Cobalt-Doped Manganese Dioxide Hierarchical Nanostructures for Enhancing Pseudocapacitive Properties

A Self-Healing and Sweat-Chargeable Hydrogel Electrolyte for All-in-One Flexible Supercapacitors

Flexible solid-state supercapacitors (SCs) with hydrogel as an electrolyte and separator combine the advantages of wearability and energy storage and exhibit a broad application prospect in wearable energy textiles. However, irreversible electrolyte damage and unstable electrode-electrolyte interfaces during mechanical deformations remain bottlenecks in realizing truly wearable applications. Herein, poly(acrylic acid) (PAA)-Fe hydrogels were prepared through a simple thermal polymerization strategy. The dynamic reversible metal coordination bonds between Fe3+ and carboxylic acids confers the hydrogels with excellent self-healing properties. As expected, the prepared hydrogels exhibited superior mechanical strength (tensile stress of 45.80 kPa), ionic conductivity (0.076 S cm-1), and self-healing properties. Subsequently, the SCs were constructed using composite hydrogel electrodes (MnO2@CC embedded in the PAA-Fe hydrogels) as symmetrical electrodes (marked as MSCs). The reversible metal coordination bonds between composite hydrogel electrodes formed an ultrastable electrode/electrolyte interface in the all-in-one MSCs, thus revealing excellent mechanical durability. The all-in-one MSCs delivered a remarkable specific capacitance (30.98 F g-1 at 0.2 A g-1), excellent cyclic stability (87.24% after 5000 cycles), outstanding mechanical deformation stability, and impressive electrochemical output stability after self-healing (capacitance retention of 85.34% after five cycles of cutting/self-healing). It is noteworthy that the all-in-one MSCs employed NaCl as an electrolyte, which can be obtained from human sweat. As a proof of the self-charged concept, the all-in-one MSCs can be reused in sweat, whose capacitance was maintained at 90.05% of the initial state after three repetitions. This work is expected to shine light into the design of all-in-one and fabric-based SCs and the development of wearable energy textiles.

Sensing of Quercetin With Cobalt-Doped Manganese Nanosystems by Electrochemical Method

BACKGROUND

The pressing need for precise, quick, and affordable detection of diverse biomolecules has led to notable developments in the realm of biosensors. Quercetin, a biomolecule rich in flavonoids predominantly found in our diet, is sensed by the electrochemical method. The electrochemical properties show remarkable improvement when Mn2O3 (MO) is doped with cobalt (Co).  Aim: This study aimed to investigate the biomolecule sensing of quercetin using Co-doped MO by electrochemical method.  Materials and methods: Co-doped MO nanospheres were prepared by hydrothermal method. The crystal structure of the synthesized material was evaluated by using X-ray diffraction analysis. The sample morphology was assessed by using field emission scanning electron microscopy (FE-SEM) techniques. The cyclic voltammetry technique was used for the detection of quercetin biomolecules.  Results: The synthesized Co-doped MO appeared to be spherical in morphology in FE-SEM. Energy-dispersive X-ray spectroscopy showed the only presence of Co, Mn, and O, which confirmed the purity of the sample. The modified electrode sensed the biomolecule with a higher current of 7.35 µA than the bare glassy carbon electrode of 6.1 µA.

CONCLUSION

The Co-doped MO exhibited enhanced conductivity, reactivity, and electrochemical performance. This tailored approach will help in the optimization of material properties toward specific biomolecule sensing applications.

Polymer-assisted synthesis of Co3O4/CoO microballs decorated N-doped carbon for symmetric supercapacitor

Cobalt oxide (Co) and cobalt oxide/N-doped carbon composites (Co-NC) were synthesized and applied as electrode materials for supercapacitors. The pristine cobalt oxide was prepared hydrothermally at 160 °C (CoH160) and further calcined at three different temperatures of 300, 400, and 500 °C (CoC300, CoC400, and CoC500). The cobalt oxide prepared at 300 °C was composited with N-doped carbon prepared from g-C3N4 at four different weight ratios 1 : 0.03, 1 : 0.06, 1 : 0.15, and 1 : 0.30 (Co-NC1, Co-NC2, Co-NC3, and Co-NC4). X-ray diffraction analysis (XRD) confirms the phase and product formation. Among all the composites, Co-NC2 showed the microball structure decorated on N-doped carbon with an average size of 4.2 μm. The X-ray photoelectron spectroscopy (XPS) of Co-NC2 confirms the presence of Co2+, Co3+, C, N, and O. The Brunauer-Emmett-Teller analysis (BET) of Co-NC2 showed a surface area of 73.06 m2 g-1 with a pore diameter of 3.39 nm. The energy storage performance of Co-NC2 exhibits a high specific capacitance of 774.38 F g-1 at a current density of 1 A g-1 in 1 M KOH electrolyte. The fabricated symmetric device showed a specific capacitance of 84.60 F g-1 at a current density of 1 A g-1. The fabricated device showed good cyclic stability with a coulombic efficiency of 92.55% and capacitance retention of 97.75% up to 4000 cycles.

Engineering work functions of cobalt-doped manganese oxide based electrocatalysts for highly efficient oxygen evolution reaction

The crystalline and electronic structures are two important factors for the design of electrocatalysts. In this work, Co-doped MnO electrocatalysts grown on nickel foam (NF) were prepared by a facile hydrothermal reaction, followed by H₂ treatment process. The electrocatalytic performance of MnO was significantly improved after doping with Co and the Co_0.1Mn_0.9O-NF sample achieved excellent oxygen evolution reaction (OER) performance with low overpotential (370 mV at 10 mA cm^-2) and reasonable Tafel slope (85.6 mV dec^-1). Significantly, the low work function was obtained in the Co_0.1Mn_0.9O-NF sample (4.37 eV), which could accelerate the charge transfer process of the OER activity. The excellent OER performance of the Co_0.1Mn_0.9O-NF sample is also attributed to the rich active sites, which improved electrical conductivity and enlarged electrochemical surface areas.

Physicochemical properties of different crystal forms of manganese dioxide prepared by a liquid phase method and their quantitative evaluation in capacitor and battery materials

Although there are many studies on the preparation and electrochemical properties of the different crystal forms of manganese dioxide, there are few studies on their preparation by a liquid phase method and the influence of their physical and chemical properties on their electrochemical performance. In this paper, five crystal forms of manganese dioxide were prepared by using manganese sulfate as a manganese source and the difference of their physical and chemical properties was studied by phase morphology, specific surface area, pore size, pore volume, particle size and surface structure. The different crystal forms of manganese dioxide were prepared as electrode materials, and their specific capacitance composition was obtained by performing CV and EIS in a three-electrode system, introducing kinetic calculation and analyzing the principle of electrolyte ions in the electrode reaction process. The results show that δ-MnO₂ has the largest specific capacitance due to its layered crystal structure, large specific surface area, abundant structural oxygen vacancies and interlayer bound water, and its capacity is mainly controlled by capacitance. Although the tunnel of the γ-MnO₂ crystal structure is small, its large specific surface area, large pore volume and small particle size make it have a specific capacitance that is only inferior to δ-MnO₂, and the diffusion contribution in the capacity accounts for nearly half, indicating it also has the characteristics of battery materials. α-MnO₂ has a larger crystal tunnel structure, but its capacity is lower due to the smaller specific surface area and less structural oxygen vacancies. ε-MnO₂ has a lower specific capacitance is not only the same disadvantage as α-MnO₂, but also the disorder of its crystal structure. The tunnel size of β-MnO₂ is not conducive to the interpenetration of electrolyte ions, but its high oxygen vacancy concentration makes its contribution of capacitance control obvious. EIS data shows that δ-MnO₂ has the smallest charge transfer impedance and bulk diffusion impedance, while the two impedances of γ-MnO₂ were the largest, which shows that its capacity performance has great potential for improvement. Combined with the calculation of electrode reaction kinetics and the performance test of five crystal capacitors and batteries, it is shown that δ-MnO₂ is more suitable for capacitors and γ-MnO₂ is more suitable for batteries.

Rational Construction of Yolk-Shell Bimetal-Modified Quinonyl-Rich Covalent Organic Polymers with Ultralong Lithium-Storage Mechanism

Covalent organic polymers are attracting more and more attention for energy storage devices due to their lightweight, molecular viable design, stable structure, and environmental benignity. However, low charge-carrier mobility of pristine covalent organic materials is the main drawback for their application in lithium-ion batteries. Herein, a yolk-shell bimetal-modified quinonyl-rich covalent organic material, Co@2AQ-MnO₂, has been designed and synthesized by in situ loading of petal-like nanosized MnO₂ and coordinating with Co centers, with the aim to improve the charge conductivity of the covalent organic polymer and activate its Li-storage sites. As investigated by in situ FT-IR, ex situ XPS, and electrochemical probing, the quinonyl-rich structure provides abundant redox sites (carbonyl groups and π electrons from the benzene ring) for lithium reaction, and the introduction of two types of metallic species promotes the charge transfer and facilitates more efficient usage of active energy-storage sites in Co@2AQ-MnO₂. Thus, the Co@2AQ-MnO₂ electrode exhibits good cycling performance with large reversible capacity and excellent rate performance (1534.4 mA h g^-1 after 200 cycles at 100 mA g^-1 and 596.0 mA h g^-1 after 1000 cycles at 1000 mA g^-1).

Influence of nickel doping on MnO2 nanoflowers as electrocatalyst for oxygen reduction reaction

Abstract

Doping is promising strategy for the alteration of nanomaterials to enhance their optical, electrical, and catalytic activities. The development of electrocatalysts for oxygen reduction reactions (ORR) with excellent activity, low cost and durability is essential for the large-scale utilization of energy storage devices such as batteries. In this study, MnO2 and Ni-doped MnO2 nanowires were prepared through a simple co-perception technique. The influence of nickel concentration on electrochemical performance was studied using linear sweep voltammetry and cyclic voltammetry. The morphological, thermal, structural, and optical properties  of MnO2 and Ni-doped MnO2 nanowires were examined by SEM, ICP-OES, FT-IR, XRD, UV–Vis, BET and TGA/DTA. Morphological analyses showed that pure MnO2 and Ni-doped MnO2 had flower-like and nanowire structures, respectively. The XRD study confirmed the phase transformation from ε to α and β phases of MnO2 due to the dopant. It was also noted from the XRD studies that the crystallite sizes of pure MnO2 and Ni-doped MnO2 were in the range of 2.25–6.6 nm. The band gaps of MnO2 and 0.125 M Ni-doped MnO2 nanoparticles were estimated to be 2.78 and 1.74 eV, correspondingly, which can be seen from UV–Vis. FTIR spectroscopy was used to determine the presence of functional groups and M–O bonds (M = Mn, Ni). The TGA/TDA examination showed that Ni-doping in MnO2 led to an improvement in its thermal properties. The cyclic voltammetry  results exhibited that Ni-doped MnO2 nanowires have remarkable catalytic performance for ORR in 0.1 M KOH alkaline conditions. This work contributes to the facile preparation of highly active and durable catalysts with improved catalytic performance mainly due to the predominance of nickel.

Article Highlights

Graphic abstract