A Reflection on Sustainable Anode Materials for Electrochemical Chloride Oxidation

Renaissance of Chlorine Evolution Reaction: Emerging Theory and Catalytic Materials

Chlorine (Cl2) is one of the most important commodity chemicals that has found widespread utility in chemical industry. Most Cl2 is currently produced via the chlorine evolution reaction (CER) at the anode of chlor-alkali electrolyzers, for which platinum group-metal (PGM)-based mixed metal oxides (MMOs) have been used for more than half a century. However, MMOs suffer from the use of expensive and scarce PGMs and face selectivity problems due to the parasitic oxygen evolution reaction. Over the last decade, the field of CER catalysis has seen dramatic advances in both the theory and discovery of new catalysts. Theoretical approaches have enabled a fundamental understanding of CER mechanisms and provided catalyst design principles. The exploration of new materials has led to the discovery of CER catalysts other than MMOs, including non-PGM oxides, atomically dispersed single-site catalysts, and organic molecules, with some of which following novel reaction pathways. This minireview provides an overview of the recent advances in CER electrocatalyst research and suggests future directions for this revitalized field.

Efficient, electrochemical degradation of organic pollutants via nanofibrous Pt/Ir-RuO2 electrode with enhanced stability

A diverse range of surfactants and chelating agents are frequently used in industrial processes, especially in the decontamination of nuclear facilities for decommissioning. To treat and degrade these organic pollutants, electrooxidation (EO) has emerged as a cost-effective method. Along these lines, in this work, a nanofibrous electrode was constructed to facilitate efficient EO. Among various metal oxide for EO, RuO2 is known for its excellent electrochemical activity, which was fabricated into a nanofiber structure with a large specific surface area and doped with IrO2 to increase its stability. In addition to the use of nanofibrous Ir-RuO2, a Pt intermediate layer was incorporated to increase both structural stability and electrical conductivity. The nanofibrous electrode with a Ru:Ir composition of 9:1 showed an electrochemical activation area 1.7 times larger and a charge transfer resistance 4.7 times smaller than a flat-type electrode with the same composition. Efficient degradation (99%) of organic pollutants (sodium dodecyl benzene sulfonate, Triton X-100, ethylenediaminetetraacetic acid, and nitrilotriacetic acid (NTA)) was successfully performed using the nanofibrous electrode. Effective decomposition of radioactive waste (NTA combined with Co ions) with 99% degradation within 4 h was achieved.

Rational surface Design and electron Regulation of co-deposition Ru and Ti on TiO2 nanotubes as self-supporting electrode for high-performance chlorine evolution reaction

The chlorine evolution reaction (CER) is essential for chlorine (Cl2) production. However, the catalyst deactivation and side reaction oxygen evolution reaction (OER) are still serious problems for CER. Improving the stability and selectivity of catalyst for long term electrocatalytic reaction in large current density is highly desired. In this work, a self-supporting Ti/RuO2@TiO2 nanotubes (NTs) electrode had been developed by co-sputtering titanium (Ti) and ruthenium oxide (RuO2) onto the high-ordered titanium dioxide (TiO2) nanotubes substrate. This process produced a well-distributed incorporation of Ti species within the RuO2 coating. It has revealed that the presence of Ti element optimizes the surface morphology of electrode and modulates the electronic state of ruthenium (Ru) species, effectively preventing over-oxidation and thereby enhancing stability. The Ti/RuO2@TiO2 NTs self-supporting electrode exhibits remarkable electrocatalytic activity for CER, evaluated by lower overpotential (58 mV) at 10 mA cm-2 and reduced Tafel slope (78 mV dec-1) than that of other electrodes. Moreover, its mass activity achieving 478 mA mgRu-1 at 1.3 V (vs SCE) is 4.3 times that of commercial dimensionally stable anodes (DSA). After operating for approximately 150 h at 100 mA cm-2, the electrode exhibits minimal performance degradation, underscoring its exceptional stability. Additionally, the large potential difference between CER and OER makes it maintain high Cl2 selectivity, achieving up to 91 % across varying current densities and reaction times. This work provides valuable strategy for designing novel Ru-based catalysts for practical CER applications in seawater and industrial environments.

C, Balcom P, Omole DO, Álvarez-Mejía C, López-Ramrez V, Gadgil A. Low-cost, local production of a safe and effective disinfectant for resource-constrained communities

Improved hygiene depends on the accessibility and availability of effective disinfectant solutions. These disinfectant solutions are unavailable to many communities worldwide due to resource limitations, among other constraints. Safe and effective chlorine-based disinfectants can be produced via simple electrolysis of salt water, providing a low-cost and reliable option for on-site, local production of disinfectant solutions to improve sanitation and hygiene. This study reports on a system (herein called "Electro-Clean") that can produce concentrated solutions of hypochlorous acid (HOCl) using readily available, low-cost materials. With just table salt, water, graphite welding rods, and a DC power supply, the Electro-Clean system can safely produce HOCl solutions (~1.5 liters) of up to 0.1% free chlorine (i.e.,1000 ppm) in less than two hours at low potential (5 V DC) and modest current (~5 A). Rigorous testing of free chlorine production and durability of the Electro-Clean system components, described here, has been verified to work in multiple locations around the world, including microbiological tests conducted in India and Mexico to confirm the biocidal efficacy of the Electro-Clean solution as a surface disinfectant. Cost estimates are provided for making HOCl locally with this method in the USA, India, and Mexico. Findings indicate that Electro-Clean is an affordable alternative to off-the-shelf commercial chlorinator systems in terms of first costs (or capital costs), and cost-competitive relative to the unit cost of the disinfectant produced. By minimizing dependence on supply chains and allowing for local production, the Electro-Clean system has the potential to improve public health by addressing the need for disinfectant solutions in resource-constrained communities.

Improved Operation of Chloralkaline Reversible Cells with Mixed Metal Oxide Electrodes Made Using Microwaves

This study focuses on the synthesis of mixed metal oxide anodes (MMOs) with the composition Ti/RuO2Sb2O4Ptx (where x = 0, 5, 10 mol) using hybrid microwave irradiation heating. The synthesized electrodes were characterized using scanning electron microscopy, X-ray energy-dispersive analysis, X-ray diffraction, cyclic voltammetry, and electrochemical impedance spectroscopy. These electrodes were then evaluated in both bulk electrolytic and fuel cell tests within a reversible chloralkaline electrochemical cell. The configurations using the electrodes Ti/(RuO2)0.7-(Sb2O4)0.3 and Ti/(RuO2)66.5-(Sb2O4)28.5-Pt5 presented lower onset potential for oxygen and chlorine evolution reactions and reduced resistance to charge transfer compared to the Ti/(RuO2)63-(Sb2O4)27-Pt10 variant. These electrodes demonstrated notable performance in reversible electrochemical cells, achieving Coulombic efficiencies of up to 60% when operating in the electrolytic mode at current densities of 150 mA cm-2. They also reached maximum power densities of 1.2 mW cm-2 in the fuel cell. In both scenarios, the presence of platinum in the MMO coating positively influenced the process. Furthermore, a significant challenge encountered was crossover through the membranes, primarily associated with gaseous Cl2. This study advances our understanding of reversible electrochemical cells and presents possibilities for further exploration and refinement. It demonstrated that the synergy of innovative electrode synthesis strategies and electrochemical engineering can lead to promising and sustainable technologies for energy conversion.

Engineering the Coordination Environment of Ir Single Atoms with Surface Titanium Oxide Amorphization for Superior Chlorine Evolution Reaction

The chlorine evolution reaction (CER) is essential for industrial Cl2 production but strongly relies on the use of dimensionally stable anode (DSA) with high-amount precious Ru/Ir oxide on a Ti substrate. For the purpose of sustainable development, precious metal decrement and performance improvement are highly desirable for the development of CER anodes. Herein, we demonstrate that surface titanium oxide amorphization is crucial to regulate the coordination environment of stabilized Ir single atoms for efficient and durable chlorine evolution of Ti monolithic anodes. Experimental and theoretical results revealed the formation of four-coordinated Ir1O4 and six-coordinated Ir1O6 sites on amorphous and crystalline titanium oxides, respectively. Interestingly, the Ir1O4 sites exhibited a superior CER performance, with a mass activity about 10 and 500 times those of the Ir1O6 counterpart and DSA, respectively. Moreover, the Ir1O4 anode displayed excellent durability for 200 h, far longer than that of its Ir1O6 counterpart (2 h). Mechanism studies showed that the unsaturated Ir in Ir1O4 was the active center for chlorine evolution, which was changed to the top-coordinated O in Ir1O6. This change of active sites greatly affected the adsorption energy of Cl species, thus accounting for their different CER activity. More importantly, the amorphous structure and restrained water dissociation of Ir1O4 synergistically prevent oxygen permeation across the Ti substrate, contributing to its long-term CER stability. This study sheds light on the importance of single-atom coordination structures in the reactivity of catalysts and offers a facile strategy to prepare highly active single-atom CER anodes via surface titanium oxide amorphization.

Defective blue titanium oxide induces high valence of NiFe-(oxy)hydroxides over heterogeneous interfaces towards high OER catalytic activity

Nickel-iron (oxy)hydroxides (NiFeOxHy) have been validated to speed up sluggish kinetics of the oxygen evolution reaction (OER) but still lack satisfactory substrates to support them. Here, non-stoichiometric blue titanium oxide (B-TiOx) was directly derived from Ti metal by alkaline anodization and used as a substrate for electrodeposition of amorphous NiFeOxHy (NiFe/B-TiOx). The performed X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations evidenced that there is a charge transfer between B-TiOx and NiFeOxHy, which gives rise to an elevated valence at the Ni sites (average oxidation state ∼ 2.37). The synthesized NiFe/B-TiOx delivers a current density of 10 mA cm-2 and 100 mA cm-2 at an overpotential of 227 mV and 268 mV, respectively, which are better than that of pure Ti and stainless steel. It also shows outstanding activity and stability under industrial conditions of 6 M KOH. The post-OER characterization studies revealed that the surface morphology and valence states have no significant change after 24 h of operation at 500 mA cm-2, and also can effectively inhibit the leaching of Fe. We illustrate that surface modification of Ti which has high corrosion resistance and mechanical strength, to generate strong interactions with NiFeOxHy is a simple and effective strategy to improve the OER activity and stability of non-precious metal electrodes.