Understanding the reaction mechanism and self-discharge of a bimetallic thermally-regenerative ammonia battery

Bimetallic thermally-regenerative ammonia batteries (B-TRABs) exhibit great potential in converting low-grade waste heat into electricity due to their high power and energy densities. However, there are still key issues such as self-discharge, low anode coulombic efficiency (ACE), and side reactions in ammonia batteries, which are not well explained but have a significant impact on performance. Here, we focus on the electrode reaction mechanism and give a universal construction criterion for the bimetallic thermally-regenerative batteries. The influence of oxygen on the electrode reactions of each process is explored, and a flow Cu/Zn-TRAB is developed to verify the electrochemical analysis. The root cause for affecting the ACE and oxygen effect are examined. The generation of self-discharge and its different effects are investigated for each electrode process. In addition, the influence of initial catholyte pH and temperature are discussed thoroughly. It is proposed to use the transition potential and pH to predict the occurrence of self-discharge. The results mainly indicate that the power production can be increased by 10–20% by removing oxygen from electrolytes. The stable existence of low-valence ions (e.g., Cu(NH₃)₄⁺) and the self-corrosion of ammonia on anode metals are the two main reasons for the low ACE. The self-discharge phenomenon is more serious in the discharge process because the reduction peak potential of Cu²⁺ is close to that of Cu(NH₃)₄²⁺. The catholyte has the strongest ability to inhibit self-discharge at 40 °C, and the self-discharge occurs when the catholyte pH rises to ≈7–8.