Proton Electroinsertion in Self-Assembled Materials forNeutralization PseudocapacitorsTiago Facci, Wellington J. A. S. Gomes, Bruno Bravin, Diogenes M. Araujo, and Fritz Huguenin*

Departamento de Química, Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto − Universidade de Sao Paulo, 14040-901Ribeirao Preto, SP, Brazil

ABSTRACT: We propose novel pseudocapacitors that can store energy related to thepartial entropy change associated with proton concentration variations followingneutralization reactions. In this situation, it is possible to obtain electrochemical energyafter the complete charge/discharge cycle conducted in electrolytic solutions with differentproton concentrations. To this end, we prepared modified electrodes from phosphomo-lybdic acid (PMA), poly(3,4-ethylenedioxythiophene/poly(styrenesulfonate) (PEDOT-PSS), and polyallylamine (PAH) by the layer-by-layer (LbL) method and investigated theirelectrochemical behavior, aiming to use them in these neutralization pseudocapacitors. Weanalyzed the potentiodynamic profile of the current density at several scan rates, to evaluatethe reversibility of the proton electroinsertion process, which is crucial to maximum energystorage efficiency. On the basis of the proposed reaction mechanism and by usingfrequency-domain measurements and models, we determined rate constants at differentpotentials. Our results demonstrated that the conducting polymer affects the self-assembledmatrixes, ensuring that energy storage is high (22.5 kJ mol−1). The process involved neutralization of a hydrochloric acid solutionfrom pH = 1 to pH = 6, which corresponds to 40% of the neutralization enthalpy.

■ INTRODUCTIONElectricity supply with minimal environmental impact isessential for sustainable growth; this goal requires thedevelopment of new energy sources. In this context, researchershave concentrated their efforts on generating electrical workfrom nonutilized energy. One example is to convert solar, wind,geothermal, and biomass power into electricity. Anotherpossibility is to take advantage of the entropic changeassociated with mixing solutions.Since the earliest studies published by Pattle in 1954,1

researchers have proposed the use of various kinds oftechnologies to obtain renewable energy by mixing solutionswith different electrolytic concentrations. More specifically,scientists have worked on the mixing of fresh and salt water inhydroelectric pile,1 osmotic pump,2 dialytic battery,3 electro-chemical concentration cells,4 devices based on differences invapor pressure,5 and electrochemical double layer capacitor.6

Recently, Mantia et al. developed a “mixing entropy battery” inwhich they employed an ion sodium intercalation electrode(consisting of Na2Mn5O10) and an electrode that selectivelyinteracts with chloride ions (Ag/AgCl) at different NaClconcentrations in aqueous medium.7 In this situation, thebattery is charged in a diluted NaCl solution (river water),whereas the discharge step is performed in a more concentratedNaCl solution (seawater). After the charge/discharge cycle, theentropic change can produce a significant amount of electricalwork. For excess seawater, 2.5 kJ per liter of river water can beextracted to generate work.7

In this study, we developed an electrochemical system(designated neutralization pseudocapacitor) that can also storeenergy after the proton insertion/deinsertion cycle. This system

can extract energy before and after the treatment of acidindustrial waste, swimming pools, and sewage, for example. Theinsertion process occurs in acidic medium; deinsertion takesplace in neutral (or slightly acidic) medium. In this system,energy storage stems from the partial entropy change related tothe variation in the proton concentration of the electrolyticsolution. This entropy change is associated with theneutralization process, and energy can be stored with selectiveproton electroinsertion in the self-assembled materials inves-tigated here: electrodes modified with phosphomolybdic acid(PMA) , po l y ( 3 , 4 - e t h y l en ed i o x y t h i ophene/po l y -(styrenesulfonate) (PEDOT-PSS), and poly(allylamine chlor-ide) (PAH).We decided to work with these host matrixes because they

can function as pseudocapacitive materials,8−10 enabling fastproton electroinsertion. This is essential to abate irreversibilityand to guarantee minimal energy loss after the completedischarge/charge cycle. Moreover, the high conductivity ofproton and charge carriers (polaron, bipolaron, and/or freecarriers) in PMA and PEDOT,11−13 respectively, tends to lowerthe overpotential during the discharge/charge processes. Weused the layer-by-layer method (LbL) to reduce the solubilityof the electroactive components in aqueous medium;polyoxometalates and PEDOT−PSS establish electrostaticinteractions with PAH.13−15 We also selected the LbL methodbecause it allowed us to control electrode thickness andnanoarchitecture; such control helped to diminish diffusion and

Received: October 4, 2013Revised: December 15, 2013Published: December 18, 2013

Article

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ohmic overpotentials and promoted synergistic effects originat-ing from the intimate contact between PMA and PEDOT−PSS.16 Together, all of these factors contributed to betterperformance of the neutralization pseudocapacitor proposed inthis work.

■ EXPERIMENTAL SECTIONTo prepare the self-assembled films, PMA (H3[P(Mo3O10)4]·xH2O(Aldrich, code 221856), PAH (Aldrich, code 283215, average MW =15 000 g mol−1), and PEDOT−PSS (Aldrich, code 483095, 1.3 wt %dispersion in water − PEDOT 0.5 wt % and PSS 0.8 wt %,conductivity 1 S cm−1) were used as received.The experimental procedure adopted here to prepare the LbL films

(Scheme 1) was based on several literature papers.17,18 LbL films were

assembled onto a glass coated with fluorine-doped tin oxide (FTO-coated glass) obtained from Flexitec (Curitiba, Brazil), with sheetresistance ≤20 Ω. The layers were achieved via ionic attraction ofoppositely charged materials, by alternately immersing the FTOsubstrate in PMA (5 g L−1 and pH = 2) and PAH (1.6 g L−1 and pH =2) aqueous dispersions for 1 min. After each layer was deposited, thesubstrates were rinsed in HCl solution for 30 s (pH = 2). The alternateimmersion procedure was repeated 15 times, and the last layer of eachLbL film was dried under nitrogen flow. The resulting material will bedesignated PMA/PAH hereafter. The same procedure was carried outfor the assembly of PEDOT/PAH and PMA/PAH/PEDOT LbLfilms. In the first case, the FTO substrate was immersed in PEDOT−PSS aqueous dispersion (1.3 g L−1 and pH = 2), followed byimmersion in the PAH (1.6 g L−1 and pH = 2) aqueous dispersions.To assemble PMA/PAH/PEDOT LbL films, the FTO substrate wasimmersed for 1 min in each of the aqueous dispersions, in thefollowing sequence: PMA (5 g L−1 and pH = 2), PAH (1.6 g L−1 andpH = 2), PEDOT−PSS (1.3 g L−1 and pH = 2), and PAH (1.6 g L−1

and pH = 2). After deposition of each layer, the substrates were rinsedunder the same conditions described above. This immersion sequencewas repeated 15 times, and the last layer was also dried under nitrogenflow.Film thickness was analyzed by specular reflectance using the

Nanocalc 2000 program coupled with a single channel 2048 pixelCCD spectrophotometer having halogen lamp as light source. Thethickness values measured for the 15-bilayer PMA/PAH, PEDOT/PAH, and PMA/PAH/PEDOT LbL films were 106 ± 4, 120 ± 4, and108 ± 4 nm, respectively. The geometrical area of the films was 1 cm2.

The electrochemical experiments were carried out using an AutolabPGSTAT30 potentiostat/galvanostat. A platinum sheet with an area of10 cm2 and a silver wire were used as the counter electrode and thequasi-reference electrode, respectively. HCl solutions with different pHvalues (0.5, 1, and 2) were used as electrolytic solution. Moreover, toobserve the changes in peak potentials as a function of pH underconstant ionic strength, KOH was added to the HCl solution, toincrease pH from 0.5 to 2.5. For the experiment simulating theneutralization pseudocapacitor, 1.15 mL of CH3COOH in 200 mL ofHCl solution (pH = 1) was added; CaO then was subsequently addedto increase the pH. This procedure maintained pH at 6 as the PMA/PAH/PEDOT electrode was immersed in the electrolytic solution. Toavoid influence of the calcium ions on the electrode potential and tomaximize the potential change as a function of the pH of theelectrolytic solution, a Nafion layer (15 μm) was deposited on theelectrode surface. For these experiments, a silver wire was used as thecounter electrode. The ac electrochemical impedance spectroscopywas performed between 100 kHz and 1 mHz, using 5 mV of acamplitude.

■ RESULTS AND DISCUSSIONFigure 1 depicts the potentiodynamic profile of the currentdensity (j) for the (a) PMA/PAH, (b) PEDOT/PAH, and (c)PMA/PAH/PEDOT films in HCl (pH = 1) at a sweeping rate(ν) of 50, 100, 150, and 200 mV s−1. The cyclic voltammo-grams reveal the low irreversibility of the proton electro-insertion process in the host matrixes: the peak potentialsremain virtually unchanged, and the peak current varies linearlywith the scan rate. The PMA/PAH/PEDOT film affords thehighest current density and volumetric charge capacity, 20.3 Ccm−3 between 0.40 and −0.20 V for the second voltammetriccycle at 50 mV s−1. Meanwhile, the volumetric charge capacitiesfor the PMA/PAH and PEDOT/PAH films are 9.5 and 4.1 Ccm−3, respectively. These data suggest that the conductingpolymer improves the electrical connection between the PMAsites,10 which may be partially isolated in the presence of PAH.Figure 1a shows three redox pairs (I−II, III−IV, and V−VI

peaks) for the PMA/PAH film, which refer to the reactionspresented below (eqs 1, 2, and 3, respectively). The two-electron and two-proton transfer taking place in this electro-chemical system is due to the disproportionation reactionsinvolving odd reduction states I, III, and V of thepolyoxometalate.19 The average cathodic and anodic peakpotentials change by approximately 59 mV per pH unit in thecase of PMA-modified electrodes.19 As for the PMA/PAH film,these peak potentials change by approximately 70 mV per pHunit (with and without added KOH to maintain the ionicstrength constant) in the pH range from 0.5 to 2.5. Theseresults are important when proposing the reaction mechanismand employing PMA-based electrodes in neutralizationpseudocapacitors.

+ + ⇌− − + −PMo O 2e 2H H PMo Mo O12 403

2 10VI

2V

403

(1)

+ +

⇌

− − +

−

H PMo Mo O 2e 2H

H PMo Mo O2 10

VI2

V40

3

4 8VI

4V

403

(2)

+ +

⇌

− − +

−

H PMo Mo O 2e 2H

H PMo Mo O4 8

VI4

V40

3

6 6VI

6V

403

(3)

The electrochemical response of the PEDOT/PAH filmexhibits rectangular shape and broad reduction and oxidationvoltammetric waves at more positive and negative potentials,

Scheme 1. (a) Assembly of the LbL Films;a and (b)Simplified Picture of the First Adsorption Steps, ShowingMaterial Deposition on the Substrate

aSteps 1 and 3 represent adsorption of PMA or PEDOT and PAH onthe FTO substrate, respectively; steps 2 and 4 are washing procedures.The sequence shown in the figure was repeated several times toprepare each of the LbL films.

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respectively (Figure 1b), as observed in several other studies onelectrodes formed from PEDOT.20,21 Although the literaturementions that anions participate in the charge compensationmechanism taking place in PEDOT films,20 cations alsoparticipate in this process in the case of PEDOT−PSSfilms.21 Moreover, C−Cl covalent bond formation in thepolymeric chain does not account for chloride ion participationin the charge compensation mechanism.22 Hence, we cansuggest that the pseudocapacitive processes in the PEDOT/PAH film are associated with proton insertion, to compensatethe electronic charge (eq 4).

− + + ⇌ −− +[PEDOT PSS]Cl e H H[PEDOT PSS]Cl(4)

Figure 1c also evidences three redox pairs associated with eqs1−3 (I−II, III−IV, and V−VI peaks). In agreement with thevoltammetric peak separation of each redox pair, the PMA/PAH/PEDOT film is more reversible than the PMA/PAH filmat more positive potentials. At 50 mV s−1, the potentials relativeto peaks I and II appear at 0.20 and 0.22 V for the PMA/PAH/PEDOT film, respectively, and at 0.18 and 0.22 V for the PMA/

PAH film, respectively. On the other hand, the potentialscorresponding to peaks V and VI lie further apart for the PMA/PAH/PEDOT film (−0.16 and −0.12 V, respectively) ascompared to the PMA/PAH film (−0.14 V and −0.12 V,respectively). These features are important when one considersapplying these films in neutralization pseudocapacitors: a smalldifference between the peak potentials of the redox pairsindicates low energy loss after the discharge/charge cycle.Figure 2 displays the cyclic voltammograms of the (d) PMA/

PAH, (b) PEDOT/PAH, and (c) PMA/PAH/PEDOT films at

the 2nd and 1000th cycles, at 50 mV s−1. The charge associatedwith the insertion process in the PMA/PAH film remainsvirtually unchanged after 1000 voltammetric cycles. For thePEDOT/PAH and PMA/PAH/PEDOT films, the chargedecreases 9% and 11%, respectively. These results indicatethat these self-assembled materials are electrochemically stable.To investigate the electrochemical steps involved in these

processes and to determine the rate constants, we conductedfrequency-domain measurements. Figure 3 illustrates theNyquist diagrams for the (a) PMA/PAH, (b) PEDOT/PAH,and (c) PMA/PAH/PEDOT films at 0.2 V; the insets display

Figure 1. Cyclic voltammograms for the (a) PMA/PAH, (b) PEDOT/PAH, and (c) PMA/PAH/PEDOT films in HCl (pH = 1) at (○) 50,(●) 100, (□) 150, and (■) 200 mV s−1. The redox pairs I−II, III−IV,and V−VI are associated with eqs 1, 2, and 3, respectively.

Figure 2. Cyclic voltammograms of the (a) PMA/PAH, (b) PEDOT/PAH, and (c) PMA/PAH/PEDOT films in HCl (pH = 1) at the (−)2nd and (−−−) 1000th cycle. ν = 50 mV s−1.

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the Nyquist diagram at 0.0 and −0.2 V. The beginning of thesemicircle observed at high frequencies results from a chargetransfer resistance (Rct) associated with the proton electro-insertion in parallel with an electrical double layer capacitance(Cdl) at the electrolytic solution/electrode interface. Mean-while, the capacitive process detected at low frequencies stemsfrom a finite proton diffusion process in the host matrixes. Wedid not verify the semi-infinite diffusion, because theseelectrodes have small thickness and/or fast proton diffusionrate.On the basis of the reactions presented above (eqs 1−4), we

propose the following elementary steps for the electroreductionmechanism:

+ →+ − + −2H 2e 2(H , e ) (for the PMA sites)i i (5)

+ →+ − + −H e (H , e ) (for the PEDOT sites)i i (6)

→+ − + −(H , e ) (H , e )i s (7)

Equations 5 and 6 represent the proton transfer at theelectrolytic solution/electrode interface at the PMA andPEDOT sites in the host matrix, respectively, which generatesthe (H+, e−)i species. Equation 7 indicates the diffusion processof these injected protons and electrons ((H+, e−)i). Thesespecies move together, to maintain the electroneutrality, whichresults in motionless species (H+, e−)s close to the reducedelectroactive sites of the host matrix. On the basis of thisreaction mechanism, one can express the oscillating componentof the current density (i) as:23

α α = + i nFkbc nFV RT V nF nFV RT cexp( / ) exp( / )P,0 P,0

(8)

= − =i nFD c y(d /d )yP P 0 (9)

where n is the number of electrons involved in the reaction, F isthe Faraday constant, cP,0 is the proton concentration in thesteady state at the electrode surface (y = 0), α is the symmetryfactor, V is the interfacial potential, V is the perturbation of thepotential, cP,0 is the oscillating component of the protonconcentration at the electrode surface, and DP is the protondiffusion coefficient. Using eqs 8 and 9, we obtain eq 10, whichfits the impedance data in the Nyquist diagrams.

ωω

ω ω= +

++ +βΩZ R

R Zj R Z C

( )( )

1 ( ) ( ( ))ct D

ct D dl (10)

where ZD(ω) is the diffusion impedance. This equationincludes the β term, which accounts for frequency dispersionsassociated with surface roughness. To also consider thefrequency dispersion observed at low frequencies, we usedthe model of anomalous diffusion. In this case, the mean squaredisplacement of the diffusing proton follows a power lawdependence on time,24 indicating that the number of protonsdiffusing in the self-assembled materials is not conserved.Equation 11 can substitute the diffusion impedance ZD(ω), torepresent the proton finite diffusion.

ωω ω

= =γ γZi C

E qi

( )1

( ) )(d /d )

( )L (11)

where CL is the limiting low frequency capacitance andcorresponds to the injected charge as a function of theequilibrium potential change (dq/dE);25 γ is a dimensionlessparameter lower than 1. In accordance with the values of thecharge capacity determined by cyclic voltammetry, the PMA/PAH/PEDOT film has higher CL values (270, 390, and 270 μFcm−2 at −0.2, 0.0, and 0.2 V, respectively) than the PMA/PAHfilm (105, 165, and 125 μF cm−2 at −0.2, 0.0, and 0.2 V,respectively). On the basis of the charge transfer resistance, eqs12 and 13 furnish the k1 (for the PMA/PAH film) and k2 (forthe PMA/PAH/PEDOT film) rate constants, respectively.

α α=

′ =

R

RTF k c

RTF c k4 ( ) 4ct 2

1 P,02 2

P,0 1 (12)

α α α=

+ ′ =

≈

R

RTF c k k c

RTF c k

RTF c k( 4 )ct 2

P,0 2 1 P,02

P,0 ap22

P,0 2

(13)

Considering that the apparent constant kap2 (=k2 + 4k1′cP,0)does not change as a function of pH, it is possible to reduce thisrate constant to k2. The k1 (= cP,0k1′) and k2 rate constants are

Figure 3. Nyquist diagrams for the (a) PMA/PAH, (b) PEDOT/PAH, and (c) PMA/PAH/PEDOT films in HCl (pH = 1) at (●) 0.2V, (▲) 0.0 V, and (△)−0.2 V. (○) Plot of the theoretical impedancedata at 0.2 V.

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4.3 × 10−7 and 7.6 × 10−4 cm s−1 at 0.2 V, respectively. At 0.0V, k1 is 1.1 × 10−5 cm s−1, while k2 is 9.7 × 10−4 and 1.2 × 10−3

cm s−1 at 0.0 and −0.2 V, respectively. The Nyquist diagramsfor the PEDOT/PAH (at −0.2 V, 0.0, and 0.2 V) and thePMA/PAH films (at −0.2 V) do not display the semicircle athigh frequencies (only the capacitive behavior associated withthe finite diffusion), indicating high proton electroinsertionrate. These data agree with the different voltammetric profilesshown above, attesting to the high reversibility of the PMA/PAH film at more negative potentials and demonstrating thehigh proton electroinsertion transfer rate in the PMA/PAH/PEDOT film in the potential range used in this work.Considering that the PMA/PAH/PEDOT film exhibits

better electrochemical performance, we chose the electrodemodified with this film to simulate the neutralizationpseudocapacitors. Figure 4a depicts the cyclic voltammogram

of the PMA/PAH/PEDOT film in HCl at pH = 1 and pH = 6at 50 mV s−1. The three voltammetric peak currents I, III, andV shift from 0.20, 0.02, and −0.16 V to −0.07, −0.28, and−0.50 V when the pH increases from 1 to 6, respectively.Meanwhile, the peak currents II, IV, and VI shift from 0.22,0.04, and −0.13 V to −0.05, −0.25, and −0.44 V, respectively.These voltammograms reveal significant potential change as afunction of pH; the PMA/PAH/PEDOT film remains highlyreversible at pH = 6. Hence, it is possible to conduct thereduction and the oxidation process at pH = 1 and 6,respectively, to extract energy from this electrochemical system.

Figure 4b contains the chronopotentiometric curvesrecorded for the same electrochemical system mentionedabove. These curves refer to the reduction (at pH = 1) and theoxidation (at pH = 6) of the PMA/PAH/PEDOT film at 5 μAcm−2. Regarding a neutralization pseudocapacitor consisting ofPMA/PAH/PEDOT and an electrode that selectively interactswith chloride ions (a pseudoreference silver electrode was usedin the three-electrode electrochemical cell, simulating a secondelectrode with overpotential tending to zero in thepseudocapacitor), the gray area corresponds to the energy(27.9 μJ) obtained after the discharge/charge cycle. Normal-izing by the amount of electroinserted proton, the experimentalconditions used in this work afford energy storage of 22.5 kJmol−1, which corresponds to 40% of the enthalpy changeinvolved in the neutralization reaction per mole of proton, asdetermined on the basis of the formation enthalpies of thereagents and products.26

■ CONCLUSIONSSelf-assembled films consisting of PMA, PAH, and PEDOTdisplay high reversibility and good charge capacity; moreover,their potential depends on the pH of the electrolytic solution.These properties are essential when one wishes to convertpartial entropic energy to electrochemical energy in neutraliza-tion pseudocapacitors. Electrochemical impedance spectrosco-py and kinetic models helped to determine the rate constants.These constants showed that the presence of PEDOT−PSS inthe self-assembled matrix containing PMA and PAH ensureshigh proton electroinsertion rate for the potential windowinvestigated here; consequently, the electrochemical process ishighly reversible. Using an electrochemical cell involving thePMA/PAH/PEDOT film, which has the highest chargecapacity among the investigated self-assembled films, and achloride ion selective electrode (simulating the secondelectrode with minimum overpotential), we were able tostore energy corresponding to 40% of the enthalpy change(normalized by the same amount of electro-inserted proton)associated with the neutralization reaction. These pseudocapa-citors can generate electrical work from neutralization reactionsand are potentially applicable in the treatment of industrialwaste and sewage before their discharge into the environment;they can also be used to treat swimming pools and waterreservoirs, among other applications.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: fritz@ffclrp.usp.br.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe are grateful to FAPESP (projects 2012/21629-1 and 2011/21545-0), NanoBiotec-Brasil (CAPES), and INCT-ADAPTA/FAPEAM/CNPq (573976/2008-2) for financial support. Weare also grateful to Prof. Rogeria Rocha Goncalves (DQ/FFCLRP/USP) for the availability of her laboratory.

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Figure 4. (a) Cyclic voltammograms and (b) chronopotentiometriccurves for the PMA/PAH/PEDOT film in HCl at (●) pH = 1 and(○) pH = 6. ν = 50 mV s−1 and j = 5 μA cm−2. The gray areacorresponds to the electrochemical energy obtained after thedischarge/charge cycle.

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