Monitoring the State of Charge of the Positive Electrolyte in a Vanadium Redox-Flow Battery with a Novel Amperometric Sensor

Abstract

Vanadium redox-flow batteries are a promising energy storage technology due to their safety, long-term stability, and independent adjustability of power and capacity. However, the vanadium crossover through the membrane causes a self-discharge, which results in a capacity shift towards one half cell. This leads to a gradual decrease in its efficiency over time. Capacity balancing methods for compensation of this effect require a reliable online state of charge (SoC) monitoring. Most common methods cannot provide exact values of the individual concentration of each species in both electrolytes. In particular, the state of the positive electrolyte cannot yet be precisely determined. In this work, an amperometric SoC monitoring is proposed as a new approach. First, the suitability of the principle is investigated with a rotating disc electrode (RDE). Then, a sensor based on a gas diffusion layer (GDL) is developed and tested in the positive electrolyte. The dependencies between oxidative current and V(IV)-concentration are examined as well as those between reduction current and V(V)-concentration. Using both relationships, a reliable measurement of all relevant concentrations is possible.

Microelectrode-Based Sensor for Measuring Operando Active Species Concentrations in Redox Flow Cells

“…[33][34][35] For example, amperometric measurements have been performed on gas diffusion electrodes to measure vanadium (IV) and vanadium (V) concentrations; but despite the robust methodology, the protocol requires specialized equipment to prepare electrodes. 36 Microelectrode voltammetry has also been demonstrated as a viable method for measuring redox species concentrations and assessing the decay of active materials ex situ in deterministically prepared electrolytes. 37 Microelectrodes have the distinct advantage of decoupling redox species concentrations, as their small active area enables steady-state voltammetric measurements at reasonably low scan rates (ca.…”

“…Most methods for monitoring SOC only measure the ratio of species concentrations in different oxidation states; however, the magnitudes of these concentrations are necessary to determine the SOH and elucidate sources of performance loss (e.g., crossover, species decay, self-discharge). While concentration measurements are often more challenging, electroanalytical techniques are well suited to quantify electroactive species in redox systems. − For example, amperometric measurements have been performed on gas diffusion electrodes to measure vanadium­(IV) and vanadium­(V) concentrations; but despite the robust methodology, the protocol requires specialized equipment to prepare electrodes . Microelectrode voltammetry has also been demonstrated as a viable method for characterizing electrochemical properties, , measuring redox species concentrations, and assessing the decay of active materials ex situ in deterministically prepared electrolytes .…”

Determination of Rate Constants and Reaction Orders of Vanadium‐Ion Kinetics on Carbon Fiber Electrodes.

“…The membrane material Nafion® 117 was soaked in 1 wt % sulfuric acid prior to use. The exact concentrations were determined via titration as described in our previous work [62] …”

Redox Flow Batteries: A Literature Review Oriented to Automatic Control

“…Spectrophotometric can be used to analyze the conductivity of the vanadium species contained in the tank, this technique is widely used in chemical studies to obtain the concentration of certain species dissolved in a solution [129]. Recently, the use of an amperiometric sensor [130] has been proposed. It estimates the vanadium species by measuring the current response at a fixed stack voltage.…”