What is over voltage in electrochemistry?

Answer 1

In electrochemistry, overvoltage is synonymous with overpotential. Look up overpotential instead of overvoltage to get details of what it is.

Basically, it is a potential that develop because of limited reaction kinetic. It works to reduce the voltage of the electrochemical cell. Think of it like this. For a given redox reaction pair that represents a chemical cell, it has a theoretical voltage determined by thermodynamics. This is typically called the open cell voltage. It's the maximum voltage that you could harness out of this reaction if you were to make this into a battery. That is all very good and all except for the little detail of reaction kinetics. In reality, you don't ever get the maximum voltage at open cell. Once current start flowing, limitations on rate of reaction at the cathode and anode develop. Reactions are not infinitely fast. Once you start the reaction, the region near the electrodes are depleted of ion species as they are oxidized or reduced into and out of the electroloyte. It takes time for more to diffuse from the bulk to the solution to the electrode/electrolyte interface. Now your reaction rate is limited. This limitation will show up as a potential, the overpotential, at the electrode. It acts to reduce the potential available to you across the entire cell. If your open cell potential of a batttery was say 2 volts, you may find a severely rate limited reaction to generate 1.9 volts of overpotential at the electrodes as you try to draw current out of the battery. When that happens, your cell actually only gives you 0.1 volts, not 2 volts. What I described above is called concentration overpotential. There are other sources of overpotential. It is much beyond the scope of Y:A to describe it. However, the phenomenon of overpotential in an electrochemical cell is a consequence of reaction kinetics. There are other concepts like equilibrium exchange current density and the use of the Tafel plot when you delve into the topic of electrochemical cells. These are things that must be considered in addition to the thermodynamic open cell potential when a cell starts to draw a current. The best batteries will be constructed out of a system in a way as to have high exchange current density and minimum overpotential. So as you draw current from the cell, you don't find yourself with vastly diminished voltage. It's ability to do work will not be self-limiting as you put it in a device that draws current.