Content Background
Non-Renewable Energy and Climate Change
Fossil fuels provide energy for nearly all of our human activities. Although our nation demands energy for a wide range of uses, the vehicles we use for transportation required approximately 28% of the energy resources in 2017. The combustion of petroleum products (gasoline, diesel, jet fuel) produced approximately 29% of the country’s greenhouse gas emissions in 2017.15
2C8H18 (l) + 25O2 (g) -> 16CO2 (g) + 18H2O (g) (Combustion of Gasoline)
The combustion of coal and natural gas for the generation of electricity required 63% of our energy resources.16 These processes produced 28% of the nation’s GHG emissions.
C (s) + O2 (g) -> CO2 (g) (Combustion of Coal)
CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (g) (Combustion of Natural Gas)
Although we believe that the coal fired power plant is in decline, many remain online, while other plants are using natural gas as a substitute. Additionally, the major portion of our energy resources is used for transportation which continues to rely heavily on petroleum as the main energy resource. Thus, it is important to note that, although there are many concerted efforts to move towards renewable energy sources to generate electricity, we continue to rely heavily on coal, petroleum, and natural gas for our energy needs.17
Electrical power plants use thermal energy from the combustion of (coal or natural gas) to convert water into steam that is then used to turn generators that convert the mechanical energy into electricity. The internal combustion engines in our vehicles convert the chemical energy in gasoline into mechanical energy. In each case, GHG emissions are produced, while much of the thermal energy is lost as waste heat.
Renewable Energy Sources
Renewable technologies convert various forms of renewable energy (solar, wind, hydro, geothermal) into electrical energy. Photovoltaic cells convert solar radiation into electrical energy while wind turbines use generators to convert the kinetic energy of the wind into electrical energy. None of these technologies emit greenhouse gases and (for the most part) they do little harm to the earth’s ecosystems. Their profitability has been increasing over the last twenty years as their efficiency has steadily increased while their production costs have dramatically declined. Wind and solar energy are the more impressive of these sources as each has the capability of providing vast amounts of energy. They are, however, limited as they are not always available, and we do not have efficient ways of storing the electricity they produce. Thus, any strategy meant to move us away from a dependence on fossil fuels depends on the availability of technologies that can efficiently and economically store energy harvested from these renewable sources.
Storage Devices
Efficient storage devices (batteries) are the key to the successful transition to renewable energy sources. In order to effectively use renewable energy, we would need to harvest and store the energy (solar or wind for example) when it was available, store it and then use it at a later time when the resource was not available. These rechargeable batteries (secondary cells) would need to efficiently store large quantities of energy, discharge their energy as needed, and recharge quickly and efficiently.
There are already many different types of rechargeable batteries that serve a wide variety of purposes in our society. They are an integral part of the information revolution as nearly all portable electronic devices are powered by a rechargeable device. Rechargeable batteries are increasingly used in the transportation industry as components of hybrid, plug in hybrid, and fully electric vehicles. Using electricity as a replacement for internal combustion engines would be extremely beneficial given that most of our energy resources (and the resulting GHG emissions) result from the combustion of petroleum products.
Over the last 30 years, the efficiency and capacity of these devices has dramatically increased as new technologies have made it possible to use them as integral components of the change towards renewable energy sources. Massive storage devices are already planned as part of a large solar farm built by the 8minute Solar Energy company near Los Angeles, California.18 The farm’s photovoltaic cells will capture and store solar energy in an array of giant batteries that will supply up to 7% of the city’s energy needs. The projected price will be 1.98 cents / kWh from the array and 1.3 cents / kWh for electricity from the battery: both prices are well below existing market prices from coal fired plants. These projected innovations in renewable energy production are possible because we have some way of storing the energy produced. Absent such a device, we would have no way of capturing this very abundant source of energy.
For this unit, I will focus on the three representative rechargeable batteries, the Ni-Cd, Ni-metal hydride (NiMH), and the Li-ion battery, as they are the more common devices in my student’s everyday lives. While each of the batteries use a similar architecture, the NiMH and the Li-ion have features that make them more practical than the Ni-Cd device. I will begin my analysis of batteries with an exploration of the basics of redox reactions in primary cells, followed by an analysis of the Ni-Cd, Ni-MH, then the Li-ion battery.
Chemistry of Batteries
Batteries (also known as electrochemical cells) are devices that can convert the stored chemical potential between two electrodes (the positive cathode and the negative anode) into electrical energy via coupled reduction / oxidation (redox) reactions. The electrodes can be made from a wide variety of materials (in any physical state); however, they must be kept apart in order to avoid a direct transfer of electrons (known as a short circuit). The separation of the electrodes forces the electrons produced at the anode (the site where oxidation occurs) to flow through an
external circuit (where they do useful work) to the cathode (where reduction occurs). Although the electrodes are not in direct contact with each other, they are connected through a conductive medium (an electrolyte) that exists between them. The electrolyte maintains the equilibrium of ions in the battery so that charge can continue to flow between the electrodes. Redox reactions are the chemical processes that convert chemical potential in the battery’s electrodes into electrical energy. There are analogous redox reactions in all types of primary and secondary batteries.
Reduction / Oxidation Reactions
When given combinations of metals interact, electrons can spontaneously flow from one metal to
the other. The coupled processes that occur during these interactions are known as reduction and oxidation (redox for short) reactions. For example, when a piece of solid aluminum (Al (s)), is placed in a solution of copper chloride (CuCl2 (aq)), the aluminum metal will donate electrons to the copper ions in the solution. Equation (1) depicts the molecular equation for this reaction.
2Al (s) + 3CuCl2 (aq) -> 2AlCl3 (aq) + 3Cu (s) (1)
During this process (known as a single displacement reaction), each aluminum atom donates three valence electrons to the copper ions in the copper chloride solution: (this changes the oxidation state of the aluminum atoms from Al0 to Al3+). Each of the 3 copper atoms receive two electrons which changes their oxidation state from Cu2+ to Cu.0 In order to analyze the flow of electrons, equation (1) can be rewritten in a series of ionic equations that show the oxidation states of the metals. Equation (2) is the total ionic equation that shows all of the ions in the reaction: (states are not included for clarity of the discussion).
2 Al0 + 3 Cu2+ + 6 Cl 1- -> 2 Al3+ + 6 Cl 1- + 3 Cu0 (2)
Eliminating the spectator ions that do not change oxidation state produces a net ionic equation (3).
2 Al0 + 3 Cu2+ -> 2 Al3+ + 3 Cu0 (3)
There are two reactions in equation (3) which can be analyzed separately in what are known as half reactions: equations (4) and (5).
2 Al0 -> 2 Al3+ (4)
3 Cu2+ -> 3 Cu0 (5)
Although the reactions have the correct mass balance, they are not balanced as to charge. Charge is balanced by adding electrons (each carrying a -1 charge) to balance the positive charges. Adding six negative charges to the appropriate side of each reaction yields equations (6) and (7).
2 Al0 -> 2 Al3+ + 6 e - (6)
3 Cu2+ + 6 e- -> 3 Cu0 (7)
The two half reactions reveal the flow of electrons. Equation (6) is known as the oxidation half reaction because each aluminum lost electrons. Equation (7) is the reduction half reaction because each of the copper atoms gained electrons. This redox reaction is representative of all the reactions in batteries. In this reaction, the metals are in close contact with each other thus the electrons flow directly from one metal to the other. If, however the reactions are placed in separate reaction vessels and connected by a conducting medium, then the electrons will flow from the anode to the cathode through the external circuit. This flow of electrons (an electric current) can be used to do useful work.
Primary cells will continue to produce electrical energy as long as the materials within them remain viable. Once they are spent, primary cells are no longer useful. In secondary cells (rechargeable batteries), the chemical potential can be restored when a reverse electrical charge is applied across the electrodes.
A secondary cell is one in which the starting materials at the anode and cathode are recovered when a reverse electrical potential is applied to the cell. For example, the Ni-Cd battery is composed of a cadmium (Cd) metal anode and a Ni(III)-oxide hydroxide (Ni(O)OH) cathode. When the battery is discharged, the Cd is oxidized to Cd(II) hydroxide (Cd(OH)2), while at the cathode the Ni(O)OH is reduced to Ni(II) hydroxide: Ni(OH)2. The reactions are as follows:
Oxidation at the anode: Cd (s) + 2OH- -> Cd(OH)2 (aq) + 2e- (8)
Reduction at the cathode: 1e- + Ni(O)OH + H2O -> Ni(OH)2 + OH- (9)
Equation (9) must be multiplied by 2 in order to balance the electron flow between the two half reactions. This yields the following:
2e- + 2Ni(O)OH + H2O -> 2Ni(OH)2 + 2OH-
Cancelling the hydroxides that appear on both sides of the reactions produces the overall net ionic reaction:
Cd (s) + 2Ni(O)OH + 2H2O -> 2Ni(OH)2 + Cd(OH)2 (aq) (10)
During the recharge period, the reactions are reversed so that Cd(OH)2 is reduced back to Cd; while the Ni(OH)2 is oxidized back to Ni(O)OH.
Reduction: Cd(OH)2 (aq) + 2e- -> Cd (s) (11)
Oxidation: 2Ni(OH)2 -> 2Ni(O)OH + 2e- (12)
In order to be considered an efficient rechargeable battery, the chemical reactions that occur during discharge must be fully reversible. This means that the reactions at each electrode must be a true reverse reaction with no additional products formed. If by products are formed, then the battery may become unstable, the electrodes may decay, or a short circuit may occur. Additionally, the starting materials must be recovered in their original physical state with no additional by products formed. If these conditions are not met, then the battery’s lifetime may be shortened, or the device will fail to deliver adequate charge.19
Each type of storage device needs to meet a different set of requirements. Given that drivers have become accustomed to quick, convenient refueling, and the driving distances provided by a tank of gasoline, researchers are working to produce rechargeable battery technologies that provide the same level of reliable (relatively inexpensive) energy that gasoline / diesel fuel provides. At present the lithium-ion battery is (for a variety of reasons) the most promising rechargeable battery technology.20
Of the various battery types available for use in our vehicles (Pb-acid; Ni-Cd; Ni-MH; and Li-ion) the lithium-ion battery provides the highest energy density and power with a relatively high miles per charge value (currently 250 mi. / charge with a goal of 350 mi. / charge). Lithium’s physical and chemical properties make it highly suited for rechargeable batteries. It has one of the lowest reduction potentials (-3.04 E0 V) and, therefore, one of the highest cell potentials. It has one of the smallest radii of any singly charged cation and as a result has as an exceptionally high-power density. Although supplies may be limited in the future, there are sufficient deposits currently available for the world’s needs. For these fundamental reasons, lithium will likely remain as the prime metal used in automobiles and portable electrochemical devices for the near future.21
Chemistry of the Li-ion Battery
Li-ion batteries rely on redox reactions at each of the electrodes to produce an electron flow. They must first be fully charged before they are used. During the charging process, the lithium ions in the lithium cobalt oxide (LiCoO2) travel through the electrolyte to the LixSi anode where Li+ combines with an electron and is reduced to Li. When no more Li can be deposited onto the anode, the battery is fully charged.
During the discharge, the lithium in the anode is oxidized to Li+ + e.- The electrons produced travel through an external circuit where they do work before returning to the cathode. At the cathode, these electrons reduce the cobalt oxide and combine with Li+ to form LiCoO2. When all the Li atoms generated during the charging process have returned to the cathode, the battery will need to be recharged.
The high charge density of the Li-ion battery results from the chemical properties of lithium and the architecture of its electrodes. Lithium atoms have the highest reduction potential of all metals; as a result, each cell of a Li-ion battery can deliver up to 3.6 V. Additionally, given its small size, many lithium atoms can be fit into the carbon matrix, giving the battery one of the highest mass to charge ratios of all batteries.
Electrodes
Lithium atoms at the anode are intercalated between successive layers of carbon atoms. Carbon compounds (graphite / graphene) can be used to intercalate lithium at the anode. Graphene is currently preferred as graphite can become unstable over time. Cathodes are typically made of a compound of lithium, oxygen and a transition metal with variable oxidation states. Cobalt is one preferred element as it can exist as Co4+ or Co3+. The compound lithium cobalt oxide (LiCoO2) is used extensively in Li-ion batteries. It has however shown to be thermally unstable as the oxide can decompose and produce oxygen if the battery overheats. Unless the oxygen is released, the battery poses a severe fire hazard.
Electrolyte
The electrolyte in the battery is usually an inert material that is a good ionic conductor that insulates the two electrodes, provides an abundance of ions that travel between anode and cathode and is thermally stable. The electrolytes can be non-aqueous solutions of lithium hexafluorophosphate (LiPF6) in a mixture of organic carbonates such as ethylene carbonate, propylene carbonate, or diethyl carbonate, aqueous solutions of lithium salts in water or ionic liquids.22
Materials Science
While lithium-ion batteries are expected to dominate the industry for the near future, the demand for electrochemical storage devices for all energy sectors (such as long-haul trucking and aviation) will require the use of new macromolecules and polymers that can be tailored to meet the specific design parameters (such as architecture, charge capacity, electrolyte components, electrodes, etc.) These innovations along with new battery technologies (i.e. the Vanadium flow, the Na-S, and Al-ion battery 23) will be necessary if we are to successfully develop electrical storage devices that “provide long-lasting solutions that can be commercialized and deployed at a cost that disrupts the market, which is currently dominated by the coal, oil, and gas industries”.24 Our ability to move away from fossil fuel technology will greatly depend on the efficiency of these future electrical storage devices.
Chemistry and Environmental Impact of Batteries
Tables 1 and 2 below provide an opportunity to evaluate the performance features of common rechargeable batteries and their environmental impact. The batteries in Table 1 are “established” devices that are currently in use. Table 2 explores future designs. Both tables are adapted from: Armand, et.al. “ Building a Better Battery” 25
Table 1: Chemistry and Environmental Impact of Established Batteries
|
Battery Type |
Features |
Environmental Impact |
|
Ni–MH (established) |
Low voltage, moderate energy density, high power density Applications: portable, large-scale |
Nickel not green (difficult extraction/ unsustainable), toxic. Not rare but limited Recyclable |
|
Lead–acid (established) |
Poor energy density, moderate power rate, low cost Applications: large-scale, start-up power, stationary |
High temperature cyclability limited Lead is toxic but recycling is efficient to 95% |
|
Lithium–ion (established) |
High energy density, power rate, cycle life, costly Applications: portable, possibly large-scale |
Depletable elements (cobalt) in most applications; replacements manganese and iron are green (abundant and sustainable) Lithium chemistry relatively green (abundant but the chemistry needs to be improved) Recycling feasible but at an extra energy cost |
|
Zinc–air (established) |
Medium energy density, high power density Applications: large-scale |
Mostly primary or mechanically rechargeable Zinc smelting not green, especially if primary Easily recyclable |
Table 2: Chemistry and Environmental Impact of Future Batteries
|
Lithium–organic (future) |
High energy density but poor energy efficiency and rate capability Technology amenable to a low-cost Applications: large-scale, preferably stationary |
Rechargeability to be proven Excellent carbon footprint Renewable electrodes Easy recycling |
|
Magnesium–Sulphur (future) |
Predicted: high energy density, power density unknown, cycle life unknown |
Magnesium and Sulphur are green Recyclable Small carbon footprint |
|
Al–CFx (future) |
Predicted: moderate energy density, power density unknown |
Aluminum and fluorine are green, but industries are not Recyclable |
|
Proton battery (future) |
Predicted: all organic, low voltage, moderate energy density, power density unknown |
Green, biodegradable |
Measuring Energy
Energy, though a central theme of this unit, cannot be easily conceptualized. It can, however, be described operationally as the “ability to do work”, (i.e. the process of changing the location of an object or altering the temperature of a substance).26
Work occurs when a force acts on an object over a distance: this is summarized in the formula W = F x d. Where the unit of force is the Newton (approximately 4.4.5 lbs. of force) and the distance is 1 M (approximately 3 feet). The SI (Systeme International) unit for this work energy is the Joule which is defined as 1 J = 1 N·m.
Thus, the energy used to exert a force of 1 N over a distance of 1 meter is equal to 1 Joule. This type of translational (mechanical energy) is but one manifestation as many forms of energy exist. All forms of energy (solar (light and thermal), wind, electrical, geothermal, nuclear, tidal, etc.) are interconvertible and can be used to “do work”. A fundamental law of thermodynamics states that in all conversions of energy the total energy is conserved; thus, while we may change its forms, energy can never be created or destroyed.
Measuring Electrical Energy
The concept of power is used to describe the rate at which energy is transformed from one form to another. The SI unit for power is the Watt (W) which is measured in Joules / second: 1 W = 1 J / s.
We are most familiar with this unit in our homes where our light bulbs are usually measured in watts. For example, a 60 W bulb converts 60 joules of electrical energy per second to either light or thermal energy. Many other electrical devices in our homes (microwaves, toasters, refrigerators) are measured in watts. (*Automobiles are rated using horsepower (hp) where 1 hp = 746 W). To describe larger quantities of power we will use the kilowatt (1 kW = 1000 watts) , or the Terawatt (1 TW = 1012 watts).
kWh
Power ratings in watts describe how much energy a device needs to function. In order to determine the amount of energy consumed, we multiply its power rating by the amount of time in use; thus,
energy = power x time.
For example, a heater that requires 1 kW of power used for one hour would use 1 kWh of energy.
E = 1 kW x 1 hr = 1 kWh.
A kWh is the standard unit of energy used to measure energy. It will be used throughout this unit to measure electrical energy.
Electric Vehicles
There are currently several variations of electric vehicles ranging from a mild hybrid (uses a Pb-acid battery in conjunction with an internal combustion engine: the battery provides power during braking or when the engine is off), a micro hybrid (a motor / generator assembly that provides power when coasting, or braking), a plug in hybrid (with the capability of the micro hybrid along with an electricity-only capacity), and finally the all-electric vehicle which relies solely on energy provided by a rechargeable battery.27
This unit will focus on the efficiency of fully electric vehicles as they depend solely on energy stored in their rechargeable batteries. Their efficiency is measured in number of miles (usually 100) per kWh. An additional factor is the range the car can travel on a fully charged battery (typically 100 -130 miles / charge). A complete listing of the efficiency of EV is available at: (www.fueleconomy.gov). To determine the cost of running an electric vehicle one can multiply the efficiency in kWh per 100 miles by the cost of electricity in a particular state. A similar calculation can be made to determine the cost to charge the onboard battery. With this information one can compare the efficiency various EVs.
Comments: