Supporting Information Spray-coated Multi-walled Carbon Nanotube Composite Electrodes for Thermal Energy Scavenging Electrochemical Cells

Transcription

1 Supporting Information Spray-coated Multi-walled Carbon Nanotube Composite Electrodes for Thermal Energy Scavenging Electrochemical Cells Nicolas E. Holubowitch, James Landon, Cameron A. Lippert, John D. Craddock, Matthew C. Weisenberger, and Kunlei Liu* 2540 Research Park Drive, Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA *Corresponding author. Tel: Fax: S-1

2 Thermocell Figure S1. Photograph of the glass thermocell used in this work to measure temperature profile across the cell. The cell can be rotated to any orientation so that the electrolyte fill level is always above the electrodes. S-2

3 a b Figure S2. (a) Voltage-time data for generating thermocell power curves in this work (potentiostat data shown). Constant current steps are applied and the voltage is sampled after steady state is achieved (~5 s for our electrodes, 120 s steps shown to demonstrate steady state behavior). Data here are for 70 wt.% CNT cell at Th = 80 C. (b) Validation of power curve acquisition methods for thermocells (data shown for 70 wt.% CNT electrodes). The resistance box data (red diamonds) represent the truest thermocell performance, where power is measured across various resistors. Other methods using a Keithley 2450 source measurement unit (SMU) or Autolab potentiostat yield power data accurate within ~10%. The most precise method, SMU current step method (solid black squares, steps from from 0 ma to short circuit current), was used in this work. S-3

4 Electrochemical materials screening Three-electrode tests were performed on various dispersible, conductive, and high-surface area carbon-based thermocell electrode candidates. Figure S3. Cyclic voltammograms at room temperature in 10 mm Fe(CN)6 3- and 0.5 M H2SO4 at 100 mv s -1. All materials except buckypaper (CNT sheet), Toray carbon paper, and xerogel were laminated to glassy carbon electrodes with 0.25 wt.% Nafion. Current is normalized by the mass of the carbon material. MWNTs are the raw CNTs eventually used in our sccnt composites. Carbon xerogel = a mesoporous material, acetylene black = a conductive additive used in battery electrode fabrication, MCMB = mesoporous carbon microbeads, a common Li-ion battery electrode material. The tests used a Pt mesh counter electrode. CAER indicates the material was synthesized in-house at the University of Kentucky Center for Applied Energy Research. The bare glassy carbon (GC) substrate itself yielded negligible redox currents due to its very low surface area; complete coverage of the GC substrate was visually confirmed after casting the carbon nanomaterials onto the GC. S-4

5 Additional sccnt micrographs Figure S4. Top-down images of sccnt composite films with 5, 10 and 20 wt.% CNTs and of buckypaper (100% CNTs). S-5

6 Figure S5. Micrographs of the 30 wt.% CNT/PVDF composite (a) as-sprayed, showing some vertical alignment and excellent substrate adhesion; (b) intentionally scraped from the SS substrate, where the higher polymer content is reflected by the pliable nature of the film; (c) cross-sectional image of composite embedded in epoxy showing relative uniformity of spraycoated thickness; and (d) close-up of CNT/PVDF layer cross-section. S-6

7 Figure S6. Images of commercial buckypaper displaying a layered morphology. S-7

8 ΔTcalc and ΔText measurements Figure S7. Relationship between ΔTcalc and ΔText in thermocells with buckypaper and a representative sccnt sample (10 wt.% CNTs in PVDF) and photograph of cell oriented at 135. Data derived from Figure 5a in the main text, where the Seebeck coefficient -1.4 mv K -1 and cell OCV were used to calculate each ΔTcalc value. S-8

9 Figure S8. Thermocell performance at all orientations studied for a representative thermocell (20 wt.% CNTs). (a) Areal peak power density and power conversion efficiency relative to a Carnot engine. 1 (b) Ohmic and charge transfer resistances derived from impedance spectroscopy. (c) Schematic showing thermal effects at play due to orientation-dependent convection currents. The >2x greater power output from 135 to 225 is a reflection of the much larger ΔTcalc values achievable at these orientations (see main text Fig. 5b and related discussion) and the trend was observed for all electrodes studied. The suppression of convection currents in those orientations increased the electrolyte s ohmic resistance which consequently reduces the power conversion efficiency. The maximum Carnot efficiency of 0.43% was observed at the 90 orientation. S-9

10 Resistance data a b Figure S9. Through-film area specific (a) resistance and (b) conductance of CNT-based electrodes. Resistance (blue bars) measured when passing 10 ma, a typical thermocell current, through 0.32 cm 2 painted Ag pads on composite. Conductivity data presented for mass of composite (red) or CNTs only (gray) underneath the Ag pad, calculated by: (1/ASR)/(mfilm) or (1/ASR)/(mCNTs). Film thicknesses in (a) are reported as ranges of total film mass loading on SS current collectors. Generally, conductivity decreased with film thickness (see 50 wt.% CNT samples) and below 20 wt.% CNTs, although film conductivity per mass of CNTs remained high down to the percolation threshold, ~2 wt.% CNTs. Buckypaper had the highest through-film resistance, more than twice as resistive as the lowest CNT content composite. S-10

11 Mass-based performance at any ΔTcalc Figure S10. CNT mass-based power output versus ΔTcalc (orientation independent) for 30 wt.% sccnt and buckypaper. S-11

12 Table 1. Summary of related thermocell research. Reference Electrode material 2 MWCNT buckypaper 3 MWNTs with Pt contacts 4 90% SWNTs, 10% reduced graphene oxide 5 MWCNT aerogel sheets Filtration Fabrication method CVD to grow super-aligned CNT forests, electrodes drawn from forest by rotor-controlled dry-state spinning around a Pt wire Vacuum filtration to form sheets followed by plasma treatment Same as ref. [3], plus additional annealing and Pt decoration CNT treatments; wrapped around tungsten plate or wire current collectors This work MWCNTs Spray-coated MWCNTs with PVDF binder onto stainless steel Note on cost and fabrication Relatively low cost, limited scalability Expensive and scalability very low HiPco SWNTs = $595 g -1 (least pure); graphene fabrication and vacuum filtration currently not scalable, plasma treatment costly Expensive and scalability very low Highly scalable (mature industrial technology) and low cost materials Note on performance Power factor a = 0.2 mw m -2 K -2 in cell with membrane and fabric separators. Seobs = 0.8 mv K -1 and mass activity not reported. 234 mw m -2 at ΔT = 21 C (power factor by area = 0.53 mw m -2 K -2 ). Optimized composition peak power single electrode = 325 mw m -2 at ΔT = 31 C (power factor by area = 0.34 mw m -2 K -2 ). 10-stacked sheet power factor by mass = 49 mw kg -1 K -2. Two designs: Planar flow-assisted cell with power factor = 0.07 mw m -2 K -2 (in electrodes most similar to ours, i.e. untreated CNTs and no Pt), Seobs = 1.0 mv K -1 ; Cylindrical cell: 6.6 W m -2 in best design with Pt. Power factor by area = 0.14 mw m -2 K - 2 ; by mass = 142 mw kg -1 K -2. Peak area power density = 930 mw m - 2 ; peak mass power density = 1 W g - a Power factor is a metric that normalizes power density by the square of the temperature differential across the cell, i.e. Pmax/ΔT 2. This provides a rough estimation of how the device would perform at other temperature differences and can help to compare data since there is no standard ΔT for thermocell performance testing. Nevertheless, caution should be taken as other factors contribute to performance as well such as inter-electrode spacing, electrolyte concentration, electrode geometry, and observed Seebeck coefficient. 1 CNT S-12

13 References (1) Quickenden, T. I.; Mua, Y. A Review of Power-Generation in Aqueous Thermogalvanic Cells. J. Electrochem. Soc. 1995, 142, (2) Hu, R.; Cola, B. A.; Haram, N.; Barisci, J. N.; Lee, S.; Stoughton, S.; Wallace, G.; Too, C.; Thomas, M.; Gestos, A.; Cruz, M. E. d.; Ferraris, J. P.; Zakhidov, A. A.; Baughman, R. H. Harvesting Waste Thermal Energy Using a Carbon-Nanotube-Based Thermo- Electrochemical Cell. Nano Lett. 2010, 10, (3) Yang, H. D.; Tufa, L. T.; Bae, K. M.; Kang, T. J. A Tubing Shaped, Flexible Thermal Energy Harvester Based on a Carbon Nanotube Sheet Electrode. Carbon 2015, 86, (4) Romano, M. S.; Li, N.; Antiohos, D.; Razal, J. M.; Nattestad, A.; Beirne, S.; Fang, S.; Chen, Y.; Jalili, R.; Wallace, G. G.; Baughman, R.; Chen, J. Carbon Nanotube - Reduced Graphene Oxide Composites for Thermal Energy Harvesting Applications. Adv. Mater. 2013, 25, (5) Im, H.; Kim, T.; Song, H.; Choi, J.; Park, J. S.; Ovalle-Robles, R.; Yang, H. D.; Kihm, K. D.; Baughman, R. H.; Lee, H. H.; Kang, T. J.; Kim, Y. H. High-Efficiency Electrochemical Thermal Energy Harvester Using Carbon Nanotube Aerogel Sheet Electrodes. Nat. Commun. 2016, 7. S-13