Research progress in conjugated polymers for photoelectrochemical hydrogen evolution reaction

The extensive use of traditional fossil fuels has led to the twin crises of energy scarcity and environmental degradation, driving the urgent search for sustainable alternatives. Photoelectrochemical (PEC) hydrogen-production technology, which converts solar energy into chemical energy, has emerged as a promising approach to producing clean, renewable fuel. Solar energy is abundant, widely available, and environmentally friendly; however, its direct utilization and storage are challenging because of its low power density at Earth’s surface. Hydrogen, by contrast, possesses a high gravimetric energy density and is therefore an ideal clean-energy carrier. PEC water splitting offers an efficient route to green-hydrogen generation by driving the thermodynamically uphill reaction under light irradiation. Continuous exploration and development of PEC materials have greatly improved hydrogen-evolution performance, yet persistent bottlenecks remain unresolved. Early work focused mainly on inorganic semiconductors. TiO2-based catalysts, however, absorb primarily ultraviolet light, leading to poor solar spectrum utilization and suboptimal solar-to-hydrogen conversion efficiency. Other alternative inorganic candidates, such as metal oxides, sulfides, and nitrides, have demonstrated moderate catalytic activity, but their narrow spectral absorption windows, limited stability, and excessive dependence on noble metal co-catalysts hinder large-scale deployment. These limitations are particularly evident in the rapid decay of catalytic activity under prolonged operation, which stems from irreversible structural changes and surface passivation caused by harsh environments. Additional drawbacks, including high material cost, potential metal toxicity, photocorrosion, and performance degradation, further restrict their development. In recent years, organic conjugated polymers have attracted considerable attention for PEC hydrogen evolution because of their distinctive advantages. Their tunable molecular structures enable precise control of optoelectronic properties, while the continuous π-conjugated backbone facilitates efficient charge transport, and their broad spectral response, which extends into the visible and near infrared, allows more effective harvesting of solar energy. Moreover, they are generally inexpensive and chemically robust, making them promising candidates for high-performance photocatalysts. This article systematically reviews progress on organic conjugated polymers, including carbon nitride (C3N4), conjugated acetylenic polymers (CAPs), covalent organic frameworks (COFs), and related hybrid composites, for PEC hydrogen evolution. We focus on molecular structure design, regulation of electron distribution and transfer, mechanisms of photogenerated-carrier separation, and strategies for enhancing catalytic activity. For C3N4, we describe its unique structure, strengths, and limitations, along with modification methods that improve performance. The highly conjugated backbones of CAPs endow these materials with excellent light-harvesting and charge-transfer capabilities; we explore structural optimizations that introduce additional active sites. COFs possess large specific surface areas and tailorable band structures, giving them great potential, and we analyze challenges and solutions associated with their synthesis and application. For composite systems, we examine how interactions among different components facilitate charge separation and boost catalytic efficiency. Finally, we discuss outstanding problems, such as the trade-off between light absorption and charge recombination in some narrow-band-gap polymers, the high cost of materials that still rely on precious metal cocatalysts, and the limited density of active sites, as well as future research directions. Strategic priorities include advancing in situ characterization techniques to unravel dynamic interfacial processes and developing scalable synthesis methods to bridge the gap between laboratory prototypes and industrial applications. We anticipate that this overview will provide strong support for the exploration of other solar-driven reactions, stimulate innovation in catalyst design, and accelerate the development of clean-energy technologies. In summary, a comprehensive understanding of organic conjugated polymers for PEC hydrogen production is essential to address global energy and environmental challenges, and it is expected to open new paths toward the large-scale implementation of sustainable energy solutions.