Abouatallah, Rami

Friedrich, Andreas

Gago, Aldo

Lettenmeier, Philipp

Wang, rainey

English

Institute of Transport Research:Publications

Durable Membrane Electrode Assemblies for Proton Exchange Membrane Electrolyzer Systems Operating at High Current Densities P. Lettenmeiera, R. Wangb, R. Abouatallahb, A. S. Gagoa, K. A. Friedricha  aInstitute of Engineering Thermodynamics, German Aerospace Center (DLR), Stuttgart 70569, Germany bHydrogenics Corporation, 220 Admiral Boulevard, Mississauga, ON L5T 2N6  Canada   • Cost reduction of the PEM electrolyzer stack • MEA tests in a 20 kWel PEM electrolyzer system • Protocol of measurements • Benchmark MEA with Ir-black catalyst • Electrochmical analysis of deagradation mechanisms • Post mortem analysis of the MEAs and water resin • Summary  Outline Cost breakdown of PEM electrolyzer system and stack Catalyst Coatings • Substitute titanium based components (bipolar plates, current collectors, PTLs, GDLs) by coated stainless stee, steel, copper or aluminium. • Use thin hydrocarbon based membranes and highly conductivenon precious metal coatings. Reduce ohmic losses. • Develop more effcient anode and cathode catalysts with low loading and improved stability. Use ceramic supports or increase activity surface area ratio • Operate at high current densities. Extend operation range from 2 (nominal) to 4 A cm-2.   Study on development of water electrolysis in the EU.  Final Report. E4tech Fuel Cells and Hydrogen Joint Undertaking; 2014 System How to reduce the stack cost? Project on degradation phenomena in PEM electrolyzer systems operating at high current densities Rainbow stack with different MEA configuration 8 Cell - 120 cm2 – 20 kWel PEM electrolyzer stack 12 kWel PEM electrolyzer Partner Tasks in the project ■ Testing of MEAs with different catalysts in a 12 kWel PEM electrolyzer system  ■ Assessment of results and post-mortem analysis ■ Construction a 12 kWel  PEM electrolyzer system  ■ Stack assembly and evaluation of the degradation tests Goal of the project: Gain knowledge about degradation mechanism of PEM electrolyzer MEAs Protocol of measurements 0 2 4 6 8 10 12 140.00.51.01.5j / A cm-2t / s10121416E Cell / V 0 500 1000 1500 200001002003004005000 500 1000 1500 20001213141516171819Stack Current / ATime / hStack Potential / VTime / h    Stack 1: Different catalyst loadings  Stack 2: MEAs from different providers Dynamic operation • There is an urgent need for accelerated stress test (AST) protocols for PEM electrolyzers • Degradation caused by operation time, current densities, voltage, temperature, water quality, etc. is not well understood  Ecell – j, EIS Ecell – j, EIS Ecell – j, EIS • Half cell measurements: OER activity of Ir-black (Umicore) is 3x higher than thermally treated IrO2 (at 1.48V, 25 °C) • MEAs with IrO2 (thermally treated) show lower performance compared to those with Ir-black • Half cell and single cell measurements correlates well with the PEM electrolyzer results • Ir-black can be considered as benchmark anode in PEM electrolyzers  Benchmark PEM electrolyzer anode 1.40 1.45 1.50 1.55 1.60050100150200IrO2 Sigma before ECM  after ECM  j / A g-1 Ir E-iR / V vs. RHEIr Black UCElectrochimica Acta, 2016, in press  Company Anode  (mg cm-2) Membrane Cathode (mg cm-2)  Wuhan WUT 2 N115 0.8  IRD 2.3 N115 0.5  FuelCellsEtc 3 N115 3  E500 (Ir-black) 1 N115 0.9 Electrochemical impedance spectroscopy (EIS) • EIS was performed before and after 500 h (T1) at 2 A cm², and before and after 250 h (T2) at 4 A cm² • MEA with Ir-black (1 mg cm-2) showed the lowest activation and ohmic resistances • EIS results correlate well with Ecell - j characteristics • At high current densities the ohmic resistance has the largest impact • No mass transport was observed   Electrochimica Acta, 2016, in press Evolution of Ecell through the time and current density • Difficult analysis of degradation rate because of temperature fluctuation.  At j = 2 A cm-2, ΔT = ± 1.5 °C caused by the addition of fresh water into the stack.  At j = 4 A cm-2 ΔT = ± 4 °C caused by the periodic turn on-off of the fan that cools down the entire system enclosure • No increase in Ecell after 4 A cm-2 test for all cells • No increase of Ecell over time for all cells   Electrochimica Acta, 2016, in press Determination of ohmic losses from EIS DLR.de  •  Chart 9 • Temperature of the stack was stricktly controlled at 29 ± 0.5 °C by shutting off completely the H2-generator.  An external pump with low flow rates was used. • The EIS were simulated using an equivalent circut • The EIS spectra at a given current density changed over time and when the current was increased • The cell resistance (ηOhm) and kinetics (Ract) were analysed   Electrochimica Acta, 2016, in press • The degradation was analyzed through changes in ohmic loses and potential over time. • The kinetic resistance (Ract) increased over time • The ohmic resistance (ηOhm) decreased at high current densities   Degradation mechanism from EIS analysis Potential  Change at 0.33 A cm-² Ecell  ↑ +10μV/h ηohm  ↓ -2μV/h ηOx  → ηAct  ↑ +13μV/h Electrochimica Acta, 2016, in press • Nafion degradation:  1. Presence of F in the DI water resin  • Degradation of intrinsic properties:  1. Significant decrease of exchange current density during time of measurement. Deactivation of the anode 2. Presence of Ir in the DI water resin   Degradation analysis and XPS on DI water resin   T / °C β /mV dec-1 jo / 10-9A mgIr-1 Ir-Black (Umicore) 25 43.1 2.5 (Half Cell, kinetic analysis) 30 43.1 3   40 43.2 5.8   50 43.6 12.0   60 43.9 22.8   70 44.6 46.2 Before 2A/cm² 30 44.3 18.1 After 2A/cm² 30 41.1 3.5 After 4A/cm² 30 41.1 2.8 Elem. Fresh [wt%] Used [wt%] Fe 0.0 2.6 F 0.0 2.9 O 10.9 21.4 Ti 0.0 2.7 N 2.4 3 C 79.6 52.9 S 7.2 9.9 Si 0.0 3.3 Ir 0.0 1.3 Post mortem analysis of the MEAs SEM AFM Anode (initial) Anode (aged) • No decrease of membrane thickness was observed from cross-section SEM images.  • Release and diffusion of Ir catalyst into the membrane.  • After operation, the conductive area of the anode increased by approximately 50% while the conductive area of the cathode remained the same.  • Surface conductivity of the catalyst layer changed due to ionomer loss.  Conductive area (%) Average Thickness / μm  unused Cell 8 (used) unused Cell 8 (used) Anode 30 ± 4 45 ± 5 5 ± 0.4 4.3 ± 0.6 Cathode 37 ± 2 39 ± 5 16.1 ± 0.6 16.7 ± 1 Membrane 121.5 ± 1.5 137.4 ± 2.9 Electrochimica Acta, 2016, in press • Investment cost can be reduced by operating the PEM electrolyzer at high current densities • The lowest Ir catalyst loading (1 mg cm-2) showed the lowest Ecell at any current density.  • Aging of the PEM electrolyzer MEAs depends on current density and operation time, but the associated degradation mechanisms are different in each case.  • EIS shows a progressive decrease in the specific exchange current, while the ohmic resistance decreases when doubling the nominal current density.  • Post mortem analysis of the MEAs (SEM and AFM) and water resin (XPS) revealed a current dependent loss of ionomer and catalyst material in the anode, which resulted in an unexpected enhancement of cell performance at high current densities.  • A first step towards developing an acelereted stress test protocol (AST) for PEM electrolyzers has been given    Summary Acknowledgements   Tobias Morawietz Michael Handl Renate Hiesgen Project No. 0325440A.  Josef Kallo Fabian Burggraf  Pawel Gazdzicki Stefan Helmly Svenja Kolb Thank you for your attention Contact: aldo.gago@dlr.de  Grand Challenges in Energy Conversion and Storage 2  Tuesday, 31 May 2016: 11:30 a.m. Aqua Salon F (Hilton San Diego Bayfront)  Novel Components for PEM Electrolysis: Status and Challenges  A. S. Gago, P. Lettenmeier, L. Wang, S. Kolb, F. Burggraf, and K. A. Friedrich 

Durable Membrane Electrode Assemblies for Proton Exchange Membrane Electrolyzer Systems Operating at High Current Densities

https://elib.dlr.de/109691/1/160630%20DLR%20229ECS%20I03-2.pdf

Durable Membrane Electrode Assemblies for Proton Exchange Membrane Electrolyzer Systems Operating at High Current Densities

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