Si/TiO_2 Tandem-Junction Microwire Arrays for Unassisted Solar-Driven Water Splitting

Tandem-junction microwire array photoelectrodes have been fabricated by coating np^+-Si radial homojunction microwire arrays sequentially with fluorine-doped tin oxide (FTO) and titanium dioxide (TiO_2). These photoelectrodes effected unassisted water splitting under simulated 1 Sun conditions with an open-circuit potential (E_(oc)) of −1.5 V vs the formal potential for oxygen evolution, E^(0′)(OH^−/O_2), a current density at E = E^(0′)(OH^−/O_2) of 0.78 mA cm^(−2), a fill factor ( ff ) = 0.51, and a photovoltaic-biased photoelectrochemical ideal regenerative cell efficiency of 0.6%

Improving O_2 production of WO_3 photoanodes with IrO_2 in acidic aqueous electrolyte

WO_3 is a promising candidate for a photoanode material in an acidic electrolyte, in which it is more stable than most metal oxides, but kinetic limitations combined with the large driving force available in the WO_3 valence band for water oxidation make competing reactions such as the oxidation of the acid counterion a more favorable reaction. The incorporation of an oxygen evolving catalyst (OEC) on the WO_3 surface can improve the kinetics for water oxidation and increase the branching ratio for O_2 production. Ir-based OECs were attached to WO_3 photoanodes by a variety of methods including sintering from metal salts, sputtering, drop-casting of particles, and electrodeposition to analyze how attachment strategies can affect photoelectrochemical oxygen production at WO_3 photoanodes in 1 M H_2SO_4. High surface coverage of catalyst on the semiconductor was necessary to ensure that most minority-carrier holes contributed to water oxidation through an active catalyst site rather than a side-reaction through the WO_3/electrolyte interface. Sputtering of IrO_2 layers on WO_3 did not detrimentally affect the energy-conversion behavior of the photoanode and improved the O_2 yield at 1.2 V vs. RHE from ~0% for bare WO_3 to 50–70% for a thin, optically transparent catalyst layer to nearly 100% for thick, opaque catalyst layers. Measurements with a fast one-electron redox couple indicated ohmic behavior at the IrO_2/WO_3 junction, which provided a shunt pathway for electrocatalytic IrO_2 behavior with the WO_3 photoanode under reverse bias. Although other OECs were tested, only IrO_2 displayed extended stability under the anodic operating conditions in acid as determined by XPS

The Influence of Structure and Processing on the Behavior of TiO_2 Protective Layers for Stabilization of n-Si/TiO_2/Ni Photoanodes for Water Oxidation

Light absorbers with moderate band gaps (1–2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO_2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO_2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO_2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated. We show herein that TiO_2 films with a variety of crystal structures and midgap defect state distributions, deposited using both ALD and sputtering, form rectifying junctions with n-Si and are highly conductive toward photogenerated carriers in n-Si/TiO_2/Ni photoanodes. Moreover, the photovoltage of these electrodes can be modified by annealing the TiO_2 in reducing or oxidizing environments. All of the polycrystalline TiO_2 films with compact grain boundaries investigated herein protected the n-Si photoanodes against photocorrosion in pH = 14 KOH(aq). Hence, in these devices, conduction through the TiO_2 layer is neither specific to a particular amorphous or crystalline structure nor determined wholly by a particular extrinsic dopant impurity. The coupled structural and energetic properties of TiO_2, and potentially other protective oxides, can therefore be controlled to yield optimized photoelectrode performance

Comparative Study in Acidic and Alkaline Media of the Effects of pH and Crystallinity on the Hydrogen-Evolution Reaction on MoS_2 and MoSe_2

Single crystals of n-type MoS_2 and n-MoSe_2 showed higher electrocatalytic activity for the evolution of H_2(g) in alkaline solutions than in acidic solutions. The overpotentials required to drive hydrogen evolution at −10 mA cm^(–2) of current density for MoS^2 samples were −0.76 ± 0.13 and −1.03 ± 0.21 V when in contact with 1.0 M NaOH(aq) and 1.0 M H_2SO_4(aq), respectively. For MoSe_2 samples, the overpotentials at −10 mA cm^(–2) were −0.652 ± 0.050 and −0.709 ± 0.073 V in contact with 1.0 M KOH(aq) and 1.0 M H_2SO_4(aq), respectively. Single crystals from two additional sources were also tested, and the absolute values of the measured overpotentials were consistently less (by 460 ± 250 mV) in alkaline solutions than in acidic solutions. When electrochemical etching was used to create edge sites on the single crystals, the kinetics improved in acid but changed little in alkaline media. The overpotentials measured for polycrystalline thin films (PTFs) and amorphous forms of MoS_2 showed less sensitivity to pH and edge density than was observed for single crystals and showed enhanced kinetics in acid when compared to alkaline solutions. These results suggest that the active sites for hydrogen evolution on MoS_2 and MoSe_2 are different in alkaline and acidic media. Thus, while edges are known to serve as active sites in acidic media, in alkaline media it is more likely that terraces function in this role

Interface engineering of the photoelectrochemical performance of Ni-oxide-coated n-Si photoanodes by atomic-layer deposition of ultrathin films of cobalt oxide

Introduction of an ultrathin (2 nm) film of cobalt oxide (CoO_x) onto n-Si photoanodes prior to sputter-deposition of a thick multifunctional NiO_x coating yields stable photoelectrodes with photocurrent-onset potentials of ~−240 mV relative to the equilibrium potential for O2(g) evolution and current densities of ~28 mA cm^(−2) at the equilibrium potential for water oxidation when in contact with 1.0 M KOH(aq) under 1 sun of simulated solar illumination. The photoelectrochemical performance of these electrodes was very close to the Shockley diode limit for moderately doped n-Si(100) photoelectrodes, and was comparable to that of typical protected Si photoanodes that contained np+ buried homojunctions

Methods for comparing the performance of energy-conversion systems for use in solar fuels and solar electricity generation

The energy-conversion efficiency is a key metric that facilitates comparison of the performance of various approaches to solar energy conversion. However, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or electrical power as the system inputs. A general expression for the system efficiency is given as the ratio of the total output power (electrical plus chemical) divided by the total input power (electrical plus solar). The solar-to-hydrogen (STH) efficiency follows from this globally applicable system efficiency but only is applicable in the special case for systems in which the only input power is sunlight and the only output power is in the form of hydrogen fuel derived from solar-driven water splitting. Herein, system-level efficiencies, beyond the STH efficiency, as well as component-level figures of merit are defined and discussed to describe the relative energy-conversion performance of key photoactive components of complete systems. These figures of merit facilitate the comparison of electrode materials and interfaces without conflating their fundamental properties with the engineering of the cell setup. The resulting information about the components can then be used in conjunction with a graphical circuit analysis formalism to obtain “optimal” system efficiencies that can be compared between various approaches. The approach provides a consistent method for comparison of the performance at the system and component levels of various technologies that produce fuels and/or electricity from sunlight

Variation of organic matter quantity and quality in streams at Critical Zone Observatory watersheds

The quantity and chemical composition of dissolved organic matter (DOM) in surface waters influence ecosystem processes and anthropogenic use of freshwater. However, despite the importance of understanding spatial and temporal patterns in DOM, measures of DOM quality are not routinely included as part of large-scale ecosystem monitoring programs and variations in analytical procedures can introduce artifacts. In this study, we used consistent sampling and analytical methods to meet the objective of defining variability in DOM quantity and quality and other measures of water quality in streamflow issuing from small forested watersheds located within five Critical Zone Observatory sites representing contrasting environmental conditions. Results show distinct separations among sites as a function of water quality constituents. Relationships among rates of atmospheric deposition, water quality conditions, and stream DOM quantity and quality are consistent with the notion that areas with relatively high rates of atmospheric nitrogen and sulfur deposition and high concentrations of divalent cations result in selective transport of DOM derived from microbial sources, including in-stream microbial phototrophs. We suggest that the critical zone as a whole strongly influences the origin, composition, and fate of DOM in streams. This study highlights the value of consistent DOM characterization methods included as part of long-term monitoring programs for improving our understanding of interactions among ecosystem processes as controls on DOM biogeochemistry

Improved Stability of Polycrystalline Bismuth Vanadate Photoanodes by Use of Dual-Layer Thin TiO_2/Ni Coatings

Ultrathin dual layers of TiO_2 and Ni have been used to stabilize polycrystalline BiVO_4 photoanodes against photocorrosion in an aqueous alkaline (pH = 13) electrolyte. Conformal, amorphous TiO_2 layers were deposited on BiVO_4 thin films by atomic-layer deposition, with Ni deposited onto the TiO_2 by sputtering. Under simulated air mass 1.5 illumination, the dual-layer coating extended the lifetime of the BiVO4 photoanodes during photoelectrochemical water oxidation from minutes, for bare BiVO4, to hours, for the modified electrodes. X-ray photoelectron spectroscopy showed that these layers imparted chemical stability to the semiconductor/electrolyte interface. Transmission electron microscopy revealed the structure and morphology of the polycrystalline BiVO_4 film as well as of the thin coating layers. This work demonstrates that protection schemes based on ultrathin corrosion-resistant overlayers can be applied beneficially to polycrystalline photoanode materials under conditions relevant to efficient solar-driven water-splitting systems