Heteroatom-doped hollow carbon spheres made from polyaniline as an electrode material for supercapacitors

In this work, novel heteroatom-doped hollow carbon spheres (HHCSs) were prepared via the carbonization of polyaniline hollow spheres (PHSs), which were synthesized by one-pot polymerization. It was found that the carbonized PHSs at 700 °C exhibit high specific capacitance of 241 F g⁻¹ at a current density of 0.5 A g⁻¹ and excellent rate capability. The excellent electrochemical performance can be attributed to the heteroatom-doping and hollow carbon nanostructure of the HHCSs electrodes. Heteroatom groups in the HHCSs not only improve the wettability of the carbon surface, but also enhance the capacitance by addition of a pseudocapacitive redox process. Their unique structure provides a large specific surface area along with reduced diffusion lengths for both mass and charge transport.

In this work, novel heteroatom-doped hollow carbon spheres (HHCSs) were prepared via the carbonization of polyaniline hollow spheres (PHSs), which were synthesized by one-pot polymerization.     

Three-dimensional graphene oxide cross-linked by benzidine as an efficient metal-free photocatalyst for hydrogen evolution

The use of low-cost photocatalysts to split water into H₂ fuel via solar energy is highly desirable for the production of clean energy and a sustainable society. Here three-dimensional graphene oxide (3DG) porous materials were prepared by cross-linking graphene oxide (GO) sheets using aromatic diamines (benzidine, 2,2′-dimethyl-4,4′-biphenyldiamine, 4,4′-diaminodiphenylmethane) that reacted with the carboxyl groups of the GO sheets at room temperature. The prepared 3DG porous materials were used as efficient metal-free photocatalysts for the production of H₂via water splitting under full-spectrum light, where the photocatalytic activity was highly dependent on the cross-linker and the 3DG reduction level. It was also found that the 3DG prepared with benzidine as the linker demonstrated a significantly higher H₂ evolution rate than the 3DGs prepared using 2,2′-dimethyl-4,4′-biphenyldiamine and 4,4′-diaminodiphenylmethane as the cross-linkers. The photoactivity was further tuned by varying the mass ratio of GO to benzidine. Among the prepared 3DG materials, 3DG-3, with an intermediate C/O ratio of 1.84, exhibited the highest H₂ production rate (690 μmol g⁻¹ h⁻¹), which was significantly higher than the two-dimensional GO (45 μmol g⁻¹ h⁻¹) and the noncovalent 3DG synthesized by a hydrothermal method (128 μmol g⁻¹ h⁻¹). Moreover, this study revealed that the 3DG photocatalytic performance was favored by effective charge separation, while it could be further tuned by changing the reduction level. In addition, these results could prompt the preparation of other 3D materials and the application of new types of photocatalysts for H₂ evolution.

Three-dimensional graphene oxide covalently linked by benzidine works as an efficient metal-free photocatalyst for H₂ evolution.     

Platinum nanoparticles on defect-rich nitrogen-doped hollow carbon as an efficient electrocatalyst for hydrogen evolution reactions

Design and synthesis of efficient electrocatalysts with low usage of precious metal and of high stability are essential for their practical applications in hydrogen evolution reactions. In this work, we synthesize an electrocatalyst through the deposition of platinum nanoparticles on defect-rich nitrogen-doped hollow carbon derived from surface-attached poly(4-vinylpyridine) monolayers. The platinum nanoparticles with an average diameter of about 1.8 nm are well dispersed on the outer surface of the pre-synthesized carbon material and the platinum loading is about 8.6 wt%. The mass activity of the as-synthesized catalyst under an overpotential of 55 mV is about 5.0 A mg_Pt⁻¹, about 4.93 times higher than that of commercial Pt/C catalysts. Moreover, the synthesized catalyst is also more electrochemically stable than commercial Pt/C catalysts as evidenced by continuous cyclic voltammetry and chronoamperometric response measurements.

Design and synthesis of efficient electrocatalysts with low usage of precious metal and of high stability are essential for hydrogen evolution reaction in their practical applications.     

Calcined chicken eggshell electrode for battery and supercapacitor applications

Biowaste eggshell can be used as a cathode while in its calcined form and it is found to be suitable as an anode in an electrochemical cell. This not only enables energy to be stored reversibly but also achieves waste management and sustainability goals by redirecting material away from landfill. Biowaste eggshell comprises 94% calcium carbonate (CaCO₃; calcite), an attractive divalent ion source as a viable option for energy storage. X-ray diffraction and electron microscopy coupled with energy dispersive analyses of the calcined (thermally decomposed) biowaste eggshell show that CaO has been formed and the reaction is topotactic. Field emission scanning electron microscopy (FESEM) images of the textural relationship show that the thermal decomposition of calcite resulted in a change in morphology. High-resolution XPS spectra of the C 1s core level from the CaCO₃ and CaO shows that there is a chemical difference in the carbon environments and the total atomic fraction of Ca for each sample with that of carbonate and oxygen varies. In a three-electrode configuration, a working electrode of CaCO₃ is found to be electrochemically active in the positive region, whereas a CaO electrode is active in the negative region. This indicates the potential use of eggshell-derived materials for both cathode and anode. Both the electrodes exhibited a quasi-box-shaped potentiostatic curve implying a capacitor-type behaviour. The CaCO₃ cathode possesses a modest discharge capacitance of 10 F g⁻¹ but the CaO anode showed excellent capacitance value of 47.5 F g⁻¹. The CaO electrode in both positive and negative regions, at a current density of 0.15 A g⁻¹ exhibited 55 F g⁻¹ with a retention of nearly 100% after 1000 cycles. At a very low sweep rate of 0.5 mV s⁻¹, the CaO electrode showed typical redox-type behaviour with well-defined peaks illustrating battery-type behaviour. The outcome of the calcite/CaO transformation, exhibiting technological importance for energy storage applications, may help to re-evaluate biowaste before throwing it away. The current work explores the viability of eggshell derived materials as a cathode/anode for use in batteries and capacitors.

Biowaste eggshell can be used as a cathode while in its calcined form and it is found to be suitable as an anode in an electrochemical cell.     

Self-supported Cu3P nanowire electrode as an efficient electrocatalyst for the oxygen evolution reaction

Hydrogen is an ideal energy carrier due to its abundant reserves and high energy density. Electrolyzing water is one of the carbon free technologies for hydrogen production, which is limited by the sluggish kinetics of the half reaction of the anode – the oxygen evolution reaction (OER). In this study, a self-supported Cu₃P nanowire (Cu₃P NWs/CF) electrode is prepared by electrodeposition of a Cu(OH)₂ nanowire precursor on conductive Cu foam (Cu(OH)₂ NWs/CF) with a subsequent phosphating procedure under a N₂ atmosphere. When used as an OER working electrode in 1.0 M KOH solution at room temperature, Cu₃P NWs/CF exhibits excellent catalytic performance with an overpotential of 327 mV that delivers a current density of 20 mA cm⁻². Notably, it can run stably for 22 h at a current density of 20 mA cm⁻² without obvious performance degradation. This highly efficient and stable OER catalytic performance is mainly attributed to the unique nanostructure and stable electrode construction. Interestingly, this synthesis strategy has been proved to be feasible to prepare large-area working electrodes (e.g. 40 cm⁻²) with unique nanowire structure. Therefore, this work has provided a good paradigm for the mass fabrication of self-supporting non-noble metal OER catalysts and effectively promoted the reaction kinetics of the anode of the electrolyzing water reaction.

We prepared Cu₃P nanowires via a simple two-step method and Cu(OH)₂ NWs/CF was converted to Cu₃P/NWs after a phosphating process. The prepared Cu₃P NWs/CF electrode shows high efficiency and excellent stability to OER in alkaline medium.     

Nickel–cobalt hydroxide: a positive electrode for supercapacitor applications

So far, numerous metal oxides and metal hydroxides have been reported as an electrode material, a critical component in supercapacitors that determines the operation window of the capacitor. Among them, nickel and cobalt-based materials are studied extensively due to their high capacitance nature. However, the pure phase of hydroxides does not show a significant effect on cycle life. The observed XRD results revealed the phase structures of the obtained Ni(OH)₂ and Co–Ni(OH)₂ hydroxides. The congruency of the peak positions of Ni(OH)₂ and Co–Ni(OH)₂ is attributed to the homogeneity of the physical and chemical properties of the as-prepared products. The obtained results from XPS analysis indicated the presence of Co and the chemical states of the as-prepared composite active electrode materials. The SEM analysis revealed that the sample had the configuration of agglomerated particle nature. Moreover, the morphology and structure of the hydroxide materials impacted their charge storage properties. Thus, in this study, Ni(OH)₂ and Co–Ni(OH)₂ composite materials were produced via a hydrothermal method to obtain controllable morphology. The electrochemical properties were studied. It was observed that both the samples experienced a pseudocapacitive behavior, which was confirmed from the CV curves. For the electrode materials Ni(OH)₂ and Co–Ni(OH)₂, the specific capacitance (C_s) of about 1038 F g⁻¹ and 1366 F g⁻¹, respectively, were observed at the current density of 1.5 A g⁻¹. The Ni–Co(OH)₂ composite showed high capacitance when compared with Ni(OH)₂. The cycle index was determined for the electrode materials and it indicated excellent stability. The stability of the cell was investigated up to 2000 cycles, and the cell showed excellent retention of 96.26%.

So far, numerous metal oxides and metal hydroxides have been reported as an electrode material, a critical component in supercapacitors that determines the operation window of the capacitor.     

A Co-MOF-derived Co9S8@NS-C electrocatalyst for efficient hydrogen evolution reaction

The exploitation of efficient hydrogen evolution reaction (HER) electrocatalysts has become increasingly urgent and imperative; however, it is also challenging for high-performance sustainable clean energy applications. Herein, novel Co₉S₈ nanoparticles embedded in a porous N,S-dual doped carbon composite (abbr. Co₉S₈@NS-C-900) were fabricated by the pyrolysis of a single crystal Co-MOF assisted with thiourea. Due to the synergistic benefit of combining Co₉S₈ nanoparticles with N,S-dual doped carbon, the composite showed efficient HER electrocatalytic activities and long-term durability in an alkaline solution. It shows a small overpotential of −86.4 mV at a current density of 10.0 mA cm⁻², a small Tafel slope of 81.1 mV dec⁻¹, and a large exchange current density (J₀) of 0.40 mA cm⁻², which are comparable to those of Pt/C. More importantly, due to the protection of Co₉S₈ nanoparticles by the N,S-dual doped carbon shell, the Co₉S₈@NS-C-900 catalyst displays excellent long-term durability. There is almost no decay in HER activities after 1000 potential cycles or it retains 99.5% of the initial current after 48 h.

A porous Co₉S₈@NS-C-900 composite was fabricated by the pyrolysis of crystal Co-MOF involving thiourea. The composite exhibits efficient electrocatalytic activities and long-term durabilities towards HER in alkaline electrolytes.     

Electrochemical properties of kenaf-based activated carbon monolith for supercapacitor electrode applications

Activated carbon monoliths of kenaf (ACMKs) were prepared by moulding kenaf fibers into a column-shape monolith and then carrying out pyrolysis at 500, 600, 700 or 800 °C, followed by activation with KOH at 700 °C. Then, the sample was characterized using thermogravimetric analyzer (TGA), field-emission scanning electron microscopy (FE-SEM), field-emission transmission electron microscopy (FE-TEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD) and N₂ sorption instruments. The prepared ACMK was subjected to electrochemical property evaluation via cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). The GCD study using a three-electrode system showed that the specific capacitance decreased with higher pyrolysis temperature (PYT) with the ACMK pyrolyzed at 500 °C (ACMK-500) exhibiting the highest specific capacitance of 217 F g⁻¹. A two-electrode system provided 95.9% retention upon a 5000 cycle test as well as the specific capacitance of 212 F g⁻¹, being converted to an energy density of 6 W h kg⁻¹ at a power density of 215 W kg⁻¹.

Monolithic carbon from kenaf-based fiber for supercapacitor electrode application provided a specific capacitance of 212 F g⁻¹via GCD at 1 A g⁻¹, converting to an energy density of 6 W h kg⁻¹ at the power density of 215 W kg⁻¹ as well as 95.9% retention upon 5000 cycling test.     

Defect-induced nucleation and epitaxial growth of a MOF-derived hierarchical Mo2C@Co architecture for an efficient hydrogen evolution reaction

The 3D hierarchical structure in catalysts not only the preserves intrinsic characteristics of each component, but also achieves increased specific surface area and active sites for the hydrogen evolution reaction (HER). Herein, we report a new strategy to synthesize efficient 3D hierarchical catalysts composed of Mo₂C nanosheets and Co nanoparticles (H-Mo₂C@Co). It was realized by using raw materials, defect-rich MoO_x, Co(NO₃)₂·6H₂O and 2-methylimidazole, to design Mo/Co bimetallic metal–organic frameworks (BMOFs), followed by pyrolysis at 800 °C. The defects in MoO_x induced preferential nucleation and growth of the BMOFs so that they can ensure the construction of a stable 3D hierarchical structure. Mo₂C and Co have a synergistic effect in improving the HER via providing large surface areas (351.5 m² g⁻¹), more active sites and optimizing charge transfer. It can achieve 10 mA cm⁻² at low overpotential over a wide pH range (144 mV in 0.5 M H₂SO₄ and 103 mV in 1.0 M KOH) and the properties can be well maintained in both acid and alkaline electrolyte after 2000 cycles. The hierarchical catalyst contains no noble metal, can be synthesized on a large scale and recycled by magnetic stirring, demonstrating great potential in water splitting, wastewater treatment, dye adsorption and other fields.

In this work, we report a new strategy to synthesize efficient 3D hierarchical catalysts composed by Mo₂C nanosheets and Co nanoparticles (H-Mo₂C@Co). The Mo₂C and Co makes a synergistic effect in improving HER via providing large surface areas.     

An efficient Au catalyst supported on hollow carbon spheres for acetylene hydrochlorination

Mesoporous hollow carbon spheres (HCSs) were prepared using SiO₂ spheres as a hard template, and Au nanoparticles were then synthesized using NaBH₄ as a reducing agent on the surface of the HCS support. Transmission electron microscopy characterization indicated that Au nanoparticles were much smaller on the HCS support than those on the active carbon (AC) support. HCl-TPD showed that the Au/HCS catalyst displayed a more active site than on Au/AC. The resulting Au/HCS catalyst showed excellent catalytic activity and stability for acetylene hydrochlorination. Acetylene conversion of Au/HCS can be maintained above 92% even after 500 h of lifetime. The excellent catalytic performance of Au/HCS was attributed to the presence of the HCS support, which limited the aggregation of Au nanoparticles.

Mesoporous hollow carbon spheres (HCS) were prepared and applied as the support of Au catalyst for acetylene hydrochlorination. Au/HCS exhibited excellent stability for acetylene hydrochlorination.     

Incorporating Ni-MOF structure with polypyrrole: enhanced capacitive behavior as electrode material for supercapacitor

Herein, Ni-MOF sheet incorporated with polypyrrole is fabricated via a simple wet-chemical approach, and the obtained PPy-MOF composite is investigated as an electrode material for supercapacitors. The composite is systematically investigated by a series of characterization studies including X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. Besides that, the electrochemical capacitive behaviors of the products are examined by electrochemical measurements. Electrochemical results show varying the ingredient ratio can lead to different electrocapacitive behavior, and PPy-MOF-0.2 is proved to possess the best performance in the investigated recipes. Furthermore, an asymmetric supercapacitor employing PPy-MOF and activated carbon as positive and negative electrodes is also assembled, which exhibits high energy density.

PPy is incorporated with a Ni-MOF sheet to achieve a significant enhancement of electro-capacitive performance for supercapacitor application.     

Improved hydrogen evolution performance by engineering bimetallic AuPd loaded on amino and nitrogen functionalized mesoporous hollow carbon spheres

A highly efficient heterogeneous catalyst was synthesized by delicate engineering of NH₂-functionalized and N-doped hollow mesoporous carbon spheres (NH₂–N-HMCS), which was used for supporting AuPd alloy nanoparticles with ultrafine size and good dispersion (denoted as AuPd/NH₂–N-HMCS). Without using any additives, the prepared AuPd/NH₂–N-HMCS catalytic formic acid dehydrogenation possesses superior catalytic activity with an initial turnover frequency value of 7747 mol H₂ per mol catalyst per h at 298 K. The excellent performance of AuPd/NH₂–N-HMCS derives from the unique hollow mesoporous structure, the small particle sizes and high dispersion of AuPd nanoparticles and the modified Pd electronic structure in the AuPd/NH₂–N-HMCS composite, as well as the synergistic effect of the modified support.

Anchoring ultrafine AuPd on NH₂-functionalized and N-doped hollow mesoporous carbon spheres for formic acid dehydrogenation.     

Electrodeposited Pd/graphene/ZnO/nickel foam electrode for the hydrogen evolution reaction

Efficient electrocatalysts are crucial to water splitting for renewable energy generation. In this work, electrocatalytic hydrogen evolution from Pd nanoparticle-modified graphene nanosheets loaded on ZnO nanowires on nickel foam was studied in an alkaline electrolyte. The high electron mobility stems from the cylindrical ZnO nanowires and the rough surface on the graphene/ZnO nanowires increases the specific surface area and electrical conductivity. The catalytic activity arising from adsorption and desorption of intermediate hydrogen atoms by Pd nanoparticles improves the hydrogen evolution reaction efficiency. As a hydrogen evolution reaction (HER) catalyst, the Pd/graphene/ZnO/Ni foam (Pd/G/ZnO/NF) nanocomposite exhibits good stability and superior electrocatalytic activity. Linear sweep voltammetry (LSV) revealed an overpotential of −31 mV and Tafel slope of 46.5 mV dec⁻¹ in 1 M KOH. The economical, high-performance, and environmentally friendly materials have excellent prospects in hydrogen storage and hydrogen production.

Efficient electrocatalysts are crucial to water splitting for renewable energy generation.     

RuxSe@MoS2 hybrid as a highly efficient electrocatalyst toward hydrogen evolution reaction

Alkaline hydrogen evolution reaction (HER) requires highly efficient and stable catalytic materials, the engineering of which needs overall consideration of the water dissociation process as well as the intermediate hydrogen adsorption process. Herein, a Ru_xSe@MoS₂ hybrid catalyst was synthesized by the decoration of MoS₂ with Ru_xSe nanoparticles through a two-step hydrothermal reaction. Due to the bifunctionality mechanism in which Ru promotes the water dissociation and the nearby Se atoms, unsaturated Mo and/or S atoms act as active sites for the intermediate hydrogen adsorption, the hybrid catalyst exhibits an exceptional HER performance in basic media with a rather low overpotential of 45 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 42.9 mV dec⁻¹. The synergetic effect between Ru_xSe and MoS₂ not only enables more catalytically active sites, but also increases the inherent conductivity of the hybrid catalyst, leading to more favorable HER kinetics under both alkaline and acidic conditions. As a result, Ru_xSe@MoS₂ also demonstrates an enhanced catalytic activity toward HER in 0.5 M H₂SO₄ in comparison with pure Ru_xSe and MoS₂, which requires an overpotential of 120 mV to deliver a 10 mA cm⁻² current density and gives a Tafel slope of 72.2 mV dec⁻¹. In addition, the hybrid electrocatalyst also exhibits superior electrochemical stability during the long-term HER process in both acidic media and alkaline media.

The bifunctionality mechanism of Ru_xSe@MoS₂ greatly enhances the alkaline HER performance, in which Ru promotes water dissociation and the nearby Se atoms, unsaturated Mo and/or S atoms act as active sites for the intermediate hydrogen adsorption.     

Electrochemical properties of an activated carbon xerogel monolith from resorcinol–formaldehyde for supercapacitor electrode applications

Activated carbon xerogel monoliths were prepared from resorcinol and formaldehyde via a catalyst-free and template-free hydrothermal polycondensation reaction, followed by pyrolysis and activation. The ratio of resorcinol (R) to distilled water (W) was varied to afford an interconnected pore structure with controlled pore size, while the pyrolysis temperature was optimized to give high specific surface area. Activation was carried out at 700 °C after soaking the samples in 6 M KOH aqueous solution. The same process, called “heat treatment”, was also carried out without soaking in KOH for comparison. The weight loss upon pyrolysis, activation and heat treatment and the weight gain via KOH soaking were measured. Field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and an N₂ sorption instrument were utilized for characterization. Additionally, electrochemical properties were evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) with a 3-electrode system, while a 2-electrode system was also employed for selected samples. The highest specific capacitance of 323 F g⁻¹via GCD at 1 A g⁻¹ was obtained at the R/W ratio of 45 and with 500 °C pyrolysis. In addition, this sample also exhibited 89.4% retention at 20 A g⁻¹ in the current density variation and 100% retention in 5000 cycling tests.

A monolithic carbon xerogel electrode for supercapacitors was prepared from resorcinol–formaldehyde, providing a specific capacitance of 323 F g⁻¹via GCD at 1 A g⁻¹ and 100% retention upon 5000 cycling tests.     

Three-dimensional flower-like NiCo2O4/CNT for efficient catalysis of the oxygen evolution reaction

The oxygen evolution reaction (OER) is an important reaction especially in water splitting and metal–air batteries. Highly efficient non-noble metal based electrocatalysts are urgently required to be developed and to replace the commercial Ru/Ir based oxide. Herein, we report the three-dimensional hierarchical NiCo₂O₄/CNT-150 composite with high activity for the OER that was synthesized via a hydrothermal reaction and subsequent annealing. Compared with CNTs, commercial RuO₂ catalysts, NiCo₂O₄/CNT, NiCo₂O₄/CNT-250, and NiCo₂O₄/CNT-150 exhibit enhanced electrocatalytic performance with a lower onset overpotential of 300 mV and the corresponding Tafel slope of 129 mV per decade. The flower-like NiCo₂O₄/CNT-150 shows an excellent catalysis performance with higher current density than the commercial RuO₂ catalyst. Moreover, the NiCo₂O₄/CNT-150 demonstrates the excellent long-term durability in 0.1 mol L⁻¹ KOH for the OER. The significant catalytic performances are ascribed to the excellent conductivity of CNTs and the high specific surface area of the three dimensional flower-like NiCo₂O₄.

Three-dimensional flower-like NiCo₂O₄ were grown on CNTs prepared by chemical vapor deposition and showed better electrochemical performances on oxygen evolution reaction.     

Reduced graphene oxide/CoS2 porous nanoparticle hybrid electrode material for supercapacitor application

Graphene/transition metal hybrid electrode materials are considered promising electrode materials for supercapacitor applications. However, the stacking of graphene sheets and agglomeration of transition metal parts are still challenging issues to overcome in order to achieve the expected theoretical performances. Herein, a reduced graphene oxide/cobalt disulphide porous nanoparticle hybrid electrode material is fabricated using sulphur as the template precursor. The unique porosity derived from the sulphur template gives favourable open structures for easy diffusion of electrolyte ions and better accessible active sites, and free space for volume changes and results in improved electrochemical performance. In this hybrid material the graphene layers serve as a conductive matrix and physical support for pours cobalt sulphide nanoparticles. On the other hand, the porous cobalt sulphide redox-active material uniformly decorated on rGO can enhance the pseudocapacitive performance of the as synthesized hybrid material. Using the combined advantage of graphene and transition metal sulphide the as synthesized composite electrode material has excellent specific capacitance, excellent rate capability and cycling stability. Thus, our design approach can be considered as a potential candidate to design advanced energy storage devices.

The unique porosity derived from sulphur templates enables easy diffusion of electrolyte ions and improved electrochemical performance is obtained.     

Nitrogen-doped hierarchical porous CNF derived from fibrous structured hollow ZIF-8 for a high-performance supercapacitor electrode

Hollow ZIF-8 was assembled into fiber to fabricate a nitrogen-doped hierarchical porous CNF electrode, which exhibits specific capacitance of 394 F g⁻¹ at 1 A g⁻¹ and excellent rate performance with a retention of up to 76.1% at 20 A ⁻¹, exceeding those of many previously reported 1D carbon materials.

Hollow ZIF-8 shells were assembled into fibers to obtain nitrogen-doped hierarchical porous carbon nanofibers for excellent supercapacitor application.     

Facile molten salt synthesis of Cs–MnO2 hollow microflowers for supercapacitor applications

A facile molten salt technique is an interesting preparation method as it enables mass production of materials. With the use of CsNO₃ salt, Cs-intercalated MnO₂ hollow microflowers are obtained in this work. δ-MnO₂ with a layered structure, instead of other allotropes with smaller structural cavities, is formed and stabilized by large Cs⁺ ions. Formation of the hollow microflowers is explained based on the Ostwald ripening process. The salt to starting agent ratio has little effect on the crystal structure and morphologies of the products but does influence the crystallinity, the interlayer distance, and the intercalating Cs⁺ content. The capacity of Cs⁺ in the structure and the interlayer distance are maximized when the weight ratio of CsNO₃ : MnSO₄ is 7 : 1. Cs–MnO₂ obtained from this optimum ratio has most suitable crystallinity and interlayer distance, and consequently shows a highest specific capacitance of 155 F g⁻¹ with excellent cycling performance. The obtained specific capacitance is comparable to that of other alkaline-intercalated MnO₂, suggesting that Cs–MnO₂ could be another interesting candidate for supercapacitor electrodes.

Hollow microflowers of Cs intercalated MnO₂ binessite are prepared by a facile molten salt method and tested for supercapacitor electrodes.