Multi-Walled Carbon Nanotube Electrode Optimization for Thermocells

There is an abundance of rejected thermal energy in industrial settings available for conversion to electricity by devices such as thermocells. Compared to solid state thermoelectric technology, thermocells scavenge low-grade heat more efficiently and offer 10-fold greater Seebeck coefficients (ferro/ferricyanide α = -1.4 mV/K). Since aqueous ferrocyanide cells are thermodynamically constrained by the Seebeck coefficient, focus has shifted to optimizing their kinetic performance.

Nanocarbons, particularly multi-walled carbon nanotubes (MWNTs) are emerging as the favored active electrode material in thermocells (1-3). These offer high conductivity and large electrochemically active surface areas. The preparation of MWNT-based electrodes, however, is no trivial task and non-electrochemically active binder materials (e.g. PVDF) are nearly always required. A variety of methods for MWNT/PVDF electrode casting have been investigated but doctor blade and spray-coat methods are currently lowest cost and most commonly employed. In this study, we contribute to the on-going optimization of spray-coated MWNT/PVDF on Al electrodes by exploring the influence of mass loading and active material-binder content via characterization in functioning full thermocells.

While varying the MWNT/PVDF ratio had little impact on peak power densities, we found that the thinnest possible coatings (i.e. one coat) yielded the highest power density per mass of MWNTs at 290 W kg^-1. This suggests that tortuosity and/or poor wettability (hydrophobicity) of the MWNT electrodes limit the penetration depth of aqueous ferro/ferricyanide ions during thermoelectric conversion. Therefore, if coating thickness is strictly controlled, only 0.08 mg cm^-2 (in our case) is needed. This minimizes inactive bulk MWNTs and brings the previously prohibitively expensive MWNTs to a fraction of the cost of the ferro/ferricyanide electrolyte for thermal to electric energy harvesting. The findings may be of broader interest for the fabrication of any electrochemical cells where integration of MWNTs is desired.

Figure 1. Potential-current and power curves of thermocells for three thicknesses of spray-coated MWNT/PVDF electrodes on Al. Current and power are normalized by MWNT mass, where the thicknesses refer to loadings of 1.0, 0.23, and 0.08 mg cm^-2 MWNTs at 50/50 MWNT/PVDF content by mass.

References

1. R. Hu, et al., Nano Letters, 10(3), 838 (2010).
2. T.J. Kang, et al., Advanced Functional Materials, 22(3), 477 (2012).
3. P.F. Salazar, S. Kumar, and B.A. Cola, Journal of the Electrochemical Society, 159(5), B483 (2012).