# Capacitive Current

Understanding capacitive charging current is important for understanding electrochemical experiments, so in this section the origin and equations for capacitive charging current will be explained. Afterwards the effect of capacitive current during Cyclic Voltammetry and Linear Sweep Voltammetry is discussed.

## Definition of the Capacitive Current

Usually, electrochemists are interested in the Faraday current which is the current caused by an electrochemical reaction. The capacitive current, caused by physics, is an unwanted side effect. The cause of this current is ions accumulating in front of the electrode.

These ions and the electrode’s charged surface form a capacitor. A capacitor will store a charge Q depending on the potential E across the capacitor and its capacitance C:

What does this mean for measurements? If the potential of the electrode is changed, for example during a potential step, the amount of charge the capacitor stores changes, and a current will flow that has no chemical but only a physical meaning. This is the current that charges or discharges the capacitor also known as capacitive charging current or short capacitive current. This current decays exponentially with time t as known from electronics (see Equation 4.2).

EC is the charging potential or voltage, I0 is the starting current, R is the resistance of the circuit around the capacitor, and C the capacitance of the capacitor. This decay is much faster than the decay of Faraday current if sufficient reactant is present.

Capacitive Current
Current flow due to a changing potential of the electrode which charges or discharges a capacitor.

It is well known that for reactions involving a free diffusing species in solution the Faraday current decays with t. This means that the capacitive current decays much faster than the Faraday current. The difference between the decay of capacitive current and the Faraday current of a free diffusing species is shown as a scheme in Figure 4.5.

## Capacitive current during sweep voltammetry and cyclic voltammetry

During linear sweep voltammetry or cyclic voltammetry, the potential of the electrode is changed continuously and linear during the whole measurement. This means during a linear sweep a constant capacitive current is flowing. This can be deducted from the definition of current I, which is charge Q per time t, and Equation 4.1: