Study of novel redox flow batteries based on double-membrane, single-membrane, and membrane-less cell configurations

Redox flow batteries (RFBs) have been identified as one of the most suitable systems for large-scale energy storage to cope with intermittent energy generation (e.g., wind and solar). Current RFBs adopt a single ion-exchange membrane as separator, which can physically separate two electrolytes but ionically conduct them with commuting ions. This dissertation explored three possible routes that provide alternative configurations to current RFBs: 1) a double-membrane RFB that could combine redox pairs with different types of charge, and of different supporting pHs; 2) a single-membrane all-iron (all-Fe) flow battery that adopts the same elements on both sides, which is immune to the crossover of metal ions; 3) a membrane-less RFB that utilizes immiscible organic and inorganic electrolytes, which thermodynamically separate two redox species and eliminate the usage of membrane in RFB.;In the double-membrane RFB design, both cation-exchange membrane and anion-exchange membrane are incorporated in cell to isolate cation and anion redox pairs respectively. Three examples have been successfully demonstrated: Zn-Ce , S-Fe and Zn-Fe RFBs. Among these three RFBs, Zn-Fe RFB has the best balance between high voltage (2.0 V) and low electrolyte cost, bringing high performance and low capital cost. In the single-membrane all-Fe RFB, the same element, iron, is used in redox pairs in both positive and negative electrolytes with different coordination chemistries. The adoption of the same element fundamentally eliminates the cross-contamination in RFBs that uses two different elements. All-Fe RFB shows good durability and stability over cycle test. In the membrane-less RFB design, a new separation method of redox pairs is introduced by employing immiscible organic and inorganic electrolytes. Redox pairs are thus thermodynamically separated and require no membrane. A zinc-ferrocene RFB was demonstrated as an example for this membrane-less design. This concept broadens the method to construct flow battery and brings more possible combinations between organic and inorganic redox pairs in RFB application.;The new designs and concepts studied in this work successfully showed that the invention of new cell structure could greatly enrich and diversify the category of RFBs, expanding new redox chemistries and enabling new redox pair combinations for RFB.