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    Synopsis

    This report thoroughly describes the functioning of a Proton-Electron-Membrane hydrogen

    fuel cell in the context of the departments HP600 Fuel cell system. Industrial equipment and

    instrument manufacturers face unprecedented demands from their customers for greater

    value in the products and services they provide. Ideally, all industrial equipment comes with

    manufacturers specifications, of which in our case, the power, current and voltage rating is

    pertinent in establishing whether or not the fuel cell is functioning properly. The performance

    of the hydrogen fuel cell against the manufacturers specification sheet was investigated by

    the means of mass balance on the hydrogen stream entering the system, an energy balance

    over the coolant recycle and by systematically inquiring into the rate law of the hydrogen fuel

    cell stack using the Butler-Volmer equation.

    Mass balance results show that the fuel cell is working properly, but under the yoke of design

    and technological constraints due to what happens at the three phase boundary.

    Since the minimum potential of the hydrogen fuel stack is constant, it was deduced that it is

    current that is directly related to the power output. The current is generated by the

    electrochemical reaction occurring at the active catalyst sites in the fuel cell. Therefore fuel

    cell performance is highly dependent on integrity of this catalyst layer. This report proposes

    that in order to ascertain the extent of compromise of active catalyst sites, current mass

    balance data must be compared to hydrogen consumption data as at factory setting dates.

    The power density curve revealed that the fuel cell was programmed to shut down before

    entering the high current density, mass transfer region of fuel cell operation. As a result,

    power output does not reach a maximum and the fuel cell was found to be performing at a

    reduced power rating of 374W. It is clear that the hydrogen fuel cell is not operating at

    optimal performance. The problem though is not so much a faulty hydrogen fuel cell, but

    rather internal design and technological constraints affecting the fuel cell.

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    Contents

    Synopsis ................................................................................................................................................ 1

    Contents ................................................................................................................................................2

    List of Figures ........................................................................................................................................ 4List of Tables.......................................................................................................................................... 4

    Nomenclature .......................................................................................................................................4

    Glossary ................................................................................................................................................5

    1. Introduction...................................................................................................................................... 6

    1.1 Subject of Report6

    1.2 Background to Report.6

    1.3 Objectives...6

    1.4 Scope and Limitations....7

    1.5 Plan of Development7

    2. Literature Review............................................................................................................................. 7

    2.1 Practical Background.....7

    2.2 Electrode kinetics.....7

    2.3 Reaction kinetics......8

    2.4 Polarization and power curves...................................................................................................... 10

    3. Experimental.......11

    3.1 Experimental Development ...11

    3.2 Apparatus......12

    3.3 Procedure .................................................................................................................................... 12

    3.4 Start-up of HP600 Fuel Cell system ..12

    3.5 External current variation and recording data for mass and energy balance...12

    3.6 Rate law data capturing.12

    3.7 Risk Assessment..12

    4. Results.....13

    4.1 Hydrogen mass balance.13

    4.2 Energy balance over cooling system.14

    4.3 Rate law: Polarisation and Power density curves...14

    4.4 Error analysis....16

    5. Discussion of results...17

    5.1 Mass balance analysis....17

    5.2 Energy balance analysis..17

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    5.3 Polarisation curves..17

    5.4 Power curve..18

    6. Conclusion19

    7. References...20

    8. Appendix.21

    8.1 Raw data21

    8.2 Sample calculations21

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    List of Figures

    Figure 1: Polarization and power curves illustration.

    Figure 2: Block flow diagram of PEM hydrogen fuel cell system.

    Figure 3: Bar graph comparing experimental and theoretical hydrogen consumption.Figure 4: Bar graph comparing experimental and theoretical heat quantity.

    Figure 5: Theoretical and experimental polarization curves.

    Figure 6: Theoretical and experimental polarization curves with power density plotted on

    secondary axis.

    List of Tables

    Table 1: Percentage difference between experimental and theoretical hydrogen consumption.

    Table 2: Percentage difference between experimental and theoretical heat quantity.

    Table 3: External and internal current settings comparison.

    Table 4: Standard deviation for theoretical and experimental data collected.

    Nomenclature

    = transfer coefficient

    COX = surface concentration of reacting species (mol/cm2)

    CRD = surface concentration of reacting species (mol/cm2

    )

    Er= reversible potential (V)

    (E- Er) = over potential (V)

    F = Faradays constant (C/mol)

    i = net current density (A/cm)

    i0 = exchange current density (A/cm)

    kf= forward reaction (reduction) rate coefficient (s-1)

    kb = backward reaction (oxidation) rate coefficient (s-1)

    nF = charge transferred (Coulombs/mol)

    n = number of electrons per molecule (H2 = 2 electrons)

    R= 8.314 J/mol.K

    T= temperature measured in Kelvins

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    Glossary

    Anode: Positive fuel cell electrode.

    Cathode: Negative fuel cell electrode.

    Current density: The quantity of current per unit area (1 fuel cell has an area of 130 cm2).

    Exchange current density: In electrochemical reactions it is analogous to the rate constant

    in chemical reactions, unlike rate constants, it is concentration dependent.

    Fuel cell stack: (In this case study) 24 fuel cells connected together in order to acquire an

    amplified amount of useful power.

    Over potential: The difference between the electrode potential and the reversible potential

    that is required to produce current.

    PEM: Proton Exchange Membrane, a fuel cell that depends on a semi permeable membraneto allow mass transport of protons through it.

    Power density: The quantity of power per unit area (Each fuel cell has an area of 130 cm 2).

    Polarisation Curve: Plot of cell voltage against current density. Used to illustrate how

    voltage losses affect the performance of a fuel cell or the deviation of fuel cell performance

    from ideality.

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    1. Introduction1.1. Subject of Report

    This report gives a comprehensive account on how a hydrogen fuel cell functions and

    through a thorough investigation by means of mass , energy and rate law findings; sets out

    to compare the specifications claimed by the manufacturer of the Chemical Engineering

    departments HP600 hydrogen fuel cell and empirical data from the experiment.Furthermore, the challenge posed is to delve into the hydrogen fuel cell stacks kinetics by

    Obtaining the rate law of the fuel cell through the derivation of the Butler- Vollmer equation

    1.2. Background to Report

    Hydrogen fuel cell technology is a more expensive source of energy than the mature oil and

    electricity commodities. The fuel cell economy is high-priced because it is a highly

    specialized industry that requires proficiency and technology at every step. Also, hydrogen

    is not readily available as a fuel; it must be obtained through means like steam reforming of

    natural gas or electrolysis. For the hydrogen fuel cell to penetrate the world energy market,

    it must start with technologies that have niche markets, like hydrogen powered vehicles.South Africa is the worlds leading producer of platinum group metals. The aforementioned

    metals are used as catalysts in the hydrogen fuel cell. In the long term, South Africa seems

    well positioned to dominate the hydrogen fuel cell economy which heavily utilises platinum

    metals. Atthis, the South African government has heavily invested in this green

    technology. It has embraced this technology by funding many research institutions

    (including HySA Catalysis at the University of Cape Town) to develop of a safe, clean and

    reliable energy source that will not only wean the world of its dependence on fossil fuels,

    but have the world depend on South Africa for its hydrogen fuel cells, catalysts or platinum.

    By this, South Africa will be able to transform itself through job creation, increasing GrossDomestic Product and knowledge based economy through research and development. This

    report has been prepared for the Department of Environmental affairs as proof of on-goingwork in green technology.

    1.3. ObjectivesIn this report we endeavour to:

    Display an understanding of how the hydrogen fuel cell works by means of a blockflow diagram.

    Determine whether the fuel cell is operating correctly using fundamental concepts inchemical engineering like mass and energy balances and obtaining the rate law

    through the derivation of the Butler-Vollmer equation.

    Prove that the fuel cell stack is operating according to manufacturer specificationsVis--vis the aforementioned mass and energy balances and rate law findings

    obtained empirically.

    The manufacturers specifications state that the fuel cell is rated as follows:

    Power rating: 600W; Voltage rating: 14.4V; Current Rating: 45A.

    As for Fuel cell kinetics, the Butler-Volmer equation is used as a model to discern the

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    relationship between current density and cell potential.

    1.4. Scope and Limitations

    There was ample time to ascertain the functionality of the HP600 hydrogen fuel cell system

    as at the date the practical was performed. However it would have been interesting tocompare the empirical data collected on the day of the practical with the empirical data

    from thefirst time the fuel cell was operatedin our departments laboratory. This would give

    us an insight into other factors that might be affecting the fuel cell like the degradation of

    the carbon support (electrodes) and perhaps the extent of competing reactions like the

    oxidation of platinum metal. Such a comparison would not only exonerate the manufacturerbut could also enlighten us on how we could account for depreciation of equipment. This is

    important because depreciation is a tax allowed deduction that not only lower income tax

    but provides for the recovery of capital that has been invested in physical property or

    assets. This might not be of interest to the faculty because it is sponsored by the

    government and mining companies but to a small business owner it would really make a

    difference.

    1.5. Plan of Development

    This report begins with an abbreviated account on how a Proton-Exchange-Membrane fuel

    cell operates. The theory and detailed account of how the fuel cell works is substantiated by

    peer reviewed journals and other credible sources of literature. A brief account of the

    experimental procedure and equipment used comes after. Then, in the context of an HP600fuel cell system, the performance of the fuel cell against manufacturers specifications is

    determined using mass balances, energy balances and rate law findings over the fuel cell

    stack. The linearization of the Butler-Volmer equation will then be tackled step by step, soas to understand the chemical kinetics within the fuel cell. As for the random error in the

    measurement the appropriate statistical tools will be used to account for it, only then canconclusions and recommendations be made about the reliability of the manufacturers

    specifications and appropriateness of the Butler-Volmer equation as a model for fuel cell

    kinetics.

    2. Literature Review2.1. Practical Background

    Miansari et al (2009: 356) state that: the Proton Exchange Membrane fuel cell is an

    electrochemical energy conversion device, which converts the energy conversion of hydrogen

    and oxygen directly and efficiently into electrical energy, with waste heat and liquid water as

    by-products. The rudimentary structure of the PEM fuel cell can be described as two

    platinum coated electrodes which are the anode and cathode of the fuel cell. The electrodesare separated by a solid membrane acting as the electrolyte (Al-Baghdadi, 2005: 1587).In the

    context of the HP600 fuel cell system, hydrogen and oxygen enter the fuel cell stack.

    Hydrogen flows through a system of channels to the positive electrode- the anode, whilst the

    oxygen in air diffuses to the negative electrode-the cathode. At the anode the hydrogen

    dissociates into protons and electrons. The electrons flow through an external circuit to the

    cathode, whereas the protons then flow through the solid membrane to the cathode, where

    they combine with the oxygen and electrons from the external circuit to form water. The

    excess water and inerts are expelled from the system via the purge stream.

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    2.2. Electrode Kinetics

    Gases are involved in fuel cell electro-chemical reactions. Therefore the electrodes are porous,

    and this allows gases to arrive to the reaction sites, as well as products to leave the reaction

    sites.

    Anode reaction (Oxidation): (Equation 1)Cathode reaction (Reduction):

    (Equation 2)Overall reaction: . (Equation 3)The above reactions occur concurrently on an interface between the solid, ionically conductive

    electrolyte and the electrically conductive platinum on carbon electrodes. In order for the reaction to

    occur, a catalyst is needed to reduce the height of the activation barrier. Therefore the hydrogen and

    the oxygen molecules must diffuse through the pores of the metal catalyst and adsorb. In order for the

    reaction to occur the protons, platinum catalyst and carbon (from the electrode) need to be in contact

    at the same time. This is referred to in literature as the Three phase boundary problem.

    2.3. Reaction Kinetics

    The Butler-Vollmer equation describes the current density at an electrode in terms of over potential.

    By linearizing this equation we want to determine the unknown activity coefficient and the exchange

    current density. The main idea behind the derivation is that the net current generated is the difference

    between electrons released at the anode and electrons consumed at the cathode. Furthermore, the

    mathematics is simplified by the fact that the over potential at the cathode is greater than that at the

    anode. Consequently the cathode reaction is regarded as the rate determining step.

    The aforementioned outline on the linearization of the Butler-Volmer equation is

    mathematically expressed as follows:

    According to (Barbir, 2005: 33), as previously mentioned, the net current generated is the

    difference between the electrons released and consumed:

    f OX b RDi nF k C k C (Equation 5)

    Net current density is determined as follows:

    0, 0,exp expOXRD

    f OX b RD

    FEFEi nF k C k C

    RT RT

    (Equation 6)

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    Taking into account that at equilibrium, the potential is Er, and net current is equal to zero:

    0 0, 0,exp expOXRD

    f OX b RD

    FEFEi nFk C nFk C

    RT RT

    (Equation 7)

    Exchange current density is the rate at which these reactions proceed at equilibrium.

    Dividing i/io:

    (Equation 8)

    This yields equation 9:

    i/i0 =exp [(-RDF (E-ER) / RT]exp [(oxF(E-ER)/RT] (Equation 9)

    And finally the relationship between current density and potential:

    0

    ( )( )exp exp OX rRD r

    F E EF E Ei i

    RT RT

    (Equation 10)

    Equation 10 is known as the Butler-Vollmer equation that relates current density and

    potential. The ER term is known as the reversible or equilibrium potential. According to

    literature the reversible or equilibrium potential at standard temperature and pressure is 0V at

    the anode and 1.229V at the cathode and this does not vary with temperature and pressure.

    As aforementioned at the beginning of this section, the over potential on the cathode side isgreater than the anode over potential. Therefore the cell potential current is given by

    (approximately):

    , ,

    0,

    ( )exp

    RD c c r c

    c c

    F E Ei i

    RT

    (Equation 11)

    Taking Equation 11 and linearizing it through the following steps:

    ()

    (Equation 12) ( )(Equation 13)

    ( ) ()(Equation 14)

    Thus the linearized Butler-Volmer equation is the means by which the activity coefficient

    (RD)and the exchange current density (io) can be determined:

    i/i0 =exp[(-RDFE/RT)/(-RDFER)/RT]-exp[([(-oxFE/RT)/( -oxFER/RT)]

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    0( ) ln( ) ln( )

    r

    RD RD

    RT RTE E i i

    F F

    (Equation 15)

    2.4.Polarisation and Power Curves

    The hydrogen fuel cells cell potential is limited by energy losses due to the kinetics. These loses areoften illustrated in polarization curves. A polarization curve is a plot of cell voltage against the cellcurrent density (Figure 1). Current density represents the pace at which the reaction is taking place.At low current densities, in the activation potential region, voltage losses are caused byelectrochemical drive to overcome the activation energy that leads to the dissociation of the oxygenand hydrogen molecules.From Srinivasan et al (1993), it is understood that the hydrogen and oxygenmolecules must diffuse through the pores of the platinum catalyst and adsorb.In literature this istermed the three phase boundary problem, where hydrogen, platinum metal catalyst and carbon mustall be in contact. The role of the platinum catalyst is to reduce the activation energy barrier.

    As current density increases, there is a gradual and continuous drop in cell potential. This is due to

    what is termed as Ohmic losses in the conduction of ions through the electrolyte membrane.

    At high current densities, mass transfer processes become the limiting factor as the supply of

    hydrogen and oxygen to electrodes becomes sluggish. The HP600 fuel cell system used in the

    experiment was programmed to shut down before entering this region of high current densities.

    Figure 1 also shows power density as afunction of current density. The power curve peaks at high

    current density, which is the mass transfer region as mentioned earlier.

    Figure 1: Polarisation curves at different temperatures (from Cell Voltage axes sloping downwards) and Power curves at

    different temperatures (from origin sloping upwards),Song et al (2007: 2552-2561).

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    3. Experimental

    3.1. Experimental Development

    Start-up of the HP600 fuel cell system entails switching on the fuel cell and opening the gas tap from

    the hydrogen container. Turning the fuel cell on, initiates the blowing of air (oxygen source) into thefuel stack. The fuel cells relative humidity and set-point temperature are then set. This particular

    fuel cell is automated, and so it runs output data like temperature, pressure, hydrogen flow rate,

    current and voltage onto a computer. This data is usually represented graphically. Steady state is

    reached when the graphs depict constant temperature, hydrogen flow rate and current. Different

    external current settings are selected and the fuel cell can be made to run recording data on an Excel

    spread sheet.

    3.2. Apparatus

    Hydrogen Fuel Cell

    Computer System

    Electronic Load

    Purge

    Coolant

    System

    Air (Oxygen source)

    Hydrogen

    Figure 2: Block flow Diagram of the PEM Hydrogen Fuel Cell System in the context of the HP600 System

    3.3. Procedure

    The first part of this practical entails adjusting the external current settings over the widest

    range permissible, in order to run recording data onto a spread sheet. The data is then used to

    perform mass and energy balances over the hydrogen fuel stack. The second part to this

    experiment involves the increasing of current from 0A, incrementally, and continuously after

    a specified amount of time. The data gleaned from the latter exercise will be necessary to

    model the rate law of the electro-chemical reaction within the fuel cell stack.

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    3.4.Start-up of HP600 Fuel cell System

    Turn on HP600 Fuel cell system at the mains (air blower starts). Open the hydrogen tap to allow flow from pressurized tank to fuel cell Adjust set point temperature to 30C and relative humidity to 80% Once Temperature, hydrogen flow rate and current are constant, record data onto the

    Excel spread sheet.

    3.5.External Current Variation and Recording Data for Mass and Energy Balances

    The HP600 fuel cell was fine-tuned by the department to shut down if it voltage goes below

    14.4V.This was done as a precaution or prudent measure to ensure that the operator of the

    fuel cell does not go below the critical voltage specified by the manufacturer: 13.3V. Based

    on this information, the practical was carried out with the 14.4V threshold in mind. The

    external current output on the electronic load was varied in steps of 5A, 10A, 15A, 20A and

    25A (internally this was 7.6A, 13.07A, 18.44A, 23.91A and 28.99A). The fine dial was used

    for currents up to 6A and above this the course dial played its part. Each interval was allowed

    to run for 2 minutes. The data was captured onto a spread sheet on a per second basis.

    3.6.Rate Law Data Capturing

    The external current was set to 0A, and then the system was allowed to reach steady state.

    Data was recorded with external current settings changing every 40 seconds in increments of

    0.3, 0.6, 1.0, 5.0 between 0A and 3A, 3A and 6A, 6A and 10A, 10A and 25A respectively.

    3.7.Health and Safety using Anglo Risk Matrix rating

    Risk of electric Shock: 8 (M). Purge stream produces enough water to cause some someone to slip and fall:

    5 (L).

    Hydrogen gas leak caused by faulty connections and piping: 12 (M).

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    4. Results4.1. Hydrogen Mass Balance

    Figure3: Experimental hydrogen consumption within the fuel cell compared to theoretical hydrogen consumption.

    Average Stack Current (A) % Difference between Experimental & Theoretical H2 Consumption

    7.66 7.1

    13.34 4.6

    18.68 5.9

    23.92 5.2

    28.93 4.5Table 1: Percentage difference between experimental and theoretical hydrogen consumption data.

    0.00

    1.00

    2.00

    3.00

    4.00

    5.00

    6.00

    7.7 13.3 18.7 23.9 28.9

    H

    ydrogenConsumption(L/Min)

    Average Stack Current (A)

    Hydrogen Mass Balance

    Experimental Hydrogen

    Consumption (L/Min)

    Theoretical Hydrogen

    Consumption (L/min)

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    4.2. Energy balance

    Figure 4: Experimental average heat quantity compared to the theoretical average heat quantity.

    Average Stack Current (A) % Difference between Experimental & Theoretical Heat Quantity

    7.7 8.6

    13.3 2.1

    18.7 1.4

    23.9 0.8

    28.9 0.7

    Table 2: Percentage difference between the experimental heat quantity and theoretical heat quantity.

    4.3. Rate law: Polarisation and Power density curves

    Given the transfer coefficient from literature as:

    Rd = 1

    And exchange current density as:

    = 3.70E-7 A/cm2The theoretical cell voltage was calculated using (Equation 14):

    ( ) ()The experimental cell voltage was calculated by dividing the average stack voltage by the number of

    fuel cell stacks, that is 24. The current density in the following graphs represents thespeed of the

    reaction. It is the number of electrons per second, divided by the facial surface area of the

    solid hydrogen fuel cell electrolyte.

    0

    100

    200

    300

    400

    500

    600

    7.7 13.3 18.7 23.9 28.9

    AverageHeat[W]

    Average Stack Current (A)

    Energy Balance over Cooling System

    Experimental Average Heat

    [W]

    Theoretical Average Heat

    [W]

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    Figure 5: Graph comparing theoretical and experimental polarisation curves.

    Current density values in Figure 5 do not start at 0A. Table 3 give us an insight into the reasons why

    this is the case:

    External Current Setting (A) Average Internal Stack Current (A) Experimental Cell Voltage [V] Theoretical Cell Voltage [V]

    0.00 2.23 0.78 0.95Table 3: External current settings and internal current settings.

    Notice that Figure 5 plots cell voltage against a current density which was computed from internal

    stack current data. This is the reason why the graph does not start at the expected zero current density.

    Table 3 shows that when the external current is set a 0A, the fuel cell has its own internal current

    reading since the system is switched on. Figure 5 and Table 3 point to a later discussion on open

    circuit potential.

    Figure 6: Polarisation curves with Power density plotted on secondary axis

    5.00E-01

    6.00E-01

    7.00E-01

    8.00E-01

    9.00E-01

    1.00E+00

    0.0200 0.0600 0.1000 0.1400 0.1800 0.2200

    CellVoltage(V)

    Current Density (A/cm2)

    Comparing Theoretical and Experimental Polarisation Curves

    Theoretical

    PolarisationCurveExperimental

    PolarisationCurve

    0.00

    0.50

    1.00

    1.50

    2.00

    2.50

    3.00

    3.50

    1.00E-01

    1.10E+00

    2.10E+00

    3.10E+00

    0.0190 0.0690 0.1190 0.1690 0.2190 0.2690

    PowerDensity(W/cm2)

    CellVoltage(V)

    Current Density (A/cm2)

    Power Density Curve and Polarisation Curves

    Theoretical Polarisation Curve Experimental Polarisation Curve Power Curve

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    The power generated by one fuel cell is calculated as follows:

    P = IV (Equation 16)

    4.4.Error Analysis

    Literature states that absolute steady state can never be attained because of what happens at the three

    phase boundary. The three phase boundary is responsible for random error in experimental data, and

    this is accounted for using error barsSrinivasan et al (1999). At low current density, what happens at

    the three phase boundary is responsible for the initial sharp decline of the slope of the polarisation

    curves. Close to zero current density the polarization curves depicted in Figure 5 illustrate this.

    The standard deviation for the experimental and theoretical hydrogen consumption curve is very small

    due to the fact all data recorded was done by the sensors connected to the computer system. Therefore

    the error associated with this is random and deviation from the mean is very small. Standard deviation

    values for theoretical heat quantity data are conspicuously larger than the rest (Table 4). This is

    because of the propagation of error due to different instrumentation used to measure flow-rate ofwater in the cooling system, inlet and outlet temperatures.

    Table 4: Standard deviation with respect to theoretical and experimental hydrogen consumption and heat quantity

    Average Stack Current (A) Experimental H2 Consumption (L/min) Standard Deviation

    7.66 1.20 0.78

    13.34 2.13 0.09

    18.68 2.95 0.74

    23.92 3.81 0.73

    28.93 4.64 0.66

    Average Stack Current (A) Theoretical H2 Consumption (L/min) Standard Deviation

    7.66 1.28 0.04

    13.3 2.23 0.04

    18.7 3.13 0.02

    23.9 4.01 0.01

    28.9 4.84 0.01

    Average Stack Current (A) Experimental Average Heat (W) Standard Deviation

    7.66 40 0.77

    13.3 193 2.3

    18.7 274 2.4

    23.9 366 2.3

    28.9 453 1.7

    Current Output (A) Theoretical Average Heat (W) Standard Deviation

    7.66 43 8.3

    13.3 197 7.9

    18.7 278 7.4

    23.9 369 7.0

    28.9 456 7.3

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    5. Discussion of Results

    5.1. Mass balance analysis

    The mass balance performed over the fuel stack shows a small discrepancy between experimental and

    calculated hydrogen consumption data, of which calculated values are greater than empirical values.

    This difference is at least 4.5% at high average stack current and at most 7.1% at low average stack

    current. One of the advantages of a PEM hydrogen fuel cell is a quick start up. The data in Table 1

    suggests thatgiven enough time the difference between experimental and calculated hydrogen

    consumption decreases. This trend leads to an interesting question: how quickly does the fuel cell

    respond to abrupt changes in external current settings?

    Figure 3 illustrates that one hundred per cent conversion of hydrogen was a huge assumption.

    According to (Barbir, 2005), this is a three phase boundary problem, since protons, platinum

    catalyst metal and carbon all have to be in contact for a reaction to occur. This notion is also highly

    supported by Zhan et al (2007) who emphasize the importance of the internal mechanisms of the fuel

    stack with regard to performance.

    It is clear that the performance of the fuel cell is highly affected by the delivery of hydrogen to the

    stack via controls such as the flow meter. However, instead of focusing on the instrument itself (flow

    meter) and any of its intrinsic errors, it is more engaging to think about the effect of increasing the

    partial pressure of hydrogen and oxygen on fuel cell performance. Song et al (2007: 2552-2561) show

    in their study that the higher the pressure the higher the performance of the fuel cell.

    Another reason for the difference between theoretical and experimental hydrogen consumption are

    competing reactions that occur at the oxygen electrode: oxidation of the platinum, corrosion of

    carbon support, and oxidation of organic impurities on the electrode Srinivasan et al (1993: 39).

    5.2.Energy balance analysis

    Figure 4 shows that theoretical heat quantity is slightly greater than experimental heat quantity. The

    graph also depicts an almost dismissible difference between the theoretical and experimental heat

    quantity values as stack current increases. Table 2 illustrates the extent and rapidity at which

    experimental heat quantity approaches theoretical heat quantity. The difference between the compared

    quantities drops from 8.6% to a very low 0.7% as average stack current increases. According to a

    formidable case study by Song et al (2007: 2552-2561), the fuel cell performance in low current

    density increases with increasing temperature from 23C to 80C.

    5.3. Polarization Curves

    Figure 5 shows that within low current density range, the Butler-Volmer equation models the

    electro-chemical kinetics of the hydrogen fuel cell appreciably. Comparing the shape of the

    two polarisation curves plotted in Figure 5, with the polarisation curve at room temperature in

    Figure 1, shows that the trend is correct.

    The theoretical polarisation curve intersects the cell potential axes at 0.95V, which is the

    potential per cell at a zero current density reading. The empirical polarisation curve intersects

    the cell potential axes at 0.78V. The theoretical zero current reading per stack is 22.8V,

    whilst the experimental zero current reading per stack is 18.7V.

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    The potential per cell at zero current density is called the open circuit potential. In practice

    the experimental open cell potential is significantly lower than the theoretical potential

    because there are some voltage losses in the fuel cell even when no external current is

    generated. Under closed circuit conditions with a load, the potential is expected to drop even

    further as a function of current being generated due to voltage losses caused by activation andOhmic over potentials (Fuel cell basics).This is the reason why the empirical data curve is

    below the theoretical data curve.

    The experiment was conducted in the range of low to medium current density, so the

    degradation of the three dimensional phase boundary and the oxidation of some platinum

    catalyst sites may be the reason for the voltage losses. These problems at the reaction sites

    lead to higher activation energies for the reactions within the fuel cell (Sepa et al, 1984:

    1169).

    5.4. Power Curve

    In the HP600 fuel cell system, 24 fuel cells are stacked to multiply the power.

    The minimum potential for the HP600s hydrogen fuel cell stack is constant; therefore it is current

    that directly affects the power output. The power curve peaks at high current density. The peak is not

    depicted in the graph above because the departments fuel cell is programmed to shut down before the

    mass transfer region is entered. Shut down occurs below 0.6V per fuel cell, which means the stack is

    operating at 14.4V at 26A. Figure 1 shows that the fuel cell, unrestrained, has a power density curve

    that peaks at high current density values.

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    6. Conclusion

    The results from the mass balance reveal that the hydrogen fuel cell does not have a hundred per cent

    conversion of hydrogen into protons and electrons. This is not only the case for old fuel cell

    equipment but this also applies to newly manufactured fuel cells. As aforementioned, literature states

    that absolute steady state can never be reached as a consequence of sluggish electrode kinetics or thethree phase boundary problem. Therefore, in order to measure the performance of the fuel cell by

    using a mass balance we need to compare the current mass balance data with hydrogen consumption

    data as at factory settings. Only then will we be able to ascertain to what extent the electrode and

    catalyst sites have deteriorated. It is not a plausible argument to make the conclusion that the fuel cell

    is not operating properly purely on the basis that there is a small difference (less than 8%, see Table 1)

    between theoretical and experimental hydrogen consumption. Therefore, instead of saying that the

    mass balance reveals that the hydrogen fuel cell is not working according to manufacturers

    specifications, it is more prudent to make the assertion that the fuel cell is working properly but

    subject to constraints that are a function of the intrinsic property and state of the internal fuel cell

    equipment.

    The energy balance shows empirical data approaching theoretical data as time increases (see Table 2).

    The fuel cell set-point temperature at the beginning of the experiment was set to 30C, however, the

    computer showed that the fuel cell temperature was slowly and gradually increasing with time. By the

    end of the experiment the temperature was above 35C. The increase in temperature was due to

    intrinsic and extrinsic factors affecting the fuel cell. From the time of fuel cell start-up, as operation

    time increases, the fuel cell stack starts to heat up due to the co-generation of current and heat.

    Internal resistances increase as current density increases; these Ohmic resistances also generate heat.

    Extrinsically, an increase in the temperature of the surroundings could also take its toll, but to a very

    small extent. As earlier mentioned in the discussion, increasing temperature improves fuel cell

    performance. This is the reason why the experimental data approached theoretical data as close as

    0.7% at the highest current density (see Table 2). The water-cooling system was designed to keep the

    fuel cell set-point temperature constant. The cooling system may be faulty or failing to keep up. The

    former, being highly improbable since a faulty cooling system would lead to run-away temperatures.

    A refrigerant based cooling system can be recommended for the latter case.

    The Butler-Volmer equation models the electro-chemical processes within the fuel well. Polarisation

    curves plotted (Figure 5) are similar to those found in literature (Figure 1) for low to medium current

    density values. As predicted by theory, the empirical curve is below the theoretical curve because of

    voltage losses in the fuel cell even when no current is running and furthermore, losses due to

    activation and Ohmic over potentials as current density increases. A good look at the chemistry showsthat these loses is unavoidable. There are irreversible losses in energy conversion due to entropy

    generation, mass transfer of reactants into catalytic pores and internal resistances.

    The power density curve in Figure 6 is expected to have a maximum but because the fuel cell is

    programmed to shut down below 0.6V (per cell); the curve is truncated as it approaches its peak. The

    power output of the HP600 hydrogen fuel cell is therefore 374V instead of 600V.

    It is clear that the hydrogen fuel cell is not operating at optimal performance. The problem though is

    not so much a faulty hydrogen fuel cell, but rather internal design and technological constraints

    affecting the fuel cell.

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    7. References

    1. Barbir, F., 2005,PEM Fuel Cells, 1st ed. Elsevier Academic Press, San Diego, pp. 17-25, pp. 33-38.

    2. Miansari, Me., Sedighi, K., Amidpour, M., Alizadeh, E., & Miansari, Mo., 2009,Experimental and thermodynamic approach on proton exchange membrane fuel cell

    performance, Journal of Power Sources, 190(2), pp. 356-361.

    3. Song, C., Tang, Y., Lu, J., Zhang, J., Wang, H., and Shen, J., 2007, PEM fuel cellreaction kinetics in the temperature range of 23120 oC, High Temperature, 52, pp.

    2552-2561.

    4. Srinivasan, S., Mosdale, R., Stevens, P., Yang, C., 1999, Fuel Cells: Reaching the Eraof Clean and Efficient Power Generation in the Twenty-First Century, Annual Review

    of Energy and the Environment, 24, pp. 227-281.

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    8. Appendix

    8.1.Raw Data

    The raw data in this experiment is too bulky to summarize, as a result, electronic copies for group B2

    are available on request.

    8.2.Sample Calculations

    Table from which data for Figure 3, Figure 4, Figure 5, and Figure 6 is derived.

    AverageStack Current(A)

    AverageStackVoltage (V)

    CurrentDensity(A/cm^2)

    E-Er(PerCell) V

    ExperimentalCell Voltage[V]

    TheoreticalCell Voltage[V]

    Power[W]

    2.23 18.76 0.0172 0.447 0.78 9.49E-01 0.32

    2.27 18.70 0.0174 0.450 0.78 9.49E-01 0.332.57 18.56 0.0198 0.456 0.77 9.46E-01 0.37

    3.36 18.19 0.0258 0.471 0.76 9.39E-01 0.47

    3.68 18.08 0.0283 0.476 0.75 9.36E-01 0.51

    4.01 17.95 0.0309 0.481 0.75 9.34E-01 0.55

    4.43 17.80 0.0341 0.487 0.74 9.31E-01 0.61

    4.56 17.75 0.0350 0.489 0.74 9.31E-01 0.62

    4.84 17.65 0.0373 0.494 0.74 9.29E-01 0.66

    5.30 17.51 0.0407 0.499 0.73 9.27E-01 0.71

    5.91 17.33 0.0455 0.507 0.72 9.24E-01 0.79

    6.66 17.12 0.0513 0.516 0.71 9.21E-01 0.88

    7.41 16.93 0.0570 0.523 0.71 9.18E-01 0.97

    7.95 16.80 0.0611 0.529 0.70 9.16E-01 1.03

    8.45 16.69 0.0650 0.533 0.70 9.15E-01 1.08

    9.66 16.43 0.0743 0.544 0.68 9.11E-01 1.22

    10.69 16.23 0.0822 0.553 0.68 9.08E-01 1.33

    11.99 15.97 0.0922 0.564 0.67 9.05E-01 1.47

    13.16 15.78 0.1012 0.572 0.66 9.03E-01 1.60

    18.10 14.97 0.1393 0.605 0.62 8.95E-01 2.08

    23.09 14.29 0.1776 0.634 0.60 8.88E-01 2.5427.91 13.73 0.2147 0.657 0.57 8.83E-01 2.95

    Excel computations were as follows:

    Current density = Average stack current/ 130cm2 ER-E= 1.229-(Average Stack Voltage/24 cells) Experimental cell voltage (V) = Average stack Voltage/24 cells Theoretical cell voltage: Ecell = ER[((RT)/f)*LN(i/io)] where ER=1.23V P= (Average stack current * Average stack voltage)/130cm2

    The table below contains extra information used in the aforementioned excel computations:

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    Below is an abbreviated table showing calculation of standard deviation values for theoretical heat

    values etc.:

    Standard deviation for 5A current output, for example:

    Table used in the calculation for experimental and theoretical hydrogen consumption:

    Sample calculations for theoretical hydrogen consumption:

    Theoretical hydrogen consumption= (Average stack current/Avogadros number/Electron Charge/2)*22.4*24*60.

    Average Stack Current (A) Experimental H2 Consumption (L/min) Standard Deviation Theoretical H2 Consumption (L/min) Standard Deviation

    7.66 1.20 0.78 1.28 0.04

    13.34 2.13 0.09 2.23 0.04

    18.68 2.95 0.74 3.13 0.02

    23.92 3.81 0.73 4.01 0.01

    28.93 4.64 0.66 4.84 0.01

    Extra information

    Avogadro's Number 6.02E+23

    Electron Charge 1.60E-19

    STP gas 22.4

    Extra Information Value units

    R 8.314 j/mol.k

    T 303.15 kF 96485.34 s.A/mol

    Io,c 3.70E-07 A/cm2

    1.00 dimensionless

    Max Theoretical Voltage 1.23 V

    5Amperes Theoretical Heat Deviation Deviation43.65 0.27 0.07

    51.27 7.89 62.23

    30.34 -13.04 170.01

    36.38 -7.00 49.07

    51.96 8.57 73.48

    44.83 1.44 2.08

    58.70 15.32 234.64

    38.42 -4.96 24.61

    Average 43.38

    sum 8266.32N-1 119.00

    SD 8.33

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    Energy Balance calculations:

    Theoretical average heat = [Cold water flow rate (l/min) / 60]*4185.5* [Tout - Tin]

    Current Output (A) Average Heat Experimental (W) Standard Deviation Average Heat Theoretical (W) Standard Deviation

    7.66 40 0.77 43 8.3

    13.34 193 2.3 197 7.9

    18.68 274 2.4 278 7.4

    23.92 366 2.3 369 7.0

    28.93 453 1.7 456 7.3