Noble Metal-Free Electrocatalysts: New Trends in Electrocatalysts for Energy Applications. Volume 2

With global energy consumption growing at an unprecedented rate and environmental concerns becoming increasingly acute, the need for clean, sustainable energy conversion and storage systems such as fuel cells, dye-sensitized solar cells, metal-air batteries and Li-CO₂ batteries is of utmost significance. The main reactions involved in these renewable energy technologies are oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and CO₂ reduction reaction (CO₂RR), which all require catalysts. An electrocatalyst is a surface where chemical energy is electrochemically converted into electrical energy in fuel cells. For the selection of an electrocatalyst and tailoring its characteristics, both stability and selectivity must be assessed. Overall fuel cell performance is mainly determined by the efficiency of the electrocatalyst. Catalysts based on metals are currently widely used, but they suffer from multiple competitive disadvantages, including their low selectivity, poor durability, and negative environmental impact. Hence, it is highly desirable to develop earth-abundant, low-cost, stable, and catalytically active metal-free alternatives for application in renewable energy. Catalysts made from carbon-based metal-free materials possess advantages such as high earth abundance, low cost, high electrical conductivity, structural tunability, good selectivity, and solid stability in acid/alkaline conditions. These characteristics explain why these catalysts are receiving increasing attention in applications related to energy and the environment. Through electrocatalysis without the use of noble metals, numerous carbon-based electrocatalysts have been developed for the storage of clean energy and the protection of the environment. This chapter aims to introduce electrocatalysts and electrocatalysis to the reader. By understanding electrocatalysis and electrocatalysts, the readers will gain a deeper understanding of the concepts. Firstly, the authors will discuss the basics of electrocatalysts, electrocatalysis, types of electrocatalysts, and electrocatalysis, and then they will review some fundamental concepts. Lastly, a brief overview of recent research and advancements in non-noble metal free electrocatalysts will be presented by using carbon-based nanomaterials in the context of energy storage and concluded with a remark.

There has been a great deal of interest in the development of electrocatalysts for energy applications that are based on materials that do not contain noble metals. Transition metal oxides, nitrides, carbides, phosphides, and chalcogenides are the primary classes of non-noble metal-based electrocatalysts. In this chapter, a compilation of the developments in the field of noble metal-free electrocatalysts for oxygen evolution reactions and hydrogen evolution reactions has been highlighted. These developments consider several optimization strategies for improving catalytic behavior and realizing effective electrocatalysis.

Electrocatalytic water splitting is critical to produce green hydrogen, which is required for the decarbonization of our society. In this regard, the development of new catalysts for its half reactions, the hydrogen evolution reaction and oxygen evolution reaction, are crucial to reduce the price of green hydrogen. This chapter describes the fundamental electrochemical methods applied in the water-splitting research community. The basic mechanisms and design principles of oxygen and hydrogen evolution reaction catalysts are presented. A substantial part of this chapter is dedicated to the structural stability and the reconstruction of materials during water-splitting conditions, and the most important active structures are described in detail. Furthermore, this chapter avoids aspects that might be familiar to graduate chemistry students, such as common synthetic and analytical methods, and focuses on aspects that are unique to water splitting. We anticipate that this chapter is an ideal introduction for students with a chemistry background who want to enter the field of water splitting but have little experience with electrochemistry and the structures of oxidic, layered, non-noble metal compounds.

The development of clean energy materials is considered an alternative to fossil fuels for the remediation of hazardous environmental pollutants. Electrocatalysis is a promising method for water splitting to produce hydrogen as a clean fuel. Noble metals are generally used as electrocatalysts in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and water splitting process. Nevertheless, its application is restricted because of being costly and offering less stability. Recently, noble metal-free metal-organic frameworks (MOFs) based materials are considered a promising electrocatalyst for water electrolysis due to their low cost, large surface area, functionality, tunable electronic properties, remarkable catalytic activities, and stability. This chapter will focus on the application of noble metal-free MOFs and MOF-based electrocatalysts in electrocatalytic water splitting for both hydrogen and oxygen evolution. It starts from the fundamentals of water electrocatalysis including HER and OER. Moreover, the unique features of noble metal-free MOFs-based electrocatalysts over noble metal-based electrocatalysts will be illustrated precisely. Finally, recent progress along with current challenges and future perspectives of noble metal-free MOFs and MOF-based electrocatalysts for water splitting will also be presented.

During continuous searching for effective alternative energy sources for replacing carbon-based fuels, it has been observed that hydrogen can play a major role, owing to its high specific energy density. Hydrogen can be generated by various means, but electrochemical water splitting is considered a greener process for its generation. Noble metal oxides (RuO₂ and IrO₂) and Pt are considered to be state-of-the-art catalysts for oxygen evolution reactions and hydrogen evolution reactions respectively, but low earth abundancy and high cost make their large-scale application difficult. Various transition metal-based hydroxides have been reported as effective electrocatalysts for generating hydrogen with long-term stability, especially in alkaline conditions. In this chapter, we summarize the developments of various strategies of hydroxide materials and discuss advancements of these materials toward the water splitting application. We also discuss the basics of water splitting, catalyst design principles, and mechanism in detail, which will help the reader understand the basic concept of a water splitting application. This chapter can familiarize the reader with various factors that determine the water splitting process. Additionally, different strategies that have been established—such as various cation doping processes, anion modulation, and defect creation to extemporize the kinetics—are discussed in detail.

Oxygen reduction reaction (ORR) is the cathode half-reaction occurring in devices like proton/anion exchange membrane fuel cells and metal-air batteries. Developing non-precious metal-based catalytic systems for these advanced energy conversion and storage devices is crucial for their commercialization and elimination of the dependence on noble metal catalysts. The transition metallic active sites in these electrocatalysts are required to be well-dispersed across the suitable electrically conducting carbon support matrices, which facilitate efficient electron and gas flow channels as required for the charge and mass transport, respectively. The hierarchical porous heteroatom doped electrically conductive carbon support can effectively serve this purpose. The morphology of these catalyst systems significantly affects the electrochemical performance. This motivated the exploration of core-shell and atomically-dispersed transition metal-carbon ORR catalysts in recent literature. Further, the proper understanding of the synergism between different components of the devices and involved degradation mechanisms leading to performance loss is of utmost importance. This has led to note-worthy improvement in designing and performing these transition metal ORR catalysts, as evident from their employment in commercial devices.

Catalysts are used in many facets of modern technology and industry, ranging from energy storage and conversion to hazardous emissions abatement to chemical and materials synthesis. The utility of electrocatalytic techniques to store and convert energies using hyper-efficient catalysts offers an attractive way to relieve the global energy crisis. For this goal, it is essential to identify electrocatalysts with high catalytic activity and selectivity. New materials, notably perovskites, provide significant advantages for use as potential hosts or carriers for energy applications. Perovskite, perovskite composites, and their derivatives have recently been adopted as efficient materials for use in electrocatalytic applications due to their unique structure and high material stability, as well as their compositional flexibility. We propose perovskite and perovskite composite catalysts for (electro)chemical energy savings and conversions in this book chapter. A framework for understanding activity trends and guiding perovskite oxide catalyst design is described, followed by examples of a thorough understanding of the synthesis technique used to develop perovskite materials in the recent past provides fundamental insights into activity, stability, and mechanism in electrocatalysis. We conclude by outlining the assets and liabilities of the synthesis and various electrocatalytic applications of perovskites and their composites.

The continuous development in energy storage and conversion fields is currently based on designing inexpensive and efficiently behaved nanomaterials to satisfy the future energy demands. Nanostructured transition metal phosphides could display numerous brilliant features enabling them as active anode materials in energy generation systems. Increased structural and thermal stability, good hardness and electrical conductivity in addition to available surfaces with enriched active sites and strong metal–phosphorous bonds could extensively characterize these fabricated metal phosphides. The short pathways of diffused ions, the increased specific capacitance and lowered charge-discharge voltage of these nanomaterials strongly elect them as promotors in energy conversion and storage systems including hydrogen (HER) and oxygen evolution (OER) reactions, water splitting, metal-ion batteries and supercapacitors. This book chapter will deal with the preparation routes of transition metal phosphides and their activity for green energy generation besides the challenges and future perspectives in this field.

The hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), along with oxygen evolution reaction (OER) are pivotal for the production of H₂ from H₂O electrolysis as well as rechargeable devices such as metal-air batteries (MABs). Electrocatalysts that efficiently promote water-splitting with zero-emission are irreplaceably desirable for clean energy technologies and systems. On the other hand, wearable, flexible, implantable, and portable electronic devices, once only seen in sci-fi, are presently being actualized. Mobile and flexible high-performing power sources for wearable technologies are fundamental for their continued growth. In recent years, exciting breakthroughs have been achieved in developing suitable electrocatalysts for energy conversion systems that support flexible devices. Even though such technology is still in its infancy, lab-scale prototypes of flexible MABs and fuel cells utilizing flexible electrocatalysts demonstrate their undeniable ripening niche. This chapter covers the fundamentals of flexible electrocatalysts for HER, OER, and ORR.