Thermally regenerative ammonia-based batteries (TRABs) can be used to harvest low-grade waste heat as electrical power. To improve TRAB performance, a series of benzyltrimethyl quaternary ammoniumfunctionalized poly(phenylene oxide) anion exchange membranes (BTMA-AEMs) were examined for their impact on performance relative to a commercial AEM (Selemion AMV). The synthesized AEMs had different degrees of functionalization (DF; 25% and 40%), and thicknesses (50, 100 and 150 mu m). Power and energy densities were shown to be a function of both DF and membrane thickness. The power density of TRAB increased by 31% using a BTMA-AEM (40% DF, 50 mu m thick; 106 +/- 7 W m(-2)) compared to the Selemion (81 +/- 5 W m(-2)). Moreover, the energy density increased by 13% when using a BTMA-based membrane (25% DF, 150 mu m thick; 350 Wh m-3) compared to the Selemion membrane (311 Wh m(-3)). The thermal-electric conversion efficiency improved to 0.97% with the new membrane compared to 0.86% for the Selemion. This energy recovery was 7.0% relative to the Carnot efficiency, which was 1.8 times greater than the highest previously reported value of a system used to capture low-grade waste heat as electricity. (C) 2017 Elsevier B.V. All rights reserved.