Have you ever wondered why society has made a huge fuss over batteries? Well, batteries are currently one of the best energy storage methods – life without it would be living in a world driven through steam and coal, and who wants to return to that? This convenient source of power drives your laptop, phone and watches, or for those who need it, our pacemakers and hearing aids. Pretty cool!
So, what actually is a battery? is a stack of galvanic cells that stores chemical energy to be converted to electrical energy through spontaneous reduction-oxidation (redox) reactions (Module 3, Reactions of Metals).
Reduction refers to the gain of electrons and oxidation refers to the loss of electrons.
Parts of an Electrochemical Cell
In a galvanic or voltaic cell, there are three main components: the anode, cathode and electrolyte. The anode is the negative terminal of the cell and is where oxidation occurs; conversely, the cathode is the positive terminal and where reduction occurs. Since electricity is essentially the flow of electrons, together they generate a current from the anode to the cathode, and if hooked up to a device such as a light bulb, the flow of energy powers the device.
For example, take the Zn-Cu battery (or Daniell cell) which was one of the earliest non-rechargeable batteries (Module 3, Galvanic half-cells).
Calculating Cell Potential
Zn and Cu act as electrodes in a half cell. Each half cell is where either oxidation or reduction occurs and by looking at our half-reactions, we can figure out which functions as the anode and cathode.
These are the standard reduction potentials, as given in the chemistry formula sheet, and since Cu has a higher reduction potential it will reduce while Zn will oxidise. Therefore, Zn is our anode while Cu functions as the cathode. Once we’ve figured that out we can add the two half-reactions gives us the overall reaction of:
In other simple cases, we would need to balance our two half-reactions by ensuring that the number of electrons oxidised equals the number of electrons reduced. One process cannot occur without the other. In this case, the half-reactions are already balanced, so we can leave it as it is. Note that some redox reactions are more complicated and involve balancing the atoms as well as the charge using species such as H2O, H+ or OH- but we’re not going to get into that.
Now that we’ve figured out which oxidises and reduces, we can calculate the electrochemical potential of the cell with the equation:
Using the values from our potential sheet with get,
And that’s it! The energy generated from this cell, using these electrodes, would be 1.1 V (Module 3, Cell potentials).
A more intuitive way of thinking about which would be the anode or cathode would be thinking about the spontaneity of our redox reactions. We know that the redox process has to be spontaneous for a battery to be functional (non-spontaneous redox reactions cannot be used to produce electricity), and this means our has to be positive. If the reaction is not spontaneous in the forward direction, it will be in the reverse direction. To illustrate this, let’s revisit our two electrodes (Module 3, Spontaneity of redox reactions).
E^0 for Zn^{2+} is negative which means the reduction half-reaction is not spontaneous. However, the reverse oxidation reaction is spontaneous and has an oxidation potential of:
The reduction potential for Cu is already positive without adjustment which means that this reduction half-reaction proceeds spontaneously.
Another way of thinking about the electrochemical potential of the cell is to just add both our spontaneous reactions. This involves flipping our Zn half reaction to get the standard oxidation potential (+0.76 V) and then we add our two positive potentials together to get the same figure of 1.1 V.
On a separate note, it is also possible to construct a cell to do work in a non-spontaneous direction. These are called electrolytic cells and need external electrical energy to drive the reactions such as electrolysis reactions.
Overall, batteries have been a game changer in improving the quality of life in our society. Different batteries will use a different combination of metals for different purposes, influencing factors such as capacity, discharge rate etc. However, they all function under the same underlying principle behind those same galvanic cells invented in the 18th century.
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