Development of in situ electro IR methodfor detection of quinones in conducting

redox polymers

Robin LundstromBachelor Degree Project in Chemistry

June 11, 2015

Mentor: Martin SjodinSubject specialist: Jacinto SaExaminer: Helena Grennberg

Abstract

Conducting redox polymers are polymers with a conductive polymer backbone and redox activependant groups. This kind of polymer could be useful in the electric energy storage field, butstill needs further research. Especially the interactions between the backbone and pendant groupsduring redox conversion must be investigated further. One observed interaction is the loss ofconductivity on the polymer backbone when the redox active pendant groups undergo redox con-version. In situ electro IR spectroscopy is an analytical method combining electrochemistry withIR spectroscopy, and this method will be developed to investigate conducting redox polymers’redox reactions. External reflectance was used during the measurements and therefore a reflectivesurface was deposited on an electrode. The gold deposition had the most reflective surface andwas robust, and was therefore chosen for the measurements. The conducting redox polymer yield-ing most promising results was poly(3,4-ethylenedioxythiophene)hydroquinone. In the spectraldata for poly(3,4-ethylenedioxythiophene)hydroquinone, indications of the redox conversion forhydroquinone was attained. The data however showed that only an intermediate state in the hy-droquinone redox conversion was reached, and the signal to noise ratio was large enough to interferewith the data. The method could identify the redox product from redox conversion, but needs fur-ther development. Firstly, figuring out a way for the hydroquinone’s redox reaction to fully convertand not halting at an intermediate state. Secondly, figuring out a way to get a better signal from theIR measurements. By improving these factors, poly(3,4-ethylenedioxythiophene)hydroquinone’sredox reaction can be investigated more precisely, and thereafter can the interactions between thepolymer backbone and pendant groups be investigated.

Contents

1 Introduction 41.1 Electrochemical polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1.1 Conducting polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.1.2 Redox polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.1.3 Conducting redox polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Electrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.1 Electrochemical cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.2 Cyclic Voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2.3 Chronocoulometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.2.4 Electrochemical polymerisation . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3 IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.4 In situ electro IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61.5 Quinone’s redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.6 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Experimental 82.1 Preparing the electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.1 Gold coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.2 Platinum coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2 Polymerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.1 Polypyrrole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.2 Polypyrrole hydroquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.3 Polypyrrole(2,5-Dimethoxy-1,4-benzoquinone) . . . . . . . . . . . . . . . . . 92.2.4 Poly(3,4-ethylenedioxythiophene) . . . . . . . . . . . . . . . . . . . . . . . . 92.2.5 Poly(3,4-ethylenedioxythiophene) hydroquinone . . . . . . . . . . . . . . . . 9

2.3 IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.4 In situ electro IR spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4.1 Polypyrrole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4.2 Polypyrrole(2,5-Dimethoxy-1,4-benzoquinone) . . . . . . . . . . . . . . . . . 102.4.3 Monolayer of hydroquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4.4 Polypyrrole hydroquinone . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.4.5 Poly(3,4-ethylenedioxythiophene) . . . . . . . . . . . . . . . . . . . . . . . . 102.4.6 Poly(3,4-ethylenedioxythiophene) hydroquinone . . . . . . . . . . . . . . . . 10

3 Results and discussion 113.1 Electrode preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2 Polymerisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.3 IR measurements on reference pellets . . . . . . . . . . . . . . . . . . . . . . . . . . 143.4 In situ electro IR measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

3.4.1 Subtraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.4.2 Normalisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.4.3 Absorbance versus Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.5 Improving the method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

4 Conclusion 21

5 Acknowledgements 22

2

Abbreviations

BQ Benzoquinone

CE Counter electrode

CP Conducting polymer

CRP Conducting redox polymers

CV Cyclic voltammetry

HQ Hydroquinone

IR Infrared

PEDOT Poly(3,4-ethylenedioxythiophene)

PEDOTHQ Poly(3,4-ethylenedioxythiophene) hydroquinone

PPy Polypyrrole

PPyDMQ Polypyrrole(2,5-Dimethoxy-1,4-benzoquinone)

PPyHQ Polypyrrole hydroquinone

Q Quinone

RE Reference electrode

RP Redox polymer

WE Working electrode

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1 Introduction

The demand of energy is growing quicker than ever before and new innovative methods to produceand store energy is needed.[1] Most batteries used for electric energy storage today are metalbased, and some of the materials necessary for producing these metal based batteries are minedin unstable parts of the world.[2] Finding new ways to produce batteries without relying on thesemining processes while also turning to greener chemistry would be optimal for the electric energystorage field. New promising materials that could be used for electric energy storage are polymerbased, which means the materials needed could be synthesised in a lab anywhere in the worldinstead of being mined. An interesting kind of polymer that could be used for electric energystorage is a conducting redox polymer (CRP), which is a conducting polymer (CP) with redoxactive pendant groups[3]. This kind of polymer has not yet been studied extensively for batteryapplications[3] and could find great use in the electric energy storage field.

In order to use CRPs, a greater understanding of them is needed, especially the interactionsbetween the pendant groups and the polymer backbone is not well understood[3] and needs to beresearched further. Some polymers of interest as polymer backbone are polypyrrole (PPy) andpoly(3,4-ethylenedioxythiophene) (PEDOT), which are CPs. An interesting redox polymer (RP)that could be used as pendant group in a CRP is hydroquinone (HQ).

To reach a greater understanding of the CRP’s electrochemical properties and the interactionbetween the polymer backbone and pendant group, new methods of analysis are needed. In orderto analyse interactions on this scale, an analytical method capable of registering the changeshappening during redox conversion is needed. Infrared (IR) spectroscopy could be used, since itis a versatile method that can be used on almost any kind of sample.[4] Since the CRP would beused as electric energy storage, it would undergo redox reactions, which then leads to the othermethod that can be used, i.e., electrochemistry. The interactions occurring when the backboneand pendant groups shift between reduced and oxidised states are of interest. In order to getmeasurements on both states, a method called in situ electro IR spectroscopy can be used, whichcombines electrochemistry with IR spectroscopy. Then measurements on the reduced and oxidisedstate can be conducted and the respective spectral data can be compared to each other.

One interaction between pendant groups and the backbone that appears during redox conversionfor previously tested CRPs is a loss of conductivity in the polymer backbone during the pendantgroups redox reaction.[5] The polymers of interest as backbone, PPy and PEDOT, form polarons1

and bipolarons2 when they are oxidised[3, 6], which makes the polymers’ conductivity higher. HQis the pendant group of interest, which oxidises into benzoquinone (BQ). A loss in conductivityin the polymer has been noticed during the oxidation process[5], i.e., the polymer backbone losesconductivity. This might be caused by the backbone twisting, leading to localisation of the polaronsand bipolarons on the polymer backbone.

1.1 Electrochemical polymers

1.1.1 Conducting polymers

CPs are polymers with chains of conjugated unsaturated carbon bonds.[7] CPs are non-conductivein their neutral state but can be made conductive by oxidising them (p-doping) or reducing them(n-doping).[8] When CPs are doped they can reach conductivities on the same level as metals.[3, 8]The CPs’ doping ability is what enables it to be used in electric energy storage. The doping processof CPs used throughout this report involves polarons and bipolarons, the more the CP gets doped,the more polarons and bipolarons will form, leading to a higher charge over the polymer chain.This raise in charge over the polymer will require the doping process to have higher potentials tocontinue, since all the charge stored on the polymer chain will repel each other. That is why theCP successively gets doped over a broad potential window.

The problem with using CP in electric energy storage is partly that they do not have a highcharge capacity due to the polarons and bipolarons covers a few moieties each. The second partbeing that the charge will be stored on the polymer chain as long as the applied potential is overthe onset potential3, instead of all charge being stored around one fixed potential.[3]

1Polaron: A positive charge on the polymer distributed over a few moieties with an unpaired electron.[3]2Bipolaron: Two positive charges on the polymer distributed over a few moieties with no unpaired electrons.[3]3Onset potential: The potential where redox reaction products form at specific conditions.[9]

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1.1.2 Redox polymers

RPs are polymers with redox sites that can be reduced and oxidised.[10] Charge can be stored inthese kind of polymers and the whole redox reaction happens around a fixed potential.[3]

The problem with RPs are that the conductivity only happens via electron hopping betweenredox sites. This process only happens when there exists both reduced and oxidised forms of thepolymer, i.e., only near the fixed potential where the redox reaction occurs.[3] This kind of electrontransport is also distance dependant and is therefore inefficient over long distances.

1.1.3 Conducting redox polymers

CRPs are made up of a CP backbone with redox active pendant groups, giving it properties ofboth a CP and a RP.[3] By combining a RP and a CP the advantage of both can be realised. Theconductivity can be kept at a metallic conductivity level, and the redox active pendant groups willnot be as soluble since the molecular weight increases with the pendant groups attached to the CPbackbone. The CRP will also get a higher charge capacity in comparison to the parent CP due tothe redox active pendant groups.

1.2 Electrochemistry

1.2.1 Electrochemical cell

Electrochemical measurements and depositions throughout this report are done in an electrochem-ical cell. The cell consists of a working electrode (WE), a reference electrode (RE) and a counterelectrode (CE) lowered into an electrolyte. For a schematic drawing of the electrochemical cell seefigure 1. The WE is the electrode that is being studied, i.e., where the polymerisation and redoxreactions occur. A RE is used since its potential is well known and stable, and can therefore beused as a reference point to the reaction occurring at the WE; a common RE is Ag/Ag+. The CEis used to direct the current past the RE. This is done by having a big surface and low resistanceon the CE; usually a Pt CE is used.

Figure 1: Schematic drawing of electrochemical cell.

1.2.2 Cyclic Voltammetry

Cyclic Voltammetry (CV) is an electrochemical method which cycles the potential in a set range,while measuring the current.[11] A cyclic voltammogram is generated after the potential cyclingand shows the current versus potential.

The method is a useful tool when an overview of the researched substance’s electrochemicalproperties are of interest. The cyclic voltammogram can be used to determine redox potentials andidentify where the oxidised and reduced forms of the substance are stable. CV can also determine ifthe substance is affected by cycling the applied potential on the substance, e.g., losing conductivityover time.

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1.2.3 Chronocoulometry

Chronocoulometry is an electrochemical method where the amount of charge passing the electro-chemical cell is measured at a constant potential for a set time.[12] A time versus current diagramis generated after the measurement where the area under the curve is the amount of charge thathave passed through the cell.

1.2.4 Electrochemical polymerisation

Electrochemical polymerisation is done in an electrochemical cell with a WE, RE and CE wherethe polymerisation happens on the WE’s surface. The three electrodes are lowered into a monomersolution consisting of the relevant monomer together with a salt diluted in an aprotic solvent. Thepolymerisation takes place when a voltage is put on the electrochemical cell, i.e., the monomersstart reacting and form a polymer film on the WE’s surface.[13]

1.3 IR spectroscopy

IR spectroscopy is a very useful technique and can be used on almost any type of sample. Thetechnique analyses the vibrations of atoms in molecules by letting IR light pass through the sampleand then measuring the absorption of the IR light at a specific energy. The absorption peak thatappears in the generated spectra from the sample absorption corresponds to the vibration frequencyof a part of the sample.[4]

There are limitations of IR spectroscopy when it comes to measurements on CRPs during redoxconversion though. The absorbance caused by the CRP’s redox processes is of interest during themeasurements. If the polymer is exposed to new surroundings in between measurements, then theredox state could be affected. Therefore ex situ methods are not preferable, since there would beunreliable factors influencing the measurements.

1.4 In situ electro IR spectroscopy

Electrochemistry can be used to gain an understanding of a system’s redox reaction, which is doneby measuring the current or charge in the electrochemical cell. This means that a change in cur-rent can be noted in, e.g., a cyclic voltammogram while the potential is being cycled. However,what reaction is causing the change can not be detected with only electrochemistry. Thereforea complementary method is used, such as IR spectroscopy, to get an understanding of what re-action is causing the change in current.[14] When combining electrochemical and spectroscopicmeasurements an in situ4 observation of the substance can be made.[14]

In situ electro IR spectroscopy is used with either the reduced or oxidised state of the sampleas background and then the measurement is done on the opposite state. This will give a signal inthe spectrum seen as a peak, due to the oxidised and reduced states most likely absorb at differentenergies. The change can be seen because the vibrational frequencies for the sample will changewhen an electron is removed by oxidation from an orbital. The intensity of the peak is proportionalto the amount of molecules absorbing at the relevant energy,[15] resulting in a lower intensity peakif the energy is changed by oxidation.

In this report only external reflectance is used, since it has more advantages than attenuatedtotal reflectance (ATR)/internal reflectance. When using external reflectance a weaker signalwill be attained than for ATR, but the whole polymer layer will be analysed instead of only thesurface or part of the bulk, as in ATR. The electrochemistry is also easier to handle with externalreflectance, and finding IR glasses which are not redox active are easier to find.

The in situ IR measurements done throughout this report should all be considered as qualitativemeasurements. This means the relative difference between reduced and oxidised state of the speciescan be seen in the spectra, but not how much of the species has been converted. The developmentof the in situ electro IR method are based on earlier work done on monolayers for HQ in [16] andthe use of IR spectroscopy on conducting polymers in [17].

4In situ: The species of interest is produced in the electrochemical cell during the measurement.

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1.5 Quinone’s redox reactions

HQ and BQ are part of organic cyclic compounds called quinones (Q), which have two carbonylgroups on a ring structure or as in the case of HQ, which is a benzene ring with two alcoholgroups in 1,4 position. The redox reaction for HQ to BQ and back are quite complicated with twoelectrons and two protons involved.[3] The redox reaction can take a number of ways from HQ toBQ as seen in figure 2.

Figure 2: Quinone’s redox process including intermediates. Reprinted from [3] with permissionfrom the author. ©2014 Christoffer Karlsson.

1.6 Aim

The aim of the project was to develop a method to investigate CRPs’ redox active pendant groups’redox reactions, by adapting in situ electro IR spectroscopy to the system. The method should beused in further research of the CRPs and be a tool to help explaining the interactions occurring inthe polymer during redox reactions.

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2 Experimental

2.1 Preparing the electrode

A carbon electrode was used as the base for every WE used in the experiments. The desiredmaterials were deposited on the electrode’s surface.

2.1.1 Gold coating

A gold surface was deposited on a carbon electrode at constant potential (Init E = 0.5V, Run Time= 500s) at 60 °C. The electrochemical cell was made up of the carbon as WE, Ag/AgCl as REand Pt wire as CE. The electrolyte was made up of 100mM AuCl3, 100mM NaNO3 and 100mMphosphate buffer in a water solution. The electrolyte was cleared from air with nitrogen gas beforethe deposition and stirred during the deposition.

The deposition was done in two 500 seconds parts were the WE was polished in between withalumina (0.3 μm). After the deposition the electrode was polished with alumina (0.3 μm and 0.05μm) and thoroughly washed with water.

2.1.2 Platinum coating

A platinum surface was deposited electrochemically on a carbon electrode. The deposition wasdone with an Ag/AgCl RE and a Pt CE. The electrolyte consisted of 5mM H2PtCl6 and 0.1MHCl in a water based solution.[18] The deposition was done in room temperature and deoxigenisedwith nitrogen gas, the electrolyte was stirred during deposition. The deposition was first done viaCV to get an overview of the platinum’s electrochemical behaviour. By slowly expanding the cyclerange (0.05V per run) to more negative potentials, it was concluded that deposition at -0.3V fora short while and then at -0.2V longer would lead to a good deposition of Pt on the WE. Thedeposition at -0.3V was necessary since the Pt nucleation process is slow.[18] A multistep potentialmethod was used to deposit the Pt. The deposition was done in multiple parts, where each partconsisted of deposition at -0.3V for 5 seconds and -0.2V for 500 seconds and then polishing the Ptsurface with 0.05μm alumina.

Another way of coating the carbon electrode was by using platinum foil. The Pt foil was tapedto the carbon electrode so only the bottom part of the electrode could be in contact with theelectrolyte. The foil had already a reflective and smooth surface and no polishing was needed.

2.2 Polymerisation

All polymerisations were done via electrochemical polymerisation onto the WE. A polymer filmformed on the electrode and the thickness could be adjusted by changing the time the reactioncould take place. The polymerisation was done in an oxygen free environment, therefore wasnitrogen gas bubbled through every solution before the polymerisation. During the polymerisationthe electrochemical cell was filled with nitrogen gas.

2.2.1 Polypyrrole

Polypyrrole (PPy), figure 3a, was polymerised from a waterbased pyrrole solution (100mM pyrrole,0.1M NaCl). The polymerisation was done with CV (Init E = 0.1V, High E = 1.0V, Low E =-0.2V, Final E = 0.1V, Scan rate = 0.10V/s, 5 segments). PPy was polymerised on both a goldsurface and a platinum foil surface.

2.2.2 Polypyrrole hydroquinone

Chronocoulometry (Init E = 0V, Final E = 1.2V, t=300s) was used to polymerise polypyrrole hy-droquinone (PPyHQ), figure 3b, from a pyrrole hydroquinone (PyHQ) solution consisting of 10mMPyHQ in tetra(n-butyl)ammonium hexafluorophosphate/acetonitrile (TBAHFP/MeCN) solvent.A two compartment cell was used where the RE was an Ag wire in AgNO3, which was in a dif-ferent compartment filled with the same solvent as for the PyHQ solution, the CE consisted of Ptwire and was in the same compartment as the WE. The polymerisation was done on a gold surface.

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2.2.3 Polypyrrole(2,5-Dimethoxy-1,4-benzoquinone)

Polypyrrole(2,5-Dimethoxy-1,4-benzoquinone) (PPyDMQ), figure 3c, was polymerised at constantpotential (0.8V) for 30 seconds times two. The monomer solution consisted of 10mM pyrrole(2,5-Dimethoxy-1,4-benzoquinone) in TBAHFP/MeCN solvent. The polymerisation was done on agold surface.

2.2.4 Poly(3,4-ethylenedioxythiophene)

Polymerisation of poly(3,4-ethylenedioxythiophene) (PEDOT), figure 3d, was done with CV (InitE = -0.156V, High E = 1V, Low E = -1.7V, Final E = -0.156V, Scan rate = 0.1V/s, 10 seg-ments). The same two compartment setup as for PPyHQ was used. The electrolyte consisted ofa monomer solution (0.1M 3,4-ethylenedioxythiophene (EDOT)) in 0.1M LiClO4/MeCN. PEDOTwas polymerised on a gold surface.

2.2.5 Poly(3,4-ethylenedioxythiophene) hydroquinone

Polymerisation of Poly(3,4-ethylenedioxythiophene) hydroquinone (PEDOTHQ), figure 3e, wasdone in an acetonitrile solution (0.1M TBAHFP, 10mM EDOTHQ) via CV(Init E = 0.1V, HighE = 1.2V, Low E = -0.2V, Scan Rate = 0.02V/s, 10 segments) on a gold surface. The EDOTHQhad an ester linker in between the EDOT and HQ. After the polymerisation, the electrode waswashed with water and put in an electrochemical cell containing a water solution (0.1M NaCl,0.1M AcOH B(OH)3 Na2HPO4, pH 0, temperature = 60 °C). The polymer was then cycled withCV (Init E = 0.1V, High E = 0.8V, Low E = -0.2V, Final E = 0.1V, Scan Rate = 0.05V/s, 41segments) in order to make the polymer film ready to be used with in situ electro IR spectroscopy.The reason for the cycling will be discussed later in the report. PEDOTHQ was polymerised on agold surface. The monomer (EDOTHQ) is a new material synthesised at the Bio Medical Centerat Uppsala University.

(a) PPy (b) PPyHQ (c) PPyDMQ (d) PEDOT (e) PEDOTHQ

Figure 3: Molecular structures of polymers used throughout the report.

2.3 IR spectroscopy

IR spectroscopy was used to get reference spectra of the relevant substances, the references wereall pellets. The pellets made for IR spectroscopy were made of 3mg relevant substance and 200mgKBr powder grounded together and pressed to pellets under 8 tons of pressure for 15 seconds.Pellets made were Q KBr, HQ KBr, DMQ KBr, EDOTHQ KBr and PEDOTHQ KBr. Since thepellets never had to undergo redox reactions, they were not pressed against the IR window in theIR cell, but instead put in a pellet holder and had IR light go through the pellet. All the pellets’spectra were done with air as background spectrum.

2.4 In situ electro IR spectroscopy

All in situ electro IR spectroscopy measurements were done in an IR chamber made of teflon witha CaF2 IR glass in the bottom. The chamber also worked as an electrochemical cell where the CEand RE were lowered into the chamber and the WE pressed against the bottom of the chamber(i.e., the CaF2 glass), a schematic drawing of the WE pressed against the IR glass with the IRlight’s path is seen in figure 4. All the polymers used in the IR chamber were first cycled to findthe oxidation and reduction peaks, and then the IR measurements were done at multiple potentialsteps. The IR light used in the measurements was non-polarised.

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Figure 4: The IR light’s path during in situ electro IR measurements.

2.4.1 Polypyrrole

The PPy coated electrode was lowered into a 1M NaCl solution together with a Pt CE and aAg/AgCl RE. The polymer was characterised in the NaCl solution with CV in the potentialwindow between the redox reactions for water (-0.7V - 0.4V) and stopped at 0V. The backgroundfor the IR spectroscopy was done at 0V and the measurement at -0.7V, then the other way around.64 scans were done per measurement with a resolution of 4cm–1.

2.4.2 Polypyrrole(2,5-Dimethoxy-1,4-benzoquinone)

PPyDMQ was measured in a 1M NaCl solution at pH 2. The polymer was measured between-0.4V and 0.4V with a 0.1V step. 256 scans were made with a resolution of 4 cm–1. A Ag/AgClreference and a Pt CE was used. The IR spectrum at -0.4V was used as a background for the restof the potential steps.

2.4.3 Monolayer of hydroquinone

A monolayer of HQ (attached to a carbon chain with a sulfur end group) was adsorbed on a goldsurface and then lowered into the IR chamber. A Ag/AgCl reference, Pt CE and 1M NaCl solutionat pH 2 were used. The monolayer was cycled between 0V and 0.7V with CV for characterisation.The IR measurement was done at 0.7V with -0.1V as background. 256 scans with a resolution of8cm–1 were used for the measurement.

2.4.4 Polypyrrole hydroquinone

PPyHQ was measured in a 1M NaCl solution at pH 2 with an Ag/AgCl RE and a Pt wire CE. Thepolymer was measured at 0.5V with -0.4V as background and the other way around. 256 scanswith a resolution of 4cm–1 were used during both the background scan and the measurement.

2.4.5 Poly(3,4-ethylenedioxythiophene)

PEDOT was measured in a 1M NaCl solution at pH 2 with Ag/AgCl as RE and Pt wire as CE. TheIR measurement was done at 0.6V with -0.4V as background. The measurement and backgroundscans used 64 scans with resolution of 4cm–1.

2.4.6 Poly(3,4-ethylenedioxythiophene) hydroquinone

PEDOTHQ was measured in a 1M NaCl solution at pH 2 with Ag/AgCl as RE and Pt wire asCE. The measurements were stepped from -0.2V (as background) up to 0.8V with 0.1V/step. 128scans were used during the measurement with a resolution of 8cm–1 for both background andmeasurements.

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3 Results and discussion

The polymer giving the most promising results throughout the development was PEDOTHQ andtherefore the results of PEDOTHQ will be shown throughout the report. Many of the triedpolymers gave next to nothing during the IR measurements, as seen in figure 5, and will thereforenot be shown in the report.

wavenumber(cm-1)

1000150020002500300035004000

Absorb

ance

×10-3

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

3

(a) PPyHQ

wavenumber(cm-1)

1000150020002500300035004000

absorb

ance

×10-3

-2

0

2

4

6

8

10

12

(b) PEDOTHQ

Figure 5: Spectrum for PPyHQ compared with spectrum for PEDOTHQ.

3.1 Electrode preparation

The coating of the electrode was of great importance, since a good reflective surface was neededfor the in situ electro IR measurements to give any useful data. The deposition done by constantpotential can be followed in figure 6. The curve is not smooth, which is probably due to the goldions being used up locally and the stirring not being constant. Therefore the amount of currentpassing the electrode changes constantly. The current decrease is probably affected by the goldsurface getting deposited on the electrode.

time(s)

0 50 100 150 200 250 300 350 400 450 500

Curr

ent(

A)

-0.0205

-0.02

-0.0195

-0.019

-0.0185

-0.018

-0.0175

-0.017

-0.0165

-0.016

Figure 6: Graph of gold deposition at constant potential.

The gold deposition gave a rough red layer on top of the gold surface. The red layer couldbe removed by polishing and the reflective gold surface that could be used for IR measurementswas attained, see figure 7a. The gold surface was not completely flat after the polishing, whichlowered the signal from the IR measurements. This was probably caused by the fact that thecarbon electrodes had a small inward bend.

The platinum deposition did not work as well as the gold deposition. The same behaviour asfor the gold deposition was observed but on a smaller scale. A rough Pt surface formed but onlyparts of the electrode were coated with Pt, see figure 7b. When the surface was polished to get a

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more reflective surface, the Pt fell off. The IR signal for the Pt electrode was lower than for thegold surface.

The use of platinum foil was a different approach and the electrode became fragile as in thefoil easily broke or dented, see figure 7c. In theory the foil would be of great use, since it’s surfaceis already very reflective and a greater signal could be attained from the IR spectroscopy. The Ptfoil was unfortunately impractical to use in the IR chamber, since the foil was pressed against theIR glass and would often break, leading to a lower unstable signal.

The gold deposition is preferred since the most reflective and robust surface is attainable. Ifthe surface was even smoother a greater signal could be attained in the IR measurements. Thiscould be done by polishing the carbon WE before the gold deposition. The carbon WE’s surfacecould be further analysed with a scanning electron microscope or profilometer in order to get aclearer picture of what the surface looks like.

(a) Gold deposition (b) Platinum deposition (c) Platinum foil

Figure 7: Surface preparations of electrode.

3.2 Polymerisation

The polymerisation process for all the different polymers gave varying results. When using onlya CP in the monomer solution (PPy and PEDOT), the polymerisation progressed fine and apolymer film form on the Au surface. Only PPy and PPyHQ were polymerised on Pt foil in orderto compare the signal strength from the IR machine between Au and Pt foil as reflective surfacewith a polymer on top of it. As mentioned above the Pt foil was fragile and the signal from the IRmachine varied greatly and never reached a stable signal, leading to the use of Au for every othermeasurement.

The polymerisation process for the CRPs went mostly fine. PPyDMQ polymerised quickly onthe gold surface and a homogeneous film could be seen. PPyHQ was slower to polymerise on agold surface, but still a thin homogeneous layer formed. Even though the PPyHQ was slower topolymerise, it was preferred over PPyDMQ, since a thin layer gave greater signal when doing theIR measurements. The polymerisation of PEDOTHQ was first tried with LiClO4 as a counterion, but then the polymer fell of the gold surface when reducing the polymer. CV was tried atsomewhat higher potentials but then there was no polymerisation at all. The counter ion may havehad an impact on the process and was therefore changed to TBAHFP. The change of the counterion worked and the PEDOTHQ polymerisation progressed and formed a film on the electrode.The polymerisation process for PEDOTHQ can be seen in figure 8. The addition to the polymerlayer is indicated by the build up of the negative current in the 0.2V to -0.2V window. Thepolymerisation seemed to happen during the first cycle and then degraded in later cycles. In thelater cycles a decrease in current can be seen, indicating a small degradation occurring. For futurepolymerisations a lower top potential (i.e., lower than 1.2V) or only one cycle could be used toavoid the degradation. The other polymers had similar cyclic voltammograms as PEDOTHQ.

12

Potential(V)

-0.2 0 0.2 0.4 0.6 0.8 1 1.2

Curr

ent(

A)

×10-4

-4

-2

0

2

4

6

8

10

12

14

Figure 8: Cyclic voltammogram of the polymerisation process for PEDOTHQ. The cycles go fromblue to red via green.

All the polymer films were characterised after the polymerisation. Characterisation was donewith CV to find out where the oxidation and reduction peaks were and also to conclude that theright polymer had formed with nothing contaminating the polymer, giving irregular results.

PPyHQ and PPyDMQ characterisation in 1M NaCl solution at pH equal to 0 followed theirrespective previous cycles and both the oxidation and reduction peaks were seen. The polymersseemed reversible and stable during the potential cycling, but their peaks were drawn out over alarge potential window. Optimally a clear oxidation och reduction peak would be seen from thependant groups. The oxidation peaks might have been thinner if the characterisation was done ina solution containing a buffer.

PEDOTHQ’s characterisation, in a 0.1M NaCl, 0.1M AcOH B(OH)3 Na2HPO4 solution at60°C, had an interesting behaviour, see figure 9. The longer the polymer was cycled, the more theoxidation peak at 0.5V grew. The oxidation peak around 0.7V grew during the first cycles andthen later shrunk during later cycles. The reduction peak at 0.4V grew with every cycle. This mayindicate that the polymer film on the electrode is packed in such a way that the whole polymerfilm can not interact with the electrolyte at the potential where HQ’s redox reactions occur. Thiswould mean only part of the polymer is getting oxidised during the first cycles at the 0.5V peakand therefore the polymer has to be cycled to get it to work fully.

Potential (V)

-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Curr

ent (A

)

×10-4

-8

-6

-4

-2

0

2

4

6

8

10

12

Figure 9: Characterisation with CV of PEDOTHQ in buffer solution. Cycles go from blue to redvia green.

Before the in situ IR measurements, PEDOTHQ was characterised again but in the solutionused in the IR chamber consisting of 1M NaCl at pH 2. Then the second oxidation peak was notas notable, and the first oxidation peak was broader, as seen in figure 10. The broader peak madeit more difficult to get good spectra over the oxidation peak, because the identification of where

13

the HQ oxidation began and ended was vague. The broader peaks in the solution without a bufferindicates that the buffer has a significant role during the oxidation and reduction, however thebuffer solution can not be used in the IR cell since it might affect the IR measurements. For futuremeasurements it would be preferable to measure over the reduction peak instead of the oxidationpeak, due to the reduction peak being smaller and clearer.

Potential(V)

-0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Absro

bance

×10-4

-4

-3

-2

-1

0

1

2

3

4

5

Figure 10: Characterisation with CV of PEDOTHQ in non buffer solution.

3.3 IR measurements on reference pellets

The IR measurements on the pellets gave a notion of what peaks were of interest when latermeasuring the CRPs. Since PEDOTHQ’s in situ electro IR results were the clearest, the pelletsfor HQ, Q, EDOTHQ and PEDOTHQ are relevant and the spectra can be seen in figure 11b and11a. The peaks throughout the report will be assigned with the help of table 1.

Table 1: Functional groups’ IR absorbtion.[19, 20, 21]

Functional group Wavenumber Type IntensityO-H 910 - 950 bend mediumC-O 1050 - 1150 stretch strongRing 1300 stretch weakC-H 1350 - 1480 bend variableC=C (Aromatic) 1400 - 1600 stretch medium-weak, multipleC-C (Aromatic) 1400 - 1500, 1585 - 1600 stretch mediumC=O 5 1670 - 1820 stretch strongC-H (Aromatic) 3000 - 3100 stretch mediumO-H 3200 - 3600 stretch (H-bonded) strong, broad

In 11a the first peak of interest lies at 3250cm–1 and only exists for HQ which indicates thatthis peak is the O-H vibration[22]. This coincide with O-H stretch in table 1. Another interestingpeak is at 1650cm–1 which is very clear for BQ and non existing for HQ, which leads to theconclusion of the peak being a carbonyl vibration, this also conform with table 1. These are themost interesting and relevant vibrations that could be seen during the PEDOTHQ redox reaction.The remaining major peaks for HQ are identified as, 1450:C-C stretch, and 1150:C-O stretch,corresponding to respective functional group in table 1. The peaks at 800 cm–1 and 650 cm–1 aremore complicated to assign, since they are in the fingerprint region, but they are probably causedby ring vibrations. The assignment for HQ have earlier been done by M. Kubinyi et al.[22]. Theremaining major peaks for BQ can be identified as 1300:Ring stretch, 1100:C-O stretch, while 880and 400 are in the fingerprint region and more complicated to assign, probably some a kind ofring vibrations. The peaks have earlier been assigned by T. Dunn and A. Francis[23]. The spectrafor EDOTHQ and PEDOTHQ differ quite a bit from the HQ pellet. The O-H stretch peak at

5Conjugation in the species lowers the wavenumber were it absorbs.

14

3250cm–1 is visible for both EDOTHQ and PEDOTHQ but is broader for EDOTHQ. The amountof peaks observed in the 500-1800cm–1 window have increased in comparison to HQ in figure 11a,presumably corresponding to PEDOT’s ring vibrations. The spectra are not very clear, but canstill be used as a reference to the IR measurements conducted on the CRPs.

wavenumber(cm-1)

05001000150020002500300035004000

Absorb

ance

0

0.5

1

1.5

BQ

HQ

(a) BQ pellet’s and HQ pellet’s spectra.

wavenumber(cm-1)

05001000150020002500300035004000

Absorb

ance

0

0.5

1

1.5

2

2.5

EDOTHQ

PEDOTHQ

(b) EDOTHQ and PEDOTHQ pellets’ spectra.

Figure 11: Reference spectra for PEDOTHQ.

3.4 In situ electro IR measurements

The spectra attained for the PEDOTHQ measurements can be seen in figure 12, where a spectrumwas generated at every potential step made in the -0.1V to 0.8V potential window, with a back-ground at -0.2V. The overall trend for the spectrum seems to be an increase in absorbance at higherpotentials, but in the 800-1600cm–1 interval it is hard to get any useful information from just thesespectra. The HQ oxidation is what is of interest and should be investigated, but something totake into consideration is that not only HQ will oxidise but also PEDOT. Therefore some of thepeaks may be caused by PEDOT and not by BQ or an intermediate in the HQ redox conversiondescribed in figure 2. The spectra in figure 12 seem to differ quite a bit from the reference pelletsin figure 11. A likely explanation is that the in situ electro IR measurements on PEDOTHQ wereconducted in a solution and not in a dry state as the pellets. Another possible explanation is thatit could be an indication of the BQ state not being reached during the measurements, resulting inthe background spectrum and measurement spectra cancelling out each other due to having peaksat same wavenumber and intensity.

In order to get data that is easier to analyse, the spectra can be modified by subtraction andnormalisation. After the modifications, the interesting peaks of the spectra can be identified andanalysed further with potential versus absorbance graphs.

The spectra for the other investigated polymers and monolayer yielded no useful spectral dataand could not be analysed.

15

wavenumber(cm-1)

1000150020002500300035004000

absorb

ance

×10-3

-2

0

2

4

6

8

10

12

-0.1V

0V

0.1V

0.2V

0.3V

0.4V

0.5V

0.6V

0.7V

0.8V

Figure 12: Spectra for PEDOTHQ in the potential window -0.1V to 0.8V with -0.2V as background.

3.4.1 Subtraction

The spectral data attained from in situ electro IR measurements most likely contains peaks fromboth oxidised PEDOT and oxidised HQ, i.e., BQ or an intermediate state seen in figure 2. ThePEDOT will start to oxidise at lower potentials than HQ and will already be oxidised when the HQoxidation peak is reached around 0.2V (see figure 10). This means the PEDOT spectral data canbe avoided to some degree, if the spectrum right after en HQ peak is subtracted by the spectrumright before the HQ peak, since mostly the HQ oxidation will occur there. As seen in figure 13,the spectrum of 0.7V subtracted by 0.3V shrinks the peaks corresponding to PEDOT more than0.8V subtracted by -0.1V. In the same figure, a PEDOT polymer exposed to the same conditionsas PEDOTHQ is used to more easily identify PEDOT’s spectral data. In the 800-1800cm–1 range,both positive and negative peaks could be seen. This indicated that a conversion had occurred inthe sample during the in situ IR measurement.

wavenumber(cm-1)

1000150020002500300035004000

Ab

so

rba

nce

×10-3

-2

0

2

4

6

8

10

12

14

16

18PEDOTHQ 0.7V sub. 0.3V

PEDOTHQ 0.8V sub. -0.1V

PEDOT 0.6V

Figure 13: Subtraction spectra for PEDOTHQ.

3.4.2 Normalisation

In the spectral data of PEDOTHQ in figure 13, indications of oxidised PEDOT are still visible overthe Q peak after the subtractions. Since it is the HQ conversion that is of interest, all peaks causedby PEDOT are interfering. The PEDOT peaks can be normalised so they will not show up onthe spectral data. This is done by first identifying a peak which is caused by a PEDOT vibration.The absorbance at the identified peak then divides the absorbance for every data point on thespectrum. Doing this for all the spectra at different potentials will change them in such a way thatthey will go through absorbance equal to 1 at the identified PEDOT peak. By doing this and then

16

subtracting a spectrum from another spectrum, results in the identified peak getting an absorbanceequal to 0, and the peak can be avoided while the HQ peaks are still intact. Normalisation andsubtraction were done at the potentials close to the HQ oxidation peak, i.e., spectrum at 0.7Vsubtracted with spectrum at 0.3V, see figure 14.

wavenumber(cm-1)

1000150020002500300035004000

No

rma

lise

d a

bso

rba

nce

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0.3V

0.7V

Subtracted 0.7V - 0.3V

Normalisation point

Figure 14: Normalised PEDOTHQ spectra at 0.3V, 0.7V and subtraction of the two.

After the normalisation and subtraction of all spectra with the 0.1V spectrum, the peaks around800 - 1800 cm–1 are of interest since the spectral line have both negative and positive peaks in thearea, see figure 15. The rest of the spectrum seems to not yield any peaks that could indicate a Qredox reaction. A notable observation is that there seems to be no carbonyl peak in the generatedspectrum, since carbonyls usually have a big intensity peak. This further indicates that the BQform was never reached during the measurements. The peak at 2360 cm–1 in figure 14 is caused byCO2 and is most likely not of interest. The CO2 peak can in future measurements be avoided bydoing the measurements with a nitrogen flow in the IR cell. In case the CO2 peak is partly causedby the redox reaction, it could be concluded with a nitrogen flow through the cell since then theCO2 could not come from the surroundings.

wavenumber(cm-1)

800900100011001200130014001500160017001800

Subtr

acte

d a

bsorb

ance

-0.4

-0.2

0

0.2

0.1V-0.1V

0.2V-0.1V

0.3V-0.1V

0.4V-0.1V

0.5V-0.1V

0.6V-0.1V

0.7V-0.1V

0.8V-0.1V

Figure 15: Spectra for PEDOTHQ in the 800-1800cm–1 range normalised at the same wavenumberas in figure 14. The spectra are subtracted by the spectrum for 0.1V.

During the HQ conversion, it is expected that the OH groups on HQ will lose their intensitywhen they are converted into a new Q product. The new Q will likely have a shifted vibrationfrequency, which would be seen as the peaks’ intensity getting higher somewhere else on thespectrum. This means that everywhere on the spectra where the curve takes on a derivativeform, it might be an indication of the Q redox conversion. Since the background was done onthe HQ state, all the negative peaks seen in figure 15 must belong to HQ and the positive peaks

17

belong to the product of the oxidation. The assignments of the peaks were not obvious. Thevibrations belonging to HQ that could be assigned were the peak around 1480 cm–1 as benzenering stretch[24] and 1150 cm–1 as C-OH stretch[25]. All negative peaks could not be identifiedwith HQ and therefore PEDOT may still be influencing the measurement. The spectra’s signalalso have a high signal to noise ratio that is influencing the spectrum and aggravate the identifyingof peaks.

3.4.3 Absorbance versus Potential

The spectra could be further analysed by identifying the peaks of interest (i.e., those that possiblycorresponded to the HQ conversion) and thereafter plot the absorbance versus potentials at thesewavenumbers. The behaviour for every peak could then be followed over the whole potentialwindow. By doing this, a behaviour similar to the Nernst equation(1) could hopefully be seenwith a bump over the HQ oxidation peak in the otherwise straight line, indicating a conversionoccurring in the polymer.

E = E0 –RT

nFln{red}{ox}

(1)

Spectra only subtracted and not normalised was investigated first, and the interesting wavenum-bers were identified. The chosen wavenumbers were 1670, 1543, 1415, 1337, 1275, 1210 and 1075cm–1 from figure 13. The absorbance versus potential curves for these wavenumbers were notstraight lines, but instead had a decreasing slope, as seen in figure 16. Indications of bumps couldbe seen in the curves for 1210 and 1075 cm–1, but since the curves had a decreasing slope, anyconclusion drawn from figure 16 was ambiguous. The slope decreasing could be caused by thePEDOT oxidising processes.

Potential(V)

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Su

btr

acte

d a

bso

rba

nce

×10-3

-2

-1

0

1

2

3

4

5

6

1670

1543

1415

1337

1275

1210

1075

Figure 16: Absorbance versus potential curves at selected wavenumbers for PEDOTHQ spectrumat 0.7V subtracted with spectrum at 0.3V.

An absorbance versus potential plot for normalised and subtracted spectra was also done, inorder to try to avoid the decreasing slopes. This time the absorbance versus potential plot weredone for wavenumbers 1540, 1477, 1408, 1280, 1219 and 1146cm–1 from figure 15. The curvesgenerated had a straighter character this time, as seen in figure 17. Bumps could be seen in everycurve and were compared with the cyclic voltammogram in figure 10. The bumps had to occurat the same potential as the HQ oxidation started for it to be an indication of a Q peak in thespectrum. The HQ oxidation peak started around 0.2V and ended around 0.65V, which also thebump in curve for 1219 cm–1 seemed to do indicating a Q redox product forming in that interval.

18

Potential(V)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Su

btr

acte

d A

bso

rba

nce

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

1540

1477

1408

1280

1219

1146

Figure 17: Absorbance versus potential curves at selected wavenumbers for spectra normalised asin figure 14 and every spectrum is subtracted with the spectrum at 0.1V.

None of the peaks had a certain compliance with the Nernst behaviour, partly due to thefact that only a few data points for the potentials were used. Even though the behaviour of thecurves did not necessarily conform with the Nernst behaviour, the curves could be compared to theincrease of charge for PEDOTHQ at each potential step, and conclusions could be further drawnfrom there. The integration of an oxidation segment for the CV measurement of PEDOTHQ, seenin figure 18, was normalised with the slope from 0.1V to 0.2V. This normalisation was done sincethe increase in charge before the HQ oxidation peak was caused by oxidising PEDOT, which werenot of interest, and also because the absorbance versus potential graphs were normalised. Theincrease of the charge on the integration plot of PEDOTHQ seemed to follow the same trend asthe absorbance change, i.e., the slope in figure 18 increased during the HQ oxidation peak, andthe curves had bumps in them in figure 17 during the HQ oxidation peak, further indicating thata reaction was taking place.

Potential(V)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Charg

e(C

)

×10-3

0

0.5

1

1.5

2

2.5

3

3.5

4

Figure 18: Integration of oxidation sweep for PEDOTHQ cyclic voltammogram normalised withthe slope between 0V and 0.1V.

19

3.5 Improving the method

The results attained from the measurements are for the moment not trailblazing, but could befurther improved by a few factors. Firstly, to get a stronger signal from the in situ electro IRmeasurements, the angle at which the IR light hits the IR glass could be optimised. Instead ofusing a flat glass (figure 4), an angled glass could be used, as in figure 19. This would make theIR light hit the IR glass perpendicular instead of angled and less light would be reflected of theglass before interacting with the sample. Secondly, to increase the signal further, polarisation oflight could be used since the Qs seems to only absorb p-polarised light up to +1.2V.[16] Thirdly,the carbon electrodes used to deposit a reflective surface on could be flatter and smoother, leadingto a better deposition and a more reflective surface. Fourthly, coating the backside of the IR glasswith a reflective surface could yield a greater signal, due to multiple bounces through the analysedspecies could be achieved. Finally, more potential steps for the absorbance versus potential plotscould be used, in order to investigate the Nernst behaviour more thoroughly.

The electrochemistry also needs to be further investigated when occurring under pressure,getting the whole polymer film to convert from HQ to BQ is essential to get as clear IR spectra aspossible.

Figure 19: New form of IR glass.

20

4 Conclusion

The BQ form of the Q redox process could not yet be identified with in situ electro IR spectroscopy.The development of the method still yielded positive results with regard to that, what is assumedto be, an intermediate state in the HQ to BQ conversion could be seen in the spectral data. Themethod needs further development, for instance figuring out a way to measure only the BQ insteadof an intermediate state of the HQ to BQ conversion. Another important aspect to improve is theIR glass, the signal for the in situ electro IR measurements was so small that the signal to noiseratio was presumably influencing the data. The glass could be improved by changing the form ofit so that the IR light enters the glass perpendicular. The use of PEDOTHQ on a gold surfacegave the most promising results and further research on the polymer could potentially bring a newcompeting material to the electric energy storage field.

21

5 Acknowledgements

Firstly, I would like to show my utmost appreciation and gratitude to my mentor Martin Sjodin,for all the explanations and discussions we have had, but above all for showing me the true side ofresearch. Secondly, thanks to Mia Sterby, for discussions, book lending and laughs. It would nothave been as fun to carry out the project without you. Thirdly, thanks to the whole Nanotechnologyand Functional Materials crew. You have shown me the dynamic a great research group possesses,and have further guided me towards the research field. And finally, my partner Linn Haggqvistfor slogging with the proof reading even though she was only given minimal time to do it. I havesince learnt a valuable lesson in sentence structure, thank you.

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