Electrochemical Power Research
- High Energy Density Carbons from Low-cost Precursors |
- Asymmetric Capacitors for Electricity Storage on a Utility Grid |
- Thermoelectrics |
- Carbon Nanospheres |
- Symmetric Carbon/carbon Electrochemical Capacitors |
- Carbon Monoflouride (CFx) for Li/CFx Batteries |
- Redox Flow Batteries for Grid Energy Storage |
- Development of Asymmetric Capacitors for Grid Energy Storage |
- Development of New Li-ion Anode Materials
The increasing demand for portable energy storage technologies, especially for electronics, transportation systems, and distributed power applications, has provided the impetus for the development of new and improved materials with enhanced volumetric and gravimetric energy densities, and device configurations that can meet these needs.
The Electrochemical Power Sources Group at the University of Kentucky Center for Applied Energy Research specializes in the development and characterization of energy storage materials for various electrochemical power sources, including fuel cells, primary and secondary batteries, and electrochemical double layer capacitors (EDLCs). A broad range of energy storage materials are synthesized and fully characterized in the Power Sources Group, and include fuel cell electrocatalysts, lithium-ion battery electrodes, thermoelectric materials, and porous, high surface area activated carbons, which can serve as electrode materials for EDLCs, as catalyst supports in fuel cell applications, or for the electrochemical storage of hydrogen.
The Electrochemical Power Sources Group also specializes in the application of electrochemical techniques to form autonomous micro-power sources, including the use of electrodeposition and electrophoretic deposition of active electrode materials onto suitable substrates. The Power Sources Group has actively developed and maintained a fruitful collaboration with government, military, industrial, and academic partners, and works closely with these entities to develop targeted solutions to specific energy storage needs.
Electrochemical double-layer capacitors (EDLCs), often referred to as supercapacitors or ultracapacitors, are electrochemical energy storage devices capable of storing and delivering energy at higher rates than most batteries including advanced Li-ion. Electrochemical capacitors typically possess lower energy densities than batteries, but have many advantages, including higher power densities (i.e. rapid charge and discharge), extended cycle life, and undergo no chemical or structural changes during charge and discharge. Electrochemical capacitors fall into two major categories, based on the charge storage mechanism. The mechanism of charge storage in electrical double layer based supercapacitors is charge-separation between ions in solution and the electrified electrode/electrolyte interface, which is known as the electric double-layer. Pseudocapacitors refer to electrodes which undergo Faradaic redox (or electron transfer) reactions upon charge or discharge of the electrochemical cell. Conducting polymers such as polypyrrole or various ruthenium- or manganese-based metal oxides serve as typical examples of pseudocapacitive materials.
The development of compact, light-weight, highly portable power sources with improved energy and power densities represent an ongoing challenge for the mission requirements of the U.S. Marine Corps. Current research by the Office of Naval Research is geared towards the development of a lighter weight alternative to the standard BA-5590 military primary battery, employing standard metal/air technology. However, the power density of the metal/air battery technology remains too low to meet all mission requirements which require high power pulses. The combination of an energy-dense metal/air battery with a high power density electrochemical double layer capacitor (EDLC) or asymmetric EDLC could provide the necessary pulse power requirements to overcome the peak power limitations of the metal/air system.
Research performed at CAER has focused on the development of energy-dense mesoporous activated carbons with size-calibrated pores for double layer charge storage. One experimental approach is to increase surface roughness and create usable external surface area by catalytic drilling of metal or metal oxide nanoparticles from the surface into the interior of the carbon precursor (i.e. pore formation) prior to or during activation. Preliminary results support the validity of this experimental approach, as various carbons have been prepared with a gravimetric capacitance of ~ 80-100 F/g, along with SBET < 1100 m
The development of utility energy storage technologies is catalyzed by a variety of reasons, including increasing fuel costs, concerns about global CO2 emissions, current strain and increased load on the aging U.S. electrical grid and infrastructure, and political unrest in the world's leading oil-producing countries. The economic cost and productivity loss caused by electricity interruptions are enormous; power outages are estimated to cost the U.S. approximately $79B/year; the majority of the electricity disruptions typically last for < 5 minutes. Energy storage research in the past has led to the development of some large scale storage technologies currently in use, including pumped hydropower and compressed air energy storage (CAES). The electricity is stored during off-peak times, and released to the grid during periods of high demand; stored electrical energy is used for load-leveling, frequency control, and spinning reserve. Energy storage accounts for only 2.5% of the total electricity capacity in the U.S., with the majority derived from pumped hydropower facilities. The development of bulk energy storage technologies is hindered by high costs and site location. Current alternatives to electricity storage to meet changing demand placed on the electrical grid include spinning reserve, when power plants operate below normal power output; gas turbine power plants brought online during periods of peak demand; and electrical devices which have a working voltage range (i.e. 110-120 V). However, the development of smaller scale utility energy storage technologies is promising, particularly distributed energy storage (DES) systems, and can assist in the improvement of generating efficiency by peak shaving and filling demand valleys.
Numerous technologies are already used for energy storage, including pumped hydropower, compressed air energy storage, flywheels, and batteries. One emerging technology for off-grid storage of electricity is the development of electrochemical capacitors. Electrochemical double-layer capacitors (EDLCs), often referred to as supercapacitors or ultracapacitors are electrochemical energy storage devices capable of storing and delivering energy at higher rates than most batteries including advanced Li-ion. Electrochemical capacitors typically possess lower energy densities than batteries, but have many advantages, including higher power densities (i.e. rapid charge and discharge), extended cycle life and undergo no chemical or structural changes during charge and discharge. Electrochemical capacitors fall into two major categories, based on the charge storage mechanism. The mechanism of charge storage in electrical double layer based supercapacitors is charge-separation between ions in solution and the electrified electrode/electrolyte interface, which is known as the electric double-layer. Pseudocapacitors refer to electrodes which undergo Faradaic redox (or electron transfer) reactions upon charge or discharge of the electrochemical cell. Conducting polymers such as polypyrrole or various ruthenium- or manganese-based metal oxides serve as typical examples of pseudocapacitive materials.
Hybrid or asymmetric capacitors are a breakthrough approach for increasing the energy content of an electrochemical capacitor. The concept of coupling a battery electrode with a carbon double-layer capacitor electrode has already been demonstrated for several aqueous systems with exceedingly high energies. Research performed at CAER will lead to the development of carbon materials and electrochemical capacitors with high energy density and enhanced discharge times. This will be accomplished by synthesizing high energy dense carbon materials and electrochemically coupling these electrode materials in an asymmetric design with appropriate electrolytes, constructing devices with single cell operating voltages > 4.2V.
Approximately 66% of heat energy is being wasted in various forms through auto exhaust pipes, thermal power plant smoke-stacks, and other sources. This wasted heat could be converted into useful electric power using thermoelectric devices. Alternatively, renewable energy source such as the solar heat energy could be converted into electrical energy using thermoelectric devices. This electrical energy could be used to power electronic devices or stored temporarily in rechargeable batteries or electrochemical double-layer capacitors. Thermoelectric devices have good reliability since they do not have any moving parts. This leads to reduced operational noise levels, and long-term maintenance-free operation. In addition, they are lightweight, and are available in small sizes that could fit in the palm of a hand. They are easy to install and act as an autonomous power harvesting device. They do not require compressed gases or hazardous chemicals to operate. Thermoelectric devices also hold the possibility of large-scale electric power generation.
The Electrochemical Power Sources Group is currently developing thermoelectric (TE) devices using doped multiwalled carbon nanotubes. Carbon nanotubes are lightweight compared to commercially available thermoelectric materials, which means that lighter TE devices can be constructed. Carbon nanotubes possess a superior mechanical property, which indicates that robust TE devices can be built. The unique one-dimensional structure of carbon nanotubes leads to quantum confinement of charge carriers. This results in ballistic electron transport through the nanotubes leading to high electrical conductivity. The semiconducting properties can be utilized to produce p-type and n-type nanotubes. By employing in-situ chemical vapor deposition methods boron- and nitrogen- doped nanotubes have been produced. The doping concentrations can be adjusted by the adjusting the CVD process parameters. The boron-doped MWCNTs offer p-type carrier conduction. The nitrogen-doped pyridine and acetonitrile-derived MWCNTs offer n-type carrier conduction. The operation of the MWCNT-based thermoelectric devices is not affected by melting or oxidation at high temperatures making it suitable for high and low temperature operations. These devices do not release toxic gases during operation making them environment friendly. The Center for Applied Energy Research has the capability to mass produce doped MWCNTs at acceptable prices for use in TE devices.
Carbons in the form of uniform micro- and nanospheres have been synthesized by various methods such as hydrothermal dehydration, chemical vapor deposition, pressure carbonization, mixed-valence oxide-catalytic carbonization, and reduction of carbides with metal catalysts. Among these, hydrothermal synthesis (HTS) has been considered to be the most efficient method for synthesis of carbon spheres. Monosaccharides (glucose, fructose, and xylose), disaccharides (sucrose, maltose), polysaccharides (starch and cellulose), and biomass derivatives like HMF and furfural have all been reported as precursors to produce carbon spheres. In addition, a variety of low-cost precursors have been used including industrial bio-waste, recycled packing materials and waste products from the food processing industry.
Our approach relies on the use of natural, inexpensive and sustainable precursors and low-cost, environmentally benign synthetic processes to prepare activated carbons for both energy storage and environmental remediation applications. Materials synthesis at the University of Kentucky Center for Applied Energy Research (UK-CAER) focuses on the implementation of hydrothermal synthesis, a facile, low environmental impact and minimal energy consumption process for preparing carbon materials using readily available organic sources, including carbohydrate precursors, industrial biomass waste derivatives and recycled materials. The simplicity, mild temperatures (ca. 200 °C), and versatility of HTS enables detailed control of the size, monodispersity, chemical composition and microscopic structure of electrode materials for this type of application, opening new prospects for decreasing manufacturing costs and improving material performance.
Hydrothermally synthesized carbons have already been prepared at UK-CAER and have shown that the size of the carbon spheres can be controlled by adjusting the concentration of the carbohydrate solution, reaction temperature and reaction dwell time. The synthetic process may be regarded as "green" as it uses no toxic organic solvents, initiators, or surfactants as is common with other methods described for the preparation of polymer micro- or nanospheres. Figure 1 shows carbon nanospheres synthesized hydrothermally and carbon nanospheres after carbonization and graphitization. Monodispersed carbon structures of similar quality and purity have also been prepared in our laboratory using industrial biomass waste obtained from local bourbon distilleries, recycled materials used in packing and shipping, and waste water collected from food preparation. Thermal processing was then performed on all as-grown spheres.
Recently, we have demonstrated the ability to control carbon sphere size by adjusting hydrothermal process parameters. The mean diameter of the carbon nanospheres can be tuned easily by adjusting the operational conditions such as precursor solution concentration, temperature and reaction time. In general, increase in the concentration of the precursor solution results in an increase in the mean diameter of the spheres. Our research group has prepared carbon spheres ranging from 60 nm to ~ 10 µm using a variety of industrial biomass products such as bourbon stillage, rice water, and potato starch and so on. The material yield can also be increased by adjusting the process conditions. The hydrothermal process has very good conversion yields (C in/out ~ 90%). Finally, functional groups can be added to the sphere surface which can greatly improve hydrophilicity and chemical reactivity.
Electrochemical capacitors (EC), also known as, "supercapacitors", "double-layer capacitors", or "ultracapacitors", are energy storage devices with long cycle life, low internal resistance, fast charge and discharge rates, and high power densities. Due to these advantages ECs have drawn much attention recently and have been applied in electric/hybrid vehicles, heavy-construction equipment, electronics, and grid utility storage. Conventional electrode materials for ECs are activated carbons with nano-porosity and high surface area. Other materials include metal oxides, graphene, carbon nanotubes, carbon aerogels, and conductive polymers. Although some of these materials show high performance, the cost may be high or the synthesis may involve using toxic or corrosive chemicals. In our present work, activated carbon materials for ECs were hydrothermally synthesized from a variety of low cost biomass precursors.
In this work, the precursors chosen for HTS included packaging peanuts, cellulose, rayon fibers, pure starch, rice starch, and bourbon waste. Water-based solutions or suspensions were sealed in a Teflon lined pressure vessel, and then heated to 200 °C for 50 hours. After the reactor cooled down, the as-synthesized carbons were harvested by filtration and dried in an oven at 120 °C over night. The morphology of the as-synthesized carbons was characterized by scanning electron microscope (SEM). Figure 2 shows the SEM images of the as-synthesized carbons derived from different biomass precursors. For packaging peanuts, rice starch, and bourbon waste micron-size spheres were formed and mixed with sub-micron-size irregular shape particles, which was possibly due to the complex chemical compositions of the precursors. For rayon fibers, uniform carbon microspheres grew on the surface of the fibers. The cellulose and pure starch gave irregular shaped nano particles.
For cell construction, 1.8M TEMABF4 in PC was used as the electrolyte, Celgard® 3501 was used as the separator, and carbon coated polymer was used as the current collector. Cyclic voltammetry and galvanostatic charge-discharge tests were run on these cells to study the electrochemical performance. The activated carbon nanospheres showed excellent capacitive performance in 1.8M TEMABF4 in PC (120 F/g, 70 F/cc) and 38 wt% H2SO4 (200 F/g, 150 F/cc), as shown in Figure 1.
Figure 3 shows the normalized CV curves of the activated carbons derived from different biomass precursors showing both gravimetric and volumetric performance. Most of the materials showed rectangular CV curves and very fast charge-discharge rates characteristic of good capacitive behavior. The specific capacitance of these materials was very high, for example, cellulose and rice starch derived activated carbons showed gravimetric capacitance of over 100 F/g and volumetric capacitance of over 50 F/cc.
Lithium/carbon monoflouride (CFx) batteries are of interest since they offer the highest theoretical energy density among commercially available primary lithium ion batteries (2180 Wh/kg) with a long storage life (20+ years), wide temperature operating range (-40°C to 150°C), and low self-discharge rate (<0.5% per year). The discharge reaction of the Li/CFx cell is:
x Li + CFx -> x LiF + C (1)
Conventional Li/CFx chemistry suffers from several shortcomings due to the fundamentally insulating nature of the CFx material. These problems include low useful rate capability (<C/50), significant self-heating during high discharge rate, and voltage delay particularly at low temperatures. An insulating LiF layer is deposited on the surface of the carbon material as well as the pores throughout discharge. This layer impedes further reaction and generates heat. Our research efforts focus on addressing these issues through the development of fluorinated carbon nanospheres (CNS) with tunable particle size that show enhanced conductivity and power performance. CNS have large regular pores (~10 nm) located in interstitial sites between adjacent particles which are capable of sequestering LiF formed during cell discharge.
Using a hydrothermal synthesis (HTS), we are able to synthesize CNS starting from inexpensive carbohydrate or biomass precursors. HTS is a facile, low environmental impact and minimal energy consumption process for preparing carbon materials using readily available sources including sugars, industrial biomass waste like bourbon stillage and recycled materials such as packaging, rice water, or potato starch. The as-grown CNS were graphitized at 2500°C under helium then fluorinated to a gravimetric ratio of 0.96 F/C (CF0.96). The scanning electron microscopy images of the fluorinated carbons are shown in Figure 1. Work is being done to decrease the graphitization temperature using a metal or carbon nanotube catalyst.
The CF0.96 electrodes were prepared by mixing fluorinated carbon with 5% carbon black and 35 Teflon® then punched into 12 mm diameter, 60 ~m thick discs. These discs were pressed onto titanium mesh and used as the cathode in a two-electrode EL-cell® with lithium metal counter electrode and Celgard® 3501 separator. Figure 2 shows the galvanostatic discharge profile conducted at 10 mA/g CF0.96 in two electrolytes: 1M LiBF4 in PC:DME (1:1) and 0.5M LiBF4 in PC:DME (2:8). The measured capacity was 836 mAh/g at 1.5 V in 1 M LiBF4 PC:DME (1:1), which is close to the theoretical capacity of 850 mAh/g (Table 1). The energy density was 1967 Wh/kg at an operating voltage of 2.60 V, which is considerably higher than that of the MnO2 and SO2 systems currently available today (Table 1).
Redox flow batteries are a class of energy storage devices suitable for stationary energy storage applications. The schematic of a redox flow battery is shown in Figure 1. Advantages of flow batteries include rapid response times, moderate cost, modularity, low maintenance, and flexible operation.
One important feature of flow batteries is the ability to independently maximize the capacity or power capability of the system.
The system capacity is dictated by ion concentration and electrolyte volume, while system power is controlled by electrode size and number of cell stacks. The all vanadium redox flow battery (VRB) is the most highly developed and characterized flow battery system currently in use, and several demonstration units are currently installed worldwide. The primary disadvantages of VRB technology are related to toxicity and cost of various cell components.
Flow battery research efforts at UK-CAER are developing lower-cost, less toxic redox chemistries based on aqueous manganese or iron redox couples with suitable energy and power densities for grid energy storage. Cyclic voltammetry and charge/discharge profiles of the proposed all-Fe redox flow battery are shown in Figures 2 and 3. New, highly conductive carbon electrode materials are also under development, using carbon nanospheres synthesized by hydrothermal synthesis (HTS) from carbohydrate precursors.
Numerous technologies are under development for storing off-grid electrical energy, including pumped hydropower, compressed air energy storage, flywheels, batteries, and electrochemical capacitors. Research efforts at UK-CAER are examining the efficacy of employing electrochemical capacitors for grid energy storage, including symmetric and asymmetric systems in aqueous and non-aqueous electrolytes. One asymmetric system of interest is based on an activated carbon positive/lithium titanate negative electrochemical couple in standard lithium-ion electrolytes. The activated double layer carbon is produced hydrothermally from inexpensive carbohydrate precursors, which results in the formation of mono-disperse carbon nanospheres. The lithium titanate negative is a zero-strain lithium-ion intercalation anode which undergoes negligible volume expansion during cycling. Significant improvements in capacitor energy densities are realized through the use of an asymmetric design.
The electrochemical performance of an asymmetric hybrid energy storage cell based on hydrothermally synthesized lithium titanate (LTO) nanospheres as the negative electrode and carbon nanospheres (CNS) as the positive electrode is presented. Non-aqueous battery electrolytes such as 1M LiPF6 (EC:DMC 1:1), 1M LiClO4 (EC:DMC 1:1), and 1M LiBF4 (EC:DMC 1:2) were used in the study. The performance of a symmetric carbon/carbon electrochemical capacitor cell is also presented for comparison. The LTO utilizes faradaic reaction to store charge and the activated CNS utilizes non-faradaic capacitive process to store charge. The main objective of this project is to develop an asymmetric hybrid supercapacitor based on non-aqueous battery electrolytes which can maintain high energy density, cycle life, and fast charge capability.
In order to improve the performance of the asymmetric capacitor the activated CNS should possess high reversible capacity in terms of anion adsorption, extremely fast electrochemical reaction with the anion, and exceptional cycle life. Figure 1 (a,b) shows the CNS prepared hydrothermally from monosaccharide sugars and CNS after activation, respectively. In order to isolate the capacitance or the capacity of the activated CNS over a defined voltage range (3-4.3V) for the anion, the electrodes were tested at a relatively slow rate of 1C in an asymmetric configuration vs. Li metal. As the cell was charged, Li+ was reduced at the Li metal negative electrode and the PF6- anion was adsorbed into a double layer on the positive electrode. The specific capacity based on anion adsorption was calculated to be 32, 29, and 34 mAh/g for LiPF6, LiClO4, and LiBF4 respectively.
For negative electrode selection in an asymmetric capacitor, the LTO should have high capacity in terms of lithium intercalation, exceptional cycle life, and rate capability. The hydrothermal synthesis of LTO spheres involves two steps, the synthesis of TiO2 spheres and  the hydrothermal synthesis of LTO from TiO2 spheres. The SEM images of TiO2 showed monodispersed spheres with a smooth exterior surface (Figure 1). The resulting LTO also showed spherical shaped particles but the exterior surface appeared rough with areas that appeared to have spalled or exfoliated off into thin nano-sheets. The surface exfoliation may be due to the fast injection of vacancies in the vicinity of the hydrous TiO2. Half-cell measurements were made with LTO as the working electrode and Li metal as the counter and tested over a voltage range of 1 and 3V at constant current charge/discharge at different rates (1C, 5C, 10C). The specific capacity of LTO was calculated to be 140 mAh/g vs Li metal.
The asymmetric cell was fabricated with activated CNS as the positive electrode and LTO as the negative electrode with a weight ratio of 5:1, respectively, in order to ensure full lithiation of LTO upon charge. The weight ratio was calculated using 30 mAh/g specific capacity for the activated carbon nanospheres and 140 mAh/g specific capacity for the lithum titanate. The LTO electrode showed two-phase lithium intercalation reaction, the activated CNS electrode shows linear voltage increase typical of capacitive storage, and the asymmetric cell reveals gradual sloping voltage profile as seen in the constant current charge/discharge voltage profiles in Figure 2. The Ragone plots in Figure 3 show the superior performance of an asymmetric cell when compared to a symmetric EDLC. Table 1 shows that an asymmetric capacitor cell had higher energy densities than a symmetric carbon/carbon electrochemical capacitor in all battery electrolytes.
Current anode materials for lithium-ion batteries are based on graphite, due to abundant supply, relatively low cost, and long cycle life. Graphite has a theoretical capacity of 372 mAh/g, but suffers from relatively low reversible capacity (~ 310 mAh/g), caused by formation of the solid electrolyte interface (SEI) during the first charge cycle of the cell (i.e. irreversible capacity loss). Electrode materials with enhanced power performance, reduced irreversible capacity loss, long cycle life, and higher capacities are required for continued improvements in energy density, power density, and cycling performance of next-generation lithium-ion cells.
Nanomaterials, including nanoparticulates and nanowires, are advantageous for lithium-ion batteries, due to reduced lithium ion diffusion lengths, maximization of the electrode/electrolyte interface, and reduced strain associated with intercalation/deintercalation processes. Anode development at UK-CAER is focused on the synthesis of nano-sized, lithiated metal oxide (Mo, Ni) anode materials with high theoretical capacities (~ 280-300 mAh/g). Metal oxide nanowires were fabricated through a combination of electrodeposition and thermal processing steps, using anodized aluminum oxide (AAO) templates for generation of the nanowire architectures. Figure 1 Shows the scanning electron microscopy images of free standing Mo nanowires that were deposited cathodically in an acidic medium, and free standing Ni nanowires, also deposited cathodically in an acidic medium. Figure 2 shows the cyclic voltammograms of as-prepared Mo nanowires and Ni nanowires at different scan rates. RE = Li metal; 1M LiPF6 in 1:1 EC/DMC.
Contact: Steve Lipka