Electrochemical Quartz Crystal Microbalance Research Articles

Exploring Metal Electroplating for Energy Storage by Quartz Crystal Microbalance: A Review

AbstractThe development and application of Electrochemical Quartz Crystal Microbalance (EQCM) sensing to study metal electroplating, especially for energy storage purposes, are reviewed. The roles of EQCM in describing electrode/electrolyte interface dynamics, such as the electric double‐layer build‐up, ionic/molecular adsorption, metal nucleation, and growth, are addressed. Modeling of the QCM sensor is introduced and its importance is emphasized. Challenges of metal electrode use, including side reactions and dendrite formation, along with their mitigation strategies are reviewed. Numerous factors affecting the electroplating processes, such as electrolyte composition, additives, temperature, and current density, and their influence on the electroplated metals’ mass, structural, and mechanical characteristics are discussed. Looking forward, the need for deeper fundamental understanding and advancing simulations of the QCM signal response as a result of electroplating metal nanostructures is stressed. Further development and integration of innovative EQCM‐strategies will provide unique future means to fundamentally understand and optimize metal electroplating for energy storage and application alike.

Sulfite permeability and sulfur accumulation in a sulfite depolarized electrolyser

Sulfur dioxide or sulfite depolarized electrolyser is considered a promising technology for green hydrogen production. A sulfite depolarized electrolyser adopting sulfite wastewater is developed for both hydrogen production and wastewater treatment in our project, e.g. flue gas desulfurization wastewater and lead plate pre-desulfurization solution. Sulfite permeability and sulfur accumulation are primary challenges of the sulfite depolarized electrolyser for long-term stability. The electrochemical behaviors of SO32− reduction are investigated particularly here, which exhibits a 3.06-electron route on the homemade Pt-Pd/C catalyst. The reduction of SO32− to S0 is observed using an electrochemical quartz crystal microbalance. The sulfite diffusion coefficients of three proton exchange membranes are also determined. A frequent polarity inverted electrolysis is developed to remove the sulfur deposits and keep cell stability. Adopting a lead plate pre-desulfurization solution as the anolyte, both the fresh and repaired sulfite depolarized electrolyser can achieve a remarkable current density of > 500 mA/cm2 at 2.0 V. 200 h long-term test indicates excellent anodic depolarization and long-term stability for hydrogen production. The strategy may shed light on the further development of industrial applications.

Air-Stable and Low-Cost High-Voltage Hydrated Eutectic Electrolyte for High-Performance Aqueous Aluminum-Ion Rechargeable Battery with Wide-Temperature Range.

Aqueous aluminum-ion batteries (AAIBs) are considered as a promising alternative to lithium-ion batteries due to their large theoretical capacity, high safety, and low cost. However, the uneven deposition, hydrogen evolution reaction (HER), and corrosion during cycling impede the development of AAIBs, especially under a harsh environment. Here, a hydrated eutectic electrolyte (AATH40) composed of Al(OTf)3, acetonitrile (AN), triethyl phosphate (TEP), and H2O was designed to improve the electrochemical performance of AAIBs in a wide temperature range. The combination of molecular dynamics simulations and spectroscopy analysis reveals that AATH40 has a less-water-solvated structure [Al(AN)2(TEP)(OTf)2(H2O)]3+, which effectively inhibits side reactions, decreases the freezing point, and extends the electrochemical window of the electrolyte. Furthermore, the formation of a solid electrolyte interface, which effectively inhibits HER and corrosion, has been demonstrated by X-ray photoelectron spectroscopy, X-ray diffraction tests, and in situ differential electrochemical mass spectrometry. Additionally, operando synchrotron Fourier transform infrared spectroscopy and electrochemical quartz crystal microbalance with dissipation monitoring reveal a three-electron storage mechanism for the Al//polyaniline full cells. Consequently, AAIBs with this electrolyte exhibit improved cycling stability within the temperature range of -10-50 °C. This present study introduces a promising methodology for designing electrolytes suitable for low-cost, safe, and stable AAIBs over a wide temperature range.

Boosting the Proton Intercalation via Crystal Plane Optimization of TiS2 for Cycling‐Stable Aqueous Zn‐Ion Batteries

AbstractTiS2 has received significant attention as a promising anode for “Rocking‐Chair”‐type aqueous Zn‐ion batteries due to the large interlayer spacing and low discharge plateau. However, the structural distortion caused by the embedding of divalent Zn2+ as well as the undesirable hydrogen evolution reaction (HER) severely affects their cycling stability. Herein, a facet‐dependent mechanism is first deeply investigated to understand charge storage behaviors of TiS2 via differential electrochemical mass spectrometry, in situ electrochemical quartz crystal microbalance, and in situ X‐ray diffraction characterizations. By regulating the exposed crystal facets of TiS2 from (001) (TS (001)) to (011) (TS(011)), HER can be effectively inhibited, and the charge storage mechanism is transformed from Zn2+ insertion/extraction dominating to H+ insertion/extraction dominating, resulting in faster charge transfer kinetics and strong structure stability during long‐term cycling. Hence, TS(011) delivers a higher reversible capacity of 212.9 mAh g−1 at 0.1 A g−1 and a strong cycling stability of 74% capacity retention over 1000 cycles, much better than that of TS (001) with a reversible capacity of 164.7 mAh g−1 at 0.1 A g−1, a capacity retention of 17% after 1000 cycles. These new findings can provide deep insight into the rational design of high‐performance intercalation‐type electrode materials for energy storage applications.

Modulating inner Helmholtz layer by electrocatalytically sieving [Zn(H2O)6]2+ for 10000-cycle zinc-ion hybrid capacitors under extremely harsh conditions

Zinc-ion hybrid capacitors (ZIHCs) are famous for potential applications in grid-scale energy storage devices with fast-charge capability. However, their industrialization is severely plagued by inferior performance caused by the sluggish Zn2+ desolvation kinetics with large spatial diffusion hinderance of [Zn(H2O)6]2+ in the inner Helmholtz plane (IHP) layer, especially under low-temperature surroundings. Herein, the simultaneous rapid desolvation strategy via pore sieving and electrocatalysis is initially proposed to promote [Zn(H2O)6]2+ dissociation, regulating the isolated Zn2+ behaviors in the IHP. Specifically, heteroatom-decorated carbon microspheres with multi-level channels modulate the local distribution of electronic density, generating abundant catalytic sites to drive the kinetics of [Zn(H2O)6]2+ desolvation and free Zn2+ diffusion. Impressively, the catalytic desolvation behaviors and storage mechanism of ZIHCs are comprehensively investigated using in-situ electrochemical quartz crystal microbalance and various ex-situ/in-situ measurements as well as theoretical simulations, revealing significant interactions of isolated Zn2+ in the IHP. Consequently, the assembled ZIHCs exhibit a superior capacity of 177.2 mAh g−1, corresponding to a high energy density of 158.8 Wh kg−1, and provide a power density as high as 15.7 kW kg−1. Exposed to extreme environment, the ZIHCs encountered with severe solvation structure stabilize for 10000 cycles withcapacity retention of 99.42%, providing new insights of catalytically sieving into modulating IHP for high-performance ZIHCs under extreme conditions.

Ultrasensitive and Highly Selective o-Phenylenediamine Molecularly Imprinted Polymer for the Detection of 2,4-Dichlorophenoxyacetic Acid

Herein, we report an ultrasensitive and highly selective analytical methods to detect 2,4-dichlorophenoxyacetic acid (2,4-D) using an o-phenylenediamine based molecularly imprinted polymer (o-PD-MIP) sensor. Electrochemical Quartz Crystal Microbalance (EQCM) is used to investigate both the kinetics and the mass of the electropolymerized o-phenylenediamine. Additionally, successful removal of the template from the imprinted cavities is confirmed by comparing the XP spectra of imprinted and o-PD-MIPs after various template removal procedures. The most effective method involves a 70:30 mixture of MeOH:H2O for 15 minutes under stirring. The o-PD-MIP sensor exhibits high sensitivity with a LoD of (3 ± 1) × 10−12 M, which is below the EU regulation limits for drinking water by six orders of magnitude, a linear range between 10 and 100 pM, and an excellent selectivity. These results proved the effectiveness of template removal procedure by using a 70:30 MeOH:H2O mixture and are a proof-of-concept for ultrasensitive and selective 2,4-D detection in real samples.

Downsizing nanoarchitectonics of multilayered MXenes electrocatalysts towards real time ion tracking via EQCM and electrocatalytic applications

Since their discovery, engineering two-dimensional (2D) MXene materials have attracted rapid interest in both energy storage and conversion applications. Among the several techniques being introduced for enhancing material properties, downsizing is one among the interesting approaches. Downsizing involves the deliberate scaling down of active 2D materials, such as MXenes, systematically. Although major studies are focused on the electrochemical applications of single layered MXene flakes, curiosity in evaluating the downsized multilayered MXenes for electrochemical applications motivates this project. Thus, in the current study, the multilayered bulk MXenes are sequentially stepped down to smaller multilayered fragments at definite time intervals, and subsequently the potential of this procured heterogeneous polydispersed solution are evaluated towards electrochemical applications without any further post-treatments. Real time monitoring of lithium-ion exchange in the downsized MXene materials at various time intervals was tracked using the electrochemical quartz crystal microbalance technique, where the downsized MXene system exhibited water-assisted lithium-ion transfer behavior. Increase in mass exchange was found to increase with increase in downsized MXene systems, thus making it very interesting towards ion storage applications. Further, downsized MXene electrocatalyst material delivered the lowest onset potential for hydrogen production among the set of catalysts studied. In short, this study outlines and pioneers some interesting observations regarding both energy storage and catalytic applications of multilayered downsized MXenes, thereby opening up new possibilities for facile, rapid and cost-effective material fabrication approaches.

Dual template (epitope) imprinted electrode for sensing bacterial protein with high selectivity.

Epitope imprinting has shown better prospects to synthesize synthetic receptors for proteins. Here, dual epitope imprinted polymer electrode (DEIP) matrix was fabricated on gold surface of electrochemical quartz crystal microbalance (EQCM) for recognition of target epitope sequence in blood samples of patients suffering from brain fever. Epitope sequences from outer membrane protein Por B of Neisseria meningitidis (MC58) bacteria predicted through immunoinformatic tools were chosen for imprinting. Self-assembled monolayers (SAM) of cysteine appended epitope sequences on gold nanoparticles were subjected to polymerization prior to electrodeposition on gold coated EQCM electrode. The polymeric matrix was woven around the cysteine appended epitope SAMs through multiple monomers (3-sulfo propyl methacrylate potassium salt (3-SPMAP), benzyl methacrylate (BMA)) and crosslinker (N, N'-methylene-bis-acrylamide). On extraction of the peptide sequences, imprinted cavities were able to selectively and specifically bind targeted epitope sequences in laboratory samples as well as 'real' samples of patients. Selectivity of sensor was examined through mismatched peptide sequences and certain plasma proteins also. The sensor was able to show specific binding towards the blood samples of infected patients, even in the presence of 'matrix' and other plasma proteins such as albumin and globulin. Even other peptide sequences, similar to epitope sequences only with one or two amino acid mismatches were also unable to show any binding. The analytical performance of DEIP-EQCM sensor was tested through selectivity, specificity, matrix effect, detection limit (0.68-1.01 nM), quantification limit (2.05-3.05 nM) and reproducibility (RSD ~ 5%). Hence, a diagnostic tool for bacterium causing meningitis is successfully fabricated in a facile manner which will broaden the clinical access and make efficient population screening feasible.

Understanding Electrolyte Ion Size Effects on the Performance of Conducting Metal-Organic Framework Supercapacitors.

Layered metal-organic frameworks (MOFs) have emerged as promising materials for next-generation supercapacitors. Understanding how and why electrolyte ion size impacts electrochemical performance is crucial for developing improved MOF-based devices. To address this, we investigate the energy storage performance of Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with a series of 1 M tetraalkylammonium tetrafluoroborate (TAABF4) electrolytes with different cation sizes. Three-electrode experiments show that Cu3(HHTP)2 exhibits an asymmetric charging response with all ion sizes, with higher energy storage upon positive charging and a greater charging asymmetry with larger TAA+ cations. The results further show that smaller TAA+ cations demonstrate superior capacitive performances upon both positive and negative charging compared to larger TAA+ cations. To gain further insights, electrochemical quartz crystal microbalance measurements were performed to probe ion electrosorption during charging and discharging. These reveal that Cu3(HHTP)2 has a cation-dominated charging mechanism, but interestingly indicate that the solvent also participates in the charging process with larger cations. Overall, the results of this study suggest that larger TAA+ cations saturate the pores of the Cu3(HHTP)2-based electrodes. This leads to more asymmetric charging behavior and forces solvent molecules to play a role in the charge storage mechanism. These findings significantly enhance our understanding of ion electrosorption in layered MOFs, and they will guide the design of improved MOF-based supercapacitors.

The Role of Side Chains and Hydration on Mixed Charge Transport in n-Type Polymer Films.

Introducing ethylene glycol (EG) side chains to a conjugated polymer backbone is a well-established synthetic strategy for designing organic mixed ion-electron conductors (OMIECs). However, the impact that film swelling has on mixed conduction properties has yet to be scoped, particularly for electron-transporting (n-type) OMIECs. Here, the authors investigate the effect of the length of branched EG chains on mixed charge transport of n-type OMIECs based on a naphthalene-1,4,5,8-tetracarboxylic-diimide-bithiophene backbone. Atomic force microscopy (AFM), grazing-incidence wide-angle X-ray scattering (GIWAXS), and scanning tunneling microscopy (STM) are used to establish the similarities between the common-backbone films in dry conditions. Electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) and in situ GIWAXS measurements reveal stark changes in film swelling properties and microstructure during electrochemical doping, depending on the side chain length. It is found that even in the loss of the crystallite content upon contact with the aqueous electrolyte, the films can effectively transport charges and that it is rather the high water content that harms the electronic interconnectivity within the OMIEC films. These results highlight the importance of controlling water uptake in the films to impede charge transport in n-type electrochemical devices.

Investigation of Copper Codeposits with SiC Nano from Ionic Liquid (Ethaline) as Function of Concentration and Micro Particles Sizes where the Working Electrode in ‘Vertical’ Position Using Acoustic Impedance Electrochemical Quartz Crystal Microbalance (EQCM)

The gravimetric inspection of co-electrodeposited copper, with hard inert second phase ceramics SiC and 0.02 M [CuCl2.2H2O] in 1 ChCl: 2 EG, based liquid for different solution-phase loadings of 1-3 \(\mu\)m silicon carbide unpolished gold AT cut working electrode placed in’Vertical’ position, revealed that the EQCM method admits the first in-situ quantification or particulate inclusion. It was found that the composition of incorporated substance was fairly dependant on the concentration of second phase particles in solution, where in this study the 5% SiC has the amount in the copper which is in a good agreement with alumina particles with the same size. Another observation was that most of the material was drawn onto the surface rather than settling on to it, however the distribution of the hard phase was found to be reasonably even throughout the coating. This technology is important because it assists deposition of bright copper coatings without adding hazard materials, such as cyanide. On the other hand the comparison between concentrations of 0.02 M and 0.2 M [CuCl2.2H2O] in 1 ChCl: 2 EG based liquid for 10% solution-phase loadings of 45-55 nm size silicon carbide was also investigated, where the working electrode is placed in’Horizontal’ postion. It is very interisting that the experimental result showed that the mass loaded by 0.2 M solution is about 10 times greater than for the mass loaded by 0.02 M solution.

Electrified Interactions of Polyzwitterions with Charged Surfaces: Role of Dipole Orientation and Surface Potentials.

The zwitterionic groups possess strong dipole moments, leading to inter- or intrachain interactions among zwitterionic polymers. This study aims to demonstrate the interaction of polyzwitterions poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), and poly(carboxybetaine methacrylate) (PCBMA) with electrified surfaces, despite their electrically neutral nature. We studied the adsorption of polyzwitterions and their monomers on electrified surfaces by using an electrochemical quartz crystal microbalance with dissipation (EQCM-D). The interaction between zwitterionic molecules and charged surfaces is explored by adjusting the surface potentials. Interestingly, the adsorption of polyzwitterions can be influenced by external potential, primarily due to the formation of polyzwitterions restricting the mobility of zwitterionic groups, affecting the adsorption behavior of polyzwitterions based on the surface potential. The impact is determined by the arrangement of positive and negative ions within the zwitterionic groups, which are the dipole orientation. Additionally, surface potentials determine the adsorption rate, amount, and chain conformation of the adsorbed thin polyzwitterion layers. The effect of ionic strength was investigated by introducing electrolytes into the aqueous solutions to assess the range of influenced surface potentials.

Li+ Cations Activate NiFeOOH for Oxygen Evolution in Sodium and Potassium Hydroxide.

The efficiency of electrolysis is reduced due to the sluggish oxygen evolution reaction (OER). Besides catalyst properties, electrocatalytic activity also depends on the interaction of the electrocatalyst with the electrolyte. Here, we show that the addition of small amounts of Li+ to Fe-free NaOH or KOH electrolytes activates NiFeOOH for the OER compared to single-cation electrolytes. Moreover, the activation was maintained when the solution was returned to pure NaOH. Importantly, we show that the origin of activation by Li+ cations is primarily non-kinetic in nature, as the OER onset for the mixed electrolyte does not change and the Tafel slope at low current density is ~30 mV/dec in both electrolytes. However, the increase of the apparent Tafel slope remains lower at increasing current densities in the presence of Li+. Based on electrochemical quartz crystal microbalance and in situ X-ray absorption spectroscopy measurements, we show that this reduction of non-kinetic effects is due to enhanced intercalation of sodium, water and hydroxide. This enhanced electrolyte penetration facilitates the OER, especially at higher current densities and for increased catalyst loading. Our work shows that mixed electrolytes where distinct cations can have different roles provide a simple and promising strategy towards improved OER rates.

Li+ Kationen aktivieren NiFeOOH für die Sauerstoffentwicklung in Natrium‐ und Kaliumhydroxid

AbstractDie Effizienz der Elektrolyse wird durch die träge Sauerstoffentwicklung (OER) verringert. Neben den Katalysatoreigenschaften hängt die elektrokatalytische Aktivität auch von der Interaktion des Elektrokatalysators mit dem Elektrolyten ab. Hier zeigen wir, dass die Zugabe kleiner Mengen Li+ zu eisenfreien NaOH‐ oder KOH‐Elektrolyten NiFeOOH für die OER aktiviert, verglichen mit monokationischen Elektrolyten. Außerdem blieb der aktivierte Zustand erhalten, wenn zu reiner NaOH zurück gewechselt wurde. Wichtig ist, dass wir zeigen, dass der Ursprung der Aktivierung durch Li+‐Kationen in erster Linie nicht‐kinetischer Natur ist, da sich der Beginn der OER für den gemischten Elektrolyten nicht ändert und die Tafelsteigung bei niedriger Stromdichte in beiden Elektrolyten ~30 mV/dec beträgt. Der Anstieg der Tafelsteigung bleibt jedoch bei steigenden Stromdichten in Gegenwart von Li+ geringer. Basierend auf elektrochemischen Quarzkristall‐Mikrowaagen‐ und In situ‐Röntgenabsorptionsspektroskopie‐Messungen zeigen wir, dass diese Verringerung der nichtkinetischen Effekte auf eine verstärkte Interkalation von Natrium, Wasser und Hydroxid zurückzuführen ist. Diese verstärkte Elektrolytpenetration erleichtert die OER, insbesondere bei höheren Stromdichten und bei erhöhter Katalysatorbeladung. Unsere Arbeit zeigt, dass gemischte Elektrolyte, in denen verschiedene Kationen unterschiedliche Rollen einnehmen können, eine einfache und vielversprechende Strategie zur Verbesserung der OER‐Raten darstellen.

The Influence of Cathode Degradation Products on the Anode Interface in Lithium-Ion Batteries.

Degradation of cathode materials in lithium-ion batteries results in the presence of transition metal ions in the electrolyte, and these ions are known to play a major role in capacity fade and cell failure. Yet, while it is known that transition metal ions migrate from the metal oxide cathode and deposit on the graphite anode, their specific influence on anode reactions and structures, such as the solid electrolyte interphase (SEI), is still quite poorly understood due to the complexity in studying this interface in operational cells. In this work we combine operando electrochemical atomic force microscopy (EC-AFM), electrochemical quartz crystal microbalance (EQCM), and electrochemical impedance spectroscopy (EIS) measurements to probe the influence of a range of transition metal ions on the morphological, mechanical, chemical, and electrical properties of the SEI. By adding representative concentrations of Ni2+, Mn2+, and Co2+ ions into a commercially relevant battery electrolyte, the impacts of each on the formation and stability of the anode interface layer is revealed; all are shown to pose a threat to battery performance and stability. Mn2+, in particular, is shown to induce a thick, soft, and unstable SEI layer, which is known to cause severe degradation of batteries, while Co2+ and Ni2+ significantly impact interfacial conductivity. When transition metal ions are mixed, SEI degradation is amplified, suggesting a synergistic effect on the cell stability. Hence, by uncovering the roles these cathode degradation products play in operational batteries, we have provided a foundation upon which strategies to mitigate or eliminate these degradation products can be developed.

Chemical stability and reactivity of electrochemically grown manganese oxide films

The fundamental scientific challenge in fully harnessing the potential of manganese oxide compounds lies in understanding the interplay between microstructure, surface properties, and electrochemical reactivity. In this study, manganese oxides were synthesized via two distinct methods: electrodeposition trough anodic oxidation of Mn2+ in solution and direct anodic oxidation of metallic Mn. Employing electrochemical quartz crystal microbalance measurements under flow conditions, the deposited compounds were successfully identified as function of the applied anodic potentials, revealing a transition from Mn3O4 to Mn2O3. Furthermore, the MnO → Mn(OH)2 conversion was determined to be a critical step in the degradation process of manganese oxides. For the anodic oxidation process, the mere alkaline "pH effect" (solid product formation predicted from Pourbaix Diagram) was demonstrated to be insufficient to form a protective oxide/hydroxide layer on top of metallic Mn. Indeed, the addition of phosphate proved necessary for achieving this protective passive layer formation, particularly in pH 12.0 Na3PO4 electrolyte solution. Potentiostatic growth in the presence of phosphates demonstrated the progressive stabilization of the protective layer consisting of a mix of manganese oxide, hydroxide, and phosphate, as confirmed by Raman and X-ray photoemission spectroscopy. To provide a comprehensive framework for our findings, a revised Pourbaix Diagram for the Mn-H2O-PO43− system is proposed and discussed.

In situ real-time investigation of ruthenium electrodeposition behavior in HCl solution using EQCM

Industrial waste solutions are rich in precious metals, which are valuable secondary resources. In order to realize the separation and recovery of ruthenium by electrodeposition, this work used a combination of cyclic voltammetry (CV), chronoamperometry (I-t), and electrochemical quartz crystal microbalance (EQCM) to study the electrochemical behavior and electrodeposition mechanism of ruthenium in HCl solution. The results show that the reduction of ruthenium in HCl solution is influenced by hydrogen evolution reaction (HER) and chlorine evolution reaction, and the reduction process of ruthenium varies at different potentials. The reduction currents of Ru(VIII)/Ru(0) and Ru(IV)/Ru(III) are observed at 0.88 V and 0.6 V. The reduction of Ru(III) to Ru(0) at negative potentials starts at −0.2 V, with peak cathodic currents between −0.2 V and −0.5 V accompanied by a significant mass increase, which is confirmed to be the electrodeposition of metallic ruthenium by SEM and EDS. The microscopic morphology of the ruthenium coating is spherical particles, and the nucleation mode of electrodeposition is progressive nucleation. The HER leads to a macroscopic morphology of a cleaved lamellar structure, which is not tightly adhered to the substrate. The metallic ruthenium starts to oxidize and dissolve rapidly after 1.12 V, leading to a rapid degradation of the electrode surface quality.

Kinetic-Thermodynamic Promotion Engineering toward High-Density Hierarchical and Zn-Doping Activity-Enhancing ZnNiO@CF for High-Capacity Desalination

HighlightsThrough facial basicity adjustment, kinetically favorable ZnxNi1-xO@CF electrode was formed with a high density hierarchical structure and three dimensional open pores.The optimal Zn-doping ratio in ZnxNi1-xO@CF has excellent sodium storage and desalination performance (128.9 mg g-1).The mechanism of Na+ intercalation process was studied by electrochemical quartz crystal microbalance with dissipation monitoring in situ test and the activation mechanism of redox-inert Zn-doping on electrode materials was reported.

Cation desolvation-induced capacitance enhancement in reduced graphene oxide (rGO)

Understanding the local electrochemical processes is of key importance for efficient energy storage applications, including electrochemical double layer capacitors. In this work, we studied the charge storage mechanism of a model material - reduced graphene oxide (rGO) - in aqueous electrolyte using the combination of cavity micro-electrode, operando electrochemical quartz crystal microbalance (EQCM) and operando electrochemical dilatometry (ECD) tools. We evidence two regions with different charge storage mechanisms, depending on the cation-carbon interaction. Notably, under high cathodic polarization (region II), we report an important capacitance increase in Zn2+ containing electrolyte with minimum volume expansion, which is associated with Zn2+ desolvation resulting from strong electrostatic Zn2+-rGO interactions. These results highlight the significant role of ion-electrode interaction strength and cation desolvation in modulating the charging mechanisms, offering potential pathways for optimized capacitive energy storage. As a broader perspective, understanding confined electrochemical systems and the coupling between chemical, electrochemical and transport processes in confinement may open tremendous opportunities for energy, catalysis or water treatment applications in the future.

An acetate electrolyte for enhanced pseudocapacitve capacity in aqueous ammonium ion batteries

Ammonium ion batteries are promising for energy storage with the merits of low cost, inherent security, environmental friendliness, and excellent electrochemical properties. Unfortunately, the lack of anode materials restricts their development. Herein, we utilized density functional theory calculations to explore the V2CTx MXene as a promising anode with a low working potential. V2CTx MXene demonstrates pseudocapacitive behavior for ammonium ion storage, delivering a high specific capacity of 115.9 mAh g−1 at 1 A g−1 and excellent capacity retention of 100% after 5000 cycles at 5 A g−1. In-situ electrochemical quartz crystal microbalance measurement verifies a two-step electrochemical process of this unique pseudocapacitive storage behavior in the ammonium acetate electrolyte. Theoretical simulation reveals reversible electron transfer reactions with [NH4+(HAc)3]···O coordination bonds, resulting in a superior ammonium ion storage capacity. The generality of this acetate ion enhancement effect is also confirmed in the MoS2-based ammonium-ion battery system. These findings open a new door to realizing high capacity on ammonium ion storage through acetate ion enhancement, breaking the capacity limitations of both Faradaic and non-Faradaic energy storage.