A Photoelectrochemical Device with Dynamic Interface Energetics: Understanding of Structural and Physical Specificities and Improvement of Performance and Stability

Abstract The basic configuration of a photoelectrochemical (PEC) water splitting device contains a semiconductor junction, which separates charge carriers by developing interface energetics. Recently, porous metal oxide/semiconductor junctions have shown that flat‐band potentials (V_fb), representing interface energetics, could be dynamically changed with PEC reactions. However, it remains unclear as to what structural and physical specificities of the porous metal oxide induce the dynamic V_fb. Herein, it is demonstrated that the electrolyte permeability and nanocrystal structure of porous NiO_x are crucial for the dynamic V_fb in porous NiO_x integrated Si photocathodes. A comparison of the dense and porous NiO_x with electrolyte impermeable and permeable features, respectively, shows that V_fb changes only in the porous NiO_x. The porous NiO_x also exhibits a nanocrystal structure and increased V_fb values with a decrease in nanocrystal size. As a result of the increased V_fb, the porous NiO_x achieves much higher PEC performance compared to that of the dense NiO_x. However, electrolyte permeability causes electrochemical decomposition of the Si component. Thus, a cointegration of the porous and dense NiO_x bilayers, which ensures stable PEC operation for 10 h while achieving high potentials of 0.2 V versus reversible hydrogen electrode at a photocurrent of 10 mA cm⁻², is proposed.