Edited by: Ramesh L. Gardas, Indian Institute of Technology Madras, India
Reviewed by: Siddharth Surajbhan Gautam, The Ohio State University, United States; Ravindra Pandey, University of Texas at Austin, United States
This article was submitted to Physical Chemistry and Chemical Physics, a section of the journal Frontiers in Physics
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Conversion of low-grade waste heat into electricity had been studied employing single thermocell or flowcells so far. Graphene coated copper electrodes based thermocells connected in series displayed relatively high efficiency of thermal energy harvesting. The maximum power output of 49.2 W/m2 for normalized cross sectional electrode area is obtained at 60°C of inter electrode temperature difference. The relative carnot efficiency of 20.2% is obtained from the device. The importance of reducing the mass transfer and ion transfer resistance to improve the efficiency of the device is demonstrated. Degradation studies confirmed mild oxidation of copper foil due to corrosion caused by the electrolyte.
Low grade waste heat (i.e., at temperatures <200°C), such as that produced via industrial or geothermal processes, in particular was a significant source of energy that could be harvested for the production of electricity. In the field of wearable devices, where the utilization of body heat could power small portable electronics, such as medical devices or sensors [
Fast charge transfer property and low resistance at the electrode/electrolyte interface were important factors for an electrode material to determine the electrochemical performance [
XRD patterns of the samples were obtained using PANalytical X-ray diffractometer, Netherlands. Electrochemical impedance measurements were conducted in the frequency range between 100 kHz and 100 mHz using a Zahner Zennium Electrochemical workstation. Cyclic Voltammetry using Zahner Zennium is carried out in aqueous 1M K3[Fe(CN)6] solution at the scan rate of 25 mV/s. Platinum and Ag/AgCl electrodes were used as counter and reference electrodes respectively for the electrochemical measurements.
Three single graphene coated copper thermocells were stacked in series configuration in such a way that the input heat flux from the hotter cell is transferred to the next cell. Illustrative representation of the thermocells were provided in Figure
Illustrative representation of the thermocells.
To obtain high power generation from the device, fast transport of redox mediator into electrodes is required. The effectiveness of ion transport in graphene coated copper electrodes is characterized by evaluating the mass transfer coefficient.
By exploiting the limiting current method, the mass transfer coefficient is estimated [
where iL being the limiting current,
LSV of the devices based on graphene coated copper from 1 to 100 mV/s at ΔT = 60°C are shown in Figure
EIS analysis is performed to support the performance improvement. The Nyquist plot of the device at ΔT = 60° is recorded employing Zahner Zennium electrochemical workstation and provided in Figure
For ΔT = 60°C, the series resistance (Rs) of the device being 1.90 Ω and correspond to the resistances at electrode/electrolyte interface which is 5.8 times lower than the series resistance of 11 Ω for the thermocells fabricated using as drawn, thermally oxidized and Pt-deposited CNT electrodes [l8]. The charge transfer resistance (R1) of the device for ΔT = 60°C is 5.12 Ω, attributed by the electron transfer reaction at electrode/electrolyte interface which is 6 times lower than the charge transfer resistance of 31 Ω for as drawn, thermally oxidized CNT and 4.5 times lower than that of the charge transfer resistance of 23 Ω for Pt-deposited CNT electrodes respectively [
The power from a single thermocell of area 0.0064 m2 [
The power and current density variation with load is shown in Table
Power and current density values of the device at ΔT = 60°C.
1 | 3916.6 | 0.5 | 0.47 | 0.235 | 1958.3 |
5 | 12583.3 | 0.5 | 1.51 | 0.755 | 6291.6 |
10 | 8250.0 | 0.5 | 0.99 | 0.495 | 4125.0 |
25 | 4500.0 | 0.5 | 0.54 | 0.270 | 2250.0 |
50 | 2916.6 | 0.5 | 0.35 | 0.175 | 1458.3 |
75 | 9250.0 | 0.5 | 1.11 | 0.550 | 4583.3 |
100 | 98333.3 | 0.5 | 11.8 | 5.900 | 49166.6 |
The energy conversion efficiency (η) of the device is given by:
where
The energy conversion efficiency is calculated [
Energy Conversion and relative carnot efficiency of the device at ΔT = 60°C.
The energy conversion efficiency (ηr), relative to the Carnot efficiency of a heat engine is given by,
where TH is the hot electrode temperature (65°C).
The relative carnot efficiency is calculated using Equation (3) for different load from 1 to 100 mV/s as shown in Figure
As the solvent used in the present study is water which boils at and above 60°C, beyond ΔT = 60°C, water vapor accumulation starts happening inside the device. Due to this, cell opening and loss of thermal energy via absorption by water vapor and oxidation of copper reduces the performance of the device. Thus, after ΔT = 60°C sudden decline in the efficiency of the device is noticed. Degradation analysis of the bare copper (BC), used graphene coated copper (UGCC) were studied employing X-ray diffraction and CV before and after the performance studies of the thermocells.
The source consisted of Cu Kα radiation (λ = 1.54 Å.). Three peaks centered at 43.4°, 50.7°, and 74.4° assigned to (111), (200), and (220) crystal planes of metallic copper, could be clearly observed in the XRD pattern of UGCC electrode as shown in Figure
XRD of UGCC electrode and electrolyte.
CV of BC and UGCC were performed employing 1M of potassium ferricyanideas electrolyte at the scan rate of 25 mV/s and temperature difference of ΔT = 60°C (Figure
where, the constant 2.69 × 105 is calculated at 25°C,
Cyclic voltammograms of BC, UGCC, GCC (inset graph) at ΔT = 60°C.
D = diffusion co-efficient in cm2/s, C = concentration of the species in mol/ cm3, ν = scan rate in mV/s. D values of BC and UGCC were close whereas GCC showed approximately 1.26 times higher value. This clearly indicated peeling of graphene coating on prolonged usage of the device and the copper getting exposed to the electrolyte solution.
In summary, we report conversion of low-grade waste heat into electricity by thermocells connected in series based on cost effective graphene coated copper electrodes displaying relatively high efficiency of thermal energy harvesting. The maximum power output of 49.2 W/m2 and relative carnot efficiency of 20.2% is obtained at 60°C of inter electrode temperature difference from the device. EIS analysis showed the importance of reducing the mass transfer and ion transfer resistance to improve the efficiency of the device. Degradation studies confirmed mild oxidation of copper foil due to corrosion in the presence of electrolyte.
MS performed EIS analysis; EI performed electrochemical experiments and fabricated the device; VS performed analysis of LSV; SH generated the idea and wrote the manuscript.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This work was supported by Department of Science and Technology-Science and Engineering Research board (EMR2014001159).
The Supplementary Material for this article can be found online at:
Electrochemical performance data, lower limit of mass transfer coefficient, the energy conversion efficiency, relative carnot efficiency of the device at different inter electrode temperature are provided in Supporting Information.