In electrolyzers, the electrocatalytic splitting of aqueous electrolytes is typically impeded by the sluggish kinetics of the Oxygen Evolution Reaction (OER) at the anode. In established alkaline electrolysis technologies, NiOOH anodes are commonly employed and have been the subject of extensive studies using various analysis methods, such as Raman spectroscopy. [1-4] In this study, we emphasize the origin of the vibrational modes of a NiOOH Raman spectrum, with the objective of attaining a more profound understanding of the surface structure of NiOOH before and during the OER. Additionally, we illustrate the impact of varying pH values on the OER activity and the surface structures of the NiOOH electrodes, with the aim of providing a comprehensive understanding of potential-dependent structural changes. The potential dependent surface structural changes of NiOOH are investigated using in situ Surface Enhanced Raman Spectroscopy (SERS) in electrolytes of varying neutral and alkaline pH. Isotope labeling experiments, employing D 2 O and H 2 18 O, were conducted to gain additional insights in the measured Raman modes, from possible peak shifts induced by the isotopes. Furthermore, Density Functional Theory (DFT) calculations on various Ni x O y (OH) z were performed to assign the peaks in the Raman spectrum. Utilizing this comprehensive set of experimental and theoretical data, we demonstrate that the surface structure of NiOOH is predominantly deprotonated under both alkaline and neutral pH conditions. Additionally, we discuss the prevailing view on the formation and detection of superoxides using Raman spectroscopy on NiOOH. [3,4] Moreover, while performing the OER on NiOOH under weak alkaline pH values, SERS reveals a band at 1040 cm-1, which has not yet been reported. Further Cyclic Voltammetry (CV) studies at various pH levels also show significant changes in OER region. Complementary Electrochemical Quartz Crystal Microbalance (EQCM) measurements reveal pH-dependent changes in the electrode mass at high potentials, which correlate with the variations observed in the CVs and SERS spectra. The variations in the CVs are rationalized in terms of local pH changes at the electrode surface. Combining the results from the various techniques we discuss the impact of pH and potential dependent surface changes on OER kinetics and its underlying mechanism. (1) Klaus, S.; Cai, Y.; Louie, M. W.; Trotochaud, L.; Bell, A. T. J. Phys. Chem. C 2015 , 119 (13), 7243–7254. (2) Merrill, M.; Worsley, M.; Wittstock, A.; Biener, J.; Stadermann, M. Journal of Electroanalytical Chemistry 2014 , 717–718 , 177–188. (3) Lee, S.; Chu, Y.-C.; Bai, L.; Chen, H. M.; Hu, X. Chem Catalysis 2023 , 3 (1), 100475. (4) Diaz-Morales, O.; Ferrus-Suspedra, D.; Koper, M. T. M. Chem. Sci. 2016 , 7 (4), 2639–2645. Figure 1

Due to their high theoretical energy density and the abundance of magnesium, rechargeable Mg batteries are promising candidate systems for future energy storage. However, finding suitable electrolytes that are compatible with the metallic Mg electrode and enable highly reversible Mg plating is still challenging. Typical electrolytes for rechargeable magnesium batteries are based on ether solvents such as tetrahydrofuran (THF), dimethoxyethane (DME), or higher glymes. Drawbacks are the high volatilites and low flashpoints of THF and DME and their harmfulness, problematic factors for industrial applicability. One potential alternative is diethylene glycol diethyl ether (DEGDEE) which is also an ether, but has significantly higher boiling and flashpoints than THF and DME, and is from today's perspective less harmful than any of the previously mentioned solvents. To test the suitability and stability of this class of electrolytes, different Mg salts in combination with DEGDEE for their electrochemical Mg plating and stripping properties are studied. Although Mg deposition needs higher overpotentials than for their DME‐based counterparts, the investigated electrolytes enable reversible Mg plating with relatively high Coulombic efficiencies, making DEGDEE a promising alternative electrolyte solvent for rechargeable Mg batteries.

Sustainable battery concepts are of great importance for the energy storage demands of the future. Organic batteries based on redox-active polymers are one class of promising storage systems to meet these demands, in particular when combined with environmentally friendly and safe electrolytes. Deep Eutectic Solvents (DESs) represent a class of electrolytes that can be produced from sustainable sources and exhibit in most cases no or only a small environmental impact. Because of their non-flammability, DESs are safe, while providing an electrochemical stability window almost comparable to established battery electrolytes and much broader than typical aqueous electrolytes. Here, we report the first all-organic battery cell based on a DES electrolyte composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and N-methylacetamide (NMA) alongside the electrode active materials poly(2,2,6,6-tetramethylpiperidin-1-yl-oxyl methacrylate) (PTMA) and crosslinked poly(vinylbenzylviologen) (X-PVBV2+). The resulting cell shows two voltage plateaus at 1.07 V and 1.58 V and achieves Coulombic efficiencies of 98 %. Surprisingly, the X-PVBV/X-PVBV+ redox couple turned out to be much more stable in NaTFSI:NMA 1:6 than the X-PVBV+/X-PVBV2+ couple, leading to asymmetric capacity fading during cycling tests.

Rechargeable magnesium batteries are promising for future energy storage. However, among other challenges, their practical application is hindered by low coulombic efficiencies of magnesium plating and stripping. Fundamental processes such as the formation, structure, and stability of passivation layers and the influence of different electrolyte components on them are still not fully understood. In this work, we gain unique insights into the initial Mg plating and stripping cycles by comparing magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2)- and magnesium tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)4]2)-based electrolytes, each with and without MgCl2, on gold electrodes by highly sensitive operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) applying hydrodynamic spectroscopy. With the stable Mg[B(hfip)4]2-based electrolytes, highly efficient and interphase-free cycling is possible and passivation layers are attributed to electrolyte contaminants. These are forming and degrading during the so-called initial conditioning process. With the more reactive Mg(TFSI)2-based electrolyte, thick passivation layers with small pores are growing during cycling. We demonstrate that the addition of chloride lowers the amount of passivated Mg deposits in these electrolytes and accelerates the currentless dissolution of the passivation layer. This has a positive effect since we observe the most efficient cycling and uniform deposition when no interphase is present on the electrode.

Rechargeable magnesium batteries are promising for future energy storage. However, among other challenges, their practical application is hindered by low coulombic efficiencies of magnesium plating and stripping. Fundamental processes such as the formation, structure, and stability of passivation layers and the influence of different electrolyte components on them are still not fully understood. Here, we gain unique insights into the initial Mg plating and stripping cycles by comparing Mg bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2)- and Mg tetrakis(hexafluoroisopropyloxy)borate (Mg[B(hfip)4]2)-based electrolytes, each with and without MgCl2, on gold electrodes by highly sensitive operando electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), applying hydrodynamic spectroscopy. With the stable Mg[B(hfip)4]2-based electrolytes, highly efficient and interphase-free cycling is possible and passivation layers are attributed to electrolyte contaminants. These are forming and degrading during the so-called conditioning process. With the more reactive Mg(TFSI)2 based electrolyte, thick passivation layers with small pores are growing during cycling. We demonstrate that the addition of chloride lowers the amount of passivated Mg deposits in these electrolytes and accelerates the currentless dissolution of the passivation layer. This has a positive effect since we observe the most efficient cycling and uniform deposition when no interphase is present on the electrode.

For sustainable storage of electrical energy, all-organic batteries based on redox-active polymers promise to become an alternative to conventional lithium ion batteries. Yet, polymers can only contribute to the goal of an all-organic cell as electrodes or as solid electrolytes. Here, we replace the electrolyte with a sustainable deep eutectic solvent (DES) composed of sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) and N-methylacetamide (NMA), while using poly(2,2,6,6-tetramethylpiperidin-1-yl-oxyl methacrylate) (PTMA) as cathode. The successful combination of a DES with a polymer electrode is reported here for the first time. The electrochemical stability of PTMA electrodes in the DES at the eutectic molar ratio of 1:6 is comparable to conventional battery electrolytes. More viscous electrolytes with higher salt concentrations can hinder charging and discharging at high rates. Lower salt concentrations on the other hand lead to decreasing capacities and faster decomposition. The used eutectic mixture of 1:6 is best suited uniting high stability and moderate viscosity.

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.

The Front Cover illustrates copper electrodeposition on a gold-coated quartz crystal from a choline chloride/ethylene glycol deep eutectic solvent (DES) in the molar ratio 1:2 with copper(II) chloride. Investigations conducted in an electrochemical quartz crystal microbalance (EQCM) revealed that the deposition occurs by a one-electron reduction from Cu+. More information can be found in the Research Article by T. Geng et al.

AbstractVollorganische Batterien auf Grundlage redoxaktiver Polymere versprechen eine Alternative zu Lithium‐Ionen‐Batterien für die nachhaltige Energiespeicherung zu werden. Polymere tragen bisher als Elektroden oder als feste Elektrolyte zum Ziel einer rein organischen Zelle bei. Hier ersetzen wir den Elektrolyten durch ein Deep Eutectic Solvent (DES), das aus Natriumbis(trifluormethansulfonyl)imid (NaTFSI) und N‐Methylacetamid (NMA) besteht, und verwenden Poly(2,2,6,6‐tetramethylpiperidin‐1‐yl‐oxylmethacrylat) (PTMA) als Kathode. Von der erfolgreichen Kombination eines DES mit einer Polymerelektrode wird hier zum ersten Mal berichtet. Die elektrochemische Stabilität von PTMA‐Elektroden im DES ist beim eutektischen Stoffmengenverhältnis von 1 : 6 mit herkömmlichen Batterieelektrolyten vergleichbar. Viskosere Elektrolyte mit höherer Salzkonzentration können das Zykeln bei hohen Raten behindern. Eine niedrigere Salzkonzentration führt zu sinkenden Kapazitäten und schnellerer Zersetzung. Die eutektische Mischung von 1 : 6 ist am besten geeignet, um hohe Stabilität und moderate Viskosität zu vereinen.

Copper electrodeposition on Au(111) from deep eutectic solvents (DESs) type III was investigated employing cyclic voltammetry as well as chronoamperometry. It was further examined on Au(poly) using the electrochemical quartz crystal microbalance (EQCM). The employed DESs are mixtures of choline chloride (ChCl) or choline nitrate (ChNO(3)) with ethylene glycol (EG) as hydrogen bond donor (HBD), each in a molar ratio of 1 : 2. CuCl, CuCl2, or Cu(NO3)(2)center dot 3H(2)O were added as copper sources. Underpotential deposition (UPD) of Cu precedes bulk deposition in chloride as well as nitrate electrolytes. Cu deposition from Cu+ in chloride media is observed as a one-electron reaction, whereas deposition from Cu2+ occurs in two steps since Cu+ is strongly stabilized by chloride. Cu+ is less stabilized by nitrate and the beginning of bulk deposition in the nitrate-containing DES with Cu2+ is shifted by several hundred mV to more positive potentials compared to the chloride DES. A diffusion-controlled, three-dimensional nucleation and growth mechanism is found by chronoamperometric measurements and analysis based on the model of Scharifker and Mostany.

Silver electrodeposition onto Au(111) has been studied as a function of composition of deep eutectic solvent (DES) type III and IV. The electrochemical investigations were performed by using cyclic voltammetry and in-situ electrochemical quartz crystal microbalance experiments. Scanning electron microscopy coupled with Auger spectroscopy and atomic force microscopy were used to characterize the deposited metal overlayers. Silver deposition from DES type III shows several analogies to the electrocrystallization from aqueous solutions, such as the presence of underpotential deposition. It is shown that the hydrogen-bond donor exerts a strong influence on the silver growth mode and on the structure of the metal deposits. In addition, the hydrogen-bond donor turns out to exert a rather strong influence on the plating process during silver deposition from DES type IV.

The Cover Feature shows an artificial representation of the Au(111) surface with silver clusters, as could be seen in the SEM. Above the striking cluster, one can find a silver nitrate, which is complexed by two urea molecules, thus forming a deep eutectic solvent surrounded by a solvation shell. The charge transfer is symbolized by a flash of lightning. More information can be found in the Article by M. U. Ceblin et al. on page 141 in Issue 1, 2019 (DOI: 10.1002/celc.201801192).