An Electrolyte-Decoupled Ammonia Battery for Enhancing Electricity Production from Low-Grade Waste Heat

Tremendous amount of fossil fuel is consumed in the form of low-grade waste heat (< 130 °C) due to inefficient energy utilization, and harvesting this waste heat as useful electricity contributes to mitigating the increasing energy crisis and the global greenhouse effect. Recently, compared with the past technologies (e.g. solid-state thermoelectric generator (s-TEG) and organic Rankine cycle (ORC)), liquid-based thermo-electrochemical batteries or systems feature more efficient and low cost for heat-to-electricity conversion. Among them, bimetallic thermally regenerative ammonia batteries (B-TRABs) based on redox reactions and ammonia thermal distillation that output the highest power density. However, all the existing ammonia batteries exhibit a common phenomenon that their discharge voltage curve basically has no stable platform and gradually decreases after peaking early, which is mainly due to the large pH difference between catholyte and anolyte, leading to self-discharge and severe energy decay. Here, an electrolyte decoupled strategy is proposed for a Cu/Zn-TRAB to restrain the self-discharge and enhance energy density as well as power generation at high temperatures. The self-discharge by ions cross contamination is observed visually as a colour evolution of the interlayer solution, and a transition of principal cathodic reactant from Cu(NH3)4 2+ to Cu2+ exists during discharging, which signals the beginning of performance degradation. The results demonstrate that the decoupled Cu/Zn-TRABs with double and triple-membrane designs improve the energy density by 45-50%, mainly due to the delay of transition region. With an energy density of 1034 W h m-3 obtained by a decoupled Cu/Zn-TRAB with double-membrane design, a thermoelectric conversion efficiency of 1.86% (15.6% relative to the Carnot efficiency) is achieved at a condenser temperature of 16 °C.