Article pubs.acs.org/JPCC
MnO2‑Based Thermopower Wave Sources with Exceptionally Large Output Voltages Sumeet Walia,† Sivacarendran Balendhran,† Pyshar Yi,† David Yao,† Serge Zhuiykov,‡ Muthu Pannirselvam,§ Rodney Weber,⊥ Michael S. Strano,∥ Madhu Bhaskaran,# Sharath Sriram,# and Kourosh Kalantar-zadeh†,* †
School of Electrical and Computer Engineering, RMIT University, Melbourne, Victoria 3001, Australia Materials Science and Engineering Division, CSIRO, Highett, Victoria 3190, Australia § School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Victoria 3001, Australia ⊥ School of Physical, Environmental and Mathematical Sciences, University of New South Wales at Australian Defence Force Academy, Canberra, BC 2610, Australia ∥ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, United States # Functional Materials & Microsystems Research Group, RMIT University, Melbourne, Victoria 3001, Australia ‡
S Supporting Information *
ABSTRACT: Miniaturization of conventional energy sources has so far proven to be a cumbersome process. A recently developed concept of thermopower waves has shown tremendous potential to reduce the dimensions of power sources while maintaining their energy generation capabilities. We demonstrate a tremendous improvement in the output for these thermopower wave-based energy generation devices by implementing manganese dioxide (MnO2) as the core thermoelectric material. In this work, the thermoelectric MnO2 layer is used as a pathway for the propagation of thermopower waves that are generated as a result of an exothermic chemical reaction of a solid fuel (nitrocellulose). Such selfpropagating thermopower waves result in exceptionally high voltage output on the order of 1.8 V and a specific power (power-tomass ratio) on the order of 1.0 kW·kg−1. The output voltage is at least 300% higher than any other thermopower wave system reported so far.
■
INTRODUCTION The commercial deployment of many micro-/nanoelectronic systems has been hampered by the lack of miniaturized power sources.1 To efficiently and reliably realize functional micro-/ nanoscale concepts such as smart dust, nanorobots, and a variety of integrated microelectromechanical systems (MEMS), power sources that match their dimensional scale need to be developed.2,3 The development of many small-scale bioelectronic devices also requires such small-scale energy sources. For commercially available energy sources such as batteries and fuel cells, the rate of energy discharge diminishes dramatically when they are miniaturized.4 Furthermore, the size of these sources cannot be reduced beyond a certain limit after which complications such as disruption of ionic flow near the electrodes impede their performance.5−7 Supercapacitors are widely used in applications where a high-energy discharge rate is required. However, they suffer from a high self-discharge rate thus requiring frequent recharging.8−10 All of the abovementioned energy sources are capable of producing only direct current (dc) power, whereas alternating current (ac) power, without a need for implementing extra electronic circuits, is © 2013 American Chemical Society
highly desirable for practical applications. Because of the aforementioned limitations of commonly used energy sources, novel approaches are required to achieve practical miniaturization of energy sources. A recently developed energy system is based on the concept of thermopower waves that can generate a high specific power (power-to-mass) ratio at small dimensions.11 The first generation of thermopower wave systems were based on thermally and electrically conductive MWCNT (multiwalled carbon nanotube) cores and have been shown to generate specific powers as large as 7 kW·kg−1.11 In these devices, a temperature gradient across a thermoelectric MWCNT core encourages the transportation of free electrical carriers. This temperature gradient is produced as a result of an exothermic chemical reaction of highly reactive solid fuels that covers the MWCNTs.11,12 A key feature of such sources is that by changing the parameters of the system the output can be kept Received: February 18, 2013 Revised: April 9, 2013 Published: April 10, 2013 9137
dx.doi.org/10.1021/jp401731b | J. Phys. Chem. C 2013, 117, 9137−9142
The Journal of Physical Chemistry C
Article
thermoelectric power factor − TPF (S2σ) are desirable. Additionally, a preferably high thermal conductivity of the core material is required in such sources to facilitate the sustained propagation of thermopower waves that is contrary to conventional TE applications for which low thermal conductivities are desired. Consequently, transition metal oxides are highly promising candidates as the core TE materials due to their high S2σ.17,18 Metal oxides such as TiO2, WO3, MnO2 are potential candidates as they demonstrate an ideal combination of these characteristics especially at elevated temperatures.19−23 Amongst them MnO2 (manganese dioxide) in a powdered form has recently been reported to exhibit an exceptionally high Seebeck coefficient.21 MnO2 is also an n-type semiconductor with an electrical conductivity that increases at elevated temperatures, thereby resulting in a high TPF in comparison to other common TE materials (Table 1). Therefore, it appears to be a plausible candidate as the core thermoelectric material in thin film thermopower devices. In this work, we show that MnO2-based thermopower wave devices are capable of generating significantly larger voltage outputs compared to previously demonstrated systems. We present a full analysis of the system by characterizing the device before and after the reaction and also the Seebeck coefficient, electrical conductiviy, output voltage profile, specific power, and thermopower wave propagation velocities.
dc or made to oscillate. However, the limitation of the MWCNT-based thermopower devices is that the output voltages (
