No abstract

Solar fuel production, mimicking natural photosynthesis of converting CO2 into useful fuels and storing solar energy as chemical energy, has received great attention in recent years. Practical large-scale fuel production needs a unique device capable of CO2 reduction using only solar energy and water as an electron source. Here we report such a system composed of a gold-decorated triple-layered ZnO@ZnTe@CdTe core-shell nanorod array photocathode and a CH3NH3PbI3 perovskite solar cell in tandem. The assembly allows effective light harvesting of higher energy photons (>2.14 eV) from the front-side photocathode and lower energy photons (>1.5 eV) from the back-side-positioned perovskite solar cell in a single-photon excitation. This system represents an example of a photocathode-photovoltaic tandem device operating under sunlight without external bias for selective CO2 conversion. It exhibited a steady solar-to-CO conversion efficiency over 0.35% and a solar-to-fuel conversion efficiency exceeding 0.43% including H2 as a minor product.

A one-dimensional zinc ferrite (ZnFe2O4) nanorod photoanode was prepared by a simple solution method on the F-doped tin oxide glass substrate. Thermal treatment under a hydrogen or vacuum atmosphere improved the photoelectrochemical water oxidation activity up to 20 times. The various physical characterization techniques used revealed that oxygen vacancies were created by the treatments in the near surface region, which increased the donor density and passivated the surface states. Hydrogen treatment was more effective and it was important to find optimum treatment conditions to take advantage of the positive role of oxygen vacancy as a source of electron donors and avoid its negative effect as electron trap sites.

Au coupled ZnTe/ZnO-NW array is a new photocathode for selective CO production from CO2. The remarkable effects of an Au are to form of a Schottky junction with ZnTe to improve band bending and provide the reaction center for CO2 reduction suppressing water reduction.

Delafossite CuFeO2 is a promising photocathode material for solar hydrogen production, but its performance is low because of poor charge transport properties. When the prepared CuFeO2 electrode is annealed by hybrid microwave annealing (HMA), its photoelectrochemical water reduction activity increases by more than 4 times (−1.3 mA cm–2 @ 0.4 VRHE), while the conventional thermal annealing (CTA) improves the performance by only 2 times (−0.62 mA cm–2 @0.4 VRHE). The postannealing of the electrode intercalates extra oxygen into the CuFeO2 lattice to form CuFeO2+1.5δ, which increases the charge carrier density and thus improves charge transport properties. The oxygen intercalation with HMA takes place more uniformly over the whole solid and is more effective than CTA. In addition, HMA post-treated CuFeO2 is modified with a NiFe-layered double hydroxide/reduced graphene oxide electrocatalyst, which exhibits a high photoactivity of −2.4 mA cm–2 @ 0.4 VRHE, unprecedented for CuFeO2-based photocathodes.

Highly efficient tree branch-shaped CuO photocathodes are fabricated using the hybrid microwave annealing process with a silicon susceptor within 10 minutes. The unique hierarchical, one-dimensional structure provides more facile charge transport, larger surface areas, and increased crystallinity and crystal ordering with less defects compared to irregular-shaped CuO prepared by conventional thermal annealing. As a result, the photocathode fabricated with the tree branch-shaped CuO produces an unprecedently high photocurrent density of -4.4 mA cm(-2) at 0 VRHE under AM 1.5 G simulated sunlight compared to -1.44 mA cm(-2) observed for a photocathode fabricated by thermal annealing. It is also confirmed that stoichiometric hydrogen and oxygen are produced from photoelectrochemical water splitting on the tree branch-shaped CuO photocathode and a platinum anode.

No abstract

Photoelectrochemical (PEC) water splitting is a promising way to produce clean and sustainable hydrogen fuel. Solar hydrogen production by using p‐type metal oxide semiconductor photocathodes has not been studied as extensively as that with n‐type metal oxide semiconductor photoanodes and p‐type photovoltaic‐grade non‐oxide semiconductor photocathodes. Copper‐based oxide photocathodes show relatively good conductivity, but suffer from instability in aqueous solution under illumination, whereas iron‐based metal oxide photocathodes demonstrate more stable PEC performance but have problems in charge separation and transport. Herein, an overview of recent progress in p‐type metal oxide‐based photocathodes for PEC water reduction is provided. Although these materials have not been fully developed to reach their potential performance, the challenges involved have been identified and strategies to overcome these limitations have been proposed. Future research in this field should address these issues and challenges in addition to the discovery of new materials.