Asymmetric Thermoelectrochemical Cell for Harvesting Low-grade Heat under Isothermal Operation

Podsumowanie

Low-grade heat is abundant, but its efficient recovery is still a great challenge. We report an asymmetric thermoelectrochemical cell using graphene oxide as a cathode and polyaniline as an anode with KCl as the electrolyte. This cell works under isothermal heating, exhibiting a high heat-to-electricity conversion efficiency in low-temperature regions.

Streszczenie

Low-grade heat is abundantly available in the environment as waste heat. The efficient conversion of low-grade heat into electricity is very difficult. We developed an asymmetric thermoelectrochemical cell (aTEC) for heat-to-electricity conversion under isothermal operation in the charging and discharging processes without exploiting the thermal gradient or the thermal cycle. The aTEC is composed of a graphene oxide (GO) cathode, a polyaniline (PANI) anode, and 1M KCl as the electrolyte. The cell generates a voltage due to the pseudocapacitive reaction of GO when heating from room temperature (RT) to a high temperature (T_H, ~40-90 °C), and then current is successively produced by oxidizing PANI when an external electrical load is connected. The aTEC demonstrates a remarkable temperature coefficient of 4.1 mV/K and a high heat-to-electricity conversion efficiency of 3.32%, working at a T_H = 70 °C with a Carnot efficiency of 25.3%, unveiling a new promising thermoelectrochemical technology for low-grade heat recovery.

Wprowadzenie

Ubiquitous low-grade heat energy (<100 °C) could be recycled and converted into electricity^1,2 but is instead wasted. Unfortunately, heat recovery is still a great challenge, because converting low-grade heat to electricity is usually inefficient due to the low temperature differential and the distributed nature of the heat sources³. Intensive research has been conducted in solid-state thermoelectric (TE) materials and devices for the past decades, but the scalable application of TE devices in a low-grade heat regime is limited by the low energy conversion efficiency (η_E) of <2%⁴.

Alternative approaches based on the effect of temperature on electrochemical cells have been suggested as a solution to this problem, because the ionic Seebeck coefficient (α) of thermoelectrochemical cells (TECs) is much higher than that of TE semiconductors^5,6. Thermogalvanic cells (TGC) utilize redox active electrolytes sandwiched between two identical electrodes to generate a voltage across the cell when a thermal gradient is applied. The commonly used aqueous Fe(CN)₆^3-/Fe(CN)₆^4- electrolyte in TGCs was reported to have an α of -1.4 mV/K and yield an η_E of <1%^7,8,9,10,11. However, TGCs suffer the drawback of the poor ionic conductivity of the liquid electrolyte, which is around three orders of magnitude smaller than the electronic conductivity in TE materials. The electric conductivity could be improved, but this improvement is always accompanied by a higher thermal conductivity, which leads to a lower temperature gradient. Therefore, the η_E of TGCs is inherently limited due to the trade-off between the liquid electrolyte conductance and the temperature requirement for the desired redox reactions in each side of the electrode.

A thermally regenerative electrochemical cycle (TREC)^12,13,14 based on a battery system using a solid copper hexacyanoferrate (CuHCF) cathode and a Cu/Cu⁺ anode was recently reported. TREC is configured as a pouch cell to improve the electrolyte conductance, showing an α of −1.2 mV/K and reaching a high η_E of 3.7% (21% of η_carnot) when operated at 60 °C and 10 °C. Nevertheless, one limit of TREC is that external electricity is required at the start of the process to charge the electrodes in each thermal cycle, leading to complicated system designs¹⁴. A TREC without this limitation can be achieved, but it suffers from a poor conversion efficiency of <1%¹³. The TREC system demonstrates that a sodium-ion secondary battery (SIB)-type thermocell consisting of two types of Prussian blue analogues (PBA) with different α values can harvest waste heat. The thermal efficiency (η) increases proportionally with ΔT. Moreover, η reaches 1.08%, 3.19% at ΔT = 30 K, 56 K separately. The thermal cyclability is improved using Ni-substituted PBA^15,16,17,18.

Alternatively, a thermally regenerative ammonia battery (TRAB) employs copper-based redox couples [Cu(NH3)₄²⁺/Cu and Cu(II)/Cu] that operate with the reverse temperature gradient by switching the temperature of electrolyte co-operated with positive and negative electrodes, which produces a η_E of 0.53% (13% of η_carnot). However, this system is configured with two tanks full of liquid electrolyte, causing sluggish heating and cooling. Also, the ammonia stream in the system creates concerns regarding safety, leakage, and stability^19,20,21.

Here we present an asymmetric thermoelectrochemical cell (aTEC) for heat-to-electricity conversion that can be thermally charged and electrically discharged by continuous isothermal heating without maintaining a temperature gradient in a geometric configuration or switching temperatures in a thermal cycle. The aTEC uses asymmetric electrodes, including a graphene oxide (GO) cathode and a polyaniline (PANI) anode, and KCl as the electrolyte. It is thermally charged via the thermo-pseudocapacitive effect of GO and then discharged with the oxidation reaction of PANI. Notably, the aTEC exhibits a high α of 4.1 mV/K and attains a η_E of 3.32%, the highest ever achieved at 70 °C (25.3% of η_Carnot).

Protokół

1. Preparation of the graphene oxide electrode

1. Synthesis of graphene oxide via the modified Hummer's method
   1. Steps 1.1.2 and 1.1.3 occur at a low temperature (<0 °C). Circulate ice water flowing through the external layer of a double wall glass beaker placed on a magnetic stirrer to create low temperature conditions for the reactants inside.
   2. Mix 1 g of sodium nitrate (NaNO₃) with 100 mL of sulfuric acid (H₂SO₄, reagent grade, 95-98%) using slow stirring in the beaker.
   3. Add 1 g of flake graphite into the sulfuric acid and stir for 1 h in the cold bath. Add 6 g of potassium permanganate (KMnO₄) gradually to the solution and stir the mixture for another 2 h.
   4. The next step of the reaction takes place at a middle temperature (~35 °C). Change the ice water to 35 °C water and continue the oxidation of the graphite by stirring for ½ h.
   5. The last step of the reaction takes place at a T_H (80-90 °C). Add 46 mL of deionized (DI) water (70 °C) into the reaction tank drop by drop. Note that that the reaction is strong. Add 140 mL of DI water and 20 mL of hydrogen peroxide (30% H₂O₂) in the reaction tank as the last step of the reaction. Make sure that golden particles of GO appear as a result.
   6. Wash the product thoroughly with dilute hydrochloric acid (HCl) and DI water several times until the GO suspension reaches pH = 7.
   7. Freeze the washed GO suspension overnight and dry it in a freeze dryer until water evaporates completely.
2. Preparation of the graphene oxide electrode
   1. Mix the graphene oxide, carbon black, and PVDF in a mass ratio of 75:15:10 and put them in a glass bottle. Drip the solvent N-methyl-2-pyrrolidone (NMP) into the solid mixture and ensure the weight ratio of solvent and solid mixture is 4:1.
   2. Prepare the paste by mixing at 2,000 rpm for 13 min and defoaming in 1,200 rpm for 2 min with a mixer.
   3. Brush coat the paste on carbon paper until the coat is ~8-15 mg/cm² and dry it for 4 h at 40 °C.

2. Preparation of the polyaniline (PANI) electrode

1. Prepare 1 wt% carboxymethyl cellulose (CMC) aqueous solution by dissolving CMC powder in DI water by stirring for 10 h.
2. Mix 50 mg of leucoemeraldine-base PANI and 10 mg of carbon black in a beaker. Add 150 μL of 1 wt% CMC solution into the beaker and mix with a magnetic stirrer for 12 h.
3. Add 6 μL of 40% styrene-butadiene (SBR) solution into the mixture and stir for another 15 min.
4. Place a piece of carbon paper on the doctor blade coater and drop the mixed PANI slurry at the leading edge of the carbon paper.
5. Blade coat the slurry to produce a film 400 μm thick on the carbon paper. Dry the coating for 4 h at 50 °C.

3. Assembling the pouch cell

1. Cut titanium foil into the approproate size and then connect each piece to a nickel tab with a 20 kHz ultrasonic spot welding machine.
2. Place the porous hydrophilic polypropylene-based separator between the GO electrode and the PANI electrode to avoid short circuits. Each electrode is paired with one current collector.
3. Package the electrodes using aluminum laminated film. Seal the sides of the aluminum laminated film with a compact vacuum sealer for 4 s. Set the temperature of the top and bottom sealing parts as 180 °C and 160 °C separately.
4. Inject 500 μL of the 1 M KCl electrolyte into the pouch cell and allow to equilibrate for 10 min.
5. Extrude the excess electrolyte and seal the last side of the pouch cell in a -80 kPa vacuum chamber.

4. Setting up the temperature controlling system

1. Stack the pouch cell between two thermoelectric modules. Place thermocouples on the top and bottom sides of the cell. Apply thermal paste to all the interfaces to ensure good thermal contact.
   NOTE: The temperature is controlled with LabVIEW code. Temperatures measured from the thermocouples are compared with the setting temperatures and the output voltage is determined by the difference between the real time temperature and setting temperature via a PID control. The voltage signals are transmitted to the power supply and are connected to the thermoelectric module. The closed-loop control guarantees a temperature measurement accuracy within ± 0.5 °C.

5. Electrochemical characterization

1. Perform the electrochemical tests of the cell using a potentiostat. Conduct the thermal charging in open circuit mode while carrying out the electrical discharging process at a constant current.

Wyniki

The aTEC pouch cell was configured with asymmetric electrodes consisting of a GO cathode, a PANI anode, and filled with the KCl electrolyte. The thickness of the pouch cell shown in Figure 1A is 1 mm, which facilitates isothermal conditions between the two electrodes as well as efficient heat conduction. The scanning electron microscopy (SEM) images of the GO cathode and the PANI anode coated on carbon paper are shown in ...

Dyskusje

The aTEC converts thermal energy into electricity via a thermal charging process when heating from RT to T_H and a successive electrical discharging process at T_H. Getting rid of the dependence on a temperature gradient or a temperature cycle like the TGC and TREC, aTEC allows isothermal heating operation during the entire charging and discharging processes. Thermal induced voltage is based on the pseudocapacitive effect of GO because heating facilitates the chemisorption of protons on the oxygen fun...

Materiały

Name	Company	Catalog Number	Comments
Alumina laminated film	Showa Denko	SPALF C4
Carbon black	Alfa Aesar	H30253.22
Carbon paper	CeTech Co. Ltd	W0S1009
Carboxymethyl cellulose (CMC)	Guidechem company
DC Power supply	B&K Precision	Model 913-B
Doctor blade coater	Shining Energy Co. Ltd
Gamry	Gamry Instruments	Reference 3000
Graphite	Sigma-Aldrich	332461-2.5KG
Mixer	Thinky	ARE-250
Nickel tab	Tianjin Iversonchem company		4 mm width
N-Methyl-2-pyrrolidone (NMP)	Sigma-Aldrich	443778-1L
Polyaniline (leucoemeraldine base)	Sigma-Aldrich	530670-5G
potassium permanganate (KMnO4)	Sigma-Aldrich	223468-500G
Separator	CLDP		25 um thickness
Sodium nitrate (NaNO3)	Sigma-Aldrich	S5506-250G
Styrene butadiene	Tianjin Iversonchem company	BM400
Sulfuric acid	Sigma-Aldrich	320501-2.5L
Thermoelectric modules	CUI Inc.	CP455535H
Titanum foil	Qingyuan metal		0.03 mm thickness