Energetic carbon precursors for micro-supercapacitor printing

A highly energetic carbon precursor suitable for soft lithographic processing based on acetylene dicarboxylic acid is presented. High-resolution micro-supercapacitors with line width down to 500 nm are produced using solvent assisted nanoimprint lithography. The resulting nonporous carbons exhibit remarkably low specific resistance down to 6.9 (cid:2) 10 (cid:3) 5 O m pyrolyzed at 800 1 C. The interdigital electrodes show excellent device capacitances of up to 1.32 mF cm (cid:3) 2 with an average capacitance retention of 97% after 10000 cycles. Tailoring the performance is achieved by variation of electrode distances and carbonization temperatures.


Introduction
The establishment of portable electronic devices in medicine, technical applications and consumer electronics also attracted an immense attention in research of miniaturized modules.2][3][4][5] In this sense micro-supercapacitors with remarkable power densities, fast charge/discharge rates and ultra-long cycle life assembled on ultra-small device areas are extensively studied. 6,76][17] Recently used inks include carbon materials like graphene and graphene oxide, 10,18,19 CNTs 20 as well as MXenes. 14,17,21anoimprint lithography (NIL) is a promising technique for the production of high-resolution interdigital carbon electrodes. 22,23The approach is using liquid carbon precursors and enables binder free direct printing of precursor structures.After pyrolysis the porous carbon electrodes adhere to the substrate.Compared to, for example, photolithographic techniques, the nanoimprint lithography facilitates fast and lowcost printing and high process throughput.A typical limiting resolution achieved via soft lithographic techniques using PDMS as stamping material is around 200 nm. 24,25he ongoing trend towards ever smaller mobile devices forces the development of flexible, stretchable and transparent devices. 26Especially in the production of e-skin, wearable devices or biomedical applications a high degree of shape stability and elasticity is required. 13,27,28In this sense polymer or silicon substrates are used to produce microsupercapacitors. 29 However, printing carbon precursors onto flexible substrates requires carbonization temperatures significantly above the melting point of the substrate. 29Hence, for porous carbon printing applications precursors transforming into conductive carbons at low temperatures are required.
1][32] The carbon properties like electronic conductivity and porosity are highly dependent on the carbonization temperature. 33Provided that the electronic conductivity has a decisive influence on the performance of highresolution micro-supercapacitors, the formation of a suitable carbon material with high electronic conductivity and appropriate surface area remains to be a challenge.Graphene and other particle dispersions afford high surface areas when deposited on substrates but continuous processing via printing technologies is hampered by clogging of nozzles and stamps.
Micro-structuring provides a good accessibility and an increased carbon surface area can even be achieved without porous carbons.A challenge is to envision molecular precursors graphitizable at low temperature (o900 1C).In this regard, energetic carbon precursors based on alkynes are promising candidates.Acetylene is a widely used gaseous precursor for CNT production and CVD processes.5][36] For the processing of a liquid carbon precursor additional functional groups like carboxylate groups are ideal as the CO 2 elimination is favorable.In this sense we envisioned acetylene dicarboxylic acid to be an ideal precursor for microcontact printing.In the following we present an acetylene dicarboxylic acid-based carbon precursor for the nanoimprint processing of micro-supercapacitor with a high resolution inplane interdigital structure (Fig. 1).

Results and discussion
A new energetic carbon precursor system is developed to produce in-plane micro-supercapacitors (MSC) with interdigital geometry using the solvent assisted nanoimprint lithography (SA-NIL) technique (Fig. 1).
A suitable precursor must fulfill various requirements regarding processability and shape stability.The selected solvent system should have a high vapor pressure to ensure a fast curing under thermal or UV-treatment, forming the desired homogeneous and stable pattern.The main focus is to reduce the necessary carbonization temperature significantly below 900 1C and guarantee a high electrical conductivity at the same time.
We study a promising energy-rich carbon precursor based on acetylene dicarboxylic acid (H 2 ADC), glycerol and sulfuric acid.During pattering of the mixture, the structure is cured through polymerization triggered by heating and UV-radiation.Three different interdigital structures are tested and compared using the same line height of 500 nm but different line width of 10 mm (IDE 500/10), 5 mm (IDE 500/5) and 1 mm (IDE 500/1).The influence of the carbonization temperature on the material and structural characteristics like conductivity, porosity and line heights as well as the electrochemical performance are explored.

Precursor characterization
The processability of the dissolved acetylene dicarboxylic acid is increased using glycerol as esterification agent in order to obtain a more homogeneous precursor.At the same time, the polymerization of these educts is promising to form stable structures during NIL-printing.
In our work, the triol glycerol serves as an optimal polymerization agent.To characterize and optimize the polymerization and esterification degree, various heat and UV-radiation treatments are analyzed by infrared spectroscopy (Fig. 2a).The treated and untreated acid-alcohol mixtures exhibit very similar IR-spectra showing mainly the characteristic stretches of the pure dicarboxylic acid and glycerol.These are namely the valence and deformation vibration of the OH-groups (B3400 cm À1 ) and the characteristic vibration of the carboxyl-group at 1650 cm À1 caused by the acid, glycerol and 20 wt% of water.
To enhance the polymerization, concentrated sulfuric acid is added to the solution, treated again with UV-light and cured at 100 1C afterwards.In the polymerized system the two characteristic bands of the ester groups are clearly visible (n as (C-C(QO)-O) at 1250 cm À1 and n as (O-C-C) at 1080 cm À1 ).Furthermore, due to the formation of the ester, the symmetric valence stretch of the CRC triple bond at 2300 cm À1 clearly emerges, because the symmetry of H 2 ADC is reduced and the stretch of the CRC triple bond becomes IR-active.A slight  shift in the stretch of the carboxylic group to higher wavenumbers can be recognized and is now observed at 1710 cm À1 , the typical range for the CQO vibration in the alpha-beta unsaturated ester group (usually at 1750-1715 cm À1 ). 37,38he TG measurements suggest that a temperature of 450 1C is sufficient to completely transform the organic precursor, because at higher temperatures only a small mass loss is observed (Fig. 2b).
To characterize the surface properties of the material, nitrogen physisorption and Raman measurements are performed using the carbon powder carbonized at varying temperature.Therefore, carbon powders are prepared at carbonization temperatures between 500 to 900 1C.The powders show no appreciable specific surface area at all analyzed pyrolysis temperatures, indicating a total absence of porosity.In the Raman spectra, the presence of highly graphitic carbon, carbonized at temperatures between 700 to 900 1C under Argon, can be observed.Due to the high reactivity, even conversion temperatures as low as 300 1C in air are sufficient to produce this allotrope of carbon (ESI, † Fig. S1).This observation is in agreement with observations by Bourlinos et al., who investigated the formation of a carbonaceous solid after pyrolysis of monopotassium salt of acetylene dicarboxylic acid at 300 1C.Besides the characteristic D-band at 1350 cm À1 for disordered carbon structures, the G-band at 1595 cm À1 assigned to the graphitic carbon is present, 37 rendering the precursor as promising for the transformation into materials for electrochemical energy storage (Fig. 2c).The powder carbonized at 800 1C showed the lowest ratio of I D /I G with 0.28 and therefore the highest amount of graphitic carbon, followed by 700 1C (0.50) and 900 1C (0.54).The drop of the I D /I G ratio at 900 1C is probably caused by the increasing degradation of the carbon material at high temperatures.
The electrochemical properties of the carbon material are investigated using compact thin films after different carbonization temperatures between 600 and 900 1C (Table 1).As expected, the electrical conductivity increases with increasing temperatures due to a higher graphitization. 39The lowest resistance is achieved for the sample carbonized at 800 1C with 6.92 Â 10 À5 Om due to the highest amount of graphitic carbon at this temperature.The specific resistance for the sample prepared at 900 1C is slightly higher probably due to the increasing carbon degradation and possible cracking occurring especially at high temperatures.A dramatic increase in the resistance emerges at temperatures below 700 1C.
After processing at 600 1C the material showed unacceptably low conductivity.The loss of conductivity could be caused by the minimal increase of organic residues in the material, which probably have a high impact on the overall resistance.However, the compact thin films pyrolyzed at 800 1C and 700 1C showed remarkably low specific resistances which are about three times lower than for comparable carbon precursors at those temperatures. 40Therefore, we focused especially on pyrolysis temperatures between 700 and 900 1C for the subsequent preparation of micro-supercapacitors.

Nanoimprint lithography
Due to the promising results of the powder characterization, the precursor was applied in the nanoimprint process.In general, the generation of homogeneous crack-free and completely separated electrode fingers is a crucial requirement to obtain high performance interdigital electrodes.The nanoimprint lithography is a suitable procedure to generate highresolution structures.The short distances between the electrodes, created with this technique, are an important factor to accelerate the ion transport and reduce the diffusion resistance.In this work, we present the printing of high-resolution interdigital micro-structures with line widths of 500 nm and distances down to only 1 mm using the investigated energy-rich carbon precursor.The characteristic structure parameters are summarized in Table 2.
The precursor is an ideal candidate for application of the SA-NIL process, because, using water as solvent, it provides an ideal viscosity and curability at moderate temperatures and UVradiation.
After the printing process at 120 1C, lines with heights of around 500 nm are produced (Fig. 3).The etching depth of the stamp is 500 nm.Typically, the line height is lower than this value because the solvent is evaporated during printing.In this case, it indicates significant swelling of the printed lines at high humidity.Only for the IDE 500/10 the line height is lower with 380 nm.In the subsequent SEM image precursor droplets along the patterned lines are visible, which might promote emergence of short circuits in the final MSCs.The formation of droplets together with the swelling of the lines is caused by the hydrophilic nature of the precursor and is especially occurring when the structures are stored at room temperature.To prevent these side effects and the appearance of short circuits of electrodes, the structures should be stored at 120 1C before carbonization.
After pyrolysis at 900 1C, almost no residues between the lines are visible, but a homogenous pattern with clearly separated and completely contacted electrode fingers is observed.During the pyrolysis high volume shrinkage and height loss of the lines down to 60-140 nm occurs.The average line height, which was achieved for all structures and all temperatures, allows an electrochemical application.Interestingly, a layered structural characteristic of the resulting carbon material can be observed in the SEM image (Fig. 3) for the pyrolyzed IDE 500/1 structure indicating the formation of graphitic domains.
In analogy to the structures pyrolyzed at 900 1C, carbonization temperatures of 800 1C and 700 1C are analyzed.Theoretically, an increase in the achieved line heights due to the decrease of graphitization is expected.However, no remarkable line height and width differences could be observed probably because the graphitization state is still very high at these temperature ranges (ESI, † Fig. S2), making the precursor an interesting material for further electrochemical applications.

Electrochemical characterization
To analyze the basic electrochemical behavior of the carbon bulk material compact thin films and sandwich type EDLCs are prepared and investigated (Fig. 4).For the precursor pyrolyzed at 900 1C, a high specific areal capacitance of 14.8 mF cm À2 is reached.The system shows a low internal series resistance of 5 O, which underlines the high conductivity of the carbon.Compared to that, the carbon material pyrolyzed at 800 1C reaches an areal capacitance of 5.3 mF cm À2 .
The CV-curves of all analyzed structures show a characteristic rectangular shape even at high scan rates.The Nyquist plot exhibits the typical form, which is expected for capacitive systems.The ESR (equivalent series resistance) ranges between 4-6 kO and is much higher compared to the thin film EDLCs caused by the small line width of the interdigital structure.Theoretically, it is expected that the capacitance increases with smaller electrode distances, because of smaller diffusion pathways.This trend can also be seen for the analyzed geometries.Here, the IDE 500/1 shows with 1.32 mF cm À2 the highest device capacitance in comparison to the device capacitance of IDE 500/5 with 0.40 mF cm À2 and IDE 500/10 with 0.1 mF cm À2 .Due to the lowest distance, IDE 500/1 shows the lowest diffusion resistance and therefor exhibits the lowest ESR with only 4 kO.Surprisingly, very high device capacitances for all structures could be reached, despite the porosity and specific surface area of the carbon material is moderate.This underlines the importance of the implementation of a highly conductive material as electrode material to achieve high capacitances, which greatly compensates a possible absence of a high surface area carbon.Moreover, the high-resolution pattern increases the outer surface area and the accessibility of the electrodes.Furthermore, the influence of the carbonization temperature on the electrochemical performance are investigated.The calculated device capacitances for all structures at different carbonization temperatures are shown in Table 3.
Because the IDE 500/1 exhibits the highest capacitance, the CV-curves, Nyquist plots and rate capability of the three analyzed temperatures are shown in detail in Fig. 6a-c.As expected, the capacitances decrease with lower carbonization temperatures, because the graphitization and therefore conductivity decreases.Because of the absence of porosity an effect on the capacitance with temperature change can be neglected.The mentioned trend can be seen for the IDE 500/5 structure as well (Fig. 6d).However, beyond the discussed temperature influence, the fabrication of defect-free structures without cracks after carbonization is another crucial challenge to achieve high capacitances.This might cause the observed reverse trend for the IDE 500/10, where the probability of line cracks is lower at reduced temperatures.
The electrode distance might play another important role when it comes to the fabrication of defect free MSCs.The interdigital structure with the smallest electrode distance (IDE 500/1) shows the highest number of electrode fingers and therefore the highest probability of line cracks and contact problems, which makes a fabrication of defect free MSCs with reproducible device capacitances more challenging.This influence, and the mentioned influence of lower conductivity, might cause the device capacitance drop from 900 1C to 800 1C for the IDE 500/1.However, high device capacitances for all structures could be reached.
To analyze the cycling stability of the fabricated MSCs, 10 000 charge and discharge cycles are performed (Fig. 6e).For all structures, an excellent cycling stability could be observed, leading to a maximal capacity retention of 92%.In the Ragone plot (Fig. 6f), all structures pyrolyzed at 900 1C showed similar power densities at different scan rates from 4.2 to 11.1 mA cm À2 , because all structures exhibit the same line width of 500 nm.However, the energy densities could be significantly improved with decreasing line distances and transport pathways, because of decreasing diffusion path length.Therefore, IDE 500/1 showed the highest energy density with an average of 0.07 mW h cm À3 .

Compact film formation
The thin films are prepared on pre-treated boro-aluminium silicate slides (Corning 1737, DELTA Technologies) (25 mm Â 25 mm Â 1.1 mm).In the pre-treatment step, the slides are stored in piranha solution (1 part 30% H 2 O 2 , 3 parts conc.H 2 SO 4 ) for at least 30 minutes, cleaned with deionized water and ethanol and dried in a nitrogen stream afterwards.Furthermore, the surface of the dried substrate is activated in argon plasma (kiNPen, Neoplas Tools).Thin films are formed by spin coating (Spin 150, ATP GmbH) 80 ml of the carbon precursor applied on the substrate using a speed of 2000 rpm for 30 seconds.The thin films are dried at 100 1C and pyrolyzed in a tube furnace at temperatures between 300 and 900 1C with a heating rate of 150 K min À1 for 2 h under argon stream.

Preparation of thin film supercapacitors
To prepare layered EDLCs the pyrolyzed compact films are placed in acetone in order to separate them from the substrate and dried at 70 1C.The two electrodes are coated with electroconductive varnish (electro-DAG) and afterwards with the dried films.To activate the electrodes, they are treated at 80 1C in vacuum overnight before additionally activation with Arplasma.Afterwards, the hydrogel electrolyte is added (see electrolyte synthesis) and a polypropylene separator placed between the electrolyte layers.After drying both electrodes at 45 1C they are built together in a Swagelok setup.

Nanoimprint lithography
For the synthesis of interdigital patterned structures the solvent assisted nanoimprint lithography is applied using a microcontact-printing System m-CP 3.0 (GeSim mbH).The substrate-material and its pre-treatment is analogous to the compact films.The preparation of the patterned PDMS stamps (Sylgard 184 elastomer kit, Dow Chemicals) is described elsewhere. 41A droplet of 4 ml precursor is applied onto the substrate before the stamp is pressed into it in order to fill the space in the stamp pattern.To receive an adequate solvent removal, the printing is carried out under constant pressure of 165 kPa and temperature of 120 1C for 900 seconds.For an improved polymerization and curing of the precursor, the structure is irradiated with UV-light during the whole printing process.After peeling of the stamp, the structure is dried at 100 1C and pyrolyzed at the same conditions as the carbon films.In this work, three different interdigital structures are prepared with line widths of 10 mm (IDE-1), 5 mm (IDE-2) and 1 mm (IDE-3) and maximum line heights and widths of 500 nm each.The structures are analyzed by atomic force microscopy (Dimensions D 3100, Digital Instruments) and SEM (DSM-982 Gemini, Zeiss) measurements.

Electrolyte synthesis
The used proton conductive electrolyte is a polymer hydrogel consisting of a mixture of polyvinyl alcohol (PVA; Merck; molecular weight 145 000) and sulfuric acid in a ratio of 1 : 1.2.For an optimal amount 1 g of PVA (0.0069 mol) is added to 14 ml deionized water and heated at 90 1C under constant stirring and reflux.Afterwards 1.2 g (0.0122 mol) conc.sulfuric acid is added drop-wise so that a clear gel-like solution is created.

Preparation of interdigital micro-supercapacitors and electrochemical characterization
Possibly attached carbon residues are carefully removed before the interdigital structure is covered with silicone to protect it from the following deposition of a chrome (10 nm) and gold (100 nm) current collector.The physical vapor deposition (B39, Malz & Schmidt) is performed with a deposition rate around 15-20 kÅ s À1 .Subsequently the silicon cover is removed, a PMMA ring is applied around the structure area to create an isolation to the metal and confine the electrolyte in a defined area.After drying, the electrode surface is activated in argon plasma to increase the hydrophilicity and remove attached particles.After that, 10 ml of the electrolyte is applied into the corresponding space.
For electrochemical characterization (VMP3 Potentiostat, Bio-Logic) cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) are performed.The CVs are recorded with a scanning rate between 5 to 100 mV s À1 in a voltage window of 0-0.8 V, while for the EIS analysis a frequency range from 5 mHz to 100 kHz is used.

Conclusions
In summary, an energy-rich carbon precursor system was successfully established as an ink in the SA-NIL process leading to highly defined interdigital structures with a line width down to 1 mm.By using pyrolysis temperatures between 700 and 900 1C, stable, highly graphitic carbon electrodes for solid-state micro-supercapacitors could be obtained.Although the material showed no porosity, very high device capacitances up to 1.32 mF cm À2 with an excellent cycling stability over 10 000 cycles could be reached.Comparable microsupercapacitors with line widths of 500 nm and 1 mm prepared via SA-NIL technique achieved areal capacitances of only 7.6 mF cm À2 and 23.1 mF cm À2 using sucrose derived carbon as precursor and a Li 2 SO 4 -solution as electrolyte. 22Other comparable interdigital micro-supercapacitors based on 8 layers of CVD graphene using a PVA/H 2 SO 4 hydrogel as electrolyte and line distances of 70 mm exhibit a device capacitance of 62.7 mF cm À2 but are difficult to print continuously. 42o further enhance the device capacitance, the specific surface area of the carbon material may be further enhanced.To increase the porosity, CO 2 -activation or the use of a template strategy are two possible options here for future development.However, an optimum between decreasing conductivity with increasing porosity should be considered.To improve the performance of the micro-supercapacitor at low carbonization temperature the reactivity of the precursor needs to be enhanced.In particular the content of polymerization agents i.e. glycerol should be minimized to achieve an enhanced reactivity of the acid.The development of new reactive precursor agents is a promising field for the SA-NIL technique to achieve defined carbonization at low temperatures for energy storage applications.

Fig. 1
Fig.1Schematic procedure of the SA-NIL process using a high energetic carbon precursor and further MSC processing.

Fig. 3
Fig.3Height profiles (after printing red lines (polymer), after pyrolysis black lines (carbon)) of the different structures and SEM images of the three different structures after printing at 120 1C and after carbonization at 900 1C.

Table 1
Specific four-point resistances of carbon thin films after different pyrolysis temperatures under Ar

Table 3
Device capacitances of all structures after pyrolysis at varying temperatures under argon