Chemical Additive-Regulated Thermocells with High Thermopower for Low-Grade Heat Harvesting

Low-grade heat (below 373 K) is ubiquitous in the human body, environment, and industry, but is usually wasted without proper recovery. Thermogalvanic cells (TGCs) are one of the most investigated technologies that can directly convert such waste heat into electricity, which present satisfactory thermopower in the magnitude of mV K-1. A typical method to improve the thermopower of aqueous TGCs is to introduce additives to change the solvation entropy or establish a concentration ratio difference between two electrodes at different temperatures. However, the introduction of additives may lead to the lower ionic conductivity of electrolyte, thus the current density of TGCs is not promising for powering electronics. From this point of view, further modification towards electrodes is required, making it complicated to optimize the performance of TGCs as a whole. Here, we first introduced a small amount of guanidine hydrochloride (GdnHCl) into 0.4 M potassium ferri/ferrocyanide (K3Fe(CN)6/K4Fe(CN)6) aqueous electrolyte. With the thermosensitive crystallization and redissolution effect between GdnHCl and Fe(CN)64-, a low local concentration of Fe(CN)64- due to the crystallization induced by GdnHCl at the cold electrode and a high local concentration of Fe(CN)64- via the redissolution process at the hot electrode were established, corresponding to a low concentration ratio of Fe(CN)64-/Fe(CN)63- at the cold side ([Fe(CN)64-/Fe(CN)63-]cold) and a high concentration ratio of Fe(CN)64-/Fe(CN)63- at the hot side ([Fe(CN)64-/Fe(CN)63-]hot), respectively. A thermopower of 1.91 mV K-1 was achieved by solely adding 0.75 M GdnHCl to the electrolyte. On the basis of thermopower being enhanced by regulating the concentration profile of Fe(CN)64- via GdnHCl, in this work, we proposed to further improve the thermopower from the perspective of Fe(CN)63-. We added the second chemical, cysteamine hydrochloride (CysHCl), into the K3Fe(CN)6/K4Fe(CN)6 aqueous electrolyte with 0.75 M GdnHCl to trigger a high temperature-favored chemical reaction with Fe(CN)63-. CysHCl was featured with temperature selectivity, the kinetic reaction rate of which was relatively slow at lower temperatures, making little contribution to the thermopower enhancement. But at temperatures above 323 K, which were within our designed temperature range for low-grade heat recovery application, the chemical interaction between CysHCl and Fe(CN)63- played a prominent part in the thermopower improvement, leading to a high ([Fe(CN)64-/Fe(CN)63-]hot ratio. As a result, a large concentration ratio difference between the cold and hot electrodes was established by GdnHCl via inducing the crystallization and redissolution of Fe(CN)64-, and was further enlarged by CysHCl via chemically regulating the concentration of Fe(CN)63-. A high thermopower of 4.32 mV K-1 and a short-circuit current density of 133.33 A m-2 were generated using planar graphite sheets as electrodes, compared with the performance of 1.0 mV K-1 and 56.44 A m-2 of pristine electrolyte in the same cell configuration. Furthermore, a maximum thermopower of 9.06 mV K-1 and a short-circuit current density of 129.24 A m-2 were achieved by optimizing the concentrations of both GdnHCl and CysHCl simultaneously. The results elucidate that the introduction of GdnHCl enables a high thermopower and current density via the thermosensitive crystallization effect, and more importantly, by interacting with Fe(CN)64-, it allows more free Fe(CN)63- to react with CysHCl, which enables the thermopower and current density being boosted to a higher level via chemical regulation effect. This work develops a new design route for TGCs to achieve high thermopower and current density, while no complex modification towards both electrolyte and electrodes is needed, which is highly important for extending the possible application fields of TGCs.