Fabrication of Resorcinol-Formaldehyde Xerogel Based High Aspect Ratio 3-D Hierarchical C-MEMS Structures
Authors: Sharma Chandra Shekhar , Katepalli Hari , Sharma Ashutosh , Teixidor Genis Turon , Madou M. 
Published: February 2020

INTRODUCTION

In Lithium ion batteries, various type of carbon materials (activated carbon, carbon nanotubes, glassy carbon, organic aerogels etc.) have been investigated as potential electrode materials (1). Further to achieve enhanced power and energy density, 3-D electrode architecture is demonstrated more useful than the conventional thin film approach (2,3). The most promising way to fabricate 3-D electrode arrays was demonstrated using photo-patterning and pyrolysis of SU-8 photoresist (3). Further it was established mathematically that a fractal electrode design constitutes a more optimal design for energy conversion devices because of the maximization of electrochemically active surface area while minimizing the electrical work involved (4). However, the fabrication of a truly 3-D multiscale or fractal carbon electrodes remains a challenging task.

In this study, we used resorcinol-formaldehyde xerogel (RFX) as an organic precursor to yield non-porous dense carbon (5). We first demonstrate the ease of fabrication of high aspect ratio (HAR) RFX 3-D structures by top-down (replica molding) followed by fabrication of arrays of carbon hierarchical microposts using bottom-up (electrospraying) approach. Finally, we show that RFX derived non-porous dense carbon films can be reversibly charged and discharged with Li ions and thus may be a potential contender as anode materials for 3-D microbattery architecture.

EXPERIMENTAL

We first fabricated 3-D HAR micro-arrays of SU-8 by photopatterning using photolithography which were then used as a master stamp to prepare negative mold in PDMS using replica molding. RF sol as synthesized by the polycondensation of resorcinol and formaldehyde in an organic solvent in the presence of an acidic catalyst was poured onto the PDMS template followed by controlled drying. Later the PDMS stamp was carefully peeled off to obtain patterned 3-D RFX HAR structures. Finally, SU-8 or RF sol based submicro- and nano- beads respectively were then conformally deposited on the 3-D RFX HAR structures by electrospraying followed by pyrolysis of the entire integrated constructs in an inert atmosphere to yield HAR 3-D hierarchical carbon structures.

Electrochemical measurements

RFX derived thin carbon films for electrochemical measurements were prepared by spin coating RF sol onto a Si wafer. Controlled drying and pyrolysis conditions were maintained to yield smooth uniform carbon films. The RFX derived carbon films so obtained were used as working electrodes while a Li foil was used as counter electrode in an electrochemical cell. The electrolyte used was 1M solution of lithium per chlorate in a 1:1 (v/v) mixture of EC:DMC. An electrochemical test cell made of Teflon was designed with an effective working electrode area of 0.654 cm2 to perform Galvanostatic (charge and discharge) experiments at two different current densities 76.4 and 152.7 µA cm-2between 0.05 and 3.0 V using a multichannel potentio/galvanostat (Gamry Instruments).

RESULTS

RFX derived HAR 3-D hierarchical structures

Fig.1 (A) SEM images in first row shows array of cylindrical and cross geometry microposts made by replica molding in RFX while second row shows array of hierarchical carbon posts obtained after conformal deposition of SU-8 and RF sol derived electrospun submicron and nano sized beads; (B) Galvanostatic charge discharge cycle behavior for RFX derived carbon film; (C) comparison of specific capacity of RFX derived carbon with SU-8 derived carbon.

Galvanostativc charge discharge cycling

Galvanostatic discharge and charge cycles experiment was carried out at C/5 rate (Fig. 1B). Initial discharge capacity was very high compare to other cycles due to the formation of solid electrolyte interphase layer. The reversible capacity of the RFX derived carbon at a current density of 76.4 µA cm-2 was found to be 195.1 mAh/g which was similar to that of SU-8 derived carbon (~220 mAh/g) in similar conditions (3). After first discharge, reversible capacity was found to be nearly constant. However for RFX derived carbon, the irreversible specific capacity was found to be significantly less (0.0636 mAh/cm2) than what has been for SU-8 derived carbon (0.0788 mAh/cm2) (3) (Fig. 1C).

SUMMARY

RFX has been introduced as a polymer precursor to fabricate 3-D HAR structures that were used to yield hierarchical array of carbon posts by integrating electrospun nanostructures. Further we have shown that RFX derived carbon can be reversibly intercalated with Li ions. Interestingly, RFX derived carbon shows a significant reduction in irreversible capacity compared to photoresist derived carbon and thus could be an important contender as an anode material for Li ion batteries.

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