Electrochemistry has a central role in our contemporary society. This is demonstrated by its profound involvement in many aspects of everyday life: from powering portable electronic devices to personal electro-mobility, passing through recycling, waste water treatment, clean energy production, water desalination, personal care, and others. The industrial development explored the limits of the current scientific knowledge in this field.
Real systems work outside ideality (i.e. in non-dilute solutions), outside thermodynamic equilibrium, and even outside stationarity. At variance, current theories cover only one of these aspects: equilibrium thermodycs easily deals with concentrated solutions (but not kinetics), while the Marcus theory of electron transfer (see attached figure 1) does not deal with concentrated solutions, particle-particle interactions, and ion transfer. A deeper understanding of these processes is necessary.
To improve our understanding of electrochemistry, it is necessary to experimentally detect and disentangle various phenomena, spanning decades in time- and length-scale (see the attached figure 2). An advanced version of impedance spectroscopy was developed in ElIonT: the dynamic multi-frequency analysis (DMFA). It makes use of multi-sine perturbation signals and inverse Fourier transform analysis coupled with quadrature filters. The technical advancements of DMFA allowed us to resolve the impedance in time, as the macroscopic state of the system evolves, thus analysing the kinetics of non-stationary systems. The non-linear behaviour is measured by observing the inter-modulation of multi-sinusoidal perturbations (see attached figure 3). DMFA was also hyphenated with the quartz crystal microbalance, getting the response of mass on DMFA perturbations.
By means of DMFA, various experimental systems were studied: i) insertion of cations in Prussian blue analogues and manganese oxide; ii) hydrogen evolution; iii) redox couples. Attention was payed to processes related to batteries. An accurate physical modelling was performed on each system.
The experiments on redox couples highlighted a complex role of the supporting electrolyte. The experiments showed a complex relation between double layers, Frumkin effect, Debye-Hückel phenomenon, electron transfer, and our analysis highlighted the role of ion pairing. The observation of this process could actually revolutionize the current view of electrochemistry.
