Roadmap drafted for research into metallic ‘sponges’ for clean hydrogen
IMAGE: THE CARTOON OF THE METAL ORGANIC FRAMEWORK STRUCTURE, DELIVERING AN EFFICIENCY BOOST TO BOTH PRODUCTION OF CLEAN HYDROGEN AND OXYGEN. view more
CREDIT: POLYOXOMETALATES, TSINGHUA UNIVERSITY PRESS
Metal organic frameworks (MOFs) could deliver a major efficiency boost to the photocatalytic production of clean hydrogen. Chemical engineers have drafted a comprehensive overview of the state of their field and a plan for where it needs to focus.
Clean hydrogen production remains an energy-intensive and therefore costly proposition, inhibiting the battle against global warming. Metal organic frameworks—in effect tiny molecular ‘sponges’—look set to radically improve the efficiency of photocatalytic production of hydrogen due to their unique structural properties, but the research into the subject faces considerable challenges. A group of chemical engineers have produced an overview of the state of the field with a roadmap of where investigations should be focused to most likely achieve progress.
Their review paper was published in the journal Polyoxometalates on August 4, 2023.
Hydrogen will be necessary for the clean transition away from fossil fuels, whether as an energy storage mechanism, an input for clean fuels or as a clean fuel directly, or for decarbonized steel and ammonia production. But the hydrogen itself must be cleanly produced, from the splitting of water into its component parts. Unfortunately, such water splitting is an energy hog, which drives up the cost of clean hydrogen production. If clean hydrogen is going to be competitive with dirty hydrogen production—typically via the splitting of methane, a greenhouse gas—then water splitting needs to achieve some significant increases to its efficiency.
One widely discussed efficiency-boosting option comes from photocatalytic water splitting with the assistance of metal organic frameworks, or MOFs.
First, the energy from sunlight activates the photocatalyst—a material that jumpstarts and speeds up the water splitting reaction. Next, imagine a Lego-like structure, but where the Lego bricks are instead made of metal clusters—a large group of metallic atoms—and the connectors (or “linkers”) between them are organic molecules. These structures form porous 3D networks that act sort of the way sponges do to absorb liquids into their pores. But these metal-organic ‘sponges,’ or more properly, metal organic frameworks (MOFs) are so small that they operate at the molecular level, allowing scientists to trap, store, or separate various gases and chemicals inside.
MOFs can be game-changers for photocatalytic water splitting due to their unique properties, particularly with respect to absorbing the sunlight that kicks off the whole photocatalytic water splitting process.
Research into the role of MOFs for photocatalytic water splitting has exploded in recent years, and so the authors felt it was time to produce a scientific review paper on the topic. Scientific review papers are like "best-of" music albums for science, gathering all the hit discoveries and insights on a topic into one comprehensive overview. They act as compasses for the scientific community, summarizing past research to guide future explorations and helping researchers build upon existing knowledge rather than reinventing the wheel.
The review paper first sets out the key advantages of MOFs here. Some MOFs can absorb sunlight and can then transfer the energy to other materials or use it directly to drive the water splitting reaction. Moreover, the efficiency of a photocatalyst largely depends on its ability to excite electrons to jump a ‘band gap’ up from the valence level of an atom to its conduction level—where these excited electrons can now flow freely in an electric current. MOFs can be designed and modified to optimize their band gaps, making them more suitable for absorbing visible light.
“MOFs also have a large surface area due to their porous nature,” said Huan Pang, one of the review paper’s authors and a chemical engineer in the School of Chemistry and Chemical Engineering, Yangzhou University. “Think of all that internal surface area encapsulating the pores.”
This extra surface area means that MOFs provide a greater number of locations where the water-splitting chemical reactions can take place—locations known as “active sites.” More places for those reactions means greater efficiency in water splitting.
MOFs can also serve as supports for other photocatalytic materials, ensuring they remain stable and dispersed. This can prevent agglomeration (clumping together) of photocatalytic particles, which can reduce their efficiency.
“And one of the biggest advantages of MOFs is their sheer versatility,” added Yang An, a co-author of the paper at the Institute for Innovative Materials and Energy at Yangzhou University. “Chemical engineers can customize the MOF structures by selecting different metals and organic linkers, allowing for the design of MOFs specifically tailored for efficient photocatalytic water splitting.”
The authors also laid out some of the most promising leads for improvement of use of MOFs for photocatalytic water splitting, in particular the development of MOFs with dual active sites—ones for both parts of the water splitting chemical reaction—the “hydrogen evolution reaction” and the “oxygen evolution reaction.”
Dual active sites can provide more active sites for the adsorption (the process where the molecules of a substance attach themselves to the surface of another substance) and activation of water molecules. The paper proposes that the dual active sites can be achieved by introducing two different types of metal ions or organic linkers into the MOF structure, or by introducing a co-catalyst (material that is used in conjunction with a photocatalyst to enhance its performance, in this case such as a noble metal) onto the MOF surface.
However, the paper also notes that the design and synthesis of MOFs with dual active sites remains still a challenging task. This is because it requires precise control over the MOF structure and composition.
In addition, the introduction of two different types of metal ions or organic linkers into the MOF structure, or the introduction of a co-catalyst onto the MOF surface, can affect the stability and activity of the MOF. Advancing the development of MOFs with dual active sites requires careful consideration of factors such as the size and shape of the MOF crystals, the authors conclude, as well as the arrangement of atoms surrounding the MOF’s central metal ion, and the interactions between the MOF and the co-catalyst.
Lastly, the paper suggests that the performance of MOFs with dual active sites can be affected by factors such as the loading amount and distribution of the co-catalyst, the surface area and porosity of the MOF, and the reaction conditions.
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Polyoxometalates is a peer-reviewed, international and interdisciplinary research journal that focuses on all aspects of polyoxometalates, featured in rapid review and fast publishing, sponsored by Tsinghua University and published by Tsinghua University Press. Submissions are solicited in all topical areas, ranging from basic aspects of the science of polyoxometalates to practical applications of such materials. Polyoxometalates offers readers an attractive mix of authoritative and comprehensive Reviews, original cutting-edge research in Communication and Full Paper formats, Comments, and Highlight.
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JOURNAL
Polyoxometalates
ARTICLE TITLE
Metal-organic Framework-based Materials for Photocatalytic Overall Water Splitting: Status and Prospects
Anode models for green hydrogen production
Peer-Reviewed PublicationIMAGE: SCHEMATIC REPRESENTATION OF AN ELECTROLYSER WITH A CRYSTALLINE ANODE view more
CREDIT: © FHI
Researchers from the Interface Science Department at the Fritz Haber Institute of the Max Planck Society conducted experiments using atomically defined model pre-catalysts to unveil intricate details of the electrocatalytic water splitting reaction, targeting the advancement of green H2 production.
The ongoing climate change poses a serious threat to humanity, affecting everybody’s life and necessitating measures to implement a more sustainable energy economy. The production of ‘green’ energy is a crucial ingredient. However, energy production must be accompanied by economic storage and transport methods. ‘Green’ hydrogen (H2) serves both as a storage medium and a means for transport, also when converted into other useful industrial products or energy carriers such as ammonia. It can be produced by electrolysis via decomposition of water molecules with ‘green’ electrical energy. In the electrocatalytic cell, molecular hydrogen is generated at the cathode, while the anode produces molecular oxygen (O2).
The O2 production at the anode is a complex multistep process, which makes it challenging to design energy-efficient anodes. As a result, most water-splitting research focuses on the anode rather than on the cathode. In real electrolysers, anodes possess intricate chemical compositions and morphologies, impeding the fundamental understanding of electrolysis processes which is very much needed for their subsequent optimization. Relevant data may be challenging to find, akin to a needle in a haystack. To address this, scientists in the Interface Science Department at the FHI have implemented an experimental approach substituting the complex anode with a simpler model pre-catalyst system.
In this approach, the anode pre-catalyst is a well-defined crystalline thin oxide film, allowing for controlled variations in its initial composition and structure. To ensure purity, the anodes are prepared under ultra-high-vacuum conditions, and all subsequent studies are conducted in the same experimental characterization system without exposing the samples to ambient air. This stringent methodology safeguards the anode from contamination throughout the experiment, preventing any adverse effects on the experimental data. Knowing the anode properties in atomic detail is a central aspect of the method. The primary focus is to investigate central aspects of water splitting catalysis, including mechanistic microscopic details of the O2 formation reaction, the active sites, electrode aging, and the role of the anode's surface structure and composition for the water-splitting performance. More specifically, it is well known in the literature that an oxyhydroxide layer is formed on the catalyst surface under operando conditions, but the characteristics of this layer and the optimum structure, thickness and composition are yet unknown. It is however recognized that there is a unifying structural transformation taking place during O2 production, regardless of the initial pre-catalyst structure. On the other hand, and as it is described in the present contribution, the specific characteristics of the pre-catalyst anode determine the transformation that takes place during operation and ultimately the long-term activity and stability of the electrocatalyst.
It is well-known that adding iron to cobalt oxide anodes significantly enhances their performance, although the underlying mechanism is still under discussion. Gaining a comprehensive understanding of the specific role of iron addition is crucial for optimizing water splitting processes. In pursuit of this goal, we conducted a study on crystalline mixed thin film oxide anodes, exploring various Co:Fe ratios. The flat and well-defined anode structure allowed us to establish a quantitative relationship between the oxide's composition, structure, and O2 formation performance, making the beneficial effect of iron addition evident. Stability studies further revealed performance improvements attributed to iron dissolution, eventually converging the catalyst towards a stable highly active anode.
The study addresses two pertinent aspects of water-splitting technology, focusing on minimizing costs associated with electrolyser fabrication and operation. Keeping these costs low by moving towards the alkali reaction conditions and Earth-abundant materials is of crucial relevance for a widespread implementation of a H2-based energy economy. Present electrolyser technology uses rare metals, iridium, and platinum for energy-efficient electrolysis. Replacement of these costly metals by the cheaper cobalt and iron-based oxides would reduce the overall water-splitting cost, increasing the economic attractivity of this process. Electrical efficiency is another crucial cost consideration, relying on the electrode's chemical composition and morphology. This study aims to enhance our understanding of structure-reactivity relationships for a rational electrocatalyst design.
This research was carried out within the framework of the Transregio 247 project funded by DFG and was also supported by the BMBF project CATLAB. The results of this study have been published in Nature Communications.
JOURNAL
Nature Communications
ARTICLE TITLE
Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction
