The massive utilization of intermittent renewables especially wind and solar energy raises an urgent need to develop large-scale energy storage systems for reliable electricity supply and grid stabilization. The iron-chromium redox flow battery (ICRFB) is a promising technology for large-scale energy storage owing to the striking advantages including low material cost, easy scalability, intrinsic safety, fast response and site independence. However, its commercialization is still hindered by the poor charge-discharge performance, fast capacity decay and high capital cost. The primary objective of this thesis is to address these issues to achieve high performance.

We first conduct a performance-cost analysis of the ICRFB to comprehensively evaluate its competitiveness for large-...[ Read more ]

The massive utilization of intermittent renewables especially wind and solar energy raises an urgent need to develop large-scale energy storage systems for reliable electricity supply and grid stabilization. The iron-chromium redox flow battery (ICRFB) is a promising technology for large-scale energy storage owing to the striking advantages including low material cost, easy scalability, intrinsic safety, fast response and site independence. However, its commercialization is still hindered by the poor charge-discharge performance, fast capacity decay and high capital cost. The primary objective of this thesis is to address these issues to achieve high performance.

We first conduct a performance-cost analysis of the ICRFB to comprehensively evaluate its competitiveness for large-scale energy storage applications. It is found that a low operating current density of less than 100 mA cm^-2 leads to the high capital cost of the conventional ICRFB, which can be significantly reduced by increasing battery charge-discharge performance. The key limiting factor of the charge-discharge performance of the conventional flow-through structured ICRFB is identified to be the ohmic resistance that closely coupled with the flow resistance of electrodes. Based on this finding, we developed a flow-field structured ICRFB with both low ohmic resistance and flow resistance. It is shown that the present flow-field structured ICRFB reaches an increased operating current density of 200 mA cm^-2 with a round-trip energy efficiency of 79.6%.

In addition to the ohmic resistance, the kinetics at the negative electrode plays an important role on the ICRFB performance. We investigate the effects of flow field designs on catalyst distribution in the porous electrode during in-situ catalyst electrodeposition and battery performance. It is found that compared to the serpentine flow field (SFF) design, the interdigitated flow field (IFF) enhances species transport during the processes of both the catalyst electrodeposition and iron/chromium redox reactions, thus enabling a more uniform catalyst distribution and higher mass transport limitation. It is demonstrated that the operating current density of the IFF-based ICRFB reaches 320 mA cm^-2 with energy efficiency of 80.7%. To further understand and identify key design factors that influence the battery performance of ICRFBs, we investigate the effects of the membrane thickness, electrode compression ratio, electrode pretreatment and catalyst loading on the battery charge-discharge performance. Results show that with a thin NR-211 membrane and a high electrode compression ratio of 62.5%, the operating current density of the ICRFB can reach as high as 480 mA cm^-2 with an energy efficiency of 80.5%. Based on the optimized cell design, the peak power density of the ICRFB is up to 1077 mW cm^-2.

The cycle performance of the ICRFB is evaluated. The energy efficiency remains stable while the battery capacity decays with decay rates of 0.25-0.31% per cycle mainly due to hydrogen evolution. To mitigate the adverse impacts of hydrogen evolution on the capacity retention and ensure safe operation, an electrochemical rebalance cell that reduces the excessive Fe(III) ions at the positive electrolyte by using the low-concentrated hydrogen evolved from the negative electrode is designed, fabricated and tested. A continuous and stable rebalance process is demonstrated with a rebalance current density of up to 60 mA cm^-2 at a low hydrogen concentration of 2.5%.

To further achieve higher capacity retention, a novel redox flow battery (RFB) with the mixed-reactant Fe/Cd electrolyte is explored. The coulombic efficiency and energy efficiency of the Fe/Cd RFB reach 98.7% and 80.2% at 120 mA cm^-2, respectively, and remain stable with a capacity decay rate of 0.13% per cycle during the cycle test. The Fe/Cd RFB is estimated to have a low capital cost of $108 kWh^-1 for 8-hour energy storage. Inexpensive active materials, high cell performance and good capacity retention offer the Fe/Cd RFB great promise for large-scale energy storage.

Keywords: Iron-chromium redox flow battery; capital cost; cell structure design; flow field; mass transport; catalyst distribution; rebalance cell