Photoelectrochemical Hydrogen Generation

A chapter on “Hydrogen: A Future Chemical Fuel” provides a basic knowledge of hydrogen energy and its importance in modern society. Nowadays, the development of hydrogen energy has shown a tremendous attraction of today’s researchers because it is a source of clean energy (i.e. green energy) and most abundant element in nature. The excellent properties of hydrogen such as high-energy content (~141.86 MJ kg⁻¹), environment friendly nature, sustainability, etc. prove it as a highly acceptable green fuel. The main idea behind this chapter is to describe the understanding of the necessity of H₂ energy, production of energy, storage, the challenges, and hopes towards the development of system infrastructure, and the most important is applications of Hydrogen Economy.

The global energy demand is growing at an alarming rate and the world is struggling with depleting sources of conventional energy along with their drawbacks, which pose serious threats to the environment. Several sources of renewable energy are being explored for their potential to bridge the energy gap, with a simultaneous contribution towards sustainable development. Following the pursuit, hydrogen due to its highest calorific value and zero emission has emerged as an ideal candidate among the clean sources of energy. It has been termed as the “future fuel” by the global research community for a long time and even the term “Hydrogen Economy” is coined, as the future economy is forecasted to be based on hydrogen. This chapter provides an in-depth study of the fascinating properties of hydrogen and a brief about the major challenges associated with its commercial use. Moreover, worldwide aspects of energy, namely, availability and sustainability in terms of energy security, efficiency, and policy. The timeline of hydrogen has also been presented in terms of production and cost competitiveness.

Development of the clean and green energy sources is the most intensive research in the present energy crisis scenario. In this approach, hydrogen (H₂) can be a promising source of clean energy due to its high energy density in molecular form. There are various methods for the H₂ production such as water splitting by providing heat energy, partial oxidation, and steam reforming. The main drawback of these methods is that they leave behind carbon emission while H₂ production. Electrolysis of water by the electrochemical hydrogen evolution reactions (HER) is more valuable and proficient method to produce H₂ because it is of low cost and pollution free. The key point of the electrochemical water splitting to produce hydrogen is that its kinetics is slow. To enhance the H₂ production, the HER kinetics need to be faster and for that an efficient electrocatalyst is required which must be earth abundant and cost effective. This book chapter covers the basics of electrochemical water splitting with providing a clear idea of the water-splitting mechanism via HER. Here, a fruitful discussion about the selection of electrocatalysts for electrochemical water splitting has been included with theoretical, computational, and experimental perspective. A detailed discussion has been carried out for the performances of various electrocatalysts for an effective water splitting in this book chapter.

Nature has evolved an intricate process called “photosynthesis” that depicts a brilliant example of solar energy conversion into fuels (energy-rich compounds), by extracting electrons from water employing solar radiation (light) as the only energy source. In this process, a huge amount of energy is stored in the form of chemical bonds (e .g. carbohydrates, etc.). These chemical transformations are facilitated by enzymes at an outstanding rate. Inspired by this biological photosynthesis process, an enormous interest has grown to mimic this process, to produce a renewable energy vector-hydrogen. There has been multifaceted development over last few decades, on the design of fuel forming photocatalysts, water oxidation catalyst, optimization of light-harvesting unit, and integration of all these to design artificial photosynthesis systems. The catalysts ranging from molecular mimics of naturally occurring enzyme activities, nanostructured semiconducting photocatalyst, cocatalysts (metal nanocrystals, etc.), and their electroactive coupling (to have seamless electron transfer across) with sensitizers ranging from organic dyes to semiconducting nanoparticles have been designed to produce hydrogen. In this chapter, the progress made in the chemistry front of designing such bio-mimetic catalysts, their integration with light-harvesting components, and the state of the art of artificial photosynthesis systems has been explicitly discussed. The focus currently lies largely on the chemistry in designing these artificial fuel forming systems and has been elaborated herein. Further, to get an overall insight into the solar H₂ generation, the underlying fundamental phenomena are also comprehensively discussed.

In the current chapter we discuss the importance of plasmonics for photoelectrochemical water splitting. A survey through plasmonics, the underlying theoretical background, and an overview of selected articles are provided highlighting the different plasmonic-enhanced systems. All relevant diagnostic parameters are explained, and simple band diagrams are chosen to provide a visual overview to explain the main mechanisms taking place. Furthermore, we provide a scope of how the field could progress and what it is still lacking.

BiVO₄ is an n-type semiconductor that has shown great potential as a photoanode in water-splitting photoelectrochemical cells due to its relatively small bandgap energy (2.4–2.5 eV), valence band energy level suitable for the water oxidation reaction, and for being formed by relatively abundant elements in the earth's crust. Despite this, BiVO₄ has poor efficiency in separating and transporting charges in bulk, and low charge transfer efficiency on the surface, which limits its application as a photoanode. In recent years, different strategies have been used to address those issues. Therefore, in this chapter, we review the latest advances in the development of BiVO₄ photoanodes involving doping with metals and non-metals, the formation of homo- and heterojunctions, control of morphology, porosity, crystal facets, annealing treatments, combination with plasmonic nanoparticles and catalysts for the oxygen evolution reaction, use of interfacial charge mediators, combination with ferroelectric materials, use of overlayers and underlayers, and electrolyte effect for water oxidation. Finally, the integration of BiVO₄-based photoanodes with photocathodes for the unbiased water splitting is also described.

Energy harvested from sunlight is providing the most abundant resource of renewable energy which reaches the earth with enough potential to meet all of essentials. The stochastic and intermittent nature of solar energy hinders the continuous supply of energy and thus storing this energy for subsequent use should be the most precise thing to do. Fuel cell is such a device which uses hydrogen and oxygen as the fuel for energy generation and these fuels can be easily produced by utilizing sunlight which breaks water into hydrogen and oxygen through water splitting. Hydrogen, a “zero-emissive fuel” is also possessing high calorific value. Among several challenges that constrain the production of hydrogen using solar energy, the performance of an electrocatalyst is the most important one. In order for mass production, electrocatalyst should be inexpensive and made up of the material which is abundantly available in earth. Defect-enriched transition metal oxides have shown excellent performance towards the efficient generation of hydrogen using solar energy. In order to create defect-rich electrocatalyst, various strategies have been opted. In this chapter, we have focussed towards the defect-enriched transition metal-based oxides as electrocatalyst for the clean source of fuel production.

In semiconductor photoelectrochemical (PEC) cells, nitride-based materials have attained immense interest because of their suitable band position and bandgap, facile and low-cost synthesis, good thermal stability, and low toxicity. Mostly, two distinct classes of nitride materials have been explored in PEC cells-(i) metal nitrides and (ii) metal-free graphitic carbon nitride (g-CN). Although the use of g-CN in PEC cells is more promising due to its low photocorrosion and long-term stability, the development of photoelectrodes with g-CN is still in its primary stage. Besides, the low photocurrent density produced by g-CN photoelectrodes restricts its application. In contrast, metal-based nitrides are widely explored as photoelectrodes, and tremendous progress in the synthetic and application front has been achieved. Doping and substitution in the materials, integration with different cocatalysts, and fabrication of composite photoelectrodes have been demonstrated to substantially improve the photocurrent density. Moreover, efforts have been made for a thorough understanding of the photochemical and photoelectrochemical processes, including charge separation, recombination, charge transport, and interfacial processes. In this chapter, we describe the basic principles of designing nitride-based PEC cells, achievements, and deficiencies in nitride-based photoelectrodes. A detailed discussion on the application of the nitride-based photoelectrodes in photoelectrochemical water splitting is included, along with a perspective for the future application of this field.

Sustainable scientific innovations hold key to a progressive world in its true sense. Steeping high energy demands and lack of universal presence of resources gravitates the need to generate alternate fuel sources to sustain human life in the near future. Splitting water over heterogeneous nanocatalysts to generate hydrogen offers an economic, green, and fairly easy replacement for clean fuel production. As conventional and one of the most sought photoelectrodes, semiconductor nanomaterials forms an integral part of PEC evolution and the prelude of this chapter. This section details semiconductor-based photoelectrode research progress over the last decade, the role of befitting optical and electronic tunability, bandgap engineering, morphology control, and high surface of these nanomaterials which make them an ideal photoelectrode material. Efficiency of light to fuel conversion predominately depends on photon absorption and the magnitude of interfacial charge transfer (ICT). While highlighting the strategies for increasing photon absorption the course of this chapter follows to focus on the role of morphology of the nanomaterial as it dictates surface area, charge separation, transportation, and mobility. In continuation, advances towards functionalizing photomaterials for making them responsive to visible light, with emphasis on harvesting solar light are cited. The chapter briefly also mentions the more recent findings on peculiar photocatalysts such as molybdenum disulphide, oxy-nitrides, and graphene based. Lastly, contemporary challenges and opportunities in the field are summarized.

The energy generation either via solar cells (SC) or photoelectrochemical (PEC) cells is becoming more and more important for catering the energy demands of the world. The PEC process conversion provides hydrogen (H₂), methanol (CH₃OH), or some other alkanes molecules as output fuels that can store a remarkable amount of energy/unit mass. Semiconductors especially Group-III nitrides (especially InGaN alloys) can offer several advantages to the design of photoelectrodes for PEC cells used for water splitting, such as it is possible to tune its bandgap from 0.65 to 3.40 eV, thus they have the potential for entire solar spectrum applications. The photocatalytic activity seems to be further improved by using nanostructures of InGaN/GaN, due to the improvement in optical absorption through light trapping, improved surface to volume ratios and swift charge carrier separation. However, to improve the PEC conversion efficiency from III-nitride materials based structures, it is required that these semiconductor layer surfaces have to be stabilized and should be engineered with catalysts, that can be either molecular or inorganic, to modify the kinetics of desired reactions pathways. During the course of the chapter, we will discuss the current state of the art in the field and strategies for designing and developing an efficient PEC solar fuel cell using III-nitride photoelectrode assisted by nanopatterning.