Recent progress in structural development and band engineering of perovskites materials for photocatalytic solar hydrogen production: A review

Photocatalytic water splitting for hydrogen production is a promising technology for the conversion of solar light to clean energy. In this perspective, several semiconductors have been under investigation, but they show less efficiency, selectivity and stability for hydrogen production. Recently, perovskites are most demanding due to their exceptional characteristics such as controlled structure and morphology, adjustable band structure, controlled valence state, adjustable oxidation state and visible light response. This review highlights structural classification of perovskites and band engineering for solar energy assisted photocatalytic hydrogen production. In the main stream, overview and fundamentals of perovskite materials for selective solar to hydrogen conversion are presented. The structural modification and band alteration to stimulate quantum efficiency and stability are specifically demonstrated. Photoactivity enhancement through metals, noble metals, non-metals doping, oxygen vacancies and fermi level adjustments are also deliberated. The role of perovskites with binary semiconductors towards hydrogen production has also been discussed. Up conversion effect of doping luminescent agents (Er, Ho, Eu, Nd) for improved photocatalytic activity by band gap narrowing is also deliberated. Various conventional and non-conventional synthesis methods for perovskites including solid-state, hydrothermal, sol-gel, co-precipitation, spray-freeze drying, microwave assisted, spray pyrolysis, low temperature combustion, pulse laser deposition and wet chemical method for enhanced photocatalytic activity are also demonstrated in this work. Finally, the key challenges and future directions for sustainable energy systems are also included.     

Hydrogen production by reforming of diesel fuel over catalysts derived from LaCo1−xRuxO3 perovskites: Effect of the partial substitution of Co by Ru (x = 0.01-0.1)

LaCo_1−xRu_xO₃ perovskites with different substitutions of Co by Ru (x = 0.01-0.1) have been investigated as precursors of catalysts for the oxidative reforming of diesel for hydrogen production. The physicochemical characterization of LaCo_1−xRu_xO₃ perovskites revealed modifications in their structure, crystalline size and surface area with the incorporation of ruthenium into the perovskite lattice. The modifications in the perovskites affect the structure and morphology of the catalysts obtained by reduction of perovskites prior the reaction. In the catalysts derived from ruthenium-containing perovskites it is observed a better reducibility, smaller particle size of La₂O₃ and Co⁰ phases and better surface concentration of Ru⁰ particles with the increase in the degree of Co substitution in the perovskite. The modifications in the characteristics of the catalysts induced by the Co substitution in perovskite directly affect their catalytic behaviour in the oxidative reforming of diesel. It is found that the greater Co⁰ + Ru⁰ exposition and the higher extension of the La₂O₂CO₃ phase achieved in catalysts derived from perovskites with higher cobalt degree of substitution produces an increase in the activity and stability of the catalysts derived.     

Cation and oxyanion doping of layered perovskite BaNd2In2O7: Oxygen-ion and proton transport

Proton-conducting electrochemical devices such as protonic ceramic fuel cells and protonic ceramic electrolysis cells play a major role in the creation of eco-friendly “green” energy systems. The most studies of proton-conducting materials for these devices are barium cerate zirconates. The layered perovskites are novel class of proton-conducting materials. In this paper, the possibility of cation and oxyanion doping of layered perovskite BaNd₂In₂O₇ was carried out for the first time. The most conductive composition BaLa_1.9Sr_0.1In₂O_6.95 demonstrates protonic conductivity value 2·10⁻⁵ S/cm at 450 °C. The acceptor-doped two-layer perovskites are the prospective class of proton-conducting materials, and further modification of their composition opens up a new way in the design of solid oxide protonic conductors.     

Water-splitting mechanism analysis of Sr/Ca doped LaFeO3 towards commercial efficiency of solar thermochemical H2 production

Solar thermochemical (STC) technology utilizes the entire spectrum of solar energy to decompose water to produce hydrogen. This technology reduces carbonic fuels, nearly only producing hydrogen rather than hydrogen-oxygen mixture. However, low water-splitting activity of redox materials restricts improvement of water-hydrogen conversion ratio and fuel production efficiency. Recently, a kind of perovskite LaFeO₃ attracts attention, because of the good performance in photocatalysis hydrogen production. Nevertheless, how LaFeO₃ system works in STC water-splitting cycle is rarely studied. In this paper, the first principle method at density functional theory level is adopted to reveal the hydrogen production mechanism of perovskite LaFeO₃ doped with 25% Sr/Ca at A site. Hydrogen migration on material surface determines hydrogen generation rate. The activation energy of 25%-Ca-doped LaFeO₃ is relatively lower 150.09 kJ/mol. In addition, fuel production efficiency has been calculated. When water to hydrogen conversion ratio is 100%, solar-to-fuel efficiency can reach maximum 0.472. The effect of water-splitting kinetics on hydrogen production is also discussed. The results indicate that when T_red = T_oxi = T = 1200K and water to hydrogen conversion ratio is 10%, the dynamic efficiency of La_0.75Ca_0.25FeO₃ can reach 20%. This research can provide index for improving the hydrogen production performance of STC technology.     

Experimental evaluation and energy analysis of a two-step water splitting thermochemical cycle for solar hydrogen production based on La0.8Sr0.2CoO3-δ perovskite

A study of the hydrogen production by thermochemical water splitting with a commercial perovskite La_0.8Sr_0.2CoO_3-δ(denoted as LSC) under different temperature conditions is presented. The experiments revealed that high operational temperatures for the thermal reduction step (>1000 °C) implied a decrease in the hydrogen production with each consecutive cycle due to the formation of segregated phases of Co₃O₄. On the other hand, the experiments at lower thermal reduction operational temperatures indicated that the material had a stable behaviour with a hydrogen production of 15.8 cm³ STP/g_material·cycle during 20 consecutive cycles at 1000 °C, being negligible at 800 °C. This results comparable or even higher than the maximum values reported in literature for other perovskites (9.80–10.50 STP/g_material·cycle), but at considerable lower temperatures in the reduction step of the thermochemical cycle for the water splitting (1000 vs 1300–1400 °C). The LSC keeps the perovskite type structure after each thermochemical cycle, ensuring a stable and constant H₂ production. An energy and exergy evaluation of the cycle led to values of solar to fuel efficiency and exergy efficiency of 0.67 and 0.36 (as a percentage of 1), respectively, which are higher than those reported for other metal oxides redox pairs commonly found in the literature, being the reduction temperature remarkably lower. These facts point out to the LSC perovskite as a promising material for full-scale applications of solar hydrogen production with good cyclability and compatible with current concentrating solar power technology.     

Effect of B-site Al substitution on hydrogen production of La0.4Sr0.6Mn1-xAlx (x=0.4, 0.5 and 0.6) perovskite oxides

Thermochemical water splitting using perovskite oxides as redox materials is one of the important way to use solar energy to produce green hydrogen. Thus, it is hence important to discover new materials that can be used for this purpose. In this regard, we focused on Al-substituted La_0.4Sr_0.6Mn_1-xAl_xO₃ (x = 0.4, 0.5 and 0.6) perovskite oxides, namely as La_0.4Sr_0.6Mn_0.6Al_0.4 (LSMA4664), La_0.4Sr_0.6Mn_0.5Al_0.5 (LSMA4655), and La_0.4Sr_0.6Mn_0.4Al_0.6 (LSMA4646) which have been successfully synthesized. Herein, synthesized LSMA4664, LSMA4655, and LSMA4646 were subjected to three consecutive thermochemical cycles in order to determine their oxygen capacity, hydrogen capacity, re-oxidation capability and structural stability following three cycles. Thermochemical cycles were carried out at 1400 °C for reduction and 800 °C for the oxidation reaction. LSMA4646 exhibited the highest O₂ production capacity with 275 μmol/g among the other perovskites employed in the study. Moreover, LSMA4646 has also the highest H₂ production, 144 μmol/g, with 90% of re-oxidation capability by the end of three thermochemical water splitting cycles. On the other hand, LSMA4664 has the lowest H₂ production and only kept approximately one-third of its hydrogen production capacity by the end of cycles. Thus, the current study provides insight that the increase in the Al-substitution enhances both oxygen and hydrogen production capacity. Besides, increasing the Al amount increases the structural stability during the redox reactions, the re-oxidation capability was also increased from 38% to 89% after thermochemical cycles.     

Syngas production by catalytic partial oxidation of methane over (La0.7A0.3)BO3 (A = Ba,Ca, Mg,Sr, and B = Cr or Fe) perovskite oxides for portable fuel cell applications

Hydrogen is a clean energy carrier for the future. More efficient, economic and small-scale syngas production has therefore important implications not only on the future sustainable hydrogen-based economy but also on the distributed energy generation technologies such as fuel cells. In this paper, a new concept for syngas production is presented with the use of redox stable lanthanum chromite and lanthanum ferrite perovskites with A-site doping of Ba, Ca, Mg and Sr as the pure atomic oxygen source for the catalytic partial oxidation of methane. In this process, catalytic partial oxidation reaction of methane occurs with the lattice oxygen of perovskites, forming H₂ and CO syngas. The oxygen vacancies due to the release of lattice oxygen ions are regenerated by passing air over the reduced nonstoichiometric perovskites. Studies by XRD, temperature-programmed reduction (TPR) and activity measurements showed the enhanced effects of alkaline element A-site dopants on reaction activity of both LaCrO₃ and LaFeO₃ oxides. In both series, Sr and Ca doping promotes significantly the activity towards the syngas production most likely due to the significantly increased mobility of the lattice oxygen in perovskite oxide structures. The active oxygen species and performance of the LaACrO₃ and LaAFeO₃ perovskite oxides with respect to the catalytic partial oxidation of methane are discussed.     

La1−xSrxMO3 (M = Mn, Fe) perovskites as materials for thermochemical hydrogen production in conventional and membrane reactors

La_1−xSr_xMO₃ (M = Mn, Fe) perovskites are investigated as potential redox materials for the thermochemical production of hydrogen. Thermogravimetric oxidation/reduction experiments indicated that the materials are able to lose and uptake oxygen reversibly from their lattice up to 5.5 wt.% for La_1−xSr_xMnO₃ with x = 1 and up to 1.7 wt.% for La_1−xSr_xFeO₃ with x = 0. Pulse reaction experiments indicated that the materials can be used as redox catalysts in a redox process where water is dissociated giving rise to the production of pure hydrogen during the oxidation step. The oxidation and reduction steps can be combined in a membrane reactor constructed from dense perovskite membranes towards a continuous and isothermal operation. The system is also able to operate on partial pressure-based desorption without the need of a carbon-containing reductant, so that a process towards hydrogen production, based only on renewable hydrogen source such as water, can be established. At steady state and 900 ^°C, 25 ± 7 cm³ (STP) H₂ m⁻² min⁻¹ is produced in purified state.     

Perovskite catalysts for methane combustion: applications,design, effects for reactivity and partial oxidation

This review underlines the importance of the developments in perovskite catalysts for methane combustion from the past up to the present. In this review, after a general and brief introduction to perovskites, the mechanisms of catalytic combustion of methane have been included. Moreover, current studies on perovskites have been summarized including the effects of substitutions, doped perovskites, perovskite preparation methods, and the effect of sulfur presence on perovskite catalysts. Besides, recent studies on perovskite oxides and phenomenon of oxygen (O₂) deficiency, porous perovskite oxides, and nanostructured perovskites have been conducted. In addition, partial oxidation of methane (POM) has been reviewed. The loss of active component during the POM reaction can take place in the nickel catalyst, in particular. Since nickel has a lower melting point than noble metals and other active components, such as Co and Fe, in general, to deactivate nickel is easier. Compared with conventional structure, the porous structure with the unique morphology significantly enhances the catalytic activity through a much larger surface area (SA) and greater reactivity of the active sites. Furthermore, the monolithic nanoarrayed perovskite presents very good results in well‐defined faceted catalysts and takes part in porous channel hydrocarbon combustion. This review study is prepared as a guide to cover the profound knowledge of perovskite oxides catalysts, considering the methane combustion reaction mechanisms, and addresses prospective studies in this field for researchers.     

All ambient environment‐based perovskite film fabrication for photovoltaic applications

Low‐temperature solution process‐able perovskite solar cells are highly desirable for future photovoltaics. Chemical root was utilized to synthesize and optimize mixed halide‐based MAPbIBr₂ light absorber perovskites on electron transport layer of TiO₂ nanoparticles in ambient atmosphere. For the first time all synthesis work was performed in an ambient environment and observe material behavioral characteristics. To accurately control the film morphology, one‐step deposition technique was applied to investigate material's optoelectronic behavior. The role of the perovskite structure, physical, and optical properties in planner device architecture was studied through ultraviolet visible, X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscope characterization techniques to confirm a band gap of 1.76 eV with cubic crystalline structure having a particle size of 12.5–13.0 nm, which is highly suitable for perovskite solar cells.     

Hydrogen-rich syngas production by chemical looping steam reforming of acetic acid as bio-oil model compound over Fe-doped LaNiO3 oxygen carriers

Ni-based perovskites are promising oxygen carriers for chemical looping steam reforming to produce H₂-rich gas from organics. In this study, a series of Fe-doped LaNiO₃ perovskites with various Ni/Fe ratios (LaNi_xFe_1-xO₃ (0 ≤ x ≤ 1)) were investigated for chemical looping steam reforming of acetic acid as a model compounds of bio-oil. Results illustrated that although LaNiO₃ showed higher activity for gas production, the Ni–Fe bimetallic perovskites were more stable during the steam reforming reactions. It was found that Fe doping can promote the content of lattice oxygen in the perovskite which could be released during the steam reforming reaction, thus coking resistant of the perovskite was effectively improved. Among the LaNi_xFe_1-xO₃ (0 ≤ x ≤ 1) perovskites, LaNi_0.8Fe_0.2O₃ exhibited the best synergistic effect between Ni and Fe to achieve the highest H₂/CO for H₂-rich gas production. Operational variables of the steam reforming reactions catalyzed by LaNi_0.8Fe_0.2O₃ for H₂ production were further optimized.     

Lanthanide based double perovskites: Bifunctional catalysts for oxygen evolution/reduction reactions

In this work, we are reporting the facile synthesis of double perovskite oxide materials LnBa0_.5Sr_0·5Co_1·5Fe_0·5O₆ (LnBSCF, Ln = Pr, Nd, Sm, and Gd) using citrate-nitrate based sol-gel method. These double perovskite oxide materials exhibit bifunctional catalytic activity for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The phase formation and structure of the prepared oxides have been determined by powder X-ray diffraction, SEM analysis. Presence of various phases is analyzed and quantified by mean of Le-Bail refinement of XRD profiles. SEM analyses confirm the morphology and composition of prepared catalysts. Electrochemical measurements, e.g. Linear Sweep Voltammetry, Cyclic Voltammetry and Electrochemical Impedance spectroscopy were used to study catalytic performance of prepared catalyst towards both oxygen evolution and oxidation reduction reactions in alkaline solution. Better catalytic performance was obtained in case of double perovskites as compared to parent perovskite for both reactions. Best catalytic performance was observed for Gd based double perovskite.     

Further studies of photodegradation and photocatalytic hydrogen production over Nafion-coated Pt/P25 sensitized by rhodamine B

TiO₂ is an excellent photocatalyst in photodegradation and hydrogen production. Herein, through simple modification of Pt/TiO₂ (P25) by Nafion (Nf), photocatalyst Nf/Pt/P25 was prepared and characterized. Photodegradation of rhodamine B (RhB) and photocatalytic hydrogen production over the Nf/Pt/P25 were investigated and compared with those of Nf/P25, P25 and Pt/P25. The results showed the Nf coating caused remarkable improvements in RhB degradation and hydrogen production with RhB as sensitizer. Especially, for the first time, the influence of sacrificial reagent (SR) on hydrogen production performance was studied in detail and the results indicated replacing disodium ethylene-diamine tetraacetate (EDTA) used in literature with triethanolamine (TEOA) leads to a significant enhancement in activity. The H₂-evolving rate of RhB-Nf/Pt/P25-TEOA system is up to 12 times that of RhB-Nf/Pt/P25-EDTA. Besides, other H₂ evolution parameters were optimized and cycling H₂ evolution tests over the Nf/Pt/P25 were conducted. The results confirmed that concurrent H₂ evolution and RhB degradation can be achieved even with very few RhB. Finally, reasons for the enhanced H₂ production performance were proposed. It addressed the importance of selecting a suitable SR and will be very helpful for further work on photocatalytic hydrogen production and photodegradation.     

Diesel fuel reforming over catalysts derived from LaCo1−xRuxO3 perovskites with high Ru loading

Catalysts derived from LaCo_1−xRu_xO₃ perovskite precursors with high Ru loading (x = 0.2 and 0.4) were studied in the oxidative reforming of diesel for hydrogen production. High partial substitution of Co by Ru in LaCoO₃ perovskite modifies its physicochemical characteristics. It was observed that the high degree of Co substitution in the perovskite changes the rhombohedral crystalline perovskite particles to particles with orthorhombic structure, lower size and higher specific surface area. These modifications affected the structure and morphology of the catalysts derived from the reduction of the perovskite precursors. Catalysts derived from LaCo_1−xRu_xO₃ perovskite precursors with high Ru loading shown particles of La₂O₃ and Co⁰ with small particle size and high surface concentration of Ru⁰ particles. The modifications in the structural characteristics of the catalysts induced by the addition of Ru in the LaCo_1−xRu_xO₃ perovskite precursor had influence on their catalytic behaviour in the oxidative reforming of diesel. The catalyst derived from the perovskite with higher degree of Co substitution (x = 0.4) showed the higher activity and stability for the production of hydrogen for long periods of time-on-stream. The greater Ru exposition achieved in this catalyst was responsible of the increase in the stability observed in this sample taking into account the lower tendency to form carbonaceous deposits of the Ru particles formed.     

Experimental,computational and thermodynamic studies in perovskites metal oxides for thermochemical fuel production: A review

Solar thermal-driven thermochemical H₂O and CO₂ splitting offers a carbon-neutral path to produce feedstocks for synthetic fuel production such as hydrogen or synthesis gas. This paper assesses research outcomes for perovskite materials in two-step thermochemical cycles. Experimental, computational and thermodynamic studies are summarized and critically discussed, identifying key attributes for future research. In addition to the critical review, a fast method for the classification of effective thermochemical properties (oxygen vacancy formation enthalpy and entropy) in a wide range of operational temperatures is provided. These properties together with a high-grade of sintering resistance and fast kinetics are the main characteristics required to maximize the solar-to-fuel efficiency of the process. The discovery of optimum material compositions for this application could be effectively achieved by a combination of machine learning, DFT, experimental testing and system modelling, and will require an extensive international research effort. If successful, this could lead to the ultimate development and practical application of thermochemical cycles for fuel production.     

Advances in the development of titanates for anodes in SOFC

Solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices with the advantage that a wide variety of hydrocarbon fuels can be used directly. Recently, the field of research on anodic materials of SOFC has advanced rapidly, with special emphasis on the development of materials with resistance to H₂S as well as to the formation of coke. Therefore, it is crucial to design new anodic materials with higher catalytic activity, stability, tolerance to coke deposition and to sulfur poisoning. Due to their stability in redox conditions, the titanates are among the most studied perovskites. Strontium titanate (SrTiO₃) is a good electronic conductor at low partial pressures of oxygen and during redox cycles and presents excellent dimensional and chemical stabilities as well as sulfur tolerance. However, for application as the anode in a SOFC, it must be doped to improve important properties such as the conductivity and power density. This article describes the progress in the knowledge of titanates with perovskite structure with potential application as anodes for SOFC.     

Characterization of novel GdBa0.5Sr0.5Co2−xFexO5+δ perovskites for application in IT-SOFC cells

Novel, Sr-substituted A-site ordered perovskites with GdBa_0.5Sr_0.5Co_2−xFe_xO_5+δ (0 ≤ x ≤ 2) chemical composition were studied, and results of measurements of their phase composition, crystal structure, oxygen content δ, transport properties and chemical stability in relation to ceria electrolyte are presented in this work. It was found that despite 50% substitution of Ba by Sr, the tendency of ordering in A-sublattice is retained in Co-rich materials, but with the increase of iron content, a significant amount of unordered, but also perovskite phase appears. Compounds with high Co content possess highest electrical conductivity, which for GdBa_0.5Sr_0.5Co₂O_5+δ greatly exceeds 1000 S cm⁻¹ at temperatures above 400 °C. Seebeck coefficient remains positive for all studied compositions in 25–850 °C temperature range, indicating dominance of holes as main charge carriers. The double perovskite structure is responsible for a high deviation from the oxygen stoichiometry in studied materials, which increases considerably above 300 °C.     

Recent development in band engineering of binary semiconductor materials for solar driven photocatalytic hydrogen production

Photocatalytic hydrogen production from water splitting is a promising approach to develop sustainable renewable energy resources and limits the global warming simultaneously. Despite the significant efforts have been dedicated for the synthesis of semiconductor materials, key challenge persists is lower quantum efficiency of a photocatalyst due to charge carrier recombination and inability of utilizing full spectrum of solar light irradiation. In this review, recent developments in binary semiconductor materials and their application for photocatalytic water splitting toward hydrogen production are systematically discoursed. In the main stream, fundamentals and thermodynamic for photocatalytic water splitting and selection of photo-catalysts has been presented. Developments in the binary photocatalysts and their efficiency enhancements though surface sensitization, surface plasmon resonance (SPR) effect, Schoktty barrier and electrons mediation toward enhanced hydrogen production has been deliberated. Different modification approaches including band engineering, coupling of semiconductor catalysts, construction of heterojunction, Z-scheme formation and step-type photocatalytic systems are also discussed. The binary semiconductor materials such as TiO₂, g-C₃N₄, ZnO, ZnS, Fe₂O₃, CdS, WO₃, rGO, V₂O₅ and AgX (Cl, Br and I) are systematically disclosed. In addition, role of sacrificial reagents for efficient photocatalysis through reforming and hole-scavenger are elaborated. Finally, future perspectives for photocatalytic water splitting towards renewable hydrogen production have been suggested.     

A critical review of metal-doped TiO2 and its structure–physical properties–photocatalytic activity relationship in hydrogen production

Hydrogen production is an effective way to replace the primary energy to provide renewable sources as well as preserve the environment. Significant efforts have been developed to increase the effectiveness of hydrogen production through many methods. However, the challenge is still on-going, which exhibits insufficient efficiency and weak selectivity toward hydrogen production. Photocatalysis is one of the best methods to produce hydrogen as well as sustained the environment. Here, modification of TiO₂ by metal doping photocatalyst is reviewed. The right conclusions only can be obtained if consistent data is used. So, in this review, the data used are only data generated from a research group, Bunsho Ohtani's research group of Hokkaido University, that used titania photocatalyst in the production of hydrogen. The photocatalytic activity of photocatalysts and their relationship with hydrogen production and the factors that affect hydrogen production are discussed critically using fuzzy graph and fuzzy logic modelling. Modification of TiO₂ photocatalyst and its application for the production of hydrogen are studied. The modification is designated as mono-, bi-, and trimetallic metal doping. Moreover, there is no clarification has been done on the factors that affect the photocatalytic activity in hydrogen production. Thus, the mathematical tool, which is the fuzzy logic controller (FLC) is introduced in photocatalysis to provide the future direction of the structure-physical properties-photocatalytic activity relationship of metal-doped TiO₂ photocatalyst. Au/TiO₂ is used as the photocatalyst model towards the production of hydrogen under UV light irradiation in the form of a fuzzy graph. It was found that the low amount of Au metal doping and high surface area are the dominant factors to obtain high-efficiency hydrogen production for Au/TiO₂ photocatalyst.