Effect of Electrolyte Additives on the Water Transfer Behavior for Alkaline Zinc-Iron Flow Batteries.

Alkaline zinc-iron flow batteries (AZIFBs) are a very promising candidate for electrochemical energy storage. The electrolyte plays an important role in determining the energy density and reliability of a battery. The substantial water migration through a membrane during cycling is one of the critical issues that affect the reliability and performance of an AZIFB. In this work, it has been proven that the reason for water migration in AZIFBs is the synergetic effect of concentration gradient, different ionic strengths of negolyte and posolyte, and the electric field. To address the issue of water migration in AZIFBs, a series of additives are employed and the effects of additives on the water transfer behavior and electrochemical performance of AZIFBs are investigated in detail. The results indicate that all investigated additives can suppress water migration through a polybenzimidazole membrane because of the shrunken gap of osmotic pressure and ionic strength between negolyte and posolyte. Moreover, organic additives such as glucose can decrease battery performance because of the increased polarizability of the electrode, whereas inorganic additives such as Na2SO4 demonstrate no distinct effect on battery performance. Specifically, an AZIFB that employs Na2SO4 as an additive in the negative electrolyte can afford a Coulombic efficiency of ∼99% and a voltage efficiency of ∼88% for 120 cycles at 80 mA cm-2, together with a good effect for inhibiting water migration behavior. This paper presents an effective way to suppress water migration and increase the reliability of AZIFBs.

Toward a Low-Cost Alkaline Zinc-Iron Flow Battery with a Polybenzimidazole Custom Membrane for Stationary Energy Storage.

Alkaline zinc-iron flow battery is a promising technology for electrochemical energy storage. In this study, we present a high-performance alkaline zinc-iron flow battery in combination with a self-made, low-cost membrane with high mechanical stability and a 3D porous carbon felt electrode. The membrane could provide high hydroxyl ion conductivity while resisting zinc dendrites well owing to its high mechanical stability. The 3D porous carbon felt could serve as a guidance for the zinc stripping/plating, which can effectively suppress zinc dendrite/accumulation as well. Thus this battery demonstrates a coulombic efficiency of 99.5% and an energy efficiency of 82.8% at 160 mA cm?2, which is the highest value among recently reported flow battery systems. The battery can stably run for more than 500 cycles, showing very good stability. Most importantly, the practicability of this battery is confirmed by assembling a kilowatt cell stack with capital cost under $90/kWh.

Negatively charged nanoporous membrane for a dendrite-free alkaline zinc-based flow battery with long cycle life.

Alkaline zinc-based flow batteries are regarded to be among the best choices for electric energy storage. Nevertheless, application is challenged by the issue of zinc dendrite/accumulation. Here, we report a negatively charged nanoporous membrane for a dendrite-free alkaline zinc-based flow battery with long cycle life. Free of zinc dendrite/accumulation, stable performance is afforded for ∼240 cycles at current densities ranging from 80 to 160 mA cm-2 using the negatively charged nanoporous membrane. Furthermore, 8 h and 7 h plating/stripping processes at 40 mA cm-2 yield an average energy efficiency of 91.92% and an areal discharge capacity above 130 mAh cm-2. A peak power density of 1056 mW cm-2 is achieved at 1040 mA cm-2. This study may provide an effective way to address the issue of zinc dendrite/accumulation for zinc-based batteries and accelerate the advancement of these batteries.

A Low-Cost Neutral Zinc-Iron Flow Battery with High Energy Density for Stationary Energy Storage.

Flow batteries (FBs) are one of the most promising stationary energy-storage devices for storing renewable energy. However, commercial progress of FBs is limited by their high cost and low energy density. A neutral zinc-iron FB with very low cost and high energy density is presented. By using highly soluble FeCl2 /ZnBr2 species, a charge energy density of 56.30 Wh L-1 can be achieved. DFT calculations demonstrated that glycine can combine with iron to suppress hydrolysis and crossover of Fe3+ /Fe2+ . The results indicated that an energy efficiency of 86.66 % can be obtained at 40 mA cm-2 and the battery can run stably for more than 100 cycles. Furthermore, a low-cost porous membrane was employed to lower the capital cost to less than $ 50 per kWh, which was the lowest value that has ever been reported. Combining the features of low cost, high energy density and high energy efficiency, the neutral zinc-iron FB is a promising candidate for stationary energy-storage applications.