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![Phase diagram of the titanium±oxygen system, showing stoichiometry of the titanium oxides against the log of conductivity of oxide phases [2].](profile/Frank-Walsh/publication/210274176/figure/fig6/AS:668675795337225@1536436141406/Phase-diagram-of-the-titaniumoxygen-system-showing-stoichiometry-of-the-titanium-oxides.png)
Phase diagram of the titanium±oxygen system, showing stoichiometry of the titanium oxides against the log of conductivity of oxide phases [2].
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![Table 2 . Comparative resistivities of some electrode materials [2]](profile/Frank-Walsh/publication/210274176/figure/tbl1/AS:668675795341327@1536436141776/Comparative-resistivities-of-some-electrode-materials-2_Q320.jpg)
Magnéli phases are a range of substoichiometric oxides of titanium of the general formula TinO2n-1, (where n is between 4 and 10) produced from high temperature reduction of titania in a hydrogen atmosphere. These blue/black ceramic materials exhibit a conductivity comparable to that of graphite and can be produced in a number of forms, such as til...
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... li phases are a substoichiometric composition of titanium oxides of the general formula Ti n O 2nA1 , where n is a number between 4 and 10 ( Fig. 1) [1]. They are identi®able compounds and not simply doped titania or casual mixtures of TiO x , where x is less than 2 [2]. The magnetic and electrical properties of these phases have been widely inves- tigated [3±7], although their use as electrode mate- rials is more recent [8±12]. Here, the structure, physical properties and ...
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... oxides from the homologous series, prin- cipally Ti 4 O 7 and Ti 5 O 9 , exhibit high electrical con- ductivity at room temperature [4,19] (Fig. 1). For example, Ti 4 O 7 , the most highly conductive phase [4], exhibits a single crystal conductivity of 1500 S cm A1 [5,8], comparable to that of graphite (Table 2) [20]. Magnetic susceptibility studies [7, 21±23], dierential thermal analysis and conductivity measurements [4] have revealed semiconductor-to-mental transitions in ...
Citations
... Titanium sub-oxides, often referred to as Magnéli phase TiO x , comprise a series of different titanium oxides that have the general formula Ti n O 2n−1 (4 ≤ n ≤ 10) [1][2][3][4][5]. It is well-known that titanium dioxide (TiO 2 ) is an electrical insulator, as it has a large band gap (anatase: 3.2 eV; 3.0: rutile) [6]. ...
... However, Magnéli phase titanium oxides are electrically conducting, and the value of electrical resistivity decreases with the increase in the oxygen deficiency [7]. Furthermore, Magnéli phase TiO x are found to be more stable than carbon in electrochemically oxidizing conditions [1,8]. These titanium sub-oxides have attracted much recent attention as promising new conducting materials because they are electrically conducting, and are highly stable towards chemical corrosion. ...
... The electrical properties of these titanium oxides were studied by Bartholomew et al., and it was found that titanium sub-oxides have semiconductor-to-metal transitions at certain temperatures and their electrical conductivities change with the oxygen content of those materials [13]. Ti 4 O 7 has the highest electrical conductivity among Magnéli phase TiO x at room temperature [1]. sequence is as follows: TiO2 → TinO2n−1 (n > 10) → TinO2n−1 (4 < n < 10) → Ti3 TiO → Ti2O [14]. ...
Magnéli phase titanium oxides, also called titanium sub-oxides (TinO2n−1, 4 < n < 9), are a series of electrically conducting ceramic materials. The synthesis and applications of these materials have recently attracted tremendous attention because of their applications in a number of existing and emerging areas. Titanium sub-oxides are generally synthesized through the reduction of titanium dioxide using hydrogen, carbon, metals or metal hydrides as reduction agents. More recently, the synthesis of nanostructured titanium sub-oxides has been making progress through optimizing thermal reduction processes or using new titanium-containing precursors. Titanium sub-oxides have attractive properties such as electrical conductivity, corrosion resistance and optical properties. Titanium sub-oxides have played important roles in a number of areas such as conducting materials, fuel cells and organic degradation. Titanium sub-oxides also show promising applications in batteries, solar energy, coatings and electronic and optoelectronic devices. Titanium sub-oxides are expected to become more important materials in the future. In this review, the recent progress in the synthesis methods and applications of titanium sub-oxides in the existing and emerging areas are reviewed.
... Magnéli phases for TiOx are a class of substoichiometric titanium oxides with a chemical formula of TinO2n−1, where n varies from 3 to 9 (and more typically between 4 and 6) [17,18]. Despite all three forms of Titanium oxides (rutile, anatase, and brookite) being composed of TiO6 octahedron basic units, their frameworks differ, resulting in distinct properties for each crystal [19]. ...
Thermoelectric materials convert heat energy into electrical energy. They have the potential to be used in a variety of applications, including power generation, waste heat recovery, and temperature control. Several transition metal suboxides, also known as Magnéli phases, are a promising class of thermoelectric materials because of their high electrical conductivity, low thermal conductivity, and chemical stability. In this review article, we discuss the Magnéli phase of five different transition metal suboxides: titanium suboxide (TiOx), tungsten suboxide (WOx), molybdenum suboxide (MoOx), vanadium suboxide (VOx), and niobium suboxide (NbOx). We focus on their crystal structure, electrical and thermal properties, and potential applications as thermoelectric materials. We conclude that the Magnéli phases have the potential to be high-performance thermoelectric materials. However, further research is needed to optimize their properties and develop new synthesis methods.
... Other prominent HOP materials are lead(IV) oxide (PbO 2 ), antimony-doped tin(IV) oxide (ATO), and Magnéli phase titanium oxides (MPTOs, especially Ti 4 O 7 ), the latter also known by the trade name Ebonex®. [14][15][16] While issues of lead and antimony toxicity have limited the use of PbO 2 and ATO, the latter also exhibiting poor stability, 17 MPTOs are emerging as a viable alternative to BDD. MPTOs are nontoxic, 18 excellent electrical conductors whose single-crystal conductivity exceeds 1,000 S cm 1 , 19 dimensionally stable in aggressive media such as nitric and hydrofluoric acid, 20 and inexpensive at <0.5 USD m 2 . ...
... A small feature at 22°can be assigned to the (102) plane of Ti 5 O 9 (CIF 539580), 54 suggesting that it may be present as a minor phase, which is typical of Ebonex® electrodes. 14,15 Electron microscopy of (Ti 1-x V x ) 4 O 7 sintered plates (0 ⩽ x ⩽ 0.05, Figs. 3a-3d) illustrate their porosity. ...
Inexpensive electrode materials and effective cell designs are needed to advance electrochemical technologies for the oxidative treatment of wastewater. Novel vanadium-doped Ti 4 O 7 porous transport layers (PTLs) used in a compact wastewater electrolyzer are developed and characterized and their performance for the electrochemical oxidation of synthetic wastewater is evaluated. An analytical model predicting performance with the apparent mass transfer coefficient and cell potential is developed. The influence of operating parameters such as volumetric flow, current density, and PTL composition on performance is investigated. Decolorization and chemical oxygen demand (COD) removal of 100 mg L ⁻¹ of methyl orange (MO), an azo dye, in 1,500 mg NaCl L ⁻¹ is rapid with mass transfer coefficients as great as 377 ± 24 µm s ⁻¹ for MO at 15 mA cm ⁻² . After 2.5 Ah L ⁻¹ at 10 mA cm ⁻² , >99% decolorization and >98% COD removal are achieved with a current efficiency of 19.2% and with specific and volumetric energy consumption of 120 and 84.1 kWh kg ⁻¹ for MO and COD, respectively, and 1.34 ± 0.09 and 6.45 ± 0.97 kWh m ⁻³ , respectively. A more energy-efficient electrochemical cell design for industrial wastewater treatment using less expensive high oxidation power electrode materials is demonstrated with these results.
... The existence of titanium interstitials and oxygen vacancies helps to reduce the band gap of TiO 2 , hence increasing their electrical conductivity. This sub-stoichiometric TiO 2 also known as the Magnéli phase possesses a higher conductivity than graphite and has a general formula of Ti n O 2n−1 (4 < n < 10) [79]. The conductivity of Ti 4 O 7 rises as high as ca. 10 3 S cm −1 at room temperature. ...
Polymer electrolyte fuel cells (PEFCs) are attractive energy resources in transportation and portable applications due to their low operational temperatures, excellent energy densities, and simplicity of storage. For the past decade, metal oxides (MO) have been identified as plentiful supplies with excellent electrochemical properties, low cost, an abundance of hydroxyl groups, and environmentally acceptable alternative to currently available materials. Their intriguing properties such as extremely large surface area, low synthesis cost, and so on, make them ideals for many applications. This review covered MO structure, characteristics, and current preparation methods. MO/metal nanoparticles and MO/carbon nanoparticles are also addressed as electrocatalysts, co-catalysts, or supports, especially in PEMFC and DMFC applications. This review also examined the effects of different MO nanoparticles in polymer and biopolymer membranes on membrane characteristics. Finally, the existing issues and future perspectives of MO nanoparticles are also discussed in this review.
... However, traditional electrode materials, such as graphite electrodes [11], metal electrodes [12], and dimensionally stable anodes (DSA) [13], exhibit several limitations, including a short service life, poor removal effect, and high cost [14]. Owing to their unique crystal structures [15], Ti 4 O 7 electronic ceramics exhibit many promising properties, such as an outstanding electrical conductivity [16], a high oxygen evolution potential [17] and activity [18], and a high corrosion resistance [19,20]. These properties render Ti 4 O 7 as a promising electrode material for electrochemical applications [21][22][23]. ...
... Although "non-active anodes" with increased O 2 generation overpotentials, like PbO 2 , boron-doped diamond (BDD) or SnO 2 can aid total combustion, they are not ideal electrodes for the entire oxidation of organics to CO 2 in the treatment of wastewater (Martínez-Huitle and Andrade, 2011). The selection depends on the usage of the anode materials (Kötz et al., 1991) or its chemical nature including the use of metal (Patra et al.) (Smith et al., 1998), metal oxides (RuO 2 ) (Drogui et al., 2007), IrO 2 (Chellammal et al., 2016), PbO 2 (Dapeng and Jiuhui, 2009), and SnO 2 . ...
The rapidly expanding global energy demand is forcing a release of regulated pollutants into water that is threatening human health. Among various wastewater remediating processes, electrocoagulation (EC) has scored a monumental success over conventional processes because it combines coagulation, sedimentation, floatation and electrochemical oxidation processes that can effectively decimate numerous stubborn pollutants. The EC processes have gained some attention through various academic and industrial publications, however critical evaluation of EC processes, choices of EC processes for various pollutants, process parameters, mechanisms, commercial EC technologies and performance enhancement via other degradation processes (DPs) integration have not been comprehensively covered to date. Therefore, the major objective of this paper is to provide a comprehensive review of 20 years of literature covering EC fundamentals, key process factors for a reactor design, process implementation, current challenges and performance enhancement by coupling EC with pivotal pollutant DPs including, electro/photo-Fenton (E/P–F), photocatalysis, sono-chemical treatment, ozonation, indirect electrochemical/advanced oxidation (AO), and biosorption that have substantially reduced metals, pathogens, toxic compound BOD, COD, colors in wastewater. The results suggest that the optimum treatment time, current density, pulse frequency, shaking speed and spaced electrode improve the pollutants removal efficiency. An elegant process design can prevent electrode passivation which is a critical limitation of EC technology. EC coupling (up or downstream) with other DPs has resulted in the removal of organic pollutants and heavy metals with a 20% improved efficiency by EC-EF, removal of 85.5% suspended solid, 76.2% turbidity, 88.9% BOD, 79.7% COD and 93% color by EC-electroflotation, 100% decolorization by EC-electrochemical-AO, reduction of 78% COD, 81% BOD, 97% color by EC-ozonation and removal of 94%ammonia, 94% BOD, 95% turbidity, >98% phosphorus by aerated EC and peroxicoagulation. The major wastewater purification achievements, future potential and challenges are described to model the future EC integrated systems.
... EO has been shown capable of destructing PFASs (Lin et al., , 2018Niu et al., 2012Niu et al., , 2016Zhuo et al., 2016;Liang et al., 2018b;Shi et al., 2019;Wang et al., 2020bWang et al., , 2021bYang et al., 2020), via direct electron transfer (DET) and ⋅OH-mediated reactions occurring in concert on the anode surface. Magnéli phase titanium suboxides (TSO) with formula Ti n O 2n-1 (4 < n < 10) have recently been explored as promising anode materials for EO applications due to its high conductivity, chemical inertness, wide working potential window, and low production cost (Smith et al., 1998;Carter and Farrell, 2008;Walsh and Wills, 2010;Guo et al., 2016;You et al., 2016;Yang et al., 2020). Among all TSO materials, Ti 4 O 7 has the highest conductivity (~1500 S cm − 1 ), comparable to graphite (Smith et al., 1998;Gusev et al., 2007). ...
... Magnéli phase titanium suboxides (TSO) with formula Ti n O 2n-1 (4 < n < 10) have recently been explored as promising anode materials for EO applications due to its high conductivity, chemical inertness, wide working potential window, and low production cost (Smith et al., 1998;Carter and Farrell, 2008;Walsh and Wills, 2010;Guo et al., 2016;You et al., 2016;Yang et al., 2020). Among all TSO materials, Ti 4 O 7 has the highest conductivity (~1500 S cm − 1 ), comparable to graphite (Smith et al., 1998;Gusev et al., 2007). EO with Ti 4 O 7 anode exhibited greater efficiency of PFAS degradation than a few other anode materials, including boron-doped diamond (BDD), Ti/TiO 2 -NTs/PbO 2 and Ti/PbO 2 anodes (Lin et al., , 2018Zhuo et al., 2020). ...
Electrooxidation (EO) has been shown effective in degrading per- and polyfluoroalkyl substances (PFASs) in water, but concurrent formation of chlorate and perchlorate in the presence of chloride is of concern due to their toxicity. This study examined EO treatment of three representative PFAS, perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and 6:2 fluorotelomer sulfonate (6:2 FTS), in chloride-containing solutions on pristine and surface-fluorinated Ti4O7 anodes having different percentage of surface fluorination. The experiment results indicate that surface fluorination of Ti4O7 anodes slightly inhibited PFAS degradation, while significantly decreased the formation of chlorate and perchlorate. Further studies with spectroscopic and electrochemical characterizations and density functional theory (DFT) computation reveals the mechanisms of the impact on EO performance by anode fluorination. In particular, chlorate and perchlorate formation were fully inhibited when fluorinated Ti4O7 anode was used in reactive electrochemical membrane (REM) under a proper anodic potential range (<3.0 V vs Standard Hydrogen Electrode), resulting from slower intermediate reaction steps and short residence time of the REM system. The results of this study provide a basis for design and optimization of modified Ti4O7 anodes for efficient EO treatment of PFAS while limiting chlorate and perchlorate formation.
... However, the valency of titanium can go less than Ti 3+ , and numerous titanium oxides (TiO x ) containing the low-valence titanium are known as titanium suboxides (TSO) Harada et al., 2010;Liborio et al., 2009;Xu et al., 2016). One of the sets of TSO is the sub-stoichiometric TiO x with the generic chemical formula Ti n O 2n-1 with 3 < n < 10 (and more typically 4 ≤ n ≤ 6) called Magnéli phases (Adamaki et al., 2014;Domaschke et al., 2019;Kasian et al., 2012;Nagao et al., 2020;Smith et al., 1998;Walsh and Wills, 2010;Yang et al., 2017). Magnéli phase TSO was first studied in the 1950s by Arne Magnéli following the construction of a phase diagram of a Ti-O system (Andersson et al., 1957). ...
... Magnéli phase TSO was first studied in the 1950s by Arne Magnéli following the construction of a phase diagram of a Ti-O system (Andersson et al., 1957). These materials have attracted much recent attention because of their high conductivity, high chemical inertness, and corrosion resistance (Arif et al., 2017;Nagao et al., 2020;Smith et al., 1998;Walsh and Wills, 2010;Xu et al., 2016). The structure of the Magnéli phase is based on the rutile TiO 2 crystal lattice, where oxygen deficiency results in delocalized electrons in the d band (Arif et al., 2017;Liborio et al., 2009;Malik et al., 2020;Xu et al., 2016). ...
... The non-active behavior of Magnéli phase-based anodes causes a weak interaction between the OH˙radicals and the surface of the electrode. These weakly adsorbed OH˙radicals, also called "free radicals, " goes into the solution and fully oxidize the organics into CO 2 and H 2 O (Geng et al., 2015;Liang et al., 2018b;Marselli et al., 2003;Smith et al., 1998). However, it is essential to evaluate the quality and quantity of hydroxyl radicals generated at the Magnéli phase anode. ...
Sub-stoichiometric titanium oxide, also called titanium suboxides (TSO), had been a focus of research for many decades with a chemical composition of TinO2n-1 (n ≥ 1). It has a unique oxygen-deficient crystal structure which provides it an outstanding electrical conductivity and high corrosion resistance similar to ceramic materials. High electrical conductivity and ability to sustain in adverse media make these phases a point of attention for researchers in energy storage and environmental remediation applications. The Magnéli phase-based reactive electroconductive membranes (REM) and electrodes have demonstrated the electrochemical oxidation of pollutants in the water in flow-through and flow by configuration. Additionally, it has also shown its potential for visible light photochemical degradation as well. This review attempts to summarize state of the art in various Magnéli phases materials synthesis routes and their electrochemical and photochemical ability for environmental application. The manuscript introduces the Magnéli phase, its crystal structure, and catalytic properties, followed by the recent development in synthesis methods from diverse titanium sources, notably TiO2 through thermal reduction. The various fabrication methods for Magnéli phase-base REMs and electrodes have also been summarized. Furthermore, the article discussed the environmental remediations via electrochemical and photochemical advanced oxidation processes. Additionally, the hybrid technology with REMs and electrodes is used to counter membrane biofouling and develop electrochemical sensing devices for the pollutants. The Magnéli phase materials have a bright future for both electrochemical and photochemical advanced oxidation of emerging contaminants in water and wastewater treatment.
... Magnéli titanium sub-oxide is a general term for a series of non-stoichiometric titanium oxides, whose generic formula is Ti n O 2n−1 (4 ≤ n ≤ 10) [57], including a series of titanium oxides, such as Ti 4 O 7 , Ti 5 O 9 , and Ti 6 O 11 . ...
To achieve low-carbon and sustainable development it is imperative to explore water treatment technologies in a carbon-neutral model. Because of its advantages of high efficiency, low consumption, and no secondary pollution, electrocatalytic oxidation technology has attracted increasing attention in tackling the challenges of organic wastewater treatment. The performance of an electrocatalytic oxidation system depends mainly on the properties of electrodes materials. Compared with the instability of graphite electrodes, the high expenditure of noble metal electrodes and boron-doped diamond electrodes, and the hidden dangers of titanium-based metal oxide electrodes, a titanium sub-oxide material has been characterized as an ideal choice of anode material due to its unique crystal and electronic structure, including high conductivity, decent catalytic activity, intense physical and chemical stability, corrosion resistance, low cost, and long service life, etc. This paper systematically reviews the electrode preparation technology of Magnéli phase titanium sub-oxide and its research progress in the electrochemical advanced oxidation treatment of organic wastewater in recent years, with technical difficulties highlighted. Future research directions are further proposed in process optimization, material modification, and application expansion. It is worth noting that Magnéli phase titanium sub-oxides have played very important roles in organic degradation. There is no doubt that titanium sub-oxides will become indispensable materials in the future.
... Non-stoichiometric titanium oxide structures containing Magnéli phases are chemically stable and rather well conducting. For these reasons, they are often applied in wastewater treatment and the design of batteries and fuel cells [57,58]; the same characteristics are required for gas sensors. ...
Nanostructured titanium compounds have recently been applied in the design of gas sensors. Among titanium compounds, titanium oxides (TiO2) are the most frequently used in gas sensing devices. Therefore, in this review, we are paying significant attention to the variety of allotropic modifications of titanium oxides, which include anatase, rutile, brukite. Very recently, the applicability of non-stoichiometric titanium oxide (TiO2−x)-based layers for the design of gas sensors was demonstrated. For this reason, in this review, we are addressing some research related to the formation of non-stoichiometric titanium oxide (TiO2−x) and Magnéli phase (TinO2n−1)-based layers suitable for sensor design. The most promising titanium compounds and hetero- and nano-structures based on these compounds are discussed. It is also outlined that during the past decade, many new strategies for the synthesis of TiO2 and conducting polymer-based composite materials were developed, which have found some specific application areas. Therefore, in this review, we are highlighting how specific formation methods, which can be used for the formation of TiO2 and conducting polymer composites, can be applied to tune composite characteristics that are leading towards advanced applications in these specific technological fields. The possibility to tune the sensitivity and selectivity of titanium compound-based sensing layers is addressed. In this review, some other recent reviews related to the development of sensors based on titanium oxides are overviewed. Some designs of titanium-based nanomaterials used for the development of sensors are outlined.
