Despite the fruitful achievements in the development of hydrogen production catalysts with record-breaking performances, there is still a lack of durable catalysts that could work under large current densities (>1000 mA cm⁻²). Here, we investigated the catalytic behaviors of Sr₂RuO₄ bulk single crystals. This crystal has demonstrated remarkable activities under the current density of 1000 mA cm⁻², which require overpotentials of 182 and 278 mV in 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively. These materials are stable for 56 days of continuous testing at a high current density of above 1000 mA cm⁻² and then under operating temperatures of 70 °C. The in-situ formation of ferromagnetic Ru clusters at the crystal surface is observed, endowing the single-crystal catalyst with low charge transfer resistance and high wettability for rapid gas bubble removal. These experiments exemplify the potential of designing HER catalysts that work under industrial-scale current density.

We have studied the stress-induced martensitic transformation behaviors and the associated elastocaloric effect (eCE) for non-textured polycrystalline all-d-metal Heusler alloys of Ni₅₀Mn₃₂Ti₁₈ and Ni₃₅Co₁₅Mn₃₅Ti₁₅ by a combination of Digital Image Correlation (DIC) and Infrared (IR) thermography techniques. A large but irreversible adiabatic temperature change (ΔT_ad) of 10.7 K at a strain level of 3.9% is observed for Ni₅₀Mn₃₂Ti₁₈, whereas Ni₃₅Co₁₅Mn₃₅Ti₁₅ exhibits a reversible eCE with ΔT_ad = 9.0 K at a strain level of 4.6%. At lower strain levels (3.2%), Ni₅₀Mn₃₂Ti₁₈ is plastically deformed with small strain variation in space from the DIC map. In contrast, Ni₃₅Co₁₅Mn₃₅Ti₁₅ can be deformed superelastically accompanied by large strain variation in space, which can be ascribed predominately to the crystalline orientation dependence of both the transformation strain and the Young's modulus from different orientated grains under mechanical loading. The improved reversibility of eCE for Ni₃₅Co₁₅Mn₃₅Ti₁₅ is supposed to be associated with the enhancement of d-d hybridization by the introduction of the element Co.

Good mechanical properties and large adiabatic temperature change render Heusler-type Ni₂FeGa-based magnetic shape memory alloys as a promising candidate material for solid-state mechanical cooling application at ambient conditions. Superelastic behavior and associated elastocaloric effect strongly reply on deformation conditions (e.g. applied strain rate and strain level) of stress-induced martensitic transformations. With the aim of developing high-performance elastic cooling materials, in this work, we have carried out a systematic study on a Ni₅₄Fe₁₉Ga₂₇ [420]-oriented single crystal by exploring the interaction between dynamic deformation parameters and thermal response. A giant and reversible adiabatic temperature change of ±7.5 K triggered by a low stress of 30 MPa was achieved. Such a high specific cooling performance thus offers the great advantage for the small scale solid-state mechanical cooling applications. Besides, a significant temporary residual strain effect has been observed at high strain rate, which is unfavorable for reversible elastocaloric effect but can be overcome by reducing stress hysteresis, and/or by elevating initial environmental temperature. The established criterion for the desirable reversible elastocaloric properties goes beyond the present system and can be applicable for other shape memory alloys used for elastic cooling techniques.

Elastocaloric cooling is regarded as one of the most promising cutting-edge alternatives to conventional vapor compression refrigeration systems. This technology is based on the temperature change of materials when being subjected to uniaxial stress, which has been observed in polymers, alloys, and ceramics. However, the existing elastocaloric prototypes have a bottleneck problem of an excessive mass ratio between the actuator and the solid-state refrigerant. To overcome this challenge, this study proposes an elastocaloric refrigerator using a single actuator with an inclined angle to produce a vertical tensile force to nickel-titanium (NiTi) shape-memory wires and a lateral motion to translate the NiTi wires between the hot and cold sides. The refrigerator can achieve a 90% improvement in the mass ratio between the solid-state refrigerant and actuator compared to the currently best-reported elastocaloric cooling prototype. The NiTi wires exhibit an adiabatic temperature change of 6.6 K during unloading at the strain of 4.8%. The proposed refrigerator can achieve a 9.2-K temperature span when the heat source and sink are insulated from ambient and has a cooling power up to 3.1 W under zero-temperature-span condition. By using thinner NiTi wires or NiTi plates, the developed elastocaloric refrigerator could be a starting point to promote applications of this technology in the future.

The clean and energy-efficient solid-state refrigeration based on magnetostructural phase transformation is a promising alternative technology for vapor-compression refrigeration. Herein, using a unique quasi-direct calorimetric method in a high hydrostatic pressure condition, we have studied the barocaloric effect for a Ni_35.5Co_14.5Mn₃₅Ti₁₅ all-d-metal Heusler alloy that undergoes a martensitic transformation accompanied by a large magnetization change. The martensitic transformation is found to be sensitive to either applied pressure or magnetic field with the large transformation temperature driving rates of 5.8 K kbar⁻¹ and 2.0 K T⁻¹. Such a sensitive response to external stimuli originates from the large lattice/volume- and magnetization-discontinuity on the phase transformation. The Ni_35.5Co_14.5Mn₃₅Ti₁₅ alloy exhibits a large barocaloric effect with an isothermal entropy change of −24.2 J kg⁻¹ K⁻¹and an adiabatic temperature change of 4.2 K, concomitant with a relatively low pressure change of 1 kbar, appearing to be the largest values among those of reported magnetic shape memory alloys. Moreover, by virtue of the strong magnetostructural coupling in the Ni_35.5Co_14.5Mn₃₅Ti₁₅ alloy, we propose a strategy of applying/releasing pressure and magnetic field in a proper sequence to eliminate the hysteresis and effectively enhance the reproducibility of the barocaloric effect.