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current collectors 014}a MWNTs dEt9so 0.14 0.6-0.4-0.2 0.20.4 1 0250300350400450 nL Cellulose with composite paper supercapacitor. (a)Cyclic voltammograms(CV) and(b) Fig 1. Fabrication of the nanocomposite paper units for supercapacitor and charge-discharge behavior of the nanocomposite supercapacitor devices attery. (a)Schematic of the supercapacitor and battery assembled by using with KOH and RTIL electrolytes. The V measurements are carried out at a scan anocomposite film units. The nanocomposite unit comprises RTIL rate of 50 mV/s temperature. The near-rectangular shape of the CV lulose covers the top of the mwnt array TiAu thin film deposited on the discharge behavior was measured at a constant current of 1 mA. (c)Power exposed MWNT acts as a current collector. In the battery, a thin Li electrode density of the supercapacitor device(using the RTIL electrolyte)as a function Im is added onto the nanocomposite. (b)Photographs of the nanocomposi operating temperature. The supercapacitor operates over a record range of units demonstrating mechanical flexibility. Flat sheet(top), partially rolled temperatures (195-450 K).(d) CV of the supercapacitor by using bioelectro- (middle), and completely rolled up inside a capillary (bottom) are shown. (c) lytes[body fluids(blood, sweat)] measured at a scan rate of 50 mV/s at roor Cross-sectional SEM image of the nanocomposite paper showing MWNT temperature. protruding from the cellulose-RTIL thin films (Scale bar, 2 um. )The schematic displays the partial exposure of MWNT. are lower(22). A power density of 1.5 kW-kg-(energy density, The paper can be rolled up, twisted, or bent to any curvature and 13 Wh/kg)is obtained at room temperature for the nanocom- is completely recoverable Fig Ic shows the cross-sectional SEM (0.01-10 kW-kg-)of commercial supercapacitors and compa The celli with randomly distributed of 50+ The performance of acitor (with RTIL) was next 5 nm(21).The nanocomposite paper, which can be typically a examined as a function of temperature to show the thermome- few tens of microns thick, contains MWNTs as the working chanical robustness of the device. The supercapacitor device electrode and the cellulose surrounding individual MWNTs, well as the extra layer as the spacer and the rTiL in cellulose as density that increases with temperature(Fig. 2c), because of the self-sustaining electrolyte. Two of the nanocomposite units enhanced ionic conductivity. Although commercially available onded back-to-back make a single supercapacitor device. The thin lightweight(=15-mg/cm)design of the device results from range of 233-358 K, our nanocomposite supercapacitor device avoiding the use of a separate electrolyte and spacer, genera ch wider record-operating temper te(s3 wt/wt of cellulose in RTIL) makes the device temperatures, and the device showed good performance over environmentally friendly. In a final package, operating devices 100 cycles of charge and discharge. When measurement was can be fabricated by laminating multiple stacks of individual performed at 77K, the device showed no capacitive behavior but promptly recovered when returned to room temperature, dem- Results and discussion onstrating it device was not damaged when kept at ultralow temperatures(77K) Electrochemical measurements were carried out for the super- In addition to using the aqueous and RTIL electrolytes, the electrolyte, 6M KOH aqueous electr the nonaqueous rTiL device operates with a suite of electrolytes based on bodily fluids the performance. Fig. 2 a and b show good capacitive behavior implant or for use under special circumstances. As a precedent, of the supercapacitor by using aqueous(6 M KOH) and non- urine-activated battery was recently demonstrated for bio- aqueous(RTIL) electrolytes. Devices using the KOH electrolyte MEMS device applications(23). Body sweat, composed of water, with RTIL electrolyte. The CNT cellulose-RTiL nanocompos- placed on the film gets sucked into the porous cellulose) for those with the KOH electrolyte(=0.9V). The calculated specific the device(specific capacitance of 12 F/g, operating voltage of capacitances were 36 F/g and 22 F/g for these devices with KOH $2.4 V; Fig 2d). Blood(human whole blood in K EDTA from nd RTiL electrolytes, respectively. The specific capacitance of Innovative Research, Southfield, MI)worked even better as an the device in the KoH solution is higher compared with RTIL, electrolyte, enhancing the capacitive behavior of the supera because the dielectric constant and ionic mobility of the latter pacitor, resulting in a specific capacitance of 18 F/g(Fig 2d) PNAs| August21.2007|vol.104|no.34|13575The paper can be rolled up, twisted, or bent to any curvature and is completely recoverable. Fig. 1c shows the cross-sectional SEM image of the nanocomposite with its corresponding structure. The cellulose is porous with randomly distributed pores of 50 5 nm (21). The nanocomposite paper, which can be typically a few tens of microns thick, contains MWNTs as the working electrode and the cellulose surrounding individual MWNTs, as well as the extra layer as the spacer and the RTIL in cellulose as the self-sustaining electrolyte. Two of the nanocomposite units bonded back-to-back make a single supercapacitor device. The thin lightweight (15-mg/cm2) design of the device results from avoiding the use of a separate electrolyte and spacer, generally used in conventional supercapacitors. The use of RTIL electrolyte (3 wt/wt % of cellulose in RTIL) makes the device environmentally friendly. In a final package, operating devices can be fabricated by laminating multiple stacks of individual nanocomposite layers (SI). Results and Discussion Electrochemical measurements were carried out for the supercapacitor and, in addition to using the nonaqueous RTIL electrolyte, 6 M KOH aqueous electrolyte was used to compare the performance. Fig. 2 a and b show good capacitive behavior of the supercapacitor by using aqueous (6 M KOH) and nonaqueous (RTIL) electrolytes. Devices using the KOH electrolyte showed lower equivalent series resistance compared with that with RTIL electrolyte. The CNT cellulose–RTIL nanocomposite showed a higher operating voltage (2.3 V) compared with those with the KOH electrolyte (0.9 V). The calculated specific capacitances were 36 F/g and 22 F/g for these devices with KOH and RTIL electrolytes, respectively. The specific capacitance of the device in the KOH solution is higher compared with RTIL, because the dielectric constant and ionic mobility of the latter are lower (22). A power density of 1.5 kWkg1 (energy density, 13 Wh/kg) is obtained at room temperature for the nanocomposite (RTIL) supercapacitor, which are within reported ranges (0.01–10 kWkg1) of commercial supercapacitors and comparable to flexible devices reported (3). The performance of supercapacitor (with RTIL) was next examined as a function of temperature to show the thermomechanical robustness of the device. The supercapacitor device, operated at different temperatures (195–450 K), shows a power density that increases with temperature (Fig. 2c), because of enhanced ionic conductivity. Although commercially available supercapacitors are reported to work typically in a temperature range of 233–358 K, our nanocomposite supercapacitor device showed a much wider record-operating temperature range (195– 423 K). Cyclability testing was also carried out at various temperatures, and the device showed good performance over 100 cycles of charge and discharge. When measurement was performed at 77 K, the device showed no capacitive behavior but promptly recovered when returned to room temperature, demonstrating it device was not damaged when kept at ultralow temperatures (77 K). In addition to using the aqueous and RTIL electrolytes, the device operates with a suite of electrolytes based on bodily fluids, suggesting the possibility of the device being useful as a dry-body implant or for use under special circumstances. As a precedent, a urine-activated battery was recently demonstrated for bioMEMS device applications (23). Body sweat, composed of water, Na, Cl, and K ions (24), used as electrolyte (a drop of sweat placed on the film gets sucked into the porous cellulose) in the RTIL-free nanocomposite affords good capacitive behavior for the device (specific capacitance of 12 F/g, operating voltage of 2.4 V; Fig. 2d). Blood (human whole blood in K2 EDTA from Innovative Research, Southfield, MI) worked even better as an electrolyte, enhancing the capacitive behavior of the supercapacitor, resulting in a specific capacitance of 18 F/g (Fig. 2d). Fig. 1. Fabrication of the nanocomposite paper units for supercapacitor and battery. (a) Schematic of the supercapacitor and battery assembled by using nanocomposite film units. The nanocomposite unit comprises RTIL ([bmIm][Cl]) and MWNT embedded inside cellulose paper. A thin extra layer of cellulose covers the top of the MWNT array. Ti/Au thin film deposited on the exposed MWNT acts as a current collector. In the battery, a thin Li electrode film is added onto the nanocomposite. (b) Photographs of the nanocomposite units demonstrating mechanical flexibility. Flat sheet (top), partially rolled (middle), and completely rolled up inside a capillary (bottom) are shown. (c) Cross-sectional SEM image of the nanocomposite paper showing MWNT protruding from the cellulose–RTIL thin films. (Scale bar, 2 m.) The schematic displays the partial exposure of MWNT. -0.6 -0.4 -0.2 0.0 0.2 0.4 -0.14 -0.07 0.00 0.07 0.14 Current (mA) Voltage (V) RTIL KOH 0 2 4 6 8 0.0 0.5 1.0 1.5 2.0 2.5 Voltage (V) Time (s) RTIL KOH 200 250 300 350 400 450 1 2 3 Power density (kW/kg) Temperature (K) sweat -0.6 -0.3 0 .0 0.3 -0 .08 -0 .04 0 .0 0 0 .0 4 0 .0 8 Current (mA) Voltage (V) blood a c b d Fig. 2. Electrochemical double-layer capacitance measurements of nanocomposite paper supercapacitor. (a) Cyclic voltammograms (CV) and (b) charge–discharge behavior of the nanocomposite supercapacitor devices with KOH and RTIL electrolytes. The CV measurements are carried out at a scan rate of 50 mV/s at room temperature. The near-rectangular shape of the CV curves indicates good capacitive characteristics for the device. Charge– discharge behavior was measured at a constant current of 1 mA. (c) Power density of the supercapacitor device (using the RTIL electrolyte) as a function of operating temperature. The supercapacitor operates over a record range of temperatures (195–450 K). (d) CV of the supercapacitor by using bioelectrolytes [body fluids (blood, sweat)] measured at a scan rate of 50 mV/s at room temperature. Pushparaj et al. PNAS August 21, 2007 vol. 104 no. 34 13575 ENGINEERING