence, unless indicated otherwise in a credit line to the
material. If material is not included in the article’s Creative Com-
4 Conclusion mons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain
permission directly from the copyright holder. To view a copy of
In summary, we show that the poor pseudocapacitive
this licence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.
behavior of supercapacitor electrodes can be strategi-
cally alleviated by incorporating a Schottky junction
next to the electrode–electrolyte interface through atomic Supplementary Information The online version contains
supplementary material available at https://​doi.​org/​10.​1007/​
layer deposition. The Schottky junction attracts and traps s40820-​023-​01016-6.
the intercalation ions (­ OH – or K
­ +) during the charging
and discharging processes, respectively, to improve the
pseudocapacitive behavior at higher current rate. With References
improved surface wettability and Schottky junction-
controlled ion diffusion, positive and negative electrodes 1. A. Noori, M.F. El-Kady, M.S. Rahmanifar, R.B. Kaner, M.F.
Mousavi, Towards establishing standard performance metrics
recorded new benchmark specific capacities. By balanc-
for batteries, supercapacitors and beyond. Chem. Soc. Rev.
ing the specific capacities of both electrodes, an energy 48(5), 1272–1341 (2019). https://​doi.​org/​10.​1039/​C8CS0​
density of 236.14 Wh k­ g −1 was achieved with a total 0581H
active mass of 15 mg ­c m −2 in a solid-state device with 2. Global electric vehicle battery industry. ReportLinker,
5798459 (2022). https://​www.​repor ​tlink​er.​com/​p0579​8459/​
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