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978-3-8439-3081-9, Reihe Verfahrenstechnik
Energy-efficient chlorine production by gas-phase hydrogen chloride electrolysis with oxygen depolarized cathode
165 Seiten, Dissertation Technische Universität Clausthal (2016), Softcover, A5
In this thesis the focus is the development of a new energy-efficiently chlorine process via a gas-phase electrolysis of HCl using an oxygen depolarized cathode (ODC). Therefore, the individual reactor components were developed, tested and optimized within a specially designed experimental plant.
For testing, a suitable reactor concept was designed and materials were selected. The reactor concept is based on a polymer membrane electrolyte reactor. The key component here is the membrane electrode assembly consisting of a catalyst coated Nafion® membrane (CCM) and a gas diffusion layer (GDL). As catalyst coating a commercially available Pt/C with Nafion® as ionomer was used. Due to the chemically harsh conditions, the other reactor components either consist of perfluorinated materials or graphite polymer composites.
In this reaction HCl(g) is electrochemically oxidized at the anode forming Cl2. The formed protons migrate through the membrane and react on the cathode with the oxygen to water. Hence, the focus in this work was to determine a suitable process window in order to run this process at a high performance with a low energy demand. The results show that the optimal process conditions are at a reaction temperature of 40 °C and a high cathode gas humidification of 60 – 80 %rH. Furthermore, cell voltages under 1 V at industrially relevant current densities of 4 kA m-2 were achieved.
A technical challenge was revealed during these experiments. The water management is crucial for an optimal performance of this process. Beside the process conditions, the contribution of each reactor component and their composition were investigated. It was found that the membrane material and the added Nafion® ionomer in the CCM play a significant role. Here, the noble metal and the ionomer loading are key parameters., which were optimized by halving the noble metal loading and adjusting the ionomer content leading to lower cell voltages. Additionally, the composition of the GDL being determined by the carbon and polymer content influences the water management considerably.
These results show that the gas-phase electrolysis of HCl with an ODC can be conducted at very low cell voltages. The energy demand is only 733 kWh t_(Cl_2)^(-1). Which corresponds to a reduction of ca. 30 % in comparison to today’s electrolysis processes. This process offers a high potential to produce chlorine in a recycling process of HCl with a lower energy demand.