Mass transfer at carbon fibre electrodes

148

176

Engineering the electrochemical reactor of a redox flow battery (RFB) is critical to delivering sufficiently high power densities, as to achieve cost-effective, grid-scale energy storage. Cell-level resistive losses reduce RFB power density and originate from ohmic, kinetic, or mass transfer limitations. Mass transfer losses affect all RFBs and are controlled by the active species concentration, state-of-charge, electrode morphology, flow rate, electrolyte properties, and flow field design. The relationship among flow rate, flow field, and cell performance has been qualitatively investigated in prior experimental studies, but mass transfer coefficients are rarely systematically quantified. To this end, we develop a model describing one-dimensional porous electrode polarization, reducing the mathematical form to just two dimensionless parameters. We then engage a single electrolyte flow cell study, with a model iron chloride electrolyte, to experimentally measure cell polarization as a function of flow field and flow rate. The polarization model is then fit to the experimental data, extracting mass transfer coefficients for four flow fields, three active species concentrations, and five flow rates. The relationships among mass transfer coefficient, flow field, and electrolyte velocity inform engineering design choices for minimizing mass transfer resistance and offer mechanistic insight into transport phenomena in fibrous electrodes. Grid-scale energy storage has been identified as a key technology for improving sustainability in the electricity generation sector 1 by increasing the efficiency of the existing fossil fuel infrastructure, 2 alleviating intermittency of renewables (e.g., wind, solar), 3 and providing regulatory services.2 In particular, redox flow batteries (RFBs) have emerged as attractive devices for grid storage.1 In these rechargeable electrochemical cells, energy is stored and released by reducing or oxidizing electroactive species that are dissolved in liquid-phase electrolyte solutions. [4][5][6][7] The electrolytes are housed in large, inexpensive tanks and pumped through a power-converting electrochemical reactor. Within the reactor, a selective membrane, which permits transport of charge-balancing ions but blocks active species, separates two porous electrodes where the respective reduction and oxidation reactions take place. The decoupling of the electrochemical reactor and the energy storage tanks enables independently scalable power (reactor size) and energy capacity (tank size), as well as simplified manufacturing, easy maintenance, and improved safety.4-7 Designing the electrochemical reactor to deliver sufficiently high power is a critical consideration toward achieving the low battery costs required for economic viability and enabling the efficient delivery of various grid services. [8][9][10] Cell-level resistive losses in RFBs can originate from one of three areas: ohmic, charge transfer, or mass transport losses. Ohmic losses arise from the current collector, the porous electrode...

Section: 50mentioning

confidence: 99%

Quantifying Mass Transfer Rates in Redox Flow Batteries

Milshtein

Tenny

Barton

et al. 2017

148

176

“…18 Because the reactants at carbon fiber surfaces are exhausted at limiting currents, the mass conservation equation in the electrolyte flow direction is given as follow:…”

Section: Resultsmentioning

confidence: 99%

“…For carbon felt electrodes, although some attention has been paid to the dependence of mass transfer coefficient on the electrolyte velocity, [16][17][18][19] correlations between these two parameters are not consistent. Moreover, it is rarely verified whether the mass transfer correlation obtained by measuring limiting currents is applicable in the condition of supplying adequate reactants.…”

mentioning

confidence: 99%

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

You

Cheng

2017

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...

“…This parameter has been determined as a function of electrolyte velocity in the work of Schmal et al, 63 the results of that work are adopted here in the form of following equation.…”

Section: Mathematical Modelmentioning

confidence: 99%

In Situ Potential Distribution Measurement and Validated Model for All-Vanadium Redox Flow Battery

Gandomi

Aaron

Zawodzinski

et al. 2015

101

An in situ, local potential measurement technique was further developed and applied to all-vanadium redox flow batteries to determine the potential distribution within multilayer electrodes of the battery. Micro-scale potential probes enabled in situ measurement of local potential in electrode layers between the cell flow field and membrane. The local redox potentials were recorded for different operating conditions and states of charges. To further analyze the behavior of potential distribution in the through-plane direction, a mathematical model was developed and the species distribution as well as the flux density of any individual component was modeled in terms of contributions from convective, diffusive and electrophoretic fluxes at each operating condition. Good agreement was achieved between the mathematical model prediction and experimental data with maximum error of 8%. Both mathematical simulation and experimental data confirmed the distribution of potential in the through plane direction as a function of discharge current density, predicting the lowest potential in a region close to the flow plate. Redox flow batteries have many benefits including energy efficiency, capital cost, and life cycle costs compared with other gridscale energy storage technologies.1 Multiple chemistries have been developed for redox flow batteries such as iron-chromium, 2 allvanadium, 3 bromine-polysulfide 4 and zinc-bromine. 5 Among these many chemistries, the all-vanadium redox flow battery (VRFB) has been studied in great detail and is relatively mature with several fullsize systems in operation around the world. The VRFB employs the V (I I )/V (I I I ) redox couple at the negative side and the V (I V )/V (V ) redox couple at the positive side, in the form of V O 2+ and V O + 2 . The kinetics associated with reduction and oxidation of vanadium species are known to be very complex 6,7 In this paper, the following simplified set of global half-reactions are adopted for the negative and positive electrodes.One advantage of VRFBs is that in utilizing the same element, vanadium, as active species in both negative and positive electrolytes, crossover of active species does not foul the electrolyte, and results only in decreased coulombic efficiency. Storage capacity can be regained through electrolyte rebalancing. The VRFB also has a wide operating temperature range from −5 • C to 50• C 8 and fast response (around 350 μs) for start-up and switching between charging and discharging processes, using a sulfuric and hydrochloric acid mixture. 9Cost is still the major barrier to VRFB commercialization. Zhang et al.10 estimated that the cell stack is one of the highest cost components with 31% of capital cost from a base case VRFB. One way to decrease the stack cost is to increase the power density of the VRFB during charge-discharge cycles, allowing for smaller stacks for the same power.11 In order to achieve high power density, the cells should operate at high current density with decreased polarization losses (activation, ohmic an...