bluuebunny/crossref_metadata_2025 huggingface dataset:
Ageing of Materials at Inlet and Outlet Fuel Manifolds in a SOFC Stack
Authors: Piccardo Paolo , Pecunia Andrea , Bongiorno Valeria , Spotorno Roberto , Wuillemin Zacharie , Ouweltjes Jan Pieter
Published: February 2020

The ageing behaviour of glass-ceramic seal materials and of alloys is a key element for a durable performance of SOFC stacks. Post-experiment analyses performed on a stack operated for more than 4000h at 750°C and at high fuel utilization (80%) reveal significant differences in ageing behaviour of such materials when comparing fuel inlet and fuel outlet areas.

Two samples were cut from the fuel inlet and outlet manifolds of an operated SOFC stack. The glass-ceramic sealing material and the alloy were exposed to a dual atmosphere (external air and internal fuel stream). The fuel composition was of dry H2/N2 (60:40 vol. %) at the inlet and of H2O/H2/N2 (48:12:40 vol. %) at the outlet. The stack was operated in co-flow configuration, with a large excess of air. The paper is focused on the sealant, on the structural steel and on the interfaces between both materials. The following figure schematically describes the samples and the zones of interest.

Both samples were mounted in epoxy-resin, polished and observed by optical microscopy and SEM-EDX.

It was found that the interfaces between the seal and the alloy were significantly affected by the difference in fuel composition, with more severe degradation at the fuel outlet side. Further, an increased porosity of the sealant was observed on the same location, while the inlet sample remained less affected. Finally, the sealant exposed to the outgoing air was significantly polluted by chromium generated in the stack and transported by the air stream.

These investigations therefore reveal that the long-term evolution of the sealing and of the metallic support depends on the local conditions in the stack, as well as on additional interactions with other degradation phenomena.

Acknowledgement: The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207.

Figure 1
Ab-Initio and Experimental Studies of Silicon Dioxide Anode Material for Lithium Ion Battery
Authors: Moon Janghyuk , Cho Kyeongjae , Cho Maenghyo
Published: February 2020

Silicon sub oxide SiOx (x ≤ 2) has been considered as a potential LIB anode. Although SiOx is thermodynamically unstable at all temperature, it consists of amorphous Si and SiO2 clusters leading to active/ inactive structures that help buffer the strain during cycling and hence much improved reversible capacities. It was thought that the role of SiO2 was buffer matrix of Si, however, some researchers reported that amorphous SiO2 reacts with Li and can be used for anode material for LIBs. And recently, communities both academy and industry have considered the idea of tailoring oxidation of proportionated SiOx to increase the initial capacity and cycle ability compared to Si. Despite these encouraging developments of Si oxide anode, the fundamental understanding is still limited and no atomistic study has been reported.

In this presentation, we study the structural stability of SiOx (0 ≤ x ≤ 2) , reaction mechanisms of Li in c-SiO2, and possibility reaction in amorphous Si sub oxide using both theoretical and experimental method. Based on the analysis of SiO2, we also discuss structural evolution and voltage profile of LiySiOx (0 ≤ y ≤ 4) during lithiation with respect to oxygen contents (0 ≤ x≤ 2).
Bio-Templating of Micro/Nanostructured Fe3O4/Carbon Composite for High-Performance Lithium-Ion Batteries
Authors: Zhang Ning , Yan Xiaohui , Li Jia , Zhang Li , NG Dickon H.L.
Published: February 2020

To meet the increasing demands on clean and efficient energy storage systems, rechargeable lithium-ion batteries (LIBs) are the most promising candidate for energy storage devices. They have attracted tremendous attention in both scientific and industrial fields. Intensive efforts have been devoted to exploring new electrode materials and to design novel nanostructures for LIBs. Among the available electrode materials, magnetite (Fe3O4) is regarded as an appealing material for LIBs due to its high theoretical storage capacity, natural abundance, as well as environmental friendiness. Herein, a green and facile strategy based on the in-vitro biomineralization of unicellular chlorella is reported to synthesize the micro/nanostructured porous Fe3O4/carbon composite for the use as anode material in high-performance lithium-ion batteries. The as-synthesized hierarchical Fe3O4/carbon composite exhibits a large reversible specific capacity (~660 mA h g-1at 100 mA g-1), excellent cyclic stability (~100% during the first 50 cycles), superior rate capacity and high Coulombic efficiency. It is found that the enhanced electrochemical performance is probably attributed to (i) the unique micro/nanostructure of Fe3O4/carbon composite having high specific surface area in comparison with other kinds of electrode materials, and (ii) the hierarchically distributed pore sizes via biomineralization of chlorella unicellular. This fabrication process is simple and low-cost, and introduces a new methodology to design and synthesize electrode materials for lithium-ion batteries.
Gallium Arsenide As a New Alloying Anode for Lithium Ion Batteries
Authors: Hays Kevin Alan , Wagner Michael J , Banek Nathan A
Published: February 2020

Li alloying metals have much greater specific and volumetric energy densities than current state of the art graphite used in commercial Li ion batteries.  However, the extremely large volumetric changes that the metals undergo when alloying with Li lead to the structural degradation of the electrode with consequent poor cycle life.   We are attempting to make stable Li alloying metal electrodes, specifically with gallium arsenide, by akalide reduction on carbon matrices.

Arsenic has been largely ignored as a candidate for Li alloying anode materials despite having the third best gravimetric capacity of the alloying metals and being abundant and inexpensive, on par with the cost of bulk silicon1. Gallium, a lithium alloying metal that melts at 29.8°C, has shown promise because of its self-healing ability at increased temperatures2,  although it suffers from a lower gravimetric capacity than some of its more popular lithium alloying counterparts. 

Here we show the synthesis of Gallium Arsenide by akalide reduction.  It is characterized by powder x-ray diffraction, transmission electron microscopy, thermo-gravimetric analysis, and x-ray fluorescence spectroscopy.  Electrochemical testing of lithium half-cells, cycled from 0.02 to 1.5V versus lithium at a rate of C/10, showed a reversible capacity of 630 mAh/g with good capacity retention.

(1) Nitta, N.; Yushin, G. Particle & Particle Systems Characterization, 31, 317-336.

(2) Deshpande, R. D.; Li, J. C.; Cheng, Y. T.; Verbrugge, M. W. Journal of the Electrochemical Society, 158, A845-A849.

Figure 1
Monodispersed Poly(ethylene oxides) and Their Role in Conductivity Mechanism in Crystalline Polymer Electrolytes
Authors: Hekselman Aleksandra K. , Bruce Peter G
Published: February 2020

Previous research in the area of polymer electrolytes has been mainly focused on complexes prepared with alkali metal salts and polydispersed poly(ethylene oxides) (abbreviated to PEO).1 Previous studies have shown that polymer chain length may play a significant part in the ion transport in the crystalline phase but high distribution of molecular weight of the commercially available polymers complicates the understanding of the conductivity mechanism.2

By employing monodispersed polymers (with a discrete chain length) we attempt to reduce a number of variables that are likely to influence the ion transport process. However, monodispersed poly(ethylene oxides) are not readily available and therefore, a synthetic method has been developed to obtain a series of monodispersed homologues in a variety of chain lengths.

These monodispersed polymers were used to prepare a series of crystalline complexes with a lithium salt (PEO6:LiPF6).  Correlation between chain length of the polymer and unit cell parameters of the crystalline complexes in combination with molecular dynamics simulations3 revealed different structural arrangements at polymer chain-end regions. These arrangements and their role in coordination of lithium ions, ion mobility and transport, and the resulting conductivities will be presented.

[1] P. G. Bruce, B. Scrosati and J. M. Tarascon, Angewandte Chemie-International Edition 2008, 47, 2930-2946. [2] E. Staunton, Y. G. Andreev and P. G. Bruce, Faraday Discussions 2007, 134, 143-156. [3]A. Liivat, D. Brandell, A. Aabloo and J. O. Thomas, Polymer 2007, 48, 6448-6456.
Proton Transport in Hydrocarbon Membranes for Fuel Cell Applications
Authors: Choe Yoong-Kee
Published: February 2020

The nature of proton transport in various hydrocarbon membranes are investigated using the first-principles molecular dynamics simulations. Three hydrocarbon membranes, sulfonated poly ether sulfones (SPES), sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (SPPBP) and sulfonated poly (arylene ether ether nitile)s (SPAEENs) were studied in detail. Especially, we focus on the effects of various native chemical structure of the membranes on the transport of protons. We found that a hydrophillic functional group in those membranes can be protonated by accepting a proton when a hydronium ion (H3O+) comes close to its vicinity. Such a result indicates that hydrophillic functional groups, such as a carbonyl group, affect the proton transport in a negative manner. The negative effects were found to be dominant under low hydration conditions.

Furthermore, it was observed that hydrophillic functional groups tend to attract water molecules, which, in turn, depletes hydration around the sulfonate group where such an effect hinders proton dissociation under low hydration conditions.

Figure 1
Pressureless Sintered Dense, Free Standing, Li+ Conducting Garnet Thin Films (< 50 µm) from Flame Made Nanoparticles
Authors: Yi Eongyu , Wang Weimin , Laine Richard
Published: February 2020

Li7La3Zr2O12 (LLZO) garnet electrolytes show the highest potential utility in future solid state Li batteries due to a wide electrochemical stability window, stability in contact with lithium metal, and high ionic conductivity. The literature is replete with methods of optimizing compositions targeting optimal conducting properties, however, no examples of thin films (< 50 µm) with ionic conductivities rivaling bulk counterparts (0.1-0.3 mS cm-1) have been reported.

To date, LLZO has been pressed into pellets (1-2 mm) and either pressureless sintered at 1100-1200 ºC for 10-40 h covered in mother powder or at 40-60 MPa at 1000-1100 ºC for 1-4 h. Both approaches are energy intensive and the scalability is questionable.

Although LiTi2(PO4)3 solid electrolytes have been studied extensively and processed by tape-casting, a low cost approach, no similar LLZO examples have been reported. Some challenges in processing LLZO thin films include high lithium loss rates, reaction of LLZO with common ceramic crucibles, and energy and equipment intensive sintering processes.

In this work, LLZO nano-powders produced by LF-FSP (liquid-feed flame spray pyrolysis) are used to formulate ball-milled suspensions that are then cast. Dried green films can be made in a process that offers potential for continuous casting. Films are cut and sintered at selected sintering schedules providing dense thin films. 30-50 wt. % excess lithium is used to compensate for lithium loss during sintering.

Green films were sintered above 900 ºC for selected dwell times to show that even a difference of 0.2 h at peak temperature undergo phase composition changes due to lithium loss. Sintered films show trans-granular fracture mode with ionic conductivities of 0.1-0.2 mS cm-1 and >95 wt. % LLZO.

The very first example of LLZO thin films with similar properties to bulk counterpart is reported. Furthermore, shorter processing steps and lower energy input for sintering compared to other reported methods make usage of LF-FSP synthesized nano-powders more desirable.
Lithium-Silica Nanosalt As an Electrolyte Additive for Lithium-Ion Batteries
Authors: Hamenu Louis , Kaner Richard B. , Ko Jang Myoun
Published: February 2020

A lithium-modified silica nanosalt (Li-SiO2, coded Li202) of hydrophobic fumed silica (R202) is synthesized to use as an electrolyte additive for lithium-ion batteries (LIBs) under low temperature conditions. The synthesis method consists of reacting the silica nanoparticles with LiH and consequently quickly reacting the conjugate silicate ions with 1,3-propanesultone as a surface stabilizer. The obtained Li202 nanosalt (2.5 wt.%) is added into an electrolyte solution of 1.0 M LiPF6 dissolved in ethylene carbonate/propylene carbonate/ethylmethyl carbonate/diethyl carbonate (20:5:55:20 vol.%) + 2 wt.% vinylene carbonate. The electrolyte solution including the Li202 nanosalt shows higher ionic conductivity and superior electrochemical stability over 5 V, which is due to the stabilized surface group. The high-rate capability at -20oC of the LiCoO2/graphite cell is particularly enhanced by adding Li202 nanosalt. In addition, excellent cycle performance at -20°C endorses the use of Li202 nanosalt as a low-temperature electrolyte additive for LIBs.
(Invited) Efficient Electrocatalytic CO2 Reduction with Dinuclear Iron Complexes
Authors: Naruta Yoshinori
Published: February 2020

Recently, reductive conversion of CO2 to useful carbon resources draws broad attentions from environmental and catalytic to biological sciences. Though many metal complexes as well as pure metals including mainly mononuclear rare metals such as Ru or Re have been proposed to date, they show rather high overpotentials and/or poor product selectivities. Thus, new catalyst design for efficient and robust CO2reduction is strongly demanded.

   In biological systems, anaerobic bacteria and archaea use oxygen-sensitive Ni-/Fe-containing carbon monoxide dehydrogenases (CODHases), which realize efficient mutual conversion between CO2 and CO at extremely low overpotentials (< 100 mV) (Fig. 1A).[1]  The dinuclear active site motif with base metal ions is attractive as chemical models. According to this strategy, we designed iron porphyrin dimers linked by 1,2-phenylene group (Fig. 1B) as the catalysts and applied them for electrochemical CO2 reduction.  The catalysts show extremely high catalytic TOF and high CO selectivity at relatively low overpotentials (η).  When electron-withdrawing meso-aryl groups are used, η
 becomes as low as 0.4 V.[2]  From the detailed CV analysis, Fe porphyrin monomer, e.g., FeTPP, shows the dischage current at its Fe0 state in CO2 atmosphere and FeII/III and FeI/II couples appear at the same potential as observed in Ar.  The FeTPP dimer, however, shows a considerable positive shift of its FeI/II couple in CO2. This suggests that the FeII state of the dimer will make strong association with CO2 molecule and the resultant complex shows its reduction peak at more positve potential than its original FeI/II couple.  Since FeII porphyrin monomesr are generally intact with CO2, this is the first example of the formation of the CO2-binding to FeII porphyrin by its dimerization. This is the reason of the considerable decrease of the CO2 reduction overpotential.  This result also supports the two nucleus motif as observed in CODHase active site promote the CO2 reduction at low η.

 When two Fe ions are separated in a larger distance as observed in 1,3-phenylene linked Fe dimer, this cooperative effect of the two FeII ions is not work any longer and the m-dimer just work as two monomers.

The present Fe porphyrin dimmr catalysts are quite robust and any current drop was not observed after 12 h at a constant potential electrolysis. High CO selectivity of these Fe catalysts is also advantage for their practical use on the aspect of easy product separation.

Regerences

[1] J.-H. Jeoung and H. Dobbek, Science, 318, 1461 (2007); J. Fesseler, J.-H. Jeoung, and H. Dobbek, Angew. Chem. Int. Ed., 54, 8560 (2015).

[2] E. Mohamed, Z. Zaharan, and Y. Naruta, Chem.
Commun, 51, 16900 (2015). (Cover article)

Figure 1
(Invited) Tetrapyrrole-Based Metal Complexes Designed for Bio-Inspired Catalysis and Solar Fuel Production
Authors: Knör Günther
Published: February 2020

Light-induced substrate transformations in artificial photosynthetic devices and functional enzyme model systems strongly depend on the feasibility of multielectron transfer catalysis [1,2]. The crucial advantage of coupling multiple redox equivalents and proton transfer steps in photocatalysis is to avoid free radical reaction pathways and energetic constraints due to charge accumulation, as these processes can decrease both the long-term stability and the efficiency of the corresponding systems. In this context, certain coordination compounds including tetrapyrrole-based derivatives such as porphyrin, corrole and phthalocyanine complexes have been shown to achieve an efficient fusion of the complementary functions of a light-harvester, a photoredox interface, a substrate recognition site and a mediator for chemical reactions [3-6]. To further improve the performance of such systems, the design of long-term stable processes for artificial photosynthetic energy storage and green synthetic chemistry using artificial (photo)enzymes has to be achieved with sustainable, environmentally benign and earth-abundant bulding blocks. Another crucial aspect in the context of solar chemistry and catalysis is not only the requirement for using visible light, but also to shift the threshold wavelength of the photosensitizers as far as possible to the red and NIR-spectral regions. Some of our recent developments in this direction will be presented in this contribution.

[1] G. Knör, Chem. Eur. J.
2009, 15, 568

[2] G. Knör, Coord. Chem. Rev.
2015, 304-305, 102

[3] M. Hajimohammadi, C. Schwarzinger, G. Knör, RSC Advances
2012, 2, 3257

[4] C. Uslan, K. T. Oppelt, L. M. Reith, B. Ş. Sesalan, G. Knör, Chem. Commun.
2013, 49, 8108

[5] K. T. Oppelt, E. Wöß, M. Stiftinger, W. Schöfberger, W. Buchberger, G. Knör, Inorg. Chem.
2013, 52, 11910

[6] M. Ertl, E. Wöß, G. Knör, Photochem. Photobiol. Sci.
2015, 14, 1826
(Invited) Metal Assembly of a Dendrimer with a Porphyrin Core
Authors: Yamamoto Kimihisa
Published: February 2020

Dendrimers are highly branched organic macromolecules with successive layers or “generations” of branch units surrounding a central core. Organic inorganic hybrid versions have also been produced, by trapping metal ions or metal clusters within the voids of the dendrimers. Their unusual, tree-like topology endows these nanometre-sized macromolecules with a gradient in branch density from the interior to the exterior. Here we show that tin ions, SnCl2 and FeCl3complexes to the imines groups of a spherical polyphenyl-azomethine dendrimer in a stepwise fashion according to an electron gradient, with complexation in a more peripheral generation proceeding only after complexation in generations closer to the core has been completed.

Many metal halides can be assembled inside the phenylazomethine dendrimer with a Zn-porphyrin core(DPA-ZnP) by the strong coordination ability of the imines in DPAs. The 60 equivalents of SnCl2 should be included in the fourth generation DPA-ZnP (DPAG4-ZnP), which has 60 imine sites, because SnCl2 is complexed with an imine at the ratio of 1:1. Complexation of DPA G4-ZnP with SnCl2 was confirmed as a shift in the absorption based on p-p* transition of imines in the UV-vis spectra. During the complexation of DPA G4-ZnP with SnCl2, four shifts in the isosbestic point were observed in the spectra. If SnCl2 molecules are complexed with the imines in DPAs at random, the isosbestic point is not shifted during the titration of SnCl2. However, the isosbestic point was observed at 370, 368, 366, and 362 nm during the addition of 0-4, 5-12, 13-28, and 29-60 equivalents of SnCl2 to the dichloromethane/ acetonitrile solution of DPA G4-ZnP, respectively. These shifts show that the complexation proceeds in four steps. The amounts of the added SnCl2 in each step, i.e., 4, 8, 16, and 32 equivalents, agree with the number of imines in the 1st to 4th shells. This result shows that SnCl2 molecules are assembled from the core to the terminals of DPA G4-ZnP in a stepwise fashion. This radial stepwise complexation was also observed for the other dendrimers with phenylazomethine dendron skeletons. This complexation behavior means that the number and the position of the metal ions in the dendrimers are controllable.
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.
Chemistry at the Dirac Point: Organometallic Hexahapto (η6) Complexation Chemistry of Graphene
Authors: Sarkar Santanu
Published: February 2020

Organometallic complexes of carbon materials are potentially useful in organometallic catalysis, as a means to increase the dimensionality of graphene and carbon nanotubes, as interconnects between graphitic surfaces and as constituents for the realization of new materials. We have recently shown that the bis-η6-bond between the benzenoid ring systems of graphene and carbon nanotubes (SWNTs), is capable of electronically conjugating these systems and preserves the delocalized band structure by the constructive rehybridization of the graphene ∏-system with the vacant chromium dΠ orbital. Bis-hexahapto organometallic chemistry offers a new approach to electrically contacting and interconnecting graphitic surfaces and may lead to new materials and phenomena with potential applications in fields such as organometallic catalysis and atomic spintronics.
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
Role of Metal Nanoparticles in Photo/ Catalytic Activation of Carbon Dioxide
Authors: Bienkowski Krzysztof , Solarska Renata , Augustynski Jan
Published: February 2020

Investigations of the photo-induced phenomena enabling to improve and understand fundamental processes involved in CO2 reduction includes co-assembly of plasmonic and catalytic metal nanoparticles to the build-up of a semiconductor photocathode. Combination of the nanostructures of plasmonic metals, such as silver or gold with other metal electrocatalysts will substantially decrease the energy barriers of the CO2 reduction process through interaction of photo-excited surface plasmon states with adsorbed reaction intermediates. The significance to investigate reduction of CO2 at gold, silver and copper surfaces enhanced by the extra activation of the process provided by the illuminated, incorporated plasmonic metallic nanostructures is supported by the occurrence of the photo-emission process associated with the decay of photo-excited surface plasmons. Of particular interest will be a strong influence of the Ag/FTO electrode roughness upon the photo-emissive cathodic currents of electro-reduction of CO2 dissolved in aqueous solution. Actually, the two metals Ag and Au, the nanostructures of which exhibit strong excitation of surface plasmon resonance through interaction with near UV and visible photons are also catalytically active toward electro-reduction of CO2. Besides the photo-emission, another property of plasmonic NPs – heat generation resulting from the absorption of incident photons and their influence on the reduction of CO2 at composite cathodes will be the objectives of this presentation.
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.
Ultrasensitive CuInS2 Quantum Dots for Detection of Nickel Ions in Drinking Water
Authors: Abdel-Fattah Tarek M , Ebrahim Shaker , Ali Magdy , El Nady Jehan , Soliman Moataz
Published: February 2020

Quantum dots (QDs) have been applied in many research fields such as chemistry, biology and medicine because of their unique physical-chemical properties such as broad absorption spectra, narrow symmetric emission bands and high photo-stability [1]. QDs showed great promises for detection of organic and inorganic analysts. For example, CuInS2 as QDs have been used for detection of ascorbic acid and folic acid [2]. In this study, a sensitive and simple method based on the fluorescence quenching of CuInS2 quantum dots (QDs) were used for the determination of nickel ions (Ni(II) ions) in drinking water samples. Water-soluble and biocompatible 3-mercaptopropionic acid-capped CuInS2 QDs was synthesized by one step aqueous route. The fluorescence intensity of synthesized QDs significantly decreased in the presence of Ni(II) ions. The emission intensity of CuInS2 QDs has an inversely proportional with the concentration of Ni(II) a linear fit with negative slope. The detection range was for Ni2+ concentration in the range of 133 to 2133 ppm. The method showed good sensitivity and was satisfactorily applied to the determination of Ni2+ contamination in real water samples, obtained from tap water.

References:

[1] Liang JG, Huang S, Zeng DY, He Z, Ji X, Ai X, et al. CdSe quantum dots as luminescent probes for spironolactone determination. Talanta 2006; 69:126–30.

[2]Liu SY, Hua JJ, Su XG. Detection of ascorbic acid and folic acid based on water-soluble CuInS2 quantum dots. Analyst 2012; 137:4598–604.