Durability Issues and Status of HT-PEM

Transcription

1 Durability Issues and Status of HT-PEM Based on Acid Doped Polybenzimidazoles Qingfeng Li dr. techn. Professor Section for Proton Conductors, Department of Energy Conversion and Storage Technical University of Denmark, Telephone: , Symposium of CISTEM in association with Hydrogen Days 2016 April , Prague, Czech Republic

2 Outline HT-PEM in Denmark Durability status and issues Membrane degradation - Polymer oxidation - Membrane failure Acid loss Catalyst degradation - Carbon corrosion - Pt dissolution/re-precipitation Conclusions

3 HT-PEM in Denmark GDL Polymer Catalyst Bipolar plates Sealing materials Red. kinetics Ox.kinetics Cathode Membrane Anode Fuel Cell modeling MEA DTU Proton conductors, Catalysts KU Catalysts SDU Components DPS Polym. Membr. MEA. Stack System simulation & integration strategies Integration AAU model & simulation Serenergy stack & modules IRD MEA, Stack, system Mobile system Stationary system Applications

4 Challenges and 4M Strategies Further challenges Membranes Proton generative functionalities Acid-base chemistry vs. protonics Immobilization of doping acids Durability orientated efforts ORR kinetics Acid anion adsorption Catalysts and electrodes Activity: Low loading Pt Stability: Support and synergies Non-precious metal catalysts Electrode engineering Fuelling strategies and its impact on lifetime Construction materials and stack engineering½½

5 Voltage(V) Durability status and issues 0,6 0,6 0,4 0,4 0,2 0,2 Performance activation - acid distribution in CL - better interfacing Steady degradation - carbon corrosion - Pt agglomeration - hydrophobicity deterioration - acid loss /dehydration (0.617 V) V) Rapid or sudden death - polymer oxidation membrane thinning final collapse - acid loss /dehydration (0,637 (0,617 V) V) (0,579 V) 0,0 0, Time(Hour) Current density 200 ma/cm 2 Temperature 160 C Relative humidity Ambient (no control) Gasses H 2 /Air ( λ= 2/4) Pressure Atmospheric - polymer oxidation (0,528 V) (0,238 V) Significant membrane degradation - thinner membranes - at high loads (currents) - at high temperatures - at high ADLs

6 Acid doping - more than immobilizing matrix > 90wt% H 3 PO 4 > 90wt% H 3 PO 4 Concentrated H 3 PO 4 PA networked by PBI thread PA/PBI = (direct casting) PA/PBI = (post doping) Self-standing & mechanically strong Essentially one phase Viscous yet flowing Extensive H-bond network - high surface tension - high dielelectric constant - nearly pure hopping - anhydrous conductivity

7 High Temperature Polymers PolybenzimidazolesT G = C (Poly (2,2 -m-(phenylene)-5,5 -bibenzimidazole (PBI) Danish pilot production Danish Power Systems Applications As seals, insulator, valves... As fibers for protective garments to astronauts, race-car drivers, firemen... As films & membranes for reverse osmosis and ultra-filtration Becoming conductive when...

8 Membrane degradation Mechanism Effect d Mechanical stress Swelling/shrinking Oxidative degradation Acid leachout (evaporation, diffusion, washout, etc.) Creep, tearing Crack formation Thinning / pinholes (crossover/ocv/final collapse) Decreased conductivity Freezing???

9 Mechanical stability tensile strength Stress at break, MPa Acid doping level 5.7, PBI at 180 C Average weight molecular weight - siginificantly improved with M W Reinforcement & integrated sealing

10 Molecular weight effect H N N H N N n PBI Mw <18,000 PBI repeating unit Mw = 25, repeating unit

11 Stress, MPa Sulfone-and Hexafluoro- PBI 20 F 6 PBI-8 PA SO 2 PBI-8 PA F 6 PBI and its blends - highest tensile strength CrL-F 6 PBI CrL-SO 2 PBI-12 PA SO 2 PBI and its blends -improved tensile strength PBI-7 PA 10 CrL-PBI-13 PA PBI - weak and less flexible PBI blends - stronger and much more flexible Strain, %

12 Covalent cross-linking F F F O O α,α -dibromo-p-xylene (DBX) OH F OH F F H H Perfluoroglutaric acid Br Br H H H HO Br P H O HO HO O O naphthalenetetracarboxylic acids H H O O Br OH HO di(chloromethyl)phosphinic acid) - Li et al. Chem. Mater. 2007; Polym. Adv. Techn O O OH OH O OH HO O

13 Stress, MPa Tensile strength Undoped Covalently-CrL ADL=8.5 Linear PBI, undoped Linear PBI, ADL= 4.5 Linear PBI, ADL = 6.9 Linear PBI, ADL = 8.9 Covalently CrL (DBX), ADL = 8.5 Covalnetly CrL (DCMP), ADL = 8.0 Ionically CrL, ADL= ADL=4.5 ADL=6.9 Covalently-CrL, ADL= Ionically-CrL, ADL=10.2 ADL= Strain, % Covalently CrL-PBI: stronger at high acid doping levels with small elongation Ionically CrL-PBI: stronger at high acid doping levels with better flexibility Li et al. Chem. Mater. 2007; Fuel cells 2008; Polym. Adv. Techn. 2008

14 Absorbance Absorbance Polymer degradation C=O C-OH Fenton test 80 hr 60 hr 40 hr 20 hr 0 hr Wave numbers,cm-1 C=N/C=C In Air 400 C, 5 hr 400 C, 2.5 hr 400 C, 1 hr 400 C, 0.5 hr 250 C, 20 hr No heat treatment Wave number, cm

15 Mw, g/mol Mass remaining, % Polymer degradation Weight loss of PBI Weight loss depedent of M W - importance of terminal groups Initial Mw = Initial Mw = Fenton test time, hr Time, hours Pristine PBI Mw decrease from 54,500 to 20,000 - ca. 1½ cuts while weight loss of 20% - chain scission and further weight loss

16 PBI Remaining of weight,% PBI Remaining of weight,% Polymer degradation Effect of Fe 2+ and PA 100 Stabilizing peroxide H 2 O 2 OH / OOH 90 Retarding the terminal oxidation Fe2+: 0, PA:0.02g/ml Fe2+: 4ppm, PA:0.02g/ml Fe2+: 16ppm, PA:0.02g/ml Fe2+: 48ppm, PA:0.02g/ml Fenton test time, hours With H 3 PO 4 H 3 PO 4 Retarding the chain scission H 3 PO 4 H 3 PO Fe: 4ppm, PA:0.02g/ml Fe: 4ppm, PA:0.04g/ml No H 3 PO 4 Fe: 4ppm, PA:0.08g/ml Fe: 4ppm, PA: Fenton test time, hours

17 Residual, % Polymer degradation Effect of PBI chemistry F 6 -PBI, IV=0.8 SO 2 -PBI, IV=1.1 PBI, IV=1.2 PBI. IV= Time, hours F 6 -PBI even better SO 2 -PBI more stable

18 Remaining membrane, % Polymer degradation Cross-linking CrL-F6PBI Nafion 117 CrL-SO2PBI CrL-PBI Linear PBI Mw =23, Time, hours CrL-PBI The weak link is nitrogencontaining heterocyclic rings cross-linking occurs an amide linkage through imidazole groups the network structure holds the membrane from being falling into pieces Compatible to Nafion

19 PBI cell durability - CrL versus pristine PBI Danish Power Systems - thermally crosslinked Little difference in electrodes catalyst surface area Large difference in Ohmic resistance of membranes

20 Membrane degradation - Acid loss Mechanisms Evaporation Washout with condensed water Migration / diffusion MEA compression FC operation, etc. Capillary transport out of cell out of cell Acid loss out of membrane from anode to cathode from membrane to catalyst layer

21 Durability issues - Acid loss out of fuel cell Mechanisms evaporation washout Staudt et al. PPA membrane, 160 o C Total: 0.6 µg m -2 s -1 DOE report 2006 Wannek et al. ABPBI, average PA loss: Total : 0.4 µg m -2 s ºC, 0.2 Acm -2, 1000 h Wannek et al. Fuel Cells 08 (2008) 87 Benicewicz et al. direct cast m-pbi: Total: 0.03 µg m -2 s ºC, 0.2 Acm -2, 1700 h Asilomar 2007 Benicewicz et al. direct cast m-pbi : Total: 0.5 μg m -2 s ºC, 0.2 Acm -2, 8000 h Mader et al. Adv. Polym. Sci. 216 (2008) 63

22 PBI cell durability -Acid loss by evaporation Acid loss in PBI cells 0.5 µg m -2 s -1 Acid loss in PBI cells 5 x g cm -2 s -1 Acid in air ppb level Air flow (0.6 A/cm²; =2) 1.6x10-3 g cm -2 s -1 Air flow (0.6 A/cm²; =2) 1.3 ml s -1 cm -2 At a rate of 0.5 µg m -2 s -1 40,000 hours: ca. 3-5% acid loss More under dynamic mode!

23 PBI cell durability - Acid loss 160 ºC, 600 ma/cm 2 Stoichiometry (flow rates) Load (current density) - atmospheric humidity - electrochemical (osmotic drag of acid) =22.6/25.2 =8.2/8.4 =2.0/1.6 =5.7/ ºC, 800 ma/cm 2 =2.0/1.6 =3.2/3.4

24 PBI cell durability - Temperature & load Temperature - vapor pressure of acid - corrosion Load (current density) - atmospheric humidity - electrochemical (osmotic drag of acid) =2.0/1.6 =2.9/3.6 =5.7/6.3 =22.6/25.2 =2.0/2.0 =4.3/5.0 =7.9/ ºC 800 ma/cm ma/cm ºC 200 ma/cm 2 =5.4/6.6 =11.8/14.5

25 Catalyst degradation Mechanism Effect a Pt dissolution / precipitation Carbon support corrosion Impurity poisoning GDL corrosion PTFE degradation Particle growth (migration through membranes) Pt detachment / growth Catalyst deactivation Change of hydrophobicity Change of hydrophobicity

26 Catalyst degradation Platinum stability Pourbaix diagramme at 25ºC Pt + H 2 O = PtO + 2H + + 2e - PtO + H 2 O = PtO 2 + 2H + + 2e - Borup et al. Chem. Rev. 107 (2007)

27 Dissolved Pt concentration, M Catalyst degradation Pt solubility in hot H 3 PO 4 From V: increased by 1000 times E RT nf ln C C 2 At < 0.7 V Pt rather stable At OCV Significant loss 1 1,E-03 1,E-04 1,E-05 1,E-06 1,E-07 1,E-08 1,E-09 Cathodic potential (V vs RHE) 96%H3PO4, 196 C, Pt foil [Bindra et al. 1979] 96%H3PO4, 176 C, Pt foil [Bindra et al. 1979] 0.57M HClO4, 23 C, Pt wire, [Wang et al. 2006] 0.5 M H2SO4, 80 C, Nano Pt [Ferreira, et al. 2005] 0,6 0,8 1 1,2 1,4 1,6 1,8 Potential, V vs. SHE Cathodic platinum losses, % of 0.5 mg/cm² PBI-6PA PBI-15PA g/cm² % g/cm² % 0.8 0,3 0,05 0,6 0, ,2 0,2 2,8 0, ,1 1,8 22,4 4, ,1 24,8 305,6 61,1

28 Corrosion current, ma/g-c Catalyst degradation Corrosion current of Vulcan XC-72 carbon: 1000 E250G, 1.2V, 85%PA, 150 C VXC-72, 1.2 V, 85%PA, 150 C VXC-72, 1.0 V, 85%PA, 150 C VXC-72, 1.0 V, 99%PA, 190 C [z] 200 o C, H 3 PO 4, 1.0 V Heat treatment temperature, C [z] Landsman et al. (2003)

29 Cell voltage, V Catalyst degradation - accelerated test 1,4 1,2 Potential cycling V Ca. 500 cycles in 17 hrs Accelerated test protocol Cycling between V 2 min/cycle Potentiostat at 0.5 V for 5-6 h Polarization curve ir measurement ECA measurement H 2 permeability Repeat the procedure 1 0,8 OCV 0,6 0,4 0,2 Potentiostat, 0,5V, ca. 6 hrs ir measurement ECA measurement Gas permeability polarisation curve Potentiostat, 0,5V, ca. 6 hrs 2nd measurement Time, min

30 Open circuit voltage, V Area specific resistance, Ohm.cm² Hydrogen permeability, mol.s -1.cm -1 Catalyst degradation - accelerated test 1 0,8 0,6 0,4 MWCNT-COOH VXC-72R G-VXC-72R H 2 OCV 1,00E-09 9,00E-10 8,00E-10 7,00E-10 6,00E-10 5,00E-10 4,00E-10 OCV H 2 permeability ASR 0,2 0 ASR Cycle number 3,00E-10 2,00E-10 1,00E-10

31 Catalyst degradation - accelerated test Relative active area, % Electrochemical active area by CV Decrease in ECA with cycle numbers - carbon support dependent CNT-OH MWCNT-COOH G-VXC-72R Cycling number

32 Relative cell performance, % OCV, V Catalyst degradation summary 1 0,8 0, Cycle number Pt/MWCNT-OH Pt/VXC-72R Pt/Chemically modified C Pt / G-VXC-72R VXC-72R VXC-72R-G3 MWCNT HiSPEC910 Ketjen E350G Modified carbon Reasonable reproducibility Performance loss - Carbon support dependent 20 0 Pt/ on Ketjen Pt/ RNSACO Cycle number

33 Durability Issues Membrane degradation Mechanical stability tensile strength; swelling; polymer chemistry Oxidative degradation endpoint oxidation / chain scission; polymer chemistry Fe 2+ and PA effect Acid leachout evaporation mechanism: 0.5 µg cm -2 h -1 ; - electrochemical mechanisms? Catalyst degradation Pt dissolution potential dependent Carbon corrosion potential dependent; variant carbon supports

34 Voltage(V) Conclusions 0,6 (0.617 V) Performance activation - acid distribution in CL - better interfacing Well demonstrated durability - up to 20,000 hours - ca. 5 µv/h rate (0.5 for CrL) o C - H 2 /air with constant load 0,4 0,2 0,0 Steady degradation - carbon corrosion - Pt agglomeration - hydrophobicity /hydrophilicity - acid loss (0,238 V) Time(Hour) Significant at higher potentials - catalyst relevance - C corrosion & Pt agglomeration - worse under dynamic operation Significant at higher loads - acid loss - temperature - water34 formation Rapid/sudden death - polymer oxidation - membr. collapse - acid loss Polymer relevant - poor polymer - thinner membrane - at high loads - at high temp - at high ADLs

35 Meet the Proton Conductors Seniors Technicians Scientists/Post docs Secretary PhD students Financial support Innovation Fund Denmark the PSO ForskEL the EUDP

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