Search

Low degradation is a key feature for a successful commercialization of solid oxide fuel cell (SOFC) systems. A variety of degradation mechanisms influences the overall degradation rate. Nickel depletion, agglomeration and coarsening in anodes during operation are considered as an important degradation mechanism. In this work, the microstructure of SOFC anodes of electrolyte-supported cells with operation times up to 20,000 hours (850 °C) were analyzed. The examined anodes consist of a porous cermet of nickel and gadolinium doped ceria. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis were used to investigate the nickel distribution in the anode. The results show nickel depletion at the electrolyte/anode interface, which becomes more noticeable for increased operation time. In addition, nickel agglomeration in the contact layer and in the functional layer was found. A relationship between nickel agglomeration and depletion was deduced.

Solid oxide fuel cells (SOFCs) are electrochemical conversion devices, which essentially consist of two porous electrodes separated by a dense, oxide ion conducting electrolyte. The performance and the durability of SOFCs strongly depend on the electrode microstructure. In this paper, the impact of a relatively long exposure time (up to 20 000 h) under realistic operation terms (temperature (T) = 850 °C, current density (J) = 190–250 mA cm−2) in the kinetics of microstructural degradation are investigated for porous nickel (Ni)/ceria gadolinium oxide (CGO) anodes, to understand the microstructural evolution in SOFC cermet anodes. A combined system of Focused Ion Beam (FIB) and Scanning Electron Microscope (SEM) tomography was used to analyze various anode microstructures aged during different operating times (2500 h, 15 000 h and 20 000 h). The methodologies of image acquisition as well as the segmentation and the object recognition were improved, offering a reliable quantification of Ni-grain growth, connectivity, tortuosity factor and triple phase boundary length (TPBL). The representative volume element (RVE) was also studied, and its dependence on aging time was confirmed. To construct a volume that can be accurate and representative for the whole sample, the necessary corresponding 3D reconstruction size was adjusted by increasing operating time, in order to suppress the influence of microstructure variation caused by Ni and CGO agglomeration. Statistically significant 3D microstructural changes were observed in the anode by increasing the operating time, including nickel particle size distribution, changes in connectivity of the ceramic part (CGO) and a significant decrease of contiguous triple phase boundary densities. Additional qualitative observations were done in order to gain a complete insight of the degradation phenomena in nickel based cermet anodes.

Fuel cells are an important technology to match the volatile energy production with the energy demand in a sustainable energy system. One crucial task is to raise the Pt mass specific activity of Pt oxygen reduction catalysts in fuel cells and to increase their stability. Therefore, different approaches are investigated like the facetting of Pt nanoparticles and the use of supporting particles like SnO2, which stabilize Pt. For the first time, the growth of facetted Pt nanoparticles on SnO2 nanoparticles is reported which combines both approaches. The synthesis is based on a polyol process and employs Ag ions to influence the shape evolution of the Pt nanoparticles. The samples were studied by transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and cyclic voltammetry (CV). Since SnO2 is etched during the original synthesis, KOH is added to compensate the protons released. The fraction of the facetted Pt nanoparticles is lowered in the presence of SnO2 likely due to adsorption of Ag on the SnO2 surface and, thus, a decrease of the Ag concentration in the bulk solution. Consequently, the Ag concentration was increased leading to the generation of a fair amount of facetted Pt nanoparticles. To avoid the occupation of Ag redeposited on the Pt catalyst's surface and, thus, lowering of its catalytic activity, a cleaning procedure consisting of potentiostatic oxidation of Ag and replacement of the electrolyte was applied.

Understanding the degradation of graphite anodes plays a major role in the improvement of Li-ion batteries. Herein, we used the nondestructive character of lock-in thermography (LIT) to investigate the aging of graphite anodes in pouch cells. The aged cells show a typical ring pattern in lock-in thermography images, which can be explained by the help of Raman spectroscopy. We attribute this pattern to the different contact due to the formation of a gas during operation, which also leads to inhomogeneous degradation of the graphite electrode. Finally, we demonstrate the detection of two-dimensional variances on the sub-mm scale in the composition of the graphite anodes in pouch cells by LIT. For this purpose, we combined LIT measurements with Raman spectroscopy mapping to link differences in the heat emission to those in the structure of the graphite.

Ternary metal tin phosphides are promising candidates for electrochemical or catalytic applications. Nevertheless, their synthesis, neither as bulk nor nanomaterials is well investigated in the literature. Here, we describe a general synthetic strategy to convert bimetallic M–Sn (M = Ni, Co, and Fe) nanoparticles to ternary metal phosphides by decomposition of tributylphosphine at 300 °C. At high phosphorus concentrations, Ni3Sn4 nanoparticles convert to hybrid structured Ni2SnP and β-Sn. The CoSn2 and FeSn2 nanoparticles undergo a phosphorization, too and form hybrid nanocrystals reported here for the first time, containing ternary or binary phosphides. We identified the crystal structure of the nanoparticles via XRD and HRTEM measurements using the diffraction data given for Ni2SnP in literature. We were able to locate the Ni2SnP and β-Sn crystal structure within the nanoparticles to demonstrate the phase composition of the nanoparticles. By transferring the synthesis to cobalt and iron, we obtained nanoparticles exhibiting similar hybrid structures and ternary element compositions for Co–Sn–P and binary Fe–P and FeSn2 compositions. In the last step, we used the given information to propose a conversion mechanism from the binary M–Sn nanoparticles through phosphorization.

For the first time, sub 10 nm sized intermetallic Ag 4 Sn particles are prepared via an aqueous synthesis route in order to improve the electrochemical performance of pure Sn nanoparticles. High-resolution transmission electron microscopy, X-ray diffraction and thermogravimetric analysis are used to investigate the morphology, crystal structure and particle surface of the as prepared Ag 4 Sn nanoparticles. In addition, galvanostatic cycling and cyclic voltammetry measurements are carried out to characterize the electrochemical behavior of the particles. Upon lithiation and de-lithiation a phase transformation from Ag 4 Sn to Ag 3 Sn is observed, which has not been reported so far. The intermetallic nanoparticle-based anode delivers a specific de-lithiation capacity of 460 mAhg −1 for more than 150 cycles.

One major challenge of proton-exchange membrane fuel cells, water electrolyzers, and unitized regenerative fuel cells is to increase the oxygen catalysts’ stability by the systematic preparation of inhomogeneous catalyst surfaces via decoration with cocatalysts. Iridium (Ir) nanodots with a typical diameter of 2 nm were colloid chemically deposited on ruthenium oxide (RuO2) nanoparticles as oxygen evolution catalyst and on platinum (Pt) nanorods to yield bifunctional oxygen catalysts. The stepwise synthesis allows a higher control of the composite nanoparticles’ morphology and thus their activity and stability. The stability toward oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) was studied with cyclic voltammetry. Structural characterization was carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) spectroscopy before and after the stability protocols. Ir nanodots as well as Ir-decorated RuO2 nanoparticles were found to be ...

The vanadium air redox flow battery is a combination of a redox flow battery and a reversible fuel cell. For the oxygen reduction during discharge, platinum (Pt) catalysts are common. During operation, vanadium (V) cations can penetrate through a proton exchange membrane into the water/air half-cell. The aim of the present work is to study whether V compounds are deposited on the Pt surface under operation conditions or whether the V ions influence the stability of Pt in any other way. Thereby, bulk platinum electrodes are compared as a simple model system to carbon-supported Pt nanoparticles via cyclic voltammetry. In the case of bulk platinum, electrochemical quartz crystal microbalance measurements showed no deposition of vanadium compounds but indicated the decrease of the (hydr)oxide layer on Pt above V3+ and VO2+ redox potentials. Cycling 100 times between oxygen reduction and oxygen evolution potentials with and without a heavy V contamination did not lead to significant degradation of the model catalyst and shows no influence of V ions. On the contrary, the nanoparticle-based catalyst significantly degraded during the same stability protocol. The V contamination lowered the degradation in this case.

In this study, the crystallographic structure of orthorhombic e-phase Ag3Sn nanoparticles with different sizes between 7 and 120 nm was studied by X-ray diffraction, and the influence of the size and the composition of the nanocrystals on the lattice parameters were investigated. Rietveld refinement was used to confirm the orthorhombic (space group Cmcm) crystal structure and to determine the lattice parameters and the size of the crystals for all nanoparticle samples. We found an anisotropic decrease of the lattice parameters which furthermore depends on the size of the crystallites. Etching the nanoparticles with sulfuric acid allows us to control the Ag/Sn ratio of the Ag3Sn nanoparticle samples. The lattice parameters decrease for higher Ag/Sn ratio.

Despite a large effort in catalyst research over the past decade, the benefit of electrocatalysts for the oxygen evolution reaction (OER) and especially the oxygen reduction reaction (ORR) in the aprotic Li/air battery system has not yet been clarified. Here, three nanostructured manganese oxide catalysts – namely Mn3O4, Mn5O8 and α-Mn2O3 – are investigated with regard to their activity for the ORR in a LiTFSI/DMSO electrolyte. In cyclic voltammetry (CV) measurements an overall decrease of potential gaps and an increase of re-oxidation efficiencies on carbon powder-based electrodes in comparison to glassy carbon (GC) was observed, which is attributed to the presence of more active centers, e.g. edges and kinks. Increased ORR potentials and the kinetic evaluation of the rate-determining step, namely the one-electron reduction of oxygen, point to a significantly enhanced activity of α-Mn2O3/C compared to pure carbon powder, Mn3O4/C and Mn5O8/C electrodes. This is discussed in terms of the electrocatalytic effect of α-Mn2O3 for aprotic ORR processes. The ORR activity is proposed to originate from a different reaction pathway due to coordinatively unsaturated Mn3+ ions on the surface of α-Mn2O3, which act as active centers for associative adsorption and reduction of molecular O2.

Tin nanoparticles can be transformed from the metallic β-Sn structure to the semiconducting α-Sn structure after electrochemical lithiation and delithiation at room temperature. Here, we studied the influence of the size of the crystallites on the β- to α-transformation in Sn nanoparticles. Differently sized Sn/SnOx nanoparticles were synthesized, processed in electrodes and cycled ten times in a lithium-ion cell at room temperature. X-ray diffraction (XRD) patterns before and after electrochemical lithium insertion/extraction reveal that samples with small particles contain the α-Sn structure. The critical size for this transformation is 17(4) nm. Smaller particles were transformed into the α-Sn structure while particles larger than 17 nm retain the β-Sn structure. Temperature dependent XRD measurements show that this α-Sn structure is stable up to 220 °C before its reflections disappear. The formation of the α-Sn structure at room temperature in small particles and the unexpected high transition point can be explained by the substantial contribution of the surface energy (facilitating formation of alloys not observed in the bulk), lithium impurities in the α-Sn structure and the Li2O shell which is formed during lithium insertion.

In situ X-ray diffraction (XRD) was performed to study the formation of the α-Sn structure in nanocrystalline Sn-based electrodes during electrochemical lithium insertion and extraction at room temperature. Therefore, pure β-Sn nanoparticles were synthesised and further processed into electrodes. The lithiation and de-lithiation process of the β-Sn nanoparticles follows the formation of discrete lithium–tin phases which perfectly fits the voltage plateaus in the charge/discharge diagram. However, unlike bulk electrodes, where no α-Sn is formed, we observed the formation of the semiconducting α-modification at 870 mV vs. Li within the first de-lithiation process. This observation explains earlier reports of an increasing internal resistance of such an electrode. Additionally, our study supports earlier suggestions that predominantly small tin crystallites are transformed from the β-Sn phase into the α-Sn phase, while larger crystallites retain their metallic β-Sn structure.

The lattice constants of metallic nanoparticles shrink with respect to that of a bulk material. This behavior affects the properties of nanoscaled crystallites and can influence their application potential. In this work, we investigate the size-dependent lattice parameters of core/shell Sn/SnOx nanoparticles, synthesized via a simple chemical reduction method. Therein, the use of appropriate surface ligands, reaction temperature, and reaction time allows us to tune the mean particle size from 6 to 104 nm. X-ray powder diffraction revealed that the s-Sn reflections shift toward higher angles for smaller particles, showing a size-dependence of the lattice constants. The change in the lattice constants varies, depending on the direction, and can be described as an inverse function of the diameter of the crystallites. The different degree of deformation can be explained by the direction dependency of the bulk modulus K and the interface energy γ of the monocrystalline tin nanoparticles.

Secondary Lithium/air batteries are of great interest because of their considerably larger theoretical energy densities (~ 11,000 Wh/kg) compared to conventional battery systems like NiCd, Ni metal hydride or Li ion accumulators (~ 300 Wh/kg) in combination with larger power densities. The current main limitation of the aprotic Lithium/air system is given by the stability of the electrolyte, which is generally based on organic media like carbonates and ethers. Recent results by Bruce et al. show the decomposition and irreversible side product formation of these organic solvents in Lithium ion-containing solutions. They suggest that the reversible Lithium oxide formation during discharge is almost completely suppressed by several side reactions, resulting in a blockage of the air cathode pores and a decrease of capacity. Moreover, the performance of the Lithium/air cell is strongly dependent on the structure and composition of the air cathode as well as on the addition of catalysts. Here, the development of bifunctional catalysts, i.e. one catalyst for the discharge (oxygen reduction) as well as for the charge (oxygen evolution) reaction, is of special interest. Therefore the colloidal synthesis of nanometerscale manganese oxide particles and their electrochemical characterization in the Lithium/air system will be discussed. Fig. 1) indicates that the use of noble metals as catalysts results in reduced stability of the electrolyte, as larger currents are measured at the Pt electrode of the RRDE in a LiTFSI/acetonitrile system than at the carbon electrode (representing the air cathode without catalyst). We report on a systematic study on the stability of different aprotic electrolyte systems containing a variety of conducting salts (e.g. LiPF6, LiTFSI), solvents (acetonitrile, DMSO, ionic liquids) and additives via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and rotating ring-disk electrode (RRDE) analysis (see Fig. 1). Furthermore the replacement of noble metal nanoparticles as catalysts for ORR/OER by considerably less expensive side group metal oxide nanoparticles will be discussed. There special attention lies on the different catalytic pathways of metal and metal oxide systems. In particular the mechanisms of side group metal oxide catalysts will be evaluated for their use in future secondary Lithium/air batteries.

Solid oxide fuel cells (SOFCs) are getting more importance with their promising future, due to their high-energy conversion efficiency, low-emissions and flexibility of usable fuel type. The performance and the lifetime of SOFCs are keenly dependent on electrode microstructure. In order to recognize the microstructural evolution and its degradation kinetics in SOFC cermet cathodes during long exposure time (up to 20 000 h) under realistic operating conditions ( T= 850 °C, J= 190-250 ), investigations on porous LSM/YSZ cathodes were conducted. The 3D-tomography technique (FIB/SEM) offers extensive data about the microstructures of various cathode aged during different operating times (2 500 h, 15 000 h and 20 000 h), which allows an exact quantification of particle size distribution, phase-connectivity, tortuosity factor and Triple Phase Boundary Length (TPBL). With the increasing operating time no significant 3D microstructural changes in the cathode were noticed in the obtained data. However, additional qualitative X-ray fluorescence measurement, indicate a clear presence of chrome contamination on aged cathodes, which may be the main degradation mechanism in the SOFC cathode. Keywords: chrome poisoning, long operating time, particle size distribution, SOFC, triple phase boundary, SEM, XRF

Tin is able to lithiate and delithiate reversibly with a high theoretical specific capacity, which makes it a promising candidate to supersede graphite as the state-of-the-art negative electrode material in lithium ion battery technology. Nevertheless, it still suffers from poor cycling stability and high irreversible capacities. In this contribution, we show the synthesis of three different nano-sized core/shell-type particles with crystalline tin cores and different amorphous surface shells consisting of SnOx and organic polymers. The spherical size and the surface shell can be tailored by adjusting the synthesis temperature and the polymer reagents in the synthesis, respectively. We determine the influence of the surface modifications with respect to the electrochemical performance and characterize the morphology, structure, and thermal properties of the nano-sized tin particles by means of high-resolution transmission electron microscopy, x-ray diffraction, and thermogravimetric analysis. The electrochemical performance is investigated by constant current charge/discharge cycling as well as cyclic voltammetry.

not Available.