17 research outputs found
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 1141Â S/m. The supercapacitor based on GH-HD delivers a high specific capacitance of 190Â F/g at a current density of 0.5 A/g and 123Â 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·gâ1) as the reduced graphene sheets partially restack through ÏâÏ 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·gâ1vs 107 F·gâ1) and volumetric capacitances that are nearly four times larger (210 F·cmâ3vs 54 F·cmâ3). 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 graphÚne pour des applications en stockage électrochimique
The expectations around graphene come from huge potentialities for various applications (RF transistor, (bio)sensorsâŠ). 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 ĂȘtre expliquĂ©es par les fortes potentialitĂ©s applicatives de ce matĂ©riau (transistors RF, (bio)capteurs, TCLâŠ). Sa grande surface spĂ©cifique, sa forte rĂ©sistance mĂ©canique ainsi que sa bonne conductivitĂ© permettraient de cibler des applications dans le domaine du stockage Ă©lectrochimique tel que les pseudo-condensateurs. Il a Ă©tĂ© mis en Ă©vidence que des structures 3D de graphĂšne dĂ©finies comme des hydro- ou des aero- gels et prĂ©sentant des surfaces spĂ©cifiques trĂšs Ă©levĂ©es peuvent ĂȘtre obtenues. Ce type dâarchitecture prĂ©sente un fort potentiel dans le domaine des matĂ©riaux dâĂ©lectrodes carbonĂ©es pour le stockage de lâĂ©nergie et notamment pour les super-condensateurs. Leur obtention repose soit sur lâutilisation de molĂ©cules pontantes et de la crĂ©ation dâinteractions covalentes ou Ă©lectrostatiques, soit sur la rĂ©alisation dâaĂ©rogel Ă partir dâoxyde de graphĂšne dans des conditions de pression/tempĂ©rature Ă©tablies. BasĂ© sur ce contexte, le sujet de thĂšse prĂ©sentĂ© propose de dĂ©velopper des matĂ©riaux structurĂ©s Ă base de graphene (Fig.) pouvant ĂȘtre utilisĂ©s pour des applications en stockage Ă©lectrochimique. La grande surface spĂ©cifique ainsi que la bonne conductivitĂ© du graphĂšne permettraient de cibler de hautes densitĂ©s dâĂ©nergie et de fortes puissances. Les diffĂ©rentes mĂ©thodes de prĂ©paration de ces architectures aĂ©rogels seront testĂ©es et les matĂ©riaux obtenus seront caractĂ©risĂ©s morphologiquement (MEB, TEM, BET), structurellement (DRX) et chimiquement (XPS, IR, Raman, ATG). Les propriĂ©tĂ©s Ă©lectrochimiques de ces espĂšces - correspondant Ă de nouvelles formes de carbones nanoporeux - seront ensuite Ă©valuĂ©es. Ces structures seront Ă©galement fonctionnalisĂ©es pour favoriser la formation dâune SEI stable et lâintercalation de certains ions. Lâimmobilisation de composĂ©s redox actifs sera testĂ©e afin dâinsĂ©rer une composante pseudocapacitive au systĂšme. Les performances de ces matrices modifiĂ©es - sâapparentant Ă des matĂ©riaux dâĂ©lectrodes pour pseudo-supercondensateur - seront enfin analysĂ©es. Les tĂąches que le thĂ©sard rĂ©alisera au cours de sa thĂšse seront donc variĂ©es et iront de la fabrication du matĂ©riau de base jusquâĂ lâĂ©valuation de son utilisation en cellule Ă©lectrochimique
Development of graphene-based composite materials for electrochemical storage applications
Les attentes autour du graphene peuvent ĂȘtre expliquĂ©es par les fortes potentialitĂ©s applicatives de ce matĂ©riau (transistors RF, (bio)capteurs, TCLâŠ). Sa grande surface spĂ©cifique, sa forte rĂ©sistance mĂ©canique ainsi que sa bonne conductivitĂ© permettraient de cibler des applications dans le domaine du stockage Ă©lectrochimique tel que les pseudo-condensateurs. Il a Ă©tĂ© mis en Ă©vidence que des structures 3D de graphĂšne dĂ©finies comme des hydro- ou des aero- gels et prĂ©sentant des surfaces spĂ©cifiques trĂšs Ă©levĂ©es peuvent ĂȘtre obtenues. Ce type dâarchitecture prĂ©sente un fort potentiel dans le domaine des matĂ©riaux dâĂ©lectrodes carbonĂ©es pour le stockage de lâĂ©nergie et notamment pour les super-condensateurs. Leur obtention repose soit sur lâutilisation de molĂ©cules pontantes et de la crĂ©ation dâinteractions covalentes ou Ă©lectrostatiques, soit sur la rĂ©alisation dâaĂ©rogel Ă partir dâoxyde de graphĂšne dans des conditions de pression/tempĂ©rature Ă©tablies. BasĂ© sur ce contexte, le sujet de thĂšse prĂ©sentĂ© propose de dĂ©velopper des matĂ©riaux structurĂ©s Ă base de graphene (Fig.) pouvant ĂȘtre utilisĂ©s pour des applications en stockage Ă©lectrochimique. La grande surface spĂ©cifique ainsi que la bonne conductivitĂ© du graphĂšne permettraient de cibler de hautes densitĂ©s dâĂ©nergie et de fortes puissances. Les diffĂ©rentes mĂ©thodes de prĂ©paration de ces architectures aĂ©rogels seront testĂ©es et les matĂ©riaux obtenus seront caractĂ©risĂ©s morphologiquement (MEB, TEM, BET), structurellement (DRX) et chimiquement (XPS, IR, Raman, ATG). Les propriĂ©tĂ©s Ă©lectrochimiques de ces espĂšces - correspondant Ă de nouvelles formes de carbones nanoporeux - seront ensuite Ă©valuĂ©es. Ces structures seront Ă©galement fonctionnalisĂ©es pour favoriser la formation dâune SEI stable et lâintercalation de certains ions. Lâimmobilisation de composĂ©s redox actifs sera testĂ©e afin dâinsĂ©rer une composante pseudocapacitive au systĂšme. Les performances de ces matrices modifiĂ©es - sâapparentant Ă des matĂ©riaux dâĂ©lectrodes pour pseudo-supercondensateur - seront enfin analysĂ©es. Les tĂąches que le thĂ©sard rĂ©alisera au cours de sa thĂšse seront donc variĂ©es et iront de la fabrication du matĂ©riau de base jusquâĂ lâĂ©valuation de son utilisation en cellule Ă©lectrochimique.The expectations around graphene come from huge potentialities for various applications (RF transistor, (bio)sensorsâŠ). 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
Dualâion intercalation and high volumetric capacitance in a twoâdimensional nonâporous 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 gâ1cathode, delivers an energy density of 765 Wh
kgâ1cathode, 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 gâ1 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