Low-Temperature Thermoelectric Nanomaterials for Waste Heat Recovery in Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are one of the most promising candidates for clean and efficient energy conversion. However, one of the challenges of PEMFCs is the management of waste heat, which accounts for a significant portion of the energy output, often close to half of the total energy converted. Recovering waste heat from PEMFCs allows for the regulation of the operating temperature. This process not only reduces the risk of thermal damage to the fuel cell components but also prevents the dry-out of the proton exchange membrane. Thus, managing waste heat leads to an enhanced overall efficiency and an extended lifetime of the PEMFC system. Thermoelectric generators (TEGs) are one of the technologies that have recently obtained notable attention due to their potential for use as waste heat recovery in PEMFCs. TEGs are solid-state devices that directly convert the temperature gradient to electricity via the Seebeck effect. By using TEGs, the waste heat produced by PEMFCs can get converted into usable electrical energy. The harvested energy can be integrated into the fuel cell s power output or utilized to provide energy to other devices within the power supply chain. The criteria for a well-performing thermoelectric material can be represented by a dimensionless figure of merit named ZT. It defines the relationship between the Seebeck coefficient (S), electrical conductivity (σ), and thermal conductivity (κ) where ZT=S^2 σTκ^(-1). Naturally to improve ZT, materials need a large S, high σ, and low κ. However, simultaneous optimization of these parameters, especially at lower temperatures, is challenging due to their interrelations. Bismuth telluride (Bi2Te3) is the most common low-temperature thermoelectric material with a ZT value ranging from 1 to 1.5 at room temperature. It has been established that nanostructuring Bi2Te3 can increase ZT via chemical synthetic route and therefore pave the way for mass usage of their thermoelectric products. Other high-ZT materials can also benefit from nanostructuring, doping, ion accumulation and composite in order to address the remaining economic and environmental limitations. Studies are now focused on emerging thermoelectric materials that are abundant, cost-effective and non-toxic. These alternative materials, with potential to be utilized for low-temperature waste heat recovery, include skutterudites, half-Heusler compounds, thermoelectric oxides, polymers, carbon nanotubes, as well as novel materials like metal-organic frameworks. For instance, materials such as carbon nanotube bundles/polystyrene composites, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), graphene/lanthanum strontium titanate oxide, α-Bi2Sn2O7 and polyvinylpyrrolidone/Ag2Se composite exhibit ZT values of 0.41, 0.42, 0.42, 0.46 and 1.1, respectively at room temperature. They can be employed for the preparation of TEGs to be used as waste heat recovery in PEMFCs. Using alternative thermoelectric materials provides a promising opportunity for the recovery of low-grade waste heat in the future. Furthermore, this approach contributes to a greener and more sustainable future by improving the efficiency and durability of PEMFC.