Open circuit voltage of vanadium redox flow batteries: Discrepancy between models and experiments

In a flow battery, the salient impact of the electrolyte velocity on the mass transfer coefficient in carbon felt electrodes is demonstrated and quantified. A lab-scale flow battery, fed with identical electrolyte solutions containing Fe 2+ /Fe 3+ as active substances in both the anode and the cathode, is used to realize stable tests free from side reactions in a broad range of current densities. The electrolyte velocities ranging from 2.5 to 15 mm s −1 are selected in this work, which are typical in flow through electrodes in most flow batteries. By measuring limiting currents at various flow rates, a correlation between the mass transfer coefficient and the velocity in dimensionless form is obtained as Sh = 1.68 Re 0.9 . Meanwhile, a 2-D numerical model incorporating this correlation and the experimentally measured electrolyte conductivity is proposed. Voltage losses of the battery fed with adequate reactants at different velocities are both experimentally measured and numerically simulated. The agreement between simulated results and experimental data verifies the applicability of this correlation under normal operating conditions below limiting currents. Owing to the exclusive advantage of decoupling power generation and energy storage, redox flow batteries (RFBs) have been considered as a critical candidate for large-scale electrical energy storage (EES). [1][2][3][4] Unlike the electrodes of conventional secondary batteries, the electrodes of RFBs do not participate in reactions but only provide electrochemical reaction sites for active ions and support electrons transfer. For numerous kinds of developed RFBs, the porous carbon felts, composed of randomly interlaced carbon fibers, are generally selected as electrodes. [5][6][7] Obtaining high voltage efficiency at high current densities is one of the major challenges in the commercialization of RFBs. Much efforts have been devoted to diminishing the activation loss by modifying the carbon fiber surface. [8][9][10][11][12] In addition, ohmic loss and mass transfer loss also lead to the reduction of the voltage efficiency. It is generally recognized that supplying high flow rate of electrolyte for the RFB is the most feasible method to minimizing mass transfer loss, but more pumping power is inevitably required for increasing flow rates. Tang et al. 6 and Ma et al. 7 carried out special research on optimizing the operating strategy of electrolyte flow rate for kilowatts class allvanadium flow battery (VRFB) systems, in which the significant impact of flow rate on cell voltage was detected during the whole charge/discharge process. In a carbon felt electrode, the mass transfer loss is caused by the species transport between the bulk solution in the pore and the carbon fiber surface, which is usually quantified by the mass transfer coefficient k m . 13 For cells operating under high current densities, the mass transfer loss also contributes a significant proportion to the voltage loss, especially when approaching the end of charge/discharge process. Considerin...

Section: Modelingmentioning

confidence: 99%

The Dependence of Mass Transfer Coefficient on the Electrolyte Velocity in Carbon Felt Electrodes: Determination and Validation

You

Cheng

2017

“…38 However, the inconsistency with thermodynamics of this equation has been stated by Pavelka et al, 42 who have introduced a OCP equation for VRFBs derived based on thermodynamic principles (Equation 5).…”

mentioning

confidence: 99%

“…14 In VRFBs, the impact of using the complete Nernst equation (Equation 4) to describe the OCP has been stated by Knehr and Kumbur 38 and used in different models.…”

mentioning

confidence: 99%

A Unit Cell Model of a Regenerative Hydrogen-Vanadium Fuel Cell

Pino-Muñoz¹,

Dewage²,

Yufit³

et al. 2017

In this study, a time dependent model for a regenerative hydrogen-vanadium fuel cell is introduced. This lumped isothermal model is based on mass conservation and electrochemical kinetics, and it simulates the cell working potential considering the major ohmic resistances, a complete Butler-Volmer kinetics for the cathode overpotential and a Tafel-Volmer kinetics near mass-transport free conditions for the anode overpotential. Comparison of model simulations against experimental data was performed by using a 25 cm 2 lab scale prototype operated in galvanostatic mode at different current density values (50 − 600 A m −2 ). A complete Nernst equation derived from thermodynamic principles was fitted to open circuit potential data, enabling a global activity coefficient to be estimated. The model prediction of the cell potential of one single charge-discharge cycle at a current density of 400 A m −2 was used to calibrate the model and a model validation was carried out against six additional data sets, which showed a reasonably good agreement between the model simulation of the cell potential and the experimental data with a Root Mean Square Error (RMSE) in the range of 0.3-6.1% and 1.3-8.8% for charge and discharge, respectively. The results for the evolution of species concentrations in the cathode and anode are presented for one data set. The proposed model permits study of the key factors that limit the performance of the system and is capable of converging to a meaningful solution relatively fast (s-min). Redox flow batteries are considered to be an exceptional candidate for grid-scale energy storage. One attractive feature is their capability to decouple power and energy.1-4 All-Vanadium Redox Flow Batteries (VRFBs) have been considered a promising system due to the limited impact of cross-contamination. However, they have faced challenges related to cost, scale-up and optimization. Current research is also focused on improvement of electrolyte stability for use over a wider temperature window and concentrations, development of electrode materials resistant to overcharge, and mitigation of membrane degradation.1,2 Cost dependency with regarding to vanadium can be mitigated through utilization of new systems that employ only half of the vanadium.1 Recently, a Regenerative Hydrogen-Vanadium Fuel Cell (RHVFC) based on an aqueous vanadium electrolyte V(V) and V(IV) and hydrogen has been introduced 5 and is illustrated schematically in Figure 1. This system contains a porous carbon layer for the positive electrode reaction, membrane and catalyzed porous carbon layer for the negative electrode reaction. Hydrogen evolution, which is an adverse reaction in VRFBs, is here the main anodic process. During discharge, V(V) is reduced to V(IV) and H 2 is oxidized, while the reverse process occurs during charge and H 2 is stored. The vanadium reaction takes place in the positive electrode (cathode), while the hydrogen reaction occurs in the catalyst layer (CL) of the negative electrode (anode). The redox reactions that occur ...

“…24 Kinetic expressions for carbon oxidation and oxygen and hydrogen evolution are excluded from the model. Our approach is to include only the main reactions and evaluate the tendencies to drive possible side reactions by examining the local overpotentials that develop to support the shunt currents.…”

mentioning

confidence: 99%

The Relationship between Shunt Currents and Edge Corrosion in Flow Batteries

Darling

Shiau

Weber

et al. 2017

Shunt currents occur in electrochemical reactors like flow batteries, electrolyzers, and fuel cells where many bipolar cells that are connected in series electrically contact a mobile electrolyte through one or more common fluid distribution manifolds. Shunt currents reduce energy efficiency, and can cause unwanted side reactions including corrosion and gas generation. Equivalent-circuit models have been widely used to examine shunt currents in multi-cell electrochemical reactors. However, a detailed investigation of the interesting electrochemical processes occurring at the edges of the active areas has not been presented. In this work, the generation of shunt currents and their tendency to drive corrosion at the edges of positive electrodes in the most positive cells in a reactor stack are investigated with a comprehensive numerical model. An analytical model based on the penetration of current into a semi-infinite electrode, that can be used in conjunction with traditional equivalent-circuit models to assess the tendency for shunt currents to drive corrosion, is developed and compared to the numerical model. The models provided here can be used to set requirements on maximum allowable port currents in order to achieve a particular durability goal. Shunt currents are an important source of inefficiency in electrochemical reactors like flow batteries, electrolyzers, and fuel cells where many bipolar cells are connected electrically in series and contact a mobile electrolyte through one or more common fluid distribution manifolds. Figure 1 shows a flow-battery stack with arrows added to illustrate how one of the two electrolyte streams enters through a single inlet manifold, distributes to the individual cells, and recombines and leaves through an exit manifold. The other electrolyte traces a similar path through the stack in a second set of inlet and exit manifolds. The fluid manifolds act as ionic short circuits between cells in the reactor stack. The shunt currents that flow through the manifolds continuously discharge the reactants and can drive parasitic reactions including corrosion that hastens battery failure and gas generation that represents a safety hazard. Shunt currents are a particularly acute concern in typical flow batteries because very conductive electrolytes circulate through the reactors. Thus, minimizing the deleterious effects of shunt currents is a primary concern of stack designers. This can be accomplished by making the liquid paths outside of the active area long and narrow to increase ionic resistance at the cost of increasing the pumping work associated with circulation. While shunt currents can be minimized in this way, they cannot be completely eliminated. Therefore, understanding how shunt currents emanate and terminate at the edges of the active areas is critical to the successful operation of bipolar electrochemical reactors. Figure 2 is a picture of a portion of the junction between the active area of a bipolar plate and its surrounding plastic frame. This picture was taken duri...