Acid Pretreatment to Enhance Proton Transport of a Polysulfone‐Polyvinylpyrrolidone Membrane for Application in Vanadium Redox Flow Batteries

Abstract

An acid pretreatment strategy is developed to enhance the proton transport of polysulfone‐polyvinylpyrrolidone (PSF‐PVP) membranes for application in vanadium redox flow batteries (VRFB). The acid pretreatment leads to the formation of ionic conducting clusters with a size of around d=15.41 nm in the membrane (p‐PSF‐PVP). As a result, the proton conductivity and proton/vanadium ion selectivity of the p‐PSF‐PVP membrane increases to 6.60×10−2 S cm−1 and 10.63×107 S min cm−3, respectively, values significantly higher than 2.30×10−2 S cm−1 and 6.67×107 S min cm−3 of the pristine PSF‐PVP membrane. Moreover, a VRFB assembled with the p‐PSF‐PVP membrane exhibits a high coulombic efficiency of 98.6 % and an outstanding energy efficiency of 88.5 %. The results indicate that treatment with either sulfuric acid or phosphoric acid leads to an improvement of membrane properties, and the acid pretreatment is a promising strategy to significantly enhance the performance of the PSF‐PVP membrane for VRFB application.

The Structure–Activity Relationship in Membranes for Vanadium Redox Flow Batteries

“…[115] Generally, the proton transports along the hydrogen bonding network via Grotthuss mechanism and transfer along adjacent free sulfuric acids via vehicle mechanism in the nonionic dense membrane. have been widely used in VRFBs.…”

“…[119c] PSF serves as the matrix to provide the mechanical stability of membrane, and PVP is responsible for the supply of ions channels. [115] The acid pretreatment enlarged the free volume of membrane and thus increased the acid uptake, leading to the high proton conductivity. The assembled VRFB showed a high CE of 98% and VE of 91%, and remained stable for 2000 h during the charge-discharge test at 80 mA cm −2 .…”

“…The key to improve the ion selectivity of membranes in VRFB depends on the effective separation of vanadium ions from protons, while the proton conductivity depends on the fast transport of protons. [115] Copyright 2018, John Wiley & Sons. Compared to the dense membrane, porous PBI membrane demonstrate 1.9-fold higher proton conductivity due to the higher sulfuric acid absorption.…”

“…The nitrogen atoms in these polymers are easily protonated to NR 3 H + species by sulfuric acid in VRFB. This leads to the formation of hydrogen bonding network between the nitrogen or oxygen atoms in polymers and the sulfuric acids, which provides the ion transport sites . Generally, the proton transports along the hydrogen bonding network via Grotthuss mechanism and transfer along adjacent free sulfuric acids via vehicle mechanism in the nonionic dense membrane .…”

“…However, the proton conductivity of the dense PSF/PVP membrane was reduced due to tortuous pathway and the blocking effect of PVP. Further development showed that phosphoric acid (PA) pretreatment is effective to enhance the proton conductivity of PSF/PVP membrane . The acid pretreatment enlarged the free volume of membrane and thus increased the acid uptake, leading to the high proton conductivity.…”