bluuebunny/crossref_metadata_2025_split huggingface dataset:
Micro- and Mesoporous Carbons and Composites: Nanopores through Salt Templating
Authors: Fechler Nina , Jordan Thomas , Mbaya Mani Christian , Antonietti Markus 
Published: February 2020

The development of more efficient and sustainable synthesis approaches for nanoporous materials is highly demanding. Specifically, porous functional carbons and their composites are very attractive. Yet, besides the quest for convenient precursors, hard templating or activation processes still have to be applied for the introduction of an adequate porosity. Major drawbacks are the use of hazardous chemicals for the template removal and multiple, rather energy consuming synthesis steps.[2, 3]
Herein, we present a new tool called “salt templating”, allowing both the synthesis of functional carbons/composites and the introduction of tunable, high porosity.[4] The general concept is based on crosslinking of a carbon precursor in the presence of a molten salt phase which simultaneously acts as solvent and template. This unique synthesis gives access to a wide variety of compositions, morphologies and porosities where the appropriate choice of salts controls the pore size and architecture on the nanoscale. The pore size can correspond to ion pairs, ion pair clusters and their geometric percolation structures. Contrary to current methods, it is possible to proceed in a single-step synthesis, while the porogen can easily be removed with water and in principal recovered afterwards for further use.
Exemplarily shown for (but not restricted to) ionic liquids (IL) as precursors, not only heteroatom-doped carbons but also composite materials can be synthesized. The IL constitutes the nitrogen-doped carbon matrix and also act as nitrogen source, e.g. for metal nitride formation. After aqueous removal of the salt phase, e.g. VN nanoparticles embedded in a nitrogen-doped carbon matrix with surface areas of up to 2400 m2g-1, are obtained.
Adding to the simplicity of the salt templating approach, examples are chosen to demonstrate the potential of the method as a platform toward the rational design of nanoporous materials in a more sustainable fashion.
References
[1] H. Wang, H. Dai Chemical Society Reviews, 2013 42, 3088.
[2] K. P. Gierszal, M. Jaroniec Journal of the American Chemical Society 2006, 128, 10026.
[3] Y. Zhu, S. Murali, et al. Science 2011 , 332, 1537.
[4] N. Fechler, T.-P. Fellinger, M. Antonietti Advanced Materials, 2013, 25, 75.
Nanosized Oer Catalysts with Superior Activity for Anodes of PEM Electrolysis Produced By a Cost-Effective Procedure
Authors: Gago Aldo Saul , Lettenmeier Philipp , Wang Li , Friedrich K. Andreas 
Published: February 2020

Proton exchange membrane (PEM) water electrolysis represents one of numerous environmentally friendly but at present economically unattractive technologies. Iridium is commonly used as oxygen evolution catalyst (OER) catalyst [1], but is highly priced and one of the rarest elements in the earth crust. Several approaches have been proposed to overcome the challenges associated with the precious metal OER catalyst such as reducing the particle size thus increasing the surface to mass ratio [2], using electroceramic supports [3] or enhancing the phase structure [4]. We present a variety of highly active nanostructured iridium based catalysts for anodes of proton exchange membrane (PEM) electrolysis: IrOx-Ir, IrPt, IrRuOx, IrV, and Ir/Ti4O7. Clusters of nanosized crystallites, Fig. 1a, are obtained by reducing surfactant stabilized IrCl3 in water free conditions [5]. The catalysts shows up to five-fold higher activity towards OER than commercial Ir-black, Fig 1c. The improved kinetics of the catalysts are reflected in the high performance of the PEM electrolyzer (1 mgIr cm-2), showing an unparalleled low over-potential and so far negligible degradation. Our results demonstrate that this enhancement cannot be only attributed to increased surface area but rather to the ligand effect and low coordinate sites resulting in a high turnover frequency (TOF). The catalysts developed herein set a benchmark and pave a strategy for the development of ultra-low loading catalyst layers for PEM electrolysis.

References

[1] M. Carmo, D.L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy 38 (2013) 4901.
[2] E. a. Paoli, F. Masini, R. Frydendal, D. Deiana, C. Schlaup, M. Malizia, T.W. Hansen, S. Horch, I.E.L. Stephens, I. Chorkendorff, Chem. Sci. 6 (2015) 190.
[3] H.-S. Oh, H.N. Nong, T. Reier, M. Gliech, P. Strasser, Chem. Sci. 00 (2015) 1.
[4] M.S. Smith, Rodney D. L. Prévot, R.D. Fagan, Z. Zhang, P.A. Sedach, M.K.J. Siu, S. Trudel, C.P. Berlinguette, Sci. Express (2015).
[5] P. Lettenmeier, L. Wang, U. Golla-Schindler, P. Gazdzicki, N. Cañas, M. Handl, R. Hiesgen, S.S. Hosseiny, A.S. Gago, A.K. Friedrich, Angew. Chemie (2015) In press.

Figure 1
Trion Dynamics in Hole-Doped (6,5) Single-Wall Carbon Nanotube Films
Authors: Eckstein Klaus , Ickert Roman , Kunkel Pascal , Schoeppler Friedrich , Hertel Tobias 
Published: February 2020

Doping of semiconductor nanostructures is essential for their application in electronic as well as photonic devices. Recent studies have focused on the assessment of electrochemical gate doping (1,2) or chemical doping (3) of semiconducting single-wall carbon nanotubes (SWNTs) by absorption and photoluminescence spectroscopy. Moderate doping levels on the order of 0.5 holes·nm-1 were found to lead to a redistribution of oscillator strength from the first subband exciton (S1) to the trion (2).
Here, we report on an investigation of trion dynamics by femtosecond pump-probe transient absorption experiments (VIS pump/NIR probe and NIR pump/NIR probe) in moderately doped (6,5)-SWNT films. SWNTs were charged by gate-doping or by interaction with redox-active gold chloride. We find that optical excitation of energetically lower lying trionic states leads to a delayed rise of the first subband exciton’s S1 photobleach (PB) signature on the ps time-scale. However, the amplitude of the S1 PB signal is surprisingly high considering that the energy separation of trion and exciton exceeds thermal energies by nearly an order of magnitude, making uphill thermal population transfer highly unlikely. Extremely weak photoluminescence from the S1 exciton following trion excitation in hole-doped films indicates that energy transfer from the trion to the first subband exciton is indeed negligible (≲ 10-3). The photobleach is thus essentially due to ground state depletion and reflects the ps-dynamics of the trion decay. Excitation energy and temperature dependent transient absorption experiments reveal that this process is thermally activated.
(1) J. Park et al., J. Am. Chem. Soc.,
134, 14461 (2012).
(2) H. Hartleb, F. Späth and T. Hertel, ACS Nano, 9, 10461 (2015).
(3) R. Matsunaga, K. Matsuda and Y. Kanemitsu, Phys. Rev. Lett., 106, 037404 (2011).
Gas Evolution at Porous Electrodes
Authors: Kadyk Thomas , Eikerling Michael 
Published: February 2020

Porous electrodes for PEM electrolyser and fuel cell technologies are optimized primarily to provide a high specific surface area for the desired electrochemical reactions. However, the increase in surface area comes at the cost of increased voltage losses due to the transport of reactant and product species through the porous medium. In gas evolving electrolyzer electrodes, the formation and transport of gas bubbles plays an important role: on the one hand, gas in the pores replaces the electrolyte phase, thus hindering ion transport and reducing the effective ionic conductivity in the porous electrode; on the other hand, gas bubbles cover and deactivate a portion of the internal surface area.
The impacts of gas formation and removal must be accounted for in the design of electrolyzer electrodes. The focus of the presented work is on the fundamental understanding of the relationships between structure, properties and performance of porous gas-evolving electrodes. The approach combines a macro-scale performance model of the electrode with a micro-scale model of gas evolution. At the macro-scale, a classic porous electrode model describes the transport of ions, electrons and produced oxygen using effective medium theory and accounting for the gas phase volume and distribution.
The micro-scale model describes the formation and growth of gas bubbles based on chemical energy considerations. It can explain the experimentally found high oversaturation that is necessary to nucleate bubbles [1]. Additionally, the size of the bubble nucleus in the experiment was estimated.
The model was used to study the influence of structural parameters on the operation of porous electrodes and to establish guidelines for an enhanced electrode design. It was found that the transport regime of the dissolved gas, viz. diffusion control vs. transfer control at the liquid-gas interface, determines the bubble growth law. Applications of the model to gas evolving porous electrodes in electrolyzers with liquid electrolyte as well as polymer electrolyte electrolyzers (PEEC) will be discussed.
[1] Chen, Q., Luo, L. and White, H. S. Electrochemical generation of a hydrogen bubble at a recessed platinum nanopore electrode. Langmuir 31, 4573–4581 (2015).
Figure 1: Bubble growth on an artificial nucleation site at 20 mA/cm2. Comparison of model results with experimental data from C. Brussieux et al., Electrochimica Acta 56 (2011) 7194-7201. Deviation at large bubble radii due to deformation of the bubbles before detachment, which is neglected in the model.

Figure 1
Synthesis of Metal-Free Electrocatalyst Obtained from Different Biomass Sources with High Performance for Oxygen Reduction Reaction in Fuel Cells
Authors: Alonso-Lemus Ivonne Liliana , Escobar-Morales Beatriz , Rodríguez-Varela Fransico Javier , González-Quijano Diego , Lardizabal Daniel , Pacheco-Saucedo Kehila , de la Torre Saenz Luis 
Published: February 2020

The development of electrocatalysts with low or zero noble metal content is a key factor in the large-scale commercialization of fuel cells. Platinum and platinum alloys are the most efficient catalysts for the oxygen reduction reaction (ORR), which occurs in the cathode of the fuel cell. Nanocarbon materials doped with nitrogen (ex. carbon nanotubes and graphene) have been recognized as a promising alternative for the development of platinum-free electrocatalysts for the ORR. However, the synthesis of these carbon materials often involves the use of harmful organic reagents, synthesis´s extreme conditions, high costs and often are restricted to small scale production. Recently, the use of biomass as source of carbon and nitrogen has been explored to obtain metal-free electrocatalysts. The results reported in the literature indicate that biomass is a very promising alternative for obtaining of electrocatalysts of low-cost and highly efficient in the ORR.
This work describes a two-step method for producing metal-free electrocatalysts from biomass. Two sources of biomass with high nitrogen content in their molecular structure were selected: sargassum and leather. First, each raw material was subjected to pyrolysis treatment at 700 C for 90 minutes under nitrogen, then an activation treatment was given with KOH in nitrogen at 750 C for 90 minutes. The materials were labeled as PAS and PAL for sargassum and leather respectively.
The structural properties of the materials were analyzed by Raman spectroscopy and X-ray diffraction. To determine the morphology scanning electron microscope was used. The electrochemical performance was evaluated by rotating disk electrode technique (RDE).
Raman spectra showed the D and G bands characteristic of carbon materials. This results indicate that after pyrolysis and activation treatments carbonaceous materials with high disorder in their molecular structure were obtained. BET analysis confirmed high surface areas (up to 2000 m2 g-1) and SEM showed the formation of larger porosity in the surface of the electrocatalysts from biomass.
Finally, electrochemical characterization showed high activity for ORR in alkaline media for both electrocatalysts (PAL and PAS). The sample PAL has major current density than platinum electrocatalyst and very close on-set potential (shift 1145 mV). The results obtaining in this work indicate that electrocatalyst from biomass as sargassum and leather are a promising alternative for cathodic reaction in fuel cells.
Quantitative Analysis of Pemel and PEMFC Electrode Drying
Authors: Scheepers Fabian , Burdzik Andrea , Stähler Markus , Lüke Wiebke , Lehnert Werner 
Published: February 2020

The establishment of a fully renewable energy system is predicated on the use of hydrogen as a fuel. Polymer electrolyte membrane (PEM-) water electrolyzers for its generation and (PEM-) fuel cells for its reconversion will be essential components of such a system. In order for this to be economically feasible, however, their costs have to be reduced. The quantity of expensive materials utilized for the construction of cells, including platinum, iridium or titanium must be reduced and overall efficiency increased. The efficiency of these devices is a function of the electrochemical processes that take place in the electrodes and is inseparably tied to their structure. The structure can be affected by controlling various steps in the manufacturing progress. Aside from the chemical composition of the wet coat (consisting of the supported catalyst, Nafion®, solvents and additives) the drying plays a significant role during the self-organization progress in the catalyst layer. Therefore, the drying aspect is one of the main focuses of interest.
Nowadays the most commonly used catalyst-containing dispersions in the manufacturing of membrane electrode assemblies (MEAs) are based on a mixture of organic solvents. In order to clarify the drying process a test rig was constructed. By using a gas phase infrared spectrometer the drying can be monitored by analyzing the waste gas. The set up enables a wide range of conditions in terms of the temperature and the type of drying gas and its preloading. Furthermore, the contact time between the drying gases with the coated layer is adjustable.
As various substances differ in their boiling points a different drying rate is observed for each organic component inside the electrode layer. Consequently, the physical and chemical parameters of the solvent mixture which evaporates during the drying step change dramatically over time. The test rig gives time-resolved information about the selectivity of drying, and the drying rates of the single mixture components in general. The measurement system provides accurate and consistent results. The low measurement uncertainty (< 2% rel.) offers an ideal tool for observing slight modifications during the drying progress.
Identification of Carbon Dioxide Electroreduction Products on Multilayered Metallic Catalysts Based on Palladium
Authors: Januszewska Aneta , Jurczakowski Rafal Robert , Kulesza Pawel J 
Published: February 2020

Atmospheric carbon dioxide levels have risen rapidly over the last few decades. Electrochemical conversion of carbon dioxide into eligible fuels has potential as a technology that may prevent from a serious environmental problems. Electroreduction of carbon dioxide to hydrocarbons is a complex process that usually involves adsorbed carbon monoxide as an intermediate. The overall electroreduction path is determined to a large extent by the catalyst’s surface affinity to carbon monoxide and hydrogen. Strongly bounded CO inhibits further reduction while weakly bounded CO easily desorbs as a final product [1]. One of primary step is CO2 adsorption followed by its reduction to absorbed carbon monoxide.
The presences of adsorbed atomic hydrogen at the catalyst surface can significantly facilitate the formation of hydrocarbons. However, within the potential range of CO2 reduction in the aqueous solution a competitive reaction of hydrogen evolution (HER) takes place, which can even dominate in the electroreduction processes. Our result points out the possibility of tuning catalyst’s affinities to both hydrogen and CO by designing multilayered structures obtained by sequential UPD strategies [2]. Results show that catalytic activity can be finely tuned by using the multilayered near-surface-alloy approach [3]. Our results clearly demonstrate that the Pd-layer thickness and location significantly changes the surface catalytic activity of palladium toward CO2 electroreduction. Interestingly, multilayered films covered with the silver monolayer largely reflect the properties of palladium deposits existing under the Ag surface.
Differential Electrochemical Mass Spectrometry method (DEMS) was used to analyze and determine gaseous products of carbon dioxide electroreduction together with evolved hydrogen. Individual molecules are recognized by ionic current mass signals. DEMS method allows to identify reduction products during faradaic reduction [4]. This technique is particularly useful for investigations of electroinactive molecules such as methane or ethylene. Ion current changes are related to applied potential, and were presented as MS-CV (mass spectrum cyclic voltammograms). MS-CV provide useful information on the formation potentials of each products.
Electroactive products and intermediates, such as carbon monoxide and formic acid were also detected by using classical electrochemical methods. For this purpose a palladium electrode was used for detection of CO2 electroreduction products obtained in macroelectrolysis.
[1]Jitaru, M., et al., Journal of Applied Electrochemistry, 1997. 27(8): p. 875-889.
[2]Januszewska, A., R. Jurczakowski, and P.J. Kulesza, Langmuir, 2014. 30(47): p. 14314-14321.
[3]Greeley, J. and M. Mavrikakis. Journal of Physical Chemistry B, 2005. 109(8): p. 3460-3471.
[4]Baltruschat, H., Journal of the American Society for Mass Spectrometry, 2004. 15(12): p. 1693-1706.
Separation of Lanthanides and Actinides in a Chloride Melt - Liquid Metal System: The Effect of Phase Composition
Authors: Volkovich Vladimir A. , Maltsev Dmitry S. , Melchakov Stanislav Yu. , Yamshchikov Leonid F. , Novoselova Alena V. , Smolensky Valery V. 
Published: February 2020

Pyrochemical processes employing high temperature molten salts can be used for treating spent nuclear fuels (SNF), especially with a high burn-up and after low cooling time. Before returning the fissile materials to the fuel cycle, fission products must be separated and the required separation factor depends on the nuclear reactor type.
Alkali metal chlorides are normally considered as the working media for the pyrochemical reprocessing. Metallic or nitride SNFs can be anodically dissolved in the melt, whereas oxide fuels can be reduced to the metallic state and then subjected to the anodic dissolution. SNF arising from molten salt reactors can be directly used for the reprocessing. Rare earth elements (yttrium, lanthanum and lanthanides) represent an important group of fission products with the electrode potentials more negative than those of uranium and plutonium. During SNF dissolution these elements will also dissolve.
Liquid metals can be effectively employed for the subsequent separation of the fissile materials from the fission products. The separation factor Θ depends on the electrode potentials (E*) of separating elements in the salt phase and their activity coefficients (γ) in the metallic alloy.
If the separating metals form in the salt melt the ions in the same oxidation state (like actinides and lanthanides), the separation factor can be expressed as
lgΘMe1/Me2 = [n·F / (2.303·R·T)]·(E*Me2 – E*Me1) + (lgγMe1 – lg γMe2).
In the present study the effect of temperature, cationic composition of the alkali chloride based melts, and composition of the liquid metal were considered, and separation of uranium from neodymium was studied as an example. Gallium, indium, tin and their mixtures of various compositions were chosen as low melting metals.
Thermodynamic and electrochemical properties of uranium and neodymium were determined between 573–1073 K thus allowing to model separation processes. Cationic composition of the salt phase has little effect on the separation factor, whilst the effect of the metallic phase nature is more pronounced. Efficiency of separation (Θ value) increases with lowering temperature.
An example of thermodynamically possible Nd/U separation factors for various liquid metals and LiCl–KCl–CsCl eutectic melt is presented in the Fig. Here experimentally determined values of E* and γ were used for the calculations.
To confirm the results of calculations, experiments on separating Nd and U were conducted. The experiments were performed under various conditions (e.g., static or with stirring the phases) and the experimentally achieved separation factors compared with the theoretical expectations.

Figure 1
Inverted Metamorphic Multijunction Semiconductors for Exceptionally High Photoelectrolysis Efficiencies: Materials Development and Measurement Challenges
Authors: Deutsch Todd G , Young James L. , Döscher Henning , Steiner Myles , Turner John A 
Published: February 2020

In order to economically generate renewable hydrogen fuel from solar energy using semiconductor-based devices, the U.S. Department of Energy Fuel Cells Technology Office has established technical targets of over 20% solar-to-hydrogen (STH) efficiency with several thousand hours of stability under operating conditions. Our goal is to improve STH efficiency from just over 10% to over 20% via novel tandem semiconductor materials and configurations. Our primary focus is to develop inverted metamorphic multijunction (IMM) III-V semiconductors that have bandgaps optimized for water splitting. The IMM devices are grown by metal organic chemical vapor deposition using a GaAs substrate and use a transparent step-graded buffer layer to allow off-lattice growth. We used IMM growth to achieve a high-quality, lattice-mismatched lower bandgap (1.2 eV) InGaAs bottom cell that captures a larger fraction of the solar spectrum. This 1.8/1.2 eV bandgap combination of the GaInP2/InGaAs configuration is able to generate higher photocurrents, and thus efficiencies, than the 1.8/1.4 eV bandgap combination of the lattice-matched GaInP2/GaAs configuration.
We will also discuss measurement challenges in appraising solar-to-hydrogen efficiency, some of which are specific to tandem absorbers. We have recently discovered several systematic errors in measurements made by our group and others that consistently overrate efficiency. Using more stringent measurement standards, we have confirmed over 15% STH on our most advanced IMM devices. Our focus in the next year will be on photon management strategies at the semiconductor/electrolyte interface. Capturing a significant portion of the ~25% of photons lost to reflection at this interface should allow the realization of devices that exceed 20% STH efficiency.