(187h) Investigating the Corrosion of MoS2 for (photo-)Electrochemical H2 Production Under Diurnal Conditions

Durable performance under outdoor and on-sun conditions is a long standing and critical challenge in the photoelectrochemical (PEC) and solar fuels community. In photoelectrochemical architectures, nm-scale thicknesses of MoS₂ have been utilized to improve the durability of photocathodes driving the H₂ evolution reaction (HER), demonstrating over 100 hours on III-V semiconductors, and for up to two months on Si-based semiconductors under constant conditions. However, the reliability of thin film protective layers is often substantially reduced under varying/diurnal-like conditions and across batches of photoelectrodes — understanding the mechanism of failure of MoS₂-protected photocathodes, as well as other thin film-protected photocathodes, remains a critical challenge to understand the opportunity for these protective layers to meet long-term DOE durability goals. Previous studies have visually observed the macroscopic degradation of MoS₂ using optical microscope imaging, but questions remain surrounding the transient, microscopic and molecular degradation pathways of MoS₂. In this work, we systematically investigated the corrosion reactions occurring at MoS₂ thin films deposited on Si substrates as a function of the electrolyte environment (e.g., acid composition, gas saturation) as well as on applied potential/current density. Using electrochemical methods, coupled to online inductively coupled plasma mass spectroscopy (ICP-MS) and electrochemical Raman spectroscopy, we probed the role of variable and reducing conditions on MoS₂ corrosion. Expanding upon this, we have probed stability of MoS₂ films under variable conditions by applying electrochemical potential profiles to simulate the operation of HER under diurnal, solar-driven electrochemical conditions. We observed corrosion under acidic conditions as a function of anion identity and applied potential, highlighting design and operational principles for durable solar fuels systems under diurnal conditions.