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Neutralization Pseudocapacitors: an Acid-Base Machine William G. Morais, Wellington J. A. Santos Gomes, and Fritz Huguenin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04480 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 26, 2016

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The Journal of Physical Chemistry

Neutralization Pseudocapacitors: an Acid-Base Machine

William G. Morais, Wellington J. A. S. Gomes, and Fritz Huguenin*

Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto – Universidade de São Paulo, 14040-901 Ribeirão Preto (SP), Brazil

*e-mail: [email protected] Phone: +55 16 3315 4862

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ABSTRACT This study proposes a thermodynamic machine that operates between acid and basic reservoirs in four stages. Two of these stages are buffered isothermal steps. The other two stages constitute an open system and allow the passage of acid and base. The machine consists of a neutralization pseudocapacitor that, after a full cycle, carries out work generated from partial change in entropy associated with change in the hydrogen potential after the neutralization process. Thermodynamic formalism is presented under reversible stages. This presentation enables determination of the maximum efficiency, related to the difference between the hydrogen potential of the acid reservoir and of the resulting solution after neutralization in the machine. Hence, the hydrogen potential scale can be defined as a function of the efficiency of the reversible acid-base machine regardless of the electrochemical cell composition. Electroactive thin films formed from phosphomolybdic acid and poly(3,4ethylenedioxythiophene) have been investigated as proof of concept in electrolytic solutions at several pH values; their efficiency was close to the efficiency predicted by the thermodynamic approach. Therefore, this model allows one to estimate the maximum energy harvesting of neutralization pseudocapacitors and financial return for the treatment of acid wastewater, contributing to sustainable growth.

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INTRODUCTION

Pollution generated by fossil fuel combustion has motivated introduction of renewable energy sources into the energy matrix.1 Therefore, improving the efficiency of energy conversion from wind, solar, geothermal, and biomass power is necessary. Another possibility is to explore the energetic potential of ionic gradient and convert it into electrical power.2,3 Recently, researchers have used appropriate electrochemical cells to deliver work in the presence of varying concentrations of electrolytic solution.4-6

Capacitive mixing (CAPIMIX) developed by Broglioli is based on the electric double-layer capacitor technology, where water of different salinity, like river and sea water, can be used to harvest energy.4 La Mantia et al. developed Mixing Entropy Batteries in which faradaic reactions involve sodium and chloride ions as well as distinct selective electrodes.5 Energy in the order of 1.6 kJ and 2.5 kJ per liter of fresh water per cycle has been harvested with CAPIMIX and Mixing Entropy Batteries, respectively. Energy harvesting during neutralization of acid solutions is also possible.7,8 This has encouraged acid wastewater treatment and contributed to sustainable growth—wastewater treatment preserves the environment and is profitable for the companies that generate the effluents.7-9 In this context, our group has recently developed electrochemical systems called “neutralization pseudocapacitors”. These systems have been able to harvest up to 40% of neutralization enthalpy for pH values between 2 and 6.7

In the present study, we propose a thermodynamic approach to these electrochemical systems. In this approach, maximum work arises from the flow of matter between an acid reservoir and a basic reservoir. By extrapolating the 3 ACS Paragon Plus Environment


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experimental procedures to ideal conditions in order to harvest maximum energy, these neutralization pseudocapacitors can operate under a thermodynamic cycle with two reversible steps. The efficiency of the machine will depend on the pH difference between the acid reservoir and the solution entering the machine. Determining the hydrogen potential without considering any specific electrochemical system will be possible. The work done after one thermodynamic cycle will also allow for measurement of the hydrogen potential on a purely mechanical or electrical basis.

Electrodes

consisting

of

phosphomolybdic

acid

(PMA),

poly(3,4-

ethylenedioxythiophene/poly(styrenesulfonate) (PEDOT-PSS), and poly(allylamine chloride) (PAH) have been investigated in acidic and neutral (or less acidic) medium and have been found to display suitable properties for use in acid-base machine; e.g., low practical irreversibility, selectivity for proton electroinsertion/electrodeinsertion, and high proton and polaron conductivity.10-15 We have used the Layer-by-Layer (LbL) method to grow thin self-assembled films, promoting low diffusion overpotential and ohmic drop as well as synergistic effects associated with the intimate contact between their components.16 Considering a full three-electrode electrochemical cell where the negative electrode is PMA/PAH/PEDOT-PSS and the positive electrode has overpotential tending to zero,7 we have simulated an acid-base machine with efficiency close to the maximum efficiency provided by the thermodynamic formalism shown below. Finally, the harvesting energy and the efficiency of a neutralization pseudocapacitor consisting of PMA/PAH/PEDOT-PSS and PEDOT as negative and positive electrodes, respectively, are also determined from the neutralization process. For a practical system, this thermodynamic approach enables estimation of an


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optimum pH range to make treatment of acid wastewater profitable, thereby contributing to environmental preservation in a sustainable way.

Working Principle and Thermodynamic Formalism: Figure 1 illustrates the working principle of the idealized acid-base machine. To deliver work, this machine operates between an acid reservoir and a basic reservoir in four steps. Two of these steps constitute reversible buffered isothermal stages. The other two steps involve an open system for the passage of acid or base. The machine comprises two insertion electrodes. One electrode is selective for the electroinsertion of protons, whereas the other electrode does not allow their electroinsertion. Therefore, the thermodynamic cycle can be described by means of the following steps: injection of an acid solution into the electrochemical cell in step 1; electroinsertion of protons in one of the electrodes and electroinsertion of other ionic species in the other electrode in step 2; addition of base to the electrochemical cell in step 3; electrodeinsertion of protons in one of the electrodes and electrodeinsertion of other ionic species in the other electrode in step 4; and exchange of the electrolytic solution with low proton activity for an acid solution. During the electrochemical cycle, protons are removed from the electrolytic solution in acid medium (step 2) and added to the electrolytic solution in less acid medium (step 4). Consequently, change in the partial entropy is converted to electric work after the thermodynamic cycle.



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Figure 1 – Scheme of the working principle of the acid-base machine, where JHX, JMOH, and JMX are the fluxes of acid, base, and salt, respectively, and w is the work. Although the proton-selective electrode can be positive or negative, the proton insertion (reduction) and the proton deinsertion (oxidation) processes must occur in more and less acid medium, respectively. In acid medium, the proton electroinsertion step in the positive electrode is spontaneous; the electrodeinsertion step in less acid medium corresponds to the non-spontaneous process. When the proton-selective host matrix corresponds to the negative electrode, non-spontaneous electroinsertion and spontaneous electrodeinsertion occur in acid and less acid medium, respectively. Considering the electrochemical cell configuration and the electroinsertion reactions described below, it is possible to demonstrate the maximum work produced by the acid-base machine and the machine efficiency. For negative (M1) and positive (M2) insertion electrodes (host matrix) selective to protons (H+) and anions (X-), respectively, ions are removed from an acidic solution by using external power sources, to promote non-spontaneous processes: M1 + H+ + e- → M1H

(1) 6 ACS Paragon Plus Environment

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M2 + X- → M2X + e-

(2)

On the other hand, spontaneous processes occur after addition of base: M1H → M1 + H+ + e-

(3)

M2X + e- → M2 + X-

(4)

On the basis of these reactions and by equating the electrochemical potentials of the reagents and products for the cell reaction, the electromotive force (Erxn) can be related to the activities of the proton (a ), anion (a ), M1 (a ), M2

(a ), M1H (a ), and M2X (a ) in acid medium (Eq. 5) and in neutral (or less acid) medium of the solution resulting from neutralization (Eq. 6),

E = −

E = − +

# !"

ln

% % % !% # % "

ln %

(5)

(6)

where R is the ideal gas constant, T1 is the temperature, F is the Faraday constant, and is the standard emf. The superscripts “a” and “n” refer to activity in acid and

neutral (or less acid) medium, respectively. The sum of the electromotive forces (Erxn,neut) in acid medium and less acid medium during the spontaneous and nonspontaneous reversible buffered and isothermal processes, respectively, corresponds to:

E , '() =

* *

ln +

+

#

(7)

In this case, the temperature (T1) during the spontaneous process is equal to the temperature during the non-spontaneous process. Considering that the ionic force 7 ACS Paragon Plus Environment


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remains constant during neutralization, and that the average activity coefficients only depend on the ionic force, according to the Debye-Hückel law, the anion activity in acid and neutral medium are canceled from Eq. 7:

E , '() =

*

ln + #

(8)

On the basis of Eq. 8, it is possible to determine the efficiency associated with the neutralization (εneut) of the idealized acid-base machine by using the work (w) and the enthalpy of neutralization (∆Hr):

ε '() = /0 = -.

1

-/2 /01

=

3415% /01

=

**

678 + 9 *

/01

(9)

Considering a unitary activity for the acid reservoir, the efficiency is a function of the pH of the resulting solution in the machine after neutralization:

ε '() =

:. T1 when Erxn (T2) > Erxn (T1). Eq. 11 gives the total efficiency:

ε))!> =

E

% *% * * * % *% % 9 -B 678 * * 9 I * * " " H H

A B -@ A B CD GB 678 ?@15% 15% F *%

/01

(11)

EXPERIMENTAL

LbL films were assembled onto a fluorine-doped tin oxide (FTO)-coated glass with sheet resistance ≤ 20 Ω obtained from Flexitec (Curitiba, Brazil). The FTO substrate was immersed for 1 min in each of the following aqueous dispersions, in the following sequence: PMA (5 g L-1 and pH = 2), PAH (1.6 g L-1 and pH = 2), PEDOTPSS (1.3 g L-1 and pH = 2), and PAH (1.6 g L-1 and pH = 2). After each layer was deposited, the substrates were rinsed in HCl solution for 30 s (pH = 2). The alternate immersion procedure was repeated 15 times, and the last layer (without immersion of the substrate in the aqueous PAH dispersion) of each LbL film was dried under nitrogen flow.

The electrochemical experiments were conducted on an Autolab PGSTAT30 potentiostat/galvanostat. A platinum sheet and Hg/Hg2SO4 in saturated H2SO4 were 9 ACS Paragon Plus Environment


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used as the counter electrode and the reference electrode, respectively. Volumes of 20 mL of electrolytic solutions of H2SO4 and K2SO4 were exchanged in a homemade electrochemical cell containing two connections for input and output electrolytic solution, by means of a four-channel peristaltic pump model BT100-1F, acquired from LongerPump. Film thickness was analyzed by specular reflectance with the aid of the Nanocalc 2000 program coupled with a single channel 2048 pixel CCD spectrophotometer equipped with halogen lamp as light source. The thickness value measured for the 15-bilayer PMA/PAH/PEDOT LbL films were 108 ± 4 nm, respectively. The geometrical area of the films was 1 cm2.

RESULTS AND DISCUSSION

Considering that the anion concentration is kept constant after the neutralization, and that this concentration consequently does not contribute to the work

performed

in

the

acid-base

machine,

we

first

investigated

the

PMA/PAH/PEDOT-PSS LbL film as a function of pH (0.0, 2.2, 3.4, 4.3, 5.0, and 6.0) for the electrolytic solution. The anodic and cathodic peak potentials in the cyclic voltammograms (Figure 2) differed slightly, indicating low irreversibility associated with proton electroinsertion/electrodeinsertion in this electrode, in agreement with electrochemical

impedance

spectroscopy

measurements.10

Moreover,

these

voltammograms revealed that the peak potentials and potential window shifted as a function of pH, indicating that it might be possible to store energy (to perform work) with the addition of base. In fact, the harvesting energy normalized by the amount of electroinserted protons corresponds to change in the potential, associated with electroreduction in more acid medium and its electrooxidation in less acid medium, multiplied by the Faraday constant (Eq. 9).


10

pH = 6 pH = 3.4 pH = 0

5 -2

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j / µ A.cm

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0

-5

-10 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

E/V Figure 2. Potentiodynamic profile of the PMA/PAH/PEDOT-PSS LbL film for several pH values of the electrolytic solution. ν = 50 mV.s-1. Figure 3 shows the proton a) electroinsertion and b) electrodeinsertion curves at several pH values for the self-assembled PMA/PAH/PEDOT-PSS electrode, obtained by using a three-electrode cell based on platinum sheet and Hg/Hg2SO4 as the counter and reference electrodes, respectively. These conditions were used to achieve an experimental approximation to an idealized acid-base machine operating between the acid (H2SO4) and the basic (KOH) reservoirs for PMA/PAH/PEDOTPSS, as negative electrode, and a positive electrode with null overpotential. In this case, we can disregard specification of the positive electrode because its contribution to the work performed in an electrochemical system that is close to the idealized acidbase machine should be null considering that the sulfate ion concentration remains constant during neutralization. Low current density (5 µA cm-2) and thickness of the self-assembled material (108 ± 4 nm) also helps to guarantee minimal overpotentials associated with electrical resistance and proton diffusion.



pH=0 pH=2.2 pH=3.4 pH=4.3 pH=5 pH=6

0.0

E/V

-0.2 -0.4 -0.6 -0.8

a -1.0 pH=0 pH=2.2 pH=3.4 pH=4.3 pH=5 pH=6

0.0 -0.2

E/V

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-0.4 -0.6 -0.8

b -1.0

0

20

40

60

80

-2

q / µC.cm

Figure 3. a) Eletroreduction and b) electrooxidation of the PMA/PAH/PEDOT-PSS LbL film for several values of pH of electrolytic solution. j = 5 µA cm-2. On basis of the area between the proton electroinsertion (at pH = 0) and electrodeinsertion (at several pH values higher than zero) curves (Figure 4a), the efficiency of the acid-base machine was determined and plotted as a function of pH (Figure 4b), according to Equation 10. The obtained values were close to the values

predicted by the thermodynamic approach shown above. Multiplying the ε '() values by the neutralization enthalpy at 298 K (55.8 kJ/mol), we determined the work as a function of the proton activity in the solution resulting from neutralization (Figure

4b). Thus, the performed work changed from 0 to 31.2 kJ per mol of electroinserted proton when the pH changed from 0 to 6.


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pH=0 pH=2.2 pH=3.4 pH=4.3 pH=5 pH=6

0.0

E/V

-0.2 -0.4 -0.6 -0.8

a -1.0

0

20

40

60

80

-2

q / µC.cm

ε / 100%

60

b

60

40

20

20

0

0

-1

40

w / kJ.mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1

2

3

4

5

6

pH

Figure 4. a) Eletroreduction at pH = 0 and electrooxidation at 2.2 ≤ pH ≤ 6 of the PMA/PAH/PEDOT-PSS LbL film at j = 5 µA cm-2. b) Theoretical (─) and experimental efficiency (○) and work (■) as a function of pH of the resulting solution after neutralization. Based on the chronopotentiometric curves shown in Figure 3a-b, Figure 5 shows the values of work for all the combinations between the pH of the acid reservoir and the pH of the resulting solution. The work values are close to the values predicted by the thermodynamic formalism (Eq. 9)



30 25

Energy

20 15 10 5 6

5

0

4

2

3

Ac id

0

6

5

4

1

pH

2

pH

3

Re su ltin g

0

1

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Figure 5. Work as a function of the pH of the acid reservoir and of the resulting solution after neutralization. We also determined the work performed by an acid-base machine consisting of the PMA/PAH/PEDOT-PSS self-assembled material as the negative electrode and electrosynthesized PEDOT as the positive electrode (Figure 6). Considering that this cell configuration is closer to an applicable system than the cell shown above, we kept the pH of the solution at 6 after neutralization, so that the resulting solution could be discharged into the environment. First, there was charging in the H2SO4 solution at pH lower than 6, followed by exchange of the acid solution with K2SO4 solution at pH = 6 (simulating the neutralization of the acid solution by addition of KOH), and the discharge in the K2SO4 solution.


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0.6 0.4

E/V

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0.2 0.0 -0.2 -0.4

0

20

40

60

80

100

-2

Q / µC.cm

Figure 6. The charge/discharge curves for the cell formed from PMA/PAH/PEDOT-

PSS LbL film and electrosynthesized PEDOT: charge at pH = 0 (∇), pH = 1 (▼), pH = 2.2 (∆), pH = 3.4 (▲), pH = 4.3 (○), pH = 5 (●) at 298 K, and discharge at pH = 6 at 298 K (□) and 318 K (■). j = 5 µA cm-2. Table 1 lists the work per mol of electroinserted proton and the efficiency after the electrochemical cycle at several pH values. The values are close to the values provided by the thermodynamic formalism. The small difference as compared with the values obtained for the three-electrode cell is associated with the overpotential related to the electroinsertion (in more acid medium)/electrodeinsertion (at pH = 6) of sulfate anions into/from the PEDOT electrode. The work normalized by the concentration of base (Cb) added to during neutralization is also important. In contrast to the work performed by the machine, the w/Cb ratio increases as a function of the pH of the acid reservoir because the work and the neutralization process are proportional to the difference in the logarithm of the proton concentration and to the proton concentration, respectively. Considering the cost of the basic reagents, the w/Cb ratio indicates that the neutralization treatment of acid solutions with higher pH values is more profitable per volume of solution. On the other hand, the energy



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obtained from solutions with very low proton concentration is not enough to compensate for the investment that is necessary to produce the neutralization pseudocapacitors. Therefore, defining a suitable pH range is essential to determine the cost to produce these devices and make wastewater treatment cost-effective. Table 1 – Work, efficiency, and work per concentration of base as a function of the pH of the acid reservoir obtained by using the resulting solution after neutralization at pH = 6.

pH

0

w/kJ.mol-1

1

2.2

3.4

4.3

5

31.167 26.0

12.979

8.219

3.977

1.495

ε

55.84

46.58

23.25

14.73

7.12

2.68

w/Cb (kJ.dm3 .mol-2)

1.95

8.67

5.57x102 8.30x103 3.90x104 6.01x104

If neutralization occurs adiabatically, the released heat increases the temperature for the discharge process and helps to increase the performed work (as in the case of thermal machines). Considering the neutralization of 1.5 mol.L-1 protons in solution, the temperature change is 20.1 K. Thus, we decided to perform the discharge step of the full cell at pH = 6 and 318 K (Figure 6). The area between the discharge (at pH = 6 and 318 K) and the charge curves (at pH = 6 and 298 K) indicates that the performed work is 812 J/mol. This work corresponds to 34.48% of the work expected for the Carnot Cycle, which is high for a practical thermal machine. However, it is implicitly assumed that all the protons in the acid solution would be electroinserted in the host matrix, which is an experimental difficulty. On the other hand, the contribution to this work (associated with the increase in 16 ACS Paragon Plus Environment

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temperature) is small as compared with the partial entropy change due to large variation in the pH. Nevertheless, this relative contribution can be significant at lower values and in a shorter pH range because the neutralization heat is proportional to the proton concentration.

CONCLUSION Thermodynamic formalism aided determination of the maximum efficiency of the acid-base machine in terms of conversion of partial change in entropy to work for the machine operating under acid and basic reservoirs. Considering that the acid reservoir is under unitary activity, the efficiency of the idealized machine defines the hydrogen potential, which can also be determined on a purely electrical or mechanical basis. By using a thin film that is selective for proton electroinsertion with low irreversibility and low current density, efficiency is close to the efficiency predicted by the thermodynamic formalism. In fact, electrochemical systems are quite efficient for energy conversion and, in this case, the selected electrodes and experimental conditions have proven to be highly efficient for conversion of entropic energy into electrical work. For a practical system, the efficiency tends to decrease according to the magnitude of overpotentials. Moreover, it is important to mention that the performed work is higher at lower pH values of the acid reservoir, which follows an opposite tendency as normalized by the amount of added base. Hence, there is a suitable pH range, obtained by addition of base to the neutralization pseudocapacitors, that makes the treatment of acid wastewater advantageous from a financial standpoint. The pH range can be estimated by the developed thermodynamic approach and the production cost of the pseudocapacitors. We hope this acid-base machine will



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encourage treatment of acid (and/or base) wastewaters, which shall contribute to sustainable growth, profitability, and environmental preservation.

ACKNOWLEDGMENTS We are grateful to FAPESP (Project 2015/ 16867-9) for financial support.

REFERENCES

(1) Yang, Z. G.; Zhang, J. L.; Kintner-Meyer, M. C. W.; Lu, X. C.; Choi, D. W.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (2) Norman, R. S. Water salination: a source of energy. Science 1974, 186, 350-352. (3) Pattle, R. E. Production of Electric Power by Mixing Fresh and Salt Water in the Hydroelectric Pile. Nature 1954, 174, 660-660. (4) Brogioli, D. Extracting Renewable Energy from a Salinity Difference Using a Capacitor. Phys. Rev. Lett. 2009, 103, 58501-58504. (5) La Mantia, F.; Pasta, M.; Deshazer, H. D.; Logan, B. E.; Cui, Y. Batteries for efficient energy extraction from a water salinity difference. Nano Lett. 2011, 11, 1810-1813. (6) Brogioli, D.; Ziano, R.; Rica, R. A.; Salerno, D.; Mantegazza, F. Capacitive mixing for the extraction of energy from salinity differences: survey of experimental results and electrochemical models. J. Colloid Interface Sci. 2013, 407, 457-466. (7) Facci, T.; Gomes, W. J.; Bravin, B.; Araujo, D. M.; Huguenin, F. Proton electroinsertion in self-assembled materials for neutralization pseudocapacitors. Langmuir 2014, 30, 426-431. (8) Gomes, W. J. A. S.; de Oliveira, C.; Huguenin, F. Energy Harvesting by Nickel Prussian Blue Analogue Electrode in Neutralization and Mixing Entropy Batteries. Langmuir, 2015, 31, 8710-8717. (9) Ye, M.; Pasta, M.; Xie, X.; Cui, Y.; Criddle, C. S. Performance of a mixing entropy battery alternately flushed with wastewater effluent and seawater for recovery of salinity-gradient energy. Energ. Environ. Sci. 2014, 7, 2295-2300.


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Neutralization Pseudocapacitors: An AcidâBase Machine - The

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