Unveiling the electronic properties of native solid electrolyte interphase layers on Mg metal electrodes using local electrochemistry.

Magnesium-ion batteries (MIBs) are of considerable interest as environmentally more sustainable, cheaper, and safer alternatives to Li-ion systems. However, spontaneous electrolyte decomposition occurs due to the low standard reduction potential of Mg, leading to the deposition of layers known as native solid electrolyte interphases (n-SEIs). These layers may inhibit the charge transfer (electrons and ions) and, therefore, reduce the specific power and cycle life of MIBs. We propose scanning electrochemical microscopy (SECM) as a microelectrochemical tool to locally quantify the electronic properties of n-SEIs for MIBs. These interphases are spontaneously formed upon contact of Mg metal disks with organoaluminate, organoborate, or bis(trifluoromethanesulfonyl)imide (TFSI)-based electrolyte solutions. Our results unveil increased local electronic and global ionic insulating properties of the n-SEI formed when using TFSI-based electrolytes, whereas a low electronically protecting character is observed with the organoaluminate solution, and the organoborate solution being in between them. Moreover, ex situ morphological and chemical characterization was performed on the Mg samples to support the results obtained by the SECM measurements. Differences in the electronic and ionic conductivities of n-SEIs perfectly correlate with their chemical compositions.

On the Tunability of Toxicity for Viologen-Derivatives as Anolyte for Neutral Aqueous Organic Redox Flow Batteries.

Viologen-derivatives are the most widely used redox organic molecules for neutral pH negative electrolyte of redox flow batteries. However, the long-established toxicity of the herbicide methyl-viologen raises concern for deployment of viologen-derivatives at large scale in flow batteries. Herein, we demonstrate the radically different cytotoxicity and toxicology of a series of viologen-derivatives in in vitro assays using model organisms representative of human and environmental exposure, namely human lung carcinoma epithelial cell line (A549) and the yeast Saccharomyces cerevisiae. The results show that safe viologen derivatives can be molecularly engineered, representing a promising family of negolyte materials for neutral redox flow batteries.

The redox mediated - scanning droplet cell system for evaluation of the solid electrolyte interphase in Li-ion batteries.

The so-called solid electrolyte interphase (SEI), a nanolayer formed on the negative electrode of lithium-ion batteries during the first cycles, largely influences some key performance indicators such as cycle life and specific power. The reason is due to the fact that the SEI prevents continuous electrolyte decomposition, making this protecting character extremely important. Herein, a specifically designed scanning droplet cell system (SDCS) is developed to study the protecting character of the SEI on lithium-ion battery (LIB) electrode materials. SDCS allows for automatized electrochemical measurements with improved reproducibility and time-saving experimentation. Besides the necessary adaptations for its implementation for non-aqueous batteries, a new operating mode, the so-called redox mediated-scanning droplet cell system (RM-SDCS), is established to investigate the SEI properties. By adding a redox mediator (e.g. a viologen derivative) to the electrolyte, evaluation of the protecting character of the SEI becomes accessible. Validation of the proposed methodology was performed using a model sample (Cu surface). Afterwards, RM-SDCS was employed on Si-graphite electrodes as a case study. On the one hand, the RM-SDCS shed light on the degradation mechanisms providing direct electrochemical evidence of the rupture of the SEI upon lithiation. On the other hand, the RM-SDCS was presented as an accelerated method capable of searching for electrolyte additives. The results indicate an enhancement in the protecting character of the SEI when 4 wt% of both vinyl carbonate and fluoroethylene carbonate were used simultaneously.

Addressing Practical Use of Viologen-Derivatives in Redox Flow Batteries through Molecular Engineering.

In practical scenarios, viologen-derivatives face an accelerated degradation in the unavoidable presence of traces of oxygen in large-scale redox flow batteries. Herein, we confirm the primary degradation mechanism and propose a straightforward, cheap, and fast method to evaluate the stability of viologen-derivatives toward this degradation. Considering that the cleavage of the N-substituent is the main proposed pathway for viologen degradation, a new viologen-derivative, bearing an alkylsulfonate chain with a secondary carbon center joined to the N atom, is synthesized to illustrate how molecular engineering can be used to improve stability.

A Systematic Study on the Redox Potentials of Phenazine-Derivatives in Aqueous Media: A Combined Computational and Experimental Work.

Phenazines are an emerging class of organic compounds that have been recently utilized in aqueous redox flow batteries, a promising technology for large-scale energy storage. A virtual screening based on density functional theory calculations is used to investigate the redox potentials of around 100 phenazine derivatives in aqueous media containing various electron-donating or electron-withdrawing groups at different positions. The calculations identify the crucial positions that should be functionalized with multiple hydroxy groups to design new anolytes. The combined experimental-computational methodology reported herein guides the development of a new molecule with a record low reversible redox potential as a potential anolyte for aqueous redox flow batteries.

Fast Li-ion Storage and Dynamics in TiO2 Nanoparticle Clusters Probed by Smart Scanning Electrochemical Cell Microscopy.

Anatase TiO2 is a promising material for Li-ion (Li+ ) batteries with fast charging capability. However, Li+ (de)intercalation dynamics in TiO2 remain elusive and reported diffusivities span many orders of magnitude. Here, we develop a smart protocol for scanning electrochemical cell microscopy (SECCM) with in situ optical microscopy (OM) to enable the high-throughput charge/discharge analysis of single TiO2 nanoparticle clusters. Directly probing active nanoparticles revealed that TiO2 with a size of ≈50 nm can store over 30 % of the theoretical capacity at an extremely fast charge/discharge rate of ≈100 C. This finding of fast Li+ storage in TiO2 particles strengthens its potential for fast-charging batteries. More generally, smart SECCM-OM should find wide applications for high-throughput electrochemical screening of nanostructured materials.

New Technique for Probing the Protecting Character of the Solid Electrolyte Interphase as a Critical but Elusive Property for Pursuing Long Cycle Life Lithium-Ion Batteries.

The formation of a protecting nanolayer, so-called solid electrolyte interphase (SEI), on the negative electrode of Li-ion batteries (LIBs) from product precipitation of the cathodic decomposition of the electrolyte is a blessing since the electrically insulating nature of this nanolayer protects the electrode surface, preventing continuous electrolyte decomposition and enabling the large nominal cell voltage of LIBs, e.g., 3.3-3.8 V. Thus, the protection performance of the nanolayer SEI is essential for LIBs to achieve a long cycle life. Unfortunately, the evaluation of this critical property of the SEI is not trivial. Herein, a new, cheap, and easily implementable methodology, the redox-mediated enhanced coulometry, is presented to estimate the protecting quality of the SEI. The key element of the methodology is the addition of a redox mediator in the electrolyte during the degassing step (after the SEI formation cycle). The redox mediator leads to an internal self-discharge process that is inversely proportional to the protecting character of the SEI. Also, the self-discharge process results in an easily measurable decrease in Coulombic efficiency. The influence of vinylene carbonate as an electrolyte additive in the resulting SEI is used as a case study to showcase the potential of the proposed methodology.

Correlative Electrochemical Microscopy for the Elucidation of the Local Ionic and Electronic Properties of the Solid Electrolyte Interphase in Li-Ion Batteries.

The solid-electrolyte interphase (SEI) plays a key role in the stability of lithium-ion batteries as the SEI prevents the continuous degradation of the electrolyte at the anode. The SEI acts as an insulating layer for electron transfer, still allowing the ionic flux through the layer. We combine the feedback and multi-frequency alternating-current modes of scanning electrochemical microscopy (SECM) for the first time to assess quantitatively the local electronic and ionic properties of the SEI varying the SEI formation conditions and the used electrolytes in the field of Li-ion batteries (LIB). Correlations between the electronic and ionic properties of the resulting SEI on a model Cu electrode demonstrates the unique feasibility of the proposed strategy to provide the two essential properties of an SEI: ionic and electronic conductivity in dependence on the formation conditions, which is anticipated to exhibit a significant impact on the field of LIBs.

A critical perspective on rechargeable Al-ion battery technology.

The development of rechargeable aluminum-ion batteries (AIBs) has recently attracted much scientific attention due to the low cost and high specific capacity of Al. Most efforts are being concentrated on enhancing the specific charge capacity of active materials for the positive electrode, while other important issues for commercial deployment of this technology have often been overlooked. The aim of this frontier article is not to systematically review the recent advances in the literature, but to bring under the spotlight the critical aspects requiring intensive research activity for paving the way toward the commercialization of AIBs. After a brief revision of the fundaments of an Al-ion battery, the discussion is classified into 5 sections: energy density, specific power, cost, cycle life and safety. Finally, a performance comparison among Al-ion, Li-ion and lead-acid battery technologies on the basis of these 5 primary parameters summarizes the strengths and limitations of Al-ion batteries.

Influence of Temperature and Electrolyte Concentration on the Structure and Catalytic Oxygen Evolution Activity of Nickel-Iron Layered Double Hydroxide.

NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (>80 °C) and high current densities (>500 mA cm-2 ) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline ß-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions.

Powder Catalyst Fixation for Post-Electrolysis Structural Characterization of NiFe Layered Double Hydroxide Based Oxygen Evolution Reaction Electrocatalysts.

Highly active electrocatalysts for the oxygen evolution (OER) reaction are in most cases powder nanomaterials, which undergo substantial changes upon applying the high potentials required for high-current-density oxygen evolution. Owing to the vigorous gas evolution, the durability under OER conditions is disappointingly low for most powder electrocatalysts as there are no strategies to securely fix powder catalysts onto electrode surfaces. Thus reliable studies of catalysts during or after the OER are often impaired. Herein, we propose the use of composites made from precursors of polybenzoxazines and organophilically modified NiFe layered double hydroxides (LDHs) to form a stable and highly conducting catalyst layer, which allows the study of the catalyst before and after electrocatalysis. Characterization of the material by XRD, SEM, and TEM before and after 100 h electrolysis in 5 m KOH at 60 °C and a current density of 200 mA cm-2 revealed previously not observed structural changes.

Polybenzoxazine-Derived N-doped Carbon as Matrix for Powder-Based Electrocatalysts.

In addition to catalytic activity, intrinsic stability, tight immobilization on a suitable electrode surface, and sufficient electronic conductivity are fundamental prerequisites for the long-term operation of particle- and especially powder-based electrocatalysts. We present a novel approach to concurrently address these challenges by using the unique properties of polybenzoxazine (pBO) polymers, namely near-zero shrinkage and high residual-char yield even after pyrolysis at high temperatures. Pyrolysis of a nanocubic prussian blue analogue precursor (Km Mnx [Co(CN)6 ]y ⋅n H2 O) embedded in a bisphenol A and aniline-based pBO led to the formation of a N-doped carbon matrix modified with Mnx Coy Oz nanocubes. The obtained electrocatalyst exhibits high efficiency toward the oxygen evolution reaction (OER) and more importantly a stable performance for at least 65 h.

Complete Prevention of Dendrite Formation in Zn Metal Anodes by Means of Pulsed Charging Protocols.

Zn metal as anode in rechargeable batteries, such as Zn/air or Zn/Ni, suffers from poor cyclability. The formation of Zn dendrites upon cycling is the key limiting step. We report a systematic study of the influence of pulsed electroplating protocols on the formation of Zn dendrites and in turn on strategies to completely prevent Zn dendrite formation. Because of the large number of variables in electroplating protocols, a scanning droplet cell technique was adapted as a high-throughput methodology in which a descriptor of the surface roughness can be in situ derived by means of electrochemical impedance spectroscopy. Upon optimizing the electroplating protocol by controlling nucleation, zincate ion depletion, and zincate ion diffusion, scanning electron microscopy and atomic force microscopy confirmed the growth of uniform and homogenous Zn deposits with a complete prevention of dendrite growth. The implementation of pulsed electroplating as the charging protocol for commercially available Ni-Zn batteries leads to substantially prolonged cyclability demonstrating the benefits of pulsed charging in Zn metal-based batteries.

Overcoming the Instability of Nanoparticle-Based Catalyst Films in Alkaline Electrolyzers by using Self-Assembling and Self-Healing Films.

Engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a challenge. We report an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on the in situ self-assembly of catalyst particles during electrolysis. The catalyst particles are added to the electrolyte forming a suspension that is pumped through the electrolyzer. Particles with negatively charged surfaces stick onto the anode, while particles with positively charged surfaces stick to the cathode. The self-assembled catalyst films have self-healing properties as long as sufficient catalyst particles are present in the electrolyte. The proof-of-concept was demonstrated in a non-zero gap alkaline electrolyzer using NiFe-LDH and Nix B catalyst nanopowders for anode and cathode, respectively. Steady cell voltages were maintained for at least three weeks during continuous electrolysis at 50-100 mA cm-2 .

Hydrogen-Treated Rutile TiO2 Shell in Graphite-Core Structure as a Negative Electrode for High-Performance Vanadium Redox Flow Batteries.

Hydrogen-treated TiO2 as an electrocatalyst has shown to boost the capacity of high-performance all-vanadium redox flow batteries (VRFBs) as a simple and eco-friendly strategy. The graphite felt-based GF@TiO2 :H electrode is able to inhibit the hydrogen evolution reaction (HER), which is a critical barrier for operating at high rate for long-term cycling in VRFBs. Significant improvements in charge/discharge and electron-transfer processes for the V3+ /V2+ reaction on the surface of reduced TiO2 were achieved as a consequence of the formation of oxygen functional groups and oxygen vacancies in the lattice structure. Key performance indicators of VRFB have been improved, such as high capability rates and electrolyte-utilization ratios (82 % at 200 mA cm-2 ). Additionally, high coulombic efficiencies (ca. 100 % up to the 96th cycle, afterwards >97 %) were obtained, demonstrating the feasibility of achieving long-term stability.

Ultrafast Dischargeable LiMn2O4 Thin-Film Electrodes with Pseudocapacitive Properties for Microbatteries.

LiMn2O4 (LMO) thin films are deposited on Si-based substrates with Pt current collector via multi-layer pulsed-laser-deposition technique. The LMO thin films feature unique kinetics that yield outstanding electrochemical cycling performance in an aqueous environment. At extremely high current densities of up to 1880 µA cm-2 (≈ 348 C), a reversible capacity of 2.6 µAh cm-2 is reached. Furthermore, the electrochemical cycling remains very stable for over 3500 cycles with a remarkable capacity retention of 99.996% per cycle. We provide evidence of significant nondiffusion-controlled, pseudocapacitive-like storage contribution of the LMO electrode.

An Intrinsic Self-Charging Biosupercapacitor Comprised of a High-Potential Bioanode and a Low-Potential Biocathode.

An intrinsic self-charging biosupercapacitor built on a unique concept for the fabrication of biodevices based on redox polymers is presented. The biosupercapacitor consists of a high-potential redox polymer based bioanode and a low-potential redox polymer based biocathode in which the potentials of the electrodes in the discharged state show an apparent potential mismatch Eanode >Ecathode and prevent the use of the device as a conventional biofuel cell. Upon charging, the potentials of the electrodes are shifted to more positive (cathode) and more negative (anode) values because of a change in the aox -to-ared ratio within the redox polymer matrix. Hence, a potential inversion occurs in the charged state (Eanode 0.4 V is achieved and the biodevice acts as a true biosupercapacitor. The bioanode consists of a novel specifically designed high-potential Os complex modified polymer for the efficient immobilization and electrical wiring of glucose converting enzymes, such as glucose oxidase and flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase. The cathodic side is constructed from a low-potential Os complex modified polymer integrating the O2 reducing enzyme, bilirubin oxidase. The large potential differences between the redox polymers and the prosthetic groups of the biocatalysts ensure fast and efficient charging of the biodevice.

Solid Electrolyte Interphase (SEI) at TiO2 Electrodes in Li-Ion Batteries: Defining Apparent and Effective SEI Based on Evidence from X-ray Photoemission Spectroscopy and Scanning Electrochemical Microscopy.

The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and effective SEI formed at graphite electrodes.

The Mechanism of the Interfacial Charge and Mass Transfer during Intercalation of Alkali Metal Cations.

Intercalation of alkali metal cations, like Li+ or Na+, follows the same three-stage mechanism of the interfacial charge and mass transfer irrespective of the nature of the electrolyte, electrolyte composition or electrode material.

Nanoelectrodes reveal the electrochemistry of single nickelhydroxide nanoparticles.

Individual Ni(OH)2 nanoparticles deposited on carbon nanoelectrodes are investigated in non-ensemble measurements with respect to their energy storage properties and electrocatalysis for the oxygen evolution reaction (OER). Charging by oxidation of Ni(OH)2 is limited by the diffusion of protons into the particle bulk and the OER activity is independent of the particle size.