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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Anthraquinone Redox Relay for Dye‐Sensitized Photo‐electrochemical H 2 O 2 Production

Abstract Anthraquinone (AQ) redox mediators are introduced to metal‐free organic dye sensitized photo‐electrochemical cells (DSPECs) for the generation of H 2 O 2 . Instead of directly reducing O 2 to produce H 2 O 2 , visible‐light‐driven AQ reduction occurs in the DSPEC and the following autooxidation with O 2 allows H 2 O 2 accumulation and AQ regeneration. In an aqueous electrolyte, under 1 sun conditions, a water‐soluble AQ salt is employed with the highest photocurrent of up to 0.4 mA cm −2 and near‐quantitative faradaic efficiency for producing H 2 O 2 . In a non‐aqueous electrolyte, under 1 sun illumination, an organic‐soluble AQ is applied and the photocurrent reaches 1.8 mA cm −2 with faradaic efficiency up to 95 % for H 2 O 2 production. This AQ‐relay DSPEC exhibits the highest photocurrent so far in non‐aqueous electrolytes for H 2 O 2 production and excellent acid stability in aqueous electrolytes, thus providing a practical and efficient strategy for visible‐light‐driven H 2 O 2 production.

Sun, Jiaonan↗
DOE OSTI2016

Covalent surface modification of gallium arsenide photocathodes for water splitting in highly acidic electrolyte

Efficient water splitting using light as the only energy input requires stable semiconductor electrodes with favorable energetics for the water-oxidation and proton-reduction reactions. Strategies to tune electrode potentials using molecular dipoles adsorbed to the semiconductor surface have been pursued for decades but are often based on weak interactions and quickly react to desorb the molecule under conditions relevant to sustained photoelectrolysis. Here, we show that covalent attachment of fluorinated, aromatic molecules to p-GaAs(1 0 0) surfaces can be employed to tune the photocurrent onset potentials of p-GaAs(1 0 0) photocathodes and reduce the external energy required for water splitting. Results indicate that initial photocurrent onset potentials can be shifted by nearly 150 mV in pH -0.5 electrolyte under 1 Sun (1000 W m -2 ) illumination resulting from the covalently bound surface dipole. Furthermore, X-ray photoelectron spectroscopy analysis reveals that the covalent molecular dipole attachment is not robust under extended 50 h photoelectrolysis, the modified surface delays arsenic oxide formation that results in a p-GaAs(1 0 0) photoelectrode operating at a sustained photocurrent density of -20.5 mA cm -2 within -0.5 V of the reversible hydrogen electrode.

36 MATERIALS SCIENCE↗
DOE OSTI2017

GaP/GaNP Heterojunctions for Efficient Solar‐Driven Water Oxidation

The growth and characterization of an n‐GaP/i‐GaNP/p + ‐GaP thin film heterojunction synthesized using a gas‐source molecular beam epitaxy (MBE) method, and its application for efficient solar‐driven water oxidation is reported. The TiO 2 /Ni passivated n‐GaP/i‐GaNP/p + ‐GaP thin film heterojunction provides much higher photoanodic performance in 1 m KOH solution than the TiO 2 /Ni‐coated n‐GaP substrate, leading to much lower onset potential and much higher photocurrent. There is a significant photoanodic potential shift of 764 mV at a photocurrent of 0.34 mA cm −2 , leading to an onset potential of ≈0.4 V versus reversible hydrogen electrode (RHE) at 0.34 mA cm −2 for the heterojunction. The photocurrent at the water oxidation potential (1.23 V vs RHE) is 1.46 and 7.26 mA cm −2 for the coated n‐GaP and n‐GaP/i‐GaNP/p + ‐GaP photoanodes, respectively. The passivated heterojunction offers a maximum applied bias photon‐to‐current efficiency (ABPE) of 1.9% while the ABPE of the coated n‐GaP sample is almost zero. Furthermore, the coated n‐GaP/i‐GaNP/p + ‐GaP heterojunction photoanode provides a broad absorption spectrum up to ≈620 nm with incident photon‐to‐current efficiencies (IPCEs) of over 40% from ≈400 to ≈560 nm. The high low‐bias performance and broad absorption of the wide‐bandgap GaP/GaNP heterojunctions render them as a promising photoanode material for tandem photoelectrochemical (PEC) cells to carry out overall solar water splitting.

Kargar, Alireza↗
DOE OSTI2025

Efficient BiVO 4 /CoFeO x H y photoanodes using controlled annealing and conformal linear-sweep electrocatalyst photodeposition

Although monoclinic bismuth vanadate (BiVO 4 ) is a promising photoanode for solar water splitting, its practical use is hindered by imperfect photocurrent generation/collection, low photovoltage compared to the bandgap, and corrosion side reactions that limit durability. Here, we introduce a controlled-annealing sol–gel process for BiVO 4 thin-film photoanodes along with an optimized linear-sweep-voltammetry photodeposition of CoFeO x H y cocatalysts. The resulting BiVO 4 films annealed at 550 °C exhibited a photocurrent density of 4.1 mA/cm 2 at 1.23 V RHE under 1 sun AM 1.5G solar simulation and a low onset potential of 0.26 V RHE due to high majority carrier conductivity, a crystalline bulk with reduced defects as evidenced by x-ray photoelectron spectroscopy and photoluminescence lifetime analysis, and thus enhanced photocarrier collection. However, significant degradation in performance was found due to interfacial photocorrosion. To protect the surface and speed the oxygen-evolution reaction CoFeO x H y cocatalyst layers were deposited. By varying the number of consecutive sweeps and adjusting the applied bias range, an ultra-thin (~15 nm) CoFeO x H y cocatalyst layer was uniformly grown deposited over 30 cycles on the BiVO 4 surface. The resulting BiVO 4 /CoFeO x H y yielded 4.03 mA/cm 2 at 1.23 V RHE and onset potential of 0.24 V RHE , with stable operation (~15 % loss in photocurrent at 1.23 V RHE relative to ~60 % loss in the uncatalyzed control sample). These conformal CoFeO x H y catalytic layers function simultaneously to selectively collect photoexcited holes from the BiVO 4 , catalyze the water-oxidation reaction, and protect the BiVO 4 from photodegradation.

42 ENGINEERING↗
DOE OSTI2011

Boron and nitrogen-codoped TiO{sub 2} nanorods: Synthesis, characterization, and photoelectrochemical properties

Boron and nitrogen codoped TiO{sub 2} nanorods (BNTRs) were synthesized via two-step hydrothermal reactions using TiN as a starting material. The as-prepared samples were characterized by X-ray diffraction, field-emission scanning electron microscope (SEM), transmission electron microscopy and X-ray photoelectron spectroscopy techniques. The results showed that TiO{sub 2} nanorods with the diameter of approximately 50-100 nm and the length of several micrometers were doped by the interstitial N and B. The nanorods were firstly formed in the hydrothermal synthesis of nitrogen doped TiO{sub 2}. The growing process of nanorods was observed by SEM and a most probable formation mechanism of the trititanate nanorods was proposed. The BNTRs showed a higher photocatalytic activity and a bigger photocurrent response than N-TiO{sub 2} nanorods under visible light irradiation. - Graphical abstract: Boron and nitrogen codoped TiO{sub 2} nanorods (BNTRs) were synthesized using TiN. The BNTRs showed a higher photocatalytic activity and a bigger photocurrent than N-TiO{sub 2} nanorods (NTRs) under visible light irradiation. Highlights: > Novel B- and N-codoped TiO{sub 2} nanorods (BNTRs) are designed and synthesized. > A most probable formation mechanism of the trititanate nanorods is proposed. > The BNTRs show an outstanding photocurrent response under visible light. > The BNTRs exhibit an improved visible light photocatalytic activity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

A facile mixed-solvothermal route to γ-Bi{sub 2}MoO{sub 6} nanoflakes and their visible-light-responsive photocatalytic activity

Graphical abstract: γ-Bi2MoO6 nanoflakes were successfully synthesized by a mixed-solvothermal process. The visible-light-responsive photocatalytic activities of the obtained γ-Bi2MoO6 nanoflakes were evaluated by photodegradation of Rhodamie B under visible light. The high degradation efficiency of RhB was attributed to the good crystallization and flake-like morphology of γ-Bi2MoO6 nanostructures, which is in accord with the photocurrent result. - Highlights: • γ-Bi{sub 2}MoO{sub 6} nanoflakes were obtained by a mixed-solvothermal process with SDBS as surfactant. • The photocurrent test was taken to characterize the photocatalytic activity. • The nanopowders exhibit high efficient photocatalytic activity under visibile-light irradiation. - Abstract: γ-Bi{sub 2}MoO{sub 6} nanoflakes were successfully synthesized by a mixed-solvothermal process with SDBS involved in the reaction. The as-prepared γ-Bi{sub 2}MoO{sub 6} was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and UV–vis absorption spectroscopy. The effect of different ratio of solvents on the resulting products was investigated. The visible-light-responsive photocatalytic activities of the obtained γ-Bi{sub 2}MoO{sub 6} nanoflakes were evaluated by photodegradation of Rhodamie B under visible light (λ > 400 nm). The high degradation efficiency of RhB was attributed to the good crystallization and flake-like morphology of γ-Bi{sub 2}MoO{sub 6} nanostructures, which is in accord with the photocurrent result.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Employing overlayers to improve the performance of Cu 2 BaSnS 4 thin film based photoelectrochemical water reduction devices

Earth-abundant copper-barium-thiostannate Cu 2 BaSnS 4 (CBTS)-based thin films have recently been reported to exhibit the optoelectronic and defect properties suitable as absorbers for photoelectrochemical (PEC) water splitting and the top cell of tandem photovoltaic solar cells. However, the photocurrents of CBTS-based PEC devices are still much lower than the theoretical value, partially due to ineffective charge collection at CBTS/water interface and instability of CBTS in electrolytes. Here, we report on overcoming these issues by employing overlayer engineering. We find that CdS/ZnO/TiO 2 overlayers can significant-ly improve the PEC performance, achieving saturated cathodic photocurrents up to 7.8 mA cm -2 at the potential of -0.10 V versus reversible hydrogen electrode (RHE) in a neutral electrolyte solution, which is much higher than the best bare CBTS film attaining a photocurrent of 4.8 mA cm -2 at the potential of -0.2 V versus RHE. Finally, our results suggest a viable approach for improving the performance of CBTS-based PEC cells.

08 HYDROGEN↗
DOE OSTI2018

Single-Nanoflake Photo-Electrochemistry Reveals Champion and Spectator Flakes in Exfoliated MoSe 2 Films

Semiconducting transition-metal dichalcogenide (TMD) nanoflake thin films are promising large-area electrodes for photo-electrochemical solar energy conversion applications. However, their energy conversion efficiencies are typically much lower than those of bulk electrodes. It is unclear to what extent this efficiency gap stems from differences among nanoflakes (e.g., area, thickness, and surface structural features). It is also unclear whether individual exfoliated nanoflakes can achieve energy conversion efficiencies similar to those of bulk crystals. Here, we use a single-nanoflake photo-electrochemical approach to show that there are both highly active and completely inactive nanoflakes within a film. For the exfoliated MoSe 2 samples studied herein, 7% of nanoflakes are highly active champions, whose photocurrent efficiency exceeds that of the bulk crystal. However, 66% of nanoflakes are inactive spectators, which are mostly responsible for the overall lower photocurrent efficiency compared to the bulk crystal. The photocurrent collection efficiency increases with nanoflake area and decreases more at perimeter edges than at interior step edges. These observations, which are hidden in ensemble-level measurements, reveal the underlying performance issues of exfoliated TMD electrodes for photo-electrochemical energy conversion applications.

14 SOLAR ENERGY↗
DOE OSTI2018

Optically Generated Free-Carrier Collection from an All Single-Walled Carbon Nanotube Active Layer

Semiconducting single-walled carbon nanotubes' (SWCNTs) broad absorption range and all-carbon composition make them attractive materials for light harvesting. We report photoinduced charge transfer from both multichiral and single-chirality SWCNT films into atomically flat SnO 2 and TiO 2 crystals. Higher-energy second excitonic SWCNT transitions produce more photocurrent, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes. A logarithmic relationship exists between photoinduced electron-transfer driving force and photocarrier collection efficiency, becoming more efficient with smaller diameter SWCNTs. Photocurrents are generated from both conventional sensitization and in the opposite direction with the semiconductor under accumulation and acting as an ohmic contact with only the p-type nanotubes. Finally, we demonstrate that SWCNT surfactant choice and concentration play a large role in photon conversion efficiency and present methods of maximizing photocurrent yields.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition

Visible-light-driven water splitting was explored in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor–π–acceptor (D–π–A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH 2 ) 3 P(O 3 H 2 ) 2 -pyr)2], 2, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (<1 nm) of TiO 2 , and catalyst binding through phosphonate linkage to the TiO 2 layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO 2 or nanoITO core materials, in acetate-buffered, aqueous solutions at pH 4.6 or 5.7. The absolute magnitudes of photocurrent changes with the core material, TiO 2 spacer layer thickness, or pH, observed photocurrents were 2.5-fold higher in the presence of catalyst. The findings of transient absorption measurements and DFT calculations reflect that electron injection by the photoexcited organic dye is ultrafast promoted by electronic interactions enabled by orientation of the dye’s molecular orbitals on the electrode surface. Rapid injection is followed by recombination with the oxidized dye which is 95% complete by 1.5 ns. While chromophore decomposition limits the efficiency of the DSPEC devices toward O 2 production, the flexibility of the strategy presented here offers a new approach to photoanode design.

14 SOLAR ENERGY↗
DOE OSTI2020

Stacking Independence and Resonant Interlayer Excitation of Monolayer WSe 2 /MoSe 2 Heterostructures for Photocatalytic Energy Conversion

Here, we report a comparison of the photocatalytic performance of WSe 2 -on-MoSe 2 and MoSe 2 -on-WSe 2 heterostructures. While built-in electric fields exist in these heterostructures on the order of 100 kV/cm due to band offsets between these two materials, the photocatalytic performance (i.e., photocurrent) is independent of the stacking order of the two materials. Solving Poisson’s equation under these conditions, we find that the built-in electric field produced in the heterostructure is at least 1 order of magnitude smaller than that produced in the electrochemical double layer (i.e., Helmholtz layer). Mott–Schottky measurements indicate that transition metal dichalcogenides (TMDCs) on ITO electrodes have similar capacitance to that of bare ITO, providing further evidence that the interfacial electric fields produced in the solid state heterostructure are negligible compared to the fields generated by the ions in solution. The photocatalytic performance of these heterostructures provided the largest relative enhancement in the heterojunction region under 920 and 785 nm irradiation compared with 532 and 633 nm wavelength excitation. Here, the 920 nm (1.35 eV) photons lie below the band gaps and produce very little photocurrent in the constituent monolayer materials but resonantly excite the interlayer optical transition in the heterostructure, producing a 5-fold enhancement in the measured photocurrent.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics

Recent efforts in organic photovoltaics (OPVs) have been devoted to obtaining low-bandgap non-fullerene acceptors (NFAs) for high photocurrent generation. However, the low-lying lowest unoccupied molecular orbital (LUMO) level in narrow bandgap NFAs typically results in a small energy difference (Δ E DA ) between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, leading to low open-circuit voltage (V OC ). The trade-off between Δ E DA and photocurrent generation significantly limits the simultaneous enhancement of both V OC and short-circuit current density ( J SC ). Here, we report a new medium-bandgap NFA, IDTT-T, containing a weakly electron-withdrawing N-ethyl thiabarbituric acid terminal group on each end of the indacenodithienothiophene (IDTT) core. When paired with a benchmark low-bandgap PTB7-th polymer donor, simultaneous enhancement of both Δ E DA and absorption spectral coverage was realized. The OPV devices yield a V OC of 1.01 V, corresponding to a low energy loss of 0.57 eV in around 10% efficiency single-junction NFA OPVs. In conclusion, the design demonstrates a working principle to concurrently increase Δ E DA and photocurrent generation for high V OC and PCE in bulk fullerene-free heterojunction OPVs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Impact of Varying the Photoanode/Catalyst Interfacial Composition on Solar Water Oxidation: The Case of BiVO 4 (010)/FeOOH Photoanodes

Photoanodes used in a water-splitting photoelectrochemical cell are almost always paired with an oxygen evolution catalyst (OEC) to efficiently utilize photon-generated holes for water oxidation because the surfaces of photoanodes are typically not catalytic for the water oxidation reaction. Suppressing electron–hole recombination at the photoanode/OEC interface is critical for the OEC to maximally utilize the holes reaching the interface for water oxidation. Here, in order to explicitly demonstrate and investigate how the detailed features of the photoanode/OEC interface affect interfacial charge transfer and photocurrent generation for water oxidation, we prepared two BiVO 4 (010)/FeOOH photoanodes with different Bi:V ratios at the outermost layer of the BiVO 4 interface (close to stoichiometric vs Bi-rich) while keeping all other factors in the bulk BiVO 4 and FeOOH layers identical. The resulting two photoanodes show striking differences in the photocurrent onset potential and photocurrent density for water oxidation. The ambient pressure X-ray photoelectron spectroscopy results show that these two BiVO 4 (010)/FeOOH photoanodes show drastically different Fe 2+ :Fe 3+ ratios in FeOOH both in the dark and under illumination with water, demonstrating the immense impact of the interfacial composition and structure on interfacial charge transfer. Using computational studies, we reveal the effect of the surface Bi:V ratio on the hydration of the BiVO 4 surface and bonding with the FeOOH layer, which in turn affect the band alignments between BiVO 4 and FeOOH. These results explain the atomic origin of the experimentally observed differences in electron and hole transfer and solar water oxidation performance of the two photoanodes having different interfacial compositions.

14 SOLAR ENERGY↗
DOE OSTI2011

Interfacial Electron Transfer Dynamics for [Ru(bpy) 2 ((4,4'-PO 3 H 2 ) 2 bpy)] 2+ Sensitized TiO 2 in a Dye-Sensitized Photoelectrosynthesis Cell: Factors Influencing Efficiency and Dynamics

Nanosecond laser flash photolysis and photocurrent measurements have been used to investigate use of [(Ru(bpy)2(4,4'-(PO3H2)2bpy)]2+ attached to TiO2 nanoparticle films, TiO2-RuII, in a dye-sensitized photoelectrosynthesis cell (DSPEC) configuration for H2 production. In these experiments, laser flash excitation of TiO2-RuII and rapid injection lead to TiO2(e-)-RuIII with subsequent TiO2(e-)-RuIII → TiO2-RuII back electron transfer monitored on the nsec time scale with and without added triethanolamine (TEOA) and deprotonated ethylenediaminetetraacetic tetra-anion (EDTA4-) as irreversible electron transfer donors. With added TEOA or EDTA4-, a competition exists between back electron transfer and scavenger oxidation with the latter leading to H2 production in the photoelectrosynthesis cell. Reduction of TiO2(e-)-RuIII by both TEOA and EDTA4- occurs with kD ~ 106 M-1 s-1. EDTA4- is a more efficient scavenger by a factor of ~3 because of a more favorable partition equilibrium between the film and the external solution. Its increased scavenger efficiency appears in incident photon-to-current conversion efficiency (IPCE) measurements, in electron collection efficiencies (ηcoll), and in photocurrent measurements with H2 production. Evaluation of electron collection efficiencies by transient current measurements gave ηcoll ~ 24% for TEOA and ~ 70% for EDTA4-. The dynamics of back electron transfer are minimized, and collection efficiencies, photocurrents, and hydrogen production are maximized by application of a positive applied bias consistent with the results of I-V measurements. A pH dependent plateau is reached at ~0 V at pH = 4.5 (EDTA4-) and at ~ -0.4 V at pH 6.7 (TEOA). The difference is qualitatively consistent with the influence of pH on electron population in trap states below the conduction band and the role they play in back electron transfer. The excitation dependence of IPCE measurements matches the spectrum of TiO2-RuII with IPCE values ~3 times higher for EDTA4- than for TEOA as noted above. Absorbed photon-to-current efficiency (APCE) values are light-intensity dependent because of the effect of multiple injection events and the influence of increasing trap site electron densities on back electron transfer. The key to efficient H2 production is minimizing back electron transfer. Application of a sufficiently positive potential relative to ECB for TiO2 accelerates loss of electrons from the film in competition with back electron transfer allowing for H2 production with efficiencies approaching 14.7% under steady-state irradiation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Substantial bulk photovoltaic effect enhancement via nanolayering

Spontaneous polarization and inversion symmetry breaking in ferroelectric materials lead to their use as photovoltaic devices. However, further advancement of their applications are hindered by the paucity of ways of reducing bandgaps and enhancing photocurrent. By unravelling the correlation between ferroelectric materials’ responses to solar irradiation and their local structure and electric polarization landscapes, here we show from first principles that substantial bulk photovoltaic effect enhancement can be achieved by nanolayering PbTiO 3 with nickel ions and oxygen vacancies ((PbNiO 2 ) x (PbTiO 3 ) 1–x ). The enhancement of the total photocurrent for different spacings between the Ni-containing layers can be as high as 43 times due to a smaller bandgap and photocurrent direction alignment for all absorption energies. This is due to the electrostatic effect that arises from nanolayering. Lastly, this opens up the possibility for control of the bulk photovoltaic effect in ferroelectric materials by nanoscale engineering of their structure and composition.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Millimetre-long transport of photogenerated carriers in topological insulators

Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in Bi 2-x Sb x Se 3 nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons. Remarkably, these excitons can transport over hundreds of micrometers along the topological insulator (TI) nanoribbons before recombination at up to 40 K. The macroscopic transport distance, combined with short carrier lifetime obtained from transient photocurrent measurements, indicates an exciton diffusion coefficient at least 36 m 2 s -1 , which corresponds to a mobility of 6 × 104 m 2 V -1 s -1 at 7 K and is four order of magnitude higher than the value reported for free carriers in TIs. The observation of highly dissipationless exciton transport implies the formation of superfluid-like exciton condensate at the surface of TIs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗