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

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

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

Time-Resolved X-Ray Microtomography Observation of Intermetallic Formation Between Solid Fe and Liquid Al

Time-resolved in situ X-ray tomography combined with scanning electron microscopy was performed on an Al-Fe diffusion system at 973K (700°C) to study the formation of the main intermetallic compounds occurring at the interface. After nucleation on the liquid side of the interface, growth occurs in both liquid and solid directions. In the direction of the solid, growth starts with a particular tongue-like feature which then progressively thickens. The thickening is linked to the deformation of the iron matrix during the formation of the intermetallic compound. Growth in the direction of the liquid is slowed down by erosio

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

Étude de Réduction et d’Oxydation de l’Anode d’une Pile à Combustible à Oxyde Solide à Support Anode (SOFC)

Le but de cette étude est de comparer la microstructure de l’anode en fonction des conditions de réduction et lors de cycles redox. Pour réaliser ce projet, un code Mathematica® est développé. Celui-ci permet de traiter et d’analyser des images SEM pour en extraire la proportion des différentes phases, leur taille de grain et leur contiguïté

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

Amorphous silicon passivated contacts for diffused junction silicon solar cells

Carrier recombination at the metal contacts is a major obstacle in the development of high-performance crystalline silicon homojunction solar cells. To address this issue, we insert thin intrinsic hydrogenated amorphous silicon [a-Si: H(i)] passivating films between the dopant-diffused silicon surface and aluminum contacts. We find that with increasing a-Si: H(i) interlayer thickness (from 0 to 16nm) the recombination loss at metal-contacted phosphorus (n(+)) and boron (p(+)) diffused surfaces decreases by factors of similar to 25 and similar to 10, respectively. Conversely, the contact resistivity increases in both cases before saturating to still acceptable values of similar to 50 m Omega cm(2) for n(+) and similar to 100 m Omega cm(2) for p(+) surfaces. Carrier transport towards the contacts likely occurs by a combination of carrier tunneling and aluminum spiking through the a-Si: H(i) layer, as supported by scanning transmission electron microscopy-energy dispersive x-ray maps. We explain the superior contact selectivity obtained on n(+) surfaces by more favorable band offsets and capture cross section ratios of recombination centers at the c-Si/a-Si: H(i) interface. (C) 2014 AIP Publishing LLC

On Potential Application of Coated Ferritic Stainless Steel Grades K41X and K44X in SOFC/HTE Interconnects

K41X is a ferritic stainless steel grade which was successfully developed in exhaust gas manifold where the temperature could reach 950°C. It contains about 18% wt of chromium and it is stabilized with both titanium and niobium to warranty a good weldability, formability and high temperature corrosion resistance. Moreover, an addition of niobium improves high temperature mechanical properties, in particular the creep resistance. K44X, an enhanced version of K41X with 19%-wt. of Cr plus niobium and molybdenum, was recently developed to be used up to 1000°C. It exhibits better high temperature properties and oxidation resistance. Thanks to their high temperature resistance and their cost competitiveness, these two grades were recently considered as potential candidates to be used as interconnects for Solid Oxide Fuel Cells (SOFC) and High Temperature Electrolysis (HTE), either bare or more certainly coated in order to increase the life duration of the SOFC or HTE systems. This paper will present the high temperature properties of K41X and K44X, in particular oxidation behavior in isothermal and cyclic conditions under operating atmosphere. The positive effect of the addition of a protective coating on these steel grades in terms of oxidation resistance will then be presented. Most of the studied coatings are Mn-Co spinels deposited by sol-gel, atmospheric plasma spray or electroplating, their aim being to limit the chromium evaporation and to fit the severe performance requirements. They lead to low and stable contact resistance, which is a requirement necessary for long-term SOFC/HTE operation: for example a contact resistance of 22 mΩ.cm2 was obtained after 3500 h at 800°C in air with MnCoFe spinel coating. In this respect, K41X was recently chosen to be tested for the 3rd generation stacks of SOFC in the European project “REAL SOFC” or the prototypes in French ANR projects