Understanding the Factors Affecting the Formation of Carbonyl Iron Electrodes in Rechargeable Alkaline Iron Batteries

Abstract

Rechargeable iron-based alkaline batteries such as iron -air and nickel -iron batteries are attractive for large-scale electrical energy storage because iron is inexpensive, globally-abundant and environmentally-friendly. Further, the iron electrode is known for its robustness to repeated charge/discharge cycling. During manufacturing these batteries are charged and discharged 20 to 50 times during which the discharge capacity of the iron electrode increases gradually and attains a stable value. This process of achieving stable capacity is called formation. In this study we have focused our efforts on understanding the effect of electrode design on formation. We have investigated the role of wetting agent, pore-former additive, and sulfide additive on the formation of carbonyl iron electrodes. The wetting agent increased the rate of formation while the pore-former additive increased the final capacity. Sodium sulfide added to the electrolyte worked as a de-passivation agent and increased the final discharge capacity. We have proposed a phenomenological model for the formation process that predicts the rate of formation and final discharge capacity given the design parameters for the electrode. The understanding gained here will be useful in reducing the time lost in formation and in maximizing the utilization of the iron electrode.Iron-based rechargeable battery systems using alkaline electrolyte such as nickel -iron and iron -air batteries are promising candidates for large-scale electrical energy storage needed for integrating renewable energy generation into the electricity grid. 1-4 Iron, the primary raw material for these systems, is globally abundant and relatively inexpensive. In addition, the iron electrode is robust and provides the battery system with long cycle life. 5-7 However, the iron electrodes suffer from poor charging efficiency and poor discharge rate capability. These limitations of electrical performance have prevented the large-scale commercialization of iron-based batteries. 1,5,6,8 There is renewed focus on addressing these limitations because of the dire need for a long-lasting and inexpensive battery system for grid-scale energy storage. [1][2][3][4]9 At the University of Southern California, we have demonstrated significant improvements to the charging efficiency and discharge rate capability of rechargeable iron electrodes. 3,4,10 These high-performance electrodes consist of carbonyl iron as the active battery material and also additives to suppress the parasitic process of hydrogen evolution. As a result, a high charging efficiency of 96% and a high utilization of 0.3 Ah/g have been realized.In the present study, we focus on understanding the process of formation of carbonyl iron-based rechargeable electrodes in aqueous alkaline batteries. A newly-prepared iron electrode in a nickel-iron or iron-air battery undergoes formation during which the electrode is charged and discharged repeatedly for 20 to 50 cycles after which a stable discharge capacity is reached. 5,11-15 The formation of...

A High-Performance Sintered Iron Electrode for Rechargeable Alkaline Batteries to Enable Large-Scale Energy Storage

“…These observations are also consistent with the model for the formation of the iron electrode that we have proposed in previous studies on the pressed-plate type iron electrode. 43 We also examined the morphology of sintered iron electrode that had passivated in the absence of sulfide under the conditions of constant current discharge. While the overall electrode structure was still porous (Figure 10a), the compact morphology (Figure 10b) suggested no significant formation of the discharged phase.…”

“…17,43,44 In our previous studies, we have demonstrated the benefits of using bismuth sulfide and iron sulfide in achieving high discharge rates on iron electrodes. 17,44 In this study of the sintered iron electrode, the effects of soluble sodium sulfide additive and iron sulfide have been studied.…”

“…17 Our recent efforts with a "pressed-plate" type iron electrode have led to remarkable advances in charging efficiency and rate capability, achieved through electrode re-design and re-formulation. 17,[43][44][45] Such "pressed-plate" electrodes were prepared by hot-pressing of a mixture of the carbonyl iron particles, sulfide-containing additives, a pore-forming additive and a polymeric binder. However, greater mechanical robustness is projected for a "sintered-type" iron electrode when compared to other types of electrodes.…”

“…In our previous studies we have discussed the formation process on a pressed-plate type iron electrode and discussed methods to shorten the number of cycles. 43 While most of the findings also apply to the sintered iron electrode, we examine in this study the role of the passivation phenomenon on the rate of formation of a sintered-type iron electrode.…”

Controlling hydrogen evolution on iron electrodes

“…As a consequence, different electrode formulation additives have been utilised to achieved that aim, including bismuth [28], bismuth sulphide [26], carbonaceous materials [29e31], iron oxide [32,33], etc. Undoubtedly, the development of sulphur based iron electrode formulations is one of the most promising alternatives [25]. Moreover, the performance of the NiFe cells can also be further improved by optimising the electrolyte composition; in fact, different electrolyte additives such as wetting agents [34], long chain thiols [35], organic acids [36], among others have been investigated.…”