Impact of electrolyte impurities and SEI composition on battery safety.

Li-ion batteries have a potential risk of thermal runaway. Current safety evaluations in academia and industry rely on experiments or semi-empirical simulations. This limits the understanding of processes leading to or occurring during thermal runaway and how chemical species and impurities can impact them. The limited (quantitative) understanding in turn hinders a holistic safety assessment and optimisation of countermeasures through design or operation. The here presented thermal-runaway model contains a detailed degradation reaction network, which allows the impact of chemical species and impurities on thermal runaway to be studied. We set a particular focus on water impurities and solid-electrolyte interphase (SEI) properties, as both are known to impact life-time of batteries. SEI composition and thickness change during ageing, which is shown here to impact battery safety significantly. The model can reproduce reported experimental behaviour: aged cells are more safe, as they start self-heating, i.e. heat production without an external heat source, at 15-20 °C higher temperatures than fresh cells. Our model suggests a thick inorganic and thus less reactive SEI as the underlying cause. Furthermore, we could show that extensive electrode drying to remove water impurities before building battery cells will not significantly improve safety characteristics. In contrast, electrodes not subjected to any drying procedure cause an earlier start of the self-heating phase, i.e. have a higher risk of thermal runaway. These insights into the sensitivity to thermal runaway allow robust methods to be tailored for its prevention, from controlling battery and SEI properties during production to adjusting safety assessment for effects of ageing.

Knowledge-driven design of solid-electrolyte interphases on lithium metal via multiscale modelling.

Due to its high energy density, lithium metal is a promising electrode for future energy storage. However, its practical capacity, cyclability and safety heavily depend on controlling its reactivity in contact with liquid electrolytes, which leads to the formation of a solid electrolyte interphase (SEI). In particular, there is a lack of fundamental mechanistic understanding of how the electrolyte composition impacts the SEI formation and its governing processes. Here, we present an in-depth model-based analysis of the initial SEI formation on lithium metal in a carbonate-based electrolyte. Thereby we reach for significantly larger length and time scales than comparable molecular dynamic studies. Our multiscale kinetic Monte Carlo/continuum model shows a layered, mostly inorganic SEI consisting of LiF on top of Li2CO3 and Li after 1 µs. Its formation is traced back to a complex interplay of various electrolyte and salt decomposition processes. We further reveal that low local Li+ concentrations result in a more mosaic-like, partly organic SEI and that a faster passivation of the lithium metal surface can be achieved by increasing the salt concentration. Based on this we suggest design strategies for SEI on lithium metal and make an important step towards knowledge-driven SEI engineering.

Microkinetic Analysis of the Oxygen Evolution Performance at Different Stages of Iridium Oxide Degradation.

The microkinetics of the electrocatalytic oxygen evolution reaction substantially determines the performance in proton-exchange membrane water electrolysis. State-of-the-art nanoparticulated rutile IrO2 electrocatalysts present an excellent trade-off between activity and stability due to the efficient formation of intermediate surface species. To reveal and analyze the interaction of individual surface processes, a detailed dynamic microkinetic model approach is established and validated using cyclic voltammetry. We show that the interaction of three different processes, which are the adsorption of water, one potential-driven deprotonation step, and the detachment of oxygen, limits the overall reaction turnover. During the reaction, the active IrO2 surface is covered mainly by *O, *OOH, and *OO adsorbed species with a share dependent on the applied potential and of 44, 28, and 20% at an overpotential of 350 mV, respectively. In contrast to state-of-the-art calculations of ideal catalyst surfaces, this novel model-based methodology allows for experimental identification of the microkinetics as well as thermodynamic energy values of real pristine and degraded nanoparticles. We show that the loss in electrocatalytic activity during degradation is correlated to an increase in the activation energy of deprotonation processes, whereas reaction energies were marginally affected. As the effect of electrolyte-related parameters does not cause such a decrease, the model-based analysis demonstrates that material changes trigger the performance loss. These insights into the degradation of IrO2 and its effect on the surface processes provide the basis for a deeper understanding of degrading active sites for the optimization of the oxygen evolution performance.

Exploring the Functional Properties of Sodium Phytate Doped Polyaniline Nanofibers Modified FTO Electrodes for High-Performance Binder Free Symmetric Supercapacitors.

The performance of high-rate supercapacitors requires fine morphological and electrical properties of the electrode. Polyaniline (PANI), as one of the most promising materials for energy storage, shows different behaviour on different substrates. The present study reports on the surface modification of fluorine doped tin oxide (FTO) with the sodium phytate doped PANI without any binder and its utilization as a novel current collector in symmetric supercapacitor devices. The electrochemical behaviour of the sodium phytate doped PANI thin film with and without a binder on fluorine doped tin oxide (FTO) as current collector was investigated by cyclic voltammetry (CV). The electrode without a binder showed higher electrocatalytic efficiency. A symmetrical cell configuration was therefore constructed with the binder-free electrodes. The device showed excellent electrochemical performance with high specific capacities of 550 Fg-1 at 1 Ag-1 and 355 Fg-1 at 40 Ag-1 calculated from galvanostatic discharge curves. The low charge transfer and solution resistances (RCT and RS) of 7.86 Ωcm² and 3.58 × 10-1 Ωcm², respectively, and superior rate capability of 66.9% over a wide current density range of 1 Ag-1 to 40 Ag-1 and excellent cycling stability with 90% of the original capacity over 1000 charge/discharge cycles at 40 Ag-1, indicated it to be an efficient energy storage device. Moreover, the gravimetric energy and power density of the supercapacitor was remarkably high, providing 73.8 Whkg-1 at 500 Wkg-1, respectively. The gravimetric energy density remained stable as the power density increased. It even reached up to 49.4 Whkg-1 at a power density of up to 20 Wkg-1.

The Effects of Gas Saturation of Electrolytes on the Performance and Durability of Lithium-Ion Batteries.

Traces of species in batteries are known to impact battery performance. The effects of gas species, although often reported in the electrolyte and evolving during operation, have not been systematically studied to date and are therefore barely understood. This study reveals and compares the effects of different gases on the charge-discharge characteristics, cycling stability and impedances of lithium-ion batteries. All investigated gases have been previously reported in lithium-ion batteries and are thus worth investigating: Ar, CO2 , CO, C2 H4 , C2 H2 , H2 , CH4 and O2 . Gas-electrolyte composition has a significant influence on formation, coulombic and energy efficiencies, C-rate capability, and aging. Particularly, CO2 and O2 showed a higher C-rate capability and a decrease in irreversible capacity loss during the first cycle compared to Ar. Similar discharge capacities and aging behaviors are observed for CO, C2 H4 and CH4 . Acetylene showed a large decrease in performance and cycle stability. Furthermore, electrochemical impedance spectroscopy revealed that the gases mainly contribute to changes in charge transfer processes, whereas the effects on resistance and solid electrolyte interphase performance were minor. Compared to all other gas-electrolyte mixtures, the use of CO2 saturated electrolyte showed a remarkable increase in all performance parameters including lifetime.

3D Polyaniline Nanofibers Anchored on Carbon Paper for High-Performance and Light-Weight Supercapacitors.

In the field of advanced energy storage, nanostructured Polyaniline (PANI) based materials hold a special place. Extensive studies have been done on the application of PANI in supercapacitors, however, the structure-property relationship of these materials is still not understood. This paper presents a detailed characterization of the novel sodium phytate doped 3D PANI nanofibers anchored on different types of carbon paper for application in supercapacitors. An excellent relationship between the structures and properties of the synthesized samples was found. Remarkable energy storage characteristics with low values of solution, charge transfer and polarization resistance and a specific capacitance of 1106.9 ± 1.5 F g-1 and 779 ± 2.6 F g-1 at current density 0.5 and 10 Ag-1, respectively, was achieved at optimized conditions. The symmetric supercapacitor assembly showed significant enhancement in both energy density and power density. It delivered an energy density of 95 Wh kg-1 at a power of 846 W kg-1. At a high-power density of 16.9 kW kg-1, the energy density can still be kept at 13 Wh kg-1. Cyclic stability was also checked for 1000 cycles at a current density of 10 Ag-1 having excellent retention, i.e., 96%.

An Amazingly Simple, Fast and Green Synthesis Route to Polyaniline Nanofibers for Efficient Energy Storage.

The major drawbacks of the conventional methods for preparing polyaniline (PANI) are the large consumptions of toxic chemicals and long process durations. This paper presents a remarkably simple and green route for the chemical oxidative synthesis of PANI nanofibers, utilizing sodium phytate as a novel and environmentally friendly plant derived dopant. The process shows a remarkable reduction in the synthesis time and usage of toxic chemicals with good dispersibility and exceedingly high conductivity up to 10 S cm-1 of the resulting PANI at the same time. A detailed characterization of the PANI samples has been made showing excellent relationships between their structure and properties. Particularly, the electrochemical properties of the synthesized PANI as electrode material for supercapacitors were analyzed. The PANI sample, synthesized at pre-optimized conditions, exhibited impressive supercapacitor performance having a high specific capacitance (Csp) (832.5 Fg-1 and 528 Fg-1 at 1 Ag-1 and 40 Ag-1, respectively) as calculated from galvanostatic charge/discharge (GCD) curves. A good rate capability with a capacitance retention of 67.6% of its initial value was observed. The quite low solution resistance (Rs) value of 281.0 × 10-3 Ohm and charge transfer resistance value (Rct) of 7.44 Ohm represents the excellence of the material. Further, a retention of 95.3% in coulombic efficiency after 1000 charge discharge cycles, without showing any significant degradation of the material, was also exhibited.

Practical ex-Situ Technique To Measure the Chemical Stability of Anion-Exchange Membranes under Conditions Simulating the Fuel Cell Environment.

Anion-exchange membrane (AEM) degradation during fuel cell operation represents the main challenge that hampers the implementation of AEM fuel cells (AEMFCs). Reported degradation values of AEMs are difficult to reproduce as no standard methods are used. The present use of different techniques based on exposure of membranes to aqueous KOH solutions under different conditions and measuring different outputs during time does not allow for a reliable and meaningful comparison of reported degradation data of different AEMs. In this study, we present a practical and reproducible ex-situ technique to measure AEM degradation in conditions that mimic an operando fuel cell environment. In this novel technique, we measure the change of the true hydroxide conductivity of the AEM over time, while exposing it to different relative humidity conditions. The technique does not make use of liquid alkaline solution, thus simulating real fuel cell conditions and providing a good baseline for comparative degradation studies.

Origin of the Drastic Current Decay during Potentiostatic Alkaline Methanol Oxidation.

The current production from the alkaline methanol electro-oxidation reaction does not reach a steady state on a smooth platinum catalyst under potentiostatic conditions. We investigated two possible explanations for this phenomenon: changes on the catalyst surface and changes in the solution near the electrode. In situ Fourier transform infrared spectroscopy experiments were conducted to evaluate the adsorbed species on the catalyst surface and a simulation model was set up to describe the changes of concentrations inside the solution. Linear- and bridge-bonded carbon monoxide are the only organic compounds which can be detected by in situ spectroscopy at fixed potentials, but their amount does not increase over time. The simulation shows that the consumption of hydroxide ions and production of carbonaceous species during alkaline oxidation causes a local pH shift near the catalyst surface. Assuming a one-electron transfer as the limiting step, this pH shift was found to contribute to the observed current loss at a potential of 0.77 V.

The origin of the hysteresis in cyclic voltammetric response of alkaline methanol electrooxidation.

The mechanism of the alkaline methanol electrooxidation reaction on platinum is complex and not fully understood. However, a better understanding will facilitate reaching the theoretical performance of an alkaline methanol fuel cell. Cyclic voltammetry is an often used method to investigate the mechanism of electrochemical reactions. The cyclic voltammogram of methanol electrooxidation shows a hysteresis in potential between the oxidation peaks in the forward and backward scans. The origin of this hysteresis has not been fully clarified. By means of parameter variations, such as the upper potential or the starting point of the CV measurements, and by physicochemical modeling, we investigate the formation of platinum oxides as a possible cause of this behaviour. The experiments show that the formation of platinum oxides is more likely to cause the hysteresis than the strongly adsorbed intermediates, which are formed during the forward scan. The simulation includes the formation of platinum oxide species and supports the experimental results that the hysteresis is due to the formation and reduction of platinum oxides. Besides, the simulation reveals that in the investigated potential area, different oxide forms are present.

Study on Direct Synthesis of Energy Efficient Multifunctional Polyaniline-Graphene Oxide Nanocomposite and Its Application in Aqueous Symmetric Supercapacitor Devices.

The synthesis of promising nanocomposite materials can always be tricky and depends a lot on the method of synthesis itself. Developing such synthesis routes, which are not only simple but also can effectively catch up the synergy of the compositing material, is definitely a worthy contribution towards nanomaterial science. Carbon-based materials, such as graphene oxide, and conjugative polymers, such as conductive polyaniline, are considered materials of the 21st century. This study involves a simple one pot synthesis route for obtaining a nanocomposite of polyaniline and graphene oxide with synergistic effects. The study was carried out in a systematic way by gradually changing the composition of the ingredients in the reaction bath until the formation of nanocomposite took place at some particular reaction parameters. These nanocomposites were then utilized for the fabrication of electrodes for aqueous symmetric supercapacitor devices utilizing gold or copper as current collectors. The device manifested a good capacitance value of 264 F/g at 1 A/g, magnificent rate performance, and capacitance retention of 84.09% at a high current density (10 A/g) when gold sheet electrodes were used as the current collectors. It also showed a capacitance retention of 79.83% and columbic efficiency of 99.83% after 2000 cycles.

Processes and Their Limitations in Oxygen Depolarized Cathodes: A Dynamic Model-Based Analysis.

Oxygen depolarized cathodes (ODCs) are key components of alkaline fuel cells and metal-air batteries or of chlor-alkaline electrolysis, but suffer from limited oxygen availability at the reaction zone. Dynamic analysis is a highly suitable approach to identify the underlying causes, especially the limiting steps and process interactions in such gas diffusion electrodes. Herein, a one-dimensional, dynamic, three-phase model for analyzing the oxygen reduction reaction in silver-based ODCs is presented. It allows for a detailed evaluation of the electrochemical reaction, the mass transport processes, and their interaction. The model also reveals that the depletion of reactant oxygen in the liquid electrolyte is caused by the current-dependent change of the gas-liquid equilibrium as the limiting subprocess. The phase equilibrium, in turn, depends on the slow mass transport of water and hydroxide ions in the liquid phase. Key parameters are the location or size of the gas-liquid interface within the electrode. Profiles of local concentrations and partial pressures of different species reveal a steep gradient of oxygen in the liquid phase, but no limitation in oxygen mass transport in the gas phase. Dynamic simulations with potential steps allowed the identification of different time constants to separate overlapping processes. Accordingly, the mass transport of water and hydroxide ions was identified as the slowest process that strongly influenced the dynamic response of all species, including oxygen, and of the current. The characteristic time constant of the mass transport of water and hydroxide ions across the liquid phase within the ODC is determined to be τ mt , H 2 O ≈0.176 s, whereas the time constant of oxygen mass transport into the reaction zone is several magnitudes smaller: τ mt , O 2 ≈1.70×10-6  s. Finally, a sensitivity analysis confirms that the overall performance is best improved by adjusting mass transport properties in the liquid phase.

A systematic reactor design approach for the synthesis of active pharmaceutical ingredients.

Today's highly competitive pharmaceutical industry is in dire need of an accelerated transition from the drug development phase to the drug production phase. At the heart of this transition are chemical reactors that facilitate the synthesis of active pharmaceutical ingredients (APIs) and whose design can affect subsequent processing steps. Inspired by this challenge, we present a model-based approach for systematic reactor design. The proposed concept is based on the elementary process functions (EPF) methodology to select an optimal reactor configuration from existing state-of-the-art reactor types or can possibly lead to the design of novel reactors. As a conceptual study, this work summarizes the essential steps in adapting the EPF approach to optimal reactor design problems in the field of API syntheses. Practically, the nucleophilic aromatic substitution of 2,4-difluoronitrobenzene was analyzed as a case study of pharmaceutical relevance. Here, a small-scale tubular coil reactor with controlled heating was identified as the optimal set-up reducing the residence time by 33% in comparison to literature values.

Operating envelope of Haber-Bosch process design for power-to-ammonia.

The power-to-ammonia concept allows for the production of ammonia, one of the most produced inorganic chemicals, from air, water and (renewable) electricity. However, power-to-ammonia requires flexible operation for use with a directly intermittent renewable energy supply. In this paper, we systematically analyse the operating envelope for steady-state operation of the three bed autothermic Haber-Bosch reactor system for power-to-ammonia by pseudo-homogeneous model. Operational flexibilities of process variables, hydrogen intake and ammonia production flexibilities are analysed, along with maximum and minimum possible changes in recycle load and recycle to feed ratio for the following process variables: reactor pressure, inert gas percentage in synthesis loop, NH3 concentration, H2-to-N2 ratio, total flow rate and feed temperature. Among the six process variables, inert gas fraction and H2-to-N2 ratio provided very high flexibilities, ca. 255% operational flexibility for Ar, up to 51 to 67% flexibility in hydrogen intake, and up to 73% reduction and 24% enhancement in ammonia production. However, a decrease in ammonia production by H2-to-N2 ratio significantly increases recycle load. Besides inert gas fraction and H2-to-N2 ratio, the total mass feed flow rate is also significant for minimum hydrogen intake and ammonia production.

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions.

In the future, (electro-)chemical catalysts will have to be more tolerant towards a varying supply of energy and raw materials. This is mainly due to the fluctuating nature of renewable energies. For example, power-to-chemical processes require a shift from steady-state operation towards operation under dynamic reaction conditions. This brings along a number of demands for the design of both catalysts and reactors, because it is well-known that the structure of catalysts is very dynamic. However, in-depth studies of catalysts and catalytic reactors under such transient conditions have only started recently. This requires studies and advances in the fields of 1) operando spectroscopy including time-resolved methods, 2) theory with predictive quality, 3) kinetic modelling, 4) design of catalysts by appropriate preparation concepts, and 5) novel/modular reactor designs. An intensive exchange between these scientific disciplines will enable a substantial gain of fundamental knowledge which is urgently required. This concept article highlights recent developments, challenges, and future directions for understanding catalysts under dynamic reaction conditions.

In operando monitoring of the state of charge and species distribution in zinc air batteries using X-ray tomography and model-based simulations.

A novel combination of in operando X-ray tomography and model-based analysis of zinc air batteries is introduced. Using this approach the correlation between the three-dimensional morphological properties of the electrode - on the one hand - and the electrochemical properties of the battery - on the other hand is revealed. In detail, chemical dissolution of zinc particles and the electrode volume were investigated non-destructively during battery operation by X-ray tomography (applying a spatial resolution of 9 µm), while simulation yielded cell potentials of each electrode and allows for the prediction of long-term operation behavior. Furthermore, the analysis of individual zinc particles revealed an electrochemical dissolution process that can be explained using an adapted shrinking-core model.

Electrochemical oxidation of carbon-containing fuels and their dynamics in low-temperature fuel cells.

Fuel cells can convert the energy that is chemically stored in a compound into electrical energy with high efficiency. Hydrogen could be the first choice for chemical energy storage, but its utilization is limited due to storage and transport difficulties. Carbon-containing fuels store chemical energy with significantly higher energy density, which makes them excellent energy carriers. The electro-oxidation of carbon-containing fuels without prior reforming is a more challenging and complex process than anodic hydrogen oxidation. The current understanding of the direct electro-oxidation of carbon-containing fuels in low-temperature fuel cells is reviewed. Furthermore, this review covers various aspects of electro-oxidation for carbon-containing fuels in non-steady-state reaction conditions. Such dynamic investigations open possibilities to elucidate detailed reaction kinetics, to sense fuel concentration, or to diagnose the fuel-cell state during operation. Motivated by the challenge to decrease the consumption of fossil fuel, the production routes of the fuels from renewable resources also are reviewed.