The photocatalytic production of H₂ using sunlight is considered a sustainable solution to fulfill the global energy demand and reduce the emission of greenhouse gases generated from the burning of fossil fuels. H₂ has the highest energy density (120–140 MJ kg⁻¹) and produces only water as a combustion product when it reacts with O₂. Therefore, it is regarded as one of the best possible contenders to meet the future energy demand. In this respect, developing efficient semiconductor-based photocatalysts is crucial to make the photocatalytic H₂ evolution commercially viable. Although a series of metal-based semiconductors have been explored for the photocatalytic H₂ evolution, photo-corrosion of these materials makes them impractical for long-term application. In contrast, utilization of the metal-free polymeric graphitic carbon nitride (g-CN) was beneficial to attain excellent photocatalytic activity and long term-stability (even for months). However, the poor electronic conductivity of g-CN results in charge recombination and poor charge transport to make the overall photocatalytic process inefficient. Nevertheless, effective utilization of cocatalyst like Pt can significantly improve the charge separation and transport. The high cost and scarcity of Pt lead to finding out transition metal-based cocatalysts, and the challenge is to reach an excellent photocatalytic activity and stability with transition metal-based cocatalysts when combined with g-CN. Hence, a tremendous effort has been provided to integrate transition metal-based cocatalysts with g-CN to achieve excellent photocatalytic activity, high quantum efficiency (QE), and long-term stability. Although non-noble metal-based cocatalysts attain major attention for the photocatalytic H₂ evolution with g-CN, its role in improving the charge separation, recombination, and hence the H⁺ reduction activity was never reviewed. This review focuses on designing transition metal-based cocatalysts and their application with g-CN to improve the H₂ evolution activity by enhancing the charge separation, transport, and minimizing the recombination of charge carriers. The basic principles of the cocatalyst design, their combination with g-CN, and the mechanism of electron-hole separation, charge transport, and recombination have been described. Besides, the challenges in this field have been addressed with a possible solution. Overall, this review deals with the fundamentals of photocatalytic hydrogen evolution with g-CN, recent progress in this field, and efficient utilization of the transition metal-based cocatalysts to boost photocatalytic activity.

利用阳光光催化生产 H ₂被认为是满足全球能源需求和减少化石燃料燃烧产生的温室气体排放的可持续解决方案。H ₂具有最高的能量密度（120-140 MJ kg ^{-1 ），并且当它与O} ₂反应时仅产生水作为燃烧产物。因此，它被认为是满足未来能源需求的最佳竞争者之一。在这方面，开发高效的基于半导体的光催化剂对于使光催化 H ₂演化具有商业可行性至关重要。尽管已经探索了一系列金属基半导体用于光催化 H ₂进化，这些材料的光腐蚀使它们不适合长期应用。相比之下，使用无金属聚合物石墨氮化碳 ( g -CN) 有利于获得优异的光催化活性和长期稳定性（甚至数月）。然而，g的电子导电性差-CN 导致电荷重组和电荷传输不良，使整个光催化过程效率低下。然而，有效利用 Pt 等助催化剂可以显着改善电荷分离和传输。Pt 的高成本和稀缺性导致寻找过渡金属基助催化剂，而挑战是与g -CN 结合使用过渡金属基助催化剂时达到优异的光催化活性和稳定性。因此，为了实现优异的光催化活性、高量子效率（QE）和长期稳定性，人们付出了巨大的努力将过渡金属基助催化剂与g -CN 结合起来。尽管非贵金属基助催化剂在光催化 H ₂与g -CN 的演变，其在改善电荷分离、重组以及 H ⁺还原活性方面的作用从未被审查过。本综述的重点是设计过渡金属基助催化剂及其与g -CN 的应用，以通过增强电荷分离、传输和最小化电荷载流子的重组来提高 H _{2析出活性。}助催化剂设计的基本原理，它们与g的结合-CN，以及电子-空穴分离、电荷传输和复合的机制已经被描述。此外，已通过可能的解决方案解决了该领域的挑战。总体而言，本综述涉及g -CN 光催化析氢的基本原理、该领域的最新进展以及过渡金属基助催化剂的有效利用以提高光催化活性。