High Performance Flow Battery with Inexpensive Inorganic Reactants: DAYS Program (Final Technical Report)

Evaluating Levelized Cost of Storage (LCOS) Based on Price Arbitrage Operations: with Liquid Air Energy Storage (LAES) as an Example

Thermo-economic assessment of sub-ambient temperature pumped-thermal electricity storage integrated with external heat sources

The objective of the proposed project is to develop and demonstrate an ultra-stable reference electrode (RE) technology as an in-situ system diagnostics tool integrated in kW scaled commercial vanadium redox flow battery (VRFB) stacks for long-duration energy storage (LDES) applications and to advance its commercialization, with expected cell reliability enhancement in stack life increase of 6-10 % and levelized cost of storage (LCOS) reduction of 5-8 %. In this project, PNNL will partner with Stryten Energy LLC, with a total budget of $250K and 50% cost share for one year.

Faradaic redox reactions facilitating electron transfer between substances have been employed in energy devices to achieve high energy density. Many active materials capable of undergoing redox reactions can achieve higher energy density per unit volume or weight when directly immobilized onto solid substrates. However, systems such as redox flow batteries (RFBs) or dye-sensitized solar cells (DSSCs) utilize species with electrochemical activity dissolved in the electrolyte, which can limit the overall capacity of the device due to the solubility of active materials in the electrolyte. Similarly, dye-sensitized photo-rechargeable battery (DSPB) systems, which utilize dye-sensitized solar cell as a generator capable of producing electricity using light and storing it in the same device, also face the same challenge.DSPB is a new hybrid energy system that matches the redox potentials of the mediator within the electrolyte of DSSC and storage material, enabling high energy conversion efficiency. This system not only harnesses solar energy but also enables electricity generation using indoor lighting and energy storage, allowing for the free use of stored energy. Moreover, the energy conversion efficiency using indoor lighting exceeds 13 %, showing great potential for energy recycling. In this study, we induced a potential difference between liquid and solid redox active materials, both initially possessing the same redox potential, arising from their redox reactions, facilitating the spontaneous transfer of electrons between them. This additional movement of electrons not only addressed the solubility issues of the mediator within the electrolyte, thereby increasing the device's capacity, but also improved storage efficiency during photo-charging, ultimately enhancing the overall energy efficiency. In particular, we were able to significantly improve the efficiency under solar irradiation, which typically exhibits much lower energy conversion efficiency compared to indoor lighting and is largely influenced by the kinetics of the redox mediators.Through this strategy, the total capacity of DSPB increased by 143 % per unit volume, with an energy conversion efficiency improvement of 140 % compared to the control group under 1 SUN solar conditions, making it more efficient than any previously reported systems when utilizing DSSC as a power generation system for energy storage.

Optimal integration of solar collectors to Carnot battery system with regenerators

This review examines the technological progress, economic viability, and growth trajectories of energy storages systems (ESSs) integrated with advanced energy management systems (AEMSs) from 2000 to 2030. This study evaluated key technologies such as battery (BESS), mechanical (MESS), and thermal (TES) storage systems via the compound annual growth rate (CAGR), net present value (NPV), and levelized cost of storage (LCOS) through a structured analysis of the peer-reviewed literature, market data, and case studies. The results show that lithium-ion batteries dominate short-term storage at a CAGR of 43% (2011–2025), driven by cost reduction and high efficiency, whereas pumped hydro remains important for long-term demand despite geographical limitations. Flow batteries and heat storage demonstrate the growth potential of grid resilience and industrial decarbonization. An advanced AEM that leverages AI and machine learning can improve operational efficiency by 10–30% in hybrid systems. This study predicts that BESSs will account for 65% of new installations by 2030, highlighting the need for policy support, material recycling, and blending solutions. This work provides actionable insights to help stakeholders align their storage deployments with global decarbonization goals.

Thermo-economic analysis of the integrated system of thermal power plant and liquid air energy storage

Aqueous organic redox flow batteries (AORFBs) represent a promising energy storage technology for the integration of renewable energy and balancing electricity grids because of their technical merits of decoupled energy and power, sustainable and tunable redox active materials, and non-flammable and low cost aqueous supporting electrolytes. This presentation will review recent efforts in developing new flow battery chemistries, including viologen (anolyte), quinone (anolyte), TEMPO (catholyte), ferrocene (catholyte), and ferrocyanide (catholyte). The merits and drawbacks of representative acidic, pH neutral, and alkaline AORFBs will be discussed and compared. A number of research strategies will be discussed to develop high-performance redox active electrolytes to enable energy dense, durable, low-cost flow battery technologies. Particularly, this presentation emphasizes that in-depth mechanistic understandings of redox active electrolytes play crucial roles in developing advanced organic redox flow batteries towards sustainable, long-duration energy storage.

The concept of redox flow batteries (RFB) was originally patented in 1949 by Dr. Kangro with inorganic active materials like iron- or chromium-ions in mind.[1] Intensive research on inorganic active materials led to the proposal of vanadium based RFBs in 1985 by the Skyllas-Kazacos group, which is the most intensively studied RFB type currently.[2],[3] Organic active materials were introduced in 2009 for RFBs and offer a potential alternative to vanadium as active species, due to potentially lower costs and the replacement of rare metal ions.[4],[5] The class of verdazyl radicals has gathered interest as organic active material for the battery application since the publication of symmetrical coin cells.[6] Verdazyl radicals can be transformed into an oxidized or reduced species by a one electron reaction in organic electrolytes, which motivates the application in symmetrical RFBs.[7] Still, degradation of the verdazyl species during operation of the symmetrical battery in organic electrolytes as well as the theoretical voltage of ≈ 1 V show the need for further optimization of the material class and electrolytes.[6] While previous electrochemical investigations of verdazyl species were limited to organic electrolytes, we used a different approach and investigated the influence of acidic electrolytes on the redox chemistry of this material class.[8] Strongly acidic electrolytes lead to a disproportionation of the radical form. The resulting species differ by two electrons and two protons and show a reversible redox reaction in cyclovoltammetry measurements. This different reaction is of interest for the RFB application due to the fast rate constant as well as the utilization of two electrons.[8] Herein, we take the next step for the possible application of the redox chemistry of verdazyl species in acidic electrolytes into aqueous RFBs by analyzing the influence of the separating membranes. We use 1,3,5-triphenylverdazyl species as model compounds in 1 M sulfuric acid. To investigate the influence of the separating membranes in aqueous RFBs, we use Nafion 211 as commercial standard and compare it to self-made mPBI and O-PBI-based polybenzimidazole (PBI) membranes with a target thickness of ≈ 25 µm. When Nafion membranes are immersed in verdazyl cation solutions, they absorb ≈ 0.5 verdazyl cations per sulfonic acid group from the solution, whereas no absorption was observed for PBI membranes. UV/vis also did not show any changes in the UV/vis spectra of the solution, indicating that the verdazyl cations did not degrade. Beside the chemical stability, the permeability through the membranes is characterized, which is detrimental for the capacity retention in an RFB. To fully characterize the membranes with verdazyl species, the conductivity of the membranes is measured and the influence of the verdazyl species onto the conductivity is investigated. As a last step, symmetrical RFBs of verdazyl species in acidic electrolytes are built and the capacity retention is compared.With this study, we show the detrimental effect of using Nafion in combination with verdazyl species. Nafion 211 shows a high affinity to verdazyl cations and absorbs them from the electrolyte, leading to fast capacity fading of the RFB. PBI membranes show chemical stability as well as low absorption and permeability.Our study presents the one of the next steps towards the implementation of verdazyl species into water-based RFBs by investigating the suitability of PBI membranes for this application. PBI membranes offer overall good stability and beneficial properties for this application, while state-of-the-art Nafion is not applicable.

Recent progress in organic redox flow batteries: Active materials, electrolytes and membranes

With the exponential growth of electricity generation using intermittent renewable power sources, there is a need for affordable energy storage. For grid level storage in past years, redox flow batteries (RFB) have drawn considerable attention. Most successful RFBs till now are all-vanadium RFB and all-iron RFB. But these RFBs have a few drawbacks associated with them like; First RFB developed by NASA[1], Iron-chromium RFB, had problems of ions crossing over; all-iron RFB [2] have a problem of hydrogen evolution reaction (HER) at the negative electrode which causes capacity decay; and the commercialized all-vanadium RFB's [3] is expensive and has limited vanadium reserves.In an effort to address these problems, we present here the initial development of an all-tungsten RFB. Its key advantage is no capacity fading with cycles due to ions crossing over. The tungsten salt used was phosphotungstic acid (PTA). As shown in cyclic voltammograms (CV) of PTA in Figure 1(a), it is clear that PTA ions exist in four different oxidation states (PTA3-, PTA4-, PTA5- and PTA6-). For all-PTA redox flow battery experiments, we used the same PTA5- active species for both negative as well as positive electrolyte. Prior to the all-PTA RFB tests, PTA3- was reduced to PTA5-. Since PTA3- is the only salt available, it was reduced to PTA5- by using it as a negative electrolyte for the iron-tungsten RFB using a procedure developed earlier [4].As shown in reaction 1, during charging on the negative side, PTA5- gains 2 electrons and reduces to PTA6- oxidation state. Simultaneously, on the positive side PTA5- loses 2 electrons and gets oxidized to PTA3-. Moreover, the maximum potential difference that can be achieved by using the highest reduced form of phosphotungstic acid as a single cell is +0.611 V vs. RHE (Eq. 4). Although we are getting half the potential when compared to an all-vanadium or an all-iron battery. But due to its extremely fast kinetics and large discharge currents makes all-PTA RFB a promising alternative. For the large capacity of the battery, one of the deciding factors is the solubility of its active species salts and the maximum solubility of PTA3- is 0.7 M.Flow cell experiments were conducted using an in-house constructed standard fuel cell setup [4]. Meanwhile, CV experiments were conducted in 3 electrode setup using glassy carbon as working electrode, platinum mesh as counter electrode and mercury/mercurous sulphate (Hg/Hg2SO4) as reference electrode. Hg/Hg2SO4 was calibrated in 0.5 M H2SO4 using an in house RHE. Also, all the potentials mentioned here are with respect to RHE. A CV (see Figure 1(a)) for phosphotungstic acid was obtained which showed 3 distinct peaks corresponding to the 3 reactions. Reactions 2 and 3 involve transfer of 1 electron each, while reaction 1 requires 2 electrons as proven from the current peaks of the CVs as well as limiting currents obtained from their RDEs [5]. The involvement of parasitic HER is clear from the 3rd reduction peak. Kinetic parameters such as rate constant (k0), Tafel slope, diffusion coefficients (Do) and transfer coefficient (α) were determined using RDE studies. From RDE analysis at different RPM, rate constant for PTA3- was found to be 4.49 x 10-3 cm s-1.Meanwhile, for the flow cell experiments we have taken electrolyte solution concentrations similar to that for the CVs. The flow rate (120 mL min-1) and volume (40 mL) for both the electrolyte were kept same. As shown in Figure 1(b), the all-PTA battery was charged and discharged using chronopotentiometry at a current density of 50 mA cm-2. Furthermore, to obtain the i-V performance curve the cell was discharged from 2 mA to 1000 mA for 10s. The potential at the end of 10th second was reported to eliminate the pseudo steady state [6]. i-V and power density performance curve was plotted from these current and potential values (see Figure 1(c)).From just 40mM concentration of PTA5- the maximum volumetric capacity we got is around 200 mAh L-1, the maximum power density obtained was 10 mW cm-2 and the maximum current that can be drawn was 85 mA cm-2. The above results strongly suggest that the all-PTA RFB could indeed be a promising alternative. Figure 1

Integration of a steam accumulator with a biomass power-generation system for flexible energy storage and discharge: Effect of the initial steam pressure on the steam discharge profile and levelized cost of storage

Adoption readiness levels (ARLs) were developed by the Department of Energy’s Office of Technology Transitions (OTT) to holistically capture barriers to market adoption for a technology. The framework consists of 17 risk dimensions falling into 4 broad categories: Value Proposition, Market Acceptance, Resource Maturity, and License to Operate. OTT’s Commercial Adoption Readiness Assessment Tool (CARAT) can be used to evaluate a technology’s ARL. This work applies CARAT to redox flow batteries to evaluate the level of risk for this technology class across the 17 dimensions. Redox flow batteries were found to bear 1-2 high risk dimensions, 10-11 medium risk dimensions, 5 low risk dimensions, and scored an overall low readiness on the CARAT scoring scale (ranges reflect variation with flow battery chemistry). Herein, we describe the factors and evaluation across the dimensions leading to this score for redox flow batteries.

: Grid-scale renewable energy installations are gaining traction around the world. The addition of these intermittent resources onto the energy grids is resulting in challenges that need to be addressed [1]. The challenge with the mismatch between supply and demand has accelerated the need for grid-scale energy storage technologies. The viability of some of these technologies from a future technical and economic requirements standpoint is being studied widely [2].Though today over 95% of the energy stored is in the form of pumped hydro, many other technologies spanning the broad areas of Mechanical (flywheel, compressed air, etc.), Thermal (molten salt, thermal dams, etc.), Batteries (lithium-based, flow batteries, etc.) and Hydrogen (electrolysis, fuel cells, etc.) are actively under development with a focus on future requirements [1-5].In this paper, we will investigate the viability of Hydrogen electrolyzer and fuel cell technologies for long duty cycle and steep ramp frequency regulation applications. We will compare the characteristics of Polymer Electrolyte Membrane (PEM) and Solid Oxide (SO) Electrolyzer Fuel Cell (EFC) combinations with Flow Batteries. The techno-economic comparison will be in the areas of (a) Levelized Cost of Energy (LCOE) and (b) Transient Capability.The LCOE models will consider round trip efficiency, equipment capital costs, storage costs, operating costs, and system durability/reliability. The key input data/parameter set for the LCOE calculations will be obtained by extrapolating real-life performance information from key Electrolyzer, Fuel Cell, and Flow Battery suppliers [3-5]. Whereas the transient capability models will be based on applicable charge and discharge duty cycle regimes as well as the transient response to a full range of variation in loads. The transient modeling will be obtained by fitting real-life electrochemical data to simple lumped parameter models [2,5].

As increasing the importance of electrical grids and utilizing renewable energy sources, large-scale energy storage systems that can effectively store and supply electrical energy have attracted great research attention in recent times. Aqueous based redox flow batteries (RFBs) are promising candidates for large-scale energy storage due to their long cycle life, high safety, and high solubility of a catholyte and anolyte. Also, unlike other battery systems such as lithium-ion batteries, the energy density and power capabilities of RFBs can be decoupled, enabling flexibility for controlling capacity and power, and scalability as an industrial perspective. Iron-chromium (Fe-Cr) RFB is a good candidate for cost-effective systems comparing to all-vanadium RFB, which is well-known as the most developed RFB. However, sluggish Cr3+/2+ redox kinetics and hydrolysis of ferrous ion have become formidable obstacles to achieving high efficiency and stability. In particular, the poor kinetics of chromium ions make the hydrogen evolution reaction more dominant and cause the state of charge of the positive and negative side to being imbalanced.Herein, we have demonstrated a new RFB system based on hexacyanometalate materials ([M(CN)6]3-/4-, M = transition metal). The cyanide ligands linked through transition metal ions play a role in mitigating the hydrolysis in the aqueous environment and enhance the electrochemical kinetics. We have investigated the stability and electrochemical kinetic information of our redox couples comparing to conventional active materials. The superior kinetics of hexacyanometalate based-redox couples contributed to improving voltage efficiency. Besides, by use of a transition metal oxide type-catalyst on a carbon-felt current collector, this RFB system can achieve higher energy efficiency than that of the pristine carbon-felt electrode, suggesting that the employing catalyst can enhance the electrochemical kinetics of the active materials.