We explore the potential of employing diketopyrrolopyrrole (DPP) based π-conjugated OSs as a hole transport layer material in heteroatom-doped hematite (Ti–Fe2O3/Ge–Fe2O3) photoanodes for efficient photoelectrochemical water splitting. The siloxane-modified π-conjugated polymer (PSi) with a high carrier mobility and crystallinity revealed great potential to extract holes by forming a built-in potential with hematite photoanodes while showing high stability in an alkaline electrolyte for photoelectrochemical water oxidation. Because of the easy hole extraction and subsequent fast hole transport property of the PSi interlayer between NiFe(OH)x and Ge-doped porous Fe2O3(Ge-PH), NiFe(OH)x/PSi/Ge-PH showed a 1.8-fold increase in photocurrent density (4.57 mA cm–2 at 1.23 VRHE) with a cathodic shift of the onset potential (0.735 VRHE) and good stability for 65 h compared to Ge-PH. This study demonstrates the successful use of inherently unstable π-conjugated OSs as a hole extracting/transport medium in a photoanode, addressing the intrinsic recombination issues of hematite for efficient and stable water splitting.
A team of researchers, led by Professor Ji-Hyun Jang from the School of Energy and Chemical Engineering at UNIST has achieved a significant breakthrough in photoelectrode development. Through collaborative research with Professor Junghoon Lee from Dongseo University and Dr. Hyo-Jin Ahn from the German Engineering Research and Development Center LSTME Busan , the team successfully created a high-performance and stable photoelectrode by incorporating organic semiconductors as an intermediate layer in existing inorganic semiconductor-based photoelectrodes.
The production of green hydrogen using solar energy involves splitting water into its constituent elements through charges generated in semiconductors that absorb sunlight. Previous studies primarily focused on utilizing inorganic semiconductors for constructing photoelectrodes. However, organic semiconductors offer several advantages such as lower costs, various process methods, and easier large-scale production. Additionally, their high solar energy conversion efficiency translates to enhanced hydrogen production efficiency. Nevertheless, the susceptibility of organic semiconductors to water damage has limited their application in photoelectrodes.
To overcome this challenge, the research team applied a coating of organic semiconductors onto the surface of conventional iron oxide-based photoelectrodes to ensure stability when exposed to water. Furthermore, they implemented a catalyst (nickel/iron double-layer hydroxide) as an additional protective layer over the coated organic semiconductor to prevent direct contact with water. This innovative approach allowed the charges generated from solar energy absorption to facilitate efficient hydrogen production reactions.
Figure 1. (a) Scheme for the fabrication of NiFe(OH)x/PSi/Ti–Fe2O3. (b) Transmission electron microscopy (TEM) image of NiFe(OH)x/PSi/Ti–Fe2O3 and (c) corresponding elemental mapping of Fe, O, Ti, Sn, N, S, Si, and C. High-resolution TEM images of (d) PSi/Ti–Fe2O3 and (e) NiFe(OH)x/PSi/Ti–Fe2O3. The inset in panel d shows a high-resolution TEM image of Ti–Fe2O3, and the scale bar is 2 nm.
Professor Jang expressed the team’s excitement about the research outcomes, by saying, “By overcoming the limitations of traditional inorganic semiconductor-oriented photoelectrodes, we have demonstrated the potential for more extensive utilization of organic semiconductors in hydrogen production through photoelectrodes.”
This breakthrough not only opens up new possibilities for enhancing efficiency and stability but also contributes to advancing sustainable energy solutions for a carbon-neutral future.
The study findings have been published ahead of their official publication in the online version of ACS Energy Letters on March 12, 2023. This work was selected for the supplementary cover of the journal and was subsequently published on June 9, 2023. The main authors were Dr. Hyo-Jin Ahn (LSTME Busan) and Ki-Yong Yoon (UNIST). Funding support was provided by the National Research Foundation of Korea (NRF) and the Ministry of Science and ICT (MSIT).
Hyo-Jin Ahn, Ki-Yong Yoon, Mingi Sung, et al., “Utilizing a Siloxane-Modified Organic Semiconductor for Photoelectrochemical Water Splitting,” ACS Energy Lett., (2023).