Wirkungsgrad Brennstoffzelle vs. Carnot-Maschine

Transkript

1 Wirkungsgrad Brennstoffzelle vs. Carnot-Maschine E rev [V] th [%] H 2 O (liquid thermodynamic fuel cell efficiency, th theoretical Carnot efficiency, Carnot (T low set as 50 C) temperature [C] Für die Berechnung wurde jeweils der Brennwert von Wasserstoff zugrundegelegt

2 PEMFC Materials Single-Cell Assembly Proton Conducting Membrane/Ionomer Catalyst and Electrode Structure

3 PEMFC Stack Single-Cell Repeating Units Bipolar Plate (BP) Diffusion Media (DM) 1-2 mm Membrane Electrode Assembly Diffusion Media (DM) e - -conducting plates H 2 & air distribution via flow-fields gas diffusion layer channel-to-land distribution (gas, e - ) MEA: electrodes on H + -conducting PEM Bipolar Plate (BP) cm full-size PEMFC stacks: s of single cells - MEA active areas of 200 to 800 cm 2

4 Single Cell Assembly & Diffusion Medium Structure M.F. Mathias et al., in: Handbook of Fuel Cells; Wiley, v.3 (2003)

5 H 2 /Air PEMFC Performance Model E cell = E rev i R (RH) HOR ORR i R H +,an&ca (RH) tx,o2(dry) tx,o2(wet) E rev : thermodynamic voltage (D.M. Bernardi and M.W. Verbrugge, J. Electrochem. Soc. 139 (1992) 2477) R : R -mem(rh) + R -el purely Ohmic resistances HOR, ORR : overpotentials of the H 2 oxidation and O 2 reduction reaction R H +,an&ca(rh) : effective proton conduction resistances within anode & cathode electrodes tx,o2(dry) : O 2 diffusion through H 2 O-free DM (accurate at <100% local relative humidty) tx,o2(wet) : additional O 2 diffusion resistances in the presence of H 2 O liquid (>100%RH)

7 Membrane material: Sulphonated fluoroethylene Main features of fluorosulphonated ionomers: chemically highly resistant mechanically strong (very thin films of some 10 m) acidic adsorb large quantities of water if hydrated, good proton conductors Sulphonated fluoroethylene

8 Proton (H + ) Exchange Membrane ionomeric membranes for PEMFCs ( 25 m) and DMFCs ( 100 m) hydrophobic backbone from: K.D. Kreuer, in: Handbook of Fuel Cells: Fundamentals, Technology & Applications (eds: W. Vielstich, A. Lamm, H.A. Gasteiger), Wiley (2003): vol. 3.

9 Conductivity (1/ohm/cm) Energiewissenschaften, Vorlesung SS 2011, Prof. K. Krischer RH Dependence of Sulfonic Acid Ionomers R H+(membrane) & R H+(electrodes) are strong functions of RH 80ºC Target Range - Nafion sulfonated polyarylenethioethersulfone (1.8 meq./g) - low-ew (<800) PFSA (M.F. Mathias et al., Interface, 14 (Fall 2005), 24) Relative Humidity (%)

11 Catalyst Layer Composite Pt Approx. Wt% 33 Carbon 33 Ionomer 33 Gas Poresmembrane 0 Ionomer Film of thickness film 20 nm Carbon Primary Particle (d=40 nm) x 25 Pt Particle (d=4 nm) 13 m Catalyst Morphology (Tanaka 46% Pt on Vulcan-XC72) 40 nm electrode x 40 Isolated catalyst agglomerate 20 µm 500 nm

12 Typical Pt/C Catalyst: Catalyst-Support Structure primary agglomerate primary C-particles (20-40nm) high structure of primary carbon agglomerates leads to highly porous packing primary agglomerates cannot be broken by typical shears/pressures agglomerates breakage occures during carbon corrosion SEM picture: Jim Mitchell, Ted Gacek, and Mike Budinski (GM Fuel Cell Activities)

13 PEFC Electrode Composition & Structure electrode (C, Pt, ionomer) structure: dominated by carbon-black structure from: Z.Y. Liu et al., J. Electrochem. Soc. 155 (2008) B979) 46% Pt/carbon 40 nm for 50% Pt/C & I/C 1/1 (g ionomer /g carbon ): %mass %volume Pt: 33 2? carbon: 33 20? ionomer: 33 20? pores: 60? highly porous catalyst/ionomer structure d pore of nm

14 PEFC Electrode Structure Model for 50% Pt/C and I/C 1/1 (g ionomer /g carbon ): membrane H + %mass %volume Pt: 33 2? carbon: 33 20? ionomer: 33 20? pores: 60? Diffusion Medium e - O 2 H 2 O O 2 + 4H + + 4e - Pt 2H 2 O electrode void volume gas transport modeling ionomer volume fraction H + transport modeling

15 Simple Ohmic Losses E cell = E rev i R (RH) HOR ORR i R H +,an&ca (RH) tx,o2(dry) tx,o2(wet)

16 Ohmic voltage losses occur due to resistance to electron and ion conduction: electron-conduction ion-conduction flow-field or bipolar plate gas-diffuser: diffusion medium - DM gas diffusion layer - GDL porous cathode: ½O 2 +2H + +2e - H 2 O membrane: H + bulk Re R contact e e R contact bulk Re contact Re charge tx R H / e cath Re charge tx R H / e cath RH contact RH membrane RH E -el =i R -el E mem(rh) =i R non-linear resistance -mem(rh) R cath e- 0 R cath H+ 0 if RH and I/C are low

17 E in PEMFC s: a) E -el (for graphite flow-fields) electronic losses (both sides) *) : a) R contact bipolar/dm: 20 m cm 2 b) R contact DM/electrode: 7 m cm 2 c) R bulk DM & electrodes: 4 m cm 2 E -el = R -el i DM: diffusion medium R -el 30 m cm 2 proportional to i (ca. 30 mv loss at 1 A/cm 2 )

18 E in PEMFC s: b) E -mem(rh) calculation of R -mem(rh) : R -mem(rh) = t membrane / ionomer = t membrane membrane e.g., for ionomer =0.1S/cm and t membrane =25 m: R -membrane =25m cm 2 calculation of E -mem(rh) : E -mem(rh) = R -mem(rh) i for ionomer >>0.1S/cm and t membrane =25 m: E -membrane <25mV at 1A/cm 2

19 Overall E in PEMFC s E (RH) = i (R -el + R -mem(rh) ) i R (RH) in-situ measurement of R : via AC-impedance at high frequency ( HFR ) via current interrupt method from: R. Makharia et al., J. Electrochem. Soc., 152 (2005) A970. commonly used resistance-corrected E cell : E -free = E cell + i R E cell +i R HFR, where R HFR R (1-10kHz) (ok at near 100%RH)

20 H 2 Oxidation Reaction (HOR) and O 2 Reduction Reaction (ORR) Kinetics E cell = E rev i R (RH) HOR ORR i R H +,an&ca (RH) tx,o2(dry) tx,o2(wet)

21 Activation losses for HOR 0.1M KOH vs. PEMFC Large variation HOR/HER in 0.05M H 2 SO 4, 60 o C: Pt face i o [ma/cm 2 ] b [V/decade] Marković et al., J. Phys. Chem. 101 (1997) 5405 from: W. Sheng et al., J. Electrochem. Soc. 157 (2010) B1529 HOR catalysis challenging in Alkaline (Membrane) Fuel Cells

22 Activation losses for ORR Exchange current densities for the best catalysts for ORR: i s (0.9V) [A/cm 2 Pt] gain reference Pt pc (sputtered) RDE V. Stamenkovic et al., J. Electroanal. Chem. 554 (2003) 191 Pt 3 Co (sputtered) RDE x V. Stamenkovic et al., J. Phys. Chem. B 106 (2002) Pt 3 Co (annealed) RDE x V. Stamenkovic et al., J. Phys. Chem. B 106 (2002) %wt Pt/C RDE U.A. Paulus et al., J. Phys. Chem. B 106 (2002) %wt. Pt 3 Co/C RDE x U.A. Paulus et al., J. Phys. Chem. B 106 (2002) %wt Pt/C MEA H.A. Gasteiger et al., Appl. Catal. B 56 (2005) 9 45%wt. Pt 3 Co/C MEA x H.A. Gasteiger et al., Appl. Catal. B 56 (2005) 9 (MEA data at 80 C, RDE data at 60 C in 0.1M HClO 4 ; both at 100kPa a O 2 ) Typical numbers (Low temperature H2-air fuel cell, ambient p): ORR: i 0 = 0.1 ma cm 2 HOR: i 0 = 200 ma cm 2 x 2000! In low-t fuel cells: Activation losses are the most important losses. They occur mainly at the cathode

23 H + Conduction Losses in Anode & Cathode and O 2 transport losses E cell = E rev i R (RH) HOR ORR i R H +,an&ca (RH) tx,o2(dry) tx,o2(wet)

24 ST19-S0559 (Nano-x coating) RC FCPM op-line MEA Performance Analysis MEA: Gore 5720 (18 m, 0.2/0.3 mg Pt /cm 2, I/C=1.2) DM/MPL: Pre-compressed SGL 25BC HOR <5mV) ORR =410 mv Voltage (V) E cell [A/cm 2 ] HFR =90 mv ( mem =30 (60mV mv) R contact ) tx,h + =18 mv tx,o2(dry) =26 mv tx,o2(wet) =18 mv undefined losses, tx,o2(wet), of only 20mV improvements require new materials need 4x better ORR catalysts to reach 0.05/0.10 mg Pt /cm 2 MEA 0.2 g Pt /kw need 10x better ORR catalysts to reach 0.05/0.04 mg Pt /cm 2 MEA 0.1 g Pt /kw from: W. Gu, D.R. Baker, Y. Liu, H.A. Gasteiger, in: Handbook of Fuel Cells, Wiley (2009): vol. 6, pp. 631.

25 Current Automotive PEMFC R&D system cost/performance minimize noble metal (platinum) loadings: understand kinetic (catalytic) performance limitations develop/test new catalysts (e.g., Pt-alloys) reduce other voltage loss-terms to increase power density (if W/cm 2, $/kw ) need to understand each of the following terms and improve catalysts: E cell = E reversible E HOR ORR H+,electrode tx,gas minimize system components (parasitic losses) operation at drier conditions (humidifier, condenser) operation at higher temperature (smaller radiator) examine system constraints an current/advanced membrane materials

26 Current Automotive PEMFC Stack Research durability understand/mitigate degradation of Pt(alloy)-crystallites and carbon-support Pt/C catalyst 15-50nm 20-50nm-sized highly structured carbon 2-5 nm sized Pt(alloy crystallites) understand/mitigate degradation of 25 m membranes nm-sized phase-segregation [relative humidity (RH)] understand/mitigate accelerated degradation for dynamic/transient operation

27 Wasserstoff-Herstellung

28 Gleichgewichtszusammensetzung in Reformer und Shift-Konverter aus: Ledjeff-Hey/Mahlendorf/Ross (Hrsg), Brennstoffzellen

29 Energieverbrauch bei der Wasserstoffproduktion aus verschiedenen Kohlenwasserstoffen aus: Kordesch, Fuel Cells and their Applications

30 Vergleich von Wirkungsgraden für die Endenergiebereitstellung verschiedener Kraftstoffe aus: Rebhan (Hrsg.), Energiehandbuch

31 Primärenergieaufwand für Kraftstoffbereitstellung aus: Rebhan (Hrsg.), Energiehandbuch

32 Brennstoffzellensysteme