Batteries play a useful, yet underappreciated, role in our lives. They power our hand-held electronic devices, are the key element in hybrid automobiles, and allow the intermittent power available from the sun and the wind to be stored and supplied when needed. So I thought a column about how the science of batteries would be a good way to kick off Common Science® in 2016.
In the most general sense, a battery is any device that has 0ne zone that can store and/or produce electrons, the anode or negative terminal, and another side that can accept electrons, the cathode or positive terminal. (This terminology can be a bit confusing. By definition, electrons have a negative charge. Therefore, the side of the battery that supplies the electrons is the negative side.) The voltage of a battery corresponds to the power with which it can deliver electrons out of the anode. That’s sort of a strange concept to ponder. Think of a battery as being like a pump with the electrons being water and the voltage being the pressure that the pump can create. A battery with higher voltage can “pump” electrons through a circuit with more electrical resistance than could a battery with lower voltage. If you connect of a battery to a circuit, provided that it has sufficient voltage, electrons will flow out of the anode, through the circuit, and then into the cathode. You can do a myriad of useful things with this flow of electrons such as making the filament in a light glow or powering the computer that you are using to read this column. With that background in hand, let’s discuss the different types of batteries and their uses.
These are your familiar Duracell® or Eveready® batteries. In a standard alkaline battery, the anode contains zinc metal in water solution with an alkaline (>7) pH. In this environment, a chemical reaction occurs between zinc (Zn) and water that makes zinc oxide (ZnO) and releases electrons. The cathode contains manganese dioxide (MnO2) and water. If you allow the electrons from the anode to flow to the cathode through a circuit, the electrons will be consumed in a process that converts the MnO2 to manganese oxide (Mn2O3). Taken together the two sides of the alkali battery make an electrochemical cell. Below is a list of key points about alkaline batteries to keep in mind as we proceed through our discussion of other battery technologies.
- Alkaline batteries generate a relatively low voltage, 1.5 V. This makes them safe for general use in things like toys, radios, and flashlights.
- If you place alkaline batteries in series, the voltages are added together. For example, two alkaline batteries in series would supply 3.0 volts. Also, if you took off the housing on a nine-volt battery, you would find six miniature, 1.5 volt batteries connected in series.
- Once the most of the zinc in the anode has converted to zinc oxide, the battery stops working and cannot be recharged.
- The rate at which electrons can flow through a circuit powered by an alkaline battery is limited by the rate at which the chemical reactions in the anode and cathode occur.
There are several different technologies used in rechargeable batteries. Since they all work on the same general principles, I will review just one, the lithium-ion battery. In lithium-ion batteries, the anode is made of graphite – a form of pure carbon – and the cathode is made of lithium cobalt oxide (LiCoO2). A solvent separates the anode and cathode. When you charge lithium-ion batteries, electricity runs “backwards” through the battery. As this happens, some lithium is liberated from the LiCoO2 cathode and diffuses through the solvent where it becomes incorporated into lithium-carbon compounds within the graphite. When you finish charging the battery and connect it to a circuit, the lithium-carbon compounds that were created when you charged the battery decompose. This process generates electrons that flow through the circuit and also allows lithium to flow back across the solvent and become reincorporated into the LiCoO2. Once most of the lithium gets back to the cathode, you need to charge the battery again.
Lithium-ion batteries can be charged and discharged many times but unfortunately, during each cycle the battery loses some capacity due to some side chemistry. For example, there are a variety of lithium-carbon compounds that can be formed in the anode, but not all of them will decompose when you use the battery. As enough side-chemistry occurs during charging and recharging the battery loses capacity and will eventually need to be replaced.
The invention of rechargeable batteries has had a tremendous impact on our world. Just imagine if you had to add new alkaline batteries to your smart phone every day and then find a way to recycle the old ones. Rechargeable batteries do have some shortcomings though. The ones used for cars are large and heavy and, as the owners of the first generation of hybrid cars have been learning of late, quite expensive to replace when the time comes.
The future of battery technology will almost certainly involve the use of advanced capacitors. So let’s start with a review of standard capacitors. A capacitor is made from two conductive surfaces, usually metal, separated by an insulating material such as ceramic. While a capacitor can’t make its own electrons like an electrochemical cell, it can store them if provided from some other source. If you expose a capacitor to a voltage, one of the two conductive plates acquires a negative charge and the other a positive charge. At this point it can be used just like a battery.
Capacitors have many useful properties and they are widely used. Putting a capacitor into a circuit dampens any fluctuations in voltage that may occur. Capacitors charge and discharge quickly which makes them useful for functions such as a flash bulb for a camera since an electrochemical cell would provide electrons too slowly to make a bright flash. Capacitors charge almost instantly and charging and discharging them does not cause any degradation, so they last a long, long time.
So the obvious question is, “If capacitors are so great, why have they not supplanted rechargeable batteries in things like phones and cars?” The answer is energy density. At least with currently available technologies, capacitors store far fewer electrons per unit volume than do rechargeable batteries. Therefore, the size of the capacitor needed to operate a car or phone would be far too large to be practical. Which brings us to . . .
There is some fascinating and promising research occurring across the country and around the world on a new class of capacitors, called super capacitors, made from of carbon nanotubes and graphene. Carbon nanotubes are just what they sound like, long thin tubes of carbon that are one atom in thickness. Graphene is a one-atom thick sheet also made from carbon. Carbon nanotubes and graphene both have novel and useful electrical properties and researchers have shown that it may be possible to build capacitors from them that have energy densities near to those of batteries.
A super capacitor made from carbon would be cheap, light, and flexible. In addition, it would last almost forever and charge in seconds. Consider how many people in this age of instant gratification would eagerly replace their current smart phone for one that would charge in five seconds. Carbon-based super capacitors could be integrated into the body panels of automobiles saving both weight and volume. In addition, the fast charging feature would make plug-in hybrid cars far more practical. Gas stations around the country are deployed using the logic that a tank of gas will last for about 300-400 miles and it will take about 5 minutes to fill the tank with gas. All of these gas stations have electricity, so the currently existing infrastructure is perfectly suited to plug-in hybrids that travel 300-400 miles per charge and charge in under a minute. And, you can still duck into the Stop-n-Go for a slushy.
Super capacitors would also be transformative in the solar and wind energy arena. However, as the length of this column is already pushing the envelope, I will omit the details. It will be interesting to watch this story continue to unfold in coming decades.
Jeff Danner discussed this week’s column on WCHL with Aaron Keck.
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