One-step synthesis of highly reduced graphene hydrogels for high power supercapacitor applications

Graphene hydrogels with high electrical conductivity were prepared by a one-step process using hydrazine hydrate as gel assembly agent (GH-HD). Conventional two-step process of gel formation and further reduction to prepare highly conducting gels was replaced by a single step involving equivalent amount of hydrazine. Optimized graphene oxide concentration was established to facilitate such monolith formation. Extensive characterization and control studies enabled understanding of the material properties and gel formation mechanism. The synthesized gel shows a high electrical conductivity of 1141 S/m. The supercapacitor based on GH-HD delivers a high specific capacitance of 190 F/g at a current density of 0.5 A/g and 123 F/g at very high current density of 100 A/g. Furthermore, excellent power capability and cyclic stability were also observed. 3D macroporous morphology of GH-HD makes it ideal for high rate supercapacitor applications

Ion Sieving Effects in Chemically Tuned Pillared Graphene Materials for Electrochemical Capacitors

Supercapacitors offer high power densities but require further improvements in energy densities for widespread commercial applications. In addition to the conventional strategy of using large surface area materials to enhance energy storage, recently, matching electrolyte ion sizes to material pore sizes has been shown to be particularly effective. However, synthesis and characterization of materials with precise pore sizes remain challenging. Herein, we propose to evaluate the layered structures in graphene derivatives as being analogous to pores and study the possibility of ion sieving. A class of pillared graphene based materials with suitable interlayer separation were synthesized, readily characterized by X-ray diffraction, and tested in various electrolytes. Electrochemical results show that the interlayer galleries could indeed sieve electrolyte ions based on size constrictions: ions with naked sizes that are smaller than the interlayer separation access the galleries, whereas the larger ions are restricted. These first observations of ion sieving in pillared graphene-based materials enable efficient charge storage through optimization of the d-spacing/ion size couple

Investigation of ion transport in chemically tuned pillared graphene materials through electrochemical impedance analysis

Chemically tuned pillared graphene structures show ability to limit restacking of graphene sheets for electrochemical energy storage in SCs. A comprehensive electrochemical characterization using various ion sizes allowed identification of ion-sieving in the cross-linked galleries of reduced pillared graphene materials (RPs). The access to the cross-linked galleries, which provide additional ion sorption sites, offered slightly increased capacitances in RPs compared to completely restacked sheets in reduced graphene oxide (RGO). We performed electrochemical impedance analyses on RPs and RGO to understand the ion transport inside the cross-linked graphene galleries. RGO adsorbs ions in the inter-particle micro/meso pores and the ion access to such sites from the bulk electrolyte occurs with relative ease. RPs sieve ions into their inter-layer gallery pores based on effective ion sizes and the ion transport process is resistive compared to RGO. A control study using 3D pillared graphene hydrogel with improved macro porosity assigns this resistive behavior and the moderate capacitances to limited ion access to the active sites due to excess number of pillars. The obtained results on the ion transport dynamics between graphene layers provide perspectives towards further optimization of these graphene materials for SCs

Sparsely Pillared Graphene Materials for High-Performance Supercapacitors: Improving Ion Transport and Storage Capacity

Graphene-based materials are extensively studied as promising candidates for supercapacitors (SCs) owing to the high surface area, electrical conductivity, and mechanical flexibility of graphene. Reduced graphene oxide (RGO), a close graphene-like material studied for SCs, offers limited specific capacitances (100 F·g–1) as the reduced graphene sheets partially restack through π–π interactions. This paper presents pillared graphene materials designed to minimize such graphitic restacking by cross-linking the graphene sheets with a bifunctional pillar molecule. Solid-state NMR, X-ray diffraction, and electrochemical analyses reveal that the synthesized materials possess covalently cross-linked graphene galleries that offer additional sites for ion sorption in SCs. Indeed, high specific capacitances in SCs are observed for the graphene materials synthesized with an optimized number of pillars. Specifically, the straightforward synthesis of a graphene hydrogel containing pillared structures and an interconnected porous network delivered a material with gravimetric capacitances two times greater than that of RGO (200 F·g–1vs 107 F·g–1) and volumetric capacitances that are nearly four times larger (210 F·cm–3vs 54 F·cm–3). Additionally, despite the presence of pillars inside the graphene galleries, the optimized materials show efficient ion transport characteristics. This work therefore brings perspectives for the next generation of high-performance SCs

Role of Serial Polio Seroprevalence Studies in Guiding Implementation of the Polio Eradication Initiative in Kano, Nigeria: 2011-2014.

BACKGROUND: Nigeria was one of 3 polio-endemic countries before it was de-listed in September 2015 by the World Health Organization, following interruption of transmission of the poliovirus. During 2011-2014, Nigeria conducted serial polio seroprevalence surveys (SPS) in Kano Metropolitan Area, comprising 8 local government areas (LGAs) in Kano that is considered very high risk (VHR) for polio, to monitor performance of the polio eradication program and guide the program in the adoption of innovative strategies. METHODS: Study subjects who resided in any of the 8 local government areas of Kano Metropolitan Area and satisfied age criteria were recruited from patients at Murtala Mohammed Specialist Hospital (Kano) for 3 seroprevalence surveys. The same methods were used to conduct each survey. RESULTS: The 2011 study showed seroprevalence values of 81%, 75%, and 73% for poliovirus types 1, 2, and 3, respectively, among infants aged 6-9 months age. Among children aged 36-47 months, seroprevalence values were greater (91%, 87%, and 85% for poliovirus types 1, 2, and 3, respectively).In 2013, the results showed that the seroprevalence was unexpectedly low among infants aged 6-9 months, remained high among children aged 36-47 months, and increased minimally among children aged 5-9 years and those aged 10-14 years. The baseline seroprevalence among infants aged 6-9 months in 2014 was better than that in 2013. CONCLUSIONS: The results from the polio seroprevalence surveys conducted in Kano Metropolitan Area in 2011, 2013, and 2014 served to assess the trends in immunity and program performance, as well as to guide the program, leading to various interventions being implemented with good effect, as evidenced by the reduction of poliovirus circulation in Kano

Développement de matériaux composites à base de graphène pour des applications en stockage électrochimique

The expectations around graphene come from huge potentialities for various applications (RF transistor, (bio)sensors…). Graphene high specific surface, mechanical resistance and conductivity make it specifically attractive for electrochemical storage applications. It has been shown that graphene sheets could be assembled to form structured graphene frameworks to translate the properties of individual sheets to functional materials and allow practical applications. The key features of these frameworks in terms of electrochemical storage applications are their graphitization level, their structural or textural disorder, and their porosity. On another hand, graphene has been interfaced with molecules to tune its electrical/optical properties or to make it more processable. The goal of the PhD project is to develop such structured graphene matrices following different methods such as the functionalization with bi-functional pilar molecules or the aerogel formation techniques. These assemblies will then be modified with different molecules to favor the formation of a stable SEI (solid electrolyte interface) and the intercalation of ions. The immobilization of redox compounds will also be attempted in order to bring a pseudo capacitive component to the system and target pseudo-capacitor applications. This thesis work will include material synthesis, assembling and functionalization steps combined to an important characterization tasks (IR, XRD, XPS, MEB, TEM, BET, electrochemistry...) The added-value of such work is that it represents an approach going from the basic material develop to the proof of concept.Les attentes autour du graphene peuvent être expliquées par les fortes potentialités applicatives de ce matériau (transistors RF, (bio)capteurs, TCL…). Sa grande surface spécifique, sa forte résistance mécanique ainsi que sa bonne conductivité permettraient de cibler des applications dans le domaine du stockage électrochimique tel que les pseudo-condensateurs. Il a été mis en évidence que des structures 3D de graphène définies comme des hydro- ou des aero- gels et présentant des surfaces spécifiques très élevées peuvent être obtenues. Ce type d’architecture présente un fort potentiel dans le domaine des matériaux d’électrodes carbonées pour le stockage de l’énergie et notamment pour les super-condensateurs. Leur obtention repose soit sur l’utilisation de molécules pontantes et de la création d’interactions covalentes ou électrostatiques, soit sur la réalisation d’aérogel à partir d’oxyde de graphène dans des conditions de pression/température établies. Basé sur ce contexte, le sujet de thèse présenté propose de développer des matériaux structurés à base de graphene (Fig.) pouvant être utilisés pour des applications en stockage électrochimique. La grande surface spécifique ainsi que la bonne conductivité du graphène permettraient de cibler de hautes densités d’énergie et de fortes puissances. Les différentes méthodes de préparation de ces architectures aérogels seront testées et les matériaux obtenus seront caractérisés morphologiquement (MEB, TEM, BET), structurellement (DRX) et chimiquement (XPS, IR, Raman, ATG). Les propriétés électrochimiques de ces espèces - correspondant à de nouvelles formes de carbones nanoporeux - seront ensuite évaluées. Ces structures seront également fonctionnalisées pour favoriser la formation d’une SEI stable et l’intercalation de certains ions. L’immobilisation de composés redox actifs sera testée afin d’insérer une composante pseudocapacitive au système. Les performances de ces matrices modifiées - s’apparentant à des matériaux d’électrodes pour pseudo-supercondensateur - seront enfin analysées. Les tâches que le thésard réalisera au cours de sa thèse seront donc variées et iront de la fabrication du matériau de base jusqu’à l’évaluation de son utilisation en cellule électrochimique

Development of graphene-based composite materials for electrochemical storage applications

Les attentes autour du graphene peuvent être expliquées par les fortes potentialités applicatives de ce matériau (transistors RF, (bio)capteurs, TCL…). Sa grande surface spécifique, sa forte résistance mécanique ainsi que sa bonne conductivité permettraient de cibler des applications dans le domaine du stockage électrochimique tel que les pseudo-condensateurs. Il a été mis en évidence que des structures 3D de graphène définies comme des hydro- ou des aero- gels et présentant des surfaces spécifiques très élevées peuvent être obtenues. Ce type d’architecture présente un fort potentiel dans le domaine des matériaux d’électrodes carbonées pour le stockage de l’énergie et notamment pour les super-condensateurs. Leur obtention repose soit sur l’utilisation de molécules pontantes et de la création d’interactions covalentes ou électrostatiques, soit sur la réalisation d’aérogel à partir d’oxyde de graphène dans des conditions de pression/température établies. Basé sur ce contexte, le sujet de thèse présenté propose de développer des matériaux structurés à base de graphene (Fig.) pouvant être utilisés pour des applications en stockage électrochimique. La grande surface spécifique ainsi que la bonne conductivité du graphène permettraient de cibler de hautes densités d’énergie et de fortes puissances. Les différentes méthodes de préparation de ces architectures aérogels seront testées et les matériaux obtenus seront caractérisés morphologiquement (MEB, TEM, BET), structurellement (DRX) et chimiquement (XPS, IR, Raman, ATG). Les propriétés électrochimiques de ces espèces - correspondant à de nouvelles formes de carbones nanoporeux - seront ensuite évaluées. Ces structures seront également fonctionnalisées pour favoriser la formation d’une SEI stable et l’intercalation de certains ions. L’immobilisation de composés redox actifs sera testée afin d’insérer une composante pseudocapacitive au système. Les performances de ces matrices modifiées - s’apparentant à des matériaux d’électrodes pour pseudo-supercondensateur - seront enfin analysées. Les tâches que le thésard réalisera au cours de sa thèse seront donc variées et iront de la fabrication du matériau de base jusqu’à l’évaluation de son utilisation en cellule électrochimique.The expectations around graphene come from huge potentialities for various applications (RF transistor, (bio)sensors…). Graphene high specific surface, mechanical resistance and conductivity make it specifically attractive for electrochemical storage applications. It has been shown that graphene sheets could be assembled to form structured graphene frameworks to translate the properties of individual sheets to functional materials and allow practical applications. The key features of these frameworks in terms of electrochemical storage applications are their graphitization level, their structural or textural disorder, and their porosity. On another hand, graphene has been interfaced with molecules to tune its electrical/optical properties or to make it more processable. The goal of the PhD project is to develop such structured graphene matrices following different methods such as the functionalization with bi-functional pilar molecules or the aerogel formation techniques. These assemblies will then be modified with different molecules to favor the formation of a stable SEI (solid electrolyte interface) and the intercalation of ions. The immobilization of redox compounds will also be attempted in order to bring a pseudo capacitive component to the system and target pseudo-capacitor applications. This thesis work will include material synthesis, assembling and functionalization steps combined to an important characterization tasks (IR, XRD, XPS, MEB, TEM, BET, electrochemistry...) The added-value of such work is that it represents an approach going from the basic material develop to the proof of concept

Dual‐ion intercalation and high volumetric capacitance in a two‐dimensional non‐porous coordination polymer

Intercalation is a promising ion-sorption mechanism for enhancing the energy density of electrochemical capacitors (ECs) because it offers enhanced access to the electrochemical surface area. It requires a rapid transport of ions in and out of a host material, and it must occur without phase transformations. Materials that fulfil these requirements are rare; those that do intercalate almost exclusively cations. Herein, we show that Ni3 (benzenehexathiol) (Ni3 BHT), a non-porous two-dimensional (2D) layered coordination polymer (CP), intercalates both cations and anions with a variety of charges. Whereas cation intercalation is pseudocapacitive, anions intercalate in a purely capacitive fashion. The excellent EC performance of Ni3 BHT provides a general basis for investigating similar dual-ion intercalation mechanisms in the large family of non-porous 2D CPs

A layered organic cathode for high-energy, fast-charging, and long-lasting Li-ion batteries

Eliminating the use of critical metals in cathode materials can accelerate global adoption of rechargeable Lithium-ion batteries. Organic cathode materials, derived entirely from earth abundant elements, are in principle ideal alternatives, but have not yet challenged inorganic cathodes due to poor conductivity, low practical storage capacity, or poor cyclability. Here, we describe a layered organic electrode material whose high electrical conductivity, high storage capacity, and complete insolubility enable reversible intercalation of Li+ ions, allowing it to compete at the electrode level, in all relevant metrics, with inorganic-based lithium-ion battery cathodes. Our optimized cathode stores 306 mAh g–1cathode, delivers an energy density of 765 Wh kg–1cathode, higher than most cobalt-based cathodes, and can charge-discharge in as little as six minutes. These results demonstrate operational competitiveness of sustainable organic electrode materials in practical batteries

High-rate, high-capacity electrochemical energy storage in hydrogen-bonded fused aromatics

Designing materials for electrochemical energy storage with short charging times and high charge capacities is a longstanding challenge. The fundamental difficulty lies in installing a high density of redox couples into a stable material that can efficiently conduct both ions and electrons. Here, we report all-organic, fused aromatic materials that store up to 310 mAh g–1 and charge in as little as 33 seconds. This performance stems from abundant quinone/imine functionalities that act as redox-active sites, engage in hydrogen bonding for outstanding stability upon cycling, and enable bulk electronic delocalization for high-rate energy storage. The hydrogen bonding-assisted bulk charge storage here contrasts with the surface-confined or hydration-dependent behavior of traditional inorganic electrodes. These materials outperform state-of-the-art faradaic and capacitive electrodes in both capacity and power capability