FULL PAPER 1700482 (1 of 9) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mme-journal.de Thermally Resistive Electrospun Composite Membranes for Low-Grade Thermal Energy Harvesting Syed Waqar Hasan, Suhana Mohd. Said,* Mohd. Faizul Mohd Sabri, Hasan Abbass Jaffery, and Ahmad Shuhaimi Bin Abu Bakar S. W. Hasan, Prof. S. M. Said Department of Electrical Engineering University of Malaya Kuala Lumpur 50603, Malaysia E-mail: smsaid@um.edu.my Prof. M. F. M. Sabri, H. A. Jaffery Department of Mechanical Engineering University of Malaya Kuala Lumpur 50603, Malaysia Dr. A. S. B. A. Bakar Low Dimensional Materials Research Centre Department of Physics University of Malaya Kuala Lumpur 50603, Malaysia The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.201700482. DOI: 10.1002/mame.201700482 the fossil fuels. It is important to gen- erate clean and scalable electricity through natural resources like solar, wind, or geo- thermal for two main reasons: (a) there is a limited amount of fossil fuel present in the earth, and (b) the environmental haz- ards obtained by burning fossil fuels are immense. Among various clean energy technologies, batteries and thermoelec- tric modules are considered as the two enormously potential candidates. [1] In batteries, the charge/discharge between the electrode/electrolyte happens owing to the potential disequilibrium between anode and cathode with respect to the electrolyte. [2] For thermoelectric mate- rials, the phenomenon of charge transfer is different; i.e., a solid-state material converts applied thermal gradient into electric potential owing to its inherent property, referred to as Seebeck coefficient (or thermoelectric power). [3] The research accomplishments of batteries are far more impressive than the thermoelectric devices. In fact, batteries are used in large- scale applications, e.g., hybrid vehicles, electronic products, and others while thermoelectric applications are still niche. Nevertheless, the charm of converting waste heat energy into electricity still attracts immense research attention on thermo- electric materials. It has been predicted that if the dimension- less figure of merit (ZT) of the thermoelectric materials may exceed 3, then the mass scale thermoelectric applications can be realized. [4] However, the ZT of the thermoelectric material is limited to ≈1 in bulk materials and around ≈2 for nanowires/ thin films. This low value of ZT is because of the interdepend- ence of electronic and thermal properties of the materials. Moreover, the state-of-the-art thermoelectric materials are toxic and are rare-earth metals like Bi 2 Te 3 or PbTe. Alternative to the solid-state thermoelectrics, redox-based liquids can also harvest electrical energy out of thermal gradient. In fact, the Seebeck coefficients of liquid electrolytes are significantly better than the solid thermoelectric materials. [5] The thermoelectric proper- ties of the liquid electrolytes are studied in special cells referred to as thermo-electrochemical cells (TEC or thermocells). It is noteworthy that the geometrical attributes of a thermocell resemble with a conventional battery; however, the driving force of the charge transfer is the thermal gradient as in thermoelec- tric materials. [6] Therefore, thermocells may be regarded as the Membrane Embedded Thermocells In this work, thermally insulating composite mats of poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) blends are used as the separator mem- branes. The membranes improve the thermal-to-electrical energy conversion efficiency of a thermally driven electrochemical cell (i.e., thermocell) up to 95%. The justification of the improved performance is an intricate relation- ship between the porosity, electrolyte uptake, electrolyte uptake rate of the electrospun fibrous mat, and the actual temperature gradient at the elec- trode surface. When the porosity is too high (87%) in PAN membranes, the electrolyte uptake and electrolyte uptake rate are significantly high as 950% and 0.53 μL s -1 , respectively. In such a case, the convective heat flow within the cell is high and the power density is limited to 32.7 mW m -2 . When the porosity is lesser (up to 81%) in PVDF membranes, the electrolyte uptake and uptake rate are relatively low as 434% and 0.13 μL s -1 , respectively. In this case, the convective flow shall be low, however, the maximum power density of 63.5 mW m -2 is obtained with PVDF/PAN composites as the aforemen- tioned parameters are optimized. Furthermore, multilayered membrane structures are also investigated for which a bilayered architecture produces highest power density of 102.7 mW m -2 . 1. Introduction In this 21st century, electricity plays a vital role in our life. We consume electricity in every sector of our life, starting from our home appliances to industries. A challenge for the scien- tists is to unlock the dependence of electricity generation on Macromol. Mater. Eng. 2018, 1700482