Akihiro Kushima

CuO and Cu(OH)₂ are active copper-based catalysts for alkaline hydrogen evolution, yet their direct synthesis often yields architectures that degrade under operating conditions. Here, copper sulfide (Cu₂S) is used as a chemically metastable precursor that undergoes rapid, water-driven electrochemical reconstruction into a dynamically re-equilibrating CuO/Cu(OH)₂ heterostructure under alkaline HER conditions. Combined ex situ and operando analyses reveal a sequential transformation from Cu₂S through Cu₂O and nanocrystalline CuO to microneedle-like Cu(OH)₂ domains intimately interfaced with defect-rich CuO. This reconstructed interface does not represent a terminal phase but instead operates in a bias-stabilized dynamic steady state, in which hydroxylation and dehydroxylation processes continuously regenerate the Cu(OH)₂ phase during catalysis. During the reconstruction, the intrinsic instability of Cu⁺ in aqueous environments accelerates sulfur depletion and oxygen incorporation, enabling water to act as a structural reactant that drives sustained interconversion between oxide and hydroxide domains. The resulting heterostructure exhibits enhanced intrinsic activity in HER, delivering an overpotential of 70 mV at 10 mA cm^–2, a Tafel slope of 78 mV dec^–1, and a forty-fold reduction in charge-transfer resistance relative to pristine Cu₂S. Temperature-programmed desorption and Auger electron spectroscopy demonstrate rapid and reversible exchange of surface hydroxyl species, while Raman isotope experiments (H₂O/D₂O) directly confirm continuous hydroxyl turnover within the Cu(OH)₂ lattice under cathodic bias. Long-term galvanostatic operation (∼138 h) reveals sustained activity with preserved microneedle morphology, indicating that electrochemical bias stabilizes the dynamic CuO/Cu(OH)₂ equilibrium. These findings establish Cu₂S as a model metastable precursor for generating self-regenerating oxyhydroxide interfaces and provide general mechanistic insight into the design of electrocatalysts that operate under nonequilibrium, water-mediated conditions.

A hierarchical heterogeneous palladium on nickel foam-based catalyst system was demonstrated for the selective hydrogenation of quinoline and quinoline derivatives under low H ₂ pressures, with green solvents (ethanol, ethanol water mixture).

In Multiscale Materials Modeling, an enduring vision is to extract the molecular mechanisms governing a certain materials phenomenon of interest in order to predict how the phenomenon will behave at a later time. This goal of predictive simulation has been discussed about a decade ago as a materials research challenge, in the Mesoscale Science Frontier, MSS. To date, it continues to motivate a growing community of computational materials science and technology. Here, we consider several materials phenomena of interest, each well known in their specific areas of application, to note that while molecular dynamics simulation is arguably the most widely used method, MD results have limitations in predicting or explaining the behavior of the phenomenon. For the type of phenomena selected here, we believe that one can raise the issue of whether MD is an appropriate method of molecular simulation in the design and performance testing of complex materials. There exists an alternative to MD, the approach of meta-dynamics simulation based on energy landscape sampling and transition state theory. This approach is notable because it allows predictive molecular simulations over timescales considerably longer than the traditional MD. We are in the process of implementing an enhanced meta-dynamics approach aimed at identifying unknown defect mechanisms, making it particularly well-suited for investigating the deformation processes in engineering alloys at timescales relevant to laboratory measurements of component performance and durability assurance. Our motivation is that such simulation capabilities will find many materials-centric applications. One such application is known as plasma-materials interactions, PMI. In PMI, the phenomenon of nuclear irradiation damage has been a practical challenge, relevant to both nuclear fission and fusion power generation systems. For the present perspective, we will focus on the use of meta-dynamics simulations in collaboration with the research activities at an academic fusion research center.

This work presents quantitative investigations into the relationships between lithium dendrite growth in the defects of Li₆PS₅Cl (LPSCl) solid electrolyte (SE), crack nucleation and propagation in the SE, and the associated mechanical forces driving these dendrites and cracks. Two different growth modes for lithium dendrites are identified by ex situ scanning electron microscopy (SEM) observation: longitudinal cracking inside pores in the SE and lateral penetration along boundaries of the SE particles. These in situ TEM tests reveal that concentrated Li plating in a nano‐sized defect on the LPSCl surface will lead to the nucleation and propagation of cracks into the LPSCl under a stress much smaller than the expected mechanical strength of the LPSCl material. This unexpected mechanical degradation is caused by a reduction in the mechanical strength of LPSCl during electrochemical charge/discharge cycling, resulting from a disorder in the crystal structure of LPSCl as revealed by DFT simulations. Due to this mechanical degradation of LPSCl, the threshold force necessary to initiate crack growth is much lower than the previously expected force to drive dendrite growth.

Layered Ta₂M₃Te₅ (M = Pd, Ni) has emerged as a platform to study 2D topological insulators, which have exotic properties such as spin-momentum locking and the presence of Dirac fermions for use in conventional and quantum-based electronics. In particular, Ta₂Ni₃Te₅ has been shown to have superconductivity under pressure and is predicted to have second-order topology. Despite being an interesting material with fascinating physics, the detailed crystalline and phononic properties of this material are still unknown. In this study, we use transmission electron microscopy (TEM) and polarized Raman spectroscopy (PRS) to reveal the anisotropic properties of exfoliated few-layer Ta₂Ni₃Te₅. An electron diffraction and TEM study reveals structural anisotropy in the material, with a preferential crystal orientation along the [010] direction. Through Raman spectroscopy, we discovered 15 vibrational modes, 3 of which are ultralow-frequency modes, which show anisotropic response with sample orientation varying with the polarization of the incident beam. Using angle-resolved PRS, we assigned the vibrational symmetries of 11 modes to A_g and two modes to B_3g. We also found that linear dichroism plays a role in understanding the Raman signature of this material, which requires the use of complex elements in the Raman tensors. The anisotropy of the Raman scattering also depends on the excitation energies. Our observations reveal the anisotropic nature of Ta₂Ni₃Te₅, establish a quick and nondestructive Raman fingerprint for determining sample orientation, and represent a significant advance in the fundamental understanding of the two-dimensional topological insulator (2DTI) Ta₂Ni₃Te₅ material.

Lithium‐sulfur battery (LSB) is a promising next generation energy storage technology due to its high theoretical capacity, low cost, and environmental friendliness surpassing current lithium‐ion batteries. However, several challenges restrict the practical applications of LSBs. A major obstacle that limits its performance is the dissolution of long‐chain lithium polysulfides into the electrolyte which results in loss of active material and capacity fade during the charge/discharge cycles. Here, we developed a novel sulfur composite with liquid‐metal (LM) nanoparticles and reduced graphene oxide (rGO) dispensed on a carbon‐cloth electrode to achieve long cycling of LSBs. The liquid nature of the LM nanoparticles results in many advantages. It provides a strong anchoring with sulfur inhibiting the polysulfide shuttle effect. In addition, it prevents the detachment of the sulfur particles from the current collector albeit large volume changes and cracking of the sulfur during lithiation and delithiation, maintaining the electron conduction paths. This innovative composite resulted in a high discharge capacity, an outstanding capacity retention, and a high Coulombic efficiency. Our results demonstrate an effective method to suppress the polysulfide shuttle effect and pave a new path towards practical applications of LSBs by using multifunctional LM nanoparticles composites.

This work presents a direct observation and quantification of the fundamental mechanism of lithium penetration in Li6PS5Cl (LPS) solid electrolyte. Lithium plating in a nano-sized defect on the LPS surface led to the nucleation and propagation of a crack without external force. It also revealed a reduction in the mechanical strength of LPS when altering the electrochemical charge/discharge bias condition. A first principles atomistic simulation was performed to confirm that the disorder in the crystal structure of the LPS, both in lithium deficient and excess states, contributes to the reduced mechanical strength, and a decrease in the modulus is observed when lithium concentration is decreased from the stoichiometric amount. The results of this study suggest the importance of minimizing defects at the surface and grain boundaries to improve the stability of the solid-state electrolyte (SSE). Interfaces and boundaries can be bottlenecks for lithium diffusion, creating the concentration gradient. This can reduce the mechanical stabilities of the SSE, accelerating lithium penetration and degradation in all-solid-state lithium batteries. The insights obtained in this study provide useful information towards understanding the mechanism and designing the materials/structures to solve this issue.

Although Li-ion battery is one of the most widely used energy storage devices, there have been extensive efforts to push its limit to meet the ever increasing demands to increase its energy density for applications such as electric vehicles, portable electronics, and grid storages. Here, lithium metal anode plays a key role in the next generation energy storage devices, ultimately enabling the anode-free configuration. However, there are major challenges that need to be overcome. These include low Coulombic efficiency and the formation of dendrites. In this work, we adopted gallium-based liquid metal (LM) as a coating layer on a copper current collector to uniformly deposit lithium to prevent the dendrite formation and improve the cycle efficiency. The LM coating effectively improved the cycle performance in the anode-free configuration combined with Li(Ni,Co,Mn)O₂ cathode. The effect of the LM coating was confirmed by in situ transmission electron microscopy and optical microscopy observations. LM reduced the charge/discharge overpotentials with its high affinity with lithium. It also contributed to decompose the dendritic lithium in the discharge process reducing the dead lithium disconnected from the current collector.