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The electrolysis of HCl to form Cl2 is an integral part of the production of polycarbonates and polyurethanes. In recent years, the direct gas-phase electrolysis was shown to be significantly more efficient than the current state-of-the-art process based on the oxidation of hydrochloric acid. Still, three phenomena significantly limit the performance and industrial applicability of this process and have so far only been investigated theoretically. Firstly, a limiting behavior in the HCl oxidation reaction was observed, which seems to be of purely kinetic origin. Secondly, also in the full-cell employing an oxygen depolarized cathode, a limiting behavior was detected, which however appears to have a different origin. Lastly, the performance of the oxygen reduction is significantly reduced in comparison to classical H2 PEM fuel cells. The present work utilizes a combined experimental and theoretical approach to confirm that the HCl oxidation is purely reaction limited while the limiting behavior in the full-cell system employing an oxygen depolarized cathode is caused by flooding at low reactor temperatures and, lastly, that the reduced performance of the oxygen reduction reaction is a consequence of significant HCl crossover that can be mitigated by means of increased cathode humidification. These insights are furthermore used to operate the HCl gas phase electrolyzer employing an oxygen depolarized cathode at current densities of more than 5000 A/m2 for the first time, while also substituting the previously employed platinum based cathode catalyst with RhxSy, decreasing the impact of HCl crossover and allowing for the lowest so far measured cell potentials over a wide interval of current densities.

The electrochemical oxidation of HCl to Cl2 plays an important role in the production of polycarbonates and polyurethanes. Recently, the gas-phase oxidation of HCl proved to be significantly more efficient than the current state-of-the-art process based on the oxidation of hydrochloric acid. In experimental investigations of this gas-phase reactor, a limiting current can be observed that is so far not understood but impedes the overall reactor performance. In the present work, a nonisothermal multiphase agglomerate model is developed to investigate the underlying reasons for this limiting behavior in more detail. It is shown that the thermal management of the cell plays a significant role and that minor changes to its thermal resistance lead to the limiting behavior being caused by either flooding of the cathode or dehydration of the membrane and anode. An optimization of operational and structural parameters of the cell based on these insights leads to an increase in the limiting current by more than 90%. Interestingly, under these conditions a third phenomenon, the rate determining Tafel step in the microkinetic reaction mechanism of the HCl oxidation, limits the overall reactor performance. These insights harbor the potential for enormous energetic savings in this industrially highly relevant process.

The electrochemical conversion of hydrogen chloride to chlorine plays a significant role within major industrial processes like the polyurethane or polycarbonate production. Our recent studies demonstrated that the direct electrolysis of gaseous HCl in combination with novel strategies for product purification leads to significant exergetic savings (36–38%) compared to the Bayer UHDENORA state-of-the art process, which employs aqueous hydrochloric acid as a feedstock. Furthermore, we showed that despite of the improvements in the efficiency of the electrochemical reactor, it still has by far the greatest exergy demand of all process units. While the oxygen depolarized cathode (ODC) utilized in both process variants has been investigated in great detail in the scientific literature, the oxidation reaction of gaseous HCl (HClOR) has not yet received much attention and is hence the major subject of this work. In earlier experimental investigations of the HClOR, a limiting current was observed that has the potential to critically reduce the performance of the overall reactor. In the past, membrane dehydration and mass transfer resistances on the anode side were suggested as possible reasons. In order to shed light on this phenomenon, a dynamic, one dimensional agglomerate model of the gas phase HCl oxidation half-cell, considering detailed micro kinetics, as well as the mass transfer of HCl in the different cell parts, is developed. The modeling results suggest that neither mass transfer resistance nor membrane dehydration but a kinetic limitation is the underlying reason for the observed limiting behavior. This has a significant impact on the strategy for further reactor optimizations and harbors the potential of major energy savings in an industrial application of this highly relevant process.