Capacitive Neutralization Dialysis for Direct Energy ... - ACS Publications

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Capacitive neutralisation dialysis for direct energy generation Yue Liu, Yi Zhang, Wei Ou-yang, Bruno Bastos Sales, Zhuo Sun, Fei Liu, and Ran Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01426 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 16, 2017

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Environmental Science & Technology

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Capacitive neutralisation dialysis for direct energy generation

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Yue Liu,a Yi Zhang,a Wei Ou-Yang,a Bruno Bastos Sales,b Zhuo Suna, Fei Liu,c Ran Zhao*a

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a

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Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, School of Physics and Materials Science, East China Normal University, 3663 North Zhongshan Road, 200062 Shanghai, China

b

School of Life Sciences and Environmental Technology, Avans University of Applied Sciences, Lovensdijkstraat 61-63, 4800 RA Breda, The Netherlands c

Department of Civil, Construction, and Environmental Engineering, North Carolina State University, 2501 Stinson Drive, Raleigh, 27695-7908, North Carolina, United States

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Corresponding author:

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E-mail: [email protected]

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Tel：+86-(0)-21-62233673

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Abstract Capacitive neutralisation dialysis energy (CNDE) is proposed as a novel energy-harvesting technique that is able to utilise waste acid and alkaline solutions to produce electrical energy. CNDE is a modification based on neutralisation dialysis. It was found that a higher NaCl concentration led to a higher open circuit potential when the concentrations of acid and alkaline solutions were fixed. Upon closing the circuit, the membrane potential was used as a driving force to move counter ions into the electrical double layers at the electrode-liquid interface, thereby creating an ionic current. Correspondingly, in the external circuit, electrons flow through an external resistor from one electrode to the other, thereby generating electrical energy directly. The influence of external resistances was studied in order to achieve greater energy extraction, with the maximum output of 110 mW/m2 obtained by employing an external resistance of 5 Ω together with the AC coated electrode.

Keywords ionic gradient energy; capacitive energy; enthalpy; entropy; Gibbs free energy; ion exchange membrane; capacitive neutralisation dialysis; nanoporous electrodes; electrical double layer.

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1. Introduction Acid and alkaline solutions are widely used in various industrial processes, including electroplating, steel manufacturing, chemical production, printing and dyeing, and papermaking. During these processes, waste solutions with either a low or high pH are generated that require proper treatment before being discharged into the environment. In the chlor-alkali industry in particular, both acidic and alkaline wastewaters are produced in large quantities 1. Conventional treatment technologies, such as evaporation 2, distillation3, neutralisation

4, 5

and membrane separation

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, without exception, are

designed to target only one of an acidic or alkaline solution, and ignore the chemical energy stored (e.g. neutralising one mole HCl and one mole NaOH generates 57.3 kJ energy) when both waste solutions are present. To exploit the energy stored in waste acid and alkaline solutions, previous work has been done by using a pseudocapacitor, where redox reactions happen on the electrodes9, 10. However, assembling of these electrode materials is not easy10. To harvest electricity with a simple manner, we developed a novel technology known as capacitive neutralisation dialysis energy (CNDE), which is based on classical neutralisation dialysis (ND)

11-21

. The ND cell, a self-powered desalination device using both

acidic and basic streams, is based on a simple three-compartment configuration consisting of one cation-exchange membrane (CEM) and one anion-exchange membrane (AEM) separating acid, salt water and alkaline compartments

11, 19, 22

. The desalination process is purely driven by the ionic

gradients between each ion exchange membrane. Our proposed CNDE technique not only desalinates salt water, but also converts the chemical potential into electrical energy at the same time. In a CNDE cell, two conductive graphite electrodes are placed on the outer flanks of the acid and alkaline compartments, sandwiching the whole stack (Fig. 1). The ionic current is therefore converted into an electrical current due to the inherent ability of ion adsorption/desorption and the form of the electric double layers (EDLs) on the surface of the capacitive electrodes

23

. Similar capacitive electrodes are widely used in many gradient energy extraction

techniques, including capacitive Donnan potential (CDP) (CDLE)

32, 33

and battery-like reactions

34, 35

24-31

, capacitive double layer expansion

. The CNDE system also contains an external circuit that 2



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comprises a resistor and a switch. Between the adjacent compartments on the two sides of the membrane (either CEM or AEM), ionic gradient energy is present due to the concentration difference of a specific ion specie across the membrane. The ionic concentration difference across the membrane formulates the membrane potential (also denoted as the membrane Donnan potential), which serves as the driving force of ion exchange over both membranes

36

. Compared to CDP studies in which only

simulated sodium chloride solutions were used, CNDE deals with multiple ion exchanges, i.e. the ion exchange of protons and cations (e.g. Na+, Ca2+) over the CEM and the ion exchange of hydroxyl ions and anions (e.g. Cl- or SO42-) over the AEM. The protons and the hydroxyl ions penetrate into the saline water compartment, forming water. In order to maintain electro-neutrality in the acid and alkaline channels, the Na+ and Cl- ions in the salt channels are exchanged for the protons and the hydroxyl ions, leading to the deionisation of the salt channel 22. An electrical voltage difference between the two electrodes Ecell that includes two membrane potentials, ECEM and EAEM, can be measured. Basically, a CNDE run consists of a charging step and a discharging step, with the charging step beginning when the circuit is closed. Driven by the membrane potential, ions in solution will move according to the direction of the electrical field towards their counter electrodes, accumulating in the electrical double layers (EDLs) formed on the solid-liquid interface and creating an EDL potential (EEDL). Accordingly, while electrons run through the resistor forming an electrical current, the Ecell is decreasing because the formed EEDL of the opposite direction is counter-balancing the membrane potential. By doing so, electrical energy is harvested. After a fixed time, the circuit is opened again. Under the open circuit condition, the cell potential gradually rises back to the initial OCP value. In the present study, we demonstrate the proof-of-principle of this new technology. We experimentally examine the influence of concentrations of acid, alkali and salt, as well as the external resistance, on maximum power, and compare the results obtained using non-coated and activated carbon (AC) coated graphite plate electrodes.

2. Materials and methods 2.1 Experimental setup The experimental CNDE contained an acid channel, a salt channel and an alkaline channel, which were separated by two homogeneous ion-exchange membranes (Neosepta® CMX and AMX, Japan, thickness δ=170 µm and 140 µm, respectively). The effective surface area of each membrane was 28 cm2 and the distance between the two membranes was 3 mm. Two electron-conducting graphite electrodes were placed on the outer sides of the cell, sandwiching the channels (Fig. 1). The acid and alkaline channels were carved from the graphite body, with the electrodes electrically connected to an external resistor in series. HCl, NaCl and NaOH solutions were used as the acid, alkaline and salt solutions, respectively; initial concentrations of both HCl and NaOH were set at 100 mM, while that of the salt solution (NaCl) was varied between 10 mM and 100 mM. The total volumes of the salt (NaCl), acid (HCl) and alkaline (NaOH) channels were 80 mL, 100 mL and 100 mL, respectively, including the interior volume in the stack, the tubing and the reservoir. All water streams were kept flowing at a constant rate (30 mL/min) by peristaltic pumps, and constantly recirculated into small reservoirs. Activated carbon (AC) coated film electrodes were used for ion adsorption and desorption. To make this AC-coated electrode, the AC paste was applied onto a graphite electrode, with the final coating layer containing 90% w/w AC (1.5 grams, Norit DLC B3) and 10% PVDF. The AC film electrode has a pore volume of 0.597 cm3/g and a surface area of 1146 m2/g (BET area). A control experiment was 3


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done using only graphite plate without any AC coating. The surface area of the graphite plate for both

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the acid and alkaline channels was 72 cm2. The capacitance of AC coated electrode and graphite

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electrode are 13.9 F/g and 11 F/m2. For the energy generation processes, a series of external resistors (ranging between 0.2 and 100 ohms) was connected electrically in series with the cell.

Fig. 1. (a) Schematic view of the experimental set-up. From left to right: acid channel, salt channel and alkaline channel. The solution in each channel is recirculated. Protons in the acid channel are exchanged with sodium ions in the salt channel through the cation-exchange membrane, and hydroxyl ions in the alkaline channel are exchanged with chloride ions in the salt channel through the anion-exchange membrane. (b) A 3-dimensional visualisation of the experimental setup. Between the graphite electrode and the membrane, a 1 mm thin gasket was used to prevent water leakage. Arrows show the pathways of the solutions.

2.2

Measurement and analysis

The cell potential, U, was measured and recorded using an on-line multimeter (Keithley 2000, U.S.A.) with an interval time of 1 s. The conductivity of the salt solution and pH were measured with a conductivity meter (DDSJ-308, Precision & Scientific Instrument) and a pH meter (PHSJ-3F, Precision & Scientific Instrument), respectively. Ionic concentrations were obtained from the measured conductivity and pH using a calibration curve incorporated with the Nernst-Einstein equation. Coated activated carbon pore volume and BET area were measured using a Quantachrome Autosorb Automated Gas Sorption System (ASIQMUTV02UT-6, Quantachrome Instruments Corporate, USA).

The current density, I, was calculated as

for each resistance, where Am = 28 cm2

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corresponding to the surface area per single membrane. The generated power density (P) was

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calculated as , where At =56 cm2 is the total active membrane area of the two membranes. The

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total energy generated (E) in one run was obtained by integrating the power curve.

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3. Results and discussion

Energy extraction was investigated using a three-stage operational mode. At open circuit, the CNDE system functions just as in ND, with deionisation processes occurring. Here it was found that an open cell potential (OCP) between the two electrodes was developed (stage 0 in Fig. 2); measurement of these OCP values is discussed further in section 3.1. It is worth emphasising that ion exchange occurs 4



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spontaneously in the presented system due to the interdiffusion between H+ and Na+, and between OHand Cl-. At stage 1, upon the closure of the circuit, the charging step occurs. Driven by the membrane potential, the ions migrate according to the membrane potential gradient and form an ionic current. The electrons move correspondingly from one electrode to the other through the external resistor, thereby creating an electrical current. Consequently, the electrode next to the HCl solution is negatively charged and that next to the NaOH solution is positively charged. Accordingly, the moving ions accumulate at the electrode-solution interface, leading to the formation of electrical double layers (EDLs) on the surface of the electrodes. The formed EDLs counter-balance the membrane potential, which causes a deterioration of the cell voltage (stage 1); cations (H+ and Na+) are adsorbed into the EDLs formed at the cathode, and anions (OH- and Cl-) are adsorbed into the EDLs of the anode. If the charging step is sufficiently long, the EDLs will eventually equal the membrane potential (stage 1’) 23. At stage 2, the circuit is opened again and discharging takes place, probably due to the self-discharge or re-balance of the ion exchange; the exact mechanism behind this discharging step still requires further investigation. The ions previously stored in the EDLs are now slowly released from the electrode regions to the acid and alkaline channels. As a result, the EDLs diminish and the cell potential increases once more, ultimately reaching a maximum as shown in stage 0.

(0)

HCl

NaCl

NaOH

Vcell

HCl

NaCl

NaOH

(1)

Vcell

HCl

NaCl

NaOH

HCl

NaCl

NaOH

(1’)

(2)

electrode

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Vcell

cation anion

exchange membrane

Fig. 2. Electrostatic potential profiles across the cell. The horizontal arrow shows the direction of the ionic current. In the initial stage (0), the cell potential equals the sum of the two membrane potentials. When the circuit is closed (1), the formation of the electrical double layers leads to a gradual decay of the cell potential. Ultimately, after sufficient time has elapsed, the cell potential will be cancelled completely. When the circuit is opened, self-discharge occurs (2) and the cell potential will increase to 5


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initial situation (0) levels.

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present. Many types of self-discharge mechanism have been proposed based on previous work 37-40. In

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As discharge occurs spontaneously during stage 2, it is suspected that the self-discharge effect is

our case, the device was not overcharged (~0.3 V) and thus it is likely that it may contain internal Ohmic leakage pathways, e.g. due to incomplete sealing of bipolar electrodes or inter-electrode contacts; self-discharge in the CNDE device therefore likely takes place via a ‘Galvanic couple’ effect 37

. According to our experimental experience, discharging steps are usually much slower than those

involving charging. This is also why in all the conducted experiments the durations of the charging step and the discharging step were fixed at 20 s and 100 s, respectively. Normally, self-discharge presents severe limitations and inconvenience for super capacitor-like devices, such as those involving CDI, CDLE and CDP, as well as for the charging step of the CNDE process. However, in our case, since the polarity of the membrane potential is irreversible, self-discharge helps to release the previously adsorbed ions and clears the surface of the electrodes for ion adsorption occurring in the charging step. In the future, research should focus on how to speed up the discharging step.

3.1 Determination of the open circuit potential in bi-ionic situations Ion exchange membranes are widely used in water purifying technologies, such as electrodialysis and membrane CDI, to separate cations from anions41, but can also be used in energy generating devices, e.g. CapMix and reverse electrodialysis (RED)42, 43. It is worthy to note that the gross power

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density per membrane area for RED is already higher than 2 W·m-2 43. Because of the concentration

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difference between the bulk and the membrane matrices, the membrane potentials formed on the

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∆φ −∆φ , + ∆φ + ∆φ, ,

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where V is the membrane potential, VT is the thermal potential (=RT/F~25.7 mV at room temperature),

interface can be used as the driving force with which to enforce ion movement to the counter electrode according to their polarities 23. The CNDE system is also a membrane process, in this case consisting of 3 channels and 2 membranes. Similarly, the driving force of the system is attributed to the difference in the ionic strengths on both sides of the membranes. It has been shown above that an open cell potential (OCP) can be measured during the operation of the experimental cell. As the OCP reveals the amount of energy that can be generated by the NDE process, and is also dictated by the membrane permselectivity and the Gibbs free energy of interdiffusion regardless of the ohmic resistivity, size and geometry of the setup, we will here first focus on the OCP. In contrast to the CapMix system, in the NDE system not only are sodium and chloride ions considered, but also hydrogen ions and hydroxyls. Thus, the OCV comprises the two bi-ionic membrane potentials

44-48

. Given that the concentration of a certain ion is different on the two sides of

the membrane and in the membrane, the classical expression of the potential difference across the membrane is the sum of two Donnan potentials and a diffusion potential, as follows: (1)

∆φ is the dimensionless electrical potential, ∆φ, refers to the Donnan potential between the bulk and the membrane on the right side, ∆φ refers to the diffusion potential within the membrane, and ∆φ, represents the Donnan potential between the bulk and the membrane on 6



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the left side. For monovalent ions, the Donnan potential on either side of the membrane is governed by

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∆φ −ln ( ),

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where "# is the activity of a certain type of ion i in the membrane phase, and "# is the corresponding

be derived 48, i.e.:

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∆φ −ln ( $,% &' (→$ (,% ),

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where subscripts A and B refer to the two types of counterion, and , - .→/

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is defined as the permselectivity between counterions A and B. ui (i = A, B) denotes the mobility of ith

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∆φ234 −ln (5

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∆φ/34 −ln (5

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where c is the concentration, subscripts H, Na, OH and Cl refer to the four different types of ion present

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∆φ>?? ∆φ234 + ∆φ/34 −ln ('

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the counter ion concentration in the two phases, given by:

(2)

activity in the liquid phase. In many cases, upon assuming the activity coefficient of the ion is the same in the liquid as it is in the membrane phase, the ratio can be further simplified. Combining the above equations and the Henderson equation for diffusion potential in the membrane, an overall equation for the voltage drop across the membrane for a system containing two types of monovalent counterion can

&'

$,+

⋅

(3)

(→$⋅ (,+

0(⋅(,% ⋅$,% 0$⋅(,%⋅1$,%

0( ⋅(,+ ⋅$,+ 0$ ⋅(,+⋅1$,+

ion in the membrane phase. ai,j (i = A, B, j=L, R) denotes the activity of ith ion in the solution, and āi,j (i = A, B, j=L, R) denotes the activity of ith ion on the two sides of the membrane. L and R denote the left and the right side of the membrane. Given that in our cell both the cation exchange membrane and the anion exchange membrane are present, and also by assuming that the activity coefficient for both ions is equal to 1, Eq. (3) can be rewritten as: 56,78 &' 97→6 ⋅597,78 6,:7; &' 97→6⋅ 5 97,:7;

),

5

Capacitive Neutralization Dialysis for Direct Energy ... - ACS Publications

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