Physicochemical properties of RuO 2 and IrO 2 electrodes affecting chlorine evolutions

Boosting selective chlorine evolution reaction: Impact of Ag doping in RuO2 electrocatalysts

The chlor-alkali process is critical to the modern chemical industry because of the wide utilization of chlorine gas (Cl2). More than 95 % of global Cl2 production relies on electrocatalytic chlorine evolution reaction (CER) through chlor-alkali electrolysis. The RuO2 electrocatalyst serves as the main active component widely used in commercial applications. However, oxygen evolution reaction (OER) generally competes with CER electrocatalysts at RuO2 electrocatalyst owing to the intrinsically scaling reaction energy barrier of *OCl and *OOH intermediates, leading to decreased CER selectivity, high energy consumption, and increased cost. Here, the effect of Ag doping on selective CER over RuO2 electrocatalysts prepared by a sol-gel method has been systematically studied. We found that Ag-doping can effectively improve the Faradaic efficiency of RuO2 electrocatalyst for CER. Furthermore, the improved CER selectivity of Ag-doped RuO2 electrocatalysts is highly dependent on the Ag-doping concentration. The optimized Ag0.15Ru0.85O2 electrocatalyst displays an overpotential of 105 mV along with a selectivity of 84.64 ± 1.84 % in 5.0 M NaCl electrolyte (pH = 2.0 ± 0.05), significantly outperforming undoped one (142 mV, 72.75 ± 1.52 %). Our experiments and density functional theory (DFT) calculations show electron transfer from Ag+ to Ru4+ suppresses *OOH intermediates desorption on Ag-doped RuO2, enabling improved CER selectivity. Such designs of Ag-doped RuO2 electrocatalysts are expected to be favorable for practical chlor-alkali applications.

Engineering decentralized electrodisinfection to sustain consistent chlorine generation under varying drinking water chloride content

In situ electrochlorination can offer an efficient and feasible solution to enable decentralized water disinfection. Unfortunately, there has been only a limited number of studies exploring single-pass flow cell systems with representative flowrates used at household level, particularly under varying chloride concentrations. This work aims to assess anode materials in a single pass and examine the impact of cross velocity, current density, and chloride concentration on various responses such as chlorine production and energy consumption. The primary objective is to determine whether the flow cell can achieve desirable chlorine levels under consistent operation while chloride content of water varies. Chlorine (Cl2/HOCl/OCl-), chlorine dioxide (ClO2) production, and toxic oxyanions (ClO3 -, ClO4 -) were assessed in a single pass setup utilizing different representative anodes including Ti/RuO2, Ti/IrO2, and Boron-doped diamond. Among these materials, the Ti/RuO2 anode emerged as the most suitable for effective chlorine generation while minimizing the formation of ClO3 - and ClO4 -. The performance of in situ electrochlorination using the Ti/RuO2 anode in the flow cell revealed that cross velocity exerted the most significant influence on chlorine generation, while chloride content and current density primarily impacted energy consumption. Optimization of the operating parameters illustrated that stable chlorine concentrations ranging from 2 to 4 mg L-1 could be maintained even with significant fluctuations in chloride concentration from 50 to 250 mg L-1, resulting in a daily energy consumption of less than 0.07 kWh per treated volume of 634 L (i.e., < 0.11 Wh L-1). These experimental findings hold promise for advancing electrodisinfection systems to higher technological readiness level.

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.

Electrochemical oxidation as a post treatment for biologically tannery wastewater in batch reactor

Tannery wastewater is known to contain high concentrations of organic compounds, heavy metals, nitrogen, sulphur, chromium, and many other chemicals. Both aerobic and anaerobic biological approaches have proven ineffective in the treatment of tannery wastewater due to the high salinity and toxic chemicals contained within the medium. Electrochemical oxidation presents a promising method for solving this problem. High pollutant removal efficiency, low energy consumption, and high electrode stability are three important factors supporting the feasibility of an efficient electrochemical treatment process. In the present study, electrochemical oxidation was performed as a post treatment for tannery wastewater (after biological pre-treatment) in a batch reactor using Ti/RuO₂, Ti/IrO₂, and Ti/BDD anodes. The effects of pH, current density, stirring rate and treatment time were studied to assess the treatment efficiency as well as the energy consumption of the process. The results showed that colour, chemical oxygen demand (COD), total organic carbon (TOC), and total nitrogen (TN) removal efficiencies on the electrodes were: Ti/RuO₂ (88.8%, 88.40%, 64.0%, 96.4%), Ti/IrO₂ (85.40%, 85.9%, 52.3%, 51.4%), Ti/BDD (90.60%, 94.7%, 90.5%, 82.7%) respectively, at a current density of 80 mA/cm². All three electrodes demonstrated optimal performance at a pH of 8, a stirring rate of 400 rpm, a current density of 80 mA/cm², and an electrolysis time of 5 h. The concentration of tri-chloromethane by-product was detected with limiting value. Electrochemical oxidation thus offers a feasible method for removing organic compounds and nutrients from tannery wastewater.

Electrochemical production of perchlorate as an alternative for the valorization of brines

In this work, the valorization of brines, with concentrations similar to those produced by reverse osmosis or electrodialysis processes, by electrolysis with diamond anodes is evaluated. To do this, synthetic brines made from solutions of NaCl (with target concentrations ranging from 1.0 to 2.0 M and an additional test at 5.0 M) were used as the raw material for the electrochemical production of perchlorate using commercial electrochemical cells equipped with boron-doped diamond (BDD) anodes. The effect of key parameters on the rate and efficiency of perchlorate production was evaluated. The results show that it is possible to transform more than 80% of the initial chloride concentration into perchlorate, with current efficiencies higher than 70% regardless of the initial concentration of sodium chloride contained in the brine. Moreover, it was observed that both hypochlorite and chlorate were produced almost simultaneously at the beginning of electrolysis, while perchlorate was only produced when a certain value of applied electric charge was passed through the system. The results obtained were essentially independent of the concentration of NaCl, as the high concentrations used in this study avoided mass transfer limitations. Moreover, the specific energy cost of perchlorate production was estimated to range from 26.14 kWh kg^-1 (for 2.0 M and 1000 A m^-2) to 56.10 kWh kg^-1 (for 1.0 M and 2000 A m^-2). According to the results obtained, the electrochemical production of perchlorate by BDD electrochemical oxidation stands out as a promising novel technology for the valorization of the brine produced in reverse osmosis or electrodialysis processes.

Effect of platinum electrode materials and electrolysis processes on the preparation of acidic electrolyzed oxidizing water and slightly acidic electrolyzed water

Electrolyzed oxidizing water (EOW) can be divided into acidic electrolyzed oxidizing water (AEOW) and slightly acidic electrolyzed water (SAEW). AEOW has the characteristics of low pH (pH < 2.7) and high oxidation-reduction potential (ORP > 1100 mV). SAEW is slightly acidic (pH = 5-6) and has an ORP of 700-900 mV. AEOW and SAEW both have a certain amount of active chlorine content (ACC), so they have the characteristics of broad spectrum, rapidity and high efficiency of sterilization. At present, there is little systematic research on AEOW and SAEW preparation. However, it is very important to study the preparation process, including electrode material and electrolytic process. First, the effects of Pt electrodes with different thermal decomposition temperatures on AEOW's pH, ORP and ACC values were investigated in detail. Next, for the SAEW preparation, the process is based on the preparation of AEOW by ion-exchange membrane electrolysis, reasonably mixing the electrolyzed cathode and anode solution. The effects of technological conditions such as electrolysis time, current density and electrolyte concentration have been systematically studied, and it is expected to get SAEW with a pH value slightly less than 7, a higher ORP value and a certain amount of ACC.

Research on the treatment of biologically treated landfill leachate by joint electrochemical system

Biologically treated landfill leachate (BTLL) is typically characterized by significantly high amount of total nitrogen (TN) and chemical oxygen demand (COD), and it has low biodegradability. In this study, a joint electrochemical system (JES) composed of iron anode reactor (IAR) and Ti/RuO₂ anode reactor (TAR) was constructed to remove both TN and COD from BTLL and improve its biodegradability. The IAR and TAR with the same structure but using different anodes. As a result, JES could simultaneously remove COD and TN by 90.9 ± 0.3% and 90.2 ± 1.0%, respectively. Reduction of nitrite-N by Cu/Zn cathode in IAR and oxidation of ammonium-N by active chlorine in TAR were the major pathways for TN removal, while the COD could be removed by coagulation of iron flocs and oxidation by hydroxyl radicals and active chlorine. Fluorescence spectrum and parallel factor analysis showed that the main components of organics in BTLL were humic-like substances, fulvic-like substances, and soluble microbial degradation products. Humic-like substances were particularly removed by JES, and the remaining organics after electrolysis were some alkanes (e.g., heptane and nonane). Furthermore, decrease in molecular weight and aromaticity and increase in biodegradable substances indicated that the biodegradability of BTLL was effectively improved by the JES. The developed JES is a promising approach for application in the BTLL treatment.

Development of Portable Flow-Through Electrochemical Sanitizing Unit to Generate Near Neutral Electrolyzed Water

We developed a portable flow-through, electrochemical sanitizing unit to produce near neutral pH electrolyzed water (producing NEW). Two methods of redirecting cathode yields back to the anode chamber and redirecting anode yields the cathode chamber were used. The NEW yields were evaluated, including: free available chlorine (FAC), oxidation-reduction potential (ORP), and pH. The performances of 2 electrodes (RuO₂ -IrO₂ /TiO₂ and IrO₂ -Ta₂ O₅ /TiO₂ ) were investigated. The unit produced NEW at pH 6.46 to 7.17, an ORP of 805.5 to 895.8 mV, and FAC of 3.7 to 82.0 mg/L. The NEW produced by redirecting cathode yields had stronger bactericidal effects than the NEW produced by redirecting anode yields or NEW produced by mixing the commercial unit's anode and cathode product (P < 0.05). Electron spin resonance results showed hydroxyl free radicals and superoxide anion free radicals were present in the NEW produced by developed unit. The NEW generator is a promising sanitizing unit for consumers and the food industry to control foodborne pathogens.

PRACTICAL APPLICATION

Current commercial NEW-producing units are quite large and are not convenient for family using. The developed portable flow-through, NEW-producing unit has great potential in a wide range of applications, such as organic farm, households, and small food industries. The examined sanitizing treatments showed effective control of Escherichia coli O157:H7 and Listeria monocytogenes.

Dissolution induced self-selective Zn- and Ru-doped TiO2 structure for electrochemical generation of KClO3

We demonstrate an electrochemical approach to prepare a highly active and stable (Zn, Ru)-doped TiO₂ (Ru_0.26Ti_0.73Zn_0.01O_x) for electrochemical generation of KClO₃.

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.

Preparation of electrolyzed oxidizing water with a platinum electrode prepared by magnetron sputtering technique

EO water has the maximum value of available chlorine content when prepared by the Pt-MS electrode due to its good selectivity for CER.

Selectivity between Oxygen and Chlorine Evolution in the Chlor-Alkali and Chlorate Processes

Chlorine gas and sodium chlorate are two base chemicals produced through electrolysis of sodium chloride brine which find uses in many areas of industrial chemistry. Although the industrial production of these chemicals started over 100 years ago, there are still factors that limit the energy efficiencies of the processes. This review focuses on the unwanted production of oxygen gas, which decreases the charge yield by up to 5%. Understanding the factors that control the rate of oxygen production requires understanding of both chemical reactions occurring in the electrolyte, as well as surface reactions occurring on the anodes. The dominant anode material used in chlorate and chlor-alkali production is the dimensionally stable anode (DSA), Ti coated by a mixed oxide of RuO2 and TiO2. Although the selectivity for chlorine evolution on DSA is high, the fundamental reasons for this high selectivity are just now becoming elucidated. This review summarizes the research, since the early 1900s until today, concerning the selectivity between chlorine and oxygen evolution in chlorate and chlor-alkali production. It covers experimental as well as theoretical studies and highlights the relationships between process conditions, electrolyte composition, the material properties of the anode, and the selectivity for oxygen formation.

The electrocatalytic activity of IrO2–Ta2O5 anode materials and electrolyzed oxidizing water preparation and sterilization effect

Ti/IrO₂–Ta₂O₅ anodes with different contents and preparation temperatures were prepared for electrolyzed oxidizing water's preparation and sterilization in this work.