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The Cu-catalyzed electrochemical CO2 reduction enables the conversion of greenhouse gas emissions to fuels or platform chemicals, with prospects of storing intermittent energy from renewable sources. While current research in tuning catalyst activity and product selectivity is often mired in finding electrode engineering solutions, the importance of electrolyte engineering is mostly overlooked. This study presents a method for measuring local pH profiles in electrode-proximity and correlating them to cation-induced buffering effects. Magnetic Resonance Imaging (MRI) techniques were applied to evaluate the local pH values using spatially resolved 13C resonances of the CO2/HCO3–/CO32– equilibrium. The buffering effect of cation hydrolysis is substantiated by local shifts of the 23Na resonance of Na+ in NaHCO3 electrolytes. Steeper local pH gradients, compared to experiments with KHCO3, account for increased selectivity for acetate formation from solution-based reaction. Proven itself capable of elucidating the effect of cations on local pH values, our presented method supports tailoring the electrode–electrolyte interface to selectively generate value-added products.

Organic radical batteries (ORBs) represent a viable pathway to a more sustainable energy storage technology compared to conventional Li-ion batteries. For further materials and cell development towards competitive energy and power densities, a deeper understanding of electron transport and conductivity in organic radical polymer cathodes is required. Such electron transport is characterised by electron hopping processes, which depend on the presence of closely spaced hopping sites. Using a combination of electrochemical, electron paramagnetic resonance (EPR) spectroscopic, and theoretical molecular dynamics (MD) as well as density functional theory (DFT) modelling techniques, we explored how compositional characteristics of cross-linked poly(2,2,6,6-tetramethyl-1-piperidinyloxy-4-yl methacrylate) (PTMA) polymers govern electron hopping and rationalise their impact on ORB performance. Electrochemistry and EPR spectroscopy not only show a correlation between capacity and the total number of radicals in an ORB using a PTMA cathode, but also indicates that the state-of-health degrades about twice as fast if the amount of radical is reduced by 15%. The presence of up to 3% free monomer radicals did not improve fast charging capabilities. Pulsed EPR indicated that these radicals readily dissolve into the electrolyte but a direct effect on battery degradation could not be shown. However, a qualitative impact cannot be excluded either. The work further illustrates that nitroxide units have a high affinity to the carbon black conductive additive, indicating the possibility of its participation in electron hopping. At the same time, the polymers attempt to adopt a compact conformation to increase radical–radical contact. Hence, a kinetic competition exists, which might gradually be altered towards a thermodynamically more stable configuration by repeated cycling, yet further investigations are required for its characterisation.

Physical chemistry, chemical physics 25(18), 12767-12776 (2023). doi:10.1039/D3CP00378G, The addition of conductive additives during electrode fabrication is standard practice to mitigate a low intrinsic electronic conductivity of most cathode materials used in Li-ion batteries. To ensure an optimal conduction pathway, these conductive additives, which generally consist of carbon particles,need to be in good contact with the active compounds. Herein, we demonstrate how a combination of pulsed electron paramagnetic resonance (EPR) relaxometry and inverse Laplace transform(ILT) can be used to study such contact. The investigated system consists of PTMA (poly(2,2,6,6–tetramethylpiperidinyloxy–4–ylmethacrylate)) monomer radicals, which is a commonly used redox unit in organic radical batteries (ORB), mixed at different ratios with Super P carbon black (CB) as the conductive additive. Inversion recovery data were acquired to determine longitudinal (T1)relaxation time constant distributions. It was observed that not only the position and relative amplitude, but also the number of relaxation modes varies as the composition of PTMA monomer and CB is changed, thereby justifying the use of ILT instead of fitting with a predetermined number of components. A hypothesis for the origin of different relaxation modes was devised. It suggests that the electrode composition may locally affect the quality of electronic contact between the active material and carbon black, Published by RSC Publ., Cambridge

The use of independent component analysis (ICA) for the analysis of two-dimensional (2D) spin-alignment echo–T 1 7Li NMR correlation data with transient echo detection as a third dimension is demonstrated for the superionic conductor Li10GeP2S12 (LGPS). ICA was combined with Laplace inversion, or discrete inverse Laplace transform (ILT), to obtain spectrally resolved 2D correlation maps. Robust results were obtained with the spectra as well as the vectorized correlation maps as independent components. It was also shown that the order of ICA and ILT steps can be swapped. While performing the ILT step before ICA provided better contrast, a substantial data compression can be achieved if ICA is executed first. Thereby the overall computation time could be reduced by one to two orders of magnitude, since the number of computationally expensive ILT steps is limited to the number of retained independent components. For LGPS, it was demonstrated that physically meaningful independent components and mixing matrices are obtained, which could be correlated with previously investigated material properties yet provided a clearer, better separation of features in the data. LGPS from two different batches was investigated, which showed substantial differences in their spectral and relaxation behavior. While in both cases this could be attributed to ionic mobility, the presented analysis may also clear the way for a more in-depth theoretical analysis based on numerical simulations. The presented method appears to be particularly suitable for samples with at least partially resolved static quadrupolar spectra, such as alkali metal ions in superionic conductors. The good stability of the ICA analysis makes this a prospect algorithm for preprocessing of data for a subsequent automatized analysis using machine learning concepts.

It is of general interest to combine the faradaic processes based high energy density of a battery with the non-faradaic processes based high power density of a capacitor in one cell. Surface area and functional groups of electrode materials strongly affect these properties, but a mechanistic explanation for the interplay between such process and how to balance them is largely missing. For the anode material Li4Ti5O12 (LTO), we suggest a polaron based mechanism that influences Li ion uptake and mobility. Electrolytes containing a lithium salt induce an observable change in the bulk NMR relaxation properties of LTO nano particles. The longitudinal 7Li NMR relaxation time of bulk LTO can change by almost an order of magnitude and, therefore, reacts very sensitively to the cation and its concentration in the surrounding electrolyte. The reversible effect is largely independent of the used anions and of potential anion decomposition products. It is concluded that lithium salt containing electrolytes increase the mobility of surface polarons. These polarons and additional lithium cations from the electrolyte can now diffuse through the bulk, induce the observed enhanced relaxation rate and enable the non-faradaic process. This picture of a Li+ ion equilibrium between electrolyte and solid may help with improving the charging properties of electrode materials.

ACS applied materials & interfaces 13(25), 30187-30197 (2021). doi:10.1021/acsami.1c06771, Published by American Chemical Society, Washington, DC

The electrochemical carbon dioxide reduction on copper attracted considerable attention within the last decade, since Cu is the only elemental transition metal that catalyses the formation of short-chain hydrocarbons and alcohols. Research in this field is mainly focused on understanding the reaction mechanism in terms of adsorbates and intermediates. Furthermore, dynamic changes in the micro-environment of the catalyst, i.e. local pH and [Formula: see text] concentration values, play an equivalently important role in the selectivity of product formation. In this study, we present an in operando [Formula: see text] nuclear magnetic resonance technique that enables the simultaneous measurement of pH and [Formula: see text] concentration in electrode vicinity during electroreduction. The influence of applied potential and buffer capacity of the electrolyte on the formation of formate is demonstrated. Theoretical considerations are confirmed experimentally and the importance of the interplay between catalyst and electrolyte is emphasised.

Advanced energy materials (2022). doi:10.1002/aenm.202200013, Published by Wiley-VCH, Weinheim

The mechanism of reversible lithium intercalation in graphite anodes is still not fully understood. In operando Raman spectroscopy provides a sensitive means to monitor structural changes during the intercalation process. Analysis of the D-band to G-band intensity ratio (D/G ratio) is a common method to study the structure of carbon materials. However, this approach is complicated for the investigation of graphite anodes during battery cycling, as the D-band disappears with the onset of lithium intercalation. To circumvent this issue, the D/G ratio can be replaced by using the G-band full-width-at-half-maximum (FWHM). In this study, an investigation of the G-band FWHM during battery cell cycling is demonstrated as an alternative to monitor the intercalation of lithium into a graphite electrode. It was observed that lithium intercalation already occurs to a small extent during solid–electrolyte interphase (SEI) formation and that the formation of staged intercalation compounds leads to a continuous deformation of the boundary graphene layer.

ChemPhysChem 10, (2021). doi:10.1002/cphc.202100602, Published by Wiley-VCH Verl., Weinheim

The use of independent component analysis (ICA) for the analysis of two-dimensional (2D) spin-alignment echo--T1 7Li NMR correlation data with transient echo detection as a third dimension is demonstrated for the superionic conductor Li10GeP2S12 (LGPS). ICA was combined with Laplace inversion, or discrete inverse Laplace transform (ILT), to obtain spectrally resolved 2D correlation maps. Robust results were obtained with the spectra as well as the vectorized correlation maps as independent components. It was also shown that the order of ICA and ILT steps can be swapped. While performing the ILT step before ICA provided better contrast, a substantial data compression can be achieved if ICA is executed first. Thereby the overall computation time could be reduced by one to two orders of magnitude, since the number of computationally expensive ILT steps is limited to the number of retained independent components. For LGPS, it was demonstrated that physically meaningful independent components and mixing matrices are obtained, which could be correlated with previously investigated material properties yet provided a clearer, better separation of features in the data. LGPS from two different batches was investigated, which showed substantial differences in their spectral and relaxation behavior. While in both cases this could be attributed to ionic mobility, the presented analysis may also clear the way for a more in-depth theoretical analysis based on numerical simulations. The presented method appears to be particularly suitable for samples with at least partially resolved static quadrupolar spectra, such as alkali metal ions in superionic conductors. The good stability of the ICA analysis makes this a prospect algorithm for preprocessing of data for a subsequent automatized analysis using machine learning concepts.

The use of independent component analysis (ICA) for the analysis of two-dimensional (2D) spin-alignment echo--T1 7Li NMR correlation data with transient echo detection as a third dimension is demonstrated for the superionic conductor Li10GeP2S12 (LGPS). ICA was combined with Laplace inversion, or discrete inverse Laplace transform (ILT), to obtain spectrally resolved 2D correlation maps. Robust results were obtained with the spectra as well as the vectorized correlation maps as independent components. It was also shown that the order of ICA and ILT steps can be swapped. While performing the ILT step before ICA provided better contrast, a substantial data compression can be achieved if ICA is executed first. Thereby the overall computation time could be reduced by one to two orders of magnitude, since the number of computationally expensive ILT steps is limited to the number of retained independent components. For LGPS, it was demonstrated that physically meaningful independent components and mixing matrices are obtained, which could be correlated with previously investigated material properties yet provided a clearer, better separation of features in the data. LGPS from two different batches was investigated, which showed substantial differences in their spectral and relaxation behavior. While in both cases this could be attributed to ionic mobility, the presented analysis may also clear the way for a more in-depth theoretical analysis based on numerical simulations. The presented method appears to be particularly suitable for samples with at least partially resolved static quadrupolar spectra, such as alkali metal ions in superionic conductors. The good stability of the ICA analysis makes this a prospect algorithm for preprocessing of data for a subsequent automatized analysis using machine learning concepts.

Lithium microstructure formation is the limiting factor preventing the application of lithium metal anodes and, to some degree, fast-charging of graphite anodes, because of enhanced cell degradation and risks of short-circuits. To improve mechanistic understanding and develop prevention strategies, a complementary analytical tool set is necessary. Contributing to non-invasive in operando techniques, we present pulse electron paramagnetic resonance (pEPR) with a practical sampling rate of 100 microseconds, providing a global view on lithium microstructure formation and evolution.[1] pEPR experiments offer an estimate of the dimension and size distribution of the morphology that is in line with post-test SEM characterization. For lithium metal anodes, the sampling rate is sufficient to observe dynamic morphology changes that evolve for several seconds after fresh lithium deposition, attributed to surface smoothening, comparable to recently observed heating-induced healing of dendrites.[2] For graphite anodes, lithium plating can be followed at C-rates up to 18C. Quantitative measurements reveal an exponential increase of deposited lithium mass upon galvanostatic charge. The plating onset is pinpointed to the characteristic voltage drop well below 0 V vs. Li/Li+.[3] After stopping the charge at intermediate states of charge, the partial intercalation of lithium can be monitored at OCV, identifying two kinetic domains and the amount of remaining dead lithium. [1] C. Szczuka, J. Ackermann, P. Philipp, M. Schleker, P. Jakes, R.-A. Eichel, J. Granwehr, Commun. Mater. 2021, 2, 20. [2] L. Li, S. Basu, Y. Wang, Z. Chen, P. Hundekar, B. Wang, J. Shi, Y. Shi, S. Narayanan, N. Koratkar, Science. 2018, 359, 1513–1516. [3] C. Uhlmann, J. Illig, M. Ender, R. Schuster, E. Ivers-Tiffée, J. Power Sources 2015, 279, 428–438. Figure 1

During operation of a Li-ion battery, layered transition metal oxides release metal ions into the electrolyte. In addition to capacity loss, dissolved metal ions are mobile in the electrolyte and can deposit at the anode, enhancing Faradaic losses and posing the risk of dendrite formation.[1] In the electrolyte solution, transition metal ions are characterized mainly by their oxidation state and complexation behavior, which dictate the transport through the electrolyte. Additionally, mechanistic insights, indirect information about electrolyte degradation, and design of counter-measures might be extracted thereof. However, particularly the coordination sphere is difficult to access, due to inherently low concentrations and scarcity of suitable analytics. In this work, the coordination sphere of dissolved transition metal ions is selectively probed by pulse Electron Paramagnetic Resonance (pEPR). pEPR selectively addresses isolated paramagnetic ions, masking analytical interference from other species in the battery. Because of its known impact on capacity fade, firstly Mn2+ is analyzed in premixed pristine and degraded electrolyte solutions. Carbonates with high dielectric constants such as ethylene carbonate (EC) are the dominant ligands, accompanied by electrolyte salt anions for charge compensation. If present, trace amounts of electrolyte degradation products such as fluorophosphates selectively form the first coordination sphere. The results from pEPR measurements, that have to be performed on frozen solutions at cryogenic temperatures, agree with paramagnetic NMR results at ambient temperatures. Furthermore, samples from batteries operating with vanadium-based cathodes were investigated. Depending on the storage conditions, V4+ ligands predominantly consist of decomposition products from the electrolyte solvent ethylene carbonate (e.g. ethylene glycol anions) or electrolyte salt LiPF6 (fluorophosphates).[2,3] [1] C. Zhan, T. Wu, J. Lu, K. Amine, Energy Environ. Sci. 2018, 11, 243–257. [2] C. Szczuka, R.-A. Eichel, J. Granwehr, ACS Appl. Energy Mater. 2022, 5, 449–460. [3] C. Szczuka, P. Jakes, R.-A. Eichel, J. Granwehr, Adv. Energy Sustain. Res. 2021, 2, 2100121.

Molecules : a journal of synthetic chemistry and natural product chemistry 26(21), 6690 (2021). doi:10.3390/molecules26216690 special issue: "Special Issue "Advances in NMR and MRI of Materials" / Special Issue Editor: Prof. Dr. Igor Ser��a, Guest Editor", Published by MDPI, Basel

This contribution addresses the experimental proof of the relaxation coupling of the 7Li (I = 3/2) longitudinal magnetization orders in the solid-state electrolyte Li10GeP2S12 (LGPS). This effect was theoretically described by Korb and Petit in 1988 but has not yet been shown experimentally. In a 2D-T1/spin-alignment echo (SAE) experiment, the inverse Laplace transformation of the spectral component over two time dimensions revealed the asymmetric course of the spin-lattice relaxation following from the coupling of all longitudinal orders. These observations were supported by Multi-quantum-filter experiments and by simulations of the 2D-T1/SAE experiment with a lithium spin system. Since the asymmetric relaxation effects are directly dependent on the velocities and degrees of freedom of ion motion they could be used especially in fast Li-ion conductors as a separation tool for environments with different mobility processes.

Electron paramagnetic resonance (EPR) spectroscopy was utilized in operando to analyze redox mechanisms in lithium cobalt oxide (LCO) cathode material during battery cycling. In the pristine battery, a considerable amount of paramagnetic Co2+ was detected that was formed during initial contact of the electrolyte with the LCO. The Co2+ got oxidized during the formation cycle. The absence of an EPR signal during subsequent battery cycles, irrespective of the state-of-charge, indicates that no EPR active Co4+ is formed. This suggests the presence of an anionic oxygen redox mechanism in the active material.

The intrinsic ionic nature of room temperature ionic liquids (RTILs) bears the potential to replace classical aqueous electrolytes in electrochemical applications, for example in metal–air batteries. For a systematic adjustment of RTIL properties in porous cathodes, the ionic arrangement under confinement is of prime importance. Using spectrally resolved pulsed gradient stimulated echo nuclear magnetic resonance (PGSTE-NMR) and spin–lattice NMR relaxation time (T1) distributions, the dynamics of 1-methyl-1-propylpyrrolidiniumbis(trifluoromethylsulfonyl)imide ([Pyr13][Tf2N]) confined to carbon black were investigated. A considerable dependence of the [PYR13] mobility on the loading fraction of the carbon black pore space was found. There is evidence for a preferential layering of the RTIL adjacent to the carbon surface and a dependence of the ionic configuration on the local structure of the carbon surface. The inversion efficiency of inversion-recovery T1 data indicates a quasi-stationary layer at the carbon surface with solid-like properties, where the bulk-like properties of the RTIL are adopted as the distance to the surface increases. From the NMR diffusion data an intermediate layer between the quasi-stationary and the bulk-like RTIL is evident. This layer shows a particularly strong pore space loading dependence. While it has an anisotropic, two-dimensional mobility with reduced diffusion perpendicular to the surface at any loading, when it interfaces a gas phase at low loading its mobility is higher than bulk diffusion by up to an order of magnitude and chemical exchange with other layers is low. This layer appears to be of particular importance for the ion exchange between RTIL environments with different spacing from the carbon surface and hence crucial for the overall dynamics of RTILs in the investigated porous environment.

The ongoing electrification of the transportation sector is triggering a continuous search for batteries with higher energy density. One approach to achieve this goal is the transition to lithium-metal anodes. However, the practical implementation brings about severe safety issues such as the dendritic deposition and growth of metallic lithium and the consequent risk of accidental short circuiting of the cell, resulting in a thermal runaway.1 Solid-state electrolytes such as polymers are considered a viable strategy to overcome this issue, especially single-ion conducting polymers, owing to the absence of the detrimental reversed cell polarization, the uniform Li+ flux, and frequently higher limiting current densities.2,3 In fact, the latter is of utmost importance for the fast charging of battery cells as well as the eventual power density. So far, however, the cycling performance at high dis-/charge rates and high current densities for battery cells comprising a polymer electrolyte remained little investigated and commonly non-satisfactory. Here, we report the development of a polysiloxane-based single-ion conducting polymer electrolyte (PSiOM) with a Li+ conductivity exceeding 10−4 S cm−1 at ambient temperature and excellent stability towards both lithium metal and high-energy LiNi0.8Co0.1Mn0.1O2 (NMC811) as the active material for the positive electrode. This new polymer electrolyte enables dendrite-free lithium deposition, thanks to the formation of a suitable electrode|electrolyte interface and interphase and the uniform Li+ flux. Moreover, PSiOM allows for an outstanding capacity retention of, e.g., 90% at 1C and 86% at 2C after 300 cycles and rapid charging and discharging at C rates as high as 5C. It is important to note that we used reasonable active material mass loadings for these tests (>7 mg cm-2), which means that the current densities applied were up to 7.20 mA cm−2 at 40 °C and 2.88 mA cm−2 at 20 °C. To the best of our knowledge, these current densities are among the highest reported so far for polymer-based electrolytes. References (1) Kalhoff, J.; Eshetu, G. G.; Bresser, D.; Passerini, S. Safer Electrolytes for Lithium-Ion Batteries: State of the Art and Perspectives. ChemSusChem 2015, 8 (13), 2154–2175. (2) Zhang, H.; Li, C.; Piszcz, M.; Coya, E.; Rojo, T.; Rodriguez-Martinez, L. M.; Armand, M.; Zhou, Z. Single Lithium-Ion Conducting Solid Polymer Electrolytes: Advances and Perspectives. Chemical Society Reviews 2017, 46 (3), 797–815. (3) Nguyen, H.-D.; Kim, G.-T.; Shi, J.; Paillard, E.; Judeinstein, P.; Lyonnard, S.; Bresser, D.; Iojoiu, C. Nanostructured Multi-Block Copolymer Single-Ion Conductors for Safer High-Performance Lithium Batteries. Energy & Environmental Science 2018, 11 (11), 3298–3309.

We report a trinuclear copper(II) complex, [(DAM)Cu3(μ3-O)][Cl]4 (1, DAM = dodecaaza macrotetracycle), as a homogeneous electrocatalyst for water oxidation to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5. Electrocatalytic water oxidation at pH 7 occurs at an overpotential of 550 mV with a turnover frequency of ∼19 s–1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs NHE confirm the evolution of dioxygen with Faradaic efficiencies of 81% and 45%, respectively. Rinse tests conducted after CPE studies provide evidence for the homogeneous nature of the catalysis. The linear dependence of the current density on the catalyst concentration indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H2O versus D2O) point to involvement of a proton in or preceding the rate-determining step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2 show no evidence of H2O2 formation and support selectivity to form dioxygen. Freeze-quench electron paramagnetic resonance studies during electrolysis provide evidence for the formation of a molecular copper intermediate. Experimental and computational studies support a key role of the phosphate as an acceptor base. Moreover, density functional theory calculations highlight the importance of second-sphere interactions and the role of the nitrogen-based ligands to facilitate proton transfer processes.

In operando NMR spectroscopy is a method for the online investigation of electrochemical systems and reactions. It allows a real-time observation of the formation of products and intermediates, and it grants insight into the interactions of substrate and catalyst. An in operando NMR setup for the investigation of the electrolytic reduction of CO2 on silver electrodes has been developed. The electrolysis cell consists of a three-electrode setup using a working electrode of pristine silver, a chlorinated silver wire as reference electrode, and a graphite counter electrode. The setup can be adjusted for the use of different electrode materials and fits inside a 5 mm NMR tube, Additionally, a shielding setup was employed to minimize noise caused by interference of external radio frequency (RF) waves with the conductive components of the setup. The electrochemical performance of the in operando electrolysis setup was investigated in comparison to a standard CO2 electrolysis cell. The small cell geometry impedes the release of gaseous products, and thus it is primarily suited for current densities below 1 mA/cm2. The effect of conductive components on 13C NMR experiments was studied using a CO2 saturated solution of aqueous bicarbonate electrolyte. Despite the B0 field distortions caused by the electrodes, line widths of ca. 1 Hz could be achieved. This enables the investigation of interactions in the sub-Hertz range by NMR spectroscopy. It was found that the dynamics of the bicarbonate electrolyte change due to interaction with the electrochemical setup, by catalyzing the exchange reaction between CO2 and HCO3− and affecting the formation of an electrical double layer.

Journal of magnetic resonance : JMR 312, 106688 (2020). doi:10.1016/j.jmr.2020.106688, Published by Elsevier, Amsterdam [u.a.]

Journal of the Electrochemical Society 167(8), 086505 (2020). doi:10.1149/1945-7111/ab8ce1, Published by IOP Publishing, Bristol

Li10GeP2S12 (LGPS) is the fastest known Li-ion conductor to date due to the formation of one-dimensional channels with a very high Li mobility. A knowledge-based optimization of such materials for use, for example, as solid electrolyte in all-solid-state batteries requires, however, a more comprehensive understanding of Li ion conduction that considers mobility in all three dimensions, mobility between crystallites and different phases, as well as their distributions within the material. The spin alignment echo (SAE) nuclear magnetic resonance (NMR) technique is suitable to directly probe slow Li ion hops with correlation times down to about 10−5 s, but distinction between hopping time constants and relaxation processes may be ambiguous. This contribution presents the correlation of the 7Li spin lattice relaxation (SLR) time constants (T1) with the SAE decay time constant τc to distinguish between hopping time constants and signal decay limited by relaxation in the τc distribution. A pulse sequence was employed with two independently varied mixing times. The obtained multidimensional time domain data was processed with an algorithm for discrete Laplace inversion that does not use a non-negativity constraint to deliver 2D SLR–SAE correlation maps. Using the full echo transient, it was also possible to estimate the NMR spectrum of the Li ions responsible for each point in the correlation map. The signal components were assigned to different environments in the LGPS structure.

Nitrogen‐containing hydrothermal carbon (N‐HTC) materials of spherical particle morphology were prepared by means of hydrothermal synthesis with glucose and urotropine as precursors. The molar ratio of glucose to urotropine has been varied to achieve a continuous increase in nitrogen content. By raising the ratio of urotropine to glucose, a maximal nitrogen fraction of about 19 wt % could be obtained. Decomposition products of both glucose and urotropine react with each other; this opens up a variety of possible reaction pathways. The pH has a pronounced effect on the reaction pathway of the corresponding reaction steps. For the first time, a comprehensive analytical investigation, comprising a multitude of analytical tools and instruments, of a series of nitrogen‐containing HTC materials was applied. Functional groups and structural motifs identified were analyzed by means of FTIR spectroscopy, thermogravimetric MS, and solid‐state NMR spectroscopy. Information on reaction mechanisms and structural details were obtained by electronic structure calculations that were compared with vibrational spectra of polyfuran or polypyrrole‐like groups, which represent structural motifs occurring in the present samples.

The ability of fast and safe charging is critical for the further success of lithium ion batteries in automotive applications. In state-of-the-art lithium ion batteries, the charging rate is limited by the onset of lithium plating on the graphite anode. Despite its high importance, so far no analytical technique has been available for directly measuring lithium plating during battery charge. Herein, we introduce operando electron paramagnetic resonance (EPR) spectroscopy as the first technique capable of time-resolved and quantitative detection of lithium metal plating in lithium ion batteries. In an exemplary study, the C-rate dependence of lithium metal plating during low-temperature charging at −20 °C is investigated. It is possible to quantify the amount of ‘dead lithium’ and observe the chemical reintercalation of plated lithium metal. In this way, it is possible to deconvolute the coulombic inefficiency of the lithium plating/stripping process and quantify the contributions of both dead lithium and active lithium loss due to solid electrolyte interphase (SEI) formation. The time-resolved and quantitative information accessible with operando EPR spectroscopy will be very useful for the optimization of fast charging procedures, testing of electrolyte additives, and model validation.

Electrochemical impedance spectroscopy of batteries and fuel cells suffers from ambiguity due to overlapping features of different electrochemical processes. One approach to mitigate this problem is the analysis of impedance data by the distribution of relaxation times (DRT) of the underlying physico-chemical processes. In this work an evolution of DRT analysis using generalized Tikhonov regularization with a uniform penalty and an RC kernel is discussed. By conducting the transformation without discriminating or constraining the sign of the distribution and by using both the imaginary and the real part of the RC kernel it can be distinguished between impedance contributions of RQ-like physico-chemical processes, pure ohmic resistances and inductances in the system under investigation. Furthermore, regularization can be simultaneously applied in a second, non-inverted dimension, which affords an increased resolution of the spectrum. The properties of the algorithm are demonstrated by analyzing synthetic impedance data.

In a typical magic-angle spinning (MAS) dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR) experiment, several mechanisms are simultaneously involved when transferring much larger polarization of electron spins to NMR active nuclei of interest. Recently, specific cross-relaxation enhancement by active motions under DNP (SCREAM-DNP) [Daube et al. JACS 2016] has been reported as one of these mechanisms. Thereby

The intrinsic ionic nature of room temperature ionic liquids (RTILs) bears the potential to replace classical aqueous electrolytes in electrochemical applications, for example in metal-air batteries. For a systematic adjustment of RTIL properties in porous cathodes, the ionic arrangement under confinement is of prime importance. Using spectrally resolved pulsed gradient stimulated echo nuclear magnetic resonance (PGSTE-NMR) and spin-lattice NMR relaxation time (T

The demand for compact benchtop NMR systems that can resolve chemical shift differences in the ppm to sub-ppm range is growing. However due to material and size restrictions these magnets are limited in field strength and thus in signal intensity and quality. The implementation of standard hyperpolarization techniques is a next step in an effort to boost the signal. Here we present a compact Overhauser Dynamic Nuclear Polarization (ODNP) setup with a permanent magnet that can resolve

The journal of physical chemistry / C 123(41), 25146-25156 (2019). doi:10.1021/acs.jpcc.9b05323, Published by American Chemical Society, Washington, DC

Journal of the Electrochemical Society 166(3), A5403-A5409 (2019). doi:10.1149/2.0641903jes special issue: "JES Focus Issue of Selected Papers from IMLB 2018", Published by Electrochemical Soc., Pennington, NJ

Physico-chemical processes in batteries are taking place on a broad time scale from fractions of seconds to the order of days [1]. Thus, for an accurate characterization of transport and mobility processes in batteries using electrochemical impedance spectroscopy (EIS), a large frequency range of up to ten decades must be covered. This implies measurement durations on the order of days for low frequency measurements, yielding the risk of distorting electrochemical instabilities in the battery and a considerable change of its state of charge (SOC) due to the probing ac current excitation. It is shown that the SOC change is frequency dependent and with 10-15% of the nominal battery capacity for the sub-millihertz range hardly a small perturbation. Nevertheless, these obstacles can be mitigated by the time domain measurement (TDM) technique [2-4]. TDM is limited to impedance measurements at low frequencies, with a small and frequency-independent SOC change. The combination of TDM and EIS, called time-domain supported electrochemical impedance spectroscopy (TD-EIS), opens up the possibility for a time-efficient implementation of impedance spectroscopy over a large frequency range down to microhertz frequencies [5]. In this work, TD-EIS at varying temperatures in combination with data fitting using an electrical equivalent circuit battery model, is used for the high-accuracy quantification of low-frequency mobility parameters in lithium-ion batteries. It is experimentally demonstrated, for the first time, that the phase of the impedance measurements converges in the sub-millihertz range, which permits a reliable quantification of diffusion kinetics. Moreover, it is shown that with TD-EIS time savings of up to 80% compared to the standard EIS measurement are feasible. From the electrical equivalent circuit battery model fit, an accurate estimate of charge transfer resistance and, in particular, also solid-state diffusion rate are obtained. Both processes follow an Arrhenius law, allowing the determination of activation energies with small variance. The results for the charge transfer process and for the solid-state diffusion process are within the range of literature values measured for similar systems. This work has been published in JECS [6]. References [1] A. Jossen, “Fundamentals of battery dynamics”, Journal of Power Sources 154 (2) (2006) 530-538 [2] S. C. Creason, J. W. Hayes, D. E. Smith, “Fourier transform faradaic admittance measurements - III. Comparison of measurement efficiency for various test signal waveforms”, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 47 (1) (1973) 9-46 [3] J.-S. Yoo, S.-M. Park, “An electrochemical impedance measurement technique employing Fourier transform”, Analytical Chemistry 72 (9) (2000) 2035-2041 [4] D. Klotz, M. Schönleber, J. Schmidt, E. Ivers-Tiffée, “New approach for the calculation of impedance spectra out of time domain data”, Electrochimica Acta 56 (24) (2011) 8763-8769 [5] J. Illig, J. Schmidt, M. Weiss, A. Weber, E. Ivers-Tiffée, “Understanding the impedance spectrum of 18650 LiFePO4-cells”, Journal of Power Sources 239 (2013) 670-679 [6] A. Mertens, I.C. Vinke, H. Tempel, H. Kungl, L.G.J. de Haart, R.-A. Eichel, J. Granwehr, "Quantitative Analysis of Time-Domain Supported Electrochemical Impedance Spectroscopy Data of Li-Ion Batteries: Reliable Activation Energy Determination at Low Frequencies", Journal of the Electrochemical Society 163 (7) (2016) H521-H527 Figure caption: Arrhenius plot of the solid-state diffusion rate obtained from battery model fits of impedance data from three batteries measured at different temperatures. The activation energies are determined from the gradients. Figure 1

The demand for compact benchtop NMR systems that can resolve chemical shift differences in the ppm to sub-ppm range is growing. However due to material and size restrictions these magnets are limited in field strength and thus in signal intensity and quality. The implementation of standard hyperpolarization techniques is a next step in an effort to boost the signal. Here we present a compact Overhauser Dynamic Nuclear Polarization (ODNP) setup with a permanent magnet that can resolve 1H chemical shift differences in the ppm range. The assembly of the setup and its components are described in detail, and the functionality of the setup is demonstrated experimentally with ODNP enhanced relaxation measurements yielding a maximal enhancement of −140 for an aqueous 4-hydroxy-TEMPO solution. Additionally, 1H spectroscopic resolution and significant enhancements are demonstrated on acetic acid as a solvent.

Nafion is the most common material used as a proton exchange membrane in fuel cells. Yet, details of the transport pathways for protons and water in the inner membrane are still under debate. Overhauser Dynamic Nuclear Polarization (ODNP) has proven to be a useful tool for probing hydration dynamics and interactions within 5-8 A of protein and soft material surfaces. Recently it was suggested that ODNP can also be applied to analyze surface water dynamics along Nafion's inner membrane. Here we interrogate the viability of this method for Nafion by carrying out a series of measurements relying on 1H nuclear magnetic resonance (NMR) relaxometry and diffusometry experiments with and without ODNP hyperpolarization, accompanied by other complementary characterization methods including small angle X-ray scattering (SAXS), thermal gravimetric analysis (TGA) of hydration, and proton conductivity by AC impedance spectroscopy. Our comprehensive study shows that commonly used paramagnetic spin probes-here, stable nitroxide radicals-for ODNP, as well as their diamagnetic analogues, reduce the inner membrane surface hydrophilicity, depending on the location and concentration of the spin probe. This heavily reduces the hydration of Nafion, hence increases the tortuosity of the inner membrane morphology and/or increases the activiation barrier for water transport, and consequently impedes water diffusion, transport, and proton conductivity.