Daniel R. Mumm

Bijel reinforcement by droplet bridging: a route to bicontinuous materials with large domains

Bijels are non-equilibrium solid-stabilized emulsions with bicontinuous arrangement of the constituent fluid phases. These multiphase materials spontaneously form through arrested spinodal decomposition in mixtures of partially miscible liquids and neutrally wetting colloids. Here, we present a new solid-stabilized emulsion with an overall bicontinuous morphology similar to a bijel, but with one continuous phase containing a network of colloid-bridged droplets. This dual morphology is the result of combined spinodal decomposition and nucleation and growth in a binary liquid mixture containing colloidal particles with off-neutral wetting properties and partial affinity for one liquid phase. The rheology of these systems, which we call bridged bijels, is nearly identical to their simple bijel counterparts, with a unique exponential dependence of the zero-shear elastic modulus on the colloid volume fraction. However, partitioning of the colloids between the spinodal surface and the fluid domains delays the onset of structural arrest, providing access to domain sizes much larger than available in simple bijels without loss of mechanical stability. This ability greatly expands the potential technological applications of these unique materials. In addition, our findings reveal new strategies for tuning the rheology of bijels and outline new directions for future fundamental research on this unique class of soft materials.

Cycling performance of LiNi1−yMyO2 (M=Ni, Ga, Al and/or Ti) synthesized by wet milling and solid-state method

The LiNi1−yMyO2 specimens with compositions of LiNiO2, LiNi0.975Ga0.025O2, LiNi0.975Al0.025O2, LiNi0.995Ti0.005O2, and LiNi0.990Al0.005Ti0.005O2 were synthesized by wet milling and a solid-state reaction method. Among all the specimens, LiNi0.990Al0.005Ti0.005O2 has the largest first discharge capacity of 196.3 mAh/g at a rate of 0.1 C. At n=50, LiNiO2 has the largest discharge capacity of 126.7 mAh/g. LiNiO2 has the best cycling performance, its degradation rate of discharge capacity being 0.73 mAh/g/cycle. LiNi0.975Al0.025O2 shows the lowest decrease rate of the first discharge capacity with C rate. An equation describing the variation of the discharge capacity with the number of charge-discharge cycles, n, is obtained. The Williamson-Hall method is applied to calculate the crystallite size and the strain of the samples before and after charge-discharge cycling.

Variations in the electrochemical properties of metallic elements-substituted LiNiO2 cathodes with preparation and cathode fabrication conditions

Variations in the electrochemical properties of LiNiO2 with preparation and cathode fabrication conditions were studied. The LiNiO2 cathode fabricated with a weight ratio of LiNiO2: acetylene black: binder = 85:10:5, after wet Spex milling for 60 min and drying in a shaking incubator showed the best cycling performance, with a discharge capacity degradation rate of 1.06 mAh/g/cycle between the first cycle and the 20th cycle and a discharge capacity at n = 20 of 143.5 mAh/g at 0.1 C rate. LiNi1−yMyO2 cathodes with active material compositions of LiNiO2, LiNi0.975Ga0.025O2, LiNi0.975Al0.025O2, LiNi0.995Ti0.005O2, and LiNi0.990Al0.005Ti0.005O2 were fabricated under these conditions. Among all the samples, LiNi0.990Al0.005Ti0.005O2 had the highest first discharge capacities at 0.1 C rate (196.3 mAh/g) and 0.5 C rate (147.3 mAh/g). This sample had the smallest R-factor value, indicating that it had the lowest degree of cation mixing. At 0.1 C rate LiNiO2 had the best cycling performance. At 0.5 C rate LiNi0.975Al0.025O2 had the best cycling performance and its discharge capacity degradation rate was 1.28 mAh/g/cycle between the first cycle and the 20th cycle.

Microstructural tunability of co-continuous bijel-derived electrodes to provide high energy and power densities

Emerging demands for national security, transportation, distributed power, and portable systems call for energy storage and conversion technologies that can simultaneously deliver large power and energy densities. To this end, here we report three-dimensional Ni/Ni(OH)2 composite electrodes derived from a new class of multi-phase soft materials with uniform, co-continuous, and tunable internal microdomains. These remarkable morphological attributes combined with our facile chemical processing techniques allow the electrodes salient morphological parameters to be independently tuned for rapid ion transport and a large volumetric energy storage capacity. Through microstructural design and optimization, our composite electrodes can simultaneously deliver energy densities equal to that of batteries and power densities equivalent to or greater than that of the best supercapacitors, bridging the gap between these modern technologies. Our synthesis procedure is robust and can be extended to a myriad of other chemistries for next generation energy storage materials.

Three Dimensional Reconstruction of Solid Oxide Fuel Cell Electrodes Using Focused Ion Beam - Scanning Electron Microscopy

Solid oxide fuel cell (SOFC) electrodes typically consist of contiguous electronically and ionically conducting solid phases in an interconnected porous structure. Quantitatively analyzing this microstructure remains a key challenge in connecting materials processing with electrode performance. This paper will describe the application of dual-beam focused ion beam – scanning electron microscopy for three-dimensional reconstruction of Ni-YSZ cermet anodes, LSM-YSZ cathodes, and (La,Sr)CoO3 cathodes. Different methods for segmentation, i.e. converting images to 3D data sets, are compared and the tradeoff between calculation accuracy and analysis time is discussed.

Characterization of a magnesium-based alloy after hydriding-dehydriding cycling (n=1–150)

The cycling performance of Mg-15 wt% Ni-5 wt% Fe2O3 alloy (named Mg-15Ni-5Fe2O3) was investigated by measuring the absorbed hydrogen quantity as a function of the number of cycles and by examining the variations in the phases and microstructures with cycling. The sample was hydriding-dehydriding cycled 150 times. The absorbed hydrogen quantity decreased as the number of cycles increased from the second to the 150th cycle. The Ha value varied almost linearly with the number of cycles. The maintainability of the absorbed hydrogen quantity was 73.8%, and the degradation rate was 0.007 wt%/cycle for the hydriding reaction time of 60 min. After the 9th hydriding-dehydriding cycle, Mg, Mg2Ni, MgO, and Fe were observed. After 150 cycles, the quantity of the MgO increased. The phases were analyzed using MDI JADE 6.5, a software system designed for XRD powder pattern processing, from the XRD pattern of the Mg-15Ni-5Fe2O3 alloy after the 9th hydriding-dehydriding cycle. The crystallite size and strain of the Mg were then estimated using the Williamson-Hall technique.

Hydrogenation reaction of Mg-Based alloys fabricated by rapid solidification

Mg-23.5wt%Ni-xwt%Cu (x=2.5, 5 and 7.5) alloys for hydrogen storage were prepared by melt spinning and crystallization heat treatment. The alloys were ground by a planetary ball mill for 2 h in order to obtain a fine powder. The Mg-23.5Ni-5Cu alloy had crystalline Mg and Mg2Ni phases. Mg-23.5Ni-5Cu had an effective hydrogen capacity of near 5 wt%. The activated Mg-23.5Ni-5Cu alloy absorbed 4.50 and 4.84 wt%H at 573K under 12 bar H2 for 10 and 60 min, respectively, and desorbed 3. 21 and 4.81 wt%H at 573K under 1.0 bar H2 for 10 and 30 min, respectively. The activated Mg-23.5Ni-5Cu alloy showed a quite high hydriding rate like Mg-10Fe2O3, and higher dehydriding rates than the activated Mg-xFe2O3−yNi. This likely resulted because the melting before melt spinning process has led to the homogeneous distribution of Ni and Cu in the melted Mg, and the Mg-23.5Ni-5Cu alloy has a larger amount of the Mg2Ni phase than the Mg-xFe2O3−yNi alloy.

Improvement of the hydrogen-storage kinetics of Mg by reactive mechanical grinding with 10 wt.% Fe2O3 and 5 wt.% Ni

The addition of Fe2O3 to Mg is believed to be able to increase the hydriding rate of Mg, and the addition of Ni is thought to be able to increase the hydriding and dehydriding rates of Mg. A sample Mg-(10wt.%Fe2O3, 5 wt.%Ni) was prepared by mechanical grinding under H2 (reactive mechanical grinding). The as-milled sample absorbed 4.61 wt.% of hydrogen at 593 K under 12 bar H2 for 60 min. Its activation was accomplished after two hydriding-dehydriding cycles. The activated sample absorbed 4.59 wt.% of hydrogen at 593 K under 12 bar H2 for 60 min, and desorbed 3.83 wt.% hydrogen at 593 K under 1.0 bar H2 for 60 min. The activated Mg-(10wt.%Fe2O3, 5 wt.%Ni) had a slightly higher hydriding rate at the beginning of the hydriding reaction but a much higher dehydriding rate compared with the activated Mg-10 wt.%Fe2O3. prepared via spray conversion. After hydriding-dehydriding cycling, Fe2O3 was reduced, Mg2Ni was formed by the reaction of Mg with Ni, and a small fraction of Mg was oxidized.

Improvement of Hydrogen Storage Properties of Mg by Addition of NbF 5 via Mechanical Milling under H 2

A 90 wt% Mg-10 wt% NbF5 sample was prepared by mechanical milling under H2 (reactive mechanical grinding). Its hydriding and dehydriding properties were then examined. Activation of the 90 wt% Mg-10 wt% NbF5 sample was not required. At n=1, the sample absorbed 3.11 wt% H for 2.5 min, 3.55 wt% H for 5 min, 3.86 wt% H for 10 min, and 4.23 wt% H for 30 min at 593K under 12 bar H2. At n=1, the sample desorbed 0.17 wt% H for 5 min, 0.74 wt% H for 10 min, 2.03 wt% H for 30 min, and 2.81 wt% H for 60 min at 593K under 1.0 bar H2. The XRD pattern of the 90 wt% Mg-10 wt% NbF5 after reactive mechanical grinding showed Mg, β-MgH2 and small amounts of γ-MgH2, NbH2, MgF2 and NbF3. The XRD pattern of the 90 wt% Mg-10 wt% NbF5 dehydrided at n=3 revealed Mg, β-MgH2, a small amount of MgO and very small amounts of MgH2 and NbH2. The 90 wt% Mg-10 wt% NbF5 had a higher initial hydriding rate and a larger quantity of hydrogen absorbed for 60 min than the 90 wt% Mg-10 wt% MnO and the 90 wt% Mg-10 wt% Fe2O3, which were reported to have quite high hydriding rates and/or dehydriding rates. The 90 wt% Mg-10 wt% NbF5 had a higher initial dehydriding rate (after an incubation period) and a larger quantity of hydrogen desorbed for 60 min than the 90 wt% Mg-10 wt% MnO and the 90 wt% Mg-10 wt% Fe2O3.

Operation of an LSGMC electrolyte-supported SOFC with composite ceramic anode and cathode

An all-perovskite-based intermediate-temperature fuel cell was fabricated from materials synthesized using glycine-nitrate process (GNP) combustion synthesis and modified Pechini synthesis routes. Yttrium-doped strontium titanate (SYT) was chosen as the conductive ceramic anode component to avoid problems associated with Ni-based anodes. La0.8 Sr0.2 Ga0.8 Mg0.1 Co0.1 O3-δ electrolyte-supported solid oxide fuel cells (SOFCs) with composite ceramic anode and cathode exhibit a relatively high power density of 0.246 Wcm at 800°C at 0.5 V.