Electrochemical Quartz Crystal Microbalance Research Articles

3D grape string-like heterostructures enable high-efficiency sodium ion capture in Ti3C2Tx MXene/fungi-derived carbon nanoribbon hybrids.

2D transition metal carbides and carbonitrides (MXenes) have emerged as promising electrode materials for electrochemistry ion capture but always suffer from severe layer-restacking and irreversible oxidation that restrains their electrochemical performance. Here we design a dual strategy of microstructure tailoring and heterostructure construction to synthesize a unique 3D grape string-like heterostructure consisting of Ti3C2Tx MXene hollow microspheres wrapped by fungi-derived N-doping carbon nanoribbons (denoted as GMNC). The 3D grape string-like heterostructure effectively avoids the aggregation of Ti3C2Tx MXene sheets and enhances the stability of MXenes, providing abundant active sites for ion capture, and an interconnected conductive bionic nanofiber network for high-rate electron transport. In consequence, GMNC achieves a superior adsorption capacity for sodium ions (Na+) in capacitive deionization (CDI) (162.37 mg gNaCl-1) with an ultra-high instantaneous adsorption rate (30.52 mg g-1 min-1) at an applied voltage of 1.6 V and satisfactory cycle stability over 100 cycles, which is a strong performer among the state-of-the-art values for MXene-based CDI electrodes. In addition, in situ electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) measurement combined with density functional theory (DFT) reveals the mechanisms of the Na+ capture process in the GMNC heterostructure. This work opens a new avenue for designing high-performance MXenes with a 3D hierarchical heterostructure for advanced electrochemical applications.

Unraveling the Effects of Carbon Corrosion on Oxygen Transport Resistance in Low Pt Loading Proton Exchange Membrane Fuel Cells.

Cost and durability have become crucial hurdles for the commercialization of proton exchange membrane fuel cells (PEMFCs). Although a continuous reduction of Pt loading within the cathode catalyst layers (CCLs) can lead to cost savings, it also increases the oxygen transport resistance, which is further compounded by key material degradation. Hence, a further understanding of the mechanism of significant performance loss due to oxygen transport limitations at the triple phase boundaries (TPBs) during the degradation process is critical to the development of low Pt loading PEMFCs. The present study systematically investigates the impact of carbon corrosion in CCLs on the performance and oxygen transport process of low Pt loading PEMFCs through accelerated stress tests (ASTs) that simulate start-up/shutdown cycling. A decline in peak power density from 484.3 to 251.6 mW cm-2 after 1500 AST cycles demonstrates an apparent performance loss, especially at high current densities. The bulk and local oxygen transport resistances (rbulk and Rlocal) of the pristine cell and after 200, 600, 1000, and 1500 AST cycles are quantified by combining the limiting current method with a dual-layer CCL design. The results show that rbulk increased from 1527 to 1679 s cm-2, Rlocal increased from 0.38 to 0.99 s cm-1, and the local oxygen transport resistance with the normalized Pt surface area (rlocal) exhibited an increase from 18.5 to 32.0 s cm-1, indicating a crucial impact on the structure collapse and changes in the chemical properties of the carbon supports in the CCLs. Further, the interaction between the ionomer and carbon supports during the carbon corrosion process is deeply studied via electrochemical quartz crystal microbalance and molecular dynamics simulations. It is concluded that the oxygen-containing functional groups on the carbon surface could impede the adsorption of ionomers on carbon supports by creating an excessively water-rich layer, which in turn aggravates the formation of ionomer agglomerations within the CCLs. This process ultimately leads to the destruction of the TPBs and hinders the transport of oxygen through the ionomer.

Interlayer Confined Water Enabled Pseudocapacitive Sodium-Ion Storage in Nonaqueous Electrolyte.

Electrochemical capacitors have faced the limitations of low energy density for decades, owing to the low capacity of electric double-layer capacitance (EDLC)-type positive electrodes. In this work, we reveal the functions of interlayer confined water in iron vanadate (FeV3O8.7·nH2O) for sodium-ion storage in nonaqueous electrolyte. Using an electrochemical quartz crystal microbalance, in situ Raman, and ex situ X-ray diffraction and X-ray photoelectron spectroscopy, we demonstrate that both nonfaradaic (surficial EDLC) and faradaic (pseudocapacitance-dominated Na+ intercalation) processes are involved in the charge storages. The interlayer confined water is able to accelerate the fast Na+ intercalations and is highly stable (without the removal of water or co-intercalation of [Na-diglyme]+) in the nonaqueous environment. Furthermore, coupling the pseudocapacitive FeV3O8.7·nH2O with EDLC-type activated carbon (FeVO-AC) as the positive electrode brings comprehensive enhancements, displaying the enlarged compaction density of ∼2 times, specific capacity of ∼1.5 times, and volumetric capacity of ∼3 times compared to the AC electrode. Furthermore, the as-assembled hybrid sodium-ion capacitor, consisting of an FeVO-AC positive electrode and a mesocarbon microbeads negative electrode, shows a high energy density of 108 Wh kg-1 at 108 W kg-1 and 15.3 Wh kg-1 at 8.3 kW kg-1. Our results offer an emerging route for improving both specific and volumetric energy densities of electrochemical capacitors.

Electrophoretic Protein Deposition as a Tool for In Situ Co-Crosslinking Enzyme Immobilization: An Electrochemical/Quartz Crystal Microbalance Study

Electrophoretic deposition is a powerful tool for depositing materials onto a substrate by using an electric field; its application in biotechnological areas, namely, electrophoretic protein deposition (EPD), is the most promising for, e.g., fabricating novel amperometric biosensors. Unfortunately, EPD suffers from several drawbacks due to coupled parasite electrochemical processes damaging the deposit; moreover, the nature of the deposition process, the deposit, and its stability are still controversial and unknown. The present research presents a deep investigation of the EPD processes conducted by using several electroanalytical techniques and an electrochemical quartz crystal microbalance (EQCM); notably, EPD was used here as a novel tool for performing an electrophoretically assisted, classical enzyme immobilization technique like co-crosslinking, thus permitting the immobilization of the desired protein in situ, i.e., exclusively onto the deposition electrode. An electrochemical study permitted the acquisition of useful insights about electrophoresis processes as well as solvent discharge and gas evolution at the deposition electrode; further, the use of appropriate current or potential pulse sequences, as investigated and improved in this study, together with fine-tuned chemical conditions, allowed the optimization of this novel EPD approach. Moreover, an EQCM study gave useful insights into the kinetics of the process, permitting a quantitative estimate of the deposit.

A Comprehensive Study of the Parameters Affecting Magnesium Plating/Stripping Kinetics in Rechargeable Mg Batteries

Beyond the lithium ion battery (LIB) technology, alkali and alkaline earth metal (Li, Na, Mg, Ca etc.) anodes boost the prospect of next generation battery chemistry through their highly promising theoretical specific capacities.1,2 Mg metal anode based battery is thus one of the strong contenders for sustainable and alternative rechargeable battery systems, with its natural abundance and eco-friendly resources.3 Despite the first proof-of-concept of a Mg battery established back in 2000,4 its complete rechargeability still remains elusive due to the lack of suitable conventional electrolytes and low mobility of Mg2+ ions in typical intercalation cathodes. Although potentially safer than Li metal (less prone to dendritic growth), reversible Mg electrodeposition in non-aqueous liquid electrolytes is a challenge due to poor salt solubility and formation of ionically blocking surface passivation films.5,6 The study of the interphase between a Mg metal anode and electrolyte is therefore of critical worth as it can dictate stability and reversibility for a particular electrode/electrolyte combination.7 In this work, we explore the role of various parameters, including electrolyte composition, concentration, solvent and substrate nature (various metals and carbon based substrates) and their impact on Mg plating/stripping kinetics and coulombic efficiency. The deposit morphology was investigated via ex situ scanning electron microscopy (SEM) and the reductive stability of the electrolytes was studied by cyclic voltammetry (CV), X-ray Photoelectron Spectroscopy (XPS) and electrochemical quartz crystal microbalance (EQCM). Overall, a comprehensive discussion about the relationship between cation solvation structure, composition and stability of passivation layer (if any) and the degree of reversibility of Mg electrodeposition will be presented. Keywords: Beyond LIB chemistry, metal anode, magnesium, EQCM, plating/stripping.

A Comprehensive Study on the Ion-Selective Behavior of MnOx for Electrochemical Deionization

Manganese oxide is an effective active material in several electrochemical systems including batteries, supercapacitors, and electrochemical deionization (ECDI). This work conducts a comprehensive study on the ion-selective behavior of MnOx to fulfill the emptiness in the energy and environmental science field. Furthermore, it broadens the promising application of MnOx in the ion-selective ECDI system. With the support of a microfluidic cell and the electrochemical quartz crystal microbalance (EQCM) analysis, we proposed a time-dependent multi-mechanism ion-selective behavior with the following guidelines. (1) Hydrated radius is the most critical factor for ions with the same valence, and MnOx tends to capture cations with a small hydrated radius. (2) The importance of charge density rises when comparing cations with different valences, and MnOx prefers to capture divalent cations with a large hydrated radius but a strong electrostatic attraction. In addition, ion-swapping may occur where divalent cations replace monovalent cations in this circumstance. (3) NH4 + triggers MnOx dissolution, leading to performance and stability decay. The clear EQCM evidence has directly verified the proposed mechanisms. Moreover, the EQCM data provide a novel but simple method to judge ion selectivity preference. The overall ion selectivity sequence is Ca2+ > Mg2+ > K+ > NH4 + > Na+ > Li+ with the highest βCa//Li and βCa//Na values around 3 at the deionization time = 10 min.

Combined Operando Investigations Reveal Correlation between Formation Parameters and Transport Mechanisms in Solid Electrolyte Interphases of Lithium-Ion Battery Anodes

Despite its thickness being only in the nanometer range, the solid electrolyte interphase (SEI) of lithium-ion battery anodes has proven to be a crucial component, contributing critically to the battery’s longevity and long-term rate capability. This is due to the SEI’s passivating properties, protecting the battery electrolyte from further decomposition during operation while maintaining a good conductivity for lithium ions to be intercalated into the active anode material. Notwithstanding prolonged scientific efforts, a reliable SEI characterization is still very challenging, due to its fragile and inherently unstable nature which can easily lead to the introduction of artifacts in its evaluation, when dried, rinsed, or exposed to ambient atmospheric conditions. Thus, the relation between SEI formation parameters and the resulting morphological, compositional, and electrochemical properties is still poorly understood.To overcome the limitations of often-employed ex-situ methods, we developed an interlocked combination of several non-destructive operando characterization methods consisting of atomic force microscopy (AFM), electrochemical quartz-crystal microbalance (EQCM) and electrochemical impedance spectroscopy (EIS) to provide a plethora of information on the SEI’s morphological evolution, mass and thickness development and its viscoelastic and electrochemical properties like charge transfer resistance. All this information is gathered in the same electrochemical cell during the SEI formation on carbon model electrodes in typical LIB electrolytes containing 1 M LiPF6 in various carbonate mixtures.The combination of AFM, EQCM and EIS data gives us the opportunity to correlate the different growth modes during formation and resulting compositions of the SEI with its dominant transport mechanisms. Additional redox probe experiments help to further distinguish the transport parameters for lithium ions, solvent molecules, and redox molecules. A better understanding of these correlations allows battery researchers to find optimized SEI formation procedures and SEI forming additives, which reduce the formation duration during battery production and can thus contribute to lowering costs of lithium-ion batteries.

Investigations on the Initial-Stages of Lithium Deposition/Dissolution Processes in Sulfolane Based Electrolytes

Li metal could be the ideal anode for rechargeable battery technologies, due to its negative redox potential (ca. -3 V vs. SHE), high specific capacity (3860 mAh g−1), and low density (0.534 g cm−3) [1-3]. Recent advances show a new design concept where there are no initial active materials in the anode (no carbon, Si, or Li metal) and the corresponding Li quantity is directly deposited on the current collector during charging. This approach will result in important practical advantages such as enhanced volumetric and gravimetric specific energies, ease of manufacturing and reduced complexity/safety concerns of the recycling process (ideally Li is completely removed in the discharged state). However, the applicability of the Li-metal batteries is constrained by the nonuniform lithium deposition, accompanied by dendrite growth and the formation of dead lithium during long-term cycling, which lead to low Coulombic efficiency and even cell failure [2]. The initial structure and morphology of the Li deposit has a vital influence on the later progression of the Li layer [4]. This underpins the importance of a fundamental understanding of the electrodeposition process. However, the presence of a solid-electrolyte interphase (SEI) makes such investigations difficult, since after the transport of the Li ions through the SEI, a subsequent charge transfer and nucleation growth process at the substrate-SEI interface, instead of substrate-liquid electrolyte interface occurs. This contribution will discuss lithium electrodeposition in a sulfolane based, localized high concentrated electrolyte system. Possible phase formation mechanisms as well as kinetic and thermodynamic aspects during the initial stages of the process are studied by classical electrochemical methods (Fig. 1, left). Furthermore, the mass-charge balance during deposition and stripping was monitored (Fig. 1, right) via in-situ microgravimetry (electrochemical quartz crystal microbalance). In the contribution we will discuss these transients in terms of lithium layer growth and SEI formation.

Unexplored Reaction Pathways of Ethylene Carbonate during Solid Electrolyte Interphase Formation

The solid electrolyte interphase (SEI) keeps eluding our grasp. Fundamental understanding of Li-ion battery performance will not be complete until the major underlying reaction pathways of SEI formation and growth have been described.1 Although the SEI has been observed to be a complex and highly diverse matrix of in-/organic crystalline and amorphous aggregates, likely only few compounds are functional, i.e. passivate the electrode while at the same time sustaining the Li+ transport. The aim of the presentation is to provide experimental evidence for hitherto largely unexplored reaction pathways of several commonly observed SEI compounds (such as lithium ethyl dicarbonate, abbr. LEDC) and prove how the formation of the same can be altered by the choice of electrolyte additives. For instance, Electrochemical Quartz Crystal Microbalance with Dissipation monitoring complemented by Online Electrochemical Mass Spectrometry is demonstrated to separate electrochemical from chemical reactions in a multistep SEI formation process.2-3 Based on our experimental findings, the role and implication of each compound (be it LiOH, Li2O, LEDC, etc.) in the SEI formation process will be discussed.4

Electrodeposition of Transition Metal Dichalcogenide Thin- Films from Non-Aqueous Solvents

Transition metal dichalcogenides (TMDCs) are an interesting group of 2D materials characterised with a layered structure analogous to graphene and they possess unique electronic and optical properties, especially when in the few- and mono-layer form. Developing scalable techniques for depositing TMDCs is a major challenge which needs to be overcome to fabricate functional devices with these materials. Electrodeposition is an industrially relevant technique that has some key advantages over other conventional deposition methods. It is a low cost and easily scalable technique and could be used for obtaining complex nanoscale features and for depositing over topologically demanding surfaces. Even though the electrodeposition of MoS2 has been achieved both in aqueous and non-aqueous electrolytes, not much progress has been made in the deposition of other TMDC materials. Tungsten based TMDCs such as WS2 and WSe2 are shown to be very promising materials in different applications, however electrodepositing them remains extremely challenging. One of the major obstacles here is developing electrochemically active precursors which are compatible with the electrolyte system and able to deliver both the tungsten and chalcogens to the electrolyte. Controlling and optimizing the deposition process to obtain few- and mono-layer TMDCs is another challenge. In addition, the choice of substrates for deposition is very important, especially for direct growth of ultra-thin TMDCs. Electrodeposition, being a bottom-up deposition method, would benefit from an atomically thin and smooth substrate such as graphene for depositing few- and mono-layer TMDCs.Here we present non-aqueous electroplating as a scalable alternative technique for tungsten-based TMDC deposition, with WS2 and WSe2 as examples. Tailored single source precursors were developed to use in non-aqueous electrolytes. WS2 was electrodeposited from dichloromethane (CH2Cl2) using the [NEt4]2[WS2Cl4] precursor. WSe2 was then electrodeposited from acetonitrile (CH3CN) electrolyte using [WSeCl4] as the precursor. Electrochemical quartz crystal microbalance (EQCM) studies were performed to optimize the deposition process and to probe the mechanism of precursor electrochemistry. Electrochemical deposition parameters were then carefully adjusted to obtain few- and mono-layer TMDCs. Patterned graphene electrodes were used as an atomically thin and smooth platform for the deposition of few- and mono-layer WS2. Few-layer TMDC films obtained on graphene were found to be much smoother than films deposited on other standard substrates such as titanium nitride (TiN) or Pt. These TMDC/graphene structures gave interesting 2D heterostructures which are technologically important for different applications.This work was funded by EPSRC grant references EP/V062689/1, EP/V062387/1 and EP/P025137/1

Is Ethanol Essential for the Lithium-Mediated Nitrogen Reduction Reaction?

Ammonia is used extensively as a fertilizer in agriculture and in the chemical industry. It also has the potential to store carbon-neutral fuel (hydrogen) in the future. To help meet the future demand for ammonia, the electrochemical nitrogen reduction reaction (NRR) has received much attention. It would serve as a decentralized method of synthesizing ammonia at low temperature and pressure. [1]To this day, the most promising NRR results have been shown via the lithium-mediated approach, in which lithium metal is electroplated in an organic electrolyte containing a lithium salt, saturated nitrogen gas, and a proton shuttle. The saturated nitrogen in the electrolyte reacts with electrodeposited lithium and protons are subsequently delivered by the shuttle. The local availabilities of protons and nitrogen are crucial for achieving a high selectivity towards ammonia rather than hydrogen evolution. The selectivity is heavily affected by the solid electrolyte interphase (SEI) which is formed between the electrodeposited lithium and bulk electrolyte. [2,3,4,5]This study investigates the role of ethanol as the proton shuttle in the lithium-mediated nitrogen reduction reaction (Li-NRR). In situ mass spectrometry and electrochemical quartz crystal microbalance are used to elucidate the importance of ethanol. Furthermore, particularly designed chronopotentiometry experiments are designed to provide additional information, allowing for a detailed discussion of ethanol’s multifaceted role in the Li-NRR. It is shown that ethanol is crucial for the formation of the SEI, but not necessary for the actual synthesis of ammonia. The SEI formation in the Li-NRR system is critically influenced by ethanol at the onset of lithium plating. Ethanol is not necessary to shuttle protons from the anode to the cathode. Furthermore, it is shown that lithium is an active species in the Li-NRR system even without electrochemistry. Finally, it is revealed that ethanol plays an important role in the context of anodic electrode potentials and electrolyte stability.References MacFarlane, D. R. et al. Adv. Mater. 32, 1904804 (2020)Du, H-L et al. Nature 609, 722 (2022)Lazaouski, N. et al. ACS Catal. 12, 5197-5208 (2022)Andersen, S. Z. et al. Energy Environ. Sci. 13, 4291-4300 (2020)Andersen, S. Z. et al. Nature 570, 504 (2019)

Role of Ionic Liquids As Modifiers of Ru-Based Catalysts for Water Electrolysis

The generation of hydrogen through electrocatalytic water splitting has attracted tremendous attention in the past years [1,2]. Although water electrolysis has been already established in industrial settings, a large amount of research is still being carried out to further improve the catalytic efficiency and stability to exploit the full potential of this technology. One approach in this endeavor is the utilization of ionic liquids (ILs) as modifiers of electrocatalysts and electrode surfaces [3, 4]. The special physicochemical properties of the IL can affect and ideally improve the characteristics of the electrocatalyst, e.g., in terms of mass and electron transport [3]. Furthermore, the hydrophilic nature of certain ILs can in addition influence gas bubbles removal on the working electrode and improve the stability of the catalysts (e.g., Ru) by acting as a monomer-like binder [4].In this study, we compared the effect of four different ILs that are based on the same cation chain (1-butyl-3-(4-methylphenethyl)-1H-imidazol-3-ium) but different anions (-OAc, -NTf2, -Br, and -MeSO3) on Ru nanoparticles as catalysts for hydrogen and oxygen evolution reactions (HER and OER). To this end, carbon nanotubes were functionalized using different ILs and subsequently decorated with 1-2 nm Ru nanoparticles (Ru NPs) following an established organometallic approach [5]. We analyze how the catalytic activity/stability for HER, as well as OER, is influenced by the metal-ILs combination by established electrochemical methods utilizing a rotational disc electrode setup, with a particular focus on the different hydrophilic natures (depending on the anion) of the four ILs. To obtain a more comprehensive picture of how the different ILs affect Ru dissolution during the reaction, the IL/catalyst systems are additionally investigated by inductively coupled plasma-optical emission spectroscopy (ICP-OES) in a flow cell setup, as well as by electrochemical quartz crystal microbalance (EQCM) measurements.In this work, we discuss the use of ILs as modifiers for catalyst/electrode systems in water-splitting reactions, highlighting the importance of careful IL selection as well as the necessity for more future research in this field to fully explore the potential of IL/NPs-systems in electrochemistry.

The Mg Electrode Cycling Mechanism in Simple Salt Glyme Electrolytes

The lithium-ion battery (LIB) has been at the forefront of innovations in energy storage since its commercialisation over 30 years ago. As we reach the practical specific capacity limit of current state-of-the-art materials, novel and innovative battery technologies are required to satiate the requirements of energy intensive applications such as electric vehicles. 1 The use of a lithium metal negative electrode has been proposed to increase the energy density, however the high reactivity causes poor cycling performance due to electrolyte degradation, dendrite formation and safety issues. 2 The magnesium electrode offers a solution that has double the theoretical volumetric capacity of the lithium electrode while simultaneously mitigating dendritic growth, reducing raw material costs and greatly increasing sustainability. 3,4 Previous work has shown that traditional LIB electrolytes are not compatible with magnesium metal, and the plating and stripping mechanism that underpins how magnesium electrodes cycle is poorly understood. 5 Here we explored the mechanism of magnesium plating and stripping in glyme-based electrolytes with a range of additional additives. We show that in the pure electrolyte solution, interphase formation is critical to stable cycling but that this is accompanied by significant degradation and accumulation of inactive Mg.6 We establish the three-dimensional chemical composition of Mg deposits in the different electrolyte formulations using a combination of focused ion beam-scanning electron microscopy (FIB-SEM), energy dispersive x-ray spectroscopy (EDX) and transmission electron microscopy (TEM). We use these findings to develop a mechanistic understanding that explains the electrochemical cycling performance of magnesium metal negative electrodes. In situ electrochemical quartz crystal microbalance (EQCM) measurements are used to evaluate the plating and stripping efficiency of the electrodes and to identify non-electrochemical degradation reactions between Mg and the electrolyte solutions.

In Situ Investigation on Cl- Repellency Effect of Electrolyte Additive on NiOOH for Alkaline Sea Water Splitting

Sea water splitting can produce green hydrogen by feeding abundant sea water directly without high-qualified water purification process. However, excess cation such as Mg2+ and Ca2+ in sea water absorbs on the cathode surface, and it causes the loss of active site and activity toward hydrogen evolution reaction. At the anode side, oxygen evolution reaction (OER) should be competed with chlorine evolution reaction due to huge amount of Cl- ion, so operating in high pH electrolyte at low overpotential has been suggested to increase the selectivity toward OER. To further inhibit the adsorption of Cl-, some researchers have suggested using anion additive such as PO4 3- and SO4 2- in the alkaline electrolyte, but both adsorption stability of additive anion and the interaction between anion and metal oxide catalyst are still needed to be studied.In this study, we investigated the Cl- repellency effect of additive anion in the electrolyte to NiOOH catalyst, known as state-of-the-art OER catalyst, for alkaline sea water splitting. To observe the change in adsorbate species by additive anion, we tracked the mass change of the catalyst during the reaction through in situ electrochemical quartz crystal microbalance (EQCM). Furthermore, Cl- can change (degrade) the NiOOH structure by formation of metal-chloride complex. Through in situ Raman spectroscopy, we observed the structural stability of NiOOH in presence of anion additive as well as the adsorption stability of additive anion depending on the applied potential.

In‐Plane Heterostructured MoN/MoC Nanosheets with Enhanced Interfacial Charge Transfer for Superior Pseudocapacitive Storage

Abstract2D transition metal carbide/nitride heterostructures are emerging pseudocapacitive materials for supercapacitors (SCs); however, the lack of efficient synthesis methods and an in‐depth understanding of the pseudocapacitive storage mechanism of these potentially important materials impede their applications in SCs. Herein, 2D MoN/MoC nanosheets with a precisely regulated interface are prepared controllably by a scalable salt‐assisted method with bulk MoS2 as the precursor. In operando infrared spectroscopy and electrochemical quartz crystal microbalance results reveal that the pseudocapacitance of the MoN/MoC nanosheets originates from the reversible reaction between Mo–N sites and H+ in the acidic electrolyte. Density‐functional theory calculations and X‐ray photoelectron spectroscopy disclose that the MoC/MoN heterointerface induces the internal electric field from the accumulated negative charges at the Mo–N sites by electron donation from MoC, leading to enhanced H+ adsorption at the Mo–N sites and superior pseudocapacitive storage. The heterostructured MoN/MoC nanosheets show a large volumetric capacity of 1045.3 F cm−3 at 1 A cm−3, high‐rate capability of 702.8 F cm−3 at 10 A cm−3, and superior cyclability with capacity retention of 98% after 10,000 cycles, which outperform reported Mo‐based carbides and nitrides. The results provide new insights into the development of high‐performance 2D heterostructured materials for superior pseudocapacitive storage.

Hydronium Intercalation Enables High Rate in Hexagonal Molybdate Single Crystals.

Rapid proton transport in solid-hosts promotes a new chemistry in achieving high-rate Faradaic electrodes. Exploring the possibility of hydronium intercalation is essential for advancing proton-based charge storage. Nevertheless, this is yet to be revealed. Herein, a new host is reported of hexagonal molybdates, (A2 O)x ·MoO3 ·(H2 O)y (A = Na+ , NH4 + ), and hydronium (de)intercalation is demonstrated with experiments. Hexagonal molybdates show a battery-type initial reduction followed by intercalation pseudocapacitance. Fast rate of 200 C (40 A g-1 ) and long lifespan of 30000 cycles are achieved in electrodes of monocrystals even over 200µm. Solid-state nuclear magnetic resonance confirms hydronium intercalations, and operando measurements using electrochemical quartz crystal microbalance and synchrotron X-ray diffraction disclose distinct intercalation behaviours in different electrolyte concentrations. Remarkably, characterizations of the cycled electrodes show nearly identical structures and suggest equilibrium products are minimally influenced by the extent of proton solvation. These results offer new insights into proton electrochemistry and will advance correlated high-power batteries and beyond.

Selective fluoride ion sensing in aqueous medium using ultrathin film of functionalized single-walled carbon nanotubes

The presence of fluoride ion (F - ) in potable water above its permissible limit (1–4 ppm) poses serious health hazards. Hence, detection of fluoride in potable water is essential. The π-electron rich single-walled carbon nanotubes can interact with F- to form semi-covalent C-F bond which can act as a basis for F - sensing in aqueous medium. Here, a single layer of octadecylamine functionalized single-walled carbon nanotubes (ODA-SWCNTs) was transferred onto solid substrates by the Langmuir–Schaefer (LS) method and employed for sensing of F- in aqueous medium by recording piezo and electrochemical responses, simultaneously using an electrochemical quartz crystal microbalance. The lowest detectable concentration and range of detectable concentration of fluoride ion were found to be 0.5 ppm and 0.5–145 ppm, respectively. The analysis of the LS film of ODA-SWCNTs before and after interaction with fluoride ion by Raman spectroscopy and grazing angle x-ray diffraction measurement reveals perturbation of π-electrons of the SWCNTs due to semi-covalent binding of the fluoride with the carbon atom of the nanotubes. The sensor showed a good selectivity towards the F- in the presence of some heavy metal ions. Testing of the sensor towards F- in tap water obtained from some local region showed a good accuracy.

Revealing the Structural Evolution of Electrode/Electrolyte Interphase Formation during Magnesium Plating and Stripping with operando EQCM-D.

Rechargeable magnesium batteries could provide future energy storage systems with high energy density. One remaining challenge is the development of electrolytes compatible with the negative Mg electrode, enabling uniform plating and stripping with high Coulombic efficiencies. Often improvements are hindered by a lack of fundamental understanding of processes occurring during cycling, as well as the existence and structure of a formed interphase layer at the electrode/electrolyte interface. Here, a magnesium model electrolyte based on magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2 ) and MgCl2 with a borohydride as additive, dissolved in dimethoxyethane (DME), was used to investigate the initial galvanostatic plating and stripping cycles operando using electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D). We show that side reactions lead to the formation of an interphase of irreversibly deposited Mg during the initial cycles. EQCM-D based hydrodynamic spectroscopy reveals the growth of a porous layer during Mg stripping. After the first cycles, the interphase layer is in a dynamic equilibrium between the formation of the layer and its dissolution, resulting in a stable thickness upon further cycling. This study provides operando information of the interphase formation, its changes during cycling and the dynamic behavior, helping to rationally develop future electrolytes and electrode/electrolyte interfaces and interphases.

Thermo-Electro-Responsive Redox-Copolymers for Amplified Solvation, Morphological Control, and Tunable Ion Interactions.

Electro-responsive metallopolymers can possess highly specific and tunable ion interactions, and have been explored extensively as electrode materials for ion-selective separations. However, there remains a limited understanding of the role of solvation and polymer-solvent interactions in ion binding and selectivity. The elucidation of ion-solvent-polymer interactions, in combination with the rational design of tailored copolymers, can lead to new pathways for modulating ion selectivity and morphology. Here, we present thermo-electrochemical-responsive copolymer electrodes of N-isopropylacrylamide (NIPAM) and ferrocenylpropyl methacrylamide (FPMAm) with tunable polymer-solvent interactions through copolymer ratio, temperature, and electrochemical potential. As compared to the homopolymer PFPMAm, the P(NIPAM0.9-co-FPMAm0.1) copolymer ingressed 2 orders of magnitude more water molecules per doping ion when electrochemically oxidized, as measured by electrochemical quartz crystal microbalance. P(NIPAM0.9-co-FPMAm0.1) exhibited a unique thermo-electrochemically reversible response and swelled up to 83% after electrochemical oxidation, then deswelled below its original size upon raising the temperature from 20 to 40 °C, as measured through spectroscopic ellipsometry. Reduced P(NIPAM0.9-co-FPMAm0.1) had an inhomogeneous depth profile, with layers of low solvation. In contrast, oxidized P(NIPAM0.9-co-FPMAm0.1) displayed a more uniform and highly solvated depth profile, as measured through neutron reflectometry. P(NIPAM0.9-co-FPMAm0.1) and PFPMAm showed almost a fivefold difference in selectivity for target ions, evidence that polymer hydrophilicity plays a key role in determining ion partitioning between solvent and the polymer interface. Our work points to new macromolecular engineering strategies for tuning ion selectivity in stimuli-responsive materials.

Epitope imprinting of Sip D protein of Salmonella Typhi bacteria through multiple monomers approach

Typhoid fever is an endemic disease in India. An early stage detection of this disease will reduce the burden on healthcare industry and prevent many fatalities. Here a MIP sensor is fabricated to detect Salmonella typhi bacteria through its antigenic epitope sequence identified through immunoinformatic tools. Epitope imprinting through multiple monomers was utilized to deposit the sensing matrix on gold coated electrochemical quartz crystal microbalance (EQCM) electrode. Methacrylic acid, 2-methacryloyloxyethyl phosphorylcholine and benzyl methacrylate were used as monomers while ethylene glycol dimethacrylate as crosslinker and azobisisobutyronitrile was used as initiator. The analytical performance of MIP-EQCM sensor was studied by electrochemical as well as piezoelectric measurements. Selectivity of sensor was examined thorugh mismatched peptide sequences and certain plasma proteins also. The detection limit was found to be 0.27 ng mL−1 by DPV and 0.87 ng mL−1 by piezoelectric measurements. The sensor exhibited high selectivity towards imprinted epitope. Additionally, the sensor demonstrated the specific and selective detection in real sample of typhoid infected patients. Thus, the proposed MIP-EQCM sensor could be proposed as diagnostic tool for typhoid fever.