Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

We present a scalable wet chemical synthesis for a catalytically active nanostructured amorphous molybdenum sulfide material. The catalyst film is one of the most active nonprecious metal materials for electrochemical hydrogen evolution, drawing 10 mA/cm2 at ∼200 mV overpotential. To identify the active phase of the material, we perform X-ray photoelectron spectroscopy after testing under a variety of conditions. As deposited, the catalyst resembles amorphous MoS3, but domains resembling MoS2 in composition and chemical state are created under reaction conditions and may contribute to this material’s high electrochemical activity. The activity scales with electrochemically active surface area, suggesting that the rough, nanostructured catalyst morphology also contributes substantially to the film’s high activity. Electrochemical stability tests indicate that the catalyst remains highly active throughout prolonged operation. The overpotential required to attain a current density of 10 mA/cm2 increases by o...

Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of their Catalytic Activity

The recent literature on photochemical and photoelectrochemical reductions of CO(2) is reviewed. The different methods of achieving light absorption, electron-hole separation, and electrochemical reduction of CO(2) are considered. Energy gap matching for reduction of CO(2) to different products, including CO, formic acid, and methanol, is used to identify the most promising systems. Different approaches to lowering overpotentials and achieving high chemical selectivities by employing catalysts are described and compared.

https://www.annualreviews.org/doi/pdf/10.1146/annurev-physchem-032511-143759

Photochemical and Photoelectrochemical Reduction of CO2

Harnessing solar energy for the production of clean hydrogen fuels by a photoelectrochemical (PEC) cell represents a very attractive but challenging alternative This review focuses on recent developments of some promising photoelectrode materials, such as BiVO4, a-Fe2O3, TaON, and Ta3N5 for solar hydrogen production Some strategies have been developed to improve PEC performances of the photoelectrode materials, including: (i) doping for enhancing visible light absorption in the wide bandgap semiconductor or promoting charge transport in the narrow bandgap semiconductor, respectively; (ii) surface treatment for removing segregation phase or surface states; (iii) electrocatalysts for decreasing the overpotentials; (iv) morphology control for enhancing the light absorption and shortening transfer distance of minority carriers; (v) other methods, such as sensitization, passivating layer, and band structure engineering using heterojunction structures, and so on Photochemical durability of the photoelectrodes is also discussed, since any potential PEC technology must balance efficiency against cost and photochemical durability Photochemical durability may be amended by optimizing the photoelectrode, electrocatalyst, and electrolyte at the same time In addition, solar seawater splitting is briefly introduced because it has received attention recently Finally, trends in research in PEC cells for solar hydrogen production are detailed

Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook

Through decades of sustained effort, researchers have made substantial progress on developing technologies for solar-driven water splitting. Nevertheless, more basic research is needed before prototype devices with a chance for commercial success can be demonstrated. In this Perspective, we summarize the major design constraints that motivate continued research in the field of solar-driven water splitting. Additionally, we discuss key device components that are now available for use in demonstration systems and prototypes. Finally, we highlight research areas where breakthroughs will be critical for continued progress toward commercial viability for solar-driven water-splitting devices.

http://pubs.acs.org/doi/pdf/10.1021/cm4021518

Will Solar-Driven Water-Splitting Devices See the Light of Day?

The improvement of H/sub 2/ evolution from two different types of catalytic p-type photocathode surfaces has been examined. p-Type Si has been platinized by phtotelectrochemically plating Pt(0) onto the Si surface. Such a photocathode shows significant improvement (compared to naked p-type Si) for photochemical H/sub 2/ evolution with respect to output photovoltage, fill factor, and overall efficiency. Such photocathodes having an optimun amount of Pt(0) give a pH-dependent output voltage with respect to the H/sub 2/O/H/sub 2/ couple, but the dependence is not a simple 59-mV/pH dependence. No pH dependence would be expected if Pt(0) formed a Schottky barrier when plated onto p-type Si. A second kind of H/sub 2/ evolution catalyst has been confined to the surface of p-type Si. Polymeric quantities of an electroactive N,N'-dialkyl-4,4'-bipridinium reagent, (PQ/sup 2 +/.)/sub n/, have been confined to the surface. The Br/sup -/ counterions of the polymer are then exchanged by PtCl/sub 6//sup 2 -/. Photoreduction then yields Pt(0) dispersed in the polymer. Such a surface is again significantly improved compared to naked p-type Si with respect to H/sub 2/ evolution. A comparison of the naked p-Si, the simply platinized, and the (PQ/sup 2 +//sup ///sup +//sub n/.nPt(0))/sub surf./ system is mademore » and contrasted to the expected behavior of an external Schottky barrier photocell driving an electrolysis cell with a Pt cathode. Experiments with n-type MoS/sub 2/, n-type Si, Pt, Au, and W cathodes functionalized with the (PQ/sup 2 +//sup ///sup +/.)sub n/.nPt(0))/sub surf./ system compared to the same surface directly platinized confirm an important difference in the mechanism of H/sub 2/ evolution catalysis for the two surface catalyst systems. p-Type Si modified with optimum amounts of Pt(0) by direct platinization appears to give improved H/sub 2/ evolution efficiency by a mechanism where the Pt(0) serves as a catalyst that does not alter the interface energetics of the semiconductor.« less

Improvement of photoelectrochemical hydrogen generation by surface modification of p-type silicon semiconductor photocathodes

Synthesis and characterization of structured interfaces for hydrogen generation. Study of an N,N'-dialkyl-4,4'-bipyridinium redox polymer/palladium catalyst system

p-Si photocathodes functionalized first with an N,N′-dialkyl-4,4′-bipyridinium redox reagent, (PQ2+/+-)surf, and then with a Pt precursor, PtCl62-, give significant efficiency (up to 5%) for photoelectrochemical H2 generation with 632.8-nm light. Naked p-Si photocathodes give nearly zero efficiency, owing to poor H2 evolution kinetics that are improved by the (PQ2+/+-)surf/Pt modification. The mechanism of H2 evolution from p-Si/(PQ2+/+-)surf/Pt is first photoexcitation of electrons to the conduction band of Si followed by (PQ2+)surf → (PQ+-)surf reduction. The dispersion of Pt then catalyzes H2O reduction to give H2 and regeneration of (PQ2+)surf. The overall energy conversion efficiency rivals the best direct optical to chemical conversion systems reported to date.

http://www.pnas.org/content/pnas/77/11/6280.full.pdf

Synthesis and characterization of a photosensitive interface for hydrogen generation: Chemically modified p-type semiconducting silicon photocathodes.

: Electrodes derivatized with electroactive polymers or with charged, nonelectroactive polymers can be significantly influenced by the nature of the electrolyte and other ions present in a solution contacted by the derivatized electrode. It has been shown that charged, nonelectroactive polymers can persistently bind significant quantities of charged, electroactive species such as Fe(CN)(6)(4-) by surface polyvinylpyridinium or Ru(bipyridine)(3)(2+) by Nafion. Theses examples illustrate how electrostatic binding may be exploited for anlaysis, preparation of a variety of modified electrodes, and study of electrocatalysis. Electroactive polymers are charged in at least one of their accessible redox states and both selectivity of counterion binding and the movement of ions in and out of the surface polymer associated with change of redox state may affect electrochemical behavior. Electrodes coated with electroactive polymers may have a number of uses, including desalting of H20, that depend on the behavior of solution ions.

Electrostatic binding of electroactive and nonelectroactive anions in a surface-confined, electroactive polymer: selectivity of binding measured by Auger spectroscopy and cyclic voltammetry

: Single crystal, n-type CdTe (E sub g = 1.4 eV) has been studied with respect to barrier height, E sub B, when contacting a liquid electrolyte solution containing a fast, one-electron, outer-sphere redox reagent. We approximate E sub B as equal to the photovoltage measured by cyclic voltammetry of various redox couples at illuminated n-CdTe vs. a reversible electrode. N-CdTe surfaces pretreated with an oxidizing etch give an E sub B of approximately 0.5 V + or - 0.1 V in H2O/0.1 M NaClO4 or CH3CN/0.1 M (n-Bu4N)ClO4 that is independent of the E sub 1/2 of the added redox couple. A reducing etch pretreatment gives an E sub B in either of the electrolyte solutions that depends on E sub 1/2 of the redox couple in a manner consistent with a nearly ideal semiconductor. The reduced CdTe exhibits an E sub B of up to 0.9 V for a redox couple having E sub 1/2 near 0.0V vs. SCE, whereas couples having E sub 1/2 negative of approx. -1.0 V vs. SCE give zero photovoltage. Auger and X-ray photoelectron spectroscopy of the reduced and oxidized surfaces are qualitatively different. The reduced surface exhibits signals for Cd and Te in relative intensities that are consistent with a close to stoichiometric (1/1) surface. The oxidized surface exhibits little or no detectable Cd signal and the Te signal is consistent with a thick overlayer of elemental Te.

James A. Bruce

Papers

Improvement of photoelectrochemical hydrogen generation by surface modification of p-type silicon semiconductor photocathodes

Synthesis and characterization of structured interfaces for hydrogen generation. Study of an N,N'-dialkyl-4,4'-bipyridinium redox polymer/palladium catalyst system

Synthesis and characterization of a photosensitive interface for hydrogen generation: Chemically modified p-type semiconducting silicon photocathodes.

Electrostatic binding of electroactive and nonelectroactive anions in a surface-confined, electroactive polymer: selectivity of binding measured by Auger spectroscopy and cyclic voltammetry

Deliberate modification of the behavior of n-type cadmium telluride/electrolyte interfaces by surface etching. Removal of Fermi level pinning