Ion-Liquid Based Supercapacitors with Inner Gate Diode-Like Separators

chemengineering

Article Ion-Liquid Based Supercapacitors with Inner Gate Diode-Like Separators

Tazima S. Chowdhury and Haim Grebel *

Electrical and Computer Engineering and Electronic Imaging Center, New Jersey Institute of Technology, Newark, NJ 07102, USA; [email protected] * Correspondence: [email protected]

Received: 11 March 2019; Accepted: 9 April 2019; Published: 14 April 2019

Abstract: In order to minimize unintentional discharge, supercapacitors are interfaced with a membrane that separates the anode from the cathode—this membrane is called the separator. We focus here on separators, which are structured as electronic diode-like. We call an electrically structured separator “the gate”. Through experiments, it was demonstrated that ionic liquid-ﬁlled supercapacitors, which were interfaced with gated separators exhibited a substantial capacitance (C) increase and reduction in the equivalent series resistance (ESR) compared to cells with ordinary separators. These two attributes help to increase the energy, which is stored in a cell, since for a given 2 2 cell’s voltage, the dissipated energy on the cell, UR = V /4(ESR) and the stored energy, UC = CV /2, would increase. These were indeed ionic diodes since the order of the diode layout mattered—the diode-like structures exhibited maximum capacitance when their p-side faced the auxiliary electrode.

Keywords: supercapacitors; gated supercapacitors; energy storage elements; diode-like separators

1. Introduction Increasingly, supercapacitors find more and more applications in short-term energy storage [1–6], namely, energy elements that can quickly handle substantial amount of current. A supercapacitor is made of two electrodes: The anode and the cathode. Both electrodes are immersed in an electrolyte. A separator membrane between the electrodes (Figure1a) was introduced to minimize unintentional electrical discharge. Thus far, research on increasing cells capacitance with carbon materials has been focused on increasing the surface area of the electrodes, decreasing the effective charge separation at the electrode-electrolyte interface, and, in some cases, on increasing the efficiency of charge transfer [7–15]. We have opted to study the less researched, yet equally important separator layer. We modified the otherwise electrically insulating or unbiased membrane [16–20] into a capacitive, diode-like, or even transistor-like porous structure [21–25]. The general idea was to place a permeable potential barrier between the anode and the cathode. The diode-like element was an ion porous structure. It was formed by layering doped films, in our case p-type and n-type layers of functionalized single-walled carbon nanotubes (SWCNTs). We call this electronically structured element “the gate”. When an ordinary supercapacitor is charged, the potential distribution is constant throughout most of its regions, with the exception of the electrolyte-electrode interfaces. This is due to a screening effect by the ions, in response to excess electronic charge on the electrodes. In our case, the electrolyte filled the space within and outside the SWCNTs. Due to the lack of charge transfer between the electrolyte and SWCNTs, the ions in the liquid screened the local electronic charges at the diode-like gate. The result was ion charge separation. Ion charge separation led to polarity increase in the mid-cell region (Figure1b,c) and might be viewed as a capacitor (the gate) within another (the supercapacitor).

ChemEngineering 2019, 3, 39; doi:10.3390/chemengineering3020039 www.mdpi.com/journal/chemengineering ChemEngineering 2018, 2, x FOR PEER REVIEW 2 of 9 supercapacitor). A capacitor-within-capacitor (CWC) has been shown to increase the overall capacitance of capacitors [26]. The concept is general and may be easily incorporated into existing supercapacitor structures. Specifically, our diode-like separators were made of two layers, each composed of functionalized SWCNT film. One layer was composed of electronically p-type tubes, and the other layer was composed of electronically n-type tubes. The SWCNTs may be viewed as nano-cylinders thatChemEngineering form a nano-contact2019, 3, 39 [27,28]. An interface (for example, when one layer is deposited on top2 ofof 9 another or when two such layers are pressed together) is composed of numerous such nano-contacts. The interface behaved as an effective diode (Figure 3). The diode-like structure was permeable to ions andA capacitor-within-capacitor exhibited resistance and (CWC) capacitance, has been as shown evidenced to increase by the the various overall capacitanceelectrochemical of capacitors techniques [26]. describedThe concept below. is general and may be easily incorporated into existing supercapacitor structures.

Vext

- + - + - + - + - + - + - + - + - + - + - + - + anode cathode

p-n gate membrane (a) (b) ordinary separator p-n separator

- + - +

double layer electrolyte electrolyte (c)

Figure 1. ((aa)) Schematics of super-capacitors, ( b) diode-like mid-cell gate structure, blue: electronic charges;charges; red: ionic ionic charges, green/blue green/blue layers: p-type/n-type p-type/n-type layers. The The counterions counterions screened screened the doped structured gate, thus forming forming a a capacitor capacitor within within the the supercapacitor, supercapacitor, ( cc)) qualitative qualitative distribution distribution of electrical potential (dash curve) within the various supercapacitor regions.

2. ExperimentSpecifically, and our Methods diode-like separators were made of two layers, each composed of functionalized SWCNT film. One layer was composed of electronically p-type tubes, and the other layer was Single wall carbon nanotubes (SWCNTs) were obtained from Nano Integris, Canada, with composed of electronically n-type tubes. The SWCNTs may be viewed as nano-cylinders that form a certified purity of better than 95%. The average tube diameter was 1.4 nm, with an average length of nano-contact [27,28]. An interface (for example, when one layer is deposited on top of another or when 1 micron. The SWCNTs may be purchased with little metallic catalyst content (<1%). The remaining two such layers are pressed together) is composed of numerous such nano-contacts. The interface ca. 4% was inert carbonaceous material. The SWCNTs were functionalized with polymers—doping behaved as an effective diode (Figure 3). The diode-like structure was permeable to ions and exhibited of either the p-type, or the n-type tubes was achieved by wrapping them with polyvinyl pyrrolidone resistance and capacitance, as evidenced by the various electrochemical techniques described below. (PVP) and polyethylenimine (PEI), respectively [29,30]. The surface area of the functionalized tubes may2. Experiment be estimated and as Methods follows: the total thickness of a wrapped tube is considered as a bundle of 20 tubes. Taking into account the larger molecular weight of the wrapping polymer, we estimated that the surfaceSingle wallarea carbonof PVPnanotubes and PEI wrapped (SWCNTs) tubes were was obtained 15 and from 7.5 m Nano2/g, respectively. Integris, Canada, After with that, certified the p- typepurity and of n-type better thannanotubes 95%. The were average suspended tube diameterin deionized was (DI) 1.4 nm,water with using an averagea horn probe length sonicator of 1 micron. for 8The hours. SWCNTs Using mayvacuum, be purchased each layer with was little then metallicdrop-cas catalystted on a content hydrophilic (<1%). filter The (TS80, remaining TriSep). ca. The 4% was inert carbonaceous material. The SWCNTs were functionalized with polymers—doping of either the p-type, or the n-type tubes was achieved by wrapping them with polyvinyl pyrrolidone (PVP) and polyethylenimine (PEI), respectively [29,30]. The surface area of the functionalized tubes may be estimated as follows: the total thickness of a wrapped tube is considered as a bundle of 20 tubes. Taking into account the larger molecular weight of the wrapping polymer, we estimated that the surface area of PVP and PEI wrapped tubes was 15 and 7.5 m2/g, respectively. After that, the p-type and n-type nanotubes were suspended in deionized (DI) water using a horn probe sonicator for 8 hours. Using ChemEngineering 2018, 2, x FOR PEER REVIEW 3 of 9

8 hours. Using vacuum, each layer was then drop-casted on a hydrophilic filter (TS80, TriSep). The thickness of each SWCNT-type layer was estimated at less than 10 μm, microns by weight and covered area. In this study and previous experiments, we found that the polymer wrapping process and sonication substantially reduced the undesired material content (non-semiconducting tubes and other carbonaceous material) in the sample, thus eliminating the need for refluxing. Due to the relatively small bandgap at the p-n interface [27] and the relatively light doping of the tubes, the depletion region extended throughout the entire gate's cross-section. As shown in Figure 2, some of the SWCNT form bundles, which did not affect the electrical properties under dry conditions (Figure 3a,b). The quality of the diode-like structure was also ascertained by a current-voltage trace, as shown in Figure 3. We comment on the performance of the diode-like gate under wet conditions in the

ChemEngineeringConclusion 20192018Section.,, 32,, 39x FOR PEER REVIEW 33 of of 9

8 hours. Using vacuum, each layer was then drop-casted on a hydrophilic filter (TS80, TriSep). The vacuum,thicknesseach of each layer SWCNT-type was then drop-casted layer was on estimated a hydrophilic at less filter than (TS80, 10 TriSep).μm, microns The thickness by weight of eachand SWCNT-typecovered area. layerIn this was study estimated and previous at less than experime 10 µm,nts, microns we found by weight that the and polymer covered wrapping area. In this process study and previoussonication experiments, substantially we reduced found thatthe undesire the polymerd material wrapping content process (non-semiconducting and sonication substantially tubes and reducedother carbonaceous the undesired material) material in content the sample, (non-semiconducting thus eliminating tubes the and need other for carbonaceousrefluxing. Due material) to the inrelatively the sample, small thus bandgap eliminating at the the p-n need interface for refluxing. [27] and Due the torelatively the relatively light smalldoping bandgap of the tubes, at the p-nthe interfacedepletion [ 27region] and extended the relatively throughout light doping the entire of the gate's tubes, cross-section. the depletion As region shown extended in Figure throughout 2, some of the SWCNT entire gate’s form cross-section. bundles, which As did shown not affect in Figure the 2electrical, some of properties the SWCNT under form dry bundles, conditions which (Figure did not3a,b). aff Theect thequality electrical of the propertiesdiode-like structure under dry was conditions also ascertain (Figureed3 bya,b). a current-voltage The quality of trace, the diode-like as shown structurein Figure was3. We also comment ascertained on the by aperformance current-voltage of the trace, diode-like as shown gate in Figureunder3 .wet We conditions comment onin thethe performanceConclusion Section. of the diode-like gate under wet conditions in the Conclusion Section. Figure 2. SEM picture of SWCNT film on top of a TS80 separator membrane.

In the following, we show that the gate was composed of p-type and n-type films. Current- voltage (I-V) measurements were conducted on each SWCNT-type film (Figure 3a) under dry conditions. A single-type film behaved as a resistor, exhibiting a linear curve. The resistance of each layer was dictated by the SWCNT doping, and Figure 3a alludes to the larger conductivity of the p- type layer. Since the SWCNTs are naturally p-type even without the wrapping by PVP, wrapping by PEI to turn them into n-type is not optimal. Hence, the n-type film would be expected to exhibit higher resistance. Linearity and symmetry in the −1, +1 V window also alludes to the ohmic contact to the samples. The I-V curve for a diode-like structure behaves nonlinearly with an accelerated behavior in a forward direction (namely, when the positive lead is on the p-type film and the negative lead on the n-type side.) Figure 2. SEM picture of SWCNT ﬁlm on top of a TS80 separator membrane. Figure 2. SEM picture of SWCNT film on top of a TS80 separator membrane.

In the following, we show that the gate was composed of p-type and n-type films. Current- voltage (I-V) measurements were conducted on each SWCNT-type film (Figure 3a) under dry conditions. A single-type film behaved as a resistor, exhibiting a linear curve. The resistance of each layer was dictated by the SWCNT doping, and Figure 3a alludes to the larger conductivity of the p- type layer. Since the SWCNTs are naturally p-type even without the wrapping by PVP, wrapping by PEI to turn them into n-type is not optimal. Hence, the n-type film would be expected to exhibit Current (uA) higher resistance. Linearity and symmetry in the −1, +1 VCurrent (uA) window also alludes to the ohmic contact to the samples. The I-V curve for a diode-like structure behaves nonlinearly with an accelerated behavior in a forward direction (namely, when the positive lead is on the p-type film and the negative lead on the n-type side.) (a) (b)

FigureFigure 3. 3.Electronic Electronic behavior. behavior. (a )(a Current-voltage) Current-voltage (I-V) (I-V) curve curve for for a dry,a dry, single-type single-type SWCNT SWCNT film film on on TS80:TS80: p-type p-type (red) (red) and and n-type n-type (blue). (blue). The The lines lines present present a lineara linear fit. fit. The The di ffdifferenceerence between between the the p-type p-type andand n-type n-type film’s film's conductivity conductivity (the (the curve’s curve's slope) slope) is is due due to to the the eff effectiveective doping doping of of the the carbon carbon tubes; tubes; (b) I-V curve for pressed p-n junction(s). In obtaining this curve, one contact was placed on the p-type side and the other contact was placed on the n-type side.

In the following, we show that the gate was composed of p-type and n-type ﬁlms. Current-voltage

(I-V)Current (uA) measurements were conducted on each SWCNT-type film (Figure3a) under dry conditions. Current (uA) A single-type film behaved as a resistor, exhibiting a linear curve. The resistance of each layer was dictated by the SWCNT doping, and Figure3a alludes to the larger conductivity of the p-type layer. Since the SWCNTs are naturally p-type even without the wrapping by PVP, wrapping by PEI to turn them into n-type is not optimal.(a) Hence, the n-type film would be expected( tob) exhibit higher resistance. Linearity and symmetry in the 1, +1 V window also alludes to the ohmic contact to the samples. Figure 3. Electronic behavior. −(a) Current-voltage (I-V) curve for a dry, single-type SWCNT film on The I-V curve for a diode-like structure behaves nonlinearly with an accelerated behavior in a forward TS80: p-type (red) and n-type (blue). The lines present a linear fit. The difference between the p-type direction (namely, when the positive lead is on the p-type film and the negative lead on the n-type side). and n-type film's conductivity (the curve's slope) is due to the effective doping of the carbon tubes; ChemEngineering 2018, 2, x FOR PEER REVIEW 4 of 9

(b) I-V curve for pressed p-n junction(s). In obtaining this curve, one contact was placed on the p-type ChemEngineering 2019, 3, 39 4 of 9 side and the other contact was placed on the n-type side.

InIn FigureFigure4 4 we we show show the the Raman Raman spectra spectra for for each each doped doped film. film. Both Both films films exhibited exhibited a a similar similar G G++ −1 - lineline atat 16161616 cmcm− .. The The broadened broadened G− line'sline’s position position was was affected affected by th thee tube's tube’s doping: the n-type filmfilm 1−1 exhibitedexhibited aa downshift downshift of of ca. ca. 10 10 cm cm− due due to to its its negative negative doping doping with with respect respect to the to p-typethe p-type film. film. A 3 mWA 3 HeNemW HeNe laser atlaser 633 at nm 633 was nm used was inused conjunction in conjunction with a with 75 cm a spectrometer75 cm spectrometer and a chargeand a charge coupled coupled device (CCD)device array,(CCD) which array, was which cooled was tocooled35 toC. − The35 °C. spectral The spectral resolution resolution was 1 cm was1. 1 cm−1. − ◦ − 1000 1616 1544 800

600

400 1554

Signal (Counts) 200 1100 1300 1500 1700 1900 -1 Raman Shift (cm ) FigureFigure 4.4. Raman spectrum for single-typesingle-type SWCNTSWCNT ﬁlms:films: n-type (blue) and p-type (red).(red).

ThermoelectricThermoelectric data are providedprovided inin TableTable1 1 for for each each single-type single-type film. film. TheThe potential potential didifferencefference whenwhen thethe sample sample is is resting resting between between a heateda heated end end (maintained (maintained at aat given a given temperature temperature of a of hot a plate)hot plate) and aand cold a endcold (maintainedend (maintained at room at room temperature) temperature) is shown. is shown. The positive The positive lead of lead the multimeterof the multimeter was placed was onplaced the hoton the end. hot The end. data The corroborated data corroborated the doping the do typeping of type the of SWCNT, the SWCNT, namely, namely, negative negative values values for a p-typefor a p-type film and film positive and positive values valu for anes n-typefor an film.n-type The film. data The also data alluded also alluded to the larger to the p-type larger doping. p-type doping. Table 1. Thermoelectric measurements for single-type SWCNT films.

Table 1. Thermoelectric measurements for single-type SWCNT films. Type ∆V at 57 ◦C ∆V at 68 ◦C Typen-type +Δ0.8V at mV 57 °C +1.2ΔV mV at 68 °C p-type 1.8 mV 2.5 mV − − n-type +0.8 mV +1.2 mV

Ionic liquid, 1-n-Butyl-3-methyl-imidazoliump-type −1.8 mV hexafluorophosphate, −2.5 mV was used as an electrolyte. It has been previously used in related experiments [26]. It does not react with carbon species, has little reaction with the copper electrodes, and has a relatively large potential window. It filled the spaceIonic between liquid, two 1-n-Butyl-3-methyl-imidazolium 25-micron-thick flat copper (Cu) hexa electrodes,fluorophosphate, of size 5 3 was cm2 usedeach. as The an ionicelectrolyte. liquid × soakedIt has been 0.1 mmpreviously thick lens used tissues in related (Bausch experiments & Lomb), which[26]. It aredoes used not herereact as with spacers. carbon The species, configurations has little forreaction the cell with with the and copper without electrodes, the gate and (namely, has a using relatively bare insulatinglarge potential TS80 window. membranes It filled to achieve the space the 2 samebetween separator two 25-micron-thick thickness) are shownflat copper in Figure (Cu)5 electrodes,a,b. The assembled of size 5 sample× 3 cm rested each. overnightThe ionic toliquid let thesoaked ion-liquid 0.1 mm soak thick in. lens In tissues order (Bausch to increase & Lomb), the cell which capacitance, are used modified here as spacers. Cu electrodes The configurations (Figure5c) werefor the prepared cell with by and placing without a double-sided the gate (namely, carbon us tapeing (typicallybare insulating used forTS80 SEM membranes samples) onto achieve the copper the filmssame andseparator spreading thickness) 1-micron are shown carbon in powder Figure 5a,b (Alpha). The onassembled the tape. sample Loose rested powder overnight was shaken to let the off. Theion-liquid thickness soak of in. the In carbon order to tape increase was 100 the microns cell capacitance, and the thickness modified ofCu the electrodes powder (Figure on the tape 5c) were was estimatedprepared by at placing 1 micron a double-sided by sample weight carbon and tape surface (typically coverage. used for The SEM cell samples) was pressed on the between copper films two 1and mm-thick spreading glass 1-micron plates to carbon ensure electrodepowder (Alpha) flatness. on An the example tape. Loose of a measurement powder was procedure shaken off. was The as follows:thickness The of cellthe wascarbon first tape tested was with 100 a bare microns separator. and the Then, thickness the cell wasof the opened powder up, interfacedon the tape with was a structuredestimated at gate, 1 micron and tested by sample again. Theweight order and of surface testing (e.g.,coverage. bare separatorThe cell was second pressed and structuredbetween two gate 1 first)mm-thick did not glass affect plates the finalto ensure outcome. electrode flatness. An example of a measurement procedure was as follows: The cell was first tested with a bare separator. Then, the cell was opened up, interfaced with a structured gate, and tested again. The order of testing (e.g., bare separator second and structured gate first) did not affect the final outcome. ChemEngineering 2019, 3, 39 5 of 9 ChemEngineering 2018, 2, x FOR PEER REVIEW 5 of 9

flat Cu electrode carbon powder spacer

p-type on TS80

n-type on TS80

bare TS80

(a) (b) (c)

Figure 5. Cell configuration conﬁguration with modified modiﬁed Cu electrodes: ( (aa)) interfaced interfaced with with a a bare separator and (b,c) interfaced with a structuredstructured separatorseparator (gate), eithereither p-n oror n-pn-p structure.structure. The thickness of the layered bare separator was exactly the samesame as the thickness ofof thethe structuredstructured gate.gate.

3. Results Results and and Discussion Discussion The 2-electrode Cyclic-Voltammetry (CV) traces and their corresponding capacitance values are shown in Figure 66.. WhileWhile ionicionic liquidliquid maymay sustainsustain aa relativelyrelatively largelarge bias,bias, thethe voltagevoltage windowwindow waswas chosen as 0–0.5 VV toto comparecompare itit with with an an aqueous aqueous cell, cell, which which is is described described elsewhere. elsewhere. Since Since the the layout layout of ofthe the structured structured separator separator (the (the gate) gate) is is asymmetric asymmetric upon upon interchanging interchanging it it withwith respectrespect toto the position of the auxiliary auxiliary electrode, electrode, one one might might expect expect a arespec respectivetive change change in inthe the cell's cell’s capacitance, capacitance, as well. as well. In FigureIn Figure 6a,b6a,b we we show show the the CV CV traces traces for for two two cases: cases: on one,e, where where the the n-side n-side of of the the gate gate faced the auxiliary electrode andand thethe other,other, where where the the p-side p-side of of the the gate gate faced faced the the auxiliary auxiliary electrode. electrode. The The blue blue trace trace for thefor thebare bare separator separator is barely is barely open, open, implying implying a very a very small small cell cell capacitance. capacitance. As observedAs observed from from Figure Figure6c, the 6c, thecell’s cell's specific specific capacitance capacitance substantially substantially increased increased upon replacing upon replacing the bare the separator bare separator with the diode-like with the diode-likegate. Maximum gate. Maximum capacitance capacitance was achieved was when achieved the p-sidewhen facedthe p-side the auxiliary faced the electrode. auxiliary electrode. Stability tests shown in Figure7 are for 100 CV cycles. The scan rate was relatively small (0.01 V /s), therefore, the obtained capacitance values were enhanced compared to Figure6c. Stability was reached after the ca. 10th cycle. We postulate that diffusion processes required a few cycles before reaching stability due to the high resistance of the ionic liquid. Thereafter, both configurations appear stable and varied within 2% and 4%, respectively, when either the p-side or the n-side was facing the auxiliary electrode. Diffusion and drift currents also played a role in energy losses: preliminary data (not shown) indicated that cells interfaced with bare membranes exhibited shorter time constants (of ca. 10 sec) than those interfaced with the diode-like gate (of ca. 20 sec). Electrochemical impedance spectroscopy (EIS) were performed for a frequency range of 50 kHz to 50 MHz with a 10 mV AC perturbation signal. In Figure8 we compare two cases of gate layout with respect to the auxiliary electrode. The knee frequency was 19 Hz (red) and 14 Hz (green) when the auxiliary electrode faced the n-side and the p-side, respectively. The equivalent series resistance (ESR) for ionic liquid-based cells was quite high and was of the order of tens of kΩ. The smaller ESR value was noted when the p-side was facing the auxiliary electrode. The smaller ESR value for the p-n layout was correlated with the largest overall cell’s capacitance. The results point to the difference in cell impedance between the two gate layouts. This is further proof that the mid-cell gate indeed acts as a diode-like and also points to ionic mid-cell charge separation that leads to an increase in the overall cell capacitance. ChemEngineering 2018, 2, x FOR PEER REVIEW 6 of 9 Current (uA) Current (uA)

ChemEngineering 2019, 3, 39 6 of 9 ChemEngineering 2018, 2, x FOR PEER REVIEW 6 of 9

(a) (b) ) 2 Current (uA) Current (uA) Ccell (uF/cm

(a) (b)

) (c) 2 Figure 6. Cyclic Voltammetry (CV) with two modified copper electrodes and a diode-like gate. Blue curve: Bare separator; red curve: p-n structured gate where the n-side was facing the auxiliary electrode; green curve p-n structured gate where the p-side faced the auxiliary electrode. (a) At scan rate of 0.5 V/s and (b) at scan rate of 0.1 V/s. The blue trace for the bare separator is barely open, implying a very small cell capacitance. (c) Specific cell capacitance as a function of scan rate. Colors, as in (b). Ccell (uF/cm Stability tests shown in Figure 7 are for 100 CV cycles. The scan rate was relatively small (0.01 V/s), therefore, the obtained capacitance values were enhanced compared to Figure 6c. Stability was reached after the ca. 10th cycle. We postulate that (diffusionc) processes required a few cycles before reaching stability due to the high resistance of the ionic liquid. Thereafter, both configurations appear stableFigure andFigure 6.variedCyclic 6. Cyclic within Voltammetry Voltammetry 2% and (CV) 4%, (CV) withrespectively, with two two modiﬁed modified when copper either copper electrodes the electrodes p-side and or and a the diode-like a n-sidediode-like gate.was gate. facing Blue Blue the auxiliarycurve:curve: electrode. Bare Bare separator; separator; Diffusion red curve: red and curve: p-n drift structured p-ncurrents structured gate also where playedgate the where n-sidea role the was in n-sideenergy facing thewas losses: auxiliary facing preliminary the electrode; auxiliary data (not greenshown)electrode; curve indicated p-n green structured curvethat cells p-n gate structuredinterfaced where the p-sidegate with wher facedbaree the themembranes p-side auxiliary faced electrode. exhibited the auxiliary (a )sh Atorter scanelectrode. time rate of constants (a 0.5) At V/ sscan (of andrate (b) atof scan0.5 V/s rate and of 0.1(b) Vat/s. scan The rate blue of trace 0.1 V/s. for theThe bare blue separator trace for isthe barely bare open,separator implying is barely a very open, ca. 10 sec) than those interfaced with the diode-like gate (of ca. 20 sec). smallimplying cell capacitance. a very small (c) Speciﬁccell capacitance. cell capacitance (c) Specific as a functioncell capacitance of scan as rate. a function Colors, asof inscan (b). rate. Colors, as in (b). ) 20 ) 20 2 2 Stability tests shown in Figure 7 are for 100 CV cycles. The scan rate was relatively small (0.01 V/s), therefore, the obtained capacitance values were enhanced compared to Figure 6c. Stability was reached15 after the ca. 10th cycle. We postulate that diffusion15 processes required a few cycles before reaching stability due to the high resistance of the ionic liquid. Thereafter, both configurations appear stable and varied within 2% and 4%, respectively, when either the p-side or the n-side was facing the auxiliary electrode. Diffusion and drift currents also played a role in energy losses: preliminary data

(not shown)Ccell (uF/cm 10 indicated that cells interfaced with bare membranesCcell (uF/cm 10 exhibited shorter time constants (of ca. 10 sec) 0than those interfaced 50 with the 100 diode-like gate (of ca.050100 20 sec). No. of Cycles No. of Cycles

) 20 (a) ) 20 (b) 2 2 Figure 7. Stability measurements with modiﬁed Cu electrodes. In (a) the p-side was facing the auxiliary electrode, whereas in (b) the n-side was facing the auxiliary electrode. The scan rate was 0.01 V/s for both cases.15 15

Ccell (uF/cm 10 Ccell (uF/cm 10 0 50 100 050100 No. of Cycles No. of Cycles (a) (b) ChemEngineering 2018, 2, x FOR PEER REVIEW 7 of 9

Figure 7. Stability measurements with modified Cu electrodes. In (a) the p-side was facing the auxiliary electrode, whereas in (b) the n-side was facing the auxiliary electrode. The scan rate was 0.01 V/s for both cases.

Electrochemical impedance spectroscopy (EIS) were performed for a frequency range of 50 kHz to 50 MHz with a 10 mV AC perturbation signal. In Figure 8 we compare two cases of gate layout with respect to the auxiliary electrode. The knee frequency was 19 Hz (red) and 14 Hz (green) when the auxiliary electrode faced the n-side and the p-side, respectively. The equivalent series resistance (ESR) for ionic liquid-based cells was quite high and was of the order of tens of kΩ. The smaller ESR value was noted when the p-side was facing the auxiliary electrode. The smaller ESR value for the p- n layout was correlated with the largest overall cell's capacitance. The results point to the difference in cell impedance between the two gate layouts. This is further proof that the mid-cell gate indeed

ChemEngineeringacts as a diode-like2019, 3, 39and also points to ionic mid-cell charge separation that leads to an increase in7 of the 9 overall cell capacitance.

(a) (b)

FigureFigure 8. 8.( a(a)) Nyquist Nyquist and and ( b(b)) Bode Bode plots plots for for modiﬁed modified Cu Cu electrodes. electrodes. Red Red curve: curve: n-side n-side is is facing facing the the auxiliaryauxiliary electrode. electrode. Green Green curve: curve: The The p-side p-side faces faces the the auxiliary auxiliary electrode. electrode.

4.4. Conclusions Conclusions OneOne can can increase increase the the capacitance capacitance of supercapacitorsof supercapacitors by incorporating by incorporating a diode-like a diode-like structure structure (gate) within(gate) them.within The them. layout The of layout the diode-like of the diode-like structure structure is asymmetric is asymmetric with respect with to therespect auxiliary to the electrode, auxiliary and,electrode, as a result, and, as the a measuredresult, the measured cell’s capacitance cell's capa is acitanceffected, is asaffected, well. Such as well. a concept Such a ofconcept gate within of gate supercapacitorswithin supercapacitors (the inner (the gate inner concept), gate orconcept), in its outer or in gate its form outer [26 gate] is generalform [26] and is can general be incorporated and can be intoincorporated many current into cells’ many design current [31 cells'–33]. design One can [31–33] coat the. One gate can films coat on the top gate of eachfilms other on top [25 of,27 each,28]; other one can[25,27,28]; also press one them can togetheralso press to achievethem together a diode-like to achieve behavior. a diode-like Pressing thebeha filmsvior. togetherPressing retain the films the diode-liketogether retain I-V curve the diode-like even within I-V a wetcurve cell even [34] within (Figure a 10). wet Further cell [34] optimization (Figure 10). ofFurther the gate optimization materials, e.g.,of the reduced gate materials, graphene oxide,e.g., redu asymmetricced graphene cells with oxide, two asymmetric types of conductive cells with polymers two types and of transitional conductive metalspolymers oxides, and aretransitional some of themetals many oxides, exciting are possibilities. some of the many exciting possibilities.

AuthorAuthor Contributions: Contributions:H.G. H.G. conceived conceived and and designed designed the the experiments; experiments; T.S.C. T.S.C. performed performed the the experiments experiments and and providedprovided data data ﬁtting fitting with with assistance assistance from from H.G.; H.G.; H.G. H.G. analyzed analyzed the the data data with with assistance assistance from from T.S.C.; T.S.C.; H.G. H.G. wrote wrote the paper with assistance from T.S.C.; All authors contributed to discussions and the production of the manuscript. the paper with assistance from T.S.C.; All authors contributed to discussions and the production of the Funding:manuscript.This research received no external funding.

ConﬂictsFunding: of This Interest: researchThe received authors no declare external no conﬂictfunding. of interest.

Declaration of interest: The authors declare no conflict of interest. References

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