Thermoelectric Power by the Diffusion of Protons in a Nanoporous Structure
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Thermoelectric Power by the Diffusion of Protons in a Nanoporous Structure Michael A. Reznikov Physical Optics Corporation, 20600 Gramercy Place, Torrance, CA 90501, U.S.A. ABSTRACT The work presents the model for the ionic thermoelectric phenomenons, which is based on the consideration of Grotthuss, hopping mechanism for the proton conductivity. The Seebeck coefficient and figure of merit are estimated in good agreement with experimental data. INTRODUCTION Most solid-state thermoelectric (TE) generators, based on the Seebeck effect, suffer from power loss due to the parasitic flow of heat. While low-dimensional structures, such as quantum wells, super-lattices, quantum wires, and quantum dots, offer new ways to decrease the thermal (phonon) conductivity while improving the electric conductivity [1,2], their energy conversion efficiency is relatively low and fabrication is very expensive. Therefore, there is a need for new TE materials for waste heat recovery and electricity generation that have a high figure of merit (ZT) and the potential for large-scale production at costs competitive with conventional technologies, considering the entire system over its lifetime. It is known that the receptors of marine sharks and skates (rays), which contain the extracellular hydrogel, are the most exquisite sensors of thermal fluctuations [3-5]. We demonstrated that these ionic structures can be mimicked by natural and synthetic gels [6], and exhibit very high TE voltage, a few mV/K at low, room temperatures. These materials currently are commercially available due to progress in fuel cell technology, where they are used as proton conductive membranes. THEORY Theoretical models for thermoelectricity [7, 8] based on electronic structure are presumably inappropriate for proton-conductive substances. First of all, the density of mobile ions depends much less on the temperature than the density of free electrons. Second, the mobility of ions is very sensitive to temperature, due to the thermally activated diffusion mechanism of charge transfer [9]. Dynamic capturing and release of protons is the base mechanism of protonic transfer in nature. A mechanism for proton mobility was suggested to be comprised of the cyclic isomerization between the two forms of protonated water, H3O+ and H5O2+, which is coupled to hydrogen-bond dynamics in the second solvation shell of the H3O+. The formation of a high fraction of pore bulk water in proton exchange membranes (PEMs) is desirable for high conductivity because of the dominance of the Grotthuss diffusion mechanism in conductivity, which occurs in bulk water rather than at the surface. The Grotthuss mechanism is the protonhopping-mechanism where each oxygen atom simultaneously passes and receives a single hydrogen atom. . Due to interaction with the wall of a pore, localized water molecules create deeper traps for protons than those in the bulk water. This is illustrated in figure 1 (courtesy of [10]).
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(a) (b) Figure 1. A simplified picture of structure and proton transfer in Nafion (a)
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