Research background:
The structure of a metal material has a significant impact on its performance. Designing and controlling the structure is a crucial way for developing a new generation of catalysts and high-strength, high-toughness alloys. The study found that the prepared dualphase new composite structure can obtain more excellent catalytic and mechanical properties after combining the crystalline and amorphous phases. According to reports, AlMn alloy catalysts exhibit good hydrogen evolution catalytic activity and mechanical properties. The corresponding Al alloy catalysts are prepared by magnetron sputtering technology and can obtain the Al-based "crystalline-amorphous" nano-dual-phase structure. In addition, the local chemical inhomogeneity, short-range order, and severe lattice distortion exhibited by the multicomponent alloys can further optimize the catalytic activity during the hydrogen evolution reaction. It can expect that by adding noble metal elements Ru or Pd to AlMn alloy and using the above advantages of nano-dualphase structure and multicomponent alloy compositions, a new type of "crystalline-amorphous" nano-dual-phase AlMnRu highperformance alloy catalyst is prepared. It will play an important role in promoting the development of a new generation of hydrogen evolution catalysts and high-strength, high-toughness alloys.
In the process of preparing thin-film materials by magnetron sputtering, the system is far from the equilibrium state, and the metastable phase is easy to obtain. Combining calculation methods such as CALculation of PHAse Diagrams (CALPHAD) with experiments can construct the metastable phase formation diagram of the system and study the influence of chemical composition and preparation process on the material structure. Then the performance of materials under different structures can be evaluated. For example, the ternary aluminum-based metastable phase formation diagram constructed according to CALPHAD and critical experiments is used to describe the phase formation of AlTiN, a hard coating material commonly used in industrial production. Similarly, the construction of the thermodynamic database of the AlMnRu thin-film system will help understand the effect of chemical composition in the alloy on the phase formation and then provide theoretical guidance for the development of high-performance Al-based nano-dual-phase alloys. If only consider traditional experimental methods, it will take more time and resources to achieve the above purposes. Hence, this project envisages expanding the existing research methods, combining theoretical calculation, experimental preparation, electrochemical testing, and advanced characterization to efficiently construct the "component-structure-property" structure-activity relationship of the (AlMn)1-xRux system. Finally, high-performance AlMn-based nano-dual-phase alloys are prepared.
Prediction of composition range of AlMnRu "crystalline-amorphous" dual-phase formation based on thermodynamic calculations (Work 1)
The binary phase diagram and thermodynamic data in the AlMnRu system already available in the literature can be used as the basis for calculating the CALPHAD phase diagram of the ternary system. Based on the CALPHAD thermodynamic calculation method, coupled with the first-principles calculations and essential experimental methods, I applied the computational simulation to the construction of the phase formation model of the aluminum-based material system. For example, the thermodynamic database and metastable phase formation diagram of ternary aluminum-based thin-film materials AlVN and AlTiN systems are constructed [Acta Mater., 2020, 196: 313-324], and the thermodynamic calculation results were highly consistent with the experimental data, which laid a theoretical foundation for the development of this project. Based on the above work, I constructed a thermodynamic database of (AlMn)1-xRux, and initially obtained the composition prediction range of the "crystalamorphous" dual-phase formation, as shown in Figure 1.
Figure 1 Thermodynamics-guided design of a crystal-glass nano-dual-phase Al-Mn-Ru system. a, Heat of mixing values between Al-Mn, Al-Ru, and Ru-Mn. The size of the sphere represents the relative sizes of Al, Mn, and Ru atoms. b, CALPHAD-calculated (Al10Mn1)1-xRux vertical section and T0 curve for the HCP (thin red line) formation of the ternary system. The calculated T0 curve reaches a minimum value at ~13 at.% Ru, revealing a weaker glass formation ability than representative glass-forming systems. The highlighted green region corresponds to the formation conditions for a crystal-glass nano-dual-phase structure.
Development of high-performance AlMnRu nanocatalysts for dualphase hydrogen evolution (Work 2)
Renewable clean energy technologies are rapidly developing, and abundant clean fuels, such as hydrogen, are expected to replace fossil fuels in the future. Electrocatalytic water splitting is a reliable hydrogen-production technique. Pt-based catalysts are widely used in hydrogen evolution reactions; however, their applications are restricted owing to a cost-efficiency trade-off. Therefore, developing low-cost and reliable electrocatalysts is crucial. Here, we present a new thermodynamics-based design strategy to synthesize an AlMnRu metal catalyst with a crystal-glass nano-dual-phase structure via combinatorial magnetron co-sputtering. The developed electrocatalyst is composed of ~2 nm medium-entropy nanocrystals surrounded by ~2 nm amorphous regions (Figure 2).
The catalyst exhibits exceptional performance, similar to that of single-atom catalysts and better than that of nanocluster-based catalysts with little noble metal loading (Figure 3). The large differences in composition and bonding structure between the crystal and glass phases considerably reduce the energy barrier for hydrogen evolution, enhancing H2O/H* adsorption/desorption. Considering costs, we use Al rather than a noble metal as the catalyst principal element, and Ru, which is cheaper than Pt, as the noble metal component. The new design strategy, based on the composition-structure-property relationship, and the onestep co-sputtering synthesis process provides an efficient route for the development of electrocatalysts for large-scale hydrogen production. Moreover, the superior hydrogen reaction evolution from the synergistic effect of the nano-dual-phase structure is expected to guide the development of high performance catalysts in other alloy systems.
Figure 2 Structure and composition of the medium-entropy crystal-glass nano-dual-phase Al-based catalyst. a, High-angle annular dark-field (HAADF) image probed from a cross-sectional sample. The z-contrast reflects the atomic weight difference (i.e., Al-enriched amorphous regions are darker). The inset shows a typical selected-area electron diffraction pattern with a halo ring feature, attributed to the extremely small-size nanocrystals and amorphous phase. b, BF-STEM image probed from the same region. Fast Fourier transform image (upper right inset) of the crystalline region (yellow dashed square) of an HCP pattern from the zone axis. By contrast, the Fast Fourier transform image (lower right inset) of the cyan dashed square region shows a diffused pattern, indicating an amorphous structure. c, 1D compositional profile, generated from (d-f) near-atomic-resolution energy-dispersive X-ray spectrometry mapping. The arrows in a-b and d-f indicate the investigated region of the 1D compositional profile.
Figure 3 Electrocatalytic performance of as-received samples in 1 M KOH solution. HER catalytic performance comparison with some previously reported noble metal-based catalysts.
Innovations
(1) Establish the phase equilibrium relationship of AlMnRu alloy through thermodynamic calculation, predict the composition range of "crystal-amorphous" dual-phase formation, and then obtain AlMnRu "crystal-amorphous" nano-dual-phase alloy by experimental means. It is the research idea and research feature of this year's work.
(2) AlMnRu "crystal-amorphous" nano-dual-phase alloys is prepared by combined magnetron sputtering experiments. It is found that the hydrogen evolution catalytic performance of AlMnRu exceeds that of most noble metal catalysts represented by Pt/C, which is an important discovery of this year's work. It is of great significance for the design and process optimization of high-performance alloy catalysts in the future.
(3) This project intends to establish the structure-activity relationship between the AlMnRu "amorphous-crystalline" nano-dual-phase structure and the catalytic performance of hydrogen evolution. And reveal the efficient hydrogen evolution catalysis mechanism under the synergistic effect of nano-dual-phase, which is expected to become an important theoretical innovation of this year's work.