Nanomaterials for Energy Conversion and Storage

Dunwei Wang, Guozhong Ca

• Publisher: WSPC (EUROPE)
• Year: 2017
• ISBN:
  9781786343642
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

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The use of nanomaterials in energy conversion and storage represents an opportunity to improve the performance, density and ease of transportation in renewable resources. This book looks at the most recent research on the topic, with particular focus on artificial photosynthesis and lithium-ion batteries as the most promising technologies to date. Research on the broad subject of energy conversion and storage calls for expertise from a wide range of backgrounds, from the most fundamental perspectives of the key catalytic processes at the molecular level to device scale engineering and optimization. Although the nature of the processes dictates that electrochemistry is a primary characterization tool, due attention is given to advanced techniques such as synchrotron studies in operando. These studies look at the gap between the performance of current technology and what is needed for the future, for example how to improve on the lithium-ion battery and to go beyond its capabilities.

Suitable for students and practitioners in the chemical, electrochemical, and environmental sciences, Nanomaterials for Energy Conversion and Storage provides the information needed to find scalable, economically viable and safe solutions for sustainable energy.

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Contents:

• The Principle of Photoelectrochemical Water Splitting (Peiyan Ma and Dunwei Wang)
• Semiconducting Photocatalysis for Solar Hydrogen Conversion (Shaohua Shen and Jie Chen)
• Visible-Light-Driven Photocatalysis (Qingzhe Zhang, Yanlong Liu, Zhenhe Xu, Yue Zhao, Mohamed Chaker and Dongling Ma)
• Metal-Nitride Nanostructures: Emerging Catalysts for Artificial Photosynthesis (Md Golam Kibria, Bandar AlOtaibi and Zetian Mi)
• Surface Engineering of Semiconductors for Photoelectrochemical Water Splitting (Gongming Wang, Yi Yang and Yat Li)
• Photoanodic and Photocathodic Materials Applied for Free-Running Solar Water Splitting Devices (Miao Zhong, Hiroyuki Kaneko, Taro Yamada and Kazunari Domen)
• Electrocatalytic Processes in Energy Technologies (Yang Huang, Min Zeng, Qiufang Gong and Yanguang Li)
• Soft X-Ray Spectroscopy on Photocatalysis (Yi-Sheng Liu, Cheng-Hao Chuang and Jinghua Guo)
• Photoelectrochemical Tools for the Assessment of Energy Conversion Devices (Isaac Herraiz-Cardona and Sixto Gimenez)
• Fundamentals of Rechargable Batteries and Electrochemical Potentials of Electrode Materials (Chaofeng Liu and Guozhong Cao)
• Revitalized Interest in Vanadium Pentoxide as Cathode Material for Alkali-Ion Batteries (Yanwei Li, Jinhuan Yao, Robert C Massé, Evan Uchaker and Guozhong Cao)
• Tin-Based Compounds as Anode Materials for Lithium-Ion Storage (Ming Zhang and Guozhong Cao)
• Beyond Li-Ion: Electrode Materials for Sodium- and Magnesium-Ion Batteries (Robert Massé, Evan Uchaker and Guozhong Cao)
• Nanomaterials and Nanostructures for Regulating Ions and Electron Transport in Advanced Energy Storage Devices (Yu Wang and Wei-Hong Zhong)

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--> Readership: Students, researchers and practitioners in the chemical, electrochemical, and environmental sciences. -->
Keywords:Nanomaterials;Lithium-Ion Batteries;Electrochemistry;Energy Conversion;Energy Storage;Artificial PhotosynthesisReview:0

CHAPTER 1

THE PRINCIPLE OF PHOTOELECTROCHEMICAL WATER SPLITTING

Peiyan Ma*^,‡ and Dunwei Wang^†,§

*School of Chemistry, Chemical Engineering and Life Science,
Wuhan University of Technology, Wuhan, China
^†Department of Chemistry, Boston College,
2609 Beacon St., Chestnut Hill, MA 02467, USA
^‡[email protected]
^§[email protected]

Photoelectrochemical (PEC) water splitting has been attracted significant attention lately due to its utilization of solar energy and H₂ production. The critical challenge in PEC research is the O₂ evolution half reaction (OER) occurring on the photoanode. This chapter consists of an introduction of PEC system, the detailed process of OER, the development of several semiconductor photoanodes, OER mechanism, high-efficient elelctrocatalysts and tandem cell based on PEC system. This chapter provides a review of the principles, research route and prospect of PEC and analysis of the microstructure, interface engineering and performance in some important samples. It will be a guide for the beginners in the PEC area.

1.Introduction

1.1.General background

Excessive exploitation of fossil fuels makes it an urgent task to develop renewable and sustainable energy sources to solve challenges that come with the energy crisis. Solar energy is expected to be the main option, especially in areas with ample sunshine. Successful utilization of solar energy relies on the development of efficient harvesting materials and effective storage technologies. As far as direct solar energy conversion and storage is concerned, hydrogen (H₂) has attracted significant attention owing to its high energy density (120–142 MJ/kg) and the ability to obtain it from H₂O. When used as an energy carrier, H₂ is highly efficient and can be used for transportation, storage, and power replacement in places where other energy sources are difficult or expensive to access. That its combustion only generates H₂O is another reason why H₂ is particularly appealing.

As a method developed by Mother Nature to harvest and store solar energy, photosynthesis is an important biochemical process. It produces biological oxygen molecules (O₂) at the protein complex of the photosystem II (PSII) in algae, cyanobacteria and plants. Water is split to O₂ and H₂ or H₂-related species by a two-step excitation process (Figure 1) in which the two half reactions (oxidation and reduction) are spatially separated and proceed in PSII and PSI (photosystem I), respectively [1]. The active site for this water splitting process is embedded in a protein complex 1, which consists of four manganese ions and one calcium ion (Figure 2) [2].

Figure 1. Schematic diagram of water splitting process in the membranes of the chloroplasts. Reprinted with permission from [1].

Figure 2. Structure of Mn₄Ca active site in the O₂ evolution catalysts (OEC). Reprinted with permission from [2].

Although it was not until 2011 that researchers successfully identify the structure of the O₂ evolution center in PSII², the natural photosynthesis route has inspired researches of a similar nature for a long time. As early as 1971, Fujishima and Honda demonstrated that TiO₂ could be used to decompose water to H₂ and O₂ under photoelectrochemical (PEC) conditions (see below), which is widely regarded as the dawn of artificial photosynthesis research [3].

The negative value of the normal electrodynamic potential indicates that water splitting is thermodynamically uphill and additional energy is necessary to power the reaction. Relative to HER (which involves 2 electrons and 2 protons), OER is far more complex because it requires 4 electrons and 4 protons. As such, it has been identified as the rate determining step (RDS) for artificial photosynthesis. Indeed, while a number of p-type semiconductors have been shown to produce H₂ with high efficiencies, n-type semiconductors for high-efficiency, stable H₂O oxidation is still missing. In recognition of this critical research need, we focus on photoanode and OER reactions for this chapter.

Figure 3. An PEC cell consists of a semiconductor photoanode and a Pt cathode. Cobalt-based catalyst was modified on the surface of photoanode. Reprinted with permission from [4]. Copyright @ American Chemical Society.

A typical water splitting cell device includes semiconductors with appropriate bandgap as photoelectrodes (photoanode or photocathode) to absorb sunlight, conductive substrates to transport charges and electrolyte to support the reactions (Figure 3) [4]. Electrons and holes are generated when the photoelectrodes are irradiated. They then transfer to the conduction and valence bands, respectively. The photogenerated electrons are required to flow to the surface of cathode, where they reduce H⁺ to H₂. At the same time, photogenerated holes are expected to move to the surface of the anode and oxidize water to O₂. Often, catalysts are needed to increase the reaction kinetics on both the photoanode and the photocathode surface, to match the rates of charge generation and separation within the photoelectrodes. In the following sections, we will first introduce the fundamental considerations of the semiconductor-based water splitting process, and then discuss material selection for the photoanode and end the chapter with a discussion on the detailed mechanisms.

1.2.Fundamental considerations in selecting a material for solar water splitting

After the introduction of TiO₂ to water splitting by Fujishima and Honda, a large number of semiconductors have been explored for the same purpose. There are several principles for selecting a semiconductor as a photoelectrode.

1.2.1.Potential requirement

Under standard conditions, the water splitting reaction can take place only when the potential difference of the two half reactions exceeds 1.23 eV (ΔG° = 237.1 kJ·mol⁻¹). That is, the combined bandgap of the semiconductors should be larger than 1.23 eV. Other energy losses have to be taken into account, especially the overpotential loss at the electrodes and ionic conductivity loss in the electrolyte. In a practical cell, the actual potential requirement typically exceeds 1.6 V.

1.2.2.Appropriate band structure

Aside from an appropriate bandgap, proper conduction and valance bandedge positions should be taken into consideration for water reduction and oxidation reactions, respectively. Values obtained by typical vacuum-based measurements are often not accurate due to effects such as solvent adsorption. To determine the relative positions of the bandedges, we often need experimental measurements by tools such as ultraviolet photoelectron spectroscopy (UPS), which reports on the ionization energy (effectively the energy of the valence band maximum, E_v).

1.2.3.High crystallization and surface area

In order to achieve efficient charge separation and collection, semiconductors with high crystallinity for charge transport and high surface activities for the OER/HER reactions are desired.

1.2.4.Stability

Stability is of paramount importance to the application of solar water splitting. The optimized photoelectrode material should be stable in electrolytes of a certain pH range, often in the extreme acidic or basic regime. Effective protection may be needed...

Table of contents

• Cover Page
• Title
• Copyright
• Preface
• About the Editors
• Contents
• Chapter 1. The Principle of Photoelectrochemical Water Splitting
• Chapter 2. Semiconducting Photocatalysis for Solar Hydrogen Conversion
• Chapter 3. Visible-Light-Driven Photocatalysis
• Chapter 4. Metal Nitride Nanostructures: Emerging Catalysts for Artificial Photosynthesis
• Chapter 5. Surface Engineering of Semiconductors for Photoelectrochemical Water Splitting
• Chapter 6. Photoanodic and Photocathodic Materials Applied for Free-Running Solar Water Splitting Devices
• Chapter 7. Electrocatalytic Processes in Energy Technologies
• Chapter 8. Soft X-ray Spectroscopy on Photocatalysis
• Chapter 9. Photoelectrochemical Tools for the Assessment of Energy Conversion Devices
• Chapter 10. Fundamentals of Rechargeable Batteries and Electrochemical Potentials of Electrode Materials
• Chapter 11. Revitalized Interest in Vanadium Pentoxide as Cathode Material for Alkali-Ion Batteries
• Chapter 12. Tin-Based Compounds as Anode Materials for Lithium-Ion Storage
• Chapter 13. Beyond Li Ion: Electrode Materials for Sodium- and Magnesium-Ion Batteries
• Chapter 14. Nanomaterials and Nanostructures for Regulating Ions and Electron Transport in Advanced Energy Storage Devices
• Index