Thermo-Electrochemical Cells Based on Carbon Nanotube Electrodes by Electrophoretic Deposition
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Drawbacks of low efficiency and high cost of the electrode materials have restricted the wide applications of the thermo-electrochemical cells (TECs). Due to high specific areas and electrical conductivities, the low cost multi-walled carbon nanotubes (MWNTs) are promising alternative electrode materials. In this work, the MWNT films of up to 16 cm2 were synthesized on stainless steel substrates by the electrophoretic deposition (EPD) to make the thermo-electrochemical electrodes. MWNT electrodes based on TECs were characterized by cyclic voltammetry and the long-term stability tests with the potassium ferri/ferrocyanide electrolyte. The TECs reached the current density of 45.2 A m−2 and the maximum power density of 0.82 W m−2. The relative power conversion efficiency of the MWNT electrode is 50 % higher than that for the Pt electrode. Meanwhile, the TECs was operated continuously for 300 h without performance degradation. With the priorities of low cost and simple fabrication, EPD-based MWNT TECs may become commercially viable.
KeywordsThermo-electrochemical cells Carbon nanotubes Electrophoretic deposition Power conversion efficiency
Harvesting of low grade heat (<130 °C) is considered an effective sustainable energy source. Thermo-electrochemical cells (TECs) utilize the temperature-dependent electrochemical redox potentials to convert the thermal energy to electrical energy. Comparing with other thermal energy harvesting techniques, such as the thermoelectrics, thermocouples, and stirling engines [1, 2, 3, 4], TECs have great potential for wide applications due to advantages of simple design, maintenance-free, environment-friendly, and low cost.
The electrode exchange current density is one of the most important factors in energy conversion for TECs. In practice, the current of TECs can be described from the relation: I = V/R, where V represents the voltage between the two working electrodes, and the resistance R consists of four parts, i.e., charge transfer, ohmic, solution diffusion, and thermal diffusion resistances [6, 7]. To get high exchange current densities, the redox couples, such as the ferri/ferrocyanide electrolyte, are commonly selected in TECs [8, 9]. In the selection of electrode materials, the fast charge transfer property and low resistance at the electrode/electrolyte interface are important factors. Platinum is the conventional electrode material due to high surface catalytic activity for oxidation and reduction reactions. However, it is hard to promote Pt-based TECs in engineering fields due to high cost and low conversion efficiency [6, 8]. With the development of nanotechnology [10, 11, 12, 13], carbon nanotubes (CNTs) have been employed in different electrochemical devices [14, 15, 16, 17, 18], such as lithium-ion batteries, supercapacitors, and fuel cells due to large specific surface area and high catalytic activity. Recently, applications of MWNTs in TECs are widely investigated [19, 20, 21, 22, 23]. In the preparation of the MWNT electrode, the chemical vapor deposition (CVD) growth is widely applied [8, 9, 22]. The MWNT TECs electrodes prepared by CVD show promising electrical contact and stability properties.
Electrophoretic deposition (EPD) is an effective technique to produce CNTs films with various advantages, including fewer requirements on the type & shape of the substrate, large-scale production capability, and low cost . In this work, we prepared the TECs electrodes by EPD of MWNTs on metal substrates. The TECs presented excellent long-term operation stability and substantial higher energy conversion efficiency than that for Pt-based TECs. This investigation suggests that EPD method may be applicable for MWNTs-based TECs.
The MWNT material, with lengths from 10 to 30 μm, outer diameters of approximately 10 nm, and purity of >90 %, was purchased from XFNANO Materials. The MWNTs were first filtered and washed with acetone, then sonicated in concentrated nitric acid for 20 h. After the processing, carboxylic and other oxygen-containing groups were decorated on MWNTs surfaces . During EPD, the carboxylic MWNTs were first dispersed in ethanol (0.1 g L−1) and sonicated for about 1 h. Then magnesium chloride powder material (MgCl2, Aladdin) was added into the suspension. Subsequently, the stainless steel substrate (SS) and counter electrode were immersed into the suspension with distance of 1 cm. Different substrates of surface areas from 0.5 to 16 cm2 were employed. After the deposition, the MWNT electrodes were annealed in vacuum at 750 °C.
The morphologies of the MWNT film were observed by scanning electron microscopy (SEM; JEOL SM-6700F). The compositions of the as-prepared products were characterized by energy-dispersive X-ray analysis (EDS), and X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe). The tensile tests of the samples were carried out by Instron 3343 instrument to investigate the adhesion between MWNT films and the substrates with the uncertainty of about 15 %. During the test, the MWNT-SS sample was fixed by a clamp, and the MWNT film was wrapped by the adhesive tape. The tape grabbing the MWNT film was pulled away until the film peeled off from the substrate.
The cyclic voltammetry (CV) measurements were conducted using a Zahner IM6 electrochemical workstation. The 3-electrode tests were conducted at room temperature with the Ag/AgCl saturated in KCl solution as the reference electrode and a platinum foil as the counter electrode. CVs were tested using 0.1 M K4Fe(CN)6 aqueous solution with 0.5 M NaCl as the supporting electrolyte at the rate of 5 mV S−1.
The characteristic performances of the MWNT-based TECs, including the open-circuit potential (V oc), the short-circuit current (I sc), and the output power, were investigated in I-shaped TECs and the stability was tested in the U-shaped TECs. The 0.4 M potassium ferrocyanide (K4Fe(CN)6·3H2O, Aladdin) and ferricyanide (K3Fe(CN)6, Aladdin) aqueous solution were employed as the electrolyte due to its high Seebeck coefficient . For the I-shaped TECs, measurements were conducted in a glass tube with the internal diameter of 8 cm and the distance of two electrode of 5 cm. The hot side temperature was controlled by the resistive heating and the cold side was immerged in an ice water with the temperature difference of 40 °C. The electrode temperatures were measured by OMEGA thermocouple probes. The maximum power (P max) generated by the MWNT electrodes could be attained when the external load resistance is equal to the internal resistance. For the U-shaped TECs, the distance between the two electrodes was 7 cm with the temperature difference of 15 °C. The hot side temperature (40 °C) was controlled by a resistive heater, and the cold side temperature (25 °C) was controlled by a recirculation water chiller. The potentials and currents generated from TECs were monitored using the KEITHLEY 2440 sourcemeter.
3 Results and Discussions
Figure 4c shows that the current densities increased with the temperature differences, and J SC of the MWNT electrode was about 50 % higher than that of the Pt electrode at the same temperature difference, contributed to good conductivity and high specific surface area for the MWNT film. Under the temperature difference of 50 °C, J SC and J SC/ΔT reached 45.2 A m−2 and 0.91 A m−2 K−1 for MWNT electrodes, comparing with 30.5 A m−2 and 0.61 A m−2 K−1 for Pt electrodes.
In our experiment, the thermal resistance at the MWNT film/substrate junction was 0.0952 cm2 K W−1 under the substrate thickness of 500 μm measured by the transient hot wire method. Such a relative high thermal resistance weakens the performance improvement comparing with CVD growth of MWNT film, as shown in SI-2 of the ESI. Further efforts on the reduction of substrate thickness and improvement of the CNT purity are expected to enhance the thermal conductivity of the junction.
Normally, it is difficult to produce large area CNT electrode by CVD method due to growth non-uniformity and facility limitation. EPD technique is able to produce CNT films with large dimensions from simple setup. In this study, different sizes of MWNT electrodes were prepared by EPD to conduct the performance investigation. As shown in Fig. 4d, the lager the electrode area, the higher the output current and power, benefited from the increase of reaction sites . When the electrode area was increased to 16 cm2, the output current and power could reach 7.0 mA and 166 μW, respectively, at the temperature difference of 40 °C. However, the increasing rates of I sc and P max, where P max was obtained by 1/4V oc × I sc, declined gradually with the increase of surface area. Two factors, i.e., the edge effect of the CNT film  and the drop of MWNT density, may cause this nonlinear relation. Interestingly, the Seebeck coefficient increased from 1.42 to 2.40 mV K−1 with raising the surface area from 1 to 16 cm2, probably due to the concentration effect of the cell . In our test system (hot-above-cold, see Fig. 4a), the buildup concentration would become obvious with increasing the surface area of the MWNT electrode, exhibiting partly the properties of the concentration cell . Therefore, further efforts should be conducted to improve the TECs performances with large electrode area, e.g., trying to overcome the concentration effect with designs such as the flowing TECs [8, 38] or the cold-above-hot TECs . The relation between the output power and the voltage is shown in the ESI as SI-1.
EPD is an efficient technique to produce MWNT electrodes for TECs applications. The relative power conversion efficiency of the MWNT TECs is 50 % higher than that of the platinum electrode-based TECs, while the excellent long-term current stability revealed the durability of the MWNT film. Furthermore, the low cost and large-scale production capabilities may promote the energy harvesting in various fields.
This work is partially financial supported by National Science Foundation of China (No. 11274244, 51302193).
- 25.A.A. Talin , K.A. Dean, S.M. O’Rourke, B.F. Coll, M. Stainer, R. Subrahmanya, FED cathode structure using electrophoretic deposition and method of fabrication, U.S. Patent 10/024,164, 7 June 2005Google Scholar
- 30.A.J. Bard, L.R. Faulkner, Electrochamical methods-fundamentals and applications (Wiley, New York, 2001), pp. 1–814Google Scholar
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