Fast Quartz Crystal Microbalance Research Articles

Scrutiny of Electrode/Electrolyte Interfaces and Electrode Degradation Mechanisms By Advanced Electrogravimetry: Implications in Energy Storage

The recent tendency in the battery field toward the use of aqueous electrolytes stimulated the pursuit for new electrode materials, compatible with narrow electrochemical stability window of aqueous electrolytes.[1, 2] The most widely studied electrode materials in lithium ion batteries using organic electrolytes (LIBs) (LiCoO2 (LCO) and LiFePO4 (LFP), etc.) have also been revisited for their potential use in their aqueous analogues (Aqu-LIB).[3]Accompanying these developments, other aspects to forge ahead include the fundamental understanding of i) the interfacial processes occurring at electrode/electrolyte interface (EEI) (dehydration and interfacial transfer of charge carriers) and ii) the interplay between the repeated ion (de)insertion and the electrode structure (affecting its mechanical integrity, causing long-term cycling performance deterioration).[3]In light of this, considerable efforts have been devoted with a number of ex situ and in situ / operando studies of the EEI of batteries.[4] As an in situ coupled electrochemical and piezoelectric probe, electrochemical quartz-crystal microbalance (EQCM) and derived methods (such as multiharmonic EQCM with dissipation monitoring) have shown great sensitivity to monitor the mass (gravimetric probe), structure (morphological probe), and viscoelastic property (mechanical probe) variation of an electrode during electrochemical processes.[5] Targeted at further understanding this fundamentally important topic, a non-classical electrochemical strategy which couples fast quartz crystal microbalance (QCM) with electro-chemical impedance spectroscopy (EIS) (the so-called AC-EQCM) has also been proposed by our group for in situ capturing of species interfacial transfer behavior.[6-8] Kinetic and gravimetric upshots can be obtained for each species involved in the charge storage process from this frequency-dependent measurement (i.e., AC-EQCM).This EQCM based instrumental platform is employed herein to study the flux of species at the EEI and to scrutinize the charge-storage and degradation mechanism of composite LiCoO2 electrodes in aqueous electrolytes, as a model system for Aqu-LIB EEI.[3] Our approach, aiming at elucidating the electrode capacity fading during cycling, will consider two main aspects: i) mechanical degradation related with failure of polymeric binder network, and ii) structural degradation of active materials affecting transfer dynamics of the species exchanged at the EEI.[3, 9] This work provides a further understanding of the underlying reasons for the electrode degradation and proposes an analytical approach for investigating fundamentals of aqueous battery chemistry.

Charge storage properties of single wall carbon nanotubes/Prussian blue nanocube composites studied by multi-scale coupled electrogravimetric methods

The gravimetric and dynamic aspects of the ions transfer at SWCNT/Prussian blue (PB) composite film electrode/electrolyte interface was investigated by the electrogravimetric impedance which couples fast quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS). In contact with an aqueous KCl electrolyte, evidence was found that the kinetic of species is controlled by Faraday and capacitive electrochemical charge storage mechanisms. It was found that the interfacial transfer of K+ was faster than the Cl− transfer in the studied potential window. Through our electrogravimetric method of investigation, it was revealed that both adsorption/desorption and the insertion/expulsion phenomena can occur for K+, while only adsorption/desorption can occur for Cl−. Indeed, for pure PB films, the anion contribution to the charge compensation process has never been observed in the studied medium and for that, it is related to only electroadsorption on the SWCNT part of the film. Present work highlighted tha fact that electrogravimetric response of the SWCNT/PB composite electrode encloses several species contribution probably due to a synergic effect of the two phases and our study contributes to disentangling it both gravimetrically and dynamically.

Dynamic Resolution of Ion Transfer in Electrochemically Reduced Graphene Oxides Revealed by Electrogravimetric Impedance

To accompany the search for optimal materials in electrochemical supercapacitors, appropriate characterization tools to assess key parameters of newly developed electrodes are required. Here we demonstrate the capabilities of ac-mode electrogravimetry to study in details the capacitive charge storage mechanisms in electrochemically reduced graphene oxide (ERGO) thin film electrodes. The coupling of electrochemical impedance spectroscopy (EIS) with fast quartz crystal microbalance (QCM) complements classical electrochemical quartz crystal microbalance (EQCM) by capturing here the dynamics of the electroadsorption process, identifying charged moieties and detecting solvation effects. We evidenced the coelectroadsorption of two types of cationic species (fully and partially hydrated cations) in the potential range studied and the indirect intervention of free solvent molecules. Further kinetic information on electroadsorption is also obtained which leads to a full deconvolution of the global EQCM response at...

Gravimetric and dynamic deconvolution of global EQCM response of carbon nanotube based electrodes by Ac-electrogravimetry

The capacitive charge storage of carbon nanotube (CNT) based electrodes was investigated by ac-electrogravimetry which couples fast quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS). In contact with an aqueous NaCl electrolyte, evidence was found that there are two types of cations (Na+·H2O and H+) electroadsorbed with different kinetics for cathodic potentials and the Cl− ions for anodic potentials together with free water molecules. The reconstruction of the total mass response from independent ac-electrogravimetry measurements agrees perfectly well with the global EQCM response. Our findings reveal the unique sensitivity of the ac-electrogravimetry to provide a fair gravimetric and dynamic deconvolution of the global EQCM responses.

Towards New Micro-Scale Supercapacitors: Pseudocapacitance of Hydrous Ruthenium Oxide Investigated By Multi-Scale Time-Resolved Electrochemical Methods

In recent years, the use of metal oxide thin films with nanoscale porosity for diverse electrochemical applications, e.g. energy storage, EC devices, etc. has become a subject of growing interest. 1 Among these materials, ruthenium oxide is a particular metal oxide which can offer a type of electrochemical capacitance termed pseudocapacitance which also involves faradaic charge-transfer reactions. RuOx. nH2O stands as an electroactive material with very high surface area which prosecute the fast reversible redox transitions in the potential window of charge and discharge. The amorphous and hydrous form exhibits noticable pseudocapacitance higher than its crystalline counterpart. 1, 2 For the improvement of the performance of metal oxide based electrodes, main attention has been attributed to their nanostructuration with various morphologies due to the unique properties and functionalities that can effectively be exploited in electrochemical devices. Among various synthesis methods such as sol-gel, spray pyrolysis and hydrothermal synthesis, electrochemical synthesis remains to be a fast, simple, low-cost and low-temperature technique which may also lead to the formation of homogenous nanoscale films with desirable qualities. Moreover, the incorporation of surfactant molecules in the electrogenerated film results in the production of thin nanostructured films by using potential-controlled self-assembly of surfactant-inorganic aggregates at solid-liquid interfaces 3 . The resulted nanostructures, with small sizes, and large surface area to volume ratios, are expected to facilitate the ion intercalation/extraction process. These improvements are attributed to a facilitated transfer and short diffusion length for ions transport, a high electrode/electrolyte contact area, and a better stress/strain management of the material during ion intercalation/adsorption. Despite the morphological information obtained by some in situ or ex situ characterization techniques such as X-ray photoelectron spectroscopy, small and wide-angle X-ray scattering, none of these methods alone provide the information on the exact identification of the transferred ionic species, their dynamics of transfer at the interfaces, as well as the role of electrolyte composition and the effect of ions solvation on the different electrochemical phenomena. Classical EQCM (Electrochemical Quartz Crystal Microbalance) was largely used to overcome these problems but its performances are not enough to extract pertinent results. Therefore, here, an alternative characterization tool was proposed to overcome the limitations of these classical EQCM. Specifically, the pseudocapacitive behavior related to the cation transfer in hydrous RuOx thin films was investigated by coupled time resolved characterization methods A c-electrogravimetry, consisting of in-situ coupling of electrochemical impedance spectroscopy (EIS) and fast quartz crystal microbalance (QCM), was used here: it measures simultaneously the electrochemical impedance, ΔE/ΔI (ω), and the mass/potential transfer functions (Δm/ΔE (ω)) It provides the access to the relevant information on the kinetics of species transferred at the solid/solution interfaces, and their transport in the bulk of the materials, the nature of these species as well as their relative concentration within the material. This coupling dominates over the limitations of QCM technique and has the ability to deconvolute the global mass variations provided by QCM measurements. Specifically, it detects the contribution of the charged or uncharged species and to identify anionic, cationic, and the free solvent contributions during various redox processes. 4,5 In the present work, this approach is exploited to study the ion transfer behaviour of amorphous hydrous RuOx in both mostly compact and rather porous structures. An electrochemical pathway was chosen, so-called surfactant-assisted electrodeposition is used for their preparation. In addition, we particularly focus on the amorphous materials rather than crystalline ones to emphasize its role in the electrochemical performance of such films. The electrochemical performance and ion intercalation/adsorption mechanisms were studied in aqueous H2SO4, Li2SO4 and Na2SO4 electrolytes by electrogravimetric methods (EQCM, and ac−electrogravimetry). Special attention is attributed to understand the nature of the ions involved in the charge compensation, solvation and the role of the electrolytes and the dynamic information of ions transfer at the electrode/electrolyte interfaces, ac−electrogravimetry as a gravimetric probe is used to study the complex H+ or Li+ intercalation/extraction mechanisms and to extract subtleties unreachable with classical tools.

New Insights into Pseudocapacitive Charge-Storage Mechanisms in Li-Birnessite Type MnO2 Monitored by Fast Quartz Crystal Microbalance Methods

Fast ionic transfer and transport properties continue to be one of the main pressing research concerns regarding energy storage materials (batteries, supercapacitors). Accompanying this search for optimal materials, appropriate characterization tools to assess key parameters of newly developed materials are required. In spite of its great relevance, fast electrogravimetric methods, i.e., coupling fast quartz crystal microbalance (QCM) and electrochemical impedance spectroscopy (EIS) (ac-electrogravimetry), have not yet been used for studying transfer and transport phenomena in materials for charge storage (except for the use of QCM along with cyclic voltammetry experiments (EQCM)). This coupled method, so-called ac-electrogravimetry, differs from classical EQCM and measures the usual electrochemical impedance, ΔE/ΔI (ω), and the mass variations of the film under a sinusoidal potential perturbation, Δm/ΔE (ω), simultaneously. This coupling has the ability to detect the contribution of the charged or uncharged species and to separate the anionic, cationic, and free solvent contributions during the various (pseudo)capacitive processes. The Li-birnessite type MnO2 thin films were studied in two different media, LiClO4 and NaClO4, by ac-electrogravimetry. The Li+ ions, Na+ ions, and their respective hydrated ionic species are detected to be involved in the pseudocapacitive charge storage of Li-birnessite type MnO2 thin films. The kinetics (fci) and resistance (Rti) of charged and noncharged species transferred at the electrode/electrolyte interfaces and the number of water molecules in the hydration shell of the ions are estimated considering integer values. The opposite flux direction of free water molecules was also detected by ac-electrogravimetry. This indicates that there is a population of hydrated Li+ or hydrated Na+ ions losing their hydration shell before being transferred at the electrode/electrolyte interfaces. Therefore, the effect of desolvation is clearly and experimentally demonstrated. The ac-electrogravimetry responses of the electrodeposited Li-birnessite type MnO2 thin films can serve as a gravimetric probe for studying the charge-storage mechanisms and extracting subtleties unreachable with classical tools.

Improved frequency/voltage converters for fast quartz crystal microbalance applications

The monitoring of frequency changes in fast quartz crystal microbalance (QCM) applications is a real challenge in today's instrumentation. In these applications, such as ac electrogravimetry, small frequency shifts, in the order of tens of hertz, around the resonance of the sensor can occur up to a frequency modulation of 1 kHz. These frequency changes have to be monitored very accurately both in magnitude and phase. Phase-locked loop techniques can be used for obtaining a high performance frequency/voltage converter which can provide reliable measurements. Sensitivity higher than 10 mVHz, for a frequency shift resolution of 0.1 Hz, with very low distortion in tracking both the magnitude and phase of the frequency variations around the resonance frequency of the sensor are required specifications. Moreover, the resonance frequency can vary in a broad frequency range from 5 to 10 MHz in typical QCM sensors, which introduces an additional difficulty. A new frequency-voltage conversion system based on a double tuning analog-digital phase-locked loop is proposed. The reported electronic characterization and experimental results obtained with conducting polymers prove its reliability for ac-electrogravimetry measurements and, in general, for fast QCM applications.

New frequency/voltage converters for ac-electrogravimetric measurements based on fast quartz crystal microbalance

A better understanding of the mechanisms located at the solid/electrolyte interface is becoming essential to the development of new applications in the electrochemical fields. The fast quartz crystal microbalance is an attractive and powerful gravimetric sensor which can be used in the dynamic regime to determine a mass/potential transfer function. The principle is equivalent to classical electrochemical impedance measurements; the only difference is the determination of mass changes given by the quartz crystal microbalance rather than current changes following sine wave modulations of the applied potential. This function appears very well adapted to characterize ionic exchanges at the electrochemical interface. Frequency/voltage converters are the key devices in translating the microbalance frequency response in terms of a continuous voltage change. The latter allows the transfer function to be obtained via a frequency response analyzer. Different converters were tested in this work in order to improve the performances of the experimental setup.

Charge compensation process in polypyrrole studied by ac electrogravimetry

Ions and solvent insertion/expulsion during the redox reaction of polypyrrole were investigated. Simultaneous measurement of impedance and mass/potential transfer function, thanks to a fast quartz crystal microbalance, allows species movement to be determined. For polypyrrole in CsCl, Na 2SO 4 and NaCl, cations and solvent were expelled and anions inserted when the film was oxidised. The insertion isotherm was obtained and it was shown that bigger ions are hosted in the polymer at cathodic potential.