Solar energy, a clean and sustainable energy source, offers a promising solution to global energy demands and environmental concerns. Photoelectrochemical (PEC) water splitting, a process that directly converts sunlight into hydrogen fuel, represents a compelling strategy for harnessing solar energy. PEC fuel cells typically consist of light-absorbing photoelectrodes (photoanode and/or photocathode) and an electrolyte. The development of highly efficient photoanodes for oxygen evolution has attracted considerable attention, as water oxidation is the rate-determining step in water splitting [1,2]. The development of highly efficient photoanodes for oxygen evolution has attracted considerable attention, as water oxidation is the rate-limiting step in water splitting. Herein, graphitic carbon nitride (g-C3N4) and FeNi NPs were successfully synthesized and hybridized into various structures, including composites, mixtures, and layer-by-layer assemblies, to enhance their PEC performance. The PEC performance of these photoelectrodes was evaluated on fluorine-doped tin oxide (FTO) substrates. Among the prepared photoelectrodes, the layer-by-layer structure exhibited the best performance for water oxidation, achieving a photocurrent density and an intrinsic solar- to-chemical conversion efficiency of 0.31 mA cm-2 at 1.23 V vs. RHE and 0.3 %, respectively. These values are 2, 3, and 6 times higher than those of the hybrid composite, mixing, pristine g-C3N4 photoanodes, respectively. Also, the photoanodes exhibited exceptional stability over a 3 h period. Photoluminescence and UV-Vis diffuse reflectance spectra, charge separation efficiency, and the efficiency of photogenerated holes that reach the electrode/electrolyte interface and are injected into the electrolyte for o injection into the electrolyte for water oxidation confirm that the presence of FeNi as active sites on the surface of the layer-by-layer photoanode enhances light absorption, promotes charge separation, and increases the efficiency of charge carrier injection at the photoanode/electrolyte interface. These findings establish this photoanode as a promising candidate for practical applications.

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