The Role of Metal Foams for Sustainability and Energy Transition

Abstract

The global pursuit of a sustainable and decarbonized energy landscape requires the development of novel materials capable of supporting lightweight construction, advanced energy conversion, storage, and thermal management technologies. Among these, metal foams have emerged as a versatile class of porous materials, offering a unique combination of low density, high surface area, three-dimensional (3D) interconnected porosity, and favorable thermal and electrical conductivities. These attributes make them highly suitable for a broad range of applications critical to the ongoing energy transition, assuming an increasingly central role in enabling clean, efficient, and resilient energy infrastructures. From this key perspective, the present review highlights the relevance of the adoption of metal foams in several fields crucial for the energy transition. By presenting methodologies and outcomes of research results, mainly from the last five years, the paper underscores the potential of low-weight, high-surface, and high-performance porous materials in contemporary and future industry, supporting sustainable development and, more generally, energy transition and circular economy. The approach also aims to minimize negative impacts and promote sustainability, for example, by recycling and transforming waste materials.

1. Introduction

A large diversity of porous materials is present in nature, such as bone, wood, and sponge [1]. Porous materials, inspired by nature, replicate these features allowing the development and manufacturing of porous materials [2,3]. Lightweight porous materials have become fundamental in modern society, representing a strategically important class of materials for the energy transition, owing to their multifunctionality and adaptability across diverse energy technologies [4]. More generally, porous materials exhibit outstanding mechanical [5] and physical properties [6], such as low density [7], high strength [8], and stiffness [9] with high energy absorption capability during deformation [10], and good vibration [11] and sound absorption [12,13], as well as thermal conductivity [3], also in anisotropic open-cell metal foams [14]. Aluminum-based foams are the most common and widely used porous structures [15] due to low cost, easy of manufacturing [16], high recyclability [17], and foamability [18]. Metal foams are not combustible and consequently can be employed for heat and flame protection [19] as well as architectural decorations, military protection [20], aerospace parts, and many other application fields. From a morphological point of view, metal foams can be classified into closed-cell [21,22] and open-cell [23], corresponding to different kinds of porous structures. Pore cell type and size, properties, and application fields are different, as well as manufacturing methods [24,25,26,27]. In a recent review [28], it has been shown that small-sized Al foams can be manufactured by processes such as casting, welding, and hot-pressing. If the average cell size of Al foams is reduced to 1 mm, the plateau stress can increase to 30 MPa. With additive manufacturing processes, the smallest cell size was 61 μm, and higher plateau stress can be achieved, further expanding the potential applications of these materials.

Several studies have investigated clean and sustainable manufacturing processes of Al foams starting from chips [22] and waste [17]. Al recycling requires just 5% of the energy necessary for the manufacturing of primary Al with consequent low environmental impact [29] for bauxite, chemicals, and electric power savings. Al waste management and its conversion into porous lightweight foams represent a key technology both for energy saving and optimal use of resources. Despite the substantial amount of research conducted in the field of metal foams, there are still numerous gaps: methodological, theoretical, and practical applications. There is a lack of standardization in some manufacturing processes of metal foams. This makes it difficult to compare results from different studies and restricts the development of a comprehensive understanding of their properties. No studies have conducted a full life-cycle assessment of metal foams, considering all stages starting from the extraction of raw material and manufacturing to use and finally to end-of-life disposal. There is limited research activity on the use of recycled materials or less energy-intensive metals in the production of metal foams. Last, but not least, there is a lack of research examining new potential applications of metal foams, particularly in terms of their sustainability.

The main aspects of metal foams investigated in this work are aimed at highlighting the potential from the perspective of energy transition and more generally, for the circular economy. A comprehensive review of studies is provided. In Section 2 the attention is focused on the structures and fabrication of metal foams. In Section 3 the topic of electrocatalysis and energy conversion based on porous materials is handled while Section 4 is fully dedicated to the energy storage systems. Section 5 deals with thermal management and heat transfer and Section 6 is focused on hydrogen technologies. Finally in Section 7 the topic of lightweight and multifunctional structural materials is highlighted.

2. Structures and Fabrication of Metal Foams

Metal foams are characterized by cellular architecture, which may be mainly categorized as either open-cell or closed-cell (Figure 1), depending on porosity morphology, fabrication method, and targeted applications. Open-cell foams, with interconnected pores, are particularly advantageous for mass and heat transfer, making them suitable for catalytic and electrochemical processes [30]. The various production processes are divided into three categories based on the state of matter: solid, liquid, and gaseous [31]. On the other hand, closed-cell foam offers several advantages due to its unique structure, primarily its water resistance, durability, and excellent insulation properties [32,33,34]. These advantages make them suitable for a wide range of applications, from insulation and waterproofing to cushioning and structural support.

Numerous fabrication techniques have been set up over decades of research to produce aluminum foam. Fabrication methods, such as template replication, gas injection, powder metallurgy, and additive manufacturing, allow for precise control over pore shape, size, and structural isotropy, enabling the tailoring of metal foams for specific functional requirements. The main production methods for metal foams are synthetically described in the following subsections. In Section 3, Section 4, Section 5, Section 6 and Section 7 the main application fields and related examples are introduced and briefly discussed, in particular: electrocatalysis and energy conversion, energy storage systems, thermal management and heat transfer, hydrogen technologies, and lightweight and multifunctional structural materials.

2.1. Gas Injection Foaming

This method was developed in the 1990s by the Alcan company in Canada by injecting gas into molten Al alloy. The addition of ceramic particles in the melt allows bubbles to rise, accumulate, and successively solidify. Porosities of foams are in the range of 3–25 mm [35] and the production method is suitable for the production of slabs in a continuous manner and at the same time with reduced cost. The adoption of ultrasonic vibration of the nozzles can decrease bubble size, as evidenced by Babcsàn [36]. Wang experimented with a high-speed horizontal oscillation system in order to reduce the pore size from 10 to 4 mm [37]. Finally revolving the gas injector can reduce the porosity size below 1 mm [38] as evidenced by Noack et al.

2.2. Powder Metallurgy Foaming

The method was developed in the 1990s at the Fraunhofer Institute in Germany. Al powders (base metal) and TiH₂ powders are mixed together and compacted to obtain a dense precursor that is subsequently heated above its melting temperature. Hydrogen release allows the molten metal to reach a foam structure that is frozen at room temperature with water quenching. Important aspects of the fabrication include the foaming process, alloy compositions, foaming agent selection, and precursor densification. To avoid gas loss during foaming, the precursors must be condensed before foaming. Different methods (extrusion, rolling, uniaxial and isostatic pressing) [39,40,41] have been set up for powder consolidation. Heating and cooling rates are important parameters to be controlled in the foaming process.

2.3. Cast Foaming Process

It was developed by Yuan [42] on the basis of the melt foaming method. The precursor alloy is near-eutectic Al-Si-Mg with the addition of oxidized TiH₂ in the melt at 600 °C as a foaming agent. Subsequently heated up to 700 °C, the melt expands, and complex near-net-shaped parts can be produced. However, some open problems are still present, mainly regarding surface shrinkage and inhomogeneous pore structure defects. As a consequence of that, this process is suitable for the manufacturing of small-sized parts.

3. Electrocatalysis and Energy Conversion

Metal foams are emerging as promising materials in electrocatalysis and energy generation due to their unique properties. Their porous structure and high surface-to-volume ratio offer significant advantages in conduction and electrochemical reactivity, improving the efficiency of energy conversion processes. Photo/electrocatalytic water-splitting systems [43,44] and supercapacitor hybrid devices [45,46] are examples of electrochemical systems that can be used for energy generation and storage as long as appropriate electroactive materials and electrolytes are incorporated. Because of their high charging capacity (e.g., 870 mAh/g for NiS₂) and considerable charge carrier density for promoting catalytic reactions under applied voltages, nickel compounds are typically used as electroactive materials for energy storage [47] and catalysts for electrocatalytic water splitting [48]. Nickel foams are usefully employed as functional conductive substrates in electrochemical electrodes, particularly for nickel electrochemical synthesis [49]. Nickel foams with a metallic and porous structure can provide strong electrical conductivity to effectively transfer charges, as well as a large specific surface area and porosity for contacting electrolytes [50,51]. More effective electroactive materials can be developed on nickel foams utilizing the “in situ” growth technique to increase the use of nickel foams and enhance the attachment of electroactive materials with more stable uses. The occurrence of redox reactions is crucial for the electrocatalytic and energy storage processes. To improve electrochemical performance, Ni can be combined with another metal to provide multiple redox states and redox processes for charge storage and electrocatalysis. However, as electrochemical reactions take place at the electrode/electrolyte contact, a substantially exposed surface area is required for electrochemical electrodes. New-generation materials known as Metal-Organic Frameworks (MOFs) have customizable pore architectures and incredibly high specific surface areas [52,53]. It is advantageous to establish nickel-based bimetallic MOFs in order to provide strong electrical conductivity, numerous redox reactions, highly exposed surface areas, and efficient charge penetration. SEM and EDX analysis on nickel foam electrode materials are reported in Figure 2 [54]: the EDX examination was utilized to ascertain the disparity in surface oxygen levels between non-oxidized and electro-oxidized nickel foam samples. The experiment has proven that the surface electro-oxidation treatment for the “as received” nickel foam electrodes implies a significant increase in the oxygen content (2.7×), as shown in Figure 2c,d. Cobalt micro- and nano-sized particles were used to modify the nickel foam electrode, resulting in technologically relevant specific capacitance values. The latter actually involved a rather basic and short experimental approach. Nanoparticle surface modifications resulted in much greater capacitances (over 100 F g⁻¹) compared to identical nickel foam baseline materials. Gopi et al. [55], Wang et al. [56], and Xu et al. [57] investigated the supercapacitor behavior of Ni foam modified with NiMoO4-CoMoO4 nanosheet arrays, Co-Fe Layered Double Hydroxide (LDH) multi-sized nanosheets, and thermally reduced graphene oxide (RGO) films (Figure 3). Comparative results are summarized in

Table 1. Comparison of electrochemical performances (from [56]).

Material	Specific Capacitance	Current Density	Capacity Retention	Cycles
Ni-Fe LDH on NF	2078 mF cm−2	5 mA cm−2	42.6%	500
Sulfidation of Ni-Fe LDHS	992 mF cm−2	2 mA cm−2	64.5%	2000
Co-Fe LDHs on NF	3340 mF cm−2	1 mA cm−2	85.1%	4000

.

Graphene oxide deposition can be suitably applied on Ni foam to improve the electrochemical characteristics. A facile electrophoretic deposition approach was established to create graphene oxide (GO) films on Ni foam frameworks in the absence of conductive agents and polymer binders. The GO was then thermally reduced into RGO at the proper temperature. Cyclic Voltammetry (CV) and galvanostatic charge/discharge were used to study the effects of deposition voltage and thermal reduction temperature on RGO’s electrochemical characteristics (Figure 3) [57].

The optimal combination of deposition voltage and thermal reduction temperature was determined. Furthermore, SEM, thermal gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and XRD were used to validate the results, which revealed that the highest specific capacitance of RGO was obtained at a deposition voltage of 60 V and a thermal reduction temperature of 300 °C. CV and galvanostatic charge/discharge measurements yielded specific capacitance values of 139 F·g⁻¹ (0.005 V·s⁻¹) and 151 F·g⁻¹ (1 A·g⁻¹), respectively. With the scan rate and current density increased to 0.3 V·s⁻¹ and 10 A·g⁻¹, the specific capacitance of RGO remained at 55% and 66% in comparison with the initial value, respectively. RGO also demonstrated outstanding cycling stability, sustaining 98% of its initial specific capacitance after 500 cycles.

4. Energy Storage Systems

Metal foams play a crucial role in energy storage systems due to their unique properties, particularly regarding heat transfer. Their open-cell structure offers a high specific surface area, which facilitates heat transmission between the phase change material (PCM) and the heat transfer fluid, reducing the charging and discharging times of thermal storage devices. Active and passive techniques of energy storage are critical for greatly boosting storage system performance. Restricting greenhouse gas emissions and exploiting renewable energy sources, particularly solar energy, have become increasingly important as a result of climate change concerns. Thermal Energy Storage (TES) has emerged as a critical solution for capturing solar energy during inaccessible hours. Phase Change Materials (PCM) are especially essential in these systems because of their high energy storage density, high latent heat, and constant temperature performance throughout phase changes. Researchers have explored passive approaches such as nanoparticles, fins, metallic foams, modifications in tank shape, and heat pipes, and active ways such as vibrations, ultrasonic waves, and magnetic and electric fields to increase energy storage in PCM. Passive solutions are appealing owing to their simplicity and inexpensive cost, but they frequently have limited thermal storage capacity and may not match certain system requirements. On the contrary, active systems are more efficient in maintaining storage capacity, allowing for exact process control. However, they are more complex and require a higher amount of energy. Porous materials (metal foams, porous polymers, carbonaceous and ceramic materials) in PCM are especially important for controlling the temperature in batteries, electronic components, building materials, solar energy conversion systems, and industrial waste heat recovery because they improve thermal conductivity, shape, and thermal stability. A lot of studies have been undertaken in this field, indicating porous materials’ strong potential to improve the efficiency of heat storage systems [58]. Variji et al. [59] explored how metallic foam affects heat transport in PCM and the electrical efficiency of photovoltaic PCM systems. Their findings revealed that using metal foam with a porosity of 0.9 resulted in a 6.8% increment in average temperature and a 9.8% increase in electrical efficiency over the PV-PCM system. Ali [60] showed that employing PCM in conjunction with nickel foam might reduce heat sink temperature by up to 25% when compared to a traditional heat sink and that if the PCM percentage is 0.8, the heat sink’s operational time is increased fourfold. Huo et al. [61] investigated the impact of fan-shaped porous geometry on the performance of energy storage systems, including PCM. They discovered that the usage of porous media can reduce heat accumulation while increasing heat transfer rates. For a porous medium with a 150-degree fill angle and porosity of 0.5, the complete melting time was reduced by 51.2% compared to pure PCM. Asefi and Wang [62] discovered that using porous materials in PCM significantly increases thermal, electrical, and exergy efficiency in photovoltaic-thermal systems, with improvements ranging from 26% to 43%, 0.8% to 1.16%, and 0.96% to 1.49%, respectively, when compared to the state without porous media. NematpourKeshteli et al. [63] investigated the effects of nanoparticles and aluminum foams combined with geometric changes and discovered that the melting time could be lowered up to 87% in comparison with pure PCM. The research discusses methods for improving the thermal performance of paraffin as a PCM in solar flat-plate collector systems for residential and industrial solar applications. These solutions are considered an effective approach to addressing the issue of energy supply and demand timing delays. Three different methods were planned, either together or separately, using 10 PPIs aluminum foams with 0.92 or 0.95 porosity, various types of 5%wt nanoparticles, and geometry modifications in three different configurations: straight (Case A), wavy wall (Case B), and wavy wall-Y-shaped fin combinations (Case C). During sun exposure, thermal energy is stored within the PCM due to melting; when the sun is not present, thermal energy is released due to PCM solidification. Numerical predictions of phase change and temperature evolution are performed and extensively confirmed using well-established methodologies such as the enthalpy-porosity method, the porous media approach, and the nanoparticles single-phase homogeneous model. In comparison to pure paraffin, nanopowders in Cases B and C lower the melting time by 18.15% and 40.70%, respectively. When simply metal foams or nanoparticles plus foams are used, there is an 86.2% and 87.2% reduction, respectively. Furthermore, Case C with nanoparticles and 0.95 or 0.92 porosity foam had a lower solidification time of 84.5% and 89.2% than Case B with pure paraffin. This shows that metal foams are the most essential factor in decreasing cycling times in such applications, although nanoparticles and wavy walls can also aid. In Case C, adding metal foam and nanoparticles to the paraffin increased the heat storage rate by an order of magnitude. Chen et al. [64] investigated the effect of pore structure on thermochemical energy storage (TCES) performance at both the particle and reactor scales, using five porous composites generated (Figure 4) via vacuum impregnation as host materials: Vermiculite (V), Expanded Perlite (EP), Pumice (Pm), Silica Gel (SG), and zeolite 13x (Z). Macroporous composites (V-CaCl₂, EP-CaCl₂) had high gravimetric salt concentrations (>65%) and moderate volumetric energy density (0.55–0.65 GJ/m) but showed longer reaction kinetics. Mesoporous SG-CaCl₂ composite demonstrated larger reaction rates and equilibrium hydration capacity, reaching sustained high-temperature output with an average temperature rise of 16 °C under Relative Humidity = 50%, even though it caused a significant pressure drop across a 5 cm thick reaction bed at a cross-sectional air velocity of 0.2 m/s (101.1 Pa). EP-CaCl₂ offered optimal heat release, low resistance (6.9 Pa), and high volumetric energy density. These findings highlight the importance of using appropriate porous materials to optimize specific performance, such as energy density, reaction kinetics, and system-level thermal output characteristics, opening the door to designing more efficient and application-specific TCES systems. Comparative results for charging and discharging performances are summarized in

Table 2. Discharging performance and charging performance of various porous composites (from [64]).

V-CaCl2	EP-CaCl2	Pm-CaCl2	SG-CaCl2	V-CaCl2
Discharging reaction rate	Slowest	Slow	Fast	Fastest
Temperature decay rate	Fast	Slow	Fast	Slow
Reaction bed pressure drop	Low	Low	Medium	Very high
Low-temperature charging reaction rate	Fast	Slow	Fastest	Fast
High-temperature charging reaction rate	Slow	Medium	Fast	Medium

.

5. Thermal Management and Heat Transfer

Metal foams play a crucial role in thermal management and heat transfer due to their porous structure and unique properties. Their high specific surface area, combined with good thermal conductivity, makes them ideal materials for optimizing heat transfer in various applications, such as heat exchangers, heat sinks, and thermal storage systems. Thin-walled copper tubes and A356 metallic syntactic foam were successfully combined by Fiedler and Movahedi to form small heat transfer components that can be used within shell-tube recuperators (Figure 5) [65]. Maximum heat transfer was increased to 1.8 kW in comparison to a single exposed copper tube (maximum 0.5 kW). Furthermore, the heat transfer performance significantly outperformed the findings of an earlier investigation that used ZA27 foam elements (maximum 0.81 kW). The use of metallic foam both inside and outside the copper tube, which reduces convective heat transfer resistance, is the cause of this enhanced performance. At the bottom volumetric flow rate of 1.0 L/min, pressure drop measurements showed moderate losses of 1.19 kPa. However, at greater flow rates—that is, 7.36 kPa at 3.0 L/min—the pressure drop accelerated quickly. At the same time, comparatively little heat transmission was gained when the volumetric flow rate was increased. All things considered, it has been demonstrated that incorporating A356 foam to improve heat transmission is appropriate for creating small heat exchangers with high heat transfer rates.

An overview of the volume-averaged models used to explain heat and momentum transfer in a metal foam has been provided in [66]. Pulvirenti et al. describe the limiting condition in which the ratio of the fluid’s thermal conductivity to the solid’s thermal conductivity is significantly less than unity. Such volume-averaged models have been validated through the use of CFD computing. A finite volume CFD code (StarCCM+ 2019.2) has been used to solve the Navier-Stokes and Fourier equations in order to numerically study the forced convection of water within a metal foam. The surface, a Triply Periodic Minimal Surface (TPMS), which is determined by a combination of trigonometric functions, has been used to define the metal foam structure. A constant temperature boundary condition has been established at the solid structure’s surface, based on the simplifying assumption that the metal foam is a perfect heat conductor. The fluid velocity distribution and the interphase heat transfer coefficient have been characterized by varying solid surface temperatures and incoming fluid velocities. The Darcy–Forchheimer model is a good estimate for the flow through periodic porous structures, as evidenced by the derived nonlinear correlation between the pressure gradient and the inlet velocity. Additionally, the volumetric interphase heat transfer coefficient was determined, and it agrees well with previous research. In general, the local thermal non-equilibrium model and the Darcy–Forchheimer model used to explain the momentum and energy transmission in metal foams have been verified for periodic configurations of metals.

6. Hydrogen Technologies

Metal foams are finding increasing exploitation in hydrogen technology, particularly in electrolysis for green hydrogen production and hydrogen storage. Their unique properties, such as high surface area, adjustable pore size, and good electrical conductivity, make them suitable for applications like electrodes, gas diffusion layers, and thermal management in hydrogen systems. In more detail, they can be usefully employed as electrode materials (hydrogen generation), metal hydride tanks (hydrogen storage), catalyst support, and fuel cell technology, as described in detail in the next subsections.

6.1. Electrode Materials

Metal-Organic Frameworks based on Ni and Fe (NiFe-MOFs) own a large number of valence states and can be employed as bifunctional electrode materials. Unannealed NiFe-MOFs are still not frequently utilized in electrode materials, such as supercapacitors, electrochemical sensors, and general water splitting. Additionally, a binder-free method for electrode fabrication has been developed: the direct growth of active material on a conductive carrier. This method enhances the conductivity and mechanical stability of the electrode, streamlines the NiFe-MOF production process, and eliminates the need for insulating binders and further electrode treatments. At the same time, excellent and stable durability has been evidenced. In order to directly generate NiFe-MOF-X (X = 4, 8, 12), nanomaterials of various sizes and morphologies on nickel foam at low reaction temperatures and varying reaction periods, a straightforward solvothermal method in conjunction with an in situ growth strategy was used in the study of Weng et al. [67]. The NiFe-MOF-8 electrode demonstrated exceptional resistance and strong capacitive characteristics, with an area-specific capacitance of 5964 mF cm⁻² at 2 mA cm⁻². However, in electrocatalytic studies conducted in a 1 M KOH aqueous solution, NiFe-MOF-12 demonstrated excellent catalytic activity, displaying both oxygen evolution reaction (η50 = 362 mV) and hydrogen evolution reaction (η10 = 150 mV) activities. The electrochemical sensing tests showed that BPA was well received. Overall, the findings of Weng et al. indicate that a straightforward solvothermal approach in conjunction with an in situ growth technique is a potential approach for the direct creation of NiFe-MOFs on nickel foam (Figure 6).

6.2. Liquid Hydrogen Fuel Tanks

The most effective way to store hydrogen is in a liquid state. A 0.7 m³ liquid hydrogen fuel tank, appropriate for small containers, was proposed in the study of Gancarczyk et al., and its integrity was evaluated by a structural analysis [68]. Vacuum insulation is crucial to reducing convectional heat transfer because of the exceptionally low liquefaction temperature of hydrogen (−253 °C) and the requirement for spatial efficiency in liquid hydrogen fuel tanks. In the vacuum annular space between the inner and outer shells, a composite insulation system consisting of MultiLayer Insulation (MLI) and Sprayed-On Foam Insulation (SOFI) was adopted (Figure 7) [68]. A tube-shaped supporter composed of a G-11 cryogenic (CR) material with high strength and low thermal conductivity was also used. Because STS 316L has minimal sensitivity to hydrogen embrittlement and appropriate ductility and strength at cryogenic temperatures, it was chosen for the inner and outer layers of the tank. The Boil-Off Rate (BOR) of the fuel tank’s design was employed to objectively evaluate the insulating performance. Using heat transfer and structural assessments in compliance with the IGF code, structural integrity assessments were carried out for nine load scenarios.

6.3. Catalyst Support

Due to their high porosity, wide specific surface area, and adequate mechanical and thermal durability, metal foams are thought to be potential catalyst carriers, as evidenced in [69]. In the work of Gancarczyk et al., heat transmission and pressure drop tests for seven foams with varying pore densities composed of various metals are presented. Characteristics of mass transport have been obtained by applying the Chilton–Colburn concept. Comparable to a packed bed, it was discovered that the foams exhibit far more intense heat/mass transmission than a monolith. The effectiveness of the foams in catalytic reactions exhibiting either slower kinetics (selective catalytic reduction of NOx) or faster kinetics (catalytic methane combustion) has then been compared using 1D reactor modeling. Achieving high process conversion and short reactor times for sluggish kinetics depends on the specific surface area of the carrier or the amount of catalyst that can be loaded into it. However, solid foams are the ideal solution when mass transfer becomes the limiting factor for fast reactions.

6.4. Fuel Cell Technology

Various chemical energy carriers can be effectively converted to electricity and vice versa by solid oxide cells. The quicker degradation rate in comparison to other fuel cell/electrolyzer technologies is now the most pressing issue. It is useful to simulate a solid oxide cell in order to comprehend the degradation mechanisms [70]. Due to licensing concerns, a knowledge transfer gap between academia and industry is progressively developing because the majority of earlier research created models using commercial tools, such as COMSOL and ANSYS Fluent. The multiphysics model presented in the work of Yu et al. was created using the openFuelCell2 (OpenCFD Limited, Bracknell, UK) computational code. OpenFOAM, an open-source library, is used to implement the code. It accounts for momentum transfer, mass transfer, electrochemical reactions, and metal interconnect oxidation. The model can precisely predict I–V curves under different temperatures, fuel humidity, and operation modes. Comparison between OpenFOAM and COMSOL simulations shows good agreement. The metal interconnect oxidation is modeled, which can predict the thickness of the oxide scale under different protective coatings. Simulations are conducted by assuming an ultra-thin film resistance on the rib surface. It was found that coatings fabricated by atmospheric plasma spraying can efficiently prevent metal interconnect oxidation, with a contribution of only 0.53% to the total degradation rate (Figure 8).

7. Lightweight and Multifunctional Structural Materials

Metal foams are lightweight materials employable in structural applications due to their unique combination of properties. They are lightweight, stiff, and can absorb energy, making them useful in a variety of applications combining lightness and strength.

Sandwich structures are a family of high-performance, multifunctional structural composites with the advantages of high specific energy absorption, high strength-to-weight ratio, and lightweight design [71]. The two criteria with encouraging areas for further exploration towards the fabrication of advanced composite sandwich structures are the innovative core design and the appropriate material selection for the face sheet and core fabrication. In addition to the various composite materials available for face sheet fabrication—such as fiber-reinforced composites, metal matrix composites, and polymer matrix composites—this work focused on a range of core designs, including truss, foam, corrugated, honeycomb, derivative, hybrid, hollow, hierarchical, gradient, folded, and smart cores (piezoelectric [72], shape memory [73], and so on).

The performance evolution of sandwich structures, which was also studied, is significantly influenced by the joining technique. In the work of Lehmhus et al. [74], the reason for a sluggish uptake of these materials is their high cost. A lot of cellular metals need expensive raw materials, intricate production processes, or both. Novel foams based on less expensive components and innovative technologies have been introduced in the effort to reduce prices in the production process. But frequently, this has resulted in materials with unpredictable qualities that prevent their full potential from being realized. In order to examine the ensuing balance between the cost and performance of cellular metals, Lehmhus et al. look at cost in proportion to performance rather than in absolute terms. In order to address this distinction, a different classification of cellular metals that focuses on structural features and the endeavor to realize them is proposed. This covers a wide range, from totally stochastic foams to cellular structures with specific functions.

8. Conclusions and Future Outlook

Growing environmental challenges and the need for sustainable and lightweight materials in various industries, from construction to transportation and from aerospace applications to hydrogen technologies, mean that metal foams can represent an effective solution. However, research, development, and production of metal foams are still at an early stage, with significant potential to be explored. On one side, these materials exhibit superior material performance, both from a structural and functional point of view, while on the other, they can contribute to reducing the environmental footprint. Recent technological advances have improved the production process of high-quality cellular materials with reproducible morphological and structural homogeneity. A uniform porosity and inter-pore size is fundamental in particular for functional applications. Additive manufacturing processes demonstrate the feasibility of this technique for the manufacturing of functionally graded foams with tailored density profiles [75]. Continuous research and development are essential to optimize recycling strategies as well as develop high-value products whose benefits are essential both for industry and the environment. For instance, in a recent work [22], the possibility of recycling Al alloy chips obtained from the milling process of aluminum alloy workpieces is a process that has been successfully investigated. The presented technique is energetically, economically, and environmentally more efficient than existing aluminum recycling and metal foam manufacturing technologies, with much lower greenhouse gas emissions.Furthermore, a comprehensive life-cycle assessment would be valuable for the full exploitation of the properties and the sustainability of these materials, taking into account production costs, barriers to adoption, and market demand. Future developments include the production of increasingly high-performance foams and their integration into advanced technologies for energy efficiency and sustainability.

As a future outlook, the following can be stated:

• Future research must focus on developing standardized protocols for manufacturing high-quality and repeatable metal foams;
• Further investigations could conduct comprehensive life-cycle assessments to provide a more accurate understanding of the environmental impacts involved in metal foam production and their full life cycle;
• Future studies must explore the feasibility and impact of using more sustainable raw materials, including waste, in the production of metal foams;
• Further research could be conducted on the eco-design of metal foams, considering factors such as recyclability, durability, and repairability.