1,721,047 research outputs found
Trends of epitaxial perovskite oxide films catalyzing the oxygen evolution reaction in alkaline media
No full textAbstractThe oxygen evolution reaction (OER) is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in electrolyzers, metal-air batteries and other applications that rely on sustainable protons from water oxidation. In this review, the prospect of epitaxial perovskite oxides for the OER at room temperature in alkaline media is reviewed with respect to fundamental insight into systematic trends of the activity. First, we thoroughly define the perovskite structure and its parameter space. Then, the synthesis methods used to make electrocatalytic epitaxial perovskite oxide are surveyed, and we classify the different kinds of electrodes that can be assembled for electrocatalytic investigations. We discuss the semiconductor physics of epitaxial perovskite electrodes and their consequences for the interpretation of catalytic results. Prototypical mechanisms of the OER are introduced and comparatively discussed. OER investigations on epitaxial perovskite oxides are comprehensively surveyed and selected trends are graphically highlighted. The review concludes with a short perspective on opportunities for future electrocatalytic research on epitaxial perovskite oxide systems.H2020 European Research Councilhttp://dx.doi.org/10.13039/10001066
Perovskite Electrocatalysts for the Oxygen Reduction Reaction in Alkaline Media
No full textOxygen reduction is considered a key reaction for electrochemical energy conversion but slow kinetics hamper application in fuel cells and metal-air batteries. In this review, the prospect of perovskite oxides for the oxygen reduction reaction (ORR) in alkaline media is reviewed with respect to fundamental insight into activity and possible mechanisms. For gaining these insights, special emphasis is placed on highly crystalline perovskite films that have only recently become available for electrochemical interrogation. The prospects for applications are evaluated based on recent progress in the synthesis of perovskite nanoparticles. The review concludes with the current understanding of oxygen reduction on perovskite oxides and a perspective on opportunities for future fundamental and applied research.</jats:p
Predicted depth profiles for nitrogen-ion implantation into gallium arsenide
No full textWe present a new method to predict the spatial variation of the band gap in nitrogen-implanted gallium arsenide. Band gap engineering of a GaAsN alloy was employed to design an emission peak at 1.3 µm. Based on SRIM simulations, we propose a concentration of 6% N in GaAs at the plateau of the trapezoidal depth profile for the desired band gap. The depth profile could be manufactured by virtue of subsequent high voltage pulses. Software based on the Lieberman model was developed to predict fluences from measurements of a simple Langmuir probe and the time dependent electrode potential. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Different Reactions Define the Electrochemical Window in Imidazolium Ionic Liquids on Gold and Platinum Electrodes
No full textDifferent Reactions Define the Electrochemical Window in 1‐Butyl‐3‐Methylimidazolium Triflate on Gold and Platinum Electrodes
No full textIonic liquids (IL) make excellent candidates for many energy storage devices due to unique and tunable properties such as a large electrochemical window (ECW). Water as an impurity in 1‐butyl‐3‐methylimidazolium (BMIM) triflate is investigated on platinum and gold electrodes in a stagnant glass cell and in a flow‐cell coupled to a differential electrochemical mass spectrometer (DEMS). It is found that the ECW closes with increasing water content on both gold and platinum electrodes in both setups. Platinum has a smaller ECW than gold, where the difference mainly stems from the limiting reduction reaction, as identified based on DEMS. Below 1.11 M H2O /L IL , the anodic reaction is predominantly IL decomposition and above the oxygen evolution reaction for both materials. The cathodic limit is given by the hydrogen evolution reaction for platinum independent of water content and gold above 1.66 M H2O /L IL , while it is IL decomposition below. The study highlights the interplay between electrode material and electrolyte for tailoring the ECW for applications involving intentional or unintentional mixing of water with IL.German Research Foundation https://doi.org/10.13039/50110000165
Prospects for band gap engineering by plasma ion implantation
No full textThe suitability of plasma ion implantation (PII) for band gap engineering will be examined by calculations of the band gap's spatial variation. Plasma Ion Implantation is a method to modify the surface and subsurface properties of materials; the ions surrounding the target are forced into all plasma exposed surfaces simultaneously by virtue of high‐voltage pulses. We calculated the fluence and the ion energy distribution from the dynamic sheath model. The distribution of the ions within the target is subsequently simulated by the TRIDYN software. The concentration profiles are converted into a spatial variation of the band gap. The challenges inherent to the method are discussed by means of the examples of carbon (C) PII in silicon (Si) as well as nitrogen (N) PII in gallium arsenide (GaAs). The ion distribution within the material of the former example is suitable for the formation of the Si‐C alloy. On the other hand, the distribution of N ions in GaAs prevents the formation of the Ga‐As‐N alloy. The discussed methods could be a powerful tool for the prediction of materials properties from the plasma processing parameters, thus helping to design materials. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim
Reversible and irreversible processes during cyclic voltammetry of an electrodeposited manganese oxide as catalyst for the oxygen evolution reaction
No full textAbstractManganese oxides have received much attention over the years among the wide range of electrocatalysts for the oxygen evolution reaction (OER) due to their low toxicity, high abundance and rich redox chemistry. While many previous studies focused on the activity of these materials, a better understanding of the material transformations relating to activation or degradation is highly desirable, both from a scientific perspective and for applications. We electrodeposited Na-containing MnOx without long-range order from an alkaline solution to investigate these aspects by cyclic voltammetry, scanning electron microscopy and x-ray absorption spectroscopy at the Mn-K and Mn-L edges. The pristine film was assigned to a layered edge-sharing Mn3+/4+ oxide with Mn-O bond lengths of mainly 1.87 Å and some at 2.30 Å as well as Mn-Mn bond lengths of 2.87 Å based on fits to the extended x-ray fine structure. The decrease of the currents at voltages before the onset of the OER followed power laws with three different exponents depending on the number of cycles and the Tafel slope decreases from 186 ± 48 to 114 ± 18 mV dec−1 after 100 cycles, which we interpret in the context of surface coverage with unreacted intermediates. Post-mortem microscopy and bulk spectroscopy at the Mn-K edge showed no change of the microstructure, bulk local structure or bulk Mn valence. Yet, the surface region of MnOx oxidized toward Mn4+, which explains the reduction of the currents in agreement with literature. Surprisingly, we find that MnOx reactivates after 30 min at open-circuit (OC), where the currents and also the Tafel slope increase. Reactivation processes during OC are crucial because OC is unavoidable when coupling the electrocatalysts to intermittent power sources such as solar energy for sustainable energy production.H2020 European Research Councilhttp://dx.doi.org/10.13039/10001066
A Modular Double Electrode Flow Cell with Exchangeable Generator and Detector Electrodes
No full textGenerator-collector experiments offer insights into the mechanisms of electrochemical reactions by correlating the product and generator currents. Most commonly, these experiments are performed using commercially-available rotating ring-disk electrodes (RRDE). We developed a modular double electrode flow cell (DEFC) with exchangeable generator and detector electrodes where the electrode width equals the channel width. As a test case, we considered the ferri-/ferrocyanide redox couple in experiments, analytical calculations and multiphysics simulations. Wall effects reduce the current density by less than 10 % in our geometry for the investigated conditions and the analytical solution for the limiting current at the generator electrode applies to widths up to 5 mm. The collection efficiency for all investigated electrode widths is close to the expected 35.4 % above a flow rate of 1.0 (mL/min)1/3 but only independent of the flow rate for electrodes with width 5 mm and larger. Kinetic constants of 1.3–1.9 ⋅ 10−3 cm/s are obtained from Koutecký-Levich analysis and 21.0–5.0 ⋅ 10−3 cm/s from Nicholson analysis for the DEFC, which falls within the range reported previously. We conclude that our DEFC with exchangeable electrodes is an attractive alternative to commercial RRDEs which offers the flexibility to optimize both the generator and collector materials for the desired reaction.Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)Germany's Excellence StrategyGerman Helmholtz AssociationHelmholtz Energy Materials Foundry (HEMF)German Helmholtz AssociationOpen-Access-Publikationsfonds 202
Increases Overpotential of Electrocatalytic Water Oxidation in Lithium Hydroxide Electrolytes
No full textChemical and structural changes preceding electrocatalysis obfuscate the nature of the active state of electrocatalysts for the oxygen evolution reaction (OER), which calls for model systems to gain systematic insight. We investigated the effect of bulk oxidation on the overpotential of ink-casted LiMn2 O4 electrodes by a rotating ring-disk electrode (RRDE) setup and X-ray absorption spectroscopy (XAS) at the K shell core level of manganese ions (Mn-K edge). The cyclic voltammogram of the RRDE disk shows pronounced redox peaks in lithium hydroxide electrolytes with pH between 12 and 13.5, which we assign to bulk manganese redox based on XAS. The onset of the OER is pH-dependent on the scale of the reversible hydrogen electrode (RHE) with a Nernst slope of -40(4) mV/pH at -5 μA monitored at the RRDE ring. To connect this trend to catalyst changes, we develop a simple model for delithiation of LiMn2 O4 in LiOH electrolytes, which gives the same Nernst slope of delithiation as our experimental data, i. e., 116(25) mV/pH. From this data, we construct an ERHE -pH diagram that illustrates robustness of LiMn2 O4 against oxidation above pH 13.5 as also verified by XAS. We conclude that manganese oxidation is the origin of the increase of the OER overpotential at pH lower than 14 and also of the pH dependence on the RHE scale. Our work highlights that vulnerability to transition metal redox may lead to increased overpotentials, which is important for the design of stable electrocatalysts.collaborative research center (CRC)HZB http://dx.doi.org/10.13039/10001311
Surface as a Bifunctional Electrocatalyst for the Oxygen Reduction Reaction and Oxygen Evolution Reaction in Alkaline Media
No full textActive and stable bifunctional electrocatalysts are required for large‐scale deployment of rechargeable metal‐air and metal‐O2 batteries. This is hindered by the large overpotentials of the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline media, where peroxide is an undesired side product. We study the suitability of epitaxial (001)‐oriented La0.6Sr0.4MnO3 perovskite surfaces as a bifunctional catalyst using a rotating‐ring disk electrode (RRDE) assembly and focus particularly on the selectivity of the ORR. The peroxide yield is above 50 % during ORR‐only investigations in the scan range of 0.69 to 0.99 V vs. RHE where the CV traces are reproducible. In contrast, the peroxide yield is drastically reduced during OER‐ORR cycling where a peroxide yield below 10 % is reached during the ORR in the scan range of 0.74 V to 1.74 V vs. RHE. Our study highlights the importance of the electrode history and thus clearly demonstrates that separate studies of the OER and ORR are insufficient to optimize bifunctional electrocatalysts
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