Carbon-Based Nanomaterials for Energy Conversion and Storage

Nanotechnology refers to the study of the properties and interactions of substances (including the manipulation of atoms and molecules) on the nanometer scale (between 1 and 100 nm), as well as the multidisciplinary science and technology that utilizes these properties, covering multiple fields such as physics, chemistry, materials, science, engineering, biology and medicine.

Carbon-based catalytic materials have the advantage of structural stability, adjustable morphology, and high tolerance to acid/alkaline media, for which they have always been a research hotspot in the field of catalysis. However, the original carbon material is inert to electrochemical reactions, so the activation and enhancement of its catalytic performance is very important. In addition to improving the carbon materials themselves, more and more researchers are devoted to promoting the development of metal carbon-based catalysts. Recently, based on the relationship between structure and performance, synthetic strategies such as defect engineering, surface engineering, and confinement effects have been proposed to modify and rationally design the morphology and structure of carbon-based nanomaterials. At the same time, with the continuous in-depth understanding of the catalytic mechanism, the carbon-based materials are doped with metal atoms, doped with heteroatoms, metal particles and hybrids are loaded, in order to obtain more excellent catalytic performance is feasible. In this chapter, the synthesis of carbon-based catalytic nanomaterials with different catalytic sites is firstly summarized. Based on the synthesis strategies and methods proposed in recent years as well as the understanding of the catalytic mechanism of the catalytic active center, the composition and the modified rational design of carbon-based catalysts have also reviewed. Hope it would provide reference and guidance for the application of carbon-based nanomaterials in the field of energy conversion.

The characterization approach is one of the most challenging aspects of researching carbon-based nanomaterials, particularly carbon-based catalysts with defects and atomically dispersion. It's difficult to precisely identify the active sites of catalysts using early characterization approaches, which makes disclosing the catalytic mechanism and constructing high-efficiency catalysts challenging. High-efficiency carbon-based catalysts have improved in recent years, thanks to extensive use of current characterization techniques (e.g., ex-situ, in-situ, and operando) and simulation calculation approaches (e.g., first-principles and molecular dynamics simulation calculations). This chapter emphasizes the characterization methodologies that reveal the true active sites, catalytic processes, and structure–activity relationships of carbon-based nanomaterials from three perspectives: direct visualization, indirect validation, and simulation calculations.

Carbon materials have been widely studied and applied as electrocatalysts in recent years due to their advantages of good stability, adjustable pores, high specific surface area, and excellent electrical conductivity. Graphene, carbon nanotubes, and mesoporous carbon are commonly used in the preparation of electrocatalysts, which are good composite matrix materials. Load different active species on the carbon nanomaterial to increase the electron asymmetry density of carbon nanomaterials or break the electrical neutrality of the carbon surface to form more adsorption active sites, which is also more conducive to the free movement of sp² hybridized π electrons on the carbon surface, so that the electronic distribution and spatial structure of the composite material are changed. In the process of synthesizing carbon-based nanomaterials, these changes will cause different catalytic effects between the substrates and the supported materials. The catalytic effect can not only change the size of supported-materials and the coordination of the chemical environment, but also use the interaction between supported-materials and the substrates as a bridge for the electrons theoretical study of heterogeneous catalytic, and can also use the structural characteristics to form a local electric field to promote transmission. Therefore, the electrocatalytic activity of carbon-based nanomaterials can be effectively improved. In this chapter, several catalytic effects of carbon-based nanomaterials have been reviewed, including confinement effect, interface engineering effect, and electric field effect, and mechanisms of different catalytic effects and electrocatalytic applications have also been systematically discussed.

The performance of proton exchange membrane fuel cell (PEMFC) and metal-air batteries are determined by the oxygen reduction reaction (ORR) on the cathode, whereas the kinetics of ORR is very sluggish. Therefore, it is urgent to design and develop efficient electrocatalysts to accelerate ORR. At present, as the highest activity catalyst, Pt-based catalysts have various issues such as high cost, easy poisoning, aggregation, and low durability. As a result, the investigation of high-activity, low-cost and stable ORR electrocatalysts is of significance to the practical implementation of fuel cell technology. The excellent electrical conductivity and high surface area of carbon-based materials make the catalysts good candidates for Pt-based catalysts. This chapter will cover the application of porous carbon-based materials in ORR from the aspects of the metal-free, single metallic atom, and metal nanoparticles (NPs), etc., reveal their deep catalytic mechanism, explain their unique structure–activity relationship, and clarify the future development direction and potential obstacles of carbon-based nanomaterials for ORR.

Developing high-efficiency and strong stability hydrogen evolution reaction (HER) electrocatalysts is the critical and promising part of reducing the catalytic energy barrier and improving the efficiency of hydrogen production. For designing prominent HER electrocatalysts, challenges remain in creating large number of effective catalytic sites for HER while maintaining their robustness at high output volumes. Therefore, the development of effective anchoring of catalytic active sites on low-cost, highly conductive carbon carriers to effectively promote metal catalytic performance through strong metal-support interactions (SMSI) is a well-established strategy that has been widely investigated. Carbon-based nanomaterials have attracted extensive attention as a promising class of HER catalysts for green sustainable energy conversion and beyond, due to their low-cost, diverse forms and highly tunable electronic structures. Herein, a summary of the advanced research progress of various types of carbon-based catalysts has been discussed, mainly including the metal-free carbon-based nanomaterials, atomically dispersed metal carbon-based materials, metal nanoparticles supported carbon-based materials, and metal nanoparticles encapsulated carbon-based materials. Finally, some notable challenges and prospects that are instructive for the design and development of next-generation high-performance carbon-based electrocatalysts have been discussed.

The oxygen evolution reaction (OER), one of the semi-reactions of water electrolysis, is expected to play an important role in the conversion and storage of energy in the future. The sluggish four-electron transfer reaction has become the primary bottleneck of electrochemical water splitting, which can be significantly alleviated with the development of low-cost and durable OER catalysts, fortunately. Carbon-based composite nanomaterials can function well in alkaline environments because of their excellent mechanical and electrical properties, low cost, high abundance, and large surface area. This chapter discusses recent breakthroughs in carbon-based OER electrocatalysts, mainly including metal-free catalysts, atomically dispersed metallic carbon, metal-encapsulated carbon nanoparticles, and carbon nanoparticles supported by metal nanoparticles. The knowledge offered in this chapter can be used to rationally design OER carbon-based composite nanomaterial catalysts, which may help shed light on the future of carbon-based OER development.

Electrochemical reduction of CO₂ with renewable electricity has attracted much attention for producing fuels and value-added chemicals while reducing carbon emissions. Carbon-based nanomaterials are of particular interest due to their earth abundance and low cost. In this chapter, the latest progress and research status of four types of nanomaterials in electrocatalytic carbon dioxide reduction reaction (CO₂RR) have been discussed in detail, including metal-free carbon-based, atomically dispersed carbon-based, metal nanoparticles encapsulated carbon-based, and metal nanoparticles supported carbon-based nanomaterials. Finally, the challenges and opportunities faced by carbon-based nanomaterials in electrochemical CO₂RR have been proposed, as well as possible solutions in the future.

Ammonia (NH₃) plays a vital role in food and industrial production and is a promising carbon-free energy storage carrier. At present, the main method of industrial synthesis of NH₃ is the Haber–Bosch method, which is carried out under high temperature and high pressure, consumes a large amount of energy and emits greenhouse gases, and this process is unsustainable. In recent years, the electrocatalytic nitrogen reduction reaction (NRR) has become a promising method for achieving green and sustainable NH₃ synthesis under ambient conditions, which has attracted the attention of researchers. However, due to the inertness of nitrogen molecules and the strong side reaction of hydrogen evolution, the catalytic activity and selectivity are low, which is still a huge challenge to the wide application of electrocatalytic NRR. Therefore, the design and development of an efficient NRR electrocatalyst is an important subject of theoretical and experimental research. Carbon-based nanomaterials have become a research hotspot in the field of electrocatalytic NRR due to their excellent electrical conductivity, chemical stability, adjustable electronic structure, and morphology characteristics. This chapter will start with the reaction mechanism of electrocatalytic NRR synthesis of NH₃, and introduce the types of carbon-based nanomaterials. The focus is on the design of various carbon-based nanomaterials and the principle of improving NRR activity. The classification mainly includes metal-free carbon-based, atomically dispersed metal carbon-based, metal nanoparticles encapsulated carbon-based and metal nanoparticles supported carbon-based electrocatalysts. Finally, the problems faced by carbon-based nanomaterial catalysts for NRR and the design of carbon-based nanomaterial catalysts in the future are discussed and prospected.

With the increasing dependence on fossil energy, environmental pollution has become a serious problem for human beings. Renewable energy has been developed to reduce the use of non-renewable sources. Clean energy, such as wind, solar and tidal power, suffers from time and space factors. Therefore, energy storage system is required to match the development of clean energy. Metal-ion batteries (MIBs) have attracted much attention due to their high-energy density and cycle life. Among MIBs, lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs) have attracted the most attention. However, metal ion batteries also face some problems. Due to the large radius of sodium/potassium ions, the active site cannot be fully utilized during the insertion/deinsertion process. In addition, charge transfer, ion transfer and volume change should be paid attention to in the exploit of electrode materials. In this regard, carbon-based nanomaterials show great potential. This chapter focuses on the application of carbon-based nanomaterials in MIBs, including metal-free carbon-based materials, atomically dispersed metal on carbon-based materials, metal nanoparticles encapsulated by carbon-based materials and metal nanoparticles supported on carbon-based materials. Finally, the application of carbon-based nanomaterials in metal batteries is briefly prospected, aiming to provide some enlightenment for the design and manufacture of MIBs.

With the rapid development of mobile electronic devices and electric vehicles, traditional lithium-ion batteries (LIBs) can no longer satisfy human’s requirement due to the limited energy density. Nowadays, the metal-sulfur/selenium (M-S/Se) batteries have attracted widespread attention due to the high theoretical energy density. Among the M-S/Se batteries, lithium-sulfur (Li-S) batteries receive more attention. Li-S batteries show a high theoretical specific capacity (1675 mA h g^–1) and high energy density (2600 W h kg^–1). However, Li-S batteries still face some problems: (i) Due to the soluble polysulfide (LiPSs), the “shuttle effect” can cause the loss of sulfur components and corrosion of Li anode. (ii) The electrical conductivity of S₈ and Li₂S₂/Li₂S is poor, lots of conductive additives need to be introduced into the sulfur cathode, making the theoretical energy density difficult to be achieved. (iii) The volume change during the charge/discharge processes is about 80%, which leads to the structural collapse. For the M-S/Se batteries, there exist the similar problems to be solved. Over the past years, efforts have been devoted to constructing conductive scaffolds to enhance the specific capacity, cycling stability and rate delivery of M-S/Se batteries. Carbon-based materials present the porous nanostructures and high conductivity, which have been employed as host materials and interlayer materials to promote the electrochemical performance. In this chapter, the investigations of carbon-based materials in M-S/Se batteries are summarized. Finally, carbon-based materials applied in M-S/Se batteries were briefly prospected, aiming at providing some thoughts for the design of electrode materials in M-S/Se batteries.

Metal-air batteries (MABs) recently have received much attention due to their possible higher energy efficiency and lower cost. However, the full utilization of the high specific capacity remains challenging and requires the exploration of appropriate electrode materials. Carbon nanomaterials simultaneously display high electrical conductivities, high specific surface areas and good stabilities with little volume expansion during the charge–discharge process. The high electrical conductivity facilitates charge transfer and high specific surface area provide channels for electrolyte and oxygen diffusion. Porous structures with high surface areas enable rapid electrolyte diffusions and charge transfers, which is beneficial for fast charge and discharge. The chemical properties of carbon nanomaterials can be varied via introducing chemical dopants. The incorporation of heteroatoms can significantly change the nanostructure and electrochemical performance of carbon nanomaterials. In this chapter, we summarize research progress on carbon-based nanomaterials with enhanced performance for rechargeable metal-air batteries. In each section, we describe the synthesis, physical and chemical characterizations, and innovation of carbon-based nanomaterials for each application. Finally, we conclude the perspectives and critical challenges that need to be addressed for designing carbon nanomaterials to improve the electrochemical performance of MABs with higher energy density and power density.

With the popularization of portable electronic products and the progress of technology, portable, flexible, portable and wearable electronic devices have attracted many scientists and developers because of their potential applications, such as artificial electronic skin, multi-dimensional energy storage devices and biocompatible electronic devices. The booming of carbon-based electrode materials affords broader options towards practical applications of energy storage and conversion under different conditions. Facing the opportunities and challenges, many researchers have devoted a lot of time and energy. However, there are still many challenges in practical industrial applications. In this chapter, the challenges as well as perspective for the advancement of the carbon-based electrode materials are discussed.