Introduction Fuel Cells

Fuel cell applications‐ PEMFC

PowerCell AB, S2 PEMFC, 5- 25 kW

Toyota Mirai – a Fuel Cell Car

A look inside

The hydrogen tank

1. InsideLayer of polymer closest to the H2 gas

2. Intermediate layerReinforced carbon fiber polymer. Gives

strength and stiffness. 3. Outside

Reinforced carbon fiber polymer. Protect from wear on the outside surface of the tank.

Toyota Fuel Cell Stack Name Toyota FC Stack

Type PEMFC

Energy density 3.1 kW/litre

Max Power 114 kW (155 horsepower (DIN))

Hydrogen tanks 2

Nominal pressure 700 bar (70 Mpa)

Material Reinforced carbon fiber ‐polymer

Volume of tank 122.4 litre (60 + 62.4), 5 kg H2

Electrical motor Syncron alternating current motor/generator

Max power 114 kW

Max angular momentum 335 Nm

Battery Type NickelmetalhydrideNiMH

Principle scheme of a single fuel cell

Principal sketch of a PEMFC

Illustration of flow, heat, mass, ion and charge transport in a PEM fuel cell

PEMFC

• Anode reaction: H2 → 2H+ + 2e‐

• Cathode reaction: (½)O2 + 2H+ + 2e‐ → H2O

• Overall: 2H2 + O2 ↔ 2H2O

SOFC• Anode reaction: H2 + O2‐ → H2O + 2e‐

• Cathode reaction: (½)O2 + 2e‐ → O2‐

• Overall: 2H2 + O2 ↔ 2H2O

Principle operation of some fuel cell types

Fuel Cell Type Mobile Ion Operating Temperature oC

Applications

Proton Exchange Membrane, PEMFC,

PEFC

H+ 30-100 Vehicles, mobile equipment, low

power CHP systemsDirect Methanol,

DMFCH+ 20-90 Portable electronic

systems with low power, long

operating timesPhosforic Acid

(PAFC)H+ ~ 200 Large numbers of

200 kW CHP systems

Alkaline, AFC OH- 50-200 Space vehiclesMolten Carbonate,

MCFCCO3

2- ~ 650 Medium to large scale CHP systems

Solid Oxide , SOFC O2- 500-1000 All sizes of CHP

Properties of some fuel cell typesFuel Cell Type Material and

CatalystOperating

Temperature oCEfficiency

%Electrolyte

Proton Exchange

Membrane, PEMFC, PEFC

Carbon and Platinum

30-100 40-50 Solid membrane

Direct Methanol,

DMFC

Platinum and Ruthenium

20-90 20-50 Solid membrane

Phosforic Acid (PAFC)

Graphite and Platinum

~ 200 40-50 Phosforic acid

Alkaline, AFC Carbon and Nickel

50-200 50-60 Potassium hydroxide

Molten Carbonate,

MCFC

Stainless steel and Nickel

~ 650 > 60 Carbonated metal

S lid O id C i d 500 1000 50 60 C i

oton Exchange Membrane Fuel Cells (PEMFC) or Polymer Electrolyte Fuel Cells (PEFC)

fuel cell operates at a relative low temperature, about 80 C. The energyversion efficiency, i.e., conversion of the hydrogen energy to electricity is inorder of 50 – 60 %. The electrolyte is a solid polymer in form of a thinmbrane in which protons are mobile. Promoted and introduced as the powerrce for vehicles on the ground. Good start up behavior at low temperatures,all and compact. Considered as the power source in portable units as well asarge stationary power plants. The surrounding catalyst is of platinum.sitive to the purity of the hydrogen fuel. PEM fuel cells were used on the firstnned spacecraft. The platinum catalyst is very costly but nickel‐tin catalystse been discovered. As an effect, fuel cells can be a possible substitute forteries in spacecraft and other applications.

Alkaline Fuel Cells (AFC)his has been used in space vehicles for generation of electricitynd drinkable water for astronauts. Conversion efficiency is about0 % and operating temperature is in the range of 150 ‐ 200 C.he electrolyte is mainly consisting of potassium hydroxide (KOH).ome interests have also been shown for ground vehicles. Andvantage of this fuel cell type is that it is not depending onlatinum as the catalyst. Carbon dioxide may affect the conversionnd it is generally required that the supplied air and fuel must beree from CO2 or otherwise pure oxygen and hydrogen must besed. This was fuel cell type was used on the Apollo and Shuttle

Orbiter craft.

Phosforic Acid Fuel Cells (PAFC)s fuel cell type has been commercially in operation for a while. Its found applications in, e.g., hospitals, hotels, offices, airportsd schools. Conversion efficiency is relatively low, about 40 ‐ 50 %.erating temperature is in the range of 150 ‐ 200 C. Bothctricity and steam can be taken care of. The electrolyte is an acidphosphor. This fuel cell type shows patience against pollutionsng accompanied with the hydrogen fuel particularly at highmperatures. At low temperature, carbon monoxide may damagee platinum layer on the catalyst. Units of this fuel cell type areen big and heavy but the technology is regarded as mature.

Solid Oxide Fuel Cells (SOFC)s fuel cell type is regarded as a candidate for large units even atmote locations. Applications as Auxiliary Power Units (APUs) inhicles. The electrolyte is in solid state and commonly made in ard ceramic material based on zirconium oxide. The operatingmperature is high, around 1000 C. Due to the high operatingmperature, high reaction rates can be achieved withoutpensive catalysts and that gases, e.g., natural gas can be usedectly or being internally reformed without the need of aparate unit. The conversion efficiency is about 60 %. It may alsointegrated with a steam turbine to generate additional

ectricity. It does not necessarily require pure hydrogen as theel.

Molten Carbonate Fuel Cells (MCFC)trolyte consists of a melt of carbonates of lithium, sodium and potassium.conversion efficiency is about 60 % and operating temperature is about 650needs carbon dioxide in the air to work. The high operating temperaturens that a good reaction rate is achieved by using a relatively cheap catalyst,ely nickel. Nickel also forms the basis of the electrode. Its simplicity is partlyet by the nature of the electrolyte which is a hot and corrosive mixture ofum, potassium and sodium carbonates. Similar to other high temperaturecells, it can be combined with a steam turbine to generate additionaltricity. It can be operated with other hydrogen carriers than pure hydrogen.high temperature operation, as for SOFCs, may have a severe effect on thecomponents.

Direct Methanol Fuel Cells (DMFC)

his is a variant of the PEMFC but methanol is reacting directly inhe anode instead of in a separate reformer upstream the fuelell. The conversion efficiency is about 30 – 40 % and theperating temperature is in the range of 50 – 100 C. The loss inhe reformer is eliminated. It may find applications in portableomputers and mobile phones.

Reversible Fuel Cellscell which can be operated both as a fuel cell and as anctrolyzer is called a reversible fuel cell. In case of surplus ofctricity from wind or solar, the cell can be operated to generatedrogen by electrolysis of water. As it operates in the fuel cellde it uses the hydrogen as the fuel and generates electricity.

Proton Ceramic Fuel Cellsnder development. The idea is that the ceramic electrolytehould be able to conduct protons. The operating temperature isbout 700 C and hydrogen is oxidized directly at the anode. Aeformer is not needed. This fuel cell type is believed to be ableo combine the advantages of the high temperature fuel cellsnd the PEMFC due to the use of a ceramic electrolyte.

FC‐Aerospace applicationsAnother type of fuel cell using novel fuel and oxidizer has also beennvestigated for space power systems. This is due to a revival interest ofhydrogen peroxide in aerospace power applications. Hydrogen peroxideH2O2) has been used directly at the cathode. The fuel on the anode sidehas been hydrogen gas and an aqueous NaBH4 solution. It has been foundhat the direct utilization of H2O2 and NaBH4 at the electrodes resulted inbout 30 % higher voltage output compared to a regular H2 – O2 fuel cell.Both NaBH4 and H2O2 are in aqueous form and the combination has someoperational advantages. A Nafion membrane is used as the electrolyte.The catalysts are commonly platinum based. The carbon substrate is herealled reactant diffusion layer instead of gas diffusion layer because theperoxide reactant is in liquid phase. The design becomes compact and it isdeal for space applications where a high energy density fuel is requirednd air is not available.

Basic transport processes and operation of a fuel cell

Electrochemical kinetics

Heat and mass transfer

Charge and water transport

Microstructure of CL‐PEMFCn: for conduction of electrons and support of the platinum nano-es ;

mer: typically Nafion®, for proton transport;

um: for electrochemical reactions;

for transport of reactant and product gases;

Illustration of Microstructure in SOFC

CFL= cathode functional layerAFL= anode functional layer

Micro-Meso-

Charge Transport

Mass Transport

Heat Transport

Layer thicknesses in PEMFC and SOFC

Electrochemical kinetics

Studies of electrochemical kinetics is important for design andoperation of fuel cells. The electron transfer rate at theelectrodes or the current produced by the fuel cell depends onhe rate of electrochemical reaction. The processes governinghe electrode reaction rates are the mass transfer between the

bulk solution and the electrode surface, the electron transfer athe electrode, and the chemical reactions involving electronransfer.

Heat and mass transfer

A number of transport processes occur in a fuel cell. Theeactant gases flow through the gas flow channels and reactant

gas species are transported from the gas flow channels andhrough the porous electrodes. Ions are transported through the

membranes or electrolyte and electrons are transported throughelectrodes and interconnects. Poor transport of heat and masscontributes to the fuel cell losses and performance. Chargeransport contributes to ohmic losses and the mass transfer ofhe reactant gases impacts the mass transfer losses.

Charge and water transport

Electrons and ions are produced and consumed in twoelectrochemical reactions at the anode-electrolyte and cathode-electrolyte interfaces. The electrons are transported through theelectrodes and interconnect to the external electric circuit. Ionsare transported through the electrolyte from the electrodewhere it is produced to the electrode where it is consumed.Ohmic voltage losses are caused by the resistances to themotion of ions through the electrolyte as well as electronshrough the electrodes, interconnect materials, and contactnterfaces. Ion transport is also important for the transport of

water in PEM fuel cells.

Length scales‐Computational Approaches

Macroscopic modelling‐CFD methods

ANSYS FLUENTCOMSOLOpenFoam

cluding porous media approachesffective thermal-physical and transport properties

The general equation

Arbitrary variable 

jj j j

u St x x x

Control volume method‐FVM

dSB

n

Vthj

j j jV V V

UdV dV S dV

x x x

S S V

U d S d S S dV

Divergence theorem

General:

Mass conservation equationMomentum conservation equationSpecies conservation equationEnergy conservation equation

Additional Submodels:

Charge conservation equationLiquid water transport equationDissolved water transport equation

Thermophysical Properties and Source terms

Depends on the variable being considered and what part of the cell 

being considered

icro- , Meso- and Nanoscale Approachesrticle based methods

Lattice based - lattice Boltzmann methodDissipative Particle DynamicsCoarse - Grained Molecular Dynamics

p 1: reconstructionp 2: analysis and modeling of various phenomena

Computational Platforms

eoretical and computational platform including a first principle approach toms that couple several scales is generally needed. This can be built up at leasty by available open–source codes or commercial ones.

Atomic Simulation Environment (ASE) is a common part of a simulationdeveloped at DTU, Denmark. Can be used to run molecular dynamiclations when the atomic numbers and positions are given (all atomelling).

Visual Molecular Dynamics (VMD) software is a molecular visualizationram for display, animation and analysis of large biomolecular systems usingraphics and built-in scripting. In our projects, the coarse graining builder

ule in VMD has been employed to transform unit structures to CG beads.

Computational PlatformsOMACS (Groningen Machine for Chemical Simulation) is a molecularmics simulation package originally developed at University of Groningen,erlands. The CG-MD method can be implemented in the GROMACSage as the coordinates of the CG beads are available.

kmol packs molecules in defined space regions and creates a starting pointMD simulations. The packing makes sure that short range repulsiveactions do not disrupt the simulations.

MMPS (large scale atomic/molecular massively parallel simulator) can inion be applied to calculate the thermal properties, thermal behavior anderature distributions inside a porous anode.

Lattice Boltzmann method (LBM) can be used as a microscale model andbined with the programs Palabos with Python and MATLAB.

he macroscale modeling approaches using continuum formulation, the sin-

Nomenclatureall atomanode functional layeratomistic simulation environmentcathode functional layercomputational fluid dynamicscoarse grainingcatalytic layerdensity functional theorydissipative particle dynamicselectromotive force (open circuit

CS Groningen machine forsimulationsS large scale atomic/moleculary parallel simulator

lattice Boltzmann methodstrontium doped lanthanum

te

MC Monte CarloMD molecular dynamicsMSR methane steam reformingN number of particlesNiO nickeloxidePEMFC polymer electrolyte membrane fuelcellSEM scanning electron microscopySOFC solid oxide fuel cellSPH smoothed particle hydrodynamicsTEM transmission electron microscopyTPB triple phase boundaryWGSR water gas shift reactionVMD visual molecular dynamicsx mole fractionYSZ yttria stabilized zirconia