• Energy Research

We highlight the status of, and propose future approaches for, rheological, electrochemical, and spectroscopic characterization of concentrated redoxmer electrolytes for energy storage, with an emphasis on nonaqueous redox flow batteries.

Abstract We characterize the performance of Pt/C-based electrodes under alkaline conditions using a microfluidic H2/O2 fuel cell as an analytical platform. Both anodes and cathodes were investigated as a function of electrode preparation procedures (i.e., hot pressing, acclimatization) and fuel cell operating parameters (i.e., electrolyte composition) via chronoamperometric and electrochemical impedance analyses. X-ray micro-computed tomography was employed to link electrode structure to performance. In addition, the flowing electrolyte stream is used to study the effects of carbonates on individual electrode and overall fuel cell performance. Our studies provide direct evidence that the performance of hydrogen-fueled room-temperature alkaline fuel cells (AFCs) is limited by transport processes to and from the anode primarily due to water formation. Furthermore, the presence of carbonate species in the electrolyte appears to impact only anode performance whereas cathode performance remains unchanged.

Increasing societal concern about carbon emissions and the concomitant emergence of inexpensive renewable resources provide growing impetus for the electrification of the chemical industry. Despite notable advances in the science and engineering of electrolytic processes, there are comparatively few engineering economic studies that outline the technical specifications needed to approach feasibility. Herein, an open‐source technoeconomic framework is introduced to quantify the economic potential of existing and conceptual electrolytic processes by connecting system price and performance goals with constituent materials property sets. To validate the outputs and demonstrate the versatility of this toolkit, three contemporary electrolyses of varying technology readiness levels are explored. Specifically, the model results are benchmarked against the Department of Energy hydrogen analysis model; the impact of mass transport and catalyst performance on the electrochemical reduction of carbon dioxide is evaluated; and a pathway to low‐cost electrolytic production of phenol from guaiacol is charted. As this model is based on generalized mass balances and electrochemical equations common to a number of electrochemical processes, it serves as an adaptable toolkit for researchers to evaluate new chemistries and reactor configurations.

The vanadium redox flow battery (VRFB) is a promising energy storage technology for stationary applications (e.g., renewables integration) that offers a pathway to cost-effectiveness through independent scaling of power and energy as well as longevity. Many current research efforts are focused on improving battery performance through electrode modifications, but high-throughput, laboratory-scale testing can be time- and material-intensive. Advances in multiphysics-based numerical modeling and data-driven parameter identification afford a computational platform to expand the design space by rapidly screening a diverse array of electrode configurations. Herein, a 3D VRFB model is first developed and validated against experimental results. Subsequently, a new 2D model is composed, yielding a computationally-light simulation framework, which is used to span bounded values of the electrode thickness, porosity, volumetric specific surface area, fiber diameter, and kinetic rate constant across six cell polarization voltages. This generates a dataset of 7350 electrode property combinations for each cell voltage, which is used to evaluate the effect of these structural properties on the pressure drop and current density. These structure-performance relationships are further quantified using Kendall $\tau$ rank correlation coefficients to highlight the dependence of cell performance on bulk electrode morphology and to identify improved property sets. This statistical framework may serve as a general guideline for parameter identification for more advanced electrode designs and redox flow battery (RFB) stacks.

Abstract Redox flow batteries (RFBs) are an emerging technology suitable for grid electricity storage. The vanadium redox flow battery (VRFB) has been one of the most widely researched and commercialized RFB systems because of its ability to recover lost capacity via electrolyte rebalancing, a result of both the device configuration as well as the symmetry of the redox chemistry. Despite broad acknowledgement of the benefits of this differentiating feature to system resilience and longevity, assessments of its economic value to the VRFB system have thus far been limited. Here we develop a techno-economic framework that incorporates a physical model of capacity fade and recovery from rebalancing and other servicing methods into a levelized cost of storage (LCOS) metric. We then evaluate the impacts of different contributing factors to the LCOS of a VRFB and identify opportunities for cost reduction through operating strategies (e.g., rebalancing schedule), performance improvements (e.g., reducing fade rates), design decisions (e.g., battery sizing), and investment approaches (e.g., electrolyte leasing). We anticipate this analysis will provide new insights into the cost-drivers for VRFBs and motivate further research efforts in understudied yet important areas.

Abstract Reducing the cost of redox flow batteries (RFBs) is critical to achieving broad commercial deployment of large-scale energy storage systems. This can be addressed in a variety of ways, such as reducing component costs or improving electrode design. The aim of this work is to better understand the relationship between electrode microstructure and performance. Four different commercially available carbon electrodes were examined – two cloths and two papers (from AvCarb® and Freudenberg Performance Materials) – and a comprehensive study of the different pore-scale and mass-transport processes is presented to elucidate their effect on the overall cell performance. Electrochemical measurements were carried out in a non-aqueous organic flow-through RFB with these different electrodes, using two supporting solvents (propylene carbonate and acetonitrile) and at a variety of flow rates. Electrode samples were scanned using X-ray computed tomography, and a customised segmentation technique was employed to extract several microstructural parameters. A pore network model was used to calculate the pressure drops and permeabilities, which were found to be within 1.26 × 10−11 and 1.65 × 10−11 m2 for the papers and between 8.61 × 10−11 and 10.6 × 10−11 m2 for the cloths. A one-dimensional model was developed and fit to polarisation measurements to obtain mass-transfer coefficients, k m , which were found to be between 1.01 × 10−6 and 5.97 × 10−4 m s−1 with a subsequent discussion on Reynolds and Sherwood number correlations. This work suggests that, for these fibrous materials, permeability correlates best with electrochemical cell performance. Consequently, the carbon cloths with the highest permeability and highest mass-transfer coefficients, displayed better performances.

Energy storage is increasingly viewed as a valuable asset for electricity grids composed of high fractions of intermittent sources, like wind turbines, or unreliable transmission and distribution services. However, the likelihood that batteries will meet the stringent cost and durability requirements of grid applications is largely unquantified. In this work, we investigate conceptual and actual aqueous and nonaqueous flow batteries designed to store energy for hours and analyze the relationships among technological performance characteristics, component cost factors, and system price. The benefit of high voltage associated with the nonaqueous approach must be balanced against the burden of high solubility of active materials. Requirements in harmony with economically effective energy storage are derived for both systems to facilitate comparison.

The complex interplay between numerous parasitic processes—voltage losses, crossover, decay—challenges interpretation of cycling characteristics in redox flow batteries (RFBs). Mathematical models offer a means to predict cell performance prior to testing and to interpret experimentally measured cycling data, however most implementations require extensive domain knowledge and computational resources. To address these challenges, we previously developed a computationally inexpensive zero-dimensional modeling approach by deriving analytical solutions to species mass balances during cell cycling. Here, we expand on this framework by deriving closed-form expressions for key performance metrics and comparing the accuracy of these simplifications to the complete analytical model. The resulting closed-form model streamlines the computational structure and allows for spreadsheet modeling of cell cycling behavior, which we highlight by developing a simulation package in Microsoft® Excel®. We then apply this model to analyze previously published experimental data from our group and others, highlighting its utility in numerous diagnostic configurations—bulk electrolysis, compositionally unbalanced symmetric cell cycling, and full cell cycling. Given the accessibility of this modeling toolkit, it has potential to be a widely deployable tool for RFB research, aiding in data interpretation, performance prediction, and electrochemistry education.