A Review of RedOx Cycling of Solid Oxide Fuel Cells Anode

Abstract: Solid oxide fuel cells are able to convert fuels, including hydrocarbons, to electricity with an unbeatable efficiency even for small systems. One of the main limitations for long-term utilization is the reduction-oxidation cycling (RedOx cycles) of the nickel-based anodes. This paper will review the effects and parameters influencing RedOx cycles of the Ni-ceramic anode. Second, solutions for RedOx instability are reviewed in the patent and open scientific literature. The solutions are described from the point of view of the system, stack design, cell design, new materials and microstructure optimization. Finally, a brief synthesis on RedOx cycling of Ni-based anode supports for standard and optimized microstructures is depicted

STABILITY OF BLACK INTERCONNECT COATINGS FOR SOLAR PHOTOVOL-TAIC MODULE APPLICATIONS

Aesthetics is crucial in the development of Building Integrated Photovoltaic (BIPV) products. Manufacturers strive to mask, typically through expensive manual processes, the reflective metallic interconnects to obtain uniform module colors. Inks offer an automated alternative but must be implemented in the production line and remain stable, maintaining their appearance over time. In this study, three black metallic ribbons were tested: one commercially pre-coated and two coated with UV-curable inkjet printing. Accelerated UV-light exposure was applied according to IEC standards on coupons mimicking glass/backsheet (G/Bs) samples including encapsulant with and without UV blockers. Additionally, one-cell modules with ink-coated ribbons were fabricated using a laboratory-designed automatic inkjet printer and exposed to accelerated UV ageing. Results showed that the commercially available coated ribbon remained stable after 120 kWh/m2 of UV exposure. However, UV-curable inkjet inks caused color changes in the encapsulant around metallic interconnects, regardless of the encapsulant used or the presence or not of UV blockers in the encapsulant. Ink #1 exhibited the most color change under UV-dose. Its main component, 2-phenoexyethyl-acrylate (2-PEA), photodegraded and caused yellowing. An early sign of degradation with a slight increase of 22% in carbonyl index (CI) was observed after 15 kWh/m2 of UV exposure. Encapsulants with UV blockers successfully mitigated 2-PEA photodegradation on G/BS laminates; however, color change occurred with ink #1 despite their application. Using this ink on PV modules results in color change, but the electrical performance remains relatively stable, with less than a 3% power loss after 360 kWh/m2 of UV exposure

Counterion effects in cyanine heterojunction photovoltaic devices

We investigated cyanine heterojunction photovoltaic devices using carbocyanine dyes as donors and buckminsterfullerene (C60) as acceptor. In particular, we focused on the influence of cyanine counterions on the photovoltaic device characteristics. It was found that counterions can be displaced in the applied electric field and give rise to important hystereses in the current-voltage characteristics, which are related to charge injection processes at electrode and organic heterointerfaces. Mobile counterions have also a drastic effect on the photocurrent spectrum and are responsible for an important C60 contribution at the organic heterojunction between cyanine and C60. If the counterion is covalently linked to the cyanine dye, the C60 contribution in the blue spectral domain can not be observed

Ni/YSZ cermet anode degradation modelling and novel image analysis method for the quantification of the three-phase-boundary length

Στην παρούσα εργασία χαρακτηρίστηκε η χρονική εξέλιξη της μικροδομής των ανόδων κυψελών καυσίμου στερεού οξειδίου (Solid Oxide Fuel Cells, SOFC) με χρήση ηλεκτρονικής μικροσκοπίας σάρωσης. Με χρήση μίας νέας τεχνικής ανάλυσης εικόνων SEM διαχωρίστηκαν και ποσοτικοποιήθηκαν οι φάσεις της ανόδου (νικέλιο, ζιρκονία και πόροι) δίνοντας την αναλογία φάσεων και μία απ’ ευθείας εκτίμηση του μήκους ορίου τριών φάσεων. Επίσης, αναπτύχθηκε ένα μαθηματικό μοντέλο που περιγράφει την υποβάθμιση της ανόδου λόγω πυροσυσσωμάτωσης της μεταλλικής φάσης και την συνεπαγόμενη μείωση του μήκους ορίου τριών φάσεων. Η χρονική μεταβολή του μήκος ορίου τριών φάσεων μπορεί να εκτιμηθεί από τον ρυθμό υποβάθμισης και τη χρήση θεμελιωδών λειτουργικών και δομικών παραμέτρων. Ο συνδυασμός του μαθηματικού μοντέλου και των πειραματικών δεδομένων επιτρέπει την διάκριση της υποβάθμισης που οφείλεται σε πυροσυσσωμάτωση επί της ολικής υποβάθμισης

In Situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Titan ETEM

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle

In situ Reduction and Oxidation of Nickel from Solid Oxide Fuel Cells in a Transmission Electron Microscope

Environmental transmission electron microscopy was used to characterize in situ the reduction and oxidation of nickel from a Ni/YSZ solid oxide fuel cell anode support between 300-500°C. The reduction is done under low hydrogen pressure. The reduction initiates at the NiO/YSZ interface, then moves to the center of the NiO grain. At higher temperature the reduction occurs also at the free NiO surface and the NiO/NiO grain boundaries. The growth of Ni is epitaxial on its oxide. Due to high volume decrease, nanopores are formed during reduction. During oxidation, oxide nanocrystallites are formed on the nickel surface. The crystallites fill up the nickel porosity and create an inhomogeneous structure with remaining voids. This change in structure causes the nickel oxide to expand during a RedOx cycle

TEM investigation on zirconate formation and chromium poisoning in LSM/YSZ cathode

Cell durability is a crucial technological issue for SOFC commercialization, and considerable progress has been made in recent years. A number of degradation pathways have been established, amongst which microstructural changes, poisoning effects and formation of less conductive phases. In this study, transmission electron microscopy was used to observe submicron-scale effects on selected cathode zones of an anode supported cell tested in SOFC stack repeat element configuration. The test has been performed with a dedicated segmented test bench, at 800 A degrees C for 1900 h, which allowed to spatially resolve degradation processes, and therefore to improve their correlation with localized post-test analysis. Evidence is presented of reaction products (mainly SrZrO(3)) at the LSM/YSZ interfaces as well as of contaminants, in particular Cr, but also Si. A polarized cell segment is compared to an unpolarized one, to assess any influence of cathode polarization

Quantitative study of anode microstructure related to SOFC stack degradation

As the performances of Solid Oxide Fuel Cells (SOFC) get attractive, long term degradation becomes the main issue for this technology. Therefore it is essential to localise the origin of degradation and to understand its processes in order to find solutions and improve SOFC durability. The electrode microstructure ageing, in particular nickel grain coarsening at the anode side, is known to be a major process to cause performance loss. The increase in nickel particle size will diminish the Triple Phase Boundary (TPB), where fuel oxidation takes place, and decrease the anode electronic conductivity. These two effects degrade the electrochemical performance of the fuel electrode. Degradation is defined as the decrease of potential at constant current density with time in %/1000h or mV/1000h. This study is based on HTceramix® anode supported cells tested in stack conditions from 100 to more than 1000 hours. The anode microstructure has been characterized by Scanning Electron Microscopy (SEM). As the back scattered electron yield coefficients of nickel and yttria stabilized zirconia (YSZ) are very close, the contrast of the different phases (Ni, YSZ and pores) is low. Various techniques are used to enhance the contrast. A new technique is presented here using impregnation and SEM observation based on secondary electron yield coefficients to separate the phases. Image treatment and analysis is done with an in-house Mathematica® code. Image analysis gives information about phase proportion, particle size, particle size distribution, contiguity and finally a new procedure is developed to compute TPB density. A model to describe the coarsening of the nickel particles is also developed. The model assumes an exponential growth of the nickel particles. Using a particle population balance, it estimates the growth of the nickel particles and the concomitant drop in the TPB length. This model is in very good agreement with experimental data, especially for relatively low fuel cell operation times (up to 100-200 hours). This model can be used in the estimation of operational parameters of the anode electrode such as the degradation rate using fundamental parameters of the cermet anode like the anode overpotential and the work of adhesion of the nickel particles on the YSZ substrate. This model gives the portion of stack degradation that corresponds to anode performance decrease due to particle sintering. Finally this study gives the possibility to isolate the degradation coming from the anode sintering and compare to the full SOFC stack degradation

On the use of cyanine dyes as low-bandgap materials in bulk heterojunction photovoltaic devices

Cyanine dyes with absorption edges of almost 1000 nm were used in combination with MEH-PPV for the fabrication of organic solar cells. For blended thin films, a pronounced phase separation between the two components occurred and resulted in photocurrents with different signs for bilayer and bulk heterojunction devices. Absorption spectra and selective dissolution experiments were used to illustrate the process of vertical phase segregation, with the preferential wetting of the polar anode by the cyanines while maintaining percolating carrier pathways between the electrodes. For a cyanine with long alkyl side chains, the compatibility with the polymer matrix was increased and the development of the effective inverted bilayer configuration was not observed. The generally low oxidative photocurrents were explained with unfavourable shifts of the highest occupied molecular orbital (HOMO) dye energy levels in the solid state