Structural transformations and adsorption properties of PtNi nanoalloy thin film electrocatalysts prepared by magnetron co-sputtering

This is the final peer-reviewed manuscript accepted for publication in Electrochimica Acta Citation of the published version is: Electrochimica Acta 251, 427–441 (2017

Norbornadiene photoswitches anchored to well-defined oxide surfaces: From ultrahigh vacuum into the liquid and the electrochemical environment

Employing molecular photoswitches, we can combine solar energy conversion, storage, and release in an extremely simple single molecule system. In order to release the stored energy as electricity, the photoswitch has to interact with a semiconducting electrode surface. In this work, we explore a solar-energy-storing model system, consisting of a molecular photoswitch anchored to an atomically defined oxide surface in a liquid electrolyte and under potential control. Previously, this model system has been proven to be operational under ultrahigh vacuum (UHV) conditions. We used the tailor-made norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) and characterized its photochemical and electrochemical properties in an organic electrolyte. Next, we assembled a monolayer of CNBD on a well-ordered Co3O4(111) surface by physical vapor deposition in UHV. This model interface was then transferred into the liquid electrolyte and investigated by photoelectrochemical infrared reflection absorption spectroscopy experiments. We demonstrate that the anchored monolayer of CNBD can be converted photochemically to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC) under potential control. However, the reconversion potential of anchored CQC overlaps with the oxidation and decomposition potential of CNBD, which limits the electrochemically triggered reconversion

Electrocatalytic Energy Release of Norbornadiene‐Based Molecular Solar Thermal Systems: Tuning the Electrochemical Stability by Molecular Design

Abstract Molecular solar thermal (MOST) systems, such as the norbornadiene/quadricyclane (NBD/QC) couple, combine solar energy conversion, storage, and release in a simple one‐photon one‐molecule process. Triggering the energy release electrochemically enables high control of the process, high selectivity, and reversibility. In this work, the influence of the molecular design of the MOST couple on the electrochemically triggered back‐conversion reaction was addressed for the first time. The MOST systems phenyl‐ethyl ester‐NBD/QC (NBD1/QC1) and p‐methoxyphenyl‐ethyl ester‐NBD/QC (NBD2/QC2) were investigated by in‐situ photoelectrochemical infrared spectroscopy, voltammetry, and density functional theory modelling. For QC1, partial decomposition (40 %) was observed upon back‐conversion and along with a voltammetric peak at 0.6 Vfc, which was assigned primarily to decomposition. The back‐conversion of QC2, however, occurred without detectable side products, and the corresponding peak at 0.45 Vfc was weaker by a factor of 10. It was concluded that the electrochemical stability of a NBD/QC couple is easy tunable by simple structural changes. Furthermore, the charge input and, therefore, the current for the electrochemically triggered energy release is very low, which ensures a high overall efficiency of the MOST system

Synthesis and Characterization of Bola‐Amphiphilic Porphyrin‐Perylenebisimide Architectures

We report on the synthesis and characterization of a family of three water‐soluble bola‐amphiphilic zinc‐porphyrin‐perylenebisimide triads containing oligo carboxylic‐acid capped Newkome dendrons in the periphery. Variations of the perylenebisimide (PBI) core geometry and dendron size (G1 and G2 dendrons with 3‐ and 9‐carboxylic acid groups respectively) allow for tuning the supramolecular aggregation behavior with respect to variation of the molecular architecture. The triads show good solubility in basic aqueous media and aggregation to supramolecular assemblies. Theoretical investigations at the DFT level of theory accompanied by electrochemical measurements unravel the geometric and electronic structure of the amphiphiles. UV/Vis and fluorescence titrations with varying amounts of THF demonstrate disaggregation.We report the synthetic approach and the characterization of a family of highly water‐soluble porphyrin‐PBI donor‐acceptor amphiphiles by the utilization of oligo‐carboxylic acid capped Newkome dendrons. The amphiphiles form stable aggregates in basic aqueous solutions. The amphiphiles can be individualized by the addition of THF, which manifests itself, for example, in the reinstatement of fluorescence emission. image Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165

Redox-mediated C–C bond scission in alcohols adsorbed on CeO2−x thin films

AbstractThe decomposition mechanisms of ethanol and ethylene glycol on well-ordered stoichiometric CeO2(111) and partially reduced CeO2−x (111) films were investigated by means of synchrotron radiation photoelectron spectroscopy, resonant photoemission spectroscopy, and temperature programmed desorption. Both alcohols partially deprotonate upon adsorption at 150 K and subsequent annealing yielding stable ethoxy and ethylenedioxy species. The C–C bond scission in both ethoxy and ethylenedioxy species on stoichiometric CeO2(111) involves formation of acetaldehyde-like intermediates and yields CO and CO2 accompanied by desorption of acetaldehyde, H2O, and H2. This decomposition pathway leads to the formation of oxygen vacancies. In the presence of oxygen vacancies, C–O bond scission in ethoxy species yields C2H4. In contrast, C–C bond scission in ethylenedioxy species on the partially reduced CeO2−x (111) is favored with respect to C–O bond scission and yields methanol, formaldehyde, and CO accompanied by the desorption of H2O and H2. Still, scission of C–O bonds on both sides of the ethylenedioxy species yields minor amounts of accompanying C2H4 and C2H2. C–O bond scission is coupled with a partial recovery of the lattice oxygen in competition with its removal in the form of water

Towards an efficient liquid organic hydrogen carrier fuel cell concept

The high temperature required for hydrogen release from Liquid Organic Hydrogen Carrier (LOHC) systems has been considered in the past as the main drawback of this otherwise highly attractive and fully infrastructure-compatible form of chemical hydrogen storage. According to the state-of-the art, the production of electrical energy from LOHC-bound hydrogen, e.g. from perhydro-dibenzyltoluene (H18DBT), requires provision of the dehydrogenation enthalpy (e.g. 65 kJ mol-1(H2) for H18-DBT) at a temperature level of 300 °C followed by purification of the released hydrogen for subsequent fuel cell operation. Here, we demonstrate that a combination of a heterogeneously catalysed transfer hydrogenation from H18-DBT to acetone and fuel cell operation with the resulting 2-propanol as a fuel, allows for an electrification of LOHC-bound hydrogen in high efficiency (> 50 %) and at surprisingly mild conditions (temperatures below 200 °C). Most importantly, our proposed new sequence does not require an external heat input as the transfer hydrogenation from H18-DBT to acetone is almost thermoneutral. In the PEMFC operation with 2-propanol, the endothermal proton release at the anode is compensated by the exothermic formation of water. Ideally the proposed sequence does not form and consume molecular H2 at any point which adds a very appealing safety feature to this way of producing electricity from LOHC-bound hydrogen, e.g. for applications on mobile platforms.Fil: Sievi, Gabriel. Forschungszentrum Jülich; AlemaniaFil: Geburtig, Denise. Universitat Erlangen-Nuremberg; AlemaniaFil: Skeledzic, Tanja. Forschungszentrum Jülich; AlemaniaFil: Bösmann, Andreas. Universitat Erlangen-Nuremberg; AlemaniaFil: Preuster, Patrick. Forschungszentrum Jülich; AlemaniaFil: Brummel, Olaf. Universitat Erlangen-Nuremberg; AlemaniaFil: Waidhas, Fabian. Universitat Erlangen-Nuremberg; AlemaniaFil: Montero, María de Los Angeles. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Khanipour, Peyman. Forschungszentrum Jülich; AlemaniaFil: Katsounaros, Ioannis. Forschungszentrum Jülich; AlemaniaFil: Libuda, Jörg. Universitat Erlangen-Nuremberg; AlemaniaFil: Mayrhofer, Karl J. J.. Forschungszentrum Jülich; AndorraFil: Wasserscheid, Peter. Universitat Erlangen-Nuremberg; Alemani

Selektivitätskontrolle in elektrokatalytischen Oxidationsreaktionen durch Ionische Flüssigkeiten

Der so genannte SCILL-Katalysator (englisch: solid catalyst with ionic liquid layer) beschreibt ein neues, äußerst erfolgreiches Konzept im Bereich der heterogenen Katalyse. Hierbei besteht die Grundidee darin, die Selektivität eines Katalysators durch die Beladung mit ionischen Flüssigkeiten drastisch zu erhöhen. In dieser Arbeit zeigen wir, dass das Konzept auf die Elektrokatalyse übertragbar ist und zur selektiven Umsetzung von organischen Verbindungen genutzt werden kann. Bei der hier untersuchten Elektrooxidation von 2,3-Butandiol können zwei Produkte entstehen. Das einfach oxidierte Acetoin und das zweifach oxidierte Diacetyl. Durch die Zugabe einer ionischen Flüssigkeit (1-Ethyl-3-methyl-imidazolium-trifluormethansulfonat, [C2C1Im][OTf]) kann die Selektivität des Katalysators zu Gunsten der Acetoinbildung drastisch erhöht werden. Der zugrundeliegende Mechanismus wurde dabei spektroskopisch in situ untersucht: Die Adsorption des Anions der ionischen Flüssigkeit verhindert die Wasseraktivierung. Dies unterbindet den zweiten Oxidationsschritt vom Acetoin zum Diacetyl und erhöht damit die Selektivität. Unsere Studie zeigt das große Potential elektrochemischer SCILL-Katalysatoren für die selektive Umsetzung von organischen Verbindungen

Strong Activity Changes Observable during the First Pretreatment Cycles of Trimetallic PtNiMo/C Catalysts

Pt‐based alloy catalysts supported on carbon are commonly characterized for oxygen reduction reaction (ORR) activity using the rotating disk electrode technique (RDE). Within this study, we show exemplarily for PtNiMo/C catalysts that the applied pretreatment influences strongly the determined activity. The classically employed descriptor of unchanged cyclic voltammetry response is insufficient to portrait completed surface restructuring, and gives an incorrect impression that stable activity can be determined. This might be one of the reasons for the strongly deviating activities reported in literature. Following the changes in activity during pretreatment also with in‐situ FTIR and online dissolution measurements gives insights to an up to now largely overseen high activity of the trimetallic catalysts. A maximum activity of 0.57 mA cmPt−2 at 0.95 VRHE is reached quickly during the first six cycles and decreases slowly subsequently. The maximum activity and change of activity over the cycle number is affected by the scan rate and electrolyte refreshing, while the gas atmosphere plays only a minor role. This exemplary study might be important for Pt alloy catalysts in general.An up to now unknown activity development is achieved during the pretreatment of alloyed trimetallic PtNiMo/C catalysts. In addition to the recording of steady state CVs under electrochemical cleaning cycles, insight into the unconditioned specific activity of the catalyst reveals a sharp increase during the first five to eight cycles and a further decrease at higher cycle numbers. image European Research Council (ERC)Deutsche Forschungsgemeinschaft (DFG)Federal Ministry of Education and Research (BMBF)Federal Ministry of Education and Research (BMBF)China Scholarship Council (CSC)National Natural Science Foundation of China http://dx.doi.org/10.13039/501100001809Shanghai Cooperation Organization Science and Technology Partnership Projec

Solar energy storage at an atomically defined organic-oxide hybrid interface

Molecular photoswitches provide an extremely simple solution for solar energy conversion and storage. To convert stored energy to electricity, however, the photoswitch has to be coupled to a semiconducting electrode. In this work, we report on the assembly of an operational solar-energy-storing organic-oxide hybrid interface, which consists of a tailor-made molecular photoswitch and an atomically-defined semiconducting oxide film. The synthesized norbornadiene derivative 2-cyano-3-(4-carboxyphenyl)norbornadiene (CNBD) was anchored to a well-ordered Co3O4(111) surface by physical vapor deposition in ultrahigh vacuum. Using a photochemical infrared reflection absorption spectroscopy experiment, we demonstrate that the anchored CNBD monolayer remains operational, i.e., can be photo-converted to its energy-rich counterpart 2-cyano-3-(4-carboxyphenyl)quadricyclane (CQC). We show that the activation barrier for energy release remains unaffected by the anchoring reaction and the anchored photoswitch can be charged and discharged with high reversibility. Our atomically-defined solar-energy-storing model interface enables detailed studies of energy conversion processes at organic/oxide hybrid interfaces

Selective electrooxidation of 2-propanol on Pt nanoparticles supported on Co3O4: an in-situ study on atomically defined model systems

2-Propanol and its dehydrogenated counterpart acetone can be used as a rechargeable electrofuel. The concept involves selective oxidation of 2-propanol to acetone in a fuel cell coupled with reverse catalytic hydrogenation of acetone to 2-propanol in a closed cycle. We studied electrocatalytic oxidation of 2-propanol on complex model Pt/Co3O4(111) electrocatalysts prepared in ultra-high vacuum and characterized by scanning tunneling microscopy. The electrocatalytic behavior of the model electrocatalysts has been investigated in alkaline media (pH 10, phosphate buffer) by means of electrochemical infrared reflection absorption spectroscopy and ex-situ emersion synchrotron radiation photoelectron spectroscopy as a function of Pt particle size and compared with the electrocatalytic behavior of Pt(111) and pristine Co3O4(111) electrodes under similar conditions. We found that the Co3O4(111) film is inactive towards electrochemical oxidation of 2-propanol under the electrochemical conditions (0.3–1.1 VRHE). The electrochemical oxidation of 2-propanol readily occurs on Pt(111) yielding acetone at an onset potential of 0.4 VRHE. The reaction pathway does not involve CO but yields strongly adsorbed acetone species leading to a partial poisoning of the surface sites. On model Pt/Co3O4(111) electrocatalysts, we observed distinct metal support interactions and particle size effects associated with the charge transfer at the metal/oxide interface. We found that ultra-small Pt particles (around 1 nm and below) consist of partially oxidized Pt δ + species which show minor activity towards 2-propanol oxidation. In contrast, conventional Pt particles (particle size of a few nm) are mainly metallic and show high activity toward 2-propanol oxidation