A quartz crystal microbalance (QCM) is a well-established technique in the field of gravimetry with a number of applications in aerosol detection, thin film deposition, biology, and electrochemistry [

1,

2,

3,

4,

5]. In various fields of electrochemistry, such as such as lithium-ion battery characterization, electrodeposition, or corrosion, QCM studies have been reported in the recent years focusing on characterization of a single resonator [

6,

7]. However, a universal understanding of intrinsic electrochemical processes, particularly in the field of battery research, often requires characterization in real or close to real conditions with simultaneous monitoring of multiple electrodes. In this study, we present an electrochemical QCM setup employing two quartz crystals, referred to as dual EC-QCM, which allows for concurrent characterization of mass deposition and dissipative loading of two quartz sensors in a three-electrode configuration.

QCM relies on the changes of the resonant piezo crystal oscillation upon deposition of mass or property changes of the surrounding medium. A common way to characterize the oscillation is by measurement of the QCM electrical response. Sauerbrey was the first one to describe the relation between the shift of the characteristic resonance frequency and the deposited mass [

8]. While the Sauerbrey relation considers the crystal mechanical properties in order to characterize the deposited mass of a thin, rigid layer, it does not account for changes due to an introduced dissipative loading. Further considerations for operation in dissipative conditions were described by Kanazawa, who has shown that the effect of liquid loading on the characteristic resonance frequency is:

where ∆

f is the resonance frequency change,

f_{0} is the resonance frequency of the unperturbed crystal,

ρ and

η represent the liquid density and viscosity, respectively,

μ_{q} is the quartz shear modulus (~2.947 × 10

^{11} g cm

^{–1} s

^{2}), and

ρ_{q} is the quartz density (~2.651 g cm

^{–3}) [

9]. This relation was further developed by Martin et al, who has shown that separation of liquid loading and mass deposition is possible with the extensive characterization of the crystal complex impedance response [

10]. In their study they showed that, in the case of a thin rigid deposited layer in liquid conditions, the following relations can be applied:

where

N indicates the order of the resonance harmonic and

ρ_{s} is the deposited mass density. As the reactance reaches zero at the resonance frequency, the absolute value of the complex impedance is dominated by the real part value:

with

Z_{Re,min} being the minimum of the real part of the complex impedance at the resonance frequency,

η_{q} the effective quartz viscosity and C

_{1} the motional capacitance of the crystal. Therefore, a deposited rigid layer also contributes to the stored kinetic energy in the oscillation. The motional capacitance can be understood from the Butterworth–van-Dyke (BVD) model that describes the crystal response as an RLC series resonator (R

_{1}, L

_{1}, C

_{1}) with an additional parallel geometrical capacitance (C

_{0}). Martin et al. also proposed additions to the BVD circuit to account for the liquid and mass interactions with an inductor (L

_{L}) and a resistor (R

_{L}) representing the liquid loading, and an additional inductor (L

_{s}) as the loading of the deposited solid film. The described study pointed out the importance of characterization of the unperturbed crystal properties in order to separate the liquid and mass loading effects. This can only be achieved after reliable calibration of the measurement system by removal of parasitic impedances arising from the fixturing, such as the introduced capacitance from connectors or additional resistance and inductance from the wiring. The above-mentioned approaches allowed for complex investigations in liquid, such as the description of molecular interactions, state-of-charge characterization in lead-acid batteries, and underpotential deposition study on gold electrodes [

1,

6,

11,

12,

13,

14]. Technical capabilities of QCM were further extended by measuring at higher harmonic frequencies of the crystal oscillation. Such characterization is possible as the electrical perturbation of a QCM results in the excitation of the odd number harmonics [

12]. The higher harmonics allow for robust separation of rigid mass deposition and dissipative loading as ∆

f varies with

Nρ_{s}, while ∆

f and

Z_{Re,min} vary with (

Nρη)

^{1/2} [

10].

In this study, the aforementioned approaches are employed to validate a dual EC-QCM configuration by characterization of underpotential deposition of Ag on two quartz sensors simultaneously. As the ability to resolve mass deposition in the range of nanograms requires high sensitivity and stability, a complex impedance correction procedure is employed to overcome practical difficulties, such as liquid dampening. The presented correction workflow allows separation of liquid loading properties and effects of rigid mass deposition with high sampling rate frequency tracking for both QCMs in the dissipative medium. Finally, multi-harmonic measurements using both working and counter electrode crystals are performed to corroborate the findings.