How Ohmic Resistance Shapes Your Voltammogram

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The iR drop, also known as the ohmic drop, refers to the reduction in effective potential at the working electrode interface due to the resistance of the electrolyte solution. This drop affects both the potential applied by the potentiostat and the potential it measures, thereby distorting the observed electrochemical response.

In the absence of mass transport limitations, the redox reaction can be modelled using an equivalent electrical circuit that describes the interfacial behaviour between the reference and working electrodes:


Simplified equivalent circuit representing an electrochemical redox reaction.

Here:

  • Rs – the solution resistance (also known as the electrolyte or uncompensated resistance).
  • Rct – the charge transfer resistance, associated with the kinetics of the electron transfer reaction.
  • Cdl – the double-layer capacitance, representing the electrochemical double layer at the electrode interface.

The potentiostat continuously regulates the potential difference between the reference electrode and the working electrode. It achieves this by supplying current between the working electrode and the counter electrode (not shown here). For example, if 1 V is applied between the reference and the working electrodes, part of this voltage is used to drive the faradaic process — distributed across the interfacial elements such as the charge transfer resistance (Rct) and the double-layer capacitance (Cdl) — while the remaining is lost across the solution resistance Rs.

Typically, the resistance of the solution is less than 100 Ω. According to Ohm’s law (V = iR), a current of 1 µA passing through this resistance results in a voltage drop of just 1 mV. This means that if we apply 1.000 V, 0.999 V effectively reaches the electrode interface to drive the faradaic process, making the ohmic drop negligible. However, as the solution resistance increases, the ohmic drop becomes more and more prominent. Consequently, only a small fraction of the applied potential contributes to the faradaic reaction.

Consequences of High Ohmic Drop

High solution resistance significantly reduces the effective potential at the working electrode. As a result, the peak separation increases, and the redox peaks become distorted. In fact, a reversible redox reaction may appear quasi-reversible or even irreversible under high ohmic drop conditions. The following animation illustrates how increasing Rs progressively affects the shape of a cyclic voltammogram for both reversible and quasi-reversible electron transfer processes. The red curve corresponds to a voltammogram recorded under negligible ohmic drop conditions, provided for comparison.

Effect of increasing ohmic resistance on a reversible voltammogram
Effect of increasing ohmic resistance on a reversible voltammogram.
Effect of increasing ohmic resistance on a quasi-reversible voltammogram
Effect of increasing ohmic resistance on a quasi-reversible voltammogram.


How to Minimize the Ohmic Drop

Although it is not possible to eliminate the ohmic drop entirely, several experimental strategies can significantly reduce the overall solution resistance:

  • Increase the concentration of the supporting electrolyte: Adding a supporting electrolyte enhances the ionic conductivity of the solution, thereby lowering the resistance.
  • Minimize the distance between the reference and working electrodes: Placing these electrodes closer together reduces the path length for current flow, which in turn decreases resistance.
  • Reduce the current density: This can be achieved by using electrodes with smaller surface areas. Lower current results in a smaller potential drop (iR) across the solution.
  • Use potentiostats with iR compensation: Many modern potentiostats include iR compensation circuits, which can correct for voltage losses when physical adjustments are not feasible.

Mathematical Considerations

In a previous blog post, we explored the kinetics of electron transfer, assuming an exponential dependence on the overpotential. The reaction rate is typically described by:

\[ r = k^0 \exp\left( \frac{\alpha F \eta}{RT} \right) \]

where \( \eta \) is the overpotential, defined as the difference between the applied potential and the formal potential:

\[ \eta = E_{\text{app}} - E_0 \]

When an ohmic drop (\( iR \)) is present due to solution resistance, the effective interfacial potential is reduced. The corrected overpotential becomes:

\[ \eta' = E_{\text{app}} - E_0 - iR \]

From a numerical perspective, the interfacial current can be approximated using a finite difference representation of the flux:

\[ i = nFAD \frac{C_2 - C_1}{\Delta x} \]

Where:

  • C₁ and C₂: concentrations at the first two nodes near the electrode surface
  • Δx: spatial step size
  • D: diffusion coefficient
  • A: electrode area

Substituting this expression for current into the corrected overpotential yields:

\[ \eta' = E_{\text{app}} - E_0 - nFAD \frac{C_2 - C_1}{\Delta x} R \]

This formulation describes the influence of the ohmic drop on electron transfer kinetics.

Voltammogram Simulator

This interactive application simulates a cyclic voltammogram for a redox system under the influence of ohmic resistance (iR drop). The black solid line represents the voltammogram affected by iR drop, while the dotted red line shows the idealised response without ohmic drop. You can explore how key parameters influence the voltammetric profile:

  • Global charge transfer constant (k0): Determines the reversibility of the electron transfer. This parameter is adjusted on a logarithmic scale.
  • Charge transfer coefficient (α): Influences the kinetics of electron transfer. Its impact is significant at low k0 values and becomes negligible at high k0.
  • Electrochemical cell length (x, in mm): Controls the thickness of the diffusion layer. When x is smaller than the characteristic diffusion length, mass transport and iR drop effects become prominent.
  • Solution resistance (Rs, in Ω): Affects the peak separation and peak current. A higher Rs increases the iR drop, distorting the voltammetric response.

By adjusting these parameters, you can gain a deeper understanding of how kinetic, transport, and ohmic factors shape a cyclic voltammogram. Additionally, simulation results can be exported as CSV files for further analysis.

References

If you want to cite this blog post, use:
Robayo, I. (2025). How Ohmic Resistance Shapes Your Voltammogram. Available at: https://electrochemeisbasics.blogspot.com/2025/05/how-ohmic-resistance-shapes-your.html [Accessed Date Accessed].

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