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Structural Hierarchy in Nanoporous Metals

Nickel Foam (5)

The concept of hierarchical organization is widespread, encompassing intentionally designed structures as well as natural substances. Instances of ordered hierarchy are most apparent in biological systems, where the arrangement of pores in trees and bones reduces the density of the structure and creates pathways for fluid and nutrient flow, thereby enhancing mechanical properties and adding functionality. Biological systems exploit hierarchy across various scales, ranging from the nanometer level to the macroscale, exhibiting a complex architecture where the structural elements themselves possess structure on a smaller scale. Human attempts to imitate sophisticated biological systems have involved designing frame structures like the Eiffel Tower, but the intricate complexity found in natural materials, such as bamboo, has yet to be fully replicated. There may be opportunities in reproducing these types of geometries in man-made materials, where hierarchy exists in the length scales occupied by structural features, such as tiered porosity in single element materials or the spatial distribution of elements in composites. However, achieving this requires the development of new strategies for advanced materials design, specifically focusing on appropriate preparation procedures and a comprehensive understanding of the effects of hierarchical structure on functional properties.

Advancements in metal or ceramic deposition methods, templating, and dealloying techniques have made it feasible to fabricate hierarchical structures in various elements and configurations. This review primarily focuses on hierarchy in porous materials that encompass characteristic pore sizes down to the nanoscale. Specifically, we examine materials in which the smallest and definitive length scale is occupied by nanoporous (np) metal produced through dealloying. Additionally, alternative processing methods that can generate additional levels of adjustable structure in single-phase materials or composites are also discussed.

Metals with pore sizes ranging from nanometers to millimeters are utilized in various structural and functional applications, including heat exchangers, filters, energy absorbers, and catalysts. These metals offer significant advantages such as a large internal surface area and high thermal and electrical conductivity. Industries such as automotive, construction, and aerospace are particularly interested in utilizing foams as lightweight structural components to enhance strength-to-weight ratios, impact absorption, and vibrational damping.

By incorporating additional length scales or architectures, the performance of materials can be greatly improved. An excellent example of this is the design of sandwich panels and their core structure, which enhances both mechanical and heat transfer properties when a fluid flows through the panel. Different core architectures, such as traditional foam core, honeycombs, or truss structures with periodic architecture, can be employed. The availability of various core designs and material options allows for the optimization of these metal cellular structures for applications in the aerospace and automotive industry.

 

Cellular materials can be broadly classified into two categories: stochastic and periodic. Stochastic materials exhibit random pore distributions, with varying degrees of short-range order and pore size distribution. On the other hand, periodic structures consist of a base cell that is repeated in two or three dimensions. Each category is associated with distinct mechanical properties, as well as differences in relative density and production effort or cost. The use of different synthesis methods can also result in characteristic pore and ligament sizes, ranging from the macroscale down to the nanoscale. Both stochastic and periodic materials can be fabricated using a wide range of base metals, from aluminum to titanium, and can possess closed or open porosity. The distinction between closed and open porosity is crucial, as the latter implies permeability to fluids, typically accompanied by a lower density.

When assessing the mechanical properties of stochastic and periodic structures, it becomes evident that ordered porous structures exhibit superior load-bearing properties compared to their stochastic counterparts, given a specific relative density. This is primarily due to the fact that periodic trusses or networks are typically materials that are dominated by stretching when subjected to loads, whereas random networks are dominated by bending. Furthermore, closed-cell materials generally possess a higher relative modulus for a given relative density, making them more desirable for applications that require load-bearing capabilities. The utilization of open porous structures in practical applications offers several key advantages. Firstly, they contribute to mass reduction, which is beneficial in terms of weight management. Additionally, open porous structures provide a large accessible surface area, which is crucial for applications such as catalysis or cooling. In these scenarios, the rate of product formation and heat transfer are directly influenced by the available surface area. It is important to note that this review solely focuses on open cell structures. The reason for this is that open cell structures possess a significant advantage in terms of their large accessible surface area, which enhances their potential for additional functional applications, including catalysis and energy storage.

The utilization of metal foams as functional materials presents numerous opportunities due to their large specific surface area in the pore space. These opportunities are exploited in various fields such as catalysis, sensing, actuation, optical switching, electropumping for microfluidic devices, integrated circuit contacting, biological implants, and as electrodes where a large surface area is required. The function of these materials is largely dependent on the transport of mass and/or electric signals through the open pore space. However, non-hierarchical nanoporosity can limit mass transport and response time. This presents a challenge as many of the applications listed above require conflicting material structures. On one hand, high surface area with many active surface sites and small pores are necessary for function. On the other hand, a network of interconnected macropores is required for fast response to external signals. Structural hierarchy is the solution to reconcile these conflicting requirements. A simple hierarchical structure with small pores at the lower hierarchy level for local function and large pores at an upper hierarchy level for fast mass transport is the obvious approach.

Numerous applications, particularly in the fields of catalysis and batteries, heavily rely on the combination of a large surface area and rapid transport. In the case of nanoporous metals, where the pore space is open, the electrochemically active surface area (EASA) concept, crucial for designing electrochemical energy storage systems, is essentially synonymous with the net surface area.

 

Nickel Foam
Nickel Foam

Metal foams have been extensively studied and proven to be effective in catalysis. One notable application is the use of nickel foam in various hydrogenation reactions that are of industrial importance. Additionally, Raney copper is widely utilized as a catalyst in the water-gas shift reaction. The recent advancements in synthetic methods, coupled with the exceptional electrical conductivity, high surface area, and abundance of reactive sites, have made metal foams highly efficient as heterogeneous catalysts and electrocatalysts. Notable examples include the electrooxidation of methanol and ethanol, as well as the electroreduction of oxygen, carbon monoxide, and carbon dioxide. Despite its high cost, platinum (Pt) remains the preferred catalyst material for fuel cell applications due to its superior catalytic activity. However, for the production and conversion of many crucial platform chemicals, metals that are more selective and less reactive than Pt are required. In this regard, recent developments in noble metal-based dilute metal foam catalysts have shown great promise. The concept behind this advancement is that the majority alloy component of the noble metal provides the desired selectivity, while the minority alloy component offers reactive sites for critical reaction steps.

Furthermore, it is crucial to recognize that the catalytic properties of these materials heavily rely on the surface chemical composition under reaction conditions. This composition can significantly differ from the surface composition during the initial preparation and characterization of the material. An interesting example is the preparation of np-Au from Ag-Au alloys, which actually forms a dilute alloy bimetallic system. Recent studies utilizing in situ microscopy and spectroscopy have demonstrated that activation by ozone enriches the surface with silver (Ag) and facilitates the formation of active sites for O2 activation.

Furthermore, the production of hierarchical composites that possess a hierarchical structure due to the spatial arrangement of their components has been incorporated. Techniques for synthesizing these composite materials involve applying coatings of metal oxides, polymers, or other metals onto the surfaces of metal foam to improve their chemical, thermal, and mechanical characteristics.

Copper Foam, What Is The Application Prospect Of This New Material-2

Hierarchical composite structures have been found to be suitable for energy storage applications such as electrodes for Li batteries or supercapacitors in certain cases where two base materials exhibit symbiotic properties. It is noteworthy that the area-specific capacitance, which is a characteristic number of the electrode material and electrolyte, is not affected by the microstructure of a porous material. Therefore, the energy density of a supercapacitor with hierarchical structure remains unchanged even if the porosity remains the same, but the power performance is enhanced. The high surface area of metal foams and their conductivity contribute to the improved abilities of these materials for a wide range of applications. The incorporation of hierarchy and other materials has broadened the working space of metal foams by enhancing their specific properties.

The extensive range of potential practical uses for metals with a structural hierarchy is immense. The ability to manipulate the sizes of the pores and the composition of the surface and interface could hold the answer to comprehensively comprehending the impact of material structure and composition on electrochemical behavior. This understanding has significant implications in the fields of catalysis and energy systems.

Picture of Lu

Lu

Our materials research team from Tsinghua University postdoctoral researcher lin and Harbin Institute of Technology researcher Mu, Nanjing University of Technology researcher Wei, they share their expertise in foam metal materials article.

About HGP

WE were established in 2003, located in the Gaoxin Zone of Guangdong-Guangxi Cooperation Special Experimental Zone, covering an area of 70 mu, with a plant of about 30,000 square meters, with more than 170 employees, is an advanced new material technology enterprise integrating research and development, production and sales.

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