Mechanical, Thermal, and Electrical Energy Storage in a Single

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Mechanical, Thermal and Electrical Energy Storage in a Single working Body: Electrification and Thermal Effects upon Pressure Induced Water Intrusion-Extrusion in Nanoporous Solids Yaroslav Grosu, Michal Mierzwa, Valentin A. Eroshenko, Sebastian Pawlus, Miros#aw Chor##ewski, Jean-Marie Nedelec, and Jean-Pierre E. Grolier ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14422 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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ACS Applied Materials & Interfaces

Mechanical, thermal and electrical energy storage in a single working body: electrification and thermal effects upon pressure induced water intrusionextrusion in nanoporous solids Yaroslav Grosu [*,a,b,c], Michal Mierzwa [d,e], Valentine A. Eroshenko [c], Sebastian Pawlus [d,e], Miroslaw Chorazewski [f], Jean-Marie Nedelec [a,b], Jean-Pierre E. Grolier [*,a,b] [a] Universite Clermont Auvergne, SIGMA Clermont, Institut de Chimie de Clermont-Ferrand, BP 10448, Clermont-Ferrand, 63000, France, [email protected]. [b] CNRS, UMR 6296, 63177 Aubiere, (France), [email protected]. [c] Laboratory of Thermomolecular Energetics, National Technical University of Ukraine “Kyiv Polytechnic Institute”, Pr. Peremogy 37, 03056 Kyiv (Ukraine), [d] Silesian Center for Education and Interdisciplinary Research, University of Silesia, 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland [e] Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice (Poland). [f] Institute of Chemistry, University of Silesia, Szkolna 9, 40-006 Katowice (Poland) KEYWORDS Interface, wetting, lyophobic, high pressure, triboelectrification

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ABSTRACT The paper presents first experimental evidence of pronounced electrification effects upon reversible cycle of forced water intrusion-extrusion in nanoporous hydrophobic materials. Recorded generation of electricity combined with high-pressure calorimetric measurements improves energy balance of {nanoporous solid + non-wetting liquid} systems by compensating mechanical and thermal energy hysteresis in the cycle. Revealed phenomena provide a novel way of ‘mechanical to electrical’ and/or ‘thermal to electrical’ energy transformation with unprecedented efficiency and additionally open a perspective to increase the efficiency of numerous energy applications based on such systems taking advantage of electricity generation during operational cycle.

INTRODUCTION Efficient energy generation and storage is one of the main problems of humanity on its way to sustainable future. Considerable efforts are being made in order to develop and improve electrical 1,2, thermal 2,3 and mechanical 4,2 energy storage materials and systems. It is most often that for energy harvesting processes, transformation of the main energy source is accompanied by implication of other types of energy. A good example would be the storage of thermal energy at concentration solar plants in order to transform it into electrical energy and thus increase the overall efficiency and smooth energy production over a day or year 3. For such cases multifunctional working bodies able to store different types of energy are highly attractive. Among others new nanofluidics methods of energy harvesting are particularly topical and involve for example electrotribology 5, streaming potential 6 and reversible electrowetting 7. For example, using electrotribology a new class of high-efficient mechanical to electrical devices called triboelectric nanogenerators were recently proposed

8,9

. Gosh et al. have shown

that the flow of a liquid on single-walled carbon nanotube (SWCNT) bundles induces a voltage


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in the sample along the direction of the flow and may be potentially used as sensors 10. Following first theoretical prediction of current generation in metallic carbon nanotubes immersed in flowing liquid

11

, in references

12,13

it was demonstrated that interactions between the water

dipole chains and charge carriers in a tube can result in charge redistribution in an SWCNT, causing a voltage difference of 17.2 mV between the two ends of the tube and that the water flow can be achieved by applying a voltage to SWCNT and can be used for energy harvesting devices 13

. Considering the non-wetting case (which corresponds to this work), the possibility to initiate

reversible wetting-dewetting transition of water in hydrophobic nanopores by electric potential was reported

14

. Effect of electric field on the water and electrolyte intrusion-extrusion was

demonstrated for carbon and silica nanotubes both theoretically and experimentally, as well as using voltage to control intrusion and extrusion of non-wetting liquid into nanopores were investigated 15 -17. It was recently demonstrated that osmotic energy should be also considered for such systems

39

. In comparison to the hydrophilic case, hydrophobic nonporous materials have

been much rarely considered probably because of the high pressure required to fill them with liquid. However, stable Heterogeneous Lyophobic Systems (HLSs) with reasonable operational pressure and negligible hysteresis are available

41

and such higher pressure is related to

simultaneous mechanical energy storage, which might be advantageous for applications where mechanical energy is used to generate electricity. Additional advantage of hydrophobic material is the slippage at the wall that may contribute to better electrical to mechanical energy conversion 42. In this work we explore electrification effects in nanoporous HLSs, which, were initially proposed as new working bodies for high-capacity mechanical energy storage or thermal energy transformation

18,19

. The operational principle of HLS, which consists of a lyophobic porous


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material and a non-wetting liquid, is based on reversible cycles of forced intrusion of the nonwetting liquid into the pronounced porosity of the matrix (energy storing) followed by its spontaneous extrusion (energy restoring), see PV-isotherms on Figure 1. A convenient property of an HLS is its ability to store and restore considerable amount of mechanical energy in a rather narrow pressure range (see the intrusion and extrusion plateaus in Fig.1). In addition, it was shown that for most of HLSs the intrusion process is endothermic (the heat of ‘solid – liquid’ interface development is supplied to the system in order to perform isothermal process) and extrusion is exothermal (the heat of reduction of ‘solid – liquid’ interface is released by the system upon extrusion)

20, 36, 21

. This property allows using HLSs not only for mechanical, but

also for latent thermal energy storage. Especially considering that intrusion-extrusion cycles can be realized not only by varying the pressure of an HLS, but also by varying its temperature 22 and hence additionally taking advantage of the sensible heat stored by its components, which are the non-wetting liquid and porous matrix. There are two main types of HLSs: 1. With a small hysteresis loop H on the pressure volume diagram (Fig. 1a), which are used for energy storage; and 2. With a large value of H used for energy dissipation (shock-absorber, bumper) (Fig. 1b). Thermal effects of intrusion and extrusion are also typically hysteretic: endothermal effect of intrusion is more pronounced compared to exothermal effect of extrusion 20, 21, 36.


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Figure 1. PV-isotherms (330K) of a) {ZIF-8 + water} and b) {WC8 + water} systems. This undesirable phenomenon for energy storage applications is very useful for energy dissipation – for example absence of overheating for a car shock-absorber operating under unprecedented frequencies of up to 22 Hz

23, 24

. But in any case described the above hysteresis

phenomenon raises a question on the energy balance of an HLS: if the system stores much more mechanical and thermal energy upon isothermal compression that it restores upon reversible isothermal decompression, what is this “lost” thermomechanical energy transformed into? In this letter we make an attempt to experimentally prove the hypothesis

25

that such energy is at least

partially transformed into electrical energy mainly due to triboelectrification and can be easily used as electric current potentially increasing the efficiency of all HLS applications, which today include not only mechanical energy storage 26,19, but also mechanical energy dissipation systems (bumpers and shock-absorbers

23,27,37

, antiseismic devices

28

), negative thermal expansion

materials 22, smart applications 29, 38. In this work we used two HLSs based on distilled water: {ZIF-8 + water} and {WC8 + water} HLSs. Both systems were proven to be stable to high-pressure intrusion-extrusion cycles in the temperature ranges used in this work

21, 30

. WC8 is a commercial mesoporous silica gel

SymmetryPrep C8, in the shape of 7 µm granules grafted with octylsilanes with density of 2.1 groups/nm2 according to the data provided by the supplier (WATERS). The average pore radius is 4.2 nm; the pore volume is 0.53 cm3/g; ZIF-8 was purchased from Sigma Aldrich as Basolite Z1200, with orifice of the pores of 0.34nm and a huge surface area of 1800 m2/g. First we conducted high-pressure calorimetry measurements using the Transitiometer ST-7M 21

in order to simultaneously measure both work and heat of intrusion-extrusion cycles, where

index “i” stands for intrusion and “e” for extrusion. Results are collected in Table 1, where


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positive values mean that energy is supplied to the system, while negative values correspond to a process where energy is released by the system. It can be seen that the sum of thermal and mechanical energies stored by HLSs upon intrusion is greater compared to the sum of thermal and mechanical energies restored by HLS upon extrusion. For the {ZIF-8 + water} HLS such difference is 3.4 J/g and for the {WC8 + water} HLS it is 16 J/g. Those values are rather large compared to the energy systems store and considering the fact that after intrusion-extrusion cycles the system returns to its initial conditions (pressure, volume, temperature and stable matrix), it is logical to assume that such thermomechanical energy unbalance is transformed into another type of energy. Below we demonstrate that at least partially it is transformed into electrical energy. In order to verify the hypothesis of electrical energy generation upon intrusion-extrusion cycle high-pressure dielectric spectroscope

31

was reconfigured (Fig. 2a) to register electrical current

generated by HLS upon compression-decompression. The scheme of the setup used for electrical measurements is presented in Fig.2a. The porous matrix in the quantity of about 50 mg was placed between two stainless steel electrodes (4mm distant by Teflon spacers). A Teflon tape was used in order to keep the matrix between electrodes (Figure 2a). Next the electrodes with HLS were placed into the flexible Teflon cell previously filled with distilled water. A constant voltage in the range of 0-4 V was applied between the electrodes, while measuring the current between them when varying the pressure in the cell, by means of compressing the flexible Teflon cell surrounded by pressure transmitting oil (Fig. 2a). Experiments with NaCl salt solution were also done to check the effect of ions presence, however, a short circuit condition was obtained between electrodes and no useful signal could be measured at this configuration.


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Figure 2. a) Scheme of experimental setup for registration of electrical effects of b) {ZIF-8 + water} and c) {WC8 + water} systems at 330K and 4V. The measurements reveal that at pressures, which corresponds to intrusion/extrusion pressure of the corresponding HLS, well pronounced peaks of current are observed upon the compression/decompression process (Fig. 2b and c). Overall 50 compression-decompression cycles exhibit repeatability of observed phenomena. Such peaks were observed only in the case of voltage between electrodes was applied. They were not observed during the blank experiment with water (without porous material) independently of applied voltage. Calculated value of the generated electrical energy per gram of material (see Table 1 and corresponding description below) seems to be not strongly dependent on the applied voltage (see Figure S2 for example), however higher values of applied voltage were favorable for registration of signal with higher precision. At the same time with voltages higher than 1.23V electrolysis of water might interfere with the measurements. The reason why we didn’t observe such interference at voltages higher than 1.23V might be due to the short time of experiment. As can be seen from figure 2, intrusion or extrusion step lasts less than 30 seconds, so electrolysis effects might be negligible. Detailed dependence and optimum voltage will be investigated in upcoming work. For the {WC8 + water} HLS the extrusion peak is much sharper compared to the intrusion one. Such result is in favor of the observation that the speed of the water flow upon extrusion


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from mesoporous grafted silica is much higher compared to intrusion case, which was previously discovered 20.

Scheme 1. Possible mechanism of electrical energy generation by HLS. We believe that the triboelectrification may be responsible for recorded intrusion/extrusion current and the possible mechanism is represented in Scheme 1. Upon intrusion/extrusion the electrification effect takes place due to mutual displacement of two phases being in contact

34, 5

.

Next charged porous grains move to oppositely charged electrode and give the charge upon direct contact, which we registered as current peak (Figure 2). The movement away from the electrodes after discharge

may be due to the collective movement of the particles: the

concentration of the porous material between electrodes is rather high, so ones some particles are in contact with an electrode, they must be pushed away by the others, which were not in contact with the electrode. That might explain why current peaks are rather broad. Such mechanism explains why some voltage is required between electrodes – it provides the moving force for grains to flow towards the electrode. It also explains why for both cases, intrusion and extrusion, the current has the same sign – the charging of the porous grain does not depend on the direction of the water flow (into the pores or from the pores). Considering that such triboelectrification


8

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effect can be observed even upon water flow through 1-meter-long dielectric tube of 3mm diameter with phases contact area of about 0.2 m2 35, it is logical to assume that the triboelectric effect upon intrusion-extrusion process with huge contact ‘porous matrix – water’ area of about 90 m2 for {ZIF-8 + water} and 20 m2 for {WC8 + water} HLSs would be much more pronounced (specific surface for ZIF-8 is ~1800 m2/g and for WC8 it is ~400 m2/g). It is important to note that a porous particle must have a net charge to flow to an electrode. It is perhaps not possible in case strongly idealized process of intrusion-extrusion under perfect quasi static conditions takes place and leads to the formation of stable and charge wise symmetrical double layer. However, upon real intrusion/extrusion process there will be a large number of phenomena, which would not allow formation of perfect double layer and as a result formation of a neutral particle. Let us consider just some of them: 1) Non-equilibrium and pore geometry. Filling of the pore is a non-equilibrium process even if the whole system is being compressed quasi statically keeping the setup in quasi equilibrium. This is due to the fact that once critical pressure of intrusion/extrusion is reached the local process of intrusion/extrusion into a single pore is not controlled since the intrusion/extrusion pressure is a threshold pressure. And it stands for any geometry except of conic pores where pore opening is the base of the cone. In fact, both of the materials used in this work are known for having cage-like topology. Hence, intrusion/extrusion process is rather complex and cannot be considered as the perfect displacement of a liquid front upon intrusion/extrusion. It is strongly non-equilibrium process at the scale of the particle, with the kinetics, which might even result in viscinal water separation from the particle at the end of the intrusion/extrusion process. And this does not contradict with the observation that the whole system demonstrates quasi static macroscopic behavior (due to finite pore size distribution, for example): the intrusion/extrusion


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plateau on the PV-isotherms (Figure 1) is indeed takes place with the same controlled speed as the compression of the bulk phases. Therefore it is worth considering the possibility of a strongly non-equilibrium process of filling/emptying of the particle, which might favor the disturbance of its neutrality. Additionally, it is interesting to note that the intrusion-extrusion pressure hysteresis (which directly disturbs the energy balance of such systems) is much lower for one dimensional cylindrical pores compared to the cage like pores 26, 41. This is a strong argument in favor of the importance of pore topology for the electrification phenomena: for perfect cylinders the electrification might follow the path described by the reviewer making charge loss of a porous particle negligible and as a result not affecting the energy balance in the intrusion-extrusion cycle; while cage like pores lead to net charge of a porous particle after intrusion/extrusion process and as a result some charge loss, hence affecting the energy balance of the system. 2) Flexibility of the porous material. Upon compression/decompression, which takes place upon intrusion/extrusion, particle's porosity changes its dimensions. This process during the triboelectrification leads to the diffusion of the charges from the pores through the movement of the water as well as to the redistribution of the charges of the solid due to decrease/increase of the solid surface area. 3) Water flow near the outer surface of the particles. Upon intrusion/extrusion water would be displaced close to the outer surface of a particle (especially close to a mouth of a pore). Such relative displacement would lead to the same phenomenon of triboelectrification outside the pores, so some charges will be taken away with the water flow away from the particle. Basically, if one gets water in contact with dielectric solid and provokes relative displacement, triboelectrification leads to charges redistribution. Next, if water is removed from the solid, both


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solid and liquid remain charged. Such phenomenon was used in one of the tribonanogenerators, taking advantage of water-solid electrification 5. In the same way water movement from a pore during the extrusion would result in charged solid surface. Additionally, water flow from the pores is more favorable for charges diffusion from the pores along with water. 4) Collective movement. We should remember that in experiments performed millions of particles form the system. Hence previously described effect of outer flow would be even more pronounced. Additionally, the collective movement of those particles would create favorable conditions to induce charges diffusion from the pores, outer surface electrification and particles migration towards the electrodes. By considering some of the examples described above, it seems that formation of perfectly neutral particle upon intrusion/extrusion is unlikely for a real system. Hence, its migration towards an electrode takes place. It is interesting to note that described above charge separation might induce a electrostatic force that would have preventing effect on the liquid to be expelled from the pore. This would mean that the extrusion pressure might be lowered by this effect. It is in agreement with PVisotherms of both systems under study: extrusion pressure is ~5 MPa lower compared to intrusion pressure for ZIF-8 and more than 10 MPa for WC8. It is, however, difficult to say how large this difference should be, as we are speaking about effect of electrowetting under extreme confinement (where we cannot even use such macroscopic terms as contact angle). Not to mention effects of topology at the nanoscale. And in general it is a possibility that intrusion/extrusion pressure is affected by these charges, however even considering macroscopic terms, extracted from intrusion/extrusion pressure contact angle would be also affected by


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extreme confinement (size effect), topology of the pores and their roughness, so it seems that such information would be difficult to interpret accurately. Also we might not exclude that observed electrical effects are additionally contributed by the appearance of convection induced by the transport of liquid in and out of the space between the electrodes during pores filling and emptying. Because of the large porosity of the particle, a none negligible convective mixing might arise during intrusion and extrusion, and maybe explain the peaks of current. If this explanation is valid, then the current peaks should change according to the speed of the intrusion and extrusion. The shorter the intrusion (extrusion) the better the mixing. This is coherent with the result obtain with WC8 : the current peak during the extrusion is much higher than the one observed during intrusion, because as was discovered earlier, extrusion for this system is faster compared to intrusion

20

. For ZIF-8 it is not the case (similar

height for the two peaks) because the hysteresis is almost negligible, so mixing during intrusion and extrusion would be similar. The identification of exact mechanism will be targeted in the upcoming work, however it is of main importance that observed electrical effects are due to nanoscale intrusion-extrusion process, which provides a new method of electrical energy generation by the working body, which simultaneously stores additionally mechanical and thermal energy. Useful electrical energy, , , generated upon intrusion/extrusion process was calculated as the difference between electrical energy of intrusion/extrusion , (current peaks on Figs.2b and 2c) required to sustain the voltage between the electrodes during and the electrical energy , intrusion/extrusion and supplied by external source: , = , − , . The intrusion/extrusion ∆,

electrical energy was calculated as the work of electrical current , = ∙

, , where

is the constant potential difference between electrodes, , is intrusion/extrusion current


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peak (integration of peaks on Fig.2b and c), ∆, is the time during which intrusion/extrusion takes place. While , = ∙ ∙ ∆, , where is the current which flows through the HLS at

atmospheric pressure due to potential difference between the electrodes (it corresponds to the base line on Figs. 2b and 2c). So basically the energy of external source is subtracted from the overall energy output. Since the external source is used to sustain the constant voltage between the electrodes, there is a current even when there is no pressure applied (no motion). That is also the reason why the absolute value of the current in Figure 2 is very high. The real value of the maximum current generated by the intrusion/extrusion is the value of the current peak minus the base line (current generated due to constant voltage of external source). After substraction of the base line the maximum current in the case of {ZIF-8 + water} is about 0.05 Å and for {WC8 + water} system it is less than 0.08 Å (Figure S1). The results are collected in Table 1. It can be seen that , values improve the energy balance for the HLS and suggests that at least partially ‘lost’ thermomechanical energy of intrusionextrusion cycle is transformed into an electrical form. Such values cannot be used directly to balance the energy equation for HLS since mechanical and thermal energy were measured at different conditions compared to the electrical ones, however it strongly suggests that electrification might be the explanation of the “lost energy”. This means that upon compression, the HLS stores mechanical and thermal energy and generates electrical energy, whereas upon extrusion it partially restores mechanical and thermal energy and again generates electricity; providing a new way to transform mechanical and/or thermal energy into electrical energy with unprecedented efficiency of = 82% for ZIF-8 based system and much lower of about = 15 % for WC8 based system, where = + /

+

+ ! + ! . Such difference is


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logical considering that ZIF-8 has almost 5 times larger surface than WC8 (1800 m2/g compared to 400 m2/g). Table 1. Mechanical (W), thermal (Q) and electrical (E) energetic characteristics of HLSs (J/g). HLS

Wi

Qi

Ei

We

Qe

Ee

{ZIF-8+water}

9.9

14.4

-1.7

-8.2

-12.7

-1.1

{WC8+water}

8.5

10.9

-1.2

-1.3

-2.1

-1.2

All values are specified to mass of porous matrices, expressed in J/g. i stands for “intrusion”, e for “extrusion”. Obtained efficiency is comparable with triboelectric nanogenerators reported previously 9, although using HLS potentially provides possibility to also transform thermal energy

25, 40

into

electricity (and not only mechanical one). CONCLUSION It is clear that water intrusion-extrusion cycle in hydrophobic porous matrix is accompanied by electrical energy release, which could be easily transformed into useful electrical current and potentially increase the efficiency of numerous HLS applications. For example, grafted silica with water was shown to be excellently working for car shock-absorbers with several advantageous 23, 27, 37. In the case that such shock absorbers additionally generate electricity their usage becomes even more beneficial, particularly in electrical vehicles. The effect of hydrophobicity and porous matrix modification should be investigated as a possible route of improving the observed efficiency. Obtained result provide a new method of ‘mechanical and/or thermal to electrical’ energy transformation in a single working body.


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Supporting Information. Figure of electrical effects with subtracted base line. The following files are available free of charge. Figure S1. (file type, i.e., docx) Corresponding Author [email protected], [email protected] † Present Addresses † CIC Energigune, Albert Einstein 48, Miñano (Álava) 01510, Spain.

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[21] Grosu, Y.; Renaudin, G.; Eroshenko,V. A.; Nedelec, J-M.; Grolier, J.-P.E. Synergetic Effect of Temperature and Pressure on Energetic and Structural Characteristics of {ZIF-8 + water} Molecular Spring. Nanoscale, 2015, 7, 8803-8810. [22] Eroshenko, V. A.; Grosu, Y.: Tsyrin, N.; Stoudenets, V.; Nedelec, J-M.; Grolier, J-P. E. Exceptionally Large and Controlled Effect of Negative Thermal Expansion in Porous Heterogeneous Lyphobic Systems. J. Phys. Chem. C, 2015, 119 (19), 10266-10272. [23] Eroshenko, V.A.; Piatiletov, I.; Coiffard, L.; Stoudenets, V. A New Paradigm of Mechanical Energy Dissipation. Part 2: Experimental Investigation and Effectiveness of a Novel Car Damper. Proc. Inst. Mech. Eng. Part D, 2007, 221 (3), 301-312. [24] Guillemot, L. ; Galarneau, A. ; Vigier, G. ; Abensur, T.; Charlaix, É. New Device to Measure Dynamic Intrusion/Extrusion Cycles of Lyophobic Heterogeneous Systems. Rev. Sci. Instrum., 2012, 83, 105105. [25] Eroshenko, V.A. Influence of the Specific Interface in a Lyophobic Heterogeneous System upon Observable Thermal Effects during the Isothermal Compression of the System. Ross Khim J. (Mendeleev Chem. J.), 2002, 46 (3), 31-38. [26] Tzanis, L.; Trzpit, M.; Soulard, M.; Patarin, J. Energetic Performances of Chanel and Cage-Type Zeosils. J. Phys. Chem. C, 2012, 116 (38), 20389-20395. [27] Suciu, C. V.; Yaguchi, K. Endurance Tests on a Colloidal Damper Destinated to Vehicule Suspension. Exp. Mech., 2009, 49, 383-393. [28] Eroshenko, V.A. Repulsive Clathrates. A New Operational Material for Efficient Seismic Isolation. Proc. Inter. SMITR Conf. “Seismic Isolation, Passive Energy Dissipation and Active


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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 ACS Paragon Plus Environment

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Figure 1 a. PV-isotherms (330 K) of a) {ZIF-8 + water} system 48x42mm (300 x 300 DPI)


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Figure 1 b PV-isotherms (330 K) of b) {WC8 + water} system 48x42mm (300 x 300 DPI)



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Figure 2 a Scheme of experimental setup for registration of electrical effects 49x41mm (300 x 300 DPI)


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Figure 2 b Registration electrical effects of b) {ZIF-8 + water} system at 330 K and 4V 287x201mm (300 x 300 DPI)



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Figure 2 c Registration of electrical effects of c) {WC8 + water} 272x208mm (300 x 300 DPI)


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system at 330 K and 4V

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Scheme 1. Possible mechanism of electrical energy generation by HLS 88x33mm (300 x 300 DPI)


Mechanical, Thermal, and Electrical Energy Storage in a Single

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