• Energy Research

Abstract Electrochemical Impedance Spectroscopy (EIS) is a powerful technique to understand the electrode-electrolyte interaction and to evaluate degradation, resistive behaviour and electrochemical activity of energy storage materials used in batteries, pseudocapacitors and supercapacitors among others. However, it can sometimes be misused or under-interpreted. To effectively acquire EIS results, the voltages imposed to the working electrode at which EIS spectra are obtained, shall be critically selected. This work follows a previous study on the EIS response of Nickel-Cobalt hydroxide, and highlights how the Mott-Schottky model can be used as a complementary tool to explain EIS results obtained at different potentials. The Mott-Schottky model is used to understand further the fundamental processes occurring at the electrode-electrolyte interface of nickel-cobalt hydroxide in alkali media and to explain the changes in conductivity of the material that ultimately determine the electrode electrochemical activity. The applicability of the model to assist in the potential selection for EIS studies on other important charge storage materials such as MnOx and MoOx is discussed too.

Nickel-cobalt oxide is synthesized in combination with electrochemically reduced graphene oxide (Er-GO) by one-step electrodeposition on stainless steel followed by thermal treatment. The presence of reduced graphene oxide leads to enhanced electrochemical response, with a capacity increase from 113 mA h g−1 to 180 mA h g−1, and to increased faradaic efficiency and rate capability. Compared to Ni-Co oxide, the addition of reduced graphene oxide increases capacity retention from 58% to 83% after 5000 cycles. The material fade during cycling is studied by means of electrochemical impedance spectroscopy, electron diffraction spectroscopy and scanning electron microscopy. As a result, different degradation mechanisms are identified as source of the capacity decay, such as microstructural cracking, phase transformation and parasitic reactions.

This work is dedicated to the memory of Patrizia Paradiso. ; International audience ; Nickel–cobalt oxyhydroxide has been delaminated by tetrabutylammonium (TBA+) intercalation in aqueous media. The electrochemical performance of the different materials obtained during delamination has been evaluated, with special emphasis on the effect of the intercalated species and their influence on the behavior of the delaminated material, toward electrochemical energy storage applications. Delamination in TBAOH, reported for the first time for nickel–cobalt oxyhydroxide in aqueous media, is an excellent route to increase the number of electrochemically active sites and to enhance the capacity of nickel–cobalt oxyhydroxide. Results show an increase in capacity from 112 mA·h·g–1 in 1 M LiOH at 1 A·g–1 for the nondelaminated precursor to 165 mA·h·g–1 after exfoliation and restacking. Thus, carefully designed exfoliation is a promising route to enhance the performance of electrode materials for electrochemical energy storage.

The introduction of pillared agents or dopants to the graphene used as the electroactive material in supercapacitor electrodes can be an efficient way to facilitate ion transfer, mitigate re-stacking, and improve electrochemical performance. We evaluated the effect of different precursors containing nitrogen (N) and sulfur (S) atoms to dope graphene flake (GF) lattices. The electrochemical performance of the doped GF was assessed in 1 M KOH and 1 M Na2SO4 electrolytes. N- and S-doped GF flakes were synthesized via mechanochemical synthesis, also known as ball milling. After being ground, the materials were calcined under N2. The physicochemical characterization of the materials evidenced the co-doping of both S and N into the graphene backbone, as corroborated by the results of Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). As shown by the results, the nature of the precursors influences the ratio of S and N in the doped graphene flakes and, consequently, the response of the electroactive electrode material. The co-doping obtained using 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole revealed a specific capacitance of 48 F.g−1 at 1.0 A∙g−1 and over 90% capacitance retention after 10,000 cycles at 10.0 A∙g−1 in Na2SO4.

Abstract High power pseudocapacitors are extremely relevant to answer specific needs in the current energy transition arena and to implement an efficient renewable energy society. However, literature shows that are still open gaps concerning improvement of their energy density at high power, conversion efficiency, cost and cycle life. Electrodes based on active transition metal compounds, and in particular metal sulphides, evidence high potential to meet these objectives. This work discusses the dependence on the synthesis route of the charge storage mechanism of manganese sulphide-based materials and relates the pseudocapacitive response of these electrodes with their polycrystalline nature. Results reveal that a manganese oxy-sulphide mixture can achieve a high specific capacitance of 231 F.g−1 at 0.5 A/g in a 0.65 V active window. These values represent a 31.5 % increase compared to pure rambergite, γ-MnS, and 436 % compared to pure hausmannite Mn3O4 prepared under the same conditions. Moreover, the results show that manganese oxy-sulphide electrodes are characterized by good charge retention (73%), and superior long term capacity retention (above 86%) after 5000 cycles, evidencing potential for high power energy storage applications.

Hybrid capacitors have been developed to bridge the gap between batteries and ultracapacitors. These devices combine a capacitive electrode and a battery-like material to achieve high energy-density high power-density devices with good cycling stability. In the quest of improved electrochemical responses, several hybrid devices have been proposed. However, they are usually limited to bench-scale prototypes that would likely face severe challenges during a scaling up process. The present case study reports the production of a hybrid prototype consisting of commercial activated carbon and nickel-cobalt hydroxide, obtained by chemical co-precipitation, separated by means of polyolefin-based paper. Developed to power a 12 W LED light, these materials were assembled and characterized in a coin-cell configuration and stacked to increase device voltage. All the processes have been adapted and constrained to scalable conditions to ensure reliable production of a pre-commercial device. Important challenges and limitations of this process, from geometrical constraints to increased resistance, are reported alongside their impact and optimization on the final performance, stability, and metrics of the assembled prototype.

The electrochemical energy storage performance of activated carbons (ACs) obtained from coffee-derived biowastes was assessed. ACs were obtained from spent coffee ground second waste, after polyphenol extraction, by means of a hydrothermal process followed by physical or chemical activation. The resulting materials exhibited microporous structures with a total specific area between 585 and 2330 m2·g-1. Scanning electron microscopy (SEM) revealed a highly porous microstructure in the case of the chemically activated carbons, while physical activation led to a cracked micro-sized morphology. The electrochemical properties of the materials for supercapacitor applications were investigated in 1 M Na2SO4. After chemical activation, the coffee-derived material displayed a capacitance of 84 F·g-1 at 1 A·g-1 in a 1.9 V voltage window, with 70% capacitance retention at 10 A·g-1 and 85% retention after 5000 cycles of continuous charge-discharge. This work demonstrates how coffee secondary biowaste can be conveniently activated to perform as electrochemical energy storage material, contributing to its revalorization and reinsertion in a circular economy.