Modeling Nucleation and Growth of Zinc Oxide During Discharge of Primary Zinc-Air Batteries

Metal-air batteries are among the most promising next-generation energy storage devices. Relying on abundant materials and offering high energy densities, potential applications lie in the fields of electro-mobility, portable electronics, and stationary grid applications. Now, research on secondary zinc-air batteries is revived, which are commercialized as primary hearing aid batteries. One of the main obstacles for making zinc-air batteries rechargeable is their poor lifetime due to the degradation of alkaline electrolyte in contact with atmospheric carbon dioxide. In this article, we present a continuum theory of a commercial Varta PowerOne button cell. Our model contains dissolution of zinc and nucleation and growth of zinc oxide in the anode, thermodynamically consistent electrolyte transport in porous media, and multi-phase coexistance in the gas diffusion electrode. We perform electrochemical measurements and validate our model. Excellent agreement between theory and experiment is found and novel insights into the role of zinc oxide nucleation and growth and carbon dioxide dissolution for discharge and lifetime is presented. We demonstrate the implications of our work for the development of rechargeable zinc-air batteries.Comment: 16 pages, 8 figures, Supplementary Information uploaded as ancillary fil

Micro-kinetic modeling of NH3 decomposition on Ni and its application to solid oxide fuel cells

This paper presents a detailed surface reaction mechanism for the decomposition of NH3 to H2 and N2 on a Ni surface. The mechanism is validated for temperatures ranging from 700 to 1500K and pressures from 5.3Pa to 100kPa. The activation energies for various elementary steps are calculated using the unity bond index-quadratic exponential potential (UBI-QEP) method. Sensitivity analysis is carried out to study the influence of various kinetic parameters on reaction rates. The NH3 decomposition mechanism is used to simulate SOFC button cell operating on NH3 fuel

Surface Reaction Kinetics of Steam- and CO₂-Reforming as Well as Oxidation of Methane over Nickel-Based Catalysts

An experimental and kinetic modeling study on the Ni-catalyzed conversion of methane under oxidative and reforming conditions is presented. The numerical model is based on a surface reaction mechanism consisting of 52 elementary-step like reactions with 14 surface and six gas-phase species. Reactions for the conversion of methane with oxygen, steam, and CO₂ as well as methanation, water-gas shift reaction and carbon formation via Boudouard reaction are included. The mechanism is implemented in a one-dimensional flow field description of a fixed bed reactor. The model is evaluated by comparison of numerical simulations with data derived from isothermal experiments in a flow reactor over a powdered nickel-based catalyst using varying inlet gas compositions and operating temperatures. Furthermore, the influence of hydrogen and water as co-feed on methane dry reforming with CO₂ is also investigated

Dynamics of CrO3–Fe2O3 catalysts during the high-temperature water-gas shift reaction: molecular structures and reactivity

A series of supported CrO3/Fe2O3 catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O═)2Cr6+O2 species and a bulk Fe2O3 phase containing some Cr3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 ,and surface dioxo (O═)2Cr6+O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3–-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe0 and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C16O2/H2 → C18O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromium–iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect

Surface Reaction Kinetics for Oxidation and Reforming of H2, CO, and CH4 over Nickel-based Catalysts

An experimental and kinetic modeling study of H2 and CO oxidation, as well as conversion of methane under oxidative and reforming conditions over nickel-based catalyst is presented. The numerical model is based on a newly developed surface reaction mechanism consisting of 52 elementary-steps reactions with 14 surface and 6 gas-phase species. The mechanism was evaluated against experimental data at varying operating conditions performed in this study and also taken from literature

A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH4)2 within Reduced Graphene Oxide.

Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH4]- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications

Process intensification of fuel synthesis and electrolysis

”As more renewable energy is added to the electric grid, energy storage becomes a high priority. Suggestions have been made for energy storage in the form of fuel and chemicals. Currently, Solid Oxide Electrolysis systems can operate in endothermic mode and reduce the electrical requirement by supplying heat. Fuel synthesis from syngas is exothermic and can supply heat. However, the temperature mismatch in the normal operation of the electrolysis step and fuel synthesis step makes the direct utilization of this heat impossible. This work explores possibilities of alternate arrangements of coupling electrochemical systems and chemical synthesis. This work also explores potential for heat integration between the electrolysis and synthesis steps. This is done through exploring higher temperature fuel synthesis systems, and a new intermediate temperature electrolysis system. The successful use of a Mo₂C/HZSM-5 catalyst for ethylene production is shown. Analysis of potential benefits and limitations of each technological approach are examined. The breakeven carbon pricing for the hybrid energy system production of chemicals to be competitive with fossil-fuel based chemical production is calculated”--Abstract, page iii

Microkinetic Model Development for Methane Oxidation over Palladium Catalysts

Der Einsatz von Erdgasmotoren für Anwendungen im Schiffs-, Lkw- und Pkw-Verkehr, stellt im Vergleich zu konventionellen Diesel- oder Benzinmotoren eine direkte Maßnahme dar, um die verkehrsbedingten CO2-Emissionen, sowie Stickoxid (NO), Kohlenmonoxid (CO) und Feinstaub (PM) Belastungen weiter zu senken. Synthetisches Methan, das unter Verwendung erneuerbarer Energien im Power-To-Gas Verfahren erzeugt wird, stellt eine erweiterte Option dar, damit die Ziele, die CO2-Emissionen, im Verkehrsbereich und bei stationären Anwendungen, zu reduzieren, sowie mittel bis langfristig vollständig zu eliminieren, erreicht werden. Typische für Erdgasmotoren eingesetzte Katalystoren verwenden Palladium (Pd) basierte Beschichtungen zur vollständigen Oxidation von Methan. Diese sind aufgrund ihrer hohen Methan Aktivität in der Lage, motorseitig unverbranntes Methan katalytisch zu konvertieren. Dies ist notwendig, weil Methan über einen Zeithorizont von 100 Jahren ein Globales Erderwärmungs Potential (GWP) von über 25 aufweist und damit als starkes Treibhausgas gilt. Ein deutlich verbessertes mechanistisches und modellbasiertes Verständnis dieser am Katalysator ablaufenden Prozesse ist deshalb insbesondere im Hinblick auf die zu erzielende System-Optimierung und Verbesserung, als auch zur Unterstützung des Entwicklungprozesses solcher katalytischen Systeme von großer Relevanz. Das Pd-Katalysator-System zeigt dabei ein äußertst komplexes Verhalten, das sowohl von der Zusammensetzung der Gasphase, der Temperatur als auch von der Vorbehandlung des Katalysators beeinflusst wird. Die Reaktionskinetik der oxidierten Pd-Phase (PdO) unterscheidet sich dabei grundlegend von der reduzierten Pd-Phase. Zusätzliche Änderungen der Pd-Partikelstruktur können während einer Phasenumwandlung ebenso auftreten. Eine damit einhergehende Veränderung spiegelt sich im Reaktionsverhalten wider. Wasser, das sich bei der Verbrennung im Erdgasmotor bildet und folglich als Nebenprodukt über den Abgasstrang in den Katalysator gelangt, wirkt sich negativ auf den Methanumsatz aus. Im Abgas auftretende Reduktionsmittel, CO und H2, sind in der Lage, die katalytisch sehr aktive PdO-Phase chemisch zu reduzieren und so die Aktivität zu mindern. Eine zusätzliche Herausforderung zur Aufrechterhaltung einer stabilen Katalysatoraktivität stellt die in Verbindung mit hohen Temperaturen eintretende thermische Reduktion der PdO-Phase dar. Im Rahmen der vorliegenden Arbeit wurden zwei detaillierte Reaktions Mechanismen entwickelt. Der Mechanismus für die Pd-Phase umfaßt dabei folgende Merkmale: • Beinhaltet folgende Gasphasen Spezies: CH4/O2/CO/CO2/H2/H2O • Berücksichtigt Total-/Partialoxidation von Methan, Wasserdampf Reformierung und Wasser-Gas-Shift Reaktionen • Energetische Berechnungen mit semi-empirischer UBI-QEP-Methode • Eine aktive Pd-Oberflächenspezies; Mean Field Näherung • Verschiedene Methan Aktivierungspfade in Abhängigkeit der Sauerstoff Oberflächenbedeckung • Voraussage ortsaufgelöster Konzentrationsprofile Die Besonderheiten des PdO-Mechanismus zeichnen sich durch folgenden Punkte aus: • Beinhaltet folgende Gasphasen Spezies: CH4/O2/CO2/H2O • Format-Oberflächenspezies; bestätigt durch in-situ DRIFTS • Berücksichtigt Total-Oxidation von Methan • Energetische Parameter basieren auf DFT–Rechnungen • Modellentwicklung als Mean Field Erweiterung für zwei koordinativ ungesättigte (cus) Pd- und O-Adsorptionszentren • Reaktionspfade gemäß Mars-van-Krevelen Mechanismus • Vorhersage des Wasser-Inhibierungseffektes • Voraussage von Light-off Verläufen Beide Mechanismen wurden mittels eines Oxidationsgrad abhängigen Ansatzes verknüpft, um so den Effekt der temperaturabhängigen Phasenumwandlung auf die Kinetik zu berücksichtigen. Eine zusätzliche Validierung des Oxidationsgrad abhängigen Ansatzes in Kombination mit den Mechanismen erfolgte dabei anhand von zyklischen Light-off und Light-out Experimenten