Noble Metal-Free Electrocatalysts: Fundamentals and Recent Advances in Electrocatalysts for Energy Applications. Volume 1

Electrocatalysis plays a crucial part in clean energy conversion, supporting numerous sustainable processes for imminent technologies. Electrocatalysts of high activity and durability are desirable for clean energy technologies such as fuel cells, rechargeable metal-air batteries, and electrolyzers. To date, significant efforts have been dedicated to discovering high-performance electrocatalysts. This has inspired the invention of new techniques to uncover structural and electrochemical properties of materials and the search for new strategies to improve their properties. This chapter begins with a discussion about the importance of electrocatalysts and their derived technologies in modern life, electrocatalyst types, approaches to enhance electrocatalysts, and recent developments in different electrocatalysts. A “paradigm shift” in how we convert and store energy has not arrived yet. However, improvements in water-splitting technologies through rapid growth in nanotechnologies are undeniably shifting the way we think about energy. With efforts from the industries and academia, research in electrocatalysis is exciting yet challenging, as they remain among the most promising alternative energy technologies in a world where fossil fuels are finite.

In the past few decades, substantial development has occurred in the field of renewable energy sources, including the synthesis and design of electrocatalysts for electrochemical water splitting to generate clean and cost-effective hydrogen fuel. The oxygen and hydrogen evolution reactions are important reactions in the overall electrochemical water-splitting process. Noble-metal-free, low-cost nanostructured electrocatalysts exhibit unique and fascinating physicochemical properties, such as low overpotentials, high turnover frequency, and long-term stability with improved efficiency, which are desirable for applications in energy conservation. Nanostructured electrocatalysts with tunable selectivity and activity have been designed using various traditional and novel synthesis methods. This chapter discusses the synthesis of noble-metal-free nanostructured electrocatalyst materials using various physical and chemical methods, along with their electrochemical activity.

In the development and designing of an efficient and stable electrocatalyst, critical mechanistic understanding is essential to mitigate the sluggish kinetics reaction processes of fuel cells. To address this issue, a brief overview has been made in the context of noble-metal-free and minimal precious metal-based electrocatalysts associated to the oxidation of small organic molecules and oxygen reduction reaction (ORR). The commercialization of either type of fuel cells like polymer electrolyte fuel cells (PEFCs) or direct liquid fuel cells (DLFCs) are highly dependent on the structural design of nanosized electrocatalysts especially noble-metal-free-based or low Pt content materials. Further, a descriptive analysis of mechanistic origin of poisoning intermediates formation and mechanistic pathways on the basis of hyphenated technique analysis has been summarized for reaction processes of fuel cells. Finally, this overview will provide insights to develop a better performing functional architectured material for catalytic electrochemical reactions in fuel cell applications.

Over the past decades, there is a significant shift from traditional fossil fuel-based power systems to sustainable and renewable energy systems. Because of the worldwide increase in energy demand, overconsumption of fossil fuels, and the associated rising negative environmental implications, the development of renewable energy sources has become a "golden standard" for researchers. Nevertheless, cost and scarcity of noble metal severely hinder their practical applications. In this regard, both the energy storage and conversion devices are the most promising towards various energy applications. This chapter gives an in-detailed overview of the various non-noble metal based-materials such as transition metals and their composites/alloys, non-noble metal based nanomaterials, 2D materials etc., as an electrocatalysts or additive for integral components and its characterization and properties are systematically organized. The diverse modification methods of non-noble metal-based materials are also well discussed in this present chapter.

Metal-air batteries (MABs), featured with eco-friendliness and high specific energy density, are seen as attractive candidates for next-generation clean and sustainable energy storage technologies to solve the energy crisis. However, the slow reaction kinetics of the oxygen reduction reaction and the oxygen evolution reaction at the cathode significantly limit the improvement of MABs’ performance. Developing the electrocatalyst with excellent activity plays a crucial role in MABs. Noble metals are not suitable for large-scale commercial applications because of their high prices. Noble metal-free catalysts with inexpensive prices exhibit excellent catalytic properties for MABs. This chapter begins with a quick overview of the development, advantages, and varieties of MABs. Then, the application of noble metal-free catalysts (transition metal oxides, transition metal macrocyclic complexes, transition metal heteroatom-doped catalysts, and carbon-based metal-free catalysts) for MABs are summarized. Finally, a general summary and perspective of noble metal-free catalysts for MABs is presented.

The next 30 years are crucial to achieving a carbon neutral economy. For such ambitious but necessary purposes, the development of high-performance green electrochemical technologies will play a key role. On the other hand, research on biomass-based carbon electrode materials has gained noteworthy attention in the last decade, especially because of their inherent advantages such as raw material abundance, cost-effectiveness, environmental friendliness, renewability, outstanding electrochemical performance, and tailoring hierarchical design. This chapter provides a review of top-performing biomass-derived electroactive carbons with applications in fuel cells, batteries, supercapacitors, and microbial fuel cells. All of them are electrochemical systems with the potential to generate and/or store clean and sustainable energy.

Graphene, a two-dimensional carbon material, has managed to attract the attention of the scientific world since its discovery because of its extraordinary properties. The number of studies on the development of graphene-based materials has increased with the achievement of significant success in different studies investigating the applicability of graphene. In particular, the preparation of graphene and its derivatives with simpler methods—and the addition of different components to its structure, thereby obtaining graphene-based composite materials—provides great advantages for many applications. Developed graphene-based composites are used as electrochemical catalysts in different application areas. The aim of this chapter is to explain the types of graphene-based composites after introducing the properties and synthesis methods of graphene and graphene-based composites. Then, the studies on graphene and graphene-based composites, which are widely used as electrocatalysts, are briefly reviewed.

In recent years, as energy security, environmental crisis and global warming have received increasing attention, the establishment of a fossil-fuel-free and renewable energy system is one of the most critical challenges facing mankind today. As a zero-carbon emission energy, hydrogen has been recognized as one of the cleanest sources, how to produce and consume hydrogen effectively and sustainably is the key for mankind to enter the hydrogen energy economy in the future, hydrogen production from water-splitting is recognized as the most efficient, promising and environmentally friendly hydrogen production method. The electrolysis water technology is based on the principle of electrochemical water splitting, using renewable electricity or solar energy to drive water-splitting into hydrogen and oxygen gas is considered to be the most sustainable way to produce hydrogen. However, the production of hydrogen by water splitting requires both active and stable cathode and anode catalysts to ensure the economy and energy saving of the entire water electrolysis reaction. At present, precious metal-based materials (such as Pt, Ru, Ir, etc.) are considered to be the most active electrocatalysts for the electrolysis of aquatic hydrogen, but their high cost and low reserves limit their large-scale application. Therefore, it is of great significance to explore highly efficient catalysts without precious metals for the industrial application of hydrogen electrolysis in aquatic products. In this chapter, the application research of metal oxides with different morphologies in the hydrogen evolution reaction in recent years is introduced in detail.

The advancement in science and technology has brought up a great revolution in the industrial sector and the day-to-day life of human beings. On one hand, the available nonrenewable sources of energy are decreasing at a greater rate because of the concomitant increase in population, while on the other side, it made scientists think about alternatives. The faster consumption of available energy resources also possesses a threat to the environment and the ecosystem by contributing to greenhouse gases. In order to overcome both of these problems, electrocatalytic green energy generation has emerged as one of the best alternatives because of the production of hydrogen energy, conversion of N₂/CO₂ to energy sources, and other methods of energy storage. These methods are environmentally benign and have high energy density. In order to have efficient energy generation, different catalysts have been developed, among which metal phosphides and their combinations have received considerable interest and attention. This chapter discusses the synthesis of different metal phosphides and their possible combinations, such as by doping or heterostructure formation, and their application in green energy generation, such as H₂ energy generation, CO₂ reduction, and so forth. The mechanism of the electrocatalytic process and the factors impacting the activity of the catalysts are also discussed in detail.

Covalent organic frameworks (COFs), owing to their greater surface area, tuneable pore structure, functionality, and unique molecular architecture, have gained tremendous interest for various applications including; catalysis, electrocatalysis, gas sensing, sensor, adsorption, etc. Further, the high structural extent and the atomic-level distribution of active sites in COF have generated considerable attention due to their principle design and rapid screening of catalyst for particular application. In this context, firstly, we have summarized the latest progress on synthesis and characteristics properties of COFs then a thorough discussion is given on fine-tuning the characteristics features. In the next part, newly emerging COFs for catalysis and electrochemical carbon dioxide reduction reaction with superior catalytic activity have been extensively studied. Finally, we have provided a brief overview of the remaining challenges and future directions for enhancing catalytic performance.