Tin Hybrid Flow Batteries with Ultrahigh Areal Capacities Enabled by Double Gradients.

Tin-based hybrid flow batteries have demonstrated dendrite-free morphology and superior performance in terms of cycle life and energy density. However, the quick accumulation of electrodeposits near the electrode/membrane interface blocks the ion transport pathway during the charging of the battery, resulting to a very limited areal capacity (especially at high current density) that significantly hinders its deployment in long-duration storage applications. Herein, a conductivity-activity dual-gradient design is disclosed by electrically passivating the carbon felt near the membrane/electrode interface and chemically activating the carbon felt near the electrode/current collector interface. In consequence, the tin metals are preferentially plated at the region near electrode/current collector, preventing the ion transport pathway from being easily blocked. The resultant gradient electrode demonstrated an unprecedentedly high areal capacity of 268 mAh cm-2 at a current density of as high as 80 mA cm-2 . Numerical modeling and experimental characterizations show that the dual-gradient electrode differs from conventional electrodes with regard to their reaction current density distribution and electrodeposit distribution during charging. This work demonstrates a new design strategy of 3D electrodes for hybrid flow batteries to induce a desirable distribution of electrodeposits and achieve a high areal capacity at commercially relevant current densities.

Flexible Carbon Sponge Electrodes for All Vanadium Redox Flow Batteries.

In the present work, a flexible carbon sponge is experimentally characterized and proposed as an alternative electrode for advanced vanadium redox flow batteries. Such an electrode is prepared via directly carbonizing the commercially-available and inexpensive melamine formaldehyde resin sponge in argon, to inherit the well-defined and three-dimensional bi-continuous architecture of the melamine sponge with 99.6% porosity and 40 µm average pore size. By applying the carbon sponges as the electrodes, it is demonstrated that the vanadium flow battery at 200 mA cm-2 can yield an energy efficiency of 77.9%, significantly higher than that with commonly-used graphite felt electrodes (72.9%). After a thermal treatment in air, the energy efficiency of carbon sponge can further be improved to 81.2% at mA cm-2 due to introduction of favorable oxygen containing functionalities. The operating stability with the carbon sponge is proven by a 200 cycling test with minor efficiency decay.