Silicon based tandem cells: novel photocathodes for hydrogen production

Design of a compact and versatile radiation heater with an additively manufactured Nb radiation shield for UHV high-temperature sample preparation

A compact, ultrahigh vacuum, radiative heater based on pyrolytic boron nitride that efficiently directs nearly all of its radiation to the sample was designed and constructed. It is shown that the heater reaches temperatures of 1300 K experimentally at 60% of its maximum power. A COMSOL Multiphysics® simulation and an analytical model predict an ultimate temperature of up to 1500 K. Furthermore, the heater does not introduce any contamination to the sample. This is accomplished by a custom-made Nb radiation shield, which was manufactured by selective laser melting and holds a flag-style sample holder. Before manufacturing, the whole assembly was simulated with COMSOL Multiphysics to validate the design of the radiation shield.

Strategies for Semiconductor/Electrocatalyst Coupling toward Solar-Driven Water Splitting

Hydrogen (H2) has a significant potential to enable the global energy transition from the current fossil-dominant system to a clean, sustainable, and low-carbon energy system. While presently global H2 production is predominated by fossil-fuel feedstocks, for future widespread utilization it is of paramount importance to produce H2 in a decarbonized manner. To this end, photoelectrochemical (PEC) water splitting has been proposed to be a highly desirable approach with minimal negative impact on the environment. Both semiconductor light-absorbers and hydrogen/oxygen evolution reaction (HER/OER) catalysts are essential components of an efficient PEC cell. It is well documented that loading electrocatalysts on semiconductor photoelectrodes plays significant roles in accelerating the HER/OER kinetics, suppressing surface recombination, reducing overpotentials needed to accomplish HER/OER, and extending the operational lifetime of semiconductors. Herein, how electrocatalyst coupling influences the PEC performance of semiconductor photoelectrodes is outlined. The focus is then placed on the major strategies developed so far for semiconductor/electrocatalyst coupling, including a variety of dry processes and wet chemical approaches. This Review provides a comprehensive account of advanced methodologies adopted for semiconductor/electrocatalyst coupling and can serve as a guideline for the design of efficient and stable semiconductor photoelectrodes for use in water splitting.

Immobilization of Reduced Graphene Oxide on Hydrogen-Terminated Silicon Substrate as a Transparent Conductive Protector

Silicon is a promising electrode material for photoelectrochemical and photocatalytic reactions. However, the chemically active surface of silicon will be easily oxidized when exposed to the oxidation environment. We immobilized graphene oxide (GO) onto hydrogen-terminated silicon (H-Si) and reduced it through ultraviolet (UV) and vacuum-ultraviolet (VUV) irradiation. This acted as an ultrathin conductive layer to protect H-Si from oxidation. The elemental evolution of GO was studied by X-ray photoelectron spectroscopy, and it was found that GO was partially reduced soon after the deposition onto H-Si and further reduced after UV or VUV light irradiation. The VUV photoreduction demonstrated ca. 100 times higher efficiency compared to the UV reduction based on the irradiation dose. The saturated oxygen-to-carbon ratio (R_O/C) of the reduced graphene oxide (rGO) was 0.21 ± 0.01, which is lower than the photoreduction of GO on SiO₂ substrate. This indicated the H-Si played an important role in assisting the photoreduction of GO. No obvious exfoliation of rGO was observed after sonicating the rGO-covered H-Si sample in water, which indicated rGO was immobilized on H-Si. The electrical conductivity of H-Si surface was maintained in the rGO-covered region while the exposed H-Si region became insulating, which was observed by conductive atomic force microscopy. The rGO was verified capable to protect the active H-Si against the oxidation under an ambient environment.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Band engineering for efficient catalyst-substrate coupling for photoelectrochemical water splitting

To achieve an overall efficient solar water splitting device, not only the efficiencies of photo-converter and catalyst are decisive, but also their appropriate coupling must be considered.

Efficient direct solar-to-hydrogen conversion by in situ interface transformation of a tandem structure

Photosynthesis is nature's route to convert intermittent solar irradiation into storable energy, while its use for an industrial energy supply is impaired by low efficiency. Artificial photosynthesis provides a promising alternative for efficient robust carbon-neutral renewable energy generation. The approach of direct hydrogen generation by photoelectrochemical water splitting utilizes customized tandem absorber structures to mimic the Z-scheme of natural photosynthesis. Here a combined chemical surface transformation of a tandem structure and catalyst deposition at ambient temperature yields photocurrents approaching the theoretical limit of the absorber and results in a solar-to-hydrogen efficiency of 14%. The potentiostatically assisted photoelectrode efficiency is 17%. Present benchmarks for integrated systems are clearly exceeded. Details of the in situ interface transformation, the electronic improvement and chemical passivation are presented. The surface functionalization procedure is widely applicable and can be precisely controlled, allowing further developments of high-efficiency robust hydrogen generators.