Emil A. Hernández-Pagán

Bio: Emil A. Hernández-Pagán is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Nanoparticle & Wurtzite crystal structure. The author has an hindex of 10, co-authored 16 publications receiving 1493 citations. Previous affiliations of Emil A. Hernández-Pagán include Vanderbilt University & Arizona State University.

Photoassisted Overall Water Splitting in a Visible Light-Absorbing Dye-Sensitized Photoelectrochemical Cell

Abstract: Iridium oxide nanoparticles stabilized by a heteroleptic ruthenium tris(bipyridyl) dye were used as sensitizers in photoelectrochemical cells consisting of a nanocrystalline anatase anode and a Pt cathode. The dye coordinated the IrO2·nH2O nanoparticles through a malonate group and the porous TiO2 electrode through phosphonate groups. Under visible illumination (λ > 410 nm) in pH 5.75 aqueous buffer, oxygen was generated at anode potentials positive of −325 mV vs Ag/AgCl and hydrogen was generated at the cathode. The internal quantum yield for photocurrent generation was ca. 0.9%. Steady-state luminescence and time-resolved flash photolysis/transient absorbance experiments were done to measure the rates of forward and back electron transfer. The low quantum yield for overall water splitting in this system can be attributed to slow electron transfer (∼2.2 ms) from IrO2·nH2O to the oxidized dye. Forward electron transfer does not compete effectively with the back electron transfer reaction from TiO2 to the ...

A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity

Abstract: Stable blue suspensions of 2 nm diameter iridium oxide (IrOx·nH2O) nanoparticles were obtained by hydrolyzing IrCl62− in base at 90 °C to produce [Ir(OH)6]2− and then treating with HNO3 at 0 °C. UV−visible spectra show that acid condensation of [Ir(OH)6]2− results in quantitative conversion to stable, ligand-free IrOx·nH2O nanoparticles, which have an extinction coefficient of 630 ± 50 M−1cm−1 at 580 nm. In contrast, alkaline hydrolysis alone converts only 30% of the sample to IrOx·nH2O at 2 mM concentration. The acidified nanoparticles are stable for at least one month at 2 °C and can be used to make colloidal solutions between pH 1 and 13. At pH 7 and above, some hydrolysis to form [Ir(OH)6]2− occurs. Uniform IrOx·nH2O electrode films were grown anodically from pH 1 solutions, and were found to be highly active for water oxidation between pH 1 and 13.

Resistance and polarization losses in aqueous buffer–membrane electrolytes for water-splitting photoelectrochemical cells

Abstract: The recent development of inexpensive catalysts for the oxygen evolution reaction has suggested that efficient photoelectrochemical cells (PECs) might be constructed from terrestrially abundant materials. Because these catalysts operate in aqueous buffer solutions at neutral to slightly basic pH, it is important to consider whether electrolytic cells can have low series loss under these conditions. Water-splitting or fuel-forming PECs will likely require porous separators or electrolyte membranes to separate the cathode products from oxygen produced at the anode. For this reason we analyze the individual potential losses in electrolytic systems of buffer solutions and commercially available anion- and cation-exchange membranes. Potentiometric analysis and pH measurements were employed to measure the potential losses associated with solution resistance, membrane resistance, and pH gradient formation at the current density (25 mA cm−2) expected for efficient PECs. The membrane pH gradient is the most problematic source of loss in these systems, but monoprotic buffers can minimize the pH gradient by diffusion of the neutral acidic or basic form of the buffer across the membrane. These results suggest that water-splitting PECs can be viable with properly chosen membrane–buffer combinations.

Anodic deposition of colloidal iridium oxide thin films from hexahydroxyiridate(IV) solutions

Abstract: A facile, in-situ deposition route to stable iridium oxide (IrO(x)·nH(2)O) nanoparticle thin films from [Ir(OH)(6)](2-) solutions is reported. The [Ir(OH)(6)](2-) solution, made by alkaline hydrolysis of [IrCl(6)](2-), is colorless and stable near neutral pH, and forms blue IrO(x)·nH(2)O nanoparticle suspensions once it is adjusted to acidic or basic conditions. IrO(x)·nH(2)O nanoparticle thin films are grown anodically on glassy carbon, fluorine-doped tin oxide, and gold electrodes by electrolyzing [Ir(OH)(6)](2-) solutions at +1.0-1.3 V versus Ag/AgCl. The thickness of the IrO(x)·nH(2)O films can be controlled by varying the concentration of [Ir(OH)(6)](2-) , the deposition potential, and/or the deposition time. These thin films are stable between pH 1 and 13 and have the lowest overpotential (η) for the oxygen evolution reaction (OER) of any yet reported. Near neutral pH, the Tafel slope for the OER at a IrO(x)·nH(2)O film/Au rotating disk electrode was 37-39 mV per decade. The exchange current density for the OER was 4-8 × 10(-10) A cm(-2) at a 4 mC cm(-2) coverage of electroactive Ir.

Electron transfer kinetics in water splitting dye-sensitized solar cells based on core–shell oxide electrodes

Abstract: Photoelectrochemical water splitting occurs in a dye-sensitized solar cell when a [Ru(bpy)3]2+-based dye covalently links a porous TiO2 anode film to IrO2·nH2O nanoparticles The quantum yield for oxygen evolution is low because of rapid back electron transfer between TiO2 and the oxidized dye, which occurs on a timescale of hundreds of microseconds, When iodide is added as an electron donor, the photocurrent increases, confirming that the initial charge injection efficiency is high When the porous TiO2 film is coated with a 1–2 nm thick layer of ZrO2 or Nb2O5, both the charge injection rate and back electron transfer rate decrease The efficiency of the cell increases and then decreases with increasing film thickness, consistent with the trends in charge injection and recombination rates The current efficiency for oxygen evolution, measured electrochemically in a generator-collector geometry, is close to 100% The factors that lead to polarization of the photoanode and possible ways to re-design the system for higher efficiency are discussed

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction

Abstract: Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm–2 per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing ...

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Abstract: Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions

Abstract: The activities of the oxygen evolution reaction (OER) on iridium-oxide- and ruthenium-oxide-based catalysts are among the highest known to date. However, the OER activities of thermodynamically stable rutile iridium oxide (r-IrO2) and rutile iridium oxide (r-RuO2), normalized to catalyst mass or true surface area are not well-defined. Here we report a synthesis of r-IrO2 and r-RuO2 nanoparticles (NPs) of ∼6 nm, and examine their OER activities in acid and alkaline solutions. Both r-IrO2 and r-RuO2 NPs were highly active for OER, with r-RuO2 exhibiting up to 10 A/goxide at 1.48 V versus reversible hydrogen electrode. When comparing the two, r-RuO2 NPs were found to have slightly higher intrinsic and mass OER activities than r-IrO2 in both acid and basic solutions. Interestingly, these oxide NPs showed higher stability under OER conditions than commercial Ru/C and Ir/C catalysts. Our study shows that these r-RuO2 and r-IrO2 NPs can serve as a benchmark in the development of active OER catalysts for electrol...

Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts

Abstract: Although sunlight-driven water splitting is a promising route to sustainable hydrogen fuel production, widespread implementation is hampered by the expense of the necessary photovoltaic and photoelectrochemical apparatus. Here, we describe a highly efficient and low-cost water-splitting cell combining a state-of-the-art solution-processed perovskite tandem solar cell and a bifunctional Earth-abundant catalyst. The catalyst electrode, a NiFe layered double hydroxide, exhibits high activity toward both the oxygen and hydrogen evolution reactions in alkaline electrolyte. The combination of the two yields a water-splitting photocurrent density of around 10 milliamperes per square centimeter, corresponding to a solar-to-hydrogen efficiency of 12.3%. Currently, the perovskite instability limits the cell lifetime.