105 research outputs found
Reporting activities for the oxygen evolution reaction: Do we compare apples to apples?
The oxygen evolution reaction (OER) is a key enabler of sustainable chemical
energy storage. Here, the author assesses the current status of protocols for
benchmarking the OER in materials- and device-centered investigations and makes
suggestions for more comparable data
The promise of nitrogen plasma implanted gallium arsenide for band gap engineering
This investigation examines band gap engineering of the GaAsN alloy by means of plasma ion implantation. The strong redshift of the alloy's band gap is suitable for telecommunication applications and thus stimulated much interest in recent years. Nitrogen (N) ion implantation into gallium arsenide (GaAs) results in a thin shallow N-rich layer below the surface. However, the violent implantation process also modifies the concentrations of gallium and arsenide. The core of this thesis is a novel method for prediction of the band gap from the conditions in the processing plasma.The first important variable, the number of implanted ions, is obtained from the Lieberman model for the current during high-voltage Plasma Ion Implantation (PII). A review of the model's assumptions is provided as well as a comprehensive discussion of the implantation which includes error boundaries. The predicted and measured ion currents agree within error boundaries. The number of implanted ions can therefore be obtained from the prediction.The distribution of the implanted ions was subsequently explored by simulations such as TRIM and TRIDYN. It was found that the nitrogen content in GaAs is limited by the sputtering of the surface atoms. Furthermore, the content of gallium increases near the surface while the content of arsenic decreases. The predicted ratios of the constituents in the implanted layer is such that the alloy cannot form by ion implantation alone; it could be reconciled by annealing.Preliminary samples were produced and tested for the formation of the GaAsN alloy by Raman spectroscopy. No evidence for bonds between N and either Ga or As was found in the as-implanted samples. The thesis concludes with a discussion of the necessary steps to synthesize the GaAsN alloy
Measurement of Enthalpy and Entropy of the Rate-Determining Step of a Model Electrocatalyst for the Oxygen Evolution Reaction
Experimentally determined thermodynamic parameters are rarely reported for
electrocatalytic reactions including the oxygen evolution reaction (OER). Yet,
they contain unique and valuable mechanistic insight and present a missing link
to theoretical investigations. Herein, a protocol for determining thermodynamic
properties of the rate determining steps (RDS) of the OER is presented. Cobalt
oxide is investigated at pH 7 as a case study. Two different approaches are
employed: steady state polarization (SSP) that uses chronopotentiometry at
different temperatures and current values, and potentiostatic electrochemical
impedance spectroscopy (PEIS) at different DC voltages and temperatures. The
data is used to fit a 3D plane from which entropy and enthalpy of the RDS are
obtained. The data analysis requires an appropriate filtering of the data.
Hence, we discuss suitable figures of merit for establishing appropriate
filtering criteria. The values obtained are 0.72 and -0.39 eV (at 298 K) for
enthalpic and entropic contributions, respectively. The obtained values are
reproducible for both approaches and consistent with literature. We further
highlight that the RDS entropy gives clue of the number of water molecules
adsorbed or protons released prior to RDS and thereby helps identifying
plausible OER mechanisms
Manganese Dissolution in alkaline medium with and without concurrent oxygen evolution in LiMnO
Manganese dissolution during the oxygen evolution reaction (OER) has been a
persistent challenge that impedes the practical implementation of Mn-based
electrocatalysts including the LiMnO system in aqueous alkaline
electrolyte. The investigated LiMnO particles exhibit two distinct Mn
dissolution processes; one independent of OER and the other associated to OER.
Combining the bulk sensitive X-ray absorption spectroscopy, surface sensitive
X-ray photoelectron spectroscopy and electrochemical detection of Mn
dissolution using rotating ring-disk electrode, we explore the less understood
Mn dissolution mechanism during OER. We correlate near-surface oxidation with
the charge attributed to dissolved Mn, which demonstrates increasing Mn
dissolution with the formation of surface Mn4+ species under anodic potential.
The observed stronger dissolution during the OER is attributed to the formation
of additional Mn from Mn during OER. We show that control over
the amount of Mn4+ in LiMn2O before the onset of the OER can partially
mitigate the OER-triggered dissolution. Overall, our atomistic insights into
the Mn dissolution processes are crucial for knowledge-guided mitigation of
electrocatalyst degradation, which can be broadly extended to manganese-based
oxide systems
Cost Analysis of Current Grids and its Implications for Future Grid Markets
Commercial Grid markets have been a topic of research for many
years. Many claims about the advantages of trading computing resources on
markets have been made. However, due to a lack of Grid computing offerings,
these claims could not be verified. This paper analyzes the question whether
using the Grid is financially advantageous, using the Amazon.com EC2 service
as a reference. To perform this analysis, the costs of computing resources in
different usage scenarios are calculated, if Grid resources and in-house
resources are used. The comparison of the costs reveals that while the Grid is
cheaper in the short term, it is not a good investment in the long term and, thus,
the existence of a Grid economy will not lead to an end of ownership but rather
to a reduction in in-house resources and more efficient resource usage
Recent Insights into Manganese Oxides in Catalyzing Oxygen Reduction Kinetics
The sluggish kinetics of the oxygen reduction reaction (ORR) limit the efficiency of numerous oxygen-based energy conversion devices such as fuel cells and metal-air batteries. Among earth abundant catalysts, manganese-based oxides have the highest activities approaching that of precious metals. In this Review, we summarize and analyze literature findings to highlight key parameters that influence the catalysis of the ORR on manganese-based oxides, including the number of electrons transferred as well as specific and mass activities. These insights can help develop design guides for highly active ORR catalysts and shape future fundamental research to gain new knowledge regarding the molecular mechanism of ORR catalysis.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR- 0819762)Skoltech-MIT CenterNational Science Foundation (U.S.). Graduate Research Fellowship (Grant no. DGE-1122374)United States. Department of Energy. Office of Basic Energy Sciences (contract no. DE-AC02- 98CH10886
in situ tracking of redox transitions and mode of catalysis
Water oxidation by amorphous oxides is of high interest in artificial
photosynthesis and other routes towards non-fossil fuels, but the mode of
catalysis in these materials is insufficiently understood. We tracked
mechanistically relevant oxidation-state and structural changes of an
amorphous Co-based catalyst film by in situ experiments combining directly
synchrotron-based X-ray absorption spectroscopy (XAS) with electrocatalysis.
Unlike a classical solid-state material, the bulk material is found to undergo
chemical changes. Two redox transitions at midpoint potentials of about 1.0 V
(CoII0.4CoIII0.6 â all-CoIII) and 1.2 V (all-CoIII â CoIII0.8CoIV0.2) vs. NHE
at pH 7 are coupled to structural changes. These redox transitions can be
induced by variation of either electric potential or pH; they are broader than
predicted by a simple Nernstian model, suggesting interacting bridged cobalt
ions. Tracking reaction kinetics by UV-Vis-absorption and time-resolved mass
spectroscopy reveals that accumulated oxidizing equivalents facilitate
dioxygen formation. On these grounds, a new framework model of catalysis in an
amorphous, hydrated and volume-active oxide is proposed: Within the oxide
film, cobalt ions at the margins of Co-oxo fragments undergo CoII â CoIII â
CoIV oxidation-state changes coupled to structural modification and
deprotonation of Co-oxo bridges. By the encounter of two (or more) CoIV ions,
an active site is formed at which the OâO bond-formation step can take place.
The Tafel slope is determined by both the interaction between cobalt ions
(width of the redox transition) and their encounter probability. Our results
represent a first step toward the development of new concepts that address the
solid-molecular Janus nature of the amorphous oxide. Insights and concepts
described herein for the Co-based catalyst film may be of general relevance
also for other amorphous oxides with water-oxidation activity
Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugĂ€nglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.In the sustainable production of non-fossil fuels, water oxidation is pivotal. Development of efficient catalysts based on manganese is desirable because this element is earth-abundant, inexpensive, and largely non-toxic. We report an electrodeposited Mn oxide (MnCat) that catalyzes electrochemical water oxidation at neutral pH at rates that approach the level needed for direct coupling to photoactive materials. By choice of the voltage protocol we could switch between electrodeposition of inactive Mn oxides (deposition at constant anodic potentials) and synthesis of the active MnCat (deposition by voltage-cycling protocols). Electron microscopy reveals that the MnCat consists of nanoparticles (100 nm) with complex fine-structure. X-ray spectroscopy reveals that the amorphous MnCat resembles the biological paragon, the water-splitting Mn4Ca complex of photosynthesis, with respect to mean Mn oxidation state (ca. +3.8 in the MnCat) and central structural motifs. Yet the MnCat functions without calcium or other bivalent ions. Comparing the MnCat with electrodeposited Mn oxides inactive in water oxidation, we identify characteristics that likely are crucial for catalytic activity. In both inactive Mn oxides and active ones (MnCat), extensive di-ÎŒ-oxo bridging between Mn ions is observed. However in the MnCat, the voltage-cycling protocol resulted in formation of MnIII sites and prevented formation of well-ordered and unreactive MnIVO2. Structureâfunction relations in Mn-based water-oxidation catalysts and strategies to design catalytically active Mn-based materials are discussed. Knowledge-guided performance optimization of the MnCat could pave the road for its technological use.DFG, EXC 314, Unifying Concepts in CatalysisEC/FP7/212508/EU/European Solar-Fuel Initiative - Renewable Hydrogen from Sun and Water. Science Linking Molecular Biomimetics and Genetics/SOLARH
Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis
In this Review, we discuss the state-of-the-art understanding of non-precious transition metal oxides that catalyze the oxygen reduction and evolution reactions. Understanding and mastering the kinetics of oxygen electrocatalysis is instrumental to making use of photosynthesis, advancing solar fuels, fuel cells, electrolyzers, and metalâair batteries. We first present key insights, assumptions and limitations of well-known activity descriptors and reaction mechanisms in the past four decades. The turnover frequency of crystalline oxides as promising catalysts is also put into perspective with amorphous oxides and photosystem II. Particular attention is paid to electronic structure parameters that can potentially govern the adsorbate binding strength and thus provide simple rationales and design principles to predict new catalyst chemistries with enhanced activity. We share new perspective synthesizing mechanism and electronic descriptors developed from both molecular orbital and solid state band structure principles. We conclude with an outlook on the opportunities in future research within this rapidly developing field.National Science Foundation (U.S.) (DMR - 0819762)National Science Foundation (U.S.) (DGE-1122374
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