Transition metal intercalated poly(heptazine imides) for electro- and photocatalytic water splitting

Overall water splitting, as a sustainable pathway to obtain clean energy, can be achieved via electrocatalytic and photocatalytic reactions. However, neither of them has yet been widely applied in the industry. Thus, the achievement of electro- and photocatalysts for overall water splitting is still in high demand. \nCarbon nitride has been commonly applied in both electro- and photocatalysis over the past years. For electrocatalysis, the abundant and well-dispersed nitrogen sites of carbon nitride can act as efficient metal coordinated sites, which is crucial for composing a type of high-performance electrocatalysts—M-Nx electrocatalysts. In the field of photocatalysis, carbon nitride, as a metal-free semiconductor, has risen as a promising candidate for multiple photocatalytic applications due to its favorable band position. In fact, many state-of-the-art photocatalysts for visible-light-driven hydrogen evolution are based on carbon nitride. \nHowever, carbon nitride has its limitations. The low conductivity and electrochemical stability of carbon nitride have restricted its performance in electrocatalysis. Therefore, carbon nitride must be mixed with carbon additives to perform as electrocatalyst. For photocatalytic water splitting, in spite of carbon nitrides’ high activity in catalyzing hydrogen evolution, it is barely active in catalyzing the other half-reaction, water oxidation. This is because the valence band of carbon nitride is usually not positive enough to drive this half-reaction. \nPoly(heptazine) imide (PHI) is an ionic carbon nitride, which not only inherits the attractive properties of carbon nitride, but also overcomes many of the aforementioned limitations. Unlike the polymeric carbon nitrides, which are prepared by condensing urea, melamine, cyanamide, dicyanamide, etc., PHI is prepared by salt-melt assisted condensation of triazole or tetrazole. The polymeric carbon nitride is composed of heptazine units connected by amine bridges. The heptazine units in PHI are linked by deprotonated imides, whose charge is counterbalanced by metal cations introduced by the salt template. \nIn this thesis, a series of transition metal, noble metal, and bi-metal intercalated poly(heptazine) imide (noted as PHI-M, M = Co, Ni, Cu, Fe, Mn, Pt…) is synthesized via facile salt melt assisted condensation of tetrazole precursor for modifying the composition of the salt template. The structures of these PHI-M’s are systematically characterized, proving that the PHI backbones stay intact after the intercalation of different metal cations. \nThe synergetic effects between the intercalated metal cations and the PHI backbone are also studied in this thesis. On the one hand, by altering the composition of the metal salt in the salt template, the metal species intercalated in the PHI backbone can be changed accordingly. This property not only makes it easy to embed different metal active sites into the PHI backbones for various catalytic reactions, but also shown to be an efficient pathway to tune the band position of PHI-based photocatalysts. On the other hand, the abundant negatively charged nitrogen sites and metal cations in the PHI backbone remarkably enhance the charge mobility of the material, enabling it to work as an electrocatalyst without conducting additives. \nThe catalytic activities of cobalt and nickel intercalated poly(heptazine) imides (PHI-Co and PHI-Ni) were evaluated, and they turned out to be highly active electro- and photocatalysts. PHI-Co shows excellent electrocatalytic OER activity and photocatalytic water oxidation activity. As an OER electrocatalyst, the overpotential (j = 10 mA/cm²) of the best-performing PHI-Co-0.5 (Co content: 0.45 wt%) is 0.324 V, standing among the best polymer-based electrocatalysts. When applied in photocatalysis, the most active PHI-Co-0.1 (Co content: 0.08 wt%) shows a visible-light-driven oxygen generation activity of 589 µmol h-1 g-1, which is one of the most active catalysts that have been reported for water oxidation. Furthermore, PHI-Ni is highly active in catalyzing visible-light-driven hydrogen evolution. PHI-Ni-0.7 (Ni content 0.74 wt%), which shows visible-light-driven hydrogen generation activity of 1996 µmol g-1 h-1, more than 14-fold higher than Ni-free PHI, and 42-fold higher than polymeric CN.