Green and Scalable Biopolymer-Based Aqueous Polyelectrolyte Complexes for Zinc-Ion Charge Storage Devices

Green and scalable materials are essential to fulfill the need of electrification for transitioning into a fossil-fuels free society, and sustainability is a requirement for all new technologies. Rechargeable batteries are one of the most important elements for electrification, enabling the transition to mobile electronics, electrical vehicles and grid storage. We here report synthesis and characterization of polyelectrolyte complexes of alginate and chitosan, both biopolymers deriving from the sea, for transport of zinc ions in hydrogel electrolytes. We have used vibrational spectroscopy, thermal measurements and microscopy, as well as transport measurements with ohmic or blocking contacts. The transference number for zinc ions is close to 1, the conductivity is � 10 mS/cm, with stability at Zn interfaces seen through 7000 cycles in symmetric zinc//zinc cell.


Homage to our predecessors
In this special issue, honoring the Swedish Chemical Society, we take the opportunity to honor those who contributed to our current state of science.The transition of chemical knowledge, from earlier forms of natural philosophy and alchemy, to a chemical science based on measurements and experiments, [1] during the Enlightenment of Europe, was brought about by many brilliant minds, and a long period of dissociation from Galenic and Paracelsian concepts of chemistry.Some of these minds -Torbern Bergman, Carl Wilhelm Scheele, Gahn -were active in Sweden in the 18 th century.This is one of the reasons for the proliferation of element names in the periodic table deriving from a Swedish locality.Ytterbium (Yb), yttrium (Y), terbium (Tb), erbium (Er), holmium (Ho), scandium (Sc), gadolinium (Gd), tulium (Tm) and tantal (Ta) were found in minerals from a mine at Ytterby in the archipelago near Stockholm; gadolinium was named in honor of Gadolin, one of the chemists of the time.The great Jöns Jacob Berzelius discovered cerium in a mine at Bastnäs, Skinnskatteberg.He may be better known for his discovery of silicon, selenium and torium, and his contributions to the study of the chemical properties of living matter, as well as proposing a theory of matter based on electrochemical phenomena.His concept of chemistry may have well been summarized in the name of this journal, ChemElectroChem.He was also a permanent secretary in the Royal Swedish Academy of Sciences in the early nineteenth century, and his writings are still very readable 200 years later and a source of inspiration for scientists of today.
Berzelius' doctoral thesis in medicine was on the topic of electrical stimulation of the human body, and his experimental subjects were retired veterans from the wars between Sweden and Russia.Electricity was at this time a mysterious phenomenon, vaguely associated with vitalism and the distinction between animate and inanimate matter.The long debate between Volta and Galvani as to whether electrical phenomena were essential to living systems was based on experiments with twitching frog legs -the most sensitive electric meter at that time -but also on the Volta pile, the electrical batteries of the day, where stacks of metals with interlaced wet tissue formed series connected primary batteries.These were curiosities, but attracted much intellectual attention and helped to shape both the theories of electricity and neurobiology. [2]Scientists such as Humphry Davy and his successor Michael Faraday, who demonstrated the laws of electrolysis in 1833, laid foundation for electrical energy storage.
This special edition honors the Swedish Chemical Society, founded much later, in 1883, and was a sign of the formation of a profession of university educated chemists, now serving also the needs of industry.The Arrhenius plaquette is given by the Society for prominent work in chemistry, named after Svante Arrhenius.He was a polymath of physics and chemistry, and awarded the Nobel prize in Chemistry in 1903 for his theory on the electrolytic dissociation of salts in solutions.This is one of the intellectual sources of our present day electrolytes in batteries, and thus part of the solution to the storage of energy.In the late 19 th century, Svante Arrhenius was among the first to calculate the greenhouse effect of carbon dioxide in the atmosphere, thus exposing one of the current truly global problems.
These eminent scientists thus helped establish theories of matter relevant for one of the currently most pressing problems; how to store the ever-increasing fraction of renewable and intermittent electrical energy from solar and wind energy, for use at night and with no wind.
Another one of these major Swedish scientists was Carl von Linné, a member of the group of Enlightenment scientists founding the Royal Swedish Academy of Sciences in 1739.Linné

Introduction
A renaissance of rechargeable aqueous Zn batteries, as an alternative to high energy density Li ion batteries, and suitable for safer and more sustainable battery chemistry, is visible.The difficulties of using zinc electrodes in aqueous electrolytes have been many, with hydrogen evolution, dendrite formation and formation of Zn oxides as main culprits in preventing stable zinc electrodes in contact with aqueous environments.This is now changing, as seen in a steady flow of new reports on the different methods to avoid dendrite formation and hydrogen evolution.A most important development is that of water-insalt electrolytes (WISE), first reported for aqueous lithium ions and batteries, [3] where the fraction of water is much smaller than in the standard dilute electrolytes previously used, so the reactivity of the water molecules is suppressed.Related examples are recently reported [4][5][6] for conduction of Zn ions in aqueous media.
The formation of a solid/electrolyte interface (SEI) prior to or during electrochemical operation, is the other main approach to stabilize the Zn electrode surface.This takes many forms, where metals, semiconductors or insulators are coated in thin films on top of the Zn electrode prior to operation, or during operation in the aqueous environment.More important are the examples where Zn metal and electrolyte react to form a thin layer of new chemical species, which help to stabilize the interface towards dendrite formation upon repeated charging/ discharging in a secondary battery.Some of these surface modifications [7] are based on thin organic or polymeric layers, such as ionic liquids [8] or polymeric adhesives, [9] and an even smaller set is based on molecules [10] and polymers [11] deriving from biological sources.The design of a WISE also reacting to form a stable SEI on Zn, using sustainable, cheap and scalable materials of biological origin is an important goal for greening of Zn batteries.
Hydrogel electrolytes have been studied for many years, [12] and are typically formed by crosslinking of polymer chains from synthetic or biological sources, by various forms of interaction.With ionic polyelectrolytes, electrostatic attraction is one of them, as well the crosslinking of chains with multivalent ions, but also π-π stacking, van der Waals interactions and more can contribute, often several in the same material.As the hydrogels contain large amounts of water, the role of the polymer chains is often that of hindering the transport of ions and solvent, but the formation of nanopores with surface charges for easy transport of ions and solvent, can add another mechanism of transport along the surface and assisted by the surface.
Polyelectrolyte hydrogels have been used in electrochemistry for a long time.[17] A major driving force in these developments have been the need to extend the electrochemical window to encompass electrode reactions for high voltage.This is a more realistic goal for zinc ion batteries.
The use of polyzwitterionic hydrogels in electrolyte [18] supplies in one chain both anionic and cationic moieties, with advantages for simultaneous optimization of conductivity and interfacial chemistry, but at the price of more complex synthesis.The need to operate electrochemical devices for stationary energy storage at cold temperatures (À 20 °C, [6a] À 40 °C [19] ) is addressed in recent reports.For a broader temperature window, a organohydrogel electrolyte was added with ethylene glycol to operate from À 30 °C to 80 °C. [20]The need to handle thermal runaway by evaporation of water in zinc ion batteries, [21] reducing conductance and current, but later condensing within the electrolyte, restoring conductance give an element of selfhealing.The needs for flexible, selfhealing and stretchable electrolytes for applications in textile electronics is yet another recent development.A review [22] combines the perspectives of flexible and stretchable polyelectrolyte hydrogels with aqueous solvent, including both synthetic and natural polyelectrolytes.Some of these use biopolymers as elements in the construction of hydrogel networks.
To create a medium for selective transport of Zn 2 + ions, the hydrogel electrolyte should allow for reversible polymer-waterion binding so the transport of the divalent ions will be favored.As the ions of a WISE are becoming more and more concentrated upon removal of the water, the shielding of ions and the crystallization of salts are inevitable effects.With polyelectrolytes both effects are evident, as in the Manning condensation of counterions on a single polyelectrolyte chain.Before a complete Manning condensation, there should be polymer sites with different local charge due to the polymer chain bound ions and the small counterions making more or less perfect matching of sites.Tunneling of ions will be irrelevant -except maybe for protons -but mediation of ion transport by interaction with charged walls in a solvation channel can help.At least this is the inspiration from biological nanopores formed out of protein molecules, ubiquitous structures found spanning biological membranes.But also free water in a wider pore may form the appropriate medium for transport of ions.It is thus attractive to introduce some element of heterogeneity in forming the hydrogels, so that surfaces with different charges are present, as well as pores filled with water.
The intrinsic disorder in this phase could be useful.Following the example of electronic transport in disordered media, where thermally assisted tunneling -hoppingbetween localized states give rise to variable range hopping [23] along a percolating path through the material, it is inspiring to follow this model also for ionic charge carriers.Similar motivations are found in recent solid state ionic conductor materials of a very different nature, in the form of high energy alloys. [24]e have used a polyanionic polyelectrolyte -alginate (AL) from the carbohydrate biopolymer mainly found in algae-and combined this with a polycationic polyelectrolyte -chitosan (CH) deriving from chitins in shells of crustaceans.When the solvated chains meet, they will form coacervates, where both polyelectrolytes help neutralize each other, and set free the small cation and anion species originally associated with the polyelectrolytes.Formation of such polyelectrolyte complexes has been studied, in the food and biotechnology literature. [25]oarcervates have also been characterized as electrolytes, [26] as binders in electrodes [27] and other electrochemical functions as reviewed in Ref. [28]. [28]e here demonstrate a solvent-based processing route to form compact polyelectrolyte complexes (COPEC) of two biological polyelectrolytes: polyanionic alginate and polycationic chitosan.By introducing Zn salts into these materials, and sometimes removing excess small ions, a hydrogel biopolymer electrolyte is obtained with adequate conductivity for ionic transport in the presence of water, and no electronic conductivity.
These hydrogels have a wide electrochemical stability window (0-2 V vs Zn) and reversible electrochemistry over 7000 galvanostatic cycles in a Zn/COPEC/Zn cell, as reported in the experimental section of this article, where extensive electrochemical stability studies are described.
To combine the Zn 2 + transport in compact polyelectrolytes complexes with Zn anodes in a rechargeable battery, we also need a suitable cathode, preferably one that allows the insertion and extraction of Zn 2 + ions.We have previously introduced carbon or polymer based electrodes with lignincontaining electrode material of biological origin, [29] our preferred solution to scalable biologically sourced materials, which are not addressed in this contribution.There are many routes available [30] for making organic cathodes for zinc insertion from aqueous electrolyte, beyond that choice.

Materials and Methods
Alginic acid sodium salt from brown algae (Brookfield viscosity: 4-12 cP; Sigma-Aldrich: ref. 71238-Lot#BCBW4678), and chitosan (low-molecular weight, deacetylated chitin, Brookfield viscosity: 20-300 cP; Sigma-Aldrich: ref. 448869 -Lot#STBF8219V) were used as received.The chitosan was protonated with acetic acid at 60 °C during 4 hours.For the preparation of the COPEC, zinc chloride (ZnCl 2 , Sigma-Aldrich) in the form of powder was dissolved in a variable volume, depending on the final biopolymer ratio, ranging from 20 to 30 ml of MilliQ water and mixed with the chitosan solution.

Preparation of zinc ion conductors from two biological polyelectrolytes of opposite charge: chitosan and sodium alginate salt
The preparation of COPEC hydrogels with different CH : AL ratios and content of ZnCl 2 takes place in two steps.First, aliquots of aqueous solutions (15 ml) of CH and AL at concentrations of 1-3 g polymer/L were mixed by means of a Gilson Miniplus 3 Peristaltic Pump, employing PTFE 3 mm tubes and a flow rate of 5 ml/min.The ZnCl 2 salt is, when required, added to the CH solution.Then, the hydrogel formed in the aqueous solution was centrifuged at 9000 rpm during 10 min in a Sigma 2-16p centrifuge.

Biobased electrolyte characterization
The elemental composition of C, H, N, and S of the synthesized COPEC hydrogels has been determined by combustion on a LECO CHNS-932 elemental microanalyzer.
A pH sensor from Eutech PHSPEAR (Model Oakton 35634-40) was used to measure the pH of the gel polymer electrolyte.
Dynamic scanning calorimetry (DSC) measurements of COPEC materials were performed at a heating rate of 5 °C/min under a nitrogen atmosphere in equipment from Netzsch (DSC 214 polyma model).The hydrogel samples were all sealed in aluminum pans.The first cooling goes from room temperature to À 40 °C, the temperature is held for 3 min, and then scanned to 250 °C.
Thermogravimetric analysis (TGA) was performed in a thermobalance from TA Instruments (TA-Q500 model) attached to a Pfeiffer Vacuum ThermoStar GSD 301 T mass analyzer.The gas used was nitrogen within a temperature interval of 25-600 °C, at a heating rate of 10 °C min À 1 and with a sample mass of 5 mg.
To capture ATR-FTIR spectra, a PIKE MIRacle accessory (PIKE Technologies, Madison, WI) attached to a Bruker Vertex 70 spectrometer, equipped with a DLaTGS detector, was employed.For the ATR measurements, a single-reflection diamond-coated ZnSe crystal was utilized.The COPEC samples were securely positioned between the prism and a microscope slide, leaving space for a nitrogen tube.Then, the COPEC electrolyte was dehydrated using a gentle flow of nitrogen gas.The results obtained from the 30-minute dehydration process indicate that the dehydration rate varied in all three samples, and time cannot be considered as an indicator for the degree of dehydration.Throughout the experiment, spectra were collected at a resolution of 2 cm À 1 , and 16 scans for each spectrum.Spectra were captured every 30 seconds, resulting in a total of 61 spectra acquired for each sample.
To investigate the temperature dependence on COPEC materials, we utilized the ATR-FTIR instrument previously employed in the dehydration experiment.We prepared a sealed cell on top of the prism, ensuring complete prevention of dehydration.The cell was heated using a temperature-controlled heater.Once the desired temperature was reached, we allowed a 15-minute period for the system to stabilize, after which we captured spectra with a resolution of 2 cm À 1 , averaging 100 scans for each spectrum.
The morphological characterization of the COPEC hydrogel was carried out through Scanning Electron Microscopy (SEM) in a Philips XL30 equipment.For the visualization of the morphology of the semi-solid gels, a solvent exchange to ethanol at ambient conditions followed by supercritical drying with carbon dioxide was applied.The final aerogel samples were sputtered with gold using a Thermo VG scientific Polaron SC7640 Sputter Coater.

Electrochemical stability
The electrochemical stability of the COPEC's materials made of different chitosan (CH) and alginate (AL) ratios and ZnCl 2 salt concentrations was studied by using a two-electrode experimental configuration in a Swagelok type-cell in a VMP3 potentiostat (Biologic).
The study of the stability of the biopolymer electrolyte during the zinc plating-stripping redox process was followed by cyclic voltammetry in an asymmetric two-electrode cell configuration.The biopolymer electrolyte was sandwiched between a titanium (Ti) collector (diameter = 12.7 mm; area = 1.26 cm 2 ) acting as a working "blocking" electrode material and a metallic zinc (Zn) disc (250 micron thickness, 146 mAh/cm 2 ) pressed on top of another Ti collector, which also served as counter and reference electrode.The voltage varied from À 0.5 V to 2 V vs. Zn 2 + /Zn 0 and the scanning rate was 10 mV/s.The terminology used for this configuration was Ti/COPEC/Zn.The electrochemical stability window of the COPEC electrolyte materials was studied by linear sweep voltammetry (LSV) in a twoelectrode configuration Ti/COPEC/Zn at room temperature (RT).The LSV measurements were carried out between the open circuit voltage (OCV) and À 2.5 V and + 4.0 V versus Zn 2 + /Zn 0 at a scan rate value of 5 mV/s.
A polarization study during long-term cycling was also done.For that, symmetric Zn cells with COPEC materials with different molar ratios of the biopolymers and water content, but with a fixed ZnCl 2 salt concentration of 0.06 M, were assembled.The symmetric cell configuration consists of two metallic Zn discs pressed on top of Ti collectors, with one of the two acting as a working electrode and the other as both counter and reference electrode.The cells were kept at RT for 5 h and then cycled galvanostatically at a constant current density of 10 and 50 mA/cm 2 , wherein the duration of each half-cycle was 0.5 h.The terminology used for this configuration was the following: Zn/COPEC/Zn.

Ionic conductivity and ion transport measurements
The influence of the water content in the hydrogel was evaluated by an open air experiment with a setup located on a balance, with a hydrogel positioned on top of two metallic zinc electrodes separated by a small gap (2-3 mm), on a Teflon substrate.In this geometry the evaporation of the water is not hindered by a top electrode, and drying occurs over time.The hydrogel shrinks as water evaporates, and the geometry changes.It is therefore not feasible to follow the conductivity, due to the changing geometry.The measurements of impedance and photos have been taken every 10 minutes over five hours, to see the water loss and follow the impedance.Initially there was a hydrogel mass of 98.3 mg, at the end of the study after 5 hours, a mass of 5.2 mg was observed.93 mg of water have been lost, corresponding to 94 % of the mass of the starting hydrogel.The average laboratory temperature was 26 °C.
Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying a low sinusoidal amplitude alternating voltage of 10 mV to the cell at frequencies from 1 MHz to 10 mHz using the above mentioned multichannel potentiostat/galvanostat.
The frequency and temperature dependence of the ionic conductivity of the COPEC biopolymer electrolytes was measured within the À 40 to 200 °C temperature range by using a Novocontrol analyser with an integrated dielectric interface ALPHA and a QUATRO temperature controller.Experiments were performed over the frequency range 10 7 -10 À 1 Hz using a non-hermetically sealed homemade stainless steel cell.The samples were located in a cavity with a 10 mm diameter and 0.78 mm height, with a SS electrode.Then a 10 mm diameter SS electrode was placed on top of the sample, which was used as the top electrode.An ac perturbation of 10 mV was applied to the cell.Data were collected during steps of 5 °C and 10 °C.The real and imaginary parts of the ionic conductivity were generated directly from the software.The thickness of the biopolymer gel electrolyte could not be accurately measured over the whole temperature range covered by the measurements, due to continuous water evaporation and complete loss beyond 100 °C.For this reason discussion of the conductivity data was based on the maxima of the conductivity curves.

Synthesis and physical-chemical characterization of COPEC hydrogels
Figure 1 shows the steps for the preparation of the biopolymerbased aqueous polyelectrolyte-Zn complexes, along with a schematic sequence of the ideal molecular structure and ionic interactions between their components in the coacervate.More details about the biopolymer structures can be found in supplemental file (Figure S1).The Syntheses of the COPEC materials were done by a microfluidic method to mix solutions of the alginate and the chitosan polymers at concentrations of 1-3 g polymer/l.(Figure 1a).After the two solutions meet in the microfluidic mixer, swollen coacervates are formed and collected from the flow systems.They are concentrated by centrifugation, and collected as an opalescent swollen hydrogel at the bottom of the centrifugation tube (see the inserted image in Figure 1a).The average thickness of the COPEC used in the different electrochemical cell configurations was within the range of 1 to 2 millimeters.These can be transferred by a spatula and positioned for insertion in an electrochemical cell, confirming the optimum interaction between the polyelectrolyte chains and the ionic species (Figure 1b) conferring compactness to the coacervate for being assessed as a solidstate Zn-ion electrolyte.
The concentrations of the bio-polyelectrolytes are shown in both molar (of the monomer) and weight percentages in Table 1.When coacervates are formed, the chain density is increased, and the water concentration reduced.This may depend on the stoichiometry CH : AL.
The elemental analysis of the COPEC materials by combustion technique has confirmed the presence of a higher content of nitrogen for those stoichometries containing a higher amount of chitosan biopolymer (Table S1).The bio-polyelectrolyte complexes have pH values between 3-3.5 for all the materials.
Figure 2 shows the thermal properties of the COPEC electrolytes with and without ZnCl 2 .The thermogravimetric analysis (TGA) shown in Figure 2a (left) reveals that the hydrogel electrolyte material with a CH : AL ratio of 2 : 1 contains more than 90 wt% of water, after an abrupt loss of weight up to 200 °C (purple line).Interestingly, without ZnCl 2 salt, the COPEC materials exhibited a much lower amount of water wt % (red line), which indicates the presence of a more dense macromolecular network structure created by the higher number of crosslinking points from the electrostatic interactions between the biopolyelectrolyte chains.We note that the starting weight in TGA measurements reflects this balance between chain density and water fraction, and may vary with stoichiometry.
The presence of Zn 2 + ions breaks apart these polyanionpolycation interactions, creating a new biopolymer network with a lower chain density, with a higher amount of water in their final porous structure.This fact is also confirmed for the other two COPEC gel compositions containing the same concentration of ZnCl 2 salt of 0.06 M, as it is shown in the supplemental file (Figure S2).From these results we can also infer that the chitosan and alginate ratio in the final COPEC composition, would affect in different degree the amount of water that can be incorporated in the hydrogel network after the introduction of the Zn salt.
The analysis by differential scanning calorimetry (DSC) of the COPEC materials is shown in Figure 1a (right).The analysis of the biopolymer electrolyte containing Zn 2 + (purple line) evidenced a set of endothermic processes between the melting temperature of water and the degradation temperature beyond 200 °C, that might be due to the modification in the state of the COPEC electrolyte material -throughout the water content change with the increase of temperature.Because these endothermic transitions have not been observed in the same extension in the COPEC materials without Zn 2 + (red line), except for the CH : AL 1 : 1 composition (Figure S3), then they do have to be associated not only with the water content as the temperature increase, but also most probably due to the presence of Zn 2 + ions.Therefore, the dehydration of the COPEC gels cause rearrangement of the network components, as a consequence of the predominant interaction between the polyelectrolyte chains, the Zn ions and the water molecules.
IR measurements have revealed more on the molecular state of the COPEC electrolyte materials with the change of  water content, as can be observed in Figure 1b and Figure S4.Hydration/dehydration processes have a significant impact on the conformation of functional moieties, such as hydroxyl groups along the biopolymer backbone, causing them to promote formation of hydrophilic domains through inter-and intramolecular hydrogen bonding.To assess the impact of dehydration on the COPEC spectra, we focus on a range within the fingerprint region (1480-920 cm À 1 ).We selected this range because the spectral regions over 1500 cm À 1 is dominated by the water spectrum.The changes are illustrated in more detail in Figures S5 and S6 where the positions and shapes of the bands associated with primary and secondary hydroxyl groups exhibit changes in response to variations in sample hydration levels.These changes in the spectra can be attributed to changes in the conformation of the carboxyl groups present in the alginate unit.Figure S7 represents the relationship between the change in hydration level and the peak positions of the secondary hydroxyl and symmetric carboxylate bands (in Table S2).The results indicate that the positions of these peaks undergo significant shifts, particularly at water content levels below 5 %.This observation suggests that the hydrogenbonding present in the material is disrupted, leading to alterations in the vibrations associated with these functional groups.Figure S8 shows the temperature dependence of a gel in a closed chamber, where water release with temperature is prevented.There were no discernible changes in the gel structure resulting from temperature changes; therefore we conclude that it is the loss of water that leads to reorganization of the COPEC, as observed in the IR spectra.The hydrogels have also been characterized by scanning electron microscopy (SEM), and sample preparation thus includes removal of the solvent from the gel via ethanol exchange and subsequent drying by using supercritical CO 2 .We observed finer details in the COPEC (Figure 1c), indicating a nanoporous material with small dimensions of pores for the three biopolymer ratios studied in this work (Figure S9).

Electrochemical stability of biopolymer electrolyte hydrogels with different stoichiometry and zinc salt concentration
Figure 3 shows the cyclic voltammetry plots corresponding to a continuous cycling for a bio-based COPEC hydrogel material with a CH : AL weight ratio of 2 : 1 and concentration values of ZnCl 2 salt of 0.06 M. The CV plots revealed a couple of redox waves split between À 0.5 V and 0.25 V vs Zn 2 + /Zn 0 .After 15 cycles, the steady state was achieved and the redox waves displayed similar anodic and cathodic current values (see inset in Figure 3a).This stability could result from both the favored transport properties of the COPEC electrolyte, and also due to the compatibility between the zinc deposits (Figure 3b) and the bio-based electrolyte material.The result of this is the creation of a stable solid electrolyte interphase (SEI), not perturbed by the compositional and volumetric changes occurring during the cycling of the electrochemical cell.A similar behavior was evidenced for 1 : 1 and 1 : 2 COPEC ratios (Figures S10-S11).
Figure 3c shows a comparison between the first voltammetry cycle corresponding to the formation and dissolution of the enriched-Zn solid deposits for the three CH : AL molar ratios containing each of the three different concentrations of ZnCl 2 salt selected in the current study.Remarkably, a correlation between the CH : AL molar ratio and the current densities of the first cycle was found for the COPEC samples containing a ZnCl 2 salt concentration of 0.06 M (left) and 0.12 M (center).The presence of a higher amount of chitosan with respect to the alginate in the biopolymer electrolyte, seems to stabilise the ion transport across the COPEC hydrogel, slowing down the formation and dissolution rate of the zinc deposits on the surface of the current collector (Ti metal surface).As the ZnCl 2 concentration increase up to 0.25 M, the biopolymer electrolyte's composition lose this impact, and the current density for the reversible plating-stripping process become independent of the CH : AL ratio as it is shown in Figure 3c (right).This effect also has a strong influence on the electrochemical stability of the COPEC materials.Figure 3d display a comparison of the linear sweep voltammetries (LSV) profile versus metallic zinc at 5 mV/s, for the three different electrolytes with 1 : 2, 2 : 1 and 1 : 1 biopolymer ratio, and containing the same concentration of ZnCl 2 salt of 0.06 M. In general, the maximum current density values observed during the oxidation process were higher for CH : AL ratios of 1 : 1 and 1 : 2 than for the CH : AL 2 : 1 ratio, the latter being electrochemically stable up to 2 V versus Zn 2 + /Zn 0 .Beyond that potential, the COPEC material suffers an oxidation process.This oxidation process could be related to the breaking of the CÀ O bonds of the biopolymers.Therefore, a dependency of the stability of the biopolyelectrolyte with the composition was observed.In any case, the three biopolyelectrolytes seem to be electrochemically stable, at least in the region covering the zinc plating at À 0.5 V versus Zn 2 + /Zn 0 .As was evidenced, the COPEC material containing the higher amount of chitosan evidenced the higher positive potential value before start of degradation/oxidation.For negative potential values, the electrochemical stability is similar for the three COPEC materials, with the dominant electrochemical process being the zinc metal plating from 0.25 V to À 0.5 V range.

COPEC Conductivity and Zn 2 + ion transport
The concentration of monomer units of COPEC polymers in the starting solutions for hydrogel is of the order of 2-3 mM (see Table 1), far from the WISE conditions.Upon formation of the hydrogel, the local concentrations must increase considerably.The transport properties of this hydrogel are retained as more than 90 % of the mass is removed by evaporation of water, rendering the concentrations closer to WISE conditions, but presumably not there.These observations come from two electrode studies with the hydrogel supported on Zn electrodes, with the assembly positioned on a balance, to follow the mass change due to evaporation of water from the hydrogel (Figure 4).This is followed by photographical imaging (Figure 4a) and impedance spectroscopy (Figure 4b) over many hours, while reducing both mass and volume of the hydrogel due to evaporation.As the change of both volume and mass leads to a different geometry of electrode/electrolyte contacts, this cannot be easily evaluated as a change of bulk conductivity, but only as change of conductance.This may be as large as an order of magnitude.
Figure 5 shows the conductivity-temperature dependence, derived from dielectric spectroscopy measurements, with the frequency and temperature dependence for the COPEC electrolyte material containing the three CH : AL ratios studied in this work.The conductivity increase abruptly for the three COPEC materials as the melting temperature of the water is achieved (~0 °C), increasing until reaching a maximum conductivity value at ~50 °C.Then, the conductivity start dropping, most probably due to the loss of water, along with the rearrangement between the Zn ions and the polymer, until a new second maximum  appears up to 80 °C.From that point, the conductivity decrease progressively until it reach a zero value just near 100 °C, when most of the water is probably gone and before the biopolymer start degrading.Interestingly, this behaviour also appears in the COPEC samples without Zn (Figure S12) so clearly the ions anchored to the polyelectrolyte chains are having a crucial role in the charge transport of the gel.
The temperature dependence of conductivity in strong electrolytes [31] is influenced by an increasing density and mobility of ions, and ionic conductivity thus typically increase with temperature.The more complex behaviour depicted here may be an indication that ion transport is mediated by more than one mechanism.The hypothesis of ion transport through bulk water and at polyelectrolyte surfaces, previously introduced, maybe help to explain this data, but there is not yet sufficient experimental data to delineate this combination in any detail.However, we note that the loss of water is affecting the COPEC complexation, as seen in the IR spectra.
After the previous characterization studies of our batch of COPEC materials, we finally choose a CH : AL 2 : 1 ratio as the biopolymer electrolyte for a potential application in energy storage, as it has a higher electrochemical stability, and the reversible formation of zinc-rich deposits might be better controlled than in the other biopolymer gel electrolyte counterparts.
Further investigation of the long-term electrochemical stability between the Zn metal and the bio-based hydrogel electrolyte, along with the ability to inhibit zinc dendrite formation is shown in Figure 6.The symmetric Zn cell plating/ stripping tests indicated that the cell with aqueous ZnCl 2 electrolyte run for less than 80 cycles at 50 mA/cm 2 (see inset Figure 6).Severe voltage oscillations were observed due to the formation of zinc dendrites and the interfacial incompatibility with the zinc metal.The COPEC materials with a 2 : 1 CH : AL ratio also suffered from voltage oscillations, caused most probably by the electrodeposition and Zn dendrite growth during the initial cycling (Figure 6), but did not short-circuit immediately, and returned to reversible and stable enriched-Zn electrodepositing/plating/dissolving/stripping redox process for more than 7000 cycles.
To assess the transport of Zn ions in the COPEC, we performed an experiment inspired by that reported by Michael Faraday in his "Experimental researches in electricity", where he used two silver electrodes connected with a silver salt electrolyte, to measure by balance the change of mass of due to electrodeposition of silver on electrodes, upon constant current electrolysis.With this method he could calculate the mass/ charge ratio of silver atoms.In our case we loaded an asymmetric Zn/COPEC/Zn cell, with a small and known amount of Zn in the form of a thin foil on the working electrode.A fixed current was applied to the two-electrode cell, and the experiment followed the transient voltage versus time occurring as Zn ions were injected and transported through the cell.This continued for a time at very moderate voltages of � 0.2 V, but eventually came to rapidly increasing voltages.These were associated with depletion of the zinc loaded on the anode.In Figure 7a we show experimental results for a single transport experiment of this kind.We have also performed experiments with alternating currents applied with the similar setup, to move zinc forward and backward through the cell, as demonstrated in the inset plot in Figure 7a.A steady state is approached, after the first 10 cycles.
In order to further explore the value of the transference number, we also simulated the experiment by considering driftdiffusion and Nernst equations by using MATLAB R2021b, assuming the charges bound to AL and CH to be immobile and not participating in transport, which is done by Zn 2 + ions and anions.The amount of Zn 2 + ions were considered to be those present in the electrolyte plus those moved from the working electrode towards the counter/reference electrode.Thus, once all of them have been consumed, the concentration of Zn 2 + becomes 0 (see simulated Zn concentration in Figure S13a in the supplementary information).By following Nernst equation, the achieved potential can be approximated as (Eq.1): [32] E t where E(t) is the potential evolution over time t, E 0 is the standard potential, R is the universal gas constant (R = 8.31 J mol À 1 K À 1 ), T is the temperature, n is the number of electrons per molecule oxidized or reduced (2 in this case for the Zn 2 + ions), F is the Faraday constant (F = 96485 C mol À 1 ), i is the current, C 0 * is the concentration of Zn 2 + ions that are to be oxidized, C R * is the concentration of Zn 2 + ions that are to be reduced and i 0 is the exchange current, and D 0 the diffusion coefficient of Zn 2 + .The fit to experiments is not impressive as it is shown in Figure 7a.Clearly, we have not been able to simulate the initial transient voltage behaviour, even though the transition into excessive voltages is easily discerned.Much more sophisticated models would be necessary for this.
However, the simulation allowed to study the amount of Zn moved from the working electrode for different transference numbers (proportion between the current carried by the Zn 2 + ions and the current carried by the rest of the ions).It is possible to observe how the time at which the potential starts to increase fast (transition time) increases for increasing amount of Zn in the working electrode (Figure S13b in the supporting Information), which is in accordance with experimental data.In Figure 7b we show the correlation between Zn mass loaded and charge flowing during the time of transport of the Zn ions, for three experimental data points, and for simulations performed with different transference numbers.
Despite the limitations of the simulation, it can be seen how the simulated charge comes closer to experimental values for very high transference numbers.It is worth noting that different parameters could affect the simulations.One of them is the diffusion coefficient used.Here, we assumed it to be close to those values reported in aqueous electrolytes (D 0 = 6.2×10À 6 cm 2 s À 1 ). [33]To check its effect on the simulation, we repeated the simulation again with a diffusion coefficient one order of magnitude lower, in case the diffusion of coefficient in the COPEC is lower.As it can be seen in Figure S13c, the diffusion coefficient has an effect on the shape of the voltage evolution, but not a major impact on the transition time.

Conclusions
We have demonstrated a route to complexes formed by coulombic self-assembly of polyanionic and polycationic polyelectrolytes, with origin in biological synthesis.The COPECs can be formed in different stoichiometries, and form gels of higher and lower chain density.Interactions with the salt ZnCl 2 disturb this organization, and the Zn 2 + cation is freely mobile in the gel with a transference number close to 1.The conductivity is high enough ( � 10 mS/cm) to be relevant for use with electrodes in supercapacitors or batteries.The temperature dependence of conductivity in the gel shows a strong increase with temperature from 0 to 50 °C, followed by decreasing conductivity, but eventually showing conductivity maximum at 80 °C.This dependence is not monotonic, as expected for a strong electrolyte, but rather indicates that more than one transport route of ions may exist in the gel, with different temperature dependence.The COPEC gel allows operation of Zn electrodes in symmetric cells for more than 7000 galvanostatic cycles, indicating that the interface to the electrolyte is rather stable.These biologically derived polyelectrolyte complexes offers many possibilities for optimizing transport and stability at electrode interfaces.

Figure 1 .
Figure 1.(A) Sequential view of the COPEC's preparation route, and (B) an ideal view of the ionic interactions between the polyelectrolyte chains and the ions involved in the synthesis of the coacervate.(X + = H + ; Y À = CH 3 COO À ).

Figure 2 .
Figure 2. (A) TGA and DSC traces of COPEC with a CH : AL ratio of 2 : 1 with a 0.06 M concentration of ZnCl 2 salt and without ZnCl 2 ; (B) 2D IR spectrum during dehydration for a COPEC electrolyte material with a CH : AL ratio of 2 : 1 and concentration of 0.06 M ZnCl 2 .(C) SEM microscopy of nanostructured COPEC aerogel with a CH : AL ratio 2 : 1 and 0.06 M ZnCl 2 .

Figure 3 .
Figure 3. (A) CV of the COPEC electrolyte material with a CH : AL ratio of 2 : 1 (scan rate: 10 mV/s).(B) electrochemical deposit of Zn obtained after 50 cycles.Inset showing the zinc deposits over the COPEC's surface after cycling.(C) CV of the COPEC materials for the three biopolymer electrolyte compositions with different CH : AL biopolymers ratio and ZnCl 2 0.06 M, 0.12 M and 0.25 M. (D) The voltage stability window of the COPEC's materials was studied by linear sweep voltammetry-LSV (scan rate: 5 mV/s).

Figure 4 .
Figure 4. (A) Photographs from the top and the side of the COPEC (2 : 1 CH : AL, 0.5 M ZnCl 2 ; pH = 3.5) drying on two zinc electrodes (zinc ribbons of 5 mm width).(B) The impedance of the COPEC was determined by electrochemical impedance spectroscopy using ionically reversible contacts (Zn electrodes).

Figure 5 .
Figure 5. 3D plots of the frequency and temperature dependence of the conductivity for the COPEC materials with CH : AL ratio of (A) 1 : 2, (B) 2 : 1 and (C) 1 : 1.

Figure 6 .
Figure 6.Symmetric cell Zn/COPEC 2 : 1/Zn exhibiting more than 7000 cycles of 1 h each.The current density applied was 50 mA/cm 2 .Inset shows the electrochemical cycling behavior for a conventional electrolyte made of ZnCl 2 salt (0.06 M) in water.

Figure 7 .
Figure 7. (A) Experimental and simulated results of a Zn/COPEC/Zn cell, with 1.7 mg known amount of Zn in the form of a thin foil on the working electrode.A fixed current of 0.32 mA was applied to the two-electrode cell.The inset shows a "back-and-forth" experiment in the same setup to evidence the motion of Zn ions in the same cell.(B) Experimental (dots) and simulated (lines) correlation between the Zn mass loaded and charge flowing during the time of transport of the Zn ions.Simulations were performed with different transference numbers.

Table 1 .
Concentration ratios of the bio-polyelectrolytes studied in this work.