Effects of Iron Species on Low Temperature CO2 Electrolyzers.

Electrochemical energy conversion devices are considered key in reducing CO2 emissions and significant efforts are being applied to accelerate device development. Unlike other technologies, low temperature electrolyzers have the ability to directly convert CO2 into a range of value-added chemicals. To make them commercially viable, however, device efficiency and durability must be increased. Although their design is similar to more mature water electrolyzers and fuel cells, new cell concepts and components are needed. Due to the complexity of the system, singular component optimization is common. As a result, the component interplay is often overlooked. The influence of Fe-species clearly shows that the cell must be considered holistically during optimization, to avoid future issues due to component interference or cross-contamination. Fe-impurities are ubiquitous, and their influence on single components is well-researched. The activity of non-noble anodes has been increased through the deliberate addition of iron. At the same time, however, Fe-species accelerate cathode and membrane degradation. Here, we interpret literature on single components to gain an understanding of how Fe-species influence low temperature CO2 electrolyzers holistically. The role of Fe-species serves to highlight the need for considerations regarding component interplay in general.

Tuning Surface Acidity of Mixed Conducting Electrodes: Recovery of Si-Induced Degradation of Oxygen Exchange Rate and Area Specific Resistance.

Metal oxides are an important class of functional materials, and for many applications, ranging from solid oxide fuel/electrolysis cells, oxygen permeation membranes, and oxygen storage materials to gas sensors (semiconducting and electrolytic) and catalysts, the interaction between the surface and oxygen in the gas phase is central. Ubiquitous Si-impurities are known to impede this interaction, commonly attributed to the formation of glassy blocking layers on the surface. Here, the surface oxygen exchange coefficient (kchem ) is examined for Pr0.1 Ce0.9 O2-δ (PCO), a model mixed ionic electronic conductor, via electrical conductivity relaxation measurements, and the area-specific resistance (ASR) by electrochemical impedance spectroscopy. It is demonstrated that even low silica levels, introduced by infiltration, depress kchem by a factor 4000, while the ASR increases 40-fold and we attribute this to its acidity relative to that of PCO. The ability to fully regenerate the poisoned surface by the subsequent addition of basic Ca- or Li-species is further shown. This ability to not only recover Si-poisoned surfaces by tuning the relative surface acidity of an oxide surface, but subsequently outperform the pre-poisoned response, promises to extend the operating life of materials and devices for which the catalytic oxygen/solid interface reaction is central.

Dominant Role of Heterojunctions in Gas Sensing with Composite Materials.

It is well known that composite materials, consisting of at least two metal oxides, show qualities and sensing behavior very different from the single components. Recently, the preparation of core-shell nanomaterials for gas sensors has become extremely popular. Specifically, these materials have been found to show desirable sensor responses. The preparation of core-shell nanomaterials is, however, complex, limiting the commercial applicability. Composite materials can be more easily attained simply through the mechanical mixing of the various components. Although some studies exist that attempt to compare mechanically mixed composites to those prepared via a synthetic route, these examinations are often flawed, as due to varying preparation methods, the basic characteristics of the materials are not the same. Here, it was possible to separate the role of the contacts between the materials from that of the secondary core-shell structure, by using a soft method to mechanically break apart the structure. This ensures that the difference in morphology is the only change in the material characteristics. It was verified that the composite materials show a different sensing behavior from that of the pure materials. It was also found that regardless of the secondary structure, the composite materials showed very similar sensor responses. By examining materials containing different ratios of Cr2O3 to SnO2, it was possible to attribute the sensor behavior changes to the contacts between the different metal oxides. It was shown that by varying the concentration of each oxide it is possible to attain either an n- or p-type response and at a certain concentration even no response. This work is significant because it identifies that the contact between the materials plays the dominant role in the sensor response and it shows the viability of mechanical mixing for composite sample preparation.

WO3-Based Gas Sensors: Identifying Inherent Qualities and Understanding the Sensing Mechanism.

Semiconducting metal oxide-based gas sensors are an attractive option for a wide array of applications. In particular, sensors based on WO3 are promising for applications varying from indoor air quality to breath analysis. There is a great breadth of literature which examines how the sensing characteristics of WO3 can be tuned via changes in, for example, morphology or surface additives. Because of variations in measurement conditions, however, it is difficult to identify inherent qualities of WO3 from these reports. Here, the sensing behavior of five different WO3 samples is examined. The samples show good complementarity to SnO2 (the most commonly used material)-based sensors. A surprising homogeneity, despite variation in morphology and preparation method, is found. Using operando diffuse reflectance infrared Fourier transform spectroscopy, it is found that the oxygen vacancies are the dominant reaction partner of WO3 with the analyte gas. This surface chemistry is offered as an explanation for the homogeneity of WO3-based sensors.

Direct Microscopic Proof of the Fermi Level Pinning Gas-Sensing Mechanism: The Case of Platinum-Loaded WO3.

It is widely known that the sensing characteristics of metal oxides are drastically changed through noble metal oxide surface additives. Using operando infrared spectroscopy it was identified that the Fermi level pinning mechanism dominates the sensor response of platinum-loaded WO3. Spectroscopy, however, provides information about the sample only on average. Traditional microscopy offers structural information but is typically done in vacuum and on unheated sensors, very different than the operation conditions of metal oxide gas sensors. Here, state-of-the-art in situ scanning transmission electron microscopy offers spatially resolved information on heated samples at atmospheric pressure in varying gas atmospheres. As a result it was possible to directly couple microscopically observed structural changes in the surface noble metal nanoclusters with IR spectra and sensor responses. On the basis of the findings, the dominant Fermi level pinning mechanism could be validated. The presented work demonstrates the benefits of coupling in situ microscopy with operando spectroscopy in order to elucidate the sensing mechanism of metal oxides.

Rhodium Oxide Surface-Loaded Gas Sensors.

In order to increase their stability and tune-sensing characteristics, metal oxides are often surface-loaded with noble metals. Although a great deal of empirical work shows that surface-loading with noble metals drastically changes sensing characteristics, little information exists on the mechanism. Here, a systematic study of sensors based on rhodium-loaded WO3, SnO2, and In2O3-examined using X-ray diffraction, high-resolution scanning transmission electron microscopy, direct current (DC) resistance measurements, operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and operando X-ray absorption spectroscopy-is presented. Under normal sensing conditions, the rhodium clusters were oxidized. Significant evidence is provided that, in this case, the sensing is dominated by a Fermi-level pinning mechanism, i.e., the reaction with the target gas takes place on the noble-metal cluster, changing its oxidation state. As a result, the heterojunction between the oxidized rhodium clusters and the base metal oxide was altered and a change in the resistance was detected. Through measurements done in low-oxygen background, it was possible to induce a mechanism switch by reducing the clusters to their metallic state. At this point, there was a significant drop in the overall resistance, and the reaction between the target gas and the base material was again visible. For decades, noble metal loading was used to change the characteristics of metal-oxide-based sensors. The study presented here is an attempt to clarify the mechanism responsible for the change. Generalities are shown between the sensing mechanisms of different supporting materials loaded with rhodium, and sample-specific aspects that must be considered are identified.

Understanding the Potential of WO3 Based Sensors for Breath Analysis.

Tungsten trioxide is the second most commonly used semiconducting metal oxide in gas sensors. Semiconducting metal oxide (SMOX)-based sensors are small, robust, inexpensive and sensitive, making them highly attractive for handheld portable medical diagnostic detectors. WO3 is reported to show high sensor responses to several biomarkers found in breath, e.g., acetone, ammonia, carbon monoxide, hydrogen sulfide, toluene, and nitric oxide. Modern material science allows WO3 samples to be tailored to address certain sensing needs. Utilizing recent advances in breath sampling it will be possible in the future to test WO3-based sensors in application conditions and to compare the sensing results to those obtained using more expensive analytical methods.