However, free energy can be stored within the chemical bonds of chemical compounds such as hydrogen, and controllably released through exothermic reactions without emitting greenhouse gases . Photoelectrochemical (PEC) water splitting is a promising technology to generate and store green energy as it directly converts solar energy into chemical energy by splitting water into hydrogen and oxygen. The PEC water splitting process consists of two half-reactions: the oxygen evolution reaction (OER), occurring through a four-electron process in the photoanode, and the hydrogen evolution reaction (HER), occurring through a two-electron process in the photocathode . The OER is the key limiting step in the PEC water splitting since it is mechanistically more complex and associated with higher overpotentials than the HER. Therefore, a lot of scientific effort has been devoted to developing more efficient photoanode materials .