Introduction to Batteries
Batteries have been an invisible yet ubiquitous part of our lives. They come in various sizes and forms ranging from tiny pencil cells used in watches to AA and AAA batteries used in most TV remotes and pocket torches to humongous rechargeable batteries used in electric vehicles and as backup storage devices for houses. As every living creature is unique with its own characteristics and traits, every type of battery invariably exhibits its distinctive features. Despite the vast differences, like every living creature shares common traits with each other, each type of battery shares some commonalities as well.
To know more about the commonalities, it is crucial to first learn about the origins of a battery starting from an electrochemical cell and its categories.
Electrochemical Cell and its Categories
As the name suggests, the cell either generates electric current from oxidation and reduction (redox) [1] chemical reactions or uses electric current to initiate and facilitate a redox chemical reaction. The best way to describe an electrochemical cell is to think of it as a combination of two bottles consisting of different liquids and are connected using electric conductors in such a way that they either use or generate electrical energy.
Electrochemical cells are broadly classified into:
Voltaic cells or Galvanic Cells — They generate electric current from redox reactions. Typically, the cell consists of two electrodes, namely, anode and cathode that enable flow of electrons from two electrolytes, namely, anolyte and catholyte respectively. Anolyte and catholyte are terms for electrolytes in anodic and cathodic chambers respectively. Daniell’s cell is an example of a voltaic cell.
Electrolytic Cells — They use electric current to initiate chemical reaction. Unlike a galvanic cell, both the electrodes are dipped into the same electrolyte. A good example of an electrolytic cell is the process of electrolysis of water. Electric current is passed through impure water to split the water molecule into its constituents, hydrogen gas and oxygen gas. Fig 2 shows production of oxygen and hydrogen gas when an external battery is applied.
Fuel Cell — They convert chemical energy of a fuel (the source) and an oxidizing agent (used to oxidize the fuel) into electrical energy. Hydrogen-Oxygen fuel cell is one of the most commonly used fuel cells to power rockets (or your favorite iron man suit’s jets).
Flow Cell — Though they also convert chemical energy into electrical energy but the major difference is the cell’s structure. Anolyte and catholyte are pumped from their respective tanks into a region containing a cation exchange membrane sandwiched between two electrodes. Fig 4 depicts a flow cell.
What do electrochemical cells have to do with electric batteries?
An electric battery is a derivative of an electrochemical cell. A good analogy is to think of a battery as a child of the electrochemical cell. The technical definition is as follows:
“An electric battery is a stack comprising one or more electrochemical cells used to supply DC power to various electrical devices.”
Note that I have specifically used “electric battery” instead of “battery” to differentiate it from a mechanical battery. Henceforth, both, “battery” and “electric battery” will be used synonymously and mean the same.
Moreover, what’s interesting to note is that the word “battery” was borrowed. Benjamin Franklin borrowed the term “battery” from the military because it refers to multiple weapons functioning together [7]. Though technically incorrect, the terms “cell” and “battery” are used synonymously and mean the same in colloquial speech.
A word of caution to the reader — Battery is not the same as fuel cell. The primary difference between a fuel cell and a battery is their purpose. A battery is used to store previously generated energy and deliver it when required. However, a fuel cell is a source of energy that actively converts chemical energy into electrical energy depending on the concentrations of the fuel and oxidizing agent. The electrical energy from the fuel cell can be stored in a battery for future use.
Now that the connection between electrochemical cells and batteries is established, the next step is to study the working principle of a voltaic cell that acts as a foundation for this connection.
Battery and Voltaic Cell
Let us study the physical structure of a very commonly used voltaic cell shown in Fig 1. The cell consists of:
- Two half cells connected to each other using conducting wires — A half cell literally means “half of the full voltaic cell”. It refers to the anode and its anolyte (here zinc sulphate solution) and cathode and its catholyte (here cupric sulphate solution).
- A voltmeter — The voltmeter measures the potential difference between the two electrodes generated due to flow of electrons.
- A Salt Bridge — Maintains charge balance or electrical neutrality in both the half cells. Without a salt bridge, the anolyte in the anodic half cell would become positively charged and the catholyte in the cathodic half cell would become negatively charged resulting in charge imbalance. This severely affects the voltaic cell as the cell cannot operate for long periods due to this charge imbalance.
As the chemical reaction progresses, metal ions at anode are oxidized to generate electrons that migrate to the cathode. Hence, anode is the electrode at which oxidation takes place (or oxidation half-reaction occurs) and cathode is the electrode at which reduction takes place (reduction-half reaction occurs). Each half-cell has a potential relative to the standard hydrogen electron referenced at 0 V [8]. The difference between the cathodic potential and anodic potential gives the open circuit cell voltage.
As you may have already guessed, a battery is a group of such cells connected in:
Depending on the configuration, the total output of the cell voltage varies.
In the odd case that this goes over your head, think of it as different ways to connect multiple water tanks. Different combinations lead to different outputs and hence serve different purposes.
Reading this article should have painted a clear picture on the basics of batteries and electrochemical cells. Moving forward, we will be talking about the two main types of batteries i.e. primary and secondary, and how their individual characteristics affect the world around us.
Thank you everyone for your time.
References
[4] P. Flowers, K. Theopold, R. Langley, et al., “Electrolysis”, Chemistry LibreTexts, Nov 13, 2018
[5] “Batteries and Fuel Cells”, Chemistry LibreText, May 25, 2020
[7] M. Bellis, “History and Timeline of the Battery”, ThoughtCo., May 3, 2019
