Periodic Reporting for period 2 - MFreePEC (Controlled Growth of Lightweight Metal-Free Materials for Photoelectrochemical Cells)
Reporting period: 2021-07-01 to 2022-12-31
In the project "Controlled Growth of Lightweight Metal-Free Materials for Photoelectrochemical Cells" we address one of the main challenges of developing sustainable energy production.
To this end, we aim to develop different metal-free materials, focusing on the family of carbon-nitride-based polymers with different heteroatoms to be used as the photoactive ingredient in photoelectrochemical cells (PEC).
These cells absorb solar light, thus reducing the required energetic input for performing liquid-phase reactions for the production of fuels. The most common PEC is the water-splitting cell, where electrons reduce water to hydrogen (the clean fuel) on the cathode, while on the other side (the photoanode), water is oxidized to oxygen.
This is a very promising concept, yet despite decades of research for real-world application, significant scientific challenges have yet to be met.
At the core of this MFREEPEC project is the development of photoactive materials. We introduce a new class of metal-free materials that are particularly suitable as semiconductors in PECs through the development of new strategies for the controlled synthesis and growth of metal-free materials on various substrates, ranging from carbon nitride to nitrogen-doped carbon and new carbon-nitrogen-phosphorus/boron/sulfur materials (referred here as CNXs, X = P, B or S).
We aim to overcome the current limitations of the traditional synthetic and growth methods for CNXs layers with controlled properties by designing and encoding the elemental composition of the final material at the molecular level. By rationally selecting the reaction monomers, we will target specific properties required for PECs: suitable optical band gap, crystal structure, porosity, layer thickness, and catalytic activity, as well as the design of a beneficial electronic structure for efficient charge separation and collection.
The overall objective is to assemble working PEC systems where the designed photoactive layer surpasses the state-of-the-art performance and allows long-term stability, which is crucial for future commercialization.
Specifically, we improve the properties of the photoactive material(s) (light absorption, charge transport characteristics, and so forth) and the PEC configuration (which layers are used, deposition of transporting/blocking layers, co-catalysts integration, interface engineering, functionalization, etc.).
This research is vital for society as it is essential to provide an alternative way of production of clean fuels (mainly hydrogen) and, if possible, achieve valuable chemical transformations as side reactions (for example, replacing oxygen evolution with other helpful products).
To this end, we aim to develop different metal-free materials, focusing on the family of carbon-nitride-based polymers with different heteroatoms to be used as the photoactive ingredient in photoelectrochemical cells (PEC).
These cells absorb solar light, thus reducing the required energetic input for performing liquid-phase reactions for the production of fuels. The most common PEC is the water-splitting cell, where electrons reduce water to hydrogen (the clean fuel) on the cathode, while on the other side (the photoanode), water is oxidized to oxygen.
This is a very promising concept, yet despite decades of research for real-world application, significant scientific challenges have yet to be met.
At the core of this MFREEPEC project is the development of photoactive materials. We introduce a new class of metal-free materials that are particularly suitable as semiconductors in PECs through the development of new strategies for the controlled synthesis and growth of metal-free materials on various substrates, ranging from carbon nitride to nitrogen-doped carbon and new carbon-nitrogen-phosphorus/boron/sulfur materials (referred here as CNXs, X = P, B or S).
We aim to overcome the current limitations of the traditional synthetic and growth methods for CNXs layers with controlled properties by designing and encoding the elemental composition of the final material at the molecular level. By rationally selecting the reaction monomers, we will target specific properties required for PECs: suitable optical band gap, crystal structure, porosity, layer thickness, and catalytic activity, as well as the design of a beneficial electronic structure for efficient charge separation and collection.
The overall objective is to assemble working PEC systems where the designed photoactive layer surpasses the state-of-the-art performance and allows long-term stability, which is crucial for future commercialization.
Specifically, we improve the properties of the photoactive material(s) (light absorption, charge transport characteristics, and so forth) and the PEC configuration (which layers are used, deposition of transporting/blocking layers, co-catalysts integration, interface engineering, functionalization, etc.).
This research is vital for society as it is essential to provide an alternative way of production of clean fuels (mainly hydrogen) and, if possible, achieve valuable chemical transformations as side reactions (for example, replacing oxygen evolution with other helpful products).
From the beginning of the project, we work in parallel on the different aspects of PEC.
We develop the synthetic pathways using supramolecular chemistry and crystallization to form various CNX materials (successes include sequence-encoded properties in final metal-free lightweight materials—allowing, amongst others, high heteroatom-content as phosphorous in carbon and nitrogen-based materials; studying the influence of alkali metals incorporation into CN materials via growth of single-crystals, especially on the surface; lowering the necessary temperatures for CNX polymer synthesis by tailoring the supramolecular precursors; incorporation of carbon doping for enhanced conductivity and light absorption by addition of carbon-rich monomers that do not participate in the supramolecular arrangement).
We develop various deposition techniques, which include electrophoretic deposition of CNX precursors, the ultrasonic spray of precursors, doctor-blading, and dip-coating, as well as the addition of precursor vapor pressure during calcination for controlled deposition of CNX layers over conductive transparent electrodes. In each method variation, we characterize the structural and photophysical properties to allow the design of better PEC devices.
Thus far, we have mainly focused on constructing photoanodes, where the active ingredient is the CNX layer(s). We have achieved very high hydrogen production rates in the presence of hole scavengers and overall water-splitting without hole scavengers where water is being oxidized. We are among the first to report the faradaic efficiencies of these reactions under long-term operational conditions.
We have successfully deposited an oxidation co-catalyst based on nickel-iron oxyhydroxide inside the porous active carbon-nitride layer, which contains reduced graphene oxide as a conduction enhancer. This system allowed very high performance with the unprecedented long-term stability of more than 35 h under operational conditions (stable and reproducible photocurrents exceeding 0.45 mA/cm2 at 1.23 V vs. NHE in an alkaline environment). A scheme of the synthetic procedure (published in Advanced Functional Materials, 31, 25, 2101724, 2021) is attached.
Another successful development uses a stable intermediate, melem, as the precursor. This precursor, combined with supramolecular chemistry, allows the synthesis of photoactive materials that may be used as a heterogeneous catalyst for hydrogen evolution and carbon dioxide reduction.
We develop the synthetic pathways using supramolecular chemistry and crystallization to form various CNX materials (successes include sequence-encoded properties in final metal-free lightweight materials—allowing, amongst others, high heteroatom-content as phosphorous in carbon and nitrogen-based materials; studying the influence of alkali metals incorporation into CN materials via growth of single-crystals, especially on the surface; lowering the necessary temperatures for CNX polymer synthesis by tailoring the supramolecular precursors; incorporation of carbon doping for enhanced conductivity and light absorption by addition of carbon-rich monomers that do not participate in the supramolecular arrangement).
We develop various deposition techniques, which include electrophoretic deposition of CNX precursors, the ultrasonic spray of precursors, doctor-blading, and dip-coating, as well as the addition of precursor vapor pressure during calcination for controlled deposition of CNX layers over conductive transparent electrodes. In each method variation, we characterize the structural and photophysical properties to allow the design of better PEC devices.
Thus far, we have mainly focused on constructing photoanodes, where the active ingredient is the CNX layer(s). We have achieved very high hydrogen production rates in the presence of hole scavengers and overall water-splitting without hole scavengers where water is being oxidized. We are among the first to report the faradaic efficiencies of these reactions under long-term operational conditions.
We have successfully deposited an oxidation co-catalyst based on nickel-iron oxyhydroxide inside the porous active carbon-nitride layer, which contains reduced graphene oxide as a conduction enhancer. This system allowed very high performance with the unprecedented long-term stability of more than 35 h under operational conditions (stable and reproducible photocurrents exceeding 0.45 mA/cm2 at 1.23 V vs. NHE in an alkaline environment). A scheme of the synthetic procedure (published in Advanced Functional Materials, 31, 25, 2101724, 2021) is attached.
Another successful development uses a stable intermediate, melem, as the precursor. This precursor, combined with supramolecular chemistry, allows the synthesis of photoactive materials that may be used as a heterogeneous catalyst for hydrogen evolution and carbon dioxide reduction.
We plan to continue pushing the envelope of water-splitting performance by designing the layers and photoactive materials alongside heteroatom incorporation.
We envision that these modifications would allow exceeding photocurrent densities of 0.5 mA per square cm while retaining low onset potentials, high faradaic efficiencies, and long-term stability.
Furthermore, we plan to explore performance in less alkaline environments, as well as replace the oxygen evolution reaction with valuable organic transformations (e.g. alcohol oxidation).
We envision that these modifications would allow exceeding photocurrent densities of 0.5 mA per square cm while retaining low onset potentials, high faradaic efficiencies, and long-term stability.
Furthermore, we plan to explore performance in less alkaline environments, as well as replace the oxygen evolution reaction with valuable organic transformations (e.g. alcohol oxidation).