Optimization of Electrodes Towards More Practical Electrochemical Water Treatment

Citation

Dong, Heng (2023) Optimization of Electrodes Towards More Practical Electrochemical Water Treatment. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/hkpg-je79. https://resolver.caltech.edu/CaltechTHESIS:03062023-153330120

Abstract

Due to water scarcity and water pollution, the world suffers from continuing water sanitation issues, which lead to billions of water-borne disease cases every year. Decentralized water treatment is regarded as an important supplement to the conventional wastewater treatment system to address the water sanitation and water pollution issues in rural, remote, and undeveloped regions. Electrochemical water treatment technology has been demonstrated to be feasible for decentralized water treatment systems because of the ambient operation conditions, robust performance, modular design, small footprint, and environmental compatibility. The performance of electrochemical water treatment systems relies heavily on the choice of electrodes. This thesis presents a comprehensive study towards understanding and optimizing the electrodes to enhance the performance and lower the cost of electrochemical water treatment systems. The research work on anodes followed an “understanding – development” approach and spanned both the scientific and engineering sides of the spectrum. Specifically, a comprehensive review was assembled through the analysis of existing literature on mixed metal oxide anodes. This review pointed towards potential future research directions. With the advancement of material sciences, it is important to focus not only on single catalytic metal elements, but also on the intermetallic electronic interaction to gain a deeper understanding of the catalytic activity of mixed metal oxides. The microscopic steric effects imposed by crystalline structures may also be a nonnegligible contributor to the catalytic properties.

Following the review, this thesis scrutinized the catalytic sites of crystalline CoSb₂O₆, an emerging anode for chlorine evolution reaction (CER) catalysis. It has been demonstrated to be a promising alternative for the conventional Ru- and Ir-based anodes based on its high activity and excellent stability, but its catalytic sites and mechanism are still unknown. By fabricating and testing a series of anodes with different Sb/Co ratios, it was discovered that the surface Sb/Co ratios in CoSb₂O₆ were ~50% higher than in the bulk. At the same time, it was surprising to find through scanning electrochemical microscopy (SECM) that Sb-rich samples showed higher catalytic activities, indicating that Sb sites may be even more active catalytic sites than the Co-sites. This was attributed to the electronic interaction between Co and Sb, as revealed by X-ray photoelectron spectroscopy (XPS).

On the engineering side, a Ni–Sb–SnO₂ reactive electrochemical membrane (REM) was developed to treat primary effluent and greywater. In 30 min, the REM removed up to 78 ± 2% COD and 94 ± 0.6% turbidity from the primary effluent. The REM had ~100% COD removal and 89 ± 4% turbidity removal from greywater, with the effluent meeting the NSF/ANSI 350 standard. Compared to the conventional plate-type electrodes under the same conditions, the REM had 36% lower energy consumption for primary effluent treatment and 22% lower energy consumption for greywater treatment while yielding better treatment results. The REM-based electrochemical system was demonstrated to be a promising solution for decentralized wastewater treatment and recycling for single households and for vehicles.

Last but not the least, this thesis presents the 3D-printing-derived carbon lattice as a monolithic electro-Fenton cathode. The Fenton reaction is one of the most important advanced oxidation processes (AOPs) that is widely used in water treatment to remove non-biodegradable pollutants, and heterogeneous electro-Fenton (HEF) process catalyzed by carbon-based cathodes has received considerable attention as an evolving branch due to its wide working pH range and independence from chemical dosing. However, the conventional carbon cathodes suffered from poorly controlled porosities, which hampered the mass transport and limited the overall catalytic performance. Three rationally-designed carbon lattice cathodes with different macroscopic porosities were fabricated and tested, showing that it was feasible to facilitate the mass transport by tuning the macroscopic electrode structure. Specifically, Grid-2% cathode, which had the largest macroscopic porosity, showed 157% higher specific activity for electrochemical H₂O₂ production and 256% higher specific activity for trimethoprim degradation than the Star-2%, the one with the smallest macroscopic porosity. Grid-2% achieved 97% aqueous trimethoprim removal in 60 min, demonstrating the potential of the carbon lattice cathode to be used for water treatment and remediation.

Thesis Files