NSF Workshop: Emerging Opportunities of Nanoscience to Energy Conversion and Storage

 

 

Section 4: Nanotechnology for Fuel Cells and Batteries

by Katsuyo Thornton, John Harb, and Liwei Lin

 

 

Introduction

 

In this section, technologies for energy conversion and storage based on electrochemistry are considered.  As electrochemical systems, fuel cells and batteries share much in common.  They convert chemical energies into electricity and vice versa.  They consist of an anode and a cathode, separated by an electrolyte (see Figure X-1).  A pair of reduction and oxidation reactions results in electric current generation.  Interfaces are important in their performance since the interfacial regions are where heterogeneous chemical reactions can take place.   Many of the governing physics of their basic cell components, such as thermodynamics and charge conservation are identical.  Therefore, many of the issues impeding improvements of fuel cells and batteries are also in common.  Nanoscience can make an impact in several aspects:

 

o New materials with unique properties resulting from nanoscale effects

o Nanofeatures and structures to enhance performance

o Improved fundamental understanding of physical mechanisms at the nanoscale

o Nano-derived methods for manufacturing and assembly

o Multifunctional materials that include energy storage and generation

 

Novel materials such as carbon nanotubes can be useful in electrochemical systems. For example, electrodes using single-walled carbon nanotubes have been shown to improve performance in both batteries (Terrones, 2003) and fuel cells (Girishkumar et al., 2005).  While this example has a cost disadvantage, such solutions may be appropriate in specialized applications.  Furthermore, interfaces of traditional materials can exhibit behaviors fundamentally different from those of the bulk due to the change in the local atomic structure. In nanostructured materials, the interfacial regions become dominant and closely spaced.  As a result, such materials possess unique properties not observed in the bulk materials. Maier (2005) recently reviewed the area of nanoionics and its significance to electrochemical systems in Nature Materials.  Indeed, it has already been demonstrated that nanoscale effects can be exploited to improve electrochemical systems, as discussed later.

 

While the issues addressed by these strategies are shared between the battery and fuel cell systems, there are fundamental differences between the two in other areas.  Since a fuel cell requires the transport of the fuel through the anode, fluid or gas dynamics are additional physics governing the system.  Understanding of nanofluidics and gas dynamics at the nanoscale thus becomes essential in optimizing the design.  There is significantly more freedom in designing the 3D architecture of batteries since, unlike fuel cells, no fuel or waste products need to be transported into/out of the cell.  For example, a battery can be designed with particles or rods of cathodic material coated with electrolyte material, embedded in a matrix of anodic material (Long, 2004).  Since battery technologies are relatively mature, especially compared to that of fuel cells, the breakthrough improvement must consider highly designed novel architectures, with precisely controlled domain sizes and patterns. Furthermore, challenges facing conventional fuel cells and non-conventional fuel cells, such as those based on biosystems or microscale devices, are also very distinct.  Therefore, we will examine the three aspects, conventional fuel cells, non-conventional fuel cells, and batteries, in turn, describing the current state-of-the-art, technological barriers, and possible nanoscience-based approaches to overcome the barriers. 

 

Conventional Fuel Cells

 

Fuel cells have the potential to offer significant efficiency and environmental benefits over todayÕs traditional energy technologies, but significant cost reductions and efficiency improvements are necessary for fuel cells to emerge as a practical energy alternative.  The recent National Research Council/National Academy of Engineering report (2003) on the hydrogen economy lists the primary barriers to full fuel cell adoption as: (1) cost, efficiencies, and lifetime of current systems, and (2) the lack of H2 infrastructure and production.  In order for an alternative energy technology to be competitive with conventional power sources, a target goal of $500/kW and twice the current efficiency must be achieved.  Nanotechnology could make a dramatic impact by improving the performance and by fulfilling materials requirements.

 

The basic principles behind fuel cells have been known for quite a long time, and the first demonstration of fuel cell was performed in the late 1830s by Sir William Robert Grove. Since then, there have been significant developments and demonstrations of various types of fuel cells. Commercialized units have been realized in some types of fuel cells (e.g., phosphoric acid fuel cells and molten carbonate fuel cells), but wider market penetration has not been achieved due to their high cost. Thus, the fate of conventional fuel cells depends not on the feasibility, but rather on cost effectiveness of the systems and their operation.  While the total cost per unit of electricity generated is the ultimate metric of the issue, there are several metrics that directly influence it.  These are:

  1. energy conversion efficiency
  2. durability of the fuel cell system
  3. material cost
  4. manufacturing cost
  5. operation and maintenance cost (other than fuel)
  6. fuel cost

Text Box: Fig. X-2. This nanostructured composite cathode was made using combustion chemical vapor deposition. The interfacial polarization resistance was measured and shown to be superior to those fabricated using conventional methods. From Liu (2004). Which of these sources hinder the success of fuel cells depend on the type of the fuel cell.  There are many conventional fuel cell approaches, and not all of them are discussed here.  Of those, the solid oxide fuel cells (SOFCs) for large-scale power generation and the proton exchange membrane fuel cells (PEMFCs, also known as the polymer electrolyte fuel cells) for smaller scale applications are considered to be promising approaches. The sources of high cost of these two types of fuel cells are quite different.  For example, a SOFC has a disadvantage of very high operation temperature, requiring expensive components especially in interconnects.  A PEMFC, on the other hand, must rely on highly efficient, expensive catalysts.  The issues associated with the SOFCs are reviewed first, followed by those of the PEMFCs.

 

Solid Oxide Fuel Cells

 

The SOFCs are fuel cells with solid, nonporous electrolyte of metal oxide such as yttria-stabilized zirconia.  They have advantages over other types of fuel cells, including their solid construction allowing design simplicity, ability to directly use unreformed fuel, and potential for very high efficiency. Thus, significant research and development efforts are underway to bring this technology to commercial reality (see, for example, Kendall (2005) and Holtappels, Vogt, & Graule (2005) for the overview).  DOEÕs Solid State Energy Conversion Alliance (SACA) Program is aiming to develop a market-ready SOFC with $400/kW cost by 2010 (Solid State Energy Conversion Alliance, 2005), a factor of ten reduction during the ten-year program.  Although sub-MW systems are possible (Minh, 2004), the SOFC technology is most advantageous in large-scale power generation because a very high efficiency (up to 70%) can be achieved when a gas turbine is combined with the fuel cell system. 

 

The major disadvantage of the SOFCs is the high operating temperature, which increases the materials cost and shortens the cell life, resulting in a high overall cost.  The currently demonstrated SOFC systems typically operate around 1000ûC, barring the use of inexpensive metal-alloy interconnects.  Therefore, the major effort has been geared toward lowering the operating temperature. The main reason for this high operating temperature stems from the low ionic conductivity of the ion-conducting materials and decreased cathodic reaction rate at low temperatures.  To address the conductivity, nanostructured fuel cell materials have recently been shown to possess enhanced conductivities of both ions and electrons at much lower temperatures (Kosacki et al., 2005).  Nanostructured electrodes are also advantageous because they provide larger reactive regions (i.e., three-phase boundaries and surface/interfacial areas).  For example, it has been demonstrated that nanostructured electrodes have increased transport efficiencies and decreased energy loss (Liu, Zha, & Liu, 2004; also see Fig. X-2.).  Thus, nanostructured composites are a promising alternative to standard electrodes.  In order to optimize the electrochemical performance, the relationship between the composite structure and the electrochemical performance must be identified.  To this end, electrochemistry at the nanoscale and the effects of microstructural characteristics on electrochemical kinetics must be understood.  Furthermore, fabrication methods must provide sufficient control of the microstructure such that desired nanoscale structures can be manufactured at a reasonable cost.

 

While nanocomposite electrodes offer significant benefits, these benefits are accompanied by enhanced diffusivity and reduced characteristic length scale of the composite structure.  This will likely affect the stability of the nanostructures during operation, and performance degradation over the expected long lifetime is still a major concern even at lower temperatures.  Therefore, various diffusion mechanisms occurring in these nanostructured materials (i.e., surface diffusion, grain boundary diffusion, etc.) must carefully be characterized, and their consequences understood.  Simulations of microstructural evolution play an important role in this area due to experimental difficulties associated with long-term usage.  Such simulations must be accompanied by experiments for validation.  Furthermore, either experiments or first principle calculation may be necessary in order to obtain realistic material properties needed in the calculation. 

 

Proton Exchange Membrane Fuel Cells

 

The PEMFCs have proton conductor membranes made of fluorinated sulfonic acid polymer or another similar polymer as the electrolyte.  This type of fuel cells is attractive for both transportation and smaller-scale stationary applications due to its high efficiency, low operation temperature, low pollutant emission, and relatively compact size.  In particular, for consumers, the PEMFCs with power output in the range of one to a few hundred kW are ideal alternatives for distributed power for either industry or residential buildings.  They are also well suited for applications where quick start-up and quick response to load changes are required (e.g., auxiliary power systems), and where high power density and low operating temperature (permitting intermittent operation without significant energy loss) are beneficial.

 

Despite the apparent benefits offered by the PEMFC technology, it requires affordable solutions for its key components.  The high capital cost for PEM fuel cells is by far the largest factor limiting market penetration.  Exploration of novel material alternatives could result for lower-cost components (cost reduction), higher power density output (increased efficiency), as well as increased durability.  Nanostructured materials, including nanoparticle catalysts, have played an important role in advancing the PEMFC technology.  However, achieving the goal of a practical PEMFC system will require further investigations of nanoscale materials for producing the membrane-electrode assembly unit, including nanocomposite-based proton-exchange membranes, low cost nanoscale low/non-Pt alloy catalysts, and nanostructured electrodes optimized for high performance, cost-effective PEMFC systems.  A variety of recently engineered nanostructured materials useful in these areas are reviewed by Malinauskas, Malinauskiene, & Ramanavicius (2005).  Fundamental understanding of nanoscale physics, especially in the area of electrochemistry and catalysis, is essential in these investigations.  First principle calculations and other types of simulations are often useful in research for catalytic materials.  For example, Kandoi et al. (2004) applied the periodic density functional theory combined with a microkinetic model to explain the selectivity of various catalysts for CO oxidation.  

 

Both hydrogen and hydrocarbon fuels may be used for PEMFCs.  Direct hydrocarbon fuel cells, such as those using methanol, are a relatively new development and face problems absent in hydrogen-fueled PEMFCs.  For example, methanol crossover through the electrolyte is a major problem especially since blocking the methanol transport can also hinder proton conduction.  Nanocomposite membranes containing nanoparticles have been intensively investigated in the recent years (see for example Choi, et al., 2004 and references therein).  While hydrogen-fueled PEMFCs do not suffer from this problem, they face challenges in hydrogen fuel generation and storage.  There are various approaches to hydrogen production, including those using nuclear energy, coal, and natural gas.  Since producing hydrogen from hydrocarbons then using the hydrogen as fuel introduces significant inefficiency, electrolysis using solar energy, and possibly wind energy to a limited scale, is likely the most environmentally friendly method, eliminating direct and indirect emission of carbon dioxide from hydrogen-fueled fuel cells.  In the short term, however, alternative sources must be identified.  The detailed examination of the technologies associated with hydrogen production is beyond the scope of this report.  A few examples of nano-based approaches are catalytic synthesis of H2 from hydrocarbons, hydrogen production based on biological systems, and nano-photoelectrochemical systems for solar-energy based production.  The opportunities in new materials for hydrocarbon processing were reviewed by Farrauto, et al. (2003).  The solar energy technology is discussed elsewhere in this report.  The challenges associated with bio-based hydrogen generation were discussed by Angenent, et al. (2004).  Possible hydrogen storage methods range widely from pressurized gas or cryogenic liquid cylinders to absorbers using novel materials such as carbon nanotubes (Terrones, 2003 and references therein), metal-organic frameworks and metal hydrides (Fichtner, 2005 and references therein).  The development of a practical system is extremely challenging due to the high diffusivity, low density, and high flammability of hydrogen.  The absorber-based storage technology is still in its exploratory phase, and its feasibility has yet to be determined.  While the success of research in this area remains to be seen, nanoscience impact on novel storage material can be the breakthrough required for the success of hydrogen-fueled fuel cells, especially for transportation applications, where hydrogen safety must meet higher standards.

 

Non-Conventional Fuel Cells

 

While exploratory, there has been research toward developing novel types of fuel cells.  As examples, two possibilities where nanoscience may play a significant role are discussed below.

 

Micro Fuel Cells

 

Most ongoing fuel cell research and development are targeted at the types of fuel cell technologies discussed above, which are limited to large-scale applications such as generation of industrial and residential power or automotive power. At much smaller scales, devices are currently powered by batteries.  The disadvantage of battery power is the need for electricity for recharging, which may not be available in some instances, and relatively large weight for both rechargeable and non-rechargeable batteries.  For micro fuel cells, possible fuels such as methanol or alcohol have high power per unit mass and per unit volume and may easily be carried.  Furthermore, micro fuel cells may offer significantly higher power densities.  The possible applications may include powering much smaller devices (such as microelectromechanical devices and Òlabs on chipsÓ), as well as standard hand-held electronic devices (such as cell phones). 

 

Micro fuel cells may be based on PEM fuel cell technology or similar, which are discussed above.  However, the design cannot simply be scaled down since additional challenges will result from much smaller construction (Maynard & Meyers, 2002).  Additionally, most of the portable applications will be for the end-users, and therefore the design requirements include a light, compact, stand-alone package that is user-friendly.  Thus, fuel, waste, and thermal management (FWTM) transparent to the users is important in this case (Cowey, Green, Mepsted, & Reeve, 2005).  Similarly, for microscale applications such as those for the lab-on-chip technology, FWTM devices, including pumps and heat exchangers, must also be designed, fabricated, and operated at the microscale.  Nanoscience will certainly play an important role in such devices.

 

As in the standard PEM fuel cells, fuel cells of this type will likely depend on highly efficient catalysts that enable the utilization of liquid fuels.  Nano-derived materials will likely be the answer to the quest for such material, as discussed earlier.  Furthermore, successful development of micro fuel cells will require fundamental understanding of fluid flow, as well as electrochemical transport, at very small scales.  For example, small-scale geometries can result in new phenomena in fluid flow (Stone, Stoock, & Ajdari, 2004 and references therein), such as the reduction of friction resulting from surface roughness of the flow boundary (Cottin-Bizonne, et al., 2003).  Consequently, the validity of the standard descriptions of underlying physics must be examined.  Although electrochemisty at the nanoscale is universally important in predicting the performance of such novel devices, the knowledge has not been fully developed.  Thus, testing and modifying the standard continuum equations and boundary conditions against experiments in micro/nanofluidic devices are essential in this endeavor.  Furthermore, understanding of electro-osmotic flows is important since removing wastewater, which is in liquid form in the low temperature environment that these systems must operate, is difficult.  As for other nanoscale devices, fabrication of nanofluidic devices poses a challenge, and both top-down and bottom-up methods of nanostructural assembly should be given considerations.  A combination of top-down and bottom-up approaches, where a top-down approach such as lithography is used to control a bottom-up self-assembly, may be particularly promising (Mijatovic, Eijkel, & van den Berg, 2005).

 

Bio-Based Fuel Cells

 

Continuous and long-term power supply generated directly from biological organisms will have great impact for systems of all dimensions, starting from nanotechnology, to microsystems, and to macro scale devices.  Nano and micro systems, such as sensors or actuators, requires power for operation.  In futuristic applications, for example, diagnosing medical problems and delivering drugs from inside the body has been a dream of doctors since Isaac Asimov's 1966 science fiction classic ÒFantastic Voyage,Ó in which a group of doctors were miniaturized and injected into a patient to remove a blood clot.  Doctors cannot be shrunk, but any future engineering device that resembles the Òfantastic voyageÓ will require power supply.  On the other hand, in the macro scale, implantable medical devices such as spinal cord stimulator and drug delivery devices used in spinal drug infusion therapy for pain relief applications require battery power.  All of the above current and future systems from nano to macro size will benefit from the generation of renewable power sources from biological processes. 

 

Figure X-3. Harness power from nature.  Photosynthesis process in green plants converts CO2 and H2O into glucose (C6H12O6) with the assistance of light.  The aerobic respiration process in living things such as the butterfly shown in the figure consume and convert glucose and oxygen into CO2 and H2O. One possibility is to learn from nature and apply engineering approaches to generate and extract energy from the natural processes as the power sources for artificial machinery or systems.

 

We can take the lesson from the energy cycle from ÒnatureÓ as illustrated in Figure X-3.  The energy of light coming from the Sun assists the photosynthetic process in green plants to convert carbon dioxide (CO2) and water (H2O) into glucose as:

                                                           (1)

Glucose is the common energy source for living things, including human, and can be considered as the ÒlargeÓ energy storage unit.  Animals take green plant as the food for glucose support and convert glucose and oxygen into carbon dioxide, water and ATP (adenosine triphospate) in the aerobic respiration process.  This can be conceptually expressed as:

                                                       (2)

ATP is considered as the ÒsmallÓ energy storage unit and acts as ÒcurrencyÓ in all energy transformation in cells.  Both the photosynthesis and aerobic respiration processes happen every second and everywhere on earth from living things as big as a whale to as small as a single cell.  They are not only the very fundamental processes for energy conversion but also indispensable processes for the supply of oxygen for living animals.  One energy possibility is to extract energy from these energy translation processes by building up artificial fuel cells to interact with living bacteria cells for engineering applications, such as to power nano or micro devices.  Moreover, because these processes actually occur at the cellular level, they are perfect for potential power sources for nano and micro systems and even for large systems.

 

Enzymatic fuel cells use enzymatic break down of fuels such as glucose rather than the metabolic activity of live microorganisms but otherwise perform the same function as microbial fuel cells (Halme, Zhang, & Ranta, 2000).  Researchers have used glucose, lactate, and even sewage as organic fuels, and glucose oxidase and lactate dehydrogenase as the enzyme biocatalyst (Liu, Ramnarayanan, & Logan, 2004; Wiltner & Katz, 2000).  Although diffusional redox mediators have been used, more recent research has centered on enzymes and redox mediators covalently linked to each other and to the anode (Katz, Shipway, & Wiltner, 2003; Wiltner & Katz, 2000; Chen, et al., 2001).  This Òbioelectrocatalytic wiringÓ of enzymes and mediators to the electrode (versus diffusional enzymes and mediators) has produced current densities as high as hundreds of microamperes per square centimeter (Katz, Shipway, & Wiltner, 2003).  A complementary bioelectrocatalytic enzyme system at the cathode accepts the electrodes from the anode to complete the circuit. 

 

In both cell- or enzyme-based bio fuel cells, two principle factors are limiting output and efficiency: (1) the ÒslowÓ and ÒrandomÓ diffusional transport of active substances and redox mediator in solution and (2) sensitivity of the redox mediator to oxidation by O2.  Nanotechnology could provide a unique solution to at least the diffusion-limited process.  In order to reduce the current-limiting effects of using a diffusional electron mediator in conjunction with the cells or enzymes that are also in a diffusional state in solution, one concept is to immobilize cells or enzymes onto a gold electrode functionalized with biological self-assembled monolayers – nanotechnology by nature.  It is postulated that such Òbio-wiringÓ of cells/enzymes to the electrode would facilitate direct transfer of electrons from cells/enzymes to the electrode, which would remedy both the current-limiting issues.  The improvement in efficiency has been demonstrated in the case of enzymatic fuel cells.  Figure 2 shows the possible architecture based on ÒnanotechnologyÓ that could improve the efficiency of cell-based fuel cells.  This architecture adopts a surface technique pioneered by Katz and Willner (Wiltner & Katz, 2000; Wiltner, 2002): covalently linking the electron mediator pyrroloquinoline quinone (PQQ) to a gold electrode functionalized by a monolayer of cystamine as illustrated in Fig. 2.  The PQQ monolayer is anchored to the gold electrode by sulfur.  We recognize that this is merely one example that nanotechnology could be used in fuel cell development; there could be many other possibilities that should be further explored.

Figure X-4. Bioelectronic wiring of cystamine/PQQ/NADP+ and cystamine/thylakoids the gold electrode; thylakoids are not shown to scale relative to PQQ, NADP+, and cystamine.  Interpreted and adapted from (Wiltner, 2000; Wiltner, 2002) for use with PFC.

 

Batteries

 

Batteries represent a critical component of almost any advanced energy strategy, whether for transportation or baseline energy from non fossil fuel sources (e.g. solar).  In addition, batteries enable mobile applications such as cell phones, lap top computers, and a host of others.  We also look to batteries to enable a new generation of technologies that include autonomous microsensing systems and micro implants for biomedical applications.   In applications for which they are the enabling factor, batteries are often the limiting factor, and frequently represent the largest and/or heaviest component of the system.  This report explores ways in which nanoscience and technology can be used to enhance battery performance. 

 

In order to understand the potential contributions of nanoscience to battery technology, it is important to understand the physical processes that determine battery performance.  Figure harb1 is a schematic diagram that illustrates a Li-ion battery.  Lithium batteries represent the largest segment of the portable battery industry and dominant battery in the computer, cell phone, and camera markets (Whittingham, 2004).  In the charged state, the amount of lithium is low in the cathode and high in the anode.  Upon discharge, lithium ions are released from the active material in the anode (typically carbon) and move through the electrode and separator to the cathode where they are incorporated into the cathode material.  The release of lithium ions at the anode during discharge is accompanied by the concurrent release of electrons, which travel through the conductive network to the anode current collector and from there to the external circuit.  Similarly at the cathode, electrons travel from the current collector through the conductive network to the active material where they are ÒconsumedÓ in the reduction reaction as lithium ions are incorporated into the cathode active material (e.g. CoO2).   Some of the physical phenomena important to this process as well as to the opposite (charging) process include:

Although their relative importance varies from system to system, the same general physics are operative for all battery systems.  As we examine ways in which nanoscience can influence battery behavior, we will look for nanoscale behavior that influences the physical processes listed above.

 

Battery performance metrics include the specific energy (Wh/kg), energy density (Wh/l), specific power (W/kg) and power density (W/l).  Values of these metrics for common primary (single use) and secondary (rechargeable) battery systems are shown in Figs harb2 and harb3, respectively (Winter, 2004).  As a rule of thumb, the practical energy density of a battery is approximately 25-50% of its theoretical value owing to the mass/volume of the inert components in a practical system, irreversible losses and incomplete utilization of the active material in the battery (Winter, 2004).   Other relevant performance characteristics include cycle life, shelf life and cost.

 

The balance of this section will examine ways in which nanoscience/technology can be used to enhance battery performance.  Strategies include: 1) the development of new materials whose properties are derived from unique nanoscale physics, 2) the use of nanofeatures and structures to improve the performance of existing materials, 3) enhancement of our fundamental understanding of nanoscale processes to enable optimization of existing energy storage devices and the development of new devices, 4) the development of new nano-derived methods for manufacturing and assembly of energy storage materials and devices, and 5) the development of multifunctional engineering materials designed for specific applications that involve electrochemical energy storage (see Section xx.xx).  

 

Materials

One opportunity for the improvement of batteries through nanoscience/technology is the exploitation of unique nanoscale physics to yield materials with improved performance and/or reduced cost.   For example, Xu et al. demonstrated qualitatively different behavior of electrode materials at the nanoscale (Xu, 2005).  In one case they demonstrated markedly improved electrochemical behavior for nanostructured a-Fe2O3 relative to microscale samples of the same material, where the amount of intercalated lithium increased from 0.03 Li per formula (micro) to 0.47 Li per formula (nano).   In another example, electrochemically inactive Li2MnO3 was made active by creating a nanostructured material.   These examples illustrate how phenomena at the nanoscale can lead to a wider selection of suitable materials, including materials that are less expensive and more environmentally friendly.

 

The unique characteristics of nanomaterials have been used in other ways to improve battery performance.  Lin and Harb used carbon nanotubes as the conductive filler in the cathodes of lithium ion batteries (Lin, 2004).  They found that the resistance of electrodes fabricated with single-walled nanotubes was significantly lower, even without compression, than that of electrodes made with a traditional carbon black filler, (Fig. harb4).  The performance enhancement due to nanotubes was attributed to the high aspect ratio of the conductive tubes which facilitated the formation of a conductive network, and the small size of the nanotubes that allowed them to surround and make multiple contacts to the larger microsized active material.  Unfortunately, the cost of nanotubes prohibits their use in large commercial batteries.  However, they may be useful for microbatteries where the materials cost is not the driving issue, and where the enhanced performance without compression is of particular benefit.  Because of the very high cost of nanotubes, the opportunity exists for the development of a low cost, highly conductive, high aspect ratio, nanosized material for use in batteries of all sizes.

 

The above materials examples were focused on improvement of todayÕs best technology.  In addition, it may be possible to use nanoscience/technology to develop completely new types of battery materials or even new types of battery systems. 

 

Nanostructures

While the previous section examined the use of materials whose unique properties were derived from the nanoscale, this section examines the use of nanofeatures or nanostructures where the physics are essentially the same as for the larger material, but the performance is enhanced due to the structure.   An example of the impact of structure on battery performance is found in the microbattery work by Humble (2001, 2003).  Optimization of deposition conditions for the cathodic material increased the capacity and rate capability of the cell by well over an order of magnitude.

 

Sides et al. (2002, 2005) prepared nanostructured electrodes for lithium-ion batteries with use of a porous template.  The resulting electrodes were highly porous, had a very high surface area, and showed dramatically improved performance on a per mass basis over traditional electrodes (Fig. harb5).    In particular, it was possible to discharge the electrodes at a very high rate relative to the amount of active material present.  Unfortunately, template synthesis is tedious and likely expensive on the large scale.  In addition, the capacity per area is small owing to the thin porous layers that are formed.  Efforts to increase the capacity have resulted in significant improvements (Sides, 2002), but have yet to reach levels practical for large cells.

 

A novel 3D nanostructured battery was recently proposed by Long et al. (2004) where a very thin electrolyte layer is formed around a random 3D network of electrode material (Fig. harb6) (see also Rhodes, 2004).   The other electrode would then fill the space that surrounds the electrolyte (see Fig. harb6).  All battery components- anode, cathode and electrolyte- are continuous throughout the Òsponge-likeÓ structure.   A key advantage of the 3D battery is the short transport distance between the anode and cathode.   However, there is likely a practical limit to how thick such a battery may be fabricated.   Effective distribution of current throughout the 3D structure will also be challenging for some battery chemistries.  In spite of the challenges, this type of battery structure represents a departure from traditional battery fabrication and, if enabled by the appropriate nanostructures, may make a significant impact.  Other types of 3D batteries with micro rather than nano structures have also been proposed (Ryan, 2003; Long, 2004).

 

The are other examples of nanostructured battery materials in the literature (e.g., Fey, 2005; Treger, 2005).  For example, LiFePO4 is a relatively new active material for Li-ion batteries whose viability depends on its nanostructure in order to overcome significant transport limitations in the solid phase (Treger, 2005). 

 

In summary, nanostructures can be used to significantly enhance battery performance.   The cost and limited capacity per area represent formidable challenges for the application of some structures to large commercial markets. 

 

Fundamental Understanding at the Nanoscale

Another way in which nanoscience/technology can help advance energy storage technologies is through the development of improved understanding of the nanoscale processes that impact the behavior of practical systems.  A fundamental understanding of these processes will lead to new strategies to enhance battery performance.

 

An example of an area where additional understanding is needed is the solid electrolyte interface (SEI) that forms primarily in the anode of lithium ion batteries (Christensen, 2004; Aurbach, 2005).  This layer forms by reaction with the electrolyte and grows until electrons can no longer effectively get to the reaction site due to the insulating nature of the layer.  Although electronically insulating, the layer readily conducts lithium ions.  Thus, lithium ions are transferred from the electrolyte, through the SEI, to the electrode where they are reduced during charging.  Failure to form a stable SEI greatly reduces the cycle life of the battery.  Changes to the SEI during cycling also reduce the battery lifetime.  A fundamental understanding of the processes that control the formation and stability of the SEI will lead to the formation of improved SEI layers, and perhaps to the development of an artificial SEI nanolayer.

 

There are other aspects of batteries where our fundamental understanding at the nanoscale is lacking.  In addition, fundamental understanding of new types of electrochemical systems that may contribute to energy storage in niche areas is needed (e.g., see Long, 2004).  

 

Nanofabrication

One of the reasons that nanotechnology has received so much attention recently is the promise that it holds for efficient cost-effective manufacturing techniques (Schulte, 2005).  The application of such techniques to battery fabrication has the potential to make a significant impact.   For example, it may be possible to utilize nanotechnology to fabricate active materials for lithium batteries.  However, the performance of these active materials not only depends on the composition of the material, but also on the material structure (Whittingham, 2004).   Thus, care must be taken to produce a material with a structure suitable for battery use.

 

Kawamura et al. (2005) used nanofabrication to produce nanostructured LiCoO2 particles (Fig. harb7), and were able to produce particles with an active crystalline phase.  As a result, they observed a two-fold increase in capacity at high rates, and significantly improved voltage profiles.   It seems possible that nanofabrication can be used to both reduce fabrication costs and enhance the performance of some battery materials.

 

Multifunctional Materials

A final area where nanotechnology may influence energy storage is in the creation of multifunctional materials.  An example of such a material is an airplane wing that also serves as the battery to store energy.  The vision of this multifunctional material is that it is more than just a wing that contains a battery.  Rather, the same materials used to form the battery are also the structural components of the wing.  There are undoubtedly other multifunctional concepts to be developed, and examples of such materials are yet to be found.

 

Summary

An understanding of the processes that govern battery behavior can be combined with recent advances in nanoscience/nanotechnology to create new and improved energy storage devices for the next generation.  Opportunities exist in at least five different areas as described above. 

 

 

Figure harb1.  Schematic diagram of lithium-ion battery.  Black- conductive carbon filler, Red Outline- anode active material, Blue Outline- cathode active material (binder not shown).

Current Collector
 
 


 

Figure harb2.  Energy storage capability of common commercial primary battery systems (Winter, 2004).

 

 

 

 

 

 

 

Figure harb3.  Energy storage capability of common rechargeable battery systems (Winter, 2004).

 

 

 

 

Figure harb4.  Influence of carbon nanotubes on internal resistance of a lithium-ion cathode (Lin, 2004).

 

 

Figure harb5.  Scanning electron micrograph of a template-synthesized V2O5 electrode prepared from 50-nm pore diameter polycarbonate template membranes (Sides, 2002).

 

 

 

Figure harb6.  ÒSpongeÓ architecture for 3D nanobattery (Long, 2004).

 

 

 

 

Figure harb7.  Nanostructured LiCoO2 powders for lithium ion batteries (Kawamura, 2005).

 

 

 

 

 

 

References for this section (Alphabetical; formats not completely consistent):

 

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