Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermo-Electrochemical Cell

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1 Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermo-Electrochemical Cell Supporting material: Electrochemical characterization Carbon multiwalled nanotube (MWNT) buckypaper was fabricated by filtration of liquiddispersed nanotubes made by chemical vapor deposition (using acetylene as the carbon source). 1 Cyclic voltammograms of these MWNT sheet electrodes were obtained and are compared with those for graphite plate and Pt foil electrodes in Figure S1. Figure S1. Cyclic voltammograms using graphite plate, MWNT buckypaper and Pt foil as electrodes in 10 mm K 4 Fe(CN) 6 solution. Working electrode: graphite plate, MWNT buckypaper, or Pt foil (1 x 0.5 cm 2 ); counter electrode: Pt (1.5 x 2.0 cm 2 ); reference electrode: Ag/AgCl in saturated NaCl solution; supporting electrolyte: 500 mm NaCl solution; scan rate: 5 mv/s. The redox reactions Fe(CN) e - <=> Fe(CN) 6 4- in 10 mm K 4 Fe(CN) 6 solution appear to be at least quasi-reversible for the graphite plate, MWNT buckypaper, and Pt foil electrodes. More

2 specifically, the ratio of peak currents for opposite direction scans are close to unity for all three electrode materials, which suggests that these electrodes can sustain self-regenerative operation. Most importantly, the separations of reduction and oxidation peaks for Pt, MWNT and graphite electrodes are 65 mv, 72 mv, and 138 mv, respectively, indicating that Pt foil and the MWNT sheets should perform better in thermocells than graphite plate electrodes. U-shaped thermocell Figure S2. U-shaped thermocell used for benchmarking different electrodes. Figure S2 is a schematic diagram of a U-shaped thermo-electrochemical harvesting cell. The distance between the two half-cells is 5 cm. One cell side (left) is cooled using circulating liquid and the other cell side (right, furnace) is heated using electric heating tape. A thermocouple is inserted in close proximity to the electrode for each half-cell. The thermocouple at the hot halfcell is connected to a temperature controller that adjusts the electric power applied to the heating tape, so as to achieve a pre-set target temperature. The electrodes were Pt foil (1.5 cm x 0.5 cm x 25 µm), graphite plate (2.0 cm x 0.5 cm x 910 µm), or MWNT buckypaper (1.5 cm x 0.5 cm) of

3 either 40 µm or 80 µm thickness. The electrolyte was 0.4M K 3 Fe(CN) 6 /K 4 Fe(CN) 6 aqueous solution. Mark I glass vial cell Figure S3. Mark I glass vial cell. Electrodes: MWNT buckypaper. Hot side and cold side temperature : 70 C and 15 C, respectively. Distance between electrodes: d = 1.5 cm. A 20 ml vial is filled with 15 ml of 0.4M K 3 Fe(CN) 6 /K 4 Fe(CN) 6 aqueous solution. As-produced MWNT buckypaper serves as electrodes. At both hot side (top electrode) and cold side (bottom electrode) the electrode has 0.35 cm 2 area and 0.5 mg weight. Both electrodes are held by Pt gauze and connected to Pt wires. The Pt gauze and wires are then covered by insulating paint to prevent possible artifacts. The electrodes are bent so that the two MWNT buckypapers face other. The distance between the two electrodes is 1.5 cm. Thermocouples are placed close to each of the electrodes. A flexible band heater covers the top part of the vial, where the hot electrode is located. The lower part of the vial is immersed in a mixture of ice/water. The heater

4 is connected to the output of a temperature controller, which sets hot side temperature at 70 C, whereas the temperature of the lower electrode is set by the ice/water bath at 15 C. For power generation applications in space, gravimetric power (watts per kilogram of device weight) is importance, given the high cost of launching equipment into space. The Mark I glass vial CNT-based thermocell can deliver up to 47.1 W/kg and 1.48 W/m 2 at a temperature difference of 55 C as shown in Table S1. The gravimetrically normalized current (by weight of the two electrodes) reaches 2455 A/kg. This suggests CNT-based thermocells can be excellent current sources, and have potential for energy generation for space exploration where the weight of the power generator is a critical technical issue. However, achieving such high specific current and power in fully packaged devices will require careful system design and optimization because the limiting factors will likely be the weight of electrolyte and packaging material instead of the weight of electrode material, which is an important concern for conventional electrochemical devices such as batteries. 2 Table S1. Performance chart of Mark I glass vial cell. Symbol* Value T ( C) 55 V OC / T (mv/k) 1.4 (76.8/55.2) I SC / electrode weight (A/kg) 2455 j SC (A/m 2 ) 77 P MAX (W/m 2 ) 1.48 P MAX /electrode weight (W/kg) 47.1 *The symbols are defined in the main text.

5 Figure S4. Two Mark I glass vial cells connected in series. Table S2. Performance chart for two Mark I cells connected in series. Cell A Cell B In Series T ( C) V OC (mv) I SC (ma) P MAX (µw) Effect of redox solution concentration The relationship between cell performance and electrolyte concentration was investigated in the U-shaped thermocell, with hot side and cold side temperatures set at 70 C and 10 C, respectively. The electrolyte consisted of different concentrations of K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] in aqueous solution. The electrodes were MWNT buckypaper (3.5 cm x 1.0 cm x 80 µm). As shown in Figure S5, V OC changes from 94 mv for a M electrolyte to 84 mv for a 0.4 M electrolyte, a drop of almost 11%. The corresponding Seebeck coefficient changes from 1.57 mv/k for the

6 0.025 M electrolyte to 1.4 mv/k for the 0.4 M electrolyte. Such trends have been previously reported for other electrodes. 3 Figure S5. Concentration dependence of short circuit current and open circuit voltage for thermocell with equal molar ratio of K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] aqueous electrolyte. The observed total decrease in V OC is relatively small considering that the electrolyte concentration changes 16 fold. This slight decease in V OC could be the result of a change in the effective redox species concentration or activity with change in total solution concentration and ionic strength. The short circuit current increased significantly with concentration from 0.3 ma for M electrolyte to 2.2 ma for a 0.4 M electrolyte a 7-fold increase. This increase is the result of increased conductivity and increased concentration of redox ions that can react at the electrodes. A concentration of 0.4 M K 4 [Fe(CN) 6 ]/K 3 [Fe(CN) 6 ] is the highest that can be dissolved in water, hence this concentration was used in subsequent experiments.

7 Effect of inter-electrode distance When electrode separation distance was decreased both j SC and P MAX increased because ions could diffuse quickly over shorter distances, i.e., ionic mass transport was enhanced (see Figure S6); yet, efficiency dropped significantly because a higher heating rate was required to maintain the same ΔT. Figure S6. I SC and maximum output power versus inter-electrode distance tested using the cell in Figure 2a. Electrodes: MWNT buckypaper. Hot-side and cold-side electrodes each weighed 1.2 mg and were 1.5 cm long and 0.6 cm wide. Materials cost analysis for CNT-based thermocells For evaluating materials costs, we consider both presently observed performance and the costs of electrochemically active materials needed for coin cells though the most important applications might be for cells having giant areas that are used for harvesting energy from waste streams and geothermal or solar sources. The cost of electrochemically active materials for CNT-based coin cells comes from the CNTs and the potassium ferri-ferrocyanide in the electrolyte. For the coin cell that continuously delivered power to a load for 3 months (the maximum investigated period) we used two MWNT buckypaper electrodes, which each weight 0.3 mg, and 0.05 ml of 0.4M K 3 Fe(CN) 6 /K 4 Fe(CN) 6 aqueous solution. From test results, for ΔT = 37 C (derived from the observed V OC and Seebeck coefficient) for a coin cell in which thermal contact is optimized

8 (which provides V OC = 51.2 mv, I = 0.77 ma, and P MAX = 9.8 µw), the target application for a ΔT of 60 C would provide V OC = 83.9 mv, I = 1.26 ma, and P MAX = 26.5 µw. The sales price of MWNTs is decreasing to ~$150/kg for semi-industrial applications and is expected to further decrease to ~$45/kg in the midterm. 4 Consequently, if the cost of MWNTs is $150/kg, the cost of MWNTs per watt of power is $3.40/W. The lower projected cost of ~$45/kg provides a MWNT cost of $1.02/W. High purity (99% or higher) K 3 Fe(CN) 6 and K 4 Fe(CN) 6.3H 2 O cost $3.7/kg 5 and $2.5/kg 6, respectively, which provides an additional $1.74/W for materials costs. Consequently, the cost of electrochemically active materials is between $5.14/W and $2.76/W depending on the above mentioned costs of MWNTs. Device packaging could increase these costs by 4X or higher. Also, when comparing these results with other energy harvesting means, it should be remembered that the longest service life so far demonstrated is 90 days. References (1) Hall, L. J.; Coluci, V. R.; Galvão, D. S.; Kozlov, M. E.; Zhang, M.; Dantas, S. O.; Baughman, R. H. Science 2008, 320, 504. (2) Modern Battery Technology; Tuck, C. S, Ed.; Ellis Horwood Ltd., (3) Quickenden, T. I.; Mua, Y. J. Electrochem. Soc. 1995, 142, (4) NanoSEE 2008: Nanomaterials Industrial Status and Expected Evolution; Publications, March (5) (6)