Chapter 1: Introduction
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Published:20 Dec 2024
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Special Collection: 2024 eBook Collection
Transition Metal-based Nanofoams for Electrochemical Systems
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The development of advanced energy devices with long-term operation characteristics has attracted increasing attention in energy conversion and storage. New goals for electrode materials and electrocatalysts with structural controllability, intrinsic conductivity and electrochemical stability have been proposed. In this chapter, the most promising energy and conversion devices are introduced along with the current state-of-the-art materials. Then, a brief introduction to metal foams, one of the emerging candidates for energy devices nowadays, is presented.
1.1 Introduction
The urgent need to address the energy crisis worldwide has led to crucial improvements in electrochemical energy conversion and storage devices. Currently, promising devices for flexible and sustainable energy solutions comprise fuel cells that involve electrocatalytic reactions, such as oxygen and hydrogen evolution reactions, and batteries and supercapacitors that involve electrochemical processes. Nowadays, there is a huge demand for more reliable, efficient and less expensive energy storage solutions and, therefore, materials tailored for the specificities of these devices have become a crucial area of work. However, the development of such materials faces many challenges, subsequently pressing scientists to develop more advanced solutions based on cutting-edge functional materials.
To date, in the domain of fuel cells, noble metal-containing electrocatalysts (Pt, RuO2 and IrO2) have been considered the most active and effective materials for electrocatalytic reactions, capable of lowering overpotential values to reach higher energy conversion efficiency.1 Nevertheless, the scarcity, geopolitical dependence, high cost and sometimes toxicity and low stability of these noble metal-based catalysts are inhibiting their application on a larger scale.1,2 Also, researchers in Europe are looking for new families of materials that are easy to produce and that can leverage the independence of the EU industry from critical raw materials and geopolitical constraints. Presently, there is an enormous interest in finding alternative noble metal-free materials to be employed in the electrocatalytic processes in fuel cells.
Concerning electrochemical energy storage systems, there is a countless portfolio of materials that can be used as electrode materials. In modern lithium-ion (Li-ion) batteries, various lithium-containing metal oxides have been widely used, for instance, materials based on cobalt, manganese and nickel (or combinations thereof). Despite being a relatively well-established technology, new compositions, especially cobalt free, are continuously under development, aiming for higher capacity, longer stability and enhanced cycling performance. Other emerging Li-free technologies, such as metal–air and sodium-ion (Na-ion) batteries, also require stable and highly effective electrode materials, some of them with radically new compositions and properties. Currently, all these technologies are evolving extremely quickly, being accelerated by the huge industrial demand and the many break-through achievements being reported.
Besides batteries, supercapacitors are another category of energy storage devices that are becoming increasingly important. Carbon and transition metal oxides (MnO2, RuO2) have been widely reported as electrode materials for ultracapacitors and pseudocapacitors, respectively.3,4 Although pseudocapacitors achieve higher capacity, due to the contribution of faradaic processes, they may present resistive features and slower ion diffusion rates that limit the high-power performance of the devices compared to conventional double-layer ultracapacitors. Consequently, nowadays, new material compositions, additives, electrolytes and electrode architectures are intensively pursued for supercapacitor-related devices4,5 and for modern batteries, including post-lithium technologies.6,7 Overall, the need for more efficient and stable materials is at the vortex of the current developments in the field of electrochemical energy conversion and storage. These technologies cannot evolve unless more reliable and tailored electrode materials are developed and introduced in the electrodes of these devices. The future of electrochemical energy storage is completely dependent on more advanced materials and surface science and engineering. This pushes scientists into new fabrication routes and functionalization strategies in close interdependence with the requirements imposed by the industry for the uptake of new material families.
Amongst the different strategies to fine-tune the properties of electrode materials, optimizing the electrochemical active surface area has been paramount for developing more efficient electrodes. For that reason, highly porous materials have been extensively investigated as electrode materials for electrochemical applications. These porous materials, with a variety of different chemical compositions, possess important advantages such as large specific surface area and low density with tailored pore size, connectivity and pore distribution. Within this classification, metal foams are described as three-dimensional (3D) frameworks with interconnected porous structures (nano-, meso- and macropores) that combine the intrinsic physical and mechanical properties of metals with a highly porous nanoarchitecture.8 These peculiar nanostructures provide increased surface area, high specific strength, decreased relative density (ρfoam/ρbulk), a rich pore structure and superior electrical conductivity,8 which are features required for several applications, particularly in energy conversion and storage electrodes. Such properties place metal foams in a very attractive and unique category, as evidenced in Figure 1.1.
Pore/cell size versus relative density plot for porous metallic materials. Reproduced from ref. 8 with permission from John Wiley and Sons, Copyright 2010.
Pore/cell size versus relative density plot for porous metallic materials. Reproduced from ref. 8 with permission from John Wiley and Sons, Copyright 2010.
Foams are composed of cells, pores and voids, as shown in Figure 1.2(a). Cells are defined as a multi-faceted polyhedral solid, while pores are the windows that appear between foam ligaments.9,10 In general, foams are classified as open- or closed-cell according to their cell structure organization. For instance, open-cell foams, as depicted in Figure 1.2(a), exhibit interconnected pore networks, while closed-cell foams show a sealed pore structure, as demonstrated in Figure 1.2(b). Foams may exhibit both pores and voids, which differ mainly in formation and size. On the one hand, pores are open regions formed near the surface, for example, by trapped gases that produce nano- and mesopores. On the other hand, voids are defined as the accumulation of vacancies in the bulk material that produce an empty space from meso- to macrostructures.
Images and nomenclature of the structural elements of a cellular metal: (a) definition of cell foam pore and cell size. Reproduced from ref. 9 with permission from NASA and the authors. (b) Schematic illustration of the definition of cell, pore and void. Example of a (c) closed-cell foam and (d) open-cell foam. Reproduced from ref. 10 with permission from Elsevier, Copyright 2006.
Images and nomenclature of the structural elements of a cellular metal: (a) definition of cell foam pore and cell size. Reproduced from ref. 9 with permission from NASA and the authors. (b) Schematic illustration of the definition of cell, pore and void. Example of a (c) closed-cell foam and (d) open-cell foam. Reproduced from ref. 10 with permission from Elsevier, Copyright 2006.
The design and preparation of 3D metal micro- and nanofoams underwent significant evolution over the last decade and, consequently, great progress has been made in the synthesis of transition metal-based foams. For example, depending on the method and experimental conditions of the synthesis, the features and key properties of the metal foams may change significantly, determining the metrics and electrochemical properties of the electrode materials in the final application. Simultaneously, it has been possible to achieve a better understanding of the correlation between the composition of the foam and its structural and physical/chemical properties, and their final electrochemical performance as electrode material. According to the literature, several parameters can affect the performance of the foam material in its final electrochemical application. In particular, the following factors must be highlighted:11–17
the chemical composition of the foam;
the crystal structure and crystalline facets;
morphology and roughness;
surface area and pore size and pore distribution;
thermal and chemical stability;
electronic conductivity;
electrochemical activity;
mechanical stability;
cost.
The literature provides excellent review articles introducing micro- and nanofoams for numerous applications.2,18–28 Nonetheless, considering the relevance that nanoporous foams are gaining, it is important to have a more comprehensive overview of the state-of-the-art progress of metal foams, especially concerning synthesis, properties, functionalization and applications for electrochemical energy storage and conversion. Therefore, this book provides an overview of the key aspects of porous metal nanofoams, discusses different synthesis routes and highlights the correlation between these routes and the required properties for higher electrocatalytic and electrochemical performance in different applications. In this way, this book fills a gap that exists in the current literature by evidencing and discussing the potential of 3D metal foams as a building block for the design of novel electrode architectures for electrochemical devices, with a particular focus on their electrocatalytic performance and application in supercapacitors, batteries and energy-related electrocatalysis.