Sustainability insights into photocatalytic CO₂-to-CH₄ conversion utilizing g-C₃N₄-based photocatalysts: Flaws, progress, and prospectives

Atmospheric carbon dioxide (CO₂) levels increased from 419.3 ppm (ppm) in 2023 to 425.4 ppm in 2024, 51 % above preindustrial levels, with emissions rising from 37.01 to 37.41 billion metric tons. This surge has driven record global temperatures, with annual mean surface temperatures reaching 1.45 °C and 1.55 °C above preindustrial averages in 2023 and 2024, intensifying glacial melt, sea-level rise, and extreme weather events. Photocatalytic CO₂ reduction to methane (CH₄) has emerged as a promising pathway to mitigate CO₂ emissions while producing an energy carrier compatible with existing infrastructure. This review focuses on advancements in photocatalysts based on graphitic carbon nitride (g-C₃N₄) for photocatalytic CO₂-to-CH₄ conversion, offering a sustainability-centered perspective often overlooked in previous reviews. In addition to summarizing progress in dopant integration, heterojunction formation, and charge separation strategies, this work critically examines the environmental footprint of precursor materials and the energy consumption of thermal, microwave, and plasma-assisted synthesis methods. It highlights the potential of solar and LED light sources as scalable, energy-efficient alternatives to conventional lamps. The review also addresses practical challenges such as CO₂ purification, impurity tolerance in exhaust-fed systems, and the feasibility of CH₄ recovery and separation. Furthermore, it explores the integration of photocatalytic CO₂ reduction with emerging approaches like direct air capture, electrocatalysis, biophotocatalysis, and computational tools including machine learning and multiphysics modeling. By aligning CH₄ production performance with system-level sustainability criteria, this review establishes a framework to guide future developments toward environmentally viable photocatalytic CO₂-to-CH₄ technologies.