High-Transconductance Graphene Solution-Gated Field Effect Transistors

In this work, we report on the electronic properties of solution-gated field effect transistors (SGFETs) fabricated using large-area graphene. Devices prepared both with epitaxially grown graphene on SiC as well as with chemical vapor deposition grown graphene on Cu exhibit high transconductances, which are a consequence of the high mobility of charge carriers in graphene and the large capacitance at the graphene/water interface. The performance of graphene SGFETs, in terms of gate sensitivity, is compared to other SGFET technologies and found to be clearly superior, confirming the potential of graphene SGFETs for sensing applications in electrolytic environments.Comment: The following article has been submitted to Applied Physics Letters. After it is published, it will be found at apl.aip.or

Hepatitis B Virus Infection Is Associated With Impaired Immunological Recovery During Antiretroviral Therapy in the Swiss HIV Cohort Study

Hepatitis B virus (HBV) infection is a major cause of morbidity and mortality in human immunodeficiency virus (HIV)-infected patients worldwide. It is unclear whether HIV-related outcomes are affected by HBV coinfection. We compared virological suppression and immunological recovery during antiretroviral therapy (ART) of patients of different HBV serological status in the Swiss HIV Cohort Study. CD4 cell recovery during ART was significantly impaired in hepatitis B surface antigen-positive patients and in those with anti-hepatitis B core antigen alone compared with HBV-uninfected patients, despite similar virological efficacy of ART. CD4 increase in patients with resolved HBV infection was similar to that in HBV-uninfected individual

Degradationsmechanismen von Elektrokatalysatoren in alkalischen Medien

Electrochemical energy conversion devices such as electrolyzers and fuel cells possess the unique ability to store and generate electricity from water, the chemical compound that covers 71 % of our planet’s surface. While the four fundamental reactions that make up the full electrocatalytic water cycle seem fairly simple to formulate, the processes involved at the electrocatalyst surface are far from comprehension. Recently, great research effort is devoted to understanding the impact of factors like material choice, pH, surface accessibility, and countless more on the turnover frequency of these reactions. Thus, leading to state of the art devices that have made it to market commercialization, by using highly active platinum group metals (PGM) in an acidic environment. However, their margin of production is still not nearly competitive to that of common fossil fuel energy converters, which is largely due to the high cost of chosen materials. Researches have long recognized that a transition to alkaline systems offers a much broader and more importantly cheaper selection of materials. This could break the niche existence of this technology and hopefully have a significant, positive impact on the inevitable climate crisis. The work at hand focuses on understanding degradation mechanisms of earth-abundant materials during alkaline electrocatalysis. To this point, not much is known about non-PGM stability other than thermodynamic data. Therefore, as the first step towards stable PGM-free or at least PGM-deficient electrocatalysts, state of the art catalysts were gathered within the CREATE project’s consortium and tested towards their stability in operando. This is done mainly by using an electrochemical scanning flow cell connected on-line to an inductively coupled plasma mass spectrometer. With the aid of advanced physical analysis techniques, various unexpected and new degradation mechanisms that are unprecedented in acidic systems were unraveled. Some alternatives, such as manganese oxides and alloying materials such as Cu and Mo are deemed to be unsuitable for the alkaline electrocatalytic water cycle. These assessments are due to low selectivity and the production of harmful intermediate products or simply due to thermodynamic instabilities. More suitable materials such as single-atom iron-nitrogen-doped carbons and NiFe-based alloys are identified as sufficiently stable. Thus clearing them for the next level, where minor optimization can lead to more applied full cell testing steps. For the hydrogen oxidation reaction at the fuel cell anode, no sufficiently active and stable non-PGM catalyst can be identified herein. However, an interesting dissolution mitigation approach is presented. Namely, through ultra-thin active passivation layers of cerium oxide, increased activity and stability of the underlying PGMs can be achieved. Such an activity increase can minder the amount of expensive PGM loadings in the future, and could elongate PGM lifetime tenfold. The increased stability is believed to be due to the semipermeable character of the thin layer that can pass H2, H2O, and OH−. Larger species however such as dissolved Mn+ or O2 are blocked by the film. To aid future catalyst development, a link is drawn between intrinsic metal properties and their impact on the metal’s dissolution behavior. A somewhat universal relation between a metal’s bond strength and its dissolution during thermodynamic transitions exists and can help to predict electrocatalyst lifetime. Last, the cornerstones for a versatile high throughput electrocatalyst synthesis system was developed to hopefully aid material discovery in the future.Energieumwandlungsprozesse durch elektrochemischen Reaktoren, wie z.B. Elektrolyseure und Brennstoffzellen, besitzen die besondere Fähigkeit, Strom in Form von Wasserstoff zu speichern und aus dem selben wieder zu erzeugen. Wasser, die chemischen Verbindung die 71 % der Oberfläche unseres Planeten bedeckt, spielt hierbei die wichtige Rolle des energiearmen Edukts. Während die vier grundlegenden Gleichgewichtsreaktionen, aus denen sich der gesamte elektrokatalytische Wasserkreislauf zusammensetzt, recht einfach zu formulieren sind, sind die Prozesse an der Oberfläche der Elektrokatalysatoren alles andere als eindeutig verstanden. Daher werden zurzeit große Forschungsanstrengungen unternommen, um die Auswirkungen von Materialzusammensetzung, pH-Wert, Oberflächenzugänglichkeit und unzähligen anderen Faktoren auf die Reaktionsrate dieser Reaktionen zu verstehen. Diese hat mittlerweile zu hochmodernen Geräten geführt, die es durch Verwendung höchst aktiver Platingruppenmetalle (PGM) in saurer Umgebung, zur Marktreife geschafft haben. Ihre Produktionsmargen sind jedoch immer noch nicht annähernd konkurrenzfähig, im Vergleich mit herkömmlichen Energiewandlern für fossile Brennstoffe. Dies ist auf die hohen Kosten der gewählten Katalysatormaterialien zurückzuführen. Forscher haben längst erkannt, dass ein Übergang zu alkalischen Systemen eine weite und vor allem kostengünstigere Materialauswahl bietet. Dies könnte die Nischenexistenz solcher Technologien beenden und hoffentlich erhebliche, positive Auswirkungen auf die unvermeidliche Klimakrise haben. Die vorliegende Dissertation beschäftigt sich hauptsächlich mit dem Verständnis von Degradationsmechanismen unedler Materialien während der alkalischen Elektrokatalyse, da bisher noch nicht viel über deren Stabilität bekannt ist außer deren thermodynamischer Stabilität. Innerhalb des Konsortiums des CREATE Projekts, wurden hochmoderne PGM-freie oder zumindest PGM-arme Elektrokatalysatoren gesammelt und auf ihre Stabilität unter Betriebsbedingungen getestet. Dies ist vor allem durch den Einsatz einer elektrochemischen Raster-Durchflusszelle, die mit einem induktiv gekoppelten Plasma-Massenspektrometer verbunden ist, möglich. Mit Hilfe weiterer physikalischer Analytikmethoden können grundlegende Zusammenhänge der Metallauflösung sowie mehrerer unerwarteter Zersetzungsmechanismen entschlüsselt werden, die in sauren Systemen bisher beispiellos sind. Einige Alternativen wie Manganoxide und Ni-Legierungen mit Cu und Mo wurden aufgrund ihrer geringen Selektivität und der damit verbundenen Produktion schädlicher Zwischenprodukte sowie wegen thermodynamischer Instabilitäten als ungeeignet eingestuft. Geeignetere Materialien wie Eisen-Stickstoff-dotierer Kohlenstoff und NiFe-Legierungen wurden als ausreichend stabil eingestuft, sodass eine geringfügige Optimierung zum nächsten Test in Vollzellen führen kann. Für die Wasserstoffoxidationsreaktion an der Brennstoffzellenanode wurde kein ausreichend aktiver und stabiler PGM-freier Katalysator identifiziert. Allerdings wird ein interessanter Lösungsansatz erörtert, der mithilfe von ultra dünnen Ceroxid Filmen, sowohl die Aktivität als auch die Lebensdauer von PGM positiv beeinträchtigt. Es wird angenommen, dass solche Filme semipermeable Eigenschaften besitzen, die das Durchdringen von H2, H2O, und OH− erlauben, aber größere Spezies wie Mn+ oder O2 blockieren. Um zukünftige Katalysatorforschung zu unterstützen, wird eine Verbindung zwischen intrinsischen Eigenschaften von Metallen und ihrem Auflösungsverhalten hergestellt. Es wurde festgestellt, dass ein universeller Zusammenhang zwischen der Bindungsstärke des Metalls und seiner Auflösung während thermodynamischen Übergängen besteht, der die Lebensdauer von Elektrokatalysators vorhersagen kann. Zuletzt wird ein vielseitiges Hochdurchsatzverfahren eingeführt, das in Zukunft die Materialentwicklung beschleunigen und unterstützen soll

Periodicity in the Electrochemical Dissolution of Transition Metals

Extensive research efforts are currently dedicated to the search for new electrocatalyst materials in which expensive and rare noble metals are replaced with cheaper and more abundant transition metals. Recently, numerous alloys, oxides, and composites with such metals have been identified as highly active electrocatalysts through the use of high-throughput screening methods with the help of activity descriptors. Up to this point, stability has lacked such descriptors. Hence, we elucidate the role of intrinsic metal/oxide properties on the corrosion behavior of representative 3d, 4d, and 5d transition metals. Electrochemical dissolution of nine transition metals is quantified using online inductively coupled plasma mass spectrometry (ICP-MS). Based on the obtained dissolution data in alkaline and acidic media, we establish clear periodic correlations between the amount of dissolved metal, the cohesive energy of the metal atoms (Ecoh), and the energy of oxygen adsorption on the metal (DHO,ads). Such correlations can support the knowledge-driven search for more stable electrocatalysts

Electrochemical copper dissolution: A benchmark for stable CO2 reduction on copper electrocatalysts

Copper is well known in fundamental electrocatalysis research due to its ability to selectively reduce CO2 to C2 products. Recent, more applied studies have revealed that electrolyzers based on Cu electrocatalysts can reach current densities of up to hundreds of mA cm−2. This opens up the opportunity for industrial application of Cu-based electrocatalysts. However, the stability of copper must first be assessed. In this communication we investigate the electrochemical corrosion behavior of copper in a broad pH window relevant to CO2 reduction applications. Using an electrochemical on-line inductively coupled plasma mass spectrometer (ICP-MS), we quantify Cu dissolution during anodic oxidation and during the reduction of electrochemically formed oxide species. We show that electrochemical oxidation of Cu leads to high dissolution in neutral and highly alkaline environments, while an intermediate pH of around 9–10 leads to minimal dissolution. The obtained results are discussed in relation to the CO2 reduction reaction to set a benchmark for stable Cu-based electrocatalysts

Periodicity in the Electrochemical Dissolution of Transition Metals

Extensive research efforts are currently dedicated to the search for new electrocatalyst materials in which expensive and rare noble metals are replaced with cheaper and more abundant transition metals. Recently, numerous alloys, oxides, and composites with such metals have been identified as highly active electrocatalysts through the use of high-throughput screening methods with the help of activity descriptors. Up to this point, stability has lacked such descriptors. Hence, we elucidate the role of intrinsic metal/oxide properties on the corrosion behavior of representative 3d, 4d, and 5d transition metals. Electrochemical dissolution of nine transition metals is quantified using online inductively coupled plasma mass spectrometry (ICP-MS). Based on the obtained dissolution data in alkaline and acidic media, we establish clear periodic correlations between the amount of dissolved metal, the cohesive energy of the metal atoms (Ecoh), and the energy of oxygen adsorption on the metal (DHO,ads). Such correlations can support the knowledge-driven search for more stable electrocatalysts

Influence of fuels and pH on the dissolution stability of bifunctional PtRu/C alloy electrocatalysts

The application of organic fuels in fuel cells is an attractive way to circumvent the major drawbacks of hydrogen as an energy carrier, yet catalysis is still a bottleneck for efficient oxidation. One of the most promising bifunctional anode catalysts is PtRu, which has proven to be the state-of-the-art electrocatalyst in alcohol oxidation processes. While plenty of works so far have addressed activity and mechanism of oxidation reactions on PtRu, less is known about the influence of organic fuels on the stability during operation. In this contribution, the effect of isopropanol, methanol, ethanol, formic acid, ammonia, and carbon monoxide on the stability of carbon-supported PtRu was studied both in acidic and alkaline media. The scanning flow cell coupled to an inductively coupled plasma mass spectrometer (on-line ICP-MS) technique allowed the tracking of dissolution events that occurred during the applied electrochemical protocol in real-time. Our main conclusion is that PtRu/C remained stable in the operation range of fuel cells. In addition, if the upper potential limit was further increased PtRu/C was less stable in alkaline medium in which, if compared to acidic electrolyte, approximately 4-times higher Ru and 14-times higher Pt dissolution was measured in the absence of the studied fuels. The onset potential of dissolution was not affected by the presence of fuels (except CO), while dissolution rates were notably affected, most visibly in the case of isopropanol and ammonia in alkaline media and carbon monoxide in both acidic and alkaline media. The observed phenomena are briefly discussed underlining the necessity of more detailed and mechanistic studies to fully understand the reason behind dissolution processes in the presence of the investigated fuels

The quasi-free-standing nature of graphene on H-saturated SiC(0001)

We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001) which was prepared by intercalation of hydrogen under the buffer layer. Using infrared absorption spectroscopy, we prove that the SiC(0001) surface is saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer layer into graphene which exhibits a slight tensile strain and short range defects. The layers are hole doped (p = 5.0 − 6.5 × 1012 cm−2) with a carrier mobility of 3100 cm2/Vs at room temperature. Compared to graphene on the buffer layer, a strongly reduced temperature dependence of the mobility is observed for graphene on H-terminated SiC(0001) which justifies the term “quasi-free-standing.