System and electrode design of new thermoelectrochemical cells for low-grade heat harvesting

Abstract

Efficient recovery of the abundantly available thermal energy is of great significance to solve the problem from the fast-growing energy consumption while the depletion of fossil fuel and the climate changes due to the emission of greenhouse gas. Waste heat is abundant and low-grade heat with the temperature lower than 100 oC takes the dominant percentage of 63% among the total amount, but the traditional technologies to fulfill the heat-to-electricity conversion are far enough away from the optimal. The emerging technologies of thermoelectrochemical cells (TECs) shed light on the low-grade heat recovery, as they achieve the superior performance of the excellent temperature coefficient (α), the great output power, the satisfying energy efficiency (ηE) and the flexible structure without moving parts. The dissertation focuses on the thorough exploration on the newly proposed direct thermal charging cell (DTCC) and the systemically survey on the thermal capacitive electrochemical cycle (TCEC). DTCC is the new thermoelectrochemical system invented to convert heat into electricity via the direct heating without building thermal gradient or thermal cycles. The cell was built up with the graphene oxide/platinum nanoparticles (GO/PtNPs) cathode, the polyaniline (PANI) anode and Ferrous/Ferric salt aqueous electrolyte. Thermal voltage was produced via the thermopseudocapacitive effect from the oxygen functional groups of GO cooperating with the thermogalvanic effect of Fe2+/Fe3+ redox couple. The voltage bias between two electrodes isothermally at the high temperature triggered the current after connecting to an electrical load, and the discharging process was conducted under the oxidation of PANI at anode and the reduction of Fe3+ at cathode. The reaction between oxidized PANI and the resultant Fe2+ regenerated the cell after cooled down. DTCC attains the temperature coefficient of 5.0 mV/K, the maximum volumetric power of 3345 W m-3 at 90 oC and the heat-to-electricity conversion efficiency of 2.8% at 70 oC (21.4% of Carnot efficiency) and 3.52% at 90 oC (19.7% of Carnot efficiency), ranking at the front of the current TECs or thermoelectric devices and holding the great promise for the development of fully-fledged and device-level waste heat recovery system. Carbon-based electrodes of different kinds and with different pretreatments are investigated to address the issue of charge leakage in TCEC. The cell operated in TCEC is normally charged at the cold condition and discharged at the hot place in one cycle. As a higher voltage is obtained due to the expansion on the electrical double layer (EDL) of the cell, extra electrical work is produced from the injection of heat. The TCEC was successfully conducted in a commercial supercapacitors (SC) but the severe self-discharging limited the efficiency. The utilization of the active carbon electrode on a pouch cell SC greatly enhanced the efficiency from 0.5% of commercial supercapacitor to 3.05% when cycling between 10 oC and 65 oC. The efficiency was further improved to 3.95% by the cell consisted of the exfoliated graphene nanosheets (MEG) electrode and the nitric acid-treated MEG (N-MEG) electrode, due to the diminishing on the charge leakage by tuning the self-potential. (494 words)