James A. Ritter

Characterization of Sol‐Gel‐Derived Cobalt Oxide Xerogels as Electrochemical Capacitors

Very fine cobalt oxide xerogel powders were prepared using a unique solution chemistry associated with the sol-gel process. The effect of thermal treatment on the surrace area, pore volume, crystallinity, particle structure, and corresponding electrochemical properties of the resulting xerogels was investigated and found to have significant effects on all of these properties. The xerogel remained amorphous as Co(OH) 2 up to 160°C, and exhibited maxima in both the surface area and pore volume at this temperature. With an increase in the temperature above 200°C, both the surface area and pore volume decreased sharply, because the amorphous Co(OH) 2 decomposed to form CoO that was subsequently oxidized to form crystalline Co 3 O 4 In addition, the changes in the surface area, pore volume, crystallinity, and particle structure all had significant but coupled effects on the electrochemical properties of the xerogels. A maximum capacitance of 291 F/g was obtained for an electrode prepared with the CoO x xerogel calcined at 150°C, which was consistent with the maxima exhibited in both the surface area and pore volume; this capacitance was attributed solely to a surface redox mechanism. The cycle life of this electrode was also very stable for many thousands of cycles

Effect of synthesis pH on the structure of carbon xerogels

Mesoporous carbon xerogels were prepared from the sol-gel polymerization of resorcinol with formaldehyde (RF) followed by carbonization. The effect of the initial pH of the RF solution on the surface area, pore volume, pore size distribution and nanostructure was studied. A brief mechanism of polymerization is discussed. Typically, a lower initial pH yielded carbon xerogels with a higher surface area and pore volume, and a broader pore size distribution. The highest surface area and pore volume were around 620 m2 g−1 and 0.8 cm3 g−1, with a mean pore radius ranging from 4 to 6 nm and with 80% of the pore volume as mesopores. Transmission electron microscopy and X-ray diffraction revealed a surface morphology consisting of 10 nm diameter particles randomly oriented to produce a partially nanocrystalline structure in between graphite and activated carbon. Thermogravimetric and differential scanning calorimetric analyses disclosed a significant weight loss (50%) during a strongly exothermic carbonization process.

State-of-the-art Adsorption and Membrane Separation Processes for Carbon Dioxide Production from Carbon Dioxide Emitting Industries

Abstract With the growing concern about global warming placing greater demands on improving energy efficiency and reducing CO2 emissions, the need for improving the energy intensive, separation processes involving CO2 is well recognized. The US Department of Energy estimates that the separation of CO2 represents 75% of the cost associated with its separation, storage, transport, and sequestration operations. Hence, energy efficient, CO2 separation technologies with improved economics are needed for industrial processing and for future options to capture and concentrate CO2 for reuse or sequestration. The overall goal of this review is to foster the development of new adsorption and membrane technologies to improve manufacturing efficiency and reduce CO2 emissions. This study focuses on the power, petrochemical, and other CO2 emitting industries, and provides a detailed review of the current commercial CO2 separation technologies, i.e., absorption, adsorption, membrane, and cryogenic, an overview of the emerging adsorption and membrane technologies for CO2 separation, and both near and long term recommendations for future research on adsorption and membrane technologies. Flow sheets of the principal CO2 producing processes are provided for guidance and new conceptual flow sheets with ideas on the placement of CO2 separations technologies have also been devised.

Determination of the Lithium Ion Diffusion Coefficient in Graphite

A complex impedance model for spherical particles was used to determine the lithium ion diffusion coefficient in graphite as a function of the state of charge (SOC) and temperature. The values obtained range from 1.12 {times} 10{sup {minus}10} to 6.51 {times} 10{sup {minus}11} cm{sup 2}/s at 25 C for 0 and 30% SOC, respectively, and for 0% SOC, the value at 55 C was 1.35 {times} 10{sup {minus}10} cm{sup 2}/s. The conventional potentiostatic intermittent titration technique (PITT) and Warburg impedance approaches were also evaluated, and the advantages and disadvantages of these techniques were exposed.

Correlation of Double‐Layer Capacitance with the Pore Structure of Sol‐Gel Derived Carbon Xerogels

Research on carbon-based electrochemical double-layer capacitors (EDLCs) has focused on developing new and improved carbon materials with high surface areas and suitable pore structures. 1-5 Both of these characteristics have been shown to control the energy and power densities of EDLCs. 6 However, the role of microporosity, i.e., pores having diameters less than 20 A, in the performance of an EDLC is still not very clear. In other words, what pore size is too small for the electrolyte to access, hence preventing it from forming a double layer? Most of the studies in the literature have attempted to correlate the double-layer capacitance (DLC) simply in terms of the total Brunauer-Emmett-Teller (BET) surface area of a carbon material, with limited success. 7-9 Some other studies have had better success by correlating the DLC in terms of the micropore and mesopore surface areas. 10-12 However, none of these studies has been able to identify the pore sizes that may not contribute to the DLC. Yet this information is crucial to understanding the performance of an ELDC from a molecular level and to designing better carbon-based EDLCs by tailoring the pore structure for optimum performance. The main reason for this lack of quantification lies in the techniques that have been employed to determine the surface areas of carbon materials. 7-9 The commonly used techniques are global in that they only provide information on the total surface area, which is sometimes divided into the total micropore and mesopore surface areas. These techniques are incapable of providing information on the pore size distributions (PSDs) and corresponding surface area distributions. However, a very promising technique, which has not been explored much in the characterization of EDLCs, is the use of density function theory (DFT) to determine the mesopore size distribution and the corresponding cumulative surface area of carbon materials. 10 Therefore, the objectives here are to demonstrate the use of DFT in determining the PSD and cumulative surface area of carbon materials that are being evaluated as EDLCs, and to show how this information can be used to identify the pore sizes that are contributing to the DLC. The carbon material chosen for this purpose is a carbonized resorcinol-formaldehyde (R-F) resin derived from a sol-gel process. This material has been receiving considerable attention in the recent literature, 1,6,11-17 In this study, the DLCs of a series of carbon xerogels fabricated from R-F resins, carbonized at different temperatures and CO2-activated to different extents, are correlated with their corresponding PSDs and cumulative surface areas determined from DFT. The contribution to the DLC of various pore sizes is revealed, including the pore size range that does not contribute to the DLC of these carbon materials. Qualitative explanations for the inactivity of these small pores are offered and contrasted with the correlation proposed by Shi. 10

Implementing a hydrogen economy

President Bush, during his State of the Union Address this year, pronounced a

State‐of‐the‐Art Adsorption and Membrane Separation Processes for Hydrogen Production in the Chemical and Petrochemical Industries

1.2 billion jump-start to the hydrogen economy. The move would represent not only freedom from US-dependence on foreign oil, which is a national security issue, but also a necessary and gargantuan step toward improving the environment by reducing the amount of carbon dioxide released into the atmosphere. However, hydrogen storage is proving to be one of the most important issues and potentially biggest roadblock for the implementation of a hydrogen economy. Of the three options that exist for storing hydrogen, in a solid, liquid, or gaseous state, the former is becoming accepted as the only method potentially able to meet the gravimetric and volumetric densities of the recently announced FreedomCar goals; and of all known hydrogen storage materials, complex hydrides may be the only hope. In recent years, months, weeks, and even days, it has become increasingly clear that hydrogen as an energy carrier is ‘in’ and carbonaceous fuels are ‘out’1. The hydrogen economy is coming, with the impetus to transform our fossil energy-based society, which inevitably will cease to exist, into a renewable energy-based one2. However, this transformation will not occur overnight. It may take several decades to realize a hydrogen economy. In the meantime, research and development is necessary to ensure that the implementation of the hydrogen economy is completely seamless, with essentially no disruption of the day-to-day activities of the global economy. The world has taken on a monumental, but not insurmountable, task of transforming from carbonaceous to renewable fuels, with clean burning, carbon dioxide-free hydrogen as the logical choice.

Development of carbon-metal oxide supercapacitors from sol-gel derived carbon-ruthenium xerogels

Abstract This review on the use of adsorption and membrane technologies in H2 production is directed toward the chemical and petrochemical industries. The growing requirements for H2 in chemical manufacturing, petroleum refining, and the newly emerging clean energy concepts will place greater demands on sourcing, production capacity and supplies of H2. Currently, about 41 MM tons/yr of H2 is produced worldwide, with 80% of it being produced from natural gas by steam reforming, partial oxidation and autothermal reforming. H2 is used commercially to produce CO, syngas, ammonia, methanol, and higher alcohols, urea and hydrochloric acid. It is also used in Fischer Tropsch reactions, as a reducing agent (metallurgy), and to upgrade petroleum products and oils (hydrogenation). It has been estimated that the reforming of natural gas to produce H2 consumes about 31,800 Btu/lb of H2 produced at 331 psig based on 35.5 MM tons/yr production. It is further estimated that 450 trillion Btu/yr could be saved with a 20% improvement in just the H2 separation and purification train after the H2 reformer. Clearly, with the judicious and further use of adsorption or membrane technology, which are both classified as low energy separation processes, energy savings could be readily achieved in a reasonable time frame. To assist in this endeavor of fostering the development of new adsorption and membrane technologies suitable for H2, CO and syngas production, the current industrial practice is summarized in terms of the key reforming and shift reactions and reactor conditions, along with the four most widely used separation techniques, i.e., absorption, adsorption, membrane, and cryogenic, to expose the typical conditions and unit processes involved in the reforming of methane. Since all of the reactions are reversible, the H2 or CO productivity in each one of them is limited by equilibrium, which certainly provides for process improvement. Hence, the goal of this review is to foster the development of adsorption and membrane technologies that will economically augment in the near term and completely revamp in the far term a typical H2, CO or syngas production plant that produces these gases from natural gas and hydrocarbon feedstocks. A review of the emerging literature concepts on evolving adsorption and membrane separations applicable to H2 production is provided, with an emphasis placed on where the state‐of‐the‐art is and where it needs to go. Recommendations for future research and development needs in adsorbent and membrane materials are discussed, and detailed performance requirements are provided. An emphasis is also placed on flow sheet design modification with adsorption or membrane units being added to existing plants for near term impact, and on new designs with complete flow sheet modification for new adsorption or membrane reactor/separators replacing current reactor and separator units in an existing plant for a longer term sustainable impact.

Carbonization and activation of sol–gel derived carbon xerogels

There has been increasing interest in electrochemical capacitors as energy storage systems because of their high power density and long cycle life, compared to battery devices. According to the mechanism of energy storage, there are two types of electrochemical capacitors. One type is based on double layer (dl) formation due to charge separation, and the other type is based on a faradaic process due to redox reactions. Sol-gel derived high surface area carbon-ruthenium xerogels were prepared from carbonized resorcinol-formaldehyde resins containing an electrochemically active form of ruthenium oxide. The electrochemical capacitance of these materials increased with an increase in the ruthenium content indicating the presence of pseudocapacitance associated with the ruthenium oxide undergoing reversible faradaic redox reactions. A specific capacitance of 256 F/g (single electrode) was obtained from a carbon xerogel containing 14 wt% Ru, which corresponded to more than 50% utilization of the ruthenium. The double layer accounted for 40% of this capacitance. This material was also electrochemically stable, showing no change in a cyclic voltammogram for over 2,000 cycles.

Approximate Solutions for Galvanostatic Discharge of Spherical Particles I. Constant Diffusion Coefficient

Abstract The effects of the carbonization temperature (in N2) and CO2-activation time (in 5% CO2 in N2) on the pore structure of carbon xerogels, derived from the sol–gel polymerization of resorcinol–formaldehyde resins, were studied in detail. As the carbonization temperature increased, the number of micropores in the 0.6 nm range decreased, with essentially no effect on the pores in the mesopore range, and the cumulative surface areas and pore volumes both decreased, but only marginally. As the CO2-activation time increased, the number of micropores and mesopores both increased, where pores in the 0.6 nm range eventually became pores in the 0.13 nm range, and the cumulative surface areas and pore volumes both increased significantly. The skeletal densities also increased significantly, approaching that of graphite, with an increase in both the carbonization temperature and CO2-activation time, but the nanoparticle size was largely unaffected. Weight loss was nearly independent of the carbonization temperature at about 50%, but it was strongly dependent on the CO2-activation time with a maximum weight loss of about 75%.