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- XANES (3)
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Organisationseinheit der BAM
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The vanadium redox flow battery (VRFB) is currently a potential candidate for stationary energy storage. A major challenge is the unintended vanadium transport through the separator, which results in a fade of capacity. To overcome this issue, it is necessary to understand the transport processes in the membrane on a more fundamental level. In this work, the vanadium species distribution in Nafion™ 117 after cyclization was investigated. Two membranes, one from a charged VRFB and another from a discharged VRFB, were analyzed using ultraviolet–visible spectroscopy (UV/VIS) and X-ray absorption near edge structure spectroscopy (XANES). Little difference between the two membranes was recognizable according to the UV/VIS results. In comparison, the XANES results showed that the membrane from the charged VRFB contains more V3+ than VO2+, whereas for the discharged case, more VO2+ is present in the membrane.
A core component of energy storage systems like vanadium redox flow batteries (VRFB) is the polymer electrolyte membrane (PEM). In this work, the frequently used perfluorosulfonic-acid (PFSA) membrane Nafion™ 117 and a novel poly (vinylidene difluoride) (PVDF)-based Membrane are investigated. A well-known problem in VRFBs is the vanadium permeation through the membrane. The consequence of this so-called vanadium crossover is a severe loss of capacity. For a
better understanding of vanadium transport in membranes, the uptake of vanadium ions from electrolytes containing Vdimer(IV–V) and for comparison also V(II), V(III), V(IV), and V(V) by both membranes was studied. UV/VIS spectroscopy, X-ray absorption near edge structure spectroscopy (XANES), total reflection X-ray fluorescence spectroscopy (TXRF), inductively coupled plasma optical emission spectrometry (ICP-OES), and micro X-ray fluorescence spectroscopy (microXRF) were used to determine the vanadium concentrations and the species inside the membrane. The results strongly support that Vdimer(IV–V), a dimer formed from V(IV) and V(V), enters the nanoscopic water-body of Nafion™ 117 as such. This is interesting, because as of now, only the individual ions V(IV) and V(V) were considered to be transported through the membrane. Additionally, it was found that the Vdimer(IV–V) dimer partly dissociates to the individual ions in the novel PVDF-based membrane.
The Vdimer(IV–V) dimer concentration in Nafion™ was determined and compared to those of the other species. After three days of equilibration time, the concentration of the dimer is the lowest compared to the monomeric vanadium species. The concentration of vanadium in terms of the relative uptake λ = n(V)/n(SO3
) are as follows: V(II) [λ = 0.155] > V(III) [λ = 0.137] > V(IV) [λ = 0.124] > V(V) [λ = 0.053] > Vdimer(IV–V) [λ = 0.039]. The results show that the Vdimer(IV–V) dimer Needs to be considered in addition to the other monomeric species to properly describe the transport of vanadium through Nafion™ in VRFBs.
A major source of capacity fade of the common vanadium redox flow battery (VRFB) is the vanadium ion transport through the separator. However, different transport models disagree significantly in the diffusion coefficient for the different V species and the influence of different transport mechanisms. The underlying hypothesis of this work is that reactions inside the membrane are partly responsible for these discrepancies. Accordingly, it was investigated if redox reactions inside the nanoscopic water body of Nafion 117 can occur. X-ray absorption near edge structure spectroscopy (XANES) was used to distinguish between the different V species inside hydrated Nafion 117 and novel PVDF-based membranes. It was validated that the speciation of vanadium can be performed using the pre-edge peak energy and intensity. The experiments were performed as follows: strips of the membrane were exposed from one site to a V3+ solution (green) and from the other site to a VO2+ solution (yellow). The ions could diffuse into the membrane from both sides. A change of color of the membrane strip was observed. The blue color in the middle of the strip indicated that VO2+ was formed where V3+ and VO2+ got in contact. Using XANES this reaction inside Nafion was proven.
Coordination chemistry was applied to deposit pyridine-functionalized gold nanoparticles on silicon substrates. The particles were synthesized through the Brust/Schiffrin route with a subsequent ligand exchange reaction yielding well-defined particles of two different sizes. Multilayer deposition was carried out on a pyridine-terminated SAM, anchored on a hydroxyl-terminated silicon surface. Analogously, Hunter/Vögtle-type tetralactam macrocycle multilayers were deposited as well as mixed layers containing both either in an alternating sequence or as a macrocycle multilayer with a terminating nanoparticle layer. These composite layers were examined with respect to their ability to bind squaraine axles in the macrocycle cavities. The amount of guest bound is higher for the composite layer with alternating macrocycles and nanoparticles.