FUEL CELL CHARGE TRANSPORT

Transcription

1 FUEL CELL CHARGE TRANSPORT M. OLIVIER 19/05/2008

2 INTRODUCTION Charge transport completes the circuit in an electrochemical system, moving charges from the electrode where they are produced to the electrode where they are consumed. They are two major types of charges species: electrons and ions. The transport of electrons versus ions is fundamentally different, primarily due to the large difference in mass between the two. In most fuel cells, ion charge transport is far more difficult than electron charge transport. Resistance to charge transport results in a voltage loss (given by Ohm s law) = ohmic, or IR, loss. These losses are minimized by making electrolytes as thin as possible and employing high-conductivity materials. 2

3 INTRODUCTION Flux J measures how much of a given quantity (ex: moles) flows through a material per unit area per unit of time. Charge flux j measures the amount of charge that flows through a material per unit area per unit of time. Typical units: C 2 = cm s A cm Charge flux = current density 2 3

4 INTRODUCTION J i j = z = k i F J M ik F k J i = flux of species i F k = the k different forces acting on i M ik = coupling coefficients which reflect the relative ability of a species to respond to a given force with movement as well as the effective strength of the driving force itself 4

5 INTRODUCTION If charge transport is dominated by electrical driving forces: dv j = σ dx 5

6 6 CHARGE TRANSPORT : VOLTAGE LOSS CHARGE TRANSPORT : VOLTAGE LOSS Why does charge transport result in a voltage loss? Because fuel cell conductors are not perfect they have an intrinsic resistance to charge flow. σ σ σ σ A L R i A L i V L j V L V j = = = = Resistance of our conductor

7 CHARGE TRANSPORT : VOLTAGE LOSS V is the voltage which must be applied in order to transport charge at a rate given by i. This voltage represents a loss (Ohmic loss)= voltage which was expended or sacrificed in order to accomplish charge transport. η ohmic ohmic ( R R ) = i R = i + elec ionic Often small compared to R ionic 7

8 8 CHARGE TRANSPORT : VOLTAGE LOSS

9 TRANSPORT RESISTANCE Fuel cell resistance scales with area and with thickness: for this reason fuel cell electrolytes are generally made as thin as possible. Fuel cell resistances are additive. Performance improvements may be won by the development of better ion conductors. 9

10 TRANSPORT RESISTANCE RESISTANCE SCALES WITH AREA Area-normalised resistance known as area-specific resistance (ASR): η ohmic ASR ASR = i ohmic ohmic R = ohmic A L = σ = fuel Cell j ( ASR ) ohmic [ 2 Acm ] R ohmic 10

11 TRANSPORT RESISTANCE RESISTANCE SCALES WITH THICKNESS The shorter the conductor length L, the lower the resistance. 11 L ASR ohmic = σ Fuel cell electrolytes are designed to be as thin as possible. The most important limitations are: - Mechanical Integrity : Ex: membrane failure can result in catastrophic mixing of the fuel and oxidant. - Nonuniformities: Thin electrolyte areas may become hot spots that are subject to rapid deterioration or failure. - Shorting: Especially when the electrolyte is on the same order of magnitude as the electrode roughness.

12 TRANSPORT RESISTANCE RESISTANCE SCALES WITH THICKNESS The shorter the conductor length L, the lower the resistance. L ASR ohmic = σ Fuel cell electrolytes are designed to be as thin as possible. The most important limitations are: - Fuel crossover : As the electrolyte thickness is reduced, the crossover of reactants may increase. - Contact resistance : Resistance associated with the interface between the electrolyte and the electrode. - Dielectric breakdown: When the electrolyte is so thin that the electric field across the membrane exceeds the dielectric breakdown field for the material. 12

13 TRANSPORT RESISTANCE RESISTANCE SCALES WITH THICKNESS Practical limitations : Limit achievable thickness : µm 13

14 TRANSPORT RESISTANCE FUEL CELL RESISTANCES ARE ADDITIVE It is extremely very difficult to distinguish between all the various sources of resistance loss. 14

15 TRANSPORT RESISTANCE IONIC RESISTANCE USUALLY DOMINATES The best electrolytes employed in fuel cell: 1 1 σ 0.1 Ω cm At a thickness of 50 µm: ASR 0,05 0,1 Ωcm 2 A 50-µm-thick porous carbon cloth electrode: ASR < Ωcm This example illustrates how electrolyte resistance usually dominates fuel cells. Developing satisfactory ionic conductors is challenging. 15

16 PHYSICAL MEANING OF CONDUCTIVITY Conductivity quantifies the ability of a material to permit the flow of charge when driven by an electric field. Two major factors: how many carriers are available to transport charge and the mobility of those carriers within the material. σ = ( z ) i F ci ui A material s conductivity is determined by carrier concentration C i and carrier mobility u i. 16

17 PHYSICAL MEANING OF CONDUCTIVITY ELECTRONIC VERSUS IONIC CONDUCTORS 17

18 REVIEW OF FUEL CELL ELECTROLYTES Three major candidate materials classes for fuel cells: aqueous, polymer, and ceramic electrolytes Any fuel cell electrolyte must meet the following requirements: - High ionic conductivity - Low electronic conductivity - High stability (in both oxidizing and reducing environments) - Low fuel crossover - Reasonable mechanical strength (if solid) - Ease of manufacturability 18

19 REVIEW OF FUEL CELL CLASSES IN AQUEOUS ELECTROLYTES/IONIC LIQUIDS Almost all aqueous/liquid electrolyte fuel cells use a matrix material to support or immobilize the electrolyte. 1. Provides mechanical strength to the electrolyte 2. Minimizes the distance between the electrodes while preventing shorts 3. Prevents crossover of reactant gases through the electrolyte Examples: Alkaline fuel cells use concentrated aqueous KOH electrolytes; phosphoric acid fuel cells use either concentrated H 3 PO 4 electrolytes or pure H 3 PO 4. Molten carbonate fuel cells use molten (K/Li) 2 CO 3 immobilized in a supporting matrix. 19

20 REVIEW OF FUEL CELL CLASSES IN AQUEOUS ELECTROLYTES/IONIC LIQUIDS σ = ( z ) i F ci ui Selected Ionic Mobilities at Infinite Dilution in Aqueous Solutions at 25 C. 20

21 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES For a polymer to be good ion conductor, at a minimum it should possess the following structural properties: 1) The presence of fixed charges sites; 2) The presence of free volume ( open space ). The fixed charge sites should be opposite charge compared to the moving ions. In a polymer structure maximizing the concentration of these charge sites is critical to ensure high conductivity. Excessive addition of ionically charged side chains will significantly degrade the mechanical stability of the polymer. 21

22 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES Schematic of ion transport between polymer chains: Polymer segments can move or vibrate in the free volume, thus inducing physical transfer of ions from one charged site to one another. 22

23 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES: Ionic Transport in Nafion Teflon backbone = mechanical strength Sulfonic acid functional groups: charge sites for proton transport 23

24 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES: Ionic Transport in Nafion In the presence of water, the protons (H + ) in the pores form hydronium complexes (H 3 O + ) and detach from the sulfonic acid side chains. When sufficient water exists in the pores, the hydronium ions can transport in the aqueous phase. -Under these circumstances, ionic conduction in Nafion is similar to conduction in liquid electrolytes. -The hydrophobic nature of the Teflon backbone accelerates water transport through the membrane, since the hydrophobic pore surfaces tend to repel water. -To maintain this extraordinary conductivity, Nafion must be fully hydrated with liquid water. 24

25 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES: Ionic Transport in Nafion The water content λ in Nafion = the ratio of the number of water molecules to the number of charged (SO 3- H + ) sites 0 < λ < 22 Completely dehydrated Nafion Full saturation 25

26 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES: Ionic Transport in Nafion Water content versus water activity for Nafion 117 at 303 K 26

27 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES: Ionic Transport in Nafion Ionic conductivity of Nafion versus water content λ at 303 K 27

28 REVIEW OF FUEL CELL CLASSES IN POLYMER ELECTROLYTES: Ionic Transport in Nafion σ 1 1 ( λ, T ) = σ 303 ( λ) exp 1268 K 303 T Ionic conductivity of Nafion versus temperature when λ= 22 28

29 REVIEW OF FUEL CELL CLASSES IN CERAMIC ELECTROLYTES SOFC electrolytes = are solid, crystalline oxide materials that can conduct ions The most popular SOFC electrolyte is yttria stabilised zirconia (YSZ) Typical YSZ electrolyte contains: 8% yttria mixed with zirconia Zirconia = ZrO 2 (zirconium oxide) Yttria = Y 2 O 3 (Yttrium oxide) Yttria stabilised the zirconia crystal structure in the cubic phase (where it is most conductive). Yttria induces high concentrations of oxygen vacancies into the zirconia crystal structure. High ion conductivity 29

30 REVIEW OF FUEL CELL CLASSES IN CERAMIC ELECTROLYTES Charge compensation effects in YSZ lead to creation of oxygen vacancies The addition of 8% (molar) yttria to zirconia causes about 4% of the oxygen sites to be vacant. 30

31 REVIEW OF FUEL CELL CLASSES IN CERAMIC ELECTROLYTES A material s conductivity is determined by the combination of carrier concentration c and carrier mobility u: σ = ( z F ) cu = c ( zf ) RT 2 D The oxygen vacancies can be considered to be ionic charge «carriers». Carrier mobility is described by D, the diffusivity of the carrier in the crystal lattice. Diffusivity describes the ability of a carrier to move, or diffuse, from site to site within a crystal lattice. 31

32 REVIEW OF FUEL CELL CLASSES IN CERAMIC ELECTROLYTES There is an upper limit to doping. Above a certain dopant or vacancy concentration, defects start to interact with each other, reducing their ability to move. 32

33 REVIEW OF FUEL CELL CLASSES IN CERAMIC ELECTROLYTES The carrier diffusivity in SOFC electrolytes is exponentially temperature dependent: D 0 = constant (cm 2 /s) D = D 0 e G act ( RT ) G act = the activation barrier for the diffusion process (J/mol) σ c ( ) 2 G ( RT ) zf D e act = 0 RT 33

34 REVIEW OF FUEL CELL CLASSES IN CERAMIC ELECTROLYTES For extrinsic carriers, c is determined by the doping chemistry of the electrolyte. In this case, c is a constant and the preceding equation can be used. For intrinsic carriers, c is exponentially dependent on the temperature and the equation becomes: σ = c sites ( ) 2 hv ( 2kT ) Gact ( RT ) zf D e e 0 RT 34