Through examination of the optoelectronic and photoelectrochemical properties of BiVO 4 and Cu 2 O photoelectrodes, we evaluate the feasibility of a BiVO 4 / Cu 2 O photoanode/photocathode tandem cell for overall unassisted solar water splitting. Using state-of-the-art photoelectrodes we identify current-matching conditions by altering the photoanode active layer thickness. By further employing water oxidation and reduction catalysts (Co-Pi and RuO x , respectively) together with an operating point analysis, we show that an unassisted solar photocurrent density on the order of 1 mA cm −2 is possible in a tandem cell and moreover gain insight into routes for improvement. Finally, we demonstrate the unassisted 2-electrode operation of the tandem cell. Photocurrents corresponding to ca. 0.5% solar-to-hydrogen conversion efficiency were found to decay over the course of minutes because of the detachment of the Co-Pi catalyst. This aspect provides a fundamental challenge to the stable operation of the tandem cell with the currently employed catalysts.
A conjugated
polymer known for high stability (poly[benzimidazobenzophenanthroline],
coded as BBL) is examined as a photoanode for direct solar water oxidation.
In aqueous electrolyte with a sacrificial hole acceptor (SO32–), photoelectrodes show a morphology-dependent
performance. Films prepared by a dispersion-spray method with a nanostructured
surface (feature size of ∼20 nm) gave photocurrents up to 0.23
± 0.02 mA cm–2 at 1.23 VRHE under
standard simulated solar illumination. Electrochemical impedance spectroscopy
reveals a constant flat-band potential over a wide pH range at +0.31
VNHE. The solar water oxidation photocurrent with bare
BBL electrodes is found to increase with increasing pH, and no evidence
of semiconductor oxidation was observed over a 30 min testing time.
Characterization of the photo-oxidation reaction suggests H2O2 or •OH production with the bare film, while
functionalization of the interface with 1 nm of TiO2 followed
by a nickel–cobalt catalyst gave solar photocurrents of 20–30
μA cm–2, corresponding with O2 evolution.
Limitations to photocurrent production are discussed.
The search for ideal semiconductors for photoelectrochemical solar fuel conversion has recently recognized the spinel ferrites as promising candidates due to their optoelectronic tunability together with superb chemical stability.
The development of solution‐processable routes to prepare efficient photoelectrodes for water splitting is highly desirable to reduce manufacturing costs. Recently, sulfide chalcopyrites (Cu(In,Ga)S2) have attracted attention as photocathodes for hydrogen evolution owing to their outstanding optoelectronic properties and their band gap—wider than their selenide counterparts—which can potentially increase the attainable photovoltage. A straightforward and all‐solution‐processable approach for the fabrication of highly efficient photocathodes based on Cu(In,Ga)S2 is reported for the first time. It is demonstrated that semiconductor nanocrystals can be successfully employed as building blocks to prepare phase‐pure microcrystalline thin films by incorporating different additives (Sb, Bi, Mg) that promote the coalescence of the nanocrystals during annealing. Importantly, the grain size is directly correlated to improved charge transport for Sb and Bi additives, but it is shown that secondary effects can be detrimental to performance even with large grains (for Mg). For optimized electrodes, the sequential deposition of thin layers of n‐type CdS and TiO2 by solution‐based methods, and platinum as an electrocatalyst, leads to stable photocurrents saturating at 8.0 mA cm–2 and onsetting at ≈0.6 V versus RHE under AM 1.5G illumination for CuInS2 films. Electrodes prepared by our method rival the state‐of‐the‐art performance for these materials.
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