First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical ...characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn3+ disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.
A critical determinant of solar-driven water splitting efficiency is the kinetic profile of the O2 evolving catalyst (OEC). We now report the kinetic profiles of water splitting by a self-assembled ...nickel–borate (NiBi) OEC. Mechanistic studies of anodized films of NiBi exhibit the low Tafel slope of 2.3 × RT/2F (30 mV/decade at 25 °C). This Tafel slope together with an inverse third order rate dependence on H+ activity establishes NiBi as an ideal catalyst to be used in the construction of photoelectrochemical devices for water splitting. In contrast, nonanodized NiBi films display significantly poorer activity relative to their anodized congeners that we attribute to a more sluggish electron transfer from the catalyst resting state. Borate is shown to play two ostensibly antagonistic roles in OEC activity: as a promulgator of catalyst activity by enabling proton-coupled electron transfer (PCET) and as an inhibitor in its role as an adsorbate of active sites. By defining the nature of the PCET pre-equilibrium that occurs during turnover, trends in catalyst activity may be completely reversed at intermediate pH as compared to those at pH extremes. These results highlight the critical role of PCET pre-equilibria in catalyst self-assembly and turnover, and accordingly suggest a reassessment in how OEC activities of different catalysts are compared and rationalized.
Strain fields in twisted bilayer graphene Kazmierczak, Nathanael P; Van Winkle, Madeline; Ophus, Colin ...
Nature materials,
07/2021, Letnik:
20, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Van der Waals heteroepitaxy allows deterministic control over lattice mismatch or azimuthal orientation between atomic layers to produce long-wavelength superlattices. The resulting electronic phases ...depend critically on the superlattice periodicity and localized structural deformations that introduce disorder and strain. In this study we used Bragg interferometry to capture atomic displacement fields in twisted bilayer graphene with twist angles <2°. Nanoscale spatial fluctuations in twist angle and uniaxial heterostrain were statistically evaluated, revealing the prevalence of short-range disorder in moiré heterostructures. By quantitatively mapping strain tensor fields, we uncovered two regimes of structural relaxation and disentangled the electronic contributions of constituent rotation modes. Further, we found that applied heterostrain accumulates anisotropically in saddle-point regions, generating distinctive striped strain phases. Our results establish the reconstruction mechanics underpinning the twist-angle-dependent electronic behaviour of twisted bilayer graphene and provide a framework for directly visualizing structural relaxation, disorder and strain in moiré materials.
Iron doping of nickel oxide films results in enhanced activity for promoting the oxygen evolution reaction (OER). Whereas this enhanced activity has been ascribed to a unique iron site within the ...nickel oxide matrix, we show here that Fe doping influences the Ni valency. The percent of Fe3+ doping promotes the formation of formal Ni4+, which in turn directly correlates with an enhanced activity of the catalyst in promoting OER. The role of Fe3+ is consistent with its behavior as a superior Lewis acid.
An oxygen evolution catalyst that forms as a thin film from Ni(aq)2+ solutions containing borate electrolyte (Ni–Bi) has been studied by in situ X-ray absorption spectroscopy. A dramatic increase in ...catalytic rate, induced by anodic activation of the electrodeposited films, is accompanied by structure and oxidation state changes. Coulometric measurements correlated with X-ray absorption near-edge structure spectra of the active catalyst show that the nickel centers in activated films possess an average oxidation state of +3.6, indicating that a substantial proportion of nickel centers exist in a formal oxidation state of Ni(IV). In contrast, nickel centers in nonactivated films exist predominantly as Ni(III). Extended X-ray absorption fine structure reveals that activated catalyst films comprise bis-oxo/hydroxo-bridged nickel centers organized into sheets of edge-sharing NiO6 octahedra. Diminished long-range ordering in catalyst films is due to their ostensibly amorphous nature. Nonactivated films display a similar oxidic nature but exhibit a distortion in the local coordination geometry about nickel centers, characteristic of Jahn–Teller distorted Ni(III) centers. Our findings indicate that the increase in catalytic activity of films is accompanied by changes in oxidation state and structure that are reminiscent of those observed for conversion of β-NiOOH to γ-NiOOH and consequently challenge the long-held notion that the β-NiOOH phase is a more efficient oxygen-evolving catalyst.
Molecular-scale manipulation of electronic and ionic charge accumulation in materials is the backbone of electrochemical energy storage
. Layered van der Waals (vdW) crystals are a diverse family of ...materials into which mobile ions can electrochemically intercalate into the interlamellar gaps of the host atomic lattice
. The structural diversity of such materials enables the interfacial properties of composites to be optimized to improve ion intercalation for energy storage and electronic devices
. However, the ability of heterolayers to modify intercalation reactions, and their role at the atomic level, are yet to be elucidated. Here we demonstrate the electrointercalation of lithium at the level of individual atomic interfaces of dissimilar vdW layers. Electrochemical devices based on vdW heterostructures
of stacked hexagonal boron nitride, graphene and molybdenum dichalcogenide (MoX
; X = S, Se) layers are constructed. We use transmission electron microscopy, in situ magnetoresistance and optical spectroscopy techniques, as well as low-temperature quantum magneto-oscillation measurements and ab initio calculations, to resolve the intermediate stages of lithium intercalation at heterointerfaces. The formation of vdW heterointerfaces between graphene and MoX
results in a more than tenfold greater accumulation of charge in MoX
when compared to MoX
/MoX
homointerfaces, while enforcing a more negative intercalation potential than that of bulk MoX
by at least 0.5 V. Beyond energy storage, our combined experimental and computational methodology for manipulating and characterizing the electrochemical behaviour of layered systems opens new pathways to control the charge density in two-dimensional electronic and optoelectronic devices.
Photovoltaic cells have considerable potential to satisfy future renewable-energy needs, but efficient and scalable methods of storing the intermittent electricity they produce are required for the ...large-scale implementation of solar energy. Current solar-to-fuels storage cycles based on water splitting produce hydrogen and oxygen, which are attractive fuels in principle but confront practical limitations from the current energy infrastructure that is based on liquid fuels. In this work, we report the development of a scalable, integrated bioelectrochemical system in which the bacterium Ralstonia eutropha is used to efficiently convert CO ₂, along with H ₂ and O ₂ produced from water splitting, into biomass and fusel alcohols. Water-splitting catalysis was performed using catalysts that are made of earth-abundant metals and enable low overpotential water splitting. In this integrated setup, equivalent solar-to-biomass yields of up to 3.2% of the thermodynamic maximum exceed that of most terrestrial plants. Moreover, engineering of R. eutropha enabled production of the fusel alcohol isopropanol at up to 216 mg/L, the highest bioelectrochemical fuel yield yet reported by >300%. This work demonstrates that catalysts of biotic and abiotic origin can be interfaced to achieve challenging chemical energy-to-fuels transformations.
Significance Renewable-fuels generation has emphasized water splitting to produce hydrogen and oxygen. For accelerated technology adoption, bridging hydrogen to liquid fuels is critical to the translation of solar-driven water splitting to current energy infrastructures. One approach to establishing this connection is to use the hydrogen from water splitting to reduce carbon dioxide to generate liquid fuels via a biocatalyst. We describe the integration of water-splitting catalysts comprised of earth-abundant components to wild-type and engineered Ralstonia eutropha to generate biomass and isopropyl alcohol, respectively. We establish the parameters for bacterial growth conditions at low overpotentials and consequently achieve overall efficiencies that are comparable to or exceed natural systems.
Transition metal dichalcogenides (TMDs) intercalated with spin-bearing transition metal centers are a diverse class of magnetic materials where the spin density and ordering behavior can be varied by ...the choice of host lattice, intercalant identity, level of intercalation, and intercalant disorder. Each of these degrees of freedom alters the interplay between several key magnetic interactions to produce disparate collective electronic and magnetic phases. The array of magnetic and electronic behavior typified by these systems renders them distinctive platforms for realizing tunable magnetism in solid-state materials and promising candidates for spin-based electronic devices. This Perspective provides an overview of the rich magnetism displayed by transition metal-intercalated TMDs by considering Fe- and Cr-intercalated NbS2 and TaS2. These four exemplars of this large family of materials exhibit a wide range of magnetic properties, including sharp switching of magnetic states, current-driven magnetic switching, and chiral spin textures. An understanding of the fundamental origins of the resultant magnetic/electronic phases in these materials is discussed in the context of composition, bonding, electronic structure, and magnetic anisotropy in each case study.
Continual improvements in solar-to-fuels catalysis require a genuine understanding of catalyst structure–function relationships, not only with respect to local order, but also intermediate-range ...structure. We report the X-ray pair distribution function analysis of the nanoscale order of an oxidic cobalt-based water-splitting catalyst and uncover an electrolyte dependence in the intermediate-range structure of catalyst films. Whereas catalyst films formed in borate electrolyte (CoBi) exhibit coherent domains consisting of 3–4 nm cobaltate clusters with up to three layers, films deposited in phosphate electrolyte (CoPi) comprise significantly smaller clusters that are not coherently stacked. These structural insights are correlated with marked differences in activity between CoPi and CoBi films.
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