Search

Photoelectrochemical (PEC) water splitting is a means to store solar energy in the form of hydrogen. Knowledge of practical limits for this process can help researchers assess their technology and guide future directions. We develop a model to quantify loss mechanisms in PEC water splitting based on the current state of materials research and calculate maximum solar-to-hydrogen (STH) conversion efficiencies along with associated optimal absorber band gaps. Various absorber configurations are modeled considering the major loss mechanisms in PEC devices. Quantitative sensitivity analyses for each loss mechanism and each absorber configuration show a profound impact of both on the resulting STH efficiencies, which can reach upwards of 25 % for the highest performance materials in a dual stacked configuration. Higher efficiencies could be reached as improved materials are developed. The results of the modeling also identify and quantify approaches that can improve system performance when working with imperfect materials.

Herein, we report the development of optically transparent high surface area electrodes (HSEs) made of indium tin oxide (ITO) using a facile, template-free, scalable wet chemical synthetic approach. The transparent HSEs are electronically conductive and physically robust, with tunable electrochemically accessible roughness factors from 1 through ∼100. These transparent HSEs can serve as a broadly functionalizable scaffold ideally suited to bridge the gap between the need to minimize ionic and electronic transport lengths within a device and the need to achieve high loadings of active material per surface area (e.g. for high capacity as needed for batteries or high optical density as required for electrochromics or photovoltaics). This gap currently stands as a major hurdle for the utilization of many nanomaterials in electronic and optoelectronic devices. The synthetic approach described here for ITO is transferable to other transparent conducting oxide (TCO) materials. This is the first report of a large...

We outline a systematic approach to develop active catalyst materials for an electrochemical reaction. The strategy allows one to tune binding energies of oxygen reaction intermediates on a catalyst surface by taking advantage of two effects: weakening oxygen binding energies by means of thin-film overlayers, and strengthening oxygen binding energies through nanoscale effects. By engineering a core–shell nanoparticle morphology with the appropriate dimensions, that is, the thickness of the overlayer and the size of the nanoparticle, bonding properties can be modified to improve the catalytic activity of the core–shell system. We demonstrate the application of this strategy for oxygen reduction by identifying Ru@Pt as a candidate material using density functional theory calculations, and then use these calculations to guide the synthesis of active Ru@Pt core–shell catalysts. The Ru@Pt particles, synthesized using a wet chemical method, exhibit ∼2 times higher specific activity (based on electrochemical active surface area) than state-of-the-art Pt/C from TKK.

It is common to electrochemically cycle catalysts prior to testing to remove contaminants that are left on the surface of the catalysts due to synthesis or exposure to ambient atmosphere. We discuss a new electrochemical pretreatment cleaning protocol for ORR catalysts that cycles the catalysts to higher potentials. We demonstrate this procedure on a class of Ru@Pt core-shell catalysts that we have recently developed. After the new cleaning protocol, the Ru@Pt catalysts show a 30mV improvement in halfwave potential, and changes in the baseline nitrogen CVs. The changes in the baseline CVs indicate a more complete removal of surface contaminants and atomic restructuring, both of which could account for the improved activity. Additionally, we show that inaccuracies in measurements of the electrochemical surface area by Hupd are rectified by the new cleaning protocol.

Molybdenite (MoS2) is a material that has been previously studied for its photoelectrochemical properties in bulk form and, more recently, for its electrocatalytic hydrogen evolution properties in various nanostructured forms. Herein, we aim to address the properties of bulk and nanostructured MoS2 in order to assess the future potential of harnessing nanostructured MoS2 as a more efficient photocathode for photoelectrochemical hydrogen production. Using an in-situ method to progressively create defects at the surface of bulk MoS2, we show that losses due to recombination of photogenerated charge carriers at surface states occur concurrently with an enhancement in catalytic activity for hydrogen evolution. We further explore how these surface defects affect the measured flat-band potential, and we discuss the consequences of the distinct electronic properties of bulk versus nanostructured MoS2. Our results for MoS2 show that differences in morphological length scales (bulk vs nanoscale) give rise to uniqu...

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...

We report a hierarchically branched TiO2 nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm2 total power density), the branched TiO2 nanorod array produces a photocurrent density of 0.83 mA/cm2 at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO2/electrolyte interface.

Irreversible, thermally induced sintering of heterogeneous catalysts is one of the most deleterious causes of activity loss during catalytic reaction and/or regeneration. Silica-coated Pd catalysts (Pd@SiO2) were prepared by a simple one-pot, water-in-oil microemulsion method and investigated as models for a general synthesis to encapsulate active nanoparticle catalysts to provide stability during extended periods of cycling and regeneration under harsh experimental conditions. An idealized stability test would involve both a catalyst whose activity changes significantly as particles sinter and test conditions that could readily induce sintering. Acetylene hydrogenation was chosen as the test reaction because its catalytic and regenerative (oxidative coke removal) cycling induce the destructive sintering conditions desired for such a test. When compared with Pd particles deposited on the outside of an identical silica support (Pd/SiO2 catalyst), the silica-encapsulated catalyst Pd@SiO2 deactivates at a much slower rate and is readily regenerated without sintering over multiple reaction and regeneration cycles. It is also found that silica encapsulation suppresses coking and insitu formation of PdC. TEM, XRD, BET, XPS, and TGA, before and after reaction and regeneration, were used to characterize the encapsulated Pd@SiO2 catalysts and compare them to conventionally supported Pd/SiO2.

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-hydrogen efficiency, as well as a flow chart with standard procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.

Using a combination of theory and experiments, we show that ZnO substitutionally doped with Ti or Al oxidizes CO by several parallel reaction pathways that differ from the traditional Mars-van Krevelen (MVK) mechanism. In one, the dopant atoms at the surface of the doped oxide adsorb and activate O 2 so that it reacts with CO. In the other, a surface oxygen atom from the lattice next to the dopant, D, moves onto the dopant and creates an O-D group and an oxygen vacancy. The O-D group is capable of oxidizing a reductant. To test these predictions made by theory, we have prepared Ti- and Al-doped ZnO and have shown that these compounds oxidize CO at temperatures at which pure-phase ZnO, Al 2O 3, or TiO 2 do not. The proposed mechanisms were made plausible by studying CO oxidation with 18O 2.

Electrochemical methods for the codeposition of Cr or Mo with α-Fe2O3 (hematite) have been developed to produce doped iron oxide films with varying compositions of Cr and Mo which are active photoanodes for photoelectrochemical (PEC) water decomposition (“water splitting”). The films were characterized by scanning electron microscopy, X-ray diffraction, UV−vis optical spectroscopy, and X-ray photoelectron spectroscopy to determine the effect of the dopants on the hematite structure and PEC performance. Upon doping, the microstructures of the films varied; however, no preferred crystallographic orientation or dopant phase segregation was observed. The best performing samples were 5% Cr and 15% Mo doped which had Incident Photon Conversion Efficiencies (IPCE’s) at 400 nm of 6% and 12%, respectively, with an applied potential of 0.4V vs Ag/AgCl. These IPCE values were 2.2× and 4× higher than the undoped sample for the 5% Cr and 15% Mo samples, respectively. The increase in performance is attributed to an imp...

In an attempt to improve photoelectrochemical (PEC) processes that is important in solar-to-chemical energy conversion, various materials have been proposed that have the potential for hydrogen production. For instance, hematite has many advantages for hydrogen production since it is somehow stable, has a relatively narrow bandgap, inexpensive, abundant and environmentally responsible. However, their use in PEC devices have been limited by several factors like poor conductivity and high electron-hole pair recombination rates. Strategies to overcome this limitation have been produced such as designing the hematite structure to permit more efficient transport and collection of photogenerated charge carriers. Others include the addition of surface electrocatalysts on iron oxide photoelectrodes, and doping the iron oxide with heteroatoms. Dopant species have also been introduced in the literature. However, there have been few reports on the synthesis of hematite films by electrodeposition and virtually no reports of electrodeposition of doped hematite. As such, an electrochemical route to prepare thin film photoanodes of nanocrystalline Pt-doped iron oxide has been proposed. It showed improvements in the performance of PEC when compared to pure iron oxide thin films.

Solar-to-hydrogen (STH) conversion efficiency is the most important figure of merit to gage the potential of a semiconductor material to photoelectrochemically split water (see Chapter “ Efficiency Definitions in the Field of PEC”). It is projected that STH conversion efficiencies in excess of 10 % will be needed for practical hydrogen production systems [1]. Taken in conjunction with gas detection measurements (see Chapter “ Stability Testing”), the photocurrent density (j SC) under short-circuited conditions (i.e., zero applied bias) in a 2-electrode measurement is critical in determining the STH conversion efficiency. Moreover, applied bias experiments using the 2-electrode configuration can shed important light on the water splitting capabilities and limits of a PEC material system.

It is important to determine the conductivity and flat-band potential (Efb) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. The three different techniques that can estimate the Efb are: Illuminated OCP, Mott–Schottky and Photocurrent Onset. The Efb should be independent of the technique used to determine it. Due to the inherent shortcomings of each technique, there is often a lack of agreement of the values determined by the various analyses. Researchers should be aware of these limitations in interpreting results.

In a UV-Vis (ultraviolet-visible light) spectroscopic measurement, light absorption as a function of wavelength provides information about electronic transitions occurring in the material. For semiconductors, UV-Vis spectroscopy offers a convenient method of estimating the optical band gap, since it probes electronic transitions between the valence band and the conduction band. Transmission UV-Vis, Diffuse Reflectance UV-Vis, and Absorption UV-Vis configurations are discussed.

The goal of PEC materials development is to design a material system that has the potential to satisfy most, if not all, of the requirements for cost-effective PEC hydrogen production. Figure 1 presents a recommended flowchart for the characterization of candidate PEC materials. The key knowledge gained as well as limitations of the different characterization techniques, highlighted in Table 1, are described in detail in following sections of this document. A material that can survive the rigorous testing set forth in this flowchart will be a particularly promising candidate for incorporation into an industrially deployable device for PEC hydrogen production. This organized approach to PEC characterization is intended to streamline the process for material screening so that discovery of promising candidates occurs at a faster and more orderly pace.

The chemical products of PEC water splitting processes are the evolved hydrogen and oxygen gases. Standard experimental methods for detecting and validating the quantity and quality of the product gases are critical. Faradaic efficiencies for the water splitting reaction in the given system can be determined. Three examples of PEC reactors are discussed, including batch, flow, and recirculating batch reactor.

The incident photon-to-current efficiency (IPCE) is a measure of the ratio of the photocurrent (converted to an electron transfer rate) versus the rate of incident photons (converted from the calibrated power of a light source) as a function of wavelength. Measuring the IPCE is also useful to determine the band gap. The band gap derived from IPCE may be higher than that obtained by optical spectroscopy techniques. Applied bias IPCE and white light bias IPCE experiments are discussed. Photocurrent spectroscopy examines the photocurrent produced by an electrochemical cell as a function of wavelength of the incident light. The optical bulk band gap of the semiconductor electrode can be determined along with information about whether it is a direct or indirect transition.

Overall solar-to-hydrogen (STH) efficiency is the most important parameter to characterize a PEC device. In fact, materials systems themselves are effectively defined by their highest-recorded STH efficiency; it is the single value by which all PEC devices can be reliably ranked against one another [1]. Unfortunately, published literature in the area of PEC sometimes contains confusing information regarding efficiency including invalid mathematical expressions for device efficiency, improper experimental methods for obtaining efficiency values, and/or wide-scale reporting of efficiencies other than STH without clear distinction. The first goal of this document is to establish proper definitions and mathematical expressions for device efficiencies. Among these definitions, we identify those that are acceptable for wide-scale benchmarking and reporting (for instance in the form of press releases to mainstream media) as well as those definitions which are helpful for their scientific value in material characterization and diagnostic testing (and suitable for journal publications). Later in this document, we overview the proper experimental procedures as well as common pitfalls that concern each type of efficiency measurement.

Herein, a high surface area electrode (HSE) consisting of indium tin oxide (ITO) as a scaffold and ultrathin Ti-doped hematite (α-Fe2O3) as the absorber material is developed. The HSE exhibits sixfold improvement in photoactivity over an analogous photoelectrode with a flat morphology. Interfacial recombination due to dopant impurities and shunting resulting from a high pinhole density in the hematite layer limit the device performance. These limitations are mitigated by introducing a tin oxide barrier layer, which reduces recombination at the solid–solid interface and mitigates shunting. Employing the HSE with an appropriate barrier layer improves charge separation efficiency and catalytic activity compared to conventional planar devices. This strategy can potentially be extended to other light absorber materials whose performance is affected by charge transport limitations.

We report a hierarchically branched TiO(2) nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm(2) total power density), the branched TiO(2) nanorod array produces a photocurrent density of 0.83 mA/cm(2) at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO(2)/electrolyte interface.

Screening of different sources of Pd/C shows reagents of highly variable nanoparticle sizes and oxidation states of the metal. Typically, catalysts with higher surface area are viewed as likely to be the more reactive. In this paper a new form of Pd/C, "UC Pd" is described that is shown to contain larger nanoparticles yet it is the most reactive catalyst of those sold commercially for Sonogashira coupling reactions. UC Pd functions efficiently in the absence of a copper co-catalyst, under very mild and "green" conditions using inexpensive 95% EtOH at 50 degrees C. It is also the only form of Pd/C that can be recycled. In side-by-side reactions with several commercially available forms of Pd/C, none compete successfully with UC Pd under standardized conditions. Physical data obtained from extensive surface analysis using TEM, XRD, XPS, and CO-TPD measurements lead to an explanation behind the unique reactivity of this new recyclable form of Pd/C.

Screening of different sources of Pd/C shows reagents of highly variable nanoparticle sizes and oxidation states of the metal. Typically, catalysts with higher surface area are viewed as likely to be the more reactive. In this paper a new form of Pd/C, "UC Pd" is described that is shown to contain larger nanoparticles yet it is the most reactive catalyst of those sold commercially for Sonogashira coupling reactions. UC Pd functions efficiently in the absence of a copper co-catalyst, under very mild and "green" conditions using inexpensive 95% EtOH at 50 degrees C. It is also the only form of Pd/C that can be recycled. In side-by-side reactions with several commercially available forms of Pd/C, none compete successfully with UC Pd under standardized conditions. Physical data obtained from extensive surface analysis using TEM, XRD, XPS, and CO-TPD measurements lead to an explanation behind the unique reactivity of this new recyclable form of Pd/C.

通讯作者地址: Wang, JF (通讯作者), Chinese Univ Hong Kong, Dept Phys, Shatin, Hong Kong Peoples R China 地址: 1. Chinese Univ Hong Kong, Dept Phys, Shatin, Hong Kong Peoples R China 2. Univ Calif Santa Barbara, Dept Chem & Biochem, Santa Barbara, CA 93106 USA 电子邮件地址: jfwang@phy.cuhk.edu.hk, stucky@chem.ucsb.edu

) that aresufficiently porous to allow unhindered mass transfer? 2) Howdoes the activity of the encapsulated catalysts compare totraditional supported Pd catalysts for CO oxidation andacetylene hydrogenation? 3) Do the shell structures providestability with respect to sintering at high temperatures?Nanoscale core/shell Pd@SiO

In an attempt to improve photoelectrochemical (PEC) processes that is important in solar-to-chemical energy conversion, various materials have been proposed that have the potential for hydrogen production. For instance, hematite has many advantages for hydrogen production since it is somehow stable, has a relatively narrow bandgap, inexpensive, abundant and environmentally responsible. However, their use in PEC devices have been limited by several factors like poor conductivity and high electron-hole pair recombination rates. Strategies to overcome this limitation have been produced such as designing the hematite structure to permit more efficient transport and collection of photogenerated charge carriers. Others include the addition of surface electrocatalysts on iron oxide photoelectrodes, and doping the iron oxide with heteroatoms. Dopant species have also been introduced in the literature. However, there have been few reports on the synthesis of hematite films by electrodeposition and virtually no reports of electrodeposition of doped hematite. As such, an electrochemical route to prepare thin film photoanodes of nanocrystalline Pt-doped iron oxide has been proposed. It showed improvements in the performance of PEC when compared to pure iron oxide thin films.

not Available.

not Available.

Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from $1.60–$10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O_2 and H_2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energy's targeted threshold cost of $2.00–$4.00 per kg H_2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met.

We report the molecular beam epitaxy growth of Al0.3Ga0.7As∕GaAs multijunction solar cells with epitaxial, semimetallic ErAs nanoparticles at the interface of the tunnel junction. The states provided by these nanoparticles reduce the bias required to pass current through the tunnel junction by three orders of magnitude, and therefore drastically reduce the voltage losses in the tunnel junction. We have measured open-circuit voltages which are 97% of the sum of the constituent cells, which result in nearly double the efficiency of our multijunction cell with a conventional tunnel junction.