Electrode For Redox Flow Battery Research Articles

Modeling Single and Two-Phase Transport in Thin Porous Layers Using a Composite Continuum-Pore Network Formulation

In this work, a composite continuum-pore network formulation is presented to model single and two-phase transport thin porous layers, such as gas diffusion layers in polymer electrolyte fuel cells (PEFCs) and active electrodes in redox flow batteries (RFBs). The formulation can be integrated into CFD codes, thus combining the ease of implementation of continuum-based modeling and the computational power of pore-network modeling. The composite model includes a control volume (CV) mesh at the layer scale, which embeds a cubic pore network [1,2]. The pore-network model is used to determine analytically local anisotropic effective transport properties (local effective diffusivity and permeability), which are mapped onto the CV mesh to simulate transport in the porous transport layer [3,4]. Good agreement is found between the predicted global effective transport properties (global effective diffusivity and permeability) under dry and wet conditions and previous experimental data reported in the literature for Toray TGP-H series carbon paper. Water saturation distributions are also compared with results obtained using X-ray computed tomography [4].[1] P.A. García-Salaberri, I.V. Zenyuk, J.T. Gostick, A.Z. Weber, Modeling Gas Diffusion Layer in Polymer Electrolyte Fuel Cells Using a Continuum-Based Pore-Network Formulation, ECS Trans. 97 (2020) 615.[2] P.A. García-Salaberri, Modeling diffusion and convection in thin porous transport layers using a composite continuum-network model: Application to gas diffusion layers in polymer electrolyte fuel cells, Int J. Heat Mass Transf. (2020), submitted.[3] P.A. García-Salaberri, J.T. Gostick, G. Hwang, A.Z. Weber, M. Vera, Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of local saturation and application to macroscopic continuum models, J. Power Sources 296 (2015) 440–453.[4] P.A. García-Salaberri, G. Hwang, M. Vera, A.Z. Weber, J.T. Gostick, Effective diffusivity in partially-saturated carbon-fiber gas diffusion layers: Effect of through-plane saturation distribution, Int. J. Heat Mass Transf. 86 (2015) 319–333.

A Pore Network Model of Carbon-Based Electrodes for Redox Flow Batteries

Electrodes constitute one of the main components of Redox Flow Batteries (RFBs) since they represent the sites where the electrochemical reaction occurs. The development of optimized electrode microstructures is therefore crucial in the design of the next generation RFBs.[1] For this purpose, there has been a surge in interest to understand the effect of the reactive and mass transport processes occurring within electrodes. Previous authors have introduced pore-scale models which perform direct numerical simulations of the multi-physics processes over reconstructed images of electrode microstructure; however, these models require high processing power and take significant time to converge.[2] In search of computationally inexpensive models the pore-scale detail of the microstructure, pore network models (PNMs) are considered a compelling alternative. [3]In this work, a computationally efficient PNM is developed to represent the multi-physics processes occurring within porous carbon electrodes at the pore-scale. The model solves the velocity field within the porous matrix and successively couples the mass transport equations (i.e. for convection, diffusion and migration) with the electrochemical reaction (i.e. Butler-Volmer equation). [4] The PNM algorithm is validated on a cubic framework to represent the cathode of a Vanadium Redox Flow Battery (VRFB) operating under discharge conditions at 400 A m-2. These results are compared against the reported values of half-cell potentials of the same system, showing a good fit [5] (Figure 1 (a)). The PNM is subsequently implemented on pore networks extracted from X-ray computed tomography (X-CT) images of two commercial porous carbon electrodes: Toray090 (Figure 1 (b)) and SGL29AA. In both cases, the results show non-uniformity in the concentration and pressure distributions across the electrode when considering the pure convective and diffusive transport processes. The existence of preferred electrolyte travel pathways through the interconnected pores and throats is simulated and visually shown. The migration and reactive processes are subsequently considered and are shown to be influenced by the rate at which the convective/diffusive flow permeates the electrode. These results demonstrate how the rate at which electrolyte is replenished is affected by the interconnectivity between pores: in both carbon electrodes, the regions with low interconnectivity induce slow replenishment, which leads to reactant. This process represents a pore-scale transport limitation that results in the localized reduction of current density upon discharge. Furthermore, the simulation was run for these two electrodes at two pressure drops () and show that, even at relatively high convective flow, the electrolyte follows preferred paths, leading to a non-uniform utilization of the electrode and the existence of “convection-limited ” where almost no electrolyte permeates, (Figure 1(c))To conclude, this PNM algorithm pore-scale transport processes within the electrode showing the effect of interpore connectivity on electrode utilization. The results suggest that an electrode with higher interpore connectivity would allow an even distribution of electrolyte, reducing transport limitations and reactant starvation. The simulations were performed on a standard single core workstation, and converged in 15 minutes, highlighting the benefits of using a computationally inexpensive pore-scale model. Chakrabarti, D. Nir, V. Yufit, F. Tariq, J. Rubio-Garcia, R. Maher, A. Kucernak, P. V. Aravind and N. P. Brandon, ChemElectroChem, 4, 194 (2017).Qiu, A.S. Joshi, C.R. Dennison, K.W. Knehr, E.C. Kumbur and Y. Sun, Electrochimica Acta, 64, 46 (2012).T. Gostick, M. A. Ioannidis, M. W. Fowler and M. D. Pritzker, Journal of Power Sources, 173(1), 277 (2007).Gayon Lombardo, B.A. Simon, O.O. Taiwo, S.J. Neethling and N.P. Brandon, Journal of Energy Storage, 24, 100736 (2019).You, H. Zhang, and J. Chen, Electrochimica Acta, 54(27), 6827 (2009). Figure 1

Combining Electrochemical, Fluid Dynamic, and Imaging Analyses to Understand the Effect of Electrode Microstructure and Electrolyte on Redox Flow Batteries

Understanding the interplay between electrode microstructure and cell performance of electrochemical devices is important both for modelling and experimental design. Redox Flow Batteries (RFBs) are an electrochemical energy storage technology with potential for grid-scale energy storage applications, although costs need to be further reduced to be competitive. One pathway to lowering the costs involves increasing the power density of each cell, such that fewer cells are required. This can be achieved in a variety of ways, including by improving the design of the electrode microstructures. The aim of this work is to better understand the relationship between electrode microstructure and RFB performance and, ultimately, to make inferences about electrode utility.The performances of a variety of commercially available carbon electrodes are examined via a series of commonly used microstructural and electrochemical analyses (Figure a-c). [1] We present a comprehensive study of pore-scale mass-transport processes occurring in each of the electrodes and rationalize their effect on the overall cell performance. A matrix of electrochemical tests were carried out in a flow-through RFB cell using incremental flow rates (Figure b-c) and two non-aqueous TEMPO electrolytes with distinct viscosity and diffusivity properties. Scanning electron microscopy (SEM) was used to image the electrodes and large 3 mm samples of each were scanned using X-ray computed tomography (Figure a). A customized segmentation technique was subsequently developed that resamples the image data to ensure the fiber dimensions agree with SEM images, improving the validity of the various extracted metrics. From these images, calculations and electrochemical tests, several microstructural parameters were extracted and a pore network model was used to calculate the permeabilities of the electrodes. A 1D model was developed across half the symmetric membrane electrode assembly and, using the parameters extracted from XCT, fit to galvanostatic polarization measurements to obtain the mass transfer coefficients at each unique operating condition – these were found to be in the range of and m s-1. Transport processes and distributions through the cell are analysed and correlations between dimensionless Reynolds and Sherwood numbers are subsequently discussed, as demonstrated in previous work. [2, 3]Given a ‘good performance’ is associated with a system being able to reach a high current with a low overpotential, the following findings were made. Firstly, while volume-specific surface area, porosity and tortuosity are useful descriptors for evaluating porous media, they are not good indicators of performance in these systems. Instead the permeability and, by extension, the ease with which convective transport can occur is found to be directly correlated with performance in this system. It is asserted that systems using solvents with higher viscosity have a reduced performance due, in part, due to regions in the electrode being starved of new electrolyte. Consistently, across all electrodes and in each electrolyte a better performance is observed when a higher flow rate is used. This effect is, in part, attributed to the reduction in size of the boundary layer at the electrode surface. Finally, in agreement with previous works, [1, 4] the carbon cloths tested in this work are shown to have higher mass-transfer coefficients and permeabilities than the carbon papers and we conclude that the best performing electrode is the thicker carbon cloth, which has the lowest resistance to convective flow, highest permeability, and highest mass-transfer coefficients in both solvents.[1]: A. Forner-Cuenca, E. E. Penn, A. M. Oliveira and F. R. Brushett, Exploring the Role of Electrode Microstructure on the Performance of Non-Aqueous Redox Flow Batteries. J. Electrochem. Soc., 166, 2230-2241 (2019).[2]: Y. A. Gandomi, D. S. Aaron, T. A. Zawodzinski and M. M. Mench, In Situ Potential Distribution Measurement and Validated Model for All-Vanadium Redox Flow Battery. J. Electrochem. Soc., 163, A5188-A5201 (2016).[3]: K. M. Tenny, A. Forner-Cuenca, Y.-M. Chiang and F. R. Brushett, Comparing Physical and Electrochemical Properties of Different Weave Patterns for Carbon Cloth Electrodes in Redox Flow Batteries. J. Electrochem. En. Conv. Stor., 17, 041108 (2020).[4]: M. A. Sadeghi, M. Aganou, M. D. R. Kok, M. Aghighi, G. Merle, J. Barralet and J. Gostick, Exploring the Impact of Electrode Microstructure on Redox Flow Battery Performance Using a Multiphysics Pore Network Model. J. Electrochem. Soc., 166, A2121-A2130 (2019). Figure 1

Carbon Electrocatalysts for High Performance Bromine and Iodine Redox Flow Battery Electrodes

Redox flow battery (RFB) electrodes act as bi-directional electrocatalysts, improving the kinetics of the electrolyte redox reactions of interest without being consumed in the reaction itself. This property of flow battery electrodes allows for the power and energy densities of RFBs to be uncoupled and designed separately. The power density is largely dependent on the activity of the electrocatalyst, whereas the energy density is determined by the solubility and volume of the electroactive species in the external electrolyte tank. Due to the advantages of RFBs, they have been heavily studied and implemented for applications in large-scale energy storage and are often used in tandem with renewable energy stations. Zinc-bromine flow batteries have arguably become the most promising and common flow battery technology behind that of the all-vanadium RFB, but are held back by low power densities that result from the slower Br2/Br- half-cell kinetics compared to that of the Zn2+/Zn half-cell. Fabrication of low-cost, high activity bromine electrodes will help increase the viability of this technology. Analogous zinc-iodine flow batteries have also been proposed for identical applications, as the electrolyte is much less toxic and corrosive to common battery materials than bromine, and a high solubility of electroactive species due to the formation of the triiodide ion (I3 -) (up to ~12 M). In addition, some scientific research has shifted towards flow batteries constructed from heteropolyhalides (such as I2Br- or Br2Cl-) by combining different halogen electrolytes.We investigated a variety of different carbon electrocatalysts for low-cost and durable electrodes for the I2/I-, Br2/Br, and mixed heteropolyhalide electrodes in RFB technology. A variety of carbon blacks, graphenes, and phosphorus-doped graphitic carbons (PCx) were drop-casted onto a glassy carbon electrode substrate and their electrocatalytic performance was assessed via a variety of techniques including cyclic voltammetry, rotating disc studies, and impedance spectroscopy. Phosphorus-doped graphitic carbon (PCx) was synthesized via the reaction of phosphorus chloride and benzene in different volumes ratios to obtain samples with variable amounts of phosphorus content (PC, PC3, PC5, PC8), characterized by X-ray diffraction analysis and 31P NMR.The electrochemical performance of various carbon blacks and graphenes were observed to have a strong dependence on their surface area and porosity. The electrocatalytic activity of the phosphorus-doped catalysts was seen to exhibit a volcano plot relationship with the degree of phosphorus-doping, where PC5 and PC3 exhibited higher electrocatalytic behaviour towards the halogen/halide redox reactions than PC and PC8. These observations can be explained by P31 NMR indicating an increased amount of P-O bonds with increasing amounts of doping, with the oxygen containing functional group being catalytic. Excessive doping leads to faults in the lattice structure of the P-doped graphite. The surface area of the electrocatalyst material had the largest impact overall on the electrocatalytic performance of the electrode.

Quantifying Electron Transfer Kinetics on Porous Carbon Electrodes for Redox Flow Batteries

Despite the critical role of electron transfer kinetics in determining the energy efficiency and the power density of a redox flow battery (RFB), there has not been much progress quantifying the kinetics on commercial porous carbon electrodes, the most common RFB electrodes with no immediate planar-electrode counterparts for cyclic voltammetry (CV) analyses. Here, we tackle this challenge by encapsulating porous carbon electrodes in epoxy, whose cross-sections are used for CV analyses. These electrodes display behaviors of micro-electrodes with length scales characteristic of fibers in the original porous structures. We thereby characterize the kinetics of ferricyanide reduction and show that its rate constants on carbon paper and pre-treated carbon felt are at the order of 10−4 cm s−1, near the low end of values reported on glassy carbon. On pre-treated carbon paper, the rate is ten times slower, attributed to the low electrocatalytic activity of the new surface accessible only after the heat treatment. We corroborate these findings with symmetric RFB tests. The method developed here provides a much-needed assessment of kinetics on porous carbon electrodes free from the impacts of mass transports in the pores, enabling the identification of performance-determining factors of RFB electrodes.

Holey Aligned Electrospun Carbon Electrodes for High-Performance Redox Flow Batteries

Recently, the electrospinning method has been employed to fabricate fibrous carbon electrodes in redox flow batteries, due to the large specific surface area of the nano-scale electrospun carbon fibers[1-3]. However, the poor transport properties of the densely packed electrospun carbon electrodes cause a large flow resistance, and hence a large concentration overpotential, which limited the battery’s performances [4,5]. The vanadium redox flow battery (VRFB) is one of the most-studied redox flow battery systems. However, the state-of-art VRFB that employed electrospun carbon fibers as the electrode can only be operated a realtively small current density (~ 100 mA cm-2), which is mainly hindered by the poor transport properties of the electrode material. Hence, the geometric structures of the electrospun carbon materials should be tailored so that the battery with the electrospun carbon fibers can be operated at a much elevated current density with high energy efficiency. Currently, several strategies have been proposed to improve the transport properties of the electrospun material, which include increasing the fiber diameter and expanding the pore sizes. However, the enhancement in the transport properties is always at the cost of sacrificing the active surface area[6].Here we report an approach to effectively enhancing the transport of the electrolyte inside the porous media while ensuring a large specific surface area of the material. Aligned electrospun polymer fiber bundles are successfully produced by properly adjusting the electrospinning conditions. By applying self-etching methods, highly holey aligned fiber bundles were fabricated. In this structure, the aligned large fiber bundles provide the pathway for the electrolyte and the large surface area enabled by the holey fibers provides sufficient active sites for the redox reactions to take place. We compared the single-cell employed with the as-prepared highly holey aligned fibers on both sides with the battery assembled with conventional electrospun materials as well as thermal-treated commercial carbon felts. The results show that the novel structure can enable an energy efficiency of around 79.3% at the current density of 400 mA cm-2.Details of fabricating and characterization of the holey aligned electrospun electrodes and more data relating to the battery tests will be disclosed at the meeting. Acknowledgement The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. T23-601/17-R) References [1] A.D. Blasi, C. Busacca, O.D. Blasi, N. Briguglio, V. Antonucci, Journal of The Electrochemical Society. 165 (2018) A147-A1485.[2] C. Busacca, O. Di Blasi, N. Briguglio, M. Ferraro, V. Antonucci, A. Di Blasi, Electrochimica Acta. 230 (2017) 174-180.[3] A. Di Blasi, C. Busaccaa, O. Di Blasia, N. Briguglioa, G. Squadritoa, V. Antonuccia, Appl.Energy. 190 (2017) 165-171.[4] S. Liu, M. Kok, Y. Kim, J.L. Barton, F.R. Brushett, J. Gostick, Journal of The Electrochemical Society. 164 (2017) A203-A2048.[5] A. Fetyan, I. Derr, M.K. Kayarkatte, J. Langner, D. Bernsmeier, R. Kraehnert, C. Roth, ChemElectroChem. 2 (2015) 2055-2060.[6] C. Xu, X. Li, T. Liu, H. Zhang, RSC Adv. 7 (2017) 45932-45937.

Carbon Electrocatalysts for High Performance Bromine and Iodine Redox Flow Battery Electrodes

Redox flow battery (RFB) electrodes act as bi-functional electrocatalysts, improving the kinetics of the redox reactions of interest without being consumed in the reaction itself. This property of flow batteries allows them to be fully discharged without damaging the electrode, and also allows for the power and energy densities of RFBs to be uncoupled and designed separately. The power density is largely dependent on the activity of the electrocatalyst, whereas the energy density is determined by the solubility of the electroactive species in the external electrolyte tank. Due to the advantages of RFBs, they have been heavily studied and implemented for applications in large-scale energy storage and are often used in tandem with renewable energy sources. Zinc-bromine hybrid flow batteries have arguably become the most promising and common flow battery technology behind that of the all-vanadium RFB, but are held back by low power densities that result from the slower Br2/Br- half-cell kinetics compared to that of the Zn2+/Zn half-cell. Analogous zinc-iodine based flow batteries have recently become the subject of much scientific interest as well, as the electrolyte is less toxic and corrosive to common battery materials compared to bromine, and a high concentration of electroactive species can be achieved due to the formation of the triiodide ion (I3 -) (up to ~12 M). In addition, some scientific research has recently shifted towards flow batteries constructed from heteropolyhalides (such as I2Br-or Br2Cl-) by combining different halogen-based electrolytes. Flow batteries constructed with Br2/Br-and I2/I-half-cells have been able to achieve some of the highest energy densities reported for aqueous flow batteries to date, but improvements still need to be made to the electrode materials.We investigated a variety of different carbon electrocatalysts for fabrication of low-cost and durable electrodes for the I2/I-, Br2/Br-, and heteropolyhalide redox couples in RFB technology. A variety of carbon blacks and phosphorus-doped graphitic carbons (PCx) were drop-casted onto a glassy carbon electrode substrate and their electrocatalytic performance was assessed via a variety of techniques including cyclic voltammetry, rotating disc electrode studies, and electrochemical impedance spectroscopy. Phosphorus-doped graphitic carbon was synthesized via the reaction of phosphorus chloride and benzene in different volumes ratios to obtain samples with variable amounts of phosphorus content (PC, PC3, PC5, PC8), characterized by X-ray diffraction analysis and 31P NMR.The electrocatalytic activity of various carbon blacks had a strong dependence on their surface area and porosity. The electrocatalytic activity of the phosphorus-doped graphitic carbons exhibit a bell curve relationship with the degree of phosphorus-doping, where PC5 and PC3 exhibited higher electrocatalytic behaviour towards the halogen/halide redox reactions than PC and PC8. These observations can be explained by 31PNMR indicating an increased amount of P-O bonds with increasing amounts of doping, with the oxygen containing functional group being catalytic. Excessive doping leads to faults in the lattice structure of the P-doped graphite. The surface area of the electrocatalyst material had the largest impact overall on the electrocatalytic performance of the electrode.

Understanding the Impact of Compression on the Active Area of Carbon Felt Electrodes for Redox Flow Batteries

The active material in the electrolyte of redox flow batteries needs to be transported without hindrance to the electrode surface to undergo reaction. This is usually ensured by utilizing high surf...

Comparing Physical and Electrochemical Properties of Different Weave Patterns for Carbon Cloth Electrodes in Redox Flow Batteries

Abstract Redox flow batteries (RFBs) are an emerging electrochemical technology suitable for energy-intensive grid storage, but further cost reductions are needed for broad deployment. Overcoming cell performance limitations through improvements in the design and engineering of constituent components represent a promising pathway to lower system costs. Of particular relevance, but limited in study, are the porous carbon electrodes whose surface composition and microstructure impact multiple aspects of cell behavior. Here, we systematically investigate woven carbon cloth electrodes based on identical carbon fibers but arranged into different weave patterns (plain, 8-harness satin, 2 × 2 basket) of different thicknesses to identify structure–function relations and generalizable descriptors. We first evaluate the physical properties of the electrodes using a suite of analytical methods to quantify structural characteristics, accessible surface area, and permeability. We then study the electrochemical performance in a diagnostic flow cell configuration to elucidate resistive losses through polarization and impedance analysis and to estimate mass transfer coefficients through limiting current measurements. Finally, we combine these findings to develop power law relations between relevant dimensional and dimensionless quantities and to calculate extensive mass transfer coefficients. These studies reveal nuanced relationships between the physical morphology of the electrode and its electrochemical and hydraulic performance and suggest that the plain weave pattern offers the best combination of these attributes. More generally, this study provides physical data and experimental insights that support the development of purpose-built electrodes using a woven materials platform.

Balancing the specific surface area and mass diffusion property of electrospun carbon fibers to enhance the cell performance of vanadium redox flow battery

Electrospun carbon fibers are featured with abundant electroactive sites but large mass transport resistances as the electrodes for vanadium redox flow battery. To lower mass transport resistances while maintaining large specific areas, electrospun carbon fibers with different structural properties, including pore size and pore distribution, are prepared by varying precursor concentrations. Increasing the polyacrylonitrile concentration from 9 wt% to 18 wt% results in carbonized fibers with an average fiber diameter ranging from 0.28 μm to 1.82 μm. The median pore diameter, in the meantime, almost linearly increases from 1.32 μm to 9.05 μm while maintaining the porosity of higher than 82%. The subsequent electroactivity evaluation and full battery testing demonstrate that the mass transport of vanadium ions through the electrode with larger fiber diameters are significantly improved but not scarifying the electrochemical activity. It is shown that the flow battery with these electrodes obtains an energy efficiency of 79% and electrolyte utilization of 74% at 300 mA cm−2. Hence, all these results eliminate the concern of mass transport when applying electrospun carbon fibers as the electrodes for redox flow batteries and guide the future development of electrospun carbon fibers.

A combined SECM and electrochemical AFM approach to probe interfacial processes affecting molecular reactivity at redox flow battery electrodes

Understanding interfacial reaction mechanisms of redoxmers at redox flow battery model carbon electrodes using insightful electrochemical scanning probe techniques enables new strategies for high-performance energy storage.

Nanoscopic and Macro-Porous Carbon Nano-foam Electrodes with Improved Mass Transport for Vanadium Redox Flow Batteries

Although free-standing sheets of multiwalled carbon nanotubes (MWCNT) can provide interesting electrochemical and physical properties as electrodes for redox flow batteries, the full potential of this class of materials has not been accessible as of yet. The conventional fabrication methods produce sheets with micro-porous and meso-porous structures, which significantly resist mass transport of the electrolyte during high-current flow-cell operation. Herein, we developed a method to fabricate high performance macro-porous carbon nano-foam free standing sheets (Puffy Fibers, PF), by implementing a freeze-drying step into our low cost and scalable surface-engineered tape-casting (SETC) fabrication method, and we show the improvement in the performance attained as compared with a MWCNT sheet lacking any macro pores (Tape-cast, TC). We attribute the higher performance attained by our in-lab fabricated PF papers to the presence of macro pores which provided channels that acted as pathways for electrolytic transport within the bulk of the electrode. Moreover, we propose an electrolytic transport mechanism to relate ion diffusivity to different pore sizes to explain the different modes of charge transfer in the negative and the positive electrolytes. Overall, the PF papers had a high wettability, high porosity, and a large surface area, resulting in improved electrochemical and flow-cell performances.

Understanding the role of the porous electrode microstructure in redox flow battery performance using an experimentally validated 3D pore-scale lattice Boltzmann model

The porous structure of the electrodes in redox flow batteries (RFBs) plays a critical role in their performance. We develop a framework for understanding the coupled transport and reaction processes in electrodes by combining lattice Boltzmann modelling (LBM) with experimental measurement of electrochemical performance and X-ray computed tomography (CT). 3D pore-scale LBM simulations of a non-aqueous RFB are conducted on the detailed 3D microstructure of three different electrodes (Freudenberg paper, SGL paper and carbon cloth) obtained using X-ray CT. The flow of electrolyte and species within the porous structure as well as electrochemical reactions at the interface between the carbon fibers of the electrode and the liquid electrolyte are solved by a lattice Boltzmann approach. The simulated electrochemical performances are compared against the experimental measurements with excellent agreement, indicating the validity of the LBM simulations for predicting the RFB performance. Electrodes featuring one single dominant peak (i.e., Freudenberg paper and carbon cloth) show better electrochemical performance than the electrode with multiple dominant peaks over a wide pore size distribution (i.e., SGL paper), whilst the presence of a small fraction of large pores is beneficial for pressure drop. This framework is useful to design electrodes with optimal microstructures for RFB applications.

Lignin-derived electrospun freestanding carbons as alternative electrodes for redox flow batteries

Redox flow batteries represent a remarkable alternative for grid-scale energy storage. They commonly employ carbon felts or carbon papers, which suffer from low activity towards the redox reactions involved, leading to poor performance. Here we propose the use of electrospun freestanding carbon materials derived from lignin as alternative sustainable electrodes for all-vanadium flow batteries. The lignin-derived carbon electrospun mats exhibited a higher activity towards the VO2+/VO2+ reaction than commercial carbon papers when tested in a three-electrode electrochemical cell (or half-cell), which we attribute to the higher surface area and higher amount of oxygen functional groups at the surface. The electrospun carbon electrodes also showed performance comparable to commercial carbon papers, when tested in a full cell configuration. The modification of the surface chemistry with the addition of phosphorous produced different effect in both samples, which needs further investigation. This work demonstrates for the first time the application of sustainably produced electrospun lignin-derived carbon electrodes in a redox flow cell, with comparable performance to commercial materials and establishes the great potential of biomass-derived carbons in energy devices.

Theoretical Description of Carbon Felt Electrical Properties Affected by Compression

Electro-conductive carbon felt (CF) material is composed by bonding together different lengths of carbon filaments resulting in a porous structure with a significant internal surface that facilitates enhanced electrochemical reactions. Owing to its excellent electrical properties, CF is found in numerous electrochemical applications, such as electrodes in redox flow batteries, fuel cells, and electrochemical desalination apparatus. CF electro-conductivity mostly arises from the close contact between the surface of two electrodes and the long carbon fibers located between them. Electrical conductivity can be improved by a moderate pressing of the CF between conducting electrodes. There exist large amounts of experimental data regarding CF electro-conductivity. However, there is a lack of analytical theoretical models explaining the CF electrical characteristics and the effects of compression. Moreover, CF electrodes in electrochemical cells are immersed in different electrolytes that affect the interconnections of fibers and their contacts with electrodes, which in turn influence conductivity. In this paper, we investigated both the role of CF compression, as well as the impact of electrolyte characteristics on electro-conductivity. The article presents results of measurements, mathematical analysis of CF electrical properties, and a theoretical analytical explanation of the CF electrical conductivity which was done by a stochastic description of carbon filaments disposition inside a CF frame.

Towards Electrode Design for Redox Flow Batteries By Using a 3D Pore-Scale Lattice Boltzmann Model: Experimental Validation and the Role of Porous Electrode Structure

Redox flow batteries (RFBs) have emerged as a promising technology for large-scale storage of intermittent power generated from renewable energy sources due to its advantages such as scalability, decoupling of energy and power, and high energy efficiency. However, to make it technologically and economically viable, advancement is needed to improve the performance (i.e. power and energy density) and reduce cost. The porous structure of electrodes in RFBs plays a critical role on performance, but the exact role remains poorly understood. In this paper, we report our efforts in understanding the fundamental mechanisms in redox flow battery electrodes and electrode design using computational modelling, which is important for improving the performance. A three-dimensional (3D) pore scale lattice Boltzmann model has been developed at University of Surrey to simulate the transport mechanisms of gases, liquid electrolyte flow, species and charge in the porous electrodes of RFBs. An electrochemical model based on the Butler-Volmer equation is used to provide species and charge coupling at the interface of active electrode materials and electrolyte. We apply this model first to simulate a vanadium based RFB and demonstrate that this model is able to capture the multiphase flow phenomenon and predict the local concentration for different species, over-potential and current density profiles under charge/discharge conditions[1]. To prove the validity of the developed LBM model, the simulated electrochemical performance are compared with the experimental measurement, based on the same electrode structures[2]. Three electrode structures (SGL paper, Freudenberg paper, Carbon Cloth) are reconstructed from X-ray computed tomography (CT). These electrodes are used in an organic aqueous RFB based on TEMPO. Excellent agreement is achieved between the simulated and experimentally measured electrochemical performance, indicating the validity of our model[2]. The three electrodes show very different pore size distribution and specific surface area, which result in significant differences in battery performance. From the simulation result, the electrode based on Carbon Cloth shows better performance comparing with other two electrodes, which is consistent with the experimental results[2]. Moreover, the developed model is used to investigate the effect of wetting area in electrode on the performance of redox flow battery. It is found that the electrochemical performance is reduced with air bubbles trapped inside the electrode[1]. The above studies confirm that the developed 3D pore scale lattice Boltzmann model can be used as a design tool to optimize the electrode structure of redox flow batteries. Acknowledgement The authors gratefully acknowledge the financial support for this work by the UK Engineering and Physical Sciences Research Council (EPSRC) projects [grant number EP/K036548/2] and [grant number EP/R021554/1]; and the EU FP7 IPACTS [grant number 268696]; [1] Duo Zhang, Qiong Cai, Oluwadamilola O. Taiwo, Vladimir Yufit, Nigel P Brandon, Sai Gu (2018) The effect of wetting area in carbon paper electrode on the performance of vanadium redox flow batteries: A three-dimensional lattice Boltzmann study, Electrochimica Acta 283 pp. 1806-1819 [2] Duo Zhang, Qiong Cai, Oluwadamilola O. Taiwo, Vladimir Yufit, Nigel P Brandon, Sai Gu (2018) Understanding the role of porous electrodes in redox flow batteries by an experimentally validated 3D pore-scale lattice Boltzmann model, Journal of Power Sources, submitted.

Twin-cocoon-derived self-standing nitrogen-oxygen-rich monolithic carbon material as the cost-effective electrode for redox flow batteries

Developing cost-effective and environmental-friend electrode material is critical to accelerate the large-scale commercialization of the all-vanadium redox flow batteries. Herein, we propose and develop a twin-cocoon-derived self-standing carbon material by a green and safe preparation method. This facile degumming and carbonization process allows the twin-cocoon-derived monolithic carbon material to be decorated with nitrogen defects and oxygen functional groups. It has been demonstrated that this nitrogen-oxygen-rich monolithic carbon electrode material yields a promoted electrochemical activity for both V2+/V3+ and VO2+/VO2+ couples, causing a 50% decrease in redox potential difference and a 192% increase in diffusion slope as compared with the commercial carbon paper, thereby improving the electrochemical reversibility and enhancing the mass transfer kinetics. It is also found that the flow battery using nitrogen-oxygen-rich monolithic carbon electrode material shows 83% higher average discharge capacity and 20% higher energy efficiency than that with carbon paper at the current density of 100 mA cm−2, promising a high-performance and low-cost all-vanadium redox flow battery. This work opens a new way to developing monolithic carbon electrode material that possesses great potential applications in flow battery and other electrochemical energy conversion and storage systems.

Comparing velocities and pressures in redox flow batteries with interdigitated and serpentine channels

AbstractSerpentine channels adjacent to a thin, porous medium are a potentially attractive alternative to a conventional thick flow‐through electrode for redox flow batteries. The hydrodynamics of serpentine flow fields were investigated with computational fluid dynamics, a two‐dimensional model of the porous electrode based on Darcy's law, and a resistance network model at the scale of the active area. Predictions from the three models were used to map the available design space. The optimal electrode thickness, in terms of minimizing nonuniformity, was identified and compared to the result for an interdigitated flow field. Serpentine favors thicker electrodes and higher flows than interdigitated, in qualitative agreement with experimental findings. Furthermore, interdigitated designs deliver more uniform intraelectrode velocities and lower overall pressure drops than serpentine flow fields.

X‐ray Nano Computed Tomography of Electrospun Fibrous Mats as Flow Battery Electrodes

AbstractMany electrochemical energy storage and conversion devices employ porous media as electrodes, gas diffusion layers or separators. Recently, electrospinning has received significant attention as a way to generate nano‐fibers of polymers with controlled morphology and properties that, once carbonised, can act as conductive and porous media for electrochemical energy devices. The recent advances in X‐ray computed tomography have led the technique to be widely used in the characterisation of energy technologies and porous media as it offers a uniquely non‐destructive insight into the 3D microstructure of materials. Here we present electrospun fibrous mats with uncontrolled, controlled and aligned morphology for use as redox flow battery electrodes and, for the first time, obtain ultra‐high resolution nano‐tomographic X‐ray imaging of the materials using a lab source. The virtual 3D volumes enable extraction of parameters that would not be possible via other characterisation routes.

Modeling flow distribution and pressure drop in redox flow batteries

The combination of interdigitated flow fields (IDFF) with porous electrodes offers lower pressure drop and better performance than conventional flow‐through porous electrodes in redox flow batteries. Comprehensive three‐dimensional and simplified one‐dimensional + two‐dimensional models describing flow uniformity and pressure losses within flow through, parallel, and interdigitated flow fields were developed and used to demonstrate the benefits of IDFF. Analytical solutions for IDFF that compare favorably with computational fluid dynamics quantify the trade between pressure loss and velocity maldistribution both along the channels and within the electrode. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3746–3755, 2018