Improved Hydrogen Peroxide Generating Gas Diffusion Electrodes Combined with Boron Doped Diamond Electrodes: A Compact in-Situ Oxidant Production Reactor for Space Application

Abstract

Electrochemical methods offer the opportunity for in-situ generation of oxidants for wastewater treatment, as disinfectant for apparatuses and production sites as well as rocket fuel. One method well known is the electrolysis of water at boron doped diamond (BDD) anodes enabling formation of highly oxidizing OH-radicals [1]. While the BDD electrode is already used as total organic carbon (TOC) sensor on the International Space Station (https://condias.de/unternehmen/chronik/), its application potential for space is even more versatile. In particular, the combination of the BDD anode and a hydrogen peroxide (H2O2) generating gas diffusion electrode (GDE) enables a simultaneous generation of oxidants at both electrodes. By this approach, the current efficiency can be theoretically improved up to 200 %. Furthermore, the formed oxidizing agents enable the formation of additional oxidation species by consecutive reactions e.g. ozone and other oxygen radicals [2,3]. Their formation is adjustable by process conditions. All together, this can be the basis of a very universal electrolysis cell for space use. BDD electrodes are available in technical dimensions and can be operated at high current densities, but H2O2-GDEs are not. Therefore, within the radar project (http://machwas-material.de/RADAR.html) carbon-based GDEs were developed and optimized in cooperation with Covestro AG, a company manufacturing GDEs for other electrochemical processes in technically relevant dimensions since many years [4]. The activity of carbon materials for the 2-electron step of H2O2 formation is been known for a long time. On the one hand, they enable the two-electron reaction step, on the other hand, they offer large BET surface areas and thus excellent electro catalyst support qualities. Moreover, they are inexpensive and offer sufficient chemical and thermal stability. In our study, the optimization of the GDEs include the variation of composition, catalyst-loading and manufacturing parameters and the performances are correlated with the pore systems. In addition to the technical electrode design, the operation mode has a great influence on H2O2 production. Therefore, the influence of the process parameters on the H2O2 yield was also investigated. During the electrolysis process the pH-value at the cathode shifts to higher values and the conditions for H2O2 formation shift as well. Without controlling the pH at the cathode it is not clear which pH value is the best for optimal H2O2 formation. [5] In our investigation, the pH value was kept constant by balancing the pH shift with controlled dosing of water. The current density was at least 0.5 kA/m2, important for the design of compact technical reactors and particular for usage in space. By this approach stationary operation was reached, allowing to identify the best conditions for H2O2 formation. Our H2O2–GDE investigations show that from 15 °C to 20 °C, H2O2 production changes only slightly, but with further temperature increase from 25 °C to 60 °C, H2O2 current efficiency decrease significantly due to H2O2 self-decomposition. Furthermore, the optimized carbon-based GDE produces H2O2 with current yields greater than 90 % at 15 °C at a current density of up to 2 kA/m2. In addition, the choice of the pH value is critical with respect to attainable H2O2 concentration. An optimized mode of operation is crucial for a high yielding GDE and the identified optimized operating point is achieved by varying the mass flow rate of the dosages. The presented electrolysis cell for simultaneous oxidant generation on both electrodes is solely fed with water on cathode and anode side – a very convenient operation method especially in space. References Samuel P. Kounaves, Total Organic Compound (TOC) Analyzer (US8216447B2) (2012). B. Marselli, J. Garcia-Gomez, P.-A. Michaud, M.A. Rodrigo and Ch. Comninellis, Journal of the Electrochemical Society(150 (3)), D79-D83 (2003). M. Sievers, Treatise on Water Science, 4 (2011). J. Kintrup, M. Millaruelo, V. Trieu, A. Bulan and E. S. Mojica, Electrochem. Soc. Interface, 26(2), 73–76 (2017). T. Muddemann, U. Kunz, D. R. Haupt and M. Sievers, ECS Trans., 86(4), 41–53 (2018).