Browsing by Author "Roth, Christina"

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    Composition‐dependent morphology, structure, and catalytical performance of nickel-iron layered double hydroxide as highly‐efficient and stable anode catalyst in anion exchange membrane water electrolysis
    (2022) Jiang, Wulyu; Faid, Alaa Y.; Gomes, Bruna Ferreira; Galkina, Irina; Xia, Lu; Lobo, Carlos Manuel Silva; Desmau, Morgane; Borowski, Patrick; Hartmann, Heinrich; Maljusch, Artjom; Besmehn, Astrid; Roth, Christina; Sunde, Svein; Lehnert, Werner; Shviro, Meital
    Water splitting is an environmentally friendly strategy to produce hydrogen but is limited by the oxygen evolution reaction (OER). Therefore, there is an urgent need to develop highly efficient electrocatalysts. Here, NiFe layered double hydroxides (NiFe LDH) with tunable Ni/Fe composition exhibit corresponding dependent morphology, layered structure, and chemical states, leading to higher activity and better stability than that of conventional NiFe LDH‐based catalysts. The characterization data show that the low overpotentials (249 mV at 10 mA cm-2), ultrasmall Tafel slopes (24 mV dec-1), and high current densities of Ni3Fe LDH result from the larger fraction of trivalent Fe3+ and the optimized local chemical environment with more oxygen coordination and ordered atomic structure for the metal site. Owing to the active intermediate species, Ni(Fe)OOH, under OER conditions and a reversible dynamic phase transition during the cycling process, the Ni3Fe LDH achieves a high current density of over 2 A cm-2 at 2.0 V, and durability of 400 h at 1 A cm-2 in a single cell test. This work provides insights into the relationship between the composition, electronic structure of the layer, and electrocatalytic performance, and offers a scalable and efficient strategy for developing promising catalysts to support the development of the future hydrogen economy.
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    Use of time domain nuclear magnetic resonance relaxometry to monitor the effect of magnetic field on the copper corrosion rate in real time
    (2022) Igreja Nascimento Mitre, Cirlei; Ferreira Gomes, Bruna; Paris, Elaine; Silva Lobo, Carlos Manuel; Roth, Christina; Colnago, Luiz Alberto
    The corrosion of metals is a major problem of modern societies, demanding new technologies and studies to understand and minimize it. Here we evaluated the effect of a magnetic field (B) on the corrosion of copper in aqueous HCl solution under open circuit potential. The corrosion product, Cu2+, is a paramagnetic ion and its concentration in the solution was determined in real time in the corrosion cell by time-domain NMR relaxometry. The results show that the magnetic field (B = 0.23 T) of the time-domain NMR instrument reduces the corrosion rate by almost 50%, in comparison to when the corrosion reaction is performed in the absence of B. Atomic force microscopy and X-ray diffraction results of the analysis of the corroded surfaces reveal a detectable CuCl phase and an altered morphology when B is present. The protective effect of B was explained by magnetic forces that maintain the Cu2+ in the solution/metal interface for a longer time, hindering the arrival of the new corrosive agents, and leading to the formation of a CuCl phase, which may contribute to the rougher surface. The time-domain NMR method proved to be useful to study the effect of B in the corrosion of other metals or other corrosive liquid media when the reactions produce or consume paramagnetic ions.