EQCM-D: why the “D” matters in electrochemical QCM

Electrochemical measurements are excellent at showing how an electrode behaves. They reveal when current flows, how much charge is transferred, how reversible a process is and how performance changes over time.

However, electrochemical data do not always show what is physically happening at the electrode surface. Is material being deposited or dissolved? Is an interfacial layer growing? Is a film becoming thicker, softer, rougher or more swollen? Is a coating still compact, or is it starting to lose mechanical stability?

This is where QCM-based techniques add important information. They connect electrochemical response to physical changes at the interface.

From QCM to QCM-D: adding mechanics to mass

Quartz crystal microbalance, QCM, is a surface-sensitive technique used to monitor nanogram-level mass changes on a sensor surface in real time. A thin quartz crystal disk is electrically excited to oscillate at its resonance frequency. When material attaches to or leaves the sensor surface, the resonance frequency shifts.

For thin, rigid layers that are firmly attached and move together with the oscillating crystal, this frequency shift can be converted into areal mass using the Sauerbrey equation. QCM is widely used to study surface processes such as adsorption, desorption, deposition, dissolution and film formation, including electrochemical processes like metal plating and stripping, catalyst dissolution, ion insertion and interphase formation.

The Sauerbrey equation only holds when the material behaves like a thin, rigid extension of the quartz crystal. This is reasonable for many compact films, but not for soft, swollen, rough, porous or viscoelastic layers, which are common in liquid environments. For these layers, the frequency shift reflects both mass and mechanical properties, and Sauerbrey mass alone can be misleading — sometimes by a large margin.

QCM-D, quartz crystal microbalance with dissipation monitoring, extends QCM by also measuring dissipation: how much energy is lost during the oscillation of the sensor. A rigid layer dissipates little energy; a layer that deforms or damps the sensor motion dissipates more. In short, frequency tells you that the load on the sensor has changed, while dissipation helps reveal what kind of layer caused that change.

From eQCM to eQCM-D: bringing the “D” into electrochemistry

When QCM is coupled with electrochemistry, the sensor electrode is used as the working electrode in an electrochemical cell. This is electrochemical QCM, or eQCM, and it allows mass changes at the electrode to be followed while potential or current is controlled.

eQCM-D adds dissipation monitoring to this experiment, so that potential, current and charge are measured together with frequency and dissipation. The result is a combined view of the electrochemical response, the mass change at the electrode surface, and the mechanical or structural character of the interfacial layer — which matters because many electrochemical interfaces change in structure and mechanics, not just in mass, during cycling..

From information to insight: what can you actually do with the “D”?

Knowing that dissipation is sensitive to layer mechanics allows you to leverage this information.

1. It tells you when your mass number is wrong — and lets you correct it.

If dissipation stays low and the frequency shift at different harmonics overlaps once normalized, the layer is behaving as a simple rigid film and a Sauerbrey mass holds true. If dissipation rises, or the harmonics disagree, that is a direct signal that a Sauerbrey conversion will misestimate the mass, sometimes substantially, because the frequency response is influenced not only by mass, but also by the layer’s material properties and mechanical behavior. In that regime, frequency and dissipation across several harmonics can be fitted to a viscoelastic (Voigt-type) model. Instead of a single areal mass number, this fitting returns a small set of physical parameters: the true areal mass and thickness of the layer, and its shear storage modulus and viscosity. This is a quantitative use of the “D”, not just a qualitative flag — it recovers a real mass value for exactly the layers where Sauerbrey would fail.

In a study developing eQCM-D with a multilayer Voigt model to probe SEI formation on Li₄Ti₅O₁₂ battery electrodes, SEI thickness and shear storage modulus were extracted cycle-by-cycle in three different electrolytes^[1]. Depending on the electrolyte, the storage modulus differed by roughly a factor of four between the stiffest and softest films, a difference frequency-only measurements could not have detected. The ranking of film quality matched the electrodes’ capacity retention trends measured separately in coin cells.

2. It tracks mechanical change over time so you can observe trends.

Often the more useful output is not an absolute modulus but a trend: is the layer stiffening, softening, swelling or restructuring as the potential is cycled? This is a qualitative-to-semi-quantitative use of dissipation, and it is often more important than absolute values. This allows you to determine whether the interphase is getting more or less stable over time, and how that compares between two samples?”

In fuel cell durability, the stability of the catalyst-ionomer interface is a key factor. In operando eQCM-D was used to track potential-dependent mass uptake and viscoelastic changes of a Nafion ionomer layer on various electrodes, and on a real spray-coated catalyst layer^[2]. The dissipation response showed how the ionomer’s mechanical coupling to the electrode changed with electrolyte anion and potential, distinguishing swelling/softening behavior that mass measurements alone would not have captured. This directly links the eQCM-D signal to catalyst-ionomer interface stability.

Together, these two uses are the added value for “D”. It flags when a mass-only interpretation is unsafe and lets you recover a corrected, physically meaningful mass; and it gives a direct, time-resolved readout of the mechanical evolution of the interface, which frequency and electrochemistry alone cannot provide.

Why this matters for electrochemical energy research

Electrochemical energy technologies are controlled by interfaces that form, change and degrade during operation. eQCM-D is especially relevant when these interfaces are thin, dynamic and mechanically complex.

Typical application areas include:

• Batteries: SEI/CEI formation, metal plating and stripping, protective coatings and electrode stability. eQCM-D with viscoelastic modeling has been used to quantify SEI thickness and stiffness on battery electrodes cycle-by-cycle, and to link electrolyte formulation directly to interphase quality^[1,4].
• Fuel cells: ionomer adsorption, catalyst-layer stability, metal dissolution and carbon corrosion. eQCM-D has been used to follow ionomer mass uptake and viscoelastic changes on catalyst surfaces and real catalyst layers under operating conditions^[2], and to compare platinum dissolution and oxidation behavior in acidic and alkaline electrolytes relevant to PEM and AEM systems^[3].
• Supercapacitors: ion and solvent uptake, swelling and structural changes in porous or pseudocapacitive electrodes.
• Electrolyzers: catalyst and protective coating stability during oxygen and hydrogen evolution reactions.

Across these applications, eQCM-D links electrochemical behavior to mass and mechanical changes at the electrode/electrolyte interface.

Conclusion: the value of the “D”

eQCM provides sensitive, in situ information about mass changes at an electrochemical interface. eQCM-D extends this by adding dissipation, which tells you whether a frequency shift can be trusted as a simple mass change or must instead be interpreted through a viscoelastic model — and, in the latter case, lets you recover a corrected mass together with the layer’s mechanical properties.

The value of the “D” is therefore twofold: it improves confidence in the interpretation of QCM mass data, and it gives direct, quantitative access to interfacial mechanical changes — such as SEI stiffening or ionomer swelling — that frequency and electrochemistry alone cannot describe.

References

1. Dargel, V., Shpigel, N., Sigalov, S., Nayak, P., Levi, M. D., Daikhin, L., & Aurbach, D. (2017). In situ real-time gravimetric and viscoelastic probing of surface films formation on lithium batteries electrodes. Nature Communications, 8, 1389. https://doi.org/10.1038/s41467-017-01722-x
2. Rieger, N., Almyren, I., Strandberg, L., Butori, M., Lindström, R. W., Eriksson, B., Jannasch, P., & Wickman, B. (2026). In-situ eQCM-D investigations of PEMFC catalyst-ionomer interactions. ChemElectroChem, 13(3). https://doi.org/10.1002/celc.202500410
3. Strandberg, L., Shokhen, V., Luneau, M., Lindbergh, G., Lagergren, C., & Wickman, B. (2022). Comparison of oxygen adsorption and platinum dissolution in acid and alkaline solutions using                                                 electrochemical quartz crystal microbalance. ChemElectroChem, 9(22). https://doi.org/10.1002/celc.202200591
4. Yang, Z., Dixon, M. C., Erck, R. A., & Trahey, L. (2015). Quantification of the mass and viscoelasticity of interfacial films on tin anodes using EQCM-D. ACS Applied Materials & Interfaces, 7, 26585–26594. https://doi.org/10.1021/acsami.5b07966