Combining phosphate species and stainless steel cathode to enhance hydrogen evolution in microbial electrolysis cell (MEC)

Microbial electrolysis cells (MEC) must work around neutral pH because of microbial catalysis at the anode. To develop a hydrogen evolution cathode that can work at neutral pH remains a major challenge in MEC technology. Voltammetry performed at pH 8.0 on rotating disk electrodes showed that the presence of phosphate species straightforwardly multiplied the current density of hydrogen evolution, through the so-called cathodic deprotonation reaction. The mechanism was stable on stainless steel cathodes whereas it rapidly vanished on platinum. The phosphate/stainless steel system implemented in a 25 L MEC with a marine microbial anode led to hydrogen evolution rates of up to 4.9 L/h/m2 under 0.8 V voltage, which were of the same order than the best performance values reported so far. Keywords: Hydrogen; Microbial electrolysis cell (MEC); Stainless steel; Phosphat

A stretchable and screen-printed electrochemical sensor for glucose determination in human perspiration

Here we present two types of all-printable, highly stretchable, and inexpensive devices based on platinum (Pt)-decorated graphite for glucose determination in physiological fluids. Said devices are: a non-enzymatic sensor and an enzymatic biosensor, the latter showing promising results. Glucose has been quantified by measuring hydrogen peroxide (H2O2) reduction by chronoamperometry at −0.35 V (vs pseudo-Ag/AgCl) using glucose oxidase immobilized on Pt-decorated graphite. The sensor performs well for the quantification of glucose in phosphate buffer solution (0.25 M PBS, pH 7.0), with a linear range between 0 mM and 0.9 mM, high sensitivity and selectivity, and a low limit of detection (LOD). Thus, it provides an alternative non-invasive and on-body quantification of glucose levels in human perspiration. This biosensor has been successfully applied on real human perspiration samples and results also show a significant correlation between glucose concentration in perspiration and glucose concentration in blood measured by a commercial glucose meter.This work is supported by the Generalitat Valenciana (Prometeo2013/038), by the Ministerio de Economia y Competitividad (MAT2016-76595-R), and the U.S. National Institute of Biomedical Imaging and Bioengineering of NIH (R21EB019698). A. Abellán also thanks the Generalitat Valenciana for her fellowship and the Universidad de Alicante for her fellowship for a research stay in University of San Diego, California

Surface-layer formation by reductive decomposition of LiPF6 at relatively high potentials on negative electrodes in lithium ion batteries and its suppression

In using a LiPF6/ethylene carbonate–dimethyl carbonate electrolyte for lithium ion batteries (LIBs), a certain reductive reaction is known to occur at a relatively high potential (ca. 2.6 V vs. Li[+]/Li) on Sn electrode, but its details are still unknown. By means of in-situ X-ray reflectometry, X-ray photoelectron spectroscopy, scanning electron microscopy observations and electrochemical measurements (by using mainly Sn electrode, and additionally Pt, graphite electrodes), we have found out that this reduction eventually forms an inactive passivation-layer consisting mainly of insulative LiF ascribed to the reductive decomposition of LiPF6, which significantly affects the battery cyclability. In contrast, a solid-electrolyte interphase (SEI) is formed by the reductive reaction of the solvent at ca. 1.5 V vs. Li[+]/Li, which is lower than the reduction potential of LiPF6. However, we have found that the formation of SEI preempts that of the passivation layer when holding the electrode at a potential lower than 1.5 V vs. Li[+]/Li. Consequently, the cyclability is improved by suppressing the formation of the inactive passivation layer. Such a pretreatment would be quite effective on improvement of the battery cyclability, especially for a relatively noble electrode whose oxidation potential is between 1.5 V and 2.6 V vs. Li[+]/Li

Melting of Amphibole-bearing Wehrlites: an Experimental Study on the Origin of Ultra-calcic Nepheline-normative Melts

Olivine + clinopyroxene ± amphibole cumulates have been widely documented in island arc settings and may constitute a significant portion of the lowermost arc crust. Because of the low melting temperature of amphibole (∼1100°C), such cumulates could melt during intrusion of primary mantle magmas. We have experimentally (piston-cylinder, 0·5-1·0 GPa, 1200-1350°C, Pt-graphite capsules) investigated the melting behaviour of a model amphibole-olivine-clinopyroxene rock, to assess the possible role of such cumulates in island arc magma genesis. Initial melts are controlled by pargasitic amphibole breakdown, are strongly nepheline-normative and are Al2O3-rich. With increasing melt fraction (T > 1190°C at 1·0 GPa), the melts become ultra-calcic while remaining strongly nepheline-normative, and are saturated with olivine and clinopyroxene. The experimental melts have strong compositional similarities to natural nepheline-normative ultra-calcic melt inclusions and lavas exclusively found in arc settings. The experimentally derived phase relations show that such natural melt compositions originate by melting according to the reaction amphibole + clinopyroxene = melt + olivine in the arc crust. Pargasitic amphibole is the key phase in this process, as it lowers melting temperatures and imposes the nepheline-normative signature. Ultra-calcic nepheline-normative melt inclusions are tracers of magma-rock interaction (assimilative recycling) in the arc crus

Liquid line of descent of a basanitic liquid at 1.5Gpa: constraints on the formation of metasomatic veins

The metasomatism observed in the oceanic and continental lithosphere is generally interpreted to represent a continuous differentiation process forming anhydrous and hydrous veins plus a cryptic enrichment in the surrounding peridotite. In order to constrain the mechanisms of vein formation and potentially clarify the nature and origin of the initial metasomatic agent, we performed a series of high-pressure experiments simulating the liquid line of descent of a basanitic magma differentiating within continental or mature oceanic lithosphere. This series of experiments has been conducted in an end-loaded piston cylinder apparatus starting from an initial hydrous ne-normative basanite at 1.5GPa and temperature varying between 1,250 and 980°C. Near-pure fractional crystallization process was achieved in a stepwise manner in 30°C temperature steps and starting compositions corresponding to the liquid composition of the previous, higher-temperature glass composition. Liquids evolve progressively from basanite to peralkaline, aluminum-rich compositions without significant SiO2 variation. The resulting cumulates are characterized by an anhydrous clinopyroxene+olivine assemblage at high temperature (1,250-1,160°C), while at lower temperature (1,130-980°C), hydrous cumulates with dominantly amphibole+minor clinopyroxene, spinel, ilmenite, titanomagnetite and apatite (1,130-980°C) are formed. This new data set supports the interpretation that anhydrous and hydrous metasomatic veins could be produced during continuous differentiation processes of primary, hydrous alkaline magmas at high pressure. However, the comparison between the cumulates generated by the fractional crystallization from an initial ne-normative liquid or from hy-normative initial compositions (hawaiite or picrobasalt) indicates that for all hydrous liquids, the different phases formed upon differentiation are mostly similar even though the proportions of hydrous versus anhydrous minerals could vary significantly. This suggests that the formation of amphibole-bearing metasomatic veins observed in the lithospheric mantle could be linked to the differentiation of initial liquids ranging from ne-normative to hy-normative in composition. The present study does not resolve the question whether the metasomatism observed in lithospheric mantle is a precursor or a consequence of alkaline magmatism; however, it confirms that the percolation and differentiation of a liquid produced by a low degree of partial melting of a source similar or slightly more enriched than depleted MORB mantle could generate hydrous metasomatic veins interpreted as a potential source for alkaline magmatism by various author

Constraints on the Parental Melts of Enriched Shergottites from Image Analysis and High Pressure Experiments

Martian basalts can be classified in at least two geochemically different families: enriched and depleted shergottites. Enriched shergottites are characterized by higher incompatible element concentrations and initial Sr-87/Sr-86 and lower initial Nd-143/Nd-144 and Hf-176/Hf-177 than depleted shergottites [e.g. 1, 2]. It is now generally admitted that shergottites result from the melting of at least two distinct mantle reservoirs [e.g. 2, 3]. Some of the olivine-phyric shergottites (either depleted or enriched), the most magnesian Martian basalts, could represent primitive melts, which are of considerable interest to constrain mantle sources. Two depleted olivine-phyric shergottites, Yamato (Y) 980459 and Northwest Africa (NWA) 5789, are in equilibrium with their most magnesian olivine (Fig. 1) and their bulk rock compositions are inferred to represent primitive melts [4, 5]. Larkman Nunatak (LAR) 06319 [3, 6, 7] and NWA 1068 [8], the most magnesian enriched basalts, have bulk Mg# that are too high to be in equilibrium with their olivine megacryst cores. Parental melt compositions have been estimated by subtracting the most magnesian olivine from the bulk rock composition, assuming that olivine megacrysts have partially accumulated [3, 9]. However, because this technique does not account for the actual petrography of these meteorites, we used image analysis to study these rocks history, reconstruct their parent magma and understand the nature of olivine megacrysts

ISOLDE target and ion source chemistry

For the production of radioactive ion beams by means of the ISOL (isotope separation on-line) method in which the nuclei of interest are stopped in a thick target, chemistry plays a crucial role. It serves to separate the nuclear reaction products in atomic or molecular form from the bulk target and to transfer them efficiently to an ion source. This article gives an overview of ISOLDE radiochemical methods where targets (liquid metals, solid metals, carbides and oxides) and ion sources are optimized with respect to efficiency, speed and chemical selectivity. Rather pure beams of non-metals and volatile metals can be obtained with a temperature-controlled transfer line acting as thermo-chromatograph. For less volatile metals the temperature of the target and ion source units needs to be kept as high as possible, but a selective ion source can be used: positive surface ionization for metals with ionization potentials below about 6 eV and the RILIS (resonance ionization laser ion source) technique for most other metals

Ultra-calcic Magmas Generated from Ca-depleted Mantle: an Experimental Study on the Origin of Ankaramites

Ultra-calcic ankaramitic magmas or melt inclusions are ubiquitous in arc, ocean-island and mid-ocean ridge settings. They are primitive in character (XMg > 0·65) and have high CaO contents (>14 wt %) and CaO/Al2O3 (>1·1). Experiments on an ankaramite from Epi, Vanuatu arc, demonstrate that its liquidus surface has only clinopyroxene at pressures of 15 and 20 kbar, with XCO2 in the volatile component from 0 to 0·86. The parental Epi ankaramite is thus not an unfractionated magma. However, forcing the ankaramite experimentally into saturation with olivine, orthopyroxene and spinel results in more magnesian, ultra-calcic melts with CaO/Al2O3 of 1·21-1·58. The experimental melts are not extremely Ca-rich but high in CaO/Al2O3 and in MgO (up to 18.5 wt %), and would evolve to high-CaO melts through olivine fractionation. Fractionation models show that the Epi parent magma can be derived from such ultra-calcic experimental melts through mainly olivine fractionation. We show that the experimental ultra-calcic melts could form through low-degree melting of somewhat refractory mantle. The latter would have been depleted by previous melt extraction, which increases the CaO/Al2O3 in the residue as long as some clinopyroxene remains residual. This finding corrects the common assumption that ultra-calcic magmas must come from a Ca-rich pyroxenite-type source. The temperatures necessary for the generation of ultra-calcic magmas are ≥1330°C, and their presence would suggest melting regimes that are at the upper temperature end of previous interpretations made on the basis of picritic magma

Equilibrium and Fractional Crystallization Experiments at 0·7 GPa; the Effect of Pressure on Phase Relations and Liquid Compositions of Tholeiitic Magmas

Two series of anhydrous experiments have been performed in an end-loaded piston cylinder apparatus on a primitive, mantle-derived tholeiitic basalt at 0·7 GPa pressure and temperatures in the range 1060-1270°C. The first series are equilibrium crystallization experiments on a single basaltic bulk composition; the second series are fractionation experiments where near-perfect fractional crystallization was approached in a stepwise manner using 30°C temperature increments and starting compositions corresponding to that of the previous, higher temperature glass. At 0·7 GPa liquidus temperatures are lowered and the stability of olivine and plagioclase is enhanced with respect to clinopyroxene compared with phase equilibria of the same composition at 1·0 GPa. The residual solid assemblages of fractional crystallization experiments at 0·7 GPa evolve from dunites, followed by wehrlites, gabbronorites, and gabbros, to diorites and ilmenite-bearing diorites. In equilibrium crystallization experiments at 0·7 GPa dunites are followed by plagioclase-bearing websterites and gabbronorites. In contrast to low-pressure fractionation of tholeiitic liquids (1 bar-0·5 GPa), where early plagioclase saturation leads to the production of troctolites followed by (olivine) gabbros at an early stage of differentiation, pyroxene still crystallizes before or with plagioclase at 0·7 GPa. The liquids formed by fractional crystallization at 0·7 GPa evolve through limited silica increase with rather strong iron enrichment following the typical tholeiitic differentiation path from basalts to ferro-basalts. Silica enrichment and a decrease in absolute iron and titanium concentrations are observed in the last fractionation step after ilmenite starts to crystallize, resulting in the production of an andesitic liquid. Liquids generated by equilibrium crystallization experiments at 0·7 GPa evolve through constant SiO2 increase and only limited FeO enrichment as a consequence of spinel crystallization and closed-system behaviour. Empirical calculations of the (dry) liquid densities along the liquid lines of descent at 0·7 and 1·0 GPa reveal that only differentiation at the base of the crust (1·0 GPa) results in liquids that can ascend through the crust and that will ultimately form granitoid plutonic and/or dacitic to rhyodacitic sub-volcanic to volcanic complexes; at 0·7 GPa the liquid density increases with increasing differentiation as a result of pronounced Fe enrichment, rendering it rather unlikely that such differentiated melt will reach shallow crustal level

Reactive Infiltration of MORB-Eclogite-Derived Carbonated Silicate Melt into Fertile Peridotite at 3GPa and Genesis of Alkalic Magmas

We performed experiments between two different carbonated eclogite-derived melts and lherzolite at 1375°C and 3 GPa by varying the reacting melt fraction from 8 to 50 wt %. The two starting melt compositions were (1) alkalic basalt with 11·7 wt % dissolved CO2 (ABC), (2) basaltic andesite with 2·6 wt % dissolved CO2 (BAC). The starting melts were mixed homogeneously with peridotite to simulate porous reactive infiltration of melt in the Earth’s mantle. All the experiments produced an assemblage of melt + orthopyroxene + clinopyroxene + garnet ± olivine; olivine was absent for a reacting melt fraction of 50 wt % for ABC and 40 wt % for BAC. Basanitic ABC evolved to melilitites (on a CO2-free basis, SiO2 ∼27–39 wt %, TiO2 ∼2·8–6·3 wt %, Al2O3 ∼4·1–9·1 wt %, FeO* ∼11–16 wt %, MgO ∼17–21 wt %, CaO ∼13–21 wt %, Na2O ∼4–7 wt %, CO2 ∼10–25 wt %) upon melt–rock reaction and the degree of alkalinity of the reacted melts is positively correlated with melt–rock ratio. On the other hand, reacted melts derived from BAC (on a CO2-free basis SiO2 ∼42–53 wt %, TiO2 ∼6·4–8·7 wt %, Al2O3 ∼10·5–12·3 wt %, FeO* ∼6·5–10·5 wt %, MgO ∼7·9–15·4 wt %, CaO ∼7·3–10·3 wt %, Na2O ∼3·4–4 wt %, CO2 ∼6·2–11·7 wt %) increase in alkalinity with decreasing melt–rock ratio. We demonstrate that owing to the presence of only 0·65 wt % of CO2 in the bulk melt–rock mixture (corresponding to 25 wt % BAC + lherzolite mixture), nephelinitic-basanite melts can be generated by partial reactive crystallization of basaltic andesite as opposed to basanites produced in volatile-free conditions. Post 20% olivine fractionation, the reacted melts derived from ABC at low to intermediate melt–rock ratios match with 20–40% of the population of natural nephelinites and melilitites in terms of SiO2 and CaO/Al2O3, 60–80% in terms of TiO2, Al2O3 and FeO, and <20% in terms of CaO and Na2O. The reacted melts from BAC, at intermediate melt–rock ratios, are excellent matches for some of the Mg-rich (MgO >15 wt %) natural nephelinites in terms of SiO2, Al2O3, FeO*, CaO, Na2O and CaO/Al2O3. Not only can these reacted melts erupt by themselves, they can also act as metasomatizing agents in the Earth’s mantle. Our study suggests that a combination of subducted, silica-saturated crust–peridotite interaction and the presence of CO2 in the mantle source region are sufficient to produce a large range of primitive alkalic basalts. Also, mantle potential temperatures of 1330–1350°C appear sufficient to produce high-MgO, primitive basanite–nephelinite if carbonated eclogite melt and peridotite interaction is taken into account