Parallel oxygen and chlorine evolution on Ru1−xNixO2−y nanostructured electrodes

Toward Realistic Models of the Electrocatalytic Oxygen Evolution Reaction

The electrocatalytic oxygen evolution reaction (OER) supplies the protons and electrons needed to transform renewable electricity into chemicals and fuels. However, the OER is kinetically sluggish; it operates at significant rates only when the applied potential far exceeds the reversible voltage. The origin of this overpotential is hidden in a complex mechanism involving multiple electron transfers and chemical bond making/breaking steps. Our desire to improve catalytic performance has then made mechanistic studies of the OER an area of major scientific inquiry, though the complexity of the reaction has made understanding difficult. While historically, mechanistic studies have relied solely on experiment and phenomenological models, over the past twenty years ab initio simulation has been playing an increasingly important role in developing our understanding of the electrocatalytic OER and its reaction mechanisms. In this Review we cover advances in our mechanistic understanding of the OER, organized by increasing complexity in the way through which the OER is modeled. We begin with phenomenological models built using experimental data before reviewing early efforts to incorporate ab initio methods into mechanistic studies. We go on to cover how the assumptions in these early ab initio simulations─no electric field, electrolyte, or explicit kinetics─have been relaxed. Through comparison with experimental literature, we explore the veracity of these different assumptions. We summarize by discussing the most critical open challenges in developing models to understand the mechanisms of the OER.

Understanding the Catalytic Active Sites of Crystalline CoSbxOy for Electrochemical Chlorine Evolution

The chlorine evolution reaction (CER) is a key reaction in electrochemical oxidation (EO) of water treatment. Conventional anodes based on platinum group metals can be prohibitively expensive, which hinders further application of EO systems. Crystalline cobalt antimonate (CoSbxOy) was recently identified as a promising alternative to conventional anodes due to its high catalytic activity and stability in acidic media. However, its catalytic sites and reaction mechanism have not yet been elucidated. This study sheds light on the catalytically active sites in crystalline CoSbxOy anodes by using scanning electrochemical microscopy to compare the CER catalytic activities of a series of anode samples with different bulk Sb/Co ratios (from 1.43 to 2.80). The results showed that Sb sites served as more active catalytic sites than the Co sites. The varied Sb/Co ratios were also linked with slightly different electronic states of each element, leading to different CER selectivities in 30 mM chloride solutions under 10 mA cm-2 current density. The high activity of Sb sites toward the CER highlighted the significance of the electronic polarization that changed the oxidation states of Co and Sb.

Probing the Effects of Electrode Composition and Morphology on the Effectiveness of Silicon Oxide Overlayers to Enhance Selective Oxygen Evolution in the Presence of Chloride Ions

Seawater electrolysis offers significant logistical advantages over freshwater electrolysis but suffers from a fundamental selectivity problem at the anode. To prevent the evolution of toxic chlorine alongside the evolution of oxygen, a promising approach is the use of electrochemically inert overlayers. Such thin films can exert a perm-selective effect, allowing the transport of water and oxygen between the bulk electrolyte and the electrocatalytic buried interface while suppressing the transport of chloride ions. In this work, we investigate thin (5-20 nm) overlayer films composed of amorphous silicon oxide (SiO x ) and their application to suppressing the chlorine evolution reaction (CER) in favor of the oxygen evolution reaction (OER) during acidic saltwater electrolysis on three different types of electrodes. While SiO x overlayers are seen to be an effective barrier against the CER on well-defined, smooth Pt thin films, decreasing the CER activity roughly 20-fold, this ability has not been previously explored on Ir-based catalysts with a higher surface area relevant to industrial applications. On amorphous iridium oxide electrodes, the selectivity toward the CER versus the OER was marginally reduced from ∼98 to ∼94%, which was attributed to the higher abundance of defects in overlayers deposited on the rougher electrode. On the other hand, Ir-based anodes consisting of thick mixed metal oxide films supported on Ti showed a significant decrease in CER selectivity, from ∼100 to ∼50%, although this came at the cost of reduced activity toward the OER. These results show that the morphology and composition of the underlying electrode play important roles in the effectiveness of the selective overlayers and provide guidance for further development of high-surface-area OER-selective anodes.

Atomically dispersed Pt-N4 sites as efficient and selective electrocatalysts for the chlorine evolution reaction

Chlorine evolution reaction (CER) is a critical anode reaction in chlor-alkali electrolysis. Although precious metal-based mixed metal oxides (MMOs) have been widely used as CER catalysts, they suffer from the concomitant generation of oxygen during the CER. Herein, we demonstrate that atomically dispersed Pt-N4 sites doped on a carbon nanotube (Pt1/CNT) can catalyse the CER with excellent activity and selectivity. The Pt1/CNT catalyst shows superior CER activity to a Pt nanoparticle-based catalyst and a commercial Ru/Ir-based MMO catalyst. Notably, Pt1/CNT exhibits near 100% CER selectivity even in acidic media, with low Cl- concentrations (0.1 M), as well as in neutral media, whereas the MMO catalyst shows substantially lower CER selectivity. In situ electrochemical X-ray absorption spectroscopy reveals the direct adsorption of Cl- on Pt-N4 sites during the CER. Density functional theory calculations suggest the PtN4C12 site as the most plausible active site structure for the CER.

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.

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.

Design Criteria, Operating Conditions, and Nickel-Iron Hydroxide Catalyst Materials for Selective Seawater Electrolysis

Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.

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.

Oxygen reduction on nanocrystalline ruthenia – local structure effects

Nanocrystalline ruthenium dioxide and doped ruthenia of the composition Ru_1−xM_xO₂ (M = Co, Ni, Zn) with 0 ≤ x ≤ 0.2 were prepared by the spray-freezing freeze-drying technique.

On the faradaic selectivity and the role of surface inhomogeneity during the chlorine evolution reaction on ternary Ti–Ru–Ir mixed metal oxide electrocatalysts

Faradaic selectivity of the chlorine and oxygen evolution (left) is linked to the spatial inhomogeneity of the surface reactivity of Ti–Ru–Ir mixed metal oxide catalysts.