Manganese-cobalt geomimetic materials for supercapacitor electrode.

A manganese-cobalt asbolane material synthesized by low-temperature cationic exchange from birnessite in cobalt nitrate solution has been comprehensively characterized and tested for the first time as a massive (with high active mass loading) positive electrode material for to asymmetric aqueous supercapacitors. The structure of this Mn-rich material, which is homologous to the natural asbolanes well known by mineralogists, consists of MnO2-type slabs with partial substitution of Co3+ for Mn; the slabs alternate with Co(OH)2 islands located in the interlayer spacing. This structural arrangement was confirmed through in-depth electronic transmission microscopy analyses, which reveal two interlocking hexagonal sublattices with distinct a lattice-cell parameters but identical c parameters. The electrochemical performance of this geomimetic phase in alkaline electrolytes is highly promising, with specific capacitance of up to 180 F g-1 at moderate current densities and 94 F g-1 at 10 A g-1. Investigation into the charge storage mechanisms indicates effective synergy between the pseudocapacitive properties of the MnO2 slabs and the Co(OH)2 islands, in which protonic conduction is suspected to play a key role. Additionally, long-term cycling and calendar aging tests suggest that the interlayer cobalt gradually migrates to the metal oxide layer upon cycling while maintaining excellent energy storage performance. This study clearly underscores the value of exploring geomimetic minerals as potential electrode materials for energy storage applications.

Kraft Black Liquor as a Carbonaceous Source for the Generation of Porous Monolithic Materials and Applications toward Hydrogen Adsorption and Ultrastable Supercapacitors.

High internal phase emulsions (HIPEs) have templated self-standing porous carbonaceous materials (carboHIPEs) while employing Kraft Black Liquor, a paper milling industry byproduct, as a carbon precursor source. As such, the starting emulsion has been prepared through a laboratory-made homogenizer, while native materials have been characterized at various length scales either with Raman spectrometry, X-ray diffraction (XRD), mercury intrusion porosimetry, and nitrogen absorption. After thermal carbonization, specific surface areas ranging from ∼600 m2 g-1 to 1500 m2 g-1 have been reached while maintaining a monolithic character. Despite a poor graphitization yield, the carbonaceous materials offer good electronic transport properties, reaching 31 S m-1. When tested toward energy storage applications, the native unwashed materials revealed a hydrogen storage of 0.07 wt % at 40 bar and room temperature (RT), while hydrogen retention is reaching 0.37 wt % at 40 bar and RT for the washed sample. When employed as supercapacitor electrodes, these carbonaceous foams are able to deliver high capacities of ∼140 F/g at 1 A/g, thereby matching the ones obtained from a commercial carbon reference, while additionally providing a restored remnant capacity of 120 F/g at 2 A/g over 5000 cycle numbers.

Composite Mn-Co electrode materials for supercapacitors: why the precursor's morphology matters!

In the energy storage field, an electrode material must possess both good ionic and electronic conductivities to perform well, especially when high power is needed. In this context, the development of composite electrode materials combining an electrochemically active and good ionic conductor phase with an electronic conductor appears as a perfectly adapted approach to generate a synergetic effect and optimize the energy storage performance. In this work, three layered MnO2 phases with various morphologies (veils, nanoplatelets and microplatelets) were combined with electronic conductor cobalt oxyhydroxides with different platelet sizes (∼20 nm vs. 70 nm wide), to synthesize 6 different composites by exfoliation and restacking processes. The influence of precursors' morphology on the distribution of the Mn and Co objects within the composites was carefully investigated and correlated with the electrochemical performance of the final restacked material. Overall, the best performing restacked composite was obtained by combining MnO2 possessing a veil morphology with the smallest cobalt oxyhydroxide nanoplatelets, leading to the most homogeneous distribution of the Mn and Co objects at the nanoscale. More generally, the aim of this work is to understand how the size and morphology of the precursor building blocks influence their distribution homogeneity within the final composite and to find the most compatible building blocks to reach a homogeneous distribution at the nanoscale.

Controlled Nanostructuration of Cobalt Oxyhydroxide Electrode Material for Hybrid Supercapacitors.

Nanostructuration is one of the most promising strategies to develop performant electrode materials for energy storage devices, such as hybrid supercapacitors. In this work, we studied the influence of precipitation medium and the use of a series of 1-alkyl-3-methylimidazolium bromide ionic liquids for the nanostructuration of ß(III) cobalt oxyhydroxides. Then, the effect of the nanostructuration and the impact of the different ionic liquids used during synthesis were investigated in terms of energy storage performances. First, we demonstrated that forward precipitation, in a cobalt-rich medium, leads to smaller particles with higher specific surface areas (SSA) and an enhanced mesoporosity. Introduction of ionic liquids (ILs) in the precipitation medium further strongly increased the specific surface area and the mesoporosity to achieve well-nanostructured materials with a very high SSA of 265 m2/g and porosity of 0.43 cm3/g. Additionally, we showed that ILs used as surfactant and template also functionalize the nanomaterial surface, leading to a beneficial synergy between the highly ionic conductive IL and the cobalt oxyhydroxide, which lowers the resistance charge transfer and improves the specific capacity. The nature of the ionic liquid had an important influence on the final electrochemical properties and the best performances were reached with the ionic liquid containing the longest alkyl chain.

Life cycle assessment of emerging Ni-Co hydroxide charge storage electrodes: impact of graphene oxide and synthesis route.

Decoupling energy supply from fossil fuels through electrification and sustainable energy management requires efficient and environmentally low-impact energy storage technologies. Potential candidates are charge storage electrodes that combine nickel and cobalt hydroxides with reduced graphene oxide (rGO) designed to achieve high-energy, high-power density and long cycling lifetimes. An early eco-efficiency analysis of these electrodes seeks to examine the impacts of materials and processes used in the synthesis, specifically while focusing on the use of rGO. The emerging electrodes synthesized by means of electrodeposition, are further compared with electrodes obtained by an alternative synthesis route involving co-precipitation. Life cycle assessment (LCA) method was applied to compare a baseline nickel-cobalt hydroxide electrode (NCED), the focal electrode integrating rGO (NCED-rGO), and the benchmark co-precipitated electrode (NCCP), for delivering the charge of 1000 mA h. Contribution analysis reveals that the main environmental hotspots in the synthesis of the NCED-rGO are the use of electricity for potentiostat, ethanol for cleaning, and rGO. Results of comparison show significantly better performance of NCED-rGO in comparison to NCED across all impact categories, suggesting that improved functionalities by addition of rGO outweigh added impacts of the use of material itself. NCED-rGO is more impactful than NCCP except for the indicators of cumulative energy demand, climate change, and fossil depletion. To produce a functional equivalent for the three electrodes, total cumulative energy use was estimated to be 78 W h for NCED, 25 W h for NCED-rGO, and 35 W h for NCCP. Sensitivity analysis explores the significance of rGO efficiency uptake on the relative comparison with NCCP, and potential impact of rGO on the category of freshwater ecotoxicity given absence of removal from the process effluent. Scenario analysis further shows relative performance of the electrodes at the range of alternative functional parameters of current density and lifetime. Lastly, the environmental performance of NCED-rGO electrodes is discussed in regard to technology readiness level and opportunities for design improvements.

Ionic liquids to monitor the nano-structuration and the surface functionalization of material electrodes: a proof of concept applied to cobalt oxyhydroxide.

This paper reports on an innovative and efficient approach based on the use of ionic liquids to govern the nano-structuration of HCoO2, in order to optimize the porosity and enhance the ionic diffusion through the electrode materials. In this work, we show that (1-pentyl-3-methyl-imidazolium bromide (PMIMBr) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)) ionic liquids (ILs) used as templates during the synthesis orientate the nanoparticle aggregation which leads to increase of the porosity and the average pore size of the electrode material. It is also demonstrated that the ILs are strongly bonded to the HCoO2 surface, leading to surface functionalized HCoO2 materials, also called nanohybrids. This surface tailoring stabilizes the material upon cycling and shifts the oxidation potential linked to the Co(iii)/Co(iv) redox couple to lower voltage in an alkaline 5 M KOH electrolyte. The surface and porosity optimizations facilitate the ionic diffusion through the material, improve the electron transfer ability within the electrode and lead to greatly enhanced specific capacity in both alkaline 5 M-KOH and neutral 0.5 M-K2SO4 aqueous electrolytes (66.7 mA h g-1 and 47.5 mA h g-1 respectively for HCoO2-PMIMBr and HCoO2-EMIMBF4 compared to 18.1 mA h g-1 for bare HCoO2 in 5 M-KOH at 1 A g-1).

Prediction of Isoelectric Point of Manganese and Cobalt Lamellar Oxides: Application to Controlled Synthesis of Mixed Oxides.

To design novel layered materials, bottom-up strategy is very promising. It consists of (1) synthesizing various layered oxides, (2) exfoliating them, then (3) restacking them in a controlled way. The last step is based on electrostatic interactions between different layered oxides and is difficult to control. The aim of this study is to facilitate this step by predicting the isoelectric point (IEP) of exfoliated materials. The Multisite Complexation model (MUSIC) was used for this objective and was shown to be able to predict IEP from the mean oxidation state of the metal in the (hydr)oxides, as the main parameter. Moreover, the effect of exfoliation on IEP has also been calculated. Starting from platelets with a high basal surface area over total surface area, we show that the exfoliation process has no impact on calculated IEP value, as verified with experiments. Moreover, the restacked materials containing different monometallic (hydr)oxide layers also have an IEP consistent with values calculated with the model. This study proves that MUSIC model is a useful tool to predict IEP of various complex metal oxides and hydroxides.