New photocatalyst boosts water splitting efficiency for clean hydrogen production
IMAGE: THE OXYGEN EVOLUTION CATALYST EFFICIENTLY REDUCES THE CONCENTRATION OF THE I3- IONS AND CHANGES TO THE HYDROGEN EVOLUTION CATALYST PREVENT ELECTRON BACK TRANSFERS (DOTTED RED LINES) AND PRIORITIZE ELECTRON TRANSFER TO PRODUCE HYDROGEN (SOLID BLACK LINES) view more
CREDIT: KAZUHIKO MAEDA FROM TOKYO TECH
In a first, a dye-sensitized photocatalyst that facilitates the most efficient solar water splitting activity recorded to date (for similar catalysts) has been optimized by researchers from Tokyo Tech. Their surface-modified, dye-sensitized nanosheet catalyst shows immense potential, as it can suppress undesirable back electron transfer and improve water splitting activity up to a hundred times!
One of the simplest ways by which water molecules can be split into hydrogen is by using photocatalysts. These materials, which are semiconductors that can absorb light and carry out water-splitting reactions simultaneously, provide a simple setup for the mass production of hydrogen. Semiconductors can generate an electron-hole pair for the water splitting reaction; however, since the charge carriers tend to recombine, a “Z-scheme” photocatalytic system involving two semiconductor materials and an electron mediator has been developed to suppress this.
In this setup, the electron mediator, which is typically a reversible electron acceptor/donor pairs (such as I3-/I-), accepts electrons from one of the photocatalysts and donates them to the other. This separates the charge carriers between the semiconductors. Despite eliminating the charge recombination within the semiconductor, the electron-accepting species (I3-) competes with the hydrogen photocatalyst for electrons, resulting in poor solar-to-hydrogen energy conversion efficiencies.
To improve hydrogen production, a team of international researchers, including Specially Appointed Assistant Professor Shunta Nishioka and Professor Kazuhiko Maeda from Tokyo Institute of Technology (Tokyo Tech) has been working on ways to prevent the unintended electron transfer. On experimenting with ruthenium (Ru) dye-sensitized niobate photocatalysts (Ru/Pt/HCa2Nb3O10), the researchers noticed that hydrogen production increases significantly at low I3- concentrations. These findings led them to develop an efficient water splitting system that consists of an oxygen evolution photocatalyst and a modified Ru dye-sensitized niobate nanosheet that functions as a better hydrogen evolution photocatalyst. “We have successfully improved the efficiency of a Z-scheme overall water splitting system by using a surface-modified dye-sensitized nanosheet photocatalyst,” says Prof. Maeda. The results of their study have been published in the journal Science Advances.
To keep the I3- concentration in the reaction system low, a PtOx/H-Cs-WO3 photocatalyst is used as the oxygen evolution catalyst. At the same time, Al2O3 and poly(styrenesulfonate) (PSS) is added to suppress the back electron transfer from the semiconductor to the oxidized Ru complex and the I3- ion, respectively. This design enables more electrons to participate in the hydrogen evolution reaction, resulting in the most efficient Z-scheme water splitting system to date (Figure 1). “The surface modification of the dye-sensitized nanosheet photocatalyst improved the solar water splitting activity by nearly 100 times, making it comparable to conventional semiconductor-based photocatalyst systems,” says Prof. Maeda.
With the back electron transfer suppressed, the developed photocatalyst could also maintain hydrogen production at low light levels, giving it an edge over other photocatalysts that require high light intensities. Moreover, by minimizing the impact of the back electron transfer reactions, the researchers have not only set a new benchmark for dye-sensitized photocatalysts for Z-scheme water splitting, but also laid the framework to improve other dye-sensitized systems that are used for other important reactions such as CO2 reduction.
CAPTION
Surface modification with an insulator and an anionic polymer improved water splitting activity up to a hundred times. This illustration was selected as a featured image in Science Advances.
CREDIT
Science Advances
About Tokyo Institute of Technology
Tokyo Tech stands at the forefront of research and higher education as the leading university for science and technology in Japan. Tokyo Tech researchers excel in fields ranging from materials science to biology, computer science, and physics. Founded in 1881, Tokyo Tech hosts over 10,000 undergraduate and graduate students per year, who develop into scientific leaders and some of the most sought-after engineers in industry. Embodying the Japanese philosophy of “monotsukuri,” meaning “technical ingenuity and innovation,” the Tokyo Tech community strives to contribute to society through high-impact research.
https://www.titech.ac.jp/english/
JOURNAL
Science Advances
METHOD OF RESEARCH
Experimental study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Surface-Modified, Dye-Sensitized Niobate Nanosheets Enabling an Efficient Solar-Driven Z-Scheme for Overall Water Splitting
New evidence shows water separates into two different liquids at low temperatures
Peer-Reviewed PublicationFresh evidence that water can change from one form of liquid into another, denser liquid, has been uncovered by researchers at the University of Birmingham and Sapienza Università di Roma.
This ‘phase transition’ in water was first proposed 30 years ago in a study by researchers from Boston University. Because the transition has been predicted to occur at supercooled conditions, however, confirming its existence has been a challenge. That’s because at these low temperatures, water really does not want to be a liquid, instead it wants to rapidly become ice. Because of its hidden status, much is still unknown about this liquid-liquid phase transition, unlike about everyday examples of phase transitions in water between a solid or vapour phase and a liquid phase.
This new evidence, published in Nature Physics, represents a significant step forward in confirming the idea of a liquid-liquid phase transition first proposed in 1992. Francesco Sciortino, now a professor at Sapienza Università di Roma, was a member of the original research team at Boston University and is also a co-author of this paper.
The team has used computer simulations to help explain what features distinguish the two liquids at the microscopic level. They found that the water molecules in the high-density liquid form arrangements that are considered to be “topologically complex”, such as a trefoil knot (think of the molecules arranged in such a way that they resemble a pretzel) or a Hopf link (think of two links in a steel chain). The molecules in the high-density liquid are thus said to be entangled.
In contrast, the molecules in the low-density liquid mostly form simple rings, and hence the molecules in the low-density liquid are unentangled.
Andreas Neophytou, a PhD student at the University of Birmingham with Dr Dwaipayan Chakrabarti, is lead author on the paper. He says: “This insight has provided us with a completely fresh take on what is now a 30-year old research problem, and will hopefully be just the beginning.”
The researchers used a colloidal model of water in their simulation, and then two widely used molecular models of water. Colloids are particles that can be a thousand times larger than a single water molecule. By virtue of their relatively bigger size, and hence slower movements, colloids are used to observe and understand physical phenomena that also occur at the much smaller atomic and molecular length scales.
Dr Chakrabarti, a co-author, says: “This colloidal model of water provides a magnifying glass into molecular water, and enables us to unravel the secrets of water concerning the tale of two liquids.”
Professor Sciortino says: “In this work, we propose, for the first time, a view of the liquid-liquid phase transition based on network entanglement ideas. I am sure this work will inspire novel theoretical modelling based on topological concepts.”
The team expect that the model they have devised will pave the way for new experiments that will validate the theory and extend the concept of ‘entangled’ liquids to other liquids such as silicon.
Pablo Debenedetti, a professor of chemical and biological engineering at Princeton University in the US and a world-leading expert in this area of research, remarks: “This beautiful computational work uncovers the topological basis underlying the existence of different liquid phases in the same network-forming substance.” He adds: “In so doing, it substantially enriches and deepens our understanding of a phenomenon that abundant experimental and computational evidence increasingly suggests is central to the physics of that most important of liquids: water.”
Christian Micheletti, a professor at International School for Advanced Studies in Trieste, Italy, whose current research interest lies in understanding the impact of entanglement, especially knots and links, on the static, kinetics and functionality of biopolymers, remarks: “With this single paper, Neophytou et al. made several breakthroughs that will be consequential across diverse scientific areas. First, their elegant and experimentally amenable colloidal model for water opens entirely new perspectives for large-scale studies of liquids. Beyond this, they give very strong evidence that phase transitions that may be elusive to traditional analysis of the local structure of liquids are instead readily picked up by tracking the knots and links in the bond network of the liquid. The idea of searching for such intricacies in the somewhat abstract space of pathways running along transient molecular bonds is a very powerful one, and I expect it will be widely adopted to study complex molecular systems.”
Sciortino adds: “Water, one after the other, reveals its secrets! Dream how beautiful it would be if we could look inside the liquid and observe the dancing of the water molecules, the way they flicker, and the way they exchange partners, restructuring the hydrogen bond network. The realisation of the colloidal model for water we propose can make this dream come true.”
The research was supported by the Royal Society via International Exchanges Award, which enabled the international collaboration between the researchers in the UK and Italy, the EPSRC Centre for Doctoral Training in Topological Design and the Institute of Advanced Studies at the University of Birmingham, and the Italian Ministero Istruzione Università Ricerca – Progetti di Rilevante Interesse Nazionale.
JOURNAL
Nature Physics
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
Not applicable
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