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Electrochemical Setup Of An Electrochemical Cell Engineering Essay

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Potentiostat can be defined as amplifiers that are used to maintain a constant voltage between the working electrode and the reference electrode by making current controlled adjustments at the counter electrode. [5]

The main function of the potentiostat include

Measurement of the potential difference between the reference electrode and the working electrode

To compare the potential difference with the pre defined voltage and forces the current through the working electrode to oppose the difference between the predefined voltage and the existing voltage across the working electrode [5].

The potentiostat behaves as a controlling and measuring device .It controls the voltage across the electrochemical cell by detecting the changes in the resistance and accordingly varying the current that is provided to the cell. The relation between the current and the resistance can be defined by Ohm's law as,[17]

R= E/I

where, [17]

R is the variable resistance

I is the output current of the potentiostat and,

E is the constant potential.

When the resistance is high the current is reduced and vice versa, so that a constant potential is maintained. [17].

A basic standardised potentiostat circuit consists of an input differential amplifier which compares the voltage between the reference electrode and working electrode to a desired biased working voltage. The error voltage obtained is amplified and is supplied to the counter electrode. The impedance change between the reference electrode and working electrode will affect the voltage supplied to the counter electrode so that constant potential is maintained between the reference and the working electrode. The output voltage is obtained which is proportional to the electrochemical cell by connecting the voltage follower circuit to the working electrode.The current follower hold the counter electrode to virtual ground. The potentiostat maintains the required reference -working voltage by comparing the reference electrode voltage to the required biased potential and making adjustments in the potential at the working electrode.[34]

The circuit of the conventional Potentiostat circuit is as shown in the figure below.[34]

Figure 1.Basic potentiostat circuit [34]

Where ,

C = counter electrode of the electrochemical cell

R=reference electrode of the electrochemical cell

W=working electrode of the electrochemical cell

Rf= feedback resistance

V= Voltage given as ,

V=I sensor/Rf [34]

CA = Controlled amplifier

APPLICATIONS:

The applications of potentiostat include

Potentiostat as Ammeter with zero resistance, controlled voltage and current source

Measurement of polarised resistance and red ox voltages

Potentiostat as a High Fidelity Amplifier

Differential voltage control

Biosensors [35].

FEATURES:

The special features of the potentiostat such as speed, stability in phase, O/P power, and high input resistance play a very important role in most of the electrochemical processes.[35]

Significant features of potentiostat are listed below.

Accuracy and wide ranges in potentials and currents.

High file size, scanning and sampling rate

Current resolution

Series of working electrodes for controlling instruments

Sweep analog generator and footprints.[35]

CHAPTER 2: OVERVIEW

Figure 2: Potentiostat Setup [19]

A basic potentiostat setup is as shown in the figure above. i(t) represents the current carrying with respect to time(t).CE,RE and WE represents counter electrode, reference electrode and working electrode respectively.

The potentiostat consists of controlling and amplifying part.

Controlling part: This part maintains the voltage of the electrochemical cell at a required voltage level. The voltage between the RE (reference electrode) and WE(working electrode)is maintained to be equal to the Bias voltage. This is done by sourcing or driving the current into CE(counter electrode).[37]

Amplifying part: This part converts the current of the counter electrode into equivalent voltage [37].

An input potential is applied to the potentiostat through a voltage source (ramp generator, PIC programmed to generate voltage waveform) or by the potentiostat internally. In the project we apply the time variant input hence we use external source. A PIC is used in the project to generate time varying potential which is applied as input to the potentiostat. The potential across the working and the reference electrode is controlled by the potentiostat [19]

The voltage applied produces current through the counter electrode which produces a desired potential across the reference and working electrode.

CHAPTER 3: PRINCIPLES OF OPERATION

CHAPTER 3.1 POTENTIOSTAT

The basic potentiostat circuit for the electrochemical cell with three electrode setup is as shown below [17].

Figure 3.Basic potentiostat circuit [17]

The input potential Ein is applied to the non inverting terminal of the opamp1 .The output of the op-amp 1 is given through the counter electrode and a feedback is given from the reference electrode through the op-amp 3.This feedback will help to reduce the difference between the inverting and non-inverting inputs of the opamp1 and thus the reference voltage will have same voltage as of Ein. The potential at the working electrode is set to the same potential that was applied to input of opamp1 since the potential difference between the reference and working electrode is zero.[17]

The reference electrode is connected to the Ein pin of the opamp1 through opamp3 of the high impedence.The current needs to flow through the counter electrode and not through the reference electrode due to high resistance and also due to the fact that it may result in uncertainty of its potential.[17]

CHAPTER 3.2 ELECTROCHEMICAL CELL

The electrochemical cell consists of 3 electrodes immersed in an electrolyte solution namely,

Working electrode

Reference electrode and ,

Counter electrode.[1]

The working electrode (WE) is the electrode which is used to analyse the electrochemical reaction. The second electrode that is used in closing the circuit is called the counter electrode (CE).The third electrode is the reference electrode that is used to measure the potential between the working electrode and the electrolyte. [2]

The experimental setup for an electrochemical cell is as shown below.

Figure 4 : Electrochemical setup of an electrochemical cell [2]

This setup contains a working electrode/electrolyte junction and the counter electrode/electrolyte junction. Assuming loss of voltage to be negligible in the electrolyte and the solid bulk, the voltage controlled by the battery and that applied to the system can be distributed as,

V=VWE+VCE [2]

Where,

VWE and VCE are the potential across the working electrode/electrolyte and counter electrode/electrolyte respectively.

This implies that the voltage supplied by the battery is not applied entirely to the working electrode/electrolyte junction.[2]

The voltage across the counter electrode will vary with time as a electrochemical reaction is stimulated by the current at the interface of the counter electrode/electrolyte(VCE)This instability will affect consistency of the voltage across the working electrode/electrolyte VWE .[2].This inconsistency can be avoided by varying the value of VCE such that VCE<< VWE and this is done by increasing the area of the counter electrode. This results in the resistivity of the Counter Electrode to be smaller as compared to the Working Electrode and thus the condition VCE<< VWE is satisfied.[2]

Choosing a very large area for the CE is always not the best solution since a significant influence is caused by VCE and VWE when high current density flows through the cell. Potentiostat can be used which controls the potential across the Working electrode and the reference electrode directly. The voltage that was supposed to be applied between the reference electrode and the solution is applied as input potential to the potentiostat [2].

There are many sensitive and information based electro analytical techniques in the field of electrochemistry that provide useful information about chemical and physical properties of analytes such as potential ,transfer of electrons,oxidation,coefficients of diffusion which are very difficult to be obtained using other techniques.

The electro analytical methods applied for quantitative measurements are,.

1. Potentiometry: Measurement of the potential in the electrochemical cell under no current flow. Examples include pH measurements, titration etc.[22][23]

2. Coulometry:Measurement of the current which is flowing through the circuit by keeping the potential constant at the working electrode.Examples:determination of thickness of film etc.[24]

3.Electrolytic method:

In this method an external energy source is applied to initiate an electrochemical reaction. If the external energy source that is applied is potential then the current that is resulting is the analytical signal and if the current is adapted then the analytical signal is the potential that is obtained. [22][30].

The approach which uses the applied potential is termed as voltammetry method and those which use current are termed as galvanostatic methods. [22][30]

In the project the voltammetry method (Cyclic voltammetry) is applied, i.e., the external applied driving force is the applied potential. [22][30]

CHAPTER 3.3 VOLTAMMETRY

It is an electro analytical process which is used in analytical chemistry. In voltammetry the information of the analyte in the electrochemical cell is obtained by varying the potential with respect to time and measuring the resultant current.[24]

There are various types of Voltammetry such as linear sweep, staircase, Squarewave,cyclic, anodic stripping, cathodic stripping, polarography, differential pulse voltammetry etc.[24]

In this project we use cyclic voltammetry for electrochemical and for electrochemical analysis.

CHAPTER 3.3.1 CYCLIC VOLTAMMETRY

Cyclic voltammetry is one of the widely used methods for analysis of electrochemical reactions. It has ability to provide rapid information about the red ox processes and chemical reactions such as red ox potential peak location, effects on the red ox processes due to media and determination of electron-transfer kinetics.[25][26]

In Cyclic voltammetry (CV), a triangular voltage waveform is applied at the working electrode linearly and the resulting current is measured .The figure below shows the triangular potential waveform applied.

Figure 5: Time varying potential signal [25]

Where, Ef= final potential

Ei= initial potential

Cycle 1, 2 .. = every cycle constitutes of forward and reverse scan.

For every cycle, the potentiostat measures the current generated due to the varying potential applied. The graph obtained by plotting the potential against the resulting current is termed as cyclic voltammogram which is dependent on time and on various chemical and physical aspects.

Figure below shows the cyclic voltammogram for a single cycle of the red ox species.

Figure 6: Cyclic voltammogram [25]

Where,

Epc = peak cathode potential

Epa=peak anode potential

Ipc=peak cathode current

Ipa=peak anode current

It is presumed that initially there is only oxidised form in the red ox species. Hence in the first half cycle negative potential scan is considered where the cathodic peak current is observed. In the next half cycle positive potential scan is considered and the anodic peak current is observed. The main parameter taken into consideration in the cyclic voltammogram are the peak potentials and the peak current at the anode and cathode sweeps.[25]

The peak current ip is directly proportional to the concentration C and can be defined by Randles-Sevcik equation,

ip = 0.4463 n F A C (n F v D / R T)1/2[25]

where,

n = number of electrons transferred/molecule

A = electrode surface area (cm2)

C = concentration (mol cm-3)

D = diffusion coefficient (cm2 s-1).

CHAPTER 4: BLOCK DIAGRAM OF THE POTENTIOSTAT CIRCUIT

The Block diagram of the Potentiostat is as shown below:

Figure 7: Block diagram of Potentiostat circuit [13]

The PIC microcontroller is used for the generation of the voltage waveform, data analysis and data acquisition. The PIC is programmed in the C language as it is the more efficient and powerful coding technique. A 10 bit counter is used in the PIC microcontroller for the voltage waveform generation.[9]

The 10 Bit counter value is fed into external D/A converter through the SPI (serial programming interface) port of the microcontroller. The voltage waveform is level shifted using the amplifier circuit and is applied to the electrochemical cell between the working and the reference electrode of the cell.[9]

The current generated from the cell is then converted into voltage and is then applied to the level shifting circuit which is used to achieve the required input voltage level that is essential for the analog input port of the PIC.The current readings are noted down for each incrementing value of the voltage.[13][9].

CHAPTER 5: SCHEMATIC OF PIC CONTROLLED POTENTIOSTAT

The Potentiostat circuit is designed using a PIC which controls the operation of the circuit. The circuit diagram of the Potentiostat circuit with PIC18F46K20 is as shown below.

Figure 8: Circuit diagram of the potentiostat circuit[3]

CHAPTER 5.1: COMPONENTS

The components used in the schematic are as below:

Item

Quantity

Reference

Part

1

4

C1,C3,C4,C5

100uF/Electrolytic

2

1

C2

2x4.7uF/Ceramic

3

1

J1

CON6

4

9

R1,R2,R4,R7,R8,R10,R11,R12,R14

R

5

3

R3,R9,R13

POT

6

1

R5

2K/0.25W

7

1

R6

1.33K/0.25W

8

1

U1

PTN78000A

9

1

U2

PTN04050

10

1

U3

LM148

11

1

U4

PIC18F46K20

12

1

U5

TLC5615

13

1

U6

SW

14

1

U7

OSC

15

1

U8

LM741/DIP8

PTN78000A: PTN78000A is a high efficiency integrated switching regulator which operates through a wide range of frequency, providing voltage conversion that ranges from negative to positive loads up to 1.[4]

PTN04050:It is an integrated switch regulator with adjustable output. The input operating voltage ranges from 2.9-7V.The output voltage ranges over a wide range of values (-15V to -3.3V) and is determined by using a single external resistor .This regulator provides a very efficient positive to negative voltage conversion for loads up to 6W.[7]

LM148: LM148 is an op-amp (operational amplifier) that has high gain and low offset current and bias. Each package of LM148 consists of four amplifier isolated from each other. This reduces the overall cost and the complexity of the circuit. The package is supplied with power rather than individual op-amps with ± 12V.[10].

TLC5615:It is a 10 bit Digital to analog converter is a product of Texas instruments with an input supply of 5V used in operations where space is critical. It uses a network of resistors which is buffered with Op-amp (gain A=2) for A/D conversions. Both the input and the output polarity of the LC5615 are same .[11]

LM741/DIP8: it is a low cost operational amplifier which has protection of excess load at the input and output .the LM741 is free from oscillations. The operating temperature ranges from 0°C- 70°C.[12].

PIC18F46K20:PIC18F46K20 is a high performance flash programming microcontroller from Microchip with optimised C complier architecture. It consists of 10 bit ADC module used for analog to digital conversion, 16 bit instruction set,8 bit data path, and interrupts with priority levels. The operating voltage ranges from 1.8V-3.6V.[31].

CHAPTER 5.2 FLOW DIAGRAM FOR THE POTENTIOSTAT SYSTEM

A/D Module setting

Timer Module Setting

SPI Module Setting

Interrupt Enable and Data collection

CHAPTER 5.3 OPERATION

The Circuit operation is as explained below:

A triangular potential waveform is generated by the PIC18F46K20 using a up down counter

The output of the PIC is in the digital form and is converted to analog from using a digital to analog converter(D/A converter) using the SPI port of the PIC.The D/A converter using in this circuit is MAX5354[3]

The variable output from the digital to analog converter is dependent of the Vref pin of the D/A converter i.e., MAX5354 .The varying output of the D/A converter is controlled by using a potentiometer which is connected to the Vref pin.[3]

The output of the D/A converter which is unipolar is converted into bipolar by using an op-amp level shifting circuit. Op amps are used to compare adc voltage and feedback voltage from electro chemical cell. A buffer is introduced between the voltage that is level shifted and counter electrode so that loading of previous op-amp phases obtained as a result of the current drawn from the electrochemical cell is prevented.[3]

The non inverting terminal of the voltage follower circuit is connected to the reference electrode so that a constant voltage is maintained by drawing very less current.[3]

The inverting terminal of the current to voltage converter is connected to the working electrode to ground the working electrode and also to obtain an equivalent voltage of the current (drawn from electrochemical cell) at the voltage converter output.[3]

The converted current as a voltage is amplified and level shifted so that the voltage is brought between the acceptable PIC voltage ranges (ie,0-5V) and is given as input through the analog port of the PIC[3].

Con6 in the circuit diagram is applied with +5V voltage externally. Pin 1, 2 and 3 of con6 are connected to the amplifiers and the output pin is connected to the electrochemical cell .Con6 provides +5V input to U2,where U2 boosts the signal from 5V to 12V that is necessary for circuit operation. Input is given to U2 from U1 which produces -12V output needed for the circuit operation.U5 does the analog to digital conversion when it gets a potential waveform from the PIC and the converted values are given as input to the amplifier circuitry.

LEVEL SHIFTING CIRCUIT:

The circuit below shows the level shifting circuit present in the Figure 3.

Figure 9 : Level shifting circuit[3]

The output of the TLC5615 (D/A Converter) is connected to the level shifting circuit. Level shifting output Vout .The level shifting output voltage Vout is a function of the output of D/A converter and the offset DC voltage and is defined by the mathematical equation as shown below.[3]

Vout= -(R4/R2) (Offset DC Voltage) + (1+R4/R2)(O/P of TLC5615) .[3]

In this project we use R4=R2=1kohm

Hence,

Vout= -1(Offset DC Voltage)+2(O/P of TLC5615) .

The output of the level shifting circuit Vout is the representation of the current that is given to the electrochemical cell. The range of voltages can be obtained by varying the R5 (potentiometer) which is connected to the ref pin of TLC5615 and the voltage polarity can be varied by changing the potentiometer R2 of the offset DC voltage.[3]

The output of the level shifting circuit is as shown.

Figure 10. Output waveforms of D/A and level shifting amplifier circuit [3]

CHAPTER 6: ELECTROCHEMICAL CELL

The RC equivalent circuit of an electrochemical cell is as shown .the components of the circuit is as explained below.

Figure 11.RC Equivalent of the electrochemical cell [15]

Rrc is the resistance between the counter electrode and the reference electrode .It is defined as the solution resistance which is the most important factor of Z(impedance) in the electrochemical cell. Rrc depends on the ion concentration the nature of ions and the area in which the current is passed.[14]

The resistance Rrc can be defined as,

Rrc = r*l/A [14]

Where, r is the is the density ,l is the length which carries a constant current and A, is the area in which the current is passed.[14]

Cd is the double layer capacitance .This capacitance is present at the electrode and the electrolytic solution interface. This is formed when the ions from the electrolyte solution resides on the surface of the electrode .the Cd depends on the type of ions, concentration of the ions, temperature, layer of oxidation, impurities present in the solution etc.[14]

Rs is the uncompensated resistance of the solution. It sometimes include the component present on the surface of the metal.[15].If the uncompensated resistance is very high then the separation between the voltage peaks is more and there is significant decrease in the peak currents.

Rd is the resistance across the interface of the metal and the solution.[15]

CHAPTER 7: DETERMINATION OF PEAK AND PEAK CURRENTS

The analog-to-digital measurements are periodically taken from the special event mode of the TC mode (timer capture mode) by setting the go/done bit of the A/D converter which starts the analog-digital measurements. Once this is done an interrupt of high level is applied. [3]

The stages involved in the interrupt subroutine are as shown below. [3]

A/D values read from the conversion register

Continuous 8 values averaged

( to eliminate noise)

Store the data in an array of length=256

Disable interrupts after 2048 counts

Once the interrupt is disabled the peak current of the anodic and cathodic sweeps are obtained.

The slope of the left side and the knee is calculated. Then the peak of both anodic and cathodic sweeps with respect to the slope is calculated. The calibration data of the curve is stored in the PIC in terms of slope and y-intercept of the best fit line. This slope and the y-intercept are used to calculate the concentration that corresponds to the peak current values and the corresponding 8 bit data value of the current is generated. [3]

CHAPTER 7.1 : BASELINE CURRENT EXTRAPOLATION

The point of maximum curvature is used to determine the baseline current for both anodic and cathodic sweeps. A line is made to pass through all the points to the left side of the point of maximum curvature. [3]

CHAPTER7.2 : L METHOD FOR THE DETERMINATION OF KNEE OF THE CURVE OR POINT OF MAXIMUM CURVATURE

The novel method (L method) finds the edge/boundary between the pair of lines that fits the cure closely. This method considers all points of data together at same time and is used where the curve includes sharp jumps. [27]

The linearity of the region to the right and left of the line is taken into consideration in the L method. If a line is drawn fitting all the points on the left hand side and another line is drawn on the right hand side, the area that is obtained in between these two lines will be same as the area of the knee. To find these 2 lines, a pair of line that fits the curve well (i.e., that covers maximum points) is obtained. [27]

Consider a graph of no of data v/s evaluation graph. Let the number of data be'd'.Hence the data range is from '1-d'.Let 'p'be the partitioning of the data points. Hence the right hand side of the data points is defined to be from 1..p and the left side of the data points from p+1...d.[27]

RMSEc= 1/d {p * RMSE (Lc) + (d-p) * RMSE (Rc)} [27]

Where,

RMSEc= the mean square error in total

RMSE (Lc) =the mean square error if the left hand side of the partition in total

RMSE (Rc)= the mean square error if the right hand side of the partition in total.

Steps to find the knee of the curve using the L method is as shown below.[28]

The data points are varied from 1 to d.

Using the points on the right hand side(1 to p), a line is fit and using the points on the left (p+1 to d),another line is fit.

The RMSEc is calculated for each value of p.

The knee of the curve is obtained by the least mean square error.

The regressive line fitting is used for fitting a line to the set of data provided.

CHAPTER 7.3 REGRESSIVE LINE FITTING

The best fit line(regression line) is used to obtain minimum value for the mean square error(i.e., the error between the expected and the obtained values if the data points).[29]

If the data points are defined by {xi,yj} then the regression line s used to obtain the equation of a straight line[29]

Y=mx+c

Where, m=slope

c =y-intercept.

The values for the m and c can be obtained by using calculus and is given as, [28]

m= covariance (xy) / {variance (x) - mean(x) mean(y)}

c = a * mean(x) - mean(y)

The peak is obtained w.r.t the baseline current and the concentration is obtained from the calibration curve. [29]

CHAPTER 8: PROGRAMMING OF PIC

MPLAB IDE v8.53 and MCC18 complier are used to program and load the program onto the PIC.PIC2 development debugger/programmer is used for interfacing.

MPLAB IDE v8.53 is the high optimised and fast complier from Microchip used for PIC18 series of microcontrollers. It is the most useful toolset used in the development of the embedded designs.[32]

MCC18 is the MPLAB C complier which is integrated into the MPLAB IDE .This provides compatibility for the MPLAB to program in C language. It controls the path and the declarations of source files header files, linker files etc that are needed in the programming.[33].

CHAPTER 8.1 MICROCONTROLLER SETTINGS

The PORTS of the PIC are configured as inputs and outputs. In this project PORTC and pin 4-7 of PORT D are configured as output and pin1-3 of PORT D as inputs. PORT A behaves as an Analog input port.

The ADC, SPI, TIMER registers for A/D conversion, Serial data transfers is as shown.

ADC Register:

The features of Analog-Digital converter module are:

Auto data acquisition capacity

External channels(13),10 bit resolution

Conversion is processes during sleep

Fixed reference voltage of 1.2V [38]

SETTINGS:

1.ADCON 0 register[38]:

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

-

-

CHS3

CHS2

CHS1

CHS0

GO/DONE

ADON

-

-

0

0

0

1

1

ADCON - A/D conversion starts when ADON=1 ,this is automatically cleared when the A/D conversion is done [38]

CHS[3:0] : Analog Channel Select bits.[38]

0001= channel 1 is selected.

2.ADCON 2 Register:

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

ADFM

-

ACQT2

ACQT1

ACQT0

ADCS2

ADCS1

ADCS0

1

-

0

0

0

0

0

1

ADFM- defines the format in which the A/D conversion values are stored.

ADFM=1 stores the values as right justified i.e. ADRESL = b7-b0 and ADRES=b9 and b8.

ADCS[2:0]- select bits for A/D clock conversion

ADCS[2:0]=001 selects FOSC/8.[38]

3.SPI (Serial Port interface)[38]:

SPI module is used to transfer / receive synchronous data of 8 bits simultaneously between the microcontrollers or peripheral devices such as A/D converters , shit registers ,EEPROM etc.[38]

The master SSP module has 4 registers assigned for SPI operation. These are:

• SSPCON1 - register for CONTRL

• SSPSTAT - register to define STATUS

• SSPBUF - Buffer to send the data in serial/parallel

• SSPSR - register for shift.[38]

SSPSTAT register (status register)[38]:

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

SMP

CKE

D/A

P

S

R/W

UA

BF

X

0

X

X

X

X

X

X

CKE - Clock bit select for SPI

CKE =0, the data O/P changes when the clock transition takes place from idle to active.[38].

4.SSPCON1 Register (control register)[38]

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

WCOL

SSPOV

SSPEN

CKP

SSPM3

SSPM2

SSPM1

SSPM0

1

X

1

1

0

0

0

0

WCOL- Write collision detect ,

WCOL = 1, the SPI buffer register is written and data is simultaneously transferred.

SSPEN- synchronous serial port enable data

SSPEN= 1 ,Serial port are enabled and pins are configured

CKP-Polarity of the clock

CKP=1 the clock is idle when it is at high

SSPM3[2:0] - selection of synchronous serial port mode

SSPM3[2:0]= 0000, Master SPI mode and the clock frequency = FOSC/4.[38]

Timer and CCP (capture compare PWM) register:

The PIC14F46K20 consists of two registers each consisting of 16 bits. Each of the register can function as PWM /capture/compare register.[38]

5.CCP2CON (CCP Control register)[38]

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

-

-

DC2B1

DC2B0

CCP2M3

CCP2M2

CCP2M1

CCP2M0

X

X

X

X

1

0

1

1

CCP2M[3:0]- Mode selection register

CCP2M[3:0]= 1011,SPI event trigger,A/D conversion, starts when COP2IF bit is set

6.T3CON (Timer 3 Control Register)[38]

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

RD16

T3CCP2

T3CKPS1

T3CKPS0

T3CCP1

T3SYNC

TMR3CS

TMR30N

1

0

1

1

0

X

0

1

RD16- Enable bit for 16 bit W/R

RD16=1,enables Timer 3 R/W for 16 bit

T3CKPS[1:0]- preset value select

T3CKPS[1:0]= 11,preset value is 1:8

T3CCP2[2:1]-clk source select

T3CCP2[2:1]= 00- Timer 1 is selected as clk source

TMR3CS -internal clk select

TMR3CS=0, internal clk (Fosc/4)

TMR30N-start/stop timer3

TMR30N=1, stop timer 3.[38]

CHAPTER 8.2 PROGRAMMING

The steps involved in the programming of the PIC are as explained.

MAIN PROGRAM[43]:

Step 1: Configure

Port A - inputs (A/D conversion)

Port B- outputs (SPI conversion)

Step 2: Enable interrupts and timers (High priority interrupts are enabled first)

Step 3: Configure SPI, ADC, Timers

Step 4: Load variables.

Step 5: Enable ADC interrupts wait till 2048 counts and disable

Step 6: Calculate the peak current and potentials

Step 7: Close SPI, ADC, Timers

Step 8: Disable all interrupts

SUB PROGRAMS:

1. Interrupts[43]:

Step 1: Interrupt priority is enabled

Step 2: Enable high priority interrupts

Step 3: Enable low priority interrupts

2. ADC sub program[43]:

Step 1: Clear all the interrupts

Step 2: Load 10 bit counter values to SPI

Step 3. Check whether 8 ADC values are obtained, if so take the average of 8 values and store the value in an array whose size=256.[43]

Step 4: Continue storing the values till the count of the ADC values reaches to 2048 and store the values in array. (Since if we take 2048 values and average them, number of values obtained in 2048/8=256.)[43]

Step 5: Once completed the counter is loaded with 1 and all the interrupts are disabled[43].

3. Peak calculation[43]:

The current peaks of the anode and the cathode is calculated by this sub program .This is done by finding the knee of the curve in anodic and cathodic directions.

Step 1: initialise all the variables

Step 2: compare the ADC stored in array 0-127 and store the highest value data and the location of it as apeak and aloc .respectively.

Step 3: Compare the next ADC values from 128-256 and store the highest value data and its location as cpeak and cloc respectively[43].

Step 4: Calculate the left hand and the right hand slope and their respective y-intercepts

Step 5: Obtain the total mean square error by adding the mean square error of the slopes on the left and right hand side.

Step 6: Obtain the least mean square error for all the iterations and slope on the left side and y interface are stored.

Step 7: calculate the peak w.r.t slope.

CHAPTER 9: OBSERVATIONS AND RESULTS

CHAPTER 9.1 : ANALYSIS OF ELECTROCHEMICAL CELL USING SPICE MODEL

PSPICE model is used for the analysis of electrochemical cell.

PSPICE (OrCAD) is a simulation program for integrated circuits, which allows the validation of the circuit which actually designing them

The circuit model of the electrochemical cell is as shown

Figure 12 .Schematics of electrochemical cell in PSPICE [40]

The RC circuit of the electrochemical cell is given an input voltage of 2.3V and the simulation results of current versus time is observed. The circuit is an extended circuit model of the Randall's model. The values assigned for the resistances and capacitances are

R1(solution resistance)=4.2kohm

R2(varying resistance between the reference and counter electrode)=2.2kohm

R3 (solution at the interface of solution and electrolyte)=3.3kohm

C1 (slow capacitance of the impedance circuit of Randall's cell)=100nF

C2 (double layer capacitance)=10nF.

The net list generated for the circuit above is shown in the APPENDIX.

The simulated output of time versus current obtained for the RC equivalent circuit of the electrochemical cell in PSPICE is shown in figure.

Figure: 13 Simulation results

The simulation result shows the graph of current plotted with respect to time. The double layer capacitance which is formed at the electrode and electrolyte interface is due to the ion and charge movement. There are two types of transfers in this capacitive layer faradic and non faradic. Faradic transfer is non reversible and is caused due to transfer of electrons which results in red ox reactions.Non Faradic transfer is reversible and is caused due to the movement of the charges resulted at the interface region where the double capacitance is formed.[41]

The graph shows the measured discharge current by an electrochemical cell after it is charged by an input potential. The potential initially at the working electrode is zero. When the input potential is applied to the cell it generates an anodic/cathodic current. This is shown in the simulation graph from point b to c and is called current clamp. The potential applied to the electrode remains same for while before changing to the next potential of opposite polarity. This result in the short circuit of the capacitance and therefore the current discharge takes place .This is shown from point b to c.This discharging is a slow process and a certain amount of charge cannot be regained since it is stored in the faradic components.

If the electrode works as an ideal capacitor then all the charge that was injected can be regained. If it is a pure resistor then no charge that is stored is regained.[40].

CHAPTER 9.2: POTENIOSTAT CIRCUIT BOARD

The layout of the hardware configured is as shown below.

Figure 14: Layout of the Hardware designed

Circuit 1: PIC Microcontroller kit

Circuit 2: amplifier circuitry, DAC

Circuit 3:RC equivalent of the electrochemical cell.

The testing of the potentiostat is carried out by applying a triangular voltage of 0-5V-0.3V.The IE (current v/s voltage) curve obtained is as shown below. These I and E values are termed as equivalent potential and current points since they are not the actual current and potential curves. These points the stored in the memory of the PIC.The potential waveform(triangular) is applied between the working electrode and the reference electrode and the resulting current values are noted for every voltage update(1024 times).

8 continuous values of the current are averaged in order to obtained improved SNR.As a result there is 1024/8=256 current points. These points are extracted using the In circuit debugger in MPLAB.[41]

The values obtained for the ADC and CNTR are as follows

The IE curve of the electrochemical cell is plotted as below by taking the current values stored in the PIC.

Figure 15: IE Curve

CHAPTER 10: FUTURE STUDY

In this project we program the potentiostat to monitor the current when there is time dependent change in the potential. Future work made be carried out on programming the potentiostat for monitoring very low currents, temperature variations and implementing them into calibration tables. The present potentiostat has fixed scan rate, future work can have varying scan rate being implemented in the potentiostat.

The Potentiostat designed is a single channel potentiostat .Multi channel potentiostat can be designed with low cost and high accuracy so that even if one substation isn't working the potentiostat can still operate using another working sub stations and multiple experiments can be carried out at the same time.

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[27] Salvador, S.; Chan, P., (2003),"Determining the Number of Clusters/Segments in Hierarchical Clustering/Segmentation Algorithms". Department of Computer Science Technical Report CS−2003-18, Florida Institute of Technology.

[28] Ashwini Vittal Gopinath, Dale Russel (2005) "An Inexpensive Field-Portable Programmable Potentiostat" Chem. Educator. Department of Chemistry. Boise State University, Pg 1- 6.

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[34] John C. Fidler, William R. Penrose, and James P. Bobis (1992)," A Potentiostat Based on a Voltage-Controlled Current Source for Use with Amperometric Gas Sensors", IEEE transactions on Instrumentation and Measurements, Vol: 42, No: 2.

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[38] MicroChip PIC 18F23K20/ 24K20/ 25K20/26K20[Online] [Accessed on 04 June 2010]. Available on World Wide Web <http://ww1.microchip.com/downloads/en/DeviceDoc/41303E.pdf >

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[40] A. van Ooyen, C. Ulrich, U. Schnakenberg (2009), "SIROF stimulation electrode evaluation using the pulse-clamp method", Institute of Materials in Electrial Engineering, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany.

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[42]. A. van Ooyen, C. Ulrich, U. Schnakenberg (2009), "SIROF stimulation electrode evaluation using the pulse-clamp method", Institute of Materials in Electrial Engineering, RWTH Aachen University, Sommerfeldstr. 24, 52074 Aachen, Germany.

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BIBILOGRAPHY

1. Joseph Anthony Von Fraunhofer, Cecil Henry Banks, "Potentiostat and its applications"

Butterworths, 1972, ISBN: 0408702397, 9780408702393

2. Carl H. Hamann, Andrew Hamnett, Wolf Vielstich ," Electrochemistry" Wiley VCH; 2nd

ed , 2007, ISBN-10: 352731069X , ISBN-13: 978-3527310692.

3. Peter T. Kissinger, William R. Heineman, "Laboratory techniques in electroanalytical

chemistry" CRC Press (1996) , 2nd ed, 1996, ISBN-10: 0824794451, ISBN-13: 978-

0824794453

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8. Jichun Zhang, Yue Huang, Nicholas Trombly, Chao Yang, and Andrew Mason(2009), "Electrochemical Array Microsystem with Integrated Potentiostat", Electrical and Computer Engineering, Michigan State University, East Lansing, MI 48823, USA

APPENDIX

APPENDIX 1

NETLIST GENERATION FOR ELECTROCHEMICAL CELL USING PSPICE.

The net list generated is given below:

**** 08/18/10 11:57:14 *********** Evaluation PSpice (Nov 1999) **************

* C:\Users\raksha\Desktop\project\Schematic1.sch

**** CIRCUIT DESCRIPTION

******************************************************************************

* Schematics Version 9.1 - Web Update 1

* Wed Aug 18 11:27:52 2010

** Analysis setup **

.ac LIN 101 10 1.00K

* From [PSPICE NETLIST] section of pspiceev.ini:

.lib "nom.lib"

.INC "Schematic1.net"

**** INCLUDING Schematic1.net ****

* Schematics Netlist *

C_C2 $N_0001 0 1n

R_R3 $N_0001 $N_0002 1k

C_C1 $N_0001 $N_0002 1n

R_R1 $N_0003 $N_0001 1k

R_R2 $N_0002 0 1k

V_V3 $N_0003 0 DC 2.3V AC 0V

**** RESUMING Schematic1.cir ****

.INC "Schematic1.als"

**** INCLUDING Schematic1.als ****

* Schematics Aliases *

.ALIASES

C_C2 C2(1=$N_0001 2=0 )

R_R3 R3(1=$N_0001 2=$N_0002 )

C_C1 C1(1=$N_0001 2=$N_0002 )

R_R1 R1(1=$N_0003 2=$N_0001 )

R_R2 R2(1=$N_0002 2=0 )

V_V3 V3(+=$N_0003 -=0 )

.ENDALIASES

**** RESUMING Schematic1.cir ****

.probe

.END

**** 08/18/10 11:57:14 *********** Evaluation PSpice (Nov 1999) **************

* C:\Users\raksha\Desktop\project\Schematic1.sch

**** SMALL SIGNAL BIAS SOLUTION TEMPERATURE = 27.000 DEG C

******************************************************************************

NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE

($N_0001) 3.3333 ($N_0002) 1.6667

($N_0003) 5.0000

VOLTAGE SOURCE CURRENTS

NAME CURRENT

V_V3 -1.667E-03

TOTAL POWER DISSIPATION 8.33E-03 WATTS

JOB CONCLUDED

TOTAL JOB TIME .02


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