Photoacid-Modified Nafion Membrane Morphology Determined by Resonant X-ray Scattering and Spectroscopy

Covalent attachment of photoacid dye molecules to perfluorinated sulfonic acid membranes is a promising route to enable active light-driven ion pumps, but the complex relationship between chemical modification and morphology is not well understood in this class of functional materials. In this study we demonstrate the effect of bound photoacid dyes on phase-segregated membrane morphology. Resonant X-ray scattering near the sulfur K-edge reveals that introduction of photoacid dyes to the end of the ionomer side chains enhances phase segregation among ionomer domains, and the ionomer domain spacing increases with increasing amount of bound dye. Furthermore, relative crystallinity is marginally enhanced within semicrystalline domains composed of the perfluorinated backbone. X-ray absorption spectroscopy coupled with first-principles density functional theory calculations suggest that above a critical concentration, the multiple hydrophilic groups on the attached photoacid dye may help increase residual water content and promote hydration of adjacent sulfonic acid side chains under dry or ambient conditions

Measurement of the Electrical Resistance of n-Type Si Microwire/p-Type Conducting Polymer Junctions for Use in Artificial Photosynthesis

The junction between n-type silicon microwires and p-type conducting polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)) was investigated using a soft contact method. Dopant levels within the microwires were varied during growth to give a highly-doped region for improved contact and a low-doped region for light absorption. The low-doped region of the microwires had a dopant density of 5 X 10(17) cm(-3) while the highly-doped region had an increased dopant density of 5 X 10(18) cm(-3) over similar to 20 mu m. Uniform, highly-doped microwires, with a dopant density of 4 X 10(19) cm(3), were used as a comparison. Regions of highly-doped n-type Si microwires (N-D = 5 X 10(18) cm(-3) and 4 X 10(19) cm(-3)) contacted by PEDOT:PSS showed a significantly lower junction resistance compared to the low-doped (3 X 10(17) cm(-3)) regions of the microwire. Junctions incorporating the metal catalyst used during growth were also investigated. Microwires with copper at the interface had similar currentvoltage characteristics to those observed for the highly-doped microwire/conducting polymer junction; however, junctions that incorporated gold exhibited significantly lower resistances, decreasing the iR contribution of the junction by an order of magnitude with respect to the total voltage drop in the entire structure

Modeling, simulation, and design criteria for photoelectrochemical water-splitting systems

A validated multi-physics numerical model that accounts for charge and species conservation, fluid flow, and electrochemical processes has been used to analyze the performance of solar-driven photoelectrochemical water-splitting systems. The modeling has provided an in-depth analysis of conceptual designs, proof-of-concepts, feasibility investigations, and quantification of performance. The modeling has led to the formulation of design guidelines at the system and component levels, and has identified quantifiable gaps that warrant further research effort at the component level. The two characteristic generic types of photoelectrochemical systems that were analyzed utilized: (i) side-by-side photoelectrodes and (ii) back-to-back photoelectrodes. In these designs, small electrode dimensions (mm to cm range) and large electrolyte heights were required to produce small overall resistive losses in the system. Additionally, thick, non-permeable separators were required to achieve acceptably low rates of product crossover

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition of n-Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I_3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm^(–2) under 100 mW cm^(–2) of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically

Maximizing value of genetic sequence data requires an enabling environment and urgency

Severe price spikes of the major grain commodities and rapid expansion of cultivated area in the past two decades are symptoms of a severely stressed global food supply. Scientific discovery and improved agricultural productivity are needed and are enabled by unencumbered access to, and use of, genetic sequence data. In the same way the world witnessed rapid development of vaccines for COVID-19, genetic sequence data afford enormous opportunities to improve crop production. In addition to an enabling regulatory environment that allowed for the sharing of genetic sequence data, robust funding fostered the rapid development of coronavirus diagnostics and COVID-19 vaccines. A similar level of commitment, collaboration, and cooperation is needed for agriculture

Photocatalytic Hydrogen Production at Titania-Supported Pt Nanoclusters that are Derived from Surface-Anchored Molecular Precursors

Degussa P-25 TiO2 bearing surface-anchored Pt(dcbpy)Cl-2 [dcbpy = 4,4\u27-dicarboxylic acid-2,2\u27-bipyridine] prepared with systematically varied surface coverage produced Pt-0 nanoparticles under bandgap illumination in the presence of methanol hole scavengers. Energy-dispersive X-ray spectroscopy confirmed the presence of elemental platinum in the newly formed nanoparticles during scanning transmission electron microscopy (STEM) eleriments. According to the statistical analysis of numerous STEM images, the Pt-0 nanoclusters were distributed in a segregated manner throughout the titania surface, ranging in size from 1 to 3 nm in diameter. The final achieved nanoparticle size and net hydrogen production were determined as a function of the Pt(dcbpy)Cl-2 surface coverage as well as other systematically varied experimental parameters. The hybrid Pt/TiO2 nanomaterials obtained upon complete decomposition of the Pt(dcbpy)Cl-2 precursor displayed higher photocatalytic activity (300 mu mol/h) for hydrogen evolution in aqueous suspensions when compared with platinized TiO2 derived from H2PtCl6 precursors (130 mu mol/h), as ascertained through gas chromatographic analysis of the photoreactor headspace under identical experimental conditions. The conclusion that H-2 was evolved from Pt-0 sites rather than from molecular Pt(dcbpy)Cl-2 entities was independently supported by Hg and CO poisoning experiments. The formation of small Pt nanopartides (1.5 nm in diameter) prevail at low surface coverage of Pt(dcbpy)Cl-2 on TiO2 (0.5 to 2% by mass) that exhibit enhanced turnover frequencies with respect to all other materials investigated, induding those produced from the in situ photochemical reduction of H2PtCl6 center dot Pt-II precursor absorption in the ultraviolet region appeared to be partially responsible for attenuation of the H-2 evolution rate at higher Pt(dcbpy)Cl-2 surface coverage. The nanoparticle size and hydrogen evolution characteristics of the surface-anchored materials generated through photodeposition were directly compared with those derived from chemical reduction using NaBH4. Finally, Degussa P-25 thin films deposited on FTO substrates enabled electrochemically induced (-1.0 V vs Ag/AgCl, pH 7.0, phosphate buffer) electron trapping (TiO2(e(-))) throughout the titania. After removal of the applied bias and the anaerobic introduction of Pt(dcbpy)Cl-2, the accumulated electrons reduce this molecular species to Pt-0 nanoparticles on the titania electrode surface, as confirmed by TEM measurements, with the concomitant production of H-2 gas. The combined experiments illustrate that TiO2(e(-)) generated with bandgap excitation or via electrochemical bias affords the reduction of Pt(dcbpy)Cl-2 to Pt-0 nanoparticles that in turn are responsible for heterogeneous hydrogen gas evolution

Unassisted solar-driven photoelectrosynthetic HI splitting using membrane-embedded Si microwire arrays

Free-standing, membrane-embedded, Si microwire arrays have been used to affect the solar-driven, unassisted splitting of HI into H_2 and I_3−. The Si microwire arrays were grown by a chemical-vapor-deposition vapor–liquid–solid growth process using Cu growth catalysts, with a radial n+p junction then formed on each microwire. A Nafion proton-exchange membrane was introduced between the microwires and Pt electrocatalysts were then photoelectrochemically deposited on the microwires. The composite Si/Pt–Nafion membrane was mechanically removed from the growth substrate, and Pt electrocatalysts were then also deposited on the back side of the structure. The resulting membrane-bound Si microwire arrays spontaneously split concentrated HI into H_2(g) and I_3− under 1 Sun of simulated solar illumination. The reaction products (i.e. H_2 and I_3−) were confirmed by mass spectrometry and ultraviolet–visible electronic absorption spectroscopy

A scanning probe investigation of the role of surface motifs in the behavior of p-WSe_2 photocathodes

The spatial variation in the photoelectrochemical performance for the reduction of an aqueous one-electron redox couple, Ru(NH_3)_6^(3+/2+), and for the evolution of H_2(g) from 0.5 M H_2SO_4(aq) at the surface of bare or Pt-decorated p-type WSe_2 photocathodes has been investigated in situ using scanning photocurrent microscopy (SPCM). The measurements revealed significant differences in the charge-collection performance (quantified by the values of external quantum yields, Φ_(ext)) on various macroscopic terraces. Local spectral response measurements indicated a variation in the local electronic structure among the terraces, which was consistent with a non-uniform spatial distribution of sub-band-gap states within the crystals. The photoconversion efficiencies of Pt-decorated p-WSe_2 photocathodes were greater for the evolution of H_2(g) from 0.5 M H_2SO_4 than for the reduction of Ru(NH_3)_6^(3+/2+), and terraces that exhibited relatively low values of Φ_(ext) for the reduction of Ru(NH_3)_6^(3+/2+) could in some cases yield values of Φ_(ext) for the evolution of H_2(g) comparable to the values of Φ_(ext) yielded by the highest-performing terraces. Although the spatial resolution of the techniques used in this work frequently did not result in observation of the effect of edge sites on photocurrent efficiency, some edge effects were observed in the measurements; however the observed edge effects differed among edges, and did not appear to determine the performance of the electrodes