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ESSEE 4 4th European Summer School on Electrochemical Engineering Palić, Serbia and Montenegro 17 – 22 September, 2006 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. 13071 Ciudad Real. Spain. Department of Chemical Engineering. Universidad de Castilla La Mancha. Spain CONTENTS 1. CURRENT DISTRIBUTION 1.1 Importance of current distribution visualization 1.2 Measurement of current distribution 1.2.1 TYPES OF MEASURING METHODS 1.2.2 PARTIAL-CELL APPROACH 1.2.3 SUBCELLS APPROACH 1.2.4 SEGMENTED ELECTRODES 1.2.5 RESISTORS NETWORK 1.2.6 PRINTED CIRCUIT BOARD APPROACH 1.2.7 TYPES OF MEASUREMENTS OF THE LOCAL CURRENT IN PASSIVE RESISTOR NETWORK 1.2.8 MATHEMATICAL MODELLING 1.2.9 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 1. CURRENT DISTRIBUTION 1.1 Importance of current distribution visualization 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 I I Through the wire flows the same current, but the current distribution on the electrode surface is different Uniform current distribution Non-uniform current distribution Some examples of the importance of uniform current distribution Electroplating: non-uniform current distribution can cause a local variation of the thickness of the deposited metal Electrolyses cell non uniform corrosion of electrodes Small contactsurface current feeder Aluminium surface after an electro-dissolution process Poor efficiency, changes in the products conversion ratio i Current efficiency 100% (if reagents arrives to the electrode at the same rate that they are consumed) 1/3i 2/3 i Current efficiency 83.3% 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 PEM fuel cell Uniform current distribution Produce maximum power densities Ensure maximum lifetime for the cell components 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. Examples of local current distribution in a circular-shape electrode uniform Current scale high low 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 1.2 Measurement of current distribution Load or power supply electrolyte electrodes Purpose of the measurement a) Current distribution in the cell anode cathode b) Current distribution in one electrode 1.2.1 TYPES OF MEASURING METHODS Invasive methods yes Partial approaches Subcells Segmented electrodes Passive resistor network Is the cell modified for the measurement? (is current distribution measurement associated with constructional modifications of the cell? ) no Mathematical modelling Non-invasive methods Magnetic measurements 1.2.2. PARTIAL-CELL APPROACH. Portions or segments of the cell are tested independently by inactivating other portions. electrode electrolyte electrode 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 subcell 3 inactive subcell 1 inactive To increase the accuracy more partial cells should be studied CELL VOLTAGE Subcell 1 Subcell 2 Subcell 3 the specific performance is determined by difference. whole cell Subcell 1 inactive Subcell 3 inactive INTENSITY Advantages: very simple, easy to manufacture Disadvantages: it can only be used as a first approach 1.2.3. SUBCELLS APROACH Several electrically isolated subcells are placed are conveniently placed at different locations in the cell a section of the anode is punched out a section of the cathode is punched out The step is repeated in several determined locations inside the cell The former anodes and cathodes are replaced with smaller ones. Main cell subcells The resulting empty space is filled with a isolating gap The subcells are separately controlled. To measure current distribution every subcell voltage has to be adjusted to fit approximately the mail cell voltage SUBCELL 5 MAIN CELL SUBCELL 3 INTENSITY 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 L1 Ln Lm Main cell Subcell m Lmain cell Subcell n CELL VOLTAGE Subcell 1 1.2.4. SEGMENTED ELECTRODES Measurement circuits Segmented electrode or segmented BPP (in a FC) isolation electrolyte Counter electrode This approach allows a very accurate current distribution mapping Coverage of the whole electrode area Good spatial resolution Example of measuring device for each piece of electrode ohmic resistor Volt-meter Piece of electrode 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 Very invasive method. It can affect significantly to the current distribution. Big differences can exist between the measure and the actual current distribution 1.2.5 RESISTORS NETWORK Buss plate Passive resistor network electrode Main problem - appearing of lateral currents Main advantage: It does not require any modification of the electrodes (or of the BPP or MEA in FC) It is less invasive Coverage of the whole electrode area Good spatial resolution current Volt-meter Drawbacks Electrical properties of the resistors depends on temperature Completely isolated resistors Buss plate Isolated wires Resistor matrix electrode Advantages Improved mechanical strength 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 interconnected resistors Buss plate Isolated wires Resistor matrix electrode Advantages Less affected by in-plane current distribution 1.2.6 PRINTED CIRCUIT BOARDS APPROACH Current collector current backside Through-holes frontside Easy to manufacture Possibility of multilayer manufacture Easy to add electrical components Can be used as BPP in FC 1.2.7 TYPES OF MEASUREMENTS OF THE LOCAL CURRENT IN PASSIVE RESISTOR NETWORK Ohmic resistors passively Hall-effect sensors (only measure) Current transformers actively Multichannel potentiostats (Measure and manipulation) Ohmic resistors current Volt-meter Very simple Frequently used Very invasive. It can affect the cell current distribution Hall-effect sensors 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. 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 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 Current follower circuit Standard operational amplifier circuit for currentto-voltage conversion To working electrode + + For very low currents To data acquisition card 1.2.8 MATHEMATICAL MODELLING Current distribution model e.g. simulation New proposal Modelled results no Experimental conditions Agreement? experiments experimental results e.g. product conversion yes 1.2.9 MAGNETOTOMOGRAPHY Non invasive method xcellvision Instrumentation for Fuel Cells and Fuel Cell System Simulation Patented technology z x y 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 Ij Hi high low H1 a11 H2 ... H n 1 H a n n1 ... a1m I1 I2 ... I n 1 a nm I n Map of the current intensity high low 2. MEASUREMENT OF MASS TRANSFER COEFFICIENTS BY ELECTROCHEMICAL TECHNIQUES 2.1 Why? Bulk solution Electrode surface high current Concentration of the electroactive species low influence the current distribution Affect to the product distribution Affect to the efficiency e- 2.2 How? relectrochem Electrode Ssurface R j·A n·F Sbulk rmass transfer k m A(Sbulk Ssurface ) The method is based on a diffusion-controlled reaction at the electrode surface: 3 6 Fe(CN ) e Fe(CN ) If the cathode is used as a probe 4 6 Typical concentration 5 mM of ferrocyanide and 20mM of ferricyanide to make sure a cathodic controlled electrochemical process The area of the anode should be larger than that of cathode for a cathodic controlled-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) 2.3 Typical setup for measuring average cell mass transfer coefficients The reservoir contains the electrolyte The electric measurement devices are used to obtain high accuracy of voltage and current values, than those provided by the power supply. The electrical energy is applied with the power supply connected to the electrodes Oxygen and hydrogen generated in the electrochemical cell can be stripped with nitrogen. V A The flow rate is measured by the rotameter. The pump propels the electrolyte through the electrochemical cell. The heat exchanger keeps the electrolyte temperature at the desired set point. The heterogeneous processes take place in the electrochemical cell, where mass transfer processes are studied. I 0 Concentration Cb 0 Current measured 2.4 Experimental procedure Distance from the electrode Applied potential V 0 a) No potential is applied to cell. No current 0 0 Current measured I Concentration Cb Distance from the electrode Applied potential V 0 a) Small potential is applied to cell. No current Concentration 0 0 Current measured I Cb Distance from the electrode Applied potential V 0 b) Potential scan begins 0 I 0 Current measured Concentration Cb Distance from the electrode Applied potential V 0 I 0 Concentration Cb 0 Current measured I limit Distance from the electrode Applied potential V 0 c) Current limit is reached 0 I Concentration Cb 0 Current measured I limit Distance from the electrode Applied potential V 0 d) Plateau zone I 0 Concentration Cb 0 Current measured I limit Distance from the electrode Applied potential V 0 I 0 Concentration Cb 0 Current measured I limit Distance from the electrode Applied potential V 0 e) Other electrochemical processes (e.g. Electrolyte decomposition) 0 I Concentration Cb 0 Current measured I limit Distance from the electrode Applied potential V 0 2.5 Calculation of the mass transfer coefficient e- relectrochem jlim·A n·F Electrode Ssurface=0 R relectrochem rmass transfer Sbulk rmass transfer k m A(Sbulk Ssurface ) k m ·A·Sbulk jlim ·A k m ·A·Sbulk n·F jlimit km n F S bulk 3. LOCAL MASS-TRANSFER DISTRIBUTION 3.1 Importance of mass-transfer distribution visualization Why? 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 … How? By measuring the limit current at different positions on the electrode By using other techniques 3.2 Limit current mapping cathode Push-button switch anode Power supply V voltmeter A A A ammeter Arrays of microelectrodes Drawback many measuring sites Corner plate centre Total current Measuring device Current of the main electrode 3.3 Measurement of mass transfer by electrochemiluminescence + N2 + light Direct electrolyses H2O2 Direct electrolyses Very slow rate Iridium tin dioxide electrode HO 2 2 3.4 Mathematical modelling Mass transfer distribution model e.g. simulation New proposal Modelled results no Experimental conditions Agreement? experiments experimental results e.g. product conversion yes 4. WALL SHEAR STRESS 4.1 Importance of wall-shear stress distribution visualization 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 liftup of wall streaks. Analyses of mass flux fluctuations I limit n· f ·J I limit (t ) I limit I limit (t ) ' In the diffusion regime Faraday’s law allows to link the mass flux to the wall of electroactive ions (J) to the limit current Statistical analyses of this parameter allows to obtain important information concerning the turbulent transfer characteristics within the viscous sublayer Turbulent flow visualization Traditional methods Laser doppler anemommetry Particle imaging velocimetry Thermoanemometry Electrochemical method Main advantage Information about the wall turbulence in the viscous sublayer u y · Oxide layer metal Schematic description of initiation of flow induced localized corrosion phenomena 4.2 Measurements of wall shear stress flow u(t) cathode A small dimension probe allows the measurement of only a local velocity gradient which can be related to local wall shear stress. anode 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 dH u(t) dN microelectrode I limit a 1/ 3 This method can be applied with high resolution using microelectrodes or microelectrodes arrays incorporated flush and isolated into flat surfaces exposed to tangential flows The time-dependent diffusion limited current density correlates with the timedependent gradient of the streamwise flow velocity perpendicular to the wall which is proportional to the wall shear stress dH dN c c c, concentration of the electroactive species microelectrode u S y y 0 S Local wall shear gradient For a newtonian fluid with dynamic viscosity the wall shear stress can be expressed I limit ( D 2 S ) 1 3 D S I 0.8075·n·F ·A·c · l 2 2 3 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 I 0.8075·n·F ·L·l ·c · D 2 S 1 3 Levêque formula (valid for a circular electrode of area A) 1 3 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 (m2s-1), n number of electrons exchanged in the electrode reaction, F Faraday constant (96500 C/mol) 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 sandwich probe consists of two active segments separated in the mean flow direction by a thin insulating gap The size of this probe should be equal or smaller to the typical size of the large flow structures to ensure homogeneity x z Photolithography probes Counter electrode 100 m X velocity component i1 + i2 To current followers Z velocity component i1 i2 Insulating gap i1 - i2 4.3 Measurement of local shear in three-phase fluidized beds Plastic sphere Support rigid tube Gold wire 4.4 Wall shear stress in multiphase flow Bubble flow Slug flow Gas slug -1 i(A) Annular flow Gas slug Gas current limit -2 -5 Liquid current limit Current collectors Shunt resistors Conductive layer (backside of the PCB) Cooling channels Printed circuit board Anodic BPP load MEA +GDL 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 1.3. Some new applications: calculation of mass diffusion overpotential distribution in a PEMFC UNIFORM OXYGEN CONCENTRATION OF OXYGEN ON THE CATHODE BY FLOW PULSE APROACH AND SEGMENTEDELECTRODE APROACH V hW Cell potential ea + h hdiff CURRENT INTERRUPTION METHOD hW ea + h + hreaction hW Electrolyte ANODE CATHODE In PEMFC uneven current distribution are caused by non uniform oxygen distribution inside the fuel cell Direction of charge flux CURRENT DISTRIBUTION MEASUREMENT WITH UNIFORM OXYGEN CONCENTRATION CELL RESISTANCE MATHEMATICAL MODEL MASS-DIFFUSION OVERPOTENTIAL DISTRIBUTION 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 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 E E 0 b ln( i j ) rji j hconc, j E 0 E rev b ln( i 0,c ) MATHEMATICAL MODEL E hom E 0 b ln( i hom, j ) rji hom, j hconc, j E hom E b ln( i hom, j ij ) rj (i hom, j i j )