Dorin Geiger

Improving the activity and stability of Ir catalysts for PEM electrolyzer anodes by SnO2:Sb aerogel supports: does V addition play an active role in electrocatalysis?

Low Ir loading oxygen evolution reaction (OER) catalysts with superior activity and durability for proton exchange membrane (PEM) electrolyzers are an important topic in industry and academia. One possible strategy for addressing this challenge is the use of support materials that are stable under highly corrosive acidic environments at a high working potential (>1.4 V). Moreover, highly porous structure is another key criteria for OER catalyst support to achieve a high electrochemical surface area. Here, we report a novel Ir supported on a SnO2:Sb aerogel OER catalyst (Ir/SnO2:Sb-mod-V), which was prepared under ambient pressure by using vanadium additives. It shows an unrivaled activity and enhanced stability, on which vanadium does not play any active role but demonstrates the influences that changes the porosity of the aerogel support and affects the impurity content of the chlorine. By taking advantage of the high porosity of the aerogel substrate, Ir/SnO2:Sb-mod-V allows a decrease of more than 70 wt% for precious metal usage in the catalyst layer while keeping a similar OER activity compared to its unsupported counterpart.

Numerical correction of anti-symmetric aberrations in single HRTEM images of weakly scattering 2D-objects

Here, we present a numerical post-processing method for removing the effect of anti-symmetric residual aberrations in high-resolution transmission electron microscopy (HRTEM) images of weakly scattering 2D-objects. The method is based on applying the same aberrations with the opposite phase to the Fourier transform of the recorded image intensity and subsequently inverting the Fourier transform. We present the theoretical justification of the method, and its verification based on simulated images in the case of low-order anti-symmetric aberrations. Ultimately the method is applied to experimental hardware aberration-corrected HRTEM images of single-layer graphene and MoSe₂ resulting in images with strongly reduced residual low-order aberrations, and consequently improved interpretability. Alternatively, this method can be used to estimate by trial and error the residual anti-symmetric aberrations in HRTEM images of weakly scattering objects.

Ultrafast Ionic Liquid-Assisted Microwave Synthesis of SnO Microflowers and Their Superior Sodium-Ion Storage Performance

Tin oxide (SnO) is considered one of the most promising metal oxides for utilization as anode material in sodium ion batteries (SIBs), because of its ease of synthesis, high specific gravimetric capacity, and satisfactory cycling performance. However, to aim at practical applications, the Coulombic efficiency during cycling needs to be further improved, which requires a deeper knowledge of its working mechanism. Here, a microflower-shaped SnO material is synthesized by means of an ultrafast ionic liquid-assisted microwave method. The as-prepared SnO anode active material exhibits excellent cycling performance, good Coulombic efficiency as well as a large capacity delivered at low potential, which is fundamental to maximize the energy output of SIBs. These overall merits were never reported before for pure SnO anodes (i.e., not in a composite with, for example, graphene). Additionally, by combining ex situ XRD and XPS, it is clearly demonstrated for the first time that the Sn-Na alloy, which is formed during the initial sodium sodiation, desodiates in two successive but fully separated steps. Totally different from the previous report, the pristine SnO phase is not regenerated upon desodiation up to 3 V vs Na/Na+. The newly disclosed reaction route provides an alternative view of the complex reaction mechanism of these families of metal oxides for sodium ion batteries.

Conversion/alloying lithium-ion anodes – Enhancing the energy density by transition metal doping

The development of alternative anodes is crucial for next generation lithium-ion batteries that can charge rapidly while maintaining high lithium storage capacity. Among the most promising candidates are conversion/alloying metal oxides like SnO2, for which, however, the irreversibility of the conversion reaction provides a great hurdle – not least with respect to the substantial charge loss and, thus, limited energy density. Herein, we report on the improved reversibility of the conversion reaction by incorporating a transition metal dopant like iron, cobalt, or manganese. While all these dopants provide substantially enhanced capacities due to their beneficial effect on the alloying and conversion reaction, a detailed comparison concerning the achievable capacity at lower voltages, i.e., less than 2.0 V, reveals that the careful selection of the dopant plays a decisive role for the achievable energy density on the full-cell level. In fact, the highest energy density is obtained when doping SnO2 with manganese rather than cobalt or iron because of its relatively lower redox potential and when setting the anodic cut-off to 1.5 V – despite the lower capacity. These results may serve as a general guideline when designing and evaluating alternatives for graphite – in particular, those including a conversion step.

Cobalt Disulfide Nanoparticles Embedded in Porous Carbonaceous Micro-Polyhedrons Interlinked by Carbon Nanotubes for Superior Lithium and Sodium Storage

Transition metal sulfides are appealing electrode materials for lithium and sodium batteries owing to their high theoretical capacity. However, they are commonly characterized by rather poor cycling stability and low rate capability. Herein, we investigate CoS2, serving as a model compound. We synthesized a porous CoS2/C micro-polyhedron composite entangled in a carbon-nanotube-based network (CoS2-C/CNT), starting from zeolitic imidazolate frameworks-67 as a single precursor. Following an efficient two-step synthesis strategy, the obtained CoS2 nanoparticles are uniformly embedded in porous carbonaceous micro-polyhedrons, interwoven with CNTs to ensure high electronic conductivity. The CoS2-C/CNT nanocomposite provides excellent bifunctional energy storage performance, delivering 1030 mAh g-1 after 120 cycles and 403 mAh g-1 after 200 cycles (at 100 mA g-1) as electrode for lithium-ion (LIBs) and sodium-ion batteries (SIBs), respectively. In addition to these high capacities, the electrodes show outstanding rate capability and excellent long-term cycling stability with a capacity retention of 80% after 500 cycles for LIBs and 90% after 200 cycles for SIBs. In situ X-ray diffraction reveals a significant contribution of the partially graphitized carbon to the lithium and at least in part also for the sodium storage and the report of a two-step conversion reaction mechanism of CoS2, eventually forming metallic Co and Li2S/Na2S. Particularly the lithium storage capability at elevated (dis-)charge rates, however, appears to be substantially pseudocapacitive, thus benefiting from the highly porous nature of the nanocomposite.

Crystallinity of Silicon-Shells Deposited onto Germanium and Silicon Nanowires for Core-Shell Nanostructures and Nanotubes

Core-shell nanowires are an essential component of numerous nanoscale device concepts. In this study, germanium and silicon nanowires were synthesized bottom-up using the vapor-liquid-solid technique and successively modified with a silicon shell. The shell synthesis was studied with focus to the crystallinity but includes as well the electrical characteristics of n- and p-doped radial core-shell nanowire devices. Furthermore, the influence of certain post-annealing procedures on the crystallinity of the core-shell nanowires are discussed. We demonstrate that the crystallinity of the silicon shell can be tuned from single-crystalline to amorphous in a controlled manner, independent on a p- or n-type doping, by deliberate adjustment of the growth parameters.

Practical aspects of the use of the X2 holder for HRTEM-quality TEM sample preparation by FIB

The X(2) holder enables the effective production of thin, electron transparent samples for high-resolution transmission electron microscopy (HRTEM). Improvements to the X(2) holder for high-quality transmission electron microscopy (TEM) sample preparation are presented in this paper. We discuss the influence of backscattered electrons (BSE) from the sample holder in determining the lamella thickness in situ and demonstrate that a significant improvement in thickness determination can be achieved by comparatively simple means using the relative BSE intensity. We show (using Monte Carlo simulations) that by taking into account the finite collection angle of the electron backscatter detector, an approximately 20% underestimation of the lamella thickness in a silicon sample can be avoided. However, a correct thickness determination for light-element lamellas still remains a problem with the backscatter method; we introduce a more accurate method using the energy dispersive X-ray spectroscopy (EDX) signal for in situ thickness determination. Finally, we demonstrate how to produce a thin lamella with a nearly damage-free surface using the X(2) holder in combination with sub-kV polishing in the Fischione Instruments׳ NanoMill(®) TEM specimen preparation system.