Studying the Interaction of Mass Transport and Electrochemical Reaction Kinetics by Species Frequency Response Analysis

Electrochemical macrokinetics contains the interaction of electrode reactions with transport phenomena. To disentangle the individual processes, dynamic techniques such as electrochemical impedance spectroscopy are widely used. Additional information can be obtained when further quantities besides current and potential are recorded. Here, we present and analyze a method to observe the dynamics of the flux of volatile species, i.e. mass transfer, in porous electrodes during electrochemical reactions with a high time resolution. We call this technique species frequency response analysis (sFRA). It is experimentally demonstrated with electrochemical methanol oxidation reaction on a porous Pt/Ru electrode. The dynamic relationship between current, potential and the flux of the gaseous reaction product CO$_{2}$ is measured by differential electrochemical mass spectrometry. The resulting transfer function that relates current density with CO$_{2}$ flux is analysed in detail by means of a one-dimensional mathematical model. It is demonstrated how the influence of reaction and transport phenomena can be separated in the sFRA Nyquist plot. Practical aspects such as sensitivity and accessible frequency range are discussed as well as the overall prospects and limitations of the technique

Poisoning of ammonia synthesis catalyst considering off-design feed compositions

Activity of ammonia synthesis catalyst in the Haber-Bosch process is studied for the case of feeding the process with intermittent and impurity containing hydrogen stream from water electrolysis. Hydrogen deficiency due to low availability of renewable energy is offset by increased flow rate of nitrogen, argon, or ammonia, leading to off-design operation of the Haber-Bosch process. Catalyst poisoning by ppm levels of water and oxygen is considered as the main deactivation mechanism and is evaluated with a microkinetic model. Simulation results show that catalyst activity changes considerably with feed gas composition, even at exceptionally low water contents below 10ppm. A decreased hydrogen content always leads to lower poisoning of the catalyst. It is shown that ammonia offers less flexibility to the operation of Haber-Bosch process under fluctuating hydrogen production compared to nitrogen and argon. Transient and significant changes of catalyst activity are expected in electrolysis coupled Haber-Bosch process

Optimisation of the Autothermal NH3 Production Process for Power-to-Ammonia

The power-to-ammonia process requires flexible operation due to intermittent renewable energy supply. In this work, we analyse three-bed autothermal reactor systems for design and off-design performance for power-to-ammonia application. The five reactor systems differ in terms of inter-stage cooling methods, i.e., direct cooling by quenching (2Q), combination of indirect and direct cooling (HQ and QH) and indirect cooling (2H) with variations. At optimum nominal operation conditions, the inter-stage indirect cooling (2H) reactor systems result in the highest NH3 production. For off-design performance analysis, NH3 production is minimised or maximised by varying one of the following process variables at a time: inert gas, feed flow rate or H2-to-N2 ratio. For each variation, the effect on H2 intake, recycle stream load and recycle-to-feed ratio is also analysed. Among the three process variables, the H2-to-N2 ratio provided ca. 70% lower NH3 production and 70% lower H2 intake than at nominal operation for all five reactor systems. Operation of autothermal reactor systems at significantly lower H2 intake makes them reliable for power-to-ammonia application; as during energy outage period, shutdown can be delayed

Analysis of Lithium‐Ion Battery State and Degradation via Physicochemical Cell and SEI Modeling

The quality of lithium-ion batteries is affected by the formation of the solid electrolyte interphase (SEI). For a better understanding of its effect on cell performance and aging, fast and economically scalable SEI diagnostics are indispensable. Battery models promise to extract hardly accessible interfacial and bulk properties of the SEI from electrochemical impedance spectra and discharge data. The common analysis of only one measurement, often with empirical models, impedes a precise localization of degradation-related and performance-limiting processes. This work offers a solution by combining physicochemical SEI and cell modeling for the joint analysis of both measurement types. Our analysis highlights the minor importance of the SEI ionic conductivity for cell performance along with a significant improvement and notable effect of its interfacial properties along aging. Such a detailed understanding of the initial SEI and its evolution over time could enable, e. g., a knowledge-based optimization of the cell formation process