Mesoporous transition metal oxides for supercapacitors

• The study focuses on mesoporous transition metal oxides (TMO) as supercapacitor electrode materials.
• The molten salt assisted self-assembly (MASA) process facilitates synthesizing mesoporous TMOs
• Nickel manganese cobaltite (Ni0.5Mn0.5C02O4) is synthesized using MASA and is evaluated as an electrode material for asymmetric supercapacitors.
• The objective is to measure performance improvement in the Ni0.5Mn0.5C02O4 electrode and reveal the improvement mechanism.
• Electrochemical performance of NiCo2O4 and MnC02O4 prepared by MASA is investigated, to understand the effect of Ni and Mn in the same cobaltite.

Asymmetric Cell Preparation

• A full asymmetric cell is prepared using Ni0.5Mn0.5C02O4 and activated carbon, based on the charge balance theory.
• Specific capacitance values are 2.62F/cm² (92.1F/g) for mesoporous NiCo2O4, 0.26F/cm² (9.8F/g) for mesoporous MnCo2O4, and 9.53F/cm² (338.5F/g) for mesoporous Ni0.5Mn0.5C02O4 at 5 mA/cm².
• The asymmetric supercapacitor with Ni0.5Mn0.5C02O4 and activated carbon has an energy density of 79.52 Wh/kg at 1 mA/cm².
• Proper surface properties and electrochemical activity enhance the specific capacitance CA (Cs) when substituting the electrode with Ni0.5Mn0.5C02O4.

Background

• There is growing interest in affordable, high-energy density, and environmentally friendly energy storage devices.
• Electrochemical capacitors (supercapacitors) are promising options due to their ability to meet rapid energy demands.
• Electrical Double-Layer Capacitors (EDLCs) store energy through ion adsorption.
• Pseudocapacitors utilize rapid surface redox reactions at the electrode-electrolyte interface.
• Pseudocapacitive materials offer higher energy and power densities than symmetrical EDLCs.
• Electrolyte formulation and electrode materials enhance device performance, including energy density, power density, and cycle life.

Mesoporous Spinel Structures

• Mesoporous spinel structures have recently drawn great interest.
• These structures exhibit stability, high theoretical capacitance, low cost, easy synthesis, environmental friendliness, good electrical conductivity, and low toxicity.
• The mesoporous structure offers increased ion-accessible surface areas, maximizing the redox activity and specific capacitance
• Redox reactions are improved by increasing the contact area between the electrode and the electrolyte interface.
• Increased surface area facilitates efficient redox reactions, improving electrochemical performance during charge-discharge cycles.
• Transition metal oxides (TMOs) improve the performance of supercapacitors by improving transition metal oxides (TMOs) and increases the number of ion diffusion nodes in the active area.
• Dag et al. introduced a one-step synthesis method for mesoporous spinel NiC02O4 and MnC02O4 in 2021.
• This method involved using metal nitrate hexahydrate precursors assembled with non-ionic and ionic surfactants in their lyotropic liquid crystalline phase, followed by high-temperature calcinations
• The authors investigated mesoporous spinel metal cobaltite as an electrocatalyst for the oxygen evolution reaction (OER) in alkaline solutions.
• Molten salt assisted self-assembly (MASA) is a technique for producing complex materials that are challenging to create using conventional methods, combining two solvents and two surfactants to induce self-assembly.
• The first solvent uniformly mixes all ingredients, while the second solvent arranges the mixture into a Lyotropic Liquid Crystal (LLC) mesophase once the first solvent evaporates.
• The MASA method uses a non-ionic surfactant to aid in forming LLC phases and an ionic surfactant to enhance the salt content in the salt-surfactant LLC mesophases.
• A clear solution is spread over a substrate to form an ordered thin LLC film, then converted to a mesoporous thin film through high temperature calcination.
• This study introduces the first synthesis of mesoporous Ni0.5Mn0.5C02O4 via the MASA method, with exceptional surface area and enhanced redox behavior, resulting in remarkable energy storage performance.

Study Details

• The electrochemical properties of Ni0.5Mn0.5CO2O4 systematically investigated to uncover its inherent energy storage capabilities
• The study reveals synergistic effects and potential enhancements in energy storage performance resulting from this novel compositional interplay.
• Ni0.5Mn0.5C02O4 offers a substantial surface area for electrode/electrolyte interaction and introduces oxidation reduction step(s) from manganese sites, contributing to improved electrochemical behavior.

Synthesis of Mesoporous Metal Oxides

• To prepare Ni0.5Mn0.5C02O4, specific amounts of Ni(NO3)2.6H2O, Mn(NO3)2.6H2O, Co(NO3)2.6H2O, and C16H33N(CH3)3Br were dissolved in ethyl alcohol and stirred.
• C12H25(OCH2CH2)10OH, a non-ionic surfactant, was added to the solution, and the mixture was mixed.
• The solution was poured into a petri dish and evaporated overnight at 60 °C.
• The Petri dish was heated to 400 °C at a rate of one degree per minute and maintained for 180 min.
• The initial solution compositions of the other samples can be found in the Supplementary File (Table S1) with the synthetic route illustrated in Fig. 1.

Activated Carbon Synthesis

• Activated carbon was used as the negative active material, with preparation and characterization in the supplementary information document.
• An experiment was conducted using 35 mg of active carbon (AC), 3 mg of polyvinylidene fluoride (PVDF), and 3 mg of carbon black with N-methyl-2-pyrrolidone (NMP) until dispersed
• The appropriate amount was plastered onto the carbon foam surface and left overnight in an oven at 70 °C to remove excess NMP.
• The porosity of activated carbon holds significan

Preparation of Active Material Coatings

• Ni0.5Mn0.5C0204 (35 mg), PVDF (5 mg), and carbon black (3 mg) were mixed, with NMP added
• The slurry-like solution was dropped onto the current collector surface, and the nickel foam was left overnight at 70 °C to remove excess NMP.
• The drop-drying process was repeated when necessary to increase the amount of active material.
• Gel phases were observed using a ZEISS Axio Scope A1 polarizing optical microscope
• X-ray diffraction (XRD) patterns obtained using a Rigaku Miniflex diffractometer with a Cu Κα X-ray source (λ = 1.54056 Å).
• Nitrogen adsorption-desorption isotherms were measured with a Micromeritics Tristar 3000, ranging from 0.01-0.99P/Po with dehydration at 35-40 mTorr and 200 °C for 2 h.
• Scanning electron microscopy (SEM) images were taken using a FEI Quanta 200F microscope on aluminum holders.
• X-ray photoelectron spectroscopy (XPS) spectra were gathered using a Thermo Scientific K-a spectrometer with an Al Ka micro-focused monochromatic source, complemented by a flood gun for charge balancing.
• Fourier transform infrared (FTIR) spectroscopy was conducted on a Thermo Scientific Nicolet IS10 device.
• Raman spectroscopy was performed using a Renishaw in via microscope spectrometer, equipped with a 532 nm laser source.

Electrochemical Measurements

• All measurements were performed at room temperature.
• A saturated calomel reference electrode (SCE), a Pt wire auxiliary electrode, and metal oxide-coated Ni foam working electrodes used in the three-electrode measurement configuration.
• Cyclic voltammetry (CV) measurements were performed from -0.5 to 0.8 V for three cycles.
• The exposed area of the working electrode is 0.48 cm².
• The CV plots of the NiCo2O4, MnC02O4, and Ni0.5Mn0.5C0204 electrodes were recorded at various scan rates in a potential range of -0.5 to 0.8 V, where the third voltammogram was obtained for each specimen.

Impedance and GCD Measurements

• Electrochemical impedance spectroscopy (EIS) measurements were performed in the 100.000 Hz to 0.1 Hz frequency range with a 10-mV amplitude.
• GCD measurements were carried out for each electrode material in the 0-0.45 V range at various current densities, where the 5th cycle was used to calculate the CA and Cs values.

Asymmetric Supercapacitor Assembly

• Mesoporous metal oxide-coated Ni foam electrodes were used as positive electrodes
• active carbon-coated carbon foam electrodes were used as negative electrodes.
• The filter paper was immersed in a 6 M KOH solution for 60 s and subsequently uses as a separator.
• Full-cell supercapacitor tests were studied between -1.5 to 1.5 V via CV measurements.
• The EIS measurements of the two-electrode system were conducted.

Long Term Analysis

• 10,000 cycles of GCD tests performed for full cells with current densities of 1, 2, 4, 8, 20 mA cm-2.
• The areal specific capacitance (CA, F-cm-2) of the supercapacitor was calculated from the GCD curves using the equations (1) and (2).
• Before assembling the full cell, the mass ratio between the positive and negative active materials was calculated based on mass balance theory using equation (3) and (4).

Energy Calculation

• The energy density (E, Wh·kg¯¹) and power density (P, kW·kg¯¹) of the full cells were calculated.
• Clear and homogeneous solutions were prepared with a 6:1 M ratio of the total metal nitrate salts to the surfactant, drop-cast onto glass slides, and ethanol evaporation and gelation (mesophase formation) was monitored.

LLC Gels

• LLC gels displayed characteristic textures that indicate anisotropic LLC mesophases.
• Fresh samples displayed diffraction lines at small angles due to the ordered structure in the mesophase where the molten salt species resides in the hydrophilic domains, losing it's at 60 °C.
• After aging and calcining at 400 °C, sharp diffraction lines were observed in the high-angle XRD patterns, indicating the formation of NiC02O4, MnCo2O4, and Ni0.5Mn0.5C02O4 with a formation of NiO.

Ni0.5Mn0.5C02O4 Structure

• The XRD patterns of Ni0.5Mn0.5C02O4 displayed lines which were indexed to the planes, respectively, of the face-centered cubic (fcc) structure of MnC02O4
• NiCO2O4 has an inverse spinel structure, with all Ni ions at the octahedral sites and Co ions evenly distributed.
• The calcination process was controlled using FTIR spectroscopy which displayed peaks related to nitrates and surfactants before, and only metal oxide peaks after, calcination.
• XPS spectra of the O 1s, Co 2p, Ni 2p, and Mn 2p regions were obtained to evaluate the chemical nature and composition of the as-synthesized mesoporous cobaltite.
• Full-scan survey XPS spectra confirmed the formation of NiCo2O4, MnC02O4, and Ni0.5Mn0.5C02O4 where The peaks were attributed to the Ni. Co and Mn.
• Transmission electron microscope/ energy-dispersive X-ray spectroscopy (TEM/EDS) analysis exhibited nearly same elements distribution for Ni, Mn, and Co.
• Raman spectroscopy was performed to further understand the composition of these structures with manganese, cobalt and nickel oxide vibrational modes visible.
• N2 adsorption/desorption isotherms of calcined samples were collected and categorized.
• All three metal cobaltite samples have a mesoporous structure, and a BET surface areas were measured.
• SEM and TEM characterizations obtained morphological information about the samples which are spherical particles that occasionally coalesce to form larger particles.
• This configuration was implemented in a 6 M KOH solution.
• Cyclic voltammograms obtained for NiCo2O4, Ni0.5Mn0.5C02O4, and MnC02O4 electrodes shown.

Voltammetry Findings

• Mesoporous Ni0.5Mn0.5C02O4 has a large surface area and electroactive sites for pseudo-capacitive reactions.
• The CVs curves of the of the Ni0.5Mn0.5C02O4 and MnC02O4 electrodes, exhibited a peak from MnOOH/MnO2 oxidation.
• the Direct contact of Ni0.5Mn0.5C02O4 supports the fast transport of electrons.
• Galvanostatic charge-discharge curves for nickel cobaltite electrodes describe the faradaic reactions that occur in the electrochemical cell.
• Ni0.5Mn0.5C02O4 exhibited the highest charge and discharge capacities and the lowest discharge capacity was obtained from the MnC02O4 material
• The CA(Cs) values exhibited from all samples.
• A potential for capacitive contribution of Ni foam without any coating was investigated, with its contribution determined to be negligible.

Energy Densitry

• The Ni0.5Mn0.5C02O4 electrode had a high energy density.
• The Nyquist plots were used to compare the impedance behavior of the Ni0.5Mn0.5C02O4, NiCO2O4, and MnC02O4 active materials
• The Nyquist plots were also used to derive a charge configuration diagram.
• GCD tests were performed to determine the service properties of the different active materials as electrodes
• The energy storage behavior of the asymmetric supercapacitor is systematically investigated based on the GCD curves, with the asymmetric device exhibiting a maximum energy density of 79.52 Wh.kg¯¹ and power density at 20 mA cm-2 .
• The Nyquist plot of asymmetrical device demonstrates a low series internal resistance.

Device Analyzation

• A charge-discharge test was conducted to observe the long-term cycling to test cycling stability by calculating capacity tests and coloumbic efficiency.
• As seen from the charge-discharge curves for the first and last 5 cycles, the shape of the curves have been unobserved for changes.
• It is observed that there are slight decreases in the specific capacity with increasing cycle numbers.
• The cyclic voltammograms (half-cell measurements) confirm the stability of these materials.
• Moreover, after the test conducted, the cell exhibited approximately a certain retention and coloumbic efficiency.
• Results underscore a high energy density within the material.
• SEM analyses after long cycling cycle confirmed that there were structural integrities for an extended period of time

Conclusion

• MASA process was employed for the first-time synthesis of mesoporous Ni0.5Mn0.5C02O4.
• MASA method ensures the synthesis of mesoporous metal oxides by simply changing the LLC mesophase.
• Half-cell measurements showed the highest values for measurements in Ni0.5Mn0.5C02O4 with the charge transfer resistance.
• These results showed there electrochemical device applications.