Two-dimensional conjugated polymer frameworks for solar fuel generation from water

Solar-to-chemical energy conversion through artificial photosynthesis is an ideal route to address the global energy crisis and realize carbon neutrality in the future. Over the past decade, two-dimensional conjugated polymer frameworks (2D CPFs), including conjugated microporous polymers, covalent organic frameworks, and covalent triazine frameworks, have emerged as a promising class of photocatalysts for solar fuel generation. They exhibit highly tunable chemical and optoelectronic structures which can be precisely controlled at the molecular level. Meanwhile, the 2D planar structure with in-plane periodicity offers many unique features for solar-driven catalytic energy conversion, including large surface areas, high absorption coefficients, efficient charge transport, and facile formation of heterostructures. In addition, their surface active sites can be rationally constructed from numerous molecular building blocks to optimize their photocatalytic performances. Herein, we comprehensively summarize recent progress in developing 2D CPFs for solar fuel generation from water, including photocatalytic overall water splitting, hydrogen peroxide production, carbon dioxide reduction, and nitrogen fixation. Basic principles in these photocatalytic reactions are described. In-depth insights into the structure-property relationships between 2D CPFs and their reaction mechanisms are discussed in detail. Moreover, recent advances in applications of 2D CPFs in photoelectrochemical energy conversion are also highlighted. Finally, the remaining challenges and research opportunities for the future development of efficient 2D CPFs toward solar fuel generation are presented.