Catalyst-free soft-template synthesis of ordered mesoporous carbon tailored using phloroglucinol/ glyoxylic acid environmentally friendly precursors

International audienceCarbon porous materials with a periodically ordered pore structure, controlled pore size and geometry and high thermal stability are synthesized using self-assembly of environmentally friendly phloroglucinol/ glyoxylic acid precursors with an amphiphilic triblock copolymer template. Glyoxylic acid, a plant-derived compound, is used for the first time as a substituent of carcinogen formaldehyde usually employed in such a synthesis. Thanks to the double functionality, i.e., aldehyde and carboxylic acid, glyoxylic acid plays not only the role of a cross-linker for the formation of the resin but also the role of a catalyst by creation of H-bonding or specific reactions between the precursors. Hence, no extra catalyst such as strong acids (HCl) or bases (NaOH) is any longer required. Carbon films and powders were successfully prepared with high surface areas (up to 800 m2 g−1), high porous volume (up to 1 cm3 g−1), tunable pore size (0.6 nm to 7 nm) and various pore architectures (hexagonal, cubic, and ink-bottle) by tuning the precursor ratio and by applying different manufacturing engineering strategies. Insights on the synthesis mechanism of the phenolic resin and carbon mesostructures were obtained using several analysis techniques, i.e., nuclear magnetic resonance (13C NMR) and FTIR spectroscopy, temperature programmed desorption coupled with mass spectrometry (TPD-MS) and thermo-gravimetric analysis (TGA)

Insights on the Na+ ion storage mechanism in hard carbon: Discrimination between the porosity, surface functional groups and defects

Sodium ion batteries (SIBs) using hard carbon as negative electrode hold the promise of being low cost alternative to lithium ion batteries (LiBs). However, the Na+ storage mechanism in hard carbons is not fully understood yet and the attribution of Na storage in the sloping and plateau regions of the sodiation/desodiation curves remains still controversial. The current work employs N-2, Kr and CO2 gases to correctly assess the changes in hard carbon porosity induced by different pyrolysis temperature of cellulose. The sloping capacity was found to decrease with the decrease of the specific area of ultramicropores measurable only by CO2 adsorption, while the plateau capacity demonstrated an opposite behavior. The high temperature derived carbons (> 1400 degrees C) present no porosity which disqualifies the attribution of plateau region to the adsorption of Na+ in the nanopores but rather the insertion between the pseudo-graphitic domains. Temperature programmed desorption coupled with mass spectrometry (TPD-MS) was performed to determine the nature and the quantity of oxygen surface functional groups followed by oxygen chemisorptions to assess the amount of carbon edge defects expressed by active surface area (ASA) values. A decrease in the amount of oxygen groups and active surface area with the increase of the pyrolysis temperature was observed which is accompanied by a decrease of the sloping capacity. These results shed light in the storage mechanisms, the sloping region being ascribed mainly to the interaction of Na+ with carbon edge defects and adsorption in the microporosity while the plateau region assigned to the intercalation of Na+ in the pseudo-graphitic nanodomains

Relationship between the carbon nano-onions (CNOs) surface chemistry/defects and their capacitance in aqueous and organic electrolytes

The effect of surface functionalities on the supercapacitors performances has been highlighted often in many works. However, studies devoted to the influence of carbon defects did not gain particular attention due to the difficulty to quantify such parameter. In this context, carbon nano-onions were used as model material in order to understand the influence of the surface chemistry (nature and amount of oxygen groups) and structural defects (active surface area, ASA) on the capacitance. Different types of thermal treatments in oxidizing or reducing atmospheres allowed to finely tune the surface chemistry and the ASA as demonstrated by temperature programmed desorption coupled with mass spectrometry (TPD-MS). For the first time, the presice control of these characteristics independently one of each other allowed to highlight an important influence of the carbon defects on the capacitance in organic and aqueous electrolytes which outbalance the oxygen functional group effect

Oxygen Reduction Reaction Promotes Li+ Desorption from Cathode Surface in Li-O2 Batteries

Li-O2 batteries are claimed to be one of the future energy storage technologies. Great number of scientific and technological challenges should be solved first to transform Li-O2 battery from a promise to real practical devices. Proposed mechanisms for oxygen reduction assume a reservoir of solved Li+ ions in the electrolyte. However, the role that adsorbed Li+ on the electrode surface might have on the overall oxygen reduction reaction (ORR) has not deserved much attention. Adsorbed Li+ consumption is monitored here using impedance measurements from extended electrochemical double layer capacitance, which depends on the carbon matrix surface area. The presence of O2 drastically reduces the amount of adsorbed Li+, signaling the kinetic competition between Li+ surface adsorption and its consumption, only for potentials corresponding to the oxygen reduction reaction. Noticeably double layer capacitance remains unaltered after cycling. This fact suggests that the ORR products (Li2O2 and Li2CO3) are not covering the internal electrode surface, but deposited on the outer electrode-contact interface, hindering thereby the subsequent reaction. Current results show new insights into the discharge mechanism of Li-O2 batteries and reveal the evidence of Li+ desorption from the C surface when the ORR starts.We thank financial support from Generalitat Valenciana (ISIC/ 2012/008 Institute of Nanotechnologies for Clean Energies). The authors acknowledge Dr. Conchi Ania from National Institute of Carbon for providing PSCo and AG carbons and their characterization

Recent advances in understanding the capacitive storage in microporous carbons

This article presents a review of our recent work on capacitance of carbide-derived carbons. Specific capacitance as high as 14 &F/cm² or 160 F/g was achieved using carbide derived carbons with tailored subnanometer pore size, which is significantly higher than 6 &F/cm² or 100 F/g) for conventional activated carbons. Such high capacitance was obtained in several types of organic electrolytes with or without solvent. A maximum is obtained for the carbons with the mean pore size close to the bare ion size, ruling out the traditional point of view that mesoporosity is highly required for maximum capacitance. Surprisingly, carbons with subnanometer porosity exhibit high capacitance retention, since only a 10% loss is measured when 6 A/g discharge is drawn. These findings show the importance of fitting the ion size with the mean pore size. The double layer theory falls short to explain such charge storage mechanisms at the nanometer scale, thus atomistic modeling is required to find out an alternative charge storage model

Tuning the porous texture and specific surface area of nanoporous carbons for supercapacitor electrodes by adjusting the hydrothermal synthesis temperature

10.1039/c3ta12649hJournal of Materials Chemistry A14112962-1297

Synergistic relationship between the three-dimensional nanostructure and electrochemical performance in biocarbon supercapacitor electrode materials

A novel study presented herein correlates the multidimensional morphology with the electrochemical performance of activated bio-carbon materials, for supercapacitor devices over multiple length scales. The optimization of the potassium hydroxide (KOH)/cellulose ratio for supercapacitor electrode materials is related to morphological characteristics and corresponding electrochemical performance, as described in terms of porosity, specific surface area, specific capacitance and electrochemical impedance. KOH/cellulose samples with ratios 0.5 : 1 and 1 : 1 exhibited the best performance, characterized by a hierarchal porous network structure, high surface area and low cell resistance. Compared with the rest of the manufactured samples and commercial activated carbons, Ketjen Black (KB), Norit activated carbon (NAC) and bead-shaped activated carbon (BAC), the former two samples showed better results in three-electrode systems and coin cells, with specific gravimetric capacitances as high as 187 F g−1 at a current density of 1 A g−1. The high performance is attributed to the morphology of the samples that constituted a combination of micro-, meso- and macroporosity which consequently gave high specific surface area, high porosity, low cell resistance and high specific capacitance. This further corroborates the structure-performance relationship observed in the author's model KOH/cellulose system, highlighting that the work can be extended to other similar systems. It is clear that the three-dimensional nanostructure of a material must be understood in its entirety in order to optimize the electrochemical performance