The Interfacial Effect on the Open Circuit Voltage of Ionic Thermoelectric Devices with Conducting Polymer Electrodes

The much-anticipated transition from fossil energy to renewable energy is governed by sustainability concepts regarding good management and efficient utilization of energy resources. According to recent reports, approximately 60% of the energy sources used globally is converted to thermal energy. Unfortunately, about 50% of the thermal energy is usually wasted because of the distributed nature of the low-temperature heat sources below 150 °C. This has raised concerns and requires effective technologies for harvesting energy from low-temperature heat sources. Among the available technologies, thermoelectric-based energy harvesting devices have drawn significant research attention owing to their simplicity in the structure and operation of non-moving parts. However, these devices are generally expensive as they require exotic materials and complex manufacturing processes, which compromise their practical applications.

The three categories of thermoelectric harvesting devices are ionic thermoelectric devices, thermogalvanic cells and electronic thermoelectric devices. In particular, the ionic thermoelectric devices are based on the ionic Seebeck effect characterized by non-electroactive ions. It is the basis of the concept of ionic thermoelectric supercapacitor/battery (ITESC), suitable for charging capacitors and batteries, which can be discharged by changing the applied temperature gradient. This concept is inspired by the high electrolyte thermo-voltage values and is suitable for intermittent energy sources. Besides, it uses low-cost organic polymer electrolytes compatible with cost-effective manufacturing techniques, which constitutes a major problem in their large-scale application.

Notably, ITESCs requires electrodes with high capacitance like conducting polymers to enhance the energy stored in them. Previous findings have shown that conducting polymers like PDAQ-BC and polyaniline are potential electrode candidates for ionic thermoelectric charged batteries and supercapacitors. Although recent research has mainly concentrated on developing new electrolytes with high Seebeck coefficients, the effects of different electrodes on the energy harvesting capacities of ionic thermoelectric devices are not fully explored.

Herein, University of Linköping researchers in Sweden: Dr. Saeed Mardi, Dr. Dan Zhao, Dr. Nara Kim, Dr. Ioannis Petsagkourakis, Dr. Klas Tybrandt and led by Professor Xavier Crispin in collaboration with Mr. Andrea Reale from the University of Rome Tor Vergata investigated the electrode/electrolyte interfacial effect in ionic thermoelectric devices based conducting polymer poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) electrode. The thermo-voltage characteristics of the resulting ionic thermoelectric supercapacitor were explored by varying the composition of the PEDOT: PSS electrode. The work is currently published in the journal, Advanced Electronic Materials.

The research team findings revealed that ionic permselectivity of the polyanion PSS content that formed part of the electrode induced a significant interfacial effect on the ionic thermoelectric devices. The thermo-induced voltage coefficient of the ITESC increased with an increase in the PSS content in the electrodes. Consequently, the permselective polyanion induced cation concentration differences at the electrolyte/electrode interface, thereby contributing to the temperature-dependent potential drop. This further suggested that the thermo-voltage of such devices possesses both bulk and interfacial contribution. Furthermore, it was worth noting that the PEDOT:PSS is also a suitable electrode for ionic thermoelectric devices used in energy harvesting as they do not require any current collectors.

In summary, the study demonstrated the effects of interfacial phenomena with conducting PEDOT: PSS-based electrodes on the thermoelectric performance of ionic thermoelectric devices. The interfacial effects exhibited great effects on the overall thermo-voltage performance of the ionic thermoelectric devices. The findings reiterated the vital role of the interfacial potential in designing high-performance ionic thermoelectric devices based on conducting polymer electrodes. In a statement to Advances in Engineering, Professor Xavier Crispin, the lead author explained that their findings provided more insights into the ionic thermoelectric effects with functional electrodes, providing a new direction for developing high-performance organic-based energy harvesting systems.