The fabrication of particle-based photoelectrodes by coating or dipping procedures-similar to the scaled fabrication of battery electrodes-can be a route to overcome the efficiency-cost tradeoff of photoelectrochemical (PEC) water splitting devices. Additional strategies for practical and economically competitive PEC approaches include the use of stable ternary metal oxides with complex mesostructures and/or the use of tunable bandgap material. Identifying and quantifying the key parameters limiting the efficiency of the photoelectrodes is fundamental to providing material and mesostructural design guidelines. This quantification is experimentally not accessible given by the multi-physical nature of the processes taking place in the photoelectrodes. Computational modeling can provide the necessary insights but requires the detailed knowledge of material parameters that are often unknown for new photoelectrode materials and requires to account for the complex mesostructured of the photoelectrode. In this thesis, the development of versatile and validated computational models that allow for the material characterization, material parameters optimization, and mesostructural optimization of photoelectrodes is presented.