Showing papers by "Nathan S. Lewis published in 2018"

Net-zero emissions energy systems

Abstract: Some energy services and industrial processes-such as long-distance freight transport, air travel, highly reliable electricity, and steel and cement manufacturing-are particularly difficult to provide without adding carbon dioxide (CO2) to the atmosphere. Rapidly growing demand for these services, combined with long lead times for technology development and long lifetimes of energy infrastructure, make decarbonization of these services both essential and urgent. We examine barriers and opportunities associated with these difficult-to-decarbonize services and processes, including possible technological solutions and research and development priorities. A range of existing technologies could meet future demands for these services and processes without net addition of CO2 to the atmosphere, but their use may depend on a combination of cost reductions via research and innovation, as well as coordinated deployment and integration of operations across currently discrete energy industries.

Net-zero Emissions Energy Systems

Abstract: Models show that to avert dangerous levels of climate change, global carbon dioxide emissions must fall to zero later this century. Most of these emissions arise from energy use. Davis et al. review what it would take to achieve decarbonization of the energy system. Some parts of the energy system are particularly difficult to decarbonize, including aviation, long-distance transport, steel and cement production, and provision of a reliable electricity supply. Current technologies and pathways show promise, but integration of now-discrete energy sectors and industrial processes is vital to achieve minimal emissions. Net emissions of CO2 by human activities - including not only energy services and industrial production but also land use and agriculture - must approach zero in order to stabilize global mean temperature. Energy services such as light-duty transportation, heating, cooling, and lighting may be relatively straightforward to decarbonize by electrifying and generating electricity from variable renewable energy sources (such as wind and solar) and dispatchable ("on-demand") nonrenewable sources (including nuclear energy and fossil fuels with carbon capture and storage). However, other energy services essential to modern civilization entail emissions that are likely to be more difficult to fully eliminate. These difficult-to-decarbonize energy services include aviation, long-distance transport, and shipping; production of carbon-intensive structural materials such as steel and cement; and provision of a reliable electricity supply that meets varying demand. Moreover, demand for such services and products is projected to increase substantially over this century. The long-lived infrastructure built today, for better or worse, will shape the future. Here, we review the special challenges associated with an energy system that does not add any CO2 to the atmosphere (a net-zero emissions energy system). We discuss prominent technological opportunities and barriers for eliminating and/or managing emissions related to the difficult-to-decarbonize services; pitfalls in which near-term actions may make it more difficult or costly to achieve the net-zero emissions goal; and critical areas for research, development, demonstration, and deployment. It may take decades to research, develop, and deploy these new technologies. DOI Link: https://doi.org/10.1126/science.aas9793

Geophysical constraints on the reliability of solar and wind power in the United States

Abstract: We analyze 36 years of global, hourly weather data (1980–2015) to quantify the covariability of solar and wind resources as a function of time and location, over multi-decadal time scales and up to continental length scales. Assuming minimal excess generation, lossless transmission, and no other generation sources, the analysis indicates that wind-heavy or solar-heavy U.S.-scale power generation portfolios could in principle provide ∼80% of recent total annual U.S. electricity demand. However, to reliably meet 100% of total annual electricity demand, seasonal cycles and unpredictable weather events require several weeks’ worth of energy storage and/or the installation of much more capacity of solar and wind power than is routinely necessary to meet peak demand. To obtain ∼80% reliability, solar-heavy wind/solar generation mixes require sufficient energy storage to overcome the daily solar cycle, whereas wind-heavy wind/solar generation mixes require continental-scale transmission to exploit the geographic diversity of wind. Policy and planning aimed at providing a reliable electricity supply must therefore rigorously consider constraints associated with the geophysical variability of the solar and wind resource—even over continental scales.

In situ recombination junction between p-Si and TiO2 enables high-efficiency monolithic perovskite/Si tandem cells.

Abstract: Increasing the power conversion efficiency of silicon (Si) photovoltaics is a key enabler for continued reductions in the cost of solar electricity. Here, we describe a two-terminal perovskite/Si tandem design that increases the Si cell’s output in the simplest possible manner: by placing a perovskite cell directly on top of the Si bottom cell. The advantageous omission of a conventional interlayer eliminates both optical losses and processing steps and is enabled by the low contact resistivity attainable between n-type TiO2 and Si, established here using atomic layer deposition. We fabricated proof-of-concept perovskite/Si tandems on both homojunction and passivating contact heterojunction Si cells to demonstrate the broad applicability of the interlayer-free concept. Stabilized efficiencies of 22.9 and 24.1% were obtained for the homojunction and passivating contact heterojunction tandems, respectively, which could be readily improved by reducing optical losses elsewhere in the device. This work highlights the potential of emerging perovskite photovoltaics to enable low-cost, high-efficiency tandem devices through straightforward integration with commercially relevant Si solar cells.

Reduction of Aqueous CO2 to 1-Propanol at MoS2 Electrodes

Abstract: Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS2 or thin-film MoS2 electrodes yields 1-propanol as the major CO2 reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and t-butanol were also produced. At an applied potential of −0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO2 to 1-propanol were ∼3.5% for MoS2 single crystals and ∼1% for thin films with low edge-site densities. Reduction of CO2 to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e– and 18 H+ in a process that involves the formation of 2 C–C bonds. NMR analyses using 13CO2 showed the production of 13C-labeled 1-propanol. In all cases, the vast majority of the Faradaic current resulted in hydrogen evolution via water reduction. H2S was detected qualitatively when single-crystal MoS2 electrodes were used, indicating that some desulfidization of si...

The Predominance of Hydrogen Evolution on Transition Metal Sulfides and Phosphides under CO2 Reduction Conditions: An Experimental and Theoretical Study

Abstract: A combination of experiment and theory has been used to understand the relationship between the hydrogen evolution reaction (HER) and CO2 reduction (CO2R) on transition metal phosphide and transition metal sulfide catalysts. Although multifunctional active sites in these materials could potentially improve their CO2R activity relative to pure transition metal electrocatalysts, under aqueous testing conditions, these materials showed a high selectivity for the HER relative to CO2R. Computational results supported these findings, indicating that a limitation of the metal phosphide catalysts is that the HER is favored thermodynamically over CO2R. On Ni-MoS2, a limitation is the kinetic barrier for the proton–electron transfer to *CO. These theoretical and experimental results demonstrate that selective CO2R requires electrocatalysts that possess both favorable thermodynamic pathways and surmountable kinetic barriers.

Relative costs of transporting electrical and chemical energy

Abstract: Transportation costs of energy resources are important when determining the overall economics of future energy infrastructure The majority of long distance energy transmission occurs via merchant ships and pipelines carrying oil or natural gas In contrast, future energy scenarios often envision vastly altered energy transportation scenarios including very high degrees of grid electrification and widespread installation of hydrogen pipelines The unit cost of energy transportation varies by over two orders of magnitude In particular, the costs of electricity and hydrogen transmission are substantially higher than the cost of oil and natural gas transportation If carbon pricing is to be used to incentivize alternative energy systems, these differences in costs will need to be reduced and used when making meaningful technology comparisons

Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense Co–P Catalyst Films on Structured p-Si Photocathodes

Abstract: Planar and three-dimensionally structured p-Si devices, consisting of an electrodeposited Co–P catalyst on arrays of Si microwires or Si micropyramids, were used as photocathodes for solar-driven hydrogen evolution in 050 M H2SO4(aq) to assess the effects of electrode structuring on parasitic absorption by the catalyst Without the use of an emitter layer, p-Si/Co–P microwire arrays produced a photocurrent density of −10 mA cm–2 at potentials that were 130 mV more positive than those of optimized planar p-Si/Co–P devices Champion p-Si/Co–P microwire array devices exhibited ideal regenerative cell solar-to-hydrogen efficiencies of >25% and were primarily limited by the photovoltage of the p-Si/Co–P junction The vertical sidewalls of the Si microwire photoelectrodes thus minimized effects due to parasitic absorption at high loadings of catalyst for device structures with or without emitters

Tin Oxide as a Protective Heterojunction with Silicon for Efficient Photoelectrochemical Water Oxidation in Strongly Acidic or Alkaline Electrolytes

Abstract: Photoelectrodes without a p–n junction are often limited in efficiency by charge recombination at semiconductor surfaces and slow charge transfer to electrocatalysts. This study reports that tin oxide (SnO_x) layers applied to n‐Si wafers after forming a thin chemically oxidized SiO_x layer can passivate the Si surface while producing ≈620 mV photovoltage under 100 mW cm^(−2) of simulated sunlight. The SnO_x layer makes ohmic contacts to Ni, Ir, or Pt films that act as precatalysts for the oxygen‐evolution reaction (OER) in 1.0 m KOH(aq) or 1.0 m H_2SO_4(aq). Ideal regenerative solar‐to‐O_2(g) efficiencies of 4.1% and 3.7%, respectively, are obtained in 1.0 m KOH(aq) with Ni or in 1.0 m H2_SO_4(aq) with Pt/IrO_x layers as OER catalysts. Stable photocurrents for >100 h are obtained for electrodes with patterned catalyst layers in both 1.0 m KOH(aq) and 1.0 m H_2SO_4(aq).

Phase Directing Ability of an Ionic Liquid Solvent for the Synthesis of HER-Active Ni2P Nanocrystals

Abstract: An ionic liquid (IL) solvent was used to synthesize small, phase-pure nickel phosphide (Ni2P) nanocrystals. In contrast, under analogous reaction conditions, substitution of the IL for the common high-boiling organic solvent 1-octadecene (ODE) results in phase-impure nanocrystals. The 5 nm Ni2P nanocrystals prepared in IL were electrocatalytically active toward the hydrogen evolution reaction. The synthesis in IL was also extended to alloyed Ni2–xCoxP nanocrystals, where 0.5 ≤ x ≤ 1.5.

Performance and failure modes of Si anodes patterned with thin-film Ni catalyst islands for water oxidation

Abstract: Silicon photoanodes patterned with thin-film Ni catalyst islands exhibited stable oxygen evolution for over 240 h of continuous operation in 1.0 mol L−1 KOH under simulated sunlight conditions. Buried-junction np+-Si(111) photoanodes with an 18.0% filling fraction of a square array of Ni microelectrodes, np+-Si(111)|NiμE18.0%, demonstrated performance equivalent to a Ni anode in series with a photovoltaic device having an open-circuit voltage of 538 ± 20 mV, a short-circuit current density of 20.4 ± 1.3 mA cm−2, and a photovoltaic efficiency of 6.7 ± 0.9%. For the np+-Si(111)|NiμE18.0% samples, the photocurrent density at the equilibrium potential for oxygen evolution was 12.7 ± 0.9 mA cm−2, yielding an ideal regenerative cell solar-to-oxygen conversion efficiency of 0.47 ± 0.07%. The photocurrent passed exclusively through the Ni catalyst islands to evolve O2 with nearly 100% faradaic efficiency, while a passivating, insulating surface layer of SiOx formed in situ on areas of the Si in direct contact with the electrolyte. The (photo)electrochemical behavior of Si electrodes patterned with varying areal filling fractions of Ni catalyst islands was also investigated. The stability and efficiency of the patterned-catalyst Si electrodes were affected by the filling fraction of the Ni catalyst, the orientation and dopant type of the substrates, and the measurement conditions. The electrochemical behavior at different stages of operation, including Ni catalyst activation, Si passivation, stable operation, and device failure, was affected by the dynamic processes of anodic formation and isotropic dissolution of SiOx on the exposed Si. Ex situ and operando microscopic and spectroscopic studies revealed that these processes were three-dimensional and spatially non-uniform across the surface of the substrate, and occurred near the active catalyst islands. The patterned catalyst/substrate electrodes serve as a model system for accelerated studies of failure mechanisms in photoanodes protected by multifunctional catalytic coatings or other hole-conductive thin-film coatings that contain defects.

Template-Free Synthesis of Periodic Three-Dimensional PbSe Nanostructures via Photoelectrodeposition.

Abstract: Highly periodic, geometrically directed, anisotropic Se–Pb films have been synthesized at room temperature from an isotropic aqueous solution without the use of physical templates by photoelectrodeposition using a series of discrete input illumination polarizations and wavelengths from an unstructured, uncorrelated, incoherent light source. Dark growth did not generate deposits with substantial long-range order, but growth using unpolarized illumination resulted in an ordered, nanoscale, mesh-type morphology. Linearly polarized illumination generated Se–Pb deposits that displayed an ordered, highly anisotropic lamellar pattern wherein the long axes of the lamellae were aligned parallel to the light polarization vector. The pitch of the lamellar features was proportional to the input light wavelength, as confirmed by Fourier analysis. Full-wave electromagnetic and Monte Carlo growth simulations that incorporated only the fundamental light–matter interactions during growth successfully reproduced the experi...

Reduction of aqueous CO_2 to 1-Propanol at MoS_2 electrodes

Abstract: Reduction of carbon dioxide in aqueous electrolytes at single-crystal MoS_2 or thin-film MoS_2 electrodes yields 1-propanol as the major CO_2 reduction product, along with hydrogen from water reduction as the predominant reduction process. Lower levels of formate, ethylene glycol, and t-butanol were also produced. At an applied potential of −0.59 V versus a reversible hydrogen electrode, the Faradaic efficiencies for reduction of CO_2 to 1-propanol were ∼3.5% for MoS2single crystals and ∼1% for thin films with low edge-site densities. Reduction of CO_2 to 1-propanol is a kinetically challenging reaction that requires the overall transfer of 18 e– and 18 H+ in a process that involves the formation of 2 C–C bonds. NMR analyses using ^(13)CO_2 showed the production of ^(13)C-labeled 1-propanol. In all cases, the vast majority of the Faradaic current resulted in hydrogen evolution via water reduction. H_2S was detected qualitatively when single-crystal MoS_2 electrodes were used, indicating that some desulfidization of single crystals occurred under these conditions.

Correction: Geophysical constraints on the reliability of solar and wind power in the United States

Abstract: Correction for ‘Geophysical constraints on the reliability of solar and wind power in the United States’ by Matthew R. Shaner et al., Energy Environ. Sci., 2018, DOI: 10.1039/c7ee03029k.

Role of Doping Dependent Radiative and Non-radiative Recombination in Determining the Limiting Efficiencies of Silicon Solar Cells

Abstract: We show that increasing the bulk doping in a silicon based solar cell can increase the fraction of photo generated carriers that recombine radiatively at open circuit condition. This increases the maximum achievable open circuit voltage (Voc) in a solar cell At higher doping levels auger recombination and band gap narrowing effects dominate leading to a reduction in Voc. Therefore radiative and non-radiative recombinations at Voc determines the optimum doping of the bulk to maximize the performance especially in thin solar cells with increased surface area due to surface texturing.

Effects of Sub-gap Absorption on Photovoltaic Performance

Abstract: The theoretical maximum photovoltaic efficiency is bounded by the Shockley-Queisser model which assumes that the photovoltaic material has step-function like absorptance, with zero absorption below its bandgap and perfect absorption above. However, typical photovoltaic materials exhibit absorption with an exponential dependence below its bandgap, known as the“Urbach tail” whose steepness is dependent on the Urbach parameter. Using a modified detailed balance model that accounts for sub-gap absorption we show that photovoltaic performance is strongly affected by the magnitude of the Urbach parameter.