’s research group at KENTECH
has been carrying out research focused on semiconductor-based photo(electro)chemical systems and environmental chemical processes to address various aspects of environmental and energy related issues. The followings are main research topics that are currently being studied.
- Semiconductor photocatalysis and photoelectrochemistry for environmental and energy applications
- Solar fuel production and artificial photosynthesis
- Photochemical and chemical purification of water and air
- Advanced redox processes for environmental applications
- Environmental chemical reactions in ice and frozen solutions
Some recent research examples are briefly described below.
Photoelectrochemical systems for water treatment:
The solar-driven photo(electro)catalytic chemical process is a key technology for sustainable utilization of solar energy. It has beena widely applied to environmental remediation and solar fuel production. The dual functional photo(electro)catalytic process can achieve the water treatment along with the simultaneous recovery of energy (e.g., H2
) or resource (e.g., metal ions). The essential feature of the process is to utilize the hole oxidation power for the degradation of water pollutants and the electron reduction power for the recovery of energy and resource from wastewaters at the same time. For example, our recent study employed vertically aligned TiO2
nanotube arrays (TNTs) for the dual-functional photoelectrochemical water treatment of organic substrates degradation and accompanying H2
generation. Various photocatalytic (PC), photoelectrochemical (PEC), and photovoltaic-driven electrochemical (PV-EC) processes with different dual functional purposes (e.g., pollutants removal combined with H2
production, heavy metal recovery, denitrification, fuel cell) can be developed. This may provide new opportunities for the development of next-generation water treatment processes based on water-solar energy nexus.
M. S. Koo, X. Chen, K. Cho, T. An, W. Choi, “In Situ Photoelectrochemical Chloride Activation Using WO3 Electrode for Oxidative Treatment with Simultaneous H2 Evolution under Visible Light”, Environ. Sci. Technol. 2019, 53, 9926-9936
T. H. Jeon, M. S. Koo, H. Kim, W. Choi, “Dual Functional Photocatalytic and Photoelectrocatalytic Systems for Energy and Resource-Recovering Water Treatment”, ACS Catal. 2018, 8, 11542-11563
M. S. Koo, K. Cho, J. Yoon, W. Choi, “Photoelectrochemical Degradation of Organic Compounds Coupled with Molecular Hydrogen Generation using Electrochromic TiO2 Nanotube Arrays”, Environ. Sci. Technol. 2017, 51, 6590-6598
Solar production of H2O2:
Hydrogen peroxide (H2
), a carbon-free energy carrier and an environment-friendly oxidant, has been widely applied in chemical and environmental processes. While the current industrial production process of H2
gas, toxic organic solvents, and high energy inputs, H2
production by photocatalytic and photoelectrochemical (PEC) method that uses sunlight, water and molecular oxygen only is a green and sustainable alternative to the conventional production method. We recently developed an efficient PEC-based H2
production system that utilizes photocurrent more efficiently by having the single charge pass in the PEC cell carry out double generation of H2
on both photoanode and cathode. The proposed system consists of a modified BiVO4
photoanode and an anthraquinone (AQ)-anchored carbon cathode. Molybdenum doping and phosphate treatment of BiVO4
facilitates the selective oxidation of H2
O to H2
and improves its durability dramatically over 100 h with preventing the dissolution of BiVO4
. On the other hand, AQ anchored on the carbon cathode catalyzes selective 2-electron reduction of O2
but suppresses the competing reaction of H2
production. The proposed PEC cell generates H2
on both photoanode and cathode with the maximum current utilization efficiency.
T. H. Jeon, H. Kim, H.-i. Kim, W. Choi, “Highly Durable Photoelectrochemical H2O2 Production via Dual Photoanode and Cathode Processes under Solar Simulating and External Bias-free Condition”, Energy Environ. Sci. 2020, DOI: 10.1039/C9EE03154E
G.-h. Moon, M. Fujitsuka, S. Kim, T. Majima, X. Wang, W. Choi, “Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Hetero-Elements”, ACS Catal. 2017, 7, 2886-2895
G.-h. Moon, W. Kim, A. D. Bokare, N.-e. Sung, W. Choi, “Solar Production of H2O2 on Reduced Graphene Oxide-TiO2 Hybrid Photocatalysts Consisting of Earth-Abundant Elements Only”, Energy Environ. Sci. 2014, 7, 4023-4028
Photocatalytic air purification:
Photocatalysis is an ideal method for removing volatile organic compounds (VOCs) present at low concentrations in indoor environment because it operates at ambient temperature and pressure to degrade them to CO2
O. Photocatalysis can be particularly suitable for removing low concentration pollutants (sub-ppm levels) in indoor environments where the conventional adsorption technologies are not very efficient. In a recent study, we synthesized 1 facet-exposed TiO2
nanotubes (001-TNT) that can be easily scaled up, and tested them for the photocatalytic removal of volatile organic compounds (VOCs) in both a laboratory reactor and a commercial air cleaner. The photocatalytic degradation activity of toluene on 001-TNT was at least twice as high as that of TNT. While the TNT experienced a gradual deactivation during successive cycles of photocatalytic degradation of toluene, the 001-TNT did not exhibit any sign of catalyst deactivation under the same test conditions. The 001-TNT filter was successfully scaled up and installed on a commercial air cleaner. The air cleaner equipped with the 001-TNT filters achieved an average VOCs removal efficiency of 72% (in 30 minutes of operation) in a 8-m3
test chamber, which satisfied the air cleaner standards protocol (Korea) to be the first photocatalytic air cleaner that passed this protocol.
S. Weon, E. Choi, H. Kim, J. Y. Kim, H.-J. Park, S.-m. Kim, W. Kim, W. Choi, "Active 1 Facet Exposed TiO2 Nanotubes Photocatalyst Filter for Volatile Organic Compounds Removal: From Material Development to Commercial Indoor Air Cleaner Application", Environ. Sci. Technol. 2018, 52, 9330-9340
H.-i. Kim, H. Kim, S. Weon, G.-h. Moon, J.-H. Kim, W. Choi, “Robust Co-Catalytic Performance of Nanodiamonds Loaded on WO3 for the Decomposition of Volatile Organic Compounds under Visible Light”, ACS Catal. 2016, 6, 8350-8360
S. Weon, W. Choi, “TiO2 Nanotubes with Open Channels as Deactivation-Resistant Photocatalyst for the Degradation of Volatile Organic Compounds”, Environ. Sci. Technol. 2016, 50, 2556-2563
Advanced redox processes:
Advanced redox processes have been intensively studied for efficient removal of recalcitrant contaminants with high chemical stability or low biodegradability in wastewater. For energy-sustainable applications of the redox processes, it is essential to develop a system that efficiently produces reactive oxygen species (ROS) such as hydroxyl radical and superoxide anion. Various methods such as photolysis, photocatalysis, electrolysis, and sonolysis have been developed to efficiently produce oxidants (e.g., ●
OH) from dioxygen and water molecule or reductants like energetic electrons. Our research group has been introducing and investigating new redox processes. In one recent example, copper phosphide (Cux
P) was synthesized and tested for its reactivity for generating H2
through spontaneous reduction of dioxygen under ambient aqueous condition. The in-situ generated H2
was subsequently decomposed to generate OH radicals, which enabled the degradation of organic compounds in water. Cux
P is proposed as a solid reagent that can activate dioxygen to generate ROS in ambient aqueous condition, which is more facile to handle and store than liquid/gas reagents (e.g., H2
H. Kim, J. Lim, S. Lee, H.-H. Kim, C. Lee, J. Lee, W. Choi, "Spontaneous Generation of H2O2 and Hydroxyl Radical through O2 Reduction on Copper Phosphide under Ambient Aqueous Condition", Environ. Sci. Technol. 2019, 53, 2918−2925
Y. Choi, M. S. Koo, A. Bokare, D.-h. Kim, D. Bahnemann, W. Choi, “Sequential Process Combination of Photocatalytic Oxidation and Dark Reduction for the Removal of Organic Pollutants and Cr(VI) using Ag/TiO2”, Environ. Sci. Technol. 2017, 51, 3973-3981
A. D. Bokare, W. Choi, “Bicarbonate-Induced Activation of H2O2 for Metal-free Oxidative Desulfurization”, J. Hazard. Mater. 2016, 304, 313-319
Environmental redox processes in ice:
Ice is one of the most ubiquitous solids on Earth, being present in the atmosphere, terrestrial surface, and ocean environment. Many environmental reactions taking place in ice are significantly different compared to aqueous counterparts. We have been investigating various homogeneous and heterogeneous chemical reactions in frozen solutions, especially those which have environmental significance and found that many environmental chemical reactions taking place in frozen solutions are significantly different compared to aqueous counterparts. Some exapmes include the photochemical and chemical dissolution of metal oxides, the reductive transformation of hexavalent chromium and bromate, the photooxidation of iodide, and the humification of phenolic compounds. Such phenomena can be related with the freezing-induced concentration of reactants in the unfrozen ice grain boundaries that should exist between ice crystals. The observed accelerated processes in frozen media may have significant effects on the chemical transformation processes in the cold environment such as polar region, upper atmosphere, and frozen soil.
D. W. Min, K. Kim, K. H. Lui, B. Kim, S. Kim, J. Cho, W. Choi, “Abiotic Formation of Humic-like Substances through Freezing-Accelerated Reaction of Phenolic Compounds and Nitrite”, Environ. Sci. Technol. 2019, 53, 7410-7418
K. Kim, S. P. M. Menacherry, J. Kim, H. Y. Chung, D. Jeong, A. Saiz-Lopez, W. Choi, “Simultaneous and Synergic Production of Bioavailable Iron and Reactive Iodine Species in Ice”, Environ. Sci. Technol. 2019, 53, 7355-7362
K. Kim, A. Yabushita, M. Okumura, A. Saiz-Lopez, C. Cuevas, C. Blaszczak-Boxe, D. W. Min, H.-I. Yoon, W. Choi, “Production of molecular iodine and triiodide in the frozen solution of iodide: implication for polar atmosphere”, Environ. Sci. Technol. 2016, 50, 1280-1287.