Summary report of few Pb-free perovskites used in SOFCs.
Perovskites possess an interesting crystal structure and its structural properties allow us to achieve various applications. Beside its ferroelectric, piezoelectric, magnetic, multiferroic, etc., properties, these branches of materials are also useful to develop materials for various environmental applications. As the population is increasing nowadays, different type of environmental pollution is one of the growing worries for society. The effort of researchers and scientists focuses on developing new materials to get rid of these individual issues. With modern advances in synthesis methods, including the preparation of perovskite nanocomposites, there is a growing interest in perovskite-type materials for environmental application. Basically, this chapter concludes with a few of the major issues in the recent environment: green energy (solar cell), fuel cell, sensors (gas and for biomedical), and remediation of heavy metals from industrial wastewater.
- fuel cell
- solar cell
- heavy metals
- wastewater treatment
Perovskites possess a very interesting crystal structure; are basically a combination of three basic crystal structures (simple cubic structure, body center cubic structure, and face-centered cubic structure). The extraordinary range of structure and properties interplay of perovskites makes them an exceptional research field for different branches like materials science, physics and solid-state chemistry. A wide range of unique functional materials and device ideas can be predicted through a basic understanding of the correlation between structural and chemical compatibility.
The perovskite structure is shown to be the single most adaptable ceramic host. Inorganic perovskite-type oxides are attractive compounds for varied applications due to its large number of compounds, they exhibit both physical and biochemical characteristics and their nano-formulation have been utilized as catalysts in many reactions due to their sensitivity, unique long-term stability, and anti-interference ability. Some perovskite materials are very hopeful applicants for the improvement of effective anodic catalysts performance. Depending on perovskite-phase metal oxides’ distinct variety of properties they became useful for various applications they are newly used in electrochemical sensing of alcohols, glucose, hydrogen-peroxide, gases, and neurotransmitters. Perovskite organometallic halide showed efficient essential properties for photovoltaic solar cells.
1.1 The basic structure of perovskites
Figure 1 depicts the ideal perovskite structure. In the ideal crystal structure of perovskite with general formula ABX3; where “A” and “B” are generally metal cations and “X” is an oxide or halide like Cl, Br, I, etc., “A” can be Ca, K, Na, Pb, Sr, and other rare-earth metals which occupy the 12-fold coordinated sites between the octahedra. “X” forms the BX6 octahedra where B located at the center of octahedra. Perovskite can be described as consisting of corner-sharing [BX6] octahedra with the A-cation occupying the 12-fold coordination site formed in the middle of the cube of eight such octahedra. In an ideal cubic unit cell of perovskite, Wyckoff positions for A- ion is at cube-corner positions (0, 0, 0); ion B sites at body center position (1/2, 1/2, 1/2) and ion X sits at face-centered positions (1/2, 1/2, 0). Figure 2 shows the elements which can be in A-site, B-site from the periodic table.
In ideal perovskite such as SrTiO3 , CsSnBr3 , etc., there is no such distortion in the unit cell. There are many different types of lattice distortions that can happen due to the flexibility of bond angles within the ideal perovskite structure .
Distortion in BX6 octahedra, by the Jahn-Teller effect.
Off-center displacement of B-cation in BX6 octahedra, this one of the causes for ferroelectricity in these type of materials.
So-called tilting in octahedra framework, usually occurring as a result of too small A-cations at cuboctahedral site.
Ordering of more than one type of cations A or B, or of vacancies.
Ordering of more than one kind of anions X, or of vacancies.
The different physical properties (mainly electronic, magnetic, dielectric, and piezoelectric properties) of perovskite materials are crucially dependent on these distortions. The distortion as a consequence of cationic substitution can be used to fine-tune physical properties exhibited by perovskite.
In the case of perovskite structure (closed packed), A-cation must fit among four BX6 octahedra. Each A-cation is surrounded by 12 nearest X-anions (12-fold coordination). Therefore A-cations have limited space to accommodate itself in the interstitial position. In the case of ideal perovskite structure, the cell axis (a) is geometrically related to the ionic radii (𝑟𝐴, 𝑟𝐵, and 𝑟𝑋) as described in the following equation.
The ratio of the two expressions for the cell length is called Goldschmidt’s tolerance factor (t) and it allows us for evaluating the degree of distortion in the unit cell. The expression for Goldschmidt’s tolerance factor  is as follows.
where 𝑟𝐴 is the radius of A-cation is, 𝑟𝐵 is the radius for B-cation and 𝑟𝑋 is the radius for X-anion.
1.2 Why lead-free?
Lead (and its oxide form) is highly hazardous and its harmfulness is further improved due to its volatilization at high temperature mainly during calcination and sintering causing environmental pollution during different sample preparation techniques . According to the European Union (EU), hazardous substances like lead and other heavy metals is planning to strictly prohibit [8, 9].
1.2.1 Toxicity effects of lead
The main indications of lead poisoning are tiredness, muscles and joints pain, abdominal uneasiness, etc. Sometimes the deposition of lead sulfide can be found out in the dental margin of the gums of the patients having poor dental hygiene. Lead harming has been considered as a health hazard, for its bad effects on neurological and cerebral development [10, 11, 12]. The main route of absorption in adults is the respiratory region where 30–70% of inhaled lead (typically the inorganic form like oxides and salts) goes into the cardiovascular system. The maximum tolerance of lead in blood ranges from 1.45 to 2.4 mol L−1 (30–50 g 100 mL−1) with a provision of 6 monthly observations . Basically, lead has few significant biochemical properties that give toxic effects on the human biological system. (i) As lead is electropositive in nature, it shows a very high affinity for the enzymes, which are necessary for the synthesis of hemoglobin. (ii) The divalent lead behaves similarly to calcium preventing mitochondrial oxidative phosphorylation as a result intelligence quotient (IQ) got reducing. (iii) The transcription of DNA can also disturb by lead by interacting with binding protein and nucleic acids [14, 15]. Figure 3 illustrated the adverse effect of lead on the human body.
Bearing in mind the hazardous effect of Pb in Pb-based compounds, the research communities focused on designing the materials which are basically Pb-free. Hence, this chapter concludes with some Pb-free perovskite-type materials for the environmental application point of view.
2. Application of Pb-free perovskites from a different aspect of environmental
2.1 Perovskites as solid oxide fuel cells
Recently, the inorganic perovskite-type of oxide nanomaterials have been widely applied in the processing of chemically modified electrodes [16, 17]. They have acknowledged considerable attention in the last few decades because of their catalytic activity in diverse processes like purification of waste gas and catalytic combustion.
In the fuel cell, there is a direct conversion of chemical energy into electrical energy similar to a battery. These are attractive because of their great efficiency, low emission, almost zero pollution (basically noise pollution). The solid oxide fuel cells (SOFCs) have come into the picture as effective substitutions to the combustion engines due to their prospective to minimize the environmental impact of the use of conventional fossil fuels. Perovskite oxides exhibited attractive properties like a high electrical and ionic conductivity similar to that of metals and the perfect mix of these two types . This mixed conduction properties of perovskite oxides are advantageous for electrochemical reaction. The working principle of a SOFC is depicted by Figure 4 . The perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-δ used as an effective cathode for intermediary SOFC reported by Shao and Haile. This cathode unveiled the maximum power density of 402 and 1010 mW cm−2 at 500 and 600°C, respectively . The combination of single and double perovskite oxide Ba0.5Sr0.5(Co0.7Fe0.3)0.6875W0.3125O3−δ (B-SCFW) was investigated by Shin et al.  for self-assembled perovskite composites for SOFC. In contrast, Goodenough reported that the double perovskite Sr2MgMnMoO6-δ can act as an anode material for SOFC with dry methane as the fuel and it shows maximum power density of 438 mW cm−2 at 800°C. This anode material exhibited long-term stability and having oxygen insufficiency, as well as some good environmental effects like tolerance to sulfur, stability in reducing atmosphere . Table 1 enlisted with some perovskites used as anode and cathode for SOFCs.
|Perovskite compositions||Anode/cathode In cell||Fuel used||Operating temperature (°C)||Maximum power density (mW cm−2)||Reference|
|Ba0.5Sr0.5Co0.8Fe0.2O3-δ||Cathode||Humidified H2 (~3% H2O)||500||402|||
|NdFeO3||Anode||Sulfur vapor or SO2||620||0.154|||
|La0.8Sr0.2Cr0.97V0.03O3||Anode||Dry methane||800||Not reported|||
|La0.75Sr0.25Cr0.5Mn0.5O3||Anode||Methane||Not reported||Not reported|||
2.2 Perovskites as sensor
2.2.1 Perovskites as glucose sensor
It is very essential to determine hydrogen peroxide (H2O2) and glucose analytically in any aspect of our daily life. In environmental waste management, chemical and food industries, and medical diagnostics H2O2 widely used as one of the most important oxidizing agents . On the other hand, glucose represents a fundamental component in human blood that delivers energy through the metabolic process. If in human blood, the glucose concentration fluctuates than the normal range of 80–120 mg dL−1 (4.4–6.6 mM) is related to the metabolic disorder from insulin insufficiency and hyperglycemia, the so-called diabetes mellitus . To perform the diagnosis and supervision of such health issue it is necessary a tight observation of glucose level of blood. Hence, it is very significant to make the biosensors for the sensitive determination of glucose and H2O2. Basically, there are two types of glucose sensors available: enzymatic and non-enzymatic. Different types of enzymatic glucose sensors were constructed and used in the literature exhibiting the advantages of simplicity and sensitivity. However, enzymatic glucose sensors suffered from the lack of stability and the difficult procedures required for the effective immobilization of the enzyme on the electrode surface. The lack of enzyme stability was attributed to its intrinsic nature because the enzyme activity was highly affected by poisonous chemicals, pH, temperature, humidity, etc. As a result, most attention was given for a sensitive, simple, stable, and selective nonenzymatic glucose sensor. Different novel materials were proposed for the electrocatalytic oxidation of glucose like noble nanometals, nanoalloys, metal oxides, and inorganic perovskite oxides. Inorganic perovskite oxides as nanomaterials exhibited fascinating properties for glucose sensing like ferroelectricity, superconductivity, charge ordering, high thermopower, good biocompatibility, catalytic.
Wang et al. utilized a carbon paste electrode (CPE) modified with LaNi0.5Ti0.5O3 (LNT) as a promising nonenzymatic glucose sensor. This glucose sensor displayed a perfect electrochemical activity and was used to quantify of glucose with great sensitivity of 1630.57 μA mM−1 cm−2 and a low detection limit of 0.07 μM. This glucose sensor also demonstrated an excellent reproducibility, long-term immovability, as well as outstanding selectivity with no interference from the common interfering substances such as dopamine, ascorbic acid, and uric acid . The perovskite-spinel type composite oxide LaNi0.5Ti0.5O3-NiFe2O4 with different compositions was demonstrated as the glucose sensor by Wang et al. This material also exhibits admirable reproducibility, stability and selectivity in glucose sensitivity with a linear signal-to-glucose concentration range of 0.5–10 mM and a detection limit (S/N = 3) of 0.04 mM . Furthermore, LaxSr1-xCoyFe1-yO3-δ (x = 0.6; y = 0.0 and 0.2) perovskites were studied as electro-catalytic materials for H2O2 and glucose electrochemical sensors by Liotta et al. . The group of Atta et al. has reformed SrPdO3 perovskite with gold nanoparticles to be employed as a non-enzymatic voltammetric glucose sensor. This nanocomposite disclosed an excellent performance to glucose sensing in terms of highly reproducible response, high sensitivity, low detection limit, appreciable selectivity, long-standing stability . He et al. depicted that the perovskite oxide La0.6Sr0.4CoO3-δ can provide superior electro-oxidation activities (to H2O2 and glucose) over La0.6Sr0.4Co0.2Fe0.8O3-δ and LaNi0.6Co0.4O3 that translates to good H2O2 or glucose detection performance. They have modified the sensor by making composite with reduced graphene oxide (RGO) and La0.6Sr0.4CoO3-δ for exhibiting higher detection properties and improved selectivity .
2.2.2 Perovskites as a gas sensor
The clean air is undoubtedly most necessary than water for human health, but unfortunately, human activities accompanying socioeconomic developments are the vital pollution sources. So, it is very important to closely observe the quality of the air, including the indoor air quality (IAQ) as we spent most of our time (~90%) of our time in the indoor climate, to prevent different unusual symptoms [35, 36, 37]. Thus, researchers and scientists across the whole globe have been developing new and advanced material based innovative methods for consistent and careful detection of gases and volatile organic compounds (VOCs) hazardous to human and environmental health [38, 39]. The environmental worries about health hazards due to the existence of poisonous gases, for example, CO, CO2, NO2, O3, etc., and subsequent safety regulations have demanded the enhanced use of sensors in various sceneries from the industrial sites to automobiles, the different workplaces and even homes. Among the several toxic gases, CO and NO2 are the most harmful air pollutants and are dangerous for animals, plants and as well as human beings. The Occupational Safety and Health Administration (OSHA) have also announced the limit lowest tolerance for these type of gases in a particular time period, for example, the limits for CO and NO2 gases are ∼20 ppm and ∼5 ppm over the period of 8 h respectively. Over-acquaintance to these gases could be the reason for diseases and in dangerous cases even loss of human life . There are a number of features that the materials can have to be utilized as gas sensors, explicitly, an excellent similitude with the target gases, easy to synthesize, thermal stability, appropriate electronic structure, and adaptation with present technologies. The perovskite oxides are interesting materials as gas sensors because of their ideal bandgap, excellent thermal stability and the size difference between the A- and B-sites cations, tolerating different dopants addition for monitoring the catalytic properties and their semiconducting properties. Lots of perovskites were synthesized to utilize as gas sensors for detecting different hazardous gases. Table 2 enlisted with different perovskite oxides for various gas sensing applications.
|Perovskites||Sensing for||Response ratio %||Reference|
|LaCoO3||CO||Under 5000 ppm CO at 500°C, the thick film sensor achieved a high sensing response of ∼279.86|||
|La0.9Ce0.1CoO3||CO||240% with respect to 100 ppm CO in air|||
|Ca modified LaFeO3||SO2||Maximum resistive response of 3 ppm SO2 was detected at ~275°C by the LaCaFeO3 samples, and it shows 15% higher efficiency than LaFeO3|||
|NdFe1–xCoxO3||CO||1215% at 170°C for 0.03% CO gas|||
|Ag-LaFeO3||Methanol||The maximum response to the other test gases is 8|||
|Ag-LaFeO3||Formaldehyde||Best response to 0.5 ppm formaldehyde (24.5) at 40°C|||
|GaFeO3||Ethanol||Ethanol sensing down to 1 ppm at 350°C|||
|SrTi1 − xFexO3 − δ||Hydrocarbons||Not reported|||
|α-Fe2O3/LaFeO3||Acetone||Response 48.3% at 100 ppm concentration at 350°C|||
|YCo1-xPdxO3||CO, NO2||Different for the different composition|||
|ZnSnO3||n-Propanol gas||The detection limit of ZnSnO3 nanospheres to 500 ppb n-propanol gas could reach 1.7|||
|LaFeO3 and rGO-LaFeO3||NO2 and CO||Response 183.4% for 3 ppm concentration of NO2 at a 250°C|||
|BaTiO3/LaFeO3 nanocomposite||Ethanol||Response 102.7% to 100 ppm ethanol at 128°C|||
|Ba-BiFeO3||Ethanol||Temperature dependent sensing performance toward 100 ppm ethanol gas; maximum sensitivity at 400°C|||
|La doped BiFeO3||Acetone||The morphotropic phase boundary (MPB) phase Bi0.9La0.1FeO3 shows ultra-low concentration detection of 50 ppb acetone|||
|Pr doped BiFeO3||Formaldehyde||50 ppm, 190°C, Rgas/Rair = 17.6|||
|BaTiO3 thick films||H2S||BaTiO3 sensor operated at 350°C|||
|Sr doped BaTiO3||NH3 and NO2||0.2 mol% doping of Sr showed enhanced performance for sensing of both NO2 and NH3 gases at room temperature|||
|BaSnO3||SO2||10 ppm of SO2|||
|BaSnO3||LPG||Addition of noble metal Pt, the operating temperature decreases and the sensitivity improved but also imparted partial selectivity for the detection of LPG|||
|LaBO3 (B = Fe, Co)||Acetone||At a low operating temperature of 120°C showed that the LaFeO3 NFs based sensor displayed high stable and selective response toward 40 ppm acetone with fast response and recovery time of 14 and 49 s|||
2.3 Perovskites as solar cell materials
The consumption of energy has been continuously increasing globally and limitations of sources of fossil fuels leading to perform the research on sustainable, environment-friendly, and renewable energy sources. Due to the abundance of sun rays on our globe, the transformation of sunlight into electricity is one of the most favorable studies for increasing energy demands without having any adverse effect on the global climate. Solar cell technology offers an eco-friendly and renewable energy path to convert photon energy into electricity openly . Nowadays a large effort has been put in the research to develop high efficiency, low-cost photovoltaic devices but regrettably did not succeed yet. During the last decade, research into perovskite solar cells (PSCs) has increased and it also been nominated as a runner-up for the top 10 breakthroughs research of 2013 by the editors of Science . The organic-inorganic perovskite having the general formula ABX3 where A is cesium (Cs), methylammonium (MA), or formamidinium (FA); B is Pb or Sn; and X is Cl, Br, or I, have recently appeared as an exciting class of semiconductors which can act as solar cell materials . These organic-inorganic halide perovskite solar cells have shown substantial improvement of power conversion efficiency (PCE) from the preliminary efficiency of 3.8%  to about 22.1% . The maximum theoretical power conversion efficiency accomplished by perovskite (CH3NH3PbI3) is about 31.4% . The organic-halide perovskites owing extraordinary performance because of some unique properties like (i) high absorbing coefficient, (ii) high charge carrier mobility, (iii) long diffusion length, (iv) direct bandgap which can be engineered easily and (v) moreover easy to fabricate . The normally used Pb-based perovskites have numerous advantages such as (i) large diffusion length, (ii) absorption range, (iii) low exciton binding energy, and (iv) high carrier mobility. Conversely, the Pb-based perovskite solar cell has a serious toxic issue on both humans and the environment . Generally, PV panels are positioned on the roof of houses or in the open field, their exposure to rainfall is inescapable. In the manifestation of rain and moisture the degradation of PbI2 may cause mild to acute health issues, like effects on cardiovascular, neurological, reproductive system  mainly it is carcinogenic [72, 74] Additionally, lead pollution has severe effects on water and soil resources and emission of greenhouse gasses [75, 76]. Hence, in PSCs, it is essential to replace Pb for economical green energy conversion devices which may use mankind in future endeavors .
To develop Pb-free PSCs, Sn2+ metal cation was the first another candidate to replace Pb2+ as of its comparable electronic configuration and effective ionic radius (Sn2+: 115 pm) to lead (Pb2+: 119 pm) . The hybrid organic-inorganic halide perovskites having the chemical formula of AMIVXVII3, where A represents a small monovalent organic molecule, MIV is a divalent group-IVA cation and XVII is a halogen anion, have recently attracted remarkable attention in the photovoltaic community MASnI3 and MASn-(I3−xBrx) have been shown to the efficiencies of 6.4%  and 5.73%  respectively. CH3NH3SnI3, HC(NH2)2SnI3, NH2NH3SnI3, and NH2(CH2)3SnI3 is the promising candidate to be a light sensitizer with suitable inorganic hole-transport material to achieve cost-effective and efficient lead-free perovskite solar cell [81, 82]. Gagandeep et al. uses the graphene as the layer for charge transport and is demonstrate the structure like n-Graphene/CH3NH3SnI3/p-Graphene which shows the efficiency of 10.67–13.28% . Giorgi et al. substituted Pb by TlBi (MATl0.5Bi0.5I3) and InBi (MAIn0.5Bi0.5I3) and depicted that these systems are quietly equivalent to MAPbI3 and can be good replacements for PSCs . Germanium is also assumed as a possible candidate for Pb substitution in halide perovskites. The theoretical structure and electronic properties of AGeI3 (A = MA, FA, Cs) were investigated by Krishnamoorthy et al. . There are several elements like: Bi [86, 87, 88], Sb [89, 90, 91], Ti , Cu [93, 94] that use to substitute Pb to decrease the lead pollution. The group of Song et al. has reported the photovoltaic application of Sn-based halide perovskite materials having the general formula ASnI3 (A = Cs, methylammonium and formamidinium tin iodide as the representative light absorbers). Among all the perovskites, CsSnI3 devices accomplished a maximum power conversion efficiency of 4.81% . Nishimura et al. synthesized GeI2 doped FA0.98EDA0.01SnI3 and GeI2 doped EA0.98EDA0.01SnI3 PSCs and shows the power conversion efficiency of 13.24% for lead-free perovskite solar cell has been demonstrated with mixed cation and surface passivation .
2.4 Removal of heavy metals from wastewater
To survive on this planet the clean air, water, and foods are essential to all forms of life. The surface and the groundwaters are only the sources of clean water which help to all living systems as well as human activities such as consuming, irrigation of crops, industrial application, etc. . Water pollution is one of the most worldwide common issues as the population outbursts and industrial evolutions are there. Day by day, the heavy metals (maybe in the form of ions) are released into water bodies by various industries  and are exceedingly water-soluble, non-decomposable, oncogenic agents and cause adverse health complications on the animals as well as human beings. Wastewaters coming out from various industries contain many heavy metal ions, for example, Cu2+, As5+, Ni2+, Sb5+, Zn2+, Cd2+, and Pb2+ . In addition to heavy metal ions, the different organic and inorganic dyes are alternative pollutant releases from different industries for example papers, textiles, and plastics where the dyes are used for coloring their product and also generate significant volumes of wastewater. Many of these dyes containing heavy metal ions have a tendency to store in the living entities causing a different type of diseases and disorders [100, 101, 102]. Hence, it is essential to purify the metal-contaminated water before its discharge to the environment. Among all compare to current methods to remove heavy metal from the contaminated water [100, 101] adsorption method is the most likely one because it low cost-effective, high efficiency, and simple to run.
2.4.1 Heavy metals from contaminated water
The group of Zhang et al. synthesized the porous nano-calcium titanate microspheres via a citric acid assisted modified sol-gel method and used for absorption of heavy metals like lead, cadmium, and zinc . Haron et al. reported that the nano-crystalline LaGdO3 perovskite was synthesized by the co-precipitation method could adsorb heavy metal ions (Cd2+ and Pb2+) which should be the attention in an application such as wastewater treatment . Zhang et al. synthesized porous nano-barium-strontium titanate via sol-gel method using sorghum straw as a template and investigate about adsorption mechanism of Pb, Zn, and Cd from contaminated water . LaFeO3 nanoparticles were synthesized by Rao et al. by the sol-gel method in presence of different chelating agents and these nanoparticles utilized for an adsorbent of the removal of heavy metal ions in particular cadmium ion. The LaFeO3 sample prepared with succinic acid (SA) as a chelating agent shows a higher removal efficiency of Cd2+ ions from aqueous systems . Zhang et al. investigated Sr modified LaFeO3 and its structural and catalytic activity. La0.8Sr0.2FeO3 contributed significantly enhanced activity in methane combustion and CO oxidation because the oxygen vacancies accelerated the dissociation of gaseous oxygen on the surface in CO oxidation and facilitated the diffusion of lattice oxygen from the bulk to the surface during CH4 combustion . The perovskite LaAlO3 was manufactured using the co-precipitation method by Haron et al. The structural and efficiency of removal of heavy metal (Cd2+ and Pb2+) were extremely investigated by them. The adsorption performance was studied which fit with the Langmuir isotherm. The results disclosed that LaAlO3 perovskite showed high efficiency as heavy metal ions remover from the contaminated water. This adsorbent could be recycled with an EDTA solution and reprocessed with only slightly less efficient than that of the fresh sample . The group of Chen et al. synthesized ternary photocatalyst ZnTiO3/Zn2Ti3O8/ZnO heterojunction which displays excellent performance for the degradation of organic pollutants as well as reduction of heavy metal Cr(VI) ions from wastewater . Figure 5 schematically represent the heavy metal ion (Cd2+) adsorb with LaFeO3 perovskite prepared with the different chelating agent.
2.4.2 Heavy metals in wastewater from the textile industry
The growing population in our globe, demands to clothe and increase with the taming sense of fashion and lifestyle thus textiles are contrived to meet the growing demands. In several countries such as India and Sri Lanka; the production of textile becomes their source of income that subsidizes their gross domestic product (GDP). However, this has brought both significances to such countries either in a positive way which is an enhancement of the economy or in a negative way indorsed to environmental pollution. The textile industries have been adapted as the worst reprobates of pollution contributors . Especially, in India, according to the Central Pollution Control Board , a total of 2324 textile industries are set up. The textile industries employ different types of dyes for the manufacturing of various fabric materials. In reality, about 1 million different dyes are found in the market  and roughly 700,000 tons of artificial dyes are produced per year . The disposal of dyes in waters exemplifies a severe environmental issue due to the coinciding presence of various types of pollutants [114, 115, 116]. All traditional methods used for the treatment of dyes and/or heavy metals have limitations because of cost, efficiency and operational complications. Among all of them, adsorption was exposed as one of the most effective methods due to its simplicity in operation, adaptability, high-treatment efficiency and low cost, and hence it is extensively applied for wastewaters treatment [117, 118, 119, 120, 121].
The perovskite oxide La0.9Sr0.1FeO3, capped with cetyl trimethyl ammonium bromide (CTAB) cationic surfactant, and used as a sorbent for the removal of the anionic Congo red (CR) dye from aqueous solutions was reported by Ali et al. . The group of Chu et al. demonstrated the efficiency of Ag-La0.8Ca0.2Fe0.94O3-δ for the removal of organic and bacterial pollutants by catalytic peroxymonosulfate (PMS) activation. The oxygen vacancies in the B-site of perovskite enhances PMS activation and The SO4• and •OH radicals enhance the biocidal activity . Nanocrystalline LaAlO3:Sm3+:Bi3+ composites are used to adsorb Direct Blue-53 (DB-53) dye was reported by Pratibha et al. . This adsorbent is good and promising in the adsorption capacity and is advantageous in the elimination of toxic and non-biodegradable pollutants from water. The group of Dong et al. hydrothermally synthesized perovskite BaZrO3 in the form of hollow micro- and nano-sphere. This size-tunable BaZrO3 hollow nanospheres exhibited an excellent adsorption performance for reactive dyes in acidic conditions and can be used as excellent circular adsorbents for removing reactive dyes. They show the adsorption capacities are over 160 mg g−1 for different investigated dyes at a pH value of 2. The adsorbents were easily recovered by using a basic solution with the adsorption performance persistent and the desorption rate is more than 97 wt% . Siddharth et al. synthesized the perovskite structure of Ti-doped BaMnO3 (BaMn0.85Ti0.15O2.93) and its enhanced photocatalytic degradation (~99%) as compared to BaMnO3 toward toxic water impurities like RhB and MB dyes within 270 and 150 min under sunlight .
Nowadays, because of the increasing population and demand for natural resources is a global concern, hence, it is one of the origins of environmental pollution. Among all the pollutants, lead is one of the most hazardous materials. For the reason that the toxic effect of lead in lead-based materials, the researcher communities and scientists concentrated on synthesized lead-free materials possessing the same properties. Since the last two decades, the investigation on such materials enhances unexpectedly. Lead-free perovskite materials with excellent dielectric and piezoelectric properties belonging to the ferroelectric family and these reduce the adverse effect on the human body as well as the environment. Here this chapter concludes with different applications like SOFCs, sensors, solar cells, wastewater treatment of lead-free perovskite materials.
I am whole heartily thankful to Dr. Sushrisangita Sahoo from Bhubaneshwar for continuous support during working in this chapter. I am thankful to Dr. S. Suganya from CSIR-Central Salt and Marine Chemicals Research Institute (CSMCRI), Bhavnagar, Gujrat, and Ms. Ananya Rout from NIT-Raipur, for their encouragement to start writing this chapter.