Post on 01-Jul-2015
description
Study And Development of Solid State Reference Electrodes: From Screen-
printed Sensors to Wearable On-Fabric Designs
Supervisors: Giusy Matzeu Prof. Dermot Diamond
Introduction
A growing awareness of personal health and fitness in today’s world has increased the demand for quicker, easier and more comfortable ways of monitoring people’s health.
Aims Measuring the potential difference
between a reference and a working electrode it is possible to measure the concentration of specific ions.
Using LiFePO4 as the active material and other additives we aimed to create and optimise reference electrodes with the potential to be used on wearable devices.
Step One: Characterizing Screen – Printed Electrodes
Before applying inks to a fabric the different inks would be applied to carbon layer screen printed
electrodes (SPEs) Inks can then easily be tested for stability, sensitivity etc. SPEs were first characterized to ensure
that the results they were producing were reproducible for all sheets of electrodes.
Cyclic Voltammetry Cyclic Voltammetry was carried out at
various scan-rates for each separate electrode.
Five electrodes were cut from one sheet and another four cut from different sheets to check reproducibility inter and intra-batch
-4.00E-06
-3.00E-06
-2.00E-06
-1.00E-06
0.00E+00
1.00E-06
2.00E-06
-0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
Current (A)
Voltage (V)
10 mV/s Electrodes from Sheet 1 1_1_a seg1
1_1_a seg2
1_1_b seg1
1_1_b seg2
1_2_a seg1
1_2_a seg2
1_2_b seg1
1_2_b seg 2
1_3_a seg1
1_3_a seg2
1_3_b seg1
1_3_b seg 2
1_4_a seg1
1_4_a seg2
1_5_a seg1
1_5_a seg2
1_5_b seg1
1_5_b seg2
10mV/s scan across electrodes from one sheet
-5.00E-06
-4.00E-06
-3.00E-06
-2.00E-06
-1.00E-06
0.00E+00
1.00E-06
2.00E-06
3.00E-06
-0.35 -0.3 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
Current (A)
Voltage(V)
10mV/s electrodes from sheets 1-4
1_3_a seg 1
1_3_a seg2
2_1_a seg1
2_1_a seg2
2_1_b seg 1
2_1_b seg2
3_1_a seg 1
3_1_a seg2
3_1_b seg1
3_1_b seg2
4_1_a
4_1_a seg 2
4_1_b seg1
10mV/s scan across 4 different sheets
Electrochemical Vs. Geometrical Area
From the CVs we could also approximate the electrochemical active area of the electrode using the Randles-Sevcik equation:
Ip = F*n 3/2 * A*D ½ * C * v ½
D = 5.48 x 10 -6 cm2/s C= 1 x 10 -6 mol/cm3
F= Faraday Constant in C/mol Graphs of peak current vs. square root
of scan rate were plotted for each electrode.
Straight line graphs confirmed the proportionality between the peak current and the square root of the scan rate.
y = -2E-05x - 7E-07 R² = 0.9984
y = 1E-05x + 1E-06 R² = 0.9886
-4.00E-05
-3.00E-05
-2.00E-05
-1.00E-05
0.00E+00
1.00E-05
2.00E-05
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
Current (A)
Scan Rate1/2 (V1/2/ s 1/2 )
Electrode 1_1_a averages
µ(Ipc) (A)
µ(Ipa) (A)
Results Electrode Average Ratio
Electrode 1_1_a 0.7212247
Electrode1_1_b 0.722780469
Electrode1_2_a 0.7212247
Electrode1_2_b 0.73163924
Electrode1_3_a 0.714583538
Electrode1_3_b 0.711550526
Electrode1_4_a 0.737783981
Electrode1_4_b 0.739965094
Electrode1_5_a 0.735668731
Electrode1_5_b 0.722780469
µ(average) 0.725920145
Stand dev. 0.009298217
Electrode Average Ratio
Electrode1_3_a 0.714583538
Electrode1_3_b 0.711550526
Electrode1_5_a 0.735668731
Electrode1_5_b 0.722780469
Electrode2_1_a 0.73163924
Electrode2_1_b 0.580394821
Electrode3_1_a 0.579881485
Electrode3_1_b 0.578210398
Electrode4_1_a 0.593285704
Electrode4_1_b 0.588569573
µ(average) 0.681948917
Stand dev. 0.068674075
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Ratios of Electrochemical Active Area to Geometrical Area
Average Ratio from one sheet
Average Ratio from 4 different sheets
Relative standard deviation between electrodes from one sheet = 1.28% Relative Standard Deviation taken between electrodes from different sheets = 10.07%
Step Two: Preparing the Ink Materials Used: Active Material: LiFePO4 - commonly used in
lithium ion batteries employed in electric cars Binding Agent: PVdF solef 5130 dissolved in
NMP Conductive Additives: C65, Graphite Materials were mixed in the ratio 80-7-5-8wt%
LFP-C65-SFG6-PVdF and ground together using a pestle and mortar to yield a homogeneous slurry .
Step Three Applying the Ink to SPE’s
Finding the easiest most reproducible way of applying the ink was important
Different techniques were experimented with.
Techniques tried: Drop- coating the ink using micro-pipette Spraying ink onto electrodes using small
spray bottle Stamping ink onto electrode
Drop Casting 0.5 µl of the ink was dropped onto a
number of SPEs one layer at a time Allowed to dry at low temp in a convection
oven to avoid cracking.
Spraying Electrodes Viscosity of solution was adjusted
appropriately for spraying – the ink was diluted by a factor of 2.
Stamping Electrodes A stamp was designed by punching
pieces of spongy material about the diameter of the electrode.
These techniques were carried out until several layers of the ink had been added, drying the electrodes between each layer.
Different sets of electrode were made each with varying amounts of layers so that the best option could be deduced.
Stamping the ink appeared to be the most reproducible option
Step 5: Checking Reference Electrodes Reference electrodes should maintain
constant voltage in the presence of the ions in sweat:
Cl – - 16mmol/L K+ - 8mmol/L Ca2+ - 0.65mmol/L Na+ - 23mmol/L
And in smaller amounts due to surface contamination of the skin:
Cu2+ - 14.4 µmol/L Fe2+ - 24.2 µmol/L Mg2+ - 0.24mmol/L Na+ - 27.6 mmol/L Zn2+ - 13.2 µmol/L
Initial Results: The data did not show stability under
varying concentrations. Voltage of electrodes were checked in solutions: 0.001M NaCl 0.1M NaCl 0.1M NaCl @ pH4 0.1M KCl 0.001M KCl
RE’s would have to be pre-conditioned electrochemically
0
20
40
60
80
100
120
140
160
180
200
0 2000 4000 6000 8000
Voltage (mV)
Time (s)
1mM NaCl
0.1M NaCl
0.1M NaCl @pH4
0.1M KCl
1mM KCl
1mM NaCl -last
Step 6: Electrochemical Conditioning Electrodes were scanned using cyclic
voltammetry at 0.01mV/s. One full cycle so that the material was fully
oxidized and reduced; then oxidized to half of its total charge and stopped.
All the electrodes were conditioned overnight in 0.01M NaCl.
Future Work and Potential Outcomes
Once the reference electrodes are showing good stability on the carbon SPE’s: Different ink recipe’s may be experimented
with to find the most cost-effective, convenient and effective option
A different binding agent such as PVdf solef 6020* can be tried and then different ratios of material.
*Polyvinylidine Fluoride
The LiFePO4 ink shows good versatility with regards to its application onto the substrate. This suggests it may have potential application in screen or ink-jet printing.
The versatility of the material may allow for
it’s direct application onto fabrics and from there lead towards a wearable sensor built directly on fabric.
Future Work and Potential Outcomes
Acknowledgements
The Hamilton-Beaufort Scholarship programme and the NCSR for the opportunity to carry out this
research.
Dr. Stefan Klink working within Professor Schumann’s group at Ruhr University, Bochum,
Germany for the provision of materials.