Direct Ink Writing of Nanocellulose and PEDOT:PSS for Flexible Electronic Patterned and Supercapacitor Papers

Printed electronic paper identifies its interest in flexible organic electronics and sustainable and clean energy applications because of its straightforward production method, cost‐effectiveness, and positive environmental impact. However, current limitations include restricted material thickness and the use of supporting substrate for printing. Here, 2D and 3D electronic patterned paper are fabricated from direct ink writing (DIW) nanocellulose and PEDOT:PSS‐based materials using syringe deposition and 3D printing. The conductor patterns are integrated in the bulk of the paper, while non‐conductive sections are used as support to form free‐standing paper. The strong interface between the patterns of electronic patterned paper gives mechanical stability for practical handling. The conductive paper‐based electrode has 202 S cm−1 and is capable of handling electric current up to 0.7 A, which can be used for high‐power devices. Printed supercapacitor papers show high specific energy of 4.05 Wh kg−1, specific power of 4615 W kg−1 at 0.06 A g−1, and capacitance retention above 95% after 2000 cycles. The new design structure of electronic patterned papers presents a solution for additive manufacturing of paper‐based composites for supercapacitors, wearable electronics, or sensors for smart packaging.


Introduction
Printed and flexible organic electronics are of large interest to industries due to their cost-effectiveness, simple processing, and environmentally friendly materials. [1]Extensive research has opened up applications such as flexible lighting and display technologies, energy storage and conversion, smart objects and textiles, and wearable sensors. [2,3]Paper electronics can be DOI: 10.1002/admt.202300652viewed as a subset of printed electronics and are being used in applications from basic electronic components to integrate platforms. [4,5]Smart solutions e.g., user interaction are emerging, such as paper displays, touchscreen, smart labels and packaging, circuit boards, RFID tags, electrochemical sensors, batteries, supercapacitors, or solar cells. [4]Flexible paper electronics are also a way to take steps toward solving the problem of environmental pollution caused by electronic waste.
Paper electronics can be classified into two categories: electronics on paper and electronics in paper.Electronics paper existed already in the late 1960s [6] in the form of electronics on paper.Brody was first to print inorganic thin-film transistors (TFTs) on paper using a stencil [7] ; unfortunately, the interest at that time was limited because of difficulties in the manufacturing process and the brittleness of the circuit.During the last decades, solution-based -conjugated organic molecules and polymers have renewed the interest in electronics manufactured on low-cost paper substrates.As an example, Berggren and co-workers fabricated an electronic smart pixel display by printing PEDOT:PSS (poly (3,4-ethylenedioxythiophene): polystyrenesulfonate) on paper. [8]Later, they integrated PEDOT:PSS-based electrochemical transistors, displays, logic, and batteries into a stand-alone label. [9]ommercial paper substrates are limited in some aspects for advanced electronic devices, due to surface roughness, opaqueness, high porosity, and humidity sensitivity, which results in low mechanical strength and influences the resolution of the produced electronic device. [10]In recent years, the next-generation high-performance nanocellulose has emerged as a material that exhibits extraordinary mechanical strength, high specific surface area and transparency, good thermal stability, and a smooth surface. [11]Owing to the potential for mass production [12] and fast nanopaper formation via papermaking, [13] nanocellulose shows great potential for flexible electronics, optoelectronics, and energy storage devices in industry applications. [5,14]Among the reported applications we find low-cost gravure printed RFID antenna patterns on transparent nanopaper, [15] with highly transparent (≈90%) [16] and low thermal expansion coefficient [17] nanopapers that were used as flexible substrates for solar cells [18] and OLED applications. [19]anocellulose can be taken from just being a substrate to serving as an active component in devices by combining with organic electronic materials. [20]Various materials such as conducting polymers and different forms of carbon can form coreshell structures on nanofibrils by blending [21][22][23] or in situ polymerization. [24]Using nanocellulose as a template for these organic electronic materials can yield combinations with excellent electrical conductivity and mechanical integrity because the conductivity extends homogeneously throughout the bulk, and thus gives high electrical current capacity compared to thin printed conductors on a paper substrate.These flexible and freestanding electronic papers (electronics in paper) can be formed by simple casting, [25] vacuum filtration, [26] or by printing techniques. [27]These electronic papers have been further processed using origami techniques, for instance, cut and stick, [21] printing-cutting-folding procedures, or lamination to produce components such as organic electrochemical transistors (OECTs), electrochemical diodes, and supercapacitors. [2,28]Besides these techniques, there is a lack of fabrication methods to pattern the electronic papers into different regions with high/low conductivity.Such patterning would enable free-standing paper substrates for electronics, with integrated "wires" of high conductivity and current capacity separated by non-conductive regions of non-conductive paper.Since the papers would be self-supporting and have no need for other substrates, this would simplify the production of paper electronics, provided that good mechanical properties can be maintained.
In this work, we fabricate electronic patterned papers from nanocellulose and PEDOT:PSS-based inks using syringe deposition and 3D printing.The patterned paper integrates conductor patterns in the bulk of nanocellulose, while non-conductive sections are used as support to form free-standing paper.Cellulose nanofiber (CNF), PEDOT:PSS, glycerol, and ethylene glycol were used in the formation of conductive ink.The nonconductive inks consist of CNF, microfibrillated cellulose (MFC), glycerol, and alginate in different concentrations for the two different methods.We utilize the ionic conductivity of the nonconductive sections, exemplified by simple supercapacitor devices where the conductor patterns are used as electrodes, and the non-conducting parts of the same paper serve as paper separator.Additionally, we develop electrolyte inks based on CNF/Glycerol/EMIM-ES (1-ethyl-3-methylimidazolium ethyl sulfate) and CNF/MFC/Alginate/CaCl 2 /PEO/NaCl for fabricating supercapacitor papers.Fully printed supercapacitor papers have low equivalent series resistance, thus give high power density and capacitance retention than our previous works. [2,27]

Pattern Formation
Direct ink writing (DIW) was successfully produced from CNF-PEDOT:PSS nanocomposite.This DIW was used to pattern conductive papers using syringe deposition and 3D printing Figure 1.From simple preparation of ink formulations using nanocellulose and PEDOT:PSS-based materials, electronic patterned papers were successfully produced using syringe deposition and 3D printing techniques.Flexible and foldable electronic patterned paper with different shape and sizes were formed using syringe deposition (Figure 1a-c), whereas 3D printing produced high-resolution structure and complex 3D objects (Figure 1d-f).The prepared inks from nanocellulose-based material were extruded on petri dish, and the droplet dried anisotropic direction.The printed structures remain unchanged in their shapes after drying (Figure S1, Supporting Information).This observation is supported by Agate [33] who explained that the prolate shape of CNF causes anisotropic Brownian motion of particles to the periphery of the droplet, and its hydrophilicity hinders water transportation, resulting in the droplet is dried from edge to inside.
The patterned paper was formed by integrating the conductor patterns in the bulk of nanocellulose, and a non-conductive section was connected between the conductor patterns to form free-standing paper.The interfaces between the patterns need to be strong enough to maintain the mechanical properties under stress.Simultaneously, diffusion of the polymers needs to be controlled to form a good resolution pattern and to prevent short-circuiting of the devices.The diffusion of polymers was investigated by mapping C, O, and S elements from EDX.There is a clear border between the patterns, defined by the elemental sulfur from PEDOT:PSS in the patterned paper (Figure S2, Supporting Information).Since the preparation of this patterned paper is in wet stage, the water osmosis phenomenon occurring during the drying process is driven through the conductor patterns and swollen the polymers molecular, and therefore PSS polyelectrolyte species can diffuse across the interface to the non-conductor patterns.It is confirmed by the small amount of S in the non-conductive channel (Figure S2d, Supporting Information).To limit the diffusion, the excess water molecule in the inks need to be removed, and polymers need to be chemically bound themselves or by crosslinking agent.We have experimented an increment of ink concentrations, and we found that 4% of ink concentration was printable, whereas higher concentration clogged the print head.In addition, 100 mm CaCl 2 was used to crosslink PSS, [34] carboxylic group on nanocellulose, [35] and alginate. [36]These crosslinking networks help to reduce the diffusion process because the mobility of polymer molecules is restricted.Figure 1g,h shows the surface morphology of antenna paper.There are clear interfaces between conductor and non-conductor patterns.Based on the cross-section image and its elemental mapping (Figure 1i,j), the distribution of sulfur composition was limited by ionic crosslinking, as no S has been found at the non-conductor region (Figure 1l,m).However, there is an overlapping area of ≈20 μm between non-conductor and conductor patterns, based on the S element in orange line in Figure 1k, which occurred during the filament extrusion in printing process.
We also studied the stability of the patterned papers in DI water.Unlike non-crosslinked paper, which was stable for only one day, the crosslinking structures could prevent the polymers swelling and therefore crosslinked papers maintain their structures in the water for more than six months (Figure S3, Supporting Information).

Tensile Strength
The interface behavior of the patterned papers was well-defined from their mechanical properties (Figure 2).Young's modulus  and c) comb structures composing of 2 mm width of each conductor and non-conductor patterns.d-h) 3D printing; d) layer by layer printing, e) 3D structure with two conductors at the bottom and one conductor (that is isolated by non-conductor) on top, and f) antenna structure with 400 μm width.SEM/EDX micrographs: (g) and (h) are morphologies, i) crosssection surface and j) its C, O, and S elemental mapping and k) point distribution, and SEM of non-conductor area of (i) and l,m) its elemental mapping.
was calculated from the selected stress-strain curve of each sample that was close to the average tensile strength and elongation at break (Figure 2a).Tensile properties of papers varied from different fabrication techniques.Nanocellulose papers from syringe deposition and 3D printing had tensile strength of 45 and 69 MPa, Young's modulus of 2.4 and 1.9 GPa, and elongation at break of 11% and 16%, respectively (Figure 2b-d).The extruded ink from 200 μm of nozzle induced alignment structuring of nanofibrils along the printing direction (Figure 3a), thus giving higher tensile strength and elongation at break, but significantly lower Young's modulus than that of syringe deposition paper.The anisotropic rod shapes of CNF and MFC contribute to hydrodynamic alignment and disentanglement themselves under a variety of external shear forces such as extrusion and spin coating. [37,38]owever, the tensile properties value of these nanocelluloseglycerol papers is relatively low in comparison to the neat nanocellulose (CNF or MFC) paper.CNF and MFC papers had high tensile strength of 220 and 206 MPa, Young's modulus of 14 and 12.6 GPa, and elongation at break of 5.5% and 7.2%, respectively due to the strong inter-and intra-molecular interaction of hydroxyl group along nanofibril chains. [22,39]The addition of glycerol into the nanocellulose network reduced the strength of hydrogen bonds between adjacent cellulose chains, instead, the interaction of hydroxyl groups between glycerol and cellulose chains altered the tensile strength of the papers.Furthermore, the conductive paper became weaker compared to the nanocellulose-glycerol papers.PEDOT:PSS possesses a poor mechanical property, and it is self-organized as a shell around the core nanofibrils, preventing the strong nanocellulose network. [40]he presence of glycerol and ethylene glycol plasticizers-like tend to separate the PEDOT:PSS and nanocellulose structure, resulting in loose and weak composites. [23]All papers from syringe deposition decreased tensile properties with increasing the number of layers (Table S1, Supporting Information).The multilayers papers suffer from internal mechanical stress, resulting in delamination and cracking, as confirmed by SEM micrograph from Figure 3b-d.
Interestingly, the fabrication of patterned papers with 1, 4, and 8 interfaces gave higher tensile strength and Young's modulus than the control conductive paper.The tensile strength of pattern paper with one interface increased up to 25.7 MPa for syringe deposition and 26.3 MPa for the printing method.This indicates good interfacial adhesion strength at the edge of the patterns, as the breaking happened in the conductive section (Figure 2b, insets numbers 3-5).The elemental mapping in Figure 1j gives evidence that more nanocellulose and less PEDOT:PSS are presented at the interfaces.Our previous work suggested that CNF-PEDOT interaction is stronger than PEDOT-PEDOT interaction. [40]Therefore, we conclude that the   strong interface between patterns is due to nanocellulose from the non-conductor pattern can interact with the PEDOT shell of CNF-PEDOT core-shell structure from the conductor pattern at the edge of the interface, as illustrated in Figure 3d,e.Based on the SEM and EDX results, we used geometry node in blender software to illustrate the distribution of CNF and CNF-PEDOT at the interface between conductive and non-conductive region (Figure 3e).
On the other hand, the weak part of the conductive section was replaced by a strong nanocellulose section that can share stress before transferring to the conductive region, and therefore strain-related polymer debonding occurs in the weak region.When the length of each pattern was reduced to 10 mm for four interfaces and to 5 mm for eight interfaces, the patterned papers become stiffer and limit the strain-to-failure because the alternating blocks of nanocellulose can respond to the stress when the force is applied.Eventually, the free-standing patterned papers are sufficient mechanical integrity for practical handling or bending.

Electrical Properties
The patterned paper consists of non-conductor and conductor patterns.Non-conductor pattern from nanocellulose-based had electrical conductivity ≈10 −7 S cm −1 , which is a characteristic of insulator material.Conductive papers were prepared to measure the conductor pattern.Different amount of ethylene glycol was added into the composite (Figure S4, Supporting Informa-tion).The conductivity of conductive paper increased with an increasing amount of ethylene glycol, and the optimum value of 202 S cm −1 was obtained with the weight ratio 1:2:6:1 of CNF-PEDOT:PSS-EG-Glycerol formulation.No obvious change of electrical conductivity has been found under bending and twisting stress (Figure S5, Supporting Information).
For some applications that need high power, thick paper is required to deliver high current capacity toward building efficient devices.A single-layer structure of patterned papers from both fabrication techniques has a limitation on thickness; however, LBL structures can increase thickness from 20 μm up to 145 μm by maintaining their flexibility.Figure 4a,b shows the increment of thickness of conductive papers with an increase in number of layers, and the relationship between the conductivity and electric current capacity of both methods with respect to the number of layers is presented in Figure 4c,d.High electrical conductivity involves with the structural properties such as chain orientation and degree of crystallinity of conductive polymers that eases charge carrier mobility and hole transport in the composites.The doping of ethylene glycol induced phase separation PEDOT-rich and PSS-rich domains in the bulk of PE-DOT:PSS and PEDOT chains conformed from mixed coils to linear/expanded coils and to aggregation structure with a slightly tight p-stacking. [41]In addition, nanocellulose acts as 3D scaffold to promote interconnected p-p stacking structure of PE-DOT aggregation to self-assemble along the surface of nanofibrils, and therefore increased internal ordering and formed PE-DOT crystallites. [23]Similar to our recipe, ethylene glycol was replaced by DMSO, Belaineh et al. [40] observed on the topography and morphology of the same composites (using DMSO instead of ethylene glycol) to fully understand the interactions between PEDOT and CNF in the systems.AFM exhibited the organization of beadlike structure of PEDOT around the fibrils.WAX and GIWAX confirmed the symmetric p-stacking of PEDOT in edgeon and face-on orientation along the CNF, while TEM showed stacks of PEDOT crystallite on the surface of the nanofibril.Recently, the computational microscopy on the morphology of the CNF:PEDOT paper showed an evidence with the experimental observations. [42]The interaction between nanocellulose and PE-DOT:PSS discussed in detail in our previous review. [43]he conductivity of printed conductive paper was 161 S cm −1 , which is lower than the deposited conductive paper.With a long period of stirring time (20-30 h) at 60 °C to get 4% of ink concentration, air bubbles were generated during the ink preparation.Attempt had been made by squeezing the cap of cartridge during ink transferring to remove the bubbles, yet some remained in the ink, leading to reduce the conductivity of conductive paper.The multilayers of printed conductive papers showed similar conductivity properties, whereas the deposited papers decreased the conductivity from 167 S cm −1 for double layers to 106 S cm −1 .The conductive ink was extruded layer by layer in the semi-solid state (viscous hydrogel) for 3D printing, and therefore the adjacent layers were compacted to form single layers after drying (Figure 4b).Inversely, each layer was first dried before adding the following layer for syringe deposition method.This created a sort of stacked layers in the composites (Figure 4c), which can reduce the efficiency of lateral charge transport mechanism through the bulk of paper.Figure 4c reveals that the conductivity slightly decreases for papers with the same thickness of 120 μm, but built with four-, six-, and seven-layers structures.This indicates that the reduction of the conductivity properties is not relevant to the thickness of paper, as explained in our previous work where the thickness of drop-casted papers was in the range from 20 to 240 μm. [23]In the contrary, electrical current capacity depends on the thickness of conductive papers.The deposited and printed papers of 145 and 90 μm can run at the maximum current density of 0.76 and 0.5 A, respectively.Figure 4c-f demonstrates that these papers can be used as electrical conductor wires to power up LED or in flexible electronic, or in PCB in general.The fabrication of fully printed conductor paper for smart packaging in Figure 4f is recorded in Video S1 (Supporting Information).

Supercapacitor Papers
The performance of supercapacitors electrode of conductive patterned papers via syringe deposition and 3D printing was first characterized using a three-electrode setup.Cyclic voltammogram (CV) curves with different scan rates, charge-discharge curves, and specific capacitance with different specific currents are presented in Figure S6 (Supporting Information).All calculated values can be found in Table S2 (Supporting Information).The rectangular boxes from the CV curve in Figure 5a and the linear triangle shape from the charge-discharge curve (Figure 5b) interpret a good capacitance behavior of electrode double layer (EDL) supercapacitor.CNF/PEDOT:PSS composite gives excellent mixed ion-electronic charge transport properties and has extremely high porosities with an interconnected 3D electrical network that can improve the ions' transportation. [21]From GCD curves (Figure S7b,e, Supporting Information), the calculated capacitance values at 1.2 A g −1 were 139 and 97 mF for deposited and printed papers electrode area of 2 cm 2 with 36 and 27 μm thickness, respectively.The increase of electrode bulk volume of deposited paper contributes to increasing its capacitance, [27] and thus requires a longer charging time (Figure 5b).Using carbon paste as a glue between electrode paper and carbon paper gave a better interface than printing electrodes on carbon paper, and thus lower ESR value.Therefore, the supercapacitor electrode from syringe deposition has higher specific capacitance than that of 3D printing (Figure 5d).The specific capacitance retention of the supercapacitor electrode remained 87% after 2000 cycles (Figure S6c, Supporting Information).
We now provide a full study of the electrochemical properties to understand the performance of the supercapacitor papers (Figure 6; Figure S7, Supporting Information).The sandwich structure of supercapacitors A and B are presented in Figure 6a.The capacitance of both supercapacitors was above 300 mF (Figure 6d), which is a double value compared to the supercapacitor electrode, resulting from using double thickness.The increase of thickness of electrode resulted in higher specific energy density but lower power density than our previous work using 7.6 μm thick electrode. [27]The nanopore-based nanocellulose separator allows the electrolyte to fill its porous structure, and the directly printed electrode on the electrolytes believe to have a good interface.This gives fast access to ionic species (ion and electron) transfer pathways to the surfaces and through the bulks of the electrode, resulting in fast charge transfer kinetics of the electrode paper. [44]Supercapacitors A and B showed fast chargedischarge time (Figure 6b) and high cycle performance of 98.2% and 95.3% after 2000 charging cycles, respectively (Figure 6f).Supercapacitor A had better electrochemical performance than that of supercapacitor B. The specific capacitance dropped from 50.63 to 39.75 F g −1 for supercapacitor A and from 47.34 to 22.39 F g −1 for supercapacitor B when the current density was increased from 0.06 to 3 A g −1 , respectively.The high reduction rate of the specific capacitance value of supercapacitor at high current density due to the high Ohmic drop (ESR) and sluggish kinetics of electrochemical activities, as Nyquist plot at 45°indicated the limitation of the mass transport in the electrode (Figure S7b,d, Supporting Information).The latter is not considered for the comparison since both supercapacitors used the same electrode.The higher value of ESR of supercapacitor B is mainly contributed to the lower ionic conductivity of electrolyte (PEO/NaCl) in comparison to that of supercapacitor A (EMIM-ES electrolyte).In addition, the crosslinked structure of the separator by ionic crosslinker (CaCl 2 ) of supercapacitor B might reduce ionic mobility and ionic conductivity of the electrolyte, which leads to increase internal resistance. [45]The increase of ESR of supercapacitor B after 2000 cycles is more pronounced than supercapacitor A (Figure S7b,d, Supporting Information).Supercapacitor B slightly decreased specific capacitance retention after 1800 cycles (Figure 6f).This is due to the number of ions that could not afford to fully charge the electrode at long cycle, as the weight ratio of CNF:PEO/NaCl to separator was 3:1, while CNF:EMIM-ES was 3:1.Further adding ionic concentration or using high ionic conductivity electrolyte can improve the performance of the supercapacitor, but it is needed to optimize its volume to maintain the viscosity and shear thinning properties of electrolyte ink.
We successfully demonstrated the performance of supercapacitor paper as the power source.Three supercapacitor B papers were formulated in series and charged up to 2.2 V in 400 s, and the supercapacitor device lighted up LED up to 30 s (see inset Figure 6f). Our supercapacitors are comparable to other flexible supercapacitors using PEDOT or hybrid PEDOT/Carbonbased, or PEDOT/metal-based electrode materials. [48]In addition to the supercapacitor performance, using hydrogel ink to print layer by layer without heating each layer will reduce the production time and cost, and it can extend to the large-scale manufacturing of supercapacitors using syringe pump deposition or roll-to-roll printing.The planar supercapacitor structure, which is crucial for flexible and portable electronics, is also possible to produce by 3D printing.

Conclusion
In this work, we fabricated electronic patterned papers from nanocellulose and PEDOT:PSS-based inks.Freestanding and flexible 2D patterned papers were produced by syringe deposition, while 3D printing allowed to design of more freedom 3D papers.The electronic patterned papers are water stable and have a strong interface between the patterns.The deposited patterning paper has a tensile strength of 25.72 MPa and Young's modulus of 1.17 GPa.The conductive parts displayed electrical conductivities of up to 202 and 165 S cm −1 and electrical current capacities up to 0.76 and 0.55 A for syringe deposition and 3D printing, respectively.The combination of high flexibility, mechanical stability, and good electrical conductivity of patterned paper is ideal for a flexible supercapacitor.As the results, printed supercapacitor paper A and B showed high specific capacitance of 50.63 and 47.34 F g −1 , specific energy of 4.05 and 3.79 Wh kg −1 , specific power of 4615 and 967 W kg −1 at 0.06 A g −1 , respectively and had high electrochemical stability with the specific capacitance retention above 95% after 2000 cycles.
The use of biocompatible and eco-friendly materials and simple processing methods, together with high electrochemical performance in one supercapacitor is a promising power source for disposable portable and wearable electronics.We believe that the unique design and properties of electronic patterned papers offer a great interest for advanced applications such as flexible organic electronic, wearable electronic, smart packaging sensor, bioelectronic, and energy storage devices or multi-functions in a single device.
Ink Formulation: CNF, MFC, and sodium alginate were diluted with DI water to 0.2%, 0.2%, and 0.8%, respectively.The materials were mixed with PEDOT:PSS glycerol, ethylene glycol, CaCl 2 , EMIM-ES, NaCl, and PEO in different weight ratios according to Table 1 to form the different inks used in this study.The mixtures were then homogenized for 5 min using a T10 basic Ultra-Turrax (Laboratory mixer) and subsequently homogenized using an ultrasonic batch for 15 min to eliminate the formation of air bubbles.Finally, the inks were heated at 60 °C and stirred using a magnetic stirrer.Inks of 1.2% concentration were used for syringe deposition.
CNF-based ink is appropriate for 3D printing since it combines high viscoelasticity with shear-thinning behavior, which allows it to flow through the narrow nozzle when air pressure is applied and to form a filament of solid gel when pressure is released. [31]Unlike low concentrations of conductive ink prepared for syringe injection, 3D printing requires highviscosity ink to improve the printing quality.For this reason, the conductive suspensions were heated at 60 °C and stirred until reaching different ink concentrations (2.5%, 3.1%, and 4%), as shown in Figure S8a-c (Supporting Information).4% conductive ink has sufficient viscoelasticity to provide printability and shape fidelity and to prevent ink viscous flow and collapse of the wet 3D printed objects.As an example of this, we used this ink to print an eight-layer star structure with rectangular infill with a 25% density using a 50 μm printing nozzle (Figure S8d, Supporting Information).
To achieve patterned samples with an even surface, the conductive and non-conductive inks should have the same concentration.Since the mixture of CNF and solvents could not dewater to reach 4% of concentration by using magnetic stirrer due to strong water binding, MFC and sodium alginate functioned as rheological modifiers to reduce the viscosity and lower gel-like characteristic of non-conductive ink.Sodium alginate also has an additional advantage in that it can act as a binder to maintain the 3D-printed structure, by forming a crosslinked structure when posttreated with drops of CaCl 2 .Inks with different weight ratios of CNF/MFC (25:75 and 50:50) were prepared, and 20 and 50 weights of sodium alginate and glycerol (Table 1) were added into both inks.50:50 CNF/MFC ratio was selected for non-conductive ink, as it gave more precise printed structure compared to 25:75 of CNF/MFC ratio (Figure S9, Supporting In-Table 1. Ink formulations for preparation of electronic patterned papers and supercapacitor.Syringe Deposition Procedure: Electronic patterned papers can be produced from water-based inks using straight forward procedure.The ink concentration 1.2% was directly deposited on substrate and dried to form electronic patterned papers.Tape was used as mask to make patterns and stuck on the plastic substrate, as shown in Figure 7A.In step 1, conductive ink was first deposited using syringe with 400 μm needle and predried at 60 °C for 2 h.This step was repeated for the building layer-by-layer (LBL) structure.The tapes were then removed and substituted by nonconductive inks (step 2), and the patterns are dried at 60 °C for 4 h (step 3).The obtained electronic patterned papers were flexible, and the conductor patterned part can be down up to 2 mm.
3D Printing Procedure: 3D structures were printed using a BioX 3D printer (Cellink, Sweden) equipped with 3 pneumatic printheads.The BioX has a custom-built extrusion with a variable speed control syringe driver mounted on a motor-driven XYZ system featuring custom RepRap/Sprinter Firmware and supporting G-code flavors.The prepared ink was loaded into 3 mL printing cartridges and printed onto petri dish substrates.The filament was extruded through a 200 μm high precision blunt needle tip at a print pressure of 150-220 kPa and print head speed of 1200-1800 mm min −1 .Depending on the shape and size of the structures and the ink formulations, the print pressure and print head speed were manually adjusted during printing for each print (Figure S10, Supporting Information).100 mm CaCl 2 was dropwise deposited onto 3D printed devices to promote crosslinking and to maintain their structures after printing.The printed structures were dried at room temperature and subsequently peeled off from the substrate.
Printed Flexible Supercapacitor Papers: CNF-based electrodes and electrolytes were used to fabricate flexible supercapacitor papers.Half-cell supercapacitor electrodes were fabricated with a dimension of 10×20 mm 2 both by syringe deposition and 3D printing to study the electrochemical behavior for both techniques.The syringe-deposited conductive paper was glued onto a carbon paper as current collector using carbon paste whereas the 3D-printed electrode was directly printed on carbon paper.This carbon paper acts as 1) substrate and has a good adhesion with 3D printed electrode and 2) biodegradable materials for transient supercapacitors.
Symmetric supercapacitor papers in sandwich structure were 3Dprinted in two versions, using the same electrode ink, but different electrolyte inks, as stated in Table 1.The electrolyte ink A was not printable, and the electrolyte paper A was formed by casting and drying at RT. Supercapacitor A was made by printing two layers of each electrode on both sides of electrolyte paper A (that measured 145 μm in thickness).Unlike supercapacitor A, supercapacitor B was fully printed in LBL (layer-by-layer) manner because electrolyte ink B is printable.Since the supercapacitor was fabricated in semi-solid state, CaCl 2 was introduced into electrolyte ink B to partially crosslink alginate and the nanocellulose network, and thus limit the diffusion of the electrode material through the separator and prevents electrical short circuits during operation.Supercapacitor B consisted of double layers of each electrode and a trilayer of electrolyte ink B between the electrodes, as illustrated in Figure S3 (Supporting Information).The supercapacitor B consisted of each electrode and electrolyte of 70 and 100 μm in thickness, respectively.
Tensile Test: Mechanical properties of the papers were evaluated using an MTS tensile tester at room temperature.Conductive and nonconductive papers (as control samples), and electronic patterned papers with one, four, and eight interfaces were fabricated by syringe deposition and 3D printing methods (Figure S4, Supporting Information).Thickness of the papers was measured using a Veeco Dektak 3ST.At least five rectangle specimens of 5×50 mm 2 of each paper were examined using tensile tester at a clamping distance of 40 mm, equipped with a 100 N load cell with a crosshead speed of 20 mm min −1 .Tensile strength, elongation at break, and Young's modulus of the papers were calculated from stressstrain curves.
Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX): Microstructure and surface morphology of the electronic patterned papers were captured using SEM Zeiss Sigma 500 Gemini, and their chemical compositions were analyzed by EDX.Cross-section images were obtained from liquid nitrogen freeze-fracture surfaces, which were coated with 20 nm of gold using an evaporator Model BA 510.The images were taken using a secondary electron detector at 5-10 kV accelerating voltage.
Measurement of Electrical Properties: Starting from conductive papers with various thicknesses, rectangular specimens of 5×25 mm 2 were prepared to measure their in-plane resistances using two crocodile clips equipped with a Keithley 2600A.The electrical conductivity (s) value was calculated by the equation: s = (L/R ´w ´t), where R, L, w, and t are the resistance (in W), length (in cm), width (in cm), and thickness (in cm) of the sample, respectively.The electrical current of conductive papers was recorded by applying a 1 V voltage with an increment of 0.1 V.The maximum electrical current of conductive paper was chosen before it was burned.
Electrochemical Properties: The electrochemical performance of supercapacitor electrodes and devices were examined through three-electrode and two-electrode measurements, using a Biologic SP-200 potentiostat.In the three-electrode system, the working, counter, and reference electrodes were formed from conductive paper, coiled platinum wire, and Ag/AgCl, respectively, and the experiment was conducted in 1 m aqueous KCl solution.Cyclic voltammetry (CV) was performed in the potential range 0-0.8 V (electrochemical side reactions occurring above 0.8 V) [2] at scan rates of 10-200 mV s −1 , and galvanostatic charge-discharge (GCD) measurement was carried out using current densities of 0.1, 0.5, 1, and 5 mA cm −2 .The supercapacitor was charged at 0.8 V and discharged at 0 V for 2000 cycles at 1.4 A g −1 specific current to study its capacitance stability.The electrode capacitance (C, in mF) and specific capacitance (C sp , in F g −1 ) from charge-discharge of three-electrode are calculated using Equations ( 1) and (2), respectively.
Where I (in A) is the constant discharge current; Δt (in s) is the discharge time; dV is the discharging voltage excluding the voltage drop (V d ), and m (in g) is the mass of the electroactive material (PEDOT) in the electrode.
For symmetrical two-electrode supercapacitor cell, the specific capacitance of the electrode material (C s ) is four times the cell-specific capacitance (C sp, cell ), see Equation ( 3) and further details in Equation S5 (Supporting Information).m t is the total mass of the active material (PEDOT) in both electrodes.
From the total cell-specific capacitance, the specific energy (E, in Wh kg −1 ) and power density (P, in W kg −1 )) can be calculated using Equations (4) and (5).Electrochemical impedance spectroscopy (EIS) before and after 2000 cycles was measured at the frequency range from 100 kHz to 100 MHz.Equivalent series resistance (ESR) of the completed supercapacitor can be calculated from the voltage drop on the current peak at the initial discharge curve using Equation ( 6). [32]It can also be deduced from the EIS curve at high frequency (>10 kHz). (5) Where V and V d are the operation voltage window (0.8 V) and the voltage drop at the onset of discharging, respectively.

Figure 1 .
Figure 1.Electronic patterned papers: a-c) syringe deposition; a) flexible, b) conformable,and c) comb structures composing of 2 mm width of each conductor and non-conductor patterns.d-h) 3D printing; d) layer by layer printing, e) 3D structure with two conductors at the bottom and one conductor (that is isolated by non-conductor) on top, and f) antenna structure with 400 μm width.SEM/EDX micrographs: (g) and (h) are morphologies, i) crosssection surface and j) its C, O, and S elemental mapping and k) point distribution, and SEM of non-conductor area of (i) and l,m) its elemental mapping.

Figure 2 .
Figure 2. Tensile properties of papers a) Stress-strain curves (S stands for paper from syringe deposition and P from 3D printing); b) tensile strength; inserted tensile specimens with breaking points; c) elongation at break; and d) Young's modulus.

Figure 3 .
Figure 3. Cross-section surface of a) printed paper after tensile test (inset a), b) two layers of deposited paper, c) three layers of printed paper, d) fractured surface between patterns (conductor pattern in the left and non-conductor pattern in the right), and e) drawing the different zones of patterned paper based on C, O, and S elementals mapping.

Figure 4 .
Figure 4. Relationship between thickness and number of layers a,d); conductivity and current capacity b,e); and demonstration of patterned paper as electrical conductor c,f) from syringe deposition and 3D printing, respectively.

Figure 5 .
Figure 5. Electrochemical properties of deposited and printed conductive papers: a) CV curves and electrode samples, b) GCD, c) capacitance, and d) specific capacitance.

Figure 6 .
Figure 6.Symmetric supercapacitor papers: a) images of supercapacitor structure, b) CV curves, c) GCD, d) specific capacitance, e) specific energy vs specific power, f) capacitance retention after 2000 cycles, and inset f) demonstration of three supercapacitor papers in series as power source for LED.

Figure 7 .
Figure 7. Schematic of fabrication of electronic patterned papers using A) syringe deposition and B) 3D printing.