Periodic array-based substrates for surface-enhanced infrared spectroscopy

At the beginning of the 1980s, the first reports of surface-enhanced infrared spectroscopy (SEIRS) surfaced. Probably due to signal-enhancement factors of only 101 to 103, which are modest compared to those of surface-enhanced Raman spectroscopy (SERS), SEIRS did not reach the same significance up to date. However, taking the compared to Raman scattering much larger cross-sections of infrared absorptions and the enhancement factors together, SEIRS reaches about the same sensitivity for molecular species on a surface in terms of the cross-sections as SERS and, due to the complementary nature of both techniques, can valuably augment information gained by SERS. For the first 20 years since its discovery, SEIRS relied completely on metal island films, fabricated by either vapor or electrochemical deposition. The resulting films showed a strong variance concerning their structure, which was essentially random. Therefore, the increase in the corresponding signal-enhancement factors of these structures stagnated in the last years. In the very same years, however, the development of periodic array-based substrates helped SEIRS to gather momentum. This development was supported by technological progress concerning electromagnetic field solvers, which help to understand plasmonic properties and allow targeted design. In addition, the strong progress concerning modern fabrication methods allowed to implement these designs into practice. The aim of this contribution is to critically review the development of these engineered surfaces for SEIRS, to compare the different approaches with regard to their performance where possible, and report further gain of knowledge around and in relation to these structures

The Bouguer-Beer-Lambert Law: Shining Light on the Obscure

The Beer-Lambert law is unquestionably the most important law in optical spectroscopy and indispensable for the qualitative and quantitative interpretation of spectroscopic data. As such, every spectroscopist should know its limits and potential pitfalls, arising from its application, by heart. It is the goal of this work to review these limits and pitfalls, as well as to provide solutions and explanations to guide the reader. This guidance will allow a deeper understanding of spectral features, which cannot be explained by the Beer-Lambert law, because they arise from electromagnetic effects/the wave nature of light. Those features include band shifts and intensity changes based exclusively upon optical conditions, i. e. the method chosen to record the spectra, the substrate and the form of the sample. As such, the review will be an essential tool towards a full understanding of optical spectra and their quantitative interpretation based not only on oscillator positions, but also on their strengths and damping constants

The Global Polarity of Alcoholic Solvents and Water – Importance of the Collectively Acting Factors Density, Refractive Index and Hydrogen Bonding Forces

The DHBD quantity represents the hydroxyl group density of alcoholic solvents or water. DHBD is purely physically defined by the product of molar concentration of the solvent (N) and the factor Σn=n×f which reflects the number n and position (f-factor) of the alcoholic OH groups per molecule. Whether the hydroxyl group is either primary, secondary or tertiary is taken into account by f. Σn is clearly linearly correlated with the physical density or the refractive index of the alcohol derivative. Relationships of solvent-dependent UV/Vis absorption energies as ET(30) values, 129Xe NMR shifts and kinetic data of 2-chloro-2-methylpropane solvolysis with DHBD are demonstrated. It can be shown that the ET(30) solvent parameter reflects the global polarity of the hydrogen bond network rather than specific H-bond acidity. Significant correlations of the log k1 rate constants of the solvolysis reaction of 2-chloro-2-methylpropane with DHBD show the physical reasoning of the approach

Untersuchung von bipolaren Membranelektrodeneinheiten für die Wasserelektrolyse

Today, water electrolyzers operating in the temperature range below 100 °C primarily employ proton exchange membranes as the solid electrolyte (PEMWE). In contrast to classical liquid alkaline water electrolyzers, these systems allow for higher current densities and (differential) pressure operation. However, one major drawback is their exorbitant demand for costly materials, namely titanium-based bipolar plates and iridium- and platinum-based anode catalysts, due to the acidic environment and the high potentials of the oxygen evolution reaction. Within the past decade, the technological readiness of anion exchange materials concerning hydroxide conductivity and chemical stability has been significantly boosted. These developments allow combining the advantages of membrane-based water electrolyzers with stable, low-cost materials for catalysts and porous transport media typically used in alkaline environments. However, a limitation of anion exchange membrane water electrolysis (AEMWE) is the slow hydrogen evolution kinetics in alkaline- compared to acidic media. As a possible solution, a bipolar membrane water electrolyzer (BPMWE) was postulated, which could combine the advantages of the acidic and the alkaline world: The bipolar membrane comprises an AEM layer facing the anode and a PEM layer facing the cathode. Consequently, high pH conditions at the anode could allow the replacement of costly Ir and Ti components, while the cathode could still operate under ideal conditions at low pH. Theoretically, it is expected that the bipolar membrane can establish a steady-state pH gradient across the membrane electrode assembly (MEA) by catalyzing the dissociation of water into protons and hydroxide at the bipolar interface. This thesis presents the first-time development of bipolar MEAs for water electrolysis with a liquid water feed. The most important steps can be summarized as the following: • Implementation and investigation of modular AEMWE and PEMWE building blocks for MEA operation in pure water based on similar catalysts and gas diffusion media with varying manufacturing approaches and ionomer materials • Development of noble metal free anode electrodes based on synthesized CuCoOx catalyst for AEMWE and their evaluation under various feed conditions considering activity and dissolution for optimum system durability • Implementation of the first liquid water fed BPMWE system and investigation of performance determining parameters such as varying AEM layer thickness and thin IrO2 layers as a water dissociation catalyst • Electrolyzers with the bipolar interface located directly between an alkaline anode and the PEM were found to outperform a PEMWE reference system under similar operating conditions, which was investigated considering local pH, reaction kinetics, and resistances in the MEA caused by the manufacturing method • Investigation of water management in BPMWEs by employing porous AEM layers and varying the hydrophobicity of the carbon-based cathode electrodeIm Temperaturbereich unter 100 °C werden heute im industriellen Maßstab hauptsächlich Protonenaustauschmembranen als Festelektrolyte für die Wasserelektrolyse eingesetzt (PEMWE). Im Gegensatz zum klassischen flüssig-alkalischen Elektrolyseur erlauben diese Systeme weitaus höhere Stromdichten und Betrieb unter (Differenzial-)Druck. Ein großer Nachteil der PEMWE ist jedoch der exorbitante Bedarf an kostspieligen Materialien wie etwa Bipolarplatten auf Titanbasis sowie Katalysatoren aus Iridium und Platin, der in der im sauren Reaktionsmillieu und den hohen Potentialen der Sauerstoffentwicklungsreaktion begründet liegt. Innerhalb des letzten Jahrzehnts war eine drastische Weiterentwicklung von Anionenaustauschermaterialien in Bezug auf Hydroxid-Leitfähigkeit und chemische Stabilität zu verzeichnen. Diese Entwicklungen ermöglichen es, die Vorteile von membranbasierten Wasserelektrolyseuren mit den typischerweise im Alkalischen verwendeten stabilen und kostengünstigen Materialien für Katalysatoren und poröse Transportmedien zu kombinieren. Ein Nachteil der Anionenaustauschmembran-Wasserelektrolyse (AEMWE) ist jedoch die langsame Kinetik der Wasserstoffentwicklung in alkalischen- im Vergleich zu sauren Medien. Als möglicher Lösungsansatz wurde ein Bipolarmembranwasserelektrolyseur (BPMWE) postuliert, der die Vorteile der sauren und der alkalischen Welt vereinen könnte: Die bipolare Membran besteht aus einer AEM-Schicht, die der Anode zugewandt ist, und einer PEM-Schicht, die der Kathode zugewandt ist. Folglich könnte ein hoher pH an der Anode den Ersatz von teuren Ir- und Ti-Komponenten ermöglichen, während die Kathode weiterhin ideal bei niedrigem pH betrieben wird. Theoretisch wird erwartet, dass die bipolare Membran einen stationären pH-Gradienten über die Membranelektrodeneinheit (MEA) hinweg aufrechterhalten kann, indem sie an der bipolaren Grenzfläche die Dissoziation von Wasser in Protonen und Hydroxid katalysiert. Diese Arbeit beschreibt die erstmalige Entwicklung von bipolaren MEAs für die Wasserelektrolyse im Betrieb mit Flüssigwasser. Die wichtigsten Schritte dabei sind folgende: • Implementierung modularer AEMWE- und PEMWE-Bausteine für den MEA-Betrieb in reinem Wasser auf Basis der gleichen Katalysatoren und Gasdiffusionsmedien mit variierenden Herstellungsmethoden und Ionomeren • Entwicklung edelmetallfreier Anodenelektroden auf Basis eines selbst synthetisierten CuCoOx Katalysators für AEMWE und Untersuchung unterschiedlicher Betriebsbedingungen unter Berücksichtigung von Aktivität und Auflösung für optimale Stabilität des Systems • Implementierung der ersten wassergespeisten BPMWE Systems und Untersuchung leistungsbestimmender Parameter wie etwa der AEM-Schichtdicke oder Einbringung von IrO2 an der bipolaren Grenzfläche als Wasserdissoziationskatalysator • Es wurde festgestellt, dass ein Elektrolyseur mit bipolarer Grenzfläche direkt zwischen der alkalischen Anode und der PEM unter analogen Betriebsbedingungen die Leistung einer PEMWE-Referenz übertreffen kann, was unter Berücksichtigung des lokalen pH Werts, der Reaktionskinetik und der unterschiedlichen Widerstände in der MEA untersucht wurde • Untersuchung des Wassermanagements in BPMWE Systemen unter Verwendung poröser AEM Schichten und Variation der Hydrophobizität der Kathod

Beyond Beer's Law: Why the Index of Refraction Depends (Almost) Linearly on Concentration

Beer's empiric law states that absorbance is linearly proportional to the concentration. Based on electromagnetic theory, an approximately linear dependence can only be confirmed for comparably weak oscillators. For stronger oscillators the proportionality constant, the molar attenuation coefficient, is modulated by the inverse index of refraction, which is itself a function of concentration. For comparably weak oscillators, the index of refraction function depends, like absorbance, linearly on concentration. For stronger oscillators, this linearity is lost, except at wavenumbers considerably lower than the oscillator position. In these transparency regions, linearity between the change of the index of refraction and concentration is preserved to a high degree. This can be shown with help of the Kramers–Kronig relations which connect the integrated absorbance to the index of refraction change at lower wavenumbers than the corresponding band. This finding builds the foundation not only for refractive index sensing, but also for new interferometric approaches in IR spectroscopy, which allow measuring the complex index of refraction function. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA