Ebbesen, Sune Dalgaard

Graves, Christopher R.

Hjelm, Johan

Online Research Database In Technology

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 19, 2017Determination of overpotential characteristics of reversible solid oxide cells viaimpedance spectroscopy and correlation with cell degradationGraves, Christopher R.; Ebbesen, Sune Dalgaard; Hjelm, JohanPublished in:ProceedingsPublication date:2011Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Graves, C. R., Ebbesen, S. D., & Hjelm, J. (2011). Determination of overpotential characteristics of reversiblesolid oxide cells via impedance spectroscopy and correlation with cell degradation. In ProceedingsDetermination of overpotential characteristics of reversible solid oxide cells via impedance spectroscopy and correlation with cell degradation  Christopher Graves, Sune D. Ebbesen, Johan Hjelm  Fuel Cells and Solid State Chemistry Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, 4000 Roskilde, Denmark  cgra@risoe.dtu.dk   Solid oxide electrochemical cells are a very promising technology for efficient, economical and sustainable energy conversion between electrical energy and chemical energy. They can be operated as fuel cells for electric power generation from fossil hydrocarbons, operated as electrolysis cells for hydrocarbon fuel production from renewable or nuclear electricity, and operated reversibly between the two modes for large-scale electricity storage (e.g. for intermittent renewable power). Today’s state-of-the-art cells have high electrochemical performance. An important goal is improving the long-term durability.   The degradation of different cell components, e.g. the electrodes, is either certainly related to or suspected to be related to the overpotential across the component, depending on the cell operating conditions. For example, too high anodic overpotential for Ni-based anodes will cause Ni oxidation, dictated simply by thermodynamics. Too high anodic overpotential for oxygen-electrodes is suspected to be responsible for build up of oxygen partial pressure in electrolyte grain boundaries near the interface of the electrolyte and oxygen electrode [1]. It is therefore of great interest to determine the overpotentials during long-term operation of full cells operated as electrolyzers.   By careful measurement and analysis of impedance data measured on technological Risø DTU cells (anode-supported with thin electrolyte), we calculate current-overpotential curves without using a reference electrode. This involves analyzing impedance spectra measured at systematically varied experimental conditions – gas compositions independently varied at the fuel electrode and the oxygen electrode, temperature variation, gas flow rates and reactant utilization, and load/current density – to determine an appropriate cell model. The operating conditions for long-term testing are then applied, and the load is then incrementally stepped up to the desired potential, measuring impedance at each step. This data is fit to the model to obtain the differential resistance versus current density. This curve gives the instantaneous slope of an overpotential-current curve, and is simply integrated to obtain the overpotential-current curve. Overlaying these overpotential-current curves obtained from the AC impedance data measured under load on top of a DC polarization curve gives identical results. A similar technique was recently performed by Leonide et al [2] to calculate Butler-Volmer charge transfer coefficients. Here we use the method to study overpotential characteristics of reversible solid oxide cells and correlation between overpotentials and degradation rates.   1. R. Knibbe, M.L. Traulsen, A. Hauch, S.D. Ebbesen, and M. Mogensen, J. Electrochem. Soc. 157, B1209-B1217 (2010). 2. A. Leonide, Y. Apel, and E. Ivers-Tiffee, ECS Transactions 19, 81-109 (2009).  

Determination of overpotential characteristics of reversible solid oxide cells via impedance spectroscopy and correlation with cell degradation

                       General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.  • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal   If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.     Downloaded from orbit.dtu.dk on: Dec 20, 2017Determination of overpotential characteristics of reversible solid oxide cells viaimpedance spectroscopy and correlation with cell degradationGraves, Christopher R.; Ebbesen, Sune Dalgaard; Hjelm, JohanPublication date:2011Link back to DTU OrbitCitation (APA):Graves, C. R., Ebbesen, S. D., & Hjelm, J. (2011). Determination of overpotential characteristics of reversiblesolid oxide cells via impedance spectroscopy and correlation with cell degradation [Sound/Visual production(digital)]. 220th ECS Meeting, Boston, MA, United States, 09/10/2011Determination of overpotential characteristics of reversible solid oxide cells via impedance spectroscopy  Christopher Graves        Sune D. Ebbesen     Johan Hjelm cgra@risoe.dtu.dk christoph.graves@gmail.com  Oct. 11th, 2011 220th ECS Meeting Boston, MA Oct 11, 2011 2 Risø DTU, Technical University of Denmark Outline • Introduction + Purpose • Experimental • Results + Modeling • Applications • Conclusions  Oct 11, 2011 3 Risø DTU, Technical University of Denmark Introduction + Purpose • Solid oxide cells promise economical + efficient sustainable energy conversion Fuel cell mode fuels  electricity Electrolysis mode electricity  fuels Oct 11, 2011 4 Risø DTU, Technical University of Denmark   High temp. co-electrolysis of H2O + CO2  • makes very efﬁcient use of electricity + heat (near-100% electricity-to-syngas efﬁciency) • provides high reaction rates • directly produces syngas (CO/H2 mixture) for use in conventional catalytic fuel synthesis reactors.  Solar PV [C. Graves, S.D. Ebbesen, M. Mogensen, K.S. Lackner, Renewable and Sustainable Energy Reviews 15 (2011) 1-23] Economical energy storage is ultimately needed to scale up renewable energy to meet a significant fraction of demand Introduction + Purpose Oct 11, 2011 5 Risø DTU, Technical University of Denmark Electrolysis cells Intermittent electricity  hydrocarbon fuels Oct 11, 2011 6 Risø DTU, Technical University of Denmark Reversible fuel cells Intermittent electricity  hydrocarbon fuels + on-demand electricity Oct 11, 2011 7 Risø DTU, Technical University of Denmark Introduction + Purpose Solid oxide cells • High performance for reversible reactions • Oxidation of H2/CO/CH4 • Reduction of H2O/CO2 • Important goal is improving long-term durability Fuel cell mode Electrolysis mode From: Graves C, Ebbesen SD, and Mogensen M, “Co-electrolysis of CO2 and H2O in Solid Oxide Cells: Performance and Durability,” Solid State Ionics 192(1): p. 398-403 (2011).  Oct 11, 2011 8 Risø DTU, Technical University of Denmark Re. choice of “safe” and “harsh” current densities:  • Graves, Ebbesen, Mogensen 2010 doi: 10.1016/j.ssi.2010.06.014 • Xiufu Sun’s talk tomorrow • Knibbe, Traulsen, Hauch, Ebbesen, Mogensen 2010 doi: 10.1149/1.3447752 • Ebbesen, Graves, Hauch, Jensen, Mogensen 2010 doi: 10.1149/1.3464804 • Ebbesen & Mogensen 2010, doi: 10.1149/1.3455882 Importance of current density Fuel production economics is promising… If we will turn around and convert it right back to electricity, need not be convenient, portable, high energy density and plug into existing infrastructure – need not be liquid hydrocarbon storage. –0.5 A/cm2 –2 A/cm2 [C. Graves, S.D. Ebbesen, M. Mogensen, K.S. Lackner, Renewable and Sustainable Energy Reviews 15 (2011) 1-23] Oct 11, 2011 9 Risø DTU, Technical University of Denmark Importance of current density & capacity factor –0.5 A/cm2 –2 A/cm2 Oct 11, 2011 10 Risø DTU, Technical University of Denmark Introduction + Purpose  Degradation of cell components is related to voltage across each component • Fuel cell mode:      too high anodic overpotential for Ni  NiO  • Electrolysis mode:  too high anodic overpotential for oxygen electrode  microstructural degradation at interface with electrolyte Normally, measuring overpotentials requires a reference electrode • Here we demonstrate calculation of component-wise overpotentials (e.g. across each electrode and the electrolyte) for fuel cell + electrolysis polarization  • without using a reference electrode • by careful measurement + analysis of impedance data • Also facilitates study of electrode kinetics See also recent work by Leonide and coworkers: Leonide A, Apel Y, and Ivers-Tiffee E, ECS Transactions, (2009). 19(20): 81-109. A. Leonide et al., Journal of Power Sources, (2011). 196(17): 7343-7346. Oct 11, 2011 11 Risø DTU, Technical University of Denmark Experimental From: Ebbesen  SD,  Graves  C,  and  Mogensen  M,  Production of Synthetic Fuels by Co-Electrolysis of Steam and Carbon Dioxide. International Journal of Green Energy, (2009). 6(6): p. 646 – 660. • Test set-up similar to • Cell type:  Most measurements on  Ni-YSZ/YSZ/CGO/LSC (16 cm2)  LSC oxygen electrode chosen because its impedance response has less overlap with other impedance processes (facilitates fitting impedance data) • Test conditions: (impedance data measured at)  650 – 850 °C  Varying H2/CO/H2O/CO2 compositions to fuel electrode and pO2 to ox. electrode  Varying current density  Oct 11, 2011 12 Risø DTU, Technical University of Denmark Results • Fuel cell mode – impedance measured under polarization 750 °C, H2/H2O=97/3, ox.el.=air Oct 11, 2011 13 Risø DTU, Technical University of Denmark Results • Reversible – impedance measured under polarization electrolysis fuel cell 750 °C, H2/H2O=50/50, ox.el.=air Oct 11, 2011 14 Risø DTU, Technical University of Denmark Results • Impedance measured under polarization Using electrolyte conductivity activation energy of 0.8-0.9 eV for RS, T accounts for RS, approximately (calculated T is slightly higher than measured T; thermo-couple is ~1mm from electrolyte which can account for this) RS is affected  by temperature changes e.g. heating especially during fuel cell polarization 750 °C, H2/H2O=50/50, ox.el=air electrolysis fuel cell electrolysis fuel cell Oct 11, 2011 15 Risø DTU, Technical University of Denmark Results • Those were ”fixed oven temperature” measurements • The heating/cooling upon polarization effects temperature activated processes e.g. electrode kinetics • This effect can be compensated for by using RS as a temperature indicator and adjusting the oven temperature at each load setting,  to obtain ”fixed cell temperature” (fixed RS) measurements: 750 °C, H2/H2O=97/3, ox.el=air Oct 11, 2011 16 Risø DTU, Technical University of Denmark Modeling the impedance data • To quantify resistance contributions from each cell component, the impedance data must be carefully modeled • Equivalent circuits   or   models based on chemical/physical processes  • Today all results used an equivalent circuit based on a previous equivalent circuit model established for Ni-YSZ/YSZ/LSM-YSZ type cells, now with a Gerischer element for oxygen electrode;  we are also fitting to a model without circuit elements • The low-frequency inductive loop observed at high current densities/overpotentials in both fuel cell & electrolysis modes has been speculated as various adsorption/surface layer processes. It affects quantifying the gas concentration process and should be included in the model Oct 11, 2011 17 Risø DTU, Technical University of Denmark LR(RQ)GW Model can be improved (see later) but reasonable estimation… Modeling the impedance data • Co-electrolysis – impedance measured under polarization 800 °C, co-electrolysis 90%-reactants (CO2+H2O), ox.el.=O2 Oct 11, 2011 18 Risø DTU, Technical University of Denmark Modeling the impedance data LR(RQ)GW 800 °C, co-electrolysis 90%-reactants (CO2+H2O), ox.el.=O2 Break down of cell resistance • Co-electrolysis – impedance measured under polarization Oct 11, 2011 19 Risø DTU, Technical University of Denmark REIS = dU/di (local slope of an i-U curve) To recreate an i-U curve, dU = REIS·di, so integrate,  or numerically each Un = Un-1+ REIS,n·(in – in-1) 00.050.10.150.20 0.2 0.4 0.6 0.8 1U or dR/diiU1R1U2R2U3R3What does it look like? Examples R’s U’s Since we have a limited number of data points, use a fit (spline) to i-R curves Meaning of RX,EIS vs i  Calculating overpotential curves from fitted data t  it l  li  l  η  and R (dU/di) Oct 11, 2011 20 Risø DTU, Technical University of Denmark Modeling the impedance data LR(RQ)GW 800 °C, co-electrolysis 90%-reactants (CO2+H2O), ox.el.=O2 Break down of overpotentials Added to OCV to obtain calculated i-U curve   Oct 11, 2011 21 Risø DTU, Technical University of Denmark Calculating overpotential curves from fitted data Overlaid on measured i-U curve → agreement (it had better agree if the method works ) 0.800.850.900.951.001.051.100.0 0.2 0.4 0.6 0.8 1.0U [V]i [A/cm2]U.revU.gas.concU.Ni-ScYSZU.sU.LSC-CGOiv_18Fuel cell mode OCV gas c fuel electrode ohmic ox. Electrode i-U curve measured  afterwards Oct 11, 2011 22 Risø DTU, Technical University of Denmark 800 °C, H2/H2O=50/50, ox.el.=O2 Reversible polarization - asymmetry Oct 11, 2011 23 Risø DTU, Technical University of Denmark Further considerations  Effect of number data points (errors that arise)  Effect of level of detail of model (errors) Applications  Overpotential vs time (degradation)   Obtaining electrode kinetics parameters  Connecting/reconciling DC and AC measurements (shown in earlier slide)  Determine safe operating conditions (avoiding Ni/NiO)  Comparing with 3-electrode measurements to study ref. electrode effects Oct 11, 2011 24 Risø DTU, Technical University of Denmark Conclusions • Analysis method demonstrated for electrolysis + fuel cell operation • careful measurement + analysis of impedance data obtained on full cells  → calculate overpotential-current curves  • Isothermal i-U measurements obtained • and procedure developed • enables more meaningful study of transport/kinetics parameters  • Method being applied to long-term durability tests to calculate overpotential across each component during testing (observe evolution of overpotentials) and correlate with cell degradation Oct 11, 2011 25 Risø DTU, Technical University of Denmark Acknowledgements • Xiufu Sun for some of the measurements • Energinet.dk • SERC.dk • Topsoe Fuel Cell  Thank you! 

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Determination of overpotential characteristics of reversible solid oxide cells via impedance spectroscopy and correlation with cell degradation

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