Unit Cell Model of a Regenerative Hydrogen-Vanadium Fuel Cell
Authors: Pino Catalina , Hewa Dewage Harini , Yufit Vladimir , Brandon Nigel P. 
Published: February 2020

All-Vanadium Redox Flow Batteries (VFBs) have been considered a promising system for the integration of renewable energy-based sources in electricity grids, but also have faced challenges related to cost, scale-up and optimization1–3. Cost dependency with regarding to vanadium price can be mitigated through utilization of new redox systems that employ only half of the vanadium1. Recently, a Regenerative Hydrogen-Vanadium Fuel Cell (RHVFC) based on an aqueous vanadium electrolyte V(V)/V(IV) and hydrogen has been introduced4. Hydrogen evolution, which is an adverse reaction in VFBs, is the main anodic process in this cell. During discharging V(V) is reduced to V(IV) and hydrogen is oxidized, while the reverse process occurs in charging mode and hydrogen is stored. The RHVFC could offer a better energy storage solution because of its fast hydrogen kinetics and absence of cross-mixing, even when the crossover of catholyte is possible, this could be collected at the anode side and pumped back to the catholyte tank4,5. Nonetheless, in order to assess key factors that limit the performance of the system a model is required that take into consideration various physical phenomena yet is capable to converge to a meaningful solution relatively fast.
In this work, a simplified unit cell model of a RHVFC is presented, which is based on mathematical phenomenological descriptions reported for VFBs6 and Polymer-Electrolyte-Membrane (PEM) Fuel Cells7. The proposed model was developed by coupling mass transport phenomena along with electrochemical processes, and is capable of predicting cell potential under different operating conditions. Comparison of simulations against experimental data was performed by means of a 25 cm2 lab scale prototype operated at galvanostatic mode with moderates values of current density, 5-10 mA cm-2, and catholyte and hydrogen flow rates of 100 mL/min. Validation of the model was performed against experimental data of the Open Circuit Potential (OCP) using a complete Nernst equation, and cell potential data considering the ohmic loss and activation overpotential. Correct estimation of the catholyte proton concentration was shown to be important in accurately determining the OCP. A good OCP fit was obtained by assuming the complete dissociation of sulphuric acid to bisulphate during the first step and further dissociation controlled by a dissociation rate constant and an overall activity coefficient term. The modelled cell potential showed a good agreement with the experimental data, and it was observed that the overpotential contribution of the cathode was more important than the anode at the tested operating conditions. This first model approximation allows simulation of the system with good accuracy, and provides a foundation to further development of RHVFC physical-based models. Future work will include testing under wider range of operating conditions and the corresponding model validation with mass transport limitations associated with diffusion in the porous media, vanadium crossover into the anode side and anode flooding. The contribution of these effects will be significant under high current densities operation.

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