Showing papers by "Nathan S. Lewis published in 2019"
TL;DR: Electrocatalysts exposed to ~3–30 ALD cycles of TiO2 exhibited overpotentials at 10 mA cm–2 of geometric current density that were several hundred millivolts lower than uncoated catalysts, with correspondingly higher specific activities, indicating a correlation between oxidation state, potential of zero charge, and activity of the tuned electrocatalyst.
Abstract: We report that TiO2 coatings formed via atomic layer deposition (ALD) may tune the activity of IrO2, RuO2, and FTO for the oxygen-evolution and chlorine-evolution reactions (OER and CER). Electrocatalysts exposed to ∼3–30 ALD cycles of TiO2 exhibited overpotentials at 10 mA cm−2 of geometric current density that were several hundred millivolts lower than uncoated catalysts, with correspondingly higher specific activities. For example, the deposition of TiO2 onto IrO2 yielded a 9-fold increase in the OER-specific activity in 1.0 M H2SO4 (0.1 to 0.9 mA cmECSA−2 at 350 mV overpotential). The oxidation state of titanium and the potential of zero charge were also a function of the number of ALD cycles, indicating a correlation between oxidation state, potential of zero charge, and activity of the tuned electrocatalysts.
67 citations
TL;DR: In this paper, electrical transport in amorphous titanium dioxide thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO2|metal substrates that had various top...
Abstract: Electrical transport in amorphous titanium dioxide (a-TiO2) thin films, deposited by atomic layer deposition (ALD), and across heterojunctions of p+-Si|a-TiO2|metal substrates that had various top ...
59 citations
TL;DR: In this paper, it was shown that NiSb_2O_x, CoSb2Ox, and MnSb-2O-x are moderately active catalysts for the chlor-alkali evolution reaction.
Abstract: The chlorine-evolution reaction (CER) is a common, commercially valuable electrochemical reaction, and is practiced at industrial scale globally. A precious metal solid solution of RuO_2 or IrO_2 with TiO_2 is the predominant electrocatalyst for the CER. Herein we report that materials comprised only of non-precious metal elements, specifically crystalline transition-metal antimonates (TMAs) such as NiSb_2O_x, CoSb_2O_x, and MnSb_2O_x, are moderately active, stable catalysts for the electrochemical oxidation of chloride to chlorine under conditions relevant to the commercial chlor-alkali process. Specifically, CoSb2Ox exhibited a galvanostatic potential of 1.804 V vs. NHE at 100 mA cm^(−2) of Cl_2(g) production from aqueous pH = 2.0, 4.0 M NaCl after 250 h of operation. Studies of the bulk and surface of the electrocatalyst and the composition of the electrolyte before and after electrolysis indicated minimal changes in the surface structure and intrinsic activity of CoSb_2O_x as a result of Cl2(g) evolution under these conditions.
57 citations
TL;DR: In this paper, a solar-driven V3+/2+(aq,H2SO4)|KOH(aq) cell, consisting of a carbon-cloth cathode in 2.36 M V2(SO4)3 (pH −0.16), and a Ni mesh anode, was used to produce H2(g) locally under pressure upon demand.
Abstract: A solar-driven V3+/2+(aq,H2SO4)|KOH(aq) cell, consisting of a carbon-cloth cathode in 2.0 M H2SO4(aq) with 0.36 M V2(SO4)3 (pH −0.16), a Ni mesh anode in 2.5 M KOH(aq) (pH 14.21) for the oxygen-evolution reaction (OER), and a bipolar membrane that sustained the pH differentials between the catholyte and anolyte, enabled water splitting with spatial and temporal decoupling of the hydrogen evolution reaction (HER) from the OER and produced H2(g) locally under pressure upon demand. Over a range of potentials and charging depths, V3+ was selectively reduced with >99.8% faradic efficiency. The V2+ species produced in the catholyte was then passed subsequently on demand over a MoCx-based HER catalyst to produce H2(g) and regenerate V3+ for subsequent reduction. Under a base hydrogen pressure of 1, 10, and 100 atm, the discharge efficiency of the V3+ to hydrogen was 83%, 65.2%, and 59.8%, respectively. In conjunction with a solar tracker and a photovoltaic device, the V3+/2+(aq,H2SO4)|KOH(aq) cell was charged ou...
33 citations
TL;DR: Femtosecond time-resolved two-photon photoemission spectroscopy is used to correlate photoexcited electron energetics and dynamics with performance losses and uncovers main factors limiting the energy conversion processes in Cu2O.
Abstract: Cuprous oxide (Cu2O) is a promising material for solar-driven water splitting to produce hydrogen. However, the relatively small accessible photovoltage limits the development of efficient Cu2O based photocathodes. Here, femtosecond time-resolved two-photon photoemission spectroscopy has been used to probe the electronic structure and dynamics of photoexcited charge carriers at the Cu2O surface as well as the interface between Cu2O and a platinum (Pt) adlayer. By referencing ultrafast energy-resolved surface sensitive spectroscopy to bulk data we identify the full bulk to surface transport dynamics for excited electrons rapidly localized within an intrinsic deep continuous defect band ranging from the whole crystal volume to the surface. No evidence of bulk electrons reaching the surface at the conduction band level is found resulting into a substantial loss of their energy through ultrafast trapping. Our results uncover main factors limiting the energy conversion processes in Cu2O and provide guidance for future material development.
27 citations
TL;DR: In this paper, the CoP-and BPMI-based membrane-electrode assemblies (MEAs) were used for vapor-fed electrolysis of water at room temperature at a current density of 10 mA cm−2.
Abstract: Vapor-fed electrolysis of water has been performed using membrane-electrode assemblies (MEAs) incorporating earth-abundant catalysts and bipolar membranes (BPMs). Catalyst films containing CoP nanoparticles, carbon black, and Nafion were synthesized, characterized, and integrated into cathodes of MEAs. The CoP-containing MEAs exhibited stable (>16 h) vapor-fed electrolysis of water at room temperature at a current density of 10 mA cm−2 with 350 mV of additional overvoltage relative to MEA's formed from Pt/C cathodic electrocatalysts due to slower hydrogen-evolution reaction kinetics under vapor-fed conditions and fewer available triple-phase boundaries in the catalyst film. Additionally, catalyst films containing a [NiFe]-layered double hydroxide ([NiFe]-LDH) as well as a hydroxide ion conductor, hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), were synthesized, characterized, and integrated into the anodes of the MEAs. The [NiFe]-LDH-containing MEAs exhibited overvoltages at 10 mA cm−2 that were similar to those of IrOx-containing MEAs for vapor-fed electrolysis of water at room temperature. A BPM was formed by pairing Nafion with HMT-PMBI, resulting in a locally alkaline environment of HMT-PMBI to stabilize the [NiFe]-LDH and a locally acidic environment to stabilize the CoP. BPM-based MEAs were stable (>16 h) for vapor-fed electrolysis of water at room temperature at a current density of 10 mA cm−2, with a change in the pH gradient of 1 unit over 16 h of electrolysis for IrOx-containing MEAs. The stability of [NiFe]-LDH-based MEAs under vapor-fed conditions was dependent on the catalyst film morphology and resulting BPM interface, with stable operation at 10 mA cm−2 achieved for 16 h. All MEAs exhibited a drift in the operating voltage over time associated with dehydration. These results demonstrate that earth-abundant catalysts and BPMs can be incorporated into stable, room-temperature, vapor-fed water-splitting cells operated at 10 mA cm−2.
13 citations
TL;DR: The first decade of Energy & Environmental Science (EES) was celebrated in this article, with a remarkable success and an outlook for the future, which can be attributed to several factors: at initiation, EES was the only high-profile journal to rapidly publish only the highest quality energy-related research.
Abstract: This Editorial commemorates the first
decade of Energy & Environmental Science
(EES), reviewing its remarkable success
as well as outlook for the future. In the
past ten years, EES filled a void by fostering
research and development in energy and
environmental science. The success of
EES can be attributed to several factors:
at initiation, EES was the only high-profile
journal to rapidly publish only the
highest quality energy-related research;
a growing global community of vibrant
energy researchers who initiated Gordon
Research Conferences, international meetings,
etc.; energy research becoming important
in the eyes of the public, due to high
oil prices and environmental issues; a
generation of scientists, especially junior
researchers and students, eager to embrace
the challenge of a transition to net-zero
emissions energy systems; increased funding
for energy research internationally as
well as in the U.S. at the National Science
Foundation and Department of Energy.
EES became the preferred option to publish
high-quality, high-impact energy
research, with success reflected by a
record-setting impact factor for a full-featured
journal.
13 citations
TL;DR: Full-wave electromagnetic simulations of light absorption indicated that the induced spatial growth anisotropy was a function of asymmetric absorption in the evolving deposit, analogous to biological systems such as palm trees that exhibit phototropic growth.
Abstract: Photoelectrochemical deposition of Se–Te on isolated Au islands using an unstructured, incoherent beam of light produces growth of Se–Te alloy toward the direction of the incident light beam. Full-...
7 citations
TL;DR: In this article, the use of arrays of antireflective, high-aspect-ratio Si microcones (μ-cones), coupled with light-blocking Pt and Co-P catalysts, as photocathodes for H2 evolution was reported.
Abstract: Microstructured photoelectrode morphologies can advantageously facilitate integration of optically absorbing electrocatalysts with semiconducting light absorbers, to maintain low overpotentials for fuel production without producing a substantial loss in photocurrent density. We report herein the use of arrays of antireflective, high-aspect-ratio Si microcones (μ-cones), coupled with light-blocking Pt and Co–P catalysts, as photocathodes for H2 evolution. Thick (∼16 nm) layers of Pt or Co–P deposited onto Si μ-cone arrays yielded absolute light-limited photocurrent densities of ∼32 mA cm−2, representing a reduction in light-limited photocurrent density of 6% relative to bare Si μ-cone-array photocathodes, while maintaining high fill factors and low overpotentials for H2 production from 0.50 M H2SO4(aq). The Si μ-cone arrays were embedded in a flexible polymeric membrane and removed from the Si substrate, to yield flexible photocathodes consisting of polymer-embedded arrays of free-standing μ-cones that evolved hydrogen from 0.50 M H2SO4(aq).
5 citations
4 citations
TL;DR: In this article, an array of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups was used to photoelectrochemically oxidize I −(aq) to I 3 −(q) in 7.6 M HI(aq).
Abstract: 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 I3–(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 ...
TL;DR: Comparison with the harmonic frequency of a methyl group in a 3-fold cosine potential suggests that the hindered rotation occurs via uncorrelated jumps of single methyl groups rather than concerted gear-like rotation.
Abstract: The methyl-terminated Si(111) surface possesses a 3-fold in-plane symmetry, with the methyl groups oriented perpendicular to the substrate. The propeller-like rotation of the methyl groups is hinde...
TL;DR: The doping-related effects of the Si substrate on the pattern fidelity of the phototropic Se-Te deposits are ascribable to an electrical effect produced by the different interfacial junction energetics between Se-te and p+-Si versus n-Si that influences the dynamic behavior during phototropic growth at the Se- Te/Si interface.
Abstract: The long-range order of anisotropic phototropic Se-Te films grown electrochemically at room temperature under uniform-intensity, polarized, incoherent, near-IR illumination has been investigated using crystalline (111)-oriented Si substrates doped degenerately with either p- or n-type dopants. Fourier-transform (FT) analysis was performed on large-area images obtained with a scanning electron microscope, and peak shapes in the FT spectra were used to determine the pattern fidelity in the deposited Se-Te films. Under nominally identical illumination conditions, phototropic films grown on p+-Si(111) exhibited a higher degree of anisotropy and a more well-defined pattern period than phototropic films grown on n+-Si(111). Similar differences in the phototropic Se-Te deposit morphology and pattern fidelity on p+-Si versus n+-Si were observed when the deposition rate and current densities were controlled for by adjusting the deposition parameters and illumination conditions. The doping-related effects of the Si substrate on the pattern fidelity of the phototropic Se-Te deposits are ascribable to an electrical effect produced by the different interfacial junction energetics between Se-Te and p+-Si versus n+-Si that influences the dynamic behavior during phototropic growth at the Se-Te/Si interface.
Patent•
31 Oct 2019TL;DR: In this article, a multi-junction photovoltaic cell that does not utilize a conventional interconnection layer and instead places a wide bandgap oxide conductor, for example, a metal oxide such as TiO2, between a top light absorption layer having a relatively large bandgap and a bottom light absorbing layer with a relatively small bandgap is presented.
Abstract: Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Disclosed herein is a multi-junction photovoltaic cell that does not utilize a conventional interconnection layer and instead places a wide bandgap oxide conductor, for example, a metal oxide such as TiO2, between a top light absorption layer having a relatively large bandgap and a bottom light absorption layer having a relatively small bandgap. The advantageous omission of a conventional interconnection layer between the two subcells is enabled by low contact resistivity between the top and bottom light absorbing layers provided by the wide bandgap oxide conductor. The absence of the conventional interconnect between the subcells significantly reduces both optical losses and processing steps. The disclosed photovoltaic cell may thus enable low-cost, high-efficiency multi-junction devices through less complex manufacturing processes and lower material costs.
01 Dec 2019
TL;DR: In this article, Dowling et al. proposed a method to solve the problem of human-computer interaction in the context of artificial neural networks (ANNs) and proposed an algorithm to solve it.
Abstract: Jacqueline A. Dowling, California Institute of Technology, 1-626-395-4575, jdowling@caltech.edu Mengyao Yuan, Carnegie Institution for Science, 1-650-319-8904, myuan@carnegiescience.edu Fan Tong, Carnegie Institution for Science, 1-650-319-8904, ftong@carnegiescience.edu Nathan S. Lewis, California Institute of Technology, 1-626-395-6335, nslewis@caltech.edu Ken Caldeira, Carnegie Institution for Science, 1-650-319-8904, kcaldeira@carnegiescience.edu