Application of a water-soluble cobalt redox couple in free-standing cellulose films for thermal energy harvesting

Abstract

Thermal energy harvesting using thermoelectrochemical cells (thermocells) is a sustainable method to produce electricity without carbon dioxide emissions. The solvent and redox couple used in the electrolyte play an important role in determining both the safety and performance of thermocells, and development of leak-free electrolytes with high performance is particularly important for transportable devices. Here, the application of aqueous and non-aqueous electrolytes containing the [Co(bpy)]^2+/3+ redox couple in both liquid and solid forms was studied. Cellulose was used as an environmentally friendly material for solidification of the different liquid electrolytes. The properties and performance of the new aqueous [Co(bpy)]^2+/3+ electrolytes was compared to those containing the Fe(CN)₆^3−/4− couple, both in liquid and quasi-solid state electrolytes. Higher diffusivity for the cobalt redox ions was observed in the aqueous electrolyte compared to the non-aqueous electrolytes, while the Seebeck coefficient of the redox couple, which determines the open circuit voltage of the thermocell, was largest in the organic solvents. No significant effect of solidification on the Seebeck coefficient was observed.

Introduction

Global energy demand is growing because of the expanding population and increasing reliance on digital technology. The environmental contamination by burning fossil fuels, and their limited availability, push us towards renewable sources of energy. Harvesting thermal energy using thermoelectrochemical cells (thermocells) is a sustainable method to produce electricity without any carbon dioxide emission. They could also be used to continuously power small electronic devices by conversion of body heat to electricity.

Thermocells are electrochemical devices that use a redox couple dissolved in an electrolyte, and two electrodes held at different temperatures, which directly converts thermal energy to electricity [1,2]. This conversion is based on the non-zero temperature dependence of the electrochemical potential of the redox couple. A potential difference (ΔV) between two electrodes is thus produced when two electrodes are kept at different temperatures [3,4]. The rate of change of redox potential with temperature is termed its “Seebeck coefficient” (S_e) and thermodynamically this is related to the change in entropy (ΔS) in the redox reaction:

$$S_e=\frac{\text{ΔV}}{\text{ΔT}}=\frac{\text{ΔS}}{nF}$$

where n is the stoichiometric number of electrons involved in the redox reaction and F is Faraday's constant.

Although the factors that determine S_e are not yet fully understood, it is known to be strongly affected by the nature of the redox couple and the solvent [[5], [6], [7], [8]]. The S_e can be positive or negative depending on the absolute charges of the reduced vs. oxidised species [9]. For individual thermocells, it is the magnitude of S_e that is of key importance. However, for the development of series-connected cells there is a significant advantage in having access to both large negative and large positive Seebeck coefficient systems as this enables an array design equivalent to n-type and p-type semiconductors in thermoelectric generators [10].

K_3/4Fe(CN)₆ and [Co(bpy)₃][NTf₂]_2/3 (where bpy = 2,2^ʹ-bipyridyl and NTf₂ = bis(trifluoromethanesulfonyl)amide) are two redox couples that have been used in thermocell research to prepare aqueous and non-aqueous electrolytes, respectively, as dictated by their respective solubilities. The cobalt-based redox couple with NTf₂₋ counter-ions is soluble in ionic liquids and high-boiling point molecular solvents such as methoxypropionitrile (MPN) and DMSO. This couple has a large and positive Seebeck coefficient (1.5–2.2 mV/K), the magnitude of which is attributed to additional entropy effects arising from the spin-state change of the central metal ion (Co^2+/3+) during the redox reaction [[11], [12], [13]]. However, the low solubility of this redox couple in water has to-date prevented its use in aqueous or hydrogel electrolytes. The highest positive Seebeck coefficient in water reported to-date is 1.02 mV/K for the FeCl₂/FeCl₃ redox couple [10]. In contrast, K_3/4Fe(CN)₆ is much more soluble in water, and has a large, negative Seebeck coefficient (−1.4 mV/K) [2,14,15]. Using water-based electrolytes is a safer and more environment friendly approach to energy harvesting than most flammable, non-aqueous electrolytes. Thus, there is a significant advantage to the development of water-based electrolytes that utilise the [Co(bpy)]^2+/3+ couple.

There have been some prior studies on the application of aqueous solutions of cobalt redox couples in dye sensitized solar cells (DSSC). These used mixtures of [Co(bpy)₃][NO₃]₂ (0.2 M) and [Co(bpy)₃][NO₃]₃ (0.04 M), [16] or mixtures of [Co(bpy)₃]Cl₂ (0.13 M) and [Co(bpy)₃]Cl₃ (0.04 M), or [Co(phen)₃]Cl₂ (0.13 M) and [Co(phen)₃]Cl₃ (0.04 M) (phen = phenanthroline) [17]. However, the use of water-soluble cobalt redox couples for thermal energy harvesting has not been reported to-date.

The second approach to the development of the practical thermocells explored here is the solidification of the electrolyte, to prevent potential leakage problems. This is important for both water-based and organic solvent-based electrolytes. Prior reports on addressing these leakage concerns for the aqueous systems containing K₃Fe(CN)₆/K₄Fe(CN)₆ or FeCl₂/FeCl₃ have used the gelation agents agar, poly (sodium acrylate), polyvinyl alcohol (PVA) and cellulose [10,18,19]. The thermocell device containing agar or poly (sodium acrylate) with 0.1 M K_3/4Fe(CN)₆ produced a maximum power density of 0.113 mW/m² (0.0003 mW) and 0.5 mW/m² (0.0012 mW) respectively, with ΔT = 20 °C [19]. These materials gave stable power outputs and decreased unwanted heat transfer across the cell, with the power output from the poly (sodium acrylate) cells similar to those of the liquid-containing cells.

Aqueous electrolytes containing K₃Fe(CN)₆/K₄Fe(CN)₆ or FeCl₂/FeCl₃ have been gelled through addition of polyvinyl alcohol (PVA), and used in a series connected thermocell system [10]. A Seebeck coefficient of −1.21 mV/K was reported for 0.1 M Fe(CN)₆^3−/4- in the polyvinyl alcohol (PVA) gel electrolyte, which is less than in the liquid (ca. −1.40 mV/K).

Apart from different gelation agents, solidification of aqueous solutions of K_3/4Fe(CN)₆ redox electrolyte has been investigated using different quantities of cellulose (2.5–20 wt%). Immersing 5 wt% porous cellulose membrane in 0.4 M K_3/4Fe(CN)₆ resulted in a leak-free redox active electrolyte, which in the thermocell produced 80% of the power of a liquid system: 14 mW/m² (0.0622 mW) with ΔT = 15 °C and cellulose thickness of 3 mm [18].

Recently, we reported the first in-situ gelation of the non-aqueous redox active electrolytes [Co(bpy)₃][NTf₂]_2/3 in methoxypropionitrile (MPN) through addition of 5 wt% PVDF or PVDF-HFP [20]. A leak-free gel was achieved with 5 wt% polymer but a higher concentration (15 wt%) was required to prepare a free-standing electrolyte, which decreased the power output due to mass transport limitations. It was shown that solidification of the liquid electrolyte limits the convection and decreases the heat transfer across the cell, providing an opportunity to reduce the electrode separation to design a thinner cell.

Here we report the first use of the water soluble [Co(bpy)₃]Cl_2/3 redox couple for thermal energy harvesting. We also combine this new electrolyte with cellulose to prepare free-standing quasi-solid state electrolytes. The thermal energy harvesting performance of these new redox-active materials is compared to two other systems: (i) a non-aqueous electrolyte containing [Co(bpy)₃][NTf₂]_2/3 redox couple in MPN, gelled with cellulose for the first time, and (ii) K_3/4Fe(CN)₆ in water, with and without solidification using cellulose. Thus, the effect of redox couple, solvent and solidification on these complimentary negative and positive Seebeck coefficient electrolytes is discussed. Finally, the effect of increasing the redox couple concentration was investigated as a route to further optimisation of thermocell performance.