The importance of non-ideality of protons and water in the open circuit potential modelling of redox flow battery aqueous electrolytes

The non-ideality of electrolytes is commonly neglected in redox flow battery (RFB) modelling work. The neglect of activity coefficients introduces a discrepancy of up to 100 mV between the experimental open circuit potential (OCP) and model predicted reversible Nernstian potential, which propagates into inaccurate state of charge estimation and misinterpretation of thermodynamic efficiency. Due to the complex chemistry and the lack of thermodynamic data, activity coefficients of individual species in highly non-ideal electrolytes are often difficult to determine. In this work, proton and water activity values were calculated using geochemical numerical codes implementing the Pitzer theory, a semi-empirical thermodynamic model that accounts for the non-ideal behavior of ions in electrolyte solutions. The completed Nernst equation with corrected activity values has been elaborated for OCP comparison. An in-operando approach was proposed to measure the proton and water activities of strongly acidic electrolytes. The cell OCP of Hydrogen-Vanadium RFB (H 2 -V RFB) and Hydrogen-Manganese RFB (H 2 -Mn RFB) was measured and calculated to assess the electrochemical significance of proton and water non-ideality to the RFB model. With the measured activity data, the error between the experimental and calculated OCP values was reduced from 88-100 mV to 17–50 mV. • Proton and water activity shape RFB electrochemistry and cell OCP response. • Proton activity in acidic RFB electrolytes is commonly underestimated by orders of magnitude. • In-operando pH-conductivity measurement tracks proton changes in RFB electrolytes. • Accurate proton and water activities halve OCP error in RFB models.