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.