Photo-electrochemical ep-oxidation using environmentally friendly oxidants

Photoelectrochemical and electrochemical (photo-electrochemical) systems for epoxidation and oxidation (ep-oxidation) are a novel, stable, and efficient methods that produce value-added products. Recently, several studies have been reported on photo-electrochemical epoxidation using environmentally friendly oxidants.

Here -> https://doi.org/10.1016/j.ccr.2023.215641, we present an overview of photo-electrochemical for ep-oxidation systems that imparts oxidizing behavior in organic molecules and relies on green oxidants, i.e., H2O and O2 as the ultimate oxygen source, which is in line with the main trend of sustainability. This review also focuses on the photo-electrochemical properties of the engineered photo-catalyst thin films, in particular synthetic outlook, efficiency, activity, and selectivity.

Creativity with Solar Energy

Research Topics

Artificial photosynthesis

Photoelectrochemical

Photocatalysis

Solar hydrogen

Solar Fuel

The world energy consumption is growing rapidly, and experts estimated that we will need ~30 terawatts (TW) in 2050 (46 TW in 2100), which is almost double the current energy requirement (~16 TW). However, fossil fuels are limited in supply to support such a high energy demand continuously, and their combustion produce large amount of carbon dioxide (CO2), which causes serious environmental effect (e.g., global warming). Hydrogen recently received much attention as a form of storing energy (i.e., fuel), as it is potentially clean (no CO2 release when used in a fuel cell) and has a very high specific energy density (142 MJ/kg). Ironically, due to economical reason, hydrogen is now mostly produced by steam reforming method, which consumes fossil fuels and releases CO2. Thus, for the premise of hydrogen being clean and sustainable to hold, renewable energy source, such as solar energy (~ 89 PW); which exceeds the world energy consumption by more than 5,000 times, needs to be utilized in the production of hydrogen. To this end, our group will work on solar water splitting for hydrogen production using photocatalyst. We will also focus on solar CO2 conversion into valuable fuels like CH4 and CH3OH and so on with it.

“Using solar energy can have a positive, indirect effect on the environment when solar energy replaces or reduces the use of other energy sources that have larger effects on the environment.”

For the solar water splitting with photocatalyst, it requires energy of 1.6~2.4 eV considering kinetic overpotential of both hydrogen and oxygen evolution reactions. And CO2 reduction even requires higher energy than water splitting. Thus semiconductor (photocatalyst) for water splitting and CO2 reduction should have the band gap above 1.6 eV (usually 2.0 eV), which cannot utilize long wavelength of visible light and infrared (IR). This portion is actually more than 50~90% of solar spectrum. While most of the research groups make efforts to increase the solar-to-fuel conversion efficiency in this restricted solar spectrum, our group will concentrate on to extend the use of solar spectrum with quantum PEC approach.

Design Photoelectrode for Solar Hydrogen Production

Solar Hydrogen Production

Photoelectrochemical (PEC) water splitting represents one of the most promising energy conversion processes capable of directly producing hydrogen from renewable energy sources. A typical PEC cell consists of a photoactive semiconductor as a working electrode, a counter electrode (usually Pt), and an appropriate supporting electrolyte. When an n-type semiconductor photoelectrode is used, photo generated holes migrate to the photoelectrode/electrolyte interface and perform water oxidation, while photoexcited electrons travel via the back contact and external circuit to the counter electrode, where they reduce water. In such a practice, the n-type semiconductor serves as the photoanode and its valence band level must be more positive than the H2O/O2 potential (+1.23 V vs RHE) to allow efficient oxygen evolution. The Fermi level of the photoanode, on the other hand, determines the potential of the photoexcited electrons at the counter electrode, which can be otherwise less negative than the H+/H2 potential (0 V vs RHE) since the potential deficiency can be compensated for by applying an external voltage. Similarly, a p-type semiconductor can function as the photocathode for practical hydrogen evolution, given that the conduction band level is more negative than the H+/H2 potential. Alternatively, a photoanode and a photocathode can be connected in a two-electrode cell without employing a counter electrode.

“The ubiquitous availability of water and its low-carbon footprint make PEC water splitting a green and sustainable approach to solve the problem of ever-rising global energy demand.”

The ubiquitous availability of water and its low-carbon footprint make PEC water splitting a green and sustainable approach to solve the problem of ever-rising global energy demand. Since Fujishima and Honda demonstrated the first PEC water splitting apparatus using TiO2, the number of related publications each year has increased significantly. The high importance and tremendous interest in this field can be highlighted by a simple search on the ISI Web of Knowledge database using “PEC” and “photocatalytic hydrogen” as title keywords. As summarized in Figure, over the past ten years, the number of publications on “PEC hydrogen” and “photocatalytic hydrogen” has both exhibited a significant growth rate, reaching approximately 1000 publications at 2018. For publications with the title of “PEC,” the yearly number has exceeded 1300 at 2018. Obviously, the interest in PEC cells continues to maintain a marvelous pace as they remain widely employed in the advancement of solar fuel systems.

How to Design an Efficient Photoelectrodes

The development of suitable techniques to fabricate high-efficiency photoelectrode for PEC water splitting is crucial. To date, there are many methods, which have been developed to prepare photoelectrodes for use in solar water oxidation.

First, the morphologies of the materials used as a photoelectrode such as shape, size, and particle contact strongly affect their interfacial energetics, kinetics, and charge transport properties as well as reactive sites.

Second, nanocomposites provide a powerful route to overcome limitations in the current studies of single material systems for water splitting, where the photoelectrochemical performance of photoelectrode can be significantly improved by the choice of proper interactions of constituents.

Third, doping with the metal (such as W, Cu, Zn, Ti, Mo, Ag, etc.) or nonmetal (such as P and N) materials can change the electrical and optical properties of the semiconductor materials used as a photoelectrode.

Finally, decoration of the surface of the photoelectrode with various oxygen evolution cocatalysts (OECs) such as Co–Pi, Co3O4, CoO, and NiFe improves the kinetic for oxygen evolution and provides unique active sites for catalytic reactions, thereby strongly enhancing the photocurrent density.