Record performance achieved from integrated bio- and photo-electrochemical H2 cells.

sunlight is sufficient to supply the ≈18 TW of energy required to meet our current global energy needs [2] and sustain our soci ety's projected energy demands (≈80 TW to carry society through the end of this century). [1][2][3][4] This enormous potential within solar energy systems has aroused great interest in the scientific commu nity. Numerous strategies to harvest solar energy have been proposed and studied, such as photovoltaics, [5] water splitting, [6][7][8][9] and artificial photosynthesis of organic molecules. [10] Of particular interest to this group is photoelectrochemical (PEC) hydrogen production, by means of photo induced electrochemical conversion. PEC systems prove to be promising and prac tical due to hydrogen's abilities as a clean chemical fuel that can be stored, trans ferred, and redistributed to meet our ever increasing energy demands.A key component of PEC systems is the presence of an efficient photoabsorber able to instantly and effectively absorb and convert solar irradiation into electrical energy. Silicon (Si) has been widely inves tigated and employed in PEC systems due to its high abundance and low cost. [11,12] Its narrow bandgap of 1.12 eV suggests that Si is suitable to absorb a wide wave length range, making it advantageous in comparison with other semiconductor materials. Si's bandgap is slightly lower than overall water splitting energetics (1.23 eV), while its conduction Photoelectrode degradation under harsh solution conditions continues to be a major hurdle for long-term operation and large-scale implementation of solar fuel conversion. In this study, a dual-layer TiO 2 protection strategy is presented to improve the interfacial durability between nanoporous black silicon and photocatalysts. Nanoporous silicon photocathodes decorated with catalysts are passivated twice, providing an intermediate TiO 2 layer between the substrate and catalyst and an additional TiO 2 layer on top of the catalysts. Atomic layer deposition of TiO 2 ensures uniform coverage of both the nanoporous silicon substrate and the catalysts.After 24 h of electrolysis at pH = 0.3, unprotected photocathodes layered with platinum and molybdenum sulfide retain only 30% and 20% of their photocurrent, respectively. At the same pH, photocathodes layered with TiO 2 experience an increase in photocurrent retention: 85% for platinum-coated photocathodes and 91% for molybdenum sulfide-coated photocathodes. Under alkaline conditions, unprotected photocathodes experience a 95% loss in photocurrent within the first 4 h of electrolysis. In contrast, TiO 2 -protected photocathodes maintain 70% of their photocurrent during 12 h of electrolysis. This approach is quite general and may be employed as a protection strategy for a variety of photoabsorber-catalyst interfaces under both acidic and basic electrolyte conditions.

Semiconductor photoelectrodes directly convert sunlight into stored chemical energy. In photoelectrochemical (PEC) devices, this photoconversion process relies on the junction between the semiconductor and catalyst to drive charge separation and generate electron/hole charge carriers. The growth of native oxides (SiOx) on the surface of semiconductors during device operation induces charge carrier recombination and photodegradation, which limit the operation lifetime of PEC devices. Likewise, the commercialization of photoelectrochemical devices is hindered by the use of expensive, rare precious metal catalysts such as platinum to enhance hydrogen evolution kinetics. This work demonstrates how drop casting zinc 1T‐phase molybdenum disulfide (Zn 1T‐MoS2) onto silicon nanowires (SiNWs) generates an interface that overcomes these challenges. This Zn 1T‐MoS2/SiNWs junction drives hydrogen evolution under acidic conditions (0.5 M H2SO4) comparably to platinum‐modified SiNWs (Pt/SiNWs) with a positive overpotential of 164 mV at 10 mA cm−2 and low Tafel slope of 42 mV dec−1. Compared to the bare SiNWs, the Zn 1T‐MoS2/SiNWs junction retains roughly 66% more photocurrent density and reduces SiOx growth by 16% after 24 h of continuous electrolysis. By developing a deep understanding of the catalyst‐semiconductor interface, photoelectrochemical devices may be effectively designed to maintain their stability over a lifetime of operation.