Improving all-phosphomolybdic acid redox flow battery performance via functional group encapsulation

초록

Phosphomolybdic acid (PMA), a representative Keggin-type polyoxometalate, is a promising active species for redox flow batteries (RFBs) due to its high energy density and multi-electron redox behavior. However, the PMARFB has been poorly studied and suffers from capacity degradation. This study addresses this limitation by investigating the distinct molecular-level effects of four organic additives, acetic acid, acetonitrile, methanol, and isopropanol, to improve the electrochemical performance and stability of the all-PMA RFB. Fourier transform infrared (FT-IR) and electrochemical impedance spectroscopy (EIS) analysis reveal that the electrolyte transport kinetics are dominated by a competitive balance between viscosity and the effective hydrodynamic radius. While polar additives (acetic acid and acetonitrile) hinder mass transport by trapping a thick hydration shell that inflates the hydrodynamic radius, amphiphilic isopropanol forms a surfactant encapsulation layer. This configuration induces localized dehydration, minimizing the cluster radius and lowering transport resistance. Consequently, adding isopropanol shows optimal performance, significantly improving both energy efficiency and capacity retention over 150 charge-discharge cycles. These results demonstrate that strategic solvation shell tuning via simple organic compounds can effectively mitigate the capacity degradation of all-PMA RFB, offering a new pathway for developing high-performance aqueous RFB systems.

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