Degradation study of a reversible solid oxide cell (rSOC) short stack using distribution of relaxation times (DRT) analysis

Reversible solid oxide cells (rSOC) can convert excess electricity to valuable fuels in electrolysis cell mode (SOEC) and reverse the reaction in fuel cell mode (SOFC). In this work, a five - cell rSOC short stack, integrating fuel electrode (Ni-YSZ) supported solid oxide cells (Ni-YSZ parallel to YSZ vertical bar CGO parallel to LSC-CGO) with an active area of 100 cm(2), is tested for cyclic durability. The fuel electrode gases of H-2/N-2:50/50 and H-2/H2O:20/80 in SOFC and SOEC mode, respectively, are used during the 35 reversible operations. The voltage degradation of the rSOC is 1.64% kh(-1) and 0.65% kh(-1) in SOFC and SOEC mode, respectively, with fuel and steam utilisation of 52%. The post-cycle steady-state SOEC degradation of 0.74% kh(-1) suggests improved lifetime during rSOC conditions. The distribution of relaxation times (DRT) analysis suggests charge transfer through the fuel electrode is responsible for the observed degradation. (C) 2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

Fuel Cells are electrochemical devices that are able to directly convert chemical energy to electrical energy, without any Carnot limitation. Hence, their energy efficiencies are relatively high. Among the various types of fuel cells, solid oxide fuel cells (SOFC) are operated at high temperatures and in principle can run on various fuels such as natural gas and hydrogen. As natural gas is sought to become one of the main fuels of the next decades, its direct feed to a SOFC is desirable as the reaction free energy is high CH4 + 2O2 = CO2 + 2H2O, ΔrG°800°C = -800.8kJ/mol At present, conventional SOFC are operated with pure hydrogen or partially to fully reformed natural gas implying an efficiency penalty. SOFC anodes are generally made of electrocatalytically active Ni-YSZ cermet. Pure CH4 is not fed directly to the anode, because of problems associated with the anode deactivation and coking as CH4 leads to the detachment of Ni particles from the YSZ support and their encapsulation by carbon. To overcome this problem, partially oxidized or reformed CH4 is used. For SOFCs running on direct CH4 feed, alternative anode materials to Ni-YSZ, or new anode formulations are necessary. However, in this case, several parameters influence the anode performance and stability. The anodes should withstand reduction at a low PO2 of around 10-24 atm, be compatible with the SOFC electrolyte, possess an acceptable conductivity and thermal expansion coefficient, and appropriate catalytic and electrocatalytic properties along with low coking activity. LaCrO3 and CeO2-based materials as well as some metal catalysts were studied here for their potential application as anodes in direct oxidation of CH4. This choice of materials was made as lanthanum chromite and ceria-based materials are mixed conductors known to resist quite well to the very reducing conditions in SOFC as they are commonly used for interconnect and electrolyte materials respectively. Moreover, these compounds possess some catalytic activity for CH4 activation and combustion. LaCrO3 and CeO2-based materials show rather low electrical conductivity (on the order of 1 S/cm in reducing conditions), necessitating the application of an extra material for proper current collection. Nevertheless, these oxides possess many of the stringent characteristics cited above. For SOFC anode purposes, LaCrO3-based compounds, substituted with Ca, Sr, Mg, Mn, Fe, Co and Ni, were synthesized by a modified citrate route from nitrate precursors. An optimal calcination temperature of 1100°C was found to be necessary to obtain XRD-pure yet sinteractive compounds. Conductivity measurements in air and in reducing atmospheres of 10-21 atm showed that the La A-site substituents (Ca and Sr) influenced the conductivity more than the Cr B-site substituents (Mg, Mn, Fe, Co and Ni). In humidifed H2, the total conductivity was maximally of 6.5 S/cm. Surface reaction with YSZ electrolyte was evidenced by SEM, XPS and SIMS, in reducing conditions, especially under current load, when these powders were applied as anodes. A current induced demixing effect was noted for heavily substituted LaCrO3. Ceria-based compounds, doped with Y, Nb, Pr and Gd, were synthesized through 3 different techniques: the solid-state method, coprecipitation and the NbCl5 route. The coprecipitation route was found to be most appropriate for Y, Pr and Gd, whereas for Nb a modified precipitation and the NbCl5 technique were the most efficient. Co-doping of Nb and Gd or Pr was studied in order to increase the electronic as well as the ionic conductivity in CeO2. Among all dopants, Nb was found to promote most the adhesion to YSZ electrolyte sheets. Adding Nb to Gd-doped ceria improved the electrode adhesion on YSZ sheets, but deteriorated its conductivity in oxidizing conditions. No interfacial reaction with YSZ was observed by HRTEM when Nb-doped ceria anodes were sintered on YSZ at 1200°C/4h. Catalytic measurements were undertaken with these materials in CH4 rich atmospheres. Different reaction mixtures were chosen to simulate the various SOFC operating conditions: partial oxidation, CO2 reforming by recycling and H2O reforming. Among the different elements, Sr and Ni were found to be the most active substituents for LaCrO3, as they promote all three types of reactions. The combination of both elements is favorable not only for catalytic use, but also for electrode application as they tend to increase the electronic conductivity of the material. Temperature programmed oxidation (TPO) and TEM made on the catalysts after runs in CH4, showed a very low carbon coverage over these oxides, except in the case of Fe substitution, over which carbon deposition was promoted. XPS analysis showed also that all B-site substituted compounds did not segregate after the catalytic runs whereas Ca and Sr (A-site substituents) tended to segregate in H2 and H2O rich atmospheres. Thermodynamic calculations, made using correlations developed in literature, show that Ca and Sr are expected to segregate in H2 and H2O rich atmospheres as they tend to form volatile hydroxyl species. Also, the thermodynamic stability of the LaCrO3 was observed to depend much on the substitution. For CeO2-based oxides, the activity for steam reforming increased from Nb

EPFL2002

(Invited) Exchanging Degradation for Durability: Accelerated Stress Test Protocols for Solid Oxide Fuel Cells and Electrolysers

Jan Van Herle

The longer lifetimes of solid oxide cells (SOC) – obtained and still required, both in fuel cell and electrolysis operation – exacerbate the need for accelerated stress tests, since neither reliable degradation models are currently available nor validated methodologies to evaluate expected stack durability in a practical timeframe. The project AD ASTRA ( "HArnessing Degradation mechanisms to prescribe Accelerated Stress Tests for the Realization of SOC lifetime prediction Algorithms") started operations to tackle this on January 1st 2019, funded by the European Fuel Cells and Hydrogen Joint Undertaking. Generally, a great number of operational parameters influence SOC degradation during lifetime, each contributing according to characteristic times and intensities, often in convoluted or even contrasting fashion. Aggravation of test input parameters can thus lead to non-representative accelerated degradation of a component where concurring effects are differently influenced, leading to a biased end state of the component investigate. To systematically address, harness and accelerate the selected failure modes, a matrix was set up of degradation mechanisms, test items, test procedures and characterization methods, that was translated to protocols in such a way that consistent measurement data are generated, helping subsequent analysis and correlation of phenomena and parameters by the project's modelling and simulation activities. The testing approach consists of a dual-focus campaign targeting: • macroscopic stack testing procedures, where short stacks will be operated in situ, in aggravated conditions with the objective of identifying test settings that will stress the stack in a representative way but minimizing testing time and resource expenditure • specific component ageing tests, to reproduce (ex-situ) the degraded condition of critical stack components or interfaces in a faster time, based on analysis of the components extracted from stacks that have been field-tested. A radical innovation of the project consists in the integration of these two methods: components specifically aged ex situ assembled into otherwise "new" stacks for in-situ testing, so that their effect on stack performance can be easily isolated from other components that in real-world conditions would also degrade. These tests allow to observe from the start (t=0) the end-of-service stack degradation, radically reducing in-situ testing time and providing powerful means to validate models simulating the long-term effects of component degradation in a stack.

2020

Cyclic degradation analysis of reversible Solid Oxide Cell short-stack

Philippe Aubin, David Constantin, Stefan Diethelm, Guillaume Jeanmonod, Suhas Nuggehalli Sampathkumar, Maria del Mar Perez Fortes, Jan Van Herle

The deployment of renewable energy systems has been increasing steadily in recent years. The intermittency associated with the renewable electricity supply can be addressed through the usage of energy storage and release systems. In this framework, reversible solid oxide cells (rSOC) integrated in power-to-gas-to-power systems can convert excess electricity into multiple useful fuels (solid oxide electrolysis (SOE) mode) and restitute it when its renewable production is too low (solid oxide fuel cell (SOFC) mode). To make rSOC competitive and durable, low degradation for the targeted lifetime must be achieved. In BALANCE project (H2020 GA no:731224), with the purpose of assessing the degradation of rSOC in both modes, a cyclic durability test as shown in the graphical abstract was conducted on a 5-cell short stack (100 cm2 active area each) designed by CEA, using H2- electrode supported Ni-YSZ cells manufactured by DTU. The composition of the inlet gases was H2/N2:50/50 in SOFC mode and H2/H2O:20/80 in SOE mode. Steady-state initial performance revealed an improvement for all the repeatable units (RUs) in SOFC mode. In SOE mode, only RU 4 and 5 showed enhanced performance while RU1-3 degraded. Afterwards, J-V characteristic curves showed loss of performances in both modes all along the experiment. A distribution of relaxation time analysis performed on the electrochemical impedance spectra measured regularly throughout the test revealed that the oxide ion transport through the H2-electrode and the O2-electrode is responsible for the observed degradation, over the 1400 h test duration.

2020

Alternative anode materials for methane oxidation in solid oxide fuel cells

Joseph Sfeir

EPFL2002

(Invited) Exchanging Degradation for Durability: Accelerated Stress Test Protocols for Solid Oxide Fuel Cells and Electrolysers

Jan Van Herle

2020