Controlled Growth of Lightweight Metal-Free Materials for Photoelectrochemical Cells

Project Information

MFreePEC

Grant agreement ID: 849068

DOI

10.3030/849068

Project closed

EC signature date 18 September 2019

Start date 1 January 2020

End date 31 December 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 495 850,00

EU contribution

€ 1 495 850,00

Coordinated by

BEN-GURION UNIVERSITY OF THE NEGEV
Israel

Periodic Reporting for period 3 - MFreePEC (Controlled Growth of Lightweight Metal-Free Materials for Photoelectrochemical Cells)

Reporting period: 2023-01-01 to 2024-06-30

Summary of the context and overall objectives of the project

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).

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

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 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.
One example of the merits of using melem is the ability to form less-volatile precursors. this allowed us to synthesize CN photoanodes at shorter times (in the 5–20 min range) and higher temperatures. these photoanodes have high activity over a wide pH range, even in acidic conditions, which are uncommon for water-splitting using CNs.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

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.
In addition, various additives would be probed as oxidation catalysts.
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, benzylamine oxidation).