After the basic physical properties are satisfied, the photocatalytic performance will be finally determined by its catalytic activity to facilitate the thermodynamics of HER and OER. The overall water splitting can proceed only if the driving force provided by the photo-generated electrons and holes (U e and U h as illustrated in Figure ) are higher than the thermal dynamic overpotential (OP) of HER and OER, respectively, so that the associated ΔG in both cases is negative. However, the sluggish 4e process of OER typically is the bottleneck of photocatalytic efficiency. Although many 2D polymers were designed and synthesized to facilitate the HER process, few of them can overcome the high OP of OER because of the unfavorable band edge alignment (contributing to insufficient value of U h ). Two well-known polymers that can photocatalyze OER are g-C 3 N 4 and CTF-1 , however, demanding the assistance of sacrificial electron acceptors.