3 November 2020

[Polymers] – How are Conjugated Microporous Polymers Used in Solar Fuel Production?

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CMPs - Codex International

Conjugated microporous polymers, CMPs, are a sub-class of porous materials linked to structures such as zeolites, metal-organic frameworks and covalent organic frameworks. They are a unique material, combining extended π-conjugation with a permanently microporous skeleton; they are amorphous rather than crystalline and possess conductivity, mechanical rigidity and insolubility.

CMPs are 3D semiconducting polymers in which firm aromatic groups, such as alkynes, are linked together in a π-conjugated fashion. It is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and double or triple bonds. This may lower the overall energy of the molecule and increase stability, with the alkynes responsible for the material’s microporosity.

The wide range of synthetic building blocks and network forming reactions offers an enormous variety of CMPs with different properties and structures. The building blocks possess a broad diversity in π units, making them attractive because they allow for tuning and optimization of the skeleton and properties of CMPs.

History of Conjugated Microporous Polymers
CMPs were discovered in 2007, the first example being microporous poly(aryleneethynylene). It was formed by Sonogashira-Hagihara cross-coupling alkynyl arene monomers with halogen-bearing aromatics. The reaction joined an aryl halide with an alkyne-containing monomer using a palladium catalyst and a copper cocatalyst in the presence of an amine base.

The first CMP had a Bunauer-Emmett-Teller (BET) surface area of up to 834 m2g-1 – this describes the physical adsorption of gas molecules on a solid surface. This serves as the basis for an important analysis technique for measuring the specific surface area of materials.

By 2008, CMPs with a BET surface area exceeding 1000 m2g-1 had been developed, and by the next year, PAFs (porous aromatic frameworks) were established. These are closely related to CMPs but do not have extended π-conjugation, rather they are linked by tetrahedral tetraphenylmethane nodes.

Also, that year, poly(p-phenylene) CMP networks were shown to exhibit photoluminescence properties, meaning they could be useful in organic light-emitting-diode (OLED) applications.

Since their discovery, many scientists worldwide have contributed to the field of CMP chemistry, resulting in a solid growth in publications over the last decade. During this time, various properties of CMPs have been revealed.

Applications of CMPs
CMPs have applications in gas storage, heterogeneous catalysis, light-emitting and harvesting, and electric energy storage. They also have usage in photoredox catalysis, energy storage, biological, and photocatalytic H2 evolution. They can be applied in areas that take advantage of their electronic properties and porous nature; for example, the pores can be filled with an inorganic material such as titanium dioxide for use in photovoltaics or processed to serve as electronic junctions.

The largest area of research so far is in the storage of gases, and adsorption – the adhesion of atoms, ions and molecules from a gas, liquid or dissolved solid to a surface. The synthetic control over structure and composition in CMPs offers approaches to increase adsorption capacity and selectivity. However, the use of expensive transition metals in many CMP syntheses prohibits their use in large-scale adsorption applications.Storage of hydrogen, methane and carbon dioxide are the most extensively studied areas. Hydrogen and methane research is driven because the gases have potential use as fuels. Carbon dioxide sorption is an important area of research because of its role as a primary greenhouse gas, and its contribution to global warming and acidification of the oceans.

The original CMP, poly(aryleneethynylene), has a role in the storage of hydrogen – it has a modest uptake of the gas, and also exhibits sorption of carbon dioxide. Tuning of the structure can lead to even higher storage capacities.

Solar Fuel Generation
The solar fuel generation has attracted vast research interest as an environmentally favorable means of producing energy from sunlight to meet the world’s ever-growing energy demand.

Solar fuel is a synthetic chemical fuel produced from solar energy either by photochemical, photobiological, thermochemical and electrochemical reactions. Light acts as the energy source, with solar energy being converted to chemical energy, typically by reducing protons to hydrogen, and carbon dioxide to organic compounds.

Hydrogen is viewed as an alternative source of energy to replace fossil fuels, especially where storage is necessary. In 2017, it was reported that CMPs could directly split water into hydrogen and oxygen using electrolysis under visible light, although the mechanism for how this works remains unclear. A photoelectrochemical cell is used, where a photosensitized electrode converts light into an electric current, which splits water into its constituent elements.

More recently, it has been noted that a copper porphyrin-based CMP might be utilized for both hydrogen evolution and oxygen evolution to allow overall water splitting. And the same can be done for carbon dioxide, splitting it into carbon monoxide or methane with the use of an appropriate photocatalyst.

Much research has focused on carbon nitride and inorganic semiconductors for photocatalytic hydrogen production, but problems with tunability and low activity in visible light restrict such materials. An organic porous push-pull polymer has been reported but requires preparation as a composite with titanium dioxide.

The microstructure and electronic properties of CMPs can be controlled by carefully designing the combination of building blocks; this provides possible advantages for water splitting photocatalysts. Such CMPs exhibit a high level of porosity.

In 2017, researchers reported the first CMP that showed overall photocatalytic water splitting in pure water. They used conventional 1,3,5-diyne-linked CMPs prepared in a nanosheet structure, which led to the concurrent generation of hydrogen and oxygen under visible light irradiation.

They believed the nanosheet structure allowed the photogenerated excitons to instantaneously reach the polymer surface to drive redox reactions. Hydrogen and oxygen were generated close to their expected 2:1 ratio for overall water splitting.

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