Square Wave Electrogravimetry Monitors Transient Submonolayer Adsorption of Redox-Active Molecules with a Precision of Less Than 1% of a Monolayer

Square wave voltammetry on electrolytes containing the reversible redox pairs flavin adenine dinucleotide (FAD) and methyl viologen (MV) was complemented by a new form of acoustic microgravimetry. The analytes FAD and MV have promising applications in bacterial fuel cells and redox flow batteries. The instrument operates similarly to a quartz crystal microbalance with dissipation monitoring (EQCM-D). It reports shifts in resonance frequency and half bandwidth on several overtones. Compared to the existing instruments, the time resolution was improved by a factor of 40 down to 2.5 ms by multifrequency lockin amplification, which is important for the analysis of the transients in electrochemistry. The frequency noise after accumulation was reduced below 10 mHz ≙ 0.12 ng/cm 2 ≙ 1.2 pm (assuming ρ = 1 g/cm 3 ) by employing modulation and averaging. Square wave modulation of the electrode potential is superior to linear ramps (cyclic voltammetry) because it reduces the contribution of non-Faraday currents and, also, improves sensitivity. The shifts of frequency,Δ f, and half bandwidth, ΔΓ, are in line with the Sauerbrey prediction (Δ f / n ≈ const. and ΔΓ/ n < -Δ f / n with n the overtone order). Small deviations (Figure B and D) presumably are caused by the layer’s softness. The apparent shear modulus, G app , of this layer is in the GPa range. The frequency difference, Δ f / n | CT (Figure E), amounts to an apparent mass transfer rate. Plots of Δ f / n | CT versus potential sometimes show curves similar to the electric current as observed for e. g. MV. For FAD, the results obtained depended on pH and the apparent mass transfer rates are much different from the current (Figure C and E). This difference is most likely caused by adsorbed molecules that are intermediates of the underlying two-electron process. The explanation is further corroborated by the response time of the QCM signals being much larger than the RC time of double layer recharging as determined with EIS. An interpretation in terms of adsorption/desorption is more plausible than an interpretation in terms of changed viscosity in the diffuse double layer. [1] Caption: Currents and frequency shifts obtained on a solution containing FAD. D f / n | av evidences the preferred adsorption of the intermediate state (FADH). References [1] Leppin, Christian; Langhoff, Arne; Johannsmann, Diethelm. Square-Wave Electrogravimetry Combined with Voltammetry Reveals Reversible Submonolayer Adsorption of Redox-Active Ions. 2022 , 94 (28), 10227–10233. Figure 1