Figure 1. The electrode used for the cathode measurements is a hundredth of the thickness of a human hair. Source: RUB/Marquard.According to the American Chemistry Council (ACC), Washington, D.C, polyvinyl chloride manufacturing consumes around 40% of the 12 million t/y of chlorine produced by the U.S. chloralkali industry. The rest goes to make basic organic chemicals, solvents and hydrochloric acid. Around two-thirds of all chemical processes use chlorine at some point, says the ACC.
However, isolating chlorine is one of the most energy-intensive processes in the chemical industry; over the years, many have attempted to improve this situation. Among the latest to emerge is the use of oxygen-depolarized cathodes (ODCs).
One of the pioneers of these is high tech polymer specialist Covestro, Leverkusen, Germany. Its ODCs have helped to reduce the energy consumed during chlorine manufacture by up to 30%.
Covestra’s method of manufacture is based on the standard membrane process for chlorine-alkaline electrolysis that uses salt and water to produce chlorine, sodium hydroxide and hydrogen.
What the company has done is replace the hydrogen-producing electrodes normally used in the membrane process with an ODC. Supplying the cathode with oxygen prevents the formation of hydrogen, leading only to the production of chlorine and sodium hydroxide.
Covestra says if introduced across the board by all chlorine manufacturers in Germany, the technology would reduce the energy consumption of the entire country by 1% — which corresponds to roughly the annual energy needs of the city of Cologne.
This method of manufacturing involves introducing oxygen into a hot (80°C), highly concentrated sodium hydroxide solution. However, oxygen is very poorly soluble under these conditions. How the reaction achieves industrial current densities remains something of a mystery.
Now, two groups of German researchers have collaborated to shine a light on ODCs in an effort to understand better what is going on.
One group consists of engineers from the Technical University of Clausthal, Clausthal-Zellerfeld, the other has researchers at the Center for Electrochemical Sciences (CES) at Ruhr-Universität Bochum (RUB), Bochum, Germany.
It was already known that three phases meet near the silver oxygen-consuming cathode: solid silver particles are bathed in highly concentrated liquid sodium hydroxide, while gaseous oxygen is forced into the system. Up until now, research has mainly focused on the concentration of the reacting oxygen in the solid-phase environment, developing models that attribute the high current density to this parameter.
What the German researchers show is that reaction conditions change constantly during chlorine production. To demonstrate this, they developed a method to analyze the processes in the liquid phase. They positioned a thin microelectrode — a mere hundredth in thickness of a human hair — directly on the surface of the working oxygen-consuming cathode. The platinum tip of the electrode tracked how hydroxide ions are formed and both oxygen and water molecules consumed as the electrocatalytic oxygen reduction reaction proceeds at the ODC. With this, they were able to follow the changes in water and hydroxide ion concentrations arising during the reaction.
Their results showed that the concentration of water and hydroxide ions on the electrode surface intensely fluctuates through the course of the reaction and isn’t uniform throughout.
“We have suspected for years that there must be significant local concentration fluctuations inside the electrode which could contribute to the high current densities,” explains Wolfgang Schuhmann, head of both the department of analytical chemistry at RUB and also the CES.
“These drastic changes have not yet been considered in the models reflecting the reaction,” adds CES researcher Alexander Botz. “The results are tremendously important for the future optimization of such electrodes.”
As part of a research group funded by the German Research Foundation, together with cooperation partners, the Bochum team now hopes to gain even more insight into the details of the reaction mechanism.
“These investigations are essential for the development of gas diffusion electrodes, which will be of great importance in the future for the binding of carbon dioxide from the air and thus contribute to a reduction in the emission of greenhouse gases,” notes Schuhmann.
The two groups have published their findings in Angewandte Chemie.