Partial Reoxidation Research Articles

Dynamic Evolution of Copper Nanowires during CO2 Reduction Probed by Operando Electrochemical 4D-STEM and X-ray Spectroscopy.

Nanowires have emerged as an important family of one-dimensional (1D) nanomaterials owing to their exceptional optical, electrical, and chemical properties. In particular, Cu nanowires (NWs) show promising applications in catalyzing the challenging electrochemical CO2 reduction reaction (CO2RR) to valuable chemical fuels. Despite early reports showing morphological changes of Cu NWs after CO2RR processes, their structural evolution and the resulting exact nature of active Cu sites remain largely elusive, which calls for the development of multimodal operando time-resolved nm-scale methods. Here, we report that well-defined 1D copper nanowires, with a diameter of around 30 nm, have a metallic 5-fold twinned Cu core and around 4 nm Cu2O shell. Operando electrochemical liquid-cell scanning transmission electron microscopy (EC-STEM) showed that as-synthesized Cu@Cu2O NWs experienced electroreduction of surface Cu2O to disordered (spongy) metallic Cu shell (Cu@CuS NWs) under CO2RR relevant conditions. Cu@CuS NWs further underwent a CO-driven Cu migration leading to a complete evolution to polycrystalline metallic Cu nanograins. Operando electrochemical four-dimensional (4D) STEM in liquid, assisted by machine learning, interrogates the complex structures of Cu nanograin boundaries. Correlative operando synchrotron-based high-energy-resolution X-ray absorption spectroscopy unambiguously probes the electroreduction of Cu@Cu2O to fully metallic Cu nanograins followed by partial reoxidation of surface Cu during postelectrolysis air exposure. This study shows that Cu nanowires evolve into completely different metallic Cu nanograin structures supporting the operando (operating) active sites for the CO2RR.

CO2 methanation under more realistic conditions: Influence of O2 and H2O on Ni-based catalysts’ performance

In this work, Ni-based catalysts (15 wt%) promoted with Y (15 wt%) and Fe (1 wt%) supported on an optimized USY zeolite were prepared by incipient wetness co-impregnation, characterized by XRD, TGA, N2 adsorption, H2-TPR, and TEM, and finally tested towards CO2 methanation (total flow of 274 mL/min, H2/CO2 = 4, 2.5 bar and 0.2 g of catalyst) including water or oxygen in the feed. On one side, water (∼5 vol%) shows a limited impact on catalysts’ performance despite being a product of the CO2 methanation reaction. These results were mainly attributed to the high hydrophobicity of the studied catalysts, as well as to the absence of additional/competitive reactions. On the other side, oxygen (∼4 vol%) negatively affected the activity of the tested catalysts, decreasing the CO2 conversion, particularly for the unpromoted catalysts. Two different aspects were found to be responsible for the decline in activity: on the one hand, the H2/O2 reaction, which is faster than the CO2 methanation and decreases H2 availability for Sabatier reaction; on the other hand, the partial re-oxidation of nickel active sites, evidenced by post-test characterization, which decreases the number of Ni0 active sites, and therefore, leads to a drop in CO2 conversion. Reversibility tests to assess O2 effect were also performed, indicating that its impact on the catalytic performances mainly arises from the decrease in H2 partial pressure, leading to operation under sub-stoichiometric CO2 methanation conditions (H2/CO2 ∼ 3). When compared to a commercial Al2O3–supported Ni catalyst, zeolite-based samples demonstrated superior resistance to H2O and O2.

The impact of light and thioredoxins on the plant thiol-disulfide proteome.

Thiol-based redox regulation is a crucial posttranslational mechanism to acclimate plants to changing light availability. Here, we conducted a biotin switch-based redox proteomics study in Arabidopsis (Arabidopsis thaliana) to systematically investigate dynamics of thiol-redox networks in response to temporal changes in light availability and across genotypes lacking parts of the thioredoxin (Trx) or NADPH-Trx-reductase C (NTRC) systems in the chloroplast. Time-resolved dynamics revealed light led to marked decreases in the oxidation states of many chloroplast proteins with photosynthetic functions during the first 10 min, followed by their partial reoxidation after 2 to 6 h into the photoperiod. This involved f, m, and x-type Trx proteins showing similar light-induced reduction-oxidation dynamics, while NTRC, 2-Cys peroxiredoxins, and Trx y2 showed an opposing pattern, being more oxidized in light than dark. In Arabidopsis trxf1f2, trxm1m2, or ntrc mutants, most proteins showed increased oxidation states in the light compared to wild type, suggesting their light-dependent dynamics were related to NTRC/Trx networks. While NTRC deficiency had a strong influence in all light conditions, deficiencies in f- or m-type Trxs showed differential impacts on the thiol-redox proteome depending on the light environment, being higher in constant or fluctuating light, respectively. The results indicate plant redox proteomes are subject to dynamic changes in reductive and oxidative pathways to cooperatively fine-tune photosynthetic and metabolic processes in the light. The importance of the individual elements of the NTRC/Trx networks mediating these responses depend on the extent of light variability, with NTRC playing a crucial role to balance protein-redox states in rapidly fluctuating light.

Fe-MOR and Fe-FER as catalysts for abatement of N2O with CH4: in situ UV–vis DRS and operando FTIR study

The catalytic reduction of N2O with CH4 in absence or in presence of O2 (CRN2O or SCRN2O) and H2O (hydrothermal conditions) was studied on Fe-MOR and Fe-FER catalysts with similar Al distribution and iron-loading. In N2O decomposition and SCRN2O Fe-FER catalysts were more active than Fe-MOR, whereas in CRN2O the catalysts exhibited similar activity. In situ FTIR and UV–vis spectroscopies of thermally activated catalysts revealed that Fe(II) species are better stabilized in FER than in MOR topology. In the presence of CH4, Fe(III)-oxo species were reduced to Fe(II) in both zeolites and became active sites for the CRN2O process. In SCRN2O the presence of O2 decreased the activity of Fe-MOR due to a partial re-oxidation of the active sites. The addition of H2O to the feed caused a poor and reversible deactivation. Operando FTIR experiments provided insights into the reaction pathways and suggested formaldehyde and formate surface species as intermediates.

Effects of thermal history on the performance of low-temperature solid oxide fuel cells with Sm0.2Ce0.8O2-δ electrolyte and LiNi0.81Co0.15Al0.04O2 electrodes

In this study, low-temperature solid oxide fuel cells with an ∼560 μm thick Sm0.2Ce0.8O2−δ (SDC) electrolyte and ∼890 μm thick LiNi0.81Co0.15Al0.04O2−δ (NCAL) electrodes are constructed and characterized under three experimental conditions. The cell with an NCAL cathode pre-reduced under an H2 atmosphere at 550 °C presents the best electrochemical performance. This is ascribed to facts that the reduction reaction generating Ni–Co alloy particles on the NCAL surface and partial reoxidation of Ni–Co to Ni(Co)O under an air atmosphere during subsequent experiments increase the triple-phase-boundary area, improve the catalytic activity for the oxygen reduction reaction, and cause a low polarization resistance (0.186 Ω cm2). Also, the reduction of the Ni foam–NCAL anode triggers the formation of H+/O2− bi-ionic SDC–Li2CO3–LiOH composite electrolytes. The LiOH–Li2CO3 melt permeates and densifies the electrolyte layer; this significantly increases the electrical conductivity of the electrolyte layer to 0.291 S cm−1 and lowers the ohmic resistance of the cell (0.170 Ω cm2). The Li2CO3–LiOH phases serve as electron-blocking layers to limit the electronic conductivity of the electrolyte layer. This induces a high open-circuit voltage of 1.024 V and a high maximum power density of 611.3 mW·cm−2.

The state of BEA zeolite supported nickel catalysts in CO2 methanation reaction

Two model nickel catalysts for CO2 methanation reaction were prepared by the impregnation method using unmodified and dealuminated BEA-type zeolite supports, denoted NiHAlBEA and NiSiBEA. Transmission electron microscopy (TEM) and X-ray diffraction studies evidenced formation of metallic nickel crystallites of different size, partially located on the external surface of zeolite grains in of NiHAlBEA catalyst and small nickel nanoparticles uniformly distributed within the zeolite grains in the NiSiBEA catalyst. Higher CO2 conversion and selectivity towards methane at low reaction temperatures were observed for NiSiBEA catalyst. This catalyst showed an improved resistance towards sintering at high reaction temperatures. The decrease of CO2 conversion with the time on stream, accompanied by the drop in methane selectivity in NiHAlBEA catalyst was attributed to the increase in Ni crystallite size, resulted from the migration of small crystallites from the inner to external surface of the zeolite grains and subsequent agglomeration. Diffuse reflectance infrared Fourier transform, and quasi in-situ X-ray photoelectron spectroscopy studies evidenced the presence of metallic nickel crystallites in the catalysts and their partial reoxidation under CO2 methanation reaction conditions. High nickel dispersion and sintering resistance of NiSiBEA led to the retardation of the deactivation of catalyst by H2S.

Paired electrochemical removal of nitrate and terbuthylazine pesticide from groundwater using mesh electrodes

Groundwater is one of the main freshwater resources on Earth, but its contamination with NO3− and pesticides jeopardizes its viability as a source of drinking water. In this work, a detailed study of single electro-oxidation (EO) and electrodenitrification and paired EO/electrodenitrification processes has been undertaken with simulated and actual groundwater matrices containing 100 mg dm−3 NO3− and/or 5 mg dm−3 terbuthylazine pesticide. Galvanostatic electrolyses were made with 500 cm3 of solutions at pH 4.0-10.5 and 250-1000 mA in tank reactors with a RuO2 or boron-doped diamond (BDD) anode and one or two Fe cathodes, all of them in the form of meshes. Most of NO3− removals agreed with a pseudo-first-order kinetics. In Cl−-free media, NH4+ predominated as electroreduction product. In chloride media, a greater amount of N-volatiles was determined alongside a slower electrodenitrification, especially with RuO2 due to the partial re-oxidation of electroreduction products like NH4+ by active chlorine. The pesticide decays were also fitted to a pseudo-first order kinetics, and its presence led to a smaller release of N-volatiles. Overall, BDD always favored the pesticide degradation thanks to the action of BDD(•OH), whereas RuO2 was preferred for electrodenitrification under some conditions. The EO/electrodenitrification of groundwater was successful once the matrix was softened to minimize its hardness. The NO3− concentration was reduced below the limit established by the WHO. Overall, the BDD/Fe cell was more suitable than the RuO2/Fe cell because it accelerated the pesticide removal with a simultaneous high degree of NO3− electroreduction. However, it produced toxic chlorate and perchlorate. A final post-treatment with an anion exchange resin ensured a significant removal of both ions, thus increasing the viability of the electrochemical approach to treat this type of water. Chromatographic analyses revealed the formation of ten heteroaromatic products like desethyl-terbuthylazine and cyanuric acid, alongside oxalic and oxamic as final short-chain carboxylic acids.

Exploring the phase transformation in ZnO/Cu(111) model catalysts in CO2 hydrogenation

The dynamic phase transformation of the ultrathin ZnO/Cu(111) film partly explained the existense of controversy of active phase in Cu/ZnO/Al 2 O 3 catalyst. Sustainable methanol production via CO 2 hydrogenation leads to increased interest in the understanding of active phase of Cu/ZnO/Al 2 O 3 (CZA) catalyst. Model catalysts of ZnO/Cu(111) with structures varied from two-dimensional planar to three-dimensional nanoparticles were prepared by atomic layer deposition (ALD) method. By combing scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) at near-ambient pressure of CO 2 hydrogenation, we reveal that the submonolayer ZnO/Cu(111) transformed into Cu-Zn alloy under 10 mbar CO 2 /H 2 at 493 K, and underwent a partial re-oxidation during evacuation. The dynamic phase transformation of ZnO/Cu(111) may partly explain the existence of differences and apparently contradictory theories to account for the origin of active phase in CZA catalysts.

Environmental evolution of the East China Sea inner shelf and its constraints on pyrite sulfur contents and isotopes since the last deglaciation

To better understand the depositional constraints on the sulfate reduction process, we present a set of sulfur isotope data for hand-picked pyrites (δ34Spyr) from the coarse fraction (63–250 μm) of bulk sediments, coupled with the contents of carbonate, total organic carbon (TOC) and nitrogen (TN), as well as δ13C values of the TOC, from sediments of core EC2005 on the inner shelf of the East China Sea (ECS) since 16.4 ka. Our results show that core sediments were deposited in a terrestrial environment before 13.1 ka (stage I) when pyrite rarely occurred. During stages II (13.1–12.3 ka) and III (12.3–12.1 ka) seawater began to influence the core location, forming a tidal flat environment where abundant pyrites are well preserved. Then the inner shelf of the ECS was fully flooded, and two intervals with high (stage IV:12.1–6.0 ka; stage VI: 5.2–1.5 ka) and low sedimentation rate (stage V: 6.0–5.2; stage VII: 1.5 ka-present) are identified respectively. When compared with previous δ34S values of chromium-reducible sulfur (δ34SCRS) derived from the bulk sediments, δ34Spyr values are usually higher, indicating the macroscopic pyrites form in the later stages of sedimentary diagenesis when ongoing sulfate reduction has distilled porewater sulfate to a high degree. Considering the wide occurrence of biogenic shallow gas (dominated by CH4) in these sediments, sulfate-driven anaerobic oxidation of methane (SO4-AOM) and organoclastic sulfate reduction (OSR) are expected to be two competitive pathways for sulfate reduction. We propose that the changes in sedimentation rate controls the pace of migration of the sulfate methane transition zone (SMTZ). When sedimentation rate is low, the SMTZ may be fixed in a certain depth (e.g., sediments deposited in stage III and V), where H2S is produced by SO4-AOM. In contrast, when sedimentation rate is very high (> 1 cm/yr), the SMTZ would be deep, and H2S is mostly produced by OSR. Further, our results imply that sulfur diagenesis in recent (since 1.5 ka) ECS mud sediments might be influenced by physical reworking (e.g., typhoon) and/or biological activity (bioturbation), resulting in partial reoxidation of pyrite crystals in fluid mud. Our findings therefore demonstrate that pyrite sulfur isotope compositions (bulk δ34SCRS & hand-picked δ34Spyr) are sensitive to local environmental evolution and the combined use of these two indicators can provide new insights into pathway of sulfate reduction in unsteady environments.

Deposition of Co onto Supported MoS2 by Water-Assisted Spreading: Increased Activity Promotion in Hydrodesulphurization of 1-Benzothiophene

Hydrodesulphurization (HDS) catalysts were newly prepared by water-assisted spreading of CoCO3.Co(OH)2 of low solubility in water onto pre-sulphided Mo species supported on several Al2O3 of surface area SBET 77-262 m2g−1, ZrO2 of SBET 108 m2g−1, and TiO2 of SBET 140 m2g−1. The spreading was followed by scanning electron microscopy (SEM-EDX). X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (H2-TPR) characterized partial re-oxidation of sulphidic Mo catalysts before Co sorption. The prepared catalysts were characterized in sulphidic form by H2-TPR. Activity of catalysts was determined in the HDS reaction of 1-benzothiophene. The spreading of Co onto sulphidic catalysts led to systematic increase of HDS activity by 16–86% in comparison to the spreading of Co onto oxide samples.

Structure and reactivity of highly reduced titanium oxide surface layers on TiO2: A first-principles study.

Titanium oxide structures featuring highly reduced TiOx films on top of nearly stoichiometric TiO2 hold promise for applications ranging from photocatalysis to resistance switching devices. Here, we focus on titanium monoxide (TiO) layers on anatase TiO2 (001) and use first principles calculations to investigate their structure and properties as well as their interface with liquid water. We find that only a single TiO layer can grow epitaxially on anatase (001) and subsequent growth leads to the formation of TiO islands. The TiO layers decrease the work function and enhance the surface conductivity in comparison to pure anatase, two features that can improve the TiO2 performance in photocatalytic hydrogen evolution but are thermodynamically unstable relative to pure TiO2 in humid/aqueous environment. Furthermore, first principles molecular dynamics simulations of the TiO (001)-water and anatase (001)-water interfaces show that unlike the multilayer structure of interfacial water on the anatase surface, a very dense and tightly packed first water layer is present on the surface of TiO, which could represent the first stage of partial surface reoxidation.

Reverse Current Treatment of Short Stacks – Experimental Results and System Considerations

Anode degradation is one of the main factors that limit the lifetime of solid oxide cells (SOCs), and thus complicate their commercial application. For nickel/yttria-stabilized-zirconia (Ni/YSZ) anodes, one possible counter measure is re-activation through reverse current treatment (RCT). This method has been described for single cells by other researchers. In order to investigate the applicability of RCT in SOC systems, we used this treatment on anode supported short stacks. Two stacks that had been exposed to different degradation phenomena were investigated. The first stack, suffering from partial re-oxidation and a cracked cell, already showed temporary performance improvement after electrolysis operation under mild conditions. The second stack had degraded over a thermal cycle, and different from the first one showed less re-activation. RCT under dry conditions was found to cause severe degradation. Furthermore, we found that, in agreement with literature, YSZ becomes electronically conductive during the RCT, meaning that electrolysis power during RCT is not limited by the amount of available steam. We discuss the issues of this behavior for SOC systems and possible solutions.

Fire Promotes Arsenic Mobilization and Rapid Arsenic(III) Formation in Soil via Thermal Alteration of Arsenic-Bearing Iron Oxides

Arsenic in oxic surface soils readily associates with Fe(III) oxide minerals such as ferrihydrite and goethite, predominantly as As(V). Fires are a common feature in many landscapes, creating high-temperature soil conditions which drive thermal transformation of these As(V)-bearing minerals. However, it is unknown whether fire-induced transformation of ferrihydrite and goethite can alter the mobility of As, or alter As(V) speciation (e.g. via pyrolysis induced electron-transfer generating the more mobile and toxic inorganic As(III) species). Here, we subject an organic-rich soil (~15% organic C) mixed (4:1) with As(V)-bearing ferrihydrite and goethite (total As of 2.8-3.8 mol g-1), to various temperatures (200-800oC) and heating durations (5-120 min) and examine the consequences for As and Fe via X-ray absorption spectroscopy, X-ray diffraction, 57Fe Mossbauer spectroscopy and selective extracts. We show that heating transformed both ferrihydrite and goethite to mainly maghemite at temperatures >400oC and tended to increase exchangeable surface-complexed As (AsEx) in ferrihydrite yet decrease AsEx in goethite. We demonstrate for the first time that ferrihydrite and goethite-bound As(V) can be rapidly reduced to As(III) during heating of organic-rich soil. Electrons were readily transferred to both Fe(III) and As(V), with reduction of As(V) to As(III) peaking at intermediate temperatures and time periods (maxima of ~88% for ferrihydrite; ~80% for goethite). Although As(III) formation was fast (within 5-10 min at temperatures >400oC), it was followed by partial re-oxidation to As(V) at higher temperatures and longer time intervals. Additionally, combusted As-bearing ferrihydrite and goethite soil-mixtures display greatly enhanced (2-3 orders of magnitude) mobilisation of inorganic As(III)aq species upon re-wetting with water. The magnitude of As(III)aq mobilisation was positively correlated with solid-phase As(III) formation and was greater for goethite than ferrihydrite. These findings challenge the current prevailing view that As(V) reduction to As(III) in soil is mainly limited to waterlogged conditions and suggest that moderate-temperature fires of short duration in oxic soils, may generate considerable labile As(III) species and cause a pulse of As(III)aq mobilisation following initial re-wetting. Further research is warranted to examine the consequences for arsenic cycling in fire-prone natural landscapes and agricultural systems which involve controlled-burn practices.

Resistive switching and charge transport mechanisms in ITO/ZnO/p-Si devices

The resistive switching properties of ITO/ZnO/p-Si devices have been studied, which present well-defined resistance states with more than five orders of magnitude difference in current. Both the high resistance state (HRS) and the low resistance state (LRS) were induced by either sweeping or pulsing the voltage, observing some differences in the HRS. Finally, the charge transport mechanisms dominating the pristine, HRS, and LRS states have been analyzed in depth, and the obtained structural parameters suggest a partial re-oxidation of the conductive nanofilaments and a reduction of the effective conductive area.

Rapid arsenic(V)-reduction by fire in schwertmannite-rich soil enhances arsenic mobilisation

Arsenic in acid sulfate soil (ASS) landscapes commonly associates with schwertmannite, a poorly crystalline Fe(III) mineral. Fires in ASS landscapes can thermally transform Fe(III) minerals to more crystalline phases, such as maghemite (γFe2O3). Although thermal genesis of maghemite requires electron transfer via organic matter pyrolysis, the possibility of fire causing concurrent transfer of electrons to schwertmannite-bound As(V) remains unexplored. Here, we subject an organic-rich soil with variable carbon content (∼9–44% organic C) mixed (4:1) with As(V)-bearing schwertmannite (total As of 4.7–5.4 μmol g−1), to various temperatures (200–800 °C) and heating durations (5–120 min). We explore the consequences for As and Fe via X-ray absorption spectroscopy, X-ray diffraction, 57Fe Mössbauer spectroscopy and selective extracts. Heating transforms schwertmannite to mainly maghemite and hematite at temperatures above 300–400 °C, with some transitory formation of magnetite, and electrons are readily transferred to both Fe(III) and As(V). As(V) reduction to As(III) is influenced by a combination of temperature, heating duration and carbon content and is significantly (P < 0.05) positively correlated with Fe(II) formation. During 2 h heating, higher carbon content favours greater As(III) and Fe(II) formation, while peak As(III) formation (∼44–70%) occurs at relatively modest temperatures (300 °C) and diminishes at higher temperatures. Kinetic heating experiments reveal fast maximum As(III) formation (∼90%) within 5–10 min at 400–600 °C, followed by partial re-oxidation to As(V) thereafter. In contrast, heating As(V)-schwertmannite in the absence of soil-organic matter did not cause reduction of As(V) or Fe(III), nor form maghemite; thus highlighting the critical role of organic matter as an electron donor. Importantly, combusted organic soil-schwertmannite mixtures display greatly enhanced mobilisation of As(III)aq species within 1 h of re-wetting with water. The magnitude of As(III)aq mobilisation is positively correlated with solid-phase As(III) formation. Overall, the results suggest that moderate fires in ASS landscapes, even of short duration, may generate considerable labile As(III) species and cause a pulse of As(III)aq mobilisation following initial re-wetting. Further research is warranted to examine if analogous As(III) formation occurs during combustion of organic-rich soil containing common As-bearing Fe(III) minerals such as ferrihydrite and goethite.

Accelerated Degradation Test in PEM Water Electrolysis

Proton exchange membrane water electrolysis (PEM-WE) is a suitable technology for producing hydrogen via electricity generated from renewable but fluctuating energy sources, such as wind or solar energy. Due to their modest activity but sufficiently high stability in the acidic environment of a PEM system, iridium oxides (IrOx) are the most commonly used catalysts for the oxygen evolution reaction (OER).1 However, recent studies revealed a strong correlation between the OER activity and the stability of IrOx depending on its surface morphology and hydration state (i.e., between highly crystalline thermal IrO2 and amorphous, hydrous oxides). Bulk IrO2 provides the best compromise regarding long lifetime requirements, since its OER activity decreases with inreaseing IrOx crystallinity, whereas its stability improves 2,3. Recent studies from our lab revealed that IrOx can easily be reduced to metallic iridium (Ir) when IrOx based membrane electrode assemblies (MEAs) are held at open circuit voltage (OCV) in a PEM-WE, where crossover H2 from the cathode side reduces the surface of the IrOx catalyst at the anode. This reduction step is indicated by the formation of H-UPD features in the recorded cyclic voltammograms (CVs). Interestingly, the polarization curve recorded directly after this reduction shifts towards lower cell voltages, corresponding to an improved OER activity (Figure 1). Moreover, in a subsequent CV (recorded after the latter polarization curve), the H-UPD features disappeared while the characteristic oxide formation and reduction peaks evolved, pointing towards a change of the catalyst surface properties to a state closer to less crystalline, hydrous IrOx. Considering the fact, that hydrous IrOx exhibits less stability4 and that this partial IrOx reduction and re-oxidation can occur during cycles of extended OCV periods, these findings must be considered as potential degradation mechanism in PEM-WE operation. During the lifetime of an electrolyzer, especially if coupled with a fluctuating power supply, operation interruptions can be expected to occur frequently, thereby altering the form of the IrOx. Therefore, we apply an accelerated test protocol cycling between operation and OCV periods to investigate this degradation mechanism further as well as the effect of potential mitigation strategies. Acknowledgements: This work was funded by the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies) and by the German Ministry of Education and Research (funding number 03SFK2V0, Kopernikus-project P2X). References (1) C. Rozain, E. Mayousse, N. Guillet and P. Millet, Appl. Catal. B, 182, 123 (2016) (2) T. Reier, D. Teschner, T. Lunkenbein, A. Bergmann, S. Selve, R. Kraehnert, R. Schlögl, and P. Strasser, J. Electrochem. Soc., 161, F876 (2014). (3) S. Cherevko, T. Reier, A. R. Zeradjanin, Z. Pawolek, P. Strasser, and K. J. J. Mayrhofer, Electrochem. Commun., 48, 81 (2014). (4) S. Geiger, O. Kasian, B. R. Shrestha, A. M. Mingers, K. J. J. Mayrhofer and S. Cherevko, J. Electrochem. Soc., 163, F3132–F3138 (2016) Figure 1 Polarization curves (A) at initial state (black) and after the reduction step (blue); IV plots were recorded galvanostatically at 80 °C and ambient pressure on a MEA (Nafion 212) with 5 cm² active area, fed with 5 mLmin-1 liquid H2O to the anode. HFR values estimated from impedance spectra recorded during the IV plots are displayed in (B). Figure 1

Reoxidation of uranium in electrolytically reduced simulated oxide fuel during residual salt distillation

We report that residual salt removal by high-temperature distillation causes partial reoxidation of uranium metal to uranium oxide in electrolytically reduced simulated oxide fuel. Specifically, the content of uranium metal in the above product decreases with increasing distillation temperatures, which can be attributed to reoxidation by Li2O contained in residual salt (LiCl). Additionally, we estimate the fractions of Li2O reacted with uranium metal under these conditions, showing that they decrease with decreasing temperature, and calculate some thermodynamic parameters of the above reoxidation.

Stability and Oer Activity of IrOx in PEM Water Electrolysis

Renewable H2 production is a prerequisite for successfully establishing fuel cell-based electromobility and hydrogen infrastructure. In this respect, proton exchange membrane water electrolysis (PEM-WE) has attracted much interest, not least due to the enormous power densities possible in these devices (1). Research is mainly focused on the anode side, where the oxygen evolution reaction (OER) takes place, causing the vast majority of kinetic losses. OER catalysts of choice are usually iridium oxide (IrOx) based, owing to its decent activity and acceptable stability (2,3). Recent studies showed a strong dependence of the OER activity and dissolution resistance of IrOx on its surface morphology and hydration state, which were controlled by the calcination temperature during the synthesis. They found that crystalline, thermal IrO2is the most stable but least active species (4,5). TGA-analysis from our lab reveals that IrOx can easily be reduced to metallic Ir in dilute H2 at a typical PEM-WE operation temperature of 80 °C. This is also observed for IrOx based membrane electrode assemblies (MEAs) held at OCV in a PEM-WE, where crossover H2 from the cathode side reduces the surface of the IrOx catalyst at the anode within hours, as evident from the formation of H-UPD features in the cyclic voltammogram (see Figure 1b, red vs. blue CV). At the same time, polarization curves after this reduction show significantly decreased cell voltage at slightly reduced Tafel slopes (Figure 1a, red vs. blue curves), corresponding to an increased OER activity. However, a subsequent CV following these polarization curves (see Figure 1b, black CV) reveals that the H-UPD features have disappeared again and that the catalyst surface properties have transformed to a state closer to the less crystalline, hydrous IrOx reported in ref. 4. This suggests that partial IrOx reduction and re-oxidation into (hydrous) IrOx can occur during cycles of extended OCV periods and electrolyzer operation. In analogy to voltage cycling degradation observed in fuel cells, the here described reduction/oxidation cycles might also lead to iridium dissolution. Therefore, we will study the effect of transient operation conditions in PEM-WEs on the OER activity, surface properties, and stability of IrOx based anodes. We will also provide a systematic analysis of OER kinetic parameters such as exchange current density and activation energy as well as their dependence on relative humidity for a commercial iridium oxide catalyst. References (1) K. A. Lewinski, D. F. van der Vliet, and S. M. Luopa, ECS Transactions, 69, 893 (2015). (2) C. Rozain, E. Mayousse, N. Guillet, and P. Millet, Appl. Catal. B, 182, 123 (2016). (3) E. Fabbri, A. Habereder, K. Waltar, R. Kötz, and T. J. Schmidt, Catal. Sci. Technol., 4, 3800 (2014). (4) T. Reier, D. Teschner, T. Lunkenbein, A. Bergmann, S. Selve, R. Kraehnert, R. Schlögl, and P. Strasser, J. Electrochem. Soc., 161, F876 (2014). (5) S. Cherevko, T. Reier, A. R. Zeradjanin, Z. Pawolek, P. Strasser, and K. J. J. Mayrhofer, Electrochem. Commun., 48, 81 (2014). Figure 1a. PEM-WE polarization curves recorded at 80 °C under dynamic O2 (anode)/H2 (cathode) at 147 kPaabs and 200 % RH (inlet). Blue curves were taken after a 12 h conditioning at 1 Acm-2 while red curves signify the status after a 15 h in-situ reduction of the anode catalyst in an H2 atmosphere. Hollow circles and crosses are subsequent repetitions. Figure 1b. Anode CVs recorded under dry N2 at 100mVs-1. Blue CV: before the polarization curves in blue, red and black CVs: before and after the polarization curves in red. The anode was loaded with 0.66 mgIrcm-2 of a commercial IrOx/TiO2 using 12 wt-% of ionomer in the catalyst layer, cathode was loaded with Pt/C at 0.35 mgPtcm-2 and a Nafion® XL membrane was used. MEAs with 5 cm2 active area were tested in a house-made single cell hardware with gold-plated titanium flowfields using porous Ti sheets and carbon fiber paper as GDLs on the anode and cathode side, respectively. Acknowledgements: P. J. Rheinländer would like to acknowledge financial support from Greenerity GmbH. M. Bernt would like to acknowledge funding from the Bavarian Ministry of Economic Affairs and Media, Energy and Technology through the project ZAE-ST (storage technologies). Figure 1

Ni cycling in mangrove sediments from New Caledonia

Covering more than 70% of tropical and subtropical coastlines, mangrove intertidal forests are well known to accumulate potentially toxic trace metals in their sediments, and thus are generally considered to play a protective role in marine and lagoon ecosystems. However, the chemical forms of these trace metals in mangrove sediments are still not well known, even though their molecular-level speciation controls their long-term behavior. Here we report the vertical and lateral changes in the chemical forms of nickel, which accumulates massively in mangrove sediments downstream from lateritized ultramafic deposits from New Caledonia, where one of nature’s largest accumulations of nickel occurs. To accomplish this we used Ni K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy data in combination with microscale chemical analyses using Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (SEM-EDXS). After Principal Component and Target Transform analyses (PCA-TT), the EXAFS data of the mangrove sediments were reliably least-squares fitted by linear combination of 3-components chosen from a large model compound spectral database including synthetic and natural Ni-bearing sulfides, clay minerals, oxyhydroxides, and organic complexes. Our results show that in the inland salt flat Ni is hosted in minerals inherited from the eroded lateritic materials, i.e. Ni-poor serpentine (44–58%), Ni-rich talc (20–31%), and Ni-goethite (18–24%). In contrast, in the hydromorphic sediments beneath the vegetated Avicennia and Rhizophora stands, a large fraction of Ni is partly redistributed into a neoformed smectite pool (20–69% of Ni-montmorillonite), and Ni speciation significantly changes with depth in the sediment. Indeed, Ni-rich talc (25–56%) and Ni-goethite (15–23%) disappear below ∼15cm depth in the sediment and are replaced by Ni-sorbed pyrite (23–52%) in redox-active intermediate depth layers and by pyrite (34–55%) in the deepest sediment layers. Ni-incorporation in pyrite is especially observed beneath an inland Avicennia stand where anoxic conditions are dominant. In contrast, beneath a Rhizophora stand closer to the ocean, where the redox cycle is intensified due to the tide cycle, partial re-oxidation of Ni-bearing pyrites favors nickel mobility, as confirmed by Ni-mass balance estimates and by higher Ni concentration in the pore waters. These findings have important environmental implications for better evaluating the protective role of mangroves against trace metal dispersion into marine ecosystems. They may also help in predicting the response of mangrove ecosystems to increasing anthropogenic pressure on coastal areas.

Improvement of performance in low temperature solid oxide fuel cells operated on ethanol and air mixtures using Cu–ZnO–Al2O3 catalyst layer

Anode-supported single-chamber solid oxide fuel cells with and without Cu–ZnO–Al2O3 catalyst layers deposited on the anode support have been operated on ethanol and air mixtures. The cells consist of gadolinia-doped ceria electrolyte, Ni-doped ceria anode, and La0.6Sr0.4CoO3-δ-doped ceria cathode. Catalyst layers with different Cu–ZnO–Al2O3 ratios are deposited and sintered at several temperatures. Since the performance of single-chamber fuel cells strongly depends on catalytic properties of electrodes for partial oxidation of ethanol, the cells are electrochemically characterized as a function of the temperature, ethanol-air molar ratio and gas flow rate. In addition, catalytic activities of supported anode, catalytic layer-supported anode and cathode for partial oxidation of ethanol are analysed. Afterwards, the effect of composition and sintering temperature of catalyst layer on the cell performance are determined. The results indicate that the cell performance can be significantly enhanced using catalyst layers of 30:35:35 and 40:30:30 wt.% Cu–ZnO–Al2O3 sintered at 1100 °C, achieving power densities above 50 mW cm−2 under 0.45 ethanol-air ratio at temperatures as low as 450 °C. After testing for 15 h, all cells present a gradual loss of power density, without carbon deposition, which is mainly attributed to the partial re-oxidation of Ni at the anode.