The 50 most-cited articles in Journal of The Electrochemical Society, 2018

In this study, the influence of catalyst loading on the performance of a proton exchange membrane (PEM) water electrolyzer is investigated (Nafion 212 membrane; IrO 2 /TiO 2 (anode) and Pt/C (cathode)). Due to the fast kinetics of the hydrogen evolution reaction (HER) on platinum (Pt), the Pt loading on the cathode can be reduced from 0.30 mg Pt cm −2 to 0.025 mg Pt cm −2 without any negative effect on performance. On the anode, the iridium (Ir) loading was varied between 0.20–5.41 mg Ir cm −2 and an optimum in performance at operational current densities (≥1 A cm −2 ) was found for 1–2 mg Ir cm −2 . At higher Ir loadings, the performance decreases at high current densities due to insufficient water transport through the catalyst layer whereas at Ir loadings <0.5 mg Ir cm −2 the catalyst layer becomes inhomogeneous, which leads to a lower electrochemically active area and catalyst utilization, resulting in a significant decrease of performance. To investigate the potential for a large-scale application of PEM water electrolysis, the Ir-specific power density (g Ir kW −1 ) for…

Layered LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) is one of the high-energy positive electrode (cathode) materials for next generation Li-ion batteries. However, compared to the structurally similar LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC111), it can suffer from a shorter lifetime due to its higher surface reactivity. This work studied and compared the formation of surface contaminations on NMC811 and NMC111 when stored under ambient conditions using electrochemical cycling, Raman spectroscopy, and X-ray photoelectron spectroscopy. NMC811 was found to develop a surface layer of up to ∼10 nm thickness that was mostly composed of nickel carbonate species mixed with minor quantities of hydroxide and water after ambient storage for 1 year, while no significant changes were observed on the NMC111 surface. The amount of carbonate species was quantified by gas chromatographic (GC) detection of carbon dioxide generated when the NMC particles were dispersed in hydrochloric acid. Surface impurity species formed on NMC811 upon ambient storage not only lead to a significant delithiation voltage peak in the…

Understanding the factors limiting Li + charge transfer kinetics in Li-ion batteries is essential in improving the rate performance, especially at lower temperatures. The Li + charge transfer process involved in the lithium intercalation of graphite anode includes the step of de-solvation of the solvated Li + in the liquid electrolyte and the step of transport of Li + in the preformed solid electrolyte interphase (SEI) on electrodes until the Li + accepts an electron at the electrode and becomes a Li in the electrode. Whether the de-solvation process or the Li + transport through the SEI is a limiting step depends on the nature of the interphases at the electrode and electrolyte interfaces. Several examples involving the electrode materials such as graphite, lithium titanate (LTO), lithium iron phosphate (LFP), lithium nickel cobalt aluminum oxide (NCA) and solid Li + conductor such as lithium lanthanum titanate or Li-Al-Ti-phosphate are reviewed and discussed to clarify the conditions at which either the de-solvation or the transport of Li + in SEI is dominating and how the…

Hybrid functional materials, constituting both inorganic and organic components, are considered potential platforms for applications in extremely diverse fields such as optics, micro-electronics, transportation, health, energy, energy storage, diagnosis, housing, environment and the highly relevant area is Internet of Things (IoT). Material properties of hybrid materials can be tuned by modification of the composition on the molecular scale to produce smart materials. Cross-cutting approaches, to synergistically couple molecular engineering and processing allows to tailor complex hybrid systems of various shapes with perfect control over size, composition, functionality, and morphology. The detailed description and discussion of variety of hybrid functional organic-inorganic materials and their contribution in the designing of specific modern technologies is the prime focus of this review.

Electrical Impedance of a Pore Wall for the FloodedAgglomerate Model of Porous Gas Diffusion Electrodes T.

Carbon corrosion is an important degradation mechanism that can impair PEMFC performance through the destruction of catalyst connectivity, collapse of the electrode pore structure, loss of hydrophobic character, and an increase of the catalyst particle size. In this study, carbon corrosion was quantified in situ by measurement of carbon dioxide in the fuel cell exhaust gases through non-dispersive infrared spectroscopy during simulated drive cycle operations consisting of potential cycling with varying upper and lower potential limits. These studies were conducted for three different types of carbon supports. A reduction in the catalyst layer thickness was observed during a simulated drive cycle operation with a concomitant decrease in catalyst layer porosity, which led to performance losses due to increased mass transport limitations. The observed thickness reduction was primarily due to compaction of the catalyst layer, with the actual mass of carbon oxidation (loss) contributing only a small fraction (< 20%). The dynamics of carbon corrosion are presented along with a model that…

This study provides a direct comparison of hydrogen crossover in PEM (Nafion 117) and alkaline water electrolysis (Zirfon) at a temperature of applying state-of-the-art separating unit materials. To this end, occurring crossover mechanisms are described first, before experimental data of the anodic hydrogen content are shown in dependence of current density, system pressure and process management strategy. The results suggest that permeation in PEM electrolyzers is mainly governed by diffusion due to a supersaturated concentration of dissolved hydrogen within the catalyst layer, showing a share of 98} of the total permeation flux at and atmospheric pressure. Permeation in alkaline electrolyzers also exhibits a significant influence of supersaturation, but the overall crossover is mainly influenced by mixing the electrolyte cycles, which makes up a share of 90} at and . Generally it becomes evident that hydrogen permeation across the separating unit is more than one order of magnitude smaller in alkaline electrolysis, which is mainly a consequence of the significantly lower hydrogen…

The reduction products of common lithium salts for lithium ion battery electrolytes, LiPF 6 , LiBF 4 , lithium bisoxalato borate (LiBOB), lithium difluorooxalato borate (LiDFOB), and lithium trifluorosulfonylimide (LiTFSI), have been investigated. The solution phase reduction of different lithium salts via reaction with the one electron reducing agent, lithium naphthalenide, results in near quantitative reactions. Analysis of the solution phase and head space gasses suggests that all of the reduction products are precipitated as insoluble solids. The solids obtained through reduction were analyzed with solution NMR, IR-ATR and XPS. All fluorine containing salts generate LiF upon reduction while all oxalate containing salts generate lithium oxalate. In addition, depending upon the salt other species including, Li x PF y O z , Li x BF y , oligomeric borates, and lithium bis[N-(trifluoromethylsulfonylimino)] trifluoromethanesulfonate are observed.

Understanding the effect of electrode-electrolyte interface (EEI) on the kinetics of electrode reaction is critical to design high-energy Li-ion batteries. While electrochemical impedance spectroscopy (EIS) is used widely to examine the kinetics of electrode reaction in Li-ion batteries, ambiguities exist in the physical origin of EIS responses for composite electrodes. In this study, we performed EIS measurement by using a three-electrode cell with a mesh-reference electrode, to avoid the effect of counter electrode impedance and artefactual responses due to asymmetric cell configuration, and composite or oxide-only working electrodes. Here we discuss the detailed assignment of impedance spectra for LiCoO 2 as a function of voltage. The high-frequency semicircle was assigned to the impedance associated with ion adsorption and desorption at the electrified interface while the low-frequency semicircle was related to the charge transfer impedance associated with desolvation/solvation of lithium ions, and lithium ion intercalation/de-intercalation into/from Li x CoO 2 . Exposure to…

Several electrochemical methods have been developed to determine the contribution of different charge storage mechanisms, such as via the electrical double layer and diffusion-limited processes, to electrochemical capacitor behavior. This includes using cyclic voltammetry (CV) data at different sweep rates to obtain the relationship between voltammetric current and sweep rate, and also the relationship between voltammetric charge and sweep rate. Step potential electrochemical spectroscopy (SPECS) also has been used to effectively differentiate between different charge storage mechanisms. Herein we compare these three methods experimentally and also discuss their advantages and limitations toward differentiating between different charge storage mechanisms. These methods have been applied to electrolytic manganese dioxide (EMD) in 0.5 M K2SO4 between 0.0-0.8 V (vs SCE). It was found that in all cases, the specific capacitance was decreased by increasing the sweep rate. The capacitance-sweep rate dependence was only found to be accurate in a short range of low sweep rates. Overall…

Multitude of complications involved with handling hydrogen fuels has propelled many to consider instead an ammonia economy. Ammonia electrooxidation reaction (AOR) herein is discussed as a means for energy application either by electrochemical decomposition for onsite hydrogen generation or by direct employment of ammonia as a fuel in a direct ammonia fuel cell. The development of a robust and stable AOR catalysts are pivotal for both applications, alongside the development of a high-performing HER and ORR catalysts. Examined here are the different operating conditions for both applications and different hurdles to overcome for the catalyst development. The kinetics and mechanism of ammonia electrooxidation are elucidated to provide a foundation to better understand the issues associated with this technology and a comprehensive review on Pt-free catalysts that have been explored for AOR is presented in order to facilitate future research direction in this promising technology.

Carbon dioxide (CO 2 ) electrolysis provides a pathway to close the anthropogenic carbon cycle and store renewable energy, but the stability, selectivity, efficiency and rate of such process needs to be improved. In this paper, we explore the use of Sustainion imidazolium-functionalized membranes and ionomers to improve the performance of that process. Potentiometric runs at a fixed current of 200 mA/cm 2 using Sustainion membranes and ionomers showed that one can maintain 98% selectivity at about 3V applied potential for five months, with a voltage increase of only 3 μV/hour. Other runs showed stable performance at 400 and 600 mA/cm². These results pave the way for commercialization of CO 2 electrolysis, providing a viable pathway to recycle CO 2 back to fuels.

Investigation of Lithium Plating-Stripping Process in Li-Ion Batteries at Low Temperature Using an Electrochemical Model, Ren, Dongsheng, Smith, Kandler, Guo, Dongxu, Han, Xuebing, Feng, Xuning, Lu, Languang, Ouyang, Minggao, Li, Jianqiu

Ni-rich transition metal layered oxide materials are of great interest as positive electrode materials for lithium ion batteries. As the popular electrode materials NMC (LiNi 1-x-y Mn x Co y O 2 ) and NCA (LiNi 1-x-y Co x Al y O 2 ) become more and more Ni-rich, they approach LiNiO 2 . Therefore it is important to benchmark the structure and electrochemistry of state of the art Li x NiO 2 for the convenience of researchers in the field. In this work, LiNiO 2 synthesized from a commercial Ni(OH) 2 precursor and modern synthesis methods shows a specific capacity close to the theoretical specific capacity of 274 mAh/g. In-situ X-ray diffraction (XRD) measurements were conducted to obtain accurate structural information versus lithium content, x. The known multiple phase transitions of Li x NiO 2 during charge and discharge were clearly observed, and the variation in unit cell lattice constants and volume was measured. Differential capacity versus voltage (dQ/dV vs. V) studies were used to investigate the electrochemical properties including regions of composition that show very slow…

Adding esters as co-solvents to Li-ion battery electrolytes can improve low-temperature performance and rate capability of cells. This work uses viscosity and electrolytic conductivity measurements to evaluate electrolytes containing various ester co-solvents, and their suitability for use in high-rate applications is probed. Among the esters studied, methyl acetate (MA) outperforms other esters in its impact on the conductivity and viscosity of the electrolyte. Therefore, viscosity and conductivity were measured as a function of temperature and LiPF 6 concentration for electrolytes ethylene carbonate (EC): linear carbonate: MA in the ratio 30:(70-x):x, where linear carbonate = {ethyl methyl carbonate (EMC), dimethyl carbonate (DMC)}, and x = {0, 10, 20, 30}. Adding MA leads to an increase in conductivity and decrease in viscosity over all conditions. Calculations of electrolyte properties from a model based on a statistical-mechanical framework, the Advanced Electrolyte Model (AEM), are compared to all measurements and excellent agreement is found. All electrolytes studied roughly…

Single crystal LiNi 0.5 Mn 0.3 Co 0.2 O 2 (NMC532) was shown to have superior stability at high voltages and elevated temperatures compared to conventional polycrystalline NMC532 by the authors. Conventional LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) usually offers more capacity than NMC532 when charged to the same upper cutoff voltage so NMC622 is attractive. It is expected that single crystal NMC622 could also provide better performance than typical polycrystalline NMC622 materials. This work explores the synthesis of single crystal LiNi 0.6 Mn 0.2 Co 0.2 O 2 and preferred synthesis conditions were found. A washing and reheating method was used to remove residual lithium carbonate after sintering. The synthesized single crystal NMC622 material worked poorly after the washing-heating treatment without the use of electrolyte additives in the electrolyte. However, with selected additives, single crystal cells outperformed the polycrystalline reference cells in cycling tests. It is our opinion that single crystal NMC622 has a bright future in the Li-ion battery field.

Review—Dynamic Models of Li-Ion Batteries for Diagnosis and Operation: A Review and Perspective, Krewer, Ulrike, Röder, Fridolin, Harinath, Eranda, Braatz, Richard D., Bedürftig, Benjamin, Findeisen, Rolf

For reliable lifetime predictions of lithium-ion batteries, models for cell degradation are required. A comprehensive semi-empirical model based on a reduced set of internal cell parameters and physically justified degradation functions for the capacity loss is developed and presented for a commercial lithium iron phosphate/graphite cell. One calendar and several cycle aging effects are modeled separately. Emphasis is placed on the varying degradation at different temperatures. Degradation mechanisms for cycle aging at high and low temperatures as well as the increased cycling degradation at high state of charge are calculated separately. For parameterization, a lifetime test study is conducted including storage and cycle tests. Additionally, the model is validated through a dynamic current profile based on real-world application in a stationary energy storage system revealing the accuracy. Tests for validation are continued for up to 114 days after the longest parametrization tests. The model error for the cell capacity loss in the application-based tests is at the end of testing…

Battery performance is strongly correlated with electrode microstructural properties. Of the relevant properties, the tortuosity factor of the electrolyte transport paths through microstructure pores is important as it limits battery maximum charge/discharge rate, particularly for energy-dense thick electrodes. Tortuosity factor however, is difficult to precisely measure, and thus its estimation has been debated frequently in the literature. Herein, three independent approaches have been applied to quantify the tortuosity factor of lithium-ion battery electrodes. The first approach is a microstructure model based on three-dimensional geometries from X-ray computed tomography (CT) and stochastic reconstructions enhanced with computationally generated carbon/binder domain (CBD), as CT is often unable to resolve the CBD. The second approach uses a macro-homogeneous model to fit electrochemical data at several rates, providing a separate estimation of the tortuosity factor. The third approach experimentally measures tortuosity factor via symmetric cells employing a blocking electrolyte.…

We present an unbalanced compositionally-symmetric flow cell method for revealing and quantifying different mechanisms for capacity fade in redox flow batteries that are based on molecular energy storage. We utilize it, accompanied in some cases by a corresponding static-cell cycling method, to study capacity fade in cells comprising anthraquinone di-sulfonate, di-hydroxy anthraquinone, iron hexacyanide, methyl viologen, and bis-trimethylammoniopropyl viologen. In all cases the cycling capacity decay is reasonably consistent with exponential in time and is independent of the number of charge-discharge cycles imposed. By introducing pauses at various states of charge of the capacity-limiting side during cycling, we show that in some cases the temporal fade time constant is dependent on the state of charge. These observations suggest that molecular lifetime is dominated by chemical rather than electrochemical mechanisms. These mechanisms include irrecoverable chemical decomposition and recoverable interactions with cell materials. We conclude with recommendations for cell cycling…

The mechanism and kinetics of hydrogen evolution (HER) and oxygen evolution (OER) reactions on nickel iron layered double hydroxide (NiFe LDH) in a basic electrolyte are investigated. The deposited film reported an overpotential of 247 and 245 mV at 10 mA/cm 2 toward the HER and OER, respectively. A least squares procedure was performed to fit a theoretical current density model with experimental linear sweep voltammetry (LSV) results, and the chemical reaction rate constants for the OER and HER steps were identified. Electrochemical impedance spectroscopy (EIS) measurements were taken at different potentials, and the resulting kinetic model demonstrates a good agreement between theoretically calculated faradaic resistance and experimental EIS results. The HER results indicated the Heyrovsky step as rate controlling, with a dependence of reaction mechanism on potential. At low potential, the mechanism begins with a Volmer step, followed by parallel Tafel and Heyrovsky steps. At higher potential, the mechanism becomes consecutive combination of the Volmer and Heyrovsky steps. The OER…

Solid electrolytes potentially enable rechargeable batteries with lithium metal anodes possessing higher energy densities than today's lithium ion batteries. To do so the solid electrolyte must suppress instabilities that lead to poor coulombic efficiency and short circuits. In this work, lithium electrodeposition was performed on single-crystal Li 6 La 3 ZrTaO 12 garnets to investigate factors governing lithium penetration through brittle electrolytes. In single crystals, grain boundaries are excluded as paths for lithium metal propagation. Vickers microindentation was used to introduce surface flaws of known size. However, operando optical microscopy revealed that lithium metal penetration propagates preferentially from a different, second class of flaws. At the perimeter of surface current collectors smaller in size than the lithium source electrode, an enhanced electrodeposition current density causes lithium filled cracks to initiate and grow to penetration, even when large Vickers defects are in proximity. Modeling the electric field distribution in the experimental cell…

The electrochemical oxidation of LiPF 6 -based electrolytes is reported to generate POF 3 gas.In order to enable a quantitative analysis of the LiPF 6 decomposition reactions, we aimed to establish calibration factors for POF 3 and PF 5 in on-line electrochemical mass spectrometry (OEMS).Thermal decomposition of dry LiPF 6 is expected to yield PF 5 , but instead all PF 5 is detected as POF 3 in our OEMS setup, rendering a differentiation of the two gases impossible and presenting an artefact which likely occurs with most on-line mass spectrometry systems due to the high reactivity of PF 5 .However, we can still determine a cumulative calibration factor for POF 3 + PF 5 (referred to as "POF 3 "), which is then used to investigate the evolution of gases during the oxidation of an EC/LiPF 6 electrolyte on a carbon black electrode.Mechanistic experiments with protons or water added to EC/LiPF 6 electrolyte show that protons trigger the formation of PF 5 , while the kinetics for the hydrolysis of LiPF 6 with water at room temperature are too slow to be detectable.These findings let us…

The intermetallic phases in AA7075-T6, including the nm-scale precipitate MgZnCu and 10-μm size Mg 2 Si, S-phase, Al 7 Fe 2 Cu(Mn), and Al 15 (FeMn) 3 (SiCu) 2, were identified and evaluated with respect to chemical composition, structure, and relative nobility.Evaluation of each major intermetallic particle (IMP) after exposure of the alloy to electrolyte allowed assessment of their roles in the localized corrosion.Different corrosion scenarios, including dealloying, trenching, particle etching out/dissolution, localized anodic and cathodic behavior of the IMPs, and corrosion sequences were studied.Combined with previous studies, the results advance the understanding of the electrochemical properties and the associated mechanisms of localized corrosion in aluminum alloys.

The standard format for cylindrical Li-ion cells is about to change from 18650-type cells (18mm diameter, 65mm height) to 21700-type cells (21mm diameter, 70mm height). We investigated the properties of five 18650 cells, three of the first commercially available 21700, and three types of the similar 20700 cells in detail. In particular, the (i) specific energy/energy density and electrode thickness, (ii) electrode area and cell resistance, (iii) specific energy as a function of discharge C-rate, as well as (iv) heating behavior due to current flow are analyzed. Finally, the production effort for cells and packs are roughly estimated for 21700 cells compared to 18650 cells.

There is a growing interest in conversion coatings based on titanium and/or zirconium as the result of the health and environmental issues associated with legacy chromate and phosphate conversion coatings. Any alternative technology should be environmentally friendly and cost effective, and also able to achieve comparable corrosion resistance and paint adhesion for ferrous and non-ferrous substrates. Conversion coatings based on titanium or zirconium seem to fulfill many of these requirements and thus offer a great potential for further applications. This literature review summarizes the scientific results in this rapidly growing area of research. Following the description of composition of conversion bath and deposition mechanism, the effects of process parameters for conversion baths such as pH, temperature, immersion time and agitation are presented together with coating characteristics. The effects of the type of substrate and substrate pre-treatment are explored for the most-studied substrates: Al alloys, zinc-coated steels and steels. Properties such as composition, morphology…

We use on-line electrochemical mass spectrometry (OEMS) to elucidate and quantify the electrolyte reduction on graphite caused by transition metal ions. To have a controlled system, we use ethylene carbonate (EC) with 1.5 M LiPF 6 and representative amounts of Ni(TFSI) 2 or Mn(TFSI) 2 as model electrolytes, combined with a 2-compartment cell in which anolyte and catholyte are separated by an impermeable solid lithium ion conductor. Focusing on C 2 H 4 evolution as a marker for EC reduction, we find that both Ni 2+ and Mn 2+ lead to enhanced gas evolution on pristine graphite electrodes once the potential is decreased to below the TM 2+ /TM 0 redox potential, demonstrating that the reduced transition metals are active toward electrolyte reduction. If the electrodes are preformed in a TM-free electrolyte and subsequently cycled in an electrolyte containing either Mn 2+ or Ni 2+ , the activity of nickel toward electrolyte decomposition is greatly reduced, whereas the electrolyte with manganese still shows a strong ongoing C 2 H 4 generation. The use of vinylene carbonate during…

This article gives a review of surface-enhanced Raman scattering (SERS) sensors for food safety and environmental pollution monitoring.It introduces the basic concepts of SERS substrates, SERS enhancement factors and mechanisms, SERS probes/labels, molecular recognition elements as well as optofluidic SERS devices (SERS sensors integrated with microfluidics).This article places an emphasis on the design strategies of various SERS sensors by utilizing fingerprinting SERS spectra or by using SERS probes.It highlights the applications of SERS sensors in detection of heavy metals (such as Hg 2+ , Pb 2+ , Cd 2+ and As 3+ ), inorganic anions (nitrite, nitrate, perchlorate and fluoride), toxic small molecules (polycyclic aromatic hydrocarbon, polychlorinated biphenyls, herbicides, pesticides, antibiotics and food additives), as well as pathogens (bacteria and viruses).

Thermal runaway is always a troublesome problem that hinders the safe application of high energy lithium-ion batteries. There is an urgent need to interpret the voltage and temperature changes and their underlying mechanisms during thermal runaway, in order to guide the safe design of a battery system. This paper is dedicated to building a coupled electrochemical-thermal model that can well predict the voltage drop and temperature increase during thermal runaway. The model can capture the underlying mechanism of ① the capacity degradation under high temperature; ② the internal short circuit caused by the thermal failure of the separator; and ③ the chemical reactions of the cell components that release heat under extreme temperature. The model is validated using by experimental data, therefore the modeling analysis has high fidelity. We employ the model to analyze 1) the capacity degradation under extreme temperature; 2) the influence of the SEI decomposition and regeneration on the thermal runaway behavior; 3) the heat generation by internal short circuit in the thermal runaway…

Promising cathode materials for Li-ion batteries are layered transition metal oxides (LiNi x Mn y Co z O 2 , NMC). Here, we will investigate the temperature dependence of oxygen release from NMC622, caused by the transformation of the near-surface structure from the layered to spinel and/or rock-salt structure. We will demonstrate that oxygen release is not a potential driven process but occurs once ∼81% of the lithium ions are removed from the NMC structure. Consequently, the onset potential for oxygen release in NMC-graphite cells decreases only by ∼60 mV from 4.42 V at 25°C to 4.36 V at 50°C, which is simply due to lower overpotentials at higher temperature. The amount of evolved oxygen increases significantly with increasing temperature, indicating the formation of thicker spinel/rock-salt surface layers. As the released oxygen causes chemical oxidation of the electrolyte, the amounts of CO 2 and CO occurring simultaneously with O 2 release also increase with temperature. Further experiments in NMC-Li cells as well as with 13 C-labelled ethylene carbonate (EC) electrolyte show…

In this contribution, we investigate the solid electrolyte interface (SEI) layers' composition depending on the spatial location within LiCoO 2 composite cathode of a commercial Li-Ion battery. The surface chemistry is analyzed by X-ray photoelectron spectroscopy (XPS), and possible SEI morphology and the differences in the SEI composition are discussed in detail. Finally, related SEI formation reactions and the controlling processes are characterized as a function of the depth in the composite cathode. The SEI is assumed to be a multi-component, layered system. The inorganic inner SEI layer consists of LiF and degraded LiCoO 2 , confirmed as Co(II,III) x O y (OH) z . The much thicker outer SEI layer is mainly composed of a poly-organic network with a significantly smaller portion of, presumably, randomly distributed macroscopic Li x PO y F z /Li x PO y-1 F z+1 and Li x PO y domains. A higher content of Co(II,III) x O y (OH) z , and especially of the poly-organic deposit, was found on the outer cathode surface compared to the analysis position near the current collector, resulting…

This perspective article formalizes our proposal that the apparent hydrogen binding energy (HBE app ), which takes into account of the water adsorption energy as well as the intrinsic hydrogen binding energy (HBE) on metal surfaces, is the descriptor for hydrogen oxidation/evolution reactions (HOR/HERs). We show that the HBE app , not HBE, is pH dependent and can be determined from the underpotential-deposited hydrogen peaks in cyclic voltammograms for platinum group metals including Pt, Ir, Pd, and Rh, and the decrease of their HOR/HER activities with pH is the result of a strengthened HBE app due to a weakened water adsorption. Also discussed are the needs of further experimental and theoretical work to prove this hypothesis.

In this investigation, it was shown that a probability of thermal runaway in commercial lithium-ion cells of the type 18650 grows with number increase of charge/discharge cycles and increase of cells state of charge (SOC).Notably, experiments in an accelerating rate calorimeter (ARC) showed that with the number growth of cells charge/discharge cycles, it is observed a considerable decline of an initiation temperature of exothermic reactions of thermal runaway and increase of released energy.Additional ARC-experiments with the following analysis of the gas released showed that in a course of cells cycling in anode graphite, hydrogen is accumulated.It was proven in experiments that a recombination of released-from-graphite-anode atomic hydrogen is exactly that powerful exothermic reaction, which increases the released energy in the beginning of the thermal runaway and decreases the temperature of its initiation.Thus, the reason for the initiation of thermal runaway in lithium-ion cells is a powerful exothermic reaction of recombination of atomic hydrogen accumulated in anode graphite…

“Water-in-Salt” for Supercapacitors: A Compromise between Voltage, Power Density, Energy Density and Stability, Lannelongue, Pierre, Bouchal, Roza, Mourad, Eléonore, Bodin, Charlotte, Olarte, Marco, le Vot, Steven, Favier, Frédéric, Fontaine, Olivier

Nickel-rich layered materials are emerging as cathodes of choice for next-generation high energy density lithium ion batteries intended for electric vehicles. This is because of their higher practical capacities compared to compositions with lower Ni content, as well as the potential for lower raw materials cost. The higher practical capacity of these materials comes at the expense of shorter cycle life, however, due to undesirable structure and chemical transformations, especially at particle surfaces. To understand these changes more fully, the charge compensation mechanism and bulk and surface structural changes of LiNi 0.6 Mn 0.2 Co 0.2 O 2 were probed using synchrotron techniques and electron energy loss spectroscopy in this study. In the bulk, both the crystal and electronic structure changes are reversible upon cycling to high voltages, whereas particle surfaces undergo significant reduction and structural reconstruction. While Ni is the major contributor to charge compensation, Co and O (through transition metal-oxygen hybridization) are also redox active. An important…

This study focuses on voltage cycling induced degradation of cathodes with different loading (0.4 and 0.1 mg Pt /cm 2 ) when applying square wave or triangular wave based accelerated stress tests (ASTs) between 0.6 and 1.0 V RHE . The degradation of the H 2 /O 2 and H 2 /air performance upon extended voltage cycling (up to 30000 cycles) was analyzed in terms of the voltage loss contributions from ORR kinetics, O 2 mass transport resistances and proton conduction resistances in the cathode. The extent of cathode thinning due to carbon support corrosion was determined by post mortem electrode thickness measurements. Square waves were found to cause a more rapid loss of ECSA and mass activity compared to triangular waves, which was shown to be due to the longer hold periods at high potentials rather than to the rate of the potential transient. The observed increase of the O 2 mass transport resistance with voltage cycling was found to mainly depend on the available Pt surface area, while mass transport resistances due to carbon corrosion were found to be insignificant. Finally, it was…

The U.S. Department of Energy (DOE) set the 2020 durability target for polymer electrolyte membrane fuel cell transportation applications at 5000 hours. Since it is impractical to test every fuel cell for this length of time, there is ever increasing interest in developing accelerated stress tests (ASTs) that can accurately simulate the material component degradation in the membrane electrode assembly (MEA) observed under automotive operating conditions, but over a much shorter time frame. In this work, a square-wave catalyst AST was examined that shows a 5X time acceleration factor over the triangle-wave catalyst AST and a 25X time acceleration factor over the modified wet drive-cycle catalyst durability protocol, significantly decreasing the testing time. These acceleration factors were correlated to the platinum (Pt) particle size increase and associated decrease in electrochemical surface area (ECSA). This square-wave AST has been adopted by the DOE as a standard protocol to evaluate catalyst durability. We also compare three catalyst-durability protocols using state-of-the-art…

One advantage of Mg batteries, which is often reported in the literature without further qualification such as the electrolyte system, is the high safety due to dendrite-free Mg deposition. Here we report results of a study on the possibility for dendrite growth in Mg metal cells using Mg(TFSI) 2 /glyme electrolytes. Dendrite growth and the subsequent internal short-circuit were proven electrochemically via cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and galvanostatic cycling. In addition, the structure and chemical composition of the globular dendrites were investigated by scanning electrode microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and in situ optical microscopy. The dendrites are structured in interconnected spherical deposits consisting of Mg metal, MgO, MgF 2 , and MgS. The results demonstrate the necessity of considering dendrite growth when developing new electrolyte systems for Mg metal batteries.

Wireless sensor networks have become integral to enabling numerous applications for the Internet of Things, such as environmental sensing or healthcare monitoring. Powering these networks has been the focus of significant research efforts due to the need for reliable and continuous operation of the sensing nodes. In this review, we outline the progress made in the sensing nodes with a particular focus on their power demands. These requirements are used as a framework for surveying the theoretical limits and developments for various power sources, including energy storage, power distribution, and power scavenging techniques. Finally, we conclude by identifying that harvesting techniques are largely insufficient for powering IoT nodes due to limited power densities or inconsistencies as to when power is harvested; accordingly, direct wiring and storage sources such as batteries are the most promising approaches for IoT applications.

Electrode/electrolyte interphase in Li-ion batteries is a critical component that ensures reversible cell reactions far from thermodynamic equilibria. The desired chemical composition and morphology for such interphase, however, remain controversial. Recent advances in battery chemistries and their electrolytes demonstrated that the presence of fluorine in the interphase, either in the form of simple inorganic fluorides (LiF) or organofluoro-moieties, brought positive impacts on the electrochemical performances. Beside the identification of chemical composition, understanding how these fluorinated species exists and distribute in the interphase might constitute the key know-how for the next generation battery chemistries. This is a topic that the battery research community should focus their attention on.

Nafion has found utility in a wide variety of applications, particularly as the most commonly used electrolyte membrane in fuel cell technology. Despite decades of characterization by X-ray Photoelectron Spectroscopy (XPS), a dispute exists within literature over the proper assignment of oxygen binding energies from the ether and sulfonate functional groups present in Nafion. Here, we have employed highly oriented pyrolytic graphite (HOPG) as an internal standard to calibrate all XPS spectra and are able to correlate binding energies from C1s, O1s, F1s and S2p to the Nafion structure. Further, microscale heterogeneities inherent to this formulation of Nafion membranes are revealed through two-dimensional XPS mapping of membrane cross-sections as well as surface ablation via Ar + ion sputtering. Results clearly show Nafion membranes are comprised of two chemically distinct layers: a surface layer several microns thick that is comprised of sulfonate groups and an inner layer that shields the more non-polar perfluoroether moieties.

Tortuosity values of porous battery electrodes determined using electrochemical impedance spectroscopy in symmetric cells with a non-intercalating electrolyte are typically higher than those values based on numerical analysis of 3D tomographic reconstructions. The electrochemical approach assumes that the electronic resistance in the porous coating is negligible and that the tortuosity of the porous electrode can be calculated from the ionic resistance determined by fitting a transmission line equivalent circuit model to the experimental data. In this work, we validate the assumptions behind the electrochemical approach. First, we experimentally and theoretically investigate the influence of the electronic resistance of the porous electrode on the extracted ionic resistances using a general transmission line model, and provide a convenient method to determine whether the electronic resistance is sufficiently low for the model to be correctly applied. Second, using a macroscopic setup with known tortuosity, we prove that the ionic resistance quantified by the transmission line model…

Physico-chemical models are key for a successful use of lithium-ion batteries, especially under extreme conditions.For correctly simulating of the internal battery states and battery aging a suitable set of material properties is needed.This work presents methods to extract these parameters from commercial cells and demonstrates them analyzing a high-power prismatic cell.In a first step, the electrolyte analysis is described, followed by an examination of the active material.The composition as well as the porous structure are measured using optical emission spectroscopy and Hg-porosimetry.To determine the electrochemical properties of the electrode materials, coin cells with lithium as counter electrode are build.With these test cells, open circuit voltage curves and galvanostatic intermittent titration technique measurements are performed to determine the electrode balancing as well as the diffusion constants of the active material.Electrochemical impedance spectroscopy experiments on the full cell are used to determine the charge transfer.In the second part of this paper, a…

This study examines the effect of ink composition on the ionomer distribution in the catalyst layer of membrane electrode assemblies (MEA) prepared by decal transfer. We combine both structural and electrochemical characterization techniques to investigate the influence of the ionomer distribution on MEA performance determined by 50 cm 2 active area single-cell proton exchange membrane (PEM) fuel cell measurements. Cathodic catalyst layers were prepared from inks with different alcohols (1-propanol or 2-propanol) and varying water content (16–65 wt% H 2 O). The H 2 /air performance of cathode catalyst layers prepared from the different inks with 700 EW ionomer differed drastically, particularly under wet operating conditions, whereby the best performance was obtained for an ink based on 16 wt% H 2 O in 1-propanol. This was successfully correlated with the observation of ionomer patches at the cathode electrode surface (i.e., the surface facing the diffusion medium) determined by scanning electron microscopy (SEM), with N 2 adsorption analysis of the electrodes using a QSDFT model,…

The energy storage performance of one of the lightest-known MXenes, Ti 2 CT x (MX) combined with carbon nanospheres (CNS) has been investigated as a symmetric electrode system in an aqueous electrolyte (1 M Li 2 SO 4 ). The energy storage properties were interrogated using cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL), electrochemical impedance spectroscopy (EIS) and voltage-holding tests. The combined material (MX/CNS) demonstrated a higher specific capacity compared to each of the individual components. The material was fabricated with relatively high and low mass loadings, assembled into a symmetric device and performance compared. Specific capacitance, specific power and specific energy for the lower electrode mass loading of 180 F⋅g −1 , 37.6 kW⋅kg −1 and 14.1 W⋅h⋅kg −1 were all higher than 86 F⋅g −1 , 20.1 kW⋅kg −1 and 6.7 W⋅h⋅kg −1 for the higher mass loading. A wide voltage window of 1.5 V was obtained, but with limited long-term cycling behavior, suggesting the need for future improvement. Mathematical modelling and simulation of the…

Further reduction of Pt in hydrogen fuel cells is hampered by reactant transport losses near the catalyst surface, especially for degraded catalysts. Strategically mitigating these performance losses requires an improved understanding of the catalyst nanostructure, which controls local transport and catalyst durability. We apply cryo-tomography in a scanning transmission electron microscope (STEM) to quantify the three-dimensional structure of carbon-supported Pt catalysts and correlate to their electrochemical accessibility. We present results for two carbon supports: Vulcan, a compact support with a large majority of Pt observed on the exterior, and HSC, a porous support with a majority of Pt observed within interior carbon pores, which have relatively constrictive openings. Increasing Pt content shifts the Pt distribution to the exterior on both carbon supports. By correlating to the electrochemical surface area, we find that all Pt surface area is accessible to protons in liquid. However, the interior Pt fraction quantitatively tracks Pt utilization losses at low humidity,…

The electrochemical performance of porous graphite anodes in lithium ion battery applications is limited by the lithium ion concentration gradients in the liquid electrolyte, especially at high current densities and for thick coatings during battery charging. Beside the electrolyte transport parameters, the porosity and the tortuosity of the coating are key parameters that determine the electrode's suitability for high power applications. Here, we investigate the tortuosity of graphite anodes using two water as well as three n-methyl-2-pyrrolidone based binder systems by analysis of symmetric cell impedance measurements, demonstrating that tortuosities ranging from ∼3–10 are obtained for graphite anodes of similar thickness (∼100 μm), porosities (∼50%) and areal capacity (∼3.4 mAh/cm 2 ). Furthermore, selected electrodes with tortuosities of 3.1, 4.3, and 10.2 were cycled in cells with reference electrode at charging C-rates from 0.1-20 1/h, illustrating the clear correlation between electrode tortuosity and its rate capability.

Potentiostatic temperature-scanned experiments were performed on 316L stainless steel and revealed that both the measured values of critical pitting temperature (CPT) and their distribution ranges increased with the decrease of the aggressiveness of exposure conditions. Previous models for CPT cannot explain these observations, so a model for CPT is proposed based on criteria for pit growth stability with added effects of the passive film breakdown. The criterion for pit stability is assumed to be the maintenance of a critical pit solution concentration at the pit surface, which requires the maximum pit dissolution current density to be equal to or greater than the diffusion current density associated with this critical concentration. The resulting model can explain many observations of nature of the CPT. The CPT is shown not to be a single-valued critical value; instead, it is a statistically distributed value based on the detailed behavior of many pitting events. For potentiostatic measurements, the measured CPT is just the temperature at which the first stable pit forms. The…

The framework for pit stability established in Part II of this series is elaborated upon and expanded. The maximum pit dissolution current density, i diff,max , which depends on temperature and maximum potential at the pit surface in a given pit environment, is proposed and compared with the diffusion current density to determine the conditions for pit stability and salt film formation. For an open pit, the criterion for pit stability is that i diss,max must be greater than or equal to the critical diffusion current density required for maintaining a critical local environment for active dissolution, i diff,crit . The critical condition of i diss,max = i diff,crit is associated with three parameters that must be exceeded for stabilization of an open pit: critical temperature T crit , critical potential E crit and critical pit depth r crit . Analogously, the criterion for salt film formation is i diss,max must further exceed the diffusion-limited current density i lim , and three parameters associated with salt film formation are: saturation temperature T sat , saturation potential E…

A combined chemical/mechanical accelerated stress test (AST) was developed for proton exchange membrane (PEM) fuel cells based on relative humidity cycling (RHC) between dry and saturated gases at open circuit voltage (OCV). Membrane degradation and failure were investigated using scanning electron microscopy and small- and wide-angle X-ray scattering. Changes to membrane thickness, hydrophilic domain spacing, and crystallinity were observed to be most similar between field-operated cells and OCV RHC ASTs, where local thinning and divot-type defects are the primary failure modes. While RHC in air also reproduces these failure modes, it is not aggressive enough to differentiate between different membrane types in >1,333 hours (55 days) of testing. Conversely, steady-state OCV tests result in significant ionomer morphology changes and global thinning, which do not replicate field degradation and failure modes. It is inferred that during the OCV RHC AST, the decay of the membrane's mechanical properties is accelerated such that materials can be evaluated in hundreds, instead of…