Understanding Surface Adsorption in Electrochemical Reactions

In many electrochemical systems, electroactive species can adsorb onto the electrode surface — especially under non-equilibrium conditions. This adsorption introduces complexity into redox processes, as charge transfer may occur from both the dissolved species in the bulk solution and those adsorbed on the electrode surface. As a result, the system exhibits dual charge-transfer pathways, requiring more sophisticated kinetic models.

Examples of Surface-Confined Redox Systems

Several well-known systems exhibit this dual charge transfer behaviour, including:

• Quinone/Hydroquinone redox couple on glassy carbon electrodes [4].
• Ferrocene derivatives on gold electrodes [5].
• Ruthenium bipyridine complexes on platinum electrodes [6].
• Hemoglobin and myoglobin on carbon electrodes [7].

These systems are particularly challenging because they involve two simultaneous charge transfer pathways:

1. Bulk charge transfer \( \boldsymbol{k_{red/ox,bulk}} \) — occurs between the species dissolved in the solution and the electrode.
2. Adsorbed charge transfer \( \boldsymbol{k_{red/ox,ads}} \) — involves the species attached to the electrode surface.

As illustrated in Figure 1, both processes can overlap, making it difficult to distinguish between contributions from the bulk and surface species. This complexity is further compounded by the fact that adsorption behaviour follows different models, each introducing new variables and parameters that must be considered during analysis.

Figure 1. Reaction scheme illustrating a surface-bulk coupled redox mechanism at an electrode interface.

Kinetics and Mechanism of Adsorption

The species that adsorbs at the electrode surface forms a surface layer that interacts differently compared to the bulk solution. To represent this, a new independent variable emerges:

• \( \boldsymbol{\Gamma_i} \): Represents the interfacial concentration of the species on the electrode surface.

To better understand the interplay between adsorbed and dissolved species, consider the following reversible reaction:

\[ A_{aq} \underset{k_{ads}}{\overset{k_{des}}{\rightleftharpoons}} A_{ads} \]

Where:

• \( \boldsymbol{A_{aq}} \): Species present in the bulk solution.
• \( \boldsymbol{A_{ads}} \): Species adsorbed on the electrode surface.

The rate of adsorption can be described as:

\[ \frac{d\Gamma_A}{dt} = \nu_{ads} - \nu_{des} \]

Where:

• \( \boldsymbol{\nu_{ads}} \): Rate of adsorption.
• \( \boldsymbol{\nu_{des}} \): Rate of desorption.

Depending on the nature of the adsorption process, different kinetic models can be applied to describe the behaviour of electroactive species on the electrode surface. Several kinetic models have been developed to describe these adsorption processes, including:

Model	\( \nu_{ads,Re} - \nu_{des,Re} \)
Langmuir	\( k_{ads}[\mathrm{A}]_{x=0}(\Gamma_{max} - \Gamma_{A}) - k_{des} \Gamma_{A} \)
Freundlich	\( k_{ads}[\mathrm{A}]^n_{x=0}(\Gamma_{max} - \Gamma_{A}) - k_{des} \Gamma_{A} \)
Temkin	\(e^{-\alpha \Gamma_{A}}k_{ads} (\Gamma_{max} - \Gamma_{A}) [\mathrm{A}]_{x=0} - k_d \Gamma_{A} \)
Frumkin	\(e^{g \Gamma_{A}} k_{ads} (\Gamma_{max} - \Gamma_{A}) [\mathrm{A}]_{x=0} - k_{des} \Gamma_{A} \)
BET	\( -\sum_{i} \left( k_{ads}^i [\mathrm{A}]_{x=0} (\Gamma_{max}^i - \Gamma_{A}^i) - k_{des} \Gamma_{A}^i \right) \)

These models can be summarized as follows: the Langmuir model assumes monolayer adsorption without interactions between adsorbed molecules, while the Freundlich model accounts for heterogeneous adsorption, where the binding strength decreases as coverage increases. The Temkin model considers adsorbate-adsorbate interactions, with the adsorption energy decreasing linearly as coverage rises. The Frumkin model includes lateral interactions between adsorbates, which can be attractive if \( g > 0 \) or repulsive if \( g < 0 \). Finally, the BET model describes multilayer adsorption, typically observed on porous surfaces.

Charge Transfer with Adsorption/Desorption Process

As illustrated in Figure 1, the time evolution of species is governed by adsorption, desorption, and interfacial electron transfer kinetics. For simplicity, the Langmuir adsorption isotherm is assumed as the basis for the numerical simulation.

The time evolution of the surface concentrations of species \(A\) and \(B\) is described by the following differential equations (figure 1):

\[ \frac{d\Gamma_A}{dt} = k_{\text{ads}}^A\left(\Gamma_{\text{max}} - \Gamma_A - \Gamma_B\right) - k_{\text{des}}^A \Gamma_A - k_{\text{red}}^{\text{ads}} \Gamma_A + k_{\text{ox}}^{\text{ads}} \Gamma_B \]

\[ \frac{d\Gamma_B}{dt} = k_{\text{ads}}^B\left(\Gamma_{\text{max}} - \Gamma_A - \Gamma_B\right) - k_{\text{des}}^B \Gamma_B + k_{\text{red}}^{\text{ads}} \Gamma_A - k_{\text{ox}}^{\text{ads}} \Gamma_B \]

Here, \( \Gamma_A \) and \( \Gamma_B \) represent the surface concentrations of adsorbed species \( A \) and \( B \), respectively. \( \Gamma_{max} \) corresponds to the maximum surface coverage, assuming a monolayer of adsorbate. The terms \( k_{ads} \) and \( k_{des} \) denote the adsorption and desorption rate constants, while \( k_{red}^{ads} \) and \( k_{ox}^{ads} \) represent the rate constants for electron transfer associated with the reduction and oxidation of the adsorbed species, respectively.

To describe the electron transfer kinetics, these constants are assumed to follow Butler–Volmer kinetics behaviour. This introduces additional parameters — including the formal potential, charge transfer coefficient, and standard rate constant — which may differ from those observed in the aqueous solution.

The current resulting from charge transfer of the adsorbed species is expressed as:

\[ j_{\text{ads}} = -F A k_{ct}^{\text{ads}} \frac{d\Gamma_A}{dt} \]

Where \( k_{ct}^{\text{ads}} \) is the global charge transfer constant for the adsorbed species, based on Butler–Volmer kinetics. In contrast, the current limited by mass transport from the bulk solution is described by:

\[ j_{\text{bulk}} = -F A k_{ct}^{\text{bulk}} D\left( \frac{\partial A}{\partial x} \right)_{x=0} \]

More details and derivations can be found here.

Effect of Adsorption on the Voltammogram

A comprehensive analysis of this system is very extensive, as several parameters influence the voltammetric response. However, to illustrate the key effects of adsorption, we will present three representative scenarios commonly observed when adsorption plays a role.

One of the most distinctive features of this system is the scan rate–dependent change in the voltammogram. When the equilibrium potentials of the adsorbed and bulk species are close, the shape of the voltammogram transitions as the scan rate increases:

• At low scan rates, the response resembles a typical diffusion-controlled process.
• At high scan rates, the voltammogram adopts a profile characteristic of adsorption-dominated systems.

This behaviour is illustrated in the following animation:

Figure: Adsorption & Charge Transfer — Effect of Scan Rate

Another scenario arises when the equilibrium potentials differ significantly, leading to notable changes in the voltammetric profile. In such cases, one may observe a broadened peak, a shoulder, or even two distinct peaks as the separation between the redox couples increases. The following animation illustrates this behaviour:

The strength of adsorption is a key parameter. In this case, the parameter \( k_{\text{ads}} \) was varied. As the adsorption rate increases, the voltammogram progressively shifts in shape, reflecting the transition towards a system increasingly governed by surface processes. The following animation illustrates this behaviour:

Finally, it was explore how the maximum coverage, \(\Gamma_{max}\), affects the voltametric response. At very low values of \(\Gamma_{max}\) the current is negligible, but as \(\Gamma_{max}\) increases, the current due to the adsorbed species becomes more relevant, and at sufficiently  (\Gamma_{max}\) the current is predominantly from the adsorbed species, highlighting the dominance of surface processes under high-coverage conditions.

Voltammogram Simulator

The following interactive app simulates a cyclic voltammogram involving a coupled adsorption process based on the scheme shown in Figure 1. In addition to the voltammogram, the app visualizes the interfacial concentrations of both species as a function of the applied potential.

You can explore the effects of various parameters on the voltammetric response and adsorption behaviour:

• Diffusion coefficient of oxidized and reduced species (\(D_{Re}\) & \(D_{Ox}\), in cm²/s): See how diffusion influences the symmetry of the voltammograms. Note that the slider values are scaled logarithmically.
• Bulk concentration of the reduced species (\([Re]\), in mM): Adjust this to observe how the initial concentration impacts the shape and magnitude of the cyclic voltammogram.
• Maximum surface coverage (\(\Gamma_{max}\), in mol/cm²): Examine how changes in adsorption capacity affect surface phenomena and peak characteristics.
• Equilibrium redox potentials of bulk and adsorbed species (\(E_q\) & \(E_q^{ads}\), in V): Define the thermodynamic driving force for electron transfer in the bulk solution and at the interface.
• Charge transfer coefficients (\(\alpha\) & \(\alpha_{ads}\)): These parameters influence electron transfer kinetics. Their effect is pronounced at low values of \(k_0\) and diminishes as \(k_0\) increases.
• Standard rate constant (\(k_0\)): Controls whether the redox process behaves reversibly or irreversibly. This parameter is also adjusted on a logarithmic scale.
• Rate constants of adsorption (\(k_{ads}^{Re}\) & \(k_{ads}^{Ox}\), in cm/s): Determines the rate at which the reduced and oxidized species attach to the surface.
• Rate constants of desorption (\(k_{des}^{Re}\) & \(k_{des}^{Ox}\)): Specifies how quickly the adsorbed species detach from the surface.

By adjusting these parameters, you can gain a deeper understanding of how adsorption affects a cyclic voltammogram. In addition, the plot can be exported as CSV files.

References

1. Compton, R.G., Laborda, E., Ward, K.R. (2014). Understanding Voltammetry: Simulation of Electrode Processes. Imperial College Press. https://doi.org/10.1142/p910
2. Bard, A.J., Faulkner, L.R., White, H.S. (2022). Electrochemical Methods: Fundamentals and Applications (3rd ed.). John Wiley & Sons. https://doi.org/10.1023/A:1021637209564
3. Britz, D., Strutwolf, J. (2016). Digital Simulation in Electrochemistry (4th ed.). Springer. https://doi.org/10.1007/978-3-319-30292-8
4. DuVall, S.H., McCreery, R.L. (1999). Control of Catechol and Hydroquinone Electron-Transfer Kinetics on Native Surfaces. Analytical Chemistry. https://doi.org/10.1021/ac990399d
5. Kondo, T., Takechi, M., Sato, Y., Uosaki, K. (1995). Adsorption behavior of functionalized ferrocenylalkane thiols and disulfide onto Au and ITO and electrochemical properties of modified electrodes: Effects of acyl and alkyl groups attached to the ferrocene ring. Journal of Electroanalytical Chemistry. https://doi.org/10.1016/0022-0728(94)03683-T
6. Elliott, C.M., Hershenhart, E.J. (1982). Electrochemical and spectral investigations of ring-substituted bipyridine complexes of ruthenium. Journal of the American Chemical Society. https://doi.org/10.1021/ja00390a022
7. Schlereth, D.D., Maentele, W. (1992). Redox-induced conformational changes in myoglobin and hemoglobin: Electrochemistry and UV-vis and FTIR difference spectroscopy at surface-modified gold electrodes in an ultra-thin-layer spectroelectrochemical cell. Biochemistry. https://doi.org/10.1021/bi00148a009

If you want to cite this blog post, use:
Robayo, I. (2025). Understanding Surface Adsorption in Electrochemical Reactions. Available at: https://electrochemeisbasics.blogspot.com/2025/04/understanding-surface-adsorption-in.html [Accessed Date Accessed].