Design Principles for Optimizing the Performance of Multicomponent Water Splitting Photoelectrocatalysts

Abstract

Affordable solar water splitting is considered the “Holy Grail” to transition our current hydrocarbon economy to a sustainable hydrogen economy. The goal is to immerse a photoelectrocatalyst in water and shine sunlight to produce hydrogen for long term energy storage and energy generation. The most common design for solar water splitting consists of light-absorbing semiconductors integrated with electrocatalysts. However, the widespread application of these multicomponent photoelectrocatalysts is limited by low efficiencies and poor stability of the materials. Throughout the past decade, research has demonstrated that insulator materials can stabilize semiconductors in metal–insulator–semiconductor (MIS) structures in which the metal layer acts as an electrocatalyst while the insulator layer protects the underlying semiconductor. Further research has demonstrated that the thickness of the insulator layer also plays a critical role in modulating the recombination rates and the performance of MIS systems. Despite significant improvements in the efficiency of MIS photoelectrocatalysts, rigorous guidelines are still needed to optimize the efficiency and approach the upper performance limits.

The overall goal of this dissertation is to shed light on the critical role of the interfaces on the performance of MIS photoelectrocatalysts and to leverage the insights to optimize their efficiency. The first scope of this dissertation focuses on the concrete example of planar n-type Si protected by a HfO2 or Al2O3 insulator layer and attached to a planar Ir electrocatalyst that completes the oxygen evolution reaction, which is one of the water splitting half-reactions. This work utilizes theory and modeling to design electrochemical experiments and quantify key parameters to evaluate the overall performance and the theoretical upper limits. The results demonstrate that the typical strategy of tuning the insulator thickness is insufficient to achieve the maximum performance. To approach the upper limits, MIS photoelectrocatalysts must overcome efficiency losses attributed to the presence of interfacial defect states, parasitic light absorption/reflection, and insufficient charge carrier selectivity of the insulator. Based on the insights from this combined experimental and modeling work, strategies such as annealing the interfaces and doping the insulator layer are implemented to optimize the performance of the MIS photoelectrocatalysts.

The second scope of this dissertation focuses on Ni nanoparticles electrodeposited on planar n-type Si which have previously demonstrated good performance and stability without an intentional insulator layer. The stability is enabled by the anodic passivation of the Si surface to form a SiO2 layer, and the high efficiency is typically attributed to “pinched-off” nanoparticles that decrease the recombination rates. Contrary to this common perception, the results herein demonstrate that an adventitious SiO2 insulator layer at the nanoparticle Ni/Si interface may be the primary explanation for the high efficiency of these photoelectrocatalysts. In other words, an interfacial insulator layer can significantly improve the performance for nanoparticle-based systems via a mechanism similar to traditional planar MIS systems. Overall, this collective work demonstrates the critical importance of the interfaces in optimizing the performance of solar water splitting systems and offers design principles that are broadly relevant to the field of photoelectrocatalysis.