CONTENTSshodhganga.inflibnet.ac.in/bitstream/10603/21262/7/07...CONTENTS Chapter I Towards a...

CONTENTS

Chapter I

Towards a sustainable Future

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1.1 An overview of energy system – past, present and future I-1

1.2 Hydrogen production methodologies I-5

1.3 Hydrogen from renewable sources I-6

1.3.1 Water splitting I-6

1.3.2 Solar energy I-7

1.3.3 Wind energy I-7

1.3.4 Geothermal energy I-7

1.3.5 Other forms of energy I-8

1.3.6 Solar powered hydrogen generation technologies I-8

1.4 Thesis overview and summary I-11

PhD Thesis/2013 Chapter I

Synthesis and Characterization of MFe2O4 (M: Zn, Ca, Mg) ferrites for photo catalytic and photo electro catalytic hydrogen generation applications

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Chapter I Towards a Sustainable Future

1.1 An overview of energy system – past, present and future The technological advancement and the economic growth during the last few decades have led to

the major improvements in the living standards of people in the developed world. In this context,

energy is one of the prominent components of productivity comparable to others such as raw

materials, capital and manpower. Our energy system stands as the backbone of the civilization

which facilitates the advancement in technology and in turn, a higher standard of living with the

discovery and use of new fuels containing higher energy content. This is indeed true as depicted

in past wherein the transition from wood to coal fuelled the industrial revolution in 18th and 19th

centuries. Further, the shift from solid to liquid state fuels i.e. coal to petroleum products

assisted the unprecedented technological developments that revolutionized the living standards in

the later half of 20th century [1]. These energy dense fossil fuels were obtained from the

biological residues which were transformed into organic minerals and compounds inside the

earth’s crust under high temperature and pressure for millions of years.

A sustainable development that reflects economic, social, political and environmental

development is the need of the hour at present. For this, it is necessary to access affordable and

reliable energy drawn from environmentally acceptable sources of supply. And this development

should meet the present needs without affecting the future generations. Hence, it remains a

challenge to the humankind to fulfill the energy demand that is required to sustain its living

standards and future developments.

Twentieth century humans used 10 times as much energy their ancestors had in 1000

years preceding 1900 [2-5]. This increase was enabled primarily by fossil fuels which accounts

for 90% of energy worldwide. For the next 20 years, global energy consumption is projected to

further rise as shown in Fig.I.1. This further enhances the utility of coal and oil [6]. As can be

understood from Fig.I.1, the energy consumption will increase at a rate of 1.4 percent per year

till 2035. The status of the current available reserves and the production rates for the primary

energy sources in the world [6] indicates an emergent need to develop alternatives to oil failing



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which would heighten growing reliance on oil imports raising the risk of political and military

conflict and economic disruption.

Further, the current energy system is mainly dependent on the extraction and combustion

of fossil fuels. This is a major threat to the environment as it produces green house gases that

misbalance the on-going carbon cycle in the atmosphere. Such a misbalance will produce

disastrous climatic effects that put into risk not only the present landscape configuration (sea

level, glacier, forest and so on) but also the human environment as in the case of agriculture

(draughts, temperature increase, ecosystem change).

More than a century ago, Arrhenius put forth the idea that CO2 from fossil fuel

combustion could cause the earth to warm as the infrared opacity of its atmosphere continued to

rise [7]. The links between fossil fuel burning, climate change and environmental impacts are

becoming better understood. The environmental concerns about climate changes and the limited

availability in the future of fossil fuels force the transformation of the energy system from a

scheme mainly based on the combustion of fossil fuels to one based on sustainable CO2-free

Fig.I.1 Projection of Global energy consumption for the period 1990-2035[Ref 6]



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sources. Hence it becomes imperative to find a suitable and sustainable energy alternative to

solve the issues related. The practical demonstration in history certainly supports creativity in

meeting this challenge. For instance, many early civilizations used the sun, water, and the wind

to meet basic needs. Even geothermal heat was used by North American Indians some 10,000

years ago for cooking. The ancient Greeks used hydro power to grind flour and the Persians used

windmills to pump water in the first millennium. This indicates that the human race is very good

at solving technological problems and we can certainly wean ourselves from fossil fuels if we

collectively put our minds to it. In this context, hydrogen appears a promising, useful candidate

to replace the “hydrocarbon society”. This idea has its origin from Rigvedic times where the

description of “vimanas” (Aircrafts) is made in the verse 6-62-6. The words like “solar energy

fueled” along with “vehicle” and “magnetic powered” appear in these verses [8]. Jules Verne

appears to be one of the earliest people to recognize, or at least articulate, the idea of splitting

water to produce hydrogen (H2) and oxygen (O2) in order to satisfy the energy requirements of

society. As early as 1874 in The Mysterious Island, Jules Verne had visioned the planet powered

by hydrogen, [9] writing: “Yes, my friends, I believe that water will someday be employed as

fuel, that hydrogen and oxygen, which constitute it, used singly or together, will furnish an

inexhaustible source of heat and light….I believe, then, that when the deposits of coal are

exhausted, we shall heat and warm ourselves with water. Water will be the coal of the future.” A

century and a quarter later, the idea of using hydrogen — the simplest, lightest, and most

abundant element in the universe — as a primary form of energy beginning to move from the

pages of science fiction and into the speeches of industry executives as explained by Frank

Ingriselli, a Texaco executive in his speech where he said “Greenery, innovation, and market

forces are shaping the future of our industry and propelling us inexorably toward hydrogen

energy” [2, 10].

The reality of an eventual transition to hydrogen becomes more evident when one takes

an atomic view of energy history. The shifting of energy from one form to another form from

solid to liquid to gas has been experienced by the world since 19th century as illustrated by Rober

Hefner of GHK Company in Fig.I.2 [11-12].

The move from solid to liquid to gas fuels involves another sort of transition: the less

visible process of “decarbonization”. From wood to coal to oil to natural gas, the ratio of



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hydrogen (H) to carbon I in the molecule of each successive source has increased. Roughly

speaking, the ratio is between 1–3 and 1–10 for wood; 1–2 for coal; 2–1 for oil; and 4–1 for

natural gas [13]. Between 1860 and 1990, the H–C ratio rose sixfold. Thus for the last 200 years,

the world has progressively favored hydrogen atoms over carbon, a trend towards

‘decarbonization’ which is at the heart of understanding the evolution of the energy system”.

Hence, hydrogen becomes the preferred logical fuel in this sequence which is the lightest and

most abundant element in the universe and also the power source of our Sun. The emergence of

hydrogen as a major energy carrier could initially build on the existing natural gas network for its

distribution, with the hydrogen derived at first from natural gas to run high-efficiency fuel cells.

The production of hydrogen from virtually limitless stores of renewable sources will free the

energy system from carbon.

Fig.I.2 Global energy systems transition, 1850–2150.[Ref 11]



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1.2 Hydrogen production methodologies The development of a clean, sustainable and cost competitive hydrogen production methodology

is always desired for the success of hydrogen powered systems. There are numerous methods of

producing hydrogen which have been well known for centuries. Most of these methods are

processed through a variety of chemical reactions. However, there are safety and environmental

issues related to the processing of these reactions. The hydrogen economy is very much

influenced by the development of safe, efficient, widely available and environmentally

responsible means for generating hydrogen. Fig.I.3 displays the schematic describing the broad

classification of various methodologies to produce hydrogen.

The seven key production technologies for hydrogen generation fall into three broad

categories namely thermal, electrolytic, and photolytic processes [11]. In the thermal processes,

one type of process uses the energy stored in such resources as coal or biomass to simply release

the hydrogen contained within their molecular structures. Another type uses heat in combination

with closed chemical cycles to produce hydrogen from feedstocks, such as water; these are

known as “thermochemical” processes. Distributed Natural Gas Reforming, Bio-Derived Liquids

Reforming Coal and Biomass Gasification and thermochemical production using a heat-driven

chemical reaction form a part of thermal process. Water electrolysis uses electricity to split water

into hydrogen and oxygen. Hydrogen produced via electrolysis can result in zero greenhouse gas

emissions, depending on the source of the electricity used. Photolytic processes use light energy

to split water into hydrogen and oxygen. Currently in the very early stages of research, these

processes offer long-term potential for sustainable hydrogen production with low environmental

impact. Photoelectrochemical Hydrogen Production (Using Solar Power to Directly Split Water)

and Biological hydrogen Production represent the important methods under this category. At

present, fossil fuel sources dominate the hydrogen production due to their abundance. However,

the aim of any realistic sustained energy system based on hydrogen necessitates the fabrication

of hydrogen from renewable sources [14]. In this regard, hydrogen using water and solar light

remains the first and the ultimate choice.



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1.3 Hydrogen from renewable sources

In recent times, focus towards the production and use of renewable fuels has accelerated to attain

cleaner and less polluting environment [15-16]. The perfect renewable sources accessible to

human kind are sun, wind and water. The use of these renewable energy sources not only helps

to reduce global CO2 [17] but also decrease our dependence on limited reserves of fossil fuels.

The constraint on these sources is the economy and in most cases, further research is needed to

make them cost-effective. We discuss water splitting and the available different renewable

sources which can be explored for the same.

1.3.1 Water splitting

Water is the most abundant source of hydrogen on the planet. However, hydrogen is strongly

bonded to oxygen and a minimum energy of 1.229 eV (at 25°C, 1 bar) is required for splitting

water into hydrogen and oxygen [1]. Although this energy can be supplied via renewable or non-

Fig.I.3 Hydrogen production paths



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renewable sources, hydrogen from water using renewable energy sources is the only permanent

solution, and one that can be obtained peacefully as well. Water splitting can be accomplished

via techniques such as electrolysis, photoelectrolysis, thermochemical, and biophotolysis.

Although each technique has its own merits and demerits and will have a role in

hydrogen economy, solar energy coupled with electrolysis and solar water photoelectrolysis

appear to be the most logical solution pathways.

1.3.2 Solar energy Solar energy is a virtually inexhaustible and freely available energy source. More sunlight (~1.2×

105 TW) falls on the earth’s surface in 1 h [9] than is used by all human activities in one year

globally. The sun is earth’s natural power source, driving the circulation of global wind and

ocean currents, the cycle of water evaporation and condensation that creates rivers and lakes, and

the biological cycles of photosynthesis and life. It is however a dilute energy source (1 kW/m2 at

noon). About 600–1000 TW strikes the earth’s terrestrial surfaces at practical sites suitable for

solar energy harvesting. Covering 0.16% of the land on earth with 10% efficient solar conversion

systems would provide 20 TW of power, nearly twice the world’s consumption rate of fossil

energy and an equivalent 20,000 1-Gwe nuclear fission plants. Clearly, solar energy is the largest

renewable carbon-free resource amongst the other renewable energy options.

1.3.3 Wind energy Wind energy represents the nearest term cost competitive renewable energy source [18-19].

Produced by the solar heating of earth, wind as an energy resource, proportional to the cube of its

velocity, presents a dual-use technology: the land can still be used for farming, ranching and

forestry, and the collection of solar energy.

1.3.4 Geothermal energy Geothermal energy is obtained by extracting heat from water or rocks deep underground. A

practical geothermal site requires that there be high temperature rocks and/or water within

approximately 300 m of the surface. There are places where the holes are drilled 1500 m deep

and 300°C superheated water pumped to the surface for electricity generation. Heat can and

typically be extracted from the rocks much more rapidly than it is restored from the ambient.



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Therefore the geothermal plants have limited lifetimes, requiring the periodic drilling of new

holes (with their associated costs) for continued operation.

1.3.5 Other forms of energy The other forms of energy are the hydro-electric, ocean and tidal energies [1]. Each type of

energy has its own merits and demerits. Hydro-electric energy has been used throughout the

world for centuries but, however, has little room for future growth. Energy from the ocean is

appealing but difficult to achieve. For example, the temperature difference between the surface

and deep-ocean can be used for thermally driven power generation. However biofouling,

corrosion, and storms make the prospect challenging. While tides have been exploited since the

18th century for power generation the small change in potential energy associated with the tides,

corresponding to a height difference of 1 m to 2 m is difficult to harvest. Small scale harvesting

of wave energy appears to be marginally feasible.

The availability of energy from renewable sources varies in space and time. The energy

may not be available as per the need of users. Hence a medium, or a buffer system, is required to

store the energy. A renewable source can be considered useful if it can deliver energy in

electrical or chemical energy form. As renewable sources are localized, typically requiring

transportation of energy to places in need, the energy carrier should be portable. Hydrogen is

considered the most appropriate medium where renewable energy can be effectively stored and

transported.

1.3.6 Solar powered hydrogen generation technologies A wide variety of solar technologies have the potential to become a large component of the

future energy portfolio. Passive technologies are used for indoor lighting, heating of buildings

for domestic use. Also, various active technologies are used to convert the solar energy into

various energy carriers for further utilization. Different types of energy conversion systems have

emerged over the years, which can be further used for hydrogen generation.

In Solar thermal conversion, the energy of direct light is converted into thermal energy

using concentrator. These systems reach temperatures of several hundred degrees with high

associated energy. Electricity can then be produced using various strategies including thermal



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engines (e.g. Sterling engines) and alternators, direct electron extraction from thermionic

devices, Seebeck effect in thermoelectric generators, conversion of IR light radiated by hot

bodies through thermo photovoltaic devices and conversion of the kinetic energy of ionized

gases through magneto-hydrodynamic converters.

Photobiological processes are based on absorption of photons by a leaf chloroplast or

algae. Photosynthetic organisms use photons of the sunlight for an energy storing reaction.

Energy storage is based on the reduction of carbon dioxide to form carbohydrates. It is possible

to modify conditions in these systems such that the photosynthetic process is coupled to a

hydrogen-generating enzyme.

In solar photon energy conversion, the photons emanated from sunlight are used as the

driving force in the energy conversion process. The photoactive materials absorb photons to

convert it into chemical or the electrical energy depending on the type of interaction.

Semiconductor systems are used in most cases for such applications. In this, the sunlight is

absorbed in a semiconductor material. The absorption of a photon results in the transfer of an

electron from its valence band to its conduction band. This electron can be used to drive a

chemical reaction. The semiconductor can be in the form of a small particle suspended in a liquid

(photocatalysis) or in the form of a film deposited on a support (photoelectrochemical cell), built

into a macroscopic unit like a photovoltaic cell or an electrochemical cell. Currently, it is the

least expensive and most effective method of hydrogen production from renewable resources.

While Technologies which have adopted these methods are already being used—such as

natural gas reforming, coal gasification, and electrolysis from both renewable and non-renewable

energy sources—others like the photolytic processes are still a long way from commercial use.

The greatest technical challenge to various renewable hydrogen production methods is cost

reduction. Current research seeks ways to reduce the cost of capital equipment, operations, and

maintenance costs, while improving hydrogen production efficiency. Fig.I.4 projects the current

and future methods for hydrogen production [19]. It is clear from the figure that while some of

the renewable technologies like solar, wind, geothermal and hydro systems are expected to

develop technologically in the near midterm and long range time span, hydrogen from biomass



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would be technologically developed in the midterm. However, certain technologies like

photobiological and photolysis which include photocatalysis, photoelectrochemical water

splitting are at the research stage and are expected to develop technologically in a long term span

of time. Further, the economical and environmental analysis of various methodologies indicates

a promising sustainable future for the photocatalytic and photoelectrochemical methods of

hydrogen generation. Hence the development of materials for the photocatalytic and

photoelectrochemical methods which are efficient in addition to being economical, eco-friendly,

and sustainable for long time is very much desired. Ferrite systems in particular MFe2O4 ferrites

exhibit suitable opto-electronic properties in addition to being economical, eco-friendly and

Fig.I.4 Current and future methods for hydrogen production Ref: [19]



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sustainable for a long time. Hence, in the present dissertation, the preparation methods of these

ferrite systems along with their photocatalytic and photoelectrocatalytic capability of hydrogen

generation are explored.

1.4 Thesis overview and summary The preceding sections presented an overview of the energy systems. Thus, the study of the

existing systems added to the environmental conditions along with the global demand force us to

think about utilizing energy from a renewable and a sustainable source such as solar energy.

Further, implementing an economic and environmental friendly method is always desirable. Of

the different methodologies available, the photocatalytic and PEC hydrogen production appears

to be the most economical and sustainable which makes them feasible for a long term

application. Thus, the main objective of the present work is to fabricate photocatalysts and

photoelectrocatalysts for solar hydrogen generation.

The present thesis entitled “Synthesis and Characterization of MFe2O4 (M: Zn, Ca, Mg)

ferrites for photocatalytic and photoelectrocatalytic hydrogen generation applications” is

organized into three sections comprising of seven chapters. The contents of each chapter are

briefly discussed in the following paragraphs.

The first section is the general introduction on the various ways of producing hydrogen

followed by specific discussion on the photoinduced hydrogen generation; this is exclusively

described in first two chapters. The first chapter “Towards a sustainable future” presents the

global energy scenario during the past, present and future era. The chapter presents the existing

scenario on the ‘hydrogen based energy system’ with respect to past, present and future times.

The study of the known existing ways which affect global environment to significant extent,

demands one to think about renewable and sustainable energy substitute. Further, the importance

of implementing an economic and environmental friendly method of hydrogen generation is

stressed in the general introduction of hydrogen production system presented in this chapter. In

this context, the feasibility of photocatalytic and photoelectrocatalytic hydrogen generation as the

most economical and sustainable methods for a long term application, has been presented.



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Chapter-2 discusses the motivation and the basic principles underlying the

photocatalytic and photoelectrocatalytic hydrogen generation methodology. In general, the

factors affecting the photocatalytic and photoelectrocatalytic performance and their evaluation

methods are explained with respect to water-splitting application. The material aspects of the

hydrogen producing materials are discussed with a specific focus on various forms of ferrites.

Accordingly, an overview of ferrites and the reported work for their photocatalytic and

photoelectrocatalytic hydrogen generation application is presented in this chapter.

The second section Part-A, presents the work on the “powder photocatalysts”. This part

comprises of two chapters, chapter 3 & 4. Chapter 3 describes the experimental and

characterization methods to fabricate the ferrite photocatalysts. Different syntheses methods

namely solid-state reaction, microwave synthesis, polymer complex and self-propagating

combustion method, adopted to obtain the three ferrite systems viz. ZnFe2O4, CaFe2O4 and

MgFe2O4 are discussed in this chapter. Further, the characterization techniques used to study the

physico-chemical properties of the as-synthesized powders are described. The details of the

parameters used to study their structural, optical, morphological, electrochemical properties are

discussed. Further, the procedure to investigate and evaluate the photocatalytic property for

methylene blue degradation and hydrogen generation is explained.

Chapter 4 discusses the results of the detailed characterization of the three ferrite

systems viz. ZnFe2O4, CaFe2O4,, MgFe2O4. The effect of syntheses methods on the physico-

chemical properties of these ferrites is investigated. Finally, the photocatalytic property of thus

obtained photocatalysts is compared for three systems from the photodegradation and

photocatalytic hydrogen generation studies. High crystallinity, smaller crystallite size, large

surface area, suitable band gap and band energetic are some of the desirable properties for an

efficient photocatalyst. The study clearly reveals that the syntheses method adopted influence

physico-chemical properties of the obtained photocatalyst material. Further, their properties can

be tuned to the desired value by appropriate optimization of the synthesis parameters. The solid

state reaction and microwave sintering were considered under the category of physical method of

material synthesis. In the chemical synthesis process, polymer complex method and self

propagating combustion methods were considered.



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The third section of the thesis i.e. part-B consists of two chapters, 5 and 6. This part

focuses on the film deposition and characterization for photoelectrochemical hydrogen

generation. Chapter 5 discusses the experimental part, describing the details on the solution

precursor plasma deposition (SPPS) of ZnFe2O4 and its composite film. Different parameters

varied during the deposition of these films are also explained. The ZnFe2O4 films were deposited

using aqueous and non-aqueous based solvents. The pH of the final precursor was varied in the

alkaline range from 7.0 to 10.0. Further, the processing parameters like plasma power, spray

distance, substrate pre-heat temperature, precursor concentration, stoichiometry, and precursor

feed rate were optimized for the deposition of these films. Thus deposited films were

characterized using different techniques.

Chapter 6 discusses the results obtained from film characterization studies discussed in

chapter 5. An effect of deposition parameters in realizing a pure phase ZnFe2O4 film is described

in detail. Further, the results with respect to the composite film are discussed. Finally, the photo

electrochemical properties of the pure and the composite films are compared so as to correlate

their hydrogen generation capability. The ZnFe2O4:Fe2O3 ferrite nano-composite photo

electrode (FNCP) was found to improve the solar hydrogen generation as compared to ZnFe2O4

photo anode.

The last chapter 7 of the thesis is the concluding part. This chapter concludes the

reported work with the brief note on the ferrite capability for photo catalytic and

photoelectrocatalytic hydrogen generation comparing the three systems obtained in part A and

pure and composite photoanodes in part B. The additional scope of the work to further improve

the efficiency is also discussed in this chapter.

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