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METHODS OF MEASUREMENTS IN ELECTROCHEMICAL ENGINEERING Dr. Manuel A. Rodrigo Department of Chemical Engineering. Facultad de Ciencias Químicas. Universidad de Castilla La Mancha. Campus Universitario s/n Ciudad Real. Spain. Department of Chemical Engineering. Universidad de Castilla La Mancha. Spain ESSEE 4 4th European Summer School on Electrochemical Engineering Palić, Serbia and Montenegro 17 – 22 September, 2006

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CONTENTS 1.CURRENT DISTRIBUTION 1.1 Importance of current distribution visualization 1.2 Measurement of current distribution TYPES OF MEASURING METHODS PARTIAL-CELL APPROACH SUBCELLS APPROACH SEGMENTED ELECTRODES RESISTORS NETWORK PRINTED CIRCUIT BOARD APPROACH TYPES OF MEASUREMENTS OF THE LOCAL CURRENT IN PASSIVE RESISTOR NETWORK MATHEMATICAL MODELLING MAGNETOTOMOGRAPHY 1.3. Some new applications: calculation of mass diffusion overpotential distribution in a PEMFC 2.MEASUREMENT OF MASS TRANSFER COEFFICIENTS BY ELECTROCHEMICAL TECHNIQUES 2.1 Why? 2.2 How? 2.3 Typical setup for measuring average cell mass transfer coefficients 2.4 Experimental procedure 2.5 Calculation of the mass transfer coefficient 3. LOCAL MASS TRANSFER DISTRIBUTION 3.1 Importance of local mass-transfer distribution visualization 3.2 Limit current mapping 3.3 Measurement of mass transfer by electrochemiluminiscence 3.4 Mathematical modelling 4.WALL SHEAR STRESS 4.1Importance of wall-shear stress distribution visualization 4.2 Measurements of wall-shear stress 4.3 Measurement of local shear in three-phase fluidized beds 4.4 Wall shear stress in multiphase flow

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1. CURRENT DISTRIBUTION It is one of the more important parameters in the performance of an electrochemical cell, but unfortunately in the electrochemical industry and in the electrochemical literature, current distribution has not received the attention that it deserves Uniform current distribution Non-uniform current distribution Through the wire flows the same current, but the current distribution on the electrode surface is different I I 1.1 Importance of current distribution visualization

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Electroplating: non-uniform current distribution can cause a local variation of the thickness of the deposited metal Some examples of the importance of uniform current distribution

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Electrolyses cell Aluminium surface after an electro-dissolution process Small contact- surface current feeder Current efficiency 100% Current efficiency 83.3% i 1/3i2/3 i Part of these electrons are consumed by an electrochemical side reaction because the desired reactant does not arrive to the anode surface at the required rate (if reagents arrives to the electrode at the same rate that they are consumed) non uniform corrosion of electrodes Poor efficiency, changes in the products conversion ratio

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Causes of non-uniform current distribution in FC during fuel cell operation: inhomogeneities in the reactant concentration, contact pressure, temperature distribution, water management along the flow field etc. Produce maximum power densities Ensure maximum lifetime for the cell components PEM fuel cell Uniform current distribution

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Current scale Examples of local current distribution in a circular-shape electrode low high uniform

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Factors affecting current distribution: Geometry of the cell system. Current feeders or collectors Conductivity of the electrolytes and the electrodes. Activation overpotentials at the electrodes which depend on the electrode kinetic. Concentration overpotentials which are mainly controlled by the mass transport processes. Other factors

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1.2 Measurement of current distribution electrodes electrolyte Load or power supply b) Current distribution in one electrode a) Current distribution in the cell anode cathode Purpose of the measurement

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Invasive methods Partial approaches Subcells Segmented electrodes Passive resistor network Non-invasive methods Mathematical modelling Magnetic measurements Is the cell modified for the measurement? (is current distribution measurement associated with constructional modifications of the cell? ) no yes TYPES OF MEASURING METHODS

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Portions or segments of the cell are tested independently by inactivating other portions PARTIAL-CELL APPROACH. The inactivation can be carried out either by masking or by other procedure (e.g. in FC some parts of the MEA can be prepared without catalyst electrode electrolyte electrode

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the specific performance is determined by difference. subcell 3 inactive subcell 1 inactive Subcell 1Subcell 2Subcell 3 To increase the accuracy more partial cells should be studied Advantages: very simple, easy to manufacture Disadvantages: it can only be used as a first approach CELL VOLTAGE INTENSITY whole cell Subcell 1 inactive Subcell 3 inactive

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Several electrically isolated subcells are placed are conveniently placed at different locations in the cell SUBCELLS APROACH a section of the anode is punched out a section of the cathode is punched out

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Main cell subcells The step is repeated in several determined locations inside the cell The former anodes and cathodes are replaced with smaller ones. The resulting empty space is filled with a isolating gap

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L main cell L1L1 LnLn LmLm Subcell 1Subcell n Subcell m Main cell The subcells are separately controlled. To measure current distribution every subcell voltage has to be adjusted to fit approximately the mail cell voltage Advantages Gives more information on a much smaller scale about the localised current density than the partial approach Disadvantages Complex manufacture. Great care has to be taken to ensure proper alignment during assembly of the cell CELL VOLTAGE INTENSITY SUBCELL 5 MAIN CELL SUBCELL 3

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electrolyte Segmented electrode or segmented BPP (in a FC) Measurement circuits isolation SEGMENTED ELECTRODES This approach allows a very accurate current distribution mapping Coverage of the whole electrode area Good spatial resolution Counter electrode

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Piece of electrode Volt-meter ohmic resistor To assume a high ratio between through-plane and in-plane conductivity segmented electrodes must be manufactured in a thin shape. This generates problems related to mechanical strength Example of measuring device for each piece of electrode Very invasive method. It can affect significantly to the current distribution. Big differences can exist between the measure and the actual current distribution

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Passive resistor network current Volt-meter Buss plate electrode Coverage of the whole electrode area Good spatial resolution Drawbacks Electrical properties of the resistors depends on temperature RESISTORS NETWORK Main advantage: It does not require any modification of the electrodes (or of the BPP or MEA in FC) It is less invasive Main problem - appearing of lateral currents

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Buss plate electrode Advantages Improved mechanical strength Resistor matrix Isolated wires To assume a high ratio between through-plane and in-plane conductivity resistor matrix must be manufacture in a thin shape. This generates problems related to mechanical strength Completely isolated resistors

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Buss plate electrode Resistor matrix Isolated wires Advantages Less affected by in-plane current distribution interconnected resistors

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1.2.6 PRINTED CIRCUIT BOARDS APPROACH Current collector Through-holes backside frontside current Easy to manufacture Possibility of multilayer manufacture Easy to add electrical components Can be used as BPP in FC

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1.2.7 TYPES OF MEASUREMENTS OF THE LOCAL CURRENT IN PASSIVE RESISTOR NETWORK passively Ohmic resistors Hall-effect sensors Current transformers actively Multichannel potentiostats (only measure) (Measure and manipulation)

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Ohmic resistors current Volt-meter Very simple Frequently used Very invasive. It can affect the cell current distribution

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The figure shows a thin sheet of semiconducting material (Hall element) through which a current is passed. The output connections are perpendicular to the direction of current. When no magnetic field is present, current distribution is uniform and no potential difference is seen across the output. When a perpendicular magnetic field is present, a Lorentz force is exerted on the current. This force disturbs the current distribution, resulting in a potential difference (voltage) across the output. This voltage is the Hall voltage (VH). Its value is directly related to the magnetic field (B) and the current (I). Hall-effect sensors Hall effect sensors can be applied in many types of sensing devices. If the quantity (parameter) to be sensed incorporates or can incorporate a magnetic field, a Hall sensor will perform the task When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the current and the field. This principle is known as the Hall effect.

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Current follower circuit To working electrode To data acquisition card Standard operational amplifier circuit for current- to-voltage conversion For very low currents

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Current distribution model simulation experiments Experimental conditions Modelled results experimental results yes no Agreement? e.g. product conversion MATHEMATICAL MODELLING e.g. New proposal

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1.2.9 MAGNETOTOMOGRAPHY xcellvision - Instrumentation for Fuel Cells and Fuel Cell System Simulation Patented technology Non invasive method

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Sensor 1 Sensor 2 Sensors are used for magnetic field data acquisition as a function of the position. The experimental setup allows the sensor to measure the magnetic field strength (H) at different positions around the cell x y z

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HiHi IjIj high low high low Map of the current intensity

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2. MEASUREMENT OF MASS TRANSFER COEFFICIENTS BY ELECTROCHEMICAL TECHNIQUES 2.1 Why? Electrode surface current Concentration of the electroactive species high low Bulk solution influence the current distribution Affect to the product distribution Affect to the efficiency

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e - Electrode e - S surface R S bulk 2.2 How?

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Typical concentration 5 mM of ferrocyanide and 20mM of ferricyanide to make sure a cathodic controlled electrochemical process A large quantity of inert electrolyte (NaOH, Na2SO4, KSO4, …) has to be added as supporting electrolyte to minimize the migration effects (to make them negligible compared to diffusion and convection) The area of the anode should be larger than that of cathode for a cathodic controlled-process The method is based on a diffusion-controlled reaction at the electrode surface: If the cathode is used as a probe

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2.3 Typical setup for measuring average cell mass transfer coefficients The flow rate is measured by the rotameter. A V The reservoir contains the electrolyte The electrical energy is applied with the power supply connected to the electrodes The pump propels the electrolyte through the electrochemical cell. The heat exchanger keeps the electrolyte temperature at the desired set point. The electric measurement devices are used to obtain high accuracy of voltage and current values, than those provided by the power supply. Oxygen and hydrogen generated in the electrochemical cell can be stripped with nitrogen. The heterogeneous processes take place in the electrochemical cell, where mass transfer processes are studied.

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 a) No potential is applied to cell. No current 2.4 Experimental procedure

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 a) Small potential is applied to cell. No current

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 b) Potential scan begins

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 I limit c) Current limit is reached

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 I limit d) Plateau zone

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 I limit

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 I limit e) Other electrochemical processes (e.g. Electrolyte decomposition)

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Current measured Applied potential Cb 0 V I Distance from the electrode Concentration 0 0 I limit

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e - Electrode e - S surface =0 R S bulk 2.5 Calculation of the mass transfer coefficient

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3. LOCAL MASS-TRANSFER DISTRIBUTION Why? How? By measuring the limit current at different positions on the electrode By using other techniques Mass transfer greatly influence current distribution Mass transfer can be easily improved in a cell by using turbulence promoters Local mass transfer distribution can depend on a lot of factors: Design of the inlet Design of the outlet Flow characteristics Turbulence promoters Smooth or uneven surfaces … Local mass transfer distribution can depend on a lot of factors: Design of the inlet Design of the outlet Flow characteristics Turbulence promoters Smooth or uneven surfaces … 3.1 Importance of mass-transfer distribution visualization

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V voltmeter A A A ammeter Push-button switch Power supply cathodeanode 3.2 Limit current mapping

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Drawback many measuring sites Arrays of microelectrodes

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Corner plate centre

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Total current Current of the main electrode Current of microelectrodes Measuring device resistor

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3.3 Measurement of mass transfer by electrochemiluminescence Direct electrolyses H2O2H2O2 + N 2 + light Iridium tin dioxide electrode Direct electrolyses Very slow rate H2O2H2O2

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3.4 Mathematical modelling Mass transfer distribution model simulation experiments Experimental conditions Modelled results experimental results yes no Agreement? e.g. product conversion e.g. New proposal

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4. WALL SHEAR STRESS Theories of wall turbulence considers the existence and interaction of turbulent bursts, ejections, sweeps and wall streaks. A turbulent bursts is an ejection of fluid from the wall, which also causes fluid to impige on the wall by simultaneous formation of sweeps, or movement of fluids towards the wall. Turbulent bursts and sweeps occur through the formation of vortices and the lift- up of wall streaks. 4.1 Importance of wall-shear stress distribution visualization

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In the diffusion regime Faraday’s law allows to link the mass flux to the wall of electroactive ions (J) to the limit current Analyses of mass flux fluctuations Statistical analyses of this parameter allows to obtain important information concerning the turbulent transfer characteristics within the viscous sublayer

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Information about the wall turbulence in the viscous sublayer Traditional methods Laser doppler anemommetry Particle imaging velocimetry Thermoanemometry Turbulent flow visualization Electrochemical method Main advantage

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Schematic description of initiation of flow induced localized corrosion phenomena metal Oxide layer

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4.2 Measurements of wall shear stress flow anode cathode Diffusion boundary layer Viscous boundary layer The electrochemical method is based on measurement of mass transfer coefficients. This coefficients are related to velocities in the proximity of the probes A small dimension probe allows the measurement of only a local velocity gradient which can be related to local wall shear stress. u(t)

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microelectrode The time-dependent diffusion limited current density correlates with the time- dependent gradient of the streamwise flow velocity perpendicular to the wall which is proportional to the wall shear stress NN HH This method can be applied with high resolution using microelectrodes or microelectrodes arrays incorporated flush and isolated into flat surfaces exposed to tangential flows u(t)

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microelectrode NN HH For a newtonian fluid with dynamic viscosity the wall shear stress can be expressed Local wall shear gradient c, concentration of the electroactive species

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Levêque formula (valid for a circular electrode of area A) Extension of the Levêque formula for a non circular electrode: L length of the electrode in the flow direction (m), l length of the electrode transverse to the flow direction (m) D, diffusion coefficient (m 2 s -1 ), n number of electrons exchanged in the electrode reaction, F Faraday constant (96500 C/mol) For a steady-state flow, the small electrode mounted flush with the insulating wall delivers a current I. This measured intensity increases with the applied potential between the two electrodes until the process becomes controlled by the diffusion of the reacting species to the surface of the working electrode. Then the value of the intensity is the limiting current. The probes behaves as a perfect mass sink

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The single wall probe is applicable only for nonreversing conditions If flow reversal occurs in the proximate wall flow region and additional information about the flow direction is needed a “sandwich probe” should be used The size of this probe should be equal or smaller to the typical size of the large flow structures to ensure homogeneity The sandwich probe consists of two active segments separated in the mean flow direction by a thin insulating gap

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Photolithography probes To current followers x z 100 m i1i1 i2i2 X velocity componenti1i1 + i 2 Z velocity componenti1i1 - i 2 Counter electrode Insulating gap

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Plastic sphere Support rigid tube Gold wire 4.3 Measurement of local shear in three-phase fluidized beds

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Gas slug i( A) Liquid current limit Gas current limit Bubble flow Annular flow Slug flow 4.4 Wall shear stress in multiphase flow

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Printed circuit board Cooling channels Shunt resistors MEA +GDL Anodic BPP Cathodic BPP Shunt resistors are integrated into the PCB using a multilayer design PCB can be easily manufactured in a way that guaranties the compatibility with the elements of the cell High flexibility to modular configuration (the same PCB can be used to study different configurations of the cell) The sense wires associated with the individual resistors can be integrated into the PCB and connected to the data acquisition system from the edge of the PCB The invasive method does not affect to the fluid dynamic properties of the reactant gases and the electrical and thermal conductivity of the cell are not importantly modified. PCB can be introduced inside a BPP. This enable to measure current distribution in a stack Shunt resistors are integrated into the PCB using a multilayer design PCB can be easily manufactured in a way that guaranties the compatibility with the elements of the cell High flexibility to modular configuration (the same PCB can be used to study different configurations of the cell) The sense wires associated with the individual resistors can be integrated into the PCB and connected to the data acquisition system from the edge of the PCB The invasive method does not affect to the fluid dynamic properties of the reactant gases and the electrical and thermal conductivity of the cell are not importantly modified. PCB can be introduced inside a BPP. This enable to measure current distribution in a stack Current collectors Conductive layer (backside of the PCB) load

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1.3. Some new applications: calculation of mass diffusion overpotential distribution in a PEMFC Direction of charge flux V Cell potential Electrolyte ANODECATHODE a + diff a + + reaction UNIFORM OXYGEN CONCENTRATION OF OXYGEN ON THE CATHODE BY FLOW PULSE APROACH AND SEGMENTED- ELECTRODE APROACH CURRENT INTERRUPTION METHOD CURRENT DISTRIBUTION MEASUREMENT WITH UNIFORM OXYGEN CONCENTRATION CELL RESISTANCE MATHEMATICAL MODEL MASS-DIFFUSION OVERPOTENTIAL DISTRIBUTION In PEMFC uneven current distribution are caused by non uniform oxygen distribution inside the fuel cell

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To ensure that the oxygen concentration along the reaction surface is uniform, the flow pulse has to be strongly over stoichiometric and long enough to remove all excess water from the electrodes. At the same time the duration of the flow pulse must be short enough in order not to change the resistance of the proton conductive phases of the MEA CONDITIONS Cell operated galvanostatically For each current the cell was allowed to stabilize and then the current distribution was measured A oxygen flow pulse of 10 s is introduced and the current distribution is measured again CONDITIONS Cell operated galvanostatically For each current the cell was allowed to stabilize and then the current distribution was measured A oxygen flow pulse of 10 s is introduced and the current distribution is measured again MATHEMATICAL MODEL

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