High-performance Ti/Sb-SnO(2)/Pb(3)O(4) electrodes for chlorine evolution: preparation and characteristics

Enhancing Ru-Cl interaction via orbital hybridization effect in Ru0.4Sn0.3Ti0.3 electrode for efficient chlorine evolution

Chlorine evolution reaction (CER) is a commercially valuable electrochemical reaction used at an industrial scale. However, oxygen evolution reaction (OER) during the electrolysis process inevitably leads to the decreased efficiency of CER. It is necessary to improve the selectivity of CER by minimizing or even eliminating the occurrence of OER. Herein, a ternary metal oxide (Ru0.4Sn0.3Ti0.3) electrode was fabricated and employed as an active and robust anode for CER. The Ru0.4Sn0.3Ti0.3 electrode exhibits an excellent CER performance in 6.0 M NaCl solution, with a low potential of 1.17 V (vs. saturated calomel electrode, SCE) at 200 mA cm-2 current density, a high Cl2 selectivity of over 90 %, and robust durability after consecutive operation for 160 h under 100 mA cm-2. The maximum O2-Cl2 potential difference between OER and CER further demonstrates the high Cl2 selectivity of Ru0.4Sn0.3Ti0.3 electrode. Theoretical studies show that the strong Ru 3d-Ti 3d orbitals hybridization effect makes the d-band center (εd) of Ru 3d and Ti 3d orbitals positively and negatively shifted, respectively, endowing Ru site with enhanced Cl adsorption ability (i.e. enhanced Ru-Cl interaction) and Ru0.4Sn0.3Ti0.3 electrode with superior CER activity. This work offers valuable insights into the development of advanced electrodes for CER in practical application.

Strain-modulated Ru-O Covalency in Ru-Sn Oxide Enabling Efficient and Stable Water Oxidation in Acidic Solution

RuO2 is one of the benchmark electrocatalysts used as the anode material in proton exchange membrane water electrolyser. However, its long-term stability is compromised due to the participation of lattice oxygen and metal dissolution during oxygen evolution reaction (OER). In this work, weakened covalency of Ru-O bond was tailored by introducing tensile strain to RuO6 octahedrons in a binary Ru-Sn oxide matrix, prohibiting the participation of lattice oxygen and the dissolution of Ru, thereby significantly improving the long-term stability. Moreover, the tensile strain also optimized the adsorption energy of intermediates and boosted the OER activity. Remarkably, the RuSnOx electrocatalyst exhibited excellent OER activity in 0.1 M HClO4 and required merely 184 mV overpotential at a current density of 10 mA cm-2 . Moreover, it delivered a current density of 10 mA cm-2 for at least 150 h with negligible potential increase. This work exemplifies an effective strategy for engineering Ru-based catalysts with extraordinary performance toward water splitting.

A Reflection on Sustainable Anode Materials for Electrochemical Chloride Oxidation

Chloride oxidation is a key industrial electrochemical process in chlorine-based chemical production and water treatment. Over the past few decades, dimensionally stable anodes (DSAs) consisting of RuO2 - and IrO2 -based mixed-metal oxides have been successfully commercialized in the electrochemical chloride oxidation industry. For a sustainable supply of anode materials, considerable efforts both from the scientific and industrial aspects for developing earth-abundant-metal-based electrocatalysts have been made. This review first describes the history of commercial DSA fabrication and strategies to improve their efficiency and stability. Important features related to the electrocatalytic performance for chloride oxidation and reaction mechanism are then summarized. From the perspective of sustainability, recent progress in the design and fabrication of noble-metal-free anode materials, as well as methods for evaluating the industrialization of novel electrocatalysts, are highlighted. Finally, future directions for developing highly efficient and stable electrocatalysts for industrial chloride oxidation are proposed.

A tubular electrode assembly reactor for enhanced electrochemical wastewater treatment with a Magnéli-phase titanium suboxide (M-TiSO) anode and in situ utilization

The electrochemical oxidation technology has been widely used for the waste water treatment and water reuse because of its easy-to-operate nature, an effective removal of pollutants and non-secondary pollution. However, the price of electrode materials, the limitation of mass transfer and the associated effects on contaminant degradation hamper its application. Within this context, an in situ utilization tubular electrode assembly reactor (TEAR) was proposed, in which a stainless steel pipe (SSP) was used as the cathode, and a tubular Magnéli-phase titanium suboxide (M-TiSO) anode was posited in the center of that pipe. Besides the cathode and anode, an integral electrochemical system to treat water pollutants was constituted with a spiral static mixer made from three-dimensional (3D) printing. A spiral static mixer was pushed into the interspace of electrodes to minimize the adverse effect caused by inhomogeneous distribution of pollutants. Here, the effects of current density and resident time on the removal of methylene blue (MB) and total organic carbon (TOC) were investigated, the corresponding hydrodynamics was studied using computational fluid dynamics (CFD), and the long-term stability of removing MB by the reactor was discussed. The results indicated that the MB and TOC removal rate was enhanced at specific current density with a static mixer and the velocity distribution tended to be more homogeneous. Moreover, the anode surface shear force and heat transfer were increased by improving the fluid state. This study proposed an in situ utilization concept and provided a potential value for feasible and efficient water treatment.

A stainless steel pipe (SSP) was used as a cathode. A tubular Magnéli-phase titanium suboxide (M-TiSO) anode was posited in the center. A spiral static mixer was used to process intensification.

Efficient electrocatalysis for denitrification by using TiO2 nanotube arrays cathode and adding chloride ions

Electrocatalysis is emerging as a promising alternative to bacterial denitrification for removing nitrate and ammonia from sewage. The technology is highly efficient and robust in actual wastewater treatment scenarios; however, there may be the generation of harmful intermediates (such as nitrite) on the traditional cathode material. In this study, we demonstrated that TiO₂ nanotube arrays can be used as an effective cathode to reduce nitrate to ammonia without generation of nitrite. Alongside this, the addition of chloride ions in the solution can further oxidize ammonia to N₂. We looked into the key factors influencing the electrocatalytic denitrification, including the current density (2-10 mA/cm²), initial pH values (3-11), and types of anions (HCO₃^-, Cl^-, SO₄^2-). The results showed that 90.8% of nitrate and 59.4% of total nitrogen could be removed in 1.5 h under optimal conditions, with degradation kinetic constants of 1.61 h^-1 and 0.79 h^-1, respectively. Furthermore, we investigated the formation of intermediate products and explored the electrocatalytic denitrification mechanism: (a) the surface oxygen vacancies and high specific surface area of TiO₂ nanotube arrays electrode promote the reduction of nitrate to ammonia and N₂; (b) the active chlorine generated at the anode surface can effectively oxidize ammonium to N₂.

Rational Surface and Interfacial Engineering of IrO2 /TiO2 Nanosheet Arrays toward High-Performance Chlorine Evolution Electrocatalysis and Practical Environmental Remediation

The chlorine evolution reaction (CER) is a critical and commercially valuable electrochemical reaction in industrial-scale utilization, including the Chlor-alkali industry, seawater electrolysis, and saline wastewater treatment. Aiming at boosting CER electrocatalysis, hybrid IrO₂ /TiO₂ nanosheet arrays (NSAs) with rational surface and interfacial tuning strategies are proposed in this study. The IrO₂ /TiO₂ NSAs exhibit superb CER electrocatalytic activity with a low overpotential (44 mV) at 10 mA cm^-2 , low Tafel slope of 40 mV dec^-1 , high CER selectivity (95.8%), and long-term durability, outperforming most of the existing counterparts. The boosting mechanism is proposed that the aerophobic/hydrophilic surface engineering and interfacial electronic structure tuning of IrO₂ /TiO₂ are beneficial for the Cl^- mass-transfer, Cl₂ release, and Volmer-Heyvrosky kinetics during the CER. Practical saline wastewater treatment by using the IrO₂ /TiO₂ NSAs electrode is further conducted, demonstrating it has a higher p-nitrophenol degradation ratio (95.10% in 60 min) than that of other electrodes. The rational surface and interfacial engineering of IrO₂ /TiO₂ NSAs can open up a new way to design highly efficient electrocatalysts for industrial application and environmental remediation.

Enhanced mass transfer and service time of mesh Ti/Sb-SnO2 electrode for electro-catalytic oxidation of phenol

Titanium-based SnO₂ with Sb dopant (Ti/Sb-SnO₂) was of interest in the field of electro-catalytic oxidation due to its high organic oxidation rates. However, the relatively poor mass transfer performance and short service time limited its practical application. To overcome this problem, Ti/Sb-SnO₂ electrode was fabricated on mesh substrate and used as the anode for electrochemical oxidization of phenol. Compared to the anode prepared on planar Ti, the mesh anode with compact and uniform catalyst surface lowered electron transfer resistance and higher O_ads content (17.41%), which benefited the generation of hydroxyl radicals (·OH) (increment of 24.5%). In addition, this structure accelerated the fluid perturbation around electrode in microscopic scale as the COMSOL simulation result indicated; the electric potential on mesh anode varied regularly along the undulant terrain of electrode so that the mass transfer coefficient was enhanced by 1.67 times. These structure-dependent characteristics contributed to the superior electro-catalytic performance toward degradation of phenol. Experimental results showed that mesh anode had a higher TOC removal efficiency of 90.6% and mineralization current efficiency of 20.1% at current density of 10 mA cm^-2, which was 9.95% and 21.6% higher than the planar anode, and the service lifetime was 1.89 times longer than planar anode. This highly electro-catalytically active and stable Ti/Sb-SnO₂ mesh electrode showed a potential application prospect toward electro-catalytic degradation process.

Construction of TiO2 nanotube clusters on Ti mesh for immobilizing Sb-SnO2 to boost electrocatalytic phenol degradation

An efficient Sb-doped SnO₂ electrode featuring superior electrocatalytic characteristic and long stability was constructed by adopting clustered TiO₂ nanotubes-covered Ti mesh as substrate (M-TNTs-SnO₂). Compared with the electrodes prepared with mere Ti mesh or Ti plate grew with TiO₂ nanotube, the M-TNTs-SnO₂ exhibited higher TOC removal (99.97 %) and mineralization current efficiency (44.0 %), and longer accelerated service lifetime of 105 h for electrochemical degradation of phenol. The enhanced performance was mainly ascribed to the introduction of mutually self-supported TiO₂ nanotube clusters in different orientations. Such unique structure not only favored a compact and smooth surface of catalyst layer which improved the stability of electrode by reinforcing the binding force between substrate and catalyst layer, but also increased the loading capacity for catalysts, leading to 1.5-2.2 times higher of ·OH generation, the main active species for indirect electrochemical oxidation of phenol. Meanwhile, the transverse electron transfer from TiO₂ nanotube to catalyst layer was possibly achieved to further prompt the generation of ·OH. This study may provide a feasible option to design of efficient electrodes for electrocatalytic degradation of organic pollutants.

A modular functionalized anode for efficient electrochemical oxidation of wastewater: Inseparable synergy between OER anode and its magnetic auxiliary electrodes

Oxygen evolution reaction (OER) anodes, (e.g., IrO₂) are well-known inefficient catalysts for electrochemical oxidation (EO) of refractory organics in wastewater due to the high energy consumption via OER. However, in this study this kind of anode participated in a very effective EO process via a specific modular anode architecture. Traces of magnetic Fe₃O₄/Sb-SnO₂ particles as auxiliary electrodes (AEs) were attracted on the surface of the two-dimensional (2D) Ti/IrO₂-Ta₂O₅ by a NdFeB magnet, and thereby constituted a new magnetically assembled electrode (MAE). MAE could be renewed by recycling its AEs. The electrochemical properties as well as the EO performances of the MAE could be regulated by adjusting the loading amount of AEs. Results showed that even a small amount of AEs could increase surface roughness and offer massive effective active sites. When removing color of azo dye Acid Red G, the optimal MAE exhibited ∼1100 % and ∼500 % higher efficiencies than 2D Ti/IrO₂-Ta₂O₅ and 2D Ti/Sb-SnO₂, respectively. The superiority of the MAE was also applicable in degrading phenol. The synergy between Ti/IrO₂-Ta₂O₅ and magnetic Sb-SnO₂ particles was therefore discussed.

Highly Selective Active Chlorine Generation Electrocatalyzed by Co3O4 Nanoparticles: Mechanistic Investigation through in Situ Electrokinetic and Spectroscopic Analyses

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co₃O₄ nanoparticles for chloride oxidation at neutral pH. In contrast with water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co-Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co₃O₄ nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO₂-based catalysts. We believe that this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled to hydrogen production.

Nanostructured 3D-porous graphene hydrogel based Ti/Sb-SnO2-Gr electrode with enhanced electrocatalytic activity

Nanostructured highly porous 3D-Ti/Sb-SnO₂-Gr electrode, based on 3D porous graphene hydrogel was fabricated via a fast-evaporation technique through layer by layer (LBL) deposition. The 3D pores are uniformly distributed on the high fidelity of substrate with pore sizes of 7-12 nm, as confirmed by SEM analysis. Compared to Ti/Sb-SnO₂ electrode, the fabricated 3D porous electrode possesses high oxygen evolution potential (2.40 V), smaller charge transfer resistance (29.40 Ω cm^-2), higher porosity (0.90), enhanced roughness factor (181), and larger voltammetric charge value (57.4 mC cm^-2). Electrocatalytic oxidation of Rhodamine B (RhB) was employed to evaluate the efficiency of the fabricated 3D-Ti/Sb-SnO₂-Gr anode. The results show that the electrochemical reaction follows pseudo first order kinetics with rate constant (k) value of 4.93 × 10^-2 min^-1, which is about 3.91 times higher compared to flat Ti/Sb-SnO₂. The fabricated electrode demonstrates better stability and low specific energy consumption signifying its potential usage in electrocatalysis.

Superaerophobic RuO2 -Based Nanostructured Electrode for High-Performance Chlorine Evolution Reaction

Constructing a nanostructured electrode with superaerophobic surface property (i.e., superlow adhesion to gas bubbles) has been strikingly highlighted as an advanced technology to minimize the energy loss during various electrochemical gas evolution reactions. Herein, aiming at enhancing the performance of chlorine evolution reaction (ClER), which holds the key for chlor-alkali industry as well as water treatment, a nanostructured RuO₂ @TiO₂ electrode is demonstrated to overcome the bubble shielding effect, thereby maximizing the working area and offering a robust working condition. Benefitting from the direct growing architecture and the superaerophobic surface property, this nanostructured RuO₂ @TiO₂ electrode exhibits an excellent ClER performance, reaching 50 mA cm^-2 at a low potential of 1.10 V (vs SCE) with a Faradaic efficiency over ≈90%. Moreover, a prominent stability (250 mA cm^-2 for 10 h) is observed for this nanostructured electrode, probably due to the small vibrations and scratching forces from gas product.

Electrocatalytic oxidation of phenol from wastewater using Ti/SnO2-Sb2O4 electrode: chemical reaction pathway study

In this study, a titanium plate was impregnated with SnO2 and Sb (Ti/SnO2-Sb2O4) for the electrocatalytic removal of phenol from wastewater, and the chemical degradation pathway was presented. The effects of various parameters such as pH, current density, supporting electrolyte, and initial phenol concentration were studied. At optimum conditions, it was found that phenol was quickly oxidized into benzoquinone because of the formation of various strong radicals during electrolysis by the Ti/SnO2-Sb2O4 anode from 100 to <1 mg/L over 1 h. The results of GC/MS analysis showed the presence of some esters of organic acid such as oxalic acid and formic acid. HPLC analysis showed only trace amounts of benzoquinone remaining in the solution. The efficiency of TOC removal at the Ti/SnO2-Sb2O4 anode surface showed a degradation rate of 49 % over 2 h. Results showed that the molecular oxygen potential at the electrode was 1.7 V. The phenol removal mechanism at the surface of the Ti/SnO2-Sb2O4 anode was influenced by the pH. Under acidic conditions, the mechanism of electron transfer occurred directly, whereas under alkaline conditions, the mechanism can be indirect. This research shows that the proposed electrolyte can significantly influence the efficiency of phenol removal. It can be concluded that the treatment using an appropriate Ti/SnO2-Sb2O4 electrode surface can result in the rapid oxidation of organic pollutants.

Preparation of porous TiO2-NTs/m-SnO2-Sb electrode for electrochemical degradation of benzoic acid

A novel porous SnO₂-Sb electrode with excellent electrocatalytic performance was fabricated through the electrodeposition method with a templating agent.

Electrochemical oxidation of guaiacol to increase its biodegradability or just remove COD in terms of anodes and electrolytes

Ti/Sb-SnO₂ and Ti/Pb₃O₄ were compared to treat or pretreat (before biotreatment) guaiacol solution by electrochemical oxidation in different electrolytes.

Highly efficient electrochemical degradation of perfluorooctanoic acid (PFOA) by F-doped Ti/SnO2 electrode

The novel F-doped Ti/SnO2 electrode prepared by SnF4 as the single-source precursor was used for electrochemical degradation of aqueous perfluorooctanoic acid (PFOA). Higher oxidation reactivity and significantly longer service life were achieved for Ti/SnO2-F electrode than Ti/SnO2-X (X=Cl, Br, I, or Sb) electrode, which could decomposed over 99% of PFOA (50 mL of 100 mg L(-1)) within 30-min electrolysis. The property of Ti/SnO2-F electrode and its electrooxidation mechanism were investigated by XRD, SEM-EDX, EIS, LSV, and interfacial resistance measurements. We propose that the similar ionic radii of F and O as well as strong electronegativity of F caused its electrochemical stability with high oxygen evolution potential (OEP) and smooth surface to generate weakly adsorbed OH. The preparation conditions of electrode were also optimized including F doping amount, calcination temperature, and dip coating times, which revealed the formation process of electrode. Additionally, the major mineralization product, F(-), and low concentration of shorter chain perfluorocarboxylic acids (PFCAs) were detected in solution. So the reaction pathway of PFOA electrooxidation was proposed by intermediate analysis. These results demonstrate that Ti/SnO2-F electrode is promising for highly efficient treatment of PFOA in wastewater.