Unveiling Nanoscale Compositional and Structural Heterogeneities of Highly Textured Mg0.7Ti0.3Hy Thin Films

Defect-Driven Evolution of Oxo-Coordinated Cobalt Active Sites with Rapid Structural Transformation for Efficient Water Oxidation

Reconstructing the surface nature of metal-organic frameworks (MOFs) as precatalytic structures is a promising methodology for improving electrocatalytic performance. However, regulating the structural evolution of MOFs during electrolysis remains highly uncontrollable and lacks an in-depth understanding of the role of in situ-derived active sites. Here, we suggest a simple approach to fine-tune the symmetry of Co-MOFs with an oxo-coordinated asymmetric coordination that acts as a prototypical structure motif for the oxygen evolution reaction (OER). Through a facile thermal treatment, the Co-N4 configuration of Co-MOFs transforms to the distorted Co-N3-oxo configuration of defective Co-ligand nanoclusters. By operando spectroscopic characterization, the reconstructed Co-N3-oxo structure enables a rapid structural transition toward homogeneous oxyhydroxides. Moreover, the defective nature of the precatalytic structure regulates the surface Co-O bonding environment with abundant μ2-O-Co3+ sites, thereby exhibiting highly enhanced OER activity with an overpotential of 256 mV at 10 mA cm-2 and excellent durability for 100 h, compared with the pristine Co-MOFs. Atomistic simulations reveal that the effect of OER intermediates on the oxyhydroxides gets distributed among neighboring Co ions, promoting balanced binding of the intermediates. This work highlights an effective strategy to design the MOF-based structure for optimizing the surface nature, thus enhancing the electrocatalytic activity.

Optical Tunability and Characterization of Mg-Al, Mg-Ti, and Mg-Ni Alloy Hydrides for Dynamic Color Switching Devices

Mg shows great potential as a metal hydride for switchable optical response and hydrogen detection due to its ability to stably incorporate significant amounts of hydrogen into its lattice. However, this thermodynamic stability makes hydrogen removal difficult. By alloying Mg with secondary elements, the hydrogenation kinetics can be increased. Here, we report the dynamic optical, loading, and stress properties of three Mg alloy systems (Mg-Al, Mg-Ti, and Mg-Ni) and present several novel phenomena and three distinct device designs that can be achieved with them. We find that these materials all have large deviations in refractive index when exposed to H₂ gas, with a wide range of potential properties in the hydride state. The magnitude and sign of the optical property change for each of the alloys are similar, but the differences have dramatic effects on device design. We show that Mg-Ti alloys perform well as both switchable windows and broadband switchable light absorbers, where Mg_0.87Ti_0.13 and Mg_0.85Ti_0.15 can achieve a 40% transmission change as a switchable window and a 55% absorption change as a switchable solar absorber. We also show how different alloys can be used for dynamically tunable color filters, where both the reflected and transmitted colors depend on the hydrogenation state. We demonstrate how small changes in the alloy composition (e.g., with Mg-Ni) can lead to dramatically different color responses upon hydrogenation (red-shifting vs blue-shifting of the resonance). Our results establish the potential for these Mg alloys in a variety of applications relating to hydrogen storage, detection, and optical devices, which are necessary for a future hydrogen economy.

Nanostructural Perspective for Destabilization of Mg Hydride Using the Immiscible Transition Metal Mn

Phase segregation in hydride-forming alloys may persist under the action of multiple hydrogenation/dehydrogenation cycles. We use this effect to destabilize metal hydrides in the immiscible Mg-Mn system. Here, in the Mg_xMn_1-x thin films, the Mg and Mn domains are chemically segregated at the nanoscale. In Mn-rich compositions, the desorption pressure of hydrogen from MgH₂ is elevated at a given temperature, indicating a thermodynamic destabilization. The increase in the desorption pressure of hydrogen reaches ∼2.5 orders in magnitude for x = 0.30 at moderate temperatures. Such large thermodynamic destabilization allows the MgH₂ to reversibly absorb and desorb hydrogen even at room temperature. Our strategy to use immiscible elements for destabilization of MgH₂ is effective and opens up the possibility for the development of advanced and low-cost hydrogen storage and supply systems.