Batteries are indispensable energy storage systems in modern society. In-depth understanding of their electrochemical dynamics under operando conditions is crucial for further optimization of component's performance and lifespan. In this context, optical microscopy (OM) has been effective to observe interfacial and microstructural processes and to acquire mechanistic insights that lead toward safer, more efficient, and longer-lasting batteries. This review summarizes recent implementation of diverse OM techniques applied to Li-ion batteries (LIB), aqueous batteries (AB), and solid-state batteries (SSB).

Coupling electrochemistry with optical techniques gives in-depth insights into the interfacial processes in action. In that context, fluorescence confocal laser scanning microscopy (F-CLSM) enables an electrode surface characterization with spatial resolution in the lateral plane (xy) as well as in the axial direction (z), perpendicular to the electrode surface. However, like most optical techniques, fluorescence microscopy has intrinsic limitations, notably in terms of resolution and sensitivity, which are investigated in this contribution by conducting F-CLSM experiments with two disk electrodes of different sizes: a large microelectrode (LME, Ø = 250 μm) and a much smaller so-called ultramicroelectrode (UME, Ø = 18 μm). We demonstrated that the diffusion layers of both microelectrodes can be imaged with sufficient resolution and sensitivity to be quantitatively compared with the simulated concentration profiles. This work highlights the intrinsic technical challenges associated with this kind of coupled experiments, and it discusses the conditions that should be fulfilled to obtain reliable results at the microscale. These results pave the way toward reaction layer imaging down to micrometric resolution and could help decipher complex electrochemical reactions possibly involving transient species.

High-entropy oxides (HEOx) constitute a new class of materials that have attracted increasing interest over the past decade. By incorporating multiple cations into a single crystalline lattice, they achieve high configurational entropy (ΔS config ). This high entropy, combined with the presence of oxygen vacancies, can significantly enhance the mobility of charge carriers, leading to improved catalytic performance. Moreover, these materials exhibit exceptional thermal, chemical, and structural stability, making them highly suitable for a variety of catalytic and functional applications [1] [2]. Despite these advantages, HEOx materials have been relatively unexplored, especially in the field of photoelectrocatalysis. In this study, (CoNiFeCuZn)WO 4 and (MgCuCoNiZn)O were synthesized using a top-down approach and thoroughly characterized using various techniques to investigate its structural and optical properties. Their electrocatalytic activity toward the oxygen evolution reaction (OER) was evaluated using cyclic voltammetry and linear sweep voltammetry (LSV), which revealed a relatively high catalytic performance. Moreover, the visible-light band gap of these materials strongly suggests their potential for solar-driven water splitting. Modeling and preliminary photocatalytic results further indicate the possibility of using these materials to generate H 2 under illumination, making them promising candidates for future sustainable energy conversion applications. References [1] B. L. Musicó et al. , “The emergent field of high entropy oxides: Design, prospects, challenges, and opportunities for tailoring material properties,” APL Mater. , vol. 8, no. 4, Apr. 2020, doi: 10.1063/5.0003149.[2] Y. Sun and S. Dai, “High-entropy materials for catalysis: A new frontier,” Sci. Adv. , vol. 7, no. 20, May 2021, doi: 10.1126/sciadv.abg1600. Figure 1 : (left), Schematic representation of water splitting by photocatalysis on HEO , (righ) LSV on (CoNiFeCuZn)WO 4 Figure 1

Glassy carbon electrodes were modified with a CeO2 film and Pt nanoparticles (Pt-CeO2) for electrocatalysis. Interestingly, the oxidation of benzyl alcohol was significantly enhanced when Pt-CeO2 films were prepared by the simultaneous electrodeposition of the two materials, indicating a significant synergistic electrocatalytic activity. Subsequently, bipolar electrochemistry was employed to prepare Pt-CeO2 gradient films. Scanning electrochemical microscopy (SECM) was employed for studying local electrochemical properties at liquid/solid interfaces. SECM allowed mapping the local electrochemical performance of the Pt-CeO2 gradient films for benzyl alcohol oxidation, showing that the reaction rate is proportional to the local Pt-CeO2 surface coverage. Therefore, Pt-CeO2 deposits with different densities along the bipolar electrode offer tunable catalytic performances for benzyl alcohol oxidation. This allows identifying in a fast and straightforward way the optimal conditions for electrocatalytic processes in a more general sense because the approach, illustrated here with one specific reaction, can be easily generalized to other catalytically-active surfaces.

The composition of essential oils varies according to culture conditions and climate, which induces a need for simple and inexpensive characterization methods close to the place of extraction. This appears particularly important for developing countries. Herein, we develop an analytical strategy to determine the thymol content in Ocimum Gratissimum, a medicinal plant from Benin. The protocol is based on electrochemical techniques (cyclic and square wave voltammetry) implemented with a low cost potentiostat. Thymol is a phenol derivative and was directly oxidized at the electrode surface. We had to resort to submillimolar concentrations (25-300 mu M) in order to minimize production of phenol oligomers that passivate the electrode. We worked first on two essential oils and realized that in one of them the thymol concentration was below our detection method. These results were confirmed by gas chromatography - mass spectrometry. Furthermore, we optimized the detection protocol to analyze an infusion made directly from the leaves of the plant. Finally, we studied whether the cost of the electrochemical cell may also be minimized by using pencil lead as working and counter electrodes.

Electrochemistry is taught in most undergraduate chemistry programs. Although this topic is important for students due to its broad interest in industry (energy, diagnostics, car industry, etc.), they often find it difficult, because it is based on a combination of various physical concepts such as electric fields, interfacial processes or charge and mass transport. Among electrochemical concepts, bipolar electrochemistry is of special interest and might be easier to teach due to a very simple general setup. In this case, an oxidation reaction and a reduction reaction occur at the two ends of a single conductive object exposed to an electric field in solution. Such an object is therefore called a bipolar electrode. The nature of the electrochemical reactions and their amplitude can be tuned by playing with the electric field. The fundamental concepts of bipolar electrochemistry are introduced here with a series of basic experiments designed to be carried out in a standard teaching laboratory. These simple and affordable experiments illustrate the key-parameters driving electrochemical reactions at a bipolar electrode. Their influence can be readily visualized using a cheap, commercially available light emitting diode (LED) acting as the bipolar electrode, which illuminates when the current generated by the electrochemical reactions flows through it. The concept of bipolar electrochemistry with eye-catching experiments enables a good introduction to general electrochemistry. It points out the importance of fundamental aspects such as the electric field or the necessity of an electrolyte and a counter-reaction.

Glassy carbon (GC) electrodes are widely used in electroanalytical applications especially in bioelectrochemistry. Their use starts with an efficient surface cleaning and activation protocol, mostly based on surface polishing steps. We studied the use of an oxygen plasma exposure of GC electrodes to replace common polishing procedures. The cyclic voltammetry (CV) responses of ferrocyanide and ferrocene-dimethanol were used to compare brand new, surface-polished and plasma-treated GC electrodes. Plasma treatment induces CV responses with improved features, close to theoretical values, as compared to other methods. The plasma effects were quasi-stable over a week when electrodes were stored in water, this being explained by increased surface energy and hydrophilicity. Furthermore, when electroreduction of diazonium was performed on GC electrodes, the surface blockade could be removed by the plasma. Thus, a short oxygen plasma treatment is prone to replace polishing protocols, that display person-dependent efficiency, in most of the experiments with GC electrodes.

Conducting polymers show attractive characteristics as electrode materials for micro-electrochemical energy storage (MEES). However, there is a lack of characterization techniques to study conjugated/conducting polymer-based nanostructured electrodes. Here, scanning electrochemical microscopy (SECM) is introduced as a new technique for in situ characterization and acceleration of degradation processes of conducting polymers. Electrodes of PEDOT:PSS on flat silicon, silicon nanowires (SiNWs) and silicon nanotrees (SiNTrs) are analyzed by SECM in feedback mode with approach curves and chronoamperometry. The innovative degradation method using SECM reduces the time required to locally degrade polymer samples to a few thousand seconds, which is significantly shorter than the time usually required for such studies. The degradation rate is modeled using Comsol Multiphysics. The model provides an understanding of the phenomena that occur during degradation of the polymer electrode and describes them using a mathematical constant A0 and a time constant τ.

Despite the dramatically increasing research effort dedicated to redox-active electrolytes towards improved supercapacitor capacitance, the research concerning charging mechanisms remains sparse. This is particularly true for porous materials such as porous carbon. Whereas some charging mechanisms have been discussed for microporous materials, the mechanisms occurring within mesoporous materials are mostly unexplored. Herein, a new redox mediator - Thionine (Th) - in highly-concentrated LiTFSI, was used as a supercapacitor electrolyte to explore these mechanisms. The addition of Th modifies the viscosity and thermal properties of the electrolyte without affecting the ionic conductivity. Electrochemical measurements confirmed that the presence of Th endows an additional pseudocapacitance in mesoporous carbon but not in microporous carbon. Based on these contrasting phenomena, the charging mechanism and solvated structure were studied by Molecular Dynamics (MD) and Density Functional Theory (DFT) simulation. DFT simulation disclosed that it has an ion association effect between TFSI and Th due to their opposite charge. Two MD models were built with different porosities: 0.9 nm (micropores) and 3 nm (mesopores). It was found that Th molecules cannot enter micropores whereas their insertion into the mesopores was possible. Once inside the mesopores, Th can interact with the inner carbon wall via hydrogen bonding thus stabilizing the redox molecule within the mesopore which can then provide a redox contribution to the charging process. The present work helps to build a deeper understanding of the activity of redox-active molecules in mesoporous materials by elucidating the exact adsorption and charging mechanisms within a microporous system. This paves the way for the design of advanced redox-active electrolytes for supercapacitors.

Water-in-salt electrolytes - WISEs - are prevailing thanks to their compelling extended voltage window due to the reduced free water molecules at the electrode interface. However, as has been reported elsewhere, free-water content still can be reduced further. In our previous work, an unstable phenomenon of solid electrolyte interphase (SEI) and salt precipitation/dissolution issue were revealed. Herein, we propose a novel approach in order to alleviate those issues using poly(ethylene glycol) diglycidyl ether (PDE) as an additive. Indeed, upon mixing LiTFSI, water and PDE at high concentrations, we observed a ring-opening reaction of PDE that was confirmed via Raman spectroscopy, FTIR and ionic conductivity measurements. These crosslinked networks could also increase the solubility limits of LiTFSI in water, which was identified by adding more LiTFSI or LiOTf. Differential scanning calorimetry (DSC) measurement demonstrated that these crosslinked electrolytes effectively suppress the crystallization of water molecules with the WISE. Linear sweep voltammetry (LSV) measurements revealed that these novel crosslinked electrolytes considerably reduce free water content which effectively drives the HER to more negative potentials. More significantly, the SEI formed with these novel electrolytes remains present and stable on the electrode surface after a resting period of 1 h. Our work herein offers a new approach to tackling SEI instability and precipitation/dissolution issues.

The effect of galvanic coupling on the corrosion behavior of Mg and Mg17Al12 in Mg-Al alloys was studied by Scanning ElectroChemical Microscopy (SECM). The effect of galvanic coupling between Mg and Mg17Al12 was investigated using a "model" Mg+Mg17Al12 material with a controlled microstructure to evaluate the hydrogen evolution at a micrometric scale. SECM maps revealed that galvanic coupling between Mg and Mg17Al12 accelerates the corrosion rate (formation of a thicker passive layer) of both components. Mg17Al12 acts controversially to a conventional cathode in galvanic system since hydrogen production by its hydrolysis reaction was found to increase due to the electron transfer with the anode (Mg). (c) 2021 Elsevier B.V. All rights reserved.

Inserting complex biomolecules such as oligonucleotides during the synthesis of polymers remains an important challenge in the development of functionalized materials. In order to engineer such a biofunctionalized interface, a single-step method for the covalent immobilization of oligonucleotides (ONs) based on novel electropolymerizable lipid thiophene-oligonucleotide (L-ThON) conjugates was employed. Here, we report a new thiophene phosphoramidite building block for the synthesis of modified L-ThONs. The biofunctionalized material was obtained by direct electropolymerization of L-ThONs in the presence of 2,2'-bithiophene (BTh) to obtain a copolymer film on indium tin oxide electrodes. In situ electroconductance measurements and microstructural studies showed that the L-ThON was incorporated in the BTh copolymer backbone. Furthermore, the covalently immobilized L-ThON sequence showed selectivity in subsequent hybridization processes with a complementary target, demonstrating that L-ThONs can directly be used for manufacturing materials via an electropolymerization strategy. These results indicate that L-ThONs are promising candidates for the development of stable ON-based bioelectrochemical platforms.

This contribution reviews a selection of the most recent studies on the use of bipolar electrochemistry in the framework of analytical chemistry. Despite the fact that the concept is not new, with several important studies dating back to the middle of the last century, completely novel and very original approaches have emerged over the last decade. This current revival illustrates that scientists still (re)discover some exciting virtues of this approach, which are useful in many different areas, especially for tackling analytical challenges in an unconventional way. In several cases, this “wireless” electrochemistry strategy enables carrying out measurements that are simply not possible with classic electrochemical approaches. This review will hopefully stimulate new ideas and trigger scientists to integrate some aspects of bipolar electrochemistry in their work in order to drive the topic into yet unexplored and eventually completely unexpected directions.

Abstract Bipolar electrochemistry (BPE) is a technique that involves two opposite chemical processes, namely an oxidation and a reduction, occurring simultaneously on the surface of a conducting object, usually without connection to a power supply. The basic phenomenon has already been described and used for many decades but regained a lot of interest in recent years, thanks to several attractive features for developing new applications in various areas ranging from materials science and analytical chemistry to catalysis and even life science. Consequently, BPE has experienced a substantial growth in the number of users and publications. This is mostly due to several advantages over classic electrochemistry, such as the absence of an ohmic contact, the generation of a directional gradient of electroactivity on the object, and the possibility to address simultaneously thousands of objects, which opens the door for high‐throughput screening of (electro)chemical properties. Also, some features of this “wireless” electrochemistry allow performing experiments that cannot be achieved with a classic electrochemical setup. Last but not least, only rather low‐cost equipment is needed. The objective of the present contribution is to introduce first some fundamental aspects of BPE, and then to illustrate its different developments, highlighting not only the historic landmark achievements, but also the most recent findings, especially in the context of micro‐ and nanoscience. This article illustrates the singularities and advantages of this straightforward approach and hopefully convinces the reader of the power of this interesting electrochemical concept.

We employed the oxidative dehydrogenation of C2H2 by CO2 and the C2H2 decomposition to prepare carbon nanotubes (CNTs) and carbon nanofibers (CNFs). The use of a small amount of Ni catalyst made it possible to produce CNTs and CNFs having a highly defective structure at a relatively low temperature compared to the one typically used for the CNT synthesis. The synthesized CNTs and CNFs were decorated with about 10 nm-gold nanoparticles (AuNPs) via an electrostatic self-attachment. The electrode with CNTs synthesized via the C2H2-CO2 reaction exhibits superior electrochemical reactivity with H2O2 when compared to the ones with CNTs synthesized via C2H2 decomposition, commercial CNTs, CNFs, and other more common carbon supports e.g. carbon black and activated charcoal. It demonstrates a rapid response, high sensitivity (104.9 mA mu M-1 cm(-2)), wide linear working range (5 mu M-23 mu M), low detection limit (0.138 mu M), good selectivity, reproducibility, and stability. The CNTs synthesized via the C2H2-CO2 reaction are suggested as energy-saving, cost-effective and environmentally friendly supporting material candidates for practical applications. The electrodes with CNTs show a high electrochemically active surface area, low charge transfer resistance and fast mass transfer at the electrode surface that are key factors for H2O2 detection. (C) 2020 Published by Elsevier Ltd.

In this work, bipolar electrochemistry is used to perform wireless indirect electrodeposition of two different polymer coatings on both sides of carbon nanotube arrays. Using a thermoresponsive hydrogel on one side and an inert insoluble polymer on the other side, it is possible to generate, in a single step, a nanoporous reservoir with Janus character closed on one side by a thermoresponsive membrane. The thermoresponsive polymer, poly(N-isopropylacrylamide) (pNIPAM), is generated by the local reduction of persulfate ions, which initiates radical polymerization of NIPAM. Electrophoretic paint (EP) is chosen as an inert polymer. It is deposited by precipitation because of a local decrease in pH during water oxidation. Both polymers can be deposited simultaneously on opposite sides of the bipolar electrode during the application of the electric field, yielding a double-modified Janus object. Moreover, the length and thickness of the polymer layers can be controlled by varying the electric field and the deposition time. This concept is applied to vertically aligned carbon nanotube arrays (VACNTs), trapped inside an anodic aluminum oxide membrane, which can further be used as a smart reservoir for chemical storage and release. A fluorescent dye is loaded in the VACNTs and its release is studied as a function of temperature. Low temperature, when the hydrogel layer is in the swollen state, allows diffusion of the molecule. Dye release occurs on the hydrogel-modified side of the VACNTs. At high temperatures, when the hydrogel layer is in the collapsed state, dye release is blocked because of the impermeability of the pNIPAM layer. This concept paves the way toward the design of advanced devices in the fields of drug storage and directed delivery.

Electrodes of TiO2-nanotubes with a gradient of length were synthesized by a simple anodization method and analyzed locally by scanning photoelectrochemical microscopy.

Gold microelectrodes decorated with nanotips were developed for spectroelectrochemical experiments. These new dual probes were fabricated by combining fabrication processes of scanning near-field optical microscopy with photolithography. A nanotip array was produced at the surface of a coherent optical fiber bundle by a wet chemical etching step. The resulting nanostructured surface was sputter-coated with a thin gold layer. This gold film conferred plasmonic properties to the sharp nanotips and served as well as the electrode material to enable electrochemical reactions. A photolithographic process was used to delimit on the bundle surface nanotips-decorated microelectrodes with tunable dimensions (radii ranging between 20 mu m and 3.5 mu m) individually or in an array format. The resulting microelectrodes with a regular nanotip pattern were characterized first by cyclic voltammetry. Numerical simulation was used to assess the electrochemical properties of these platforms and the influences of the recessed geometry and of the nanotips. Approach curves were recorded in negative and positive feedback modes of scanning electrochemical microscopy (SECM) on insulating and conducting substrates. Finally, spatially resolved Raman imaging allowed us to detect a mercaptobenzoic acid monolayer adsorbed on the microelectrode surface, demonstrating a surface-enhanced Raman scattering (SERS) effect induced by the gold-coated nanotips with a typical enhancement factor of similar to 7 x 10(4). Such an approach introduces a reproducible method to fabricate promising SERS-active platforms with microelectrode behavior for SECM experiments. (C) 2019 Elsevier Ltd. All rights reserved.

The Cover Feature illustrates some of the efforts performed to investigate enzymatic electrodes at the micro- and nanoscale. Enzymes are extremely interesting molecules in the design of powerful bioelectrochemical devices. Among various techniques used to characterize their immobilization and behavior at the electrode surface, microscopy is the most adequate to offer spatially resolved information. This Review describes the microscopy methods used so far, ranging from scanning probe microscopy to study enzymes at the molecule level to fluorescence microscopy to characterize the whole bioelectrode. More information can be found in the Review by D. Zigah et al. on page 5524 in Issue 22, 2019 (DOI: 10.1002/celc.201901065).

The Cover Feature illustrates some of the efforts performed to investigate enzymatic electrodes at the micro- and nanoscale. Enzymes are extremely interesting molecules in the design of powerful bioelectrochemical devices. Among various techniques used to characterize their immobilization and behavior at the electrode surface, microscopy is the most adequate to offer spatially resolved information. This Review describes the microscopy methods used so far, ranging from scanning probe microscopy to study enzymes at the molecule level to fluorescence microscopy to characterize the whole bioelectrode.More information can be found in the Review by D. Zigah et al.