Dalton Transactions

(1)

Dalton

Transactions

An international journal of inorganic chemistry

rsc.li/dalton

ISSN 0306-0012

COMMUNICATION Douglas W. Stephen et al.

N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl, and tellurenyl cations (XH⁺, X = S, Se, Te)

Volume 46 Number 10 14 March 2017 Pages 3073-3412

Dalton

Transactions

An international journal of inorganic chemistry

This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication.

Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available.

You can find more information about Accepted Manuscripts in the Information for Authors.

Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Accepted Manuscript

This article can be cited before page numbers have been issued, to do this please use: T. V. Nguyen, T. S.

Le, V. V. Thuy, V. D. Thao, M. Hatsukano, K. Higashimine, S. Maenosono, S. Chun and T. V. Thu, Dalton Trans., 2020, DOI: 10.1039/D0DT01177K.

(2)

Facile synthesis of Mn-doped NiCo

2

O

4

nanoparticles with enhanced electrochemical performance for battery-type

supercapacitor electrode

To Van Nguyen¹, Le The Son^1,2, Vu Van Thuy¹, Vu Dinh Thao¹, Masahito Hatsukano², Koichi Higashimine², Shinya Maenosono², Sang-Eun Chun^3, and Tran Viet Thu^4

1Department of Chemical Engineering, Le Quy Don Technical University, 236 Hoang Quoc Viet, Hanoi 100000, Vietnam

2School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan

3School of Materials Sciences and Engineering, Kyungpook National University, Daegu 41566, Republic of Korea

4Institute of Research and Development, Duy Tan University, Danang 550000, Vietnam

Corresponding authors: E-mail: sangeun@knu.ac.kr; tranvietthu1@duytan.edu.vn;Tel: +84 964 793 732

Abstract

We report the synthesis of manganese-doped nickel cobalt oxide (Mn-doped NiCo2O4) nanoparticles (NPs) by an efficient hydrothermal and subsequent calcination route. The material exhibits a homogeneous distribution of Mn dopants and a battery-type behavior when tested as a supercapacitor electrode material. Mn-doped NiCo2O4 NPs show an excellent specific capacity of 417 C g^-1 at scan rate of 10 mV s⁻¹and 204.3 C g^-1 at current density of 1 A g⁻¹ in standard three- electrode configuration, ca. 152-466 % higher than that of the pristineNiCo2O4 or MnCo2O4. In addition, Mn-doped NiCo2O4 NPs showed excellent capacitance retention of 99% after 1,000 charge- discharge cycles at current density of 2 A g⁻¹. The symmetric solid-state supercapacitor device assembled using this material delivered an energy density of 0.87 Wh cm⁻² at a power density of 25

Wh cm⁻², and 0.39 Wh cm⁻² at a high power density of 500 Wh cm⁻². The cost-effective synthesis and high electrochemical performance suggest that Mn-doped NiCo2O4 is a promising material for supercapacitors.

Keyword: Energy storage materials; NiCo2O4; chemical synthesis; doping; supercapacitor.

Dalton Transactions Accepted Manuscript

Published on 29 April 2020. Downloaded by Université de Paris on 4/29/2020 3:31:30 PM.

View Article Online DOI: 10.1039/D0DT01177K

(3)

1. Introduction

The rapid development of portable electronics and electric vehicles has recently imposed additional requirements for power sources, such as enhanced durability and safety, improved mechanical strength, smaller size, and lower weight [1-3]. Popular power sources such as lithium-ion batteries have numerous advantages, including high energy density and low self-discharge; however, they also suffer several disadvantages, including low power density (ca. 101000 W kg^-1), short lifecycle (ca. 4001200 cycles), high cost, and limited resource that restrict their range of applications [4]. Supercapacitors have emerged as a promising complimentary power source for batteries because of their numerous advantages, such as high power density, excellent durability, and high Coulombic efficiency [5]. The specific power of supercapacitors is typically 10 to 100 times greater that of batteries and can reach values up to 15 kW kg^-1 [4].

Supercapacitor electrodes are generally classified into three main categories according to their energy storage mechanism: (1) Electrochemical double-layer electrodes, in which charge storage is relied on the absorption/desorption of electrolyte ions at the electrolyte/electrode interface. Typical electrodes are based on carbonaceous materials with large specific surface area [6-8] for attaining high energy density. (2) Pseudocapacitive electrodes, which rely on the transfer of electrical charges through redox reactions and intercalation/de-intercalation processes of electrode material beyond electrostatic adsorption, typically RuO2 and MnO2 [9-11]. Their capacitance values are relatively constant within potential window. (3) Battery-type electrodes, in which the charge storage is also faradaic in nature, but their specific capacity values vary with voltage window and their specific capacity is better expressed in C g^-1 or mAh g^-1 [12]. Typical battery-type electrode materials are transitional metal oxides/sulfides [13-16], and they are frequently coupled with pseudocapacitive electrodes in hybrid supercapacitors to obtain high specific power and high specific energy [15, 17].

Therefore, the electrode material is a critical factor that determines the charging mechanism, electrochemical performance, and practical applications of a supercapacitor in which the electrode material is incorporated. Over the past decade, numerous efforts have been dedicated to developing novel materials as well as to improving the performance of existing electrode materials [14, 18].

Spinel-type metal oxides with general formula AB₂O₄ (where A and B represent first-row transition metals such as Co, Fe, Ni, and Mn) have received intensive attention as a promising material for energy storage because of their high specific capacity, cost-effectiveness, and high earth- abundance [18-23]. Among them, NiCo2O4 is a very promising candidate because of their excellent charge storage capacity which is much higher than that of both neat NiO and Co3O4 [24-27]. The developments of high-performance NiCo2O4-based electrodes for supercapacitors have included different strategies such as tuning the morphology, forming the hetero-structures, and hybridizing

Dalton Transactions Accepted Manuscript

(4)

with high surface area conductive support [23, 26-28]. Recently, it was shown that the composition of nickel-cobalt oxides has significant effects on their microstructures and electrochemical properties [29]. A substitution of Ni with Cu of up to 50% could increase the Cs of NiCo2O4 from 280.3 F g⁻¹ to 367.4 F g⁻¹ at a discharge rate of 1 A g⁻¹ [30]. Whereas, the substitution of Ni with Mo could double the Cs of NiCo2O4 [31], and the substitution of Co with Fe could increase the Cs of NiCo2O4

up to 45.7 % [32]. Doping has also been demonstrated as a promising strategy for tuning the chemical composition and modulating the electrochemical property of NiCo2O4 materials [24, 33]. Among various dopants, Mn has many advantages such as multi-valence states, earth-abundance and environmental benign [34]. Mn-doped NiMoO4 nanorods as positive electrode in supercapacitor exhibit an improved electrochemical performance and cycle life due to synergistic effects of Ni and Mn elements [35]. 10% Mn-doped ZnCo2O4 synthesized by hydrothermal route exhibited a maximum capacitance of 707.4 F g⁻¹ at a current density of 0.5 A g⁻¹, and a high Coulombic efficiency of 96.3%

after 500 CV cycles in KOH electrolyte [34]. 5% Mn-doped NiCo2O4 nanosheets were found to show excellent lithium storage due to hierarchical structure and the combination of Mn dopants and the NiCo2O4 host structure [36]. However, the study of Mn-doped NiCo2O4 for supercapacitor application has not been reported.

In this work, we report a facile combined hydrothermal treatment/calcination route to prepare Mn-doped NiCo2O4 nanoparticles (NPs) with a doping content as high as 10 at%. The crystal structure, morphology, chemical composition, and chemical state of the as-prepared Mn-doped NiCo2O4 NPs was systematically characterized. We found that Mn-doped NiCo2O4 NPs display battery-type behavior with enhanced specific capacity compared with those of pristine NiCo2O4 and MnCo2O4, indicating a synergistic effect of Mn dopant and NiCo2O4 host structure. The cycling test indicated that the Mn-doped NiCo2O4 NPs demonstrate a long-term cycle stability with 99% capacity retention after 1000 charge–discharge cycles.

2. Experimental

2.1. Chemicals: Nickel(II) chloride hexahydrate (NiCl2·6H2O), cobalt(II) chloride hexahydrate (CoCl2·6H2O), manganese(II) chloride (MnCl2), urea (CO(NH2)2), hydrochloric acid (HCl), Ni foam, acetylene black, polyvinylidene fluoride (PVDF), N-methyl-2-pyrrolidone (NMP), and polyvinyl alcohol (PVA) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. All of the reagents were of analytical grade and were used without further purification. All solutions in the experiments were prepared with deionized (D.I.) water (18 M cm).

2.2. Materials synthesis: NiCo2O4 (NCO), Mn-doped NiCo2O4 (NMCO), and MnCo2Ox (MCO) NPs were synthesized by a combined hydrothermal treatment/calcination route (Fig. 1a). In a typical

Dalton Transactions Accepted Manuscript

(5)

procedure, 0.9 g of urea (15 mmol) and predetermined amounts of M²⁺ salts (M = Ni, Mn, Co) were dissolved into 75 mL of D.I. water by magnetic stirring for 30 min, resulting in the formation of a pinkish solution. The total amount of M²⁺ salts was fixed at 15 mmol, where Ni²⁺/Mn²⁺/Co²⁺ (molar ratio) was varied: 5/0/10 (NCO); 4/1/10 (NMCO); or 0/5/10 (MCO). The resulting mixture was transferred to a 150 mL Teflon-lined autoclave and heated to 120 ^oC for 6 h in air for hydrothermal treatment. After the autoclave was cooled to room temperature, the product in it was washed with D.I. water and ethanol several times. The washed product was calcined at 400 ^oC in air for 2 h. The amount of each precursor in synthesized samples are given in Table 1.

Table 1. Ratio between precursor chemicals for different samples Amount (mmol)

NiCl2∙6H2O MnCl2 CoCl2∙6H2O Total

NCO 5 0 10 15

NMCO 4 1 10 15

MCO 0 5 10 15

2.3. Characterization: All synthesized cobaltites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), scanning TEM (STEM) on an instrument equipped with a high-angle annular dark-field (HAADF) detector, energy- dispersive X-ray spectroscopy (EDS) elemental mapping, X-ray photoelectron spectroscopy (XPS), X-ray fluorescence (XRF) spectroscopy, and inductively coupled plasma (ICP). XRD measurements were conducted on a Siemens D5005 X-ray diffractometer equipped with a Cu-Kα radiation source (wavelength 1.5418 Å, acceleration voltage/current 30 kV/40 mA, scan rate 0.9 ô min⁻¹, scan step 0.03 ô, 2θ range from 10 ô to 70 ô). SEM analysis was conducted on a Hitachi S4800 field-emission scanning electron microscope. TEM observations were performed on a Jeol JEM-1010 microscope.

STEM-HAADF imaging and EDS elemental mapping were carried out on a JEOL JEM-ARM200F microscope operated at an acceleration voltage of 200 kV and equipped with a spherical aberration corrector; the nominal resolution was 0.8 Å. XPS analysis was performed on a Shimadzu Kratos AXIS-ULTRA DLD high-performance XPS system. Monochromated Al Kα radiation was used to excite photoelectrons. The chemical composition was estimated by XRF spectroscopy (X-MET 5100, Oxford Instruments). The elemental compositions of the synthesized samples were further confirmed by ICP conducted on a Optima 7300DV. Each sample was quantified twice to ensure reproducibility.

2.4. Electrochemical measurements: To evaluate the electrochemical behavior and performance of the synthesized materials, a homogeneous slurry was prepared by mixing active material, acetylene black, and PVDF (weight ratio 75:12.5:12.5) in a small amount of NMP solvent. The working

Dalton Transactions Accepted Manuscript

(6)

electrode was prepared by pasting the prepared slurry onto a Ni foam sheet (coating area 1.0 × 1.0 cm²) and drying in a vacuum oven at 60 °C for 5 h. The mass loading of active materials was approximately 2 mg cm⁻². Cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) measurements were performed on Metrohm Autolab PGSTAT 302N using a standard three-electrode cell system with a Ag/AgCl reference electrode (in 3 M KCl solution) and Pt counter electrode (sheet). In all of the electrochemical measurements, a 3 M KOH aqueous solution was used as the electrolyte. CV measurements were performed between

−0.1 and 0.5 V at different scan rates (10, 20, 40, 60, 80, and 100 mV s⁻¹). GCD measurements were performed at different current densities (1.0, 1.5, 2.0, 2.5 and 3.0 A g⁻¹) in the potential range from

−0.1 to 0.45 V. EIS analyses were carried out in the frequency range from 0.01 Hz to 100 kHz at open-circuit potential with an AC amplitude of 10 mV. The specific capacity (Cs, C g^-1) of the electrochemically active material was subsequently calculated from cyclic voltammograms using the formula Cs = (IdV)/(×m), where I (A), V (V),  (V s^-1), and m (g) represent the current, potential, scan rate, and mass of active material, respectively. Cs was also calculated from the GCD curves at different current densities using Cs = (I×t)/m, where I is the discharge current (A) and t is the discharge time (s).

2.5. Fabrication of supercapacitor device: The symmetric all-solid-state supercapacitor device was fabricated with NMCO as the active material and PVA–KOH as a gel-type electrolyte. First, an electrode was prepared by pasting the prepared slurry of NMCO onto nickel sheets (width × length × thickness = 2.0 × 2.5 × 0.15 mm³) and dried in a vacuum oven at 60 ^oC for 5 h. The mass loading of active material on each electrode was approximately 10 mg cm⁻². The gel-type electrolyte was prepared by dissolving 3 g of PVA in 30 mL of D.I. water with constant stirring at 100 ^oC for 2 h.

After the PVA solution became clear, 1.5 g of KOH in 10 mL of D.I. water was added to the solution at 80 ^oC under vigorous stirring. Two electrodes prepared with NMCO were immersed in PVA–KOH solution for 15 min. The electrodes were assembled with a PVA–KOH gel electrolyte between them.

Finally, the PVA–KOH gel was solidified by evaporating the contained water at room temperature, resulting in all-solid-state supercapacitor. Furthermore, the electrochemical characteristics of the supercapacitor device were studied using CV and GCD. The areal capacitance (Ca, F m^-2) of the supercapacitor device was calculated from GCD using the formula Ca = (It)/(SV), where I (A), t (s), S (m²), and V (V) represent discharge current, discharge time, geometrical area of the electrode, and potential window (V). The specific energy density (E) and specific power density (P) were calculated using the equations E = Ca×(V)²/2 and P = E/t, respectively.

3. Results and Discussion

Dalton Transactions Accepted Manuscript

(7)

3.1. Structural and morphological characterization

Phase identification: As shown in Fig. 1b, the XRD patterns of the as-synthesized cobaltites are almost identical and can be assigned to NiCo2O4 (JCPDS #20-0781) and/or MnCo2O4.5 (JCPDS

#32−0297); both have the same cubic spinel structure and crystallize in space group Fd-3m (227).

The observed diffraction peaks at 18.6, 31.4, 36.7, 44.3, 59.1, and 64.6 ^o are appropriately indexed to the (111), (220), (311), (422), (511), and (440) planes, respectively. No diffraction peaks due to impurities are observed in any of the XRD patterns. The similarity in both the positions and relative intensities of the peaks of these cobaltites indicates that their crystal structures and lattice parameters do not substantially differ. A very small deviation from each other’s diffraction peak is due to the difference in atomic radii of Ni (0.135 nm) and Mn (0.140 nm). The structural representation of the typical cobaltite (NiCo2O4, Fig. 1c) shows that Ni (or Mn) ions occupy octahedral sites, whereas Co ions occupy both octahedral and tetragonal sites.

Stirring 30 min

pink powder black powder

Calcination 400^oC, 2h autoclave

NiCl2.6H2O CoCl₂.6H₂O MnCl₂

Urea Hydrothermal

120^oC, 6h

 JCPDS #20-0781

 JCPDS #32-0297 a b

c OCo NiCo

(a)

(b) (c)

10 20 30 40 50 60 70

MCO

NMCO

Normalized intensity

NCO

440

511

422

400

222311

220

2-theta (degree)

111

Figure 1. (a) Synthesis flowchart for NCO, MCO, and NMCO. (b) XRD patterns of the as- synthesized cobaltites along with those of reference materials of NiCo2O4 (JCPDS #20-0781, black

bars) and MnCo2O4.5 (JCPDS #32-0297, red bars). (c) Illustration of spinel-structured NiCo2O4. Morphology: Figs. 2(a,d,g) and 2(b,e,h) show SEM and TEM images of NCO, NMCO, and MCO, respectively. As shown in the SEM images, all of the obtained cobaltites have polyhedral

Dalton Transactions Accepted Manuscript

(8)

shapes, and their agglomeration is likely resulting from the calcination step (at 400 ^oC). Other authors have found that urea acts as a facet-directing agent in the hydrothermal crystallization of binary and ternary oxides when KOH is used as a mineralizer [37, 38]. In our synthesis route, KOH was not used and urea was hydrolyzed during the thermal treatment to generate carbonate ions [39-41]. Metal ions then combined with carbonate ions to form metal carbonates (pink powder), which were ultimately transformed to metal cobaltites (black powder) during calcination. Although these polyhedra are not very well defined, their average size was typically smaller than 100 nm. Despite huge differences in their elemental compositions, the NMCO exhibits a morphology similar to that of NCO and MCO.

The selected-area electron diffraction (SAED) patterns, shown in Figs. 2(c,f,i) can be indexed as the (311), (222), (511), and (440) planes of spinel-structured NiCo2O4, consistent with the previously shown XRD results.

200 nm

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

Figure 2. SEM image, TEM image, and SAED patterns of NCO (a, b, c) NMCO (d, e, f), and MCO (g, h, i).

The distribution of Ni, Mn, Co, and O across the as-synthesized cobaltite particles was characterized using STEM-EDS elemental mapping. The STEM-HAADF images and EDS elemental mapping images of NCO, NMCO, and MCO are shown in Fig. 3(a–c), respectively. These images and mapping images confirm the presence of all of the aforementioned elements as well as their

Dalton Transactions Accepted Manuscript

(9)

homogeneous distribution throughout the cobaltite particles. These results thus demonstrate that well- structured NMCO were successfully synthesized for all combinations of the precursors.

(a) (c)

(b)

Figure 3. STEM-HAADF images and corresponding STEM-EDS elemental mappings of typical (a) NCO, (b) NMCO, and (c) MCO. Scale bar for STEM: 20 nm.

Elemental composition and chemical state: The XPS survey spectra (Fig. S1) assert the presence of Ni, Co, O in NCO; Mn, Co, O in MCO; and Ni, Mn, Co, O in NMCO. The presence of other elements such as C (ca. 286–288 eV) and Fe (ca. 720 eV) is due to adventitious carbon contamination and iron-based sample holder used for the sample preparation, respectively. NMCO was selected as a representative cobaltite sample for further XPS studies. All core-level XPS spectra of Ni 2p, Mn 2p, Co 2p, and O 1s were calibrated (reference: C 1s line), background subtracted (linear-type), deconvoluted, and fitted (Gaussian fitting method). The Ni 2p spectrum (Fig. 4a) was deconvoluted into six peaks, four of which correspond to two characteristic spin–orbit doublets of Ni²⁺ (853.1 and 872.3 eV) and Ni³⁺ (856.4 and 873.9 eV) [42]. The two remaining peaks (861.2 and 879.7 eV) correspond to shake-up satellites of nickel [33]. Similar to Ni spectrum, the Mn 2p spectrum (Fig. 4b) was also deconvoluted into six peaks, corresponding to Mn²⁺ (641.3 and 652.5 eV), Mn³⁺ (642.6 and 653.8 eV) [43], and shake-up satellites (643.9 and 655.4 eV) of manganese.

The XPS spectrum of Co 2p (Fig. 4c) exhibits peaks at 781.1 and 796.2 eV ascribed to Co²⁺, two

Dalton Transactions Accepted Manuscript

(10)

peaks at 779.6 and 794.6 eV corresponding to Co³⁺, and two pairs of shake-up satellites corresponding to Co²⁺ (785.2 and 799.6 eV) and Co³⁺ (790 and 804.3 eV) [21]. In addition, the XPS spectrum of Co 2p includes two small peaks at 777.9 and 792.9 eV, suggesting the presence of a small amount of Co⁰ in the surface materials. The XPS O 1s spectrum (Fig. 4d) indicates two O species: the peak at 529.3 eV corresponds to O involved in M–O bonding (M is the metal), and the peak at 531.0 eV corresponds to defect sites on the surface of the synthesized cobaltite[44]. Ni²⁺, Ni³⁺, Co⁰, Co²⁺, Co³⁺, and O species on the surface of NCO, and Mn²⁺, Mn³⁺, Co⁰, Co²⁺, Co³⁺, and O species on the surface of MCO were verified in the XPS spectra of NCO (Fig. S2) and MCO (Fig. S3), respectively. These data clearly indicate that the NCO, MCO, and NMCO cobaltites were successfully synthesized according to our proposed hydrothermal and calcination routes. Both XRF and ICP (Table S1) were used to quantify the elemental composition of NMCO sample, and the results were consistent. The Mn:Ni:Co atomic ratio was calculated to be 0.3:3:10, which indicates a Mn doping content of approximately 10 % (as compared to Ni content).

Figure 4. XPS spectra of (a) Ni 2p, (b) Mn 2p, (c) Co 2p, and (d) O 1s in NMCO.

3.2. Electrochemical characteristics

Dalton Transactions Accepted Manuscript

(11)

CV analysis: Fig. 5(a–c) show the CV curves of NCO, NMCO, and MCO, respectively. In all of the CV curves, a pair of well-defined redox peaks is clearly observed, which is attributed to redox reactions involved in AO/AOOH (A = Ni, Mn, Co) conversions[19]. This suggests the materials display battery-type rather than pseudocapacitance-type behavior; accordingly, their specific capacity is better presented as C g^-1 (or mAh g^-1) instead of F g^-1 due to the charge storage is non-constant over the working potential [12, 45, 46]. Therefore, the supercapacitor electrodes fabricated using these materials mainly store and release electrical charges through Faradaic reactions.

No additional redox peaks are detected in the CV curves of NMCO, indicating that the redox processes in NMCO are very similar to those in NCO and MCO. Fig. 5d shows CV curves for all of the samples, as recorded at a scan rate of 100 mV s⁻¹. The CV curve for NMCO shows a much larger internal area than those of NCO and MCO, indicating that NMCO exhibits the largest C_s. The Randles–Sevcik plots of NMCO sample are shown in Fig. 5e. It can be seen that, as the scan rate is increased from 10 to 100 mV s⁻¹, both the anodic peak current (Ipa) and the cathodic peak current (Ipc) increase. More specifically, both Ipa and Ipc vary linearly with the square root of the scan rate (^1/2), as well described by the Randles–Sevcik equation[47, 48]. These observations indicate that the redox events occurring at the electrode–electrolyte interface are fast and quasi-reversible and are only limited by electrolyte diffusion[49]. Simultaneously, the separation between anodic and cathodic peaks also increases with increasing scan rate, which is attributed to the equivalent series resistance.

The Cs of these materials at different scan rates was calculated, and the results are plotted in Fig. 5f.

The Cs of all of the materials increase with decreasing scanning rate. At the same scan rate, the Cs

values increase in the order MCO < NCO < NMCO. Specifically, the Cs value of NMCO obtained at 10 mV s⁻¹ was calculated to be 417.0 C g^-1, much greater than that of NCO (274 C g^-1) and MCO (98.0 C g^-1). The enhancement of Cs in NMCO suggests that the Ni and Mn in NMCO synergistically behave as a faradaic charging site compared with the Ni or Mn sites alone in NCO and MCO.

Dalton Transactions Accepted Manuscript

(12)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5 -60

-40 -20 0 20 40 60 80

Current (A g-1 )

Potential vs. Ag/AgCl (V) 100 mV s^-1

80 mV s^-1 60 mV s^-1 40 mV s^-1 20 mV s^-1 10 mV s^-1

NCO

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

-60 -30 0 30 60

90 100 mV s^-1 80 mV s^-1 60 mV s^-1 40 mV s^-1 20 mV s^-1 10 mV s^-1

Current (A g-1 )

Potential vs. Ag/AgCl (V)

NMCO

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

-30 -20 -10 0 10 20

30 ^{100 mV s}^-1 80 mV s^-1 60 mV s^-1 40 mV s^-1 20 mV s^-1 10 mV s^-1

Current (A g-1 )

MCO

0 20 40 60 80 100

50 100 150 200 250 300 350 400 450

Specific Capacity (C g-1 )

Scan rate (mV s-1)

NCO NMCO MCO

(a) (b)

(c) (d)

-0.1 0.0 0.1 0.2 0.3 0.4 0.5

-80 -60 -40 -20 0 20 40 60 80 100

Current (A g-1)

Potential vs Ag/AgCl (V) NCO NMCO MCO

3 4 5 6 7 8 9 10 11

-60 -40 -20 0 20 40 60 80

y = -6.7063x + 7.7012 R² = 0.9982 y = 8.9765x - 5.5143 R² = 0.9971

Current (A/g)

v1/2 Ipa

Ipc

NMCO (e) (f)

Figure 5. CV curves at different scan rates of NCO (a), NMCO (b), and MCO (c); CV curves of NCO, NMCO, and MCO at 100 mV s⁻¹ (d); Randles–Sevcik plots of NMCO (e); Cs of all of the

synthesized materials at different scan rates (f).

GCD analysis: To evaluate the Cs and the rate performance of the obtained cobaltites, GCD measurements were conducted in the potential range from −0.1 to 0.45 V at different current densities ranging from 1 to 3 A g⁻¹. Fig. 6(a–c) presents the GCD voltage profiles of NCO, NMCO, and MCO, respectively. The observed voltage plateaus in the discharging profiles match well with the peak potentials observed in the cyclic voltammograms, confirming their battery-type behavior and that their main mechanism for energy storage is based on redox reactions. The difference between the discharge and charge potential plateau in the GCD profile originates from the series resistance of the electrode. A smaller deviation indicates greater electrochemical reversibility; the GCD results in the current range from 1 to 3 A g⁻¹ show that the NMCO exhibit excellent reversibility similar to that of neat NCO and MCO. Fig. 6d presents the GCD profiles for all of the samples, as recorded at a current

Dalton Transactions Accepted Manuscript

(13)

density of 1 A g⁻¹. The calculated Cs of NMCO is 204.3 C g⁻¹, much higher than those of NCO (127 C g⁻¹) and MCO (43.9 C g⁻¹). It is also observed that the Cs of NMCO is always greater than those of NCO and MCO at other current densities, as shown in Fig. 6e. For all of the samples, the minimal decrease of Cs with increasing current density reveals remarkable rate capability, indicating fast kinetics of the faradaic reaction of the electrode materials. These results agree well with the results of the CV analysis and indicate the synergy of faradaic conversions between AO/AOOH (A = Ni, Mn, Co) pairs in NMCO. Given the similar structure and morphology of NMCO and NCO, the higher Cs of NMCO obviously arises from the composition difference due to Mn doping. It is therefore concluded that the Mn doping greatly enhanced the electrochemical properties of NiCo2O4 NPs.

0 50 100 150 200 250 300

-0.1 0.0 0.1 0.2 0.3

0.4 NCO

Time (sec)

1 A g^-1 1.5 A g^-1 2 A g^-1 2.5 A g^-1 3 A g^-1

0 100 200 300 400

-0.1 0.0 0.1 0.2 0.3

0.4 1 A g^-1

1.5 A g^-1 2 A g^-1 2.5 A g^-1 3 A g^-1

NMCO

Time (sec)

0 20 40 60 80 100

-0.1 0.0 0.1 0.2 0.3

0.4 MCO ^{1 A g}^-1

1.5 A g^-1 2 A g^-1 2.5 A g^-1 3 A g^-1

Time (sec)

(a) (b)

(c) (d)

0 200 400 600 800 1000

0 20 40 60 80 100 120

Capacitance retention (%)

Cycle number

NMCO

(e) (f)

1.0 1.5 2.0 2.5 3.0

0 30 60 90 120 150 180 210

Specific Capacity (C g-1 )

Current (A g^-1)

NMCO

NCO

MCO

0 100 200 300 400

-0.1 0.0 0.1 0.2 0.3 0.4

Potential vs Ag/AgCl (V)

Time (sec)

NCO NMCO MCO

Figure 6. GCD curves of NCO, NMCO, and MCO at different current densities (a–c) and at 1 A g⁻¹ (d); rate capability of all samples (e); and cycling performance of NMCO for 1,000 cycles at a

current density of 2 A g⁻¹ (f).

Dalton Transactions Accepted Manuscript

(14)

For comparison, the Cs of several supercapacitor electrodes fabricated from NiCo2O4

nanomaterials is summarized in Table 2. The specific capacity of NMCO NPs was 204.3 C g^-1, which was much higher than that of NiCo2O4 nanosheets (92.1 F) [50], NiCo2O4 nanoflowers (122.5 C g⁻¹) [12], NiCo2O4 nanosheets (145 C g⁻¹) [51], NiCo2O4 hollow spheres (171.2 C g⁻¹) [45], NiCo2O4

microspheres (112-202 C g⁻¹) [52], and MnCo2O4 nanofibers (52 C g⁻¹) [53] at the same discharge current of 1 A g^-1, indicating the excellent performance of NMCO NPs. However, the Cs of NMCO NPs was still lower than that of NiCo2O4 nanoflowers grown on Ni foam (348.9 C g⁻¹) [54] or rGONiCo2O4 nanocomposites (427 C g⁻¹) [45]. This comparison suggests that further developments of this work should include the deposition of NMCO NPs on three-dimensional conductive scaffold or combination with other materials to form heterostructures.

Table 2. Comparison of NiCo₂O₄–based electrode materials for supercapacitors

Electrode material Method of synthesis Electrolyte Cs from GCD

(at 1.0 A g⁻¹) Ref.

NiCo2O4 nanosheets Hydrothermal 1 M KOH 92.1 C g⁻¹ [50]

NiCo2O4 nanoflowers Hydrothermal 6 M KOH 122.5 C g⁻¹ [12]

NiCo2O4 nanosheets Co-precipitation 2 M KOH 145 C g⁻¹ [51]

NiCo2O4 hollow spheres Hydrothermal 3 M KOH 171.2 C g⁻¹ [45]

NiCo2O4 microspheres Co-precipitation 6 M KOH 112-202 C g⁻¹ [52]

MnCo2O4 nanofibers Hydrothermal 1 M KOH 52 C g⁻¹ [53]

Mn-doped NiCo2O4

nanoparticles Hydrothermal/Calcination 3 M KOH 204.3 C g⁻¹ This study

Cycling stability: The cycling performance of the cobaltites was evaluated by repeating GCD measurements for 1000 cycles at a current density of 2 A g⁻¹ for NMCO, which exhibited the highest specific capacity among the synthesized materials. The Cs of NMCO increases upon cycling and reaches the maximum at approximately the 50^th cycle (Fig. 6f). This maximum corresponds to full activation of the cobaltite. After 1,000 consecutive cycles, the NMCO sample maintained 99 % of its initial C_s. This high capacitance retention indicates that faradaic charging/discharging reactions in the NMCO is highly reversible. The overall electrochemical performance of the NMCO clearly demonstrates that it is attractive for supercapacitor applications and that its composition is critical for obtaining a large capacitance.

EIS analysis: EIS analysis was conducted to assess the electrochemical behavior of the electrode materials, and the results are presented in Fig. 7. In the high-frequency region, the intercept of the semicircle in EIS spectra with the real axis depicts the equivalent series resistance (ESR), which

Dalton Transactions Accepted Manuscript

(15)

is contributed from several factors including the resistance of the electrode material/current collector, the resistance of the bulk electrolyte solution, and the resistance of the electrode material[55]. Since we used the same procedure for the preparation of electrodes and the identical geometrical arrangement of the electrochemical cell in EIS measurements, the difference in ESR is mainly originated from the resistance of the electrode material. As shown in the inset of Fig. 7 and Table S2, the ESR of NCO, MCO, and NMCO are 0.702, 0.663, and 0.649 , respectively. Here, NMCO exhibited the lowest ESR, which is agree well the maximum Cs according to the CV and GCD results.

In addition, NMCO might have unique MnO bondings and NiCo2O4MnCo2O4 interfaces which provide additional redox-active sites for charge storage. Moreover, EIS results also indicated that the NMCO has the lowest ESR. Therefore, Mn doping is beneficial for enhancing the performance of NiCo₂O₄ supercapacitor electrodes.

0 40 80 120 160 200

0 100 200 300 400 500

-Z" (Ohm)

Z' (Ohm) NCO

NMCO MCO

0.64 0.67 0.70 0.73 0.76 0.79 0.00

0.06 0.12 0.18 0.24 0.30

-Z" (Ohm)

Z' (Ohm)

Figure 7. Overall Nyquist plots of NCO, NMCO, and MCO. Inset: Enlarged Nyquist plots in the high-frequency region.

Electrochemical performance of the solid-state supercapacitor made from NMCO

To further evaluate the practical applicability of the NMCO materials, we constructed an all- solid-state, symmetric supercapacitor with NMCO as the active material and PVA–KOH as the gel electrolyte. The CV curves of the supercapacitor device recorded at different scan rates (Fig. 8a) exhibit a quasi-rectangular shape, and their internal areas increase with increasing scan rate, indicating that the device possesses pseudocapacitive behavior and that the faradaic reactions on its electrode surfaces are fast. The inset in Fig. 8a shows an image of the actual device during testing.

The GCD profiles measured at current densities of 0.1 to 2.0 mA cm⁻² (Fig. 8b) are nearly symmetric, reflecting the electrochemical reversibility of the supercapacitor device. The areal capacitance of the supercapacitor device was calculated from the GCD measurements; the results are shown in Fig. 8c.

Dalton Transactions Accepted Manuscript

(16)

Remarkably, the areal capacitance only decreased ca. 55% (from 25.07 to 11.2 mF cm⁻²) as the current density was increased 20-fold (from 0.1 to 2.0 mA cm⁻²), which is an indication of excellent performance at high current density. The relationship between the power density and energy density of the supercapacitor device (Ragone plot) was calculated from GCD profiles; the results are shown in Fig. 8d. Our supercapacitor device delivers an energy density of 0.87 Wh cm⁻² at a power density of 25 W cm⁻², and 0.39 Wh cm⁻² at a high-power density of 500 W cm⁻². The energy density of our device is several times higher than that of ZnO/MnO2 [56], Mn3(PO4)2/graphene [17], NiCoSx/Cu/Ni [57], graphene fiber [58], carbon nanotube (CNT) fiber [49] and is comparable to that of graphene oxide/polyaniline (GO/PANI) [48]. Although the energy density of our device is slightly lower than that of NiO/MnO2 [59] and carbon-based hybrids such as mesoporous carbon/CNTs) [47], rGO/CNTs [60], Co₃O₄/CNTs [14], and carbon–carbon composites [47], its power density is one to two orders of magnitude higher than those of the aforementioned hybrids. In comparison to MnCo2O4/Ni/Cu [61], NiCo(OH)x/Ni/C [62], and MnCo(OH)x/NiO/Ni [63], our device has nearly comparable powder density but lower energy density. Therefore, combining NMCO with other metal/metal oxides could be a promising route for improvement of its energy density, especially for high power-supplying applications. Long-term GCD test of the supercapacitor device at 1.0 mA/cm² (Fig. S4) reveals a capacity retention of 98.1 % after 1000 cycles, demonstrating its excellent stability.

0.0 0.1 0.2 0.3 0.4 0.5

-4 -2 0 2 4 6

Current (mA)

30 mV/s 20 mV/s 10 mV/s

0.0 0.5 1.0 1.5 2.0

0 4 8 12 16 20 24

Areal capacitance (mF/cm2 )

Current density (mA/cm²)

0 70 140 210 280 350

0.0 0.1 0.2 0.3 0.4 0.5

Time (sec)

0.1 mA/cm² 0.25 mA/cm² 0.5 mA/cm² 1 mA/cm² 2 mA/cm²

(a) (b)

(c) (d)

0.1 1 10

1 10 100 1000

Power density (W cm-2 )

Energy density (Wh cm^-2)

ZnO/MnO₂

Graphene fiber CNT fiber

Mn-doped NiCo₂O₄ (This work) Mn₃(PO₄)₂/

graphene

C-C composite Co₃O₄/CNT

C/CNT rGO/CNT

NiO/MnO₂ MnCo(OH)_x/NiO/Ni

NiCo(OH)_x/Ni/C MnCo₂O₄/Ni/Cu

NiCoS_x/Cu/Ni

GO/PANI

Dalton Transactions Accepted Manuscript

(17)

Figure 8. Electrochemical performances of the symmetric supercapacitor device: (a) CV curves (inset: a photo of the device); (b) GCD curves; (c) areal capacitance vs. current density; and (d) Ragone plot. The mass loading of active material on each electrode was approximately 10 mg cm⁻².

4. Conclusions

In summary, Mn-doped NiCo2O4 NPs have been successfully prepared by a facile hydrothermal/calcination route and thoroughly characterized. The XRD, SEM, TEM, STEM, and XPS results confirmed the spinel structure, polyhedral shape, and chemical composition of the as- prepared Mn-doped NiCo₂O₄ NPs. CV and GCD measurements revealed battery-type behavior of all as-synthesized cobaltites and nearly two-fold enhancement in capacity of Mn-doped NiCo2O4 NPs as compared to that of the pristine NiCo2O4 NPs, which indicated the positive role of Mn doping. Mn- doped NiCo2O4 NPs exhibited an impressive Cs of 417 C g^-1 at scan rate of 10 mV s g⁻¹, and 204.3 C g^-1 at discharge current density of 1 A g⁻¹. EIS analysis supported that the enhanced Cs is related to decreased ESR. Mn-doped NiCo2O4 NPs also showed remarkable capacitance retention of ~99 % after 1,000 cycles at current density of 2 A g⁻¹. The all-solid-state symmetric supercapacitor device fabricated from Mn-doped NiCo2O4 NPs exhibited promising electrochemical performance, resulting in enhanced power density at a comparable energy density. The cost-effective synthesis and high electrochemical performance suggest that Mn-doped NiCo2O4 is a promising material for supercapacitors. Further developments are being made including deposition of NiCo2O4 NPs on three-dimensional conductive scaffold and assembly of an asymmetric device for enhanced capacity.

Associated content

Supporting Information

XPS survey spectra of all samples (Fig. S1), core-level spectra of NCO (Fig. S2) and MCO (Fig. S3), cycling test of supercapacitor device (Fig. S4), results of ICP analysis (Table S1), and EIS fitting parameters (Table S2).

Author information

Corresponding Authors

E-mail: tranvietthu1@duytan.edu.vn (T. V. T); sangeun@knu.ac.kr (S.-E. C)

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgments

Dalton Transactions Accepted Manuscript

(18)

This research was partly supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (NRF- 2017R1C1B2005470) (NRF-2018R1A4A1022260).

References

[1] C. Meng, C. Liu, L. Chen, C. Hu, S. Fan, Highly Flexible and All-Solid-State Paperlike Polymer Supercapacitors, Nano Letters, 10 (2010) 4025-4031.

[2] S. Park, S. Jayaraman, Smart Textiles: Wearable Electronic Systems, MRS Bulletin, 28 (2011) 585-591.

[3] B.S. Shim, W. Chen, C. Doty, C. Xu, N.A. Kotov, Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring made by Carbon Nanotube Coating with Polyelectrolytes, Nano Letters, 8 (2008) 4151-4157.

[4] H.D. Abruña, Y. Kiya, J.C. Henderson, Batteries and electrochemical capacitors, Physics Today, 61 (2008) 43-47.

[5] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature, 414 (2001) 359.

[6] Y.J. Lee, H.W. Park, U.G. Hong, I.K. Song, Mn-doped activated carbon aerogel as electrode material for pseudo-capacitive supercapacitor: Effect of activation agent, Current Applied Physics, 12 (2012) 1074-1080.

[7] J. Chen, J. Xu, S. Zhou, N. Zhao, C.-P. Wong, Nitrogen-doped hierarchically porous carbon foam:

A free-standing electrode and mechanical support for high-performance supercapacitors, Nano Energy, 25 (2016) 193-202.

[8] W. Yang, M. Ni, X. Ren, Y. Tian, N. Li, Y. Su, X. Zhang, Graphene in Supercapacitor Applications, Current Opinion in Colloid & Interface Science, 20 (2015) 416-428.

[9] Z. Peng, X. Liu, H. Meng, Z. Li, B. Li, Z. Liu, S. Liu, Design and tailoring of the 3D macroporous hydrous RuO2 hierarchical architectures with a hard-template method for high-performance supercapacitors, ACS Applied Materials & Interfaces, 9 (2017) 4577-4586.

[10] K.R. Prasad, N. Miura, Electrochemically synthesized MnO2-based mixed oxides for high performance redox supercapacitors, Electrochemistry Communications, 6 (2004) 1004-1008.

[11] Y. Jiang, J. Liu, Definitions of pseudocapacitive materials: a brief review, Energy &

Environmental Materials, 2 (2019) 30-37.

[12] W. Jiang, F. Hu, Q. Yan, X. Wu, Investigation on electrochemical behaviors of NiCo₂O₄ battery- type supercapacitor electrodes: the role of an aqueous electrolyte, Inorganic Chemistry Frontiers, 4 (2017) 1642-1648.

[13] B. Wang, T. Zhu, H.B. Wu, R. Xu, J.S. Chen, X.W. Lou, Porous Co3O4 nanowires derived from long Co(CO3)0.5(OH)·0.11H2O nanowires with improved supercapacitive properties, Nanoscale, 4 (2012) 2145-2149.

[14] F. Su, X. Lv, M. Miao, High‐Performance Two‐Ply Yarn Supercapacitors Based on Carbon Nanotube Yarns Dotted with Co3O4 and NiO Nanoparticles, Small, 11 (2015) 854-861.

[15] H. Liu, X. Liu, S. Wang, H.-K. Liu, L.J.E.S.M. Li, Transition Metal based Battery-Type Electrodes in Hybrid Supercapacitors: A Review, (2020).

[16] T.V. Nguyen, L.T. Son, P.M. Thao, L.T. Son, D.T. Phat, N.T. Lan, N.V. Nghia, T.V. Thu, One- step solvothermal synthesis of mixed nickel–cobalt sulfides as high-performance supercapacitor electrode materials, Journal of Alloys and Compounds, (2020) 154921.

[17] C. Yang, L. Dong, Z. Chen, H. Lu, High-performance all-solid-state supercapacitor based on the assembly of graphene and manganese (II) phosphate nanosheets, The Journal of Physical Chemistry C, 118 (2014) 18884-18891.

Dalton Transactions Accepted Manuscript

(19)

[18] T.V. Thu, T.V. Nguyen, L.X. Duong, L.T. Son, V.V. Thuy, T.Q. Huy, T.Q. Duc, Graphene- MnFe2O4-polypyrrole ternary hybrids with synergistic effect for supercapacitor electrode, Electrochimica Acta, 314 (2019) 151-160.

[19] H. Wang, Q. Gao, L. Jiang, Facile Approach to Prepare Nickel Cobaltite Nanowire Materials for Supercapacitors, Small, 7 (2011) 2454-2459.

[20] T.Y. Wei, C.H. Chen, H.C. Chien, S.Y. Lu, C.C. Hu, A cost‐effective supercapacitor material of ultrahigh specific capacitances: spinel nickel cobaltite aerogels from an epoxide‐driven sol–gel process, Advanced Materials, 22 (2010) 347-351.

[21] F. Yunyun, L. Xu, Z. Wankun, Z. Yuxuan, Y. Yunhan, Q. Honglin, X. Xuetang, W. Fan, Spinel CoMn2O4 nanosheet arrays grown on nickel foam for high-performance supercapacitor electrode, Applied Surface Science, 357 (2015) 2013-2021.

[22] C. Liu, W. Jiang, F. Hu, X. Wu, D. Xue, Mesoporous NiCo2O4 nanoneedle arrays as supercapacitor electrode materials with excellent cycling stabilities, Inorganic Chemistry Frontiers, 5 (2018) 835-843.

[23] D. Zhao, H. Liu, X. Wu, Bi-interface induced multi-active MCo2O4@MCo2S4@PPy (M=Ni, Zn) sandwich structure for energy storage and electrocatalysis, Nano Energy, 57 (2019) 363-370.

[24] Y. Li, X. Han, T. Yi, Y. He, X. Li, Review and prospect of NiCo2O4-based composite materials for supercapacitor electrodes, Journal of Energy Chemistry, 31 (2019) 54-78.

[25] J. Xiao, S. Yang, Sequential crystallization of sea urchin-like bimetallic (Ni, Co) carbonate hydroxide and its morphology conserved conversion to porous NiCo2O4 spinel for pseudocapacitors, RSC Advances, 1 (2011) 588-595.

[26] D.P. Dubal, P. Gomez-Romero, B.R. Sankapal, R. Holze, Nickel cobaltite as an emerging material for supercapacitors: an overview, Nano Energy, 11 (2015) 377-399.

[27] X. Wu, Z. Han, X. Zheng, S. Yao, X. Yang, T. Zhai, Core-shell structured Co3O4@NiCo2O4

electrodes grown on flexible carbon fibers with superior electrochemical properties, Nano Energy, 31 (2017) 410-417.

[28] D. Zhao, M. Dai, H. Liu, L. Xiao, X. Wu, H. Xia, Constructing high performance hybrid battery and electrocatalyst by heterostructured NiCo2O4@NiWS nanosheets, Crystal Growth & Design, 19 (2019) 1921-1929.

[29] J. Zhang, F. Liu, J. Cheng, X. Zhang, Binary nickel–cobalt oxides electrode materials for high- performance supercapacitors: influence of its composition and porous nature, ACS Applied Materials

& Interfaces, 7 (2015) 17630-17640.

[30] S. Samanta, A.K. Nayak, A. Mukherji, D. Pradhan, B. Satpati, R. Srivastava, Flower-Shaped Self-Assembled Ni0.5Cu0.5Co2O4 Porous Architecture: A Ternary Metal Oxide as a High-Performance Charge Storage Electrode Material, ACS Applied Nano Materials, 1 (2018) 5812-5822.

[31] Y.-Y. Huang, L.-Y. Lin, Synthesis of Ternary Metal Oxides for Battery-Supercapacitor Hybrid Devices: Influences of Metal Species on Redox Reaction and Electrical Conductivity, ACS Applied Energy Materials, 1 (2018) 2979-2990.

[32] Y. Yuan, D. Long, Z. Li, J. Zhu, Fe substitution in urchin-like NiCo2O4 for energy storage devices, RSC Advances, 9 (2019) 7210-7217.

[33] L. Liu, H. Zhang, L. Fang, Y. Mu, Y. Wang, Facile preparation of novel dandelion-like Fe-doped NiCo2O4 microspheres@nanomeshes for excellent capacitive property in asymmetric supercapacitors, Journal of Power Sources, 327 (2016) 135-144.

[34] A.J.C. Mary, A.C. Bose, Hydrothermal synthesis of Mn-doped ZnCo2O4 electrode material for high-performance supercapacitor, Applied Surface Science, 425 (2017) 201-211.

[35] Y. Li, S. Zhang, M. Ma, X. Mu, Y. Zhang, J. Du, Q. Hu, B. Huang, X. Hua, G. Liu, Manganese- doped nickel molybdate nanostructures for high-performance asymmetric supercapacitors, Chemical Engineering Journal, 372 (2019) 452-461.

[36] J. Ma, E. Guo, L. Yin, Porous hierarchical spinel Mn-doped NiCo2O4 nanosheet architectures as high-performance anodes for lithium-ion batteries and electrochemical reaction mechanism, Journal of Materials Science: Materials in Electronics, 30 (2019) 8555-8567.