Enhancing the Selectivity between Oxygen and Chlorine towards Chlorine during the Anodic Chlorine Evolution Reaction on a Dimensionally Stable Anode

Abstract

The selectivity of the chlorine evolution reaction over the oxygen evolution reaction during the electrolysis of aqueous NaCl is, despite being very high, still insufficient to prevent expensive separation of the formed Cl2 and O2 by means of liquefaction. We hypothesize that, by decreasing the local activity of H2O near the anode surface by substantially increasing the ionic strength of the electrolyte, the oxygen evolution reaction would be suppressed, leading concomitantly to a higher selectivity of Cl2 over O2 formation. Hence, the influence of the ionic strength on the competition between electrochemical evolution of O2 and Cl2 at dimensionally stable anodes (DSAs) was investigated. Addition of a high concentration of NaNO3, an inert electrolyte additive, increases the selectivity for chlorine at high current density, as determined by means of online electrochemical mass spectrometry and UV‐vis spectroscopy. We propose conditions in which free water is suppressed, owing to under‐coordination of the solvation shells of ions, as a general concept to modulate the selectivity of competing electrochemical reactions.

Overpotential‐Dependent Volcano Plots to Assess Activity Trends in the Competing Chlorine and Oxygen Evolution Reactions

“…It needs to be stressed that the volcano concept, as used in this study, is generally limited in its outreach: it allows assessing activity trends, but relies on several assumptions, for instance constant activities. Recently, it has been reported that local activities of H 2 O and Cl À and local pH are important factors that govern the CER vs. OER selectivity: [12] these quantities allow tuning CER selectivity, but are not captured by the simplified volcano method.…”

“…Regrettably, the oxygen evolution reaction (OER) constitutes a detrimental anodic side reaction under CER conditions . Even though conventionally applied dimensionally stable anodes (DSAs) reveal CER selectivity of about 97–99 %, the gaseous products Cl 2 and O 2 need to be separated from each other, corresponding to a significant fraction of the overall process costs . Therefore, the development of an electrode material that is as stable as DSAs under the harsh anodic reaction conditions, but reveals higher CER selectivity than DSAs, is an ultimate dream of chemical industry.…”

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

“…The electrolytes were prepared by diluting 70% HClO 4 and by adding 99% NaCl in 18.2 MΩ cm Millipore water. For experiments under different NaCl concentrations, NaClO 4 •H 2 O was added into the electrolyte to compensate the total ionic strength 23 . The pH values of the acidic electrolytes were adjusted to 0.90 ± 0.05 by adding a few drops of 70% HClO 4 .…”

“…However, computational and experimental works revealed that MMO catalysts are also highly active for the OER, exhibiting a scaling relationship between the CER and OER 13,[17][18][19][20] . This relationship suggests that two reactions are catalysed on a similar active site of the MMOs or form a common surface intermediate species [20][21][22][23][24] . The oxidative water activation and concomitant surface oxidation on MMOs [25][26][27] are, therefore, unavoidable, leading to a decrease in active site density for the CER 25 .…”

Photo-Chlorine Production with Hydrothermally Grown and Vacuum-Annealed Nanocrystalline Rutile

“…Chlorine is an essential chemical for modern society. For example, pure chlorine gas is used in a wide range of industrial manufacturing techniques, such as disinfection [24] and polymer production [25]. The dissociation of dissolved chlorine in aqueous solution forms hypochlorite (ClO − ), which is a key component in disinfectants, bleaches, deodorisers, oxidising agents, and water treatment formulations.…”