Thermal Capacitive Electrochemical Cycle on Carbon-Based Supercapacitor for Converting Low-grade Heat to Electricity

Abstract

It is a great challenge to efficiently convert low-grade heat (<100°C) to electricity. Currently available heat-to-current converters, such as thermoelectric generators, operating in a low-grade heat regime reach efficiencies no higher than a few percent (<3%). Herein, we illustrated a thermal capacitive electrochemical cycle (TCEC) using electrochemical cell, where the connection to the hot or cold reservoirs alternates in a cyclic chargingheating-discharging-cooling mode to convert heat into electricity, which performs as an electrochemical heat engine. TCEC technology is a cost-effective method for exploiting the temperature-dependent electrostatic potential in an electric double layer (EDL) at carbon electrode/electrolyte interfaces; it produces net electricity by altering the EDL thickness via heating and cooling. In this paper, TCEC on supercapacitor was confirmed on commercial supercapacitor, which showed a poor conversion efficiency. To improve the performance, we redesigned the cell by employing the pouch cell setup with activated carbon as electrode materials and homemade temperature controlling system, which boosted the efficiency from 0.5% of commercial supercapacitor to 3.05% when cycling between 10 and 65°C. A higher efficiency of 3.95% could be reached by using microwaved exfoliated graphene nanosheets (MEG) and nitric acid-treated MEG, which could help in decreasing the energy loss caused by charge leakage.

Optimising nanoporous supercapacitors for heat-to-electricity conversion

“…We find there that S > 0 if the electrolyte's relative permittivity ε r satisfies ∂ε r /∂T < ε r /T , which is the case for most electrolytes [44]. Meanwhile, experiments on pseudocapacitors [23] and electric double-layer capacitors [19,20,23] yielded positive Seebeck coefficients of the order of S = 0.5 mV K −1 . Conversely, negative Seebeck coefficients were found for batteries [1,3] and supercapacitors based on Li-ion intercalation [23].…”

“…(This thermal voltage rise can also boost the work output of blue-energy devices [14][15][16][17][18].) Later studies of HEC with supercapacitors included theoretical works to improve power output and efficiency [20,21] as well as experimental work improving the design of the HEC apparatus [22]. In an extensive recent work, Kim et al compared HEC charging cycles of a commercial pseudocapacitor, Li-ion supercapacitor, and an electric double-layer capacitor [23].…”