Materials Today Energy
Can CuFeS2 be used in a sensitized thermal cell?
Graphical abstract
Introduction
In a world where energy consumption is on the rise, our only hope is to develop new energy-generation technologies. Although the currently used renewable energy sources (e.g. wind [1] and solar energy [2]) have their merits, there is a large, permanent, and untapped energy source that is quite literally under our feet, i.e. geothermal energy.
Recently, we have developed a new type of battery, named sensitized thermal cell (STC) [[3], [4], [5], [6]], which can reliably generate electric power from heat in the environments with temperatures above 80 °C, which is sufficiently low to mimic geothermal heat. In an earlier experiment, we developed STCs that employed dye-sensitized solar cells [7] to convert light into electric power. We replaced the dye with a semiconductor to enable the cells to operate using heat instead of light.
Several methods for converting heat into electric power already exist, including redox batteries [8] (which employ the flow of hot and cold chemical liquids to create electricity) and thermoelectric batteries (which use the Seebeck [9] effect to generate electricity when a temperature gradient is applied along a conductor). However, an STC battery can be literally buried in the ground and work as is; other devices would face major physical and operational issues if required to operate in such a way.
STC consists of a working electrode, a counter electrode, and electrolyte. An electron transport layer and a semiconductor layer, which together comprise the working electrode. Sandwiched between these layers and the counter electrode is a solid electrolyte layer. When heat is applied, electrons in the semiconductor are thermally excited and rise from a low-energy state to the high-energy state and are then injected into the electron transport layer. Then, they pass through an external circuit to the counter electrode and, finally, to the electrolyte. A reduction reaction at the counter electrode and an oxidation reaction at the working electrode involving electrolyte ions occur by the thermally excited carriers [3].
Thus far, STCs using β-FeSi2 [3,10], Ge [6], and Ag2S [5] as the semiconductor have been reported; however, these materials are relatively expensive. Ge, which is currently regarded as the most promising material, has the advantage that Ge wafer is commercially available. However, Ge is unstable in water, and there is concern about deterioration owing to the moisture contained in the atmosphere. In this study, we examined the possibility of CuFeS2 as an STC material. CuFeS2 is well known material in opto-electronic field, for example, as a photothermal material [11] and as an interface layer in photovoltaics [12]. This material also used in thermoelectric field [13]. The CuFeS2, called chalcopyrite, is a natural mineral and abundant on the Earth, and stable in water. It is promising semiconductor for STC.
Section snippets
Fabrication of the CuFeS2 powder
CuCl (5.04 mmol, Fujifilm Wako Pure Chemical), FeCl3·6H2O (5.04 mmol, Fujifilm Wako Pure Chemical), citric acid (0, 5.04, 10.08, and 15.12 mmol, Fujifilm Wako Pure Chemical), deionized water (39 mL) was mixed and stirred for 1 h at room temperature with a stirrer. Then, (NH4)2S (10.08 mmol, Sigma-Aldrich) was added to the mixture, and the hydrothermal synthesis reaction was conducted at 180 °C for 2, 8, 9, 10, and 12 h. The resulting solution was filtered to remove the residue and then
Characterization of the CuFeS2 powder
Fig. 1 shows the XRD pattern change depending on the hydrothermal time and citric acid amount (sintering temperature was fixed at 150 °C). Under all conditions, the peaks of CuFeS2 (chalcopyrite type) were confirmed. Basically, the more citric acid is present, the better the crystallinity. This occurs probably because citric acid formed a complex with iron and copper and promoted a reaction with sulfur [14]. The best crystallinity in the case of citric acid amount of 10.08 mmol was obtained
Conclusions
The power generation by the CuFeS2 STC using a ferrocene electrolyte was observed. The theoretical open circuit voltage of STC is the difference between the Fermi level of the working electrode and the redox potential of electrolyte ions [25]. Thus, the theoretical open circuit voltage in this manuscript was 190 mV, while the acquired voltage was only a few tens of mV. The reason for low voltage may be the large internal resistance caused by the impurities inside fabricated CuFeS2. The 35 nA/cm2
Author contributions
Sekiya Hayato: Methodology, Validation, Formal analysis, Investigation, Writing-Original Draft, Visualization. Sachiko Matsushita: Conceptualization, Methodology, Resources, Data Curation, Writing-Review & Editing, Visualization, Supervision, Project administration, Funding acquisition. Toshihiro Isobe: Project administration. Akira Nakajima: Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by Sanoh Industrial Corporation, Japan, Tohnic Corporation, Japan, and Sumitomo Foundation, Japan, and also technically supported by Prof. Hiroshi Funakubo, Tokyo Tech., Riken Keiki, Co., Ltd., and Ookayama Analysis Division, Tech. Dept., Tokyo Tech.
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