In our efforts to address the issues of Si based anodes for Li ion batteries, such as limited active mass loading, rapid capacity degradation and low scalability in manufacturing, we reported a scalable, high mass loading, and additive-free Si nanoparticles (NP) deposition based electrode, but the achieved capacity and cycle life were still limited. In order to improve the reversible capacity and cycling stability of this Si NP deposition electrode, in this work, we have investigated various substrates for Si deposition, including carbon paper (CP), preheated CP and stainless steel felt/mesh (SSF/SSM), and their influences on the electrochemical Li-ion storage performance of the Si NP electrodes. Meanwhile, protective encapsulations of amorphous carbon or silicon nitride on Si NP has been performed and the capabilities of these coatings in improving the cycling stability of Si NP electrodes have been researched. It is found that a carbon-coated Si NP deposition on an SSM substrate achieves an extraordinary cycling stability in electrochemical Li-ion storage for 500 cycles with an average capacity loss of 0.09% per cycle, showing significantly improved commercial viability of Si NP deposition based electrodes in high-energy-density Li-ion batteries.

Nanostructured silicon has been intensively investigated as a high capacity Li-ion battery anode. However, the commercial introduction still requires advances in the scalable synthesis of sophisticated Si nanomaterials and electrodes. Moreover, the electrode degradation due to volume changes upon de-/lithiation, low areal electrode capacity, and application of large amounts of advanced conductive additives are some of the challenging aspects. Here we report a Si electrode, prepared from direct deposition of Si nanoparticles on a current collector without any binder or conducting additives, that addresses all of the above issues. It exhibits an excellent cycling stability and a high capacity retention taking advantages of what appears to be a locally protective, yolk-shell reminiscent, solid electrolyte interphase (SEI) formation. Cycling an electrode with a Si nanoparticle loading of 2.2 mg cm⁻² achieved an unrivalled areal capacity retention, specifically, up to 4.2 mAh cm⁻² and ~ 1.5 mAh cm⁻² at 0.8 mA cm⁻² and 1.6 mA cm⁻², respectively.

A detailed structural analysis of Mg-Ti-H thin films reveals the presence of a chemically partially segregated but structurally coherent metastable phase. By combining X-Ray Diffraction and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy on Mg_yTi_1-yH_x thin films we find non-zero Chemical Short-Range Order (CSRO) parameters for all the compositions measured. Despite the positive enthalpy of mixing of Mg and Ti the degree of ordering does not increase upon loading and unloading with hydrogen. The robustness of this system and the fast and reversible kinetics of hydrogen loading and unloading are caused by the formation of nanoscale compositional modulations in the intermetallic alloy. This microstructure is responsible for the exceptional properties of Mg_yTi_1-yH_x thin films. It also shows that reversible metastable metal-hydrides offer new possibilities for hydrogen storage, beyond the limits imposed by thermodynamic equilibrium.

The reflection and transmission spectra of Pd capped Mg_y Ti_{1 - y} thin films (y = 0.7, 0.8 and 0.9) are measured in the 0.5-5.5 eV energy range, both in the as-prepared and hydrogenated states. Upon hydrogenation these films switch reversibly from a shiny metallic state into a "black" absorbing one. The composition and thicknesses can be tailored to achieve high solar absorptance and low thermal emittance in the hydrogenated state. The combination of these two characteristics is interesting for the application of this material as switchable absorber in solar collectors. The use of a Mg_y Ti_{1 - y} switchable absorber in solar collectors allows to lower the stagnation temperature from 180 to 80 °C. The collector efficiency is affected only minimally.

X-Ray diffraction and electrochemical (de)hydrogenation were performed in situ to monitor the symmetry of the unit cells of Mg_yTi _100-y thin film alloys (70 ≤ y ≤ 90 at.% Mg) along the pressure-composition isotherms at room temperature. The diffraction patterns show that the crystal structures of all as-deposited alloys have a hexagonal closed packed symmetry. Inserting hydrogen transforms the structure to a body centered tetragonal structure for Mg₉₀Ti₁₀, whereas the unit cells of Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ transform into a face centered cubic symmetry. The structural change of the hydrides along with the ability to rapidly (de)hydrogenate the films emphasize the influence of the symmetry of the host lattice on the hydrogen transport properties. The lattice spacings of the unit cell of Mg₉₀Ti ₁₀H_x as a function of hydrogen content do not change much in the phase transformation region, indicating that only the fractions of the phases change. Remarkably, the lattice spacings found for Mg₇₀Ti ₃₀H_x in the two phase coexistence region reveal that not only the fractions but also the Mg-to-Ti ratio of both phases continuously change. Evidently, there is a large spread in the thermodynamic stability of the available sites for hydrogen. Since the X-ray diffraction patterns rule out large scale segregation, the results imply a nanostructured alloy with Ti-poor and Ti-rich regions and illustrate that the Mg and Ti atoms in Mg ₇₀Ti₃₀ are not randomly distributed.

The structural, optical, and electrical transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepared by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent Mg H2 and Ti H2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The average structure resembles rutile Mg H2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and Ti H2. The present results point toward a rich and unexpected chemistry of Mg-Ti-H compounds.

We report on the implementation of Pd-capped chemo-chromic metal hydrides as a sensing layer in fiber optic hydrogen detectors. Due to the change in optical properties of Mg-based alloys on hydrogen absorption, a drop in reflectance by a factor of 10 is demonstrated at hydrogen levels down to 15% of the lower explosion limit. The switching takes place in only a few seconds. Comparing Mg-Ni and Mg-Ti based alloys, we find that the latter has superior optical and switching properties. Using a 50 nm thick Mg₇₀Ti₃₀ film capped with a 30 nm Pd catalytic layer as the sensing layer, we report on the sensitivity of the fiber optic detector for hydrogen both in argon and oxygen, the temperature behavior and the reversibility.

Hydrogenography is an advanced combinatorial and standard method used for the search of new hydrogen-storage materials to synthesize bulk samples and to use volumetric or gravimetric techniques to follow their hydrogenation reaction. Hydrogenography, with a straightforward optical setup, makes it possible to monitor hydrogen absorption and desorption simultaneously on thousands of samples under exactly the same experimental conditions. Hydrogenography is much more than a monitoring technique, as it also provides a high-throughput method to measure quantitatively the key thermodynamic properties of hydride formation. The continuous change of optical transmission with hydrogen concentration was used to measure the pressure-concentration isotherms and determine the enthalpy of hydride formation. Hydrogenography is valuable for the search for catalytic caplayers promoting hydrogen uptake, electrode materials for batteries, and smart coatings for adaptive solar collectors.

Mg-Ti-H thin films are found to have very attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. Furthermore, in the absorbing state Mg-Ti-H has a low emissivity; at 400 K only 10% of blackbody radiation is emitted. The transition between both optical states is fast, robust, and reversible. The sum of these properties highlights the applicability of such materials as switchable smart coatings in solar collectors.

In order to develop optical fiber hydrogen sensors, thin film materials with a high optical contrast between the metallic and hydrided states are needed. Magnesium exhibits such a contrast but cannot be easily hydrogenated at room temperature. However, thin films of Pd-doped Mg (MgPd_y with 0.023 ≤ y ≤ 0.125) prepared by magnetron dc sputtering can easily be hydrided at room temperature and 0.5 bar H₂ within a few minutes. The rate of first hydrogenation increases linearly with increasing Pd concentration. Hydrogenation induces high variations of transmission (ΔT up to 20%) and reflection (ΔR up to 70%) of light (0.5 eV ≤ℏω≤6.0 eV corresponding to 2500 nm≥λ ≥ 210 nm). The optical properties can be understood by considering Pd as a deep donor in semiconducting MgH₂.

Mg₂NiH₄ thin films have been prepared by activated reactive evaporation in a molecular beam epitaxy system equipped with an atomic hydrogen source. The optical reflection spectra and the resistivity of the films are measured in situ during deposition. In situ grown Mg₂NiH ₄ appears to be stable in vacuum due to the fact that the dehydrogenation of the Mg₂NiH₄ phase is kinetically blocked. Hydrogen desorption only takes place when a Pd cap layer is added. The optical band gap of the in situ deposited Mg₂NiH₄ hydride, 1.75 eV, is in good agreement with that of Mg₂NiH₄ which has been formed ex situ by hydrogenation of metallic Pd capped Mg₂Ni films. The microstructure of these in situ grown films is characterized by a homogeneous layer with very small grain sizes. This microstructure suppresses the preferred hydride nucleation at the film/substrate interface which was found in as-grown Mg₂Ni thin films that are hydrogenated after deposition.