Photocatalysis Lecture 4 | Fundamentals of Overpotential

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Overpotential, also known as overvoltage or superpotential, refers to the part where the actual voltage required exceeds the theoretical voltage during the process of electrocatalysis or photocatalysis when a certain current density is reached. 

In an ideal scenario, the operational potential required for electrocatalysis or photocatalysis is the same as the potential at equilibrium. However, in practical reactions, the working potential often needs to overcome the barriers of kinetic processes, resulting in a numerical value higher than the equilibrium potential. The voltage exceeding the theoretical value is called overpotential, mainly used to overcome activation resistance and charge transfer resistance, among other resistances^[1]. 

Simplistically, overpotential refers to the difference between the actual potential and the equilibrium potential when the current density reaches a specified level. It directly reflects the catalytic activity of electrocatalytic or photocatalytic reactions. 

According to the Nernst equation^[2], the actual working potential E can be represented as:

E: Actual working potential of the reaction 

E₀: Standard potential of the reaction 

T: Absolute zero (273.15°C) 

R: Ideal gas constant 

F: Faraday's constant (96485 C/mol) 

N: Number of electrons transferred in the reaction 

C₀: Concentration of the oxidized product 

C_R: Concentration of the reduced product

The basic formula for overpotential is:

: Overpotential 

: Actual potential 

: Theoretical potential

In theory, as overpotential approaches 0 V, the better the performance of the catalyst. A lower actual voltage is required to achieve the relative current density, resulting in lower energy consumption and higher catalytic activity^[3].

It's important to note that when comparing the overpotentials of different catalysts, the specific current density must be specified; otherwise, the results of the comparison will not have much significance. 

Typically, at a current density of 10 mA/cm^-2, lower overpotential indicates a stronger catalytic ability for the target reaction, making it a common condition for assessing catalyst performance.

Fig.1 (a) and (c) Polarization curves^{[4, 5]}; (b) LSV curves^[6]; (d) Overpotentials (at 10 mA·cm^-2) ^[5].

References