MoS2 is an earth-abundant and low-cost HER electrocatalyst that can substitute noble metal catalysts. Here, we develop the atomic layer deposition (ALD) of MoS2 nanomaterials on p-Si photocathodes for highly efficient and stable PEC water reduction reactions.

Photoelectrochemical (PEC) cells have attracted much attention as a viable route for storing solar energy and producing value-added chemicals and fuels. However, the competition between light absorption and electrocatalysis at a restrained cocatalyst area on conventional planar-type photoelectrodes could limit their conversion efficiency. Here, we demonstrate a new monolithic photoelectrode architecture that eliminate the optical-electrochemical coupling by forming locally nanostructured cocatalysts on a photoelectrode. As a model study, Ni inverse opal (IO), an ordered three-dimensional porous nanostructure, was used as a surface-area-controlled electrocatalyst locally formed on Si photoanodes. The optical-electrochemical decoupling of our monolithic photoanodes significantly enhances the PEC performance for the oxygen evolution reaction (OER) by increasing light absorption and by providing more electrochemically active sites. Our Si photoanode with local Ni IOs maintains an identical photolimiting current density but reduces the overpotential by about 120 mV compared to a Si photoanode with planar Ni cocatalysts with the same footprint under 1 sun illumination. Finally, a highly efficient Si photoanode with an onset potential of 0.94 V vs reversible hydrogen electrode (RHE) and a photocurrent density of 31.2 mA/cm at 1.23 V vs RHE in 1 M KOH under 1 sun illumination is achieved with local NiFe alloy IOs.

Cobalt oxide (CoO x ), an earth-abundant and low-cost oxygen evolving catalyst (OEC), has notable advantages as a top protection layer of photoanodes for solar-driven water oxidation because of its desirable durability. However, cobalt oxides exist as various phases, such as Co(II)O, Co2(III)O3, Co3(II,III)O4, and the (photo)­electrochemical properties of CoO x are significantly governed by its phase. Atomic layer deposition (ALD) is a suitable method to form a multifunctional layer for photoelectrochemical (PEC) water splitting because it allows direct growth of a conformal high-quality film on various substrates as well as facile control over its chemical phases by adjusting the deposition conditions. Here, a well-controlled CoO x /SiO x /n-Si heterojunction prepared by ALD is demonstrated for solar-driven water splitting. The phase of the ALD CoO x films can be easily controlled from CoO to Co3O4 by varying the deposition temperature. In addition, this systematic study reveals that its energetic as well as electrochemical properties are changed significantly with the phase. Whereas CoO grown at 150 °C produces high photovoltage by building desirable hole-selective heterojunctions with n-Si, Co3O4 formed at 300 °C has a better catalytic property for water oxidation. To address this competitive correlation, we developed a double-layered (DL) ALD CoO x film that has advantages of both CoO and Co3O4. The DL ALD CoO x /SiO x /Si heterojunction photoanode produces a photocurrent density of 3.5 mA/cm2 without a buried junction and maintains a saturating current density of 32.5 mA/cm2 without noticeable degradation during 12 h in 1 M KOH under a simulated 1 sun illumination.

A new passivation strategy for an oxygen evolving Si photoanode for efficient and stable photoelectrochemical (PEC) water splitting is presented. In our Si photoanode structure, to eliminate the Si/water interface, the Si is stabilized with a thick insulating and chemically inert SiO2 film and locally defined electrocatalysts on the Si surface. The stabilized p+n-Si photoanode with SiO2 film and micropatterned Ni catalysts produced photocurrent densities of 27 mA cm–2 at water oxidation potential without corrosion for 24 h under water oxidation conditions. In addition, we provide a device modeling of our Si cell, i.e., an integrated photovoltaics–electrolyzer cell, to quantitatively assess the interplay between optical shadowing and catalytic performance, as a function of the coverage and characteristics of electrocatalysts on the water oxidation reaction during PEC water splitting. Design principles for high performance oxygen evolving Si photoanodes are presented based on the device modeling.

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The strong metal–oxide interaction of platinum nanoparticles (PtNPs) deposited on two types of cobalt oxides, CoO and Co3O4, was investigated using CO oxidation.

We report on the photoelectrochemical (PEC) performance and stability of Cu(In,Ga)Se (CIGS)-based photocathodes for photocatalytic hydrogen evolution from water. Various functional overlayers, such as CdS, TiO, ZnSnO, and a combination of the aforementioned, were applied on the CIGS to improve the performance and stability. We identified that the insertion of TiO overlayer on p-CIGS/n-buffer layers significantly improves the PEC performance. A multilayered photocathode consisting of CIGS/CdS/TiO/Pt exhibited the best current-potential characteristics among the tested photocathodes, which demonstrates a power-saved efficiency of 2.63%. However, repeated linear sweep voltammetry resulted in degradation of performance. In this regard, we focused on the PEC durability issues through in-depth chemical characterization that revealed the degradation was attributed to atomic redistribution of elements constituting the photocathode, namely, in-diffusion of Pt catalysts, out-diffusion of elements from the CIGS, and removal of the metal-oxide layers; the best-performing CIGS/CdS/TiO/Pt photocathode retained its initial performance until the TiO overlayer was removed. It was also found that the durability of CIGS photocathodes with a TiO-coated metal-oxide buffer layer such as ZnSnO was better than those with a TiO-coated CdS, and the degradation mechanism was different, suggesting that the stability of a CIGS-based photocathode can be improved by careful design of the structure.

Hydrogen energy has been drawing much attention in renewable energy technologies. Hydrogen production by water splitting reaction has especially been widely studied as an environmentally friendly and sustainable energy source. Realization of cost‐effective hydrogen production by water splitting requires electrolysis or photoelectrochemical cells decorated with highly efficient cocatalysts. Here, we briefly summarize the theory of water splitting reaction and discuss various types of catalysts for water splitting reaction (hydrogen evolution reaction and oxygen evolution reaction). Principles of photoelectrochemical hydrogen evolution from light absorbing semiconductors will be described.