Unleashing solar energy's full potential: Synergetic thermo-photo catalysis for enhanced hydrogen production with metal-free carbon nitrides

•Mesoporous carbon nitride with 2.3 eV band gap is synthesized.•Full-spectrum solar energy is efficiently converted to hydrogen at relatively mild temperature.•The microscopic intricacies of the synergistic interaction between photonic and thermal energies are analyzed.•The thermo-photo reactions with and without an external heat source are compared. Conventional photocatalytic hydrogen production processes are limited by their dependency on photons possessing energy levels surpassing the material's bandgap, thus restricting their ability to utilize the full solar spectrum. Conversely, in thermal catalysis, although the entire solar spectrum is accessible, high-energy photons remain underutilized, and reaction temperatures are often elevated. To address these issues, thermo-photo catalysis is proposed to effectively harness the full spectrum of solar energy under gentle conditions. In this study, a narrow-bandgap carbon nitride catalyst with a bandgap of 2.3 eV was synthesized for the specific purpose of hydrogen production from water through thermo-photo catalysis. Results revealed that under thermo-photo catalytic (TPC) conditions, impressive hydrogen generation rates of 3009.6 μmol·g−1·h−1 and 7090.7 μmol·g−1·h−1 were achieved without and with the application of an external heat source, respectively. Both of them substantially surpassed the cumulative hydrogen production rates achieved via photocatalytic (PC) and thermocatalytic (TC) pathways, demonstrating a remarkable synergy between photonic and thermal energies in the hydrogen evolution process. The synergetic factor with an external heat source applied (2.29) was lower than that without an external heat source (3.02), indicating that there is a better synergy between the photo energy and the thermal energy provided by photothermal conversion than the thermal energy supplied by external heat sources. The properties of photo-generated carriers under various temperature regimes were further examined by photoelectrochemically tests, and electron paramagnetic resonance experiments elucidated the pivotal influence of both photo and thermal energy on radical species, offering a microscopic understanding of the mechanism responsible for the enhanced hydrogen production rate in thermo-photo catalysis.