Chemical Energy as a Source of Electrical Energy
This series provides a look at different phenomena that can produce electrical energy In this article, the fifth in our series, we’ll discuss the basics and applications of producing electrical energy through chemical reactions.
Chemical reaction was the very first accurate and usable process devised to generate electrical energy. Luigi Galvani first discovered the process and published his results in 1792. In 1800, Allesandro Volta constructed the voltaic pile, the first battery, based on Galvani’s findings. Volta documented his findings with the Royal Society of London. This ready and reliable source of electrical energy spurred research into the nature of electricity.
The voltaic pile consists of a series of silver discs separated from zinc discs by a porous material soaked in a solution of saltwater. A conductor is connected at each end. The silver terminal is the positive electrode, and the zinc terminal is the negative electrode. Volta later developed a more efficient combination using a copper plate and a zinc plate in a lye solution. See Figure 1.
Figure 1. The voltaic pile was the first reliable and usable source of electrical energy. The voltaic cell uses a copper plate and a zinc plate in a lye solution.
A cell is a device that produces electricity at an almost fixed voltage. A cell consists of a combination of two metallic electrodes in an acid or alkaline solution. For example, copper and zinc in a lye solution is a chemical cell. A battery is a DC voltage source that converts chemical energy to electrical energy. A battery is formed when two or more cells are combined to provide a higher potential or current than a single cell provides. The voltaic pile is a battery. Note: The cells in the voltaic pile are combined in series.
When cells are connected in series, their voltages are added together. Cells connected in series will have the same current because the ability to deliver current to a load is limited to the current rating of the lowest cell. When cells are connected in parallel, their currents are added together. Cells connected in parallel should all have the same voltage rating. If one of the batteries in the parallel combination has a higher voltage than others, a discharge current will flow to reduce its output voltage to the rated voltage of the other cell. For example, three 1.5 V/1.0 A cells produce 4.5 V/1.0 A when connected in series and 1.5 V/3.0 A when connected in parallel. A series/parallel combination of nine 1.5 V/1.0 A cells can be used to produce a 4.5 V/3.0 A battery. See Figure 2.
Figure 2. Cells may be wired in series to produce higher voltages, in parallel to produce higher current, or in series/ parallel to provide higher voltage and current.
A chemical cell is formed when two electrodes of dissimilar metals are immersed in an electrolyte. The electrolyte may be an acid, alkaline, or salt solution. The purpose of the electrolyte is to refine or produce usable materials.
When an acid, such as sulfuric acid (H2SO4), is mixed in solution with water (H2O), chemical ionization occurs. Ionization is the separation of atoms and molecules into particles that have electrical charges. These charged particles are called ions. The particles may be either negatively (excess of electrons) or positively (deficiency of electrons) charged. When water and sulfuric acid are mixed as an electrolyte, the sulfuric acid molecules split into three ions: two hydrogen ions (H+ H+) and one sulfate ion (SO42−). An ionic balance is established in the solution when the number of negative and positive charges is equal. See Figure 3.
Figure 3. When water and sulfuric acid are mixed as an electrolyte, hydrogen, and sulfate ions are formed. When a zinc electrode is added to the electrolyte, the zinc is dissolved as positively charged zinc ions, leaving the zinc electrode with a negative potential.
When a zinc (Zn) electrode is immersed in the electrolyte, the sulfuric acid dissolves some of the zinc into the solution, producing positively charged zinc ions (Zn++). For each zinc ion formed, two free electrons remain on the zinc electrode. In time, the zinc electrode becomes negatively charged while the electrolyte becomes positively charged. An equilibrium is established. The positive zinc ions collect near the negative zinc electrode.
When a copper electrode is immersed in the electrolyte, the positive hydrogen ions are attracted to the copper electrode, where they receive an electron, neutralizing their charge. The neutral hydrogen atoms become atoms of hydrogen gas (H). The hydrogen gas collects around the copper electrode and can be seen as bubbles rising to the top of the solution. Due to the loss of electrons, the copper electrode becomes positively charged, and an equilibrium is established. See Figure 4.
Figure 4. When a copper electrode is immersed in an electrolyte, electrons are transferred to the positive hydrogen ions, leaving the copper electrode with positive potential. Connecting a load across the two electrodes allows electrons to flow from the zinc electrode to the copper electrode
An electrical potential now exists between the copper and zinc electrodes. The amount of the potential depends on the metal used. Electrodes are chosen for chemical cells based on the ease with which one metal gives up electrons compared with another metal. For a zinc-copper cell, the potential is approximately 1.08 V. Once this potential is achieved, the chemical action stops until a load is connected to the electrodes of the cell.
When a load is connected to the electrodes, electrons travel from the negative zinc electrode, through the load, and to the positive copper electrode. This upsets the chemical equilibrium of the cell. The zinc electrode now has a deficiency of electrons, and the copper electrode has an excess of electrons. To compensate for this change, the positive hydrogen ions in the electrolyte flow to the copper electrode, where they accept the excessive electrons and are neutralized. The neutral hydrogen ions travel up the copper electrode and leave the solution as a gas. Due to the loss of the negative particles on the zinc electrode, the negative sulfate molecules are attracted to the zinc electrode. Zinc atoms again enter the electrolyte. This allows the zinc electrode to accumulate some negative charges.
. As long as the zinc is not consumed, the charges on the terminals are replaced as the load draws the current. Therefore, the potential remains almost constant.
Our Everyday Devices Convert Chemical Energy into Electricity
A cell or battery chemically stores and releases its electrical potential when a load is placed across its electrodes. In the 1830s, telegraph systems became a major consumer of batteries, making their manufacture a profitable commercial venture. By the 1870s, circuits were being produced for electric bells, an item with high consumer demand. By 1900, the flashlight had been invented, and more than two million batteries were manufactured yearly. The growth of technology has continued to provide a high demand for batteries. The electrical energy produced by chemical reactions is used in a variety of applications in modern society. See Figure 5.
Figure 5. Applications for electricity produced by chemical reactions include flashlights, lift trucks, portable audio systems, electronic test instruments, watches, and computers.
Most vehicles depend on batteries to start their gasoline and diesel engines. Forklifts used in areas where fumes from gasoline engines could be harmful are often battery-powered. Research is continuing on more efficient batteries for EV applications. Portable audio systems such as CD players use batteries. Most portable electronic test instruments use batteries to power their electronic circuits. Common electric wristwatches and desktop computers rely on energy supplied by batteries to maintain date and time functions. Portable (laptop) computers can be operated entirely on battery power.