Structural dependence of diamond composites on electrocatalytic performance of small molecules

Electrocatalysis of small molecules is a sustainable solution to help the global energy crisis and meanwhile to reduce environmental pollution. The key to reduce energy losses during the electrocatalytic processes is the rational design of efficient and robust electrocatalysts under practical operation conditions. However, commercial metal based catalysts such as Pt/C and RuO2 are suffer from their high costs, scarcity, susceptibility to poisoning, and single reaction sites, limiting their large-scale applications. Moreover, the most common catalytic reactions of small molecules such as oxygen reduction/evolution reaction (ORR/OER), CO2 reduction reaction (CO2RR) and nitrate reduction reaction (NITRR) are multi-electron reactions, where sluggish kinetics exist together with complex reaction pathways. Such traditional single metal active sites are not sufficient to overcome such challenges. Therefore, it is urgent to deeply understand the reaction mechanisms, to design the heterogeneous catalysts, to tune the structure and composition of the multiple components.
Diamond-based electrodes have been widely employed in electrochemical sensing and advanced electrocatalytic oxidation of pollutants, due to their broad potential windows, low background currents, high chemical stability, and resistance to toxication. As for the electrocatalysis of small molecules, the broad potential window means the thermodynamic inhibition of hydrogen evolution, which ensures the selectivity of ORR, CO2RR and NITRR. Meantime, the robust structure provides sufficient stability and recycling possibilities in extreme environments. However, the diamond-based electrodes feature poor electrocatalytic performance, due to the lack of active sites. Creation of or the increase of active sites together with the balance of their activity and stability still remains challenging.
This thesis combines the density functional theory (DFT) calculations with experiments to construct efficient and robust diamond composites, which are employed as the catalysts for the electrocatalysis of small molecules such as oxygen and nitrate. Deep understanding of the relationship between catalyst structures and electrocatalytic performance is further desired to develop their practical application in energy devices.
Concerning the oxygen electrocatalysis, namely, the ORR and OER, the nitrogen doped carbon nanowalls/diamond (N-CNWs/D) composites are synthesized by means of microwave plasma enhanced chemical vapor deposition (MPCVD) technique, consisting of a thin nanodiamond layer at the bottom and a vertically aligned multigraphene on the surface. After annealing post-treatment, the composites feature superior ORR performance (high onset potential of 835 mV vs. Standard hydrogen electrode, RHE) and excellent stability (90% current retention after 20 h). It is attribute to the activated and exposed edge sites and a bonded nanodiamond layer. The Co4N@d-NCNWs/D composites are further synthesized and employed as the ORR/OER bifunctional catalysts. In more detail, the DFT calculations predict that the enhanced metal-supporting interaction between the Co4N and d-NCNWs/D promotes the anchoring and dispersion of the Co4N nanoparticles. It also adjusts the electronic structures of Co and C atoms at the interface, reducing the overpotential of ORR/OER from 0.67 to 0.23 eV. Experimentally, the composites feature superior ORR/OER bifunctional performance (e.g., a small potential gap of 0.75 V). Since these composites are directly grow on carbon cloth collectors, they are utilized as air electrodes. The assembled flexible zinc-air batteries exhibit a high open circuit voltage of 1.41 V and excellent bending stability.
With respect to the nitrate electrocatalysis, the copper nanoparticles coated boron-doped diamond mesh (Cu-np@BDD-m) composites are synthesized by means of hot-filament chemical vapor deposition (HFCVD) and magnetron sputtering. After the application of an etching process at a low pressure, Cu nanoparticles are embedded in the BDD-m surface, leading to a strong metal-supporting interaction or a nanoconfined trapping area. The DFT calculations reveal that this strong interaction enhances the adsorption of nitrogen-contained intermediates (e.g., *NO3, *NO2, *N), minimizes the reaction energy through NITRR from 1.06 to 0.91 eV, and raises the energy barrier of competing HER from 0.50 to 0.97 eV at a neutral medium. Consequently, the composite features superior NITRR performance (e.g., nitrate removal of 90.2%and ammonia selectivity of 76.8%). The resultant nano-confined area stabilizes the active sites and enhanced the local adsorption of nitrate, eventually upshifting the diffusion-controlled first-step onset potential (from -0.25 to 0.1 V vs. RHE). In this way, the NITRR mechanism is elaborated in thermodynamic and kinetic processes. Such an electrode promises to assemble liquid flow reactors for sustainable applications.
In summary, efficient and robust diamond composites based catalysts are fabricated for the electrocatalysis of small molecules such as oxygen and nitrate. The balance of their activity and stability is achieved via the construction of the rational support structure, the modification of the active sites on the surface, and the enhancement of the metal-supporting interactions. The catalyst design approach proposed in this thesis is promising to be applied for the electrocatalysis of other small molecules and further for the assembly of energy devices of practical applications in future.