Nanostructured diamond-based materials for the photoelectrochemical reduction of molecular nitrogen

• Applicants: Professor Dr.-Ing. Elias Klemm; Professorin Dr. Anke Krüger; Dr. Marcel Risch
• Subject Area: Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 501932605

The sustainable synthesis of ammonia from nitrogen is one of the topical challenges of the chemical industry. In this project, we develop a carbon-based electron source for the photo(electro)chemical reduction of N2 to NH3. The material of choice is diamond. Recently, it has become a widely available material that is being produced on industrial scale by high-temperature and high-pressure synthesis for particles and chemical vapour deposition for extended thin films on substrates up to m2 scale. Diamond is chemically inert, has a high overpotential for the hydrogen evolution reaction, and in doped form possesses a high mobility for electrons and holes. Furthermore, engineered diamond materials offer a unique opportunity to adjust and control the electronic properties for photoelectrochemical nitrogen reduction reaction (N2RR). Namely, the negative electron affinity of diamond enables the emission of highly reductive electrons from the conduction band, which will be the focus of the 2nd funding period. The large bandgap currently requires deep UV excitation to create the desired solvated electrons. Doping, thermal annealing, surface termination/modification, and extensive nanostructuration will be used for knowledge-based engineering of the bandgap and charge transfer for visible light driven N2RR as well as of the conductivity and the surface with the goal to enhance the product yield and selectivity of the electrochemical N2RR. Our work on product analysis, synthesis and cell development in the 1st funding period lays the ground for further development toward the proposed photoelectrochemical studies. First, the design and synthesis of modified diamond materials will aim at adjusting and understanding structure-activity relationships separately for electrochemical and photochemical N2RR. Therefore, a thorough characterization of these materials is necessary, especially an ex situ and in situ characterization by laboratory-based UV-Vis, electrochemical impedance spectroscopy (EIS), XPS and synchrotron-based X-ray spectroscopy to map the density of states near the Fermi level which can be populated by light irradiation. Adsorption of nitrogen and its intermediates will be studied using electrochemical gravimetry and EIS to elucidate whether the solvated electrons react as an outer-sphere process as expected. The population of adsorbed intermediates and hence product yield for ammonia will be optimized by pulsed potential operation. With these experimental approaches at hand, structure-activity relationships can be identified and rationalized for knowledge-guided optimization of the N2RR. These results will be used to implement tailored diamond materials in the photo(electro)chemical reduction of nitrogen and the identification of optimal material-reactor combinations.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2370: Interlinking catalysts, mechanisms and reactor concepts for the conversion of dinitrogen by electrocatalytic, photocatalytic and photoelectrocatalytic methods (“Nitroconversion”)