Cost-effective, large-scale stationary storage systems are crucial for grid stability and the integration of renewable energy sources into the utility market. Long-duration electricity storage (LDES) systems with multi-day or seasonal storage capabilities are particularly advantageous for enabling deeper penetration of low-cost wind and solar power. 1-6 While most commercial electricity storage deployments and research and development efforts have focused on systems with durations of around 10 h at rated power, there is a growing recognition of the need for longer-duration storage solutions. 7,8 Existing technologies like pumped-hydro storage (PHS) can provide storage for up to 10 h but are limited in their ability to leverage the full benefits of LDES. Although widely used, conventional lithium-ion batteries face challenges in scaling up to longer durations due to high costs and safety concerns associated with large-scale agglomerated systems. Redox flow batteries offer scalability and safety advantages, but their low energy density and efficiency have limited their application in LDES scenarios. The lack of viable LDES technologies in the utility market is a significant barrier to achieving resilient grid stability and maximizing the potential of renewable energy.
In recent years, we have been researching a new type of metal-air battery for LDES applications. The battery consists of a solid oxide-ion electrolyte and porous electrodes and operates based on oxide-ion chemistry. In this battery, the chemical energy of oxygen is transported in the form of O2- ions through the oxide ion conductor as an electrolyte. The reversible storage of this oxygen chemical energy occurs in an energy-dense Fe/FeOx bed integrated within the anode chamber of a reversible solid oxide cell (RSOC). 9-12 During the battery’s operation, the oxygen electrode (OE) is open to an unlimited oxygen source, typically air while the hydrogen electrode (HE) is enclosed in a low-cost Fe bed chamber. The RSOC alternates between fuel cell mode during discharge and electrolyzer mode during charge. In the fuel cell mode, the battery generates electricity as the oxygen is transferred via a gas phase H2/H2O shuttle and stored within the Fe bed through the Fe-O redox reaction. The unique feature of the SOIAB is its direct access to oxygen in the air, eliminating the need for oxygen storage in the OE and making it well-suited for LDES applications.
From our early work, we have identified two major problems limiting the overall performance of a SOIAB: 1) Fe-bed’s sluggish FeOx-to-Fe reduction kinetics; 13 2) RSOC’s high electrode overpotentials. 9 To address these issues, we have previously shown that the synthesis of nanostructured Fe-bed materials 11 and the addition of catalyst (e.g., Pd) nanoparticles can boost the FeOx-to-Fe reduction kinetics. 9 However, our effort to further improve RSOC’s electrochemical performance has been very limited until very recently we demonstrated that adding Ir into Fe-bed can significantly improve the kinetics of FeOx reduction. 14
To understand the slow kinetic issue, we have also previously measured the rate constant of the H2 reduction process of FeOx-ZrO2. 15,16 These kinetic data are very useful for our Multiphysics modeling effort. In addition, we have also demonstrated the benefits of replacing ZrO2 with a proton containing perovskite BaZr-0.4Ce0.4Y0.1Yb0.1O3 (BZC4YYb). 17,18 However, there is a lack of reduction kinetics datasets for Ir-catalyzed FeOx-ZrO2 and FeOx-BZCYYb systems. Therefore, the goal of this study is to acquire this set of kinetic data for comparison with the baseline FeOx-ZrO2 and future Multiphysics modeling of the battery. The obtained results also provide fundamental insights into the reduction mechanisms as well as engineering data for other large-scale iron-based redox chemical systems such as chemical looping hydrogen, and “green steel” production.
