Metal-Free Polymeric Carbon Nitride (CN)-Based Materials
Photoelectrochemical cells (PECs), which directly convert sunlight into chemical fuels, are among the most promising and economically feasible future technologies for producing alternative energy sources. At the heart of a PEC lies a light-harvesting semiconductor that, under solar illumination, converts absorbed photons into excited electronic states (holes and electrons). Depending on the pH conditions, the excited charged states can then be used to electrochemically produce fuels, such as hydrogen, by reducing protons from water to H2(g). PECs based on a semiconductor/liquid electrolyte junction can compete commercially with traditional fossil-fuel-based production methods because they are relatively simple and convert solar energy into fuels. Production of solar fuel utilizing PECs requires low-cost, robust, and highly efficient semiconductors, which should have good conductivity and be able to transfer charge rapidly at the semiconductor/liquid electrolyte interface, display long-term stability, possess good light-harvesting properties, and have a suitable energy band position for the desired reaction. Up to now, the PEC field has been dominated mostly by metal-based materials (oxides, sulfides, (oxy)nitrides, and organo(metallic) dyes), and despite the significant progress in this field, semiconductors that fulfill all the aforementioned requirements as PEC semiconductors do not exist today, and novel materials are still much sought after. Thus, developing suitable semiconductor materials is an immense challenge for allowing PECs to fulfill their role in the energy-devices landscape.
Metal-free light element materials based only on carbon, nitrogen, phosphorus, sulfur, and boron – referred to here as CNs and CNXs (X = P, B, or S) – exhibit electronic, chemical, and photophysical properties that range from insulators through semiconductors to semi-metals. As such, they have the potential to dramatically accelerate the development of numerous fields, such as electronic devices, energy-related applications, lubricants, and fire-retardant materials. The electronic, photophysical, and catalytic properties of these CNX materials depend primarily on the ratios of the different elements and their spatial organization. Simple carbon nitride (CN) materials have been explored as promising photo- and electrocatalysts for various reactions, including water splitting and carbon dioxide reduction, with potential applications in fuel cells, pollutant degradation, and other areas. The incorporation of heteroatoms into their structure results in a remarkable combination of low cost, tunable electronic and catalytic properties, environmental friendliness, and stability under harsh conditions, which positions CNXs as ideal materials for photoelectrochemical applications. However, to date, progress on the utilization of CN and CNX materials for PECs has been limited by the fact that no method is currently available for the preparation of high-quality, large-scale, and homogenous CN layers on a conductive substrate with simultaneous control over the chemical, electronic, and catalytic properties and our lack of basic knowledge of the layer’s photophysical and photoelectrochemical properties. Moreover, the understanding of the physical and catalytic properties of these materials through structure-activity relationships remains poor.
This research was of the ERC Horizon 2020 StG #MFreePEC for CN-based photoelectrochemical cells—see updates in the online blog: 
MFreePEC ERC StG blog
We develop new strategies that allow the controlled synthesis and growth of metal-free materials on various substrates for photoelectrochemical cells, ranging from carbon nitride to nitrogen-doped carbon and new carbon-nitrogen-phosphorus/boron/sulfur materials. We aim to gain a fundamental understanding of their growth mechanisms and the resulting structural, chemical, photophysical, and photoelectrochemical properties. Our approach enables us to overcome the limitations of traditional synthetic and growth methods for CNX layers with controlled properties by designing and encoding the elemental composition of the final material at the molecular level. Our new synthetic path, based on the rational selection of the reaction monomers, taking into account their intermolecular interactions prior to their calcination at high temperatures, allows us to target specific required properties for PEC: 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. This research is highly interdisciplinary, combining materials science, photoelectrochemistry, and supramolecular chemistry. It will open up new avenues in these fields, in particular in the synthesis and deposition of metal-free materials, which hopefully will result in the integration of lightweight materials into energy conversion and other devices.
