Development of a Simple and Rapid Diagnostic Method for Polymer-Electrolyte Fuel Cells J. Electrochem. Soc. (IF 3.259) Pub Date : 2018-01-13 Lalit M. Pant, Zhiwei Yang, Michael L. Perry, Adam Z. Weber
A simple and fast diagnostic tool has been developed for analyzing polymer-electrolyte fuel-cell degradation. The tool is based on analyzing changes in polarization curves of a cell over its lifetime. The shape of the polarization-change curve and its sensitivity to oxygen concentration are found to be unique for each degradation pathway based on analysis from a detailed 2-D numerical model of the cell. Using the polarization-change curve methodology, the primary mechanism for degradation (kinetic, ohmic, and/or transport related) can be identified. The technique is applied to two sets of data to explain performance changes after two different cells undergo voltage-cycling accelerated stress test, where it is found that changes are kinetic and then ohmic or transport in nature depending on the cell type. The diagnostic tool provides a simple method for rapid determination of primary degradation mechanisms. Areas for more detailed future investigations are also summarized.
Unsupported Pt3Ni Aerogels as Corrosion Resistant PEFC Anode Catalysts under Gross Fuel Starvation Conditions J. Electrochem. Soc. (IF 3.259) Pub Date : 2018-01-06 Sebastian Henning, Ryo Shimizu, Juan Herranz, Laura Kühn, Alexander Eychmüller, Makoto Uchida, Katsuyoshi Kakinuma, Thomas J. Schmidt
Mitigating catalyst corrosion is crucial for the commercial success of polymer electrolyte fuel cells (PEFCs). Novel catalysts that can withstand the harsh conditions in case of gross fuel (i.e. H2) starvation events at the PEFC anode are needed to increase the fuel cell stack's service life and to meet the durability targets set for automotive applications. To make progress in this respect, we have tested an unsupported, bimetallic Pt3Ni alloy (aerogel) catalyst at the PEFC anode and subjected it to a stress test that mimics the high potentials (≥ 1.5 V vs. the reversible hydrogen electrode) encountered upon fuel starvation. In contrast to commercial carbon-supported platinum catalysts (Pt/C), the Pt3Ni aerogel displays excellent durability and performance retention in end-of-life fuel cell polarization curves. Additionally, the aerogel catalyst shows ≈35% higher surface-specific activity for the hydrogen oxidation/evolution reaction than Pt/C. These results highlight the great potential of using novel unsupported catalysts at the anode of PEFCs.
Electrochemical Microscopy Based on Spatial Light Modulators: A Projection System to Spatially Address Electrochemical Reactions at Semiconductors J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-30 Yan B. Vogel, Vinicius R. Gonçales, J. Justin Gooding, Simone Ciampi
Here we describe a “light projector” system that can address, i.e. “read and write” electrochemical reactions on a non-structured macroscopic semiconducting electrode with spatial and temporal resolution. In our approach the illumination of an amorphous silicon electrode/electrolyte interface is spatially defined by means of a ferroelectric micromirror system that gives total freedom on both the two-dimensional light profile (illumination shapes) as well as on the transient times of the projected images. The device has no moving parts and allows for spatial and temporal control of the illumination stimulus driving local changes to the rate of an electrochemical reaction. The performance of the system is assessed by generating microscale patterns of Cu2O on the electrode (“electrochemical writing”) followed by their 2D current mapping (“electrochemical reading”) using methanol electro-oxidation and carbon dioxide electro-reduction. The latter illustrate the electrochemical imaging aspects of the device using two technologically relevant examples.
Study of Cu Film Surface Treatment Using Formic Acid Vapor/Solution for Low Temperature Bonding J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-28 Wenhua Yang, Yangting Lu, Chenggong Zhou, Jian Zhang, Tadatomo Suga
In this paper, the surface of Cu film was treated with formic acid vapor and solution for Cu low temperature bonding. After formic acid vapor/solution treatment, Cu film surface oxide was reduced, and surface became rougher. For formic acid vapor treatment, with the increase of treatment time, Cu film surface was reduced gradually at 200°C, surface roughness and particle size became bigger. For formic acid solution treatment, the reduction of the surface of the copper film was the most abundant when the solution concentration is 50%. After the treatment with formic acid solution, the surface roughness and particle size of copper film increase significantly. As the formic acid solution on the surface of copper film has a certain role in corrosion, using formic acid solution treatment, better surface reduction and rougher surface for Cu film were obtained. By contrast, using formic acid vapor treatment, surface reduction is not so good, but surface is smoother than that treated by formic acid solution. Cu/Cu direct bonding was realized at 200°C after formic acid vapor/solution treatment. Using formic acid vapor treatment, Cu/Cu bond strength is about 18.2 MPa, which is higher than that using formic acid vapor treatment.
Role of Hydrogen Evolution during Epitaxial Electrodeposition of Fe on GaAs J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-20 Karin Leistner, Kenny Duschek, Jonas Zehner, Mingze Yang, Andreas Petr, Kornelius Nielsch, Karen L. Kavanagh
The electrode reactions during the initial stages of Fe electrodeposition on GaAs from a sulfate-based aqueous electrolyte, were investigated. Electrochemical quartz microbalance measurements were carried out to distinguish hydrogen evolution from Fe deposition. For conditions with a lower hydrogen evolution rate, hemispherical Fe nanoparticles with negligible in-plane magnetic anisotropy are obtained. In contrast, when hydrogen evolution dominates over Fe electrodeposition, the deposited nanoparticles exhibit a defined faceted shape, crystallographic alignment and magnetic in-plane anisotropy. This beneficial impact of hydrogen evolution on the epitaxy is discussed with regard to the role of hydrogen adsorption during Fe/GaAs interface formation.
The Application of Heterostructured SrTiO3-TiO2 Nanotube Arrays in Dye-Sensitized Solar Cells J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-19 Qiong Sun, Yong Hong, Tao Zang, Qiuhong Liu, Liyan Yu, Lifeng Dong
Heterostructured SrTiO3-TiO2 nanotube arrays (ST-TNTAs) on fluoride doped SnO2 conductive glass (FTO) were synthesized through a three-step in-situ hydrothermal reaction. TiO2 nanotubes in ST-TNTAs vertically grew on the FTO substrate in single crystallized rutile phase, while SrTiO3 grains in cubic perovskite phase dispersed evenly on the surface of TiO2. The sandwich shaped all-solid-state dye-sensitized solar cells (DSSCs) were assembled with TiO2 nanorods, nanotubes and ST-TNTAs as the photoanode, respectively. Once SrTiO3 deposited, the position of Fermi level of the composited semiconductor raised, resulting in the increase of open circuit voltage (VOC). Meanwhile, both short-circuit current density (JSC) and photoelectrical conversion efficiency (η) increased first and then decreased with the amount of SrTiO3. In comparison to TiO2 nanorods and nanotubes, ST-TNTAs demonstrated the highest photoelectrical conversion efficiency (5.42%) under the irradiation of solar simulator and external quantum efficiency (EQE) at visible region, and also the lowest electron transfer resistance, which further proved that SrTiO3 acted as a good medium for electron transfer between TiO2 and photosensitizer. As a result, both the increased surface area of the nanotube relative to the nanorod and the matched bandgap structure in the composited structure of TiO2 and SrTiO3 improve the performance of the DSSCs.
Controlled and Selective Growth of 1D and 3D CdTe Nanostructures through a Structurally Engineered Porous Alumina Template for Enhanced Optical Applications J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-15 Harsimran Singh Bindra, Subish John, Somnath C. Roy, Om Prakash Sinha, S. S. Islam, Ranu Nayak
Current manuscript describes porous alumina (PA) template assisted electrodeposition of high aspect ratio nanowires and dense hierarchical structures of CdTe. We demonstrate here for the first time that simple structural engineering of a PA template can lead to electrochemical growth of diverse shapes of CdTe nanostructures. Facile and cost-effective modifications have been implemented for the fabrication of self-organized through-hole PA membrane and its transfer onto any rough substrate. These modifications have facilitated extended duration (30 minute to 1 hour) electrodeposition of CdTe nanostructures at high bath temperature of 60°C without delaminating the PA membrane. High aspect ratio nanowires of 60 nm diameter and 2.8 μm length were growth through the self-ordered PA membrane without any underlying metal coating i.e. without altering its optical properties. An average of 56% optical absorption (within 350 nm – 1400 nm wavelength) and a moderate photoluminescence was observed for the CdTe nanowires. Minor variation in the anodization process resulted into a non-uniform/branched PA template that enabled the formation of dense 3D hierarchical structures of CdTe using similar electrodeposition conditions as that used for CdTe nanowires. The hierarchical CdTe nanostructures exhibited very high total optical absorption of ∼90% within 350 nm – 1400 nm wavelength and a strong photoluminescence was also demonstrated that was almost 10 fold more intense than the CdTe nanowires.
Rapid Thermal Processed CuInSe2 Layers Prepared by Electrochemical Route for Photovoltaic Applications J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-08 Ashwini B. Rohom, Priyanka U. Londhe, Nandu B. Chaure
Impact of rapid thermal (RT) annealing and normal selenization process on the properties of CuInSe2 (CIS) layers prepared by electrochemical route is reported. Cyclic voltammetric measurement was carried out to optimize the co-deposition potentials. A range of characterization techniques were employed to study the properties. Three prominent reflections(112),(204/220) and (312/116) of tetragonal CIS were exhibited in as-deposited CIS layers. Upon selenization, the crystallinity was found to be improved. Uniform, compact, densely packed surface morphology was observed in as-prepared sample. Large grains are developed upon RT annealing due to recrystallization. Elemental composition obtained by EDAX confirms the growth of stoichiometric layers. Photo-electrochemical study demonstrates the p-type conductivity. Current-voltage, capacitance-voltage, electrochemical impedance spectroscopy were conducted to investigate the influence of the grain size and crystallinity on electrical properties. Energy band-gap estimated from absorption spectra were 1.18, 1.04 and 0.98 eV for as-deposited, selenized, RT annealed samples, respectively. X-ray Photoelectron spectroscopy confirms the presence of Cu+, In3+ and Se2− oxidation states in all CIS layers. Power conversion efficiency of 3.05% and 5.94% were achieved for selenized and RT annealed samples, respectively. The improved efficiency measured for RT annealed sample is proposed due to the growth of highly crystalline, large grain and compact surface morphology.
On The Mechanism of the Anisotropic Dissolution of Silicon in Chlorine Containing Hydrofluoric Acid Solutions J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-12-06 André Stapf, Peter Nattrodt, Edwin Kroke
The wet-chemical acidic treatment of silicon wafer surfaces is very important in photovoltaic, microelectronic and further industries. Recent works report on new mixtures for acidic anisotropic etching mixtures based on hydrofluoric acid HF and hydrochloric acid HCl with an added oxidant. The aim of this work was to get an insight into the reactions during the etching process of silicon in the system HF-HCl-Cl2. The etching mixtures, gaseous reaction products, as well as the generated silicon surfaces were investigated by 19F, 29Si, and 35Cl NMR, ion chromatography (IC), iodometric titration, FT-IR spectroscopy, diffuse reflectance FT-IR spectroscopy (DRIFT) as well as scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX). A reaction scheme for the anisotropic dissolution of silicon in chlorine containing aqueous HF-solutions is proposed, which involves dissolved Cl2 as the oxidizing agent, coordination of fluoride/chloride ions and formation of a hydrophilic surface. These steps are similar to the well known alkaline anisotropic etching of silicon.
Electrocatalytic Water Oxidation by Single Site and Small Nuclearity Clusters of Cobalt J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-11-30 John R. Swierk, T. Don Tilley
Cobalt oxides are an earth abundant material that exhibits high electrocatalytic activity for the oxygen evolution reaction (OER) across a wide pH range. Recent studies suggest that OER catalysis can proceed through an active site comprised of one or two cobalt atoms but that multiple adjacent cobalt centers are preferred to stabilize high valent cobalt oxo-intermediates by delocalization. Utilizing molecular precursors to prepare single, isolated cobalt atoms (SS-Co) and small clusters of Co3O4 we find that OER proceeds more efficiently on Co3O4. Using electrochemical impedance spectroscopy (EIS), these results were rationalized at an atomic level. The EIS results support a hypothesis that charge transfer related to the formation of reaction intermediates proceeds more easily on Co3O4 than on SS-Co, which is attributed to the difficulty in forming Co(IV) = O and unlikely nucleophilic attack by water to form Co(II)-OOH.
ZnO Nanorod-Arrays as Photo-(Electro)Chemical Materials: Strategies Designed to Overcome the Material's Natural Limitations J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-11-30 Jan Kegel, Ian M. Povey, Martyn E. Pemble
The urgent need for clean and storable energy drives many currently topical areas of materials research. Among the many materials under investigation zinc oxide is one of the most studied in relation to its use in photo-(electro)chemical applications. This study aims to give an overview of some of the main challenges associated with the use of zinc oxide for these applications: the high density of intrinsic defects which can lead to fast recombination, low visible light absorption and the occurrence of photo-corrosion. Employing simple low-temperature solution based methods; it is shown how defect-engineering can be used to increase the photo-electrochemical performance and how doping can strongly increase the visible light absorption of zinc oxide nanorod-arrays. Furthermore the deposition of ultra-thin titanium dioxide layers using atomic layer deposition is investigated as possible route for the protection of zinc oxide against photo-corrosion.
Enriched Photoelectrochemical Performance of Phosphate Doped BiVO4 Photoelectrode by Coupling FeOOH and rGO J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-11-28 Qin Shi, Xiaozhe Song, Hui Wang, Zhaoyong Bian
Bismuth vanadate is a promising photoanode with a set of intrinsic limitations for water oxidation and photoelectrochemical degradation of organic pollution. FeOOH/P:BiVO4/rGO composite with a considerably small charge-transfer resistance was successfully developed by doping the BiVO4 lattice with phosphate (P:BiVO4), photo-depositing FeOOH nanoparticles on P:BiVO4 nanoparticles, and grafting reduced graphene oxide (rGO) onto the surface of P:BiVO4, in that order. This composite photoelectrode significantly improves photoelectrochemical performance originating from its effective suppression on electron-hole recombination and charge transfer at the semiconductor/electrolyte interface. The photoelectrocatalysts were systematically characterized by FTIR, XPS, SEM, TEM, UV-vis and XRD. The characterization results show that FeOOH/P:BiVO4/rGO consisted of spherical agglomerates comprising a large number of P:BiVO4 nanoparticles with an average size of approximately 10 nm. FeOOH nanoparticles were successfully loaded onto the surface of P:BiVO4 nanoparticles, and rGO layers with a thickness of approximately 4 nm were coated onto the P:BiVO4 particles. The enhanced photoelectrochemical properties were observed using linear sweep voltammetry. The mechanism underlying the observed photoelectrocatalytic activity enhancement was determined using Mott-Schottky analysis and electrochemical impedance spectroscopy. The photocurrent density of FeOOH/P:BiVO4/rGO in a Na2SO4 solution with 2,4-dichlorophenol (2,4-DCP) at 0.6 VAg/AgCl is approximately 100 times higher than that of P:BiVO4; the onset potentials of FeOOH/P:BiVO4/rGO (0.008 VAg/AgCl) is 5 times lower than that of P:BiVO4 (0.043 VAg/AgCl). It is suggested that FeOOH/P:BiVO4/rGO obtains the highest photoelectrocatalytic performance for 2,4-DCP degradation. The proposed mechanism is that the synergistic effect between FeOOH and rGO can alleviate two main limitations of P:BiVO4: suppression on the bulk recombination and interfacial recombination formed at the P:BiVO4-FeOOH junction and effective charge transfer at the semiconductor/electrolyte interface.
Surface States- and Field-Effects at GaAs(100) Electrodes in Sodium Dodecyl Sulfate Acid Solution J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-11-22 Mirela Enache, Catalin Negrila, Mihai Anastasescu, Gianina Dobrescu, Mihail Florin Lazarescu, Valentina Lazarescu
Sodium dodecyl sulfate (SDS) effects at the electrified p- and n-GaAs(100)/H2SO4 interfaces were investigated by EIS, XPS and AFM. XPS data revealed that under the open circuit conditions, SDS adsorption on GaAs(100) results in a protective overlayer which prevents the further oxidation in air of both types of semiconductor surfaces. The dopant nature is, however, decisive for the way of bonding the surfactant molecule to the surface. At the p-doped substrate, SDS adsorbs mainly at As sites by its hydrocarbon tail and by the anion head to the Ga sites at the n-doped one. Although the surfactant behaves as a dipole under the applied potential control, the dopant type plays a key role in the SDS interaction with GaAs(100) electrodes too. EIS data evidenced that SDS interaction with n-GaAs(100) electrode brings a pronounced decrease of the capacitive contribution of the surface states and a shift of the flatband potential to less negative values, unlike the p-doped one, where no significant change in its electronic properties was observed. These results were rationalized in terms of surface states- and field-effects operating at the electrified interfaces under discussion.
Electrochemical UV Sensor Using Carbon Quantum Dot/Graphene Semiconductor J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-10-03 Yuxuan Wang, Morgan Myers, John A. Staser
Our group has developed an electrochemical UV sensor utilizing carbon quantum dots as the photoactive material functionalizing a graphene semiconductor layer. When used as a photoactive electrode in contact with a solid polymer electrolyte in a photoelectrochemical cell, illumination under UV radiation at 365 nm induces a photocurrent with a corresponding change in device voltage. The time-dependent change in voltage is a function of UV radiation intensity. Varying the UV LED power density from 26.6 mW/cm2 (approximately 100% intensity) to 5.1 mW/cm2 (approximately 20% intensity) results in time-dependent potential changes (dU/dt) ranging from approximately 4.0 mV/s to 0.5 mV/s. The dU/dt vs. LED power density trend is nearly linear (r2 = 0.97). Similarly, when a constant bias potential is applied to the cell, a sustained photocurrent is observed under UV illumination, with the magnitude of the photocurrent a linear function of the LED power density. In that case, the coefficient of determination r2 = 0.98. These results indicate that a graphene semiconductor, when functionalized with a photoactive material like carbon quantum dots, has application as a UV sensor with the ability to quantify the intensity of UV radiation.
Viability of Polysulfide-Retaining Barriers in Li-S Battery J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-09-30 Erik J. Berg, Sigita Trabesinger
Lithium–sulfur (Li–S) batteries are among the most promising candidates for future high-energy, low-cost energy-storage systems. However, still many challenges have to be solved on the way to their commercialization. One of the most prominent of those is related to the polysulfide shuttle. In recent years, various approaches have been developed to contain, control or eliminate its effects, and thus to achieve higher specific charge, higher coulombic efficiencies and longer cycling life. One of recurring approaches is best described as introducing ‘polysulfide barriers’, either inorganic or polymeric membranes with lithium-ion conduction or interlayers with adsorptive properties, preventing polysulfides from reaching the lithium metallic anode. All of these approaches result in improved performance and longer cycling life of the Li–S battery. However, little attention has been given to the commercial viability of such solutions. Here we present a simple model to evaluate the practicability of polysulfide barriers in terms of gravimetric and volumetric energy densities as well as cost. We take into account the effects of barrier thickness, the physical properties and cost of the materials they are made of, as well as account for sulfur loading when assessing the viability of polysulfide barrier implementation into a practical Li–S cell.
Perspective--Lithium-Sulfur Batteries J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-09-29 Patrick Bonnick, Erika Nagai, John Muldoon
Despite immense effort to solve the problem of lithium polysulfide dissolution in Li-S batteries, only partially successful solutions have been found for liquid-based electrolytes. Further research efforts to showcase new sulfur, positive electrode technologies should ensure they demonstrate commercially applicable sulfur loadings of > 3 mg/cm2, electrolyte to sulfur weight ratios < 3 and current densities > C/2. Alternatively, by using solid electrolytes lithium polysulfide dissolution is eliminated and research can be refocused on the cell-limiting factor of reversibly stripping and plating Li metal without dendrite growth.
What Limits the Rate Capability of Li-S Batteries during Discharge: Charge Transfer or Mass Transfer? J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-14 T. Zhang, M. Marinescu, S. Walus, P. Kovacik, G. J. Offer
Li-S batteries exhibit poor rate capability under lean electrolyte conditions required for achieving high practical energy densities. In this contribution, we argue that the rate capability of commercially-viable Li-S batteries is mainly limited by mass transfer rather than charge transfer during discharge. We first present experimental evidence showing that the charge-transfer resistance of Li-S batteries and hence the cathode surface covered by Li2S are proportional to the state-of-charge (SoC) and not to the current, directly contradicting previous theories. We further demonstrate that the observed Li-S behaviors for different discharge rates are qualitatively captured by a zero-dimensional Li-S model with transport-limited reaction currents. This is the first Li-S model to also reproduce the characteristic overshoot in voltage at the beginning of charge, suggesting its cause is the increase in charge transfer resistance brought by Li2S precipitation.
The Dependence of Mass Transfer Coefficient on the Electrolyte Velocity in Carbon Felt Electrodes: Determination and Validation J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-10 Xin You, Qiang Ye, Ping Cheng
In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe2+/Fe3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s−1 are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re0.9. Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents.
One-Dimensional Porous Electrode Model for Predicting the Corrosion Rate under a Conductive Corrosion Product Layer J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-08 Maalek Mohamed-Said, Bruno Vuillemin, Roland Oltra, Laurent Trenty, Didier Crusset
General corrosion is the main form of corrosion likely to affect carbon steels in an anoxic and near neutral environment such as encountered in the context of long term storage of steel canisters in a deep geological repository. This paper aims at studying the influence of the electrical and geometrical properties of a siderite corrosion product layers (CPL) formed in such conditions on its stability and on its subsequent protective properties against corrosion. A 1-D numerical model describing general corrosion under a porous conductive CPL and accounting for chemical evolution in the electrolyte is presented. It is demonstrated that a conductive layer with a cathodic activity increases the corrosion rate and the Fe2 + ions concentration. Otherwise, a conductive layer leads to high saturation levels of siderite and high pH values within the CPL and consequently to a stabilization of the CPL. It is shown also that the stability of the CPL is promoted when it is initially thick and/or when it has a low porosity.
Micro-Scale Analysis of Liquid Water Breakthrough inside Gas Diffusion Layer for PEMFC Using X-ray Computed Tomography and Lattice Boltzmann Method J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-08 P. Satjaritanun, J. W. Weidner, S. Hirano, Z. Lu, Y. Khunatorn, S. Ogawa, S. E. Litster, A. D. Shum, I. V. Zenyuk, S. Shimpalee
The main objective of this work is to predict the breakthrough pressure of liquid water transport through the gas diffusion layer (GDL) and/or micro porous layer (MPL) used in polymer electrolyte membrane fuel cells. The integration of structural GDL and MPL with Lattice Boltzmann Method is primary focused. The numerical predictions are also compared with experimental data. The interaction between liquid phase and different surface treatments of solid structures controls the evolution of liquid water and the change of capillary pressure. The geometries of GDLs and MPLs were obtained by three dimensional reconstructed micro-structure images from both nanometer and micrometer-scaled high spatial resolution X-ray computed tomography (CT). The predictions of water breakthrough pressure agree with the data observed in the experiment. They also reveal that the breakthrough pressure and liquid water evolution inside the GDL samples are different when the wetting properties of GDL and/or MPL are changed. The detailed microporous property can be obtained using high spatial resolution image from nanometer-scaled X-ray CT, a.k.a. Nano X-ray CT. Meanwhile, images from micrometer-scaled X-ray CT, a.k.a. Micro X-ray CT, give proper field of view to cover complete vision of porous materials, including cracks in the MPL.
Analysis of Ge-Si Heterojunction Nanowire Tunnel FET: Impact of Tunneling Window of Band-to-Band Tunneling Model J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-07 Erry Dwi Kurniawan, Shang-Yi Yang, Vasanthan Thirunavukkarasu, Yung-Chun Wu
Tunnel FET (TFET) has potential applications in the next generation ultra-low power transistor to substitute the conventional FETs. It can offer very steep inverse subthreshold swing slope to maintain a low leakage current, thus it can be very essential for limiting power consumption in MOSFETs. The carriers in TFET transport from source to channel by the band-to-band tunneling (BTBT) mechanisms. To realize high saturation currents of TFET, it critically depends on the transmission probability, TWKB. In indirect semiconductor, such as Si and Ge, the BTBT model is very crucial for designing and predicting the device performance. In this paper, we employed the nonlocal BTBT model applied to three-dimensional Ge-Si heterojunction TFET with gate length 10 nm compare with Si TFET by including quantum effects simulation. The results show that the Ge-Si TFET outperforms Si TFET because of the lower bandgap and larger tunneling windows. BTBT generation rates of Ge-Si TFET are higher than Si TFET in the on-state condition. The highest BTBT generation rates are located in the source and channel junction and its peaks close to the gate dielectric.
Simulations of Turbulent Flow, Mass Transport, and Tertiary Current Distribution on the Cathode of a Rotating Cylinder Electrode Reactor in Continuous Operation Mode during Silver Deposition J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-06 Mario Rosales, José L. Nava
This work presents numerical simulations of turbulent flow, mass transport and tertiary current distribution on the cathode of a rotating cylinder electrode reactor (RCE) in a continuous operation mode. A configuration of a RCE with electrolyte inlet at the bottom and the electrolyte exit at the top was employed. Silver electrodeposition (12.15 mol m−3 (1300 ppm) Ag(I), 883.5 mol m−3 (23000 ppm) CN−, pH 13 and 150 mS cm−1 conductivity) was used as a test system. Bulk electrolysis in the RCE was performed at a constant potential of − 1.2 V vs. SCE, which ensured complete mass transport control. A constant volumetric inflow rate of 0.1 L min−1 at the RCE inlet was employed. CFD simulations were obtained solving the RANS equations with the standard k − ε turbulence model. For mass transport simulations, the averaged diffusion-convection equation was solved. For the simulations of tertiary current distribution, wall functions were employed. The tertiary current distribution on the RCE interface along the z-coordinate presented one border effect close to the electrolyte inlet, afterwards, even current distribution was obtained. The border effect is created by the abruptly silver concentration depletion at the electrolyte inlet. Good agreement between mass transport correlation and current distribution simulations with experimental data were attained.
Multi-Scale Simulation of Heterogeneous Surface Film Growth Mechanisms in Lithium-Ion Batteries J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-03 Fridolin Röder, Richard D. Braatz, Ulrike Krewer
A quantitative description of the formation process of the solid electrolyte interface (SEI) on graphite electrodes requires the description of heterogeneous surface film growth mechanisms and continuum models. This article presents such an approach, which uses multi-scale modeling techniques to investigate multi-scale effects of the surface film growth. The model dynamically couples a macroscopic battery model with a kinetic Monte Carlo algorithm. The latter allows the study of atomistic surface reactions and heterogeneous surface film growth. The capability of this model is illustrated on an example using the common ethylene carbonate-based electrolyte in contact with a graphite electrode that features different particle radii. In this model, the atomistic configuration of the surface film structure impacts reactivity of the surface and thus the macroscopic reaction balances. The macroscopic properties impact surface current densities and overpotentials and thus surface film growth. The potential slope and charge consumption in graphite electrodes during the formation process qualitatively agrees with reported experimental results.
Determination of Tortuosity Using Impedance Spectra Analysis of Symmetric Cell J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-03 Simon Malifarge, Bruno Delobel, Charles Delacourt
An approach to increase the autonomy of batteries developed for transportation applications, without changing currently-used positive and negative active materials, is to increase the battery energy density by increasing the active material loading (mg.cm−²) of the electrodes. A direct consequence of a higher loading is the increase of mass transport-related issues across the electrode porosity. Therefore, the optimization of the porous electrode structure is mandatory to facilitate the access of lithium ions to the whole electrode volume. In this regard, pore tortuosity is a key parameter whose determination is not so straightforward. Although tomography techniques and corresponding analyses are promising methods to acquire precise geometrical information about porous electrode, they hardly can be used as a routine technique. In this work, a transmission-line-model analysis of the electrochemical impedance diagram of symmetric cells containing porous electrodes in blocking condition, i.e. without any charge transfer reaction, is proposed in order to readily derive pore tortuosity. The method is applied to a set of graphite electrodes composed of anisotropic particles.
Extending Newman's Pseudo-Two-Dimensional Lithium-Ion Battery Impedance Simulation Approach to Include the Nonlinear Harmonic Response J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-02 Murbach, M. D., Schwartz, D. T.
The pseudo-two-dimensional (P2D) model of lithium-ion batteries couples a volume-averaged treatment of transport, reaction, and thermodynamics to solid-state lithium diffusion in electrode particles. Here we harness the linear and nonlinear physics of the P2D model to evaluate the fundamental (linear) and higher harmonic (nonlinear) response of a LiCoO2|LiC6 cell subject to moderate-amplitude sinusoidal current modulations. An analytic-numeric approach allows the evaluation of the linearized frequency dispersion function that represents electrochemical impedance spectroscopy (EIS) and the higher harmonic dispersion functions we call nonlinear electrochemical impedance spectroscopy (NLEIS). Base case simulations show, for the first time, the full spectrum second and third harmonic NLEIS response. The effect of kinetic, mass-transport, and thermodynamic parameters are explored. The nonlinear interactions that drive the harmonic response break some of the degeneracy found in linearized models. We show that the second harmonic is sensitive to the symmetry of the charge transfer reactions in the electrodes, whereas EIS is not. At low frequencies, NLEIS probes aspects of the cell thermodynamics that are not accessible with EIS. In short, NLEIS has the potential to increase the number of physicochemical parameters that can be assessed in experiments similar in complexity to standard EIS measurements.
The Kinetic Parameters of the Oxygen Evolution Reaction (OER) Calculated on Inactive Anodes via EIS Transfer Functions: *OH Formation J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-02 Garcia-Osorio, D. A., Jaimes, R., Vazquez-Arenas, J., Lara, R. H., Alvarez-Ramirez, J.
The electrocatalytic behavior of "inactive" (Boron Doped Diamond (BDD), SnO2-Sb and PbO2) anodes toward the oxygen evolution reaction (OER) is evaluated using dc and ac techniques, under controlled experimental conditions (e.g. air bubbling, ohmic drop correction). Tafel slopes estimated from anodic polarization curves for all catalysts are located above 100 mV dec–1, suggesting that the rate-controlling step is similar for these materials at determined overpotentials. In agreement with the literature, it could be associated with the •OH generation since "active catalysts" displays slopes below 80 mV dec–1. A transfer function model is derived to account for the kinetic parameters of the OER mechanism for each anode, throughout its fitting to experimental electrochemical impedance spectroscopy (EIS) spectra. The model considers the kinetics of each elementary reaction and the material balances for the rates of formation of the adsorbates (H2Oads, •OHads, •Oads, •OOHads, and O2ads) involved in the OER. It is found that the rate-controlling step on SnO2-Sb (≤2.14 V), PbO2 (≤1.74 V) and BDD (2.54–2.71 V) is associated with the •OH formation, while this control is only modified when the potential becomes more positive for the oxide catalysts, thus, being determined by the production of adsorbed oxygen O2ads which on its turn promotes the O2 evolution. On the other hand, the rate-control step remains similar for BDD over the entire analyzed potential range, standing out its unique properties as inactive catalyst of the OER.
Multiphase Porous Electrode Theory J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-06-01 Smith, R. B., Bazant, M. Z.
Porous electrode theory, pioneered by John Newman and collaborators, provides a macroscopic description of battery cycling behavior, rooted in microscopic physical models. Typically, the active materials are described as solid solution particles with transport and surface reactions driven by concentration fields, and the thermodynamics are incorporated through fitting of the open circuit potential. However, this approach does not apply to phase separating materials, for which the voltage is an emergent property of inhomogeneous concentration profiles, even in equilibrium. Here, we present a general framework, "multiphase porous electrode theory", based on nonequilibrium thermodynamics and implemented in an open-source software package called "MPET". Cahn-Hilliard-type phase field models are used to describe the active materials with suitably generalized models of interfacial reaction kinetics. Classical concentrated solution theory is implemented for the electrolyte phase, and Newman’s porous electrode theory is recovered in the limit of solid solution active materials with Butler-Volmer kinetics. More general, quantum-mechanical models of faradaic reactions are also included, such as Marcus-Hush-Chidsey kinetics for electron transfer at electrodes, extended for concentrated solutions. The full model and implementation are described, and a variety of example calculations are presented to illustrate the novel features of the software compared to existing battery models.
Quantifying Mass Transfer Rates in Redox Flow Batteries J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-31 Milshtein, J. D., Tenny, K. M., Barton, J. L., Drake, J., Darling, R. M., Brushett, F. R.
Engineering the electrochemical reactor of a redox flow battery (RFB) is critical to delivering sufficiently high power densities, as to achieve cost-effective, grid-scale energy storage. Cell-level resistive losses reduce RFB power density and originate from ohmic, kinetic, or mass transfer limitations. Mass transfer losses affect all RFBs and are controlled by the active species concentration, state-of-charge, electrode morphology, flow rate, electrolyte properties, and flow field design. The relationship among flow rate, flow field, and cell performance has been qualitatively investigated in prior experimental studies, but mass transfer coefficients are rarely systematically quantified. To this end, we develop a model describing one-dimensional porous electrode polarization, reducing the mathematical form to just two dimensionless parameters. We then engage a single electrolyte flow cell study, with a model iron chloride electrolyte, to experimentally measure cell polarization as a function of flow field and flow rate. The polarization model is then fit to the experimental data, extracting mass transfer coefficients for four flow fields, three active species concentrations, and five flow rates. The relationships among mass transfer coefficient, flow field, and electrolyte velocity inform engineering design choices for minimizing mass transfer resistance and offer mechanistic insight into transport phenomena in fibrous electrodes.
A Non-Electroneutral Model for Complex Reaction-Diffusion Systems Incorporating Species Interactions J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-31 Minton, G., Purkayastha, R., Lue, L.
In this study we develop a general framework for describing reaction-diffusion processes in a multi-component electrolyte in which multiple reactions of different types may occur. Our motivation for this is the need to understand how the interactions between species and processes occurring in a complex electrochemical system. We use the framework to develop a modified Poisson-Nernst-Planck model which accounts for the excluded volume interaction (EVI) and incorporates both electrochemical and chemical reactions. Using this model, we investigate how the EVI influences the reactions and how the reactions influence each other in the contexts of the equilibrium state of a system and of a simple electrochemical device under load. Complex behavior quickly emerges even in relatively simple systems, and deviations from the predictions of ideal solution theory, together with how they may influence the behavior of more complex system, are discussed.
Optimal Design of Li-Ion Batteries through Multi-Physics Modeling and Multi-Objective Optimization J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-26 Liu, C., Liu, L.
Battery design variable optimization can significantly affect battery capacity, discharge specific power, and discharge specific energy. However, many design variables need to be taken into consideration, which requires intensive computation and simulation. Our previously developed comprehensive battery degradation model is utilized in this optimization study via parallel computing. A three-electrode cell is developed for model validation over long term cycling. The objectives of optimization are maximizing discharge specific power and specific energy as well as minimizing capacity loss. Several design variables (e.g., thickness, particle size, and porosity) are optimized through a modified Elitist Non-Dominated Sorting Genetic Algorithm (NSGA-II). The obtained Pareto-optimal solutions that show electrode thickness, particle sizes, porosity, and conductivity are the battery design variables that can significantly affect battery performance. In addition, a sensitivity analysis suggests that a thicker electrode and a smaller particle size can improve battery performance. The optimized batteries have a better performance over 750 cycle's simulation: less SOC swing and less reduction of capacity. The design optimization framework developed herein can be modified and applied to various type of batteries with different optimization objectives and battery design variables.
Adjoint Method for the Optimization of the Catalyst Distribution in Proton Exchange Membrane Fuel Cells J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-25 Lamb, J., Mixon, G., Andrei, P.
In this article we present an adjoint method for the optimization of the catalyst distribution in proton exchange membrane fuel cells (PEMFCs). By using the theory of functional analysis we derive analytical equations for the sensitivity functions of the cell voltage with respect to the catalyst distribution in a very general framework, independent on the transport model used to simulate the PEMFC. Then we present an efficient numerical algorithm to calculate the sensitivity functions using the adjoint method. The adjoint method has the advantage that it can be applied to the optimization of systems with a large (>104) number of optimization variables that are computed simultaneously and independently to maximize the objective function. Finally, we apply the method to the optimization of 2-D platinum distribution in PEMFCs. We show that the optimum platinum distribution varies with the operating conditions, position of landings and openings, cell geometry, and dimensions of the catalyst layers. The method presented in this work can be naturally extended to the optimization of other 2-D and 3-D field variables such as the porosity of catalyst and gas diffusion layers, particle size distribution, or microstructure of the cell.
Thermodynamic Model for Substitutional Materials: Application to Lithiated Graphite, Spinel Manganese Oxide, Iron Phosphate, and Layered Nickel-Manganese-Cobalt Oxide J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-25 Verbrugge, M., Baker, D., Koch, B., Xiao, X., Gu, W.
We derive and implement a method to describe the thermodynamics of electrode materials based on a substitutional lattice model. To assess the utility and generality of the method, we compare model results with experimental data for a variety of electrode materials: lithiated graphite and layered nickel-manganese-cobalt oxide (Chevrolet Bolt Electric Vehicle negative and positive electrode materials, respectively), manganese oxide (in the positive electrodes of the Gen 1 and Gen 2 Chevrolet Volt Extended Range Electric Vehicle and the positive electrode of many high-power-density batteries), and iron phosphate (Gen 1 Chevrolet Spark Electric Vehicle positive electrode material and of immediate interest for 12 and 48 V applications). An early version of the model has been applied to lithiated silicon (Li-Si). As was found in the Li-Si study, the model enables one to quantitatively represent experimental data from these different electrode materials with a small number of parameters, and, in this sense, the approach is both general and efficient. An open question is the utility of controlled-potential vs. controlled-current experiments for the elucidation of the system thermodynamics. We provide commentary on this question, and we highlight other open questions throughout this work.
Nonlinear State-Variable Method (NSVM) for Li-Ion Batteries: Finite-Element Method and Control Mode J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-24 Guo, M., Jin, X., White, R. E.
The finite element method (FEM) was used in our nonlinear state-variable method (NSVM) presented recently (J. Electrochem. Soc., 164, E3001 (2017)). The details of the application of the FEM to solve the lithium ion pseudo-2D (P2D) model equations using the NSVM are presented here for several control modes (constant current, voltage, power, or load). Validation of the method was performed by comparison to rigorous full-order models and experimental data. The FEM based NSVM shows excellent performance, and the estimated cell parameters are determined with a high confidence level.
Considering Photon Scattering and Harmonics for Synchrotron X-ray Radiographic Imaging of Polymer Electrolyte Membrane Fuel Cells J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-24 Ge, N., George, M. G., Lee, J., Muirhead, D., Chevalier, S., Banerjee, R., Liu, H., Wysokinski, T. W., Belev, G., Webb, M. A., Zhu, N., Bazylak, A.
We predicted the attenuated, undesired secondary scattered, and undesired harmonic components of measured X-ray intensities from synchrotron X-ray radiographic visualizations of liquid water in an operating polymer electrolyte membrane (PEM) fuel cell. The undesired secondary scattered component of the measured intensity increased as a function of the liquid water thickness (traversed by the X-ray beam). This increase in the secondary scattered component led to a decrease in the calibrated attenuation coefficient for liquid water, decreasing the accuracy of water quantification. We recommend calibrating the attenuated coefficient with a range of water thicknesses defined by the maximum expected water thickness present in the PEM fuel cell. The undesired harmonic component of the measured intensity also increased as a function of liquid water thickness, which led to a decrease in the accuracy of the measured liquid water thickness.
Improving the Lithium Ion Transport in Polymer Electrolytes by Functionalized Ionic-Liquid Additives: Simulations and Modeling J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-24 Diddens, D., Paillard, E., Heuer, A.
We present a theoretical study combining molecular dynamics (MD) simulations with an analytical lithium ion transport model [Maitra and Heuer, Phys. Rev. Lett. 2007, 98, 227802] to highlight a novel strategy to increase the lithium mobility in polymer electrolytes based on poly(ethylene oxide) (PEO). This is achieved by using a pyrrolidinium-based ionic liquid (IL) where the cation has been chemically functionalized by a short oligoether side chain [von Zamory et al., Phys. Chem. Chem. Phys. 2016, 18(31), 21539] as an additive. Since the oligoether moieties at the pyrrolidinium cations form pronounced coordinations to the lithium ions for sufficiently long side chains, the ions can be detached from the PEO backbone. In this way, a fundamentally new lithium ion transport mechanism is established (shuttling mechanism), in which the lithium dynamics is decoupled from the polymer dynamics, the latter typically being slow under experimental conditions. Based on our simulations, we incorporate this novel mechanism into our existing model, which accurately reproduces the observed lithium dynamics. We demonstrate that the use of oligoether-functionalized IL additives significantly increases the lithium diffusivity. Finally, we show that for experimentally relevant electrolytes containing long polymer chains, an even stronger increase of the lithium mobility can be expected.
Equilibration Process in Response to a Change in the Anode Gas Using Thick Sm-Doped Ceria Electrolytes in Solid-Oxide Fuel Cells J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-23 Miyashita, T.
The electronic conductivity of Sm-doped ceria is very low in air but increases substantially in H2 gas. Conventional models can explain the equilibration processes of yttria-stabilized zirconia electrolytes and thin mixed ionic-electronic conducting electrolytes in response to variations in the fuel composition. However, the equilibration processes of thick samaria-doped ceria electrolytes have not yet been explained. We measured and attempted to explain the equilibration process of a very thick (6.6 mm) samaria-doped ceria electrolyte in response to a change in the anode gas. The measured open-circuit voltage gradually increased to an equilibrium voltage of 0.80 V within 5 min. However, based on the chemical diffusion coefficient equation for the electron diffusion current, the equilibrium time should have been much longer than 5 min. When we assumed a current-independent constant anode voltage loss (0.35 V), the calculations were substantially improved for determining the experimental results.
A Study on the Effect of Porosity and Particles Size Distribution on Li-Ion Battery Performance J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-19 Taleghani, S. T., Marcos, B., Zaghib, K., Lantagne, G.
A pseudo two-dimensional model (P2D) is presented that describes the effect of the structural properties of the positive electrode on Li-ion cell performance during discharge. The validation of the mono-modal model was done by using Doyle's experiment and results [C.M. Doyle, University of California, Berkeley (1995)]. A large increase or decrease in the porosity beyond a specific value led to a sharp change in the cell voltage curve and lower cell capacities. The maximum specific energy was obtained in the porosity range of 0.55, while the specific power still had a high value. Furthermore, different particle size distribution models, including mono-modal, bi-modal and 3-particle models, were compared to each other. The mono-modal model was the ideal state with the lowest total polarization. The bi-modal and 3-particle models approached this ideal state when the volume fraction of the smallest particles in their structures increased. This structural arrangement in these models led to more uniform local current density distribution profiles resulting in a greater decrease in cell polarization. Different discharge current densities were applied to different particle size distribution models, and the results showed that the particle size distribution has a greater effect at higher discharge current densities.
Electrochemical Hydrogen Evolution: H+ or H2O Reduction? A Rotating Disk Electrode Study J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-18 Grozovski, V., Vesztergom, S., Lang, G. G., Broekmann, P.
We study the effect of H+ and OH– diffusion on the hydrogen evolution reaction in unbuffered aqueous electrolyte solutions of mildly acidic pH values. We demonstrate that the cathodic polarization curves measured on a Ni rotating disk electrode in these solutions can be modeled by assuming two irreversible reactions, the reduction of H+ and that of water molecules, both following Erdey-Grúz–Volmer–Butler kinetics. The reduction of H+ yields a transport-limited and thus, rotation rate-dependent current at not very negative potentials. At more cathodic potentials the polarization curves are dominated by the reduction of water and no mass transfer limitation seems to apply for this reaction. Although prima facie the two processes may seem to proceed independently, by the means of finite-element digital simulations we show that a strong coupling (due to the recombination of H+ and OH– to water molecules) exists between them. We also develop an analytical model that can well describe polarization curves at various values of pH and rotation rates. The key indication of both models is that hydroxide ions can have an infinite diffusion rate in the proximity of the electrode surface, a feature that can be explained by assuming a directed Grotthuss-like shuttling mechanism of transport.
Ion Diffusivity through the Solid Electrolyte Interphase in Lithium-Ion Batteries J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-17 Benitez, L., Seminario, J. M.
Understanding the transport properties of the solid electrolyte interphase (SEI) is a critical piece in the development of lithium ion batteries (LIB) with better performance. We studied the lithium ion diffusivity in the main components of the SEI found in LIB with silicon anodes and performed classical molecular dynamics (MD) simulations on lithium fluoride (LiF), lithium oxide (Li2O) and lithium carbonate (Li2CO3) in order to provide insights and to calculate the diffusion coefficients of Li-ions at temperatures in the range of 250 K to 400 K, which is within the LIB operating temperature range. We find a slight increase in the diffusivity as the temperature increases and since diffusion is noticeable at high temperatures, Li-ion diffusion in the range of 1300 K to 1800 K was also studied and the diffusion mechanisms involved in each SEI compound were analyzed. We observed that the predominant mechanisms of Li-ion diffusion included vacancy assisted and knock-off diffusion in LiF, direct exchange in Li2O, and vacancy and knock-off in Li2CO3. Moreover, we also evaluated the effect of applied electric fields in the diffusion of Li-ions at room temperature.
Modeling of Effect of Double-Layer Capacitance and Failure of Lead-Acid Batteries in HRPSoC Application J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-04-14 Gandhi, K. S.
Lead-acid batteries fail faster in partial state-of-charge start-stop technology than in SLI application. Accumulation of lead sulfate on negative electrode’s surface has been identified as the cause. It is also known that life can be enhanced by increasing capacitance of negative electrode. A bench-marking test cycle is used to explain these observations through a one-dimensional model. It is shown that, at the large discharge current densities used, faradaic reactions in the electrodes are spatially inhomogeneous, and charging is unable to reverse its effects. Consequently, lead sulfate deposit is larger on electrode’s surface than at its center. Model uses a rate expression for charging modified to include diffusion of Pb2 +, and predicts that sulfate continues to accumulate with cycling. A portion of electrode becomes inactive when volume fraction of sulfate reaches a critical value there. Battery fails when inactive area becomes large. It is shown that double-layer capacitance suppresses the non-uniformity in the faradaic reaction and alters the pattern of accumulation of sulfate. Negative electrode does not benefit from this since its capacitance is low. Sulfate accumulates in positive electrode also, but does not reach critical levels since positive electrode’s capacitance is large. This explains the life enhancing effect of capacitance.
Communication--Modeling Polymer-Electrolyte Fuel-Cell Agglomerates with Double-Trap Kinetics J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-04-14 Pant, L. M., Weber, A. Z.
A new semi-analytical agglomerate model is presented for polymer-electrolyte fuel-cell cathodes. The model uses double-trap kinetics for the oxygen-reduction reaction, which can capture the observed potential-dependent coverage and Tafel-slope changes. An iterative semi-analytical approach is used to obtain reaction rate constants from the double-trap kinetics, oxygen concentration at the agglomerate surface, and overall agglomerate reaction rate. The analytical method can predict reaction rates within 2% of the numerically simulated values for a wide range of oxygen concentrations, overpotentials, and agglomerate sizes, while saving simulation time compared to a fully numerical approach.
Reducing Inhomogeneous Current Density Distribution in Graphite Electrodes by Design Variation J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-04-28 Kindermann, F. M., Osswald, P. J., Ehlert, G., Schuster, J., Rheinfeld, A., Jossen, A.
Inhomogeneous utilization of electrodes and consequent limitations in the operating conditions are a severe problem, reducing lifetime and safety. By using a previously developed laboratory cell setup, we are able to show an inhomogeneous retrieval of lithium-ions from a graphite electrode throughout the layer with spatial resolution for two different graphites. After provoking inhomogeneities via constant current operations, equilibration processes are recorded and are assigned to two different effects. One effect is an equilibration inside the particles (intra-particle) from surface to bulk whereas the second effect is an equalization between the particles (inter-particle) to reach a homogeneous degree of lithiation in each particle throughout the electrode layer. With the recorded data, we implemented a P2D model with multiple particle sizes and considered the electrode thickness in several separate domains. Using the relaxation data of intra- and inter-particle relaxation for parametrizing the model, we investigated the influence of different solid and liquid phase parameters. As the liquid phase parameters scaled via porosity and tortuosity showed the biggest impact, we performed a design variation study to achieve a more homogeneous utilization of the electrode. Structuring the electrode to lower tortuosity is identified as the most promising design variation for homogeneous utilization.
Model Based Analysis of One-Dimensional Oriented Lithium-Ion Battery Electrodes J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-04 Chadha, T. S., Suthar, B., Rife, D., Subramanian, V. R., Biswas, P.
Oriented one-dimensional nanostructures have been of substantial interest as electrodes for lithium-ion batteries due to the better performance both in terms of initial capacity and lower capacity fade compared to powder pressed electrodes. This paper focuses on a model driven approach to understanding the relationship between the morphology of these oriented nanostructures to the performance of the battery. The Newman-type P2D modeling technique is applied to a porous electrode made up with solid continuous cylinders that extends from the current collectors to separator. TiO2 columnar nanostructures of varying heights were synthesized using the aerosol chemical vapor deposition (ACVD) and their performance as electrodes in a lithium-ion battery was measured. This electrochemical transport model was validated with the experimental data. This model was used to understand the role of transport parameters, including the diffusivity of lithium in the TiO2 and the electronic conductivity of the TiO2 columns, and structural parameters, including the height of the columns and the porosity of the electrode, on the areal capacity of a lithium ion battery at different rates of discharge. The model enables for the prediction of optimized structural parameters of one-dimensional electrodes tailored to the desired application of lithium and sodium-ion batteries.
Performance Modeling and Design of Ultra-High Power Microbatteries J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-04 Pikul, J. H., Braun, P. V., King, W. P.
High power density microbatteries could enable new capabilities for miniature sensors, radios, and industrial electronics. There is, however, a lack of understanding on how battery architecture and materials limit power performance when battery discharge rates exceed 100 C. This paper describes the development and application of an electrochemical model to predict the performance of microbatteries having interdigitated bicontinuous microporous electrodes, discharged at up to 600 C rates. We compare predicted battery behavior with measurements, and use the model to explore the underlying physics. The model shows that diffusion through the solid electrodes governs microbattery power performance. We develop design rules that could guide the development of improved batteries.
Revealing SEI Morphology: In-Depth Analysis of a Modeling Approach J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-05 Single, F., Horstmann, B., Latz, A.
In this article, we present a novel theory for the long term evolution of the solid electrolyte interphase (SEI) in lithium-ion batteries and propose novel validation measurements. Both SEI thickness and morphology are predicted by our model as we take into account two transport mechanisms, i.e., solvent diffusion in the SEI pores and charge transport in the solid SEI phase. We show that a porous SEI is created due to the interplay of these transport mechanisms. Different dual layer SEIs emerge from different electrolyte decomposition reactions. We reveal the behavior of such dual layer structures and discuss its dependence on system parameters. Model analysis enables us to interpret SEI thickness fluctuations and link them to the rate-limiting transport mechanism. Our results are general and independent of specific modeling choices, e.g., for charge transport and reduction reactions.
Probing the Role of Electrode Microstructure in the Lithium-Ion Battery Thermal Behavior J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-05-06 Chen, C.-F., Verma, A., Mukherjee, P. P.
Safety and performance of lithium-ion batteries over a wide temperature window are of paramount importance, especially for electric vehicles. The safety concerns are predicated on the thermal behavior as the occurrence of local temperature excursions may lead to thermal runaway. In this work, the role of electrode microstructure and implications on the cell thermal behavior are examined. A microstructure-aware electrochemical-thermal coupled model has been proposed, which delineates the electrode-level thermal complexations due to the structure-transport-electrochemistry interactions. Detailed analysis of the spatio-temporal variation of the heat generation rates (ohmic, reaction and reversible contributions) for different electrode microstructural configurations is presented to explain the dominant factors causing temperature rise. The tradeoff between the cell performance and safety is discussed from an electrode-level, bottom-up view point. This study aims to provide valuable insights into potentially tuning electrode-level structural features as an internal safety switch toward limiting the Li-ion cell temperature rise during operation.
The Relationship between Shunt Currents and Edge Corrosion in Flow Batteries J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-04-04 Darling, R. M., Shiau, H.-S., Weber, A. Z., Perry, M. L.
Shunt currents occur in electrochemical reactors like flow batteries, electrolyzers, and fuel cells where many bipolar cells that are connected in series electrically contact a mobile electrolyte through one or more common fluid distribution manifolds. Shunt currents reduce energy efficiency, and can cause unwanted side reactions including corrosion and gas generation. Equivalent-circuit models have been widely used to examine shunt currents in multi-cell electrochemical reactors. However, a detailed investigation of the interesting electrochemical processes occurring at the edges of the active areas has not been presented. In this work, the generation of shunt currents and their tendency to drive corrosion at the edges of positive electrodes in the most positive cells in a reactor stack are investigated with a comprehensive numerical model. An analytical model based on the penetration of current into a semi-infinite electrode, that can be used in conjunction with traditional equivalent-circuit models to assess the tendency for shunt currents to drive corrosion, is developed and compared to the numerical model. The models provided here can be used to set requirements on maximum allowable port currents in order to achieve a particular durability goal.
Macro-Scale Analysis of Large Scale PEM Fuel Cell Flow-Fields for Automotive Applications J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-03-31 Shimpalee, S., Hirano, S., DeBolt, M., Lilavivat, V., Weidner, J. W., Khunatorn, Y.
The objective of this work is to establish the design principles for a proton exchange membrane fuel cell in automotive applications. In this work, the macro-scale analysis was considered to create the overall design principle. A combination of experiments and numerical simulations were carried out and the results analyzed to enhance understanding of the behavior of the large-scale 300-cm2 proton exchange membrane fuel cell under automotive operations. A three-dimensional computational fluid dynamics-based methodology was used to predict such as the current and temperature distributions of this design as a function of anode relative humidity. The effect of flow direction and the cooling pattern on this design was also taken into account to enhance the understanding for this selected flow-field design. The predictions show that the gas flow and cooling directions are important dependent variables that can impact the overall performance and local distributions.
Impedance Spectroscopy Study of the PEM Fuel Cell Cathode with Nonuniform Nafion Loading J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-02-23 Reshetenko, T., Kulikovsky, A.
We report modeling and experimental study of impedance of the PEM fuel cell cathode with nonuniform ionomer loading. A physics–based model for the high–frequency impedance is developed and analytical solution for impedance is derived. Assuming that the CCL proton conductivity p exponentially decays from the membrane surface, we fit the model to experimental spectra of the cell measured at the open circuit conditions. Fitting gives the characteristic scale of the p decay, the average CCL proton conductivity and the double layer capacitance.
Theoretical Analysis of Microelectrode Arrays under Forced Convection J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-02-23 Georgescu, N. S., Scherson, D. A.
Numerical solutions to the equation that governs steady-state mass transport to a hexagonal array of small redox-active disks embedded in an otherwise inert rotating disk electrode, RDE, under both diffusion-limited and mixed, first order kinetic control were obtained using COMSOL. Analytical expressions were found, which accurately reproduced the simulations, yielding, for limiting cases, a behavior in agreement with that reported in the literature. This formalism was applied to the analysis of thin films of nanoparticles dispersed in inert high area supports attached to the surface of an inactive RDE. The results obtained made it possible to verify that, at loadings within the range of relevance to electrocatalyic materials for low temperature fuel cells, the use of a modified Koutecky-Levich-like equation for determining rate constants of first order redox processes is indeed warranted.
Direct, Efficient, and Real-Time Simulation of Physics-Based Battery Models for Stand-Alone PV-Battery Microgrids J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-02-23 Lee, S. B., Pathak, C., Ramadesigan, V., Gao, W., Subramanian, V. R.
With renewable energy based electrical systems becoming more prevalent in homes across the globe, microgrids are becoming widespread and could pave the way for future energy distribution. Accurate and economical sizing of stand-alone power system components, including batteries, has been an active area of research, but current control methods do not make them economically feasible. Typically, batteries are treated as a black box that does not account for their internal states in current microgrid simulation and control algorithms. This might lead to under-utilization and over-stacking of batteries. In contrast, detailed physics-based battery models, accounting for internal states, can save a significant amount of energy and cost, utilizing batteries with maximized life and usability. It is important to identify how efficient physics-based models of batteries can be included and addressed in current grid control strategies. In this paper, we present simple examples for microgrids and the direct simulation of the same including physics-based battery models. A representative microgrid example, which integrates stand-alone PV arrays, a Maximum Power Point Tracking (MPPT) controller, batteries, and power electronics, is illustrated. Implementation of the MPPT controller algorithm and physics-based battery model along with other microgrid components as differential algebraic equations is presented. The results of the proposed approach are compared with the conventional control strategies and improvements in performance and speed are reported.
Determination of the Resistance of Cone-Shaped Solid Electrodes J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-03-11 Frandsen, H. L., Hendriksen, P. V., Koch, S., Hansen, K. K.
A cone-shaped electrode pressed into an electrolyte can with advantage be utilized to characterize the electro-catalytic properties of the electrode, because it is less dependent on the electrode microstructure than e.g. thin porous composite electrodes, and reactions with the electrolyte occurring during processing can be avoided. Newman's formula for current constriction in the electrolyte is then used to deduce the active contact area based on the ohmic resistance of the cell, and from this the surface specific electro-catalytic activity. However, for electrode materials with low electrical conductivity (like Ce1-xPrxO2-), the resistance of the cell is significantly influenced by the ohmic resistance of the cone electrode, wherefore it must be included. In this work the ohmic resistance of a cone is modelled analytically based on simplified geometries. The two analytical models only differ by a model specific pre-factor, which is consequently determined by a finite element model. The model was applied to measurements on cones of Ce1-xPrxO2- characterized on an YSZ electrolyte. Conclusively, the finite element model was used to obtain a formula for the resistance for different cone angles with a small contact area. This reproduces Newman's formula for a cone angle equal to 90°, i.e. a semi-infinite body.
Mesoscopic Modeling of a LiFePO4 Electrode: Experimental Validation under Continuous and Intermittent Operating Conditions J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-03-16 Farkhondeh, M., Pritzker, M., Fowler, M., Delacourt, C.
The previously presented mesoscopic model [Phys. Chem. Chem. Phys., 16, 22555, (2014)] for battery electrodes consisting of phase-change insertion materials is incorporated into porous-electrode theory and validated by comparing the simulation results with experimental data from continuous and intermittent galvanostatic discharge of a LiFePO4 electrode under various operating conditions. The model features mesoscopic LiFePO4 units that undergo non-equilibrium lithiation/delithiation and fast solid-state diffusion. Good agreement with the experimental data supports the validity of this model. GITT analysis suggests that the slow evolution of the electrode polarization during each pulse and the subsequent relaxation period is due to Li transport between LiFePO4 units rather than diffusion within the units. Galvanostatic pulse techniques commonly used to determine diffusivities of inserted species in solid-solution systems may also be used to estimate the equilibrium potential of individual mesoscopic units for which no actual measurement has been reported to date. Further analysis of the GITT experiments suggests an alternative pathway for the intermittent charge/discharge of LFP electrodes. Depending on the overall depth-of-discharge/charge of the electrode, relaxation time and the incremental depth-of-discharge/charge of each pulse, the solid-solution capacity available in the Li-rich/Li-poor end-member may be able to accommodate Li insertion/extraction entirely without phase transformation during each pulse.
Understanding Power Enhancement of SOFC by Built-in Chemical Iron Bed: A Computational Approach J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-03-17 Jin, X., Guo, M., White, R. E., Huang, K.
Solid oxide fuel cells (SOFCs) with enhanced fast ramping power capability and overload tolerance can find important applications in grid stability management and critical data center overload protection. Recently, we have demonstrated a new SOFC configuration featuring a built-in chemical Fe-bed in the anode chamber of a tubular SOFC with exceptional fast power ramping capability and overload tolerance. In the present study, we showed our theoretical understanding of the enhanced performance through a two-dimensional axial symmetrical numerical model. The model couples the charge and mass transport in the tubular SOFC with chemical reaction kinetics in the Fe-bed, producing longitudinal distributions of Nernst potential, H2O/H2 molar ratio, local current density and fuel utilization under various operating conditions. The crucial role of Fe-bed in providing instant H2 to support fast ramping and overload currents has been explicitly explained by this computational model.
In Situ Observation and Mathematical Modeling of Lithium Distribution within Graphite J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-03-18 Thomas-Alyea, K. E., Jung, C., Smith, R. B., Bazant, M. Z.
Lithium forms ordered stages when it reacts with graphite. These stages have distinct colors; therefore, optical microscopy gives direct information about the lithium concentration in the graphite. Here we present in situ optical images during charging and discharging of a graphite electrode. Stages are observed to coexist with each other even after extended rest. There is considerable spatial nonuniformity on the microscale. To predict this concentration distribution, we employ a model which combines porous-electrode theory and Cahn-Hilliard phase-field theory to describe the flux of lithium within the graphite. The model closely matches the experimental voltage and concentration distribution. The spatial nonuniformity can be approximated with a relatively simple model of distributed resistances. Finally, we discuss the implications of using the phase-field model instead of a solid-solution model for prediction of lithium plating. The two models give similar predictions of cell voltage and risk of lithium plating under many operating conditions, with the main difference being the relaxation of concentration gradients within particles during rest. The distributed-resistance model shows a higher risk of lithium plating because well-connected particles are overworked as their more-resistive neighbors require a higher driving force for passage of current.
Nonlinear State-Variable Method for Solving Physics-Based Li-Ion Cell Model with High-Frequency Inputs J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-02-17 Guo, M., Jin, X., White, R. E.
A nonlinear state-variable method is presented and used to solve the pseudo-2D (P2D) Li-ion cell model under high-frequency input current and temperature signals. The physics-based governing equations are formulated into a nonlinear state variable method (NSVM), in which the mass transfer variables are evaluated using a 1st order exponential integrator approach at each discrete time point and the electrochemical kinetics (Butler-Volmer) equations are solved by either an iterative or an explicit method. This procedure provides an accurate, computationally efficient method to develop physics-based simulations of the performance of a dual-foil Li-ion cell during practical drive cycles.
Evaluation of an Electrodeposited Bimetallic Cu/Ag Nanostructured Screen Printed Electrode for Electrochemical Surface-Enhanced Raman Spectroscopy (EC-SERS) Investigations J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-02-11 Clarke, O. J. R., St. Marie, G. J. H., Brosseau, C. L.
The field of plasmonics has experienced rapid growth over the past decade with a host of emerging applications including single molecule sensing and plasmon-assisted catalysis. The vast majority of these applications use either silver or gold as the plasmonic metal, which are both high cost and face earth-abundance limitations in the next 100 years. Recent efforts have focused on taking advantage of the plasmonic properties of copper, a more abundant and low cost coinage metal as a sustainable route for plasmonic applications. In particular, there has been great interest in developing copper substrates capable of reliable and efficient enhancement of Raman signals for use in surface-enhanced Raman spectroscopy (SERS) sensing. Herein we describe a sequential electrodeposition technique whereby highly functional and robust Cu/Ag bimetallic SERS-active screen printed electrodes can be produced rapidly and at low cost, which display excellent plasmonic performance and are capable of supporting surface-plasmon assisted catalysis (SPAC). This modified screen printed electrode allows for the in situ spectroelectrochemical investigation of surface redox processes using a sustainable alternative to traditional monometallic electrodes.
Clogging-Free Irreversible Bonding of Polycarbonate Membranes to Glass Microfluidic Devices J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-02-10 Wang, C., Gao, X., Mawatari, K., Kitamori, T.
An irreversible bonding method for bonding porous polycarbonate membranes to glass microfluidic devices is demonstrated. The membrane surfaces were modified with an ammonia solution that contained amino hydrophilic groups. Additionally, the glass substrates were terminated with hydroxyl groups after exposure to an oxygen plasma. Based on the dehydration reaction, reliable bonding between the glass and the porous membrane was achieved at 110°C and was verified by the fluidic leakage tests for burst pressure (>500 kPa) and long-term durability (~5 days). In particular, chemical modification by small ammonia molecules allowed bonding of the porous membranes without clogging to the pores. Therefore, this method has great potential for use in nanofluidic channels integrated with nanoporous membranes. Moreover, a simple disassembly strategy for the sandwich-structured microfluidic devices was proposed and realized for the reuse and recycling of glass substrates with microchannels in the event that the membrane morphology changes after long-term use.
Enhancement of Surface Raman Spectroscopy Performance by Silver Nanoparticles on Resin Nanorods Arrays from Anodic Aluminum Oxide Template J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-01-31 Tu, K. T., Chung, C. K.
The surface enhanced Raman spectroscopy (SERS) substrates were fabricated by many methods including the AAO molding and high-cost nanolithography-etching-deposition process. In this article, we have demonstrated the SERS substrate fabricated with silver nanoparticles (Ag-NP) electroless deposition on highly ordered dense hexagonal arrays of UV curable resin nanorods (R-NR) arrays which was reproducible from a precisely controlled anodic aluminum oxide (AAO) template. This method could improve the non-flatness induced defect problem in the conventional nanoimprinting technique and form high-quality duplications of nanometer templates. The periodic hierarchical double-structured nanorods arrays with small nanorods (diameter about 30~40 nm) in large nanorods (diameter about 100~110 nm) were obtained by the UV curable resins molding from the AAO template. Two kinds of electroless deposition i.e. self-assembly and ultrasonic vibration methods were performed for Ag-NP formation on R-NR. The enhancement factors of SERS using 10–5 M methylene blue (MB) as probing molecules were 104~106 for the self-assembled method and 105~107 for the ultrasonic vibration one, respectively. This was because the ultrasonic vibration deposition could result in Ag nanorods arrays with higher regularity and contact area than the self-assembly one.
Theoretical Studies of Cortisol-Imprinted Prepolymerization Mixtures: Structural Insights into Improving the Selectivity of Affinity Sensors J. Electrochem. Soc. (IF 3.259) Pub Date : 2017-01-25 Manickam, P., Arizaleta, F., Gurusamy, M., Bhansali, S.
The binding affinity of molecularly imprinted polymers (MIPs) relies on the mechanisms and the extent of the functional monomer-template interactions present in the prepolymerization mixture. Thus, a clear understanding and optimizing the physiochemical parameters governing these interactions is key in designing and modeling MIPs with good selectivity. Quantum chemical method was applied here for the theoretical investigation into the interaction between cortisol and pyrrole in a molecularly imprinted prepolymerization mixtures. Since polypyrrole (PPy) is one of the most extensively used conducting polymers in design of bioanalytical sensors, pyrrole is chosen as a functional monomer. The pre-assembly system of possible conformations of cortisol/pyrrole monomer systems have been optimized with the use of density functional theory (DFT) at B3LYP/6-311G (d) level using Gaussian 09 software. The binding energy calculations of a range of structurally related steroids (cortisol, progesterone, prednisolone, 21-deoxycortisol and 6-methyprednisolone) with functional monomer have been analyzed through computational modelling. The most stable configurations of cortisol/functional monomer complexes have been optimized and selected. Based on the conformational analysis and the calculated binding energies of steroid/pyrrole molecular imprinted systems, we have concluded that the interactions between cortisol and pyrrole are more specific and stronger in comparison to the interactions between other steroid hormones (progesterone, prednisolone, 21-deoxycortisol and 6-methyprednisolone) and pyrrole.
Some contents have been Reproduced by permission of The Royal Society of Chemistry.
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