Electrode-integrated bipolar plate structure for multi-cells in vanadium redox flow batteries

Enhancing Vanadium redox flow batteries performance through local compression ratio adjustment using stiffness gradient carbon felt electrodes

Renewables sources of energy like solar and wind are expected to supply ~35% of the total electricity demand by 2050.1 The intermittent nature of solar and wind necessitates the development of energy storage technologies. Redox flow batteries (RFBs) are a promising technology for storing energy over several hours to days; however, RFBs suffer from inefficiencies in part due to the overvoltages at the electrode surface. Vanadium redox flow batteries (VRFBs) are the most commercialized RFB chemistry till date due to the use of same element both sides of battery, eliminating the issue of cross-contamination. Carbon felts (CFs) are the most commonly used electrodes in RFBs due to their high conductivity and porous nature. More than 70 electrode treatments for VRFBs alone have been previously shown to reduce the overvoltages and improve performance of VRFBs. Some of these CF electrode treatments include thermal, acid/base, elemental doping, and loading with electrocatalysts.2–5 However, the differences in the operating conditions between treatment studies in literature makes the comparison among different treatments very difficult. Additionally, the complexity associated with each treatment makes identification of industrially implementable treatments extremely challenging.In this talk, I will discuss our efforts to tackle the above problem by comparing treatments under similar operating conditions and accounting for the treatment complexity. We compare the VRFB performance, stability, and economic feasibility with various CF treatments, and treatment complexity to identify the most promising industrially implementable treatments. We extract the round-trip energy efficiencies (EE) of VRFBs using CFs with (and without) various treatments from literature at laboratory and industrial scale. The performance is evaluated using the EE at various operating current densities and stability is identified based on the change in EE per cycle. Economic feasibility of a treatment is measured based on the affordable capital cost (ACC) obtained from an in-house techno-economic model. ACC is defined as the maximum capital cost that can be invested in a treatment without an increase in the overall capital cost of VRFB without treated electrodes. The treatment complexity is measured using the number of steps and the process units required in a treatment. Based on carefully chosen metrics for satisfying the performance, stability, economic feasibility, and treatment complexity criteria, we identify the high performing, stable, and economic feasible treatments that are easy to implement industrially. We conduct a cost analysis of these promising treatments by utilizing the similarities in production of carbon fibers and CFs. We encourage the RFB community to standardize of RFB testing procedures for fast industrial implementation of CF treatments as we move towards a society powered by renewable energy.

Insights into energy efficiency for vanadium redox flow battery (VRFB) using the artificial intelligence technique

Electrical, mechanical and morphological properties of compressed carbon felt electrodes in vanadium redox flow battery

The carbon/epoxy composite bipolar plate is a promising substitute for the conventional graphite bipolar plate for the vanadium redox flow battery due to its high mechanical property and productivity. The carbon/epoxy composite bipolar plate, a multifunctional structure, requires expanded graphite coating or additional surface treatments to decrease the interfacial contact resistance. However, expanded graphite coating has low durability under vanadium redox flow battery operating condition and surface treatments are costly to implement. In this work, excess resin-absorbing method is developed with polyester fabric to uniformly remove the resin-rich layer and expose carbon fibers on the surface of the carbon/epoxy composite bipolar plate. This method not only decreases the interfacial contact resistance by exposing carbon fibers but also forms a unique ditch pattern which is desirable to fix carbon felt electrode in place. Durability against acidic environment, mechanical property, and gas permeability of the composite bipolar plate manufactured by excess resin-absorbing method is investigated.

The Redox Flow Battery (RFB) systems are unique chemically, mechanically, and electrically compared to other kinds of batteries. Among RFBs, the Vanadium Redox Flow Batteries (VRFBs) are the most commercialized type. VRFBs are a suitable option for large-scale energy storage with exceptional advantages like the decoupled power and energy design (scalability), long life-time, safe battery chemistry (non-toxic, and non-flammable), etc. The power and energy designs in VRFBs are decoupled, known as the scalability benefit of the VRFBs. This implies more output energy from the battery is possible only by adding more electrolytes to the reservoir tanks.There are only a few research studies in the literature about the optimized operation of the Vanadium redox flow batteries. Most of these papers are not applicable to develop in real practice. The introduced optimized operation algorithms of VRFBs in this lecture can help the participants learn how to improve the performance of the battery and operate the battery more efficiently. Different objective functions are considered in this lecture to be optimized, e.g. minimizing the time duration of battery charging (for fast charging), energy loss, voltage loss, and capacity loss of the battery. The participants can find the results of this useful in optimized operation and implementation of the VRFBs.High charging current density results in faster charging and reduces the capacity fading in Vanadium Redox Flow Batteries (VRFB). On the other hand, it leads to the reduced energy efficiency of the battery. Also, the lower electrolyte flow rate in VRFBs results in less energy consumption by the pumps leading to the higher energy efficiency of the VRFBs. However, higher flow rates have the benefit of reducing voltage loss of VRFBs. To address these trade-offs, closed-loop charge control and flow management in VRFBs are necessary. In this lecture, a multi-objective optimization is proposed in first section to optimize the charging duration and flow management of the VRFB simultaneously during its charging. An innovative method is proposed for modeling pump consumption based on affinity laws for centrifugal pumps, which leads to new electrolyte flow management. Further, three case studies are defined in charging mode on a nine-cell VRFB unit laboratory prototype to validate the proposed optimization's performance, involving the duration of charging, flow management, and energy efficiency of the VRFB. The method is compared with previously published research studies on the optimal operation of VRFBs, which shows the uniqueness and consistency of the proposed optimization method for simultaneous controlling VRFB's charging duration and flow management.Capacity fade in Vanadium Redox Flow Batteries (VRFB) relies on the loss of electrolyte volume in each of charge and discharge cycles. The loss of volume in each cycle, also known as the bulk electrolyte osmosis, is due to Vanadium ions' diffusion from the membrane. The lower electrolyte flow rate in VRFB can reduce capacity fade as the electrolyte's velocity across the membrane decreases. However, the lower electrolyte flow increases the battery’s voltage loss. A new electrolyte flow management is introduced in second section of the lecture to address this trade-off, which considers the decrease of both capacity fade and voltage loss in VRFBs simultaneously. The proposed multi-objective flow management shows a significant reduction of both capacity and voltage losses in VRFBs.Moreover, typically complex electrochemical models and equations are needed to model capacity fade in VRFBs, which are not straightforward to model. The capacity fade modeling can lead to the estimation of available capacity and the battery’s State of Health (SoH). Therefore, a new mathematical model is proposed for the VRFB’s available capacity based on the electrochemical-based capacity fade model results. The model further is developed to estimate the State of Charge (SoC) and the SoH of VRFBs per cycle of charge and discharge.

An increasing number of studies focus on organic flow batteries (OFBs) as possible substitutes for the vanadium flow battery (VFB), featuring anthraquinone derivatives, such as anthraquinone-2,7-disulfonic acid (2,7-AQDS). VFBs have been postulated as a promising energy storage technology. However, the fluctuating cost of vanadium minerals and risky supply chains have hampered their implementation, while OFBs could be prepared from renewable raw materials. A critical component of flow batteries is the electrode material, which can determine the power density and energy efficiency. Yet, and in contrast to VFBs, studies on electrodes tailored for OFBs are scarce. Hence, in this work, we propose the modification of commercial carbon felts with reduced graphene oxide (rGO) and poly(ethylene glycol) for the 2,7-AQDS redox couple and to preliminarily assess its effects on the efficiency of a 2,7-AQDS/ferrocyanide flow battery. Results are compared to those of a VFB to evaluate if the benefits of the modification are transferable to OFBs. The modification of carbon felts with surface oxygen groups introduced by the presence of rGO enhanced both its hydrophilicity and surface area, favoring the catalytic activity toward VFB and OFB reactions. The results are promising, given the improved behavior of the modified electrodes. Parallels are established between the electrodes of both FB technologies.

Carbon felt supported carbon nanotubes catalysts composite electrode for vanadium redox flow battery application

Enhancing energy efficiency and long-term durability of vanadium redox flow battery with catalytically graphitized carbon fiber felts as electrodes by boron doping

All-vanadium redox flow batteries (VRFBs) have been identified as a promising technology for bridging the gap to integrating renewables onto grids as well as improving grid-level power generation efficiency. VRFBs are prime candidates for this role on account of their modularity, scalability, and relatively high performance. [1] There are still, however, many design parameters that can be more highly optmized to improve and specialize the aspects of this technology. Capacity fade is a parasitic loss that occurs in all electrochemical enery storage systems and is one of the parameters that can be most improved in VRFBs. Fading capacity is due to a host of variables in the system, such as membrane permeability and electroactive species concentration.[2] Degradation of cell components (electrodes and membranes) can also contribute to capacity decay in the form of increased overpotentials throughout the systems operating range. The kinetics at the negative electrode (V(II) ↔ V(III)) are generally understood to be more sluggish than at the positive electrode [3] as well as being less stable (i.e. degrading more rapidly with respect to time). Several high performance materials have been identified for the negative electrode [4] [5], however extended cycling data on the order of years is uncommon in the literature. Mitigation of this degradation on this time scale is one such optimization that can improve voltage efficiency and mitigate capacity fade. In this work we present our most recent experimental work in which the Faradaic current that can be supported by the plates is quantified on different sets of VRFB hardware and isolated from the current on the combined plates with GFD 2.5 mm carbon felt electrodes. For this work symmetric cells were utilized, to isolate crossover effects, in conjunction with electrochemical impedance spectroscopy and polarization curves to quantify the contributions from the different processes at each overpotential. Likewise, experiments were also performed on a symmetric cell where the plates served as the electrodes to quantify the parallel contribution. The data were assessed via statistical tests in order to determine significance. The results of this study show that, graphitic bipolar plates can support 35-50% of the current density on the negative electrode at low overpotentials and 20 and 30% at higher overpotentials. The open circuit impedance data also show that the ASR of the cell with felt electrodes is increasing at a rate of around 25 mΩ cm2 min−1 at this stage in the electrode’s life. This degradation is nonlinear and asymptotes to a stable ASR, however, this implies that the percentage measured in this context is smaller than the steady state percentage. These combined results suggests a material oriented approach to improving the bipolar plate material in order to achieve a more stable negative electrode.

Recent research has focused on vanadium redox flow batteries (VRFBs) to address the short lifetimes and fire risks associated with lithium battery systems. While VRFBs offer advantages in safety, they suffer from low energy density and efficiency compared with lithium batteries. To improve VRFB performance, studies are exploring improvements in materials such as anodes, cathodes, and separators and optimizing operations by controlling electrolyte flow rates. However, the impact of current magnitude on VRFB efficiency has been less studied, with few analyses addressing both current and flow rate effects. This research proposes an experimental procedure to evaluate charge/discharge efficiency, energy efficiency, and system efficiency across varying current magnitudes and electrolyte flow rates, using a 40 W VRFB stack composed of four 10 W cells in series. In addition, we introduce a design method for an electrical equivalent circuit model that simulates the VRFB stack, reflecting experimental findings. The model’s accuracy was validated by comparing it with data from 11 full charge/full discharge cycle tests, which varied current and electrolyte amounts.

Highly porous zeolitic-imidazole frameworks (ZIFs) are synthesized to produce N-doped mesoporous carbon electrocatalysts via calcination. The N-doped carbon (m-NC) and carbon nanotubes (m-NCNT) are obtained from ZIF-8 and ZIF-67, while the core-shell structure of ZIF-8@ZIF-67 produced with ZIF-8 seeds (m-NC@NCNT) is prepared by hydrothermal method. Chemical and optical evaluations of the catalysts are characterized using BET, FT-IR, XPS, XRD, Raman spectroscopy and SEM/STEM and they are used as the catalysts for redox reactions of vanadium ions and redox flow battery (VRFB) performance. In the utilization, m-NC@NCNT and m-NCNT are effective for improving VO2+/VO2+ redox reaction, although m-NC does not influence that. Even in VRFB tests using the catalysts, charge/discharge potential and energy efficiency (EE) of m-NC@NCNT and m-NCNT are highest, not to mention excellent EE resilience after undergoing tougher cycling condition. These results are due to the large graphitic-N portion of the two catalysts. Namely, electrons produced by the graphitic-N are delocalized, forming pi-conjugated system and vanadium–nitrogen transition state. This state then promotes electron transfer during VO2+/VO2+ redox reaction and VRFB performance.

The worldwide effort to decarbonize the electricity grid has made battery energy storage system (BESS) an essential technology. A promising technology for long duration storage on the grid is Vanadium Redox Flow Batteries (VRFBs), which can support the grid by regulating peak load, reducing intermittency of large-scale renewables, and as emergency power supply. There are two primary methods of integrating a BESS to the grid. Frist, the BESS is directly connected to the DC-link of a DC-AC converter that interfaces with the grid. Second, the BESS is connected to a DC-DC converter that in turn interfaces with the DC-link of a DC-AC converter. These converters all have a characteristic ripple voltage and current during operation. The magnitude and frequency of the ripples depend on topology and switching frequency of a converter, controller design and protocol, type of connection to the grid, voltage magnitude and load characteristics. Currently, studies on the impact of ripples have been limited to Lithium-ion and Lead-Acid batteries. The impact of ripple current and voltage has not been studied in VRFBs. In a VRFB, potential impact includes excessive gassing of hydrogen and carbon corrosion of the bipolar plate and porous electrode and cell heating. To better integrate and operate VRFBs to the electricity grid, the impact of ripples needs to be studied in detail.Using a novel experimental platform, where both magnitude and frequency of ripples will be incorporated, we will emulate the performance of different power electronic converter configurations. It will comprise standard 5cm2 VRFB cells with a carbon felt electrode and Nafion ion exchange membrane. The various converter ripples in an electricity grid setting will be emulated by superimposing the signals from a potentiostat (constant current) and a high frequency transformer (modulated ripples), which will be connected to the cathode of the cells.The experimental testbed will be used to cycle the VRFB cells at various states of charge with ripples that emulate the performance of power electronic converters during grid operations and disturbances. Varied parameters will include ripple amplitude, ripple frequency, electrolyte flow rate, applied current density and state of charge. Measurement data from monitoring voltage of each electrode, cell voltage and current, gas generation, electrolyte crossover and electrolyte loss will be analyzed to determine the impact of ripples on VRFB cells. Finally, the cells will be monitored for signs of gas generation and postmortem signs of carbon corrosion.Our analysis will determine the impact of various ripple magnitude and frequencies on VRFBs in a simulated grid integration environment. This work will facilitate the development of design standards for power electronic converters that are intended for battery applications, while also informing future efforts to optimize VRFB cell and stack performance.

With dwindling fossil resources and growing environmental concerns, clean and efficient energy conversion and storage technologies are getting more and more attention from the public. Lithium ion batteries, redox flow batteries as well as fuel cells hold the promise and burden to sustain our energy-demanding society. In recent years, several efforts have been addressed to vanadium redox flow batteries (VRFB) which show great promise for the economical storage of electrical energy intermittently available from wind and solar. The excess energy is stored in liquid electrolytes using the four oxidation states of Vanadium, V2+/3+ at the negative, V4+/5+at the positive side. The liquid electrolytes flow through porous carbon felt electrodes, which provide a heterogeneous reaction surface. The main reasons to use carbon felts are their corrosion resistance in acidic electrolyte, high overpotential towards the unwanted hydrogen evolution reaction (HER) and low material costs. In addition, their activity towards the positive and negative reaction, respectively, can be modified using thermal oxidation in air atmosphere, acid treatment, heteroatom doping, and others. With respect to stationary applications with projected system runtimes of 10˙000 hours and more, not only activity, but also durability of the electrode materials moves into the focus of current research. It has been reported [1] that the carbon felts undergo significant degradation when cycled for a certain number of times. However, already the electro-less contact with the highly acidic electrolytes seems to affect the felts properties. In this study, static electro-less ageing and dynamic degradation of heat-treated carbon felt electrodes were investigated by combining different electrochemical and analytical methods. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to monitor the performance loss of the carbon-based electrodes, whereas scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to study cause and effects of material’s degradation. In this respect, oxygen functional groups, electron conductivity, wettability, accessible surface area and other characteristics play an important role. In general, the HER is a parasitic reaction, which should be hindered in favor of the V3+ reduction. It has been reported that bismuth additions to the electrolyte enhance the activity of the felt for the negative reaction without enhancing the HER [2]. However, the competition between HER and V3+reduction can also be used as an indicator of the felt’s state of health when probed at different temperatures and in different electrolyte concentrations. We will discuss this for differently degraded felts. Furthermore, we use our battery tester with implemented reference electrodes to compare the individual half-cell potentials with full cell potentials and to study the effect of different cut-off voltages on the projected electrode degradation. Higher cut-off potentials might prove effective for the system’s operation strategy, if they do not lead to the expected increased degradation phenomena. [1] I. Derr, M. Bruns, J. Langner, A. Fetyan, J. Melke, C. Roth „Degradation of all-vanadium redox flow batteries (VRFB) investigated by electrochemical impedance and X-ray photoelectron spectroscopy: Part 2 electrochemical degradation, Journal of Power Sources 325 (2016) 351-359. [2] B. Li, M. Gu, Z. Nie, Y. Shao, Q. Luo, X. Wei, X. Li, J. Xiao, C. Wang, V. Sprenkle, W. Wang Nano Letters. 13 (2013) 1330-1335.

Coating developments towards enabling aluminum as a bipolar plate material for PEM fuel cells