Superabsorbent polymer binder for achieving MnO2 supercapacitors of greatly enhanced capacitance density

One common dilemma encountered in designing a supercapacitor electrode is that the specific capacitance (C_s) of the active material decreases significantly as the active-material loading (mass area^? 1) increases. As a result, the geometric capacitance density (GCD; Farad area^? 1) of the electrode does not scale up linearly but gradually levels off with increasing loading. For MnO₂ supercapacitors, this problem has been solved to a great extent by introducing a superabsorbent polymer (SAP) binder, namely polyacrylic acid (PAA), to form composite particles with MnO₂. Other than acting as a binder to bound together MnO₂ particles, the SAP is believed to facilitate distribution of electrolyte throughout the active layer owing to its electrolyte-absorbing and swelling behaviors. The C_s of MnO₂ remains almost unchanged as the oxide loading varies over a wide range (1.5–6.5 mg cm^? 2) of heavy active-material loading. In addition, putting PAA throughout the entire active layer helps to magnify the specific interaction between PAA and MnO₂ that is known to enhance the capacitance of individual MnO₂ particles. The success in combining both high C_s and high active-material loading results in GCD of ca. 1.8–1.4 F cm^? 2 even under very high current densities (ca. 35–260 mA cm^? 2 or 5–40 A g^? 1-MnO₂).     

Achieving high-rate capacitance of multi-layer titanium carbide (MXene) by liquid-phase exfoliation through Li-intercalation

The stacks of multi-layer Ti₃C₂T_x and other types of MXene materials limit their electrochemical performance. Herein, we report a facile exfoliation technique to improve the exfoliation efficiency through Li-intercalation into Ti₃C₂T_x interlayers in isopropyl alcohol (IPA) with LiOH as intercalant. This de-intercalation method presented here not only effectively delaminates the stacked Ti₃C₂T_x multi-layers into separate few-layer MXene sheets, but also achieves high-rate supercapacitive performance of Ti₃C₂T_x electrode. The as-produced delaminated Ti₃C₂T_x shows highly improved electrochemical capacitive properties from 47 to 115 F g^− 1 at 200 mV s^− 1. Even at extremely high scan rate of 1000 mV s^− 1, a specific capacitance of 82 F g^− 1 is still obtained. The high-rate capability can be attributed to improved ions accessibility into the few-layer structures. This study offers a new and simple exfoliation pathway for MXenes materials to exploit their full potential in energy storage applications.     

Preparation of monodisperse carbon nanospheres for electrochemical capacitors

The efficiently hydrothermal route using sucrose without any catalysts is employed to prepare the uniform carbon spheres. The monodisperse 100–150 nm carbon spheres are obtained with the activation treatment in molten KOH. The carbon spheres are characterized by transmission electron microscope, X-ray diffraction, N₂ adsorption, Raman spectroscopy and electrochemical techniques. The relationships of specific capacitance and surface properties of carbon spheres are investigated. A single electrode of carbon nanosphere materials performs excellent specific capacitance (328 F g⁻¹), area capacitance (19.2 μF cm⁻²) and volumetric capacitance (383 F cm⁻³).     

Highly mesoporous and high surface area carbon: A high capacitance electrode material for EDLCs with various electrolytes

Activated carbon fibers (ACFs) with high surface area and highly mesoporous structure for electrochemical double layer capacitors (EDLCs) have been prepared from polyacrylonitrile fibers by NaOH activation. Their unique microstructural features enable the ACFs to present outstanding high specific capacitance in aqueous, non-aqueous and novel ionic liquid electrolytes, i.e. 371 F g⁻¹ in 6 mol L⁻¹ KOH, 213 F g⁻¹ in 1 mol L⁻¹ LiClO₄/PC and 188 F g⁻¹ in ionic liquid composed of lithium bis(trifluoromethane sulfonyl)imide (LiN(SO₂CF₃)₂, LiTFSI) and 2-oxazolidinone (C₃H₅NO₂, OZO), suggesting that the ACF is a promising electrode material for high performance EDLCs.     

Potentiostatically deposited nanostructured α-Co(OH)2: A high performance electrode material for redox-capacitors

A high specific capacitance was obtained for α-Co(OH)₂ potentiostatically deposited onto a stainless-steel electrode in 0.1 M Co(NO₃)₂ electrolyte at −1.0 V vs. Ag/AgCl. The structure and surface morphology of the obtained α-Co(OH)₂ were studied by using X-ray diffraction analysis and scanning electron microscopy. A network of nanolayered α-Co(OH)₂ sheets was obtained; the average thickness of individual α-Co(OH)₂ sheets was 10 nm, and the thickness of the deposit was several micrometers. The capacitive characteristics of the α-Co(OH)₂ electrodes were investigated by means of cyclic voltammetry and constant current charge–discharge cycling in 1 M KOH electrolyte. A specific capacitance of 860 F g⁻¹ was obtained for a 0.8 mg cm⁻² α-Co(OH)₂ deposit. The specific capacitance did not decrease significantly for the active mass loading range of 0.1–0.8 mg cm⁻² due its layered structure, which allowed easy penetration of electrolyte and effective utilization of electrode material even at a higher mass. This opens up the possibility of using such materials in supercapacitor applications.     

Electrodeposited PbO2 thin film on Ti electrode for application in hybrid supercapacitor

PbO₂ thin films were prepared by pulse current technique on Ti substrate from Pb(NO₃)₂ plating solution. The hybrid supercapacitor was designed with PbO₂ thin film as positive electrode and activated carbon (AC) as negative electrode in the 5.3 M H₂SO₄ solution. Its electrochemical properties were determined by cyclic voltammetry (CV), charge–discharge test and electrochemical impedance spectroscopy (EIS). The results revealed that the PbO₂/AC hybrid supercapacitor exhibited large specific capacitance, high-power and stable cycle performance. In the potential range of 0.8–1.8 V, the hybrid supercapacitor can deliver a specific capacitance of 71.5 F g^?1 at a discharge current density of 200 mA g^?1(4 mA cm^?2) when the mass ratio of AC to PbO₂ was three, and after 4500 deep cycles, the specific capacitance remains at 64.4 F g^?1, or 32.2 Wh Kg^?1 in specific energy, and the capacity only fades 10% from its initial value.     

Tube-covering-tube nanostructured polyaniline/carbon nanotube array composite electrode with high capacitance and superior rate performance as well as good cycling stability

The combination of a vertically aligned carbon nanotube array (CNTA) framework and electrodeposition technique leads to a tube-covering-tube nanostructured polyaniline (PANI)/CNTA composite electrode with hierarchical porous structure, large surface area, and superior conductivity. PANI/CNTA composite electrode has high specific capacitance (1030 F g⁻¹), superior rate capability (95% capacity retention at 118 A g⁻¹), and high stability (5.5% capacity loss after 5000 cycles). Energy storage characteristics of the PANI/CNTA composite are presented in this paper.     

Electronic structure and Raman spectroscopy study of dibarium magnesium orthoborate,Ba2Mg(BO3)2

The samples of dibarium magnesium orthoborate Ba₂Mg(BO₃)₂ were synthesized by solid-state reaction. The X-ray diffraction (XRD) patterns and Raman spectra of the samples were collected. Electronic structure and vibrational spectroscopy of Ba₂Mg(BO₃)₂ were systematically investigated by first principle calculation. A direct band gap of 4.4 eV was obtained from the calculated electronic structure results. The top valence band is constructed from O 2p states and the low conduction band mainly consists of Ba 5d states. Raman spectra for Ba₂Mg(BO₃)₂ polycrystalline were obtained at ambient temperature. The factor group analysis results show the total lattice modes are 5E_u + 4A_2u + 5E_g + 4A_1g + 1A_2g + 1A_1u, of which 5E_g + 4A_1g are Raman-active. Furthermore, we obtained the Raman active vibrational modes as well as their eigenfrequencies using first-principle calculation. With the assistance of the first-principle calculation and factor group analysis results, Raman bands of Ba₂Mg(BO₃)₂ were assigned as E_g (42 cm⁻¹), A_1g (85 cm⁻¹), E_g (156 cm⁻¹), E_g (237 cm⁻¹), A_1g (286 cm⁻¹), E_g (564 cm⁻¹), A_1g (761 cm⁻¹), A_1g (909 cm⁻¹), E_g (1165 cm⁻¹). The strongest band at 928 cm⁻¹ in the experimental spectrum is assigned to totally symmetric stretching mode of the BO₃ units.     

In-plane chemical pressure in (U1−yCey)O2 studied by Raman spectroscopy

We investigate the nature of bonding and charge states in (U_1−yCe_y)O₂ (y = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) by Raman spectroscopy. Raman spectrum of UO₂ exhibits two prominent bands below 1000 cm⁻¹, a F_2g mode at 446 cm⁻¹ and a F_1u LO mode at 578 cm⁻¹. As y is increased from 0 to 0.6, the F_1u exhibits a large blue shift of 90 cm⁻¹, and from y = 0.6 to 1.0, a red shift of 54 cm⁻¹. We show that our results can be interpreted as arising from anisotropic compression/relaxation of the lattice under Ce substitution and this can give an indication of its charge states. Alternate interpretations have been given in the literature on the effect of substituents and dopants to the Raman spectra of UO₂ and CeO₂. The present interpretation of chemical stress effects can be taken as another plausible explanation.     

Bioelectrocatalytic formate oxidation and carbon dioxide reduction at high current density and low overpotential with tungsten-containing formate dehydrogenase and mediators

We show a great possibility of mediated enzymatic bioelectrocatalysis in the formate oxidation and the carbon dioxide (CO₂) reduction at high current densities and low overpotentials. Tungsten-containing formate dehydrogenase (FoDH1) from Methylobacterium extorquens AM1 was used as a catalyst and immobilized on a Ketjen Black-modified electrode. For the formate oxidation, a high limiting current density (j_lim) of ca. 24 mA cm^− 2 was realized with a half wave potential (E_1/2) of only 0.12 V more positive than the formal potential of the formate/CO₂ couple (E°′_CO2) at 30 °C in the presence of methyl viologen (MV^2 +) as a mediator, and j_lim reached ca. 145 mA cm^− 2 at 60 °C. Even when a viologen-functionalized polymer was co-immobilized with FoDH1 on the porous electrode, j_lim of ca. 30 mA cm^− 2 was attained at 60 °C with E_1/2 = E°′_CO2 + 0.13 V. On the other hand, the CO₂ reduction was also realized with j_lim ≈ 15 mA cm^− 2 and E_1/2 = E°′_CO2 − 0.04 V at pH 6.6 and 60 °C in the presence of MV^2 +.     

Enhanced supercapacitive behaviors of birnessite

The birnessite type manganese dioxide electrode was prepared by the electrochemical stimulation as we recently described. It showed 190 F g⁻¹ in a Na₂SO₄ aqueous solution between −0.1 and 0.9 V versus Ag/AgCl at 1 A g⁻¹. The specific capacitance of birnessite was decreased by the manganese dissolution when the reduction and oxidation were repeated. By adding small amounts of Na₂HPO₄ or NaHCO₃ into the electrolyte, the capacitance increased to 200–230 F g⁻¹ and the manganese dissolution was successfully suppressed. Thanks to the additives, the birnessite demonstrated the much improved cycleability over >1800 cycles.     

Correlation between capacitances of porous carbons in acidic and aprotic EDLC electrolytes

A study based on a total of 41 nanoporous carbons shows that there exists a good correlation between the limiting gravimetric capacitances C_o at low current densities j (1 mA cm⁻²) measured in aprotic (1 M (C₂H₅)₄NBF₄ in acetonitrile) and in acidic (2 M aqueous H₂SO₄) electrolytes. The comparison of the surface-related capacitances (F m⁻²) of well characterized samples with the amount of thermodesorbed CO suggests a strong contribution of CO generating surface groups to charge storage in the acidic electrolyte, but a negligible contribution in the aprotic medium. It also appears that the decrease of the capacitance with current density is similar in both electrolytes. This confirms that the average micropore width and the CO₂ generating surface groups are the main factors which limit the ionic mobility in both electrolytes.     

Outstanding room-temperature capacitance of biomass-derived microporous carbons in ionic liquid electrolyte

A remarkable capacitance of 180 F·g^− 1 (at 5 mV·s^− 1) in solvent-free room-temperature ionic liquid electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was achieved in symmetric supercapacitors using microporous carbons with a specific surface area of ca. 2000 m²·g^− 1 calculated from gas sorption by the 2D-NLDFT method. The efficient capacitive charge storage was ascribed to textural properties: unlike most activated carbons, high specific surface area was made accessible to the bulky ions of the ionic liquid electrolyte thanks to micropores (1–2 nm) enabled by fine-tuning chemical activation. From the industrial perspective, a high volumetric capacitance of ca. 80 F·cm^− 3 was reached in neat ionic liquid due to the absence of mesopores. The use of microporous carbons from biomass waste represents an important advantage for large-scale production of high energy density supercapacitors.     

Influence of chemical composition of electrode material on the differential capacitance characteristics of the ionic liquid | electrode interface

The electrical double layer structure at polycrystalline metal | ionic liquid interface has been studied using cyclic voltammetry, electrochemical impedance spectroscopy and in situ infrared methods. Polycrystalline Bi(PC), Pb(PC), Au(PC) and Pt(PC) electrodes have been prepared using ultra-high vacuum magnetron sputtering method. Noticeable dependence of differential capacitance on the electrode potential has been observed. For all electrodes, a wide well-expressed minimum in capacitance, potential (C, E) curve has been shown. For graphene, C(0001), carbide-derived carbon and Bi(PC) U-shaped curves and for Pb(PC), Au(PC) and Pt(PC) M-shaped C, E curves have been measured. Dependence of the C, E curve shape on the electrode chemical composition has been explained by the different position of the image plane of surface charge, dependent on the electronic characteristics of the electrodes under study.     

Vibrational spectroscopic study of syngenite formed during the treatment of liquid manure with sulphuric acid

Syngenite (K₂Ca(SO₄)₂·H₂O), formed during treatment of manure with sulphuric acid, was studied by infrared, near-infrared (NIR) and Raman spectroscopy. C_s site symmetry was determined for the two sulphate groups in syngenite (P2₁/m), so all bands are both infrared and Raman active. The split ν₁ (two Raman+two infrared bands) was observed at 981 and 1000 cm⁻¹. The split ν₂ (four Raman+four infrared bands) was observed in the Raman spectrum at 424, 441, 471 and 491 cm⁻¹. In the infrared spectrum, only one band was observed at 439 cm⁻¹. From the split ν₃ (six Raman+six infrared) bands three 298 K Raman bands were observed at 1117, 1138 and 1166 cm⁻¹. Cooling to 77 K resulted in four bands at 1119, 1136, 1144 and 1167 cm⁻¹. In the infrared spectrum, five bands were observed at 1110, 1125, 1136, 1148 and 1193 cm⁻¹. From the split ν₄ (six infrared+six Raman bands) four bands were observed in the infrared spectrum at 604, 617, 644 and 657 cm⁻¹. The 298 K Raman spectrum showed one band at 641 cm⁻¹, while at 77 K four bands were observed at 607, 621, 634 and 643 cm⁻¹. Crystal water is observed in the infrared spectrum by the OH-liberation mode at 754 cm⁻¹, OH-bending mode at 1631 cm⁻¹, OH-stretching modes at 3248 (symmetric) and 3377 cm⁻¹ (antisymmetric) and a combination band at 3510 cm⁻¹ of the H-bonded OH-mode plus the OH-stretching mode. The near-infrared spectrum gave information about the crystal water resulting in overtone and combination bands of OH-liberation, OH-bending and OH-stretching modes.     

Electrochemical supercapacitor application of pervoskite thin films

We have explored electrochemically deposited pervoskite nanocrystalline porous bismuth iron oxide (BiFeO₃) thin film electrode from alkaline bath for electrochemical supercapacitors. The pervoskite BiFeO₃ nanocrystalline thin film electrode showed comparable specific capacitance of 81 F g⁻¹ and electrochemical supercapacitive performance and stability in an aqueous NaOH electrolyte to that of commonly used ruthenium based pervoskites.     

Cathodic carboxylation of gold in thick {Au-CO2}n layers. A model for reversible electrochemical sequestration of CO2

Catalytic reduction of CO₂ (saturated in organic polar solvents, e.g. N,N-dimethylfomamide, containing Me₄NX or NaBF₄) was achieved at smooth gold electrodes and at glassy carbon electrodes galvanostatically capped with a thin layer of gold. Under these quite explicit conditions, very sharp reduction steps were observed near − 1.5 V vs. Ag/AgCl. With small cations listed above, an unexpected behavior was observed, a progressive electrode inhibition occurring upon several scans or after a fixed-potential electrolysis at E < − 1.7 V. This phenomenon could be attributed to the insertion of CO₂ into gold, leading to the formation of a thick iono-metallic multi-strata layer (less conducting than pure metal) that grows with the electrode charge. The formation of this new interface is due to the concur of three elements: transient CO₂ anion radical, the metal, and rather small-sized cations (M⁺ = Na⁺ or TMA⁺), the three possibly associated in a form {Au-CO₂⁻,M⁺} apparently very reactive with oxygen, moisture, and with some organic π-acceptors. Upon multi-scans up to − 2.2 V, the thickness of formed layer progressively increases reaching more than 10^− 7 to 10^− 6 mol cm^− 2. Such multi-layers undergo decomposition in the anodic domain at about + 1.7 V liberating CO₂ beforehand trapped in Au. Coulometric analyses demonstrated that insertion (cathodic) and release (anodic) steps are quite equivalent, which permits to consider this process as chemically reversible sequestration of carbon dioxide.     

Electrochemical characterization on cobalt sulfide for electrochemical supercapacitors

High capacitance at a high charge–discharge current density of 50 mA/cm² for a new type of electrochemical supercapacitor cobalt sulfide (CoS_x) have been studied for the first time. The CoS_x was prepared by a very simply chemical precipitation method. The electrochemical capacitance performance of this compound was investigated by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge–discharge tests with a three-electrode system. The results show that CoS_x has excellent electrochemical capacitive characteristic with potential range −0.3 ∼ 0.35 V (versus SCE) in 6 M KOH solution. Charge–discharge behaviors have been observed with the highest specific capacitance values of 475 F/g at the current density of 5 mA/cm², even at the high current density of 50 mA/cm², CoS_x also shows the high specific capacitance values of 369 F/g.     

Development of micro-tubular SOFCs with an improved performance via nano-Ag impregnation for intermediate temperature operation

Micro-tubular solid-oxide fuel cell consisting of a 10-μm thick (ZrO₂)_0.89(Sc₂O₃)_0.1(CeO₂)_0.01 (ScSZ) electrolyte on a support NiO/(ScSZ) anode (1.8 mm diameter, 200 μm wall thickness) with a Ce_0.8Gd_0.2O_1.9 (GDC) buffer-layer and a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)/GDC functional cathode has been developed for intermediate temperature operation. The functional cathode was in situ formed by impregnating the well-dispersed nano-Ag particles into the porous LSCF/GDC layer using a citrate method. The cells yielded maximum power densities of 1.06 W cm⁻² (1.43 A cm⁻², 0.74 V), 0.98 W cm⁻² (1.78 A cm⁻², 0.55 V) and 0.49 W cm⁻² (1.44 A cm⁻², 0.34 V), at 650, 600 and 550 °C, respectively.     

Vibrational spectroscopic characterization of the phosphate mineral series eosphorite–childrenite–(Mn,Fe)Al(PO4)(OH)2·(H2O)

The phosphate mineral series eosphorite–childrenite–(Mn,Fe)Al(PO₄)(OH)₂·(H₂O) has been studied using a combination of electron probe analysis and vibrational spectroscopy. Eosphorite is the manganese rich mineral with lower iron content in comparison with the childrenite which has higher iron and lower manganese content. The determined formulae of the two studied minerals are: (Mn_0.72,Fe_0.13,Ca_0.01)(Al)_1.04(PO₄, OHPO₃)_1.07(OH_1.89,F_0.02)·0.94(H₂O) for SAA-090 and (Fe_0.49,Mn_0.35,Mg_0.06,Ca_0.04)(Al)_1.03(PO₄, OHPO₃)_1.05(OH)_1.90·0.95(H₂O) for SAA-072. Raman spectroscopy enabled the observation of bands at 970 cm⁻¹ and 1011 cm⁻¹ assigned to monohydrogen phosphate, phosphate and dihydrogen phosphate units. Differences are observed in the area of the peaks between the two eosphorite minerals. Raman bands at 562 cm⁻¹, 595 cm⁻¹, and 608 cm⁻¹ are assigned to the ν₄ bending modes of the PO₄, HPO₄ and H₂PO₄ units; Raman bands at 405 cm⁻¹, 427 cm⁻¹ and 466 cm⁻¹ are attributed to the ν₂ modes of these units. Raman bands of the hydroxyl and water stretching modes are observed. Vibrational spectroscopy enabled details of the molecular structure of the eosphorite mineral series to be determined.