Characterization of a Regenerative Hydrogen-Vanadium Fuel Cell Using an Experimentally Validated Unit Cell Model

Abstract

A hydrogen-vanadium electrochemical system was characterized using extensive experimental tests at different current densities and flow rates of vanadium electrolyte. The maximum peak power density achieved was 2840 W m −2 along with a limiting current density of over 4200 A m −2. The cycling performance presented a stable coulombic efficiency over 51 cycles with a mean value of 99.8%, while the voltage efficiency decreased slowly over time from a value of 90.3% to 87.0%. The capacity loss was of 5.6 A s per cycle, which could be related to crossover of ionic species and liquid water. A unit cell model, previously proposed by the authors, was modified to include the effect of species crossover and used to predict the cell potential. Reasonable agreement between the model simulations and the experimental charge-discharge data was observed, with Normalized Root-Mean-Square Errors (NRMSEs) within the range of 0.8-5.3% and 2.9-19.0% for charge and discharge, respectively. Also, a good degree of accuracy was observed in the simulated trend of the polarization and power density, with NRMSEs of 3.1% and 1.0%, and 1.1% and 1.9%, for the operation at a flow rate of vanadium electrolyte of 100 and 50 mL min −1 , respectively, while the voltage efficiency during the cycling test were estimated within a Root-Mean-Square Error (RMSE) of 1.9%. A study of the effect of the component properties on the cell potential was carried out by means of a model sensitivity analysis. The cell potential was sensitive to the cathodic transfer coefficient and the cathode porosity, which are directly related to the cathodic overpotential through the Butler-Volmer equation and the cathodic ohmic overpotential. It was recognized that a kinetic study for the cathodic reaction is needed to obtain more reliable kinetic parameters at practical vanadium concentrations, as well as reliable microstructural parameters of carbon electrodes.

Evaluation of a Non-Aqueous Vanadium Redox Flow Battery Using a Deep Eutectic Solvent and Graphene-Modified Carbon Electrodes via Electrophoretic Deposition

“…An in-house designed and assembled reactor having a 5 cm 2 active area was employed for RFB testing. The zero-gap flow-by RFB cell [5,31,32] is illustrated in Figure 2a along with a photo of the operational cell (Figure 2b). Graphite plates with a double serpentine flow-channel (CNC (Computer numerical control, and commonly) machined in-house) were employed for distributing the electrolyte across the active area and to electrically connect to the Cu current collector as for a previous study on aqueous-based VRFBs [31].…”

“…RFB cell charge and discharge cycles along with the anion exchange membrane were conducted galvanostatically at 10 mA cm −2 . The first charge was performed up to a 90% theoretical state of charge (SOC) followed by electrochemical impedance spectroscopy, as described elsewhere [5]. Then, the RFB was charged and discharged to about a 70% theoretical SOC (20-95%) unless the cell voltage approached the cut-off value.…”

“…Electrochemical impedance spectroscopy (EIS) was performed at the open-circuit voltage with an AC current root-mean-square value of 5 mA over a frequency range from 1 MHz to 100 mHz, and with 6 points per decade of frequency as reported elsewhere [5]. EIS measurements were conducted at the end of RFB charging as in a previous study [32].…”

“…The semi-depressed circle diameter is the lowest for the N-G-modified carbon paper (CP) electrode (Figure 9b). The EIS responses of pristine CP tested in IL, pristine CP tested in DES, and N-G-modified CP tested in DES have been fitted with an equivalent circuit in Figure 9c [5]. As the CP samples are porous in nature, the capacitor is replaced by a constant-phase element (CPE) in this equivalent circuit (may also be referred to as a modified Randles circuit).…”

“…The aqueous-based all-vanadium redox flow battery (VRFB) is one of the most investigated and commercially available grid-scale electrochemical energy storage devices for potential renewable energy applications [1][2][3][4][5][6][7]. Despite its myriad of advantages, which include the use of a single element in both half-cells, thereby eliminating cross-contamination problems [8], one of its drawbacks involves low energy and power densities that limit its potential commercial uptake in areas such as transportation or energy storage for home appliances [9].…”