Metal Aerogels: Controlled Synthesis and Applications

Emerging as one of the youngest members in the family of porous materials, metal aerogels (MAs) are a new class of aerogels entirely built of nanostructured metals such as gold, silver, palladium, platinum, ruthenium, rhodium, osmium, copper, and nickel. They are typically fabricated via a sol-gel process coupled with special drying techniques (e.g., supercritical drying and freeze-drying). Combining the unique physicochemical properties of various nanostructured metals with the structural attributes of aerogels, MAs mark rapid mass transfer channels, highly conductive three-dimensional (3D) networks, and optical and magnetic properties. In this regard, MAs outperform conventional materials in various territories such as electrocatalysis, enzyme-like catalysis, surface-enhanced Raman scattering, diverse sensing, and actuators. Additionally, a substantial number of metal elements can offer vast opportunities for creating numerous MAs featuring desired properties, which is critical for a deep exploration and releases the full potential of aerogels. Consequently, MAs have received wide attention since their discovery in 2009. However, compared with conventional aerogels, MAs only appeared around a decade ago. A short research history challenges their fundamental studies, including controlled synthesis and structure-performance investigations, thereby retarding on-target materials design for practical applications. Currently, the majority of studies are restricted to MAs based on noble metals. This fact is ascribed to both their intrinsically high catalytic activity and simple fabrication due to the relatively high redox potential. In contrast, reports on low-cost non-noble metal aerogels are largely constrained, not to mention controlled synthesis as well as practical applications. As a result, the compositional and structural diversity of MAs is highly limited. Furthermore, the scope of the application of MAs is still constrained and is mostly restricted to electrocatalysis. Though certain remarkable MA-based electrocatalysts have been reported in the last few years, their limited composition and structure have retarded the systematic investigations into correlations between the material parameters and electrocatalytic properties. This discourages on-demand material design and performance optimization. Hence, fundamental studies and application attempts need to be strengthened to allow further development of this new material system. On this occasion, it is essential to thoroughly summarize the knowledge and design principles of MAs that have been presented in the past ten years. This study comprehensively introduces the state-of-the-art research progress in this field, which includes synthesis strategies, potential gelation mechanisms, and diverse applications of MAs. Additionally, the challenges and opportunities presented by MAs will be drawn from aspects of synthesis and applications. This review expects to attract widespread scientists from different fields (e.g., chemistry, physics, materials science, and life science) to join the area of MAs, thus jointly promoting the development of this young and promising field.