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aMawson Institute, University of South Aust

E-mail: pejman.hojatitalemi@unisa.edu.au

23147bDepartment of Chemistry, University of Bat

† Electronic supplementary information (detailed XPS analysis results and addisamples are available. See DOI: 10.1039/c

Cite this: RSC Adv., 2014, 4, 9819

Received 28th October 2013Accepted 23rd January 2014

DOI: 10.1039/c3ra46167j

www.rsc.org/advances

This journal is © The Royal Society of C

Metal-free oxygen reduction electrodes based onthin PEDOT films with high electrocatalyticactivity†

Philip P. Cottis,ab Drew Evans,a Manrico Fabretto,a Samuel Pering,ab Peter Murphya

and Pejman Hojati-Talemi*a

Oxygen reduction reaction (ORR) electrodes play an important role in the development of new battery and

fuel cell technologies. However most of the presented electrode materials cannot provide the efficiency

required for these applications, and/or they are based on economically unfavourable noble metals. In

this article, multi-layer electrodes of high conductivity PEDOT prepared by vacuum vapour phase

polymerization in the presence of a PEG–PPG–PEG triblock copolymer are used to fabricate a metal-

free oxygen reduction electrode. After optimizing the main production parameters, measuring ORR

performance of the metal-free PEDOT based electrodes confirms that they have the ability to deliver a

stable electrocatalytic activity. A chemical treatment is also used for further enhancing the

electrocatalytic activity of these electrodes. Depending on pH, the electrocatalytic activity of these

treated electrodes reaches a higher or the same level as platinum based electrodes.

1. Introduction

The global movement towards sustainable power suggests thatpower storage devices such as batteries and fuel cells will have agreater impact on the development of electronic devices andpower generation technologies. Although some of the currenttechnologies may help to provide clean energy, many are stillexpensive due to their reliance on noble metal catalysts. The air-electrode is one of the main components of fuel cells andmetal–air batteries and is essential for the reduction of O2. It isoen regarded as the most complicated component of thesetechnologies.1 To date, many materials have been put forwardas candidates which display the electrocatalytic activity requiredfor an air-electrode, including materials such as: carbon nano-tubes,2 nitrogen-doped graphene3 and platinum–gold nano-particles.4 Despite these studies, no air-electrode has yet beendeveloped that has forgone the use of noble metals somewherein its architecture, and can provide enough efficiency to warrantcomparison to platinum (Pt), which is the current benchmark insuch systems. Pt in its own right, besides being expensive, hasknown disadvantages including CO poisoning5,6 and the driphenomenon, wherein particles build up over time through

ralia, Mawson Lakes, SA 5092, Australia.

; Fax: +61-8-830-25639; Tel: +61-8-830-

h, Bath, BA2 7AY, UK

ESI) available: The experimental setup,tional data on ORR performance of3ra46167j

hemistry 2014

diffusion causing a loss of performance in fuel cells.7 Conse-quently, a high level of research has been carried out in an effortto reduce the amount of Pt used in oxygen reduction reaction(ORR) electrodes.8–10 As a result the development of noble metal-free air-electrodes has gained signicant recent attention.1,11–14

Intrinsically conducting polymers have recently been reportedas effective electrocatalytic materials for oxygen and protonreduction.11,12 Recent studies of poly(3,4-ethylenedioxythiophene)(PEDOT) have shown the polymer to produce high rates of oxygenreduction,12–14 but this has not been achieved without the use of ametal (gold).13 Although it is claimed that role of this gold underlayer is to provide a conduction path,13 it is shown that thismetallic under layer plays a signicant role in increasing theelectrocatalytic activity of PEDOT electrodes.15 Thus developing anew-generation of PEDOT with high conductivity and electro-catalytic activity can be seen as a feasible approach towards thefabrication of efficient and economical ORR electrodes.

Previous work carried out by our group using the in situvacuum-VPP technique16 to synthesize PEDOT has resulted inthe fabrication of PEDOT with metal-like properties.17 Thepolymer, which grows with the aid of a templating glycol tri-block copolymer, PEG–PPG–PEG, within the oxidant layer, hasopened up the possibility of using PEDOT in modern elec-tronics as a viable alternative to expensive inorganic materials.The high conductivity, metal-like property, of this PEDOT–PEG–PPG–PEG alloy may provide the means to achieving higherelectrocatalytic activity, and remove the need for a gold sub-layer. In this paper, we present a technique for developing air-electrodes based on the use of this highly active PEDOT. Byoptimizing the fabrication parameters, an economically

RSC Adv., 2014, 4, 9819–9824 | 9819

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favourable, stable, and high performance alternative to the useof Pt in oxygen reduction electrodes is proposed. The methodpresented herein requires no additional adhesion or conduc-tion layer (typically a noble metal) unlike previous examples.13

Due to production of OH� ions in the ORR process, theconcentration of H+ ions in solution strongly affects theperformance of the cell. As a method to improve ORR perfor-mance further, it was hypothesized that attaching moleculesthat can attract protons to the surface could potentially enhancethe ORR performance. Most amines or amides, depending onpH, can adsorb a proton and form a quaternary amine or amide,and then release it upon a change in the concentration ofprotons in solution. It was previously shown than amines oramides are able to be chemisorbed to PEDOT,18 thus by usingthis mechanism, we demonstrate how chemical modication ofPEDOT electrodes can lead to even greater efficiency comparedto the unmodied PEDOT.

2. Experimental

To prepare the air-electrodes, a hydrophobic PVDF membrane(220 or 450 nm pores, Durapore, Millipore) was saturated withparaffin wax dissolved in a minimal volume of n-hexane andthen dried at room temperature (i.e. lling the membrane withthe wax). PEDOT was polymerized using oxidant solutionsbased on 2900 or 5800 Da PEG–PPG–PEG triblock copolymers(Sigma-Aldrich) in accordance with methods reported previ-ously.17,19 Aer polymerization of the PEDOT, the membraneswere soaked in n-hexane for 15 minutes to remove the wax,dried at 70 �C for 30 min, and then soaked in ethanol for30 minutes to remove any oxidant and residual monomer,before being dried again at 70 �C for 30 min. Our previousstudies have shown that washing PEDOT sample with ethanol issufficient for removing residual Fe2+ and Fe3+ ions fromsamples.20,21 To achieve PEDOT multi-layered electrodes, onlythe ethanol rinse and drying was carried out. Aer this, thevacuum-VPP process was repeated using fresh oxidant lmscoated onto the underlying PEDOT layer. Once the desirednumber of PEDOT layers was achieved, the wax was removedfrom the membrane using the n-hexane soak. Further treatmentof the PEDOT air-electrodes with urea was performed by soak-ing the electrodes in a solution of 1 M urea in water for 30 min.For comparative purposes Pt electrodes were prepared bysputtering a 40 nm Pt layer onto both 220 nm and 450 nmmembranes.

The efficiency of the electrodes was examined in a three-electrode cell (Fig. S1†) using a Voltalab PGZ 100. A platinumwire and an Ag/AgCl electrode (3 M KCl with saturated AgCl)were used as the counter and reference electrodes respectively.Copper strips were used as connectors between the workingelectrode and source; these were attached to the working elec-trode using conductive silver paste and taped to the cell havinga working area of 1 cm2. Chrono-amperometry was performedfor 30 minutes at potentials ranging from 0.3 V to �0.9 V (in100 mV steps) with the values reported for the steady stateconversion currents. All electrodes were analysed in phosphatebuffer solutions of pH 7, with the best performing electrode also

9820 | RSC Adv., 2014, 4, 9819–9824

examined at pH 1 and pH 13. To ensure that airborne oxygen iseffectively involved in the process and able to diffuse throughthe membrane and the measured current is not only due todissolved oxygen, an inert gas was blown on the window duringthe measurement. This resulted in a rapid drop in currentwhich recovered upon cessation of the application of the inertgas.

Scanning Electron Microscopy (SEM) was performed usingan FEI Quanta 400 microscope under an operating voltage of 20kV. The thickness of the Pt and PEDOT layers were estimated bymeasuring the lm thickness (DektakXT, Bruker, 25 mm tip,0.03 g normal loading) when deposited onto glass microscopeslides using the same protocol.

3. Results and discussion

On the path to developing high efficiency air-electrodes fromPEDOT, it is necessary to explore how some of the fabricationparameters inuence PEDOT's ability in the electrocatalysis ofoxygen reduction. Firstly the effect of the triblock copolymeremployed in the vacuum-VPP process, the amount of depositedPEDOT (number of layers), and the pore size of the supportingmembrane were examined. These three aspects were seen as themost inuential parameters on the ORR performance of theelectrodes.

It has been previously shown that the triblock copolymer(PEG–PPG–PEG) can have a structure directing effect on PEDOT,allowing for the nal PEDOT morphology to be manipulated,and thus leading to PEDOT with superior electrical and elec-trochemical properties.17,19,20 The combination of morphology-control, along with changes in the electrical property, was a keyto addressing the performance of the PEDOT air-electrode. Inorder to evaluate these concomitant effects, electrodes based ontwo different (and previously studied17,20) triblock copolymerswere fabricated by depositing three layers of PEDOT onto thetwo different PVDF (polyvinylidene diuoride) membranes(220 nm and 450 nm pores sizes). The PVDF membraneprovides an inert porous support material upon which thePEDOT thin lms were deposited. This allows air to passthrough the membrane allowing access to the active PEDOT–electrolyte interface. Due to its hydrophobic nature it also servesto prevent the aqueous electrolyte from passing through andleak from the cell. The ORR performances of cells were evalu-ated in a three-electrode cell (Fig. S1†). Fig. 1 shows the steady-state measurement of conversion current versus appliedpotential for the different PEDOT electrodes in a PBS solution atpH 7. It can be seen that, independent of the pore size, thePEDOT electrode based on the 5800 Da triblock copolymerexhibits greater ORR performance. At applied potentials morenegative than �300 mV the difference in conversion currentbetween the two PEDOT variants increases, while at voltagesmore positive than �300 mV there appears to be only a slightoffset in the measured current. This is mainly because, thispotential is not enough for oxygen reduction to occur, and thecurrent before this point is simply because of ionic conductionof solution and probably some proton reduction. Thus in theabsence of any true oxygen reduction, both the 2900 Da and

This journal is © The Royal Society of Chemistry 2014

Fig. 1 Steady-state conversion current density versus appliedpotential for three layers of PEDOT based on oxidant solutions con-taining on 5800 Da or 2900 Da triblock copolymers on 220 nm and450 nm membranes.

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5800 Da based PEDOT samples produce similar results in thispotential range. The increase in current that is observed atpotentials more negative than 300 mV for the 5800 Da copolymer,compared to the 2900 Da copolymer based PEDOT is an indica-tion of a higher ORR rate. Determining the doping level of thesetwo PEDOT samples (Fig. S2 and Table S1†) using X-ray photo-electron spectroscopy (XPS) reveals a higher doping level (33.14%)for PEDOT based on the 5800 Da copolymer compared to the onebased on the 2900 Da copolymer (28.61%). As the doping sites areresponsible for catalysing the ORR process by becoming oxidizedor reduced, a higher doping level as well as the higher conduc-tivity (>3000 S cm�1 compared to �1600 S cm�1) are the mostlikely the candidates for the higher electrocatalytic activityexhibited by this sample. An essential part of the air-electrode isthe three-phase interface that exists between the electrolyte,electrocatalytic material and air. This interface is necessary for theORR to proceed.13 Fig. 2a shows that the vacuum-VPP depositedPEDOT was able to forming a thin continuous layer over theporous membrane. It can also be noted that the high surface areaof the under-laying porous membrane is not reected in thestructure of PEDOT layer. However as both water and oxygen canpenetrate into the lm to some extent, a large volume of PEDOTcan be used for the ORR process, while in case of most metalliccatalysts the reaction is limited to the surface. Due to the

Fig. 2 SEM images of PVDF membrane (450 nm) coated with (a)PEDOT and with (b) 50 nm of platinum.

This journal is © The Royal Society of Chemistry 2014

electrocatalytic activity of the PEDOT throughout the electrode,one may reasonably expect that depositing a thicker PEDOT layerwould result in higher ORR performance. However, very thicklayers limit the diffusion of oxygen or electrolyte through thePEDOT resulting in a poor three-phase interface, and loss of ORRperformance. Thus it is important to nd the amount of PEDOTrequired for optimum ORR performance. To test this, PEDOTlms with different thicknesses were deposited on the 450 nmpore size membrane, and the conversion current versus appliedpotential for these samples were measured (Fig. 3a). Consideringthat the VPP technique is not able to deposit thick lms ofPEDOT, the only way for fabricating considerably thicker lmswas through depositingmulti-layers of PEDOT, thus the thicknessvariations were achieved by depositing 1 to 9 individual PEDOTlayers (60 min. for each vacuum-VPP layer). Increasing thenumber of PEDOT layers resulted in an increase in the measuredcurrent, up to approximately 4 layers. The addition of more layers,up to 7, yielded a current plateau, aer which the conversioncurrent decreased. The electrode comprising 4 individual PEDOTlayers was dened as being the most efficient, because of the factthat the conversion current per unit deposited mass was thegreatest for this multi-layered electrode. In order to convert thenumber of layers into an effective thickness, PEDOT depositedonto a glass slide under the same vacuum-VPP conditions wasmeasured by prolometery. The thickness of the PEDOT layer,formed aer one hour in the vacuum-VPP chamber was found tobe 210� 14 nm, with the 4 layer structure resulting in a thicknessof 915 � 17 nm. The approximate 75 nm of additional thickness(i.e. 4 � 210 ¼ 840 � 56 compared to 915 � 17) can be attributedto small deviations in thickness measurement or the actualthickness variation of each of the 4 deposited layers as they arestacked on top of each other. However, considering that thesesamples are prepared by collapsing one layer on top of theprevious layer, the possibility that gaps or bubbles may be trappedbetween these layers cannot be discounted.

The results presented in Fig. 1 show that there is little to noinuence of the membrane pore size on the measured conver-sion currents. This indicates that with the amount of polymerdeposited in this experiment, both of the pore sizes were largeenough to allow a sufficient amount of oxygen to transport tothe PEDOT layer for unrestricted ORR to occur. However,depositing multilayers of PEDOT on the 220 nm pore size PVDF

Fig. 3 (a) Conversion current density versus number of PEDOT layerson 450 nm membrane, at various potentials, the thickness of eachlayer is 210 � 14 nm, and (b) changes in pH versus time as a result ofoxygen reduction reaction.

RSC Adv., 2014, 4, 9819–9824 | 9821

Fig. 4 XPS survey spectra of PEDOT samples before (a) and after (b)urea treatment.

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membrane produced a peak in the electrode efficiency at 3deposited PEDOT layers, however the ORR performanceobserved was well below that of the 4 layered PEDOT on the450 nm membrane (Fig. S3†). Therefore based on the resultspresented herein it was concluded that the optimum ORRelectrodes were produced using 4 PEDOT layers in conjunctionwith the 5800 Da triblock copolymer and the membrane with anaverage pore size of 450 nm. Aer optimising these parameters,the question arises if this system can further benet from a goldconduction layer. In order to test this, 30 nm of gold wasdeposited on the membrane followed by deposition of 4 layersPEDOT. Evaluation of ORR performance (Fig. S4†) shows thataddition of a gold under-layer in this case has a detrimentaleffect on ORR performance of electrodes. This hypothesized tobe caused by lling of the pores in the PVDF by gold, thusinhibiting air from reaching the electrocatalytic PEDOT.

It has been shown recently that vapour phase polymerisedPEDOT is able to electrocatalyse ORR through a direct fourelectron path way producing OH� ions.21 However such exper-iments require a different conguration of the electrodes, andexclude the use of the breathable electrodes for determining thereaction mechanism. Thus, the reaction products were inferredby monitoring the pH as a function of time (pH changes causedby the release of OH� ions over time in a non-buffered solutionis monitored and presented in Fig. 3b). The observed rise in pH(production of OH�) as a result of oxygen reduction is inagreement with the reported works on VPP–PEDOT,21 conrm-ing a 4 electron mechanism for ORR in our system. Theproduced OH� ions are then neutralized by H+ ions in thesolution, which in turn drives the reaction forward and incontrast, under basic conditions the excess OH� ions reduce theORR efficiency. This pH dependency of ORR process has beenshown before.13 On the other hand, due to the application ofORR electrodes in a variety of technologies that require opera-tion at different pH levels, it is important to obtain a high levelof ORR performance at acidic, basic and neutral pH. In order tomake use of this pH dependency and improve ORR perfor-mance, attempts were made to form a buffer-like layer on thesurface of the PEDOT electrodes to maintain a high concen-tration of H+ ions on the surface. The nitrogen atom in theamines or amides has the ability to accept an additional protonand convert into a quaternary amine or amide. This protonuptake may increase the pH, but should be balanced by thesurrounding buffer solution and the system reaches an equi-librium state between, the quaternary amine/amid and H3O

+

ions. Recent studies18 have shown that through a nucleophilicattack amines or amides such as urea can attach to the surfaceof PEDOT. In this proposed method, while the ORR reaction isrunning, generation of OH� momentarily shis this equilib-rium state, thus the quaternary amine releases the proton tobalance this shi, resulting in a lowering of the pH in thevicinity of the electrode and removing the reaction product bythe formation of water. However, as the system is operating in abuffer solution, the amine/amides bound on the surface acceptanother proton from the buffer solution, thus returning to thequaternary form and maintain the equilibrium state. Thus, inpractice, the surface the lone pair electrons of nitrogen atoms

9822 | RSC Adv., 2014, 4, 9819–9824

may dynamically attract protons from the solution and increasethe proton concentration in the vicinity of the PEDOT electrodesurface.

In order to test ORR performance of our samples and theeffect of this chemical treatment on them, steady state conver-sion current density of samples versus applied potential weremeasured and compared to a platinum electrode deposited onthe same membrane (prepared by magnetron sputtering). SEMimages of the platinum coated membrane (Fig. 2b) shows auniform coating over the porous structure of the membrane,which results in a platinum electrode with an effective surfacearea much higher than that of the PEDOT coated electrode.Urea was used as the chemical treatment for the samples. Ureawas chosen because its low pKa indicates that it can easilyrelease the absorbed protons. It was also known to form a stablebond with PEDOT.18 However, many other amines, polyaminesor amides may also be suitable for this proposed method, whichis the subject of future studies. Urea treatment was performedby soaking PEDOT electrodes in a 1 M urea solution in water for30 min followed by washing the electrodes with water andethanol. Fig. 4 shows the X-ray photo electron spectroscopy(XPS) survey spectra of PEDOT electrodes before and aer ureatreatment. Appearance of the N1s peak in the spectrum of theurea treated samples conrms attachment of urea to the PEDOTsurface.

In order to quantify the effect of this treatment on perfor-mance of electrodes, the steady state conversion current densityversus applied potential at various pH levels (Fig. 5a–c) havebeen measured. It should be noted that these data points pre-sented in these graphs are the current value aer 30 min ofapplying a constant potential. Thus, considering the smallamount of urea adsorbed on the surface, any possible sidereaction (i.e. oxidation or reduction of urea), would consumethe adsorbed urea in the rst few seconds of applying thevoltage. Thus the measured current values are a safe andaccurate indication of ORR reaction rate. Fig. 5a–c show thatdespite the higher surface area and similar mass loading(�0.1 mg cm�2) of the Pt catalyst, in both acidic and neutral

This journal is © The Royal Society of Chemistry 2014

Fig. 5 Steady-state conversion current density versus appliedpotential for PEDOT (dotted line), urea treated PEDOT (solid line) andPt electrode (dashed line) at (a) pH 1, (b) pH 7, and (c) pH 13.

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conditions the PEDOT electrode outperformed Pt, while theperformance at basic pH was comparable. Thus, not only doesthe urea treatment further enhance the performance at acidicand neutral pH, the ORR performance of these samples at highpH matches the performance of the Pt electrode. However, themagnitude of enhancement is more signicant in acidic elec-trolyte. This is most probably because urea in the basic pH

This journal is © The Royal Society of Chemistry 2014

mainly exists in its neutral form and cannot absorb a proton toform a quaternary ammonium ion. In the absence of thisprocess, urea cannot exhibit their positive effect on ORRperformance. The dependency of this positive effect to pH thusprovides supporting evidence that conrms the hypothesispresented here. Estimating the doping level of urea treatedPEDOT shows that the treatment reduces the doping level from33.14% to 29.94% (Fig. S1 and Table S1†). This chemicaltreatment also results in a loss in conductivity (i.e. from >3000to 1550 S cm�1). The observed enhanced electrocatalyticactivity, despite the lower doping level and electrical conduc-tivity, is a good indication of the effectiveness of this treatmentbased on themechanism proposed herein. Comparison of theseresults with other reported ORR electrodes based on PEDOT13,14

shows that despite the removal of the noble metal from theelectrode in this study (i.e. no adhesion layer or gold underlayer), the PEDOT electrodes prepared herein exhibited similaror higher ORR efficiency compared to those reports. Long termORR stability carried out at pH 7 for over 60 hours revealed nodecay in the measured conversion current density (Fig. S5†).

4. Conclusions

We have developed a highly active, PEDOT-based electrode forthe ORR process and optimized the amount of PEDOT neededfor this purpose. The metal-free, polymer electrode, exhibits astable oxygen reduction performance that outperformed Pt foracidic and neutral conditions, while being comparable to Pt inbasic media. The performance of this polymer electrode wasfurther enhanced by binding urea to the surface of PEDOT. Theurea treated electrodes exhibited better ORR performance thanplatinum in all pH levels examined.

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