In the current work, the heterostructures highly PEC activity photoelectrodes composed of mixed metal oxides (TiO2-RuO2-Ta2O5 (TRT)), polypyrroleN doped reduced graphene oxide (PPy-NrGO) were prepared by electrodeposition of PPy-NrGO films on the TRTcoated titanium (Ti) plates as substrates. The TRT coatings were deposited on Ti plates via the sol-gel technique. The effects of the Ta2O5 molar ratios were followed using physical and PEC measurements. Morphology studies showed that the surface structure of photoelectrodes alters remarkably by changing the Ta2O5 content in the mixture of metal oxides. TRT/PPy-NrGO photoelectrodes indicated to have a decreasing impact on the charge transfer resistance at the photoelectrode/electrolyte interfaces. The overall PEC water splitting performances of photoelectrodes were compared by measuring photocurrent density at 0 and 1.23 V vs. RHE, Tafel slopes, charge carrier density, stability, hydrogen evaluation reaction (HER) and oxygen evaluation reaction (OER) efficiencies. The highest HER (43.68% at -0.4 V) and OER (17.51% at 0.45 V) efficiencies were obtained in photoelectrodes comprising of 20 mol% Ta2O5 which possessed charge carrier density (5.9 x 1018) of about 1.7 times higher than that considered for TRT (0%)/PPy-NrGO with no Ta2O5 in the mixture of metal oxides. (C) 2020 Elsevier Inc. All rights reserved.