Energy Saving Chlorine Electrolysis with Gas Diffusion Electrodes

Abstract

Chlorine is needed for some two-thirds of all chemical products, including plastics. And that comes at a cost. The production of this important basic chemical is one of the most energy-intensive processes in the chemical industry. In Germany the annual production of around 4.5 million tons of chlorine accounts to about 3% of the overall German electricity consumption. Thus, process innovation in chlorine manufacturing provides an important lever for tremendous savings of energy and their related carbon dioxide emissions. The existing state-of-the-art chlorine processes of brine or hydrochloric acid electrolysis are already highly developed. A significant improvement is only possible by a major technology change. For conventional chlor-alkali electrolysis the reactions can be described as: Anode: 2 NaCl = Cl2 + 2 e- + 2 Na+ (E0 = +1.36 V) Cathode: 2 H2O + 2 Na+ + 2 e- = H2 + 2 NaOH (E0 = -0.83 V) Replacing the cathode reaction of hydrogen evolution reaction by oxygen reduction reaction (ORR) would bring a drastic decrease of the overall electrolysis voltage due to thermodynamics. Cathode: ½ O2 + H2O + 2 Na+ + 2 e- = 2 NaOH (E0 = +0.4 V) Electrodes for efficient ORR are well known from fuel cell technology in form of gas diffusion electrodes. However, the implementation of gas diffusion electrodes in a chlorine evolving electrolysis is a challenge when it comes to industrial scale. By interdisciplinary cooperation of industry partners and research institutes, significantly supported by public funding, the development of suited gas diffusion electrodes for chlorine electrolysis - also known as oxygen depolarized cathodes (ODC) - was successfully realized. The process has been operating on an industrial scale at Covestro’s plant in Krefeld-Uerdingen in Germany since 2011 and has been commercialized worldwide since 2013. As a result, up to 30 percent less electricity than for the conventional process is required. The talk will highlight the technical requirements and the corresponding developments as well as the ecological and economic aspects of the use of gas diffusion electrodes for chlorine production. Furthermore, the developed gas diffusion electrodes are the platform for future applications in the field of power generation, energy storage, water purification/disinfection and electrosynthesis. Figure 1