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Investigation into Pseudo-Capacitance Behavior of Glycoside-containing Hydrogels Nachiket R Raravikar, Andrew Dobos, Eshwaran Narayanan, Taraka Sai Pavan Grandhi, Saurabh Mishra, Kaushal Rege, and Michael Goryll ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11113 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Investigation into Pseudo-Capacitance Behavior of Glycoside-containing Hydrogels Nachiket Raravikar*†‖, Andrew Dobos‡┴, Eshwaran Narayanan†¶, Taraka Sai Pavan Grandhi†, Saurabh Mishra§, Kaushal Rege†, and Michael Goryll*§ †

School of Engineering of Matter, Transport and Energy, Arizona State University, Tempe, AZ 85287, USA. ‡

School of Biological and Health Systems Engineering, Arizona State University, Tempe, AZ 85287, USA. §

School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ 85287, USA. ‖

former affiliation, during the span of this research: adjunct faculty at the School of Engineering of Matter, Transport and Energy, Arizona State University ┴

Current affiliation: Stryker Corporation, Tempe, AZ 85281, USA.

¶

Current affiliation: Insys Therapeutics Inc., Chandler, AZ 85286, USA.

*corresponding authors: [email protected], [email protected]

Keywords: Hydrogels, Amikagels, Biocompatibility, Pseudocapacitors, Energy Storage, Cyclic Voltammetry, Impedance Spectroscopy

ACS Paragon Plus Environment


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Abstract

Electrochemical pseudo-capacitors are an attractive choice for energy storage applications because they offer higher energy densities than electrostatic or electric double layer capacitors. They also offer higher power densities in shorter durations of time, as compared to batteries. Recent efforts on pseudo-capacitors include biocompatible hydrogel electrolytes and transition metal electrodes for implantable energy storage applications. Pseudo-capacitive behavior in these devices has been attributed to the redox reactions that occur within the electric double layer, which is formed at the electrode-electrolyte interface. In the present study, we describe a detailed investigation on redox reactions responsible for pseudo-capacitive behavior in glycosidecontaining hydrogel formulations. Pseudo-capacitive behavior was compared among various combinations of biocompatible hydrogel electrolytes, using carbon tape electrodes and transition metal electrodes based on fluorine-doped tin oxide. The hydrogels demonstrated a pseudocapacitive response only in the presence of transition metal electrodes but not in presence of carbon electrodes. Hydrogels containing amine moieties showed greater energy storage than gels based purely on hydroxyl functional groups. Furthermore, energy storage increased with greater amine content in these hydrogels. We claim that the redox reactions in hydrogels are largely based on Lewis acid-base interactions, facilitated by amine and hydroxyl side groups along the electrolyte chain backbones, as well as hydroxylation of electrode surfaces. Water plays an important role in these reactions, not only in terms of providing ionic radicals but also in assisting ion transport. This understanding of redox reactions will help determine the choice of transition metal electrodes, Lewis acid-base pairs in electrolytes, and medium for ionic transport in future bio-compatible pseudo-capacitors.


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INTRODUCTION Electrochemical capacitors, also known as pseudo-capacitors, demonstrate greater energy density than electric double layer (EDL) capacitors because of the additional reduction-oxidation (redox) reactions within the electric double layer [1-5]. While this mechanism of charge storage through redox reactions is similar to that in batteries, redox reactions in pseudo-capacitors occur locally near the electrode surface, within the electric double layer between electrodes and electrolyte. However, the charge storage density in pseudo-capacitors is limited compared to that in batteries because of the localized nature of redox reactions. On the other hand, charging and discharging can occur fast; thus, pseudo-capacitors can provide higher power density than batteries [1-5]. Research on pseudo-capacitors has picked up significantly in the last few years because pseudocapacitors may complement batteries by providing high power densities where needed [6-27].

A pseudo-capacitor requires electrodes based on multivalent ions, a dielectric which acts as an electrolyte, along with a medium for ion diffusion, thereby allowing redox reactions to occur at the electrode-electrolyte interface. The possibility of using pseudo-capacitors for in-vivo energy storage has been explored recently [23-27]. Mercier et al. demonstrated an in vivo energy harvester chip that extracted a minimum of 1.12 nW from the endocochlear potential for up to 5 hours, enabling a 2.4 GHz radio transmit measurement of the endocochlear potential every 40360 seconds [23]. In vivo environments involve ionic fluids, which can act as electrolytes if appropriate bio-compatible electrode assemblies are used. Indeed, such possibility has been demonstrated as well [24-25]. Makino et al. explored the pseudocapacitive behavior of ruthenium dioxide nanoparticles and nanosheets as electrodes in near neutral pH as an environmentally benign electrolyte [25]. The ruthenium nanosheets in acetic acid-lithium acetate



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buffered solution showed ~44% greater capacitance than the benchmark RuO2.nH2O in H2SO4 electrolyte [25]. The in vivo environment limits the use of strongly dissociative electrolytes, such as sulfuric acid, or electrodes such as activated carbons, because of their toxicity risk.

In light of such constraints, it will be possible to improve the energy efficiency of pseudocapacitors only with biocompatible materials, through a control over material composition, morphology and hierarchy of electrode-electrolyte assembly. In order to achieve such control over the pseudo-capacitor structure, it is essential to first delineate fundamental redox mechanisms in biocompatible materials and identify material formulations responsible for triggering such mechanisms. In the present study, we have used biocompatible hydrogels as electrolytes, along with two different electrodes: a transition metal oxide-based electrode and a non-activated carbon electrode as control. Cyclic voltammetry responses of the various combinations of electrode-electrolyte groups have been compared, and possible mechanisms of redox reactions at the electrode-electrolyte interface have been proposed. We also describe new insights into material design for realizing efficient biocompatible supercapacitors.

EXPERIMENTAL Two types of glycoside-containing biocompatible hydrogel-electrolytes were used in this study, along with their derivative formulations: a recently developed hydrogel, Amikagel [28], and agarose. The synthesis of gels and the subsequent pseudo-capacitor assembly as well as measurements are described below.


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Synthesis of Amikagel: Amikacin hydrate was purchased from TOKU-E (Bellingham, WA). Poly(ethylene glycol) diglycidyl ether (PEGDE) was purchased from Sigma-Aldrich Inc. (St. Louis, MO). A pre-gel solution was formed by dissolving amikacin hydrate (100 mg, 0.17 millimoles) in 0.5 mL of nanopure water (NPW) followed by the addition of PEGDE in various molar ratios. Both mixtures were combined and vortexed to form a uniform pre-gel solution. The derivative formulation with a molar ratio between Amikacin to PEGDE of 1:1.5 is termed as ‘AM1’, and the one with molar ratio of 1:3 is called as ‘AM3’.

Cell culture and viability: NIH3T3 murine fibroblasts were obtained from Professor David Capco, School of Life Sciences, Arizona State University, Tempe, AZ, as part of an existing collaboration. Cell culture media - DMEM with L-glutamine and Pen-Strep solution (10000 units/mL penicillin and 10000 µg/mL streptomycin in 0.85% NaCl) were purchased from Hyclone (Logan, UT). Fetal bovine serum (FBS) was purchased from Atlanta Biologicals (Flowery Branch, GA). Cell culture-treated 96 well plates were purchased from Corning Life Sciences (Corning, NY). Live-Dead® (Calcein AM-Ethidium homodimer) stain was purchased from ThermoFisher Scientific (Waltham, MA). Nanopure water was used in all formulations.

For cell culture studies, amikacin hydrate and PEGDE were dissolved in 1 mL of nanopure water at AM:PEGDE mole ratios of 1:1.5 for generating AM1 and 1:3 for generating AM3 hydrogels, respectively. After complete dissolution, the pre-gel solution was filtered through a 0.2 m filter. 40 L of filtered AM1 or AM3 pregels were added to the wells of the 96 well plate. The 96 well plate with Amikagel pre-gels were incubated at 40oC for 7.5 hours in order to induce gelation. AM1 and AM3 gels were washed with 250 L of sterile nanopure water for 12 hours. 10,000



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NIH3T3 cells in 150 uL cell culture media were added to the washed gels and maintained for 96 hours in a humidified chamber (95% humidity) and 5% CO2. 50% of the cell culture media was replaced with fresh media after 48 hours of incubation. Qualitative cell viability was estimated after 96 hours using Live-Dead® (Calcein AM-Ethidium homodimer) staining. Briefly, fresh serum-free DMEM media, containing a final concentration of 2 μM of ethidium homodimer-1 (EthD-1) and 1 μM of calcein AM, was added to the cultured cells for 20 minutes. Fluorescence imaging was carried out using a Zeiss fluorescence microscope. Green fluorescence emission of calcein-AM was detected using 38 HE filter set (Excitation: 470/40; Emission: 525/50) and red fluorescence of nucleic acid bound-EthD-1 was detected using a 43 HE filter set (Excitation: 550/25; Emission: 605/70). The extent of red and green fluorescence was indicative of the extent of viable and dead cells, respectively.

Generation of Agarose Gels: Genetic grade agarose powder was purchased from Fisher Scientific (Fair Lawn, NJ). Mineral oil was purchased from Acros Organics (Morris Plains, NJ). Syringes (3 mL) were purchased from Ultra Cruz – Santa Cruz Biotechnology (Santa Cruz, CA). Nanopure water was used in all experiments. A single 250 mL beaker was obtained and filled with 50 mL of NPW. A 1% w/w agarose solution was prepared by adding 0.5 g agarose powder to the NPW, while the stiffer, 10 weight % gel was formed by adding 5 g agarose. The solution was mixed and microwaved for 40 seconds using a microwave oven from SHARP (Output 1200 W). The boiling liquid was removed and allowed to cool briefly. The precursor gel was then drawn into the previously prepared syringe. Gel-containing syringes were left undisturbed at room temperature to cool. Gelation occurred in approximately 15 minutes. After gelation, a razor was used to cut the barrel of the syringe at the first tick mark and the tip was completely


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removed. The plunger was used to push the cylindrical gel out of the newly formed opening. Gel discs were cut to the desired thickness, while each disc possessed the same diameter as that of the syringe.

All fully prepared hydrogels (Amikagels and agarose) were clear, transparent, and had absorbed water content. Final gel discs in all of the above cases had a diameter of ~9 mm and a thickness of ~4 mm. Discs were stored in 20 mL scintillation vials until testing. A single Kimwipe® was placed in each vial to remove excess moisture.

Electrodes: Pilkington TEC 15 Fluorine-doped Tin Oxide [FTO] glass slides were used as electrodes in this study. FTO is a readily available material that can be deposited onto large area substrates such as glass. The electrode is composed of an oxide of Tin, which is multivalent, and is also cheaper than the conventionally used pseudo-capacitor electrode material, RuO2. Carbon tapes, commonly used in Scanning Electron Microscopy (SEM) for grounding samples, were used as ‘control’ electrodes.

Pseudo-capacitor Assembly: A schematic of pseudo-capacitor assembly is shown in Figure 1. The gel discs were sandwiched between electrodes with gentle pressure from the top, so as to ensure complete contact between the electrodes and the gels, and to keep the distance between electrodes to approximately 3 mm in all cases.

Electrochemical Measurements: Cyclic voltammetry was conducted using an Autolab PGStat 302 (Metrohm Instruments, NL), where current was measured across the voltage range of -5 V to



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+5 V, using step potential of 2.4 mV at the scan rate of 99.9 mV/sec with equilibration time of 5 seconds. Each sample was cycled three times as part of the measurement experiment. In addition, cyclic stability was tested on AM1 and AM3 samples for up to 300 consecutive cycles. Measurements were repeated over 4 to 5 hydrogel samples of each composition, and repeated on the same samples at least twice on different days. Energy stored during charging was calculated from the area under the curve of these voltammograms over a voltage range from 0 to +2.19 V. The resulting power, P = V x I, was then multiplied by time interval of 21.9 seconds estimated to reach from 0 V to 2.19 V, and thus the energy storage was estimated. In addition to cyclic voltammetry, electro-chemical impedance was measured versus frequency, on some of the same hydrogel samples, over the frequency range of 1 mHz - 1 MHz. EIS-based equivalent circuit models were constructed in order to investigate mechanisms behind the observed electrochemical impedance behavior.

RESULTS Amikagel [28] is a hydrogel derived from polymerization between a small-molecule aminoglycoside antibiotic amikacin and epoxide groups in poly(ethylene glycol) diglycidyl ether (PEGDE). Amikagel contains both amine and hydroxyl groups in the chain as shown in Figure 2. Details on Amikagel structure and preparation have already been published elsewhere [28]. Seeding of NIH3T3 cells on different formulations of Amikagels induced minimal toxicity after 96 hours of incubation as indicated by high green (live) fluorescence and minimal red (dead) fluorescence of cells, which in turn is reflective of the overall biocompatibility of these gels (Figure 3). Agarose is a polysaccharide derived from seaweed, contains a large number of


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hydroxyl groups and was investigated as another biocompatible hydrogel due to its extensive use in cellular and biomolecular studies.

To our knowledge, the pseudo-capacitive responses of Amikagel and agarose have not yet been demonstrated. Typical cyclic voltammetry curves are shown in Figure 4, where the X-axis displays applied voltage, and the Y-axis shows current in response to the applied voltage. The existence of redox reactions becomes obvious in the peaks superposed onto this curve. A truly electrostatic capacitive response would appear as a square or rectangular hysteresis loop. A transmission line, such as a conducting wire, would show up as linear, non-hysteretic curve. Figure 5 shows Bode EIS plots with the Y-axes displaying phase and magnitude, and the applied frequency shown on the X-axis. This measurement was used to determine the parameters in the subsequent equivalent circuit model, which is discussed in detail in the next section. Using a non-linear least squares algorithm, the parameters were modified from their initial values to provide an optimal match between measured data and data generated from the equivalent circuit model. These parameters are useful in determining the mechanisms of charge storage as explained later in this paper.

Figure 4 shows the cyclic voltammetry curves for all hydrogels. In presence of FTO electrodes, the hydrogels show a small but pronounced peak in current at around 1.8 V. No such peak is observed in presence of carbon electrodes, confirming the redox based pseudo-capacitive charge storage behavior of these hydrogels in presence of FTO electrodes [1-27]. To our knowledge, the pseudo-capacitive response from FTO has not been demonstrated yet.



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The relative comparison of energy storage between different formulations of Amikagel and agarose hydrogels [e.g. 1% vs 10% Agarose and AM1 vs AM3 Amikagel] is shown in Tables 12. Results in Table 1 and Figure 4 a-h indicate that low amounts of energy and hence, charge, are stored in hydrogels in the presence of carbon electrodes. The lack of a peak feature in the cyclic voltammogram, but presence of a narrow hysteresis loop, indicates that a small amount of charge is stored in such systems, which can be attributed to the electric double layer between the carbon electrode and the hydrogel electrolytes with virtually no redox contributions. However, in presence of FTO electrodes, redox peaks appear on the voltammogram for the same hydrogel disc. The possible mechanisms behind the observed redox peak are discussed in the next section.

Amikagel, AM1, shows greater energy storage capacity than AM3 and both agarose derivatives. The amount of energy stored in the two agarose formulation derivatives is not statistically different. This relative difference in energy density among various gels, as derived from cyclic voltammograms, was validated by adding additional discs of the same gel between the same two electrodes and repeating the cyclic voltammetry experiments. It was observed that the stored energy doubled with two AM3 discs as compared to a single AM3 disc (Table 2). Similarly, the stored energy increased by a factor 3 and 4, for two and three discs, as compared to a single disc, respectively in case of 10% agarose. This study helps validate the relative energy storage comparison trends among the discs of AM1, AM3 and agarose. Thus, based on Tables 1 and 2, it is safe to conclude that the average energy stored due to redox-based charge storage is two orders of magnitude greater than that stored electrostatically within an electric double layer. In addition, the average energy stored in AM1 gels is greater than all other gels investigated. Modeling


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equivalent circuit elements based on the EIS plots of hydrogels in presence of FTO electrodes (Figure 6) indicates a capacitance approximately in the range of 10-14 F.

DISCUSSION The above results indicate that a redox-based pseudo-capacitive response is observed in glycoside-containing hydrogels measured against FTO electrodes. The redox reactions in biological systems have been explained in literature [2, 29-30], and were employed to further elucidate potential mechanisms contributing to pseudo-capacitance in the current biocompatible hydrogel-electrode system. The EIS approach has been successfully used in the past in biological systems, to understand foreign body response using implanted biosensors [31], and also, to separate Faradaic contributions from non-Faradaic contributions of a pseudo-capacitor [32-33].

It is important to note that all hydrogels, by definition, contain a significant amount of water. Thus, an abundance of protons [H+] and hydroxyl groups [OH-] is available to participate in biochemical reactions, as shown in equation 1 in Appendix I, Supporting Information section. Amikagels contains nitrogen atoms in the side groups [-R-NH2] and also within the chain backbone [R-N(H)-R]. Amikagels also contain oxygen atoms present in hydroxyl side-groups [R-OH] and also in the chain backbone as R-O-R. Agarose, on the other hand, contains only hydroxyl groups, but nitrogen atoms are not present in its chain structure. Lewis bases result in loan pairs of electrons which participate in a redox reaction by electron donation. Nitrogen and oxygen are Lewis bases; a bound nitrogen atom contains one lone pair of electrons and a bound oxygen atom contains two lone pairs of electrons that are available for a redox reaction. When one of the FTO electrodes is positively biased, the hydrogel monolayer adjacent to that electrode



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will be negatively charged, thus forming an electric double layer. Both Lewis bases within the hydrogel, N and O, can then protonate by forming coordinate bonds with the free H+ radicals from water, according to equations 2-5, Appendix I, Supporting Information section. Under equilibrium, the charge and mass transfer due to depletion of protons within the electric double layer must be balanced, and hence the hydroxyl radicles from water must also react. We hypothesize that the hydroxyl radicals may react with Sn2+ from the FTO electrode to form hydrated tin [II] oxide residue, according to equations 6-7, Appendix I. Indeed, visible staining of an originally transparent FTO electrode was noticed after multiple cyclic voltammetry tests, thus supporting the hypothesis about a permanent tin [II] oxide precipitation at the electrodehydrogel interface, as result of these redox reactions through multiple cycles of charging. The FTO electrode is doped with fluorine, which may dissociate in aqueous medium as anions, F-. When the counter FTO electrode in this configuration is negatively charged, the fluorine anions can protonate at the electrode (equation 8, Appendix I, Supporting Information section). The hydrogel monolayer adjacent to the electrode will now be positively charged. In this case, the bound amines in hydrogels will hydrolyze (equation 9, Appendix I, Supporting Information section) in order to balance the charge within this electric double layer.

All these reactions can occur if the activation energy barrier for them can be overcome in the system. Above a sufficient voltage of approximately 1.8 V, a rather ‘broad’ peak in current is observed when most of the above reactions would complete. It is interesting to note that a broad peak or ‘hump’ is observed rather than a sharp peak in cyclic voltammograms of the hydrogelFTO system. We hypothesize that such a broad peak could be composed of several peaks, from


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multiple redox reactions such as shown in equations 1-9 in Appendix I, Supporting Information section [2].

The peaks in the cyclic voltammograms are not independent of the electrode material; these peaks are only observed with FTO electrodes but not with carbon electrodes, in case of all hydrogels investigated. Also, the fact that FTO electrodes participate in the redox reaction is evident from the noticeable discoloration of the originally clear, transparent electrode after multiple measurements. No such discoloration of gels themselves or the carbon electrodes was observed despite multiple measurements. Thus, it is clear that redox reactions occur between the FTO electrode and the hydrogel electrolyte, confirming the pseudo-capacitive behavior.

The above reaction mechanisms also help explain the observed enhancement in energy storage in case of AM1 as compared to the rest of hydrogels. As described earlier, Amikagel formulations contain amines along with hydroxyl groups, whereas both agarose gel formulations contain only hydroxyl groups and no amines. Due to the presence of both, amines and hydroxyl groups in Amikagels, the redox carriers add up from all of the reactions as shown in equations 1-9. Moreover, AM1 has a higher amount of free amines than AM3 due to a lower degree of crosslinking; thus, more current is expected for a given applied voltage range, resulting in greater energy storage than AM3 and other gels. On the other hand, only hydroxyl carrier based redox reactions are likely to occur (equations 1 and 4-8, Appendix I, Supporting Information section) in case of agarose derivatives. As a result, lower current and lower energy density are observed within a given voltage range, for agarose gel derivatives than for the Amikagel derivatives. This



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mechanism demonstrates an important formulation parameter for improving the energy density of future biocompatible hydrogel-based pseudo-capacitors.

The cyclic voltamogramms were determined up to fairly high voltages in order to observe the proposed redox reactions. Since the hydrogels contain a significant amount of water, the electrolyte can be considered aqueous despite the solid support matrix. Thus, deterioration of the aqueous electrolyte is likely to occur at voltages above 1V applied bias and certainly at voltages around 1.8V where the redox reaction is observed. The deterioration of the electrolyte can be observed in cyclic stability tests, which show a reduction in the energy storage capacity over the course of several hundred cycles (cyclic voltammograms can be found in Supporting Information Figure S1). After repeated cycling, the hydrogels appear to have lost moisture, which is consistent with electrochemical reactions above 1V applied bias. These experiments demonstrate that in order to enhance energy storage via redox reactions, a material system is necessary which exhibits a redox reaction at 1V applied bias or below.

The proposed mechanisms of energy storage based on redox reactions among the various anions and cations are further validated through the equivalent circuit model of the hydrogel-electrode system. The equivalent circuit for the hydrogel-FTO psuedo-capacitor assembly is shown in Figure 6. Besides the electrode and solution resistance being modeled by the series resistance R1, the contribution of the electric double layer and the pseudo-capacitance are modeled by a parallel combination of a capacitance C1 in series with a diffusion-related constant phase element (CPE) W1 and a Faradaic resistance R2 in series with a second CPE W2, respectively. The equivalent


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circuit models for all gels are similar and only differ in values for the individual circuit parameters.

The various equivalent circuit parameters based on iterative non-linear least squares fitting of the equivalent circuit model to the EIS measurements are shown in Table 3 for two bias voltages. At 0V bias, all hydrogels are expected to exhibit a behavior dominated by the electrochemical double layer (EDL). However, at 2V bias, evidence of redox reactions should manifest itself in a change in the redox-related circuit parameters. All parameters are shown in Appendix II of Supporting Information Section. While there is some sample-to-sample variation in circuit parameters present due to variations in sample-electrode contact area and the change of the FTO electrodes due to the redox reaction, certain trends can be observed when comparing different gels and electrode combinations. In case of the hydrogel-FTO combination, a decrease in EDLrelated capacitance C1 can be observed along with an increase in redox-related admittance W2 when the bias is increased. No such behavior is observed in case of carbon-based electrodes. This clearly confirms the hydrogel-FTO combination as redox-based and the hydrogel-carbon combination as purely electrostatic double layer based. Further, general trends among various hydrogels indicate a mild increase in admittance for gels with amines compared to agarose gels with no amines. The observed trends as explained above hold true despite this statistical variation.

Biocompatible hydrogel-based pseudo-capacitors may find applications in the future electronic devices where in vivo energy harvesting and storage is required. However, in order to serve as an energy storage device, the energy density and thus the charge density of the device must be



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comparable to the state-of-the-art devices. However most devices are not biocompatible and use toxic electrolytes such as sulfuric acid. The challenge, therefore, is to make high energy density biocompatible pseudo-capacitors using biocompatible electrode and electrolyte materials. Based on the knowledge generated from the present work, biocompatible electrolytes with high concentrations of anions and cations along with highly dissociative electrolytes would help improve the charge density. Nanoparticles and nanosheets may be employed to improve energy density by providing high electrode surface area [24-25]. In addition, new manufacturing methods, including layered printing or self-assembly approaches, will be needed in order to achieve the best combination of electrolytes and high surface area electrodes.

CONCLUSIONS We have investigated the electrochemical behavior of glycoside-containing hydrogels for potential applications as pseudo-capacitors. We confirm the conventional understanding that a transition metal-based electrode and an electrolytic dielectric are essential to the redox reactions in pseudo-capacitors. Moreover, we propose possible redox reactions within the electric double layers formed near the electrodes of both polarities. A positively biased electrode is likely to undergo tin hydrolysis leading to the formation of tin oxide residue at the electrode surface, and protonation of Lewis bases - O and N - in the hydrogels. A negatively charged electrode is likely to undergo protonation of fluorine cations, and hydroxylation of amines and free protons within the electrolyte. The presence of hydroxyls and amines helps enhance the overall redox reaction energy exchange thus providing more current for a given voltage. These proposed redox reaction mechanisms are further validated by equivalent circuit models. Our results indicate that it is


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essential to have redox species containing Lewis acids and Lewis bases in order to maximize the charge carrier concentration, and thus improve charge storage density. Ultra-high surface area electrodes of transition metal oxides help improve the charge storage further by improving redox efficiency, such as electrodes based on mesoporous or nano-porous electrode structures. Further, hierarchically controlled assemblies of electrodes and electrolytes will help maximize the redox efficiency, and therefore, charge storage. These hierarchically assembled materials will lead to new generations of biocompatible super capacitors with significantly improved performance for bio-electronic device energy storage applications.

ACKNOWLEDGEMENTS The authors thank Mr. Karthik Pushpavanam, Ms. Maryam Ridha and Mr. Xiaofeng Wang for several technical discussions and help with sample preparation and measurements. This work was funded in part by the National Science Foundation (Grant CBET-1067840 to KR and Grant ECCS-1150024 to MG).

SUPPORTING INFORMATION Supporting Information is available on the equations for the chemical reactions described in the manuscript, the raw data of all Equivalent Circuit Parameters referred to in Table 3 and the cyclic stability plots.



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References 1. Conway, B. E. Transition from Supercapacitor to Battery Behavior in Electrochemical Energy-Storage J Electrochem Soc 1991, 138, 1539-1548. 2. Simon, P.; Gogotsi, Y. Materials for electrochemical capacitors Nat Mater 2008, 7, 845854. 3. Brousse, T.; Belanger, D.; Long, J. W. To Be or Not To Be Pseudocapacitive? J Electrochem Soc 2015, 162, A5185-A5189. 4. Long, J. W.; Brousse, T.; Belanger, D. Electrochemical Capacitors: Fundamentals to Applications J Electrochem Soc 2015, 162, Y3-Y3. 5. Choudhury, N. A.; Sampath, S.; Shukla, A. K. Hydrogel-polymer electrolytes for electrochemical capacitors: an overview Energ Environ Sci 2009, 2, 55-67. 6. Nohara, S.; Wada, H.; Furukawa, N.; Inoue, H.; Morita, M.; Iwakura, C. Electrochemical characterization of new electric double layer capacitor with polymer hydrogel electrolyte Electrochim Acta 2003, 48, 749-753. 7. Choudhury, N. A.; Sampath, S.; Shukla, A. K. Gelatin hydrogel electrolytes and their application to electrochemical supercapacitors J Electrochem Soc 2008, 155, A74-A81. 8. Sampath, S.; Choudhury, N. A.; Shukla, A. K. Hydrogel membrane electrolyte for electrochemical capacitors J Chem Sci 2009, 121, 727-734. 9. Stepniak, I.; Ciszewski, A. Electrochemical characteristics of a new electric double layer capacitor with acidic polymer hydrogel electrolyte Electrochim Acta 2011, 56, 24772482. 10. Brezesinski, T.; Wang, J.; Tolbert, S. H.; Dunn, B. Next generation pseudocapacitor materials from sol-gel derived transition metal oxides J Sol-Gel Sci Techn 2011, 57, 330335. 11. Senthilkumar, S. T.; Selvan, R. K.; Ponpandian, N.; Melo, J. S. Redox additive aqueous polymer gel electrolyte for an electric double layer capacitor Rsc Adv 2012, 2, 89378940. 12. Chinnathambi, S.; Ammam, M. A molecular hybrid polyoxometalate-organometallic moieties and its relevance to supercapacitors in physiological electrolytes J Power Sources 2015, 284, 524-535. 13. Hur, J.; Im, K.; Hwang, S.; Choi, B.; Kim, S.; Hwang, S.; Park, N.; Kim, K. DNA hydrogel-based supercapacitors operating in physiological fluids Sci Rep-UK 2013, 3, 1282. 14. Wu, X. L.; Wen, T.; Guo, H. L.; Yang, S. B.; Wang, X. K.; Xu, A. W. Biomass-Derived Sponge-like Carbonaceous Hydrogels and Aerogels for Supercapacitors ACS Nano 2013, 7, 3589-3597. 15. Hall, P. J.; Mirzaeian, M.; Fletcher, S. I.; Sillars, F. B.; Rennie, A. J. R.; Shitta-Bey, G. O.; Wilson, G.; Cruden, A.; Carter, R. Energy storage in electrochemical capacitors: designing functional materials to improve performance Energ Environ Sci 2010, 3, 12381251. 16. Zhao, X.; Sanchez, B. M.; Dobson, P. J.; Grant, P. S. The role of nanomaterials in redoxbased supercapacitors for next generation energy storage devices Nanoscale 2011, 3, 839-855. 17. Pan, L. J.; Yu, G. H.; Zhai, D. Y.; Lee, H. R.; Zhao, W. T.; Liu, N.; Wang, H. L.; Tee, B. C. K.; Shi, Y.; Cui, Y.; Bao, Z. N. Hierarchical nanostructured conducting polymer hydrogel with high electrochemical activity P Natl Acad Sci USA 2012, 109, 9287-9292.


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18. Hatzell, K. B.; Beidaghi, M.; Campos, J. W.; Dennison, C. R.; Kumbur, E. C.; Gogotsi, Y. A high performance pseudocapacitive suspension electrode for the electrochemical flow capacitor Electrochim Acta 2013, 111, 888-897. 19. Xiong, G. P.; Meng, C. Z.; Reifenberger, R. G.; Irazoqui, P. P.; Fisher, T. S. A Review of Graphene-Based Electrochemical Microsupercapacitors Electroanal 2014, 26, 30-51. 20. Iavicoli, I.; Leso, V.; Ricciardi, W.; Hodson, L. L.; Hoover, M. D. Opportunities and challenges of nanotechnology in the green economy Environ Health-Glob 2014, 13. 21. Kurra, N.; Alhebshi, N. A.; Alshareef, H. N. Microfabricated Pseudocapacitors Using Ni(OH)(2) Electrodes Exhibit Remarkable Volumetric Capacitance and Energy Density Adv Energy Mater 2015, 5. 22. Zhou, M. Q.; Glushenkov, A. M.; Kartachova, O.; Li, Y. C.; Chen, Y. Titanium Dioxide Nanotube Films for Electrochemical Supercapacitors: Biocompatibility and Operation in an Electrolyte Based on a Physiological Fluid J Electrochem Soc 2015, 162, A5065A5069. 23. Mercier, P. P.; Lysaght, A. C.; Bandyopadhyay, S.; Chandrakasan, A. P.; Stankovic, K. M. Energy extraction from the biologic battery in the inner ear Nat Biotechnol 2012, 30, 1240-1244. 24. Lv, Z. S.; Xie, D. H.; Li, F. S.; Hu, Y.; Wei, C. H.; Feng, C. H. Microbial fuel cell as a biocapacitor by using pseudo-capacitive anode materials J Power Sources 2014, 246, 642-649. 25. Makino, S.; Ban, T.; Sugimoto, W. Towards Implantable Bio-Supercapacitors: Pseudocapacitance of Ruthenium Oxide Nanoparticles and Nanosheets in Acids, Buffered Solutions, and Bioelectrolytes J Electrochem Soc 2015, 162, A5001-A5006. 26. Bettinger, C. J. Materials Advances for Next-Generation Ingestible Electronic Medical Devices Trends Biotechnol 2015, 33, 575-585. 27. Li, L. L.; Wang, Y. Q.; Pan, L. J.; Shi, Y.; Cheng, W.; Shi, Y.; Yu, G. H. A Nanostructured Conductive Hydrogels-Based Biosensor Platform for Human Metabolite Detection Nano Lett 2015, 15, 1146-1151. 28. Grandhi, T. S. P.; Mallik, A.; Lin, N.; Miryala, B.; Potta, T.; Tian, Y.; Rege, K. Aminoglycoside Antibiotic-Derived Anion-Exchange Microbeads for Plasmid DNA Binding and in Situ DNA Capture ACS Appl Mater Inter 2014, 6, 18577-18589. 29. Forman, H. J.; Ursini, F.; Maiorino, M. An overview of mechanisms of redox signaling J Mol Cell Cardiol 2014, 73, 2-9. 30. Dupont, M. F.; Donne, S. W. Separating the Faradaic and Non-Faradaic Contributions to the Total Capacitance for Different Manganese Dioxide Phases J Electrochem Soc 2015, 162, A5096-A5105. 31. Karp, F. B.; Bemotski, N. A.; Valdes, T. I.; Bohringer, K. F.; Ratner, B. D. Foreign body response investigated with an implanted biosensor by in situ electrical impedance spectroscopy IEEE Sens J 2008, 8, 104-112. 32. Brezesinski, T.; Wang, J.; Senter, R.; Brezesinski, K.; Dunn, B.; Tolbert, S. H. On the Correlation between Mechanical Flexibility, Nanoscale Structure, and Charge Storage in Periodic Mesoporous CeO2 Thin Films ACS Nano 2010, 4, 967-977. 33. Miller, J. M.; Dunn, B. Morphology and electrochemistry of ruthenium/carbon aerogel nanostructures Langmuir 1999, 15, 799-806.



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Figure 1. Schematic drawing of the assembly and measurement setup of pseudo-capacitor configurations.

Figure 2. Structure of Amikagel electrolyte used in pseudo-capacitors [Reproduced with permission from reference 28. Copyright 2014 American Chemical Society.]


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Figure 3. AM1 (a) and AM3 gels (b) induced minimal toxicity in NIH3T3 murine fibroblast cells after 96 hours of seeding. Representative phase contrast and fluorescence (Live-Dead®) images of the NIH3T3 murine fibroblast cells on AM1 and AM3 Amikagels are shown.

a. AM1 gels

b. AM3 gels



Figure 4. Cyclic voltammograms of Amikagels (a-d) and Agarose (e-h). Panels (a,c,e and g) show the cyclic voltammograms measured using a redox-active electrode (FTO glass) while panels (b, d, f and h) show the voltammograms measured using carbon electrodes. The energy storage capability is higher in AM 1 hydrogels (a) than in AM 3 hydrogels (b). The energy storage capability of the gels is smaller using carbon electrodes than FTO electrodes.

(a)

AM1 FTO

300µ

(b) Current (A)

Current (A)

AM1 Carbon

3µ 2µ

200µ 100µ 0 -100µ

1µ 0 -1µ

-200µ

-2µ

-300µ

-3µ

-6

-4

-2

0

2

4

-6

6

-4

-2

AM3 FTO

(c)

300µ

(d) Current (A)

Current (A)

100µ 0 -100µ

AM3 Carbon

3µ

1µ 0 -1µ

-4

-2

0

2

4

-6

6

-4

-2

Agarose 1% FTO

2

4

6

300µ

Agarose 1% Carbon

(f)

3µ 2µ

Current (A)

200µ

Current (A)

0

Voltage (V)

Voltage (V)

100µ 0 -100µ

1µ 0 -1µ -2µ

-200µ

-3µ

-300µ -4

-2

0

2

4

-6

6

-4

-2

0

2

4

6

Voltage (V)

Voltage (V) Agarose 10% FTO

(g)

300µ

(h)

200µ

Agarose 10% Carbon

3µ

Current (A)

2µ

100µ 0 -100µ -200µ

1µ 0 -1µ -2µ

-300µ -6

6

-3µ

-300µ

-6

4

-2µ

-200µ

(e)

2

2µ

200µ

-6

0

Voltage (V)

Voltage (V)

Current (A)

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-3µ -4

-2

0

Voltage (V)

2

4

6

-6

-4

-2

0

Voltage (V)


2

4

6

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Figure 5. EIS Impedance results of Amikagels (a-c) and Agarose (d). Panels (a, b and d) report measurements performed on FTO electrodes while panel (c) shows the impedance spectrum of carbon electrodes. The simulations have been performed using the equivalent circuit shown in Figure 6 and the resulting circuit parameters have been listed in Table 3.

AM1 FTO Electrodes

AM3 FTO Electrodes

Z (Experiment) Z (Simulation) Theta (Experiment) Theta (Simulation)

100k

70 60

10k

50

1k

40 30

100 20 10

0

10

1

10

2

10

3

4

10

10

5

10

100k

0 6 10

50

1k

40 30

100 20

1 -1 10

10 0

10

1

10

2

10

3

AM1 Carbon Electrodes 1M

50 40

1k

30 100 20 10

10 1

10

2

10

3

4

10

10

5

10

0 6 10

- Phase (deg)

60

Impedance (Ohm)


0

0 6 10

90

(d)


70

10

5

10

10M

80

(c)

1 -1 10

10

Agarose 10% FTO Electrodes 90

10k

4

10

Frequency (Hz)

10M

100k

70

10k

Frequency (Hz)

1M

80

60

10

10

1 -1 10


1M

- Phase (deg)

Impedance (Ohm)

1M

90

(b)

80

100k

80 70 60

10k

50

1k

40 30

100 20 10 1 -1 10

10 0

10

Frequency (Hz)


1

10

2

10

3

10

Frequency (Hz)

4

10

5

10

0 6 10

- Phase (deg)

(a)

10M

- Phase (deg)

90

Impedance (Ohm)

10M

Impedance (Ohm)

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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

Figure 6. Equivalent circuit for hydrogel-FTO electrode assembly of gel-based pseudocapacitors used to determine the parameters reported in Table 3.

W1

C1 R1 W2

R2 R1 = solution resistance, i.e. resistance to ionic diffusion, offered by the entire electrolyte Electric double layer parameters: C1 and W1 C1 = Electric double layer capacitance W1 = Electric double layer admittance Pseudo-capacitance parameters: R2, W2 R2 = Resistance to redox reactions within the electric double layer W2 = Redox reactions admittance


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Table 1. Comparison of energy storage among various pseudo-capacitor electrode-electrolyte combinations, as derived from cyclic voltammetry curves. Samples AM1 with FTO electrodes AM1 with carbon electrodes AM3 with FTO electrodes AM3 with carbon Agarose 1% [FTO] Agarose 1% [carbon] Agarose 10% [FTO] Agarose 10% [carbon]

Energy [mJ] 2.1681 0.0055 0.7884 0.0059 0.9636 0.0074 0.7884 0.0039

Standard Deviation 1.0950 0.0015 0.3723 0.0024 0.1752 0.0004 0.2628 0.0007



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Table 2. Comparison of energy storage with multiple discs of the same gel under the same FTO electrodes, as derived from cyclic voltammetry curves. Samples [all with FTO electrodes] AM3-1 disc AM3-2 discs Agarose 10%-1 disc Agarose 10%-2 discs Agarose 10%-3 discs

Energy [mJ] 0.79 1.71 0.79 2.83 3.26

Standard Deviation 0.37 0.50 0.26 1.03 1.03


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Table 3. Pseudo-capacitive redox admittance [W2] comparison based on EIS curves and equivalent circuit models

C1 0V 14.1 5.75 6.19 0.05

AM1-FTO [2 samples tested] AM3-FTO [3 samples tested] Agarose 10%-FTO [1 sample] AM1-Carbon [1 sample] R1 [kΩ] 0V 2V 1.85 1.67 AM1-FTO 6.18 6.10 AM3-FTO 6.72 Agarose 10%- 4.95 FTO 4.88 AM1-Carbon 4.88

W2 0V

2V 1.68 1.39 1.90 0.05

5.93 3.41± 0.31 2.28 0.22

2V 6.73 5.63±1.4 2.57 0.68

C1 [µF] 0V 2V 8.14 2.83 9.29 1.80 6.19 1.90

W1 [µS*s½] 0V 2V 63.3 43.5 22.9 10.9 53.7 26.7

R2 [kΩ] 0V 2V 0 620 0 374 0 0

W2[µS*s½] 0V 2V 5.39 28.1 3.13 6.81 2.28 2.57

0.05

0.001

59.8

0.22

0.05

0.001


61.4

0.68


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