New photocatalysts have been synthesized with inorganic clusters of polyoxometalates (POMs) and graphitic carbon nitride (g-C3N4) under hydrothermal conditions. In this research, for the first time, a Preyssler-type polyoxometalate (Na14[NaP5W30O110]·xH2O) with g-C3N4 was applied to design P5W30/g-C3N4, and photoelectrocatalytic water splitting with a g-C3N4/P5W30 photoanode was investigated. All of applied characterization methods verified the preparation of g-C3N4 nanorods and the successful compositing of POMs with g-C3N4. The photoelectrocatalytic results reveal that the photocurrent density of P5W30/g-C3N4 (0.3 : 1), of around 44 μA cm−2, is greater than those of g-C3N4, P5W30/g-C3N4 (0.1 : 1), or P5W30/g-C3N4 (0.5 : 1). The lower slope of the transient open circuit potential decay for P5W30/g-C3N4 (0.3 : 1) in comparison with other photocatalysts demonstrates more efficient separation and lower recombination rate of photo-induced charge carriers. Also, the constant photocurrent density in chronoamperometry analysis confirms a stable photoelectrocatalytic performance during three cycles under light irradiation for all of the prepared photoanodes. Mott–Schottky plots displayed a deeper band bending for P5W30/g-C3N4 (0.3 : 1) compared to that of g-C3N4, implying a greater charge carrier density, which leads to greater electron accumulation on its surface and simplifies electron transfer at the photoelectrode/electrolyte interface.

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