Enhanced thermal energy harvesting performance of a cobalt redox couple in ionic liquid–solvent mixtures

Abstract

Thermoelectrochemical cells are increasingly promising devices for harvesting waste heat, offering an alternative to the traditional semiconductor-based design. Advancement of these devices relies on new redox couple/electrolyte systems and an understanding of the interplay between the different factors that dictate device performance. The Seebeck coefficient (Se) of the redox couple in the electrolyte gives the potential difference achievable for a given temperature gradient across the device. Prior work has shown that a cobalt bipyridyl redox couple in ionic liquids (ILs) displays high Seebeck coefficients, but the thermoelectrochemical cell performance was limited by mass transport. Here we present the Se and thermoelectrochemical power generation performance of the cobalt couple in novel mixed IL/molecular solvent electrolyte systems. The highest power density of 880 mW m(-2), at a ΔT of 70 °C, was achieved with a 3 : 1 (v/v) MPN-[C2mim][B(CN)4] electrolyte combination. The significant power enhancement compared to the single solvent or IL systems results from a combination of superior ionic conductivity and higher diffusion coefficients, shown by electrochemical analysis of the different electrolytes. This is the highest power output achieved to-date for a thermoelectrochemical cell utilising a high boiling point redox electrolyte.

High Power Density Electrochemical Thermocells for Inexpensively Harvesting Low‐Grade Thermal Energy

“…We will start by describing the performance realized for the thermocell, and then discuss the advances we have obtained in order to realize this performance. Figure e compares the areal power densities presently achieved, for different values of Δ T , with those reported in the literature for planar thermocells . This power density is here and elsewhere normalized with respect to the planar area of the thermocell electrodes, since this is the key performance metric when thermal energy is abundant.…”

“…d) Optical image of a typical cellulose sponge thermal separator. e) Comparison of presently realized power densities for different values of Δ T with record and typical values that are reported in the literature for planar thermocells that operate at ambient pressure.…”

“…One of the problems is that the Seebeck coefficient for a highly conducting electrolyte has not exceeded 1.4 mV K −1 , even though less ionically conductive organic electrolytes have provided high S = 7.2 mV K −1 . Hence, the power density for an organic‐electrolyte thermocell has not exceeded 0.9 W m −2 . Regarding the thermocell electrode materials, recent efforts have been devoted to improving the surface area of carbon electrodes, such as by using multilayer graphene/carbon nanotube composites and carbon nanotube aerogels .…”

Can charged colloidal particles increase the thermoelectric energy conversion efficiency?

“…To enhance the thermocell performance (e.g., higher thermoelectric coefficient and larger electrical conductivity) various improvements are made through electrode materials, redox-couples, and electrolyte types, as well as the natural convection of liquids and the diffusion of dissolved ionic species. 8,10,11,12,13 So far, the highest power output reaching over 10 W m À2 has been reported very recently by Zhang et al 14 using a highly concentrated ferri-/ferrocyanide electrolyte.…”

Diglyme‐Based Thermoelectrochemical Cells Operable at Both Subzero and Elevated Temperatures

“…To ease the intrinsic drawbacks of ILs, the use of a low‐viscosity cosolvent has been suggested. For example, 3‐methoxypropionitrile/1‐ethyl‐3‐methylimidazolium tetracyanoborate (3/1, v/v) electrolyte with a Co 3+/2+ tris(bipyridyl) redox couple gave the highest power density (780 mW m −2 for T cold and T hot of 60 and 130 °C, respectively), but its poor performance at room temperature still needs to be resolved.…”