Yaxiong Yang

Tailoring Thermodynamics and Kinetics for Hydrogen Storage in Complex Hydrides towards Applications

Solid-state hydrogen storage using various materials is expected to provide the ultimate solution for safe and efficient on-board storage. Complex hydrides have attracted increasing attention over the past two decades due to their high gravimetric and volumetric hydrogen densities. In this account, we review studies from our lab on tailoring the thermodynamics and kinetics for hydrogen storage in complex hydrides, including metal alanates, borohydrides and amides. By changing the material composition and structure, developing feasible preparation methods, doping high-performance catalysts, optimizing multifunctional additives, creating nanostructures and understanding the interaction mechanisms with hydrogen, the operating temperatures for hydrogen storage in metal amides, alanates and borohydrides are remarkably reduced. This temperature reduction is associated with enhanced reaction kinetics and improved reversibility. The examples discussed in this review are expected to provide new inspiration for the development of complex hydrides with high hydrogen capacity and appropriate thermodynamics and kinetics for hydrogen storage.

Tuning Surface Structure and Strain in Pd–Pt Core–Shell Nanocrystals for Enhanced Electrocatalytic Oxygen Reduction

Icosahedral, octahedral, and cubic Pd@Pt core-shell nanocrystals with two atomic Pt layers are epitaxially generated under thermodynamic control. Such icosahedra exhibit remarkably enhanced catalytic properties for oxygen reduction reaction compared to the octahedra and cubes as well as commercial Pt/C, which can be attributed to ligand and geometry effects, especially twin-induced strain effect that is revealed by geometrical phase analysis.

Achieving ambient temperature hydrogen storage in ultrafine nanocrystalline TiO2@C-doped NaAlH4

Sodium alanate (NaAlH4) has attracted tremendous interest as a prototypical high-density complex hydride for on-board hydrogen storage. However, poor reversibility and slow kinetics limit its practical application. In this paper, we propose a novel strategy for the preparation of an ultrafine nanocrystalline TiO2@C-doped NaAlH4 system by first calcining the furfuryl alcohol-filled MIL-125(Ti) at 900 °C and then ball milling with NaAlH4 followed by a low-temperature activation process at 150 °C under 100 bar H2. The as-prepared NaAlH4-9 wt% TiO2@C sample releases hydrogen starting from 63 °C and re-absorbs starting from 31 °C, which are reduced by 114 °C and 54 °C relative to those of pristine NaAlH4, respectively. At 140 °C, approximately 4.2 wt% of hydrogen is released within 10 min, representing the fastest dehydrogenation kinetics of any presently known NaAlH4 system. More importantly, the dehydrogenated sample can be fully hydrogenated under 100 bar H2 even at temperatures as low as 50 °C, thus achieving ambient-temperature hydrogen storage. The synergetic effect of the Al–Ti active species and carbon contributes to the significantly reduced operating temperatures and enhanced kinetics.

Mg2Si anode for Li-ion batteries: Linking structural change to fast capacity fading

This work reports on the underlying mechanism of the fast capacity fading of Mg2Si. The linking of the structural change and degradation behavior that occurs during cycling shows that the dissociation and irreversible lithiation of Mg is the critical factor for the capacity fading of a Mg2Si anode. This mechanism was further proven by designing a ternary Li2MgSi that exhibited significantly improved cycling stability because of the elimination of the intermediate Mg upon charging/discharging. This finding is useful as a general guideline and inspiration for improving the cycling stability of Mg2Si anodes and designing anode materials with long-term cyclability.

Insights into the dehydrogenation reaction process of a K-containing Mg(NH2)2–2LiH system

The thermal dehydrogenation process of the KOH-containing Mg(NH2)2-2LiH system was systematically investigated by identifying changes in the structure and composition of its components by XRD and FTIR. During ball milling, the added KOH reacts with Mg(NH2)2 and LiH to produce MgO, KH and Li2K(NH2)3. During the initial heating process (<120 °C), the newly formed KH and Li2K(NH2)3 react with Mg(NH2)2 and LiH to yield MgNH, LiNH2 and Li3K(NH2)4 along with hydrogen release. Raising the temperature to 185 °C results in a reaction between Mg(NH2)2, MgNH and LiH that gives Li2Mg2N3H3 as the product and further releases hydrogen. As the temperature is increased to 220 °C, Li2Mg2N3H3 reacts with LiNH2 and LiH to produce Li2MgN2H2 and H2. Meanwhile, two parallel reactions between Li2Mg2N3H3, Li3K(NH2)4 and LiH also generate additional hydrogen. Specifically, the KH and Li2K(NH2)3, formed in situ during ball milling, serve as reactants in the dehydrogenation reaction of the Mg(NH2)2-2LiH system, which is responsible for the significantly improved thermodynamics and kinetics of hydrogen storage.

Linking particle size to improved electrochemical performance of SiO anodes for Li-ion batteries

We demonstrate a first attempt to understand the particle size-dependence of electrochemical Li storage properties of silicon monoxide (SiO). SiO powder particles of different sizes are obtained by planetary ball milling at 300 rpm for 0–12 h. The 10 h-milled SiO sample exhibits relatively uniform particle morphology with significantly reduced particle size, which induces optimal electrochemical Li storage properties. The specific surface area of the 10 h-milled SiO sample is determined to be approximately 20.1 m2 g−1, which is more than 22 times that of pristine SiO (∼0.9 m2 g−1). The first discharge and charge capacities of the 10 h-milled SiO sample are 2684 and 2091 mA h g−1, respectively, at 100 mA g−1. After 150 cycles, the discharge capacity of the 10 h-milled sample remains at 1159 mA h g−1, while the discharge capacity is only 777 mA h g−1 for the pristine SiO sample. The mechanism of the capacity loss upon cycling is also analysed and discussed.

Reaction-Ball-Milling-Driven Surface Coating Strategy to Suppress Pulverization of Microparticle Si Anodes

In this work, we report a novel reaction-ball-milling surface coating strategy to suppress the pulverization of microparticle Si anodes upon lithiation/delithiation. By energetically milling the partially prelithiated microparticle Si in a CO2 atmosphere, a multicomponent amorphous layer composed of SiO x, C, SiC, and Li2SiO3 is successfully coated on the surface of Si microparticles. The coating level strongly depends on the milling reaction duration, and the 12 h milled prelithiated Si microparticles (BM12h) under a pressure of 3 bar of CO2 exhibit a good conformal coating with 1.006 g cm-3 of tap density. The presence of SiC remarkably enhances the mechanical properties of the SiO x/C coating matrix with an approximately 4-fold increase in the elastic modulus and the hardness values, which effectively alleviates the global volume expansion of the Si microparticles upon lithiation. Simultaneously, the existence of Li2SiO3 insures the Li-ion conductivity of the coating layer. Moreover, the SEI film formed on the electrode surface maintains relatively stable upon cycling due to the remarkably suppressed crack and pulverization of particles. These processes work together to allow the BM12h sample to offer much better cycling stability, as its reversible capacity remains at 1439 mAh g-1 at 100 mA g-1 after 100 cycles, which is nearly 4 times that of the pristine Si microparticles (381 mAh g-1). This work opens up new opportunities for the practical applications of micrometer-scale Si anodes.