Photoelectrochemistry of core–shell tandem junction n–p⁺-Si/n-WO₃microwire array photoelectrodes

Abstract

Tandem junction (n-p + -Si/ITO/WO 3 /liquid) core-shell microwire devices for solar-driven water splitting have been designed, fabricated and investigated photoelectrochemically. 0.0068% and 0.0019% when the cathode compartment was saturated with Ar or H 2 , respectively, due to the non-optimal photovoltage and band-gap of the WO 3 that was used in the demonstration system to obtain stability of all of the system components under common operating conditions while also insuring product separation for safety purposes. Broader contextDirect photoelectrochemical conversion of sunlight into a storable, energy-dense fuel has the opportunity to provide a predictable, carbon-neutral energy source to displace current carbon-based technologies. Solar hydrogen generation via water splitting is an important goal because the voltage requirements for this process are well matched to the maximum power point of high-efficiency tandem photovoltaics. In addition to including light-absorbing materials that provide sufficient voltage for water splitting, an integrated solar fuels device requires catalysts connected to the light absorbers and an ionic transport pathway between the anode and cathode to complete the circuit while maintaining product separation below the lower explosive limits, for safety purposes. Integration of these different active materials is important to further development of this technology. Single-crystalline Si microwire arrays represent an architecture that can allow the system operation and integration requirements to be met, because photoactive Si microwires have been previously embedded into ionically conductive, gasblocking membranes. However, Si microwires do not produce enough photovoltage for unassisted water splitting even in a tandem Si-based structure. We describe a Si microwire based tandem junction device that produces sufficient photovoltage for unassisted water splitting, by use of WO 3 in a core-shell tandem structure. This system provides a proof-of-principle for this design, which can be improved signicantly through the incorporation of higher efficiency wide band-gap semiconductors as they become available and are stable under the same conditions as the rest of the components of the device.

Modeling, Simulation, and Implementation of Solar‐Driven Water‐Splitting Devices

“…[55] However, challenges include the increased complexity of epitaxial growth on the nontraditional crystallographic surface terminations present, as has been used for state-of-the-art planar designs, and the increased fabrication complexity, in general.…”

“…Similar absorber-inmembrane architectures that incorporate semiconducting microwires have also been proposed, where the electrical connectivity of the device is achieved through the microwires that cross through the membrane. [55,74] Recent modeling studies have also provided guidelines for the design of efficient vapor-fed solar-hydrogen devices, where the ionomer encapsulates the photoelectrochemical components (Figure 4 d2). Critical device dimensions that lead to optimal water, hydrogen, and oxygen transport have been identified in various cell configurations.…”

Photochemical Charge Separation at Particle Interfaces: The n-BiVO4–p-Silicon System

“…11−13 For example, Shaner et al reported a n-p(+)-Si/n-WO 3 microwire junction obtained by successive vacuum deposition of BCl 3 to achieve Si p-doping, DC sputter coating of an ITO ohmic contact, and electrodeposition of n-WO 3 , followed by annealing at 400°C for 2 h. The device supported overall water splitting with 0.0019% solar to hydrogen (STH) efficiency. 12 Recently, there is also increasing interest in tandem junctions formed between suspended semiconductor particles. Such tandem or "z-scheme photocatalysts" usually employ soluble redox couples for charge transfer.…”

Particle Suspension Reactors and Materials for Solar-Driven Water Splitting

“…81 Under this condition the initial potential required to impart water splitting was also not 1.229 V but rather approached ÀN V because no H 2 was present and therefore E bias -N V. It is often assumed that during rapid water splitting under an ambient atmosphere, enough H 2 and O 2 exist near the surface of the electrodes (or particles) so that generally the standard potential, E 0 fuel , is an accurate approximation of E fuel . 82 It is unclear if this approximation holds when H 2 and/or O 2 are present at reduced partial pressures. To assess whether particle suspensions are suitable for use in an actual solar PEC water-splitting reactor, they should be evaluated in the presence of H 2 at 1 bar pressure above the HER reactor and O 2 at 1 bar pressure above the OER reactor.…”