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.
Usually electrochemists are interested in the Faraday current that 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.
What does this mean for measurements? If the potential of the electrode is changed, for example during a potential step, 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 exponential 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 capacity of the capacitor. This decay is much faster than the decay of Faraday current, if sufficient reactant is present.
It is well known that for reactions involving a free diffusing species in solution the Faraday current decays with t-½. 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.
During linear sweep voltammetry or cyclic voltammetry the potential of the electrode is changed continuously 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 the Equation 4.1:
Equation 4.3 shows the higher the capacity C is the higher is the capacitive current. The capacity C for a plate capacitor can be calculated with
where ε0 is the electric field constant, εr is the relative permittivity of the medium between the plates, d is the distance between the two plates and A is the surface area of the two plates.
Most factors influencing the capacity cannot be altered in an electrochemical experiment. The constant ε0 cannot be changed. The distance d and the relative permittivity εr can only be changed by changing the solution, because d is defined by the distance between electrode’s surface and the layer of ions. The area A is influenced by the surface roughness. The rougher a surface is the higher is its area. If a reusable electrode is used, a proper polishing that leads to a smooth surface can reduce capacitive current drastically.
Fortunately, digital potentiostats don’t provide a true linear sweep. The linear increase of potential is done step wise. These small steps are approximately a line. However, this means capacitive current behaves under these conditions according to Equation 4.2 and not Equation 4.3. Only the last quarter of a step is used for the measurement and most of the capacitive current is already decayed there.
Measurements with digital potentiostats that don’t offer a true linear option will always show a significantly smaller capacitive current than the theory predicts. The disadvantage is that digital potentiostat can’t perform measurements where the exact capacitive current needs to be measured.
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