FUEL CELLS: INTRODUCTION

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1 FUEL CELLS: INTRODUCTION M. OLIVIER 19/5/8

2 A SIMPLE FUEL CELL Two electrochemical half reactions : H 1 O H + + H + e + + e H O These reactions are spatially separated: Electrons: flow through an external circuit (electrical current) Electrolyte : allows ions to flow but not electrons At a minimum: Fuel cell = two electrodes separated by an electrolyte

3 DEFINITIONS Energy = the ability to do work - [J] or [cal] Power= rate at which energy is expended or produced Power is a rate = amount of energy used or produced per second - [W = J/s] Volumetric power density = amount of power that can be supplied by a device per unit volume [W/cm 3 ] or [kw/m 3 ] Gravimetric power density = amount of power that can be supplied by a device per unit mass [W/g] or [kw/kg] Volumetric energy density = amount of energy that can be supplied by a device per unit volume [Wh/cm 3 ] or [kwh/m 3 ] Gravimetric energy density = amount of energy that can be supplied by a device per unit mass [Wh/g] or [kwh/kg] 3

4 FUEL CELL ADVANTAGES - Producing electricity as long as they are supplied with fuel - Far more efficient than combustion engines - No moving parts and so silent - Undesirable products such as NO x, SO x and particulate emissions are virtually zero - Unlike batteries, fuel cells allow easy independent scaling between power (determined by the fuel cell size) and capacity (determined by the fuel reservoir size). - Higher energy densities compared to batteries and can be quickly recharged by refueling (no recharge) 4

5 FUEL CELL DISADVANTAGES - Cost - Power density - Fuel availability and storage: hydrogen or alternatives such as gasoline, methanol (difficult to use directly) - Operational temperature compatibility concerns: susceptibility to environmental poisons and durability under start-stop cycling 5

6 FUEL CELL TYPES Type PEMFC DMFC DEFC PAFC AFC MCFC SOFC Electrolyte Proton exchange membrane fuel cell Direct methanol fuel cell Direct ethanol fuel cell Phosphoric acid fuel cell Alkaline fuel cell Molten carbonate fuel cell Solid oxide fuel cell Same electrochemical principles but operate at different temperature regimens, incorporate different materials and often differ in their fuel tolerance and performance characteristics 6

7 ELECTROCHEMICAL CONCEPTS ANODE = OXYDATION CATHODE = REDUCTION OXIDATION = process where electrons are liberated by the reaction REDUCTION = process where electrons are consumed by the reaction H 1 O H + + H + e + + e H O For hydrogen-oxygen fuel cell: - The anode is the electrode where the hydrogen oxidation reaction (HOR) takes place - The cathode is the electrode where the oxygen reduction reaction (ORR) takes place 7

8 ELECTROCHEMICAL CONCEPTS Be carefull Anodes and cathodes can be either positive or negative Galvanic cell = Anode is negative and cathode is positive Electrolytic cell = Anode is positive and cathode is negative 8

9 BASIC FUEL CELL OPERATION Major steps involved in producing electricity: 1) Reactant delivery (transport) ) Electrochemical reaction 3) Ionic conduction through the electrolyte and electron transport through the external circuit 4) Product removal 9

Current voltage (i-v) curve: voltage output of the fuel cell for a given current output Irreversible losses: - Activation losses (due to electrochemical

10 Current voltage (i-v) curve: voltage output of the fuel cell for a given current output Irreversible losses: - Activation losses (due to electrochemical reaction) - Ohmic losses (due to ionic and electronic conduction) - Concentration losses (due to mass transport) FUEL CELL PERFORMANCE V = E thermo η act η ohmic η conc 1

11 FUEL CELL PERFORMANCE Power density curve: power density delivered by a fuel cell as a function of the current density P = iv [ ] W cm 11

12 1 FUEL CELL AND THE ENVIRONMENT

13 FUEL CELL THERMODYNAMICS Key to understand the conversion of chemical energy into electrical energy Thermodynamics yields the theoretical boundaries of what is possible with a fuel cell = «ideal case» Understanding real fuel cell performance requires a knowledge of kinetics in addition to thermodynamics 13

14 FUEL CELL THERMODYNAMICS Internal Energy (U): The energy needed to create a system in the absence of changes in temperature and volume Enthalpy (H): The energy needed to create a system plus the work needed to make room for it (from zero volume) Helmholtz Free Energy (F): The energy needed to create a system minus the energy that you can get from the system s environment due to spontaneous heat transfer (at constant temperature) Gibbs Free Energy (G): The energy needed to create a system and make room for it minus the energy that you can get from the environment due to heat transfer 14

15 15 FUEL CELL THERMODYNAMICS

16 REVERSIBILITY Implies equilibrium FUEL CELL THERMODYNAMICS A reversible fuel cell voltage = voltage produced by a fuel cell at the thermodynamic equilibrium To distinguish between reversible and nonreversible fuel cell voltages E = reversible (thermodynamically predicted) fuel cell voltage V = operational (nonreversible) fuel cell voltage 16

17 The maximum heat energy that we can extract from a fuel is given by the fuel s enthalpy of reaction For a general reaction: FUEL CELL THERMODYNAMICS WORK POTENTIAL OF A FUEL : ENTHALPY OF REACTION a A+ b B m M + n N h rxn = [ ( ) ( ) ] [ ( ) ( ) ] m h M + n h N a h A + b h B f f f f Enthalpy of reaction (in STP) = computed from the difference between the molar formation enthalpies of the products and the reactants 17

18 FUEL CELL THERMODYNAMICS WORK POTENTIAL OF A FUEL : ENTHALPY OF REACTION 18

19 FUEL CELL THERMODYNAMICS WORK POTENTIAL OF A FUEL : GIBBS FREE ENERGY G : the net energy you had to transfer to create the system = the maximum energy that you could ever get back out of the system Gibbs free energy = the exploitable energy potential (Work potential) At T = constant g = h T s W = g elec rxn 19

20 FUEL CELL THERMODYNAMICS GIBBS FREE ENERGY AND REACTION SPONTANEITY G > : Nonspontaneous (energitically unfavourable) G = : Equilibrium G < : Spontaneous (energetically favourable) Spontaneity is no guarantee that a reaction will occur, nor does it indicate fast a reaction will occur STUDY OF THE KINETIC BARRIERS Ex: Diamond from graphite

21 W elec = E Q and Q = n F 1 H + O H O FUEL CELL THERMODYNAMICS GIBBS FREE ENERGY AND VOLTAGE g = n F E grxn 37 E = = = 1, 3V nf 964 g = -37, kj mol -1 (liquid water - STP) g = -8,6 kj mol -1 (gaseous water - STP). The highest voltage attainable from H -O fuel cell at STP Most feasible fuel cell reactions have reversible voltage in the range of,8 1,5 V 1

22 FUEL CELL THERMODYNAMICS STANDARD ELECTRODE POTENTIALS STANDARD ELECTRODE POTENTIAL TABLES + 1 = E Cell E half reactions H + e =, + ( O + 4 H + 4e H O) E = + 1, 9 H 1 + H + O H O Ecell = + 1,9 Electrode potential tables list all reactions as reduction reactions. Any thermodynamically spontaneous electrochemical reaction will have a positive cell potential. To obtain the reverse reaction: an external voltage must be applied. E

23 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE CONDITIONS Reversible voltage variation with temperature g = n F E E dg dt de dt T p p = E = S s = nf s + nf ( T T ) ( g) d dt p = s For the familiar H -O cell, s rxn = - 44,43 J/mol For every 1 C increase in cell temperature, there is an approximate 3 mv decrease in cell voltage Should we operate a fuel cell a the lowest temperature? The answer is NO. Kinetic losses tend to decrease with increasing temperature. 3

24 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE CONDITIONS Reversible voltage variation with temperature 4

25 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE CONDITIONS Reversible voltage variation with temperature Temperature ( C) Water G (kj.mol -1 ) E (V) 5 Liquid -37, 1,3 1 Gaseous -5,3 1,17 Gaseous -,4 1,14 6 Gaseous -199,6 1,4 1 Gaseous -177,4,9 5

26 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE CONDITIONS Reversible voltage variation with pressure dg = V dp T g = n F E de dp T v = nf ( g) d dp T = v If the volume change of the reaction is negative, then the cell voltage will increase with increasing pressure. Pressure has a minimal effect on reversible voltage. Pressurizing a H -O fuel cell to 3 atm H and 5 atm O increases the reversible voltage by only 15 mv. 6

27 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE CONDITIONS 7

28 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE WITH CONCENTRATION Nernst Equation Concept of chemical potential α G µ i = ni T, P, n j i When we change the concentration of chemical species in a fuel cell, we are changing the free energy of the system. This change in turn changes the reversible voltage. µ = µ + RT ln i i a i µ i = the reference chemical potential of species i at standard conditions a i = the activity of the species i 8

29 9 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE WITH CONCENTRATION Nernst Equation Concept of chemical potential dg = E = E 1A+ bb mm g = g g = n F E i µ dn i RT nf i ( µ + RT ln a ) This Nernst equation is the centerpiece of fuel cell thermodynamics. = + RT ln a ln a i + nn a a m M 1 A m M 1 A a a n N b B i a a n N b B = E i RT nf dn ln i a a ν i products ν i reactants

30 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE WITH CONCENTRATION Nernst Equation For the familiar hydrogen-oxygen fuel cell reaction Below 1 C 1 H + O H O E = E E = E RT F RT F ln ln Pressurizing the fuel cell in order to increase the reactant gas partial pressures will increase the reversible voltage. a p a H H H O a 1 p 1 O 1 O 3

31 FUEL CELL THERMODYNAMICS UNDER NON-STANDARD-STATE WITH CONCENTRATION Nernst Equation Perhaps, we are worried that almost fuel cells operate on air instead of pure oxygen. How much does this affect the reversible voltage of a room temperature H -O fuel cell? ( 8,314)( 98,15 ) 1 ln ( )( 964) ( 1)(,1) E =,9 1, 19V 1 = 1 Operation in air drops the reversible voltage by only 1 mv. 31

32 FUEL CELL THERMODYNAMICS IDEAL REVERSIBLE FUEL CELL EFFICIENCY EFFICIENCY ε = The amount of useful energy that can be extracted from the process relative to the total energy evolved by that process: ε = useful energy total energy = work h g = h 37,3 86 ε thermo, fc = =,83 3

33 FUEL CELL THERMODYNAMICS REAL (PRACTICAL) FUEL CELL EFFICIENCY The real efficiency of a fuel cell is always less than the reversible thermodynamic efficiency. 1) Voltage losses ) Fuel utilization losses ε =ε real thermo ε voltage ε fuel The voltage efficiency of the fuel cell ε voltage = the ratio of the real operating voltage of the fuel cell (V) to the thermodynamics reversible voltage (E) ε voltage = V E 33

34 FUEL CELL THERMODYNAMICS REAL (PRACTICAL) FUEL CELL EFFICIENCY The fuel utilization efficiency ε fuel = accounts that not all of the fuel provided to a fuel cell will participate in the electrochemical reaction. The ratio of the fuel used by the cell to generate electric current versus the total fuel provided to the cell. i nf 1 = = ε fuel υ fuel ν fuel = rate at which fuel is supplied to the fuel cell (mol/s) λ = stoichiometric factor λ ε real = g h V E i nf ν fuel = g h V E 1 λ 34

Basic overall reaction for hydrogen powering

Fuel Cell Basics Basic overall reaction for hydrogen powering 2H 2 + O 2 2H 2 O Hydrogen produces electrons, protons, heat and water PEMFC Anode reaction: H 2 2H + + 2e Cathode reaction: (½)O 2 + 2H +