Successful Thesis Defense of Graduate Student Tran Thi Thuy Kieu

We are pleased to announce the successful defense of the master's thesis by Tran Thi Thuy Kieu on May 18, 2024, under the supervision of Assoc. Prof. Pham Van Viet (HUTECH University) and Assoc. Prof. Tran Van Man (APCLab, University of Science, VNU-HCM). Her thesis, titled "Synthesis and Study on the Water-Splitting Activity of Co₃O₄ Materials Deposited on Carbon Cloth," addresses critical advancements in hydrogen energy production.

Thesis Summary

In the context of global efforts to reduce CO₂ emissions, hydrogen energy is seen as a strategic solution for green, safe, and efficient renewable energy in the future. However, current water electrolysis systems for producing green hydrogen still face significant limitations. For instance, alkaline electrolysis technology using nickel electrodes often suffers from low current density, while proton (H⁺) exchange membrane electrolysis in acidic environments requires the use of expensive and noble metals such as Pt, RuO₂, or IrO₂, which impedes the scalability of production. Therefore, researching catalyst materials to overcome these limitations in water electrolysis technologies is an important issue that has attracted significant attention from the scientific community.

In this study, the Au/Co₃O₄/CC materials were synthesized through a two-step process involving electrodeposition and photoreduction methods, aiming to create an electrocatalyst with good catalytic activity to overcome the low current density issue in alkaline electrolysis and replace Pt and noble metal-based oxides in proton exchange membrane electrolysis. To achieve the best catalytic activity for the electrode material, the fabrication process of Au/Co₃O₄/CC material was optimized based on three factors: (i) electrodeposition time, (ii) volume ratio of ethylene glycol (EG) to deionized water (DI) in the electrodeposition process for Co₃O₄/CC synthesis, and (iii) ultraviolet light exposure time to carry out the reduction reaction for Au nanostructure formation on the Co₃O₄/CC material.

Based on the obtained results, the best Co₃O₄/CC electrode is CEC5.2, prepared by electrodeposition method in 5 minutes of electrodeposition and a volume ratio of 2/3 between EG and DI. The CEC5.2 sample exhibited a porous structure consisting of ultrathin Co₃O₄ layers arranged perpendicular to the carbon cloth substrate, resulting in a large electrochemical surface area (937.5 cm²/1,0 cm² physical surface area). Among all the Co₃O₄/CC samples, CEC5.2 also demonstrated the best catalytic activity for hydrogen evolution reaction (HER) in an acidic environment and oxygen evolution reaction (OER) in an alkaline environment, with overpotentials of 291 mV and 357 mV at a current density of 10 mA/cm², respectively.

Regarding the photoreduction process, the exposure time to UV light not only affected the amount of deposited Au nanoparticles on the surface of the Co₃O₄/CC sample but also significantly influenced the degradation of Co₃O₄ due to the occurrence of photochemical corrosion. With longer exposure time, the rate of photochemical corrosion increased, causing the loss of the initial porous coating of Co₃O₄/CC. For shorter light exposure times (< 40 minutes), the deposition of Au nanoparticles on Co₃O₄/CC not only improved the conductivity and electrochemical surface area of the material but also provided active sites for HER catalysis, while supporting the electrochemical kinetics of Co₃O₄ to enhance the OER efficiency.

The CEC5.2.30 electrode exhibited the best catalytic activity, especially in an alkaline environment, with very low overpotentials of approximately 121 mV for HER and 160 mV for OER, compared to the commercial Ni electrode with overpotentials of 146 mV and 210 mV, respectively. Furthermore, in an overall water-splitting system, the CEC5.2.30 electrode showed higher current density than the commercial Ni electrode during 24 hours. Additionally, CEC5.2.30 also demonstrated superior catalytic durability in an alkaline environment, as it only experienced a 4.52% increase in voltage to maintain a current density of 10 mA/cm² over 100 hours of operation. In an acidic environment, with the catalytic support from Au, the CEC5.2.30 material exhibited superior OER catalytic activity, achieving a lower overpotential (198 mV) compared to the commercial RuO₂ electrode (254 mV). However, CEC5.2.30 could not replace commercial electrodes in acidic environments such as Pt and RuO₂ due to its poor stability in H₂SO₄ environment through stability test of chronopotentiometry.

Achievements

During her master's program, Tran Thi Thuy Kieu has published three papers in high quality journals:

1. Phan La Phuong Ha^#, Tran Thi Thuy Kieu^#, Nguyen Hoang Bao Long, Tran Van Man, Pham Van Viet, "Development of Co₃O₄ Nanomaterials on Flexible Carbon Cloth Substrates for Hydrogen and Oxygen Evolution Reactions," International Journal of Hydrogen Energy, 2024, 52, 482-493. (#: co-first)
2. Tran Thi Thuy Kieu, Nguyen Nhat Khang, Tran Van Man, "Acid-Enabled Carbon Cloth Boosts Activity and Durability of Au/Co₃O₄ Catalysts for Acidic Water Electrolysis," Advances in Natural Sciences: Nanoscience and Nanotechnology, 2024, accepted for publication.
3. Khang Nhat Nguyen, Thy Nha Ngoc Tran, Kieu Thuy Thi Tran, Man Van Tran, Nam Hoang Vu, Tien-Thanh Nguyen, Yoshiyuki Kawazoe, Viet Van Pham, "Mott-Schottky Au/Co₃O₄ Decorated on Carbon Cloth Substrate as an Efficiently Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution Reactions," Energy Material Advances, 2024, accepted for publication.

Additionally, she received the Domestic Postgraduate Scholarships Program from the Vingroup Innovation Foundation (VinIF) for her master's studies, where she successfully completed her program with a distinction rating of "Outstanding".

Currently, Tran Thi Thuy Kieu is a recipient of the prestigious Faculty of Engineering Research Scholarship for her PhD studies at The University of Sydney, ranked 18th globally in the QS World University Rankings 2024. Her research will continue to focus on developing low-cost and high-performing electrocatalysts for water electrolysis.