Electrochemical cyclability mechanism for MnO2 electrodes utilized as electrochemical supercapacitors

The electrochemical cyclability mechanism of nanocrystalline MnO₂ electrodes with rock salt-type and hexagonal ?-type structures was investigated to determine the relationship between physicochemical feature evolution and the corresponding electrochemical behaviour of MnO₂ electrodes. Rock salt MnO₂ and hexagonal ?-MnO₂ electrodes, with fibrous and porous morphologies, evolve into the antifluorite-type MnO₂ with a petal-shaped nanosheet structure after electrochemical cycling, similar to that observed in nanocrystalline antifluorite-type MnO₂ electrodes after electrochemical cycling. However, a different impedance response was observed for the rock salt MnO₂ and hexagonal ?-MnO₂ electrodes during the charge–discharge cycles, compared with the improved impedance response observed for the cycled antifluorite-type MnO₂. A dissolution–redeposition mechanism is proposed to account for the impedance response of the MnO₂ electrodes with different morphologies and crystal structures.     

Improving hydrogen evolution reaction and capacitive properties on CoS/MoS2 decorated carbon fibers

We report a facile method to transform abundantly dumped banana stem fibers into carbon fibers (CFs) useful for energy applications. The CFs surface area is increased by varying the quantity of KOH activation to 488 m²g^-1. The solvothermal method is used to synthesize CoS, CoS/MoS₂ and also grown on the activated carbon fibers (ACFs). Nano nodules of CoS arranged into sheets and layers of MoS₂ stacked together were found in FESEM analysis. The morphology of the CoS/MoS₂ differs when grown on ACFs. The growth of CoS/MoS₂ along the ACFs length prevents any stacking of the pseudocapacitance materials. The ternary composite ACFs/CoS/MoS₂ exhibits superior supercapacitor behavior as well as hydrogen evolution reaction (HER) due to the synergetic effect of the conducting ACF surface and redox active CoS/MoS₂. A maximum specific capacitance of 733 Fg^-1, energy and power density of 33 WhKg⁻¹ and 999 WKg^-1 respectively are obtained. A low Tafel slope value of 61 mVdec⁻¹ is obtained for the ACFs/CoS/MoS₂ ternary composite electrode. The present work therefore offers a fresh insight into the effective conversion of waste materials into electrode material for energy storage and conversion applications.     

Electrochemical behaviour of single walled carbon nanotubes – Hydrogen storage and hydrogen evolution reaction

The electrochemical behaviour of single walled carbon nanotubes (SWCNT) related to the mechanism involved in the hydrogen electrode reaction applying electrochemical and spectroscopic techniques is studied. Cyclic voltammetry applied to electrodes containing different percentages of SWCNT demonstrates that this material can behave as efficient capacitor and that the hydrogen electrode reaction develops through the H-electrosorption followed by the formation of molecular H₂ and its evolution. Also, SWCNT are able to storage hydrogen within their porous structure. This is confirmed through the galvanostatic charge and discharge experiments. Electrochemical impedance spectroscopy allowed calculating the real area that takes part in the electrode reaction and the main and valuable conclusion is that the hydrogen electrode reaction consists of a simple charge transfer reaction and that the H adatom relaxation or diffusion processes can be disregarded. Furthermore, a model proposed for their structure which was validated through impedance experiments confirms those conclusions. Results of Raman spectra allowed identifying the nature of the electrodes confirming that after purification the material is composed of single walled carbon nanotubes.     

Effects of tetrabutoxytitanium on photoelectrochemical properties of plastic-based TiO2 film electrodes for flexible dye-sensitized solar cells

The flexible DSSCs based on conducting plastic substrates are fabricated using electrodes made of tetrabutoxytitanium (TBOT) mixed with P25 TiO₂ nanoparticles at low temperature. To investigate the effects of TBOT on the flexible dye-sensitized solar cells, electrochemical impedance spectroscopy (EIS) is performed in the dark and under illumination conditions. Resistances for electron transport through TiO₂, charge-transfer resistance related to the TiO₂/redox electrolytes interface recombination, electron transport time and electron lifetime are quantified under different weight ratios of TBOT/P25. Additionally, the photovoltaic characteristics I-V curves and incident photon to current conversion efficiencies (IPCE) of flexible anodes made of different weight ratios of TBOT/P25 are obtained as well. It is found that the electrode under weight ratio 0.17 has the smallest inherent resistance, longest electron transport time and electron lifetime, lowest recombination rate and best performance with conversion efficiency 3.94%. These results indicate that after the weight ratios of TBOT/P25 is optimized, TBOT could enhance the interconnection between the TiO₂ particles, improve the conductivity of the electrode and decrease the charge recombination. Above results demonstrate that adding TBOT to TiO₂ is an easy and efficient method to improve the performance of the flexible DSSC fabricated at low temperature.     

Electrochemical investigation of polarization phenomena and intercalation kinetics of oxidized graphite electrodes coated with evaporated metal layers

The electrochemical behavior of partially oxidized graphite electrodes coated with 50 Å thick Au, Cu, In, Pb or Sn layers has been studied by slow scan rate cyclic voltammetry and galvanostatic charge–discharge. Electrochemical impedance spectroscopy (EIS) has also been applied to Cu- and Sn-coated electrodes in order to study the effect of the metal coating on the interfacial intercalation/deintercalation kinetics.

The results demonstrate that certain metallic layers produce remarkable improvements of intercalation kinetics of graphite electrodes by reducing the charge-transfer and the solid–electrolyte interface (SEI) resistance making this type of surface modification attractive for the development of high rate anodes for lithium-ion batteries.     

Efficient and stable electrocatalyst for hydrogen evolution reaction prepared by hybrid technique in surface engineering: Electrochemical and magnetron sputtering methods

Developing low-cost, stable, and robust electrocatalysts is significant for high effective hydrogen evolution reaction (HER). In this work, a coating system with Cu₂O/NiMoCu on stainless steel (SS) is employed as a highly active and stable catalyst for HER in acidic solutions. Electrochemical measurements for as-designed system on SS show a low onset overpotential, small Tafel slope of ～32 mV/decade and long-term durability over 7 days of HER operation. To further inspections of electrocatalytic behavior of as-prepared system in HER, the EIS measurements are performed at several overpotentials and temperatures. It is found that high hydrogen evolution activity and stability of Cu₂O/NiMoCu hybrid is likely due to special morphology of Cu₂O which result in large number of active sites for hydrogen adsorption, and a synergetic effect giving electronic structure suitable for the HER.     

Electrochemical activity and stability of Pt catalysts on carbon nanotube/carbon paper composite electrodes

This article reports a facile microwave-assisted approach to synthesize Pt catalysts on carbon nanotube (CNT)/carbon paper (CP) composite through catalytic chemical vapor deposition. The Pt deposits, with an average size of 3–5 nm were uniformly coated over the surface of oxidized CNTs. The electrochemical activity and stability of the Pt–CNT/CP electrode were investigated in 1 M H₂SO₄ using cyclic voltammetry (CV) and ac electrochemical impedance spectroscopy. The Pt catalysts showed not only fairly good electrochemical activity (electrochemically active surface area) but also durability after a potential cycling of >1000 cycles. The analysis of ac impedance spectra associated with equivalent circuit revealed that the presence of CNTs significantly reduced both connect and charge transfer resistances, leading to a low equivalent series resistance ˜0.22 Ω. With the aid of CNTs, well-dispersed Pt catalysts enable the reversibly rapid redox kinetic since electron transport efficiently passes through a one-dimensional pathway. Thus, the CNTs do not only serve as carbon support, they also charge transfer media between the Pt catalysts and the gas diffusion layer. The results shed some light on the use of CNT/CP composite, offering a promising tool for evaluating high-performance gas diffusion electrodes.     

Electrochemical behavior of the carbon black Vulcan XC-72R: Influence of the surface chemistry

The effect of chemical surface oxidation of the carbon black Vulcan XC-72R on the capacitance, tolerance to corrosion and electrochemical activity toward the hydrogen evolution reaction (HER) has been studied in 0.1 M NaHCO₃. Acid treatments with HNO₃ or a HNO₃H₂SO₄ mixture resulted in a progressive introduction of oxygen-containing groups which led to a strong increase of the capacitance and a higher tendency to carbon corrosion. In contrast, an inhibition of the hydrogen evolution current (at potentials more negative than ?1.0 V) was observed for oxidized samples in comparison to the un-treated material. HER was also tested in the presence of dissolved CO₂ to study the influence of the surface chemistry on the CO₂ electroreduction. An inhibition of the H₂ evolution current was evidenced in the CO₂ saturated electrolyte due to the adsorption of species from CO₂ reduction. A strong hydrogen current decrease (65–78%), and thus a higher tendency to adsorb (CO₂)_red species, was obtained on the original Vulcan and the HNO₃-treated samples, in comparison to the carbon oxidized with HNO₃H₂SO₄ (15%), which could be related to the highest content of basic groups of the last carbon.     

Electrochemical micro-capacitors of patterned electrodes loaded with manganese oxide and carbon nanotubes

MnO₂ and carbon nanotubes (CNT) composite electrodes have been built on the interdigital stack layers of Fe-Al/SiO₂ and Fe-Al/Au/Ti/SiO₂ for the electrochemical micro-capacitors, using photolithography and thin-film technologies. The electrode properties and the performance of micro-cells are measured and analyzed with cyclic voltammetry (CV), impedance spectroscopy, and galvanostatic charge/discharge test in 0.1 M Na₂SO₄ electrolyte. The vertically aligned CNT, grown on Fe-Al/SiO₂, is more suitable for supporting the pseudocapacitive MnO₂ than the random CNT on Fe-Al/Au/Ti/SiO₂, but ohmic resistance of the former electrode is higher. We have prepared three cells on each stack layer with different electrode materials. The Ragone plot shows systematic variations in power and energy performance, reflecting their differences in electrode structure and polarization loss. The asymmetric cell of a pseudocapacitive positive electrode, loaded with MnO₂ and CNT, exhibits a small IR drop and a high specific energy during discharge. Built on Fe-Al/SiO₂, this asymmetric cell discharges at specific power 0.96 kW kg⁻¹ with specific energy 10.3 Wh kg⁻¹; while on Fe-Al/Au/Ti/SiO₂, the asymmetric cell discharges at power 1.16 kW kg⁻¹ with energy 5.71 Wh kg⁻¹.     

Preparation and characterization of platinum/iron contained hydroxyapatite/carbon black composites

In this report, calcium ions in the porous hydroxyapatite (HAp) microspheres are partially exchanged with ferrous ions to form iron contained hydroxyapatite (FeHAp) on which Pt ions in H₂PtCl₆ solution are reduced to form Pt/FeHAp catalyst and finally mixed with carbon blacks to derive Pt/FeHAp/C catalysts. They exhibit the characteristics of Pt (1 1 0) facet with a sharp desorption peak at −0.109 V (vs. Ag/AgCl), the electrochemical surface area (ECSA) ranging from 73 to 224 m² g⁻¹ with little CO poisoning effect on Pt, and the mass activity ranging from 6.88 to 28.99 A g_Pt⁻¹ in methanol oxidation reaction (MOR) at 0.4 V (vs. Ag/AgCl). Besides, Pt/FeHAp reveals the lower onset potential in CO-stripping than Pt/C. These better performances of Pt/FeHAp/C catalysts, compared with Pt/C, are also related to the Pt (1 1 0) facet, the content of Fe, and the coexistence of Pt⁰ and Pt²⁺ in Pt/FeHAp.     

Carbon xerogels as electrochemical supercapacitors. Relation between impedance physicochemical parameters and electrochemical behaviour

The electrochemical behavior of carbon xerogels was studied with the aim of analyzing the performance of the materials used as electrochemical supercapacitors (SC) and to relate with physicochemical parameters. These materials have areas involving 1500–2000 m²/g measured with the BET equation and a range of pore size distributions.     

Electrochemical hydrogen storage properties of ball-milled multi-wall carbon nanotubes

The structure changes of multi-wall carbon nanotubes (MWNTs) processed by mechanical ball milling and the influence on their electrochemical hydrogen storage capacities were studied. TEM micrographs show that MWNTs are shortened and open-ended after ball milling. The effects of different MWNT type and ball milling time on the discharging capacity were investigated. Among all the samples examined, the sample of short MWNTs with diameter of 5 nm and ball milling time of 12 h has the largest discharge capacity (741.1 mAh/g). According to the analysis of Raman spectra and nitrogen adsorption experiments, it can be inferred that the micropore volume, specific surface area and appropriate defects are crucial to the storage capacity. In the cyclic voltammograms, the hydrogen desorption peak appears prior to hydrogen oxidation peak, which is attributed to the slow reaction of hydrogen oxidation at MWNTs. The results also suggest the possible existence of the strong chemisorption of hydrogen.     

Electrochemical properties of manganese oxide coated onto carbon nanotubes for energy-storage applications

Birnessite-type manganese dioxide (MnO₂) is coated uniformly on carbon nanotubes (CNTs) by employing a spontaneous direct redox reaction between the CNTs and permanganate ions (MnO₄⁻). The initial specific capacitance of the MnO₂/CNT nanocomposite in an organic electrolyte at a large current density of 1 A g⁻¹ is 250 F g⁻¹. This is equivalent to 139 mAh g⁻¹ based on the total weight of the electrode material that includes the electroactive material, conducting agent and binder. The specific capacitance of the MnO₂ in the MnO₂/CNT nanocomposite is as high as 580 F g⁻¹ (320 mAh g⁻¹), indicating excellent electrochemical utilization of the MnO₂. The addition of CNTs as a conducting agent improves the high-rate capability of the MnO₂/CNT nanocomposite considerably. The in situ X-ray absorption near-edge structure (XANES) shows improvement in the structural and electrochemical reversibility of the MnO₂/CNT nanocomposite after heat-treatment.     

Comparison of multiwalled carbon nanotubes and carbon black as percolative paths in aqueous-based natural graphite negative electrodes with high-rate capability for lithium-ion batteries

The effects of multiwalled carbon nanotubes (MWNTs) and carbon black (CB) as conducting additives on the rate capability of natural graphite negative electrodes in lithium-ion (Li-ion) batteries is investigated within concentration ranges where no degradation of anode capacity is observed. MWNT or CB solutions prepared with Nafion in an 80:20 volume mixture of water:1-propanol are incorporated into graphite precursor suspensions consisting of graphite particulates, carboxymethyl cellulose, and styrene butadiene rubber prepared in an aqueous medium. While negative electrodes with MWNTs demonstrate much better rate behaviour than those without MWNTs at a high C-rate, the rate capability of negative electrodes with MWNTs is not much different from that with CB. The reason for this similar behaviour is investigated with respect to the structural changes and aspect ratio of MWNTs, as well as the density difference between MWNTs and carbon black. Scanning electron microscopy images and Raman spectra for the dispersed MWNTs indicate that MWNTs are significantly damaged and shortened during dispersion, which reduces their electrical conductivity and increases their percolation threshold. This damage negatively affects the rate capability of graphite-nanotube composite electrodes.     

Ageing behaviour of electrochemical double layer capacitors: Part I. Experimental study and ageing model

Different types of commercially available electrochemical double layer capacitors (EDLCs) were analysed in accelerated ageing tests by impedance spectroscopy. From these measurements the parameters of an impedance model were determined. The characteristic change of the impedance parameters is discussed and an ageing model for EDLCs is developed.     

Significantly enhanced charge conduction in electric double layer capacitors using carbon nanotube-grafted activated carbon electrodes

Carbon nanotube (CNT)-grafting by chemical vapor deposition was conducted to reduce the resistance of activated carbon fiber serving as an electrode for electric double layer capacitors. Sputtering deposition of Ni catalyst particles led to a uniform growth of CNTs on the carbon fiber surface through the tip-growth mechanism. Because sputtering deposition ensures little pore blockage (in comparison with wet-impregnation), the surface area decrease of the carbon fiber due to Ni loading was minimized. By using H₂SO₄ aqueous solution as the electrolyte, a capacitor cell assembled with the CNT-grafted fiber showed higher electron and electrolyte-ion conductivities relative to a cell assembled with the bare fiber. By increasing the discharging current density from 1 to 150 mA cm⁻², the bare fiber exhibited a capacitance loss of 17% while the CNT-grafted fiber showed a mitigated capacitance loss of only 7%. This developed CNT-grafting technique renders activated carbon fiber a promising electrode material for a variety of electrochemical applications.     

Electrochemical activity and durability of Pt-Sn alloys on carbon-based electrodes prepared by microwave-assisted synthesis

The microwave-assisted polyol (MP) reduction was used as the deposition technique for preparing binary Pt-Sn alloys with a mean size of 5 nm on carbon nanotubes (CNTs). A large number of CNTs with an average diameter of 40-50 nm were attached to carbon paper substrate using a catalytic chemical vapor deposition. The fast MP synthesis enabled the formation of Pt-Sn nanoalloys within 6 min. X-ray diffraction and X-ray photoelectron spectroscopy were applied to analyze the crystalline alloy structure. Postcalcination at 600 °C showed a positive effect in improving the activity and the long-term durability (i.e., 1000 cycles) of Pt-Sn catalysts toward the H₂SO₄ electrolyte, as demonstrated by cyclic voltammetry test and AC impedance spectroscopy. An equivalent circuit also was proposed to analyze the equivalent serial resistance of the electrocatalysts after the potential cycling. This improvement can be attributed to the reasons that the calcination process is prone to the following: (i) the reinforcement of the adherence between the catalyst particle and the carbon support, (ii) the transformation of crystalline phase on Pt-Sn alloy, and (iii) the decrease of Pt (II) content and chemisorbed oxygen species. The Pt-Sn crystallographic phase on the CNT-based support shows an enhanced activity and more corrosion resistance in acidic electrolyte.     

Enhanced membrane electrode assembly performance by adding PTFE/Carbon black for high temperature polymer electrolyte membrane fuel cell

Phosphoric acid used as a proton-conductive medium in high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs) poisons the Pt surface and prevents oxygen transport in the cathode catalyst layer. The hydrophobic binders in the catalyst not only maintain the catalyst layer structure but also control the phosphoric acid distribution. In this study, polytetrafluoroethylene (PTFE)/carbon black (Vulcan XC-72R) added to the catalyst layer generates an oxygen transport channel. The catalyst layers coated on the gas diffusion layer by the bar-coating method serve as the cathode. High PTFE content causes hydrophobicity in the catalyst layer. The membrane electrode assembly (MEA) with 6 wt% PTFE/Vulcan results in the highest peak power density (0.347 W cm⁻²) and voltage (0.653 V) at 0.2 A cm⁻². A critical reason for its high performance is having the lowest R_ct + R_mt values measured at 0.6 V and 0.4 V. These results could contribute to improving the MEA performance for HT-PEMFCs.     

Bimetallic Pt-M electrocatalysts supported on single-wall carbon nanotubes for hydrogen and methanol electrooxidation in fuel cells applications

A series of PtRu and PtMo bimetallic catalysts were prepared via a chemical reduction method by bubbling CO to form carbonyl compounds as metal precursors. In both cases the PtRu and PtMo bimetallic electrocatalysts achieved the maximum activity when the amount of Ru and Mo in the material was 50%wt. The physicochemical characterization of the electrocatalytic materials through X-ray diffraction (XRD) and transmission electron microscopy (TEM) has determined the presence of bimetallic structures. The electrochemical characterization using cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and polarization curves in Proton Exchange Membrane Fuel Cells (PEMFC) and Direct Methanol Fuel Cell (DMFC) allowed to systematically investigate the electrocatalytic activity of the synthesized materials for the electrooxidation of hydrogen and methanol. The PtRu/SWCNT electrocatalysts showed a higher current density at least 7-fold and 3-fold compared with Pt/SWCNT and PtMo/SWCNT electrocatalysts, respectively. Besides, the Pt50%–Ru50%/SWCNT exhibited a shifting to negative values in the onset potential reaction for the electrooxidation of methanol of 200 mV in comparison with Pt100%/SWCNT and Pt50%–Mo50%/SWCNT electrocatalysts. The experimental and simulated polarization curves obtained from DMFC show that PtRu/SWCNT and PtMo/SWCNT electrocatalysts exhibited higher power and current densities values compared with the Pt/SWCNT electrocatalyst. The membrane-electrode assembly (MEA) with Nafion^® and the PtRu/SWCNT electrocatalysts showed an open-circuit voltage value of 0.730 V, significantly higher than that the values for the MEAs with Pt/SWCNT (0.663 V) and PtMo/SWCNT (0.633 V), respectively.