Sludge reduction and hydrogen production in a microbial photoelectrochemical cell with a g-C3N4/CQDs/BiOBr composite photocathode

Hydrogen (H2) remains a pivotal clean energy source, and the emergence of Solar-powered Microbial Photoelectrochemical Cells (MPECs) presents promising avenues for H2 production while concurrently aiding organic matter degradation. This study introduces an MPEC system employing a g-C3N4/CQDs/BiOBr photocathode and a bioanode, successfully achieving simultaneous H2 production and sludge reduction. The research highlights the effective formation of a Z-type heterojunction in the g-C3N4/CQDs/BiOBr photocathode, substantially enhancing the photocurrent response under light conditions. Operating at - 0.4 V versus RHE, it demonstrated a current density of - 3.25 mA·cm-2, surpassing that of g-C3N4/BiOBr (-2.25 mA·cm-2) by 1.4 times and g-C3N4 (-2.04 mA·cm-2) by 1.6 times. When subjected to visible light irradiation and a 0.8 V applied bias voltage, the MPEC system achieved a current density of 1.0 mA·cm-2. The cumulative H2 production of the MPEC system reached 8.9 mL, averaging a production rate of 0.13 mL·h-1. In the anode chamber, the degradation rates of total chemical oxygen demand (TCOD), soluble chemical oxygen demand (SCOD), total suspended solids (TSS), volatile suspended solids (VSS), proteins, polysaccharides, and volatile fatty acids (VFA) in the sludge were recorded at 57.18%, 82.64%, 64.98%, 86.39%, 42.81%, 67.34%, and 29.01%, respectively.

Graphitic carbon nitride (g-C3N4)-based photocatalytic materials for hydrogen evolution

The semiconductors, such as TiO₂, CdS, ZnO, BiVO₄, graphene, produce good applications in photocatalytic water splitting for hydrogen production, and great progress have been made in the synthesis and modification of the materials. As a two-dimensional layered structure material, graphitic carbon nitride (g-C₃N₄), with the unique properties of high thermostability and chemical inertness, excellent semiconductive ability, affords good potential in photocatalytic hydrogen evolution. However, the related low efficiency of g-C₃N₄ with fast recombination rate of photogenerated charge carriers, limited visible-light absorption, and low surface area of prepared bulk g-C₃N₄, has called out the challenge issues to synthesize and modify novel g-C₃N₄-block photocatalyst. In this review, we have summarized several strategies to improve the photocatalytic performance of pristine g-C₃N₄ such as pH, morphology control, doping with metal or non-metal elements, metal deposition, constructing a heterojunction or homojunction, dye-sensitization, and so forth. The performances for photocatalytic hydrogen evolution and possible development of g-C₃N₄ materials are shared with the researchers interested in the relevant fields hereinto.