What Are Batteries, Fuel Cells, and Supercapacitors?
Chem. Rev. 2004, 104, 4245-4269
Dr. Martin Winter is currently University Professor for Applied Inorganic Chemistry and Electrochemistry at the Institute for Chemistry and Technology of Inorganic Materials, Graz University of Technology (Austria). His fields of specialization are applied electrochemistry, chemical technology and solid state electrochemistry with special emphasis on the development and characterization of novel materials for rechargeable lithium batteries.
Dr. Ralph J. Brodd is President of Broddarp of Nevada. He has over 40 years of experience in the technology and market aspects of the electrochemical energy conversion business. His experience includes all major battery systems, fuel cells, and electrochemical capacitors. He is a Past President of the Electrochemical Society and was elected Honorary Member in 1987. He served as Vice President and National Secretary of the International Society of Electrochemistry as well as on technical advisory committees for the National Research Council, the International Electrotechnic Commission, and NEMA and on program review committees for the Department of Energy and NASA.
While this article is more than 10 years old, I chose this as part of my reading list because it gives a thorough but clear overview of the different modes of energy storage and conversion. While the materials and chemistries involved have progressed beyond the examples provided here, the fundamental principles have not changed much and still provide a context from which more current technical developments can be understood and appreciated. Because it is a general overview, the article was very helpful in explicitly reviewing and defining many of the common terms used in electrochemistry. For instance, it thoughtfully pointed out the difference between a cell and a battery in clarifying the confusing language used in “fuel cell”: a cell is a single electrochemical power system also called an element while cells arranged in parallel constitute a battery. Therefore, the use of “cell” in fuel cells is misleading. Unless it is of relevance (labeled with ATTOW or at the time of writing), I skipped note-taking on discussions of what were then, at the time of writing, “current” technology and development as the article is old and a lot have happened in the more than ten years since the article was published. My notes are limited to relevant fundamental content. In retrospect, it would have been helpful to have read this article before reading the article on the Li-ion battery perspective as this article explains in a more explicit way many of the key terms and parameters discussed in the perspective.
Notes taken verbatim from the article to preserve the precision of the content are in italics.
1. Introduction
1.1. Batteries versus Fuel Cells versus Electrochemical Capacitors
Batteries, fuel cells, and capacitors are all forms of energy storage and conversion. In these three devices, “the energy-providing processes take place at the phase boundary of the electrode/electrolyte interface and that electron and ion transport are separated.” All three consist of two electrodes immersed in an electrolyte solution (see Figures 1 and 2).
In both batteries and fuel cells, electrical energy is derived from the chemical energy of spontaneous redox reactions. A battery, however, is a closed system with the charge transfer taking place between the anode and cathode which participate in the redox reactions as active masses (participating in the chemical reactions that produce the current); the energy conversion and storage are integrated in the same compartment. In a fuel cell, the redox reactants are delivered externally (as fuel) and the electrodes act as charge transfer media and not active masses. The energy is stored in the fuel tank and the conversion takes place in the fuel cell assembly. As a footnote, the authors point out that a cell is a single electrochemical power system also called an element while cells arranged in parallel constitute a battery. Therefore, the use of “cell” in fuel cells is misleading.
In defining what electrochemical capacitors are, I use the authors’ description verbatim from the article:
In electrochemical capacitors (or supercapacitors), energy may not be delivered via redox reactions and, thus the use of the terms anode and cathode may not be appropriate but are in common usage. By orientation of electrolyte ions at the electrolyte/electrolyte interface, so-called electrical double layers (EDLs) are formed and released, which results in a parallel movement of electrons in the external wire, that is, in the energy-delivering process.
Of these three, batteries have the most commercial applications and market base. Supercapacitors are used in memory devices while fuel cells have found more exotic uses in space shuttles.
Metrics used for comparisons include:
The terms “specific energy” [expressed in watthours per kilogram (Wh/kg)] and “energy density” [in watt-hours per liter (Wh/L)] are used to compare the energy contents of a system, whereas the rate capability is expressed as “specific power” (in W/kg) and “power density” (in W/L). Alternatively, the attributes “gravimetric” (per kilogram) and “volumetric” (per liter) are used.
Technologies of existing batteries, fuel cells, and supercapacitors ATTOW put fuel cells in the high -energy category, the supercapacitors in the high-power category, and the batteries intermediate in both energy and power:
A plot of specific energy density places gasoline in the highest category of kWh/m3 and hydrogen gas with the highest kWh/tonne (Figure 4). In both cases, current batteries much lower figures.
The authors provide the following approximations for comparing theoretical versus practical energy capacities: “The theoretical values in Figure 4 are an indication for the maximum energy content of certain chemistries. However, the practical values differ and are significantly lower than the theoretical values. As a rule of thumb, the practical energy content of a rechargeable battery is 25% of its theoretical value, whereas a primary battery system can yield >50% of its theoretical value in delivered energy.” Fuel cells hold the best promise of >70% efficiency in providing electrical energy. These differences in theoretical and practical energy storage capacities can be attributed to (verbatim from article):
(1) inert parts of the system such as conductive diluents, current collectors, containers, etc., that are necessary for its operation,
(2) internal resistances within the electrodes and electrolyte and between other cell/battery components, resulting in internal losses, and
(3) limited utilization of the active masses, as, for example, parts of the fuel in a fuel cell leave the cell without reaction or as, for example, passivation of electrodes makes them (partially) electrochemically inactive.
However, as batteries and fuel cells are not subject to the Carnot cycle limitations, they may operate with much higher efficiencies than combustion engines and related devices.
1.2 DEFINITIONS
The following terms used in the discussion of batteries, fuel cells, and capacitors are defined: battery, primary battery, secondary battery (or rechargeable or accumulator), specialty battery, anode, cathode, active mass, electrolyte, separator, fuel cell, electrochemical capacitor, open- and closed-circuit voltage, discharge, charge, internal resistance or impedance, Faraday constant F, thermal runaway,
Below are the verbatim definitions for the three devices discussed in this article for reference:
A battery is one or more electrically connected electrochemical cells having terminals/contacts to supply electrical energy.
A fuel cell is an electrochemical conversion device that has a continuous supply of fuel such as hydrogen, natural gas, or methanol and an oxidant such as oxygen, air, or hydrogen peroxide. It can have auxiliary parts to feed the device with reactants as well as a battery to supply energy for start-up.
An electrochemical capacitor is a device that stores electrical energy in the electrical double layer that forms at the interface between an electrolytic solution and an electronic conductor. The term applies to charged carbon-carbon systems as well as carbon battery electrode and conducting polymer electrode combinations sometimes called ultracapacitors, supercapacitors, or hybrid capacitors.
1.3 THERMODYNAMICS
Basic thermodynamic considerations apply to these electrochemical systems in the form of DG = DH – TDS. TDS is the “heat associated with the organization/disorganization of materials”.
Applied to electrochemical systems: DG = -nFE, the net useful energy. The amount of electricity produced, nF, is determined by the total amount of materials available for reaction and can be thought of as a capacity factor; the cell voltage can be considered to be an intensity factor.
In a more precise expression (concentrations are used in General Chemistry in place of activities):
The van’t Hoff isotherm identifies the free energy relationship for bulk chemical reactions as
where R is the gas constant, T the absolute temperature, AP the activity product of the products and AR the activity product of the reactants. This gives rise to the well-known Nernst equation:
Faraday’s laws, as summarized below, give the direct relationship between the amount of reaction and the current flow. There are no known exceptions to Faraday’s laws.
g is the grams of material transformed, I is the current flow (amps), t is the time of current flow (seconds, hours), MW is the molecular or atomic weight of the material being transformed, and n is the number of electrons in the reaction.
The reversible heat effect is given by:
Measuring the voltage as a function of temperature (dE/dT) allows prediction of heat change upon charge and discharge: a positive dE/dT indicates heating on charge and cooling on discharge. Lead acid batteries have negative dE/dT while Ni-Cd have positive dE/dT.
The total heat released during cell discharge is the sum of the thermodynamic entropy contribution plus the irreversible contribution. This heat is released inside the battery at the reaction site on the surface of the electrode structures.
Heat release is an important consideration for high-rate operations requiring heat dissipation to avoid thermal runaway.
1.4 KINETICS
Thermodynamics describe reactions at equilibrium and the maximum energy release for a given reaction. Compared to the equilibrium voltage (= open circuit voltage, EOCV), the voltage drops off ( = “electrode polarization” or “overvoltage”) when current is drawn from the battery because of kinetic limitations of reactions and of other processes must occur to produce current flow during operation.
Kinetics of electrode reactions differ from general chemical reaction kinetics in two ways both stemming from structural factors:
(1) the influence of the potential drop in the electrical double layer at an electrode interface as it directly affects the activated couples and
(2) the fact that reactions at electrode interfaces proceed in a two-dimensional, not three-dimensional, manner.
The detailed kinetics of electrochemical systems involve consideration of the individual steps of the mechanisms which may include physical, chemical, and electrochemical steps, including charge-transfer and transport processes. Polarization processes affect the kinetics in three different ways:
(1) activation polarization is related to the kinetics of the electrochemical redox (or charge-transfer) reactions taking place at the electrode/electrolyte interfaces of anode and cathode;
(2) ohmic polarization is interconnected to the resistance of individual cell components and to the resistance due to contact problems between the cell components;
(3) concentration polarization is due to mass transport limitations during cell operation.
The polarization can be quantified by:
where EOCV is the open-circuit voltage of the cell and ET is the terminal cell voltage with a flowing current I. Mathematical details of these polarization effects are given in the article.
Most battery electrodes are porous structures in which an interconnected matrix of small solid particles, consisting of both nonconductive and electronically conductive materials, is filled with electrolyte. Porous electrode structures are used to extend the available surface area and lower the current density for more efficient operation.
1.5 EXPERIMENTAL TECHNIQUES
Below are some of the experimental techniques employed to study electrochemical reactions in batteries:
1) An instantaneous current-voltage measurement upon discharge illustrates the different polarization effects on the voltage (see figure 6) and is useful in determining cell capacity, effect of charge-discharge rate, and temperature and information on state of the battery
2) Impedance behavior (see article for the detailed explanation)
Both of these are non-destructive methods. Spectroscopic methods can be used for a more detailed characterization of material change but requires “tearing down” the battery components.
1.6 CURRENT DISTRIBUTION AND POROUS ELECTRODES
Most practical electrodes are a complex composite of powders composed of
particles of the active material,
a conductive diluent (usually carbon or metal powder), and
a polymer binder to hold the mix together and bond the mix to a conductive current collector.
A typical composite battery has 30% porosity to increase the surface area for reactions and decrease polarization effects. The pores are filled with electrolytes shortening diffusion path lengths to reaction sites on the electrode.
Ideally there is uniform distribution of current production on the surface to maximize efficiency and performance. Pure metallic electrodes such as zinc or lithium require minimum supporting conductive structures. See article for the key parameters determining reaction sites in porous electrodes.
2.1 INTRODUCTION AND MARKET PROSPECTS
There are three classes of batteries: primary (non-rechargeable), secondary (rechargeable), and specialty batteries (built for a special purpose). The advantages and disadvantages of batteries are summarized in a table in the article copied below:
Table 1 in the article gives the market share in dollars of the different batteries available then (2004 values).
2.2 BATTERY OPERATIONS
Basic elements and operation are discussed in this section.
Electrodes:
The negative electrode (anode) is a good reducing agent (source of electrons) like reactive metals lithium, zinc, or lead. The positive electrode (cathode) should be a good oxidizing agent or electron acceptor such as lithium cobalt oxide, manganese dioxide, or lead oxide.
Electrolyte:
The electrolyte should be a pure ionic conductor separating the anode from the electrode. If the anode and the cathode come in contact, the battery shorts and the full energy from the redox reactions is released as heat in the battery.
The chemical stability of the electrolyte is limited to within certain voltage ranges beyond which (called the “window”), the electrolyte may start to decompose. This window depends on the electrolyte composition and its purity.
Aqueous solvent-based systems generally have high conductivities (~1 S/cm) due to dielectric constant values that stabilized dissociated ions and high solvation capacities that produce hydrogen bridge bonds that promote hydrogen ion conductivity. The thermodynamic voltage stability window for aqueous systems is ~1.23 V which can go up to ~2 V due to kinetic effects.
Non-aqueous organic solvent-based system of lithium batteries have very low conductivities in the order of 10-3 to 10-2 S/cm due to lower dielectric constants and solvating power of organic solvents which promote ion pair formation even at low salt concentrations. They are also more viscous. The voltage stability window for organic solvents can be as high as ~4.6 V, beyond which decomposition or polymerization may occur.
2.3 CHARACTERISTICS OF COMMON BATTERY SYSTEMS
The article provides a list of the most common commercial battery systems that exist in 2004. Figures 13 and 14 show the energy storage capabilities for common primary and secondary batteries. In the next sections, the authors describe the charge/discharge mechanisms for common battery systems.
2.4 PRIMARY BATTERIES
Figure 15 shows schematic diagrams of the charge/discharge mechanisms of some common batteries. In primary batteries, the products are stable and the reaction is not easily reversed and the battery is not rechargeable.
Li-CuS in 15a
During the cell reaction, Cu is displaced by Li to form stable Li2S and Cu.
Li battery of 15b
The Li electrode is discharged by oxidation to Li+ which goes into solution. The battery can be recharged because Li+ can be reduced back to Li metal and redeposited. The Li redeposition is hampered by non-uniform, dendritic formations that can cause safety problems.
Lead battery of 15c
The lead electrode is discharged by oxidation to Pb2+. Because of its low solubility in a sulfuric acid electrolyte solution, it precipitates out as PbSO4 at the reaction site on the electrode surface. In the charge reaction, the PbSO4 dissolves and the Pb2+ is converted back to metallic Pb on the electrode surface.
Li ion battery of 15d
An insertion electrochemical reaction takes place which is “a solid-state redox reaction involving electrochemical charge transfer, coupled with insertion of mobile guest ions (in this case Li+ cations) from an electrolyte into the structure of a solid host, which is a mixed, that is, electronic and ionic, conductor (in this case graphite).” This type has high reversibility because of structure and shape stability. Good insertion electrodes must have high electronic and ionic conductivity. For poor conductors like MnO2, highly conductive C can be mixed with the electrode matrix (15e).
Zn-MnO2 batteries
A detailed technical description of zinc manganese batteries (dominating the market on primary batteries) follow the brief summary above. The mutli-step discharge redox mechanisms for the alkaline type (KOH) are diagrammed in Figure 17.
Zn-air batteries
The Zn-air battery system has the highest energy density of all aqueous systems because only the zinc powder anode is contained in the cell and the oxygen is extracted from the surrounding air. The Zn-air energy density equals that of the lithium thionyl chloride battery which is the highest of the lithium batteries. The air electrode is a polymer bonded carbon sometimes with a manganese dioxide catalyst. Aspirin-size batteries of this type are used in hearing aids.
Primary lithium metal batteries
This battery uses a reactive lithium metal anode that requires a solvent that would form a solid electrolyte interphase (SEI) protective layer. The SEI layer should allow selective Li+ transport. The anode undergoes a displacement reaction (Li «Li+) like in 15b and can generate voltages of up to ~3.7 V or higher (lithium has a strong negative potential). Low lithium ion transport rate through the SEI and low conductivity of the nonaqueous electrolyte lower the rate capability [?]. Commercial types can have solid or liquid cathodes. Examples of solid cathodes are carbon monofluoride, manganese dioxide, iron (IV) sulfide, and copper (II) sulfide. Examples of electrolytes include propylene carbonate – dimethyl ether, lithium triflate (LiSO3CF3), and lithium perchlorate.
Lithium thionyl chloride batteries
Thionyl chloride is both the electrolyte solvent and the soluble cathode. The inorganic electrolyte us LiAlCl4 dissolved in SOCl2 (thionyl chloride). Reaction between the lithium metal and the electrolyte produces an SEI layer of LiCl and S which are also the discharge reaction products at the carbon positive electrode (cathode) where thionyl chloride is reduced. See Figure 18 for a diagram. Once the carbon electrode is completely covered with the electronically insulating discharge products, the reaction stops.
Lithium-sulfur dioxide battery
This battery also uses a liquid cathode: sulfur dioxide dissolved in propylene carbonate or acetonitrile for example. Or, SO2 can be liquid at very high pressures. It follows a similar reaction mechanism as in figure 18 but the SEI is Li2S2O4 which is also the cathode discharge product.
Other lithium batteries include lithium-silver-vanadium oxide systems in heart defibrillators and lithium iodine in pacemakers.
2.5 RECHARGEABLE BATTERIES
Rechargeable batteries generally have lower energy storage capacities than primary batteries. The materials used are limited as they need to be optimized for longer operational lives and more robust construction.
Lead acid battery
This dominates the market. The mechanism is shown in 15c and the complete reaction in Figure 19. The chemical components are the lead and lead oxide electrodes and aqueous sulfuric acid electrolyte solution. Because of the heavy electrodes and electrolyte solution, the specific energy is low. In addition, factors such as excess acid requirements to maintain ionic conductivity of the electrolyte at charged and discharged states, low mass utilization, and use of grids, separators, cell containers etc. lower the practical value of the specific energy (in Wh/kg) down to 25% of theoretical for rechargeables. It is low cost and recyclable (up to ~98% as quoted in the article). Footnote on specific energy: a 30 Wh/kg lead acid battery literally means that 1 kg of lead can power a 60 W bulb for 0.5 hours.
Nickel-cadmium (Ni-Cd) batteries
The reversible anode is cadmium, the cathode is nickel hydroxide (NiOH)2, and the electrolyte is alkaline KOH solution. The discharge product at the anode is Cd(OH)2. At the nI(OH)2 cathode, reversible proton insertion/deinsertion takes place during charge/discharge. This is the first small sealed rechargeable battery.
Nickel metal hydride (Ni-MH) batteries
The development of alloys for storing hydrogen made the construction of this battery possible. The anode is the hydrogen-storing alloy that undergoes proton insertion, replacing the Cd anode of the Ni-Cd system. It uses the same cathode and electrolyte as the Ni-Cd battery. This has a higher energy storage capacity and lighter weight than N-Cd which it replaced. ATTOW, it was the battery of choice for hybrid electric vehicles.
Li ion battery
The current lithium ion battery technology at that time had a carbon-graphite anode, a lithium cobalt oxide cathode, and a non-aqueous electrolyte solution of lithium hexafluorophosphate (LiPF6) salt with ethylene carbonate organic solvent. Li+ ion is inserted/de-inserted analogous to the H+ ion in the NiMH battery. This battery would have the highest energy except for it being plagued with mossy and dendritic lithium metal formation in the electrolyte at charging.
2.6 SELECTION CRITERIA FOR COMMERCIAL BATTERY SYSTEMS
1) Mechanical and chemical stability: “Mechanical and chemical stability limitations arise from reaction with the electrolyte, irreversible phase changes and corrosion, isolation of active materials, and local, poor conductivity of materials in the discharged state, etc.”
2) Energy storage capability: “The reactants must have sufficient energy content to provide a useful voltage and current level, measured in Wh/L or Wh/kg. In addition, the reactants must be capable of delivering useful rates of electricity, measured in terms of W/L or W/kg. This implies that the kinetics of the cell reaction are fast and without significant kinetics hindrances.”
3) Temperature range of operation: “For military applications, the operational temperature range is from -50 to 85 °C. Essentially the same temperature range applies to automotive applications. For a general purpose consumer battery, the operating temperature range is 0-40 °C, and the storage temperatures range from -20 to 85 °C.”
4) Self-discharge: “Self-discharge is the loss of performance when a battery is not in use…Li-MnO2 primary cells will deliver 90% of their energy even after 8 years on the shelf; that is, their self-discharge is low. Some military batteries have a 20-year storage life and still deliver their rated capacity. On the other hand, rechargeable batteries can be electrically restored to their operating condition and generally have more rapid loss of capacity on storage. The rechargeable Ni-MH cell, for instance, will lose up to 30% of its capacity in a month. Usually, self-discharge increases with temperature.”
5) Shape of the discharge curve: “For operation of an electronic device, a flat, unchanging, discharge voltage is preferred. A sloping discharge is preferred for applications when determining the state-of-charge is important. This may be modified somewhat by the impact of cost. Although a constant brightness is preferred in a flashlight, the user may select carbon-zinc with a sloping discharge for its lower cost.”
6) Cost
7) Safety
Other criteria for rechargeables:
Ability to recharge and deliver power: “The rechargeable battery systems place a severe added requirement. The active materials must be capable of being restored exactly to their original condition (crystal structure, chemical composition, etc.) on reversal of the current flow (charging). After being recharged by current reversal, the electrode materials must be able to deliver the same rate of discharge while maintaining their voltage level. Very few chemical systems exhibit this characteristic.”
Cycle life: “A commercial cell must be capable of completely discharging its energy and then fully recharging a minimum of 300 times and not lose >20% of its capacity. This requires a very robust system and reversible electrode reactions. There can be no side reactions that result in the loss of the active materials during the charge-discharge cycle.”
Charge time: “For convenience, recharging in 15 min is accepted for many consumer applications. However, fast charging places a stress on the robustness of the electrode reactions and may result in shortened cycle life. Most batteries require 3-8 h to recharge completely and maintain their required cycle life. This slower charge rate allows time for the atoms and molecules to find their correct positions in the charged material.”
Overcharge/overdischarge proteiction: “When a battery is forced outside its thermodynamic voltage levels, the reaction path becomes unstable; irreversible new reactions can occur, and new compounds can form. These events harm the active material and either reduce the capacity or render the system inoperable. In addition, unsafe battery conditions may occur under overcharge/overdischarge conditions. The Ni-Cd, Ni-MH, and lead acid have a built-in overcharge and overdischarge characteristic based on an oxygen recombination mechanism. Cell designs often use the ratio of the capacities of each electrode (cell balance) to accomplish protection of the battery system. It is also possible to use electronic controls to control the charge and discharge voltage limits within safe limits. The lithium-cobalt oxide cathode in the Li ion system is protected from overvoltage and overdischarge by electronic means. Voltage excursions outside its operating range can cause irreversible changes in its crystal structure and damage cell operations.”
3. FUEL CELLS
3.1 INTRODUCTION AND MARKET ASPECTS
Hydrogen and hydrocarbons store considerably more energy than materials used in common batteries.
Figure 21 shows the reactions in some common fuels
Figure 22 shows the components of a functioning fuel cell
Figure 23 shows the components of a complete fuel cell system
In Table 3, the authors provide advantages, disadvantages, and other notable comments about their history and operations for different types of fuel cells: alkaline fuel cell, polymer electrolyte membrane fuel cell, direct methanol fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, and solid oxide fuel cell.
3.2 FUEL CELL OPERATION
Similar to batteries in that they convert the chemical energy in the fuel to electrical energy. They also operate with an anode, a cathode, and an electrolyte for ion conductivity. Unlike batteries, however, the fuel and oxidant are delivered externally and are not contained within the cell itself.
Table 4 gives information about the anode and cathode compositions, the chemicals that are delivered at each electrode, the electrolyte composition, and the operating temperatures.
Fuel cells follow the fundamental thermodynamics, kinetics, and operational characteristics discussed for electrochemical cells previously. For a H2 – O2 fuel cell, the overall reaction and thermodynamic parameters are:
Electrolytes can be basic or acidic. The reaction in acidic electrolytes are as follows:
Catalysts help accelerate the dissociation reaction of hydrogen at the anode. They also help decompose the intermediate hydrogen peroxide faster to avoid its corrosive effect on the carbonaceous electrode material and its effect on reducing the voltage from the OCV.
In fuel cells, the electrodes have more complex structures and function in three ways:
1) to ensure a stable interface between the reactant gas and the electrolyte,
(2) to catalyze the electrode reactions, and
(3) to conduct the electrons from or to the reaction sites.
Problem arise in controlling the interactions at the 3-phase boundary where the reactant gas, the solid electrode, and the liquid electrolyte interphase.
Fuel cells can operate at very high efficiencies up to 60-70% rising to 90% if the waste heat is used.
As with batteries, the electrolyte must be a pure ionic conductor to prevent shorting issues. These electrolytes can be liquid, molten salt, polymer, or ceramic.
For low-T fuel cells, the preferred gas is hydrogen gas. For high-T fuel cells, hydrocarbon fuels such as methane or gasoline can be delivered directly to the cell. For low – T fuel cells, the hydrocarbon must first be converted to hydrogen, a process that can produce CO, H2S, and CO2 that can irreversibly block the Pt catalyst.
Other possible fuels are hydrazine, methanol, and ammonia.
One disadvantage of fuel cells is the need for supporting devices which consume current thus lowering the overall efficiency.
The fuel cell stack makes up about 50% of the overall volume of the system and has very low energy density compared to batteries.
The slow kinetics, especially at the oxygen cathode, means low power capability, lower than that of batteries and gasoline engines.
3.3 CHARACTERISTICS OF VARIOUS TYPES OF FUEL CELLS
Many of these fuel cell types had their beginnings on space shuttles.
Alkaline fuel cell
The cell reactions are:
· 
· can have efficiencies as high as 60%.
· uses a KOH-based electrolyte.
· Both electrodes use noble catalysts
· Can achieve higher voltages in alkaline electrolyte due to the more facile oxygen reaction via the HO2- intermediate. In this set-up, non-noble catalysts (nickel, silver) work well because of the better kinetics.
· Pure hydrogen and oxygen are required as the electrode pores are susceptible to clogging by CO2 and catalyst poisoning by CO and sulfide impurities
Polymer electrolyte fuel cell
The electrodes are formed on a thin layer on each side of a proton-conducting polymer membrane, used as electrolyte… It consists of a solid polymer PTFE backbone with a perfluorinated side chain that is terminated with a sulfonic acid group.
Direct methanol fuel cell
· The fuel is a liquid methanol – water mixture fed at the anode (easy to transport and store because it is liquid)
· The reactions are:
· 
Phosphoric acid fuel cell
· Operates in acidic media
· A SiC matrix holds the acid
· The reactions are:
· 
Molten carbonate fuel cell
· Works best at 560 C and the waste heat can be used in cogeneration
· 
Solid oxide fuel cell
· Operates at ~800 – 1000 C; limitations arise from the very high operating temperatures, including the choice of materials
· Materials must have the same expansion coefficients and stable under oxidizing and reducing conditions
· Operates at close to 96% thermodynamic efficiency
· Uses exotic metal electrodes and electrolytes
· 
4. ELECTROCHEMICAL CAPACITORS (ECs)
4.1 INTRODUCTION AND MARKET ASPECTS
Electrochemical capacitors are sometimes referred to as supercapacitors, ultracapacitors, or hybrid capacitors. They use the same nomenclature as batteries in terms of anode, cathode, and electrolyte.
Capacitors that have the same anode and cathode configuration are referred to as asymmetric capacitors.
Ultracapacitors have referred to capacitors that have high surface area carbon at both electrodes.
Supercapacitors have been used to refer to symmetric capacitors where the carbon surface is coated with a ruthenium dioxide catalyst. The RuO2 presents a redox coupling between the two valence states of ruthenium.
Asymmetric capacitors usually have a battery or redox electrode (e.g. nickel hydroxide) with a carbon electrode.
Capacitors are energy storage device based on charge stored in the electrical doubly layer of a high surface are carbon immersed in aqueous electrolyte. They have been mainly used for memory protection in electronic devices.
4.2 CHARACTERISTICS OF THE ELECTRICAL DOUBLE LAYER (EDL)
· An electrical double layer is an interface of charges between the electrode (an electron conductor) and the electrolyte (an ionic conductor) separated by a distance of molecular dimensions.
· The properties of this layer depend on the electrode and electrolyte material, the electrode surface structure, and the potential field between the charges at the interface.
· If the electrode has a negatively charged electrode surface will interface with positively charged ions in the electrolyte solution.
· The interface time formation and relaxation is in the order of 10-8 seconds. This is considerably faster than the time scale for redox reactions in batteries.
· Only a charge rearrangement takes place and not a chemical reaction.
4.3 EC OPERATION
· High-surface-area carbon is the material of choice, as it combines a large surface area wetted by the electrolyte, high electronic conductivity, and chemical and electrochemical stabilities with low cost.
· The voltage for capacitors with an aqueous electrolyte is about 1 V. With organic solvent, this can go up to as high as about 2.7 V.
· The energy stored is given by ½ QV2
· Figure 29 shows a schematic diagram of the charging process for capacitors.
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