Periodic Reporting for period 4 - ElIonT (Electron- and Ion Transfer at the Interface: a Hyphenated Dynamic Multi-Frequency Approach)
Periodo di rendicontazione: 2022-11-01 al 2023-10-31
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.
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.
The activitWP 1: The DMFA experimental technique and the analysis techniques were fully developed. The acquired spectra are fitted with equivalent circuits or Padè approximants; proper procedures enable the evaluation of the error and of the optimal number of free paramters of the fit. The inter-modulation (non-linearity) is analyzed. The quartz crystal microbalance is successfully hyphenated with DMFA. A mathematical foundation of the techniques is given in terms of Volterra series expansion.
WP 2: The multi-physics numerical model includes ion transport phenomena, electric field equations, and electron transfer at the surface of the electrode, in realistic geometries. The calculation time has been decreased by analytically solving part of the system and by linearization of the perturbation part.
WP 3: Three redox couples were analyzed: iron ferrocyanide, ferrocene, ammonia-ruthenium complex. The equilibrium potential and the reaction rate are sensitive to supporting electrolyte concentration. Our model shows a significant role of ion pairing. This phenomenon opens an interesting research field and is a potential revolution in electrochemistry.
WP 4: The effect of ion concentration in electrochemical systems was analyzed. While the thermodynamic parameters properly account for the equilibrium behaviour, it does not fully define the kinetics. We developed a simplified statistical mechanics model, which shows the presence of characteristic quantities that are not directly connected to the thermodynamic parameters.
WP 5: Insertion of ions was monitored in various Prussian blue derivatives (copper, zinc, and manganese hexacyanoferrates) and in lithium manganese oxide. The work was relevant for batteries and supercapacitors and attention was payed to insertion of zinc and lithium, for energy applications.
WP 6: The effect of insertion of multiple cation species was analyzed in the solids investigated in WP 5. The kinetic parameters and their dependence on state of charge were evaluated by means of DMFA.
WP 7: Hydrogen evolution was investigated with DMFA and modelled with the Volmer-Heyrovsky-Tafel mechanism. The process kinetics has been analyzed in TRIS and phosphate buffer, around pH 7: the role of the buffer on The mass transport is visible. The results could have practical relevance in hydrolyzers.
The dissemination of the results has been performed in several publications, presentations in conferences and seminars (see attached figure 4).
WP 2: The multi-physics numerical model includes ion transport phenomena, electric field equations, and electron transfer at the surface of the electrode, in realistic geometries. The calculation time has been decreased by analytically solving part of the system and by linearization of the perturbation part.
WP 3: Three redox couples were analyzed: iron ferrocyanide, ferrocene, ammonia-ruthenium complex. The equilibrium potential and the reaction rate are sensitive to supporting electrolyte concentration. Our model shows a significant role of ion pairing. This phenomenon opens an interesting research field and is a potential revolution in electrochemistry.
WP 4: The effect of ion concentration in electrochemical systems was analyzed. While the thermodynamic parameters properly account for the equilibrium behaviour, it does not fully define the kinetics. We developed a simplified statistical mechanics model, which shows the presence of characteristic quantities that are not directly connected to the thermodynamic parameters.
WP 5: Insertion of ions was monitored in various Prussian blue derivatives (copper, zinc, and manganese hexacyanoferrates) and in lithium manganese oxide. The work was relevant for batteries and supercapacitors and attention was payed to insertion of zinc and lithium, for energy applications.
WP 6: The effect of insertion of multiple cation species was analyzed in the solids investigated in WP 5. The kinetic parameters and their dependence on state of charge were evaluated by means of DMFA.
WP 7: Hydrogen evolution was investigated with DMFA and modelled with the Volmer-Heyrovsky-Tafel mechanism. The process kinetics has been analyzed in TRIS and phosphate buffer, around pH 7: the role of the buffer on The mass transport is visible. The results could have practical relevance in hydrolyzers.
The dissemination of the results has been performed in several publications, presentations in conferences and seminars (see attached figure 4).
The potential of DMFA technique has been fully developed thanks to ElIonT project, allowing us to acquire impedance spectra and intermodulation data (non-linear part of the response) under non-stationary conditions, with excellent time resolution. Some of the developed analysis tools are included in a freely-distributed software. Analysis of non-linearity of the system is now possible by inter-modulation analysis.
A new technique for quartz crystal microbalance measurements was developed. It is based on the demodulation in quadrature of the output of the quartz oscillator; the procedure leads to two demodulated (low-frequency) signals which are digitally acquired and analyzed to extract the signal frequency. This innovative approach allowed us to couple the microbilance to the DMFA technique: we are now able to detect the mass response of the electrochemical system to simultaneous sinusoidal perturbations introduced by DMFA.
The full physical modelling of the systems has been developed, with techniques to shorten the calculation time have been developed, including the analytical solution of parts of the equations and the linearization of the perturbation and response signals.
The physical processes involved in complex reactions have been detected and disentangled, thanks to the developed techniques. The experimental systems include the Insertion of ions in various Prussian blue derivatives (copper, zinc, and manganese hexacyanoferrates, and thin films) and in lithium manganese oxide, with relevance for battery applications.
The hydrogen evolution has been successfully analyzed by DMFA and modelled. Experiments in buffer solutions, around pH 7, enabled the measurement of the reaction kinetics and the role of the buffer on the mass transport. The results could have significant practical relevance in hydrolyzers.
Redox couples (iron ferrocyanide, ferrocene, ammonia-ruthenium complex) were accurately analyzed, thanks to the advancements of DMFA. Experiments showed interesting effects: the equilibrium potential and the electron transfer resistance significantly depend on the concentration of the supporting electrolyte. The overall analysis revealed an intricate relation between Frumkin effect and double layers, Debye-Hückel phenomenon, and electron transfer; we highlighted the relevance of ion pairing. These results, enabled by the improvement of the DMFA technique, can lead to a revolution in electrochemistry.
A new technique for quartz crystal microbalance measurements was developed. It is based on the demodulation in quadrature of the output of the quartz oscillator; the procedure leads to two demodulated (low-frequency) signals which are digitally acquired and analyzed to extract the signal frequency. This innovative approach allowed us to couple the microbilance to the DMFA technique: we are now able to detect the mass response of the electrochemical system to simultaneous sinusoidal perturbations introduced by DMFA.
The full physical modelling of the systems has been developed, with techniques to shorten the calculation time have been developed, including the analytical solution of parts of the equations and the linearization of the perturbation and response signals.
The physical processes involved in complex reactions have been detected and disentangled, thanks to the developed techniques. The experimental systems include the Insertion of ions in various Prussian blue derivatives (copper, zinc, and manganese hexacyanoferrates, and thin films) and in lithium manganese oxide, with relevance for battery applications.
The hydrogen evolution has been successfully analyzed by DMFA and modelled. Experiments in buffer solutions, around pH 7, enabled the measurement of the reaction kinetics and the role of the buffer on the mass transport. The results could have significant practical relevance in hydrolyzers.
Redox couples (iron ferrocyanide, ferrocene, ammonia-ruthenium complex) were accurately analyzed, thanks to the advancements of DMFA. Experiments showed interesting effects: the equilibrium potential and the electron transfer resistance significantly depend on the concentration of the supporting electrolyte. The overall analysis revealed an intricate relation between Frumkin effect and double layers, Debye-Hückel phenomenon, and electron transfer; we highlighted the relevance of ion pairing. These results, enabled by the improvement of the DMFA technique, can lead to a revolution in electrochemistry.