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1.
ACS Omega ; 5(27): 16455-16459, 2020 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-32685809

RESUMO

The recirculation of gases in a sealed reactor system is a broadly useful method in catalytic and electrocatalytic studies. It is especially relevant when a reactant gas reacts slowly with respect to residence time in a catalytic reaction zone and when mass transport control through the reaction zone is necessary. This need is well illustrated in the field of electrocatalytic N2 reduction, where the need for recirculation of 15N2 has recently become more apparent. Herein, we describe the design, fabrication, use, and specifications of a lubricant-free, readily constructed recirculating pump fabricated entirely from glass and inert polymer (poly(ether ether ketone) (PEEK), poly(tetrafluoroethylene) (PTFE)) components. Using these glass and polymer components ensures chemical compatibility between the piston pump and a wide range of chemical environments, including strongly acidic and organic electrolytes often employed in studies of electrocatalytic N2 reduction. The lubricant-free nature of the pump and the presence of components made exclusively of glass and PEEK/PTFE mitigate contamination concerns associated with recirculating gases saturated with corrosive or reactive vapors for extended periods. The gas recirculating glass pump achieved a flow rate of >500 mL min-1 N2 against atmospheric pressure at 15 W peak power input and >100 mL min-1 N2 against a differential pressure of +6 in. H2O (∼15 mbar) at 10 W peak power input.

2.
ACS Appl Mater Interfaces ; 12(5): 5901-5908, 2020 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-31971770

RESUMO

Silicon has shown promise for use as a small band gap (1.1 eV) absorber material in photoelectrochemical (PEC) water splitting. However, the limited stability of silicon in acidic electrolyte requires the use of protection strategies coupled with catalysts. Herein, spin coating is used as a versatile method to directly coat silicon photoanodes with an IrOx oxygen evolution reaction (OER) catalyst, reducing the processing complexity compared to conventional fabrication schemes. Biphasic strontium chloride/iridium oxide (SrCl2:IrOx) catalysts are also developed, and both catalysts form photoactive junctions with silicon and demonstrate high photoanode activity. The iridium oxide photoanode displays a photocurrent onset at 1.06 V vs reversible hydrogen electrode (RHE), while the SrCl2:IrOx photoanode onsets earlier at 0.96 V vs RHE. The differing potentials are consistent with the observed photovoltages of 0.43 and 0.53 V for the IrOx and SrCl2:IrOx, respectively. By measuring the oxidation of a reversible redox couple, Fe(CN)63-/4-, we compare the charge carrier extraction of the devices and show that the addition of SrCl2 to the IrOx catalyst improves the silicon-electrolyte interface compared to pure IrOx. However, the durability of the strontium-containing photoanode remains a challenge, with its photocurrent density decreasing by 90% over 4 h. The IrOx photoanode, on the other hand, maintained a stable photocurrent density over this timescale. Characterization of the as-prepared and post-tested material structure via Auger electron spectroscopy identifies catalyst film cracking and delamination as the primary failure modes. We propose that improvements to catalyst adhesion should further the viability of spin coating as a technique for photoanode preparation.

4.
Nature ; 570(7762): 504-508, 2019 06.
Artigo em Inglês | MEDLINE | ID: mdl-31117118

RESUMO

The electrochemical synthesis of ammonia from nitrogen under mild conditions using renewable electricity is an attractive alternative1-4 to the energy-intensive Haber-Bosch process, which dominates industrial ammonia production. However, there are considerable scientific and technical challenges5,6 facing the electrochemical alternative, and most experimental studies reported so far have achieved only low selectivities and conversions. The amount of ammonia produced is usually so small that it cannot be firmly attributed to electrochemical nitrogen fixation7-9 rather than contamination from ammonia that is either present in air, human breath or ion-conducting membranes9, or generated from labile nitrogen-containing compounds (for example, nitrates, amines, nitrites and nitrogen oxides) that are typically present in the nitrogen gas stream10, in the atmosphere or even in the catalyst itself. Although these sources of experimental artefacts are beginning to be recognized and managed11,12, concerted efforts to develop effective electrochemical nitrogen reduction processes would benefit from benchmarking protocols for the reaction and from a standardized set of control experiments designed to identify and then eliminate or quantify the sources of contamination. Here we propose a rigorous procedure using 15N2 that enables us to reliably detect and quantify the electrochemical reduction of nitrogen to ammonia. We demonstrate experimentally the importance of various sources of contamination, and show how to remove labile nitrogen-containing compounds from the nitrogen gas as well as how to perform quantitative isotope measurements with cycling of 15N2 gas to reduce both contamination and the cost of isotope measurements. Following this protocol, we find that no ammonia is produced when using the most promising pure-metal catalysts for this reaction in aqueous media, and we successfully confirm and quantify ammonia synthesis using lithium electrodeposition in tetrahydrofuran13. The use of this rigorous protocol should help to prevent false positives from appearing in the literature, thus enabling the field to focus on viable pathways towards the practical electrochemical reduction of nitrogen to ammonia.

5.
Nano Lett ; 16(7): 4082-6, 2016 07 13.
Artigo em Inglês | MEDLINE | ID: mdl-27322181

RESUMO

The behavior of n-Si(111) photoanodes covered by monolayer sheets of fluorinated graphene (F-Gr) was investigated under a range of chemical and electrochemical conditions. The electrochemical behavior of n-Si/F-Gr and np(+)-Si/F-Gr photoanodes was compared to hydride-terminated n-Si (n-Si-H) and np(+)-Si-H electrodes in contact with aqueous Fe(CN)6(3-/4-) and Br2/HBr electrolytes as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. Illuminated n-Si/F-Gr and np(+)-Si/F-Gr electrodes in contact with an aqueous K3(Fe(CN)6/K4(Fe(CN)6 solutions exhibited stable short-circuit photocurrent densities of ∼10 mA cm(-2) for 100,000 s (>24 h), in comparison to bare Si electrodes, which yielded nearly a complete photocurrent decay over ∼100 s. X-ray photoelectron spectra collected before and after exposure to aqueous anodic conditions showed that oxide formation at the Si surface was significantly inhibited for Si electrodes coated with F-Gr relative to bare Si electrodes exposed to the same conditions. The variation of the open-circuit potential for n-Si/F-Gr in contact with a series of nonaqueous electrolytes of varying reduction potential indicated that the n-Si/F-Gr did not form a buried junction with respect to the solution contact. Further, illuminated n-Si/F-Gr electrodes in contact with Br2/HBr(aq) were significantly more electrochemically stable than n-Si-H electrodes, and n-Si/F-Gr electrodes coupled to a Pt catalyst exhibited ideal regenerative cell efficiencies of up to 5% for the oxidation of Br(-) to Br2.

6.
J Phys Chem Lett ; 6(4): 592-8, 2015 Feb 19.
Artigo em Inglês | MEDLINE | ID: mdl-26262472

RESUMO

Semiconductors with small band gaps (<2 eV) must be stabilized against corrosion or passivation in aqueous electrolytes before such materials can be used as photoelectrodes to directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of photoelectrodes is required for efficient oxidation of H2O to O2(g) and reduction of H2O or H2O and CO2 to fuels. We report herein the stabilization of np(+)-Si(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O2(g) in 1.0 M KOH(aq) by an earth-abundant, optically transparent, electrocatalytic, stable, conducting nickel oxide layer. Under simulated solar illumination and with optimized index-matching for proper antireflection, NiOx-coated np(+)-Si(100) photoanodes produced photocurrent-onset potentials of -180 ± 20 mV referenced to the equilibrium potential for evolution of O2(g), photocurrent densities of 29 ± 1.8 mA cm(-2) at the equilibrium potential for evolution of O2(g), and a solar-to-O2(g) conversion figure-of-merit of 2.1%.

7.
J Am Chem Soc ; 135(46): 17246-9, 2013 Nov 20.
Artigo em Inglês | MEDLINE | ID: mdl-24125019

RESUMO

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K3[Fe(CN)6]/K4[Fe(CN)6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm(-2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si-H electrodes yielded a nearly complete decay of the current density within ~100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact.

8.
J Am Chem Soc ; 130(50): 16844-5, 2008 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-19053427

RESUMO

Pyridine borane is combined with TpW(NO)(PMe(3))(eta(2)-benzene) to form a complex of the heterocycle, which upon treatment with acetone and acid yields the pyridinium complex [TpW(NO)(PMe(3))(eta(2)-pyH(+))]OTf. Deprotonation in the presence of acetic anhydride delivers the N-acetylpyridinium complex as a 10:1 mixture of coordination diastereomers. This acylpyridinium resists reaction with water or oxygen but readily reacts with acetone, pyrrole, indole, or acrolein and a weak base to stereoselectively form 1,2-dihydropyridine complexes. Treatment of the indole-derived analogue with CuBr(2) results in liberation of 3-(pyridin-2-yl)-1H-indole.

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