EQCM-D Measurement

Electrochemical Quartz Crystal Microbalance with Dissipation monitoring (EQCM-D) is a real-time surface-sensitive analytical technique that combines electrochemistry with quartz crystal microbalance with dissipation (QCM-D). The technique allows for the simultaneous monitoring of mass changes, viscoelastic properties, and changes in current and/or potential at the electrode surface. This information is used to characterize molecular interactions and reactions at surfaces and interfaces and to study interfacial processes in various applications, including battery research, corrosion studies, fuel cell development as well as in biological applications.

Fundamentals and Working Principles

EQCM-D measures two parameters via QCM-D, the resonance frequency (f) and energy dissipation (D) of the quartz crystal sensor, where the frequency relates to mass changes at the surface and the dissipation relates to the softness or viscoelasticity of the surface-adhering layer. By applying an electrical potential, electrochemical reactions can be induced, and the resulting mass and viscoelastic changes can be monitored in real-time.

The two methods, electrochemistry and QCM-D are ideally suited to be paired since both are surface-based techniques. QCM-D can provide real-time information on mass and structure of thin films in the form of changes in frequency or dissipation, while electrochemistry can be the stimulus of an interaction or provide information about interfacial charge transfer.

QSense EQCM-D is a so-called multi-harmonic EQCM-D, which measures f and D at multiple harmonics, and allows for analysis of interfacial processes with nano-level sensitivity.

Design of the Electrochemical Cell

The electrochemical cell used in EQCM-D experiments typically consists of three electrodes: the working electrode (WE), the reference electrode (RE), and the counter electrode (CE), Fig. 1. The quartz crystal sensor acts as the working electrode, where the electrochemical reactions and mass changes are monitored.

• Working Electrode (WE): The quartz crystal sensor serves as the working electrode. It is coated with a conductive material, such as gold, to facilitate electrochemical reactions.
• Reference Electrode (RE): The reference electrode is usually a standard electrode, such as Ag/AgCl, which provides a stable reference potential against which the working electrode's potential is measured.
• Counter Electrode (CE): The counter electrode is typically made of an inert material, such as platinum, and completes the electrical circuit by allowing current to flow between the working and counter electrodes.

Figure 1: Schematic illustration of the cross section of QSense EQCM-D module showing the 3-electrode configuration.

Voltammetry

A typical EQCM-D setup combines cyclic voltammetry (CV) with QCM-D, enabling simultaneous measurements of frequency (f), dissipation (D), and current (I) in response to a varying potential (E) on the sensor surface.

In CV, the potential of the working electrode is cycled, and the resulting current is measured, allowing for the study of redox reactions and the corresponding mass changes at the electrode surface. This approach allows for a direct correlation between QCM-D and electrochemical results, Fig. 2. For example, in the study of battery electrode materials, CV can help identify the specific ions involved in the charge storage mechanism and their impact on the electrode's mass and viscoelastic properties.

Figure 2. EQCM-D measurement of copper being deposited onto and stripped from an Au sensor. Copper sulfate solution (10 mM CuSO₄ in 0.1 M H₂SO₄) is injected into the QEM module whereafter five CV cycles are performed, starting at +0.3 V and cycling back and forth from -0.5 V to +0.5 V at 50 mV/s. The QCM-D data shows a reversible process where the decrease in f indicates mass building up on the surface while the increase in D indicates the film’s viscoelastic character.

Electrochemical Impedance Spectroscopy (EIS)

Another technique that can be combined with QCM-D is Electrochemical Impedance Spectroscopy (EIS). EIS measures the impedance of the electrochemical system over a range of frequencies, providing insights into the charge transfer processes and the properties of the interfacial layer. This technique is particularly useful for studying the properties of thin films and interfacial layers in various electrochemical systems.

In the study of lipid membranes, for example, the combination of QCM-D and EIS can provide a comprehensive understanding of the structural and functional properties of the membrane and reveal for example membrane disruption.

In addition to the electrochemical information provided by the selected method, for example cyclic voltammetry (CV), galvanostatic charge-discharge, or electrochemical impedance spectroscopy (EIS), EQCM-D provides information on mass, thickness, and viscoelastic properties of interfacial layers at the electrode surface. Combined, the time-resolved electrochemical, gravimetric, and mechanical information provides insight into the mechanisms of various electrochemical processes with nano-sensitivity.

EQCM-D is a powerful tool for studying a wide range of interfacial processes in various applications, including battery research, fuel cell development, electrochemical deposition, polyelectrolyte multilayers, biomolecular interactions, and membrane potential measurements. A few application examples are provided below.

1. Battery Research

EQCM-D provides insights into various battery phenomena, such as phase transformations, electrode surface morphology evolution, and intermediate species formation during electro-deposition.

The method has for example been used to investigate the Charge Storage Mechanisms of battery electrode materials where it provided insight into how ions are stored in redox-active materials and the voltage-dependent storage mechanisms.

EQCM-D has also been employed to study the formation, evolution, and mechanical properties of the Solid Electrolyte Interphase (SEI) Formation in different electrolytes. Aspects of interest included monitoring the build-up process and the viscoelastic properties of the formed layers.

2. Fuel Cell Development

EQCM-D can be used to analyze the corrosion of fuel cell electrodes, such as those in Polymer Electrolyte Membrane Fuel Cells (PEMFCs) where it helps in understanding the mass loss due to corrosion and the temperature dependence of these reactions.

3. Biomolecular Interactions and biomembranes

EQCM-D can be used to study the electrostatic interactions of biomolecules with surfaces, such as redox proteins, cells, and DNA. This is useful for understanding the binding and interaction mechanisms at the molecular level.

EQCM-D can also be used to measure membrane potentials and study the structural and functional properties of lipid membranes. This includes monitoring the disruption of lipid membranes by enzymes.

EQCM vs EQCM-D

While EQCM measures only the mass changes, EQCM-D provides additional information on the viscoelastic properties of the adsorbed layer. This makes EQCM-D a more comprehensive tool for studying complex interfacial processes. For example, in battery research, EQCM-D can provide insights into both the mass changes and the mechanical properties of the SEI layer, which are crucial for optimizing battery performance. Also, since QSense EQCM-D measures f and D at both the fundamental resonance frequency several harmonics, you have the possibility to perform viscoelastic modelling which you do not have using an EQCM setup.