Cross Section of Proton Exchange Membrane Fuel Cell

Transcription

1 PEMFC Electrodes 1

2 Cross Section of Proton Exchange Membrane Fuel Cell Anode Cathode 2

3 Typical PEMFC Electrodes: - Anode Hydrogen Oxidation - Pt Ru / C - Cathode Oxygen reduction - Pt / C Pt is alloyed with Ru to enhance CO tolerance of Anode electrocatalyst 3

4 Blacks vs. supported catalysts Pt black large particle sizes difficult to prevent agglomeration - Leads to low surface area (SA) Supported Pt: - High SA carbon used as support - Smaller Pt particle size - Better dispersion - Larger SA 4

5 Dispersion Ratio of surface atoms to bulk atoms - higher dispersion, smaller particle size -larger SA Assuming spherical particles: SA = 6 / (ρ pt x d) (1) Where ρ pt = density of Pt, d = particle diameter 5

6 Pt particle size vs. SA 6

7 Anode Catalysis H 2 electrooxidation in absence of impurities H 2H + 2e; E = 0V 2 o + Net reaction at anode (2) 7

8 Proposed mechanism In acidic electrolytes, a two step mechanism is thought to occur 1. Dissociative adsorption of hydrogen 2. Charge transfer (oxidation) H 2 adsorption is the rate limiting step 8

9 Mechanism H 2 + 2Pt 2Pt H ads (3) 2 Pt H 2Pt + 2H + + 2e ads (4) 9

10 Characteristics High exchange current density (i o ~ 1mA/cm 2 ) on Pt single crystals Therefore no significant overpotential upon increased load Reaction proceeds efficiently with very low Pt loadings 0.05 mg/cm 2 is sufficient 10

11 Problems? Pure H 2 anode feed unrealistic Typically at least 10 ppm (upto 20,000ppm) of CO present in feed stream CO acts as a poison: adsorbs on Pt reduces active sites for H 2 adsorption Increases anode overpotential CO Tolerant Fuel Cells Discussed in separate lecture 11

12 Effect of CO in Reformate on Performance Data obtained at LANL 12

13 Cathode electrolysis Oxygen reduction reaction + O + 4H + 4e 2H O 2 2 (5) 13

14 Characteristics Lower exchange current density (10-4 to 10-6 ma/cm 2 ) on Pt Much higher overpotential (compared to pure H 2 oxidation) for a given current density Higher loadings (0.4 mg/cm 2 ) needed Kinetics strong function of oxygen partial pressure (0.7 1 order) 14

15 Effect of oxygen concentration 15

16 Components of an oxygen reduction electrode - Pt catalyst - Carbon support - Ionomer (to facilitate proton conduction) - Teflon (optional) to create hydrophobic channels permits gas diffusion through unwetted pores 16

17 Need for ionomer contact with electrocatalyst Protons need to: (a) get to anode / membrane interface from anode electrocatalyst oxidation site (b) get to the cathode Pt site from membrane / cathode interface Ionomer / catalyst contact is essential to minimize resistive losses 17

18 Oxygen transport Reactant interaction with Pt crucial (recall strong kinetic dependence on concentration of Oxygen) Thus viable path for oxygen transport needed Places limitation on electrode thickness and composition 18

19 The three phases Catalyst Ionomer Reactant Intimate 3 phase contact essential for low overpotential 19

20 Typical model Catalyst Particle Ionomeric Skin Reactant diffuses through the skin, and interacts with the catalyst 20

21 Optimization Factors to be considered include: - catalyst loading - supported / unsupported - ionomer loading - electrode thickness - electrode porosity - ratio of hydrophobic / hydrophilic pores Note each optimized electrode will only be optimal at a given operating condition different conditions result in different optima! 21

22 Interesting tradeoffs Catalyst loading: - Kinetics vs. Mass transport Catalyst Ionomer loading: - Catalytic activity vs. mass transport - Ionic resistance vs. mass transport - Electronic vs. ionic resistance 22

23 Kinetics vs. transport Larger catalyst loading thicker electrode (for a given catalyst): - Decrease in kinetic losses to a certain loading (thickness) - Increase in mass transport losses (increased thickness) - above a certain thickness (loading) Increasing catalyst loading only works until mass transport effects negate gains in kinetics 23

24 A caveat Recall catalysts are typically Pt supported on carbon Increasing the % of Pt/C higher Pt density per gram of catalyst For given Pt loading (and ionomer content): - lower electrode thickness with increasing Pt content in Pt/C catalyst Note that Pt surface area decreases with increasing Pt loading in Pt/C - lower catalyst utilization 24

25 Note that: Ionomer loading - Ionomer presence is essential - Homogeneous distribution required - Must intertwine with Pt particles Also recall: - Good ionomer does not permit large reactant crossover - Low oxygen permeability through ionomer - Diffusional problems! - Good ionomer does not conduct electrons 25

26 Catalytic activity vs. mass transport Large ionomer loading: - Good contacting with Pt particles - High catalyst utilization - High catalytic activity However: - Large ionomeric skin thickness (slide 20) - Enhanced diffusional losses 26

27 Ionic resistance vs. mass transport Large ionomer loading: - Good ionomeric network between catalyst site and membrane electrode interface - High proton conductivity / transport - Low resistive losses However: - Large ionomeric skin thickness (slide 20) - Enhanced diffusional losses 27

28 Ionic vs. electronic resistance Large ionomer loading: - Good ionomeric network between catalyst site and membrane electrode interface - High proton conductivity / transport - Low resistive losses However: - Extensive coating of carbon network with non electron conducting ionomer - Enhanced electronic resistance losses 28

29 Complex interplay Catalyst loading Pt content in Pt/C Electrode thickness Ionomer loading Kinetic losses Ohmic losses Transport losses 29

30 Optimization Extremely challenging difficult to manipulate single parameters Optimal compositions change with type of catalyst even from manufacturer to manufacturer Optimal compositions certainly change with operating conditions 30

31 Typical catalyst ink manufacture Catalyst + solvent (methanol) Stirring / sonication Additional steps may include: Addition of ionomer - Drying and redissolution - Heat treatment - Addition of Teflon Stirring / sonication 31

32 Added complexity Variables in catalyst ink manufacture: - Temperature of ink manufacture - Type / extent of agitation - Heat treatment - Solvent used Any change in above results in different optimal compositions for any given condition 32

33 Yet another parameter MEA manufacturing technique Several techniques of applying catalyst to membrane Broad subdivision: - Catalyst applied to membrane - Catalyst applied to gas diffusion layer 33

34 Catalyst applied to membrane Spraying onto membrane Silk screening onto membrane Decal transfer 34

35 Spraying onto membrane Preparation of Catalyst Ink: low viscosity;good particle dispersion needed Use of ultrasonic, magnetic stirring, homogenizers etc. preferred dispersion techniques Variables include dispersion time, solution viscosity, dispersion temperature, solvent used etc. 35

36 Spraying onto membrane - setup IR lamp Membrane (active area exposed) The lamp enables solvent evaporation The carrier gas propels the dispersion on to the membrane surface Caution carrier gas pressure must be low enough to prevent hole formation Can be done by hand. Spray gun (paint brush) Better control obtained using computer controlled X-Y recorder based spraying instrument Carrier Gas (N 2 ) 36

37 Silk screening Preparation of Catalyst Ink: higher viscosity;good particle dispersion needed Use of ultrasonic, magnetic stirring, homogenizers etc. preferred dispersion techniques viscosity controlled by external agents such as ethylene glycol Variables include dispersion time, solution viscosity, dispersion temperature, solvent used etc. 37

38 Silk Screening - setup Brush to spread the ink Screen known mesh size Masks to expose desired active area Membrane Substrate Note all pneumatically controlled! 38

39 Decal transfer Preparation of Catalyst Ink: highest viscosity;good particle dispersion needed Use of ultrasonic, magnetic stirring, homogenizers etc. preferred dispersion techniques viscosity controlled by external agents such as ethylene glycol Variables include dispersion time, solution viscosity, dispersion temperature, solvent used etc. 39

40 Decal transfer - methodology blank Paint (or silk screen / spray) ink onto blank desired quantity heat treat to evaporate solvent Blank + ink Blank + ink Place painted blank onto membrane - Hot press membrane blank membrane + ink Peel off blank Repeat procedure on other side different blank blank + membrane + ink 40

41 Catalyst applied to GDL Prepare catalyst ink desired viscosity Apply on to porous gas diffusion layer Spraying Silk screening Dry the ink evaporate solvent 41

42 Making the MEA Catalyst coated GDL Membrane Catalyst coated GDL Hot press 42

43 Catalyst onto membrane Merits High catalyst utilization Intimate interfacial contact Demerits Can be complicated Can lead to wastage especially if spraying is used The other two techniques do not lead to wastage but are difficult to use with thin membranes 43

44 Catalyst onto GDL Merits Simple technique Less wastage of catalyst especially if catalyst is screened onto GDL Demerits Low catalyst utilization catalyst lost in GDL pores Interfacial contact may be poor Risk of short circuiting especially with thin membranes 44

45 Remarks On the whole, applying the catalyst onto the membrane is the preferred technique for high performance Decal transfer with catalyst inks screened on to the surface of the blank most elegant technique good reproducibility 45